WO2005062564A1 - Residual frequency error estimation in an ofdm receiver - Google Patents
Residual frequency error estimation in an ofdm receiver Download PDFInfo
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- WO2005062564A1 WO2005062564A1 PCT/US2004/035418 US2004035418W WO2005062564A1 WO 2005062564 A1 WO2005062564 A1 WO 2005062564A1 US 2004035418 W US2004035418 W US 2004035418W WO 2005062564 A1 WO2005062564 A1 WO 2005062564A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
- H04L27/266—Fine or fractional frequency offset determination and synchronisation
Definitions
- the present invention relates to estimation of frequency error caused due to oscillator differences between an IEEE 802.11a based Orthogonal Frequency Division Multiplexing (OFDM) transmitter and an OFDM receiver.
- OFDM Orthogonal Frequency Division Multiplexing
- BACKGROUND ART Local area networks historically have used a network cable or other media to link stations on a network. Newer wireless technologies are being developed to utilize OFDM modulation techniques for wireless local area networking applications, including wireless LANs (i.e., wireless infrastructures having fixed access points), mobile ad hoc networks, etc..
- the IEEE Standard 802.11a entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in tlie 5 GHz Band", specifies an OFDM PHY for a wireless LAN with data payload communication capabilities of up to 54 Mbps.
- the IEEE 802.11a Standard specifies a PHY system that uses fifty-two (52) subcarrier frequencies that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16- quadrature amplitude modulation (QAM), or 64-QAM.
- BPSK/QPSK binary or quadrature phase shift keying
- QAM 16- quadrature amplitude modulation
- 64-QAM 64-QAM
- FIG. 1 is a diagram of a typical direct conversion receiver.
- the direct conversion receiver 10 includes an antenna 12, a low noise amplifier 14, a local oscillator 16 tuned to a prescribed carrier frequency, mixers 18a and 18b, and lowpass channel filters 20a and 20b.
- I and Q channel signals are generated based on modulating a signal by a first carrier and a second carrier phase-shifted by B/2 (i.e., 90 degrees), respectively.
- the received signal is supplied to tlie mixers 18a and 18b.
- the mixer 18a outputs a first demodulated signal that includes the I component and a first carrier component (e.g., a sine wave); the mixer 18b, having received a phase-shifted carrier signal from the phase shifter 22, outputs a second demodulated signal that includes the Q component and a second carrier component (e.g., a cosine wave).
- the low pass filters 20a and 20b remove the respective carrier components and output the I and Q components, respectively.
- a particular concern involves frequency differences (f E ) between the transmit frequency (f ⁇ ) generated by local crystal oscillator in the OFDM transmitter and the receive frequency (f ⁇ ) generated by the local crystal oscillator 16 in tlie OFDM receiver 10.
- the IEEE Standard 802.11 specifies a short preamble and a long preamble that may be used by the OFDM receiver 10 for generating an estimated f equency error (f ES ⁇ ).
- the estimated frequency error (f E s ⁇ ) does not equal the actual frequency error (f E ) because both the short preamble and long preamble contain noise components from transmission between the OFDM transmitter and the OFDM receiver 10.
- the short preamble and long preamble received by the OFDM receiver differs from the short preamble and long preamble output by the OFDM transmitter.
- the short and long preamble output by the OFDM transmitter do not address frequency errors encountered due to oscillator drift, where the transmit frequency (f ⁇ ) and the receive frequency ( ⁇ ) independently vary over time.
- residual frequency error (f ⁇ s f ⁇ ST - 4)
- an OFDM receiver has a frequency error detector configured for measuring frequency error based on comparing prescribed pilot tones from a prescribed group of consecutive symbols in a received OFDM signal.
- the prescribed group includes a first subgroup of the consecutive symbols and a second subgroup of the consecutive symbols.
- the first and second subgroups each have an equal number of symbol subgroup positions.
- the frequency error detector includes a complex conjugate generator, a multiplier, a complex summation circuit, and an error calculator.
- the complex conjugate generator is configured for generating complex conjugates of the prescribed pilot tones of the first subgroup of the consecutive symbols.
- the multiplier is configured for generating a complex pilot product, for each symbol subgroup position, by multiplying the pilot tones of a second subgroup symbol at the corresponding symbol subgroup position with the respective complex conjugates of the first subgroup symbol at the corresponding symbol subgroup position.
- the complex summation circuit sums the complex pilot products of the symbol subgroup positions to obtain an accumulated complex value.
