Intermodulation Noise in FM-FDM Radio Link on Radio Sections with Direct Sight as Result of Interference Fading . Intermodulationsgeraeusche im FM-FDM-Richtfunk Auf Funkfeldern MIT Direkter Sicht als Folge von Interferenzschwund

摘要

The intermodulation noise on direct sight sections in FM-FDM radio relay systems is calculated as a result of interference fading produced by two-path propagation. It is assumed that the group delay difference between both waves is very small, approximately 1 nanosec. but that the amplitude ratio of both waves is near to 1. It is shown that strong intermodulation noise only appears simultaneously with fading breaks of more than 20 db. A relationship exists between the maximum possible intermodulation noise and the fading depth, with which the statistical distribution of the overall noise can be calculated as the sum of thermal and intermodulation noise, when the transmission attenuation is known. Because of the coupling of intermodulation noise and fading, the fading dependent intermodulation noise can be reduced by switching diversity. This makes it possible to comply with CCIR conditions.