Cnc-Apc Framing; Solving The Problem Of Self-Locking - Comtech EF Data CDM-570A Installation And Operation Manual

The depth of fade that the CnC-APC algorithm can tolerate is highly dependent on a
number of factors, including:
•
The CnC Ratio and Eb/No margins following a re-balance. In general, with a highly
asymmetric link, where CnC Ratio is not close to zero, the tolerance to fade depth on
one side will be less than the other.
•
The maximum permitted level of power increase that you have defined. If, for
example, you have programmed a TPLImax value of 2 dB (because that's the only
headroom the RF chain has) you should not expect the link to tolerate a fast 10 dB
fade at Ku-Band.
G.6.4

CnC-APC Framing

In order to provide a two-way communications link between the two ends, CnC-APC requires an
overhead channel (very much like EDMAC, or one of its variants, which is used in AUPC).
Recognizing that this overhead needs to have a minimal impact on occupied bandwidth and
Eb/No performance, the overhead has been kept as low as possible:
•
The overhead rate is fixed at 0.5%, causing the symbol rate of the carrier to grow by an
almost imperceptible amount (no need to re-calculate channel spacing) and causing a
loss of Eb/No performance of only 0.02dB.
•
This choice of overhead rate effectively gives you the functionality of the CnC-APC
channel without having to pay a penalty in power or bandwidth – it's essentially free.
The CnC-APC channel constantly passes a number of metrics and parameters, including CnC
Ratio, Eb/No, Receive signal level, power level and max power increase, as well as information
required for the initial set-up, including the modem operating band (L-Band or 70/140 MHz), and
the satellite band in use.
In addition to the information fields in the frame, there is a control field that permits commands to
be sent from one side or the other during initial re-balancing, and re-balancing during fade events.
Note that the CnC-APC frame size (and the latency associated with it) limits the lower end of the
range of bit rates that are supported to 64 kbps. If you select the CnC-APC mode at a data rate
below 64 kbps, an error message will be generated if a re-balance operation is requested.
G.6.5

Solving the Problem of Self-Locking

Self-locking is a problem that has been previously reported to Comtech EF Data Product Support
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.
DoubleTalk Carrier-in-Carrier Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
G–26
Revision 5
MN-CDM570A