Impulse-based Ultra Wideband (UWB) radio technology has recently gained significant research attention for various indoor ranging, sensing and communications applications due to the large amount of allocated bandwidth and desirable properties of UWB signals (e.g., improved timing resolution or multipath fading mitigation). However, most of the applications have focused on indoor environments where the UWB channel is characterized by tens to hundreds of resolvable multipath components. Such environments introduce tremendous complexity challenges to traditional radio designs in terms of signal detection and synchronization. Additionally, the extremely wide bandwidth and shared nature of the medium means that UWB receivers must contend with a variety of interference sources. Traditional interference mitigation techniques are not amenable to UWB due to the complexity of straight-forward translations to UWB bandwidths. Thus, signal detection, synchronization and interference mitigation are open research issues that must be met in order to exploit the potential benefits of UWB systems. This thesis seeks to address each of these three challenges by first examining and accurately characterizing common approaches borrowed from spread spectrum and then proposing new methods which provide an improved trade-off between complexity and performance. / Ph. D.

Abstract High radio capacity is one of the main targets in wireless network planning. The characteristics of the broadband radio channel pose serious challenges for achieving this goal. This thesis views the capacity problem from two frameworks. In the first, the effective user capacity in advanced direct sequence wideband code-division multiple-access (DS-CDMA) radio networks is evaluated. Sensitivity to various imperfections in key system parameters is studied. The analysis is based on a mathematical foundation that presents complex signal models and enables evaluation of the performance losses due to parameter estimation errors and multipath fading. The effective number of users supported in a cell is restricted by the multiple access interference (MAI) in the same cell (intracell interference) and overall background noise. The studied wideband CDMA receiver structures comprise conventional rake receivers with both the maximal ratio combining (MRC) and equal gain combining (EGC) schemes that can be supplemented with either linear decorrelating or nonlinear successive cancellation-based multiuser detectors and M-antenna spatial diversity. The second framework focuses on direct sequence spread spectrum-based ultra wideband (UWB) indoor communications. Cochannel interference limited capacity is evaluated against the outage probability criterion in exponentially decaying lognormal multipath fading channels. Distance-dependence and spatial distribution of users is taken into account at different spatial cell configurations. Only moderate complexity partial rake receivers with noncoherent combining are employed. Total interference is composed of interpath, multipath, intracell, and intercell interference contributions. Lognormal sum approximations and simulations are used to evaluate distributions of the desired and interfering signals. The impact of the timing errors at the receiver monopulse correlation is studied. The numerical results for the wideband CDMA framework show that effective user capacity and sensitivity depend critically on the joint impact of nonidealities in system parameters (e.g., channel profile, severity of fading, receiver algorithms). User capacities of the studied multiuser enhanced receivers were more prone to these impairments than those of the simpler single user receivers. The results should be used for network planning and optimization. The numerical results of the UWB framework suggest that, even in the multipath rich channel, the optimal number of rake fingers can be less than half of the significant multipaths. Differences between circular, square, and hexagonal cell models proved to be minor with respect to link distance distributions. The derived link distance statistics are useful tools in the analytic piconet dimensioning and optimization.

This thesis examines various signal processing techniques that are required for establishing efficient (near optimal) communications in multiuser multiple-input multiple-output (MIMO) environments. The central part of this thesis is dedicated to acquisition of information about the MIMO channel state - at both the receiver and the transmitter. This information is required to organize a communication set up which utilizes all the available channel resources. Realistic channel model, i.e., the spatial channel model (SCM), has been used in this study, together with modern long-term evolution (LTE) standard. The work consists of three major themes: (a) estimation of the channel at the receiver, also known as tracking; (b) quantization of the channel information and its feedback from receiver to the transmitter (feedback quantization); and (c) reconstruction of the channel knowledge at the transmitter, and its use for data precoding during communication transmission. / Communications

In this thesis, the problem of Angle of Arrival estimation of radar signals with Time Difference of Arrival method in an outdoor land multipath environment with limited line of sight is analyzed. A system model is proposed. Effects of system, channel and radar parameters on the Angle of Arrival estimation performance are investigated through Monte Carlo simulations. Improving effect of utilization of diversity on the estimation performance is observed. Performances of the space diversity with noncoherent and selective combining are compared. Finally a realistic scenario is studied and performance of the proposed system is investigated.

No description available.

