In this thesis new techniques are presented to achieve performance enhancement in wireless cooperative networks. In particular, techniques to improve transmission rate and maximise end-to-end signal-to-noise ratio are described. An offset transmission scheme with full interference cancellation for a wireless cooperative network with frequency flat links and four relays is introduced. This method can asymptotically, as the size of the symbol block increases, achieve maximum transmission rate together with full cooperative diversity provided the destination node has multiple antennas. A novel full inter-relay interference cancellation method that also achieves asymptotically maximum rate and full cooperative diversity is then designed for which the destination node only requires a single antenna. Two- and four-relay selection schemes for wireless cooperative amplify and forward type networks are then studied in order to overcome the degradation of end-to-end bit error rate performance in single-relay selection networks when there are feedback errors in the relay to destination node links. Outage probability analysis for a four-relay selection scheme without interference is undertaken. Outage probability analysis of a full rate distributed transmission scheme with inter-relay interference is also studied for best single- and two-relay selection networks. The advantage of multi-relay selection when no interference occurs and when adjacent cell interference is present at the relay nodes is then shown theoretically. Simulation results for outage probability analysis are included which support the theoretical expressions. Finally, outage probability analysis of a cognitive amplify and forward type relay network with cooperation between certain secondary users, chosen by best single-, two- and four-relay selection is presented. The cognitive amplify and forward relays are assumed to exploit an underlay approach, which requires adherence to an interference constraint on the primary user. The relay selection scheme is performed either with a max-min strategy or one based on maximising exact end-to-end signal-to-noise ratio. The outage probability analyses are again confirmed by numerical evaluations.

In this thesis new methods are presented to achieve performance enhancement in wireless cooperative networks. In particular, techniques to improve diversity gain, throughput and minimise the transmission delay are described. A buffer-aided amplify-and-forward max-link relay selection scheme for both symmetric and asymmetric channels is introduced. This approach shows that the max-link scheme is most effective over the traditional max-SNR scheme when the source-to-relay and relay-to-destination links are symmetric. The closed form expressions for the outage probability and average packet delay of the proposed scheme under both symmetric and asymmetric channel configurations is derived. The diversity order and the coding gain of the AF max-link scheme is analytically provided. Then a novel relay selection scheme with significantly reduced packet delay is proposed. Both the outage performance and average packet delay of the proposed scheme are analysed. The analysis shows that, besides the diversity and coding gains, the proposed scheme has average packet delay similar to that of a non buffer-aided relay system when the channel SNR is sufficiently high thereby. A novel buffer-aided link selection scheme based on network-coding in a multiple hop relay network is proposed. Compared with existing approaches, the proposed scheme significantly increases the system throughput. This is achieved by applying data buffers at the relays to decrease the outage probability and using network-coding to increase the data rate. The closed-form expressions of both the average throughput and packet delay are derived. The proposed scheme has not only significantly higher throughput than both the traditional and existing buffer-aided max-link scheme, but also smaller average packet delay than the max-link scheme. A decode-and-forward buffer-aided relay selection for the underlay cognitive relay networks in the presence of both primary transmitter and receiver is presented. A novel buffer aided relay selection scheme for the cognitive relay network is proposed, where the best relay is selected with the highest signal-to-interference-ratio among all available source-to-relay and relay-to-destination links while keeping the interference to the primary destination within a certain level. A closed-form expression for the outage probability of the proposed relay selection scheme is obtained. Finally, A novel security buffer-aided decode-and-forward cooperative wireless networks is considered. An eavesdropper which can intercept the data transmission from both the source and relay nodes is considered to threaten the security of transmission. Finite size data buffers are assumed to be available at every relay in order to avoid having to select concurrently the best source-to-relay and relay-to-destination links. A new max-ratio relay selection policy is proposed to optimise the secrecy transmission by considering all the possible source-to-relay and relay-to-destination links and selecting the relay having the link which maximises the signal to eavesdropper channel gain ratio. Two cases are considered in terms of knowledge of the eavesdropper channel strengths: exact and average gains, respectively. Closed-form expressions for the secrecy outage probability for both cases are obtained. The proposed max-ratio relay selection scheme is shown to outperform one based on max-min-ratio relay scheme.

