Patent Publication Number: US-2013244659-A1

Title: Radio communication system, base station and relay station

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority of the prior Japanese Patent Application No. 2012-062887, filed on Mar. 19, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a radio communication system. 
     BACKGROUND 
     In the 3rd Generation Partnership Project (3GPP), a study of Long Term Evolution-Advanced (LTE-Advanced) has been proceeded with which realizes higher speed and larger volume communication than LTE that is an evolved standard of the  3 rd generation radio communication system. 
     According to LTE-Advanced, in addition to high speed and large volume communication by improving the efficiency of using frequencies, an improvement of throughput for a cell end user and an easy increase of a coverage area are aimed at. As one method of realizing this object, a study has been carried out for a relay configuration of defining two types, i.e., a donor base station (DeNB: Donor eNB) and a relay node (RN) in addition to a radio base station (eNB: eNodeB). 
     As one advantage of using the relay configuration, being able to achieve area expansion at low cost may be cited. This is because, by being able to eliminate a backhaul link that connects apparatuses included in a radio communication system such as switching stations, base stations and so forth, it is possible to reduce the physical network cost. By using a relay node, it is possible to remarkably reduce the limitations on an installation position and so forth in comparison to a case of using a repeater or the like. Employing the relay configuration is advantageous in particular for a disaster-stricken area or the like. 
     In a case of not using the relay configuration, there is a likelihood that plural user terminals (User Equipment (UE)) carry out handover processes almost simultaneously. For example, in a case where a movable body having plural users therein moves to another cell or the like, handover processes of the plural user terminals are carried out almost simultaneously. It is noted that the “movable body” may be an electric train, a bus or the like. In a case where handover processes of plural user terminals are carried out almost simultaneously, there is a likelihood that the success rate of the handover processes may be degraded due to a lack of the resources of the radio section or the like. 
     A case of using the relay configuration will now be described. A relay node functions as a user terminal for a donor base station, and thus, can carry out a handover process. 
     For example, by mounting a relay node in a movable body, a handover process can be carried out between a donor base station and the relay node in a case where the movable body moves to another cell. As a result of the handover process being thus carried out between the donor base station and the relay node, it is possible to deal with the movement of the movable body to the other cell by the handover process of a single radio circuit. Thus, improvement in the success rate of handover processes can be expected. Further, by using the relay configuration, the respective user terminals do not carry out handover processes. Thus, it is possible to reduce the power consumption in comparison to a case of not using the relay configuration. 
     As described above, by using the relay configuration, it is possible to share the resources of the radio section between the donor base station and the relay node. However, the Evolved Packet Core (EPC) carries out processes for the user terminals that are under control of the relay node at a time of the handover process. Thus, the processing load of the EPC may be increased. 
     Further, when the plural users ride on or exit the movable body, a tracking area update (TAU) procedure may be carried out using a NAS protocol by the user terminals of the plural users almost simultaneously. Since the TAU procedure will be thus carried out by the plural user terminals almost simultaneously, the processing load of the EPC may be increased. 
     PRIOR ART REFERENCE 
     Non-Patent Reference 
     NON-PATENT REFERENCE NO. 1: 3GPP TS36. 300 V10.4.0, “4.7 Support for relaying”, June 2011 
     NON-PATENT REFERENCE NO. 2: 3GPP TS29.281 V10.2.0, “4.2 GTP-U Tunnels”, “7.2.1 Echo Request”, “7.2.1 Echo Response”, June 2011 
     NON-PATENT REFERENCE NO. 3: 3GPP TS36.423 V10.2.0, “9.1.1.1 HANDOVER REQUEST”, June 2011 
     NON-PATENT REFERENCE NO. 4: RFC4960, “1.5.5. Chunk Bundling”, “6.10. Bundling”, September 2007 
     NON-PATENT REFERENCE NO. 5: 3GPP TS36.413 V10.2.0, “9.1.5.8 PATH SWITCH REQUEST”, “9.1.5.9 PATH SWITCH REQUEST ACKNOWLEDGE”, June 2011 
     SUMMARY 
     A radio communication system discussed herein has a first base station and a relay station that carries out radio communication with the first base station. The first base station includes a control part that sets an interface between the first base station and a second base station. The second base station has information to be used for adjusting between a protocol that is set between a host station and the first base station and a protocol that is set between the first base station and a relay station. The first base station further includes a communication module that carries out communication between the first base station and the host station through the interface that is set by the control part. The relay station includes a control part that sets a protocol between the relay station and the second base station through an interface that is set between the relay station and the second base station. 
     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. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts one embodiment of a radio communication system; 
         FIGS. 2A and 2B  depict one example of protocol stacks; 
         FIG. 3  depicts one embodiment of the radio communication system; 
         FIG. 4  depicts a setting example of a GTP-U tunnel; 
         FIG. 5  depicts one embodiment of a donor base station; 
         FIGS. 6A and 6B  depict one embodiment of S1proxy information; 
         FIG. 7  is a functional block diagram depicting one embodiment of the donor base station; 
         FIG. 8  depicts one embodiment of a relay node; 
         FIG. 9  is a functional block diagram depicting one embodiment of a relay node; 
         FIG. 10  is a flowchart depicting one embodiment of operations of the radio communication system; 
         FIG. 11  is a flowchart depicting one embodiment of delay amount measurement; 
         FIG. 12  is a sequence chart depicting one embodiment of the radio communication system; 
         FIG. 13  is a flowchart depicting one embodiment of a bundling process; 
         FIG. 14  is a sequence chart depicting one embodiment of the radio communication system; 
         FIG. 15  depicts one embodiment of a handover request; 
         FIG. 16  depicts one embodiment of a routing request; 
         FIG. 17  depicts one embodiment of a routing response; 
         FIG. 18  depicts one embodiment of a process of setting handover; 
         FIG. 19  is a sequence chart depicting one embodiment of the radio communication system; 
         FIG. 20  depicts one embodiment of a group path switch request; 
         FIG. 21  depicts one embodiment of a group path switch response; and 
         FIG. 22  is a sequence chart depicting one embodiment of the radio communication system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Below, the embodiments will be described using figures. 
     It is noted that in all the figures that will be used to explain the embodiments, the same reference numerals are given to the elements having the same functions, and duplicate descriptions will be omitted. 
     &lt;Radio Communication System&gt; 
       FIG. 1  depicts one embodiment of a radio communication system. 
     The radio communication system includes a base station  400  that carries out radio communication according to LTE. The base station  400  may also be referred to as a E-UTRAN Node-B. The base station  400  covers one or more cells. Although  FIG. 1  depicts the single base station  400 , a plurality of the base stations  400  may be included in the radio communication system. The base station  400  and an MME/S-GW  500   n  are connected through an interface. The interface is an interface between a base station and a MME/S-GW and may be referred to as S1. 
     The radio communication system further includes a donor base station  100 . Although  FIG. 1  depicts the single donor base station  100 , a plurality of the donor base stations  100  may be included in the radio communication system. The donor base station is obtained from adding a function of carrying out radio communication with a relay node  200  to the base station  400 . The donor base station  100  covers one or more cells. The donor base station  100  and the base station  400  are connected through an interface. The interface is an interface between a donor base station and a base station and may be referred to as X2. The donor base station  100  and the MME/S-GW  500   n  are connected through an interface. The interface is an interface between a donor base station and a MME/S-GW and may be referred to as S1. Further, the donor base station  100  and the MME/S-GW  500   n  are connected through an interface. The interface is an interface between a donor base station and a MME/S-GW and may be referred to as S11. 
     The radio communication system further includes a user terminal  300  that carries out radio communication according to LTE. Although  FIG. 1  depicts the single user terminal  300 , a plurality of the user terminals  300  may be included in the radio communication system. The user terminal  300  carries out radio transmission according to Single Carrier-Frequency Division Multiple Access (SC-FDMA) for uplink. The user terminal  300  receives a transmitted radio signal according to Orthogonal Frequency Division Multiple Access (OFDMA) for downlink. The donor base station  100  and the user terminal  300  are connected through an interface, the base station  400  and the user terminal  300  are connected through an interface, and these interfaces may be referred to as Uu. 
     The radio communication system further includes the relay node  200 . The relay node  200  is a node that relays communication between the donor base station  100  and the user terminal  300  at the layer  3  level. Although  FIG. 1  depicts the single relay node  200 , a plurality of the relay nodes  200  may be included in the radio communication system. The relay node  200  carries out a process of demodulation and a process of decoding of a downlink radio signal from the donor base station  100 . Further, the relay node  200  reproduces and/or keeps confidentiality of the user data, carries out a process of separation, combination and/or the like of the user data, and carries out radio transmission of the user data to the user terminal  300  after coding and modulating the user data. The relay node  200  not only transmits a signal that is received from the donor base station  100  through the interface Uu to the terminal  300  but also transmits an uplink signal from the terminal  300  to the donor base station  100  through the interface Uu. 
     The relay node  200  covers a cell. The donor base station  100  and the relay node  200  are connected through interfaces. The interfaces may be referred to as S1, X2 and Un. The relay node  200  and the user terminal  300  are connected through an interface. The interface may be referred to as Uu. 
     The radio communication system includes the Mobility Management Entity/Serving Gateways (MME/S-GW)  500   n  (n denotes an integer greater than 0 (n&gt;0)).  FIG. 1  depicts a case where n=2. The MME/S-GW  500   n  may be realized by switching equipment. The switching equipment may include a node that manages C-Plane and a node that manages U-Plane. The node that manages C-Plane may be referred to as a Mobility Management Entity (MME). The node that manages C-Plane manages and stores a UE context. The UE context includes a user identifier, a mobility state, a security state and/or the like. The node that manages U-plane may be referred to as a Serving Gateway (S-GW). The node that manages U-Plane manages and stores a UE context. The UE context includes an IP bearer service parameter, routing information and/or the like. The MME/S-GW  500   n  is connected with the donor base station  100  and the base station  400  through an IP network. 
