Content for  TS 38.300  Word version:  18.3.0

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 direct path under the same gNB. And also, the L2 MP Remote UE using PC5 link adds the direct path on top of the indirect path under the same gNB. Meanwhile, the L2 MP Remote UE adds the indirect path using N3C link on top of the only direct path under the same gNB. But it is not allowed that the L2 MP Remote UE using N3C indirect path adds the direct path on top of the indirect path. The MP Relay UE using N3C indirect path is restricted to serve only one L2 MP Remote UE. For L2 MP Remote UE, the direct path and the indirect path should be in the same gNB. 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 top of 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).

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/resume 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, the L2 MP Remote UE sends the RRCReconfigurationComplete message via the L2 MP Relay UE. The RRCReconfigurationComplete message triggers that the L2 U2N Relay UE in RRC_IDLE or RRC_INACTIVE initiates 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 a 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).

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 L2 MP Remote UE and L2 MP Relay UE are connected by 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/resume procedure. 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, the L2 MP Remote UE sends the RRCReconfigurationComplete message via the target L2 MP Relay UE. The RRCReconfigurationComplete message triggers that the L2 U2N Relay UE in RRC_IDLE or RRC_INACTIVE 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 16.21.3.1-4 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.

If the direct path addition/change is failed, the L2 U2N Remote UE always shall trigger RRCReestablishment.

The L2 MP Remote UE in RRC_CONNECTED performs Uu RLM (as described in clause 9.2.7). When the L2 MP Remote UE detects Uu Radio Link Failure (RLF) on the direct path, the L2 MP Remote UE triggers path failure reporting through the indirect path via a RRC message if split SRB1 is configured and the indirect path is not suspended. Otherwise, RRC connection re-establishment is initiated. When the L2 MP Remote UE using PC5 indirect path detects PC5 Radio Link Failure (RLF) and/or Uu link failure on the indirect path, the L2 MP Remote UE triggers path failure reporting through the direct path via a RRC message, if the direct path is not suspended. When the L2 MP Remote UE using N3C indirect path detects N3C link failure and/or Uu link failure on the indirect path, the L2 MP Remote UE triggers path failure reporting through the direct path via a RRC message, if the direct path is not suspended.

The L2 MP Remote UE can acquire any necessary SIB(s) over Uu interface or indirect path. If common search space for system information is configured with the active BWP on PCell, the L2 MP Remote UE can perform direct system information acquisition on PCell. The L2 MP Remote UE can also receive system information at lease PBCH/MIB on the direct path and directly acquires SFN from MIB on the direct path. Besides, the L2 MP Remote UE can receive the system information via dedicated RRC signalling via SRB1.