This thesis work is directed at estimating gas hydrate and free gas distribution and saturation in local structures in the Ulleung Basin, East Sea offshore Korea. The estimates are obtained from a 2-D multi-channel seismic (MCS) reflection profile from the basin. Firstly, structures of locally focused upwelling fluid and gas flow were imaged using time-migrated sections and seismic attributes, and secondly seismic velocities were obtained to estimate gas hydrate and free gas saturations. The structures investigated are up to 1 km across, and are characterized by reduced reflectivity (‘blank zones’) and pulled-up sediment reflectors on the seismic sections. Throughout the study, a comparison is made between the blank zones and areas outside them where not much gas hydrate or gas is expected, to examine their peculiar characteristics as related to the formation of gas hydrate and underlying free gas. The regional depth of possible occurrence of gas hydrate and free gas is determined by predicting the base of the gas hydrate stability zone (BGHSZ) from sediment properties and heat flow estimates calibrated by a few bottom-simulating reflectors (BSRs) from outside the analyzed seismic section. A large number of normal moveout (stacking) velocity profiles were obtained within and outside the blank zones. Interval velocities were then derived by applying the commonly used unconstrained Dix equation as well as by applying constraints to inversion using regularized linear inversion and non-linear Bayesian inversion. The latter method fully explores the uncertainty of the interval velocity estimates. Compared to areas outside the blank zones, the velocities within the blank zones are up to 30% larger at about 30 m above the BGHSZ and up to 65% smaller immediately below the BGHSZ. The velocity increase implies a gas hydrate saturation of 10-40% of the pore space. The velocity decrease implies a free gas saturation of 1-4% of the pore space. Their detailed distribution within individual structures cannot be resolved. Reflector pull-up in time sections in the hydrate zone allows an independent velocity estimate, assuming the pull-up is solely a velocity effect. The implied velocity is much higher than the interval velocity estimates, so there also must be physical deformation. The heat flow estimated depth of the BGHSZ is in good agreement with the transition from gas hydrate to free gas as inferred from seismic velocities. The general conclusion of the thesis work is that a variety of careful analyses of MCS data that characterize the seismic signal and estimate the seismic velocity structure can provide insight into gas hydrate and free gas occurrences. The large amounts of gas hydrate and free gas associated with the blank zones inferred by this study should draw special attention to future energy and climate effects in this area and other similar regions.

Η παρούσα διπλωματική εργασία πραγματεύεται τη μελέτη, το σχεδιασμό και την κατασκευή ενός ηλεκτρονικού μετατροπέα ισχύος ανύψωσης συνεχούς μεταβαλλόμενης τάσης εισόδου σε σταθερή συνεχή τάση εξόδου. Η εργασία αυτή εκπονήθηκε στο Εργαστήριο Ηλεκτρομηχανικής Μετατροπής Ενέργειας του τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών της Πολυτεχνικής Σχολής του Πανεπιστημίου Πατρών. Σκοπός της εργασίας είναι η κατασκευή ενός ηλεκτρονικού μετατροπέα συνεχούς τάσης σε συνεχή, ο οποίος θα τροφοδοτείται από ένα σύστημα ενεργειακών κυψελών και θα παρέχει σταθερή συνεχή τάση εξόδου 50V, ούτως ώστε στην συνέχεια να τροφοδοτείται ένας αντιστροφέας, ο οποίος θα οδηγεί ένα dc brushless κινητήρα. Το συγκεκριμένο σύστημα ενεργειακών κυψελών έχει μεταβαλλόμενη τάση εισόδου από 26-43Vdc και μέγιστη ισχύ 1200W. Αρχικά, παρουσιάζεται μια συνοπτική θεωρία, η οποία αναλύει την χαρακτηριστική V-I μιας ενεργειακής κυψέλης. Στην συνέχεια αναλύεται το σύστημα ενεργειακών κυψελών, το οποίο θα χρησιμοποιηθεί στην εφαρμογή μας. Στην συνέχεια, ακολουθεί η θεωρητική ανάλυση του απλού μετατροπέα ανύψωσης συνεχούς τάσης σε συνεχή αρχικά και έπειτα η θεωρητική ανάλυση του μετατροπέα ανύψωσης συνεχούς τάσης σε συνεχή με διαδοχική αγωγή τεσσάρων φάσεων, που κατασκευάστηκε για την συγκεκριμένη εφαρμογή. Ο συγκεκριμένος μετατροπέας της εφαρμογής προσομοιώθηκε στο πρόγραμμα PSpice ούτως ώστε να ελεγχθεί η συμπεριφορά του. Στο επόμενο βήμα, γίνεται αναλυτική παρουσίαση όλων των στοιχείων και κυκλωμάτων που χρησιμοποιήθηκαν για την κατασκευή του ηλεκτρονικού μετατροπέα. Τέλος αναλύονται τα δυο προγράμματα ελέγχου που