Performance of multiuser detection in multirate DS-CDMA systems

The performance of multirate direct-sequence code-division multiple-access (CDMA) systems is considered. We compare two multirate schemes: variable spreading length (VSL-CDMA) and multicode (MC-CDMA). The performance in terms of asymptotic multiuser efficiency (AME) and near-far resistance (NFR) for various detectors are evaluated. Analytical and numerical results demonstrate that in multirate systems, MC-CDMA has a similar performance to that of VSL-CDMA employing low-rate detection in terms of multirate AME (MAME) and multirate NFR (MNFR). A lower bound for the optimal MNFR is also obtained and is shown to be that of the linear decorrelator in multirate systems. Thus, this implies that the decorrelator is no longer optimal in the sense of MNFR.