Avec l'introduction des plate-formes CubeSat, le nombre de petits satellites lancés dans l'espace a grandi de manière importante pendant les deux dernières décennies.Étant développés initialement par des universités et des centres de recherche pour des simples tests technologiques ou des expériences académiques, ces plate-formes aujourd'hui permettent d'envisager de nouvelles applications et services.Dans cette thèse, nous nous intéressons à l'usage de petits satellites à défilement pour des réseaux globaux de collecte de données et, plus généralement, pour des applications de type machine-to-machine (M2M).En raison des contraintes existantes tant au segment sol comme au segment spatial, la capacité du canal de transmission est fortement limitée---notamment celle du lien montant, qui correspond à un canal à accès multiple.Ces réseaux sont aussi caractérisés par des très petits messages arrivant au système de manière imprévisible, ce qui implique que toute redondance liée au protocole a un impact important sur l’efficacité spectrale. Ainsi, des méthodes d'accès aléatoires sont souvent préférés pour le lien montant.Relever ces défis nécessite d'aborder l'optimisation de la transmission de manière holistique. Plus spécifiquement, la conception des couches physiques (PHY) et de contrôle d'accès au support (MAC, de l'anglais Media Access Control) doit être menée de manière conjointe.Les principales contributions de cette thèse portent sur l'étude du protocole Time-- and Frequency--Asynchronous ALOHA (TFAA), une technique d'accès aléatoire utilisée dans des réseaux terrestres à modulation de bande étroite. En réduisant significativement le débit binaire de transmission, TFAA permet notamment d'établir des liaisons à longue portée et/ou à faible consommation énergétique, dont des systèmes M2M par satellite sont un exemple.D'abord, nous évaluons les performances au niveau MAC (i.e., le taux d'utilisation de canal et la probabilité d'erreur de packet) sous trois différents modèles de réception: le modèle de collisions, le modèle de capture et un modèle plus détaillé qui prend en compte les paramètres de la couche PHY.À partir de ce dernier modèle, nous étudions ensuite l'impact de certains paramètres de la couche PHY sur les performances au niveau MAC.Afin d'améliorer la performance de TFAA, nous proposons Contention Resolution Time-- and Frequency--Asynchronous ALOHA (CR-TFAA), une solution plus sophistiquée intégrant des techniques de suppressions successives d'interférences.Enfin, nous étudions les bénéfices obtenus en exploitant le compromis <<performance--délai de bout-en-bout>> en utilisant des techniques simples telles qu'un système de contrôle de transmission et le codage au niveau packet. / With the introduction of the CubeSat standard, the number of small-satellite missions has increased dramatically over the last two decades.Initially developed by universities and research centres for technology validation and academic experiments, these low-cost platforms currently allow to perform a variety of advanced, novel applications.In this thesis we are interested in the use of small satellites for global data collection and, more generally, for Internet of Things (IoT) and machine-to-machine (M2M) applications.Since both the space and ground segments are subject to stringent constraints in terms of size and mass, the overall capacity of the communications channel is highly limited, specially that of the uplink, which is a multi-access channel.These systems are also characterised by bursty, short messages, meaning that any protocol overhead may have a significant impact on the bandwidth efficiency. Hence, a random access approach is usually adopted for the uplink.Facing these challenges requires to optimize the communication system by taking an holistic approach. In particular, a joint design of both the physical (PHY) and Medium Access Control (MAC) layers is needed.The main contributions of this thesis are related to the study of Time-- and Frequency--Asynchronous ALOHA (TFAA), a random access approach adopted in terrestrial ultra narrowband (UNB) networks. By trading data rate for communication range or transmission power, TFAA is particularly attractive in power constrained applications such as low power wide area networks and M2M over satellite. First, we evaluate its MAC performance (i.e., its throughput and packet error