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Network coding has emerged as an attractive alternative to routing because of the through put improvement it provides by reducing the number of channel uses. In a wireless scenario, in addition, further improvement can be obtained through Physical layer Network Coding (PNC), a technique in which nodes are allowed to transmit simultaneously, instead of transmitting in orthogonal slots. In this thesis, the design and analysis of network coding schemes are considered, for wireless two-way relaying, multi-user Multiple Access Relay Channel (MARC) and wireline networks. In a wireless two-way relay channel with PNC, the simultaneous transmissions of user nodes result in Multiple Access Interference (MAI) at there lay node. The harmful effect of MAI is the presence of signal set dependent deep channel fade conditions, called singular fade states, under which the minimum distance of the effective constellation at the relay become zero. Adaptively changing the network coding map used at the relay according to channel conditions greatly reduces the impact of this MAI. In this work, we obtain these adaptive PNC maps, which are finite in number ,by completing partially filled Latin Squares and using graph vertex coloring. Having obtained the network coding maps, the set of all possible channel realizations is quantized into a finite number of regions, with a specific network coding map chosen in a particular region and such a quantization is obtained analytically for 2λ-PSK signal set. The performance of the adaptive PNC scheme for two-way relaying is analyzed and tight high SNR upper bounds are obtained for the average end-to-end symbol error probability, in terms of the average error probability of a point-to-point fading channel. The adaptive PNC scheme is generalized for two-way relaying with multiple antennas at the nodes. As an alternative to the adaptive PNC scheme for two-way relaying, a Distributed Space Time Coding (DSTC) scheme is proposed, which effectively re-moves the effect of singular fade states at the transmitting nodes itself without any Channel State Information at the Transmitter (CSIT), and without any need to change the PNC map as a function of channel fade conditions. It is shown that the singular fade states can be viewed equivalently as vector subspaces of C2, which are referred to as the singular fade subspaces. DSTC design criterion to minimize the number of singular fade subspaces and maximize the coding gain is formulated and explicit low decoding complexity DSTC designs are provided. For the K-user MARC, in which K source nodes want to transmit messages to a destination node D with the help of are lay node R, a new PNC scheme is proposed. Use of a many-to-one PNC map with conventional minimum squared Euclidean distance decoding at D, results in a loss of diversity order due to error propagation from the relay node. To counter this, we propose a novel low complexity decoder which offers the maximum diversity order of two. Next, we consider wire line networks and explore the connections between linear network coding, linear index coding and discrete polymatroids, which are the multi-set analogue of matroids. We define a discrete polymatroidal network and show that a fractional vector linear solution over a field Fq exists for a network if and only if the network is discrete polymatroidal with respect to a discrete polymatroid representable over Fq.An algorithm to construct networks starting from certain class of discrete polymatroids is provided. Every representation over Fq for the discrete polymatroid, results in a fractional vector linear solution over Fq for the constructed network. It is shown that a linear solution to an index coding problem exists if and only if there exists a representable discrete polymatroid satisfying certain conditions which are determined by the index coding problem considered. El Rouayheb et. al. showed that the problem of finding a multi-linear representation for a matroid can be reduced to finding a perfect linear index coding solution for an index coding problem obtained from that matroid. Multi-linear representation of a matroid can be viewed as a special case of representation of an appropriate discrete polymatroid. We generalize the result of El Rouayheb et. al. by showing that the problem of finding a representation for a discrete polymatroid can be reduced to finding a perfect linear index coding solution for an index coding problem obtained from that discrete polymatroid.

Secret key generation from reciprocal multi-antenna channels is an interesting alternative to cryptographic key management in wireless systems without infrastructure access. In this work, we study the secret key rate for the basic source model with a MIMO channel. First, we derive an expression for the secret key rate under spatial correlation modelled by the Kronecker model and with spatial precoding at both communication nodes. Next, we analyze the result for uncorrelated antennas to understand the optimal precoding for this special case, which is equal power allocation. Then, the impact of correlation is characterized using Majorization theory. Surprisingly for small SNR, spatial correlation increases the secret key rate. For high SNR, the maximum secret key rate is achieved for uncorrelated antennas. The results indicate that a solid system design for reciprocal MIMO key generation is required to establish the secret key rate gains.

