Ching-eh Li

TIME DOMAIN INVERSE SCATTERING OF A TWO-DIMENSIONAL HOMOGENOUS DIELECTRIC OBJECT WITH ARBITRARY SHAPE BY PARTICLE SWARM OPTIMIZATION

This paper presents a computational approach to the two- dimensional time domain inverse scattering problem of a dielectric cylinder based on the finite difference time domain (FDTD) method andthe particle swarm optimization (PSO) to d etermine the shape, location and permittivity of a dielectric cylinder. A pulse is incident upon a homogeneous dielectric cylinder with unknown shape and d ielectric constant in free space andthe scatteredfieldis record ed outside. By using the scattered field, the shape and permittivity of the dielectric cylinder are reconstructed. The subgridding technique is implemented in the FDTD code for modeling the shape of the cylinder more closely. In order to describe an unknown cylinder with arbitrary shape more effectively, the shape function is expandedby closedcubic- spline function insteadof frequently usedtrigonometric series. The inverse problem is resolvedby an optimization approach, andthe global searching scheme PSO is then employedto search the parameter space. Numerical results demonstrate that, even when the initial guess is far away from the exact one, goodreconstruction can be obtained . In addition, the effects of Gaussian noise on the reconstruction results are investigated. Numerical results show that even the measured scattered E fields are contaminated with some Gaussian noise, PSO can still yield goodreconstructedquality.

Time-Domain Inverse Scattering of a Two-Dimensional Metallic Cylinder in Slab Medium Using Asynchronous Particle Swarm Optimization

This paper presents asynchronous particle swarm opti- mization (APSO) applied to the time-domain inverse scattering prob- lems of two-dimensional metallic cylinder buried in slab medium. For this study the flnite-difierence time-domain (FDTD) is employed for the analysis of the forward scattering part, while for the APSO is ap- plied for the reconstruction of the two-dimensional metallic cylinder buried in slab medium, which includes of the location and shape the metallic cylinder. For the forward scattering, conceptually several elec- tromagnetic pulses are launched to illuminate the unknown scatterers, and then the scattered electromagnetic flelds around are measured. In order to e-ciently describe the details of the cylinder shape, sub- gridding technique is implemented in the flnite difierence time domain method. Then, the simulated EM flelds are used for inverse scattering, in which APSO is employed to transform the inverse scattering prob- lem into optimization problem. By comparing the simulated scattered flelds and the calculated scattered flelds, the shape and location of the metallic cylinder are reconstructed. In addition, the efiects of Gaussian noises on imaging reconstruction are also investigated.

Electromagnetic Transverse Electric Wave Inverse Scattering of a Partially Immersed Conductor by Steady-State Genetic Algorithm

Abstract In this article, we present a computational approach to the imaging of a partially immersed perfectly conducting cylinder by the steady-state genetic algorithm (GA). A conducting cylindrical section of unknown shape scatters the incident transverse electric wave in free space, while the scattered field is recorded outside. Based on the boundary condition and the measured scattered field, a set of nonlinear integral equations is derived, and the imaging problem is reformulated into an optimization problem. An improved steady-state GA is employed to search for the global extreme solution. Numerical results demonstrate that, even when the initial guess is far away from the exact one, good reconstruction can be obtained.

Time Domain Image Reconstruction for Homogenous Dielectric Objects by Dynamic Differential Evolution

Abstract This article presents the studies of the microwave image reconstruction of two-dimensional homogeneous dielectric cylinders that are based on the finite difference time domain method and dynamic differential evolution. For forward scattering, the finite difference time domain method is employed to calculate the scattered E-fields; for inverse scattering, the dynamic differential evolution scheme is utilized to determine the shape, location, and permittivity of the cylindrical scatterers with an arbitrary cross-section. The subgirdding technique is implemented for the finite difference time domain code in order to more smoothly model the shape of the cylinder. In addition, in order to more effectively describe an unknown cylinder with an arbitrary cross-section during the course of searching, the closed cubic-spline expansion is adopted to represent the scatterer contour instead of the frequently used trigonometric series. Numerical results demonstrate that, even when the initial guess is far from exact, good reconstruction can be obtained. In addition, the effects of Gaussian noise on the reconstruction results are investigated. Numerical results show that even the measured scattered fields are contaminated with Gaussian noise, and dynamic differential evolution is able to yield good reconstructed quality.

Comparative Study of Some Population-Based Optimization Algorithms on Inverse Scattering of a Two-Dimensional Perfectly Conducting Cylinder in Dielectric Slab Medium

The application of four techniques for the shape reconstruction of a 2-D metallic cylinder buried in dielectric slab medium by measured the scattered fields outside is studied in the paper. The finite-difference time-domain (FDTD) technique is employed for electromagnetic analyses for both the forward and inverse scattering problems, while the shape reconstruction problem is transformed into optimization one during the course of inverse scattering. Then, four techniques including asynchronous particle swarm optimization (APSO), PSO, dynamic differential evolution (DDE) and self-adaptive DDE (SADDE) are applied to reconstruct the location and shape of the 2-D metallic cylinder for comparative purposes. The statistical performances of these algorithms are compared. The results show that SADDE outperforms PSO, APSO and DDE in terms of the ability of exploring the optima. However, these results are considered to be indicative and do not generally apply to all optimization problems in electromagnetics.

