UHF SATCOM slot-connection conflicts caused by half-duplex radio equipment

The two military standards that specify multiple-access communications on UHF SATCOM channels, MIL-STD-188-182 and MIL-STD-188-183, employ TDMA. When TDMA is used, a terminal that has half-duplex radio equipment (half-duplex terminal), cannot both receive a signal in time slot X and transmit a signal in time slot Y if any part of the burst in the received time slot X precedes any part of the burst in the transmitted time slot Y by the terminal's round-trip satellite delay. This study considers two types of half-duplex terminals, one that adjusts transmit timing in an attempt to avoid user time-slot conflicts and one that does not. An analytical evaluation of slot-connection conflicts is performed for each type of terminal. The study concentrates on the conflicts for single-port terminal operation. Both types of terminals are found to have very similar half-duplex conflicts. Most of the user time slots at the higher user rates have such conflicts, at least under some conditions, illustrating the importance of full-duplex terminal capabilities for short frame-time TDMA systems having significant satellite throughput delays. The conditions under which the conflicts exist for each type of terminal are evaluated. For many of the user time slots, a conflict occurs each time the terminal performs ranging, or each time it sends an orderwire to the controller, or both. This can cause relatively frequent interruptions in user data transfers. Techniques that may be used to avoid slot-connection conflicts are discussed.