331 research outputs found
Protocole de routage à chemins multiples pour des réseaux ad hoc
Ad hoc networks consist of a collection of wireless mobile nodes which dynamically exchange data without reliance on any fixed based station or a wired backbone network. They are by definition self-organized. The frequent topological changes make multi-hops routing a crucial issue for these networks. In this PhD thesis, we propose a multipath routing protocol named Multipath Optimized Link State Routing (MP-OLSR). It is a multipath extension of OLSR, and can be regarded as a hybrid routing scheme because it combines the proactive nature of topology sensing and reactive nature of multipath computation. The auxiliary functions as route recovery and loop detection are introduced to improve the performance of the network. The usage of queue length metric for link quality criteria is studied and the compatibility between single path and multipath routing is discussed to facilitate the deployment of the protocol. The simulations based on NS2 and Qualnet softwares are performed in different scenarios. A testbed is also set up in the campus of PolytechâNantes. The results from the simulator and testbed reveal that MP-OLSR is particularly suitable for mobile, large and dense networks with heavy network load thanks to its ability to distribute the traffic into different paths and effective auxiliary functions. The H.264/SVC video service is applied to ad hoc networks with MP-OLSR. By exploiting the scalable characteristic of H.264/SVC, we propose to use Priority Forward Error Correction coding based on Finite Radon Transform (FRT) to improve the received video quality. An evaluation framework called SVCEval is built to simulate the SVC video transmission over different kinds of networks in Qualnet. This second study highlights the interest of multiple path routing to improve quality of experience over self-organized networks.Les rĂ©seaux ad hoc sont constituĂ©s dâun ensemble de nĆuds mobiles qui Ă©changent des donnĂ©es sans infrastructure de type point dâaccĂšs ou artĂšre filaire. Ils sont par dĂ©finition auto-organisĂ©s. Les changements frĂ©quents de topologie des rĂ©seaux ad hoc rendent le routage multi-sauts trĂšs problĂ©matique. Dans cette thĂšse, nous proposons un protocole de routage Ă chemins multiples appelĂ© Multipath Optimized Link State Routing (MP-OLSR). Câest une extension dâOLSR Ă chemins multiples qui peut ĂȘtre considĂ©rĂ©e comme une mĂ©thode de routage hybride. En effet, MP-OLSR combine la caractĂ©ristique proactive de la dĂ©tection de topologie et la caractĂ©ristique rĂ©active du calcul de chemins multiples qui est effectuĂ© Ă la demande. Les fonctions auxiliaires comme la rĂ©cupĂ©ration de routes ou la dĂ©tection de boucles sont introduites pour amĂ©liorer la performance du rĂ©seau. Lâutilisation de la longueur des files dâattente des nĆuds intermĂ©diaires comme critĂšre de qualitĂ© de lien est Ă©tudiĂ©e et la compatibilitĂ© entre routage Ă chemins multiples et chemin unique est discutĂ©e pour faciliter le dĂ©ploiement du protocole. Les simulations basĂ©es sur les logiciels NS2 et Qualnet sont effectuĂ©es pour tester le routage MP-OLSR dans des scĂ©narios variĂ©s. Une mise en Ćuvre a Ă©galement Ă©tĂ© rĂ©alisĂ©e au cours de cette thĂšse avec une expĂ©rimentation sur le campus de PolytechâNantes. Les rĂ©sultats de la simulation et de lâexpĂ©rimentation rĂ©vĂšlent que MP-OLSR est particuliĂšrement adaptĂ© pour les rĂ©seaux mobiles et denses avec des trafics Ă©levĂ©s grĂące Ă sa capacitĂ© Ă distribuer le trafic dans des chemins diffĂ©rents et Ă des fonctions auxiliaires efficaces. Au niveau application, le service vidĂ©o H.264/SVC est appliquĂ© Ă des rĂ©seaux ad hoc MP-OLSR. En exploitant la hiĂ©rarchie naturelle dĂ©livrĂ©e par le format H.264/SVC, nous proposons dâutiliser un codage Ă protection inĂ©gale (PFEC) basĂ© sur la Transformation de Radon Finie (FRT) pour amĂ©liorer la qualitĂ© de la vidĂ©o Ă la rĂ©ception. Un outil appelĂ© SVCEval est dĂ©veloppĂ© pour simuler la transmission de vidĂ©o SVC sur diffĂ©rents types de rĂ©seaux dans le logiciel Qualnet. Cette deuxiĂšme Ă©tude tĂ©moigne de lâintĂ©rĂȘt du codage Ă protection inĂ©gale dans un routage Ă chemins multiples pour amĂ©liorer une qualitĂ© dâusage sur des rĂ©seaux auto-organisĂ©s
Project OASIS: The Design of a Signal Detector for the Search for Extraterrestrial Intelligence
An 8 million channel spectrum analyzer (MCSA) was designed the meet to meet the needs of a SETI program. The MCSA puts out a very large data base at very high rates. The development of a device which follows the MCSA, is presented
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Asymptotic theory for Bayesian nonparametric inference in statistical models arising from partial differential equations
Partial differential equations (PDEs) are primary mathematical tools to model the behaviour of complex real-world systems. PDEs generally include a collection of parameters in their formulation, which are often unknown in applications and need to be estimated from the data. In the present thesis, we investigate the theoretical performance of nonparametric Bayesian procedures in such parameter identification problems in PDEs. In particular, inverse regression models for elliptic equations and stochastic diffusion
models are considered.
