Abstract:
A base station includes a memory that stores identification information of a cell of the base station and identification information of one or more neighboring cells. The base station also includes a receiver that receives a neighboring cell information message. The received neighboring cell information message includes source-indicating data that indicates a source base station and indicates cell identification information of a respective cell of the source base station; neighboring cell information that includes received identification information of one or more neighbor cells adjacent to the respective cell of the source base station; and sender-indicating data that indicates a sender of the neighboring cell information message. The base station also includes a controller adapted to (1) analyze the received identification information to detect any instance in which the stored identification information of the cell of the base station is present more than one time in the received identification information; and (2) in response to detecting the instance, to carry out an update function of the stored identification information using the received identification information. The sender-indicating data of the received neighboring cell information message indicates a sender other than the source base station.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2012-136782, filed on Jun. 18, 2012, the disclosure of which is incorporated herein in its entirety by this reference. 
       BACKGROUND 
       [0002]    Exemplary embodiments of the present invention relate to a base station, a wireless communication system and an identification information change method and program and, more particularly, to a base station, a wireless communication system and an identification information change method and program capable of resolving a conflict between items of cell identification information. 
       Description of the Related Art 
       [0003]    In a wireless communication system using mobile stations such as portable terminals, a base station performs wireless communication with a mobile station that is located in the cell of its base station. 
         [0004]    Cells are each assigned a physical cell ID (PCI) for identification of the cell. PCIs are assigned so that no PCI conflict (no coincidence between PCIs) occurs between the cells adjacent to each other or between the cells that are both adjacent to one of the other cells. 
         [0005]    However, the state of adjacency between the cells is changed if the electric wave environment is changed by construction of a high-rise building or installation of a new base station after assignment of the PCIs. If the state of adjacency between the cells is changed, there is a possibility of a PCI conflict between cells that are adjacent to each other or between the cells that are both adjacent to one of the other cells. Occurrence of a PCI conflict causes difficulty in discrimination between the cells adjacent to each other or between the cells that are both adjacent to one of the other cells. 
         [0006]    Japanese Patent Laid-Open JP 2012-19468A describes a wireless communication system in accordance with the Long Term Evolution (LTE) standard capable of resolving a PCI conflict. 
         [0007]    In this wireless communication system, a base station receives cell information through an X2 interface from each of other two base stations adjacent to it (hereinafter referred to as “neighboring base stations”). The cell information includes a PCI of a cell belonging to the neighboring base station, from which the cell information is transmitted. The X2 interface is an interface between the base stations. A link between the X2 interfaces is referred to as “X2 link” below. 
         [0008]    When the base station detects a PCI conflict between cells that respectively belong to the two neighboring base stations by referring to the items of cell information, the base station transmits a PCI change designating request, which indicates the conflicting PCI, to one of the neighboring base stations through the X2 link. 
         [0009]    Upon receipt of the PCI change designating request through the X2 link, one of the base stations changes the PCI indicated by the PCI change designating request, i.e., the conflicting PCI. 
       SUMMARY 
       [0010]    In the wireless communication system, X2 links are not established between all the base stations. 
         [0011]    The base station to which the X2 link is not established cannot receive cell information from the neighboring base stations and cannot transmit a PCI change designating request to the neighboring base stations. Therefore the base station to which the X2 link is not established cannot resolve a PCI conflict that occurs between the neighboring base stations. In this case, in order to resolve the PCI conflict, an operator is required to manually change the conflicting PCI. 
         [0012]    An object of certain exemplary embodiments is to provide a base station, a wireless communication system and an identification information change method and program capable of resolving the above-described problem. 
         [0013]    A base station, which communicates with a controller covering a plurality of base stations, according to certain exemplary embodiments includes: storage means that stores neighboring cell information, which indicates identification information of a cell of the base station and identification information of at least one adjacent cell adjacent to the cell of the base station; transmitting means that transmits the neighboring cell information in said storage means to the controller; and changing means that replaces the identification information of the cell of the base station in said storage means with cell identification information that is different from any identification information indicated in neighboring cell information stored in another of the plurality of base stations when the neighboring cell information stored in the other of the plurality of base stations is received from the controller and if the identification information of the cell of the base station is designated two or more times in the received neighboring cell information. 
         [0014]    A base station, which executes handover with respect to a mobile station by using identification information of a cell belonging to the base station, according to certain exemplary embodiments includes: count means that counts the number of times handover ends in failure; and changing means that changes the identification information of the cell belonging to the base station when the count made by said count means is equal to or larger than a predetermined value. 
