Mobile communication system detectable mobile station moving out of communication range

1. A mobile communication system comprising a plurality of base stations, each of which performs wireless communication with mobile stations located within a cell formed thereby, and a base station controller connected to the plurality of base stations, wherein

each of said base stations includes:

a system parameter storage for storing a base station identifier and a border flag indicative of whether the base station is a border cell base station forming a border cell located on a boundary portion of a service area or a normal cell base station forming a normal cell located on the inward side than the border cell; and

a distance calculation unit for calculating a distance to each of the mobile stations connected via wireless channels and notifying said base station controller of distance information of each of the mobile stations in conjunction with the base station identifier and the border flag as handoff information, and

said base station controller includes:

a handoff information management table for storing the base station identifier, the border flag, and the mobile station distance information notified from each of the base stations in association with each of mobile station identifiers; and

an out-of-range detection unit for updating said handoff information management table each time new handoff information is received from each of said base stations and detecting any of the mobile stations moving out of the service area into an out-of-communication region, wherein

said out-of-range detection unit performs out-of-range detection by selectively monitoring any of the mobile stations that is currently communicative only with one or more of said border cell base stations discriminated by a state of the border flag as a detection target and comparing the distance information of the mobile station indicated by said handoff information management table with a specified threshold value;

wherein said out-of-range detection unit performs the out-of-range detection of each of the mobile stations by using a threshold value common to the plurality of base stations; and

wherein said out-of-range detection unit performs the out-of-range detection of each of the mobile stations by selecting the distance information with a minimum value from among the distance information on the same mobile station measured at the plurality of border cell base stations and compares the selected minimum distance information with said threshold value.

2. A mobile communication system according to claim 1 , wherein

said handoff information management table stores an individual threshold value for each of the base stations, and

said out-of-range detection unit performs the out-of-range detection by comparing the mobile station distance information with the individual threshold value for each of the base stations.

3. A mobile communication system according to claim 1 , wherein said out-of-range detection unit transmits an out-of-range warning message to any of the mobile stations detected to be at a distance exceeding said threshold value.

4. A mobile communication system according to claim 3 , wherein

said handoff information management table stores, in association with each of the mobile station identifiers, a warning flag indicative of whether the out-of-range warning message has been transmitted, and

said out-of-range detection unit clears the warning flag when it detects based on said handoff information management table that any of the mobile stations, for which the warning flag indicates that the out-of-range warning message has been transmitted, has become communicative with one of said normal cell base stations or that the distance between the mobile station and one of said border cell base stations has fallen within a range of a second threshold smaller than said threshold.