The last three decades have seen significant advances in the wireless communication field. As the data rates of wireless systems are increasing, the demand for mobile services also is increasing rapidly. Like other natural resources, radio spectrum suitable for mobile wireless communications is also limited. In order to keep up with this ever increasing demand, there is a requirement of new signal processing algorithms. Diversity is a technique used in wireless systems to combat the effects of fading and thereby improve reliability of data transfer. There are many ways in which algorithms can exploit diversity in wireless channels. Hybrid-automatic repeat request (H-ARQ) schemes and relaying mechanisms are two such diversity extracting techniques. Even though these diversity achieving techniques have been well understood in theory, there are many ways in which one can optimize these techniques for specific application scenarios.In this thesis, we focus on improving the performance of retransmission schemes and relaying systems. In the first part of the thesis, we improve the performance of H-ARQ schemes in the 3GPP- long term evolution (LTE) system by improving the performance of feedback signaling. We employ complex-field coding to extract the inherent frequency diversity available in the resources. Next, we provide a sub-optimal solution to the outage-optimal power allocation problem in incremental redundancy based H-ARQ system, whose performance is practically the same as that of the optimal solution. In the later part of the thesis, we propose a retransmission scheme based on superposition coding (SPC) for the symmetric relaying scenario. We provide packet error probability (PEP) expressions and solutions for the optimal fraction of power allocated for the partners' data. Finally, we study the optimal bits-to-symbol mappings for SPC and its effect on an H-ARQ scheme and the symmetric relaying scenario using SPC.

With the fast development of today’s multimedia services, engineers face a huge hurdle that is, the overwhelming need of highly reliable communication over long distances. Cooperative communication is a novel concept which tackles this problem effectively. The direct link is assisted by nodes called relays, which also reduce shadowing and pathloss effects in wireless networks. An added advantage of such a cooperative communication network is that when combined with multiple-input multiple-output (MIMO) antenna systems and cognitive radio networks (CRN), the system performance in terms of spectral efficiency and reliability, can be extremely enhanced without any extra power and spectrum.The concept of cooperative communications in MIMO and CRN systems has gained immense interest in the literature. Most of the research works have assumed Rayleigh fading conditions. In this thesis, the performance of cooperative communications with practical constraints of shadowing is studied. Analytical expressions for the outage probability of cooperative networks under different relaying protocols with selection combining are presented under the assumption of κ − µ shadowing fading. Specifically, the relaying protocols that are investigated are incremental relaying, opportunistic relaying, adaptive amplify-and-forward and decode-and-forward. Furthermore, this system model is simulated and the simulation results are compared with the analytical results. Mathematica, a technical computing tool, is used for numerical estimations using stochastic processes and probability theory. Simulation is done in MATLAB.In this thesis, along with the analytical framework for evaluating outage probability for the system is presented. Simulations are performed for various fading parameters and the results closely match with analytical results which validate the derivations.

"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."

We consider a system with a single base station communicating with multiple users over orthogonal channels while being assisted by multiple relays. Several recent works have suggested that, in such a scenario, selection, i.e., a single relay helping the source, is the best relaying option in terms of the resulting complexity and overhead. However, in a multiuser setting, optimal relay assignment is a combinatorial problem. We formulate a related convex optimization problem that provides an extremely tight upper bound on performance and show that selection is, almost always, inherent in the solution. We also provide a heuristic to find a close-to-optimal relay assignment and power allocation across users supported by a single relay. Simulation results using realistic channel models demonstrate the efficacy of the proposed schemes, but also raise the question as to whether the gains from relaying are worth the additional costs.

Recent developments in data packets based high speed digital communications have opened the door for new types of applications in power system protection and control. Intelligent Electronic Devices (IEDs) are equipped with various communication capabilities that make their functional integration a natural next step. Existing integration of substation equipment is not capable of clustering with the purpose of pooling hardware resources. <p> Presently, every electric device requiring protection has its dedicated hardware performing the predetermined set of protective functions. A new function-based protection and control philosophy is proposed, based on an open-system solution. In the proposed system, the resources of the protective and control hardware are pooled, and as a clustered system provide each protected unit (line, transformer, breaker, etc) with functions required for complete direct and backup protection. <p> The work presented in this thesis identifies the performance requirements of a digital relaying algorithm for processing samples that are sent across Ethernet-based communication channels. The work shows the shortcomings and unstable performance of widely used protective algorithms in accommodating data samples that are out of step from their proper position due to variable time delays of the communications media. A new digital relaying algorithm was developed that is able to extract the amplitude and phase angle of signals from data samples received across Ethernet networks with variable jitter. <p> The performance of the algorithm was tested by using the recovered phasor amplitude and phase angle information in protective solutions. The results show that there is significant flexibility in the algorithm that can be used to facilitate less performant communication channels, or, to take advantage of faster communications channels by reducing the response time of the protective function. <p> The results show that the algorithm works well with variable length data windows, and variable sampling frequencies. Higher sampling rates make communications problems more visible, but the presented algorithm is able to compensate for wide variations in network performance, effectively maintaining sampled signal phase and amplitude information during network performance fluctuations.

