Decorrelating Detectors for a Dual Rate Synchronous DS/CDMA Channel

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. All users have the same BPSK modulation technique and the same chip rate. The diierences in bit transmission rates result in diierent processing gains for each class of user. We assume that in an interval of duration T 0 , a low rate user transmits one bit while a high rate user transmits M bits. Applying a standard decorrelator to the interval of duration T 0 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 T 0 =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. This paper proves that the asymptotic eeciency of the standard decorrelator is greater than or equal to the proposed decorrelator. However, 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 while retaining the near-far resistance property.