Abstract:
A mobile communication system includes first and second switching apparatuses, an information channel provided between the first and second switching apparatuses, a plurality of base stations managed by each of the first and second switching apparatuses, and a mobile station. The mobile station can communicate with the first switching apparatus through one of the plurality of base stations managed by the first switching apparatus and with the second switching apparatus through one of the plurality of base stations managed by the second switching apparatus. The first and second switching apparatuses detect traffics therethrough as first and second traffics, respectively, and the second switching apparatus notifies the second traffic to the first switching apparatus via the information channel. The first switching apparatus controls the mobile station and the second switching apparatus based on the first and second traffics such that the mobile station which is currently communicating with the first switching apparatus communicates with the second switching apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mobile communication system, and more particularly, to a mobile communication system in which traffics are equalized between switching apparatuses. 
     2. Description of the Related Art 
     Conventionally, various mobile telephone systems are known such as a PHS (personal handy phone system) in which a minimal cell method is adopted, an automobile telephone system in which a small cell method is used, and a PDS (portable digital phone system). Each of the mobile phone systems have a merit and a defect. 
     For example, in the PHS system, the area which a radio wave reaches has the radius about 100 m from a base station. Therefore, in order to form a wide service area, a lot of base stations are densely installed. As a result, there is a problem in that there is an area where the radio wave is difficult to be received, depending on the installed state of the base stations. On the other hand, there is an advantage in low communication charge in the PHS system, compared with the other mobile phone systems. 
     On the other hand, in the PDS system, because the small cell method is adopted such that the radio wave can reach to a relatively wide area, there is a low possibility of the above-mentioned problem that the radio wave is difficult to be received. However, there is a problem in a high communication charge in the PDS system, compared with the PHS system. 
     In order to eliminate such a conventional problem, a dual band portable phone is discussed in recent years to be possible to be applied to two mobile phone systems such as the PHS system and the PDS system. In the following description, an operation mode is referred to as a “PHS mode” when the dual band portable phone operates as a mobile station of the PHS system, and as a “PDS mode” when the dual band portable phone operates as the mobile station of the PDS system. 
     The switching of the operation mode in the dual band portable phone, i.e., the switching of the mobile communication system is conventionally carried out on the dual band portable phone. That is, a control unit of the dual band portable phone operates in the PHS mode when the dual band portable phone is in a service area of the PHS system and out of any service area of the PDS system. Oppositely, the operation mode of the dual band portable phone operates is automatically switched to the PDS mode when the dual band portable phone is out of any service area of the PHS system and in a service area of the PDS system. Also, the control unit of the dual band portable phone automatically switches the operation mode to be PHS mode with low communication charge, when the dual band portable phone is in an area which is covered by the PHS system and the PDS system. 
     Also, the dual band portable phone is possible to operate in either of the PHS mode or the PDS mode in response to the operation of a predetermined button by a user in addition to the automatic switching. 
     Conventionally, the mobile communication system to which the dual band portable phone belongs is selectively switched based on the control of the dual band portable phone itself, as mentioned above. In this case, the traffic of a base station of each mobile telephone system is not considered. Therefore, the dual band portable phone sometimes operates in the PHS mode with a large traffic quantity while the traffic of the PDS system is small. Or, the opposite situation occurs. As a result, there is a problem in degradation of communication quality due to traffic jam. 
     It should be noted that “a digital radio communication system and a digital radio communication apparatus” is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-65606) as the technique to improve the communication quality. In the reference, the communication system is composed of base stations A 1  to C 1  and terminal stations A 2  to C 2 . Radio communications between the base station and the terminal station and between the base stations are carried out. In such a digital radio communication system, the base station selects a combination of the frequency with a smallest transmission error to noise around the base station and a convolution code, and communicates using the frequency and the convolution code of the selected combination. 
     In this way, an error correcting code used for the communication is appropriately selected in accordance with the radio wave situation around the communication environment. As a result, the deterioration of the communication quality which is caused by the transmission path error can be restrained because the transmission path error becomes little. However, in this digital radio communication system, it is not possible to prevent the deterioration of the communication quality which is caused by the large traffic. 
