With advances in communications technologies, there is a constant need for higher data rates. One possible solution to overcome this need is to allocate additional bandwidth. However, due to spectrum scarcity this is no longer feasible. In addition, the results of spectrum measurement campaigns discovered the fact that the available spectrum is under-utilized. One of the most significant solutions to solve the under- utilization of radio-frequency (RF) spectrum is the cognitive radio (CR) concept. A valid mathematical model that can be applied for most practical scenarios and also captures the random fluctuations of the spectrum is necessary. This model provides a significant insight and also a better quantitative understanding of such systems and this is the topic of this dissertation. Compact mathematical formulations that describe the realistic spectrum usage would improve the recent theoretical work to a large extent. The data generated for such models, provide a mean for a more realistic evaluation of the performance of CR systems. However, measurement based models require a large amount of data and are subject to measurement errors. They are also likely to be subject to the measurement time, location, and methodology. In the first part of this dissertation, we introduce cognitive radio networks and their role on solving the problem of under-utilized spectrum. In the second part of this dissertation, we target the random variable which accounts for the fraction of available subcarriers for the secondary users in an OFDMA based CR system. The time and location dependency of the traffic is taken into account by a non-homogenous Poisson Point Process (PPP). In the third part, we propose a comprehensive statistical model for user activity, spectrum occupancy, and system throughput in the presence of mutual interference in an OFDMA-based CR network which accounts for the sensing procedure of spectrum sensor, spectrum demand-model and spatial density of primary users, system objective for user satisfaction which is to support as many users as possible, and environment-dependent conditions such as propagation path loss, shadowing, and channel fading. In the last part of this dissertation, unlike the second and the third parts that the modeling is theoretical and based on limiting assumptions, the spectrum usage modeling is based on real data collected from an extensive measurement.

Cognitive radio (CR) networks and heterogeneous cellular networks are promising approaches to satisfy the demand for higher data rates and better connectivity. A CR network increases the utilization of the radio spectrum by opportunistically using it. Heterogeneous networks provide high data rates and improved connectivity by spatially reusing the spectrum and by bringing the network closer to the user. Interference presents a critical challenge for reliable communication in these networks. Accurately modeling it is essential in ensuring a successful design and deployment of these networks. We first propose modeling the aggregate interference power at a primary receiver (PU-Rx) caused from transmissions by randomly located cognitive users (CUs) in a CR network as a shifted lognormal random process. Its parameters are determined using a moment matching method. Extensive benchmarking shows that the proposed model is more accurate than the lognormal and Gaussian process models considered in the literature, even for a relatively dense deployment of CUs. It also compares favorably with the asymptotically exact stable and symmetric truncated stable distribution models, except at high CU densities. Our model accounts for the effect of imperfect spectrum sensing, interweave and underlay modes of CR operation, and path-loss, time-correlated shad-owing and fading of the various links in the network. It leads to new expressions for the probability distribution function, level crossing rate (LCR), and average exceedance duration (AED). The impact of cooperative spectrum sensing is also characterized. We also apply and validate the proposed model by using it to redesign the primary exclusive zone to account for the time-varying nature of interference. Next we model the uplink inter-cell aggregate interference power in homogeneous and heterogeneous cellular systems as a simpler lognormal random variable. We develop a new moment generating function (MGF) matching method to determine the lognormal’s parameters. Our model accounts for the transmit power control, peak transmit power constraint, small scale fading and large scale shadowing, and randomness in the number of interfering mobile stations and their locations. In heterogeneous net-works, the random nature of the number and locations of low power base stations is also accounted for. The accuracy of the proposed model is verified for both small and large values of interference. While not perfect, it is more accurate than the conventional Gaussian and moment-matching-based lognormal and Gamma distribution models. It is also performs better than the symmetric-truncated stable and stable distribution models, except at higher user density.

