Application Requirements; J.2 Application Requirements - Comtech EF Data CDM-760 Installation And Operation Manual

Appendix J
CDM-760 Advanced High-Speed Trunking Modem
CnC was first introduced in Comtech EF Data products such as the CDM-Qx Satellite Modem,
CLO-10 Link Optimizer, and CDM-625 Advanced Satellite Modem. The implementation of
DoubleTalk Carrier-in-Carrier in the CDM-760 has been further refined, and some of the
limitations that existed in the prior offerings have been overcome.
CnC 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.
Conclusion: When using the DoubleTalk Carrier-in-Carrier (CnC) option, up to 50% savings in
transponder utilization is possible when carriers share common bandwidth.
J.2

Application Requirements

These conditions are necessary in order to operate DoubleTalk Carrier-in-Carrier:
•
Link must be full duplex.
•
A CDM-760 must be used at the end of the link where the cancellation needs to take place.
Normally, this is both ends of the link.
•
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 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 J-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.
This performance is achieved through advanced signal processing algorithms that provide superior
cancellation while tracking and compensating for these common link impairments:
1) Time varying delay: In addition to the static delays of the electronics and the round-trip
delay associated with propagation to the satellite and back, there is a time-varying
component due to movement of the satellite. The CnC module tracks and compensates for
this variation.
2) Frequency offset and drift: Common sources are satellite Doppler shift, up and down converter
frequency uncertainties, and other drift associated with the electronics in the CDM-760 itself.
The CnC module tracks and compensates for this frequency offset and drift.
J–2
Revision 4
MN-CDM760