Η παρούσα διδακτορική διατριβή ασχολείται με τη μελέτη της τεχνικής μετάδοσης OFDM. Η τεχνική OFDM είναι πια μια ώριμη τεχνολογία με αποδεδειγμένη ικανότητα να προσφέρει υψηλούς ρυθμούς μετάδοσης δεδομένων με ιδιαίτερα μεγάλη αποτελεσματικότητα ακόμα και σε δύσκολα περιβάλλοντα μετάδοσης. Αυτό αντικατοπτρίζεται και από την πολύ μεγάλη διείσδυσή της στην αγορά με ενσωμάτωσή της σε πολλά εμπορικά προϊόντα. Όμως οι σύγχρονες ανάγκες επικοινωνιών επιτάσσουν ακόμα πιο μεγάλη αποτελεσματικότητα στην μετάδοση δεδομένων και πλέον συστήματα και λύσεις που προτάθηκαν πριν 2 με 3 χρόνια θεωρούνται αναποτελεσματικά. Κύριο σημείο για την αύξηση της αποτελεσματικότητας αποτελεί η δυνατότητα προσαρμογής του συστήματος στις συνθήκες μετάδοσης. Οι συνθήκες μετάδοσης χαρακτηρίζονται κυρίως από το ασύρματο κανάλι. Για να επιτευχθεί η μεγαλύτερη δυνατή αποτελεσματικότητα το σύστημα θα πρέπει να είναι ικανό να εκτιμά τα κύρια χαρακτηριστικά του ασύρματου καναλιού και στην συνέχεια να προσαρμόζει ανάλογα τα χαρακτηριστικά μετάδοσης. Ένα τέτοιο προσαρμοστικό σύστημα έχει σημαντικό πλεονέκτημα απέναντι σε ένα μη προσαρμοστικό σύστημα το οποίο αντιμετωπίζει την κάθε περίπτωση με έναν ενιαίο μη βέλτιστο τρόπο. Ένα άλλο σημαντικό πρόβλημα, το οποίο συναντάται σε όλα τα συστήματα και στο OFDM, είναι η διαφορά της απόδοσης των αλγορίθμων ανάμεσα στην εξομοίωση τους σε λογισμικό και στην υλοποίηση τους σε υλικό. Η παρούσα διατριβή έχει τρεις στόχους. Ο πρώτος στόχος είναι να μελετηθεί το κανάλι μετάδοσης και να διερευνηθούν τα κύρια χαρακτηριστικά του. Η εκτίμηση των χαρακτηριστικών αυτών θα αποτελέσει την προσεγγιστική εκτίμηση καναλιού. Στην συνέχεια το σύστημα βασιζόμενο σε αυτά τα χαρακτηριστικά θα μπορεί να προσαρμοστεί κάθε φορά με τον βέλτιστο τρόπο. Αποτέλεσμα αυτής της μελέτης είναι να αποφασιστεί ότι οι συντελεστές που περιγράφουν προσεγγιστικά το κανάλι είναι ο λόγος σήματος προς θόρυβο, η συχνότητα Doppler, η εξάπλωση καθυστέρησης και ο συντελεστής Κ του καναλιού Ricean και να αναπτυχθούν αλγόριθμοι εκτίμησης για κάθε έναν. Μέσω των προτεινόμενων αλγορίθμων το σύστημα θα μπορεί να εκτιμήσει τις συνθήκες μετάδοσης και να προσαρμοστεί κατάλληλα. Ο δεύτερος στόχος της διατριβής είναι η μελέτη της ακριβούς εκτίμησης καναλιού και η ενίσχυση της απόδοσης του συστήματος μέσω αυτής. Για την μελέτη αυτή επιλέχθηκαν δύο διαφορετικοί τρόποι προσέγγισής της. Η πρώτη προσέγγιση ασχολείται με την λεπτομερή εκτίμηση χαρακτηριστικών του καναλιού όπως ο αριθμός των διαδρομών του και η χρονική τοποθέτησή τους. Στην συνέχεια αυτά τα στοιχεία χρησιμοποιούνται για την εκτίμηση των συντελεστών του καναλιού. Η δεύτερη προσέγγιση ασχολείται με την εκτίμηση καναλιού με την χρήση τεχνικών παρεμβολής. Η μελέτη των τεχνικών παρεμβολής οδήγησε στον προσδιορισμό των βασικών προβλημάτων τους και στην πρόταση νέων τεχνικών εκτίμησης καναλιού. Αρχικά προτείνεται μία νέα μέθοδος εκτίμησης μέσω αντικατάστασης των εικονικών υποφορέων του συστήματος με πιλότους. Για την μέθοδο αυτή δίνεται μια νέα μαθηματική ανάλυση η οποία προσδιορίζει τα χαρακτηριστικά της απόδοσής του. Τέλος η απόδοση της εξετάζεται σε πρακτικό OFDM σύστημα σε ιδιαίτερα δύσκολες συνθήκες. Στην συνέχεια προτείνεται μία νέα μέθοδος εκτίμησης καναλιού μέσω παρεμβολής η οποία χρησιμοποιεί παραπάνω από ένα σύμβολα OFDM για την εκτίμηση του καναλιού. Για την μέθοδο αυτή δίνεται μία νέα μαθηματική ανάλυση η οποία εμπεριέχει την επίδραση της χρονικής μεταβολής του καναλιού στην απόδοση του συστήματος. Η απόδοση και αυτής