CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-005759, filed on Jan. 15, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a base station, a wireless communication system, and a wireless communication method. 
       BACKGROUND 
       [0003]    Currently, a mobile phone system or a wireless communication system of a wireless local area network (LAN) and the like has been widespread used. In the field of wireless communication, the next generation communication technology has been continuously discussed in order to further improve a communication speed or communication capacity. For example, in 3rd Generation Partnership Project (3GPP) which is a standardization organization, standardization of a communication standard referring to long term evolution (LTE) or a communication standard referring to LTE-Advanced (LTE-A) which uses LTE as a base is completed or examined. 
         [0004]    Regarding such a wireless communication system, there is a technology referring to a heterogeneous network (HetNet). The HetNet is a technology in which systems having different radiuses of a cell or different wireless communication method are mixed in the same wireless communication area. Thus, for example, it is possible to improve capacity of the entirety of a network in comparison to a wireless communication system other than the HetNet. However, the HetNet is currently used as a wireless communication system in which a small-cell base station having a service area (which may be referred to as “a small cell” below) which has a wireless communication area narrower than that of the macro cell is disposed in a service area (which may be referred to as “a macro cell” below) of a macro-cell base station. 
         [0005]    Since the small cell has a service area narrower than that of the macro cell, even when a terminal device moves in a macro cell, handover from the macro-cell base station to the small-cell base station, or handover from the small-cell base station to the macro-cell base station may occur. Accordingly, in a wireless communication system having a configuration of the HetNet, the frequency of occurrence of handover is increased in comparison to a wireless communication system having a configuration other than the HetNet. Thus, processing load in the wireless communication system is also increased. 
         [0006]    A technology referring to a C/U separation HetNet has attracted attention. The C/U separation HetNet is, for example, a technology in which regarding a terminal device under the small-cell base station, the small-cell base station performs user data processing (which may refer to “U-Plane processing”, for example) and the macro-cell base station performs processing on a control signal (which may refer to “C-Plane processing”, for example). 
         [0007]    In the C/U separation HetNet, since the macro-cell base station performs the C-Plane processing, even when a terminal moves between the macro cell and the small cell, the macro-cell base station and the small-cell base station may not perform processing relating to handover. Accordingly, in the C/U separation HetNet, a control signal relating to handover is not exchanged between the macro-cell base station and the small-cell base station, and the terminal. Thus, it is possible to increase a speed and capacity of data communication. 
         [0008]    As a technology of the related art relating to the wireless communication system, for example, there is a technology as follows. 
         [0009]    That is, there is a wireless control device in which when congestion occurs, position information and a movement speed of a mobile station are detected, a mobile station which is determined to enable handover is forcibly subjected to handover to a wireless communication system which has a wireless communication area different from a wireless communication area in a congestion state. 
         [0010]    According to this technology, portable terminal in the cell can be efficiently distributed and congestion can be settled when congestion occurs or in a state where occurrence of congestion is predicted. 
         [0011]    An example of the related art includes Japanese Laid-open Patent Publication No. 2008-270919. 
       SUMMARY 
       [0012]    According to an aspect of the invention, a base station includes an antenna configured to form a second cell, and a processor configured to: process a control signal with a terminal that is located in an overlapping area of a first cell and the second cell, the first cell being formed by another base station that processes a data signal with the terminal, and transfer processing of the control signal with the terminal to the other base station when a load of the processing of the control signal is more than a first threshold in the base station and when the terminal is located in the overlapping area for more than a given length of time. 
         [0013]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0014]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a diagram illustrating a configuration example of a wireless communication system; 
           [0016]      FIG. 2  is a diagram illustrating a configuration example of the wireless communication system; 
           [0017]      FIG. 3  is a diagram illustrating CA between base stations; 
           [0018]      FIG. 4  is a diagram illustrating a configuration example of a macro-cell base station; 
           [0019]      FIG. 5  is a diagram illustrating a configuration example of a small-cell base station; 
           [0020]      FIG. 6  is a diagram illustrating a configuration example of a terminal; 
           [0021]      FIG. 7A  is a diagram illustrating a configuration example of an MME; 
           [0022]      FIG. 7B  is a diagram illustrating a configuration example of a S-GW; 
           [0023]      FIG. 8  is a flowchart illustrating a monitoring example of a processing quantity of a C-Plane; 
           [0024]      FIG. 9  is a flowchart illustrating an operation example of C-Plane processing transition; 
           [0025]      FIG. 10  is a sequence diagram illustrating an operation example of C-Plane processing transition; 
           [0026]      FIG. 11  is a sequence diagram illustrating an operation example of C-Plane processing transition; 
           [0027]      FIG. 12  is a sequence diagram illustrating an operation example of C-Plane processing transition; 
           [0028]      FIG. 13  is a sequence diagram illustrating an operation example of C-Plane processing transition; 
           [0029]      FIG. 14  is a diagram illustrating a division example of a service area; 
           [0030]      FIG. 15  is a diagram illustrating an example of an HO determination matrix; 
           [0031]      FIG. 16  is a diagram illustrating a setting example of an SRB and a DRB; 
           [0032]      FIG. 17  is a diagram illustrating a setting example of an SRB and a DRB; 
           [0033]      FIG. 18  is a diagram illustrating a setting example of an SRB and a DRB; 
           [0034]      FIG. 19  is a flowchart illustrating an example of HO frequency calculation processing; 
           [0035]      FIG. 20  is a flowchart illustrating an example of the HO frequency calculation processing; 
           [0036]      FIG. 21  is a flowchart illustrating an operation example of C-Plane processing transition; 
           [0037]      FIG. 22  is a diagram illustrating a configuration example of the macro-cell base station; 
           [0038]      FIG. 23  is a diagram illustrating a configuration example of the small-cell base station; and 
           [0039]      FIG. 24  is a diagram illustrating a configuration example of the terminal. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0040]    However, in the C/U separation HetNet, C-Plane processing congestion may have an influence on the terminal device under the small-cell base station. 
         [0041]    That is, in the C/U separation HetNet, the macro-cell base station performs the C-Plane processing on a terminal device under the small-cell base station, in addition to a terminal device under the macro-cell base station. For example, a case where many users (or terminal devices) who (which) move at a high speed on Shinkansen (a bullet train) and the like pass through the macro cell is considered. In this case, the macro-cell base station performs handover processing on the many terminal devices. In the macro-cell base station, an allowable range of the handover processing may be exceeded due to the many terminal devices. In this case, the C-Plane processing for all terminal devices under the macro-cell base station is congested. In this case, since the macro-cell base station performs the C-Plane processing on the terminal under the small-cell base station, congestion has an influence on the terminal device under the macro-cell base station and on the C-Plane processing for the terminal device under the small-cell base station. Thus, in the macro-cell base station, the C-Plane processing may have an influence on the terminal device under the small-cell base station. 
         [0042]    In a HetNet configuration which is not a C/U separation type, the macro-cell base station performs the C-Plane processing on a terminal device under the macro-cell base station, and the small-cell base station performs the C-Plane processing on a terminal device under the small-cell base station, and thus the processing is performed on the terminal devices which are separated from each other. Therefore, congestion of the C-Plane processing in the macro-cell base station does not have an influence on congestion of the C-Plane processing in the small-cell base station. 
         [0043]    In the technology in which handover is forcibly performed on a wireless communication system having a wireless communication area different from a wireless communication area in the above-described congestion state, a congestion state of a handover source in the wireless communication system can be avoided, but congestion of a handover destination in the wireless communication system has no consideration. Accordingly, in this technology, the C-Plane processing of the handover destination in the wireless communication system may be congested. 
         [0044]    The disclosure is to provide a base station device and a wireless communication system in which congestion for processing on the control signal is avoided. 
         [0045]    Hereinafter, embodiments will be described. 
       First Embodiment 
       [0046]    A first embodiment will be described. 
         [0047]      FIG. 1  is a diagram illustrating a configuration example of a wireless communication system  10 . The wireless communication system  10  includes a base station device  100 - 1 , another base station device  100 - 2 , and a terminal  200 - 2 . 
         [0048]    The base station device  100 - 1  has a second service area  100 -M. The other base station device  100 - 2  has a first service area  100 -S. The second service area  100 -M includes the first service area  100 -S and has an area wider than the first service area  100 -S. 
         [0049]    The base station device  100 - 1  exchanges a control signal with the terminal device  200 - 2  which stays in the first service area  100 -S. The other base station device  100 - 2  exchanges user data with the terminal device  200 - 2  which stays in the first service area  100 -S. 
         [0050]    The wireless communication system  10  is a C/U separation HetNet, for example. 
         [0051]    The base station device  100 - 1  includes a control unit  125 . The control unit  125  causes processing on a control signal of a terminal device  200 - 2  to be transitioned to the base station device  100 - 2  from the base station device  100 - 1  when the terminal device  200 - 2  has a processing quantity for the control signal which is equal to or greater than a first threshold, and stays in the first service area  100 -S for a period which is equal to or longer than a predetermined period of time. 
         [0052]    Thus, the processing on the control signal for the terminal device  200 - 2  staying in the first service area  100 -S is transitioned to the other base station device  100 - 2  from the base station device  100 - 1 . Accordingly, in the base station device  100 - 1 , the processing on the control signal is not performed for the terminal device  200 - 2 , and thus it is possible to avoid congestion on control signal processing in the base station device  100 - 1 . 
       Second Embodiment 
       [0053]    Next, a second embodiment will be described. 
