CDM-625 Advanced Satellite Modem
Revision 1
DoubleTalk
®
Carrier-in-Carrier
®
Option
MN-0000036 (Ref MN/CDM625.IOM)
10.8 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.
Eb/No Degradation For Asymmetric 4.5-Meter Antenna At A and 2.4 Meter Antenna At B
Eb/No Degradation At Site A
Parameter
Due To Fade At A (dB)
Due To Fade At B (dB)
CnC Ratio @ Site A
+5.3
+5.3
Uplink Fade @ Site A
6
-
Uplink Fade @ Site B
-
3
Faded CnC @ Site A
-0.7
+8.3
Eb/No Degradation (dB) @ Site A
0.0
-0.1
Eb/No Degradation At Site B
Parameter
Due To Fade At B (dB)
Due To Fade At A (dB)
CnC Ratio @ Site B
-5.3
-5.3
Uplink Fade @ Site B
3
-
Uplink Fade @ Site A
-
6
Faded CnC @ Site B
-8.3
+0.7
Eb/No Degradation (dB) @ Site B
0.0
-0.1
10–11
