Patent Publication Number: US-2009239538-A1

Title: Base station and handover control method

Description:
The present invention generally relates to a handover control technique for providing lossless handover when a mobile station performs handover. More particularly, the present invention relates to a base station and a handover control method when the mobile station enters a tunnel and the like and handover fails. 
     BACKGROUND ART 
     In a system to which IMT-2000 (International Mobile Telecommunication-2000) is adopted, when a mobile station (UE: User Equipment) moves and handover for going across radio network control apparatuses (RNC: Radio Network Controller) is performed, a procedure called “SRNS Relocation” is performed. 
     The “SRNS Relocation” procedure includes functions for exchanging a transmission sequence number and a reception sequence number between UE and RNC, checking statuses of the transmission side and the reception side, determining a transmission start sequence number, and transferring PDCP PDU (protocol data unit) from a source RNC to a target RNC using packet data convergence protocol (PDCP) that is terminated between UE and RNC. At this time, the source RNC does not locally buffer the PDCP PDU which the source RNC starts to transfer to the target RNC. 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, in the above-mentioned background art, there are following problems. 
     In the system to which IMT-2000 is adopted, recovery procedure when handover fails due to circumstances, for example, retransmission of PDCP PDU is not concretely described. 
     In an actual environment, when a mobile station enters an area such as a tunnel and the like where radio wave does not reach while the mobile station is moving, some cases in which the handover procedure fails can be considered due to reception error of a handover command (HO command) from the source NB (sNB) (handover source base station) e.g. a resource block reconfiguration (RB reconfiguration) message, transmission error of handover command complete (HO Command Complete) to the target NB (tNB) (handover destination base station) e.g. a resource block reconfiguration complete (RB reconfiguration Complete), or reception error of HO command complete at the target NB and the like. 
     In those cases, UE performs cell search, transmits a handover request to a target NB from which radio wave can be received so as to perform error avoidance procedure. In this case, the target NB may be different from a target NB of original handover. Also, the target NB may be the same as the target NB of original handover. 
     In this procedure, the target NB sends a transfer request of context to the source NB. In the SRNS Relocation in the system to which IMT-2000 is applied, since the source NB does not store the user data packet for which transfer has been completed, data loss occurs when handover is performed in reconnection, and there is a fear in that a problem of decrease of throughput in upper layer e.g. transport layer (TCP: Transmission Control Protocol layer) may occur. 
     The present invention is contrived in view of the above-mentioned problem, and the object is to provide a base station and a handover control method that can realize lossless handover even though handover is failed. 
     Means for Solving the Problem 
     For solving the problem, as one of features, the base station of the present invention includes: 
     data storing means configured to store data addressed to a mobile station transmitted from a transfer route control apparatus; 
     data transfer means configured to transfer the data addressed to the mobile station to a handover destination base station according to a request of the data addressed to the mobile station from the handover destination base station, 
     wherein, when the base station receives a handover confirm complete report for reporting handover completion from the handover destination base station, the data storing means ends storing the data addressed to the mobile station. 
     By configuring like this, the handover source base station stores user data addressed to the mobile station so as to be able to transfer the user data to the handover destination base station until handover completes at the handover destination base station. 
     In addition, as one of features, the handover control method includes: 
     a data storing step in which a handover source base station stores data addressed to a mobile station transmitted from a transfer route control apparatus; 
     a data transfer step in which the handover source base station transfers the data addressed to the mobile station to a handover destination base station according to a request of the data addressed to the mobile station by the handover destination base station; 
     a handover confirm complete report reception step in which the handover source base station receives, from the handover destination base station, a handover confirm complete report for reporting handover completion; and 
     a data storing end step in which the handover source base station ends storing the data addressed to the mobile station according to the reception of the handover confirm complete report. 
     By configuring like this, the handover source base station stores user data addressed to the mobile station so as to be able to transfer the user data to the handover destination base station until handover completes at the handover destination base station. 
