Collision-type multiple-user communications

A collision-type multiple access system is investigated in which every user transmits symbols from a common N-ary frequency-shift-keyed signal alphabet. We present a series of information-theoretic properties of the associated mathematical channel model. In the absence of noise, we calculate a large system approximation to the sum capacity, which is used to show that, in the limit, the combining of the multiple-accessing and coding results in no loss in capacity, compared to a fixed-allocation scheme. The presence of thermal noise or undetected users influences the capacity. Bounds on the capacity of the channel in the presence of thermal noise are calculated, as well as the capacity of the system in the presence of interfering users. Finally, a new iterative multiuser detector, the consensus decoder, is described and simulation performance results are shown. It is demonstrated that this decoder can operate to within approximately 70% of the channel capacity.