Adjustment For Data Rate - Comtech EF Data Vipersat CDM-570 User Manual

Table C-1
Target Range
Speed Up
Eb/No
The start point for all enabled modulators is the Nominal Power Level. This
level is measured during initial system turn-up and is the power level at which
all modems will initially transmit when a call is established. Once communica-
tion is established and data starts to flow, the modulator begins receiving DPC
packets from the demodulator and adjusting its transmit power level accord-
ingly.
The transmit power level is compared to the received Eb/No. Depending on
whether the transmit power is high or low, the modulator transmit output level
is adjusted in steps (step size set by parameters Max Step Up/Down Power)
until the Eb/No is within the range set by the Target Range for no power adjust-
ment.
If the received Eb/No should, for any reason, fall below a minimum value set by
the Speed Up Eb/No parameter then the PLDM messages will be transmitted at
a faster rate until the nominal level is restored.

Adjustment for Data Rate

The maximum output power level is scaled according to the data rate at which
the modulator is transmitting. Consider, for example, a system that normally
runs at a data rate of 512 kbps with a Nominal Power Level of -10 dBm and a
Maximum Power Level of -5 dBm. If a voice switch occurs that sets the data
rate to 32 kbps, the power requirements are much lower than the nominal -10
dBm level. With the above process, it is theoretically possible for the power to
be raised all the way to -5 dBm, which is past the saturation point for such a
small data rate. To prevent the use of excessive power that will drown out the
signal and cause damage to the satellite, the modulator calculates a maximum
power that is appropriate for its current data rate.
The net result of the DPC algorithm is that the transmit power of all video carri-
ers is adjusted such that the receiving modems see a constant Eb/No value. This
optimizes receive signal quality and use of system resources since it can lead to
a reduction in power amplifier intermodulation and also a conserving of satellite
transmit power.
A p p e n d i x D y n a m i c P o w e r C o n t r o l
C -
Dynamic Power Control Parameters
Parameter Default
0.2 dB Target range for no power adjustment; No
adjustment occurs when power is within this range
5 dB DPC and PLDM process speeded up from default
(1/min) if Eb/No is below this amount
Significance
C-3