σχεδιάστηκαν για τον έλεγχο της τάσης εξόδου στα 50V dc, το πρόγραμμα ανοικτού και κλειστού βρόγχου και παρουσιάζονται οι μετρήσεις που διεξήχθησαν στον μετατροπέα για την επιβεβαίωση και αξιολόγηση της θεωρητικής ανάλυσης. / The thesis deals with the design, study and construction of a dc/dc boost converter, having a variable input voltage and a constant output voltage. The work was conducted in the Laboratory of Electromechanical Energy Conversion, Department of Electrical and Computer Engineering School of Engineering, University of Patras. The aim is to construct a dc/dc converter, which will be supplied by a fuel cell system and will provide a constant dc output voltage at 50V, in order to supply then an inverter, which will drive a dc brushless motor. In the specific fuel cells system the voltage varies from 26-43V dc and it has maximum output power 1200W. First, a small theory is presented, which analyzes the polarization curve of a fuel cell. Then we analyze the fuel cell system, which will be used in this application. Next, a theoretical analysis of the simple boost converter and then the theoretical analysis of the interleaved multi-(four) channel boost converter, which was constructed, are presented. This converter is simulated with the program PSpice, in order to check its operation. In the next step, a comprehensive presentation of all of the chips and circuits which were used for the construction of the converter is done. Finally, the two programs designed for the control of the output voltage at 50V dc, the open and close loop programs, are analyzed and the experimental measurements carried out in the converter are displayed, so as to confirm and evaluate the theoretical analysis

L’utilisation des communications multi-canaux au niveau des protocoles MAC (MAC multi-canaux) dans les réseaux sans fil peut améliorer significativement le débit du réseau par rapport aux protocoles MAC mono-canal en permettant à plusieurs transmissions concurrentes d’avoir lieu simultanément sans interférence sur différents canaux non recouverts. Cependant, l’environnement multicanaux ajoute une nouvelle dimension aux protocoles MAC qui nécessite un nouveau mécanisme appelé "le mécanisme d’allocation des canaux" pour coordonner l’utilisation des canaux. Le rôle de ce mécanisme est de permettre aux nœuds de s’accorder sur le canal qui doit être utilisé et à quel moment pour la transmission de données dans un contexte distribué. L’absence de ce mécanisme ou une mauvaise coordination de l’utilisation des canaux peut produire l’effet inverse de celui recherché. Ainsi, l’efficacité d’un protocole MAC multi-canaux dépend strictement de l’efficacité du mécanisme d’allocation des canaux, qui par ailleurs, doit offrir les garanties nécessaires au fonctionnement de la communication multi-canaux. Dans cette thèse, nous nous intéressons à la modélisation, l’analyse et la conception de nouveaux protocoles MAC multi-canaux dans les réseaux sans fil. Dans un premier temps, nous nous focalisons sur la spécification formelle et la vérification des propriétés fondamentales inhérentes aux activités de l’allocation des canaux et son impact sur le transfert de données des principales approches MAC multi-canaux proposées dans la littérature. Dans un second temps, nous visons à proposer, modéliser et à analyser formellement deux nouveaux protocoles MAC multicanaux résultants de l’analyse qualitative réalisée. Enfin, une étude quantitative et comparative est menée afin d’analyser les performances des protocoles proposés en terme de débit et d’impact du temps de commutation entre les canaux. / The use of multi-channel communications in MAC protocols (multi-channel MAC) for wireless networks can significantly improve the network throughput compared with single channel MAC protocols since concurrent transmissions can take place simultaneously on distinct non-overlapping channels without interfering. However, due to the multi-channel environment, a novel issue must be addressed in MAC protocols, which is called "the channel assignment mechanism" in order to coordinate the channels access. The role of this mechanism is to enable nodes to agree on which channel should be used and when for data transmission in a distributed way. The absence of this mechanism or a bad coordination of the channel usage can incur the opposite effect to that intended. Thus, the efficiency of a multi-channel MAC protocol depends strictly on the efficiency of the channel assignment mechanism, which