Current technological advances have caused an exponential growth of the number of mobile Internet-connected devices, along with their respective traffic demands. To cope with this increase of traffic demands, fifth generation (5G) network architectures will need to provide multi-gigabit capacity at the access base stations (BSs), through the deployment of ultra-dense small cells (SCs) operating with millimeter-wave (mmWave) frequencies, e.g. 60 GHz. To connect the BSs to the core network, a robust and high capacity backhaul infrastructure is required. As it is unfeasible to connect all the SCs through optical fiber links, a solution for the future 5G backhaul relies on the usage of mmWave frequencies to interconnect the SCs, forming multi-hop wireless mesh topologies. In this thesis, we explore the application of the Software-defined Networking (SDN) paradigm for the management of a SC wireless backhaul. With SDN, the data and control planes are separated and the network management is done by a centralized controller entity that has a global network view. To that end, we provide multiple contributions. Firstly, we provide an SDN-based architecture to manage SC backhaul networks, which include an out-of-band Long Term Evolution (LTE) control channel and where we consider aspects such as energy efficiency, resiliency and flexible backhaul operation. Secondly, we demonstrate the benefit of the wireless backhaul configuration using the SDN controller, which can be used to improve the wireless resource allocation and provide resiliency mechanisms in the network. Finally, we investigate how a SC mesh backhaul can be optimally reconfigured between different topologies, focusing on minimizing the network disruption during the reconfiguration. / The growth of mobile devices, along with their traffic demands, is expected to saturate the current mobile networks soon. To cope with such demand increase, fifth generation (5G) network architectures will need to provide multi-gigabit capacity at the access level, through the deployment of a massive amount of ultra-dense small cells (SCs). To connect the access and core networks, a robust and high capacity backhaul is required. To that end, mmWave links that operate at e.g. 60 GHz, can be used to interconnect the SCs, forming multi-hop wireless mesh topologies.   In this thesis, we study the application of the Software-defined Networking (SDN) paradigm for the management of a SC wireless backhaul. Firstly, we provide an SDN-based architecture to manage SC backhaul networks, which includes an out-of-band control channel and where we consider aspects such as energy efficiency, resiliency and flexible backhaul operation. Secondly, we show the benefits of the wireless backhaul configuration using the SDN controller, which can be used to improve the wireless resource allocation and provide network resiliency. Finally, we investigate how a SC mesh backhaul can be optimally reconfigured between different topologies, while minimizing the network disruption during the reconfiguration.

The exponential increase of mobile users and the demand for high-speed data services has resulted in significant congestions in cellular backhaul capacity. As a solution to satisfy the traffic requirements of the existing 4G network, the 5G network has emerged as an enabling technology and a fundamental building block of next-generation communication networks. An essential requirement in 5G backhaul networks is their unparalleled capacity to handle heavy traffic between a large number of devices and the core network. Microwave and optic fiber technologies have been considered as feasible solutions for next-generation backhaul networks. However, such technologies are not cost effective to deploy, especially for the backhaul in high-density urban or rugged areas, such as those surrounded by mountains and solid rocks. Additionally, microwave technology faces alarmingly challenging issues, including limited data rates, scarcity of licensed spectrum, advanced interference management, and rough weather conditions (i.e., rain, which is the main weather condition that affects microwave signals the most). The focus of this work is to investigate the feasibility of using free-space-optical (FSO) technology in the 5G cellular backhaul network. FSO is a cost-effective and wide-bandwidth solution as compared to traditional backhaul solutions. However, FSO links are sensitive to atmospheric turbulence-induced fading, path loss, and pointing errors. Increasing the reliability of FSO systems while still exploiting their high data rate communications is a key requirement in the deployment of an FSO backhaul network. Overall, the theoretical models proposed in this work will be shown to enhance FSO link performance. In the experimental direction, we begin by designing an integrated mobile FSO system. To the best of our knowledge, no work in the literature has addressed the atmospheric path loss characterization of mobile FSO channels in a coastal environment. Therefore, we investigate the impact of weather effects in Thuwal, Saudi Arabia, over FSO links using outdoor and indoor setups. For the indoor experiments, results are reported based on a glass climate chamber in which we could precisely control the temperature and humidity.