         [0015]    A wireless communication system according to certain exemplary embodiments includes a plurality of base stations and a controller that covers said plurality of base stations, each of said plurality of base stations includes: storage means that stores neighboring cell information, which indicates identification information of a cell of the base station and identification information of at least one adjacent cell adjacent to the cell of the base station; transmitting means that transmits the neighboring cell information in the storage means to the controller; and changing means that replaces the identification information of the cell of the base station in the storage means with cell identification information that is different from any identification information indicated in neighboring cell information stored in another of said plurality of base stations when the neighboring cell information stored in the other of said plurality of base stations is received from the controller and if the identification information of the cell of the base station is designated two or more times in the received neighboring cell information, wherein when said controller receives the neighboring cell information, said controller transmits the neighboring cell information to one of said plurality of base stations different from the base station that has transmitted the neighboring cell information. 
         [0016]    An identification information change method according to certain exemplary embodiments carried out by a base station that communicates with a controller that manages a plurality of base stations includes: storing in storage means neighboring cell information, which indicates identification information of a cell of the base station and identification information of at least one adjacent cell adjacent to the cell of the base station; transmitting the neighboring cell information in the storage means to the controller; and replacing the identification information of the cell of the base station in the storage means with cell identification information that is different from any identification information indicated in neighboring cell information stored in another of the plurality of base stations when the neighboring cell information stored in the other of the plurality of base stations is received from the controller and if the identification information of the cell of the base station is designated two or more times in the received neighboring cell information. 
         [0017]    An identification information change method according to certain exemplary embodiments carried out by a base station that executes handover with respect to a mobile station by using identification information of a cell belonging to the base station includes: counting the number of times that handover ends in failure; and changing the identification information of the cell belonging to the base station when the count is equal to or larger than a predetermined value. 
         [0018]    A program according to certain exemplary embodiments for operating a computer, which communicates with a controller that manages a plurality of base stations, makes the computer execute: a storage process of storing in storage means neighboring cell information, which indicates identification information of a cell of a base station which includes the computer and identification information of at least one adjacent cell adjacent to the cell of the base station; a transmission process of transmitting the neighboring cell information in the storage means to the controller; and a change process of replacing the identification information of the cell of the base station in the storage means with cell identification information that is different from any identification information indicated in neighboring cell information stored in another of the plurality of base stations when the neighboring cell information stored in the other of the plurality of base stations is received from the controller and if the identification information of the cell of the base station is designated two or more times in the received neighboring cell information. 
         [0019]    A program according to certain exemplary embodiments for operating a computer, which executes handover with respect to a mobile station by using identification information of a cell that belongs to a base station which includes the computer, makes the computer execute: a count process of counting the number of times that handover ends in failure; and a change process of changing the identification information of the cell that belongs to the base station when the count is equal to or larger than a predetermined value. 
         [0020]    Exemplary embodiments ensure that a PCI conflict can be automatically resolved even in a situation where the X2 link is not established. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a diagram showing wireless communication system  100  in a first exemplary embodiment; 
           [0022]      FIG. 2  is a diagram showing an example of base stations  201 ,  202 , and  203 ; 
           [0023]      FIG. 3  is a diagram showing an example of neighboring cell information “cell  201   a”;    
           [0024]      FIG. 4  is a diagram showing an example of neighboring cell information “cell  202   a ” in storage section  202   b;    
           [0025]      FIG. 5  is a diagram showing an example of neighboring cell information “cell  203   a ” in storage section  203   b;    
           [0026]      FIG. 6  is a sequence diagram for explaining the operation of wireless communication system  100 ; 
           [0027]      FIG. 7  is a diagram showing wireless communication system  100 A in a second exemplary embodiment; 
           [0028]      FIG. 8  is a diagram showing an example of base stations  701 ,  702 , and  703 ; 
           [0029]      FIG. 9  is a sequence diagram for explaining the operation of wireless communication system  100 A; and 
           [0030]      FIG. 10  is a flowchart for explaining the operation of control section  703   d  in base station  703 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Exemplary embodiments will be described with reference to the drawings. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
       First Exemplary Embodiment 
       [0032]      FIG. 1  is a diagram showing a wireless communication system  100  according to a first exemplary embodiment. 
         [0033]    Referring to  FIG. 1 , wireless communication system  100  is, for example, a wireless communication system in accordance with the LTE standard. Wireless communication system  100  includes base stations  201 ,  202 , and  203 , mobile management entity (MME or core network node)  301 . The number of base stations is not limited to three. Any number of base stations that is no smaller than two may exist. 
         [0034]    Each of base stations  201 ,  202 , and  203  has one or more cells. 
         [0035]    For ease of description of the present exemplary embodiment, it is assumed that base station  201  has cell  201   a;  base station  202 , cell  202   a;  and base station  203 , cell  203   a.    
         [0036]    Each of cells  201   a,    202   a,  and  203   a  is assigned PCI (identification information). In the present exemplary embodiment, PCI#1 is assigned as PCI to cell  201   a;  PCI#3, to cell  202   a;  and PCI#3, to cell  203   a.    
