Comtech EF Data SLM-5650A Vipersat - Manual (Page 123)

Appendix C - Dynamic Power Control C-3
Description
Description
The behavior of the Dynamic Power Control feature is designed to regulate
power by utilizing the uplink margin to limit the maximum effective transmitted
power (EIRP
C
) of the terminal. The DPC function in the IP-enabled Vipersat-
optioned modems provides controls allowing an operator to input site budget
numbers that compensate and limit modulator output power. This power is
based on link margin and maximum transmission parameter settings (Data Rate,
Modulation, and Code Rate), as referenced to Home State base configurations.
The starting point for all enabled modulators is the Home State Power Level.
This modulator output power is applied during initial system startup and is the
baseline power at which the remote site will initially transmit. The modulator
will continue to transmit at the baseline power as long as the site remains in
STDMA mode.
When the modem is switched out of STDMA mode and into SCPC mode, the
modulator will begin to receive DPC packets from the allocated demodulator
(DPC must be enabled on both ends of the link). The packets are unicast to the
modulator via the IP address. Once the packet is received, the modem then
compares the received E
b
/N
0
value to the stored value, immediately trimming
the transmit power level accordingly. Depending on whether the received value
is low or high, the modulator transmit output level is adjusted up or down until
the E
b
/N
0
meets the defined target for no power adjustment.
If the received E
b
/N
0
should, for any reason, fall below a minimum value set by
the Speed Up Eb/No parameter, then the DPC messages will be transmitted at a
faster rate until the nominal level is restored.
BER Waterfall Mapping
DPC Target Eb/No values are automatically adjusted using the higher order
modulation BER waterfall curves. These BER curves, as defined by the modem
specifications, provide greater accuracy of the DPC function throughout all TPC
modes. The compensated calculations are based on the received VMS multi-
command message for an SCPC switch that configures the frequency, power,
bit rate, modulation, and code rate. If the configuration differs from the base
settings as compared to the BER table, the Target E
b
/N
0
is modified to sustain
an acceptable bit performance over all possible waveform configurations.
Delta Power Compensation
When DPC is adjusting power because of environmental conditions, such as
rain fade, the adjusted power becomes a delta DPC Offset from the BaseLine
Power parameter setting. This offset is applied to any power value received
through remote dynamic switching commands. Briefly described, any DPC