rate) under three different reception models: the collision channel, the capture channel and a more detailed model that takes into account the PHY layer design.Then, we study the impact of PHY layer parameters, such as forward error correction (FEC), pulse shaping filter and modulation order, on the MAC performance.We show that, due to the characteristics of the multiple access interference, significant improvements can be obtained by applying low-rate FEC.To further improve TFAA's performance, we propose Contention Resolution Time-- and Frequency--Asynchronous ALOHA (CR-TFAA), a more advanced design which is in line with recent developments such as Asynchronous Contention Resolution Diversity ALOHA (ACRDA).Under the same set of hypothesis, we see that CR-TFAA provides similar and even better performance than ACRDA, with a decrease in the packet error rate of at least one order of magnitude.Finally, we study the benefits that can be obtained by trading delay for MAC performance and energy efficiency, using simple techniques such as transmission control and packet-layer erasure coding.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Educação, Programa de Pós-Graduação em Educação, 2016. / Submitted by Nayara Silva (nayarasilva@bce.unb.br) on 2016-06-24T15:20:42Z No. of bitstreams: 1 2016_ThalitaAndressaBarbosaPaesLandim.pdf: 6970063 bytes, checksum: 419c1b84d963b43d18df2607d5f3e99f (MD5) / Approved for entry into archive by Raquel Viana(raquelviana@bce.unb.br) on 2016-07-07T21:38:03Z (GMT) No. of bitstreams: 1 2016_ThalitaAndressaBarbosaPaesLandim.pdf: 6970063 bytes, checksum: 419c1b84d963b43d18df2607d5f3e99f (MD5) / Made available in DSpace on 2016-07-07T21:38:03Z (GMT). No. of bitstreams: 1 2016_ThalitaAndressaBarbosaPaesLandim.pdf: 6970063 bytes, checksum: 419c1b84d963b43d18df2607d5f3e99f (MD5) / A presente dissertação buscou compreender como a formação inicial ofertada pela Universidade de Brasília influência o trabalho do professor em sala de aula com estudantes em inclusão. Deste modo ela tem como objetivo principal compreender como o Pedagogo egresso do curso de pedagogia da UnB desenvolve sua prática pedagógica em escolas inclusivas nos anos iniciais do ensino fundamental. E possui como objetivos específicos: analisar o Projeto Acadêmico do curso de Pedagogia da UnB para a formação inicial do Pedagogo; analisar os fundamentos teórico- metodológicos dos planos de ensino das disciplinas: “O Educando com Necessidades Educacionais Especiais” e “Aprendizagem e Desenvolvimento do PNEE” que delimitam a formação do Pedagogo que atuará nos anos iniciais do ensino fundamental inclusivo; analisar as práticas pedagógicas inclusivas desenvolvidas pelo professor em sala de aula, e analisar as percepções das crianças da escola inclusiva com relação a prática pedagógica do professor dos anos iniciais do ensino fundamental. Foi utilizado como metodologia a pesquisa qualitativa e os seus instrumentos como: análise documental, questionário, entrevista semiestruturada, observação de campo e grupo focal. Sendo este último realizado com os estudantes das escolas observadas. As escolas selecionadas estavam localizadas nas regiões administrativas da Ceilândia e Asa Norte. Os professores atuavam em turmas do quinto ano do Ensino Fundamental e ambos tinham em suas salas de aula estudantes em inclusão. Por intermédio da comparação das análises da legislação vigente sobre a Educação Especial e Inclusiva, análise da literatura acadêmica e do observado em sala de aula foi possível visualizar que existe uma falha entre a formação inicial e o vivenciado em sala de aula em classes inclusivas, ocasionando assim uma ruptura entre a teoria e a prática. As reflexões e sinalizações desta pesquisa busca contribuir para uma mudança na formação inicial, onde a teoria e a prática caminhem juntas. _______________________________________________________________________________________________ ABSTRACT / The present research aims to understand how the early formation offered by Brasília University (Universidade de Brasília – UnB) leading the teacher works in the classroom with special needs students. Thus, it aims to know how the new pedagogue professionals, graduated at UnB, develop your pedagogic practices in inclusive schools in the early years of the elementary school. In this context, the specifics subjects in this project are: the analysis of the