In wireless communications systems, the transmission channel between the transmitter and the receiver antennas is one of the main sources of disruption for the signal. The multicarrier modulations, such as the orthogonal frequency division multiplexing (OFDM), are very robust against the multipath effect, and allow to recover the transmitted signal with a low error rate, when they are combined with a channel encoding. The channel estimation then plays a key role in the performance of the communications systems. In this PhD thesis, we study techniques based on least square (LS) and minimum mean square error (MMSE) estimators. The MMSE is optimal, but is much more complex than LS, and requires the a priori knowledge of the second order moment of the channel and the noise. In this presentation, two methods that allow to reach a performance close to the one of LMMSE while getting around its drawback are investigated. In another way, a third part of the presentation investigates the errors of estimation due to the interpolations.

Orthogonal frequency division multiplexing (OFDM) systems with cyclic prefix (CP) can be used to protect signal from the time-variant multipath channel induced distortions. However, the presence of CP could greatly decrease the effective data rate, thus many recent research works have been focused on the multiple-input multiple-output (MIMO) OFDM systems without CP (CP-free), equipped with the space-time block codes (ST-BC). The constraint of the conventional MIMO-OFDM (without using the ST-BC) system is that the number of receive-antenna has to be greater than the transmit-antenna. In this thesis, we first consider the ST-BC MIMO-OFDM system and show that the above-mentioned constraint can be removed, such that the condition become that the receive antenna should be greater than one, that is the basic requirement for MIMO system. It is particular useful and confirm to the recently specification, e.g., 3GPP LTE (Long Term Evolution) where the system deploy the 2¡Ñ2 or 4¡Ñ4 antennas systems. This thesis also considers the effects of peak-to-average power ratio (PAPR) in the transmitter and In-phase/ Quadrature-phase (IQ) imbalance in the receiver, and solves them by using the adaptive Volterra predistorter and blind adaptive filtering approach of the nonlinear parameters estimation and compensation, along with the power measurement, respectively. After the compensator of IQ imbalance in the receiver, an equalizer under the framework of generalized sidelobe canceller (GSC) is derived for interference suppression. To further reduce the complexity of receiver implementation, the partially adaptive (PA) scheme is applied by exploiting the structural information of the signal and interference signature matrices. As demonstrated from computer simulation results, the performance of the proposed CP-free ST-BC MIMO-OFDM receiver is very similar to that obtained by the conventional CP-based ST-BC MIMO-OFDM system under either the predistortion or compensation scenario.

Dans les systèmes de communications sans fil, le canal de transmission entre les antennes d’émission et de réception est l’une des principales sources de perturbation pour le signal. Les modulations multiporteuses, telles que l’OFDM (pour orthogonal frequency division multiplexing), sont très robustes contre l’effet des multi-trajets, et permet de retrouver le signal émis avec un faible taux d’erreur, quand elles sont combinées avec un codage canal. L’estimation de canal joue alors un rôle clé dans les performances des systèmes de communications. Dans cette thèse, on étudie des techniques fondées sur les estimateurs LS (pour least square, ou moindres carrés) et MMSE (pour minimum mean square error, ou erreur quadratique moyenne minimum). La technique MMSE est optimale, mais est beaucoup plus complexe que LS, et nécessite la connaissance a priori des moments de second ordre du canal et du bruit. Dans cette présentation, deux méthodes permettant d’atteindre des performances proches de LMMSE en évitant ses inconvénients sont étudiées. Une troisième partie étudie quant à elle les erreurs d’estimation dues aux interpolations. / In wireless communications systems, the transmission channel between the transmitter and the receiver antennas is one of the main sources of disruption for the signal. The multicarrier modulations, such as the orthogonal frequency division multiplexing (OFDM), are very robust against the multipath effect, and allow to recover the transmitted signal with a low error rate, when they are combined with a channel encoding. The channel estimation then plays a key role in the performance of the communications systems. In this PhD thesis, we study techniques based on least square (LS) and minimum mean square error (MMSE) estimators. The MMSE is optimal, but is much more complex than LS, and requires the a priori knowledge of the second order moment of the channel and the noise. In this presentation, two methods that allow to reach a performance close to the one of LMMSE while getting around its drawback are investigated. In another way, a third part of the presentation investigates the errors of estimation due to the interpolations.

