Echo Cancellation in DMT Modems With Frame-Asynchronous Operation

Echo cancellation in DMT modems has been long studied as a means to improve transmission performance. Most of the current echo-canceller structures make use of FIR filters that emulate the echo-path adaptively. In frame-asynchronous operation, the misalignment of the TX and RX DFT windows (combined with other modem and channel parameters like: the transmit tone-mask and the echo-path impulse response) determines the echo input statistics and can worsen the conditioning of the coefficient update formula. This effect is analyzed in the paper, within the context of two specific algorithms: the circulant decomposition canceller (CDC) and also the time-domain canceller (TD-EC). Self-orthogonalizing (S-O) update formulas, implemented through compensation matrices, and their respective low-rank approximations are proposed as a method to improve the convergence rate and, more generally, to render the echo-canceller performances less sensitive to the input statistics. The method introduced does not employ additional power on the unsent tones.