Patent Publication Number: US-2018041909-A1

Title: Mobile communication terminal, communication control apparatus, communication control method, and non-transitory computer readable medium

Description:
TECHNICAL FIELD 
     The present invention relates to a mobile communication terminal, a communication control apparatus, a communication control method and a program. More particularly, the present invention relates to the mobile communication terminal, the communication control apparatus, the communication control method and the program which can use a plurality of frequency bands. 
     BACKGROUND ART 
     In recent years, mobile telephone terminals supplied in the market have supported a plurality of RATs (Radio Access Technology). Further, each mobile telephone can perform communication by using one RAT of a plurality of RATs. The RATs include, for example, GSM (registered trademark) (Global System for Mobile communications), W-CDMA (Wideband Code Division Multiple Access) and LTE (Long Term Evolution) which are radio access schemes defined by 3GPP (3rd Generation Partnership Project). Further, in addition to the radio access schemes defined by 3GPP, mobile telephone terminals which can perform wireless LAN communication are also increasing. 
     Generally, each RAT uses a radio wave of a frequency band different from those of other RATs. Hence, a propagation characteristic of the radio wave of each RAT differs from those of other RATs. Further, each RAT uses radio waves of a plurality of frequency bands in some cases. For example, there are also standards which support a 2.4 GHz frequency band and a 5 GHz frequency band for wireless LAN (Local Area Network) communication. 
     Generally, a mobile telephone terminal performs communication by using a RAT of the latest wireless communication scheme when detecting a plurality of RATs. This is because the RAT of the latest wireless communication scheme can provide the highest speed transmission. Therefore, the mobile telephone terminal selects the RAT of the latest wireless communication scheme to improve service of the mobile communication terminal. 
     Further, a mobile telephone terminal measures a signal strength of each RAT, and selects a RAT which transmits a signal of the highest reception quality. 
     Non-Patent Literature 1 discloses an ISR (Idle mode Signaling Reduction) function of defining a flow of incoming call processing when a communication terminal can use communication according to a plurality of RATs. When, for example, a RNC (Radio Network Controller) and an eNB (evolved NodeB) which uses a wireless communication scheme different from that of the RNC perform paging, a communication terminal performs response processing of responding to one paging. Further, a network side also continues subsequent communication by using a wireless communication scheme of the RNC or the eNB to which the communication terminal has responded. 
     Patent Literature 1 discloses deciding whether or not it is possible to perform handover between different RATs according to whether communication is voice communication or data communication. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: U.S. Pat. No. 7,257,403 
       
    
     Non-Patent Literature 
     
         
         NPL 1: 3GPP TS23.401 V13.1.0 (2014-12) 5.3.4 Service Request procedures 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     When processing disclosed in Non-Patent Literature 1 is executed in an environment where a communication terminal can perform communication by using a plurality of RATs as in Non-Patent Literature 1 and Patent Literature 1, if different RATs are available, the communication terminal performs communication by using one of the RATs. In this case, there are problems that a plurality of communication terminals are likely to perform communication while intensively using the latest RAT and are likely to concentrate on a high frequency band which enables high speed communication. Further, even when whether or not to it is possible to connect to a different RAT is decided by using an information element used during handover for deciding whether communication is voice communication or data communication as in Patent Literature 1, there is the following problem. When whether or not it is possible to connect to the different RAT is decided by using the information element used during handover for deciding whether or not communication is voice communication, a communication terminal is likely to continue using the same frequency band after connecting to the different RAT. Therefore, there is a problem that intensive use of a given frequency band is not reduced, and radio resources cannot be effectively used. 
     An object of the present invention is to provide a mobile communication terminal, a communication control apparatus, a communication control method, and a program which can use an appropriate frequency band to effectively make use of radio resources. 
     Solution to Problem 
     A mobile communication terminal according to a first aspect of the present invention includes: a first communication unit that connects to a mobile communication network by using a first frequency band; a second communication unit that connects to the mobile communication network by using a second frequency band; and a determining unit that determines to connect to the mobile communication network by using a service type and using at least one of the first communication unit and the second communication unit when performing communication via the mobile communication network, wherein the service type indicates service executed during the communication via the mobile communication network. 
