Patent Publication Number: US-2021168624-A1

Title: Communication system, base station device and communication terminal device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 from U.S. application Ser. No. 16/183,307 filed Nov. 7, 2018 which is a continuation of Ser. No. 14/903,519, filed Jan. 7, 2016, now U.S. Pat. No. 10,149,174 issued Dec. 4, 2018, which is a national stage of International Application No. PCT/JP2014/064904, filed Jun. 5, 2014, which is based upon and claims the benefit of priority under 35 U.S.C. § 119 from Japanese Patent Application No. 2013-148201, filed Jul. 17, 2013. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a communication system that performs radio communication between a communication terminal device and a base station device. 
     BACKGROUND ART 
     The 3rd generation partnership project (3GPP) that is a standard organization regarding a mobile communication system is studying new communication systems referred to as long term evolution (LTE) regarding radio sections and system architecture evolution (SAE) regarding the overall system configuration including a core network and a radio access network (hereinafter, collectively referred to as a network as well). This communication system is also referred to as 3.9 generation (3.9 G) system. 
     As an access scheme of the LTE, orthogonal frequency division multiplexing (OFDM) is used in a downlink direction, and single carrier frequency division multiple access (SC-FDMA) is used in an uplink direction. Further, differently from wideband code division multiple access (W-CDMA), circuit switching is not provided but a packet communication system is only provided in the LTE. 
     The current decisions by 3GPP regarding the frame configuration in the LTE system described in Non-Patent Document 1 (Chapter 5) will be described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating the configuration of a radio frame used in the LTE communication system. With reference to  FIG. 1 , one radio frame is 10 ms. The radio frame is divided into ten equally sized subframes. The subframe is divided into two equally sized slots. The first and sixth subframes contain a downlink synchronization signal (SS) per each radio frame. The synchronization signals are classified into a primary synchronization signal (P-SS) and a secondary synchronization signal (S-SS). 
     Non-Patent Document 1 (Chapter 5) describes the current decisions by 3GPP regarding the channel configuration in the LTE system. It is assumed that the same channel configuration is used in a closed subscriber group (CSG) cell as that of a non-CSG cell. The CSG cell will be described below (see Chapter 3.1 of Non-Patent Document 2). 
     A physical broadcast channel (PBCH) is a channel for downlink transmission from a base station to a mobile terminal. A BCH transport block is mapped to four subframes within a 40 ms interval. There is no explicit signaling indicating 40 ms timing. 
     A physical control format indicator channel (PCFICH) is a channel for downlink transmission from the base station to the mobile terminal. The PCFICH notifies the number of OFDM (Orthogonal Frequency Division Multiplexing) symbols used for PDCCHs from the base station to the mobile terminal. The PCFICH is transmitted in each subframe. 
     A physical downlink control channel (PDCCH) is a channel for downlink transmission from the base station to the mobile terminal. The PDCCH notifies the resource allocation information for a downlink shared channel (DL-SCH) being one of the transport channels described later, resource allocation information for a paging channel (PCH) being one of the transport channels described later, and HARQ (Hybrid Automatic Repeat reQuest) information related to DL-SCH. The PDCCH carries an uplink scheduling grant. The PDCCH carries acknowledgement (Ack)/negative acknowledgement (Nack) that is a response signal to uplink transmission. The PDCCH is referred to as an L1/L2 control signal as well. 
     A physical downlink shared channel (PDSCH) is a channel for downlink transmission from the base station to the mobile terminal. A downlink shared channel (DL-SCH) that is a transport channel and a PCH that is a transport channel are mapped to the PDSCH. 
     A physical multicast channel (PMCH) is a channel for downlink transmission from the base station to the mobile terminal. A multicast channel (MCH) that is a transport channel is mapped to the PMCH. 
     A physical uplink control channel (PUCCH) is a channel for uplink transmission from the mobile terminal to the base station. The PUCCH carries Ack/Nack that is a response signal to downlink transmission. The PUCCH carries a channel quality indicator (CQI) report. The CQI is quality information indicating the quality of received data or channel quality. In addition, the PUCCH carries a scheduling request (SR). 
     A physical uplink shared channel (PUSCH) is a channel for uplink transmission from the mobile terminal to the base station. An uplink shared channel (UL-SCH) that is one of the transport channels is mapped to the PUSCH. 
     A physical hybrid ARQ indicator channel (PHICH) is a channel for downlink transmission from the base station to the mobile terminal. The PHICH carries Ack/Nack that is a response signal to uplink transmission. A physical random access channel (PRACH) is a channel for uplink transmission from the mobile terminal to the base station. The PRACH carries a random access preamble. 
     A downlink reference signal (RS) is a known symbol in the LTE communication system. The following five types of downlink reference signals are defined: cell-specific reference signals (CRS), MBSFN reference signals, data demodulation reference signal (DM-RS) being UE-specific reference signals, positioning reference signals (PRS), and channel-state information reference signals (CSI-RS). The physical layer measurement objects of a mobile terminal include reference signal received power (RSRP). 
     The transport channels described in Non-Patent Document 1 (Chapter 5) will be described. A broadcast channel (BCH) among the downlink transport channels shown in  FIG. 5(A)  is broadcast to the entire coverage of a base station (cell). The BCH is mapped to the physical broadcast channel (PBCH). 
     Retransmission control according to a hybrid ARQ (HARD) is applied to a downlink shared channel (DL-SCH). The DL-SCH can be broadcasted to the entire coverage of the base station (cell). The DL-SCH supports dynamic or semi-static resource allocation. The semi-static resource allocation is also referred to as persistent scheduling. The DL-SCH supports discontinuous reception (DRX) of a mobile terminal for enabling the mobile terminal to save power. The DL-SCH is mapped to the physical downlink shared channel (PDSCH). 
     The paging channel (PCH) supports DRX of the mobile terminal for enabling the mobile terminal to save power. The PCH is required to be broadcast to the entire coverage of the base station (cell). The PCH is mapped to physical resources such as the physical downlink shared channel (PDSCH) that can be used dynamically for traffic. 
     The multicast channel (MCH) is used for broadcast to the entire coverage of the base station (cell). The MCH supports SFN combining of MBMS services (MTCH and MCCH) in multi-cell transmission. The MCH supports semi-static resource allocation. The MCH is mapped to the PMCH. 
     Retransmission control according to a hybrid ARQ (HARQ) is applied to an uplink shared channel (UL-SCH) among the uplink transport channels. The UL-SCH supports dynamic or semi-static resource allocation. The UL-SCH is mapped to the physical uplink shared channel (PUSCH). 
     A random access channel (RACH) is limited to control information. The RACH involves a collision risk. The RACH is mapped to the physical random access channel (PRACH). 
     The HARQ will be described. The HARQ is the technique for improving the communication quality of a channel by combination of automatic repeat request (ARQ) and error correction (forward error correction). The HARQ is advantageous in that error correction functions effectively by retransmission even for a channel whose communication quality changes. In particular, it is also possible to achieve further quality improvement in retransmission through combination of the reception results of the first transmission and the reception results of the retransmission. 
     An example of the retransmission method will be described. If the receiver fails to successfully decode the received data, in other words, if a cyclic redundancy check (CRC) error occurs (CRC=NG), the receiver transmits “Nack” to the transmitter. The transmitter that has received “Nack” retransmits the data. If the receiver successfully decodes the received data, in other words, if a CRC error does not occur (CRC=OK), the receiver transmits “AcK” to the transmitter. The transmitter that has received “Ack” transmits the next data. 
     The logical channels described in Non-Patent Document 1 (Chapter 6) will be described. A broadcast control channel (BCCH) is a downlink channel for broadcast system control information. The BCCH that is a logical channel is mapped to the broadcast channel (BCH) or downlink shared channel (DL-SCH) that is a transport channel. 
     A paging control channel (PCCH) is a downlink channel for transmitting paging information and system information change notifications. The PCCH is used when the network does not know the cell location of a mobile terminal. The PCCH that is a logical channel is mapped to the paging channel (PCH) that is a transport channel. 
     A common control channel (CCCH) is a channel for transmission control information between mobile terminals and a base station. The CCCH is used in the case where the mobile terminals have no RRC connection with the network. In a downlink direction, the CCCH is mapped to the downlink shared channel (DL-SCH) that is a transport channel. In an uplink direction, the CCCH is mapped to the uplink shared channel (UL-SCH) that is a transport channel. 
     A multicast control channel (MCCH) is a downlink channel for point-to-multipoint transmission. The MCCH is used for transmission of MBMS control information for one or several MTCHs from a network to a mobile terminal. The MCCH is used only by a mobile terminal during reception of the MBMS. The MCCH is mapped to the multicast channel (MCH) that is a transport channel. 
     A dedicated control channel (DCCH) is a channel that transmits dedicated control information between a mobile terminal and a network on a point-to-point basis. The DCCH is used if the mobile terminal has an RRC connection. The DCCH is mapped to the uplink shared channel (UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) in downlink. 
     A dedicated traffic channel (DTCH) is a point-to-point communication channel for transmission of user information to a dedicated mobile terminal. The DTCH exists in uplink as well as downlink. The DTCH is mapped to the uplink shared channel (UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) in downlink. 
     A multicast traffic channel (MTCH) is a downlink channel for traffic data transmission from a network to a mobile terminal. The MTCH is a channel used only by a mobile terminal during reception of the MBMS. The MTCH is mapped to the multicast channel (MCH). 
     A CGI represents a cell global identifier. An ECGI represents an E-UTRAN cell global identifier. A closed subscriber group (CSG) cell is introduced in the LTE, and the long term evolution advanced (LTE-A) and universal mobile telecommunication system (UMTS) described below. 
     The closed subscriber group (CSG) cell is a cell in which subscribers who are allowed to use are specified by an operator (hereinafter, also referred to as a “cell for specific subscribers”). The specified subscribers are allowed to access one or more cells of a public land mobile network (PLMN). One or more cells in which the specified subscribers are allowed access are referred to as “CSG cell(s).” Note that access is limited in the PLMN. 
     The CSG cell is part of the PLMN that broadcasts a specific CSG identity (CSG ID; CSG-ID) and broadcasts “TRUE” in a CSG indication. The authorized members of the subscriber group who have registered in advance access the CSG cells using the CSG-ID that is the access permission information. 
     The CSG-ID is broadcast by the CSG cell or cells. A plurality of CSG-IDs exist in the LTE communication system. The CSG-IDs are used by mobile terminals (UEs) for making access from CSG-related members easier. 