         [0054]    Configuration Example of Wireless Communication System 
         [0055]    A configuration example of the wireless communication system will be described.  FIG. 2  is a diagram illustrating the configuration example of the wireless communication system  10 . The wireless communication system  10  includes the macro-cell base station device (or a first base station device, which may refer to “a macro-cell base station” below)  100 - 1  and the small cell base station device (or a second base station device, which may refer to “a small-cell base station” below)  100 - 2 . The wireless communication system  10  includes the terminal devices (which may refer to “a terminal” below)  200 - 1  and  200 - 2 , a mobility management entity (MME)  300 , and a Serving Gateway (S-GW)  400 . 
         [0056]    The service area  100 -M of the macro-cell base station  100 - 1  is wider than the service area  100 -S of the small-cell base station  100 - 2 . The service area  100 -S of the small-cell base station  100 - 2  is hierarchically disposed in the service area  100 -M of the macro-cell base station  100 - 1 . In this manner, the wireless communication system in which the service area  100 -S is hierarchically disposed in the service area  100 -M may refer to a HetNet, for example. The wireless communication system  10  in  FIG. 2  is an example of the HetNet. 
         [0057]    The macro-cell base station  100 - 1  performs wireless communication with the terminals  200 - 1  and  200 - 2  which stay in the service area  100 -M, and exchanges user data, a control signal, and the like with the terminals  200 - 1  and  200 - 2 . 
         [0058]    The macro-cell base station  100 - 1  is connected to the small-cell base station  100 - 2 , the MME  300 , and the S-GW  400 . The macro-cell base station  100 - 1  exchanges user data, a control signal, and the like with the small-cell base station  100 - 2  by using an X2 interface. The macro-cell base station  100 - 1  exchanges a control signal, user data, and the like with the MME  300  or the S-GW  400  by using an S1 interface. 
         [0059]    In the following descriptions, a function group relating to an exchange of user data may refer to a U-Plane (or user plane), and a function group which relates to call control and relates to an exchange of a control signal may refer to a C-Plane (or control plane). User data such as sound data and text data is exchanged (transmitted) by using the U-Plane. A control signal relating to call control is exchanged by using the C-Plane. 
         [0060]    The small-cell base station  100 - 2  performs wireless communication with the terminal  200 - 2  staying in the service area  100 -S so as to exchange user data, a control signal and the like. 
         [0061]    The small-cell base station  100 - 2  is connected to the macro-cell base station  100 - 1 , the MME  300 , and the S-GW  400 . The small-cell base station  100 - 2  exchanges user data or a control signal with the macro-cell base station  100 - 2  by using the X2 interface. The small-cell base station  100 - 2  exchanges a control signal or user data with the MME  300  or the S-GW  400  by using the S1 interface. 
         [0062]    In this wireless communication system  10 , the small-cell base station  100 - 2  performs the U-Plane processing for the terminal  200 - 2  which stays in the service area  100 -S of the small-cell base station  100 - 2  and the macro-cell base station  100 - 2  performs the C-Plane processing. 
         [0063]    In this manner, a HetNet in which the small-cell base station  100 - 2  performs the U-Plane processing, and the macro-cell base station  100 - 1  performs the C-Plane processing may refer to a C/U separation HetNet, for example.  FIG. 2  illustrates an example of the wireless communication system  10  of the C/U separation HetNet. 
         [0064]    The terminals  200 - 1  and  200 - 2  are, for example, portable mobile terminals such as a feature phone, a smart phone, and a personal computer. The terminal  200 - 1  exchanges user data with the macro-cell base station  100 - 1 , and thus it is possible to receive provision of various services such as a call service and an image distribution service. The terminal  200 - 2  exchanges user data with the small-cell base station  100 - 2 , and thus it is possible to receive provision of various services such as a call service from the small-cell base station  100 - 2 . 
         [0065]    The MME  300  is a management apparatus, for example, which performs position management of the terminals  200 - 1  and  200 - 2  or performs bearer control and the like. A bearer is, for example, a logical path which is established between the terminals  200 - 1  and  200 - 2 , and the S-GW  400 , and on which user data or a control signal is transmitted. For example, a logical path for transmitting user data may refer to a data bearer (or a data radio bearer (DRB)), and a logical path for transmitting a control signal may refer to a signal bearer (or a signal radio bearer (SRB)). 
         [0066]    The DRB is set between the S-GW  400 , and the terminals  200 - 1  and  200 - 2 , and thus user data is exchanged (or transmitted) between the S-GW  400  and the terminals  200 - 1  and  200 - 2  by using the U-Plane. The SRB is set between the S-GW  400 , and the terminals  200 - 1  and  200 - 2 , and thus a control signal is exchanged between the S-GW  400  and the terminals  200 - 1  and  200 - 2  by using the C-Plane. 
         [0067]    The MME  300  leads setting of a bearer. For example, the MME  300  sets the signal bearer and instructs the S-GW  400  of setting of the data bearer. The S-GW  400  receives this instruction and sets the data bearer. 
         [0068]    The S-GW  400  exchanges user data and a control signal with the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 . The S-GW  400  transmits user data to the macro-cell base station  100 - 1  or the small-cell base station  100 - 2  in accordance with the set DRB. The S-GW  400  transmits a control signal to the macro-cell base station  100 - 1  or the small-cell base station  100 - 2  in accordance with the set SRB. 
         [0069]    As described above, this wireless communication system  10  is the C/U separation HetNet. As one form of the C/U separation HetNet, for example, there is carrier aggregation (CA) (or Inter-eNB CA) between the base stations. The HetNet wireless communication system  10  may be, for example, the C/U separation HetNet by setting CA between the base stations. 
         [0070]    CA between the base stations is, for example, a technology in which a frequency band used in transmission and reception of a radio signal in the macro-cell base station  100 - 1  and a frequency band used in transmission and reception of a radio signal in the small-cell base station  100 - 2  are put together (or collected) and used as one frequency band. For example, in CA between the base stations, since the frequency bands of both of the macro-cell base station  100 - 1  and the small-cell base station  100 - 2  may be simultaneously used, it is possible to increase capacity and a speed in communication. 
         [0071]      FIG. 3  illustrates an example of the wireless communication system  10  when CA between the base stations is performed. In this case, two DRBs which are a DRB via the macro-cell base station  100 - 1 , and a DRB via the small-cell base station  100 - 2  are set for the terminal  200 . The SRB is set between the terminal  200  and the macro-cell base station  100 - 1 . The U-Plane is set in the small-cell base station  100 - 2 , and the C-Plane is set in the macro-cell base station  100 - 1 . The wireless communication system  10  illustrated in  FIG. 3  includes the C/U separation HetNet. 
         [0072]    Next, configuration examples of the macro-cell base station  100 - 1 , the small-cell base station  100 - 2 , the terminal  200 , the MME  300 , and the S-GW  400  will be described. 
         [0073]    Configuration Example of Macro-Cell Base Station 
         [0074]      FIG. 4  is a diagram illustrating a configuration example of the macro-cell base station  100 - 1 . The macro-cell base station  100 - 1  includes a plurality of antennae  101 - 1 ,  101 - 2 , . . . , a plurality of wireless units  110 - 1 ,  110 - 2 , . . . , and a control and baseband unit  120 . 
         [0075]    Since all of the plurality of wireless units  110 - 1 ,  110 - 2 , . . . , have the same configuration, representatively, descriptions will be made by using the wireless unit  110 - 1  as an example. 
         [0076]    The wireless unit  110 - 1  includes an orthogonal modulation/demodulation unit  111 , a transmission unit  112 , a power amplifier (PA)  113 , a duplexer (DUP)  114 , a low noise amplifier (LNA)  115 , and a reception unit  116 . 
         [0077]    The orthogonal modulation/demodulation unit  111  modulates a baseband signal output from the control and baseband unit  120  and outputs the modulated baseband signal to the transmission unit  112 . The orthogonal modulation/demodulation unit  111  demodulates a baseband signal output from the reception unit  116  and outputs the demodulated baseband signal to the control and baseband unit  120 . 
         [0078]    The transmission unit  112  performs conversion into a radio signal in a wireless band by performing frequency conversion processing and the like on the demodulated baseband signal. 
         [0079]    The PA  113  is an amplifier and amplifies the radio signal output from the transmission unit  112 . 
         [0080]    The DUP  114  outputs the radio signal output from the PA  113  to the antenna  101 - 1  and outputs the radio signal (reception signal) which is received by the antenna  101 - 1  to the LNA  115 . 
         [0081]    The LNA  115  is a low-noise amplifier and amplifies the reception signal output from the DUP  114 . 
         [0082]    The reception unit  116  performs conversion into a baseband signal in a baseband band by performing frequency conversion processing and the like on the reception signal output from the LNA  115 . 
         [0083]    The control and baseband unit  120  includes a baseband unit  121 , a transmission channel interface unit  122 , a timing control unit  123 , a power source unit  124 , a control unit  125 , and a memory  126 . 
         [0084]    The baseband unit  121  receives user data from the transmission channel interface unit  122 , performs error correction encoding processing and the like on the received user data, and performs conversion into a baseband signal. The baseband unit  121  outputs the baseband signal to the orthogonal modulation/demodulation unit  111 . The baseband unit  121  receives a control signal from the control unit  125 , performs the error correction encoding processing and the like on the control signal, and outputs the baseband signal to the orthogonal modulation/demodulation unit  111 . 
         [0085]    The baseband unit  121  performs error correction decoding processing and the like on the baseband signal output from the orthogonal modulation/demodulation unit  111 , and extracts user data, a control signal, or the like. The baseband unit  121  outputs the extracted user data to the memory  126  or the transmission channel interface unit  122 , and outputs the extracted control signal to the control unit  125 . 
         [0086]    The transmission channel interface unit  122  exchanges a message and the like in an S1 format with the MME  300  or the S-GW  400  and exchanges a message and the like in an X2 format with the small-cell base station  100 - 2 . 
