Les réseaux réflecteurs imprimés connaissent un fort développement depuis la fin des années 80. Ce type d’antenne offre la possibilité de former des diagrammes de rayonnement complexes avec une relative simplicité, un faible coût de réalisation, de faibles pertes et un volume réduit. Cependant, il souffre encore de quelques défauts : - La non régularité de la géométrie de la cellule sur la surface du réseau, dans le cas d’une antenne passive, peut engendrer des dégradations sur le diagramme de rayonnement, surtout à la transition entre deux géométries extrêmes, lorsqu’un nouveau cycle de phase commence. - Le nombre relativement élevé de composants utilisés pour contrôler la phase de l’onde réfléchie, dans le cas d’une antenne reconfigurable, augmente le coût de fabrication de l’antenne et complexifie le circuit de commande des éléments reconfigurables. - La limitation en bande passante, qui a longtemps cantonné ce type d’antenne à des applications bande étroite, est principalement liée au comportement de la cellule unitaire constitutive du réseau. Dans cette thèse, nous nous intéressons donc à la conception de nouvelles topologies de cellules déphaseuses, passives et surtout reconfigurables, qui permettent, tout en conservant une relative simplicité de réalisation, d’offrir une large bande passante. De plus, le contrôle de la phase, dans le cas des cellules reconfigurables, doit être réalisé avec un nombre réduit de composants afin de respecter la contrainte de faible coût de fabrication. / Microstrip printed reflectarrays experience strong development since the late 80s. This type of antenna has the potential to form complex radiation patterns with relative simplicity, low cost, low losses and low profile. However, it still has some shortcomings: - The non-regularity of the geometry of the cell on the surface of the array, in the case of passive antenna, may cause degradation on the radiation pattern, especially at the transition between two extreme geometries, when a new phase cycle begins. - The high number of components used to control the phase of the reflected wave, in the case of reconfigurable antenna, increases the manufacturing cost of the antenna and complicates the control circuit of the reconfigurable elements. - The limited bandwidth has long confined this type of antenna to narrowband applications and is mainly due to the intrinsic bandwidth of the unit cell. In this thesis, we therefore focus on the design of new, passive and reconfigurable, phase-shifting cells that can provide a wide bandwidth while maintaining a simple implementation. In addition, the control of the phase, in the case of reconfigurable cells, must be made with a reduced number of components in order to comply the constraint of low manufacturing cost.