In Chapter 2, we study the statistical inverse problem of recovering an unknown function from a linear indirect measurement corrupted by additive Gaussian white noise. We employ a nonparametric Bayesian approach with standard Gaussian priors, for which the posterior-based reconstruction corresponds to a Tikhonov regulariser with a reproducing kernel Hilbert space norm penalty. We prove a semiparametric Bernsteinâvon Mises theorem for a large collection of linear functionals of the unknown, implying that semiparametric posterior estimation and uncertainty quantification are valid and optimal from a frequentist point of view. The general result is applied to three concrete examples that cover both the mildly and severely ill-posed cases: specifically, elliptic inverse problems, an elliptic boundary value problem, and the recovery of the initial condition of the heat equation. For the elliptic boundary value problem, we also obtain a nonparametric version of the theorem that entails the convergence of the posterior distribution to a prior-independent infinite-dimensional Gaussian probability measure with minimal covariance. As a consequence, it follows that the Tikhonov regulariser is an efficient estimator, and we derive frequentist guarantees for certain credible balls centred around it.
Chapter 3 is concerned with statistical nonlinear inverse problems. We focus on the prototypical example of recovering the unknown conductivity function in an elliptic PDE in divergence form from discrete noisy point evaluations of the PDE solution. We study the statistical performance of Bayesian nonparametric procedures based on a flexible class of Gaussian (or hierarchical Gaussian) process priors, whose implementation is feasible by MCMC methods. We show that, as the number of measurements increases, the resulting posterior distributions concentrate around the true parameter generating the data, and derive a convergence rate, algebraic in inverse sample size, for the estimation error of the associated posterior means.
Finally, in Chapter 4 we extend the posterior consistency analysis to dynamical models based on stochastic differential equations. We study nonparametric Bayesian models for reversible multi-dimensional diffusions with periodic drift. For continuous observation paths, reversibility is exploited to prove a general posterior contraction rate theorem for the drift gradient vector field under approximation-theoretic conditions on the induced prior for the invariant measure. The general theorem is applied to Gaussian priors and p-exponential priors, which are shown to converge to the truth at the minimax optimal rate over Sobolev smoothness classes in any dimension.
Chapter 1 is dedicated to introducing the statistical models considered in Chapters 2 - 4, and to providing an overview of the theoretical results derived therein. The main theorems of Chapter 2 and Chapter 3 are illustrated via the results of simulations, and detailed comments are provided on the implementation.Richard Nicklâs ERC grant No. 647812; EPSRC grant EP/L016516/1 for the
Cambridge Centre for Analysi
Entropies from coarse-graining: convex polytopes vs. ellipsoids
We examine the Boltzmann/Gibbs/Shannon and the
non-additive Havrda-Charv\'{a}t / Dar\'{o}czy/Cressie-Read/Tsallis \
\ and the Kaniadakis -entropy \ \
from the viewpoint of coarse-graining, symplectic capacities and convexity. We
argue that the functional form of such entropies can be ascribed to a
discordance in phase-space coarse-graining between two generally different
approaches: the Euclidean/Riemannian metric one that reflects independence and
picks cubes as the fundamental cells and the symplectic/canonical one that
picks spheres/ellipsoids for this role. Our discussion is motivated by and
confined to the behaviour of Hamiltonian systems of many degrees of freedom. We
see that Dvoretzky's theorem provides asymptotic estimates for the minimal
dimension beyond which these two approaches are close to each other. We state
and speculate about the role that dualities may play in this viewpoint.Comment: 63 pages. No figures. Standard LaTe
Evolution-Operator-Based Single-Step Method for Image Processing
This work proposes an evolution-operator-based single-time-step
method for image and signal processing. The key component of the
proposed method is a local spectral evolution kernel (LSEK) that
analytically integrates a class of evolution partial differential
equations (PDEs). From the point of view PDEs, the LSEK provides
the analytical solution in a single time step, and is of spectral
accuracy, free of instability constraint. From the point of
image/signal processing, the LSEK gives rise to a family of
lowpass filters. These filters contain controllable time delay and
amplitude scaling. The new evolution operator-based method is
constructed by pointwise adaptation of anisotropy to the
coefficients of the LSEK. The Perona-Malik-type of anisotropic
diffusion schemes is incorporated in the LSEK for image denoising.
A forward-backward diffusion process is adopted to the LSEK for
image deblurring or sharpening. A coupled PDE system is modified
for image edge detection. The resulting image edge is utilized for
image enhancement. Extensive computer experiments are carried out
to demonstrate the performance of the proposed method. The major
advantages of the proposed method are its single-step solution and
readiness for multidimensional data analysis
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Partielle Differentialgleichungen
The workshop dealt with partial diïŹerential equations in geometry and technical applications. The main topics were the combination of nonlinear partial diïŹerential equations and geometric problems, regularity of free boundaries, conformal invariance and the Willmore functional
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