DMET: a computer package for the design of digital microwaveline-of-sight communication routes

The computer-aided design software package DMET (Digital Microwave Engineering Tool), which has been developed to meet the needs of the telecommunication engineering community involved in the design of line-of-sight digital microwave communication routes, is described. DMET can be used either by professionals or engineering students in such a way that efforts can be concentrated on the design objectives of the link rather than on cumbersome and tedious computations. This software package is particularly useful as a tool in teaching communication engineering students the basic practical steps involved in the design of line-of-sight radio communication links. Examples of antenna height computation and radio link budget computation using DMET are presented     

Minimum variance prediction and control for adaptive optics

Adaptive Optics (AO) systems enable to compensate the adverse effects of atmospheric turbulence on ground-based telescopes’ images in real time, using a deformable mirror (DM) inserted in the telescope’s optical path, and measurements provided by a wavefront sensor (WFS). This paper revisits minimum-variance (MV) control design for astronomical AO systems in a state-space framework. It presents a survey of the modeling and control issues arising in this multi-variable disturbance rejection problem. In a linear time-invariant framework, and under some mild assumptions, the optimal solution to MV control for AO systems is shown to be a discrete-time LQG controller. This result holds for a DM with instantaneous response, and for a fairly general class of DM’s dynamics. The state-space approach is extended to Wide-field Adaptive Optics (WfAO) configurations involving several DMs and/or WFSs. Integral-action control used in existing AO systems is compared with the LQG controller. Experimental WfAO results obtained on a laboratory test bench are presented, showing significant improvement in performance. Finally, open issues and perspectives of applicative and/or theoretical interests are discussed.     

Etude des conditions pour optimiser le calcul des frigorifères et des condenseurs à air ou évaporatifs

From the basis of an example of design calculations for refrigerating equipment for a low temperature cold store, the author raises a number of problems in relation to the search for improved conditions, both for design and operation, with regard to energy savings. The analysis is made without recourse to current practice. The load calculation is criticized and remarks are made on the method of determining optimum operating conditions for direct expansion evaporators. The method of reasoning developed here is considered applicable to most refrigeration problems.

Résumé

Très souvent les paramètres essentiels servant à la définition des matériels importants des circuits frigorifiques sont quelque peu traditionnels (écarts de température des évaporateurs, vitesses de circulation d'air, écarts d'approche des condenseurs évaporatifs, etc.).Cela ne veut pas dire qu'ils n'ont pas, en leur temps, été retenus comme de bonne pratique sur la base de considérations économiques raisonnées, mais la remise en cause des bases n'a sans doute pas été faite réguliérement. Les événements récents ou en cours touchant l'énergie et son coût incitent à vérifier la validité des idées recues. L'article présente une méthode d'évaluation des paramétres essentiels pour la définition des évaporateurs refroidisseurs d'air et des condenseurs refroidis par air en fonction des rapports des coûts de l'energie et des matières. Une application aux conditions françaises permet de l'illustrer.     

Evaluating location management schemes for third generation mobile networks

Several strategies have been proposed recently to improve the performance of the IS-41 location management scheme. A forwarding pointers' strategy and a built-in memory strategy are proposed to reduce the signaling cost for location update and improve the IS-41 location update procedure. In this paper, we present a performance analysis of each strategy in an arbitrary time interval. In this analysis, users are classified by their call to mobility ratio which is defined as the call arrival rate divided by the mobility rate. We evaluate each of these strategies using this call to mobility ratio in order to come up with a set of recommendations that determine when each strategy is beneficial and for which class of users. We provide also a simplified analysis of the database loads generated by each strategy.     

Noise propagation in wave-front sensing with phase diversity

The phase diversity technique is studied as a wave-front sensor to be implemented with widely extended sources. The wave-front phase expanded on the Zernike polynomials is estimated from a pair of images (in focus and out of focus) by use of a maximum-likelihood approach. The propagation of the photon noise in the images on the estimated phase is derived from a theoretical analysis. The covariance matrix of the phase estimator is calculated, and the optimal distance between the observation planes that minimizes the noise propagation is determined. The phase error is inversely proportional to the number of photons in the images. The noise variance on the Zernike polynomials increases with the order of the polynomial. These results are confirmed with both numerical and experimental validations. The influence of the spectral bandwidth on the phase estimator is also studied with simulations.     

