Abstract:
A mobile communication system, a radio switching method, a mobile base station and a mobile terminal in which the mobile terminal does not perform handover to a fixed base station while moving in/on a moving body to avert an adverse affect of variation of received field intensity on communication. Fixed base stations that are connected to a public telephone network are disposed along a traveling direction of a train. The train which a user who possesses a mobile terminal gets on is provided with a mobile base station capable of being connected to the fixed base stations. The mobile base station detects the mobile terminal entering into the communication area covered by the mobile base station and the train starting moving to the next station. Triggered by the two kinds of detection, the mobile base station takes over processes of handover to the fixed base station from the mobile terminal. While moving, the mobile base station performs handover to a next base station one after another. When the train approaches the next station, the mobile terminal performs handover to the fixed base station that covers the communication area around the station of arrival in behalf of the mobile base station.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a mobile communication system, a radio switching method, a mobile base station and a mobile terminal, and in particular, to a mobile communication system, a radio switching method, a mobile base station and a mobile terminal for performing smooth handover to a fixed base station when a user uses a mobile terminal in/on a moving body such as an express train.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    The number of mobile phone users is increasing year by year along with the popularization of mobile phones. Additionally, it is definite in the future that mobile-phone markets will enter into a next generation of mobile phones based on CDMA (Code Division Multiple Access), in which codes are used for multiplexing. A CDMA system has some advantages of reduction of transmission power, concurrent use of the same frequency band by a plurality of mobile phones, and the like. A spread spectrum communication system is employed in the CDMA system for continuous and consistent transmission of data, not for burst transmission, thereby generating less electromagnetic disturbance. Owing to this system, it is anticipated that more users will be able to use the mobile phones on/in a moving body such as an express train. In the existing PDC (Personal Digital Cellular) communication system, however, it is difficult to execute smooth handover to a base station when the mobile phone is used in/on a moving body such as an express train traveling at a high speed. Further, it is often the case that communication by the mobile phone becomes impossible. Therefore, satisfactory service cannot be provided for users on a train or in a car.  
           [0003]    In Japanese Patent Application Laid-Open No. HEI 10-322747, for example, there is disclosed a mobile communication system for comfortable communication on the occasion of high-speed moving. In this system, when a mobile terminal of a user receives a position control signal from a radio base station set to an entrance gate of a station as the user intends to get on a train, a control section of the mobile terminal (mobile phone) switches the operation mode from a normal operation mode to a specific space mode (high-speed moving mode) and controls a measuring section to shorten averaging time of measuring a received field intensity. By this means, the measuring section can measure the received field intensity in less time. Accordingly, it becomes possible for the user to communicate by the mobile terminal even on the train moving at a high speed.  
           [0004]    In Japanese Patent Application Laid-Open No. HEI 9-84126, for another example, there is disclosed a mobile communication system. In this system, a high-speed moving body M is provided with a relay station RS to form a micro cell of a wireless area therein. When a mobile terminal in/on the moving body M communicates with a base station, the mobile terminal is always connected to the relay station RS in/on the moving body M. The relay station RS sequentially performs handover to an adjacent base station as the moving body M travels. Accordingly, it becomes possible to communicate by a mobile terminal even in/on a high-speed moving body M in the same manner as on the ground.  
           [0005]    However, in the system disclosed in Japanese Patent Application Laid-Open No. HEI 10-322747, the mode of the mobile terminal in/on the moving body is switched to the specific space mode only to shorten the averaging time of measuring the received field intensity. Furthermore, the high-speed moving body is not provided with a relay station. Under the condition of high-speed moving, frequent handover is supposed to be executed. Accordingly, the mobile terminal becomes more under the influence of the variation of the received field intensity at the time of handover. Moreover, there is a need to set up dedicated equipment as described above, which results in an increase of equipment costs.  
           [0006]    Further, in Japanese Patent Application Laid-Open No. HEI 9-84126, the mobile terminal always communicates with the base station (network) via the relay station RS on the high-speed moving body M. Even when there is no need to interpose the relay station RS for communication, communication is executed via the relay station RS. Accordingly, the relay station RS has a bigger processing burden. Further, handover processing is generated when the user gets off the train at the destination station as he/she talks through the mobile terminal and when the high-speed moving body M leaves the station at an accelerated pace. Accordingly, the mobile terminal becomes affected by the variation of the received field intensity.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the present invention to provide a mobile communication system and a radio switching method, in which a mobile terminal can communicate with a fixed base station without being affected by variation of received field intensity due to no execution of handover to a fixed base station when moving in/on a moving body.  
