Method and apparatus for stabilizing a decision feedback...

Pulse or digital communications – Equalizers – Automatic

Reexamination Certificate

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C375S229000, C375S232000

Reexamination Certificate

active

06442198

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to filters and, more particularly, to adaptive equalizers.
BACKGROUND OF THE INVENTION
Tap drifting in an adaptive equalizer has been a problem from the very beginning of their use. A number of solutions have been proposed by those in the art for solving the tap drifting problem. For example, the use of the tap-leakage method (e.g., see R. D. Gitlin, H. C. Meadors, and S. B. Weinstein “
The Tap
-
Leakage Algorithm: An Algorithm for the Stable Operation of a Digitally Implemented, Fractionally Spaced, Adaptive Equalizer
,” B. S. T. J. Vol. 61 No. 8 (October 1982) pp. 1817-1839); adding out-of-band noise (e.g., see U.S. Pat. No. 4,384,355 issued May 17, 1983, entitled “Control of coefficient drift for fractionally spaced equalizers”, and U.S. Pat. No. 4,376,308 issued Mar. 8, 1983, entitled “Control of Coefficient Drift For Fractionally Spaced Equalizers”); and processing the received signal with a whitening filter before equalization (e.g., see U.S. Pat. No. 5,751,768 issued May 12, 1998, entitled “Fractionally spaced equalizing circuits and method”).
SUMMARY OF THE INVENTION
Unfortunately, I have observed that tap drifting seems to be compounded in filter structures that incorporate a feedback filter such as an adaptive decision feedback equalizer (DFE). As such, I have realized that the above-mentioned solutions to tap drifting in an equalizer may not always yield the best results in the case of a DFE. Therefore, I have discovered a new method for stabilizing an adaptive filter such as a DFE. In particular, and in accordance with the principles of the invention, taps of a DFE are stabilized by adding a small amount of noise to the feedback data symbols.
In an embodiment of the invention, a decision feedback equalizer structure comprises a feedforward filter, slicer, feedback filter and further includes a noise source. The noise source adds noise to the sliced data signal (provided by the slicer) before further processing by the feedback filter. This is sufficient to stop the tap drifting and stabilize the DFE. The amount of noise added is small enough that it has a negligible effect on overall system performance.


REFERENCES:
patent: 4376308 (1983-03-01), McNair
patent: 4384355 (1983-05-01), Werner
patent: 5414733 (1995-05-01), Turner
patent: 5751768 (1998-05-01), Guglielmi et al.
patent: 6069917 (2000-05-01), Werner et al.
R.D. Gitlin, H. C. Meador, and S. B. Weinstein “The Tap-Leakage Algorithm: An Algorithm for the Stable Operation of a Digitally Implemented, Fractional Spaced, Adaptive Equalizer,” B.S.T.J. vol. 61, No. 8 (Oct. 1982) pp. 1817-1839.

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