Patent Publication Number: US-2018041932-A1

Title: Base station and communication control method

Description:
RELATED APPLICATION 
     This application is a continuation application of international application PCT/JP2016/061674, filed Apr. 11, 2016, which claims the benefit of U.S. Provisional Application No. 62/148,953 (filed on Apr. 17, 2015), the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a base station and a communication control method in a mobile communication system. 
     BACKGROUND ART 
     Dual connectivity communication is specified in 3GPP (3rd Generation Partnership Project) which is a standardization project for mobile communication systems. The dual connectivity communication is a communication mode in which a master cell group (MCG) and a secondary cell group (SCG) are configured to a user terminal in a RRC (Radio Resource Control) connected mode. The MCG is a serving cell group managed by a master base station. The SCG is a serving cell group managed by a secondary base station. 
     There are also three types of bearers in total of an MCG bearer, an SCG bearer, and a split bearer for a user data transfer method of dual connectivity communication. The MCG bearer is a bearer whose corresponding radio protocol exists only in the master base station and uses only the resources of the master base station. The SCG bearer is a bearer whose corresponding radio protocol exists only in the secondary base station and uses only resources of the secondary base station. The split bearer is a bearer whose corresponding radio protocol exists in both the master base station and the secondary base station and uses the resources of both the master base station and the secondary base station. 
     SUMMARY 
     A target base station according to an embodiment is a base station to which a user terminal is handed over from a source base station. The target base station includes a controller configured to, when the target base station and a secondary base station perform dual connectivity communication with the user terminal accompanying the handover, notify the secondary base station of identification information related to a serving gateway connected to the target base station. 
     A secondary base station according to an embodiment is configured to perform dual connectivity communication with a user terminal together with a target base station in a case where the user terminal is handed over from a source base station. The secondary base station includes a controller configured to perform a process of receiving, from the target base station, identification information related to a serving gateway connected with the target base station. 
     A communication control method according to an embodiment includes the steps of: handing over a user terminal from a source base station to a target base station; and notifying a secondary base station of identification information from the target base station when the target base station and the secondary base station perform dual connectivity communication with the user terminal accompanying the handover, the identification information being related to a serving gateway connected to the target base station. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a configuration of an LTE system. 
         FIG. 2  is a protocol stack diagram of a radio interface of the LTE system. 
         FIG. 3  is a block diagram of a UE (user terminal). 
         FIG. 4  is a block diagram of an eNB (base station). 
         FIGS. 5A and 5B  are views for explaining an outline of dual connectivity communication according to the embodiment. 
         FIG. 6  is a view illustrating an example of operation environment according to the embodiment. 
         FIG. 7  is a view illustrating another example of the operation environment according to the embodiment. 
         FIG. 8  is a sequence diagram illustrating an example of an operation sequence according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Outline of Embodiment 
     The embodiment assumes dual connectivity communication of an SCG bearer scheme. According to the dual connectivity communication of the SCG bearer scheme, a bearer which does not pass through a master base station is established between a user terminal and a serving gateway. In this case, it is demanded that the serving gateway connected to the master base station and the serving gateway connected to the secondary base station are the same. 
     For dual connectivity communication of the SCG bearer scheme, it is studied to enable inter master base station handover which changes the master base station without changing the secondary base station. However, according to the inter master base station handover, the serving gateway connected to the master base station may be changed. Consequently, it is concerned that the serving gateways do not match between the master base station and the secondary base station. 
     The following embodiment discloses a technique which enables smooth dual connectivity communication after the handover. 
     A target base station according to an embodiment is a base station to which a user terminal is handed over from a source base station. The target base station includes a controller configured to, when the target base station and a secondary base station perform dual connectivity communication with the user terminal accompanying the handover, notify the secondary base station of identification information related to a serving gateway connected to the target base station. 
     A secondary base station according to an embodiment is configured to perform dual connectivity communication with a user terminal together with a target base station in a case where the user terminal is handed over from a source base station. The secondary base station includes a controller configured to perform a process of receiving, from the target base station, identification information related to a serving gateway connected with the target base station. 
