CDM-570A/570AL Satellite Modem with Optional Packet Processor
Revision 5
DoubleTalk Carrier-in-Carrier Option G–14 MN-CDM570A
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.
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 3 Mbps / 1 Mbps
While the traditional link was based on QPSK, TPC 3/4 for the Hub to Remote and QPSK TPC
7/8 for Remote to Hub and required 4.0 MHz of leased bandwidth, the Carrier-in-Carrier link was
based on QPSK, VersaFEC 0.631 and QPSK, VersaFEC 0.533 and required 2.9 MHz of leased
bandwidth.
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