Performance of turbo coded quaternary CPM over AWGN channels

This paper investigates the performance of a turbo-coded system transmitted over AWGN channels using quaternary partial response Continuous Phase Modulation (CPM). The system provides an attractive option for spectral efficient communications systems. We compare the performances of a Maximum A Posteriori (MAP) algorithm and a Soft Output Viterbi Algorithm (SOVA). The paper considers the hybrid concatenation of a parallel designed turbo encoder and a partial response quaternary CPM modulator. Based on the decomposition theory of CPM, the quaternary CPM modulator is decomposed into a ring Continuous Phase Encoder (CPE) and a Memoryless Modulator (MM). This allows a quaternary convolutional encoder to be combined with the CPE without the need of a mapper. The hybrid design combined with iterative decoding yield very good performances. The results show the considerable superiority of the MAP algorithm to that of the SOVA.