Comtech EF Data CDM-625 - Manual (Page 352)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–22
• If the power changes calculated would put the CnC Ratio out of its working range.
• If the Rx signal level is above its minimum level – if it is not, the CnC Ratio monitor will
not be accurate, which is essential correct calculations.
• If the power change would result in the Eb/No margin on either side becoming negative
(see below).
At the end of the rebalance procedure, the modem will display the result:
 Full rebalance.
 Partial rebalance.
 Some issue was found that prevents a rebalance.
In the case of a partial or no rebalance, 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 rebalance 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 rebalance if it is known with certainty that
there are CLEAR SKY conditions at BOTH SIDES OF THE LINK.
It is not necessary for there to be a person at each end – the rebalance can
be initiated from either side, with the other side unattended.
Following the rebalance, the CDM-625 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.