ront. The variations of the local tangential is implemented in a 3D RT model and results are com- planes are directly related to the statistical behaviour of the pared with measurements. According to the ER approach roughness of the corresponding surface. These tangential diffuse scattering, although mainly due to surface and planes are 'local' in the sense that for each discrete ray volume irregularities (windows, balconies, indentations, a different tangential plane is generated. decorative elements, rain-pipes, internal reinforcements, The new technique is compared to the analytical power lines, heating pipes, etc.), is assumed originating Kirchhoff model in Figure 4.7-15. A relatively rough from surface roughness scattering. An effective roughness surface with a standard deviation of the surface height is associated to each wall in order to take into account not oh = 0.710, correlation length L = 410, relative permit- only real surface roughness but also the mean effects of tivity Er = (10-j3) and relative permeability Mr = 1, of the above-mentioned elements. A Lambertian scattering size Lx = Ly = 820 is considered in this example. For the pattern is ascribed to each wall (if the wall is far enough ray-optical stochastic scattering approach, the isotropic from TX and RX) or to each surface element (if the wall source is situated at Ai = 60° and = 0°, at a distance of is close). The scattering contribution of each wall is ri = 200020 from the plane. The rays impinge separated by computed directly from wall distance and orientation 20/10 in the x-and y-direction on the surface, resulting in with respect to the TX and the RX using simple, analytic N = 81 X 81 = 6561 rays per realisation. The resulting formulas derived from power balance considerations, field is determined in the upper hemisphere at a distance which depend on only one parameter, the scattering pa- rs = 200020 from the centre of the plane, corresponding rameter S. S is defined as the ratio between the ampli- to approximately eight times the far-fi