     In conjunction with the above description, a mobile communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 4-70094). In this reference, a service area is composed of a plurality of zones, in which base stations are respectively provided. A mobile station communicates with one of the base stations. When the mobile station moves to an adjacent area during the communication, the base station is switched to another base station for the adjacent area. Each base station has notifying means for notifying a control channel frequency information. of the other base station for the adjacent area and a channel operation percentage of the base station. The mobile station has monitoring means and a selecting means. The monitoring means monitors the channel operation percentage notified from each base station and reception levels from each base station. The selecting means selects one of the base stations which has the reception level higher than a predetermined value and the channel operation percentage lower than a predetermined value as a switching destination base station. 
     Also, a traffic distributing system in a mobile communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 5-75530). In the reference, a traffic distribution start threshold value T 1 , a traffic distribution stop threshold value T 2 , and a channel selection re-execution instruction transmission period τ are stored in a main control unit of a radio base station. When a communication channel usage percentage exceeds the traffic distribution start threshold value T 1 , the transmission of a channel selection re-execution instruction is started at the transmission period Tτ. When the communication channel usage percentage is decreased below the traffic distribution stop threshold value T 2 , the transmission of the channel selection re-execution instruction is stopped. Thus, a mobile terminal communicating with a radio base station which has no empty communication channel because of a high traffic quantity is made to communicate with another radio base station. 
     Also, a mobile communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 7-226973). In the reference, permission or inhibition of mode switching between one mode and another mode is set for each of zones which corresponds to at least one mode. An unnecessary mode switching is restrained in the mobile communication system in which communication is carried out using one selected from a plurality of modes. 
     Also, a communication channel switching system is described in Japanese Laid Open Patent Application (JP-A-Heisei 8-70479). In the reference, a control station ( 5 ) notifies a usable communication channel to a mobile station ( 1 ) via base station ( 2 ,  3 ). The mobile station monitors the present communication condition. When the communication condition is degraded, the mobile station independently switches to a new channel by which good communication condition is attained. The mobile station notifies the new channel to the control station ( 5 ) via the base station ( 2 ,  3 ). 
     Also, a communication restriction control system is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-9340). In the reference, threshold values a′ and b′ are predetermined based on a use percentage of a communication channel and stored in a communication restriction control section ( 33 ). The communication restriction control section inputs a traffic quantity of a communication channel from a call control section ( 32 ) and calculates the use percentage. When the calculated use percentage exceeds the threshold value a′, a notice a is sent out to carry out restriction to roaming subscribers such as call origination restriction. When the calculated use percentage exceeds the threshold value b′, a notice b is sent out to carry out restriction to all subscribers such as call origination restriction. 
     Also, a method of implementing soft hand-off in a radio communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 10-145834). In the reference, a communication channel is provided between adjacent base stations to be logically separated from a mobile switching station. Traffic information is exchanged between the base stations via the communication channel. Thus, the soft hand-off by the base station can be realized. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a mobile communication system in which traffics are equalized between switching apparatus so that the communication quality can be improved. 
     In order to achieve an aspect of the present invention, a mobile communication system includes first and second switching apparatuses, an information channel provided between the first and second switching apparatuses, a plurality of base stations managed by each of the first and second switching apparatuses, and a mobile station. The mobile station can communicate with the first switching apparatus through one of the plurality of base stations managed by the first switching apparatus and with the second switching apparatus through one of the plurality of base stations managed by the second switching apparatus. The first and second switching apparatuses detect traffics therethrough as first and second traffics, respectively, and the second switching apparatus notifies the second traffic to the first switching apparatus via the information channel. The first switching apparatus controls the mobile station and the second switching apparatus based on the first and second traffics such that the mobile station which is currently communicating with the first switching apparatus communicates with the second switching apparatus. 
