A decorrelating decision-feedback detector for dual rate synchronous DS/CDMA communications

We consider a dual rate synchronous DS/CDMA system in which the length of each signature sequence is inversely proportional to the corresponding data rate. Two types of decorrelators that employ the difference in processing gains have been proposed: 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. It has been observed that the LRD provides better performance than the HRD for all the users at the expense of higher complexity and longer demodulation delay for high-rate users. To improve the performance and reduce the demodulation delay, a decorrelating decision-feedback detector is proposed and its asymptotic multiuser efficiency (AME) is analyzed. Because of the high computational complexity in calculating the exact AME, both upper and lower bounds of the AME are developed. In some special cases, the complexity of these bounds is linear in the number of users. 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 powers of the interfering users are comparable to that of the desired user.