     In the embodiment of the radio communication system, the radio interface referred to as Un is added between the donor base station  100  and the relay node  200 . Through the radio interface Un, a signal that is transmitted between the MME/S-GW  500   n  and the donor base station  100  through the interface S1 is transmitted to the user terminal  300  that is under control of the relay node  200 . 
     In the relay configuration, it is preferable to reduce the impact to the EPC and the user terminal to a minimum. In order to reduce the impact to the EPC and the user terminal to a minimum, the relay node  200  and the user terminal  300  that is under control of the relay node  200  are recognized by the MME as a user terminal that is under control of the donor base station  100 . Further, the relay node  200  is recognized by the user terminal  300  as a base station. During communication with the MME, a fact of connecting with the EPC through the donor base station  100  is prevented from being recognized by the user terminal  300 . 
       FIGS. 2A and 2B  depict protocol stacks in the embodiment of the radio communication system. Details of the protocol stacks are described in NON-PATENT REFERENCE No. 1. 
     The base station  400  provides E-UTRA U-Plane and C-Plane. When providing U-plane, the base station  400  carries out processes concerning Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHY. When providing C-plane, the base station  400  carries out a process concerning RRC. The base station  400  carries out processes as corresponding to a Node-B and a RNC of a Universal Terrestrial Radio Access Network (UTRAN). 
     An S1 Application Protocol (S1AP) is set in C-Plane and a GPRS Tunneling Protocol User plane (GTP-U) (referred to as “GTP”) is set in U-Plane between the MME/S-GW  500   n  and the donor base station  100  and between the donor base station  100  and the relay node  200 . Further, a S1proxy function that is mounted in the donor base station  100  adjusts and coordinates between the ME/S-GW  500   n  and the relay node  200 . A radio bearer is mapped between the relay node  200  and the donor base station  100  as the interface Un. The radio bearer is a radio bearer for a case where the relay node  200  is regarded as a user terminal. 
     Further, in a case where the relay node  200  is regarded as a user terminal, the function of S-GW/P-GW (PDN-GW) will be mounted in the donor base station  100  in a pseudo manner. For this purpose, a function of terminating an interface S11 (GTP-C) between the donor base station  100  and the MME will be provided. 
     The protocol stacks of C-Plane are depicted in  FIG. 2A . 
     The S1AP is mounted in the relay node  200 . In the S1AP, functions to be used between the donor base station  100  and the MME are prescribed. The S1AP is terminated between the donor base station  100  and the MME. The S1AP has functions of establishing, changing and releasing a communication link for transmitting user data for a user terminal, a handover control function, a function of arriving at a user terminal that has been waiting and so forth. 
     The S1AP is transmitted between the relay node  200  and the MME using a Stream Control Transmission Protocol (SCTP) and an IP. The S1AP is once terminated at the donor base station  100  to which the relay node  200  is connected. As a result of the S1AP being thus terminated at the donor base station  100 , the MME will establish a communication link with the donor base station  100  for transmitting the S1AP without regard to the number of relay nodes. 
     The protocol stacks of U-Plane are depicted in  FIG. 2B . 
     The GTP-U is mounted in the relay node  200 . The GTP-U transfers user data from the S-GW to the relay node  200 . 
     &lt;Donor Base Station  100 &gt; 
     The relay node  200  will carry out handover from a donor base station that is a handover source (hereinafter, referred to as a “serving donor base station”) to a donor base station that is a handover destination (hereinafter, referred to as a “target donor base station”). When the relay node  200  will thus carry out handover from the serving donor base station to the target donor base station, the serving donor base station stores S1proxy information concerning the S1proxy function. The serving donor base station that stores the S1proxy information will be referred to as an “anchor donor base station”. 
     As a result of the serving donor base station thus storing the S1proxy information at a time of the relay node  200  carrying out handover, it is possible that the relay node carries out handover without giving impact to the side of the user terminal  300  and the EPC. That is, it is possible to reduce the process of the side of the user terminal  300  and the EPC as a result of the serving donor base station thus storing the S1proxy information in comparison to moving the S1proxy information from the serving donor base station to the target donor base station. 
       FIG. 3  depicts one embodiment of a radio communication system. 
     In the example depicted in  FIG. 3 , the radio communication system has plural donor base stations  100   m  (m denotes an integer greater than 0 (m&gt;0).  FIG. 3  depicts a case of, as one example, m=4. 
     The relay node  200  will carry out handover from the donor base station  100   1  to the donor base station  100   2 . As a result of relay node  200  thus carrying out handover to the donor base station  100   2 , the relay node  200  will carry out radio communication with the donor base station  100   2 . When radio communication is thus carried out between the relay node  200  and the donor base station  100   2 , the donor base station  100   2  carries out communication with the EPC  600  through the donor base station  100   1 . As a result of the S1proxy information being stored by the donor base station  100   1 , this communication mode is made possible. Between the donor base station  100   2  and the donor base station  100   1 , a path (GTP-U tunnel) is established for allowing the corresponding signal to pass therebetween. 
     The relay node  200  will carry out handover from the donor base station  100   2  to the donor base station  100   3 . As a result of relay node  200  thus carrying out handover to the donor base station  100   3 , the relay node  200  will carry out radio communication with the donor base station  100   3 . When radio communication is thus carried out between the relay node  200  and the donor base station  100   3 , the donor base station  100   3  carries out communication with the EPC  600  through the donor base station  100   1 . As a result of the S1proxy information being stored by the donor base station  100   1 , this communication mode is made possible. Between the donor base station  100   3  and the donor base station  100   1 , a path (GTP-U tunnel) is established for allowing the corresponding signal to pass therebetween. 
     A case of handover of the relay node  200  from the donor base station  100   3  to the donor base station  100   4  will now be considered. In a case where, also the same as the above, the donor base station  100   1  stores the S1proxy information, there will be a likelihood that the processing delay is increased because the distance between the donor base station  100   1  and the donor base station  100   4  may be large. In particular, there will be a likelihood that the processing delay of U-Plane may become problematic. 
     The donor base station  100   4  determines whether a transmission delay between the donor base station  100   4  and the donor base station  100   1  becomes greater than or equal to a predetermined threshold. In a case where an X2AP connection has been established between the donor base station  100   4  and the donor base station  100   1 , one measured as the traffic information may be used as the transmission delay. In a case where an X2AP connection has not been established between the donor base station  100   4  and the donor base station  100   1 , it is possible to determine that the transmission delay is greater than or equal to the predetermined threshold. 
     In a case of having determined that the transmission delay between the donor base station  100   4  and the donor base station  100   1  will be greater than or equal to the predetermined threshold, the donor base station  100   4  carries out a process of moving the S1proxy information from the donor base station  100   1  to the donor base station  100   4 . Thus, when radio communication will be carried out between the relay node  200  and the donor base station  100   4 , the donor base station  100   4  obtains the S1proxy information from the donor base station  100   1 . When the donor base station  100   4  will obtain the S1proxy information from the donor base station  100   1 , the donor base station  100   4  may obtain the S1proxy information through the interface X2. Further, when the donor base station  100   4  will obtain the S1proxy information from the donor base station  100   1 , it is also possible that the donor base station  100   4  obtains the S1proxy information through the interface S1. 
     Further, in a case of having determined that the transmission delay between the donor base station  100   4  and the donor base station  100   1  will be less than the predetermined threshold, the donor base station  100   4  does not carry out the process of moving the S1proxy information from the donor base station  100   1  to the donor base station  100   4 . Thus, when radio communication will be carried out between the relay node  200  and the donor base station  100   4 , the donor base station  100   4  carries out communication with the EPC  600  through the donor base station  100   1 . 
     Further, in a case where the process of moving the S1proxy information is thus not carried out and the S1proxy information is stored in the anchor donor base station when the relay node  200  will carry out handover, a GTP-U tunnel may be set between the target donor base station and the anchor donor base station. Details of the GT-U tunnel are described in NON-PATENT REFERENCE NO. 2. 
     The GTP-U tunnel thus set between the target donor base station and the anchor donor base station will be for the exclusive use of transferring C-Plane and U-Plane between the user terminal  300  and the MME. As a result of the GTP-U tunnel being thus set, the target donor base station can transmit data that is received by a radio frame of the interface Uu to the anchor donor base station through the GTP-U tunnel without regard to the type of the user terminal that is the transmission source. The anchor donor base station transmits the received payload data of GTP-U to the EPC  600  after carrying out an information conversion process based on the S1proxy information. 
     Further, the anchor donor base station transmits a signal to be transmitted to the relay node  200  to the target donor base station through the GTP-U tunnel. The target donor base station can transmit the data thus received from the GTP-U tunnel using a radio frame of the interface Uu by the same method as that used before the handover. 
       FIG. 4  depicts an example of setting the GTP-U tunnel. 
     The upper side of  FIG. 4  depicts a state of a normal relay configuration before the GTP tunnel is set. That is, the state before the relay node  200  carries out handover is depicted. In the state before the relay node  200  carries out handover, S1AP connection and a GTP tunnel are set between the relay node  200  and the serving donor base station  100   1 . Further, a radio bearer of an interface Un is set between the relay node  200  and the serving donor base station  100   1 . Further, in the state before the relay node  200  carries out handover, a S1AP connection and a GTP tunnel are set between the serving donor base station  100   1  and the EPC. 