Cette thèse, menée en partenariat avec le CNES et la DGA s’inscrit dans un contexte international très actif sur les réseaux réflecteurs ou reflectarrays. Un reflectarray est constitué d’une source primaire placée au-dessus d’un réseau de cellules contrôlant les propriétés du champ réfléchi. Une reconfiguration du diagramme de rayonnement peut être réalisée de manière électronique en introduisant des éléments actifs dans chacune des cellules. Dans ce contexte, le travail réalisé a consisté à étudier une nouvelle topologie de cellule permettant de réfléchir indépendamment deux ondes incidentes, en polarisations circulaires gauche et droite, en bande X. Cette cellule, basée sur la superposition d’une cellule sélective en polarisation et d’une cellule simple polarisation, est conçue de façon à être compatible avec une reconfiguration électronique à l’aide de commutateurs. Après un important travail de simulation électromagnétique, une première validation expérimentale, menée à l’échelle de la cellule unitaire, a mis en évidence de très bonnes performances dans les deux polarisations (pertes inférieures à 1dB, déphasage atteignant 2 bits de résolution). A cette occasion, un banc de caractérisation spécifique a été développé. Une étude de faisabilité de la cellule reconfigurable a également été menée afin d’identifier les technologies de commutateur les plus pertinentes et de quantifier les perturbations apportées par la circuiterie de commande. Finalement, un démonstrateur de réseau à états figés a été réalisé et mesuré. Constitué de 97 cellules, il a permis de démontrer les potentialités de la structure développée, pour une application spatiale réaliste : dépointage jusqu’à 26°, bande passante de 800 MHz en bande X, réjection de polarisation croisée>20dB correspondant à une pureté de polarisation circulaire satisfaisante (TE<2dB). Il s’agit de la première antenne à réseau réflecteur qui permette de gérer indépendamment et simultanément les deux orientations de la polarisation circulaire à la même fréquence tout en offrant des capacités de reconfigurabilité. / This thesis done in partnership with the French Space Agency (CNES) and the French Defense Agency (DGA) is placed in a very active international context on reflectarrays antennas. A reflectarray consists of a primary source located above microstrip elements on a grounded substrate. The microstrip elements are designed to reradiate the incident wave. A reconfiguration of the radiation pattern can be electronically achieved by introducing switches in each element. In this context, for space applications in X-band, the objective of this thesis is to propose a dual-circular polarization (CP) unit-cell able to separate at the same frequency, the two incident circular polarizations. This unit-cell, made of two layers with reconfigurable capabilities, is based on a circular polarization selective surface (CPSS) and on a single polarization cell. After intensive electromagnetic simulations, the unit-cell in dual-circular polarization with reconfigurable capabilities has been experimentally validated using a specific waveguide measurement. In fact, the unitcell reflects independently and simultaneously the two incidents circular polarizations for a phase resolution around 2 bits in LHCP and in RHCP. A feasibility study of the reconfigurable cell was also carried out to identify the most relevant technologies. Then, a reflectarray in X-band has been designed, fabricated and measured. Made up of 97 cells, it has demonstrated the potentialities of the structure for a realistic space application: scan angle up to 26 °, bandwidth of 800MHz in X-band, cross-polarization rejection>20dB and good polarization purity (AR<2dB). This is the first time that a dual circular polarization reflectarray with reconfigurable capabilities has been validated with the unique capability to reflect independently and simultaneously the two incident circular polarization at the same frequency.

The rapid growth in demand for cellular mobile communications and emerging fixed wireless access has created the need to increase system capacity through more efficient utilization of the frequency spectrum, and the need for better grade of service. In cellular systems, capacity improvement can be achieved by reducing co-channel interference. Several techniques have been proposed in literature for mitigating co-channel interference, such as adaptive antennas and power control. Also, by allocating transmitter power and communication channels efficiently (resource allocation), overall co-channel interference can be maintained below a desired maximum tolerable level, while maximizing the carried traffic of the system. This dissertation presents investigation results on the performance of base station adaptive antennas, power control and channel allocation, as techniques for capacity improvement. Several approaches are analyzed. Firstly, we study the combined use of adaptive antennas and fractional loading factor, in order to estimate the potential capacity improvement achieved by adaptive antennas. Next, an extensive simulation analysis of a cellular network is carried out aiming to investigate the complex interrelationship between power control, channel allocation and adaptive antennas. In the first part of this simulation analysis, the combined use of adaptive antennas, power control and reduced cluster size is analyzed in a cellular system using fixed channel allocation. In the second part, we analyze the benefits of combining adaptive antennas, dynamic channel allocation and power control. Two representative channel allocation algorithms are considered and analyzed regarding how efficiently they transform reduced co-channel interference into higher carried traffic. Finally, the spatial filtering capability of adaptive antennas is used to allow several users to share the same channel within the same cell. Several allocation algorithms combined with power control are analyzed. / Ph. D.

Adaptive antenna arrays have tremendous potential for increasing the capacity of mobile communications, by reducing co-channel interference, multipath, and noise. Blind adaptive algorithms, that is, algorithms which do not require a training sequence, are investigated and compared in this study. These algorithms are tested for common cellular signals. The performances of three blind adaptive algorithms: the Constant Modulus Algorithm (CMA), the Spectral self-COherence Restoral Algorithm (SCORE), and the spectral correlation predictor using a Time-Dependent Adaptive Array (TDAA), are studied. The TDAA is introduced as a new blind algorithm that exploits the cyclostationary property of the signal. Results show that the TDAA is able to out-perform the other blind algorithms for most of the test conditions and provides the optimal MSE solution. / Master of Science

No description available.

Thesis (M. Eng.)--Carleton University, 2000. / Includes bibliographical references. Also available in electronic format on the Internet.

