Marco Bonaventura

Echo cancellation in DMT-receivers: circulant decomposition canceler

Asymmetric digital subscriber lines (ADSLs) employ discrete multitone modulation (DMT) as transmission format, where subcarriers are assigned to the up- and/or downstream transmission direction. 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. In DMT-based modems, typically, the mixed time/frequency (MTF) domain echo canceling scheme, as proposed by Ho et al., is implemented. This technique estimates the echo filter in the frequency domain using the least mean square (LMS) algorithm with the transmitted echo symbols as update directions. Since not every tone of the transmitted echo signal will carry data, i.e., will be excited, the MTF adaptation process does not lead to a good estimate for the echo channel, unless extra power on unused echo tones is transmitted. However, transmitting extra power on such tones is often undesired. In this paper, we present an alternative echo canceling scheme referred to as the circulant decomposition canceler (CDC), which works without extra power requirements and with comparable complexity as the method of Ho et al. Similar to MTF echo canceling, the CDC scheme can easily be incorporated into a multirate environment with different transmit and receive rates and can also cheaply be combined with per-tone equalization and double talk cancellation to allow fast tracking and/or convergence in the presence of a far-end signal.

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

A multi-carrier echo canceller based on symmetric decomposition

Echo cancellation in DMT modems is a means to improve transmission performance. It consists in extending the downstream (DS) and upstream (UP) frequency bands so that they become adjacent or even overlap, and relaxing at the same time the existing filters. Modern cancellers make use of FIR filters that emulate the echo-path, and adaptive algorithms that estimate the correspondent coefficients. In this article, we start from the circulant decomposition canceller (CDC), an efficient algorithm recently developed for ADSL, and explore new kinds of decomposition, based on the diagonalization properties of the discrete cosine and sine transforms (DCT, DST).