Abstract:
An interface establishing method in a handover procedure, includes: managing an occurrence status of handover for a plurality of radio node apparatuses; transmitting an establishment instruction from the management apparatus to a first radio node apparatus to establish a first link between the first radio node apparatus and a second radio node apparatus according to the occurrence status of handover, the first radio node apparatus and the second radio node apparatus being accommodated in the management apparatus; and establishing the first link between the first radio node apparatus and the second radio node apparatus based on the establishment instruction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-080905, filed on Mar. 30, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The invention relates to an interface establishing method, a management apparatus, and a radio node apparatus used in radio communication system. 
       BACKGROUND 
       [0003]    A standardization organization 3GPP (3rd Generation Partnership Project) is developing a radio access network technique, which is called UTRAN LTE (Universal Terrestrial Radio Access Network Long Term Evolution), for the next-generation mobile communication system. For the core-network, SAE (System Architecture Evolution) is being developed. 
         [0004]    The LTE is being developed with an expectation of realizing the functions of a conventional radio base station (Node-B) and a radio base station controller (RNC: Radio Network Controller) in a single node, which is called eNB (evolved Node B). 
         [0005]      FIG. 10  is a diagram illustrating a configuration example of the IMT-2000 LTE/SAE system based on the above development. The system includes a UE (User Equipment)  1001 , an LTE  1002 , an SAE  1003 , and an IP service network  1004 . The LTE  1002  is called E-UTRAN, in which eNBs  1002 - 1  are disposed. In the SAE  1003 , an MME (Mobility Management Entity)  1003 - 1 , a Serving Gateway  1003 - 2 , an HSS (Home Subscriber Server)  1003 - 3 , a PDN (Packet Data Network) Gateway  1003 - 4 , and a PCRF (Policy Charging Rule Function)  1003 - 5  are disposed. The IP service network  1004  is a function unit that provides application services (multimedia communication, packet communication), and includes various server groups that provide application services such as IMS (IP Multimedia Service) and PSS (Packet Switching Service). 
         [0006]    For the IMT-2000 LTE/SAE system, the interface between eNBs is called an X2 interface, and the interface between an eNB and an MME/SAE is called an S1-AP interface. The protocol stacks of the X2 interface and the S1-AP interface are illustrated in  FIG. 11 . 
         [0007]    A mobile phone or a PHS (Personal Handyphone System) with mobile communication that adopts the cell system cannot continue communication when the signal from a base station weakens as the terminal moves to the cell boundary or due to other reasons. Therefore, when the signal weakens or before the signal weakens, a procedure called handover (H/O, H. O.) is performed to switch the connection to a base station of another cell with a strong signal. 
         [0008]    For the handover in the IMT2000 LTE/SAE system, there are two types, namely, (1) handover using the X2 interface and (2) handover using the S1-AP interface. 
         [0009]    The handover using the X2 interface is a handover procedure performed when the X2 interface between a handover-source eNB  1002 - 1  (Source eNB) and a handover-destination eNB  1002 - 1  (Target eNB) is established. The MME  1003 - 1  is not involved in the handover control. The operation sequence of the handover using the X2 interface is illustrated in  FIG. 12 . 
         [0010]    The handover using the S1-AP interface is a handover procedure performed when the X2 interface between a Source eNB  1002 - 1  and a Target eNB  1002 - 1  are not established. This procedure is performed in different ways depending on whether the Source eNB  1002 - 1  and the Target eNB  1002 - 1  belong to the same MME  1003 - 1  or to different MMEs  1003 - 1 . The operation sequence of the handover using the S1-AP interface (where the Source eNB  1002 - 1  and the Target eNB  1002 - 1  belong to the same MME  1003 - 1 ) is illustrated in  FIG. 13 . 
         [0011]    Respective procedures in using the handover procedure of an S1-AP message during the handover between eNBs  1002 - 1  that do not go beyond the MME  1003 - 1  are described below. 
         [0012]      FIG. 14  is an operation sequence diagram of the HANDOVER PREPARATION procedure (S1-AP). 
         [0013]    In the handover between eNBs  1002 - 1  that do not go beyond the MME  1003 - 1 , first, the Source eNB  1002 - 1  receives a handover request (RRC: MEASUREMENT REPORT) from the UE  1001 . Then, a check is performed, from Target eNB information in the handover request, as to whether the X2 interface between the Source eNB and the Target eNB has been activated. When it is determined that the X2 interface has not been activated yet as a result of the check, the Source eNB  1002 - 1  sends a handover request (S1-AP: HANDOVER REQUIRED) to the MME  1003 - 1 . ( FIG. 14 , S 1401 ). The response (success or failure) is determined depending on the success (S 1402 ) or failure (S 1403 ) of the HANDOVER RESOURCE ALLOCATION procedure illustrated in  FIG. 15 . 
