Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix F F–15 MN-CDM-425
F.2.5.3 Power Limited Links
Carrier-in-Carrier can provide substantial savings even when the original link is power limited.
Spreading the carrier by using a lower modulation and/or FEC along with latest FEC such as
VersaFEC can substantially reduce the total power, which can then be traded with bandwidth
using Carrier-in-Carrier. The concept is illustrated with the following examples:
The conventional link is using 8-PSK, TPC 3/4:
Switching to VersaFEC and using a lower order modulation – e.g.,
QPSK, VersaFEC 0.803 increases the total occupied bandwidth, while
reducing the total power equivalent bandwidth:
Now using DoubleTalk Carrier-in-Carrier, the second QPSK,
VersaFEC 0.803 carrier can be moved over the first carrier – thereby
significantly reducing the total occupied bandwidth and total power
equivalent bandwidth when compared to the original side-by-side
8PSK, TPC 3/4 carriers:
To continue, consider this example shown in Table F-9.
Table F-9. Power Limited Links Example
Satellite & Transponder IS-901 @ 342º W, 22/22 (EH/EH)
Earth Station 1 Africa – 9.2 m
Earth Station 2 Africa – 4.5 m
Data Rate 2.048 Mbps / 2.048 Mbps