Patent Publication Number: US-2018049200-A1

Title: Communication system, network device, base station, mobile station, frequency band control method, and non-transitory computer readable medium

Description:
TECHNICAL FIELD 
     The present invention relates to a communication system, a network device, a base station, a mobile station, a frequency band control method and a program and, particularly, relates to a communication system, a network device, a base station, a frequency band control method, a frequency band determination method and a program using a plurality of frequency bands. 
     BACKGROUND ART 
     With the proliferation of smartphones, services using a large volume of data such as video viewing are being provided to users today. In order to send a large volume of data to smartphones at high speed, carrier aggregation is used. The carrier aggregation is the communication standard defined in the 3GPP (3rd Generation Partnership project) TS36.101 V12.6.0, clause 5.6A (2014-12). The carrier aggregation is a technique to achieve high-speed communications by ensuring a wide band using frequencies in different frequency bands such as, for example, the 800 MHz frequency band and the 2 GHz frequency band. 
     Non Patent Literature 1 discloses the carrier aggregation between a UE (User Equipment) and an eNB (evolved NodeB). The UE is used as a general term for mobile stations in the 3GPP. The eNB is a base station that uses LTE (Long Term Evolution), which is the wireless communication standard defined in the 3GPP. 
     Patent Literature 1 discloses controlling the carrier aggregation operation by exchanging information between a terminal device and a base station device. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL1: Japanese Unexamined Patent Application Publication No. 2013-229943 
       
    
     Non Patent Literature 
     
         
         NPL1: 3GPP TS 36.300 V12.4.0 (2014-12) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2, clauses 5.5, 7.5 (Release 12) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Non Patent Literature 1 and Patent Literature 1 describe that an eNB or a base station device (hereinafter referred to collectively as “eNB etc.”) determines a frequency band or the like to be used in the carrier aggregation. Thus, in the case where a UE or a terminal device (hereinafter referred to collectively as “UE etc.”) is a device to which the carrier aggregation is applicable, the eNB etc. uses the carrier aggregation in communication with the UE etc. However, the eNB etc. determines whether or not to apply the carrier aggregation only by using information sent and received to and from the UE etc. Therefore, there is a problem that the carrier aggregation is applied to the UE etc. even when application of the carrier aggregation is not unauthorized under subscriber contract conditions, usage conditions and the like of that UE etc. 
     An exemplary object of the present invention is to provide a communication system, a network device, a base station, a mobile station, a frequency band control method and a program that can more appropriately determine whether the carrier aggregation is applicable or not per UE. 
     Solution to Problem 
     A communication system according to a first exemplary aspect of the present invention includes a base station that wirelessly communicates with a mobile station by using at least one of a first frequency band and a second frequency band, the mobile station being located concurrently in a first cell using the first frequency band and a second cell using the second frequency band; and a network device that sends, to the base station, information to be used for determining a frequency band to be used in wireless communications with the mobile station. 
     A network device according to a second exemplary aspect of the present invention includes a communication unit for sending, to a base station that wirelessly communicates with a mobile station by using at least one of a first frequency band and a second frequency band, the mobile station being located concurrently in a first cell using the first frequency band and a second cell using the second frequency band, information to be used for determining a frequency band to be used in wireless communications with the mobile station. 
     A base station according to a third exemplary aspect of the present invention is a base station that forms a first cell using a first frequency band and a second cell using a second frequency band, the base station including a determination unit for determining a frequency band to be used in communication with a mobile station located concurrently in the first cell and the second cell by using information, sent from a network device, to be used for determining a frequency band to be used in wireless communications with the mobile station. 
     A mobile station according to a fourth exemplary aspect of the present invention is a mobile station that is located concurrently in a first cell using a first frequency band and a second cell using a second frequency band and wirelessly communicates with a base station by using at least one of the first frequency band and the second frequency band, wherein the mobile station is configured to receive information to be used for determining a frequency band to be used in wireless communications with the base station and determine whether or not to use the first frequency band and the second frequency band by using the received information. 