The goal of this thesis is to analyze the current state of the customer's computer network and his requirements. Then the changes and the way of their realization were proposed. The next part of the thesis are the description of appropriate components, selected technologies and the detailed description of the implementation process, including blueprints.

Le but de cette thèse est d'étudier la mise en œuvre de services de type Internet of Thing (IoT) ou Machine to Machine (M2M) par satellite. Ce type de système pose une double problématique: d'une part au niveau couche physique : les contraintes liées au terminal (limité en puissance, énergie, taille d'antenne), au canal (potentiellement avec masquage et multitrajet) et au segment spatial impliquent la mise en œuvre de différentes techniques (entrelacement, suppression d'interférents, ...) permettant d'assurer le bilan de liaison adéquat pour le service. D'autre part, le besoin d'offrir un accès à la ressource à un grand nombre de terminaux demandant un débit faible implique l'utilisation de techniques d'accès à contention optimisées, ainsi que la prise en compte au niveau accès des problématiques d'économie d'énergie. Cette couche accès doit également être en mesure de s'interfacer avec des architectures réseaux plus vastes. On peut citer par exemple les architectures Internet afin de supporter des services IP pour l'IoT, avec des notions de services intermittents, telles qu'on les retrouve dans les réseaux DTN, ou bien les architectures 4G/5G pour la mise en œuvre de services mobiles. Cette thèse va investiguer deux approches systèmes innovantes ainsi que différentes techniques aussi bien couche physique que couche accès (potentiellement couplée) permettant leur mise en œuvre. Le premier scénario système consiste à l'utilisation d'un terminal satellite relais très bas débit (contrairement au cas classique traité dans la littérature reposant sur des terminaux broadband), s'interfaçant avec des capteurs en technologie accès terrestres. Des techniques innovantes de gestion des ressources et d'économie d'énergie au travers d'une couche accès dédiée (non DVB) pourraient permettre de supporter le nombre très important de terminaux dans ce type de système. Le second scénario repose sur une communication directe avec des capteurs/objets via une constellation satellite. Cette approche pose le problème de l'efficacité de la forme d'onde pour des services extrêmement sporadique et de la fiabilisation de la communication. Il existe de nombreux travaux coté DLR sur ce type de forme d'onde avec notamment la définition de S-MIM. Néanmoins, cette solution semble complexe et de nombreuses optimisations pourraient être apportées. Coté accès, E-SSA (communication asynchrone à spectre étalé avec SIC) défini par l'ESA est également une piste de travail intéressante même si sa mise en œuvre au niveau système et sa complexité doivent être consolidées. / The aim of this thesis is to study the implementation of Internet-based services of Thing (IoT) and Machine to Machine (M2M) through a satellite link. This type of system have to deal with two issues: first the physical layer level: terminal related constraints (limited in power, energy, and antenna size), channel (potentially with masking and multipath) and the space segment involve the implementation of different techniques (interleaving, interference cancellation,) to ensure proper link budget allowing the communication. On the other hand , the need to provide access to the resource to a large number of terminals requiring low throughput involves the use of optimized contention access techniques , as well as taking into account the level of access issues energy saving. The access layer should also be able to interface with larger networks architectures. Internet architectures for example include supporting IP services for Iota, with sporadic services, such as the ones found in the DTN networks, or 4G architectures / 5G for the implementation of mobile services. This thesis will investigate two innovative approaches and different techniques as well as physical layer access layer (potentially coupled) to their implementation. The first scenario involves the use of a very low throughput satellite relay terminal (unlike in the conventional case found in the literature based on broadband terminals), interfacing with terrestrial access technology sensors. Innovative resource management and energy saving techniques through a dedicated access layer (not DVB) could absorb the large number of terminals in this type of architecture. The second scenario is based on direct communication with sensors / objects via satellite constellation. This approach raises the question of the efficiency of the waveform for extremely sporadic services and the reliability of communication. DLR works on this type of waveform including the definition of S -MIM. However, this solution seems to be complex and many optimizations can be made. From the access layer point of view, E -SSA (asynchronous spread spectrum communication with SIC) defined by the ESA is also interesting even if its implementation to the system and its complexity level should be consolidated.

