Comtech EF Data CDM-570A - Manual (Page 530)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix G Revision 2
G–12
Total Leased Resource Usage
LST Calculated
(MultiCarrier Transponder Lease)
Total BW Allocated 1.0319 MHz
TotalBW PEB* .9755 MHz
Total EIRP Utilized 12.8 dBw Total BW Utilized 1.0319 MHz
Total EIRP Available 13.3 dBw Total BW Available 1.1000 MHz
Margin (available-utilized) .5 dBw Margin (available-utilized) 0.681 MHz
*PEB = Power Equivalent Bandwidth
CnC Allocated BW = 1.0319 MHz
CnC PEB = 0.9755 MHz
CnC Leased BW = 1.03196 MHz
The link budget summary for the different ModCod combinations is as follows:
S.No. Modulation & FEC (CnC Carrier)
Allocated
BW
(MHz)
PEB
(MHz)
Leased
BW
(MHz)
CnC Leased
BW Savings
PSD Ratio
(dB)
1 QPSK, VersaFEC 0.533, 10% ROF 1.0319 0.9755 1.0319 36% 2.2
2 QPSK, VersaFEC 0.631, 15% ROF 0.9113 1.1069 1.1069 31% 2.2
3 QPSK, VersaFEC 0.706, 25% ROF 0.8859 1.3223 1.3223 17% 2.2
Based on this analysis, QPSK, VersaFEC 0.533 with Carrier-in-Carrier provides savings of 36%. In
addition to this 36% reduction in Leased Bandwidth, using Carrier-in-Carrier also reduced the
BUC requirement from 10W to 5W:
BUC Size
Traditional Link
(QPSK, TPC 7/8)
CnC Link
(QPSK, VersaFEC 0.533)
BUC Size Reduction
BUC @ 4.5 m 10 W 5 W 50%
BUC @ 3.0 m 10 W 5 W 50%
G.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.