WO2000052498A1 - Method and apparatus combining a tracking system and a wireless communication system - Google Patents

Method and apparatus combining a tracking system and a wireless communication system Download PDF

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Publication number
WO2000052498A1
WO2000052498A1 PCT/US2000/005493 US0005493W WO0052498A1 WO 2000052498 A1 WO2000052498 A1 WO 2000052498A1 US 0005493 W US0005493 W US 0005493W WO 0052498 A1 WO0052498 A1 WO 0052498A1
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WO
WIPO (PCT)
Prior art keywords
tag
signal
frequency
radio frequency
antenna
Prior art date
Application number
PCT/US2000/005493
Other languages
French (fr)
Inventor
James Goldinger
Colin Lanzl
Jay Werb
Original Assignee
Pinpoint Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Pinpoint Corporation filed Critical Pinpoint Corporation
Priority to EP00916010A priority Critical patent/EP1212635A1/en
Priority to AU37181/00A priority patent/AU3718100A/en
Publication of WO2000052498A1 publication Critical patent/WO2000052498A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/87—Combinations of radar systems, e.g. primary radar and secondary radar
    • G01S13/878—Combination of several spaced transmitters or receivers of known location for determining the position of a transponder or a reflector
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/74—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
    • G01S13/82—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein continuous-type signals are transmitted
    • G01S13/825—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein continuous-type signals are transmitted with exchange of information between interrogator and responder
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K7/00—Methods or arrangements for sensing record carriers, e.g. for reading patterns
    • G06K7/0008—General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer

Definitions

  • the invention relates to tracking systems and, more particularly, to systems designed to track articles and personnel.
  • Global positioning systems and local positioning systems are examples that may be used depending on the particular
  • the hardware may be used, for example, to collect data about the various entities being tracked or located.
  • these tags may be e usually low-powered, for example, emitting in the range of 1 - 10 mW.
  • read ranges up to about 100 meters can be realized. Alterntatively, in a more cluttered indoor environment, read ranges more like 25 meters is typical. As a practical
  • the system includes a radio
  • the antenna moduel is used in transmitting and receiving signals from the radio
  • the antenna module is also used in transmitting and receiving signals used in the wireless communication system.
  • the controller is coupled to the antenna module. The controller is used in communications with the radio frequency identification tag and the wireless communication system.
  • Signals are transmitted and received using an
  • antenna module from the radio frequency identification tag and from the wireless communication
  • N controller is coupled to the antenna module and used in communications with the radio frequency identification tag and the wireless communication system.
  • Figure 1 shows an overview of a positioning system
  • Figure 2 shows several cell controllers deployed in a multi-story building
  • FIG. 3 is a block diagram of a tag RF design
  • Figure 4 is a block diagram of an alternative embodiment of a tag
  • Figures 5N-5G are diagrams of a signal as it passes through various stages of the system
  • Figure 6 is a block diagram of the cell controller RF design
  • Figure 7 is a block diagram of a cell controller active antenna module
  • Figure 8 is a block diagram of a modulator RF design
  • Figure 9 is a block diagram of a cell controller cable extender module
  • Figure 10 is a block diagram of a cell controller
  • Figure 11 illustrates extraction of tag data from a series of correlations
  • Figures 12N-C are diagrams of tag datagrams
  • Figure 13 shows a tag incorporating a delay element
  • Figure 14 shows several cell controller receive chains operating in parallel
  • Figure 15N is a block diagram of an embodiment of the signal processing hardware of Figure 10.
  • Figure 15B is a flowchart depicting method steps of one embodiment of the Signal
  • Figure 16 is a block diagram of an embodiment of a signal filtering technique
  • Figure 17 is a block diagram of an embodiment of the transversal filter of Figure 16.
  • FIG 18 is a flowchart depicting method steps of one embodiment of the recursive-least squares (RLS) technique as used in a method step of Figure 15B;
  • RLS recursive-least squares
  • Figures 19A-19E are diagrams of sample waveforms in embodiments of the system of Figure 1 ;
  • Figure 20 is a flowchart depicting method steps of one embodiment of approximating a
  • Figure 21 is an example of an embodiment of a block diagram of a cell controller
  • Figure 22 is an example of an embodiment of a block diagram of a cell controller active
  • Figure 23 is an example of an embodiment of a block diagram of a tag
  • Figure 24 is an example of an embodiment of tag specifications
  • Figure 25 is an example of another embodiment of a position system
  • Figure 26 is an example of an architecture of a local positioning system (LPS) cell
  • Figure 27 is an example of an embodiment of a "fixed" portion of a digital enhanced cordless telecommunications (DECT) installation
  • Figure 28 is an example of an embodiment of a single base station supporting DECT and
  • Figure 29 is an example of the Second LF Carrier Recovery, Demodulator, and NGC
  • Figure 30 is an example of another embodiment of a system including integration of LPS
  • This system may be used, for example, to track and locate objects indoors. This system may be
  • radio-frequency identification system characterized as a radio-frequency identification system.
  • a radio-frequency identification system may also be
  • Radio Frequency Identification (RFLD) products typically have three components: (1) a tag (the item being identified), (2) an interrogator (a device which detects the presence of a RFID (RFLD) tag), (2) an interrogator (a device which detects the presence of a RFID (RFLD) tag, and (3) an interrogator (a device which detects the presence of
  • RFLD products are typically designed to detect
  • RFLD systems are usually deployed as high-end replacement technology for bar coding.
  • RFLD and related systems include passive RFLD systems, active RFLD systems, infrared LD
  • the tags in a passive RFLD system do not carry on-board power.
  • the interrogator in such a passive RFLD system do not carry on-board power.
  • Such systems transmits operating power for the tags. Such systems generally have a detection range of a meter or less, although somewhat longer ranges have been achieved. Typically, these
  • a coil in the tag is powered by the electromagnetic
  • tags used in other passive RFLD systems have a limited ability to accept
  • Passive RFLD tags have been employed in conjunction with access control, smart cards,
  • NBI vehicle identification
  • waste management item tracking
  • animal identification animal identification
  • manufacturing control materials handling
  • materials handling materials handling
  • Circuitry in the tag receives a carrier from the interrogator, translates
  • the signal to another frequency, and emits a response modulated onto that second frequency.
  • the tag modulates its antenna cross section to identify itself to
  • SAW Surface acoustic wave
  • interrogator's carrier after a delay.
  • the tag's identity is indicated by time variations in the delayed
  • Active RFLD systems require battery-powered tags.
  • the battery permits a longer
  • Active tags tend to enable multiple tags to be within range of an interrogator
  • Some active RFLD tags "chirp” (transmit) a signal spontaneously at
  • a tag's chirped signal is detected by the interrogator if the tag is in range of the interrogator.
  • Infrared systems While not RFLD systems, also endeavor to detect and identify
  • a typical IRLD system includes a tag that chirps its identity at randomized intervals. Infrared readers located in the ceiling detect these transmissions, and
  • the transmission rate from the tag to reader is typically about 600 baud.
  • Motion detectors in the tags enable the tags to transmit more frequently when in motion.
  • tags are typically about the size of dominos.
  • ENS systems are intended to deter theft in retail environments. ENS tags are fairly
  • ENS tags are not generally considered to be identification products, because ENS tags are uncoded and cannot be distinguished from one another.
  • an article tracking system 100 contains the following general
  • Inexpensive miniature radio frequency transponding tags lOla-c are attached to
  • Tags lOla-c "wake up” periodically, and "chirp” (transmit)
  • the tags 101 a-c are designed so that their range is 50- 100 meters in a typical indoor environment, the range mostly being limited by a need to conserve
  • Cell controllers 102a-c detect the chirps of tags 101 a-c and calculate
  • Each antenna module preferably has a transmit antenna and a receive
  • the antenna modules connected to cell controllers 102b and 102c are
  • a cell controller 102a is typically contained in a case and is mounted behind a hung ceiling.
  • the cell controller 102a can receive power from a conventional wall
  • the cell controller 102a is attached through coaxial cables 103a-d to the antenna modules 104a-d, respectively, which provide coverage of an area of the indoor
  • a tag signal 107, transmitted by a tag 101a, is received by one or more antenna modules 104a-d, and is processed by chips in the cell controller 102a, such as digital signal
  • DSP digital signal processing
  • Host Computer Cell controllers 102a-c are in data communication with a host computer
  • client workstations 120a-c communicate with
  • the client workstations 120a-c can access the SQL Server and presents the data in a way that
  • the tag 101a does not generate its own radio signal. Rather, an antenna module, for
  • example antenna module 104a continuously transmits a direct sequence spread spectrum
  • the interrogator signal 106 at a first frequency, for example 2440-megahertz.
  • the tag 101a receives
  • this signal 106 modulates its ULD code onto the signal 106, and immediately transmits back a
  • frequency-shifted signal 107 at, for example, 5800-megahertz to, for example, antenna 104a.
  • the distance from the antenna module 104a to the tag 101a can then be determined by the cell
  • the cell controller 102a can quickly switch among antenna modules 104a-d to obtain the distance from the tag 101a to each of antenna modules 104a-d
  • the system 100 is designed to be scaleable, allowing addition of cell controllers to
  • Figure 2 shows how a collection
  • cell controllers 102a-c can be deployed in the large multistory building 110. As shown in
  • multiple cell controllers 102a-c feed data to a single host computer 105, typically through a TCP/IP communications network.
  • a single host computer 105 typically through a TCP/IP communications network.
  • connection to the host can be accomplished via RS485, RS232, RS422, power line modem, or
  • Each of the cell controllers 102a-c can be installed to cover a separate floor 130a-c,
  • cell controller 102a On floor 130a, cell controller 102a, with its collection of antenna modules 104a-d, is installed in the
  • Antenna modules 104a-d are designed to provide good gain downwardly and horizontally, and
  • a ground plane is placed behind each antenna to reflect signals downward.
  • the horizontal location of the tag 101a with respect to an antenna module 104a can be determined more precisely by estimating the distance from the tag 101a to each antenna module 104a-d, based on the combined time of flight of the
  • each cell operates independently, enabling more cells to be added without affecting the performance of
  • wishing to triangulate upon the location of a tag 101a may install enough antenna modules such
  • the tag 101a will be in range of at least two or three of the antenna modules, depending on
  • antenna type may be based on a variety of
  • a tag RF circuitry 300 receives signal 106 at a tag receive antenna
  • the circuitry 300 is to transpond the incoming spread spectrum signal 106 by frequency translation.
  • the secondary function of the tag RF circuitry 300 is to modulate tag data onto the emitted tag
  • 107 includes, in a preferred embodiment of the invention, the serial number of the tag, datagram
  • tag data 309 such as that derived from a motion indicator or a low power indicator.
  • the incoming signal 106 is preferably a direct sequence spread spectrum signal, biphase or quadrature modulated from the cell controller 102a, in the 2440 megahertz band.
  • the tag receive antenna 301 which collects the signal 106 and feeds it into the
  • filter 302 ensures that the tag is receiving signals only in the 2440 megahertz ISM band, rejecting
  • the filter 302 is configured to filter the filter 302
  • the signal 106 is implemented as an etched coupled stripline filter embedded in the circuit board.
  • the signal 106 is then amplified by an amplifier 303 to ensure that the received signal can be mixed, in a
  • the frequency mixer 304 translates or shifts the carrier frequency from 2440 megahertz
  • the incoming signal with a center frequency of 2440 megahertz, is mixed
  • phase locked oscillator (PLO) 305 with a center frequency of 3340
  • the PLO 305 consists of a phase locked loop (PLL) chip with three inputs: (1) a sampled output from a voltage controlled oscillator (VCO); (2) a reference tone from a 10
  • VCO voltage controlled oscillator
  • the PLO 305 outputs a 1670-megahertz tone, which is then doubled to give the desired 3340-megahertz result.
  • the next element of the tag RF circuitry 300 is a biphase modulator 307 which, under
  • control of the microprocessor 308, can either pass the 5800-megahertz signal unaltered, or change the phase of the signal by 180 degrees.
  • the modulator 307 is implemented as a single
  • pole double throw RF switch 801 that feeds a 180 degree hybrid, as shown in Figure 8.
  • modulation forms of modulation can be used, including on-off keyed (OOK) modulation, binary phase-shift
  • BPSK BPSK modulation
  • MPK multiple phase-shift keyed
  • modulator 307 is fed into an amplifier 310, then is filtered by a transmitter bandpass filter 311,
  • the amplifier 310 operates at high frequency, it consumes significant power, and alternative embodiments (such as that shown in Figure 4) that make this amplifier 310 unnecessary are
  • the Tx Filter 31 implemented as a 5-pole filter, is necessary to ensure tag
  • the tag RF circuitry 300 shown in Figure 3 is intended to illustrate the general functions
  • Figure 4 shows an alternative embodiment 400 which fulfills the same basic functions as that shown in Figure 3, but with fewer components and using less power.
  • the essential difference between the circuitry 400 shown in Figure 4 and the circuitry 300 shown in Figure 3 is that the modulator 404 in Figure 4 is placed before the frequency mixer 406 in order to reduce the
  • the amplifier 310 is eliminated
  • a tag such as the tag 101 a, can transpond using backscatter, frequency translation by
  • timing is an important, if not critical, feature of the system, enabling the cell controllers 102a-c
  • PLO 407 and in the microprocessor 405 clock timing allows the cell controller 102a to accurately calibrate the source by measuring phase shifts in the received signal (as described hereinafter),
  • transmit antenna 409 and receive antenna 401 are combined into a single element, and which
  • tags 101 a-c are powered depends on the application. (Note that Figure 3 and 4 omit the tag power source.) Typically, a tag 101a will be battery powered, with
  • the RF stage turned on and off under control of the microprocessor 405.
  • the microprocessor 405 goes into a low power state, where it merely waits until it
  • all tag circuitry 400 cycles on and off under analog control, using an RC time constant in the circuit 400 as the timing
  • the cell controllers 102a-c and those cell controllers are sending pseudonoise with low cross
  • the tag 101a will correctly transpond both signals simultaneously.
  • Tags 101 a-c require a period of time, on the order of a millisecond, to charge up and
  • the tags 101 a-c will not be stable enough to use, but will nonetheless emit RF into the radio channel through the transmit antenna 409.
  • the tag RF circuitry 300, 400 shown in Figure 3 and 4 can be used in conjunction with
  • Spread spectrum operation is not required; two licensed narrow bands can be used. However, spread spectrum operation in the 2440 and 5800 megahertz bands is assumed for the remainder of the discussion.
  • the tag RF circuitry 300, 400 shown in Figures 3 and 4 use frequency division multiple
  • the tag circuitry 300, 400 receives and emits signals on different frequencies.
  • alternative embodiment 1500 uses time division multiple access, as shown in Figure 13.
  • tag circuitry 1500 shown in Figure 13 takes as an input at
  • a receive antenna 1501 a signal at one frequency, such as 915mHz, and emits the same signal
  • a cell controller such as cell controller 102a, transmits an interrogation signal 106 in bursts
  • a tag such as tag 101a, takes this signal as an input through the receive
  • delay element 1505 is then used to delay for a microsecond.
  • the signal then passes through a
  • N SAW device can be used as the time delay element 1505. During the period of the delay, the cell controller ceases
  • the delay-based tag is capable of modulating the response signal by a 180-degree
  • tag design 1500 shown in Figure 13 is similar to those shown
  • Figure 6 shows the radio stage of a cell controller 102a.
  • the architecture of an antenna module, such as the antenna module 104, is shown in Figure 7. Together, the cell controller 102a
  • Figure 10 shows the main components of the cell controller digital subsystem 650.
  • the digital subsystem 650 provides a baseband input signal 601, and some number of
  • microprocessor nanoseconds later receives a demodulated response 107 from a tag 102a.
  • 1001 can change the behavior of the radio system by (a) modifying the baseband
  • pseudonoise sequence code (c) modifying the transmit frequency 610 of radio transmitter 1002 and the receive frequency of radio receiver 1003 within a narrow range; (d) modifying the transmit gain of radio transmitter 1002 and the receive gain of radio receiver 1003; and (e) by
  • the demodulated response 107 from the tag 102a is split into I (Inphase) and Q
  • processing hardware 1004 for example a combination of DSP and FPGA components, reduces
  • each channel must be correlated twice, once with each sequence.
  • the correlated data from the signal processing hardware 1004 is processed by a microprocessor 1001, such as a Pentium processor.
  • Ethernet being preferred.
  • the data that is input to the transmit chain is a baseband input signal 601 which is a
  • pseudonoise spreading sequence The length of the sequence and the code encoded in the
  • a cell controller microprocessor 1001 sets a cell controller microprocessor 1001 and can be varied depending on signal processing requirements. Thirty-one or 127 bit sequences are typical, giving about 15 dB and
  • the 2440 megahertz and 5780 megahertz bands can support a 40 megahertz baseband input signal 601, and the cell controller 102a is designed to
  • Figures 5A-5G show an interrogation signal 106 as it passes through various stages of
  • Figure 5A shows a square wave baseband input to the
  • FIG. 5B shows this baseband input digitally correlated 510.
  • Figure 5C shows an output 520 from a-modulator 602, viewed through a spectrum analyzer centered at 2440
  • Figure 5D shows a spectrum analyzer view 530 of the tag signal 107, centered at
  • Figure 5E shows the demodulated response from tag 107, separated into its I
  • Figure 5G shows the negative of the second derivative of the correlated
  • the modulator 602 ( Figure 6) modulates the baseband input 601 onto a 2440 megahertz carrier.
  • Various forms of modulation are available and well-known to those skilled in the art.
  • the modulator 602 is implemented as a single pole double throw RF switch 801 that feeds a 180-degree hybrid combiner 803, as shown in Figure 8.
  • 602 is preferably implemented as a QPSK modulator, which duplicates the BPSK modulator with one channel offset by 90 degrees from the other, each channel driven by a different
  • Modulation by the modulator 602 results in sidelobes extending for hundreds
  • a SAW filter 607 that combines wide passbands with tight stopbands.
  • a wider passband supports a faster chipping rate in the baseband input signal
  • the modulator 602 is configured to avoid jammers and/or support advanced signal processing techniques.
  • the modulator 602 is configured to avoid jammers and/or support advanced signal processing techniques.
  • a preamplifier 606 is necessary prior to the SAW LF filter 607, and
  • the 10 megahertz source 603 needs to be
  • the output from the LF filter 607 (from amplifier 608) is then mixed by a mixer 609 with
  • phase locked oscillator (PLO) 611 and is converted to a carrier frequency of 2440 megahertz.
  • the frequency of the PLO 611 can be modified within a narrow range under
  • microprocessor control 610 in order to provide the frequency diversity needed to avoid jammers and/or for various advanced signal processing techniques.
  • a driver amplifier 612 which raises the power level of the
  • the RF bandpass filter 613 which buffers the output of the mixer 609 for a bandpass filter 613.
  • An attenuator 614 under microprocessor control 615 allows the signal processing
  • the signal is then fed into a diplexer 618, which combines the transmitted 106 and
  • the diplexer 618 is a highpass/lowpass filter
  • bandpass filters 613, 624, the specifications of the diplexer 618 are not very stringent.
  • the cell controller RF stage 600 shown in Figure 6 supports one remote antenna module 104a-d at a time. In order to support multiple antennas from the same cell controller, the system
  • the switch 619 needs a switch 619, which enables a microprocessor control 620 to rapidly switch from one antenna to the next.
  • the switch 619 takes RF and passes it into one of n cables, where n is, for
  • the switch 619 also provides DC power to the selected line.
  • the RF switch 619 also provides DC power to the selected line.
  • the DC power is coupled into the cables with RF chokes to provide RF isolation.
  • the DC power is coupled into the cables with RF chokes to provide RF isolation.
  • the rise time of the DC in an antenna is in the range of 20 microseconds, limited by the
  • an antenna is preloaded before the RF is switched.
  • the DC signal 712 is sent to Tx/Rx power
  • control logic 702 which, in the simplest embodiment is a filter to remove noise from the line and provide a clean 5 volt power source.
  • the RF output 710 from the bias tee 701 is fed into a
  • diplexer 715 which is identical to the diplexer 618 in the cell controller 102a. This is then
  • the transmit antenna 705 and receive antenna 706 are, in this embodiment, wall mounted
  • the 5780-megahertz response 107 from the tag 101a is filtered by a filter 707, amplified by an amplifier 708, and sent back down the cable 103a to the cell controller 102a.
  • the system is designed to use cables 103a-d of a standard length, for example, 20 meters.
  • a cable extender module 900 connects two lengths of cable and supports an extended cable
  • the elements of the module 900 use the DC power 910 from the
  • Bias tees 906, 907 separate the DC power 910 from the RF signals, and
  • diplexers 908, 909 operate to separate the transmit signal 106 from the receive signal 107.
  • controller 102a passes through the switch elements 621, 619 and diplexer 618 to the cell
  • controller receive RF chain 622.
  • the signal passes through a combination of a preamplifier 623
  • bandpass filter 624 the exact arrangement of which varies based on the parts selected.
  • the signal then enters an I-Q zero LF demodulator circuitry 627-633. As noted
  • microprocessor Rx frequency control 635 must be set in tandem with its
  • controller can send and receive from only one antenna at a given time. Improved performance can be achieved by selecting send and receive antennas independently of each other. Software in the cell controller determines which antenna module receives the best signal from the tag. For
  • the antenna 104 will receive a strong signal from the tag 101a.
  • the cell controller 102a can then
  • the design 1600 shown in Figure 14 provides multiple receive chains 1610a-1610n
  • Each of the receive chains 1610a-1610n includes an IQ demodulator, a digitizer, and a correlating element, shown as integer DSPs, for example, integer DSP 1620.
  • processing software to employ spatial processing techniques to isolate multipath effects. These techniques take advantage of the fact that the multipath-corrupted response will have different
  • a simple triangular correlation peak can be derived from a
  • Figure 11 shows how tag data is extracted from a series of correlations. In the left half of the chart 1110 shown in Figure 11, the tag is transmitting a "one.” This is
  • the tag flips the phase of the modulator by 180
  • 1120d is corrupted, and is best ignored.
  • the fifth and sixth correlation peaks 1120e-f cleanly reflect the 180-degree shift.
  • Pseudonoise sequences can be varied under microprocessor control at the cell controller.
  • the 10-megahertz sources in the cell controller 102a and the tag 101a will differ typically by a
  • This factor can be calibrated by noting the phase difference between contiguous
  • Each tag is a stand-alone unit that is unaware in any way of the outside world.
