Showing papers by "Polytechnic University of Turin published in 1999"

Optimum power control over fading channels

Abstract: We study optimal constant-rate coding schemes for a block-fading channel with strict transmission delay constraint, under the assumption that both the transmitter and the receiver have perfect channel-state information. We show that the information outage probability is minimized by concatenating a standard "Gaussian" code with an optimal power controller, which allocates the transmitted power dynamically to the transmitted symbols. We solve the minimum outage probability problem under different constraints on the transmitted power and we derive the corresponding power-allocation strategies. In addition, we propose an algorithm that approaches the optimal power allocation when the fading statistics are not known. Numerical examples for different fading channels are provided, and some applications discussed. In particular, we show that minimum outage probability and delay-limited capacity are closely related quantities, and we find a closed-form expression for the delay-limited capacity of the Rayleigh block-fading channel with transmission over two independent blocks. We also discuss repetition diversity and its relation with direct-sequence or multicarrier spread-spectrum transmission. The optimal power-allocation strategy in this case corresponds to selection diversity at the transmitter. From the single-user point of view considered in this paper, there exists an optimal repetition diversity order (or spreading factor) that minimizes the information outage probability for given rate, power, and fading statistics.

Probabilistic modelling of water balance at a point: the role of climate, soil and vegetation

Abstract: The soil moisture dynamics under seasonally fixed conditions are studied at a point. The water balance is described through the representation of rainfall as a marked Poisson process which in turn produces an infiltration into the soil dependent on the existing level of soil moisture. The losses from the soil are due to evapotranspiration and leakage which are also considered dependent on the existing soil moisture. The steady–state probability distributions for soil moisture are then analytically obtained. The analysis of the distribution allows for the assessment of the role of climate, soil and vegetation on soil moisture dynamics. Further hydrologic insight is obtained by studying the various components of an average water balance. The realistic representation of the processes acting at a site and the analytical tractability of the model make it well suited for further analyses which consider the spatial aspect of soil moisture dynamics.

Stability and Stabilization of Discontinuous Systems and Nonsmooth Lyapunov Functions

Abstract: We study stability and stabilizability properties of systems with discontinuous righthand side (with solutions intended in Filippov's sense) by means of locally Lipschitz continuous and regular Lyapunov functions. The stability result is obtained in the more general context of differential inclusions. Concerning stabilizability, we focus on systems affine with respect to the input: we give some sufficient conditions for a system to be stabilized by means of a feedback law of the Jurdjevic-Quinn type.

Central WENO schemes for hyperbolic systems of conservation laws

Abstract: We present a family of high-order, essentially non-oscillatory, central schemes for approx- imating solutions of hyperbolic systems of conservation laws. These schemes are based on a new centered version of the Weighed Essentially Non-Oscillatory (WENO) reconstruction of point-values from cell-averages, which is then followed by an accurate approximation of the fluxes via a natural con- tinuous extension of Runge-Kutta solvers. We explicitly construct the third and fourth-order scheme and demonstrate their high-resolution properties in several numerical tests. R esum e. Nous pr esentons une famille de sch emas centr es ENO d'ordre elev e pour des solutions approch ees de syst emes hyperboliques de lois de conservation. Ces sch emas reposent sur une nouvelle version centr ee de la reconstruction ENO a poids (WENO) des valeurs ponctuelles a partir des moyennes sur les cellules, ce qui conduit a une approximation precise des flux gr^ ace a une extension naturelle continue des solveurs Runge-Kutta. Nous construisons explicitement les sch emas d'ordre trois et quatre et nous provons leurs propri et es de haute pr ecision a travers des essais num eriques.