           [0008]    It is another object of the present invention to provide a mobile base station for performing handover to a fixed base station in behalf of a mobile terminal that is moving in/on a moving body, thereby realizing stable communication without being affected by variation of received field intensity.  
           [0009]    It is further another object of the present invention to provide a mobile terminal for stable communication without being affected by variation of received field intensity when moving in/on a moving body.  
           [0010]    According to a first aspect of the present invention, for achieving the objects mentioned above, there is provided a mobile communication system comprising:  
           [0011]    a plurality of fixed base stations that are serially disposed along a moving direction of a moving body capable of high-speed moving and that are connected to a public telephone network;  
           [0012]    a mobile terminal that is possessed by a user who gets on the moving body; and  
           [0013]    a mobile base station that is capable of being connected to the plural fixed base stations and that is put in or on the moving body, wherein:  
           [0014]    triggered by an entrance of the mobile terminal into a communication area of the mobile base station and by a start of the moving body, the mobile terminal performs handover from a fixed base station that has been connected to the mobile terminal to the mobile base station;  
           [0015]    while the moving body is moving, the mobile base station sequentially performs handover to an adjacent fixed base station when the moving body enters into a communication area of the adjacent fixed base station; and  
           [0016]    when the moving body approaches a stop, the mobile terminal performs handover from the mobile base station to a fixed base station that covers a communication area around the stop.  
           [0017]    According to the system, the mobile terminal performs handover from the fixed base station to the mobile base station put in/on the moving body when the mobile terminal enters into the communication area of the mobile base station and when the moving body starts moving. Further, while the moving body is moving, the mobile base station performs handover to a fixed base station one after another. Moreover, when the moving body approaches a next stop such as a station, the mobile base station gets the mobile terminal to perform handover to a fixed base station that covers a communication area around the stop. By this means, the mobile terminal does not have to perform handover to a fixed base station while the moving body is moving, which results in less effects of variation of received field intensity on communication by the mobile terminal.  
           [0018]    According to a second aspect of the present invention, there is provided a radio switching method for sequentially performing handover to an adjacent fixed base station among a plurality of fixed base stations that covers respective communication areas divided into cells according to a move of a mobile terminal, comprising the steps of:  
           [0019]    performing, by the mobile terminal in or on a moving body, handover from a fixed base station that covers a communication area where the moving body is located to a mobile base station that is put in or on the moving body, which is triggered by an entrance of the mobile terminal possessed by a user into a communication area of the mobile base station and when the moving body starts moving;  
           [0020]    sequentially performing, by the mobile base station, handover to an adjacent fixed base station when the moving body enters into a communication area of the adjacent fixed base station while the moving body is moving; and  
           [0021]    performing, by the mobile terminal, handover from the mobile base station to a fixed base station that covers a communication area around a stop when the moving body approaches the stop.  
           [0022]    According to the method, the mobile terminal performs handover from the fixed base station to the mobile base station when the mobile terminal enters into the communication area of the mobile base station put in/on the moving body and when the moving body starts moving. The mobile base station performs handover to a fixed base station while the moving body is moving. When the moving body approaches a stop such as a station, the mobile terminal takes over, from the mobile base station, the processes of handover to the fixed base station that covers the communication area around the stop. By this means, the mobile terminal does not have to perform handover to a fixed base station while the moving body is moving, which results in less effects of variation of received field intensity on communication by the mobile terminal.  
           [0023]    According to a third aspect of the present invention, there is provided a mobile base station that is put in or on a moving body, comprising:  
           [0024]    a first detecting means for detecting a mobile terminal entering into a communication area of the mobile base station;  
           [0025]    a second detecting means for detecting the moving body which a user of the mobile terminal gets on starting moving; and  
           [0026]    a control means for:  
           [0027]    connecting to the mobile terminal and to a fixed base station that has been connected to the mobile terminal to continue communication between the mobile terminal and the fixed base station on the basis of the detection by the first and the second detecting means;  
           [0028]    performing handover to an adjacent fixed base station when the moving body enters into a communication area of the adjacent fixed base station while the moving body is moving; and  
           [0029]    getting the mobile terminal to perform handover to a fixed base station that covers a communication area around a stop when the moving body approaches the stop.  
