Evaluation des performances du réseau en anneau AMBRE pour distribution d’abonnés

ambre is an integrated services digital loop-like network for public local communication. Each communication is a sequence of fixed-length packets separated by fixed-length silences. Performance evaluation is carried through in terms of access-time and transport delays with emphasis on inter packet delay fluctuations.     

Potentialités des séquences d’étalement chaotiques pour l’amélioration duTEEB d’un systèmeDS-CDMA asynchrone

The use of chaotic sequences as spectral spreading sequences in a Direct Sequence-Code Division Multiple Access is proposed in this paper. The simulation of the system is performed with the system simulator Omnisys. The chaotic spreading sequences are generated from a non-linear recursive equation. The Bit Error Rate of this system is compared with theBER of the same system using binary Gold sequences. The results show that chaotic sequences can improve performances in terms of allowable number of users.     

Reconnaissance des défauts et traitement d’images pour les composés III–V

This paper resumes the most recent results in the field of defect detection and analysis in III–V compounds by image analysis. The correlation of the defects with the specifications of the integrated circuits is especially emphasized. The production of integrated circuits or optoelectronic integrated circuits from III–V compounds is intended to bring large improvements in speed, power and radiation or thermal hardness. Such a development requires a precise control of the material quality and we will consider the relation between microscopic imperfections and device specifications through imaging techniques. This paper contents five chapters respectively devoted to: crystal growing and dislocation imaging; possible role of the deep centerEl2 in GaAs; correlation of defects withFet specifications; infrared transmission and scattering imaging; numerical processing and instruments. Computer assisted processing of image now gives a large aptitude for image transformation synthesis and recording; it allows also to conceive numerical processing and instruments. Computer assisted processing of images gives a large aptitude for image transformation, synthesis and recording; it allows to conceive a diagnostic of the quality of the wafer with respect to the structure that is be elaborated on.     

Étude du comportement d’une boucle de Costas en modulation MAQ 16 en présence d’évanouissements sélectifs

This paper gives the variance of the phase recovered by a conventional quadriphase Costas loop in 16 Qam modulation in the presence of a selective fading, represented as a three rays simplified model. The power spectral density of the phase error is computed using a spectral estimation technique. The phase error variance is evaluated for different values of the fade depth, the location of the maximum attenuation in the frequency band and the delay of the reflected ray. Finally the relation between the performance degradation and phase noise is given, assuming a gaussian p.d.f. of the phase error.     

Codage pour modulation d’amplitude en quadrature à grand nombre d’états avec partition à plusieurs niveaux

We present a new multilevel coding technique for the quadrature amplitude modulation with a high number of states (64-QAM and 256-QAM). The constellation is partitioned into four subsets, two bits coded with the samebch code selecting the label of each subset. We show that our system, with a double error correctingbch code, gives better results, considering redundancy, decoding simplicity and performance, than the systems known bx now (using the Reed-Solomon code (64,62), the Lee-Nakamura and Hamming codes). Preliminary differential encoding of the two coded bits and utilisation of a transparentbch code can solve the phase ambiguity problem in a simple way.     

Antennes en réseau à bas niveau de lobes secondaires par commutation de la loi d’alimentation. Optimisation par l’algorithme génétique

The paper presents an approach of synthesis from a new thinned microstrip antennas array with binary feeding law. The operating principle of this type of antennas array consists in establishing a number of elements supplied at the center of the array as well as the feeding of the extreme elements. The problem of synthesis with constraints is dealt with only by the research of the binary feeding law of the array eccentric elements, thus allowing to reduce to the maximum the sidelobes level. To solve this type of problem, the genetic algorithm is proposed.     

La compression d’images embarquée pour les missions spatiales

On-board space instruments produce more and more high amount of data due to the increase in both resolution and number of bits per pixel, not compensated by the reduced swath. Due to the stringent limitations (in terms of mass and power) which apply to on-board equipment, it is essential to reduce to a minimum the on-board sto-rage capacity and the on-board transmission rate needed to fullfill the mission: on-board image compression is a very powerful tool to optimise the resources. The availability, for spaceborne applications, of highly integrated circuits allows to implement on board very sophisticated real-time compression schemes and then to improve the performances of on-board compression. The whole image chain (from the sensor to the on-ground post-processing such as déconvolution and denoising) has to be considered in order to select the compression algorithm and the compression ratio. This article presents the compression algorithms, selected or currently studied, for space missions, and the methods of image quality evaluation.     

Synthèse sur la réduction conjointe de bruit et d’écho pour lés systèmes mains-libres

In the context of modern telecommunications, hands-free systems allow to talk in a more natural way, without handling a telephone. This leads to new problems which were negligible in classical telephony: the superposition of energetic noise and echo on the speech signal. To solve these problems and to provide a sufficient speech quality, a joint reduction of these perturbations is needed. This paper presents a synthesis of solutions retained to achieve this reduction in the context of mono-channel and two-channel observations.