     The first switching apparatus may control the mobile station and the second switching apparatus based on the first and second traffics and first and second threshold values. In this case, the first switching apparatus compares the first traffic and the first threshold value, and the second traffic and the second threshold value. Then, the first switching apparatus controls the mobile station and the second switching apparatus when the first traffic is equal to or larger than the first threshold value and when the second traffic is smaller than the second threshold value. Also, the first switching apparatus continues the communication with the mobile station when the first traffic is smaller than the first threshold value or when the second traffic is larger than the second threshold value. 
     In the above, the first and second switching apparatuses may operate under a same communication system. Instead, the first and second switching apparatuses may operate under different communication systems from each other. In this case, it is preferable that the first and second switching apparatuses operate under two of a PHS communication system, a PDS communication system, and an automobile communication system. 
     The first switching apparatus may issue a switching request to the mobile station via the base station and to the second switching apparatus via the information channel, when the mobile station should communicate with the second switching apparatus. When a switching confirmation is received from the mobile station and the second switching apparatus, the first switching apparatus stops the communication with the mobile station. 
     In order to achieve another aspect of the present invention, a switching apparatus includes a traffic detecting unit and a traffic detecting unit. The traffic detecting unit detects a traffic through the switching apparatus and transmits the detected traffic to other switching apparatuses. The control unit issues a switching request to one of the other switching apparatuses and a mobile station communicating with the switching apparatus based on the detected traffic and a traffic received from the one switching apparatus. The control unit stops the communication with the mobile station when a switching confirmation is received from the mobile station and the one switching apparatus. 
     The control unit issues the switching request to the one switching apparatus and the mobile station based on the detected traffic and the received traffic and first and second threshold values such that the mobile station which is currently communicating with the switching apparatus communicates with the one switching apparatus. The first and second threshold values are allocated to the switching apparatus and the one switching apparatus. In this case, the control unit compares the first traffic and the first threshold value, and the second traffic and the second threshold value. Then, the control unit issues the switching request to the mobile station and the one switching apparatus when the first traffic is equal to or larger than the first threshold value and when the second traffic is smaller than the second threshold value. In this case, the switching apparatus continues the communication with the mobile station when the first traffic is smaller than the first threshold value or when the second traffic is larger than the second threshold value. 
     In order to achieve still another aspect of the present invention, a mobile terminal includes first and second communication radio sections corresponding to first and second communication systems, respectively, and a control unit. The control unit issues a switching confirmation via the first communication radio section when a switching request is received through the first communication radio section, and then starts a communication through the second communication radio section. 
     In order to achieve yet still another aspect of the present invention, a method of averaging traffics between a plurality of switching apparatuses, includes: 
     detecting traffics as first and second traffics by first and second switching apparatuses of the plurality of switching apparatuses, respectively; 
     notifying the second traffic from the second switching apparatus to the first switching apparatus; 
     issuing a switching request to the second switching apparatus and a mobile station currently communicating with the first switching apparatus, based on the first and second traffics; 
     stopping the communication with the mobile station in response to a switching confirmation from the mobile station and a switching confirmation from the second switching apparatus such that communication is carried out between the second switching apparatus and the mobile station. 
     The switching request may be issued to the second switching apparatus and the mobile station based on the first and second traffics and first and second threshold values. In this case, for the issuing the switching request, the first traffic and the first threshold value and the second traffic and the second threshold value are compared. The switching request is issued to the second switching apparatus and the mobile station based on the first and second traffics and first and second threshold values, when the first traffic is equal to or larger than the first threshold value and when the second traffic is smaller than the second threshold value. The issue of the switching request is stopped when the first traffic is smaller than the first threshold value or when the second traffic is larger than the second threshold value. 
     Also, the first and second switching apparatuses may operate under a same communication system. Instead, the first and second switching apparatuses may operate under different communication systems from each other. In this case, it is preferable that the first and second switching apparatuses operate under two of a PHS communication system, a PDS communication system, and an automobile communication system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating the structure of a mobile communication system according to an embodiment of the present invention; 
     FIG. 2 is a block diagram illustrating the structure of a switching apparatus used in the mobile communication system according to the embodiment of the present invention; 
     FIG. 3 is a block diagram illustrating the structure of a dual band radio communication apparatus as a mobile station used in the mobile communication system according to the embodiment of the present invention; 
     FIG. 4 is a flow chart illustrating a system switching process in the switching apparatus shown in FIG. 2; and 
     FIG. 5 is a flow chart illustrating a system switching process in the dual band radio communication apparatus shown in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a mobile communication system of the present invention will be described in detail with reference to the attached drawings. 
