Las técnicas de estimación de canal y de adaptación de la transmisión a las condiciones del entorno son temas de interés actual al estudiar la aplicación de técnicas de diversidad en transmisión en la tercera y cuarta generación de sistemas inalámbricos. En esta tesis se realiza un análisis del impacto del error de estimación de canal y la correlación en sistemas OFDM con diversidad en transmisión basados en codificación espacio-tiempo por bloques (STBC), se proponen técnicas de estimación de canal para estos sistemas y se propone una técnica de adaptación de la transmisión mediante la selección de código espacio-tiempo. En primer lugar, una técnica sencilla de mínimos cuadrados en el dominio de la frecuencia permite la estimación de canal en sistemas con dos antenas y constelaciones complejas, y con tres o cuatro antenas y constelaciones reales o complejas, utilizando STBCs ortogonales como bloques de entrenamiento. En segundo lugar, una representación 'sobre-completa' permite hacer una estimación diferencial de canal para un sistema con tres antenas transmisoras mediante la selección a partir de un banco de posibles estimadores, basándose en la redundancia provista por la matriz de transmisión no cuadrada del código ortogonal esporádico de tasa 3/4 para tres antenas transmisoras.En el contexto de sistemas con adaptación del transmisor, la técnica propuesta de diversidad por selección adaptativa de código espacio-tiempo se basa en el estado instantáneo del vector de canal y en un conjunto de niveles umbrales hallados fuera de línea en función del período de realimentación. Los resultados indican que esta técnica proporciona buenas prestaciones en canales correlados e incorrelados. Su aplicación a sistemas OFDM ha sido estudiada, superando a técnicas de selección de antena y a otras técnicas de transmisión adaptativa. / Channel estimation and adaptive transmission techniques are areas of increasing interest these days when considering transmit diversity systems for the 3G and 4G wireless communication systems. In this thesis an analysis of the channel estimation and channel correlation impact on transmit diversity OFDM systems based on space-time block coding (STBC) is presented, two channel estimation techniques are outlined and an adaptive space-time code selection technique is proposed. First, a simple frequency domain least square technique allows channel estimation for two transmitter systems with complex constellation, and three or four transmitter systems with real or complex constellation, using orthogonal STBCs as training blocks. Second, an 'overcomplete' representation allows a di.erential channel estimation for three transmitter systems through the instantaneous selection from a bank of estimators, based on the redundacy provided by the non-square transmission matrix of the sporadic 3/4-rate STBC for three transmitters.In the context of transmit adaptive systems, the proposed adaptive space-time code selection technique is based on both the instantaneous channel vector state and a set of predetermined threshold levels found o.-line as a function of the feedback period. Analytical and simulation results show that the proposed technique has a good performance in the presence of correlated and uncorrelated channels. Its application to OFDM systems has been considered, outperforming classical antenna selection techniques and other closed-loop adaptive transmission techniques.

As wireless communication becomes an ever-more important and pervasive part of our everyday life, system capacity and quality of service issues are becoming more critical. In order to increase the system capacity and improve the quality of service, it is necessary that we pay closer attention to bandwidth and power efficiency issues. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique for high speed data transmission and is generally regarded as bandwidth efficient. However, OFDM signals suffer from high peak-to-average power ratios (PARs) which lead to power inefficiency in the RF portion of the transmitter. Moreover, in OFDM, the well-known pilot tone assisted modulation (PTAM) technique utilizes a number of dedicated training pilots to acquire the channel state information (CSI), resulting in somewhat reduced bandwidth efficiency. In this dissertation, we will address the above mentioned bandwidth and power efficiency issues in wireless transmissions. To avoid bandwidth efficiency loss due to dedicated training, we will first develop a superimposed training framework that can be used to track the frequency selective as well as the Doppler shift characteristics of a channel. Later on, we will propose a generalized superimposed training framework that allows improved channel estimates. To improve the power efficiency, we adopt the selected mapping (SLM) framework to reduce the PARs for both OFDM and forward link Code Division Multiple Access (CDMA). We first propose a dynamic SLM algorithm to greatly reduce the computational requirement of SLM without sacrificing its PAR reducing capability. We propose a number of blind SLM techniques for OFDM and for forward link CDMA; they require no side information and are easy to implement. Our proposed blind SLM technique for OFDM is a novel joint channel estimation and PAR reduction algorithm, for which bandwidth efficiency power efficiency - complexity - bit error rate tradeoffs are carefully considered.

