分层煤质中探测浅二维目标的新方法

本文提出利用时域散射信息对埋于分层媒质中的无耗二维电磁目标进行微波成像的新方法一时城变分迭代法（TVIM）。这是一种基于电磁体等效原理、变分原理和傅立叶变换的迭代方法。我们列举一些典型的数值反演实例,并与有关文献结果作了比较,考察TVIM的收敛性能、反演复杂目标能力、抗随机噪声等反演性能,并从理论上简要分析形成TVIM良好反演性能的原因。     

半无限大空间中无耗二维电磁目标的微波成像新方法

本文提出利用模拟得到的时域散射信息对埋于半无限大空间中的无耗二维电磁目标进行微波成像的新方法－－时域变分迭代法，这是一种基于电磁体等效原理，变分原理和傅立叶变换的迭代方法，每次迭代各计算正问题和逆问题一次，正问题采用时域有限差分方法处理，列举了一些典型的数值反演实例，并与有关文献结果作了比较。考察了ＴＶＩＭ的收敛性能，反复杂目标能力、抗随机噪声等反性能，并从理论上简要分析了形成ＴＶＩＭ良好反演性能     

分层媒质中探溅浅二维目标的新方法

本文提出利用时域散射信息对埋于分层媒质中的无耗二维电磁目标进行微波成像的新方法－时域变分迭代法（ＴＶＩＭ）。这是一种基于电磁体等效原理、变分原理和傅立叶变换的迭代方法。我们列举一些典型的反演实例，并与有关文献结果作了比较，考察ＴＶＩＭ的收敛性能、反演复杂目标能力、抗随机噪声等反演性能，并从理论上简要分析形成ＴＶＩＭ良好反演性能的原因。     

自由空间中二维电磁目标的时域非线性反演

提出一种基于电磁等效原理的一组积分方程，应用变分方法和傅立叶变换理论，得到了时域散射场与目标电磁参数之间非线性关系的频域表示式，即反演自由空间中二维电磁目标的时域非线性迭代方法的反演方程。列举了一些典型的数值反演实例，对反演性能进行了多方面的考察，指出本法是一种具有良好反演性能的时域非线性反演方法。     

分层媒质中浅二维目标的探测

本文探讨利用时域散射场信息探测分层媒质中法二维目标、分层媒质中目标的探测具有明显的应用价值．但相对自由空间而言，这种探测要困难得多。本文给出一种基于体等效原理的探测分层媒质中钱二维目标的方法。这是一种选代方法．从一阶玻恩近似开始迭代．每次迭代过程中正问题和逆问题各计算一欢，正问题采用FDTD方法。利用体等效原理，得到反映时域散射场和目标介电常数分布之间关系的体积分方程，经傅立叶变换并离散化后得到反演方程。为了克服问题的病态．采用了伪求逆技术、文中列举了几个具有代表性的例子，最后讨论了有关的几个问题。     

乳腺癌检测的三维时域微波断层成像方法

应用泛函分析和变分法,改进拉格朗日(Lagrange)乘子算法为一种三维时域微波断层成像方法,用于检测早期乳腺癌。该方法首先以最小二乘准则构造目标函数,将反演问题表示为约束最小化问题;接着应用罚函数法转化为无约束最小化问题;然后基于变分计算导出闭式的拉格朗日函数关于相对介电常数和电导率的Fréchet导数;最后借助梯度算法和时域有限差分(FDTD)法迭代求解。为了对抗噪声污染和逆问题的病态特性,采用了一阶的吉洪诺夫(Tikhonov)正则化方法。利用FDTD和PRP共轭梯度(CG)法,对三维半球乳房模型进行了仿真计算,仿真结果显示了方法的可行性。     

时域反演德鲁色散媒质的电磁逆散射技术

德鲁(Drude)经验模型常用于描述等离子体、金属等媒质的电色散特性.利用宽带的时域测量数据直接反演电参数,相比单频(频域)技术而言,具有信息量大、成像分辨率高的优势.时域直接反演色散媒质电参数的主要困难在于它们是频率相关的.为了克服该困难,本文提出了一种时域电磁(EM)逆散射新技术:转而同时反演德鲁模型的4类频率无关的模型参数.该技术的主要环节为:(1)描述为含正则化项的约束最小化问题;(2)转化为无约束最小化问题;(3)解析导出梯度;(4)分别利用时域有限差分(FDTD)法、共轭梯度(CG)法迭代求解正演、反演子问题.在一维(1-D)、二维(2-D)两个数值算例中,所需的测量数据也由FDTD仿真值代替,并加入了加性高斯白噪声(AWGN).反演结果初步证实了该技术的性能.     

多极德拜色散媒质的时域电磁逆散射改进技术

在已有的经验模型中,多极德拜(Debye)模型最适合高精地描述生物组织、土壤、水等媒质的色散特性.为了同时反演这类媒质的电磁参数,本文提出了一种时域逆散射改进技术:分别应用迭代法和吉洪诺夫(Tikhonov)正则化技术克服逆问题的非线性和病态性困难;解析导出了目标泛函关于目标参数的梯度;迭代重建过程中,产生的正演、反演子问题分别选用时域有限差分(FDTD)法、共轭梯度(CG)法求解.噪声环境下,通过两个一维(1-D)的数值算例,初步证实了该技术的可行性和鲁棒性.     

基于STL文件的FDTD网格剖分算法

针对复杂目标的时域有限差分(finite-difference time-domain,FDTD)法计算,提出了一种将非结构化网格直接转化为Yee网格的改进算法.该算法在射线求交方法的基础上对射线方程与三角面元求交公式进行优化,实现了快速的目标网格生成,有效降低了计算复杂度并提升了剖分效率.通过介质球、军舰和F22飞机...     

