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4Satellite LinksA satellite link consists <strong>of</strong> an uplink (transmit earth station to satellite) <strong>and</strong> adownlink (satellite to the receive earth station); see Fig. 4.1. Signal qualityover the uplink depends on how strong the signal is when it leaves the sourceearth station <strong>and</strong> how the satellite receives it. Also, on the downlink side, thesignal quality depends on how strongly the satellite can retransmit the signal<strong>and</strong> how the receiving earth station receives the signal.Satellite link design involves a mathematical approach to the selection <strong>of</strong>link subsystem variables in such a way that the overall system performancecriteria are met. The most important performance criterion is the signalquality; that is, the energy per bit noise density ratio ðE b =N 0 Þ in theinformation channel, which carries the signal in the form in which it isdelivered to the user(s). As such, in designing a satellite communicationsystem, the designer must attempt to ensure a minimum E b =N 0 in the receiver’sCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.

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