Abstract:
Wireless Local Area Networks (WLANs), commonly referred to as Wi-Fi, serve the increasing demands of data-centric Internet applications. The IEEE 802.11ax standard, also known as Wi-Fi 6, introduces the High-Effciency WLAN (HEW) concept to enhance network capacity, spectral effciency, and user experience, particularly in dense user environments, by implementing Orthogonal Frequency Division Multiple Access (OFDMA)-based Random Access (UORA). Despite the spectral utilization improvements offered by OFDMA, dense scenarios such as urban streets, shopping malls, sports stadiums, and conference centers still pose effciency challenges to Wi-Fi 6 due to increased contention at the Medium Access Control (MAC) layer. This thesis addresses the channel access issues for both single-channel and multiple-channel random access stations in up-link transmission mode within dense Wi-Fi 6 networks. A detailed examination of spectrum or channel allocation techniques is presented to explore various solutions ranging from distributed to centralized approaches for uplink single-channel and multi-channel random access in dense Wi-Fi networks. A distributive channel Collision-based Window Scaled Back-off (CWSB) mechanism is proposed to improve channel resource allocations for single-channel random access stations in dense WLANs. It optimizes contention window sizes for each back-off stage based on the recent data frame’s transmission status and is analytically evaluated using an Iterative Discrete-Time Markov Chain (I-DTMC) model. This approach signifcantly improves network performance in terms of improved throughput and delay reduction. In addition to the distributed solution for single-channel random access, a centralized heuristic OFDMA back-off (HOBO) UORA mechanism is proposed for multiple-channel random access stations in Wi-Fi 6. The HOBO scheme adjusts the OFDMA back-off procedure globally using only resource units, controlled by the access point, to minimize collision probabilities based on network density or user congestion level. Simulations demonstrate enhanced channel effciency and performance metrics in Wi-Fi 6 networks. Lastly, a Collision-based Distributed OFDMA Back-Off Control (CODOBO_CTRL) scheme is proposed for multiple-channel random access stations in UORA mode in Wi-Fi 6. This scheme dynamically adjusts the OFDMA back-off counter based on the recent transmission success or failure and network parameters, aiming to mitigate collisions in highly dynamic environments. Simulation-based evaluation confrms its effectiveness in managing OFDMA back-off operations in highly dynamic and congested Wi-Fi 6 environments. v
