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


PhD Thesis Defence


"Mixed-Mode Wireless Networks: Framework and Power Control Issues"

By

Mr. Jiancong Chen


Abstract

Wireless networks have been widely deployed in recent years to provide
high-speed Internet access to mobile users. In traditional IEEE 802.11
wireless LAN using infrastructure mode, all users share the same channel
and all packets are forwarded by an access point (AP). As a result, when
the number of users increases, the throughput of each user degrades
correspondingly.

If users within a cell communicate with each other (as in some
conferencing or file exchange scenarios), the throughput of the users
could be improved by their using ad hoc connections instead of the AP. In
such a network, a node can dynamically switch between the two modes with
the help of the AP, transparent to the users. There are two important
issues to be addressed in the system: i) the framework on how the system
is designed, and ii) as regards to the ad hoc networks, unfairness due to
channel capture.

With respect to the first issue, we propose and present a framework for
mixed-mode wireless LAN (MM-WLAN) to seamlessly integrate infrastructure
and ad hoc modes. We also investigate the use of ad hoc connection to
divert traffic from one AP to another, hence alleviating the hot-spot
problem in public wireless networks.   Using simulation, we show that
MM-WLAN can substantially improve system throughput or capacity without
user's manual configuration.

With respect to the second issue, we note that due to the existence of
hidden terminal and absence of central administration, IEEE 802.11 ad hoc
networks may lead to capture problem. Capture problem refers to that flows
of a few hops monopolize, or so-called "capture" the channel while others
suffer from starvation.  We propose and investigate a distributed power
control scheme termed "Power Adaptation for Starvation Avoidance" (PASA)
to mitigate the problem. PASA dynamically adjusts the transmission power
so as to break starvation, hence improving short-term fairness among
contending flows. We also investigate the fairness and throughput issues
on delivering multimedia over ad hoc networks. Via extensive simulations,
PASA is shown to be able to achieve substantially better fairness without
compromising TCP and UDP throughput.


Date:			Friday, 28 May 2004

Time:			10:00a.m.-12:00noon

Venue:			Room 1402
			Lifts 25-26

Chairman:		Prof. Grafton Hui (MATH)

Committee Members:	Prof. Gary Chan (Supervisor)
			Prof. Mounir Hamdi
			Prof. Jogesh Muppala
			Prof. Danny Tsang (ELEC)
			Prof. Soung-Chang Liew (Inf. Engg., CUHK)


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