         [0014]      FIG. 15  is an operation diagram of the HANDOVER RESOURCE ALLOCATION procedure (S1-AP). 
         [0015]    First, the MME  1003 - 1  receives an S1-AP: HANDOVER REQUIRED ( FIG. 14 , S 1401 ). Then, the MME  1003 - 1  checks, from I.E (information of the Target eNB and so on) in the HANDOVER REQUIRED, whether the Target eNB  1002 - 1  is not beyond the MME  1003 - 1  (the Target eNB  1002 - 1  exists under the MME) or the Target eNB  1002 - 1  is beyond the MME  1003 - 1  (the Target eNB  1002 - 1  does not exist under the MME). When it is determined that the Target eNB  1002 - 1  is not beyond the MME  1003 - 1  as a result of the check, the MME  1003 - 1  sends a handover request (d2S1-AP: HANDOVER REQUEST) to the Target eNB  1002 - 1  ( FIG. 15 , S 1501 ). 
         [0016]    Upon receiving the S1-AP: HANDOVER REQUEST, the Target eNB  1002 - 1  performs the resource capture at the Target eNB side for the handover, and the like, and returns the response (success (S 1502 ) or failure (S 1503 )) on the request to the MME  1003 - 1 . Then the MME  1003 - 1  returns, based on the response result on the request, the response ( FIG. 14 , S 1402  or S 1403 ) in the HANDOVER PREPARATION procedure to the Source eNB  1002 - 1 . 
         [0017]      FIG. 16  is an operation sequence diagram of the HANDOVER NOTIFICATION procedure (S1-AP). 
         [0018]    The Target eNB  1002 - 1  gives, to the MME  1003 - 1 , a notification that the UE  1001  has been connected to the cell of the Target eNB  1002 - 1 , and the S1-AP handover has been completed ( FIG. 16 , S 1601 ). 
         [0019]      FIG. 17  is an operation sequence diagram of the X2 setup procedure (X2-IF). 
         [0020]    In this procedure, data exchange at the application level that is required between the eNBs  1002 - 1  for the accurate interoperation is performed. First, an X2 SETUP REQUEST is transmitted from the transmission-source eNB  1002 - 1  to the transmission-destination eNB  1002 - 1  (S 1701 ). Then, a success response X2 SETUP RESPONSE (S 1702 ) or a failure response X2 SETUP FAILURE (S 1703 ) is returned from the transmission-destination eNB  1002 - 1  to the transmission-source eNB  1002 - 1 . 
         [0021]      FIG. 18  is an operation sequence diagram of the S1 setup procedure (S1-AP). 
         [0022]    In this procedure, data exchange at the application level that is required for the eNB  1002 - 1  and the MME  1003 - 1  for realizing the appropriate communication on the S1-AP interface is performed. First, an S1 SETUP REQUEST is transmitted from the transmission-source eNB  1002 - 1  to the transmission-destination MME  1003 - 1  (S 1801 ). Then, a success response S1 SETUP RESPONSE (S 1802 ) or a failure response S1 SETUP FAILURE (S 1803 ) is returned from the transmission-destination MME  1003 - 1  to the transmission-source eNB  1002 - 1 . 
         [0023]    The message formats used in the S1-AP handover and X2 setup procedure (X2-IF) are illustrated in  FIG. 19  through  FIG. 25 .  FIG. 19  is the format of the HANDOVER REQUIRED (S1-AP) message.  FIG. 20  is the format of the HANDOVER REQUEST (S1-AP) message.  FIG. 21  is the format of the HANDOVER COMMAND (S1-AP) message.  FIG. 22  is the format of the S1-AP I.E Criticality Diagnostics message.  FIG. 23  is the format of the X2 SETUP REQUEST (X2-IF) message.  FIG. 24  is the format of the X2 SETUP RESPONSE (X2-IF) message.  FIG. 25  is the format of the S1 SETUP REQUEST (S1-AP) message. 
         [0024]    In the handover between eNBs  1002 - 1 , in order to prevent the data loss, the Source eNB  1002 - 1  may perform Data forwarding of the user data to the Target eNB  1002 - 1 . 
         [0025]      FIG. 26  illustrates the data forwarding with the handover using the X2 interface. 
         [0026]    It is assumed that before the handover, data d 1 , d 2 , d 3 , d 4 , d 5 , d 6  were sent from a Serving Gateway  1003 - 2  to a Source eNB  1002 - 1 . Here, if the handover (H.O.) occurs in a state in which the data d 1 , d 2 , d 3  have reached the UE  1001 , the Source eNB  1002 - 1  performs the Data forwarding, to the Target eNB  1002 - 1  by means of the X2 interface, of the data d 4 , d 5 , d 6  that have not been transmitted to the UE  1001 . During the handover procedure, the path switch from the Source eNB  1002 - 1  to the Target eNB  1002 - 1  is performed, and the subsequent data d 7 , d 8 , d 9  are transmitted from the Serving Gateway  1003 - 2  to the Target eNB  1002 - 1 . 
