|dc.description.abstract||This dissertation studies fundamental limits of modern digital communication
systems in presence/absence of delay and secrecy constraints.
In the first part of this dissertation, we consider a typical time-division wireless
communication system wherein the channel strengths of the wireless users vary with
time with a power constraint at the base station and which is not subject to any
delay constraint. The objective is to allocate resources to the wireless users in an
equitable manner so as to achieve a specific throughput. This problem has been
looked at in different ways by previous researchers. We address this problem by
developing a systematic way of designing scheduling schemes that can achieve any
point on the boundary of the rate region. This allows us to map a desired throughput
to a specific scheduling scheme which can then be used to service the wireless users.
We then propose a simple scheme by which users can cooperate and then show that a
cooperative scheduling scheme enlarges the achievable rate region. A simple iterative
algorithm is proposed to find the resource allocation parameters and the scheduling
scheme for the cooperative system.
In the second part of the dissertation, a downlink time-division wireless sys-
tem that is subject to a delay constraint is studied, and the rate region and optimal
scheduling schemes are derived. The result of this study concludes that the achievable throughput of users decrease as the delay constraint is increased. Next, we consider
a problem motivated by cognitive radio applications which has been proposed as a
means to implement efficient reuse of the licensed spectrum. Previous research on this
topic has focussed largely on obtaining fundamental limits on achievable throughput
from a physical layer perspective. In this dissertation, we study the impact of im-
posing Quality of Service constraints (QoS) on the achievable throughput of users.
The result of this study gives insights on how the cognitive radio system needs to be
operated in the low and high QoS constraint regime.
Finally, the third part of this dissertation is motivated by the need for commu-
nicating information not only reliably, but also in a secure manner. To this end, we
study a source coding problem, wherein multiple sources needs to be communicated
to a receiver with the stipulation that there is no direct channel from the transmitter
to the receiver. However, there are many \agents" that can help carry the information
from the transmitter to the receiver. Depending on the reliability that the transmit-
ter has on each of the agents, information is securely encoded by the transmitter and
given to the agents, which will be subsequently given to the receiver. We study the
overhead that the transmitter has to incur for transmitting the information to the
receiver with the desired level of secrecy. The rate region for this problem is found
and simple achievable schemes are proposed. The main result is that, separate secure
coding of sources is optimal for achieving the sum-rate point for the general case of
the problem and the rate region for simple case of this problem.||