No description available.

En raison des conflits maritimes qui s'étendent (piraterie, embargo,...), les besoins de communiquer et de détecter les menaces sont de plus en plus importants. Ceci conduit irrémédiablement à l'augmentation du nombre d'aériens à bord des plateformes navales. Afin de gérer au mieux cet accroissement du nombre d'antennes, qui conduit à des effets de couplage et à une augmentation de la signature radar du navire, les principales entreprises du secteur ont mis en place des structures de mâts intégrés. C'est dans un contexte d'amélioration de leur mâture intégrée compacte (Cmast™) que les Constructions Mécaniques de Normandie (CMN) de Cherbourg en collaboration avec l'IETR de Rennes et INEO Défense ont proposé cette thèse. Ces travaux ont pour objectifs le développement de deux systèmes antennaires intégrables au sein de cette mâture intégrée compacte : une antenne de communication en bande V/UHF et une antenne de radar de navigation à balayage électronique en bande X. Une première étude a permis le développement d'une antenne conique large bande (225-400MHz) dont les paramètres géométriques ont été optimisés pour assurer une communication entre les navires et des aéronefs. Un prototype de cette antenne a été réalisé et a permis une validation expérimentale de ses performances. Une seconde étude a permis le développement d'une antenne d'un radar de navigation à balayage électronique en bande X. Cette antenne est basée sur une technologie transmit-array comprenant une source illuminante et un réseau permettant la formation du diagramme. Un effort particulier a été porté sur la source illuminante qui doit éclairer, à des distances très courtes (<550mm), un réseau qui présente des dimensions particulières (1530mmx100mm). Plusieurs sources utilisant un principe de focalisation en zone champ proche ont été développées et validées expérimentalement. Enfin deux architectures d'antennes transmit-array ont été étudiées, utilisant respectivement une technologie imprimée et une technologie en guide. Le fonctionnement de l'antenne complète (source illuminante + réseau transmit-array) a été étudié théoriquement. / The need to communicate and detect potential enemies increases with the extension of maritime conflicts. This need impacts directly the number of antennas on naval platforms. However, this increase of aerials leads to several damaging effects: like coupling or high radar signature. To limit this effect, some companies have developed integrated mast design. This structure limits the coupling effect between aerials by a subdivision of the mast and provides an omnidirectional coverage for all antennas inserted inside the mast. The objective of the thesis is to design two antenna systems for the compact integrated mast (CmastTM) developed by the Constructions Mécanique de Normandie (CMN): a communication antenna in the V/UHF band and an electronically scanning antenna for a maritime navigation radar in X-band. For the communication in the V/UHF band, a broadband conical antenna has been developed (225-400MHz). This antenna provides an optimized radiation pattern to insure the communications between the ship and the aircrafts. An experimental validation has been done with a prototype. In the second study, we have developed an electronically scanning antenna for a navigation radar. This antenna is based on a transmit-array technology including an illuminating feed and an antenna which generates the appropriate radiation pattern. A large part of the study has been done on the feed, which illuminates an array with specific dimensions (1530mmx100mm). Three different near-field focusing feeds have been developed and some of them have been validated experimentally. Then, two architectures of transmit-array antennas have been studied, using two different technologies: printed technology and a mixed technology with waveguide and horn.

