Comtech EF Data CDM-625A - Manual (Page 629)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix F Revision 3
F–23
The algorithm uses a number of inputs, from both sides of the link, including knowledge of
the satellite band in use (which you are required to know at the time of set-up). The
characteristics of rain fading changes with frequency, and there are different models for
C-Band, X-Band, Ku-Band, and Ka-Band. Not only does downlink attenuation vary as a
function rain rate, but in some cases (depending on the G/T of the terminals at each end,
and the satellite transponder itself) may be dominated by an increase in receive noise
temperature when the antenna is pointing through rain and clouds, rather than ‘cold’ sky.
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.