Method and system for compensation of a carrier frequency offset in an OFDM receiver

• US 8,184,523 B2
• Filed: 06/22/2001
• Issued: 05/22/2012
• Est. Priority Date: 06/22/2001
• Status: Active Grant

• Current Assignee: InterDigital Madison Patent Holdings (InterDigital, Inc.)
• Original Assignee: Thomson Licensing SA (Vantiva SA)
• Inventors: Belotserkovsky, Maxim Borisovich, Demoulin, Vincent, Litwin, Louis Robert Jr.
• Primary Examiner(s): Shah, Chirag
• Assistant Examiner(s): Kang, Suk Jin
• Application Number:
  US10/481,061
• Publication Number:
  US 20040156349A1
• Time in Patent Office: 3,987 Days
• Field of Search: 370/203, 370/215
• US Class Current: 370/208
• CPC Class Codes: H03L 7/07 using several loops, e.g. for redundant clock signal generation
• CPC Class Codes: H03L 7/093 using special filtering or amplification characteristics in the loop H03L7/087 - H03L7/091 take precedence
• CPC Class Codes: H03L 7/0991 the oscillator being a digital oscillator, e.g. composed of a fixed oscillator followed by a variable frequency divider H03L7/0995 takes precedence; fixed oscillators with means for selecting among various phases H03L7/0814
• CPC Class Codes: H04L 2027/0026 Correction of carrier offset
• CPC Class Codes: H04L 2027/0044 Control loops for carrier regulation
• CPC Class Codes: H04L 2027/0067 Phase error detectors
• CPC Class Codes: H04L 27/261 Details of reference signals
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols

Abstract

An Orthogonal Frequency Division Multiplexing (OFDM) receiver that employs N second-order phase-lock loops sharing a common integrator (where N is the number of pilots in the system). The N second order phase-lock loops track out independent pilot phase rotations to facilitate the constructive averaging of the pilots'"'"' phase information. At the same time, by sharing a common integrator, the OFDM receiver takes advantage of noise averaging over multiple pilots to obtain a cleaner frequency offset estimation. The OFDM receiver may also compensate for FFT window drift by calculating a phase difference between a selected pair of pilots and tracking the rate of change of the calculated phase difference over time. The calculated phase difference is used to control the position of an upstream FFT window after a predetermined phase difference threshold is exceeded. The tracked rate of change is used to continuously adjust the phase of downstream equalizer taps.

Claims

1. A method for processing an Orthogonal Frequency Division Multiplexing (OFDM) signal in an OFDM receiver, the method comprising the steps of:

receiving the OFDM signal;

extracting a first plurality of pilots from a first user data segment of the received OFDM signal;

calculating an independent phase error for each pilot of the first plurality of pilots and a first common phase error based upon an average of said independent phase errors for each pilot of said first plurality of pilots;

extracting a second plurality of pilots from a second user data segment of the received OFDM signal, each pilot of said second plurality of pilots associated with one pilot from said first plurality of pilots;

separately derotating each pilot of said second plurality of extracted pilots to reduce the independent phase error of each pilot of said second plurality of pilots using the calculated first common phase error and the calculated independent phase error for the associated pilot from said first plurality of pilots;

calculating a phase error for each separately derotated pilot of said second plurality of pilots and a second common phase error based in part upon an average of said phase errors for each separately derotated pilot of said second plurality of extracted pilots;

integrating the second common phase error to reduce the effects of noise; and

deriving a carrier frequency offset estimate from the integrated second common phase error.

2. The method of claim 1, further comprising the step of:

correcting for a carrier frequency offset using the carrier frequency offset estimate.

3. The method of claim 1, wherein the step of derotating includes passing each pilot of the first plurality of pilots through a distinct phase-lock loop.

4. The method of claim 1, wherein the step of derotating includes passing each pilot of the plurality of pilots through a time-shared phase-lock loop.

5. The method of claim 1, wherein said deriving a carrier frequency offset estimate is performed after the phase errors of each pilot of the second plurality of pilots has been reduced below a predetermined value.

