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