Page 400 - Satellite Communications, Fourth Edition

380 Chapter Twelve

For clear-sky conditions, [C/N] CS 17.4 dB, which gives an N/C ratio of 0.0182.
Substituting these values in Eq. (12.60) gives

sA 2 1d 3 272
0.1 5 0.0182 3 aA 1 b
544
Solving this equation for A gives A 4, or approximately 6 dB. From the curve
4
of Fig. 12.8, the probability of exceeding the 6-dB value is 2.5 10 , and there-
fore, the availability is 1 2.5 10 4 0.99975, or 99.975 percent.
For digital signals, the required [C/N 0 ] ratio is determined by the
acceptable BER, which must not be exceeded for more than a specified
percentage of the time. Figure 10.17 relates the BER to the [E b /N 0 ]
ratio, and this in turn is related to the [C/N 0 ] by Eq. (10.24), as discussed
in Sec. 10.6.4.
For the downlink, the user does not have control of the satellite
[EIRP], and thus the downlink equivalent of uplink power control,
described in Sec. 12.9.1, cannot be used. In order to provide the rain-fade
margin needed, the gain of the receiving antenna may be increased by
using a larger dish and/or a receiver front end having a lower noise
temperature. Both measures increase the receiver [G/T] ratio and thus
increase [C/N 0 ] as shown by Eq. (12.53).

12.10 Combined Uplink and Downlink
C/N Ratio
The complete satellite circuit includes an uplink and a downlink, as
sketched in Fig. 12.9a. Noise will be introduced on the uplink at the
satellite receiver input. Denoting the noise power per unit bandwidth
by P NU and the average carrier at the same point by P RU , the carrier-to-
noise ratio on the uplink is (C/N 0 ) U (P RU /P NU ). It is important to note
that power levels, and not decibels, are being used here.
The carrier power at the end of the space link is shown as P R , which
of course is also the received carrier power for the downlink. This is
equal to times the carrier power input at the satellite, where is the
system power gain from satellite input to earth-station input, as shown
in Fig. 12.9a. It includes the satellite transponder and transmit antenna
gains, the downlink losses, and the earth-station receive antenna gain
and feeder losses.
The noise at the satellite input also appears at the earth station input
multiplied by , and in addition, the earth station introduces its own
noise, denoted by P ND . Thus the end-of-link noise is P NU P ND .
The C/N 0 ratio for the downlink alone, not counting the P NU contri-
bution, is P R /P ND , and the combined C/N 0 ratio at the ground receiver is