Sidelink supports SL DRX for unicast, groupcast, and broadcast. Similar parameters as defined in clause 11
for Uu (on-duration, inactivity-timer, retransmission-timer, cycle) are defined for SL to determine the SL active time for SL DRX. During the SL active time, the UE performs SCI monitoring for data reception (i.e., PSCCH and 2nd stage SCI on PSSCH). The UE may skip monitoring of SCI for data reception during SL DRX inactive time.
The actual parameters supported for each cast type (unicast, groupcast, broadcast) are specified in the following clauses.
The SL active time of the RX UE includes the time in which any of its applicable SL on-duration timer(s), SL inactivity-timer(s) or SL retransmission timer(s) (for any of unicast, groupcast, or broadcast) are running. In addition, the slots associated with announced periodic transmissions by the TX UE and the time in which a UE is expecting CSI report following a CSI request (for unicast) are considered as SL active time of the RX UE.
The TX UE maintains a set of timers corresponding to the SL DRX timers in the RX UE(s) for each pair of source/destination L2 ID for unicast or destination L2 ID for groupcast/broadcast. When data is available for transmission to one or more RX UE(s) configured with SL DRX, the TX UE selects resources taking into account the active time of the RX UE(s) determined by the timers maintained at the TX UE.
The UE can determine from SIB12 whether the gNB supports SL DRX or not.
A default SL DRX configuration for groupcast/broadcast can be used for discovery message in sidelink discovery in clause 16.9.5
and for relay discovery messages in clause 16.12.3
For unicast, SL DRX is configured per pair of source L2 ID and destination L2 ID.
The UE maintains a set of SL DRX timers for each direction per pair of source L2 ID and destination L2 ID. The SL DRX configuration for a pair of source/destination L2 IDs for a direction may be negotiated between the UEs in the AS layer. For SL DRX configuration of each direction, where one UE is the TX UE and the other is the RX UE:
RX UE may send assistance information, which includes its desired SL on-duration timer, SL DRX start offset, and SL DRX cycle, to the TX UE and the mode 2 TX UE may use it to determine the SL DRX configuration for the RX UE;
Regardless of whether assistance information is provided or not, the TX UE in RRC_IDLE/ RRC_INACTIVE/ OOC, or in RRC_CONNECTED and using mode 2 resource allocation, determines the SL DRX Configuration for the RX UE. For a TX UE in RRC_CONNECTED and using mode 1 resource allocation, the SL DRX configuration for the RX UE is determined by the serving gNB of the TX UE;
TX UE sends the SL DRX configuration to be used by the RX UE to the RX UE;
The RX UE may accept or reject the SL DRX configuration.
A default SL DRX configuration for groupcast/broadcast can be used for DCR messages.
When the TX UE is in RRC_CONNECTED, the TX UE may report the received assistance information to its serving gNB and sends the SL DRX configuration to the RX UE upon receiving the SL DRX configuration in dedicated RRC signalling from the gNB. When the RX UE is in RRC_CONNECTED, the RX UE can report the received SL DRX configuration to its serving gNB, e.g. for alignment of the Uu and SL DRX configurations.
SL on-duration timer, SL inactivity-timer, SL HARQ RTT timer, and SL HARQ retransmission timer are supported in unicast. SL HARQ RTT timer and SL HARQ retransmission timer are maintained per SL process at the RX UE. In addition to (pre)configured values for each of these timers, SL HARQ RTT timer value can be derived from the retransmission resource timing when SCI indicates more than one transmission resource.
SL DRX MAC CE is introduced for SL DRX operation in unicast only.
For groupcast/broadcast, SL DRX is configured commonly among multiple UEs based on QoS profile and Destination L2 ID. Multiple SL DRX configurations can be supported for each of groupcast/broadcast.
SL on-duration timer, SL inactivity-timer, SL HARQ RTT and SL retransmission timers are supported for groupcast. Only SL on-duration timer is supported for broadcast. SL DRX cycle, SL on-duration, and SL inactivity timer (only for groupcast) are configured per QoS profile. The starting offset and slot offset of the SL DRX cycle is determined based on the destination L2 ID. The SL HARQ RTT timer (only for groupcast) and SL HARQ retransmission timer (only for groupcast) are not configured per QoS profile or per destination L2 ID. For groupcast, the RX UE maintains a SL inactivity timer for each destination L2 ID, and selects the largest SL inactivity timer value if multiple SL inactivity timer values associated with different QoS profiles are configured for that L2 ID. For groupcast and broadcast, the RX UE maintains a single SL DRX cycle (selected as the smallest SL DRX cycle of any QoS profile of that L2 ID) and single SL on-duration (selected as the largest SL on-duration of any QoS profile of that L2 ID) for each destination L2 ID when multiple QoS profiles are configured for that L2 ID.
For groupcast, SL HARQ RTT timer and SL retransmission timer are maintained per SL process at the RX UE. SL HARQ RTT timer can be set to different values to support both HARQ enabled and HARQ disabled transmissions.
A default SL DRX configuration, common between groupcast and broadcast, can be used for a QoS profile which is not mapped onto any non-default SL DRX configuration(s).
In-coverage TX and RX UEs in RRC_IDLE/RRC_INACTIVE obtain their SL DRX configuration from SIB. UEs (TX or RX) in RRC_CONNECTED can obtain the SL DRX configuration from SIB, or from dedicated RRC signalling during handover. For the out of coverage case, the SL DRX configuration is obtained from pre-configuration.
For groupcast, the TX UE restarts its timer corresponding to the SL inactivity timer for the destination L2 ID (used for determining the allowable transmission time) upon reception of new data with the same destination L2 ID.
TX profile is introduced to ensure compatibility for groupcast and broadcast transmissions between UEs supporting/not-supporting SL DRX functionality. A TX profile is provided by upper layers to AS layer and identifies one or more sidelink feature group(s). Multiple TX profiles with the support of SL DRX and without the support of SL DRX can be associated to a destination L2 ID. A TX UE only assumes SL DRX for the destination L2 IDs when all the associated TX profiles correspond to support of SL DRX. A Tx UE assumes no SL DRX for the destination L2 ID if there is no associated TX profile. An RX UE determines that SL DRX is used if all destination L2 IDs of interest are assumed to support SL DRX. For groupcast, the UE reports each destination L2 ID and associated SL DRX on/off indication to the gNB.
Alignment of Uu DRX and SL DRX for a UE in RRC_CONNECTED is supported for unicast, groupcast, and broadcast. Alignment of Uu DRX and SL DRX at the same UE is supported. In addition, for mode 1 scheduling, the alignment of Uu DRX of the TX UE and SL DRX of the RX UE is supported.
Alignment may comprise of either full overlap or partial overlap in time between Uu DRX and SL DRX. For SL RX UEs in RRC_CONNECTED, alignment is achieved by the gNB.