     A frequency band control method according to a fifth exemplary aspect of the present invention includes sending, to a base station that wirelessly communicates with a mobile station by using at least one of a first frequency band and a second frequency band, the mobile station being located concurrently in a first cell using the first frequency band and a second cell using the second frequency band, information to be used for determining a frequency band to be used in wireless communications with the mobile station. 
     A frequency band determination method according to a sixth exemplary aspect of the present invention is a frequency band determination method used in a base station that forms a first cell using a first frequency band and a second cell using a second frequency band, the method including determining a frequency band to be used in communication with a mobile station located concurrently in the first cell and the second cell by using information, sent from a network device, to be used for determining a frequency band to be used in wireless communications with the mobile station. 
     A program according to a seventh exemplary aspect of the present invention includes sending, to a base station that wirelessly communicates with a mobile station by using at least one of a first frequency band and a second frequency band, the mobile station being located concurrently in a first cell using the first frequency band and a second cell using the second frequency band, information to be used for determining a frequency band to be used in wireless communications with the mobile station. 
     Advantageous Effects of Invention 
     According to the exemplary aspects of the present invention described above, it is possible to provide a communication system, a network device, a base station, a mobile station, a frequency band control method and a program that can more appropriately determine whether the carrier aggregation is applicable or not per UE. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a communication system according to a first exemplary embodiment. 
         FIG. 2  is a block diagram of a communication system according to a second exemplary embodiment. 
         FIG. 3  is a block diagram of an MME according to the second exemplary embodiment. 
         FIG. 4  is a block diagram of an eNB according to the second exemplary embodiment. 
         FIG. 5  is a view showing a flow of an Attach process according to the second exemplary embodiment. 
         FIG. 6  is a view illustrating an Initial Context Setup Request message according to the second exemplary embodiment. 
         FIG. 7  is a view illustrating an RRC Connection Reconfiguration/NAS:Attach Accept message according to the second exemplary embodiment. 
         FIG. 8  is a view showing a flow of a UE triggered Service Request process according to a third exemplary embodiment. 
         FIG. 9  is a view showing a flow of a Network triggered Service Request process according to the third exemplary embodiment. 
         FIG. 10  is a block diagram of a communication system according to a fourth exemplary embodiment. 
         FIG. 11  is a view showing a flow of an Attach process according to a fifth exemplary embodiment. 
         FIG. 12  is a block diagram of a UE according to each exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
     Exemplary embodiments of the present invention are described hereinafter with reference to the drawings. First, a configuration example of a communication system according to a first exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 1 . The communication system in  FIG. 1  includes a base station  10 , a mobile station (mobile terminal)  20 , and a network device  30 . The base station  10  uses a plurality of frequency bands and forms a cell with a different communication range for each of the frequency bands. For example, the base station  10  forms a cell  100  and a cell  110 . A frequency band (frequency band A) that is used in the cell  100  is different from a frequency band (frequency band B) that is used in the cell  110 . 
     It is assumed that the mobile station  20  is located concurrently in the cell  100  and the cell  110 . In other words, it is assumed that the mobile station  20  is located in the area where the cell  100  and the cell  110  overlap. In this case, the base station  10  wirelessly communicates with the mobile station  20 . For example, the base station  10  may communicate with the mobile station  20  by using LTE, which is the communication standard defined in the 3GPP, or may communicate with the mobile station  20  by using another communication standard. 
     The network device  30  sends, to the base station  10 , information to be used for determining the frequency band to be used in wireless communications with the mobile station  20 . The information to be used for determining the frequency band to be used may be, for example, information indicating whether both the frequency band A and the frequency band B can be used, only the frequency band A can be used, or only the frequency band B can be used when communicating with the mobile station  20 . 