En Imagerie par Résonance Magnétique (IRM), l’augmentation du champ magnétique statique permet en théorie de fournir un rapport signal sur bruit accru, améliorant la qualité des images. L’objectif de l’IRM à ultra haut champ est d’atteindre une résolution spatiale suffisamment haute pour pouvoir distinguer des structures si fines qu’elles sont actuellement impossibles à visualiser de façon non-invasive. Cependant, à de telles valeurs de champs magnétiques, la longueur d’onde du rayonnement électromagnétique envoyé pour basculer les spins des protons de l’eau est du même ordre de grandeur que l’objet dont on souhaite faire l’image. Des phénomènes d’interférences sont observés, ce qui se traduit par l’inhomogénéité de ce champ radiofréquence (RF) au sein de l’objet. Ces interférences engendrent des artefacts de signal et/ou de contraste dans les images IRM, et rendent ainsi leur exploitation délicate. Il est donc crucial de fournir des solutions pour atténuer la non-uniformité de l’excitation des spins, à défaut de quoi de tels systèmes ne pourront atteindre leurs pleins potentiels. Pour obtenir des diagnostics pertinents à très haut champ, il est donc nécessaire de créer des impulsions RF homogénéisant l'excitation de l'ensemble des spins (ici du cerveau humain), optimisées pour chaque individu. Pour cela, un système de transmission parallèle (pTX) à 8 canaux a été installé au sein de notre imageur à 7 Tesla. Alors que la plupart des systèmes IRM cliniques n’utilisent qu’un seul canal d’émission, l’extension pTX permet de jouer différentes formes d’impulsions RF de concert. La somme résultante de ces interférences doit alors être optimisée pour atténuer la non-uniformité observée classiquement. L’objectif de cette thèse est donc de synthétiser ce type d’impulsions, en utilisant la pTX. Ces impulsions auront pour contrainte supplémentaire le respect des limitations internationales concernant l'exposition à des champs radiofréquence, qui induit une hausse de température dans les tissus. En ce sens, de nombreuses simulations électromagnétiques et de températures ont été réalisées en introduction de cette thèse, afin d’évaluer la relation entre les seuils recommandés d’exposition RF et l’élévation de température prédite dans les tissus. Cette thèse porte plus spécifiquement sur la conception de l’ensemble des impulsions RF refocalisantes utilisées dans des séquences IRM non-sélectives, basées sur l’écho de spin. Dans un premier temps, seule une impulsion RF a été générée, pour une application simple : l’inversion du déphasage des spins dans le plan transverse. Dans un deuxième temps, sont considérées les séquences à long train d’échos de refocalisation appliquées à l’in vivo. Ici, l’opérateur mathématique agissant sur la magnétisation, et non pas son état final comme il est fait classiquement, est optimisé. Le gain en imagerie à très haut champ est clairement visible puisque les opérations mathématiques (la rotation des spins) voulues sont réalisées avec plus de fidélité que dans le cadre des méthodes de l’état de l’art. Pour cela, la génération de ces impulsions RF combine une méthode d’excitation des spins avec navigation dans l’espace de Fourier, les kT-points, et un algorithme d’optimisation, appelé Gradient Ascent Pulse Engineering (GRAPE), utilisant le contrôle optimal. Cette conception est rapide grâce à des calculs analytiques plus directs que des méthodes de différences finies. La prise en compte d’un grand nombre de paramètres nécessite l’usage de GPUs (Graphics Processing Units) pour atteindre des temps de calcul compatibles avec un examen clinique. Cette méthode de conception d’impulsions RF a été validée expérimentalement sur l’imageur 7 Tesla de NeuroSpin, sur une cohorte de volontaires sains. / In Magnetic Resonance Imaging (MRI), the increase of the static magnetic field strength is used to provide in theory a higher signal-to-noise ratio, thereby improving the overall image quality. The purpose of ultra-high-field MRI is to achieve a spatial image resolution sufficiently high to be able to distinguish structures so fine that they are currently impossible to view in a non-invasive manner. However, at such static magnetic fields strengths, the wavelength of the electromagnetic waves sent to flip the water proton spins is of the same order of magnitude than the scanned object. Interference wave phenomena are then observed, which are caused by the radiofrequency (RF) field inhomogeneity within the object. These generate signal and/or contrast artifacts in MR images, making their exploitation difficult, if not impossible, in certain areas of the body. It is therefore crucial to provide solutions to mitigate the non-uniformity of the spins excitation. Failing this, these imaging systems with very high fields will not reach their full potential.For relevant high field clinical diagnosis, it is therefore necessary to create RF pulses homogenizing the excitation of all spins (here of the human brain), and optimized for each individual to be imaged. For this, an 8-channel parallel transmission system (pTX) was installed in our 7 Tesla scanner. While most clinical MRI systems only use a single transmission channel, the pTX extension allows to simultaneously playing various forms of RF pulses on all channels. The resulting sum of the interference must be optimized