     The locations of mobile terminals are tracked on the basis of an area composed of one or more cells. The locations are tracked for enabling tracking the locations of mobile terminals and calling mobile terminals, in other words, incoming calling to mobile terminals even in an idle state. An area for tracking locations of mobile terminals is referred to as a tracking area. 
     3GPP is studying base stations referred to as Home-NodeB (Home-NB; HNB) and Home-eNodeB (Home-eNB; HeNB). HNB/HeNB is a base station for, for example, household, corporation, or commercial access service in UTRAN/E-UTRAN. Non-Patent Document 3 discloses three different modes of the access to the HeNB and HNB. Specifically, an open access mode, a closed access mode, and a hybrid access mode are disclosed. 
     The respective modes have the following characteristics. In the open access mode, the HeNB and HNB are operated as a normal cell of a normal operator. In the closed access mode, the HeNB and HNB are operated as a CSG cell. The CSG cell is a CSG cell where only CSG members are allowed access. In the hybrid access mode, the HeNB and HNB are operated as CSG cells where non-CSG members are allowed access at the same time. In other words, a cell in the hybrid access mode (also referred to as a hybrid cell) is the cell that supports both the open access mode and the closed access mode. 
     In 3GPP, among all physical cell identifiers (PCI), there is a range of PCIs reserved by the network for use by CSG cells (see Chapter 10.5.1.1 of Non-Patent Document 1). Division of the PCI range is also referred to as PCI split. The information about PCI split (also referred to as PCI split information) is broadcast in the system information from a base station to mobile terminals being served thereby. To be served by a base station means to take the base station as a serving cell. 
     Non-Patent Document 4 discloses the basic operation of a mobile terminal using PCI split. The mobile terminal that does not have the PCI split information needs to perform cell search using all PCIs, for example, using all 504 codes. On the other hand, the mobile terminal that has the PCI split information is capable of performing cell search using the PCI split information. 
     Further, 3GPP is pursuing specifications standard of long term evolution advanced (LTE-A) as Release 10 (see Non-Patent Documents 5 and 6). The LTE-A is based on the LTE communication system regarding radio sections and is configured by addition of several new techniques thereto. 
     Carrier aggregation (CA) is studied in the LTE-A system, in which two or more component carriers (CCs) are aggregated to support wider transmission bandwidths up to 100 MHz. 
     In the case where a CA is configured, a UE has a single RRC connection with a network (NW). In RRC connection, one serving cell provides NAS mobility information and security input. This cell is referred to as a primary cell (PCell). In downlink, a carrier corresponding to a PCell is a downlink primary component carrier (DL PCC). In uplink, a carrier corresponding to PCell is an uplink primary component carrier (UL PCC). 
     A secondary cell (SCell) is configured to form a serving cell group with a PCell, in accordance with the UE capability. In downlink, a carrier corresponding to SCell is a downlink secondary component carrier (DL SCC). In uplink, a carrier corresponding to SCell is an uplink secondary component carrier (UL SCC). 
     A serving cell group of one PCell and one or more SCells is configured for one UE. 
     Further, new techniques in the LTE-A include the technique of supporting wider bands (wider bandwidth extension) and the coordinated multiple point transmission and reception (CoMP) technique. The CoMP studied for LTE-A in 3GPP is described in Non-Patent Document 7. 
     Further, 3GPP is pursuing specifications standard of Release 12. Among the specifications, in order to satisfy a tremendous volume of traffic in the future, the use of small eNBs configuring a small cell is studied. Examples of the study include the technique of increasing spectral efficiency by configuring a large number of small cells by installing a large number of small eNBs to increase communication capacity. 
     A traffic amount of a mobile network is in an increasing tendency, and increasing a communication speed is also progressed. When the application of the LTE and the LTE-A is started on a full scale, the communication speed is further increased, and an increase of the traffic amount is expected. 
     A conventional mobile communication system has a problem that the probability of occurrence of a delay in the network and loss of data may further increase (hereinafter, also referred to as an “occurrence probability”) as a traffic amount increases. A technique to solve such a problem is disclosed in Patent Document 1, for example. 
     Patent Document 1 discloses a user device having means for selecting a data transmission cell based on a measurement value of a congestion level in a cell. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Application Laid-Open No. 2003-134550 
     Non-Patent Documents 
     Non-Patent Document 1: 3GPP TS 36.300 V11.4.0 
     Non-Patent Document 2: 3GPP TS 36.304 V11.1.0 Chapter 3.1 
     Non-Patent Document 3: 3GPP S1-083461 
     Non-Patent Document 4: 3GPP R2-082899 
     Non-Patent Document 5: 3GPP TR 36.814 V9.0.0 
     Non-Patent Document 6: 3GPP TR 36.912 V10.0.0 
     Non-Patent Document 7: 3GPP TR 36.819 V11.1.0 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     According to the technique disclosed in Patent Document 1, a user device selects a cell. In the case where a plurality of user devices exist in the communication system, the user devices select cells individually. In this case, there is a risk that a cell having a relatively low congestion level at a time point of measuring the congestion level is selected by a plurality of user devices so that load of the cell increases. Thus, with the technique disclosed in Patent Document 1, it is not possible to reduce the occurrence probability of a delay in the network and the loss of data. 
     Further, according to the technique disclosed in Patent Document 1, the user device selects a cell having a low congestion level in downlink. Patent Document 1 fails to expressly disclose uplink. With the technique disclosed in Patent Document 1, it is not possible to select a cell in consideration of congestion in uplink. 
     An object of the present invention is to provide a communication system capable of reducing the occurrence probability of a delay in the network and loss of data. 
     Means for Solving the Problems 
     A communication system according to the present invention includes a communication terminal device, and one or a plurality of base station devices that perform radio communication with the communication terminal device. The communication system includes a plurality of cells that are configured by the one or the plurality of base station devices, and perform radio communication with the communication terminal device by being connected to the communication terminal device, and a higher-level device that is provided in a higher level of the base station device. A cell to which the communication terminal device is to be connected is selected from among the plurality of cells, based on a flow of at least either one of received data that each cell has received from the communication terminal device and transmission data that each cell has transmitted to the higher-level device. 
     Effects of the Invention 
     According to the communication system of the present invention, a cell to which the communication terminal device is to be connected is selected from among the plurality of cells, based on a flow of at least either one of received data that each cell has received from the communication terminal device and transmission data that each cell has transmitted to the higher-level device. Accordingly, in the case where a traffic volume in the communication system has increased, load can be dispersed to each cell. Therefore, because concentration of traffic in a specific cell can be prevented, it is possible to reduce the occurrence probability of a delay in the entire network including a core network and loss of data. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of a radio frame used in an LTE communication system. 
         FIG. 2  is a block diagram showing the overall configuration of an LTE communication system  700  currently under discussion of 3GPP. 
         FIG. 3  is a block diagram showing the configuration of a user equipment  71  of  FIG. 2  being a user equipment according to the present invention. 
         FIG. 4  is a block diagram showing the configuration of a base station  72  of  FIG. 2  being a base station according to the present invention. 
         FIG. 5  is a block diagram showing the configuration of an MME according to the present invention. 
         FIG. 6  is a flowchart showing an outline from a cell search to an idle state operation performed by a user equipment (UE) in the LTE communication system. 
         FIG. 7  is a diagram showing the concept of the configuration of cells in which macro eNBs and small eNBs coexist. 
         FIG. 8  is a block diagram showing the configuration of a communication system  1400  according to a first embodiment of the present invention. 
         FIG. 9  is a diagram showing a flow direction of data in the communication system  1400  according to the first embodiment of the present invention. 
         FIG. 10  is a diagram showing a configuration of a protocol stack in a communication system  1410  according to a conventional technique. 
         FIG. 11  is a diagram showing the configuration of a protocol stack in the communication system  1400  according to the first embodiment of the present invention. 
         FIG. 12  is a block diagram showing the configuration of a communication system  1500  according to a second embodiment of the present invention. 
         FIG. 13  is a block diagram showing the configuration of a communication system  1600  according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 2  is a block diagram showing an overall configuration of an LTE communication system  700 , which is currently under discussion of 3GPP.  FIG. 2  will be described. A radio access network will be referred to as an evolved universal terrestrial radio access network (E-UTRAN)  70 . A mobile terminal device (hereinafter, referred to as a “mobile terminal (user equipment (UE))”  71  being a communication terminal device is capable of performing radio communication with a base station device (hereinafter, referred to as a “base station (E-UTRAN NodeB: eNB)”  72 , and transmits and receives signals through radio communication. 
     A control protocol RRC between the user equipment  71  and the base station  72  performs broadcast, paging, RRC connection management, and the like. The states of the base station  72  and the user equipment  71  in RRC are classified into RRC_IDLE and RRC_CONNECTED. 
     In RRC_IDLE, public land mobile network (PLMN) selection, system information (SI) broadcast, paging, cell re-selection, mobility, and the like are performed. In RRC_CONNECTED, the user equipment has RRC connection and is capable of transmitting/receiving data to/from a network. In RRC_CONNECTED, for example, handover (HO) and measurement of a neighbor cell are performed. 
     The base stations  72  are classified into eNBs  76  and Home-eNBs  75 . The communication system  700  includes an eNB group  72 - 1  including a plurality of eNBs  76 , and a Home-eNB group  72 - 2  including a plurality of Home-eNBs  75 . A system configured by an EPC (Evolved Packet Core) being a core network and an E-UTRAN  70  being a radio access network will be referred to as an EPS (Evolved Packet System). The EPC being a core network and the E-UTRAN  70  being the radio access network are also collectively referred to as a “network”. 
     The eNB  76  is connected to an MME/S-GW unit (hereinafter, also referred to as an “MME unit”)  73  including an MME, or an S-GW, or an MME and an S-GW by means of an S1 interface, and control information is communicated between the eNB  76  and the MME unit  73 . A plurality of MME units  73  may be connected to one eNB  76 . The eNBs  76  are connected to each other by means of an X2 interface, and control information is communicated between the eNBs  76 . 
     The Home-eNB  75  is connected to the MME unit  73  by means of an S1 interface, and control information is communicated between the Home-eNB  75  and the MME unit  73 . A plurality of Home-eNBs  75  are connected to one MME unit  73 . Alternatively, the Home-eNBs  75  are connected to the MME units  73  through a HeNBGW (Home-eNB GateWay)  74 . The Home-eNB  75  is connected to the HeNBGW  74  by an S1 interface, and the HeNBGW  74  is connected to the MME unit  73  by means of an S1 interface. 
     One or a plurality of Home-eNBs  75  are connected to one HeNBGW  74 , and information is communicated therebetween by means of an S1 interface. The HeNBGW  74  is connected to one or a plurality of MME units  73 , and information is communicated therebetween by means of an S1 interface. 
