The cooperative relaying scheme is a promising technique for increasing the capacity and reliability of wireless communication. Even though extensive research has performed in information theoretical aspect, there are still many unresolved practical problems of cooperative relaying system. This dissertation analyzes the performance of cooperative decode-and-forward (DF) relaying systems in the presence of multiple interferers and improve network throughput for these systems. We propose and summarize various systems in the view of network topology, transmission structure, and slot allocation. We present closed-form expressions for the end-to-end outage probability, average symbol-error-probability, average packet-error-probability, and network throughput of the proposed systems. This dissertation shows that the robustness of the destination against interference is more important than robustness of the relay against interference from an interference management perspective, and increasing the number of branches yields better outage and error performance improvements against shadowing than increasing the number of hops. In cellular networks, the cooperative diversity systems can outperform the dual-Rx antenna system, but only when the relay is located in a relatively small portion of the total cell area with respect the the destination mobile terminal. The results also show that since the effective regions of the uplink and the downlink do not overlap, different relays should be utilized for cell sectorization in the uplink and the downlink. Finally, the proposed variable-slot selection DF scheme can reduce the system complexity and make the maximum throughput point in the low and moderate signal-to-interference-plus-noise ratio region.

This dissertation develops optimal algorithms for distributed detection and estimation in static and mobile sensor networks. In distributed detection or estimation scenarios in clustered wireless sensor networks, sensor motes observe their local environment, make decisions or quantize these observations into local estimates of finite length, and send/relay them to a Cluster-Head (CH). For event detection tasks that are subject to both measurement errors and communication errors, we develop an algorithm that combines a Maximum a Posteriori (MAP) approach for local and global decisions with low-complexity channel codes and processing algorithms. For event estimation tasks that are subject to measurement errors, quantization errors and communication errors, we develop an algorithm that uses dithered quantization and channel compensation to ensure that each mote's local estimate received by the CH is unbiased and then lets the CH fuse these estimates into a global one using a Best Linear Unbiased Estimator (BLUE). We then determine both the minimum energy required for the network to produce an estimate with a prescribed error variance and show how this energy must be allocated amongst the motes in the network. In mobile wireless sensor networks, the mobility model governing each node will affect the detection accuracy at the CH and the energy consumption to achieve this level of accuracy. Correlated Random Walks (CRWs) have been proposed as mobility models that accounts for time dependency, geographical restrictions and nonzero drift. Hence, the solution to the continuous-time, 1-D, finite state space CRW is provided and its statistical behavior is studied both analytically and numerically. The impact of the motion of sensor on the network's performance is also studied.

The future success of communication networks hinges on the ability to overcome the mismatch between requested quality of service (QoS) and limited network resources. Spectrum is a natural resource that cannot be replenished and therefore must be used efficiently. On the other hand, energy efficiency (EE) is also becoming increasingly important as battery technology has not kept up with the growing requirements stemming from ubiquitous multimedia applications. The qualities of wireless channels vary with both time and user. We use channel state information (CSI) to dynamically assign wireless resources to users to improve spectral and energy efficiency. We first investigate a series of general treatments of exploiting CSI in a distributed way to control the medium access to maximize spectral efficiency for networks with arbitrary topologies and traffic distributions. As the first step, we propose decentralized optimization for multichannel random access (DOMRA), which uses local CSI and two-hop static neighborhood information to achieve performance comparable with the global optimal channel-aware Aloha. The generic framework developed in DOMRA proved to be very useful in improving cellular networks as well. We develop cochannel interference avoidance (CIA) medium access control (MAC), which is optimized by DOMRA, to mitigate the downlink severe cochannel interference that is usually experienced by cell-edge users. Aloha-based schemes have low channel utilization efficiency because of the collision of entire data frames. We further develop channel-aware distributed MAC (CAD-MAC), which avoids collision through signaling negotiation ahead of data transmission. CAD-MAC completely resolves the contention of networks with arbitrary topologies, achieves throughput close to centralized schedulers, and is robust to any channel uncertainty. Then we address energy-efficient wireless communications while emphasizing orthogonal frequency multiple access (OFDMA) systems. We first discover the global optimal energy-efficient link adaptation in frequency-selective channels using the strict quasiconcavity of energy efficiency functions. This link adaptation optimally balances the power consumption of electronic circuits and that of data transmission on each subchannel. The global optimal energy-efficient transmission can be obtained using iterative operations, which may be complex to be implemented in a practical system. Besides, running iterative algorithms consumes additional energy. Hence, we further develop a closed-form link adaptation scheme, which performs close to the global optimum. Besides, since subchannel allocation in OFDMA systems determines the energy efficiency of all users, we develop closed-form resource allocation approaches that achieve near-optimal performance too. In an interference-free environment, a tradeoff between EE and spectral efficiency (SE) exists, as increasing transmit power always improves SE but not necessarily EE. We continue the investigation in interference-limited scenarios and show that since increased transmit power also brings higher interference to the network, SE is not necessarily higher and the tradeoff is improved. Especially, in interference-dominated regimes, e.g., local area networks, both spectral- and energy-efficient communications desire optimized time-division protocols and the proposed DOMRA, CIA-MAC, and CAD-MAC can be used to improve both spectral and energy efficiency.