Cooperative relaying is a promising diversity achieving technique to provide reliable transmission, high throughput and extensive coverage for wireless networks in a variety of applications. Two-way relaying is a spectrally efficient protocol, providing one solution to overcome the half-duplex loss in one-way relay channels. Moreover, incorporating the multiple-input-multiple-output (MIMO) technology can further improve the spectral efficiency and diversity gain. A lot of related work has been performed on the two-way relay network (TWRN), but most of them assume perfect channel state information (CSI). In a realistic scenario, however, the channel is estimated and the estimation error exists. So in this thesis, we explicitly take into account the CSI error, and investigate its impact on the performance of amplify-and-forward (AF) TWRN where either multiple distributed single-antenna relays or a single multiple-antenna relay station is exploited. For the distributed relay network, we consider imperfect self-interference cancellation at both sources that exchange information with the help of multiple relays, and maximal ratio combining (MRC) is then applied to improve the decision statistics under imperfect signal detection. The system performance degradation in terms of outage probability and average bit-error rate (BER) are analyzed, as well as their asymptotic trend. To further improve the spectral efficiency while maintain the spatial diversity, we utilize the maximum minimum (Max-Min) relay selection (RS), and examine the impact of imperfect CSI on this single RS scheme. To mitigate the negative effect of imperfect CSI, we resort to adaptive power allocation (PA) by minimizing either the outage probability or the average BER, which can be cast as a Geometric Programming (GP) problem. Numerical results verify the correctness of our analysis and show that the adaptive PA scheme outperforms the equal PA scheme under the aggregated effect of imperfect CSI. When employing a single MIMO relay, the problem of robust MIMO relay design has been dealt with by considering the fact that only imperfect CSI is available. We design the MIMO relay based upon the CSI estimates, where the estimation errors are included to attain the robust design under the worst-case philosophy. The optimization problem corresponding to the robust MIMO relay design is shown to be nonconvex. This motivates the pursuit of semidefinite relaxation (SDR) coupled with the randomization technique to obtain computationally efficient high-quality approximate solutions. Numerical simulations compare the proposed MIMO relay with the existing nonrobust method, and therefore validate its robustness against the channel uncertainty. / Graduate

Τα σύγχρονα τηλεπικοινωνιακά συστήματα, καθώς επίσης και οι επόμενες γενιές τους, πρέπει να προσαρμόζονται για να υποστηρίζουν ένα μεγάλο αριθμό από υπηρεσίες με διαφορετικές απαιτήσεις ποιότητας. Για παράδειγμα, στα κυψελικά συστήματα, οι κυψέλες μικραίνουν σε μέγεθος και αυξάνονται σε πλήθος για να υποστηρίζουν ένα συνεχώς αυξανόμενο πλήθος χρηστών. Επίσης, σε μια άλλη κατεύθυνση, τα δίκτυα αισθητήρων αποτελούνται από μικρές συσκευές που εισάγουν περιορισμούς μεγέϑους, ενέργειας και επεξεργαστικής ισχύος. Αυτά τα δυο παραδείγματα επιδεικνύου τόσο την αυξανόμενη πολυπλοκότητα των τηλεπικοινωνιακών συστημάτων όσο και τις ιδιαίτερες απαιτήσεις που υπάρχουν στους μεμονωμένους κόμβους τους. Τα τηλεπικοινωνιακά συστήματα πολλαπλών εισόδων και εξόδων έχουν την δυνατότητα να προσφέρουν αυξημένη χωρητικότητα και αξιοπιστία στην μετάδοση δεδομένων μέσω της έννοιας της χωρικής ποικιλομορφίας (space diversity). Συγκεκριμένα, αυτό επιτυγχάνεται με την μετάδοση της ζητούμενης πληροφορίας μέσω ενός αριθμού από διαφορετικά χωρικά μονοπάτια τα οποία δημιουργούνται από την ύπαρξη πολλαπλών κεραιών στον πομπό ή/και στον δέκτη. Ωστόσο, η προαναφερόμενη πολυπλοκότητα στα τηλεπικοινωνιακά συστήματα και οι ιδιαίτερες απαιτήσεις των κόμβων έχουν ως αποτέλεσμα να μην επαρκούν οι τεχνικές που έχουν αναπτυχθεί. Μια πιθανή διέξοδο έρχεται να δώσει η ιδέα της συνεργασίας. Η έννοια της συνεργασίας έχει διάφορες οπτικές γωνίες σε ένα τηλεπικοινωνιακό σύστημα. Πρώτον, αν οι συσκευές δεν μπορούν να υποστηρίξουν πολλαπλές κεραίες (π.χ. λόγω μεγέϑους όπως στα δίκτυα αισθητήρων και στα κινητά τηλέφωνα), σίγουρα μπορούν να συνεργαστούν ώστε με έναν κατανεμημένο τρόπο να προσφέρουν σε επίπεδο συστήματος τα απαραίτητα διαφορετικά χωρικά μονοπάτια. Δεύτερον, ακόμη και αν είναι δυνατή η χρήση πολλαπλών κεραιών σε κάποιον κόμβο ενός δικτύου, π.χ. σε σταθμούς ϐάσης κυψελικών συστημάτων, ο αριθμός τους μπορεί απλώς να μην αρκεί λόγω της αυξημένης πολυπλοκότητας και του μεγέθους του δικτύου. Η κατάλληλη χρήση συνεργατικών κόμβων μπορεί να δώσει επίσης λύση στον εν λόγω περιορισμό.Η παρούσα διδακτορική διατριβή ϑα ϐασιστεί πάνω σε συνεργατικά συστήματα υπό την πρώτη οπτική γωνία που παρουσιάστηκε παραπάνω. Συγκεκριμένα, ϑα ϑεωϱηθεί ένα συνεργατικό δίκτυο με τρεις κόμβους, δηλαδή μια πηγή, έναν αναμεταδότη και έναν προορισμό. ϑα μελετηθούν τεχνικές εκτίμησης των καναλιών που συμμετέχουν στην μετάδοση της πληροφορίας αναδεικνύοντας τα ϐασικά χαρακτηριστικά που εισάγει η έννοια της συνεργασίας στις εν λόγω τεχνικές. Επίσης, ϑα παρουσιαστούν υλοποιήσεις διαφόρων συνεργατικών πρωτοκόλλων μετάδοσης σε ένα πραγματικό τηλεπικοινωνιακό σύστημα προσφέροντας έτσι την απαραίτητη πρακτική διαίσθηση πίσω από αυτά τα συστήματα. Συγκεκριμένα, αφού παρουσιαστούν κάποιες ϐασικές έννοιες για τις συνεργατικές επικοινωνίες και την λειτουργία της εκτίμησης καναλιών, ϑα μελετηθεί το πρόβλημα εκτίμησης με μερική επίβλεψη σε σχέση με το μοντέλο του συνεργατικού συστήματος που ϑεωρήθηκε. Προτείνονται εναλλακτικά σχήματα για την υλοποίηση του εκτιμητή καθώς επίσης και ένας απλός σχεδιασμός της ακολουθίας συμβόλων που υποβοηθάει το εφαρμοζόμενο κριτήριο ετεροσυσχέτισης. ΄Ολες οι έννοιες που παρουσιάζονται