| Abstract |
Cooperative communications are widely acknowledged to provide higher
communication rates and reliability in a wireless network with time varying channels,
due to their ability to overcome fading and signal attenuation. While most
cooperative research in this area concern gains achieved by cooperation on the
physical layer, recent works suggest that similar gains can be achieved by Network –
Level cooperation. By Network – Level cooperation we refer to plain relaying without
any physical layer considerations.
In this thesis, we first provide the basic background behind the relay channel
which is the basic paradigm of cooperative communications. We present the
fundamental cooperative architectures, relaying and multiple access protocols and
we discuss the advantages and disadvantages of cooperative systems. Furthermore,
we discuss several system tradeoffs that arise and we present some basic application
scenarios along with the standardization of new technologies.
In the second part of the thesis, we study the impact of the insertion of a
second relay node in a network where the relay nodes relay packets from a number
of users to a destination node. We consider the case of a discrete-time slotted
system in which the users have saturated queues and random access to the medium.
Simultaneous transmission attempts by two or more nodes of the network result in a
collision. The two relays do not have packets of their own, but assist the users by
relaying their packets when necessary. We obtain analytical expressions for the
arrival and service rates of the queues of the two relays and also the stability
conditions. In addition, we present a topology of the system in which the users are
divided into two clusters and study the impact of the two relay nodes of the two
models (with and without clustering) on the aggregate throughput and the
throughput per user. We show that the probabilities of the two relays to attempt
transmission do not depend on each other when the queues are stable. Another
important observation is that the insertion of a second relay in a system generally
does not offer higher throughput per user in comparison to a system with one relay,
but the system in which the users are divided into two clusters separated by some
distance offers significant advantage. Moreover, there is an optimum number of
users that maximizes the aggregate throughput of the clustered system. These
results could be used to allocate the users among the relays for example in cellular
and sensor networks.
In the third part of the thesis, we study a similar model as in the second part, with
two relays that relay packets from a number of users to a destination, with the main
difference being that the relays and the destination are equipped with multiuser
detectors, so that they can decode packets successfully from more than one
transmitter at a time (multi-packet reception - MPR capability). We present two
variations of a system with two relays, the first in which if both relays receive the
same packet from a user, they both store it and forward it to the destination, and
the second in which the relay with the smaller queue size stores the packet in its
queue and is responsible for forwarding it to the destination. We also, present a
topology of the system in which the users are divided into two distant clusters and
study the impact on the aggregate and per user throughput compared to the cases
of no relay, one relay and two variations (as above) of two relays in the system.
Moreover, we find that there is an optimum number of users that maximizes the
aggregate throughput of the systems with two relays. We show by extensive
simulations that under certain circumstances, the use of two relays offers significant
advantage as per aggregate and per user throughput compared to systems with one
and no relay. Furthermore, we present a way to verify if the queues of the relays are
stable and compare the average queue size and the average delay per packet of all
the systems presented. We also show that although the average queue size of the
clustered system is higher, the average delay per packet is much lower and in
combination with the higher aggregate and per user throughput that it can provide,
the clustered system is the most appropriate solution. Finally, we present the
conditions under which an interference cancellation technique, in the systems with
two relays, can provide higher aggregate throughput and what are the effects of the
distance between the two clusters in terms of aggregate throughput.
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Performance Issues of Network - Level Cooperation with Multiple - Relays