     The MME units  73  and HeNBGW  74  are higher-level devices, specifically, higher nodes, and control connections between the eNB  76  and the Home-eNB  75  being base stations, and the user equipment (UE)  71 . The MME unit  73  configures an EPC being a core network. The base station  72  and the HeNBGW  74  configure the E-UTRAN  70 . 
     Further, 3GPP has studied the configuration below. The X2 interface between the Home-eNBs  75  is supported. In other words, the Home-eNBs  75  are connected to each other by means of an X2 interface, and control information is communicated between the Home-eNBs  75 . The HeNBGW  74  appears to the MME unit  73  as the Home-eNB  75 . The HeNBGW  74  appears to the Home-eNB  75  as the MME unit  73 . 
     The interfaces between the Home-eNBs  75  and the MME units  73  are the same, which are the S1 interfaces, in the both cases where the Home-eNB  75  is connected to the MME unit  73  through the HeNBGW  74  and the Home-eNB  75  is directly connected to the MME unit  73 . 
     The base station device  72  may configure a single cell, or may configure a plurality of cells. Each cell has a range predetermined as a coverage in which the cell can communicate with a communication terminal device, and the cell performs radio communication with the communication terminal device within the coverage. In the case where one base station device configures a plurality of cells, every cell is configured to communicate with a mobile terminal. 
       FIG. 3  is the block diagram showing the configuration of the user equipment  71  of  FIG. 2  being a user equipment according to the present invention. The transmission process of the user equipment  71  illustrated in  FIG. 3  will be described. First, a transmission data buffer unit  803  stores the control data from a protocol processing unit  801  and the user data from an application unit  802 . The data stored in the transmission data buffer unit  803  is passed to an encoding unit  804  and is subjected to an encoding process such as error correction. There may exist the data output from the transmission data buffer unit  803  directly to a modulating unit  805  without the encoding process. The data encoded by the encoding unit  804  is modulated by the modulating unit  805 . The modulated data is converted into a baseband signal, and the baseband signal is output to a frequency converting unit  806  and is then converted into a radio transmission frequency. After that, a transmission signal is transmitted from an antenna  807  to the base station  72 . 
     The user equipment  71  executes the reception process as follows. The radio signal from the base station  72  is received through the antenna  807 . The received signal is converted from a radio reception frequency into a baseband signal by the frequency converting unit  806  and is then demodulated by a demodulating unit  808 . The demodulated data is passed to a decoding unit  809  and is subjected to a decoding process such as error correction. Among the pieces of decoded data, the control data is passed to the protocol processing unit  801 , while the user data is passed to the application unit  802 . A series of processes of the user equipment  71  is controlled by a control unit  810 . This means that, though not illustrated in  FIG. 3 , the control unit  810  is connected to the respective units  801  to  809 . 
       FIG. 4  is a block diagram showing the configuration of the base station  72  of  FIG. 2  being a base station according to the present invention. The transmission process of the base station  72  illustrated in  FIG. 4  will be described. An EPC communication unit  901  performs data transmission and reception between the base station  72  and the EPC (such as the MME unit  73 ) and the HeNBGW  74 . A communication with another base station unit  902  performs data transmission and reception to and from another base station. The EPC communication unit  901  and the communication with another base station unit  902  each transmit and receive information to and from a protocol processing unit  903 . The control data from the protocol processing unit  903 , and the user data and control data from the EPC communication unit  901  and the communication with another base station unit  902  are stored in a transmission data buffer unit  904 . 
     The data stored in the transmission data buffer unit  904  is passed to an encoding unit  905  and is then subjected to an encoding process such as error correction. There may exist the data output from the transmission data buffer unit  904  directly to a modulating unit  906  without the encoding process. The encoded data is modulated by the modulating unit  906 . The modulated data is converted into a baseband signal, and the baseband signal is output to a frequency converting unit  907  and is then converted into a radio transmission frequency. After that, a transmission signal is transmitted from an antenna  908  to one or a plurality of user equipments  71 . 
     The reception process of the base station  72  is executed as follows. A radio signal from one or a plurality of user equipments  71  is received through the antenna  908 . The received signal is converted from a radio reception frequency into a baseband signal by the frequency converting unit  907 , and is then demodulated by a demodulating unit  909 . The demodulated data is passed to a decoding unit  910  and is then subjected to a decoding process such as error correction. Among the pieces of decoded data, the control data is passed to the protocol processing unit  903 , the EPC communication unit  901 , or the communication with another base station unit  902 , while the user data is passed to the EPC communication unit  901  and the communication with another base station unit  902 . A series of processes by the base station  72  is controlled by a control unit  911 . This means that, though not illustrated in  FIG. 4 , the control unit  911  is connected to the respective units  901  to  910 . 
       FIG. 5  is a block diagram showing the configuration of the MME according to the present invention.  FIG. 5  illustrates the configuration of an MME  73   a  included in the MME unit  73  illustrated in  FIG. 2  described above. A PDN GW communication unit  1001  performs data transmission and reception between the MME  73   a  and a PDN GW. A base station communication unit  1002  performs data transmission and reception between the MME  73   a  and the base station  72  by means of the S1 interface. If the data received from the PDN GW is user data, the user data is passed from the PDN GW communication unit  1001  to the base station communication unit  1002  via a user plane communication unit  1003  and is then transmitted to one or a plurality of base stations  72 . If the data received from the base station  72  is user data, the user data is passed from the base station communication unit  1002  to the PDN GW communication unit  1001  via the user plane communication unit  1003  and is then transmitted to the PDN GW. 
     If the data received from the PDN GW is control data, the control data is passed from the PDN GW communication unit  1001  to a control plane control unit  1005 . If the data received from the base station  72  is control data, the control data is passed from the base station communication unit  1002  to the control plane control unit  1005 . 
     A HeNBGW communication unit  1004  is provided in the case where the HeNBGW  74  is provided, which performs data transmission and reception between the MME  73   a  and the HeNBGW  74  by means of the interface (IF) according to an information type. The control data received from the HeNBGW communication unit  1004  is passed from the HeNBGW communication unit  1004  to the control plane control unit  1005 . The processing results of the control plane control unit  1005  are transmitted to the PDN GW via the PDN GW communication unit  1001 . The processing results of the control plane control unit  1005  are transmitted to one or a plurality of base stations  72  by means of the S1 interface via the base station communication unit  1002 , and are transmitted to one or a plurality of HeNBGWs  74  via the HeNBGW communication unit  1004 . 
     The control plane control unit  1005  includes a NAS security unit  1005 - 1 , an SAE bearer control unit  1005 - 2 , an idle state mobility managing unit  1005 - 3 , or other unit, and performs an overall process for the control plane. The NAS security unit  1005 - 1  provides, for example, security of a non-access stratum (NAS) message. The SAE bearer control unit  1005 - 2  manages, for example, a system architecture evolution (SAE) bearer. The idle state mobility managing unit  1005 - 3  performs, for example, mobility management of an idle state (LTE-IDLE state, which is merely referred to as idle as well), generation and control of a paging signal in the idle state, addition, deletion, update, and search of a tracking area of one or a plurality of user equipments  71  being served thereby, and tracking area list management. 
     The MME  73   a  distributes a paging signal to one or a plurality of base stations  72 . In addition, the MME  73   a  performs mobility control of an idle state. When the user equipment is in the idle state and an active state, the MME  73   a  manages a list of tracking areas. The MME  73   a  begins a paging protocol by transmitting a paging message to the cell belonging to a tracking area in which the UE is registered. The idle state mobility managing unit  1005 - 3  may manage the CSG of the Home-eNBs  75  to be connected to the MME  73   a,  the CSG-IDs, and a whitelist. 
     An example of a cell search method in a mobile communication system will be described next.  FIG. 6  is the flowchart showing an outline from a cell search to an idle state operation performed by a user equipment (UE) in the LTE communication system. When starting a cell search, in Step ST 1201 , the user equipment synchronizes slot timing and frame timing by a primary synchronization signal (P-SS) and a secondary synchronization signal (S-SS) transmitted from a neighbor base station. 
     The P-SS and S-SS are collectively referred to as a synchronization signal (SS). Synchronization codes, which individually correspond to physical cell identities (PCIs) assigned per cell, are assigned to the synchronization signal (SS). The number of PCIs is currently studied in  504  ways. The  504  ways of PCIs are used for synchronization, and the PCIs of the synchronized cells are detected (specified). 
     In Step ST 1202 , next, with respect to the synchronized cells, the user equipment detects a cell-specific reference signal (CRS) being a reference signal (RS) transmitted from the base station per cell and measures the reference signal received power (RSRP). The codes individually corresponding to the PCIs are used for the reference signal RS. Separation from another cell is enabled by correlation using the code. The code for RS of the cell is derived from the PCI specified in Step ST 1201 , which makes it possible to detect the RS and measure the RS received power. 
     In Step ST 1203 , next, the user equipment selects the cell having the best RS reception quality, for example, the cell having the highest RS received power, that is, the best cell, from one or more cells that have been detected up to Step ST 1202 . 
     In Step ST 1204 , next, the user equipment receives the PBCH of the best cell and obtains the BCCH that is the broadcast information. A master information block (MIB) containing the cell configuration information is mapped to the BCCH over the PBCH. Accordingly, the MIB is obtained by obtaining the BCCH through reception of the PBCH. Examples of the MIB information include the downlink (DL) system bandwidth (also referred to as transmission bandwidth configuration (dl-bandwidth)), the number of transmission antennas, and system frame number (SFN). 
     In Step ST 1205 , next, the user equipment receives the DL-SCH of the cell on the basis of the cell configuration information of the MIB, to thereby obtain a system information block (SIB) 1 of the broadcast information BCCH. The SIB 1  contains information about the access to the cell, information about cell selection, and scheduling information on other SIB (SIBk; k is an integer equal to or larger than two). In addition, the SIB 1  contains a tracking area code (TAC). 
     In Step ST  1206 , next, the user equipment compares the TAC of the SIB 1  received in Step ST  1205  with the TAC portion of a tracking area identifier (TAI) in the tracking area list that has already been possessed by the user equipment. The tracking area list is also referred to as a TAI list. The TAI is identification information for identifying a tracking area, and is composed of an MCC (Mobile Country Code), an MNC (Mobile Network Code), and a TAC (Tracking Area Code). MCC is a country code. MNC is a network code. TAC is the code number of a tracking area. 
     If the TAC received in Step ST 1205  is identical to the TAC included in the tracking area list as a result of the comparison of Step ST 1206 , the user equipment enters an idle state operation in the cell. If the TAC received in Step ST 1205  is not included in the tracking area list as a result of the comparison, the user equipment requires a core network (EPC) including MME and the like to change a tracking area through the cell for performing tracking area update (TAU). 