Fixed wireless communication is a cost-efficient solution for flexible and rapid front-/backhaul deployments. Technologies including dual polarization, carrier aggregation, and higher order modulation schemes have been developed for enhancing its throughput. In order to better support the massive traffic increment during network evolution, novel wireless backhaul solutions with possible new dimensions in increasing the spectral efficiency are needed. Line-of-Sight (LoS) Multiple-Input-Multiple-Output (MIMO) communication is such a promising candidate allowing the throughput to scale linearly with the deployed antenna pairs. Spatial multiplexing with sub-channels having approximately equal quality exists within a single LoS direction. In addition, operating at millimeter wave (mmWave) frequencies or higher, the abundantly available bandwidth can further enhance the throughput of LoS MIMO communication. The mmWave LoS MIMO communication in this work exploits the spatial multiplexing from the structured phase couplings of a single path direction, while most of the state-of-the-art works in mmWave communication focus on the spatial multiplexing from the spatial signature of multiple path directions. Challenges: The performance of a LoS MIMO system is highly dependent on the antenna topology. Topologies resulting in theoretically orthogonal channels are considered as optimal arrangements. The general topology solution from a unified viewpoint is unknown. The known optimal arrangements in the literature are rather independently derived and contain restrictions on their array planes. Moreover, operating at mmWave frequencies with wideband signals introduces additional challenges. On one hand, high pathloss is one limiting factor of the received signal power. On the other hand, high symbol rates and relatively high antenna numbers create challenges in signal processing, especially the required complexity for compensating hardware imperfections and applying beamforming. Targets: In this thesis, we focus on antenna topologies and signal processing schemes to effectively handle the complexity challenge in LoS MIMO communications. Considering the antenna topology, we target a general solution of optimal arrangements on any arbitrarily curved surface. Moreover, we study the antenna topologies with which the system gains more streams and better received signals. Considering the signal processing, we look for low complexity schemes that can effectively compensate the hardware impairments and can cope with a large number of antennas. Main Contributions: The following models and algorithms are developed for understanding mmWave LoS spatial multiplexing and turning it into practice. First, after analyzing the relation between the phase couplings and the antenna positions in three dimensional space, we derive a channel factorization model for LoS MIMO communication. Based on this, we provide a general topology solution from a projection point of view and show that the resulting spatial multiplexing is robust against moderate displacement errors. In addition, we propose a multi-subarray LoS MIMO system for jointly harvesting the spatial multiplexing and array gains. Then, we propose a novel algorithm for LoS MIMO channel equalization, which is carried out in the reverse order w.r.t. the channel factorization model. The number of multiplications in both digital and analog implementations of the proposed solution is found to increase approximately linearly w.r.t. the number of antennas. The proposed algorithm thus potentially reduces complexity for equalizing the channel during the system expansion with more streams. After this, we focus on algorithms that can effectively estimate and compensate the hardware impairments. A systolic/pipelined processing architecture is proposed in this work to achieve a balance between computational complexity and performance. The proposed architecture is a viable approach that scales well with the number of MIMO streams. With the recorded data from a hardware-in-the-loop demonstrator, it is shown that the proposed algorithms can provide reliable signal estimates at a relatively low complexity level. Finally, a channel model is derived for mmWave systems with multiple widely spaced subarrays and multiple paths. The spatial multiplexing gain from the spatial signature of multiple path directions and the spatial multiplexing gain from the structured phase couplings of a single path direction are found simultaneously at two different levels of the antenna arrangements. Attempting to exploit them jointly, we propose to use an advanced hybrid analog/digital beamforming architecture to efficiently process the signals at reasonable costs and complexity. The proposed system can overcome the low rank property caused by the limited number of propagation paths.

