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6-41
D-620003-0-20 Rev J
CONFIDENTIAL
Connecting a Main Hub to a Base Station
6.6.3
RAU Attenuation and ALC
The RAU attenuation and Automatic Level Control (ALC) are set using the OpsCon-
sole or AdminManager Advanced RAU Settings command.
Embedded within the uplink RF front-end of each Unison Remote Access Unit is an
ALC circuit. This ALC circuit protects the Unison system from overload and exces-
sive intermodulation products due to high-powered mobiles or other signal sources
that are within the supported frequency band and are in close proximity to the RAU.
The Unison uplink ALC circuit operates as a feedback loop. A power detector mea-
sures the level of the uplink RF input and if that level exceeds –30 dBm, an RF atten-
uator is activated. The level of attenuation is equal to the amount that the input
exceeds –30 dBm. The following sequence describes the operation of the ALC cir-
cuit, as illustrated in Figure 6-4.
1.
The RF signal level into the RAU rises above the activation threshold
(–30 dBm), causing the ALC loop to enter into the attack phase.
2.
During the attack phase, the ALC loop increases the attenuation (0 to 30 dB) until
the detector reading is reduced to the activation threshold. The duration of this
attack phase is called the attack time.
3.
After the attack time, the ALC loop enters the hold phase and maintains a fixed
attenuation so long as the high-level RF signal is
present
.
4.
The RF signal level drops below the release threshold (–45 dBm) and the ALC
loop enters the release phase.
5.
During the release phase, the ALC loop holds the attenuation for a fixed period
then quickly releases the attenuation.
An important feature of the ALC loop is that in Step 3, the attenuation is maintained
at a fixed level until the signal drops by a significant amount. This prevents the ALC
loop from tracking variations in the RF signal itself and distorting the waveform
modulation.
Figure 6-4
ALC Operation
Input Signal
Level
Activation
Level
-30dBm
Release
Level
-45dBm
Output Signal
Level
Time
Hold
Phase
Release
Phase
Attack
Phase
3
1
2
4
5
