Equalization in orthogonal frequency domain multiplexing

• US 20040146123A1
• Filed: 01/28/2003
• Published: 07/29/2004
• Est. Priority Date: 01/28/2003
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Agere Systems Incorporated (Broadcom, Inc.)
• Inventors: Lai, Yhean-Sen
• Granted Patent: US 7,277,493 B2
• Time in Patent Office: Days
• US Class Current: 375/329
• CPC Class Codes: H04L 1/20 using signal quality detector
• CPC Class Codes: H04L 2025/03414 Multicarrier
• CPC Class Codes: H04L 2027/003 at baseband only
• CPC Class Codes: H04L 2027/0055 single phase
• CPC Class Codes: H04L 2027/0065 Frequency error detectors H04L2027/0067 takes precedence
• CPC Class Codes: H04L 25/03159 operating in the frequency domain H04L25/03165, H04L25/03178 take precedence
• CPC Class Codes: H04L 25/067 providing soft decisions, i.e. decisions together with an estimate of reliability H04L25/068 and H04L25/069 take precedence; sequence estimation techniques H04L25/03178
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

A COFDM demodulator providing independent DQPSK symbol amplitude and phase equalization provides improved soft-symbol estimates by reducing the possibility of clipping in the differential demodulator, improving the tolerance of the differential demodulator to finite word length effects, correcting for phase drifting between adjacent subcarriers, and improving signal-to-noise ratio (SNR) calculation accuracy for maximal ratio combining applications. Separate acquisition and tracking modes for the amplitude and phase equalizers are provided that handle start-up, reacquisition, and steady-state operation of the demodulator.

Claims

1. A method for demodulating OFDM signals, comprising:

(a) adjusting amplitude of symbols in an OFDM signal;

(b) differentially demodulating the amplitude-adjusted symbols; and

(c) adjusting phase of the demodulated symbols.

2. The invention of claim 1, wherein the amplitude adjustment and the phase adjustment are independently optimized.

3. The invention of claim 1, wherein the amplitude is adjusted such that clipping is avoided when differentially demodulating the amplitude-adjusted symbols.

4. The invention of claim 1, wherein the OFDM signal is a COFDM signal and the symbols are DQPSK symbols.

5. The invention of claim 1, wherein adjusting the amplitude comprises:

(1) applying an amplitude gain to symbols in each carrier of the OFDM signal; and

(2) computing an amplitude error based on comparing the amplitudes of amplitude-adjusted symbols to an amplitude reference, wherein the amplitude error is used to adjust the amplitude gain.

6. The invention of claim 1, wherein adjusting the phase comprises:

(1) rotating the demodulated symbols based on a phase error; and

(2) comparing the phases of the rotated symbols to the phases of reference symbols to update the phase error.

7. The invention of claim 6, wherein the phase error is low-pass filtered before being used to rotate the demodulated symbols.

8. The invention of claim 1, wherein the OFDM demodulation includes an acquisition mode and a tracking mode, wherein:

(a) during the tracking mode, adjusting the amplitude comprises;

(1) applying an amplitude gain to symbols in each carrier of the OFDM signal; and

(2) computing an amplitude error based on comparing the amplitudes of amplitude-adjusted symbols to an amplitude reference, wherein the amplitude error is used to adjust the amplitude gain, and

(b) during the acquisition mode, the amplitudes of the symbols are not adjusted.

9. The invention of claim 1, wherein the OFDM demodulation includes an acquisition mode and a tracking mode, wherein:

(a) during the tracking mode, adjusting the phase comprises;

(1) rotating the demodulated symbols based on a phase error; and

(2) comparing the phases of the rotated symbols to the phases of reference symbols to update the phase error, and

(b) during the acquisition mode, the phases of the demodulated symbols are not adjusted.

10. The invention of claim 1, further comprising processing the phase-adjusted demodulated symbols using a soft-symbol, Viterbi decoder.

11. The invention of claim 1, further comprising processing the phase-adjusted demodulated symbols using a Reed-Solomon decoder.

12. The invention of claim 1, further comprising processing the phase-adjusted demodulated symbols to determine signal-to-noise ratio (SNR).

13. The invention of claim 12, wherein the SNR is used in a maximal ratio combiner.

14. The invention of claim 1, comprising using the phase-adjusted demodulated signals in a receiver that utilizes one or more diversity types corresponding to one or more of space, time, and frequency.

15. An OFDM signal demodulator comprising:

(a) an amplitude equalizer adapted to adjust amplitude of symbols in an OFDM signal;

(b) a differential demodulator adapted to differentially demodulate the amplitude-adjusted symbols; and

(c) a phase equalizer adapted to adjust phase of the demodulated symbols.

16. The invention of claim 15, wherein:

the amplitude equalizer is adapted to equalize the amplitude of the symbols independent of the phase of the symbols; and

the phase equalizer is adapted to equalize the phase of the demodulated symbols independent of the amplitude of the demodulated symbols.

17. The invention of claim 15, wherein the amplitude equalizer is adapted to:

(1) apply an amplitude gain to symbols in each carrier of the OFDM signal; and

(2) compute an amplitude error based on comparing the amplitudes of the amplitude-adjusted symbols to an amplitude reference, wherein the amplitude error is used to adjust the amplitude gain.

18. The invention of claim 15, wherein the phase equalizer is adapted to:

(1) rotate the demodulated symbols based on a phase error; and

(2) compare the phases of the rotated symbols to the phases of reference symbols to update the phase error.

19. The invention of claim 18, wherein the phase error is low-pass filtered before being used to rotate the demodulated symbols.

20. The invention of claim 15, wherein the OFDM demodulator is adapted to operate in an acquisition mode and a tracking mode, wherein:

(a) during the tracking mode, the amplitude equalizer is adapted to;

(1) apply an amplitude gain to symbols in each carrier of the OFDM signal; and

(2) compute an amplitude error based on comparing the amplitudes of amplitude-adjusted symbols to an amplitude reference, wherein the amplitude error is used to adjust the amplitude gain, and

(b) during the acquisition mode, the amplitude equalizer is adapted to leave the amplitudes of the symbols unchanged.

21. The invention of claim 15, wherein the OFDM demodulator is adapted to operate in an acquisition mode and a tracking mode, wherein:

(a) during the tracking mode, the phase equalizer is adapted to;

(1) rotate the demodulated symbols based on a phase error; and

(2) compare the phases of the rotated symbols to the phases of reference symbols to update the phase error, and

(b) during the acquisition mode, the phase equalizer is adapted to leave the phases of the demodulated symbols unchanged.

22. A receiver comprising:

(a) at least two demodulators adapted to demodulate time, frequency, or space diverse versions of substantially identical transmitted data, wherein at least one demodulator is an OFDM signal demodulator comprising;

(1) an amplitude equalizer adapted to adjust amplitude of symbols in an OFDM signal;

(2) a differential demodulator adapted to differentially demodulate the amplitude-adjusted symbols; and

(3) a phase equalizer adapted to adjust phase of the demodulated symbols; and

(b) a combining circuit adapted to process (i) outputs of the demodulators and (ii) signal-to-noise (SNR) estimates calculated from the outputs of the demodulators to recover the transmitted data.