5.21 LBT operation

38.3213GPPMedium Access Control (MAC) protocol specificationNRRelease 17TS

5.21.1 General

The lower layer may perform an LBT procedure, see TS 37.213 [18], according to which a transmission is not performed by lower layers if the channel is identified as being occupied. When lower layer performs an LBT procedure before a transmission and the transmission is not performed, an LBT failure indication is sent to the MAC entity from lower layers. Unless otherwise specified, when LBT procedure is performed for a transmission, actions as specified in this specification are performed regardless of if an LBT failure indication is received from lower layers. When LBT is not performed by the lower layers, LBT failure indication is not received from lower layers.

5.21.2 LBT failure detection and recovery procedure

The MAC entity may be configured by RRC with a consistent LBT failure recovery procedure. Consistent LBT failure is detected per UL BWP by counting LBT failure indications, for all UL transmissions, from the lower layers to the MAC entity.

RRC configures the following parameters in the lbt-FailureRecoveryConfig:

– lbt-FailureInstanceMaxCount for the consistent LBT failure detection;

– lbt-FailureDetectionTimer for the consistent LBT failure detection;

The following UE variable is used for the consistent LBT failure detection procedure:

– LBT_COUNTER (per Serving Cell): counter for LBT failure indication which is initially set to 0.

For each activated Serving Cell configured with lbt-FailureRecoveryConfig, the MAC entity shall:

1> if LBT failure indication has been received from lower layers:

2> start or restart the lbt-FailureDetectionTimer;

2> increment LBT_COUNTER by 1;

2> if LBT_COUNTER >= lbt-FailureInstanceMaxCount:

3> trigger consistent LBT failure for the active UL BWP in this Serving Cell;

3> if this Serving Cell is the SpCell:

4> if consistent LBT failure has been triggered in all UL BWPs configured with PRACH occasions on same carrier in this Serving Cell:

5> indicate consistent LBT failure to upper layers.

4> else:

5> stop any ongoing Random Access procedure in this Serving Cell;

5> switch the active UL BWP to a UL BWP, on same carrier in this Serving Cell, configured with PRACH occasion and for which consistent LBT failure has not been triggered;

5> initiate a Random Access Procedure (as specified in clause 5.1.1).

1> if all triggered consistent LBT failures are cancelled in this Serving Cell; or

1> if the lbt-FailureDetectionTimer expires; or

1> if lbt-FailureDetectionTimer or lbt-FailureInstanceMaxCount is reconfigured by upper layers:

2> set LBT_COUNTER to 0.

The MAC entity shall:

1> if consistent LBT failure has been triggered, and not cancelled, in the SpCell; and

1> if UL-SCH resources are available for a new transmission in the SpCell and these UL-SCH resources can accommodate the LBT failure MAC CE plus its subheader as a result of logical channel prioritization:

2> instruct the Multiplexing and Assembly procedure to generate the LBT failure MAC CE.

1> else if consistent LBT failure has been triggered, and not cancelled, in at least one SCell:

2> if UL-SCH resources are available for a new transmission in a Serving Cell for which consistent LBT failure has not been triggered and these UL-SCH resources can accommodate the LBT failure MAC CE plus its subheader as a result of logical channel prioritization:

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

2> else:

3> trigger a Scheduling Request for LBT failure MAC CE.

1> if a MAC PDU is transmitted and LBT failure indication is not received from lower layers and this PDU includes the LBT failure MAC CE:

2> cancel all the triggered consistent LBT failure(s) in SCell(s) for which consistent LBT failure was indicated in the transmitted LBT failure MAC CE.

1> if consistent LBT failure is triggered and not cancelled in the SpCell; and

1> if the Random Access procedure is considered successfully completed (see clause 5.1) in the SpCell:

2> cancel all the triggered consistent LBT failure(s) in the SpCell.

1> if lbt-FailureRecoveryConfig is reconfigured by upper layers for a Serving Cell:

2> cancel all the triggered consistent LBT failure(s) in this Serving Cell.