Comtech EF Data Vipersat 564L User Manual page 118

D e s c r i p ti o n
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 E
whether the transmit power is high or low, the modulator transmit output level
is adjusted until the E
power adjustment.
If the received E
the Speed Up Eb/No parameter, then the DPC messages will be transmitted at a
faster rate until the nominal level is restored.
Higher Order Modulation BER Waterfall Mapping
DPC target E
curves stored in the CDM-570/570L modems. The calculations are based on the
received VMS multi-command message configuration (i.e., bit rate, modulation,
FEC) lookup per BER table and used to modify the target E
acceptable bit performance over all possible waveform configurations. This
feature is an embedded function and is automatic.
Delta Rain Fade Power Compensation
DPC offsets in modem power that are necessary during rain fade conditions are
now applied to incoming switch commands from the VMS. This prevents possi-
ble link failures due to power value changes associated with these switch
commands.
A parameter called BaseLine Power is a function of the power given the modem
by the VMS for the last switch command. At boot-up, and prior to receiving
switch commands from the VMS, this parameter will match the Nominal Power
Level value. If DPC adjusts due to changes in environment or other factors, the
adjustment will appear in the DPC Offset, and will remain until environmental
conditions change or a unit reset occurs. The resultant modem power will be
the addition of the BaseLine Power and the DPC offset.
C-4
/N
b
/N should, for any reason, fall below a minimum value set by
b o
/N values are automatically adjusted using the BER waterfall
b o
Example
A Remote modem has the home state bit rate set at 128 kbps, with a
BaseLine Power value of -15 dBm. When the VMS sends a switch command
for a new bit rate of 256 kbps, it will send a corresponding power value of
-12 dBm (new BaseLine) to compensate for the additional energy required to
maintain C/N.
If, prior to a switch, a fade condition causes the power to increase by 2 dB at
the home state bit rate, the resulting modem power will be -13 dBm, creating
a DPC Offset value of 2 dB. Now when the VMS sends a switch command
to 256 kbps, the modem power will be adjusted to -10 dBm (-12 dBm plus
is within the range set by the Target Range for no
o
V i p e r s a t C D D - 5 6 X S e r i e s U s e r G u i d e
/N . Depending on
b o
/N to sustain an
b o