Le codage réseau est apparu comme une technique alternative au routage au niveau de la couche réseau permettant d'améliorer le débit et d'optimiser l'utilisation de la capacité du réseau. Récemment, le codage réseau a été appliqué au niveau de la couche physique des réseaux sans-fil pour profiter de la superposition naturelle des signaux effectuée par le lien radio. Le codage réseau peut être vue comme un traitement interne du réseau pour lequel différentes techniques de relayage peuvent être utilisées. Cette thèse étudie un ensemble de traitements ayant des compromis variés en terme de performance et complexité. Nous considérons le canal bidirectionnel à relais, un modèle de canal de communication typique dans les réseaux coopératifs, où deux terminaux s'échangent mutuellement des messages par l'intermédiaire d'un relais. La communication se déroule en deux phases, une phase à accès multiple et une phase de broadcast. Pour ce scénario, nous analysons, dans une première partie, une stratégie de "decode-and-forward". Nous considérons, pour cette étude, des alphabets de taille finie et nous calculons les probabilités moyennes d'erreur de bout-en-bout en se basant sur la métrique d'exposant d'erreur du codage aléatoire. Puis, nous dérivons les régions des débits atteignables par rapport à une probabilité d'erreur maximale tolérable au niveau de chaque nœud. Dans une deuxième partie de la thèse, nous proposons deux schémas de codage réseau pratiques, avec complexité réduite, qui se basent sur la stratégie de relayage "compress-and-forward" (CF). Le premier schéma utilise un codage en réseau de points imbriqués (nested lattices). Le deuxième schéma est une version améliorée qui permet d'atteindre des débits de données supérieurs pour l'utilisateur qui a les meilleures conditions canal. Nous construisons les régions des débits atteignables par les deux schémas proposés tout en optimisant la répartition du temps alloué à chacune des deux phases de transmission. Après l'étude du régime asymptotique, nous analysons le schéma de codage CF avec des réseaux de points de dimension finie. Nous nous concentrons sur le problème de la transmission analogique où la distorsion est optimisée. Enfin, nous étudions l'application d'un schéma de codage, basé sur la stratégie CF avec des réseaux de points imbriqués, pour le canal bidirectionnel à canaux parallèles. Ainsi, nous présentons deux régions de débits atteignables selon la technique de traitement, conjoint ou séparé, des sous-canaux par le relais. / Network coding has emerged as an alternative technique to routing that enhances the throughput at the network layer. Recently, network coding has been applied at the physical layer to take advantage of the natural signal superposition that occurs in the radio link. In this context, the physical-layer network coding can be seen as an in-network processing strategy for which multiple forwarding schemes can be proposed. This thesis investigates a set of processing schemes tailored to the network coding at the physical layer with various compromises between performance and complexity. We consider a two-way relay channel, a typical communication system in cooperative networks, where two terminals communicate with each other via a relay node. This communication occurs during two transmission phases, namely a multiple-access phase and a broadcast phase. For TWRC scenario, we first analyze a decode-and-forward strategy with finite size alphabets. We calculate the end-to-end average error probabilities based on random coding error exponents. Then, we derive the achievable rate regions with respect to a maximal probability of error allowed at each terminal. Next, we propose two low-complexity and practical schemes based on compress-and-forward relaying strategy. The first scheme employs nested lattice coding. The second is an improved version which enables higher data rates for the user experiencing the best channel conditions. We present an information-theoretic framework to reconstruct the achievable rate regions of both schemes by considering optimal time division between both transmission phases. After the asymptotic regime analysis, we study single-layer lattice coding scheme with finite dimension lattices. We focus on the analog transmission problem where the distortion is optimized. Finally, we investigate single-layer lattice coding scheme for parallel Gaussian two-way relay channel. We present two achievable rate regions based on whether the relay processes all the sub-channels jointly or separately.