also must provide the guarantees necessary to the good behavior of the multi-channel communications. In this thesis, we focus on the modeling, analysis, and design of new multi-channel MAC protocols in wireless networks. Firstly, we focus on the formal specification and verification of fundamental properties inherent to the activities of the channel assignment and its impact on the data transfer for the main multi-channel MAC approaches proposed in the literature. Secondly, we aim to propose, model and analyze formally two new multichannel MAC protocols, resulting from the qualitative analysis conducted. Finally, a quantitative and comparative study is conducted to analyze the performance of the proposed protocols in terms of the throughput and the impact of the switching time between channels.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Wireless sensor and actuator networks are expected to grow quickly in size due to the decreasing cost of nodes. For such large scale networks, the geographic routing algorithm is a suitable approach. It finds a route for the packet based on the positions of the nodes. However, it often finds longer routes than the shortest possible ones, increasing the probability of packet loss. Moreover, the low cost wireless devices usually operate in the unlicensed bands, which is a crowded portion of the spectrum where several networks may be operating. Furthermore, the expected increase in the network density will also increase the competition for the wireless channel. Thus, intra and inter-network interferences play a major role in the reliability of the communication. In order to increase it, communication over multiple channels can be used. It exploits the capability of the radios that equip the nodes to be tuned over different frequencies. However, the number of available channels is limited in practice. Therefore, an allocation protocol must employ the channels eficiently. However, it was shown that to find an optimal allocation is NP-hard. Thus, one can use game theory to design a protocol that sub-optimally assigns channels in a distributed way. Game theory allows to model the problem as a game, where nodes are players and channels are their actions. According to the actions played, every node receives a payoff. The task of the protocol designer is to assign to every player a payoff function and a learning algorithm. This algorithm allows a player to play accordingly its observations of the game in order to maximize its payoff. If the designed game is potential, the algorithm can be used such that repeated playing converge to a steady state, known as Nash Equilibrium (NE). In a NE, no player can be better of by switching its current action and its received payoff can be bounded by a constant factor of the optimal payoff. By following such approach, a protocol called GBCA has been proposed, which allocates channels by exploiting topology and routing information. However, GBCA was designed for networks where routes are known in advance and static during a relatively long time. Geographic routing, by the other hand, finds the route for every packet on-demand. Therefore, in order to apply GBCA in networks that use geographic routing, this thesis proposes suitable modifications to the payoff functions. Using this modified protocol, which is called GBCA-G, it is shown, by simulations, the improvement of the network perfomance according to the metrics of delivery ratio, average delay and average number of trials per successfull transmission. / Espera-se que as redes de sensores e atuadores sem fio cresçam rapidamente em tamanho devido à contínua redução de custo dos dispositivos. Para tais redes de larga escala, os algoritmos de roteamento geográfico são adequados. Estes encontram a rota com base nas posições dos nós. Contudo, geralmente as rotas encontradas são mais longas do que as rotas possíveis, aumentando a probabilidade de perda do pacote. Ademais, os dispositivos sem fio de baixo custo operam nas faixas livres do espectro e estão sujeitos a interferências causadas por outras redes que operam nessas mesmas faixas. Ainda, com o aumento da densidade da rede, aumenta também a competição pelo canal. Portanto, tanto interferências internas quanto externas prejudicam a confiabilidade da comunicacão. Para aumentá-la pode-se utilizar múltiplos canais, uma vez que os rádios que equipam os nós podem ser sintonizados em diferentes frequências. Contudo, o número de canais disponíveis é limitado, tornando