     A communication control method according to an embodiment includes the steps of: handing over a user terminal from a source base station to a target base station; and notifying a secondary base station of identification information from the target base station when the target base station and the secondary base station perform dual connectivity communication with the user terminal accompanying the handover, the identification information being related to a serving gateway connected to the target base station. 
     Embodiment 
     (1) Configuration of Mobile Communication System 
       FIG. 1  is a view illustrating a configuration of an LTE system which is the mobile communication system according to the embodiment. As illustrated in  FIG. 1 , the LTE system according to the first embodiment includes UEs (User Equipment)  100 , an E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network)  10  and an EPC (Evolved Packet Core)  20 . 
     Each UE  100  corresponds to a user terminal. The UE  100  is a mobile communication apparatus, and performs radio communication with cells (serving cells). A configuration of each UE  100  will be described below. 
     The E-UTRAN  10  corresponds to a radio access network. The E-UTRAN  10  includes eNBs  200  (evolved Node-B). Each eNB  200  corresponds to a base station. The eNBs  200  are connected with each other via an X2 interface. A configuration of each eNB  200  will be described below. 
     The eNB  200  manages one or a plurality of cells, and performs radio communication with the UEs  100  which have established connection with the cell of this eNB  200 . The eNB  200  includes a radio resource managing (RRM) function, a user data (simply referred to as “data” below) routing function and a measurement control function for mobility control and scheduling. The “cell” is used not only as a term which indicates a minimum unit of a radio communication area, and but also as a term indicating a function of performing radio communication with the UEs  100 . 
     The EPC  20  corresponds to a core network. The EPC  20  includes MMEs (Mobility Management Entity)/S-GW (Serving-Gateway)  300 . The MME corresponds to a mobility management device, and performs various types of mobility control on each UE  100 . Each S-GW controls data transfer. Each MME/S-GW  300  is connected with each eNB  200  via an S1 interface. The E-UTRAN  10  and the EPC  20  configure a network. 
     (2) Configuration of Radio Interface 
       FIG. 2  is a protocol stack diagram of a radio interface of the LTE system. As illustrated in  FIG. 2 , a radio interface protocol is partitioned into a first layer to a third layer of an OSI reference model, and the first layer is a physical (PHY) layer. The second layer includes an MAC (Medium Access Control) layer, a RLC (Radio Link Control) layer and a PDCP (Packet Data Convergence Protocol) layer. The third layer includes a RRC (Radio Resource Control) layer. 
     In the physical layer, encoding, decoding, modulation, demodulation, antenna mapping, antenna demapping, resource mapping and resource demapping are performed. Data and control signals are transmitted between the physical layer of each UE  100  and the physical layer of each eNB  200  via a physical channel. 
     In the MAC layer, data prioritization control, a retransmission process according to hybrid ARQ (HARQ), and a random access procedure are performed. Data and control signals are transmitted between the MAC layer of each UE  100  and the MAC layer of each eNB  200  via a transport channel. The MAC layer of each eNB  200  includes a scheduler which determines a transport format (a transport block size and a modulating/encoding method (MCS)) in uplink and downlink, and allocated resource blocks for each UE  100 . 
     In the RLC layer, data is transmitted to the RLC layer at a reception side by using functions of the MAC layer and the physical layer. Data and control signals are transmitted between the RLC layer of each UE  100  and the RLC layer of each eNB  200  via a logical channel. 
     In the PDCP layer, header compression, header extension, encryption and decoding are performed. 
     The RRC layer is defined only in a control plane which handles a control signal. A message (RRC message) for various configurations is transmitted between the RRC layer of each UE  100  and the RRC layer of each eNB  200 . In the RRC layer, a logical channel, a transport channel and a physical channel are controlled in response to establishment, reestablishment and release of a radio bearer. When the RRC of each UE  100  and the RRC of each eNB  200  are connected (RRC connection), each UE  100  is in a RRC connected mode and, when this is not a case, each UE  100  is in a RRC idle mode. 
