Why S1 handover prepapration fails with cause unknown target?

Question

This is observed in intra MME handover

MME sends handover preparation failure to source eNB with cause unknown target ID
Both eNBs are under same TAC

What could be reason for this failure with this cause?

Answer 1

As you have mentioned that both the eNodeBs are under same TAC but it is not necessary that both the eNodeBs have connection with same MME. Please verify , Is s1 setup done for both the eNodeBs using wireshark or tcpdump ? I am suspecting that target eNodeB is not registered with MME.

Comments

Thank you.

Target nodeB is registered with MME.

Have tried handover in reverse order where target eNB is source and source is target eNB. in this case also  same failure - unknown target ID

In  handover required message - target eNB is detected correctly but failure happens in preparation phase

Answer 2

Are you adding neighbor information in NRT using ANR or manually. If you are adding neighbor information in NRT manually then i think need to check the neighbor cell id in the NRT. It may be possible that neighbor cell id in NRT is different from the actual neighbor cell id. Due to this mismatch MME can not find the target eNB so that MME can send MME preparation failure to source eNB.

Comments

yes neighbor was added manually.

issue resolved. TAC conflict was causing the issue

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Hi Alkesh, I am facing same issue but in my case "LTE intra frequency HO execution failure rate" this KPi is very Poor, Neighbor are created using ANR.
NOKIA RL60 . Could you please suggest some solution. 40-50 Cells are having >50 Failure. Thanks

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I think you need to synchronise the a2/a3 event threshold so that HO should trigger at correct time to the target.

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@Alkesh - can you tell me what do you mean by TAC conflict...Is it that the TAC reported from the enodeb to the mme in the S1setup request not matching in the mme...can you provide more information on this..

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This was the case where two MMEs were using the same TAC

So when MME get HO prep message it was not able to route correctly to target eNB

There should be unique TAC

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If MMEs are in pooling then there might be possiblilities of same TAC in both MME , if we have two MMEs in NW . Then what need to do