της μεθόδου εξετάζεται σε πρακτικό OFDM σύστημα σε δύσκολες συνθήκες. Τέλος ο τρίτος στόχος της διατριβής είναι η μελέτη της διαδικασίας υλοποίησης και βελτιστοποίησης των προτεινόμενων αλγορίθμων εκτίμησης ρεαλιστικά συστήματα υλικού. Για τον λόγο αυτό επιλέχθηκε η υλοποίηση μέρους των προηγούμενων προτεινόμενων αλγορίθμων σε ένα σύστημα ψηφιακής επεξεργασίας, το οποίο είναι πολύ κοντά στο να θεωρείται μια ρεαλιστική υλοποίηση. Έτσι είναι εφικτή η μελέτη των επιλογών της υλοποίησης και των συμβιβασμών της για την βέλτιστη απόδοση του συστήματος. / This dissertation deals with the study of Orthogonal Frequency Division Multiplexing (OFDM) transmission. OFDM is a mature technology with a proven ability to offer high data transmission rates with particularly big effectiveness even in difficult transmission environments. This is also reflected by its very big infiltration in the market with its incorporation in a lot of commercial products. However the modern needs of communications ordain even bigger effectiveness in data transmission and henceforth systems and solutions that have been proposed 2 or 3 years before are considered ineffective. Main point for the achievement of increase in the effectiveness of the system constitutes the ability of adaptation of the system in the transmission conditions. The transmission conditions are mainly characterized by the wireless channel. In order to achieve the highest effectiveness the system must be able to estimate the channel characteristics and to adjust its transmission characteristics accordingly. Such an adaptive system has a significant advantage compared to a non adaptive system which faces each case with a uniform and not optimal way. Another important problem that all communication systems face, including OFDM, is the difference in the performance of an algorithm between its software simulation and it hardware implementation. The present dissertation has three objectives. The first objective is to study the transmission channel and investigate its main characteristics. The estimation of these characteristics will constitute the coarse channel estimation. Then the system will be able to adapt itself optimally. The result of this study is to decide that the factors that coarsely describe the channel are the signal to noise ratio, the Doppler frequency, delay spread and the Ricean channel K factor. Then new algorithms were developed to estimate each one of these factors. Through the proposed algorithms the system will be able to estimate the transmission conditions and be adapted suitably. The second objective of this dissertation is to study the exact channel estimation and the way that the system performance can be enhanced through it. For this study two different approaches were selected. The first approach deals with the detailed estimation of the channel characteristics such as the number of the paths and their placement in time. Then these elements are used to get the channel estimation coefficients. The second approach deals with the channel estimation through interpolation. The study of interpolation methods led to the determination of their basic problems and the proposal of new channel estimation techniques. Firstly a novel channel estimation is proposed through the replacement of virtual subcarriers with pilots. For this method a novel mathematic analysis is given that determines its performance characteristics. Finally its performance is examined in a practical OFDM