Multiuser detection of quasisynchronous CDMA signals using linear decorrelators

A linear decorrelator detector is proposed for a quasisynchronous code-division multiple-access (QS-CDMA) cellular system. It is assumed that each of N users has a Global Positioning System (GPS) generated local clock and attempts to transmit synchronously 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, Doppler shifts, and variable distances from the mobiles to the base. 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. A noncoherent version of the decorrelator detector is presented, along with a maximum-likelihood scheme for synchronizing to the desired signal. Bounds and analytical results for the bit-error rate (BER) are obtained for various interferer conditions.