Comtech EF Data SLM-5650A - Manual (Page 300)

SLM5650A_CDM625 Satellite Modem Revision 8
DoubleTalk Carrier-in-Carrier Option MN-SLM5650A
15–2
CnC was first introduced in Comtech EF Data’s CDM-Qx Satellite Modem and, more recently, in
the CLO-10 Link Optimizer. The implementation of CnC in the SLM-5650A has been further
refined to address some of the limitations that existed in the CDM-Qx implementation.
This technology improves bandwidth and power utilization beyond what is possible with FEC
and modulation alone. This allows unprecedented user savings and, 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.
15.3 Application Requirements
The following conditions are necessary in order to operate DoubleTalk Carrier-in-Carrier:
• Link must be full duplex.
• An SLM-5650A 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. CnC
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 155-1 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 optimal
cancellation while tracking and compensating for the following 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