Mixed-mode next/echo canceller for pulse amplitude modulated (pam) signals

Abstract

A mixed-mode crosstalk canceller is disclosed that performs crosstalk cancellation in the continuous time domain. The disclosed mixed-mode crosstalk canceller processes the pulse amplitude modulated (PAM) digital signal to be transmitted as well as the received signal to compensate for the crosstalk between the transmit and receive signals. The output of the crosstalk canceller is subtracted from the received signal in the continuous time domain. Thus, the transmit symbol clock and the receive symbol clock can be asynchronous. The tap weights for the crosstalk cancellation are illustratively obtained using a modified version of the least mean square (LMS) algorithm for discrete time signals. The modified least mean square (LMS) algorithm is applied for continuous time signals that are derived from different clocks. Since, the LMS algorithm requires a costly multiplication of the error signal, e(t), and the digital transmit signal, x (t), and it is only necessary to go in the direction of the gradient with the steepest decent, computational gains are achieved using a correlation multiplier that quantizes e(t) and x (t) with only one or two bits. The quantized version of the error signal, e(t), indicates the sign of the error (positive or negative) and is obtained in the illustrative embodiment by comparing the error signal to zero. A crosstalk canceller updates each tap weight utilizing the disclosed correlation multiplier that provides a signal indicating whether the tap weight needs to be increased or decreased to a charge pump that produces a current in the proper direction.