     In a NAS (Non-Access Stratum) layer is a higher layer than the RRC layer, session management and mobility management are performed. 
     (3) Configuration of User Terminal 
       FIG. 3  is a block diagram of the UE  100  (user terminal). As illustrated in  FIG. 3 , the UE  100  includes a receiver  110 , a transmitter  120 , and a controller  130 . 
     The receiver  110  performs various types of reception under control of the controller  130 . The receiver  110  includes an antenna and a receiver. Further, the receiver converts a radio signal received at the antenna into a baseband signal (received signal) to output to the controller  130 . 
     The transmitter  120  performs various types of transmission under control of the controller  130 . The transmitter  120  includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) outputted from the controller  130  into a radio signal to transmit from the antenna. 
     The controller  130  performs various types of control in the UE  100 . The controller  130  includes a processor and a memory. The memory stores programs executed by the processor and information used for a process performed by the processor. The processor includes a baseband processor which modules, demodulates, encodes and decodes baseband signals, and a CPU (Central Processing Unit) which executes the programs stored in the memory to execute various types of processes. The processor may further include a codec which encodes and decodes audio and video signals. The processor executes the above-described various communication protocols and processes described later. 
     (4) Configuration of Base Station 
       FIG. 4  is a block diagram of the eNB  200  (base station). As illustrated in  FIG. 4 , the eNB  200  includes a transmitter  210 , a receiver  220 , a controller  230 , and a backhaul communication unit  240 . 
     The transmitter  210  performs various types of transmission under control of the controller  230 . The transmitter  210  includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) outputted from the controller  230  into a radio signal to transmit from the antenna. 
     The receiver  220  performs various types of reception under control of the controller  230 . The receiver  220  includes an antenna and a receiver. The receiver converts a radio signal received at the antenna into a baseband signal (received signal) to output to the controller  230 . 
     The controller  230  performs various types of control in the eNB  200 . The controller  230  includes a processor and a memory. The memory stores programs executed by the processor and information used for a process performed by the processor. The processor includes a baseband processor which modules, demodulates, encodes and decodes baseband signals, and a CPU (Central Processing Unit) which executes the programs stored in the memory to execute various types of processes. The processor executes the above-described various communication protocols and processes described later. 
     The backhaul communication unit  240  is connected with the neighboring eNBs  200  via an X2 interface, and is connected with the MME/S-GWs  300  via the S1 interface. The backhaul communication unit  240  is used for communication performed on the X2 interface, and communication performed on the S1 interface. 
     (5) Outline of Dual Connectivity Communication 
     The dual connectivity communication is a communication mode in which a master cell group (MCG) and a secondary cell group (SCG) are configured in the UE  100  in the RRC connected mode. The MCG is a serving cell group managed by a master base station (MeNB). The SCG is a serving cell group managed by a secondary base station (SeNB). Radio resources are allocated to the UE  100  from each eNB  200 , so that improvement of throughput is expected. 
     In dual connectivity communication, among a plurality of eNBs  200  which establishes connection with the UE  100 , only the MeNB  200 M establishes RRC connection with the UE  100 . In contrast, an SeNB  200 S provides additional radio resources to the UE  100  without establishing RRC connection with the UE  100 . There is the X2 interface between an MeNB  200 M and the SeNB  200 S. 
       FIGS. 5A and 5B  are views for explaining an outline of dual connectivity communication according to the embodiment. The embodiment assumes the dual connectivity communication of the SCG bearer scheme (SCG bearer option).  FIGS. 5A and 5B  illustrate a downlink data path. However, an uplink data path is also configured similar to the downlink. 
     As illustrated in  FIG. 5A , the UE  100  includes two bearers (an EPS bearer # 1  and an EPS bearer # 2 ). The EPS bearer # 1  is a bearer which passes through an S-GW  300 U and the MeNB  200 M. Such a bearer will be referred to as an MCG bearer. The EPS bearer # 2  is a bearer which passes through the S-GW  300 U and the SeNB  200 S. Such a bearer will be referred to as an SCG bearer. 