Τα συστήματα που έχουν πολλαπλές κεραίες σε πομπό και δέκτη (ΜΙΜΟ) έχουν την δυνατότητα να επιτύχουν υψηλούς ρυθμούς δεδομένων και αυξημένη αξιοπιστία χωρίς να απαιτείται επιπλέον εύρος ζώνης ή ισχύς μετάδοσης. Η βασική αρχή στην οποία βασίζονται είναι ότι το σήμα που εκπέμπεται από τον πομπό διέρχεται από περισσότερα του ενός ανεξάρτητα κανάλια. Ο δέκτης εκμεταλλευόμενος τις πολλαπλές λήψεις του ίδιου σήματος μπορεί να βελτιώσει την απόδοσή του. Η εισαγωγή πολλαπλών κεραιών σε κάποια συστήματα δεν είναι δυνατό να συμβεί συνήθως λόγω του χώρου που απαιτείται. Παρόλα αυτά, σε αυτό το περιβάλλον επικοινωνίας υπάρχουν συνήθως πολλαπλοί χρήστες που μπορούν να συνεργαστούν και να δημιουργήσουν ένα κατανεμημένο σύστημα ΜΙΜΟ, που αναφέρεται στην βιβλιογραφία ως συνεργατικό (cooperative). Στα συστήματα MIMO έχουν προταθεί τεχνικές προσανατολισμένης επικοινωνίας (beamforming) με σκοπό την ακύρωση των παρεμβολών και του θορύβου στο δέκτη. Οι τεχνικές αυτές απαιτούν την γνώση της κατάστασης του καναλιού αφού πραγματοποιούν διάσπαση της μήτρας των συντελεστών του καναλιού κατά παράγοντες ώστε να εξάγουν τα διανύσματα βάρους με τα οποία θα πολλαπλασιάσουν τις ακολουθίες των συμβόλων που λαμβάνει ο δέκτης ή /και των συμβόλων που μεταδίδει ο πομπός. Συγκεκριμένα, μελετήθηκε η περίπτωση της τεχνικής προσανατολισμένης επικοινωνίας που στηρίζεται στην μέθοδο SVD η οποία πολλαπλασιάζει το διάνυσμα των προς μετάδοση συμβόλων στον πομπό και το διάνυσμα των ληφθέντων συμβόλων στο δέκτη με τα κατάλληλα ιδιάζοντα διανύσματα, επιτυγχάνοντας προσανατολισμένη επικοινωνία λήψης και εκπομπής (transmit και receive beamforming). Όπως αναφέρθηκε, η συγκεκριμένη μέθοδος απαιτεί την γνώση της κατάστασης του καναλιού σε πομπό και σε δέκτη. Για αυτό το σκοπό αρχικά θα μελετηθεί η επίδραση της εκτίμησης του καναλιού στην επίδοση της μεθόδου η οποία διεξάγεται από ένα εκτιμητή μέγιστης πιθανοφάνειας από ακολουθίες συμβόλων εκμάθησης που έχει μεταδώσει ο πομπός. Στην συνέχεια θα εξεταστεί η περίπτωση που τα κανάλια μπορούν να περιγραφούν από ένα μοντέλο πολλαπλών μονοπατιών. Ένα τέτοιο μοντέλο είναι δυνατό να εκτιμηθεί από δείγματα της κρουστικής απόκρισης του καναλιού, βελτιώνοντας περαιτέρω την επίδοση του συστήματος. Επίσης, παρέχει και την δυνατότητα της πρόβλεψης των μεταγενέστερων καταστάσεων του καναλιού, μειώνοντας αρκετά τον αριθμό των συμβόλων εκμάθησης που απαιτούνται. Η εκτίμηση των παραμέτρων του μοντέλου του καναλιού θα γίνει με την βοήθεια της μεθόδου ESPRIT. Τελικά, θα ενσωματωθεί η εκτίμηση του μοντέλου και στην τεχνική προσανατολισμένης επικοινωνίας και θα μελετηθεί η επίδοσή της. Στην συνέχεια θα εφαρμόσουμε την προηγούμενη διάταξη στην περίπτωση των συνεργατικών συστημάτων. Θα εφαρμοστεί λοιπόν, σε ένα τέτοιο σύστημα η τεχνική με την εκτίμηση του μοντέλου του καναλιού και θα αναλυθεί η επίδοσή του για τα δύο πιο γνωστά πρωτόκολλα επικοινωνίας μεταξύ του κόμβου-πηγής και των κόμβων-συνεργατών, το ενίσχυσης και προώθησης και το αποκωδικοποίησης και προώθησης. / The systems that have multiple transmit and receive antennas (MIMO) can achieve high data rates and increased reliability without the need for additional bandwidth or transmission power. The aforementioned is based on the transmission of the signal of the transmitter via multiple independent channels. The receiver can use the multiple versions of the same signal to improve its performance. The introduction of multiple antennas in some systems it is not possible due to the lack of space. On contrast, in a multi-user environment there are users of a single antenna that can cooperate to construct a distributed MIMO system, which are called in the bibliography as a cooperative system. A number of beamforming schemes have already proposed in MIMO systems with the view of interference and noise cancellation. These schemes compute the singular value decomposition of the channel matrix and use the singular vectors to extract the weight vectors that are used to multiply the sequences of symbols that transmitter transmits and the symbols that receiver receives. This scheme achieves transmit and receive beamforming and transmitter and receiver must have full channel state information (CSI). The next step is to examine the performance of this method under channel estimation errors. The estimation of the channel is carried out with a maximum likelihood estimator from training sequences that were transmitted from the transmitter. After that, we examine the case in which the channel taps can be modeled by a multipath model. The parameters of a model of this kind can be computed from noise corrupted samples with sub-space methods. In this thesis we use the ESPRIT method for the estimation of these parameters. After the estimation of the model’s parameters we can use the model to predict future values of channel taps, decreasing with this way the number of the training symbols that are needed. Then, we will use this method in the system with the beamforming scheme and we will evaluate its performance. Finally, the complete infrastructure will be applied to a cooperative system and its performance will be tested for the two most popular cooperation protocols, the amplify and forward protocol and the decode and forward protocol.