in order to reduce the non-uniformity typically seen.The objective of this thesis is to synthesize this type of tailored RF pulses, using parallel transmission. These pulses will have as an additional constraint the compliance with the international exposure limits for radiofrequency exposure, which induces a temperature rise in the tissue. In this sense, many electromagnetic and temperature simulations were carried out as an introduction of this thesis, in order to assess the relationship between the recommended RF exposure limits and the temperature rise actually predicted in tissues.This thesis focuses specifically on the design of all RF refocusing pulses used in non-selective MRI sequences based on the spin-echo. Initially, only one RF pulse was generated for a simple application: the reversal of spin dephasing in the transverse plane, as part of a classic spin echo sequence. In a second time, sequences with very long refocusing echo train applied to in vivo imaging are considered. In all cases, the mathematical operator acting on the magnetization, and not its final state as is done conventionally, is optimized. The gain in high field imaging is clearly visible, as the necessary mathematical operations (that is to say, the rotation of the spins) are performed with a much greater fidelity than with the methods of the state of the art. For this, the generation of RF pulses is combining a k-space-based spin excitation method, the kT-points, and an optimization algorithm, called Gradient Ascent Pulse Engineering (GRAPE), using optimal control.This design is relatively fast thanks to analytical calculations rather than finite difference methods. The inclusion of a large number of parameters requires the use of GPUs (Graphics Processing Units) to achieve computation times compatible with clinical examinations. This method of designing RF pulses has been experimentally validated successfully on the NeuroSpin 7 Tesla scanner, with a cohort of healthy volunteers. An imaging protocol was developed to assess the image quality improvement using these RF pulses compared to typically used non-optimized RF pulses. All methodological developments made during this thesis have contributed to improve the performance of ultra-high-field MRI in NeuroSpin, while increasing the number of MRI sequences compatible with parallel transmission.

Motivated by the increasing demand for higher data rates, coverage, and spectral efficiency, current and next generation wireless systems adapt transmission parameters and even who is being transmitted to, based on the instantaneous channel states. For example, frequency-domain scheduling(FDS) is an instance of adaptation in orthogonal frequency division multiple access(OFDMA) systems in which the base station opportunistically assigns different subcarriers to their most appropriate user. Likewise ,transmit antenna selection(AS) is another form of adaptation in which the transmitter adapts which subset of antennas it transmits with. Cognitive radio(CR), which is a next generation technology, itself is a form of adaptation in which secondary users(SUs) adapt their transmissions to avoid interfering with the licensed primary users(PUs), who own the spectrum. However, adaptation requires channel state information(CSI), which might not be available apriori at the node or nodes that are adapting. Further, the CSI might not be perfect due to noise or feedback delays. This can result in suboptimal adaptation in OFDMA systems or excessive interference at the PUs due to transmissions by the SUs in CR. In this thesis, we focus on adaptation techniques in current and next generation wireless systems and evaluate the impact of CSI –both perfect and imperfect –on it. We first develop a novel model and analysis for characterizing the performance of AS in frequency-selective OFDMA systems. Our model is unique and comprehensive in that it incorporates key LTE features such as imperfect channel estimation based on dense, narrow band demodulation reference signal and coarse, broad band sounding reference signal. It incorporates the frequency-domain scheduler, the hardware constraint that the same antenna must be used to transmit over all the subcarriers that are allocated to a user, and the scheduling constraint that the allocated subcarriers must all be contiguous. Our results show the effectiveness of combined AS and FDS in frequency-selective OFDMA systems even at lower sounding reference signal powers. We then investigate power adaptation in underlay CR, in which the SU can transmit even when the primary is on but under stringent interference constraints. The nature of the interference constraint fundamentally decides how the SU adapts its transmit power. To this end, assuming perfect CSI, we propose optimal transmit power adaptation policies that minimize the symbol error probability of an SU when they are subject to different interference and transmit power constraints. We then study the robustness of these optimal policies to imperfections in CSI. An interesting observation that comes out of our study is that imperfect CSI can not only increase the interference at the PU but can also decrease it, and this depends on the choice of the system parameters, interference, and transmit power constraints. The regimes in which these occur are characterized.

