Studying the Effect of Delay Diversity on a DS-CDMA Downlink

In this paper, we study the error rate performance of a direct sequence CDMA downlink similar to the CDMA2000 standard. While analytical computation of the error rate is possible in DS-CDMA systems under certain simplifying assumptions, computer simulations are perhaps the only means to quantify the error rate for realistic channel models. Since the delay spread of the multipath channel varies with distance, we investigate the error performance as a function of the distance from the base station. The computer simulations reveal certain interesting results which to our knowledge have not explicitly appeared in open literature.We compare the RAKE error rate performance under situations of no handoff, soft handoff, independent fading, correlated fading, chip-spaced sampling, sub-chip sampling, and pilot channel cancellation at the mobile receiver. These results indicate that narrow-band DS-CDMA downlink has certain severe shortcomings, and that it may be vital to ensure line of sight propagation for the “near” users, and exploit base-station (macro) diversity for the “far” users (in soft handoff) to ensure adequate error rate performance.