         [0027]      FIG. 27  illustrates the data forwarding with the handover using the S1 interface. 
         [0028]    It is assumed that before the handover, the data d 1 , d 2 , d 3 , d 4 , d 5 , d 6  were sent from the Serving Gateway  1003 - 2  to the Source eNB  1002 - 1 . Here, if the handover occurs in the state in which the data d 1 , d 2 , d 3  have reached the UE  1001 , the Sour eNB  1002 - 1  performs the Data forwarding, to the Target eNB  1002 - 1  via the Serving Gateway  1003 - 2 , of the data d 4 , d 5 , d 6  that have not been transmitted to the UE  1001 . During the handover procedure, the path switch from the Source eNB  1002 - 1  to the Target eNB  1002 - 1  is performed, and the subsequent data d 7 , d 8 , d 9  are transmitted from the Serving Gateway  1003 - 2  to the Target eNB  1002 - 1 . 
         [0029]    Note that related art may be described, for example, in Japanese Laid-open Patent Publication No. 2008-103865 and Japanese Laid-open Patent Publication No. 2008-227772. 
         [0030]    For the handover between LTEs  1002  illustrated in  FIG. 10 , the 3GPP defines that when it is made via the SAE  1003 , the S1-AP interface is used, and when it is not made via the SAE  1003 , the X2 interface is used. The S1-AP interface corresponds to the inter-station link between the eNB  1002 - 1  and the SAE  1003 , and the X2 interface corresponds to the inter-station link between the eNBs  1002 - 1 . 
         [0031]    In the conventional art, the inter-station link is set up or cancelled with the addition or reduction of an eNB  1002 - 1 , and is set up to all adjacent radio base stations. 
         [0032]    However, since resources that are required for the setting of the inter-station link are limited, if the inter-station link is set up for all eNBs  1002 - 1 , there is a risk of resource shortage. Here, the number of adjacent eNBs  1002 - 1  may increase due to the operation of a Home eNB (home base station) and the like. 
         [0033]    In addition, since the inter-station link is set on SCTP (Stream Control Transmission Protocol), the exchange of a message with an adjacent eNB  1002 - 1  takes place due to the HEATBEAT function of the SCTP, even in a state under which no communication is performed. As a result, the processing load may increase. 
         [0034]    Therefore, in the IMT-2000 LTE/SAE system, it is expected that setting the X2 interface (inter-station link) for all adjacent eNB  1002 - 1  at the time with resume or addition is not practical. However, if many handovers occur while X2 interface is not set between eNBs  1002 - 1 , the handover using the S1-AP interface takes place many times. As a result, the load for SAE may increase and available line band and resources for the S1-AP interface may be reduced, which is expected to cause the degradation of the handover service. 
       SUMMARY 
       [0035]    According to an aspect of the invention, an interface establishing method in a handover procedure to perform a handover using a first link established between radio node apparatuses or a second link established between a radio node apparatus and a management apparatus that manages a plurality of radio node apparatuses in a radio communication system, the method includes: managing an occurrence status of handover for the plurality of radio node apparatuses; transmitting an establishment instruction from the management apparatus to a first radio node apparatus to establish the first link between the first radio node apparatus and a second radio node apparatus according to the occurrence status of handover, the first radio node apparatus and the second radio node apparatus being accommodated in the management apparatus; and establishing the first link between the first radio node apparatus and the second radio node apparatus based on the establishment instruction. 
         [0036]    According to another aspect of the invention, a management apparatus that accommodates a plurality of radio node apparatuses and controls a handover between the plurality of radio node apparatuses in a radio communication system, includes: a processing unit configured to issue an establishment instruction to establish a link for a handover between a first radio node apparatus and a second radio node apparatus according to occurrence status of handover for the plurality of radio node apparatuses, and to transmit the establishment instruction to the first radio node apparatus. 
         [0037]    According to still another aspect of the invention, a radio node apparatus that performs a handover with another radio node apparatus using a first link established between the radio node apparatus and the other radio node apparatus or a second link established between the radio node apparatus and a management apparatus that accommodates the radio node apparatus in a radio communication system, includes: a processing unit configured to establish the first link with the other radio node apparatus based on an establishment instruction received from the management apparatus. 
         [0038]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0039]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0040]      FIG. 1  is a diagram illustrating a configuration of an embodiment of the radio communication system. 
           [0041]      FIG. 2  and  FIG. 3  are sequence diagrams of the establishing operation of the X2 interface according to the embodiment. 
           [0042]      FIG. 4  is a sequence diagram of the cancellation operation of the X2 interface according to the embodiment. 
           [0043]      FIG. 5  is a diagram illustrating a configuration example of an inter-station link status table held by an eNB. 
