Section: 16.21.3.1 Path Management
Content: The L2 MP Remote UE needs to establish both a direct path and an indirect path. The L2 MP Remote UE adds the indirect path using PC5 link on top of the only direct path under the same gNB. And also, the L2 MP Remote UE using PC5 link adds the direct path on top of the only indirect path under the same gNB. Meanwhile, the L2 MP Remote UE adds the indirect path using N3C link on the only direct path under the same gNB. But it is not allowed that the L2 MP Remote UE using N3C link adds the direct path on the indirect path. The MP Relay UE using N3C indirect path is restricted to serve only one MP Remote UE. For L2 MP Remote UE, only a single cell group is configured (i.e., only MCG is configured for the direct path and indirect path). The primary path of split SRB1 and SRB2 is always configured on direct path. In the L2 MP Remote UE, non-split SRB1/2 is allowed to be configured only on direct path. Figure 16.21.3.1-1 describes the procedures for the indirect path addition on the direct path for the L2 MP relaying. This procedure is applicable to the L2 MP Remote UE using SL indirect path or N3C indirect path except step 5. Figure 16.21.3.1-1: Procedure for indirect path addition on top of direct path 0. The L2 MP Remote UE performs data transmission and reception by using direct path on PCell. 1. If the L2 MP Remote UE will be connected with L2 MP Relay UE using PC5 link, the L2 MP Remote UE reports at least the list of the candidate L2 MP Relay UE ID and the cell ID of the candidate L2 MP Relay UEs. Meanwhile, if the MP Remote UE will be connected with L2 MP Relay UE using N3C link, the MP Remote UE reports at least the list of the C-RNTI and the cell ID of the connected candidate MP Relay UEs. NOTE 1: The C-RNTI and cell ID of MP Relay UE using N3C link can be reported only if the secure connection between MP Remote UE and MP Relay UE is established. 2. The gNB decides to add the indirect path for the L2 MP Remote UE. The cell serving the direct path and the cell serving the L2 MP Relay UE on the indirect path belong to the same gNB but can be same or different. 3. The gNB sends an RRCReconfiguration message to the L2 MP Relay UE to configure the indirect path of the L2 MP Remote UE, if the L2 MP Relay UE is in RRC_CONNECTED. 4. The gNB sends the RRCReconfiguration message to the L2 MP Remote UE. 5. The L2 MP Remote UE establishes a PC5 unicast link with the target L2 MP Relay UE. NOTE 2: For the N3C indirect path addition, step 5 is omitted. It is L2 MP Remote UE's implementation how to make N3C indirect path between L2 MP Remote UE and L2 MP Relay UE. 6a. The L2 MP Remote UE sends the RRCReconfigurationComplete message to the gNB at least via the direct path in order to complete the indirect path addition procedure. 6b. If a split SRB1 with duplication is configured, the L2 MP Remote UE also sends the RRCReconfigurationComplete message to the gNB via the indirect path served by the L2 MP Relay UE. NOTE 3: Step 5 can be executed after step 6a. Step 5 is independent of step 6a. 7. The L2 MP Remote UE performs data transmission and reception by using both the direct path on PCell and the indirect path served by a L2 MP Relay UE. In the case that the selected L2 MP Relay UE for indirect path addition is in RRC_IDLE or RRC_INACTIVE, after receiving the path addition command, the L2 MP Remote UE should trigger the L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE to move to RRC_CONNECTED. If the L2 MP Remote UE and L2 MP Relay UE are connected by using N3C link, it is L2 MP Remote/Relay UE's implementation on how to trigger the RRC_IDLE/RRC_INACTIVE L2 MP Relay UE to initiate RRC connection establishment procedure. If the split SRB1 with duplication is not configured at the L2 MP Remote UE, the L2 MP Remote UE using SL indirect path sends PC5-RRC message to the L2 MP Relay UE. Upon receiving the PC5-RRC message, the L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE initiates a Uu RRC connection establishment or an RRC connection resume. If the split SRB1 with duplication is configured at the L2 MP Remote UE, the L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE received RRCReconfigurationComplete message