Littérature scientifique sur le sujet « Random spreading »

Since direct-sequence code-division multiple-access (DS-CDMA) is an interference-limited random multiple-access scheme, the reduction of co-channel interference with either interference suppression or interference cancellation multiuser receivers and/or power control to prevent detrimental near-far situations is vital for improved performance. Up to date, some contributions investigated randomly-spread asymptotically - large number of users and large bandwidth - large CDMA systems with multiuser receivers and power control via random matrix theoretic and free probability theoretic tools especially over Gaussian and single-path fading channels. As complement within this thesis, we analyze also within the generalized random spreading framework but at finite system sizes and without power control the capacity achievable with linear multichannel multiuser receivers<br>i.e. RAKE, zero-forcing decorrelator, linear minimum mean-squared error (LMMSE) multiuser receivers, within a single-cell setting over generalized time-varying GWSSUS - Rayleigh/Ricean - fading channels via random matrix theoretic tools. Assuming maximal-ratio combining (MRC) of resolved frequency - multipath - diversity channels due to wideband transmission, the signal-to-interference ratios (SIRs) with multichannel multiuser receivers that set the basis for further derivations are statistically characterized. The information-theoretic ergodic and outage sum-rates spectral efficiencies are then derived and analyzed.

This thesis investigates the structural properties of graph models of wireless networks, where autonomous agents communicate using radios in order to accomplish a predefined task. Ad hoc, sensor, and vehicle networks are perhaps the most familiar examples. The goal of this thesis is the analytical characterization of information spreading in graph models of wireless networks, since this fundamental process is a primitive needed to accomplish more complex tasks. The well-established graph-based approaches adopted when analyzing the behavior of “classical” distributed systems (e.g., P2P networks, computing clusters, etc.) fail to generalize to wireless networks, due to several causes, including the stricter physical constraints governing the operation of these systems (e.g., interference on the physical channel or scarce energy/computational resources) and the fact that the topology of the network might be unknown at design time or it might evolve over time. This thesis shows how to tackle these problems by suitably defining and rigorously analyzing graph models and graph processes capturing the structure, evolution and operation of these networks. We present two reference scenarios. In the first one we study a family of random graphs known as Bluetooth Topology, which closely model the connectivity of a network built by the device discovery phase of Bluetooth-like protocols, largely employed in wireless networks. Formally, the Bluetooth Topology generalizes the well-known Random Geometric Graph model, introducing a distributed pruning of the edge set. We investigate the expansion and the diameter of these graphs, as they quantify the bandwidth and the latency of a wireless network. We give tight bounds on the expansion and, leveraging on these, we prove nearly-tight bounds on the diameter. Our results show that the Bluetooth Topology features the same global level of connectivity of the Random Geometric Graph but requires maintaining much fewer communication links. Motivated by the recent and rapidly growing interest in mobile systems, in the second part of the thesis we turn our attention to the dynamics of information dissemination between agents performing random walks on a planar grid and communicating over short distances. This setting can also be employed to study phenomena like the spreading of a disease, where infections are the result of local interactions between agents. We prove that, for a sufficiently sparse system, the broadcast time of a message is independent from the transmission radius; indeed, we show that the broadcast time is dominated by the time needed for many agents to meet. Our findings nicely complements previous results that dealt with dense systems, where there is dependency from the transmission radius. Moreover, our analysis techniques extend to similar mobility-communication models, suggesting some interesting further research directions.<br>Questa tesi studia le proprieta' strutturali di alcuni modelli a grafo di reti di agenti autonomi che comunicano via radio per completare un prefissato compito. Reti ad hoc, di sensori e veicolari sono forse gli esempi piu' immediati. Lo scopo di questa tesi e caratterizzare la diffusione dell’infor- mazione in questi modelli a grafo di reti wireless, considerata l’importanza di questo processo come primitiva fondamentale per realizzare protocolli piu' complessi. Gli approcci basati su tecniche combinatorie adottati per l’analisi di sistemi distribuiti “classici”, come le reti P2P o i cluster di calcolo, non possono essere estesi alle reti wireless, per varie ragioni: ad esempio a causa dei vincoli fisici che governano il funzionamento di questi sistemi (interferenza sul canale radio, scarse risorse energetiche/computazionali, ecc.) e per il fatto che la topologia della rete puo' essere ignota in fase di progettazione o puo' evolvere nel tempo. Questa tesi suggerisce come sia possibile affrontare tali problemi tramite l’opportuna definizione e l’analisi rigorosa di modelli a grafo (o processi su grafi) che catturino l’evoluzione e il funzionamento delle reti wireless. Mostriamo come sia possibile applicare quest’approccio a due scenari di riferimento. Innanzitutto studiamo una famiglia di grafi random nota come Bluetooth Topology, che ben rappresenta la connettivita' della rete creata dalla fase di device discovery in protocolli simili al Bluetooth, largamente utilizzati nelle reti wireless. Dal punto di vista formale, la Bluetooth Topology generalizza il ben noto modello Random Geometric Graph, introducendovi una selezione distribuita degli archi. Studiamo l’espansione e il diametro di questi grafi, poiche' quantificano la banda e la latenza della rete. Dimostriamo limiti stretti all’espansione e, sfruttando questa caratterizzazione, diamo dei limiti quasi stretti al diametro. I nostri risultati provano che la Bluetooth Topology presenta lo stesso livello globale di connettivita' del Random Geometric Graph, pur richiedendo molti meno link di comunicazione. Graph, pur richiedendo molti meno link di comunicazione. Motivati dal recente crescente interesse verso i sistemi mobili, nella seconda parte della tesi concentriamo la nostra attenzione sulle dinamiche di disseminazione dell’informazione tra agenti che effettuano random walk su una griglia planare e che comunicano su brevi distanze. Questo scenario puo' essere utilizzato per studiare fenomeni come la diffusione di malattie, dove le infezioni sono il risultato di interazioni locali tra gli agenti. Proviamo che, per un sistema sufficientemente sparso, il tempo di broadcast di un messaggio e indipendente dal raggio di trasmissione, dimostrando che esso e' dominato dal tempo necessario affinche' molti agenti si incontrino. I nostri risultati completano l’analisi, apparsa in lavori precedenti, di sistemi densi, dove viceversa vi e' dipendenza del tempo di broadcast dal raggio di trasmissione. Inoltre le nostre tecniche di analisi possono essere estese a modelli di mobilita'-comunicazione simili, suggerendo alcune interessanti linee di ulteriore ricerca.

