PATENT ABSTRACT
Even when a terminal goes out of a macro cell, data communication in the small cell can be caused to continue while the mobile management in a macro cell is continued. When a first determination unit ( 205 ) determines that a terminal exists outside the cell of a base station ( 200 ), an existence recognition packet transmission/reception unit ( 207 ) transmits a recognition packet, which is used for recognition, to the terminal via a small cell, and thereafter receives a response to the recognition packet from the terminal via the small cell. When a second determination unit ( 206 ) determines, because of the reception of the response, that the terminal exits inside the cell of the base station ( 200 ), a radio resource control unit ( 204 ) causes the connection of the control plane between the terminal and the base station ( 200 ) to continue.

PATENT DESCRIPTION
TECHNICAL FIELD 
       [0001]    The present invention relates to a base station, a terminal and a communication method. 
       BACKGROUND ART 
       [0002]    “LTE-Advanced,” which is an evolved version of Long Term Evolution (LTE), is currently under study in 3GPP. In LTE-Advanced, a heterogeneous network (HetNet) is under study as a technique for improving the frequency utilization efficiency or the like. The heterogeneous network disposes a cell (small cell) which is a base station having low transmission power such as a pico cell or femt cell in a macro cell in addition to a cell arrangement centered on the macro cell which is a conventional base station having high transmission power. 
         [0003]    LTE-Advanced is proposing to separate, in a system constituting a heterogeneous network, a C-Plane (also referred to as “control plane”) from a U-Plane (also referred to as “data plane” or “user plane”) (hereinafter, referred to as “C/U separation”) (e.g., see Non-Patent Literature (hereinafter, simply referred to as “NPL”) 1). More specifically, during C/U separation, the macro cell performs movement management using the control plane to maintain connectivity and the small cell handles only the user plane using a wideband to thereby provide high throughput. 
         [0004]    Furthermore, when the user operating environment is taken into consideration, it is expected that small cells are more likely to be arranged indoors. On the other hand, macro cells are expected to maintain connectivity in a wide range under the control of control planes. Thus, indoors where a small cell is disposed, there can be a situation where while a terminal (which may also be referred to as “UE”) can receive a user plane favorably, the terminal cannot receive a control plane. That is, there can be a situation where although it is possible to perform communication using a user plane, it is possible neither to perform communication using a control plane nor to maintain connectivity. 
         [0005]    In contrast, NPL 1 discloses that both of a macro cell and a small cell include a control plane control section (RRC: radio resource control) and when it is not possible to maintain connectivity by the macro cell, the small cell transmits a control plane message to a terminal to thereby maintain connectivity. That is, the terminal continues communication even when it is outside a service area of the macro cell if it is located within the service area of the small cell and determines to be outside the communication range when links with both the macro cell and the small cell are outside the service areas. 
       CITATION LIST 
     Non-Patent Literature 
     NPL 1 
       [0006]    3GPP TR 36.842 V12.0.0 (2013-12), Study on Small Cell enhancements for E-UTRA and E-UTRAN, Higher layer Aspects 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    However, in consideration of the fact that the small cell handles only a user plane as in the aforementioned case of C/U separation, it may be possible to introduce low-priced devices dedicated to data transmission/reception as a small cell. In this case, the small cell is not provided with any control plane control section that processes a control plane message as in the case of NPL 1. Thus, when it is not possible to perform communication using a control plane between the macro cell and the terminal during C/U separation, even if communication using a user plane between the small cell and the terminal is possible, the terminal cannot receive the control plane. 
         [0008]    Furthermore, when communication between the macro cell and the terminal using the control plane is not possible within the same radio system, traditional handover to the small cell may be performed to shift to communication without C/U separation. However, when the aforementioned small cell dedicated to data transmission/reception is introduced, handover to the small cell is not possible. 
         [0009]    In the future, it is expected that the macro cell and the small cell constituting different radio systems; for example, a macro cell may constitute an ESE-Advanced system and a small cell may constitute another system such as WiFi/WiGig. However, during C/U separation, when communication between macro cell and the terminal using a control plane is possible, handover processing becomes necessary between different radio systems, causing the processing to become complicated. 
         [0010]    Thus, when the terminal is located outside the service area of the macro cell during C/U separation, there is a problem that the processing for continuing data communication while continuing movement management becomes more difficult or more complicated. 
         [0011]    An object of the present invention is to provide a base station, a terminal and a communication method capable of continuing, even when a terminal is located outside the service area of a macro cell, movement management in the macro cell and continuing data communication in a small cell in a heterogeneous network in which a macro cell performs movement management using a control plane and a small cell handles a user plane. 
       Solution to Problem 
       [0012]    A base station according to an aspect of the present invention is a base station in a macro cell that performs communication using a control plane in a communication system in which the macro cell performs communication using the control plane and a small cell performs communication using a user plane with respect to a terminal, the base station including: a first determining section that determines, when there is no response to data of the control plane transmitted to the terminal, that the terminal is located outside a service area of the base station; a transmitting and receiving section that transmits, when the first determining section determines that the terminal is located outside the service area of the base station, a confirmation packet that confirms connection between the terminal and the small cell as data of the user plane to the terminal via the small cell and receives a response to the confirmation packet as the data of the user plane from the terminal via the small cell; a second determining section that assumes, upon receiving the response, that the terminal is located inside the service area of the base station; and a control section that causes, when the second determining section assumes that the terminal is located inside the service area of the base station, the connection of the control plane between the terminal and the base station to continue. 
         [0013]    A terminal according to an aspect of the present invention is a terminal in a communication system in which a macro cell performs communication using a control plane and a small cell performs communication using a user plane with respect to the terminal, the terminal including: a first determining section that determines, when a reception level of a signal transmitted from the macro cell is less than a predetermined threshold, that the terminal is located outside a service area of the macro cell; a transmitting and receiving section that transmits, when the first determining section determines that the terminal is located outside the service area of the macro cell, a confirmation packet that confirms connection between the terminal and the small cell as data of the user plane to the macro cell via the small cell and receives a response to the confirmation packet as the data of the user plane from the macro cell via the small cell; a second determining section that assumes, upon receiving the response, that the terminal is located inside the service area of the macro cell; and a control section that causes, when the second determining section assumes that the terminal is located inside the service area of the macro cell, the connection of the control plane between the terminal and the macro cell to continue. 
         [0014]    A communication method according to an aspect of the present invention is a communication method in a base station in a macro cell that performs communication using a control plane in a communication system in which the macro cell performs communication using the control plane and a small cell performs communication using a user plane with respect to a terminal, the method including: a first determining step of determining, when there is no response to data of the control plane transmitted to the terminal, that the terminal is located outside a service area of the base station; a transmitting and receiving step of transmitting, when the first determining step determines that the terminal is located outside the service area of the base station, a confirmation packet that confirms connection between the terminal and the small cell as data of the user plane to the terminal via the small cell and receiving a response to the confirmation packet as the data of the user plane from the terminal via the small cell; a second determining step of assuming, upon receiving the response, that the terminal is located inside the service area of the base station; and a controlling step of causing, when the second determining section assumes that the terminal is located inside the service area of the base station, the connection of the control plane between the terminal and the base station to continue. 
