WO2002027983A2

WO2002027983A2 - Distance to fault measurement using multi-path cdma signals

Info

Publication number: WO2002027983A2
Application number: PCT/US2001/026135
Authority: WO
Inventors: Cory R. Christensen; Jon Q. St. Clair; Jeffrey N. Highley
Original assignee: Motorola, Inc.
Priority date: 2000-09-26
Filing date: 2001-08-21
Publication date: 2002-04-04
Also published as: WO2002027983A3; AU2001286594A1

Abstract

The present invention is for measuring a distance to a fault (26) in a direct sequence spread spectrum communications link such as a cell site antenna (12) in a CDMA cellular system (10). Specifically, a time offset of a forward path signal (22) is measured with respect to a second correlated reference signal. A timing difference (t1-t2) between the time offset of the forward path signal (22) and the reflected path signals (24) is then estimated to determine a distance to the communications link fault (26). The present invention therefore minimized cell site down time and eliminates the need for external test signals.

Description

DISTANCE TO FAULT MEASUREMENT USING MULTI-PATH CDMA

SIGNALS

Background of the Invention Field of the Invention

The present invention relates generally to telecommunications networks, and more particularly to the measurement of the distance to a transmission line fault in a cell site antenna using time offsets of an existing multi-path CDMA signal.

Description of Related Art

Conventional cellular communications stations transmit and receive signals through antennas that may be up to, and even more than, 150 feet tall to facilitate communication between mobile system communications devices or between mobile system communications devices and wireline communications systems. Because of the height of the antennas, troubleshooting an antenna to locate a faulty section of the antenna or antenna feedline when the antenna is not operating correctly becomes problematic, as a technician must access the antenna by physically scaling the antenna or by reaching each section through means such as a cherry picker. Therefore, the distance to the faulty section from a base reference is preferably estimated before the technician accesses the antenna so that the technician need only troubleshoot a specific antenna section or sections.

Present distance to fault techniques each provide a general estimation as to the location of faulty antenna sections. According to one technique, a time domain reflectometer (TDR) sends an RF pulse up an antenna transmission line and displays a subsequent reflected pulse, caused by a faulty antenna section, on a time scale. The difference in time between the transmitted and reflected pulses is then used to calculate the distance to an antenna section that caused the reflected pulse. According to another technique, the antenna return loss is measured using an external narrow band signal over multiple frequencies. A standing wave resulting from the multi-frequency scan is then measured, and the distance to fault is computed using well-known transform techniques. The present techniques, however, have certain associated drawbacks. For example, the TDR-based technique requires that an external signal be generated and transmitted up the antenna transmission lines, thereby limiting utilization of the antenna during testing. Also, because the frequency domain- based technique requires that the antenna be scanned with an external narrow band signal, the application of this technique to sites using wide bandwidth spread spectrum interfaces, such as CDMA sites, is limited because the site must be taken out of service during testing.

Brief Description of the Drawings Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments thereof when taken together with the accompanying drawings in which:

FIG. 1 is a block diagram of a cell site of the type in which a distance to fault measurement technique in accordance with a first embodiment of the present invention is implemented;

FIG. 2 is a block diagram of the correlation tracking loop and time offset calculator of the RF diagnostic subsystem shown in FIG. 1;

FIG. 3 is a graphical illustration of an exemplary signal received at the rake receiver shown in FIG. 1 and a locally generated synchronous reference signal with the same spreading sequence illustrating how time domain correlations are detected by time shifting the reference signal with respect to the received signal; FIG. 4 is a graphical illustration of the result of a pseudorandom noise (PN) correlation function that combines the receive signal mixed with the synchronous clock reference signal shown in FIG. 3;

FIG. 5 illustrates two PN correlation function peaks generated by the RF diagnostic subsystem in the first embodiment, with a first peak representing the combination of a forward path signal with a reference signal with a continuously sweeping time offset, and a second peak representing the combination of a reflected path signal with the same reference signal;

FIG. 6 is a block diagram of a cell site of the type in which a distance to fault measurement technique in accordance with a second embodiment of the present invention is implemented;

FIG. 7 is a graphical illustration of a PN correlation function peak generated by an RF diagnostic subsystem in the second embodiment combining a reflected path signal with a reference PN sequence as well as an absolute time reference and against which the PN correlation function peak is compared to determine distance to fault;

FIG. 8 is a block diagram of a cell site of the type in which a distance to fault measurement technique in accordance with a third embodiment of the present invention is implemented; and FIG. 9 is a graphical illustration of a PN correlation function peak generated by the RF subsystem in the third embodiment by combining a forward path signal a reference PN sequence where the chip time is sufficiently small and the bandwidth is sufficiently high such that the correlation function produces two distinct peaks with times that can be measured relative to one another to determine distance to fault.