Academic Project from the UnB Pedagogy course to the initial pedagogue formation; analyze the theoretical-methodological from the teaching projects in these subjects: “the student with special educational needs” (O Educando com Necessidades Educacionais Especiais) e “The PNEE knowledge and Development” (Aprendizagem e Desenvolvimento do PNEE) who delimiting the Pedagogue formation who will act in the early years of the inclusive elementary school; analyze the inclusive pedagogic practices developed by the teacher in the classroom, and analyze the perceptions of the students from the early years of the basic educational. It was used as methodology the qualitative survey and theirs instruments, like: documental analysis, questionnaires, half structured interviews, field observational and a focus group. Being the last one realized with the students from the observed schools. The selected schools were located in the administrative regions from Ceilândia and Asa Norte. The teachers worked in fifth grade class from the basic educational, and the both have in their classrooms special needs students. With the comparison between the analysis of the actual legislation about the Special and Inclusive Education, the academic literature analysis and the classroom observation was possible to visualized the existence of a failure between the beginning formation and what is experiencing in the inclusive special classes, promoting a rupture between theory and practice. The reflections and signs of this research seek to contribute for a change in the early formation, where the theory and practice can walk together.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Educação, Programa de Pós-Graduação em Educação, 2018. / Submitted by Raquel Viana (raquelviana@bce.unb.br) on 2018-07-17T20:18:15Z No. of bitstreams: 1 2018_LuanaChavesMartins​.pdf: 1364134 bytes, checksum: 3958e155a05f5b7e90fdfa8dc1f36712 (MD5) / Approved for entry into archive by Raquel Viana (raquelviana@bce.unb.br) on 2018-07-20T21:33:38Z (GMT) No. of bitstreams: 1 2018_LuanaChavesMartins​.pdf: 1364134 bytes, checksum: 3958e155a05f5b7e90fdfa8dc1f36712 (MD5) / Made available in DSpace on 2018-07-20T21:33:38Z (GMT). No. of bitstreams: 1 2018_LuanaChavesMartins​.pdf: 1364134 bytes, checksum: 3958e155a05f5b7e90fdfa8dc1f36712 (MD5) Previous issue date: 2018-07-17 / A pesquisa desenvolvida nesta dissertação de mestrado, a partir das articulações da Psicanálise com a educação, tem por objetivo principal investigar se a formação em nível superior em Pedagogia na Universidade de Brasília é capaz de subjetivar politicamente seus futuros pedagogos no intuito de minimizar a exclusão de crianças socioeconomicamente vulneráveis na educação básica quando estes se tornarem professores. Entre os objetivos específicos, destacamos a análise da efetivação das políticas públicas de acesso à educação superior, análise do percurso educativo de estudantes de Pedagogia da UnB em vulnerabilidade social no intuito de investigar se eles já sofreram discriminação por sua condição socioeconômica, investigar se eles pretendem atuar como professores na educação básica e identificar e analisar se ocorreram experiências na Universidade que operaram para que se transformassem em sujeitos implicados politicamente com o coletivo e não somente engajados em seus interesses individuais. Sob a perspectiva da abordagem qualitativa, foram escolhidos como procedimentos metodológicos a pesquisa documental acerca da quantidade de matrículas em cursos de educação superior nos últimos dezesseis anos e a entrevista semiestruturada com oito estudantes participantes da assistência estudantil. Dessas pesquisas surgiram cinco categorias de análises, a saber: Pedagogia, uma escolha (in) consciente; a origem que envergonha; o eu professor e o professor coletivo em formação; o profissional do futuro; e inclusão pela (des) igualdade. Para dar conta da discussão dos resultados, por meio da análise de conteúdo, foram escolhidas obras de Freud, bem como seus autores contemporâneos abordados no referencial teórico. Os resultados desta pesquisa indicam que a UnB propicia vivências que possibilitam uma formação auto participada que instigam seus futuros pedagogos a buscarem atuar de forma crítica e implicados politicamente em seu ambiente de trabalho. / The research developed in this master's thesis, based on the articulations between