19 research outputs found
Super-orthogonal space-time turbo coded OFDM systems.
Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.The ever increasing demand for fast and efficient broadband wireless communication
services requires future broadband communication systems to provide a high data rate,
robust performance and low complexity within the limited available electromagnetic
spectrum. One of the identified, most-promising techniques to support high
performance and high data rate communication for future wireless broadband services
is the deployment of multi-input multi-output (MIMO) antenna systems with
orthogonal frequency division multiplexing (OFDM). The combination of MIMO and
OFDM techniques guarantees a much more reliable and robust transmission over a
hostile wireless channel through coding over the space, time and frequency domains.
In this thesis, two full-rate space-time coded OFDM systems are proposed. The first
one, designed for two transmit antennas, is called extended super-orthogonal space-time
trellis coded OFDM (ESOSTTC-OFDM), and is based on constellation rotation. The
second one, called super-quasi-orthogonal space-time trellis coded OFDM (SQOSTTCOFDM),
combines a quasi-orthogonal space-time block code with a trellis code to
provide a full-rate code for four transmit antennas. The designed space-time coded
MIMO-OFDM systems achieve a high diversity order with high coding gain by
exploiting the diversity advantage of frequency-selective fading channels.
Concatenated codes have been shown to be an effective technique of achieving reliable
communication close to the Shannon limit, provided that there is sufficient available
diversity. In a bid to improve the performance of the super orthogonal space-time
trellis code (SOSTTC) in frequency selective fading channels, five distinct
concatenated codes are proposed for MIMO-OFDM over frequency-selective fading
channels in the second part of this thesis. Four of the coding schemes are based on the
concatenation of convolutional coding, interleaving, and space-time coding, along
multiple-transmitter diversity systems, while the fifth coding scheme is based on the
concatenation of two space-time codes and interleaving. The proposed concatenated
Super-Orthogonal Space-Time Turbo-Coded OFDM System I. B. Oluwafemi 2012 vii
coding schemes in MIMO-OFDM systems achieve high diversity gain by exploiting
available diversity resources of frequency-selective fading channels and achieve a high
coding gain through concatenations by employing the turbo principle. Using computer
software simulations, the performance of the concatenated SOSTTC-OFDM schemes is
compared with those of concatenated space-time trellis codes and those of conventional
SOSTTC-OFDM schemes in frequency-selective fading channels. Simulation results
show that the concatenated SOSTTC-OFDM system outperformed the concatenated
space-time trellis codes and the conventional SOSTTC-OFDM system under the
various channel scenarios in terms of both diversity order and coding gain
Coherent and Non-coherent Techniques for Cooperative Communications
Future wireless network may consist of a cluster of low-complexity battery-powered nodes or mobile stations (MS). Information is propagated from one location in the network to another by cooperation and relaying. Due to the channel fading or node failure, one or more relaying links could become unreliable during multiple-hop relaying. Inspired by conventional multiple-input multiple-output (MIMO) techniques exploiting multiple co-located transmit antennas to introduce temporal and spatial diversity, the error performance and robustness against channel fading of a multiple-hop cooperative network could be significantly improved by creating a virtual antenna array (VAA) with various distributed MIMO techniques. In this thesis, we concentrate on the low-complexity distributed MIMO designed for both coherent and non-coherent diversity signal reception at the destination node.
Further improvement on the network throughput as well as spectral efficiency could be achieved by extending the concept of unidirectional relaying to bidirectional cooperative communication. Physical-layer network coding (PLNC) assisted distributed space-time block coding (STBC) scheme as well as non-coherent PLNC aided distributed differential STBC system are proposed. It is confirmed by the theoretical analysis that both approaches have the potential for offering full spatial diversity gain.
