ing from building tops and upper parts, which are both with S = 0.4 and with S = 0.6. On the contrary, often directly illuminated by the BS, plays a key role in results without scattering obtained with an ordinary determining the RtS contribution. 2D RT model are unacceptable. Note that 2D RT results do not include the first peak, which must of course be computed using an ORT model. Therefore, using a tra- UWB propagation modelling by RT ditional 2D RT- ORT model the overall received power UWB is a very promising technique for high-data rate, would indeed be predicted with a good accuracy while short range transmission and is claimed to permit very wideband parameters such as DS would come out heavily low power consumption and therefore very high battery underestimated due to the strong underestimation of the duration in portable terminals. In [LoGC05] a 3D RT 'tail' of the PDP. Similar results but relative to a more model is applied to the modelling of UWB propagation complete characterisation in terms of both delay spread in an indoor environment. The application of ray models and angle spread of the multipath channel in the city of to UWB propagation prediction is quite straightforward. Helsinki are reported in [DEESGK02a] (see Section Since the geometry of the multipath pattern does not 4.7.5). depend on frequency, the rays are generated by the RT An interesting study on the role of scattering in ORT algorithm in a similar manner to the narrowband case. propagation is reported in [DeFG03]. In dense urban On the contrary, the amplitude and the phase/delay of environment, where the average building height is each ray do depend on frequency because of the de- comparable to or even greater than street width, multi- pendence on frequency of reflection and diffraction co- screen diffraction only partially accounts for the overall efficients and of the emission/reception characteristics ORT propagation mechanism. In particular, the last part of the antennas. A UWB time-domain-pulse emitted by of the ORT path, the so