"Fading is one of the most fundamental impairments to wireless communications. The standard approach to combating fading is by adding redundancy - or diversity - to help increase coverage and transmission speed. Motivated by the results in multiple-input multiple-output technologies, which are usually used at base stations or access points, cooperation commutation has been proposed to improve the performance of wireless networks which consist of low-cost single antenna devices. While the majority of the research in cooperative communication focuses on flat fading for its simplicity and easy analysis, in practice the underlying channels in broadband wireless communication systems such as cellular systems (UMTS/LTE) are more likely to exhibit frequency selective fading. In this dissertation, we consider a frequency selective fading channel model and explore distributed diversity techniques in broadband wireless relay networks, with consideration to practical issues such as channel estimation and complexity-performance tradeoffs. We first study a system model with one source, one destination and multiple decode-and-forward (DF) relays which share a single channel orthogonal to the source. We derive the diversity-multiplexing tradeoff (DMT) for several relaying strategies: best relay selection, random relay selection, and the case when all decoding relays participate. The best relay selection method selects the relay in the decoding set with the largest sum-squared relay-to-destination channel coefficients. This scheme can achieve the optimal DMT of the system at the expense of higher complexity, compared to the other two relaying strategies which do not always exploit the spatial diversity offered by the relays. Different from flat fading, we find special cases when the three relaying strategies have the same DMT. We further present a transceiver design and prove it can achieve the optimal DMT asymptotically. Monte Carlo simulations are presented to corroborate the theoretical analysis. We provide a detailed performance comparison of the three relaying strategies in channels encountered in practice. The work has been extended to systems with multiple amplify-and-forward relays. We propose two relay selection schemes with maximum likelihood sequential estimator and linear zero- forcing equalization at the destination respectively and both schemes can asymptotically achieve the optimal DMT. We next extend the results in the two-hop network, as previously studied, to multi-hop networks. In particular, we consider the routing problem in clustered multi-hop DF relay networks since clustered multi-hop wireless networks have attracted significant attention for their robustness to fading, hierarchical structure, and ability to exploit the broadcast nature of the wireless channel. We propose an opportunistic routing (or relay selection) algorithm for such networks. In contrast to the majority of existing approaches to routing in clustered networks, our algorithm only requires channel state information in the final hop, which is shown to be essential for reaping the diversity offered by the channel. In addition to exploiting the available diversity, our simple cross-layer algorithm has the flexibility to satisfy an additional routing objective such as maximization of network lifetime. We demonstrate through analysis and simulation that our proposed routing algorithm attains full diversity under certain conditions on the cluster sizes, and its diversity is equal to the diversity of more complicated approaches that require full channel state information. The final part of this dissertation considers channel estimation in relay networks. Channel state information is vital for exploiting diversity in cooperative networks. The existing literature on cooperative channel estimation assumes that block lengths are long and that channel estimation takes place within a fading block. However, if the forwarding delay needs to be reduced, short block lengths are preferred, and adaptive estimation through multiple blocks is required. In particular, we consider estimating the relay-to-destination channel in DF relay systems for which the presence of forwarded information is probabilistic since it is unknown whether the relay participates in the forwarding phase. A detector is used so that the update of the least mean square channel estimate is made only when the detector decides the presence of training data. We use the generalized likelihood ratio test and focus on the detector threshold for deciding whether the training sequence is present. We also propose a heuristic objective function which leads to a proper threshold to improve the convergence speed and reduce the estimation error. Extensive numerical results show the superior performance of using this threshold as opposed to fixed thresholds."

