Introduction to "Synchronous Communications"

The paper "Synchronous Communications," originally published in this journal in December 1956 by John P. Costas, has had a profound effect on modem digital communications. At the time of its publication, virtually all communications were analog in nature and, in fact, the stated goal of the paper was to demonstrate an amplitude-modulated (AM) system that could provide significantly improved performance for analog signaling and to dispel the notion that single sideband (SSB) had inherent performance advantages over properly constructed AM signaling systems. This paper is particularly significant because it was the first to demonstrate that carrier phase could be reliably recovered from the received signal using a structure that has come to be known as the Costas Loop.