     As described above, with using the communication system in  FIG. 1 , the base station  10  can determine whether or not to use a plurality of frequency bands in wireless communications with the mobile station  20  by using the information sent from the network device  30 . Therefore, a mobile telecommunications carrier that manages the base station  10  and the network device  30  can determine whether or not to perform communications using a plurality of frequency bands for each mobile station  20 ; that is, whether or not to apply the carrier aggregation per mobile station  20 . A mobile telecommunications carrier can thereby appropriately determine whether the carrier aggregation can be applied or not per mobile station  20 . 
     Second Exemplary Embodiment 
     A configuration example of a communication system according to a second exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 2 . The communication system in  FIG. 2  includes an eNB  40 , an UE  50 , an MME (Mobility Management Entity)  60 , an MME  70 , an HSS (Home Subscriber Server)  71 , an S-GW (Serving-Gateway)  72 , and a P-GW (Packet Data Network Gateway)  73 . The eNB  40  is connected to the UE  50  through a Uu interface. 
     The MME  60  accommodates the eNB  40  through an S 1 -MME interface. Further, the MME  70  may accommodate another eNB which is different from the eNB  40 . The MME  60  and the MME  70  perform UE mobility management, authentication, user data transfer path setting and the like. Further, the MME  60  is connected to the MME  70  through an external MME interface. 
     The HSS  71  manages subscriber information of the UE  50 . For example, the subscriber information includes static information such as contract information between the UE  50  and a mobile telecommunications carrier and dynamic information such as position information of the UE  50 . The static information is information that is not frequently modified, and the dynamic information is information that is more frequently modified compared with the static information. The HSS  71  sends the subscriber information about the UE  50  to the MME  60  through an S 6   a  interface when the MME  60  performs, for example, authentication of the UE  50 . 
     The S-GW  72  performs transmission of user data. The S-GW  72  is connected to the MME  60  through an external MS interface, and sends and receives control data. The P-GW  73 , like the S-GW  72 , performs transmission of user data. Further, the P-GW  73  performs allocation of an IP address or the like for the UE  50 . The P-GW  73  is connected to the S-GW  72  through an external SP interface, and sends and receives control data. The P-GW  73  may be connected to an external network, the Internet or the like (not shown) through another interface. 
     A configuration example of the MME  60  according to the second exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 3 . The MME  60  includes a signal generation unit  61  and a communication unit (sending and receiving unit)  62 . The signal generation unit  61  generates a signal that contains information about whether the eNB  40  applies the carrier aggregation or not in wireless communications with the UE  50 . To be specific, the MME  60  acquires the subscriber information about the UE  50  from the HSS  71  through the communication unit  62 . The subscriber information about the UE  50  contains information about whether or not to apply the carrier aggregation in wireless communications with the UE  50 . The signal generation unit  61  sends the generated signal to the eNB  40  through the communication unit  62 . 
     The information about whether or not to apply the carrier aggregation which is contained in the subscriber information may be, for example, contract information about whether the UE  50  has applied for using the carrier aggregation or not. Further, the information about whether or not to apply the carrier aggregation which is contained in the subscriber information may be for example, information about the communication traffic capacity available for the UE  50  according to the contract. When the available communication traffic capacity according to the contract of the UE  50  is a large capacity that exceeds a predetermined threshold, the eNB  40  may determine to apply the carrier aggregation in communication with the UE  50 , and when it is a capacity that falls below a predetermined threshold, the eNB  40  may determine not to apply the carrier aggregation in communication with the UE  50 . 
     Further, the information about whether or not to apply the carrier aggregation which is contained in the subscriber information may be information about a service which the UE  50  has a contract for. For example, the eNB  40  may determine to apply the carrier aggregation in communication with the UE  50  when high-capacity communications are required for a service which the UE  50  has a contract for. Further, the eNB  40  may determine not to apply the carrier aggregation when the UE  50  subscribes to a voice service only. Furthermore, when the UE  50  subscribes to a premium contract or the like at a higher fee than that of a basic contract, the eNB  40  may allocate a frequency that is more suitable for high-speed communications and high-capacity communications as the frequency band to be allocated to the carrier aggregation. Further, the information about whether or not to apply the carrier aggregation which is contained in the subscriber information may be information that combines pieces of subscriber information related to the UE  50  managed by the HSS  71 . 