Menschliche Systeme werden seit einiger Zeit modelliert und analysiert auf der Basis der Theorie komplexer Netzwerke. Dies erlaubt es quantitativ zu untersuchen, welche strukturellen und zeitlichen Merkmale eines Systems Ausbreitungsprozesse beeinflussen, z.B. von Informationen oder von Infektionskrankheiten. Im ersten Teil der Arbeit wird untersucht, wie eine modular-hierarchische Struktur von statischen Netzwerken eine schnelle Verbreitung von Signalen ermöglicht. Es werden neue Heuristiken entwickelt um die Random-Walk-Observablen “First Passage Time” und “Cover Time” auf lokal geclusterten Netzwerken zu ermitteln. Vergleiche mit der Approximation eines gemittelten Mediums zeigen, dass das Auftreten der beobachteten Minima der Observablen ein reiner Netzwerkeffekt ist. Es wird weiterhin dargelegt, dass nicht alle modular-hierarchischen Netzwerkmodelle dieses Phänomen aufweisen. Im zweiten Teil werden zeitlich veränderliche face-to-face Kontaktnetzwerke auf ihre Anfälligkeit für Infektionskrankheiten untersucht. Mehrere Studien belegen, dass Menschen vornehmlich Zeit in Isolation oder kleinen, stark verbundenen Gruppen verbringen, und dass ihre Kontaktaktivität einem zirkadianen Rhythmus folgt. Inwieweit diese beiden Merkmale die Ausbreitung von Krankheiten beeinflussen, ist noch unklar. Basierend auf einem neuen Modell wird erstmals gezeigt, dass zirkadian variierende Netzwerke Trajektorien folgen in einem Zustandsraum mit einer strukturellen und einer zeitlichen Dimension. Weiterhin wird dargelegt, dass mit zunehmender Annäherung der zeitlichen Dimension von System und Krankheit die systemische Infektionsanfälligkeit sinkt. Dies steht in direktem Widerspruch zu Ergebnissen anderer Studien, die eine zunehmende Anfälligkeit vorhersagen, eine Diskrepanz, die auf die Ungültigkeit einer weit verbreiteten Approximation zurückzuführen ist. Die hier vorgestellten Ergebnisse implizieren, dass auf dem Gebiet die Entwicklung neuer theoretischer Methoden notwendig ist.<br>Human systems have been modeled and analyzed on the basis of complex networks theory in recent time. This abstraction allows for thorough quantitative analyses to investigate which structural and temporal features of a system influence the evolution of spreading processes, such as the passage of information or of infectious diseases. The first part of this work investigates how the ubiquitous modular hierarchical structure of static real-world networks allows for fast delivery of messages. New heuristics are developed to evaluate random walk mean first passage times and cover times on locally clustered networks. A comparison to average medium approximations shows that the emergence of these minima are pure network phenomena. It is further found that not all modular hierarchical network models provide optimal message delivery structure. In the second part, temporally varying face-to-face contact networks are investigated for their susceptibility to infection. Several studies have shown that people tend to spend time in small, densely-connected groups or in isolation, and that their connection behavior follows a circadian rhythm. To what extent both of these features influence the spread of diseases is as yet unclear. Therefore, a new temporal network model is devised here. Based on this model, circadially varying networks can for the first time be interpreted as following trajectories through a newly defined systemic state space. It is further revealed that in many temporally varying networks the system becomes less susceptible to infection when the time-scale of the disease approaches the time-scale of the network variation. This is in direct conflict with findings of other studies that predict increasing susceptibility of temporal networks, a discrepancy which is attributed to the invalidity of a widely applied approximation. The results presented here imply that new theoretical advances are necessary to study the spread of diseases in temporally varying networks.

