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An approach to eigenvalue assignment for systems of linear time-invariant (LTI) delay differential equations (DDEs), based upon the solution in terms of the matrix Lambert W function, is applied to the problem of robust control design for perturbed LTI systems of DDEs, and to the problem of time-domain response specifications. Robust stability of the closed-loop system can be achieved through eigenvalue assignment combined with the real stability radius concept. For a LTI system of DDEs with a single delay, which has an infinite number of eigenvalues, the recently developed Lambert W function-based approach is used to assign a dominant subset of them, which has not been previously feasible. Also, an approach to time-domain specifications for the transient response of systems of DDEs is developed in a way similar to systems of ordinary differential equations using the Lambert W function-based approach. [DOI: 10.1115/1.4001339]

Low-pressure discharge lamps obey a set of physical laws that are different from those of high-pressure discharge lamps. In this paper, these differences are addressed. Based on a recently developed HID lamp model frame, a semi-theoretical fluorescent lamp model that can be determined by genetic algorithms and simple electrical measurements is presented. This model does not require any lamp data from lamp manufacturers. Its parameters can be determined from electrical voltage and current measurements of the lamps under ac operation at mains frequency. With the same set of parameters, the model can predict the lamp terminal characteristics accurately under low, medium and high frequency operations. Good simulation results were achieved when the lamp power was reduced to 60% of rated power and when the lamp was operated under step-up and step-down transient processes. Simulation results for different sizes of tubular and compact fluorescent lamps agree well with their experimental results. Particularly, the differences between simulation results and experimental results under rated power are less than 10%. Hence, the proposed model shows a good degree of accuracy: 1) for different types of fluorescent lamps; 2) at different operating frequencies; 3) under different dimming levels; and 4) during step-up and step-down transient processes. Index Terms--Fluorescent lamp modeling, gas discharge, genetic algorithms, simulation.

A reduced-complexity time-domain equalization scheme for wideband turbo-multiple-input multiple-output (turbo-MIMO) systems is presented. This scheme, called iterative trellis search equalization, is based on a modified version of the M-BahlCocke--Jelinek--Raviv (M-BCJR) algorithm, applied to a suitably chosen trellis representation of the wideband MIMO channel process. Exploiting the properties of quadrature amplitude modulation (QAM) signal constellations with block-partitionable labels, this modified M-BCJR algorithm has complexity per bit that is independent of the constellation size, and polynomial in the number of transmit antennas and channel memory. Results from computer simulations show that the new scheme successfully mitigates intersymbol interference even if only a very small fraction of trellis state transitions is considered. It is also demonstrated that asynchronous transmission of the spatially multiplexed symbol streams can result in considerable performance improvement compared to synchronous MIMO systems. Index Terms--Complexity reduction, iterative detection/ decoding, multiple-input multiple-output (MIMO) systems, trellis searching.

The novel multidomain pseudospectral time domain method based on a symplectic integrator scheme is presented in this paper. A symplectic integrator scheme is a decomposition of the exponential operator or a general propagation technique. The scheme is nondissipative and does not require additional memory for temporary storage of data for the internal stages. The good performance of the scheme is demonstrated by numerical examples. Index Terms--Multidomain pseudospectral time domain (PSTD), symplectic integrator.

This note presents a robust moving horizon [H.sub.[infinity]] control scheme that achieves disturbance attenuation with performance adaptation or for a prescribed level. The on-line solved optimization problem is a Lagrange relaxation of the constrained minimax problem, with the benefit of achieving feasibility in the moving horizon implementation. Uncertainties are incorporated by the use of the full block multiplier technique. Simulation and comparison results are given. Index Terms--Disturbance attenuation, feasibility, full block multiplier, moving horizon control, time-domain constraint.

A direct time-domain near-field-to-far-field transform (NFFFT) is developed that is applicable to the finite-difference time-domain analysis of radiation in the presence of a non-dispersive, infinite, grounded dielectric slab. With minor modification and/or simplification, the results can also be applied to the dielectric half-space and the infinite ungrounded dielectric slab. A few time-domain radiation examples involving microstrip transmission lines are used to illustrate the method. Index Terms--Finite-difference time-domain (FDTD), microstrip, nonhomogeneons media.

This paper explores global time frequency approaches to EEG data analysis with the Wigner Distribution Function and the Symmetric Ambiguity Function. The task chosen was to characterize the activity profile of EEG signals in sample Frontal, Central and Occipital electrodes from human subjects, coincident with the perception of a reversal in the orientation of a bistable Necker cube figure. The result of this analysis has implications for blind signal processing as the goal was to identify an unknown input source eliciting the observed EEG signals. The methods demonstrate an internally initiated EEG signal source not tied to a regularly anticipated external source. The results demonstrate the general applicability of the methods for a wide variety of neural and biological signals and systems. The findings can be summarized as the observation of high energy activity patterns in terms of significant dissimilarities in the waveform, both in time and frequency, in the Frontal and Occipital electrodes, approximately 200-600 ms prior to the appearance of the premotor potentials in the medial electrodes. Index Terms--Ambiguity function diagrams, bistable images, EEG, time frequency analysis, Wigner distribution function.

