5.17 Beam Failure Detection and Recovery procedure

38.3213GPPMedium Access Control (MAC) protocol specificationNRRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The MAC entity may be configured by RRC per Serving Cell with a beam failure recovery procedure which is used for indicating to the serving gNB of a new SSB or CSI-RS when beam failure is detected on the serving SSB(s)/CSI-RS(s). Beam failure is detected by counting beam failure instance indication from the lower layers to the MAC entity. If beamFailureRecoveryConfig is reconfigured by upper layers during an ongoing Random Access procedure for beam failure recovery for SpCell, the MAC entity shall stop the ongoing Random Access procedure and initiate a Random Access procedure using the new configuration. When the SCG is deactivated, the UE performs beam failure detection on the PSCell if bfd-and-RLM is set to true.

RRC configures the following parameters in the beamFailureRecoveryConfig, beamFailureRecoverySpCellConfig, beamFailureRecoverySCellConfig and the radioLinkMonitoringConfig for the Beam Failure Detection and Recovery procedure:

– beamFailureInstanceMaxCount for the beam failure detection (per Serving Cell or per BFD-RS set of Serving Cell configured with two BFD-RS sets);

– beamFailureDetectionTimer for the beam failure detection (per Serving Cell or per BFD-RS set of Serving Cell configured with two BFD-RS sets);

– beamFailureRecoveryTimer for the beam failure recovery procedure;

– rsrp-ThresholdSSB: an RSRP threshold for the SpCell beam failure recovery;

– rsrp-ThresholdBFR: an RSRP threshold for the SCell beam failure recovery or for the beam failure recovery of BFD-RS set of Serving Cell;

– powerRampingStep: powerRampingStep for the SpCell beam failure recovery;

– powerRampingStepHighPriority: powerRampingStepHighPriority for the SpCell beam failure recovery;

– preambleReceivedTargetPower: preambleReceivedTargetPower for the SpCell beam failure recovery;

– preambleTransMax: preambleTransMax for the SpCell beam failure recovery;

– scalingFactorBI: scalingFactorBI for the SpCell beam failure recovery;

– ssb-perRACH-Occasion: ssb-perRACH-Occasion for the SpCell beam failure recovery using contention-free Random Access Resources;

– ra-ResponseWindow: the time window to monitor response(s) for the SpCell beam failure recovery using contention-free Random Access Resources;

– prach-ConfigurationIndex: prach-ConfigurationIndex for the SpCell beam failure recovery using contention-free Random Access Resources;

– ra-ssb-OccasionMaskIndex: ra-ssb-OccasionMaskIndex for the SpCell beam failure recovery using contention-free Random Access Resources;

– ra-OccasionList: ra-OccasionList for the SpCell beam failure recovery using contention-free Random Access Resources;

– candidateBeamRSList: list of candidate beams for SpCell beam failure recovery;

– candidateBeamRS-List-r16: list of candidate beams for SCell beam failure recovery or beam failure recovery of BFD-RS set one of Serving Cell;

– candidateBeamRS-List2-r17: list of candidate beams for beam failure recovery of BFD-RS set two of Serving Cell.

The following UE variables are used for the beam failure detection procedure:

– BFI_COUNTER (per Serving Cell or per BFD-RS set of Serving Cell configured with two BFD-RS sets): counter for beam failure instance indication which is initially set to 0.

The MAC entity shall for each Serving Cell configured for beam failure detection:

1> if the Serving Cell is configured with two BFD-RS sets:

2> if beam failure instance indication for a BFD-RS set has been received from lower layers:

3> start or restart the beamFailureDetectionTimer of the BFD-RS set;

3> increment BFI_COUNTER of the BFD-RS set by 1;

3> if BFI_COUNTER of the BFD-RS set >= beamFailureInstanceMaxCount:

4> trigger a BFR for this BFD-RS set of the Serving Cell;

2> if BFR is triggered for both BFD-RS sets of the SpCell and the Beam Failure Recovery procedure is not successfully completed for any of the BFD-RS sets:

3> initiate a Random Access procedure (see clause 5.1) on the SpCell;

2> if the Serving Cell is SpCell and the Random Access procedure initiated for beam failure recovery of both BFD-RS sets of SpCell is successfully completed (see clause 5.1):

3> set BFI_COUNTER of each BFD-RS set of SpCell to 0.

3> consider the Beam Failure Recovery procedure successfully completed.

2> if the beamFailureDetectionTimer of this BFD-RS set expires; or

2> if beamFailureDetectionTimer, beamFailureInstanceMaxCount, or any of the reference signals used for beam failure detection is reconfigured by upper layers or by the BFD-RS Indication MAC CE associated with a BFD-RS set of the Serving Cell:

3> set BFI_COUNTER of the BFD-RS set to 0.

2> if a PDCCH addressed to C-RNTI indicating uplink grant for a new transmission is received for the HARQ process used for the transmission of the Enhanced BFR MAC CE or Truncated Enhanced BFR MAC CE which contains beam failure recovery information of this BFD-RS set of the Serving Cell:

3> set BFI_COUNTER of the BFD-RS set to 0;

3> consider the Beam Failure Recovery procedure successfully completed for this BFD-RS set and cancel all the triggered BFRs of this BFD-RS set of the Serving Cell.