           [0044]      FIG. 6  is a diagram illustrating a configuration example of an inter-station link status table held by an SAE. 
           [0045]      FIG. 7  is a diagram illustrating a configuration example of a handover occurrence management table held by the SAE. 
           [0046]      FIG. 8  is a diagram illustrating a configuration example of a handover occurrence management table held by the eNB. 
           [0047]      FIG. 9  is a diagram illustrating an example of data structure of the S1-AP: HANDOVER COMMAND according to the embodiment. 
           [0048]      FIG. 10  is a diagram illustrating a configuration example of the IMT-2000 LTE/SAE system. 
           [0049]      FIG. 11  is a diagram illustrating the protocol stacks of the X2 interface and the S1-AP interface. 
           [0050]      FIG. 12  is an operation sequence diagram of the handover using the X2 interface. 
           [0051]      FIG. 13  is an operation sequence diagram of the handover using the S1-AP interface (where the eNBs belong to the same MME). 
           [0052]      FIG. 14  is an operation sequence diagram of the HANDOVER PREPARATION procedure (S1-AP). 
           [0053]      FIG. 15  is an operation sequence diagram of the HANDOVER RESOURCE ALLOCATION procedure (S1-AP). 
           [0054]      FIG. 16  is an operation sequence diagram of the HANDOVER NOTIFICATION procedure (S1-AP). 
           [0055]      FIG. 17  is an operation sequence diagram of the X2 setup procedure (X2-IF). 
           [0056]      FIG. 18  is an operation sequence diagram of the S1 setup procedure (S1-AP). 
           [0057]      FIG. 19  is a diagram illustrating the format of the HANDOVER REQUIRED (S1-AP) message. 
           [0058]      FIG. 20  is a diagram illustrating the format of the HANDOVER REQUEST (S1-AP) message. 
           [0059]      FIG. 21  is a diagram illustrating the format of the HANDOVER COMMAND (S1-AP) message. 
           [0060]      FIG. 22  is a diagram illustrating the format of the S1-AP I.E Criticality Diagnostics message. 
           [0061]      FIG. 23  is a diagram illustrating the format of the X2 SETUP REQUEST message. 
           [0062]      FIG. 24  is a diagram illustrating the format of the X2 SETUP RESPONSE message. 
           [0063]      FIG. 25  is a diagram illustrating the format of the S1 SETUP REQUEST message. 
           [0064]      FIG. 26  is an illustration of the data forwarding in the handover using the X2 interface. 
           [0065]      FIG. 27  is an illustration of the data forwarding in the handover using the S1 interface. 
           [0066]      FIG. 28  is a diagram illustrating the hardware configuration of a computer to realize the radio communication system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0067]    Hereinafter, embodiments are described in detail. 
         [0068]    The radio communication system of the embodiment is realized, for example, on the IMT-2000 LTE/SAE system illustrated in  FIG. 10 . 
         [0069]      FIG. 1  is a diagram illustrating a configuration of an embodiment of the radio communication system. An SAE  100  in  FIG. 1  corresponds to the MME  1003 - 1  in the SAE  1003  in  FIG. 10 . Meanwhile, an eNB  200  in  FIG. 1  corresponds to the eNB  1002 - 1  in the LTE  1002  in  FIG. 10 . The SAE  100  may accommodate and manages a plurality of eNBs  200 . That is, the SAE  100  may operate as an upper layer apparatus for the eNBs  200 . In the radio communication system of the embodiment, an inter-station link may be established between eNBs  200  according to the X2 interface, and an inter-station link may be established between the eNB  200  and the SAE  100  according to the S1-AP interface. 
         [0070]    The SAE  100  has a signal transmission/reception unit  101 , a signal analysis unit  102 , a handover processing unit  103 , a new information management unit  104 , an inter-station link establishing unit  105 , an inter-station link status table  106 , and a handover occurrence management table  107 . 
         [0071]    The eNB  200  has a signal transmission/reception unit  201 , a signal analysis unit  202 , a handover processing unit  203 , a new information management unit  204 , an inter-station link establishing unit  205 , an inter-station link status table  206 , and a handover occurrence management table  207 . 
         [0072]    The establishing operation of the X2 interface in the embodiment having the above configuration is described below. 
         [0073]      FIG. 2  and  FIG. 3  are sequence diagrams of the establishing operation of the X2 interface in the embodiment. Hereinafter, the explanation is made with these operation sequence diagrams. The eNB 0  and eNB 1  illustrated in  FIG. 2  and  FIG. 3  may operate as a source eNB  200  and a target eNB  200 , respectively. 
