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This paper introduces two important enhancements to the IEEE 802.11 medium access control (MAC) protocols which are not considered in the current IEEE 802.11 MAC protocol: the channel state between the transmitter and the receiver and the node queue state. Our objective is to characterize the impact of these parameters on medium access regulation, and ultimately rely on them in order to enhance the sum throughput compared to the legacy 802.11x MAC protocols. We consider the scenario of several nodes attempting to access an access point (AP). Each node is characterized by: (i) a link quality to the AP which is essentially mapped to the PHY-layer rate that can be supported; (ii) a queue length depending on the packet arrival process at that node. We make the contention window dependent on queue size (backlog) and PHY-rate. The key idea is that a node with large PHY transmission rate and large queue size should tend to use smaller contention window, so that it gets higher chances for accessing the channel. We suggest heuristic queue and link state-aware rules for defining the contention window. We demonstrate with numerical arguments a significant enlargement in the capacity region under our modified access control policies and substantial performance improvement in terms of sum throughput for this system, compared to legacy IEEE 802.11 protocols.
Date of Conference: 5-8 April 2009