     EFFECT OF THE INVENTION 
     According to an embodiment of the present invention, a base station and a handover control method that can realize lossless handover even when handover fails can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a radio access network of an embodiment of the present invention; 
         FIG. 2  is a block diagram showing a base station of an embodiment of the present invention; 
         FIG. 3  is a flow diagram showing operation of the radio access network of an embodiment of the present invention; 
         FIG. 4  is a flow diagram showing operation of a source base station of an embodiment of the present invention; 
         FIG. 5  is a flow diagram showing operation of a target base station of an embodiment of the present invention; and 
         FIG. 6  is a flow diagram showing operation of a radio access network of an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           100  radio access network 
           200  core network 
           300 ,  300   1 ,  300   2 ,  300   3  access gateway (aGW) 
           400 ,  400   1 ,  400   2 ,  400   3  base station (eNB: eNode B) 
           500  mobile station (UE: User Equipment) 
       
    
     Preferred Embodiments for Carrying Out the Invention 
     In the following, embodiments of the present invention are described with reference to figures. 
     By the way, in all figures for explaining the embodiments, parts having the same function are assigned the same reference symbols and repeated descriptions are not given. 
     A radio access network of an embodiment of the present invention is described with reference to  FIG. 1 . 
     In the present embodiment, a case is described as an example in which the system is applied to E-UTRA (Evolved-UMTS Terrestrial Radio Access) and E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) that are being standardized in 3GPP. 
     The radio access network  100  of the present embodiment includes a core network  200 , access gateways (aGW: Access Gateway)  300  ( 300   1 ,  300   2 ,  300   3 ) as transfer route control apparatuses that function as gateways between the core network  200  and the radio access network, a plurality of base stations (eNodeB)  400  ( 400   1 ,  400   2 ,  400   3 ) connected under the access gateway  300 , and a mobile station (UE)  500 . 
     The core network  200  includes an IP backbone  250 , for example. 
     The access gateway  300  is a node integrating at least a part of functions of a gateway GPRS support node (GGSN) and RNC in the IMT-2000. Therefore, the access gateway  300  is a node including a function of the core network  200  and a function of the RNC. 
     The base station  400  is a node integrating at least a part of functions of the RNC and NB (Node B) in IMT-2000 aiming dramatic decrease of connection time and decrease of latency. The NB is provided with MAC (Media Access Control) protocol, RLC (Radio Link Control) protocol and the like as functions of U plane, and with RRC (Radio Resource Control) protocol as a function of C plane. 
     In this embodiment, although E-UTRAN is described as an example of radio access network, the radio access network is not limited to this, and it may be other system architecture configuration. 
     The NB is provided with a function to be able to forward, to a target NB, user data buffered when the mobile station  500  performs handover by Outer ARQ function unit of a source NB e.g. outer ARQ SDU (service data unit) that is unsent data and unacknowledged data. In addition, a transmission route for forwarding data is established. The transmission route may be a wireless line or a cable line, and is not limited to the wireless line or the cable line. 
     Next, the base station  400  of the present embodiment is described with reference to  FIG. 2 . 
     The base station  400  of the present embodiment includes a control unit  402  as data transfer means and data request means, a cable transmission unit  404 , a radio transmission unit  406 , a data storing unit  408 , a C-plane storing unit  410 , a HO source base station analyzing unit  414  and a HO control unit  412  as transfer route change request means that are connected to the control unit  402 . 
     The control unit  402  performs control for each function entity provided in the Node B to control the whole operation of the Node B. Detailed functions are described later. 
     The cable transmission unit  404  transfers user data transmitted from the access gateway  300  to the data storing unit  408 . In addition, the cable transmission unit  404  transmits and receives control signals for handover via an interface between base stations. In addition, the cable transmission unit  404  transfers user data stored in the data storing unit  408  via the interface between base stations. 
     The radio transmission unit  406  transmits and receives control signals for handover transmitted from the mobile station  500 . 
     The data storing unit  408  stores user data transmitted from the access gateway  300 . In addition, the data storing unit  408  discards stored user data when handover completes or a time predetermined by a timer elapses. 
     The C-plane storing unit  410  stores control information necessary for setting of U-plane. 
     The HO control unit  412  recognizes information indicating handover completion transmitted from the mobile station  500  so as to request the access gateway  300  to change transfer route based on the information. In addition, the HO control unit  412  sends, to the base station of the source of handover, a handover confirmation completion report for reporting completion of handover. 
     The HO source base station analyzing unit  414  analyzes a handover request that is received from the mobile station  500  via the radio transmission unit  406  to obtain an address, for example, IP address on the network of the handover source base station from an ID indicating the handover source base station included in the handover request. 
