In a mobile communication system, a mobile station (Mobile Station: which is also referred to as User Equipment (UE)) can perform radio communication in an area which is covered by a base station (Base Station: BTS/BS). The area covered by the base station is referred to as a cell. The size of the cell may be large or small. For example, there are types of cells such as a macrocell, a microcell, a picocell, and a femtocell. In the present application, a cell that covers a relatively narrow area compared to that of the macrocell is referred to as the “femtocell.”
FIG. 1 shows a conceptual diagram of an area where macrocells and femtocells coexist. User equipment (UE) can perform radio communication in both the macrocell and the femtocell, or in one of the macrocell and the femtocell. A base station of a femtocell is referred to as a “femto base station (a femto BTS)” or a “Home Node B (HNB).” A base station of a macrocell is referred to as a “macro base station (a macro BTS or simply a BTS).” In addition to a communication carrier and an operator, a general user can set up a femto BTS. By establishing a femtocell, a load to a macrocell can be reduced while expanding coverage to an area in which communications can be performed.
When the macrocell and the femtocell coexist, a handover may be performed between them. When a user moves from a macrocell to a femtocell, or when the user moves from the femtocell to the macrocell, in a method which has conventionally been defined, it may be required to pass through a switching center (MMS/xGSN), which is an upper layer node of a macro base station (BTS), a radio network controller (RNC), a Home Node B (HNB), and a Home Node B Gateway (HNB-GW). This type of a handover method has been disclosed in Non-Patent Document 1.