           [0030]    According to the configuration, the mobile terminal performs handover from the fixed base station to the mobile base station put in/on the moving body when the mobile terminal enters into the communication area of the mobile base station and when the moving body starts moving. When the moving body approaches a next stop, the mobile terminal performs handover from the mobile base station to the fixed base station that covers the communication area around the stop. By this means, the mobile terminal does not have to perform handover to a fixed base station while the moving body is moving, which results in less effects of variation of received field intensity on communication by the mobile terminal.  
           [0031]    According to a fourth aspect of the present invention, there is provided a mobile terminal capable of communicating with a fixed base station connected to a public telephone network by radio, including a communication means capable of communicating with a mobile base station put in or on a moving body when the mobile terminal is in or on or in the vicinity of the moving body, wherein:  
           [0032]    the mobile terminal communicates with the mobile base station while the mobile terminal is moving in or on the moving body; and  
           [0033]    the mobile terminal performs handover from the mobile base station to a fixed base station that covers a communication area around a stop when the moving body approaches the stop.  
           [0034]    According to the configuration, when the mobile terminal is moving in/on the moving body, the mobile terminal communicates with (or is connected to) the mobile base station put in/on the moving body. Further, when the moving body approaches a next stop such as a station, the mobile terminal performs handover to the fixed base station that covers the communication area around the stop to communicate with the fixed base station. By this means, the mobile terminal does not have to perform handover to a fixed base station while the moving body is moving, which results in less effects of variation of received field intensity on communication by the mobile terminal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0036]    [0036]FIG. 1 is a diagram showing a configuration of a mobile communication system according to the present invention;  
         [0037]    [0037]FIG. 2 is a diagram for explaining positional situation of each unit shown in FIG. 1;  
         [0038]    [0038]FIG. 3 is a timing chart showing processes of the mobile communication system when the train shown in FIG. 2 starts at the point A of the station  201 ;  
         [0039]    [0039]FIG. 4 is a timing chart showing processes of the mobile communication system when the train shown in FIG. 2 passes through the boundary point B between communication areas of two adjacent fixed base stations; and  
         [0040]    [0040]FIG. 5 is a timing chart showing processes of the mobile communication system when the train shown in FIG. 2 passes through the point C of the station  202 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]    Referring now to the drawings, an embodiment of the present invention is explained in detail.  
         [0042]    [0042]FIG. 1 is a diagram showing a configuration of a mobile communication system, and FIG. 2 is a diagram for explaining positional situation of each unit shown in FIG. 1.  
         [0043]    In FIG. 1, the mobile communication system according to the present invention comprises a train  101  such as an express train, a mobile base station  102 , a mobile terminal  103 , fixed base stations  104  and  105 , a base station control apparatus  106 , and a network (public telephone network)  107 . The mobile base station  102  is put in/on the train  101 . The mobile terminal  103  is used by a user inside or outside the train  101 . The fixed base stations  104  and  105  communicate with the mobile base station  102  or the mobile terminal  103  by radio. The base station control apparatus  106  is connected to the fixed base stations  104  and  105 . The network  107  is connected to the base station control apparatus  106 . Incidentally, the mobile terminal  103  is a mobile phone. Further, the network  107  comprises a mobile telephone network, a fixed-line telephone network and the Internet.  
         [0044]    As shown in FIG. 2, the train  101  runs on a track  203  laid via stations  201  and  202 . The fixed base station  104  is placed so as to cover a communication area (cell) around the station  201 . On the other hand, the fixed base station  105  is placed so as to cover a communication area (cell) around the station  202 . The mobile terminal  103  in/on the train  101  is normally connected to the fixed base station  104  that covers the communication area around the station  201 . When the train  101  starts running, the mobile terminal  103  is connected to the mobile base station  102 . By this means, the mobile terminal  103  can communicate with the side of the fixed base station  104  without executing handover.  
