and required in output treat reflection and transmission on plane surfaces and prediction is usually not SO high as to justify the adoption diffraction on rectilinear edges (see Section 4.7.2). GO of sophisticated numerical methods. Electromagnetic theory is based on the so-called ray approximation, which methods require a detailed description of both the ge- is valid when the wavelength is sufficiently small com- ometry and the electromagnetic parameters of the pared to the dimensions of the environment obstacles. water water water (a) Complete map (183 buildings). (b) Simplified map (118 buildings). (c) Guessed map (69 buildings). Figure 4.7-8 Central Helsinki maps; black dots = radio terminals. Propagation modelling and channel characterisation CHAPTER decorrelation, fast fading, etc.) can in principle be repro- duced by ray models. However, due to limitations in en- vironment description and/or in the modelling of the propagation process, multidimensional performances of today's models still need to be improved [Bert02]. High computation time, which depends nearly expo- nentially on both input database size and prediction order, represents the main drawback of ray models. Therefore a great effort has been made in trying to speed up ray models either reducing the dimension of the problem from 3D to 2D or 2.5D, or somehow limiting ray/beam tracing to a subset of dominant rays. In 2D models, propagation is assumed to take place on a hori- zontal plane almost parallel to ground and containing the Frequency in MHz radio terminals. This is strictly true, however, only if the radio terminals are located at the same height and if Figure 4.7-9 Comparison of the measured and simulated room the Over Roof Top (ORT) paths are negligible. In 2.5D attenuations. Drawn through: measurement, dashed: simulation. Squares: MRTo room, circles: operating room, x-marks: X-ray models instead of performing a full 3D prediction, RT room. is on two or more two-dimensional planes. Usually two planes are considered: the horizontal plane a