from the L2 MP Remote UE may a Uu RRC connection establishment or an RRC connection resume for sending the RRCReoncfigurationComplete message to the gNB. Figure 16.21.3.1-2 describes the procedures for the indirect path change under the single direct path in the L2 MP Relay operation. This procedure is applicable to the L2 MP Remote UE using SL indirect path or N3C indirect path except step 5. Figure 16.21.3.1-2: Procedure for indirect path change under a single procedure 0. The L2 MP Remote UE performs data transmission and reception by using both the direct path on PCell and the indirect path served by a source L2 MP Relay UE. 1. The L2 MP Remote UE and the L2 MP Relay UE perform measurements based on measurement configuration. When the measurement reporting is triggered, the L2 MP Remote UE using SL indirect link reports at least signal strength (e.g., SD-RSRP/SL-RSRP) of the serving indirect path, the list of the candidate L2 MP Relay UE ID and the cell ID of the candidate L2 MP Relay UEs. Meanwhile, the MP Remote UE using N3C link reports at least the list of the C-RNTI and the cell ID of the connected candidate MP Relay UEs. 2. The gNB decides to change the indirect path of L2 MP Remote UE from the source L2 MP Relay UE to a target L2 MP Relay UE. The cell serving the direct path and the cell serving the source/target L2 MP Relay UE on the indirect path belong to the same gNB but can be same or different. 3a. The gNB sends an RRCReconfiguration message to the source L2 MP Relay UE to release the indirect path of the L2 MP Remote UE. 3b. The gNB sends an RRCReconfiguration message to the target L2 MP Relay UE to add the indirect path for the L2 MP Remote UE. 4. The gNB sends the RRCReconfiguration message to the L2 MP Remote UE on the direct path and/or the indirect path for indirect path change. If SRB1 is configured either direct path or indirect path to the L2 MP Remote UE, the RRCReconfiguration message is sent via one of the path on which the SRB1 is configured. If split SRB1 is configured, it is up to gNB implementation whether the RRCReconfiguration is sent via one of the paths or both paths. 5. The L2 MP Remote UE establishes a PC5-RRC connection with the target L2 MP Relay UE for using SL indirect path. NOTE 4: For the N3C indirect path addition, step 5 is omitted. It is L2 MP Remote UE's implementation how to make N3C indirect path between L2 MP Remote UE and L2 MP Relay UE. 6a. The L2 MP Remote UE sends the RRCReconfigurationComplete message to the gNB at least via the direct path in order to complete the indirect path change procedure. NOTE 5: Step 5 can be executed after step 6a. Step 5 is independent of step 6a. 6b. If a split SRB1 with duplication is configured, the L2 MP Remote UE also sends the RRCReconfigurationComplete message to the gNB via the indirect path served by the target L2 MP Relay UE. 7. The L2 MP Remote UE performs data transmission and reception by using both the direct path on PCell and the indirect path served by a target L2 MP Relay UE. In the case that the selected target L2 MP Relay UE for indirect path change is in RRC_IDLE or RRC_INACTIVE, after receiving the path change command, the L2 MP Remote UE should trigger the target L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE to be in RRC_CONNECTED. If the target L2 MP Relay UE is not in RRC_CONNECTED in step 3, the gNB sends an RRCReconfiguration message to the target L2 MP Relay UE after the target L2 MP Relay UE enters RRC_CONNECTED. If the split SRB1 with duplication is not configured at the L2 MP Remote UE, the L2 MP Remote UE sends the RRCReconfigurationComplete message only on the direct path in Step 6a. The L2 MP Remote UE using SL indirect path can send a PC5-RRC message to the target L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE after or during Step 5. The target L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE initiates an RRC connection establishment or an RRC connection resume upon receiving the PC5-RRC message from the L2 MP Remote UE. If the target L2 MP Relay UE is in RRC_IDLE or RRC_INACTIVE, the RRCReconfigurationComplete message at Step 6b is sent to the gNB after the target L2 MP Relay UE enters RRC_CONNECTED. If the split SRB1 