Acquisition of direct sequence spread spectrum signals with correlated fading

Antenna diversity is an effective way of improving the performance of a communication system, especially for initial acquisition in a narrowband code division multiple access (CDMA) system that operates in a flat fading environment. Earlier attempts to analyze such systems assume independent fading at various antenna elements. However, the diversity gain predicted by these analyses is often unrealizable, since the assumption about the independence of the fades is unrealistic in a spatially constrained system. We relax the assumption about the spatial independence of the fades by employing a realistic model for spatial correlation. Also, we obtain an optimum receiver for initial acquisition that takes into account the particulars of the spatial correlation and present a technique for the analysis of this receiver. We also study the diversity gain available for various parameters of the spatial correlation model and conclude that there exists an optimum number of the antenna elements for an initial acquisition system, based on the spatial correlation of the fades. Further, we consider a wideband multicarrier CDMA system with correlated fades among its subcarriers, which also employs antenna diversity, and apply the techniques used in the narrowband case to obtain an optimum receiver and analyze its performance. Finally, we study the tradeoff between the diversity gain available and the noncoherent loss incurred as the numbers of subcarriers and antenna elements are changed.