Section: 9.2.3.5.2 C-Plane Handling
Content: Cell switch command is conveyed in a MAC CE, which contains the necessary information to perform the LTM cell switch. The overall procedure for LTM is shown in Figure 9.2.3.5.2-1 below. Subsequent LTM is done by repeating the early synchronization, LTM cell switch execution, and LTM cell switch completion steps without releasing other LTM candidate configurations after each LTM cell switch completion. The general procedure over the air interface is applicable to SCG LTM. Further details of SCG LTM can be found in TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [21]. Figure 9.2.3.5.2-1. Signalling procedure for LTM The procedure for LTM is as follows: 1. The UE sends a MeasurementReport message to the gNB. The gNB decides to configure LTM and initiates LTM preparation. 2. The gNB transmits an RRCReconfiguration message to the UE including the LTM candidate configurations. 3. The UE stores the LTM candidate configurations and transmits an RRCReconfigurationComplete message to the gNB. 4a. The UE performs DL synchronization with the candidate cell(s) before receiving the cell switch command. 4b. When UE-based TA measurement is configured, UE acquires the TA value(s) of the candidate cell(s) by measurement. UE performs early TA acquisition with the candidate cell(s) as requested by the network before receiving the cell switch command as specified in clause 9.2.6. This is done via CFRA triggered by a PDCCH order from the source cell, following which the UE sends preamble towards the indicated candidate cell. In order to minimize the data interruption of the source cell due to CFRA towards the candidate cell(s), the UE does not receive random access response from the network for the purpose of TA value acquisition and the TA value of the candidate cell is indicated in the cell switch command. The UE does not maintain the TA timer for the candidate cell and relies on network implementation to guarantee the TA validity. 5. The UE performs L1 measurements on the configured candidate cell(s) and transmits L1 measurement reports to the gNB. L1 measurement should be performed as long as RRC reconfiguration (step 2) is applicable. 6. The gNB decides to execute cell switch to a target cell and transmits a MAC CE triggering cell switch by including the candidate configuration index of the target cell. The UE switches to the target cell and applies the configuration indicated by candidate configuration index. 7. The UE performs the random access procedure towards the target cell, if UE does not have valid TA of the target cell as specified in clause 6.1.3.xy of TS 38.321[ NR; Medium Access Control (MAC) protocol specification ] [6]. 8. The UE completes the LTM cell switch procedure by sending RRCReconfigurationComplete message to target cell. If the UE has performed a RA procedure in step 7 the UE considers that LTM cell switch execution is successfully completed when the random access procedure is successfully completed. For RACH-less LTM, the UE considers that LTM cell switch execution is successfully completed when the UE determines that the network has successfully received its first UL data. The steps 4-8 can be performed multiple times for subsequent LTM using the LTM candidate configuration(s) provided in step 2. The procedure over the air interface described in Figure x is applicable to both intra-gNB-DU LTM and inter-gNB-DU LTM. The overall LTM procedures over F1-C interface are captured in TS 38.401[ NG-RAN; Architecture description ] [4].