imperativo um protocolo de alocação eficiente. Porém foi demonstrado que encontrar uma solução ótima é inviável. Por isso, a Teoria dos Jogos pode ser empregada para se encontrar alocações sub-ótimas de forma distribuída. A Teoria dos Jogos permite modelar o problema como um jogo, onde os nós são os jogadores; os canais, suas ações; e conforme suas escolhas, cada não recebe uma recompensa. Um projetista deve definir as funções de recompensa e um algoritmo de aprendizado, que permite ao jogador determinar a ação que maximiza sua recompensa através da observação do jogo. Se o jogo for potencial, o algoritmo pode levá-lo a um equilíbrio, conhecido por Equilíbrio de Nash (NE), após repetidas rodadas. Em um NE, todo jogador estão satisfeito e sua recompensa pode ser limitada por uma constante multiplicativa da máxima recompensa possível. Seguindo esta abordagem, um protocolo denominado GBCA foi proposto na literatura, o qual aloca canais com base na topologia da rede e nas rotas utilizadas. Contudo, o GBCA necessita conhecer as rotas antecipadamente e estas devem ser estáticas. Por outro lado, algoritmos geográficos determinam a rota sob demanda para cada pacote. Portanto, para tornar o GBCA adequado para redes com roteamento geográfico, propõe-se, nesta Tese, modificações ao GBCA. Utilizando-se o protocolo proposto, denominado GBCA-G, demonstra-se, por simulação, que é possível obter resultados melhores quanto à taxa de entrega de pacotes, ao atraso médio de entrega e ao número médio de transmissões efetuadas por transmissão bem-sucedida.

Les réseaux de capteurs suscitent un engouement croissant du fait du grand nombre d'applications mais également des défis inhérents à ce genre de réseaux. Le standard IEEE 802.15.4 a été proposé afin de standardiser les couches physique et MAC. Dans ce travail nous avons dans un premier temps proposé une variante multi-canal pour le standard 802.15.4 permettant de résoudre le problème de collisions de supertrames. Pour cela nous proposons de construire un réseau en arbre avec la particularité que chaque cellule du réseau utilise un canal différent permettant ainsi une réduction conséquente des interférences et augmente la capacité du réseau. Nous avons également introduit un nouveau mécanisme de construction de topologie, d'allocation de canal et d'ordonnancement de supertrames nécessaire au bon fonctionnement d'une telle solution. Dans un deuxième temps nous avons analysé l'impact des différents paramètres de la méthode d'accès du standard. Nous avons mis en exergue les faiblesses de la méthode d'accès dont les performances baissent drastiquement pour des réseaux trop importants. Partant de ce constat, nous avons proposé des mécanismes d'auto-adaptation pour la méthode d'accès du standard. Ces derniers permettent d'adapter dynamiquement la taille des fenêtres de contention en fonction des conditions de trafic observées sur le canal. Le calcul des valeurs optimales est exécuté par chaque coordinateur pour résoudre le problème de surdité. Ces mécanismes sont distribués et convergent rapidement même en cas de trafic en rafales. / An increasing interest has been observed in Wirless Sensor Network that can be explained by wide range of WSN applications as well as by the challenges involving the constraints of this type of networks. The IEEE 802.15.4 standard has been proposed with the objective of standardizing the physical and MAC layers. In this work, we have firstly proposed an alternative multichannel scheme for the IEEE 802.15.4 standard. It is able to solve the problem of superframe collisions based on a cluster-tree topology approach in which each cluster uses a different channel allowing the reduction of the interference and increased network capacity. We have also difined a novel mechanism for topology construction, channel allocation, and superframe scheduling. Secondly, we have analyzed the impact of different parameters on the medium access control. We have shown the weakness of the medium access method proposed in the standard. For instance, we have observed a decrease in performance when the standard is applied in large networks. Motivated by the analysis and its results we have then proposed auto-adaptive mechanisms for the medium access control. They allow to dynamically adapt the size of the contention window according to the observed traffic conditions. Each coordinator computes the optimal values to avoid deafness. The proposed mechanisms rapidly converge even in the case of bursty traffic.

