Comtech EF Data CDM-625 - Manual (Page 342)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–12
The link budget summary for the different ModCod combinations is as follows:
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%:
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%
10.4.5.2 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 IS-901 @ 342º W, 22/22 (EH/EH)
Earth Station 1 Africa – 4.5 m
Earth Station 2 Africa – 3.0 m
Data Rate 3000 Mbps / 1000 Mbps
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.