     The S-GW  300 U sorts the two bearers of the UE  100  to the MeNB  200 M and the SeNB  200 S. Therefore, it is demanded that an S-GW having a data path (S1 bearer) to the MeNB  200 M, and an S-GW having a data path (S1 bearer) to the SeNB  200 S are the same. 
     As illustrated in  FIG. 5B , the MeNB  200 M processes data belonging to the EPS bearer # 1  in each layer of PDCP, RLC, and MAC. The SeNB  200 S processes data belonging to the EPS bearer # 2  in each layer of PDCP, RLC, and MAC. 
     (6) Operation Environment According to Embodiment 
     The embodiment assumes that inter MeNB handover (inter MeNB handover without SeNB change) which changes the MeNB  200 M without changing the SeNB  200 S is performed for the dual connectivity communication of the SCG bearer scheme. 
       FIG. 6  is a view illustrating an example of the operation environment according to the embodiment. 
     As illustrated in  FIG. 6 , the secondary base station (SeNB  200 S) is located near a boundary of a coverage of the source base station (S-MeNB  200 M 1 ) and a coverage of a target base station (T-MeNB  200 M 2 ).  FIG. 6  illustrates an example where the S-MeNB  200 M 1  and the T-MeNB  200 M 2  are macrocell base stations and the SeNB  200 S is a small cell base station. 
     The S-MeNB  200 M 1 , the T-MeNB  200 M 2 , and the SeNB  200 S are connected with each other via the X2 interface. In addition, each of the S-MeNB  200 M 1 , the T-MeNB  200 M 2 , and the SeNB  200 S is connected to the same S-GW  300 U via an S1-U interface (S1 bearer). The S-MeNB  200 M 1  and the T-MeNB  200 M 2  are connected to the same MME (not illustrated) via an S1-MME interface. 
     In such operation environment, the UE  100  first performs dual connectivity communication with the S-MeNB  200 M 1  and the SeNB  200 S at a point A in the coverage of the SeNB  200 S. Next, the UE  100  moves toward a point B in the coverage of the SeNB  200 S and is handed over from the S-MeNB  200 M 1  to the T-MeNB  200 M 2 . The UE  100  performs dual connectivity communication with the T-MeNB  200 M 2  and the SeNB  200 S without changing the SeNB  200 S at point B. 
       FIG. 7  is a view illustrating another example of the operation environment according to the embodiment. 
     As illustrated in  FIG. 7 , the S-MeNB  200 M 1  and the T-MeNB  200 M 2  are connected to different S-GWs. More specifically, the S-MeNB  200 M 1  is connected with an S-GW  300 U 1  via the S1-U interface (S1 bearer), and the T-MeNB  200 M 2  is connected with an S-GW  300 U 2  via the S1-U interface (S1 bearer). The S-MeNB  200 M 1  and the T-MeNB  200 M 2  are connected to the same MME (not illustrated) via the S1-MME interface. 
     In such operation environment, when the UE  100  is handed over similar to the above, relocation of the S-GWs (S-GW relocation) occurs before and after the handover. As a result, the S-GWs do not match between the T-MeNB  200 M 2  and the SeNB  200 S. In this case, when the SeNB  200 S cannot acquire information (identification information) related to the new S-GW  300 U 2 , it is not possible to appropriately perform dual connectivity communication. 
     In the embodiment, when the T-MeNB  200 M 2  and the SeNB  200 S perform the dual connectivity communication with the UE  100  accompanying the handover, the T-MeNB  200 M 2  notifies the SeNB  200 S of the identification information related to the S-GW  300 U 2  connected to the T-MeNB  200 M 2 . The identification information related to the S-GW  300 U 2  includes at least one of an “E-RAB (E-UTRAN Radio Access Bearer) ID”, a “Transport Layer Address”, and a “GTP-TEID (GPRS Tunneling Protocol-Tunnel Endpoint Identifier)”. Consequently, it is possible to smoothly perform dual connectivity communication after the handover. 