     Next, a handover sequence in the radio access network  100  of the present embodiment is described with reference to  FIG. 3 . 
     In this embodiment, a case is described in which the mobile station  500  performs handover from a source base station  400 , connected under the access gateway  300 , to a target base station  400   2 . 
     The mobile station  500  reports, to the source base station  400   1 , a measurement report storing information indicating communication quality of base stations that are candidates of handover destination such that the source-base station  400 , can select a proper handover destination base station (step S 302 ). 
     The source base station  400 , selects (HO decision) a proper target base station for which resource unoccupied state is checked based on the measurement report from the mobile station  500  (step S 304 ). For example, the base station  400   2  is selected as a target base station. 
     Next, the source base station  400 , transfers (context transfer) context that is information related to C-plane, for example, to the base station  400   2  (step S 306 ). 
     Next, the base station  400   2  allocates resources (resource allocation) based on QoS (Quality of Service) information included in the context transferred from the source base station  400   1 . In addition, the target base station  400   2  reserves the allocated resources (step S 308 ). 
     Next, when reservation of necessary resources succeeds, the base station  400   2  reports a context transfer response indicating that preparation for handover is completed to the source base station  400 , (step S 310 ). 
     When the necessary resources cannot be reserved, the base station  400   2  reports that resources cannot be reserved for the mobile station  500  to the source base station  400   1 . 
     When receiving the context transfer response, the source base station  400   1  determines the base station  400   2  as the handover destination base station that is the target base station. 
     On the other hand, when the source base station  400   1  is notified that resources for the mobile station  500  cannot be reserved, the source base station  400   1  selects another proper target base station for which resource available state is checked based on the measurement report from the mobile station  500 . That is, operation similar to that in step S 304  is performed. 
     Next, the source base station  400   1  starts forwarding, to the target base station  400   2 , user data that should be forwarded to the mobile station  500  for which ACK (Acknowledge) is not received from the mobile station  500 , and user data that is received from the access gateway  300   1  and that is not yet transmitted to the mobile station  500  (steps S 312 , S 314 ). 
     The source base station  400   1  reports, to the mobile station  500 , cell specific information necessary for reestablishing a radio line between the mobile station  500  and the target base station  400   2  (resource block (RB) reconfiguration). 
     The mobile station  500  establishes radio synchronization with the target base station  400   2  so as to establish a radio channel. After setting the radio channel between the mobile station  500  and the target base station  400   2 , the mobile station  500  reports a handover complete command (RB Reconfiguration complete) indicating that handover completes to the target base station  400   2 . 
     However, handover does not complete successfully when the mobile station goes into a tunnel and the like, and the electric field becomes weak so that the connection to the source base station  400 , is disconnected, or, when transmission of handover complete command (RB Reconfiguration complete) to the target base station  400   2  fails (HO failure) (step S 316 . 
     In such a case, after the connection with the source base station  400 , is lost, the mobile station  500  autonomously launches handover error recovery procedure using cell search and the like. The mobile station  500  sends a handover request (HO request) to a base station having the best communication quality as a result of the cell search (step S 318 ). For example, the mobile station  500  sends a handover request to the base station  400   2 . 
     The handover request includes identification information such as an ID of the source base station  400 , that disconnected the connection, RNTI (Radio Network Temporary Identifier) used in the source base station  400 , and the like. The RNTI is a temporary identifier for identifying a user. 
     Next, after receiving the handover request, the base station  400   2  analyzes the address e.g. IP address of the source base station  400 , from the ID of the source base station  400   1  included in the handover request to establish a (security) tunnel for transmitting context. By doing that, even when a security tunnel with the source base station  400   1  is not established, the base station that receives the handover request can establish the security tunnel. Thus, the base station that receives the handover request can communicate data between the base station and the source base station  400 , safely. 
     Next, when the base station  400   2  receives the handover request, the base station  400   2  sends a context transfer request to the source base station  400   1  (step S 320 ). 
     Next, the source base station  400   1  copies context and user data for the mobile station  500  and sends them to the target base station  400   2  (step S 322 , step S 324 , step S 326 ). The source base station  400   1  stores these pieces of data until receiving the handover confirm complete report, from the handover destination base station, for reporting that handover completes. 