         [0015]    A communication method according to an aspect of the present invention is a communication method in a terminal in a communication system in which a macro cell performs communication using a control plane and a small cell performs communication using a user plane with respect to the terminal, the method including: a first determining step of determining, when a reception level of a signal transmitted from the macro cell is less than a predetermined threshold, that the terminal is located outside a service area of the macro cell; a transmitting and receiving step of transmitting, when the first determining step determines that the terminal is located outside the service area of the base station, a confirmation packet that confirms connection between the terminal and the small cell as data of the user plane to the terminal via the small cell and receiving a response to the confirmation packet as the data of the user plane from the terminal via the small cell; a second determining step of assuming, upon receiving the response, that the terminal is located inside the service area of the macro cell; and a controlling step of causing, when the second determining step assumes that the terminal is located inside the service area of the macro cell, the connection of the control plane between the terminal and the macro cell to continue. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention, in a heterogeneous network in which a macro cell performs movement management using a control plane and a small cell handles a user plane, it is possible to continue, even when the terminal is located outside the service area of the macro cell, movement management in the macro cell and continue data communication in the small cell. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a diagram illustrating a configuration example of a communication system according to Embodiment 1 of the present invention; 
           [0018]      FIG. 2  is a block diagram illustrating a configuration of a terminal according to Embodiment 1 of the present invention; 
           [0019]      FIG. 3  is a block diagram illustrating a configuration of a macro cell and a small cell according to Embodiment 1 of the present invention; 
           [0020]      FIG. 4  is a flowchart illustrating an operation of the terminal according to Embodiment 1 of the present invention; 
           [0021]      FIG. 5  is a flowchart illustrating an operation of the base station according to Embodiment 1 of the present invention; 
           [0022]      FIG. 6  is a sequence diagram illustrating an operation of the communication system according to Embodiment 1 of the present invention; 
           [0023]      FIG. 7  is a diagram illustrating protocol stacks of the macro cell and the small cell according to Embodiment 1 of the present invention; 
           [0024]      FIG. 8  is a block diagram illustrating a configuration of a terminal according to Embodiment 2 of the present invention; 
           [0025]      FIG. 9  is a block diagram illustrating a configuration of a macro cell and a small according to Embodiment 2 of the present invention; 
           [0026]      FIG. 10  is a flowchart illustrating an operation of the terminal according to Embodiment 2 of the present invention; 
           [0027]      FIG. 11  is a flowchart illustrating an operation of the base station according to Embodiment 2 of the present invention; 
           [0028]      FIG. 12  is a sequence diagram illustrating an operation of a communication system according to Embodiment 2 of the present invention; and 
           [0029]      FIG. 13  is a diagram illustrating protocol stacks of the macro cell, the small cell and the terminal according to Embodiment 2 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
     [Overview of Communication System] 
       [0031]    A communication system according to the present embodiment includes, as shown in  FIG. 1 , terminal  100 , base station  200  in a macro cell and base station  300  in a small cell. In the communication system, the macro cell controls a control plane (C-plane) to perform movement management and the small cell handles only a user plane (U-plane). That is, C/U separation is applied to the communication system shown in  FIG. 1 , in which base station  200  performs communication with terminal  100  using the control plane and base station  300  performs communication using the user plane. 
         [0032]    A case will be described below where a radio system of the macro cell (base station  200 ) (hereinafter referred to as “first radio communication system”) is different from a radio system of the small cell (base station  300 ) (hereinafter referred to as “second radio communication system”). For example, the macro cell supports an LTE-Advanced system and the small cell supports a radio system such as WiFi/WiGig. Terminal  100  adopts a configuration communicable in both the radio system of the macro cell and the radio system of the small cell. 
         [0033]    Note that, the present embodiment is suitable for not only a case where the radio system of the macro cell is different from the radio system of the small cell in the communication system shown in  FIG. 1  but also a case where the radio system of the macro cell is the same as the radio system of the small cell and the small cell is a base station that can communicate using only the user plane, for example. 
       [Configuration of Terminal  100 ] 
       [0034]      FIG. 2  is a block diagram illustrating a configuration of terminal  100  according to the present embodiment. In  FIG. 2 , terminal  100  includes receiving section  101 , transmitting section  102 , receiving-quality measuring section  103 , first determining section  104 , radio-resource control section  105 , second determining section  106 , keep-alive-packet transmitting/receiving section  107 , application section  108 , data processing section  109 , receiving section  110 , transmitting section  111 , receiving-quality measuring section  112  and data processing section  113 . 
         [0035]    In terminal  100 , receiving section  101 , receiving-quality measuring section  103 , data processing section  109  and transmitting section  102  constitute first-communication processing section  150  corresponding to a first radio system identical to that of the macro cell. On the other hand, receiving section  110 , receiving-quality measuring section  112 , data processing section  113  and transmitting section  111  constitute second-communication processing section  151  corresponding to a second radio system identical to that of the small cell (that is, different from the macro cell). 
         [0036]    Receiving section  101  demodulates a received signal received via an antenna and restores the signal. Examples of the received signal include a data signal and a reference signal transmitted from base station  200  (macro cell). During C/U separation, the received signal includes only control plane related data (control plane data). Receiving section  101  outputs the data signal or reference signal to receiving-quality measuring section  103  and data processing section  109 . 
         [0037]    Transmitting section  102  modulates a signal received from data processing section  109  and transmits the modulated signal via an antenna. For example, during C/U separation, the transmission signal includes only control plane data. 
         [0038]    Receiving-quality measuring section  103  measures receiving quality (e.g., reception level) of a signal transmitted from base station  200  using a reference signal received from receiving section  101  and outputs a receiving-quality measured value to first determining section  104 . Receiving-quality measuring section  103  may also measure an error rate (e.g., rate of occurrence of NG in CRC) of received data (restored information) as the receiving-quality measured value. 
         [0039]    First determining section  104  determines whether or not terminal  100  is located in a service area of the cell (e.g., base station  200  which is a macro cell) currently being measured based on the receiving-quality measured value received from receiving-quality measuring section  103 . For example, when the reception level of a signal transmitted from base station  200  is less than a predetermined threshold, first determining section  104  determines that terminal  100  is located outside the service area of base station  200 . First determining section  104  outputs the determination result (inside or outside the service area) to radio-resource control section  105  and second determining section  106 . 
         [0040]    Radio-resource control section  105  controls establishment of a connection of radio resources between terminal  100  and base station  200  (macro cell) or base station  300  (small cell) based on the control plane data received from data processing section  109  and the determination result received from first determining section  104 . Radio-resource control section  105  outputs the control plane data including control information on the connection establishment to data processing section  109 . For example, when the determination result from first determining section  104  shows that the terminal is located inside the service area, radio-resource control section  105  continues the connection with the cell as the determination target and cuts the connection with the determination target cell when the terminal is located outside the service area. 
         [0041]    During C/U separation, when the determination result on base station  200  (macro cell) received from first determining section  104  shows that the terminal is outside the service area, radio-resource control section  105  controls establishment of a connection of radio resources with the macro cell based on the determination result (re-determination result) received from second determining section  106 . More specifically, when the determination result received from second determining section  106  shows that the terminal is inside the service area, radio-resource control section  105  continues the connection with the macro cell (base station  200 ) and cuts the connection with the macro cell when the terminal is outside the service area. 
         [0042]    During C/U separation, second determining section  106  determines whether or not terminal  100  is located inside the service area of the small cell (base station  300 ) based on the receiving-quality measured value (receiving quality of signal transmitted from the small cell) received from receiving-quality measuring section  112  of second-communication processing section  151 . Furthermore, second determining section  106  receives the determination result (inside or outside the service area) on the macro cell from first determining section  104 . 