system in particularly difficult conditions. Then another novel channel estimation method through interpolation is proposed that uses more than one OFDM symbol to estimate the channel. For this novel method a novel mathematic analysis is given which includes the effect of the time variant channel. The performance of this method is also examined in a practical OFDM system in particularly difficult conditions. Finally the third objective is to study the implementation and optimization process of the proposed estimation algorithms in realistic hardware systems. For this it was selected to implement a part of the previously proposed algorithms in a digital signal processing system that is very close to be considered a realistic implementation. This way it is feasible to study the implementation choices that have to be made and the trade offs for the optimum performance of the system.

H επιστήμη των τηλεπικοινωνιών τα τελευταία χρόνια έχει κάνει μεγάλα τεχνολογικά άλματα, σε σύγκριση με τις προηγούμενες δεκαετίες. Το ασύρματο κανάλι επικοινωνίας συχνά θεωρείται ο αδύναμος κρίκος της τηλεπικοινωνιακής αλυσίδας. Στην παρούσα διπλωματική εργασία εξετάζεται το πρόβλημα του διαμοιρασμού των διαθέσιμων πόρων ενός ασύρματου συστήματος επικοινωνίας στους χρήστες μέσα στην περιοχή μιας κυψέλης. Γίνεται η υπόθεση ότι το ασύρματο σύστημα κάνει χρήση της τεχνικής πολλαπλής πρόσβασης (multiple–access technique) OFDMA, δηλαδή ότι χρησιμοποιεί έναν αριθμό ορθογώνιων υποφορέων τον οποίο αναθέτει στους χρήστες για τη μετάδοση της πληροφορίας. Οι διαθέσιμοι πόροι του συστήματος είναι οι ορθογώνιοι υποφορείς και η συνολική διαθέσιμη ισχύς στο σταθμό βάσης. Θεωρούμε ότι ο σταθμός βάσης έχει στη διάθεσή του τέλεια γνώση του καναλιού που τον συνδέει με κάθε χρήστη (channel state information – CSI) μέσω καναλιών ανάδρασης. Η πληροφορία κατάστασης του καναλιού χρησιμοποιείται από τον σταθμό βάσης για την κατανομή των πόρων προς τους χρήστες με δυναμικό τρόπο με χρήση κατάλληλων αλγορίθμων. Στην παρούσα διπλωματική εργασία εξετάζονται διαφορετικά σενάρια κατανομής των πόρων που περιλαμβάνουν αλγορίθμους για την ανάθεση των υποφορέων και το διαμοιρασμό των κατάλληλων ποσοτήτων ισχύος στους υποφορείς. Ανάλογα με το στόχο και τις ανάγκες των χρηστών του συστήματος, καθώς και με το ποσό της διαθέσιμης πληροφορίας καναλιού στο σταθμό βάσης, χρησιμοποιούνται διαφορετικοί αλγόριθμοι. / The telecommunications science the last years has made big technological vaults, compared to the previous decades. Wireless communication channel is often considered as the weakest link of telecommunications' chain. In this diploma thesis, the downlink of a single-cell system is considered in the downlink transmission. The multiple–access technique that is used in this system is OFDMA and the objective is the allocation of the resources among users that access the system. The available resources to be distributed among the users of the OFDMA system comprise the subcarriers over which the signals of the users are transmitted and the available power that is allocated among subcarriers. It is assumed that users estimate and feedback perfectly the channel state information (CSI) to the base station of the cell, where subcarrier and power allocation are determined according to the CSI of the users and the resource–allocation algorithm. In this diploma thesis, different resource allocation strategies for the downlink of an OFDMA system are compared. Each algorithm has a different objective.

