Modified quasi-orthogonal space-time block coding in distributed wireless networks

Cooperative networks have developed as a useful technique that can achieve the same advantage as multi-input and multi-output (MIMO) wireless systems such as spatial diversity, whilst resolving the difficulties of co-located multiple antennas at individual nodes and avoiding the effect of path-loss and shadowing. Spatial diversity in cooperative networks is known as cooperative diversity, and can enhance system reliability without sacrificing the scarce bandwidth resource or consuming more transmit power. It enables single-antenna terminals in a wireless relay network to share their antennas to form a virtual antenna array on the basis of their distributed locations. However, there remain technical challenges to maximize the benefit of cooperative communications, e.g. data rate, asynchronous transmission and outage. In this thesis, therefore, firstly, a modified distributed quasi-orthogonal space-time block coding (M-D-QO-STBC) scheme with increased code gain distance (CGD) for one-way and two-way amplify-and-forward wireless relay networks is proposed. This modified code is designed from set partitioning a larger codebook formed from two quasi-orthogonal space time block codes with different signal rotations then the subcodes are combined and pruned to arrive at the modified codebook with the desired rate in order to increase the CGD. Moreover, for higher rate codes the code distance is maximized by using a genetic algorithm to search for the optimum rotation matrix. This scheme has very good performance and significant coding gain over existing codes such as the open-loop and closed-loop QO-STBC schemes. In addition, the topic of outage probability analysis in the context of multi-relay selection from $N$ available relay nodes for one-way amplify-and-forward cooperative relay networks is considered together with the best relay selection, the $N^{th}$ relay selection and best four relay selection in two-way amplify-and-forward cooperative relay networks. The relay selection is performed either on the basis of a max-min strategy or one based on maximizing exact end-to-end signal-to-noise ratio. Furthermore, in this thesis, robust schemes for cooperative relays based on the M-D-QO-STBC scheme for both one-way and two-way asynchronous cooperative relay networks are considered to overcome the issue of a synchronism in wireless cooperative relay networks. In particular, an orthogonal frequency division multiplexing (OFDM) data structure is employed with cyclic prefix (CP) insertion at the source in the one-way cooperative relay network and at the two terminal nodes in the two-way cooperative network to combat the effects of time asynchronism. As such, this technique can effectively cope with the effects of timing errors. Finally, outage probability performance of a proposed amplify-and-forward cooperative cognitive relay network is evaluated and the cognitive relays are assumed to exploit an overlay approach. A closed form expression for the outage probability for multi-relay selection cooperation over Rayleigh frequency flat fading channels is derived for perfect and imperfect spectrum acquisitions. Furthermore, the M-QO-STBC scheme is also proposed for use in wireless cognitive relay networks. MATLAB and Maple software based simulations are employed throughout the thesis to support the analytical results and assess the performance of new algorithms and methods

Novel transmission schemes for application in two-way cooperative relay wireless communication networks