     The lower side of  FIG. 4  depicts a state of the GTP-U tunnel being set when the relay node carries out handover without moving the S1proxy information. That is, the state is depicted of handover of the relay node  200  having been carried out from the serving donor base station  100   1  to the target donor base station  100   2  without moving the S1proxy information. In this state, a GTP tunnel is set between the serving donor base station  100   1  and the target donor base station  100   2 . Further, in this state, a S1AP connection and a GTP tunnel are set between the relay node  200  and the target donor base station  100   2 . Further, a radio bearer of an interface Un is set between the relay node  200  and the target donor base station  100   2 . Further, the S1AP connection and the GTP tunnel that have been set between the serving donor base station  100   1  and the EPC  600  are maintained. 
     Between the serving donor base station  100   1  and the target donor base station  100   2 , information that is transmitted by communication between the relay node  200  and the target donor base station  100   2  and information that is transmitted by communication between the serving donor base station  100   1  and the EPC  600  are set in GTP payload parts of the GTP tunnel. 
     &lt;Donor Base Station  100 &gt; 
       FIG. 5  depicts one embodiment of the donor base station  100 . 
       FIG. 5  mainly depicts a hardware configuration. 
     The donor base station  100  includes an optical module  102 , a RF module  104 , a CPU  106  and a DSP  108 . The optical module  102 , RF module  104 , CPU  106  and DSP  108  are mutually connected by a bus  150 . 
     The optical module  102  converts an electric signal into an optical signal and outputs the optical signal to the MME/S-GW  500   n  according to control of the CPU  106 . Further, the optical module  102  converts an optical signal that is input from the MME/S-GW  500   n  into an electric signal and inputs the electric signal to the CPU  106  according to control of the CPU  106 . The optical module  102  connects this donor base station  100  and the MME/S-GW  500   n . 
     The RF module  104  converts a baseband signal from the DSP  108  into a radio signal and transmits the radio signal to the relay node  200  or the user terminal  100  according to control of the CPU  106 . Further, the RF module  104  converts a radio signal from the relay mode  200  or the user terminal  300  and converts the radio signal into a baseband signal and inputs the baseband signal to the CPU  106  according to control of the CPU  106 . The RF module  104  connects this donor base station  100  and the relay node  200  or the user terminal  300 . 
     The CPU  106  stores the S1proxy information. The CPU  106  manages the S1proxy information separately for the respective user terminals that are under control of the relay node(s) that is under control of this donor base station  100 . Further, the CPU  106  converts node identification information and UE identification information based on the S1proxy information. 
       FIGS. 6A and 6B  depict one embodiment of the S1proxy information. 
     The S1proxy information of the embodiment includes a S1-AP message conversion table and a GTP message conversion table. That is, the S1proxy information is used to adjust between a protocol(s) that is(are) set between the EPC and the donor base station and a protocol(s) that is(are) set between the donor base station and the relay node. 
       FIG. 6A  depicts one embodiment of the S1-AP message conversion table. The S1-AP message conversion table includes a table (hereinafter referred to as a “first S1-AP message conversion table”) in which a list of node information (IP addresses) and UE information (UE context IDs) is described to be used for communication of S1-MME between the relay node  200  and the donor base station  100 . The donor base station  100  and the relay node  200  have the first S1-AP message conversion tables, respectively. 
     According to the first S1-AP message conversion table, the IP address of the RN#1 is 192.168.2.10. The IP address of the DeNB is 192.168.2.1. 
     The UE#1 is under control of the RN#1. The radio interface Un between the DeNB and the RN#1 to be used for allowing the S1-AP signal for the UE#1 to pass is RB#1. The UE context ID on the side of the RN# 1  is 1. The UE context ID on the side of the DeNB is 5001. 
     The UE#2 is not under control of an RN. That is, the UE#2 is a UE that is directly managed by the DeNB without using an RN. Thus, the field concerning the UE#2 in the first S1-AP message conversion table is blank. 
     Further, the S1-AP message conversion table includes a table (hereinafter referred to as a “second S1-AP message conversion table”) in which a list of node information (IP addresses) and UE information (UE context IDs) is described to be used for communication of S1-MME between the donor base station  100  and the MME. The donor base station  100  and the MME have the second S1-AP message conversion tables, respectively. 
     According to the second S1-AP message conversion table, the IP address of the DeNB is 192.168.1.10. The IP address of the MME is 192.168.1.20. 
     Further, concerning the UE#1, the UE context ID on the side of the DeNB is 1. The UE context ID on the side of the MME is 5001. 
     In a case where the CPU  106  has received the S1-AP message concerning the UE#1 received from the RN#1 (i.e., the CPU  106  has received a packet in which the source IP address in the IP header is 192.168.2.10, the target IP address in the IP header is 192.168.2.1, the UE context ID of counterpart node management in the S1-AP message is  1  and the UE context ID of own node management in the S1-AP message is 5001), the CPU  106  uses the S1proxy information, converts the various sorts of information (i.e., the CPU  106  converts the source IP address in the IP header into 192.168.1.10, converts the target address in the IP header into 192.168.1.20, converts the UE context ID of counterpart node management in the S1-AP message into  1  and converts the UE context ID of own node management in the S1-AP message into 5001), and transmits the converted information to the side of the MME. Also in a case of having received the S1-AP message concerning the UE#1 from the MME, the CPU  106  converts the information in the same or a similar method and transmits the converted information to the RN#1 (RB#1 of the interface Uu). 
       FIG. 6B  depicts one embodiment of the GTP message conversion table. The GTP message conversion table includes a table (hereinafter referred to as a “first GTP message conversion table”) in which a list of node information (IP addresses) and UE information (TE IDs) is described to be used for communication of S1-u between the relay node  200  and the donor base station  100 . The donor base station  100  and the relay node  200  have the first GTP message conversion tables, respectively. 
     According to the first GTP message conversion table, the IP address of the RN#1 is 192.168.2.10. The IP address of the DeNB is 192.168.2.1. 
     The radio interface Un between the DeNB and the RN# 1  to be used for allowing the S1-u signal for the UE#1/RB#1 to pass is RB#2. The TEID on the side of the RN#1 is 1. The TEID on the side of the DeNB is 5001. 
     The UE#2 is not under control of an RN. That is, the UE#2 is a UE that is directly managed by the DeNB without using an RN. Thus, the field concerning the UE#2 in the first GTP message conversion table is blank. 
     Further, the GTP message conversion table includes a table (hereinafter referred to as a “second GTP message conversion table”) in which a list of node information (IP addresses) and UE information (TE IDs) is described to be used for communication of S1-u between the donor base station  100  and the S-GW. The donor base station  100  and the S-GW have the second GTP message conversion tables, respectively. 
     According to the second GTP message conversion table, the IP address of the DeNB is 192.168.1.10. The IP address of the S-GW is 192.168.1.20. 
     Concerning the UE#1/RB#1, the TEID on the side of the DeNB is 1. The TEID on the side of the S-GW is 5001. 
     In a case where the CPU  106  has received the S1-u signal received from the RN#1 (i.e., the CPU  106  has received a packet in which the source IP address in the IP header is 192.168.2.10, the target IP address in the IP header is 192.168.2.1 and the TEID in the GTP header is 5001), the CPU  106  uses the S1proxy information, converts the various sorts of information (i.e., the CPU  106  converts the source IP address in the IP header into 192.168.1.10, converts the target address in the IP header into 192.168.1.20 and converts the TEID in the GTP header into 5001) and transmits the converted information to the side of the S-GW. Also in a case of having received the S1-u signal concerning the UE#1/RB#1 from the S-GW, the CPU  106  converts the information in the same or a similar method and transmits the converted information to the RN#1 (RB#2 of the interface Uu). 
     The CPU  106  carries out call processing control. That is, the CPU  106  terminates RRC between this donor base station  100  and the user terminal  300 . The CPU  106  terminates S1AP between this donor base station  100  and the relay node  200 . The CPU  106  terminates S1AP between this donor base station  100  and the MME. The CPU  106  terminates X2AP between this donor base station  100  and another donor base station. The CPU  106  terminates X2AP between this donor base station  100  and the relay node  200 . 
     The CPU  106  carries out a transmission process. The CPU  106  terminates a GTP-U layer between this donor base station  100  and the relay node  200 . The CPU  106  terminates a GTP-U layer between this donor base station  100  and the MME. The CPU  106  terminates SCTP between this donor base station  100  and the relay node  200 . The CPU  106  terminates SCTP between this donor base station  100  and the MME. The CPU  106  terminates MAC between this donor base station  100  and the relay node  200 . The CPU  106  terminates IP between this donor base station  100  and the relay node  200 . The CPU  106  terminates IP between this donor base station  100  and the MME. The CPU  106  terminates UDP between this donor base station  100  and the relay node  200 . The CPU  106  terminates UDP between this donor base station  100  and the MME. 
     The DSP  108  carries out a baseband process. The DSP  108  carries out control to terminate a L1 layer according to control of the CPU  106 . The DSP  108  carries out control to terminate a MAC layer according to control of the CPU  106 . 
     The DSP  108  carries out a baseband process. The DSP  108  terminates a MAC layer according to control of the CPU  106 . The DSP  108  terminates a RLC layer according to control of the CPU  106 . The DSP  108  terminates a PDCP layer according to control of the CPU  106 . 
     The RF module  104  converts a baseband signal from the DSP  108  into a radio signal. 
     &lt;Function of Donor Base Station  100 &gt; 
       FIG. 7  is a functional block diagram depicting one embodiment of the donor base station  100 . 