     FIG. 1 is a diagram illustrating the structure of the mobile communication system according to an embodiment of the present invention. It should be noted that a case where two kinds of mobile telephone systems such as an A telephone system and a B telephone system are provided to have an overlapping service area will be described below for simplification of the description. However, the mobile communication system to which the present invention is applied may be three kinds or more. Also, as the A telephone system and the B telephone system, various mobile communication systems can be used such as an automobile telephone system, a PDS system or a portable digital phone system, a PHS system or a personal handy phone system, a cordless telephone system, a train/ship/aircraft telephone system, and a satellite mobile communication system. 
     Referring to FIG. 1, the A telephone system is composed of a switching apparatus  10 , base stations  11   1  to  11   3 , radio communication apparatuses  13   1  and  13   2  as mobile stations. 
     The switching apparatus  10  controls the whole of A telephone system. The base stations  11   1  to  11   3  are connected with the switching apparatus  10 . The base stations  11   1  to  11   3  transmit and receive radio waves to and from radio communication apparatuses which exist in zones  12   1  to  12   3  in a service area of the A telephone system. Thus, a radio communication is carried out between the radio communication apparatus and the switching apparatus via the base station. The radio communication apparatuses  13 , and  13   2  can transmit and receive the radio waves to and from only the base stations  11   1 , to  11   3  of the A telephone system. 
     The B telephone system is composed of a switching apparatus  20 , base stations  21   1 , to  21   3 , and radio communication apparatuses  23   1 , and  23   2  The switching apparatus  20  controls the whole of B telephone system. The base stations  21   1 , to  21   3  are connected with the switching apparatus  20 . The base station  21   1  to  21   3  transmit and receive radio waves to and from the radio communication apparatuses which exist in zones  22   1 , to  22   3  in a service area of the B telephone system. Thus, a radio communication is carried out between the radio communication apparatus and the switching apparatus  20  via the base station. The radio communication apparatuses  23   1 , and  23   2  can transmit and receive the radio waves only to and from the base stations  21   1 , to  21   3  of the B telephone system. 
     The dual band radio communication apparatuses  30   1 , and  322  can carry out radio communication with the switching apparatus  10  of the A telephone system and the switching apparatus  20  of the B telephone system. One of the switching apparatuses  10  and  20  to be communicated is determined based on traffic quantities, as described in detail later. 
     Also, an information exchange line  40  as a communication channel is operatively connected between the switching apparatus  10  and the switching apparatus  20 . The traffic quantity data indicative of current traffics the A telephone system and B telephone system are exchanged through the information exchange line  40 . It should be noted that it is possible to operatively connect between the switching apparatus  10  and the switching apparatus  20  by a radio channel. 
     Next, the structures of above-mentioned switching apparatuses  10  and  20  will be described with reference to the block diagram shown in FIG.  2 . 
     The switching apparatus  10  of the A telephone system is composed of a control unit  100 , a traffic detecting section  101  and a transmitting and receiving section  102 . The control unit  100  may be composed of a computer to control the whole A telephone system or the whole of switching apparatus  10 . The base stations  11   1 , to  11   3  and a communication network are connected with the control unit  100 . The control unit  100  sends out to the communication network, signals which are sent from the base stations  11   1  to  11   3  which have received radio waves from the radio communication apparatuses  13   1 , and  13   2  and the dual band radio communication apparatuses  30   1 , and  30   2 . Also, the control unit  100  sends out signals from the communication network to the radio communication apparatuses  13   1 , and  13   2  and the dual band radio communication apparatuses  30   1 , and  30   2  through the base stations  11   1  to  11   3 . In this manner, a user can communicate with the desired destination of using the A telephone system. 
