9.2.8 Beam failure detection and recovery

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

For beam failure detection, the gNB configures the UE with beam failure detection reference signals (SSB or CSI-RS) and the UE declares beam failure when the number of beam failure instance indications from the physical layer reaches a configured threshold before a configured timer expires. For beam failure detection in multi-TRP operation, the gNB configures the UE with two sets of beam failure detection reference signals each associated with a TRP, and the UE declares beam failure for a TRP when the number of beam failure instance indications associated with the corresponding set of beam failure detection reference signals from the physical layer reaches a configured threshold before a configured timer expires.

SSB-based Beam Failure Detection is based on the SSB associated to the initial DL BWP and can only be configured for the initial DL BWPs and for DL BWPs containing the SSB associated to the initial DL BWP. Besides, SSB-based Beam Failure Detection can be also performed based on the non-cell defining SSB, if configured for RedCap UEs. For other DL BWPs, Beam Failure Detection can only be performed based on CSI-RS.

After beam failure is detected on PCell, the UE:

– triggers beam failure recovery by initiating a Random Access procedure on the PCell;

– selects a suitable beam to perform beam failure recovery (if the gNB has provided dedicated Random Access resources for certain beams, those will be prioritized by the UE).

– includes an indication of a beam failure on PCell in a BFR MAC CE if the Random Access procedure involves contention-based random access.

Upon completion of the Random Access procedure, beam failure recovery for PCell is considered complete.

After beam failure is detected on an SCell, the UE:

– triggers beam failure recovery by initiating a transmission of a BFR MAC CE for this SCell;

– selects a suitable beam for this SCell (if available) and indicates it along with the information about the beam failure in the BFR MAC CE.

Upon reception of a PDCCH indicating an uplink grant for a new transmission for the HARQ process used for the transmission of the BFR MAC CE, beam failure recovery for this SCell is considered complete.

After beam failure is detected for a TRP of Serving Cell, the UE:

– triggers beam failure recovery by initiating a transmission of a BFR MAC CE for this TRP;

– selects a suitable beam for this TRP (if available) and indicates whether the suitable (new) beam is found or not along with the information about the beam failure in the BFR MAC CE for this TRP.

Upon reception of a PDCCH indicating an uplink grant for a new transmission for the HARQ process used for the transmission of the BFR MAC CE for this TRP, beam failure recovery for this TRP is considered complete.

After beam failure is detected for both TRPs of PCell, the UE:

– triggers beam failure recovery by initiating a Random Access procedure on the PCell;

– selects a suitable beam for each failed TRP (if available) and indicates whether the suitable (new) beam is found or not along with the information about the beam failure in the BFR MAC CE for each failed TRP;

– upon completion of the Random Access procedure, beam failure recovery for both TRPs of PCell is considered complete.