Showing papers by "Polytechnic University of Milan published in 1994"

The wavenumber shift in SAR interferometry

Abstract: SAR surveys from separate passes show relative shifts of the ground wavenumber spectra that depend on the local slope and the off-nadir angle. The authors discuss the exploitation of this spectral shift for different applications: 1) generation of "low noise" interferograms benefiting phase unwrapping, 2) generation of quick-look interferograms, 3) decorrelation reduction by means of tunable SAR systems (TINSAR), 4) range resolution enhancement, and 5) the combination of SAR data gathered by different platforms (airborne and satellite) for a "long-time coherence" study. >

Ant system for Job-shop scheduling

Abstract: The study of natural processes has inspired several heuristic optimization algorithms which have proved to be very effective in combinatorial optimization. In this paper we show how a new heuristic called ant system, in which the search task is distributed over manysimple, loosely interacting agents, can be successfully applied to find good solutions of job-shop scheduling problems.

Genetic evolution of the topology and weight distribution of neural networks

Abstract: This paper proposes a system based on a parallel genetic algorithm with enhanced encoding and operational abilities. The system, used to evolve feedforward artificial neural networks, has been applied to two widely different problem areas: Boolean function learning and robot control. It is shown that the good results obtained in both cases are due to two factors: first, the enhanced exploration abilities provided by the search-space reducing evolution of both coding granularity and network topology, and, second, the enhanced exploitational abilities due to a recently proposed cooperative local optimizing genetic operator. >

Capture and retransmission control in mobile radio

Abstract: We consider the slotted ALOHA protocol with capture, in a mobile communication environment, in the presence of Rayleigh fading and log-normal shadowing. The capture probabilities and their asymptotic value, as the number of colliding packets tends to infinity, are computed. A sufficient condition, under which this limit is positive, is identified and expressed in terms of the spatial distribution of the users. Also, various models for this latter distribution are discussed. These results help identify the region in which the system may achieve a positive stable throughput. Finally, a retransmission control scheme, that enhances the throughput, is considered, and the stability of the system is rigorously proven. We also show that the use of retransmission control is unavoidable if the network performance predicted in some recent literature is to be achieved. >

Genetic-algorithm programming environments

Abstract: This review classifies genetic-algorithm environments into application-oriented systems, algorithm-oriented systems, and toolkits. It also presents detailed case studies of leading environments. Following Holland's (1975) original genetic algorithm proposal, many variations of the basic algorithm have been introduced. However. an important and distinctive feature of all GAs is the population-handling technique. The original GA adopted a generational replacement policy, according to which the whole population is replaced in each generation. Conversely, the steady-state policy used by many subsequent GAs selectively replaces the population. After we introduce GA models and their programming, we present a survey of GA programming environments. We have grouped them into three major classes according to their objectives: application-oriented systems hide the details of GAs and help users develop applications for specific domains; algorithm-oriented systems are based on specific GA models; and toolkits are flexible environments for programming a range of GAs and applications. We review the available environments and describe their common features and requirements. As case studies, we select some specific systems for more detailed examination. To conclude, we discuss likely future developments in GA programming environments. >

Model for the assessment of heart period and arterial pressure variability interactions and of respiration influences.

Abstract: A model which assesses the closed-loop interaction between heart period (HP) and arterial pressure (AP) variabilities and the influence of respiration on both is applied to evaluate the sources of low frequency (LF approximately 0.1 Hz) and high frequency (HF, respiratory rate approximately 0.25 Hz) in conscious dogs (n = 18) and humans (n = 5). A resonance of AP closed-loop regulation is found to amplify LF oscillations. In dogs, the resonance gain increases slightly during baroreceptor unloading (mild hypotension obtained with nitroglycerine (NTG) i.v. infusion, n = 8) and coronary artery occlusion ((CAO), n = 6), and it is abolished by ganglionic transmission blockade ((ARF), Arfonad i.v. infusion, n = 3). In humans, this gain is considerably increased by passive tilt. Different, possibly central, sources of LF oscillations are also evaluated, finding a strong rhythmic modulation of HP during CAO. At HF, a direct respiratory arrhythmia is dominant in dogs at control, while it is considerably reduced during CAO. On the contrary, in humans, a strong influence of respiration on AP is shown which induces a reflex respiratory arrhythmia. An index of the gain of baroreceptive response, alpha cl, was decreased by NTG and CAO, and virtually abolished by chronic arterial baroreceptive denervation (TABD, n = 4) and ARF.