     Also, the traffic detecting section  101  detects the current traffics of the base stations  11   1  to  11   3  which are under the control of the control unit  100 . A traffic data indicative of the detected current traffics is possible to express a ratio (%) of the number of current communications to the number of maximum communications permitted to process in the A telephone system. When the traffic data is equal to 100%, the call origination and the call arrival are made to wait. The traffic data detected by the traffic detecting section  101  is supplied to the transmitting and receiving section  102  through the control unit  100 . 
     The transmitting and receiving section  102  modulates and outputs the traffic data received from the control unit  100  to other switching apparatuses (the switching apparatus  20  in this example) through the information exchange line  40 . Also, the transmitting and receiving section  102  demodulates and supplies the traffic data received from the other switching apparatuses through the information exchange line  40  to the control unit  100 . 
     The switching apparatus  20  of the B telephone system is composed of a control unit  200 , a traffic detecting section  201  and a transmitting and receiving section  202 . The control unit  200  can be composed of a computer to control the whole of B telephone system or the whole switching apparatus  20 . 
     The base stations  21   1  to  21   3  and the communication network are connected with the control unit  200 . The control unit  200  receives the signals from the radio communication apparatuses  23   1  and  23   2  and the dual band radio communication apparatuses  30   1  and  30   2  through the base stations  21   1  to  21   3  to send out to the communication network. Also, the control unit  200  sends out the signals received from the communication network to the radio communication apparatuses  23   1  and  23   2  and the dual band radio communication apparatuses  30   1  and  30   2  through the base stations  21   1  to  21   3 . In this way, the user can communicate with a desired destination using the B telephone system. 
     Also, the traffic detecting section  201  detects the current traffics of the base stations  21   1  to  21   3  under the control of the control unit  200 . The traffic data indicative of the detected current traffics is possible to express a ratio (%) of the number of current communications in the B telephone system to the number of maximum communications permitted to process, like the above. The traffic data detected by the traffic detecting section  201  is supplied to the transmitting and receiving section  202  through the information exchange line  40 . 
     The transmitting and receiving section  202  modulates and outputs the traffic data received from the control unit  200  to other switching apparatuses through the information exchange line  40 . Also, the transmitting and receiving section  202  demodulates and supplies the traffic data received through the information exchange line  40  to the control unit  200 . 
     Next, the structures of the dual band radio communication apparatuses  30   1 , and  30   2  will be described with reference to the block diagram shown in FIG.  3 . The dual band radio communication apparatus is composed of a common antenna  300 , a radio section  301 , a control unit  302 , a receiver  303 , a transmitter  304  and an operation section  305 . 
     The common antenna  300  converts a received radio wave into an electric signal to supply to the radio section  301 . Also, the common antenna  300  converts the signal sent from the radio section  301  into a radio wave to radiate in the air. 
     The radio section  301  is composed of an A telephone system radio section  301 A and a B telephone system radio section  301 B. The A telephone system radio section  301 A and the B telephone system radio section  301 B demodulate a signal component taken out from the radio frequency band signal which has been received by the common antenna  300 , respectively. A sound data and a control data obtained through the demodulation are supplied to the control unit  302 . The data indicative of “system switching request” to be mentioned later is contained in the control data. 
     Also, the A telephone system radio section  301 A and the B telephone system radio section  301 B modulate and supply a sound data and a control data supplied from the control unit  302  to the common antenna  300 . The signal indicative of “system switching confirmation” to be mentioned later is contained in the modulated control data. Which of the A telephone system radio section  301 A and the B telephone system radio section  301 B is used for the above mentioned modulation and demodulation is determined based on the system switching process, to be mentioned later. 
     The control unit  302  controls the whole of dual band radio communication apparatus. The control unit  302  performs the system switching control (to be mentioned later in detail) such that the dual band radio communication apparatus operates under the A telephone system or the B telephone system. Also, the control unit  302  controls the receiver  303  to generate sound by supplying the sound data received from the radio section  301 . Also, the control unit  302  supplies the sound data from the transmitter  304  to the radio section  301 . Moreover, the control unit  302  controls the operation section  305 . 