电磁场时域数值算法的新进展

电磁场时域计算方法由于一次计算可以获得目标的时域响应,结合傅里叶变换得到宽带信息等的优势越来越受到关注.本文介绍了近年来时域有限差分（finite-difference time-domain,FDTD）法和时域有限元（finite element time-domain,FETD）无条件稳定算法方面的研究进展以及FETD算法的更新方案——时域非连续伽辽金（discontinuous Galerkin time-domain,DGTD）方法的新进展.     

Analysis of multilayer planar circuits by a hybrid method

This letter proposes a hybrid method for the modeling of multilayer planar structures with locally inhomogeneous penetrable objects. By using the equivalence principle, the original problem can be decomposed into external and internal problems, which are formulated by the mixed-potential integral equation (MPIE) method and finite-difference time-domain (FDTD) method, respectively. Instead of directly implementing the continuity boundary conditions, an iterative approach is used to solve the hybrid FDTD-MPIE problem. Numerical results are presented to validate the hybrid method.     

A Single-Boundary Implicit and FFT-Accelerated Time-Domain Finite Element-Boundary Integral Solver

A novel time-domain finite-element boundary integral (FE-BI) solver for analyzing broadband scattering and radiation from free-standing electromagnetically large and perfect electrically conducting platforms supporting inhomogeneous and geometrically intricate structures is presented. The solver has three distinctive features that render it especially attractive for broadband analysis of installed antennas. i) The FE and BI solver components are hybridized using a single-surface interface. ii) The hybrid equations are solved by an implicit time-marching scheme accelerated by an (outer) Jacobi iterative solver that leverages (inner) direct FE and iterative BI solvers. iii) The BI solver component is accelerated by a distributed memory parallel implementation of the time-domain adaptive integral method based on the message-passing interface. The accuracy, late-time stability, and performance of the proposed time-domain FE-BI solver are demonstrated via its application to various scattering and radiation problems; moreover, the solver is used to characterize conformally mounted antennas on several platforms including an aircraft     

时域玻恩迭代的改进方法

本文提出了一种能够加速收敛的助推法，把助推法应用于时域非线性优化方法求解二维有耗介质体的逆散射问题，获得了很好的结果。     

Nonlinear two-dimensional velocity profile inversion using timedomain data

An iterative algorithm is developed to solve the nonlinear inverse scattering problem for two-dimensional lossless dielectric inhomogeneities using time-domain scattering data. The method is based on performing Born-type iterations on a volume integral equation and, hence, successively calculating higher-order approximations to the unknown object profile. Both the full-angle and the limited-angle problems are considered. Solutions are obtained for cases where the first-order Born approximation is severely violated. Wideband time-domain scattered field measurements make it possible to use sparse data sets and thus reduce experimental complexity and computation time. Several examples are given to show the ability of this method to invert arbitrarily shaped permittivity profiles using few transmitters and receivers. The high-resolution capability of the algorithm is also demonstrated     

Blind Receiver Design for OFDM Systems Over Doubly Selective Channels

We develop blind data detectors for orthogonal frequency-division multiplexing (OFDM) systems over doubly selective channels by exploiting both frequency-domain and time-domain correlations of the received signal. We thus derive two blind data detectors: a time-domain data detector and a frequency-domain data detector. We also contribute a reduced complexity, suboptimal version of a time-domain data detector that performs robustly when the normalized Doppler rate is less than 3%. Our frequency-domain data detector and suboptimal time-domain data detector both result in integer least-squares (LS) problems. We propose the use of the V-BLAST detector and the sphere decoder. The time-domain data detector is not limited to the Doppler rates less than 3%, but cannot be posed as an integer LS problem. Our solution is to develop an iterative algorithm that starts from the suboptimal time-domain data detector output. We also propose channel estimation and prediction algorithms using a polynomial expansion model, and these estimators work with data detectors (decision-directed mode) to reduce the complexity. The estimators for the channel statistics and the noise variance are derived using the likelihood function for the data. Our blind data detectors are fairly robust against the parameter mismatch     

Ground effects for VHF/HF antennas on helicopter airframes

In this paper, the finite element method (FEM) is used to predict the space and surface wave radiation patterns of VHF/HF antennas mounted on a helicopter in the presence of a lossy ground. The equivalent sources of the radiation system are obtained by solving an FEM problem in conjunction with an absorbing boundary condition (ABC) or an impedance boundary condition (IBC). From the equivalent sources, the total radiated field is calculated using the equivalence principle and superposition; the original problem is converted into a set of properly combined Hertzian dipoles referred to as the Sommerfeld problem. Instead of evaluating the Sommerfeld integral rigorously, Norton's approximation is used to improve the overall computational efficiency. The validation of this method is accomplished in two steps: first, the FEM is compared with the finite-difference time-domain method (FDTD) in the absence of a lossy ground; second, the Hertzian dipole problem is solved in the presence of a lossy ground and the results are compared with analytic solutions. Finally, this technique is extended to analyze an antenna on a helicopter above a lossy ground     

TIME-DOMAIN VOLUME INTEGRAL EQUATION FOR TRANSIENT SCATTERING FROM INHOMOGENEOUS OBJECTS-2D TM CASE

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse magnetic case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.     

Time-domain volume integral equation for transient scattering from inhomogeneous objects—2D TE case

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse electric case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.     

Time-domain volume integral equation for transient scattering from inhomogeneous objects—2D TM case

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse magnetic case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.