σε αυτό το κεφάλαιο υποστηρίζονται με πειραματικά και ημιαναλυτικά επιχειρήματα. Στην συνέχεια, παρουσιάζεται το πρόβλημα σχεδιασμού της κατανομής ενέργειας σε σύμβολα εκμάθησης για την εκτίμηση συσχετισμένων καναλιών. Αφού περιγραφεί το προς μελέτη πρόβλημα, ϑα επικεντρωθούμε στο κριτήριο ελαχίστων τετραγώνων για το οποίο παρουσιάζονται η ϐέλτιστη και τρεις υποβέλτιστες λύσεις που συνοδεύονται από χρήσιμα συμπεράσματα και παρατηρήσεις. ΄Επειτα, μελετάται το κριτήριο ελάχιστου μέσου τετραγωνικού σφάλματος για δυο περιπτώσεις. Στην πρώτη, παρουσιάζεται μια ανάλυση χειρότερης περίπτωσης και γίνεται η σύνδεση των λύσεων του προβλήματος με τις λύσεις του προηγούμενου κριτηρίου. Επίσης, υπό την υπόθεση των καναλιών χωρίς συσχέτιση, παρουσιάζεται η ϐέλτιστη λύση για τον σχεδιασμό της ακολουθίας των συμβόλων εκμάθησης. Στην τρίτη κατεύθυνση, ϑα παρουσιαστεί αρχικά το σύστημα στο οποίο ϑα υλοποιηθούν και εκτελεστούν τα πρωτόκολλα συνεργατικής επικοινωνίας. Στην συνέχεια, παρουσιάζονται τα εν λόγω σχήματα και το κεφάλαιο καταλήγει με την πειραματική διαδικασία, την παρουσίαση και αξιολόγηση των αποτελεσμάτων καθώς και την εξαγωγή χρήσιμων συμπερασμάτων. Στο τέλος της διατριβής περιγράφονται συνοπτικά τα ϐασικά συμπεράσματα που έχουν προκύψει και παρουσιάζονται κάποιες ενδιαφέρουσες νέες κατευθύνσεις. / Contemporary communication systems, as well as their next generations, are expected to adapt to a rapidly increasing number of desired applications and quality of service levels. For example, in cellular systems, the cells are getting smaller in size and larger in numbers in order to support the increasing number of users. Also, towards another direction, wireless sensor network consist of small devices that comply with stringent constraints such as size, consumed energy and computational power. These examples demonstrate both the high complexity of communication networks and the specific requirements that exist in individual communication nodes. Multiple input multiple output systems are capable of offering high capacity and reliable data communications utilizing the notion of spatial diversity. This is achieved by transmitting the desired information through different spatial paths that are created because of multiple antennas at the transmitter and/or the receiver side. However, the aforementioned complexity of communication networks and the specific requirements of the nodes have as a result that currently proposed techniques, for such systems, are inadequate. A possible solution to this dead end is the idea of cooperation. Cooperation has several aspects in a communication system. Firstly, if the nodes cannot support multiple antennas (e.g. due to size restriction as in sensor networks and mobile phones), they can cooperate in order to provide, in a distributed manner, the desired spatial paths. Secondly, even if multiple antennas can be used, as in base stations, their number might not be good enough because of the increased complexity and size of the network. The appropriate use of cooperative nodes can provide a solution to this problem, too. This dissertation has been focused on cooperative systems that are viewed according to the first aspect. Specifically, it has been assumed that the cooperative network consists of three nodes, a source, a relay and a destination. On this network, channel estimation techniques have been studied pointing out the main characteristics that are inherent to cooperation. Moreover, test-bed implementations have been provided for several well known cooperative schemes and protocols pointing out the practical aspects of such systems. In more detail, after the presentation of some introductory notions on cooperation and channel estimation, a semi-blind technique has been studied that is based on the so called cross-relation criterion. Two alternative schemes for constructing the channel estimator have been proposed as well as a simple training design procedure for improving the estimation performance has been devised. The results that have been produced are supported by semi analytic arguments and computer simulations. Then, a training design problem has been studied for a training based channel estimator. The design has been focused on the energy allocation of training symbols under the assumption that channel taps are correlated. After the description of the problem, the least squares criterion has been utilized and the optimal solution, along with three suboptimal ones, has been presented and useful conclusions have been drawn. Also, the problem has been studied under the minimum mean square error criterion for two cases. In the first one, a worst case analysis has been presented. There, a connection to the least squares solution was provided. In the second case, relaxing the assumption of correlated channel taps, the optimal solution has been presented. In the third direction, a number of well known protocols have been implemented in a test-bed system. A measurement campaign has been conducted to acquire the bit error performance and the computational complexity of the protocols. The protocols have been compared according to three different metrics and useful insights have been identified. The dissertation is concluded with a brief presentation of the main points that have been raised in the aforementioned directions. Moreover, new interesting research directions have been provided.