Dans les systèmes de communications sans fil, le canal de transmission entre les antennes d’émission et de réception est l’une des principales sources de perturbation pour le signal. Les modulations multiporteuses, telles que l’OFDM (pour orthogonal frequency division multiplexing), sont très robustes contre l’effet des multi-trajets, et permet de retrouver le signal émis avec un faible taux d’erreur, quand elles sont combinées avec un codage canal. L’estimation de canal joue alors un rôle clé dans les performances des systèmes de communications. Dans cette thèse, on étudie des techniques fondées sur les estimateurs LS (pour least square, ou moindres carrés) et MMSE (pour minimum mean square error, ou erreur quadratique moyenne minimum). La technique MMSE est optimale, mais est beaucoup plus complexe que LS, et nécessite la connaissance a priori des moments de second ordre du canal et du bruit. Dans cette présentation, deux méthodes permettant d’atteindre des performances proches de LMMSE en évitant ses inconvénients sont étudiées. Une troisième partie étudie quant à elle les erreurs d’estimation dues aux interpolations. / In wireless communications systems, the transmission channel between the transmitter and the receiver antennas is one of the main sources of disruption for the signal. The multicarrier modulations, such as the orthogonal frequency division multiplexing (OFDM), are very robust against the multipath effect, and allow to recover the transmitted signal with a low error rate, when they are combined with a channel encoding. The channel estimation then plays a key role in the performance of the communications systems. In this PhD thesis, we study techniques based on least square (LS) and minimum mean square error (MMSE) estimators. The MMSE is optimal, but is much more complex than LS, and requires the a priori knowledge of the second order moment of the channel and the noise. In this presentation, two methods that allow to reach a performance close to the one of LMMSE while getting around its drawback are investigated. In another way, a third part of the presentation investigates the errors of estimation due to the interpolations.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2015-12-01T12:45:13Z No. of bitstreams: 1 robertomussideoliveira.pdf: 1293292 bytes, checksum: 78c6c9fd9415c0b3990a1aaf55b842a2 (MD5) / Made available in DSpace on 2015-12-01T12:45:13Z (GMT). No. of bitstreams: 1 robertomussideoliveira.pdf: 1293292 bytes, checksum: 78c6c9fd9415c0b3990a1aaf55b842a2 (MD5) Previous issue date: 2015-03-11 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesse trabalho, discutimos o uso de protocolos de coopera¸c˜ao na subcamada de controle de acesso ao meio (CMAC) para reduzir a taxa de perda de pacote e aumentar o goodput em um sistema de comunica¸c˜ao banda larga via rede el´etrica (PLC) de ambientes residenciais. Para apoiar essa discuss˜ao, n´os, pela primeira vez, apresentamos uma an´alise estat´ıstica da taxa de erro de pacote (PER) de canais PLC residenciais a partir de dados medidos em um modelo com um ´unico relay. Adicionalmente, n´os esbo¸camos um simples protocolo CMAC capaz de explorar a diversidade oferecida por uma rede el´etrica dom´estica. Usando esse protocolo, nosso objetivo ´e mostrar o impacto da varia¸c˜ao da largura de banda, da varia¸c˜ao da PER e da posi¸c˜ao relativa do relay no desempenho do sistema. Sendo assim, n´os mostramos que a taxa de perda de pacote e o goodput melhoram quando a largura de banda de frequˆencia aumenta. Al´em disso, resultados mostram que a coopera¸c˜ao na camada de enlace n˜ao oferece vantagens caso os valores de PER do enlace direto e do enlace intermediado pelo relay sejam muito altos ou muito baixos. N´os tamb´em notamos que as melhorias estudadas dependem da posi¸c˜ao do n´o relay em rela¸c˜ao ao n´o fonte e ao n´o destino (i.e., notamos melhoras na rede nos casos em que o relay estava situado pr´oximo `a fonte e no meio do caminho entre a fonte e o destino). Finalmente, uma compara¸c˜ao entre os esquemas de acesso m´ultiplo por divis˜ao de frequˆencias ortogonais - acesso m´ultiplo por divis˜ao de tempo (OFDMA-TDMA) e acesso m´ultiplo por divis˜ao de tempo - multiplexa¸c˜ao por divis˜ao ortogonal de frequˆencia (TDMA-OFDM) mostra que o simples protocolo CMAC ´e mais eficaz quando usado juntamente com o primeiro esquema do que com o ´ultimo. Em suma, a nossa contribui¸c˜ao ´e dividida em duas etapas: primeiramente, desenvolvemos um simples protocolo MAC de coopera¸c˜ao que traz melhorias de desempenho na rede quando comparado com um sistema sem a coopera¸c˜ao; em segundo lugar, n´os realizamos uma an´alise sistem´atica de diferentes cen´arios, mostrando os benef´ıcios e limita¸c˜oes da coopera¸c˜ao na camada de enlace de redes PLC. / In this work, we discuss