La croissance phénoménale du trafic pousse les opérateurs mobiles à différencier leurs plans de tarification en se basant sur la bande passante consommée. Afin de maximiser la monétisation du trafic de données, les opérateurs devront envisager des approches plus intelligentes tout en améliorant leurs réseaux actuels ou en déployant de nouvelles infrastructures. Les Small Cells sont une partie intégrante des réseaux cellulaires matures 3G/4G et futurs 5G. Les Small Cells peuvent être de facto déployées dans des architectures hétérogènes pour la densification des réseaux macrocellulaires, ou de façon homogène pour une couverture en haut débit. Pour le deuxième cas de déploiement, de nouveaux défis doivent être résolus: un réseau de collecte fiable et économique est vital pour les déploiements des Small Cells. Le réseau de collecte est spécifiquement plus contraignant pour les déploiements des Small Cells dans les zones dites green-field, où les infrastructures de transport sont absentes ou présentes mais ne peuvent être contrôlées par l'opérateur. En d'autres termes, l'opérateur mobile souhaite garantir une bonne qualité d'accès aux services haut débit en se basant uniquement sur des Small Cells, tout en réduisant le coût global de l'installation. Dans cette thèse, nous nous focalisons sur des solutions de réseau de collecte rentables qui peuvent fournir les capacités minimales requises par les utilisateurs finaux. Notre première contribution vise à assurer une capacité suffisante aux réseaux Small Cells 4G. Tout d'abord, nous proposons une méthode rentable qui minimise les coûts du réseau de collecte tout en respectant les contraintes de : 1) demande de trafic dans le réseau d'accès, et de 2) caractéristiques technologiques des liens de collecte. Cette méthode permet d'obtenir des solutions sur mesure de réseau de collecte à coûts optimal pour un réseau d'accès donné, basé sur des Small Cells; ces solutions sont constituées de différentes technologies de liaison. Deuxièmement, nous analysons l'impact de l'activité des utilisateurs finaux sur le trafic généré à la fois sur les deux interfaces logiques S1 et X2 d'une Small Cell, tout en tenant compte les différentes composantes de trafic moyen d'un utilisateur final. Cette analyse permet d'avoir un aperçu très utile pour la sélection des solutions nécessaires au réseau de collecte. Dans notre deuxième contribution, nous nous focalisons sur l'amélioration des capacités des systèmes WLAN. Nous concevons un protocole d'ordonnancement MAC pour les transmissions uplink multi-utilisateurs : il permet un échange minimal des trames de contrôle requises pour la mise en place des transmissions entre les multiples émetteurs et le récepteur. Les résultats d'analyse et de simulations révèlent des performances améliorées, d'un point de vue du système et de l'utilisateur. / The recent phenomenal traffic growth is driving mobile operators to tier their pricing plans based on consumed bandwidth. To maximize data traffic monetization, operators will need to consider smarter approaches while upgrading their current networks or deploying new ones. Small Cells are an integral part of both mature 3G/4G and future 5G cellular networks. Small Cells may be de facto deployed in heterogeneous architectures for Macro cells densification, or homogeneously for minimum broadband coverage. In this respect, emerging challenges must be tackled: a reliable and economical backhaul is vital for Small Cells deployments. It is specifically more constraining for Small Cells deployments in green-field areas, where transport infrastructure are absent or non-owned. In other words, the mobile operator wants to ensure good quality access to broadband services based only on Small Cells, while reducing overall installation cost. In this thesis, we focus on cost-efficient backhaul solutions that may provide the minimum capacities required by end users. Our first contribution targets the provisioning of 4G Small Cells networks with sufficient capacity. Firstly, we provide a cost-efficient method that minimizes backhaul cost while respecting the constraints of access network traffic demand and connecting technologies characteristics. This method provides with customized cost-optimal backhaul solutions for a given Small Cells access network; those solutions are made up of different linking technologies. Secondly, we analyze the impact of end users activity -i.e. data exchange- on generated traffic on both a Small Cell logical interfaces S1 and X2; by taking into account different traffic components of an end user device. The analysis supplies with valuable insights on selecting the needed backhaul solutions. In our second contribution, we focus on improving capacity in WLAN systems. We design a MAC scheduling scheme for uplink multi-users transmissions: it enables to exchange minimal control frames required for the establishment of transmissions between the multiple transmitters and the receiver. Both analytic results and conducted proof-of-concept simulations show improved efficiency for both system and user oriented performances.

