Decorrelating Detectors for a Dual Rate Synchronous DS/CDMA System

This paper addresses the use of decorrelating detectors for a dual rate DS/CDMA system that serves both low bit rate and high bit rate users. We assume that in an interval of duration T0, a low rate user transmits one bit while a high rate user transmits M bits. Applying a standard decorrelator to the interval of duration T0 yields an M bit processing delay for high rate users and a computational complexity that grows with M. In this paper, we propose a decorrelator that generates bit decisions for each high rate user in every subinterval of duration T0/M. To decode a low rate user, a soft decoding rule applies maximal ratio combining on M separate decorrelated outputs of each low rate user. The soft decoding dual rate decorrelator eliminates the bit processing delay for high rate users and also reduces the computational complexity of a standard decorrelator. Further, it is shown that the asymptotic efficiency of the standard decorrelator is greater than or equal to the proposed decorrelator. By evaluation, it is observed that when the signature sequences have good correlation properties, the proposed soft decoding decorrelator is found to perform nearly as well as the standard decorrelator. However, when the signature sequences have high cross-correlation, then the proposed decorrelator suffers in terms of bit error rate while retaining the near-far resistance property.