A single-frequency communications system

In those radio communications applications where area coverage is not desired, a series of repeaters, each operating at low power, can be used to extend the radio over long distances. Microwave relay systems operate on this principle with each node representing a point of potential local distribution. Where it is desirable to have the nodes closely spaced, such as along a highway, it becomes desirable to utilize a single frequency for all transmissions in one direction in the interest of conserving spectrum. This paper covers such a system operating in the 935-MHz band. The system described provides both the "backbone" communications system which employs single-frequency repeaters and the "roadside" communications from the repeater node points to users in their vehicles. The functioning of the repeaters is described, and the relationship between signal levels, isolation, external noise levels, and thresholds is presented. Both analog and digital modulations are employed, and it is shown how each repeater is remotely switched between "backbone" and "roadside" communications modes. Extensions from current single-channel to multiple-channel operation is described in terms of the application to a variety of highway-related uses. The results of propagation experiments both along the "backbone" and on the "roadside" are described and related to theoretical models. From these data, conclusions can be drawn about the relationship between repeater spacing, height, and power level, as well as their relationship to user antenna height, output power, and receiver sensitivity.