Radio frequency spectrum is a scarce natural resource that is utilized for many services including surveillance, navigation, communication, and broadcasting. Recent years have seen tremendous growth in use of spectrum especially by commercial cellular operators. As a result, cellular operators are experiencing a shortage of radio spectrum to meet bandwidth demands of users. Spectrum sharing is a promising approach to solve the problem of spectrum congestion as it allows cellular operators access to more spectrum in order to satisfy the ever growing bandwidth demands of commercial users. The US spectrum regulatory bodies are working on an initiative to share 150 MHz of spectrum, held by federal agencies, in the 3.5 GHz band with commercial wireless operators. This band is primarily used by radar systems that are critical to national defense. Field tests have shown that spectrum sharing between radars and communication systems require large separation distance in order to protect them from harmful interference. Thus, novel methods are required to ensure spectrum sharing between the two systems without the need of large protection distances. In order to efficiently share spectrum between radars and communication systems at the same time and in the same geographical area, a novel method is proposed that transforms radar signal in such a way that it does not interfere with communication systems. This is accomplished by projecting the radar signal onto null space of the wireless channel between radar and communication system. In order to understand the effects of the proposed sharing mechanism -- in urban, sub-urban, and littoral areas -- new channel models, specifically, two- and three-dimensional channel models are designed that capture azimuth and elevation angles of communication systems and helps in placing accurate nulls. In addition, interference coming from communication systems into radar receivers is analyzed and radar performance is accessed. Using this information, resource allocation schemes are designed for communication systems that take advantage of the carrier aggregation feature of the LTE-Advanced systems. This further helps in dynamic sharing of spectrum between radars and communication systems. The proposed signal projection approach not only meets radar objectives but also meets spectrum sharing objectives. However, there is a trade-off as signal projection results in some performance degradation for radars. Performance metrics such as probability of target detection, Cramer Rao bound and maximum likelihood estimate of target's angle of arrival, and beampattern of radar are studied for performance degradation. The results show minimal degradation in radar performance and reduction in exclusion zones, thus, showing the efficacy of the proposed approach. / Ph. D.

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Les travaux présentés dans cette thèse portent sur l'étude et la réalisation d'un système d'imagerie SAR (synthetic aperture radar) exploitant deux techniques provenant des communications numériques: la configuration MIMO et les signaux OFDM. Dans la première partie de cette étude, différentes méthodes de focalisation des signaux reçus pour la configuration MIMO sont proposées afin de mesurer l'impact de la configuration MIMO sur la robustesse du système d'imagerie SAR par rapport aux bruits. Par ailleurs, on mesure aussi l'impact de la configuration MIMO sur la résolution en azimut. Finalement, un système expérimental est développé au sein du laboratoire afin de confirmer les résultats obtenus par simulation. Dans la deuxième partie de cette étude, une méthode de réduction de l'ambiguïté en distance est proposée et validée par simulation. Cependant, l'utilisation de signaux classiques de type \textit{chirps} montre ses limites pour la réduction de l'ambiguïté en distance. Ainsi, une méthode de conception de signaux OFDM est développée afin de résoudre ce problème. Une dernière étude sur les signaux OFDM est mené dans le cadre de son utilisation dans la configuration MIMO pour l'imagerie SAR. L'impact des signaux OFDM sur la résolution azimutale ainsi que sur les différents paramètres de qualité images est étudié. / The work presented in this thesis focuses on the design and implementation of a SAR system operating with two Digital Communications technology: MIMO configuration and OFDM signals. In the first part of this study, various methods for focusing received signals for MIMO configuration are proposed in order to measure the impact of the MIMO configuration on the robustness. In addition, the impact of the MIMO configuration on the azimuth resolution is measured. Finally, an experimental system is developed in order to validate the results obtained by simulation. In the second part of this study, a range ambiguity suppression method is proposed and validated by simulation. However, the use of conventional chirp signals showed the limits of its use for the range ambiguity suppression. Thus, a design method of OFDM signals is developed in order to solve this problem. The last study on the OFDM signals is carried out in the context of its use with the MIMO configuration. The impact of the OFDM signals on the azimuth resolution and the imaging quality parameters are studied.

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With the recent advancements of wireless networks to satisfy new requirements, the investigation of novel transmission schemes to improve the link level performance is of major importance. A very common technique utilized in nowadays systems is the Orthogonal frequency division multiplexing (OFDM) waveform, which has been adopted by several standards, including WiFi, LTE, and more recently 5G, due to its simple equalization process. Despite its success, this dissertation shows that OFDM is a sub-optimal scheme under frequency-selective channel (FSC), when channel state information (CSI) is available at the receiver only. Based on the coded modulation capacity approach, this work demonstrates that the data symbols should experience the same channel gain in order to achieve the best performance, leading to the equal gain criterion (EGC). However, this comes at a cost in terms of losing orthogonality among data symbols. The result is valid for linear modulation matrices under the assumptions of CSI at only at the receiver with perfect feedback equalization. In order to attain the EGC for doubly-dispersive channels, the block multiplexing (BM) waveform is proposed in this thesis, where the data symbols are spread in frequency and time. For instance, the recently conceived orthogonal time frequency space (OTFS) is shown to be a particular case of BM with the classical single-carrier (SC). Regarding the equalization for the robust waveforms, it is shown that the minimum mean squared error with parallel interference cancellation (MMSE-PIC) employed together with convolutional encoder and soft decoder can completely remove the inter-symbol interference (ISI), where a low-complexity implementation is designed. In addition, a waveform with decreased complexity based on the sparse Walsh-Hadamard (SWH) is proposed for two reasons, i) sparse spreading requires a transform with lower size, ii) the Walsh-Hadamard transform is implemented with 1s and −1s, which requires less complexity than fast Fourier transform (FFT) based waveforms. Furthermore, the problem of estimating the time varying channel is considered, where a unique word (UW) or (pilot block) based approach is studied. In this regard, another main contribution of this dissertation is to develop an optimization framework, where the combination of channel estimation plus Doppler spread error is minimized. In particular, the composite error minimization is achieved by properly setting the FFT size of the system, for a fixed data length. Lastly, cyclic prefix (CP)-free system is considered such that the transmission time is decreased, and therefore provides a better channel estimation. Naturally, the CP-free system has undesirable interference, which is resolved by an iterative CP-Restoration algorithm. In this case, we extend the EGC to equal reliability criterion (ERC), i.e., the data symbols should be equally reliable and not only have equal gain. As a consequence, the BM with orthogonal chirp division multiplexing (OCDM) waveform has the best performance due to equal time and frequency spreading. In conclusion, the coded modulation capacity approach of this dissertation provides new insights and solutions to improve the performance of wireless systems.

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.