Σκοπός αυτής της εργασίας είναι η μελέτη τεχνικών κατανεμημένου προσανατολισμού σε πραγματικές συνθήκες. Πιο συγκεκριμένα σε αυτά στα συστήματα θεωρείται ότι ο κόμβος-πομπός δεν έχει καλή σύνδεση με το δέκτη και κατά συνέπεια δεν μπορεί να επικοινωνήσει απευθείας με τον κόμβο-δέκτη χωρίς δραματική αύξηση της ενέργειας μετάδοσης. Παρόλα αυτά η χρήση κατανεμημένου προσανατολισμού δίνει τη δυνατότητα να βελτιωθεί σημαντικά η κατανάλωση ενέργειας. Το σχήμα που θα χρησιμοποιηθεί είναι ενίσχυση και προώθηση (AF) 2 βημάτων, με το οποίο οι συνεργατικοί κόμβοι απλώς ενισχύουν και στην συνέχεια επαναμεταδίδουν το μήνυμα. Συνεπώς, ζητούμενο είναι η εύρεση των μιγαδικών βαρών με τα οποία πρέπει ο κάθε συνεργαζόμενος κόμβος χωριστά να ενισχύσει το σήμα. Οι τεχνικές που θα χρησιμοποιηθούν έχουν ως κριτήρια την ελαχιστοποίηση της ενέργειας μετάδοσης με ταυτόχρονη ικανοποίηση του SNR, μεγιστοποίηση του SNR με περιορισμένη ολική ενέργεια μετάδοσης και μεγιστοποίηση του SNR με περιορισμένη ενέργεια μετάδοσης ανά συνεργαζόμενο κόμβο. Το πρώτο κριτήριο θα εξεταστεί, επίσης, και σε συστήματα με πολλαπλούς πομπούς και δέκτες. Λόγω της φύσης του προβλήματος, ο κατανεμημένος προσανατολισμός αναμένεται να έχει μεγάλη απήχηση σε συστήματα με πολλούς διασκορπιστές και εμπόδια, όπως σε ένα αστικό περιβάλλον, και, επομένως, είναι λογικό να θεωρηθεί ότι τα κανάλια του συστήματος είναι Rayleigh, δηλαδή ασυσχέτιστα χωρίς οπτική επαφή (LOS). Για να προσομοιωθεί το σύστημα σε πραγματικές συνθήκες οι μέθοδοι που θα υλοποιήσουμε στην εργασία χρησιμοποιούν τα στατιστικά του καναλιού. Επιπλέον, η εκτίμηση καναλιού εφόσον θεωρούμε ότι έχουμε Gaussian λευκό θόρυβο θα γίνει με την χρήση του βέλτιστου γραμμικού εκτιμητή (BLUE). Η επίδραση της εκτίμησης του καναλιού θα μελετηθεί για δύο περιπτώσεις: με αμοιβαία και χωρίς αμοιβαία κανάλια. / The purpose of this thesis is the study of methods of distributed beamforming under real circumstances. More specifically, these systems are considered that the transmitter must increase tremendously the required transmit energy to communicate with the receiver. However the use of the distributed beamforming allows the system to improve the energy consumption. The scheme that is used from relays is amplify and forward of two steps, where the relays only amplify and then forward the message to the destination. That is, the purpose is to find the complex weights to be used by the corresponding relay so as to amplify the message of the transmitter. The methods that are implemented have as criterions the minimization of transmit energy while satisfying the SNR, maximization of SNR while limiting the system's transmit energy and maximization of SNR while limiting transmit energy of each relay individually. The first criterion is also studied at systems with more than one pair transmitter-receiver. Due to the nature of the problem, distributed beamforming is expected to be used at environments with many obstacles and scatterers, like urban environment, and so it is rationale to suppose that the channels should be Rayleigh, meaning uncorrelated without line of sight. To simulate the system under real circumstances the methods that we will implement shall use the second order statistics of the channels. Moreover, due to Gaussian white noise, channels are estimated using the Best Linear Unbiased Estimator. The impact of channel estimation is studied in two cases: "reciprocal" and "not reciprocal".

