Réflexions partielles dans l’atmosphère et propagation a grande distance

Annals of Telecommunications - L’auteur commence par donner un bref aperçu des théories susceptibles d’expliquer le champ électrique observé à grande distance...     

Modélisation d’un anneauSDH pour les besoins de la gestion de fautes

When an anomaly occurs in aSDH network, many notifications are emitted towards the supervision center. The emission of these notifications is the result of the signals treatment on the level of the objects describing the network. This document describes the modeling process of the behavior and the interaction between the objects intervening in the data processing of faults management. It gives a methodology ofSDH network model construction for the needs for faults situations diagnosis.     

Modèles de télétrafic pour le réseau à intégration de services du satellite TELECOM 1

The author discusses models of multiservice circuit switched communications systems where, in particular, the amount of transmission capacity (e.g. bit rate) depends on the call type. Two modes of call set up are considered: «on demand» with blocked calls cleared and «reservation» where capacity is previously reserved for planned calls. The latter mode is described generally and, for particular cases, we suggest interpretations leading to birth process and multiservice queue models. The models have been studied for evaluating the performance of the telecom 1business communication network but derived exact and approximate results should be useful in other applications.     

Sur l’élaboration par jets moléculaires et les propriétés optiques d’hétérojonctions Ga Al Sb/Ga Sb

In this paper the authors present the molecular beam epitaxy preparation of GaSb and GaAlSb layers. They describe briefly the experimental technique and report on the growth characterization of the layers. They have used reflection high-energy electron diffraction (rheed)to study the epitaxial growth of GaSb and GaAlSb. The evolution of the rheed pattern was then followed,for given growth conditions, during the formation of GaSb-GaSb, GaSb-GaAs, GaAISb-GaSb and GaAlSb-GaAs interfaces. By electroreflectance study energy positions of the main direct interband transitions for GaSb and GaAISb are obtained.     

Le système cellulaire de télécommunication avec les mobiles

Cept has undertaken to standardize a new land mobile cellular system in the 900 MHz band, which will enable the compatibility of services throughout Europe. The specifications of this second generation cellular system are under study by the special group « Systèmes mobiles publics de radiocommunication » (gsm). This paper intends to give a general overview of this system, based on the work going on withingsm. It concentrates mainly on the services aspects, and the system architecture, interfaces and protocols. In order to illustrate the global working of the system, the example of a mobile terminated call establishment is given.     

Synthèse optimale d’un réseau de télécommunication avec contraintes de sécurité

A commonly used method for achieving a prescribed security level in a telecommunication network is to manage extra capacities all over the network. This paper describes a number of computational methods, both exact and approximate, for determining a minimum cost network of extra capacities needed to achieve security under any given trunk dimensioning, routing and grouping policy. From a mathematical point of view, it is shown that the problem reduces to the determination of a minimum cost network subject to a number of non simultaneous multicommodity flow requirements. Since real life instances of such problems commonly result in very large linear programs of many thousands of variables and constraints, the techniques of linear programming would be unacceptably time and space-consuming. Instead, it is shown how the use of Lagrangean relaxation in conjunction with sub-gradient optimization schemes can lead to efficient and easy-to-implement solution methods. The various procedures described in this paper form the heart of a general software product for long and medium term planning applications (Supersec program) which has already been used to investigate the economical implications of security requirements in telecommunication networks.     

Modélisation des discontinuités d’un objectif radar par des points brillants

In order to study the scattering of a monochromatic electromagnetic wave by a complex radar target, specialists usually modelize the target by a discrete scatterers model. The majority of the scatterers are represented by specular points of reflexion on the surface of the target, but the radar cross section (rcs) is also a function of surface discontinuities. In this article, we develop a method which allows us to replace the discontinuities by groups of point contributors of which the positions and thercs are given. Eventhough thercs are generally functions which decrease as the frequency increases, which can at times be neglected, it can be shown there exists quite frequentlyrcs which are independent of the wavelength. General formulae are given. It is emphasized that the physical optic integral may be used to calculatercs eventhough the conditions of application are not correct. For this reason a number of coefficients including the polarisations are used. The formulae of these coefficients are given. As a result an evaluation of thercs may be simplified, and the same method may be used to study the specular regions and the surface discontinuities.     

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.     

Amélioration des performances des systèmes à haut débit par une méthode de réduction de la diaphonie dans les câbles multiconducteurs de télécommunications

dsl (Digital Subscriber Line) technology provides low cost access to the consumers and sources of information for any one subscribing to classical analogical telephony. Indeed xdsl services enable high-speed digital transmission on conventional telephone lines. For sixteen years that the internet has risen, a great number of offers proposing increasing bit rate have been developed. Considering this increasing demand of high bit rate access, telecommunication operators perpetually work on finding solutions to optimize the copper lines channel. In that way, we present here a new method of crosstalk reduction in a multiconductor telecommunication cable. Interests and limitations of this method are presented and some simulations on a frequency band extending to 30 MHz will picture those theoretical results.     

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.