Limiting performance of block-fading channels with multiple antennas

Abstract: We derive the performance limits of a radio system consisting of a transmitter with t antennas and a receiver with r antennas, a block-fading channel with additive white Gaussian noise (AWGN), delay and transmit-power constraints, and perfect channel-state information available at both the transmitter and the receiver. Because of a delay constraint, the transmission of a codeword is assumed to span a finite (and typically small) number M of independent channel realizations; therefore, the relevant performance limits are the information outage probability and the "delay-limited" (or "nonergodic") capacity. We derive the coding scheme that minimizes the information outage probability. This scheme can be interpreted as the concatenation of an optimal code for the AWGN channel without fading to an optimal beamformer. For this optimal scheme, we evaluate minimum-outage probability and delay-limited capacity. Among other results, we prove that, for the fairly general class of regular fading channels, the asymptotic delay-limited capacity slope, expressed in bits per second per hertz (b/s/Hz) per decibel of transmit signal-to-noise ratio (SNR), is proportional to min (t,r) and independent of the number of fading blocks M. Since M is a measure of the time diversity (induced by interleaving) or of the frequency diversity of the system, this result shows that, if channel-state information is available also to the transmitter, very high rates with asymptotically small error probabilities are achievable without the need of deep interleaving or high-frequency diversity. Moreover, for a large number of antennas, delay-limited capacity approaches ergodic capacity.

Loop-star decomposition of basis functions in the discretization of the EFIE

Abstract: A general and readily applicable scheme is presented for the determination of the basis functions that allow the decomposition of the surface current into a solenoidal part and a nonsolenoidal remainder. The proposed approach brings into correspondence these two parts with two scalar functions and generates the known loop and star basis functions. The completeness of the loop-star basis is discussed, employing the presented scheme; the issue of the irrotational property of the nonsolenoidal functions is addressed.

Gait abnormalities in minimally impaired multiple sclerosis patients.

Abstract: Subclinical evidence of gait abnormalities were identified in a group of seven patients with multiple sclerosis, EDSS scored 0 - 2, without functional limitations. A movement analysis technique was used to identify gait parameters indicative of impaired motor function during walking. Abnormalities related primarily to time-distance parameters (reduced speed of progression, shorter strides, prolonged double support phase) and muscular function (premature recruitment of gastrocnemius and late relaxation of tibialis anterior during stance phase) were identified regardless the severity of the clinical score. The gait analysis procedure was able to provide the clinician with evidence of motor abnormalities prior to functional disturbance observable by a trained physician. These minimal dysfunctions may have resulted from reflex mechanisms impaired by delayed transmission through long loop pathways or else as a result of a nonspecific protective gait strategy to improve balance control. The technique described in this study may be useful to identify earlier starting points for follow-up and physiotherapy.

Non-Abelian Berry connections for quantum computation

Abstract: In the holonomic approach to quantum computation, information is encoded in a degenerate eigenspace of a parametric family of Hamiltonians and manipulated by the associated holonomic gates. These are realized in terms of the non-Abelian Berry connection and are obtained by driving the control parameters along adiabatic loops. We show how it is possible for a specific model to explicitly determine the loops generating any desired logical gate, thus producing a universal set of unitary transformations. In a multipartite system unitary transformations can be implemented efficiently by sequences of local holonomic gates. Moreover, a conceptual scheme for obtaining the required Hamiltonian family, based on frequently repeated pulses, is discussed, together with a possible process whereby the initial state can be prepared and the final one can be measured.

Compact Central WENO Schemes for Multidimensional Conservation Laws

Abstract: We present a new third-order central scheme for approximating solutions of systems of conservation laws in one and two space dimensions. In the spirit of Godunov-type schemes,our method is based on reconstructing a piecewise-polynomial interpolant from cell-averages which is then advanced exactly in time. In the reconstruction step, we introduce a new third-order as a convex combination of interpolants based on different stencils. The heart of the matter is that one of these interpolants is taken as an arbitrary quadratic polynomial and the weights of the convex combination are set as to obtain third-order accuracy in smooth regions. The embedded mechanism in the WENO-like schemes guarantees that in regions with discontinuities or large gradients, there is an automatic switch to a one-sided second-order reconstruction, which prevents the creation of spurious oscillations. In the one-dimensional case, our new third order scheme is based on an extremely compact point stencil. Analogous compactness is retained in more space dimensions. The accuracy, robustness and high-resolution properties of our scheme are demonstrated in a variety of one and two dimensional problems.