In this paper, a probe-fed rectangular ring with a cross-strip for wireless local-area network system is proposed. The antenna is analyzed by using the three-dimensional finite-difference time-domain method. The effects of various parameters on the characteristics of the antenna are studied. The computed and measured results for the return loss and co-and crosspolarization patterns are presented and compared. A prototype antenna operating at 2.44 GHz shows an impedance bandwidth (2:1 VSWR) of 13.06 % and a gain of 9.5 dBi. Good radiation characteristics, including a cross-polarization level in both E- and H-planes greater than 26 dB and a front-to-back ratio (F/B) of radiation pattern greater than 16 dB, have been obtained. Index Terms--Bandwidth enhancement, cross-strip, finite-difference time-domain (FDTD) method, low cross-polarization, ring antenna, unidirectional antenna, wireless local-area network (WLAN).

The perfectly matched layer (PML) technique is extended for a cell-centered finite-volume time-domain (FVTD) method. A step-by-step procedure for the performance characterization of the FVTD PML is presented for both structured and unstructured finite-volume meshes. The FVTD PML is compared with the standard first-order Silver-Miiller absorbing boundary condition (SM ABC) for practical applications. It is found that the FVTD PML for an unstructured grid achieves a reflection coefficient lower than--40 dB for incident angles up to 45[degrees] and outperforms the SM ABC by 15-20 dB. Index Terms--Absorbing boundary condition (ABC), computational electromagnetics (CEM), finite volume time domain (FVTD), Maxwell's equations, perfectly matched layer (PML).

The aim of this study was to identify conditions under which the presence of rail strike object can serve to increase or decrease lightning-induced voltages on a nearby overhead wire. We examined the ratios of magnitudes of lightning-induced voltages on the overhead wire for the cases of strikes to a tall object and to flat ground as a function of distance from the lightning channel d, current reflection coefficients at the top of the strike object [rho]top and at the bottom of the strike object [rho]bot, the current reflection coefficient at the channel base (in the case of strikes to flat ground) [rho]gr, and the return stroke speed v. Lightning-induced voltages were computed using the finite-difference time-domain (FDTD) method. The transmission line (TL) model was used to find the distribution of current along the lightning channel and the strike object. The ratio of magnitudes of lightning-induced voltages for tall-object and flat-ground cases increases with increasing d (ranging from 40-200 m), decreasing [rho]bot ( < 1), decreasing [rho]top (< 0, except for the case of [rho]bot = 0), and decreasing v (< c, speed of light). Also, the ratio increases with decreasing the lightning current rise time. Under realistic (expected) conditions such as [rho]bot = 1, [rho]top = -0.5, and v = c/3, the ratio is larger than unity (the tall strike object serves to enhance lightning-induced voltages), but it becomes smaller than unity (the tall object serves to decrease lightning-induced voltages) under some special conditions, such as [rho]bot = 1, [rho]top = 0, and V:C. Index Terms--FDTD method, lightning, lightning-induced voltage, return stroke model, tall object, transmission line (TL) model.

This paper details the impact of partial element accuracy on quasi-static partial element equivalent circuit (PEEC) model stability in the time domain. The potential sources of in-accurate partial element values are found to be poor geometrical meshing and the use of unsuitable partial element calculation routines. The impact on PEEC model stability of erroneous partial element values, and the coefficients of potential and partial inductances, are shown as theoretical constraints and practical results. Projection meshing, which is a discretization strategy suitable for the PEEC method, is shown to improve calculated partial element values for the same number of unknowns, thus improving model stability. Index Terms--Partial element equivalent circuit (PEEC).

This paper presents a finite element method with high spatial order for solving the Maxwell equations in the time domain. In the first part, we provide the mathematical background of the method. Then, we discuss the advantages of the new scheme compared to a classical finite-difference time-domain (FDTD) method. Several examples show the advantages of using the new method for different kinds of problems. Comparisons in terms of accuracy and CPU time between this method, the FDTD and the finite-volume time-domain methods are given as well. Index Terms--Finite difference method, finite element method (FEM), finite volume method, high spatial order method, Maxwell equations.

A new method is proposed in this paper to design the time-domain equalizer for orthogonal frequency division multiplexing systems based on maximizing signal-to-interference-plus-noise ratio (SINR) at the output of the equalizer. The method called Maximum SINR Time-domain Equalization (MSINR-TEQ) is derived based on a new formulation of SINR. Computer simulation and analytical results show the performance of the MSINR-TEQ method and its superiority in comparison with the performance of the much-used method of channel impulse response shortening. Index Terms--Channel shortening, discrete multitone, minimum intersymbol interference equalizer, orthogonal frequency-division multiplexing (OFDM) systems, time-domain equalization.

