Modeling of frequency-dependent magnetized plasma in hybrid symmetrical condensed TLM method

In this paper, magnetized plasma media are modeled using the time-domain TLM method with hybrid symmetrical condensed node (HSCN). The proposed technique consists in adding voltage sources characterizing dispersive media in the HSCN. Numerical results are presented for reflection and transmission coefficients for a magnetized plasma wall, proving the efficiency of the proposed model.     

A General Three-Dimensional Tensor FDTD-Formulation for Electrically Inhomogeneous Lossy Media Using the Z-Transform

A general three-dimensional tensor finite-difference time-domain (TFDTD) formulation is derived to model electrically inhomogeneous lossy media of arbitrary shapes. The time domain representation of electric losses is achieved using Z-transforms. The regular cubical grid structure is maintained everywhere in the calculation domain by defining a 3-D face-fraction based 3 x 3 permittivity tensor on the interfaces that describes the relationship between the (known) average flux density vector and the (unknown) local electric field vector. For electrically lossy media, this tensor is complex in the frequency domain. However, it can be modified for use with the Z-transform. Only this modified real form is inverted, then transformed from the frequency into the Z-domain, and finally into the time domain. Furthermore, a local interface matrix is used to describe the relationship between the local electric field in the grid node and its counterpart on the other side of the interface. This matrix is complex in the frequency domain for lossy media. By applying the Z-transform, this matrix can also be transformed into the time domain using only real modified matrix elements. The accuracy of the method is confirmed by comparisons with analytical solutions.     

一种随机媒质时间反转成像算法

提出在随机媒质中采用时间反转镜对主动换能器阵列超声成像的一种新算法。这种算法主要特征是：对散射中接收的信号进行到达时间分解；响应矩阵频域奇异值分解；时城内目标函数的构建是统计稳定的并且对散射形成峰值。     

A comprehensive study of numerical anisotropy and dispersion in 3-DTLM meshes

This paper presents a comprehensive analysis of the numerical anisotropy and dispersion of 3-D TLM meshes constructed using several generalized symmetrical condensed TLM nodes. The dispersion analysis is performed in isotropic lossless, isotropic lossy and anisotropic lossless media and yields a comparison of the simulation accuracy for the different TLM nodes. The effect of mesh grading on the numerical dispersion is also determined. The results compare meshes constructed with Johns' symmetrical condensed node (SCN), two hybrid symmetrical condensed nodes (HSCN) and two frequency domain symmetrical condensed nodes (FDSCN). It has been found that under certain circumstances, the time domain nodes may introduce numerical anisotropy when modelling isotropic media     

An SCN-TLM model for the analysis of ferrite media

The modeling of ferrite media using the symmetrical condensed node (SCN) of transmission line matrix (TLM) method and current sources is developed. The proposed approach allows one to study the interaction between electromagnetic (EM) waves and gyromagnetic media. The obtained results are in good agreement with the analytical ones.     

Analysis of Millimeter Wave Microstrip Circulator with a Magnetized Ferrite Sphere by FDTD Method with Modified Matrix Pencil Method

In this paper, the modified matrix pencil (MMP) method is used in the finite difference time domain (FDTD) method to eliminate the late time instability of time domain responses when analyzing the novel microstrip circulator with a magnetized ferrite sphere. The frequency dependent scattering parameters: reflection, isolation and insertion parameters of the microstrip circulator calculated over a wide band of frequencies with this hybrid method are compared with that obtained by direct FDTD method which agree well with the experimental results.     

New wavelet based approach for time domain simulations

A new numerical method for the time domain solution of Maxwell's equations in linear media is proposed in this paper. The field quantities derived from the spatial discretization of Maxwell's equations are expanded in the time domain by wavelets on the interval. This choice yields a new arrangement of the unknowns into a matrix (instead of the usual vector) and transforms the differential equations in time in a Sylvester (1983) matrix equation. The memory requirements are proportional to the number of spatial unknowns and the time evolution of the space quantities is obtained with better accuracy than in conventional marching-on-time techniques.     