The focus of this thesis is peak-to-average power ratio (PAPR) reduction in cooperative wireless networks which exploit orthogonal frequency division multiplexing in transmission. To reduce the PAPR clipping is employed at the source node. The first contribution focuses upon an amplify-and-forward (AF) type network with four relay nodes which exploits distributed closed loop extended orthogonal space frequency block coding to improve end-to-end performance. Oversampling and filtering are used at the source node to reduce out-of-band interference and the iterative amplitude reconstruction decoding technique is used at the destination node to mitigate in-band distortion which is introduced by the clipping process. In addition, by exploiting quantized group feedback and phase rotation at two of the relay nodes, the system achieves full cooperative diversity in addition to array gain. The second contribution area is outage probability analysis in the context of multi-relay selection in a cooperative AF network with frequency selective fading channels. The gains of time domain multi-path fading channels with L paths are modeled with an Erlang distribution. General closed form expressions for the lower and upper bounds of outage probability are derived for arbitrary channel length L as a function of end-to-end signal to noise ratio. This analysis is then extended for the case when single relay selection from an arbitrary number of relay nodes M is performed. The spatial and temporal cooperative diversity gain is then analysed. In addition, exact form of outage probability for multi-path channel length L = 2 and selecting the best single relay from an arbitrary number of relay nodes M is obtained. Moreover, selecting a pair of relays when L = 2 or 3 is additionally analysed. Finally, the third contribution context is outage probability analysis of a cooperative AF network with single and two relay pair selection from M available relay nodes together with clipping at the source node, which is explicitly modelled. MATLAB and Maple software based simulations are employed throughout the thesis to support the analytical results and assess the performance of algorithms and methods.

This thesis presents new methodologies to improve performance of wireless cooperative networks using the Golden Code. As a form of space-time coding, the Golden Code can achieve diversity-multiplexing tradeoff and the data rate can be twice that of the Alamouti code. In practice, however, asynchronism between relay nodes may reduce performance and channel quality can be degraded from certain antennas. Firstly, a simple offset transmission scheme, which employs full interference cancellation (FIC) and orthogonal frequency division multiplexing (OFDM), is enhanced through the use of four relay nodes and receiver processing to mitigate asynchronism. Then, the potential reduction in diversity gain due to the dependent channel matrix elements in the distributed Golden Code transmission, and the rate penalty of multihop transmission, are mitigated by relay selection based on two-way transmission. The Golden Code is also implemented in an asynchronous one-way relay network over frequency flat and selective channels, and a simple approach to overcome asynchronism is proposed. In one-way communication with computationally efficient sphere decoding, the maximum of the channel parameter means is shown to achieve the best performance for the relay selection through bit error rate simulations. Secondly, to reduce the cost of hardware when multiple antennas are available in a cooperative network, multi-antenna selection is exploited. In this context, maximum-sum transmit antenna selection is proposed. End-to-end signal-to-noise ratio (SNR) is calculated and outage probability analysis is performed when the links are modelled as Rayleigh fading frequency flat channels. The numerical results support the analysis and for a MIMO system maximum-sum selection is shown to outperform maximum-minimum selection. Additionally, pairwise error probability (PEP) analysis is performed for maximum-sum transmit antenna selection with the Golden Code and the diversity order is obtained. Finally, with the assumption of fibre-connected multiple antennas with finite buffers, multiple-antenna selection is implemented on the basis of maximum-sum antenna selection. Frequency flat Rayleigh fading channels are assumed together with a decode and forward transmission scheme. Outage probability analysis is performed by exploiting the steady-state stationarity of a Markov Chain model.

In the wireless sensor network environment (WSN), the system transmits signals often need to rely on the stability and reliability of the relay node of each path of cooperation with each other to achieve balance between leisure and stability. In general, relay adopted Amplify-and-Forward (AF) and Decode-and-Forward (DF) to relaying the signal to destination. Unfortunately, in reality, the relay node itself had a problem of limited energy supplies, would make the overall performance degrade before reaching the optimal performance. Therefore, we propose two novel relay selection schemes and through the multi-hop transmission with cooperation. We also derived the optimal power allocation algorithms for all relay nodes. Finally, simulation results show that our proposed scheme obtained the better lifetime and performance where compared with the traditional schemes in a fair environment.

