Effects of Multipath Propagation on Digital Radio

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 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.