High stability fast tracking adaptive equalizer for use with time varying communication channels

• US 6,240,133 B1
• Filed: 02/04/1999
• Issued: 05/29/2001
• Est. Priority Date: 02/05/1998
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Shalvi, Ofir, Sommer, Naftali, Segal, Mordechai
• Primary Examiner(s): Ghebretinsae, Temesghen
• Application Number:
  US09/245,117
• Time in Patent Office: 845 Days
• Field of Search: 375/232, 375/233, 375/229, 333/18, 333/28 R
• US Class Current: 375/232
• CPC Class Codes: H03H 21/0012 Digital adaptive filters
• CPC Class Codes: H04L 2025/03382 Single of vestigal sideband
• CPC Class Codes: H04L 2025/0342 QAM
• CPC Class Codes: H04L 2025/0349 as a feedback filter
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence
• CPC Class Codes: H04N 21/426 Internal components of the client ; Characteristics thereof H04N5/44 takes precedence
• CPC Class Codes: H04N 5/211 Ghost signal cancellation H04N23/81 takes precedence

Abstract

An adaptive equalizer capable of tracking rapid channel variations while maintaining high stability and low jitter, and a receiver constructed therefrom. A novel feature of the invention is that is that the equalizer is sectioned, that is constructed from a plurality of feed-forward sections and decision-feedback sections, where these sections comprise a cascade of an adaptive linear filter and an adaptive multiplier. This structure is effective at combating rapid channel variations, which are a result of delay variations of the reflections of the signal, e.g., airplane flutter, without sacrificing the stability and the accuracy of the equalizer even in cases where the equalizer has a large number of taps. The different equalizer sections may have different step size parameters. A controller monitors the channel variations and adjusts the step size parameters of each section accordingly. The controller increases the step size of the equalizer sections that are able to compensate for the variation of the channel and does not increase the step sizes of the other sections. This allows the equalizer to combat rapid channel variations without sacrificing stability and accuracy. The present invention is particularly suitable to high speed wireless RF digital communications applications, e.g., to DTV broadcast applications, where the equalizer is characterized by a large number of taps and the signal is subject to rapid time varying reflections due to movement of objects such as cars and airplanes.

Claims

1. An adaptive equalizer having an input and an output, said equalizer for equalizing a receiver signal input thereto, comprising:

a plurality of feed forward equalizer (FFE) sections, at least one feed forward equalizer section having tap coefficients and step size parameters that are adjustable separately for each said feed forward equalizer section independent of the tap coefficients and step size parameters of other feed forward equalizer sections, wherein said feed forward equalizer section comprises;

a linear adaptive digital filter operating on said input of said equalizer delayed in time and generating an output therefrom, said linear adaptive digital filter including a plurality of coefficients;

an adaptive gain multiplier operating on said output of said linear adaptive digital filter, said adaptive gain multiplier including a gain coefficient;

a first step size parameter for adapting said plurality of coefficients of said linear adaptive digital filter;

a second step size parameter for adapting said gain coefficient of said adaptive gain multiplier; and

wherein said first step size parameter and said second step size parameter are set independently for each said FFE section;

a plurality of decision feedback equalizer (DFE) sections, at least one decision feedback equalizer section having tap coefficients and step size parameters that are adjustable separately for each said decision feedback equalizer section independent of the tap coefficients and step size parameters of other decision feedback equalizer sections;

a summation unit operatively coupled to the outputs of said plurality of FFE sections and said plurality of DFE sections, said summation unit adapted to add said outputs so as to generate a soft decision output;

a symbol slicer operatively coupled to the output of said summation unit and adapted to generate estimated symbols yielding a hard decision output;

an error calculation unit operatively coupled to the output of said summation unit adapted to generate an estimated error signal; and

a controller operatively coupled to said plurality of FFE sections and said plurality of DFE sections, said controller adapted to generate said step size parameters for each FFE section and DFE section in response to sensing variations in the linear distortion of said receiver signal which can be compensated for by said FFE and said DFE sections.

2. The adaptive equalizer according to claim 1, wherein said linear adaptive digital filter comprises a plurality of tapped delay line cells.

3. The adaptive equalizer according to claim 1, wherein said decision feedback equalizer section comprises:

a linear adaptive digital filter operating on said estimated symbols delayed in time and generating an output therefrom, said linear adaptive digital filter including a plurality of coefficients;

an adaptive gain multiplier operating on said output of said linear adaptive digital filter, said adaptive gain multiplier including a gain coefficient;

a first step size parameter for adapting said plurality of coefficients of said linear adaptive digital filter;

a second step size parameter for adapting said gain coefficient of said adaptive gain multiplier; and

wherein said first step size parameter and said second step size parameter are set independently for each said DFE section.

