Comtech EF Data CDM-570A Installation And Operation Manual page 590

To recap, consider the following diagram:
Whenever you adjust power on Carrier 1, you need to adjust the power in Carrier 2 so the
composite carrier power remains constant (or does not exceed its allocated limit), while keeping
the CnC ratio within limits.
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:
•
The minimum and maximum power levels possible in the modem (which is affected
by the 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-570A/AL 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 TPLI
Power Level Increase, max).
•
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.
DoubleTalk Carrier-in-Carrier Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
G–24
Revision 5
(Transmit
max
MN-CDM570A