Small cells are deployed in a crowded areas with a high demand for both coverage and capacity. It is hard to address both of these requirements simultaneous with a conventional mobile network architecture based on macro cells. In many case a wire is not available to connect the small cell to the core of the mobile network. Under these circumstances a wireless link could be a convenient solution for the backhaul. In this master’s thesis IEEE 802.11n technology was evaluated to assess its suitability for backhaul from a small wireless cell. The performance of wireless equipment manufactured by several vendors has been measured. The results of these measurements were analyzed and compared to a set of requirements established for small cell backhaul. The analysis has affirmed that IEEE 802.11n is capable of providing sufficient performance to be used for small cell backhaul in various deployment scenarios. Note that in this thesis we include femtocells, picocells, wireless LAN access points, and other technologies in the category of "small cells". Another research questions of this master’s thesis is security of small cell backhaul. In addition to protecting the backhaul link itself, the security research investigated the safety of the whole mobile network architecture remodeled with the introduction of small cells. A mechanism to integrate secure small cells into a mobile network was developed. The results obtained during the project will be used as an input for product development activities in the company hosting the project. The resulting product could become the target of future wireless system performance measurements. / Små celler sätts ut i områden med höga krav på täckning och kapacitet. Det är svårt att adressera båda dessa krav samtidigt med en konventionell mobil nätverksarkitektur baserad på makro-celler. I många fall finns ingen kabel tillgänglig att koppla den lilla cellen till kärnan i det mobila nätverket. Under dessa omständigheter kan en trådlös länk vara en lämplig lösning för backhaul. I denna avhandling utvärderas IEEE 802.11n-teknikens lämplighet för backhaul av små celler. Prestandan hos trådlös utrustning tillverkad av flera olika tillverkare har mätts. Resultaten av dessa mätningar analyserades och jämfördes med en mängd krav uppsatta för backhaul av små celler. Analysen har förankrat att IEEE 802.11n är kapabel till att tillhandahålla tillräcklig prestanda för backhaul av små celler i diverse miljöer. Notera att i denna avhandling så inkluderas femto-celler, pico-celler, Wireless LAN-åtkomstpunkter, och andra teknologier i kategorin små celler". Andra forskningsfrågor berörda i avhandlingen är säkerhet vid backhaul av små celler. Utöver att skydda backhaul-länken själv så undersökte säkerhetsforskningen säkerheten av hela mobilnätsarkitekturen när små celler används i arkitekturen. En mekanism för att integrera säkra små celler i ett mobilnät utvecklades. De resultat som införskaffades under projektets genomförande kommer att användas som input till produktutvecklingsaktiviteter hos företaget som sponsrade projektet. Den resulterande produkten skulle kunna bli mål för framtida prestandamätningar av trådlösa system.

With the increasing maturity of unmanned aerial vehicles (UAVs), also known as drones, wireless ecosystem is experiencing an unprecedented paradigm shift. These aerial platforms are specifically appealing for a variety of applications due to their rapid and flexible deployment, cost-effectiveness, and high chance of forming line-of-sight (LoS) links to the ground nodes. As with any new technology, the benefits of incorporating UAVs in existing cellular networks cannot be characterized without completely exploring the underlying trade space. This requires a detailed system-level analysis of drone cellular networks by taking the unique features of UAVs into account, which is the main objective of this dissertation. We first focus on a static setup and characterize the performance of a three-dimensional (3D) two-hop cellular network in which terrestrial base stations (BSs) coexist with UAVs to serve a set of ground user equipment (UE). In particular, a UE connects either directly to its serving terrestrial BS by an access link or connects first to its serving UAV which is then wirelessly backhauled to a terrestrial BS (joint access and backhaul). We consider realistic antenna radiation patterns for both BSs and UAVs using practical models developed by the third generation partnership project (3GPP). We assume a probabilistic channel model for the air-to-ground transmission, which incorporates both LoS and non-LoS links. Assuming the max-power association policy, we study the performance of the network in both amplify-and-forward (AF) and decode-and-forward (DF) relaying protocols. Using tools from stochastic geometry, we analyze the joint distribution of distance and zenith angle of the closest (and serving) UAV to the origin in a 3D setting. Further, we identify and extensively study key mathematical constructs as the building blocks of characterizing the received