Cytotoxicity of some catalysts commonly used in the synthesis of copolymers for biomedical use

Abstract: The cytotoxicity of four catalysts commonly used for the synthesis of copolymers for biomedical use, such as segmented polyurethanes, was evaluated towards two types of cells, the first being the well-characterized cell line Swiss 3T3 mouse fibroblasts, the second the actual living system that faces any device in contact with blood, i.e. human endothelial cells (HEC). The catalysts were two tertiary aliphatic amines: TMBDA (tetramethylbutanediamine), and DABCO (1–4 diazo (2,2,2) octane); two alkyl tin compounds: DBTDL (dibutyl-tin-dilaurate), and SnOct (stannous octoate). Cytotoxicity tests were carried out by adding to the culture medium, after cell adhesion, different concentrations of each catalyst in dimethylsulphoxide, and keeping them in contact with the monolayer for 72 h. All the catalysts proved to be cytotoxic, although at different extent (in the order: DABCO < TMBDA < SnOct < DBTDL); their dose inhibiting 50% of cell growth (IC50) came out to be lower for 3T3 fibroblasts than for HEC, with the exception of DBTDL, which showed a similar toxicity for both the cell lines. As an example, the cytotoxicity of a polyurethane-amide, laboratory synthesized with DBTDL as catalysts, was checked with fibroblasts. By using both the method of the extract, and that of the direct contact (through a microporous membrane), a moderate to severe cell growth inhibition, related to the Sn content in the material, was observed.

Confinement potential and pi -electron delocalization in polyconjugated organic materials.

Abstract: Frequency dispersions with chain length have been experimentally determined from Raman-scattering data published previously on a series of oligomers and polymers of paraphenylene, paraphenylene vinylene, thiophene, N-protected pyrrole, pyrrole, and furan. The dispersion behavior changes noticeably in the different series of compounds. Conformational flexibility and the confinement of \ensuremath{\pi} electrons within each aromatic ring are the two factors considered for the explanation of such an observation. The \ensuremath{\pi}-electron confinement is analyzed in terms of the effective conjugation coordinate theory which is related to the amplitude mode theory. Ab initio calculations performed on model compounds are used to support the experimental evidence of the competition between \ensuremath{\pi}-electron confinement within the rings and delocalization along the chain.

Bifurcation analysis of periodic SEIR and SIR epidemic models

Abstract: The bifurcations of the periodic solutions of SEIR and SIR epidemic models with sinusoidally varying contact rate are investigated. The analysis is carried out with respect to two parameters: the mean value and the degree of seasonality of the contact rate. The corresponding portraits in the two-parameter space are obtained by means of a numerical continuation method. Codimension two bifurcations (degenerate flips and cusps) are detected, and multiple stable modes of behavior are identified in various regions of the parameter space. Finally, it is shown how the parametric portrait of the SEIR model tends to that of the SIR model when the latent period tends to zero.

C-PRMA: The centralized packet reservation multiple access for local wireless communications

Abstract: Packet-switched technology has been demonstrated as effective in cellular radio systems with short propagation delay, not only for data, but also for voice transmission. In fact, packet voice can efficiently exploit speech on-off activity to improve bandwidth utilization over time division multiple access (TDMA). Such an approach has been first suggested in the packet reservation multiple-access (PRMA) technique, an adaptation of the reservation ALOHA protocol to the cellular environment. However, being PRMA-based on a fixed frame scheme, it cannot thoroughly take advantage of the very short propagation delays encountered in microcellular systems that allow, for instance, the immediate retransmission of packets lost because of the interference noise from adjacent cells. We present the centralized PRMA, a natural enhancement of PRMA, in which the base station (BS) plays a central role in scheduling the transmissions of mobile stations (MSs). As a consequence, the transmission scheduling is very flexible and can account for the different traffic rate and delay constraints that emerge from voice and data integration. A packet retransmission policy to recover corrupted packets can be implemented and operated efficiently to provide an acceptable grade of service, even in a very noisy environment. The simulation results presented show the quantitative improvements of the centralized packet reservation multiple-access (C-PRMA) performance with respect to PRMA.

