Opportunistic CSMA/CA for achieving multi-user diversity in wireless LAN

The throughput of an IEEE 802.11 network decreases as the number of users increases because of the increased collision probability. To improve the throughput in the presence of a large population, this paper presents an opportunistic carrier-sense multiple-acces/collision-avoidance (CSMA/CA) scheme that exploits multi-user diversity while minimally modifying the IEEE 802.11 medium-access control. This method prioritizes those users having high signal-to-noise-ratio by granting earlier access to these users, thus improving the throughput. This prioritization, however, can cause repeated collisions of same users, which is similar to so-called head-of-line (HOL) blocking. To reduce the probability of the occurrence of the HOL blocking, a simple randomization is proposed. A probabilistic analysis shows that this scheme significantly reduces the HOL blocking probability. The simulation results show that the proposed method improves the throughput by as much as 100%, while achieving long-term fairness among users with different channel conditions.