     Further, the information about whether or not to apply the carrier aggregation which is contained in the subscriber information may be information about whether or not to subscribe to MVNO (Mobile Virtual Network Operator). For example, the eNB  40  may determine not to apply the carrier aggregation when the UE  50  subscribes to the MVNO, and the eNB  40  may determine to apply the carrier aggregation when the UE  50  subscribes to the MVNO. 
     Further, the information about whether or not to apply the carrier aggregation which is contained in the subscriber information may be information about whether the UE  50  is under roaming or not. For example, the eNB  40  may determine not to apply the carrier aggregation when the UE  50  is under roaming. For example, the MME  60  determines to allow roaming of the UE  50  when a roaming-partner HSS, which is in a roaming source, is set, and determines not to allow roaming of the UE  50  when a roaming-partner HSS is not set. Alternatively, the MME  60  determines to allow roaming of the UE  50  when it is set to allow roaming of the UE  50  in the subscriber information, and the MME  60  determines not to allow roaming of the UE  50  when it is not set to allow roaming of the UE  50  in the subscriber information. 
     A configuration example of the eNB  40  according to a second exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 4 . The eNB  40  includes a determination unit  41  and a communication unit (sending and receiving unit)  42 . The determination unit  41  determines whether or not to apply the carrier aggregation in wireless communications with the UE  50  by using information sent from the MME  60 . In other words, the determination unit  41  determines whether or not to apply the carrier aggregation per UE by using information sent from the MME  60 . 
     When the contract information about whether the UE  50  has applied for using the carrier aggregation or not is sent from the MME  60 , the determination unit  41  determines whether or not to apply the carrier aggregation to the UE  50  according to the details of the contract information. 
     The determination unit  41  may previously set a threshold in preparation for the case where information about the communication traffic capacity available for the UE  50  according to the contract is sent from the MME  60 . The determination unit  41  may determine whether or not to apply the carrier aggregation to the UE  50  depending on whether the communication traffic capacity available for the UE  50  according to the contract exceeds a preset threshold or not. The determination unit  41  may previously set, for each service, whether it is the service to which the carrier aggregation is applied or not in preparation for the case where information about the service for which the UE  50  has a contract is sent from the MME  60 . The determination unit  41  may determine whether or not to apply the carrier aggregation to the UE  50  depending on whether the service to which the carrier aggregation is applied is contained in the information about the service. 
     Further, when the determination unit  41  determines to apply the carrier aggregation in communication with the UE  50 , it may determine the frequency band and the frequency width to be used by the communication unit  42 . The determination unit  41  may determine to use the 800 MHz frequency band and the 2.4 GHz frequency band as the frequency band, for example. When applying the carrier aggregation, the determination unit  41  selects two or more frequency bands from a plurality of frequency bands available for the eNB  40 . Further, the determination unit  41  may determine 20 MHz or the like as the frequency width. The determination unit  41  may determine the total of frequency widths to be used in a plurality of frequency bands or may determine a frequency width to be used in each frequency band. 
     The determination unit  41  may previously specify the frequency bands that are available between the eNB  40  and the UE  50  in communication with the UE  50 . Alternatively, the determination unit  41  may previously specify the frequency bands that are available between the eNB  40  and the UE  50  according to the place where the UE  50  is located. 
     The communication unit  42  wirelessly communicates with the UE  50  by using the frequency band and the frequency width specified by the determination unit  41 . Further, the communication unit  42  may send and receive data to and from the MME  60  also. 