Asymmetrical coplanar waveguide with finite metallization thicknesscontaining anisotropic media

The spectral-domain approach (SDA) is extended to symmetrical and asymmetrical coplanar waveguides with anisotropic media. The quasi-static and hybrid-mode analytical methods are developed in the spectral domain taking the metallization thickness effect into consideration. Numerical computations include the quasi-static and frequency-dependent hybrid-mode values of the phase constants and characteristic impedances for the symmetrical and asymmetrical CPWs and metallization thickness effects in CWPs. Increased metallization thickness significantly reduces the nonreciprocal properties in CPWs with magnetized ferrite and additional dielectric layers     

Analysis on the Calculation of Plasma Medium with Parallel SO‐FDTD Method

This paper introduces a novel parallel shift operator finite‐difference time‐domain (SO‐FDTD) method for plasma in the dispersive media. We calculate the interaction between the electromagnetic wave of various frequencies and non‐magnetized plasma by using the parallel SO‐FDTD method. Then, we compare the results, which are calculated with serial and parallel SO‐FDTD executions to obtain the speedup ratio and validate the parallel execution. We conclude that the parallel SO method has almost the same precision as the serial SO method, while the parallel approach expands the scope of memory and reduces the CPU time.     

A spectral recursive transformation method for electromagneticwaves in generalized anisotropic layered media

A transition-matrix method is commonly used to deal with the problems of plane wave scattering from and the Green's function for multilayered generalized anisotropic media. The boundary conditions at the source interfaces are matched numerically. This method, although rigorous analytically, causes numerical singularities in the matrix inversion when the spectral fields are highly attenuating. A recursive variable transformation method is developed to deal with the exponentially growing or decaying terms associated with the spectral matrix method. The proposed scheme is suitable for numerical analysis of generalized anisotropic layers including uniaxial and biaxial materials, biased ferrites, magnetoplasmas, chiral and bi-anisotropic materials without increasing computer time. Applications of the recursive method are highlighted through examples of radiation and scattering from a three-layer ferrite structure and a conductor-backed magnetoplasma layer     

一维不均匀介质中波传播瞬态响应的传输线模型分析

提出了用传输线模型分析一维不均匀介质中波传播的瞬态响应。这种方法将一维不均匀介质用多层介质近似，并进一步用级联的传输线模型等效。分析的过程是：首先对局限在某一时间段内的输入电磁信号进行傅里叶变换得出其频谱，然后研究该频谱的主要分量在所研究介质中的传播，并得出波传播方向任一截面上瞬态信号的频谱，最后通过快速傅里叶变换得出时域瞬态响应。这种方法适用范围广，数值计算方便，只要离散时间间隔足够小，抽样点数足够大，就能达到一定的精度。     

Off‐grid DoA estimation method of wideband signals

In this paper, an off‐grid direction of arrival (DoA) estimation method is proposed for wideband signals. This method is based on the sparse representation (SR) of the array covariance matrix. Similar to the time domain DoA estimation methods, the correlation function of the sources was assumed to be the same and known. A new measurement vector is obtained using the lower‐left triangular elements of the covariance matrix. The DoAs are estimated by quantizing the entire range of continuous angle space into discrete grid points. However, the exact DoAs may be located between two grid points; therefore, this estimation has errors. The accuracy of DoA estimation is improved by the minimization of the difference between the new measurement vector and its estimated values. Simulation results revealed that the proposed method can enhance the DoA estimation accuracy of wideband signals.     

基于特征子空间的快速多径时延测量

提出一种基于特征子空间的快速多径时延测量方法.新方法对参考信号与接收信号的互相关函数做离散傅里叶反变换,将时延测量问题转换成具有低通包络的正弦信号频率估计问题.重构互相关函数的离散傅里叶反变换序列,形成的信号矩阵用于构造低阶协方差矩阵,使新方法的运算量与传统MUSIC算法相比大幅减小.理论分析、仿真结果验证了新方法的正确性,表明新方法以稍微降低时延测量性能为代价换取运算量的大幅减小.     