4. The adaptive equalizer according to claim 3, wherein said linear adaptive digital filter comprises a plurality of tapped delay line cells.

5. The adaptive equalizer according to claim 1, wherein said controller comprises processing means programmed to:

sense the magnitudes of gradient outputs generated by each said FFE section and each said DFE section; and

adjust the step size parameters within those FFe and DFE sections having high gradient magnitudes so as to allow fast adjustment of tap coefficients.

6. The adaptive equalizer according to claim 1, wherein said controller comprises processing means programmed to:

monitors the tap coefficients associated with each said FFE section and each said DFE section;

detect the time variation of said coefficients;

normalize the time variation with the step size parameters for a section;

adjust the step size parameters within those FFE and DFE sections having large time variations so as to allow fast adjustment of tap coefficients.

7. The adaptive equalizer according to claim 1, wherein said controller comprises processing means programmed to:

set large values for the step size parameters in a FFE or DFE section that is expected to have large or fast time varying tap coefficients; and

set small values for the step size parameters in a FFE or DFE section that is expected to have small or slow time varying tap coefficients.

8. The adaptive equalizer according to claim 1, wherein the tap coefficients of each FFE section are adapted in accordance with the cross correlation between the input to said FFE section and said estimated error signal.

9. The adaptive equalizer according to claim 1, wherein said gain coefficient of each FFE section is adapted in accordance with the cross correlation between the input to said adaptive gain multiplier and said estimated error signal.

10. The adaptive equalizer according to claim 1, wherein the tap coefficients of each DFE section are adapted in accordance with the cross correlation between the input to said DFE section and said estimated error signal.

11. The adaptive equalizer according to claim 3, wherein said gain coefficient of each DFE section is adapted in accordance with the cross correlation between the input to said adaptive gain multiplier and said estimated error signal.

12. The adaptive equalizer according to claim 1, wherein the tap coefficients in each FFE section are adapted such that they converge to a solution wherein the magnitudes of the tap coefficients are close to a predetermined value.

13. The adaptive equalizer according to claim 1, wherein the tap coefficients in each DFE section are adapted such that convergence is achieved to a solution wherein their magnitudes are close to a predetermined value.

15. The adaptive equalizer according to claim 3, wherein the gain coefficient of said adaptive gain multiplier is adapted such that convergence is achieved to a solution wherein its magnitude is close to a predetermined value.

16. The adaptive equalizer according to claim 1, wherein the plurality of coefficients of said linear adaptive digital filter and said gain coefficient of said adaptive gain multiplier comprise real values.

17. The adaptive equalizer according to claim 3, wherein the plurality of coefficients of said linear adaptive digital filter and said gain coefficient of said adaptive gain multiplier comprise real values.

18. The adaptive equalizer according to claim 1, wherein the plurality of coefficients of said linear adaptive digital filter and said gain coefficient of said adaptive gain multiplier comprise complex values.

19. The adaptive equalizer according to claim 3, wherein the plurality of coefficients of said linear adaptive digital filter and said gain coefficient of said adaptive gain multiplier comprise complex values.

14. The adaptive equalizer according to clam 1, wherein the gain coefficient of said adaptive gain multiplier is adapted such that convergence is achieved to a solution wherein its magnitude is close to a predetermined value.

20. An adaptive filter having an input and an output, said filter for filtering an input signal, comprising:

a plurality of filter sections, at least one filter section having tap coefficients and step size parameters that are adjustable separately for each said filter section independent of the tap coefficients and step size parameters of other filter sections, wherein said filter section comprises;

a linear adaptive digital filter operating on said input of said adaptive filter delayed in time and generating an output therefrom, said linear adaptive digital filter including a plurality of coefficients;

an adaptive gain multiplier operating on said output of said linear adaptive digital filter, said adaptive gain multiplier including a gain coefficient;

a first step size parameter for adapting said plurality of coefficients of said linear adaptive digital filter;

a second step size parameter for adapting said gain coefficient of said adaptive gain multiplier; and

wherein said first step size parameter and said second step size parameter are set independently for each said filter section;

a summation unit operatively coupled to the outputs of said plurality of filter sections, said summation unit adapted to add said outputs so as to generate a soft decision output;

an error calculation unit operatively coupled to the output of said summation unit adapted to generate an estimate error signal; and

a controller operatively coupled to said plurality of filter sections, said controller adapted to generate said step size parameters for each filter section in response to sensing variations in the linear distortion of said input signal which can be compensated for by said filter sections.