     The target base station  400   2  receives the context transmitted from the source base station  400   1 . In addition, when handover completes, the target base station  400   2  transmits the handover confirm complete report (HP confirm) for reporting completion of handover to the source base station  400   1  (step S 328 ). 
     The source base station  400   1  that receives the handover confirm complete report ends storing data addressed to the mobile station so as to discard the copied context (step S 332 ). 
     Next, the target base station  400   2  transmits, to the access gateway  300   1 , a request (Path switch) for switching a path from the access gateway  300   1  set in the source base station  400   1  to the target base station  400   2  (step S 330 ). 
     The access gateway  300   1  that receives the Path switch reports path release to the source base station  400   1  (step S 334 ). 
     As mentioned above, the source base station  400   1  copies original context, transfers the copy to the target base station  400   2 , and discards the original context after receiving the handover confirm complete report from the target base station  400   2 . By doing the procedure, since the original context can be held until receiving the handover confirm complete report from the target base station  400   2 , loss-less handover can be realized even when the mobile station  500  moves into a tunnel so that handover does not complete successfully, for example. 
     Next, operation of the base station  400  in the radio access network  100  of the present embodiment is described. 
     In the present embodiment, operation is described for the source base station and the target base station. 
     Operation of the source base station is described with reference to  FIG. 4 . 
     User data addressed to the mobile station transmitted from the access gateway  300  is stored in the data storing unit  408 . In addition, control information necessary for setting U-plane is stored in the C-plane storing unit  410 . 
     The control unit  402  determines whether there is a transfer request of context, for example, C-plane control information and user data from the target base station  400   2  (step  402 ). The transfer request of the context is received via the cable transmission unit  404 . 
     When it is determined that there is no transfer request of the context from the target base station  400   2  (step S 402 :NO), the process ends. 
     On the other hand, when it is determined that there is the transfer request of the context from the target base station  400   2  (step S 402 :YES), the control unit  402  copies control information, stored in the C-plane storing unit  410 , necessary for setting U-plane so as to start forwarding the copy. In addition, the control unit  402  copies the user data stored in the data storing unit  408  to start forwarding the copy (step S 404 ). As a result, copied control information necessary for setting U-plane and copied user data are forwarded to the target base station  400   2  instead of original control information necessary for setting U-plane and original user data 
     Next, the control unit  402  determines whether the handover confirm-complete report is received from the target base station  400   2  (step S 406 ). The handover confirm complete report is received via the cable transmission unit  404 . 
     When the handover confirm complete report is not yet received (step S 406 :NO), the process returns to step S 404 . 
     On the other hand, when the handover confirm complete report is received (step S 406 :YES), the control unit  402  discards context, for example, discards control information necessary for setting U-plane stored in the C-plane storing unit  410  and user data stored in the data storing unit  408 , wherein copies of the control information and the user data have been sent to the target base station  400   2  (step S 408 ). 
     Operation of the target base station is described with reference to  FIG. 5 . 
     After the connection between the mobile station  500  and the source base station  400 , is lost, the mobile station  500  autonomously launches a handover error recovery procedure by cell search and the like. The mobile station  500  transmits a handover request to a base station, for example, the base station  400   2  by which the communication quality is the best as a result of the cell search. 
     The control unit  402  determines whether the handover request is received from the mobile station  500  (step S 502 ). The handover request is received via the radio transmission unit  406 . 
     When it is determined that the handover request is not yet received (step S 502 :NO), the process ends. 
     On the other hand, when it is determined that the handover request is received (step S 502 :YES), the HO source base station analyzing unit  414  obtains the address of the source base station from the ID of the source base station included in the handover request message to establish a tunnel for transferring context (step S 503 . 
     Next, the control unit  402  transmits a transfer request of the context to the source base station (step S 504 ). The transfer request of the context is transmitted to the source base station via the cable transmission unit  402 . 
     Next, the control unit  402  determines whether the context is received from the source base station (step S 506 ). 
     When it is determined that the context is not yet received from the source base station (step S 506 :NO), the process returns to step S 504 . 
     On the other hand, when it is determined that the context is received from the source base station (step S 506 :YES), the HO control unit  412  transmits the handover confirm complete report to the source base station. In addition, the HO control unit  412  transmits a path switch request to the access gateway (step S 508 ). 