the use of cooperative medium access control (CMAC) protocols to reduce packet loss rate and to improve goodput of in-home broadband power line communication (PLC) systems. To support this discussion, we, for the first time, present a statistical packet error rate (PER) analysis of measured in-home PLC channels by adopting a single relay model. Additionally, we outline a simple CMAC protocol that is capable of exploiting the diversity offered by in-home electric power grids. Using this protocol, we aim to show the impact of bandwidth variation, PER variation and of relative relay location on system performance. Thus, we show that packet loss rate and goodput improve when frequency bandwidth increases. Also, results show that cooperation at the link layer does not offer advantages if the PER values of direct and relayed links are very high or very low. Furthermore, we note that the improvements depend on the location of the node relay in relation to the nodes source and the destination (i.e., network improves if the relay is located near the source or in the midway between the source and the destination). Finally, a comparison between orthogonal frequency division multiple access - time division multiple access (OFDMA-TDMA) and time division multiple access - orthogonal frequency division multiplexing (TDMA-OFDM) schemes show that the simple CMAC protocol is more effective when it is used together with the former scheme than the latter.

Submitted by Carolina Neves (carolinapon@uefs.br) on 2016-08-09T01:01:22Z No. of bitstreams: 1 Karine_Souza_de_Almeida_Dissertacao_Final.pdf: 13569698 bytes, checksum: 81752dfa3be4ee9493654f5c61524c5a (MD5) / Made available in DSpace on 2016-08-09T01:01:22Z (GMT). No. of bitstreams: 1 Karine_Souza_de_Almeida_Dissertacao_Final.pdf: 13569698 bytes, checksum: 81752dfa3be4ee9493654f5c61524c5a (MD5) Previous issue date: 2016-05-24 / Funda??o de Amparo ? Pesquisa do Estado da Bahia - FAPEB / By using the Orthogonal Frequency Division Multiplexing (OFDM), it is possible to transmit by a channel a more robust signal and less susceptible to interferences. When such technique, based on orthogonal subcarriers, is combined with the use of propagation models, which are relevant tools for simulating the signal behavior in respect to environmental parameters and antenna configurations, it is feasible to have an evaluation about the performance of these systems, improving their project phase. Therefore, this work presents the use of OFDM technique applied to two propagation models: a two-ray geometric model and a microcellular model adapted to the Paulista Avenue, S?o Paulo, SP, having as objective the analysis of a mobile communication system through Bit Error Rate (BER). In the developed application using the MAtrix LABoratory (MATLAB) software, the mobile communication system based on OFDM admits several scenarios during the simulation, so that scenarios are related to positioning of the antennas, heights, type of modulation, size of the bit sequence to be sent, and others. The adopted OFDM architecture has been the IFFT/FFT and based on it, the system performs the transmission and reception of a bit set through a channel with interference by using different digital modulations, making appropriate comparisons. / Atrav?s da utiliza??o da t?cnica de Multiplexa??o por Divis?o de Frequ?ncias Ortogonais (OFDM, do ingl?s, Orthogonal Frequency Division Multiplexing), ? poss?vel transmitir por um canal um sinal mais robusto e menos propenso a interfer?ncias. Quando tal t?cnica, baseada na utiliza??o de subportadoras ortogonais, ? aliada ao uso de modelos de propaga??o, que s?o ferramentas relevantes para a simula??o do comportamento do sinal em rela??o aos par?metros do ambiente e as configura??es das antenas, ? poss?vel ter uma avalia??o a respeito do desempenho desses sistemas, melhorando a fase de projeto dos mesmos. Sendo assim, este trabalho apresenta o uso da t?cnica OFDM aplicada a dois modelos de propaga??o: um modelo geom?trico de dois raios e um modelo microcelular adaptado para a Avenida Paulista, S?o Paulo, SP, possuindo como objetivo a an?lise de um sistema de comunica??o m?vel atrav?s do c?lculo da Taxa de Erro de Bit (BER, do ingl?s, Bit Error Rate). Na aplica??o desenvolvida utilizando o software MATLAB (MATrix LABoratory), o sistema de comunica??o m?vel baseado em OFDM assume diversos cen?rios durante as simula??es, de forma que esses cen?rios s?o relacionados ao posicionamento das antenas, alturas, tipo de modula??o, tamanho da sequ?ncia de bits a ser enviada, entre outras. A arquitetura OFDM adotada foi a IFFT/FFT e baseado nela, o sistema realiza a transmiss?o e a recep??o de um conjunto de bits por um canal com interfer?ncias utilizando diferentes modula??es digitais, fazendo as devidas compara??es.