         [0043]    When terminal  100  is located outside the service area of the macro cell and terminal  100  is inside the service area of the small cell, second determining section  106  requests keep-alive-packet transmitting/receiving section  107  to transmit a keep-alive packet. Upon receiving a notice that a response (ACK) to the requested keep-alive packet has been received from keep-alive-packet transmitting/receiving section  107 , second determining section  106  determines that terminal  100  is inside the service area of the macro cell. On the other hand, upon receiving a notice that no response to the requested keep-alive packet has been received for a predetermined period from keep-alive-packet transmitting/receiving section  107 , second determining section  106  determines that terminal  100  is outside the service area of the macro cell. That is, upon receiving a response to the keep-alive packet, although second determining section  106  determines, based on the receiving quality, that terminal  100  is outside the service area of the macro cell, second determining section  106  assumes that terminal  100  is located inside the service area of the macro cell. Second determining section  106  outputs the determination result (re-determination result) to radio-resource control section  105 . 
         [0044]    Keep-alive-packet transmitting/receiving section  107  generates a keep-alive packet at the request from second determining section  106  and outputs the keep-alive packet to data processing section  109 . The keep-alive packet is a packet to confirm a connection between terminal  100  and the small cell, and is transmitted to the macro cell (base station  200 ) via the small cell (base station  300 ) as user plane-related data (user plane data). That is, when first determining section  104  determines that terminal  100  is located outside the service area of the macro cell, keep-alive-packet transmitting/receiving section  107  transmits a keep-alive packet that confirms the connection between terminal  100  and the small cell to the macro cell via the small cell as user plane data. While a response to the keep-alive packet is returned from base station  200 , keep-alive-packet transmitting/receiving section  107  may, for example, periodically transmit keep-alive packets. 
         [0045]    Keep-alive-packet transmitting/receiving section  107  receives a response (ACK) to the keep-alive packet transmitted to the macro cell from macro cell (base station  200 ) via the small cell (base station  300 ) as user plane data. Keep-alive-packet transmitting/receiving section  107  confirms whether or not a response (ACK) to the transmitted keep-alive packet is received and outputs the confirmation result (the presence or absence of a response) to second determining section  106 . 
         [0046]    Upon receiving the keep-alive packet transmitted from the macro cell from data processing section  109 , keep-alive-packet transmitting/receiving section  107  generates a response (ACK) to the keep-alive packet and outputs the response to the keep-alive packet to data processing section  109 . The response to the keep-alive packet is transmitted to the macro cell via the small cell as user plane data. 
         [0047]    Application section  108  processes normal application data (user plane data). Application section  108  processes a signal (keep-alive packet or response to the keep-alive packet) received from keep-alive-packet transmitting/receiving section  107  via data processing section  109  in the same way as normal application data and outputs the processed data (user plane data) to data processing section  109 . Application section  108  processes a signal (keep-alive packet or response to the keep-alive packet) received by data processing section  109  from the macro cell or the small cell in a manner similar to that of normal application data, and outputs the processed data to keep-alive-packet transmitting/receiving section  107  via data processing section  109 . 
         [0048]    Data processing section  109  outputs the control plane data out of the information received from receiving section  101  or data processing section  113  to radio-resource control section  105  and outputs the user plane data to application section  108 . Furthermore, data processing section  109  multiplexes the control plane data received from radio-resource control section  105  and the user plane data received from application section  108  and outputs the multiplexed signal to transmitting section  102 . 
         [0049]    During C/U separation, data processing section  109  outputs the user plane data received from application section  108  to data processing section  113  of second-communication processing section  151  and outputs the user plane data received from data processing section  113  to application section  108 . During C/U separation, data processing section  109  outputs the control plane data received from radio-resource control section  105  to transmitting section  102  and outputs the control plane data received from receiving section  101  to radio-resource control section  105 . 
         [0050]    Receiving section  110  demodulates a received signal received via an antenna and restores the signal. Examples of the received signal include a data signal and a reference signal transmitted from base station  300  (small cell). During C/U separation, the received signal includes only the user plane data. Receiving section  110  outputs the data signal or reference signal to receiving-quality measuring section  112  and data processing section  113 . 
         [0051]    Transmitting section  111  modulates a signal received from data processing section  113  and transmits the modulated signal via an antenna. For example, during C/U separation, the transmission signal includes only user plane data. 
         [0052]    As in the case of receiving-quality measuring section  103 , receiving-quality measuring section  112  measures receiving quality (e.g., reception level) of a signal transmitted from base station  300  using a reference signal received from receiving section  110  and outputs the receiving-quality measured value to second determining section  106 . 
         [0053]    Data processing section  113  outputs the signal received from receiving section  110  to data processing section  109 . Data processing section  113  outputs the signal received from data processing section  109  to transmitting section  111 . For example, during C/U separation, data processing section  113  outputs the user plane data received from data processing section  109  to transmitting section  111  and outputs the user plane data received from receiving section  110  to data processing section  109 . 
       [Configurations of Base Station  200  and Base Station  300 ] 
       [0054]      FIG. 3  is a block diagram illustrating configurations of base station  200  (macro cell) and base station  300  (small cell) according to the present embodiment. For example, base station  200  supports a first radio system and base station  300  supports a second radio system. 
       [Configuration of Macro Cell (Base Station  200 )] 
       [0055]    In  FIG. 3 , base station  200  includes receiving section  201 , transmitting section  202 , data processing section  203 , radio-resource control section  204 , first determining section  205 , second determining section  206 , keep-alive-packet transmitting/receiving section  207 , application section  208 , inter-base-station I/F control section  209 . 
         [0056]    Receiving section  201  demodulates a received signal received via an antenna and restores the signal. For example, during C/U separation, the received signal includes only control plane data. Receiving section  201  outputs the restored signal to data processing section  203 . 
         [0057]    Transmitting section  202  modulates a signal received from data processing section  203  and transmits the modulated signal via an antenna. For example, during C/U separation, the transmission signal includes only control plane data. 
         [0058]    Data processing section  203  outputs control plane data out of information received from receiving section  201  or inter-base-station I/F control section  209  to radio-resource control section  204  and outputs user plane data to application section  208 . Data processing section  203  multiplexes the control plane data received from radio-resource control section  204  and the user plane data received from application section  208  and outputs the multiplexed signal to transmitting section  202 . 
         [0059]    During C/U separation, data processing section  203  transmits user plane data received from application section  208  to base station  300  via inter-base-station I/F control section  209  and outputs user plane data received from base station  300  via inter-base-station I/F control section  209  to application section  208 . During C/U separation, data processing section  203  outputs control plane data received from radio-resource control section  204  to transmitting section  202  and outputs control plane data received from receiving section  201  to radio-resource control section  204 . 
         [0060]    Radio-resource control section  204  generates signaling (control plane data) relating to control of radio resources and transmits the signaling to terminal  100  via data processing section  203 . Radio-resource control section  204  outputs the presence or absence of a response (ACK) to the signaling to first determining section  205 . 
         [0061]    Radio-resource control section  204  controls establishment of a connection of radio resources between terminal  100  and base station  200  based on the control plane data received from data processing section  203  and determination result (whether or not terminal  100  is located inside the service area of base station  200 ) received from first determining section  205 . Radio-resource control section  204  outputs control plane data including control information relating to connection establishment to data processing section  203 . More specifically, radio-resource control section  204  continues the connection between terminal  100  and base station  200  when the determination result from first determining section  205  shows that terminal  100  is located inside the service area and cuts the connection between terminal  100  and base station  200  when the determination result shows that terminal  100  is located outside the service area. 