         [0087]    Thus, the transmission channel interface unit  122  converts user data or a control signal which is received from the baseband unit  121  or the control unit  125  into a message and the like of the S1 format, and transmits the converted message to the MME  300  or the S-GW  400 . The transmission channel interface unit  122  converts user data or a control signal which is received from the baseband unit  121  or the control unit  125  into a message and the like of the X2 format, and transmits the converted message to the small-cell base station  100 - 2 . 
         [0088]    The transmission channel interface unit  122  extracts user data or a control signal from the message of the S1 format received from the MME  300  or the S-GW  400 , and outputs the extracted user data or control signal to the baseband unit  121 , the control unit  125 , or the memory  126 . The transmission channel interface unit  122  extracts user data or a control signal from the message of the X2 format received from the small-cell base station  100 - 2 , and outputs the extracted user data or control signal to the baseband unit  121 , the control unit  125 , or the memory  126 . 
         [0089]    The timing control unit  123  synchronizes other devices such as the terminal  200 , the small-cell base station  100 - 2 , and the S-GW  400  and operates each of the units  110 - 1  and the like in the macro-cell base station  100 - 1 , and the like. 
         [0090]    The power source unit  124  causes power of the macro-cell base station  100 - 1  to turn ON or OFF, for example. 
         [0091]    The control unit  125  performs, for example, scheduling for allocation of radio resources, an error correction encoding method, a modulation method, and the like on the terminal  200  under the macro-cell base station  100 - 1 , and generates a control signal including a result of scheduling. 
         [0092]    The control unit  125  monitors a processing quantity of the control signal, and causes processing on a control signal of the terminal  200 - 2  which stays in the service area  100 -S for a period which is equal to or longer than the predetermined period of time to be transitioned to the small-cell base station  100 - 2  from the macro-cell base station  100 - 1 . At this time, the processing quantity of the control signal is equal to or greater than the first threshold. Details thereof will be described in an operation example. 
         [0093]    The memory  126  stores user data and the like output from the baseband unit  121  or the transmission channel interface unit  122 . An HO determination matrix  1261  is stored in the memory  126 . Details of the HO determination matrix  1261  will be described later. 
         [0094]    Configuration Example of Small-Cell Base Station 
         [0095]      FIG. 5  is a diagram illustrating a configuration example of the small-cell base station  100 - 2 . The small-cell base station  100 - 2  includes an antenna  101 , a wireless unit  110 , and a control and baseband unit  130 . 
         [0096]    The antenna  101  and the wireless unit  110  have the same configurations as those in the macro-cell base station  100 - 1 . The macro-cell base station  100 - 1  includes a plurality of wireless units  110 . However, the small-cell base station  100 - 2  includes “one” wireless unit  110 . 
         [0097]    The control and baseband unit  130  includes a transmission baseband unit  131 - 1 , a reception baseband unit  131 - 2 , a transmission channel interface unit  132 , a timing control unit  133 , a control unit  134 , a power source unit  135 , a memory  136 , and an RTT measurement unit  137 . 
         [0098]    The transmission baseband unit  131 - 1  performs error correction encoding processing and the like on user data or a control signal received from the transmission channel interface unit  132  or the control unit  134 , and outputs a result of the error correction encoding processing as a baseband signal to the orthogonal modulation/demodulation unit  111 . 
         [0099]    The reception baseband unit  131 - 2  performs error correction decoding processing and the like on the baseband signal received from the orthogonal modulation/demodulation unit  111 . The reception baseband unit  131 - 2  extracts and outputs user data or a control signal to the transmission channel interface unit  132 , the control unit  134 , or the memory  136 . 
         [0100]    The transmission channel interface unit  132 , the timing control unit  133 , the power source unit  135 , and the memory  136  have the same functions as those of the transmission channel interface unit  122 , the timing control unit  123 , the power source unit  124 , and the memory  126  of the macro-cell base station  100 - 1 . The transmission channel interface unit  122  exchanges user data, a control signal, or the like with the macro-cell base station  100 - 1  in the X2 format, and exchanges user data, a control signal, or the like with the MME  300  or the S-GW  400  in the S1 format. 
         [0101]    The RTT measurement unit  137  measures round trip time (RTT; period of time from transmission of a radio signal until reception) for a radio signal which is transmitted and received between the small-cell base station  100 - 2  and the terminal  200 . A measuring method will be described in an operation example. The RTT measurement unit  137  outputs the measured RTT to the control unit  134 . 
         [0102]    The control unit  134  estimates a straight distance from the small-cell base station  100 - 2  to the terminal  200  based on the RTT. The small-cell base station  100 - 2  estimates a straight distance to the terminal  200 , and thus estimates (or determines) a position of the terminal  200 , for example. Details thereof will be described later. 
         [0103]    Configuration Example of Terminal 
         [0104]      FIG. 6  is a diagram illustrating a configuration example of the terminal  200 . The terminal  200  includes an antenna  201 , a wireless unit  210 , a baseband unit  220 , an application control unit  221 , a video coding unit  222 , a charge coupled device (CCD)  223 , and a liquid crystal device (LCD)  224 . The terminal  200  includes a sound coding unit  225 , a speaker  226 , a microphone  227 , a power source unit  230 , a battery  231 , a speed sensor  232 , a control unit  233 , and a key  234 . 
         [0105]    The antenna  201  transmits a radio signal output from the wireless unit  210  to the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 . The antenna  201  receives a radio signal transmitted from the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 , and outputs the received radio signal to the wireless unit  210 . 
         [0106]    The wireless unit  210  includes an orthogonal modulation/demodulation unit  211 , a transmission unit  212 , a PA  213 , a DUP  214 , an LNA  215 , and a reception unit  216 . The functions of the orthogonal modulation/demodulation unit  211 , the transmission unit  212 , the PA  213 , the DUP  214 , the LNA  215 , and the reception unit  216  are the same as that of the wireless unit  110  in the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 , for example. The functions of the baseband unit  220  and the power source unit  230  are the same as those of the baseband unit  121  and the power source unit  124  in the macro-cell base station  100 - 1 , for example. 
         [0107]    For example, the application control unit  221  receives user data from the baseband unit  220 , outputs image data in the received user data to the video coding unit  222 , and outputs sound data in the received user data to the sound coding unit  225 . The application control unit  221  outputs, for example, image data or sound data which is output from the video coding unit  222  or the sound coding unit  225 , as user data to the baseband unit  220 . 
         [0108]    The video coding unit  222  performs image processing such as compression encoding processing on image data output from the CCD  223 , and outputs the image data subjected to the image processing to the application control unit  221 . The video coding unit  222  performs expansion processing on the compressed image data which is received from the application control unit  221 , and outputs the expanded image data to the LCD  224 . 
         [0109]    The CCD  223  is an imaging element, for example. The CCD  223  generates image data by photographing a subject, and outputs the generated image data to the video coding unit  222 . The LCD  224  is a display unit, for example. The LCD  224  displays image data from the video coding unit  222 . 
         [0110]    The sound coding unit  225  performs sound processing such as compression encoding on sound data received from the microphone  227 , and outputs the sound data subjected to the sound processing to the application control unit  221 . The sound coding unit  225  performs expansion processing and the like on sound data received from the application control unit  221 , and outputs the sound data subjected to the expansion processing to the speaker  226 . 
         [0111]    The speaker  226  outputs sound corresponding to the sound data received from the sound coding unit  225 . The microphone  227  outputs acquired sound as sound data to the sound coding unit  225 . 
         [0112]    The power source unit  230  supplies power to each of the units in the terminal  200 . The battery  231  stores electricity supplied from the power source unit  230 . The battery  231  supplies the stored electricity to each of the units in the terminal  200  when power from the power source unit  230  is not supplied. The speed sensor  232  is a sensor of detecting a movement speed of the terminal  200 . 
         [0113]    The control unit  233  appropriately controls the wireless unit  210  and the baseband unit  220 , for example, so as to transmit an instruction of a coding rate of error correction encoding processing in the baseband unit  220 , or an instruction of a modulation method of orthogonal modulation encoding in the wireless unit  210 , and the like. The control unit  233  receives information regarding the SRB or the DRB which is set by the MME  300  or the S-GW  400 . At this time, the control unit  233  controls the wireless unit  210  and the like to transmit a control signal or user data to the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 , in accordance with the information regarding the SRB or the DRB. The control unit  233  controls the wireless unit  210  and the like to receive a control signal or user data from the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 , in accordance with the information regarding the SRB or the DRB. 
         [0114]    Configuration Example of MME 
         [0115]      FIG. 7A  is a diagram illustrating a configuration example of the MME  300 . The MME  300  includes an S1 interface unit  310 , a control unit  320 , and a memory  330 . 
         [0116]    The S1 interface unit  310  is connected to the macro-cell base station  100 - 1 , the small-cell base station  100 - 2 , the S-GW  400 , and the like, and exchanges a message of the S1 format with these devices. In this case, the S1 interface unit  310  extracts a control signal and the like from the message of the S1 format which is transmitted from the two base stations  100 - 1  and  100 - 2  or the S-GW  400 . The S1 interface unit  310  outputs the extracted control signal and the like to the control unit  320 . The S1 interface unit  310  converts a control signal and the like output from the control unit  320  into a message of the S1 format, and outputs the converted control signal and the like to the two base stations  100 - 1  and  100 - 2  or the S-GW  400 . 
         [0117]    The control unit  320  leads setting of a bearer, for example. For example, the control unit  320  sets the SRB and asks the S-GW  400  to set the DRB. In the second embodiment, the MME  300  sets the SRB and the S-GW  400  sets the DRB. 
