Eavesdropping on the IS-95 downlink: reduced complexity optimum and suboptimum multiuser detectors

We consider multiuser detection techniques for an eavesdropping receiver in an IS-95 cellular communication system observing interfering downlink transmissions from a cluster of B base stations. The receiver desires to accurately estimate the symbols transmitted from these base stations to their active local users. For a system with K total asynchronous users and an observation of 2L+1 bits, the optimum detector is reviewed and shown to have complexity on the order of 2^K(2L+1) . Taking advantage of the structure of the IS-95 downlink, we develop a reduced complexity optimum detector with exponentially lower complexity. In typical scenarios with two or three base stations, the reduced complexity optimum detector is significantly less complex than the brute-force optimum detector. We also consider a suboptimum, low complexity IS-95 downlink eavesdropping detector with connections to reduced complexity optimum detection as well as group detection and parallel interference cancellation. We demonstrate via simulation that the performance of this receiver can be near-optimum while offering very low computational complexity