which thanks Figure 4.7-12 RMS delay spread: comparison between simula- to their adherence to the actual physical process can be tions and measurements. used also to model and study multipath propagation. In [TaLH02] an interesting method based on a Gaussian BL technique is presented as an alternative to RT for 3D surprisingly, simulations seem to underestimate channel physical modelling of multipath propagation. The dispersion, but the relative difference between measured and simulated results, which is minimum for the LoS case method is applied to the indoor case. By adopting the Gabor frame-based decomposition of source fields it is (RX1), is coherent with previously published results in possible to represent radiated fields as a superposition of similar environments and frequency range. elementary Gaussian functions which are translated in the spatial and angular-spectral domains. Gaussian beam 4.7.3.5 Advanced techniques for ray tracking through multiple reflections and transmissions is models performed in a similar way to conventional BL methods. Fields can then be evaluated by summation of analytical As highlighted in previous subsections, ray models are terms representing transformed Gaussian beams. Dif- generally characterised by a high computation time, es- fraction is not taken into account, but due to the paraxial pecially when large input databases are needed, as for the concentration of the field in a Gaussian beam, diffraction case of macrocellular urban environments. Moreover, it is should have less influence with respect to the case of known that conventional ray models are not able to fully a plane or spherical wave. It must be noted that the pa- reproduce a correct space and angle distribution of rays, rameters of a given transformed Gaussian beam have to with a consequent detriment to wideband and multidi- be calculated only once for all observation points: com- mensional prediction performance. Besides the studies putation time is therefore independent of the number of described in previous s