         [0062]    During C/U separation, when the determination result on base station  200  (macro cell) received from first determining section  205  shows that terminal  100  is outside the service area, radio-resource control section  204  controls establishment of a connection of radio resources between terminal  100  and base station  200  based on the determination result (re-determination result) received from second determining section  206 . More specifically, when the determination result received from second determining section  206  shows that terminal  100  is inside the service area, radio-resource control section  204  continues the connection between terminal  100  and base station  200 , and cuts the connection between terminal  100  and base station  200  when the determination result shows that terminal  100  is outside the service area. 
         [0063]    First determining section  205  determines whether terminal  100  is inside or outside the service area of base station  200  using data about the presence or absence of a response to signaling (control plane data) received from radio-resource control section  204 . For example, when a period during which radio-resource control section  204  receives no response exceeds a predetermined period (upon expiration of the timer), first determining section  205  determines that terminal  100  is not in the service area of base station  200  (outside the service area). On the other hand, when radio-resource control section  204  receives a response within a predetermined period, first determining section  205  determines that terminal  100  is inside the service area of base station  200 . First determining section  205  outputs the determination result (inside or outside the service area) to radio-resource control section  204  and second determining section  206 . 
         [0064]    When the determination result on base station  200  received from first determining section  205  shows that terminal  100  is outside the service area, second determining section  206  requests keep-alive-packet transmitting/receiving section  207  to transmit a keep-alive packet. Upon receiving a notice that a response (ACK) to the requested keep-alive packet has been received from keep-alive-packet transmitting/receiving section  207 , second determining section  206  determines that terminal  100  is inside the service area of base station  200 . On the other hand, upon receiving a notice that no response to the requested keep-alive packet has been received for a predetermined period from keep-alive-packet transmitting/receiving section  207 , second determining section  206  determines that terminal  100  is outside the service area of base station  200 . That is, upon receiving a response to the keep-alive packet, although second determining section  206  determines, based on the presence or absence of a response to the control plane data, that terminal  100  is outside the service area of base station  200 , second determining section  206  assumes that terminal  100  is located inside the service area of base station  200 . Second determining section  206  outputs the determination result (re-determination result) to radio-resource control section  204 . 
         [0065]    At the request from second determining section  206 , keep-alive-packet transmitting/receiving section  207  generates a keep-alive packet and outputs the keep-alive packet to data processing section  203 . The keep-alive packet is transmitted to terminal  100  via the small cell (base station  300 ) as user plane data. That is, when first determining section  205  determines that terminal  100  is located outside the service area of base station  200 , keep-alive-packet transmitting/receiving section  207  transmits a keep-alive packet that confirms the connection between terminal  100  and the small cell to terminal  100  via the small cell as user plane data. For example, keep-alive-packet transmitting/receiving section  207  may periodically transmit keep-alive packets while a response to the keep-alive packet is returned from terminal  100 . 
         [0066]    Keep-alive-packet transmitting/receiving section  207  receives a response (ACK) to the keep-alive packet transmitted to terminal  100  from terminal  100  via the small cell (base station  300 ) as user plane data. Keep-alive-packet transmitting/receiving section  207  confirms whether or not a response (ACK) to the transmitted keep-alive packet is received and outputs the confirmation result (presence or absence of a response) to second determining section  206 . 
         [0067]    Upon receiving the keep-alive packet transmitted from terminal  100  from data processing section  203 , keep-alive-packet transmitting/receiving section  207  generates a response (ACK) to the keep-alive packet and outputs the response to data processing section  203 . The response to the keep-alive packet is transmitted to terminal  100  via the small cell as the user plane data. 
         [0068]    Application section  208  processes normal application data (user plane data). Application section  208  processes the signal (keep-alive packet or response to the keep-alive packet) received from keep-alive-packet transmitting/receiving section  207  via data processing section  203  in a manner similar to that of the normal application data and outputs the processed data to data processing section  203 . On the other hand, application section  208  processes the signal (keep-alive packet or response to the keep-alive packet) received by data processing section  203  from terminal  100  via the small cell in a manner similar to that of the normal application data and outputs the processed data to keep-alive-packet transmitting/receiving section  207  via data processing section  203 . 
         [0069]    Inter-base-station I/F control section  209  controls an interface that performs communication between base station  200  and base station  300 . For example, inter-base-station I/F control section  209  performs inter-base-station transfer using an IP (internet protocol) layer. 
         [0070]    Note that a case has been described in  FIG. 2  and  FIG. 3  where terminal  100  is provided with second determining section  106  and keep-alive-packet transmitting/receiving section  107 , and base station  200  (macro cell) is provided with second determining section  206  and keep-alive-packet transmitting/receiving section  207 . That is, in  FIG. 2  and  FIG. 3 , both terminal  100  and base station  200  have configurations capable of transmitting a keep-alive packet. However, in the present embodiment, only one of terminal  100  and base station  200  may transmit a keep-alive packet and the other may have a configuration or operation capable of transmitting only a response to the keep-alive packet. 
       [Configuration of Base Station  300  (Small Cell)] 
       [0071]    In  FIG. 3 , base station  300  includes receiving section  301 , transmitting section  302 , data processing section  303  and inter-base-station I/F control section  304 . 
         [0072]    Receiving section  301  demodulates a received signal received via an antenna and restores the signal. For example, during C/U separation, the received signal includes only user plane data transmitted from terminal  100 . Receiving section  301  outputs the restored signal to data processing section  303 . 
         [0073]    Transmitting section  302  modulates the signal received from data processing section  303  and transmits the modulated signal via an antenna. During C/U separation, the transmission signal includes only user plane data. 
         [0074]    Data processing section  303  transmits the signal received from receiving section  301  to base station  200  via inter-base-station I/F control section  304 . Data processing section  303  outputs a signal received from base station  200  to transmitting section  302  via inter-base-station I/F control section  304 . For example, during C/U separation, data processing section  303  outputs user plane data received from base station  200  to transmitting section  302  and outputs user plane data received from receiving section  301  to base station  200 . 
         [0075]    Inter-base-station I/F control section  304  controls the interface that performs communication between base station  300  and base station  200 . For example, inter-base-station I/F control section  304  performs inter-base-station transfer using an IP layer. 
       [Operations of Terminal  100  and Base Station  200 ] 
       [0076]    Operations of terminal  100  and base station  200  having the above-described configurations will be described. 
         [0077]      FIG. 4  is a flowchart illustrating a processing flow when terminal  100  transmits a keep-alive packet and base station  200  returns a keep-alive packet.  FIG. 5  is a flowchart illustrating a processing flow when base station  200  transmits a keep-alive packet and terminal  100  returns a response to the keep-alive packet. 
         [0078]    Note that terminal  100  and base station  200  may perform processes shown in  FIG. 4  and  FIG. 5  simultaneously or may perform only processes on one side. That is, only one of terminal  100  and base station  200  may transmit a keep-alive packet and the other may return a response to the keep-alive packet. 
         [0079]    First, a case shown in  FIG. 4  where terminal  100  transmits a keep-alive packet will be described. 