abstract: Underwater acoustic communications face significant challenges unprecedented in radio terrestrial communications including long multipath delay spreads, strong Doppler effects, and stringent bandwidth requirements. Recently, multi-carrier communications based on orthogonal frequency division multiplexing (OFDM) have seen significant growth in underwater acoustic (UWA) communications, thanks to their well well-known robustness against severely time-dispersive channels. However, the performance of OFDM systems over UWA channels significantly deteriorates due to severe intercarrier interference (ICI) resulting from rapid time variations of the channel. With the motivation of developing enabling techniques for OFDM over UWA channels, the major contributions of this thesis include (1) two effective frequencydomain equalizers that provide general means to counteract the ICI; (2) a family of multiple-resampling receiver designs dealing with distortions caused by user and/or path specific Doppler scaling effects; (3) proposal of using orthogonal frequency division multiple access (OFDMA) as an effective multiple access scheme for UWA communications; (4) the capacity evaluation for single-resampling versus multiple-resampling receiver designs. All of the proposed receiver designs have been verified both through simulations and emulations based on data collected in real-life UWA communications experiments. Particularly, the frequency domain equalizers are shown to be effective with significantly reduced pilot overhead and offer robustness against Doppler and timing estimation errors. The multiple-resampling designs, where each branch is tasked with the Doppler distortion of different paths and/or users, overcome the disadvantages of the commonly-used single-resampling receivers and yield significant performance gains. Multiple-resampling receivers are also demonstrated to be necessary for UWA OFDMA systems. The unique design effectively mitigates interuser interference (IUI), opening up the possibility to exploit advanced user subcarrier assignment schemes. Finally, the benefits of the multiple-resampling receivers are further demonstrated through channel capacity evaluation results. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

abstract: The recent proposal of two-way relaying has attracted much attention due to its promising features for many practical scenarios. Hereby, two users communicate simultaneously in both directions to exchange their messages with the help of a relay node. This doctoral study investigates various aspects of two-way relaying. Specifically, the issue of asynchronism, lack of channel knowledge, transmission of correlated sources and multi-way relaying techniques involving multiple users are explored. With the motivation of developing enabling techniques for two-way relay (TWR) channels experiencing excessive synchronization errors, two conceptually-different schemes are proposed to accommodate any relative misalignment between the signals received at any node. By designing a practical transmission/detection mechanism based on orthogonal frequency division multiplexing (OFDM), the proposed schemes perform significantly better than existing competing solutions. In a related direction, differential modulation is implemented for asynchronous TWR systems that lack the channel state information (CSI) knowledge. The challenge in this problem compared to the conventional point-to-point counterpart arises not only from the asynchrony but also from the existence of an interfering signal. Extensive numerical examples, supported by analytical work, are given to demonstrate the advantages of the proposed schemes. Other important issues considered in this dissertation are related to the extension of the two-way relaying scheme to the multiple-user case, known as the multi-way relaying. First, a distributed source coding solution based on Slepian-Wolf coding is proposed to compress correlated messages close to the information theoretical limits in the context of multi-way relay (MWR) channels. Specifically, the syndrome approach based on low-density parity-check (LDPC) codes is implemented. A number of relaying strategies are considered for this problem offering a tradeoff between performance and complexity. The proposed solutions have shown significant improvements compared to the existing ones in terms of the achievable compression rates. On a different front, a novel approach to channel coding is proposed for the MWR channel based on the implementation of nested codes in a distributed manner. This approach ensures that each node decodes the messages of the other users without requiring complex operations at the relay, and at the same time, providing substantial benefits compared to the traditional routing solution. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