         [0118]    For example, the SRB is set as follows. That is, in the example of  FIG. 2 , as a path on which a control signal is transmitted by using the C-Plane, there are two paths of a path from the S-GW  400  to the terminal  200 - 2  through the macro-cell base station  100 - 1 , and a path from the S-GW  400  to the terminal  200 - 2  through the small-cell base station  100 - 2 . The control unit  320  may set either of the two paths as a path on which the control signal is transmitted by using the C-Plane. In this case, the control unit  320  assigns identification (ID or “identifier”) relating to the SRB to either of the paths and causes the assigned identification to be stored in the memory  330 .  FIG. 16  is a diagram illustrating an example of setting the SRB. In the example of  FIG. 16 , the MME  300  assigns “1” as an ID of the SRB to an ID “xxx” of the terminal  200 - 2  and an ID “yyy” of the macro-cell base station  100 - 1 . Thus, for example, the bearer of the SRB “1” is set in a path from the terminal  200 - 2  to the S-GW  400  through the macro-cell base station  100 - 1 . The control signal is transmitted on this path by using the C-Plane. The control unit  320  transmits information regarding the set SRB to the terminal  200 . 
         [0119]    The memory  330  stores information regarding the set SRB, for example. In the example of  FIG. 16 , a UEID “xxx”, a base station ID “yyy”, and an SRB ID “bbb” are stored in the memory  330 . 
         [0120]    Configuration Example of S-GW 
         [0121]      FIG. 7B  is a diagram illustrating a configuration example of the S-GW  400 . The S-GW  400  includes an S1 interface unit  410 , a control unit  420 , a memory  430 , a data transmission unit  440 , and an external interface unit  450 . 
         [0122]    The S1 interface unit  410  is connected to the macro-cell base station  100 - 1 , the small-cell base station  100 - 2 , and the MME  300 , and receives a message of the S1 format which is transmitted from these devices. The S1 interface unit  410  extracts a control signal from the received message, and outputs the extracted control signal to the control unit  420 . The S1 interface unit  410  extracts transmission destination information of the received message from the message including user data, and outputs the transmission destination information to the data transmission unit  440 . 
         [0123]    The S1 interface unit  410  converts the control signal output from the control unit  420  into a message of the S1 format and transmits the converted message to the macro-cell base station  100 - 1 , the small-cell base station  100 - 2 , or the MME  300 . The S1 interface unit  410  transmits a message including the received user data to a transmission destination in accordance with an instruction from the data transmission unit  440 . 
         [0124]    The control unit  420  sets a call, for example. Generation of a control signal relating to the C-Plane, generation of a response signal to the control signal relating to the C-Plane, and the like are included in setting of a call. If a control signal indicating an instruction of setting the DRB is received from the MME  300 , the control unit  420  sets the DRB in accordance with the instruction. 
         [0125]    For example, the DRB is set as follows. That is, in the example of  FIG. 2 , as a path on which user data is transmitted by using the U-Plane, there are two paths of a path from the S-GW  400  to the terminal  200 - 2  through the macro-cell base station  100 - 1 , and a path from the S-GW  400  to the terminal  200 - 2  through the small-cell base station  100 - 2 . The control unit  420  may set either of the two paths as a path on which the user data is transmitted by using the U-Plane. In this case, the control unit  420  assigns an ID relating to the DRB to either of the paths and causes the assigned ID to be stored in the memory  430 .  FIG. 16  is a diagram illustrating an example of setting the DRB. In the example of  FIG. 16 , the S-GW  400  assigns “1” as an ID of the DRB to an ID “xxx” of the terminal  200 - 2  and an ID “yyy” of the macro-cell base station  100 - 1 . Thus, for example, the bearer of the DRB “1” is set in a path from the terminal  200 - 2  to the S-GW  400  through the macro-cell base station  100 - 1 . The user data is transmitted on this path by using the U-Plane. The control unit  420  transmits information regarding the set DRB to the terminal  200 . 
         [0126]    The control unit  420  may set the DRB for a higher device regarding setting of the DRB. 
         [0127]    The memory  430  stores information regarding the set DRB, for example. In the example of  FIG. 16 , a UEID “xxx”, a base station ID “yyy”, and a DRB ID “aaa”, and the like are stored in the memory  430 . 
         [0128]    The data transmission unit  440  confirms a transmission destination in accordance with the information regarding the DRB stored in the memory  430 , based on the transmission destination information. The data transmission unit  440  instructs the S1 interface unit  410  to perform transmission to the transmission destination through the macro-cell base station  100 - 1  or the small-cell base station  100 - 2 . 
         [0129]    Operation Example 
         [0130]    Next, an operation example will be described. The operation example will be described by using an example of  FIG. 2 . The service area  100 -M of the macro-cell base station  100 - 1  and the service area  100 -S of the small-cell base station  100 - 2  are hierarchically disposed. The terminal  200 - 2  stays in the service area  100 -S of the small-cell base station  100 - 2 . In this case, the macro-cell base station  100 - 1  determines whether or not the C-Plane processing for the terminal  200 - 2  is transitioned to the small-cell base station  100 - 2 . 
         [0131]    The operation example includes 1) monitoring of a C-Plane processing quantity, 2) transition determination and transition processing of the C-Plane processing. The operation example will be described in this order. 
         [0132]    1. Monitoring of Processing Quantity of C-Plane 
         [0133]      FIG. 8  is a flowchart illustrating a monitoring example of the processing quantity in the C-Plane processing. 
         [0134]    The macro-cell base station  100 - 1  sets a message threshold for the number of RRC messages (S 10 ). For example, the message threshold is set through an input device such as a keyboard by a manager who manages the macro-cell base station  100 - 1 , or an operator. The message threshold is recorded in the memory  126 . The message threshold may be appropriately corrected or changed. 
         [0135]    Then, the macro-cell base station  100 - 1  measures the number of RRC messages per unit time (S 11 ). For example, the control unit  125  measures the number of baseband signals relating to the RRC messages output from the wireless units  110 - 1 , . . . , for a predetermined period of time. Thus, the control unit  125  measures the number of RRC messages per unit time. 
         [0136]    Then, the macro-cell base station  100 - 1  determines whether or not the number of RRC messages is equal to or greater than the message threshold (S 12 ). For example, the control unit  125  performs determination by comparing the measured number of RRC messages per unit time and the message threshold read from the memory  126 . 
         [0137]    When the number of RRC messages is equal to or greater than the message threshold (YES in S 12 ), the macro-cell base station  100 - 1  performs transition determination and transition processing of the C-Plane processing as a condition of moving to the neighboring small-cell base station  100 - 2  (S 13 ). The process of S 13  corresponds to the above-described “2) transition determination and transition processing of the C-Plane”. 
         [0138]    When the number of RRC messages is less than the message threshold (NO in S 12 ), the macro-cell base station  100 - 1  releases the condition of moving to the neighboring small-cell base station  100 - 2  (S 14 ). In this case, the macro-cell base station  100 - 1  does not perform “2) transition determination and transition processing of the C-Plane”. 
         [0139]    2. Transition Determination And Transition Processing of C-Plane 
         [0140]    Next, the transition determination and the transition processing of the C-Plane will be described.  FIGS. 9 to 18  are diagrams illustrating the transition determination and the transition processing of the C-Plane. Among these drawings,  FIG. 9  illustrates an example of the entire processing of the transition determination and the transition processing. 
         [0141]    In  FIG. 9 , processing for selecting a method used for narrowing the terminal  200 - 2  which is a target causing the C-Plane processing to be transitioned is also included. A narrowing method may be one method or a method in combination of methods. In the second embodiment, narrowing is performed by combining a location and mobility of the terminal  200 - 2 . For example, it is possible to set the terminal  200 - 2  which continuously stays in the service area of the small-cell base station  100 - 2  for a period which is equal to or longer than the predetermined period of time, as a target terminal which causes the C-Plane processing to be transitioned. 
         [0142]    As illustrated in  FIG. 9 , if the process is started (S 20 ), the macro-cell base station  100 - 1  confirms a communication status of the terminal  200  (S 21 ), and determines whether or not the wireless communication system  10  has the C/U separation HetNet configuration (S 22 ). 
         [0143]    When it is determined that the wireless communication system  10  does not have the C/U separation HetNet configuration (NO in S 22 ), the macro-cell base station  100 - 1  causes the process to proceed to S 21  and waits until the wireless communication system  10  has the C/U separation HetNet configuration. 
         [0144]    When it is determined that the wireless communication system  10  has the C/U separation HetNet configuration (YES in S 22 ), the macro-cell base station  100 - 1  selects a narrowing method for the terminal  200  which is a C-Plane transition target (S 23 ). 
         [0145]    As the narrowing method, a position of the terminal  200  (“narrowing by using a UE location”), mobility of the terminal  200 - 2  (“narrowing by using mobility”), and other conditions (“narrowing by using other conditions”) are used. As the narrowing method, any of the three methods may be used or combination of these methods may be used (S 24  to S 25  and S 29  to S 32 ). 
         [0146]    Narrowing may be performed by using other conditions (YES in S 30 , S 32 ). This method will be described with reference to  FIGS. 19 and 20 . 
         [0147]    If the narrowing method is selected, the macro-cell base station  100 - 1  determines whether or not the terminal  200 - 2  is set as a transition target of the C-Plane processing (S 26 ). Details of the determination method will be described later with reference to  FIGS. 10 to 13 . 
         [0148]    When the terminal  200 - 2  is set as the transition target of the C-Plane processing (YES in S 27 ), the macro-cell base station  100 - 1  performs transition of the C-Plane processing and ends a series of processes (S 28 ). When the terminal  200 - 2  is not set as the transition target of the C-Plane processing (NO in S 27 ), the macro-cell base station  100 - 1  causes the process to proceed to S 21  and repeats the above-described processes. 