     A communication control apparatus according to a second aspect of the present invention is a communication control apparatus that is installed in a mobile communication network, and includes: a communication unit that receives information when a mobile communication terminal performs communication via the mobile communication network, wherein the information is transmitted from the mobile communication terminal and relates to a frequency band that can be used by the mobile communication terminal; and a selecting unit that determines a frequency band used by the mobile communication terminal by using a service type, and transmitting information related to the determined frequency band to the mobile communication terminal, wherein the service type indicates service executed when the mobile communication terminal performs the communication via the mobile communication network. 
     A communication control method according to a third aspect of the present invention includes determining to connect to a mobile communication network by using a service type and using at least one of a first communication unit and a second communication unit when performing communication via the mobile communication network, wherein the service type indicates service executed during the communication via the mobile communication network, the first communication unit connects to the mobile communication network by using a first frequency band, and the second communication unit connects to the mobile communication network by using a second frequency band. 
     A program according to a fourth aspect of the present invention causes a computer to execute determining to connect to a mobile communication network by using a service type and using at least one of a first communication unit and a second communication unit when performing communication via the mobile communication network, wherein the service type indicates service executed during the communication via the mobile communication network, the first communication unit connects to the mobile communication network by using a first frequency band, and the second communication unit connects to the mobile communication network by using a second frequency band. 
     Advantageous Effects of Invention 
     The present invention can provide a mobile communication terminal, a communication control apparatus, a communication control method and a program which enable communication terminals to perform communication in a frequency band matching a band which is necessary for the communication terminals to perform communication. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a mobile communication terminal according to a first embodiment. 
         FIG. 2  is a configuration diagram of a communication system according to a second embodiment. 
         FIG. 3  is a configuration diagram of a UE according to the second embodiment. 
         FIG. 4  is a view for explaining service areas of a plurality of RATs which use different frequency bands according to the second embodiment. 
         FIG. 5  is a view for explaining a relationship between a frequency band and a band which can be secured in this frequency band according to the second embodiment. 
         FIG. 6  is a configuration diagram of a cell according to the second embodiment. 
         FIG. 7  is a view illustrating a flow of RAT selection processing according to the second embodiment. 
         FIG. 8  is a view illustrating a flow of RAT selection processing according to the second embodiment. 
         FIG. 9  is a configuration diagram of a MME according to a third embodiment. 
         FIG. 10  is a view illustrating a flow of RAT selection processing according to the third embodiment. 
         FIG. 11  is a view illustrating a flow of carrier aggregation processing according to a fourth embodiment. 
         FIG. 12  is a view illustrating a flow of the carrier aggregation processing according to the fourth embodiment. 
         FIG. 13  is a view illustrating a flow of carrier aggregation processing according to a fifth embodiment. 
         FIG. 14  is a view illustrating a flow of network connection processing according to a sixth embodiment. 
         FIG. 15  is a view illustrating a flow of the network connection processing according to the sixth embodiment. 
         FIG. 16  is a view illustrating a flow of network connection processing according to a seventh embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Embodiments of the present invention will be described below with reference to the drawings. A configuration example of a mobile communication terminal  10  according to the first embodiment of the present invention will be described with reference to  FIG. 1 . The mobile communication terminal  10  may be a mobile telephone terminal, a smartphone or a tablet terminal. Further, the mobile communication terminal  10  may be a computer apparatus which operates when a processor executes programs stored in a memory. 
     The mobile communication terminal  10  in  FIG. 1  includes a communication unit (transmitting/receiving unit)  11 , a communication unit (transmitting/receiving unit)  12  and a determining unit  13 . The communication unit  11 , the communication unit  12  and the determining unit  13  may be software or modules which execute processing when the processor executes programs stored in the memory. Alternatively, the communication unit  11 , the communication unit  12  and the determining unit  13  may be configured as circuits. 
