Communications by Means of Low Earth Orbiting Satellites

This tutorial paper describes the current development in the use of low earth orbiting satellites (LEOS) for global personal communications. The basic architectures and rationales for design choices are discussed. Representative TDMA and CDMA systems are outlined and a summary of the diierences between LEOS communications and terrestrial cellular is presented along with the prospects for CDMA bandsharing in LEOS systems. Many organizations have proposed satellite-based personal communications providing global coverage for voice and data to hand-held subscriber units with a direct link to a satellite. Such systems will ooer the ultimate promise of personal communications of allowing communications to the person at any time and in any place, literally, rather than to a physical location. The projected system capital costs are estimated variously as being between $1-4 thousand million dollars. While satellite communications for trunked transoceanic telephone traac has seen successful operation for more than a quarter of a century, the application of satellites to mobile communications applications and for direct broadcasting is a relatively recent development. To some extent, the change in focus was due as much to the "push" of competition from wideband digital, transoceanic optical ber cable as to the "pull" of satellites being able to service a new market of mobile communications and, it is hoped, multimedia, nomadic personal communications, and computing. The unique opportunities that satellites have are for covering entire globe with minimal terrestrial infrastructure. Thus, initially, at least, satellites can be used for obtaining telephone and data communications in parts of the world where none exists at present. Traditional communication satellites reside in a geosynchronous equatorial orbit (inclination 0 o) at an altitude of 35,784 Km. While such systems ooer signiicant advantages in terms of coverage per satellite and other beneets, this is not the typical approach being taken by current developers of mobile satellite communication systems in general, and for Personal Communications in particular. In the next section, we discuss, brieey, the issues that drive towards Low Earth Orbiting Satellites (LEOS).