We consider a system with a single base station communicating with multiple users over orthogonal channels while being assisted by multiple relays. Several recent works have suggested that, in such a scenario, selection, i.e., a single relay helping the source, is the best relaying option in terms of the resulting complexity and overhead. However, in a multiuser setting, optimal relay assignment is a combinatorial problem. We formulate a related convex optimization problem that provides an extremely tight upper bound on performance and show that selection is, almost always, inherent in the solution. We also provide a heuristic to find a close-to-optimal relay assignment and power allocation across users supported by a single relay. Simulation results using realistic channel models demonstrate the efficacy of the proposed schemes, but also raise the question as to whether the gains from relaying are worth the additional costs.

Recent developments in data packets based high speed digital communications have opened the door for new types of applications in power system protection and control. Intelligent Electronic Devices (IEDs) are equipped with various communication capabilities that make their functional integration a natural next step. Existing integration of substation equipment is not capable of clustering with the purpose of pooling hardware resources. <p> Presently, every electric device requiring protection has its dedicated hardware performing the predetermined set of protective functions. A new function-based protection and control philosophy is proposed, based on an open-system solution. In the proposed system, the resources of the protective and control hardware are pooled, and as a clustered system provide each protected unit (line, transformer, breaker, etc) with functions required for complete direct and backup protection. <p> The work presented in this thesis identifies the performance requirements of a digital relaying algorithm for processing samples that are sent across Ethernet-based communication channels. The work shows the shortcomings and unstable performance of widely used protective algorithms in accommodating data samples that are out of step from their proper position due to variable time delays of the communications media. A new digital relaying algorithm was developed that is able to extract the amplitude and phase angle of signals from data samples received across Ethernet networks with variable jitter. <p> The performance of the algorithm was tested by using the recovered phasor amplitude and phase angle information in protective solutions. The results show that there is significant flexibility in the algorithm that can be used to facilitate less performant communication channels, or, to take advantage of faster communications channels by reducing the response time of the protective function. <p> The results show that the algorithm works well with variable length data windows, and variable sampling frequencies. Higher sampling rates make communications problems more visible, but the presented algorithm is able to compensate for wide variations in network performance, effectively maintaining sampled signal phase and amplitude information during network performance fluctuations.

Wireless cellular networks, which serve a large area by geographically partitioning users, suffer from interference from adjacent cells transmitting in the same frequency band. This interference can theoretically be completely mitigated via transceiver cooperation in both the uplink and downlink. Optimally, base stations serving the users can utilize high-capacity backbones. to jointly transmit and receive all the data in the network across all the base stations. In reality, the backbone connecting the base stations is of finite capacity, limiting joint processing to localized clusters. Even with joint processing on a small scale, the overhead involved in sharing data between multiple base stations is large and time-sensitive. Other forms of cooperation have been shown to require less overhead while exhibiting much of the performance benefit from interference mitigation. One particular strategy, called interference alignment (IA), has been shown to exploit all the spatial degrees of freedom in the channel provided data cannot be shared among base stations. Interference alignment was developed for the multi-user interference channel to exploit independent channel observations when all of the links in the network have high signal-to-noise ratio, and assumes all the nodes utilizing the physical resources are participating in the cooperative protocol. When some or all of the links are at moderate signal-to-noise ratio, or when there are non-cooperating users, IA is suboptimal. In this dissertation, I take three approaches to addressing the drawbacks of IA. First, I develop cooperative transmission strategies that outperform IA in various operationg regimes, including at low-to-moderate SNR and in the presence of non-cooperating users. These strategies have the same complexity and overhead as IA. I then develop algorithms for network partitioning by directly considering the overhead of cooperative strategies. Partitioning balances the capacity gains of cooperation with the overhead required to achieve them. Finally, I develop the shared relaying model, which is equivalent to the interference channel but with a single multi-antenna relay mediating communications between transceivers. The shared relay requires less overhead and cooperation than interference alignment but requires added infrastructure. It is shown to outperform conventional relaying strategies in cellular networks with a fixed number of total relay antennas. / text

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abstract: In the future electrical distribution system, it can be predicted that local power generators such as photovoltaic panels or wind turbines will play an important role in local distribution network. The local energy generation and local energy storage device can cause indeterminable power flow, and this could cause severe protection problems to existing simple overcurrent coordinated distribution protection system. An accurate, fast and reliable protection system based on pilot protection concept is proposed in this thesis. A comprehensive protection design specialized for the FREEDM system - the intelligent fault management (IFM) is presented in detail. In IFM, the pilot-differential protective method is employed as primary protection while the overcurrent protective method is employed as a backup protection. The IFM has been implemented by a real time monitoring program on LabVIEW. A complete sensitivity and selectivity analysis based on simulation is performed to evaluate the protection program performance under various system operating conditions. Followed by the sensitivity analysis, a case study of multiple-terminal model is presented with the possible challenges and potential limitation of the proposed protection system. Furthermore, a micro controller based on a protection system as hardware implementation is studied on a scaled physical test bed. The communication block and signal processing block are accomplished to establish cooperation between the micro-controller hardware and the IFM program. Various fault cases are tested. The result obtained shows that the proposed protection system successfully identifies faults on the test bed and the response time is approximately 1 cycle which is fast compared to the existing commercial protection systems and satisfies the FREEDM system requirement. In the end, an advanced system with faster, dedicated communication media is accomplished. By verifying with the virtual FREEDM system on RTDS, the correctness and the advantages of the proposed method are verified. An ultra fast protection system response time of 4ms is achieved, which is the fastest protection system for a distribution level electrical system. / Dissertation/Thesis / M.S. Electrical Engineering 2012