An Improved High-Frequency Radiotelephone System Featuring Constant Net Loss Operation

The first transatlantic telephone cable was laid in 1956. For almost, thirty years prior to that time, transoceanic telephone communication was provided almost exclusively via high-frequency radiotelephone circuits. Numerous submarine cables have since been laid, and HF radio is no longer used for telephone service over routes of heaviest traffic, e.g., New York-London, where large numbers of cable and satellite circuits exist. Many long transoceanic routes, however, are 677 678 TIIE BELL SYSTEM TECHNICAL J O U R N A L , APRIL 1967 not served by submarine cables or satellites; direct telephone service between the United States and forty foreign countries and areas (see Fig. 1) is still provided by means of HF radio facilities exclusively. Although a change in the composition of transoceanic telephone facilities is expected as satellites become more fully utilized, it is likely that the HF services will hold their own for economic reasons, particularly for small circuit groups. For some time the HF radio spectrum, a rather limited resource to begin with, has been nearing the point of saturation. Thus, as new submarine cable and land* facilities make it possible to suspend existing HF routes, the frequencies are reassigned to provide service to new areas, typically to the developing nations of the world. Thus, the total number of HF circuits is expected to remain about the same well into the communication satellite era. Since high frequencies propagate via the ionosphere, which is in a continual state of change, HF transmission is highly variable and requires special equipment and procedures to cope with the problems created by this variability.