16.12.5.1 RRC Connection Management

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:

1. The L2 U2N Remote and L2 U2N Relay UE perform discovery procedure, and establish a PC5-RRC connection using the NR sidelink PC5 unicast link establishment procedure.

2. The L2 U2N Remote UE sends the first RRC message (i.e., RRCSetupRequest) for its connection establishment with gNB via the L2 U2N Relay UE, using a specified PC5 Relay RLC channel configuration. The L2 U2N Relay UE sends the SidelinkUEInformationNR message to request for the dedicated configurations required to support the relay operation for the L2 U2N Remote UE. If the L2 U2N Relay UE is not in RRC_CONNECTED, it needs to do its own Uu RRC connection establishment upon reception of a message on the specified PC5 Relay RLC channel. After L2 U2N Relay UE's RRC connection establishment procedure and sending the SidelinkUEInformationNR message, gNB configures SRB0 relaying Uu Relay RLC channel to the U2N Relay UE. The gNB responds with an RRCSetup message to L2 U2N Remote UE. The RRCSetup message is sent to the L2 U2N Remote UE using SRB0 relaying Uu Relay RLC channel over Uu and a specified PC5 Relay RLC channel over PC5.

NOTE 1: Void.

3. The gNB and L2 U2N Relay UE perform relaying channel setup procedure over Uu. According to the configuration from gNB, the L2 U2N Relay/Remote UE establishes a PC5 Relay RLC channel for relaying of SRB1 towards the L2 U2N Remote/Relay UE over PC5.

4. The RRCSetupComplete message is sent by the L2 U2N Remote UE to the gNB via the L2 U2N Relay UE using SRB1 relaying channel over PC5 and SRB1 relaying channel configured to the L2 U2N Relay UE over Uu. Then the L2 U2N Remote UE is as in RRC_CONNECTED with the gNB.

5. The L2 U2N Remote UE and gNB establish security following the Uu security mode procedure and the security messages are forwarded through the L2 U2N Relay UE.

6. The gNB sends an RRCReconfiguration message to the L2 U2N Remote UE via the L2 U2N Relay UE, to setup the end-to-end SRB2/DRBs of the L2 U2N Remote UE. The L2 U2N Remote UE sends an RRCReconfigurationComplete message to the gNB via the L2 U2N Relay UE as a response. In addition, the gNB may configure additional Uu Relay RLC channels between the gNB and L2 U2N Relay UE, and PC5 Relay RLC channels between L2 U2N Relay UE and L2 U2N Remote UE for the relaying traffic.

The following high level connection establishment procedure in Figure 16.12.5.1-2 applies to a multi-hop L2 U2N Relay(s) and L2 U2N Remote UE:

1. The L2 U2N Remote and the first L2 U2N Relay UE, and between L2 U2N Relay UEs, perform a discovery procedure and establish a PC5-RRC connection using the NR sidelink PC5 unicast link establishment procedure.

2. The L2 U2N Remote UE sends the first RRC message (i.e., RRCSetupRequest) for its connection establishment with gNB via the first L2 U2N Relay UE, using a specified PC5 Relay RLC channel configuration. The first L2 U2N Relay UE sends the SidelinkUEInformationNR message to request for the dedicated configurations required to support the relay operation for the L2 U2N Remote UE. If the first L2 U2N Relay UE is not in RRC_CONNECTED, it performs its connection establishment procedure with the gNB. In this case, the first L2 U2N Relay UE acts as a L2 U2N Remote UE to establish its own relay connection. The first L2 U2N Relay UE stores the first RRC message received from the L2 U2N Remote UE and sends its own first RRC message (i.e., RRCSetupRequest) via the parent UE, using a specified PC5 Relay RLC channel configuration. If the parent UE is not the last L2 U2N Relay UE and not in RRC_CONNECTED, the same connection establishment procedure as the first L2 U2N Relay UE is performed. If the parent UE is the last L2 U2N Relay UE and not in RRC_CONNECTED, the same connection establishment procedure as the L2 U2N Relay UE in the single-hop is performed. After the first L2 U2N Relay UE’s RRC connection establishment procedure and sending the SidelinkUEInformationNR message, gNB configures SRB0 relaying PC5 Relay RLC channel and UE ID of the remote UE to the first L2 U2N Relay UE. The first L2 U2N Relay UE sends the stored first RRC message of the L2 U2N Remote UE to the gNB via the parent UE. The gNB responds with an RRCSetup message to L2 U2N Remote UE. The RRCSetup message is delivered toward the L2 U2N Remote UE using SRB0 relaying Uu Relay RLC channel over Uu or PC5 Relay RLC channel over PC5 through multiple U2N Relay UEs. The RRCSetup message for the L2 U2N Remote UE is delivered via a specified PC5 Relay RLC channel over PC5 from the first L2 U2N Relay UE to the L2 U2N Remote UE.

3. The gNB and the last L2 U2N Relay UE perform relaying channel setup procedure over Uu. The gNB and the intermediate L2 U2N Relay UE(s) perform relaying channel setup procedure over PC5. According to the configuration from gNB, the L2 U2N Relay UEs/Remote UE establishes a PC5 Relay RLC channel for relaying of SRB1 towards the L2 U2N Remote UE/Relay UEs over PC5.

4. The RRCSetupComplete message is sent by the L2 U2N Remote UE to the gNB via intermediate L2 U2N Relay UE(s) using SRB1 relaying channel over PC5 and SRB1 relaying channel configured to the last L2 U2N Relay UE over Uu. Then the L2 U2N Remote UE is as in RRC_CONNECTED with the gNB.

5. The L2 U2N Remote UE and gNB establish security following the Uu security mode procedure and the security messages are forwarded through the multiple L2 U2N Relay UEs.

6. The gNB sends an RRCReconfiguration message to the L2 U2N Remote UE via multiple L2 U2N Relay UEs, to setup the end-to-end SRB2/DRBs of the L2 U2N Remote UE. The L2 U2N Remote UE sends an RRCReconfigurationComplete message to the gNB via multiple L2 U2N Relay UEs as a response. In addition, the gNB may configure additional Uu Relay RLC channels between the gNB and the last L2 U2N Relay UE, and PC5 Relay RLC channels between L2 U2N Relay UEs, and between the first L2 U2N Relay UE and L2 U2N Remote UE for the relaying traffic.