  • ULD Unique Identifying Code
  • the tag does not explicitly respond to an interrogation signal, but merely transponds
  • any incoming signal 106 in the 2440-megahertz band which may or may not include a
  • tags "wake up" and chi ⁇ their ULDs at randomized times, which can be calculated (by both the tag and the cell
  • controller based on a pseudorandom number generator which inco ⁇ orates the tag's ULD.
  • the tag For a tag which chi ⁇ s approximately every 5 seconds, the tag generates pseudorandom
  • analog inputs such as an input from an internal clock or the delay of
  • a typical pseudorandom number generator has the form:
  • the resulting N is used as the seed for the next pseudorandom number in the
  • pseudorandom number sequence When using a pseudorandom number generator of this kind,
  • Delay Xor(N, BitRotate (ULD, AND(N, 1111 2 )) Formula 3
  • N Xor(Delay, BitRotate(ULD, And(N,l 111 2 )).
  • one embodiment of the tag datagram 1400 contains a header
  • the header 1401 can be of zero length.
  • the identifier preamble 1402 can be implemented, for example, as a validity check such as a cyclic redundancy check (CRC). Given a sufficiently simple Delay function and high clock stability, the cell controller can infer the tag's chi ⁇ ing sequence by noting the timing of a series of chi ⁇ s of the
  • the tag adds
  • Delay information 1414 thus enabling the cell controller to forecast the transmission time of the tag's next and subsequent chi ⁇ s of the datagram 1410.
  • the tag will next chi ⁇ its datagram 1420, even if the cell controller does not have enough time
  • the cell controller can then anticipate this next chi ⁇ and ascertain the identity of the tag at that time. Once the tag is identified, the cell controller can duplicate the tag's pseudorandom number
  • 5 1423 must be defined so that it does not include the synchronization sequence or its inverse.
  • Figures 12a, 12b, and 12c include optional data sections 1404, 1415, 1426, which allow
  • These sections 1404, 1415, 1426 can include data
  • An identifier preamble related to the tag ULD, precedes the tag ULD. This identifier preamble enables the cell controller to quickly verify that a tag is chi ⁇ ing as expected, without
  • the identifier preamble
  • the ULD of a tag may be hardcoded into the tag (e.g., as a serial number).
  • Tags may be
  • Each cell controller contains information (received from another source) about
  • the cell controller can extract the ULD information from the tag signal to determine
  • 1414, 1425 and data fields 1404, 1415, 1426 can also include error correction bits.
  • data can be reduced to a stream of half-bytes.
  • the tag can look up the half-byte's value in a table which
  • a single cell controller can handle all three types of datagrams 1400,
  • the amount of time it takes for a cell controller to detect the presence of a tag may vary depending on the nature of the cell controller design. For example, a 100-microsecond time to
  • switch antennas may be significant when the cell controller is cycling among 16 antennas.
  • the tag datagram header must be long enough to give the cell controller time to try all
  • the cell controller can collect
  • tag datagram in order to be reasonably certain that it is receiving a signal from the correct tag.
  • a correct identifier preamble arriving exactly on schedule is almost certain to be from the expected tag. This provides an opportunity for the cell controller to try a variety of antennas that
  • the tag can be any suitable tag. If it is necessary to track tags between the tag datagram transmissions, the tag can be any suitable tag.
  • the cell controller can anticipate the exact timing of each such transmission, thus matching each
  • Error correction codes can be arranged such that long chi ⁇ s from one tag will not typically be corrupted by quick chi ⁇ s
  • the cell controller has the data to forecast most of such collisions.
  • the cell controller can select an antenna such that the signal the antenna receives from one tag
  • the cell controller has a means for sending information
  • Such instructions can include commands to be passed on to a device attached to the tag.
  • the cell controller is capable of downloading such information, most simply by on-off
  • a single cell controller can support read-only tags, read/write
  • Timing of transmission from tag to cell controller depends on the item being tagged. Inventory and equipment can be set to transmit relatively infrequently, such as once per minute.
  • motion detectors may be placed in a tag to decrease the time between
  • a tag might transmit more
  • a tag might inco ⁇ orate a slightly modified Electronic Article Surveillance (EAS) device, which
  • transmission interval can be modified under the control of that device.
  • Tag Power Tags 101 a-c transmit a low RF power level in order to increase their portability and
  • tag signal transmissions 107 are designed to be only a few milliseconds in
  • a motion detector can be inco ⁇ orated into the tag so that, for example,
  • battery replacement may be accomplished by inco ⁇ orating the
  • re-usable tag electronics may be attached to a disposable patient bracelet, with the battery included in the bracelet.
  • a battery may be inco ⁇ orated into the clip of an LD bracelet. More generally, a battery may be
  • the tag could tap into that
  • a tag signal 107 is received at a time that is the sum of (1) known fixed delays in the cell
  • controller 102a that transmitted the interrogator signal 106, due to its circuitry and the wiring to
  • the cell controller 102a can derive a simple triangular correlation peak from the received
  • Tag signal 107 As shown in Figure 5B. But in most indoor environments, the actual received tag signal looks more like that shown in Figures 5D-5G. Indoor radio signals are subject to substantial multipath effects, due to reflection from a variety of surfaces, such as whiteboards, fluorescent lights, file cabinets, elevator shafts, steel beams, and the like.
  • a 40-megahertz chip rate results in a correlation peak with a rise time of 25 nanoseconds
  • Approximate location of a tag can be calculated by noting when correlated signal-to-noise
  • the MUSIC algorithm known to those in this field, can be used, for which
  • MUSIC requires frequency diversity
  • motion detectors can be inco ⁇ orated into the tag, which would then
  • Story-by-story discrimination in a building can be accomplished by mounting antennas in the
  • antennas mounted sideways on horizontal partitions can determine location relative to those partitions. Relatively narrow beamwidth antennas, which are less sensitive to multipath
  • a cell controller antenna 104a can be mounted near a computer screen, with coverage
  • a single antenna module can include three separate antennas placed in a triangle. By comparing phase difference using the Inphase and Quadrature components of the returning signal, an indication of the tag's angle can be determined. In high frequency embodiments, such
  • such antennas could be within inches of each other and be quite effective.
  • Heuristic techniques can be used to analyze the correlation profile to estimate the time at which the correlation began, that is, the time at which the correlation peak begins to be
  • Frequency diversity can provide a variety of samples, the best of which can be chosen. Improved estimates can be achieved by pattern matching the
  • tags can be placed at known fixed locations, and tags passing near those
  • Antenna diversity is the most important tool for improving the accuracy of tag location
  • antennas can be placed so that only one antenna is in range of a given tag. In this case there is insufficient data for triangulation, and only enough
  • bearing of a tag can be estimated from the signal strength of antennas designed for the pu ⁇ ose
  • Antenna diversity also provides system scaleability. For facilities or sections within facilities, that do not require calculation of tag location with high accuracy, substantially omnidirectional and/or ceiling mounted antennas can be installed relatively far from each other, for a relatively low cost per square foot of coverage. For facilities, or sections within facilities,
  • a diversity of closely-spaced and/or directional antennas can provide high accuracy at an increased cost.
  • the tag's transmission interval varies according to an algorithm
  • the tag may for example, include an element, such as a
  • a photo LD with an inco ⁇ orated tag might be reprogrammed each time the person wearing the photo LD
  • multiple cell controllers can be installed, covering
  • each cell controller will be operating according to a
  • codes with known cross-correlation characteristics such as Gold Codes
  • cell controllers can switch choice of pseudonoise codes on a randomized
  • each cell controller reports the tag's
  • the central host 105 assembles this data to calculate the distances from its antenna modules.
  • the cell controller can choose another code.
  • the tag being in essence a transponder, does not need to know the
  • center frequency can be adjusted somewhat if another user
  • the signal processing hardware 1004 performs operations
  • the signal processing hardware 1004 generally functions to perform operations upon the signal from digitizer 636 such as, for example, the previously
  • signal processing hardware 1004 may perform a filtering
  • the signal processing hardware may include one or more hardware components that collectively perform digital signal processing of the received signals. Recall that the signal processing hardware 1004 was previously described in
  • a correlator unit 1800 connected to other DSP
  • DSP hardware components 1802 vary with the type of processing done in each particular
  • connections 1804a and 1804b may be controlled by the microprocessor 1001, such as by using connections 1804a and 1804b.
  • signal processing hardware 1004 uses only the correlator 1800.
  • data is directly output by the correlation unit, as indicated by the path 1804a.
  • Nt step 2000 a number of samples of received signals are taken. The number of samples to be taken varies with each
  • the sampling rate is typically twice the transmission rate. For example,
  • 254 points or received signals are sampled where each of the 254 signals is
  • the system has a chipping rate of 40 megahertz.
  • the signal transmitter rate is 127
  • a first range of the sample received signals is determined. The first range
  • One factor that may be considered is the anticipated arrival time of a signal returned by a tag. This relates to, for example, anticipated delays in the transmission circuitry and may be used in determining a starting point of the first
  • Another factor that may be considered relates to the tag transmission range and the
  • the first range as well as the size of the first range. For example, if objects in one system are
  • the earliest possible time which a signal may be received by an antenna of a cell controller is earlier than a time of a different tag in another
  • the actual span or size of the first range is 26 which was determined in accordance
  • Nt step 2004 auto-correlation is performed for the first range of samples producing a
  • One preferred embodiment implements the auto-correlation portion using
  • FPGN field programmable gate array
  • sample waveform 254 consecutive samples of received data (“sample waveform") are compared with an idealized version of the same data (“reference waveform"). If a coherent demodulation is available, then a real correlation may be
  • Autocorrelation may generally
  • An autocorrelation function is a plot of autocorrelation over all phase shifts (t-r) of the
  • this number of calculations may cause a "bottleneck"
  • a correlator implemented in hardware can generally make a quick estimate of tag
  • the reference waveform generally includes l's and -l's, 2's complement arithmetic
  • FPGA field programmable array
  • custom or custom ASICs enabling operations to take place in parallel and with generally high
  • the tag's radio has a maximum range of 100 meters, there is no reason to perform the autocorrelation function with phase shifts corresponding to the
  • half-chip intervals may be used to search for the signal, and then half-chip intervals may be used.
  • a subset of the 254 autocorrelation offsets may be used in the search for the peak or
  • heuristics are used to select a sample point which approximates where in
  • step 2006 produces a
  • One heuristic or technique which may be used to approximate the location in the first range of the received tag signal is related to the strength or magnitude of
  • corresponding signal may be used in approximating where the received tag signal may be
  • the rising or leading edge detection technique is a second heuristic that may be used to determine the rising or leading edge detection technique.
  • One technique used with the rising or leading edge detection may include using
  • the rising edge detection technique is based upon the assumption that the first peak is the line-of-
  • a threshold value is determined
  • the threshold value chosen varies with environmental and
  • Running trials of the system is one suggested method for choosing a threshold
  • a second range of samples is determined which is a subset of the first range
  • step 2008 the precise starting and end point as well as the size or span of the
  • ending points of the second range may be related to or dependent upon the heuristic used to
  • One preferred embodiment may use the rising edge detection technique
  • a range is determined having a
  • the second factor to be considered is the actual size of the second range. It
  • the size of the second range in one particular embodiment is 12. The reasons and factors that may be considered when choosing the size of the second range will be considered.
  • step 2010 the recursive least squares (RLS) technique using the second range of
  • the RLS technique used in step 2010 is used to filter out the noise component of received signals.
  • the RLS technique is used to filter out the
  • this vector contains values which correspond to filtered received signals.
  • the approximate peak co ⁇ esponding to the filtered received tag signal is determined using the values included in the vector. Detail is described in the paragraphs that
  • step 2016, the tag distance is determined using the time of the filtered received tag
  • One of the functions of the signal processing hardware 1004 is to filter the noise component out of a received signal.
  • Figure 16 depicts a feedback control system
  • transversal filter 2022 which includes a transversal filter 2022 and an adaptive weight control mechanism 2024.
  • the transversal filter 2022 operates upon an input signal u and represented as an input
  • the transversal filter produces an output
  • This estimation e ⁇ or (n) is the difference between the desired response signal d(n)
  • the adaptive weight control mechanism 2024 is a mechanism for performing the adaptive
  • control process by varying certain parameters which feed back into the transversal filter 2022.
  • the transversal filter 2022 may also be referred to as a tap delay line filter.
  • transversal filter such that, given the least squares estimate of the tap weight vector, w(n-l) of the filter at time n-1, the updated estimate of this vector at time n may be computed.
  • estimated signal E(n) 2026 is denoted as w H (n-1) u(n). The precise notation of this will be
  • transversal filter 2022 Generally, a transversal filter includes three basic elements: unit delay
  • unit delay operator 2031a operates on input u(n)
  • the resulting output is u(n)
  • the role of the multiplier 2032a in the filter is to multiply the tap input 2030a by a filter coefficient refe ⁇ ed to as a tap weight denoted w * 0 (n). It should be noted that the asterisk in
  • Figure 17 denotes complex conjugation which assumes that the tap inputs, and therefore the tap
  • weights are all complex values.
  • the combined function of the adders in the filter is to sum the
  • multipliers 2032b-2032m-l operate similarly to the multiplier 2032a.
  • the input signal u(n), and the tap weights denoted w(n) are represented as vectors with each element of the vectors co ⁇ esponding to various components.
  • the recursive least squares or RLS technique attempts to choose a tap vector
  • the e ⁇ or is determined as the sum of the
  • the adaptive weight control mechanism 2024 of Figure 17 is used to determine the weighting
  • weighting factor allows additional weight to be given to the e ⁇ or values which are most recent
  • the adaptive weight control mechanism 2024 provides a co ⁇ ection factor
  • co ⁇ ection factor determined at time n is applied to the tap weight at a time of n+1.
  • correction factor which is applied to the tap weights.
  • the error factor provides for adjustment or co ⁇ ection of the various tap weights. This is the nature of the feed back mechanism of the system 2018 of Figure 16.
  • step 2034 variables are initialized to initial
  • control variable n Some of the variables used in Figure 18 processing steps are initialized to a
  • n is initialized to one in the method described in Figure
  • the tap weight vector w and the tap input vector u may be initialized to 0.
  • the co ⁇ elation matrix denoted P(0) may be initialized to the inverse of a small positive constant time the identity matrix denoted I.
  • the recommended choice of the small positive constant A is
  • ⁇ is a small positive constant which may be refe ⁇ ed to as the "forgetting
  • ⁇ should generally be chosen such that:
  • the former condition generally reflects the fact that the RLS technique provides a converged
  • a starting value for P(0) a starting value should be chosen which assures the non-singularity of the co ⁇ elation matrix.
  • control proceeds to step 236 where a determination is made whether or not the loop control variable n is less than or equal to the number of desired iterations. If the
  • n is greater than the number of desired iterations, meaning that execution of the method steps of Figure 18 is complete, control proceeds to step 238 where the
  • step 2036 If a determination is made at step 2036 that n is less than
  • step 2040 the gain denoted k(n) is computed as:
  • ⁇ (n) is computed as:
  • step 2044 the tap weight vector for a particular instance in time
  • w(n) is computed as:
  • Control proceeds to step 2046 where the inverse of the co ⁇ elation matrix denoted P(n) is
  • Control then proceeds to step 2048 where the loop control variable n is incremented by 1.
  • Control proceeds to the top of the loop at step 2036 where a determination again is made whether or not the loop formed by steps 236-248 has been performed the desired number of iterations.
  • step 2044 describes the adaptive operation of this
  • (n) represents the a priori estimation error.
  • the a priori estimation e ⁇ or refers to an
  • ⁇ is a positive constant close to but less than 1.
  • ⁇ is used in determining
  • w(n) is computed as being dependent upon the value w(n-l). It should also be noted that in the flowchart of Figure 18, the asterisk denotes the complex conjugate of a number. Additionally, the superscript of H, as depicted in step 2042 when associated with the tap weight
  • a complex valued matrix such as the tap weight vector w(n) is Hermitian if it is equal to its conjugate transpose, as known to those skilled in the art.
  • step 2010 performs the RLS technique just described using the second range of sample receive signals. Also recall that a note was made that the size or span
  • the RLS technique performs matrix operations which are generally expensive in terms of computing time and
  • the noise component of the receive signal includes tonal frequencies or jammers
  • the model may follow the peaks of the jammer signals rather than properly fit a curve identifying the filtered received signal. Additionally, if the size of the second range is too small,
  • the RLS technique used in this embodiment assumes that the observed sequence includes a linear combination of a known number of the data sequence. Generally, the RLS technique
  • the RLS technique assumes a wide-sense stationary process. Generally, the RLS technique also presumes the presence of random additive noise which is uncorrelated with the data sequence. With the presence of "white noise", the sum process
  • the sum process is no longer wide-sense stationary.
  • corrective technique that may be considered is in choosing the number of coefficients or taps.
  • the noise input to the co ⁇ elator may be modeled as the
  • the number of coefficients or taps should generally span the main peak of the impulse response magnitude. However, the number of taps
  • the number of taps may vary with embodiment and application. For each
  • the observed process should be modeled to take into account all
  • the size of the second range varies with the environment in which this application will be used. If there will be tonal frequencies, such as a microwave oven within an indoor environment, this should be considered when choosing an appropriate value for the second range. In one preferred
  • a value of 12 was used for the size of the second range for an indoor article
  • This indoor system may possibly have tonal frequencies
  • one data set of 254 received signals was recorded. Rather than perform the method
  • the number of iterations of the RLS algorithm should be as large as possible in order to meet the
  • each element in this vector co ⁇ esponds to a component of a received signal which represents a filtered signal.
  • each element of the vector co ⁇ esponds to a filtered signal with the noise portion removed.
  • the vector produced in step 2012 is a vector
  • step 2010 removes the noise component and returns a
  • step 2014 is set forth in paragraphs that follow in connection with Figure
  • Figure 19B shows a waveform with a low degree of indoor
  • Figure 19C illustrates a
  • Each of the waveforms shown in Figures 19B-D are waveforms that may
  • model in Figure 19E, is an example of a waveform resulting from graphing the 12 points included in the second range of samples which is output from the RLS technique in one
  • this vector may be produced in step 2012 after performing the RLS technique in step 2010.
  • the peak of the actual waveform for example, may be approximated in step 2014, as the 4 th data point of the "model" waveform of Figure 19E.
  • the transmission rate is 127 chips per data bit.
  • bit pattern of the transmitted signal repeats itself every 127 bits. N lesser number bit
  • sequence such as 31 chips, may also be used in a particular implementation which requires less
  • a longer bit sequence generally allows for
  • a longer sequence generally enables a stronger signal at the output of the co ⁇ elator than that produced by a shorter sequence.
  • the processing gain indicates this signal strength enhancement and refers to the number of bits
  • co ⁇ elation is required. For example, in an embodiment where one is required to detect or reduce
  • sequence length as well as the various properties of the sequence sent in the transmission signals may vary with application and each particular implementation.
  • processing may be performed using the techniques previously described. Since it is the atypical
  • the system may devote
  • correlation function performed in step 2004 on a first range of samples is implemented in a
  • the same data set was used when performing multiple iterations of the RLS algorithm of Figure 18.
  • a single data set set may be "reused" on subsequent iterations
  • step 2100 if the first or last element of the
  • a threshold value is determined which, in one embodiment
  • step 2100 is equal to 62.5 percent of this largest magnitude previously determined in step 2100.
  • threshold is refe ⁇ ed to as the X. It should be noted that in one prefe ⁇ ed embodiment, if the
  • Xth element is the second to last vector element.
  • the actual received tag signal is determined based on the expected shape of the received signal as being a co ⁇ elated signal with a triangular peak.
  • weight v[j] MNX(0, magnitude(vector element j) - (largest magnitude of all vector elements/4) )
  • Received signal [X+l - (weightv[X]/Total) /** if element X+2 is not used and is the second to last element of the vector **/
  • the autocorrelation (step 2004) is performed by the correlator 1800.
  • a first heurisitic determines the received tag signal to be the signal in the first range with the maximum magnitude of all the signals in the first range.
  • a second or alternate heurisitic that may be included in an embodiment is to choose a threshold value. The first signal included in the first range having a magnitude equal
  • choosing the threshold value may vary with environmental and other factors particular to each
  • this threshold value is selected in accordance with trial test runs of
  • received tag signal is then used (step 2016) in determining the tag distance.
  • signal processing hardware 1004 may include other hardware
  • Each particular embodiment may be implemented in using a variety of
  • code may be executed on a computer system, such as the microprocessor 1001 or the host computer 105 or yet another computer component included in the signal processing hardware
  • the components may vary with application and design choices associated with a particular implementation.
  • Another embodiment of the signal processing hardware includes hardware and/or
  • This embodiment may perform, for example, all the
  • chassis RF subsystem consists of three of the five major cell controller components:

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Abstract

System for tracking mobile tags. Cell controllers with multiple antenna modules generate a carrier signal which is received by the tags. Tags shift the frequency of the carrier signal, modulate an identification code onto it, and transmit the resulting tag signal at randomized intervals. The antennas receive and process the response, and determine the presence of the tags by proximity and triangulation. The recursive-least squares (RLS) technique is used in filtering received signals. Distance of a tag from an antenna is calculated by measuring the round trip signal time. The cell controllers send data from the antenna to a host computer. The host computer collects the data and resolves them into positional estimates. A combined infrastructure including a wireless communication system and the local positioning system is also disclosed.

Description

METHOD AND APPARATUS COMBINING A TRACKING SYSTEM AND A WIRELESS COMMUNICATION SYSTEM
Background of the Invention
Field of the Invention:
The invention relates to tracking systems and, more particularly, to systems designed to track articles and personnel.
Description of Related Art:
Various techniques may be used to track articles and personnel. Global positioning systems and local positioning systems are examples that may be used depending on the particular
items or persons being located. Included as part of these systems are various embodiments of hardware and software. The hardware may be used, for example, to collect data about the various entities being tracked or located.
Long range identification or tracking systems are designed to read large numbers of tags every few seconds, wherever they may roam in a bounded environment. For low cost and small
size, these tags may be e usually low-powered, for example, emitting in the range of 1 - 10 mW.
Under ideal conditions, read ranges up to about 100 meters can be realized. Alterntatively, in a more cluttered indoor environment, read ranges more like 25 meters is typical. As a practical
matter, an antenna placed every 20 meters or so is often needed for reliable coverage of a
complete indoor space. In such instances, installing an antenna every 20 meters can pose a
substantial installation challenge. In some cases, cable installation costs can exceed hardware
costs, and the installation process can cause significant business disruption. Therefore, it is desirable to find additional uses for the same antennas such as enabling one installation to be
used for several purposes.