2> if the Serving Cell is SCell and the SCell is deactivated as specified in clause 5.9:

3> set BFI_COUNTER of each BFD-RS set of SCell to 0;

3> consider the Beam Failure Recovery procedure successfully completed and cancel all the triggered BFRs of all BFD-RS sets of the Serving Cell.

1> else:

2> if beam failure instance indication has been received from lower layers:

3> start or restart the beamFailureDetectionTimer;

3> increment BFI_COUNTER by 1;

3> if BFI_COUNTER >= beamFailureInstanceMaxCount:

4> if the Serving Cell is SCell:

5> trigger a BFR for this Serving Cell;

4> else if the Serving Cell is PSCell and, the SCG is deactivated:

5> if beam failure of the PSCell has not been indicated to upper layers since the SCG was deactivated or since the deactivated SCG was last reconfigured with BFD-RS:

6> indicate beam failure of the PSCell to upper layers.

NOTE: After beam failure is indicated to upper layers, the UE may stop the beamFailureDetectionTimer and lower layer beam failure indication while BFI_COUNTER >= beamFailureInstanceMaxCount for the deactivated SCG.

4> else:

5> initiate a Random Access procedure (see clause 5.1) on the SpCell.

2> if the beamFailureDetectionTimer expires; or

2> if beamFailureDetectionTimer, beamFailureInstanceMaxCount, or any of the reference signals used for beam failure detection is reconfigured by upper layers associated with this Serving Cell:

3> set BFI_COUNTER to 0.

2> if the Serving Cell is SpCell and the Random Access procedure initiated for SpCell beam failure recovery is successfully completed (see clause 5.1):

3> set BFI_COUNTER to 0;

3> stop the beamFailureRecoveryTimer, if configured;

3> consider the Beam Failure Recovery procedure successfully completed.

2> else if the Serving Cell is SCell, and a PDCCH addressed to C-RNTI indicating uplink grant for a new transmission is received for the HARQ process used for the transmission of the MAC CE for BFR which contains beam failure recovery information of this Serving Cell; or

2> if the SCell is deactivated as specified in clause 5.9:

3> set BFI_COUNTER to 0;

3> consider the Beam Failure Recovery procedure successfully completed and cancel all the triggered BFRs for this Serving Cell.

The MAC entity shall:

1> if the Beam Failure Recovery procedure determines that at least one BFR has been triggered and not cancelled for an SCell for which evaluation of the candidate beams according to the requirements as specified in TS 38.133 [11] has been completed and if none of the Serving Cell(s) of this MAC entity are configured with two BFD-RS sets:

2> if UL-SCH resources are available for a new transmission and if the UL-SCH resources can accommodate the BFR MAC CE plus its subheader as a result of LCP:

3> instruct the Multiplexing and Assembly procedure to generate the BFR MAC CE.

2> else if UL-SCH resources are available for a new transmission and if the UL-SCH resources can accommodate the Truncated BFR MAC CE plus its subheader as a result of LCP:

3> instruct the Multiplexing and Assembly procedure to generate the Truncated BFR MAC CE.

2> else:

3> trigger the SR for SCell beam failure recovery for each SCell for which BFR has been triggered, not cancelled, and for which evaluation of the candidate beams according to the requirements as specified in TS 38.133 [11] has been completed.

1> if the Beam Failure Recovery procedure determines that at least one BFR for any BFD-RS set has been triggered and not cancelled for an SCell for which evaluation of the candidate beams according to the requirements as specified in TS 38.133 [11] has been completed; or

1> if the Beam Failure Recovery procedure determines that at least one BFR for only one BFD-RS set has been triggered and not cancelled for an SpCell for which evaluation of the candidate beams according to the requirements as specified in TS 38.133 [11] has been completed; or

1> if the Beam Failure Recovery procedure determines that at least one BFR has been triggered and not cancelled for an SCell for which evaluation of the candidate beams according to the requirements as specified in TS 38.133 [11] has been completed and if at least one Serving Cell of this MAC entity is configured with two BFD-RS sets:

2> if UL-SCH resources are available for a new transmission and if the UL-SCH resources can accommodate the Enhanced BFR MAC CE plus its subheader as a result of LCP:

3> instruct the Multiplexing and Assembly procedure to generate the Enhanced BFR MAC CE.

2> else if UL-SCH resources are available for a new transmission and if the UL-SCH resources can accommodate the Truncated Enhanced BFR MAC CE plus its subheader as a result of LCP:

3> instruct the Multiplexing and Assembly procedure to generate the Truncated Enhanced BFR MAC CE.

2> else:

3> trigger the SR for beam failure recovery of each BFD-RS set for which BFR has been triggered, not cancelled, and for which evaluation of the candidate beams according to the requirements as specified in TS 38.133 [11] has been completed;

All BFRs triggered for an SCell shall be cancelled when a MAC PDU is transmitted and this PDU includes a MAC CE for BFR which contains beam failure information of that SCell. All BFRs triggered for a BFD-RS set of a Serving Cell shall be cancelled when a MAC PDU is transmitted and this PDU includes an Enhanced BFR MAC CE or Truncated Enhanced BFR MAC CE which contains beam failure recovery information of that BFD-RS set of the Serving Cell.