- the error calculator calculates the frequency error from the accumulated complex value for use in correcting frequency offset.
- One aspect of the present invention provides a method in an OFDM direct conversion receiver.
- the method includes receiving a prescribed group of consecutive symbols in a received OFDM signal, and identifying within the prescribed group a first subgroup of the consecutive symbols and a second subgroup of the consecutive symbols, the first and second subgroups each having an equal number of symbol subgroup positions.
- the method also includes generating complex conjugates of the prescribed pilot tones of the first subgroup of the consecutive symbols.
- a complex pilot product is generated, for each symbol subgroup position, by multiplying the pilot tones of a second subgroup symbol at the corresponding symbol subgroup position with the respective complex conjugates of a first subgroup symbol at the corresponding symbol subgroup position.
- the method also includes obtaining an accumulated complex value by summing the complex pilot products of the symbol subgroup positions, and calculating the frequency error from the accumulated complex value for use in correcting frequency offset.
- Figure 1 is a diagram illustrating a conventional (PRIOR ART) direct conversion receiver configured for recovering I and Q components from a received IEEE 802.11 OFDM wireless signal.
- Figure 2 is a diagram illustrating the receiver portion of an IEEE 802.11 OFDM transceiver according to an embodiment of the present invention.
- Figure 3 is a block diagram illustrating a frequency error detector within the frequency tracking block of Figure 2, according to an embodiment of the present invention.
- Figure 4 is a diagram illustrating the method of calculating the frequency error, according according to an embodiment of the present invention.
- FIG. 2 is a diagram illustrating an architecture of a receiver module 50 of an IEEE 802.11 Orthogonal Frequency Division Multiplexing (OFDM) transceiver, according to an embodiment of the present invention.
- the receiver module 50 implemented as a digital circuit, includes an I/Q mismatch compensation module 52 that receives detected wireless signal samples (in digital form) from an R/F analog front end (AFE) amplifier 40 having an analog to digital (A/D) converter.
- AFE analog front end
- A/D analog to digital
- the detected wireless signal samples include an I component and Q component: these I and Q components, which ideally should be orthogonal to each other and have a uniform relative gain, may in fact have a non-orthogonal phase difference (i.e., other than 90 degrees) and have an unequal gain.
- the I/Q mismatch compensation module 52 is configured for compensating the mismatched I/Q components to generate compensated signal samples having matched I/Q components with orthogonal phase difference and a uniform relative gain.
- the receiver module 50 also includes a dynamic range adjustment module 54.
- the dynamic range adjustment module 54 is configured for adjusting the gain of the compensated signal samples to a prescribed dynamic range for optimized signal processing, thereby outputting adjusted signal samples according to the prescribed dynamic range.
- the rotor circuit 56 is configured for compensating between a local receiver carrier frequency (i.e., local oscillator) and the remote transmitter carrier frequency (i.e., remote oscillator) used to transmit the wireless signal.
- the course/fine frequency offset estimator 58 is configured for estimating the difference in the frequency between the local receiver carrier frequency and the remote receiver carrier frequency, and supplying this difference to a phasor circuit 60; the phasor circuit 60 converts the difference value to a complex phasor value (including angle information) which is supplied to the rotor circuit 56.
- the rotor circuit 56 rotates the adjusted signal samples based on the complex phasor value, and outputs rotated signal samples.
- the circular buffer 62 is configured for buffering the rotated signal samples.
- the beginning of a data packet is not guaranteed to be located at the same position within the sequence of rotated signal samples.
- the rotated signal samples are stored in the circular buffer 62 in a manner such that any data sample within a prescribed duration (e.g., one maximum-length data packet) can be located and retrieved from the circular buffer 62.
- a prescribed duration e.g., one maximum-length data packet
- any new signal sample to be stored in the circular buffer 62 is overwritten over the oldest stored signal sample.
- the circular buffer 62 enables the receiver 50 to adjust the "starting point" of the data packet within the sequence of rotated signal samples.
- the Fast Fourier Transform (FFT) circuit 64 is configured for converting the time-based sequence of rotated signal samples into a frequency domain-based series of prescribed frequency points (i.e., "tones"); according to the disclosed embodiment, the EFT circuit 64 maps the rotated signal samples to a frequency domain of fifty-two (52) available tones.
- the available fifty-two (52) tones are used to transport information: four (4) tones are used as pilot tones, and the remaining forty-eight (48) tones are data tones, where each tone may carry from one to six (1-6) bits of information.
