MIMO-Based Enhancement to the IEEE 802.11 Distributed Coordination Function

In this paper, we propose an enhancement to the distributed coordination function (DCF) of the IEEE 802.11 MAC protocol, called the MIMO-based DCF (MBD). The MBD scheme exploits the multiple spatial channels (MSC) created by multiple inputs multiple outputs (MIMO) systems to enhance the IEEE 802.11 DCF collision-avoidance mechanism. The key idea is the sharing of the spatial channels during the contention period, i.e., instead of accessing the channel using the total spatial channels, the contending nodes only use a set of the available spatial channels. As the total concurrent used spatial channels are fewer than or equal to the degree of freedom (DOF), receivers can receive multiple contention attempts and hence they can coordinate their responses to avoid collisions. The spatial channel sharing produces other properties such as medium contention termination and medium contention selection. The former refers to the accessing node´s capability to terminate its contention upon detecting other ongoing medium access attempts to avoid potential collisions. Contention selection happens when receivers detect multiple medium access requests initiated by different transmitters. Consequently, receivers either respond to some requests or keep silent to avoid further collisions. Simulation results obtained using OPNET modeler show that the MBD scheme substantially reduces the medium access collisions. Actuating the medium contention termination and medium contention selection mode further reduces the medium contention collisions. The results of the MBD based on an adaptive MSC sharing scheme demonstrate that with different network loads using the adaptive MSC sharing scheme further reduces medium access collisions.