Dans les décennies à venir, la connaissance d’informations très précises concernant la position d’un objet permettra de créer des applications révolutionnaires dans les domaines sociaux, médicaux, commerciaux et militaires. La technologie Ultra-Wideband (UWB) est considérée comme un bon candidat permettant de fournir des capacités de localisation précise grâce à la mesure de l’estimation du temps d'arrivée (TOA). Dans cette thèse, des algorithmes de mesure de distance dans le cas multi-utilisateurs pour des systèmes UWB sont étudiés afin d'atteindre une bonne précision pour une faible complexité, avec de la robustesse aux interférences multi-utilisateur et dans le cas d’un grand nombre d'utilisateurs. Au cours de la dernière décennie, les signaux chaotiques ont reçu une attention significative en raison d'un certain nombre de caractéristiques intéressantes. Les signaux chaotiques sont des signaux non périodiques, déterministes ou considérés comme pseudo-aléatoires provenant de systèmes dynamiques non linéaires. Leur bonne autocorrélation et leurs faibles propriétés d’inter corrélation les rendent particulièrement résistants aux évanouissements par trajets multiples et capables d'atténuer les interférences multi-utilisateur (MUI). En raison de leur grande sensibilité aux conditions initiales, il est possible de générer un grand nombre de signaux chaotiques pour accroître la capacité globale du système. Dans cette thèse, deux nouveaux algorithmes d'estimation de TOA sont proposés dans un cadre multi-utilisateur avec une faible complexité et une bonne robustesse. Le nombre d'utilisateurs pris en charge par ces deux algorithmes est beaucoup plus grand que dans le cas des estimateurs de TOA actuels. Cependant, l'utilisation de séquences d'étalement classique et d’impulsion limite l'amélioration des performances et la capacité du système. Afin d’apporter des améliorations, des signaux chaotiques sélectionnés sont utilisés comme séquences d'étalement ou impulsion dans les algorithmes proposés. Grâce à l'utilisation de signaux chaotiques, notre algorithme est non seulement amélioré, mais permet également l’utilisation d’un plus grand nombre d'utilisateurs par comparaison avec l’algorithme utilisant des signaux classiques / In the coming decades, highly accurate position information has the potential to create revolutionary applications in the social, medical, commercial and military areas. Ultra-Wideband (UWB) technology is considered as a potential candidate for enabling accurate localization capabilities through Time-of-Arrival (TOA) based ranging techniques. Over the past decade, chaotic signals have received significant attention due to a number of attractive features. Chaotic signals are aperiodic, deterministic, and random-like signals derived from nonlinear dynamical systems whose good autocorrelation, low cross-correlation and sensitivity to the initial conditions make them particularly suitable to ranging systems. In this thesis, two new multiuser TOA estimation algorithms are proposed with low complexity and robustness to MUI, the number of users supported by which is much larger than current multiuser TOA estimators. While, the use of classic spreading sequences and ranging pulse constrain the further improvement of ranging performance and system capacity. For breaking through the limit brought by the classic signals, the selected chaotic signals are employed as the spreading sequences or ranging pulse in our proposed algorithms. With the use of chaotic signals, our proposed algorithm not only obtains the additional improvement, but also with capability to support larger number of users comparing with its counterpart using classic signals