         [0037]    Each of cells  201   a,    202   a,  and  203   a  is also assigned EUTRAN Cell Global Identity (ECGI) (identification information). In the present exemplary embodiment, ECGI#100 is assigned as ECGI to cell  201   a;  ECGI#101, to cell  202   a;  and ECGI#102, to cell  203   a.    
         [0038]    Each of base stations  201 ,  202 , and  203  performs wireless communication with a mobile station that is located in the cell belonging to itself (for example, base station  201  performs wireless communication with mobile stations  401  and  402 ). 
         [0039]      FIG. 2  is a diagram showing an example of base stations  201 ,  202 , and  203 . 
         [0040]    Referring to  FIG. 2 , base station  201  includes storage section  201   b  and control section  201   c.  Control section  201   c  includes transmitting section  201   c   1  and changing section  201   c   2 . Base station  202  includes storage section  202   b  and control section  202   c.  Control section  202   c  includes transmitting section  202   c   1  and changing section  202   c   2 . Base station  203  includes storage section  203   b  and control section  203   c.  Control section  203   c  includes transmitting section  203   c   1  and changing section  203   c   2 . 
         [0041]    Base station  201  will first be described. 
         [0042]    Storage section  201   b  is an example of storage means. Storage section  201   b  stores neighboring cell information for handover with respect to cell  201   a.  Neighboring cell information for handover with respect to cell  201   a  is referred to as neighboring cell information “cell  201   a ” below. 
         [0043]      FIG. 3  is a diagram showing an example of neighboring cell information “cell  201   a”.    
         [0044]    Referring to  FIG. 3 , neighboring cell information “cell  201   a ” has a combination of “PCI#1” and “ECGI#100” for identification of cell  201   a,  a combination of “PCI#3” and 
         [0045]    “ECGI#102” for identification of cell  203   a  adjacent to cell  201   a,  and a combination of “PCI#3” and “ECGI#101” for identification of cell  202   a  adjacent to cell  201   a.    
         [0046]    In the present exemplary embodiment, information in the adjacent cell information in an upper position on the neighboring cell information (the combination of “PCI#3” and “ECGI#102” for identification of adjacent cell  203   a  in  FIG. 3 ) has a higher priority. 
         [0047]    Control section  201   c  communicates with the mobile station located in cell  201   a  and also communicates with MME  301  through an S1 interface. No X2 link is established between control section  201   a  (base station  201 ) and base station  202  and between control section  201   a  (base station  201 ) and base station  203 . 
         [0048]    Control section  201   c  controls the operation of base station  201 . 
         [0049]    Transmitting section  201   c   1  is an example of transmitting means. Transmitting section  201   c   1  transmits neighboring cell information “cell  201   a ” in storage section  201   b  to MME  301 . 
         [0050]    For example, transmitting section  201   c   1  receives a radio resource control (RRC):MEASUREMENT REPORT, which is a handover (HO) trigger message, from mobile station  401 . 
         [0051]    After receiving RRC:MEASUREMENT REPORT, transmitting section  201   c   1  identifies the base station as an HO destination based on the PCI contained in RRC:MEASUREMENT REPORT and neighboring cell information “ 201   a ” in storage section  201   b.  A base station identified as an HO destination is referred to as “target base station” below. 
         [0052]    After identifying the target base station, transmitting section  201   c   1  prepares an S1 application protocol (S1AP):HANDOVER REQUIRED including the information for identification of the target base station. S1AP:HANDOVER REQUIRED is an example of a predetermined message for urging MME  301  to transmit S1AP:HANDOVER REQUEST to the other base station. S1AP:HANDOVER REQUEST is an example of a particular message. 
         [0053]    After preparing S1AP: HANDOVER REQUIRED, transmitting section  201   c   1  adds neighbor information (NI), which is neighboring cell information “ 201   a ”, to S1AP:HANDOVER REQUIRED. 
         [0054]    Transmitting section  201   c   1  transmits to MME  301  S1AP:HANDOVER REQUIRED to which NI has been added. S1AP:HANDOVER REQUIRED having NI added to it is an example of first information. 
         [0055]    Changing section  201   c   2  is an example of changing means. Changing section  201   c   2  receives from MME  301  neighboring cell information stored in the other base stations (the base stations other than base station  201 ). If PCI#1 for the cell of base station  201  is designated two or more times in the received neighboring cell information, changing section  201   c   2  replaces PCI#1 for the cell of base station  201  in storage section  201   b  with a PCI (cell identification information) that is different from any of the items of identification information designated in the received neighboring cell information. 
         [0056]    For example, when changing section  201   c   2  receives from MME  301  S1AP:HANDOVER REQUEST including NI that the other base stations have, it determines whether or not there is a conflict between PCI#1 that is assigned to “cell  201   a ” by referring to the NI that the other base stations have and neighboring cell information “cell  201   a ” in storage section  201   b.  S1AP: HANDOVER REQUEST including the NI that the other base stations have received is an example of second information. 
