A Method for Predicting Interchannel Modulation due to Multipath Propagation in FM and PM Tropospheric Radio Systems
01 January 1963
8 9 9 10 11 13 13 17 18 19 24 26 28 29 29 29 30 30 32 33 34 35 Various theories have been proposed to explain the mechanism by which energy radiated from a transmitting antenna is reflected or refracted in the atmosphere and ultimately arrives at a receiving antenna beyond the horizon.1 2 3 Apart from the merits of any particular theory of propagation, the geometry of a typical over-the-horizon, or "tropospherie scatter," path suggests an inherent multipath transmission problem. As illustrated by Fig. 1, components of signal reflected to the receiving antenna from successively higher points in the atmosphere are delayed by increasing amounts behind the earliest arriving component. When such time-delayed RF signals -- called echoes henceforth -- are demodulated by an FM or PM receiver, they are converted into energy at undesired regions of the baseband. In a frequency-division multiplexed, multi-channel voice system, this undesired energy is commonly called "interchannel modulation," or simply "intermodulation." It adds to other distortion energy generated by equipment nonlinearities and to the ever-present white noise, to produce the total background of noise faced by the desired speech signal. Whether or not "path intermodulation" noise is objectionable in a given system depends on its magnitude relative to these other components of noise. The general theory of interchannel modulation due to transmission echoes in angle-modulated systems has been treated by Bennett, Curtis,