A Practical Approach Toward Maximum Likelihood Sequence Estimation for Band-Limited Nonlinear Channels

Abstract

A receiver structure, called a maximum likelihood (pseudo MLSE), which approximates MLSE with a simple hardware configuration, was derived. By introducting a tentatively estimated sequence, the pseudo MLSE detects the received symbol by symbol, retaining the MLSE optimum decision property. The number of arithmetic operations required in one symbol duration is reduced from M^{L + 1} to (L + 1)M in an M -ary signaling case with channel memory length L . An adaptation algorithm for the variation in the channel characteristics was also developed. Pseudo MLSE application to quadrature phase shift keying (QPSK) for a band-limited nonlinear channel is described. The most practical application of pseudo MSLE, named the adaptive threshold detector with estimated sequence (ATDES), detects symbols with threshold detection and is suitable for high bit rate operation. For both the pseudo MLSE processor and ATDES, most of the hardware is occupied by a replica memory stored in the receiver. Performance in a typical nonlinear satellite channel model is evaluated by computer simulation. Simulation results show a 0.8 dB improvement by ATDES with 64 replica memories and 1.3 dB improvement by the pseudo MLSE processor with 3072 replica memories. The tentative estimation error effect is estimated to be less than 0.1 dB in the simulated satellite channel.