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vol. 39 num. 3 lang. es<![CDATA[SciELO Logo]]>http://www.scielo.org.ar/img/en/fbpelogp.gif
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<![CDATA[Guest editorial]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300001&lng=es&nrm=iso&tlng=es
<![CDATA[Security and BER performance trade-off in wireless communication systems applications]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300002&lng=es&nrm=iso&tlng=es
There is nowadays a strong need of designing communications systems with excellent BER performance and high levels of privacy, specially in wireless networking and mobile communications. The transmission of encrypted information over a noisy channel presents an error propagation effect, which degrades the BER performance of the system. In this paper, we present combined error-control coding and encryption schemes based on iteratively decoded error-control codes like LDPC and turbo codes and AES algorithm. We show that the proposed schemes strongly reduce this degradation effect. The increase of the level of privacy is obtained by using procedures of pseudo random nature over the encoders and decoders of the error-control code. Thus, the proposed schemes provide a given communication system with excellent BER performance and encryption capabilities.<![CDATA[Sensorless speed controller for induction motors]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300003&lng=es&nrm=iso&tlng=es
This paper presents a novel sensorless strategy for controlling speed in AC drives containing induction motors. The controller uses field oriented control strategy calculated with estimated rotor flux and speed. These estimates are obtained via a robust estimator.<![CDATA[Non-punctured non-systematic ½-rate turbo codes]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300004&lng=es&nrm=iso&tlng=es
This paper presents non-punctured non-systematic ½-rate turbo codes designed using Finite State Sequential Machines (FSSMs) that operate over the Finite Field GF(4). It is shown that these schemes do not show convergence problems in their iterative decoding, as it happens to similar schemes defined over the binary Field. Non-systematic turbo codes can be useful for the design of communication systems for which error correction and encryption are two important goals. A relationship is found between the state transitions structure of the FSSM and the BER performance (or equivalently the form of the EXIT chart) of the designed turbo code. FSSMs defined over GF(4) with all-zero input responses of the form of closed cycles are the best options as constituent encoders of non-punctured non-systematic ½-rate turbo codes. A close relationship is also found between the BER performance and the coefficients that define the structure of those FSSMs.<![CDATA[Partial differential equations for missing boundary conditions in the linear-quadratic optimal control problem]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300005&lng=es&nrm=iso&tlng=es
New equations involving the unknown final states and initial costates corresponding to families of LQR problems are found, and their solutions are computed and validated. Having the initial values of the costates, the optimal control can then be constructed, for each particular problem, from the solution to the Hamiltonian equations, now achievable through on-line integration. The missing boundary conditions are obtained by solving (off-line) two uncoupled, first-order, quasi-linear, partial differential equations for two auxiliary n × n matrices, whose independent variables are the time-horizon duration T and the final-penalty matrix S. The solutions to these PDEs give information on the behavior of the whole two-parameter family of control problems, which can be used for design purposes. The mathematical treatment takes advantage of the symplectic structure of the Hamiltonian formalism, which allows to reformulate one of Bellman's conjectures related to the "invariant-imbedding" methodology. Results are tested against solutions of the differential Riccati equations associated with these problems, and the attributes of the two approaches are illustrated and discussed.<![CDATA[Optical pulse compression by photonic devices]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300006&lng=es&nrm=iso&tlng=es
By combining phase modulation and dispersive transmission on a given time-varying in-put signal, we propose an optical device for produc-ing ultrashort light pulses of high optical power. We derive the relationship between the required proper-ties of the input signal and the device parameters in order to optimize the energy concentration in the output optical pulses. The high-order aberrations effects of the time lens which perform the phase modulation are considered. Besides, the differences on the obtained pulse shape when the input signal has a gaussian or a super-gaussian envelope are illus-trated. Finally, some numerical simulations are shown which illustrate the feasibility of the method.<![CDATA[Block-based transceivers for frequency selective channels with reduced redundancy]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300007&lng=es&nrm=iso&tlng=es
We propose a transceiver structure for a frequency selective channel that allows the introduction of reduced redundancy. We optimize jointly the transmitter and receiver in this structure to maximize the information rate. The simulation results show that the proposed design has good performance while increasing the bandwidth efficiency.<![CDATA[Automatic marker determination algorithm for watershed segmentation using clustering]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300008&lng=es&nrm=iso&tlng=es
Biomedical image processing is a difficult task because of the presence of noise, textured regions, low contrast and high spatial resolution. The objects to be segmented show a great variability in shape, size and intensity whose inaccurate segmentation conditions the ulterior quantification and parameter measurement. The partition of an image in regions that allow the experienced observant to obtain the necessary information can be done using a Mathematical Morphology tool called the Watershed Transform (WT). This transform is able to distinguish extremely complex objects and is easily adaptable to various kinds of images. The success of the WT depends essentially on the existence of unequivocal markers for each of the objects of interest. The standard methods of marker detection are highly specific, they have a high computational cost and they determine markers in an effective but not automatic way when processing highly textured images. This paper proposes the use of clustering techniques for the automatic detection of markers that allows the application of the WT to biomedical images. The results allow us to conclude that the method proposed is an effective tool for the application of the WT.<![CDATA[Relative error control in quantization based integration]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300009&lng=es&nrm=iso&tlng=es
