lone mode of operation between LTE and NR unlike for dual connectivity where there is simultaneous transmission across both RATs most of the time. Interworking between LTE and NR is not expected to be significantly 168 Chapter 1 E-UTRA Handover RRC_CONNECTED NR RRC_CONNECTED RAN paging/RAN update/ connection resume Connection establishment Connection establishment and release NR RRC_INACTIVE and release RAN-based notification area Cell reselection update E-UTRA RRC_IDLE Cell reselection NR RRC_IDLE Figure 1.82 LTE-NR mobility state diagram [16,35]. different from what is defined in LTE specifications for interworking with other 3G net- works. The inter-RAT mobility is expected to be supported both in idle mode as well as in the connected mode. Fig. 1.82 illustrates possible mobility scenarios across LTE and NR. RRC_INACTIVE is a new RRC state in NR, in addition to RRC_IDLE and RRC_CONNECTED. It is a state where a UE remains in CM-CONNECTED and is able to move within an area configured by NG-RAN (i.e., RAN-based notification area or RNA) without notifying NG-RAN. The RNA can cover a single cell or multiple cells. In RRC_INACTIVE, the last serving NG-RAN node retains the UE context and the UE- associated NG connection with the serving AMF and UPF. The UE notifies the network via RAN-based notification area update procedure, if it has moved out of the configured RNA. If the last serving gNB receives downlink data from the UPF or downlink signaling from the AMF while the UE is in RRC_INACTIVE state, it pages the UE in the cells correspond- ing to the RNA and may forward the paging message to the neighbor gNB(s) if the RNA includes cells of neighbor gNB(s) [16]. Connected mode mobility is enabled using Xn inter- face between LTE eNB and NR gNB where both eNB and gNB are connected to 5G core network. S1/N2-based handover is supported when LTE eNB is connected to EPC and NR gNB is connected to 5GC. Xn and core network handover in 5GC, when both eNB and gNB are connected to 5GC, is transparent to the UE. Seamless handove