A Method for Predicting Interchannel Modulation due to Multipath Propagation in FM and PM Tropospheric Radio Systems*

This paper describes a method for predicting the magnitude of interchannel modulation due to multipath propagation on angle-modulated tropospheric scatter radio systems. Values of signal-to-intermodulation ratio, S/I, are calculated for various pairs of signal reflections in the troposphere, taking into account the base bandwidth and frequency deviation of the system, the antenna patterns, the path geometry, and climatic conditions during the “worst month” of propagation. The lowest value of S/I (worst intermodulation) computed for such pairs of signal reflections is then corrected empirically to account for multiple reflections. The result represents the median value of S/I expected during the worst month of transmission on a specified path. The method yields results that, when compared to measured results from four widely different paths normalized to worst-month conditions, have a standard error of estimate of about 2.6 db.