Skip to main content

Intermodulation Distortion in Analog FM Troposcatter Systems

01 January 1964

New Image

An analysis has been made elsewhere1 of error probabilities in highspeed digital transmission over idealized troposcatter paths, considering both random noise and intersymbol interference owing to pulse distortion caused by selective fading. The above analysis indicated that a principal cause of intersymbol interference is a quadratic component of phase distortion, or linear delay distortion. On the same basic premise an evaluation is made herein of intermodulation noise in analog transmission by frequency modulation, as now used for transmission of voice channels in frequency division multiplex. Expressions and curves are given of intermodulation noise in an idealized troposcatter channel for a signal with the properties of random noise, as related to certain basic system parameters and comparisons are made with the results of measurements on four troposcatter systems.2-3 In random multipath transmission the received wave can be considered the sum of a plurality of echoes, arriving over the various paths with varying amplitudes and different delays. Although this view is conceptually simple, it does not facilitate analysis of the statistical properties of the received signal and of signal distortion. In the combination of a number of time functions, such as echoes, the analysis is greatly facilitated by the use of Fourier transformation to determine the corresponding spectra. The latter can in turn be combined directly with appropriate attention to phase relations to obtain the resultant wave.