Optimal decision strategies for acquisition of spread-spectrum signals in frequency-selective fading channels

The focus of this paper is on the initial acquisition of a direct sequence (DS) spread-spectrum signal utilizing a purely parallel search strategy. A parallel search strategy is utilized because it reduces the acquisition time compared to either serial or partially parallel strategies. In particular, the purpose of this paper is to derive the optimal decision rule, based on the maximum-likelihood criterion, for frequency-selective fading channels. The performance of the conventional decision rule, optimized for additive white Gaussian noise and flat-fading channels, the optimal decision rule, derived in this paper, and a suboptimal decision rule, also presented in this paper, are compared, It is shown that the optimal and suboptimal decision rules for Rayleigh-fading channels outperform the conventional decision rule. For Rician-fading channels, it is shown both that the optimal decision rule outperforms the conventional decision rule, and that the optimum decision rule for Rayleigh-fading channels, when implemented on a Rician-fading channel, yields approximately optimum performance.