This thesis describing basic concepts and parts of OFDM modulator, showing their implementation in Matlab. It’s focused on how to use USRP with Matlab to create fully functional OFDM modulator.

Master’s thesis belongs to the Cognitive radio network sphere. These networks utilize frequency spectrum more effectively than networks used in present radio communications. The Cognitive radio concept makes coexistence of classic and cognitive radio networks possible. Attention is aimed at spectrum sensing as the key task of the Cognitive radio. Main properties of the cyclostationary detector, as the detector, that reaches high probability of the detection at a very low signal to noise ratio with apriori knowledge of the transmitted signal's cyclic frequency, are examined in this paper. The OFDM signals, that inherit cyclostationarity from cyclic prefix, used in the real systems have been chosen for testing the properties of the detector. The influences of decimation and multipath propagation on the probability of detection are quantitatively expressed. The optimal values for the weights of the multicycle detector are determined.

This thesis describing basic concepts and parts of OFDM modulator, showing their implementation in Matlaba. It's also focusing on how to use USRP with Matlab and creating functional OFDM modulator. Main efford is forwarded to practical impelementation of OFDM using available hardware.

[pt] Esta dissertação enfoca a combinação de duas conhecidas e eficazes tecnologias usadas em muitas redes de comunicação atuais, as técnicas de transmissão Orthogonal Frequency Division Multiplexing (OFDM) e Multiple-Input Multiple-Output (MIMO). Sistemas MIMO-OFDM unem as vantagens conhecidas da comunicação MIMO, como altas taxas de dados e ampliação da confiabilidade do link, com as características marcantes da transmissão OFDM, como comunicação livre de interferência entre símbolos e a equalização simples no domínio da frequência. Este trabalho apresenta em detalhes a modelagem de sinais do sistema MIMO-OFDM e o processamento adequado no receptor para realizar detecções desacopladas dos sub-símbolos OFDM. Além disso, estratégias de seleção de antenas são propostas para aumentar o desempenho do sistema, explorando cenários em que o transmissor ou receptor está equipado com mais antenas do que o número de cadeias de radiofrequência (RF). Esses procedimentos são baseados em métricas específicas propostas para cada cenário. A avaliação de desempenho inclui sistemas equipados com técnicas de equalização linear e sistemas com pré-codificadores, como Zero-Forcing (ZF) e Minimum Mean Square Error (MMSE). Os resultados de simulação evidenciam que as estratégias de seleção da antena resultam em uma melhoria significativa no desempenho do sistema. / [en] This thesis focuses on the combination of two well-known effective technologies used in many of today s communication networks, Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple- Output (MIMO). It unites the well-known advantages of MIMO communication, such as high data rates and improved link reliability to the characteristics of OFDM, known as intersymbol-interference-free communication and simple frequency-domain equalization. This work presents in detail the signal modelling of MIMO-OFDM system and the adequate processing at the receiver to perform decoupled detections per OFDM subsymbol. In addition, antenna selection strategies are proposed to increase system performance by exploiting some scenarios when the transmitter or receiver is equipped with more antennas than the number of radiofrequency (RF) chains. These procedures are based on the specific metric for the current scenario. The performance of the system is evaluated employing linear equalization or precoding techniques, such as Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE). Simulation results evidence that antenna selection strategies result in significant system performance improvement.