Modeling of surface myoelectric signals. I. Model implementation

Abstract: The relationships between the parameters of active motor units (MU's) and the features of surface electromyography (EMG) signals have been investigated using a mathematical model that represents the surface EMG as a summation of contributions from the single muscle fibers. Each MU has parallel fibers uniformly scattered within a cylindrical volume of specified radius embedded in an anisotropic medium. Two action potentials, each modeled as a current tripole, are generated at the neuromuscular junction, propagate in opposite directions and extinguish at the fiber-tendon endings. The neuromuscular junctions and fiber-tendon endings are uniformly scattered within regions of specified width. Muscle fiber conduction velocity and average fiber length to the right and left of the center of the innervation zone are also specified. The signal produced by MU's with different geometries and conduction velocities are superimposed. Monopolar, single differential and double differential signals are computed from electrodes placed in equally spaced locations on the surface of the muscle and are displayed as functions of any of the model's parameters. Spectral and amplitude variables and conduction velocity are estimated from the surface signals and displayed as functions of any of the model's parameters. The influence of fiber-end effects, electrode misalignment, tissue anisotropy, MU's location and geometry are discussed. Part II of this paper will focus on the simulation and interpretation of experimental signals.

Distributed-Memory Model Checking with SPIN

Abstract: The main limiting factor of the model checker SPIN is currently the amount of available physical memory. This paper explores the possibility of exploiting a distributed-memory execution environment, such as a network of workstations interconnected by a standard LAN, to extend the size of the verification problems that can be successfully handled by SPIN. A distributed version of the algorithm used by SPIN to verify safety properties is presented, and its compatibility with the main memory and complexity reduction mechanisms of SPIN is discussed. Finally, some preliminary experimental results are presented.

Pulsed battery discharge in communication devices

Abstract: The overall objective of this work is to explore ways in which the energy efficiency of communications can be enhanced through the use of communication protocols that exploit the charge recovery mechanism inherent to many secondary storage batteries. In the first part of this paper, we summarize the behavior of electrochemical energy cells. We compile results that pertain to the capacity of a cell and its dependence on the intensity of the discharge current. The phenomenon of charge recovery that takes place under bursty or pulsed discharge conditions is identified as a mechanism that can be exploited to enhance the capacity of a cell. The bursty nature of many data trafllc sources suggests that there may be a natural fit between the two. In the second part of this manuscript, we explore thii synergy by developing a stochastic model that tracks charge recovery in conjunction with bursty discharges due to transmissions driven by Bernoulli arrivals. We derive the resulting capacity advantage relative to constant discharge as a function of the burstiness of the arrival for two discharge scenarios.

VLSI architectures for turbo codes

Abstract: A great interest has been gained in recent years by a new error-correcting code technique, known as "turbo coding", which has been proven to offer performance closer to the Shannon's limit than traditional concatenated codes. In this paper, several very large scale integration (VLSI) architectures suitable for turbo decoder implementation are proposed and compared in terms of complexity and performance; the impact on the VLSI complexity of system parameters like the state number, number of iterations, and code rate are evaluated for the different solutions. The results of this architectural study have then been exploited for the design of a specific decoder, implementing a serial concatenation scheme with 2/3 and 3/4 codes; the designed circuit occupies 35 mm/sup 2/, supports a 2 Mb/s data rate, and for a bit error probability of 10/sup -6/, yields a coding gain larger than 7 dB, with ten iterations.

Multilayered Shell Theories Accounting for Layerwise Mixed Description, Part 1: Governing Equations

