Fig: SS/TDMA operation:

The above figure shows a simple SS/TDMA system serving two areas, each with two stations. As with ordinary TDMA, only one station at a time may transmit within an area.Thus, within area A, either station 1 or 2 may transmit in any given time slot. Similarly, either station 3 or station 4 may transmit in area B at anyone time. Stations from the two areas do not interfere either through the use of polarized signals or different frequencies. At the satellite, data that are received are immediately retransmitted on a downlink frequency. Two separate downlink beams are used. The satellite contains a switch for interconnecting input beams and output beams. The connections through the switch may change over time. In the figure, downlink beam A repeats uplink beam A during periods 1 and 3 and repeats uplink beam B during period 2. Thus any station in any area can send data to any other station in any area.

For a satellite serving N areas, there are N TDM input streams. At any given time, the switch is configured to route these uplink beams in a particular fashion to the N downlink beams. Each configuration is referred to as a mode and N! different modes are required for full connectivity.

Table: SS-TDMA Modes (three beams):

Table given above shows the modes for a three area system. For example, stations in area A can communicate with each other during modes 1 and 2, communicate with stations in area B during modes 3 and 5, and so on. The satellite will change from mode to mode periodically. At most, a mode change would occur once per slot time. The mode pattern and duration are nor- mally adjustable by ground command to meet changing traffic requirements.