         [0149]      FIGS. 10 to 13  are sequence diagrams illustrating the operation example of the C-Plane processing transition.  FIGS. 10 to 13  illustrate details of the processes of S 26  and S 27  in  FIG. 9 , for example. 
         [0150]    As a whole, the processes are performed in an order of 1) registration processing of the terminal  200 - 2  to the macro-cell base station  100 - 1  ( FIG. 10 ), 2) setting processing for CA between the macro-cell base station  100 - 1  and the small-cell base station  100 - 2  ( FIG. 11 ), 3) narrowing processing according to the selected narrowing method (or C-Plane transition determination processing) ( FIG. 12 ), and 4) transition processing of the C-Plane at last ( FIG. 13 ). The processes will be described below in this order. 
         [0151]    2.1 Registration Processing to Macro-Cell Base Station 
         [0152]      FIG. 10  illustrates an example of the registration processing to the macro-cell base station  100 - 1 . The terminal  200 - 2  transmits Attach Request to the macro-cell base station  100 - 1  (S 40 ). This transmission causes the terminal  200 - 2  to request registration (or connection) to the macro-cell base station  100 - 1 . 
         [0153]    Then, the macro-cell base station  100 - 1  transmits Attach Request to the MME  300  (S 41 ). For example, the macro-cell base station  100 - 1  transmits the request and thus requests setting of the C-Plane for the terminal  200 - 1  to the MME  300 . 
         [0154]    Then, the macro-cell base station  100 - 1  transmits Create Session Request to the MME  300  (S 42 ). For example, the macro-cell base station  100 - 1  requests setting of the U-Plane for the terminal  200 - 1  to the MME  300 . 
         [0155]    Then, the MME  300  transmits Create Session Response to the macro-cell base station  100 - 1  (S 43 ). For example, the MME  300  transmits a response to Create Session Request to the macro-cell base station  100 - 1 . 
         [0156]    Then, the MME  300  transmits Initial ContextSetup Request/Attach Accept to the macro-cell base station  100 - 1  (S 44 ). For example, the MME  300  transmits a response message indicating permission for the connection request to the macro-cell base station  100 - 1 . 
         [0157]    Then, the macro-cell base station  100 - 1  transmits RRC Connection Reconfiguration to the terminal  200 - 2  (S 45 ). For example, the macro-cell base station  100 - 1  transmits a response message indicating permission for the connection request (S 40 ) to the terminal  200 - 2 . 
         [0158]    Then, the terminal  200 - 2  transmits RRC Connection Reconfiguration Complete to the macro-cell base station  100 - 1  (S 46 ). For example, the terminal  200 - 2  requests the U-Plane to be used to the macro-cell base station  100 - 1  allowed to be connected (S 46 ). 
         [0159]    Then, the terminal  200 - 2  transmits Direct Transfer to the macro-cell base station  100 - 1  (S 47 ). For example, the terminal  200 - 2  declares using of a radio section between the terminal  200 - 2  and the macro-cell base station  100 - 1  to the macro-cell base station  100 - 1 . 
         [0160]    Then, the macro-cell base station  100 - 1  transmits Attach Complete to the MME  300  (S 48 ). For example, the macro-cell base station  100 - 1  notifies the MME  300  of completion of connection processing with the terminal  200 - 2 . 
         [0161]    Then, the terminal  200 - 2  transmits user data to the macro-cell base station  100 - 1 , and the macro-cell base station  100 - 1  transmits the user data to the S-GW  400  (S 49 ). 
         [0162]    The MME  300  receives Attach Complete, sets an SRB of the terminal  200 - 2  and transmits Bearer Request to the S-GW  400  so as to request setting of a DRB for the terminal  200 - 2  to the S-GW  400  (S 50 ). 
         [0163]    For example, as illustrated in  FIG. 16 , the MME  300  sets an SRB ID “bbb” for the ID “xxx” of the terminal  200 - 2  and the ID “yyy” of the macro-cell base station  100 - 1 . The MME  300  may obtain the IDs of the terminal  200 - 2  and the macro-cell base station  100 - 1  in S 40 , S 41 , and the like, for example. Setting of the SRB as illustrated in  FIG. 16  causes, for example, a path for the C-Plane from the terminal  200 - 2  to the S-GW  400  through the macro-cell base station  100 - 1  to be set. The control signal is exchanged by using the path. 
         [0164]    Returning to  FIG. 10 , if Bearer Request is received, the S-GW  400  sets a DRB for the terminal  200 - 1  and transmits Bearer Response to the MME  300  (S 51 ). The S-GW  400  sets the DRB, for example, based on the IDs and the like of the macro-cell base station  100 - 1  and the terminal  200 - 2 , which are included in Bearer Request, and sets each of the IDs as illustrated in  FIG. 16 . Thus, for example, a path for the U-Plane from the terminal  200 - 2  to the S-GW  400  through the macro-cell base station  100 - 1  is set. The user data is exchanges by using the path. Information regarding the DRB which is set by the S-GW  400  may be transmitted to the MME  300  by Bearer Response (S 51 ). 
         [0165]    Returning to  FIG. 10 , then, the S-GW  400  transmits the user data to the macro-cell base station  100 - 1  in accordance with the set DRB, and the macro-cell base station  100 - 1  transmits the user data to the terminal  200 - 2  (S 52 ). 
         [0166]    2.2 Setting Processing of CA Between Base Stations 
         [0167]      FIG. 11  illustrates an example of the setting processing of CA between the base stations. The process continues from the process of  FIG. 10 . 
         [0168]    The terminal  200 - 2  transmits Measurement Report to the macro-cell base station  100 - 1  (S 55 ). For example, the terminal  200 - 2  requests performing of CA between the base stations by transmitting Measurement Report. 
         [0169]    Then, the macro-cell base station  100 - 1  determines whether or not performing of CA between the base stations for the small-cell base station  100 - 2  is capable (S 56 ). For example, the macro-cell base station  100 - 1  determines whether or not an installation location of the small-cell base station  100 - 2  is in a range of the service area  100 -M of the macro-cell base station  100 - 1 . 
         [0170]    When it is determined that performing of CA between the base stations for the small-cell base station  100 - 2  is impossible (NO in S 56 ), the macro-cell base station  100 - 1  causes the process to proceed to S 56  and repeats the above-described processes. 
         [0171]    When it is determined that performing of CA between the base stations for the small-cell base station  100 - 2  is capable (YES in S 56 ), the macro-cell base station  100 - 1  transmits CA Request relating to CA between the base stations to the small-cell base station  100 - 2  (S 57 ). For example, the macro-cell base station  100 - 1  requests performing of CA between the base stations to the small-cell base station  100 - 2 . 
         [0172]    The small-cell base station  100 - 2  which receives CA Request transmits CA Acknowledge to the macro-cell base station  100 - 1  (S 58 ). For example, the small-cell base station  100 - 2  notifies the macro-cell base station  100 - 1  to permit CA between the base stations. 
         [0173]    Then, the macro-cell base station  100 - 1  transmits RRC Connection Reconfiguration to the terminal  200 - 2  (S 59 ). For example, the macro-cell base station  100 - 1  notifies the terminal  200 - 2  to permit the request for CA between the base stations. 
         [0174]    Then, the terminal  200 - 2  transmits RRC Connection Reconfiguration Complete to the macro-cell base station  100 - 1  (S 60 ). For example, the terminal  200 - 2  notifies the macro-cell base station  100 - 1  of the U-Plane to be used for the small-cell base station  100 - 2 . 
         [0175]    Then, the macro-cell base station  100 - 1  transmits SN Status Transfer to the small-cell base station  100 - 2  (S 61 ). For example, the macro-cell base station  100 - 1  transmits information regarding CA between the base stations to the small-cell base station  100 - 2 . 
         [0176]    Then, the small-cell base station  100 - 2  transmits the user data to the terminal  200 - 2  (S 62 ). The terminal  200 - 2  transmits the user data to the small-cell base station  100 - 2  (S 63 ). 
         [0177]    Next, the small-cell base station  100 - 2  transmits Path Switch Request to the MME  300  (S 64 ). For example, the small-cell base station  100 - 2  requests performing of CA between the base stations with the macro-cell base station  100 - 1  to the MME  300  by transmitting a path change request to the MME  300 . The small-cell base station  100 - 2  may transmit Path Switch Request by using reception of CA Request (S 57 ), SN Status Transfer (S 61 ), or the user data (S 63 ) as a trigger. 
         [0178]    Then, the MME  300  transmits Modify Bearer Request to the S-GW  400  (S 65 ). For example, the MME  300  requests setting of the DRB between the small-cell base station  100 - 2  and the terminal  200 - 2  to the S-GW  400 . 
         [0179]    In this process (S 65 ), the MME  300  may cause information regarding the set SRB to be included in Modify Bearer Request and transmit the information to the S-GW  400 . The S-GW  400  may exchange the control signal with the terminal  200 - 2  in accordance with the information regarding the set SRB. 
         [0180]    Then, the S-GW  400  performs setting relating to the DRB of the terminal  200 - 2  and transmits Modify Bearer Response to the MME  300  (S 66 ). 
         [0181]      FIG. 17  illustrates a setting example of the DRB and the SRB after the DRB is set by the terminal  200 - 2 . As illustrated in  FIG. 17 , the S-GW  400  newly sets a DRB ID “ccc” for the ID “xxx” of the terminal  200 - 2 , the ID “zzz” of the small-cell base station  100 - 2 . Thus, for example, a path for the U-Plane from the terminal  200 - 2  to the S-GW  400  through the small-cell base station  100 - 2  may be added, and a state where CA between the base stations is capable may occur. 