Le réseau de radio communications mobiles professionnelles (PMR), qui est utilisé pour le fonctionnement de la sécurité publique, doit évoluer pour les solutions à large bande pour satisfaire les demandes des utilisateurs à l’avenir. Dans les technologies à large bande actuels, Long Term Evolution (LTE), développé par le 3GPP (3rd Generation Partnership Project), est considéré comme l’un des candidats potentiels pour la prochaine génération de PMR. Malgré le fait que la technologie LTE en charge la transmission de données à haute vitesse et prend en charge différentes tailles de paquets en utilisant la modulation et le codage adaptatifs (AMC), le LTE n’est pas encore optimisée pour la communication vocale à bas débit, en particulier en cas d’utilisation LTE pour la radio communications mobiles professionnelles (PMR ) contexte. Par conséquent, dans cette thèse, nous présentons des solutions pour renforcer la capacité de la voix de la technologie LTE dans le cadre PMR à la fois la liaison montante et la transmission de liaison descendante. Les nouvelles propositions, basées sur la technologie de norme LTE existant avec des adaptations, permettent la réduction de les frais généraux de données et les frais généraux de contrôle sur LTE (VoLTE) dans le contexte PMR / The Professional Mobile Radio (PMR) network, which is used for public safety operation, has to evolve to the broadband solutions to satisfy the user demands in the future. In the current broadband technologies, Long Term Evolution (LTE) standard, developed by the 3GPP (3rd Generation Partnership Project), is considered one of the potential candidates for the next generation of PMR. Despite the fact that LTE supports high-speed data transmission and supports different packet sizes by using Adaptive Modulation and Coding (AMC), LTE is not yet optimized for low bit rate voice communication, especially in case of using LTE in Professional Mobile Radio (PMR) context. Therefore, in this dissertation, we present several new solutions for enhancing the voice capacity of LTE in the PMR context for both uplink and downlink transmission. The new propositions, based on existent LTE standard technology with adaptations, allow reducing both control and data overhead issues of Voice over LTE (VoLTE) in PMR context

Aujourd'hui, le format « Quadrature Phase Shift Keying » avec multiplexage de polarisation (DP-QPSK) opérant à 100 Gbps est devenue un standard pour la transmission WDM longue distance. Une alternative au format DP-QPSK permettant d’atteindre des débits de 100 Gbps et plus (400 G & 1Tbps) est l’ « Orthogonal Frequency Division Multiplexing » (OFDM). Mais, des interrogations subsistent quant à sa robustesse aux effets non linéaires. Dans cette thèse nous avons étudié le potentiel de la technologie OFDM pour la transmission WDM longue distance à 100 Gbps. Le traitement du signal est détaillé ainsi que la mise en œuvre du transmetteur et récepteur OFDM cohérent. Nous présentons aussi les résultats expérimentaux de la transmission obtenus dans plusieurs configurations. Dans l’une de ces configurations, le canal modulé avec le format DP-OFDM coherent (Co-DP-OFDM) est multiplexé avec 40 canaux modulés en DP-QPSK à 100 Gbps. Les canaux ont ensuite été transmis sur 1000 km de fibre G.652 sans gestion de dispersion chromatique. Dans une autre configuration, les canaux Co-DP-OFDM et Co-DP-QPSK sont combinés avec 78 canaux 10 Gbps NRZ-OOK et transmis sur 1000 km de fibre G.652 avec gestion de dispersion. Nous avons montré que le Co-DP-OFDM et Co-DP-QPSK ont des performances similaires après une transmission de 1000 km sur une ligne sans gestion de dispersion, et nous avons aussi montré que la transmission de ces formats sur une infrastructure de fibre deployée est possible à condition de réduire de 5 dB la puissance des canaux 10 Gbps NRZ-OOK par rapport aux canaux à100 Gbps. Ces résultats sont précieux pour la prochaine génération de systèmes WDM à 400 Gbps ou 1 Tbps. / Today the 100 Gbps coherent dual polarization quadrature phase shift keying (Co-DP-QPSK) is standardized as the industrial solution for long-haul WDM transmission. Another alternative format to DP-QPSK that permits also to reach a data rate of 100 Gbps and beyond is the coherent orthogonal frequency division multiplexing (OFDM) format. However a doubt exists over the ability of OFDM to be as efficient as QPSK for long-haul WDM transmission due to its supposed higher sensitivity to nonlinear effects . In this thesis, we have investigated the potential of Co-DP-OFDM for 100 Gbps WDM transport. The digital signal processing algorithms are detailed as well as the various experimental set-ups required to carry out and validate the 100 Gbps transceiver. We also present the transmission results obtained with several configurations. In one of these configurations, the 100 Gbps Co-DP-OFDM channel is multiplexed with forty 100 Gbps DP-QPSK channels and all these channels are transmitted over 1000 km of DCF-free G.652 fiber, while in another configuration, the Co-DP-OFDM and Co-DP-QPSK channels are combined with seventy eight 10 Gbps NRZ-OOK channels and transmitted over 1000 km of dispersion managed G.652 fiber line. We have demonstrated that OFDM and QPSK have nearly the same performance after a transmission over 1000 km, and also we have demonstrated that the transmission of these two formats over legacy fiber infrastructure is possible under the condition of decreasing by 5 dB the 10 Gbps NRZ-OOK channel power with respect to the 100 Gbps channels. The results presented in this thesis are very valuable when considering the next generation of 400 Gbps or 1 Tbps for WDM systems.