     As described above, in the present embodiment, even though handover fails for some reasons, lossless handover can be realized by forwarding context from the handover destination base station. 
     In addition, since the handover source base station can hold the context until receiving the handover confirm complete report from the handover destination base station, even when entering a tunnel so that handover does not complete successfully, loss-less handover can be realized. 
     A radio access network according to another embodiment of the present invention is described. 
     The configuration of the radio access network of the present embodiment is similar to the configuration described with reference to  FIG. 1 . 
     The base station  400  is a node integrating at least a part of functions of the RNC and NB (Node B) in IMT-2000 aiming dramatic decrease of connection time and decrease of latency. The NB is provided with MAC (Media Access Control) protocol, RLC (Radio Link Control) protocol, PDCP (Packet Data Convergence Protocol) and the like as functions of U plane, and with RRC (Radio Resource Control) protocol, S1-AP (S1-Application Part), X2-AP (X2-Application Part) and the like as functions of C plane. 
     The NB is provided with a function to be able to forward, to a target NB, user data buffered when the mobile station  500  performs handover by PDCP function unit of a source NB e.g. service data unit (SDU) that is unsent data and unacknowledged data. In addition, a transmission route for forwarding data is established. The transmission route may be a wireless line or a cable line, and is not limited to the wireless line or the cable line. 
     Configuration of the base station  400  of the present embodiment is similar to the configuration described with reference to  FIG. 2 . 
     The HO control unit  412  recognizes information indicating handover complete transmitted from the mobile station  500 , and transmits, to the access gateway  300 , handover complete report for reporting that handover is completed in order to request the access gateway  300  to switch transfer route. In addition, the HO control unit  412  reports, to the handover source base station, that handover is completed using a resource release message. 
     Next, a handover sequence in the radio access network  100  of the present embodiment is described with reference to  FIG. 6 . 
     In this embodiment, a case is described in which the mobile station  500  performs handover from the source base station  400 , connected under the access gateway  300 , to the target base station  400   2 . 
     The mobile station  500  reports, to the source base station  400   1 , a measurement report storing information indicating communication quality of base stations that are candidates of handover destinations such that the source base station  400   1  can select a proper handover destination base station (step S 602 ). 
     The source base station  400 , selects (HO decision) a proper target base station for which resource unoccupied state is to be checked based on the measurement report from the base station  500  (step S 604 ). For example, the base station  400   2  is selected as a target base station. 
     Next; the source base station  400 , sends handover request to transfer context that is information related to C-plane, for example, to the base station  400   2  (step S 606 ). 
     Next, the base station  400   2  allocates resources (Admission Control) based on QoS (Quality of Service) information included in the context transferred from the source base station  400   1 . In addition, the target base station  400   2  reserves the allocated resources (step S 608 ). 
     Next, when reservation of necessary resources succeeds, the base station  400   2  reports, to the source base station  400   1 , handover request acknowledgment (Handover Request ACK), indicating that preparation for handover completes, including cell specific information necessary for resetting radio line (step S 610 ). 
     When the necessary resources are not reserved, the base station  400   2  reports that resources cannot be reserved to the mobile station  500 . 
     When receiving the handover request acknowledgment, the source base station  400 , determines the base station  400   2  as the handover destination base station that is the target base station. On the other hand, when the source base station  400   1  is notified that resources from the base station  400   2  to the mobile station  500  cannot be reserved, the source base station  400 , selects another proper target base station for which resource available state is checked based on the measurement report from the mobile station  500 . That is, operation similar to that in step S 604  is performed. 
     Next, the source base station  400   1  starts forwarding, to the target base station  400   2 , user data that should be forwarded to the mobile station  500  for which ACK (Acknowledge) is not yet received from the mobile station  500 , and user data that is received from the access gateway  300   1  and that is not yet transmitted to the mobile station  500  (start user data forwarding) (steps S 612 , S 614 ). 
     The source base station  400 , reports, to the mobile station  500 , cell specific information necessary for reestablishing a radio line between the mobile station  500  and the target base station  400   2  (Handover Command). 
     The mobile station  500  establishes radio synchronization with the target base station  400   2  to establish a radio channel. After setting the radio channel to the target base station  400   2 , the mobile station  500  reports a handover confirm command (Handover Confirm) indicating that handover completes to the target-base station  400   2 . 