The brief describes synthesis of pulse-forming reactance networks with output response of quasi-rectangular shape. Three cases of input excitation are considered, namely, an impulse, a step, and a sinusoid. The derivative of network output response is approximated using half-periods of sine-squared function. The real and imaginary parts of the Laplace transform of this derivative are expanded in infinite products. Then, using a finite amount of terms in these products as further approximation step. one finds the rational function from which one derives realizable transfer functions for each case of input excitation. The obtained transfer functions are realized as reactance networks loaded by resistors. Index Terms--Linear network theory, network synthesis, pulse-forming networks, time-domain approximation, transient electronics.

This study aims at describing the field propagation in terms of pulsed rays, that are particularly advantageous when dealing with short-pulse excitations. In the framework of the Geometrical Theory of Diffraction we augment Geometrical Optics and uniform singly diffracted field solutions available in the time domain (TD), by TD doubly diffracted (DD) rays, that are expressed in simple closed forms. Impulsive double diffraction at a pair of coplanar edges is here formulated directly in the TD, as a double superposition of impulsive spherical waves. Non-uniform and uniform wavefront approximations for TD-DD fields are determined in closed form, defining two novel TD transition functions. The scalar case with either hard or soft boundary conditions is analyzed first, and then used to build an electromagnetic dyadic DD coefficient for a pair of coplanar edges with perfectly conducting faces. Particular attention is given to the definition of TD transition regions, i.e., the elliptical regions where the TD-DD field does not exhibit a ray optical behavior. The compensation mechanism by which the TD-DD fields repair the discontinuity introduced by singly diffracted fields at their shadow boundaries is also analyzed in detail. Our result for the TD-DD field excited by an impulsive spherical wave is valid only for early times, at and close to (behind) the DD ray wavefront. The TD-DD field response to a more general pulsed excitation is obtained via convolution, and if the exciting signal has no low-frequency components the range of validity of the resulting pulsed response is enlarged to later observation times behind the wavefront. Index Terms--Diffraction, electromagnetic transient analysis, ray tracing, time domain (TD) analysis, transient propagation, transient scattering, wedges.

The higher order (2, 4) scheme optimized in terms of Taylor series in the finite-difference time-domain method is often used to reduce numerical dispersion and anisotropy. This paper investigates optimization of the numerical dispersion behavior for a square Yee mesh based on the (2, 4) computational stencil. It is shown that, for one designated frequency, numerical dispersion can be eliminated for some directions of travel, such as the coordinate axes or the diagonals, or numerical anisotropy can be eliminated entirely, resulting in a constant 'residual' numerical dispersion. Using a coefficient-modification technique, the residual numerical dispersion can then be completely eliminated at that frequency, or for a wide-band signal, the numerical dispersion error and the averaged-accumulated phase error can be minimized. The stability of the method is analyzed, the numerical dispersion relation is given and validated using numerical experiments, and the relative rms errors are compared to the standard (2, 4) scheme for the proposed methods. The optimized methods are second-order accurate in space and have higher accuracy than the standard (2, 4) scheme. It has been found that the dispersion error of the (2, 4) scheme is like that of a second-order accurate method, though it behaves like a fourth-order accurate method in terms of anisotropy. Index Terms--Computational electromagnetics, finite-difference time-domain (FDTD) method, higher order method, numerical anisotropy, numerical dispersion.

A recently proposed high power switch-mode rectifier topology offers advantages over the conventional line-commutated thyristor rectifiers in terms of size and cost. This paper presents modeling and analysis of the high power switch-mode rectifier configuration, operated under a newly developed digital control strategy. The developed strategy provides a fast transient response to disturbances and achieves equal current sharing between the buck converter modules which are paralleled within the switch-mode rectifier. A continuous-time state-space model of the switch-mode rectifier and analyses of both the steady-state and the transient operation modes of the digitally controlled rectifier are presented. The continuous-time model is used as a basis for time-domain simulations of the rectifier's behavior in the MATLAB/Simulink environment. The validity of the presented model is demonstrated based on comparison between the simulation results and the corresponding experimental results. The experimental results are obtained from a 2-kW scaled-down laboratory setup of the rectifier. The stability of the digitally controlled rectifier system is also examined based on an overall small-signal sampled-data model of the rectifier system including the controls. Index Terms--Switch-mode rectifier topology, time-domain simulations.

This paper presents a method to evaluate harmonic distortion due to space vector-based pulse-width modulation (PWM) strategies for ac drives. The proposed method is general enough to deal with division of zero vector time as well as division of active vector time within a subcycle. The method is based on the notion of stator flux ripple, which is a measure of line current ripple. Expressions for RMS ripple over a subcycle are derived for six switching sequences in terms of magnitude and angle of the reference vector, and subcycle duration. The sequences considered include those involving division of active vector time within a subcycle. Further, analytical closed form expressions are derived for the total RMS harmonic distortion factor corresponding to six space vector-based synchronized PWM strategies, proposed recently, for high power drives. The square of the distortion factor turns out to be a quadratic polynomial in modulation index (M), and the coefficients differ with PWM strategies and pulse numbers. These expressions are validated through Fourier analysis as well as experimental measurements. The concept of stator flux ripple provides insight into current ripple as well as torque ripple corresponding to different sequences and strategies. Index Terms--Harmonic analysis, harmonic distortion, induction motor drives, inverters, pulse width modulation (PWM), space vector, stator flux ripple, switching sequence, time domain analysis, vectors.