     The communication unit  11  connects to a mobile communication network  20  by using a frequency band A. The frequency band A may be, for example, one frequency band of an 800 MHz band, a 1.5 GHz band, a 2 GHz band and a 5 GHz band. The mobile communication network  20  is a network which is managed by a mobile telecommunication carrier. Connection of the communication unit  11  to the mobile communication network  20  means that, for example, the communication unit  11  can communicate with a base station which configures the mobile communication network  20 . 
     The communication unit  12  connects to the mobile communication network  20  by using a frequency band B different from the frequency band A. The frequency band B may be one frequency band of the 800 MHz band, the 1.5 GHz band, the 2 GHz band and the 5 GHz band. When, for example, the frequency band A is the 800 MHz band, the frequency band B is the 2 GHz band. 
     When the mobile communication terminal  10  performs communication via the mobile communication network  20 , the determining unit  13  determines to connect to the mobile communication network  20  by using a service type indicating service executed during communication via the mobile communication network  20 , and using at least one of the communication unit  11  and the communication unit  12 . 
     The communication via the mobile communication network  20  may mean that, for example, the mobile communication terminal  10  performs voice communication or data communication with another mobile communication terminal via the mobile communication network  20 . The service type is information for identifying service determined according to a type of data transmitted and received from and by the mobile communication terminal  10 , and may be, for example, voice communication, Web access, text message communication or moving image distribution. 
     Generally, as a bandwidth to be used widens, a communication speed increases. Consequently, a high frequency band which makes it possible to secure a wide bandwidth can increase the communication speed compared to a low frequency band which can only secure a narrow bandwidth. Hence, the determining unit  13  may determine to select the communication unit  11  which uses the 800 MHz band when, for example, the service indicated by the service type does not need high speed communication. Further, when the service indicated by the service type needs high speed communication (e.g., in a case of moving image distribution), the determining unit  13  may determine to select the communication unit  12  which uses the 2 GHz band or determine to select both the communication unit  11  and the communication unit  12 . 
     As described above, the mobile communication terminal  10  in  FIG. 1  can determine the frequency band to use according to a service executed during communication via the mobile communication network  20 . Hence, when, for example, executing a service which does not need high speed communication, the mobile communication terminal  10  can perform control of selecting a communication unit which uses a lower frequency band. Consequently, the mobile communication network  20  can secure appropriate radio resources for the mobile communication terminal  10 , and prevent a plurality of mobile communication terminals from intensively using a high frequency band which enables high speed communication. 
     Second Embodiment 
     Next, a configuration example of a communication system according to the second embodiment of the present invention will be described with reference to  FIG. 2 . The communication system in  FIG. 2  includes a UE (User Equipment)  30 , a BSC (Base Station Controller)  41 , a RNC (Radio Network Controller)  42 , an eNB (evolved NodeB)  43 , an eeNB (enhanced eNB)  44 , a SGSN (Serving GPRS Support Node)  50 , a MME (Mobility Management Entity)  60 , a eMME (evolved MME)  70 , a SGW (Serving Gateway)  80 , a PGW (Packet Date Network Gateway)  81 , a PCRF (Policy and Charging Rules Function)  82 , a service server  83  and an external network  84 . 
     The BSC  41 , the RNC  42 , the eNB  43 , the eeNB  44 , the SGSN  50 , the MME  60 , the eMME  70 , the SGW  80 , the PGW  81  and the PCRF  82  operate as defined by 3GPP and are node apparatuses which configure a mobile communication network. The mobile communication network configured by these components, i.e., the BSC  41 , the RNC  42 , the eNB  43 , the eeNB  44 , the SGSN  50 , the MME  60 , the eMME  70 , the SGW  80 , the PGW  81  and the PCRF  82  correspond to the mobile communication network  20  in  FIG. 1 . 