In this work, a multilayer security solution for digital communication systems is provided by considering the joint effects of physical-layer security channel codes with application-layer cryptography. We address two problems: first, the cryptanalysis of error-prone ciphertext; second, the design of a practical physical-layer security coding scheme. To our knowledge, the cryptographic attack model of the noisy-ciphertext attack is a novel concept. The more traditional assumption that the attacker has the ciphertext is generally assumed when performing cryptanalysis. However, with the ever-increasing amount of viable research in physical-layer security, it now becomes essential to perform the analysis when ciphertext is unreliable. We do so for the simple substitution cipher using an information-theoretic framework, and for stream ciphers by characterizing the success or failure of fast-correlation attacks when the ciphertext contains errors. We then present a practical coding scheme that can be used in conjunction with cryptography to ensure positive error rates in an eavesdropper's observed ciphertext, while guaranteeing error-free communications for legitimate receivers. Our codes are called stopping set codes, and provide a blanket of security that covers nearly all possible system configurations and channel parameters. The codes require a public authenticated feedback channel. The solutions to these two problems indicate the inherent strengthening of security that can be obtained by confusing an attacker about the ciphertext, and then give a practical method for providing the confusion. The aggregate result is a multilayer security solution for transmitting secret data that showcases security enhancements over standalone cryptography.

There has recently been growing interest in using physical layer network coding techniques to facilitate information transfer in wireless relay networks. The physical layer network coding technique takes advantage of the additive nature of wireless signals by allowing two terminals to transmit simultaneously to the relay node. This technique has several performance benefits, such as improving utilization and throughput of wireless channels and reducing delay. In this thesis, we present an algorithm for joint decoding of two unsynchronized transmitters to a modulo-2 sum of their transmitted messages. We address the problems that arise when the boundaries of the signals do not align with each other and when their phases are not identical. Our approach uses a state-based Viterbi decoding scheme that takes into account the timing offsets between the interfering signals. As a future research plan, we plan to utilize software-defined radios (SDRs) as a testbed to show the practicality of our approach and to verify its performance. Our simulation studies show that the decoder performs well with the only degrading factor being the noise level in the channel.

Within this thesis, we investigate the possibilities of physical layer secrecy for two special system models. In detail, we study beamforming and protection strategies in the Multiple-Input Single-Output (MISO) Gaussian Wiretap Channel (WTC) and the Gaussian two-hop relay WTC with multiple antennas at transmitter and receiver. In both system models, we examine the influence of partial Channel State Information (CSI) on the link to the eavesdropper and compare the achievable secrecy rates with the case of full CSI. We show for the MISO WTC that in the fast fading scenario the Beamforming Vector (BV) can be optimized such that the ergodic secrecy rate is maximized with regard to the degree of channel knowledge. Further we show that the ergodic secrecy rate can be significantly increased by usage of Artificial Noise (AN), if applied in a smart way. This means that the degree of channel knowledge on the link to the eavesdropper influences the portion of power that is spent for AN at the transmitter as well as the direction, in which the AN signal is sent. In addition, we apply the same beamforming and protection strategies to the slow fading scenario and find that these techniques also reduce the secrecy outage probability. For the two-hop relay WTC, we introduce Information Leakage Neutralization (IN) as a new