A new signal classification approach is presented that is based upon modeling the dynamics of a system as they are captured in a reconstructed phase space. The modeling is done using full covariance Gaussian Mixture Models of time domain signatures, in contrast with current and previous work in signal classification that is typically focused on either linear systems analysis using frequency content or simple nonlinear machine learning models such as artificial neural networks. The proposed approach has strong theoretical foundations based on dynamical systems and topological theorems, resulting in a signal reconstruction, which is asymptotically guaranteed to be a complete representation of the underlying system, given properly chosen parameters. The algorithm automatically calculates these parameters to form appropriate reconstructed phase spaces, requiring only the number of mixtures, the signals, and their class labels as input. Three separate data sets are used for validation, including motor current simulations, electrocardiogram recordings, and speech waveforms. The results show that the proposed method is robust across these diverse domains, significantly outperforming the time delay neural network used as a baseline. Index Terms--Signal classification, reconstructed phase spaces, Gaussian mixture models.

We have simulated and constructed a one-dimensional metamaterial composed of a periodically loaded transmission line that exhibits both negative and positive group velocities in a band of effective negative index of refraction. The negative group velocity or, equivalently, the negative group delay, is demonstrated theoretically and experimentally in the time domain using modulated Gaussian pulses. Due to this negative delay, we can show an output pulse peak emerging from the loaded transmission line prior to the input peak entering the line, i.e., the output pulse precedes the input pulse. The fact that this surprising behavior does not violate the requirements of relativistic causality is illustrated with time-domain simulations, which show that discontinuities in the pulse waveforms are traveling at exactly the speed of light in vacuum. The pulse-reshaping mechanism underlying this behavior is also illustrated using time-domain simulations. Index Terms--Anomalous dispersion, coplanar waveguide (CPW), metamaterials, negative group delay (NGD), negative group velocity (NGV), negative refractive index (NRI).

An improved expression of the thin-slot formalism has been proposed to alleviate the gridding of the finite-difference time-domain (FDTD) solution for electromagnetic problems involving narrow slots. By applying the conformal mapping technique, the varying field distribution near the slots is analytically derived and fully cast into a unit FDTD cell with effective permittivity and permeability. As a result, this formalism permits narrow slots to be modeled without reducing spatial grid size, even when the slot gap width is much smaller than a one space grid. The method has been applied to several transmission line problems and the results are compared with traditional FDTD and other thin-slot formalisms. It is verified that the proposed method is easy to implement, numerically efficient, and accurate. Index Terms--Conformal mapping technique, finite-difference time-domain (FDTD), thin slot formalism.

In this paper, we present an efficient near-to-far-field transformation technique to be used in conjunction with the body of revolution finite-difference time-domain (BOR/FDTD) algorithm. The efficiency is gained by factoring out the azimuthal angle [theta] from the two-dimensional (2-D) far-field integration using an analytical approach, thereby reducing the 2-D integral on the surface of the Huygens' box to a 1-D version in the process. To illustrate the application of the proposed algorithm, the Potter horn is analyzed. The numerical results show good agreement with those obtained by using either analytical (where possible), or 2-D integration schemes. Index Terms--Body of revolution, finite-difference time-domain (FDTD), near-to-far-field transformation.

In this communication, the application of a new causal anisotropic absorber for the truncation of time domain finite element meshes is presented. Numerical results are illustrated in order to show the performances of the absorber. Both homogeneous and stratified absorbers are considered and their performances compared. The computational efficiency of the finite element code implementing this absorber is also proven by comparing its computational cost with that one of a conventional time domain finite element code with boundary conditions given by perfect conductors and no absorber. Index Terms--Anisotropic absorbers, finite element method (FEM), time domain.

A new numerical finite-difference time-domain (FDTD) model of interfaces with metals and semiconductors is developed. The model uses a higher order impedance boundary condition to simulate the fields in conductive media. The approach has been found to be considerably more accurate than the known models based on the simple Leontovich impedance boundary condition. This is because the new model takes the incidence angle of the incident waves into account, and it is valid for all values of conductivity. The derivation of the method is presented in the two-dimensional (2-D) case, and the performance is studied over a wide range of conductivity. The validation of the method is made by comparing with the exact results in a 2-D example problem. The proposed method is also compared with some other methods available in the literature. Index Terms--Finite-difference time-domain (FDTD) techniques, higher-order impedance boundary conditions, recursive convolution techniques.

We present a two-dimensional higher order FDTD algorithm to minimize the numerical dispersion error at arbitary, preassigned angles. Filtering schemes are further employed to optimize the FDTD frequency response at a given frequency range. A stability analysis of the resulting FDTD algorithms is included. Numerical dispersion properties are compared against the Yee's scheme and a higher order (2,4) FDTD scheme. Numerical results demonstrate that the dispersion error around a preassigned angle (and, with filtering, around a frequency band of choice) can be significantly reduced. Index Terms--finite-difference time-domain (FDTD) method, numerical dispersion, optimization.

