On the optimisation of a noncoherent CDMA system based on M-ary orthogonal modulation

This paper concerns the optimisation of a DS-CDMA based mobile communication system with respect to the transmission quality in the case of data and video services respectively. For combatting transmission errors, forward error protection based on convolutional codes and interleaving in connection with soft decision maximum-likelihood (ML) decoding through the Viterbi algorithm is applied. In this context, a technique to generate post-decision reliability information in the case of a noncoherent DS-CDMA receiver for a given ML metric is presented. The proposed error control strategy involves the application of equal error protection for raw data transmission with the average post-decoding BER as an optimisation criterion. In the case of a video transmission, a-priori knowledge regarding the error sensitivity of individual parameter sets suggests an unequal error protection strategy based on punctured convolutional codes, where the segmental SNR at the source codec output represents the optimisation criterion. Due to the high complexity of the system under investigation, corresponding results are obtained through simulation of the point-to-point transmission link. A significant gain regarding the achievable transmission quality with respect to conventional approaches can be achieved in connection with a compound design of source coding, channel coding and spectral spreading.