Summary of the Invention
In accordance with one aspect of the invention is a system. The system includes a radio
frequency identification tag, a wireless communication system, an antenna module, and a
controller. The antenna moduel is used in transmitting and receiving signals from the radio
frequency identification tag. The antenna module is also used in transmitting and receiving signals used in the wireless communication system. The controller is coupled to the antenna module. The controller is used in communications with the radio frequency identification tag and the wireless communication system.
In accordance with another aspect of the invention is a method for locating objects and
operating a wireless communication system. Signals are transmitted and received using an
antenna module from the radio frequency identification tag and from the wireless communication
system. N controller is coupled to the antenna module and used in communications with the radio frequency identification tag and the wireless communication system.
Brief Description of the Drawings
The above-mentioned and other features of the invention will now become apparent by
reference to the following description taken in connection with the accompanying drawings, in which:
Figure 1 shows an overview of a positioning system; Figure 2 shows several cell controllers deployed in a multi-story building;
Figure 3 is a block diagram of a tag RF design;
Figure 4 is a block diagram of an alternative embodiment of a tag;
Figures 5N-5G are diagrams of a signal as it passes through various stages of the system;
Figure 6 is a block diagram of the cell controller RF design;
Figure 7 is a block diagram of a cell controller active antenna module;
Figure 8 is a block diagram of a modulator RF design;
Figure 9 is a block diagram of a cell controller cable extender module;
Figure 10 is a block diagram of a cell controller;
Figure 11 illustrates extraction of tag data from a series of correlations;
Figures 12N-C are diagrams of tag datagrams;
Figure 13 shows a tag incorporating a delay element;
Figure 14 shows several cell controller receive chains operating in parallel;
Figure 15N is a block diagram of an embodiment of the signal processing hardware of Figure 10;
Figure 15B is a flowchart depicting method steps of one embodiment of the Signal
processing hardware unit of Figure 10;
Figure 16 is a block diagram of an embodiment of a signal filtering technique;
Figure 17 is a block diagram of an embodiment of the transversal filter of Figure 16;
Figure 18 is a flowchart depicting method steps of one embodiment of the recursive-least squares (RLS) technique as used in a method step of Figure 15B;
Figures 19A-19E are diagrams of sample waveforms in embodiments of the system of Figure 1 ;
Figure 20 is a flowchart depicting method steps of one embodiment of approximating a
peak of the filtered tag signal;
Figure 21 is an example of an embodiment of a block diagram of a cell controller;
Figure 22 is an example of an embodiment of a block diagram of a cell controller active
antenna radio frequency block diagram;
Figure 23 is an example of an embodiment of a block diagram of a tag;
Figure 24 is an example of an embodiment of tag specifications;
Figure 25 is an example of another embodiment of a position system;
Figure 26 is an example of an architecture of a local positioning system (LPS) cell
controller;
Figure 27 is an example of an embodiment of a "fixed" portion of a digital enhanced cordless telecommunications (DECT) installation;
Figure 28 is an example of an embodiment of a single base station supporting DECT and
LPS systems;
Figure 29 is an example of the Second LF Carrier Recovery, Demodulator, and NGC
component of Figure 21; and
Figure 30 is an example of another embodiment of a system including integration of LPS
technology. Detailed Description of Preferred Embodiment(s)
The local positioning system described in paragraphs that follow is an article tracking
system that may be used, for example, to track and locate objects indoors. This system may be
characterized as a radio-frequency identification system. Generally, such a system may also be
used in a variety of other local positioning applications, such as outdoor tracking of objects or
locating personnel indoors as well as outdoors. Limitations specific to the embodiments
described herein are not meant to imply limitations to the claimed invention.
Radio Frequency Identification (RFLD) products typically have three components: (1) a tag (the item being identified), (2) an interrogator (a device which detects the presence of a
tag), and (3) a system (typically including cabling, computers, and software which tie together the tags and interrogators into a useful solution). RFLD products are typically designed to detect
tags when they pass within range of a few fixed or handheld interrogators.
RFLD systems are usually deployed as high-end replacement technology for bar coding.
RFLD and related systems include passive RFLD systems, active RFLD systems, infrared LD
systems, and Electronic Article Surveillance (ENS) systems.
The tags in a passive RFLD system do not carry on-board power. The interrogator in such
systems transmits operating power for the tags. Such systems generally have a detection range of a meter or less, although somewhat longer ranges have been achieved. Typically, these
systems operate in the 125-kilohertz or 13.56 megahertz radio band.
Most passive RFLD systems work as follows. An interrogator emits an electromagnetic
field for the purpose of powering the tag. A coil in the tag is powered by the electromagnetic
field, causing the tag's circuitry to "wake up." The tag uses this power to send an identifying signal back to the interrogator. Although some passive RFLD systems are read-only (that is, the tags in such system
respond to a query by reading information from their memory and sending the information back
to the interrogator), the tags used in other passive RFLD systems have a limited ability to accept
information and instructions from the interrogator, for example read/write capabilities in smart
cards (electronic money) and "electronic manifests" in industrial applications.
Passive RFLD tags have been employed in conjunction with access control, smart cards,
vehicle identification (NVI), waste management, item tracking, animal identification, manufacturing control, materials handling, and a variety of other purposes.
One fundamental design goal of any RFLD system is for the weak signal emitted from the tag to be distinguishable from the much stronger signal emitted by the interrogator. Some
strategies for doing this include:
Frequency shifting. Circuitry in the tag receives a carrier from the interrogator, translates
the signal to another frequency, and emits a response modulated onto that second frequency.
Half duplex operation. The tag is charged by the interrogator. When the interrogator's
charging circuit turns off, the tag uses the stored power to respond.
Modulated backscatter. The tag modulates its antenna cross section to identify itself to
the interrogator.
Delayed retransmission. Surface acoustic wave (SAW) devices retransmit the
interrogator's carrier after a delay. The tag's identity is indicated by time variations in the delayed
response.
Active RFLD systems require battery-powered tags. The battery permits a longer
detection range of between 3 and 100 meters. These systems are capable of locating tags with
higher accuracy than passive RFLD systems and typically operate in the 400, 900, 2440, or 5800 megahertz bands. Active tags tend to enable multiple tags to be within range of an interrogator
by the use of "handshaking" between the tags and interrogator, so that each tag transmits its
signal in turn. Communication between tag and interrogator in active RFLD systems is also
typically faster than with passive tags.
Most active RFLD tags respond to the interrogator when polled, in accordance with a
communications protocol. Some active RFLD tags "chirp" (transmit) a signal spontaneously at
predetermined intervals. A tag's chirped signal is detected by the interrogator if the tag is in range of the interrogator.
Infrared systems (IRLD), while not RFLD systems, also endeavor to detect and identify
the position of mobile tags. A typical IRLD system includes a tag that chirps its identity at randomized intervals. Infrared readers located in the ceiling detect these transmissions, and
report them to a host. The transmission rate from the tag to reader is typically about 600 baud.
Motion detectors in the tags enable the tags to transmit more frequently when in motion. The
tags are typically about the size of dominos.
ENS systems are intended to deter theft in retail environments. ENS tags are fairly
unreliable, very low in cost, and limited in capabilities. Although they track mobile tags, they
are not generally considered to be identification products, because ENS tags are uncoded and cannot be distinguished from one another.
Referring to Figure 1, an article tracking system 100 contains the following general
components:
Tags: Inexpensive miniature radio frequency transponding tags lOla-c are attached to
people and/or items being tracked. Tags lOla-c "wake up" periodically, and "chirp" (transmit)
a radio-coded unique identity code (ULD). The tags 101 a-c are designed so that their range is 50- 100 meters in a typical indoor environment, the range mostly being limited by a need to conserve
the life of the tag battery, and the requirement that the tag 101a and tag battery be small and thin.
Cell Controllers: Cell controllers 102a-c detect the chirps of tags 101 a-c and calculate
the distances of those tags 101 a-c to active antenna modules 104a-d connected to the cell
controllers 102a-c. Each antenna module preferably has a transmit antenna and a receive
antenna. In Figure 1, the antenna modules connected to cell controllers 102b and 102c are
omitted for simplicity. A cell controller 102a is typically contained in a case and is mounted behind a hung ceiling. The cell controller 102a can receive power from a conventional wall
outlet or the equivalent. The cell controller 102a is attached through coaxial cables 103a-d to the antenna modules 104a-d, respectively, which provide coverage of an area of the indoor
facility 110. A tag signal 107, transmitted by a tag 101a, is received by one or more antenna modules 104a-d, and is processed by chips in the cell controller 102a, such as digital signal
processing (DSP) chips as may be included in the signal processing hardware. The information
resulting from this processing is used to identify both the identity of the transmitting tag 101a
and the distance between the tag 101a and each, for example, of the receiving antenna modules
104a-d.
Host Computer: Cell controllers 102a-c are in data communication with a host computer
105, which collects data and information available from the cell controllers 102a-c and archives the data into an open format database, such as an SQL Server.
User Application: In a preferred option, client workstations 120a-c communicate with
the host computer 105 over a network, such as a LAN 115. A client application running on each
of the client workstations 120a-c can access the SQL Server and presents the data in a way that
is useful to an end-user. The tag 101a does not generate its own radio signal. Rather, an antenna module, for
example antenna module 104a, continuously transmits a direct sequence spread spectrum
interrogator signal 106 at a first frequency, for example 2440-megahertz. The tag 101a receives
this signal 106, modulates its ULD code onto the signal 106, and immediately transmits back a
frequency-shifted signal 107 at, for example, 5800-megahertz to, for example, antenna 104a.
The distance from the antenna module 104a to the tag 101a can then be determined by the cell
controller 102a from the round trip time of transmission, accounting for fixed and known delays in the wiring and electronics. The cell controller 102a can quickly switch among antenna modules 104a-d to obtain the distance from the tag 101a to each of antenna modules 104a-d
(which receive the return signal 107), and from that information determines the tag's location by triangulation techniques.
The system 100 is designed to be scaleable, allowing addition of cell controllers to
existing cell controllers 102a-c and antenna modules to existing antennas modules 104a-d to
improve the precision with which tag location is determined. Figure 2 shows how a collection
of cell controllers 102a-c can be deployed in the large multistory building 110. As shown in
Figure 2, multiple cell controllers 102a-c feed data to a single host computer 105, typically through a TCP/IP communications network. A variety of data protocols and transfer mechanisms
can be used in preferred embodiments. For example, if a local area network is not available,
connection to the host can be accomplished via RS485, RS232, RS422, power line modem, or
a dedicated phone line. Alternatively, specialized modems designed for use on such cables can
be employed.
Each of the cell controllers 102a-c can be installed to cover a separate floor 130a-c,
respectively, with the exact configuration being modifiable by a system administrator. On floor 130a, cell controller 102a, with its collection of antenna modules 104a-d, is installed in the
ceiling 140a. The same configuration of equipment is used on the remaining floors 130b-c.
Antenna modules 104a-d are designed to provide good gain downwardly and horizontally, and
poor gain upwardly, so that a vertical location (that is, the floor) of a tag 101a can be determined
by noting which antenna modules 104a-d receive the strongest signals from the tag 101a.
Structurally, a ground plane is placed behind each antenna to reflect signals downward. The
horizontal location of a tag 101a is then roughly determined by noting which antenna modules
104a-d receive a strong signal from the tag 101a. The horizontal location of the tag 101a with respect to an antenna module 104a can be determined more precisely by estimating the distance from the tag 101a to each antenna module 104a-d, based on the combined time of flight of the
interrogation signal 106 and the tag signal 107. Each "cell," consisting of a cell controller 102a
and its antenna modules 104a-d, covers several thousand square feet of floor space. Each cell operates independently, enabling more cells to be added without affecting the performance of
existing cells. If a user desires to locate tags by "zone," one antenna per zone can be installed. Users
wishing to track one or more tags 101 a-c moving down hallways, can install antenna modules
104a-d every 20 or so meters along a hallway 130a-c, and calculate the linear location of a tag
101a by measuring the distance from the tag 101a to those antenna modules 104a-d. Customers
wishing to triangulate upon the location of a tag 101a may install enough antenna modules such
that the tag 101a will be in range of at least two or three of the antenna modules, depending on
the coverage of the antenna modules and the geometry of the facility. A typical installation will
cover a complete facility 110 with a combination of "zone" and "hallway" coverage at a relatively low cost per square foot, and, over time, upgrade certain areas with enough antenna modules to
triangulate upon a tag's position.
Other preferred embodiments may include a wall mounted antenna radiating generally
horizontally, rather than vertically. The selection of antenna type may be based on a variety of
functional factors familiar to one of ordinary skill in the art.
Tag RF Design
Referring to Figure 3, a tag RF circuitry 300 receives signal 106 at a tag receive antenna
301 and transmits tag signal 107 at a tag transmit antenna 312. The function of the tag RF
circuitry 300 is to transpond the incoming spread spectrum signal 106 by frequency translation. The secondary function of the tag RF circuitry 300 is to modulate tag data onto the emitted tag
signal 107, under the control of a microprocessor 308. The information emitted on the tag signal
107 includes, in a preferred embodiment of the invention, the serial number of the tag, datagram
headers, and tag data 309 such as that derived from a motion indicator or a low power indicator.
The incoming signal 106 is preferably a direct sequence spread spectrum signal, biphase or quadrature modulated from the cell controller 102a, in the 2440 megahertz band. The signal
106 is received by the tag receive antenna 301, which collects the signal 106 and feeds it into the
tag RF circuitry 300.
After the signal 106 is received by the tag receive antenna 301, an Rx (receive) bandpass
filter 302 ensures that the tag is receiving signals only in the 2440 megahertz ISM band, rejecting
radar signals, electronic newsgathering signals, and the like. In one embodiment, the filter 302
is implemented as an etched coupled stripline filter embedded in the circuit board. The signal 106 is then amplified by an amplifier 303 to ensure that the received signal can be mixed, in a
frequency mixer 304, without degrading the signal to noise ratio (SNR).
The frequency mixer 304 translates or shifts the carrier frequency from 2440 megahertz
to 5800 megahertz. The incoming signal, with a center frequency of 2440 megahertz, is mixed
with the output of a phase locked oscillator (PLO) 305 with a center frequency of 3340
megahertz. This results in a sum frequency of 5800, along with a difference frequency and a
variety of harmonics and subharmonics which are removed with a bandpass filter 306. In one embodiment, the PLO 305 consists of a phase locked loop (PLL) chip with three inputs: (1) a sampled output from a voltage controlled oscillator (VCO); (2) a reference tone from a 10
megahertz oscillator; and (3) a frequency programming interface to a microprocessor 308. This generates a pure tone with good phase noise at the 3340-megahertz tag LO frequency. In an
alternative embodiment, the PLO 305 outputs a 1670-megahertz tone, which is then doubled to give the desired 3340-megahertz result.
The next element of the tag RF circuitry 300 is a biphase modulator 307 which, under
control of the microprocessor 308, can either pass the 5800-megahertz signal unaltered, or change the phase of the signal by 180 degrees. The modulator 307 is implemented as a single
pole double throw RF switch 801 that feeds a 180 degree hybrid, as shown in Figure 8. Several
forms of modulation can be used, including on-off keyed (OOK) modulation, binary phase-shift
keyed (BPSK) modulation, multiple phase-shift keyed (MPK) modulation, and quadrature
amplified (QAM) modulation. BPSK is the preferred form of modulation. The output from the
modulator 307 is fed into an amplifier 310, then is filtered by a transmitter bandpass filter 311,
and the output of filter 311 is emitted from a transmit antenna 312 as the tag signal 107. Since
the amplifier 310 operates at high frequency, it consumes significant power, and alternative embodiments (such as that shown in Figure 4) that make this amplifier 310 unnecessary are
preferred. The Tx Filter 311, implemented as a 5-pole filter, is necessary to ensure tag
compliance with FCC Part 15 requirements.
The tag RF circuitry 300 shown in Figure 3 is intended to illustrate the general functions
of a tag 101 a-c, with an embodiment that is workable and self-explanatory. Those skilled in the
art will be able to combine multiple functions into single elements in order to conserve power
and take full advantage of available parts, or implement the same functions with a custom ASIC.
Figure 4 shows an alternative embodiment 400 which fulfills the same basic functions as that shown in Figure 3, but with fewer components and using less power. The essential difference between the circuitry 400 shown in Figure 4 and the circuitry 300 shown in Figure 3 is that the modulator 404 in Figure 4 is placed before the frequency mixer 406 in order to reduce the
number of components (for example, the amplifier 310 is eliminated) and to conserve power.
In the place of the frequency mixer 304 (Figure 3) or the time delay element 1505 (Figure
13), other signal transmission discriminators may be used to transpond by other methods. For
example, a tag, such as the tag 101 a, can transpond using backscatter, frequency translation by
mixing, frequency translation by taking a harmonic, frequency translation by taking a subharmonic, or by signal delay (such as via a SAW device).
Not shown in Figure 4, but desirable for the tag RF circuitry, is the use of a common
crystal reference for both the PLO 407 and clock timing in the microprocessor 405. Accurate
timing is an important, if not critical, feature of the system, enabling the cell controllers 102a-c
to predict when a tag 101 a will transmit a tag signal 107. Using the same crystal reference in the
PLO 407 and in the microprocessor 405 clock timing allows the cell controller 102a to accurately calibrate the source by measuring phase shifts in the received signal (as described hereinafter),
and to synchronize its clock timing accordingly.
Not shown in Figure 4, but desirable for some applications, is an embodiment where the
transmit antenna 409 and receive antenna 401 are combined into a single element, and which
uses a diplexer with the single antenna structure.
The manner in which the tags 101 a-c are powered depends on the application. (Note that Figure 3 and 4 omit the tag power source.) Typically, a tag 101a will be battery powered, with
the RF stage turned on and off under control of the microprocessor 405. In a preferred embodiment, the microprocessor 405 goes into a low power state, where it merely waits until it
is time to power up the tag 101 a again. In an alternative embodiment, all tag circuitry 400 cycles on and off under analog control, using an RC time constant in the circuit 400 as the timing
source.
Using the tag RF circuitry 300 or 400 of Figure 3 or 4, if a tag 101a is in range of two of
the cell controllers 102a-c, and those cell controllers are sending pseudonoise with low cross
correlation characteristics, the tag 101a will correctly transpond both signals simultaneously.
Tags 101 a-c require a period of time, on the order of a millisecond, to charge up and
charge down. During these brief periods, typically, the tags 101 a-c will not be stable enough to use, but will nonetheless emit RF into the radio channel through the transmit antenna 409. For
high-performance applications, where radio bandwidth is limited, a microprocessor-controlled
switch can be added to the tag's transmit chain to eliminate such spurious emissions.
The tag RF circuitry 300, 400 shown in Figure 3 and 4 can be used in conjunction with
different pairs of frequencies. The general approach described above works for any two allowable FCC spread spectrum bands. For example, the following combinations are permissible
for license-free radio under FCC regulation Part 15.247:
915 megahertz translated to 2440 megahertz.
915 megahertz translated to 5800 megahertz.
2440 megahertz translated to 915 megahertz.
5800 megahertz translated to 915 megahertz.
5800 megahertz translated to 2440 megahertz.
Spread spectrum operation, however, is not required; two licensed narrow bands can be used. However, spread spectrum operation in the 2440 and 5800 megahertz bands is assumed for the remainder of the discussion.
Tag With Time Delay
The tag RF circuitry 300, 400 shown in Figures 3 and 4 use frequency division multiple
access, i.e., the tag circuitry 300, 400 receives and emits signals on different frequencies. An
alternative embodiment 1500 uses time division multiple access, as shown in Figure 13. For illustrative purposes, assume that the tag circuitry 1500 shown in Figure 13 takes as an input at
a receive antenna 1501 a signal at one frequency, such as 915mHz, and emits the same signal
through a transmit antenna 1508 at the same frequency after a delay of a microsecond. Assume
that a cell controller, such as cell controller 102a, transmits an interrogation signal 106 in bursts
every 2 microseconds. A tag, such as tag 101a, takes this signal as an input through the receive
antenna 1501. The signal then passes through elements 1502-1504, as in Figures 3 and 4. A time
delay element 1505 is then used to delay for a microsecond. The signal then passes through a
transmit bandpass filter 1507 and is emitted from the transmit antenna. N SAW device can be used as the time delay element 1505. During the period of the delay, the cell controller ceases
transmission, and reflections of the interrogation signal 106 in the environment die down to a
minimal level. This half-duplex approach allows single frequency operation, although with
lower bandwidth than with a full-duplex frequency shifting approach. As in the frequency-
changing tag, the delay-based tag is capable of modulating the response signal by a 180-degree
phase shift. In other respects, the tag design 1500 shown in Figure 13 is similar to those shown
in Figures 3 and 4.
Cell Controller RF Design
Figure 6 shows the radio stage of a cell controller 102a. The architecture of an antenna module, such as the antenna module 104, is shown in Figure 7. Together, the cell controller 102a
and its remote antenna modules 104a-d modulate a baseband square wave input onto a 2440-
megahertz carrier, filter the resulting 2440 megahertz signal to comply with FCC transmission
requirements, transmit that filtered 2440-megahertz signal through a selected antenna module,
receive a returning 5800-megahertz tag response through the same antenna module, extract the
1 (Inphase) and Q (Quadrature) components of the demodulated baseband signal, and digitize the result for subsequent processing.
Figure 10 shows the main components of the cell controller digital subsystem 650. In summary, the digital subsystem 650 provides a baseband input signal 601, and some number of
nanoseconds later receives a demodulated response 107 from a tag 102a. The microprocessor
1001 , as noted above, can change the behavior of the radio system by (a) modifying the baseband
input signal 601; (b) modifying the chip rate, pseudonoise sequence length, and/or the
pseudonoise sequence code; (c) modifying the transmit frequency 610 of radio transmitter 1002 and the receive frequency of radio receiver 1003 within a narrow range; (d) modifying the transmit gain of radio transmitter 1002 and the receive gain of radio receiver 1003; and (e) by
switching antenna modules 104a-d.
The demodulated response 107 from the tag 102a is split into I (Inphase) and Q
(Quadrature) components by the receiver Radio 1003, and digitized by a digitizer 636. Signal
processing hardware 1004, for example a combination of DSP and FPGA components, reduces
the output from the digitizer 636, performing correlation operations at high speed. If binary phase-shift keyed (BPSK) modulation is used on the transmitting side, the I and Q channels are
correlated separately and combined. For quadrature phase-shift keyed (QPSK) modulation, each channel must be correlated twice, once with each sequence. The correlated data from the signal processing hardware 1004 is processed by a microprocessor 1001, such as a Pentium processor.