Magnetresonanztomographie (MRT) ist eine nichtinvasive Bildgebungsmethode, die in der Medizin sowie in der Forschung eingesetzt wird und auf der magnetischen Kernresonanz beruht. Die Erforschung der Ultrahochfeld (UHF) MRT ab Magnetfeldstärken von 7.0 Tesla und darüber ist durch einen intrinsischen Signalgewinn hin zu hohen Magnetfeldstärken motiviert und beschäftigt sich mit den dabei auftretenden physikalischen Effekten ebenso wie mit den dazu notwendigen neuartigen Technologien. Die vorliegende Arbeit untersucht Mehrkanalantennen zur Anregung der magnetischen Kernresonanz sowie zum Empfang des resultierenden Signals bei 7.0 T. Für die magnetische Kernresonanz von Protonen ergibt sich eine Resonanzfrequenz von 300 MHz. Die zugehörige Wellenlänge in menschlichem Gewebe verlässt in diesem Frequenzbereich im Verhältnis zu den Körperabmessungen den quasistatischen Bereich. Die sich ergebende Wellenausbreitung hat Interferenzmuster in den erzeugten Bildern zur Folge, die zu klinisch nicht verwertbaren Bildinformationen führen können. Vor diesem Hintergrund wurden in dieser Arbeit Mehrkanalantennen mit 4, 8 und 16 unabhängigen Elementen zur Signalanregung und zum Empfang konzipiert, aufgebaut und untersucht. Die Erkenntnisse mündeten in der erfolgreichen Implementierung der weltweit ersten 32-Kanal Antenne zur kardiovaskulären Bildgebung bei 7.0 T. Darüber hinaus wurde eine Antenne entwickelt, welche die ersten auf der Natriumkonzentration beruhenden bewegten MRT Bilder des menschlichen Herzens bei 7.0 T ermöglichte. Der Zusammenhang zwischen Natriumkonzentration und Zellintegrität ermöglicht direkte und ortsaufgelöste Einblicke in physiologische Prozesse. Die Ergebnisse dieser Arbeit belegen die breite Anwendbarkeit von Mehrkanalantennen in der UHF MRT zur Protonen-und Natriumbildgebung und bilden eine solide technologische Basis für breitere klinische Studien, um die Ultrahochfeld MRT reif für den routinemäßigen Einsatz im Gesundheitswesen zu machen. / Magnetic resonance imaging (MRI) is a non-invasive imaging method based on the effect of nuclear magnetic resonance. It is used in healthcare as well as in research. MRI at magnetic field strengths of 1.5 Tesla and 3 Tesla is well established. The gain in signal-to-noise ratio (SNR) intrinsic to higher magnetic field strength fuels the vigorous research field of Ultrahigh field (UHF) MRI at 7.0 T and above. Nevertheless for MRI based upon proton imaging the wavelength of the transmitted electro-magnetic fields slowly departs from the semi-static regime and reaches the dimension of the transection of the human body at 7.0 T. This gives rise to constructive and destructive interferences that potentially render image quality non-diagnostic for clinical use. Therefore is work proposes the worlds’ first 32 channel antenna array for cardiovascular MRI at 7.0 T. Electro-magnetic field simulations are utilized to study the capabilities of multi-channel RF antenna arrays to mitigate destructive interferences and provided the basis for a workflow towards homogenization of the electromagnetic radio-frequency field. Pre-clinical studies showed the capabilities and limits of translating the SNR gain of UHF MRI into clinical beneficial numbers, namely increased spatial or temporal resolution or scan time shortening. To make further use of the benefits of UHR MRI and to make a step towards first-hand spatial resolved information of biological processes in human tissue sodium imaging of the human heart was enabled with the design of a tailored antenna array. The results were reconstructed into the first movies of the human heart at 7.0 T based on sodium signal. This profound technological basis for radio frequency excitation and reception in UHF MRI can be expected to pave the way for broader clinical studies at 7.0 T with the ultimate goal to improve the quality and the earliness of treatment decisions in future clinical practice.

