Digital noise cancellation for xDSL.

Abstract

Digital subscriber line (DSL) modems operate in frequency bands which coincide with many significant radio-frequency interference sources, particularly commercial AM radio. In these bands, the balance of most twisted-pair cables is low enough to allow substantial interference to transfer to differential mode, disrupting the transmitted information signal. To compensate for this handicap, xDSL receivers will require a front-end circuit which uses the common-mode signal as a reference to cancel out as much differential interference as possible. Unfortunately, design of such a canceller is complicated by the statistical properties of the interference, as well as the complex interactions between the differential and common-mode signals on the loop. If a digital canceller is desired, compensation for finite-precision effects poses an additional challenge. This thesis examines the feasibility of wideband digital noise cancellation for xDSL. Emphasis is placed on the stability of the adaptive algorithm, the number of bits of precision required, the adaptive filter parameters which lead to the best noise reduction, and the number of filter taps required. Simulation results indicate that digital cancellation is possible, giving a maximum noise reduction of 30--40 dB. Some hardware implementation problems are also identified in the course of the simulations.