Circulant decomposition canceler for echo cancellation in DMT-based receivers

Asymmetric digital subscriber line (ADSL) employs the discrete multi-tone modulation (UMT) as the transmission format. To separate up- and downstream signals, the ADSL standard allows the use of echo cancellation resulting in improved bit rates, reach and/or noise margins. DMT-based modems typically use the mixed time/frequency (MTF) domain echo canceling (EC) scheme of M. Ho et al. (Jul. 1996), where the echo-filter is estimated in the frequency domain by means of the least mean square (LMS) algorithm with the transmitted echo symbols as update directions. Since not every tone of the transmitted echo signal is excited, the MTF adaptation process leads to an inferior estimate of the echo channel, unless extra power on unused echo tones is transmitted. Since transmitting extra power on such tones is often undesired, we present in this paper an alternative EC scheme, the circulant decomposition canceler (CDC), which works without extra power requirements and which exhibits comparable complexity as M. Ho et al. (Jul. 1996). In addition, we have shown how the CDC scheme can be incorporated into a receiver structure using per tone equalization and double talk cancellation to allow fast tracking and/or convergence in the presence of a far end signal.