Optimal bandwidth allocation to coding and spreading in DS-CDMA systems using LMMSE front-end detector

In code-division multiple-access (CDMA) systems, it is interesting to study the optimal bandwidth allocation to coding and spreading in order to maximize the number of users that the system can accommodate. This optimal bandwidth allocation is referred to as the optimal allocation point (OAP). In this brief, a practical CDMA system with a fixed total-bandwidth expansion factor that employs convolutional codes and random spreading is considered. The receiver consists of a multiuser linear-minimum-mean-square-error (lmmse) detector front end followed by autonomous single-user decoders. Intuitive reasoning is applied for the existence of an OAP in such a system. Also, we present a theoretical formulation to estimate the OAP in the aforementioned system. Simulations confirm the correctness of results obtained. Further, the paper investigates system behavior at different values of information-bit signal-to-noise ratio (SNR). At high values of SNR, within the range considered, the system favors spreading only. However, at relatively lower SNRs, channel coding is required to improve the system performance, and it is important that we operate at the OAP obtained.