Adapted finite-difference time-domain (FDTD) update equations exist for a number of objects that are smaller than the grid step, such as wires and thin slots. In this contribution we provide a technique that automatically generates new FDTD update equations for small objects. Our presentation will be focussed on 2-D-FDTD. We start from the FDTD equations in a fine grid where the time derivative is not discretised. This yields a large state-space model that is drastically reduced with a reduced order modeling technique. The reduced state-space model is then translated into new FDTD update equations that can be used in an FDTD simulation in the same way as the existing update equations for wires and thin slots. This technique is applied to a number of numerical problems showing the accuracy and versatility of the proposed method. Index Terms--Finite-difference time-domain (FDTD) methods, finite-difference methods, reduced order systems, transfer functions.

A general and consistent integral finite-difference time-domain (FDTD) formulation on cubical grids for modeling of cylindrical antennas with or without dielectric coating is derived. No additional grid points or modifications of the integral paths are necessary. Instead, effective material properties are modified in the FDTD grid. Thus, even for insulated antennas, the simple cubical structure is maintained. Special integral factors are defined on cubical elements, which take into account the behavior of fields in all directions in the neighborhood of the antenna. Applying these factors to the gap region and along the antenna's axis allows a correct modeling of the influence of the antenna's thickness. Furthermore, integral factors derived for the antenna's ends improve the modeling of the antenna's length. The accuracy of the method is confirmed by a systematic comparison with analytical and numerical results. Index Terms--Dielectric coating, finite-difference time-domain (FDTD) method, input impedance, thin-wire antenna.

We introduce dispersion-relation-preserving (DRP) algorithms to minimize the numerical dispersion error in large-scale three-dimensional (3-D) finite-difference time-domain (FDTD) simulations. The dispersion error is first expanded in spherical harmonics in terms of the propagation angle and the leading order terms of the series are made equal to zero. Frequency-dependent FDTD coefficients are then obtained and subsequently expanded in a polynomial (Taylor) series in the frequency variable. An inverse Fourier transforation is used to allow for the incorporation of the new coefficients into the FDTD updates. Butterworth or Chebyshev filters are subsequently employed to fine-tune the FDTD coefficients for a given narrowband or broadband range of frequencies of interest. Numerical results are used to compare the proposed 3-D DRP-FDTD schemes against traditional high-order FDTD schemes. Index Terms--Finite-difference time-domain (FDTD) method, numerical dispersion, optimization.

The finite-difference time-domain (FDTD) method is routinely used to calculate the input admittance/impedance of simple antennas. The value of the input admittance/impedance depends on the level of discretization used in the method, and should converge to a final value as the discretization becomes finer. In this paper, the level of discretization necessary for convergence is studied using two common feed models: the hard-source feed and the transmission-line feed. First, the simplest and most naive methods for introducing the voltage and the current in these models are considered, and the results for the admittance are shown not to converge. Next, improved methods for introducing the voltage and current in these models are constructed. The results for the admittance are then shown to converge, and guidelines are offered for the level of discretization needed for convergence. In addition, two general problems associated with the computation of the admittance are discussed: the agreement between admittances computed with different simple feed models, and the agreement between these admittances and measurements. Index Terms--Convergence, discretization, finite-difference time domain (FDTD), hard-source feed, transmission-line feed.

A general integral finite-difference time-domain (FDTD) formulation on cubical grids for the modeling of electrically inhomogeneous media of arbitrary shape is derived. The simple cubical structure is maintained and no modifications of the integral paths are necessary. Instead, a three-dimensional tensor relationship between the average electric flux density and electric field is determined beforehand and used during the simulation to account for discontinuities in the neighborhood of sloped interlaces. The accuracy of the method is confirmed by comparisons with analytical solutions. Index Terms--Finite-difference time-domain (FDTD) method, inhomogeneous media, permittivity tensor, sloped interfaces.

This paper proposes a new method of S-parameter extraction from finite-difference time-domain simulations. Unlike the previously published methods, the present method extracts the frequency-dependent mode impedance and propagation constant directly from the three-dimensional simulations. This makes it accurate and computationally effective in wide-band analysis. This paper provides examples of calculations including difficult cases of inhomogeneous or lossy structures, with the frequency band spanning below and above the cutoff frequency. Special attention is given to the S-parameter extraction for evanescent modes. It is shown that the available literature provides insufficient and sometimes confusing background in this regard. Thus, a new consistent theoretical background is presented. Index Terms--Circuit simulation, computer-aided analysis, electromagnetic analysis, scattering parameters.