     A device that configures a core network (hereinafter, also referred to as a “core network side device”) updates the tracking area list on the basis of an identification number (such as a UE-ID) of the user equipment transmitted from the user equipment together with a TAU request signal. The core network side device transmits the updated tracking area list to the user equipment. The user equipment rewrites (updates) the TAC list of the user equipment on the basis of the received tracking area list. After that, the user equipment enters the idle state operation in the cell. 
     Widespread use of smartphones and tablet terminals explosively increases traffic in cellular radio communications, causing a fear of insufficient radio resources all over the world. To increase spectral efficiency, accordingly, it is studied to downsize cells for further spatial separation. 
     In the configuration of a conventional cell, a cell configured by an eNB has a relatively-wide-range coverage. Conventionally, a cell is configured such that relatively-wide-range coverages of a plurality of cells configured by a plurality of eNBs cover a certain area. 
     In the case of the downsized cells, the cell configured by an eNB has a narrow-range coverage as compared with the coverage of a cell configured by a conventional eNB. Therefore, in order to cover a certain area as in the conventional case, a larger number of downsized eNBs than the conventional eNBs are necessary. 
     In the following description, like the cell configured by a conventional eNB, a cell configuring a relatively-wide-range coverage, that is, a cell having a relatively wide coverage area, will be referred to as a “macro cell”, and an eNB that configures the macro cell will be referred to as a “macro eNB”. Further, like a downsized cell that configures a relatively-narrow-range coverage, that is, a cell having a relatively narrow coverage area will be referred to as a “small cell”, and an eNB configuring the small cell will be referred to as a “small eNB”. 
     The macro eNB may be, for example, a “wide area base station” described in 3GPP TS 36.141 V11.1.0 (hereinafter, referred to as “Non-Patent Document 8”). 
     The small eNB may be, for example, a low power node, local area node, or hotspot. Alternatively, the small eNB may be a pico eNB configuring a pico cell, a femto eNB constituting a femto cell, HeNB, RRH, RRU, RRE, or RN. Still alternatively, a small eNB may be a “local area base station”, or a “home base station” as described in Non-Patent Document 8. 
       FIG. 7  is a diagram showing the concept of the configuration of cells in which macro eNBs and small eNBs coexist. A macro cell configured by a macro eNB has a relatively-wide-range coverage  1301 . A small cell configured by small eNBs has a narrow range coverage  1302  as compared with the coverage  1301  of a macro cell configured by macro eNBs. 
     In the case where a plurality of eNBs coexist, a coverage of a cell configured by a certain eNB may be included in the coverage of a cell configured by other eNB. In the configuration of a cell illustrated in  FIG. 7 , as indicated by a reference “ 1304 ” or “ 1305 ”, the coverage  1302  of a small cell configured by small eNBs may be included in the coverage  1301  of a macro cell configured by macro eNBs. 
     As indicated by a reference “ 1305 ”, the coverages  1302  of a plurality of, for example, “two”, small cells may be included in the coverage  1301  of one macro cell. The user equipment (UE)  1303  is included in the coverage  1302  of a small cell, for example, and performs communication through the small cell. 
     In the configuration of a cell illustrated in  FIG. 7 , as indicated by a reference “ 1306 ”, the coverage  1301  of a macro cell configured by macro eNBs and the coverage  1302  of a small cell configured by small eNBs may overlap each other in a complicated manner. 
     Further, as indicated by a reference “ 1307 ”, there also occurs a case where the coverage  1301  of a macro cell configured by macro eNBs and the coverage  1302  of a small cell configured by small eNBs may not overlap each other. 
     Further, as indicated by a reference “ 1308 ”, the coverage  1302  of a large number of small cells configured by a large number of small eNBs may be configured in the coverage  1301  of one macro cell configured by one macro eNB. 
     The problems solved and a solution in the first embodiment will be described below. In the present embodiment, considered here is a configuration that a coverage of small cells configured by small eNBs is included in the coverage of a macro cell configured by macro eNBs. 
     As described above, in order to satisfy an enormous traffic in the future, examples of the study include the technique of increasing spectral efficiency through installation of a large number of small cells by installing a large number of small eNBs to increase a communication capacity. 
     In the case where a large number of small cells are installed, it becomes a problem about to which small cell the UE is to be connected. 
     For example, considered here is a case where a small cell in which received power in the UE becomes maximum is selected like the conventional technique. In this case, when the load of the selected small cell is already excessively large, radio resources given to the UE connected to the small cell are reduced, and high throughput cannot be obtained. Therefore, there occurs a problem such as the occurrence of a high delay in the data transmission. 
     In the present embodiment, in order to solve such a problem, in selecting a small cell to be connected to the UE, information about a flow of data (hereinafter, also referred to as a “data flow”) is used. Thus, it is an object to improve a delay in the entire network by dispersing the process load of a small cell. 
       FIG. 8  is a block diagram showing the configuration of the communication system  1400  according to the first embodiment of the present invention. The communication system  1400  is configured to include a UE  1401 , a small cell cluster  1402 , an MME  1403 , an S-GW  1404 , a macro cell  1406 , and a concentrator  1407 . The small cell cluster  1402  includes one or a plurality of small cells  1405 . 
     The MME  1403  and the S-GW  1404  correspond to higher-level devices. In this case, the higher-level device refers to a device provided in a higher level of the base station device. That is, the higher-level device is provided on a core network side based on the base station device. As the higher-level devices, there are a higher-level entity and a higher-level node. 
     The base station device is a macro eNB and a small eNB, for example, and configures a cell like a macro cell and a small cell. In the case where a plurality of cells are installed, the plurality of cells may be configured by one base station device, or may be configured by a plurality of base station devices. 
       FIG. 8  illustrates a case where the small cell cluster  1402  includes three small cells  1405 . In the following description, in the case of distinctly indicating three small cells  1405 , by adding suffixes “a”, “b”, and “c” to the reference “ 1405 ”, the small cells  1405  will be respectively indicated as a first small cell # 1   1405   a,  a second small cell # 2   1405   b,  and a third small cell # 3   1405   c.  In the case of indicating three small cells  1405  without distinction, the small cell will be indicated by adding a reference “ 1405 ”. 
     The small cell  1405  is configured by a small eNB. The eNB that configures the small cell  1405  corresponds to Home-eNB  75  illustrated in  FIG. 2  described above, for example. The macro cell  1406  is configured by a macro eNB. The macro eNB that configures the macro cell  1406  corresponds to an eNB  76  illustrated in  FIG. 2  described above, for example. 
     The communication system  1400  according to the present embodiment is configured to include the concentrator  1407  between the macro cell  1406  and the small cell  1405 , unlike the architecture of the communication system of the conventional technique. The concentrator  1407  is an entity that collectively controls the small cells  1405  in the small cell cluster  1402  consisting of one or a plurality of small cells  1405 . 
     The macro cell  1406  and the concentrator  1407  are connected to each other by means of a new interface. The small cell  1405  and the concentrator  1407  are connected to each other by means of a new interface. A plurality of small cells  1405  are connected to one concentrator  1407 . In the example illustrated in  FIG. 8 , three small cells  1405  are connected to one concentrator  1407 . The small cells  1405  are connected to each other by means of an interface. In the case where each small cell is configured by one small eNB, an x2 interface is used as an interface between the small cells  1405 . 
       FIG. 9  is a diagram showing the flow direction of data in the communication system  1400  according to the first embodiment of the present invention. In  FIG. 9 , an uplink (UL) traffic is indicated by an arrow of a reference “ 41 ”, and a downlink (DL) traffic is indicated by an arrow of a reference “ 42 ”. 
     The concentrator  1407  has the function of obtaining a total of data flow in the small cell  1405  (hereinafter, also referred to as a “total data flow”) regarding each small cell  1405  in the small cell cluster  1402  as the own management item. Further, the concentrator  1407  has the function of selecting the small cell  1405  to which the UE  1401  is connected. 
     Each small cell  1405  has the function of measuring a data flow in the own cell, and the function of notifying the measured data flow in the own cell to the concentrator  1407 . It is preferable that when notifying the concentrator  1407  of the measured data flow in the own cell, each small cell  1405  notifies the data flow together with the cell identification information of the own cell. The cell identification information of each small cell  1405  is useful when the concentrator  1407  selects the small cell  1405 . 
     As the data flow, for example, information expressing a size of data (hereinafter, also referred to as a “data size”) or an amount of data (hereinafter, also referred to as a “data amount”) of data handled in each small cell  1405  may be used, and information expressing the data size of the data handled by the transmission data buffer unit when each small cell  1405  transmits the data to the UE  1401  may be used. 
     As detailed examples of the data flow concerning uplink of each small cell  1405  (hereinafter, also referred to as an “uplink data flow”), the following four examples (1) to (4) will be disclosed.
         (1) The data size of the data that each small cell  1405  has transmitted to a higher-level entity or a higher-level node (hereinafter, also referred to as “transmission data”).   (2) The data size of the data that each small cell  1405  has received from a lower-level entity or a lower-level node (hereinafter, also referred to as “received data”).   (3) Buffer status report (BSR) information that each small cell  1405  has received from the UE  1401  connected to the own cell  1405 .   (4) Combination of the examples (1) to (3).       

     BSR information (hereinafter, also simply referred to “BSR”) indicates a state of a buffer unit that temporarily stores data transmitted from the UE  1401  to each small cell  1405 . The buffer unit corresponds to the transmission data buffer unit  803  illustrated in  FIG. 3  described above. The BSR expresses a flow of received data. 
     More specifically, the BSR includes information expressing the data size of the data handled by the transmission data buffer unit of the UE  1401 . Therefore, by using the BSR, it becomes possible to measure the uplink data flow of each small cell  1405 . 
     In the present embodiment, each small cell  1405  has the function of measuring the data flow. Therefore, in the detailed example (1) of the uplink data flow, the data size may be that of the data that the small cell  1405  has transmitted to the concentrator  1407  being the higher-level node. In the detailed example (2), because the lower-level node becomes the UE  1401 , the data size may be that of the data that the small cell  1405  has received from the UE  1401  connected to the own cell  1405 . In the detailed example (3), the data size may be the BSR information that the small cell  1405  has received from the UE  1401  connected to the own cell  1405 . 
     In both the detailed examples (1) and (2), the data size may be that for each UE  1401  connected to the small cell  1405 , or may be a total data size of all the UEs  1401  connected to the small cell  1405 . 