the Psychoanalysis and education, has as main objective to investigate if the formation in Pedagogy at the University of Brasilia is capable of politically subjecting its future pedagogues in order to minimize the exclusion of socioeconomically vulnerable children in basic education when they become teachers. Among the specific objectives, we highlight the analysis of the effectiveness of public policies for access to higher education, the analysis of the educational pathway of Pedagogy students from UnB in social vulnerability in order to investigate whether they have already suffered discrimination due to their socioeconomic status, to investigate if they intend to act as teachers in basic education and to identify and analyze if there were experiences in the University that operated to turn them into subjects politically involved with the collective and not only engaged in their individual interests. From the perspective of the qualitative approach, the documentary research on the number of enrollments in higher education courses in the last sixteen years and the semi-structured interview with eight students who participated in student assistance were the methodological procedures selected. From these researches came five categories of analysis, namely: Pedagogy, an (in) conscious choice; the origin that shames; the I teacher and the collective teacher in formation; the professional of the future; and inclusion by (un) equality. In order to account for the discussion of the results, through the analysis of content, Freud's works, as well as his contemporary authors, were chosen in the theoretical framework. The results of this research indicate that UnB provides experiences that allow a self-participation training that instigates future educators to seek to act critically and politically involved in their work environment.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Educação, 2006. / Submitted by Thaíza da Silva Santos (thaiza28@hotmail.com) on 2009-11-30T21:48:23Z No. of bitstreams: 1 2006_Patrícia Neves Raposo.pdf: 1002304 bytes, checksum: cd66ab67f5747dca9890b8d6629fdb64 (MD5) / Approved for entry into archive by Lucila Saraiva(lucilasaraiva1@gmail.com) on 2009-12-04T03:26:55Z (GMT) No. of bitstreams: 1 2006_Patrícia Neves Raposo.pdf: 1002304 bytes, checksum: cd66ab67f5747dca9890b8d6629fdb64 (MD5) / Made available in DSpace on 2009-12-04T03:26:55Z (GMT). No. of bitstreams: 1 2006_Patrícia Neves Raposo.pdf: 1002304 bytes, checksum: cd66ab67f5747dca9890b8d6629fdb64 (MD5) Previous issue date: 2006-12-02 / O presente trabalho tem como objetivo compreender o impacto do sistema de apoio da Universidade de Brasília na aprendizagem de universitários com deficiência visual. Esse sistema desenvolve distintas ações de apoio, por meio de tutores especiais e das tecnologias assistivas de que se pode dispor no meio acadêmico. No ensino superior, a aprendizagem de conceitos científicos caracteriza-se como um processo complexo e diverso, que não pode ser definido apenas como um conjunto de aquisições e de recursos instrumentais organizados como estratégias de apoio. Compreendemos que esse processo constitui a dimensão subjetiva do sujeito, que imprime um caráter singular ao curso de sua aprendizagem. Desse modo, a aprendizagem está implicada pelos sujeitos e pelos espaços em que atuam concretamente. Como referencial teórico norteador do nosso trabalho, optamos pela perspectiva histórico-cultural do desenvolvimento humano, particularmente aportada por Vigotski e González Rey, que nos proporciona uma nova compreensão sobre o sujeito e sua constituição subjetiva. Vigotski inaugura uma concepção sobre o sujeito com deficiência visual e compreende a unidade entre aprendizagem e desenvolvimento como fundamental para o campo educacional. González Rey nos mostra um sujeito reflexivo e ativo que interage dinamicamente com os sujeitos e os contextos dos quais participa. A aprendizagem é compreendida como um processo da subjetividade, nas suas dimensões individual e social, que envolve os significados e emoções produzidos nos distintos espaços. A metodologia utilizada teve como base a epistemologia qualitativa proposta por González Rey, que oferece, nos seus pressupostos, elementos fundamentais para viabilizar o estudo dos fenômenos humanos complexos. Optamos pelo estudo de caso, utilizando instrumentos abertos e semi-estruturados, tais