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FIGURE 4.3A model <strong>of</strong> the combined-link carrier-to-noise ratio.Combined link carrier power-to-noise ratio:Having determined each link C=N, we are in a position to establish thecombined carrier-to-noise ratio ðC=NÞ c . As seen in Fig. 4.1, each single-hopearth station to earth station via a satellite consists <strong>of</strong> an uplink <strong>and</strong> downlinkin cascade, which can be modelled as in Fig. 4.3. G s represents the effectivetransponder gain. The C s <strong>and</strong> N s are the carrier <strong>and</strong> the effective noise powers<strong>of</strong> associated links, respectively.From Fig. 4.3, the following expressions can be written.1. Output carrier power <strong>of</strong> the satellite transponder:C d ¼ G s C uð4:19Þ2. Combined carrier power <strong>of</strong> the single hop:C c ¼ G d C d ¼ G s G d C uð4:20Þ3. Combined noise power:N c ¼ N u G s G d þ N d G dIn view <strong>of</strong> (4.19)–(4.21), the combined power ratio in cascade isð4:21ÞN cC c¼ N uG s G d þ N d G dC u G s G d¼ N uC uþ N dC u G sð4:22Þ¼ N uC uþ N dC dThus, the combined carrier-to-noise ratio is C1¼Nc ðC=NÞ 1u þðC=NÞ1dð4:23ÞCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.

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