Figure D-10. Link With Fading At Site A - Comtech EF Data CDM-Qx Installation And Operation Manual

CDM-Qx Satellite Modem
CnCPerformance Characterization
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 sine.
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.
In links without rain fade, the CnC ratio is constant and only the asymmetry of link due to
satellite footprint, different antenna sizes, different symbol rates, or modulation and code
rates alter the ratio.
Uplink Fade
-6 dB
Downlink Fade
Rain
Rain
Fade
Fade
Site A

Figure D-10. Link With Fading At Site A

D–10
-4 dB
0 dB
Revision 6
MN/CDMQx.IOM
-6 dB
Site B