         [0045]    It is easy to locate the position of the train  101  because the train  101  travels on the fixed track  203  at a predetermined speed. Thereby, the mobile base station  102  can uniquely identify the next fixed base station to be accessed (connected) according to the positional information of the train  101 . Moreover, the mobile terminal  103  has only to be connected to the mobile base station  102  to communicate with the fixed base station  104  or  105  while moving. On this occasion, the mobile terminal  103  does not have to execute handover. Subsequently, when the train  101  approaches the station  202  next to the station  201 , the mobile base station  102  performs handover to the fixed base station  105  that covers the communication area around the station  202  on the basis of the positional information of the train  101  for continuous communication.  
         [0046]    In the following, an explanation will be given of an operation of the mobile communication system according to the present invention referring to FIGS.  1  to  5 .  
         [0047]    [0047]FIG. 3 shows processes of the mobile communication system when the train  101  shown in FIG. 2 starts at the point A of the station  201 . FIG. 4 shows processes of the mobile communication system when the train  101  passes through the boundary point B between communication areas of the fixed base stations  104  and  105 . Moreover, FIG. 5 shows processes of the mobile communication system when the train  101  passes through the point C of the station  202 . Incidentally, the capital “S” in FIGS.  3  to  5  denotes a step.  
         [0048]    In FIG. 3, when a user of the mobile terminal  103  uses the mobile terminal  103  on the platform or the like in the station  201 , the mobile terminal  103  is connected to the network  107  via the fixed base station  104  that covers the communication area around the station  201  (Steps S 301  and S 302 ). After that, when the user gets on the train  101  (Step S 300 A), the mobile terminal  103  detects the mobile base station  102  put in/on the train  101  (Step S 303 ), and notifies the fixed base station  104  of the detected result (Step S 304 ). When receiving the detected result, the fixed base station  104  informs the network  107  of the information acquired at Step S 304  (Step S 305 ).  
         [0049]    When the train  101  starts running, the mobile base station  102  detects the move (Step S 300 B) and informs the fixed base station  104  of the information (Step S 306 ). Further, the fixed base station  104  informs the network  107  of the information acquired at Step S 306  (Step S 307 ). Subsequently, the network  107  informs the fixed base station  104  of a direction that the mobile terminal  103  is to perform handover (Step S 308 ). The fixed base station  104  forwards the direction to the mobile terminal  103  (Step S 309 ).  
         [0050]    Subsequently, the mobile terminal  103  switches the connected point from the fixed base station  104  to the mobile base station  102  (Step S 300 C). By this means, the mobile terminal  103  is connected to the network  107  via the mobile base station  102  and the fixed base station  104  for continuous communication (Steps S 310 , S 311  and S 312 ).  
         [0051]    In the following, an explanation will be given of processes of the mobile communication system when the train  101  passes through the boundary point B between the communication areas of the fixed base stations  104  and  105  in the course of its traveling in reference to FIG. 4. When the train  101  passes through the boundary point B shown in FIG. 2, the mobile base station  102  detects the next fixed base station to be connected on the basis of the positional information of the train  101  (Step S 400 A). Here, the mobile base station  102  detects the fixed base station  105  that covers the communication area around the next station  202  (Step S 401 ). The mobile base station  102  informs the fixed base station  104  of the detected information about the fixed base station  105  (Step S 402 ). The fixed base station  104  informs the network  107  of the received detected information (Step S 403 ). Subsequently, the network  107  informs the fixed base station  104  of a direction that the mobile base station  102  is to perform handover (Step S 404 ). The fixed base station  104  informs the mobile base station  102  of the direction (Step S 405 ). The mobile base station  102  activates handover function to switch the connected point from the fixed base station  104  to the fixed base station  105  (Step S 400 B). Subsequently, the mobile base station  102  communicates with the network  107  via the fixed base station  105  (Steps S 406  and S 407 ). Hereat, the mobile terminal  103  on the train  101  constantly communicates via the onboard mobile base station  102 . Thereby, the mobile terminal  103  does not have to execute handover processes.  
