9.3
SATELLITE COMMUNICATIONS
227
First calculate the free-space loss. Use Eq. (9.4):
L
dB
= 96.6 + 20 log F
GHz
+ 20 log D
sm
= 96.6 + 20 log 4.0 + 20 log 25, 573
= 96.6 + 12.04 + 88.16
= 196.8 dB.
Example Link Budget: Downlink
EIRP of satellite
+30 dBW
Free-space loss
-196.8 dB
Satellite pointing loss
-0.5 dB
Off-contour loss
0.0 dB
Excess attenuation rainfall
0.0 dB
Gaseous absorption loss
-0.5 dB
Polarization loss
-0.5 dB
Terminal pointing loss
-0.5 dB
Isotropic receive level
-168.8 dBW
Terminal
G/T
+20.0 dB/K
Sum
-148.8 dBW
Boltzmann's constant (dBW)
-(-228.6 dBW)
C/N
0
79.8 dB
On repeatered satellite systems, sometimes called "bent-pipe satellite systems" (those
that we are dealing with here), the link budget is carried out only as far as
C/N
0
, as
we did above. It is calculated for the uplink and for the downlink separately. We then
calculate an equivalent
C/N
0
for the system (i.e., uplink and downlink combined). Use
the following formula to carry out this calculation:
C
N
0
(s)
=
1
1
/(C/N
0
)
(u)
+ 1/(C/N
0
)
(d)
(9.24)
Example. Suppose that an uplink has a
C/N
0
of 82.2 dB and its companion downlink
has a
C/N
0
of 79.8 dB. Calculate the
C/N
0
for the system,
(C/N
0
)
s
. First calculate the
equivalent numeric value (NV) for each
C/N
0
value:
NV
(1) = log
-1
(79.8/10) = 95.5 × 10
6
,
NV
(2) = log
-1
(82.2/10) = 166 × 10
6
,
C/N
0
= 1/[(10
-6
/95.5) + (10
-6
/166)]
= 1/(0.016 × 10
-6
) = 62.5 × 10
6
= 77.96 dB.
This is the carrier-to-noise ratio in 1 Hz of bandwidth. To derive C/N for a particular RF
bandwidth, use the following formula:
C/N = C/N
0
- 10 log BW
Hz
.
Summary :
First calculate the equivalent numeric value (NV) for each C/N 0 value: NV (1) = log -1 (79.8/10) = 95.5 × 10 6 , NV (2) = log -1 (82.2/10) = 166 × 10 6 , C/N 0 = 1/[(10 -6 /95.5) + (10 -6 /166)] = 1/(0.016 × 10 -6 ) = 62.5 × 10 6 = 77.96 dB.
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