Tuesday, 3 November 2015

RF interference trouble shooting


1     Introduction
RF interference is disturbance that affects an electrical circuit due to either
electromagnetic induction or electromagnetic radiation emitted from an external
source. The disturbance may interrupt, obstruct, or otherwise degrade or limit
the effective performance of the circuit. These effects can range from a simple
degradation of data to a total loss of data. The source may be any object,
artificial or natural, that carries rapidly changing electrical currents, such
as an electrical circuit, the Sun or the Northern Lights.
Because of widespread use of wireless communication, the interference source could be
anywhere to affect the performance of a commercial network. Therefore PMI (Passive
Intermodulation) test must be finished before the deployment of commercial
network.
However we still cannot avoid the RF interference completely, because it is easy to launch an
interference source, and the small operators are not willing to do PMI
test due to stingy manner. Therefore it is necessary to check and locate the
interference in the equipment side such as eNB
BTS.
2     Analysis and Trouble shooting procedure
2.1              RSSI and TSG
1.1.1         2.1.1 RSSI
RSSIreceived signal strength indicatoris a measurement of the power present  received signal in the antenna input port. When there is no traffic in the eNB, the value of RSSI is considered as floor noise. In the commercial scenario, floor noise must be equal to or less than -95dbm, for example – 100dbm or -105dbm.
When it is detected that the floor noise is larger than -95dbm such as -80dbm, trouble shooting needs to process. Generally there are 2 possible reasons. One is internal interference ( within the equipment and feeder) and the other is external interference ( normally from noise source outside).
1.1.2                2.1.2 Internal signal source---TSG
TSG is an internal signal source and a basic function of RRU version, which is supported by each types of
RRU. It is a group of constant data, which sends stable signals and power. It is useful for the checking of output power, VSWR characteristics among different paths and interference as well.
2.2     Trouble shooting
1.1.1         2.2.1 Turn on TSG
When we make sure there is no traffic in the eNB, TSG can be turned on by the following
command.
TSG on
TX1 TSG BspTsgSwitch 0,1
TX2 TSG BspTsgSwitch 1,1
TSG off and status recover
TX1 TSG BspTsgSwitch 0,0
TX2 TSG BspTsgSwitch 1,0
Check status:  showrru
1.1.2         2.2.2 RSSI Checking
1.     Turn on the PA and record the RSSI value for each path
2.     Turn off the PA and record the RSSI value for each path
3.     Command is shown as follows
PA0 offpasw 0,0
PA1 offpasw 1,0
PA0 offpasw 0,1
PA1 offpasw 1,1
check RSSi: showrru
2.3    Analysis
1.2.1         2.3.1 External interference
If the RSSI value is still higher than -95dbm with PA off, it is concluded that there is an
interference resource outside the LTE system. It is necessary to process the
frequency scanning with spectrum analyzer and locate the resource.
1.2.2         2.3.2 Internal interference
If the RSSI value returns to normal with PA off, it is concluded that there is an interference
resource inside the LTE system. It is probably caused by the following reasons:
1.     The downlink output influences the uplink receiver, and DPD alarm may occur in RRU.
2.     Turn off the PA and record the RSSI value for each path
3.     There is something wrong with feeder or the connector. For example, if combiner is used for both TDD and FDD power input, the combiner or the connection between combiner and feeder need to be checked.
1.2.3         2.3.3 Example
An example is shown in the table-1. It is concluded that
1.     For ant-1, there is an external interference resource with the power of -81dbm.
2.     For ant-1, there is internal interference (intermodulation) with the power of -64dbm
3.    For ant-0, there is internal interference (intermodulation) with the power of -75dbm
PA0 status
PA1 statu
Rx0 BbRssi[0][0](dBm)
Rx2
  BbRssi[0][0](dBm)
off
Off
-96.88
-81.49
On
Off
-75.88
-81.29
Off
On
-98.88
-64.09
On
On
-75.68
-63.09
 
Note: TSG must be turned off after trouble shooting. Otherwise the service will not recover.

Budi Prasetyo

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