         [0057]    In a case where a conflict of PCI#1 that is assigned to the cell (cell  201   a ) exists, changing section  201   c   2  replaces PCI#1 that is assigned to the cell (cell  201   a ) with a value not described in the NI that the other base stations have. 
         [0058]    Base stations  202  and  203  will next be described. 
         [0059]    Each of storage section  202   b  in base station  202  and storage section  203   b  in base station  203  has the same function as that of storage section  201   b  in base station  201 . The description of storage section  202   b  or  203   b  may be carried out by replacing “ 201 ” in the description of storage section  201   b  with “ 202 ” or “ 203 ”. 
         [0060]    Each of neighboring cell information “cell  202   a ” in storage section  202   b  and neighboring cell information “cell  203   a ” in storage section  203   b  differs from neighboring cell information “cell  201   a ” in storage section  201   b.    
         [0061]      FIG. 4  is a diagram showing an example of neighboring cell information “cell  202   a ” in storage section  202   b.    
         [0062]    Referring to  FIG. 4 , neighboring cell information “cell  202   a ” has a combination of “PCI#3” and “ECGI#101” for identification of cell  202   a  and a combination of “PCI#1” and 
         [0063]    “ECGI#100” for identification of adjacent cell  201   a.    
         [0064]      FIG. 5  is a diagram showing an example of neighboring cell information “cell  203   a ” in storage section  203   b.    
         [0065]    Referring to  FIG. 5 , neighboring cell information “cell  203   a ” has a combination of “PCI#3” and “ECGI#102” for identification of cell  203   a  and a combination of “PCI#1” and “ECGI#100” for identification of adjacent cell  201   a.    
         [0066]    Each of control section  202   c  and control section  203   c  has the same functions as those of control section  201   c  in base station  201 . The description of control section  202   c  or  203   c  may be carried out by replacing “ 201 ” in the description of control section  201   c  with “ 202 ” or “ 203 ”. 
         [0067]    Each of transmitting section  202   c   1  and transmitting section  203   c   1  has the same function as that of transmitting section  201   c   1  in base station  201 . The description of transmitting section  202   c   1  or  203   c   1  may be carried out by replacing “ 201 ” in the description of transmitting section  201   c   1  with “ 202 ” or “ 203 ”. 
         [0068]    Each of changing section  202   c   2  and changing section  203   c   2  has the same function as that of changing section  201   c   2  in base station  201 . The description of changing section  202   c   2  or  203   c   2  may be carried out by replacing “ 201 ” in the description of changing section  201   c   2  with “ 202 ” or “ 203 ”. 
         [0069]    MME  301  is an example of a controller. 
         [0070]    MME  301  covers base stations  201 ,  202 , and  203  and communicates with base stations  201 ,  202 , and  203  through S1 interfaces. 
         [0071]    MME  301  includes processing section (processing means)  301   a.    
         [0072]    When processing section  301   a  receives S1AP:HANDOVER REQUIRED to which NI has been added, for example, it transmits S1AP:HANDOVER REQUEST to which the received NI has been added to the target base station designated by the S1AP:HANDOVER REQUIRED. 
         [0073]    Mobile stations  401  and  402  are portable terminals such as portable telephones, smart phones, tablet terminals or portable game machines. Each of mobile stations  401  and  402  communicates with, for example, another mobile station or a server (not shown) that provides a predetermined service through wireless communication system  100 . The number of mobile stations is not limited to 2. The number of mobile stations can be changed as required. 
         [0074]    The operation will next be described. 
         [0075]    The operation of wireless communication system  100  in a situation where mobile station  401  moves out of cell  201   a  into cell  202   a  will be described below. 
         [0076]      FIG. 6  is a sequence diagram for explaining the operation of wireless communication system  100  in a situation where mobile station  401  moves out of cell  201   a  into cell  202   a.    
         [0077]    Mobile station  401  that is present at about the center of cell  201   a  moves toward cell  202   a . When mobile station  401  reaches area  500  (see  FIG. 1 ), which is an overlap between cell  201   a  and cell  202   a,  mobile station  401  receives notice information transmitted from base station  202 . The notice information transmitted from base station  202  includes PCI#3 and ECGI#101 for cell  202   a.    
         [0078]    Mobile station  401  receives the notice information transmitted from base station  202  and detects PCI#3 from the notice information. 
         [0079]    After detecting PCI#3, mobile station  401  transmits RRC:MEASUREMENT REPORT including PCI#3 to base station  201  (step S 601 ). 
         [0080]    In base station  201 , transmitting section  201   c   1  receives RRC:MEASUREMENT REPORT including PCI#3 and reads out PCI#3 from RRC:MEASUREMENT REPORT. 
