109 research outputs found
Spectral density matrix estimation method: bias-variance trade-off optimization
A non-parametric method is presented for the computation of the spectral density matrix of stationnary signais, intended ta
minimize the total error, al least in the case of slowly varying spectra . The method is basent on a recursive algoritlnn tvhich
achieves, by successive linear transforms, whitening and decorrelation of the signais . Each step involves the estimation of tivo
spectral densities and one cross-spectral riensity, using frequency smoothing with an auloinatic choice of the stnoothing
window. This method preserves the definite non-negative property of the matrix and achieves an optimisation of the trade-off
between bias and variance .Nous présentons une méthode non paramétrique pour le calcul de la matrice de densité spectrale de signaux stationnaires
permettant de minimiser l'erreur totale d'estimation, au moins dans le cas de signaux Ă spectres lentement variables . La
mĂ©thode repose sur un algorithme rĂ©cursif de blanchiment et de dĂ©corrĂ©lation de signaux stationnaires qui se ramĂšne Ă
chaque étape au calcul des autospectres et de l'interspectre de deux signaux par un lissage fréquentiel avec adaptation
automatique de la fenĂȘtre de pondĂ©ration . Cette mĂ©thode tout en assurant le caractĂšre dĂ©fini non nĂ©gatif de la matrice de
densité spectrale permet d'optimiser le compromis biais-variance sur chacun de ses éléments
Estimation des caractéristiques de sources diffuses
Les algorithmes de localisation en traitement d'antenne utilisent souvent l'hypothÚse de sources ponctuelles. Or, de récentes études montrent que dans un milieu riche en obstacles, le front d'onde se distord et les trajets subissent une dispersion angulaire. Aussi cet article propose-t-il une méthode de type sous-espace robuste à une distribution azimutale de puissance, et dont nous analysons les performances. Il motive de plus l'utilisation de techniques de Prédiction Linéaire, alternatives de poids à la localisation Haute Résolution (HR) de sources diffuses
Iterative sequential algorithm for inversion of an operator. Application to the restoration of the images given by a multi-element eddy current sensor
Iterative methods are commonly used in signal processing, to restore a
signal smoothed by a linear operator H, in presence of noise. In this paper,
we present an original sequential method which relies on a recursive feed
back control loop including the operator H. It is based on the use of only a
small fixed number of iterations, with a predetermined sequence of gain
factors k,, . We demonstrate that the sequence kâ can be choosen in order to
approximate the optimal Wiener inverse filter corresponding to a particular
crude assumption on the input signal statistics . This is ofparticular interest
if H is a non stationary filter, or a non linear filter with a varying local
linear approximation . It is this kind of non linear filtering that we encountered in a practical
situation in the design of a multi-element eddy carrent sensor. The
performance of our algorithm is illustrated in this application, in order to
improve the depth image of a metallic body given by the sensor .Les techniques d'estimation utilisées en traitement du signal nécessitent
fréquemment l'emploi de méthodes itératives, qui permettent de restituer
au mieux le signal d'entrée d'un systÚme à partir de l'observation . Dans
cet article, nous effectuons une brÚve synthÚse des méthodes d'inversion,
et prĂ©sentons un algorithme original mettant en jeu l'opĂ©rateur H Ă
inverser et une séquence de gains de correction k,,, choisie pour obtenir la
meilleure approximation du filtre optimal de Wiener inverse correspondant
à des hypothÚses assez simples sur les propriétés statistiques du
signal d'entrée. L'analyse détaillée des performances de ce schéma itératif est menée dans le cas d'un opérateur H linéaire en présence de
bruit, mais nous montrons qu'elle s'Ă©tend sous certaines conditions Ă des
systÚmes non linéaires . Les développements théoriques sont ensuite
illustrés par une application en restauration de profil pour un multicapteurs
Ă courants de Foucault
Estimation des paramĂštres spatio-temporels d'un canal de propagation Ă trajets multiples
