Frequency Division Multiplexing on Transoceanic Cables

Heretofore cable, land-line telegraph, and microwave channeling has been limited to approximately 1100 bits per second (bauds), utilizing 22 subcarriers spaced 120 cps apart. Frequency shift keying, together with more sophisticated filter design, has enabled this capacity to be doubled. High data transmission speeds require frequency division multiplexing in order to reduce the distortion in the received signal. A certain degree of time division multiplexing is desirable although it is more economical and otherwise required to utilize frequency division multiplexing to a greater degree. This paper describes means for accommodation of traffic over the transatlantic telephone cable voice channels at a speed of more than 2200 bits per second with reasonably low error rates. Techniques for this utilize sine wave keying pulses at the fundamental keying rate over subcarriers spaced every 120 cps from 420 cps to 2940 cps, and time division multiplex of the slower input information channels. Cable outages require adequate, not necessarily 100 per cent, backup by independent sideband HF radio circuits using similar subcarrier modulation with tones spaced at 170-cps intervals instead of 120-cps by reason of the additional distortion-due multipath delay. Examples of filter characteristics for reception and transmission to accomplish this are shown, as well as typical equipment for each tone channel. Curves presented give distortion at various speeds per subcarrier and illustrate the effects of cross-talk and tone jitter as per cent distortion vs keying rate.