A differential error reference adaptive echo canceller for multilevel PAM line codes

The objective of any local echo canceller connected to any end of a digital subscriber line (DSL) is to compensate the outgoing transmit signal energy in the receiving path (commonly called near-echo) that the hybrid circuit leaks. The observed signals that usually play the role of error references for adaptive echo cancellers (EC) in data communications are basically composed by far-end data plus a residual echo. The EC goal is to eliminate that residual echo. However, depending on the convergence stage, the presence of data in the error reference may mask the effective error reference (the residual echo). We show a new adaptive EC that offers better convergence properties than those associated to the simplest EC algorithms (standard LMS and decision-directed) for multilevel PAM signals. It exploits the discrete nature of the far-end signal to self-cancel the masking effect by adding (or subtracting) a weighted old version of the error reference to the current one. We show that the new error cost function is now closer to the instantaneous optimal one: its corresponding MLE. This new error surface presents a more robust (or uniform) behavior during the convergence period.