Will RRC connection re-establishment be successful to other than source eNB after Ho failure?

Question

How to inform MME about UE movement from Source eNB to Target eNB after UE sending Re-establishment with HO failure to target eNB during handover. In this case HO notify or path switch is applicable ?

Answer 1

Yes, it will. For the successful re-establishment procedure , target cell should be prepared. As part of handover either X2 or S1-AP, target first prepares resource for UE and then only source eNodeB sends RRC connection reconfiguration message to UE. In that case if handover gets failed and re-establishment (due to HO failure) reaches at the target cell or back to source cell, it will be handled.

As per my understanding, If UE sends re-establishment request message to other than these two cells, re-establishment procedure fails for that UE.

Looking for others to respond.

Answer 2

Hi,
RRC Connection reestablishment procedure is to reestablish AS security and resume the SRB1 configuration.
Now there are scenarios
Consider two ENBs are there ENB-A and ENB-B and UE initially attached to ENB-A
1 > UE sends the RRCConnectionReestablishmentRequest to the ENB-A with cause OtherFailure
In this case UE sends the C-RNTI(generated by ENB-A), phyCellID of ENB-A, ShortMAC-I.
Now at ENB-A side after receiving reestablishment request will try to get the UE context based on the C-RNTI and matching the shortMAC-I,since UE initially attached to the ENB-A,ENB-A will have UE context and sends reestablishment to the UE so that UE will resume SRB1 configuration and reactivate the AS security.
2 > UE sends the RRCConnectionReestablishmentRequest to the ENB-B with cause OtherFailure
In this case also UE will send the C-RNTI(generated by ENB-A), phyCellID of ENB-A, ShortMAC-I
Now at ENB-B side after receiving reestablishment request,ENB-B also will try to get UE context based on the CRNTI and shortMAC-I
but at ENB-B side UE context with respect this UE will not be present so ENB-B will send RLF indication(MRO case) to ENB-A if there is X2AP setup in between ENB-A and ENB-B else ENB-B will respond with reestablishment reject to the UE so that further UE can go for suitable cell selection.

Comments

Well explained !!!

Answer 3

Thanks Vimal

I am observing that RRCconnection re-establishment is not getting successful to the target eNB when HO is getting failed due to t304 timer expire. UE is trying re-establishment to the target eNB but UE is sending source eNB information in the re-establishment. Due to this it is getting failed. Is this the issue of UE or is this correct behavior?

Comments

RRCConnectionReestablishmentRequest-r8-IEs ::= SEQUENCE {
    ue-Identity                            ReestabUE-Identity,
    reestablishmentCause                ReestablishmentCause,
    spare                                BIT STRING (SIZE (2))
}

ReestabUE-Identity ::=                SEQUENCE {
    c-RNTI                                C-RNTI,
    physCellId                            PhysCellId,
    shortMAC-I                            ShortMAC-I
}

I have just copied from spec for your reference. Here C-RNTI which is allocated by source cell at the time of normal attach. Physical Cell Id (source cell) and shortMAC-I for (authentication purpose). If you have wireshark logs for S1-AP then please go to the handover-request message, mainly source to target transparent container. This container carries the information which may be required by target eNodeB/cell in case of handling of re-establishment procedure for an UE at target eNodeB.

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Answer 4

As per the 3gpp 36.331 if t304 is getting expired then UE needs to revert back to the configuration of source cell. Then try to send the reestablishement to the cell which UE is selecting as per the cell selection procedure.

UE can select source cell/HO prepared target cell other any other new cell for reestablishment. RRCConnection reestablishment at source cell/HO prepared target cell will be successful if UE context is not yet released by eNB. RRCConnection reestablishment to any other cell will fail.