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FIGURE 5.2 Principle <strong>of</strong> FDMA system: (a) multiple access by frequency; (b)frequency partition with guard b<strong>and</strong>s.signals between adjacent carriers; see Fig. 5.2(b). The number <strong>of</strong> subdivisionspermissible, which in turn determines the transmission capacity, depends onthe satellite transponder’s channels <strong>and</strong> b<strong>and</strong>width (this dependence willbecome obvious by Eq. (5.7)).The FDMA scheme has some advantages <strong>and</strong> disadvantages. A majorlimitation arises from the need for guard b<strong>and</strong>s between adjacent channels inorder to avert interference from adjacent channels. These guard b<strong>and</strong>s imposea practical limit on the efficiency <strong>of</strong> an FDMA system. A secondarydisadvantage is the need to control the transmitting power <strong>of</strong> earth stationsin such a way that the carrier powers at the satellite input are the same in orderto avoid the capture effect. Despite these disadvantages, FDMA is the oldestCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.

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