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F.4.4.1
CnC-APC Framing and the Self-Locking Problem
Self-locking is a problem that has been previously reported to Comtech EF Data Customer
Service on CnC links that have identical data rates, FEC and modulation.
In the case where both ends of the link are up and running, the CnC canceller performs as
intended. However, if the carrier at the remote side stops transmitting a carrier, the CnC canceller
will still cancel the interferer (its own outbound Tx signal), but the cancelation is not perfect (it
may be in the order -30 dB) and hence a weak signal appears at the input to the demodulator
that, because of the correct settings of data rate, FEC and modulation, is sometimes acquired by
the demod. This can sometimes be an issue because, although the Tx carrier from the distant
side is not transmitting, no demodulator alarm is generated and hence the local end is not alerted
– and data being transmitted is looped back to the receiver, and back into the network.
The use of CnC-APC framing provides the means to eliminate the self-locking problem. Rather
than having an identical framing structure for the local and distant sides of the link, by designating
each side as Side 'A' and Side 'B', each side has a different framing structure and is therefore
non-identical and incompatible. This makes it impossible for a modem in this CnC mode to lock to
itself. With this approach, there is no 'master-slave' relationship – both sides are peers, and either
side can be designated as 'A' or 'B'.
F.4.5
CnC-APC Response Time
After CnC-APC is activated, power may be modified at a rate that does exceed once every 1.75
seconds. This rate has been determined by taking into account the round trip satellite delay, the
time taken to accurately determine CnC Ratio and Eb/No following a power adjustment, and the
latency of the CnC-APC frame itself. The overall response time of the control system is therefore
limited by these factors, and deep fading events that exceed approximately 1 dB/second are likely
to be too fast for CnC-APC (or any other 'classic' scheme) to respond to. Users should bear in
mind that these limitations in response time are inherent in schemes involving round-trip delays
from geostationary satellites.
Appendix F
CDM-425 Advanced Satellite Modem
F–26
Revision 0
MN-CDM-425
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