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, 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.

The use of antenna diversity is investigated for improving the parallel acquisition performance of a direct-sequence spread-spectrum signal in both flat and frequency-selective fading channels. Noncoherent combining techniques are analyzed because coherent diversity techniques, such as maximal ratio combining, cannot be employed prior to initial acquisition. A major application of this work is to cellular communications.

This paper presents the performance of a direct sequence spread spectrum acquisition scheme in a mobile terrestrial communications system. The effects of fading, multipath, power control, shadowing, multiple access interference, out-of-cell interference, vehicle speed, voice activity, and sectorization are examined. The acquisition scheme uses noncoherent detection and a parallel search strategy. The analysis is done for the reverse link of a mobile code-division multiple-access (CDMA) system. The purpose of this paper is to derive the acquisition performance of a mobile communications system under practical assumptions, and give realistic capacity estimates based on acquisition performance criteria.