6. The method of claim 5, wherein the predetermined value is selected such that the derived carrier frequency offset estimate is substantially constant.

7. The method of claim 1, further comprising the steps of:

calculating a phase difference between a predetermined pair of extracted pilots;

comparing the calculated phase difference to a threshold value, the threshold value representative of a predetermined FFT window offset; and

adjusting an FFT window of an upstream FFT module if the calculated phase difference exceeds the threshold value.

8. The method of claim 7, wherein the step of deriving a carrier frequency offset estimate further includes the steps of:

tracking the rate of change of the calculated phase difference; and

adjusting the derived carrier frequency offset estimate to compensate for the tracked rate of change, the tracked rate of change being representative of a phase rotation due to an FFT window offset.

9. The method of claim 1, wherein the OFDM receiver is implemented in one of a wireless LAN adapter, a home networking terminal, a portable terminal, and a desktop terminal.

10. A system for processing an Orthogonal Frequency Division Multiplexing (OFDM) signal, the system comprising:

a processing module for receiving a first, second, and third plurality of pilots from an OFDM signal and for reducing an independent phase error of each pilot of said second and third plurality of pilots through derotation based upon an independent phase error determined in part from a corresponding pilot of a prior plurality of pilots; and

a carrier frequency offset estimator module coupled to the processing module, the carrier frequency offset estimator module deriving a carrier frequency offset estimate from the third plurality of pilots after the independent phase error of each pilot of the third plurality of pilots is reduced below a predetermined value, the carrier frequency offset estimator module including;

an averaging unit for calculating a first common phase error for the first plurality of pilots used by the processing module to reduce the independent phase error of each pilot and for calculating a second common phase error for the plurality of pilots after the independent phase error of each pilot of the third plurality of pilots is reduced below a predetermined value, the second common phase error representative of an average pilot phase rotation caused by a carrier frequency offset, and

an integrator unit for integrating the first common phase error and the second common phase error to reduce the effects of noise.

11. The system of claim 10, wherein the OFDM receiver is implemented in one of a wireless LAN adapter, a home networking terminal, a portable terminal, and a desktop terminal.

12. The system of claim 10, wherein the predetermined value is substantially zero.

13. The system of claim 10, wherein the predetermined value is selected such that the derived carrier frequency offset estimate is substantially constant.

14. The system of claim 10, wherein the processing module includes a plurality of phase-lock loops, each phase-lock loop being dedicated for a predetermined pilot within a user data segment.

15. The system of claim 10, wherein the processing module includes a time-shared phase-lock loop clocked for processing all the pilots within a user data segment.

16. The system of claim 10, wherein the processing module further includes:

a phase difference calculator for calculating a phase difference between two predetermined pilots within a user data segment; and

an FFT window synch unit for adjusting the positioning of an upstream FFT window if the calculated phase difference exceeds a predetermined value.

17. The system of claim 16, wherein the carrier frequency offset module further includes:

a rate of phase difference estimator coupled to the phase difference calculator for tracking the change of the calculated phase difference over multiple user data segments, the estimator compensating the derived carrier frequency offset for phase rotations due to a drifting FFT window.

18. A system for processing an Orthogonal Frequency Division Multiplexing (OFDM) signal, the system comprising:

means for extracting a first plurality of pilots from a first user data segment of an OFDM signal;

means for calculating an independent phase error for each pilot of the first plurality of pilots and a first common phase error based upon an average of said independent phase errors for each pilot of said first plurality of pilots;

means for extracting a second plurality of pilots from a second user data segment of the OFDM signal, each pilot of said second plurality of pilots associated with one pilot from said first plurality of pilots;

means for separately derotating each pilot of the second plurality of extracted pilots, said derotation based upon the calculated first common phase error and the calculated independent phase error for the associated pilot from said first plurality of pilots;

means for processing the extracted pilots to compensate for an FFT window drift;

means for calculating a phase error for each separately derotated pilot of said second plurality of pilots and a second common phase error based in part upon an average of said phase errors for each separately derotated pilot of said second plurality of extracted pilots; and

means for deriving a carrier frequency offset estimate from the second common phase error.