A measurement campaign has been carried out in the Berlin subway to characterize electromagnetic wave propagation in underground railroad tunnels. The received power levels at 945 and 1853.4 MHz are used to evaluate the attenuation and the fading characteristics in a curved arched-shaped tunnel. The measurements are compared to ray-optical modeling results, which are based on ray density normalization. It is shown that the geometry of a tunnel, especially the cross-sectional shape and the course, is of major impact on the propagation behavior and thus on the accuracy of the modeling, while the material parameters of the building materials have less impact.
A new method to include stochastic rough surface scattering into deterministic ray-optical wave propagation modeling is derived. It can be utilized in conjunction with the concept of ray launching. Similar to the Kirchhoff formulations, the approach is based on a tangential plane approximation, i.e., it is applicable to surfaces with gentle undulations, whose horizontal dimensions are large compared to the incident wavelength. However, in contrast to the Kirchhoff models, which are only valid for either slightly rough or very rough surfaces, the proposed stochastic scattering approach includes both the coherent and incoherent components at the same time. The purely deterministic ray-based modeling is expanded by a "stochastic" component, allowing for the first time to account for nondeterministic scattering in ray-optical wave propagation modeling.
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