CSMA-based random access networks with multi-user MIMO

Abstract

The IEEE 802.11 wireless local area network (WLAN) standard has recently included multi-user multiple-input multiple-output (MU-MIMO) schemes both in uplink and downlink. Through the downlink MU-MIMO, an access point (AP) can simultaneously transmit multiple streams for different stations (STAs), while the uplink MU-MIMO enables multi-packet reception (MPR) with which the AP can successfully receive simultaneous transmissions from different STAs. In this dissertation, we focus on the medium access control (MAC) layer issues in carrier sense multiple access (CSMA) networks, e.g., WLANs with MU-MIMO considering physical layer behaviors. When each STA has a single antenna, we compare the system performance of downlink MU-MIMO with another downlink user multiplexing in time domain: a frame aggregation. The numerical results show a tradeoff of these schemes according to signal-to-noise ratio (SNR) region, and the differences of SNRs and packet length of multiplexed STAs. For uplink MU-MIMO, we discuss an unfairness problem in terms of channel access opportunity due to different frame error rates among STAs. We propose a fair channel access scheme and the performance evaluation results show that it significantly improves the fairness in terms of throughput as well as channel access opportunity. Besides, when each STA has multiple antennas, we propose a new backoff algorithm which adaptively selects the number of transmit antennas according to the queue state of backlogged STAs in uplink MPR-capable CSMA networks under an unsaturated traffic condition, in order to efficiently utilize the multiple transmit antennas. Moreover, to resolve a performance bottleneck of the proposed backoff algorithm in physical layer due to a per-STA power constraint, we propose a transmission scheme which opportunistically determines to transmit based on the channel condition. The performance evaluation results show that it efficiently resolve the performance bottleneck in physical layer while maintaining the benefit of using multiple transmit antennas in MAC layer.