     In the case of the data size of each UE  1401  connected to the small cell  1405 , the concentrator  1407  being the entity that obtains the data flow and selects the cell of a target UE  1401  may derive a total data size of all the UEs  1401  connected to each small cell  1405 . Accordingly, it becomes possible to drive the data flow of each small cell  1405 . In this case, because it becomes unnecessary for the small cell  1405  to derive the total data size of all the UEs  1401 , it becomes possible to simplify the control. 
     In the case of the total data size of all the UEs  1401  connected to the small cell  1405 , the concentrator  1407  being the entity that obtains the data flow and selects the cell of the target UE  1401  may set the total data size as the flow of each small cell  1405 . Because the small cell  1405  notifies the total data size of all the UEs  1401  to be connected, as compared with the case of notifying the data size of each UE  1401 , it becomes possible to decrease a calculation volume of the concentrator  1407 . 
     Also in the detailed example (3), the data size may be the BSR information of each UE  1401  connected to the small cell  1405 , or may be the BSR information of all the UEs  1401  connected to the small cell  1405 . Accordingly, it becomes possible to obtain the effects similar to those in the detailed examples (1) and (2). 
     The BSR information of all the UEs  1401  connected to the small cell  1405  may be a list that expresses a correspondence between the BSR information of each UE  1401  and the identification information of the UE  1401 , for example the UE-ID, or may be an uplink data size derived from the BSR information of all the UEs  1401  that the small cell  1405  has received. 
     Also, considered here is a case where a large number of small cells  1405  are used and the UE  1401  is connected to a plurality of cells. In this case, like the conventional technique, when the BSR is notified to the cell to which the control plane (C-plane) is to be connected, in other cells to which the UE  1401  is connected, the data flow attributable to the UE  1401  cannot be recognized. 
     In the present embodiment, the data size of each entity to which the UE  1401  is connected, that is, the data size of each cell, or the BSR is notified. The notification of the data size of each entity to which the UE  1401  is to be connected, or the BSR, may be performed on the entity or on the cell to which the control plane (C-plane) is to be connected. In the case where the notification is performed with respect to the cell to which the control plane (C-plane) is to be connected, for the data size or the BSR, a list that expresses a correspondence between the cell identification information and the data size or the BSR may be notified. 
     By configuring in this way, unlike the case where the BSR is notified to only the cell to which the conventional control plane (C-plane) is to be connected, even in the case where the UE  1401  is connected to a plurality of cells, the network side can recognize the data flow of each entity, that is, each cell. 
     Further, the data size may be the data size or the BSR information in the user plane (U-plane) connection. 
     In the case where a large number of small cells  1405  are used and the UE  1401  is connected to a plurality of cells, unlike the conventional technique, there is a case where the cell to which the control plane (C-plane) is to be connected and cell to which the user plane (U-plane) is to be connected are different. 
     Therefore, in the present embodiment, the data size or the BSR information specified for the entity or the node in the user plane (U-plane) connection is the data flow. Accordingly, it becomes possible to measure the data flow in the user plane (U-plane) connection. 
     For the measurement of the data flow, the flow during a predetermined period may be measured. As the predetermined period, a start time and a period may be set, or a start time and an end time may be set. The predetermined period may be statically predetermined, or may be set appropriately changeable semi-statically or dynamically. The data flow may be a packet number, a bit number, or a byte number per a predetermined period. Alternatively, the data flow may be a packet number, a bit number, or a byte number per a unit time obtained from the measurement of the data flow during a predetermined period. Change of the predetermined period may be performed by an instruction of the concentrator  1407 . 
     A node or an entity having the function of measuring the data flow notifies an entity or a node having the function of selecting the small cell  1405  to which the target UE  1401  is to be connected, of the information about the data flow. 
     In the present embodiment, each small cell  1405  is the node having the function of measuring the data flow, and the concentrator  1407  is the entity having the function of selecting the small cell  1405  to which the target UE  1401  is to be connected. In this case, each small cell  1405  notifies the concentrator  1407  of the information about the data flow. The concentrator  1407  receives the information about the data flow from each small cell  1405 . 
     As detailed examples of trigger of notifying the information about the data flow, the following three examples (1) to (3) will be disclosed.
         (1) Cyclical or periodical.   (2) By providing a predetermined threshold value, in the case where the data flow becomes equal to or higher than the threshold value, or in the case where the data flow exceeds the threshold value.   (3) In the case where a request has been received from the entity having the function of selecting a small cell.       

     As the interface to be used to notify the information about the data flow, a new interface may be provided between the small cell  1405  and the concentrator  1407 . 
     In order to decrease the information volume of the data flow, by dividing the data flow into one or a plurality of predetermined ranges, an index may be provided in each range. The index is set as information about the data flow. By such arrangement, it becomes possible to decrease the information volume notified between the nodes, such as the bit number, for example. Therefore, in the case where a large number of small cells  1405  are applied, because the information volume signaled between the nodes can be reduced, it becomes possible to reduce congestion. 
     In  FIG. 8  described above, a case where the BSR is used in the uplink data flow will be described (see Chapter 5.4.5 of 3GPP TS 36.321 V11.2.0 (hereinafter, referred to as “Non-Patent Document 9”)). The BSR notified from the UE  1401  is terminated by the small cell  1405  in the standard defined in Non-Patent Document 9. 
     The BSR is MAC information (see Non-Patent Document 9). According to the conventional base station, for example, the macro cell, a control protocol to the user equipment, for example, a radio resource control (RRC) and a user plane, for example, a packet data convergence protocol (PDCP), a radio link control (RLC), a medium access control (MAC), and a physical layer (PHY) are terminated in the base station. Accordingly, the BSR being the MAC information also becomes the information from the user equipment to the base station. 
     Therefore, according to the conventional method, in  FIG. 8 , the BSR becomes the information from a UE  1401  to the small cell  1405  connected to the UE  1401 . 
     According to the conventional method, the entity other than the eNB being the base station directly connected to the UE  1401  cannot realize selection of a connection cell based on the data flow of the cell. 
     Therefore, in the present embodiment, the BSR information is arranged to be notified from the small cell  1405  to the concentrator  1407 . 
     By using the data size of the transmission data notified from the BSR, the concentrator  1407  selects the small cell  1405  to which the UE  1401  is to be connected. 
     The concentrator  1407  notifies the small cell  1405  to which the target UE  1401  is currently connected, of the information of the selected small cell  1405  as the information of the small cell  1405  to which the UE  1401  is to be connected. The information of the selected small cell  1405  may be notified together with the identifier of the target UE  1401 . 
     The small cell  1405  to which the UE  1401  is currently connected receives the information of the small cell  1405  to which the target UE  1401  is to be connected, and transmits an RRC reconfiguration message, so that the information of the small cell  1405  to be connected is notified to the UE  1401 . 
     The UE  1401  executes connection to the small cell  1405  which the concentrator  1407  has selected, by using the received RRC reconfiguration message. 
     In the case where the RRC connection is not being performed between the small cell  1405  to which the UE  1401  is currently connected and the UE  1401 , the concentrator  1407  may notify the cell that has the RRC connection with the UE  1401  of the information of the small cell  1405  to which the UE  1401  is to be connected. The information of the small cell  1405  to which the UE  1401  is to be connected may be notified together with the identifier of the target UE  1401 . 
     The cell that has the RRC connection with the UE  1401  receives the information of the small cell  1405  to which the target UE  1401  is to be connected, and transmits an RRC reconfiguration message, so that the information of the small cell  1405  to be connected is notified to the UE  1401 . 
     For example, in the case where the cell that has the RRC connection with the UE  1401  is the macro cell  1406 , the concentrator  1407  notifies the macro cell  1406  of the information of the small cell  1405  to which the UE  1401  is to be connected. The macro cell  1406  receives the information of the small cell  1405  to which the target UE  1401  is to be connected, and transmits the RRC reconfiguration message, so that the information of the small cell  1405  to be connected is notified to the UE  1401 . 
     The UE  1401  executes connection to the small cell  1405  which the concentrator  1407  has selected, by using the received RRC reconfiguration message. 
     In the case where a connection destination has been notified for a target cell of handover, the UE  1401  executes handover to the notified small cell  1405  to be connected. 
     In the case where a connection destination has been notified as a secondary cell of carrier aggregation, the UE  1401  configures the notified small cell  1405  to be connected, as the secondary cell. 
       FIG. 10  is a diagram showing the configuration of the protocol stack in the communication system  1410  according to the conventional technique.  FIG. 11  is a diagram showing the configuration of the protocol stack in the communication system  1400  according to the first embodiment of the present invention. 
     According to the macro cell  1406  being the conventional base station, the user plane terminates up to PDCP, that is, the PDCP, RLC, MAC, and PHY. On the other hand, in the present embodiment, the small cell  1405  terminates the RLC, MAC, and PHY, and the macro cell  1406  terminates the PDCP. 
     The concentrator  1407  has the function of selecting the small cell  1405  connected to the target UE  1401 . 
     The BSR information is notified from the UE  1401  to the small cell  1405 . The small cell  1405  further notifies the concentrator  1407  of the BSR information. For example, the BSR information is notified from the UE  1401  to a third small cell # 3   1405   c , and is notified from the third small cell # 3   1405   c  to the concentrator  1407 . 
     Accordingly, the concentrator  1407  can obtain an uplink data flow of each small cell  1405 , and can select the small cell  1405  to be connected to the target UE  1401 , by using the uplink data flow. 
     The uplink data from the target UE  1401  is to be transmitted to the macro cell  1406  through the small cell  1405  selected by the concentrator  1407 . For example, in the case where the small cell  1405  selected by the concentrator  1407 , to be connected to the target UE  1401  is the first small cell # 1   1405   a,  the uplink data from the UE  1401  is transmitted to the macro cell  1406  through the first small cell # 1   1405   a.    
     In this way, in selecting the small cell  1405  to be connected to the UE  1401 , by using the information about the data flow, it becomes possible to avoid selecting the small cell  1405  of a large uplink data flow, for example. Accordingly, the process load of the small cell  1405  can be dispersed. Therefore, it becomes possible to improve a delay in the entire network. 
     When the BSR has been received from the UE  1401 , the small cell  1405  may use the difference between the Short BSR and the Long BSR, for deciding whether the BSR is to be transmitted to the concentrator  1407 . 
     For example, by including in the Short BSR the information of the data amount of the transmission data of the DCCH, and by including in the Long BSR the information of the data amount of the transmission data of the DTCH, the small cell  1405  may notify only the information of the Long BSR to the concentrator  1407 . 
