User Manual

DELTANODE FIBER DAS MANUAL
©DeltaNode Solutions 2012
30
Revision 12-03
Antenna
RU
Remote Unit Fiber
FOI
FOI Card
POI
POI unit
BIU
BIU
BTS
Base Station
50
dB
-40
dB 20 dB -35 dB 7 dB
-80 dBm -30 dBm
-70
dBm -
50
dBm -85 dBm -78 dBm
-2 dB
-80 dBm
In the illustration above there is an input signal to the remote of -80 dBm. Then each step of the chain
has gain or attenuation as shown, the fiber loss is in total 40 dB, the FOI is set to 20 dB gain, the loss
through the POI is 35 dB, the BIU is set to 7 dB gain and the loss on the jumper between the Master Unit
and the Radio Base Station is 2 dB.
This means that the signal level entered into the Remote Unit is seen by the base station. This is
considered an optimal point setting for the uplink when it comes to signal level. If the net gain in the
uplink is positive, we also put noise on to the base station and will desensitize its receiver. This may not
be a problem, if the Base Station is dedicated to only the Fiber-DAS system then a positive net gain is not
a problem because any desensitization caused by increased gain is compensated by an equal increase in
the useful signals level. Thus maintaining the same C/I.
However if the Base Station is also covering an outdoor area or has other antennas connected to the
same sector then a positive net gain will cause a desensitization of the receiver for the other antenna
and this is generally speaking a bad thing.
The system also has a thermal noise load that it will put on the base station, just like an antenna. The
noise figure of the system can be determined by using the Fiber-DAS calculator excel sheet detailed in
chapter using the Fiber-DAS calculator detailed later in this manual. The total noise is also an
accumulation of the noise posed by each chain and the net gain of the system.
If we have a system with 4 equally set up chains, and each chain has a noise load of 3 dB and the net gain
is 0 dB then the noise load on the system will be around 9 dB.
If we decrease the net gain in the uplink we can lower this noise as the system NF can be construed as
the NF + Net gain uplink times the number of equal chains. Since the chains are not in fact equal, they
will have different NF and different gain slightly it may be a good idea to calculate the total noise load:
Chain
NF
Gain
N. Load
Comment
1
3,2
-1,2
2,0
1
2,3
+2,3
4,6
2
2,8
+4,0
6,8
This is very high
3
4,0
-2,8
1,2
Total
10,2
By lowering the net gain to -5 dB on all chains we get the following:
Chain
NF
Gain
N. Load
Comment
1
3,2
-5
-1,8
1
2,3
-5
-2,7
2
2,8
-5
-2,2