Date: Tuesday, April 29th, 2003
Special University Ph.D. Oral Examination
Time: 4:15 pm (refreshments 4:00)
Location: Packard #101
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Capacity of Wireless Networks
and Design Principles
of Electrical Engineering, Stanford University
We define and study capacity regions for wireless ad hoc
networks with an arbitrary number of nodes and topology.
These regions describe the set of achievable rate combinations
between all source-destination pairs in the network under
various transmission strategies, such as variable-rate transmission,
single hop or multihop routing, power control and successive
interference cancellation. Multihop cellular networks and
networks with energy constraints are studied as special
cases. With slight modifications, the developed formulation
can handle node mobility and time-varying flat fading channels.
Using this framework, we then concentrate
on the joint-layer design of wireless ad hoc networks. We
show that using single routes for each data stream might
significantly reduce the capacity of the network. We then
study CSMA/CA, and find that its performance strongly depends
on the choice of the accompanying routing protocol. We introduce
two protocols that outperform CSMA/CA, both in terms of
energy efficiency and achievable throughput. The *Progressive
Back Off Algorithm (PBOA)* performs medium access jointly
with power control. The *Progressive Ramp Up Algorithm*
sacrifices energy efficiency in favor of higher throughput.
Both protocols slot time, and are integrated with queuing
disciplines that are more relaxed than the First In First
Out (FIFO) rule. They are totally distributed and the overhead
they require does not increase with the size and node density
of the network.
We conclude with a brief discussion on
the capacity of ad hoc networks with an asymptotically large
number of nodes. We establish a lower bound on the aggregate
throughput as a function of the number of nodes, under a
general model for fading, and assuming the nodes to be immobile.
We then assume node mobility, showing a fundamental trade-off
between the packet delay and the aggregate throughput.