     The operation section  305  contains switches and a display unit (both not shown). The switches are used for input of a telephone number, setting of various modes, and registration of various data, for example. Also, the display unit displays messages such as date, time, already registered data, and radio wave state with character or a picture. A data inputted using the switches of the operation section  305  is supplied to the control unit  302 . Also, the display data is supplied to the display unit of the operation section  305  from the control unit  302 . 
     Next, the system switching process in the mobile communication system structured as mentioned above will be described with reference to the flow charts of FIG.  4  and FIG.  5 . 
     In the following description, it is supposed that the dual band radio communication apparatus  30   1  is set to operate under the A telephone system. Also, it is supposed that the dual band radio communication apparatus  30   1  notifies that it is possible to communicate with both of the A telephone system and the B telephone system, to the switching apparatus  10  through the base station  112 , in the start of communication. As a result, information indicative of the notice is stored in a storage unit (not shown) of the switching apparatus  10 . 
     FIG. 4 is a flow chart illustrating the system switching process which is carried out by the control unit  100  of the switching apparatus  10  of the A telephone system. The system switching process routine is called at every predetermined time period from a main processing routine to control the whole of switching apparatus  10 . Therefore, the system switching process routine is executed at every predetermined time period. 
     In the system switching process, whether or not the traffic of the A telephone system is large is first determined (Step S 10 ). This process is carried out by determining whether or not the traffic data detected by the traffic detecting section  101  is equal to or larger than a predetermined value α. Here, as the predetermined value α, an optional value can be used in consideration of the waiting time to be permissible in the A telephone system for call origination and call arrival. The function to always monitor the traffic of the A telephone system by the switching apparatus  10  of the A telephone system is realized through the processing of the step S 10 . 
     When it is determined at the step S 10  that the traffic of the A telephone system is not larger than the predetermined value α, it is recognized that the load of the switching apparatus  10  is light. The process is ended without being carried out the subsequent system switching process. In this case, the dual band radio communication apparatus continues the communication under the A telephone system. 
     On the other hand, when it is determined at the above step S 10  that the traffic of the A telephone system is larger than the predetermined value α, it is recognized that the load of the switching apparatus  10  is heavy. Next, it is determined whether or not the traffic of the B telephone system is large (Step S 11 ). This process carried out by determining whether or not the traffic data sent from the B telephone system to the transmitting and receiving section  102  through the information exchange line  40  is equal to or larger than a predetermined value β. Here, as the predetermined value beta, an optional value can be used in consideration of the waiting times to be permissible in the B telephone system for the call origination and call arrival. The function to monitor the traffic of the B telephone system by the switching apparatus  10  of the A telephone system when the traffic of the A telephone system is large is realized through the process of the step S 11 . 
     When it is determined at the step S 11  that the traffic of the B telephone system is equal to or larger than the predetermined value β, it is recognized that the loads of both of the switching apparatus  10  and the switching apparatus  20  are heavy. As a result, the system switching process is ended without carrying out it. In this case, the dual band radio communication apparatus continues the communication under the A telephone system. 
     On the other hand, when it is determined at the above step S 11  that the traffic of the B telephone system is not larger than the predetermined value β, it is recognized that the load of the switching apparatus  20  is light. Next, it is determined whether a radio communication apparatus possible to communicate using the B telephone system, i.e., any dual band radio communication apparatus exists (Step S 12 ). This is carried out by referring to the information indicating whether the radio communication apparatus is possible to communicate using both of the A telephone system and the B telephone system and stored in the storage of switching apparatus  10  in the start of the communication. 
     When it is determined at the step S 12  that the dual band radio communication apparatus possible to communicate using the B telephone system does not exist, the subsequent system switching process is ended with no further operation. On the other hand, when it is determined that the dual band radio communication apparatus possible to communicate under the B telephone system exists, a “system switching request” is transmitted to the dual band radio communication apparatus  30   1  possible to communicate using the B telephone system (Step S 13 ). At the same time, the system switching request is sent from the switching apparatus  10  to the switching apparatus  20  through the information exchange line  40 . This is attained by producing the data indicating the system switching request by the control unit  100  of the A telephone system and by transmitting the data to the dual band radio communication apparatus  301  through the base station  112 . It should be noted that the switching apparatus  10  of the A telephone system always grasps that the dual band radio communication apparatus  301  exists in the zone of the base station  21   2 . 
