5G NR Mobility- Radio link failure

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 only be configured for the initial DL BWP and for DL BWPs containing the SSB associated to the initial DL BWP. For other DL BWPs, RLM can only be performed based on CSI-RS.

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

– Expiry of a 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

– Random access procedure failure; or

– RLC failure.

After RLF is declared, the UE:

– stays in RRC_CONNECTED;

– 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.

Reference – 3GPP