Le milieu sous-marin est doublement dispersif, en temps et en fréquence. L'utilisation récente de signaux de communication multi-porteuses offre une solution pour lutter contre la sélectivité fréquentielle. Mais ce type de signaux reste très sensible à la dispersivité fréquentielle. Cette thèse se concentre sur le problème de la dispersivité fréquentielle qu'est l'effet Doppler. Des méthodes d'estimation du paramètre Doppler sont développées pour des signaux multi-porteuses DMT. Est également abordé le problème de la compensation Doppler dans les cas mono et multitrajets. Une méthode générale et une méthode spéci que au signaux multiporteuses sont présentées et testées sur simulation. Cette thèse se distingue par son modèle du canal sous-marin qui considère un paramètre Doppler différent sur chaque trajet. Ceci résulte de la géométrie du problème et des vitesses émetteur/récepteur considérées. Nous présentons nalement des méthodes pour séparer les différents trajets arrivant sur une antenne a n de pouvoir considérer un paramètre Doppler unique sur chacun. / The underwater channel is doubly dispersive, in time and freqeuncy. In the last decade, multicarrier communication signal , like OFDM signal, has offer a solution in order to struggle aginst multipath propagation. Unfortunatly, these kinds of signal is strongly sensitive to Doppler effect. This thesis focus on frequential dispersivity involved by Doppler effect. Several Doppler estimation methods are developped for multicarrier signals. Doppler compensation for one path and multipath channel is also considered. For this problem, we developped general method but also speci c method for DMT signals. Each of them have been tested on simulations. This thesis uses an underwater channel model presenting different Doppler parameter for each path. This approach is a consequence of the problem geometry, considering emitter/receiver velocity. Finaly, using an antenna, we present several methods in order to separate efficiently each arriving path ; allowing us to consider each separated path with a single Doppler parameter to compensate.

Orientador: Yuzo Iano / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-08T10:21:08Z (GMT). No. of bitstreams: 1 Arthur_Rangel_D.pdf: 3675976 bytes, checksum: 65350df75e5a9588b1366325ac95ef62 (MD5) Previous issue date: 2007 / Resumo: Esta tese trata da otimização de receptores de TV Digital baseados em OFDM, com avaliação de desempenho em redes de retransmissão em freqüência única (SFN . Single Frequency Networks) regionais. Tal ambiente facilita a distribuição de canais, porém possui características que dificultam o trabalho do receptor. São tratados, inicialmente, de projetos de filtros canceladores de elos de realimentação em estações retransmissoras, que ocorrem quando a antena de transmissão interfere na antena de recepção. Um novo filtro, baseado em técnicas que utilizam informação temporal é proposto. Novas propostas são feitas para as partes de sincronismo, estimação e equalização de canal, e codificação/decodificação. Uma técnica, vinda da teoria de reconhecimento de padrões, é aplicada para diminuição da complexidade no processo de sincronismo temporal. Um sistema de estimação de canal 2D e equalização adaptativa, usando o LMS (Least Mean Square), é comparado com técnicas clássicas da literatura, e um ganho significativo é encontrado. Como novo esquema de codificação e decodificação é proposto um esquema iterativo, baseado em códigos turbo, com número reduzido de iterações. Tal código melhora o desempenho do sistema em relação ao uso combinado dos decodificadores Viterbi e Reed Solomon. Todas as propostas são combinadas para se avaliar o desempenho do receptor diante de condições típicas de SFN e multicaminhos típicos em recepção de TV do Brasil / Abstract: This thesis deals with the optimization of Digital TV receivers based on OFDM, with performance evaluation in regional single frequency networks (SFN). Such environment facilitates the channel distribution, however its characteristics degrade the receiver operation. Initially, projects of loop canceller filters in relay stations are treated, and they are necessary when the transmission antenna causes interference on reception antenna. A new filter, based on time information is proposed. New proposals are made for