Abstract

Spatial diversity can be employed by sending copies of the transmitted signal using
multiple antennas at the transmitter/receiver, as implemented in multiple-input multipleoutput
(MIMO) systems. Spatial receive diversity has already been used in many applications
with centralized systems where base station receivers are equipped with multiple
antennas. However, due to the power constraints and the small size of the mobile terminal,
it may not be feasible to deploy multiple transmit antennas. User cooperation
diversity, a new form of space diversity, has been developed to address these limitations.
Recently, user cooperative diversity has gained more attention as a less complex alternative
to centralized MIMO wireless systems. It revealed the ability to improve wireless
communications through reliable reception.
One common network of the user cooperation diversity is the direct sequence code
division multiple access (DS-CDMA) in which the Rayleigh fading channels are adopted
and the orthogonality between users is assumed. The Rayleigh fading channels are unrealistic
since they cannot represent the statistical characteristics of the complex indoor
environments. On the other hand, Nakagami-m fading model is well known as a generalized
distribution, where many fading environments can be modeled. It can be used to
model fading conditions ranging from severe, light to no fading, by changing its fading parameter m.
The bit-error-rate (BER) and outage probability of uplink cooperative DS-CDMA over
Nakagami-m has not been addressed in the literature. Thus, in this thesis, the performance
of both decode-and-forward (DF) and amplify-and-forward (AF) cooperative
asynchronous DS-CDMA system over Nakagami-m fading channels is investigated. The
Rake receiver is used to exploit the advantages of multipath propagation. Besides, multiuser
detection (MUD) is used to mitigate the effect of multiple-access interference (MAI).
We show that our proposed multi-user system achieves the full system diversity gain.
The first part of the thesis introduces a new closed-form expression for the outage
probability and the error probability of the DF cooperative DS-CDMA over asynchronous
transmission over independent non-identical Nakagami-m fading channels. The underlying
system employs MUD such as minimum mean square error (MMSE) and decorrelator
detector (DD) to achieve the full diversity. The aforementioned closed-form expression
is obtained through the moment generating function (MGF) for the total signal-to-noise
ratio (SNR) at the base station where the cumulative density function (CDF) is obtained.
Furthermore, we investigate the asymptotic behavior of the system at high SNR to calculate
the achievable diversity gain. The results demonstrate that the system diversity gain
is fulfilled when MUD is used to mitigate the effect of MAI.
In the second part of the thesis, we study the performance of cooperative CDMA
system using AF relaying over independent non-identical distribution (i.n.i) Nakagami-m
fading channels. Using the MGF of the total SNR at the base station, we derive the outage
probability of the system. This enables us to derive the asymptotic outage probability for
any arbitrary value of the fading parameter m.
The last part of the thesis investigates the optimum power allocation and optimum
relay location in AF cooperative CDMA systems over i.n.i Nakagami-m fading channels.
Moreover, we introduce the joint optimization of both power allocation and relay location
under the transmit power constraint to minimize the outage probability of the system.
The joint optimization of both power allocation and relay location is used to minimize
the outage performance of the system, thereby achieving full diversity gain.