     However, handover does not complete successfully when the mobile station  500  moves into a tunnel and the like so that the electric field becomes weak and the connection to the source base station  400 , is disconnected, or, when transmission of handover confirm command (Handover confirm) to the target base station  400   2  fails (HO failure) (step S 616 ). 
     In such a case, after the connection with the source base station  400   1  is lost, the mobile station  500  autonomously launches handover error recovery procedure using cell search and the like. The mobile station  500  sends a handover request (HO request) to a base station having the best communication quality as a result of the cell search (step S 618 ). For example, the mobile station  500  sends a handover request to the base station  400   2 . 
     The handover request includes identification information such as an ID of the source base station  400   1  that disconnected the connection, RNTI (Radio Network Temporary Identifier) used in the source base station  400   1  and the like. The RNTI is a temporary identifier for identifying a user. 
     Next, after receiving the handover request, the base station  400   2  analyzes the address e.g. IP address of the source base station  400 , from the ID e.g. cell ID of the source base station  400   1  included in the handover request to establish a (security) tunnel e.g. IPsec (Security Architecture for Internet Protocol) for transmitting context as necessary. By doing that, even when a security tunnel is not established with the source base station  400   1 , a base station that receives the handover request can establish a security tunnel. Thus, the base station that receives the handover request can perform communication of data between the base station and the source base station  400   1  safely. 
     Next, when the base station  400   2  receives the handover request, the base station  400   2  sends a context transfer request to the source base station  400   1  (step S 620 ). 
     Next, the source base station  400   1  copies context and user data for the mobile station  500  and sends them to the target base station  400   2  (step S 622 , step S 624 , step S 626 ). 
     The target base station  400   2  receives the context transmitted from the source base station  400   1 . 
     Next, the target base station  400   2  transmits, to the access gateway  300   1 , a handover complete report (Handover Complete) for reporting that handover is completed in order to switch a path from the access gateway  300   1  set in the source base station  400 , to the target base station  400   2  (step S 628 ). 
     The access gateway  300 , that receives the handover complete report reports, to the target base station  400   2 , a handover complete acknowledgement (Handover Complete ACK) for reporting path switch complete (step S 630 ). 
     The target base station  400   2  that receives the handover complete acknowledgement sends resource release message to the source base station  400   1  (step S 632 ). 
     The source base station  400   1  that receives the resource release message discards the copied context (Delete context) (step S 634 ). 
     As mentioned above, the source base station  400   1  copies original context, transfers the copy to the target base station  400   2 , and discards the original context after receiving the resource release message from the target base station  400   2 . By doing that procedure, since the original context can be held until receiving the resource release message from the target base station  400   2 , even when the mobile station  500  moves into a tunnel so that handover does not complete successfully, for example, lossless handover can be realized. 
     Next, operation of the base station  400  in the radio access network  100  of the present embodiment is described. 
     In the present embodiment, operation is described for the source base station and the target base station. 
     The operation of the source base station is different from the operation of the source base station described with reference to  FIG. 4  in the process of step S 406 . In step S 406 , it is determined whether the resource release message is received from the target base station  400   2 . When receiving the resource release message is received from the target base station  400   2 , in step S 408 , the control unit  402  discards context, for example, discards control information necessary for setting U-plane stored in the C-plane storing unit  410  and user data stored in the data storing unit  408 , wherein copies of the control information and the user data have been sent to the target base station  400   2 . 
     The operation of the target base station is different from the operation of the source base station described with reference to  FIG. 5  in the process of step S 508 . In step S 508 , when determining that context is received from the source base station (step S 506 :Yes), the HO control unit  412  sends the resource release message to the source base station and sends the handover complete report to the access gateway. 
     As described above, in the present embodiment, even though handover fails for some reason, lossless handover can be realized by forwarding context from handover destination base station. 
     In addition, since the context can be held at the handover source base station until receiving the resource release message from the handover destination base station, lossless handover can be realized even though handover does not complete successfully when entering a tunnel. 
     The present international application claims priority based on Japanese patent application No. 2006-57979, filed in the JPO on Mar. 3, 2006 and the entire contents of the Japanese patent application No. 2006-57979 is incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The base station and the handover control method of the present invention can be applied to a radio communication system.