Les Réseaux de capteurs sans fil (WSN) sont une technologie émergente avec des applications potentielles dans divers domaines de la vie quotidienne, tels que la surveillance structurelle et environnementale, la médecine, la surveillance militaire, les explorations robotisées, etc. Les nœuds de capteurs doivent fonctionner pendant une longue période avec des batteries capacité limitée, par conséquent le facteur plus important dans les WSN est la consommation d'énergie. Dans cette thèse, nous proposons des techniques d'optimisation algorithmiques dynamiques, et adaptative pour la réduction de l'énergie. Tout d'abord, un modèle énergétique précis est présenté. Ce modèle repose sur des mesures réelles de courant consommé pour différents scénarios qui peuvent se produire lors de la communication entre les nœud. Il en est conclu que la couche MAC joue un rôle essentiel dans la réduction de l'énergie consommée. Ensuite, un protocole MAC dynamique est présenté. Il adapte de manière dynamique l’intervalle de réveil des nœuds de capteurs à partir d’une estimation du trafic. L’algorithme adaptatif modélisé de façon heuristique pour comprendre le comportement de convergence des paramètres algorithmiques. Le protocole est appliqué sur des réseaux de capteurs corporels et il surclasse les autres protocoles MAC en termes de latence ainsi que de consommation d'énergie ce qui permet donc d'augmenter la durée de vie de trois à six fois. Enfin, une technique basée sur l’optimisation adaptative de la puissance d'émission radio est appliquée sur des canaux variant dans le temps. La puissance de sortie est réglée dynamiquement au meilleur niveau de puissance selon l’état du canal, ce qui diminue la consommation d’un facteur deux. / Wireless Sensor Networks (WSN) are a fast emerging technology with potential applications in various domains of daily-life, such as structural and environmental monitoring, medicine, military surveillance, robotic explorations etc. WSN devices are required to operate for a long time with limited battery capacity, therefore, the most important constraint in WSN is energy consumption. In this thesis, we propose algorithmic-level dynamic and adaptive optimization techniques for energy reduction in WSN. First, an accurate energy model is presented. This model relies on real-time power measurements of various scenarios that can occur during communication between sensor nodes. It is concluded that MAC layer plays a pivotal role for energy reduction. Then, a traffic-aware dynamic MAC protocol is presented which dynamically adapts the wake-up schedule of sensor nodes through traffic estimation. An adaptive algorithm is designed for this purpose that is heuristically modeled to understand the convergence behavior of algorithmic parameters. The proposed protocol is applied to body area networks and it outperforms other low-power MAC protocols in terms of latency as well as energy consumption and consequently increases the lifetime from three to six times. Finally, an SNR-based adaptive transmit power optimization technique is applied under time-varying channels. The output power is dynamically tuned to best power level under slow varying channel, which results in an average gain by two times.

The interest in the development of wireless high-speed data services is in response to the strong market demand for high-speed wireless Internet access. Current standards aim at delivering a peak data rate greater than 2Mbps on the forward link. Since data services and voice services are fundamentally different, new concepts were introduced in the design of the forward data channel. In addition, methods of evaluating the performance of a cellular CDMA system have to be revisited. This thesis proposes a method which can be used to find the forward link peak and average data rates, throughput and coverage of a cellular CDMA system which is capable of delivering high-speed wireless data. A summary of changes in design philosophy and recent advances in technologies which enable high-speed wireless data delivery are presented. The proposed method takes into account major aspects commonly found in the forward data channel and applies the generalized Shannon capacity formula for multi-element antenna (MEA) systems. The analysis focuses on the physical layer and is flexible enough to be adapted to various propagation environments, antenna configurations, multicode allocations, user distributions and cell site configurations. Sample numerical results for various multicode allocations are shown using a system model with two-tier interfering cells with one transmit antenna and two receive antennas operating under a frequency selective slow fading channel with propagation environment described by the Recommendation ITU-R M. 1225 indoor office, outdoor to indoor and pedestrian and vehicular test environments. Different transmit / receive antenna configurations and multicode allocations and their impact on the average data rate is also explored.