Les réseaux de télécommunications sont soumis à des processus d'amélioration et d'optimisation continue. Chaque nouvelle itération apporte son lot de défis et limites. En effet, la croissance exponentielle des appareils de télécommunication, des stations de base aux équipements utilisateurs conduisent à de sérieux problèmes d'economie d'énergie. En plus des menaces à la vie privée, en particulier pour les réseaux sans fil car les canaux utilisés par les opérateurs peuvent également être utilisés par une oreille indiscrète quelconque. L'optimisation de l'utilisation du spectre est également un défi en raison du fait que le spectre disponible dans les systèmes de communication sans fil est devenu une ressource très rare en raison de la demande croissante. Les réseaux émergents, tels que les femtocells, souffrent également des défis mentionnés précédemment. Le travail de thèse actuel se concentre sur la proposition de solutions aux défis cités précédemment: l'efficacité énergétique, le partage du spectre et la sécurité. Le travail de recherche présenté dans cette thèse a porté sur trois axes principaux: Premièrement, trouver un moyen de réduire au minimum la consommation d'énergie des femtocellules dans les reseaux BWA femto/macro-cellulaire en diminuant le nombre d'événements de mobilité non désirées et l'introduction de nouveaux états de puissance pour la femtocellule. En second lieu, proposer une solution qui vise à réduire le temps de transmission prévu dans le temps de séjour de l'utilisateur secondaire (SU) dans la couverture d'une femtocellule en utilisant un algorithme basé sur le temps minimum prévu de transmission dans le temps de séjour de l'équipement utilisateur (UE). Enfin, introduire un nouveau modèle qui basé sur la sélection du meilleur relais qui maximise le taux de confidentialité et les avantages de l'augmentation du nombre de relais sous la contrainte de qualité de service à la destination. / Telecommunications networks are subject to continuous improvement and enhancement processes. Every new iteration brings its set of challenges and limitations. In fact, the exponential growth in telecommunication devices, from base stations to user equipments lead to serious energy efficiency issues. Along with the privacy threats, especially for wireless networks as the channels used by operators can also be used by any eavesdropper. Spectrum usage optimization is also a challenge due to the fact that the available spectrum in wireless communications systems has been a very rare resource because of the increasing demand. Emerging networks, such as femtocells, suffer also from the previously mentioned challenges. The current thesis work focuses on proposing several solutions to the previously cited challenges: energy efficiency, spectrum sharing and security. The research work introduced in this thesis has focused on three main axes: First, find a way to minimize the energy consumption of femtocells in macro/femto-cellular BWA networks by decreasing the number of unwanted mobility events and introducing new power states for the femtocell device. Second, propose a solution that aims to reduce the expected transmission time within the dwell time of Secondary User (SU) in the coverage of a femtocell using an algorithm based on the minimum expected transmission time within the dwell time of the User Equipment (UE) in the coverage of the femtocell. Finally, introduce a new scheme that is based on best relay selection method that maximizes the secrecy rate and benefits from increasing the number of relays under QoS constraint at the destination.

This thesis addresses the problem of building a Cognitive Radio that has the ability to interact with human users in a better way by making use of Quality of Experience (QoE) as its basis and marshalling its resources optimally around the user. Salient activities of this thesis include: Analysis of CR leads to the definition of its basic functional blocks such as cognition, learning and adaptation of radio behaviour in a multi-disciplinary manner. CR tracts signal processing for radio and sensors, cognitive and behavioural psychology for user intelligence, machine learning and AI for decision systems and optimization etc. Therefore it provides a rich, fertile area to make lateral connections across diverse helds. This thesis proposes a broad definition for CR (called as Unifed Cognitive Radio) inspired by key foundation works described in literature. Besides, it also describes its functionality and its ecosystem. Taking cue from the definition of UCR, this thesis proposes architectural frame-works for various sub-systems. Also their design and implementation is achieved with the aid of a comprehensive tested setup and is tested using realistic scenarios. Builds a set of intelligent decision systems that help to achieve the set goal. This involves various design decisions with a set of diverse algorithms from the world of signal processing, machine learning and articial intelligence. Transitioning disparate small functional entities (mostly built around experiments) into an integrated system that works in real-world environment is the key aspect of this thesis. It is definitely a challenging task. Therefore, starting from deterring the architectural reference frameworks for realizing various sub-systems of UCR to an evaluation based on integrated scenario, this being an important final step constitutes a sign cant amount of work. Analysis and implementation of the integrated system to meet the desired end functionality - QoE centricity of the CR system to satisfy the needs of the end user better is the contribution of this thesis

