Rain-Scatter Interference in Terrestrial Microwave Systems

The potential cochannel interference between two stations, illustrated on Fig. la, is usually assumed to occur along the shortest path (dotted line) between stations. The transmission loss along the dotted path includes not only the free-space loss, but also any additional loss caused by obstructions or by site shielding. Transmission is further modified by the antenna patterns in the direction toward the other site. In addition, reflections from nearby trees and terrain may reduce the expected suppression of the antenna in directions away from the main beam. If the two antenna beams intercept, any reflection from rain, aircraft, or birds in or near their common volume can provide another path by which interference can occur. While the magnitude of such reflections is normally small, this "sneak" path has free-space transmission with full antenna gain at one end and, thus, bypasses the suppression that is normally provided by the antenna patterns and shielding losses.