Symbol-by-Symbol Map Demodulation of CMP signals Transmitted on Rayleigh Flat-Fading Channels

by

Michael J. Gertsman

M. Eng., 1996

Abstract

Demodulation using the symbol-by-symbol MAP algorithm is presented. The algorithm is derived for the case of CMP signals transmitted over Rayleigh flat-fading channels and a corresponding receiver structure is specified. It is shown that the MAP algorithm requires computing, for each trellis branch, the sum of the products of the weights of all paths through the trellis which pass through that branch and that this generic computational problem can be efficiently solved by an approach that uses a forward and backward recursion through the trellis. Simulation results are presented which show both the hard and soft decision performance of the MAP receiver to be robust even in the presence of fade rates of up to 30% of the symbol rate. The MAP algorithm is also shown to produce accurate BER estimates and to be amenable to reduced state implementation. The application of the receiver concept to QPSK signals is also discussed and then evaluated via simulation.

The concept of joint demodulation and decoding using iterative processing techniques is introduced. It is shown that the MAP receiver is well suited fro iterative processing applications due to its use of a priori symbol probabilities  and its production of optimal soft decisions. Simulation results for the reception of QPSK show that the BER performance of the iterative MAP receiver can approach that of a receiver can approach that of a receiver operating with perfect knowledge of the fading process.

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