Simulation in Urban Environment of a 3d Ray Tracing Propagation Model Based on Building Database Preprocessing

The analysis and simulation of electromagnetic (EM) wave propagation represent a crucial task in planning cellular networks for mobile communication. When an adequate building database is available, deterministic 3D ray tracing techniques are quite useful and accurate to face this problem, but since their computational complexity is time consuming, a preprocessing step is necessary. Such a step allows the practical use of 3D ray tracing simulations so as to achieve the same accuracy of the ray optical approach in short simulation time. The preprocessing phase is independent on the position of the transmitter’s antenna, consequently it can be run once and for all, independently on EM field strength simulations. This strategy makes the technique practical and useful in common cellular network design, such as UMTS cellular planning. During the preprocessing phase, the mutual visibility between the walls and edges of the buildings are pre-evaluated and stored: the mutual visibility is described by a so called “visibility tree”. In this study a new simulation software is presented. It uses a new technique of preprocessing, which is able to reduce the visibility tree building time of an order of magnitude or more (if compared with a “brute force” approach). In order to evaluate reflections and diffractions, the walls of buildings need to be splitted in smaller faces whose typical size should be about 10 wavelengths. Since the number of visibility relationships increases with the square of the number of faces, the splitting phase makes the time necessary to build the visibility tree unacceptably long. This problem was solved through the following two steps: first, the visibility among the main (large) faces is evaluated; then the obtained raw tree is used to create the full and accurate visibility tree. The output of such a technique is the same as a “brute force” approach, but the computational time needed is one order of magnitude smaller. The final EM prediction can take into account any number of reflections and diffractions: Geometric Optics allows to evaluate specular reflections while the Uniform Theory of Diffraction is used to estimate multiple diffractions from vertical edges and roof-tops. Because of unavoidable uncertainties affecting building positioning and dimension in the database (root mean square error of ~6-7 wavelengths at 2 GHz), phase estimate cannot be accurate. Consequently, reflection and diffraction contributions are added up using a MonteCarlo technique, so as to estimate at least the expected value of field strength.