Nonlinear adaptive filtering for echo cancellation

An examination is made of the problem of nonlinear adaptive filtering for echo cancellation. The high-speed requirements of digital subscriber loops and voiceband data modems place constraints on the design of adaptive echo cancellers due to the presence of nonlinearities. The authors consider table look-up structures and nonlinear filters based on the Volterra series for general nonlinearities, and nonlinear compensators for specific practical configurations. For the first category, means to speed up the initial convergence of an adaptive table look-up structure are suggested. The configurations involve two table-driven structures, one for cancellation and one to form the reference signal. It is shown how the second structure can also be used for decision-feedback equalization. A combined linear and nonlinear structure with improved convergence behavior is also proposed. Theoretical convergence rate results are presented for such structures. In the second category, nonlinear compensators are cascaded with linear filters to combat nonlinearities for specific channel models.<>