como: entrevista, técnica de completamento de frases, redação, observação e análise documental. Três alunos com deficiência visual, seis alunos tutores especiais e sete professores dos cursos de Pedagogia e de Ciências Contábeis participaram da pesquisa. Como conclusão, percebemos de modo evidente que os recursos técnicos e tecnológicos proporcionaram a acessibilidade dos universitários com deficiência visual à informação. Destacamos, de modo semelhante, a importância do papel instrumental do aluno / tutor especial nas diferentes ações de apoio desenvolvidas, dentro e fora da sala de aula. Consideramos que o aspecto relacional que implicou os sujeitos em uma complexa rede envolve experiências e vivências além do próprio conhecimento científico. A facilitação da aprendizagem de conceitos científicos por meio dos apoios proporcionados gerou oportunidades de acessibilidade aos sujeitos com deficiência visual. No entanto, os distintos tipos de aprendizagem identificados não se caracterizaram, somente, pela quantidade e qualidade da instrumentalização proporcionada. Vários aspectos articularam-se na constituição das aprendizagens e dos elementos da subjetividade que influenciam a aprendizagem de cada sujeito. ________________________________________________________________________________________ ABSTRACT / The present work discusses the impact of the University of Brasília's support system on the learning process of its visually impaired students. This system involves various support actions employing students' tutors and assistive technologies available in the academic environment. At the undergraduate level, the learning of scientific concepts is a complex process that goes beyond the simple acquisition of skills and instrumental resources organized only as support strategies. We assume that the learning process constitutes the subjective dimension of the individual, and this assumption defines the singularity of a learning process designed by the individuals in the contexts where they act. The historical-cultural perspective of human development advanced by Vigotsky's and Gozález Rey's theory guides our work. This perspective brings a new understanding about the learning of scientific concepts specially its subjective dimension. Vigotski proposes a new understanding of the visually impaired individual that includes the importance of the unity between the development and learning process as the fundamental basis for education. González Rey shows us a reflexive and active subject that dynamically interacts with the other subjects and its contexts. Learning is understood as a process of subjectivity in its individual and social dimensions that comprehends the meaningfulness and the emotions produced in the environment. The adopted methodology was based on the Qualitative Epistemology proposed by Gozález Rey which offers as assumptions some fundamental elements that allow the study of the complex human phenomena. The adopted methods were the case study with open and semi-structured interviews, technique sentences completion, written texts, content analysis of documents and field observations. Three students with visual impairment, six tutor students and seven teachers of the Pedagogy and Accounting courses participated in the research. In sum, we observed that the technical and technological resources allowed the accessibility to information of university students with visual impairment. We also noted the importance of the instrumental role of the student tutor in different support actions developed inside and outside the classroom. We consider that the relational characteristics that introduced the subjects in a complex web involve experiences that go beyond the acquisition of scientific knowledge. The learning of scientific concepts was helped through support actions which created accessibility to the subjects with visual impairment. Nevertheless the different observed types of learning were not marked by the quantity and quality of the instrumental support. Many aspects integrated the formation of the learning process and participated of the elements of the subjectivity that affected each subject's learning.

