15.5.2.2.2 Connection failure due to intra-system mobility

One of the functions of Mobility Robustness Optimization is to detect connection failures that occur due to Too Early or Too Late Handovers, or Handover to Wrong Cell. These problems are defined as follows:

• Intra-system Too Late Handover: an RLF occurs after the UE has stayed for a long period of time in the cell; the UE attempts to re-establish the radio link connection in a different cell.
• Intra-system Too Early Handover: an RLF occurs shortly after a successful handover from a source cell to a target cell or a handover failure occurs during the handover procedure; the UE attempts to re-establish the radio link connection in the source cell.
• Intra-system Handover to Wrong Cell: an RLF occurs shortly after a successful handover from a source cell to a target cell or a handover failure occurs during the handover procedure; the UE attempts to re-establish the radio link connection in a cell other than the source cell and the target cell.

In the definition above, the "successful handover" refers to the UE state, namely the successful completion of the RA procedure.

In case of MCG LTM cell switch, the "successful handover" in the definitions above refers to a successful completion of the RA procedure in case of RACH-based LTM or when the UE determines that the network has successfully received its first UL data in case of RACH-less LTM.

In case of CHO, the Too Late Handover, Too Early Handover and Handover to Wrong Cell in the definition above means Too Late CHO Execution, Too Early CHO Execution and CHO Execution to Wrong Cell.

In case of LTM Cell switch, the Too Late Handover, Too Early Handover and Handover to Wrong Cell in the definition above means Too Late LTM cell switch, Too Early LTM cell switch and LTM cell switch to Wrong Cell.

In case of CHO with candidate SCG(s), the Too Late Handover, Too Early Handover and Handover to Wrong Cell in the definition above means Too Late CHO with candidate SCG(s), Too Early CHO with candidate SCG(s) and CHO with candidate SCG(s) Execution to Wrong Cell.

Detection mechanism

A failure indication may be initiated after a UE attempts to re-establish the radio link connection at NG-RAN node B after a failure at NG-RAN node A. NG-RAN node B may initiate the Failure Indication procedure towards multiple NG-RAN nodes if they control cells which use the PCI signalled by the UE during the re-establishment procedure. The NG-RAN node receiving this selects the UE context that matches the received Failure Cell ID and C-RNTI, and, if available, uses the shortMAC-I to confirm this identification, by calculating the shortMAC-I and comparing it to the received IE.

A failure indication may also be sent to the node last serving the UE when the NG-RAN node fetches the RLF REPORT from UE by triggering:

• The Failure Indication procedure over Xn;
• The Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer procedure over NG.

The detailed detection mechanisms for too late handover, too early handover and handover to wrong cell are carried out through the following in the NG-RAN node that served the UE before the reported connection failure:

• Intra-system Too Late Handover: there is no recent handover for the UE prior to the connection failure e.g. the UE reported timer is absent or larger than the configured threshold (e.g. Tstore_UE_cntxt), or if CHO/LTM or CHO with candidate SCG(s) is configured but the CHO/LTM execution is not initiated for the UE prior to the connection failure, e.g. the UE reported timer is absent or larger than the configured threshold (e.g. Tstore_UE_cntxt).
• Intra-system Too Early Handover: there is a recent handover/LTM cell switch for the UE prior to the connection failure e.g. the UE reported timer is smaller than the configured threshold (e.g. Tstore_UE_cntxt), and the first re-establishment attempt cell/the successful re-connect cell/the cell UE attempts LTM recovery is the cell that served the UE at the last handover/LTM initialisation or fall back to the source cell configuration in case of DAPS HO.
• Intra-system Handover to Wrong Cell: there is a recent handover/LTM cell switch for the UE prior to the connection failure e.g. the UE reported timer is smaller than the configured threshold (e.g. Tstore_UE_cntxt), and the first re-establishment attempt cell/ the cell UE attempts to re-connect/the cell UE attempts CHO recovery/the cell UE attempts LTM recovery is neither the cell that served the UE at the last handover/LTM initialisation nor the cell that served the UE where the RLF happened or the cell that the handover/LTM was initialized toward.

The "UE reported timer" above indicates the time elapsed since the last handover initialisation until connection failure or the time elapsed since the CHO/LTM execution until connection failure.

In case of Too Early Handover or Handover to Wrong Cell, the NG-RAN node receiving the failure indication may inform the NG-RAN node controlling the cell where the mobility configuration caused the failure by means of the Handover Report procedure over Xn or the Uplink RAN Configuration Transfer procedure over NG. This may include the RLF report.

In case of Too Late Handover when UE is configured with CHO with candidate SCG(s), the NG-RAN node receiving the failure indication may send a Handover Report over Xn to the candidate NG-RAN node(s) controlling the candidate PSCell(s) whose mobility configuration caused the failure.

For MRO analysis, in case of RLFs or handover failures, the source gNB may take into account the information regarding the LBT failures occurred in UL and DL for a specific UE. For UL, the information may be collected and provided by the UE to the network in the RLF report. For DL, the information may be collected at the source and target gNBs. The target gNB may provide the information to the source gNB via the Access and Mobility Indication procedure over Xn, if requested by the source gNB during the handover preparation.

Retrieval of information needed for problem analysis

In order to retrieve relevant information collected at the network side as part of the UE context, the UE provides C-RNTI used in the last serving cell. If the cause for the failure is identified as a "Too Early HO" or a "HO to Wrong Cell", the NG-RAN node controlling the last serving cell shall, include in the HANDOVER REPORT message the C-RNTI used in the source cell of the last completed handover before the failure. If the NG RAN node controlling that source cell provided the Mobility Information, it is also included in the HANDOVER REPORT message. If used, the Mobility Information is prepared at the source NG RAN node of a handover and may refer to or identify any handover-related data at this NG RAN node. In the Handover Preparation procedure, the source gNB may request the target gNB to provide information on DL LBT failures at the target gNB and to report it in case of mobility failure of the UE.

Handling multiple reports from a single failure event

In case the RRC re-establishment fails and the RRC connection setup succeeds, MRO evaluation of intra-RAT mobility connection failures may be triggered twice for the same failure event. In this case, only one failure event should be counted.