The recent growth in demand for wireless services coupled with the limited spectrum available for these services has spawned new efforts to increase the spectral efficiency of wireless links. Recent research has shown that in multipath propagation environments, the spatial characteristics of the propagation channel can be exploited to increase spectral efficiency through the use of multiple antennas at the transmitting and receiving nodes. Such multiple-input multiple-output (MIMO) systems show promise for dramatic performance gains over their single-antenna counterparts. However, MIMO system performance is influenced by many different factors. Antenna array configuration directly contributes to MIMO system performance. The ability to build and integrate adaptive antenna arrays into MIMO systems requires the development of strategies for determining which antenna array configuration best enhances performance. Since an exhaustive search of all configurations is computationally prohibitive, this dissertation develops information theoretic based, computationally tractable solutions for determining favorable array configurations. The characteristics of the MIMO receiver front-end also play a large role in determining how well the system performs. Where portable MIMO devices will be forced to closely space antenna elements, mutual coupling can greatly impact both capacity and diversity performance. To study strategies for mitigating mutual coupling performance degradation, an accurate receiver front-end model is necessary. This work realistically models amplifier noise in the receiver and determines how matching networks may be used to improve system performance in the presence of antenna mutual coupling and amplifier coupling. Since MIMO systems operate by identifying optimal antenna array weights for the channel of interest, it is surprising that array superdirectivity has yet to be observed in theoretical solutions to the problem. When formulating system capacity using a radiated power constraint, the capacity is shown to be overestimated due to superdirectivity. Since superdirectivity provides for elegant theoretical results and poor realistic performance, this work incorporates constraints into the formulation of system capacity to arrive at phyically achievable capacity values.

On considère les paquets d'ondes hydrodynamiques comme mécanismes de génération de bruit des jets subsoniques. Cette approche résulte tout d'abord de l'analyse de données numériques - DNS d'une couche de mélange (Wei et Freund 2006) et LES d'un jet à Mach 0,9 (Daviller 2010) - permettant de déterminer les propriétés des sources en termes de compacité, d'intermittence et de structure azimutale. L'identification d'un rayonnement intermittent associé aux modifications des structures cohérentes des écoulements permet de proposer un modèle de paquet d'onde pour représenter ce phénomène dans l'analogie de Lighthill, dont l'enveloppe présente des variations temporelles d'amplitude et d'étendue spatiale. Celles-ci sont tirées de données de vitesse de simulations numériques de jets subsoniques, et un accord de l'ordre de 1,5dB entre le champ acoustique simulé et le modèle confirme sa pertinence. L'exploration du concept proposé est ensuite poursuivie expérimentalement, avec des mesures de pression acoustique et de vitesse de jets turbulents subsoniques, permettant la décomposition des champs en modes de Fourier azimutaux. On observe l'accord des directivités des modes 0, 1 et 2 du champ acoustique avec le rayonnementd'un paquet d'onde. Les modes 0 et 1 du champ de vitesse correspondent également à des paquets d'onde, modélisés comme des ondes d'instabilité linéaires à partir des équations de stabilité parabolisées. Finalement, des corrélations de l'ordre de 10% entre les modes axisymétriques de vitesse dans le jet et de pression acoustique rayonnée montrent un lien clair entre les paquets d'onde et l'émission acoustique du jet. / Hydrodynamic wavepackets are studied as a sound-source mechanism in subsonic jets. We first analyse numerical simulations to discern properties of acoustic sources such as compactness, intermittency and azimuthal structure. The simulations include a DNS of a two-dimensional mixing layer (Wei and Freund 2006) and an LES of a Mach 0.9 jet (Daviller 2010). In both cases we identify intermittent radiation, which is associated with changes in coherent structures in the flows. A wave-packet model that includes temporal changes in amplitude and axial extension is proposed to represent the identified phenomena using Lighthill's analogy. These parameters are obtained from velocity data of two subsonic jet simulations, and an agreement to within 1.5dB between the model and the acoustic field of the simulations confirms its pertinence. The proposed mechanism is then investigatedexperimentally, with measurements of acoustic pressure and velocity of turbulent subsonic jets, allowing the decomposition of the fields into azimuthal Fourier modes. We find close agreement of the directivities of modes 0, 1 and 2 of the acoustic field with wave-packet radiation. Modes 0 and 1 of the velocity field correspond also to wavepackets, modelled as linear instability waves using parabolised stability equations. Finally, correlations of order of 10% between axisymmetric modes of velocity and far-field pressure show the relationship between wavepackets and sound radiated by the jet.

