Enhanced echo canceler

Details Enhanced echo canceler Enhanced echo canceler

1998-06-11

2001-05-29

Bocure, Tesfaldet (Department: 2631)

Pulse or digital communications

Transceivers

Modems

C375S285000, C375S229000, C375S232000

Reexamination Certificate

active

06240128

ABSTRACT:

TECHNICAL FIELD
This invention relates to modems and, more particularly, to echo cancelers employed in modems.
BACKGROUND OF THE INVENTION
Echo cancelers are employed in modems in order to achieve full-duplex data communications. A number of different echo cancellation techniques have been employed in order to simplify the echo canceler structure and/or improve overall data communications. These prior techniques have employed echo cancelers at various positions in the receive path of the modem in attempting to improve over-all echo cancellation and, in particular, adaptation speed in synthesizing an echo. Indeed, it is desirable to have the echo canceler transversal filter impulse response rapidly converge to the impulse response of the echo path. To this end, the prior techniques “trained” the echo canceler during the so-called “half-duplex” operation of the modem. Modems are typically connected to a two-wire telephone line through a hybrid, i.e., 2-to-4-wire conversion, network, whose characteristics tend to change over time and, especially, because of temperature variations. Consequently, the echo canceler impulse response synthesis capability must be updated in order to track changes in the echo path impulse response characteristic. This is especially true in modems, employing the so-called PCM technique, i.e., pulse code modulation technique, which are embedded in personal computers, and particularly, laptop computers. In these and similar applications, because of the use of smaller so-called form factors in the circuitry, there are more rapid circuit characteristic changes caused by rapid temperature variations, which translate into faster impedance drift of the hybrid network. Thus, it is desirable to rapidly track variations in the echo path impulse response in order to enhance modem performance. This leads to a well known dichotomy of noise and rapid response. To achieve rapid response in synthesizing the echo path impulse response a large adaptation step size is desirable but causes noise which degrades data communication capability. To minimize noise a small adaptation step size is desirable but results in a slow convergence when synthesizing the echo path impulse response. This problem is acute in PCM modems because the values of PCM symbols vary significantly from one symbol to another and a very small adaptation step size is required in order to avoid adverse effects in the modem receiver performance. The small adaptation step size, of course, translates into too poor of a tracking capability for practical applications.
SUMMARY OF THE INVENTION
These and other problems and limitations of prior known modem echo canceler arrangements are overcome by utilizing a so-called post equalization echo canceler, and transmitter and receiver data timing synchronization to enhance tracking of the echo path impulse response and convergence of the transversal filter in the post equalization echo canceler. This is realized by employing the equalization error in the receiver to adapt coefficients of the post equalization echo canceler transversal filter, in conjunction, with the transmitter and receiver data timing synchronization. The timing synchronization is realized by using sample rate conversion of the transmit sample rate to the receive sample rate and, in one example, variable phase interpolation of the converted timing signal.
More specifically, the receiver timing is recovered and a phase error signal generated by the timing recovery unit is advantageously employed to adjust a variable phase interpolator in the receiver and a variable phase interpolator in a path supplying the transmitter signal to an input of the post equalization echo canceler. This insures that both the adaptive transversal filter of the post equalization echo canceler and a transversal filter in an equalizer in the receiver are operating on data having the same timing. In this example, the timing is that of the received data signal.
In an embodiment of the invention, the post equalization echo canceler is utilized in conjunction with a so-called conventional, e.g., a primary, echo canceler. The conventional echo canceler is employed before the equalizer to cancel a major portion of any echo signal, while the post equalization echo canceler is employed after the equalizer to cancel primarily echo signals caused by drift in the hybrid network. To this end, the conventional echo canceler is “trained” during the initial half-duplex operation of the modem and, then, updating of its impulse response is inhibited, while the post equalization echo canceler is allowed to continue adapting.


REFERENCES:
patent: 4535206 (1985-08-01), Falconer
patent: 4924492 (1990-05-01), Gitlin et al.
patent: 5005168 (1991-04-01), Cummiskey et al.
patent: 5095497 (1992-03-01), Aman et al.
patent: 5132963 (1992-07-01), Ungerboeck
patent: 5162812 (1992-11-01), Aman et al.
patent: 5163044 (1992-11-01), Golden
patent: 5319636 (1994-06-01), Long et al.
patent: 5631899 (1997-05-01), Duttweiller
patent: 5864545 (1999-01-01), Gonikberg et al.
patent: 6018755 (2000-01-01), Gonikberg et al.
Sylvie Marcos and Odile Macchi, “Joint Adaptive Echo Cancellation and Channel Equalization For Data Transmission”,Signal Processing, vol. 20, No.1, May 1990, pp. 43-65.
K.H. Muller, “Combining Echo Cancellation and Decision Feedback Equalization”,B.S.T.J., vol. 58, No. 2, Feb. 1979, pp. 491-500.

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