     The UE  30  is a generic term for a mobile communication terminal according to 3GPP, and corresponds to a mobile communication terminal  10  in  FIG. 1 . The BSC  41  is an apparatus which performs communication by using a radio access scheme which is called a so-called second generation (2G). The RNC  42  is an apparatus which performs communication by using a radio access scheme which is called a so-called third generation (3G). The eNB  43  is an apparatus which performs communication by using a radio access scheme which is called a so-called fourth generation (4G) or LTE (Long Term Evolution). The eeNB  44  is an apparatus which performs communication by using a radio access scheme which is called a so-called fifth generation (5G). The eeNB  44  is a base station which can perform communication by using a radio access scheme newly defined subsequently to LET. 
     The SGSN  50  is an apparatus which integrates the BSC  41  and the RNC  42 , and relays U (User)-Plane data and C (Control)-Plane data transmitted and received between the BSC  41  and the SGW  80 . The U-Plane data may be referred to as user data, for example, and the C-Plane data may be referred to as control data. The MME  60  relays C-Plane data transmitted and received between the eNB  43  and the SGW  80 . The eMME  70  relays the C-Plane data transmitted and received between the eeNB  44  and the SGW  80 . 
     The SGW  80  relays the U-Plane data transmitted and received between the RNC  42 , the SGSN  50 , the eNB  43  and the eeNB  44 , and the PGW  81 . The PGW  81  is a gateway apparatus connected with the external network  84  different from the mobile communication network  20  configured by the UE  30 , the BSC  41 , the RNC  42 , the eNB  43 , the eeNB  44 , the SGSN  50 , the MME  60 , the eMME  70 , the SGW  80 , the PGW  81  and PCRF  82 . The external network  84  may be a mobile communication network which is managed by another mobile telecommunications carrier, may be a network which performs point-to-point communication or may be a network which is managed by an ISP (Internet Service Provider). 
     The PCRF  82  is an apparatus which manages a communication policy or a charging policy of the mobile communication network  20 . The service server  83  is a server managed by the mobile communication network  20  or the external network  84 , and provides communication service to the UE  30 . 
     Next, a configuration example of the UE  30  according to the second embodiment of the present invention will be described with reference to  FIG. 3 . The UE  30  includes an application  31 , a RAT selecting unit  32  and communication units  33  to  36 . The application  31 , the RAT selecting unit  32  and the communication units  33  to  36  may be software or modules which execute processing when a processor executes programs stored in a memory. Alternatively, the RAT selecting unit  32  and the communication units  33  to  36  may be configured as circuits. 
     The communication units  33  to  36  communicate with other party apparatuses by using different frequency bands. The other party apparatuses may be, for example, the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44 . 
     For example, the communication unit  33  may perform wireless communication by using the 800 MHz band. The communication unit  34  may perform wireless communication by using the 1.5 GHz band. The communication unit  35  may perform wireless communication by using the 3 GHz band. The communication unit  36  may perform wireless communication by using the 30 GHz band. 
     The application  31  may be, for example, an application which performs voice communication, Web access, music playback or moving image playback. The RAT selecting unit  32  receives information related to a service type transmitted from the mobile communication network  20  via the communication unit  33  to the communication unit  36 . The RAT selecting unit  32  receives, for example, the information related to the service type as C-Plane data when incoming call processing is performed for the UE  30 . 
     Further, the RAT selecting unit  32  determines a RAT to use and a frequency band according to a service type. Hereinafter, service areas of a plurality of RATs which use different frequency bands will be described with reference to  FIG. 4 . 
     In  FIG. 4 , a horizontal axis indicates service area sizes. Further, 2G is a communication area which is formed by using the BSC  41 . 3G is a communication area which is formed by using the RNC  42 . 4G is a communication area which is formed by using the eNB  43 . 5G is a communication area which is formed by using the eeNB  44 . 
     In this regard, a relationship between a frequency band and a band which can be secured in this frequency band will be described with reference to  FIG. 5 . As illustrated in  FIG. 5 , a RAT of an older communication scheme or generation uses a lower frequency band and use radio waves of longer wavelengths. Consequently, it is possible to widen a communication area formed by the one BSC  41  or the one RNC  42 . However, by lowering the frequency band to use, the band which can be secured narrows, and therefore it is not possible to perform high speed communication. 