Pour faire cohabiter les nombreuses technologies radios, les terminaux mobiles nécessitent une miniaturisation de plus en plus poussée des antennes. Toutefois, les performances d'antennes ont des limites fondamentales liées à leurs dimensions physiques. La littérature met en évidence que les réseaux superdirectifs permettent de dépasser la limite de Harrington sur la directivité et que des antennes adaptées par des circuits non-Foster peuvent dépasser la limite de Bode-Fano sur la bande passante. Les contributions essentielles de ce travail de thèse consistent en la conception deréseaux d'antennes superdirectifs et d'antennes adaptées par des circuits non-Foster comme solutions possibles pour l'amélioration des performances des Antennes Electriquement Petites (AEP). Dans une première partie, un convertisseur d'impédance négative est réalisé pour obtenir des condensateurs de valeurs négatives de façon à adapter des antennes miniatures sur une large bande de fréquence. Dans la deuxième partie de ces travaux, les limites théoriques des réseaux d'antennes superdirectifs sont évaluées et une approche simple et pratique permettant la conception de ces réseaux à partir d'éléments parasites est proposée. L'intégration des AEP superdirectives sur des cartes de circuit imprimé est étudiée et les difficultés de mesure de ce type d'antenne sont évaluées. A partir de ces résultats, une nouvelle stratégie pour réaliser des réseaux compactes 3D ou planaires à polarisation linéaire ou circulaire en utilisant des éléments superdirectifs est présentée. / For supporting different wireless technologies, mobile terminals require significant miniaturization of antennas. However, antennas performance has some fundamental limits related to their physical dimensions. The available theory shows that superdirective arrays can exceed Harrington’s limit on antenna directivity and non-Foter matched antennas can surpass Bode-Fano limit on antenna bandwidth. Therefore, this work focuses on the design of superdirective antenna arrays and non-Foster matched antennas as possible solutions for improving the performance of Electrically Small Antennas (ESAs). In the first part: a Negative Impedance Converter (NIC) is designed to have a very small negative capacitor. The circuit is evaluated in terms of gain, stability and linearity. Then, the circuit is used to match several small antennas in the UHF band. In the second part: the theoretical limits of superdirective antenna arrays are studied. A simple and practical approach to design parasitic antenna arrays is proposed. The integration of superdirective ESAs in Printed Circuit Boards (PCBs) is studied and the difficulties of measuring this type of antennasare evaluated. A new strategy for the design of 3D or planar compact arrays, with linear or circular-polarization, using superdirective elements is presented.

While the capacity of multiple-input multiple-output (MIMO) systems has been explored in considerable detail, virtually all literature on this topic ignores electromagnetic considerations. This dissertation explores electromagnetic effects on the capacity performance of these multi-antenna architectures. Specifically, it examines the impact of superdirectivity for compact antenna arrays, the effect of antenna mutual coupling, and MIMO performance of multi-mode optical fiber with non-linear detection. Superdirectivity can lead to abnormally large capacity bounds in a MIMO communication system, especially when the antennas are placed close together. Because superdirective behavior is difficult to achieve in practice, this work formulates an approach for limiting the impact of superdirectivity by introducing finite ohmic loss into the capacity expressions. Results show that even a small amount of ohmic loss significantly affects the achievable system capacity and suppresses superdirective solutions. This formulation allows a more detailed examination of the capacity of MIMO systems for compact arrays. For channels which do not vary in time, placing antennas closer together generally reduces the system capacity. However, recent work has demonstrated that for a MIMO system operating in a fast fading environment where the transmitter and receiver know the channel covariance information, the capacity increases as antennas are placed near each other due to an increase in spatial correlation. Analysis of this behavior illustrates that when these capacity gains (due to closely spaced antennas) are observed the radiated power is also increased. Constraining the radiated power leads to superdirective solutions in which the ohmic loss constraint developed must be used to properly determine the capacity behavior of this system. Application of this constraint then leads to an optimum antenna spacing in contrast to the findings of previous research which indicate that antennas should be as close together as possible. Additionally, this section provides an analysis regarding the number of spatial modes that can be used for various system configurations. Recent research has shown that it is possible for MIMO communication techniques to be used with multimode optical fibers to increase the available distance-bandwidth. However, implementation of traditional MIMO schemes requires the use of coherent optical detection which can lead to high system complexity and cost. This dissertation proposes a multimode fiber MIMO system architecture which allows simultaneous transmission of unique streams to different users on the same fiber while using incoherent detection with amplitude and phase modulation at the transmitter. The resulting capacity scales nearly linearly with the number of transmitters and receivers. Because the architecture requires channel state information at the transmitter, a training scheme appropriate for use with optical intensity detection is also discussed.