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-04-20T12:09:28Z No. of bitstreams: 1 thiagorodriguesoliveira.pdf: 2429204 bytes, checksum: b1904f2bde31b546890c5bfa77d58c80 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-04-20T12:44:42Z (GMT) No. of bitstreams: 1 thiagorodriguesoliveira.pdf: 2429204 bytes, checksum: b1904f2bde31b546890c5bfa77d58c80 (MD5) / Made available in DSpace on 2017-04-20T12:44:42Z (GMT). No. of bitstreams: 1 thiagorodriguesoliveira.pdf: 2429204 bytes, checksum: b1904f2bde31b546890c5bfa77d58c80 (MD5) Previous issue date: 2010-09-28 / Esta dissertação apresenta, de forma detalhada, um conjunto de metodologias e técnicas destinadas à análise de redes de energia elétrica como meio de transmissão de dados (power line communication - PLC). As características das redes elétricas que influenciam um sistema de comunicação de dados consideradas neste trabalho são as seguintes: a impedância de acesso à rede elétrica, a resposta ao impulso e o ruído. Para tanto, técnicas de processamento de sinais para estimação da resposta em frequência, estimação do comprimento efetivo da resposta ao impulso, detecção e segmentação de ruídos impulsivos e análise espectral de ruídos aditivos são propostas e discutidas na presente contribuição. Os desempenhos objetivos e a apreciação subjetiva das técnicas propostas, a partir de dados sintéticos e medidos, evidenciam a adequação destas técnicas para a análise em questão. Além disso, formulações matemáticas para a resposta ao impulso de canais PLC invariantes, variantes e periodicamente variantes no tempo, derivadas a partir do modelo de multi-propagação para canais PLC, são apresentadas. Tais formulações proporcionam de forma simples e objetiva a emulação dos possíveis comportamentos temporais de canais PLC reais e, portanto, podem se constituir como ferramentas de grande utilidade para o projeto e a avaliação de sistemas de comunicações baseados na tecnologia PLC. / This thesis addresses a set of methodologies and techniques for the analysis of electric grids as a medium for data communications (power line communications - PLC). The main features influencing a communication system that are considered in this work are the input impedance, the channel impulse response, and the noise. In this regards, signal processing-based techniques are investigated, proposed and analyzed for the estimations of the channel frequency response and the effective length of the channel impulse response; the detection and segmentation of impulsive noise; and the power spectral analysis of the additive noise at the channel output. The numerical performance and subjective analysis regarding the use of the proposed techniques in synthetic and measured data indicate that those techniques fit well in the thesis purposes. In addition, mathematical formulation for invariant, time-varying, and periodically time-varying PLC channel models, which are based on multi-path channel model approach, are presented. These formulations are simple and elegant ones for the emulation of possible temporal behavior of existing PLC channels and, as a result, can constitute a useful tool for the design and analysis of PLC systems.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-04-25T12:36:15Z No. of bitstreams: 1 antonioangelomissiaggiapicorone.pdf: 6590019 bytes, checksum: 115ef6e90742599945ac5e9c8c44cb2c (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-04-25T15:26:22Z (GMT) No. of bitstreams: 1 antonioangelomissiaggiapicorone.pdf: 6590019 bytes, checksum: 115ef6e90742599945ac5e9c8c44cb2c (MD5) / Made available in DSpace on 2017-04-25T15:26:22Z (GMT). No. of bitstreams: 1 antonioangelomissiaggiapicorone.pdf: 6590019 bytes, checksum: 115ef6e90742599945ac5e9c8c44cb2c (MD5) Previous issue date: 2014-08-22 / Esta tese discute a medição e a caracterização das redes de distribuição de energia elétrica de baixa tensão (RDE-BT) e externa (outdoor) como meio de comunicação de dados para sistemas power line communication (PLC). São apresentados e discutidos diversos parâmetros obtidos a partir de uma campanha de medição realizada na RDE-BT de uma concessionária brasileira. Dentre os parâmetros analisados destacam-se o tempo de coerência, banda de coerência, espalhamento do atraso, comprimento da resposta ao impulso do canal e capacidade do canal, quando se considera as bandas de frequências de 1,7 a 30 MHz, 1,7 a 50 MHz e 1,7 a 100 MHz. As análises estatísticas evidenciam o potencial da rede de energia elétrica como meio de comunicação e discute os limites dessa potencialidade. Além disso, são propostos modelos de densidade espectral de potência dos ruídos, caracterização esparsa de canais PLC e proposto um modelo para geração de canais PLC variantes no tempo, cujo tempo de coerência entre as realizações dos canais é controlado. A estimação de canais PLC é apresentada como uma oportunidade de aplicação das técnicas de amostragem compressiva. Os resultados relacionados com as modelagens propostas constituem um ferramental de grande utilidade para projetar e analisar o desempenho de sistemas PLC. / This dissertation aims at discussing the measurement and characterization of outdoor electric power grid of low voltage as communication medium (power line communication - PLC). Statistical analyses carried out on several parameters such as average channel gain, coherence time, coherence bandwidth, delay spread, length of the channel impulse response, and channel capacity, when it is considered the frequency bands from 1.7 to 30 MHz, from 1.7 up to 50 MHz, and from 1.7 up to 100 MHz, are presented. These analyses reveal the circumstances in which outdoor and low-voltage electric power grids can be advantageous to support a reliable and efficient PLC system operation. Moreover, power spectral density models of additive noise and sparse representation of PLC channels are proposed. Also, a PLC time-varying channels model generator that makes use of coherence time is introduced. The proposed model generator, which is validated with measured PLC channels, is an effective tool to carry out performance analysis of PLC systems. Finally, PLC channel estimation is highlighted as an opportunity for the application of compressive sampling-based techniques. The use of traditional OFDM - based channel estimation techniques and the ones based on compressive sensing gives some directions for advancing channel estimation techniques for PLC channels.