L’objectif de cette thèse concerne l’allocation des canaux fréquentiels dans les réseaux ad hoc organisés en clusters. Les terminaux du réseau sont assemblés localement en clusters afin de garder les avantages des réseaux ad hoc tout en réduisant la quantité de signalisation nécessaire à son fonctionnement. Dans chaque cluster, un chef de cluster (CH en anglais) est désigné parmi les terminaux dont le rôle est de gérer localement les ressources ainsi que les communications. Un des problèmes concerne l'allocation des bandes de fréquence de manière distribuée à chaque cluster pour leur permettre d'opérer correctement. Les fréquences sont une ressource rare ce qui implique que plusieurs clusters sont amenés à utiliser les mêmes et donc à interférer entre eux. Le CH base ses décisions sur une fonction d'utilité qui prend en compte des mesures de performance des communications. Ces dernières peuvent être perturbées à cause des diverses variations dynamiques auxquels sont soumis les réseaux ad hoc. Parmi les algorithmes d'apprentissage distribués, nous avons identifié des méthodes basées sur le paradigme "d'essais erreur'' (TE en anglais) comme des solutions potentielles. Ces algorithmes ont la particularité d'avoir des propriétés de convergence globale intéressantes bien que le problème soit non coopératif.Dans un premier temps, nous avons étudié la convergence théorique de ces algorithmes en réalisant des approximations de chaînes de Markov dans des cas particuliers. Ensuite, nous avons montré théoriquement et numériquement que le principal défaut des approches TE est leur sensibilité aux variations aléatoires de la mesure d'utilité. Nous avons donc proposé des solutions, avec des preuves théoriques à l’appui, pour adapter ces algorithmes aux cas où l'utilité serait perturbée par des phénomènes aléatoires. Enfin, nous avons analysé de manière plus approfondie l’influence des évanouissements de Rayleigh sur les statistiques de l’utilité. / This thesis deals with the fully distributed allocation of channels in clustered ad hoc networks. Nodes are gathered locally into clusters in order to keep the advantage of the no infrastructure of ad hoc networks, and to reduce the amount of signalling. In each cluster a node is elected as the Clustered Head (CH) whose role is to manage the resources and the transmissions locally. One of the major problem is to allocate in a distributed way spectrum bands to the clusters in order to make them able to operate. Bandwidth is a scarce resource which implies that several clusters may use the same frequency and hence interfere among each other. The CH realizes the frequency allocation based on a utility function that uses measurements as inputs. These measurements are possibly disturbed due to the wide variety of dynamic changes that face ad hoc networks. Among the distributed learning algorithms, we have identified approaches based on "trial and error'' (TE) paradigm that could solve the channel allocation problem. These approaches possess very attractive global convergence behavior despite the non-cooperativeness of the problem and thus in a broad class of games.First, based on a specific utility model, we analyse the performance of these algorithms using Markov chains approximations in order to reduce numerical computations complexity. Then, we assess theoretically and numerically that a drawback of TE algorithms is their sensitivity to disturbances. We propose modifications with supporting theoretical proofs in order to adapt the TE algorithms to disturbances of the utility. Furthermore, we study the impact of Rayleigh fading on the utility by deriving its probability density function (pdf) in various contexts.