         [0080]    In step (hereinafter simply denoted as “ST”)  101 , terminal  100  determines whether or not the reception level of a signal transmitted from base station  200  (macro cell) (e.g., receiving-quality measured value) satisfies a predetermined condition (reception level condition such as a predetermined threshold). When the reception level satisfies the predetermined condition (ST 101 : Yes, that is, when terminal  100  is inside the service area of base station  200 ), terminal  100  returns to the process in ST 101 . 
         [0081]    On the other hand, when the reception level does not satisfy the predetermined condition (ST 101 : No, that is, terminal  100  is outside the service area of base station  200 ), terminal  100  determines in ST 102  whether or not the communication mode with respect to terminal  100  is in a C/U separation state. For example, terminal  100  is notified in advance of whether C/U separation is applied or not through signaling of a higher layer. When a C/U separation state is in progress (ST 102 : Yes), terminal  100  proceeds to a process in ST 103  and when a C/U separation state is not in progress (ST 102 : No), terminal  100  proceeds to a process in ST 109 . 
         [0082]    In ST 103 , terminal  100  determines whether or not terminal  100  is located inside the service area of base station  300  (small cell) (whether or not terminal  100  satisfies the condition for terminal  100  to be located inside the service area of the small cell). For example, when the reception level (receiving-quality measured value) of a signal transmitted from base station  300  satisfies a predetermined threshold, terminal  100  determines that terminal  100  is located inside the service area of base station  300 . When terminal  100  is located inside the service area of the small cell (ST 103 : Yes), terminal  100  proceeds to a process in ST 104  and proceeds to a process in ST 109  when terminal  100  is not located inside the service area of the small cell (ST 103 : No). 
         [0083]    In ST 104 , terminal  100  stops communication (communication of control plane data) with base station  200  while maintaining the connection with base station  200  (macro cell). 
         [0084]    In ST 105 , terminal  100  transmits a keep-alive packet to base station  200 . Note that since direct communication is not possible between terminal  100  and base station  200 , terminal  100  transmits a keep-alive packet to base station  200  via base station  300  as user plane data. 
         [0085]    Upon receiving the keep-alive packet from terminal  100  in ST 105 , base station  200  returns a response (ACK) to the keep-alive packet to terminal  100  via base station  300  as user plane data. That is, when communication is possible between terminal  100  and base station  300  (small cell), ACK is returned from base station  200  to terminal  100 . 
         [0086]    In ST 106 , terminal  100  determines whether or not the response (ACK) to the keep-alive packet transmitted in ST 105  has been received within a predetermined period. When terminal  100  has received ACK (ST 106 : Yes), terminal  100  proceeds to a process in ST 107  and when terminal  100  has not received ACK (ST 106 : No), terminal  100  proceeds to a process in ST 109 . 
         [0087]    In ST 107 , terminal  100  determines to continue the connection state (connection) with base station  200  (macro cell). That is, although communication with base station  200  is actually stopped, if the response to the keep-alive packet is received in ST 106 , that is, communication between terminal  100  and base station  300  is possible, terminal  100  assumes that terminal  100  is located inside the service area of base station  200  and continues the connection with base station  200 . 
         [0088]    In ST 108 , terminal  100  determines whether the elapsed time of the timer that has started counting after the process in ST 107  exceeds a predetermined period or not (expiration of the timer or not). Upon expiration of the timer (ST 108 : Yes), terminal  100  returns to the process in ST 101 . That is, terminal  100  continues the connection with the macro cell until the timer expires (ST 108 : during “No”). 
         [0089]    When C/U separation is not applied (ST 102 : No) or when terminal  100  is outside the service areas of both the macro cell and the small cell (ST 103 : No or ST 106 : No), terminal  100  cuts communication with base station  200  in ST 109  and transitions to outside the service area of base station  200  (inside a service area of another cell). 
         [0090]    Next, a case shown in  FIG. 5  where base station  200  transmits a keep-alive packet will be described. 
         [0091]    In ST 201 , base station  200  determines whether or not a response to control plane data (message) transmitted by base station  200  is received from terminal  100 . When a response is received (ST 201 : Yes, that is, terminal  100  is inside the service area of base station  200 ), base station  200  returns to the process in ST 201 . 
         [0092]    On the other hand, when no response is received (ST 201 : No, that is, terminal  100  is outside the service area of base station  200 ), base station  200  determines in ST 202  whether or not the communication mode with respect to terminal  100  is a C/U separation state. Base station  200  knows beforehand whether C/U separation is applicable to terminal  100  or not. Base station  200  proceeds to a process in ST 203  when the C/U separation state is in progress (ST 202 : Yes) and proceeds to a process in ST 208  when the C/U separation state is not in progress (ST 202 : No). 
         [0093]    In ST 203 , base station  200  stops communication (communication of control plane data) with terminal  100  while maintaining the connection state with terminal  100 . 
         [0094]    In ST 204 , base station  200  transmits a keep-alive packet to terminal  100 . Note that since direct communication between terminal  100  and base station  200  is not possible, base station  200  transmits a keep-alive packet to terminal  100  via base station  300  as user plane data. 
         [0095]    Upon receiving the keep-alive packet from base station  200  in ST 204 , terminal  100  returns a response (ACK) to the keep-alive packet to base station  200  via base station  300  as user plane data. That is, when communication between terminal  100  and base station  300  (small cell) is possible, ACK is returned from terminal  100  to base station  200 . 
         [0096]    In ST 205 , base station  200  determines Whether or not a response (ACK) to the keep-alive packet transmitted in ST 204  has been received within a predetermined period. Upon receiving ACK (ST 205 : Yes), base station  200  proceeds to a process in ST 206  and proceeds to a process in ST 208  upon receiving no ACK (ST 205 : No). 
         [0097]    In ST 206 , base station  200  determines to continue the connection state with terminal  100 . That is, although communication with terminal  100  is actually stopped, if a response to the keep-alive packet is received in ST 205 , that is, when communication between terminal  100  and base station  300  is possible, base station  200  assumes that terminal  100  is inside the service area of base station  200  and base station  200  continues the connection with terminal  100 . 
         [0098]    In ST 207 , base station  200  determines whether or not the elapsed time of the timer that has started counting after a process in ST 206  exceeds a predetermined period (expiration of the timer or not). Upon expiration of the timer (ST 207 : Yes), base station  200  returns to the process in ST 201 . That is, base station  200  continues the connection with terminal  100  until the timer expires (ST 207 : during “No”). 
         [0099]    When C/U separation is not applied (ST 202 : No) or when terminal  100  is outside the service areas of both the macro cell and the small cell (ST 205 : No), base station  200  cuts communication with terminal  100  in ST 208  and causes terminal  100  to transition to outside the service area of base station  200  (inside the service area of another cell). 
         [0100]    As described above, when terminal  100  is located outside the service area of the macro cell (base station  200 ), terminal  100  and/or base station  200  transmit(s) a keep-alive packet, as long as there is a response (ACK) to the keep-alive packet, base station  200  assumes that terminal  100  is located inside the service area of base station  200  and continues communication using a user plane while maintaining the connection of the control plane. When communication between terminal  100  and base station  200  becomes possible while continuing the connection of the control plane, terminal  100  and base station  200  resume communication of control plane data (C-Plane). 
         [0101]    Next,  FIG. 6  is a sequence diagram illustrating exchange of signals among terminal  100 , base station  200  (macro cell) and base station  300  (small cell). 
         [0102]    In  FIG. 6 , C/U separation is applied to terminal  100 , base station  200  performs communication using a control plane and base station  300  performs communication using a user plane. 
