nd the ver- tical plane, the latter being a plane perpendicular to the This is usually the case in urban radio propagation ground, containing the radio terminals and the main ORT problems. Under this assumption, the electromagnetic radio paths. An interesting overview of different kinds of field and its multipath propagation can be expressed in ray models is reported in [Bert02]. In particular an terms of a set of beams or a set of rays, depending on the original 2.5D method called Vertical Plane Launch (VPL) transverse extension of them. Rays have a null transverse is described in the paper. Since rays satisfying GO rules dimension and therefore can in principle describe the belong to folded vertical planes (see Figure 4.7-10), and field with infinite resolution. Beams (tubes of flux) have the projection of these planes onto the ground plane re- a finite transverse dimension because a space discretisa- sults in piecewise linear lines also satisfying GO rules as tion is usually adopted. Gaussian beams, however, have rays in 2D, the basic idea is to perform a 2D RT in the an infinite extension, but still the field is concentrated horizontal plane and then to find the actual 3D paths of around an axis [TaLH02]. If beams are adopted, a limit to the rays in the unfolded vertical planes by analytical space resolution is set. Models adopting rays are usually treatment. VPL is shown to perform well in field strength referred to as RT models. Models adopting beams are predictions when compared with measurements carried usually referred to as BL models or sometimes beam out in Helsinki. In particular the mean error is of a frac- tracking models. In RT algorithms the position of both tion of dB and the standard deviation of the error is of terminals is specified from the beginning at the generic about 4 dB for all measurement routes. Performance is iteration. Then, by means of geometric considerations, all not as good as regards delay spread prediction. the possible rays that reach the RX from the TX A sophisticated ray to