Collective cell spreading is frequently observed in development, tissue repair and disease progression. Mathematical modelling used in conjunction with experimental investigation can provide key insights into the mechanisms driving the spread of cell populations. In this study, we investigated how experimental and modelling frameworks can be used to identify several key features underlying collective cell spreading. In particular, we were able to independently quantify the roles of cell motility and cell proliferation in a spreading cell population, and investigate how these roles are influenced by factors such as the initial cell density, type of cell population and the assay geometry.

Biological processes underlying skin cancer growth and wound healing are governed by various collective cell spreading mechanisms. This thesis develops new statistical methods to provide key insights into the mechanisms driving the spread of cell populations such as motility, proliferation and cell-to-cell adhesion, using experimental data. The new methods allow us to precisely estimate the parameters of such mechanisms, quantify the associated uncertainty and investigate how these mechanisms are influenced by various factors. The thesis provides a useful tool to measure the efficacy of medical treatments that aim to influence the spread of cell populations.

This thesis is concerned with studying the behavior of a gossiping protocol in the specific sense meant by Ericsson; in the following pages I’ll introduce a Markov process which models the spread of information in such systems. The results will be verified by means of a discreet-event simulation.<br>Gossiping Protocols, are inherently random in behavior.Nonetheless, they are not structure-less. Their asymptotic behavior when implemented in large scales is the matter of focus in this thesis.<br>Tel: +46709700505 Address: Pinnharvsgatan 3 E lgh 1202 43147 Mölndal Sweden

