OFDM pilot tone tracking for wireless LAN

• US 6,633,616 B2
• Filed: 08/21/2001
• Issued: 10/14/2003
• 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 TOI
• Application Number:
  US09/935,081
• Publication Number:
  US 20020159533A1
• Time in Patent Office: 784 Days
• Field of Search: 375/134, 375/137, 375/144, 375/145, 375/147, 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: 375/326
• 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 pilot phase tracking loop for an OFDM receiver including a phase rotator receiving an incoming signal, a fast Fourier transform coupled to a phase rotator output, and a pilot phase error metric including a discrete Fourier transform portion coupled to the phase rotator output. The pilot phase error metric determines a phase error estimate associated with a received OFDM symbol, e.g., a data symbol, from the phase rotator output. A loop filter is coupled to the pilot phase error metric output and an oscillator is coupled to the loop filter output. The oscillator output is coupled to the phase rotator to adjust the phase of subsequent OFDM symbols of the incoming signal. Phase noise introduced by a radio portion of the OFDM receiver and OFDM transmitter is reduced by the baseband portion of the OFDM receiver improving OFDM signal tracking under poor SNR conditions.

Claims

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

a discrete Fourier transform portion for receiving an incoming signal corresponding to an OFDM waveform, wherein the discrete Fourier transform portion outputs complex signal measurements corresponding to each of a plurality of pilots of a preamble portion of the OFDM waveform and complex signal measurements corresponding to each of a plurality of pilots of a subsequent OFDM symbol of the OFDM waveform, wherein the discrete Fourier transform portion is separate from a fast Fourier transform operation of the OFDM receiver;

a maximum likelihood phase error/weighting processor coupled to the discrete Fourier transform portion for processing the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol in comparison to pilot reference points; and

a phase error estimator coupled to the maximum likelihood phase error/weighting processor for estimating a phase error of the subsequent OFDM symbol relative to a phase corresponding to the preamble portion based on the processed complex signal measurements and the pilot reference points.

2. The pilot phase error metric of claim 1 wherein the phase error estimator estimates an aggregate phase error of the subsequent OFDM symbol relative to the pilot reference points based on the processed complex signal measurements and the pilot reference points.

3. The pilot phase error metric of claim 1 further comprising a reference point storage coupled to the discrete Fourier transform portion for storing the pilot reference points corresponding to each of the plurality of pilots of the OFDM preamble waveform.

4. The pilot phase error metric of claim 1 further comprising a pseudo random pilot phase modulation removal coupled to the phase error estimator.

5. The pilot phase error metric of claim 4 further comprising a pseudo random generator coupled to the pseudo random pilot phase modulation removal.

6. The pilot phase error metric of claim 1 further comprising a multiplexer coupling the discrete Fourier transform portion to the maximum likelihood phase error/weighting processor for buffering the complex signal measurements corresponding to each of the plurality of pilots of the OFDM symbol to the maximum likelihood phase error/weighting processor.

7. The pilot phase error metric of claim 1 wherein the discrete Fourier transform portion comprises a respective discrete Fourier transform module corresponding to each of the plurality of pilots of the OFDM waveform.

8. The pilot phase error metric of claim 1 wherein the discrete Fourier transform portion comprises a respective discrete Fourier transform module for respective pairs of the plurality of pilots of the OFDM waveform due to symmetries in the frequency locations of the respective pairs of the plurality of pilots.

9. The pilot phase error metric of claim 1 wherein the discrete Fourier transform portion has a plurality of frequency bin outputs each corresponding to a respective one of the plurality of pilots of the OFDM waveform.

10. The pilot phase error metric of claim 1 wherein the pilot reference points comprise the complex signal measurements corresponding to each of the plurality of pilots of the preamble portion of the OFDM waveform.

11. The pilot phase error metric of claim 1 wherein the pilot reference points comprise complex signal measurements corresponding to each of the plurality of pilots of an OFDM symbol occurring after the preamble portion of the OFDM waveform and prior to the subsequent OFDM symbol.

12. 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;

processing, in a parallel path to the determining step, the OFDM preamble waveform with a fast Fourier transform;

determining a phase error estimate of a subsequent OFDM symbol relative to the pilot reference points; and

processing, in the parallel path to the determining step, the subsequent OFDM symbol with the fast Fourier transform;

wherein the determining the phase error estimate step is completed prior to the completion of the processing the subsequent OFDM symbol with the fast Fourier transform in the parallel path.

13. The method of claim 12 wherein the determining the phase error estimate step comprises determining an aggregate phase error estimate of the subsequent OFDM symbol relative to the pilot reference points using complex signal measurements corresponding to each of a plurality of pilots of the subsequent OFDM symbol and the pilot reference points.

14. The method of claim 12 further comprising filtering the phase error estimate of the subsequent OFDM symbol relative to the pilot reference points.

15. The method of claim 12 further comprising rotating a phase of an incoming signal by the phase error estimate, prior to processing, in the parallel path, OFDM symbols of the incoming signal occurring after the subsequent OFDM symbol with the fast Fourier transform.

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

determining, in a separate processing path parallel to a fast Fourier transform processing path of the OFDM receiver, pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform;

estimating, in the separate processing path, a phase error of a subsequent OFDM symbol relative to the pilot reference points using complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol and the pilot reference points, the estimating the phase error for the purpose of phase tracking; and

rotating a phase of OFDM symbols occurring after the subsequent OFDM symbol according to the phase error.

17. The method of claim 16 wherein the estimating step is completed prior to the completion of processing the subsequent OFDM symbol in the fast Fourier transform processing path of the OFDM receiver.

18. The method of claim 16 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 symbol and the pilot reference points.

19. The method of claim 16 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.

20. The method of claim 19 wherein the determining step further comprises processing the long symbol portion of the OFDM preamble waveform with a discrete Fourier transform producing the complex signal measurements corresponding to respective ones of the plurality of pilots of the long symbol portion of the OFDM preamble waveform.

21. The method of claim 16 further comprises determining, in the separate processing path, the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol.

22. The method of claim 21 wherein the determining the complex signal measurements comprises processing the subsequent OFDM symbol with a discrete Fourier transform producing the complex signal measurements corresponding to respective ones of the plurality of pilots of the subsequent OFDM symbol.

23. The method of claim 21 wherein the estimating step further comprises performing pre-signal detection combining to produce a complex composite signal.

24. The method of claim 23 wherein the performing step comprises calculating a complex signal for each of the plurality of pilots of the subsequent OFDM 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.

25. The method of claim 24 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 symbol.

26. The method of claim 25 wherein the estimating step comprises estimating the phase error of the subsequent OFDM symbol from the complex composite signal.

27. The method of claim 25 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 a maximum likelihood-based estimation.

28. The method of claim 27 wherein the determining the argument comprises estimating the phase error using a cordic-based arctangent on the real and the imaginary parts of the complex composite signal.

29. The method of claim 21 wherein the estimating step is represented mathematically as:

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

where {circumflex over (θ

)}_m is an aggregate phase error for the subsequent OFDM 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 symbol.