Three radio pilot tones, available on in-service AR6A single sideband radios, have been used to characterize the dispersive nature of radio fading. The paper presents new results on how three tones can be used to determine the delay and amplitude of the second ray and the notch frequency in a two-ray fading model. The model chosen is justified on physical and practical grounds. Measurements on radio hops yield histograms of second ray delay and amplitude. It is found that most of the fading events can be described by the two-ray model. The measurements are compared to distributions, assumed or measured, given in the literature. The experimental technique used lends itself easily to the measurement of other propagation characteristics like the cumulative statistics of single and average tone power and the in-band power difference.

This paper summarizes experimental work at Bell Laboratories in the field of multipath propagation as it affects digital radio. Field and laboratory measurements are presented on commercial-quality high-speed digital radio systems working in the 4, 6, and 11 GHz common carrier bands. Circuits and equipment used in these systems are all state of the art, but no attempt has been made to describe them in any detail. Modulation types are offset-4 PSK, conventional 8 PSK, and 16 QAM. Outages(BER > 10^{-3})vastly exceeding accepted objectives are measured on unprotected radio hops. A very effective method for reducing outages has been the use of a space diversity combiner followed by an amplitude slope equalizer. Results from single, as well as dual-polarized digital radio relay systems, are presented. The paper also describes the laboratory measurement of an equipment "signature" as a means to compare different digital radio designs in their sensitivity to multipath fading.