Costas D. Sarris

Periodic FDTD analysis of leaky-wave structures and applications to the analysis of negative-refractive-index leaky-wave antennas

The determination of the attenuation constants of periodic leaky-wave structures via the finite-difference time-domain (FDTD) method has been pursued so far via the simulation of a number of unit cells that is large enough to guarantee the convergence of the computed value. On the other hand, Brillouin diagrams of periodic structures can be readily extracted via the simulation of a single unit cell, terminated in periodic boundary conditions. This paper demonstrates a methodology that enables the concurrent extraction of leaky-wave attenuation constants and Brillouin diagrams of periodic structures, through the FDTD simulation of a unit cell. The proposed methodology is first validated and then employed to model leaky-wave radiation from a two-dimensional negative-refractive-index transmission-line (NRI-TL) medium. Apart from evaluating the characteristics of forward and backward leaky-wave radiation from such a medium, a lumped-element macro-model, with element values determined from the FDTD simulation, is extracted. The FDTD analysis, combined with this equivalent circuit, is used to investigate theoretically the possibility of the NRI-TL medium, as a leaky-wave antenna, to achieve continuous scanning from backward to forward end-fire and broadside radiation.

A Perfectly Matched Layer for Subcell FDTD and Applications to the Modeling of Graphene Structures

This letter presents a perfectly matched layer (PML) for the subcell finite-difference time-domain (FDTD) method for thin dispersive layers. Such a PML is useful when thin layers extend to the boundaries of the computational domain. Emphasis is given on the case of graphene thin layers, whose modeling with conventional FDTD produces significant computational cost. The proposed formulation is accompanied by detailed validation and error analysis studies.

Efficient Analysis of Geometrical Uncertainty in the FDTD Method Using Polynomial Chaos With Application to Microwave Circuits

A novel finite-difference time-domain (FDTD)-based method is developed to analyze 3-D microwave circuits with uncertain parameters, such as variability and tolerances in the physical dimensions and geometry introduced by manufacturing processes. The proposed method incorporates geometrical variation into the FDTD algorithm by appropriately parameterizing and distorting the rectilinear and curvilinear computational lattices. Generalized polynomial chaos is used to expand the time-domain electric and magnetic fields in terms of orthogonal polynomial chaos basis functions of the uncertain mesh parameters. The technique is validated by modeling several microstrip circuits with uncertain physical dimensions and geometry. The computed S-parameters are compared against Monte Carlo simulations, and good agreement for the statistics is observed over 0-25 GHz. A considerable computational advantage over the Monte Carlo method is also achieved.

Fundamental gridding-related dispersion effects in multiresolution time-domain schemes

The effect of electric and magnetic node arrangement on the dispersion characteristics of the multiresolution time domain technique is investigated in this paper. It is first noted that by multiresolution analysis principles, introducing one wavelet level refines the resolution of a numerical scheme based on scaling functions only, by a factor of two. However, the dispersion analysis of recently formulated MRTD schemes shows that this is not always the case. The apparent contradiction is resolved by indicating that MRTD does achieve its predicted dispersion performance under certain meshing conditions that are outlined here.

Rigorous and Efficient Time-Domain Modeling of Electromagnetic Wave Propagation and Fading Statistics in Indoor Wireless Channels

Conventional numerical electromagnetic methods are known to provide accurate means of characterizing wireless channel transfer functions. However, their practical utilization is hampered by their typically large computational cost compared to empirical, measurement-based or ray-tracing techniques. In this paper, a full-wave, time-domain technique, stemming from the spatial expansion of electromagnetic field components in smooth, spline-type basis functions, is shown to provide a rigorous, yet efficient tool for site-specific indoor channel modeling. Based on this method, wireless propagation across indoor channel geometries can be accurately characterized and signal fading statistics can be extracted. Numerical examples, indicating the significantly improved efficiency of the proposed approach, compared to the standard finite-difference time-domain method, are given. Moreover, important wave propagation effects on indoor channel performance, readily accounted for by our full-wave analysis, are demonstrated.

Convex Optimization of Wireless Power Transfer Systems With Multiple Transmitters

Wireless power transfer systems with multiple transmitters promise advantages of higher transfer efficiencies and focusing effects over single-transmitter systems. From the standard formulation, straightforward maximization of the power transfer efficiency is not trivial. By reformulating the problem, a convex optimization problem emerges, which can be solved efficiently. Further, using Lagrangian duality theory, analytical results are found for the achievable maximum power transfer efficiency and all parameters involved. With these closed-form results, planar and coaxial wireless power transfer setups are investigated.

Fundamental gridding related dispersion effects in multiresolution time domain schemes

The effect of electric and magnetic node arrangement on the dispersion characteristics of the multiresolution time domain technique is investigated in this paper. It is first noted that by multiresolution analysis principles, introducing one wavelet level refines the resolution of a numerical scheme based on scaling functions only, by a factor of two. However, the dispersion analysis of recently formulated MRTD schemes shows that this is not always the case. The apparent contradiction is resolved by indicating that MRTD does achieve its predicted dispersion performance under certain meshing conditions that are outlined here.

A Spatially Filtered Finite-Difference Time-Domain Scheme With Controllable Stability Beyond the CFL Limit: Theory and Applications

A 3-D finite-difference time-domain (FDTD) scheme with controllable stability beyond the stability limit of FDTD is presented. The key to controlling the FDTD stability limit is a spatial filter applied to the electromagnetic field vectors in order to ensure that unstable spatial harmonics are eliminated. A significant advantage of this approach is the simplicity of its incorporation into existing FDTD codes. Moreover, this method is employed as a means to implement a late-time stable 3-D sub-gridding scheme. Applications include waveguide filters and the analysis of screens for near-field sub-wavelength focusing at microwave frequencies.

Efficient Evaluation of the Terminal Response of a Twisted-Wire Pair Excited by a Plane-Wave Electromagnetic Field

Twisted-wire pair (TWP) loops are used as one of the primary communication channels in digital subscriber line (DSL) networks. As part of the development of channel noise models, a transmission-line-based model for predicting the terminal response of a single TWP to an illuminating plane-wave electromagnetic field is presented. Closed-form analytic approximations of the terminal response valid for an arbitrary direction of incidence and polarization of the incoming plane-wave field are provided. A worst-case model with the very same purpose has been previously proposed in the DSL literature; however, a worst-case approach only provides partial information about the field coupling mechanism. The goal of the presented transmission-line model is to introduce a more robust yet efficient approach for characterizing radio noise interference in DSL systems. Computed results for two TWP configurations of interest are presented together with measured results for one of them.

Ray-tracing based modeling of ultra-wideband pulse propagation in railway tunnels

Ray tracing based on geometric optics can be utilized for generating propagation models for arbitrary and complex environments. These methods can be employed to determine important wireless channel characteristics such as coherence and delay spread. In this paper, an image theory based ray tracing method is used to study ultra-wideband propagation in complex tunnel environments such as curved tunnels and bifurcations. To validate the model, simulation results were compared to the experimental measurements performed in a hallway of an office building.