signal-to-interference-plus-noise ratio (SINR) distribution. Using these results, we obtain exact mathematical expressions for the coverage probability in both AF and DF relaying protocols. Furthermore, considering the fact that backhaul links could be quite weak because of the downtilted antennas at the BSs, we propose and analyze the addition of a directional uptilted antenna at the BS that is solely used for backhaul purposes. The superiority of having directional antennas with wirelessly backhauled UAVs is further demonstrated via extensive simulations. Second, we turn our attention to a mobile setup and characterize the performance of several canonical mobility models in a drone cellular network in which UAV base stations serve UEs on the ground. In particular, we consider the following four mobility models: (i) straight line (SL), (ii) random stop (RS), (iii) random walk (RW), and (iv) random waypoint (RWP), among which the SL mobility model is inspired by the simulation models used by the 3GPP for the placement and trajectory of UAVs, while the other three are well-known canonical models (or their variants) that offer a useful balance between realism and tractability. Assuming the nearest-neighbor association policy, we consider two service models for the UEs: (i) UE independent model (UIM), and (ii) UE dependent model (UDM). While the serving UAV follows the same mobility model as the other UAVs in the UIM, it is assumed to fly towards the UE of interest in the UDM and hover above its location after reaching there. We then present a unified approach to characterize the point process of UAVs for all the mobility and service models. Using this, we provide exact mathematical expressions for the average received rate and the session rate as seen by the typical UE. Further, using tools from the calculus of variations, we concretely demonstrate that the simple SL mobility model provides a lower bound on the performance of other general mobility models (including the ones in which UAVs follow curved trajectories) as long as the movement of each UAV in these models is independent and identically distributed (i.i.d.). Continuing our analysis on mobile setups, we analyze the handover probability in a drone cellular network, where the initial positions of the UAVs serving the ground UEs are modeled by a homogeneous Poisson point process (PPP). Inspired by the mobility model considered in the 3GPP studies, we assume that all the UAVs follow the SL mobility model, i.e., move along straight lines in random directions. We further consider two different scenarios for the UAV speeds: (i) same speed model (SSM), and (ii) different speed model (DSM). Assuming nearest-neighbor association policy, we characterize the handover probability of this network for both mobility scenarios. For the SSM, we compute the exact handover probability by establishing equivalence with a single-tier terrestrial cellular network, in which the BSs are static while the UEs are mobile. We then derive a lower bound for the handover probability in the DSM by characterizing the evolution of the spatial distribution of the UAVs over time. After performing these system-level analyses on UAV networks, we focus our attention on the air-to-ground wireless channel and attempt to understand its unique features. For that, we first study the impact of UAV wobbling on the coherence time of the wireless channel between UAVs and a ground UE, using a Rician multi-path channel model. We consider two different scenarios for the number of UAVs: (i) single UAV scenario (SUS), and (ii) multiple UAV scenario (MUS). For each scenario, we model UAV wobbling by two random processes, i.e., the Wiener and sinusoidal processes, and characterize the channel autocorrelation function (ACF) which is then used to derive the coherence time of the channel. For the MUS, we further show that the UAV-UE channels for different UAVs are uncorrelated from each other. One key observation that is revealed from our analysis is that even for small UAV wobbling, the coherence time of the channel may degrade quickly, which may make it difficult to track the channel and establish a reliable communication link. Finally, we develop an impairments-aware air-to-ground unified channel model that incorporates the effect of both wobbling and hardware impairments, where the former is caused by random physical fluctuations of UAVs, and the latter by intrinsic radio frequency (RF) nonidealities at both the transmitter and receiver, such as phase noise, in-phase/quadrature (I/Q) imbalance, and power amplifier (PA) nonlinearity. The impact of UAV wobbling is modeled by two stochastic processes, i.e., the canonical Wiener process and the more realistic sinusoidal process. On the other hand, the aggregate impact of all hardware impairments is modeled as two multiplicative and additive distortion noise processes, which is a well-accepted model. For the sake of generality, we consider both wide-sense stationary (WSS) and nonstationary processes for the distortion noises. We then rigorously