Recently, cooperative relay networks have emerged as an attractive communications technique that can generate a new form of spatial diversity which is known as cooperative diversity, that can enhance system reliability without sacrificing the scarce bandwidth resource or consuming more transmit power. To achieve cooperative diversity single-antenna terminals in a wireless relay network typically share their antennas to form a virtual antenna array on the basis of their distributed locations. As such, the same diversity gains as in multi-input multi-output systems can be achieved without requiring multiple-antenna terminals. However, there remain technical challenges to maximize the benefit of cooperative communications, e.g. data rate, asynchronous transmission, interference and outage. Therefore, the focus of this thesis is to exploit cooperative relay networks within two-way transmission schemes. Such schemes have the potential to double the data rate as compared to one-way transmission schemes. Firstly, a new approach to two-way cooperative communications via extended distributed orthogonal space-time block coding (E-DOSTBC) based on phase rotation feedback is proposed with four relay nodes. This scheme can achieve full cooperative diversity and full transmission rate in addition to array gain. Then, distributed orthogonal space-time block coding (DOSTBC) is applied within an asynchronous two-way cooperative wireless relay network using two relay nodes. A parallel interference cancelation (PIC) detection scheme with low structural and computational complexity is applied at the terminal nodes in order to overcome the effect of imperfect synchronization among the cooperative relay nodes. Next, a DOSTBC scheme based on cooperative orthogonal frequency division multiplexing (OFDM) type transmission is proposed for flat fading channels which can overcome imperfect synchronization in the network. As such, this technique can effectively cope with the effects of fading and timing errors. Moreover, to increase the end-to-end data rate, a closed-loop EDOSTBC approach using through a three-time slot framework is proposed. A full interference cancelation scheme with OFDM and cyclic prefix type transmission is used in a two-hop cooperative four relay network with asynchronism in the both hops to achieve full data rate and completely cancel the timing error. The topic of outage probability analysis in the context of multi-relay selection for one-way cooperative amplify and forward networks is then considered. Local measurements of the instantaneous channel conditions are used to select the best single and best two relays from a number of available relays. Asymptotical conventional polices are provided to select the best single and two relays from a number of available relays. Finally, the outage probability of a two-way amplify and forward relay network with best and Mth relay selection is analyzed. The relay selection is performed either on the basis of a max-min strategy or one based on maximizing exact end-to-end signal-to-noise ratio. MATLAB and Maple software based simulations are employed throughout the thesis to support the analytical results and assess the performance of new algorithms and methods

Adaptive Equalization and Capacity Analysis for Amplify-and-Forward Relays

Recent research has shown that multiple-input multiple-output (MIMO) systems provide high spectral efficiencies and error performance gains. However, the use of multiple antennas in mobile terminals may not be very practical. Certainly there is limited space and other implementation issues which make this a challenging problem. Therefore, to harness the diversity gains afforded by MIMO transmitter diversity techniques, while maintaining a minimal number of antennas on each handset, cooperative diversity techniques have been proposed. In addition, attention has also been given to combining wireless relaying systems with MIMO techniques to improve capacity, coverage, and obtain better diversity at the expense of increased node complexity. This thesis considers the design and analysis of cooperative diversity systems and MIMO amplify-and-forward relaying systems. In particular, we investigate adaptive time- and frequency-domain equalization techniques for cooperative diversity systems using space-time block codes (STBC). For MIMO relaying systems, we analyze the ergodic capacity of various systems and compare different amplify-and-forward methods in terms of system capacity performance. We propose a new block time-domain adaptive equalization structure for time reversal-space time block coding (TR-STBC) systems, which eliminates the separate decoder and also the need for explicit channel state information (CSI) estimation at the receiver. Our simulation results show that the time-domain adaptive block equalizer performs better than the frequency-domain counterpart but at the cost of increased complexity. Then, we extend this time-domain adaptive equalization scheme to distributed TR-STBC systems. We also develop a frequency-domain counterpart for the distributed systems. Our simulation results show that the adaptive algorithms work well for Protocols I and III proposed by Nabar et al. The time-domain adaptive algorithms perform better than the frequency-domain algorithms, and overall the Protocol I receivers outperform the Protocol III receivers. We also show that, if only the Protocol III receiver is used, it can be susceptible to noise amplification due to a weaker source-to-relay link compared to the relay-to-destination link. This problem can be mitigated by using the Protocol I receivers with some extra complexity but much superior diversity performance. We also present an ergodic capacity analysis of an amplify-and-forward (AF) MIMO two-hop system including the direct link and validate the analysis with simulations. We show that having the direct link improves the capacity due to diversity and quantify this improvement. We also present an ergodic capacity analysis of an AF MIMO two-hop, two relay system. Our results verify the capacity gain of relaying systems with two relays due to the extra diversity compared to a single relaying system. However, the results also show that when one of the source-to-relay links has a markedly higher SNR compared to the other, a single relay system has better capacity than a two relay system. Finally, we compare three types of relay amplification methods: a) average amplification, b) instantaneous channel amplification, and c) instantaneous power amplification. The instantaneous power amplification method has a higher mean capacity but with a higher variance. Also, it requires additional information at the destination and would create enormous overheads compared to the other methods. We also find that the instantaneous channel amplification method has almost no advantage in terms of the mean capacity but its capacity is less variable than the average amplification method. On the other hand, the average amplification method is simpler to implement as it does not require channel estimation at the relaying terminal

