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The depth of fade that the CnC-APC algorithm can tolerate is highly dependent on a
number of factors, including:
•
•
F.4.4
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.
Appendix F
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.
CDM-425 Advanced Satellite Modem
F–25
Revision 0
MN-CDM-425
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