2 research outputs found
Cooperative diversity techniques for future wireless communications systems.
Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2013.Multiple-input multiple-output (MIMO) systems have been extensively studied in the past
decade. The attractiveness of MIMO systems is due to the fact that they drastically reduce
the deleterious e ects of multipath fading leading to high system capacity and low error rates.
In situations where wireless devices are restrained by their size and hardware complexity, such
as mobile phones, transmit diversity is not achievable. A new paradigm called cooperative
communication is a viable solution. In a cooperative scenario, a single-antenna device is
assisted by another single-antenna device to relay its message to the destination or base
station. This creates a virtual multiple-input multiple-output (MIMO) system.
There exist two cooperative strategies: amplify-and-forward (AF) and decode-and-forward
(DF). In the former, the relay ampli es the noisy signal received from the source before forwarding
it to the destination. No form of demodulation is required. In the latter, the relay
rst decodes the source signal before transmitting an estimate to the destination. In this
work, focus is on the DF method. A drawback of an uncoded DF cooperative strategy is
error propagation at the relay. To avoid error propagation in DF, various relay selection
schemes can be used. Coded cooperation can also be used to avoid error propagation at
the relay. Various error correcting codes such as convolutional codes or turbo codes can
be used in a cooperative scenario. The rst part of this work studies a variation of the
turbo codes in cooperative diversity, that further reduces error propagation at the relay,
hence lowering the end-to-end error rate. The union bounds on the bit-error rate (BER) of
the proposed scheme are derived using the pairwise error probability via the transfer bounds
and limit-before-average techniques. In addition, the outage analysis of the proposed scheme
is presented. Simulation results of the bit error and outage probabilities are presented to
corroborate the analytical work. In the case of outage probability, the computer simulation
results are in good agreement with the the analytical framework presented in this chapter.
Recently, most studies have focused on cross-layer design of cooperative diversity at the
physical layer and truncated automatic-repeat request (ARQ) at the data-link layer using the
system throughput as the performance metric. Various throughput optimization strategies
have been investigated. In this work, a cross-relay selection approach that maximizes the
system throughput is presented. The cooperative network is comprised of a set of relays and
the reliable relay(s) that maximize the throughput at the data-link layer are selected to assist
the source. It can be shown through simulation that this novel scheme outperforms from
a throughput point of view, a system throughput where the all the reliable relays always
participate in forwarding the source packet.
A power optimization of the best relay uncoded DF cooperative diversity is investigated.
This optimization aims at maximizing the system throughput. Because of the non-concavity
and non-convexity of the throughput expression, it is intractable to derive a closed-form
expression of the optimal power through the system throughput. However, this can be done
via the symbol-error rate (SER) optimization, since it is shown that minimizing the SER of
the cooperative system is equivalent to maximizing the system throughput. The SER of the
retransmission scheme at high signal-to-noise ratio (SNR) was obtained and it was noted that
the derived SER is in perfect agreement with the simulated SER at high SNR. Moreover, the
optimal power allocation obtained under a general optimization problem, yields a throughput
performance that is superior to non-optimized power values from moderate to high SNRs.
The last part of the work considers the throughput maximization of the multi-relay adaptive
DF over independent and non-identically distributed (i.n.i.d.) Rayleigh fading channels,
that integrates ARQ at the link layer. The aim of this chapter is to maximize the system
throughput via power optimization and it is shown that this can be done by minimizing the
SER of the retransmission. Firstly, the closed-form expressions for the exact SER of the
multi-relay adaptive DF are derived as well as their corresponding asymptotic bounds. Results
showed that the optimal power distribution yields maximum throughput. Furthermore,
the power allocated at a relay is greatly dependent of its location relative to the source and
destination
Repeat-punctured turbo coded cooperation.
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2008.Transmit diversity usually employs multiple antennas at the transmitter. However, many
wireless devices such as mobile cellphones, Personal Digital Assistants (PDAs), just to name
a few, are limited by size, hardware complexity, power and other constraints to just one
antenna. A new paradigm called cooperative communication which allows single antenna
mobiles in a multi-user scenario to share their antennas has been proposed lately. This
multi-user configuration generates a virtual Multiple-Input Multiple-Output system, leading
to transmit diversity. The basic approach to cooperation is for two single-antenna users to use
each other's antenna as a relay in which each of the users achieves diversity. Previous
cooperative signaling methods encompass diverse forms of repetition of the data transmitted
by the partner to the destination. A new scheme called coded cooperation [15] which
integrates user cooperation with channel coding has also been proposed. This method
maintains the same code rate, bandwidth and transmit power as a similar non-cooperative
system, but performs much better than previous signaling methods [13], [14] under various
inter-user channel qualities.
This dissertation first discusses the coded cooperation framework that has been proposed
lately [19], coded cooperation with Repeat Convolutional Punctured Codes (RCPC) codes
and then investigates the application of turbo codes in coded cooperation.
In this dissertation we propose two new cooperative diversity schemes which are the
Repeat-Punctured Turbo Coded cooperation and coded cooperation using a Modified
Repeat-Punctured Turbo Codes. Prior to that, Repeat-Punctured Turbo codes are introduced.
We characterize the performance of the two new schemes by developing the analytical bounds
for bit error rate, which is confirmed by computer simulations. Finally, the turbo coded
cooperation using the Forced Symbol Method (FSM) is presented and validated through
computer simulations under various inter-user Signal-to-Noise Ratios (SNRs)