5.28 Sidelink Discontinuous Reception (DRX)
38.3213GPPMedium Access Control (MAC) protocol specificationNRRelease 17TS
5.28.1 General
The MAC entity may be configured by RRC with an SL DRX functionality that controls the UE’s SCI (i.e., 1st stage SCI and 2nd stage SCI) monitoring activity for unicast, groupcast and broadcast. When using SL DRX operation, the MAC entity shall also monitor SCI (i.e., 1st stage SCI and 2nd stage SCI) according to requirements found in other clauses of this specification.
RRC controls Sidelink DRX operation by configuring the following parameters:
– sl-drx-onDurationTimer/sl-DRX-GC-BC-OndurationTimer: the duration at the beginning of an SL DRX cycle;
– sl-drx-SlotOffset: the delay before starting the sl-drx-onDurationTimer/sl-DRX-GC-BC-OndurationTimer;
– sl-drx-InactivityTimer/sl-DRX-GC-InactivityTimer (except for the SL broadcast communication): the duration after the first slot of SCI (i.e., 1st stage SCI and 2nd stage SCI) reception in which an SCI indicates a new SL transmission for the MAC entity;
– sl-drx-RetransmissionTimer/sl-DRX-GC-RetransmissionTimer (per Sidelink process except for the SL broadcast process): the maximum duration until an SL retransmission is received;
– sl-drx-StartOffset: the slot where the SL DRX cycle starts;
– sl-drx-Cycle/sl-DRX-GC-BC-Cycle: the Sidelink DRX cycle;
– sl-drx-HARQ-RTT-Timer/sl-DRX-GC-HARQ-RTT-Timer (per Sidelink process except for the SL broadcast process): the minimum duration before an SL HARQ retransmission is expected by the MAC entity.
5.28.2 Behaviour of UE receiving SL-SCH Data
When SL DRX is configured, the Active Time includes the time while:
– sl-drx-onDurationTimer/sl-DRX-GC-BC-OndurationTimer or sl-drx-InactivityTimer/sl-DRX-GC-InactivityTimer is running; or
– sl-drx-RetransmissionTimer/sl-DRX-GC-RetransmissionTimer is running; or
– period of sl-LatencyBoundCSI-Report configured by RRC in case SL-CSI reporting MAC CE is not received; or
– the time between the transmission of the request of SL-CSI reporting and the reception of the SL-CSI reporting MAC CE in case SL-CSI reporting MAC CE is received; or
– Slot(s) associated with the announced periodic transmission(s) by the UE transmitting SL-SCH Data; or
– the time between transmission/reception of Direct Link Establishment Request message (TS 24.587 [28]) or ProSe Direct Link Establishment Request message (TS 24.554 [29]) and reception of RRCReconfigurationSidelink message including initial DRX configuration or the link establishment procedure being aborted by upper layer; or
– the time between transmission of RRCReconfigurationSidelink message including initial DRX configuration and reception of corresponding RRCReconfigurationCompleteSidelink or RRCReconfigurationFailureSidelink message.
When one or multiple SL DRX is configured, the MAC entity shall:
1> if a single sl-DRX-GC-BC-Cycle that is mapped with one or multiple SL-QoS-Profile is configured to a Destination and interested cast type is associated to groupcast or broadcast:
2> select the sl-DRX-GC-BC-Cycle that is mapped with one or multiple SL-QoS-Profile associated with the Destination:
2> select the sl-DRX-GC-BC-OndurationTimer that is mapped with one or multiple SL-QoS-Profile associated with the Destination.
1> else if multiple sl-DRX-GC-BC-Cycle that are mapped with multiple SL-QoS-Profile are associated to a Destination Layer-2 ID and interested cast type is associated to groupcast or broadcast:
2> select the sl-DRX-GC-BC-Cycle whose length is the shortest one among multiple sl-DRX-GC-BC-Cycle that are mapped with multiple SL-QoS-Profile associated with the Destination Layer-2 ID:
2> select the sl-DRX-GC-BC-OndurationTimer whose length is the longest one among multiple sl-DRX-GC-BC-OndurationTimer that are mapped with multiple SL-QoS-Profile associated with the Destination Layer-2 ID.