     Accordingly, the concentrator  1407  can recognize the data amount of the transmission data of the user plane (U-plane) from the information of the Long BSR, and can select the small cell  1405 , by using the data amount of the transmission data of the user plane (U-plane). Therefore, it becomes possible to decrease the data amount of communication between the small cell  1405  and the concentrator  1407 . 
     As detailed examples of trigger of notifying the information about the data flow, in place of the above detailed examples (1) to (3), the following method may be employed. 
     In  FIG. 8  described above, when the concentrator  1407  has received the BSR, in the case where the small cell  1405  not transmitting the BSR exists during a predetermined time in the small cell cluster  1402  that the concentrator  1407  manages, the concentrator  1407  may inquire the small cell  1405  about the total data flow. 
     Accordingly, the concentrator  1407  can select the optimum small cell  1405 , after recognizing the data flow in the small cell  1405  included in the small cell cluster  1402 . 
     Further, the small cell  1405  may periodically transmit the BSR to the concentrator  1407 , regardless of presence or absence of the reception of the B SR from the UE  1401 . 
     Next, as detailed examples of the data flow concerning downlink (hereinafter, also referred to as a “downlink data flow”) of each small cell  1405 , the following three examples (1) to (3) will be disclosed.
         (1) A data size of the data that each small cell  1405  has received from the higher-level entity or the higher-level node.   (2) A data size of the data that each small cell  1405  has transmitted to the lower-level entity or the lower-level node.   (3) Combination of the examples (1) and (2).       

     In the present embodiment, the small cell  1405  has the function of measuring the data flow. Therefore, in the detailed example (1) of the downlink data flow, the data size may be that of the data that the small cell  1405  has received from the concentrator  1407  being the higher-level node. In the detailed example (2), because the lower-level node becomes the UE  1401 , the data size may be that of the data that the small cell  1405  has transmitted to the UE  1401  connected to the own cell  1405 . 
     In both the detailed examples (1) and (2), the data size may be that for each UE  1401  connected to the small cell  1405 , or may be a total data size of all the UEs  1401  connected to the small cell  1405 . This is similar to the case of the uplink data flow. 
     The data size may be that in the user plane (U-plane) connection. This is similar to the case of the uplink data flow. 
     For the measurement period of the data flow, notification of the data flow, trigger of notification of the data flow, the interface for the notification of the data flow, and the like, the interface and the like disclosed concerning the uplink can be appropriately applied. By appropriately applying in this way the interface and the like disclosed concerning uplink, the effect similar to that in the case of uplink can be obtained. 
     In  FIG. 8 , as the downlink data flow, a case where the data size of the data transmitted to the lower-level entity or the lower-level node is used will be described. 
     The small cell  1405  measures a data size of the data transmitted from the own cell  1405  to the UE  1401  to be connected to the own cell  1405 , and notifies the concentrator  1407  of the measured data size of the data transmitted to the UE  1401 . The concentrator  1407  selects the small cell  1405  to which the target UE  1401  is to be connected, by using the downlink data flow of each small cell  1405  in the small cell cluster  1402  that the concentrator  1407  manages. 
     As the method for the concentrator  1407  to notify the target UE  1401  of the small cell  1405  which has been selected as the small cell  1405  to which the UE  1401  is to be connected, it is possible to apply the method disclosed by the method of notifying the uplink data flow. 
     In the case where a connection destination has been notified for a target cell of handover, the UE  1401  executes handover to the notified small cell  1405  to be connected. 
     In the case where a connection destination has been notified as a secondary cell of the carrier aggregation, the UE  1401  configures the notified small cell  1405  to be connected as the secondary cell. 
     The entity having the function of selecting the small cell  1405  to which the target UE  1401  is to be connected may select the small cell  1405 , by using both the uplink data flow and the downlink data flow, or by using one of the data flow. By using both the uplink data flow and the downlink data flow, in the case where the target UE  1401  performs bidirectional communication, it becomes possible to select the optimum small cell  1405 . 
     In the case where the link that is required to have high throughput and the like to the target UE  1401  is one of uplink and downlink, the small cell  1405  may be selected by using the data flow of one link. Accordingly, it becomes possible to select the small cell  1405  that is optimum for communication of the UE  1401 . 
     The entity having the function of measuring the data flow may notify the entity having the function of selecting the small cell  1405  to which the target UE  1401  is connected, of the uplink data flow and the downlink data flow together. The data flow of each link may be notified in the list format, or may be a total value. Accordingly, it becomes possible to decrease the message volume to be notified. 
     The small cell  1405  to which the UE  1401  is to be connected may be selected independently of uplink and downlink. By using the uplink data flow and the downlink data flow in uplink and downlink respectively, it becomes possible to select the optimum small cell  1405  in each link. 
     For example, the small cells  1405  in the small cell cluster  1402  are configured by the same frequency band and the same TAG. In the case of selecting the small cell  1405  to which the target UE  1401  is connected in the small cell cluster  1402 , the small cell  1405  to which the UE  1401  is connected may be different between downlink (DL) and uplink (UL). The concentrator  1407  executes selection of the small cell  1405  independently of downlink (DL) and uplink (UL). 
     Because the small cells  1405  in the small cell cluster  1402  are configured by the same frequency band and the same TAG, transmission timings in uplink to the small cell  1405  selected in downlink become the same. Therefore, even when any small cell  1405  is used in uplink, it is not necessary to change the transmission timing, and it becomes possible to simplify the control in the UE  1401 . 
     Further, in both uplink and downlink, because the optimum small cell  1405  can be selected, it becomes possible to realize high throughput and a low delay in both links. 
     For example, the small cells  1405  in the small cell cluster  1402  are configured by the same TA (Tracking Area). In the case of selecting the small cell  1405  to which the target UE  1401  is connected in the small cell cluster  1402 , the small cell  1405  to which the UE  1401  is connected may be different between downlink (DL) and uplink (UL). The concentrator  1407  executes selection of the small cell  1405  independently of downlink (DL) and uplink (UL). Because the small cells  1405  in the small cell cluster  1402  are configured by the same TA and TAC, it is not necessary to execute the procedure of TAU (Tracking Area Update), and it becomes possible to simplify the control. 
     In the function of selecting the small cell  1405  to which the UE  1401  is to be connected, as detailed examples of information used for the selection, the following five examples (1) to (5) will be disclosed.
         (1) Data flow   (2) Load   (3) Communication quality   (4) Quality of service (QoS)   (5) Combination of the examples (1) to (4).       

     In selecting the cell, a load state of the small cell  1405  and communication quality may be considered. Considered here is a case where transfer load is high because communication quality is low although the flow is small. 
     In selecting the cell, the QoS required in the data may be considered. For example, as the data that is required to have a lower packet error loss rate as the QoS, the small cell  1405  of high communication quality may be selected. 
     The data flow in the detailed example (1) described above includes at least either one of the uplink data flow and the downlink data flow. Because this has been described above, the description will be omitted here. 
     The load in the detailed example (2) described above includes a data delivery delay time, throughput, the number of UEs  1401  connected to the small cell  1405 , a using rate of the CPU (Central Processing Unit), a using rate of radio resources, and a buffer stay amount in at least either one of the uplink data and the downlink data, for example. 
     Each small cell  1405  has the function of measuring the load, and each small cell  1405  notifies the concentrator  1407  of the information about the load (hereinafter, also referred to as “load information”). The concentrator  1407  receives the load information from each small cell  1405 . The concentrator  1407  selects the small cell  1405  to which the target UE  1401  is to be connected, by using the notified load information. 
     For the measurement period of load, notification of load information, the trigger of notification of the load information, the interface for the notification of the load information, and the like, the method disclosed concerning the data flow can be appropriately applied. Accordingly, it becomes possible to obtain the effects similar to those in the method disclosed concerning the data flow. 
     As detailed examples of communication quality in the detailed example (3) above, the following nine examples (3-1) to (3-9) will be disclosed. 
     First, concerning downlink, the following four examples (3-1) to (3-4) will be disclosed.
         (3-1) The number of times of ACK/NACK of the HARQ to the downlink data   (3-2) The number of times of ACK/NACK of the RLC to the downlink data   (3-3) CQI   (3-4) CSI       

     Next, concerning uplink, the following five examples (3-5) to (3-9) will be disclosed.
         (3-5) The number of times of ACK/NACK of the HARQ to the uplink data   (3-6) The number of times of ACK/NACK of the RLC to the uplink data   (3-7) The received power of the sounding reference signal (SRS) transmitted from the UE to the small cell   (3-8) Reception quality of the PUCCH. This may be the reception quality of the DM-RS of the PUCCH.   (3-9) Reception quality of the PUSCH. This may be the reception quality of the DM-RS of the PUSCH.       

     The number of times of ACK/NACK of the HARQ to the downlink data, the CQI, and the CSI are the information that the physical layer provides to the MAC layer (see Chapter 4.3.2 of Non-Patent Document 9). The MAC operates the HARQ (see Chapter 5.3.2 of Non-Patent Document 9). 
     According to the conventional base station, for example, in the macro cell, a control protocol to the user equipment, for example, a radio resource control (RRC) and a user plane, for example, a packet data convergence protocol (PDCP), a radio link control (RLC), a medium access control (MAC), and a physical layer (PHY) terminate in the base station. Accordingly, the number of times of ACK/NACK of the HARQ to the downlink data, the CQI, and the CSI being the information that the physical layer provides to the MAC layer also become the information from the user equipment to the base station. 
     Therefore, the number of times of the ACK/NACK of the RLC becomes the information of the base station that terminates the RLC protocol. 
     The number of times of ACK/NACK to the uplink data of the HARQ that the MAC operates becomes the information of the base station. 
     The received power of a sounding reference signal (SRS), the reception quality of the PUCCH, and the reception quality of the PUSCH, being the measurement information of the reception quality of the radio sections, become the information of the base station. 
     Therefore, according to the conventional method, it is impossible for the entity other than the eNB being the base station directly connected to the UE  1401  to realize the selection of the connection cell based on the communication quality. 
     Then, in the present embodiment, the small cell  1405  notifies the entity or the node having the function of selecting the small cell  1405  to which the target UE  1401  is to be connected, of the communication quality. 
     The entity or the node having the function of selecting the small cell  1405  to which the target UE  1401  is connected, selects the small cell  1405  to which the UE  1401  is to be connected, by using the notified communication quality. 
     For the measurement period of communication quality, notification of communication quality information, trigger of notification of the communication quality information, the interface for the notification of the communication quality information, and the like, the method disclosed concerning the data flow can be appropriately applied. Accordingly, it becomes possible to obtain the effects similar to those in the method disclosed concerning the data flow. 
     As detailed examples of the QoS in the detailed example (4) described above, the following six examples (4-1) to (4-6) will be disclosed.