     The donor base station  100  includes a baseband processing part  1082 , a call processing control part  1062 , a transmission processing part  1064  and a memory  1066 . 
     The function of the baseband processing part  1082  is executed by the DSP  108 . The baseband processing part  1082  inputs data to and outputs data from the RF module  104 . The baseband processing part  1082  inputs information to and outputs information from the CPU  106 . The baseband processing part  1082  monitors traffic. Specifically, the baseband processing part  1082  monitors a state(s) of the cell(s). The baseband processing part  1082  carries out setting and canceling confidentiality. The baseband processing part  1082  carries out MAC multiplexing and demultiplexing. The baseband processing part  1082  carries out synchronous processing. The baseband processing part  1082  carries out paging processing. 
     The function of the call processing control part  1062  is executed by the CPU  106 . The call processing control part  1062  is connected with the baseband processing part  1082 . The call processing control part  1062  carries out call processing. The call processing control part  1062  carries out control for when transferring the S1proxy information. The call processing control part  1062  carries out control connection between this donor base station  100  and the user terminal  300 . Further, the call processing control part  1062  manages connection between this donor base station  100  and the user terminal  300 . 
     The function of the transmission processing part  1064  is executed by the CPU  106 . The transmission processing part  1064  is connected with the baseband processing part  1082  and the call processing control part  1062 . The transmission processing part  1064  carries out resource management. The transmission processing part  1064  analyzes the reception quality of downlink notified of by the user terminal  300 . Specifically, the transmission processing part  1064  determines whether to cause the user terminal  300  to carry out handover. 
     The memory  1066  is mounted in the CPU  106 . The memory  1066  is connected with the call processing control part  1062 . In the memory  1066 , the S1proxy information is stored. 
     &lt;Relay Node  200 &gt; 
       FIG. 8  depicts one embodiment of the relay node  200 . 
       FIG. 8  mainly depicts a hardware configuration. 
     The relay node  200  includes a RF module  204 , a CPU  206  and a DSP  208 . The RF module  204 , CPU  206  and DSP  208  are mutually connected by a bus  250 . 
     The RF module  204  converts a baseband signal from the DSP  208  into a radio signal and transmits the radio signal to the user terminal  300  or the donor base station  100  according to control of the CPU  206 . Further, the RF module  204  converts a radio signal from the user terminal  300  into a baseband signal and inputs the baseband signal to the CPU  206  according to control of the CPU  206 . The RF module  204  connects this relay node  200  and the user terminal  300 . Further, the RF module  204  converts a radio signal from the donor base station  100  into a baseband signal and inputs the baseband signal to the CPU  206  according to control of the CPU  206 . The RF module  204  connects this relay node  200  and the donor base station  100 . 
     The CPU  206  stores the S1proxy information. The CPU  206  manages the S1proxy information separately for the respective user terminals that are under control of this relay node  200 . Further, the CPU  206  carries out NAT conversion based on the S1proxy information. The CPU  206  stores the first S1-AP message conversion table and the first GTP message conversion table depicted in  FIGS. 6A and 6B . 
     The CPU  206  carries out call processing. That is, the CPU  206  terminates RRC between this relay node  200  and the user terminal  300 . The CPU  206  terminates S1AP between this relay node  200  and the donor base station  100 . The CPU  206  terminates X2AP between this relay node  200  and another donor base station. The CPU  206  terminates X2AP between this relay node  200  and the donor base station  100 . 
     The CPU  206  carries out transmission processing. The CPU  206  terminates a GTP-U layer between this relay node  200  and the donor base station  100 . The CPU  206  terminates SCTP between this relay node  200  and the donor base station  100 . The CPU  206  terminates IP between this relay node  200  and the donor base station  100 . The CPU  206  terminates UDP between this relay node  200  and the donor base station  100 . 
     The DSP  208  carries out baseband processing. The DSP  208  carries out control to terminate a L1 layer according to control of the CPU  206 . The DSP  208  carries out control to terminate a MAC layer according to control of the CPU  206 . 
     The DSP  208  carries out baseband processing. The DSP  208  terminates a MAC layer according to control of the CPU  206 . The DSP  208  terminates a RLC layer according to control of the CPU  206 . The DSP  208  terminates a PDCP layer according to control of the CPU  206 . 
     The RF module  104  converts a baseband signal from the DSP  208  into a radio signal. 
     &lt;Function of Relay Node  200 &gt; 
       FIG. 9  is a functional block diagram depicting one embodiment of the relay node  200 . 
     The relay node  200  includes a first baseband processing part  2082 , a second baseband processing part  2084 , a call processing control part  2062 , a transmission processing part  2064  and a memory  2066 . 
     The function of the first baseband processing part  2082  is executed by the DSP  208 . The first baseband processing part  2082  inputs data to and outputs data from the RF module  204 . The first baseband processing part  2082  inputs information to and outputs information from the CPU  206 . The first baseband processing part  2082  has a function as a user terminal. The first baseband processing part  2082  monitors traffic and monitors a state(s) of the cell(s). The first baseband processing part  2082  carries out control for when searching for a cell. The first baseband processing part  2082  sets and cancels confidentiality. The first baseband processing part  2082  carries out MAC multiplexing and demultiplexing. The first baseband processing part  2082  carries out synchronous processing. 
     The function of the second baseband processing part  2084  is executed by the DSP  208 . The second baseband processing part  2084  inputs data to and outputs data from the RF module  204 . The second baseband processing part  2084  inputs information to and outputs information from the CPU  206 . The second baseband processing part  2084  has a function as a base station. The second baseband processing part  2084  manages traffic. Specifically, the second baseband processing part  2084  manages GTP. Further, the second baseband processing part  2084  carries out traffic transfer. Specifically, the second baseband processing part  2084  carries out a process of transferring GTP. The second baseband processing part  2084  monitors traffic and monitors a state(s) of the cell(s). Further, the second baseband processing part  2084  sets and cancels confidentiality. The second baseband processing part  2084  carries out MAC multiplexing and demultiplexing. The second baseband processing part  2084  carries out synchronous processing. The second baseband processing part  2084  carries out paging processing. 
     The function of the call processing control part  2062  is executed by the CPU  206 . The call processing control part  2062  is connected with the first baseband processing part  2082  and the second baseband processing part  2084 . The call processing control part  2062  has a function as a user terminal and a function as a base station. The call processing control part  2062  carries out call processing between this relay node  200  and the donor base station  100  when executing the function as a user terminal. The call processing control part  2062  carries out call processing between this relay node  200  and the user terminal  300  when executing the function as a base station. The call processing control part  2062  manages connection between this relay node  200  and the donor base station  100  when executing the function as a user terminal. The call processing control part  2062  manages connection between this relay node  200  and the user terminal  300  when executing the function as a base station. Further, the call processing control part  2062  controls handover of the user terminal  300  that is connected with this relay node  200  when executing the function as a base station. 
     The function of the transmission processing part  2064  is executed by the CPU  206 . The transmission processing part  2064  is connected with the first baseband processing part  2082 , the second baseband processing part  2084  and the call processing control part  2062 . The transmission processing part  2064  has a function as a user terminal and a function as a base station. The transmission processing part  2064  carries out resource management when executing the function as a user terminal. The transmission processing part  2064  carries out resource management when executing the function as a base station. The transmission processing part  2064  controls handover of the user terminal  300  that is connected with this relay node  200  when executing the function as a base station. 
     The memory  2066  is mounted in the CPU  206 . The memory  2066  is connected with the call processing control part  2062 . In the memory  2066 , the S1proxy information is stored. 
     &lt;Operations of Radio Communication System&gt; 
       FIG. 10  depicts one embodiment of operations of the radio communication system. 
     In an example depicted in  FIG. 10 , a case will be described where a movable body in which the relay node  200  is mounted moves. 
     The donor base station  100  periodically measures a delay amount between this donor base station  100  and another donor base station between which the interface X2 has been established (step S 1002 ). That is, the transmission processing part  1064  of the donor base station  100  periodically measures the delay amount. The delay amount is generated due to the physical distance between the donor base station  100  and another donor base station and network traffic, and varies due to a difference of a target donor base station that is a handover destination. Thus, according to the embodiment, any one of different handover procedures is selected based on the delay amount different depending on a target donor base station that is handover destination. 
     As a user rides in a movable body, the user terminal  300  carries out handover between the donor base station  100  and the relay node  200  (step S 1004 ). 
     As the movable body moves, the relay node  200  moves. As the relay node  200  thus moves, the serving donor base station starts a handover process (step S 1006 ). 
     The target donor base station determines whether the delay amount is greater than or equal to a threshold (step S 1008 ). 
     In a case where it has been determined that the delay amount is less than the threshold (step S 1008  NO), the target donor base station carries out a handover process without taking over the S1proxy information (step S 1010 ). 
     In a case where it has been determined that the delay amount is greater than or equal to the threshold (step S 1008  YES), the target donor base station carries out a handover process with taking over the S1proxy information (step S 1012 ). 
     As the user exits the movable body, the user terminal  300  carries out handover between the relay node  200  and the donor base station  100  (step S 1014 ). 
     In the flowchart depicted in  FIG. 10 , the process of steps S 1006  through S 1012  may be repeated a plural number of times as appropriate until the movable body reaches a destination. 
     &lt;Process of Calculating Delay Amount&gt; 
       FIG. 11  depicts one embodiment of a process of calculating the delay amount.  FIG. 11  depicts details of a process corresponding to step S 1002  of  FIG. 10 . 
