Meeting the challenge of applying cellular concepts to LEO SATCOM systems

The authors discuss challenges encountered in satellite-based cellular systems that will employ low Earth orbiting (LEO) constellations to provide continuous worldwide coverage, with some minimum grazing angle. Motorola´s Iridium system will utilize spot beams to form cells on the Earth´s surface similar to terrestrial cellular radio. However, for LEO constellations, the geometric relationship between cells will no longer be constant, as with terrestrial systems. To execute channel reuse, it will be necessary to maintain sufficient distance between reuse cells to assure that interference levels do not degrade performance. Due to the dynamics of the cell movement, relative to the subscribers, calls must be handed off, from cell-to-cell and/or from space vehicle-to-space vehicle, during a call. Cell hand-off will be transparent to the subscribers at both ends of the call, and can occur several times during a normal call. The Iridium program will operate with a seven-cell frequency reuse pattern. On a global basis, there will typically be 1628 cells on to ensure worldwide coverage, and the seven-cell pattern will reuse each frequency over two hundred times. Within the contiguous US alone, Iridium will achieve more than five times frequency reuse