Optimum phase error metric for OFDM pilot tone tracking in wireless LAN

• US 6,549,583 B2
• Filed: 02/21/2001
• Issued: 04/15/2003
• Est. Priority Date: 02/21/2001
• Status: Expired due to Term

• Current Assignee: Fleet Connect Solutions LLC (Empire IP LLC)
• Original Assignee: Magis Networks, Inc. (ON Semiconductor Corporation)
• Inventors: Crawford, James A.
• Primary Examiner(s): Tse, Young T.
• Application Number:
  US09/790,429
• Publication Number:
  US 20020150168A1
• Time in Patent Office: 783 Days
• Field of Search: 375/130, 375/134, 375/137, 375/144, 375/145, 375/148, 375/149, 375/226, 375/260, 375/326, 375/327, 375/340, 375/341, 375/349, 370/206, 370/207, 370/208, 370/210
• US Class Current: 375/260
• CPC Class Codes: H04L 2027/0067 Phase error detectors
• CPC Class Codes: H04L 2027/0087 Out-of-band signals, (e.g. pilots)
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols
• CPC Class Codes: H04L 27/2679 Decision-aided

Abstract

A method and apparatus of pilot phase error estimation in an orthogonal frequency division multiplexed (OFDM) receiver including the steps of: determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; and estimating an aggregate phase error of a subsequent OFDM data symbol relative to the pilot reference points using complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFOM data symbol and the pilot reference points. For example, a maximum likelihood based estimation is performed using the complex signal measurements and the pilot reference points. Thus, the poor phase performance in a radio portion of the OFDM receiver is compensated for by the pilot phase error estimation in the baseband portion of the OFDM receiver and improved OFDM signal tracking accomplished under poor SNR conditions.

Claims

1. A method of pilot phase error estimation in an orthogonal frequency division multiplexed (OFDM) receiver comprising:

determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; and

estimating an aggregate phase error of a subsequent OFDM data symbol relative to the pilot reference points using complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM data symbol and the pilot reference points;

wherein the estimating step comprises performing a maximum likelihood-based estimation using the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM data symbol and the pilot reference points.

2. The method of claim 1 wherein the estimating step is represented mathematically as:

${\widehat{\mathrm{\theta <br><br>}}}_m={\tan }^{-1}<br><br>\left[\frac{\underset{k=0}{\overset{n-1}{\sum <br><br>}}<br><br>\left(u_k<br><br>Q_{k,m}-v_k<br><br>I_{k,m}\right)}{\underset{k=0}{\overset{n-1}{\sum <br><br>}}<br><br>\left(u_k<br><br>I_{k,m}+v_k<br><br>Q_{k,m}\right)}\right]<br><br>\text{\hspace{1em}<br><br>}$

where {circle around (θ

)}_m is the aggregate phase error for the subsequent OFDM data symbol having a time index m, wherein u_k and u_k represent in-phase (I) and quadrature (Q) values, respectively, for the pilot reference points for n pilots of the OFDM preamble waveform, and where I_k,m and Q_k,m represent the complex signal measurements corresponding to the k^th pilot of the m^th subsequent OFDM data symbol.

3. The method of claim 1 wherein the determining step comprises determining the pilot reference points corresponding to the plurality of pilots of a long symbol portion of the OFDM preamble waveform.

4. The method of claim 3 wherein the determining step further comprises obtaining complex signal measurements from bins of a Fourier transform corresponding to respective ones of the plurality of pilots of the long symbol portion of the OFDM preamble waveform.

5. The method of claim 1 further comprises determining the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM data symbol.

6. The method of claim 5 wherein the determining the complex signal measurements comprises obtaining the complex signal measurements from bins of a Fourier transform corresponding to respective ones of the plurality of pilots of the subsequent OFDM data symbol.

7. The method of claim 5 wherein the estimating step further comprises performing pre-signal detection combining to produce a complex composite signal.

8. The method of claim 7 wherein the performing step comprises calculating a complex signal for each of the plurality of pilots of the subsequent OFDM data symbol, wherein the complex signal for each of the plurality of pilots is calculated from a respective one of the pilot reference points and a respective one of the complex signal measurements.

9. The method of claim 8 wherein the performing step further comprises calculating the complex composite signal by summing complex signals for each of the plurality of pilots of the subsequent OFDM data symbol.

10. The method of claim 9 wherein the estimating step comprises estimating the aggregate phase error of the subsequent OFDM data symbol from the complex composite signal.

11. The method of claim 9 wherein the estimating step further comprises determining an argument of the complex composite signal, wherein the argument of the complex composite signal is based upon the maximum likelihood-based estimation.

