In this dissertation, the performance of cooperative cognitive systems with spectrum sharing is investigated. A low-complexity and high performance node selection strategy is proposed for two different of cooperative cognitive systems models. In the first model, the secondary network is composed by one source node that communicates with one among L destinations through a direct link and also assisted by one among N AF or DF relays nodes. The selected secondary destination employs a selection combining technique for choosing the best link (direct or dual-hop link) from the secondary source. Considering an underlay spectrum sharing approach, the secondary communication is performed taking into account an interference constraint, where the overall transmit power is limited by the interference at the primary receiver as well as by the maximum transmission power available at the respective nodes. An asymptotic analysis is carried out, revealing that the diversity order of the considered system is not affected by the interference, and equals to L + N. In the second model, by its turn, the secondary network is composed by one source, N AF or DF relays, and one destination. However, it is assumed the presence of M primary receivers. A relay selection strategy is proposed with the aim of maximing the end-to-end signal-to-noise ratio and, at the same time, to satisfy the interference constraints imposed by these primary receivers. After the relay selection procedure is performed, the secondary destination chooses the best path (direct link or relaying link) by employing a selection combining scheme. An asymptotic analysis is carried out, revealing that the system diversity order equals to N + 1, and showing that it is not affected neither by the number of primary receivers nor by the interference threshold. A close-form expression and an approximation for the outage probability is derived for the DF and AF protocols, respectively. / Nesta dissertaÃÃo, o desempenho de sistemas cooperativos cognitivos com compartilhamento espectral à investigado. Uma estratÃgia de seleÃÃo de nÃs de baixa complexidade e alto desempenho à proposta para dois modelos distintos de redes cooperativas cognitivas. No primeiro modelo, a rede secundÃria à composta por um nà fonte que comunica-se com um dentre L nÃs destinos atravÃs de um link direto e atravÃs de um dentre N nÃs relays decodifica-e-encaminha (DF) ou amplifica-e-encaminha (AF). O nà destino secundÃrio selecionado emprega uma tÃcnica de combinaÃÃo por seleÃÃo para selecionar o melhor link (direto ou auxiliar) a partir da fonte secundÃria. Considerando um ambiente com compartilhamento espectral, tem-se que a comunicaÃÃo secundÃria à realizada levando em consideraÃÃo uma restriÃÃo de interferÃncia, na qual a potÃncia de transmissÃo à governada pela interferÃncia no receptor primÃrio bem como pela mÃxima potÃncia de transmissÃo dos respectivos nÃs secundÃrios. Uma anÃlise assintÃtica à realizada, revelando que a ordem de diversidade do sistema nÃo à afetada pela interferÃncia, sendo igual a L + N. Jà no segundo modelo, a rede secundÃria à composta por uma fonte, N relays DF ou AF e um nà destino, no entanto assume-se a presenÃa de M receptores primÃrios. A seleÃÃo do relay deve satisfazer as restriÃÃes de interferÃncia impostas por estes Ãltimos. ApÃs a seleÃÃo de relay ser realizada, o nà destino seleciona o melhor caminho (link direto ou link via relay) proveniente da fonte utilizando um combinador por seleÃÃo. Uma anÃlise assintÃtica à realizada, revelando que a ordem de diversidade do esquema proposto iguala a N + 1, o que mostra que a mesma nÃo à afetada nem pelo nÃmero de receptores primÃrios nem pelo limiar de interferÃncia. Uma expressÃo em forma fechada para a probabilidade de outage à obtida para ambos protocolos cooperativos. SimulaÃÃes Monte Carlo sÃo apresentadas com o intuito de validar as anÃlises propostas.