     In an example depicted in  FIG. 11 , the donor base station  100   1  calculates the delay amount between the donor base station  100   1  and the donor base station  100   2 . Further, in the example depicted in  FIG. 11 , the delay amount is calculated using an Echo Request and an Echo Response based on a period of time from when the message has been transmitted until the message is received. The Echo Request and the Echo Response may be carried out periodically for the purpose of confirming a connection of GTP. The Echo Request and the Echo Response are described in NON-PATENT REFERENCE NO. 2. 
       FIG. 11  mainly depicts a process executed by the transmission processing part  1064  of the donor base station  100   1 . 
     In the donor base station  100   1 , a GTP-U message transmission period has begun (step S 1102 ). 
     The donor base station  100   1  transmits an “Echo Request” using the GTP-U protocol (step S 1104 ). 
     The donor base station  100   1  receives an “Echo Response” using the GTP-U protocol from the donor base station  100   2  (step S 1106 ). 
     The donor base station  100   1  determines the time of having transmitted the GTP-U message (Echo Request) as “T 2 ” and the time of having received the GTP-U response message (Echo Response) as “T 1 ” (step S 1108 ). 
     The donor base station  100   1  measures the delay amount ΔT (step S 1110 ). Specifically, the donor base station  100   1  calculates ΔT=(T 1 −T 2 )/2, and thus calculates the delay amount ΔT. The donor base station  100   1  stores the delay amount ΔT. 
     The donor base station  100   1  determines whether to have calculated the delay amounts k times (step S 1112 ). 
     In a case of not having determined to have calculated the delay amounts k times (step S 1112  NO), the process is returned to step S 1102 . 
     In a case of having determined to have calculated the delay amounts k time (step S 1112  YES), the donor base station  100   1  averages the delay amounts calculated k times and obtains the average thereof as the delay amount (step S 1114 ). 
     &lt;Operations When Riding in Movable Body&gt; 
       FIG. 12  depicts operations for when the user terminal  300  that is under control of the source donor base station  100   2  carries out handover to the relay node  200 .  FIG. 12  depicts details of a process corresponding to step S 1004  of  FIG. 10 .  FIG. 12  depicts a case where the anchor donor base station  100   1  has the S1proxy information of the user terminal  300 . That is, communication between the relay node  200  and the anchor donor base station  100   1  is carried out through a GTP tunnel that is for the exclusive use and set between the source donor base station  100   2  and the anchor donor base station  100   1 . The parts provided by the GTP tunnel for the exclusive use are expressed by thick lines in  FIG. 12 . There is a case where the source donor base station  100   2  and the anchor donor base station  100   1  are the same base station. In this case, communication between the source donor base station  100   2  and the anchor donor base station  100   1  will be omitted from the following description. 
     Whether a passenger is in a movable body in which the relay node  200  is mounted at a rail station or the like is determined by whether handover from the donor base station to the relay node has occurred first. It is also possible to determine whether a passenger is in a movable body in which the relay node  200  is mounted at a rail station or the like as a result of being notified of by the relay node. For example, the relay node  200  may carry out notification using the S1AP protocol. Further, It is also possible to determine whether a passenger is in a movable body in which the relay node  200  is mounted at a rail station or the like by the donor base station. For example, the donor base station may determine it using Operations, Administration, and Maintenance (OAM) information that includes operation information of the movable body and/or the moving speed thereof. 
     The user terminal  300  transmits a “measurement report (MEASUREMENT REPORT)” to the source donor base station  100   2  using the RRC protocol (step S 1202 ). 
     The source donor base station  100   2  transmits a “handover request (HANDOVER REQUEST)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1204 ). The call processing control part  1062  of the source donor base station  100   2  transmits the handover request to the anchor donor base station  100   1  through the transmission processing part  1064  and the optical module  102 . An adjacent node recognizes as if the cell(s) of the relay node  200  is(are) managed by the anchor donor base station  100   1 . Thus, the source donor base station  100   2  transmits the handover request to the anchor donor base station  100   1 . 
     The anchor donor base station  100   1  transmits a “handover request (HANDOVER REQUEST)” to the relay node  200  using the X2AP protocol (step S 1206 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the handover request to the source donor base station  100   2  through the transmission processing part  1064  and the optical module  102 . The handover request is then received by the transmission processing part  1064  of the source donor base station  100   2  through the optical module  102 , and is transmitted to the relay node  200  from the baseband processing part  1082  through the RF module  104 . 
     The relay node  200  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1208 ). The call processing control part  2062  of the relay node  200  carries out a process of transmitting the handover request acknowledge through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . The handover request acknowledge is then received by the baseband processing part  1082  of the source donor base station  100   2  through the RF module  104 , and is transmitted to the anchor donor base station  100   1  from the transmission processing part  1064 . 
     The anchor donor base station  100   1  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the source donor base station  100   2  using the X2AP protocol (step S 1210 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the handover request acknowledge to the source donor base station  100   2  through the transmission processing part  1064  and the optical module  102 . 
     The source donor base station  100   2  transmits a “SN status transfer (SN STATUS TRANSFER)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1212 ). The call processing control part  1062  of the source donor base station  100   2  transmits the SN status transfer to the anchor donor base station  100   1  through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  transmits a “SN status transfer (SN STATUS TRANSFER)” to the relay node  200  using the X2AP protocol (step S 1214 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the SN status transfer to the source donor base station  100   2  through the transmission processing part  1064  and the optical module  102 . The SN status transfer is then received by the transmission processing part  1064  of the source donor base station  100   2  through the optical module  102 , and is transmitted to the relay node  200  from the baseband processing part  1082  through the RF module  104 . 
     The source donor base station  100   2  transmits a “RRC connection reconfiguration (RRC CONNECTION RECONFIGURATION)” to the user terminal  300  using the RRC protocol (step S 1216 ). The call processing control part  1062  of the source donor base station  100   2  transmits the RRC connection reconfiguration to the user terminal  300  through the baseband processing part  1082  and the RF module  104 . 
     The user terminal  300  transmits a “RRC connection reconfiguration complete (RRC CONNECTION RECONFIGURATION COMPLETE)” to the relay node  200  using the RRC protocol (step S 1218 ). 
     The relay node  200  transmits a “path switch request (PATH SWITCH REQUEST)” to the EPC  600  using the S1AP protocol (step S 1220 ). The call processing control part  2062  of the relay node  20  transmits the path switch request to the source donor base station  100   2  through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . The path switch request is then transmitted to the EPC  600  through the source donor base station  100   2  and the anchor donor base station  100   1 . 
     The EPC  600  transmits a “path switch request acknowledge (PATH SWITCH REQUEST ACKNOWLEDGE)” to the relay node  200  using the S1AP protocol (step S 1222 ). 
     The relay node  200  transmits a “UE context release (UE CONTEXT RELEASE)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1224 ). The call processing control part  2062  of the relay node  200  transmits the UE context release to the source donor base station  100   2  through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . The UE context release is then transmitted to the anchor donor base station  100   1  through the source donor base station  100   2 . 
     The anchor donor base station  100   1  transmits a “UE context release (UE CONTEXT RELEASE)” to the source donor base station  100   2  using the X2AP protocol (step S 1226 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the UE context release to the source donor base station  100   2  through the transmission processing part  1064  and the optical module  102   
     The user terminal  300  transmits a “tracking area update request (TAU Req)” to the EPC  600  using the NAS protocol (step S 1228 ). The tracking area update request transmitted from the user terminal  300  is transmitted to the EPC  600  through the source donor base station  100   2  and the anchor donor base station  100   1 . The anchor donor base station  100   1  carries out a bundling process of the SCTP layer on uplink messages that are transmitted using the NAS protocol. That is, the anchor donor base station  100   1  makes the header information common concerning layers lower than and equal to the SCTP layer. By thus carrying out the process of bundling of the SCTP layer, it is possible to reduce the processing load of the EPC  600 . 
     The EPC  600  transmits a “tracking area update accept (TAU Accept)” to the user terminal  300  using the NAS protocol (step S 1230 ). 
     The user terminal transmits a “tracking area update complete (TAU Complete)” to the EPC  600  using the NAS protocol (step S 1232 ). 
     &lt;Bundling Process of Uplink Messages&gt; 
       FIG. 13  depicts one embodiment of the process of bundling uplink NAS messages carried out by the anchor donor base station  100   1 .  FIG. 13  depicts a process corresponding to step S 1228  of  FIG. 12 . 
     The anchor donor base station  100   1  determines whether the user is in the movable body (step S 1302 ). That is, the call processing control part  1062  of the anchor donor base station  100   1  determines whether the user is in the movable body in which the relay node that is managed by the anchor donor base station  100   1  is mounted. 
     In a case of not having determined that the user is in the movable body (step S 1302  NO), the process is finished. That is, bundling uplink NAS messages is not carried out. 
     In a case of having determined that the user is in the movable body (step S 1302  YES), the anchor donor base station  100   1  starts a timer T 1  (step S 1304 ). That is, the call processing control part  1062  of the anchor donor base station  100   1  starts the timer T 1 , and also gives an instruction to the transmission processing part  1064  to start bundling uplink NAS messages. The timer T 1  is previously set to have a time length on the order of a period of time for which the movable body is standing. 
     The anchor donor base station  100   1  starts a timer T 2  (step S 1306 ). That is, the transmission processing part  1064  of the anchor donor base station  100   1  starts the timer T 2 . The timer T 2  is set to have a period of time on the order for which no problem will occur in the NAS communication. 
