Optimization of code rate and spreading factor for direct-sequence CDMA systems

In direct-sequence CDMA, code rate and spreading factor are parameters of the transmission system that can be traded with each other in order to optimize its performance and implementation complexity. In this paper, we investigate the bit error rate performance of convolutionally encoded direct-sequence spread spectrum transmission schemes where the code rate and spreading factor are varied while the transmission bandwidth and interleaving delay are kept constant. It is shown that for coherent demodulation schemes decreasing code rate improves the performance even if very simple channel estimation is used. In contrast, for noncoherent and differential demodulation schemes no gain can be achieved when the code rate is decreased below 1/3.