Comtech EF Data CDM-Qx, Installation And Operation Manual

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CDM-Qx Satellite Modem Revision 6
CnCPerformance Characterization MN/CDMQx.IOM
D.9 3
rd
CnC Example: Asymmetric Link With Rain Fade
A reasonable question to ask about the previous example is the impact rain fade has on
the link. This example combines parts of the previous two examples using the same
symbol rates and asymmetric antennas and adds in fade at Site A, with the larger antenna
and examines the CnC ratio at both sites. The impact on both sites is also estimated due
to a fade at Site B. Just as in the earlier example, the larger fade (6 dB up and 4 dB down)
occurs at Site A (hub), and the smaller fade (3 dB up and 2 dB down) is allocated to Site
B (remote).
Treating the fades in this way is convenient as a first order approximation, but other
factors influence performance such as noise increase and G/T degradation at the receive
site.
From the previous examples there are some characteristics worth summarizing:
At the same symbol rate with the same modulation and code rate:
•
The CnC ratio is highest at the site with the larger antenna (Site A)
•
The CnC ratio for the site with the smaller antenna (Site B) is the same
magnitude but opposite sign: CnC(site B) = -CnC(Site A).
For rain fade:
•
The CnC ratio changes by the same amount as the uplink fade
•
Uplink fades at the near end decrease the CnC ratio
•
Uplink fades at the far end increase the CnC ratio.
The tables below summarize the Eb/No degradation at Site A when a fade is introduced
first at Site A and then at Site B. Next, the degradation at Site B is evaluated when a fade
appears at Site B and then at Site A. For this asymmetric case, the estimated CnC
degradation is 0.1 dB despite the significant fade.
D–13