Effects of Channel Impairments on the Performance of an In-band Data-Driven Echo Canceler

Abstract

High-speed (≤ 4.8 kb/s) echo-cancellation-based full-duplex direct distance dialing modems usually have to deal with two echos: the near echo, which is generated at the modem location, and the far, or talker, echo, which has been looped back to the modem after passing through a carrier system. The near echo propagates through a channel that is essentially linear, and thus it can, at least in theory, be perfectly canceled by an echo canceler. On the other hand, the far-echo channel is generally plagued by impairments that can seriously degrade the performance of an echo canceler. In this paper we study the effects of these channel impairments on the performance of an in-band data-driven echo canceler. This echo canceler has been found to be particularly well suited for full-duplex voice-grade data transmission applications. Both analytical and real-time experimental results are presented. It is shown that frequency offset, even in small amounts, is by far the most damaging of the channel impairments that are commonly encountered in carrier systems. The degradation of performance due to phase jitter can be significant. However, this can only happen under simultaneous worst-case conditions of phase jitter and signal power levels, and these cases might not be statistically significant. Worst-case nonlinearities in the echo channel do not degrade substantially the performance of the echo canceler.