Abstract: A Reissner mixed variational equation is employed in this paper to derive the differential governing equations of multilayered, double curved shells made of orthotropic laminae In linear static cases. A layerwise description is referred to by assuming two independent fields in the thickness direction for the transverse stress (both shear and normal components) and displacement variables in each layer. Interlaminar values are used as the unknown variables of the introduced expansions. The continuity conditions of displacements and transverse shear and normals stresses at the interfaces between two consecutive layers, referred to as C 0 z requirements, have been a priori fulfilled. These have been used to drive the governing equations from a layer to a multilayered level. Classical displacement formulations and related equivalent single-layer equations have been derived for comparison purposes. No assumptions have been made concerning the terms of type thickness to radii shell ratio h/R. Donnell's shallow shell-type equations are given as particular cases for all of the considered theories. Indicial notations and arrays have been used extensively to handle the presented developments in a concise manner. Numerical evaluations and comparisons to exact and other available two-dimensional solutions are given in a companion paper (E. Carrera, Multilayered Shell Theories Accounting for Layerwise Mixed Description, Part 2: Numerical Evaluations, AIAA Journal, Vol. 37, No. 9, 1999, pp. 1117-1124).

Methane combustion on Mg-doped LaMnO3 perovskite catalysts

Abstract: LaMn1−xMgxO3 perovskite catalysts (x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO3 and LaMn0.8Mg0.2O3). The major results obtained are: (i) Mg substitution in the basic LaMnO3 perovskite has a positive effect on the catalytic activity only at low x values (x≤0.2); (ii) as opposed to the results of previous studies on the LaCr1−xMgxO3 system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO3, the catalytic combustion over LaMn0.8Mg0.2O3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.

Soft-error detection through software fault-tolerance techniques

Abstract: The paper describes a systematic approach for automatically introducing data and code redundancy into an existing program written using a high-level language. The transformations aim at making the program able to detect most of the soft-errors affecting data and code, independently of the Error Detection Mechanisms (EDMs) possibly implemented by the hardware. Since the transformations can be automatically applied as a pre-compilation phase, the programmer is freed from the cost and responsibility of introducing suitable EDMs in its code. Preliminary experimental results are reported, showing the fault coverage obtained by the method, as well as some figures concerning the slow-down and code size increase it causes.

Optimal Low-Thrust Escape Trajectories Using Gravity Assist

Abstract: Electric propulsion provides a spacecraft with continuous steering capabilities, which can be used to approach a planet with hyperbolic excess velocity that enhances the gravity assist. Low-thrust trajectories to escape from the solar system are considered in the present paper, which searches for the strategy that maximizes the spacecraft energy for assigned payload and engine operating time. The optimal conditions to escape using electric propulsion and gravity assist are presented for the cases of free-height and minimum-height  ybys. Optimal trajectories that exploit Jupiter or Venus  ybys have been computed for constant exhaust power with either constant or variable speciŽ c impulse; the procedure is also able to determine the optimal power level and to suggest when it is convenient to switch the engine on and off. The beneŽ t that system performance can receive by increasing the number of controls, i.e., by adding the possibility of coast arcs and engine throttling to the thrust direction control, is also noted.

A deadlock detection tool for concurrent Java programs

Abstract: This paper presents some issues related to the design and implementation of a concurrency analysis tool able to detect deadlock situations in Java programs that make use of multithreading mechanisms. An abstract formal model is generated from the Java source using the Java2Spin translator. The model is expressed in the PROMELA language, and the SPIN tool is used to perform its formal analysis. The paper mainly focuses on the design of the Java2Spin translator. A set of experiments, carried out to evaluate the performances of the analysis tool, is also presented. Copyright © 1999 John Wiley & Sons, Ltd.

Modeling of surface myoelectric signals. II. Model-based signal interpretation

Abstract: For pt. I see ibid., vol. 46, no. 7, p. 810-20 (1999). Experimental electromyogram (EMG) data from the human biceps brachii were simulated using the model described in pt. I of this work. A multichannel linear electrode array, spanning the length of the biceps, was used to detect monopolar and bipolar signals, from which double differential signals were computed, during either voluntary or electrically elicited isometric contractions. For relatively low-level voluntary contractions (10%-30% of maximum force) individual firings of three to four-different motor units were identified and their waveforms were closely approximated by the model. Motor unit parameters such as depth, size, fiber orientation and length, location of innervation and tendonous zones, propagation velocity, and source width were estimated using the model. Two applications of the model are described. The first analyzes the effects of electrode rotation with respect to the muscle fiber direction and shows the possibility of conduction velocity (CV) over- and under-estimation. The second focuses on the myoelectric manifestations of fatigue during a sustained electrically elicited contraction and the interrelationship between muscle fiber CV, spectral and amplitude variables, and the length of the depolarization zone. It is concluded that a) surface EMG detection using an electrode array, when combined with a model of signal propagation, provides a useful method for understanding the physiological and anatomical determinants of EMG waveform characteristics and b) the model provides a way for the interpretation of fatigue plots.