Face à une demande sans cesse croissante de haut débit et d’adaptabilité des systèmes existants, qui à son tour se traduit par l’encombrement du spectre, le développement de nouvelles solutions dans le domaine des communications sans fil devient nécessaire afin de répondre aux exigences des applications émergentes. Parmi les innovations récentes dans ce domaine, l’ultra large bande (UWB) a suscité un vif intérêt. La radio impulsionnelle UWB (IR-UWB), qui est une solution intéressante pour réaliser des systèmes UWB, est caractérisée par la transmission des impulsions de très courte durée, occupant une largeur de bande allant jusqu’à 7,5 GHz, avec une densité spectrale de puissance extrêmement faible. Cette largeur de bande importante permet de réaliser plusieurs fonctionnalités intéressantes, telles que l’implémentation à faible complexité et à coût réduit, la possibilité de se superposer aux systèmes à bande étroite, la diversité spatiale et la localisation très précise de l’ordre centimétrique, en raison de la résolution temporelle très fine.Dans cette thèse, nous examinons certains éléments clés dans la réalisation d'un système IR-UWB intelligent. Nous avons tout d’abord proposé le concept de radio UWB cognitive à partir des similarités existantes entre l'IR-UWB et la radio cognitive. Dans sa définition la plus simple, un tel système est conscient de son environnement et s'y adapte intelligemment. Ainsi, nous avons tout d’abord focalisé notre recherché sur l’analyse de la disponibilité des ressources spectrales (spectrum sensing) et la conception d’une forme d’onde UWB adaptative, considérées comme deux étapes importantes dans la réalisation d'une radio cognitive UWB. Les algorithmes de spectrum sensing devraient fonctionner avec un minimum de connaissances a priori et détecter rapidement les utilisateurs primaires. Nous avons donc développé de tels algorithmes utilisant des résultats récents sur la théorie des matrices aléatoires, qui sont capables de fournir de bonnes performances, avec un petit nombre d'échantillons. Ensuite, nous avons proposé une méthode de conception de la forme d'onde UWB, vue comme une superposition de fonctions B-splines, dont les coefficients de pondération sont optimisés par des algorithmes génétiques. Il en résulte une forme d'onde UWB qui est spectralement efficace et peut s’adapter pour intégrer les contraintes liées à la radio cognitive. Dans la 2ème partie de cette thèse, nous nous sommes attaqués à deux autres problématiques importantes pour le fonctionnement des systèmes UWB, à savoir la synchronisation et l’estimation du canal UWB, qui est très dense en trajets multiples. Ainsi, nous avons proposé plusieurs algorithmes de synchronisation, de faible complexité et sans séquence d’apprentissage, pour les modulations BPSK et PSM, en exploitant l'orthogonalité des formes d'onde UWB ou la cyclostationnarité inhérente à la signalisation IR-UWB. Enfin, nous avons travaillé sur l'estimation du canal UWB, qui est un élément critique pour les récepteurs Rake cohérents. Ainsi, nous avons proposé une méthode d’estimation du canal basée sur une combinaison de deux approches complémentaires, le maximum de vraisemblance et la décomposition en sous-espaces orthogonaux,d’améliorer globalement les performances. / Faced with an ever increasing demand of high data-rates and improved adaptability among existing systems, which inturn is resulting in spectrum scarcity, the development of new radio solutions becomes mandatory in order to answer the requirements of these emergent applications. Among the recent innovations in the field of wireless communications,ultra wideband (UWB) has generated significant interest. Impulse based UWB (IR-UWB) is one attractive way of realizing UWB systems, which is characterized by the transmission of sub nanoseconds UWB pulses, occupying a band width up to 7.5 GHz with extremely low power density. This large band width results in several captivating features such as low-complexity low-cost transceiver, ability to overlay existing narrowband systems, ample multipath diversity, and precise ranging at centimeter level due to extremely fine temporal resolution.In this PhD dissertation, we investigate some of the key elements in the realization of an intelligent time-hopping based IR-UWB system. Due to striking resemblance of IR-UWB inherent features with cognitive radio (CR) requirements, acognitive UWB based system is first studied. A CR in its simplest form can be described as a radio, which is aware ofits surroundings and adapts intelligently. As sensing the environment for the availability of resources and then consequently adapting radio’s internal parameters to exploit them opportunistically constitute the major blocks of any CR, we first focus on robust spectrum sensing algorithms and the design of adaptive UWB waveforms for realizing a cognitive UWB radio. The spectrum sensing module needs to function with minimum a-priori knowledge available about the operating characteristics and detect the primary users as quickly as possible. Keeping this in mind, we develop several spectrum sensing algorithms invoking recent results on the random matrix theory, which can provide efficient performance with a few number of samples. Next, we design the UWB waveform using a linear combination of Bsp lines with weight coefficients being optimized by genetic algorithms. This results in a UWB waveform that is spectrally efficient and at the same time adaptable to incorporate the cognitive radio requirements. In the 2nd part of this thesis, some research challenges related to signal processing in UWB systems, namely synchronization and dense multipath channel estimation are addressed. Several low-complexity non-data-aided (NDA) synchronization algorithms are proposed for BPSK and PSM modulations, exploiting either the orthogonality of UWB waveforms or theinherent cyclostationarity of IR-UWB signaling. Finally, we look into the channel estimation problem in UWB, whichis very demanding due to particular nature of UWB channels and at the same time very critical for the coherent Rake receivers. A method based on a joint maximum-likelihood (ML) and orthogonal subspace (OS) approaches is proposed which exhibits improved performance than both of these methods individually.

