Ioannis T. Rekanos

Shape Reconstruction of a Perfectly Conducting Scatterer Using Differential Evolution and Particle Swarm Optimization

The shape reconstruction of a perfectly conducting 2-D scatterer by inverting transverse magnetic scattered field measurements is investigated. The reconstruction is based on evolutionary algorithms that minimize the discrepancy between measured and estimated scattered field data. A closed cubic B-spline expansion is adopted to represent the scatterer contour. Two algorithms have been examined the differential-evolution (DE) algorithm and the particle swarm optimization (PSO). Numerical results indicate that the DE algorithm outperforms the PSO in terms of reconstruction accuracy and convergence speed. Both techniques have been tested in the case of simulated measurements contaminated by additive white Gaussian noise.

Detection of explosive lung and bowel sounds by means of fractal dimension

An efficient technique for detecting explosive lung sounds (LS) (fine/coarse crackles and squawks) or bowel sounds (BS) in clinical auscultative recordings is presented. The technique is based on a fractal-dimension (FD) analysis of the recorded LS and BS obtained from controls and patients with pulmonary and bowel pathology, respectively. Experimental results demonstrate the efficiency of the proposed method, since it clearly detects the time location and duration of LS and BS, despite their variation either in time duration and/or amplitude. A noise stress test justifies the noise robustness of the FD-based detector, indicating its potential use in everyday clinical practice.

Reconstruction of One-Dimensional Dielectric Scatterers Using Differential Evolution and Particle Swarm Optimization

A comparison between differential evolution (DE) and particle swarm optimization (PSO) in solving 1-D small-scale inverse scattering problems is presented. In this comparison, the efficiency of both aforementioned optimization techniques is examined for permittivity and conductivity profile reconstruction problems. The comparison is carried out under the same conditions of initial population of candidate solutions and number of iterations. Numerical results indicate that both optimization methods are reliable tools for inverse scattering applications even when noisy measurements are considered. In the particular case of small-scale problems investigated in this letter, DE outperforms the PSO in terms of reconstruction accuracy. This is considered an indicative result and not generally applicable.

Inverse scattering using the finite-element method and a nonlinear optimization technique

A new spatial-domain technique for the reconstruction of the complex permittivity profile of unknown scatterers is proposed in this paper. The technique is based on a combination of the finite-element method (FEM) and the Polak-Ribiere nonlinear conjugate gradient optimization algorithm. The direct scattering problem is explicitly dealt with by means of the differential formulation and it is solved by applying the FEM. The inversion methodology is oriented to minimizing a cost function, which consists of a standard error term and regularization term. A sensitivity analysis, which is carried out by an elaborate finite-element procedure, results in the determination of the direction required for correcting the profile. Significant reduction of the computation time is obtained by introducing the adjoint state vector methodology. The efficiency of the presented inversion technique is validated by applying it to the inversion of synthetic scattered far-field measurements, which are corrupted by additive noise.

A finite element based technique for microwave imaging of two-dimensional objects

In this paper, a microwave imaging technique for estimating the spatial distributions of the permittivity and the conductivity of a scatterer, by post-processing electromagnetic scattered field data, is presented. For the description of the direct scattering problem, the differential formulation is applied. This allows the use of the finite element method. During the inversion, the computation of the derivative of the finite element solution with respect to the parameters, which describe the scatterer, is required. This task is performed by a finite element-based sensitivity analysis scheme, which is enhanced by applying the adjoint state vector methodology. The merits of the proposed technique are examined by applying it to both transverse magnetic and transverse electric polarization cases. Finally, the technique is adopted by a frequency-hopping approach to cope with multifrequency inverse scattering problems.

Thinned Planar Array Design Using Boolean PSO With Velocity Mutation

The design of thinned planar microstrip arrays under specific constraints concerning the impedance-matching condition of the array elements and the radiation pattern is presented. The radiation characteristics of the structure are extracted by applying the method-of-moments. The array design is based on a novel optimization method, which is a modified version of the boolean particle swarm optimization that employs velocity mutation (BPSO-vm). Apart from the optimization of the array geometry, the proposed method is applicable to other discrete-variable optimization problems. Moreover, the planar array design is coped with by means of other techniques, namely, a binary coded Genetic Algorithm, the binary Particle Swarm Optimization, and the Boolean PSO. The comparison of the above methods and the BPSO-vm shows the efficiency of the proposed technique.

