A reduced complexity multi-rate echo canceller for DMT based DSL systems

In the past years, a number of researchers have investigated the use of digital echo cancellation systems for transmission technologies based on discrete multi-tone (DMT) modulation such as used in asymmetrical digital subscriber lines (ADSL) and the likes. This paper reports on a novel set of techniques that reduces the computational complexity of the multi-rate echo canceller structure at the lower transmit rate side of the link, e.g. the remote terminal (RT) side in ADSL. In this case, the echo canceller located in the terminal processes the signals into the entire frequency bandwidth (higher rate). In fact, the echo channel estimated during the processing corresponds to the imperfect hybrid channel combined with the low pass filtering action of the analog-to-digital converter (ADC). In such a situation, we can exploit the special structure of the echo channel to reduce the computational complexity of the echo cancellation processing. Specifically, this is achieved by zeroing the high frequency coefficients of the echo channel estimate. The frequency representation of the echo allows the exploitation of certain properties of the inverse fast Fourier transform (IFFT) algorithm to further reduce the complexity. Experimental simulations for an ADSL like configuration show a computational complexity reduction factor of as much as 67% without significantly affecting the residual echo level