OFDM pilot tone tracking for wireless lan

• US 20020159533A1
• Filed: 08/21/2001
• Published: 10/31/2002
• Est. Priority Date: 02/21/2001
• Status: Active Grant

• Current Assignee: Fleet Connect Solutions LLC (Empire IP LLC)
• Original Assignee: Sanyo Semiconductor Company Limited (ON Semiconductor Corporation)
• Inventors: Crawford, James A.
• Granted Patent: US 6,633,616 B2
• Time in Patent Office: Days
• 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 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 tracking loop for an orthogonal frequency division multiplexed (OFDM) receiver comprising:

a phase rotator for receiving and phase de-rotating an incoming signal;

a fast Fourier transform coupled to an output of the phase rotator for processing a signal output from the phase rotator;

a pilot phase error metric including a discrete Fourier transform portion, the discrete Fourier transform portion coupled to the output of the phase rotator, wherein the pilot phase error metric determines a phase error estimate associated with a received OFDM symbol of the signal output from the phase rotator;

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

an oscillator coupled to an output of the loop filter and having an output coupled to the phase rotator such that the phase rotator adjusts the phase of subsequent OFDM symbols of the incoming signal arriving after the received OFDM symbol by the phase error estimate.

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

3. The pilot phase tracking loop of claim 2 wherein the phase error estimate comprises an aggregate phase error estimate of the received OFDM symbol relative to the pilot reference points corresponding to the OFDM preamble waveform.

4. The pilot phase tracking loop of claim 2 wherein the discrete Fourier portion of the pilot phase error metric determines the pilot reference points corresponding to the plurality of pilots of the OFDM preamble waveform and determines the complex signal measurements corresponding to each of the plurality of pilots of the received OFDM symbol and of the subsequent OFDM symbols.

5. The pilot phase tracking loop of claim 2 further comprising a pilot reference storage coupled to the pilot phase error metric for storing the pilot reference points.

6. The pilot phase tracking loop of claim 1 wherein the wherein the pilot phase error uses a maximum likelihood estimation that processes complex signal measurements corresponding to each of a plurality of pilots of the received OFDM symbol in comparison to pilot reference points corresponding to each of a plurality of pilots of an OFDM symbol received prior to the received OFDM symbol.

7. 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, 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.

8. The pilot phase tracking loop of claim 1 wherein a phase noise of the signal output from the phase rotator after adjustment is reduced.

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

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

11. The pilot phase tracking loop of claim 8 wherein the phase noise of the signal output from the phase rotator after adjustment is reduced to less than about 0.5 degrees rms.

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

13. The pilot phase tracking loop of claim 8 wherein the phase noise performance of a local oscillator of the radio portion is greater than about −

80 dBc/Hz at a 10 kHz offset.

14. 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.

15. The pilot phase tracking loop of claim 1 wherein, after adjustment of the phase rotator, a phase noise of the subsequent OFDM symbols at the output of the phase rotator is minimized.

17. The method of claim 16 wherein small frequency errors remaining after a coarse and fine frequency estimation occurring during the OFDM preamble waveform are tracked out.

18. The method of claim 16 wherein the determining the phase error estimate step 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.

19. The method of claim 18 wherein the determining the aggregate phase error estimate step comprises performing a maximum likelihood-based estimation using the complex signal measurements for the OFDM symbol and the pilot reference points.

20. The method of claim 19 wherein the determining the aggregate phase error estimate step is represented mathematically as:

21. The method of claim 16 wherein the determining the pilot reference points step comprises determining the pilot reference points corresponding to the plurality of pilots of a long symbol portion of the OFDM preamble waveform.

22. The method of claim 16 wherein the determining the pilot reference points and determining the complex signal measurements steps comprise processing the baseband signal corresponding to the long symbol portion and the baseband signal corresponding to the OFDM symbol with the discrete Fourier transform.

23. The method of claim 16 wherein the rotating compensates for phase noise introduced by a radio portion of the OFDM receiver that provides the incoming signal and phase noise introduced by a transmitting radio portion of an OFDM transmitter communicating with the OFDM receiver.

24. The method of claim 16 wherein, after the rotating step, the phase noise of the incoming signal corresponding to the subsequent OFDM symbols is reduced to less than about 1 degree rms.

