S1AP HO - HO FAILURE ISSUES

Question

In S1 based HO -
FIRST SCENARIO -> enodeB receives a HO command message and delivers the HO command mesesage in RRCconnection reconfiguration but due to poor radio condition or interference this message is not received , under this scenario will it be a HO failure and therefore for a RRC connection reastablishment procedure,

SECOND SCENARIO -> Say there is a S1ap HO preparation failure message from MME to source enodeB under this situation what will be next step..Will the UE remain to be served under the source enodeB - what will be the RRC message transfer on the air interface (if any)

Answer 1

In first scenario - eNodeB is sending the HO command through RRC reconfiguration to UE but UE is not receiving this. It means RLF is happening at UE before receiving the HO command. So I think this will not be HO failure case. In this case UE should trigger rrc connection re-establishment with cause other.

In second scenario - UE will continue with serving cell and no message will be send by eNOdeB to UE if HO preparation is getting failed.

Comments

Hi veer

Thanks for the answer - for the first scenario if i consider as a RLF with a reastablishment cause other then can anyone tell me the 1 or 2 scenarios when the cause will be stated as handover failure

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After receiving HO command through RRC Connection reconfiguration from eNodeB, UE need to try sync with target eNodeB. Suppose UE is not  able to get RACH successful with target eNodeB then UE will try RRC connection re establishment with cause HO failure.

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Thanks for the prompt reply... now my confusion is to which cell UE will send  RRC reastablishment request if the serving cell has a bad RLF and there is a HO failure with the target enodeB

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UE sends rrc reestablishment to the cell for which he is getting good RSRP. It may be serving or target or any other cell. RRC reestablishment will be successful with serving cell or with target cell if target cell still have the UE context prepared during HO preparation. For other cell rrc reestablishment will be unsuccessful.

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thanks for the reply.. correct me if i am wrong by the understanding - RRC reastablishment procedure means reestablishment of srb1 and AS security , so if the reastablishment is done on the target enodeB in this context this is not a HO , now under this scenario where HO has failed due to sync failure on target enodeB but UE already received the HO command( RRC connection reconfiguration) in which all the configurations prepared in target enodeB is given to UE, and UE and enodeB both has the relevant context established like the radio bearers, under this scenario will the radio bearers remain suspended and only SRB1 will be established through rrc reastablishment procedure...

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radio bearers will be resumed if rrc reestablishment is successful

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Hi
slight confusion regarding the fact that if all the radio bearers reestablish in target enodeB through the successful commencement of the reastablishment procedure even after the failure of HO by not getting synced at the target , will this be considered as a successful HO to the target enodeB.

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Yes this will be considered successful HO at target.