User's Manual Part 5

ROI-S05749 SYSTEM SETUP
3-23
3.5.3 Automatic Transmitter Power Control
The automatic transmit power control (ATPC) function automatically
varies the TX output power according to path conditions. In the SHF and
EHF band, fading exerts heavy influences on propagation, causing the
receive signal level at the opposite station to vary. The ATPC function
operates by controlling the transmit output power of the opposite station
according to the variation of the received signal level at the local station.
ATPC provides the following advantages:
• Improvement in up fading characteristics
• Improvement in residual BER characteristics
• Reduction of interference to intra system
• Reduction of interference to inter system
Note: In the XPIC configuration, ATMC/MTPC setup and action
control in the Sub Master station are applied from the Main
Master station.
A functional block diagram of the ATPC operation is shown in Fig.
3-4.
ATPC improves the BER characteristics under adverse changes in
climatic conditions and reduces the possibility of interference. To
implement ATPC, the received level (RX LEV) is detected by the
Receiver (RX) in the ODU and passed to the CPU in the CTRL
module. The CPU then determines whether the transmit output
power needs to be controlled. This is based on the transmit output
power and the minimum and maximum values of the output control
range (ATPC range). ATPC is relevant for the receiving threshold
level that were previously specified using the LCT or PNMT (as
ATPC Threshold Level).
A control signal (POWER CTRL), whose function is to maintain the
received RX signal level (RSL) by decreasing or increasing the TX
output power of the opposite station, is generated by the CTRL
module through the MD Unit. This control signal is based on the
result of comparison between the current receiver input level and
the preset receiving threshold level. This control signal is sent to the
opposite station to control its transmit output power.
At the opposite station, this control signal is detected by the CTRL
module. The TR Unit, in accordance with this control signal,
produces a control that will either raise, lower or maintain the
current TX output power.