     The anchor donor base station  100   1  queues the uplink NAS messages while the timer T 2  is measuring a time (step S 1308 ). That is, the transmission processing part  1064  of the anchor donor base station  100   1  queues the uplink NAS messages. Specifically, the transmission processing part  1064  queues the tracking area update request, the tracking area update complete and so forth from the user terminal  300 . 
     The anchor donor base station  100   1  determines whether at least one of either the expiration of measuring of time of the timer T 2  or the Maximum Transmission Unit (MTU) being exceeded has occurred (step S 1310 ). That is, the transmission processing part  1064  of the anchor donor base station  100   1  determines whether at least one of either the expiration of measuring a time of timer T 2  or the MTU being exceeded has occurred. 
     In a case where neither the expiration of measuring a time of the timer T 2  nor the MTU being exceeded has occurred (step S 1310  NO), the process is returned to step S 1308 . 
     At least one of either the expiration of measuring a time of the timer T 2  or the MTU being exceeded has occurred (step S 1310  YES), the anchor donor base station  100   1  carries out bundling the queued uplink NAS messages and transmits them using the SCTP protocol (step S 1312 ). That is, the transmission processing part  1064  of the anchor donor base station  100   1  carries out bundling the queued uplink NAS messages and transmits them using the SCTP protocol. It is noted that in a case where the MTU has been exceeded, the transmission processing part  1064  of the anchor donor base station  100   1  forcibly causes the measuring of time of the timer T 2  to expire. 
     The anchor donor base station  100   1  determines whether the measuring of time of the timer T 1  expires (step S 1314 ). That is, the transmission processing part  1064  of the anchor donor base station  100   1  determines whether the expiration of measuring of time of the timer T 1  has occurred. 
     In a case of having determined that the expiration of measuring of time of the timer T 1  has occurred (step S 1314  YES), the anchor donor base station  100   1  finishes the process. 
     In a case of not having determined that the expiration of measuring of time of the timer T 1  has occurred (step S 1314  NO), the anchor donor base station  100   1  returns to step S 1306 . 
     &lt;Handover Process without Transferring S1proxy Information&gt; 
       FIG. 14  depicts operations for when the relay node  200  that is under control of the source donor base station  100   2  carries out handover to the target donor base station  100   3 .  FIG. 14  depicts a case where the anchor donor base station  100   1  has the S1proxy information even after the relay node  200  carries out the handover. 
     The user terminal  300  that is under control of the relay node  200  carries out communication of U-Plane with the EPC  600  through the source donor base station  100   2  and the anchor donor base station  100   1  (step S 1402 ). 
     Below, terminations of the S1AP and X2AP protocols are carried out by the call processing control part. However, in a case where the donor base station relays communication between the anchor donor base station and the relay node (using a GTP tunnel for the exclusive use), usage of the call processing control part of the donor base station may be omitted. 
     The relay node  200  transmits a “measurement report (MEASUREMENT REPORT)” using the RRC protocol to the source donor base station  100   2  (step S 1404 ). The call processing control part  2062  of the relay node  200  transmits the measurement report using the RRC protocol through the transmission processing part  2064 , the first baseband processing part  2082  an the RF module  204 . The measurement report is then received by the baseband processing part  1082  of the source donor base station  100   2  through the RF module  104 . 
     The source donor base station  100   2  transmits a “handover request (HANDOVER REQUEST)” to the target donor base station  100   3  using the X2AP protocol (step S 1406 ). The handover request includes the IP address of the anchor donor base station  100   1  and the IP address of the relay node  200 . The call processing control part  1062  of the source donor base station  100   2  transmits the handover request using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . The handover request is then received by the transmission processing part  1064  of the target donor base station  100   3  through the optical module  102 . 
       FIG. 15  depicts one embodiment of information included in the handover request. A handover request is described in NON-PATENT REFERENCE NO. 3. 
     According to the embodiment of the handover request, the IP address of the donor base station that has the S1proxy information (DeNB with S1 proxy information IP address), the IP address of the relay node (RN IP address) and the S1proxy information (S1 proxy Information) are included in a handover request of NON-PATENT REFERENCE NO. 3. 
     Further, the S1proxy information includes a UE information list (UE information list). The UE information list includes one or more UE information elements (UE information IEs). 
     The UE information elements include “Source MME UE S1AP ID”, “Related MME UE S1AP ID on Un”, “Related eNB UE S1AP ID on Un” and “ERAB information list”. “Source MME UE S1AP ID” indicates the ID of S1AP between the MME that has the source donor base station under control thereof and the UE. “Related MME UE S1AP ID on Un” indicates the ID of S1AP of Un between an associated MME and the UE. “Related eNB UE S1AP ID on Un” indicates the ID of S1AP of Un between an associated base station and the UE. “ERAB information list” indicates a list of sets of radio bearer information. 
     “ERAB information list” includes one or more ERAB item information elements (ERAB item IEs). 
     The ERAB item information elements include “ERAB ID”, “Related UL GTP Tunnel Endpoint on Un” and “Related DL GTP Tunnel Endpoint on Un”. 
     The target donor base station  100   3  determines whether to carry out handover with transferring the S1proxy information or handover without transferring the S1proxy information (step S 1408 ). The call processing control part  1062  of the target donor base station  100   3  determines whether to carry out handover with transferring the S1proxy information or handover without transferring the S1proxy information when causing the relay node  200  to carry out the handover. Here, it is determined to carry out handover without transferring the S1proxy information. 
     The target donor base station  100   3  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the source donor base station  100   2  using the X2AP protocol (step S 1410 ). The call processing control part  1062  of the target donor base station  100   3  carries out a process of transmitting the handover request acknowledge using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . The handover request acknowledge is then received by the transmission processing part  1064  of the source donor base station  100   2  through the optical module  102 . 
     The source donor base station  100   2  transmits a “SN status transfer (SN STATUS TRANSFER)” to the target donor base station  100   3  using the X2AP protocol (step S 1412 ). The call processing control part  1062  of the source donor base station  100   2  carries out a process of transmitting the SN status transfer using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . The SN status transfer is then received by the transmission processing part  1064  of the target donor base station  100   3  through the optical module  102 . 
     The source donor base station  100   2  transmits a “RRC connection reconfiguration (RRC CONNECTION RECONFIGURATION)” to the relay node  200  using the RRC protocol (step S 1414 ). The call processing control part  1062  of the source donor base station  100   2  transmits the RRC connection reconfiguration through the baseband processing part  1082  and the RF module  104 . 
     The relay node  200  transmits a “RRC connection reconfiguration complete (RRC CONNECTION RECONFIGURATION COMPLETE)” to the target donor base station  100   3  using the RRC protocol (step S 1416 ). The call processing control part  2062  of the relay node  200  transmits the RRC connection reconfiguration complete through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . 
     The target donor base station  100   3  transmits a “relay node info routing request (RN info Routing Request)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1418 ). The call processing control part  1062  of the target donor base station  100   3  transmits the relay node info routing request using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . That is, the target donor base station  100   3  makes a request for setting a X2AP connection and a GTP tunnel between the target donor base station  100   3  and the anchor donor base station  100   1  for transferring data between the relay node  200  and the anchor donor base station  100   1 . 
       FIG. 16  depicts one embodiment of the relay node info routing request (RN info Routing Request). 
     The “RN info Routing Request” includes a message type (Message Type), the IP address of the relay node to be switched (RN IP address to be switched) and transport network load information of the donor base station (Donor eNB TNL (Transport Network Load) information). 
     The transport network load information of the donor base station includes the transport layer address (Transport layer address) and the GTP-TEID. 
     The anchor donor base station  100   1  transmits a “relay node info routing response (RN info Routing Response)” to the target donor base station  100   3  using the X2AP protocol (step S 1420 ). 
       FIG. 17  depicts one embodiment of the relay node info routing response (RN info Routing Response). 
     The “RN info Routing Response” includes a message type (Message Type), the IP address of the relay node to be switched (RN IP address to be switched) and transport network load information of the donor base station (Donor eNB TNL (Transport Network Load) information). 
     The transport network load information of the donor base station includes the transport layer address (Transport layer address) and the GTP-TEID. 
     A GTP tunnel is set between the anchor donor base station  100   1  and the target donor base station  100   3 . By using the GTP tunnel, the anchor donor base station  100   1  transmits C-Plane and U-Plane to be transmitted to the relay node  200  to the target donor base station  100   3 . The target donor base station  100   3  transmits information included in the payload parts of C-Plane and U-Plane from the anchor donor base station  100   1  to the relay node  200 . The target donor base station  100   3  transmits the information included in the payload parts to the relay node  200  using the DRB(Un) protocol. 
     Further, by using the GTP tunnel, the target donor base station  100   3  transmits the C-Plane and the U-Plane from the relay node  200  to the anchor donor base station  100   1 . The anchor donor base station  100   1  receives information included in the payload parts of the C-Plane and the U-Plane from the relay node  200 . The anchor donor base station  100   1  thus receives the information included in the payload parts of the C-Plane and the U-Plane from the relay node  200  using the DRB(Un) protocol. 
     The relay node  200  transmits a “S1 setup request (S1SETUP REQUEST)” to the anchor donor base station  100   1  using the S1AP protocol (step S 1422 ). The relay node  200  requests to change the anchor donor base station. The call processing control part  2062  of the relay node  200  carries out a process of transmitting the S1 setup request using the S1AP protocol through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . 
