Comtech EF Data CDM-625A Installation And Operation Manual page 618

CDM-625A Advanced Satellite Modem
Appendix F
The link budget summary for the different ModCod combinations is as follows:
S. No. Modulation & FEC
1 8-QAM, LDPC 2/3
2 QPSK, LDPC 3/4
3 QPSK, LDPC 2/3
4 QPSK, LDPC 1/2
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%:
HPA Power
HPA @ 4.6 m
HPA @ 2.4 m
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
Consider the following example:
Satellite & Transponder
Earth Station 1
Earth Station 2
Data Rate
While the traditional link was based on QPSK, TPC 3/4 and required 3.9 MHz of leased
bandwidth, the Carrier-in-Carrier link was based on QPSK, LDPC 3/4 and QPSK, LDPC 1/2 and
required 2.8 MHz of leased bandwidth.
Allocated
PEB (MHz)
BW (MHz)
0.3584 1.1468
0.47785 0.6734
0.53735 0.5777
0.7168 0.5184
Traditional Link
(QPSK, TPC 3/4)
0.7 W
1.5 W
IS-901 @ 342º W, 22/22 (EH/EH)
Africa – 4.5 m
Africa – 3.0 m
3000 Mbps / 1000 Mbps
F–12
Leased BW Savings Compared
(MHz) to Original
1.1468 -20%
0.6734 30%
0.5777 40%
0.7168 25%
CnC Link
HPA Power Reduction
(QPSK, LDPC 2/3)
0.5 W
1.1 W
MN-CDM625A
Revision 4
PSD
Ratio
(dB)
2.1
2.1
2.1
2.1
40%
36%