1> if a single sl-DRX-GC-InactivityTimer that is mapped with one or multiple SL-QoS-Profile is configured to a Destination and interested cast type is associated to groupcast:
2> select the sl-DRX-GC-InactivityTimer that is mapped with one or multiple SL-QoS-Profile associated with the Destination:
1> else if multiple sl-DRX-GC-InactivityTimer that are mapped with multiple SL-QoS-Profile of a Destination Layer-2 ID and interested cast type is associated to groupcast:
2> select sl-DRX-GC-InactivityTimer whose length is the longest one among multiple sl-DRX-GC-InactivityTimer that are mapped with multiple SL-QoS-Profile associated with the Destination Layer-2 ID.
1> if an sl-drx-HARQ-RTT-Timer/sl-drx-HARQ-RTT-Timer1/sl-drx-HARQ-RTT-Timer2 expires:
2> if the data of the corresponding Sidelink process was not successfully decoded or if the HARQ feedback (i.e., negative acknowledgement) is not transmitted for unicast due to UL/SL prioritization:
3> start the sl-drx-RetransmissionTimer/sl-DRX-GC-RetransmissionTimer for the corresponding Sidelink process in the first slot after the expiry of sl-drx-HARQ-RTT-Timer1 or sl-drx-HARQ-RTT-Timer2.
When the cast type is groupcast or broadcast as indicated by upper layer, the sl-drx-StartOffset and sl-drx-SlotOffset are derived from the following equations:
sl-drx-StartOffset (ms) = Destination Layer-2 ID modulo sl-DRX-GC-BC-Cycle (ms).
sl-drx-SlotOffset (ms) = Destination Layer-2 ID modulo the number of slots in one subframe (ms).
1> if the SL DRX cycle is used, and [(DFN × 10) + subframe number] modulo (sl-drx-Cycle or sl-DRX-GC-BC-Cycle) = sl-drx-StartOffset:
2> start sl-drx-onDurationTimer/sl-DRX-GC-BC-OndurationTimer after sl-drx-SlotOffset from the beginning of the subframe.
1> if an SL DRX is in Active Time:
2> monitor the SCI (i.e., 1st stage SCI and 2nd stage SCI) in this SL DRX.
2> if the SCI indicates a new SL transmission:
3> if Source Layer-1 ID of the SCI is equal to the 8 LSB of the intended Destination Layer-2 ID and Destination Layer-1 ID of the SCI is equal to the 16 LSB of the intended Source Layer-2 ID and the cast type indicator in the SCI is set to unicast:
4> start or restart sl-drx-InactivityTimer for the corresponding Source Layer-2 ID and Destination Layer-2 ID pair in the first slot after SCI reception.
3> if Destination Layer-1 ID of the SCI (i.e., 2nd stage SCI) is equal to the 16 LSB of the intended Destination Layer-1 ID and the cast type indicator in the SCI is set to groupcast:
4> start or restart sl-drx-InactivityTimer for the corresponding Destination Layer-2 ID in the first slot after SCI reception.
2> if the SCI indicates an SL transmission:
3> if a next retransmission opportunity is indicated in the SCI:
4> derive the sl-drx-HARQ-RTT-Timer from the retransmission resource timing of the next retransmission resource in the SCI.
3> else if PSFCH resource is configured for the SL grant associated to the SCI:
4> set the sl-drx-HARQ-RTT-Timer based on sl-drx-HARQ-RTT-Timer1 configured by upper layer when HARQ feedback is enabled, or based on sl-drx-HARQ-RTT-Timer2 configured by upper layer when HARQ feedback is disabled, for resource pool configured with PSFCH.
3> else (i.e., if PSFCH resource is not configured for the SL grant associated to the SCI):
4> set the sl-drx-HARQ-RTT-Timer as 0 slots.
3> if PSFCH resource is not configured for the SL grant associated to the SCI:
4> start the sl-drx-HARQ-RTT-Timer2 for the corresponding Sidelink process in the slot following the end of PSSCH transmission (i.e., currently received PSSCH).