- the physical layer data packet should include a short training sequence, a long training sequence, a signal field (indicating the data rate and length of the payload, and coded at the lowest data rate of 6Mbps), and the payload data symbols encoded in one of eight data rates from 6Mbps to 54Mbps.
- the FFT circuit 64 determines the data rate from the signal field, and recovers the data tones.
- the FFT circuit 64 outputs a group of tone data to a buffer 66, illustrated as a first buffer portion 66a, a second buffer portion 66b, and a switch 66c: tlie FFT circuit 64 alternately outputs the groups of tone data between the buffer portions 66a and 66b, enabling the switch 66 to output one group of tone data from one buffer portion (e.g., 66a) while the FFT circuit 64 is outputting the next group of tone data into the other buffer portion (e.g., 66b).
- Note actual implementation may utilize addressing logic to execute the functions of the switch 66c. Since certain tones output by the FFT 64 may have encountered fading due to signal attenuation and distortion on the wireless channel, equalization is necessary to correct the fading.
- the frequency domain equalizer 68 is configured for reversing the fading encountered by the tones in order to provide equalized tones.
- Channel information is obtained by the channel estimator 70 from the long training sequence in the IEEE 802.11 preamble; the channel information is used by the channel estimator 70 to estimate the channel characteristics; the estimated channel characteristics are supplied to the frequency equalizer 68 to enable equalization of each tone.
- the receiver module 50 also includes a timing synchronization module 72, a frequency tracking block 74, a channel tracking block 76, and a timing correction block 78 for controlling signal conditioning to ensure the received signal samples are decoded properly to accurately recover the data symbols.
- the decoding portion 80 includes a digital slicer module 82, a deinterleaver 84, and a Viterbi decoder 86.
- the digital slicer module recovers up to 6 bits of symbol data from each tone, based on the data rate specified in the signal field in the preamble.
- the deinterleaver 84 performs the converse operation of the transmitter interleaver circuit, and rearranges the data back into the proper sequence of deinterleaved data.
- the Viterbi decoder 86 is configured for decoding the deinterleaved data into decoded data, in accordance with the IEEE 802.11 specification.
- the descrambler circuit 90 is configured for recovering the original serial bit stream from the decoded data, by descrambling a 127-bit sequence generated by the scrambler of the transmitter, according to the IEEE 802.11 specification.
- the descrambler circuit 90 utilizes a scrambling seed, recovered from the service field of the data packet by the seed estimation circuit 92, for the descrambling operation.
- the signal field information from the preamble also is stored in a signal field buffer 94, configured for storing the length and data rate of the payload in the data packet.
- Overall control of the components of the receiver 50 is maintained by the state machine 96.
- the serial bit stream recovered by the descrambler circuit 90 is output to an IEEE 802.11 compliant Media Access Controller (MAC).
- MAC Media Access Controller
- FIG. 3 is a block diagram illustrating a frequency error detector 100 within the frequency tracking block 74 of Figure 2, according to an embodiment of the present invention.
- the frequency error detector 100 includes a buffer 102, a complex conjugate generator 104, a multiplier 106, a complex summation circuit 108, and an error calculator 110.
- the frequency error detector 100 is configured for measuring frequency error (Freq. Error) based on determining phase differences between pilot tones over a period of time.
- the disclosed embodiment compares the pilot tones 114 within a prescribed group (N) of consecutive OFDM symbols 112 in order to derive an accurate frequency error estimation.
- each OFDM symbol 112 transmitted according to IEEE 802.11a protocol includes fifty-two (52) tones 116, which includes four (4) pilot tones 114a, 114b, 114c, and 114d.
- the pilot tones 114 for each of the symbols 112 should be identical.
- noise components and oscillator drift may cause residual frequency errors that may be undetectable by conventional frequency tracking systems.
- the prescribed (N) group of consecutive symbols 112 (i.e., 112 ⁇ through 112 N ) are stored in the buffer 102 and divided into two first and second subgroups 118a and 118b, where each subgroup 118a and 118b has the same number of symbols N/2.
- each symbol has a corresponding subgroup position within its subgroup: in the case of subgroup 118a, the symbol 112 ⁇ (Syml) is at position "1", the symbol 112 2 (Sym2) is at position "2", and the symbol 112 N/2 (SymN/2) is at position "N/2"; in the case of subgroup 118b, the symbol 112 N 2+ ⁇ (SymN/2+1) is at position "1", the symbol 112 N 2+2 (SymN/2+2) is at position "2", and the symbol 112 N/2 (SymN/2) is at position "N/2".