In recent years, the increase in the population of mobile users and the advances in computational capabilities of mobile devices have led to an exponentially increasing traffic load on the wireless networks. This trend is foreseen to continue in the future due to the emerging applications such as cellular Internet of things (IoT) and machine type communications (MTC). Since the spectrum resources are limited, the only promising way to keep pace with the future demand is through aggressive spatial reuse of the available spectrum which can be realized in the networks through dense deployment of small cells. There are many challenges associated with such densely deployed heterogeneous networks (HetNets). The main challenges which are considered in this research work are capacity enhancement, velocity estimation of mobile users, and energy efficiency enhancement. We consider different approaches for capacity enhancement of the network. In the first approach, using stochastic geometry we theoretically analyze time domain inter-cell interference coordination techniques in a two-tier HetNet and optimize the parameters to maximize the capacity of the network. In the second approach, we consider optimization of the locations of aerial bases stations carried by the unmanned aerial vehicles (UAVs) to enhance the capacity of the network for public safety and emergency communications, in case of damaged network infrastructure. In the third approach, we introduce a subsidization scheme for the service providers through which the network capacity can be improved by using regulatory power of the government. Finally, we consider the approach of device-to-device communications and multi-hop transmissions for enhancing the capacity of a network. Velocity estimation of high speed mobile users is important for effective mobility management in densely deployed small cell networks. In this research, we introduce two novel methods for the velocity estimation of mobile users: handover-count based velocity estimation, and sojourn time based velocity estimation. Using the tools from stochastic geometry and estimation theory, we theoretically analyze the accuracy of the two velocity estimation methods through Cramer-Rao lower bounds (CRLBs). With the dense deployment of small cells, energy efficiency becomes crucial for the sustained operation of wireless networks. In this research, we jointly study the energy efficiency and the spectral efficiency in a two-tier HetNet. We optimize the parameters of inter-cell interference coordination technique and study the trade-offs between the energy efficiency and spectral efficiency of the HetNet.

Desde comienzos del siglo XXI, los sistemas de radiodifusión terrestre han sido culpados de un uso ineficiente del espectro asignado. Para aumentar la eficiencia espectral, los organismos de estandarización de TV digital comenzaron a desarrollar la evolución técnica de los sistemas de TDT de primera generación. Entre otros, uno de los objetivos principales de los sistemas de TDT de próxima generación (DVB-T2 y ATSC 3.0) es proporcionar simultáneamente servicios de TV a dispositivos móviles y fijos. El principal inconveniente de esta entrega simultánea son los diferentes requisitos de cada condición de recepción. Para abordar estas limitaciones, se han considerado diferentes técnicas de multiplexación. Mientras que DVB-T2 acomete la entrega simultánea de los dos servicios mediante TDM, ATSC 3.0 adoptó la Multiplexación por División en Capas (LDM). LDM puede superar a TDM y a FDM al aprovechar la relación de Protección de Error Desigual (UEP), ya que ambos servicios, llamados capas, utilizan todos los recursos de frecuencia y tiempo con diferentes niveles de potencia. En el lado del receptor, se distinguen dos implementaciones, de acuerdo con la capa a decodificar. Los receptores móviles solo están destinados a obtener la capa superior, conocida como Core Layer (CL). Para no aumentar su complejidad en comparación con los receptores de capa única, la capa inferior, conocida como Enhanced Layer (EL), es tratada como un ruido adicional en la decodificación. Los receptores fijos aumentan su complejidad, ya que deben realizar un proceso de Cancelación de Interferencia (SIC) sobre la CL para obtener la EL. Para limitar la complejidad adicional de los receptores fijos, las capas de LDM en ATSC 3.0 están configuradas con diferentes capacidades de corrección, pero comparten el resto de bloques de la capa física, incluido el TIL, el PP, el tamaño de FFT, y el GI. Esta disertación investiga tecnologías avanzadas para optimizar el rendimiento de LDM. Primero se propone una optimización del proceso de demapeo para las dos capas de LDM. El algoritmo propuesto logra un aumento de capacidad, al tener en cuenta la forma de la EL en el proceso de demapeo de la CL. Sin embargo, el número de distancias Euclidianas a computar puede aumentar significativamente, conduciendo no solo a receptores fijos más complejos, sino también a receptores móviles más complejos. A continuación, se determina la configuración de piloto ATSC 3.0 más adecuada para LDM. Teniendo en cuenta que las dos capas comparten el mismo PP, surge una contrapartida entre la densidad de pilotos (CL) y la redundancia sobre los datos (EL). A partir de los resultados de rendimiento, se recomienda el uso de un PP no muy denso, ya que ya han sido diseñados para hacer frente a ecos largos y altas velocidades. La amplitud piloto óptima depende del estimador de canal en los receptores (ej., se recomienda la amplitud mínima para una implementación Wiener, mientras que la máxima para una implementación FFT). También se