Detailed Description of the Presently Preferred Embodiments Referring now to the drawings in which like numerals reference like parts, FIG. 1 shows a cell site 10 including a direct sequence spread spectrum communications link such as an antenna 12 that typically is between 50 and 150 feet tall. The antenna 12 transmits pseudorandom noise (PN) sequences that preferably form a direct sequence spread spectrum communications signal, such as a CDMA signal, generated by a base station 14 and amplified by a power amplifier 16 located within the base station 14.

According to a first embodiment, the base station 14 is connected to and receives user voice and data signals from the telephony switch (not shown) from which it produces a CDMA signal in a well known manner. As the signal is transmitted over the transmission line 20 as a forward path signal 22, a variable portion of the signal reflects from all faults or discontinuities in the antenna and or the transmission line, thereby resulting in one or more fault- induced reflected path signals, represented generally at 24, that propagate toward the site 14 in a direction opposite that of the forward path signal 22. If an antenna fault such as the fault 26 is present, the forward path signal 22 is reflected at the fault 26 back down the transmission line 20 and through a coupler 30, which is preferably a dual directional coupler that reflects approximately 0.1% of the reflected path signals 24 out onto a receiver line 31a and approximately 0.1% of the forward path signals 22 out onto a receiver line 31b to a rake receiver 32. The rake receiver 32, although shown as a separate component, is preferably incorporated as part of the base station 14, and is capable of receiving and time aligning several different independent paths of a multi-path CDMA signal. As will be discussed below in more detail, an RF diagnostic subsystem referenced generally at 34 and located in the receiver 32 is for comparing the reflected path signals 24 with the forward path signal 22 using a precision clock reference signal 36 generated by and input from the based station 14 to determine the distance to the fault 26 relative to the position of the dual directional coupler 30.

It should be noted at this point that the distance to fault measurement in accordance with the present invention can be executed using an existing CDMA communications signal, thereby eliminating the need for external test equipment and the need to generate and transmit test signals over the antenna 12, as is often necessary in conventional distance to fault testing. As a result, cell site operating capacity is maintained while site downtime is minimized. The RF diagnostic subsystem 34 includes several components connected in circuit, including a power meter 37, a controller 38, a correlation-tracking loop and time offset calculator 40, and a switch 42. The power meter 37 is for measuring the severity of the fault 26 by determining the power of the forward and reflected path signals 22, 24. The controller 38 is programmed to implement the correlation tracking loop and time offset calculator 34 to determine a timing difference between the forward and reflected path signals 22, 24 for fault location purposes in a manner described below in more detail. The switch 42 enables the controller 38 to toggle the correlation-tracking loop and time offset calculator 40 to alternately receive the forward and reflected path signals 22, 24.

FIGs. 2-4 provide further detail as to the structure and operation of the correlation tracking loop and time offset calculator 40. As mentioned above, the correlation tracking loop and time offset calculator 40 is for receiving either the forward or reflected path CDMA signals 22, 24 depending on the position of the switch 42. A mixer 50, a peak detector 56, a variable timing offset control 60 and a reference PN sequence generator 54 together form a feedback loop similar to a phase lock loop, albeit in the time domain. The mixer 50 delivers the product of the received signal PN sequence and the reference PN sequence signal 36 to the peak detector 56, which filters and measures the two mixed signals using a PN correlation function and then determines the location of a PN correlation function peak of the filtered mixed signals representing the point at which the filtered mixed signals become temporally aligned. The timing offset of the reference PN sequence generator 54 is controlled by the variable offset timing control 60, and an absolute timing reference 62 is disciplined to a precise timing reference source such as a global positioning system (GPS) source.

FIG. 3 shows an exemplary received path signal transmitted on the antenna 12, such as the reflected path signal 24, and the precision clock reference signal 36 generated by the reference PN sequence generator 54. The present invention exploits the fact that CDMA protocol incorporates direct sequence PN sequences to spread and encode unique subscriber information, and is capable of determining the distance to an antenna fault or faults based on the pilot PN sequence and associated parameters well known to those skilled in the art.

Specifically, the correlation tracking loop and timing offset calculator 40 effectively locks the reflected path signal 24 to the precision clock reference signal 36 via the PN correlation function. The correlation tracking loop and timing offset calculator 40 locks onto and measures the timing offset of the two signals by shifting the reference signal TS with respect to the received reflected path signal 24 to cause the reference signal to lead, lag or lock onto the received signal in an "as is" condition to align the two signals, as shown at 64 in FIG. 3. The correlator peak detector 56 then detects a peak, such as the peak 66 shown in FIG. 4, resulting from alignment of the two mixed signals by the variable offset timing calculator 60. The peak 66 has a width shown in

FIG. 4 as It, where t is the chipping rate (the smallest increment in signal shift possible at the variable timing offset calculator 60). Therefore, as is well known in CDMA technology, a variable timing offset calculator with a chipping rate t of, for example, 5 Mchips/s would result in a PN correlation function peak that would be twice as wide, and thus half as accurate, as a variable timing offset calculator with a chipping rate t of 10 Mchips/s.

