Content for  TS 38.401  Word version:  18.3.0

The U2N Remote UE and the U2N Relay UE perform discovery procedure, and establish PC5 connection using NR ProSe procedure.

The U2N Remote UE sends an RRCSetupRequest message to the U2N Relay UE via PC5 Relay RLC Channel.

The U2N Relay UE withholds the received RRC message and sends the SidelinkUEInformationNR message to the gNB-DU. Before that, if the U2N Relay UE is in RRC_IDLE/RRC_INACTIVE state, it should trigger the RRC establishment/resume procedure to enter RRC_CONNECTED state upon reception of the RRC message.

The gNB-DU sends the UL RRC MESSAGE TRANSFER message of the U2N Relay UE by encapsulating the SidelinkUEInformationNR message to gNB-CU, and gNB-CU allocates the local ID of U2N Remote UE.

The gNB-CU sends the UE CONTEXT MODIFICATION REQUEST message of the U2N Relay UE to gNB-DU. Such message may request the establishment of Uu Relay RLC channel(s) for the transmission of U2N Remote UE's SRB0.

The gNB-DU sends the UE CONTEXT MODIFICATION RESPONSE message of the U2N Relay UE to gNB-CU.

The gNB-CU sends the DL RRC MESSAGE TRANSFER message of the U2N Relay UE to gNB-DU by encapsulating the RRCReconfiguration message, which contains the local ID allocated to the U2N Remote UE. The RRCReconfiguration message shall also contain the Uu Relay RLC channel(s) configuration if not configured and bearer mapping for relaying of U2N Remote UE's SRB0.

The gNB-DU sends the RRCReconfiguration message to the U2N Relay UE to configure the local ID of the U2N Remote UE, the Uu Relay RLC channel(s) configuration and bearer mapping for relaying of U2N Remote UE's SRB0.

The U2N Relay UE sends the RRCReconfigurationComplete message to gNB-DU.

The gNB-DU sends the UL RRC MESSAGE TRANSFER message of the U2N Relay UE by encapsulating the RRCReconfigurationComplete message to gNB-CU.

After receiving the local ID of the U2N Remote UE and the Uu Relay RLC channel(s) configuration and bearer mapping for relaying of U2N Remote UE's SRB0, the U2N Relay UE sends the RRCSetupRequest message of the U2N Remote UE to gNB-DU. The local ID of the U2N Remote UE and RB ID for SRB0 are conveyed in the SRAP header.

The gNB-DU allocates a C-RNTI and a gNB-DU UE F1AP ID for the U2N Remote UE and sends the INITIAL UL RRC MESSAGE TRANSFER message to gNB-CU by encapsulating the RRCSetupRequest message of the U2N Remote UE. In addition, the local ID of the U2N Remote UE , the gNB-DU UE F1AP ID of the U2N Relay UE and the sidelink configuration container for at least the PC5 Relay RLC channel configuration for relaying of U2N Remote UE's SRB1 are included in the INITIAL UL RRC MESSAGE TRANSFER message.

The gNB-CU allocates a gNB-CU UE F1AP ID for the U2N Remote UE and generates a RRCSetup message towards the U2N Remote UE. The RRC message is encapsulated in the DL RRC MESSAGE TRANSFER message, and includes the configurations of PC5 Relay RLC channel and bearer mapping at least for the transmission of U2N Remote UE's SRB1.

The gNB-DU sends the RRCSetup message to the U2N Remote UE via the U2N Relay UE.

The gNB-CU configures the U2N Relay UE with PC5 Relay RLC channel, Uu Relay RLC channel and bearer mapping for relaying of U2N Remote UE's SRB1. According to the configuration from gNB-CU, the U2N Relay UE establishes a PC5 Relay RLC channel for relaying of U2N Remote UE's SRB1 over PC5 and establishes a Uu Relay RLC channel for relaying of U2N Remote UE's SRB1 towards gNB-DU if not configured yet.

The U2N Remote UE sends the RRCSetupComplete message to the gNB-DU via the U2N Relay UE.

The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and sends it to the gNB-CU.

Upon receiving the RRCSetupComplete message of U2N Remote UE, the gNB-CU sends the INITIAL UE MESSAGE message to the AMF.

The AMF sends the INITIAL CONTEXT SETUP REQUEST message to the gNB-CU.

The gNB-CU sends the UE CONTEXT SETUP REQUEST message to establish the U2N Remote UE context in the gNB-DU. Such message may request the configuration of PC5 Relay RLC channels for the transmission of U2N Remote UE's SRB2 and DRBs, and may also encapsulate the SecurityModeCommand message.

The gNB-DU sends the SecurityModeCommand message to the U2N Remote UE via U2N Relay UE.

The gNB-DU sends the UE CONTEXT SETUP RESPONSE message of the U2N Remote UE to the gNB-CU, which contains the configuration of PC5 Relay RLC channels for the transmission of U2N Remote UE's SRB2 and DRBs.

The U2N Remote UE responds with the SecurityModeComplete message.

The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and sends it to the gNB-CU.

The gNB-CU generates the RRCReconfiguration message for U2N Remote UE and encapsulates it in the DL RRC MESSAGE TRANSFER message. The RRCReconfiguration message contains the configuration of PC5 Relay RLC channels and bearer mapping for the transmission of U2N Remote UE's SRB2 and DRBs.

The gNB-DU sends RRCReconfiguration message to the U2N Remote UE via the U2N Relay UE.

The U2N Remote UE sends RRCReconfigurationComplete message to the gNB-DU via the U2N Relay UE.

The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and send it to the gNB-CU.

The gNB-CU sends the INITIAL CONTEXT SETUP RESPONSE message to the AMF.

The gNB-CU configures additional Uu Relay RLC channels between the gNB-DU and the U2N Relay UE, and additional PC5 Relay RLC channels for the U2N Relay UE for relaying of U2N Remote UE's DRBs and SRBs. Also, such step may configure the bearer mapping between U2N Remote UE's DRB/SRB and PC5/Uu Relay RLC channel at the U2N Relay UE.