 the changes in initial plane and adequate boundary conditions on the the evaporation duct thickness on two links at 2.5 and 5.8 scattering objects and at the outer boundaries of the in- GHz. Examples of range dependent ducting (corre- tegration domain are given, it is easy to proceed (march) sponding to refractivity profiles along a mixed land-sea in range in the propagation direction. Here, no heavy path) are also reported. For this last case between 5 and integrals and matrices are involved. This explains the 10 dB variation in the losses can be accounted for. In both success of this method for propagation purposes, spe- above-mentioned papers a smooth perfectly conducting cially if the equation is reduced to its 2D version. It is underlying surface is assumed. The influence of the ter- very accurate at angles within 15° of this propagation rain is introduced in [SiMi04] and [SiMi05]. Assessment direction. Several contributions apply the 2D version of the combined effect of terrain and ducting on UMTS of this method to ducting phenomena on microwave path losses at UMTS frequencies (2.1674 GHz) is propagation. Indeed, ducting is present in 15% of the reported in [SiMi04]. Ducting can help to increase signal time all over the world, specially over humid environ- in valleys. This is seen when comparing the standard case, ments like coastal zones, and hence being able to Figure 4.7-5, with the ducting case, Figure 4.7-6. Relying compute the propagation in those circumstances is im- on the precision of the PE and its ability to provide portant. In [SiMi03] it is shown that surface-based ducts quantitative assessment of the losses, in [SiMi05] the affect the propagation in short distances (up to 3 km) by importance of possible inaccuracies in the terrain profile provoking the shift of the location of the interference data for the Sofia region is discussed. The influence of the Range km Figure 4.7-6 Path loss under a 100 m ducting layer between 450 and 550 m. Propagation modelling and channel characterisation CHAPTER