Abstract High radio capacity is one of the main targets in wireless network planning. The characteristics of the broadband radio channel pose serious challenges for achieving this goal. This thesis views the capacity problem from two frameworks. In the first, the effective user capacity in advanced direct sequence wideband code-division multiple-access (DS-CDMA) radio networks is evaluated. Sensitivity to various imperfections in key system parameters is studied. The analysis is based on a mathematical foundation that presents complex signal models and enables evaluation of the performance losses due to parameter estimation errors and multipath fading. The effective number of users supported in a cell is restricted by the multiple access interference (MAI) in the same cell (intracell interference) and overall background noise. The studied wideband CDMA receiver structures comprise conventional rake receivers with both the maximal ratio combining (MRC) and equal gain combining (EGC) schemes that can be supplemented with either linear decorrelating or nonlinear successive cancellation-based multiuser detectors and M-antenna spatial diversity. The second framework focuses on direct sequence spread spectrum-based ultra wideband (UWB) indoor communications. Cochannel interference limited capacity is evaluated against the outage probability criterion in exponentially decaying lognormal multipath fading channels. Distance-dependence and spatial distribution of users is taken into account at different spatial cell configurations. Only moderate complexity partial rake receivers with noncoherent combining are employed. Total interference is composed of interpath, multipath, intracell, and intercell interference contributions. Lognormal sum approximations and simulations are used to evaluate distributions of the desired and interfering signals. The impact of the timing errors at the receiver monopulse correlation is studied. The numerical results for the wideband CDMA framework show that effective user capacity and sensitivity depend critically on the joint impact of nonidealities in system parameters (e.g., channel profile, severity of fading, receiver algorithms). User capacities of the studied multiuser enhanced receivers were more prone to these impairments than those of the simpler single user receivers. The results should be used for network planning and optimization. The numerical results of the UWB framework suggest that, even in the multipath rich channel, the optimal number of rake fingers can be less than half of the significant multipaths. Differences between circular, square, and hexagonal cell models proved to be minor with respect to link distance distributions. The derived link distance statistics are useful tools in the analytic piconet dimensioning and optimization.

Information security has been a very important issue in wireless networks. With the ever-increasing amount of data being exchanged over wireless networks, the confidentiality of information needs to be protected from unauthorized users called eavesdropper. Due to the broadcast nature of the wireless medium, the transmissions between legitimate users maybe overheard and intercepted by the unauthorized parties, which makes wireless transmission vulnerable to potential eavesdropping attacks. The security of wireless communications plays an increasingly important role in the cybercrime defense against unauthorized activities.     We consider the wireless physical layer security which has been explored for the sake of enhancing the protection of wireless communications against eavesdroppers. We consider the problem of secret communication through Rayleigh fading channel in the presence of an eavesdropper in which the transmitter knows the channel state information of both the main and eavesdropper channel. Then, we analyze the average capacity of the main channel and eavesdropper channel from which an expression of secrecy capacity is derived based on the cumulative distribution function and probability density function of the signal to noise ratio. We also analyze an expression for the symbol error rate of the main channel, and the outage probability is obtained for the main transmission. Finally, we perform the numerical results in MATLAB.

The aggregate interference distribution in cognitive radio networks is studied in a rigorous and analytical way using the popular Poisson point process model. While a number of results are available for this model for non-cognitive radio networks, cognitive radio networks present extra levels of difficulties for the analysis, mainly due to the exclusion region around the primary receiver, which are typically addressed via various ad-hoc approximations (e.g., based on the interference cumulants) or via the large-deviation analysis. Unlike the previous studies, we do not use here ad-hoc approximations but rather obtain the asymptotic interference distribution in a systematic and rigorous way, which also has a guaranteed level of accuracy at the distribution tail. This is in contrast to the large deviation analysis, which provides only the (exponential) order of scaling but not the outage probability itself. Unlike the cumulant-based analysis, our approach provides a guaranteed level of accuracy at the distribution tail. Additionally, our analysis provides a number of novel insights. In particular, we demonstrate that there is a critical transition point below which the outage probability decays only polynomially but above which it decays super-exponentially. This provides a solid analytical foundation to the earlier empirical observations in the literature and also reveals what are the typical ways outage events occur in different regimes. The analysis is further extended to include interference cancelation and fading (from a broad class of distributions). The outage probability is shown to scale down exponentially in the number of canceled nearest interferers in the below-critical region and does not change significantly in the above-critical one. The proposed asymptotic expressions are shown to be accurate in the non-asymptotic regimes as well.