with duplication is configured at the L2 MP Remote UE, the target L2 MP Relay UE in RRC_IDLE or RRC_INACTIVE receiving RRCReconfigurationComplete message from the L2 MP Remote UE initiates an RRC connection establishment or an RRC connection resume for sending the RRCReconfigurationComplete message to the gNB at Step 6b. Figure 16.21.3.1-3 describes the procedures for the direct path addition on top of the indirect path for the L2 MP Relay operation. This procedure is only applicable to the L2 MP Remote UE using SL indirect path. Figure 16.21.3.1-3: Procedure for direct path addition on top of indirect path 0. The L2 MP Remote UE performs data transmission and reception by using indirect path via PC5 link. 1. The L2 MP Remote UE perform measurements based on measurement configuration. When the measurement reporting is triggered, the L2 MP Remote UE reports at least the Uu signal strength of the serving cell and neighbour cells with the cell IDs (i.e., NCGI/NCI). 2. The gNB decides to add the direct path for the L2 MP Remote UE. The cell serving the direct path and the cell serving the L2 MP Relay UE on the indirect path belong to the same gNB but can be same or different. 3. The gNB sends an RRCReconfiguration message to the L2 MP Relay UE to update the indirect path configuration, if necessary. 4. The gNB sends the RRCReconfiguration message to the L2 MP Remote UE via the L2 MP Relay UE. The contents in the RRCReconfiguration message includes at least a target cell within direct path addition configuration. 5. The L2 MP Remote UE synchronizes to DL of the target cell serving the direct path and performs random access procedure towards the cell serving the direct path. The L2 MP Remote UE configures the target cell as PCell. 6a. The L2 MP Remote UE sends the RRCReconfigurationComplete message to the gNB at least via the direct path in order to complete the direct path addition procedure. 6b. If a split SRB1 with duplication is configured, the L2 MP Remote UE also sends the RRCReconfigurationComplete message to the gNB via the indirect path served by the L2 MP Relay UE. 7. The L2 MP Remote UE performs data transmission and reception by using both the direct path on PCell and the indirect path served by a L2 MP Relay UE. Figure 16.21.3.1-4 describes the procedures for the direct path change on top of the indirect path for the L2 MP Relay operation. This procedure is only applicable to the L2 MP Remote UE using SL indirect path. Figure 16.21.3.1-4: Procedure for direct path change under a single procedure 0. The L2 MP Remote UE performs data transmission and reception by using both the direct path on the source PCell and the indirect path served by a L2 MP Relay UE. 1. The L2 MP Remote UE and the L2 MP Relay UE perform measurements based on measurement configuration. The L2 MP Remote UE may report measurement results. 2. The gNB decides to change the direct path of the L2 MP Remote UE from the PCell (i.e. source PCell) to a new cell (i.e. target PCell). The source/target PCell serving the old/new direct path and the cell serving the L2 MP Relay UE on the indirect path belong to the same gNB but can be same or different. 3. The gNB sends an RRCReconfiguration message to the L2 MP Relay UE to update the indirect path configuration, if necessary. 4. The gNB sends the RRCReconfiguration message to the L2 MP Remote UE on the direct path and/or the indirect path for direct path change. 5. The L2 MP Remote UE synchronizes to DL of the target PCell serving the new direct path and performs random access procedure towards the target PCell serving the new direct path. 6a. The L2 MP Remote UE sends the RRCReconfigurationComplete message to the gNB at least via the direct path in order to complete the direct path change procedure. 6b. If a split SRB1 with duplication is configured, the L2 MP Remote UE also sends the RRCReconfigurationComplete message to the gNB via the indirect path served by the L2 MP Relay UE. 7. The L2 MP Remote UE performs data transmission and reception by using both the direct path on the target PCell and the indirect path served by a L2 MP Relay UE. If the direct path addition/change is failed, the L2 U2N Remote UE always shall trigger RRCReestablishment.