         [0081]    Subsequently, transmitting section  201   c   1  refers to neighboring cell information “ 201   a ” in storage section  201   b  and identifies the adjacent cell information as having the highest priority (hereinafter referred to as “high-priority cell information”) in the items of adjacent cell information including PCI#3. In the present exemplary embodiment, transmitting section  201   c   1  identifies as high-priority cell information the adjacent cell information indicating cell  203   a  identified through the combination of “PCI#3” and “ECGI#102”. 
         [0082]    Subsequently, transmitting section  201   c   1  identifies, as a target base station, base station  203  to which cell  203   a  that has been identified through the high-priority cell information belongs (step S 602 ). 
         [0083]    Subsequently, transmitting section  201   c   1  prepares S1AP:HANDOVER REQUIRED including the information for identification of the target base station (base station  203 ) and adds NI, which is neighboring cell information “ 201   a ”, to S1AP:HANDOVER REQUIRED. 
         [0084]    Subsequently, transmitting section  201   c   1  transmits S1AP:HANDOVER REQUIRED to which the NI has been added to MME  301  through the S1 interface (step S 603 ). 
         [0085]    In MME 301 , processing section  301   a  receives S1AP:HANDOVER REQUIRED having the NI added to it and transmits S1AP:HANDOVER REQUEST to which the NI added to S1AP:HANDOVER REQUIRED has been added to the target base station (base station  203 ) indicated in S1AP:HANDOVER REQUIRED through the S1 interface (step S 604 ). 
         [0086]    In base station  203 , changing section  203   c   2  receives S1AP:HANDOVER REQUEST to which the NI has been added and determines whether or not there is a conflict of the PCI#3 that is assigned to “cell  203   a ” by referring to the NI (see  FIG. 3 ) and neighboring cell information “cell  203   a ” in storage section  203   b  (see  FIG. 5 ). 
         [0087]    In the present exemplary embodiment, since the received NI signifies that PCI#3 is used for cell  203   a  having ECGI#102 and for cell  202   a  having ECGI#101, changing section  203   c   2  determines that there is a conflict of PCI#3 that is assigned to the cell (cell  203   a ). After recognizing the conflict of PCI#3 that is assigned to the cell (cell  203   a ), changing section  203   c   2  replaces PCI#3 that is assigned to the cell (cell  203   a ) with a value not described in the received NI (e.g., PCI#5) (step S 605 ). 
         [0088]    If there is no conflict of PCI#3 that is assigned to the cell (cell  203   a ), changing section  203   c   2  does not change PCI#3 that is assigned to the cell (cell  203   a ). 
         [0089]    Subsequently, changing section  203   c   2  transmits S1AP:HANDOVER REQUEST ACK to MME  301  (step S 606 ). 
         [0090]    In MME  301 , processing section  301   a  receives S1AP:HANDOVER REQUEST ACK and then transmits S1AP:HANDOVER COMMAND to base station  201  (step S 607 ). 
         [0091]    In base station  201 , transmitting section  201   c   1  receives S1AP:HANDOVER COMMAND and transmits RRC:RRC Connection Reconfiguration to mobile station  401  (step S 608 ). 
         [0092]    Mobile station  401  receives RRC:RRC Connection Reconfiguration and transmits RRC:RRC Connection Reconfiguration Complete in an attempt to achieve synchronization with base station  203 . However, since mobile station  401  is not present in cell  203   a  that belongs to base station  203  but is in area  500 , RRC:RRC Connection Reconfiguration Complete does not reach base station  203 , and the handover ends in failure (step S 609 ). 
         [0093]    Thereafter, when mobile station  402 , for example, moves toward cell  203   a  and reaches area  600  (see  FIG. 1 ), which overlaps between cell  201   a  and cell  203   a,  mobile station  402  receives notice information transmitted from base station  203 . The notice information transmitted from base station  203  includes new PCI#5 and ECGI#102 for cell  203   a.    
         [0094]    Mobile station  402  receives the notice information transmitted from base station  203  and detects PCI#5 from the notice information. 
         [0095]    After detecting PCI#5, mobile station  402  transmits RRC:MEASUREMENT REPORT including PCI#5 to base station  201  (step S 610 ). 
         [0096]    In base station  201 , transmitting section  201   c   1  receives RRC:MEASUREMENT REPORT including PCI#5 and reads out PCI#5 from RRC:MEASUREMENT REPORT. 
         [0097]    Subsequently, transmitting section  201   c   1  refers to neighboring cell information “ 201   a ” in storage section  201   b  and attempts to identify the highest-priority cell information in the items of adjacent cell information including PCI#5. 
         [0098]    However, PCI#5 is not in the adjacent cell information. Therefore, transmitting section  201   c   1  transmits to mobile station  402  a message that transmitting station  201   c   1  is to obtain the ECGI for the cell to which PCI#5 has been assigned (e.g., RRC:RRC Connection Reconfiguration with the PCI value (PCI#5) designated). 