Traditional Ad Hoc network wireless communication in the past years have contributed tremendously to the dawn of wireless mesh networks (WMNs) which have so far been able to provide a significant improvement in capacity and scalability. Routing metrics which form the basic element for the routing protocol in this innovative communication technology is a call for concern as they must take into consideration the wireless medium network characteristics in order to provide a optimum appreciable QoS performance. In the past many single-radio routing metrics have been proposed for Ad Hoc networks which are not compatible with the multi-radio routing scenario demanded by WMNs. In our work, we provide a comparative analysis of most recently proposed multi-radio routing metrics for WMNs. We begin by providing an overview of the features of a wireless mesh network thereby presenting a better understanding of some of the research challenges of WMNs. Also, since single-radio routing forms the basis of multi-radio routing, we in this regard provide a review of some single-radio routing metrics. In our comparative analysis, an overview of routing protocols for WMNs is provided enabling an understanding of the demands to be included in a routing metric to ensure efficient routing in WMNs since different routing protocols may impose different demands; we then identify the requirements of multi-radio routing metrics from which we base our comparative analysis.

Le phénomène du cocktail party fait référence à notre remarquable capacité à nous concentrer sur une seule voix dans des environnements bruyants. Dans cette thèse, nous concevons, implémentons et évaluons une approche computationnelle nommée GCC-NMF pour résoudre ce problème. GCC-NMF combine l’apprentissage automatique non supervisé par la factorisation matricielle non négative (NMF) avec la méthode de localisation spatiale à corrélation croisée généralisée (GCC). Les atomes du dictionnaire NMF sont attribués au locuteur cible ou à l’interférence à chaque instant en fonction de leurs emplacements spatiaux estimés. Nous commençons par étudier GCC-NMF dans le contexte hors ligne, où des mélanges de 10 secondes sont traités à la fois. Nous développons ensuite une variante temps réel de GCC-NMF et réduisons par la suite sa latence algorithmique inhérente de 64 ms à 2 ms avec une méthode asymétrique de transformée de Fourier de courte durée (STFT). Nous montrons que des latences aussi faibles que 6 ms, dans la plage des délais tolérables pour les aides auditives, sont possibles sur les plateformes embarquées actuelles. Nous évaluons la performance de GCC-NMF sur des données publiquement disponibles de la campagne d’évaluation de séparation des signaux SiSEC. La qualité de séparation objective est quantifiée avec les méthodes PEASS, estimant les évaluations subjectives humaines, ainsi que BSS Eval basée sur le rapport signal sur bruit (SNR) traditionnel. Bien que GCC-NMF hors ligne ait moins bien performé que d’autres méthodes du défi SiSEC en termes de métriques SNR, ses scores PEASS sont comparables aux meilleurs résultats. Dans le cas de GCC-NMF en ligne, alors que les métriques basées sur le SNR favorisent à nouveau d’autres méthodes, GCC-NMF surpasse toutes les approches précédentes sauf une en termes de scores PEASS globaux, obtenant des résultats comparables au masque binaire idéale. Nous montrons que GCC-NMF augmente la qualité objective et les métriques d’intelligibilité STOI et ESTOI sur une large gamme de SNR d’entrée de -30 à 20 dB, avec seulement des réductions mineures pour les SNR d’entrée supérieurs à 20 dB. GCC-NMF présente plusieurs caractéristiques souhaitables lorsqu’on le compare aux approches existantes. Contrairement aux méthodes d’analyse de scène auditive computationnelle (CASA), GCC-NMF ne nécessite aucune connaissance préalable sur la nature des signaux d’entrée et pourrait donc convenir aux applications de séparation et de débruitage de source dans un grand nombre de domaines. Dans le cas de GCC-NMF en ligne, seule une petite quantité de données non étiquetées est nécessaire pour apprendre le dictionnaire NMF. Cela se traduit par une plus grande flexibilité et un apprentissage beaucoup plus rapide par rapport aux approches supervisées, y compris les solutions basées sur NMF et les réseaux neuronaux profonds qui reposent sur de grands ensembles de données étiquetées. Enfin, contrairement aux méthodes de séparation de source aveugle (BSS) qui reposent sur des statistiques de signal accumulées, GCC-NMF fonctionne indépendamment pour chaque trame, ce qui permet des applications