Automatic generation of production rules for integrity maintenance

Abstract: In this article we present an approach to integrity maintenance, consisting of automatically generating production rules for integrity enforcement. Constraints are expressed as particular formulas of Domain Relational Calculus; they are automatically translated into a set of repair actions, encoded as production rules of an active database system. Production rules may be redundant (they enforce the same constraint in different ways) and conflicting (because repairing one constraint may cause the violation of another constraint). Thus, it is necessary to develop techniques for analyzing the properties of the set of active rules and for ensuring that any computation of production rules after any incorrect transaction terminates and produces a consistent database state.Along these guidelines, we describe a specific architecture for constraint definition and enforcement. The components of the architecture include a Rule Generator, for producing all possible repair actions, and a Rule Analyzer and Selector, for producing a collection of production rules such that their execution after an incorrect transaction always terminates in a consistent state (possibly by rolling back the transaction); moreover, the needs of applications are modeled, so that integrity-enforcing rules reach the final state that better represents the original intentions of the transaction's supplier. Specific input from the designer can also drive the process and integrate or modify the rules generated automatically by the method. Experimental results of a prototype implementation of the proposed architecture are also described.

Three-dimensional optical analysis of chest wall motion

Abstract: A method for kinematic analysis of chest wall motion is presented, based on a television-image processor that allows a three-dimensional assessment of volume change of the trunk by automatically computing the coordinates of several passive markers placed on relevant landmarks of the thorax and abdomen. The parallel computation used for the image processing allows for a real time recognition of the passive markers with the necessary accuracy. A geometric model also allows the online computation of the contribution to the chest volume by the different parts. For this purpose, the model presented here is based on 54 tetrahedrons that can be grouped into 9 compartments and 3 sections representing 1) upper thorax (mainly reflecting the action of neck and parasternal muscles and the effect of pleural pressure), 2) lower thorax (mainly reflecting the action of diaphragm and the effect of pleural and abdominal pressure), and 3) abdomen (mainly reflecting the actions of diaphragm and abdominal muscles). By this model, the volume can also be split into three vertical sections pointing out asymmetries between the right and left sides. The method is noninvasive, nonionizing, and leaves the subject maximum freedom of movement during the test, thus being suitable for routine clinical analysis. The monitoring of the subject can be prolonged in time and can be performed in different postures: standing, sitting, and supine. The method was tested on 12 healthy subjects showing its good accuracy, reliability, and reproducibility.

A fuzzy-set approach to fault-type identification in digital relaying

Abstract: Sometime the operations that a relay is required to perform cannot be easily described in a deterministic way. A significant example of this situation is given by the operations a line relay must perform in order to detect the type of fault (line-to-ground, line-to-line, line-to-line involving ground). Recently, the mathematical theory of fuzzy sets featured many practical applications, mainly in industrial controls, and fuzzy-set processors are available to allow real time applications. This paper shows that a fuzzy-set approach can be useful even in digital relaying, whenever "fuzzy" decisions have to be under-taken. A possible application to the detection of the type of fault when symmetrical component relaying techniques are adopted is proposed and the results of simulation tests are given. >

Weighted k-cardinality trees: complexity and polyhedral structure

Abstract: We consider the k-CARD TREE problem, i.e., the problem of finding in a given undirected graph G a subtree with k edges, having minimum weight. Applications of this problem arise in oil-field leasing and facility layout. Although the general problem is shown to be strongly NP hard, it can be solved in polynomial time if G is itself a tree. We give an integer programming formulation of k-CARD TREE and an efficient exact separation routine for a set of generalized subtour elimination constraints. The polyhedral structure of the convex hull of the integer solutions is studied. © 1994 by John Wiley & Sons, Inc.

Outage probability in multiple access packet radio networks in the presence of fading

Abstract: The outage probability in a mobile communications environment, where the interferers are randomly distributed on the ground, taking into account the background noise, the cochannel interference and the random effects of propagation (log-normal shadowing and Rayleigh fading), is evaluated. Although not feasible in closed form, the computation can be performed numerically in a very fast and accurate manner. A comparison to the results reported in the recent literature is presented, along with a sensitivity analysis and an example of application. >

Proving properties of real-time systems through logical specifications and Petri net models

Abstract: Addresses the problem of formally analyzing the properties of real-time systems. We propose a method based on modeling the system as a timed Petri net and on specifying its properties in TRIO, an extension of temporal logic suitable for dealing explicitly with time and for measuring it. Timed Petri nets are axiomatized in terms of TRIO, so that their properties can be derived as theorems in the same spirit as the classical Hoare method allows one to prove properties of programs coded in a Pascal-like language. The method is also illustrated through an example. >

Robust design of countercurrent adsorption separation processes: 2. Multicomponent systems

Abstract: The adsorption separation of a multicomponent mixture in a four-section countercurrent separation unit is analyzed. A procedure for the optimal and robust design of the unit is developed in the frame of equilibrium theory, where adsorption equilibria are described through the constant selectivity stoichiometric model, while mass-transfer resistances and axial dispersion are neglected. A set of conditions obtained defines a region in the operating parameters space where the unit achieves complete separation. In the case of binary separations, with a desorbent having any adsorptivity with respect to the components to be separated, the boundaries of this region are obtained explicitly. In the general multicomponent case, a numerical procedure is needed. An approximate shortcut method devised allows to obtain explicit and reliable relationships for estimating the boundaries of the exact region. The results provide a very convenient tool not only to analyze the role of the desorbent in determining the separation performance, but to find both optimal and robust operating conditions. Comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical findings.

