User's Manual

5-44 InterReach Unison Accel Installation, Operation, and Reference Manual PN 9000-10
620021-0 Rev. A
5.5.2 Uplink Attenuation
The attenuation between the Main Hub’s uplink port and the base station does two
things:
1. It attenuates the noise coming out of Unison.
2. 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 Hub and 8
RAUs (1-8) with uplink NF=22 dB. (See Table 5-32 on page 5-35.) 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 will be 34.3 dB (refer to the
formula on page 5-33) which is 12.3 dB worse than Unison by itself. That causes a
12.3 dB reduction in the uplink coverage distance. Now, if the attenuation instead is
10 dB, the cascaded noise figure is NF=22.6 dB, which implies that the uplink sensi-
tivity 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 5-3 on
page 5-43, 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.