For example, in a mobile radio communication system for mobile phones, an area where an electric wave from one base station can reach are of the order of about several hundred meters to several kilometers. Therefore, when a mobile station moves from the area of a base station to the area of another base station, the mobile station performs an operation of changing the base station on the other end of the communications, i.e., a so-called handover operation. When performing this handover operation, the mobile station performs a search operation of measuring the receive level (the field strength) of the base station with which the mobile station is currently communicating and the receive level of each base station located in an adjacent area, and detects a base station whose receive level is the highest from among base stations whose receive levels have been received. As a technology for such a handover operation, a method of a mobile station measuring the reception quality of a neighboring base station, and increasing or decreasing the frequency of a searching operation according to whether or not a searching operation is newly required, thereby reducing the power consumption of the mobile station is proposed by patent reference 1.
In a mobile radio communication system which implements this conventional technology, the length of time intervals at which candidates for handover destination base station are detected is determined dependently upon the original design of mobile stations. On the other hand, because as base stations for use in a mobile radio communication system, asynchronous base stations also exist in addition to synchronous base stations, and an influence of a drift of a system clock, and so on is added to this type of base stations, it is necessary to reduce the length of time intervals at which candidates for handover destination base station are detected for asynchronous base stations as compared with that for synchronous base station. A problem is therefore that when an asynchronous base station becomes a candidate for handover destination base station, a mobile station which independently determines the detection time intervals as mentioned above, like conventional mobile stations, cannot catch the candidate for handover destination base station.
[Patent reference 1] JP, 2001-28778, A
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a mobile radio communication system which enables a mobile station to catch a candidate for handover destination base station certainly even if asynchronous base stations coexist in a mobile radio network in addition to synchronous base stations, a base station apparatus, a radio network controller, and a mobile station.