Comtech EF Data CDD-880, Инструкция по установке и эксплуатации

Страница: 197 / 234

CDD-880 Multi Receiver Router MN-CDD880
Appendix G Revision 2
G–3
G.2.2 DPC Functionality
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
S
/N
0
value
as the input function; the modulators transmit power attenuator as the output function. You
control modulator transmit power only by the algorithm, assuming that the transmit up-
conversion chain (BUC) is a fixed gain block.
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
S
/N
0
measurements, which is critical in closing and maintaining a stable power control loop
mechanism. The “Power Hunt” function is used in place of the closed loop control, and is
described later in this appendix.
G.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
much attenuation. This can possibly limit the terminal from reaching budgeted margin and/or
terminal maximum power.
Power Reference calibration returns an error if the modulator output level is >0 dBm. If this
happens, you are notified to reevaluate budget values or cable attenuation before Power