     (7) Identification Information Related to Serving Gateway 
     Hereinafter, the identification information related to the S-GW  300 U 2  (serving gateway) will be described in detail. 
     The “E-RAB ID” is an ID for identifying an E-RAB. 
     The “Transport Layer Address” is an IP address of the S-GW. This information makes it possible to grasp which S-GW to connect. When the S-GW is changed, this IP address will be naturally changed, too. In the future, it is also possible to assume a flow that the S-GW is virtualized and plays a role of a plurality of S-GWs in one server. In such a case, there is a possibility that the IP address is not changed even when the S-GW is changed, so that a next GTP-TEID can be used. 
     The “GTP-TEID” indicates the GTP-TEID of the S1-U, and, more specifically, is an ID assigned to both ends of the S1 bearer (S1 transport bearer). That is, the “GTP-TEID” is assigned to both ends (an eNB side and an S-GW side) of one S1-U bearer. When the eNB knows the ID of the S-GW side, the UL data transmission destination can be identified, and when the S-GW knows the ID assigned by the eNB, the DL data transmission destination can be identified. Each ID is allocated as an address of a data transmission destination of each data transmission destination node. 
     (8) Operation Sequence According to Embodiment 
       FIG. 8  is a sequence diagram illustrating an example of an operation sequence according to the embodiment. Hereinafter, a handover sequence in the operation environment illustrated in  FIG. 7  will be described. In the initial state of  FIG. 8 , the UE  100  performs dual connectivity communication with the S-MeNB  200 M 1  and the SeNB  200 S. 
     As illustrated in  FIG. 8 , in step  1 , the S-MeNB  200 M 1  transmits a handover request (Handover Request) message for requesting the handover of the UE  100  to the T-MeNB  200 M 2 . The “Handover Request” includes identification information related to the S-GW  300 U 1  connected to the S-MeNB  200 M 1 . The identification information related to the S-GW  300 U 1  includes at least one of the “E-RAB ID”, the “Transport Layer Address” and the “GTP-TEID”. Further, the “Handover Request” may include identification information related to the SeNB  200 S. The identification information related to the SeNB  200 S includes an eNB ID (Global eNB ID). Further, the identification information related to the SeNB  200 S may include at least one of the “E-RAB ID”, the “Transport Layer Address” and the “GTP-TEID” as information of bearers established for the SeNB  200 S. 
     In step  2 , in response to reception of the “Handover Request” message, the T-MeNB  200 M 2  transmits an addition request (SeNB Addition Request) message for requesting addition of the SeNB  200 S to the SeNB  200 S. 
     In step  3 , in response to the reception of the “SeNB Addition Request” message, the SeNB  200 S transmits an “SeNB Addition Request Ack” message to the T-MeNB  200 M 2 . The “SeNB Addition Request Ack” message includes SCG configuration information. 
     In step  4 , the T-MeNB  200 M 2  transmits a handover acknowledgment response (Handover Request Ack) message to the “Handover Request” message to the S-MeNB  200 M 1 . The “Handover Request Ack” message includes MCG configuration information and SCG configuration information. 
     In step  5 , the S-MeNB  200 M 1  transmits an SeNB release request (“SeNB Release Request) message to the SeNB  200 S. 
     In step  6 , in response to reception of the “Handover Request Ack” message, the S-MeNB  200 M 1  transmits a RRC connection reconfiguration (RRC Connection Reconfiguration) message to the UE  100 . The “RRC Connection Reconfiguration” message corresponds to a handover command for instructing handover to the T-MeNB  200 M 2 , and includes the SCG configuration information and the MCG configuration information. 
     In step  7 , in response to reception of the “RRC Connection Reconfiguration” message, the UE  100  performs a random access procedure (Random Access Procedure) on the T-MeNB  200 M 2 . 