the synchronism, channel estimation and equalization, and coding/decoding. One technique, coming from the pattern recognition theory, is applied for complexity reduction in the process of time synchronism. A 2D channel estimation system and adaptive equalization, using LMS (Least Mean Square), is compared to classical techniques in the literature, and a significant gain is achieved. As a new coding and decoding scheme, an iterative system based on turbo codes is used with reduced number of iterations. Such code improves the system performance when compared to the Viterbi and Reed Solomon concatenated decoders. The proposals are combined and the performance of the proposed receiver is evaluated on typical conditions of SFN and on typical multipaths for TV reception in Brazil / Doutorado / Telecomunicações e Telemática / Doutor em Engenharia Elétrica

Made available in DSpace on 2015-04-22T22:00:47Z (GMT). No. of bitstreams: 1 jose.pdf: 2255469 bytes, checksum: cebe37770c4719e5ebf2b60e4ae0d2a6 (MD5) Previous issue date: 2013-06-07 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Currently, the majority of multicarrier transmission systems are based on the Fourier transform, which is used as frequency multiplexing technique. The main advantages of this approach include multipath immunity and low computational complexity, due to the use of a family of fast algorithms, known as Fast Fourier Transform (FFT). However, when used to provide a robust transmission, this technique presents some disadvantages, like the need for a cyclic prefix, which demands part of the useful datarate. In this work, a study for verifying the possibility of replacing the FFT, by the wavelet transform, was carried out, which has the potential to raise some advantages, such as the removal of the cyclic prefix and a simpler synchronization procedure. A computational model for the transmission layer of the Integrated Services Digital Broadcasting Terrestrial, Brazilian version (ISDB-Tb) was created, aiming to provide a direct comparison between two versions: the traditional one, which is based on FFT, and a new proposal, which is based on wavelets. Such an evaluation was performed through the relation between error bit rate and signal-to-noise ratio, in additive white gaussian noise, Rayleigh and Rician fading and also in frequency-selective Rayleigh fading channels. Similar comparisons were also performed for the ETSI TS 101 475 (HIPERLAN-2) and Digital Video Broadcasting Terrestrial (DVB-T) systems. The simulation results show that the wavelet-based systems present a similar performance, when compared with the FFT-based ones, for the chosen channel models, and without employing a cyclic prefix, which allows a more efficiente spectrum use. / Atualmente, uma grande parte dos sistemas de transmissão multiportadoras é baseada na transformada de Fourier, como técnica de multiplexação em frequência. Dentre as suas principais vantagens, encontram-se a imunidade a multipercursos e a baixa complexidade computacional, através de algoritmos rápidos conhecidos como Fast Fourier Transform (FFT). Esta técnica, entretanto, para proporcionar uma transmissão robusta, apresenta a desvantagem de exigir prefixo cíclico, o que resulta no consumo de uma fatia do espectro útil. Neste trabalho, um estudo para se verificar a viabilidade de substituição da FFT pela transformada wavelet foi realizado, o que promete trazer algumas vantagens, tais como a não exigência de prefixo cíclico e uma sincronização mais simples. Um modelo computacional para a camada de transmissão do Integrated Services Digital Broadcasting - Terrestrial, Brazilian version (ISDB-Tb) foi criado, com o objetivo de proporcionar uma comparação entre duas versões: a atual, baseada em FFT, e uma nova proposta, baseada em wavelets. Tal avaliação foi realizada em termos das curvas de taxa de erro de bit versus relação sinal ruído, em canais com adição de ruído gaussiano branco, com desvanecimento plano de Rayleigh e Rice e também com desvanecimento seletivo em frequência de Rayleigh. Comparações similares também foram realizadas para os sistemas ETSI TS 101 475 (HIPERLAN-2) e Digital Video Broadcasting Terrestrial (DVB-T). Os resultados obtidos mostram que os sistemas baseados em wavelets apresentam desempenhos similares aos dos baseados em FFT, para os modelos de canal simulados, com a vantagem adicional de não utilizarem prefixo cíclico, o que promove um uso mais eficiente do espectro.