25. The method of claim 24 wherein the phase noise of the incoming signal corresponding to the subsequent OFDM symbols is less than about 2.5 degrees rms prior to the rotating step.

26. The method of claim 16 wherein, after the rotating step, the phase noise of the incoming signal corresponding to the subsequent OFDM symbols is reduced to less than about 0.5 degrees rms.

27. The method of claim 26 wherein the phase noise of the incoming signal corresponding to the subsequent OFDM symbols is less than about 2.5 degrees rms prior to the rotating step.

28. The method of claim 16 wherein the phase noise performance of a local oscillator of the radio portion is greater than about −

80 dBc/Hz at a 10 kHz offset.

30. The method of claim 29 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 an OFDM preamble portion of the OFDM waveform.

31. The method of claim 30 wherein the determining further comprises:

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

determining complex signal measurements corresponding to each of a 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.

32. The method of claim 29 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.

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

receiving a baseband signal corresponding to an OFDM preamble waveform at a discrete Fourier transform portion of the OFDM receiver, wherein the discrete Fourier transform is a separate processing operation than a fast Fourier transform of the OFDM receiver;

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

receiving a baseband signal corresponding to an OFDM symbol at the discrete Fourier transform portion;

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

determining a phase error estimate corresponding to the OFDM symbol based on the pilot reference points and the complex signal measurements;

filtering the phase error estimate; and

rotating a phase of an incoming signal corresponding to subsequent OFDM symbols to be received at the fast Fourier transform after the OFDM symbol by a filtered phase error estimate;

wherein a phase noise of the incoming signal corresponding to the subsequent OFDM symbols to be received at the fast Fourier transform is reduced.

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

receiving a signal representing an OFDM waveform at a discrete Fourier transform portion of the OFDM receiver, wherein the discrete Fourier transform is a separate processing operation than a fast Fourier transform of the OFDM receiver that also receives the signal;

determining a phase error estimate corresponding to an OFDM symbol of the OFDM waveform;

filtering the phase error estimate; and

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

33. 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 the 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.

34. The pilot phase error metric of claim 33 wherein the phase 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.

35. The pilot phase error metric of claim 33 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.

36. The pilot phase error metric of claim 33 further comprising a pseudo random pilot phase modulation removal coupled to the phase error estimator.

37. The pilot phase error metric of claim 36 further comprising a pseudo random generator coupled to the pseudo random pilot phase modulation removal.

38. The pilot phase error metric of claim 33 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.

39. The pilot phase error metric of claim 33 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.

40. The pilot phase error metric of claim 33 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.

41. The pilot phase error metric of claim 33 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.

42. 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 steps, 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.

43. The method of claim 42 wherein the determining the phase error step comprises determining an aggregate phase error 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.

44. The method of claim 42 further comprising filtering the phase error estimate of the subsequent OFDM symbol relative to the pilot reference points.

45. The method of claim 42 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.

47. The method of claim 46 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.

48. The method of claim 46 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.

49. The method of claim 46 wherein the determining step comprises determining the reference points corresponding to the plurality of pilots of a long symbol portion of the OFDM preamble waveform.

50. The method of claim 49 wherein the determining step further comprises processing the long symbol portion of the OFDM preamble waveform with a discrete Fourier transform producing complex signal measurements corresponding to respective ones of the plurality of pilots of the long symbol portion of the OFDM preamble waveform.

51. The method of claim 46 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.

52. The method of claim 51 wherein the determining the complex signal measurements comprises processing the subsequent OFDM symbol with a discrete Fourier transform producing complex signal measurements corresponding to respective ones of the plurality of pilots of the subsequent OFDM symbol.

53. The method of claim 51 wherein the estimating step further comprises performing pre-signal detection combining to produce a complex composite signal.

54. The method of claim 53 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.

55. The method of claim 54 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.

56. The method of claim 55 wherein the estimating step comprises estimating the aggregate phase error of the subsequent OFDM symbol from the complex composite signal.

57. The method of claim 55 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.

58. The method of claim 57 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.

59. The method of claim 51 wherein the estimating step is represented mathematically as:

46. 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; and

estimating, in the separate processing path, an aggregate 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.