The Uu measurement configuration and measurement report signalling is performed between U2N Remote UE and gNB-CU to evaluate both relay link measurement and Uu link measurement. The U2N Remote UE may report one or multiple candidate U2N Relay UE(s) and Uu measurement results after it measures/discovers the candidate U2N Relay UE(s).
The gNB-CU decides to switch the U2N Remote UE to a target U2N Relay UE under a different gNB-DU (i.e., target gNB-DU).
The reconfiguration to target U2N Relay UE is performed among U2N Relay UE, the target gNB-DU and gNB-CU, if the U2N Relay UE is in RRC_CONNECTED state. The gNB-CU sends an RRCReconfiguration message to the target U2N Relay UE. If the target Relay UE is in RRC_IDLE/INACTIVE state, this step is skipped and the configuration to the target U2N Relay UE is performed in Step 10.
gNB-CU sends the UE CONTEXT SETUP REQUEST message for the U2N Remote UE to the target gNB-DU, which contains the path switch configuration at least.
gNB-DU responds with the UE CONTEXT SETUP RESPONSE message to gNB-CU.
gNB-CU sends the UE CONTEXT MODIFICATION REQUEST message by including the RRCReconfiguration message to the source gNB-DU. The contents in the RRCReconfiguration message may include at least path switch configuration, PC5 Relay RLC channel configuration for relaying traffic, bearer mapping configuration and the associated radio bearer(s).
The source gNB-DU sends the RRCReconfiguration message to the U2N Remote UE. The U2N Remote UE stops UP and CP transmission over Uu after reception of RRCReconfiguration message from the gNB.
The source gNB-DU sends the UE CONTEXT MODIFICATION RESPONSE message to the gNB-CU.
The U2N Remote UE establishes PC5 connection with target U2N Relay UE.
he U2N Remote UE completes the path switch procedure by sending the RRCReconfigurationComplete message to the target gNB-DU via the target U2N Relay UE. In case the U2N relay UE is in RRC_IDLE/ INACTIVE state, when the U2N relay UE receives the RRCReconfigurationComplete message, the reception of the RRCReconfigurationComplete message will first trigger RRC setup/resume procedure for the U2N relay UE to enter RRC_CONNECTED state.
The target gNB-DU sends the UL RRC MESSAGE TRANSFER message to gNB-CU by including the RRCReconfigurationComplete message.
The gNB-CU sends an UE CONTEXT RELEASE COMMAND message to the source gNB-DU.
The source gNB-DU releases the UE context and responds the gNB-CU with an UE CONTEXT RELEASE COMPLETE message.