M.2.2 Dpc Functionality; Entrance Link Cabling - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix M
The CDM-840 takes receipt of these messages via the outbound channel in order to take the
appropriate actions. Thus, depending on the link estimation compared to internal power curves,
the link is adaptive through configuration limits that maintain power budgets on the satellite.

M.2.2 DPC Functionality

DPC can be applied to any or all of the CDM-840s with the exception of the Entry Channel Mode
(ECM) control CDD-880s in Dynamic Single Carrier Per Channel (dSCPC) networks. Since the ECM
burst CDD-880s are dSCPC fast acquisition receivers, they cannot provide accurate signal quality
E /N measurements, which is critical in closing and maintaining a stable power control loop
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mechanism. The "Power Hunt" function is used in place of the closed loop control, and is
described later in this appendix.
The CDM-840 receives each message and compares it to tables, automatically adjusting the
uplink modulator transmit power to maintain a target signal-to-noise ratio (SNR) as measured by
the downlink signal quality at the corresponding receive demodulator(s). This exchange provides
a mechanism to compensate for varying signal levels during transmit uplink/downlink fade
conditions.
The power control algorithm is a closed loop servomechanism linked to the received E
as the input function; the modulators transmit power attenuator as the output function. Only
modulator transmit power is controlled by the algorithm, assuming that the transmit up-
conversion chain (BUC) is a fixed gain block.
M.2.2.1

Entrance Link Cabling

DPC relies on basic fundamental settings before it can operate properly. These parameters build
the foundation on which the control can reference all power variations. One significant but
often overlooked parameter is the transmit cable attenuation between the CDM-840 and the
BUC.
The entrance link cable run can be problematic when trying to balance the modulator's output
signal level to the output gain power of the BUC. If the mismatch of balance is not calculated
during site commissioning, this can result in:
• Too much cable attenuation and not enough power from the modulator to drive the BUC
output to achieve required uplink power, or
•
Not enough cable attenuation and the minimum output power of the modulator is still
overdriving the BUC, or worse, PEB on the satellite.
To help manage this problem, DPC Power Reference calibration takes into consideration
budgeted parameters, maximum data rate, and power margin to determine rated and/or
terminal maximum power. The Power Reference function uses the budgeted values, calculating
from current power determining the amount of power required to achieve maximum. If the
calculated power exceeds the modulator's maximum output power range, the cable run has too
M–3
MN-CDM840
Revision 3
/N value
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