         [0103]    In ST 11 , base station  200  transmits a control plane message to terminal  100 . However, the control plane message does not reach terminal  100  in ST 11 . 
         [0104]    In this case, base station  200  cannot receive a response to the control plane message transmitted in ST 11  from terminal  100 . Thus, base station  200  determines that terminal  100  is located outside the service area of base station  200 . Terminal  100  determines that the reception level of the signal from base station  200  does not satisfy a predetermined condition. Here, suppose terminal  100  is located inside the service area of base station  300 . Thus, in ST 12 , terminal  100  detects that terminal  100  is located inside the service area of base station  300  and located outside the service area of base station  200 . 
         [0105]    In ST 13  and ST 14 , base station  200  and terminal  100  stop communication of the control plane while continuing communication of the user plane. 
         [0106]    In ST 15 , base station  200  transmits a keep-alive packet to terminal  100  via base station  300 . Upon receiving a keep-alive packet in ST 15 , terminal  100  transmits a response to the keep-alive packet to base station  200  via base station  300  in ST 16 . 
         [0107]    Note that the macro cell transmits a keep-alive packet in  FIG. 6  as an example, but terminal  100  may also transmit a keep-alive packet and base station  200  may transmit a response to the keep-alive packet in ST 15  and ST 16 . 
         [0108]    Upon receiving the response to the keep-alive packet in ST 16 , base station  200  determines in ST 17  that communication is possible using the user plane between terminal  100  and base station  300 . In this case, although base station  200  cannot communicate with terminal  100  using the control plane, base station  200  maintains the connection using the control plane. Thus, even when terminal  100  is located outside the service area of base station  200  which is a macro cell, base station  200  continues movement management on terminal  100 , and base station  300  which is a small cell can continue data communication. 
         [0109]    While communication using the control plane between base station  200  and terminal  100  is stopped but the connection is continued, if it is detected that terminal  100  is located again inside the service area of base station  200  (ST 18 ), terminal  100  resumes communication using the control plane in ST 19 . 
         [0110]      FIG. 7  is a diagram provided for describing exchange of information using protocol stacks between base station  200  and base station  300  (small cell) while communication using the control plane between base station  200  (macro cell) and terminal  100  is stopped but the connection is continued. 
         [0111]    Note that in  FIG. 7 , base station  200  applies, for example, an LTE-Advanced system. In this case, base station  200  includes, for the control plane, at least an RF (radio frequency) layer, a PHY (physical) layer, a MAC (medium access control) layer, an RLC (radio link control) layer, a PDCP (packet data control protocol) layer and an RRC (radio resource control) layer. Furthermore, base station  200  includes at least a PDCP layer for the user plane. The RF layer and the PHY layer correspond to layer 1 (L1), the MAC layer, the RLC layer and the PDCP layer correspond to layer 2 (L2) and the RRC layer corresponds to layer 3 (L3). For example, in base station  200  shown in  FIG. 3 , receiving section  201  and transmitting section  202  correspond to layer 1, data processing section  203  corresponds to layer 2, and first determining section  205  and second determining section  206  correspond to layer 3. 
         [0112]    In  FIG. 7 , base station  300  applies a WiGig system as a radio system different from base station  200 . In this case, base station  300  includes at least an RF layer, a PHY layer, a MAC layer, and an LLC (logical link control) layer for the user plane. For example, in base station  300  shown in  FIG. 3 , receiving section  301  and transmitting section  302  correspond to the RF layer and PHY layer and data processing section  303  corresponds to the MAC layer and the LLC layer. Note that a case will be described in  FIG. 7  where the small cell applies a radio system different from that of the macro cell, but the small cell is not limited to this, and the small cell may be a WiFi system or have a configuration in the same radio system as that of the macro cell in which only communication using a user plane can be implemented. 
         [0113]    In  FIG. 7 , a core network (CN) is provided with MME/S-GW (mobility management entity/serving gateway). 
         [0114]    For example, when it is determined that terminal  100  is outside the service area of base station  200 , base station  200  generates a keep-alive packet in the IP layer and transmits the keep-alive packet generated using the user plane. Base station  200  receives a response to the keep-alive packet using the user plane. That is, terminal  100  and base station  200  exchange keep-alive packets via the small cell and using the user plane. Thus, when base station  200  can obtain a response to the keep-alive packet, it is determined that communication is possible between terminal  100  and base station  300 . 
         [0115]    At this time, as long as a response to the keep-alive packet is obtained, base station  200  continues the connection of the control plane. More specifically, as shown in  FIG. 7 , although base station  200  stops communication using the control plane, base station  200  does not report to the core network (e.g., MME) that terminal  100  is outside the service area, but keeps the control plane “attached.” When communication between terminal  100  and base station  200  is made possible again by continuation of the connection of the control plane, terminal  100  and base station  200  are allowed to resume transmission/reception of control plane data without performing further connection processing. 
         [0116]    By this means, according to the present embodiment, when the terminal is outside the service area of the macro cell during C/U separation, the terminal and the macro cell transmit/receive a keep-alive packet using the user plane. When a response to the keep-alive packet is obtained, the terminal and the macro cell continue the connection of the control plane between the terminal and the macro cell. That is, as long as communication is possible between the terminal and the small cell using the user plane, it is possible to maintain the connection between the terminal and the macro cell even when communication between the terminal and the macro cell is not possible. 
         [0117]    Transmission/reception of a keep-alive packet between the terminal and the macro cell is performed using the user plane. Thus, even when the macro cell and the small cell are mutually different radio systems, the terminal and the macro cell can perform control to maintain the connection between the terminal and the macro cell using the user plane. Similarly, even when the macro cell and the small cell are the same radio system, and the small cell is provided with only a communication function using a user plane, the terminal and the macro cell can likewise perform control to maintain the connection between the terminal and the macro cell using the user plane. 
         [0118]    As described above, according to the present embodiment, in a heterogeneous network in which a macro cell performs movement management using a control plane and a small cell handles a user plane, even when the macro cell is located outside the service area, it is possible to continue data communication in the small cell while continuing movement management in the macro cell. 
       Embodiment 2 
     [Configuration of Terminal  400 ] 
       [0119]      FIG. 8  is a block diagram illustrating a configuration of terminal  400  according to the present embodiment. Note that in  FIG. 8 , components identical to those in Embodiment 1 ( FIG. 2 ) are assigned identical reference numerals and the description thereof will be omitted. 
         [0120]    In terminal  400  shown in  FIG. 8 , when first determining section  104  determines that terminal  400  is outside the service area of a macro cell (base station  500  which will be described later) and that terminal  400  is inside the service area of a small cell (base station  300 ), second determining section  106  instructs C-Plane encapsulated packet transmitting/receiving section  401  to encapsulate control plane data. 
         [0121]    Upon receiving a notice that a response (ACK) to the encapsulated control plane data is received from C-Plane encapsulated packet transmitting/receiving section  401 , second determining section  106  re-determines that terminal  100  is inside the service area of the macro cell. On the other hand, upon receiving a notice that no response to the encapsulated control plane data is received for a predetermined period from C-Plane encapsulated packet transmitting/receiving section  401 , second determining section  106  re-determines that terminal  100  is outside the service area of the macro cell. That is, upon receiving a response to the encapsulated control plane data, second determining section  106  assumes, based on receiving quality, that terminal  400  is located inside the service area of the macro cell although terminal  400  is determined to be outside the service area of the macro cell. Second determining section  106  outputs the determination result (re-determination result) to radio-resource control section  105 . 