protection strategy. If applied to a system model, where the transmitter has full CSI, the instantaneous secrecy rate performs almost as well as the instantaneous capacity of the peaceful system without an eavesdropper. The IN protected scheme outperforms the AN protected approach and performs much better than any beamforming scheme without additional protection mechanism. Another positive aspect of the IN protected scheme in the case of full CSI is that conventional channel codes can be applied instead of wiretap codes. For the case of partial CSI, where the transmitter has only an outdated estimate on the channel between relay and the eavesdropper, we show that the IN protected scheme can also be applied. Here, it strongly depends on the channel realizations and the delay of the estimate, whether the IN or the AN protection scheme should be applied. / In dieser Arbeit wird das Leistungsvermögen der Sicherheit auf der physikalischen Schicht anhand von zwei speziellen Systemmodellen untersucht. Im Detail werden Beamforming- und Absicherungsstrategien im gaußschen Multiple-Input Single-Output (MISO) Wiretap Channel (WTC) und dem gaußschen Two-hop Relay WTC mit mehreren Antennen am Sender und Empfänger studiert. In beiden Systemmodellen wird der Einfluss von partieller Kanalkenntnis zum Abhörer betrachtet und die so erreichbaren Sicherheitsraten mit denen verglichen, die bei voller Kanalkenntnis erreichbar sind. Für den MISO WTC kann gezeigt werden, dass für Kanäle mit schnellem Schwund der Beamforming-Vektor in Hinblick auf die ergodische Sicherheitsrate unter Berücksichtigung des Grades der Kanalkenntnis optimiert werden kann. Zudem kann durch die intelligente Verwendung von künstlichem Rauschen (Artificial Noise, AN) die ergodische Sicherheitsrate signifikant erhöht werden. Hierbei nimmt der Grad der Kanalkenntnis direkt Einfluss auf die Aufteilung der Leistung zwischen Daten- und AN-Signal am Sender sowie auch auf die Richtung, in der das AN-Signal gesendet wird. Zudem kann gezeigt werden, dass dieselben Beamforming- und Absicherungsstrategien ebenfalls die Sicherheitsausfallwahrscheinlichkeit für Kanäle mit langsamem Schwund minimieren. Im gaußschen Two-hop Relay WTC wird Information Leakage Neutralization (IN) als neuartige Absicherungsstrategie eingeführt. Diese Absicherungsstrategie erreicht nahezu dieselben instantanen Raten wie ein friedvolles System ohne Abhörer, wenn es bei voller Kanalkenntnis am Sender eingesetzt wird. Weiterhin sind durch die IN-Absicherungsstrategie höhere Raten erreichbar als durch den Einsatz von AN. Zusätzlich kann im Fall von voller Kanalkenntnis auf den Einsatz von Wiretap-Codes verzichtet werden. Auch im Fall partieller Kanalkenntnis, wo der Sender nur eine veraltete Schätzung des Kanals zwischen Relay und Abhörer besitzt, kann gezeigt werden, dass die IN-Absicherungsstrategie angewendet werden kann. Hierbei hängt es jedoch stark von den Kanalrealisierungen und dem Alter der Kanalschätzung ab, ob die IN- oder die AN-Absicherungsstrategie bessere Ergebnisse bringt und daher angewandt werden sollte.

Το πρόβλημα της μοντελοποίησης του ασύρματου διαύλου συνίσταται σε ένα ολοένα και αυξανόμενο πλήθος παραμέτρων και ιδιαίτερων χαρακτηριστικών που πρέπει να λαμβάνονται υπόψη και να ενσωματώνονται στα πλαίσια του RF σχεδιασμού (Radio-Frequency planning) ενός ασύρματου δικτύου εύρωστης παροχής υπηρεσιών. Ο χαρακτηρισμός του ασύρματου καναλιού προϋποθέτει κάτι περισσότερο από μία ντετερμινιστική εξίσωση που υπολογίζει τις απώλειες οδεύσεως ελεύθερου χώρου συναρτήσει της απόστασης πομπού-δέκτη και της συχνότητας. Ένα αξιόπιστο μοντέλο απωλειών σε μία τέτοια περίπτωση απαιτεί τον συνυπολογισμό των απωλειών λόγω των διάφορων μηχανισμών διάδοσης και εξασθένησης της Η/Μ ακτινοβολίας, συμπεριλαμβανομένων των απωλειών λόγω εμποδίων, αλλά και των φαινομένων σκίασης λόγω ανθρώπινης παρεμβολής. Η σημασία της συμβολής της διατριβής έγκειται στην συγκριτική αντιπαραβολή των βασικότερων μοντέλων απωλειών οδεύσεως και στην αριθμητική διόρθωση και επαναξιολόγηση του μοντέλου της ITU για ένα σύνολο τοπολογιών σύνθετης ράδιο-διάδοσης. Οι προτεινόμενες μας αλλαγές στο μοντέλο ITU αύξησαν σημαντικά την αξιοπιστία του όχι μόνο σε σχέση με την αρχική του απόδοση αλλά και συγκριτικά με τα υπόλοιπα