Transmission Unavailability of Frequency-Diversity Protected Microwave FM Radio Systems Caused by Multipath Fading

Transmission Unavailability of FrequencyDiversity Protected Microwave FM Radio Systems Caused by Multipath Fading By A. VIGANTS and M. V. PURSLEY (Manuscript received April 2, 1979) Estimates of transmission unavailability caused by multipath fading are needed to determine the adequacy of diversity protection arrangements. A computer program producing such estimates was made available for general Bell System use in 1977. In this paper, we summarize the underlying mathematical model of frequency diversity operation and atmospheric multipath fading and use it to study frequency diversity behavior. I. INTRODUCTION Microwave FM radio systems utilized in the Bell System provide protected broadband communications channels that generally have a capacity, depending on the frequency band and on the type of equipment, of 1200 to 2400 message circuits per channel. The protected channels, with protection provided by radio switching systems, are referred to as "working" channels, to distinguish these from the actual radio channels, which are assigned as either "regular" or "protection" channels. The number of regular channels in a radio system equals the number of working channels. Equipment failures, atmospheric fading, or other impairments in a regular channel cause temporary transfers of the communications traffic to a protection channel at a different radio frequency, if an unoccupied and unimpaired protection channel is available. In the case of fading, such frequency-diversity protection switching capitalizes on the frequency selectivity of the multipath fading process, where, at an instant in time, the strength of the received signal is a function of frequency.