         [0099]    Mobile station  402  receives the message that transmitting station  201   c   1  is to obtain the ECGI for the cell to which PCI#5 has been assigned, detects ECGI#102 from the notice information, and transmits to base station  201  a message that the ECGI value associated with PCI#5 is ECGI#102 (e.g., RRC:RRC Connection Reconfiguration Complete including ECGI#102 associated with PCI#5) (step S 612 ). 
         [0100]    In base station  201 , transmitting section  201   c   1  receives the message that the ECGI value associated with PCI#5 is ECGI#102, and replaces PCI#3, which is associated with ECGI#102 in neighboring cell information “cell  201   a ” in storage section  201   b,  with PCI#5 (step S 613 ). 
         [0101]    Therefore, when transmitting section  201   c   1  in base station  201  thereafter receives RRC:MEASUREMENT REPORT including PCI#3 from mobile station  401  that is in area  500 , transmitting section  201   c   1  can identify base station  202  as a target base station since the conflict of PCI#3 has been resolved. 
         [0102]    The effects of the present exemplary embodiment will next be described. 
         [0103]    In the present exemplary embodiment, storage section  201   b  stores neighboring cell information “cell  201   a ” (see  FIG. 3 ). Transmitting section  201   c   1  transmits neighboring cell information “cell  201   a ” in storage section  201   b  to MME  301 . Processing section  301   a  in MME  301  receives neighboring cell information “cell  201   a ” and transmits neighboring cell information “cell  201   a ” to a base station different from base station  201  of a plurality of base stations that has transmitted the information (e.g., base station  203 ). Changing section  203   c   2  receives from MME  301  neighboring cell information “cell  201   a ” that is stored in base station  201  and, if PCI#3 that is assigned to the cell is designated two or more times in the received neighboring cell information “cell  201   a ”, replaces PCI#3 that is assigned to the cell in storage section  203   b  with PCI that is different from any of the items of identification information designated in the received neighboring cell information “cell  201   a”.    
         [0104]    Therefore, even in a case where the X2 link as a link between the base stations is not established, neighboring cell information is communicated between the base stations by using the links provided between the base stations and the MME to enable detection of a PCI conflict, thus enabling resolving a PCI conflict. 
         [0105]    In the present exemplary embodiment, transmitting section  201   c   1  transmits to MME  301  S1AP:HANDOVER REQUIRED having NI, i.e., neighboring cell information “cell  201   a ”, added to it. Changing section  203   c   2  receives S1AP:HANDOVER REQUEST including NI that the other base stations have and, if PCI#3 that is assigned to the cell (cell  203   a ) is designated two or more times in the received NI, replaces PCI#3 that is assigned to the cell in storage section  203   b  with PCI that is different from any of the PCI items designated in the received NI. 
         [0106]    Thus, transmission of NI with S1AP:HANDOVER REQUIRED or S1AP:HANDOVER REQUEST is enabled. 
         [0107]    In the present exemplary embodiment, S1AP:HANDOVER REQUIRED and S1AP:HANDOVER REQUEST are used as a predetermined message and a particular message. The predetermined message and the particular message can be changed as desired. 
         [0108]    For example, S1AP:HANDOVER REQUEST ACKNOWLEDGE and S1AP:HANDOVER COMMAND, S1AP:3NB STATUS TRANSFER and S1AP:MME STATUS TRANSFER or S1AP:ENB CONFIGURATION TRANSFER and S1AP:MME CONFIGURATION TRANSFER may be used as a predetermined message and a particular message. 
         [0109]    In a case where S1AP:HANDOVER REQUIRED and S1AP:HANDOVER REQUEST are used as a predetermined message and a particular message, NI is inserted into IE/Group Name: “Source to Target Transparent Container” of the messages, for example. 
         [0110]    In a case where S1AP:HANDOVER REQUEST ACKNOWLEDGE and S1AP:HANDOVER REQUEST COMMAND are used as a predetermined message and a particular message, NI is inserted into IE/Group Name: “Target to Source Transparent Container” of the messages, for example. 
         [0111]    In a case where S1AP:3NB STATUS TRANSFER and S1AP:MME STATUS TRANSFER are used as a predetermined message and a particular message, NI is inserted into IE/Group Name: “eNB Status Transfer Transparent Container” of the messages, for example. 
         [0112]    In a case where S1AP:ENB CONFIGURATION TRANSFER and S1AP:MME CONFIGURATION TRANSFER are used as a predetermined message and a particular message, NI is inserted into IE/Group Name: “SON Configuration Transfer”, for example. 
