16.21.2.2 L2 MP Relay using N3C indirect path

For the multi-path relay using N3C indirect path between the L2 MP Remote UE and L2 MP Relay UE(s), at least one MP Relay UE(s) can be configured with MP Remote UE via N3C interface. The protocol stacks for the user plane and control plane of L2 MP Relay architecture are illustrated in Figure 16.21.2.2-1 and Figure 16.21.2.2-2.

In the multi-path relay using N3C indirect path, the SRAP sublayer does not exist on the protocol stack. For the direct path, the Uu SDAP, PDCP, RLC, MAC, PHY and RRC are terminated at gNB and L2 MP Remote UE. While RLC, MAC, and PHY in Uu hop of indirect path are terminated at L2 MP Relay UE and gNB. Without the SRAP sublayer over N3C, an UL PDCP PDU in the L2 MP Remote UE is delivered to RLC entity in the L2 MP Relay UE via N3C link based on UE implementation. It is supported for more than one RB over the Uu link of the L2 MP Relay UE by configuring 1:1 bearer mapping between the Radio bearer in the L2 MP Remote UE and Uu Relay RLC channel in the L2 MP Relay UE. The Uu logical channels for the PDU delivery of the L2 MP Relay UE's local traffic and relay traffic are configured differently. Bearer identification except LCID is not needed in L2 PDU over the Uu link. If split bearer is configured, it is up to MP Remote UE implementation to decide which MP Relay UE is used for transmission of a UL PDCP PDU, when more than one MP Relay UEs are configured to MP Remote UE in N3C interface. If the split bearer is configured and the PDCP duplication is activated, the duplicated PDCP PDUs are delivered via both direct path and indirect path, and it is up to MP Remote UE implementation to decide which MP Relay UE(s) is(are) used for transmission of a UL PDCP PDU when more than one MP Relay UEs are configured to MP Remote UE in N3C interface.