| Many urban communication systems in the UHF and microwave bands, such as cellular radio, depend on radio propagation between an elevated antenna and low antennas located at or below roof-top level. The development of a theoretical model that explains the effect of buildings on the propagation has been the subject of extensive investigation.; In cities, buildings are adjacent to each other or have only narrow passages between them, so that propagation must take place over the roof tops. The buildings, therefore, act as connected rows or blocks that can be treated as cylindrical diffracting obstacles. For simplicity, each row is approximated by a thin absorbing half-screen, so that propagation over the roofs is modeled as a process of multiple diffraction past a series of parallel half-screens.; In our studies diffraction by as many as about 150 half-screens has been considered. A Kirchoff-huygens integration is used to find the field incident on the plane of one half-screen due to the field illuminating previous half-screen, and the process is repeated in sequence for all screens. Choice of the integration aperture and truncation strategy are discussed.; For low glancing angles, in the range of interest for communication systems, it is found that the field at the roof tops varies as the glancing angle to the 0.9 power, in addition to the l/R dependence appropriate to free space propagation. Path loss predictions are in good agreement with the extensive measurements of Okumura, except for high fixed antennas.; The theoretical model shows how the path loss depends on frequency, as well as building and street geometry. The model also gives the size of the region in front of the mobile radio that was a significant influence on the received signal. Knowledge of this significant foreground permits the definition of a local slope in hilly terrain that can be used to predict the received signal. |
