CDM-625 Advanced Satellite Modem
Revision 1
DoubleTalk
®
Carrier-in-Carrier
®
Option
MN-0000036 (Ref MN/CDM625.IOM)
10–8
Site A
Site B
Parameter
dB
Comment
Parameter
dB
Comment
Relative Level of
Carrier A @ Site A
-10
Due to fade @ A
Relative Level of
Carrier B @ Site B
0
Due to fade @ A
Relative Level of
Carrier B @ Site A
-4
Due to fade @ A
Relative Level of
Carrier A @ Site B
-6
Due to fade @ A
CnC Ratio @ Site A
-6
CnC Ratio @ Site B
+6
Degradation @ Site A
-0.1
Per Figure 10-7
8-PSK
Degradation @ Site B
-
0.3
Per Figure 10-7
8-PSK
Notice from the table, the CnC change is proportional to the uplink fade. The CnC ratio decreases
by the amount of the uplink fade at the near end while the CnC ratio increases by the amount of
uplink fade at the distant end. Also, the CnC ratio at opposite ends of the link has the same
magnitude but opposite sign.
Figure 10-7. Link with Fading at Site A
eaving the demodulator to recover the desired carrier nearly absent the undesired
interferer.
tio
(more de dation). In links similar to these, the unfaded end of the link has the highest CnC ratio.
nt, different antenna sizes, different symbol rates, or modulation and code rates
alter the ratio.
As shown, the interfering carrier at Site
A
is attenuated twice passing through both the uplink and
returning on the downlink back to Site
A
. Since the carrier transmitted and then received at Site
A
is the interfering carrier this extra attenuation is much less of an issue because it makes the CnC
ratio more negative (less degradation). In a practical link the interfering carrier might drop into
the noise l
At Site
B
, the desired carrier from Site
A
is received and attenuated thereby increasing the CnC ra
gra
In links without rain fade, the CnC ratio is constant and only the asymmetry of link due to
satellite footpri
Rain
Fade
Rain
Fade
Site A
Site B
Uplink Fade
-6 dB
Downlink Fade
-4 dB
-6 dB
0 dB