Time Domain Image Reconstruction for a Buried 2D Homogeneous Dielectric Cylinder Using NU-SSGA

This article presents an image reconstruction approach for a buried homogeneous cylinder with arbitrary cross-section in half space. The computational method combines the finite difference time domain (FDTD) method and non-uniform steady state genetic algorithm (NU-SSGA) to determine the shape and location of the subsurface scatterer with arbitrary shape. The FDTD-subgirdding technique is implemented for modeling the shape of the cylinder more closely. The inverse problem is reformulated into an optimization problem and the global searching scheme NU-SSGA with closed cubic-spline is then employed to search the parameter space. A set of representative numerical results is presented for demonstrating that the proposed approach is able to efficiently reconstruct the electromagnetic properties of homogeneous dielectric scatterer even when the initial guess is far from the exact one. In addition, the effects of Gaussian noises on imaging reconstruction are also investigated.

Application of finite-difference time domain and dynamic differential evolution for inverse scattering of a two-dimensional perfectly conducting cylinder in slab medium

We apply the dynamic differential evolution (DDE) algorithm to solve the inverse scattering problem for which a two-dimensional perfectly conducting cylinder with unknown cross section is buried in a dielectric slab medium. The finite-difference time domain method is used to solve the scattering electromagnetic wave of a perfectly conducting cylinder. The inverse problem is resolved by an optimization approach, and the global searching scheme DDE is then employed to search the parameter space. By properly processing the scattered field, some electromagnetic properties can be reconstructed. One is the location of the conducting cylinder, the others is the shape of the perfectly conducting cylinder. This method is tested by several numerical examples, and it is found that the performance of the DDE is robust for reconstructing the perfectly conducting cylinder. Numerical simulations show that even when the measured scattered fields are contaminated with Gaussian noise, the quality of the reconstructed results obtained by the DDE algorithm is very good.

Image Reconstruction of the Buried Metallic Cylinder Using FDTD Method and SSGA

This paper presents an image reconstruction approach based on the time-domain and steady state genetic algorithm (SSGA) for a 2-D perfectly conducting cylinder buried in a half-space. The computational method combines the finite difference time domain (FDTD) methodandthe stead y state genetic algorithms (SSGA) to determine the shape and location of the subsurface scatterer with arbitrary cross section. The subgirdding technique is implemented in the FDTD code for modeling the shape of the cylinder more closely. In order to describe a a unknown 2-D cylinder with arbitrary cross section more effectively, the shape function is expanded by closed cubic-spline function insteadof frequently usedtrigonometric series. The inverse problem is reformulatedinto an optimization problem and the global searching scheme SSGA with closedcubic-spline is then employedto search the parameter space. Numerical results show that the shadowing effect for the inverse problem in a half space results in poor image reconstruction on the backside of the cylinder. We propose the two-step strategy to overcome the shadowing effect. It is found that goodimaging quality couldbe attainedbasedon the proposedstrategy.

Time Domain Inverse Scattering for a Homogenous Dielectric Cylinder by Asynchronous Particle Swarm Optimization

In this paper, we propose a time domain inverse scattering technique for reconstructing the electromagnetic properties of a homogeneous dielectric cylinder based on the finite difference time domain method and the asynchronous particle swarm optimization (APSO). The homogeneous dielectric cylinder with unknown electromagnetic properties is illuminated by transverse magnetic pulse and the scattered field is recorded outside. By minimizing the discrepancy between the measured and estimated scattered field data, the location, shape, and permittivity of the dielectric cylinder are reconstructed. The inverse problem is resolved by an optimization approach and the global searching scheme APSO is then employed to search the parameter space. A set of representative numerical results is presented for demonstrating that the proposed approach is able to efficiently reconstruct the electromagnetic properties of the homogeneous dielectric scatterer even when the initial guess is far away from the exact one. In addition, the effects of Gaussian noises on imaging reconstruction are also investigated.

Image reconstruction for 2D homogeneous dielectric cylinder using FDTD method and SSGA

This paper presents an image reconstruction approach for a 2-D homogeneous cylinder with arbitrary cross section in free space. The computational method combines the finite difference time domain (FDTD) method and non-uniform steady state genetic algorithm (NU-SSGA) to determine the shape and location of the scatterer with arbitrary shape. The subgirdding technique is implemented for modeling the shape of the cylinder more closely. The inverse problem is reformulated into an optimization problem and the global searching scheme NU-SSGA with closed cubic-spline is then employed to search the parameter space. A set of representative numerical results is presented for demonstrating that the proposed approach is able to efficiently reconstruct the electromagnetic properties of homogeneous dielectric scatterer even when the initial guess is far away from the exact one. In addition, the effects of Gaussian noises on imaging reconstruction are also investigated.