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6-39
D-620003-0-20 Rev J
CONFIDENTIAL
Connecting a Main Hub to a Base Station
6.6.2
Uplink Attenuation
The attenuation between the Main Hub’s uplink port and the base station does two
things:
• It attenuates the noise coming out of Unison.
• It attenuates the desired signals coming out of Unison.
Setting the attenuation on the uplink is a trade-off between keeping the noise and
maximum signal levels transmitted from Unison 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 can not be better than the SNR of Unison by itself,
although it can be significantly worse.
For example, suppose we have a GSM Unison system consisting of one Main Hub,
four Expansion Hubs, and 32 RAUs (1-4-32) with uplink NF=22 dB. (Refer to
Table 6-28 on page 6-30.) If we use 30 dB of attenuation between the Main Hub’s
uplink port and the base station (which has its own noise figure of about 4 dB), the
overall noise figure is 34.3 dB (refer to the formula on page 6-28) which is 12.3 dB
worse than Unison by itself. That causes a 12.3 dB reduction in the uplink coverage
distance. If the attenuation is 10 dB instead, the cascaded noise figure is NF=22.6 dB,
which implies that the uplink sensitivity is limited by Unison, a desirable condition.
Rule of Thumb
A good rule of thumb is to set the uplink attenuation, A2+A3 in Figure 6-3 on
page 6-38, as follows:
A2+A3
≈
Unison uplink NF + uplink gain (0 dB for reverse port) – BTS NF – 10dB
and round A2
down
to the nearest convenient attenuation value.