Thesis (Ph.D.)--University of Delaware, 2005. / Principal faculty adviser: Xiang-Gen Xia, Dept. of Electrical & Computer Engineering. Includes bibliographical references.

This thesis examines the problem of providing high data-rate wireless connectivity to users in indoor environments. The goal is to be able to reach Gbps/user rates even when there are multiple users present. The technology that we study is to use the 60 GHz spectrum whose special propagation properties make it ideally suited to this task. The approaches developed include using multiple spatially distributed smart antennas in a room or multiple co-located antennas to provide coverage where needed and when needed. All the antennas are connected to a single access point which allows us to dynamically change spectrum and link allocation among the users (as they move or as their needs change). The innovations in this work include the exploitation of the special properties of 60 GHz and the corresponding design of algorithms for efficient spectrum allocation. We use detailed simulations to demonstrate that very high data rates are indeed achievable.

OFDM is a promising technique for high-data-rate wireless communications because it can combat inter-symbol interference (ISI) caused by the dispersive fading of wireless channels. The proposed research focuses on techniques that improve the performance of OFDM-based wireless communications and its commercial and military applications. In particular, we address the following aspects of OFDM: inter-channel interference (ICI) suppression, interference suppression for clustered OFDM, clustered OFDM based anti-jamming modulation, channel estimation for MIMO-OFDM, MIMO transmission with limited feedback. For inter-channel interference suppression, a frequency domain partial response coding (PRC) scheme is proposed to mitigate ICI. We derive the near-optimal weights for PRC that is independent on the channel power spectrum. The error floor resulting from ICI can be reduced significantly using a two-tap or a three-tap PRC. Clustered OFDM is a new technique that has many advantages over traditional OFDM. In clustered OFDM systems, adaptive antenna arrays are used for interference suppression. To calculate weights for interference suppression, we propose a polynomial-based parameter estimator to combat the severe leakage of the DFT based estimator due to the small size of the cluster. An adaptive algorithm is developed to obtain optimal performance. For high data rate military communications, we propose a clustered OFDM base spread spectrum modulation to provide both anti-jamming and fading suppression capability. We analyze the performance of uncoded and coded system. Employing multiple transmit and receive antennas in OFDM systems (MIMO-OFDM) can increase the spectral efficiency and link reliability. We develop a minimum mean-square-error (MMSE) channel estimator that takes advantage of the spatial-frequency correlations in MIMO-OFDM systems to minimize the estimation error. We investigate the training sequence design and two optimal training sequence designs are given for arbitrary spatial correlations. For a MIMO system, the diversity and array gains can be obtained by exploiting channel information at the transmitter. For MIMO-OFDM systems, we propose a subspace tracking based approach that can exploit the frequency correlations of the OFDM system to reduce the feedback rate. The proposed approach does not need recalculate the precoding matrix and is robust to multiple data stream transmission.

As the size of an adaptive antenna array grows, the system is able to resist interference signals of increasing bandwidth. This is a result of the transmit pattern gain increasing, which raises the target's return power, and a greater number of degrees of freedom. However, once the interference signal decorrelates completely from one channel to the next, increasing array size will cease to improve detection capability. The use of tapped delay-line processing to improve correlation between channels has been studied for smaller arrays with single element antennas, but previous analyses have not considereded larger systems that are partitioned into subarrays. This thesis quantifies the effect that subarrays have on performance, as measured by the interference bandwidth that can be handled, and explains how tapped delay-line processing can maintain the ability to detect targets in an environment with high bandwidth interference. The analysis begins by deriving equations to estimate the half-power bandwidth of an array with no taps. Then we find that a single delay with optimal spacing is sufficient to completely restore performance if the interference angle is known exactly. However, in practice, the tap spacing will never be optimal because this angle will not be known exactly, so further consideration is given to this non-ideal case and possible solutions for arbitrary interference scenarios are presented. Simulations indicate that systems with multiple taps have more tolerance to increasing interference bandwidth and unknown directions of arrival. Finally, the tradeoffs between ideal and practical configurations are explained and suggestions are given for the design of real-world systems.