Linear multiuser detectors for quasi-synchronous CDMA systems

A linear decorrelator detector is proposed for a quasi-synchronous code-division multiple-access (QS-CDMA) cellular system. It is assumed that each of N users has a GPS generated local clock and attempts to transmit in synchrony with the other users in its cell. The base station thus receives N signature waveforms which are approximately synchronized, but with a small timing error due to local oscillator drift and Doppler shifts. The residual timing offset is discretized, and a decorrelator is constructed which attempts to reject the multiuser interference vectors corresponding to the set of possible offsets. Bounds and analytical results for the bit error rate (BER) are obtained for various interferer conditions