investiga la potencial transmisión conjunta de LDM con tres tecnologías avanzadas adoptadas en ATSC 3.0: las tecnologías de agregación MultiRF, los esquemas de MISO distribuido y los de MIMO colocalizado. Se estudian los potenciales casos de uso, los aspectos de implementación del transmisor y el receptor, y las ganancias de rendimiento de las configuraciones conjuntas para las dos capas de LDM. Las restricciones adicionales de combinar LDM con las tecnologías avanzadas se consideran admisibles, ya que las mayores demandas ya están contempladas en ATSC 3.0 (ej., una segunda cadena de recepción). Se obtienen ganancias significativas en condiciones de recepción peatonal gracias a la diversidad en frecuencia proporcionada por las tecnologías MultiRF. La conjunción de LDM con esquemas de MISO proporciona ganancias de rendimiento significativas en redes SFN para la capa fija con el esquema de Alamouti. / Since the beginning of the 21st century, terrestrial broadcasting systems have been blamed of an inefficient use of the allocated spectrum. To increase the spectral efficiency, digital television Standards Developing Organizations settled to develop the technical evolution of the first-generation DTT systems. Among others, a primary goal of next-generation DTT systems (DVB-T2 and ATSC 3.0) is to simultaneously provide TV services to mobile and fixed devices. The major drawback of this simultaneous delivery is the different requirement of each reception condition. To address these constraints different multiplexing techniques have been considered. While DVB-T2 fulfilled the simultaneous delivery of the two services by TDM, ATSC 3.0 adopted the LDM technology. LDM can outperform TDM and FDM by taking advantage of the UEP ratio, as both services, namely layers, utilize all the frequency and time resources with different power levels. At receiver side, two implementations are distinguished, according to the intended layer. Mobile receivers are only intended to obtain the upper layer, known as CL. In order not to increase their complexity compared to single layer receivers, the lower layer, known as EL is treated as an additional noise on the CL decoding. Fixed receivers, increase their complexity, as they should performed a SIC process on the CL for getting the EL. To limit the additional complexity of fixed receivers, the LDM layers in ATSC 3.0 are configured with different error correction capabilities, but share the rest of physical layer parameters, including the TIL, the PP, the FFT size, and the GI. This dissertation investigates advanced technologies to optimize the LDM performance. A demapping optimization for the two LDM layers is first proposed. A capacity increase is achieved by the proposed algorithm, which takes into account the underlying layer shape in the demapping process. Nevertheless, the number of Euclidean distances to be computed can be significantly increased, contributing to not only more complex fixed receivers, but also more complex mobile receivers. Next, the most suitable ATSC 3.0 pilot configuration for LDM is determined. Considering the two layers share the same PP a trade-off between pilot density (CL) and data overhead (EL) arises. From the performance results, it is recommended the use of a not very dense PP, as they have been already designed to cope with long echoes and high speeds. The optimum pilot amplitude depends on the channel estimator at receivers (e.g. the minimum amplitude is recommended for a Wiener implementation, while the maximum for a FFT implementation). The potential combination of LDM with three advanced technologies that have been adopted in ATSC 3.0 is also investigated: MultiRF technologies, distributed MISO schemes, and co-located MIMO schemes. The potential use cases, the transmitter and receiver implementations, and the performance gains of the joint configurations are studied for the two LDM layers. The additional constraints of combining LDM with the advanced technologies is considered admissible, as the greatest demands (e.g. a second receiving chain) are already contemplated in ATSC 3.0. Significant gains are found for the mobile layer at pedestrian reception conditions thanks to the frequency diversity provided by MultiRF technologies. The conjunction of LDM with distributed MISO schemes provides significant performance gains on SFNs for the fixed layer with Alamouti scheme. Last, considering the complexity in the mobile receivers and the CL performance, the recommended joint configuration is MISO in the CL and MIMO in the EL. / Des de començaments del segle XXI, els sistemes de radiodifusió terrestre han sigut culpats d'un ús ineficient de l'espectre assignat. Per a augmentar l'eficiència espectral, els organismes d'estandardització de TV digital van començar a desenvolupar l'evolució tècnica dels sistemes de TDT de primera generació. Entre altres, un dels objectius principals dels sistemes de TDT de pròxima generació (DVB-T2 i el ATSC 3.0) és proporcionar simultàniament serveis de TV a dispositius mòbils i fixos. El principal inconvenient d'aquest lliurament simultani són els diferents requisits de cada condició de recepció. Per a abordar aquestes limitacions, s'han considerat diferents tècniques de multiplexació. Mentre que DVB-T2 escomet el lliurament simultani dels dos serveis mitjançant TDM, ATSC 3.0 va adoptar la Multiplexació per Divisió en Capes (LDM). LDM pot superar a TDM i a FDM en aprofitar la relació de Protecció d'Error Desigual (UEP), ja que tots dos serveis, cridats capes, utilitzen tots els recursos de freqüència i temps amb diferents nivells de potència. En el costat del receptor, es distingeixen