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Whenever power is adjusted on Carrier 1, the power in Carrier 2 needs to be adjusted so the
composite carrier power remains constant (or does not exceed its allocated limit), while keeping
the CnC ratio within limits, as shown in Figure F-9.
There are two distinct phases to the CnC-APC algorithm:
1. In order for the CnC-APC algorithm to work effectively, it must first analyze the CnC Ratio
and Eb/No margins on each side of the link, and based on the starting conditions, re-
distribute power between the two ends so that good protection against fades can be
achieved. This process is referred as re-balancing, and is done so that total composite
power (TCP) in the two carriers remains constant (within ±0.5 dB).
In this process, both sides of the link calculate power changes, based on their ability to
see not only local parameters, including CnC Ratio, Eb/No, Receive signal level, power
level and max power increase, but those of the modem at the other end of the link. After
these calculations are performed, a comparison of the results is performed, and if they
are in close agreement, the power changes are implemented.
It may not always be possible to drive the powers to an optimum point. The calculations
take into account:
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Appendix F
Figure F-9. CnC Automated Power Control
The minimum and maximum power levels possible in the modem (as affected by
the selected operating band – 70/140 MHz IF versus L-Band).
If the user-programmed value that determines the maximum permitted increase
in Tx power for the terminal would be exceeded. This is a very important value,
and you must know exactly how far from compression his transmit RF chain is
operating. As an example, if, under normal clear sky conditions, you determine
that the backoff from compression is -3 dB, and under no circumstances must the
RF chain ever go past -1 dB, the maximum power increase that can be permitted
is 2 dB. You enter this into the CDM-425 Advanced Sattelite Modem as a setup
parameter. If this value is ignored, or incorrectly entered, compression or
saturation could occur, either during the initial re-balance, or during a fade event.
This parameter is referred to as Transmit Power Level Increase, max (TPLI
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.
CDM-425 Advanced Satellite Modem
F–23
Revision 0
).
max
MN-CDM-425
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