         [0122]    Upon receiving an instruction for encapsulation from second determining section  106 , C-Plane encapsulated packet transmitting/receiving section  401  receives control plane data from radio-resource control section  105 . C-Plane encapsulated packet transmitting/receiving section  401  encapsulates (tunneling) the received control plane data as an IP packet and outputs the encapsulated data to application section  108 . C-Plane encapsulated packet transmitting/receiving section  401  extracts control plane data from the IP packet (encapsulated data) received from application section  108  and outputs the control plane data to radio-resource control section  105 . Upon receiving a response (ACK) to the control plane data as the IP packet from application section  108  or upon receiving a response to the control plane data from radio-resource control section  105 , C-Plane encapsulated packet transmitting/receiving section  401  outputs a notice that a response to the control plane data is received to second determining section  106 . 
         [0123]    Thus, C-Plane encapsulated packet transmitting/receiving section  401  encapsulates the control plane data, transmits the encapsulated control plane data (IP packet) as a confirmation packet for confirming a connection between terminal  400  and the small cell and receives a response to the control plane data encapsulated in the macro cell. 
         [0124]    During C/U separation, when the determination result on base station  500  (macro cell) received from first determining section  104  shows that terminal  400  is outside the service area, radio-resource control section  105  controls establishment of a connection of radio resources with the macro cell based on the determination result (re-determination result) received from second determining section  106 . More specifically, radio-resource control section  105  continues the connection with the macro cell (base station  500 ) when the determination result received from second determining section  106  shows that terminal  400  is inside the service area and disconnects the connection with the macro cell when the determination result shows that terminal  400  is outside the service area. 
         [0125]    During the continuation of the connection with the macro cell (base station  500 ), radio-resource control section  105  outputs control plane data directed to base station  500  to C-Plane encapsulated packet transmitting/receiving section  401  and receives control plane data from base station  500  from C-Plane encapsulated packet transmitting/receiving section  401  based on the determination result of second determining section  106 . 
         [0126]    Application section  108  outputs the encapsulated data received from C-Plane encapsulated packet transmitting/receiving section  401  to data processing section  109  as user plane data. Furthermore, upon receiving user plane data including the encapsulated data from data processing section  109 , application section  108  outputs the data to C-Plane encapsulated packet transmitting/receiving section  401 . 
       [Configuration of Macro Cell (Base Station  500 )] 
       [0127]      FIG. 9  is a block diagram illustrating a configuration of base station  500  according to the present embodiment. Note that in  FIG. 9 , components identical to those in Embodiment 1 ( FIG. 3 ) are assigned identical reference numerals and the description thereof will be omitted. 
         [0128]    In base station  500  shown in  FIG. 9 , when first determining section  205  determines that terminal  400  is not located in the service area of base station  500 , second determining section  206  instructs C-Plane encapsulated packet transmitting/receiving section  501  to encapsulate control plane data. 
         [0129]    Upon receiving a notice that a response (ACK) to the encapsulated control plane data is received from C-Plane encapsulated packet transmitting/receiving section  501 , second determining section  206  re-determines that terminal  100  is located in the service area (inside the service area) of base station  200 . On the other hand, upon receiving a notice that no response to the encapsulated control plane data is received for a predetermined period from C-Plane encapsulated packet transmitting/receiving section  501 , second determining section  206  re-determines that terminal  100  is not located in the service area (outside the service area) of base station  200 . That is, upon receiving a response to the encapsulated control plane data, although it is determined, based on receiving quality, that terminal  400  is outside the service area of base station  500 , second determining section  206  assumes that terminal  400  is located inside the service area of base station  500 . Second determining section  206  outputs the determination result (re-determination result) to radio-resource control section  204 . 
         [0130]    Upon receiving an instruction for encapsulation from second determining section  206 , C-Plane encapsulated packet transmitting/receiving section  501  receives the control plane data from radio-resource control section  204 . C-Plane encapsulated packet transmitting/receiving section  501  encapsulates the received control plane data as an IP packet and outputs the encapsulated data to application section  208 . C-Plane encapsulated packet transmitting/receiving section  501  extracts the control plane data from the IP packet (encapsulated data) received from application section  108  and outputs the control plane data to radio-resource control section  204 . Upon receiving a response (ACK) to the control plane data from application section  208  as an IP packet, or upon receiving a response to the control plane data from radio-resource control section  204 , C-Plane encapsulated packet transmitting/receiving section  501  outputs a notice of such receipt of the response to the control plane data to second determining section  206 . 
         [0131]    Thus, C-Plane encapsulated packet transmitting/receiving section  501  encapsulates the control plane data, transmits the encapsulated control plane data (IP packet) as a confirmation packet for confirming a connection between terminal  400  and the small cell and receives a response to the control plane data encapsulated by terminal  400 . 
         [0132]    During C/U separation, when the determination result on base station  500  (macro cell) received from first determining section  205  shows that terminal  400  is outside the service area, radio-resource control section  204  controls establishment of a connection of radio resources between terminal  400  and base station  500  based on the determination result (re-determination result) received from second determining section  206 . More specifically, when the determination result received from second determining section  206  shows that terminal  400  is inside the service area, radio-resource control section  204  continues the connection between terminal  400  and base station  500 , and disconnects the connection between terminal  400  and base station  500  when the determination result shows that terminal  400  is outside the service area. 
         [0133]    While the connection between terminal  400  and base station  500  is continued based on the determination result of second determining section  206 , radio-resource control section  204  outputs control plane data directed to terminal  400  to C-Plane encapsulated packet transmitting/receiving section  501  and receives control plane data from terminal  400  from C-Plane encapsulated packet transmitting/receiving section  501 . 
         [0134]    Application section  208  outputs the encapsulated data received from C-Plane encapsulated packet transmitting/receiving section  501  as user plane data to data processing section  203 . Upon receiving the user plane data including the encapsulated data from data processing section  203 , application section  208  outputs the data to C-Plane encapsulated packet transmitting/receiving section  501 . 
       [Operations of Terminal  400  and Base Station  500 ] 
       [0135]    Operations of terminal  400  and base station  500  having the above-described configurations will be described. 
         [0136]      FIG. 10  is a flowchart illustrating a processing flow when terminal  400  encapsulates and transmits control plane data (C-Plane message).  FIG. 11  is a flowchart illustrating a processing flow when base station  500  encapsulates and transmits control plane data. 
         [0137]    Note that in  FIG. 10  and  FIG. 11 , processes identical to those in Embodiment 1 ( FIG. 4 ,  FIG. 5 ) are assigned identical reference numerals and the description thereof will be omitted. 
         [0138]    First, a case will be described where terminal  400  shown in  FIG. 10  encapsulates and transmits control plane data. 
         [0139]    In  FIG. 10 , in ST 301 , terminal  400  encapsulates control plane data and transmits the encapsulated data to base station  500  via base station  300  (small cell) as user plane data. Upon receiving the encapsulated control plane data from terminal  400  in ST 301 , base station  500  encapsulates a response (ACK) to the control plane data and returns the encapsulated response to terminal  400  via base station  300  as user plane data. 
         [0140]    In ST 302 , terminal  400  determines whether or not terminal  400  has received a response (ACK) to the control plane data transmitted in ST 301  within a predetermined period. When terminal  400  has received ACK (ST 302 : Yes), terminal  400  proceeds to a process in ST 107  and proceeds to a process in ST 109  when terminal  400  has not received ACK (ST 302 : No). 
