Showing papers by "Polytechnic University of Turin published in 1984"

A class of generalized stochastic Petri nets for the performance evaluation of multiprocessor systems

Abstract: Generalized stochastic Petri nets (GSPNs) are presented and are applied to the performance evaluation of multiprocessor systems. GSPNs are derived from standard Petri nets by partitioning the set of transitions into two subsets comprising timed and immediate transitions. An exponentially distributed random firing time is associated with each timed transition, whereas immediate transitions fire in zero time. It is shown that GSPN are equivalent to continuous-time stochastic processes, and solution methods for the derivation of the steady state probability distribution are presented. Examples of application of gspn models to the performance evaluation of multiprocessor systems show the usefulness and the effectiveness of this modeling tool. 15 references.

Analytical formulas for coplanar lines in hybrid and monolithic MICs

Abstract: Some analytical formulas for the parameters of coplanar lines are discussed and validated; a chart is given for the design of coplanar waveguides on GaAs. The formulas discussed here, together with those presented previously by us (1983) represent a suitable set for the design of coplanar lines for hybrid and monolithic MICs (microwave integrated circuits).

Adaptive Cancellation of Nonlinear Intersymbol Interference for Voiceband Data Transmission

Abstract: Two related cancellation techniques based on "tentative" decisions are proposed for removing intersymbol interference, due to channel nonlinearities, for voiceband data transmission. Cancellation of intersymbol interference avoids the need for equalizing or "inverting" analog nonlinear operations as in the decision-feedback method previously proposed by Falconer. The nonlinear cancellation approach removes nonlinear interference terms without excessive noise enhancement and allows effective implementation with ROM's to generate the needed nonlinear signal variables that are to be weighted and summed. The first technique can be incorporated as an "add-on" unit to a present-day modem; the second technique requires a different receiver architecture but offers somewhat enhanced improvement. Simulation results for 9.6 kbit/s data transmission over nonlinear channel models show that both techniques can offer substantial performance improvements for 16-point QAM systems.

High-Level Modulation and Coding for Nonlinear Satellite Channels

Abstract: Currently, 4-PSK is the prevalent modulation format in use for digital satellite communications. To improve bandwidth efficiency, 8PSK could be used instead, but a higher power would be needed; to improve power efficiency, error-correcting codes could be used, but at the expense of a larger bandwidth. Recently, Ungerboeck [1] has proposed a class of codes in which a constellation of 2M signals is used to transmit information at the rate of log_{2} M bits per symbol, and has shown that coding gains of up to several decibels can be achieved on the additive white Gaussian noise (AWGN) channel with no increase in bandwidth occupancy and a relatively small added complexity. Thus, these codes seem to be particularly attractive for application in the band-limited environment typical of satellite communication systems, provided that the performance gain that they provide on the AWGN channel is not lost over a satellite channel. The goal of this work is to assess the performance of this class of codes when used to transmit 3 information bits per symbol on a band-limited, nonlinear satellite channel. Three modulation formats are considered, namely 16-PSK, 16-QAM, and a 16-ary amplitude-phase keying scheme with two amplitude levels. It is found that 16-PSK outperforms the two other modulation formats, and that the use of these codes can provide a substantial performance improvement even on a satellite channel.

A Contribution to the Numerical Prediction of Unsteady Flows

Abstract: Methods based on "flux-differe nce splitting" are investigated with reference to the prediction of unsteady compressible flows. They combine the feature of taking into account the wake-like nature of these flows and the capability of correctly capturing shock waves. Two recently proposed approaches and a third one suggested here are compared for one-dimensional flow, with particular reference to the means of operating the splitting. They are extended to the cases of variable-area ducts and a general transformation of the independent variables. Firstand second-order accurate schemes are presented and procedures for the computation at the boundaries are shown. Finally, numerical experiments are reported and discussed.

