Adaptive interference cancellation using common-mode information in DSL

Exploiting the common-mode (CM) receive signal in wireline communication can yield significant improvements in terms of channel capacity compared to using only the differential-mode (DM) signal. Recently published, independent, scientific work proposed the employment of an adaptive CM-reference based interference canceller and reported performance improvements based on simulation results. Adaptive processing of correlated receive signals, however, bears the potential danger of cancelling the useful component-an undesired effect we will address. We present an analysis of the linear adaptive cancellation approach in this application. For a large class of practically relevant cases, it can be shown that a canceller, whose coefficients are adapted while the far-end transmitter is silent, yields a signal-to-noise-and-interference power ratio (SNIR) which is higher than the SNIR of the DM-only channel output. Moreover, the performance of a canceller with this tap-setting is close to the performance of the front-end that is optimum in the sense of maximising the SNIR at its output. Adaptation while the useful far-end signal is present yields a front-end whose output SNIR is considerably lower compared to the SNIR of the DM channel output. The results and their practical impact are demonstrated by an example.