High-frequency / Short Pulse Methods for Wave Propagation

High frequency / short pulse wave propagation modeling and simulation strategies will be reviewed in this presentation invited for the special session honoring Prof. Leopold B. Felsen on his 80 birthday. INTRODUCTION Narrow and broad band wave propagation either natural or man-made environments are of interest for many decades, since the success and quality of any communication depends directly on the propagation characteristics. The applications may range from wireless communication (indoor/outdoor propagation) to satellite communication, from subsurface imaging to radars, waveguides, optical fibers, microstrip networks, etc. The availability of massive computational resources has made it possible to go from the early, idealized, analytically tractable models to the new, sophisticated numerical models with realistic approaches for nearly a decade. This presentation aims to summarize these methods , their current status, and challenges, and to discuss future trends. The focus will be on the propagation scenarios pictured in Fig. 1; propagation through guiding environments with transversely as well as longitudinally varying refractivity, and with varying boundary contours (rural propagation) , and verticaland/or horizontal-plane propagation along a chosen street among the buildings (urban propagation). Figure1: (a) Rural (b) urban radiowave propagation scenarios. The problem is to predict path loss between any specified two points (a) above the 3D, spherical Earth’s surface having lossy, irregular terrain, and above which exists inhomogeneous, time-varying atmosphere layers, from troposphere up to ionosphere and above, (b) among the buildings with imperfect, penetrable boundaries, including edges and tips that cause reflection, refraction, diffraction, surface and traveling waves, and shadowing. Because of the difficulty and impossibility of the inclusion and citation of all of the studies on this subject, here, only a short list with a few classical books [1-5], and papers are given in the references from which one can reach almost all major contributions in this area. MODELING AND SIMULATION STRATEGIES Major modeling and numerical simulation strategies are listed in Fig. 2 [6], which varies from mathematical exact representations for idealized geometries, to pure numerical ones, from purely empirical ones, based on extensive path loss measurements to hybrid techniques that combine two or more methods to extend their range of validity and accuracy. Early efforts for determining field behavior in a guiding environment were to express propagation characteristics in terms of progressing (ray-type) or oscillatory (mode type) constituents, from which rigorous analytical algorithms can be developed. Normal modes (NM) are the solution of strictly-separable wave equations [6]. They are orthonormal wave functions with finite energy and are tagged by distinct longitudinal propagation constants. The mode concept can be extended to weakly non-separable environments with slowly varying longitudinal characteristics. This gives rise to adiabatic modes (AM), D E F Above50 MHz 1 0 0 k m 4 0 0 k m NVI