         [0074]    Upon receiving a signal sent from an UE (not specifically illustrated in  FIG. 1 ), the signal transmission/reception unit  201  of the source eNB  200  transmits a notification of the reception signal to the signal analysis unit  202 . The signal analysis unit  202  analyzes the transmitted reception signal. When a handover request (RRC: MEASUREMENT REPORT) is detected as a result of the signal analysis, the signal analysis unit  202  transmits the handover request (RRC: MEASUREMENT REPORT) to the handover processing unit  203  ( FIG. 2 , S 1 ). 
         [0075]    Based on Target eNB information in the RRC: MEASUREMENT REPORT, the handover processing unit  203  sends, to the inter-link establishing unit  205 , an inquiry to check whether the X2 interface with the Target eNB  200  has been established ( FIG. 2 , S 2 ). The inter-station link establishing unit  205  refers to the inter-station link status table  206  to check whether the X2 interface has been established, and sends the result to the handover processing unit  203 . 
         [0076]      FIG. 5  is a diagram illustrating a configuration example of the inter-station link status table  206  held by the eNB  200 . As illustrated in  FIG. 5 , the inter-station link status table  206  stores, for every connectable eNB  200 , the ID, IP address, port number of the eNB, and the status indicating whether the X2 interface (X2 IF) has been established (established/not established). The inter-station link establishing unit  205  searches, in the inter-station link status table  206  illustrated in  FIG. 5 , the ID corresponding to the Target eNB information sent from the handover processing unit  203 , and checks whether the X2 interface corresponding to the ID has been established. The inter-station link establishing unit  205  sends a notification indicating whether the X2 interface has been established to the handover processing unit  203 . 
         [0077]    Upon receiving a notification that the X2 interface has not been established from the inter-station link establishing unit  205 , the handover processing unit  203  transmits a handover request (S1-AP: HANDOVER REQUIRED) to the SAE  100  via the signal transmission/reception unit  201  ( FIG. 2 , S 3 ). 
         [0078]    Upon receiving a signal transmitted from the signal transmission/reception unit  201  of the Source eNB  200 , the signal transmission/reception unit  101  of the SAE  100  transmits the reception signal to the signal analysis unit  102 . The signal analysis unit  102  analyzes the reception signal. When a handover request (S1-AP: HANDOVER REQUIRED) is detected as a result of the signal analysis, the signal analysis unit  102  sends the handover request (S1-AP: HANDOVER REQUIRED) to the handover processing unit  103 . 
         [0079]    Based on Target eNB information in the S1-AP: HANDOVER REQUIRED, the handover processing unit  103  sends, to the inter-station link establishing unit  105 , an inquiry to check whether or not the Target eNB  200  exists under the SAE  100  (whether or not the S1 interface has been activated) ( FIG. 2 , S 4 ). The inter-station link establishing unit  105  refers to the inter-station link status table  106  to check whether or not the Target eNB  200  exists under the SAE  100 , that is, inter-station link establishing unit  105  detects the presence/absence of the involvement of another SAE  100  by checking the inter-station link status table  106 . 
         [0080]      FIG. 6  is a diagram illustrating a configuration example of the inter-station link status table  106  held by the SAE  100 . As illustrated in  FIG. 6 , the inter-station link status table  106  stores, for every connectable eNB  200 , the ID, IP address, port number of the eNB, and the status indicating whether the S1-AP interface (S1-AP IF) has been established (established/not established). The inter-station link establishing unit  105  searches, in the inter-station link status table  106  illustrated in  FIG. 6 , the ID corresponding to the Target eNB information sent from the handover processing unit  103 , and checks whether the S1-AP interface corresponding to the ID has been established. The inter-station link establishing unit  105  sends a notification indicating whether the S1-AP interface has been established to the handover processing unit  203 . 
         [0081]    The description below is about the control sequence for the case without the involvement of another SAE  100 . For the case with the involvement of another SAE  100 , the conventional handover processing between SAEs  100  is performed. 
         [0082]    Upon receiving a notification from the inter-station link establishing unit  105  that the S1-AP interface has been established for a Target eNB  200 , that is, there is no involvement of another SAE  100 , the handover processing unit  103  performs the following process. 
         [0083]    The handover processing unit  103  accumulates the number of the occurrence of the handover (hereinafter, handover occurrence count HOC) to the Target eNB  200  in the entry corresponding to the Target eNB  200  in the handover occurrence management table  107  ( FIG. 2 , S 5 ).  FIG. 7  illustrates an example of the handover occurrence management table  107  held by the SAE  100 . As illustrated in  FIG. 7 , the handover occurrence management table  107  stores, for every eNB  200  under the SAE  100 , the ID of the eNB  200 , and the number of the S1-AP interfaces (S1-AP IF). That is, the handover occurrence management table  107  manages, for each eNB  200 , the occurrence frequency of the handover using the S1-AP interface with which another SAE  100  is not involved. Now, when a handover to the Target eNB  200  is newly requested, the handover processing unit  103  increments the handover occurrence count HOC by “1” in the entry of the ID corresponding to the Target eNB  200  in the handover occurrence management table  107 . Meanwhile, the handover processing unit  103  decrements the handover occurrence count HOC by “1” in the entry after the completion of the handover process (including the failure of the handover). 
