Reduced complexity soft-output maximum Likelihood Sequence Estimation of 4-ary CPM signals transmitted over Rayleigh flat-fading channels

Abstract

Maximum Likelihood Sequence Estimation (MLSE) of Continuous Phase Modulated (CPM) signals can be realized in a few different manners. For the special case of transmission over a Rayleigh flat-fading channel, an attractive receiver is in the form of the Viterbi algorithm with the trellis updates being computed using a bank of FIR filters and square operations. The FIR filters are functions of the CPM pulse shape, of the channel model and of the transmitted sequence. The channel modeling being performed by using a finite order linear predictor, the receiver has been called a linear predictive receiver. For signaling with a symbol set larger than two symbols, this type of receiver can become too complex to even contemplate a practical implementation. The problems of complexity reduction and Viterbi soft-output decoding for quaternary CPM signals are examined. State-reduction techniques coupled with a q-ary Soft-Output Viterbi Algorithm (QSOVA) are presented. The performance of the algorithms is evaluated by simulations, showing the trade-off between performance and complexity reduction in some important practical cases.