         (4-1) QoS class identifier (QCI)   (4-2) Allocation and retention priority (ARP)   (4-3) Guaranteed bit rate (GBR)   (4-4) Maximum bit rate (MBR)   (4-5) Packet delay   (4-6) Packet error loss rate       

     The entity having the QoS information notifies the entity having the function of selecting the small cell  1405  to which the target UE  1401  is to be connected, of the information of the QoS. 
     For the notification of the QoS information, trigger of notification of the QoS information, the interface for the notification of the QoS information, and the like, the method disclosed concerning the data flow can be appropriately applied. Accordingly, it becomes possible to obtain the effects similar to those in the method disclosed concerning the data flow. 
     In the detailed example (5) described above, the small cell  1405  may have the function of measuring the data flow by the combination of the pieces of information. 
     The small cell  1405  may notify the concentrator  1407  of the pieces of information at different timings, or may notify one or a plurality of pieces of information at the same timing. 
     Further, the concentrator  1407  may select the small cell  1405  to which the target UE  1401  is to be connected, by using a part or the whole of the information received from each small cell  1405 . 
     Accordingly, depending on the wave environment between the target UE  1401  and the small cell  1405  in the small cell cluster  1402 , it becomes possible to appropriately change the small cell  1405  to be selected. Therefore, it becomes possible to select the optimum small cell  1405 . 
     By employing the method disclosed in the present embodiment while the concentrator  1407  uses the information about the data flow when selecting the small cell  1405  to be connected to the UE  1401 , it becomes possible to disperse the process load of the small cell  1405 . Therefore, it becomes possible to improve a delay in the entire network. 
     When the concentrator  1407  selects the small cell  1405  to be connected to the UE  1401 , it becomes possible to use other various pieces of information, and becomes possible to select the small cell  1405  which is more optimum for communication. Accordingly, communication with high throughput and a low delay becomes possible. 
     As described above, according to the present embodiment, the small cell  1405  to which the UE  1401  is to be connected is selected from among a plurality of small cells  1405 , based on the flow of at least either one of the received data that each small cell  1405  receives from the UE  1401  and the transmission data that each small cell  1405  transmits to the UE  1401 . Accordingly, in the case where the traffic volume in the communication system  1400  has increased, load can be dispersed to each cell  1405  that configures the small cell cluster  1402 . Therefore, because concentration of traffic in a specific small cell  1405  can be prevented, it is possible to reduce the occurrence probability of a delay in the network and loss of data. 
     In the present embodiment, the communication system  1400  includes the concentrator  1407 . The concentrator  1407  selects the small cell  1405  to which the UE  1401  is to be connected from among a plurality of the small cells  1405 , based on the flow of at least either one of the received data that each small cell  1405  receives from the UE  1401  and the transmission data that each small cell  1405  transmits to the UE  1401 . 
     By providing the concentrator  1407 , it becomes possible to realize, in a simple configuration, the communication system  1400  capable of reducing the occurrence probability of a delay in the network and loss of data as described above. 
     In the present embodiment, the flow of the received data is expressed by the BSR information. The BSR information is transmitted from the UE  1401  to the small cell  1405 , and is also transmitted from the small cell  1405  to the concentrator  1407  or other higher-level device. The concentrator  1407  or other higher-level device that has received the BSR information from the small cell  1405  selects the small cell  1405  to which the UE  1401  is to be connected, from among a plurality of small cells  1405 , based on the BSR information. 
     By selecting the small cell  1405  that the UE  1401  is to be connected based on the BSR information in this way, it becomes possible to select the small cell  1405  which is more optimum for communication. Accordingly, communication with high throughput and a low delay becomes possible. 
     Further, in the present embodiment, the communication system  1400  includes the macro cell  1406  and a plurality of the small cells  1405 . From among a plurality of small cells  1405 , the small cell  1405  to which the UE  1401  is to be connected is selected based on the flow of at least either one of the received data and the transmission data of each small cell  1405 . Accordingly, in the case where a large number of small cells  1405  are installed, the process load can be properly dispersed to each small cell  1405 . Therefore, it becomes possible to more securely improve a delay in the entire network. 
     First Embodiment Modification 1 
     In the example disclosed in the first embodiment, the small cell  1405  has the function of measuring the data flow. In the present modification, the concentrator  1407  has the function of measuring the data flow of each small cell  1405 . 
     In this case, concerning uplink, the data size of the data received from each small cell  1405  being the lower-level entity may be measured. Concerning downlink, the data size of the data transmitted to each small cell  1405  being the lower-level entity may be measured. By such arrangement, the concentrator  1407  can measure the data flow of each small cell  1405 . 
     By arranging such that the concentrator  1407  has the function of measuring the data flow of each small cell  1405 , it becomes possible to make the entity of measuring the data flow the same as the entity of selecting the small cell  1405  to be connected to the UE  1401 . 
     Accordingly, because it becomes unnecessary for each small cell  1405  to notify the concentrator  1407  of a measured result of the data flow, it becomes possible to decrease the signaling volume. Further, because measurement and selection can be unified in the concentrator  1407 , a measurement timing can be flexibly configured. Therefore, it becomes possible to select more timely a dynamic small cell  1405 . 
     First Embodiment Modification 2 
     In the example disclosed in the first embodiment, the concentrator  1407  has the function of selecting the small cell  1405  to which the UE  1401  is to be connected. In the present modification, the macro cell  1406  has the function of selecting the small cell  1405  to which the UE  1401  is to be connected. 
     In this case, the entity having the function of measuring the data flow notifies the macro cell  1406  of the data flow of each small cell  1405 . The macro cell  1406  obtains the data flow of each small cell  1405 . The macro cell  1406  selects the small cell  1405  to be connected to the UE  1401 , by using the data flow of each small cell  1405 . 
     The macro cell  1406  notifies the UE  1401  of the information of the selected small cell  1405 . This information may be notified through the small cell  1405  connected to the concentrator  1407  and the UE  1401 , and when the UE  1401  is connected to the macro cell  1406 , the information may be directly notified from the macro cell  1406  to the UE  1401 . For the method of notification from the small cell  1405  or the macro cell  1406  to the UE  1401 , the method disclosed in the first embodiment can be applied. 
     For example, in the case where each small cell  1405  has the function of measuring the data flow of the own cell  1405 , each small cell  1405  may notify the measured data flow of the own cell  1405  to the macro cell  1406  through the concentrator  1407 . 
     By such arrangement, it becomes unnecessary to increase the function of the concentrator  1407 . Therefore, in the case of configuring the concentrator  1407  separately from the macro cell  1406 , it becomes possible to facilitate the configuration of the concentrator  1407 . 
     First Embodiment Modification 3 
     According to the example disclosed in the first embodiment, in the user plane (U-plane) connection between the UE  1401  to be connected to the small cell  1405  and the S-GW  1404 , the S-GW  1404  is connected to the concentrator  1407  through the macro cell  1406 . 
     In the present modification, in the user plane (U-plane) connection between the UE  1401  to be connected to the small cell  1405  and the S-GW 1404 , the S-GW 1404  is directly connected to the concentrator  1407 . Accordingly, the S-GW 1404  and the small cell  1405  are connected to each other through the concentrator  1407  without through the macro cell  1406 . 
     Therefore, because the user plane (U-plane) data handled in the small cell  1405  does not pass through the macro cell  1406 , it becomes possible to decrease the process in the macro cell  1406 . 
     In this case, the concentrator  1407  may have the PDCP to the UE  1401 , or the small cell  1405  may have the PDCP. 
     Second Embodiment 
       FIG. 12  is a block diagram showing the configuration of a communication system  1500  according to a second embodiment of the present invention. The communication system  1500  according to the present embodiment is configured to include an UE  1501 , a small cell cluster  1502 , an MME  1503 , an S-GW  1504 , and a macro cell  1506 . 
     The small cell cluster  1502  includes one or a plurality of small cells  1505 .  FIG. 12  illustrates a case where the small cell cluster  1502  includes three small cells  1505 . 
     In the following description, in the case of distinctly indicating three small cells  1505 , by adding suffixes “a”, “b”, and “c” to the reference “ 1505 ”, the small cells  1505  will be respectively indicated as a first small cell # 1   1505   a,  a second small cell # 2   1505   b , and a third small cell # 3   1505   c.  In the case of indicating three small cells  1505  without distinction, the small cell will be indicated by adding a reference “ 1505 ”. 
     In the first embodiment, as illustrated in  FIG. 8 , the concentrator  1407  is configured separately from the macro cell  1406 . On the other hand, in the present embodiment, the macro cell  1506  has the function of a concentrator  1507  as well. That is, the macro cell  1506  includes the concentrator  1507 . 
     The function of the concentrator  1507  is the function of collectively controlling the small cells  1505  in the small cell cluster  1502  including one or a plurality of small cells  1505 , in a similar manner to that of the concentrator  1407  disclosed in the first embodiment. 
     In the present embodiment, the function of the concentrator  1507  is added to the macro cell  1506 . The user plane (U-plane) connection between the UE  1501  to be connected to the small cell  1505  and the S-GW  1504  is performed through the macro cell  1506 . 
     The macro cell  1506  has the function of obtaining the data flow and the function of selecting the small cell  1505  to which the UE  1501  is to be connected, for each small cell  1505  in the small cell cluster  1502  being the management item of the function of the concentrator  1507  that the macro cell  1506  has. 
     Each small cell  1505  has the function of measuring the data flow, and the function of notifying the macro cell  1506  having the function of the concentrator  1507 , of the measured data flow. 
     In the configuration illustrated in  FIG. 12 , by appropriately employing the method disclosed in the first embodiment while the macro cell  1506  uses the information about the data flow when selecting the small cell  1505  to be connected to the UE  1501 , it becomes possible to disperse the process load of the small cell  1505 . Therefore, it becomes possible to improve a delay in the entire network. 
     When the macro cell  1506  selects the small cell  1505  to be connected to the UE  1501 , it becomes possible to use other various pieces of information, and becomes possible to select the small cell  1505  which is more optimum for communication. Accordingly, communication with high throughput and a low delay becomes possible. 
     Further, in the case where the UE  1501  is connected to not only one or more small cells  1505  but also to the macro cell  1506 , the macro cell  1506  can select the small cell  1505  to be connected to the UE  1501 . Accordingly, it becomes possible to put together in the macro cell  1506  the control to the UE  1501 . Therefore, it becomes possible to reduce malfunction. 
     The macro cell  1506  having the function of the concentrator  1507  may have the function of measuring the data flow of each small cell  1505 . 