Communications between the microprocessor 1001 and the host computer 105 is accomplished
using a TCP/IP protocol, with Ethernet being preferred.
The data that is input to the transmit chain is a baseband input signal 601 which is a
pseudonoise spreading sequence. The length of the sequence and the code encoded in the
sequence are set by a cell controller microprocessor 1001, and can be varied depending on signal processing requirements. Thirty-one or 127 bit sequences are typical, giving about 15 dB and
20 dB of compression gain, respectively. The 2440 megahertz and 5780 megahertz bands can support a 40 megahertz baseband input signal 601, and the cell controller 102a is designed to
enable this full bandwidth to be utilized.
Figures 5A-5G show an interrogation signal 106 as it passes through various stages of
the cell controller RF circuitry 600. Figure 5A shows a square wave baseband input to the
modulator 500. Figure 5B shows this baseband input digitally correlated 510. Figure 5C shows an output 520 from a-modulator 602, viewed through a spectrum analyzer centered at 2440
megahertz. Figure 5D shows a spectrum analyzer view 530 of the tag signal 107, centered at
5780 megahertz. Figure 5E shows the demodulated response from tag 107, separated into its I
(Inphase) 545 and Q (Quadrature) 540 components. Figure 5F shows the I and Q components,
digitally correlated 550. Figure 5G shows the negative of the second derivative of the correlated
waveform, combining the I and Q components 560.
The modulator 602 (Figure 6) modulates the baseband input 601 onto a 2440 megahertz carrier. Various forms of modulation are available and well-known to those skilled in the art.
For BPSK modulation, the modulator 602 is implemented as a single pole double throw RF switch 801 that feeds a 180-degree hybrid combiner 803, as shown in Figure 8. The modulator
602 is preferably implemented as a QPSK modulator, which duplicates the BPSK modulator with one channel offset by 90 degrees from the other, each channel driven by a different
baseband sequence with acceptable cross-correlation characteristics. Higher order modulation
is also possible. Modulation by the modulator 602 results in sidelobes extending for hundreds
of megahertz, which need to be filtered to comply with FCC requirements. The 2440 megahertz
band has an adjacent band, which imposes very strong filtering requirements, best addressed
using in the illustrated embodiment using a SAW filter 607 that combines wide passbands with tight stopbands. A wider passband supports a faster chipping rate in the baseband input signal
601 , but a narrower passband provides an opportunity to use a wider range of frequency diversity
to avoid jammers and/or support advanced signal processing techniques. The modulator 602
must operate at the same frequency as the available LF filter 607, typically in the range of 200
megahertz to 400 megahertz. A preamplifier 606 is necessary prior to the SAW LF filter 607, and
the output of the filter needs to be amplified by an amplifier 608. N transmit LF oscillator 605, like all other RF oscillators in the cell controller circuitry
600, is phase locked to a 10 megahertz crystal source 603, which is distributed through a filter
and splitter network 604 to each of the oscillators. The 10 megahertz source 603 needs to be
within a few kilohertz of the 10 megahertz sources on the tags to avoid excessive baseband phase
shift.
The output from the LF filter 607 (from amplifier 608) is then mixed by a mixer 609 with
the output from a phase locked oscillator (PLO) 611 and is converted to a carrier frequency of 2440 megahertz. The frequency of the PLO 611 can be modified within a narrow range under
microprocessor control 610, in order to provide the frequency diversity needed to avoid jammers and/or for various advanced signal processing techniques. The degree of frequency diversity
available is related to the specifications of the LF filter 607, with narrower filters permitting a slower chip rate but having more frequency flexibility. Not shown in Figure 6 is a filter typically
needed to remove undesirable harmonic and difference frequencies from the output of the mixer
609. Following the mixer 609 is a driver amplifier 612 which raises the power level of the
signal 106, so that it can he driven down the cable 103a to the remote antenna module 104a, and
which buffers the output of the mixer 609 for a bandpass filter 613. The RF bandpass filter 613
is needed to remove FCC non-compliant outputs from the mixer 609. A directional coupler 616
provides a port to examine the signal 106 before it is transmitted to the remote antenna modules, for example antenna modules 104a-d.
An attenuator 614 under microprocessor control 615 allows the signal processing
software to decrease output power when a tag 101 a-d is known to be nearby. This is helpful in circumstances when a nearby tag is known to be over-driven by the cell controller, and/or the
signal processing software needs the tag to operate in a more linear range.
The signal is then fed into a diplexer 618, which combines the transmitted 106 and
received 107 signals onto a single wire. The diplexer 618 is a highpass/lowpass filter
combination 619a that attenuates a received signal 107 relative to the transmit side and attenuates
the transmit signal 106 relative to the receive side. Because of the presence of the Tx and Rx
bandpass filters 613, 624, the specifications of the diplexer 618 are not very stringent.
The cell controller RF stage 600 shown in Figure 6 supports one remote antenna module 104a-d at a time. In order to support multiple antennas from the same cell controller, the system
needs a switch 619, which enables a microprocessor control 620 to rapidly switch from one antenna to the next. The switch 619 takes RF and passes it into one of n cables, where n is, for
example either 8 or 16. The switch 619 also provides DC power to the selected line. The RF
signal is coupled into the cables with a capacitor (not shown), which provides DC isolation, and
the DC power is coupled into the cables with RF chokes to provide RF isolation. Thus, the DC
and RF travel together through a single coaxial cable to the selected antenna.
The rise time of the DC in an antenna is in the range of 20 microseconds, limited by the
effective resistance of the circuitry in and characteristics of the antenna and the capacitors needed
for operation. To provide antenna switching time in the microsecond range, the DC power to
an antenna is preloaded before the RF is switched.
Referring to Figure 7, in an antenna system 700, the combined DC and RF signals arrive
through a coaxial cable, such as the cable 103a from the cell controller 102a. A bias-tee 701
separates the RF signal 710 from the DC signal 712. The DC signal 712 is sent to Tx/Rx power
control logic 702 which, in the simplest embodiment is a filter to remove noise from the line and provide a clean 5 volt power source. The RF output 710 from the bias tee 701 is fed into a
diplexer 715, which is identical to the diplexer 618 in the cell controller 102a. This is then
amplified by an amplifier 703 to the power level allowed by the FCC, and filtered by a filter 704
to remove line and amplifier noise in compliance with FCC regulations. The resulting signal is
then sent to a transmit antenna 705.
The transmit antenna 705 and receive antenna 706 are, in this embodiment, wall mounted
patch arrays, providing reduced energy in the vertical direction and spreading energy laterally, so that power is not wasted in the floor and ceiling, and so that minimal power is radiated upward. The 5780-megahertz response 107 from the tag 101a is filtered by a filter 707, amplified by an amplifier 708, and sent back down the cable 103a to the cell controller 102a.
The system is designed to use cables 103a-d of a standard length, for example, 20 meters.
A cable extender module 900 connects two lengths of cable and supports an extended cable
length. Referring to Figure 9, the elements of the module 900 use the DC power 910 from the
cable 103 to drive low noise amplifiers 903, 904, which provide enough gain to drive the next
section of cable. Bias tees 906, 907 separate the DC power 910 from the RF signals, and
diplexers 908, 909 operate to separate the transmit signal 106 from the receive signal 107.
Referring to Figure 6, the signal returning from the antenna module 104a to the cell
controller 102a passes through the switch elements 621, 619 and diplexer 618 to the cell
controller receive RF chain 622. The signal passes through a combination of a preamplifier 623
and bandpass filter 624, the exact arrangement of which varies based on the parts selected. A
digitally controlled receive attenuator 625 under microprocessor control 626 is used to avoid
saturating the receive chain when the tag 101a is known to be nearby. This is necessary to avoid losing the relationship between the I and Q components of the received signal 107, necessary for proper correlation and other signal processing.
The signal then enters an I-Q zero LF demodulator circuitry 627-633. As noted
previously, the microprocessor Rx frequency control 635 must be set in tandem with its
counterpart in the transmit chain. The resulting signal, an example of which is shown in Figure
5E, is fed into a digitizer 636 (Figure 10) in preparation for digital signal processing.
The embodiment described above is simplified, based on an assumption that the cell
controller can send and receive from only one antenna at a given time. Improved performance can be achieved by selecting send and receive antennas independently of each other. Software in the cell controller determines which antenna module receives the best signal from the tag. For
example, if a particular tag, such as tag 101a, is close to an antenna, such as antenna 104a, then the antenna 104 will receive a strong signal from the tag 101a. The cell controller 102a can then
transmit a signal, such as signal 106, from antenna 104a, and receive the transponded response
107 at antennas 104b, 104c, and 104d in turn. This can result in a stronger signal received at the
antennas 104b-d, compared to the signal the antennas 104b-d would have received had the signal
106 been transmitted and the signal 107 received from each of the antenna modules 104b-d independently.
The design 1600 shown in Figure 14 provides multiple receive chains 1610a-1610n
operating in parallel. Each of the receive chains 1610a-1610n includes an IQ demodulator, a digitizer, and a correlating element, shown as integer DSPs, for example, integer DSP 1620.
Implementing each receive chain on a separate card provides scaleability. The use of multiple
receive antenna modules for the same transmitted signal allows the cell controller signal
processing software to employ spatial processing techniques to isolate multipath effects. These techniques take advantage of the fact that the multipath-corrupted response will have different
characteristics at each antenna.
Bit Detection
In an ideal environment, a simple triangular correlation peak can be derived from a
received tag signal 107, as shown in Figure 5B. Distortions introduced in the radio chain,
particularly caused by indoor multipath effects, result in a distorted but nonetheless distinct correlation peak, a function of which is shown in Figure 5G. For the purpose of bit detection,
the essential point is to reliably detect the existence of a series of correlations, which indicates the operation of a tag. Figure 11 shows how tag data is extracted from a series of correlations. In the left half of the chart 1110 shown in Figure 11, the tag is transmitting a "one." This is
accomplished by setting the tag's modulator 307 to pass the interrogator signal 106 unaltered.
When the received tag signal 107 is correlated with the transmitted pseudonoise sequence,
essentially identical correlation peaks result. Three such peaks 1120a-c are illustrated here.
During the time of the fourth correlation 1120d, the tag flips the phase of the modulator by 180
degrees, indicating a "zero," as shown in the chart 1110 (Zero is encoded as a 180 degree phase
change). Since the modulation is changed in the middle of a bit, the fourth correlation data peak
1120d is corrupted, and is best ignored. The fifth and sixth correlation peaks 1120e-f cleanly reflect the 180-degree shift.
Pseudonoise sequences can be varied under microprocessor control at the cell controller.
when a tag's presence is first detected, relatively short sequences must be used, as shown in
Figure 11. Once the tag's bit timing is ascertained, it is possible to use longer sequences for
improved SNR, which is helpful in distance measurement. An important consideration, not shown in Figure 11, is that the balance between the
Inphase and Quadrature components of the received signal will drift over time. This is because
the 10-megahertz sources in the cell controller 102a and the tag 101a will differ typically by a
few kilohertz. This factor can be calibrated by noting the phase difference between contiguous
correlations, detectable at the baseband by noting changes in Inphase and/or Quadrature
components of the received signal. As noted previously, this same calibration process can be
used to calibrate the tag clock in reference to the cell controller clock, allowing for precise prediction of tag chiφing time, without needing to precisely measure the timing of tag bit transitions.
Interaction Between Cell Controllers and Tags
Each tag is a stand-alone unit that is unaware in any way of the outside world. Each tag
has a Unique Identifying Code (ULD) associated with the tag when it is manufactured. A tag
wakes up periodically and, for a short period of time, converts any incoming 2440-megahertz
signal 106 to an outgoing 5780-megahertz signal 107, while modulating its ULD and other data onto the outgoing signal 107 which it chiφs (transmits). The tag does not communicate with
other tags. The tag does not explicitly respond to an interrogation signal, but merely transponds
any incoming signal 106 in the 2440-megahertz band, which may or may not include a
pseudonoise sequence from a nearby cell controller antenna module 104a. This approach greatly
simplifies the design and fabrication of the tag 101a.
Some portion of the time, two or more tags will transpond simultaneously. In many
cases, one of the two tags will return a stronger signal than the other tag and some data will be
lost in such a collision. To avoid collisions occurring in a repeating pattern, tags "wake up" and chiφ their ULDs at randomized times, which can be calculated (by both the tag and the cell
controller) based on a pseudorandom number generator which incoφorates the tag's ULD. For
example, for a tag which chiφs approximately every 5 seconds, the tag generates pseudorandom
numbers between 0.0 and 2.0, and adds these to a 4.0-second minimum delay time, resulting in
a sequence of delay times uniformly distributed between 4.0 and 6.0 seconds.
It is possible to use analog inputs, such as an input from an internal clock or the delay of
an RC circuit, to the pseudorandom number generator to vary the seed over time, but a purely digital scheme is preferred to enable the cell controller 102a to accurately forecast a known tag's chiφing time. A typical pseudorandom number generator has the form:
N = rand(seed) Formula 1
The resulting N is used as the seed for the next pseudorandom number in the
pseudorandom number sequence. When using a pseudorandom number generator of this kind,
it is possible that two tags will use the same seed, resulting in their tag signals repetitively
colliding. Further, with small differences in tag clocks, all pairs of tags will eventually drift through this synchronized state for some amount of time. To avoid these conditions it is
desirable to incoφorate each tag's ULD, as noted above, into the delay time for that tag, resulting in a different pseudorandom sequence for each tag, that is:
Delay = f(N,ULD) Formula 2
One simple example of such a function is: Delay = Xor(N, BitRotate (ULD, AND(N, 11112)) Formula 3
Referring to Formula 3, it is possible to reconstruct seed from ULD, Delay and
And(N,l 1112), by calculating N = Xor(Delay, BitRotate(ULD, And(N,l 1112)).
Referring to Figure 12a, one embodiment of the tag datagram 1400 contains a header
5 1401 to enable the cell controller to detect the tag's presence, followed by an identifier preamble
1402, followed by the tag's ULD 1403. The header 1401 can be of zero length. The identifier preamble 1402 can be implemented, for example, as a validity check such as a cyclic redundancy check (CRC). Given a sufficiently simple Delay function and high clock stability, the cell controller can infer the tag's chiφing sequence by noting the timing of a series of chiφs of the
0 datagram 1400.
Referring to Figure 12b, in another embodiment of the tag datagram 1410, the tag adds
Delay information 1414, thus enabling the cell controller to forecast the transmission time of the tag's next and subsequent chiφs of the datagram 1410. In the example of Formula 3, this
information would include the data: Delay and And(N,l 1112). 5 Referring to Figure 12c, in another embodiment of the tag datagram 1420, a shorter
header is used than in the datagrams 1400, 1410 of Figures 12a and 12b, such that the cell
controller is not guaranteed to have enough time to detect the tag's presence before the ULD 1423
contained in the tag datagram 1420 is transmitted. Appended to the datagram 1420 is the
transmission delay 1425 of the next chiφ, enabling the cell controller to anticipate the time that
0 the tag will next chiφ its datagram 1420, even if the cell controller does not have enough time
to identify the identity of the tag from the first received chiφ of the datagram 1420. The cell
controller can then anticipate this next chiφ and ascertain the identity of the tag at that time. Once the tag is identified, the cell controller can duplicate the tag's pseudorandom number
generator to calculate the times of all future chiφs by the tag. In the tag datagram 1420 of Figure
12c, a series of special synchronization bits 1424 are inserted between the ULD 1423 and the
delay information 1425, to reliably determine when the ULD 1423 ends; in this case, the ULD
5 1423 must be defined so that it does not include the synchronization sequence or its inverse.
Figures 12a, 12b, and 12c include optional data sections 1404, 1415, 1426, which allow
a tag to transmit data to the cell controller. These sections 1404, 1415, 1426 can include data
from within the tag, such as from a motion detector or a low power indicator, or from an external device attached to the tag, such as metabolic information for medical telemetry if the tag is
10 attached to a person.
An identifier preamble, related to the tag ULD, precedes the tag ULD. This identifier preamble enables the cell controller to quickly verify that a tag is chiφing as expected, without
needing to decode the tag's complete ULD. This frees the cell controller for other activities, such as communicating with different tags in proximity to other antennas. The identifier preamble
15 1402, 1412, 1422 and tag ULDs 1403, 1413, 1423 are set externally, and if appropriate can be defined to include error correction bits.
The ULD of a tag may be hardcoded into the tag (e.g., as a serial number). Tags may be
grouped based on their ULDs, and different groups may be associated with different cell
controllers. Each cell controller contains information (received from another source) about
20 which tags are in the group associated with the cell controller. When a cell controller receives
a tag signal, the cell controller can extract the ULD information from the tag signal to determine
whether the tag signal was sent by a tag in the group associated with the cell controller. In the tag datagrams 1400, 1410, 1420 of Figures 12a-c, the delay information fields
1414, 1425 and data fields 1404, 1415, 1426 can also include error correction bits. For
simplified processing, data can be reduced to a stream of half-bytes. To determine what value
to send for a particular half-byte, the tag can look up the half-byte's value in a table which
contains, for example, 8-bit values, which represent the value of the half-byte plus error correction information. A single cell controller can handle all three types of datagrams 1400,
1410, 1420 shown in Figures 12a-c. The choice of datagram type would depend on the application requirements for a particular tag.
The amount of time it takes for a cell controller to detect the presence of a tag may vary depending on the nature of the cell controller design. For example, a 100-microsecond time to
switch antennas may be significant when the cell controller is cycling among 16 antennas. In
order to be assured that a tag will be identified the first time its tag signal is received by the cell controller, the tag datagram header must be long enough to give the cell controller time to try all
of its antennas. If the performance requirement is in the range of 100 tags per second, 2 or 3
extra milliseconds in the header can be tolerated. But for higher performance requirements, or
when tag power consumption must be minimized, it is necessary to either improve the
performance of the cell controller or to use a tag datagram 1420 of the type shown in Figure 12c.
By anticipating time of transmission from a particular tag, the cell controller can collect
tag information from a variety of antennas in an organized way, in order to better calculate tag
location by using antenna and/or frequency diversity. If a tag is responding exactly when it is
expected to respond, it is not necessary for a cell controller to detect every bit transmitted in the
tag datagram in order to be reasonably certain that it is receiving a signal from the correct tag.
A correct identifier preamble arriving exactly on schedule is almost certain to be from the expected tag. This provides an opportunity for the cell controller to try a variety of antennas that
may or may not be able to communicate with the tag.
If it is necessary to track tags between the tag datagram transmissions, the tag can be
configured to send shorter transmissions more frequently. For example, if a tag is configured
to chiφ its datagram on average every 10 seconds, it may also be configured to transmit a much
shorter code more frequently, such as every half-second. This shorter code might be as short as
one bit long, and take just a few microseconds to transmit. Thus, even hundreds of such transmissions per second would consume only a small percentage of the communication channel. The cell controller can anticipate the exact timing of each such transmission, thus matching each
signal with the originating tag based on the time of the transmission. Error correction codes can be arranged such that long chiφs from one tag will not typically be corrupted by quick chiφs
from other tags. The cell controller has the data to forecast most of such collisions.
When a tag is first introduced into a cell controller's range, collisions between datagram transmissions of different tags will occur in an unpredictable way. Transmission by a tag newly
introduced into the area, or by a tag which spontaneously increases its transmission rate (such as in response to a motion detector or "panic button"), cannot be forecast by the cell controller
102a, and might cause data corruption. However, once the tag is identified, previous collisions
can be modeled and questionable data discarded. Alternatively, if signals from two tags collide,
the cell controller can select an antenna such that the signal the antenna receives from one tag
will be stronger than the signal the antenna receives from the other tag.
In a more advanced tag design, the cell controller has a means for sending information
and instructions to the tag during the times that the tag is known by the cell controller to be in
operation. Such instructions can include commands to be passed on to a device attached to the tag. The cell controller is capable of downloading such information, most simply by on-off
keying, or for more advanced tags by flipping the pseudonoise bit sequences to indicate one or
zero. In general, the downlink (downloading) approach is driven by tag cost and feature
requirements, with higher bit downlink transmission rates requiring more expensive receivers
that consume more power. Thus, a single cell controller can support read-only tags, read/write
tags, and high-speed read write tags simultaneously, with the cell controller adapting its behavior depending on the features supported in a particular tag.
Timing of transmission from tag to cell controller depends on the item being tagged. Inventory and equipment can be set to transmit relatively infrequently, such as once per minute.
More frequent transmissions would be required, for example, for tags on people in a secure facility. For read/write versions of tags, timing of transmission could be modified on command
from the cell controller.
Alternative tag designs enable variation in transmission time based on environmental
factors. For example, motion detectors may be placed in a tag to decrease the time between
transmissions when the tag is in motion. As another example, a tag might transmit more
frequently and with higher power when the tag has been tampered with. As another example,
a tag might incoφorate a slightly modified Electronic Article Surveillance (EAS) device, which
would cause the tag to transmit its ULD more frequently when in range of a standard EAS
detector. More generally, if a tag is attached to another electronic device, transmission interval can be modified under the control of that device.
Tag Power Tags 101 a-c transmit a low RF power level in order to increase their portability and
lifespan. In addition, tag signal transmissions 107 are designed to be only a few milliseconds in
duration. Therefore, even if a tag transponds its ULD every few seconds, careful tag design
allows the battery life of the tag to approximate the shelf life of the battery itself. For even lower
power usage, a motion detector can be incoφorated into the tag so that, for example,
transmissions can be less frequent when the tag is stationary.
For some situations, battery replacement may be accomplished by incoφorating the
battery in an attachment mechanism. For example, re-usable tag electronics may be attached to a disposable patient bracelet, with the battery included in the bracelet. As another example, a battery may be incoφorated into the clip of an LD bracelet. More generally, a battery may be
incoφorated into an inexpensive disposable portion of an active RFLD tag, with the electronics in the other, more expensive, portion.
If the item to which a tag is attached is a power source itself, the tag could tap into that
power source. This approach is most practical in situations where the tag can he designed into the equipment itself (such as a handheld computer), or where the equipment and power source
are large (such as a forklift). A larger power source allows for longer tag range.