en temps réel à faible latence. / Abstract: The cocktail party phenomenon refers to our remarkable ability to focus on a single voice in noisy environments. In this thesis, we design, implement, and evaluate a computational approach to solving this problem named GCC-NMF. GCC-NMF combines unsupervised machine learning via non-negative matrix factorization (NMF) with the generalized cross-correlation (GCC) spatial localization method. Individual NMF dictionary atoms are attributed to the target speaker or background interference at each point in time based on their estimated spatial locations. We begin by studying GCC-NMF in the offline context, where entire 10-second mixtures are treated at once. We then develop an online, instantaneous variant of GCC-NMF and subsequently reduce its inherent algorithmic latency from 64 ms to 2 ms with an asymmetric short-time Fourier transform (STFT) windowing method. We show that latencies as low as 6 ms, within the range of tolerable delays for hearing aids, are possible on current hardware platforms. We evaluate the performance of GCC-NMF on publicly available data from the Signal Separation Evaluation Campaign (SiSEC), where objective separation quality is quantified using the signal-to-noise ratio (SNR)-based BSS Eval and perceptually-motivated PEASS toolboxes. Though offline GCC-NMF underperformed other methods from the SiSEC challenge in terms of the SNR-based metrics, its PEASS scores were comparable with the best results. In the case of online GCC-NMF, while SNR-based metrics again favoured other methods, GCC-NMF outperformed all but one of the previous approaches in terms of overall PEASS scores, achieving comparable results to the ideal binary mask (IBM) baseline. Furthermore, we show that GCC-NMF increases objective speech quality and the STOI and ETOI speech intelligibility metrics over a wide range of input SNRs from -30 dB to 20 dB, with only minor reductions for input SNRs greater than 20 dB. GCC-NMF exhibits a number of desirable characteristics when compared existing approaches. Unlike computational auditory scene analysis (CASA) methods, GCC-NMF requires no prior knowledge about the nature of the input signals, and may thus be suitable for source separation and denoising applications in a wide range of fields. In the case of online GCC-NMF, only a small amount of unlabeled data is required to pre-train the NMF dictionary. This results in much greater flexibility and significantly faster training when compared to supervised approaches including NMF and deep neural network-based solutions that rely on large, supervised datasets. Finally, in contrast with blind source separation (BSS) methods that rely on accumulated signal statistics, GCC-NMF operates independently for each time frame, allowing for low latency, real-time applications.

In a time of pervasive digitalization, technology is constantly reshaping contemporary society. Because digital product architecture allows for unprecedented flexibility in terms of design, production, distribution and use, significant shifts are seen in how firms organize to create value. As disruptive technologies enable novel ways of operating in the media industry, new actors are entering the stage, capitalizing on the affordances associated with digitalization of content. Established in 2013, multi-channel network United Screens build a business around online video creators posting original material to social media platforms such as YouTube, helping them with anything from audience growth to rights management. Connecting content creators with brands looking to market themselves through online video, United Screens provide an illustrative example of how relative positions of new and established actors are changing as an effect of digitalization. Because little research has previously been directed at multi-channel networks, this study set out to explore how United Screens organize themselves and their partnerships to create value in the digital media landscape. By interviewing employees and partners of the company, themes covering roles, offerings, affiliation and control were identified and subsequently discussed in relation to previous knowledge of business logic and digital product architecture. The study provides an empirical illustration of how digitalization enables new ways of creating value, and how this affects organizational arrangements. Concluding that more research is required in the field, the study serves as a starting point for theory generation concerning emergent actors and their organizing logic following pervasive digitalization.

