LTE: Incase of handover, when will UE recieves Target eNB systemInformation(SIB,MIB) ?

Question

Lets suppose UE has sent mesurementReport to Source eNB and Source eNB respond with rrcConnectionReconfiguration including Target eNB's infor( PCI..etc). My question is whether UE will first sync in DL direction with target eNB and then sends rrcConnectionReconfiguationComplete. Then UE will read MIB and SIB's ?

Answer 1

Yes After sending rrcConnection Reconfiguration complete to target eNB then UE reads target eNB MIB/SIBS.

Answer 2

Before UE sends RRC Reconfiguration Complete , it should be in sync with target ENB, as without sync UE can not send any message to ENB.

So Flow will be

i> UE Gets RRC Reconfiguration from Source with Target CRNTI, Target ENB Security Info ...)
ii> After that UE disconnects with the Source ENB.
iii> Then UE try to sync with Target ENBs, by reading MIB and SIB info of target ENBs then UE is synchronized with the network.
iv> After in sync with Target ENB , UE does Contention free RACH based on the info shared by ENB in Reconfiguration message.
v> Now After RACH , Ue is gets UL Allocation to send any Uplink Message.
vi> After UL Allocation UE sends RRC Reconfiguration complete message to Target ENB.

Comments

point iii - UE try to sync to target eNB but does not reads MIB and SIB of target eNB. For sync UE needs to read PSS and SSS of the target cell.

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Sorry,in the step iv,where is the Reconfiguration message from,the source ENB?or the Target ENB?

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UE receives RRC Reconfiguration from source eNB which acts as handover command to UE and ue latches to target eNB and sends RRC Reconfiguration complete to Target eNB.

Answer 3

As Veer Pal also mentioned , point iii) is wrong , UE will not read MIB and SIB first for synchronization . UE will read PSS and SSS for DL synchronization first and then will receive MIB and then SIB.Then after getting RACH related info from SIB2 , UE will attempt for RA process and will get UL synchronization if successful.

Comments

If it is send before read mib and sib, what is the initial rach power and ramp step?

Answer 4

Hi ,
As far as I know both EnodeB should be in sync with the help of GPS.
If both ENBs are sync over a GPS and UE receives mobility control info from source ENB,UE tries to do contention free rach to the target cell with the help of preamble index(RACH-ConfigDedicated) and prach ,rach configuratio(same as SIB2 of target cell) received in mobility control info.
Observation : If both ENBs are not in sync UE will fail to do rach on the target cell and after maximum retransmission of preamble UE will send re-establishment to target cell with reestablishment cause "handover failure".

Since dedicated rach preamble is optional field in mobility control info,if it is not present as part of mobility control info UE may go for contention based rach procedure during handover which may cause handover failure if contention is not resolved for UE.