Performance Study on a CSMA/CA-Based MAC Protocol for Multi-User MIMO Wireless LANs

A multi-antenna access point (AP) can communicate simultaneously with multiple clients, however, this multi-user MIMO (MU-MIMO) capability is underutilized in conventional 802.11 wireless LANs (WLANs). To address this problem, researchers have recently developed a CSMA/CA-based MAC protocol to support concurrent transmissions from different clients. In this paper, we propose an analytical model to characterize the saturation throughput and mean access delay of this CSMA/CA-based MAC protocol operating in an MU-MIMO WLAN. We also consider and model a distributed opportunistic transmission scheme, where clients are able to contend for the concurrent transmission opportunities only when their concurrent rates exceed a threshold. Comparisons with simulation results show that our analytical model provides a close estimation of the network performance. By means of the developed model, we evaluate the throughput and delay performance with respect to different network parameters, including the backoff window sizes, the number of AP's antennas, the network size, and the threshold of the opportunistic transmission scheme. Performance optimization over key parameters is also conducted for the transmission schemes.