Adaptive chip-equalizers for synchronous DS-CDMA systems with pilot sequences

It is well known that in a multiple-user synchronous DS-CDMA system with multipath fading, the widely used RAKE receiver is suboptimal. Recently there has been interest in the use of chip-equalizers before despreading in order to restore the orthogonality of the transmitted sequences at the receiver. However, the issue of adapting these equalizers fast enough to track a wideband fading channel is still an open problem. In this paper we first show that if the optimal MMSE equalizer taps for a pilot sequence are used for equalizing other users, the MSE performance is very close to that obtained by using the optimal taps for that user, i.e. the equalizer taps that are optimal for the pilot sequence are, when multiplied by a gain factor, close to optimal for any other user in the system. We then go on to show that a DS-CDMA system that uses multiple pilot sequences can very effectively track fast-fading channel variations using least-squares adaptation on the known pilot sequences.