16.9.6.2 Unicast

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, SL DRX slot offset, and SL DRX cycle, to the TX UE and the TX UE using mode 2 resource allocation 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.

When the TX UE is in RRC_CONNECTED and using mode 1 resource allocation, the TX UE may report the received assistance information or the received SL DRX configuration reject information to its serving gNB supporting SL DRX 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 and using mode 1 resource allocation, the RX UE can report the received SL DRX configuration to its serving gNB supporting SL DRX, 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 HARQ RTT timer can be set to different values to support both HARQ enabled and HARQ disabled transmissions.

SL DRX MAC CE is introduced for SL DRX operation in unicast only.