Optimal control law for classical and multiconjugate adaptive optics

Classical adaptive optics (AO) is now a widespread technique for high-resolution imaging with astronomical ground-based telescopes. It generally uses simple and efficient control algorithms. Multiconjugate adaptive optics (MCAO) is a more recent and very promising technique that should extend the corrected field of view. This technique has not yet been experimentally validated, but simulations already show its high potential. The importance for MCAO of an optimal reconstruction using turbulence spatial statistics has already been demonstrated through open-loop simulations. We propose an optimal closed-loop control law that accounts for both spatial and temporal statistics. The prior information on the turbulence, as well as on the wave-front sensing noise, is expressed in a state-space model. The optimal phase estimation is then given by a Kalman filter. The equations describing the system are given and the underlying assumptions explained. The control law is then derived. The gain brought by this approach is demonstrated through MCAO numerical simulations representative of astronomical observation on a 8-m-class telescope in the near infrared. We also discuss the application of this control approach to classical AO. Even in classical AO, the technique could be relevant especially for future extreme AO systems.     

Acid and base catalyzed ring-opening polymerization of 2,2,4,4,6,6-hexamethyl-8,8-diphenylcyclotetrasiloxane

Triflic acid and P₄-t-Bu superbase both catalyze the ring-opening polymerization (ROP) of 2,2,4,4,6,6-hexamethyl-8,8-diphenylcyclotetrasiloxane (I) to yield a mixture of linear copoly(dimethylsiloxane/diphenylsiloxane) (3:1 mole ratio), low molecular weight oligomer and cyclic siloxanes. The molecular weight distribution, microstructure and thermal properties of the copolymers have been determined. The composition of the low molecular weight fractions have been analyzed. Triflic acid catalyzed ROP of I yields a copolymer whose microstructure can be accounted for by chemoselective ring opening of a single silyloxonium ion intermediate. In addition, 40% of I is recovered. On the other hand, P₄-t-Bu superbase catalyzed ROP of I yields a copolymer whose microstructure is random. The low molecular weight fraction, in this case, is made up of oligomer and a number of cyclotetra-, cyclopenta- and cyclohexasiloxanes.     

Using situation calculus for e-business agents

As the Internet grows, it is becoming less feasible for customers and merchants to manually visit each web site, analyze the information there, and make sound business decisions regarding the trading of goods or services. To cope with this evolution, software agents can be designed that are capable of automating the more routine, tedious, and time-consuming tasks involved in current trading processes. At a higher level agents may also be able to negotiate and make autonomous decisions and commitments on behalf of their owners.

This paper describes an agent implementation using the situation calculus, which offers a possibly unifying paradigm for dynamic agents. Interesting applications are currently being developed. Our contribution is a situation calculus agent system developed for e-business. Ongoing work is focused on implementing this system in an open marketplace environment.     

Error correction/masking for digital voice transmission over theland mobile satellite system

The development of a 2400-b/s speech digitizer which provides an acceptable level of intelligibility and quality over land mobile satellite channels is described. Performance tests over simulated channels in the UHF band (800 MHz) are presented. The voice digitizer is a linear prediction (LPC) vocoder which uses a channel error correction and concealment procedure tailored to error statistics for a minimum-shift keyed (MSK) downlink to a moving vehicle. The error-handling technique is based on perceptual criteria and utilizes the parametric nature of LPC representation of speech. A single-error-correcting, single-burst-detecting (28, 20) fire code is shown to be the best choice for the application. The intelligibility of the vocoder is measured and compared to the standard LPC-10 algorithm. The major remaining sources of speech quality degradation due to channel errors are determined and ranked