characterize the ACF of the wireless channel, using which we provide a comprehensive analysis of four key channel-related metrics: (i) power delay profile (PDP), (ii) coherence time, (iii) coherence bandwidth, and (iv) power spectral density (PSD) of the distortion-plus-noise process. Furthermore, we evaluate these metrics with reasonable UAV wobbling and hardware impairment models to obtain useful insights. Similar to our observation above, this work again demonstrates that the coherence time severely degrades at high frequencies even for small UAV wobbling, which renders air-to-ground channel estimation very difficult at these frequencies. / Doctor of Philosophy / With the increasing maturity of unmanned aerial vehicles (UAVs), also known as drones, wireless ecosystem is changing dramatically. Owing to their ease of deployment and high chance of forming direct line-of-sight (LoS) links with the other UAVs and ground users, they are very appealing for numerous wireless applications. As with any new technology, exploring the full extent of the benefits of UAVs requires careful exploration of the underlying trade space. Therefore, in this dissertation, our main focus is on the analysis of such aerial networks, their interplay with the current terrestrial networks, and the unique features of UAVs that make them different from conventional ground nodes. One important aspect of aerial communication systems is their integration into our current cellular networks. Clearly, the addition of these new aerial components has the potential of benefiting both the ground users (such as mobile users watching a concert who need cellular connectivity to share the moments) and the cellular base station (BS). Therefore, careful analysis of these ``aerial-terrestrial" networks is of utmost importance. In the first phase of this dissertation, we perform this analysis by interpreting the network as a combination of one-hop (from the BS to the user) and two-hop (from the BS to the UAV and then from the UAV to the UE) links. Since the locations of BSs, UAVs, and users are irregular in general, we use tools from stochastic geometry to carry out our analysis, which is a field of mathematics that studies random shapes and patterns. Also, because existing terrestrial BSs are primarily designed to serve the ``ground", we propose the addition of a separate set of antennas at the BS site that is solely used to serve the ``air", i.e., to communicate with the UAVs, and demonstrate the benefits of this additional infrastructure in detail. One of our assumptions in the first phase of this dissertation was that the considered network was static, i.e., the UAVs were hovering in the air and the BSs/users were also not moving. In the second phase, on the other hand, we explore the benefits and challenges of a mobile network of UAVs and characterize the performance of several canonical mobility models in a drone cellular network. In particular, one of the models that we studied extensively is the so-called straight line (SL) mobility model, which was inspired by the simulation models used by the third generation partnership project (3GPP) for the placement and trajectory of UAVs. Since the locations of UAVs could be assumed random in general, we use tools from stochastic geometry and present a unified approach to characterize the point process of UAVs, using which we obtained exact mathematical expressions for the average received rate (i.e., throughput) as seen by the users. Continuing our analysis on mobile setups and using the SL mobility model, we also analyze the handover probability in a drone cellular network, which is defined as the event when the serving UAV of a user changes. By establishing equivalence between our aerial setup with a terrestrial cellular network, we compute the exact handover probability in drone cellular networks. In the final phase of this dissertation, we focus our attention on the air-to-ground wireless channel and attempt to understand its unique features. For that, we propose an impairments-aware unified channel model for an air-to-ground wireless communication system and extensively analyze the link between a hovering UAV in the air and a static user on the ground. In particular, we consider two different types of impairments: (i) UAV wobbling, and (ii) hardware impairments, where the former is caused by random physical fluctuations, and the latter by intrinsic radio frequency (RF) nonidealities at both the transmitter and receiver. Using appropriate models for each type of impairment, we rigorously characterize the autocorrelation function (ACF) of the wireless channel, using which we provide a comprehensive analysis of key channel-related metrics, such as coherence time and coherence bandwidth. One key observation that is revealed from our analysis is that even for small UAV wobbling and low hardware impairment levels, the coherence time of the channel may degrade quickly at high frequencies, which could make it difficult to track the channel and establish a reliable communication link at these frequencies.