Intermodulation distortion due to fading in frequency-modulation frequency-division multiplex trunk radio systems

Increased intermodulation distortion in frequency-modulation frequency-division multiplex trunk radio systems will be generated under fading conditions when the fade is due to the presence of one or more echoes. The distortion level is not simply related to the fading depth, but depends in a rather complicated way on the parameters of the echoes responsible for the fade. As a guide for future experimental work, theoretical investigation has been made of the nature of this dependence, both for single-echo and for multi-echo fading. In the latter case, a simple formula has been developed for the worst intermodulation distortion due to a system of echoes of given amplitudes and delay times. Numerical results are given for a particular 22.5 dB fade whose echo structure was derived experimentally.2 It is concluded that nothing can be deduced from measurements of fading depth alone about the magnitude of intermodulation distortion associated with fades generated by cancelling echoes. Measurements are required either of the parameters of the echoes or, directly, of the distortion.