         [0084]    Note that the process of S 5  in  FIG. 2  described above is an example of the handover occurrence status management step or the handover occurrence status management unit. 
         [0085]    Next, the handover processing unit  103  performs the handover procedure by means of the S1-AP interface for the Target eNB  200  ( FIG. 2 , S 7 , S 8 ), and terminates the handover process for the Target eNB  200 . This handover procedure may be realized by the conventional sequence, for example, illustrated in  FIG. 13  and so on. 
         [0086]    In parallel with the above process, the handover processing unit  103  sends, to the new information management unit  104 , a handover request (S1-AP: HANDOVER REQUIRED)from the Source eNB  200  to the Target eNB  200 . 
         [0087]    Upon receiving the transmission instruction, the new information management unit  104  operates asynchronously with the handover process ( FIG. 2 , S 7 , S 8 ) for the Target eNB  200  performed by the handover processing unit  103 . The new information management unit  104  refers to the handover occurrence management table  107  to obtain the handover occurrence count HOC for the Target eNB  200 , and compares the obtained handover occurrence count HOC with a predetermined threshold value ( FIG. 2 , S 8 ). 
         [0088]    When the handover occurrence count HOC is smaller than the threshold value, the new information management unit  104  terminates the current process without doing anything. 
         [0089]    When the handover occurrence count HOC is equal to or larger than the threshold value, the new information management unit  104  determines that the occurrence frequency of the handover between the Source eNB  200  and the Target eNB  200  is increasing, and performs the control to establish the X2 interface between the Source eNB  200  and the Target eNB  200 . 
         [0090]    Specifically, the new information management unit  104  extracts the ID, IP address and port number of the Target eNB  200  from the inter-station link status table  106 , and sends the extracted information to the handover processing unit  103 . When the handover processing unit  103  transmits the S1-AP: HANDOVER COMMAND with respect to the handover process to the Source eNB  200 , the handover processing unit  103  adds the ID, IP address and port number of the Target eNB  200  to the HANDOVER COMMAND message ( FIG. 3 , S 9 ). Then, the handover processing unit  103  transmits the S1-AP: HANDOVER COMMAND to the Source eNB  200  through the signal transmission/reception unit  101  ( FIG. 3 , S 10 ). 
         [0091]      FIG. 9  illustrates an example of data structure of the S1-AP: HANDOVER COMMAND in the embodiment. In the message of the embodiment, the ID, IP address and port number of the Target eNB  200  are added. The ID, IP address and port number of the Target eNB  200  may be added, for example, to the last three rows of the message. 
         [0092]    Note that the processes in S 9  and S 10  in  FIG. 3  described above are examples of the inter-station link establishment instructing step or an inter-station link establishment instructing unit. 
         [0093]    Upon receiving a signal transmitted from the signal transmission/reception unit  101  of the SAE  100 , the signal transmission/reception unit  201  of the Source eNB  200  transmits the reception signal to the analysis unit  202 , The analysis unit  202  analyzes the reception signal. When an S1-AP: HANDOVER COMMAND is detected as a result of the signal analysis, the signal analysis unit  202  sends the S1-AP: HANDOVER COMMAND to the handover processing unit  203 . 
         [0094]    Upon receiving the S1-AP: HANDOVER COMMAND, the handover processing unit  203  instructs the new information management unit  204  to check the presence/absence of the setting of the Target eNB information in the S1-AP: HANDOVER COMMAND message. The new information management unit  204  checks the presence/absence of the setting of the Target eNB information in the S1-AP: HANDOVER COMMAND message. If the information has been set, the new information management unit  204  sends a notification of the presence of the setting to the inter-station link establishing unit  205  ( FIG. 3 , decision in S 11  is YES). 
         [0095]    The inter-station link establishing unit  205  searches in the linter-station link status table  206  illustrated in  FIG. 5 , based on the ID of the Target eNB  200  transmitted from the new information management unit  104 . That is, the inter-station link establishing unit  205  checks whether the X2 interface for the Target eNB  200  has been established. Then, if the link has not been established, the inter-station link establishing unit  205  establishes the inter-station link between the Source eNB  200  and the Target eNB  200  based on the IP address and the port number transmitted from the new information management unit  104  ( FIG. 3 , S 12 ). At this time, if ARP (Address Resolution Protocol) for the Target eNB  200  has not been resolved, the resolution protocol of ARP is performed before the communication between the Source eNB and the Target eNB is started. The setup procedure of the X2 interface may be similar to the conventional example, for example, illustrated in  FIG. 17 . 
         [0096]    Note that the process in S 12  of  FIG. 3  is an example of the inter-station link establishing procedure execution step or an inter-station link establishing procedure execution unit. 