     The flow of an Attach process according to the second exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 5 . First, the UE  50  sends a NAS (Non Access Stratum):Attach Request message to the eNB  40  (S 1 ). The NAS:Attach Request message is sent when the UE  50  transitions from the power-off state to the power-on state, for example. The NAS:Attach Request message is used in the protocol called NAS, which is specified between a UE and a core network device. The core network device is an MME, for example. 
     Next, the eNB  40  transfers the NAS:Attach Request message sent from the UE  50  to the MME  60  (S 2 ). The MME  60  receives the NAS:Attach Request message sent from the UE  50  and then sends an Update Location Request message to the HSS  71  (S 3 ). The MME  60  sends the Update Location Request message to the HSS  71  in order to acquire subscriber information from the HSS  71 . For example, the MME  60  sends the Update Location Request message to the HSS  71  by setting an identifier for identifying the UE  50 . 
     Then, the HSS  71  sends an Update Location Ack message to the MME  60  as a response message to the Update Location Request message (S 4 ). The HSS  71  sends the Update Location Ack message to which the subscriber information of the UE  50  is set to the MME  60 . 
     The subscriber information that is sent from the HSS  71  to the MME  60  may be contract information about whether the UE  50  has applied for using the carrier aggregation or not, contract information about the communication traffic capacity available for the UE  50  according to the contract, or contract information about the service for which the UE  50  has a contract. 
     Further, the HSS  71  may indicate the frequency band in the case where the eNB  40  uses the carrier aggregation in communication with the UE  50  by using SPID (Subscriber ProfileID for RAT/frequency priority) indicating the priority of a communication method to be used. For example, the priority of each of communication methods LTE (E-UTRAN), 3G (UTRAN) and 2G (GERAN) is specified by the SPID. The eNB  40  determines a communication method to be used according to the priority specified by the SPID. 
     The HSS  71  may specify, by the SPID, the use of the communication methods LTE and 3G as the highest priority, for example. By setting the SPID in this manner, the eNB  40  can determine to carry out the carrier aggregation by using the frequency band used in LTE and the frequency band used in 3G when communicating with the UE  50 . A combination of various communication methods may be set to the SPID, and, for example, the use of LTE and an LTE communication method may be set as the highest priority. In this case, it is specified to carry out the carrier aggregation by combining two frequency bands that are used in LTE. 
     Then, the MME  60  extracts information to be used for the eNB  40  to determine whether the carrier aggregation is applicable or not from the subscriber information set to the Update Location Ack message. The MME  60  sets the extracted information to an Initial Context Setup Request message. The MME  60  sends, to the eNB  40 , the Initial Context Setup Request message to which the information to be used for the eNB  40  to determine whether the carrier aggregation is applicable or not is set (S 5 ). 
     The Initial Context Setup Request message is described hereinafter with reference to  FIG. 6 . The MME  60  sends the Initial Context Setup Request message to the eNB  40  through the S 1 -MME interface. The Initial Context Setup Request message indicates a parameter to be set for each bearer that is set in the UE  50 . For example, in the case where the UE  50  sets two bearers, two E-RABToBeSetupListCtxtSUReq are set as shown in  FIG. 6 . E-RAB-ID or the like is set in the E-RABToBeSetupListCtxtSUReq. 
     The MME  60  may set the information to be used for the eNB  40  to determine whether the carrier aggregation is applicable or not as an arbitrary parameter described for GTP(General Packet Radio Service Tunneling Protocol)-TEID(Tunnel Endpoint Identifier). To be specific, the MME  60  may set the contract information, the information indicated by the SPID or the like as an arbitrary parameter to be set for each bearer. 
     Referring back to  FIG. 5 , the eNB  40  receives the Initial Context Setup Request message, and determines whether or not to apply the carrier aggregation in communication with the UE  50 . The eNB  40  sends, to the UE  50 , an RRC(Radio Resource Control) Connection Reconfiguration/NAS:Attach Accept message that reflects the determination result as to whether or not to apply the carrier aggregation (S 6 ). 
