Frequency domain echo cancellation for discrete multitone asymmetric digital subscriber line transceivers

An asymmetric digital subscriber line (ADSL) is an emerging technology for the transport of video dial tone and other new services over the telephone copper loop plant. Discrete multitone (DMT) has been selected by ANSI as the modulation scheme for ADSL transmission. Echo cancellation can be used to increase the reach or noise margin of an ADSL system by allowing both upstream and downstream channels to use the low frequency portion of the twisted pair spectrum. Previous papers have described an efficient time and frequency domain echo canceler (TAFDEC) for potential DMT ADSL application. This paper presents a generalization of this previously described canceler which, when properly optimized, results in significant complexity reduction. The generalization applies to the cyclic echo synthesizer (CES), which forms the time-domain processing portion of the canceler. By expanding the achievable CES input range and optimizing the temporal alignment between transmitter and echo canceler, ADSL CES complexity is reduced to approximately one-half of that previously described.<>