Low-complexity receivers for multiuser detection with an unknown number of active users

In multiuser detection, the set of users active at any time may be unknown to the receiver. A two-step detection procedure, in which multiuser detection is preceded by active-user identification, is suboptimum. The optimum solution consists of detecting simultaneously the set of active users and their data, problem that can be solved exactly by applying random-set theory (RST). However, implementation of optimum detectors can be limited by their complexity, which grows exponentially with the number of potential users. In this paper we illustrate how the complexity of optimum can be reduced. In particular, sphere detection (SD) techniques (possibly in an approximate version) are examined.