3> if PSFCH resource is configured for the SL grant associated to the SCI:
4> if HARQ feedback is enabled by the SCI and the cast type indicator in the SCI is set to unicast; or
4> if HARQ feedback is enabled by the SCI and the cast type indicator in the SCI is set to groupcast and positive-negative acknowledgement is selected;
5> start the sl-drx-HARQ-RTT-Timer1 for the corresponding Sidelink process in the first slot after the end of the corresponding PSFCH transmission carrying the SL HARQ feedback; or
5> start the sl-drx-HARQ-RTT-Timer1 for the corresponding Sidelink process in the first slot after the end of the corresponding PSFCH resource for the SL HARQ feedback when the SL HARQ feedback is not transmitted due to UL/SL prioritization;
4> if HARQ feedback is enabled by the SCI and the cast type indicator in the SCI is set to groupcast and negative-only acknowledgement is selected;
5> start the sl-drx-HARQ-RTT-Timer1 for the corresponding Sidelink process in the first slot after the end of the corresponding PSFCH transmission carrying the SL HARQ feedback; or
5> start the sl-drx-HARQ-RTT-Timer1 for the corresponding Sidelink process in the first slot after the end of the corresponding PSFCH resource for the SL HARQ feedback when the SL HARQ feedback is not transmitted due to UL/SL prioritization; or
5> start the sl-drx-HARQ-RTT-Timer1 for the corresponding Sidelink process in the first slot after the end of the corresponding PSFCH resource for the SL HARQ feedback when the SL HARQ feedback is a positive acknowledgement.
4> if HARQ feedback is disabled by the SCI and the resource(s) for one or more retransmission opportunities is not scheduled in the SCI:
5> start the sl-drx-HARQ-RTT-Timer2 for the corresponding Sidelink process in the slot following the end of PSFCH resource.
4> if HARQ feedback is disabled by the SCI and the resource(s) for one or more retransmission opportunities is scheduled in the SCI:
5> start the sl-drx-HARQ-RTT-Timer2 for the corresponding Sidelink process in the slot following the end of PSSCH transmission (i.e., currently received PSSCH).
NOTE: Void.
3> stop the sl-drx-RetransmissionTimer/sl-DRX-GC-RetransmissionTimer for the corresponding Sidelink process.
1> if the SCI indicates a new transmission where the cast type is set to groupcast is transmitted:
2> start or restart sl-DRX-GC-InactivityTimer for the corresponding Destination Layer-2 ID in the first slot after SCI transmission.
1> if an SL DRX Command MAC CE is received for the Source Layer-2 ID and Destination Layer-2 ID pair of a unicast:
2> stop sl-drx-onDurationTimer for the Source Layer-2 ID and Destination Layer-2 ID pair of a unicast;
2> stop sl-drx-InactivityTimer for the Source Layer-2 ID and Destination Layer-2 ID pair of a unicast.
5.28.3 Behaviour of UE transmitting SL-SCH Data
The UE transmitting SL-SCH Data should keep aligned with its intended UE receiving the SL-SCH Data regarding the SL DRX Active time as specified in clause 5.28.2.
Furthermore, the UE transmitting SL-SCH Data determines the SL DRX Active time based on SL DRX timers that are running (e.g., sl-drx-onDurationTimer, sl-drx-InactivityTimer, sl-drx-RetransmissionTimer) or will be running in the future (e.g., sl-drx-onDurationTimer, sl-drx-InactivityTimer, sl-drx-RetransmissionTimer) at the UE(s) receiving SL-SCH data. The UE may select resource for the initial transmission of groupcast within the time when sl-drx-onDurationTimer or sl-drx-InactivityTimer of the destination is running.
NOTE 1: A UE may assume that a resource for retransmission is in the Active time if an initial transmission causes the sl-drx-RetransmissionTimer to be started at the receiving UE.
NOTE 2: A UE may send SL DRX Command MAC CE to receiving UE for unicast and when to send SL DRX Command MAC CE is up to UE implementation.
The MAC entity shall for each pair of the Source Layer-2 ID and the Destination Layer-2 ID corresponding to each PC5-RRC connection which has been established by upper layers:
1> if the SL DRX Command indication has been triggered by the UE:
2> if the MAC entity has SL resources allocated for new transmission and the SL-SCH resources can accommodate the SL DRX Command MAC CE and its subheader as a result of logical channel prioritization:
3> instruct the Multiplexing and Assembly procedure to generate a Sidelink DRX Command MAC CE as defined in clause 6.1.3.52;
3> cancel the triggered SL DRX Command indication.
2> else if the MAC entity has been configured with Sidelink resource allocation mode 1:
3> trigger a Scheduling Request.