Technique to design spreading sequences for the uplink of DS-CDMA systems in frequency selective fading channels

We propose a novel technique to design a set of spreading sequences for the uplink of a DS-CDMA system in the presence of frequency selective fading. The focus is on the case when channel state information (CSI) is available at the transmitter and when the chips in the spreading sequences are confined to be from a finite alphabet We consider two cases: (i) knowledge of the CSI of all the users is available centrally during the design process, such as in a base station; (ii) the design is at the mobile unit when each user only knows the CSI of his/her frequency selective channel. In both cases, we propose novel techniques to design spreading sequences whose spectra are well matched to that of the frequency selective channel. In the former case, a set of short spreading sequences is designed that is both well matched to the frequency selective channels and possesses low cross correlation. In the latter case, the use of long spreading sequences and error control coding is proposed. Results show that the proposed techniques provide significantly improved performance over the use of Gold codes and that they outperform single user performance over an additive white Gaussian noise channel. The designed short spreading sequences are also well suited to be used with low complexity multiuser detectors.