Method and apparatus for fault recovery in a decision feedback equalizer

• US 6,608,864 B1
• Filed: 05/26/1999
• Issued: 08/19/2003
• Est. Priority Date: 05/26/1999
• Status: Expired due to Fees

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: 3Com Corporation (HP Inc.)
• Inventors: Strait, Jeffrey C.
• Primary Examiner(s): Le, Amanda T.
• Application Number:
  US09/320,258
• Time in Patent Office: 1,546 Days
• Field of Search: 375/233, 375/232, 375/229, 375/350, 375/260, 708/322, 708/323
• US Class Current: 375/233
• CPC Class Codes: H04L 2025/03414 Multicarrier
• CPC Class Codes: H04L 2025/03522 Frequency domain
• CPC Class Codes: H04L 2025/03617 Time recursive algorithms H04L2025/03643 takes precedence
• CPC Class Codes: H04L 2025/03687 of step size
• CPC Class Codes: H04L 25/03159 operating in the frequency domain H04L25/03165, H04L25/03178 take precedence

Abstract

A method and apparatus for frequency domain equalization. The method and apparatus are particularly well suited for use in a receiver in a multicarrier transmission system that has predetermined symbols periodically embedded in the transmissions, such as in an ADSL system. The equalizer apparatus includes a digital filter having a plurality of single-tap filters. The filters operate on the symbols in the frequency domain, accepting frequency domain representations of the received symbols. The equalizer also includes a reference symbol generator that provides reference symbols, a coefficient generator that accepts the equalized frequency domain symbols from the digital filter and the reference symbols and updates the filter taps using a received predetermined symbol and the reference symbol. The coefficient generator includes a generator filter having a first and a second adaptation increment, an error generator, and a threshold detector that controls the coefficient generator in response to said error signal. Preferably, the threshold detector determines whether the equalizer output would have been decoded improperly during a synchronization symbol, and if so, enables the use of the second update increment in the generator filter.

Claims

1. A frequency domain equalizer for use in a receiver in a multicarrier transmission system characterized by predetermined symbols being periodically embedded in the transmissions, said equalizer comprising:

a digital filter having a plurality of single-tap filters, wherein said digital filter accepts frequency domain representations of received symbols and provides equalized frequency domain symbols;

a reference symbol generator providing reference symbols;

a coefficient generator connected to said digital filter and said reference symbol generator, said coefficient generator accepting the equalized frequency domain symbols from said digital filter and said reference symbols, wherein the coefficient generator further comprises a generator filter; and

wherein said coefficient generator updates said plurality of filter taps in response to the equalized frequency domain symbols and wherein said coefficient generator updates said plurality of taps also in response to said reference symbols when said received symbol is a predetermined symbol.

2. The frequency domain equalizer of claim 1, wherein the generator filter generates new coefficients using an adaptive updating algorithm.

3. The frequency domain equalizer of claim 1, wherein the coefficient generator further comprises an error generator, said error generator providing an error signal indicative of the convergence of the frequency domain equalizer.

4. The frequency domain equalizer of claim 3, wherein the coefficient generator further comprises an error threshold detector connected to said error generator wherein said error threshold detector directs the coefficient generator to perform an aggressive update.

5. The frequency domain equalizer of claim 4 wherein said threshold detector directs the coefficient generator to perform an aggressive update only when a synchronization symbol was received.

6. The frequency domain equalizer of claim 4 wherein said threshold detector directs the coefficient generator to perform an aggressive update only when a synchronization symbol was received and the equalized synchronization symbol would have been decoded improperly.

7. The frequency domain equalizer of claim 3, wherein said digital filter receives said predetermined symbols during a synchronization interval, and wherein said coefficient generator updates said plurality of filter taps during the synchronization symbol interval.

8. The frequency domain equalizer of claim 7, wherein the error generator determines that an error would have occurred if the equalized frequency domain symbols were decoded, the error determination made by calculating a magnitude of spacing between constellation points during the synchronization symbol interval and comparing the magnitude of spacing between constellation points to the error signal to determine if an error threshold has been satisfied.

9. The frequency domain equalizer of claim 1 further comprising a reference selector, said reference selector selectively connecting said symbol reference generator to said coefficient generator.

10. The frequency domain equalizer of claim 1 further comprising a synchronization symbol gain compensator connected to said digital filter for imposing gain factors on the frequency domain representations of received symbols.

11. A method of equalizing a sequence of received symbols including predetermined symbols comprising the steps of:

adaptively filtering the symbols in the frequency domain using at least one single-tap filter, wherein the adaptive filtering is based in part on decision feedback information, and wherein the update filter increment value is μ

₁;

determining whether a predetermined symbol has been received, and if so;

generating at least one reference symbol;

calculating an error in response to the filtered predetermined symbol and the at least one reference symbol;

updating the at least one single-tap filter in response to the error using an aggressive update algorithm.

12. The method of claim 11, wherein the at least one single-tap filter comprises a plurality of single-tap filters, one for each of a plurality of carriers in a multi-carrier communication system.

13. The method of claim 11, further comprising the step of determining an error threshold, and wherein the step of updating the at least one single-tap filter is performed only if the error exceeds the threshold.

14. The method of claim 13, wherein the calculated error is the difference between the equalized predetermined symbol and the reference symbol.

15. The method of claim 11, wherein the step of adaptively filtering includes performing an adaptive gradient algorithm update.

16. The method of claim 11, wherein the step of updating the at least one single-tap filter includes performing an LMS algorithm update.

17. The method of claim 11, wherein the aggressive update is performed with a filter increment μ

₂, wherein μ

₂ is greater than μ

₁.

18. The method of claim 17, wherein μ

₂ is four to eight times greater than μ

₁.

19. The method of claim 11, further comprising the step of modifying the scale of at least one of the reference symbol and the received symbol to ensure comparable scaling.

20. The method of claim 11, wherein the step of updating the at least one single-tap filter includes performing two update iterations using a single received predetermined symbol.

21. The method of claim 20, further comprising the step of determining an error threshold for each update iteration, and wherein each of the update iterations are performed only if the corresponding error exceeds the threshold.

22. A computer readable medium containing instructions that will cause a microprocessor to perform the steps of claim 11.

23. The method of claim 11, further comprising determining an error threshold, and wherein the step of updating the at least one single-tap filter is performed if the error does not exceed the threshold.

24. A system comprising:

a processing unit; and

machine language instructions stored in data storage executable by the processing unit to perform functions including;

adaptively filtering received symbols in the frequency domain using at least one single-tap filter, wherein the adaptive filtering is based in part on decision feedback information, and wherein the update filter increment value is μ

₁;

determining whether a predetermined symbol has been received, and if so;

generating at least one reference symbol;

calculating an error in response to the filtered predetermined symbol and the at least one reference symbol; and

updating the at least one single-tap filter in response to the error using an aggressive update algorithm.

25. The system of claim 24, wherein the machine language instructions further perform functions including determining an error threshold, and wherein the function of updating the at least one single-tap filter is performed only if the error exceeds the threshold.

26. The system of claim 24, wherein the machine language instructions further perform functions including modifying the scale of at least one of the reference symbol and the received symbol to ensure comparable scaling.