Once the correlator peak detector 56 determines the point 66 at which the two signals become aligned, it transmits the resulting timing data to the controller 38. As shown graphically in FIG. 5, the controller 38 then determines a timing offset 70 between the peak 66 of the reflected path signal 24 occurring at time tj and received during a current tracking period corresponding to a position A of the switch 42, and a peak 72 of the forward path signal 22 occurring at time t2 and received during a previous tracking period corresponding to a position B of the switch 42. After determining the timing offset 70, the controller 38 then can determine a round trip signal delay associated with the reflected path signals 24 that is indicative of the location of the distance from the receiver 32 to the antenna fault 26.

More specifically, the controller 38 calculates the distance from the receiver 32 to the fault 26, which is one-half the total signal path of the reflected path signals 24, by utilizing the propagation delay of the reflected path signals 24. The resulting calculation has an accuracy, or fault resolution, equal to the propagation velocity divided by one-half the product of the chipping rate and the chipping resolution. Therefore, if the signal propagation velocity is determined from the forward path signal 22 by transmission line characteristics known to be 0J5 x 3 x 10⁸ m/s, the chipping rate is 10 Mchips/s, the chip resolution is 1/10 chip, and the fault resolution would be 1.125 meters. As the receiver 32 is capable of decoding several components of a communications signal, several reflected path signals 24 can be distinguished and corresponding delays calculated in this manner by the controller 38.

It should be noted that there will be no correlated energy on the reflected path of the reflected path signals 24, aside from path leakage, if an antenna fault such as the fault 26 does not exist, as all of the transmitted power of the forward path signal 22 would be transmitted over the antenna 12 and would not be reflected back into the cell site 10 and, more specifically, into the receiver 32.

FIG. 6 shows a cell site 10' including a rake receiver 32' with an RF diagnostics subsystem 34' for determining distance from the receiver 32' to the fault 26' in a second embodiment in accordance with the present invention. While the components of the RF diagnostic subsystem 34' are identical to those of the RF diagnostic subsystem 34 of the first embodiment, the distance to fault measurement technique is different. Specifically, while the PN correlation function peak for the reflected path signal 24' is generated in the same manner as in the first embodiment using a correlated reference signal 36', the correlation tracking loop and timing offset calculator 40' can sweep past the first peak and then the second peak. Because the peaks are narrow enough to be individually resolved without interfering with one another, the correlation tracking loop and timing offset calculator 40' can utilize the delay between the two peaks to calculate distance to fault. Because the absolute timing reference signal is utilized, the correlation tracking loop and time offset calculator 40' does not need to determine a timing offset value for a forward path signal such as the signal 22 in the first embodiment. Consequently, the RF diagnostic subsystem 34' also does not need a switch, such as the switch 42 in the first embodiment, as it does not need to switch between forward and reflected path signals.

As shown graphically in FIG. 7, the controller 38' in the second embodiment determines the distance from the receiver 32' to the fault 26' by calculating the timing offset 70' as the difference tj - t₂, where t is the timing offset of the reflected path signal(s) 24' compared to the absolute reference signal 64', and tj is the point in the absolute timing reference signal against which the reflected path signals 24' are measured.

FIG. 8 shows a cell site 10" including a rake receiver 32" with an RF diagnostics subsystem 34" for detennining distance from the receiver 32" to the fault 26" in a third embodiment in accordance with the present invention.

All components of the RF diagnostic subsystem 34" are identical to those of the RF diagnostic subsystem 34 of the first embodiment, except that the RF subsystem 34" does not include a switch such as the switch 42. Instead, the correlation tracking loop and timing offset calculator 40" simultaneously receives and measures forward and reflected path signals 22", 24" against the same precision clock reference signal 36", which is generated in the same manner as is the precision clock reference signal 36 in the first embodiment. As shown in FIG. 9, the timing difference tj - t₂ at 70" between the two peaks 66", 72" of the forward and reflected path signals 22", 24", respectively, is at least one peak width because of a high associated chipping rate. Consequently, the correlation tracking loop and timing offset calculator 40" can resolve the time offsets tj , t₂ and the controller 38 can determine the distance to the fault 26" without the two peaks 66", 72" interfering with one another. The distance to fault measurement executed in the third embodiment is more practical in 3X and 5X third generation networks due to the faster chipping rates of such networks and the narrower correlator fingers and resulting narrower peaks such as the peaks 66", 72". Further, the distance to fault measurement executed in the third embodiment is also highly effective and accurate for faults that are located relatively far from the base station rake receiver, such as faults occurring at a distance of at least one-half the inverse of the chipping rate times the propagation velocity of 3 x 10⁸ m/s.