     Meanwhile, a RAT of a newer communication scheme or generation uses a higher frequency band to use, and uses radio waves of shorter wavelengths. Hence, a communication area formed by the one eNB  43  or the one eeNB  44  narrows. However, by raising the frequency band to use, the band which can be secured widens, so that it is possible to perform high speed communication. 
     For example, a 5 to 10 MHz frequency band may be secured in the 800 MHz band. A 20 MHz frequency band may be secured in the 1.5 GHz band. A 40 MHz frequency band may be secured in the 3 GHz band. A 100 MHz frequency band may be secured in the 30 GHz band. 
     Further, a RAT of a newer communication scheme or generation is not prevented from using a lower frequency band, and a RAT of an older communication scheme or generation is not prevented from using a high frequency band. In other words, a plurality of RATs can also use one frequency band. 
       FIG. 6  is a configuration diagram of cells whose communication areas become smaller as frequency bands become higher. Further,  FIG. 6  illustrates that RATs of 2G to 5G are available in the 800 MHz band, RATs of 3G to 5G are available in the 1.5 GHz, RATs of 4G and 5G are available in the 3 GHz, and a RAT of 5G is available in the 30 GHz band. 
     Back to  FIG. 4 , a Zone 1 included in a service area on the horizontal axis in  FIG. 4  enables communication by using RATs related to 2G, 3G, 4G and 5G. Zones indicate specific areas included in the service area. A Zone 2 enables communication by using RATs related to 2G and 4G. A Zone 3 enables communication by using a RAT related to 3G. Further, as is clear from  FIG. 4 , service areas narrow in order of 2G, 3G, 4G and 5G. 
     Back to  FIG. 3 , selection processing of the RAT selecting unit  32  will be described. A case where the UE  30  locates in the Zone 1 will be described. First, a case where the RAT selecting unit  32  receives a service type indicating voice communication will be described. For the voice communication, a narrow band which can realize a communication speed of approximately 20 kbps to 100 kbps may be used. Hence, the RAT selecting unit  32  may select a RAT which uses a low frequency band when the service type indicates the voice communication. A wavelength of a radio wave in a low frequency band is long and, even when there is an obstacle such as a building, and when a radio wave of a long wavelength can go around the building, the radio wave easily keeps connection and hardly disconnects communication even during movement. The voice communication needs real time data communication, and therefore it is preferable to use a low frequency band. When, for example, receiving the service type indicating the voice communication, the RAT selecting unit  32  may select 2G or 3G which uses a low frequency as a RAT. Further, when the RATs of 4G and 5G are available in a plurality of frequency bands, the RAT selecting unit  32  may select the lowest frequency band of a plurality of frequency bands in which RATs of 4G and 5G are available. 
     Next, a case where the RAT selecting unit  32  receives a service type indicating moving image distribution will be described. For the moving image distribution, it is generally necessary to perform high speed communication which uses a wider band as image resolution becomes higher. Hence, when the service type indicates the moving image distribution, the RAT selecting unit  32  may select a RAT which uses a high frequency band. When, for example, receiving the service type indicating the moving image distribution, the RAT selecting unit  32  may select 4G or 5G which uses a high frequency band as a RAT. Further, when the RAT of 5G is available in a plurality of frequency bands, the RAT selecting unit  32  may select the highest frequency band of a plurality of frequency bands in which the RAT of 5G is available. 
     The RAT selecting unit  32  connects to the mobile communication network  20  via one communication unit of the communication units  33  to  36  according to a frequency band used by the selected RAT. Alternatively, the RAT selecting unit  32  may select two or more communication units from the communication units  33  to  36 , and connect to the mobile communication network  20  via the two or more communication units. 
     Next, a flow of RAT selection processing according to the second embodiment of the present invention will be described with reference to  FIGS. 7 and 8 .  FIGS. 7 and 8  illustrate a flow of processing when the UE  30  receives an incoming call. 