Cette dissertation concerne l'analyse des problématiques économiques introduites par la numérisation et par la parution des nouveaux réseaux et supports numériques dans l'économie des médias. Le passage du modèle analogique et physique au modèle numérique, en opérant une séparation du concept d’expression signifiante et du support qui permet sa circulation auprès d'un large public, impose un recadrage des procédés de création et production dans plusieurs domaines de l'industrie des médias. En utilisant le cas d'étude de l’industrie de l'édition des magazines, l'introduction de cette dissertation analyse les enjeux économiques plus urgents pour une marque éditoriale face à la numérisation, au niveau d'entreprise et du secteur industriel. Dans le premier chapitre, on va approfondir l’analyse en explorant la littérature existante dans le domaine des effets de la numérisation et de la régulation dans l'économie des médias. Dans le deuxième chapitre, nous analysons les dynamiques d'adoption d'une technologie numérique associée avec un réseau, dans contexte où les contenus media sont des biens complémentaires essentiels et en présence d'externalités positives associées avec le déploiement du réseau. La contribution originale du modèle que nous allons montrer est la possibilité d'évaluer différentes qualités de subsides au déploiement et différentes valeurs de l'innovation technologique. dans le troisième et dernier chapitre, nous développons un modèle pour analyser la tarification des produits soumis au droit d'auteur dans un marché biface avec réseaux de distribution multiples. / This dissertation deals with the economic issues introduced by digitization and by the roll-out of innovative networks and supports in the economics of Media goods. The switch from a “physical analogic model” to a “digitized model”, by separating the concepts of meaningful expression from the traditional support allowing for publishing contents, imposes a rethinking of many media industries' business models. Using the publishing industry as a study case, in the introduction we analyze the key economic issues emerging for magazine publishers facing digitization, both at the firm level and at the industry level, where new strategic interactions may occur, changing the traditional dynamics in the value chain. In the first chapter we then proceed to explore recent research efforts focusing on the effects of digitization and regulation on Media economics. The main contribution of this analysis is that it tries to conjugate the economic literature that analyses technology related issues generated by digitization with contributions analysing the impacts of new publishing protocols, following a value-chain based approach.In fact, the originality of the problem is that digitization is quite not just a technologic innovation, improving efficiency through the value chain but it is also a new written language. The study of all the different implications of digitiziation is thus essential in order to fully understand the emerging economic models in media industries. Economists will find in this chapter not only the more important contributions on pricing and bundling in digital two-sided markets or multi-channel distribution models, but also interesting contributions from other specialists, analysing questions related to regulatory framework and even epistemologic issues. In the second chapter we analyse the adoption dynamics of a digital technology associated with a network , when media goods are essential complements and there are positive externalities associated to a succesfull adoption. The original contribution of this model is that it allows for different types of subsidies and different stand-alone values for technology. In the third and last chapter, we develop a pricing model for copyrighted contents in a two-sided market with multi-channel distribution. We analyze both the case of a firm producing only on one market (digital or physical) and of a firm producing the substitutable versions in the two markets. We find that: (i) in the digital market the “free dailies” business model is sustainable only if the number of publishers is limited. (ii) Publishers that are active already in the traditional channel with relevant market shares should “defend” their market share setting higher prices for the digital versions of their products. (iii) Unless both the advertising revenues per copy and the total sales in the digital markets grow larger than the traditional market, a traditional publisher should keep operating in the traditional market. (iv) If the total cannibalization grows to be more than proportional the optimal strategy for a publisher can be not to produce the digital version of a given product.

