Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix F F–13 MN-CDM-425
The link budget summary for the different ModCod combinations is shown in Table F-4.
Table F-4. Link Budget Summary for ModCod Combinations
S. No. Modulation & FEC
Allocated
BW (MHz)
PEB (MHz) Leased BW
(MHz)
Savings Compared
to Original
PSD
Ratio
(dB)
1 8-QAM, LDPC 2/3 0.3584 1.1468 1.1468 -20% 2.1
2 QPSK, LDPC 3/4 0.47785 0.6734 0.6734 30% 2.1
3 QPSK, LDPC 2/3 0.53735 0.5777 0.5777 40% 2.1
4 QPSK, LDPC 1/2 0.7168 0.5184 0.7168 25% 2.1
Based on this analysis, QPSK, LDPC 2/3 with Carrier-in-Carrier provides the maximum savings of
40%. In addition to 40% reduction in Leased Bandwidth, using Carrier-in-Carrier also reduced the
required HPA Power by almost 40%:
Table F-5. HPA Power Reduction
HPA Power
Traditional Link
(QPSK, TPC 3/4)
CnC Link
(QPSK, LDPC 2/3)
HPA Power Reduction
HPA @ 4.6 m 0.7 W 0.5 W 40%
HPA @ 2.4 m 1.5 W 1.1 W 36%
F.2.5.2 Asymmetric Data Rate Link
As occupied (or allocated) bandwidth of a Carrier-in-Carrier circuit is dictated by the larger of the
two carriers, it is strongly recommended that the smaller carrier be spread as much as possible
using a lower order modulation and/or FEC, while meeting the PSD ratio spec. Spreading the
smaller carrier using a lower order modulation has multiple benefits:
•
Lower order modulation is always more robust;
•
Lower order modulation uses less transponder power – this reduces total transponder,
and increases available link margin;
•
Lower order modulation uses less transmit power on the ground – this can significantly
reduce the BUC/SSPA size by not only reducing the transmit EIRP, but also reducing the
BUC/SSPA backoff