By E. Guthrie and F. Ivanek, Fairchild Microwave & Optoelectronics
Single-sideband amplitude modulation (SSB-AM) is well established as the signal processing method which assures minimum spectrum occupancy. Already four decades ago it became the generally accepted standard of multichannel telephone transmission in the form of frequency-division multiplex, first used on open-wire lines and cables, and later on microwave links. The last decade witnessed the transition to virtually exclusive use of SSB-AM on a world-wide basis in another segment of communications where frequency spectrum is at a premium, namely, short-wave radio telephony. This became feasible after an impressive arsenal of technological solutions for the rather difficult inherent problems of SSB-AM radio transmission became available. The December 1956 special issue of the Proceedings of the IRE on single-sideband techniques makes one of the most interesting readings on this subject. It might come as a surprise to some that one of the articles in that issue describes an experimental 24-channel telephone system using SSB-AM for beyond-the-horizon UHF transmission [1]. Not too long after that, in 1960, came a proposal for the use of SSB-AM on line of- sight radio relay links [2] .. This would quadruple the transmission capacity as compared to the most advanced FM radio relay telephone systems in use today. The main problem to be solved to this end is that of linear power amplification at microwave frequencies. Technological implementation, therefore, came first at lower frequencies where linear power amplifiers are available at higher output levels. SSB-AM 120-channel telephone systems operating in the 400-410 and 420-430 MHz bands were installed in the mid 60 1s to establish a high-quality commercial telephone link between West Berlin and the Federal Republic of Germany.