     The anchor donor base station  100   1  transmits a “S1 setup response (S1SETUP RESPONSE)” to the relay node  200  using the S1AP protocol (step S 1424 ). Since transfer of the S1proxy information is not carried out here, the anchor donor base station  100   1  transmits the response to the S1 setup request. The call processing control part  1062  of the anchor donor base station  100   1  carries out a process of transmitting the S1 setup response using the S1AP protocol through the transmission processing part  1064 , the baseband processing part  1082  and the RF module  104   
     The user terminal  300  that is under control of the relay node  200  carries out communication of U-Plane with the EPC  600  through the relay node  200 , the target donor base station  100   3  and the anchor donor base station  100   1  (step S 1426 ). 
     The target donor base station  100   3  transmits a “UE context release (UE CONTEXT RELEASE)” to the source donor base station  100   2  using the X2AP protocol (step S 1428 ). The call processing control part  1062  of the target donor base station  100   3  transmits the UE context release using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     &lt;Process of Determining Type of Handover&gt; 
       FIG. 18  depicts one embodiment of a process of determining the type of handover carried out by the target donor base station  100   3 . 
     The target donor base station  100   3  measures the delay amount between the target donor base station  100   3  and an adjacent donor base station between which the interface X2 has been established (step S 1802 ). That is, the transmission processing part  1064  measures the delay amount between the target donor base station  100   3  and the adjacent donor base station that is connected through the optical module  102 . Specifically, the transmission processing part  1064  measures the delay amount according to the flowchart depicted in  FIG. 11 . 
     The target donor base station  100   3  receives a handover request from the source donor base station  100   2  (step S 1804 ) as the relay node  200  has moved. That is, the source donor base station  100   2  transmits the handover request to the target donor base station  100   3  when causing the relay node  200  that is under control of the source donor base station  100   2  to carry out handover. The handover request is input to the transmission processing part  1064  through the optical module  102  of the target donor base station  100   3 . 
     The target donor base station  100   3  determines whether there is information of the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  (step S 1806 ). That is, the transmission processing part  1064  of the target donor base station  100   3  determines whether the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  has been measured. 
     In a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  has been measured (step S 1806  YES), the target donor base station  100   3  determines whether the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  is less than the threshold (step S 1808 ). That is, the call processing control part  1062  of the target donor base station  100   3  determines whether the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  is less than a threshold in a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  has been measured. 
     In a case where the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  is less than the threshold (step S 1808  YES), the target donor base station  100   3  carries out a process according to the handover request (step S 1810 ). That is, the call processing control part  1062  of the target donor base station  100   3  carries out a process according to the handover request in a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  is less than the threshold. 
     In a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  has not been measured (step S 1806  NO), the target donor base station  100   3  carries out a handover preparation process (step S 1812 ). That is, the call processing control part  1062  of the target donor base station  100   3  carries out a handover preparation process in a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  has not been measured. 
     In a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  is greater than or equal to the threshold (step S 1808  NO), the target donor base station  100   3  carries out a handover preparation process (step S 1812 ). That is, the call processing control part  1062  of the target donor base station  100   3  carries out a handover preparation process in a case of having determined that the delay amount between the target donor base station  100   3  and the anchor donor base station  100   1  is greater than or equal to the threshold. 
     &lt;Handover Process in Case of Transferring S1proxy Information&gt; 
       FIG. 19  depicts operations for when the relay node  200  that is under control of the source donor base station  100   2  carries out handover to the target donor base station  100   3 .  FIG. 19  depicts a case where the S1proxy information is moved from the anchor donor base station  100   1  to the target donor base station  100   3  in a case where the relay node  200  has thus carried out handover. 
     The user terminal  300  that is under control of the relay node  200  carries out communication of U-Plane with the EPC  600  through the relay node  200 , the source donor base station  100   2  and the anchor donor base station  100   1  (step S 1902 ). 
     The relay node  200  transmits a “measurement report (MEASUREMENT REPORT)” to the source donor base station  100   2  using the RRC protocol (step S 1904 ). The call processing control part  2062  of the relay node  200  carries out a process of transmitting the measurement report using the RRC protocol through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . 
     The source donor base station  100   2  transmits a “handover request (HANDOVER REQUEST)” to the target donor base station  100   3  using the X2AP protocol (step S 1906 ). The handover request includes the IP address of the anchor donor base station  100   1  and the IP address of the relay node  200 . The call processing control part  1062  of the source donor base station  100   2  transmits the handover request using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The target donor base station  100   3  determines whether to carry out handover with transferring the S1proxy information or handover without transferring the S1proxy information (step S 1908 ). The transmission processing part  1064  of the target donor base station  100   3  determines whether to carry out handover with transferring the S1proxy information or handover without transferring the S1proxy information when causing the relay node  200  to carry out handover. Here, it is determined to carry out handover with transferring the S1proxy information. 
     The target donor base station  100   3  transmits a “handover preparation failure (HANDOVER PREPARATION FAILURE)” to the source donor base station  100   2  using the X2AP protocol (step S 1910 ). The call processing control part  1062  of the target donor base station  100   3  transmits the handover preparation failure through the transmission processing part  1064  and the optical module  102 . 
     The source donor base station  100   2  transmits a “handover request (HANDOVER REQUEST)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1912 ). That is, the call processing control part  1062  of the source donor base station  100   2  transmits the handover request using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . The handover request includes the IP address of the anchor donor base station  100   1 . Further, the handover request may also include the IP address of the target donor base station  100   3  and the IP address of the relay node  200 . 
     The anchor donor base station  100   1  has the IP address of the relay node  200  managed by the anchor donor base station  100   1  and the IP address of the relay node  200  from the source donor base station  100   2 . The anchor donor base station  100   1  can recognize an opportunity to transmit a handover message to transfer the S1proxy information as a result of the IP address of the relay node  200  being thus notified of by the source donor base station  100   2 . 
     The anchor donor base station  100   1  transmits a “handover request (HANDOVER REQUEST)” to the target donor base station  100   3  using the X2AP protocol (step S 1914 ). That is, the call processing control part  1062  of the anchor donor base station  100   1  transmits the handover request using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . The handover request includes the S1proxy information and the IP address of the relay node  200 . 
     The target donor base station  100   3  understands that handover with transferring the S1proxy information will be carried out (step S 1916 ). The call processing control part  1062  of the target donor base station  100   3  understands that handover with transferring the S1proxy information will be carried out. 
     The target donor base station  100   3  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1918 ). The call processing control part  1062  of the target donor base station  100   3  transmits the handover request acknowledge using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the source donor base station  100   2  using the X2AP protocol (step S 1920 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the handover request acknowledge using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The source donor base station  100   2  transmits a “SN status transfer (SN STATUS TRANSFER)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1922 ). The call processing control part  1062  of the source donor base station  100   2  transfers the SN status transfer using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  transmits a “SN status transfer (SN STATUS TRANSFER)” to the target donor base station  100   3  using the X2AP protocol (step S 1924 ). The call processing control part  1062  of the anchor donor base station  100   1  transfers the SN status transfer using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The source donor base station  100   2  transmits a “RRC connection reconfiguration (RRC CONNECTION RECONFIGURATION)” to the relay node  200  using the RRC protocol (step S 1926 ). The call processing control part  1062  of the source donor base station  100   2  transmits the RRC connection reconfiguration using the RRC protocol through the transmission processing part  1064 , the baseband processing part  1082  and the RF module  104 . The RRC connection reconfiguration includes the IP address of the target donor base station  100   3 . 
     The relay node  200  transmits a “RRC connection reconfiguration complete (RRC CONNECTION RECONFIGURATION COMPLETE)” to the target donor base station  100   3  using the RRC protocol (step S 1928 ). The call processing control part  2062  of the relay node  200  transmits the RRC connection reconfiguration complete using the RRC protocol through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . 
     The relay node  200  transmits a “S1 setup request (S1 SETUP REQUEST)” to the target donor base station  100   3  using the S1AP protocol (step S 1930 ). The call processing control part  2062  of the relay node  200  transmits the S1 setup request using the S1AP protocol through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . 
     The target donor base station  100   3  transmits a “S1 setup response (S1 SETUP RESPONSE)” to the relay node  200  using the S1AP protocol (step S 1932 ). The call processing control part  1062  of the target donor base station  100   3  transmits the S1 setup response using the S1AP protocol through the transmission processing part  1064 , the baseband processing part  1082  and the RF module  104 . 
     A case where the tracking area of the relay node  200  is new will now be described. 
     The target donor base station  100   3  transmits a “eNB configuration update (ENB CONFIGURATION UPDATE)” to the EPC  600  using the S1AP protocol (step S 1934 ). The call processing control part  1062  of the target donor base station  100   3  transmits the eNB configuration update using the S1AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The EPC  600  transmits a “ENB configuration update acknowledge (eNB CONFIGURATION UPDATE ACKNOWLEDGE)” to the target donor base station  100   3  using the S1AP protocol (step S 1936 ). 
     The target donor base station  100   3  transmits a “ENB configuration update (eNB CONFIGURATION UPDATE)” to the anchor donor base station  100   1  and the source donor base station  100   2  using the X2AP protocol (step S 1938 ). The call processing control part  1062  of the target donor base station  100   3  transmits the eNB configuration update using the S1AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  and the source donor base station  100   2  transmit “eNB configuration update acknowledges (ENB CONFIGURATION UPDATE ACKNOWLEDGE)” to the target donor base station  100   3  using the X2AP protocol, respectively (step S 1940 ). The call processing control parts  1062  of the anchor donor base station  100   1  and the source donor base station  100   2  transmit the eNB configuration update acknowledges using the S1AP protocol through the transmission processing parts  1064  and the optical modules  102 , respectively. 
     In a case where the tracking area of the relay node  200  is not new, the processes of steps S 1934  through S 1940  may be omitted. 