     The RRC Connection Reconfiguration/NAS:Attach Accept message is described hereinafter with reference to  FIG. 7 . The RRC Connection Reconfiguration/NAS:Attach Accept message in  FIG. 7  indicates that the Primary cell that is set in #1 and the Secondary cell set that is in #2 are specified. For example, the Primary cell may be the cell  100  in  FIG. 1 , and the Secondary cell may be the cell  110  in  FIG. 1 .  FIG. 7  shows that the eNB  40  sets the Primary cell and the Secondary cell and thereby causes the carrier aggregation to operate in communication with the UE  50 . In the case of applying the carrier aggregation using three or more frequency bands in communication with the UE  50 , the eNB  40  may set three or more cells. 
     The eNB  40  specifies carrierFreq in mobilityControlInfo of #1 and thereby specifies the frequency to be used in the Primary cell. Further, the eNB  40  specifies dl-CarrierFreq-r10 in sCellToAddModList-r10 of #2 and thereby specifies the frequency to be used in the Secondary cell. Furthermore, the eNB  40  may specify the frequency width by specifying carrierBandwidth. 
     The frequency band, the frequency width and the like to be used when the eNB  40  communicates with the UE  50  by using the carrier aggregation may be predetermined between the UE  50  and the eNB  40 . For example, after the eNB  40  receives the NAS:Attach Request message in Step S 1 , it may send a UE Capability Enquiry message to the UE  50  in order to make inquiries about information such as the frequency band and the frequency width to be used in the carrier aggregation. As a response to the UE Capability Enquiry message, the UE  50  may send a UE Capability Information message where the frequency band, the frequency width and the like to be used in the carrier aggregation are specified. It is assumed that sending and receiving of the UE Capability Enquiry message and the UE Capability Information message between the UE  50  and the eNB  40  have completed before the eNB  40  sends the RRC Connection Reconfiguration/NAS:Attach Accept message in Step S 6 . 
     Further, in the case where the eNB  40  determines not to use the carrier aggregation, only #1 indicating the setting of the Primary cell may be set in  FIG. 7 . 
     Referring back to  FIG. 5 , the UE  50  sends an RRC Connection Reconfiguration Complete message to the eNB  40  as a response message to the RRC Connection Reconfiguration/NAS:Attach Accept message (S 7 ). 
     Then, the eNB  40  sends an Initial Context Setup Response message to the MME  60  as a response message to the Initial Context Setup Request message (S 8 ). After that, the UE  50  sends a Direct Transfer/NAS:Attach Complete message to the eNB  40  in order to notify the completion of processing related to Attach Request (S 9 ). The eNB  40  then sends a NAS:Attach Complete message to the MME  60  (S 10 ). 
     As described above, with using the communication system according to the second exemplary embodiment of the present invention, the MME, which is the core network device, can send, to the eNB  40 , the information to be used for determining whether the carrier aggregation is applicable or not. The eNB can determine whether the carrier aggregation is applicable or not per UE by using the information sent from the MME. A mobile telecommunications carrier can thereby flexibly set the carrier aggregation per UE. 
     Third Exemplary Embodiment 
     The flow of a UE triggered Service Request process according to a third exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 8 . A precondition for performing the UE triggered Service Request process is that a bearer is previously set in the UE  50  and radio resources are released after the lapse of a specified period of time. In other words, the UE triggered Service Request process is a process that the UE  50  requests the resetting of a bearer, which is, the activation of a bearer. 
     First, the UE  50  sends a NAS:Service Request message to the eNB  40  (S 21 ). For example, the NAS:Service Request message is sent to the eNB  40  when an arbitrary operation is performed in the UE  50  in the state where the Attach process of the UE  50  has completed. Next, the eNB  40  transfers the received NAS:Service Request message to the MME  60  (S 22 ). 