The objective of this research is to investigate improved methods for distributing certificate revocation lists (CRLs) in vehicular ad hoc networks (VANETs). VANETs are a subset of mobile ad hoc networks composed of network-equipped vehicles and infrastructure points, which will allow vehicles to communicate with other vehicles and with roadside infrastructure points. While sharing some of the same limitations of mobile ad hoc networks, such as lack of infrastructure and limited communications range, VANETs have several dissimilarities that make them a much different research area. The main differences include the size of the network, the speed of the vehicles, and the network security concerns. Confidentiality, authenticity, integrity, and availability are some of the standard goals of network security. While confidentiality and authenticity at times seem in opposition to each other, VANET researchers have developed many methods for enhancing confidentiality while at the same time providing authenticity. The method agreed upon for confidentiality and authenticity by most researchers and the IEEE 1609 working group is a public key infrastructure (PKI) system. An important part of any PKI system is the revocation of certificates. The revocation process, as well as the distribution of revocation information, is an open research problem for VANETs. This research develops new methods of CRL distribution and compares them to existing methods proposed by other researchers. The new methods show improved performance in various vehicle traffic densities.

This work focuses on the performance characterization of distributed collaborative ad-hoc networks, focusing on such metrics as the lifetime, latency, and throughput capacity of two such classes of networks. The first part concerns modeling and optimization of static Wireless Sensor Networks, specifically dealing with the issues of energy efficiency, lifetime, and latency. We analyze and characterize these performance measures and discuss various fundamental design tradeoffs. For example, energy efficiency in wireless sensor networks can only be improved at the cost of the latency (the delay incurred during communication). It has been clearly shown that improvement in energy efficiency through data aggregation increases the latency in the network. In addition, sleep-active duty cycling of nodes (devices constituting the network), a commonly employed mechanism to conserve battery lifetime in such networks, has adverse effects on their functionality and capacity. Hence these issues deserve a detailed study. The second part of this work concerns performance modeling of Delay Tolerant Networks (DTNs) and Sparse Mobile Ad-Hoc Networks (SPMANETs) in general. We first investigate the effect of modern coding, such as the application of packet-level rateless codes, on the latency, reliability, and energy efficiency of the network. These codes provide us the means to break large messages into smaller packets thereby enabling efficient communication. The work then focuses on developing and formalizing an information-theoretic framework for Delay Tolerant- and other Sparse Mobile Networks. This is enabled by the use of an embedded-Markov-chain approach used for complex queuing-theoretic problems. An important goal of this work is to incorporate a wide range of mobility models into the analysis framework. Yet another important question will be the effect of changing the mobility on the comparative performance of networking protocols. Lastly, the framework will be extended to various communication paradigms such as two-hop vs multi-hop routing, unicast, and multicast.

In the wake of major disasters, the failure of existing communications infrastructure and the subsequent lack of an effective communication solution results in increased risks, inefficiencies, damage and casualties. Currently available options such as satellite communication are expensive and have limited functionality. A robust communication solution should be affordable, easy to deploy, require little infrastructure, consume little power and facilitate Internet access. Researchers have long proposed the use of ad hoc wireless networks for such scenarios. However such networks have so far failed to create any impact, primarily because they are unable to handle network transience and have usability constraints such as static topologies and dependence on specific platforms. LifeNet is a WiFi-based ad hoc data communication solution designed for use in highly transient environments. After presenting the motivation, design principles and key insights from prior literature, the dissertation introduces a new routing metric called Reachability and a new routing protocol based on it, called Flexible Routing. Roughly speaking, reachability measures the end-to-end multi-path probability that a packet transmitted by a source reaches its final destination. Using experimental results, it is shown that even with high transience, the reachability metric - (1) accurately captures the effects of transience (2) provides a compact and eventually consistent global network view at individual nodes, (3) is easy to calculate and maintain and (4) captures availability. Flexible Routing trades throughput for availability and fault-tolerance and ensures successful packet delivery under varying degrees of transience. With the intent of deploying LifeNet on field we have been continuously interacting with field partners, one of which is Tata Institute of Social Sciences India. We have refined LifeNet iteratively refined base on their feedback. I conclude the thesis with lessons learned from our field trips so far and deployment plans for the near future.

