Comtech EF Data CDM-570A - Manual (Page 540)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix G Revision 2
G–22
Following the re-balance, the CDM-570A/AL stores key parameters, such as Tx Power level,
that are referred to as the Home State. In the event of a loss of communication between the
two sides, the modem will revert to Home State, which is a known safe condition that will
not cause TCP to be exceeded. Once the link conditions have improved so that
communication is re-established, CnC-APC will automatically re-activate.
2) In the second phase of the algorithm, Eb/No, CnC ratio, and RSL are continuously
monitored. For every combination of the modulation, FEC type and FEC code rate, the
modem calculates an Eb/No margin. An Eb/No margin of 0 dB is arbitrarily determined to be
the Eb/No value that produces a BER of 1 x 10 -7 at the output of the decoder, plus 1 dB. If
the algorithm senses that Eb/No margin is dropping, power is increased at one or both ends
of the link, but in a manner that ensures that TCP is not exceeded. In the vast majority of
cases the TCP during a fade will not exceed its clear-sky value by more than +0.5 dB.
The algorithm uses a number of inputs, from both sides of the link, including a 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.
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.