9.2.7 Radio Link Failure

38.3003GPPNRNR and NG-RAN Overall descriptionRelease 17Stage 2TS

In RRC_CONNECTED, the UE performs Radio Link Monitoring (RLM) in the active BWP based on reference signals (SSB/CSI-RS) and signal quality thresholds configured by the network. SSB-based RLM is based on the SSB associated to the initial DL BWP and can be configured for the initial DL BWP and for DL BWPs containing the SSB associated to the initial DL BWP. Besides, SSB-based RLM can be also performed based on the non-cell defining SSB, if configured for RedCap UEs. For other DL BWPs, RLM can only be performed based on CSI-RS. In case of DAPS handover, the UE continues the detection of radio link failure at the source cell until the successful completion of the random access procedure to the target cell.

The UE declares Radio Link Failure (RLF) when one of the following criteria are met:

– Expiry of a radio problem timer started after indication of radio problems from the physical layer (if radio problems are recovered before the timer is expired, the UE stops the timer); or

– Expiry of a timer started upon triggering a measurement report for a measurement identity for which the timer has been configured while another radio problem timer is running; or

– Random access procedure failure; or

– RLC failure; or

– Detection of consistent uplink LBT failures for operation with shared spectrum channel access as described in 5.6.1; or

– For IAB-MT, the reception of a BH RLF indication received from its parent node.

After RLF is declared, the UE:

– stays in RRC_CONNECTED;

– in case of DAPS handover, for RLF in the source cell:

– stops any data transmission or reception via the source link and releases the source link, but maintains the source RRC configuration;

– if handover failure is then declared at the target cell, the UE:

– selects a suitable cell and then initiates RRC re-establishment;

– enters RRC_IDLE if a suitable cell was not found within a certain time after handover failure was declared.

– in case of CHO, for RLF in the source cell:

– selects a suitable cell and if the selected cell is a CHO candidate and if network configured the UE to try CHO after RLF then the UE attempts CHO execution once, otherwise re-establishment is performed;

– enters RRC_IDLE if a suitable cell was not found within a certain time after RLF was declared.

– otherwise, for RLF in the serving cell or in case of DAPS handover, for RLF in the target cell before releasing the source cell:

– selects a suitable cell and then initiates RRC re-establishment;

– enters RRC_IDLE if a suitable cell was not found within a certain time after RLF was declared.

When RLF occurs at the IAB BH link, the same mechanisms and procedures are applied as for the access link. This includes BH RLF detection and RLF recovery.

The IAB-DU can transmit a BH RLF detection indication to its child nodes in the following cases:

– The collocated IAB-MT initiates RRC re-establishment;

– The collocated IAB-MT is dual-connected, detects BH RLF on a BH link, and cannot perform UL re-routing for any traffic. This includes the scenario of an IAB-node operating in EN-DC or NR-DC, which uses only one link for backhauling and has BH RLF on this BH link;

– The collocated IAB-MT has received a BH RLF detection indication from a parent node, and there is no remaining backhaul link that is unaffected by the BH RLF condition indicated.

Upon reception of the BH RLF detection indication, the child node may perform local rerouting for upstream traffic, if possible, over an available BH link.

If the IAB-DU has transmitted a BH RLF detection indication to a child node due to an RLF condition on the collocated IAB-MT’s parent link, and the collocated IAB-MT’s subsequent RLF recovery is successful, the IAB-DU may transmit a BH RLF recovery indication to this child node.

If the IAB-DU has transmitted a BH RLF detection indication to a child node due to the reception of a BH RLF detection indication by the collocated IAB-MT, and the collocated IAB-MT receives a BH RLF recovery indication, the IAB-DU may also transmit a BH RLF recovery indication to this child node.

Upon reception of the BH RLF recovery indication, the child node reverts the actions triggered by the reception of the previous BH RLF detection indication.

In case the RRC re-establishment procedure fails, the IAB-node may transmit a BH RLF indication to its child nodes. The BH RLF detection indication, BH RLF recovery indication and BH RLF indication are transmitted as BAP Control PDUs.