LTE: What the eNodeB does when there is no target eNodeB is available to handover an UE ?

Question

Need clarity on following items:
1. What is the use of "UE selected plmn" information for an eNodeB ?
2. Is the ECGI/TAI constructed based on the primary plmn (PLMN broadcasted in 0th index in SIB1 ) or based on the UE selected PLMN ?

Answer 1

1. UE will do intra frequency measurements if there is a suitable cell then UE will gives the Target cell id based on that enodeb will decide to do s1 or x2 handover.
2. If there is no intra frequency cell then UE will search for inter frequency cell even if there no inter frequency cell then enodeb asks to measure inter RAT.. In all the cases UE will report about neighbour cell id.
   If there is no suitable, inter frequency and inter RAT CELLS then UE will report 1F event and if radio link is very poor then UE Will lost radio connection.

Answer 2

If UE is sending meas for HO and eNB does not have any neighbour in nrt corresponding to PCI send in meas by UE . Depends upon implementation below can be performed by eNB ...
1. If anr is supported or not enabled at eNB then eNB will not perform HO preparation.
2. If anr is enabled then eNB can request to UE to reportcgi. Then eNB will upstate the nrt and can perform S1 HO .

Answer 3

If I understood your question correctly then my answer is as below :
Here I assumed that all eNodeBs of the network have X2 connection with HO enabled.
As UE moves away from serving eNodeB/cell, radio signal strength gets weak. UE reports to eNodeB by sending event A2 for the serving cell. If UE is unable to detect and report possible target cell(s) and serving cell is not able to deliver data due to poor radio channel condition, source cell/eNodeB send Error Indication (say failure in radio interface) to MME.

MME may initiate detach procedure towards the UE.
Whatever I explained above is an example when UE is moving out of coverage of a serving network. In case when UE is moving out of coverage then only UE would not be able to report any target cell/eNodeB .