From Satellite Communications, Fourth Edition

14.9 Satellite-Switched TDMA

More efficient utilization of satellites in the geostationary orbit can be achieved through the use of antenna spot beams. The use of spot beams is also referred to as space-division multiplexing. Further improvements can be realized by switching the antenna interconnections in synchronism with the TDMA frame rate, this being known as satellite-switched TDMA (SS/TDMA).

Figure 14.27a shows in simplified form the SS/TDMA concept (Scarcella and Abbott, 1983). Three antenna beams are used, each beam serving two earth stations. A 3 3 satellite switch matrix is shown. This is the key component that permits the antenna interconnections to be made on a switched basis. A switch mode is a connectivity arrangement. With three beams, six modes would be required for full interconnectivity, as shown in Fig. 14.27b, and in general with N beams, N! modes are required for full interconnectivity. Full interconnectivity means that the signals carried in each beam are transferred to each of the other beams at some time in the switching sequence. This includes the loopback connection, where signals are returned along the same beam, enabling intercommunications between stations within a beam. Of course, the uplink and downlink microwave frequencies are different.


Figure 14.27: ( a) Satellite switching of three spot beams; ( b) connectivities or modes.

Because of beam isolation, one frequency can be used for all uplinks, and a different frequency for all downlinks (e.g., 14 and 12 GHz in the Ku band).


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