In addition to the above first, second and third embodiments, an alternative embodiment is also contemplated in which a reference signal would be generated using a highly stabilized reference signal oscillator. In such an embodiment, neither the precision clock reference signal in the first and third embodiments nor the absolute timing reference signal in the second embodiment would be necessary, thereby simplifying overall system design.

In view of the foregoing discussion, it should be appreciated that, because the distance to fault measurement in accordance with the present invention can be determined utilizing existing paths of a multi-path signal, a cell site does not need to be taken out of service to perform distance to fault measurement with external measurement devices such as a time domain reflectometer (TDR) or with test signals. It should also be appreciated that the distance to fault measurement in accordance with the present invention can also be executed when the source of the reflected path signal is a test subscriber unit (TSU) containing the necessary amplifiers, filters and correlator, with a call linked through the cell site 10, and when the rake receiver 32 is implemented on a base station transceiver. Implementation of the present invention on both the TSU and the base station transceiver provides distance to fault measurement capabilities at both transmit and receive antennas, as the reference signal will be a normally existing CDMA signal regardless of whether the base station transceiver or the TSU is the host.

While the above description is of the preferred embodiment of the present invention, it should be appreciated that the invention may be modified, altered, or varied without deviating from the scope and fair meaning of the following claims.

Claims

ClaimsWhat is claimed is:

1. A method of measuring a distance to a fault in a direct sequence spread spectrum communications link, comprising: measuring a forward path signal time offset of a directly received forward path signal and using the forward path signal time offset as a reference point;

measuring a reflected path signal time offset of a fault-induced reflected path signal; and calculating a difference between the forward path signal time offset and the reflected path signal time offset to determine distance to fault.

2. The method of claim 1, wherein the measuring of a forward path signal time offset comprises correlating the directly-received forward path signal with a known sequence derived from a precision clock reference to enable a precise time offset to be recorded.

3. The method of claim 1, further comprising switching from the measuring of a forward path signal time offset to enable the measuring of a reflected path signal time offset.

4. The method of claim 1, wherein the measuring of a reflected path signal time offset comprises correlating the fault-induced reflected path signal with a known sequence derived from a precision clock reference to enable a precise time offset to be recorded.

5. The method of claim 4, wherein the calculating of a difference between the forward path signal time offset and the reflected path signal time offset further comprises: mixing the directly-received forward path signal and a known spreading sequence derived from a precision reference signal; and locking the directly-received forward path signal to the known spreading sequence derived from the precision reference signal to locate a signal peak indicating temporal alignment of the directly-received forward path signal and the known spreading sequence derived from the precision reference signal.

6. A diagnostic subsystem for locating a fault in a direct sequence spread spectrum communications link, comprising: a receiver for receiving a reflected path communications signal if the communications signal is reflected at a communications link fault; and a correlator comprising a mixer for mixing the reflected path communications signal with a known reference signal, a peak detector for measuring a time offset of the reflected path communications signal, and a controller for determining a distance to a link fault based on a peak of the mixed reflected path communications signal relative to the known reference signal.

7. The diagnostic subsystem of claim 6, wherein the receiver is further for receiving a forward path communications signal, locking the forward path communications signal to a forward path correlated reference signal to create a mixed forward path reference signal, and for filtering the mixed forward path reference signal to generate a mixed forward path signal peak; and the timing offset calculator is further for determining the distance to the link fault based on a timing difference between the mixed forward path communications signal peak and the mixed reflected path communications signal peak.

8. The diagnostic subsystem of claim 7, further comprising a switch for switching the receiver between receipt of the reflected path communications signal and receipt of the forward path communications signal.

9. The diagnostic subsystem of claim 7, wherein the known reference signal and the forward path correlated reference signal comprise a single reference signal.

10. The diagnostic subsystem of claim 6, wherein the known reference signal comprises a communications link absolute time reference based on GPS timing data.

11. The diagnostic subsystem of claim 6, further comprising a dual directional coupler for coupling the receiver to the communications link.

12. The diagnostic subsystem of claim 6, wherein the communications signal is an existing CDMA signal.

13. The diagnostic subsystem of claim 6, further comprising a variable offset timing generator for aligning the communications signal and the known reference signal for the correlator.