Dans cette thèse, nous abordons le problème fondamental de l'accès au spectre opportuniste dans un système de communication multi-canal. Plus précisément, nous considérons un système de communication dans lequel un utilisateur a accès à de multiples canaux, tout en étant limité à la détection et la transmission sur un sous-ensemble de canaux. Nous explorons comment l'utilisateur intelligent exploite ses observations passées et les propriétés stochastiques de ces canaux afin de maximiser son débit. Formellement, nous fournissons une analyse générique sur le problème d'accès au spectre opportuniste en nous basant sur le problème de `restless multi-bandit’ (RMAB), l'une des généralisations les plus connues du problème classique de multi-armed bandit (MAB), un problème fondamental dans la théorie de décision stochastique. Malgré les importants efforts de la communauté de recherche dans ce domaine, le problème RMAB dans sa forme générique reste encore ouvert. Jusqu'à aujourd'hui, très peu de résultats sont connus sur la structure de la politique optimale. L'obtention de la politique optimale pour un problème RMAB général est intraçable dû la complexité de calcul exponentiel. Par conséquent, une alternative naturelle est de se focaliser sur la politique myopique qui maximise la récompense à immédiate, tout en ignorant celles du futur. Donc, nous développons trois axiomes caractérisant une famille de fonctions que nous appelons fonctions régulières, qui sont génériques et pratiquement importantes. Nous établissons ensuite l'optimalité de la politique myopique lorsque la fonction de récompense peut être exprimée comme une fonction régulière et le facteur de discount est borné par un seuil déterminé par la fonction de récompense. Nous illustrons également l'application des résultats pour analyser une classe de problèmes RMAB dans l'accès opportuniste. Ensuite, nous étudions un problème plus difficile, où l'utilisateur doit configurer le nombre de canaux à accéder afin de maximiser son utilité (par exemple, le débit). Après avoir montré la complexité exponentielle du problème, nous développons une stratégie heuristique v-step look-ahead. Dans la stratégie développée, le paramètre v permet de parvenir à un compromis souhaité entre l'efficacité sociale et de la complexité de calcul. Nous démontrons les avantages de la stratégie proposée via des simulations numériques sur plusieurs scénarios typiques. / In the thesis, we address the fundamental problem of opportunistic spectrum access in a multi-channel communication system. Specifically, we consider a communication system in which a user has access to multiple channels, but is limited to sensing and transmitting only on one at a given time. We explore how the smart user should exploit past observations and the knowledge of the stochastic properties of these channels to maximize its transmission rate by switching channels opportunistically. Formally, we provide a generic analysis on the opportunistic spectrum access problem by casting the problem into the restless multi-armed bandit (RMAB) problem, one of the most well-known generalizations of the classic multi-armed bandit (MAB) problem, which is of fundamental importance in stochastic decision theory. Despite the significant research efforts in the field, the RMAB problem in its generic form still remains open. Until today, very little result is reported on the structure of the optimal policy. Obtaining the optimal policy for a general RMAB problem is often intractable due to the exponential computation complexity. Hence, a natural alternative is to seek a simple myopic policy maximizing the short-term reward. Therefore, we develop three axioms characterizing a family of functions which we refer to as regular functions, which are generic and practically important. We then establish the optimality of the myopic policy when the reward function can be expressed as a regular function and the discount factor is bounded by a closed-form threshold determined by the reward function. We also illustrate how the derived results, generic in nature, are applied to analyze a class of RMAB problems arising from multi-channel opportunistic access. Next, we further investigate the more challenging problem where the user has to decide the number of channels to sense in each slot in order to maximize its utility (e.g., throughput). After showing the exponential complexity of the problem, we develop a heuristic v-step look-ahead strategy. In the developed strategy, the parameter v allows to achieve a desired tradeoff between social efficiency and computation complexity. We demonstrate the benefits of the proposed strategy via numerical experiments on several typical settings.