Differentially coherent combining for slot synchronization in inter-cell asynchronous DS/SS systems

Slot combining is a method to achieve fast code acquisition in inter-cell asynchronous DS/SS systems by employing multiple observations. The optimal slot combining is difficult to implement, since it requires information on channel and frequency offset. Two practical combining schemes, a noncoherent combining and a coherent combining, are found to be special cases of the optimal combining. In this paper, a novel differentially coherent combining scheme is proposed. This combining scheme may be viewed as a combination of differential detection and coherent combining techniques. The performances of the differentially coherent combining as well as the optimal combining, noncoherent combining, and coherent combining are analyzed for Rayleigh fading environments with frequency offset. It is found that the differentially coherent combining scheme significantly outperforms the noncoherent combining and coherent combining schemes for a typical range of Doppler spread and frequency offset