 + and the initial investments for 5G service are expected to be limited due to lack of 5G user equipment. As a result, the 5G networks need to be able to interwork with the exist- ing LTE networks. The interworking solution can provide seamless service to the users. In this section, we discuss the solutions for LTE-NR interworking and we will compare the solutions in terms of performance, features and ease of migration to a full 5G network. If 5G cells are not deployed with full coverage, a seamless service can be provided to the users by interworking with the existing LTE networks, which are already deployed with full coverage. For LTE-NR interworking, two types of solutions namely access-network-level interworking and core-network-level interworking, have been studied in 3GPP. In RAN-level interworking solutions, the interworking service between LTE and 5G is made possible by using a direct interface between LTE eNB and NR gNB. The RRC mes- sages are transmitted over the LTE radio interface, thus the connection and the mobility of UE are controlled by the LTE eNB. The user traffic is simultaneously transmitted through LTE eNB and NR gNB either by PDCP aggregation or using NR gNB split bearer. Although the RRC messages can be processed by the LTE eNBs that provide larger cover- age than NR gNBs, LTE radio interface always remains connected, even though user traffic is transmitted over the NR. RAN-level interworking is necessary in non-stand-alone archi- tecture, where the NR cannot be used without overlaid LTE network. Note that when the LTE EPC is used, only EPC-based services can be provided, even though 5G radio 5G Network Architecture 159 technology is used. Two different core networks can be used for RAN-level interworking, as shown in Fig. 1.79. LTE and NR interworking can be achieved by upgrading some LTE eNBs connected to NR gNBs and by increasing the gateway capacity in EPC. The new 5G core network, 5GC, has been designed to support RAN-level interworking. In this solution, the new 5GC network slicing 