12. The method of claim 11 wherein the determining the argument comprises estimating the aggregate phase error using a cordic-based arctangent on the real and the imaginary parts of the complex composite signal.

13. A pilot phase error metric in an orthogonal frequency division multiplexed (OFDM) receiver comprising:

means for determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; and

means for estimating an aggregate phase error of a subsequent OFDM data symbol relative to the pilot reference points using complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM data symbol and the pilot reference points;

wherein the means for estimating comprise means for performing a maximum likelihood-based estimation using the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM data symbol and the pilot reference points.

14. The pilot phase error metric of claim 13 wherein the means for determining comprise means for determining the pilot reference points corresponding to the plurality of pilots of a long symbol portion of the OFDM preamble waveform.

15. The pilot phase error metric of claim 14 wherein the means for determining further comprise means for obtaining complex signal measurements from bins of a Fourier transform corresponding to respective ones of the plurality of pilots of the long symbol portion of the OFDM preamble waveform.

16. The pilot phase error metric of claim 13 further comprising means for determining the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM data symbol.

17. The pilot phase error metric of claim 16 wherein the means for determining the complex signal measurements comprises obtaining the complex signal measurements from bins of a Fourier transform corresponding to respective ones of the plurality of pilots of the subsequent OFDM data symbol.

24. The pilot phase error metric of claim 13 wherein the means for estimating is represented mathematically as:

${\widehat{\mathrm{\theta <br><br>}}}_m={\tan }^{-1}<br><br>\left[\frac{\underset{k=0}{\overset{n-1}{\sum <br><br>}}<br><br>\left(u_k<br><br>Q_{k,m}-v_k<br><br>I_{k,m}\right)}{\underset{k=0}{\overset{n-1}{\sum <br><br>}}<br><br>\left(u_k<br><br>I_{k,m}+v_k<br><br>Q_{k,m}\right)}\right]$

where {circle around (θ

)}_m is the aggregate phase error for the subsequent OFDM data symbol having a time index m, wherein u_k and v_k represent in-phase (I) and quadrature (Q) values, respectively, for the pilot reference points for n pilots of the OFDM preamble waveform, and where I_k,m and Q_k,m represent the complex signal measurements corresponding to the k^th pilot of the m^th subsequent OFDM data symbol.

18. The pilot phase error metric of claim 18 wherein the means for estimating further comprise means for performing pre-signal detection combining to produce a complex composite signal.

19. The pilot phase error metric of claim 18 wherein the mean for performing comprise means for calculating a complex signal for each of the plurality of pilots of the subsequent OFDM data symbol, wherein the complex signal for each of the plurality of pilots is calculated from a respective one of the pilot reference points and a respective one of the complex signal measurements.

20. The pilot phase error metric of claim 19 wherein the means for performing further comprise means for calculating the complex composite signal by summing complex signals for each of the plurality of pilots of the subsequent OFDM data symbol.

21. The pilot phase error metric of claim 20 wherein the means for estimating comprise means for estimating the aggregate phase error of the subsequent OFDM data symbol from the complex composite signal.

22. The pilot phase error metric of claim 20 wherein the means for estimating further comprise means for determining an argument of the complex composite signal, wherein the argument of the complex composite signal is based upon the maximum likelihood-based estimation.

23. The pilot phase error metric of claim 22 wherein the means for determining the argument comprise means for estimating the aggregate phase error using a cordic-based arctangent on the real and the imaginary parts of the complex composite signal.

25. A pilot phase error metric for an orthogonal frequency division multiplexed (OFDM) receiver comprising:

a reference point storage for storing pilot reference points corresponding to each of a plurality of pilots of an OFDM preamble waveform;

a maximum likelihood phase error/weighting processor coupled to the reference point storage for processing complex signal measurements corresponding to each of a plurality of pilots of a subsequent OFDM data symbol in comparison to the pilot reference points from the reference point storage; and

a phase error estimator coupled to the maximum likelihood phase error/weighting processor for estimating an aggregate phase error of the OFDM data symbol relative to the pilot reference points from the processed complex signal measurements and the pilot reference points.

26. The pilot phase error metric of claim 25 further comprising a pseudo random pilot phase modulation removal coupled to the phase error estimator.

27. The pilot phase error metric of claim 26 further comprising a pseudo random generator coupled to the pseudo random pilot phase modulation removal.

28. The pilot phase error metric of claim 25 further comprising a multiplexer coupled the maximum likelihood phase error/weighting processor for buffering the complex signal measurements corresponding to each of the plurality of pilots of the OFDM data symbol to the maximum likelihood phase error/weighting processor.