Abstract available in digital copy. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2003.

An Opportunistic Large Array (OLA) is a form of cooperative diversity in which a large group of simple, inexpensive relays operate without any mutual coordination, but naturally fire together in response to the energy received from a single source or another OLA. The main contributions of this thesis are the introduction of two OLA-based routing protocols: OLA Concentric Routing Algorithm (OLACRA), which is an upstream routing algorithm suitable for static wireless sensor networks (WSNs), and OLA Routing On-Demand (OLAROAD), which is a robust reactive routing scheme suitable for mobile ad hoc networks (MANETs). In fixed multi-hop wireless sensor networks with a single sink, where energy conservation is often a concern, simulations of the new algorithms show as much as 80% of the transmit energy required to broadcast data can be saved, relative to existing OLA-based broadcasting approaches. In MANETs, where robustness of the routes is an important performance indicator, OLAROAD-based cooperative routes last much longer compared to their state-of-art multi-hop non-cooperative transmission (CT)-based counterparts. However, OLACRA and OLAROAD have higher node participation, and thereby lower throughput, in comparison with the non-CT schemes. To improve the throughput, and thereby bandwidth utilization, the properties of uplink OLAs and their suppression regions are carefully studied. Based on the observations, Hop-Optimized OLACRA (HOLA), which is a variant of OLACRA, and has the maximum bandwidth utilization amongst all the OLA unicast schemes studied, is proposed. HOLA routes have bandwidth utilization comparable to non-CT schemes, but a much lower (~10 dB less) transmit power per node. The last section of this thesis treats the MAC design for OLA-based networks. In contrast to non-CT networks, a 802.11-based RTS/CTS MAC scheme is shown to reduce the reliability in OLA unicast schemes. A distributed cluster-head-based MAC scheme for channel reservation and OLA Size Adaptation Mechanism for link repair/maintenance are proposed for OLA-based networks. The performances of these protocols are shown to be comparable to a non-CT multihop scheme using the RTS/CTS/DATA/ACK handshake-based link layer design.

Currently there is ongoing research in the field of Mobile Ad-hoc Networks (MANET) for several different scenarios. Research has focused on topology related challenges such as routing mechanisms or addressing systems, as well as security issues like traceability of radio communication or encryption. In addition, there are very specific research interests such as the effects of directional antennas for MANETs or optimized transmission techniques for minimal power consumption or range optimization. Unmanned aerial vehicles (UAVs), and unmanned aerial systems (UAS) in general, need wireless systems in order to communicate. Current UAS are very flexible and allow for a wide spectrum of mission profiles by means of utilizing different UAVs, according to the requirements at hand. Each mission poses special needs and requirements on the internal and external UAS communication and special mission scenarios calling for UAV swarms increase the complexity and require specialized communication solutions. UAS have specific needs not provided by the general research, but are, on the other hand, to diversified to make much use of narrowly focused developments; UAS form a sufficiently large research area for application of MANETs to be considered as an independent group with specialized needs worthy of tailored implementations of MANET principles. MANET research has not tackled a general approach to UAS although some sources show specific applications involving UAVS. This work presents some new aspects for the development of of ad-hoc wireless networks for UAVs and UAS and focuses on their specialties and needs. A general framework for MANET development is proposed. Furthermore, the proposed specific evaluation scenarios provide for a UAS focused comparison of MANET performance.