Dissertação (mestrado)—Universidade de Brasília, Centro de Desenvolvimento Sustentável, Programa de Pós-Graduação em Desenvolvimento Sustentável, 2014. / Submitted by Ana Cristina Barbosa da Silva (annabds@hotmail.com) on 2015-02-06T16:20:32Z No. of bitstreams: 1 2014_GiseleCarneirodeLemosPalmeiraodeAlvarenga.pdf: 3738115 bytes, checksum: 0a5fa360f5c1cb1cff9cbd1da1865105 (MD5) / Approved for entry into archive by Ruthléa Nascimento(ruthleanascimento@bce.unb.br) on 2015-02-13T14:36:08Z (GMT) No. of bitstreams: 1 2014_GiseleCarneirodeLemosPalmeiraodeAlvarenga.pdf: 3738115 bytes, checksum: 0a5fa360f5c1cb1cff9cbd1da1865105 (MD5) / Made available in DSpace on 2015-02-13T14:36:08Z (GMT). No. of bitstreams: 1 2014_GiseleCarneirodeLemosPalmeiraodeAlvarenga.pdf: 3738115 bytes, checksum: 0a5fa360f5c1cb1cff9cbd1da1865105 (MD5) / O objetivo deste estudo foi identificar as redes de pesquisa existentes no Centro de Desenvolvimento Sustentável da Universidade de Brasília - CDS/UnB utilizando os dados registrados no Diretório dos Grupos de Pesquisa – DGP, com vistas a contribuir para melhorar o acesso às informações coletadas e oferecer sugestões que aprimorem o funcionamento do Diretório. A pesquisa evidenciou desconhecimento de uma parte substancial da comunidade científica quanto ao potencial que o Diretório comporta. Isso foi reconhecido pelos próprios respondentes, que sugeriram dentre outras ações, uma maior divulgação sobre os benefícios que este banco de dados pode oferecer, tornando o seu manuseio mais abrangente. Revelou, ainda, a real percepção dos dirigentes de pesquisa sobre o funcionamento do Diretório, o que facilitará sua utilização pela comunidade científica. Também, observou-se que muitas das sugestões propostas já vêm sendo incorporadas, tais como: o registro dos egressos, a inclusão de titulação dos membros do grupo e a possibilidade de inserir instituições parceiras no desenvolvimento da pesquisa. Tais iniciativas bem como a realização desta pesquisa contribuem para tornar o DGP mais conhecido e acessível à sociedade. ________________________________________________________________________________ ABSTRACT / The aim of this study was to identify existing networks of research at the Centre for Sustainable Development at the University of Brasilia - CDS/UnB using the data registered in the Directory of Research Groups – DGP, in order to contribute to improving access to information collected and offer suggestions that will improve the operation of the Directory. The research showed lack of knowledge of a substantial part of the scientific community about the Directory potential’s. This was acknowledged by the respondents themselves, which suggested among other things, greater disclosure about the benefits that this database can provide, making handling more comprehensive. Also brought up the real perception of the managers about the operation of the directory, which will make easier its use by the scientific community. Also, it was observed that many of the proposed suggestions have already been incorporated, such as the registration of graduates, including titration of group members and the possibility of inserting partner institutions in the research. Such initiatives as well as the realization of this research contribute to make the Directory best known and accessible to society.

La compagnie Sigfox est reconnue comme un acteur prometteur pour des transmissions de longue-distance et faible consommation, dans le contexte de l'IoT. La modulation à bande ultra étroite (Ultra Narrow Band (UNB)), la technologie de communication choisie par Sigfox, permet de transmettre des informations dans des bandes de signal très étroites (typiquement 100 Hz). A cause de l'imprécision fréquentielle causée par les oscillateurs générateurs de fréquence, il n'est pas réaliste de transmettre des signaux UNB dans des canaux parfaitement orthogonaux. L'accès naturel au canal radio pour le système de UNB est de type ALOHA, avec un aspect aléatoire à la fois en en temps et en fréquence. Cet accès aléatoire peut introduire des collisions qui dégradent la performance du réseau. Le but de cette thèse est de caractériser la capacité des réseaux basés sur UNB, ainsi que d’améliorer la performance en considérant l'aspect aléatoire en temps et en fréquence. La première contribution de cette thèse, est une évaluation de la capacité en théorie et en simulation pour une seule station de base (BS), sous des conditions de canal idéaliste ou réaliste. En conditions idéalistes, nous avons exprimé la capacité pour le cas de l'ALOHA généralisé, et l'avons étendu