         [0097]    The inter-station link establishing unit  205  updates the status of the X2 interface in the entry having the ID corresponding to the Target eNB  200  in the inter-station link status table  206  in  FIG. 23  to “established” ( FIG. 3 , S 13 ). 
         [0098]    Thus, according to this embodiment, it becomes possible, when the occurrence frequency of the handover from the Source eNB  200  to the Target eNB  200  increases, to automatically establish the X2 interface between the Source eNB  200  and the Target eNB  200  and to directly perform the handover process between the Source eNB  200  and the Target eNB  200  using the X2 interface. 
         [0099]    Next, the cancellation operation of the X2 interface in the embodiment in  FIG. 1  is described below. 
         [0100]      FIG. 4  illustrates a sequence of the cancellation operation of the X2 interface in the embodiment. Hereinafter, the explanation is made in accordance with the sequence diagram. 
         [0101]    Upon receiving a signal transmitted from a UE (not specifically illustrated in  FIG. 1 ), the signal transmission/reception unit  201  of the source eNB  200  transmits the received signal to the signal analysis unit  202 . The signal analysis unit  202  analyzes the reception signal. When a handover request (RRC: MEASUREMENT REPORT) is detected as a result of the signal analysis, the signal analysis unit  202  sends the handover request (RRC: MEASUREMENT REPORT) to the handover processing unit  203  ( FIG. 4 , S 1 ). 
         [0102]    Based on Target eNB information in the RRC: MEASUREMENT REPORT, the handover processing unit  203  sends, to the inter-link establishing unit  205 , an inquiry to check whether the X2 interface with the Target eNB  202  has been established ( FIG. 4 , S 2 ). The inter-station link establishing unit  205  searches, in the inter-station link status table  206  illustrated in  FIG. 5 , the ID corresponding to the Target eNB information transmitted from the handover processing unit  203 , and checks whether the X2 interface for the ID has been established. The inter-station link establishing unit  205  sends a notification indicating whether the X2 interface has been established to the handover processing unit  203 . 
         [0103]    Upon receiving a notification that the X2 interface has not been established from the inter-station link establishing unit  205 , the handover processing unit  203  transmits a handover request (S1-AP: HANDOVER REQUIRED) to the SAE  100  via the signal transmission/reception unit  201  ( FIG. 2 , S 3 ). By this, the X2 interface is established between the Source eNB  200  and the Target eNB  200  after the handover process for the Target  2 NB  200 , based on the operation sequences in  FIG. 2  and  FIG. 3 . 
         [0104]    Meanwhile, upon receiving a notification from the inter-station link establishing unit  205  that the X2 interface has been established, the handover processing unit  203  performs the following process. 
         [0105]    The handover processing unit  203  increments the number of the occurrence of the handover for the Target eNB  200  in the entry corresponding to the Target eNB  200  in the handover occurrence management table  207  ( FIG. 4 , S 3 ).  FIG. 8  illustrates an example of the handover occurrence management table  207  held by the eNB  200 . As illustrated in  FIG. 8 , the handover occurrence management table  207  stores, for every eNB  200  that has a possibility of becoming the target, the ID of the eNB  200 , and the number of handover with the X2 interface (X2 IF). That is, the handover occurrence management table  207  manages, for each eNB, the occurrence frequency of the handover using the X2 interface. Now, when a handover to the Target eNB  200  is newly requested, the handover processing unit  203  increments the handover occurrence count HOC by “1” in the entry of the ID corresponding to the Target eNB  200  in the handover occurrence management table  207  illustrated in  FIG. 8 . Meanwhile, the handover processing unit  203  decrements the handover occurrence count HOC by “1” in the entry after the completion of the handover process (including the failure of the handover). 
         [0106]    Next, the handover processing unit  203  transmits a handover request (HANDOVER REQUEST) through the signal transmission/reception unit  201  to the Target eNB  200 , using the X2 interface. Then, the handover processing unit  203  starts the handover procedure using the X2 interface ( FIG. 4 , S 4 ). This procedure may be realized by, for example, the sequence illustrated in  FIG. 12  and the like. 
         [0107]    Next, the handover processing unit  203  refers to the handover occurrence management table  207  illustrated in  FIG. 8 , to obtain the handover occurrence count HOC corresponding to the Target eNB  200 , and compares the handover occurrence count HOC with a predetermined threshold value ( FIG. 4 , S 5 ). 
         [0108]    When the handover occurrence count HOC is equal to or larger than the threshold value, the handover processing unit  203  terminates the current process after the handover process for the Target eNB  200  using the X2 interface. 
         [0109]    Meanwhile, if the handover occurrence count HOC is smaller than the threshold value, the handover processing unit  203  determines that the number of occurrence of the handover between the Source eNB  200  and the Target eNB  200  has decreased. In this case, the handover processing unit  203  performs the control to cancel the X2 interface between the Source eNB  200  and the Target eNB  200 . 
