[8]: Source Target 1: Handover request Admission control 2: Handover request acknowledge 3. RRC reconfiguration Switch to new cell 4. RRC reconfiguration complete Figure 2.27 Inter-gNB handover procedure [8]. 260 Chapter 2 1. The source gNB initiates handover and issues a Handover Request over Xn interface. 2. The target gNB performs admission control and provides the RRC configuration as part of the Handover ACK. 3. The source gNB provides the RRC configuration to the UE in the Handover Command message, which includes the cell ID and all information required to access the target cell, SO that the UE can access the target cell without detecting that cell's SI. In some cases, the information required for contention-based and contention-free random-access procedure can be included in the Handover Command message. The access information to the target cell may include beam-specific information. 4. The UE moves the RRC connection to the target gNB and replies with the Handover Complete message. The user data can be sent in step 4, if the grant allows. The handover mechanism triggered by RRC signaling requires the UE to reset the MAC entity and reestablish RLC entity. The RRC-triggered handovers with and without PDCP entity reestablishment are both supported in NR. For DRBs using RLC-AM mode, the PDCP entity can either be reestablished along with a security key change or initiate a data recovery procedure without a key change. For DRBs using RLC-UM mode and for SRBs, the PDCP entity can either be reestablished in conjunction with a security key change or to remain as it is without a key change. Data for- warding, in-sequence delivery, and duplication avoidance at handover can be guaranteed when the target gNB uses the same DRB configuration as the source gNB. Timer-based handover failure procedure is supported in NR where an RRC connection reestablishment procedure is used for recovering from handover failure [8]. The beam-level mobility does not require explicit RRC signaling in order to be triggered. The gNB provides