- the complex representations of the pilot tones 114 (114a, 114b, 114c, 114d) (i.e., constellation values) are compared between N/2 OFDM symbols by multiplying the pilot tone 114 of a symbol from the second subgroup 118b with the complex conjugate (represented by "*") of the pilot from the corresponding symbol subgroup position of the first subgroup 118a, resulting in a phase difference.
- the complex conjugate generator 104 outputs the complex conjugates of the pilot tones 114 of the first subgroup 118a of the consecutive symbols to the multipliers 106.
- the multipliers 106 (e.g., 106a) generate a complex pilot product 120 (e.g., 120a) for each symbol subgroup position (e.g., position "1") based on multiplying the pilot tones 114 (114a, 114b, 114c, 114d) from a second subgroup symbol (e.g., 112 N/2+ ⁇ ) at the corresponding position with the complex conjugate output by the corresponding conjugate generator (e.g., 104a).
- the complex pilot product 120 represents the phase difference between the pilot tones 114 for the symbols separated by the interval N/2.
- the complex products for all the symbol subgroup positions are accumulated (i.e., summed) by the complex summation circuit 108, resulting in an accumulated complex value 122. Also note that the different complex products 120a, 120b, etc., may cancel out noise components.
- the accumulated complex value 122 represents the accumulated phase difference.
- the frequency error 124 is obtained by the error calculator 110 calculating the inverse tangent of the accumulated complex value 122. The frequency error can then be supplied to tlie frequency offset estimator 58 of Figure 2 for frequency adjustment that eliminates the residual frequency error.
- Figure 4 is a diagram illustrating the method by the state machine 96 of controlling the frequency error detector according to an embodiment of the present invention.
- the state machine 96 sets the value of N symbols to be stored and compared for generation of the frequency error 124.
- a prescribed threshold equivalent to an adjustment parameter in the estimator 58
- the frequency tracking module 74 obtains channel quality information for the pilot tones (114a, 114b, 114c, 114d) from the FEQ 68 and Channel-Estimation module 70. If the estimated channel signal quality on any one of the pilot tones is worse than a pre-determined threshold, where the signal on that particular tone(s) is severely attenuated by the channel, resulting in a lower signal-to-noise ratio, the frequency estimation module 100 (under the control of the state machine 96) will discard the "noisy" tones and not sum the corresponding products of these tones with the remaining tones.
- frequency tracking is improved by providing more accurate frequency error estimation as frequency error becomes smaller during frequency tracking convergence.
- the present invention is applicable to networked computers, and wireless network computer systems.
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Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
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DE112004002397T DE112004002397B4 (en) | 2003-12-05 | 2004-10-26 | Residual frequency error estimation in an OFDM receiver |
GB0609637A GB2424157B (en) | 2003-12-05 | 2004-10-26 | Residual frequency error estimation in an ofdm receiver |
JP2006542573A JP4647616B2 (en) | 2003-12-05 | 2004-10-26 | Residual frequency error estimation in OFDM receiver |
CN2004800362045A CN1890937B (en) | 2003-12-05 | 2004-10-26 | Residual frequency error estimation in an OFDM receiver |
KR1020067011091A KR101091100B1 (en) | 2003-12-05 | 2004-10-26 | Residual frequency error estimation in an ofdm receiver |
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US10/727,670 | 2003-12-05 | ||
US10/727,670 US7292527B2 (en) | 2003-12-05 | 2003-12-05 | Residual frequency error estimation in an OFDM receiver |
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PCT/US2004/035418 WO2005062564A1 (en) | 2003-12-05 | 2004-10-26 | Residual frequency error estimation in an ofdm receiver |
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US (1) | US7292527B2 (en) |
JP (1) | JP4647616B2 (en) |
KR (1) | KR101091100B1 (en) |
CN (1) | CN1890937B (en) |
DE (1) | DE112004002397B4 (en) |
GB (1) | GB2424157B (en) |
TW (1) | TWI359585B (en) |
WO (1) | WO2005062564A1 (en) |
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Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
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WO2007079680A1 (en) * | 2006-01-12 | 2007-07-19 | Shanghai Ultimate Power Communications Technology Co., Ltd. | Frequency offset correction method for wideband time division duplex cell communication system and method for cell first searching |
CN101001231B (en) * | 2006-01-12 | 2011-11-30 | 上海原动力通信科技有限公司 | Frequency deviation correction method and cell initial search method of broadband time-division duplex cellular system |
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US20050122895A1 (en) | 2005-06-09 |
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