A mobile communication system such as a mobile phone system generally includes multiple cells having different sizes. For example, a cell (hereinafter, referred to as “large cell” for convenience) formed by a general base station for public communications and a cell (hereinafter, referred to as “small cell” for convenience) covering a particular spot generally coexist.
Such a mobile communication system is required to perform mobility control for controlling which cell a mobile station is to camp on. As a method for the mobility control, a method of using a difference in a path loss value between cells is widely used. Specifically, each cell notifies a transmission power value of a common control channel (Common Pilot Channel (CPICH) in W-CDMA system) to a mobile station. The mobile station calculates the path loss value based on the transmission power value notified from each cell and the reception power value of the common control channel form the cell. The mobile station requests handover to the cell neighboring the current cell when the difference in the path loss value between the cells converges to a predetermined range (for example Non-Patent Document 1).
However, multiple cells of the different sizes generally coexist in the mobile communication system as described above. Thus, the transmission power value of the common control channel differs between the cells. Accordingly, in some cases, the mobility control cannot be carried out appropriately with the mobility control based on the path loss value as described above.
Generally, the transmission power value of the common control channel of the small cell is smaller than the transmission power value of the large cell. Accordingly, with the simple mobility control in which the mobile station requests a handover when the difference in the path loss value converges to the predetermined range, the handover might be requested at a position outside the area covered by the small cell. Thus, smooth handover of the mobile station between the cells is hindered.
Thus, to solve such a problem, another method is specified in which each cell notifies its power offset (Cell Individual Offset (CIO) in a W-CDMA system) to neighboring cells in the neighborhood thereof (see Non-Patent Document 1). The mobile station calculates the path loss value with the transmission power value to which the power offset is added.