Simplified Adaptive IIR Filters Based on Optimized Orthogonal Prefiltering

In order to reduce the circuit complexity associated with the estimation of echoes coming from systems with a long impulse response, we consider an estimator which is based on prefiltered input data. We propose a design of this prefilter which is optimal for a given system environment. In doing so, we represent the unknown discrete-time system by a set of characteristic impulse responses, which adequately describe the variety of the system. For such an environment we determine the optimum poles of a recursive prefilter. These poles are assumed to be fixed during the on-line LMS estimation process, which estimates the unknown echo by linearly weighting the prefilter states. An echo canceler for a typical European telephone subscriber-loop environment is used as a practical example. For this example the prefilter is optimized and realized with an orthogonal-state (lattice) filter. This not only reduces the computational costs—if compared to a conventional FIR filter design—but also permits a substantial speed-up of the on-line LMS adaptation process.