     Next, it is determined whether or not a “system switching confirmation” is received from both of the dual band radio communication apparatus  301  and the switching apparatus  20  (Step S 14 ). When it is determined that the signal indicating a system switching confirmation is not received from one of :the both, the control waits the reception of the signal while the step S 14  is repeatedly executed. When it is determined that the signal indicating the system switching confirmation is received in the repetitive process, the system switching process is ended. After that, the dual band radio communication apparatus  301  operates using the B telephone system. It should be noted that the above mentioned system switching process is carried out for the A telephone system. However, the same system switching process can be carried out in the switching apparatus  20  of the B telephone system. 
     Next, the system switching process which is carried out in the control unit  302  of the dual band radio communication apparatus will be described with reference to the flow chart shown in FIG. 5. A telephone system switching process routine is called at ever predetermined time period from a main processing routine so as to control the whole of dual band radio communication apparatus. Therefore, the system switching process is executed at every predetermined time period. 
     In the system switching process, it is determined whether or not the signal “system switching request” is received (Step S 20 ). This is carried out by determining whether the control data indicating the system switching request is contained in the data which is obtained by receiving the radio wave by the common antenna  300  and by demodulating the received radio wave by the radio section  301 . Because the system switching process routine is executed at every predetermined time period, the dual band radio communication apparatus always monitors whether the “system switching request” is received. 
     When it is determined at the step S 20  that the “system switching request” is not received, the system switching process is ended with no operation. On the other hand, when it is determined that the “system switching request” is received, it is determined whether or not the dual band radio communication apparatus is in the state in which the system can be switched at present (Step S 21 ). When it is determined that the dual band radio communication apparatus is not in the state which the system can be switched, the system switching process is ended with no further operation. 
     On the other hand, when it is determined at the above step S 21  that the dual band radio communication apparatus is in the state in which the system can be switched, the signal indicating the “system switching confirmation” is sent out to the switching apparatus  10  of the A telephone system through the base station  11   2  (Step S 22 ). That is, the control unit  302  generates and supplies the data indicating the system switching confirmation to the radio section  301 . The radio section  301  modulates and supplies the data indicating the system switching confirmation to the common antenna  300 . In this way, the data indicating the system switching confirmation is transmitted as a radio wave from the common antenna  300  to the base station  11   2 . 
     Next, the communication is started using the B telephone system (Step S 23 ). That is, the B telephone system radio section  301 B is made active and the A telephone system radio section  301 A is made inactive. In this way, the communication with the base station  11   2  which sends and receives the radio wave of the A telephone system is stopped, and the communication with the base station  21   2  which sends and receives the radio wave of the B telephone system is started. 
     In the above description, the switching apparatus  10  operates under the A telephone system and the switching apparatus  20  operates under the B telephone system. Also, the radio communication apparatus  301  is of a dual band type. However, the present invention can be applied even in a case where the switching apparatuses  10  and  20  operate under the same telephone system. In this case, the radio communication system may adopt the above telephone system. Also, a “switching request” is sent to the radio communication apparatus and a “switching confirmation” is received from the radio communication apparatus. 
     As described above, according to the mobile communication system according to the present invention, when the load becomes heavy in one of the mobile communication systems, the operation mode of the dual band radio communication apparatus is switched. That is, the operation mode of the dual band radio communication apparatus is switched in response to a control signal from the base station such that the communication is carried out using the other telephone system. As a result, the traffic of the telephone system with the large load is reduced so that the traffics of the plurality of telephone system are equalized. In this way, it is possible to prevent the degradation of the communication quality which is caused by the large traffic. 
     Also, the mobile communication system is switched under the control of the base station so that it is possible to switch the telephone system in the dual band radio communication apparatus in consideration of the traffic of the telephone system. Therefore, the reliability of the mobile communication system can be increased.