Urban communications systems in the UHF band, such as cellular mobile radio, depend on propagation between an elevated antenna and antennas located at street level. While extensive measurements of path loss have been reported, no theoretical model has been developed that explains the effect of buildings on the propagation. The development of such a model is given in which the rows or blocks of buildings are viewed as diffracting cylinders lying on the earth. Representing the buildings as absorbing screens, the propagation process reduces to multiple forward diffraction past a series of screens. Numerical computation of the diffraction effect yields a power law dependence for the field that is within the measured range. Accounting for diffraction down to street level from the rooftops gives an overall path loss whose absolute value is in good agreement with average measured path loss.
A vertical-plane-launch (VPL) technique for approximating a full three dimensional (3-D) site-specific ray trace to predict propagation effects in cities for frequencies in the 300-MHz-3-GHz band is described and its predictions are compared with measurements for Rosslyn, VA. The VPL technique employs the standard shoot and bounce method in the horizontal plane while using a deterministic approach to find the vertical displacement of the unfolded ray paths. This approximation is valid since buildings walls are almost always vertical. The VPL method shows significant improvement compared with the slantplane/vertical-plane (SP/VP) method for rooftop antennas. For a base station located at street level, the VPL method gives better predictions than the two-dimensional (2-D) method in locations where propagation over buildings is significant.
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