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CDM-625 Advanced Satellite Modem
®
DoubleTalk
Carrier-in-Carrier
Parameter
Relative Level of
Carrier A @ Site A
Relative Level of
Carrier B @ Site A
CnC Ratio @ Site A
Degradation @ Site A
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.
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
eaving the demodulator to recover the desired carrier nearly absent the undesired
interferer.
At Site B, the desired carrier from Site A is received and attenuated thereby increasing the CnC ra
(more de
gra
dation). In links similar to these, the unfaded end of the link has the highest CnC ratio.
In links without rain fade, the CnC ratio is constant and only the asymmetry of link due to
satellite footpri
alter the ratio.
®
Option
Site A
dB
Comment
-10
Due to fade @ A
-4
Due to fade @ A
-6
-0.1
Per Figure 10-7
8-PSK
Uplink Fade
-6 dB
Rain
Rain
Fade
Fade
Site A
Figure 10-7. Link with Fading at Site A
nt, different antenna sizes, different symbol rates, or modulation and code rates
Parameter
Relative Level of
Carrier B @ Site B
Relative Level of
Carrier A @ Site B
CnC Ratio @ Site B
Degradation @ Site B
Downlink Fade
-4 dB
10–8
MN-0000036 (Ref MN/CDM625.IOM)
Site B
dB
Comment
0
Due to fade @ A
-6
Due to fade @ A
+6
-
Per Figure 10-7
0.3
8-PSK
-6 dB
0 dB
Site B
Revision 1
tio
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