The ever-growing demands for wireless technologies necessitate the evolution of next generation wireless networks that fulfill the diverse wireless users requirements. However, upscaling existing wireless networks implies upscaling an intrinsic component in the wireless domain; the aggregate network interference. Being the main performance limiting factor, it becomes crucial to develop a rigorous analytical framework to accurately characterize the out-of-cell interference, to reap the benefits of emerging networks. Due to the different network setups and key performance indicators, it is essential to conduct a comprehensive study that unifies the various network configurations together with the different tangible performance metrics. In that regard, the focus of this thesis is to present a unified mathematical paradigm, based on Stochastic Geometry, for large-scale networks with different antenna/network configurations. By exploiting such a unified study, we propose an efficient automated network design strategy to satisfy the desired network objectives. First, this thesis studies the exact aggregate network interference characterization, by accounting for each of the interferers signals in the large-scale network. Second, we show that the information about the interferers symbols can be approximated via the Gaussian signaling approach. The developed mathematical model presents twofold analysis unification for uplink and downlink cellular networks literature. It aligns the tangible decoding error probability analysis with the abstract outage probability and ergodic rate analysis. Furthermore, it unifies the analysis for different antenna configurations, i.e., various multiple-input multiple-output (MIMO) systems. Accordingly, we propose a novel reliable network design strategy that is capable of appropriately adjusting the network parameters to meet desired design criteria. In addition, we discuss the diversity-multiplexing tradeoffs imposed by differently favored MIMO schemes, describe the relation between the diverse network parameters and configurations, and study the impact of temporal interference correlation on the performance of large-scale networks. Finally, we investigate some interference management techniques by exploiting the proposed framework. The proposed framework is compared to the exact analysis as well as intensive Monte Carlo simulations to demonstrate the model accuracy. The developed work casts a thorough inclusive study that is beneficial to deepen the understanding of the stochastic deployment of the next-generation large-scale wireless networks and predict their performance.

It has long been recognized that in multi-cell WCDMA networks the admission of a new session into the system can have undesirable impact on the neighboring cells. Although admission control algorithms that take into account such multi-cell impact have been studied in the past, little attention has been paid to multi-cell admission and rate control algorithms when traffic is elastic. In this thesis, we propose a model for multi-cell multi-service WCDMA networks to study the impact of multi-cell admission and rate control algorithms on key performance measures such as the class-wise blocking and outage probabilities, block error rates, and the noise rise violation probabilities. By means of simulation we compare the performance of load based multi-cell algorithms with that of a single cell algorithm. We find that with multi-cell based algorithms the system capacity and performance (in terms of the above mentioned measures) are (in some cases significantly) better in homogeneous load scenarios as well as in the heterogeneous ’hotspot’ and ’hotaround’ scenarios. / Det har länge varit känt att i multi-cellulära WCDMA nät så kan insläppandet av en ny användarei systemet ha en icke önskvärd effekt på intilliggande celler. Fastän insläppskontrollalgoritmer (AC)som tar hänsyn till sådana multi-cellulära effekter har studerats tidigare, så har endast begränsaduppmärksamhet ägnatsåt multi-cellulär insläpps- och bittaktskontrollalgoritmer när trafiken är elastisk.I detta arbete föreslår vi en modell för WCDMA-nät med multipla celler och multipla tjänster ochsom är applicerbar för studier av av hur multi-cellulär insläpps- och bittaktskontroll inverkar påviktiga prestandamått som klassvisa spärr- och utslagningssannolikheter, blockfelssannolikheter, ochsannolikheten för överträdande av tillåten interferensnivå. Med simuleringar jämför vi prestanda förlastbaserade multi-cellalgoritmer med prestanda för singel-cellalgoritmer. Vi har funnit att med multicellalgoritmerså är systemskapacitetet och prestanda (i termer av tidigare nämnda mått) i några fallbetydligt bättre i homogena lastscenarier, samt i heterogena lastscenarier av typerna ’hotspot’ och’hotround’.

The usage of wireless communication has increased over the past few years. Most wired communications are replaced by wireless communication for ease of use. Wireless communication transfers confidential information like personal information and credentials between two entities. We can't probably say that it is safe to send this information via wireless communication. As more data is sent, more attacks happen to steal the data. Hence, it is necessary to implement secure methods to transfer the data between source and receiver. In this communication channel, we use secrecy capacity as a parameter to measure how data is sent securely between source and destination. Generally, to achieve high system performance, the information is sent with low power, but this reduces the signal efficiency at the receiver. So, in this thesis, we have implemented cooperative transmission to increase the efficiency of low power signals by adding the relays between source and destination. This thesis consists of two relays. The relay that obtains the maximum signal-to-noise ratio is selected for the primary communication link. The other relay sends the signal to the eavesdropper to confuse the eavesdropper. In this thesis, we have derived the mathematical expression for SNR at receiver eavesdropper, and also we have derived a word for outage probability and secrecy capacity. Then, we simulated the Matlab code to obtain results on how the secrecy capacity affects by changing the various parameters like path loss exponent and fading severity parameter and suggests which environment is better to maintain high secrecy capacity. We also analysed the system performance and secrecy capacity in the presence of eavesdropper as well.