         [0113]    In the present exemplary embodiment, each of transmitting sections  201   c   1 ,  202   c   1 , and  203   c   1  may transmit the neighboring cell information that corresponds to any one of storage sections  201   b,    202   b,  and  203   b  to MME  301  in a case where the same PCI is designated two or more times in the neighboring cell information in storage section  201   b,    202   b,  or  203   b.  In such a case, when no PCI conflict exists, that is, the need to transmit the neighboring cell information is low, transmission of the neighboring cell information can be stopped. 
       Second Exemplary Embodiment 
       [0114]    In the first exemplary embodiment, a base station provides an SiAP message, to which NI has been added, to other base stations through MME  301 . In some cases, however, NI cannot be added to an S1AP message and a PCI conflict cannot be noticed. 
         [0115]    The second exemplary embodiment includes an example of detection of a PCI conflict that is performed even when NI cannot be added to an SLAP message. 
         [0116]      FIG. 7  is a diagram showing wireless communication system  100 A in a second exemplary embodiment. 
         [0117]    Referring to  FIG. 7 , wireless communication system  100 A is, for example, a wireless communication system in accordance with the LTE standard. Wireless communication system  100 A includes base stations  701 ,  702 , and  703 , and MME  800 . The number of base stations is not limited to three. Any number of base stations that is not smaller than two may exist. 
         [0118]    Each of base stations  701 ,  702 , and  703  has one or more cells. 
         [0119]    For ease of description of the present exemplary embodiment, it is assumed that base station  701  has cell  701   a;  base station  702 , cell  702   a;  and base station  703 , cell  703   a.    
         [0120]    Each of cells  701   a,    702   a,  and  703   a  is assigned PCI. In the present exemplary embodiment, PCI#71 is assigned as PCI to cell  701   a;  PCI#72, to cell  702   a;  and PCI#72, to cell  703   a.    
         [0121]    Each of cells  701   a,    702   a,  and  703   a  is also assigned ECGI. In the present exemplary embodiment, ECGI#701 is assigned as ECGI to cell  701   a;  ECGI#702, to cell  702   a;  and ECGI#703, to cell  703   a.    
         [0122]    Each of base stations  701 ,  702 , and  703  performs wireless communication with a mobile station positioned in the cell belonging to itself (for example, base station  701  performs wireless communication with mobile stations  401  and  402 ). 
         [0123]      FIG. 8  is a diagram showing an example of base stations  701 ,  702 , and  703 . Referring to  FIG. 8 , base station  701  includes storage section  701   b,  PCI changing counter  701   c  and control section  701   d.  Base station  702  includes storage section  702   b,  PCI changing counter  702   c  and control section  702   d.  Base station  703  includes storage section  703   b,  PCI changing counter  703   c  and control section  703   d.    
         [0124]    Storage sections  701   b,    702   b,  and  703   b  respectively store neighboring cell information with respect to cell  701   a,    702   a,  and  703   a.    
         [0125]    Each of PCI changing counters  701   c,    702   c,  and  703   c  is an example of count means. Each of PCI changing counters  701   c,    702   c,  and  703   c  counts the number of times that handover ends in failure in the station to which it belongs. 
         [0126]    Each of control sections  701   d,    702   d,  and  703   d  is an example of changing means. 
         [0127]    When receiving S1AP:HANDOVER REQUEST from MME  800 , each of control sections  701   d,    702   d,  and  703   d  transmits S1AP:HANDOVER REQUEST ACK to MME  800 . 
         [0128]    If RRC:RRC Connection Reconfiguration Complete is not received from the mobile station even after a lapse of a predetermined time period from the time at which S1AP:HANDOVER REQUEST ACK is transmitted, each of control section  701   d,    702   d,  and  703   d  determines that the station to which it belongs failed to perform handover. Time information that indicates the predetermined time period is stored in control sections  701   d,    702   d , and  703   d.    
         [0129]    Each of control sections  701   d,    702   d,  and  703   d  adds 1 to the count made by PCI changing counter  701   c,    702   c,  or  703   c  in the case of failure to perform handover in the station to which it belongs. 
         [0130]    Control sections  701   d,    702   d,  and  703   d  determine that a PCI conflict exists when the respective counts made by PCI changing counters  701   c,    702   c,  and  703   c  become equal to or larger than a predetermined value, and change the respective PCIs in storage sections  701   b,    702   b , and  703   c  for the cells that belong to the stations to which the control sections belong. The predetermined value is stored in control section  701   d,    702   d,  and  703   d.    
         [0131]    Each of control sections  701   d,    702   d,  and  703   d  resets PCI changing counter  701   c,    702   c , and  703   c  when a certain time period, after the time at which the count was made by PCI changing counter  701   c,    702   c,  and  703   c,  is exceeded. Time information indicating the certain time period is stored in control sections  701   d,    702   d,  and  703   d.  The certain time period is longer than the predetermined time period. 
         [0132]    The operation will next be described. 
         [0133]    The operation of wireless communication system  100 A in a situation where mobile station  401  moves from cell  701   a  to cell  702   a  will be described below. 