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-04-24T14:08:37Z No. of bitstreams: 1 fabriciopablovirginiodecampos.pdf: 1190225 bytes, checksum: 6bd805e035e5d8b4716929ad23f416a2 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-04-24T16:57:44Z (GMT) No. of bitstreams: 1 fabriciopablovirginiodecampos.pdf: 1190225 bytes, checksum: 6bd805e035e5d8b4716929ad23f416a2 (MD5) / Made available in DSpace on 2017-04-24T16:57:44Z (GMT). No. of bitstreams: 1 fabriciopablovirginiodecampos.pdf: 1190225 bytes, checksum: 6bd805e035e5d8b4716929ad23f416a2 (MD5) Previous issue date: 2007-08-27 / Nesta dissertação, é discutido e analisado o uso de sistemas COFDM (Coded-Ortogonal Frequency Division Mulitplexing) para aplicações de transmissão banda-larga de dados via rede elétrica (Power Line Communications -PLC) de baixa tensão externa. A partir desta discussão e análise, o uso de sistemas clustered-COFDM é proposto e, a seguir, analisado para minimizar os efeitos devido à presença de ruídos impulsivos e a ocorrência de nulos espectrais nas respostas em frequência dos modelos lineares e invariantes no tempo das redes elétricas de baixa tensão, os quais são derivados para aplicações de transmissão de dados. A principal diferença entre os sistemas COFDM e clustered-COFDM é que o segundo é um conjunto de sistemas COFDM com menor número de sub-portadoras operando em paralelo e na mesma banda de frequência de operação do sistema COFDM. A comparação de desempenho dos sistemas COFDM e clustered-COFDM sob as mesmas condições de operação é apresentada quando o transmissor dispõe de informações parciais e completas do canal de comunicação. Aqui, é denominado informações parciais o conhecimento da distância entre o transmissor e o receptor e, desta forma, a abordagem para simulação é a mesma implementada em sistemas multi-portadoras aplicados para a comunicação sem fio. Por outro lado, as informações completas do canal estão associadas ao conhecimento das relações sinal-ruído (Signal to Noise Ratio - SNR) nas sub-portadoras pelo transmissor e, neste contexto, tem-se a abordagem de simulação aplicada a sistemas multi-portadoras para comunicação via cabos. Os resultados obtidos em ambas as abordagens de simulação revelam que, em relação ao sistema COFDM, o sistema clustered-COFDM reduz a taxa de bits errados (Bit Error Ratio - BER) quando as informações parciais do canal são disponíveis. Quando as informações completas do canal de comunicação são disponíveis, o sistema clustered-COFDM é capaz de não apenas reduzir a BER, mas também de aumentar a taxa de transmissão. Assim sendo, é possível concluir, através dos resultados de simulação indicados, que o sistema clustered-COFDM é o mais apropriado para aplicações PLC. / In this dissertation, is discussed and analyzed the use of coded- ortogonal frequency division multiplexing (COFDM) for broadband data transmission through outdoor low-voltage power electric network (Power Line Communications - PLC). Based on such discussion and analysis, the use of clustered-COFDM system is proposed and investigated to minimize the effects due to the presence of impulsive noise and the occurrence of spectral nulls in the frequency response of linear and time-invariant channels models of low-voltage electric networks, which are derived for data transmission applications. The main difference between COFDM and clustered-COFDM system is that the latter is a set of COFDM systems with a reduced number of tones that operates in parallel and in the same frequency band of operation of the COFDM system. A performance comparison between COFDM and clustered-COFDM systems under the same operation condition and when the transmitter makes use of partial and complete information of the channel is presented. Here, the term partial information means that the distance from the transmitter to the receiver is available. In this situation, the approachfor simulation is the same implemented to mutlicarrier systems applied to wireless communication. On the other hand, the term complete information about the channel is associated with the availability of each tone’s SNR (Signal to Noise Ratio) at the transmitter side. In this context, the approach for simulation is the one applied to multicarrier systems for wireline communications. The results obtained with simulations reveal that the clustered-COFDM offer lower BER (Bit Error Ratio) than the COFDM when partial information about the channel is available. If complete information of the channel is available, the clustered-COFDM allows lower BER and higher data transmission than the COFDM. Based on the simulation results, we conclude that the clustered-COFDM system is more appropriate that the COFDM one for PLC applications.

