Consideration of Topography

The terrain profile should be considered for propagation modeling when the considered urban area is not flat. Include terrain information in preprocessing and prediction.

The criterion taken into account is the standard deviation of the terrain heights in comparison to the standard deviation of the building heights. For large standard deviations of the terrain heights databases in pixel format are required with resolutions of 20-30 m. This is a higher resolution compared to terrain models.

To reduce the additional computational effort the terrain information is included in preprocessing and prediction. At the preprocessing, all buildings are shifted in the Z direction according to the terrain height at their centers of gravity. With this approach, the shapes of the buildings remain constant while the arrangement of the buildings among each other is changed. If the intelligent ray-tracing is used, the topography also has to be considered for the determination of the visibility relations between the elements of the building database. Therefore the shadowing by hills has to be evaluated for all visibility relations. Additional ray paths because of the topography (for example, diffraction from the ground) are not taken into account as their influence can be neglected in urban environments.

For the prediction again the shifted buildings have to be used to determine the elements (tiles, segments, and receiver points) that have line-of-sight to the transmitter. Because of the topography, it is possible that the line-of-sight area seen from the transmitter may have changed considerably.

Figure 1. Topographical extension to the COST-Walfisch-Ikegami model.

The topography has a significant influence on the wave propagation from the transmitting to the receiving antenna. Therefore a topographical extension of the empirical COST-Walfisch-Ikegami model is necessary. This can be done by calculating the empirical parameters (mean value of building heights, the height of transmitter, and height of receiver) according to a new reference level which is determined by the footprints of transmitter and receiver (see Figure 1). As a result of the formulas of the COST-Walfisch-Ikegami model implicitly consider the terrain profile for the prediction.