     The target donor base station  100   3  transmits a group path switch request (GROUP PATH SWITCH REQUEST)” to the EPC  600  using the S1AP protocol (step S 1942 ). The call processing control part  1062  of the target donor base station  100   3  transmits the group path switch request using the S1AP protocol through the transmission processing part  1064  and the optical module  102 . The target donor base station  100   3  carries out a handover procedure using the S1proxy information. Further, in order to carry out the path switch procedures for the user terminals that are under control of the relay node  200 , an interface (IF) in which plural sets of user terminal information can be designated is defined. By using the IF, the target donor base station  100   3  carries out switching into the side of the EPC  600 . 
     Further, also an IF for simultaneously changing plural user terminals may be defined on the side of the EPC  600 . By using the IF, the EPC  600  may carry out a MODIFY bearer procedure of GTP-C. 
       FIG. 20  depicts one embodiment of the group path switch request. In the embodiment of the group path switch request, path switch requests are set for plural user terminals. A path switch request is described in NON-PATENT REFERENCE NO. 5. 
     The group path switch request is different from a path switch request in that the group path switch request includes a “UE information list” and a “UE information IEs”. 
     The EPC  600  transmits a “create session request (for relay node) (Create Session Request (for RN))” to the target donor base station  100   3  using the GTP-C protocol (step S 1944 ). 
     The target donor base station  100   3  transmits a “create session response (for relay node) (Create Session Response (for RN))” to the EPC  600  using the GTP-C protocol (step S 1946 ). The call processing control part  1062  of the target donor base station  100   3  transmits the create session response using the GTP-C protocol through the transmission processing part  1064  and the optical module  102 . 
     The EPC  600  transmits a “group path switch request acknowledge (GROUP PATH SWITCH REQUEST ACKNOWLEDGE)” to the target donor base station  100   3  using the S1AP protocol (step S 1948 ). 
       FIG. 21  depicts one embodiment of the group path switch request acknowledge. In the embodiment of the group path switch request acknowledge, path switch request acknowledges are set for plural user terminals. A path switch request acknowledge is described in NON-PATENT REFERENCE NO. 5. 
     The group path switch request acknowledge is different from a path switch request acknowledge in that the group path switch request acknowledge includes a “UE information list” and a “UE information IEs”. 
     The user terminal  300  that is under control of the relay node  200  carries out communication of U-Plane with the EPC  600  through the relay node  200  and the target donor base station  100   3  (step S 1950 ). 
     The target donor base station  100   3  transmits a “UE context release (UE CONTEXT RELEASE)” to the anchor donor base station  100   1  using the X2AP protocol (step S 1952 ). The call processing control part  1062  of the target donor base station  100   3  transmits the UE context release using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  transmits a “UE context release (UE CONTEXT RELEASE)” to the source donor base station  100   2  using the X2AP protocol (step S 1954 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the UE context release using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The EPC  600  transmits a “delete session request (for relay node) (Delete Session Request (for RN))” to the anchor donor base station  100   1  using the GTP-C protocol (step S 1956 ). 
     The anchor donor base station  100   1  transmits a “delete session response (for relay node) (Delete Session Response (for RN))” to the EPC  600  using the GTP-C protocol (step S 1958 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the delete session response using the GTP-C protocol through the transmission processing part  1064  and the optical module  102 . 
     &lt;Operations for When Exiting Movable Body&gt; 
       FIG. 22  depicts operations for when the user terminal  300  that is under control of the relay node  200  carries out handover to the target donor base station  100   3 .  FIG. 22  depicts a case where the anchor donor base station  100   1  has the S1proxy information of the user terminal  300 . 
     The user terminal  300  transmits a “measurement report (MEASUREMENT REPORT)” to the relay node  200  using the RRC protocol (step S 2202 ). 
     The relay node  200  transmits a “handover request (HANDOVER REQUEST)” to the anchor donor base station  100   1  using the X2AP protocol (step S 2204 ). The call processing control part  2062  of the relay node  200  transmits the handover request using the X2AP protocol through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . The relay node  200  knows the cell information that is under control of the target donor base station  100   3  as an adjacent cell. Since an interface X2 is established through a donor base station, the relay node  200  transmits the “handover request” to the anchor donor base station  100   1  using the X2AP protocol. 
     The anchor donor base station  100   1  transmits a “handover request (HANDOVER REQUEST)” to the target donor base station  100   3  using the X2AP protocol (step S 2206 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the handover request using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The target donor base station  100   3  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the anchor donor base station  100   1  using the X2AP protocol (step S 2208 ). The call processing control part  1062  of the target donor base station  100   3  transmits the handover request acknowledge using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  transmits a “handover request acknowledge (HANDOVER REQUEST ACKNOWLEDGE)” to the relay node  200  using the X2AP protocol (step S 2210 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the handover request acknowledge using the X2AP protocol through the transmission processing part  1064 , the baseband processing part  1082  and the RF module  104 . 
     The relay node  200  transmits a “SN status transfer (SN STATUS TRANSFER)” to the anchor donor base station  100   1  using the X2AP protocol (step S 2212 ). The call processing control part  2062  of the relay node  200  transmits the SN status transfer using the X2AP protocol through the transmission processing part  2064 , the first baseband processing part  2082  and the RF module  204 . 
     The anchor donor base station  100   1  transmits a “SN status transfer (SN STATUS TRANSFER)” to the target donor base station  100   3  using the X2AP protocol (step S 2214 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the SN status transfer using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The relay node  200  transmits a “RRC connection reconfiguration (RRC CONNECTION RECONFIGURATION)” to the user terminal  300  using the RRC protocol (step S 2216 ). The call processing control part  2062  of the relay node  200  transmits the RRC connection reconfiguration using the RRC protocol through the transmission processing part  2064 , the second baseband processing part  2084  and the RF module  204 . 
     The user terminal  300  transmits a “RRC connection reconfiguration complete (RRC CONNECTION RECONFIGURATION COMPLETE)” to the target donor base station  100   3  using the RRC protocol (step S 2218 ). 
     The target donor base station  100   3  transmits a “path switch request (PATH SWITCH REQUEST)” to the EPC  600  using the S1AP protocol (step S 2220 ). The call processing control part  1062  of the target donor base station  100   3  transmits the path switch request using the S1AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The EPC  600  transmits a “path switch request acknowledge (PATH SWITCH REQUEST ACKNOWLEDGE)” to the target donor base station  100   3  using the S1AP protocol (step S 2222 ). 
     The target donor base station  100   3  transmits a “UE context release (UE CONTEXT RELEASE)” to the anchor donor base station  100   1  using the X2AP protocol (step S 2224 ). The call processing control part  1062  of the target donor base station  100   3  transmits the UE context release using the X2AP protocol through the transmission processing part  1064  and the optical module  102 . 
     The anchor donor base station  100   1  transmits a “UE context release (UE CONTEXT RELEASE)” to the relay node  200  using the X2AP protocol (step S 2226 ). The call processing control part  1062  of the anchor donor base station  100   1  transmits the UE context release using the X2AP protocol through the transmission processing part  1064 , the baseband processing part  1082  and the RF module  104 . 
     The user terminal  300  transmits a “tracking area update request (TAU Req)” to the EPC  600  using the NAS protocol (step S 2228 ). The tracking area update request transmitted from the user terminal  300  is transmitted to the EPC  600  through the target donor base station  100   3  and the anchor donor base station  100   1 . The anchor donor base station  100   1  carries out a bundling process of the SCTP layer on the uplink messages that are transmitted using the NAS protocol. That is, the anchor donor base station  100   1  makes the header information common concerning layers lower than and equal to the SCTP layer. By thus carrying out bundling of the SCTP layer, it is possible to reduce the processing load of the EPC  600 . 
     The EPC  600  transmits a “tracking area update accept (TAU Accept)” to the user terminal  300  using the NAS protocol (step S 2230 ). 
     The user terminal transmits a “tracking area update complete (TAU Complete)” to the EPC  600  using the NAS protocol (step S 2232 ). The anchor donor base station  100   1  carries out a bundling process of the SCTP layer on the uplink messages that are transmitted using the NAS protocol. That is, the anchor donor base station  100   1  makes the header information common concerning layers lower than and equal to the SCTP layer. By thus carrying out bundling of the SCTP layer, it is possible to reduce the processing load of the EPC  600 . 
     According to the embodiments, it is possible to measure the delay amount between the base station and the adjacent base station using the GTP-U layer that is the highest layer of U-Plane. 
     Further, when the relay node will carry out handover between the donor base stations, the S1proxy information is not moved while the handover is carried out in a case where the delay is small between the donor base station that has the S1proxy information and the donor base station that is the handover destination. An interface Uu is terminated between the base station that is the handover destination and the relay node. 
     Further, when the relay node will carry out handover, the S1proxy information is moved to the donor base station that is the handover destination in a case where the delay is large between the donor base station that has the S1proxy information and the donor base station that is the handover destination. An interface Uu is terminated between the base station that is the handover destination and the relay node. In this case, the procedure to be carried out for the EPC and the procedure(s) associated therewith are carried out almost simultaneously for the user terminals that are under control of the relay node. 
     When the user terminal carries out handover from the relay node to the donor base station, the donor base station transmits NAS signals that will be transmitted in an uplink manner after carrying out bundling of SCTP, during a predetermined period of time. That is, the header information is made to be common concerning layers lower than and equal to the SCTP layer. 
     According to the embodiments discussed above, it is possible to reduce an EPC&#39;s processing load at a time of a handover process of a relay node and achieve the most suitable load sharing in the entirety of the radio communication system. 
     All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as 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 one or more embodiments of the present invention 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.