Estimating Tag Location
A tag signal 107 is received at a time that is the sum of (1) known fixed delays in the cell
controller 102a that transmitted the interrogator signal 106, due to its circuitry and the wiring to
and from its antenna modules 104a-d, (2) fixed time delays in the antenna module 104a and tag
101a, and (3) the time for the interrogator signal 106 and tag signal 107 to travel through the air. Since (1) and (2) are fixed, attention can now be turned to (3), the travel time of the
interrogator signal 106 and tag signal 107 through the air. The duration of the pseudonoise
sequence modulated onto the 2440MHz carrier signal 106 by the cell controller 102a must be
greater than the combined travel time of the signal 106 and the tag signal 107. Techniques for
correlating a pseudonoise sequence are well known in the art. In the absence of multipath
effects, the cell controller 102a can derive a simple triangular correlation peak from the received
tag signal 107, as shown in Figure 5B. But in most indoor environments, the actual received tag signal looks more like that shown in Figures 5D-5G. Indoor radio signals are subject to substantial multipath effects, due to reflection from a variety of surfaces, such as whiteboards, fluorescent lights, file cabinets, elevator shafts, steel beams, and the like. When a tag 101a
transmits a tag signal 107, the sum of the direct tag signal 107 and reflected signals is received at the cell controller antenna module 104a. A variety of approaches can be employed in order
to extract a correlation peak from such information, with the particular approach chosen depending on the available signal quality, processing power, and required performance.
A 40-megahertz chip rate results in a correlation peak with a rise time of 25 nanoseconds,
corresponding to a rise time distance of about 25 feet. Since tag location is calculated using
round-trip travel time, single-chip accuracy therefore allows tag distance to be calculated within
about 12 feet, without any advanced signal processing.
Approximate location of a tag can be calculated by noting when correlated signal-to-noise
ratio rises above a predefined level. Improved accuracy can be achieved by trying a small variety
of carrier frequencies and choosing the one that rises the earliest; such frequency diversity is
supported by the radio system shown in Figures 4-6. This approach is sensitive to the system
signal-to-noise ratio. An alternative approach finds the peak of the correlation function. For an improved
result, the signal delay is measured by taking the negative of the cross-correlation function's
second derivative and finding the location of its peak, as shown in Figure 5G.
For highest accuracy, the MUSIC algorithm, known to those in this field, can be used, for which
accuracy in the 0.01 chip range has been reported. MUSIC requires frequency diversity, which
is supported by the radio system herein disclosed and shown in Figure 6. The method is based
on a decomposition of the eigenvector space of the pseudonoise correlation matrix of the delay profile data vector. Frequency diversity is required, where each distinct frequency provides the information to solve for an additional multipath component. For tags that are mostly stationary,
necessary data can be collected and the calculation completed as a background process. For inventory applications, motion detectors can be incoφorated into the tag, which would then
inform the cell controllers whenever their locations need to be recalculated.
In situations where adequate data collection time, antenna diversity, frequency diversity,
or processing power are not available, a variety of heuristic techniques can be used to calculate
tag location, even in the presence of severe multipath effects. A variety of techniques well- known in the art can be used to estimate bearing to each antenna, which can be employed as
required.
In many circumstances, although precise accuracy in calculating tag location is not
required, it may still be important to calculate tag location with reference to a floor or partition.
Story-by-story discrimination in a building can be accomplished by mounting antennas in the
ceiling that radiate downward (or antennas in the floor that radiate upward), as shown in Figure
2. Similarly, antennas mounted sideways on horizontal partitions can determine location relative to those partitions. Relatively narrow beamwidth antennas, which are less sensitive to multipath
effects, can be directed at doorways and the like.
A cell controller antenna 104a can be mounted near a computer screen, with coverage
corresponding to viewing angle of the screen. Software can then be arranged to automatically
configure the operating system for the person in range, or to blank the screen for security
puφoses depending on who is or is not in range. Similar concepts can be used in conjunction with copying machines, microfilm readers, restricted equipment, and the like.
A single antenna module can include three separate antennas placed in a triangle. By comparing phase difference using the Inphase and Quadrature components of the returning signal, an indication of the tag's angle can be determined. In high frequency embodiments, such
as at 2.45 gigahertz, such antennas could be within inches of each other and be quite effective.
Heuristic techniques can be used to analyze the correlation profile to estimate the time at which the correlation began, that is, the time at which the correlation peak begins to be
distinguishable from the "noise" baseline. Frequency diversity can provide a variety of samples, the best of which can be chosen. Improved estimates can be achieved by pattern matching the
correlation peak with one of a vocabulary of well-studied typical correlation profiles. For
calibration puφoses, tags can be placed at known fixed locations, and tags passing near those
locations will likely demonstrate similar correlation profiles. Such fixed tags can also be utilized
to detect the presence of jammers (objects which emit, intentionally or unintentionally,
interfering signals) and to provide a real-time testbed for trying a variety of antijamming
techniques.
Antenna diversity is the most important tool for improving the accuracy of tag location
calculation. If low accuracy is required, antennas can be placed so that only one antenna is in range of a given tag. In this case there is insufficient data for triangulation, and only enough
information to detect tag presence and estimate tag distance from that antenna. Approximate
bearing of a tag can be estimated from the signal strength of antennas designed for the puφose
and well-known to those skilled in the art; with the caveat that such bearing tends to reflect the
strongest signal received, which might include a substantial multipath component. Conversely,
for areas requiring high accuracy, a diversity of relatively narrow beamwidth antennas can be
installed, for example, at entrances, which together provide a clear picture of location.
Antenna diversity also provides system scaleability. For facilities or sections within facilities, that do not require calculation of tag location with high accuracy, substantially omnidirectional and/or ceiling mounted antennas can be installed relatively far from each other, for a relatively low cost per square foot of coverage. For facilities, or sections within facilities,
that require high positional accuracy, a diversity of closely-spaced and/or directional antennas can provide high accuracy at an increased cost.
Cell Controller Operation
The design of the overall system, and the fact that the intervals between tag signal transmissions are generated pseudo-randomly, provide an opportunity for secure operation. Tags
dispersed at the entrance of a secure facility can be tracked through that facility and a special
code can be emitted when the tag is tampered with. Although the tag's code can be determined
by monitoring the tag's response, the tag's transmission interval varies according to an algorithm
that can be arranged to be known only to the tag and the host, and impossible to directly
determine without destroying the tag. The tag may for example, include an element, such as a
physical element, for reprogramming its code and transmission interval. For example, a photo LD with an incoφorated tag might be reprogrammed each time the person wearing the photo LD
passes a security checkpoint, potentially in conjunction with biometrics technology.
For full coverage of a facility, multiple cell controllers can be installed, covering
somewhat overlapping areas. Although each cell controller will be operating according to a
search and data collection method, rapid movement between antennas, the pseudonoise code, the
changing chipping rate, and so forth, will appear as random noise to another cell controller. In
addition, codes with known cross-correlation characteristics, such as Gold Codes, can be allocated to various cell controllers by the host computer, particularly the codes used to search for tags. Alternatively, cell controllers can switch choice of pseudonoise codes on a randomized
basis.
For tags on the border between two cell controllers, each cell controller reports the tag's
distances from its antenna modules. The central host 105 assembles this data to calculate the
tag's location.
A variety of pseudonoise codes are available for use by a cell controller. Thus, if one
code seems to be receiving interference from other users of the spectrum, the cell controller can choose another code. The tag, being in essence a transponder, does not need to know the
particular code being used. Likewise, center frequency can be adjusted somewhat if another user
is causing difficulty.
Digital Signal Processing
Referring back to Figure 10, the signal processing hardware 1004 performs operations
upon the output of the digitizer 636. The signal processing hardware 1004 generally functions to perform operations upon the signal from digitizer 636 such as, for example, the previously
described correlation. Additionally, signal processing hardware 1004 may perform a filtering
process to filter out the noise components of received data signals. Various filtering techniques
are known to those skilled in the art, such as the use of an adaptive transversal filter. Following
is a description of functions that may be performed by the signal processing hardware 1004 in
a preferred embodiment of the article tracking system of depicted in Figure 1.
Referring now to Figure 15 A, shown is a block diagram of a preferred embodiment of the signal processing hardware 1004. Generally, the signal processing hardware may include one or more hardware components that collectively perform digital signal processing of the received signals. Recall that the signal processing hardware 1004 was previously described in
connection with Figure 10. The processing set forth below may be implemented using any
combination of conventional off-the-shelf hardware and/or software, utilizing DSP and/or FPGA
hardware and/or semi-custom or custom ASICs, that may be configured by one of ordinary skill in the art using the description set forth herein.
Shown in Figure 15N is signal processing hardware 1004 which is connected to the
microprocessor 1001 and the digitizer 636, as previously described in connection with Figure
10. Included in the signal processing hardware is a correlator unit 1800 connected to other DSP
hardware components 1800. The number and type of components that are included in the other
DSP hardware components 1802 vary with the type of processing done in each particular
implementation. Each of the hardware components included in the signal processing hardware
maybe controlled by the microprocessor 1001, such as by using connections 1804a and 1804b.
The connections required vary with each particular implementation and associated processing. One embodiment of signal processing hardware 1004 uses only the correlator 1800. In
this instance, data is directly output by the correlation unit, as indicated by the path 1804a.
Generally, the other components 1802 and associated connections, such as 1804b, would not be
used in this embodiment. As will be described in paragraphs that follow in connection with
Figure 15B, only a portion of the method steps of Figure 15B are performed by the signal
processing hardware in this embodiment. In an alternate embodiment of the signal processing
hardware which will also be described in paragraphs that follow, all of the method steps of Figure 15B are performed as part of digital signal processing.
Referring to Figure 15B, shown are method steps of an embodiment for filtering received
signals to enable determination of when a tag signal has been received. Nt step 2000, a number of samples of received signals are taken. The number of samples to be taken varies with each
particular implementation. Generally, in accordance with well accepted principles known to
those skilled in the art, the sampling rate is typically twice the transmission rate. For example,
in one embodiment 254 points or received signals are sampled where each of the 254 signals is
provided every 12.5 nanoseconds corresponding to the 80 megahertz sampling rate in which the
system has a chipping rate of 40 megahertz. In this system, the signal transmitter rate is 127
chips per bit, or rather, every 3.2 microseconds a bit of data is transmitted. Generally, "chipping
rate" and "chip time" are described in "The Practical Engineer", IEEE Spectrum, Vol. 35, No.
9, September 1998. At step 2002, a first range of the sample received signals is determined. The first range
is a subset of the samples recorded, as in step 2000. Generally, certain factors which are
dependent upon each particular implementation may be considered when determining the starting
point and size of the first range of step 2002. One factor that may be considered is the anticipated arrival time of a signal returned by a tag. This relates to, for example, anticipated delays in the transmission circuitry and may be used in determining a starting point of the first
range.
Another factor that may be considered relates to the tag transmission range and the
distance at which tags may be expected to be located. This may affect both the starting point of
the first range as well as the size of the first range. For example, if objects in one system are
known to be located within a small range, then the earliest possible time which a signal may be received by an antenna of a cell controller is earlier than a time of a different tag in another
system in which objects are known to be located farther away from the transmission source. This may affect how many of the received signals which occur earlier in the sampling may be
disregarded.
Generally, the data collection event, the transmission of the signals, and the anticipated
hardware delays and other timing delays may be calibrated in accordance with the starting point
of the collection or sampling. In one particular embodiment, due to the calibration of the
transmitting and the data collection as well as the anticipated delays, approximately the 70th
sample was determined to be the beginning of the first range because this was one of the earliest
points at which a return signal may be expected in a particular embodiment. Also in this
embodiment, the actual span or size of the first range is 26 which was determined in accordance
with the anticipated range of distances of objects to be located.
Nt step 2004 auto-correlation is performed for the first range of samples producing a
magnitude for each sample in the first range. The performing of auto-correlation upon received
signals was previously described. For example, Figure 5B shows the results of performing auto¬
correlation upon the wave form 500 as shown in Figure 5 A. However, the waveform 510 of Figure 5B does not include any "noise" in addition to the originally transmitted signal 500 as
shown in Figure 5N. One preferred embodiment implements the auto-correlation portion using
a field programmable gate array (FPGN). The ability of the FPGN to perform massively parallel
operations, such as matrix operations, efficiently in hardware is one factor in considering using
the FPGN to perform the auto-correlation function of step 2004. Generally, use of particular
hardware, such as the FPGN, may reduce the real time calculations and computational costs
associated with performing expensive matrix operations, as in the method steps of Figure 15B.
Generally, in the calculation of the autocorrelation function, 254 consecutive samples of received data ("sample waveform") are compared with an idealized version of the same data ("reference waveform"). If a coherent demodulation is available, then a real correlation may be
performed. Otherwise, a complex correlation may be performed. Autocorrelation may generally
be defined as the integral:
Figure imgf000042_0001
The above equation is a measure of the similarity between a signal and a phase shifted replica of itself. An autocorrelation function is a plot of autocorrelation over all phase shifts (t-r) of the
signal, where ±t, the change in time, is in half-chiφ intervals.
For a 127-chip sequence, sampled once every half chip, each correlation calculation takes
254 multiply-and-add operations, and calculating the entire autocorrelation function takes 2542
which is approximately 64,000 multiply and add operations. If a complex reference waveform
is used, computational complexity is increased by a factor of 4. Even with very fast hardware
or specialized signal processing hardware, this number of calculations may cause a "bottleneck"
due to the amount of time required to perform the calculations. A correlator implemented in hardware can generally make a quick estimate of tag
location by combining various techniques, some as described above and other which will be
described in the paragraphs that follow.
Since the reference waveform generally includes l's and -l's, 2's complement arithmetic
may be used for the multiplication operation. With this simplification, one of ordinary skill in
the art may implement the foregoing using a field programmable array (FPGA) and/or semi-
custom or custom ASICs, enabling operations to take place in parallel and with generally high
throughput.
It is generally not necessary to calculate each interval of the autocorrelation function. Some of the range may be ignored because the tag is low-powered and can only be detected at
a limited distance. For example, if the tag's radio has a maximum range of 100 meters, there is no reason to perform the autocorrelation function with phase shifts corresponding to the
distances in excess of 100 meters. Additionally, in searching for the leading edge, phase shifts
of a full chip or more may be used to search for the signal, and then half-chip intervals may be
used in the neighborhood corresponding to the time when the first signal is detected. More
generally, a subset of the 254 autocorrelation offsets may be used in the search for the peak or
the rising edge of the autocorrelation function This is described in more detail in paragraphs below.
At step 2006, heuristics are used to select a sample point which approximates where in
the first range of received signals is the received tag signal. Generally, step 2006 produces a
rough estimate as to the timing of the returned signal. This selected sample point is used, in
connection with step 2008, to further refine and limit the sampled data points considered in
determining the returned signal. One heuristic or technique which may be used to approximate the location in the first range of the received tag signal is related to the strength or magnitude of
the signals within the first range. By looking for the autocorrelation peak in the first subset, this
corresponding signal may be used in approximating where the received tag signal may be
located. Generally, this is based upon the premise that the strongest received signal corresponds
to the direct path of the received tag signal.
The rising or leading edge detection technique is a second heuristic that may be used to
approximate where in the first range of receive signals is the actual receive tag signal. Generally,
the samples are observed until a large or significant change in slope is detected. The actual determination of what is "large" or "significant" is relative to each system and varies with each implementation. One technique used with the rising or leading edge detection may include using
a normalized value from 0.0 to 1.0 where 1.0 corresponds to the signal with the maximum
amplitude received. When two points are encountered in which the slope of the line formed between these two points is greater than, for example, 20 percent of the normalized value, then
this change may be considered large enough to signal a significant change in slope. Generally,
the rising edge detection technique is based upon the assumption that the first peak is the line-of-
sight returned signal. Note that this is different than the premise or assumption of the first
technique which is based upon the assumption that the strongest returned signal is the returned
tag signal.
Yet another heuristic is a threshold detection technique. A threshold value is determined,
and the first sample point having a magnitude greater than or equal to this threshold is the
selected sample point. The threshold value chosen varies with environmental and
implementation. Running trials of the system is one suggested method for choosing a threshold
value. At step 2008, a second range of samples is determined which is a subset of the first range
of samples using the approximated location as determined in step 2006. In determining the
second range of step 2008, the precise starting and end point as well as the size or span of the
range must be determined. Both the span of the second range as well as the precise starting and
ending points of the second range may be related to or dependent upon the heuristic used to
approximate the location of the received tag signal in step 2006. For example, if the peak or
maximum amplitude of the received signal were used in determining the approximate location of the receive tag signal, one common technique would be to take a specified number of equal
points to the right and to the left of this peak and use this to correspond to the span and beginning and end points of the second range. If a different technique were used, such as the leading or rising edge detection technique, a varying number of points before and after the rising edge may
be used. One preferred embodiment, for example, may use the rising edge detection technique
or the threshold detection technique. In this embodiment, a range is determined having a
starting point which is three to the left of the rising edge, and eight points to the right of the
rising edge. The second factor to be considered is the actual size of the second range. It
should be noted that the size of the second range in one particular embodiment is 12. The reasons and factors that may be considered when choosing the size of the second range will
become more apparent in light of following paragraphs describing the different operations which
are performed upon the second range of data.
In step 2010, the recursive least squares (RLS) technique using the second range of
samples is performed. Generally, the RLS technique is used in the design of adaptive transversal
filters and is based upon the least-means square adaptation method, as generally known to those
skilled in the art. Functionally, the RLS technique used in step 2010 is used to filter out the noise component of received signals. In this embodiment, the RLS technique is used to filter out the
noise components of the second range of sampled receive signals. The precise steps and how
the RLS algorithm works are disclosed in the paragraph that follows.
At step 2012, a vector of filtered samples corresponding to the second range of samples
being filtered are produced. As previously described, this vector contains values which correspond to filtered received signals.
At step 2014, the approximate peak coπesponding to the filtered received tag signal is determined using the values included in the vector. Detail is described in the paragraphs that
follow regarding how the peak is approximated using the values included in the vector produced
by step 2012.
In step 2016, the tag distance is determined using the time of the filtered received tag
signal. The precise details of how to determine tag distance using the time of the filtered
received tag signal were previously described based upon the difference between transmission
time and receipt time of the tag signal.
One of the functions of the signal processing hardware 1004 is to filter the noise component out of a received signal. Before describing detailed steps of how the RLS technique
is used in performing this filtering process, a general description of how the RLS technique is
used in a feedback control system to perform this filtering function is described.
Referring now to Figure 16, shown is an example of an embodiment of a block diagram
of a feedback control system which filters out noise components of received signals. Generally,
the function of the block diagrams of Figure 16 combined filter out the noise components of a
received signal. In this particular embodiment, Figure 16 depicts a feedback control system
which includes a transversal filter 2022 and an adaptive weight control mechanism 2024. Generally, the transversal filter 2022 operates upon an input signal u and represented as an input
vector u(n) with a varying number of components. The transversal filter produces an output
signal E(n) which provides an estimate of the desired response d(n) 2028. In this instance, the
desired response or signal d(n) is the actual received signal. The estimated signal E(n) produced
by the filter is compared with the desired response signal 2028 to produce an error estimation
(n) 2030. This estimation eπor (n) is the difference between the desired response signal d(n)
and the estimated signal E(n) 2026 as produced as an output by the transversal filter. This error value (n) 2030 is used as an input and feeds back into the adaptive weight control mechanism 2024. The adaptive weight control mechanism 2024 is a mechanism for performing the adaptive
control process by varying certain parameters which feed back into the transversal filter 2022.
Generally, the transversal filter 2022 may also be referred to as a tap delay line filter.
Further description and details of the transversal filter is described in paragraphs that follow in
connection with Figure 17. In this particular embodiment, the RLS algorithm extends the use
of the method of least squares to provide a recursive algorithm for the design of adaptive
transversal filter such that, given the least squares estimate of the tap weight vector, w(n-l) of the filter at time n-1, the updated estimate of this vector at time n may be computed. The
estimated signal E(n) 2026 is denoted as wH (n-1) u(n). The precise notation of this will be
described also in paragraphs that follow.
Referring now to Figure 17, shown is an example of one preferred embodiment of a
transversal filter 2022. Generally, a transversal filter includes three basic elements: unit delay
elements (2031a-2031m), multipliers (2032a-2032m-l), and adders (2033a-2033m-l). In
particular, when unit delay operator 2031a operates on input u(n), the resulting output is u(n),.
The role of the multiplier 2032a in the filter is to multiply the tap input 2030a by a filter coefficient refeπed to as a tap weight denoted w* 0 (n). It should be noted that the asterisk in
Figure 17 denotes complex conjugation which assumes that the tap inputs, and therefore the tap
weights, are all complex values. The combined function of the adders in the filter is to sum the
individual multiplier outputs and produce an overall filter component denoted y(i). In this
particular embodiment shown in Figure 17, the number of delay elements is shown as m- 1. This
is commonly refeπed to as the order of the filter. Each of the components 2031b-2031m-l
operates in a manner similar to 2031a. Similarly, multipliers 2032b-2032m-l operate similarly to the multiplier 2032a. Generally, the input signal u(n), and the tap weights denoted w(n) are represented as vectors with each element of the vectors coπesponding to various components.
When the various components are summed, they produce an estimated signal denoted y(i) in Figure 17. u(n) denotes the tap input vector at a particular time n. The tap weight vector w(n)
defines the tap weight vector at a particular time n.
Generally, the recursive least squares or RLS technique attempts to choose a tap vector
w(n) which minimizes the expected squared eπor. The eπor is determined as the sum of the
differences between the expected or actual signal and the estimated signal output from the filter. The adaptive weight control mechanism 2024 of Figure 17 is used to determine the weighting
factor associated with each of the eπors at a particular point in time where the eπor is the
difference between the desired response or the actual output signal, and the estimated signal
produced by the filter. The use of the weighting factor is intended to ensure that data in the
distant past is "forgotten" in order to afford the possibility of following the statistical variations
of the observable data when the filter operates in a non-stationary environment. Thus, use of the
weighting factor allows additional weight to be given to the eπor values which are most recent
in time, and give less weight to those eπor values which are earliest in time. The precise use of a weighting factor and how it relates to the measure of the "memory" of the RLS technique will
become more apparent in following text.
Generally, the adaptive weight control mechanism 2024 provides a coπection factor
which is applied to the tap weights upon subsequent processing of data. In other words, a
coπection factor determined at time n is applied to the tap weight at a time of n+1. As known
to those skilled in the art, a scaled version of the inner product of the estimation eπor and the tap input denoted u(n-k) is computed for k=0,l,2... to m-1. The result obtained defines this
correction factor which is applied to the tap weights. Thus, the error factor provides for adjustment or coπection of the various tap weights. This is the nature of the feed back mechanism of the system 2018 of Figure 16.
