Exploiting Multiuser Diversity in Wireless ALOHA Networks

In wireless networks, diversity techniques are widely used to compensate for the unstable environment. Multiuser diversity has recently received attention in the literature. The idea of multiuser diversity has its roots in [1], where a centralized power control scheme for maximizing the information theoretic capacity of the uplink of a single cell is studied. The optimal strategy is shown to be to transmit to the user with the best channel at any given time. In this work we also consider a group of users accessing the uplink of a single cell. However we focus on the case where no centralized scheduler is available; specifically we assume each user has knowledge of its own fading level, but no knowledge of the fading levels of the other users in the cell. This precludes an approach as in [1], for each user will not be able to tell at which times its channel gain is the best. We consider a variation of a slotted ALOHA random access technique in which a user’s transmission probability in a slot is a function of the user’s channel gain. For a simple model of such a system, we show that users can still take advantage of multi-user diversity to improve the overall system capacity. Consider a model of a slotted ALOHA network with n users. Assume a back-logged system where all n users always have packets to send and transmit independently in each slot with probability p. For such a system, the maximum total throughput, s∗, is given by (