Le cadre général de cette thèse porte sur les communications véhicule à véhicule (V2V). L'objectif principal de ce type de communication est d'améliorer la sécurité routière et d'optimiser la fluidité du trafic. Les performances de systèmes de communication embarqués dépendent principalement de la configuration des antennes, du type de récepteurs utilisés (modem) et du canal de propagation. Le sujet de la thèse se concentre précisément sur l'élaboration d'un outil de simulation robuste et fiable pour optimiser les choix technologiques d'antennes et leurs implantations à bord. Il permettra de limiter ainsi le recours aux prototypes et aux essais de roulage. Dans un premier temps, nous avons proposé une méthode de type géométrique-stochastique pour modéliser des canaux de propagation d'un système de communication V2V. Cette méthode permet de définir des scénarios dynamiques et sa flexibilité nous offre l'opportunité de paramétrer les caractéristiques d'un canal de propagation. Nous montrons pour l'essentiel de quelle manière l'agencement et le nombre de diffuseurs de forme simple peuvent contribuer à influencer les statistiques de l'étalement des retards, la distribution des angles d'arrivée et les spectres Doppler. Dans la deuxième partie de ces travaux, nous nous focalisons sur la problématique d'intégration d'antenne. Il a été constaté qu'il est particulièrement utile de simplifier la maquette numérique du véhicule complet lors d'une simulation électromagnétique de l'antenne intégrée à bord de ce véhicule. De plus, dans cette section, nous avons étudié la limite de la validité de substitution d'une antenne intégrée à bord par son seul diagramme de rayonnement dans les modèles de canaux de propagation. A partir de cette étude, nous en avons déduit, que les interactions électromagnétiques avec les éléments se situant dans le champ-proche n'étaient plus valides. Ainsi, une stratégie de correction a été trouvée en considérant l'élément en champ proche dans la simulation initiale de l'antenne intégrée sur le véhicule. L'évaluation des performances d'antennes, en termes de taux d'erreurs paquet, après l'implémentation de la couche physique du standard 802. 11 p, est au cœur de la troisième partie de cette thèse. Des résultats de simulation, pour chaque configuration d'antenne, ont été donnés pour plusieurs combinaisons de paramètres de couche PHY tels que le débit et la longueur de paquet. La dernière partie de ce manuscrit traite de la présentation d'une campagne de mesure qui a été réalisée principalement dans un environnement de type rural. Les essais de roulage avaient pour but de démontrer la capacité des méthodes de simulation à faire un choix approprié d'installation d'antenne. Nous avons constaté que les tendances attendues des performances de communication en termes de PER sont globalement prévisibles à partir de notre modèle bien que nous ayons obtenu des résultats surprenants dans certains cas. Ce qui nécessitera une enquête plus approfondie. / The general framework of this Ph.D. thesis deals with Vehicle to Vehicle (V2V) communications. This communication is principally dedicated to reinforce security through exchange of information between vehicles in case of unexpected events. The development of V2V solutions requires the understanding and merging of a number of techniques from several communication technology areas. Indeed, the performance of these systems are related to three main blocks: the implementation of the modems, the antenna configuration and the fast varying electromagnetic environment surrounding the transmitter and the receiver. The purpose of this PhD thesis is precisely to elaborate a design strategy to investigate all the different blocks constituting the communication chain. The ultimate goal is to optimize antenna selection and location on board the vehicle. In the first place, we proposed a geometry-based stochastic channel modelling approach to develop virtual dynamic scenarios. It was shown that the most common propagation environments such as rural, highway or urban-like propagation channels are reached through adjustment or numbers and location of simple metallic plates. Using this method, we are able to emulate the characteristics, such as the delay spread, angles of arrival distribution and the Doppler spectrum, of these typical driving scenarios. Moreover, we dealt with antenna integration problems in the second part of this work. A study on the simplification of numerical models of vehicles was carried out. Furthermore in this section, we investigated the limit of validity of representing the integrated antenna on board vehicle solely by its radiation pattern. We deduced from this study that the interactions with the elements present in the near-field is no longer valid. Thus, a strategy to alleviate the degree of error was found upon consideration of the near-field scatter in the a-priori simulation of the antenna integrated on the vehicle. In the third part of this thesis, we evaluated the performance of different antenna configurations in terms of packet error rate after the embodiment of the PHY layer of the LEEE 802.11p standard. Simulation results, for each antenna configurations, was given for several combinations of PHY layer parameters such as data rate and packet length. The last part of this manuscript is dedicated to the presentation of a measurement campaign that was carried out in a rural driving environment. Results obtained from these functional tests were confronted with simulation results to demonstrate the strength of the latter. We found that the expected trends in communication performance in terms of PER are globally predictable from our model though we obtained surprising results in some cases, needing further investigation.