Referring to FIG. 3C, if DC is used, a UE may connect to and use two eNBs at the same time. FIG. 3C shows an example in which a UE 3c-05 connects to a macro base station 3c-00 using the LTE technology and a small base station 3c-10 using the NR technology at the same time, and transmits/receives data. This is referred to as E-UTRAN-NR dual connectivity (EN-DC). The macro base station 3c-00 is an MeNB, and the small base station 3c-10 is an SeNB. Multiple small cells may be present in the service area of the MeNB 3c-00. The MeNB 3c-00 is connected to SeNBs (e.g., SeNB 3c-10) over a wired backhaul network 3c-15. A set of serving cells provided by the MeNB 3c-00 is referred to as an MCG 3c-20. Only one serving cell of the MCG 3c-20 is a PCell 3c-25 having all of the functions performed by the existing cell, such as connection establishment, connection re-establishment and handover.