aux cas de réplications. Pour les conditions réalistes, nous avons pris en compte l'interférence spectrale d'UNB et le path loss (sans et avec Rayleigh fading) afin de caractériser la performance des réseaux UNB, avec l'outil géométrie stochastique. La deuxième contribution est d'appliquer l’annulation successive d'interférence (SIC), qui nous permet d'atténuer les interférences, dans des réseaux de UNB. Nous avons fourni une analyse théorique de la performance des réseaux en considérant le SIC et l'interférence spectrale de UNB, pour le cas de mono-BS. La troisième contribution est l'amélioration de la performance des réseaux UNB, en exploitant la diversité de multi-BS. Nous avons fait une analyse théorique de performance en considérant multi-BS et selection combining (SC). En particulier, nous avons considéré que l’interférence vue par chaque BS est corrélée. Nous avons ainsi démontré mathématiquement que cette corrélation ne peut pas être supprimée dans des systèmes UNB. Ensuite, nous avons appliqué les technologies de la combinaison des signaux plus complexes comme MRC (max ratio combining) et EGC (equal gain combining), ainsi que le SIC à travers multi-BS. Nous avons évalué l'amélioration de performance que chaque technologie apporte, et les avons comparées. Nous avons souligné l'efficacité de ces technologies qui nous permettent d’obtenir des gains importants comparés au cas mono-BS (e.x. 125 fois plus de réduction d'erreur avec SIC globale). La dernière contribution est une validation expérimentale du modèle d'interférence spectrale de UNB, ainsi que la capacité des réseaux UNB, sur un testbed de radio FIT/Cortexlab. / Sigfox rises as a promising candidate dedicated for long-distance and low-power transmissions in the IoT backgrounds. Ultra Narrow Band (UNB), being the communication technology chosen by Sigfox, allows to transmit information through signals whose bandwidth is very limited, typically 100 Hz. Due to the imprecision restraint on electronic devices, it is impossible to transmit UNB signals in orthogonal channels. The natural radio access for this kind of system is thus random ALOHA, in both time and frequency domain. This random access can induce collisions which degrades the networks performance. The aim of this thesis is to characterize the capacity of UNB based networks, as well as to enhance its performance, by considering the randomness in time and frequency. The first contribution of the thesis, is the theoretical and numerical capacity evaluation under idealized and realistic channel conditions, for mono base station (BS) case. Under idealized conditions, we have quantified this capacity for generalized ALOHA case and extended for replications. We highlight the time-frequency duality in UNB systems, and that there exists an optimum replication number for a given network parameter set. Under realistic conditions, we have taken into account the specific spectral interference of UNB systems and propagation path loss (without and with Rayleigh fading) to characterize the performance, with the aid of stochastic geometry. The second contribution is the enhancement of UNB network performance in single BS case. We propose to use successive interference cancellation (SIC) in UNB networks, which allows to mitigate the interference. We have provided a theoretical analysis by considering both SIC and the spectral interference, for mono-BS case. We bring to light the efficiency of SIC in enhancing UNB system performance. The third contribution is the improvement of UNB systems, by exploiting the multiple BS diversity. An analytical performance evaluation considering the simplest selection combining is conducted. In particular, we consider the interference viewed by all the BSs are correlated. Then we apply more complex signal combining technologies such as MRC (max ratio combining) and EGC (equal gain combining), and even interference cancellation across multi-BS in UNB networks. We evaluate the performance improvement that each technology can bring, and compare them with each other. We highlight the efficiency of these multi-BS technologies which allow us to achieve significant performance enhancement compared to mono-BS (e.x. 125 times better performance with global SIC). Last but not least, we experimentally verify the the spectral interference model and network capacity on a cognitive radio testbed.