Single-photon detection beyond 1 µm: performance of commercially available germanium photodiodes.

Abstract: Germanium avalanche photodiodes (APD’s) working biased above the breakdown voltage detect single optical photons in the near-infrared wavelength range. We give guidelines for the selection of devices suitable for photon-counting applications among the commercial samples, and we discuss in detail how the devices should be operated to achieve the best performance, both in terms of noise-equivalent power (NEP) and the timing-equivalent bandwidth. We introduce the driving electronics and we show that, in the measurements of fast optical signals, the adoption of single-photon techniques is very favorable, notwithstanding that presently available photodiodes are not designed for this purpose. On the contrary, in the detection of cw signals, the lower NEP values achieved in photon counting may not be sufficient to justify the replacement of conventional analog p-i-n germanium detectors, which offer comparable performance with a definitely larger sensitive area. Finally, we show that, by properly choosing the operating conditions, some selected APD’s achieve an 85-ps time resolution in the detection of optical photons at a 1.3-μm wavelength, which corresponds to a timing-equivalent bandwidth of 1.8 GHz. To the best of our knowledge, this time resolution is the lowest reported to date for single-photon detectors in the near infrared.

Towards an infrastructure for temporal databases: report of an invitational ARPA/NSF workshop

Abstract: the discussion. Specifically, an initial glossary of temporal database concepts and a. test suite of temporal queries were distributed before the workshop. Both of these document*s were amended based on the analysis and critique of the workshop. A language design committee was constituted after the workshop to develop a consensus temporal query la,nguage extension to SQL-92; this design also benefited from the discussion at the workshop. This report documents the discussions and consensus reached at the workshop. The report. reflects the conclusions rea.ched at the workshop in June, 1993 and further discussions amongst the group participants through electronic mail. In preparing this report, each group coordinator assembled ideas and prepared an initial draft, which was then reviewed by a.11 the workshop participants. The record of the deliberations of these four groups, in the following four sections, forms t.he bulk of this report. Each of these sections begins with the group’s charter and a brief snapshot of the status of the field and ends with a list of follow-on efforts. The last, section identifies the workshop pa.rticipants. The full report’ provides more discussion and many additional references t,o the literature.

Defining and validating high-level design metrics

Abstract: The availability of significant metrics in the early phases of the software development process allows for a better management of the later phases, and a more effective quality assessment when software quality can still be easily affected by preventive or corrective actions. In this paper, we introduce and compare four strategies for defining high-level design metrics. They are based on different sets of assumptions (about the design process) related to a well defined experimental goal they help reach: identify error-prone software parts. In particular, we define ratio-scale metrics for cohesion and coupling that show interesting properties. An in-depth experimental validation, conducted on large scale projects demonstrates the usefulness of the metrics we define.

Phase composition and mechanism of formation of Ba-β-alumina-type systems for catalytic combustion prepared by precipitation

Abstract: A novel preparation method is proposed for Ba-β-Al203-type systems to be used in high-temperature catalytic combustion, consisting of precipitation in aqueous medium. Differential thermal analysis-thermogravimetry, X-ray diffraction, Fourier transform-infrared, TEM and surface area data are presented for Ba-Al-O samples with Al/Ba atomic ratios in the range 14-9 calcined at different temperatures up to 1670 K. The final materials consist of a barium-poor Ba-β1Al2O3 phase together with 1% α-Al2O3 in the case of Al/Ba = 14 and of a barium-rich Ba-β11-Al2O3 phase together with 2% BaAl2O4 in the case of Al/Ba = 9. A mono-phasic sample with a Ba-β-Al2O3 structure is obtained in the case of Al/Ba = 12; this phase is constituted by the simultaneous presence of both β1 and β11 structure types. Two routes operate above 1370 K in the formation of the Ba-β-Al2O3 phases involving solid-state reactions between γ-Al2O3 and BaAl2O4, and γ-Al2O3 and dispersed barium compounds. Based on the analogies between the structures of γ-Al2O3 and of the Ba-β-Al2O3 phases it is suggested that the formation of the latter occurs via diffusion of barium ions within oxygen close-packed planes of the γ-Al2O3-type spinel structure.