16.12.6.1 Switching from single/multi-hop indirect to direct path

For service continuity of L2 U2N Relay, the following procedure is used, in case of L2 U2N Remote UE switching from indirect to direct path under the same gNB. The Figure 16.12.6.1-1a describes a single-hop indirect path to direct path switching and Figure 16.12.6.1-1b describes a multi-hop indirect path to direct path switching:

1. The Uu measurement configuration and measurement report signalling procedures are performed to evaluate both relay link measurement and Uu link measurement. The measurement results from L2 U2N Remote UE are reported when configured measurement reporting criteria are met. The sidelink relay measurement report shall include at least L2 U2N Relay UE's source L2 ID, serving cell ID (i.e., NCGI/NCI), and sidelink measurement quantity result. The sidelink measurement quantity can be SL-RSRP of the serving L2 U2N Relay UE, and if SL-RSRP is not available, SD-RSRP is used.

2. The gNB decides to switch the L2 U2N Remote UE onto direct Uu path.

3. The gNB sends the RRCReconfiguration message to the L2 U2N Remote UE. The L2 U2N Remote UE stops User Plane and Control Plane transmission via the L2 U2N Relay UE(s) after reception of the RRCReconfiguration message with the path switch configuration.

4. The L2 U2N Remote UE synchronizes with the gNB and performs Random Access.

5. The UE (i.e., L2 U2N Remote UE in previous steps) sends the RRCReconfigurationComplete message to the gNB via the direct path, using the configuration provided in the RRCReconfiguration message. From this step, the UE (i.e., L2 U2N Remote UE in previous steps) uses the RRC connection via the direct path to the gNB.

6. The gNB sends the RRCReconfiguration message to the L2 U2N Relay UE(s) on the source path to reconfigure the connection between the L2 U2N Relay UE and the gNB. The RRCReconfiguration message to the L2 U2N Relay UE(s) on the source path can be sent any time after step 3 based on gNB implementation (e.g., to release Uu Relay RLC Channel and PC5 Relay RLC channel configuration for relaying, and bearer mapping configuration related to the L2 U2N Remote UE).

7. Either L2 U2N Relay UE or L2 U2N Remote UE's AS layer indicates upper layers to release PC5 unicast link after receiving the RRCReconfiguration message from the gNB. The timing to execute link release is up to UE implementation.

8. The data path is switched from indirect path to direct path between the UE (i.e., previous L2 U2N Remote UE) and the gNB. The PDCP re-establishment or PDCP data recovery in uplink is performed by the UE (i.e., previous L2 U2N Remote UE) for lossless delivery during path switch if gNB configures it.

NOTE 1: Step 8 can be executed any time after step 4. Step 8 is independent of step 6 and step 7.

For service continuity of L2 U2N Relay, the following procedure is used, in case of L2 U2N Remote UE switching from indirect to direct path under another gNB:

1. The Uu measurement configuration is configured by the source gNB, and measurement report signalling procedures are performed by the L2 U2N Remote UE to evaluate both relay link measurement and Uu link measurement. The measurement results from L2 U2N Remote UE are reported when configured measurement reporting criteria are met. The sidelink relay measurement report shall include at least L2 U2N Relay UE's source L2 ID, serving cell ID (i.e., NCGI/NCI), and sidelink measurement quantity result. The sidelink measurement quantity can be SL-RSRP of the serving L2 U2N Relay UE, and if SL-RSRP is not available, SD-RSRP is used.

2. The source gNB decides to trigger path switch for the L2 U2N Remote UE, onto direct path.

3. The source gNB sends the HANDOVER REQUEST message to the target gNB with necessary information to prepare the handover at the target side.

NOTE 2: In order to support the DL lossless path switch for the L2 U2N Remote UE, the source gNB may not discard the DL data even though the delivery of the data has been acknowledged by the L2 U2N Relay UE based on the gNB implementation. Then, the source gNB forwards the buffered DL data to the target gNB during the data forwarding procedure.

4. Admission Control may be performed by the target gNB.

5. The target gNB sends the HANDOVER REQUEST ACKNOWLEDGE message to the source gNB, which contains RRC configuration for the L2 U2N Remote UE at the target side.

6. The source gNB triggers the path switch by sending an RRCReconfiguration message to the L2 U2N Remote UE, containing at least cell ID and the information required to access the target cell. The L2 U2N Remote UE stops User Plane and Control Plane transmission via the L2 U2N Relay UE after reception of the RRCReconfiguration message.

7. The source gNB sends the SN STATUS TRANSFER message to the target gNB to convey the uplink PDCP SN receiver status and the downlink PDCP SN transmitter status of the L2 U2N Remote UE's DRBs for which PDCP status preservation applies (i.e. for RLC AM).

8. The L2 U2N Remote UE synchronizes with the target gNB and performs Random Access.

9. The L2 U2N Remote UE sends RRCReconfigurationComplete message to target gNB via the direct path.

10. The target gNB sends the UE CONTEXT RELEASE message to inform the source gNB about the success of the path switch.

11. The source gNB sends RRCReconfiguration message to the L2 U2N Relay UE to reconfigure the connection between the L2 U2N Relay UE and the source gNB. The RRCReconfiguration message to the L2 U2N Relay UE can be sent any time after step 6 based on source gNB implementation (e.g., to release Uu Relay RLC channel and PC5 Relay RLC channel configuration for relaying, and bearer mapping configuration related to the L2 U2N Remote UE).

12. Either L2 U2N Relay UE or L2 U2N Remote UE's AS layer indicates upper layer to release PC5 unicast link after receiving the RRCReconfiguration message from the source gNB. The timing to execute link release is up to UE implementation.