         [0134]      FIG. 9  is a sequence diagram for explaining the operation of wireless communication system  100 A in a situation where mobile station  401  moves from cell  701   a  to cell  702   a.    FIG. 10  is a flowchart for explaining the operation of control section  703   d  in base station  703 . 
         [0135]    Mobile station  401  that is present in cell  701   a  moves toward cell  702   a.  When mobile station  401  reaches area  900  (see  FIG. 7 ), which overlaps between cell  701   a  and cell  702   a , mobile station  401  receives notice information transmitted from base station  702 . The notice information transmitted from base station  702  includes PCI#72 and ECGI#702 for cell  702   a.    
         [0136]    Mobile station  401  receives the notice information transmitted from base station  702  and detects PCI#72 from the notice information. 
         [0137]    After detecting PCI#72, mobile station  401  transmits RRC:MEASUREMENT REPORT including PCI#72 to base station  701  (step S 901 ). 
         [0138]    In base station  701 , control section  701   d  receives RRC:MEASUREMENT REPORT including PCI#72 and reads out PCI#72 from RRC:MEASUREMENT REPORT. 
         [0139]    Subsequently, control section  701   d  refers to neighboring cell information “ 701   a ” in storage section  701   b  and identifies high-priority cell information in the items of adjacent cell information including PCI# 72 . In the present exemplary embodiment, control section  701   d  identifies as high-priority cell information the adjacent cell information that indicates cell  703   a  that was identified through the combination of “PCI#72” and “ECGI#703”. 
         [0140]    Subsequently, control section  701   d  identifies, as a target base station, base station  703  to which cell  703   a  that was identified through the high-priority cell information belongs (step S 902 ). 
         [0141]    Subsequently, control section  701   d  transmits S1AP:HANDOVER REQUIRED including the information for identification of the target base station (base station  703 ) to MME  800  through the S1 interface (step S 903 ). 
         [0142]    MME  800  receives S1AP:HANDOVER REQUIRED and transmits S1AP:HANDOVER REQUEST to the target base station (base station  703 ) indicated in S1AP:HANDOVER REQUIRED through the S1 interface (step S 904 ). 
         [0143]    In base station  703 , control section  703   d  receives S1AP:HANDOVER REQUEST, then transmits S1AP:HANDOVER REQUEST ACK to MME  800  (step S 905 ) and starts, on a mobile station basis, a timer incorporated in control section  703   d.    
         [0144]    MME  800  receives S1AP:HANDOVER REQUEST ACK and transmits S1AP:HANDOVER COMMAND to base station  701  (step S 906 ). 
         [0145]    In base station  701 , control section  701   d  receives S1AP:HANDOVER COMMAND and transmits RRC:RRC Connection Reconfiguration to mobile station  401  (step S 907 ). 
         [0146]    Mobile station  401  receives RRC:RRC Connection Reconfiguration and transmits to base station  703  RRC:RRC Connection Reconfiguration Complete in an attempt to achieve synchronization with base station  703 . However, since mobile station  401  is not in cell  703   a  that belongs to base station  703  but is in area  900 , RRC:RRC Connection Reconfiguration Complete does not reach base station  703 , and the handover ends in failure (step S 908 ). 
         [0147]    In this situation, when the timer corresponding to mobile station  401  indicates the predetermined time period, control section  703   d  determines that the handover has ended in failure (step S 1001 ), and adds 1 to the count made by PCI changing counter  703   c  (step S 1002 ). 
         [0148]    Subsequently, if the count made by PCI changing counter  703   c  is equal to or larger than the predetermined value (step S 1003 ), control section  703   d  determines that a PCI conflict exists, replaces PCI#72 that is assigned to the cell in storage section  703   b  with a different PCI value, and initializes the count made by PCI changing counter  703   c  to 0 (step S 1004 ). 
         [0149]    The effects of the present exemplary embodiment will next be described. 
         [0150]    In the present exemplary embodiment, PCI changing counter  703   c  counts the number of times handover ends in failure. When the count made by PCI changing counter  703   c  exceeds the predetermined value, control section  703   d  changes the PCI that is assigned to the cell that belongs to the station to which control section  703   d  belongs. 
         [0151]    Thus, a PCI conflict can be resolved even in a case where NI cannot be added to an S1AP message. 
         [0152]    In each of the exemplary embodiments, each base station may be implemented by means of a computer. In such a case, a computer reads and executes a program recorded on a recording medium such as a compact disk read only memory (CD-ROM) readable with the computer to realize the functions of each base station. The recording medium is not limited to the CD-ROM. The recording medium can be changed as required. 
         [0153]    In each of the exemplary embodiments described above, the illustrated configuration is only an example. It should be noted that the present inventive concept is not limited to the above exemplary embodiments but modification can be made as needed without deviating from the spirit and scope as defined by the claims.