User's Manual

Designing a Fusion Wideband Solution
Page 102 InterReach Fusion Wideband Installation, Operation, and Reference Manual
© 2015 TE Connectivity D-620616-0-20 Rev K • TECP-77-044 Issue 9 • March 2015
Uplink Attenuation
The attenuation between the Main Hub’s uplink port and the associated band’s Base Station
reduces both the noise level and the desired signals out of Fusion Wideband. Setting the
attenuation on the uplink is a trade-off between keeping the noise and maximum signal levels
transmitted from Fusion Wideband to the Base Station receiver low while not reducing the SNR
(Signal-to-Noise Ratio) of the path from the RAU inputs to the Base Station inputs. This SNR
cannot be better than the SNR of Fusion Wideband by itself, although it can be significantly worse.
A good rule of thumb is to set the uplink attenuation so that the noise level o
ut of Fusion
Wideb
and is within 10 dB of the Base Station’s sensitivity.
RAU Attenuation and ALC
The RAU attenuation and ALC are set using the AdminBrowser Edit Unit Properties screen.
Embedded within the uplink RF front-end of each Fusion Wideband RAU b
and is an ALC circuit.
This ALC circuit protects the Fusion Wideband system from overload and excessive
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.
Each individual Band (1or 2) of a Fusion Wideband RAU has an uplink ALC circu
it that operates
as a feedback loop. A power detector measures the level of each band’s uplink RF input and if that
level exceeds –30 dBm, an RF attenuator 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 circuit, as illustrated in Figure 16 on page 103.
1 The
RF signal level into either Band of the RAU rises above the activation thre
shold (–30
dBm), causing that 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) a
nd 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.