Comtech EF Data CDM-Qx, Installation And Operation Manual

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CDM-Qx Satellite Modem Revision 6
CnCPerformance Characterization MN/CDMQx.IOM
Referring back to Figure D-9, the degradation due to adjacent carrier spacing is negligible
when spacing is 1.3 x Symbol Rate and 0 db is allocated for adjacent carrier degradation.
Desired
Carrier(s)
Carrier
Spacing
Figure D-9. CnC Example
Initially the CnC ratio is 0 dB, and the desired and interfering carriers are operating at the
same power level. At one end of the link (Site A), a downlink fade of 4 dB is expected
and an uplink fade of 6 dB. The other end of the link (Site B) is allocated 2 dB for
downlink and 3 dB for the uplink:
DL Fade UL Fade
Site A
4 dB 6 dB
Site B
2 dB 3 dB
When a rain fade occurs at one site, the effect is felt at both sites as outlined in
Figure D-10, which diagrams the worst case fade at Site A. The interfering carrier at Site
A is attenuated twice, once due to the uplink and the second time due to the downlink on
the return path. The carrier transmitted from Site B sees only the downlink fade when it is
received at Site A. The resulting power level changes at each site due to the rain fade and
the resulting CnC ratio and Eb/No degradation is summarized in the following table:
Site A Site B
Parameter dB Comment Parameter dB Comment
Relative Level of
Carrier A @ Site A
-10 Due to fade at
A
Relative Level of
Carrier B @ Site B
0 Due to fade at A
Relative Level of
Carrier B @ Site A
-4 Due to fade at
A
Relative Level of
Carrier A @ Site B
-6 Due to fade at A
CnC Ratio At
Site A
-6 CnC Ratio At
Site B
+6
Degradation At
Site A
-0.1 Per Figure D-10
8-PSK
Degradation At
Site B
-0.3 Per Figure D-10
8-PSK
D–9