         [0182]    Returning to  FIG. 11 , then, the S-GW  400  transmits user data to the small-cell base station  100 - 2  in accordance with the set DRB, and the small-cell base station  100 - 2  transmits the user data to the terminal  200 - 2  (S 67 ). In this case, the S-GW  400  may transmit the user data to the macro-cell base station  100 - 1  in accordance with the set DRB. 
         [0183]    Then, the MME  300  transmits Path Switch Request Acknowledge to the small-cell base station  100 - 2  (S 68 ). For example, the MME  300  transmits a permission response message to the path change request (S 64 ) to the small-cell base station  100 - 2 . 
         [0184]    2.3 Narrowing Processing (or Transition Determination Processing of C-Plane) 
         [0185]      FIG. 12  is a diagram illustrating an example of transition determination processing of the C-Plane. This processing is performed after the setting processing (for example,  FIG. 11 ) of CA between the base stations is performed. 
         [0186]    As described above, the narrowing processing is performed based on determination of a position of the terminal  200 - 2  (“narrowing and mixing of UE locations”) and mobility of the terminal  200 - 2  (“narrowing of UE mobility”). Position determination is performed in the small-cell base station  100 - 2  and corresponds to the processes of S 71  to S 75 . Mobility determination is performed in the macro-cell base station  100 - 1  and corresponds to the processes of S 76  to S 79 . 
         [0187]    The position determination is performed as follows. That is, when user data is transmitted to the terminal  200 - 2 , the small-cell base station  100 - 2  starts a timer (S 70 , S 71 ). For example, the RTT measurement unit  137  of the small-cell base station  100 - 2  starts the timer from a point of time when a baseband signal is transmitted to the terminal  200 - 2  from the transmission baseband unit  131 - 1 . 
         [0188]    Then, the small-cell base station  100 - 2  receives a channel quality indictor (CQI or transmission quality) for the transmitted user data (S 70 ) (S 72 ) and stops the timer at a time of reception (S 73 ). For example, the RTT measurement unit  137  stops the timer at a point of time when a CQI signal is received from the reception baseband unit  131 - 2 . 
         [0189]    Then, the small-cell base station  100 - 2  measures an RTT value based on a measured timer value (S 74 ). For example, the RTT measurement unit  137  measures a period from a start of the timer to a stop of the timer, and sets a half of the period (or half time in transmission and reception) as an RTT. In this case, the RTT measurement unit  137  may measure the RTT considering an offset value, an offset frame, or the like for a slot position of the user data in a radio frame. 
         [0190]    Then, the small-cell base station  100 - 2  determines the service area  100 -S of the small-cell base station  100 - 2  in which the terminal  200 - 2  stays, by using the RTT value (S 75 ). 
         [0191]    For example, a position of the terminal  200 - 2  staying in the service area  100 -S is determined as follows. That is, the small-cell base station  100 - 2  and the terminal  200  perform sampling on a radio signal which is transmitted and received, at a sampling frequency of 30.72 MHz. In this case, a transmission period of time corresponding to one cycle of the radio signal is 32.55 ns (=1/30.72M). If a transmission period of time of an electromagnetic wave in the air is set to 5 ns/m, accuracy of about 6.5 m (≅32.55/5) for the radio signal may be secured. If the radius of the service area  100 -S is set to 300 m, the radium of the service area  100 -S is divided into 46 (≅300/6.5) areas, and the small-cell base station  100 - 2  may determine whether or not the terminal  200 - 2  is positioned in the divided areas.  FIG. 14  illustrates a division example of the service area  100 -S, in which division is performed in this manner. For example, the RTT measurement unit  137  calculates a straight distance (=5 ns×RTT) from the small-cell base station  100 - 2  to the terminal  200 - 2 , based on the RTT value, and calculates a position of the calculated straight distance in the  46  divided areas. Thus, the RTT measurement unit  137  outputs the calculated position information (for example, the area # 1 , the area # 2 , and the like as illustrated in  FIG. 14 ) to the control unit  134 . The control unit  134  temporarily stores the position information in an internal memory and the like. Accordingly, the small-cell base station  100 - 2  can acquire a position of the terminal  200 - 2 . 
         [0192]    Returning to  FIG. 12 , after the CQI information is transmitted to the small-cell base station  100 - 2  (S 72 ), the terminal  200 - 2  operates the timer for 10 seconds (S 76 ). For example, the control unit  233  of the terminal  200 - 2  performs this processing by holding the timer therein and operating the timer. 
         [0193]    Then, the terminal  200 - 2  measures a movement speed of the terminal  200 - 2  by using the speed sensor  232  (S 77 ). 
         [0194]    Then, the terminal  200 - 2  determines whether or not 10 seconds elapses (S 78 ). When 10 seconds does not elapse (NO in S 78 ), the terminal  200 - 2  causes the process to proceed to S 76  and repeats the above-described processes. 
         [0195]    When 10 seconds elapses (YES in S 78 ), the terminal  200 - 2  calculates an average value of movement speeds for 10 seconds (S 79 ). For example, the speed sensor  232  immediately outputs the measured movement speed to the control unit  233  and the control unit  233  calculates the average value of movement speeds for 10 seconds. 
         [0196]    Then, the terminal  200 - 2  transmits Measurement Report to the macro-cell base station  100 - 1  (S 80 ). For example, the terminal  200 - 2  transmits a movement average speed to the macro-cell base station  100 - 1  by using Measurement Report. For example, the control unit  233  generates Measurement Report including the average value of the calculated movement speeds, and instructs the baseband unit  220  and the wireless unit  210  to transmit generated Measurement Report. 
         [0197]    If Measurement Report is received (S 80 ), the macro-cell base station  100 - 1  determines whether or not the movement speed of the terminal  200 - 2  is equal to or faster than a speed threshold (S 81 ). For example, the control unit  125  of the macro-cell base station  100 - 1  reads a movement speed threshold stored in the memory  126 , and performs determination by comparing the movement speed threshold and a movement speed average included in Measurement Report. 
         [0198]    When the movement speed is faster than the speed threshold (NO in S 81 ), the macro-cell base station  100 - 1  causes the process to proceed to S 80  and repeats the above-described processes. 
         [0199]    When the movement speed is equal to or less than the speed threshold (YES in S 81 ), the macro-cell base station  100 - 1  sets the terminal  200 - 2  as a C-Plane processing transition target terminal candidate (S 82 ). For example, when the movement speed of the terminal  200 - 2  is equal to or less than the movement speed threshold, the control unit  125  stores information of the terminal  200 - 2  as the C-Plane processing transition target terminal candidate, in the memory  126 . 
         [0200]    Then, the macro-cell base station  100 - 1  inquires a position of the terminal  200 - 2  to the small-cell base station  100 - 2  by using the X2 interface (S 83 ). 
         [0201]    The small-cell base station  100 - 2  notifies the macro-cell base station  100 - 1  of the position information of the terminal  200 - 2  as a response to the inquiry (S 83 ) of the position of the terminal  200 - 2  (S 84 ). For example, the control unit  134  of the small-cell base station  100 - 2  reads the position information (S 75 , for example, area # 1  and the like) of the terminal  200 - 2 , which is stored in the memory  136 , and transmits the read position information to the macro-cell base station  100 - 1  through the transmission channel interface unit  132 . 
         [0202]    Then, the macro-cell base station  100 - 1  determines whether or not the C-Plane is transitioned, based on the mobility (or movement speed average value (S 79 )) of the terminal  200 - 2  and the position (or position information (S 75 )) of the terminal  200 - 2  (S 85 ). In the second embodiment, the macro-cell base station  100 - 1  performs determination by using the HO determination matrix. 
         [0203]      FIG. 15  is a diagram illustrating an example of the HO determination matrix  1261 . For example, the HO determination matrix  1261  is stored in the memory  126  of the macro-cell base station  100 - 1 . The HO determination matrix  1261  has an item of “cell area” and an item of “movement speed”. For example, the “cell area” corresponds to the position information of the terminal  200 - 2 , and the “movement speed” corresponds to the movement speed average value of the terminal  200 - 2 . In the HO determination matrix  1261 , “S” and “M” are stored in correspondence with the two items. “S” indicates handover to the small-cell base station  100 - 2 , for example. “M” indicates that handover to the small-cell base station  100 - 2  is not performed and the terminal  200 - 2  is held to the macro-cell base station  100 - 1 , for example. 
         [0204]    For example, the control unit  125  acquires the corresponding items “S” or “M” of the HO determination matrix  1261 , based on the movement speed average value (S 80 ) acquired from the terminal  200 - 2  and the position information (S 84 ) of the terminal  200 - 2  acquired from the small-cell base station  100 - 2 . In a case of “S”, the control unit  125  determines that the C-Plane transition processing is performed to the small-cell base station  100 - 2 . In a case of “M”, the control unit  125  determines that the transition processing of the C-Plane is not performed. 
         [0205]    The HO determination matrix  1261  illustrated in  FIG. 15  is as follows, for example. 
         [0206]    That is, the terminal  200 - 2  which stays at a cell edge (area number “ 46 ”, “ 45 ”, and the like) in the service area  100 -S of the small-cell base station  100 - 2  has a probability of moving the macro-cell base station  100 - 1  to the service area  100 -M, higher than that in other cases regardless of the movement speed. Thus, the C-Plane processing of the terminal  200 - 2  which stays at the cell edge in the service area  100 -S is held to the macro-cell base station  100 - 1 . 