L'identification active de canaux de propagation à trajets multiples réduit le débit de la transmission. Nous proposons ici des algorithmes passifs, qui n'utilisent pas de séquence d'apprentissage. Nous avions proposé, dans une publication antérieure, des algorithmes procédant en deux étapes à partir des méthodes de déconvolution autodidacte. Nous utilisons ici la forme particuliÚre des critÚres pour estimer conjointement les paramÚtres spatio-temporels (angles d'incidence et retards de groupe), et donnons leurs bornes de Cramer-Rao. Les propriétés de ces méthodes, ainsi que leurs liens avec l'approche au sens du maximum de vraisemblance, sont démontrées puis analysées par simulations
Analyse de textures à l'aide de modÚles anisotropes à longue dépendance
Dans ce travail, nous introduisons des modĂšles discrets de textures de type "1/f", permettant de caractĂ©riser l'anisotropie souvent prĂ©sente dans les images rĂ©elles. Ces modĂšles peuvent ĂȘtre conçus comme des Ă©quivalents 2D des processus 1D discrets ARIMA fractionnaires. Ils constituent aussi une alternative anisotrope aux bruits gaussiens fractionnaires 2D. Deux mĂ©thodes sont proposĂ©es pour identifier ces nouveaux modĂšles, en l'absence de bruit. Une application de l'algorithme EM est ensuite proposĂ©e pour traiter le cas oĂč les donnĂ©es sont bruitĂ©es
Generalised array manifold model for wireless communication channels with local scattering
Identification and characterization of secreted and pathogenesis-related proteins in Ustilago maydis
Interactions between plants and fungal pathogens require a complex interplay at the plantâfungus interface. Extracellular effector proteins are thought to play a crucial role in establishing a successful infection. To identify pathogenesis-related proteins in Ustilago maydis we combined the isolation of secreted proteins using a signal sequence trap approach with bioinformatic analyses and the subsequent characterization of knock-out mutants. We identified 29 secreted proteins including hydrophobins and proteins with a repetitive structure similar to the repellent protein Rep1. Hum3, a protein containing both, a hydrophobin domain and a repetitive Rep1-like region, is shown to be processed during passage through the secretory pathway. While single knock-outs of hydrophobin or repellent-like genes did not affect pathogenicity, we found a strong effect of a double knock-out of hum3 and the repetitive rsp1. Yeast-like growth, mating, aerial hyphae formation and surface hydrophobicity were unaffected in this double mutant. However, pathogenic development in planta stops early after penetration leading to a complete loss of pathogenicity. This indicates that Hum3 and Rsp1 are pathogenicity proteins that share an essential function in early stages of the infection. Our results demonstrate that focusing on secreted proteins is a promising way to discover novel pathogenicity proteins that might be broadly applied to a variety of fungal pathogens
Comparative Genome Analysis of Filamentous Fungi Reveals Gene Family Expansions Associated with Fungal Pathogenesis
Fungi and oomycetes are the causal agents of many of the most serious diseases of plants. Here we report a detailed comparative analysis of the genome sequences of thirty-six species of fungi and oomycetes, including seven plant pathogenic species, that aims to explore the common genetic features associated with plant disease-causing species. The predicted translational products of each genome have been clustered into groups of potential orthologues using Markov Chain Clustering and the data integrated into the e-Fungi object-oriented data warehouse (http://www.e-fungi.org.uk/). Analysis of the species distribution of members of these clusters has identified proteins that are specific to filamentous fungal species and a group of proteins found only in plant pathogens. By comparing the gene inventories of filamentous, ascomycetous phytopathogenic and free-living species of fungi, we have identified a set of gene families that appear to have expanded during the evolution of phytopathogens and may therefore serve important roles in plant disease. We have also characterised the predicted set of secreted proteins encoded by each genome and identified a set of protein families which are significantly over-represented in the secretomes of plant pathogenic fungi, including putative effector proteins that might perturb host cell biology during plant infection. The results demonstrate the potential of comparative genome analysis for exploring the evolution of eukaryotic microbial pathogenesis
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