Monday, 14 September 2015

LTE can't SETUP

Problem Description
MBTS is configured with BTS and eNodeB.
Software version:
BTS: V100R012C00SP48
eNodeB: V100R003C00SPC430

BTS and eNodeB have both 3 shared sectors, all of them composed by 2 MRFUs.
Thus, we have the total amount of 3 sectors X 2 MRFUs/sector = 6 MRFUs.

Sector 3 has configured:
1 GCELL, composed by 3 TRX (TRX ID 5, 6, 7) in first MRFU and 3 TRX (TRX ID 8, 9 y 10) in second MRFU.
2 LTE Cell, using MIMO technique, so it has 1 LTE carrier in first MRFU and 1 LTE carrier in second MRFU.


First step is activation of GCELL: OK.
Second step is activation of LTE Cell: NOK. Alarm of " Cell Unavailable" is reported by eNodeB.

According to MSR Table combining different technologies in the same RF board (in this case, MRFU V2),
this configuration is OK: 3 GTRX + 1 LTE Cell on each MRFU.

Power configuration for each carrier matches the next table.


 



After activation of LTE Cell, " Cell Unavailable" alarm is reported in eNodeB.

No alarms are reported by BTS on that moment.





Handling Process
Next steps show the troubleshooting process executed:

1) Check power configuration: LST GTRXDEV 4G --> LST PDSCHCFG
     The power configured in 2G side and LTE Cell must match the MSR table or be below those values.

2) Check frequency planning: 2G -->LST GCELLFREQ  4G --> LST CELL
     Frequencies must be compatible.

3) Check transceivers modes in both ends of CPRI cable, this means, check they are 850nm and 2.5 Gb in LBBP and MRFU.

4) Check from time to time the status of the LTE Cell.
On the next day in the morning, before keeping on troubleshooting we found the LTE Cell was successfully setup from 3:05 a.m. 
We found this information executing DSP CELL and checking the last column showed on the next picture:



5) Check Main Board Log of GTMU.
    After checking main board log, we found something really interesting. At the beginning of the activation of the GCELL,
just second MRFU was connected. After some minutes, first MRFU connected to BBU
.
Due to have BCCH configured on TRX located in first MRFU, all carriers of first MRFU were automatically migrated to second MRFU,
so second MRFU had 6 GTRX configured as a consequence of first MRFU board disconnected.



This behaviour is caused by BTS function called "TRX Aid Function". When TRX with BCCH is down,
then BCCH is automatically re-configured on a different TRX available. However, if the GCELL contains two RF boards
(as in our case: 2 MRFU boards) and the whole board is down due to disconnection,
all GTRX will be migrated to another available board in the same sector.

After this automatic migration, second MRFU had 6 GTRX configured. Thus, LTE Cell could not setup.
Solution
There is no handling process for this issue because we found it was solved automatically.
However, there some important recommendations and suggestions in the next section.

Suggestions and Summary
To avoid this behaviour, you must always take care for next details:
1) Always connect all RF Boards before activating the respective cell, no matter LTE Cell or GSM Cell.
2) Before activating the GCELL, check the configuration of the TRX Aid Function. If it's active, you have two options:
a) Allow Aid Function and move back BCCH to its original position as soon as possible.
b) Allow Aid Function and move back BCCH after automatic Resource Check. This Resource Check is executed everyday at 3:00 a.m.  
This is why our LTE Cell could setup at 3:05: 5 minutes before, Resource Check was done by BTS and
all GTRX belonging to first MRFU moved back to first MRFU. After that, LTE Cell Setup succeeded.

To modify TRX Aid Function configuration in GCELL, please execute next commands:
          Configuration a) --> SET GCELLCCACCESS: IDTYPE=BYID, CELLID=100, TRXAIDSWITCH=ALLOWREIMMED;
          Configuration b) --> SET GCELLCCACCESS: IDTYPE=BYID, CELLID=100, TRXAIDSWITCH=ALLOWRECHECKRES;

Budi Prasetyo

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