     Then, the MME  60  sends, to the eNB  40 , an Initial Context Setup Request message that specifies a bearer to be reset in the UE  50  (S 23 ). The MME  60  specifies the bearer that has been set in the Attach process of the UE  50  as the bearer to be reset, for example. At this time, the MME  60  sets the same information as the information that has been set to the arbitrary parameter in Step S 5  of  FIG. 5  to the Initial Context Setup Request message in Step S 23 . 
     The eNB  40  receives the Initial Context Setup Request message to which the information to be used for determining whether the carrier aggregation is applicable or not is set in Step S 23  and then determines whether or not to apply the carrier aggregation in communication with the UE  50 . 
     Then, the eNB  40  establish a radio bearer with the UE  50  (S 24 ). At this time, when it is determined to use the carrier aggregation, the eNB  40  may send, to the UE  50 , information about the frequency band, the frequency width and the like to be used in the carrier aggregation. 
     After that, the eNB  40  sends an Initial Context Setup Complete message as a response message to the Initial Context Setup Request message (S 25 ). 
     Hereinafter, the flow of a Network triggered Service Request process according to the third exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 9 . The Network triggered Service Request process is performed when an incoming message is sent to the UE  50 . First, the P-GW  73  sends, to the S-GW  72 , Downlink Data that is addressed to the UE  50  (S 31 ). Next, the S-GW  72  sends a Downlink Data Notification message to the MME  60  in order to notify the MME  60  that the Downlink Data addressed to the UE  50  has been sent (S 32 ). Then, the MME  60  sends a Paging message to the eNB  40  (S 33 ), and the eNB  40  performs a Paging process (S 34 ). After the Paging process in Step S 34 , the process of Steps S 21  to S 25  in  FIG. 8  is performed. 
     As described above, in the UE triggered Service Request process and the Network triggered Service Request process that are performed after the Attach process is completed also, the eNB  40  can determine whether the carrier aggregation is applicable or not in the same way as in the second exemplary embodiment. 
     Fourth Exemplary Embodiment 
     A configuration example of a communication system according to a fourth exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 10 . The communication system in  FIG. 10  includes an eNB  200 , an eNB  220 , a UE  240 , an MME  250  and an HSS  71 . The eNB  200  forms a cell  210 . The eNB  220  forms a cell  230 . The frequency band that is used in the cell  210  is different from the frequency band that is used in the cell  230 . The UE  240  is located concurrently in the cell  210  and the cell  230 . In other words, the UE  240  is located at the position where the cell  210  and the cell  230  overlap. Further, the MME  250  accommodates the eNB  200  and the eNB  220 . 
     In the configuration of the communication system as shown in  FIG. 10 , the UE  240  may carry out the carrier aggregation by using the frequency band that is used in the cell  210  and the frequency band that is used in the cell  230 . 
     Then MME  250  receives the subscriber information from the HSS  71  or the like, and it may then determine whether or not to apply the carrier aggregation to the UE  240  based on the received subscriber information, for example. Specifically, the MME  250  may include the determination unit  41  in the eNB  40 . 
     For example, when the MME  250  determines to use the carrier aggregation in wireless communications with the UE  240  by using the subscriber information sent from the HSS  71 , it may send an Initial Context Setup Request message to the eNB  200  and the eNB  220  that form cells where the UE  240  is located. The UE  240  may receive an RRC Connection Reconfiguration/NAS:Attach Accept message that specifies a frequency to be used from both of the eNB  200  and the eNB  220 . The UE  240  can thereby carry out the carrier aggregation by using the frequency band that is used in the cell  210  and the frequency band that is used in the cell  230 . 
     As described above, the UE  240  can use the carrier aggregation by simultaneously communicating with a plurality of eNBs using different frequency bands, in addition to the case where one eNB uses a plurality of frequency bands. Further, the MME  250  can determine whether the carrier aggregation using a plurality of eNBs is applicable or not by using the subscriber information sent from the HSS  71 . 
