User's Manual

Designing an LGCell Solution
7-46 LGCell 4.0 Installation, Operation, and Reference Manual PN 8100-40
620004-0 Rev. B
7.5.2 Uplink Attenuation
The attenuation between the LGCell’s REVERSE port and the base station does two
things:
1. It attenuates the noise coming out of the LGCell.
2. It attenuates the desired signals coming out of the LGCell.
Setting the attenuation on the uplink is a trade-off between keeping the noise and
maximum signal levels transmitted from the LGCell to the base station receiver low
while not reducing the SNR (signal-to-noise ratio) of the path from the LGCell RAU
inputs to the base station inputs. This SNR can not be better than the SNR of the
LGCell by itself, although it can be significantly worse.
For example, suppose we have a GSM LGCell system consisting of one Main Hub,
four Expansion Hubs, and 16 RAUs (1-4-16) with uplink NF=28 dB. (See Table 7-30
on page 7-37.) If we use 30 dB of attenuation between the LGCell’s reverse port and
the base station (which has its own noise figure of about 4 dB), the overall noise fig-
ure will be 35 dB. (Refer to the formula on page 7-35.) Thus, by using this amount of
attenuation, the SNR is reduced by 7 dB. That causes a 7 dB reduction in the uplink
coverage distance. Now, if the attenuation instead is 0 dB, the cascaded noise figure
is NF=28.01 dB, which implies that the uplink sensitivity is limited by the LGCell, a
desirable condition. But now the maximum signal from the LGCell into the base sta-
tion is as high as –40 dBm. This can cause problems for some base stations. We can
reduce the maximum received signal levels by using some attenuation. For instance,
if the attenuation is 10 dB, the maximum received signal is –50 dBm and the noise
level is reduced by 10 dB but the cascaded noise figure is still only 28.16 dB (for a
SNR reduction of only 0.15 dB). Even with a 20 dB attenuator, the cascaded noise
figure is 29.45 dB. This is an SNR reduction of 1.44 dB. So, in this situation it would
be good to use at least 10 dB of uplink attenuation but not more than 20 dB.
Rule of Thumb
A good rule of thumb is to set the uplink attenuation, A2+A3 in Figure 7-3 on
page 7-45, as follows:
A2+A3 < LGCell uplink NF + uplink gain (0 dB for reverse port) – BTS NF – 10dB
and round A2 down to the nearest convenient attenuation value.