         [0207]    The terminal  200 - 2  which stays at the center (area number “ 1 ”, “ 2 ”, and the like) of the service area  100 -S of the small-cell base station  100 - 2  has a probability of staying in the service area  100 -S for a period which is equal to or longer than the predetermined period of time, higher than that in other cases regardless of the movement speed. Thus, the C-Plane processing of such a terminal  200 - 2  is transitioned to the small-cell base station  100 - 2 . It is possible to reduce a C-Plane processing load of the macro-cell base station  100 - 1  and to avoid occurrence of congestion in the C-Plane processing by transitioning the C-Plane processing to the small-cell base station  100 - 2  from the macro-cell base station  100 - 1 . 
         [0208]    When the movement speed is slower than that in other cases, the C-Plane processing of the terminal  200 - 2  in the vicinity (area number “ 18 ”, “ 19 ”, and the like) of the intermediate position of the service area  100 -S is transitioned to the small-cell base station  100 - 2 . This is, for example, because the terminal  200 - 2  staying at such a position has a probability of staying in the service area  100 -S for a period which is equal to or longer than the predetermined period of time, higher than that in other cases. 
         [0209]    When the movement speed is slower than that in other cases in the vicinity of the intermediate position of the service area  100 -S, the C-Plane processing of the terminal  200 - 2  is not transitioned and the terminal  200 - 2  is held to be under the macro-cell base station  100 - 1 . This is, for example, because the terminal  200 - 2  staying at such a position has a probability of moving the terminal  200 - 2  from the service area  100 -S of the small-cell base station  100 - 2  to the service area  100 -M of the macro-cell base station  100 - 1 , higher than that in other cases. 
         [0210]    Returning to  FIG. 12 , when it is determined that handover is performed, that is, the C-Plane of the terminal  200 - 2  is transitioned to the small-cell base station  100 - 2 , based on the HO determination matrix  1261  (YES in S 86 ), the macro-cell base station  100 - 1  causes the process to proceed to the processes of  FIG. 13 . 
         [0211]    The macro-cell base station  100 - 1  does not perform handover, based on the HO determination matrix  1261 . That is, when it is determined that the C-Plane processing is not transitioned (NO in S 86 ), the macro-cell base station  100 - 1  causes the process to proceed to S 80  and repeats the above-described processes. 
         [0212]    2.4 Transition Processing of C-Plane 
         [0213]      FIG. 13  illustrates transition processing of the C-Plane. This processing is realized through handover (which may refer to “HO” below) processing for the terminal  200 - 2  from the macro-cell base station  100 - 1  to the small-cell base station  100 - 2 . 
         [0214]    The macro-cell base station  100 - 1  transmits HandOver Request to the small-cell base station  100 - 2  (S 90 ). For example, the macro-cell base station  100 - 1  requests transition of the C-Plane processing to the small-cell base station  100 - 2 . Such a request is generated, for example, by the control unit  125  in the macro-cell base station  100 - 1  and is transmitted to the small-cell base station  100 - 2  through the transmission channel interface unit  122 . 
         [0215]    Then, the small-cell base station  100 - 2  transmits HandOver Acknowledge to the macro-cell base station  100 - 1  (S 91 ). For example, the small-cell base station  100 - 2  transmits a permission response to the transition request of the C-Plane processing to the macro-cell base station  100 - 1 . The permission response is used for permitting the transition request. 
         [0216]    Then, the macro-cell base station  100 - 1  transmits RRC Connection Reconfiguration to the terminal  200 - 2  (S 92 ). For example, the macro-cell base station  100 - 1  notifies the terminal  200 - 2  to transition the C-Plane processing to the small-cell base station  100 - 2  from the macro-cell base station  100 - 1 . 
         [0217]    Then, the terminal  200 - 2  transmits RRC Connection Reconfiguration Complete to the macro-cell base station  100 - 1  (S 93 ). 
         [0218]    Then, the macro-cell base station  100 - 1  transmits SN status Transfer to the small-cell base station  100 - 2  (S 94 ). For example, the macro-cell base station  100 - 1  notifies the small-cell base station  100 - 2  of information regarding handover (information regarding the ID of the terminal  200 - 2  or transmission packets, and the like). 
         [0219]    Then, the small-cell base station  100 - 2  transmits Path Switch Request to the MME  300  (S 95 ). For example, the small-cell base station  100 - 2  requests the C-Plane processing of the terminal  200 - 2  to the MME  300  so as to be set in the small-cell base station  100 - 2 . 
         [0220]    For example, a request is performed by Path Switch Request. The request indicates that the small-cell base station  100 - 2  transitions the C-Plane processing for the terminal  200 - 2  to the small-cell base station  100 - 2  from the macro-cell base station  100 - 1 . In this case, the small-cell base station  100 - 2  notifies the MME  300  of the transition request by using the handover request (S 90 ) or reception of notification (S 94 ) of information regarding handover as a trigger. Thus, the macro-cell base station  100 - 1  requests transition of the C-Plane processing to the small-cell base station  100 - 2 , to the MME  300  by the handover request or the notification of the information regarding handover. These messages (S 90  or S 94  and S 95 ) represent a request message for requesting transition of the C-Plane processing, for example. 
         [0221]    Then, the MME  300  changes the setting of the SRB so as to cause the C-Plane to be transitioned to the small-cell base station  100 - 2 , and transmits Path Switch Request Acknowledge to the small-cell base station  100 - 2  (S 96 ). For example, the MME  300  changes the setting of the SRB of the terminal  200 - 2  so as to be a path from the terminal  200 - 2  through the macro-cell base station  100 - 1 , and be a path through the small-cell base station  100 - 2 . 
         [0222]      FIG. 18  illustrates a setting example of the SRB and the DRB after the C-Plane processing is transitioned. As illustrated in  FIG. 18 , regarding the SRB, the SRB ID “bbb” is set for the ID “xxx” of the terminal  200 - 2  and the ID “zzz” of the small-cell base station  100 - 2 . Regarding the DRB, the DRB ID “ccc” is also set for the ID “xxx” of the terminal  200 - 2 , the ID “zzz” of the small-cell base station  100 - 2 . The DRB may be set, for example, by the MME  300  transmitting Modify Bearer Request to the S-GW  400 . The above setting is performed, for example, by the control unit  320  of the MME  300 . 
         [0223]    Returning to  FIG. 13 , if Path Switch Request Acknowledge is received from the MME  300  (S 96 ), the small-cell base station  100 - 2  transmits UE Context Release to the macro-cell base station  100 - 1  (S 97 ). For example, the small-cell base station  100 - 2  notifies the macro-cell base station  100 - 1  of ending of transition of the C-Plane processing, and notifies the macro-cell base station  100 - 1  to enable deletion of information regarding the terminal  200 - 2  which has been maintained. 
         [0224]    2.5 Other Conditions of Narrowing Method 
         [0225]    Next, other conditions of the narrowing method will be described. The conditions described herein correspond to “other conditions” (YES in S 30 , S 32 ) in  FIG. 9 , for example. For example, in the macro-cell base station  100 - 1 , the terminal  200 - 2  which is a transition target of the C-Plane processing is set based on the movement speed (for example, S 82  in  FIG. 12 ) or is determined based on the HO determination matrix  1261  (for example, S 85  in  FIG. 12 ). Here, an example in which such a terminal  200 - 2  is excluded from the transition target of the C-Plane processing when the terminal  200 - 2  is selected (for example, S 82  or S 85  in  FIG. 12 ) will be described. It is possible to reduce, for example, processing load of the macro-cell base station  100 - 1  by such determination. 
         [0226]    For example, when the handover frequency is greater than a predetermined frequency threshold, in such a terminal  200 - 2 , handover from the macro-cell base station  100 - 1  to the small-cell base station  100 - 2  or the reverse handover occurs more than that in other cases. In addition, the staying position of the terminal  200 - 2  is immediately changed in many cases. In the second embodiment, regarding the frequency of handover, the terminal  200 - 2  which performs repetition the frequency threshold or more times may be excluded from terminals  200 - 2  which are set as the transition target of the C-Plane processing. Such a terminal  200 - 2  has a probability of continuously staying in the service area  100 -S of the small-cell base station  100 - 2  for a period which is equal to longer than the predetermined period of time, smaller than that in other cases. 
         [0227]      FIG. 19  illustrates an example of calculation processing of the handover frequency. For example, the calculation processing is performed by the control unit  233  of the terminal  200 . 
         [0228]    If the process is started (S 100 ), the terminal  200  confirms an HO frequency report timing (S 101 ) and determines to perform reporting at the HO frequency report timing (YES in S 102 ). The terminal  200  determines that reporting is not performed at timings other than the HO frequency report timing (NO in S 102 ). 
         [0229]    The terminal  200  confirms the HO frequency report timing (YES in S 102 ), and a transmission message of RRC Connection Reconfiguration Complete (S 103 ), and determines whether or not the message is transmitted (S 104 ). 
         [0230]    When the message is transmitted, the terminal  200  increments the HO frequency (S 105 ). As the message, a message transmitted when the terminal  200  performs handover is used. 
         [0231]    When the message is not transmitted (NO in S 104 ), the terminal  200  causes the process to proceed to S 101  and repeats the above-described processes. 
         [0232]    If the HO frequency is incremented, the terminal  200  determines whether or not the processing is ended (S 106 ). When the processing is not ended (NO in S 106 ), the terminal  200  causes the process to proceed to S 101  and repeats the above-described processes. When the processing is ended (YES in S 106 ), the terminal  200  ends a series of processes. 
         [0233]    When the current timing is not the HO frequency report timing (NO in S 102 ), the terminal  200  transmits the HO frequency which is obtained by performing counting until now to the macro-cell base station  100 - 1  (S 108 ). For example, the terminal  200  transmits the HO frequency by using an uplink dedicated control channel (UL DCCH). 