μοντέλα. Επιπρόσθετα, υπολογίσαμε αναλυτικά για κάθε τοπολογία την εξασθένηση ανά απόσταση και προβήκαμε στην μοντελοποίηση της γεωγραφικής διασποράς της εξασθένησης ανά απόσταση με τρόπο που μπορεί να αποτελέσει το θεμέλιο ενός προγνωστικού εργαλείου για την παράμετρο αυτή. Επίσης, εξετάσαμε τις διαλείψεις μεγάλης κλίμακας και καταλήξαμε σε μία καινοτόμο μέθοδο για τον υπολογισμό του βάθους σκίασης απευθείας από τα εμπόδια της εκάστοτε τοπολογίας. Η μέθοδος αυτή προτάθηκε και ελέγχθηκε για την ακρίβειά της στα 2.4 GHz, αλλά μπορεί να εφαρμοστεί για κάθε συχνότητα ενδιαφέροντος εφόσον ακολουθηθούν οι διαδικασίες καταμέτρησης των απωλειών σκίασης των εμποδίων κάθε τύπου και πλήθους. Με τη μέθοδο αυτή μπορεί να υπολογιστεί και η εξασθένηση ανά απόσταση μέσω του βάθους σκίασης, συνεπώς είναι εφικτός ο χαρακτηρισμός της εξασθένησης εντός της τοπολογίας χωρίς την αναγκαιότητα πραγματοποίησης εκτεταμένων μετρήσεων. Επιπρόσθετα, εξετάσαμε πώς διαφορετικοί μηχανισμοί ράδιο-διάδοσης οδηγούν σε διαφορετική γεωγραφική διασπορά των τοπικών τιμών της λαμβανόμενης ισχύος. Τέλος, παρουσιάστηκε η λύση της Ασφάλειας Φυσικού Επιπέδου (Wireless Information-Theoretic Security, WITS) με διερεύνηση της επίδρασης των ρεαλιστικών απωλειών οδεύσεων ενός ασύρματου διαύλου με εμπόδια και διαλείψεις στα όρια των τιμών των παραμέτρων, ενώ επίσης εξετάστηκε για πρώτη φορά σε κλειστή μορφή η Αποκοπή Χωρητικότητας Ασφαλείας (Outage Secrecy Capacity). Μελετήσαμε την επίδραση της κινητικότητας των χρηστών στην αξιοπιστία της μεθόδου σε επίπεδο προσομοιώσεων αλλά και με πειραματικές μετρήσεις, αξιολογώντας ποσοτικά και ποιοτικά την επίδρασή τους στα όρια τιμών των παραμέτρων υπό την παρουσία ενός ωτακουστή, και εν συνεχεία σε καθεστώς πολλαπλών ωτακουστών, θεωρώντας μέθοδο συνεργασίας ωτακουστών SC και MRC. / Wireless Channel Modeling consists of an even more increasing number of factors and parameters that need to be accounted for, in the context of RF planning. Wireless Channel Characterization requires more than a deterministic formula for calculating free space path loss in relation to frequency and distance. A reliable path loss model needs to incorporate in its formula the various propagation mechanisms that influence signal attenuation, including losses due to obstacles and human body shadowing. The importance of our contribution lies in the comparative evaluation and validation of the most fundamental RF path loss models and in the numerical adjustment and re-evaluation of the ITU path loss model for a number of different complex propagation topologies. Our suggested correction to the ITU model has significantly increased its prediction precision compared not only to the initial ITU model but also to all other path loss models in question. In addition, we have calculated the attenuation over distance and modeled the distribution of the local mean values versus distance throughout each topology, in a way that can serve as the foundation of a prediction method for this parameter. The large-scale fading of the received signal has also been characterized and a new empirical method for the calculation of the shadow depth has been introduced. This method has been validated for the 2.4 GHz frequency, but it can be applied to any frequency of interest as long as the respective obstacle-caused losses are measured. Extending this method can be applied in order to calculate the attenuation over distance as a function of shadow depth, therefore allowing us to predict this parameter without extensive measurements. The impact of different propagation mechanisms on the variation of distribution of local mean values of received signal power throughout the topology has also been studied. Finally, the concept of Wireless Information-Theoretic Security (WITS) was discussed, by investigating the impact of channel-dependent variation of path loss on the boundaries of secure communications as defined by WITS parameters, whereas a closed-form expression for Outage Secrecy Capacity was introduced. The impact of user mobility on the range of these parameters was investigated in terms of simulations and experimental measurements, in the presence of a single eavesdropper and also for a multiple eavesdroppers scenario, assuming SC and MRC schemes.