Content for  TS 38.300  Word version:  18.4.0

The L2 U2N Remote UE needs to establish its own PDU sessions/DRBs with the network before user plane data transmission. The NR sidelink PC5 unicast link establishment procedures can be used to setup a secure unicast link between L2 U2N Remote UE and L2 U2N Relay UE before L2 U2N Remote UE establishes a Uu RRC connection with the network via L2 U2N Relay UE. The establishment of Uu SRB1/SRB2 and DRB of the L2 U2N Remote UE is subject to Uu configuration procedures for L2 UE-to-Network Relay. The following high level connection establishment procedure in Figure 16.12.5.1-1 applies to a L2 U2N Relay and L2 U2N Remote UE:

The L2 U2N Remote UE in RRC_CONNECTED suspends Uu RLM (as described in clause 9.2.7) when connected to the gNB via a L2 U2N Relay UE. The L2 U2N Relay UE declares Uu Radio Link Failure (RLF) following the same criteria as described in clause 9.2.7. After Uu RLF is declared, the L2 U2N Relay UE takes the following action on top of the actions described in clause 9.2.7:

• a PC5-RRC message can be used for sending an indication to its connected L2 U2N Remote UE(s), which may trigger RRC connection re-establishment for L2 U2N Remote UE; or
• indicating to upper layer to trigger PC5 unicast link release.

Upon detecting PC5 RLF, the L2 U2N Remote UE may trigger RRC connection re-establishment.

The L2 U2N Remote UE may perform the following actions during the RRC connection re-establishment procedure:

• If only suitable cell(s) are available, the L2 U2N Remote UE initiates RRC re-establishment procedure towards a suitable cell;
• If only suitable L2 U2N Relay UE(s) are available, the L2 U2N Remote UE initiates RRC re-establishment procedure towards a suitable relay UE's serving cell via selected suitable L2 U2N Relay;
• If both a suitable cell and a suitable relay are available, the L2 U2N Remote UE can select either one to initiate RRC re-establishment procedure based on implementation.

The RRC connection resume procedure described in clause 9.2.2 is applied to L2 U2N Remote UE.

The in-coverage L2 U2N Remote UE is allowed to acquire any necessary SIB(s) over Uu interface irrespective of its PC5 connection to L2 U2N Relay UE. The L2 U2N Remote UE can also receive the system information from the L2 U2N Relay UE after PC5 connection establishment with L2 U2N Relay UE. The L2 U2N Remote UE in RRC_CONNECTED can use the on-demand SIB framework as specified in TS 38.331 to request the SIB(s) via L2 U2N Relay UE. The L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE can inform L2 U2N Relay UE of its requested SIB type(s) via PC5-RRC message. Then, L2 U2N Relay UE triggers on-demand SI/SIB acquisition procedure as specified in TS 38.331 according to its own RRC state (if needed) and sends the acquired SI(s)/SIB(s) to L2 U2N Remote UE via PC5-RRC message. Any SIB that the RRC_IDLE or RRC_INACTIVE L2 U2N Remote UE has a requirement to use (e.g., for relay purpose) can be requested by the L2 U2N Remote UE (from the L2 U2N Relay UE or the network). For SIBs that have been requested by the L2 U2N Remote UE from the L2 U2N Relay UE, the L2 U2N Relay UE forwards them again in case of any update for requested SIB(s). In case of RRC_CONNECTED L2 U2N Remote UE(s), it is the responsibility of the network to send updated SIB(s) to L2 U2N Remote UE(s) when they are updated. The L2 U2N Remote UE de-configures SI request with L2 U2N Relay UE when entering into RRC_CONNECTED state. For SIB1 forwarding, for L2 U2N Remote UE, both request-based delivery (i.e., SIB1 request by the U2N Remote UE) and unsolicited forwarding are supported by L2 U2N Relay UE, of which the usage is left to L2 U2N Relay UE implementation. If SIB1 changes, for L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE, the L2 U2N Relay UE always forwards SIB1. For the L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE, the short message over Uu interface is not forwarded by the L2 U2N Relay UE to the L2 U2N Remote UE. The L2 U2N Relay UE can forward PWS SIBs to its connected L2 U2N Remote UE(s). RAN sharing is supported for L2 U2N Relay UE. In particular, the L2 U2N Relay UE may forward, via discovery message, cell access related information before the establishment of a PC5-RRC connection.

When both L2 U2N Relay UE and L2 U2N Remote UE are in RRC IDLE or RRC INACTIVE, the L2 U2N Relay UE monitors paging occasions of its connected L2 U2N Remote UE(s). When a L2 U2N Relay UE needs to monitor paging for a L2 U2N Remote UE, the L2 U2N Relay UE should monitor all POs of the L2 U2N Remote UE. When L2 U2N Relay UE is in RRC_CONNECTED and L2 U2N Remote UE(s) is in RRC_IDLE or RRC_INACTIVE, there are two options for paging delivery:

• The L2 U2N Relay UE monitors POs of its connected L2 U2N Remote UE(s) if the active DL BWP of the L2 U2N Relay UE is configured with common search space including paging search space;
• The delivery of the L2 U2N Remote UE's paging can be performed through a dedicated RRC message from the gNB to the L2 U2N Relay UE. The dedicated RRC message for delivering L2 U2N Remote UE paging to the RRC_CONNECTED L2 U2N Relay UE may contain one or more Remote UE IDs (5G-S-TMSI or I-RNTI).

It is up to network implementation to decide which of the above two options to use. The L2 U2N Relay UE in RRC_CONNECTED, if configured with paging search space, can determine whether to monitor POs for a L2 U2N Remote UE based on the indication within the PC5-RRC signalling received from the L2 U2N Remote UE. The L2 U2N Remote UE in RRC_IDLE provides 5G-S-TMSI and UE specific DRX cycle (if configured by upper layer) to the L2 U2N Relay UE for requesting to perform PO monitoring. The L2 U2N Remote UE in RRC_INACTIVE provides the minimum value of two UE specific DRX cycles (if configured respectively by upper layer and NG-RAN), 5G-S-TMSI and I-RNTI to the L2 U2N Relay UE for PO monitoring. The L2 U2N Relay UE in RRC_CONNECTED can notify the L2 U2N Remote UE information (i.e. 5G-S-TMSI/I-RNTI) to the gNB via the SidelinkUEInformationNR message for paging delivery purpose. The L2 U2N Relay UE receives paging messages to check the 5G-S-TMSI/I-RNTI and sends relevant paging record to the L2 U2N Remote UE accordingly. The L2 U2N Relay UE uses unicast signalling to send paging to the L2 U2N Remote UE via PC5.

The L2 U2N Remote UE performs unified access control as defined in TS 38.331. The L2 U2N Relay UE does not perform UAC for L2 U2N Remote UE's data.

The L2 U2N Remote UE performs Mobility Registration Update/RNAU based on the L2 U2N Relay UE's serving cell when it is connected with the L2 U2N Relay UE. A L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE initiates Mobility Registration Update/RNAU procedure if the serving cell changes (due to cell change by the L2 U2N Relay UE) and the new serving cell is outside of the L2 U2N Remote UE's configured RNA/TA.