This paper introduces a method to achieve relative error control in Quantized State System (QSS) methods. Based on the use of logarithmic quantization, the proposed methodology solves the problem of quantum selection.<![CDATA[Input-to-output stability properties of switched perturbed nonlinear control systems]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300010&lng=es&nrm=iso&tlng=es
In this paper we obtain the extension of results on input-to-output stability properties of switched systems to switched systems whose dynamics are described by perturbed forced differential equations and whose outputs are obtained via switched perturbed functions. We also present Lyapunov characterizations of these input-output stability properties, obtained in terms of certain conceptual output functions.<![CDATA[Linearly implicit discrete event methods for stiff ODE's.]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300011&lng=es&nrm=iso&tlng=es
This paper introduces two new numerical methods for integration of stiff ordinary differential equations. Following the idea of quantization based integration, i.e., replacing the time discretization by state quantization, the new methods perform first and second order backward approximations allowing to simulate stiff systems. It is shown that the new algorithms satisfy the same theoretical properties of previous quantization-based integration methods. The translation of the new algorithms into a discrete event (DEVS) specification and its implementation in a DEVS simulation tool is discussed. The efficience of the methods is illustrated comparing the simulation of two examples with the classic methods implemented by Matlab/Simulink.<![CDATA[Analysis of ventricular late potentials in high resolution ecg records by time-frequency representations]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300012&lng=es&nrm=iso&tlng=es
This study proposes the application of different linear and quadratic Time-Frequency Representations (TFR) for the analysis of Ventricular Late Potentials (VLP) in High-Resolution Electrocardiographic (HRECG) records. The TFR used here were Short-Time Fourier Transform (STFT), Wigner-Ville (WV) and Choi-Williams (CW) distributions. These techniques were used to evaluate the risk of Ventricular Tachycardia (VT) in post-infarction patients. For comparison, the same analysis was made on HRECG records of subjects presenting Low Arrhythmia Risk (LAR). For all TFR and both populations, two indexes on the Time-Frequency (T-F) plane were considered: a) Energy of VLP normalized with respect to the total energy of the beat (EN TOTAL) and b) Energy of VLP normalized with respect to the energy of QRS complex (EN QRS). The results indicate that the two T-F indexes of the LAR group show higher mean values than those of the VT group, for all TFR used. In addition, the EN QRS index for the WV distribution obtained the best valuation as a diagnostic test, showing the highest sensitivity and specificity values. It can be concluded that T-F analysis of HRECG is a promissory technique of diagnosis to identify post-infarct patients with high risk of VT.<![CDATA[Estimation of the particle size distribution of a dilute latex from combined elastic and dynamic light scattering measurements: A method based on neural networks]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300013&lng=es&nrm=iso&tlng=es
A method for estimating the particle size distribution (PSD) of a dilute latex from light scattering measurements is presented. The method utilizes a general regression neural network (GRNN), that estimates the PSD from 2 independent sets of measurements carried out at several angles: (i) light intensity measurements, by elastic light scattering (ELS); and (ii) average diameters measurements, by dynamic light scattering. The GRNN was trained with measurements simulated on the basis of typical asymmetric PSDs (unimodal normal-logarithmic distributions of variable mean diameters and variances). First, the ability of the method was tested on the basis of two synthetic examples. Then, the obtained GRNN was used for estimating the PSD of a narrow polystyrene (PS) latex standard of nominal diameter 111 nm. The standard was also characterized by 2 independent techniques: capillary hydrodynamic fractionation, and transmission electron microscopy (TEM). The PSD predicted by the GRNN resulted close to that obtained by TEM. The estimated PSDs were better than those obtained through standard numerical techniques for 'ill-conditioned' inverse problems.<![CDATA[Control of a production-inventory system using a pid and demand prediction based controller]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932009000300014&lng=es&nrm=iso&tlng=es
The need of reducing inventory levels as much as possible without loosing sales opportunities is an important goal not only for small but also for mid-size and large companies, on account of the high costs associated with large inventory stocks. In general, the performance of inventory systems is also affected by the Bullwhip effect caused, among other factors, by non-zero lead times. This paper proposes an automatic pipeline feedback order-based production control system (APIOBPCS) considering a demand with cyclic and stochastic components. The dynamics and delays of the production process are modeled as a pure delay. The control system structure consists of a PID controller and demand prediction based on an Extended Kalman Filter (EKF). The main objective of the controller is to stabilize and regulate the inventory levels about a desired set-point. The extended Kalman Filter estimates the parameters of a Volterra time-series model to predict future values of the demand. The control system is evaluated by simulations, showing a good performance and better results than those achieved by using traditional inventory control techniques.