Satellite Technology: Principles and Applications, 3rd Edition

6Multiple Access Techniques

Multiple access means access to a given facility or a resource by multiple users. In the context of satellite communication, the facility is the transponder and the multiple users are various terrestrial terminals under the footprint of the satellite. The transponder provides the communication channel(s) that receives the signals beamed at it via the uplink and then retransmits the same back to Earth for intended users via the downlink. Multiple users are geographically dispersed and certain specific techniques, to be discussed in this chapter, are used to allow them a simultaneous access to the satellite's transponder. The text matter is suitably illustrated with the help of a large number of problems.

6.1 Introduction to Multiple Access Techniques

Commonly used multiple access techniques include the following:

1. Frequency division multiple access (FDMA)
2. Time division multiple access (TDMA)
3. Code division multiple access (CDMA)
4. Space domain multiple access (SDMA)

In the case of frequency division multiple access (FDMA), different Earth stations are able to access the total available bandwidth in the satellite transponder(s) by virtue of their different carrier frequencies, thus avoiding interference amongst multiple signals. The term should not be confused with frequency division multiplexing (FDM), which is the process of grouping multiple base band signals into a single signal so that it could be transmitted over a single communication channel ...