OFDM pilot tone tracking for wireless LAN

• US 6,549,561 B2
• Filed: 08/21/2001
• 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 TOI
• Application Number:
  US09/935,243
• Publication Number:
  US 20020176483A1
• Time in Patent Office: 602 Days
• Field of Search: 375/134, 375/137, 375/144, 375/145, 375/148, 375/149, 375/226, 375/326, 375/327, 375/328, 375/340, 375/341, 375/349, 375/260, 370/206, 370/207, 370/208, 370/210
• US Class Current: 375/137
• 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 for receiving an incoming signal, which is coupled to a Fourier transform, which is coupled to a pilot phase error metric for determining a phase error estimate associated with a received OFDM symbol, e.g., a data symbol. The pilot phase error metric is coupled to a loop filter, which is coupled to an oscillator, which is coupled back to the phase rotator. The phase rotator rotates the incoming signal by the filtered phase error estimate for subsequent OFDM symbols to reduce the phase noise of the signaling output from the phase rotator. Thus, the phase noise introduced by a radio portion of the OFDM receiver and an OFDM transmitter is compensated for by the pilot phase error estimation in the baseband portion of the OFDM receiver and improved OFDM signal tracking is accomplished under poor SNR conditions.

Claims

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

a phase rotator for receiving an incoming signal;

a Fourier transform coupled to an output of the phase rotator;

a pilot phase error metric coupled to an output of the Fourier transform for determining a phase error estimate associated with a received OFDM symbol;

a loop filter coupled to an output of the pilot phase error metric; and

an oscillator coupled to an output of the loop filter, the oscillator having an output coupled to the phase rotator for causing the phase rotator to rotate the phase of the incoming signal by the filtered phase error estimate for subsequent OFDM symbols such that the phase noise of the signaling output from the phase rotator is reduced;

wherein the pilot phase error metric uses a maximum likelihood estimation that processes complex signal measurements from the Fourier transform corresponding to each of a plurality of pilots of the received OFDM symbol in comparison to pilot reference points obtained from each of a plurality of pilots of a previous OFDM symbol.

2. The pilot phase tracking loop of claim 1 wherein the Fourier transform comprises a fast Fourier transform.

3. The pilot phase tracking loop of claim 1 wherein the pilot phase error metric uses the maximum likelihood estimation that processes the complex signal measurements from the Fourier transform corresponding to each of the plurality of pilots of the received OFDM symbol in comparison to the pilot reference points for each of the plurality of pilots of an OFDM preamble waveform.

4. The pilot phase tracking loop of claim 1 further comprising a pilot reference point storage coupled to the output of the Fourier transform and coupled to the pilot phase error metric for storing the pilot reference points corresponding to each of the plurality of pilots.

5. The pilot phase tracking loop of claim 1 further comprising a pseudo random pilot modulation generator coupled to the pilot phase error metric for removal of a priori known pseudo-random pilot modulation.

6. The pilot phase tracking loop of claim 1 further comprising a radio portion of the OFDM receiver that provides the incoming signal to the phase rotator, the radio portion including a local oscillator, wherein the pilot phase tracking loop compensates for phase noise introduced by the radio portion and phase noise introduced by a transmitting radio portion of an OFDM transmitter communicating with the OFDM receiver.

7. The pilot phase tracking loop of claim 1 wherein a phase noise of the signaling output from the phase rotator after rotation is reduced.

8. The pilot phase tracking loop of claim 7 wherein the phase noise of the signaling output from the phase rotator after rotation is reduced to less than about 1 degree rms.

9. The pilot phase tracking loop of claim 8 wherein the phase noise of the incoming signal received at the phase rotator is less than about 2.5 degrees rms.

10. The pilot phase tracking loop of claim 7 wherein the phase noise of the signaling output from the phase rotator after rotation is reduced to less than about 0.5 degrees rms.

11. The pilot phase tracking loop of claim 10 wherein the phase noise of the incoming signal received at the phase rotator is less than about 2.5 degrees rms.

12. The pilot phase tracking loop of claim 7 wherein the phase noise performance of a local oscillator of a radio portion of the OFDM receiver is greater than about −

80 dBc/Hz at a 10 kHz offset.

13. A pilot phase tracking loop for an orthogonal frequency division multiplexed (OFDM) receiver comprising:

a phase rotator for receiving and adjusting the phase of an incoming signal corresponding to an OFDM waveform;

a Fourier transform coupled to an output of the phase rotator;

a pilot phase error metric coupled to an output of the Fourier transform;

a loop filter coupled to the pilot phase error metric; and

an oscillator coupled to the loop filter and having an output coupled to the phase rotator;

wherein the pilot phase error metric uses a maximum likelihood estimation that processes complex signal measurements from the Fourier transform corresponding to each of a plurality of pilots of an OFDM symbol in comparison to pilot reference points obtained from each of the plurality of pilots of a previous OFDM symbol.

