Performance Optimization in IEEE 802.11 WLANs with Co-Channel Interference

The Hong Kong University of Science and Technology
Department of Computer Science and Engineering

PhD Thesis Defence


Title: "Performance Optimization in IEEE 802.11 WLANs with Co-Channel 
Interference"

By

Mr. Ka Lok HUNG

Abstract

IEEE 802.11 wireless local area networks (WLANs) are increasingly invoked 
to provide broadband internet access to mobile and wireless devices. 
Today, the ubiquity of WLANs is such that WLAN access points (APs) are 
deployed densely, especially in indoor environments; so much so that often 
several APs operating in close geographical proximity must contend for 
channel bandwidth due to the  limited number of orthogonal channels 
available for WLANs. This so-called co-channel interference, nowadays 
inevitable in WLANs, causes severe throughput degradation, and mitigating 
this problem has become an important step towards improving the network 
performance. In this thesis, motivated by real world experiments and 
simulation results, we study how co-channel interference impacts the 
network throughput, and propose solutions to alleviate it.

To begin 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 the AP's and stations' contention windows.

Then, we extend this study to multi-cell WLANs, demonstrating via 
simulation and real world test-bed experiments, that a severe throughput 
imbalance occurs between downlink TCP flows even in the simplest of 
multi-cell WLANs where only two mutually hidden APs compete for channel 
access. 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 of the TCP sender.

Finally, we generalize our study to multi-cell WLANs with arbitrary 
topologies in the presence of the hidden terminals and spatial unfairness. 
Combining the rate control and contention resolution, we formulate the 
optimal rate allocation problem on top of the CSMA/CA protocol as a 
non-convex optimization problem. 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 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 derive a distributed 
algorithm to obtain the maximum transmission rate and the optimal 
contention window setting. To further improve the network performance, we 
incorporate association control into the cross-layer optimization problem, 
reducing the number of hidden terminals by discarding problematic 
associations and optimally re-associating APs and clients. We formulate 
such a problem as a non-convex mixed integer programming problem, which is 
known to be NP-hard and propose a distributed algorithm to approximate the 
optimal solution of this problem.


Date:			Wednesday, 19 January 2011

Time:			11:00am – 1:00pm

Venue:			Room 3501
 			Lifts 25/26

Chairman:		Prof. Richard So (IELM)

Committee Members:	Prof. Brahim Bensaou (Supervisor)
 			Prof. Lin Gu
 			Prof. Jogesh Muppala
                      	Prof. Danny Tsang (ECE)
                         Prof. Chun Tung Chou (Comp. Sci. & Engg., Univ. of NSW)


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