Comtech EF Data CDM-425, Инструкция по установке и эксплуатации

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix F F–25 MN-CDM-425
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.
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.