14. The pilot phase tracking loop of claim 13 wherein the Fourier transform comprises a fast Fourier transform.

15. The pilot phase tracking loop of claim 13 further comprising a pilot reference point storage coupled to the output of the Fourier transform and coupled to the pilot phase error metric.

16. The pilot phase tracking loop of claim 13 wherein the pilot phase error metric uses the maximum likelihood estimation that processes the complex signal measurements from the Fourier transform corresponding to each of the plurality of pilots of the OFDM symbol in comparison to the pilot reference points corresponding to each of the plurality of pilots of an OFDM preamble waveform.

17. The pilot phase tracking loop of claim 13 further comprising a radio portion of the OFDM receiver that provides the incoming signal to the phase rotator, wherein the radio portion includes a local oscillator, wherein the pilot phase tracking loop compensates for phase noise introduced in the radio portion and phase noise introduced by a transmitting radio portion of an OFDM transmitter communicating with the OFDM receiver.

18. The pilot phase tracking loop of claim 17 wherein the phase noise of the incoming signal output from the phase rotator is reduced.

19. The pilot phase tracking loop of claim 18 wherein as a result of the reduced phase noise of the incoming signaling output from the phase rotator, a phase noise performance of the OFDM receiver using M-ary QAM, M-PSK, and other digital modulation waveforms is improved.

20. A method for tracking pilot phase in an orthogonal frequency division multiplexed (OFDM) receiver comprising:

receiving an incoming signal corresponding to an OFDM preamble waveform at a Fourier transform of the OFDM receiver;

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

receiving the incoming signal corresponding to an OFDM symbol at the Fourier transform;

determining complex signal measurements corresponding to each of the plurality of pilots of the OFOM symbol;

determining a phase error estimate corresponding to the OFDM symbol;

filtering the phase error estimate; and

rotating a phase of the incoming signal for subsequent OFDM symbols to be received at the Fourier transform after the OFDM symbol by the filtered phase error estimate, wherein a phase noise of the incoming signal for the subsequent OFDM symbols is reduced;

wherein the determining the phase error estimate comprises determining an aggregate phase error estimate of the OFDM symbol relative to the pilot reference points using the complex signal measurements corresponding to each of the plurality of pilots of the OFDM symbol and the pilot reference points; and

wherein the determining the aggregate phase error estimate step comprises performing a maximum likelihood-based estimation using the complex signal measurements and the pilot reference points.

21. The method of claim 20 wherein the receiving the incoming signal representing the OFDM preamble waveform comprises receiving the incoming signal representing the OFDM preamble waveform at a fast Fourier transform of the OFDM receiver.

22. The method of claim 20 wherein the receiving the incoming signal representing the OFDM symbol comprises receiving the incoming signal representing the OFDM symbol at a fast Fourier transform.

23. The method of claim 20 further comprising updating an oscillator with the filtered phase error estimate, wherein the oscillator causes the rotating step.

24. The method of claim 20 wherein the determining the aggregate phase error estimate 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 {circumflex over (θ

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

25. The method of claim 20 wherein the determining the pilot reference points comprises determining the pilot reference points corresponding to the plurality of pilots of a long symbol portion of the OFDM preamble waveform.

26. The method of claim 25 wherein the determining the pilot reference points and the complex signal measurements steps comprise processing the incoming signal corresponding to the long symbol portion and the incoming signal corresponding to the OFDM symbol with the Fourier transform.

27. A method of pilot phase tracking in an orthogonal frequency division multiplexed (OFDM) receiver comprising:

receiving an incoming signal representing an OFDM waveform at a Fourier transform of the OFDM receiver;

determining a phase error estimate corresponding to an OFDM symbol of the OFDM waveform based upon the output of the Fourier transform;

filtering the phase error estimate; and

rotating a phase of the incoming signal for subsequent OFDM symbols to be received at the Fourier transform after the OFDM symbol by the filtered phase error estimate, wherein the phase noise of the incoming signal for the subsequent OFDM symbols is reduced;

wherein the determining the phase error estimate step comprises determining an aggregate phase error estimate of the OFDM symbol relative to a pilot phase corresponding to a previous OFDM symbol;

wherein the determining the phase error estimate step further comprises;

determining pilot reference points corresponding to a plurality of pilots corresponding to the previous OFDM symbol;

determining complex signal measurements corresponding to the plurality of pilots of the OFDM symbol;

wherein the determining the aggregate phase error estimate includes processing the complex signal measurements and the pilot reference points using a maximum likelihood-based estimation.

28. The method of claim 27 wherein the determining the phase error estimate comprises determining the aggregate phase error estimate of the OFDM symbol relative to the pilot phase corresponding to an OFDM preamble portion of the OFDM waveform.

29. The method of claim 28 wherein the determining the phase error estimate step further comprises:

determining the pilot reference points corresponding to the plurality of pilots corresponding to the OFDM preamble portion of the OFDM waveform;

determining the complex signal measurements corresponding to the plurality of pilots of the OFDM symbol;

wherein the determining the aggregate phase error estimate includes processing the complex signal measurements and the pilot reference points using the maximum likelihood-based estimation.