Microwave imaging in the time domain of buried multiple scatterers by using an FDTD-based optimization technique

In this paper, a microwave imaging technique for reconstructing underground multiple scatterers is presented. The electromagnetic properties of buried objects are estimated by postprocessing total-field measurements, which are obtained when the domain of investigation is illuminated by wide-band electromagnetic waves. The solution of this limited-angle inverse scattering problem is based on the differential formulation of the direct problem. The scatterers are reconstructed by applying an iterative technique, which combines the finite-difference time-domain (FDTD) method and the Polak-Ribiere optimization algorithm. An augmented cost functional is defined taking into account the fulfillment of the Maxwells curl equations by means of Lagrange multipliers. The Frechet derivatives of the functional with respect to the scatterer properties are derived from the stationary condition. Moreover, it is proven that the Lagrange multipliers fulfill the Maxwells curl equations. In numerical results, the presented technique is applied to the reconstruction of scatterers buried in earth. In general, these scatterers can be dielectric, lossy, or magnetic.

Time-domain inverse scattering using Lagrange multipliers: An iterative FDTD-based optimization technique

In this paper, we present an electromagnetic inverse scattering technique in the time domain, which is based on the minimization of an augmented cost functional. This functional is composed of an error term representing the discrepancy between the measured and the estimated values of the electromagnetic field, a regularization term related to the first-order Tikhonovs regularization scheme and, finally, an equality-constraints term associated with the fulfillment of the Maxwells curl equations. These equality constraints are introduced by applying Lagrange multipliers. The minimization of the functional is carried out by applying the Polak-Ribière nonlinear conjugate-gradient algorithm. The required Fr´echet derivatives of the augmented cost functional with respect to the functions that describe the scatterer properties are derived by an analysis based on the calculus of variations. Actually, it is proven that the Lagrange multipliers are waves satisfying the Maxwells curl equations. Consequently, during each iteration of the minimization procedure, we apply the finite-difference time-domain method and the perfectly-matched-layer absorber to calculate both the electromagnetic fields and the Lagrange multipliers. The presented technique is successfully applied to the reconstruction of multiple two-dimensional scatterers, which can be dielectric, lossy, and magnetic.

Two-Dimensional Microwave Imaging Based on Hybrid Scatterer Representation and Differential Evolution

A hybrid method for solving two-dimensional inverse scattering problems is proposed. The method utilizes differential evolution as a global optimizer and is based on two alternative representations of the unknown scatterer. Initially, the scatterer properties are represented by means of truncated cosine Fourier series expansion that involves limited number of unknown expansion coefficients. Then, the reconstructed profile obtained is used as an initial estimate and the differential evolution is further applied to a scatterer representation based on pulse function expansion. In this representation, the scatterer region is subdivided by a fine grid and the scatterer properties are considered constant within each cell. When the truncated cosine Fourier expansion representation is adopted, the dimension of the solution space can be reduced and the instabilities caused by the ill-posedness of the problem are suppressed. In the second step of the hybrid method, where the pulse functions representation is considered, the scatterer reconstruction is finer and more accurate due to its quite accurate initial estimate. Numerical results show that the hybrid method results in lower reconstruction error compared to above-mentioned representations. Also, the hybrid method outperforms the other two representations, even in the presence of noisy field measurements.

Influence of diffraction coefficient and corner shape on ray prediction of power and delay spread in urban microcells

For a low base-station (BS) antenna located on one street, signals propagate into crossing and parallel streets by reflection and diffraction at corners of buildings. Therefore, in order to accurately predict the received signals, it is necessary to properly model the diffraction coefficient at the building edge and to accurately represent the shape and the electrical properties of the building near the corner. This paper compares ray-tracing predictions to measurements of received power and root mean square (rms) delay spread and shows the need for a diffraction coefficient having larger values than suggested by the commonly used heuristic diffraction coefficient. A new heuristic diffraction coefficient is proposed that has higher diffracted field strength in the deep shadow region and in the region between the two shadow boundaries. The proposed diffraction coefficient shows better agreement with measurements of both received power and delay spread compared to the commonly used heuristic diffraction coefficient. The influence of building shape near the corner and its electrical properties on the ray-tracing predictions are also presented. The shape is shown to have an important role in accurately predicting both received power and delay spread.