Band Width and Transmission Performance

B ARRIER systems for the transmission of many telephone channels on a single metallic circuit have grown to be very important in the telephone network. Since the development of the coaxial cable system in which 480 channels are transmitted in a 2-mc baseband, advances in high frequency techniques, including the war-accelerated microwave art, have inspired efforts to utilize the broad band capabilities of high transmission frequencies. Some of the efforts have related to the wave-guide conductor but mainly they relate to radio relay transmission. As a consequence of these efforts a considerable number of new multiplex methods for use at microwave frequencies have been devised. All of these methods employ bandwidth more liberally than the 4 kc per channel rate associated with single sideband carrier systems, in return for which various transmission advantages are obtained. Theoretically, transmission advantages can be sacrificed to permit bandwidth reduction but the transmission requirements then become very severe. Bandwidth as a transmission parameter has grown to a prominent position in modern communication theory as set forth by Shannon et al. 1, 2 - 3 The liberal use of bandwidth, employed in an effective way, operates to permit higher noise and distortion within a system and, in the case of radio relay systems, operates to permit higher interfering signals from other radio systems. When all the frequency space necessary to avoid mutual inter1 C. E. Shannon, "A Mathematical Theory of Communication," Bell Sys.