Time-domain simulation of dynamic circuits and, in general, of any physical model characterized by ordinary differential equations or differential algebraic equations, implies the use of some numerical integration method to find an approximate solution in a discrete set of time points. Among these methods, the class known as linear multistep includes many we!i-known formulas such as the backward Euler method, the trapezoid method, and the implicit backward differentiation formulas used in most circuit simulators. All these methods introduce a very subtle effect that is, here, called the warping error. As shown, it is equivalent to a perturbation of the eigenvalues of the linearized ordinary differential problem. The perturbation introduced depends on the integration time step; it is often very small and in most cases irrelevant or even not noticeable. Nevertheless an exception to this situation is found when simulating high-quality factor circuits where even very small warping errors can lead to qualitatively wrong solutions. In this paper, we demonstrate that higher order linear multistep methods, while characterized by weaker stability properties, introduce less of a warping error and are well suited to the simulation of high-quality factor circuits. Index Terms--Frequency warping, linear multistep methods, numerical integration methods.

A new time domain wave propagator (TDWP) based on two-dimensional finite-difference time-domain (FDTD) technique (that was introduced in May 2000 issue) has been augmented in this paper so that it deals with impedance boundary condition or varying terrain heights and an example of each, with comparisons to other methods is presented. Index Terms--EM Propagation, FDTD wave propagator, groundwaves, mixed-path propagation, surface impedance modeling, surface wave propagation, terrain modeling.

In the use of the time-domain integral equation (TDIE) method for the analysis of layered media, it is important to have the time-domain layered medium Green's function computed for many source-to-field distances [rho] and time instants t. In this paper, a numerical method is used that computes the mixed potential Green's functions [G.sub.v]([rho], t) and [G.sub.A] ([rho], t) for a multilayered medium for many [rho]'s and t's simultaneously. The method is applicable to multilayered media and for lossless or lossy dispersive media. Salient features of the method are: 1) the use of complex [omega] so that the surface wave poles are lifted off the real [k.sub.[rho]] axis such that pole extractions are not required; 2) the use of half-space extraction so that the integrand for the Sommerfeld integral decays exponentially along the [k.sub.[rho]] axis to obtain fast convergence of the integral; and 3) the use of the fast Hankel transform so that the Green's function is calculated for many values of [rho] simultaneously. For a four-layer medium, we illustrate the numerical results by a three-dimensional plot of [rho][G.sub.v]([rho], t) versus [rho] and t and demonstrate the space-time evolution of these Green's functions. For a maximum frequency range of 8 GHz, the method requires only a few CPU minutes to compute a table of 100 (points in [rho]) x 168 (points in t) uniformly spaced values of [G.sub.v]([rho], t) on an 867-MHz Pentium PC. Index Terms--Discrete transforms, Green function, nonhomogenous media, time-domain analysis.

A model for the transient response of a distributed resistance-inductance-capacitance (RLC) interconnect is described and discussed.

Accurate measurement of profile soil water content is essential to many areas of environmental and agricultural research. In this study five methods were evaluated for determining volumetric profile soil water content from time domain reflectometry (TDR) data at nine locations within the Little Washita River Experimental Watershed (LWREW) in south central Oklahoma. Soil compositions for the sites ranged between 24.6 to 86.4% for sand and 5.2 to 29.2% for clay. Comparisons were made between gravimetric soil sample data and soil water content as determined by the TDR factory instrument calibration, two methods of site-specific calibration, a regional calibration technique, and an empirically derived universal approach. Method 1 is the factory calibration, which uses average values for model coefficients that were derived from extensive laboratory work and theoretical analysis. Method 2 fits a site-specific linear regression of TDR time delay on measured soil-core water content. Method 3 uses the factory calibration equation and site-specific values for the ratio of TDR time delay in dry soil, to that in air ([T.sub.s]/[T.sub.a]). In Method 4, a regional linear regression equation was developed from an analysis combining data from all study sites. Method 5 applies a universal equation based on the linear relationship between soil water content and the apparent dielectric constant of soil ([K.sub.a]) measured by TDR. Statistical analysis of the data showed that of the five methods, only the mean root mean square error (RMSE) value for Method 2 was significantly different from all other methods ([alpha] = 0.05). Method 2 also had the smallest standard deviation and standard error, and the narrowest range of RMSE values. AH field calibration methods show that it is necessary to include very low water content data in determining absolute water content. When compared with the factory calibration, all other methods improved the measurement of soil water content, with Method 2 providing the most accurate results at the site-specific level of analysis.

In a paper presented by Heimovaara (1993) a method of calibrating TDR sensors was presented using air and water. Time has moved on but time domain reflectometry (TDR) sensors are still calibrated in a number of different ways. In this article we present a rigorous investigation of the method proposed by Heimovaara and demonstrate its accuracy. We demonstrate that the placement of a starting point in any place other than the one determined using Heimovaara's method results in erroneous permittivity measurement. This will be most significant at low values of permittivity. We propose that Heimovaara's method be adopted as a standard method for calibrating TDR sensors for measuring permittivity. The discussion centers on the placement of the first time marker used to measure the signal travel time from which permittivity is measured. Our modeling results suggest that this point is slightly forward of the apex of the bump on the waveform which corresponds to the impedance increase as the wave travels from the cable into the TDR sensor head. We also demonstrate that using the apex of this bump as a starting point reference can lead to erroneous measurements of travel time in layered dielectric media. Finally we examine the use of long cables to connect sensors to the TDR. We demonstrate that the travel time in the cable changes as a function of temperature and that fixed travel time markers based on cable length cause error in the measurement of travel time. For a 2.6-m cable the error was 1.6% at 50[degrees]C, and 4.7% for a 10.3-m cable, relative to calibration at 25[degrees]C. Software that tracks the sensor head either through the impedance mismatch caused by the head or using an electrical marker eliminates this source of error.