New techniques for constructing asymptotic stability regions for nonlinear systems

Abstract: The paper deals with the problem of the estimation of regions of asymptotic stability for continuous, autonomous, nonlinear systems. After an outline of the main approaches available in the literature, the "trajectory reversing method" is presented as a. powerful numerical technique for low order systems. Then, an analytical procedure based on the same topological approach is developed, and a comparison is made with the classical Zubov method, noting the possibility of overcoming some of the classical method's drawbacks (e.g., its nonuniform convergence). Several examples of applications of the "trajectory reversing method" both in the numerical and analytical formulation are reported.

Some results on the asymptotic stability of second-order nonlinear systems

Abstract: This note gives some stability results concerning second-order systems \dot{x} = f(x) , where f(x) contains either linear and quadratic or linear and cubic terms in x . Following a Lyapunov-like approach, a closed-form estimate for asymptotic stability regions of such systems is derived in terms of quadratic functions and then it is optimized with respect to its area. Some application examples show the good results of the proposed method, in comparison to those obtained by the classical numerical approaches.

Digital Simulation of Communication Systems with TOPSIM III

Abstract: TOPSIM III is a flexible, easy to use, stand-alone Fortran package for the digital simulation of communication systems in the time domain TOPSIM III provides a simple high-level language for the description of the system Programs can be written following the same structure used in drawing the system block diagram The paper discusses the time domain simulation approach used in the design of the package and describes its capabilities The contents of the subprogram libraries are presented, describing some of the building blocks that can be used in the simulation as well as the performance evaluation facilities available in the package Some simple examples illustrate the use of TOPSIM III as a tool for analysis and design of digital communication systems

Convergence of product formulas for the numerical evaluation of certain two-dimensional Cauchy principal value integrals

Abstract: We consider product rules of interpolatory type for the numerical approximation of certain two-dimensional Cauchy principal value integrals We present convergence results which generalize those known in the one-dimensional case

Determination of optical properties of SnO2 films

Abstract: The optical properties of conductive transparent thin films of undoped SnO2, prepared by using magnetron supttering, were studied by measuring the transmittance and the reflectance between λ=0.25 μm and λ=3μm. The extracted optical constants are interpreted to give values of a direct band gap of the order of 4 eV and of an indirect band gap of the order of 3 eV. Typical SnO2 films transmit ≈85% of visible light, have sheet resistanceR∂ (100÷800) Ω∂ and resistivities of (2.4·10−3÷1.8·10−2) Ω cm.

Oxidation of sulfur dioxide in a trickle‐bed reactor: A study of reactor modelling

Abstract: Introduction a co-courants de l'eau et divers melanges gazeux de SO 2 , air et O 2 , l'ecoulant vers le bas dans un lit fixe de petites particules de carbone a differents degres d'humidification. Interpretation des resultats experimentaux au moyen de deux modeles: dans le 1er, on a considere que la surface externe du catalyseur etait divisee en 2 zones (mouillee et seche), toutes deux actives pour le transfert de matiere; dans le second, on a suppose qu'une seule zone de la surface externe participe au transfert de matiere et que le coefficient de transfert de matiere doit etre calcule d'apres la theorie du film

Using ADA for Discrete Event Simulation

Abstract: The process interaction approach is proposed for developing a discrete simulation environment in Ada. The introduction of simulation facilities in Ada not only concerns the classical aspect of model building, but allows a new class of problems to be tackled, that is the testing of correctness of programs intended for real-time applications. In this paper attention is focused on the presentation of the process scheduling in the simulation context and on the definition of standard forms of interactions among processes. Simulation facilities are organized by making use of Ada's structuring concepts.