  
Furthermore, differential encoding and non-coherent detection techniques are generally associated with performance degradation due to the doubled noise variance. More importantly, conventional differential schemes suffer from the incapability of recovering the source information in time-varying channels owing to the assumption of static channel model used in the derivation of non-coherent detection algorithm. Several low-complexity solutions are proposed and studied in this thesis, which are able to compensate the performance loss caused by non-coherent detection and guarantee the reliable recovery of information in applications with high mobility. A substantial amount of iteration gain is achieved by combining the differential encoding with error-correction code and sufficient interleaving, which allows iterative possessing at the receiver
Spatial diversity in MIMO communication systems with distributed or co-located antennas
The use of multiple antennas in wireless communication systems has gained much attention during the last decade. It was shown that such multiple-input multiple-output (MIMO) systems offer huge advantages over single-antenna systems. Typically, quite restrictive assumptions are made concerning the spacing of the individual antenna elements. On the one hand, it is typically assumed that the antenna elements at transmitter and receiver are co-located, i.e., they belong to some sort of antenna array. On the other hand, it is often assumed that the antenna spacings are sufficiently large, so as to justify the assumption of independent fading. In this thesis, the above assumptions are relaxed. In the first part, it is shown that MIMO systems with distributed antennas and MIMO systems with co-located antennas can be treated in a single, unifying framework. In the second part this fact is utilized, in order to develop appropriate transmit power allocation strategies for co-located and distributed MIMO systems. Finally, the third part focuses on specific synchronization problems that are of interest for distributed MIMO systems
Distributed convolutional-based coding for cooperative systems
Whenever size, power, or other constraints preclude the use of multiple-input multiple-output (MIMO) systems, wireless systems cannot benefit from the well-known advantages of space-time coding (STC) methods. Also the complexity (multiple radio-frequency (RF) front ends at both the transmitter and the receiver), channel estimation, and spatial correlation in centralized MIMO systems degrade the performance. In situations like these, the alternative would be to resort to cooperative communications via multiple relay nodes. When these nodes work cooperatively, they form a virtual MIMO system. The destination receives multiple versions of the same message from the source and one or more relays, and combines these to create diversity. There are two main cooperative diversity techniques for transmission between a pair of nodes through a multiple relay nodes: decode-and-forward (DF) and amplify-and-forward (AF) modes. In the DF mode, the signal received from the source node is demodulated and decoded before retransmission. In the AF mode, the relay node simply amplifies and retransmits the signal received from the source node. No demodulation or decoding of the received signal is performed in this case. In encoded cooperative communication networks, the diversity of the system degrades significantly. This diversity degradation is attributed to the errors made at the relay nodes. Consequently, if better reliability is achieved at the relay nodes, the diversity may improve. or even may be preserved. as compared to the error-free case. In light of this, the objective of this thesis is to devise coding schemes suitable for relay channels that aim at improving the end-to-end performance of such systems. In this thesis, we present a coding scheme suitable for cooperative networks where the source and relays share their antennas to create a virtual transmit array to transmit towards their destination. We focus on the problem of coding for the relay channels. While the relays may use several forwarding strategies, including AF and DF, we focus on coded DF relaying. We derive upper bounded expressions for the bit error rate (BER) assuming M -ary phase shift keying ( M -PSK) transmission and show that the proposed scheme achieves large coding gains and frill diversity relative to the coded non-cooperative case for a wide range of signal-to-noise ratio (SNR) of interest. To improve the detection reliability further, we consider antenna/relay selection on the performance of cooperative networks in conjunction with the distributed coding scheme proposed. For simplicity, we assume that there is one relay that is equipped with n R antennas and only the best antenna is selected. For this scenario, assuming DF and AF relaying, we derive upper bounds on the BER for M -PSK transmission. Our analytical results show that the proposed scheme achieves full diversity for the entire range of BER of interest, unlike the case without antenna selection. In the last part of the thesis, we consider the same system considered in the ideal case but now with system imperfections. In particular, we consider the case when the channel state information is estimated at all nodes involved in the transmission process. We derive upper bounds on the performance with imperfect channel estimation. Our results show that there is a performance degradation due to the presence of channel estimation error. However, the observations made in the case of ideal channel state information still hold for the non-ideal case
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
The Multi-Input Multi-Output (MIMO) Channel Modeling, Simulation and Applications