Abstract During the last couple of decades a lot of research efforts have been spent on developing different spectrum sharing concepts. As the traditional regulatory methods for spectrum allocation are proving inadequate in responding to a growing need for mobile spectrum in a timely manner and finding spectrum for exclusive use is getting increasingly difficult, the political atmosphere is also becoming more and more receptive to new innovative spectrum sharing concepts that increase the efficiency of spectrum use. These concepts also provide regulatory authorities an opportunity to fundamentally change the current major operator driven mobile market and thereby to allow new players and innovative services to surface. However, there is still a gap between the work done by the research community and the work of the regulatory authorities. In this thesis, the aim is to clarify the reasons behind this gap by analysing three prevailing regulatory spectrum sharing concepts: Licensed Shared Access, the three-tier model and TV white space concept. As different stakeholders involved in spectrum sharing – the incumbent user, the entrant user and the regulatory authority – have very diverse roles in spectrum sharing, their incentives and key criteria may vary significantly. In order for a spectrum sharing concept to have a chance in a real life deployment, all these perspectives need to be carefully considered. In fact, a feasible spectrum sharing concept is a delicate balance between the viewpoints of different stakeholders, not necessarily the one offering the most efficient spectrum utilization. This thesis analyses spectrum sharing concepts from all these perspectives and as a consequence unveils the common process model for implementing a spectrum sharing concept in real life, highlighting the distinct roles of different stakeholders in its phases. / Tiivistelmä Parin viimeisen vuosikymmenen aikana tutkimusyhteisö on kehittänyt valtavasti eri käsitteitä ja tuloksia taajuuksien yhteiskäyttöön. Matkapuhelinoperaattoreiden nopeasti kasvavan tiedonsiirto- ja taajuustarpeen myötä myös poliittinen ilmapiiri on muuttunut vastaanottavaisemmiksi uusille jaetuille taajuuksienkäyttömalleille, joilla voidaan sekä lisätä taajuuksien käytön tehokkuutta että mahdollistaa uusien toimijoiden ja innovatiivisten palvelujen kehitys. Taajuuksien yhteiskäyttömalleihin liittyvä tutkimustyö ei kuitenkaan usein palvele suoraan taajuusviranomaisten tarpeita. Tämän työn tavoitteena on selvittää syitä tähän kolmen vallitsevan joustavan taajuuksienkäyttökonseptin avulla: lisensoidun taajuuksien yhteiskäyttömallin, kolmitasoisen taajuuksien yhteiskäyttömallin ja TV kaistojen yhteiskäyttömallin. Eri toimijoilla on hyvin erilaiset roolit taajuuksien yhteiskäytössä ja siksi sekä heidän kannustimensa että keskeiset kriteerit voivat olla hyvin erilaiset. Käytäntöön sopiva jaetun taajuudenkäytön malli onkin usein kompromissi eri näkemysten omaavien toimijoiden kesken, ei välttämättä taajuuksien käytöltään tehokkain vaihtoehto. Tässä työssä analysoidaan eri taajuuksien yhteiskäyttömalleja sekä eri toimijoiden näkökulmia. Analyysin tuloksena luodaan yleinen prosessimalli taajuuksien yhteiskäyttömallien toteutukselle sekä identifioidaan eri toimijoiden roolit sen eri vaiheissa.

To explore spectral opportunities in wideband regime for cognitive radio we need a wideband spectrum sensing receiver. Current wideband receiver architectures need wideband analog to digital converter (ADC) to sample wideband signal. As current state-of-art ADC has limitation in terms of power and sampling rate, we need to explore some alternative solutions. Compressive sampling (CS) data acquisition method is one of the solutions. Cognitive Radio signal, which is sparse in frequency domain can be sampled at Sub-Nyquist rate using low rate ADC. To relax the receiver complexity in terms of performance requirement we can use Modulated Wideband Converter (MWC) architecture, a Sub-Nyquist sampling method. In this thesis circuit design of this architecture covers signal within a frequency range of 500 MHz to 2.1 GHz, with a channel bandwidth of 1600 MHz. By using 8 parallel lines with channel trading factor of 11, effective sampling rate of 550 MHz is achieved for successful support recovery of multi-band input signal of size N=12.