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 ****