This thesis mainly focus on the Multi-Input Multi-Output (MIMO) channel modeling, simulation and applications. There are several ways to design a MIMO channel. Most of the examples are given in Chapter 2, where we can design channels based on the environments and also based on other conditions. One of the new MIMO channel designs based on physical and virtual channel design is discussed in Unitary-Independent- Unitary (UIU) channel modeling. For completeness, the different types of capacity are discussed in details. The capacity is very important in wireless communication. By understanding the details behind different capacity, we can improve our transmission efficiently and effectively. The level crossing rate and average duration are discussed.One of the most important topics in MIMO wireless communication is estimation. Without having the right estimation in channel prediction, the performance will not be correct. The channel estimation error on the performance of the Alamouti code was discussed. The design of the transmitter, the channel and the receiver for this system model is shown. The two different types of decoding scheme were shown - the linear combining scheme and the Maximum likelihood (ML) decoder. Once the reader understands the estimation of the MIMO channel, the estimation based on different antenna correlation is discussed. Next, the model for Mobile-to-Mobile (M2M) MIMO communication link is proposed. The old M2M Sum-of-Sinusoids simulation model and the new two ring models are discussed. As the last step, the fading channel modeling using AR model is derived and the effect of ill-conditioning of the Yule-Walker equation is also shown. A number of applications is presented to show how the performance can be evaluated using the proposed model and techniques
Self-interference cancellation for full-duplex MIMO transceivers
PhD ThesisIn recent years, there has been enormous interest in utilizing the full-duplex
(FD) technique with multiple-input multiple-output (MIMO) systems to complement
the evolution of fifth generation technologies. Transmission and reception
using FD-MIMO occur simultaneously over the same frequency band
and multiple antennas are employed in both sides. The motivation for employing
FD-MIMO is the rapidly increasing demand on frequency resources,
and also FD has the ability to improve spectral efficiency and channel capacity
by a factor of two compared to the conventional half-duplex technique.
Additionally, MIMO can enhance the diversity gain and enable FD to acquire
further degrees of freedom in mitigating the self-interference (SI). The
latter is one of the key challenges degrading the performance of systems operating
in FD mode due to local transmission which involves larger power
level than the signals of interest coming from distance sources that are significantly
more attenuated due to path loss propagation phenomena. Various
approaches can be used for self-interference cancellation (SIC) to tackle SI
by combining passive suppression with the analogue and digital cancellation
techniques. Moreover, active SIC techniques using special domain suppression
based on zero-forcing and null-space projection (NSP) can be exploited
for this purpose too. The main contributions of this thesis can be summarized
as follows. Maximum-ratio combining with NSP are jointly exploited in order
to increase the signal-to-noise ratio (SNR) of the desired path and mitigate
the undesired loop path, respectively, for an equalize-and-forward (EF) relay
using FD-MIMO. Additionally, an end-to-end performance analysis of the
proposed system is obtained in the presence of imperfect channel state information
by formulating mathematically the exact closed-form solutions for
the signal-to-interference-plus-noise ratio (SINR) distribution, outage probability,
and average symbol-error rate for uncoded M-ary phase-shift keying
over Rayleigh fading channels and in the presence of additive white Gaussian
noise (AWGN). The coefficients of the EF-relay are designed to attain
the minimum mean-square error (MMSE) between the transmission symbols.
Comparison of the results obtained with relevant state-of-the-art techniques
suggests significant improvements in the SINR figures and system capacity.
Furthermore, iterative detection and decoding (IDD) are proposed to mitigate
the residual self-interference (SI) remaining after applying passive suppression
along with two stages of SI cancellation (SIC) filters in the analogue
and digital domains for coded FD bi-directional transceiver based multiple
antennas. IDD comprises an adaptive MMSE filter with log-likelihood ratio
demapping, while the soft-in soft-out decoder utilizes the maximum a posteriori
(MAP) algorithm. The proposed system’s performance is evaluated in
the presence of AWGN over non-selective (flat) Rayleigh fading single-input
multiple-output (SIMO) and MIMO channels. However, the results of the
analyses can be applied to multi-path channels if orthogonal frequency division
multiplexing is utilised with a proper length of cyclic prefix in order to
tackle the channels’ frequency-selectivity and delay spread. Simulation results
are presented to demonstrate the bit-error rate (BER) performance as a
function of the SNR, revealing a close match to the SI-free case for the proposed
system. Furthermore, the results are validated by deriving a tight upper
bound on the performance of rate-1=2 convolutional codes for FD-SIMO and
FD-MIMO systems for different modulation schemes under the same conditions,
which asymptotically exhibits close agreement with the simulated BER
performance.Ministry of Higher Education and Scientific Research
(MoHESR), and the University of Mosul and to the Iraqi Cultural Attache in
London for providing financial support for my PhD scholarship