Comtech EF Data CDM-425, Installation And Operation Manual

Page: 626 / 744

CDM-425 Advanced Satellite Modem
Revision 0
Appendix F F–24 MN-CDM-425
At the end of the re-balance procedure, the modem will display the result:
•
Full re-balance.
•
Partial re-balance.
•
Some issue was found that prevents a re-balance.
In the case of a partial or no re-balance, the modem displays the reason for this result;
you can then adjust an operating parameter and try again.
CAUTION – READ THESE IMPORTANT NOTES:
•
You should be aware that after the re-balance has been carried out,
it is very likely that the CnC Ratio will not be at 0dB, and the Eb/No
margins will not be equal. This is particularly true for cases where
there is an asymmetry in antenna size, terminal G/T, symbol rate
etc. This is an inherent function of the re-balancing algorithm, and
is not a cause for concern.
•
You should only attempt to do a re-balance if it is known with
certainty that there are CLEAR SKY conditions at BOTH SIDES OF
THE LINK.
It is not necessary that there be a person at each end – the re-
balance can be initiated from either side, with the other side
unattended.
Following the re-balance, the CDM-425 Advanced Satellite Modem 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 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.