Fluid antenna systems (FAS) are an emerging technology that promises a signif icant diversity gain even in the smallest spaces. Motivated by the groundbreaking potentials of liquid antennas, researchers in the wireless communication community are investigating a novel antenna system where a single antenna can freely switch positions along a small linear space to pick the strongest received signal. However, the FAS positions do not necessarily follow the ever-existing rule separating them by at least half the radiation wavelength. Previous work in the literature param eterized the channels of the FAS ports simply enough to provide a single-integral expression of the probability of outage and various insights on the achievable perfor mance. Nevertheless, this channel model may not accurately capture the correlation between the ports, given by Jake’s model. This work builds on the state-of-the-art and accurately approximates the FAS channel while maintaining analytical tractabil ity. The approximation is performed in two stages. The first stage approximation considerably reduces the number of multi-fold integrals in the probability of outage expression, while the second stage approximation provides a single integral represen tation of the FAS probability of outage. Further, the performance of such innovative technology is investigated under a less-idealized correlation model. Numerical results validate our approximations of the FAS channel model and demonstrate a limited performance gain under realistic assumptions. Further, our work opens the door for future research to investigate scenarios in which the FAS provides a performance gain compared to the current multiple antennas solutions.

L’étude de dimensionnement d’un réseau cellulaire est une phase de conception qui doit permettre de déterminer les performances d’un système dans une configuration donnée. Elle inclut l’étude de couverture et l’analyse de trafic. De complexes simulations sont possibles pour connaître les paramètres de performances d’un réseau mais seules les études analytiques fournissent des résultats rapides. Par ailleurs, pour faire face à la demande de hauts débits, à la rareté du spectre fréquentiel et à l’impossibilité d’émettre à de plus fortes puissances, de nouvelles techniques de transmissions sont apparues. Nous sommes ainsi passés d’un système classique à une seule antenne à des systèmes à multiple antennes et même à des scénarios de coopération entre stations de base. Dans cette thèse, nous proposons des modèles analytiques pour l’étude des performances, notamment en termes de probabilités de coupure, de ces évolutions des réseaux cellulaires. Dans une première phase, nous considérons des systèmes multicellulaires à une antenne émettrice et une antenne réceptrice (SISO). Nous proposons deux méthodes d’étude de l’impact conjoint de l’affaiblissement de parcours, de l’effet de masque et des évanouissements rapides. Nous étudions, par la suite, un système à large bande utilisant le retournement temporel comme technique de transmission. Dans une deuxième phase, nous considérons des systèmes multicellulaires à antennes multiple à l’émission ou à la réception (MISO/MIMO) implémentant les schémas de diversité Alamouti et de combinaison par rapport maximal (MRC). Ensuite, nous considérons un système multicellulaire multi-utilisateurs à précodage de forçage à zéro (ZFBF). / The implementation of cellular systems have aroused issues related to the design of cellular networks termed to as network dimensioning. It includes the coverage estimation and thetraffic analysis. Simple models and methods are required to reduce the time consumption of these two analysis. At the same time, the growing demand for higher data rates constrained by the scarcity of frequency spectrum, and the requirements in terms of power consumption reduction make the telecommunication community think about new transmission techniques moving from the classical single antenna systems to multiple antenna systems and even the newly envisaged cooperative systems. In this thesis, we provide analytical models to assess the performance of these different cellular network evolutions in terms of outage probabilities. In a first study, we consider multicellular single input single output (SISO) systems. First, we propose two accurate methods to study the joint impact of path-loss, shadowing and fast fading. This system has so far been studied either considering the only impact of path-loss and Rayleigh fading, or considering the same channel model as in our case but providing very complex outage probability expressions. Then, we provide an outage probability expression in a wideband communication context implementing the Time Reversal (TR) transmission technique considering the impact of fast fading. In a second study, we focus on multiple antenna systems. We study the performance of a Multiple Input Multiple Output (MIMO) system implementing a transmit and a receivediversity schemes namely the Alamouti code and the Maximum Ratio Combining (MRC).

Multiple Input and Multiple Output (MIMO) is one of the techniques used tiimprove the system performance. In this thesis, we improve and evaluate the system performance of MIMO deploying MRT technique at the transmitter and MRCtechnique at the receiver with an assuming that the system can obtain perfect CSI.We derived expressions for outage probability and symbol error rate and simulationgraphs for SISO, SIMO, MISO, and MIMO in terms of outage probability. Our results show that the MIMO system performance is better than other configurations.We also observed that SISO have the least performance where SIMO and MISOhave same performance. The performance also increases as the number of antennasincrease in the system and it is suggested to use MIMO when performance is givehigh priority than cost.