The previous descriptions regarding Figures 16 and 17 present a general description of the transversal filter and its corresponding adaptive control mechanism as generally known to
those skilled in the art of adaptive filter theories as may be used in digital signal processing, for
example.
Now what will be described is the RLS algorithm as generally set forth in the textbook
entitled "Adaptive Filter Theory", by Simon Haykin, ©1986 by Prentice Hall, Inc.
Referring now to Figure 18, shown are method steps of an embodiment for performing the recursive least squares (RLS) method. At step 2034, variables are initialized to initial
conditions. Generally, this includes variables used in subsequent method steps such as the loop
control variable n. Some of the variables used in Figure 18 processing steps are initialized to a
set of initial conditions. Specifically, n is initialized to one in the method described in Figure
18. The tap weight vector w and the tap input vector u may be initialized to 0. The inverse of
the coπelation matrix denoted P(0) may be initialized to the inverse of a small positive constant time the identity matrix denoted I. The recommended choice of the small positive constant A is
that it should be small when compared to 0.01 times the variance of the data sample u(n).
Generally, λ is a small positive constant which may be refeπed to as the "forgetting
factor". Since this constant is between zero and unity, multiplying a variable by λ reduces the
magnitude of the variable. Using the RLS technique, a weighted sum of square errors is
minimized by choosing a vector of coefficients. In this summation, the squared errors in the
"past" are weighted by higher powers of λ that are more "recent" squared errors. Thus, the coefficients produced by the RLS technique is generally chosen with less regard for eπors in the past.
There are many "rules of thumb" for choosing λ which may depend upon the variance of the data, the variance of the additive noise, the number of components to be determined, and the rate of change of the underlying system to be estimated. It may generally be helpful to note
that the effective number of errors which are not yet forgotten is approximately l/(l-λ). A forgetting factor of 1/100, for example, yields roughly 100 significant errors. If the underlying
system to be estimated is time invariant, then λ should be set as close as possible to unity. In the
instance of a time- varying system, λ should generally be chosen such that:
> 3 *(tbe number of coefficients estimated)
X -λ
and such that generally:
1 .< t/?e number of samples for which system is approximately constant.
1 -λ
The former condition generally reflects the fact that the RLS technique provides a converged
solution in 3 to 5 times the number of coefficients for cases in which the additive noise is not too severe. The latter condition reflects the fact that the RLS technique attempts to find, for each
sample input, a single vector of coefficients which may be used to approximate the last 1/1-λ
outputs of the time-varying system.
In accordance with principles described in the Haykin textbook, certain factors should
be considered when initializing values for use with the RLS method. In particular, regarding a
starting value for P(0), a starting value should be chosen which assures the non-singularity of the coπelation matrix.
After initialization, control proceeds to step 236 where a determination is made whether or not the loop control variable n is less than or equal to the number of desired iterations. If the
determination is made that n is greater than the number of desired iterations, meaning that execution of the method steps of Figure 18 is complete, control proceeds to step 238 where the
method depicted in Figure 18 stops. If a determination is made at step 2036 that n is less than
or equal to the number of iterations, then control proceeds to step 2040. Nt step 2040, the gain denoted k(n) is computed as:
Figure imgf000051_0001
Control proceeds to step 2042 where the a priori estimation eπor denoted α(n) is computed. α(n) is computed as:
a(n) = d(n)-wH(n-X) u(n) Control proceeds to step 2044 where the tap weight vector for a particular instance in time
denoted w(n) is computed as:
w(n) = w(n-X) + k(n) *(n)
Control proceeds to step 2046 where the inverse of the coπelation matrix denoted P(n) is
computed as:
P(n) = λ"1 P(n-X) -λ"1
Control then proceeds to step 2048 where the loop control variable n is incremented by 1.
Control proceeds to the top of the loop at step 2036 where a determination again is made whether or not the loop formed by steps 236-248 has been performed the desired number of iterations.
It should be noted that the method step 2044 describes the adaptive operation of this
method whereby the tap weight vector w is updated by incrementing its old value by an amount
equal to the complex conjugate (denoted by an *) of the a priori estimation error (n) times the
time varying gain vector k(n), hence the name "gain vector". The a priori estimation, denoted
(n) represents the a priori estimation error. Generally the a priori estimation eπor refers to an
estimate of the eπor based on a tap weight vector that was made at time n-1. The constant λ is
a value close to 1. 1-λ "represents a measure of the memory of the algorithm". As previously described, this is a weighted value introduced in a definition of the cost function based on the
eπor at time n. Generally, λ is a positive constant close to but less than 1. When λ equals 1 , we
have the ordinary method of least squares. The inverse of 1 minus λ, generally speaking, is a
measure of the memory of the algorithm. The special case where λ equals 1 coπesponds to having infinite memory. As previously described in accordance with the use of a weighting
factor being associated with the eπor at each particular point in time, λ is used in determining
the actual weight given to particular values of the eπor determined at different points in time.
It should generally be noted that this algorithm is deemed to be "recursive" for the fact
that updating the tap weight vector at time n, a prior value for the tap weight vector at time n-1
is used. This becomes apparent when method step 2044 is examined where the tap weight vector
denoted w(n) is computed as being dependent upon the value w(n-l). It should also be noted that in the flowchart of Figure 18, the asterisk denotes the complex conjugate of a number. Additionally, the superscript of H, as depicted in step 2042 when associated with the tap weight
vector w, implies that the tap weight vector w has the Hermitian property. Generally, a complex valued matrix such as the tap weight vector w(n) is Hermitian if it is equal to its conjugate transpose, as known to those skilled in the art.
Referring back to Figure 15, step 2010 performs the RLS technique just described using the second range of sample receive signals. Also recall that a note was made that the size or span
of the second range may generally be related to use of the RLS technique. The RLS technique performs matrix operations which are generally expensive in terms of computing time and
resources. Thus, this expense is often a factor to be considered when determining the size of the
second range in that the size of the second range affects the dimensions of the matrix and hence
the number of matrix operations which are performed in computations of the RLS technique for
the second range of samples. Additional factors should also be considered when choosing the
size of the second range. Generally, if the number of points considered in the second range is
too large, and the noise component of the receive signal includes tonal frequencies or jammers,
then the model may follow the peaks of the jammer signals rather than properly fit a curve identifying the filtered received signal. Additionally, if the size of the second range is too small,
then enough points may not be considered to properly fit the curve. It should generally be noted
that the number of points or data samples to be included in the second range varies with the
system and implementation.
The RLS technique used in this embodiment assumes that the observed sequence includes a linear combination of a known number of the data sequence. Generally, the RLS technique
attempts to find the combining coefficients for this linear combination which best fits the
observation. Generally, the RLS technique assumes a wide-sense stationary process. Generally, the RLS technique also presumes the presence of random additive noise which is uncorrelated with the data sequence. With the presence of "white noise", the sum process
remains generally wide-sense stationary. However, with the addition of a tonal frequency or
jammer frequency, the sum process is no longer wide-sense stationary. To use the RLS technique when the sum process is not wide-sense stationary, such as may be in the application
of this embodiment as used indoors, a coπective factor should generally be considered. One
corrective technique that may be considered is in choosing the number of coefficients or taps.
In this instance, with the sum process not being wide-sense stationary, care should generally be taken to insure that the number of coefficients are not overspecified when using the RLS
technique. In this embodiment, the noise input to the coπelator may be modeled as the
superimposition of two components: a wide-sense stationary white noise process, and a non-
stationary intermittent plurality of tones of unknown frequencies and amplitudes. Based on this
model for this particular embodiment, it is found that the number of coefficients or taps should generally span the main peak of the impulse response magnitude. However, the number of taps
should generally not span more than this in this particular embodiment.
Generally, the number of taps may vary with embodiment and application. For each
application and embodiment, the observed process should be modeled to take into account all
factors and a number of taps chosen in accordance with these considerations as described herein.
As just described, factors that may be considered when choosing a size for the second
range include consideration of the time complexity regarding computational expense as well as
application for real time considerations when performing complex calculations. Additionally, the size of the second range varies with the environment in which this application will be used. If there will be tonal frequencies, such as a microwave oven within an indoor environment, this should be considered when choosing an appropriate value for the second range. In one preferred
embodiment, a value of 12 was used for the size of the second range for an indoor article
tracking system which included the previously described transmission rate of 127 bits per second
in a sample of 254 points of received. This indoor system may possibly have tonal frequencies
and other jammer signals since it is an indoor application as well as multipath noise. Thus, a
range size of 12 for the second range was used considering these factors.
Generally, the steps describing the RLS technique of Figure 18 may be performed using
a different data sample set for each execution or iteration of the loop. In one particular
embodiment, one data set of 254 received signals was recorded. Rather than perform the method
steps of Figure 18 with a different set of 254 data samples each time the method steps of Figure
18 were performed for a particular iteration, the same data set is used for each iteration. Ideally,
the number of iterations of the RLS algorithm should be as large as possible in order to meet the
mathematical convergence for simulating when n goes to infinity. However, in one prefeπed embodiment, the method steps of Figure 18, as described in the loop formed by steps 236
through 248, are performed for 84 iterations. Through experimentation for this particular
embodiment, it was determined that this was an optimal value to be used in the tradeoff of real
time application, computational complexity, and accuracy of locating an object. For other
systems and other applications, this number may vary.
As an output of the RLS method performed in step 2010, a vector of filtered samples is
produced in step 2012. Each element in this vector coπesponds to a component of a received signal which represents a filtered signal. In other words, each element of the vector coπesponds to a filtered signal with the noise portion removed. The vector produced in step 2012 is a vector
of filtered signals in which each element of the vector corresponds to an element of the second range. The RLS technique performed in step 2010 removes the noise component and returns a
filltered signal.
At step 2014, using the data points included in the vector of step 2012, a technique is
applied which approximates the peak corresponding to the filtered received tag signal. A more
detailed description of step 2014 is set forth in paragraphs that follow in connection with Figure
20.
Referring now to Figures 19A-19E, shown are sample waveforms of received signals for
a number of sample points. In Figure 19A, shown is an example of a coπelated received signal
with a small amount of "white noise". It should be noted that the waveform of Figure 19A
generally does not include a multipath component, as would be seen in the system of the
embodiment described herein. Figure 19B shows a waveform with a low degree of indoor
multipath components as may be included in a received signal. Figure 19C illustrates a
wavefonn that includes a medium degree of severity of indoor multipath noise in addition to a transmitted signal, and Figure 19D shows a severe amount of indoor multipath noise added to
a transmitted signal. Each of the waveforms shown in Figures 19B-D are waveforms that may
be received in different environments within which a prefeπed embodiment of the invention
operates. Shown in Figure 19E is a coπelated waveform, labeled "coπelation", which is also
typical as the output waveform after performing step 2004 of Figure 15. The second waveform,
labeled "model" in Figure 19E, is an example of a waveform resulting from graphing the 12 points included in the second range of samples which is output from the RLS technique in one
prefeπed embodiment. As previously described, this vector may be produced in step 2012 after performing the RLS technique in step 2010. The peak of the actual waveform, for example, may be approximated in step 2014, as the 4th data point of the "model" waveform of Figure 19E.
In this particular embodiment, the transmission rate is 127 chips per data bit. In other
words, the bit pattern of the transmitted signal repeats itself every 127 bits. N lesser number bit
sequence such as 31 chips, may also be used in a particular implementation which requires less
processing time and less number of samples to be taken. However, the gain is lost as you
decrease the number of patterns in the bit sequence. A longer bit sequence generally allows for
a greater distribution of energy over a larger period of time. A longer sequence generally enables a stronger signal at the output of the coπelator than that produced by a shorter sequence.
The processing gain indicates this signal strength enhancement and refers to the number of bits
in the sequence.
Other factors regarding a bit sequence which should be considered when implementing
the techniques described herein relate to the properties of the bit sequence regarding auto-
coπelation and cross-coπelation. For example, a maximal sequence has good auto-coπelation
but bad cross-coπelation. Thus, a transmitted signal which is a maximal sequence would produce an idealized peak waveform, for example as shown in Figure 5A. However, the cross-
correlation of such a sequence may not be desirable in an embodiment where better cross-
coπelation is required. For example, in an embodiment where one is required to detect or reduce
the interference between multiple transmitted signals, good cross-correlation is often needed.
For applications such as this where one is required to distinguish between multiply transmitted
signals, a sequence such as the Gold code may also be used. It should be noted that the precise
sequence length as well as the various properties of the sequence sent in the transmission signals may vary with application and each particular implementation.
In the previously described embodiment, a local positioning system was described in which the assumption is made that the articles or persons being tracked by the system may be at
a different position at any particular point in time. In other words, there is no assumption that the person or object will remain stationary for a majority of time. In an application of a system
which includes the local positioning system and the techniques described herein, if the objects
being located are primarily stationary for a majority of the time, then special processing may be
performed in the atypical case when an object is determined or sensed to have moved, as by a
motion detector. When motion of a primarily stationary device has been detected, special
processing may be performed using the techniques previously described. Since it is the atypical
case in which the location of an object is to be determined, in the small number of instances when the location of an object or person needs to be determined, the system may devote
additional processing time to locating the object. Thus, in an application of article tracking in
which the objects tend to remain stationary, minimizing the amount of computational time to the
extent as previously described may not be a factor in selecting, for example, the size or span of
the second range. It should be noted that various portions of the signal processing hardware 1004 may be
implemented in varying combinations of software and/or hardware dependent upon the particular
application and system implemented. For example, in one prefeπed embodiment, the auto¬
correlation function performed in step 2004 on a first range of samples is implemented in a
FPGA, as previously described. Dependent upon each particular system, application, and
requirements of each system, other functions of the signal processing hardware 1004 as described herein may be implemented in varying combinations of software and/or hardware.
In one of the previously described embodiments, the same data set was used when performing multiple iterations of the RLS algorithm of Figure 18. As an alternative to using the same data set for each iteration, a single data set set may be "reused" on subsequent iterations
with slight modifications, such as rotating or shifting to the right or to the left by one data element for each iteration.
Referring now to Figure 20, shown are example steps of a method for approximating the
peak corresponding to the filtered receive tag signal using the values included in the output
signal vector produced as a result of step 2014. The method steps shown in Figure 20 are more
detailed steps of the method step previously described for step 2014 of Figure 15. For all of the
magnitudes of the signals included in the output signal vector, determine the largest magnitude,
as in step 2100. It should be noted that in one embodiment, if the first or last element of the
vector has the largest magnitude, then an assumption is made that the received signal cannot reliably be determined, and the method stops execution.
Subsequently, in step 2102, a threshold value is determined which, in one embodiment
is equal to 62.5 percent of this largest magnitude previously determined in step 2100. At step
2104, it is then determined which element in the output signal vector is first in time to exceed the threshold value . This signal determined to be first in time to exceed the established
threshold is refeπed to as the X. It should be noted that in one prefeπed embodiment, if the
"X+l,h" element of the vector is smaller than the magnitude of the "Xth' element, then the "X-lsl"
element is used rather than the "Xth" element.
At step 2106, other elements of the vector are selected to be used in subsequent
processing steps to determine the actual received signal. In one embodiment, two other elements
are generally chosen. These are the vector elements denoted by the indices "X+l" and "X+2". It should be noted that the element indicated by the index value "X+2" is not used if the the
"Xth" element is the second to last vector element. In step 2108, using a weighted average formula of these 3 points, the actual received tag signal is determined based on the expected shape of the received signal as being a coπelated signal with a triangular peak. One embodiment
calculates or estimates the received tag signal as indicated in the pseudo-code type description
below. It should be noted that the description below generally summarizes that which is set forth
and previously described in conjunction with the method steps of Figure 20.
Total = 0
FOR each element in the vector, v, DO
{
Determine the weight of cuπent vector element, denoted by index^j, as: weight v[j] = MNX(0, magnitude(vector element j) - (largest magnitude of all vector elements/4) )
Total = total + weight v[j]
}
Received signal = X+l - (weightv[X]/Total) + (weight v[X+2]/Total) or Received signal = [X+l - (weightv[X]/Total) /** if element X+2 is not used and is the second to last element of the vector **/ The following points are worth noting regarding the previously described embodiments
of the signal processing hardware 1004 and previously described method steps of Figure 15B.
In one of the previously described embodiments of the signal processing hardware which
includes only the coπelator 1800, method steps 2008-2014 of Figure 15B are not performed.
Rather, only method steps 2000-2006, and 2016 of Figure 15B are performed. Specifically, in
this embodiment the autocorrelation (step 2004) is performed by the correlator 1800. The
approximation produced as a result of using the heuristics at step 2006 is considered to be the
received tag signal. A first heurisitic, as used in step 2006, determines the received tag signal to be the signal in the first range with the maximum magnitude of all the signals in the first range. A second or alternate heurisitic that may be included in an embodiment (at step 2006) is to choose a threshold value. The first signal included in the first range having a magnitude equal
to or exceeding this magnitude is determined to be the received tag signal. The method for
choosing the threshold value may vary with environmental and other factors particular to each
implementation. Generally, this threshold value is selected in accordance with trial test runs of
particular implementations to allow for "tuning" the threshold value. Using either heuristic, this
received tag signal is then used (step 2016) in determining the tag distance.
Other embodiments of the signal processing hardware 1004 may include other hardware
components in accordance with the particular digital signal processing requirements in a
particular embodiment. Each particular embodiment may be implemented in using a variety of
combinations of hardware components, including, but not limited to, gate arrays and read-only-
memory. Additionally, other embodiments of the signal processing hardware may be
implemented as some combination of hardware and software in which the machine executable
code may be executed on a computer system, such as the microprocessor 1001 or the host computer 105 or yet another computer component included in the signal processing hardware
1004 as a dedicated processing unit. The components may vary with application and design choices associated with a particular implementation.
Another embodiment of the signal processing hardware includes hardware and/or
software in addition to the coπelator. This embodiment may perform, for example, all the
method steps of Figure 15B, rather than only some of the steps of Figure 15B, as in the embodiment with signal processing hardware that includes only the coπelator.
The foregoing description sets forth a technique using an RLS method that affords a flexible and efficient way of filtering noise from a received signal as used in an article tracking system. Both multipath noise and tonal frequencies may be taken into consideration as factors
when using the foregoing techniques in various environments within which an article tracking system as described herein may operate.
The foregoing technique for filtering a received signal is scalable for use in applications
with both large and small sample sets in a variety of different environments each having
different "noise" considerations while generally providing a high degree of accuracy in locating objects as required in article tracking systems as described herein.
Other embodiments are within the scope of the following claims. For example, the order of performing steps of the invention may be changed by those practiced in the field and still achieve desirable results.
An alternative embodiment includes modifications to the previously described cell
controller and tag that are described in paragraphs that follow.
Chassis General RF description The chassis RF subsystem consists of three of the five major cell controller components:
the cell controller transmitter module, cell controller receive module and the single-pole, 4 throw
switch modules. The digital section and the controller section are described elsewhere. This
description outlines the operation of all three elements and presents performance data.
The function of the cell controller transmitter is to modulate the digital baseband
spreading signals onto the transmitter carrier to generate the direct-sequence spread-spectrum
RF signal. This transmitted signal must comply with the FCC Part 15 regulations, specifically with sections 15.247, 15.209 and 15.205. The transmitted RF signal power level must be digitally adjustable over a range commensurate with the variation in path loss of the transmitted signal to the tag (about 32dB at 2.44GHz over 125 feet). The final radiated signal should have an effective isotropic radiated power level of about 30dBm (1 watt).
The function of the cell controller receiver is to convert the received signal delivered by
the antenna to an intermediate frequency, demodulate the spread-spectrum signal from the tag
and deliver a filtered baseband signal to the analog to digital converter. The receiver also
implements automatic gain control over a 50dB range at the intermediate frequency. The
function of the single-pole, four throw switch module is to properly route the combined transmit / receive / DC signals to the coπect antenna.
The cell controller RF elements have some common features that are digitally controlled
by signals from the microprocessor. The transmit and receive local oscillators (which control
upconversion and downconversion carriers) are set digitally. The transmit power control is set
with digital signals so that the total received signal SNR can be maximized over the entire path
traverse of a tag (from right on top of a particular cell controller antenna to maximum distance
from that antenna). The receiver gain level is digitally measured and delivered to the baseband signal processor. Finally, the transmit and receive signals are combined onto one wire (which
includes DC power) and this combined signal is switched from antenna to antenna by digital
control from the microprocessor. Up to sixteen antennas may be installed on each cell controller.
Cell Controller RF Description
General Description
Referring now to Figure 21, shown is a block diagram of an embodiment of a cell controller. The cell controller RF elements can best be understood by referring first to the block diagram of Figure 21. The first item to note is the cell controller RF elements include active components in the antenna. This is because the signals sent up to the antenna are low in level and
widely spaced in frequency. The RF cable transports the low-level Tx signal at 2442MHz to the
antenna as well as the low-level Rx signal at 5800MHz from the antenna to the receive module
in the cell controller. The cable also supplies DC power to the antenna from the chassis. Note
that the transmitter module provides the first stage of RF switching to the antennas: it incoφorates the "bank" switches that select which of the four SP4T (Single Pole, Four Throw)
switch modules are to be used. Ndditionally, it incoφorates the diplexer that separates
transmitted signals from received signals and also a stage of Rx signal amplification.
The cell controller receive module will be briefly presented to show the frequency plan
and signal flow. Also, the operation of the active antenna will be presented in detail.
Transmitter Module In one embodiment, the transmitter module may be included in a card that fits into the
main backplane of a 3D-iD chassis and provides the necessary transmitter signals. It receives DC
power from the backplane. It uses input signals from the digital module and provides signals to
the receive module and the digital module. It provides transmitted signals to the active antennas
through SP4T switch modules installed in the chassis.
Modulator 2200
The digital baseband signal that represents spread data is an input to the transmit module. It modulates a 36OMHz signal (provided by the 360MHz LO synthesizer module) in the biphase modulator consisting of a driver and a mixer. This modulator generates a classic Sin(x)/x spectrum at 360MHz and provides a signal level of about -7dBm to the next stage.
IF Local Oscillator 2204
The intermediate frequency (360MHz) local oscillator provides a carrier for the
modulator. It consists of a commercial voltage-controlled oscillator, a buffer amplifier, a
programmable synthesizer and a lowpass filter. The synthesizer is programmed by the
microprocessor and returns its lock-detect status. The nominal output level for this LO is +6dBm.
