(1) Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to a mobile communication system which allows high-accuracy detection of a mobile station (mobile terminal) currently moving out of a communication service area into an out-of-communication range.
(2) Description of the Related Art
As an out-of-range detection method used in a conventional mobile communication system, there has been known one which calculates the approximate position of a mobile station from the placement location of a base station currently communicating with the mobile station and from the power level of a radio wave received from the mobile station and thereby detects the movement of the mobile station out of communication range. Examples of such a method are proposed in Japanese Unexamined Patent Publication Nos. Hei 6-86353 and Hei 11-243586. According to the method disclosed in Japanese Unexamined Patent Publication No. 2004-364223, an application server has map information on the service area of a mobile communication system and detects the movement of a mobile station moving toward out of communication range by plotting positional information calculated by the mobile station equipped with a GPS (Global Positioning System) on the map of the service area.
On the other hand, a signal round-trip delay between a base station and a mobile station has been described in detail in 3GPP2 C.S0024 cdma2000 “High Rate Packet Data Air Interface Specification” (Non-Patent Document). In this document, the phase difference between the mobile station and the base station is defined as “On-Way Delay” with accuracy represented by the chips of the CDMA spread code as units. The document also shows that, for the mobile station having set a wireless channel with the base station, the “One-Way Delay” can be measured at any time. In addition, the document graphically illustrates that the round-trip delay between the base station and the mobile station is double the “One-Way Delay”.
Each of the prior art technologies described above has attempted to detect the mobile station moving out of the service area by judging whether an adjacent cell exists in the direction of progress of the mobile station. However, these prior art technologies, in reality, can merely detect a mobile station having moved into a cell which has no adjacent cells in the direction of progress thereof and the accuracy with which the movement of the mobile station toward the outside of the service area is detected is not high. In order to recognize the direction of progress of the mobile station, it is a precondition that a handover history is left for each of the mobile stations. However, in the case of a mobile communication system which is large in cell size and low in the placement density of the base stations, e.g., the frequency of the occurrence of handover is low so that the accuracy of prediction as to the direction of progress of the mobile station based on the handover history deteriorates. As a result, the problem is encountered that a sudden change in the direction of progress of the mobile station cannot be recognized properly.
Moreover, because out-of-range detection based on the power level of the radio wave received from a mobile station is susceptible to the influence of variations in an environment in which the radio wave propagates, there is the possibility of erroneous warning given to the mobile station moving within the service area. In a mobile communication system of a code division multiple access (CDMA) type, in particular, the level of an output signal from each of the mobile stations is power-controlled under an instruction from the base station such that the reception signal at the base station is kept at a constant level. Therefore, it is impossible to calculate the position of the mobile station based on the reception signal level.
A system using the GPS and the service area map information can perform the out-of-range detection of the mobile station with high accuracy. However, an approach using the system presents the problem that the structure of the mobile communication system is complicated and not only the mobile stations but also each of the base stations become higher in cost. In addition, the region where a GPS radio wave can be received is different from the range where a mobile radio wave reaches. Therefore, even within the service area of mobile communication, there exists a region where the out-of-range detection of the mobile station is impossible for the system because the GPS radio wave does not reach there, such as, e.g., an underground mall where a wireless base station is placed.