Interstitial Channels for Doubling TD-2 Radio System Capacity

The TD-2 microwave radio relay system at the present time provides six broadband channels in each direction of transmission in the f requency band between 3700 and ^200 mc, each channel being capable of transmitting several hundred voice circuits. This paper describes arrangements and some of the technical problems involved whereby the number of broadband channels can be doubled with no significant penalty to the existing channels. I. USE AND GROWTH OF TD-2 SYSTEMS TD-2 routes were installed at a rapid rate after the first TD-2 system 1 was placed in service between New York and Chicago in September 1950. During the following eight years there was a large demand for additional intercity television, message and private line circuits to meet the expanding business requirements. Because the TD-2 system gives excellent transmission performance and also has economic advantages over other systems, especially where new routes are needed, a large part of the additional Bell System intercity facilities provided during the eight-year period consisted of TD-2 channels. Long video network circuits were obtained almost entirely from TD-2 channels, and side connections to the backbone video networks were usually TD-2 facilities. These channels were also widely used to obtain intercity voice and private line circuits. By using the type L carrier terminal equipment, up to 600 voice circuits were assigned to one TD-2 radio channel. This technique uses baseband frequencies only up to about 3 mc for the G O voice circuits, although the baseband capability O of the TD-2 channel is considerably higher.