     In step  8 , the UE  100  transmits a RRC connection reconfiguration complete (RRC Connection Reconfiguration Complete) message to the T-MeNB  200 M 2 . 
     In step  9 , the UE  100  performs the “Random Access Procedure” on the SeNB  200 S. However, instead of the “Random Access Procedure”, RRC connection re-establishment (RRC Connection Re-establishment) may be performed on the SeNB  200 S. 
     In step  10 , the T-MeNB  200 M 2  transmits an SeNB reconfiguration complete (SeNB Reconfiguration Complete) message to the SeNB  200 S. 
     In step  11 , the S-MeNB  200 M 1  transmits an “SN Status Transfer” to the T-MeNB  200 M 2 . 
     In step  12 , the S-MeNB  200 M 1  performs “Data Forwarding” of forwarding data received from the S-GW  300 U 1  to the T-MeNB  200 M 2 . 
     In step  13 , the T-MeNB  200 M 2  transmits a data path switch request (Path Switch Request) message to an MME  300 C. 
     In step  14 , in response to reception of the “Path Switch Request” message, the MME  300 C transmits a create session request (Create Session Request) message to the S-GW  300 U 2 . 
     In step  15 , the S-GW  300 U 2  transmits a response (Response) to the “Create Session Request” message to the MME  300 C. 
     In step  16 , in response to reception of a response (Response) from the S-GW  300 U 2 , the MME  300 C transmits a path switch response (Path Switch Request Acknowledge) message indicating the data path switch from the S-MeNB  200 M 1  to the T-MeNB  200 M 2  to the T-MeNB  200 M 2 . The “Path Switch Request Acknowledge” message includes identification information related to the S-GW  300 U 2 . As described above, the identification information related to the S-GW  300 U 2  includes at least one of the “E-RAB ID”, the “Transport Layer Address” and the “GTP-TEID”. The “E-RAB ID”, the “Transport Layer Address” and the “GTP-TEID” are included in “E-RABs Switched in Uplink Item IEs” in the “Path Switch Request Acknowledge” message. 
     In step  17 , the T-MeNB  200 M 2  compares the “Transport Layer Address” in the “Handover Request” message and the “Transport Layer Address” in the “Path Switch Request Acknowledge” message. The “Transport Layer Address” in the “Handover Request” message corresponds to the address information of the S-GW  300 U 1 . The “Transport Layer Address” in the “Path Switch Request Acknowledge” message corresponds to the address information of the S-GW  300 U 2 . 
     In addition, the T-MeNB  200 M 2  may compare the “GTP-TEID” (the “GTP-TEID” related to the S-GW  300 U 1 ) in the “Handover Request” message and the “GTP-TEID” (the “GTP-TEID” related to the S-GW  300 U 2 ) in the “Path Switch Request Acknowledge” message. In addition, the T-MeNB  200 M 2  may compare the “E-RAB ID” (the “E-RAB ID” related to the S-GW  300 U 1 ) in the “Handover Request” message and the “E-RAB ID” (the “E-RAB ID” related to the S-GW  300 U 2 ) in the “Path Switch Request Acknowledge” message. 
     When the match is not found as a result of the comparison, the T-MeNB  200 M 2  notifies the SeNB  200 S of the identification information related to the S-GW  300 U 2 . The identification information related to the S-GW  300 U 2  includes at least one of the “E-RAB ID”, the “Transport Layer Address” and the “GTP-TEID”. When making comparison with the “Transport Layer Address”, the T-MeNB  200 M 2  preferably notifies the SeNB  200 S of the “Transport Layer Address”. Also, when making comparison with the “GTP-TEID”, the T-MeNB  200 M 2  preferably notifies the SeNB  200 S of the “GTP-TEID”. In this regard, the T-MeNB  200 M 2  may make comparison with the “Transport Layer Address” and comparison with the “GTP-TEID”. Further, the T-MeNB  200 M 2  may notify both of the “Transport Layer Address” and the “GTP-TEID”. 