SAW Bandpass Filter 2202
The function of the surface acoustic wave bandpass filter is to pass through only the main
lobe of the modulated signals and to remove any sideband signals. This device is a passive filter and is used unmatched. This filter provides most of the suppression of out-of-band signals
needed to comply with the Part 15 regulations.
Transmit Level Control Attenuator 2206
The function of the transmit level control is to set the final antenna radiated output power
to the desired level. This can be done antenna by antenna and can be tailored to the installation
site. The microcontroller allows the installer to adjust each antenna's output power and stores these settings in a configuration table. As the cell controller operates each antenna in turn, it sets the transmit attenuator to the configuration value determined at installation. The transmit level
control is implemented with a digitally controlled attenuator that has a 31dB range in 1 dB steps. The attenuator is followed with a buffer amplifier.
Transmit Mixer and Bandpass Filter 2208
The transmitter intermediate frequency signal is upconverted from the intermediate
frequency to the final RF frequency in the transmit mixer. This mixer uses a local oscillator
signal at 2082MHz to convert the 360MHz transmit LV signal to 2442MHz, the final RF
frequency. The nominal LO level is +9dBm and the mixer suffers about 6dB of conversion loss.
The mixer is followed by a 3-pole ceramic bandpass filter that serves to pass only the desired
upper sideband and to reject the lower sideband and spurious signals.
RF Transmit Local Oscillator 2210 The RF local oscillator operates at 2082MHz and provides a carrier for the transmit
mixer. It consists of a commercial voltage-controlled oscillator, a buffer amplifier, a
programmable synthesizer and a power divider. The synthesizer is programmed by the
microprocessor and returns its lock-detect status. The nominal output level for this LO is +9dBm.
This synthesizer also provides the input tone for the receive module downconverter local oscillator (hence the power divider).
Transmit Preamplifier and Bandpass Filter 2212
The preamplifier and bandpass filter serve to boost the RF transmit signal to a
level sufficient to drive the final transmit amplifier without compromising signal linearity. The
bandpass filter serves to help clean up mixer spurious and to help remove harmonics. The
preamplifier consists of two stages that have a total gain of about 33dB. The amplifiers used in the preamp are identical to the buffer amplifier used in the transmit local oscillator. The bandpass
filter is identical to the filter following the transmit mixer.
Transmit Final Amplifier 2214
The final transmit amplifier serves to boost the transmitted RF signal to prepare for the
ride through the various switches and the RF cable on its way to the active antenna. It consists of a pair of amplifiers in a balanced amplifier configuration using hybrid couplers. The gain of
this stage is about 17dB and this amplifier provides a signal of about +10dBm (at the maximum
Tx level setting). The ldB compression point of this amplifier is about +19dBm. The amplifier
is implemented with devices identical to the Tx preamp and the LO buffer. The hybrid couplers
are implemented with the same power divider used in the local oscillator. Diplexer 2216
The function of the diplexer is to provide separate paths for the transmit and receive
signals so they can be combined onto one coaxial cable. The diplexer is implemented with
microstrip technology as a lowpass-high pass filter set with a common feedpoint. The isolation
requirement of each branch of the diplexer is eased by the extensive use of other bandpass filters
in the transmit, receive and antenna modules. This filter is fabricated on the printed circuit board.
Single Pole, Four Throw Bank Switch
The circuitry in this section 2230 serves to route the combined Rx / Tx signals to one of four switch banks. This switch is implemented as a cascade of three GaAs single-pole,
double-throw RF switches that are controlled by digital signals from another cell controller
module (digital module).
Single Pole, Four Throw Switch Module
The SP4T module is a separate plug-in unit that fits into a small backplane in the 3D-iD chassis. One module is required for the chassis, enabling 4 antennas to be connected 2232a-d to
the system. Up to 4 SP4T modules may be installed in the chassis, providing service for up to 16 antennas. The switch module RF configuration is identical to the single-pole, four throw bank
switch described above. Ndditionally, the SP4T module implements switching to impose DC
on the output ports after the RF signal has been routed. Bias tee circuitry is used to isolate the
DC switches from the RF circuitry (to prevent unnecessary RF loading). Note that the number of these modules varies in accordance with the number of antennae in a particular embodiment.
Receive Preamplifier 2220
The transmit module contains the first receive preamplifier following the diplexer. This
architecture was chosen because the receive signal needs to leave the diplexer and go to the
receiver module through a cable, so some signal stabilization was needed. This preamplifier is
implemented as a balanced amplifier with GaAs units. It has a gain of about 19dB and the balanced configuration guarantees a good impedance to the diplexer. The hybrids for this amplifier are implemented as traces on the printed circuit board.
lOLMHz Reference Generator 2222
Each of the phase-locked local oscillators used in the 3D-iD chassis requires a
high-stability reference signal. This section provides a reference signal for the 360MHz T LO,
for the 2082MHz LO and for synthesizers in the receive module. This reference generator is implemented with a commercial, high-stability crystal oscillator at 10MHz followed by buffer
amplifiers, a bandpass filter and a four-way power divider. The nominal output level on each of the four ports is OdBm.
Antenna Module 2300
Referring now to Figure 22, shown is a block diagram of an embodiment of an antenna
module. The antenna module 2300 includes a unit that mounts on the wall or the ceiling to
provide the necessary radiated transmit and receive signals for a 3D-iD tag. In addition to the radiating antenna aπays, the module has amplifiers and filters mounted on a printed circuit board
that also provides the ground plane for the antenna arrays. Nil inputs and outputs for this module
arrive on a 0.25" coaxial RF cable. The printed circuit board is mounted on an aluminum backing
plate and has a thermoplastic radome over the antenna aπays.
Antenna Bias Tee 2302
The bias tee extracts the DC current from the coaxial cable to power the active components on the antenna module. It provides a nominal 12 volts to the regulators on the antenna module that in turn provide 8 volts and 5 volts to the appropriate amplifiers.
Antenna Diplexer 2304
The diplexer is identical to the diplexer used in the transmit module.
Antenna Transmit Preamplifier 2306
The transmit preamplifier serves to boost the low-level transmitter signal delivered by
the coaxial cable to a level sufficient to properly drive the antenna final transmit amplifier. The
preamp is implemented as two amplifiers that are identical to the LO buffer amplifier in the transmit module.
Antenna Transmit Final Amplifier 2308
The antenna transmit final amplifier is implemented as a balanced amplifier using
a topology and components identical to the transmit module final amplifier described above. Antenna Transmit Final Bandpass Filter 2310
The antenna transmit final bandpass filter is a haiφin bandpass filter centered at 5800
MHz with a passband bandwidth of about 100MHz. The combination of the responses of this
filter and the filters in the transmitter module ensure that the radiated transmitted RF signal
conforms to the requirements of Part 15.247 and 15.209, especially at the allocated band edges.
Antenna Transmit Antenna Array 2312
The transmit radiating elements of the antenna module are a series-fed patch aπay (2 elements) whose center frequency is 2440MHz. The 3D-iD tag uses a low-cost, linearly polarized
patch antenna. To prevent tag signal loss due to antenna cross-polarization for tags that are not aligned with the active antenna, the radiating elements here are circularly polarized. This choice
of antenna polarization results in an additional 3dB of effective path loss between the active
antenna and the tag antenna. The gain of the transmit antenna array is about 1 ldBi.
Antenna Receive Antenna Array 2314
The receive radiating elements of the antenna module are a series-fed patch aπay (3 elements) whose center frequency is 5770MHz. The 3D-iD tag uses a low-cost, linearly polarized
patch antenna. To prevent tag signal loss due to antenna cross-polarization for tags that are not aligned with the active antenna, the radiating elements here are circularly polarized. This choice
of antenna polarization results in an additional 3dB of effective path loss between the active
antenna and the tag antenna. The gain of the receive antenna aπay is about 12dBi. O 00/52498
Antenna Receive Bandpass Filter 2316
The antenna receive bandpass filter is a 3-pole ceramic bandpass filter centered at
5770MHz with a passband bandwidth of about lOOMHz. This filter serves primarily to ensure
that only the desired tag signals are brought into the receive chain.
Antenna Receive Low Noise Amplifier (LNA) 2318
The antenna receive low noise amplifier is identical to the receive preamplifier described above. It is implemented as a balanced amplifier to present a good impedance to the Rx bandpass filter. The hybrids for this amplifier are implemented as traces on the printed circuit board.
Antenna Receive Buffer Amplifier
The antenna receive buffer amplifier provides enough gain to compensate for the loss of
the coaxial cable that ferries the received signal to the receiver module in the cell controller. It
is implemented with two stages, each having an active device identical to the one used in the
low-noise amplifier. The final stage has an impedance good enough to drive the diplexer without mismatch.
Receiver Module
The receiver module is a card that fits into the main backplane of a 3D-iD chassis. It
receives DC power from the backplane. It uses input signals from the digital module and the
transmit module. It provides signals demodulated baseband signals to the digital module for
correlation. Receive Final RF Amplifier
The receive final RF amplifier recovers the signal level lost over the RF coaxial cable
between the antenna and the chassis. It is preceded by a 5800MHz bandpass filter that is printed
on the circuit board; its puφose is to remove any unwanted signals before the receive signal is
5 demodulated. A lowpass filter follows the amplifier to remove any harmonic spurious from the
demodulation chain; this filter 1 s also printed on the circuit board. The active device used here is the same as the active devices in the Rx chain on the antenna.
Receive First Mixer and LO Tripler
The amplified receive signal is downconverted to an intermediate frequency of 446MHz 0 in the first mixer. The mixer is driven by an LO signal at a level of about +9dBm. The lower
sideband output from the mixer is then filtered and sent on to the first IF AGC amplifier chain.
Receive First LO
The first LO signal is generated by amplifying the third harmonic of the transmitter LO
signal, sent by coaxial cable from the transmitter module. The transmitter LO frequency at
15 2082MHz is bandpass filtered and amplified and the third harmonic is picked off by the bandpass
filter (at 6246MHz) following the tripler. This signal is then amplified and filtered.
Receive AGC Amplifier and Gain Chain
The first LF AGC amplifier is controlled by the magnitude of the baseband signal sent on
to the analog to digital converter. The puφose of the NGC signal is to keep the baseband signal
20 at a level of about 800mV into the ADC. This function is implemented with a cascade of two amplifiers driven by the same control voltage. The ADC amplifier is followed by a bandpass
filter and gain block to ensure enough signal is present in the demodulator chain.
Receive Second Mixer
The first LF amplified signal is downconverted to the second LF in the receive
second mixer. This mixer is driven by an LO signal at 616MHz at a level of
about +7dBm. The lower sideband of the resulting signal is filtered and sent to the second LF demodulator chain.
Receive Second LO
The receive second LO at 616MHz provides a carrier for the downconversion of the
receive signal to the second LF at 170MHz. It consists of a commercial voltage-controlled oscillator, a buffer amplifier and a programmable synthesizer. The synthesizer is programmed
by the microprocessor and returns its lock-detect status. The nominal output level for this LO is
+7dBm.
Receiver Carrier Recovery
The receiver demodulator implements a data-directed caπier recovery loop that is
ultimately used in the baseband downconverter. This loop performs two basic functions: it tracks
the phase transitions generated in the tag and it also tracks the variation in phase of the received
signal due to tag movement and tag crystal offset relative to, the cell controller reference signal.
Receive Baseband Downconverter The second LF signal is downconverted to baseband at the baseband mixer. The LO for
this mixer is derived from the receiver carrier recovery section and is at 170MHz at a level of
about OdBm.
Receive Baseband AGC Signal Conditioner
The baseband signal is amplified and sent off to the digital board to be digitized in the
analog to digital converter. A replica of this signal is averaged and used to set the gain of the
AGC amplifiers in the first LF section. This signal is also sampled by another analog to digital converter (on command from the digital module) and sent to the digital module.
3D- ID Tag (T20 Edition)
General
Referring now to Figure 23, shown is an embodiment of a tag that may operate in the
system of Figure 1 and Figure 25, that will be described further in paragraphs that follow. The tag is a low cost RF device which transponds radio signals from the cell controller transmit
antenna to the cell controller receive antenna. Ndditionally, it modulates tag information such
as the tag ID onto the signal sent to the cell controller receive antenna. The tag has three
important design parameters: cost, battery life and size. Low tag cost is achieved by making the
tag RF and digital design as simple as possible while using off the shelf components. Battery life
is achieved with low-cuπent RF and digital designs and by utilizing a small duty cycle. Size is
achieved by a combination of surface-mount technology and careful antenna design. The tag is
compliant with FCC Part 15.249 regulations. Figure 23 is a block diagram of the tag, showing
all the major elements of the tag. Figure 24 is an overall specification of the RF elements of one embodiment of the tag of Figure 23. The following sections will describe the various elements
of the tag in detail.
RF Input Section
The RF input section consists of four basic structures: an antenna 2402, an input
bandpass filter 2404, a voltage variable attenuator 2406 and amplifier structures. The antenna
element is external to the tag circuitry and for the T20 edition of the tag is a single patch having a gain of about 4.5dB. The input bandpass filter passes RF signals over a bandwidth of 80MHz centered at 2440MHz with a loss of less than 2dB and a delay nonlinearity of less than 4nsec
peak to valley. This filter is nominally a three-pole response with at least 40dB of rejection in the tag output band of 5725-5815MHz. The amplifier structure consists of two gain stages, each
having about 18dB of gain at 2442MHz.
Modulator and Mixer Section
This section consists of two basic structures: the biphase modulator 2408 and the mixer
2410. In the T20 edition of the tag, the biphase modulator is implemented with a pair of
single-pole, double throw switches and transmission lines that differ in phase by 180 degrees.
The mixer is implemented as a double-balanced diode quad. The sum and difference signals are
present at the mixer output. The low frequency result (difference) is rejected at a highpass filter,
so only the high frequency (sum) products are sent to the output amplifier.
Local Oscillator Section This section consists of two basic structures: the phase-locked oscillator operating at half
the desired frequency and the doubler. The phase-locked oscillator (at 1679MHz) consists of a
VCO 2412 centered at the output frequency, a buffer amplifier, a standard fixed frequency PLL
chip 2414, a passive loop filter and a fixed crystal. The PLL chip implements a ÷512 prescaler,
a phase-frequency detector and a charge pump in a single package. The doubler is implemented as an amplifier followed by a filter.
RF Output Section
The RF output section is implemented with three basic structures: an amplifier, output filters and the transmit antenna 2416. The antenna element is external to the tag circuitry and for
the T20 edition of the tag is a single patch having a gain of about 3dB. The output bandpass filter
2418 passes RF signals over a bandwidth of lOOMHz centered at 5800MHz with a loss of less than 3dB and a delay nonlinearity of less than 4nsec peak to valley. This filter is nominally a
three-pole response with ay least 40dB of rejection in the tag input band of 2400-2484MHz. The amplifier structure consists of a single gain stage having about 18dB of gain at 5800MHz.
Microcontroller and Power Control Section
This section consists of two major elements: the microcontroller 2420 and the power
control circuitry. The tag protocol requires the microcontroller to turn on the local oscillator
section: long enough for it to stabilize fully and then to turn on the RF section. The loop
bandwidth of the local oscillator is wide enough that the LO lock-time is a maximum of
0.75msec. There is also time required for the crystal oscillator in the PLL chip to stabilize: the
sum of the lock-time and the crystal stabilization time is about 3.5msec. Once the LO is stable, during transmit the protocol requires that the microcontroller turn on the RF section and proceed
to phase modulate the transmitted signal according to the requirements of the tag protocol. This
protocol requires a tag bit time of about 19 μsec and an overall tag transmit time of about
2.3msec.
In the on state, the microcontroller samples the tag low battery indicator and latches this
onto the tag housekeeping data bits. These bits form part of the tag datagram sent during
transmit. After the tag LD, tag data bits and checksum have been modulated onto the transmit signal, the microcontroller then shuts off the RF section followed by the LO section. Finally, the microcontroller determines the off time (typically 5 seconds), loads the off time counter and puts
itself into a very low-power state in which it simply counts down the off time counter. When the counter expires, the microcontroller wakes up and repeats the on cycle.
Tag RF System Description
This section describes in detail the operation of the RF elements of the tag, following the
general outline given above. It consists of detailed descriptions of the RF input section, the modulator and mixer section, the local oscillator section' and the RF output section.
RF Input Section
The RF input section describes all the elements from the antenna to the modulator. This
section consists of the Rx antenna, the input bandpass filter (Rx BPF), the voltage-variable
attenuator and the low-noise amplifier (LNA). All of the elements of this system have a nominal
impedance of 50 ohms and are connected to one another with 50 ohm transmission lines. The
puφose of the input section is to make sure the incoming signal (nominally a spread-spectrum- O 00/52498 signal centered at 2.442GHz) provides enough drive for the mixer and RF output sections
without driving them into non-linearity that would violate the requirements of 15.249.
To this end, the nominal tag gain of 35 dB is adjusted in the RF input section to provide
a constant, linear level using a hardware implementation of automatic gain control. The AGC
is implemented with a detector diode at the RF output stage, which provides a DC voltage that
is proportional to the RF output power. This DC voltage is amplified and smoothed in the AGC amplifier and then delivered to a combination of two voltage-variable attenuators: one GaAs
MMIC in front of the LNN and a PLΝ diode attenuator, following the LΝA. This combination is designed to have 35-40dB of attenuation range. This range is designed to allow tag linear operation from about 1 meter away from a cell controller antenna to the maximum range.
For tag ranges inside the 1 meter from a cell controller antenna, the circuitry associated
with the LΝA shutdown, controller monitors the AGC voltage. When this voltage reaches a
value corresponding to the tag RF output of about 3dB below the point where the spectral
re-growth of the desired spread-spectrum signal reaches the limits of 15.249 at the band edges,
the amplifier output transitions, signaling the microcontroller that the low-noise amplifier needs to be shut down. This signal generates an interrupt in the microcontroller that forces the assertion
of the OFF line to a switch, which removes power from the LΝA bias line. This effectively adds
40dB of signal attenuation into the RF input chain. The hardware AGC then readjusts the
voltage-variable attenuators to provide the best tag RF output level possible consistent with
linear operation (now is back at the bottom end of the hardware AGC range).
Rx Antenna The function of the receive antenna is to capture radiated signals at the desired input
frequency over a spatial range as close to half-plane omnidirectional as possible. The receive
antenna is a low-cost quarter-wave shorted patch antenna constructed of sheet metal. The antenna
structure uses air as a dielectric and operates over the ground plane on the backside of the tag
printed circuit board. The antenna makes connection to the rest of the RF input section with a simple RF feedthrough pin. The antenna has a midband gain of 4.6dBi and is linearly polarized.
Rx Input Bandpass Filter
The function of the input bandpass filter is to pass only the input signals of interest and to suppress signals that are not part of the MD system. The input receive bandpass filter is a 3-pole monolithic ceramic dielectric resonator bandpass filter. It has a nominal insertion loss of
2.3dB and a nominal 3dB bandwidth of -100MHz.
Voltage- Variable Attenuator
The function of the WA is to implement the AGC scheme described above. The
voltage-variable attenuator is implemented as a combination of two elements: a monolithic
microwave integrated circuit and a shunt PIN diode attenuator. The first element is placed in
front of the LNA and has an effective dynamic range of about 33dB. The second element is
placed after the LNA and has an effective dynamic range of about 7dB. The combination of the
two on either side of the LNA results in a control range of about 40dB.
Low-Noise Amplifier The low-noise amplifier is a GaAs monolithic amplifier. It exhibits about 20dB of gain
with a noise figure of about 3dB. It serves to set the tag noise figure and to provide enough gain
at maximum range for suitable tag operation.
Modulator and Mixer Section
The modulator and mixer section provides two of the essential system functions for a 3D- iD tag: the ability to identify a tag and the conversion of the input frequency to the output
frequency. The modulator input comes from the microcontroller and is the digital representation of the tag datagram that the tag sends to the cell controller. The mixer inputs come from the RF
input section and from the LO section and its output goes to the RF output section. Each of the
elements of this system has a nominal impedance of 50 ohms and is connected to one another with 50 ohm transmission lines.
Modulator
The function of the modulator is to change the representation of the tag datagram from
a digital signal to a biphase modulated RF signal. The modulator is implemented as a pair of GaAs non-reflective, single-pole, double throw switches that switch between two transmission
lines whose difference in length at the tag receive center frequency (2.442GHz) is 180°. The
particular switches chosen require differential drive, so a CMOS inverter is used to derive a pair
of differential drive signals from a single digital control from the microcontroller.
Mixer The function of the mixer is to translate the incoming 2.442GHz signal to the proper
output center frequency (5.800GHz). The mixer is implemented a commercial double-balanced
mixer. The input local oscillator signal to the mixer is at 3.358GHz and a filter in the RF output
section passes only the sum signals and suppresses the difference signals from the mixer.
Local Oscillator Section
The local oscillator simply supplies the single tone used in the mixer to upconvert the
incoming 2.442GHz signal to a 5.800GHz frequency. The oscillator is implemented as a phase-locked oscillator derived from the output of a voltage-controlled oscillator stabilized by a phase-locked loop. The final output of the local oscillator is 3.358GHz. This is obtained by
filtering the second harmonic of the phase-locked oscillator that is running at 1.679GHz. The
phase-locked loop is implemented in a standard CMOS RF chip and the loop filter is implemented as a 4-pole passive filter. The reference signal for the PLL comes from an onchip
oscillator that uses an external crystal at 6.5MHz.
The VCO output level is at about -5dBm for the fundamental signal. An amplifier whose
gain is about 13dB and whose output feeds both the PLL chip and the next amplifier stage
buffers this output signal. The final amplifier stage is tuned to reject the fundamental signal and
amplify the second harmonic at 3.358GHz to about -8dBm. A bandpass filter to remove
everything but the desired second harmonic follows the final amplifier.
RF Output Section The RF output section simply provides the proper radiated output signal for proper tag
operation. The output section consists of a cleanup bandpass filter following the mixer, an
amplifier, a final bandpass filter and the Tx antenna.
Tx Bandpass filters
5 The bandpass filters are identical 3-pole haiφin filters having about 3dB of insertion loss
and a 3dB bandwidth of about 300MHz centered on 5.8GHz. The filters are implemented directly on the printed circuit substrate.
Tx Final Amplifier
The amplifier is a single monolithic microwave integrated circuit having a gain of about
10 18dB and a third-order intercept point of about +15dBm so that the output signal can be as linear as possible at the desired nominal output level of -7dBm.
