Information Rate of a Coaxial Cable with Various Modulation Systems

In the very early days of information theory, it was proposed that broadband signals might be sent over a narrow-band medium by using more power. Most media (such as radio) are inherently broadband, and it has turned out that for broadband media the advantage lies in the other direction. Broadband modulation systems, such as FM or PCM transmitted by means of binary pulses, increase the signal-to-noise ratio 1197 1198 T H E BELL SYSTEM TECHNICAL J O U R N A L , OCTOBER 1 9 ( 5 0 for a given power and help to guard against distortion and interference. Indeed, there are strong arguments for the advantage of broadband modulation systems for any broadband medium, including radio, TE 0J waveguide and guided optical transmission. With the rising importance of digital transmission, there are of course very strong arguments for digital forms of modulation, such as binary pulse transmission. Coaxial cable (and other transmission lines) are unique in that the attenuation arises extremely rapidly with increasing frequency. Qualitatively, this suggests that broadband modulation systems may be unsuited to coaxial cable. What do the numbers show? The purpose of this paper is to illustrate strong effects, not to make exhaustive comparisons or optimizations or to take the practical matters of details of circuit use and limitations of circuit art into account. To this end, a simple, particular case will be considered -- a system using standard f-inch coaxial cable, with a repeater spacing of two miles.