     Fifth Exemplary Embodiment 
     The flow of an Attach process according to a fifth exemplary embodiment of the present invention is described hereinafter with reference to  FIG. 11 .  FIG. 11  shows the flow of a process where Step S 11  and Step S 12  are added between Step S 5  and Step S 6  in  FIG. 5 . In  FIG. 11 , the process which is common to that in  FIG. 5  is not redundantly described. 
     In Step S 11 , the eNB  40  receives an Initial Context Setup Request message and then sets information to be used for determining whether the carrier aggregation is applicable or not to an arbitrary parameter of a UE Capability Enquiry message and sends it to the UE  50  (S 11 ). 
     The UE  50  receives the UE Capability Enquiry message and determines whether or not to apply the carrier aggregation by using the information sent from the eNB  40  and sends, to the eNB  40 , a UE Capability Information message that reflects the determination result as to whether or not to apply the carrier aggregation (S 12 ). The eNB  40  receives the UE Capability Information message and sends, to the UE  50 , an RRC Connection Reconfiguration/NAS:Attach Accept message that reflects the determination result as to whether or not to apply the carrier aggregation (S 6 ). 
     As described above, by performing the Attach process in  FIG. 11 , the UE  50  can make determination about whether or not to apply the carrier aggregation. 
     Further, the UE  50  that is used in the above-described exemplary embodiment may include a communication unit  51  and a determination unit  52  as shown in  FIG. 12 . The communication unit  51  receives information to be used for determining the frequency band to be used in wireless communications with the eNB  40 . Further, by using the received information, the determination unit  52  determines whether or not to perform wireless communications with the eNB  40  by using two or more frequency bands. 
     Although the present invention is described as a hardware configuration in the above exemplary embodiments, the present invention is not limited thereto. The present invention may be implemented by causing a processor such as a CPU (Central Processing Unit) to execute a computer program to perform processing in the core network device (e.g., MME), the base station (e.g., eNB) and the mobile station (e.g., UE). 
     In the above example, the program can be stored and provided to the computer using any type of non-transitory computer readable medium. The non-transitory computer readable medium includes any type of tangible storage medium. Examples of the non-transitory computer readable medium include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g. magneto-optical disks), CD-ROM (Read Only Memory), CD-R , CD-R/W, DVD-ROM (Digital Versatile Disc Read Only Memory), DVD-R (DVD Recordable)), DVD-R DL (DVD-R Dual Layer)), DVD-RW (DVD ReWritable)), DVD-RAM), DVD+R), DVR+R DL), DVD+RW), BD-R (Blu-ray (registered trademark) Disc Recordable)), BD-RE (Blu-ray (registered trademark) Disc Rewritable)), BD-ROM), and semiconductor memories (such as mask ROM, PROM (Programmable ROM), EPROM (Erasable PROM), flash ROM, RAM (Random Access Memory), etc.). The program may be provided to a computer using any type of transitory computer readable medium. Examples of the transitory computer readable medium include electric signals, optical signals, and electromagnetic waves. The transitory computer readable medium can provide the program to a computer via a wired communication line such as an electric wire or optical fiber or a wireless communication line. 
     It should be noted that the present invention is not limited to the above-described exemplary embodiments and may be varied in many ways within the scope of the present invention. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-048210 filed on Mar. 11, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           10  BASE STATION 
           20  MOBILE STATION 
           30  NETWORK DEVICE 
           40  eNB 
           41  DETERMINATION UNIT 
           42  COMMUNICATION UNIT 
           50  UE 
           51  COMMUNICATION UNIT 
           52  DETERMINATION UNIT 
           60  MME 
           61  SIGNAL GENERATION UNIT 
           62  COMMUNICATION UNIT 
           70  MME 
           71  HSS 
           72  S-GW 
           73  P-GW 
           100  CELL 
           110  CELL 
           200  eNB 
           210  CELL 
           220  eNB 
           230  CELL 
           240  UE 
           250  MME