La popularité des téléphones portables, des appareils photo numériques et des ordinateurs personnels a modifié la création, la consommation et le partage des contenus multimédias. Cela pose le problème de la duplication à l’infini, du téléchargement non autorisé et de la redistribution illégale. À l’ère de la distribution généralisée des informations numériques, il est plus important que jamais de développer des solutions fiables et solides pour éviter la distribution illégale. Un système d’empreinte multimédia est un moyen efficace de protéger le contenu multimédia et d’empêcher la distribution illégale. L’objectif de cette thèse est de trouver les individus qui ont participé à la production et à la distribution illégale de contenus multimédias. Nous avons proposé un système de filigrane vidéo aveugle par transformée en ondelettes discrète associé à des codes d’empreintes probabilistes pour contrer les attaques de collusion parmi les vidéos à haute résolution. Le filigrane robuste et aveugle conduit à un taux d’erreur binaire plus élevé, nous devons ajouter une redondance supplémentaire aux codes d’empreintes digitales pour obtenir des taux de traçage plus élevés. Pour cela, nous proposons un schéma de codage dans lequel l’étalement aléatoire est utilisé avec des codes correcteurs d’erreurs. Nous avons utilisé FFMpeg pour intégrer l’image du filigrane dans la vidéo, ainsi que pour mener une série d’attaques de collusion (par exemple, moyenne, assombrissement et éclaircissement) sur des vidéos à haute résolution et nous avons comparé les générateurs-décodeurs de codes d’empreintes digitales les plus souvent suggérés dans la littératurepour trouver l’auteur de l’attaque de collusion. L’étude expérimentale montre que notre conception est très performante en termes de repérage des fraudeurs et de temps<br>The popularity of mobile phones, digital cameras, and personal computers has changed multimedia content creation, consumption, and sharing. This raises the issues of endless duplication, unauthorized uploading, and unlawful redistribution. In this era of widespread digital information distribution, it is more important than ever to develop dependable and strong solutions to avoid illegal distribution. A multimedia fingerprinting scheme is an efficient means of protecting multimedia content and preventing illegal distribution. The goal of this thesis is to find individuals who were engaged in the production and illegal distribution of multimedia content. We proposed a Discrete Wavelet Transform blind video watermarking scheme tied with probabilistic fingerprinting codes to counter collusion attacks among higher-resolution videos. The robust and blind watermarking leads to a higher bit error rate, we need to add extra redundancy to fingerprinting codes to obtain greater tracing rates. For that, we propose a coding scheme, in which the random spreading is utilized with error-correcting codes. We utilized FFMpeg to embed the watermark image into the video, as well as to conduct a range of collusion attacks (e.g., average, darken, and lighten) on high-resolution video and compared the most often suggested fingerprinting code generator-decoders in the literature to find the colluder. The experimental investigation shows that our design has high performance in terms of colluder tracing, and time

Heterogeneity in lattice potentials (like random or quasiperiodic) can localize linear, non-interacting waves and halt their propagation. Nonlinearity induces wave interactions, enabling energy exchange and leading to chaotic dynamics. Understanding the interplay between the two is one of the topical problems of modern wave physics. In particular, one questions whether nonlinearity destroys localization and revives wave propagation, whether thresholds in wave energy/norm exist, and what the resulting wave transport mechanisms and characteristics are. Despite remarkable progress in the field, the answers to these questions remain controversial and no general agreement is currently achieved. This thesis aims at resolving some of the controversies in the framework of nonlinear dynamics and computational physics. Following common practice, basic lattice models (discrete Klein-Gordon and nonlinear Schroedinger equations) were chosen to study the problem analytically and numerically. In the disordered linear case all eigenstates of such lattices are spatially localized manifesting Anderson localization, while nonlinearity couples them, enabling energy exchange and chaotic dynamics. For the first time we present a comprehensive picture of different subdiffusive spreading regimes and self-trapping phenomena, explain the underlying mechanisms and derive precise asymptotics of spreading. Moreover, we have successfully generalized the theory to models with spatially inhomogeneous nonlinearity, quasiperiodic potentials, higher lattice dimensions and arbitrary nonlinearity index. Furthermore, we have revealed a remarkable similarity to the evolution of wave packets in the nonlinear diffusion equation. Finally, we have studied the limits of strong disorder and small nonlinearities to discover the probabilistic nature of Anderson localization in nonlinear disordered systems, demonstrating the finite probability of its destruction for arbitrarily small nonlinearity and exponentially small probability of its survival above a certain threshold in energy. Our findings give a new dimension to the theory of wave packet spreading in localizing environments, explain existing experimental results on matter and light waves dynamics in disordered and quasiperiodic lattice potentials, and offer experimentally testable predictions.