         [0052]    In the following, an explanation will be given of processes of the mobile communication system when the train  101  passes through the point C of the station  202  in reference to FIG. 5. When the train  101  is about to arrive at the station  202  (point C) shown in FIG. 2, the mobile base station  102  detects the train  101  approaching the station  202  on the basis of the positional information (Step S 500 A). Subsequently, the mobile base station  102  informs the connected fixed base station  105  about the detected result (Step S 501 ). When receiving the detected result, the fixed base station  105  informs the network  107  of the information received at Step S 501  (Step S 502 ). Subsequently, the network  107  notifies the fixed base station  105  of a direction that the mobile terminal  103  is to perform handover (Step S 503 ). When receiving the direction, the fixed base station  105  informs the mobile base station  102  of the received information (Step S 504 ). Subsequently, the mobile base station  102  informs the mobile terminal  103  of the direction received from the fixed base station  105  (Step S 505 ). After that, the mobile terminal  103  fulfills the handover function (Step S 500 B) and switches the connected point from the currently connected mobile base station  102  to the fixed base station  105  for communication with the network  107  (Steps S 506  and S 507 ).  
         [0053]    Chart 1 shows connection relationships among the mobile terminal  103 , mobile base station  102  and fixed base stations  104  and  105  described hereinbefore.  
                       CHART 1                       Point A   Point B   Point C                   Mobile Terminal 103   Mobile Terminal 103   Mobile Terminal 103       ↓   ↓   ↓       Mobile Base Station   Mobile Base Station       102   102   ↓       ↓   ↓   ↓       Fixed Base Station 104   Fixed Base Station 105   Fixed Base Station 105       ↓   ↓   ↓       Network 107   Network 107   Network 107                  
 
         [0054]    By this means, while the train  101  is moving, the mobile terminal  103  is connected via the mobile base station  102  to a fixed base station that covers the communication area where the train  101  is located now. Whenever the train  101  arrives at the next station, the connected point is switched from the mobile base station  102  to a fixed base station that covers the communication area around the station of arrival. Namely, whenever the train  101  arrives at the next station, the mobile terminal  103  is directly connected to the fixed base station that covers the communication area around the station of arrival not via the mobile base station  102 . Accordingly, there is no need to execute handover by the mobile terminal  103  during communication on the train. Subsequently, when the train  101  approaches a station, the connected point is switched from the mobile base station  102  to a fixed base station that covers the communication area around the station. Consequently, when a user gets off the train  101  at the station of arrival as he/she talks by the mobile terminal  103  and when the train  101  departs from the station, there is no need to execute extra handover processing. Furthermore, at this point, the mobile terminal  103  is connected to the fixed base station not via the mobile base station  102  as in normal communication circumstances. Accordingly, variation of received field intensity accompanied by handover processing has no effect on communication via the mobile terminal  103 .  
         [0055]    Incidentally, while a train (an express train) is taken as an example of a moving body in the above-described embodiment, the present invention is applicable to any object that moves along a predetermined track. Examples of the moving body include an express bus that travels along a freeway or expressway. In this case, bus stops function as the stations  201  and  202 .  
         [0056]    Moreover, the mobile terminal  103  includes, in addition to a mobile phone, a PHS (Personal Handyphone System), a PDA (Personal Digital Assistant) with wireless communication functions capable of being connected to a public telephone network, and the like.  
         [0057]    As set forth hereinbefore, according to the mobile communication system, the radio switching method and the mobile base station of the present invention, when a mobile terminal enters in a communication area covered by a mobile base station put in/on a moving body and when the moving body starts moving, the mobile terminal performs handover from a fixed base station to the mobile terminal. By this means, communication between the mobile terminal and the fixed base station is executed via the mobile base station. Further, while the moving body is moving, the mobile base station sequentially performs handover to the next fixed base station one after another. Moreover, when the moving body approaches a stop such as a station, the mobile terminal takes over the processes of handover from the mobile base station to perform handover to a fixed base station that covers the communication area around the stop. By this means, the mobile terminal does not have to execute handover to a fixed base station while the moving body is moving. Furthermore, it becomes possible to eliminate adverse effects of variation of received field intensity on communication via the mobile terminal.  
         [0058]    Further, according to the present invention, the mobile terminal communicates with a fixed base station via the mobile base station put in/on the moving body while the mobile terminal is moving on the moving body. Subsequently, when the moving body approaches a stop such as a station, the mobile terminal performs handover to a fixed base station that covers the communication area around the stop. By this means, communication via the mobile terminal is not affected by the variation of received field intensity.  
         [0059]    While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by the embodiment but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.