A method is presented to mathematically characterize a wide range of solute transport processes in softs from time-domain-reflectometry (TDR)-based breakthrough curve (BTC) measurements. To do this, we combined the flexible generalized transfer function model (GTF) with the definition of the time normalized resident concentrations [C.sup.rt] *. The GTF is a four-parameter flexible transfer function able to describe both the convection-dispersive (CD) and the stochastic-convective (SC) process of dispersion in a soil. In addition, it allows other dispersion processes typical for heterogeneous soils to be modeled. To obtain robust estimations of the GTF model parameters, closed-form expressions of the GTF time normalized resident concentrations [C.sup.rt] * and temporal moments of [C.sup.rt] * were defined. These expressions allow the transport parameters to be estimated from TDR-based BTCs without relying on problematic probe calibrations and mass recovery assumptions. Since GTF is a four-parameter model, the robustness of the parameterization procedure was tested by fitting the [C.sup.rt] * of the GTF model to numerically generated, error-contaminated BTCs. Using the least-square optimization technique, robust estimates of the GTF parameters could be obtained from TDR observations at two different soil depths. Application of the proposed method was also tested for a real undisturbed sandy subsoil and compared with the convective lognormal transfer (CLT) and CD models. For this case, it was shown that the GTF model improved greatly the goodness of prediction of the BTCs. The proposed method offers a new powerful tool for analyzing transport processes of nonreactive solutes in soil.

The channel estimation techniques for OFDM systems based on pilot arrangement are investigated. The channel estimation based on comb type pilot arrangement is studied through different algorithms for both estimating channel at pilot frequencies and interpolating the channel. The estimation of channel at pilot frequencies is based on LS and LMS while the channel interpolation is done using linear interpolation, second order interpolation, low-pass interpolation, spline cubic interpolation, and time domain interpolation. Time-domain interpolation is obtained by passing to time domain through IDFT (Inverse Discrete Fourier Transform), zero padding and going back to frequency domain through DFT (Discrete Fourier Transform). In addition, the channel estimation based on block type pilot arrangement is performed by sending pilots at every sub-channel and using this estimation for a specific number of following symbols. We have also implemented decision feedback equalizer for all sub-channels followed by periodic block-type pilots. We have compared the performances of all schemes by measuring bit error rate with 16QAM, QPSK, DQPSK and BPSK as modulation schemes, and multi-path Rayleigh fading and AR based fading channels as channel models. Index Terms--Cochannel interference, communication channels, data communication, digital communication, frequency division multiplexing, frequency domain analysis, time domain analysis, time-varying channels.

In this paper, we consider a solution method of the inverse problem of imaging two-dimensional (2-D) objects buried underground by cross-hole radar data in the time domain. In addition to less information on the targets due to restriction on the arrangement of transmitters and receivers than for full-view cases such as imaging of objects in free space, the large search region between boreholes makes solving the inverse problem difficult. Although iterative optimization approaches take long computing time, these approaches give much better image qualities for high-contrast objects than linear inversions such as a diffraction tomography. However, the reconstruction in a large search region with limited-view measurements often fails trapped in a local minimum. To overcome this difficulty, we propose a two-step iterative approach: the first step is to reduce the search region to a smaller one and the second step is the accurate reconstruction of the targets in the small region. Both steps are based on an iterative optimization approach, i.e., the forward-backward time-stepping method previously proposed. This two-step approach is tested for detection of tunnel-like objects surrounded by a heterogeneous background medium to evaluate its performance. Numerical results indicate the efficiency of the approach and its ability of circumventing local minima. Index Terms--Borehole radar, cross-hole measurements, detection of buried objects, inverse scattering, time domain.

A fast algorithm for accelerating the time-marching solution of time-domain integral equations pertinent to the analysis of free-space electromagnetic scattering from perfectly conducting, platelike, and uniformly meshed structures is presented. The matrix-vector multiplications required by the time-marching scheme are accelerated by use of the fast Fourier transform (FFT). This acceleration is achieved in a multilevel fashion by hierarchically grouping sparse interactions to extract denser pieces that are efficiently evaluated by the FFT. The total computational cost and storage requirements of this algorithm scale as O([N.sub.t][N.sub.s] [log.sup.2] [N.sub.s]) and O([N.sup.1.5.sub.s]), respectively, as opposed to O([N.sub.t] [N.sup.2.sub.s]) and O([N.sup.2.sub.s]) for classical time-marching methods ([N.sub.s] and [N.sub.t] denote the total number of spatial unknowns and time steps, respectively). Simulation results demonstrate the accuracy and efficiency of the algorithm. Index Terms--Algorithms, electromagnetic transient scattering, integral equations, time-domain analysis, Volterra integral equations.

