Métodos assintóticos para predição banda larga da cobertura radioelétrica em ambientes urbanos

Abstract

This work deals with radio-propagation prediction in urban environments by means of the Uniform Theory of Diffraction (UTD), both in frequency and time domains. The UTD is an asymptotic technique that uses the optical paths between the transmitter and receiver to estimate the electromagnetic-field propagation. The time domain version of the UTD uses these optical paths to provide the early-time solution of the scattered field in time domain. The optical paths of the reflected and diffracted fields are determined by a ray tracing algorithm based on the Image Theory (IT) on a three-dimensional environment. The algorithm uses several accelerating techniques, as the determination of the visible facets from a source; environment division, that optimizes the several ray intersection tests; and the consideration of multiple reflections and diffractions by separating the optical paths into several ray classes. The consideration of three-dimensional environments is made by the conversion of the 2D ray paths into 3D ones, using the total distance of the trajectory and the obstacles heights. In the evaluation of the scattered field, perfect electric conductor and finite conductivity surfaces are considered both in the time and frequency domains. In frequency domain, losses are treated in the UTD formulation by the definition of new heuristic diffraction coefficients. In time domain, losses are treated in an approximate manner by the application of a time transform on asymptotic expansions of the Fresnel reflection coefficients. In the frequency domain, the results obtained by the UTD were compared with the ones obtained by the Moment Method and through measures presented in the literature. For the time domain, the proposed formulation was compared with the application of an inverse Fourier transform in the frequency domain UTD results.