Maximizing multiuser diversity gains in IEEE 802.11 WLANs

Opportunistic scheduling monitors the receivers’ channel states and schedules packets to the receivers in relatively good channel conditions. Opportunistic scheduling can be easily implemented in cellular network systems that have embedded channel state report functions. To apply opportunistic scheduling to wireless LANs, deficient of channel report functions, we first devise an efficient channel probing mechanism. Prior opportunistic scheduling methods for WLANs limit the number of probed receivers and may not fully utilize the potential multiuser diversity gains. In this paper, we develop a new opportunistic scheduling scheme called WDOS (WLAN Distributed Opportunistic Scheduling). WDOS invites all receivers to channel probing and has a potential to fully realize the multiuser diversity gains. To reduce the overhead of probing all active receivers, we devise a contention based RTS/CTS exchange with early termination. A sender initiates channel probing by multicasting an MRTS (modified RTS) frame and each receiver responds with a CTS frame after a backoff delay. Hearing the first CTS frame, the sender terminates channel probing and transmits data frames to the first responder. WDOS attains the multiuser diversity gains by allocating a shorter backoff delay to a receiver in better relative channel conditions. We evaluate the performance of WDOS both via an analytic method and computer simulations. Our performance study shows that WDOS achieves the performance near optimal.