This paper reports on a general approach for the time-domain simulation of electromagnetic wave propagation in frequency-dependent nonlinear dielectric materials using transmission-line modeling (TLM). The computational schemes are formulated in detail and results are presented for three classes of nonlinear dielectric materials. Index Terms--Frequency-dependent materials, nonlinear dielectrics, time-domain electromagnetics, transmission-line modeling (TLM).

Modeling a three-winding transformer gives occasionally a negative reactance that may result in an unstable circuit. This unstable condition can be eliminated by neglecting the magnetizing losses or taking them into account in a different way as shown in this paper. Unstable responses are, under certain conditions, extremely dependent on the time step when using EMTP or similar programs. This paper shows why simulated terminal voltages do not always reveal an unstable circuit condition. Index Terms--Modeling, negative inductance, time domain analysis, transformers, unstable circuit.

Printed circuit board transmission lines propagating high-speed digital signals are susceptible to nonideal return paths which may affect signal integrity. In an attempt to clarify this complex issue, this paper presents a study of differential signal integrity issues for the familiar situation in which two coupled microstrip traces are in close proximity to a common reference plane. Differential signaling on coupled microstrip lines and stripline configurations are investigated. Finite-difference time domain simulations are used to demonstrate the impact a nonideal return path can have on differential signal integrity. Index Terms--Differential signaling, finite-difference time domain, nonideal return paths, signal integrity.

A systematic method is presented for determining the optimal pulsewidth and variance of a Gaussian excitation function in the finite difference time domain (FDTD) method. We highlight the interaction of several criteria, such as the stability condition, machine precision limits, the numerical grid cutoff frequency, and the dispersion relation, that play crucial roles in the design of the initial pulse. Optimal Gaussian pulse design is desirable if numerical dispersion, an inherent yet unavoidable property of the standard second-order FDTD Yee algorithm, is to be minimized. A method for determining the phase error of a Gaussian pulse is also presented. Index Terms--finite difference time domain (FDTD), Gaussian excitation, numerical dispersion, phase error.

The forced response of a low aspect-ratio transonic fan due to different inlet distortions was predicted using an integrated time-domain aeroelasticity model A time-accurate, nonlinear viscous, unsteady flow representation was coupled to a linear modal model obtained from a standard finite element formulation. The predictions were checked against the results obtained from a previous experimental program known as 'Augmented Damping of Low-aspect-ratio Fans' (ADLARF). Unsteady blade surface pressures, due to inlet distortions created by screens mounted in the intake inlet duct, were measured along a streamline at 85 percent blade span. Three resonant conditions, namely 1F/3EO, IT & 2F/8EO and 2S/8E0, were considered. Both the amplitude and the phase of the unsteady pressure fluctuations were predicted with and without the blade flexibility. The actual blade displacements and the amount of aerodynamic damping were also computed for the former case. A whole-assembly mesh with about 2,000,000 points was used in some of the computations. Although there were some uncertainties about the aerodynamic boundary conditions, the overall agreement between the experimental and predicted results was found to be reasonably good. The inclusion of the blade motion was shown to have an effect on the unsteady pressure distribution, especially for the 2F/1T case. It was concluded that a full representation of the blade forced response phenomenon should include this feature. [DOI: 10.1115/1.1416151]

In this paper, we present three-dimensional finite-difference time-domain (FDTD) algorithms for transient simulation of electromagnetic-wave propagation in arbitrary inhomogeneous media, which incorporate the perfectly matched layer (PML) absorbing boundary condition. We discuss the choice of constitutive parameters inside the PML layers to match the interior inhomogeneous media for planar interfaces and corner regions. We illustrate the method using both a split-field PML-FDTD formulation in Cartesian coordinates and an anisotropic medium (unsplit) PML-FDTD formulation in cylindrical coordinates. Index Terms--Absorbing boundary condition, finite-difference time domain, inhomogeneous media, perfectly matched layer.

This paper presents a finite difference time domain solution for the electromagnetic fields in ferromagnetic conducting steel pipes of the type used to deliver large currents for in situ heating of heavy oil reservoirs and for in situ environmental decontamination. A method is described whereby a single measured hysteresis loop can be used to deduce the family of hysteresis loops that governs the variable magnetic behavior throughout the pipe wall. Hysteresis and eddy current losses are calculated, and it is shown that hysteresis effects greatly alter the eddy current distribution and can more than triple the total power losses in the steel pipe when compared to the power losses that would be present if hysteresis effects are ignored and magnetic permeability is assumed constant. Index Terms--Eddy currents, finite difference time domain methods, hysteresis modeling, hysteresis nonlinearities, losses, time dependent magnetic fields.