Future mobile communications systems reaching for ever increasing data rates require higher bandwidths than those typical used in today’s cellular systems. By going to higher bandwidth the (for low bandwidth) flat fading radio channel becomes frequency selective and time dispersive. Due to its inherent robustness against time dispersion Orthogonal Frequency Division Multiplex (OFDM) is an attractive candidate for such future mobile communication systems. OFDM partitions the available bandwidth into many subchannels with much lower bandwidth. Such a narrowband subchannel experiences now due to its low bandwidth an almost flat fading leading in addition to above mentioned robustness also to simple implementations. However, one potential drawback with OFDM modulation is the high Peak to Average Ratio (PAR) of the transmitted signal: The signal transmitted by the OFDM system is the superposition of all signals transmitted in the narrowband subchannels. The transmit signal has then due to the central limit theorem a Gaussian distribution leading to high peak values compared to the average power. A system design not taking this into account will have a high clip rate: Each signal sample that is beyond the saturation limit of the power amplifier suffers either clipping to this limit value or other non-linear distortion, both creating additional bit errors in the receiver. One possibility to avoid clipping is to design the system for very high signal peaks. However, this approach leads to very high power consumption (since the power amplifier must have high supply rails) and also complex power amplifiers. The preferred solution is therefore to apply digital signal processing that reduces such high peak values in the transmitted signal thus avoiding clipping. These methods are commonly referred to as PAR reduction. PAR reduction methods can be categorized into transparent methods – here the receiver is not aware of the reduction scheme applied by the transmitter – and non-transparent methods where the receiver needs to know the PAR algorithm applied by the transmitter. This master thesis would focus on transparent PAR reduction algorithms. The performance of PAR reduction method will be analysed both with and without the PSD constrained. The effect of error power on data tones due to clipping will be investigated in this report.

A multi-hop ad hoc network requires no infrastructure, like base stations or routers to function. This means that it can quickly be deployed and in movement, making it ideal for scenarios like natural disasters or battlefield communication. A single-frequency network (SFN) is the idea that transmitters can cooperate to send the same signals simultaneously over the same frequency channel, such that the signals are amplified. It was previously proposed that SFNs could be created in multi-hop ad hoc networks. According to simulations, this could improve the energy consumption, signal reachability, and data transfer rate. As this has only been simulated, the purpose of this project is to experimentally verify that SFN is attainable in a multi-hop ad hoc network, and to assess what the difficulties are of an actual implementation. The method involved synchronizing the transmission of two devices operating under the 802.11a standard. A multi-hop ad hoc network was created by changing the settings of the network interface cards. The devices used were mainly laptops and transmissions were monitored in Wireshark. The laptops were forced to send on one frequency channel to make interference possible, and identical packets were sent. Experiments were made to assess whether SFN was attained. The packet loss rate and distance were used to evaluate the results. The results suggest that a synchronized transmission off by less than 2 μs was attained. However, the error of these measurements were neither known nor approximable. This meant it was hard to know when a SFN was formed. The results indicate that SFN was attained, as the packet loss rate decreased significantly when employed. The effectiveness of SFN was hard to assess because the results were not comparable with the simulation values. The difficulty of an actual implementation is deemed to be the synchronization of transmissions.