dues implementacions, d'acord amb la capa a decodificar. Els receptors mòbils solament estan destinats a obtenir la capa superior, coneguda com Core Layer (CL). Per a no augmentar la seua complexitat en comparació amb els receptors de capa única, la capa inferior, coneguda com Enhanced Layer (EL), és tractada com un soroll addicional en la decodificació. Els receptors fixos augmenten la seua complexitat, ja que han de realitzar un procés de Cancel·lació d'Interferència (SIC) sobre la CL per a obtenir l'EL. Per a limitar la complexitat addicional dels receptors fixos, les capes de LDM en ATSC 3.0 estan configurades amb diferents capacitats de correcció, però comparteixen la resta de blocs de la capa física, inclòs el TIL, el PP, la grandària de FFT i el GI. Aquesta dissertació investiga tecnologies avançades per a optimitzar el rendiment de LDM. Primer es proposa una optimització del procés de demapeo per a les dues capes de LDM. L'algoritme proposat aconsegueix un augment de capacitat, en tenir en compte la forma de l'EL en el procés de demapeo de la CL. No obstant açò, el nombre de distàncies Euclidianes a computar pot augmentar significativament, conduint NO sols a receptors fixos més complexos, sinó també a receptors mòbils més complexos. A continuació, es determina la configuració de pilot ATSC 3.0 més adequada per a LDM. Tenint en compte que les dues capes comparteixen el mateix PP, es produeix una contrapartida entre la densitat de pilots (CL) i la redundància sobre les dades (EL). A partir dels resultats de rendiment, es recomana l'ús d'un PP no gaire dens, ja que ja han sigut dissenyats per a fer front a ecos llargs i altes velocitats. L'amplitud pilot òptima depèn de l'estimador de canal en els receptors (ex., es recomana l'amplitud mínima per a una implementació Wiener, mentre que la màxima per a una implementació FFT). També s'investiga la potencial transmissió conjunta de LDM amb tres tecnologies avançades adoptades en ATSC 3.0: les tecnologies d'agregació de MultiRF, els esquemes de MISO distribuït i els de MIMO colocalitzat. S'estudien els potencials casos d'ús, els principals aspectes d'implementació del transmissor i el receptor, i els guanys de rendiment de les configuracions conjuntes per a les dues capes de LDM. Les restriccions addicionals de combinar LDM amb les tecnologies avançades es consideren admissibles, ja que les majors demandes ja estan contemplades en ATSC 3.0 (ex., una segona cadena de recepció). S'obtenen guanys significatius per a la capa mòbil en condicions de recepció per als vianants gràcies a la diversitat en freqüència proporcionada per les tecnologies MultiRF. La conjunció de LDM amb esquemes MISO distribuïts proporciona guanys de rendiment significatius en xarxes SFN per a la capa fixa amb l'esquema d'Alamouti. / Garro Crevillén, E. (2018). Advanced Layered Divsion Multiplexing Technologies for Next-Gen Broadcast [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/105559 / TESIS

Tato práce je rozdělena do šesti kapitol. První kapitola vysvětluje rozdíly mezi fyzickou vrstvou uplinků systémů LTE a LTE Advanced, zkoumá charakteristiky kanálu komunikace D2D v rámci sítě LTE Advanced, a navrhuje mechanismy optimalizace interferencí. Rovněž je v práci prezentována struktura softwarově definované rádiové platformy, která může být využita pro vyhodnocení rádiových kanálů. Druhá kapitola hodnotí a porovnává výkony uplinkové části fyzické vrstvy systému LTE a LTE Advanced. V prostředí MATLAB je implementována struktura LTE Advanced vysílače a přijímače se všemi stupni zpracování signálu. Generované signály obou výše uvedených systémů jsou přenášeny přes různé modely kanálu ITU-R. Byly používány různé techniky odhadu kanálů a detekce signálu pro obnovení vysílaného signálu. Výsledky jsou prezentovány formou křivek BER a křivek datové prostupnosti. Třetí kapitola navrhuje způsob opakování frekvencí v celulární síti (frequency-reuse) se třemi úrovněmi výkonu, který se využívá jako typ pokročilé metody snižování interferencí. Jsou ukázány normalizované kapacitní hustoty buněk a jejich podoblastí se třemi různými případy distribuce uživatelů uvnitř buněk. Je zobrazena korelace mezi celkovou kapacitou a poloměrem každého regionu. Dosažené výsledky navrhovaného schématu jsou porovnávány s tradiční technikou opakovaného použití frekvence (Reuse-3). Čtvrtá kapitola se zabývá výzkumem alternativní metody optimalizace interferencí. Bylo provedeno ověření kooperačních metod snímání rádiového spektra ve čtyřech různých reálných prostředích: venkovní-interiérové, vnitřní-venkovní, venkovní-vnitřní a venkovní-venkovní. Navržený systém je testován pomocí zařízení Universal Software Radio Peripheral (USRP) a obsahuje dva typy detektorů; energetický detektor a statistický detektor založený na Kolmogorově-Smirnovově testu, které byly implementovány na přijímací straně. Jedním z hlavních požadavků komunikace D2D je znalost charakteristik impulzních odezev rádiového kanálu. Pátá kapitola proto představuje metodu měření kanálu pomocí Zadoff-Chu sekvencí ve frekvenční oblasti jako alternativní techniku k měření kanálu v časové nebo frekvenční doméně. Pomocí navržené metody se základní charakteristiky kanálu, jako je časové rozšíření kanálu (RMS delay spread, mean excess delay), útlum šířením a koherenční šířka pásma extrahují v (až 20x) kratším čase ve srovnání s klasickou metodou měření kanálu ve frekvenční doméně. Jsou také zkoumány charakteristiky venkovních statických rádiových kanálů na vzdálenost několika kilometrů pro pásma UHF a SHF s ko-polarizovanou horizontální a vertikální konfigurací antény. Šestá kapitola uzavírá tezi a shrnuje závěry.