Comtech EF Data CDM-625 - Manual (Page 332)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–2
has been further refined, and some of the limitations that existed in the CDM-Qx
implementation have been overcome.
This innovative technology provides a significant improvement in bandwidth and power
utilization, beyond what is possible with FEC and modulation alone, allowing users to achieve
unprecedented savings. When combined with advanced modulation and FEC, it allows for multi-
dimensional optimization:
• Reduced operating expense (OPEX) – e.g., Occupied Bandwidth & Transponder Power;
• Reduced capital expenditure (CAPEX) – e.g., Block Up Converter/High-Power Amplifier
(BUC/HPA) size and/or antenna size;
• Increased throughput without using additional transponder resources;
• Increased link availability (margin) without using additional transponder resources;
• A combination of any of the above to meet different objectives.
Summary: When carriers share common bandwidth, up to 50% savings in transponder
utilization is possible.
10.3 Application Requirements
The following conditions are necessary in order to operate DoubleTalk Carrier-in-Carrier:
• Link must be full duplex.
• A CDM-625 must be used at the end of the link where the cancellation needs to take place.
• The transponder is operated as Loopback. That is, each end of the link must be able to see a
copy of its own signal in the return (downlink) path from the satellite. The looped back
signal is then subtracted which leaves the signal from the distant end of the link. DoubleTalk
Carrier-in-Carrier cannot be used in spot beam systems.
• The transponder needs to be “bent-pipe” – meaning no on-board processing, demodulation,
regeneration can be employed. Demodulation/remodulation does not preserve the linear
combination of the forward and return signals and the resulting reconstituted waveform
prevents recovery of the original constituent signals.
Figure 10-1 shows a simplified conceptual block diagram of CnC processing. The two ends of the
link are denoted ‘A’ and ‘B’ and the uplink and downlink are shown.