5.27 Small Data Transmission

38.3213GPPMedium Access Control (MAC) protocol specificationNRRelease 17TS

5.27.1 General

The MAC entity may be configured by RRC with SDT and the SDT procedure may be initiated by RRC layer. The SDT procedure can be performed either by Random Access procedure with 2-step RA type or 4-step RA type (i.e., RA-SDT) or by configured grant Type 1 (i.e., CG-SDT).

RRC configures the following parameters for SDT procedure:

– sdt-DataVolumeThreshold: data volume threshold for the UE to determine whether to perform SDT procedure;

– sdt-RSRP-Threshold: RSRP threshold for UE to determine whether to perform SDT procedure;

– cg-SDT-RSRP-ThresholdSSB: an RSRP threshold configured for SSB selection for CG-SDT.

The MAC entity shall, if initiated by the upper layers for SDT procedure:

1> if the data volume of the pending UL data across all RBs configured for SDT is less than or equal to sdt-DataVolumeThreshold; and

NOTE: For SDT procedure, the MAC entity also considers the suspended RBs configured with SDT for data volume calculation. It is up to the UE’s implementation how the UE calculates the data volume for the suspended RBs. Size of the CCCH message is not considered for data volume calculation

1> if the RSRP of the downlink pathloss reference is higher than sdt-RSRP-Threshold; or

1> if sdt-RSRP-Threshold is not configured:

2> if the Serving Cell for SDT is configured with supplementary uplink as specified in TS 38.331 [5]; and

2> if the RSRP of the downlink pathloss reference is less than rsrp-ThresholdSSB-SUL:

3> select the SUL carrier.

2> else:

3> select the NUL carrier.

2> if CG-SDT is configured on the selected UL carrier, and TA of the configured grant Type 1 resource is valid in the first available CG occasion according to clause 5.27.2; and

2> if, for each RB having data available for transmission, configuredGrantType1Allowed, if configured, is configured with value true for the corresponding logical channel; and

2> if at least one SSB configured for CG-SDT with SS-RSRP above cg-SDT-RSRP-ThresholdSSB is available:

3> indicate to the upper layers that the conditions for initiating SDT procedure are fulfilled;

3> perform CG-SDT procedure on the selected UL carrier according to clause 5.8.2.

2> else if a set of Random Access resources for performing RA-SDT are selected according to clause 5.1.1b on the selected UL carrier:

3> if cg-SDT-TimeAlignmentTimer is running, consider cg-SDT-TimeAlignmentTimer as expired and perform the corresponding actions in clause 5.2;

3> indicate to the upper layers that the conditions for initiating SDT procedure are fulfilled.

2> else:

3> indicate to the upper layers that the conditions for initiating SDT procedure are not fulfilled.

1> else:

2> indicate to the upper layers that the conditions for initiating SDT procedure are not fulfilled.

If RA-SDT is selected above and after the Random Access procedure is successfully completed (see clause 5.1.6), the UE monitors PDCCH addressed to C-RNTI until the RA-SDT procedure is terminated. If CG-SDT is selected above and after the initial transmission for CG-SDT is performed, the UE monitors PDCCH addressed to C-RNTI and CS-RNTI until the CG-SDT procedure is terminated.

5.27.2 TA Validation for CG-SDT

RRC configures the following parameters for validation for CG-SDT:

– cg-SDT-RSRP-ChangeThreshold: RSRP threshold for the increase/decrease of RSRP for time alignment validation.

The MAC entity shall, upon the reception of CG-SDT configuration:

1> store the RSRP of the downlink pathloss reference derived based on the measObject configured for the Serving Cell as in TS 38.331 [5].

The MAC entity shall consider the TA of the initial CG-SDT transmission with CCCH message to be valid when the following conditions are fulfilled:

1> The RSRP values for the stored downlink pathloss reference and the current downlink pathloss reference are valid according to TS 38.133 [11]; and

1> Compared to the stored downlink pathloss reference RSRP value, the current RSRP value of the downlink pathloss reference calculated as specified in TS 38.133 [11] has not increased/decreased by more than cg-SDT-RSRP-ChangeThreshold, if configured; and

1> cg-SDT-TimeAlignmentTimer is running.