Complex ray analysis of radiation from large apertures with tapered illumination

Abstract: Near and far zone calculation of the radiation characteristics of large apertures with tapered illumination generally requires time-consuming numerical integraton over the aperture plane because geometrical theory of diffraction (GTD) asymptotic techniques are difficult to apply to focused beam systems. Asymptotic methods do, however, become feasible for complex ray systems since ray fields originating in a complex coordinate space yield beam type fields in physical space. In particular, rays emanating from a complex source point are known to generate a real beam type field without sidelobes, very similar to that produced by a Gaussian amplitude taper. These considerations are generalized here to one-dimensional taper profiles of the form exp [ -kI ], where k is the wavenumber and I has a polynomial dependence on the aperture coordinate. Profile shapes ranging from Gaussian to rectangular are accommodated thereby. The analysis is performed by complex ray tracing from the analytically extended complex aperture plane to the observer. Criteria are developed for inclusion of all relevant rays, and only these, and for their domains of validity. These criteria are based on saddle point analysis of the exact field integral, whose direct numerical evaluation furnishes the reference solution. Saddle points and steepest descent paths corresponding to various envelope polynomials are traced in detail for near and far zone fields. Numerical comparisons between saddle point asymptotics (completely equivalent to complex ray tracing) and the reference solution establish that simple complex ray theory is useful and accurate for Gaussian-like profiles with moderate envelope gradients but that the strong gradients in almost square profiles require an excessively large number of contributing complex rays. For the latter profiles, where edge-diffraction-like phenomena predominate, modification of conventional GTD would provide a better alternative. Thus, this study establishes the complementary utility of complex ray tracing for smoothly tapered, and of GTD for almost rectangular, aperture distributions.

Nonlinear dynamic models for spatial interaction: The results of some empirical experiments

Abstract: The content of this paper is related to the field of urban dynamic modelling, which has been greatly stimulated recently by the use of nonlinear methods. Some results of the application of an S.I.A. model of this kind to the Rome metropolitan area are described here. The experiments are mainly concerned with the analysis of the effects produced on the territorial diffusion of places of residence and services by variations in the mobility of the population in relation to journeys to work and journeys to services and by variations in economies of scale of consumers. The analysis is carried out for residential location and service location separately, and for the interacting Lowry-type system.

Optical study of the molecular alignment in a nematic liquid crystal in an oblique magnetic field

Abstract: The light intensity I transmitted through a homeotropically oriented nematic liquid crystal slab held between crossed polarizers, as a function of the angle 03B8H of an applied magnetic field, is calculated for oblique light incidence. The presence of singular points in the I vs. 03B8H curve which are related to the elastic constants of the liquid crystal is demonstrated One of these points, found here for the first time, for small angles of incidence depends only on the ratio K33/~a, and allows a direct determination of this ratio. The comparison of the calculated curve I vs. 03B8H with the experimental points taken in a sufficiently wide range of 03B8H values, using K11/~a, K33/~a and An = ne2014 no as best fit parameters, allows a very precise determination of all these constants. The method turns out to be much more sensitive than the usual ones, mostly based on capacitance measurements and it is thus very suitable for determining the behaviour of the elastic constants when the temperature is changed, particularly near the transition point.

New analytical approach for computing DC unipolar corona losses

Abstract: New analytical expressions for ionised electric field, emitter charge and current densities and voltage-current characteristics for DC unipolar corona are presented for conductor-to-plane geometry. The hypotheses normally considered in the literature, including that of Deutsch, are used in the integration procedure which is based on a Fourier series expansion of charge and current densities on the emitter wire. Different unipolar configurations are considered for the computation of the emitter charge density distribution and of corona losses. The results obtained, compared with those in the References, show the accuracy of the analytical approach.

Comparison between Measured and Calculated Reflectance Spectra from a Monodomain Cholesteric Liquid Crystal

Abstract: The general theory developed by the authors for the light propagation in cholesteric liquid crystals has been applied to the analysis of the high resolution reflectance spectra measured in a monodomain sample, in various optical conditions and in a wide wavelength range by Takezoe et al. The dispersion relation for the refractive indices obtained by the same authors has been taken into account. The agreement between theory and experiment is excellent. The Oseen's optical model and the local uniaxiality of the medium are confirmed. The polarization properties of the first order Bragg reflection peaks and the general behaviour of the interference curves are related to the properties of the internal eigenmodes. A full explanation of the beat or swell structure of the interference fringes is given.

