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. All users have the same BPSK modulation technique and the same chip rate. The differences in bit transmission rates result in different processing gains for each class of user. 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 is considered. The soft decoding rule will apply 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. The proposed soft decoding decorrelator is found to perform nearly as well as the standard decorrelator while retaining the near-far resistance property.