On the Start-Up Problem in Digital Echo Cancelers

Copyright 0 I983 American Telephone and Telegraph Company. Printed in U.S.A. On the Start-Up Problem in Digital Echo Cancelers By J. SALZ* (Manuscript received October 5, 1982) Digital echo cancellation techniques make it possible to realize efficient full-duplex data transmission over a single loop. The purpose of this paper is to elucidate the solution to the start-up problem in these devices and to present a new, fast, and simple tap-adjustment procedure. The theory indicates that a modified stochastic gradient tap-adjustment algorithm, using pseudorandom input data sequences for the initial training period, converges in N steps, where N is the total number of canceler taps, and that this is the fastest possible convergence time. I. INTRODUCTION Two-way voice communication over a single loop is made possible by the use of a hybrid bridge. However, the suppression of echoes by fixed hybrids is insufficient to support full-duplex data transmission, and therefore makes adaptive data echo cancelers necessary. * Bell Laboratories. °Copyright 1983, American Telephone & Telegraph Company. Copying in printed form for private use is permitted without payment of royalty provided that each reproduction is done without alteration and that the Journal reference and copyright notice are included on the first page. The title and abstract, but no other portions, of this paper may be copied or distributed royalty free by computer-based and other informationservice systems without further permission.