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Performance Optimization in IEEE 802.11 WLANs with Hidden Terminals
PhD Thesis Proposal Defence Title: "Performance Optimization in IEEE 802.11 WLANs with Hidden Terminals" by Mr. Ka-Lok Hung Abstract: Due to the growing prevalence of mobile wireless devices, IEEE 802.11 WLANs have been densely deployed in the indoor environment, providing ubiquitous internet access. Numerous APs working in close proximity contend channel bandwidth through limited number of orthogonal channels, making the hidden terminal problem inevitable, causing severe throughput degradation. Mitigating this problem becomes an important step to improve the network performance. In this proposal, motivated by real world experiments and simulation results, we study how hidden terminals impacting the network throughput, and propose solutions to alleviate it. To start with, we model the MAC layer activities of a single cell WLAN under the influence of hidden terminals, and identify the causes of unfair bandwidth allocation. Unlike existing models, our model can accommodate different numbers of hidden nodes without increasing the model complexity. Given any number of hidden nodes, only four constraints are needed to describe the interaction between stations and the AP with the consideration of both uplink and downlink traffic. Based on our model, we formulate a bandwidth allocation problem to optimize the network throughput and fairness under some predefined requirements by systematically tuning AP's and stations' contention window. Then, we extend our study to multi-cell WLANs, demonstrating that a severe throughput imbalance occurs between downlink TCP flows even in the simplest of multi-cell WLANs which contains only two mutual hidden APs via simulation and real world test-bed experiments. To solve this unfairness problem, we derive an analytical model that describes the interaction between TCP flows at the MAC layer, and formulate the throughput allocation problem as a nonlinear optimization problem subject to certain fairness requirements. Our formulation considers real world complexity such as hidden terminals, packet transmission retry limit,and the unique characteristics of TCP traffic. Solving our optimization problem yields the optimal MAC layer contention window settings that can lead each TCP flow to its target end-to-end throughput without any per-flow queuing or modification on the TCP sender. Finally, we generalize our study to multi-cell WLANs of arbitrary topologies in presence with hidden terminals. Combining the rate control and contention resolution, we formulate a non-convex optimization on top of the CSMA/CA protocol. Unlike previous approaches that require maximal weight scheduling or ignore the hidden terminal problem, our formulation considers a realistic IEEE 802.11-based MAC layer model including the random backoff, carrier sensing, frame retransmission and contention window (CW) setting. We propose a simple scheme to transform this non-convex problem into a convex one, and derived a distributed algorithm to obtain the maximum transmission rate and the optimal contention window setting. Date: Friday, 8 October 2010 Time: 11:00am - 1:00pm Venue: Room 4483 lifts 25/26 Committee Members: Dr. Brahim Bensaou (Supervisor) Dr. Jogesh Muppala (Chairperson) Dr. Lin Gu Prof. Danny Tsang (ECE) **** ALL are Welcome ****