In the mobile communications, such as cellular phone communications, the control of switching of radio base stations to communicate with in response to the movement of a mobile station, i.e., handover, is indispensable. FIG. 21 shows a conventional system that realizes handover. The system includes a mobile station 30, two or more radio base stations (here, a case with two radio base stations is shown) 21 and 22, and an RNC (Radio Network Controller: radio network control unit) 10. The RNC 10 includes an electric field intensity measurement control unit 11 and the radio resources management unit 12 and a switch 13.
Next, FIG. 22 shows the flow of the handover operation. The mobile station 30 is connected to a backbone network through the radio base station 1 (21) and the switch 13, and communicates through this backbone network with terminals that belong to the Internet, the PSTN (Public Switched Telephone Network) and the ISDN (Integrated Service Digital Network) which are operated by communication carriers other than the mobile communication carrier that manages the backbone network. When the mobile station 30 moves away from the radio base station 1 (21) and approaches the radio base station 2 (22), the electric field intensity from the radio base station 1 (21) becomes weaker, and, conversely, the electric field intensity from the radio base station 2 (22) becomes stronger. Since the electric field intensity measurement control unit 11 requires the mobile station 30 to measure and report the electric field intensities of nearby radio base stations including the radio base station 1 (21) (S1 of the operation flow), the mobile station 30 measures the electric field intensities of two or more radio base stations around itself, and transmits the information to the electric field intensity measurement control unit 11 through the radio base station 1 (21) on the communication line (S2 and S3 of the operation flow). The electric field intensity measurement control unit 11 informs the radio resources management unit 12 that handover will take place, when the electric field intensity of the radio base station 2 (22) becomes higher than the electric field intensity of the radio base station 1 (21) (S4 of the operation flow).
The radio resource management unit 12 controls radio resources required for communication between the mobile station and the radio base station. So to speak, radio resources are physical parameters, types of which are dependent on multiple accessing methods and duplexing methods that are used by a radio system. Specifically, in FDMA (Frequency Division Multiple Access), it is the bandwidth of a carrier wave, and the appropriated time length of the carrier wave to a certain communication, and, in TDMA (Time Division Multiple Access), it is the bandwidth of the carrier wave, and a number of time slots that are slices in a fixed interval of an appropriated time of the carrier wave. Further, in the case of CDMA (Code Division Multiple Access), it is the bandwidth, and transmission electric power density of the carrier wave, which is dependent on the spread factor. The maximum values of these values are determined by the maximum transmission capacity of each radio base station and a mobile station.
The radio resource management unit which manages radio resources such as above assigns the amount of radio resources equivalent to the radio resources which were being assigned to the radio base station 1 (21) to the radio base station 2 (22), in order to communicate with the mobile station 30, and sets up a new radio channel (S5 of the operation flow). That is, even if the communicating counterpart of the mobile station 30 is switched from the radio base station 1 (21) to the radio base station 2 (22), the transmission capacity of the mobile station 30 and the radio base station does not change. However, when an amount of the radio resources equivalent to prior to the switching was not available (S6 of the operation flow), or the electric field intensity of the radio channel newly set up was insufficient and not suitable for communication (S8 of the operation flow), re-selection of radio resources, or handover is suspended (S11 of the operation flow). Simultaneously, the radio resources management unit 12 controls the switch 13, and switches the connection between the backbone and the radio base station 1 (21) to the connection between the backbone and the radio base station 2 (22) (S9 of the operation flow). Further, the radio resources management unit 12 controls the mobile station 30 through the radio base station 1 (21) so as to switch the communicating counterpart from the radio base station 1 (21) to the radio base station 2 (22) (S10 of the operation flow). Since the switching control is performed synchronously, there is almost no communication interruption during the handover.
The control procedure mentioned above is called Mobile-Assisted Handover and used by PDC (Personal Digital Cellular Telecommunication System). Explanation of this procedure is detailed in “Personal Digital Cellular Telecommunication System ARIB Standard RCR STD-27H edition”, published by Association of Radio Industries and Businesses.
As mentioned above, in the conventional handover system, the handover is triggered by monitoring and checking the electric field intensity or the amount of radio wave interference. For this reason, in conventional systems, the handover is not triggered when an amount of traffic which goes via a specific radio base station increases because of increase in the user's data volume of the mobile station 30. For this reason, a problem occurs that the radio resources of the radio base station run short, and the available communication bandwidth decreases. Although the communication itself can be secured in such a system that offers the packet telecommunication service which communicates user information in packets only when the user data to be transmitted is generated, for example, PDC-P (PDC mobile packet communications Systems) even if the radio resources run short, a sharp fall in a throughput is caused and there is a possibility of producing an inconvenience to the user. Further, it has been difficult to efficiently utilize radio resources of a mobile communications network as a whole by making a mobile station to communicate with another radio base station that has a margin in the radio resources, when the shortage of radio resources of a certain radio base station takes place.
Further, a minimum bandwidth guarantee type IP (Internet Protocol) communication service which guarantees only the minimum of a throughput is currently offered as a cheap communication service in a wired communications network. However, in the conventional mobile communications, since there was a possibility of causing the sharp fall in the throughput by the increase in traffic as mentioned above, it was difficult to apply such bandwidth guarantee service to mobile communications.
Further, conventionally, handover systems have been proposed (for example, JP, 2000-175243) to cope with a traffic congestion condition in the communication of a radio base station by switching a mobile station from the station with which the communication has been made to an adjacent radio base station. In such a handover system, directions of handover are made to all the mobile stations that communicate with the radio base station which is experiencing the traffic congestion condition. In addition, the handover takes place to each adjacent base station from all the mobile stations that are connected to the radio base station in the traffic congestion condition and that are capable of communicating with adjacent radio base stations.
If all the mobile stations are switched to the adjacent radio base stations from the radio base station that is in a traffic congestion condition in this way, the traffic amount in the congested radio base station will be lowered. However, depending upon traffic situation of the adjacent radio base stations to which the handover has been made, some of handed-over mobile stations may experience such a situation that it would have had a wider bandwidth available if it stayed with the original radio base station. In this case, the quality of the communication service which can be received by the mobile station will deteriorate.