         [0141]    Next, a case will be described where base station  500  shown in  FIG. 11  encapsulates and transmits control plane data. 
         [0142]    In  FIG. 11 , in ST 401 , base station  500  encapsulates control plane data and transmits the encapsulated data to terminal  400  via base station  300  as user plane data. Upon receiving the control plane data encapsulated in ST 401  from base station  500 , terminal  400  encapsulates a response (ACK) to the control plane data and returns the encapsulated response to base station  500  via base station  300  as user plane data. 
         [0143]    In ST 402 , base station  500  determines whether or not base station  500  has received a response (ACK) to the keep-alive packet transmitted in ST 401  within a predetermined period. Base station  500  proceeds to a process in ST 206  when base station  500  has received ACK (ST 401 : Yes) and proceeds to a process in ST 208  when base station  500  has not received ACK (ST 401 : No). 
         [0144]    As described above, when terminal  400  is located outside the service area of the macro cell (base station  500 ), terminal  400  and/or base station  500  encapsulate(s) and transmit(s) control plane data, assume(s), as long as there is a response (ACK) to the control plane data, that terminal  400  is located inside the service area of base station  500  and continue(s) communication using a user plane while maintaining the connection of the control plane. During the continuation of the connection of the control plane, if communication between terminal  400  and base station  500  is made possible, terminal  400  and base station  500  resume communication of control plane data (C-Plane). 
         [0145]    Next,  FIG. 12  is a sequence diagram illustrating exchange of signals among terminal  400 , base station  500  (macro cell) and base station  300  (small cell). Note that in  FIG. 12 , processes identical to those in Embodiment 1 ( FIG. 6 ) are assigned identical reference numerals and the description thereof will be omitted. 
         [0146]    In  FIG. 12 , C/U separation is applied to terminal  400  as in the case of  FIG. 6  and base station  500  performs communication using a control plane and base station  300  performs communication using a user plane. 
         [0147]    In  FIG. 12 , in ST 21 , base station  500  encapsulates control plane data (control plane message) and transmits the encapsulated control plane data to terminal  400  via base station  300 . When terminal  400  has received the encapsulated control plane data in ST 21 , terminal  400  transmits a response to the control plane data to base station  500  via base station  300  in ST 22 . Note that although  FIG. 12  shows a case where the macro cell transmits control plane data as an example, terminal  400  may also transmit the encapsulated control plane data and base station  500  may transmit a response to the control plane data. 
         [0148]    Thus, even when terminal  400  is located outside the service area of base station  500  which is a macro cell, base station  500  transmits/receives encapsulated control plane data using the user plane, and base station  500  can thereby continue movement management on terminal  400 , and base station  300  which is a small cell can continue data communication. 
         [0149]    As described above,  FIG. 13  is a diagram provided for describing exchange of information using protocol stacks among terminal  400 , base station  500  (macro cell) and base station  300  (small cell) while communication using the control plane between base station  500  (macro cell) and terminal  400  is stopped but the connection is still continued. 
         [0150]    Note that in  FIG. 13 , as in the case of  FIG. 7 , base station  500  applies, for example, an LTE-Advanced system, and base station  300  applies a WiGig radio system as a radio system different from that of base station  200 . As shown in  FIG. 13 , terminal  400  has functions of both the LTE-Advanced system and the WiGig system. More specifically, terminal  400  has at least an RF layer, a PHY layer, a MAC layer, an RLC layer, a PDCP layer, an RRC layer, and an NAS (non access stratum) layer for the control plane of the LIE-Advanced system. Moreover, terminal  400  has at least an RF layer, a PHY layer, a MAC layer and an LLC layer for the user plane of the WiGig radio system. 
         [0151]    For example, when it is determined that terminal  400  is outside the service area of base station  500 , base station  500  encapsulates the control plane data (RRC message) as an IP layer packet and transmits the generated control plane data using the user plane. Base station  500  receives a response to the encapsulated control plane data using the user plane. That is, terminal  400  and base station  500  exchange the control plane data and its response via the small cell using the user plane. Thus, when base station  500  can obtain a response to the control plane data transmitted/received using the user plane, it is determined that communication is possible between terminal  400  and base station  300 . 
         [0152]    At this time, as long as a response to the encapsulated control plane data is obtained, base station  500  continues the connection of the control plane. More specifically, as shown in  FIG. 13 , although base station  500  stops communication of the control plane data using the control plane, base station  500  keeps the control plane “attached” without notifying the core network (e.g., MME) that terminal  400  is outside the service area. Since the connection of the control plane is continued, if communication between terminal  400  and base station  500  is made possible again, terminal  400  and base station  500  can resume transmission/reception of control plane data using the control plane without performing further connection processing. 
         [0153]    By this means, according to the present embodiment, during C/U separation, when the terminal is located outside the service area of the macro cell, the terminal and the macro cell transmit/receive the encapsulated control plane data using the user plane. When a response to the control plane data transmitted/received using the user plane is obtained, the terminal and the macro cell continue the connection of the control plane between the terminal and the macro cell. That is, when communication of the user plane between the terminal and the small cell is possible, it is possible to maintain the connection between the terminal and the macro cell even when communication between the terminal and the macro cell is not possible. 
         [0154]    Even when direct communication between the terminal and the small cell is not possible, the control plane data is transmitted/received using the user plane. Thus, even when the macro cell and the small cell are different radio systems, the macro cell can communicate the control plane data using the user plane and can execute movement management uninterruptedly. Similarly, even when the macro cell and the small cell are the same radio system and the small cell is provided with only the communication function of the user plane, the macro cell can communicate the control plane data using the user plane and can thereby execute movement management uninterruptedly. 
         [0155]    Thus, according to the present embodiment, as in the case of Embodiment 1, in the heterogeneous network in which the macro cell executes movement management using the control plane and the small cell handles the user plane, it is possible to continue data communication in the small cell while continuing movement management in the macro cell even when the terminal is located outside the service area of the macro cell. Furthermore, according to the present embodiment, even when direct communication between the terminal and the macro cell is not possible, it is possible for the macro cell to appropriately perform movement management compared to Embodiment 1. 
         [0156]    The embodiments of the present invention have been described so far. 
         [0157]    Note that although cases have been described with the above embodiments as examples where the present invention is configured by hardware, the present invention can also be realized by software in cooperation with hardware. 
         [0158]    Each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
         [0159]    Further, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible. After LSI manufacture, utilization of a programmable FPGA (field programmable gate array) or a reconfigurable processor where connections and settings of circuit cells within an LSI can be reconfigured is also possible. 
         [0160]    Further, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
         [0161]    The disclosure of Japanese Patent Application No. 2014-058089 filed on Mar. 20, 2014, the contents of Which including the specification and drawings are incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0162]    The present invention is suitable for use in mobile communication systems. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Terminal 
           200 ,  300  Base station 
           101 ,  110 ,  201 ,  301  Receiving section 
           102 ,  111 ,  202 ,  302  Transmitting section 
           103 ,  112  Receiving-quality measuring section 
           104 ,  205  First determining section 
           105 ,  204  Radio-resource control section 
           106 ,  206  Second determining section 
           107 ,  207  Keep-alive-packet transmitting/receiving section 
           108 ,  208  Application section 
           109 ,  113 ,  203 ,  303  Data processing section 
           150  First-communication processing section 
           151  Second-communication processing section 
           209 ,  304  Inter-base-station I/F control section