Adaptive OFDM Cooperative Systems

Cooperative communication is a promising technique for wireless communication systems where wireless nodes cooperate together in transmitting their information. Such communication transmission technique, which realizes the multiple antenna arrays in a distributed manner over multiple wireless nodes, succeeds in extending the network coverage, increasing throughput, improving both link reliability and spectral efficiency. Available channel state information at the transmitting nodes can be used to design adaptive transmission schemes for improving the overall system performance. Throughout our work, we adaptively change loaded power and/or bit to the Orthogonal Frequency Division Multiplexing (OFDM) symbol in order to minimize bit error rate or maximize the throughput. In the first part of this dissertation, we consider single-relay OFDM system with amplify-and-forward relaying. We propose three algorithms to minimize the bit error rate under total power constraint and fixed transmission rate. These algorithms are optimal power loading, optimal bit loading and optimal bit and power loading. Through Monte Carlo simulations we study the proposed system performance and discuss the effect of relay location and channel estimation. This study shows that the proposed algorithms result in exploiting the multi-path diversity and achieving extra coding gain. In the second part, we extend the problem to a multi-relay OFDM network but with decode-and-forward relaying. We propose an adaptive power loading algorithm to minimize the bit error rate under total power constraint based on two relay selection strategies. The proposed system leads to achieve both multi-path and cooperative spatial diversity using maximal-ratio combiner for the detection. In the last part, we consider also multi-relay network but with amplify and forward relaying. We optimize the bit loading coefficients to maximize the throughput under target bit error rate constraint. The proposed algorithm is considered more practical since it takes into consideration the channel estimation quality. The considered adaptive system has less complexity compared with other adaptive systems through reducing the feedback amount. Furthermore, the full network channel state information is needed only at the destination

Optimum Power Allocation for Cooperative Communications

Cooperative communication is a new class of wireless communication techniques in which wireless nodes help each other relay information and realize spatial diversity advantages in a distributed manner. This new transmission technique promises significant performance gains in terms of link reliability, spectral efficiency, system capacity, and transmission range. Analysis and design of cooperative communication wireless systems have been extensively studied over the last few years. The introduction and integration of cooperative communication in next generation wireless standards will lead to the design of an efficient and reliable fully-distributed wireless network. However, there are various technical challenges and open issues to be resolved before this promising concept becomes an integral part of the modern wireless communication devices. A common assumption in the literature on cooperative communications is the equal distribution of power among the cooperating nodes. Optimum power allocation is a key technique to realize the full potentials of relay-assisted transmission promised by the recent information-theoretic results. In this dissertation, we present a comprehensive framework for power allocation problem. We investigate the error rate performance of cooperative communication systems and further devise open-loop optimum power allocation schemes to optimize the performance. By exploiting the information about the location of cooperating nodes, we are able to demonstrate significant improvements in the system performance. In the first part of this dissertation, we consider single-relay systems with amplify-and-forward relaying. We derive upper bounds for bit error rate performance assuming various cooperation protocols and minimize them under total power constraint. In the second part, we consider a multi-relay network with decode-and-forward relaying. We propose a simple relay selection scheme for this multi-relay system to improve the throughput of the system, further optimize its performance through power allocation. Finally, we consider a multi-source multi-relay broadband cooperative network. We derive and optimize approximate symbol error rate of this OFDMA (orthogonal frequency division multiple access) system

