Maximum likelihood acquisition of time and frequency shift in land mobile DS-CDMA systems

Today code division multiple access based on direct sequence spread spectrum (DS-CDMA) is one of the most promising multiple access techniques for land mobile communication systems. Systems like IS-95, Globalstar or UMTS will use DS-CDMA. To allow fast network access and high system capacity, these systems use pilot signals. In this paper maximum likelihood acquisition of the time and frequency uncertainty of the received signal is investigated. The purpose is to obtain methods for fast initial synchronization in the presence of multipath propagation, Doppler shift, and noise. The maximum likelihood acquisition detector is derived and approximations, which are better suited for implementations, are given and discussed. Finally, the mean acquisition time of the maximum likelihood acquisition detector is determined and compared with the threshold detector.