Tx Antenna
The function of the transmit antenna is to emit radiated signals at the desired output
frequency over a spatial range as close to half-plane omnidirectional as possible. The transmit
15 antenna is a low-cost quarter-wave shorted patch antenna constructed of sheet metal. The antenna
structure uses air as a dielectric and operates over the ground plane on the backside of the tag
printed circuit board. The antenna makes connection to the rest of the RF input section with a
simple RF feedthrough pin. The antenna has a midband gain of 3.0dBi and is linearly polarized.
Microcontroller Section The microcontroller section operates the protocol for the tag. It accepts user information
and combines this with information about the tag state (such as battery voltage is low). The
microcontroller then provides this digital stream to the biphase modulator. The microcontroller
also monitors the state of the AGC shutdown input and commands the LNA bias off at the
properties. It also directs the sequence of RF element power at the beginning of a datagram (LO
on first, check for phaselock, then RF on) and controls the off-time under software direction. The processor controls DC power with FET switches.
Microcontroller
The microcontroller is a PIC16F84 CMOS low-power, general puφose digital processor. This processor derives all timing from a 4MHz ceramic resonator that establishes the internal
clock stream. The internal clock is derived from the ceramic resonator frequency by dividing it by 4: thus, the internal clock operates at 1 MHz.
DC switches
The switches accept logic inputs from the microcontroller and turn DC power on and off to various system elements. One controls DC power to the LO section and to the majority of the
RF elements. Another controls DC power to the LNA section, implementing level 2 AGC.
As in previously described embodiments included herein, an example embodiment of a
single RFLD system embodiment is shown in Figure 25, which is similar to Figure 1. Figure 25
shows a 3D-ID Cell Controller, which is a sophisticated digital radio that controls a set of antennas that cover a cell within a building. In an open indoor environment, each cell may be
configured to cover about 10,000 square meters.
Like GPS satellites, 3D-LD Cell Controllers emit direct sequence spread spectrum signals
that are received by the tags. Unlike GPS, 3D-LD tags do not include sophisticated circuitry and
software to decode this signal; instead, they simply change the signal's frequency and transpond
it back to the Cell Controller. Tag LD information is phase modulated onto the return signal.
The Cell Controller extracts the Tag LD from this return signal, and also determines the tag's distance from the antenna by measuring the round trip time of flight of the radio signal. Advanced numerical filtering provides approximately one meter distance measurement accuracy.
Up to 16 antennas can be attached to a single Cell Controller via coaxial cable. (For simplicity, 4 antennas are shown in Figure 25). Both power and radio signals travel through this
single cable, so no additional power source is necessary for the antenna. Both the Cell Controller
and the tag are carefully designed to comply with FCC Part 15 regulations, so no license is needed for operation of this embodiment.
One version of the active tag emits Vi milliwatt of radio energy, enabling detection of the
tag at a distance of 50 meters. This allows a tag about the size of a PCMCIA card to operate for
over a year with a small battery. The tag transmits a "low battery" signal well before replacement
is required. For applications where battery life isn't critical, higher power tags can support
substantially longer range. The Cell Controller quickly cycles among antennas, determining the distance between
all of its antennas and a given tag. Once the distance to 3 antennas is found, the tag's location
in space can be estimated. In many situations, it's possible to get a good estimate of tag location
from fewer than 3 antennas. For example, in a warehouse an aisle between racks can be covered
by two antennas — one at each end. Most hallways can be similarly covered.
Tags 'wake up' spontaneously, 'chirp' their unique codes, and then go back to 'sleep'.
Each chiφ takes 2.3 milliseconds. The chiφing interval can vary based on application
requirements. For example, tags attached to personnel might chiφ every two seconds, while tags attached to inventory might be set to chiφ once per minute. For infrequently used physical assets, special tags with motion detectors can be provided, that chiφ infrequently when stationary and more frequently when in motion.
The tag data protocol includes a capability to report information provided by a closely
integrated device. The types of data that might be reported through a tag are limited only by the
creativity of the customer. For example, a specialized tag on a guard could transmit pulse rate,
and a "panic button" could also be included. More generally, tag electronics could be used as
an inexpensive RF stage for a handheld data collection device. Future versions of the tag will
support two-way communication, implementing a low-cost form of "wireless LAN." Thus, this 3D-ID embodiment is not only a system to track people and assets, it also provides a cost-
effective infrastructure to collect a wide variety of data throughout the enteφrise.
The 3D-LD embodiment was designed with careful attention to minimizing cabling and
installation costs of the Cell Controllers. The link from a Cell Controller to its antennas requires
the installation of dedicated coaxial cable. Both power and RF signals are transmitted to the antenna across this cable, so no additional power source is necessary. This may be helpful for
a situation where no power source is available close to the desired antenna location.
Cell Controllers, and by extension their tags, are designed as network devices. Inside the
Cell Controller, an embedded version of Unix supports communication with the coφorate
network via TCP/LP. For example, an application programming interface (API) enables an
application developer to extract data from the cell controllers using a publish/subscribe model
via TCP/LP.
The 3D-LD system bring the benefits of GPS indoors, in a package that is accessible to a wide range of businesses. It also provides a cost-effective wireless infrastructure to collect a variety of data throughout the enteφrise.
The foregoing descriptions of an identification system may be integrated with systems
embodying other technologies. This results in a single infrastructure supporting more than one
technology. As will be described in paragraphs that follow, in one embodiment, a technological
candidate that may be integrating with a long-range RFLD system may have similar antenna
density, operate at non-interfering frequencies, and provide complementary functions to a long-
range RFLD system. One such candidate includes cordless PBX products, such as those systems
operating in accordance with the DECT standard, such as the commercially available Ericsson
cordless PBX system. Other such candidate systems may operate in accordance with other
wireless in-building telephone standards, for example, such as those operating in the 1.9 Ghz
spectrum recently allocated for use by the FCC. It should be noted that because DECT is a standard air interface, DECT products are
increasingly competing on the basis of price. Adding the ability to read long-range RFLD tags
to these systems provides competitive advantage for the Cordless PBX vendors providing this
functionality. In combination systems such as these, the Cordless PBX provides convenient
communication in conjunction with the long-range RFLD system providing enteφrise-wide
peφetual inventory, and an inexpensive radio link to a wide range of embedded devices. A
single infrastructure incoφorating both capabilities may be used in a wide range of markets.
The embodiment that will be described focuses on the integration of PinPoint's 3D-LD Local Positioning System, and DECT cordless phone Fixed Portion (FP). For various commercial and technical reasons, this is a description one embodiment. However, this should not be construed as a limitation.
Referring to Figure 26, shown is an example of an embodiment of an identification system that includes an LPS cell controller. The system 3700 LPS Cell Controller 3702 is
attached to one or more antenna modules 3706 via high-quality coaxial cables 3710a-c. Each
antenna module 3706 sends 2.44 GHz signals to tag, such as 3708, in range, and receives 5.8
GHz signals from the tags. In this embodiment, the cell controller 3702 reports tag information
to a host 3704 via a communication connection 372, such as one that operates in accordance with
the TCP/IP standard. This tag information may be communicated to and stored upon the host
3704 using any one of a variety of software techniques and commercially available programming
languages known to those skilled in the art. For example, in one embodiment, the software
ViewPoint V2.0, from PinPoint Coφoration, may be used in communicating tag information. „ „„,.„„„ PCT/USOO/05493
WO 00/52498
It should be noted that the host 3704 may be any one of a variety of commercially
available computer systems. For example, in one embodiment, the host system is a Vectra
computer system by Hewlett Packard runnning the Windows NT operating system by Microsoft.
Additionally, the host 3704 may support any one of a variety of commercially available operating
5 systems, such as the Windows NT operating system. This may vary in accordance with each
implementation and application.
Referring to Figure 27, shown is an example of an embodiment of a fixed or stationary portion of a DECT installation 3720. The DECT Controller 3724 is connected via twisted pair
cables 37030a-d to "Base Stations" 3726a-d, which in turn communicate with DECT phones
10 3728a-b over the air at 1.9 GHz. This DECT Controller 3724 also connects with an in-house
PBX 3722. Note that the basic architecture of the system 3720 is similar to the foregoing LPS
system of Figure 26. It should be noted that in this particular embodiment, the cable used in each
of the foregoing systems differs since the LPS system uses a higher grade cable to support its less
"intelligent" antenna modules. However, other embodiments may use different cables than the
15 previously described cable of system 3700. It should be noted that the DECT controller is
commercially available, as are other components of the foregoing.
Referring to Figure 28, shown is an example of an embodiment of that includes a base
station used in communications of both DECT and LPS systems. The system 3740 includes a
controller 3742 connected to a base station 3744 used to communicate with DECT phone 3748
20 and LPS tag 3746. In this embodiment, the Controller 3742 may include a single processor
having portions of the two systems, the DECT and LPS systems, each as a plug-in card to a single device. In other embodiments, the Controller 3742 may also be implemented as having
more than one processor rather than using a single processor that operates with the two cards.
In this example, the LPS system uses two antennas, a first 3744a at 2.4 GHz for transmission
and a second 3744b at 5.8 GHz for reception. The DECT system uses one or more additional
antenna structures, such as 3744c, operating at 1.9 GHz. Since the DECT and LPS systems
operate at different frequencies, they are included in a combined implementation having a single infrastructure in which the operation of one system does not interfere with the other system's operation.
In the foregoing description, the connection 3743 between the controller 3742 and the base station 3744 may be any one of a variety of cables or cable combinations. For example, in
one embodiment, this connection 3743 may be the previously described two cables, one from each system 3700 and 3720, bundled together.
Referring back to Figure 21, shown is an example of a block diagram of an embodiment
of a cell controller. This cell controller block diagram includes details regarding a modulator in the transmit chain and a demodulator in the receive chain as included in the commercially
available 3DLD system by PinPoint Coφoration.
In the transmit chain, a 40 megachip baseband input signal is modulated via modulator
2200 onto a 360 MHz carrier from a TX LF Oscillator 2204. The resulting 360 MHz signal is
filtered with a TX IF Filter 2202, for example, implemented as a SAW device. It is then mixed with the output of a 2082 MHz Tx RF PLO, resulting in a 2442 MHz signal which is then
transmitted through the cable(s) to the antenna module.
In an alternative embodiment, the 2082 MHz mixer may be included in the
antenna module rather than in the cell controller. In one embodiment, for example, an ASIC
circuit may be used to reduce the cost of including this in each of the antennas.
Referring back again to Figure 21, in the receive chain, the 5800 MHz response from the tag is first mixed with the output of a 6246 MHz Rx LO Generator (Tripler), resulting in a 446
MHz signal. In this example, the 6246 MHz Rx LO (Tripler) runs at exactly triple the rate of
the 2082 MHz Tx RF PLO. Thus, in an embodiment in which the 2082 MHz Tx RF PLO is
included in the antenna module, the 6246 MHz Rx LO is also easily included in the antenna
module. The result is a signal that is 360 MHz to the antenna module, and 446 MHz from the antenna module, thus relaxing the cabling requirements at the cost of increased complexity in
the antenna module.
Referring again to the receive chain, the 446 signal is again mixed with the output of a
616 MHz Rx 2nd LFOscillator, resulting in a 170 MHz signal which is then demodulated. If such
mixing is alternatively performed in the antenna module, the result is a signal that is 360 MHz
to the antenna module, and 170 MHz from the antenna module, further relaxing the cabling
requirements at the cost of increased complexity in the antenna module. By relaxing the cabling requirements in an embodiment that includes the foregoing
increased complexity, standard coaxial cable may also be included in an embodiment at a grade
commonly used for video or Ethernet.
In view of the foregoing description, an embodiment may include a wide range of different implementations, for example, varying the quality of the cable and the complexity of the antenna module. In particular, the choice of frequencies and stages may be driven by the
availability of cost-effective parts. For example, in the system of Figure 21 , the choice of a 2082
MHz oscillator was driven by the commercial availability of a 360 MHz SAW filter.
It should also be noted that the foregoing LPS system may be a radio frequency
identification system operating at a microwave frequencies with remote antenna modules that use a high quality cable to carry the microwave frequencies. Mixers may be included in the
antenna modules to enable a lower grade of cable than that used with the microwave frequency
operation. Cable may also be run over longer distances without the use of additional repeaters.
Referring to Figure 29, shown is an example of an embodiment of component 4062 of
system of Figure 21. Generally, this is a block diagram for the IF Carrier Recovery and
Demodulator 4062. In this embodiment, a closed-loop "decision-directed" demodulator is
implemented at 170 MHz to recover digital data. PN modulated 170 MHz from one leg of a ΛΛ in ΛΛO PCT/USOO/05493
WO 00/52498
Power Divider 4080 is amplified (4082) and mixed (4084) with recovered baseband data to
produce a low-noise CW tone. Two cascaded 110 degree phase shifters 4092 trim the phase
difference set by the delay-line/filter detector output 4098 for maximum output, thus offsetting
the mismatch of the delay contributors in the delay line filter. A 170M/1M BPF 4090 reduces
the
noise floor.
Referring to Figure 30, shown is an example of an embodiment of an LPS system with another product class that may use wireless technology. The system 4100 includes a host
computer 4102 that communicates with a controller 4104. In this embodiment, the controller
4104 may be implemented as the previously described LPS cell controller as included in the
commercially available PinPint 3DLD system. In this example application, a medical telemetry
system included in the combination system 4100 uses wireless technology to report real-time
medical information. In operation, the monitor 4118 communicates with the cell controller 4104 through the antenna module 4116 using cabling 4109.
The telemetry signals at 620 MHz pass through the telemetry receiver. Signals to the tag
at 24 MHz are transmitted using connection 4109. Signals from the tag at 5800 MHz are also
transmitted using connection 4109. The telemetry monitor 4108 emits is a signal at 620 MHz,
which is picked up by the telemetry antenna module and sent across connection 4109 at 620 MHz. The diplexer 4108 sends the 620 MHz signal to the telemetry receiver 4110 and the 5800
MHz signal to the LPS Cell Controller 4104.
As previously described, the publish/subscribe software included in 4104 may be the
ViewPoint V2.0 software by PinPoint Coφoration or other software used in collecting and processing information in the system 4100.
In the foregoing example of system 4100, the telemetry receiver is a radio that may share the network connection and the software used in the LPS cell controller. Generally, details
regarding the telemetry portion of the system 4100 may vary in accordance with vendor and
product.
It should be noted that medical systems such as the one included in the system 4100, may
operate at unoccupied VHF or UHF television bands. A dedicated frequency band, for example
around 620 MHz, may be dedicated for this particular medical application. As shown in the
system 4100, the medical system operating at 620 MHz may be designed to use the same cabling 4109 as the LPS system. Additionally, both the LPS and medical systems included in the
combination system 4100 may report their data to the host 4102 using the same hardware and
software. Further levels of integration are possible, similar to the approaches discussed above
with regard to Cordless PBXs. In other words, using approaches described above, an
embodiment may include an implementation of cabling that varies in accordance with other complexities, such as the antenna module or other hardware. For example, in another
embodiment, the three applications, that is medical telemetry, wireless phone communications,
and a local positioning system, may co-exist in one infrastructure.
Having described prefeπed embodiments of the invention, it will now become apparent
to those of skill in the art that other embodiments incoφorating its concepts may be provided. It is felt, therefore, that this invention should not be limited to the disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.

Claims

What is Claimed is:
1. A system comprising:
a radio frequency identification tag;
a wireless communication system;
an antenna module used in receiving signals from said radio frequency identification tag, said antenna module also used in transmitting and receiving signals used in said wireless communication system; and
a controller coupled to said antenna module, said controller being used in
communications with said radio frequency identification tag and said wireless communication system.
2. The system of Claim 1 further including a first transmitter for transmitting signals to
said radio frequency identification tag and a second transmitter for transmitting signals to said wireless communication system.
3. The system of Claim 2, wherein said first and second transmitters transmit at different
frequencies.
4. The system of Claim 3, wherein said antenna module is also used in transmitting
signals to said radio frequency identification tag.
5. The system of Claim 2 further including a first receiver for receiving signals from said radio frequency identification tag and a second receiver for receiving signals from said wireless communication system.
6. The system of Claim 5, wherein said first and second receivers operate at different
frequencies.
7. The system of Claim 6, wherein said wireless communication system operates in
accordance with a DECT wireless communication standard.
8. The system of Claim 5, wherein said radio frequency identification tag receives a signal at a first frequency and transmits a second signal at a second frequency.
9. The system of Claim 8, wherein said first frequency and said second frequency are
different.
10. The system of Claim 8, wherein said radio frequency identification tag operates at
a first frequency and said wireless communication system operates at a second frequency that is
non-interfering with operation of said radio frequency identification tag at said first frequency.
11. The system of Claim 10, further including a tag reader for detecting a presence of
said radio frequency identification tag and for use in operation of said wireless communication
system.
12. The system of Claim 10, wherein said wireless communication system is a cordless
PBX product.
13. The system of Claim 1, wherein said tag reader operates in a first mode for detecting
said presence of said radio frequency identification tag and a second mode for use in said
wireless communication system.
14. The system of Claim 13, wherein said tag reader includes a transmitter and a receiver.
15. The system of Claim 1, wherein said radio frequency identification tag is used in locating an object associated with said radio frequency identification tag.
16. The method of Claim 15, wherein said antenna module is also used in transmitting
signals to said radio frequency identification tag.
17. A radio frequency identification system comprising:
a radio frequency identification tag transmitting a first signal at a first frequency; a remote antenna module for receiving said first signal at said first frequency; a frequency converter included in said remote antenna module for converting said first
signal at said first frequency to a second signal at a second frequency that is less than said first frequency; and
a cable for transmitting said second signal at said second frequency from said remote
antenna module to a tag reader.
18. The radio frequency identification system of Claim 17, wherein said cable is a first
cable, said frequency converter is a first frequency converter, and further comprising:
a transmitter included in said tag reader for transmitting a third signal at a third frequency;
a second cable for transmitting said third signal at said third frequency to said remote
antenna module;
a second frequency converter included in said remote antenna module for converting said
third frequency to a fourth signal at a fourth frequency that is greater than said third frequency; an antenna included in said remote antenna module for transmitting said fourth signal at
said fourth frequency; and
a receiver included in said radio frequency identification tag for receiving said fourth
signal at said fourth frequency.
19. The radio frequency identification system of Claim 17, wherein said tag reader transmits and receives signals from said radio frequency identification tag.
20. The radio frequency identification system of Claim 17, wherein said radio frequency
identification tag transmits and receives signals.
21. A method for locating objects and operating a wireless communication system
comprising:
receiving signals using an antenna module from a radio frequency identification tag; transmitting and receiving signals using said antenna module from said wireless
communication system; and
using a controller that is coupled to said antenna module in communications with said
radio frequency identification tag and said wireless communication system.
22. The method of Claim 21 further including: transmitting, using a first transmitter, signals to said radio frequency identification tag
and; transmitting, using a second transmitter, signals to said wireless communication system.
23. The method of Claim 22, wherein said first and second transmitters transmit at
different frequencies.
24. The method of Claim 23, further including:
transmitting signals, using said antenna module, from said radio frequency identification
tag.
25. The method of Claim 22 further including:
receiving, at a first receiver, signals from said radio frequency identification tag; and
receiving, at a second receiver, signals from said wireless communication system.
26. The method of Claim 25, wherein said first and second receivers operate at different
frequencies.
27. The method of Claim 26, wherein said wireless communication system operates in
accordance with a DECT wireless communication standard.
28. The method of Claim 25, wherein said radio frequency identification tag receives a
signal at a first frequency and transmits a second signal at a second frequency.
29. The method of Claim 28, wherein said first frequency and said second frequency are
different.
30. The method of Claim 28, wherein said radio frequency identification tag operates at a first frequency and said wireless communication system operates at a second frequency that
is non-interfering with operation of said radio frequency identification tag at said first frequency.
31. The method of Claim 30, further including:
detecting, using a tag reader, a presence of said radio frequency identification tag wherein
said tag reader is also used in operation of said wireless communication system.
32. The method of Claim 30, wherein said wireless communication system is a cordless
PBX product.
33. The method of Claim 21 , wherein said tag reader operates in a first mode for
detecting said presence of said radio frequency identification tag and a second mode for use in
said wireless communication system.
34. The method of Claim 33, wherein said tag reader includes a transmitter and a
receiver.
35. The method of Claim 21, further including:
using said radio frequency identification tag to locate an object associated with said radio frequency identification tag.
36. The method of Claim 35, further including:
transmitting signals from said radio frequency identification tag using said antenna
module.
37. A method for locating objects comprising:
transmitting a first signal at a first frequency from a radio frequency identification tag; receiving said first signal at said first frequency at a remote antenna module;
converting, by a converter included in said remote antenna module, said first signal at
said first frequency to a second signal at a second frequency that is less than said first frequency
and
transmitting said second signal at said second frequency from said remote antenna
module to a tag reader using a cable.
38. The method of Claim 37, wherein said cable is a first cable, said converter is a first converter, and further comprising:
transmitting, using said tag reader, a third signal at a third frequency;
transmitting, using a second cable, said third signal at said third frequency to said remote antenna module; converting, using a second converter included in said remote antenna module, said third
signal at said third frequency to a fourth signal at a fourth frequency that is greater than said third frequency;
transmitting said fourth signal at said fourth frequency to said remote antenna module;
and
receiving said fourth signal at said fourth frequency using a receiver included in said radio frequency identification tag.
39. The method of Claim 37, wherein said tag reader transmits and receives signals from said radio frequency identification tag.
40. The method of Claim 37, wherein said radio frequency identification tag transmits and receives signals.
PCT/US2000/005493 1999-03-03 2000-03-02 Method and apparatus combining a tracking system and a wireless communication system WO2000052498A1 (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP00916010A EP1212635A1 (en) 1999-03-03 2000-03-02 Method and apparatus combining a tracking system and a wireless communication system
AU37181/00A AU3718100A (en) 1999-03-03 2000-03-02 Method and apparatus combining a tracking system and a wireless communication system

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US12272099P 1999-03-03 1999-03-03
US60/122,720 1999-03-03
US37841799A 1999-08-20 1999-08-20
US09/378,417 1999-08-20

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US7676380B2 (en) 2005-02-11 2010-03-09 Nortel Networks Limited Use of location awareness to establish and suspend communications sessions in a healthcare environment
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US7707044B2 (en) 2005-02-11 2010-04-27 Avaya Inc. Use of location awareness to transfer communications sessions between terminals in a healthcare environment
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