         [0234]    Then, the terminal  200  resets the HO frequency (S 109 ), and causes the process to proceed to S 101 . 
         [0235]      FIG. 20  illustrates an example in which the HO frequency is counted in the macro-cell base station  100 - 1 . In this case, the macro-cell base station  100 - 1  determines whether or not a selection of the terminal  200  among terminal candidates of the C-Plane processing transition is confirmed (S 121 , S 122 ), instead of the HO frequency report timing (S 102 , S 103  in  FIG. 19 ). The macro-cell base station  100 - 1  determines a count timing of the HO frequency at a timing when the selection is confirmed. 
         [0236]    Instead of transmission (S 104  in  FIG. 19 ) of RRC Connection Reconfiguration Complete, reception (S 134 ) of the message causes the HO frequency to be incremented (S 135 ). 
         [0237]    As another example of the narrowing method, it may be determined whether or not the macro-cell base station  100 - 1  performs transition of the C-Plane processing, based on the attribute of the terminal  200 . For example, the macro-cell base station  100 - 1  may perform determination based on attribute information of the terminal  200  (for example, the person having the terminal  200  is a very important person (VIP) user), which is received as a notification from the terminal  200 . 
         [0238]    For example, there is an example in which the terminal  200  is a smart phone. The terminal  200  may notify the macro-cell base station  100 - 1  of being a smart phone as the attribute information. In addition, instead of transmission of a movement speed average value as mobility information by the terminal  200  (S 76  to S 79  in  FIG. 12 ), the terminal  200  may transmit a fixed value to the macro-cell base station  100 - 1  such that the transition processing of the C-Plane to the small-cell base station  100 - 2  is not performed. 
         [0239]      FIG. 21  illustrates an operation example of the case. That is, the terminal  200 - 2  sets a DRB between two base stations  100 - 1  and  100 - 2  by CA between the base stations (S 150 , S 151 ), and an SRB is set in the macro-cell base station  100 - 1  as the C/U separation HetNet (S 151 ). 
         [0240]    The terminal  200 - 2  determines the mobility (S 152 ) and reports the fixed value as a smart phone (S 153 , S 154 ). The macro-cell base station  100 - 1  acquires a position of the terminal  200 - 2  (S 155 ). The position may be acquired through the processes of S 71  to S 75  in  FIG. 12 . 
         [0241]    The macro-cell base station  100 - 1  determines whether or not the terminal is the C-Plane processing transition target, based on the reported mobility information (for example, fixed value) of the terminal  200 - 2  and the position information of the terminal  200 - 2  (S 156 , S 157 ). In this case, since the fixed value is set to have an extent that transition to the small-cell base station  100 - 2  is impossible, the macro-cell base station  100 - 1  determines that transition of the C-Plane processing to the small-cell base station  100 - 2  is not performed (NO in S 157 ). Thus, the C-Plane processing is maintained to be performed by the macro-cell base station  100 - 1  (S 158 , S 159 ). 
         [0242]    In this manner, in the second embodiment, the processing quantity for the control signal is equal to or greater than the threshold, and the processing for the control signal of the terminal  200 - 2  which stays in the service area  100 -S for a period which is equal to or longer than the predetermined period of time is transitioned to the small-cell base station  100 - 2  from the macro-cell base station  100 - 1 . Thus, for example, the C-Plane processing of the terminal  200 - 2  in the macro-cell base station  100 - 1  is performed by the small-cell base station  100 - 2  and the processing load of the C-Plane processing is reduced. Accordingly, it is possible to avoid occurrence of congestion of the C-Plane processing in the macro-cell base station  100 - 1 . 
       OTHER EMBODIMENTS 
       [0243]    Next, other embodiments will be described. 
         [0244]    In the above-described second embodiment, determination based on the RTT value is performed as the position of the terminal  200  (S 71  to S 75  in  FIG. 12 ). Additionally, for example, a reception power value of a reference signal received by the terminal  200  may be reported to the small-cell base station  100 - 2 , and the areas # 1  to # 46  in the service area  100 -S may be distinguished from each other based on the reception power value. In this case, the macro-cell base station  100 - 1  may exclude the terminal  200  which is determined to be at an area edge (for example, area # 45 , # 46 , or the like) of the service area  100 -S, from transition targets of the C-Plane processing. 
         [0245]    In addition, instead of the reception power value, the terminal  200 - 2  may acquire position information of the terminal  200 - 2  by using Global Positioning System (GPS) and report the position information to the small-cell base station  100 - 2 . 
         [0246]    Regarding such position information of the terminal  200 - 2 , the macro-cell base station  100 - 1  may exclude the terminal  200 - 2  staying at the cell edge of the service area  100 -S of the small-cell base station  100 - 2  from transition targets of the C-Plane processing. 
         [0247]    In the above-described second embodiment, an example of the average movement speed for the mobility information of the terminal  200  is described (S 76  to S 79  in  FIG. 12 ). Instead of the average movement speed, the mobility information of an UE may be acquired by using a Hold button of the terminal  200 - 2 . For example, the Hold button indicates that the terminal  200 - 2  stays in the field and does not move. If the Hold button is pressed, the terminal  200 - 2  may notify the macro-cell base station  100 - 1  of pressing of the Hold button and a period of time when the Hold button is pressed. In such a case, the macro-cell base station  100 - 1  expects that such the terminal  200 - 2  does not move from the small-cell base station  100 - 2 . Thus, it is possible to transition the C-Plane processing to the small-cell base station  100 - 2  may be performed. 
         [0248]    The terminal  200 - 2  may acquire GPS information, report the acquired GPS information to the macro-cell base station  100 - 1 , and use the reported information in determination of the mobility information of the terminal  200 . In addition, information of an acceleration sensor, a pedometer, or the like of the terminal  200 - 2  may be reported to the macro-cell base station  100 - 1  as the mobility information of the terminal  200 . 
         [0249]    In the second embodiment, a case where the C-Plane processing of the terminal  200 - 2  is transitioned to the small-cell base station  100 - 2  is described. After transition, in the small-cell base station  100 - 2 , the mobility information of the terminal  200 - 2  may be acquired or the position information may be acquired in the macro-cell base station  100 - 1 . The transition of the C-Plane processing to the macro-cell base station  100 - 1  may be determined by using the HO determination matrix  1261 . In this case, in the HO determination matrix  1261 , “M” indicates performing of HO from the small-cell base station  100 - 2  to the macro-cell base station  100 - 1  (transition of the C-Plane processing to the macro-cell base station  100 - 1 ), and “S” indicates that HO is not performed (where the C-Plane processing is still performed in the small-cell base station  100 - 2 ) in  FIG. 15 . 
         [0250]    In the second embodiment, the configuration examples of the macro-cell base station  100 - 1 , the small-cell base station  100 - 2 , and the terminal  200  are described.  FIG. 22  illustrates a configuration example of hardware of the macro-cell base station  100 - 1 .  FIG. 23  illustrates a configuration example of hardware of the small-cell base station  100 - 2 .  FIG. 24  illustrates a configuration example of hardware of the terminal  200 . 
         [0251]    The macro-cell base station  100 - 1  includes a central processing unit (CPU)  150 , a read only memory (ROM)  151 , a random access memory (RAM)  152 , and an internal bus  153 . 
         [0252]    The CPU  150  reads a program stored in the ROM  151  and loads the program onto the RAM  152  so as to conduct the loaded program. Thus, the CPU  150  can perform the functions of the baseband unit  121  and the control unit  125 . The CPU  150  corresponds to the baseband unit  121  and the control unit  125  in the second embodiment, for example. 
         [0253]    The small-cell base station  100 - 2  includes a CPU  160 , a ROM  161 , a RAM  162 , and an internal bus  163 . 
         [0254]    The CPU  160  also reads a program stored in the ROM  161  and loads the program onto the RAM  162  so as to conduct the loaded program. Thus, for example, the CPU  160  can perform the functions of the transmission baseband unit  131 - 1 , the reception baseband unit  131 - 2 , the timing control unit  133 , the control unit  134 , and the RTT measurement unit  137 . The CPU  160  corresponds to the two baseband units  131 - 1  and  131 - 2 , the timing control unit  133 , the control unit  134 , and the RTT measurement unit  137  in the second embodiment, for example. 
         [0255]    The terminal  200  includes a first CPU  250 , a ROM  251 , a RAM  252 , a memory  253 , and a second CPU  254 , and an internal bus  255 . 
         [0256]    The first CPU  250  and the second CPU  254  read a program stored in the ROM  251  and loads the program onto the RAM  252  so as to conduct the loaded program. Thus, for example, the first CPU  250  and the second CPU  254  can perform the functions of the baseband unit  220 , the control unit  233 , the application control unit  221 , and the like. For example, the first CPU  250  corresponds to the baseband unit  220  and the control unit  233 , and the second CPU  254  corresponds to the application control unit  221 . 
         [0257]    The CPU  150 ,  160 , and  250  illustrated in  FIGS. 22 to 24  may be other controllers such as a micro processing unit (MPU) and a field programmable gate array (FPGA). The control unit  320  of the MME  300  or the control unit  420  and the data transmission unit  440  of the S-GW  400  may be controllers such as the CPU and the MPU. 
         [0258]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
A base station including: an antenna configured to form a second cell, and a processor configured to: process a control signal with a terminal that is located in an overlapping area of a first cell and the second cell, the first cell being formed by another base station that processes a data signal with the terminal, and transfer processing of the control signal with the terminal to the other base station when a load of the processing of the control signal is more than a first threshold in the base station and when the terminal is located in the overlapping area for more than a given length of time.