MAC Efficiency Improvement of IEEE 802.11 Wireless Local Area Networks

Abstract

In this dissertation, we develop medium access control (MAC) efficiency improvement schemes for IEEE 802.11 wireless local area networks.

In part I of this dissertation, we develop a contention window (CW) control scheme for practical IEEE 802.11 wireless local area networks (WLANs) that have node heterogeneity in terms of the traffic load, transmission rate, and packet size. We introduce activity probability, i.e., the probability that a node contends for medium access opportunities at a given time. We then newly develop a performance analysis model that enables analytic estimation on the contention status including the collision probability, collision time, back-off time, and throughput with comprehensive consideration of node heterogeneity. Based on the newly developed model, we derive the theoretically ideal contention status, and develop a CW control scheme that achieves the ideal contention status in an average sense. We perform extensive NS-3 simulations and real testbed experiments for evaluation of both the proposed performance analysis model and CW control scheme. The results show that the proposed model provides accurate prediction on the contention status, and the proposed CW control scheme achieves considerable throughput improvement compared to the existing schemes which do not comprehensively consider node heterogeneity.

In part II of this dissertation, we propose a sounding control scheme for IEEE 802.11ac multi-user multiple-input multiple-output (MU-MIMO). The proposed scheme comprehensively considers the long-term characteristics of a network environment including the downlink traffic loads and channel coherence times of wireless links, and jointly determines the sounding node set and sounding interval to maximize the long-term expected MU-MIMO throughput gain in consideration of sounding overhead. To this end, we analytically formulate an MU-MIMO throughput gain maximization problem considering the network environment and sounding overhead. We conduct MIMO channel measurement in practical WLAN environments, and evaluate the performance of the proposed scheme by employing the real channel data traces. Simulation results verify that the proposed scheme adaptively determines the sounding node set and sounding interval according to the network environment, and outperforms the existing scheme which considers the channel coherence times only.

In part III of this dissertation, we develop an adaptive group ID (GID) control scheme to mitigate idle power consumption at nodes in IEEE 802.11ac wireless local area networks (WLANs) supporting multi-user multiple input multiple output (MU-MIMO). We analytically derive the expected idle power consumption at nodes sharing common GIDs, revealing that it has relations with their downlink (DL) traffic loads. Based on the analysis, we formulate an idle power consumption minimization problem, and develop an efficient algorithm to reduce the computational complexity. Simulation results reveal that idle power consumption becomes extremely severe when an access point (AP) has a large number of associated nodes. The proposed scheme assigns GIDs in consideration of DL traffic loads, thus considerably mitigating idle power consumption compared to random GID overloading.