Mocomtech CDM-QX - Manual (Page 197)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–11
Table 9-1. Rain Fade Degradation
Site ‘A’ Site ‘B’
Parameter dB Comment Parameter dB Comment
Relative Level of Carrier ‘A’
at Site ‘A’
-10
Due to fade at
Site ‘A’
Relative Level of Carrier ‘B’
at Site ‘B’
0
Due to fade at
Site ‘A’
Relative Level of Carrier ‘B’
at Site ‘A’
-4
Due to fade at
Site ‘A’
Relative Level of Carrier ‘B’
at Site ‘B’
-6
Due to fade at
Site ‘A’
CnC Ratio at Site ‘A’ -6 CnC Ratio At Site ‘B’ +6
Degradation at Site ‘A’ -0.1
8-PSK
per Fig. 9-12
Degradation at Site ‘B’ -0.3
8-PSK
per Fig. 9-12
Notice from the table that 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 sine.
Figure 9-12. Link With Fading At Site ‘A’
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 leaving 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 ratio (more degradation). In links similar to these, the unfaded end of the link has the highest
CnC ratio.