This thesis proposes resource allocation algorithms for multi-input multi-output (MIMO) relay and Scalable H.264/AVC video transmission over cooperative communication networks. For MIMO relay, we explore the reception diversity with maximal ratio combining (MRC) and transmission diversity with space-time block codes (STBC) respectively. Then, a condition is proposed to maximize the overall output signal-to-noise ratio (SNR). In this condition, the ineffective relays will be excluded in sequence from the cooperation. Simulation results indicate that the effect of bit error rate (BER) through the relay selection is similar to the scheme which applies all relays, but the amounts of used relay decreased. For Scalable H.264/AVC video, by introducing frame significance analysis, the video quality dependency between coding frame and its references is investigated for temporal layers and quality layers. The proposed algorithm allocates the relay and sub-band to each layer based on channel conditions and the priority of classified video packets. Experimental results indicate that the proposed algorithm is superior to the temporal-based allocation and quality-based allocation cooperative schemes.

Cooperative communication is a promising technique to provide spatial diversity in a virtual multi-input and multi-output (MIMO) manner. However, as application evolves toward a more practical situation, realistic constraints and issues such as channel state information (CSI) assumption must be accounted when developing appropriate cooperative schemes. In this thesis, we have addressed delay related problems in both single user pair cooperation (SUPC) and multiple user pair cooperation (MUPC) networks. In SUPC, realizing that the outdated CSI caused by delay between relay selection instant and transmission instant can impair diversity order severely, we propose an opportunistic multiple relay selection (MRS) scheme to achieve desired diversity order and combat the variation of the wireless environment. On the other hand, for multiple user pairs cooperation (MUPC), we start from one of the notable work, two hop opportunistic relaying (THOR), and analyze its the delay related problems. We propose an opportunistic pair scheduling (OPS) scheme which can get rid of the buffer requirement at the relay nodes of THOR and incurs no loss in terms of throughput scaling. Furthermore, we extend OPS to a general scheduling scheme, $L$ scheduling, which can achieve controllable throughput-and-delay trandeoffs. / Graduate

In this dissertation, we study cross-layer design for cooperative wireless data communication networks. Based on the characteristics of cooperative wireless communications, and the requirement of Quality of Service (QoS) provisioning for data networks, we consider cross-layer system design for cooperative wireless networking. Three major design issues which cover cooperative link establishment, information security of cooperative communications, and cross-layer cooperative transmission scheduling, are investigated. Specifically, we follow the communication procedure in cooperative wireless systems and investigate several cross-layer design problems. Considering the queueing behavior of data buffers at the candidate relays, we study relay selection from a queue-aware perspective which takes into account the queueing systems at both the source and the potential relays. With the cooperative link established, we then study the secret key establishment problem by cross-layer cooperative discussion. Then cross-layer transmission scheduling is investigated from two perspectives. We first look at cross-layer adaptive modulation and coding (AMC), which takes both the channel condition and traffic intensity into consideration in the scheduling design. A more general queue-aware scheduler state selection mechanism based on buffer queue occupancy is studied, and optimization by nonlinear integer programming is presented. / Graduate / 0544

In this work, the problem of relay selection and resource power allocation in one- way and two-way cognitive relay networks using half duplex channels with different relaying protocols is investigated. Optimization problems for both single and multiple relay selection that maximize the sum rate of the secondary network without degrading the quality of service of the primary network by respecting a tolerated interference threshold were formulated. Single relay selection and optimal power allocation for two-way relaying cognitive radio networks using decode-and-forward and amplify-and-forward protocols were studied. Dual decomposition and subgradient methods were used to find the optimal power allocation. The transmission process to exchange two different messages between two transceivers for two-way relaying technique takes place in two time slots. In the first slot, the transceivers transmit their signals simultaneously to the relay. Then, during the second slot the relay broadcasts its signal to the terminals. Moreover, improvement of both spectral and energy efficiency can be achieved compared with the one-way relaying technique. As an extension, a multiple relay selection for both one-way and two-way relaying under cognitive radio scenario using amplify-and-forward were discussed. A strong optimization tool based on genetic and iterative algorithms was employed to solve the 
formulated optimization problems for both single and multiple relay selection, where discrete relay power levels were considered. Simulation results show that the practical and low-complexity heuristic approaches achieve almost the same performance of the optimal relay selection schemes either with discrete or continuous power distributions while providing a considerable saving in terms of computational complexity.