The interest in the development of wireless high-speed data services is in response to the strong market demand for high-speed wireless Internet access. Current standards aim at delivering a peak data rate greater than 2Mbps on the forward link. Since data services and voice services are fundamentally different, new concepts were introduced in the design of the forward data channel. In addition, methods of evaluating the performance of a cellular CDMA system have to be revisited. This thesis proposes a method which can be used to find the forward link peak and average data rates, throughput and coverage of a cellular CDMA system which is capable of delivering high-speed wireless data. A summary of changes in design philosophy and recent advances in technologies which enable high-speed wireless data delivery are presented. The proposed method takes into account major aspects commonly found in the forward data channel and applies the generalized Shannon capacity formula for multi-element antenna (MEA) systems. The analysis focuses on the physical layer and is flexible enough to be adapted to various propagation environments, antenna configurations, multicode allocations, user distributions and cell site configurations. Sample numerical results for various multicode allocations are shown using a system model with two-tier interfering cells with one transmit antenna and two receive antennas operating under a frequency selective slow fading channel with propagation environment described by the Recommendation ITU-R M. 1225 indoor office, outdoor to indoor and pedestrian and vehicular test environments. Different transmit / receive antenna configurations and multicode allocations and their impact on the average data rate is also explored.

It is well known that multiple antennas at the transmitter and receiver are imperative for reliable and high data-rate communication over wireless channels. However, these systems essentially need multiple radio frequency (RF) chains owing to multiple antennas, and hence pose challenges for applications with limited form-factor. Antenna Selection (AS) techniques alleviate this problem by using only a subset of the total available antennas and hence require only a few RF chains compared to the number of antennas. These systems operate in a closed-loop scenario, where the information fed back from the receiver is used for the transmit antenna subset selection. In contrast to this, a novel open-loop technique known as spatial modulation (SM) was recently proposed that uses a single RF-chain at the transmitter and achieves a higher spectral efficiency compared to single-input and AS based systems. The work in the thesis mainly focuses on the following aspects of SM system: Study of Mutual Information in SM systems operating in open-loop and closed-loop scenarios: We study the achievable mutual information in the SM system operating with finite and Gaussian input alphabet, and compare the results with that of the SIMO and AS based systems. Reduced-complexity maximum-likelihood (ML) decoding algorithms for SM systems: We propose ML-optimal sphere decoders for SM systems with arbitrary number of transmit antennas. Furthermore, a reduced-complexity ML detector is also proposed whose computational complexity is lowest among the known existing detectors in the literature. Transmit diversity techniques for SM systems: The conventional SM system achieves a transmit diversity order of one. We propose a complex interleaved orthogonal design baaed SM scheme that achieves a transit diversity order of two, while offering symbol-by- symbol ML decodability. Transmit antenna subset selection algorithms for SM systems: The SM system is considered in the closed-loop scenario, where only a subset of the total number of transmit antennas is chosen based on the information fed back by the receiver. Specifically, the Euclidean distance and capacity optimized antenna selection algorithms are studied in comparison with the conventional AS based systems. SM system operating in dispersive channels: The SM system operating in a dispersive channel with the aid of zero-padding is studied. It is shown that the SM system achieves full receive-diversity and multipath-diversity with ML decoding, but offers a decoding complexity that is exponential in the number of multipaths. Furthermore, a reduced complexity linear receiver is proposed that achieves achieves full multipath as well as receive-diversity, while offering a decoding complexity order same as that of the SM system operating in a frequency-flat channel.