Comtech EF Data SLM-5650A Vipersat - Manual (Page 124)

Description
C-4 Vipersat SLM-5650A User Guide
Offset adjustments to the modulator transmit power during rain fade conditions
are applied to incoming switch commands from the VMS. This prevents possi-
ble link failures due to power value changes associated with changes in band-
width, modulation, etc.
In more detail, the BaseLine Power obtains its value from either the Home State
Power Level setting or the power given to the modem by the VMS during 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 a DPC
adjustment occurs due to changes in environment or other factors, the adjust-
ment will appear in the DPC Offset, and will remain until environmental condi-
tions change or a unit reset occurs. The resultant modem power will be the
combined sum of the BaseLine Power and the DPC Offset.
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
2 dB). This prevents loss of the link due to power value changes based on
fade conditions.
This feature is automatic and requires no operator involvement.
Theory of Operation
DPC is referenced from the commissioned Home State settings (Data Rate and
Modulator Power Level). The Home State calibration point relies on the basic
antenna commissioning procedures and relative link budgetary numbers. After
antenna alignment, the base (clear-sky) modulator power output is adjusted to
achieve the desired BER (E
b
/N
0
), typically while operating in switched SCPC
mode at the base rate. This commissioned power and the data rate make up part
of the Home State parameters which determine the BaseLine Power value that is
also set and matched in the VMS site policies.
The Max Power value is converted to power Margin control. This positive inte-
ger sets the upper limit on any data rate, modulation, and code rate, only allow-
ing power increases per the site Margin level. The example below, figure C-1,
shows the Max Power levels for a range of data rates from 64 kbps to 4.096