Joint detection based on exploiting differences among the channels employed by several users allows a receiver to distinguish cochannel signals without reliance on spectrum spreading. This paper makes a number of new contributions to the topic: it provides an analytical expression for the union bound on average symbol error rate for an arbitrary number of users and diversity antennas in a fading environment, for both perfect and imperfect CSI, and it compares the performance of joint detection with diversity antennas against classical MMSE combining. The performance is remarkable. With accurate CSI, several users can experience good performance with only a single antenna; moreover, for perfect CSI, only a 2 dB penalty is incurred for each additional user. With several antennas, many more users than the number of antennas may be supported with a slow degradation in performance for each additional user. Furthermore, high accuracy is not required from the channel estimation process. In all cases, theperformance of joint detection exceeds that of MMSE combining by orders of magnitude.
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