FDTD modeling of lumped ferrites

Implementing ferrites in finite-difference time-domain (FDTD) modeling requires special care because of the complex nature of the ferrite impedance. Considerable computational resources and time are required to directly implement a ferrite in the FDTD method. Fitting the ferrite impedance to an exponential series with the generalized-pencil-of-function (GPOF) method and using recursive convolution is an approach that minimizes the additional computational burden. An FDTD algorithm for a lumped ferrite using GPOF and recursive convolution is presented herein. Two different ferrite impedances in a test enclosure were studied experimentally to demonstrate the FDTD modeling approach. The agreement is generally good     

用谱域导抗法计算铁氧体加载鳍线的传输特性

本文从Maxwell方程出发,经过Fourier变换和坐标旋转,将铁氧体中的非寻常波分解成TE和TM波,并给出它们的波导纳。利用边界条件,忽略TE和TM波间耦合的影响后,将鳍线用一等效网络表示。本文方法与T.Itoh提出的谱域导抗法比较,在等效网络中引入了理想变压器元件。最后对铁氧体加载单侧鳍线的正反向传播常数进行了数值计算,其结果与有关文献中数据吻合得很好。     

铁氧体介质基片微带线色散特性的研究

利用扩展的二维FD-TD方法研究了包含铁氧体介质的均匀微带线的色散特性.给出了铁氧体介质的厚度,微带线导带宽度以及铁氧体的外加恒磁场对其传输特性的影响.并对这类传输线在禁带附近模式容易传播的模式进行了讨论.     

FDTD Modeling for Dispersive Media Using Matrix Exponential Method

This letter presents a finite-difference time-domain formulation to model electromagnetic wave propagation in dispersive media using matrix exponential method. The Maxwell's curl equations and the time domain relations between electric fields and auxiliary variables are formulated as a first order differential matrix system. The fundamental solution to such a system is derived in terms of matrix exponential and the update equations can be extracted conveniently from the solution. Numerical results show that this formulation yields higher accuracy compared to many other previous methods, without incurring additional auxiliary variable and complexity.      

A wavelet-based multiresolution regularized least squaresreconstruction approach for optical tomography

The authors present a wavelet-based multigrid approach to solve the perturbation equation encountered in optical tomography. With this scheme, the unknown image, the data, as well as the weight matrix are all represented by wavelet expansions, thus yielding a multiresolution representation of the original perturbation equation in the wavelet domain. This transformed equation is then solved using a multigrid scheme, by which an increasing portion of wavelet coefficients of the unknown image are solved in successive approximations. One can also quickly identify regions of interest (ROI's) from a coarse level reconstruction and restrict the reconstruction in the following fine resolutions to those regions. At each resolution level a regularized least squares solution is obtained using the conjugate gradient descent method. This approach has been applied to continuous wave data calculated based on the diffusion approximation of several two-dimensional (2-D) test media. Compared to a previously reported one grid algorithm, the multigrid method requires substantially shorter computation time under the same reconstruction quality criterion     

一种改进的MIMO-OFDM系统Steiner估计算法

鉴于MIMO-OFDM（多输入多输出-正交频分复用）系统中时域信道方法的灵活性和低带宽开销的特点,本文采用Steiner训练符号设计方法设计MIMO-OFDM系统中的时域训练符号,给出传统CDMA系统的Steiner信道估计器在MIMO-OFDM系统中的实现过程,解决了MIMO-OFDM时域信道估计矩阵求逆的问题,降低了时域信道估计的实现复杂度。并进一步提出了采用接收天线DOA（波达方向）改进的Steiner信道估计方法,提高了信道估计的精度,获得了一定程度的性能增益,增强了时域信道估计方案的实际可行性。