     The SeNB  200 S performs a process of receiving the identification information related to the S-GW  300 U 2  from the T-MeNB  200 M 2 . Then, the SeNB  200 S may establish a data path to the S-GW  300 U 2  based on the identification information related to the S-GW  300 U 2 . The SeNB  200 S may establish the data path to the S-GW  300 U 2  by switching to the S-GW  300 U 2  a path to the S-GW  300 U 1 . Alternatively, the SeNB  200 S may store the identification information related to the S-GW  300 U 2  as the context information of the UE  100 . 
     In step  18 , the T-MeNB  200 M 2  transmits a “UE Context Release” message indicating release of UE context information to the S-MeNB  200 M 1 . In this regard, the process in step  18  may be performed before notification of the identification information related to the S-GW  300 U 2 . 
     In step  19 , in response to the reception of the “UE Context Release” message, the S-MeNB  200 M 1  transmits a “UE Context Release” message to the SeNB  200 S. 
     Although the embodiment including step  1  to step  19  has been described. However, the present disclosure is not limited to this, and part of steps may be omitted. 
     (9) Summary of Embodiment 
     When the T-MeNB  200 M 2  according to the embodiment and the SeNB  200 S perform the dual connectivity communication with the UE  100  accompanying the handover of the UE  100 , the T-MeNB  200 M 2  notifies the SeNB  200 S of the identification information related to the S-GW  300 U 2  connected to the T-MeNB  200 M 2 . As a result, it is possible to smoothly perform dual connectivity communication after the handover. 
     Modified Example of Embodiment 
     In the above embodiment, when deciding that an S-GW (S-GW  300 U 2 ) connected to a T-MeNB  200 M 2  is different from an S-GW (S-GW  300 U 1 ) connected to an S-MeNB  200 M 1 , the T-MeNB  200 M 2  notifies the SeNB  200 S of identification information related to the S-GW  300 U 2 . 
     However, irrespectively of whether or not the S-GW connected to the T-MeNB  200 M 2  is different from the S-GW connected to the S-MeNB  200 M 1 , the T-MeNB  200 M 2  may notify the SeNB  200 S of the identification information related to the S-GW  300 U 2  notified from the MME  300 C. In this case, the T-MeNB  200 M 2  may notify the SeNB  200 S of information indicating the decision result as to whether or not the S-GW connected to the T-MeNB  200 M 2  is different from the S-GW connected to the S-MeNB  200 M 1  together with the identification information. Also, the T-MeNB  200 M 2  may omit the decision on whether or not the S-GW connected to the T-MeNB  200 M 2  is different from the S-GW connected to the S-MeNB  200 M 1 , and notify of the identification information related to the S-GW  300 U 2  notified from the MME  300 C. In that case, the SeNB  200 S may decide whether or not the identification information related to the S-GW  300 U 2  notified from the MME  300 C the identification information related to the S-GW  300 U 1  match. 
     Other Embodiments 
     The above embodiment assumes inter master base station handover which changes a master base station from an S-MeNB  200 M 1  to a T-MeNB  200 M 2  without changing an SeNB  200 S. However, instead of the inter master base station handover, the present disclosure may be applied to “Handover with SeNB Addition”. The “Handover with SeNB Addition” is a method for starting dual connectivity communication during handover although the dual connectivity communication is not performed before the handover. In the case of the “Handover with SeNB Addition”, step  5  and step  18  illustrated in  FIG. 8  are unnecessary. 
     In the above embodiment, an LTE system has been exemplified as a mobile communication system. However, the present disclosure is not limited to the LTE system. The present disclosure may be applied to systems other than the LTE system. 
     The above-described embodiment assumes that the SeNB  200 S is not changed during the inter master base station handover. However, the present disclosure is not limited to this. For example, the embodiment may include that the SeNB  200 S (SeNB  200 S 1 ) is changed to the another SeNB  200 S (SeNB  200 S 2 ).