Cooperative Symbol-Based Signaling for Networks with Multiple Relays

Wireless channels suffer from severe inherent impairments and hence reliable and high data rate wireless transmission is particularly challenging to achieve. Fortunately, using multiple antennae improves performance in wireless transmission by providing space diversity, spatial multiplexing, and power gains. However, in wireless ad-hoc networks multiple antennae may not be acceptable due to limitations in size, cost, and hardware complexity. As a result, cooperative relaying strategies have attracted considerable attention because of their abilities to take advantage of multi-antenna by using multiple single-antenna relays. This study is to explore cooperative signaling for different relay networks, such as multi-hop relay networks formed by multiple single-antenna relays and multi-stage relay networks formed by multiple relaying stages with each stage holding several single-antenna relays. The main contribution of this study is the development of a new relaying scheme for networks using symbol-level modulation, such as binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). We also analyze effects of this newly developed scheme when it is used with space-time coding in a multi-stage relay network. Simulation results demonstrate that the new scheme outperforms previously proposed schemes: amplify-and-forward (AF) scheme and decode-and-forward (DF) scheme

A Simple Cooperative Diversity Method Based on Network Path Selection

Cooperative diversity has been recently proposed as a way to form virtual antenna arrays that provide dramatic gains in slow fading wireless environments. However most of the proposed solutions require distributed space-time coding algorithms, the careful design of which is left for future investigation if there is more than one cooperative relay. We propose a novel scheme, that alleviates these problems and provides diversity gains on the order of the number of relays in the network. Our scheme first selects the best relay from a set of M available relays and then uses this best relay for cooperation between the source and the destination. We develop and analyze a distributed method to select the best relay that requires no topology information and is based on local measurements of the instantaneous channel conditions. This method also requires no explicit communication among the relays. The success (or failure) to select the best available path depends on the statistics of the wireless channel, and a methodology to evaluate performance for any kind of wireless channel statistics, is provided. Information theoretic analysis of outage probability shows that our scheme achieves the same diversity-multiplexing tradeoff as achieved by more complex protocols, where coordination and distributed space-time coding for M nodes is required, such as those proposed in [7]. The simplicity of the technique, allows for immediate implementation in existing radio hardware and its adoption could provide for improved flexibility, reliability and efficiency in future 4G wireless systems.Comment: To appear, IEEE JSAC, special issue on 4

On Outage Probability and Diversity-Multiplexing Tradeoff in MIMO Relay Channels

Fading MIMO relay channels are studied analytically, when the source and destination are equipped with multiple antennas and the relays have a single one. Compact closed-form expressions are obtained for the outage probability under i.i.d. and correlated Rayleigh-fading links. Low-outage approximations are derived, which reveal a number of insights, including the impact of correlation, of the number of antennas, of relay noise and of relaying protocol. The effect of correlation is shown to be negligible, unless the channel becomes almost fully correlated. The SNR loss of relay fading channels compared to the AWGN channel is quantified. The SNR-asymptotic diversity-multiplexing tradeoff (DMT) is obtained for a broad class of fading distributions, including, as special cases, Rayleigh, Rice, Nakagami, Weibull, which may be non-identical, spatially correlated and/or non-zero mean. The DMT is shown to depend not on a particular fading distribution, but rather on its polynomial behavior near zero, and is the same for the simple "amplify-and-forward" protocol and more complicated "decode-and-forward" one with capacity achieving codes, i.e. the full processing capability at the relay does not help to improve the DMT. There is however a significant difference between the SNR-asymptotic DMT and the finite-SNR outage performance: while the former is not improved by using an extra antenna on either side, the latter can be significantly improved and, in particular, an extra antenna can be traded-off for a full processing capability at the relay. The results are extended to the multi-relay channels with selection relaying and typical outage events are identified.Comment: accepted by IEEE Trans. on Comm., 201