Induction motor high frequency model

Abstract: In this paper, an accurate high frequency model of an induction motor is presented. The proposed model allows analysis, at the same time, of both high frequency phenomena up to some MHz due to the static supply, and low frequency phenomena usually analyzed by means of the dq motor model. The high frequency model has been obtained by means of a frequency and time domain analysis and has been verified on a wide spread of induction motors starting from 4 up to 55 kW. The proposed model can be used to evaluate the high frequency leakage currents, which are the cause of electromagnetic interference to electronic and electric equipment.

Multilayered Shell Theories Accounting for Layerwise Mixed Description, Part 2: Numerical Evaluations

Abstract: The mixed layerwise shell theories that are presented in the companion article (E. Carrera, Multilayered Shell Theories Accounting for Layerwise Mixed Description, Part 1: Governing Equations' AIAA Journal, Vol. 37, No. 9, 1999, pp. 1107-1116) are evaluated here by solving several problems related to orthotropic cross-ply laminated, circular, cylindrical, and spherical shells subjected to static loadings for which closed-form solutions are given. Particular cases related to layerwise and equivalent single-layer models, based on classical displacement formulations, are evaluated for comparison purpose. A further comparison with three-dimensional elasticity exact solutions and to other higher-order shear deformations studies have been made. Results are given in the form of tables and diagrams. Approximations introduced by Donnell's shallow shell theories are evaluated for most of the problems. It has been concluded that the proposed mixed layerwise theories leads to a better description than the related analyses, which are based on displacement formulations. An excellent agreement, with respect to the exact solution, has been found for displacement and transverse stress components. These stresses have been herein calculated a priori. The importance of an adequate description of curvature terms related to the shell thickness to radii ratio h/R is also underlined. These effects have been contrasted by extensive use of fictitious interfaces in the conduced layerwise investigations.

Magnetic hysteresis based on dipolar interactions in granular magnetic systems

Abstract: The magnetic hysteresis of granular magnetic systems is investigated in the high-temperature limit $(T\ensuremath{\gg}$ blocking temperature of magnetic nanoparticles). Measurements of magnetization curves have been performed at room temperature on various samples of granular bimetallic alloys of the family ${\mathrm{Cu}}_{100\ensuremath{-}x}{\mathrm{Co}}_{x} (x=5--20 \mathrm{at}.%)$ obtained in ribbon form by planar flow casting in a controlled atmosphere, and submitted to different thermal treatments. The loop amplitude and shape, which are functions of sample composition and thermal history, are studied taking advantage of a novel method of graphical representation, particularly apt to emphasize the features of thin, elongated loops. The hysteresis is explained in terms of the effect of magnetic interactions of the dipolar type among magnetic-metal particles, acting to hinder the response of the system of moments to isothermal changes of the applied field. Such a property is accounted for in a mean-field scheme, by introducing a memory term in the argument of the Langevin function which describes the anhysteretic behavior of an assembly of noninteracting superparamagnetic particles. The rms field arising from the cumulative effect of dipolar interactions is linked by the theory to a measurable quantity, the reduced remanence of a major symmetric hysteresis loop. The theory's self-consistence and adequacy have been properly tested at room temperature on all examined systems. The agreement with experimental results is always striking, indicating that at high temperatures the magnetic hysteresis of granular systems is dominated by interparticle, rather than single-particle, effects. Dipolar interactions seem to fully determine the magnetic hysteresis in the high-temperature limit for low Co content $(xl~10).$ For higher concentrations of magnetic metal, the experimental results indicate that additional hysteretic mechanisms have to be introduced.