Analysis of decorrelator-based receivers for multirate DS/CDMA communications

Decorrelator-based receivers have been investigated for demodulating data in a dual-rate synchronous direct-sequence code-division multiple-access (DS/CDMA) system. The proposed receivers were a high-rate decorrelator (HRD) matched to the data rate of the high-rate users and a low-rate decorrelator (LRD) matched to the data rate of the low-rate users. In this paper, the probabilities of error attainable for high-rate and low-rate users with the use of the HRD and the LRD are analyzed. It is proven that the LRD offers superior performance to that of the HRD for all users. Asymptotic analysis provides bounds for the performance difference for high-rate users. The results are generalized to multirate systems. To increase fidelity, a sliding-window decorrelator (SWD) is proposed which demodulates a high-rate user's data by a soft-decoding rule from the outputs of several decorrelators sliding along the received signal sequence. The results show that it performs better than the HRD while maintaining smaller demodulation delay and computational complexity than the LRD. To further exploit the characteristics of multirate systems, a decorrelating decision-feedback detector is proposed and its asymptotic multiuser efficiency is analyzed. It is shown that this detector incurs little demodulation delay for high-rate users and provides better performance for low-rate users than that of the LRD when the energies of the interfering users are comparable to that of the desired user.