R. J. Marhefka's research while affiliated with The Ohio State University and other places

A high-frequency asymptotic technique based on the Uniform Geometric Theory of Diffraction (UTD) is employed for building interior imaging. The analysis is implemented using a ray-tracing technique to account for multiple scattering interactions in a building, along with a set of heuristic diffraction coefficients for dielectric wedges and corners. Imaging of the synthetic aperture radar data is carried out by the conventional fast Fourier transform method to transform to the downrange domain, and combined with a coherent near-zone imaging function for cross-range. Comparisons with experimental data for a scaled-down building model are given to demonstrate the suitability and efficacy of our analysis for through-wall building imaging. The UTD ray mechanisms account for the dominant scattering features observed in the image.

Through-wall radar imaging is a challenging area of research due to the complex multi-layer and inhomogeneous structure of building walls. The wall distorts and attenuates the radar signal in a way that is not easy to predict, except in the most simple of cases. In this paper a general periodic model is developed and applied to the imaging algorithm. Periodic dielectric wall models for microwave transmission and reflection have been investigated by several researchers [1-5]. Floquet mode theory [6] is used to find the discrete plane wave directions that are uniquely determined by the period. Below a certain frequency there are no propagating Floquet modes, so only the specularly transmitted and reflected plane waves are present. In this case it is often possible to use an equivalent 3-layer homogeneous model as described in [4]. This makes it much easier to use a ray tracing code such as the NEC-Basic Scattering Code (NEC-BSC) [7] because the Fresnel plane wave reflection and transmission coefficients may be applied directly. Here we compare the 3-layer model with a volume periodic moment method solution. The goal is to generate model-based images using both approaches to see the effect of the periodicity on the image.

Radar imaging studies of full-scale buildings present serious electromagnetic modeling challenges. The large electrical size makes numerical methods impractical, and the complexity makes ray methods difficult to apply. In this paper we adapt the NEC-basic scattering code (NEC-BSC) [1] to generate simulated radar scattering data for building imaging purposes. The NEC-BSC is based on the uniform geometrical theory of diffraction (UTD) ray tracing method [2,3]. The NEC-BSC combines geometrical optics (GO) ray tracing with the edge and corner diffraction contributions of UTD by constructing models build up of basic canonical shapes, such as plates, cylinders, ellipsoids, cone frustums and wires. For buildings we are primarily interested in models made from multi-layered dielectric plates which represent walls, floors and ceilings. The code traces rays through multiple reflections and diffractions to include the dominant building scattering mechanisms shown in Figure 1, such as dihedrals, trihedrals, direct reflection, and edge and corner diffraction. Higher order combinations of reflections and diffractions are also included, as well as transmission through dielectric plates.

Using classical array theory this paper presents directivity, sidelobe, and bandwidth performance analysis over narrow and wide bandwidths in the microwave frequency region for several large cylindrical array configurations comprised of panel subarrays. The degree of main beam spreading, grating lobe levels, and directivity loss is presented for several designs involving trade-offs between true time delay beam scanning and phase shifter beam scanning. It is also shown that the grating lobes may be reduced with an amplitude taper across the array.

This paper demonstrates a ray-tracing method for modeling indoor propagation channels at 60 GHz. A validation of the ray-tracing model with our in-house measurement is also presented. Based on the validated model, the multipath channel parameter such as root mean square (RMS) delay spread and the fading statistics at millimeter wave frequencies are easily extracted. As such, the proposed ray-tracing method can provide vital information pertaining to the fading condition in a site-specific indoor environment.

Through-wall radar imaging has received much attention recently for discerning objects inside of a building. Most conventional imaging approaches are based on Fourier spectral transforms and/or far-field approximations, such as inverse synthetic aperture radar (ISAR) and tomographic imaging. These methods assume a simple point-scattering model. On the other hand, model-based imaging is a more general approach that may utilize a more appropriate scattering model based on the physics of the scenario of interest. In this paper model-based imaging is applied to through-wall imaging with a near-field sensor. The imaging function is presented in a very general form which allows arbitrary sensor movement for collecting the scattering data. A numerical simulation is presented of a drive-by radar scan of a small 1-room building.

Hybrid combinations of numerical and asymptotic methods are utilized to evaluate in-situ antenna performance, and coupling to other systems on a shared platform such as a ship topside. This paper describes a combination of the finite element-boundary (FE-BI) method with ray techniques for evaluating antenna patterns in the presence of complex platforms. Specifically, a very complex array antenna may be modeled with FE-BI, and interfaced to the platform via the use of equivalent currents. For the case considered here, the FE-BI is accelerated with the array decomposition fast multipole method (AD-FMM) so that large arrays may be considered. A novel discrete Fourier transform method is also introduced to provide a greatly reduced representation of the fields over a planar array aperture and the uniform theory of diffraction (UTD) along with iterative physical optics (IPO) are used to characterize the platform. To tie it all together, a matrix framework is formulated to iteratively increment the higher order interactions between antennas and platform.

In this paper, based on a 3D ray-tracing analysis, statistical parameters (i.e. mean excess delay and rms delay spread) for site-specific indoor environments were extracted. The fading statistics of these indoor environments were shown to obey a Weibull distribution. As is well-known, fading statistics are necessary for predicting channel capacity limit and this were presented in the conference

The modeling of complex environments can be very challenging. The uniform geometrical theory of diffraction (UTD) has proven to be very useful in this endeavor especially for large scalable geometries in terms of a wavelength. Most of the time including only a few ray types produces very good engineering results. However, in some cases the need for higher order terms becomes necessary. For example, double diffraction can become important near box like objects which can occur in vehicle or indoor and outdoor propagation situations. Tiberio et al. have recently developed improvements leading to a general uniform theoretical solution near shadow boundaries. This paper discusses implementation of enhancements related to multiple interactions with surrounding structures, i.e., through wall propagation