Microscopic and macroscopic aspects of the magnetization process in SiFe single crystals

Abstract: On effectue des experiences de bruit de Barkhausen sur Fe-3% Si monocristallin dans le but de clarifier les relations entre les aspects microscopiques du processus d'aimantation et les proprietes magnetiques macroscopiques du materiau

Some New Techniques for Modelling Non-Linear Economic Fluctuations: A Brief Survey

Abstract: Despite the dynamic nature of classical theories (Smith, Ricardo, Marx), for many years the mathematical tools employed in economics have been essentially static. It is plain that in such a way it is not possible to get beyond equilibrium analysis. However, when we try to understand the nature of the equilibria (substantially of their stability), the inadequacy of this approach becomes apparent (Hicks (1939),Samuelson (1947)).

Existence of a Critical Tilt Angle for the Optical Properties of Chiral Smectic Liquid Crystals

Abstract: The optical properties of single-domain chiral smectic liquid crystals have been theoretically studied as a function of the tilt angle. It is shown that structures with a particular value ${\ensuremath{\theta}}_{\mathrm{rev}}$ of the tilt angle, which depends only on the principal values of the local dielectric tensor, have very peculiar behavior. In passage through ${\ensuremath{\theta}}_{\mathrm{rev}}$ a reversal of the polarization states of the eigenfunctions and a drastic change of many optical properties occur.

Isotopic shift in the3P1-3P0 Mg transition

Abstract: High precision frequency measurements of the3P1-3P0 transitions have been performed for the three natural abundant Mg-isotopes in a metastable atomic beam. A second order hyperfine theory for two-electron atoms allows the determination of the centre-of-gravity for25Mg and then the evaluation of the isotopic shift displacements. The high resolution frequency measurements (10−8) show a noticeable departure of the total shift from a linear dependence with respect to the atomic mass.

Optimization of optical parameters and electric field distribution in multilayers.

Abstract: Multilayer coatings for high-power laser mirrors and lenses have been optimized by a particular constrained gradient method to reach together with a reduction of damage threshold (i.e., suppression of the peak electric field), good values of optical parameters (reflectance or transmittance). The problem has been solved for normal and oblique incident radiation of λ = 10.6 μm. Reflectance values of more than 0.99 have been achieved with only two layers deposited on a metal mirror.

Dyadic Green's function in bounded media

Abstract: A new theory for the evaluation of the dyadic Green's functions in a homogeneous medium bounded by perfectly conducting walls is presented. The peculiarities of this theory are 1) the Green's functions are systematically and explicitly decomposed in a singular and an analytical part, and 2) these two parts are formulated in a general manner, i.e., independent on the boundaries. As an application, the Green's functions in a conical guide are evaluated.

Ethylene oxide reactor: safety according to operability analysis

Abstract: A detailed account is given of operability analysis of the cooling system of an ethylene oxide reactor (30,000 t/yr) as a typical example of a particularly critical process. Although this method is purely qualitative, it proved very effective in revealing the more critical parts of the plant at the design stage, with the result that suitable modifications were introduced. This assertion is supported by the results of a fault tree analysis, which is also described with reference to a detailed example of a reactor feed and control circuit. It was found that operability analysis is of more general and ready applicability than fault tree analysis, since it does not require failure rate data.

Optimal sampling schedule with unknown but bounded measurement errors: Families of linear models

Abstract: The problem of optimal sampling design for parameter estimation when data are generated by linear models is addressed. The measurements are assumed to be corrupted by unknown-but-bounded additive noise. The sampling design assumes that the samples number is free and no replication is allowed. Two main results are shown: 1) for particular classes of linear models, the optimal measurements number is equal to the parameters number, as in the statistical context; 2) the parameters uncertainty intervals of an actual realization are bounded from above by quantities that can be computed a priori, knowing only the model and the error structure.