         [0110]    The handover processing unit  203  performs, after the handover process for the Target eNB  200 , the following process. The handover processing unit  203  transmits a SHUTDOWN message based on the SCTP (Stream Control Transmission Protocol) through the signal transmission/reception unit  201  to the Target eNB  200 , using the X2 interface ( FIG. 4 , S 6 ). As a result, a SHUTDOWN ACK message is returned from the handover processing unit  103  in the Target eNB  200  to the handover processing unit  103  in the Source eNB  200  ( FIG. 4 , S 7 ), and the X2 interface between the Source eNB  200  and the Target eNB  200  is cancelled. 
         [0111]    Note that the processes in S 5  through S 7  in  FIG. 4  described above are examples the inter-station link cancellation step or the inter-station link cancellation unit. 
         [0112]    Then the handover processing unit  203  updates the status of the X2 interface of the entry having the ID corresponding to the Target eNB  200  in the inter-station link status table  206  illustrated in  FIG. 5  to “not established” ( FIG. 4 , S 8 ). 
         [0113]    Thus, according to the embodiment, it becomes possible, when the occurrence frequency of the handover from the Source eNB  200  to the Target eNB  200  decreases, to automatically cancel the X2 interface between the Source eNB  200  and the Target eNB  200 , and to release the resource of the X2 interface. 
         [0114]    As described above, according to the embodiment, it becomes possible to establish or cancel an inter-station link between eNBs  200  in accordance with the frequency of the occurrence of the handover between the eNBs  200 . Therefore, the resources required for the setting of the inter-station link is utilized effectively, and the increase of the number of adjacent eNBs  1002 - 1  due to the operation of a Home eNB (home base station) and the like in the future may be handled efficiently. 
         [0115]    In addition, as a result of the assignment of the handover process to the direct communication between the eNBs, the frequency of handover on the S1-AP interface may be reduced, making it possible to avoid decreasing available line band and resources and to prevent the degradation of the handover service. 
         [0116]      FIG. 28  is a diagram illustrating the hardware configuration of a computer to realize the radio communication system in the embodiment in  FIG. 1 . The hardware realizes the SAE  100  or the eNB  200  in  FIG. 1 . 
         [0117]    The computer illustrated in  FIG. 28  has a CPU  2801 , a memory  2802 , an input device  2803 , an output device  2804 , a storage device  2805 , a portable recording medium drive device  2806  to which a portable recording medium  2809  is inserted, and a network connection device  2807 . These units  2801 - 2807  are connected to each other by means of a bus  2808 . The configuration illustrated in  FIG. 28  is an example of the computer to realize the system described above, and such a computer is not limited to this configuration. 
         [0118]    The CPU  2801  performs the control of the computer as a whole. The memory  2802  includes a memory such as a RAM that temporality stores program or data stored in the storage device  2805  (or in the portable recording medium  2809 ). The CPU  2801  performs the overall control by executing the program by reading it onto the memory  2802 . 
         [0119]    The input device  2803  includes, for example, a keyboard, a mouse and the like and interface control devices for them. The input device  2803  detects input operations using the keyboard, mouse and the like by a user and sends a notification of the detection result to the CPU  2801 . 
         [0120]    The output device  2804  includes a display device, a printing device and the like and interface control devices for them. The output device  2804  outputs data to the display device and the printing device in accordance with the control by the CPU  2801 . 
         [0121]    The storage device  2805  is, for example, a hard-disc storage device, which is used for saving various data and programs. 
         [0122]    The portable recording medium drive device  2806  accommodates and accesses the portable recording medium  2809  such as an optical disc, SDRAM, compact flash and the like. 
         [0123]    The network connection device  2807  is a device for connecting the communication line of, for example a LAN (local area network) or a WAN (wide area network). 
         [0124]    The SAE  100  or the eNB  200  according to the embodiment in  FIG. 1  is realized by the execution of a program having the functions required for the SAE  100  or the eNB  200  by the CPU  2801 . The program may be provided by the storage device  2805  or the portable recording medium  2809 , or may be obtained from a network by means of the network connection device  2807 . 
         [0125]    According to the disclosed method or configuration, it becomes possible to automatically establish or cancel an inter-station link between the radio communication nodes in accordance with the occurrence frequency of the handover between the nodes. Therefore, the resources used for the inter-station link is utilized effectively, and the number of radio communication nodes may be increased without lowering the performance of the handover operation. 
         [0126]    In addition, since the handover process is assigned to the direct communication between the radio communication nodes, the frequency of handover using the inter-station link with an upper layer apparatus may be reduced, it is possible to increase available line band and resources, and to prevent the degradation of the handover service. 
         [0127]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions has (have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.