An adaptive contention window algorithm to hybrid OFDMA/CSMA mechanism in WLANs

The Distributed Coordination Function (DCF) in the IEEE 802.11 protocol is a random access scheme based on the carrier sense multiple access with collision avoidance (CSMA/CA) for wireless local area networks (WLANs). Recently, a novel MAC design based on orthogonal frequency division multiple access (OFDMA) and CSMA/CA has been proposed. And this design, by dividing the entire channel into a number of sub-channels and thereby enabling nodes to simultaneously transmit packets in different sub-channels, to some extent, helps improve the performance of MAC layer in WLANs. However, with simply combining the OFDMA technique with CSMA/CA scheme, it withholds the defects of low utilization in terms of bandwidth and time resources, involved in the original CSMA/CA mechanism. In this paper, we propose an adaptive contention window algorithm based on the conflict weights in order to mitigate the wastage of unnecessary bandwidth and time of the conventional binary exponential backoff (BEB) rules and further enhance the performance of the hybrid OFDMA/CSMA mechanism. The crucial idea of this adaptive algorithm lies in that each node would choose different values of Contention Window (CW) value according to its conflict weights, which are modified on the basis of channel situation whenever necessary. Simulation results validate that the proposed algorithm, dynamically as well as properly, sets CW value in proportion to the current channel situation, consequently enhancing the network performance.