     In this case, concerning uplink, the data size of the data received from each small cell  1505  being the lower-level entity may be measured. Concerning downlink, the data size of the data transmitted to each small cell  1505  being the lower-level entity may be measured. 
     By such arrangement, the macro cell  1506  can measure the data flow of each small cell  1505 . Accordingly, because it becomes unnecessary for each small cell  1505  to notify the macro cell  1506  of a measured result of the data flow, it becomes possible to decrease the signaling volume. Therefore, it becomes possible to obtain the effects similar to those in the first embodiment. 
     In the present embodiment, as illustrated in  FIG. 12 , the user plane (U-plane) connection between the UE  1501  to be connected to the small cell  1505  and the S-GW  1504  is performed through the macro cell  1506 . 
     In the user plane (U-plane) connection between the UE  1501  to be connected to the small cell  1505  and the S-GW  1504 , the S-GW  1504  may be directly connected to the small cell  1505 . In this case, the S-GW  1504  is connected to the small cell  1505  without through the macro cell  1506 . 
     Therefore, because the user plane (U-plane) data handled in the small cell  1505  does not pass through the macro cell  1506 , it becomes possible to decrease the process in the macro cell  1506 . In this case, the small cell  1505  may have the PDCP to the UE  1501 . 
     In the user plane (U-plane) connection between the UE  1501  to be connected to the small cell  1505  and the S-GW  1504 , in the case where the S-GW  1504  is directly connected to the small cell  1505 , the S-GW  1504  may have the function of measuring the data flow of each small cell  1505 . 
     In this case, concerning uplink, the data size of the data received from each small cell  1505  being the lower-level entity may be measured. Concerning downlink, the data size of the data transmitted to each small cell  1505  being the lower-level entity may be measured. 
     By such arrangement, the S-GW  1504  can measure the data flow of each small cell  1505 . 
     The S-GW  1504  notifies the entity that selects the small cell  1505  to be connected to the UE  1501 , of the measured data flow of each small cell  1505 . For example, in the case where the entity that selects the small cell  1505  to be connected to the UE  1501  is the macro cell  1506 , the S-GW 1504  notifies the macro cell  1506  of the data flow. The S-GW  1504  may notify the macro cell  1506  of the data flow through the MME  1503 . 
     Accordingly, because it becomes unnecessary for the small cell  1505  to notify the macro cell  1506  of a measured result of the data flow, it becomes possible to decrease the signaling volume between the small cell  1505  and the macro cell  1506 . 
     As described above, according to the present embodiment, the macro cell  1506  has the function of the concentrator  1507 . Accordingly, because it becomes unnecessary for each small cell  1505  to notify the macro cell  1506  of a measured result of the data flow, it becomes possible to decrease the signaling volume. Therefore, it is possible to further improve a delay in the entire network. 
     Third Embodiment 
       FIG. 13  is a block diagram showing the configuration of a communication system  1600  according to a third embodiment of the present invention. The communication system  1600  according to the present embodiment is configured to include a UE  1601 , a small cell cluster  1602 , an MME  1603 , and an S-GW  1604 . The small cell cluster  1602  includes one or a plurality of small cells  1605 .  FIG. 13  illustrates a case where the small cell cluster  1602  includes three small cells  1605 . 
     In the following description, in the case of distinctly indicating three small cells  1605 , by adding suffixes “a”, “b”, and “c” to the reference “ 1605 ”, the small cells  1605  will be respectively indicated as a first small cell # 1   1605   a,  a second small cell # 2   1605   b , and a third small cell # 3   1605   c.  In the case of indicating three small cells  1605  without distinction, the small cell will be indicated by adding a reference “ 1605 ”. 
     In the second embodiment, as illustrated in  FIG. 12 , the macro cell  1506  has the function of the concentrator  1507  as well. On the other hand, in the present embodiment, the MME  1603  has the function of a concentrator  1607  as well. That is, the MME  1603  includes the concentrator  1607 . 
     The function of the concentrator  1607  is the function of collectively controlling the small cells  1605  in the small cell cluster  1602  including one or a plurality of small cells  1605 , in a similar manner to that of the concentrator  1407  disclosed in the first embodiment. 
     In the present embodiment, the function of the concentrator  1607  is added to the MME  1603 . The MME  1603  is directly connected to each small cell  1605 . Further, the S-GW 1604  is directly connected to each small cell  1605 . 
     The MME  1603  has the function of obtaining the data flow and the function of selecting the small cell  1605  to which the UE  1601  is to be connected, for each small cell  1605  in the small cell cluster  1602  being the management item of the function of the concentrator  1607  that the MME  1603  has. 
     Each small cell  1605  has the function of measuring the data flow, and the function of notifying the MME  1603  having the function of the concentrator  1607 , of the measured data flow. 
     In the present embodiment, in the configuration illustrated in  FIG. 13 , the method disclosed in the first embodiment may be appropriately used. The MME  1603  may notify the UE  1601  of the information of the selected small cell  1605 , through the small cell  1605  connected to the UE  1601 . 
     By such arrangement, when the MME  1603  selects the small cell  1605  to be connected to the UE  1601 , it becomes possible to use the information about the data flow. Accordingly, the process load of the small cell  1605  can be dispersed. Therefore, it is possible to improve a delay in the entire network. 
     When the MME  1603  selects the small cell  1605  to be connected to the UE  1601 , it becomes possible to use other various pieces of information, and becomes possible to select the small cell  1605  which is more optimum for communication. Accordingly, communication with high throughput and a low delay becomes possible. 
     Because the MME  1603  has the function of selecting the small cell  1605  to be connected to the UE  1601 , the information of the small cell  1605  to be connected to the UE  1601  can be directly notified to the S-GW  1604 . Therefore, a signaling volume required by the network side can be small. 
     Further, after the MME  1603  has selected the small cell  1605  to which the UE  1601  is to be connected, the S-GW  1604  can perform with a low-level delay time the routing of data to one or more small cells  1605  to which the UE  1601  is to be connected. Therefore, it becomes possible to flexibly change the small cell  1605  to which the UE  1601  is to be connected with a lower delay. Therefore, it becomes possible to select the small cell  1605  corresponding to the continuously changing surrounding wave environment. 
     According to the first embodiment to the third embodiment described above, one small cell cluster  1402 ,  1502 , or  1602  exists in the communication system  1400 ,  1500 , or  1600 . The number of small cell clusters  1402 ,  1502  and  1602  that exist in the communication systems  1400 ,  1500 , and  1600  is not limited to one. In the communication systems  1400 ,  1500 , and  1600 , a plurality of small cell clusters  1402 ,  1502 , and  1602  may exist. 
     In the case where a plurality of small cell clusters  1402 ,  1502 , and  1602  exist in the communication systems  1400 ,  1500 , and  1600 , a plurality of concentrators  1407 ,  1507 , and  1607  are configured corresponding to the small cell clusters  1402 ,  1502 , and  1602 . In this case, by applying the method disclosed in each embodiment, it is possible to select the concentrators  1407 ,  1507 , and  1607  that control the small cells  1405 ,  1505 , and  1605  to be connected to the UEs  1401 ,  1501 , and  1601 . 
     For example, considered here is a case where the macro cell  1406  is connected to the concentrator  1407  corresponding to each small cell cluster  1402 , like the first embodiment illustrated in  FIG. 8 . In this case, the macro cell  1406  is configured to have the function of obtaining the data flow in each small cell cluster  1402 , and the function of selecting to which small cell  1405  in the small cell cluster  1402  the UE  1401  is to be connected. 
     Further, in this case, each small cell cluster  1402  is configured to have the function of measuring the data flow, and the function of notifying the macro cell  1406  of the measured data flow, through the concentrator  1407 . 
     Without being limited to the above, by providing each small cell  1405  with the function of measuring the data flow, each small cell  1405  may notify the small cell cluster  1402  to which the UE  1401  is connected, of the measured data flow. Each small cell cluster  1402  derives the data flow of the entire small cells  1405  belonging to the own cluster. 
     In the above configuration, by appropriately employing the method disclosed in the first embodiment while the macro cell  1406  uses the information about the data flow of each small cell cluster  1402  when selecting the small cell  1405  to be connected to the UE  1401 , it becomes possible to disperse the process load of the small cell cluster  1402 . Therefore, it becomes possible to improve a delay in the entire network. 
     The embodiments and the modifications thereof described above are merely exemplifications of the present invention, and the embodiments and the modifications thereof can be freely combined without departing from the scope of the present invention. Further, an arbitrary component in the embodiments and the modifications thereof can be appropriately changed or omitted. Accordingly, by installing small eNBs configuring a small cell, a communication system that obtains a high communication capacity can be provided. 
     For example, according to another embodiment of the present invention, the communication system may be a communication system including a communication terminal device, and one or a plurality of base station devices that perform radio communication with the communication terminal device. The communication system includes a plurality of cells that are configured by the one or the plurality of base station devices, and perform radio communication with the communication terminal device by being connected to the communication terminal device, and a higher-level device that is provided in a higher level of the base station device. A cell to which the communication terminal device is to be connected is selected from among the plurality of cells, based on a flow of received data that each cell has received from the communication terminal device. 
     Accordingly, as in the case with each of the embodiments and the modification thereof described above, when the traffic volume in the communication system has increased, the load can be dispersed to each cell. Therefore, because concentration of traffic in a specific cell can be prevented, it is possible to reduce the occurrence probability of a delay in the network and loss of data. 
     Further, according to still another embodiment of the present invention, the communication system may be a communication system including a communication terminal device, and one or a plurality of base station devices that perform radio communication with the communication terminal device. The communication system includes a plurality of cells that are configured by the one or the plurality of base station devices, and perform radio communication with the communication terminal device by being connected to the communication terminal device, and a higher-level device that is provided in a higher level of the base station device. A cell to which the communication terminal device is to be connected is selected from among the plurality of cells, based on a flow of transmission data which each cell has transmitted to the higher-level device. 
     Accordingly, as in the case with each of the embodiments and the modification thereof described above, when the traffic volume in the communication system has increased, the load can be dispersed to each cell. Therefore, because concentration of traffic in a specific cell can be prevented, it is possible to reduce the occurrence probability of a delay in the network and loss of data. 
     While the present invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the present invention. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1400 ,  1500 ,  1600  communication system,  1401 ,  1501 ,  1601  mobile terminal (UE),  1402 ,  1502 ,  1602  small cell cluster,  1403 ,  1503 ,  1603  MME,  1404 ,  1504 ,  1604  S-GW,  1405 ,  1505 ,  1605  small cell,  1406 ,  1506  macro cell,  1407 ,  1507 ,  1607  concentrator.