     First, the PGW  81  receives a download packet (DL Packet) addressed to the UE  30  (S 11 ). Next, the PGW  81  transmits the received download packet to the SGW  80  (S 12 ). The PGW  81  transmits to the SGW  80  the download packet via a GTP (General packet radio service Tunneling Protocol)-U tunnel set between the PGW  81  and the SGW  80 . The PGW  81  transmits the download packet and information related to a service type indicating service executed by the UE  30 , to the SGW  80  in step S 12 . 
     For example, the PGW  81  may decide the service type of the download packet by executing DPI (Deep Packet Inspection) with respect to the download packet received in step S 11 . The DPI means that, for example, the PGW  81  analyzes contents of the download packet received in step S 11 . The PGW  81  can decide which data related to which service the download packet is by executing the DPI. 
     Alternatively, PGW  81  may decide a service type indicating service executed by the incoming call destination UE  30  by using the C-Plane data transmitted from the PCRF  82 . When, for example, VoIP (Voice over IP) service for performing call control in an IMS (IP Multimedia Subsystem) is executed, the PGW  81  receives C-Plane data indicating that the UE  30  executes the VoIP, from the IMS via the PCRF  82 . Thus, the PGW  81  may decide the service type as voice communication by receiving the C-Plane data transmitted from the IMS. 
     Next, the SGW  80  transmits a DDN (Downlink Data Notification) message to the SGSN  50 , the MME  60  and the eMME  70  to notify the UE  30  of the incoming call (S 13 ). The SGW  80  transmits a DDN message to which the service type has been set, to the SGSN  50 , the MME  60  and the eMMB  70  in which subscriber information of the UE  30  is registered. 
     When, for example, locating in the Zone 1 in  FIG. 4 , the UE  30  can perform communication by using the RATs of 2G to 5G. Hence, in such a case, the subscriber information related to the UE  30  is assumed to be registered in the SGSN  50 , the MME  60  and the eMME  70 . Further, when the UE  30  locates in the Zone 2, the UE  30  can perform communication by using the RATs of 2G and 4G. In such a case, the subscriber information related to the UE  30  is assumed to be registered in the SGSN  50  and the MME  60 . 
     For example, the UE  30 , the SGSN  50 , the MME  60  and the eMME  70  register the subscriber information related to the UE  30  in the SGSN  50 , the MME  60  and the eMMB  70  by activating the ISR function. 
     Next, the SGSN  50  transmits a Page message for instructing paging to the BSC  41  and the RNC  42  (S 14 ). Further, the MME  60  transmits the Page message for instructing paging to the eNB  43  (S 15 ), and the eMME  70  transmits the Page message for instructing paging to the eeNB  44  (S 16 ). The SGSN  50 , the MME  60  and the eMME  70  transmit the Page messages. 
     Next, when receiving the Page message, the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44  perform paging processing on the UEs located in communication areas of the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44  (S 17 ). 
     Next, the UE  30  transmits a NAS (Non-Access Stratum): Service Request message as a response message to the paging processing to the SGSN  50 , the MME  60  or the eMME  70  via one of the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44  (S 18 ).  FIG. 7  illustrates an example where the UE  30  transmits the NAS: Service Request message to the MME  60  via the eNB  43 . Further,  FIG. 7  illustrates that, in step S 18 , the eNB  43  transmits the received NAS: Service Request message to the MME  60 . The NAS: Service Request message is a message transmitted to the MME  60  via a S1 interface set between the eNB  43  and the MME  60 . The UE  30  may set information related to migratable RATs and a radio wave situation of each RAT to the NAS: Service Request message. The information related to the migratable RATs is, for example, communicable RAT information. 
     In step S 18 , for example, the UE  30  may decide that radio wave propagation environment between the UE  30  and the eNB  43  is the best, and transmit the NAS: Service Request message to the eNB  43 . Alternatively, when the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44  execute the paging processing, the UE  30  may transmit the NAS: Service Request message to one of the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44  which executes the paging processing which the UE  30  recognizes the earliest. The UE  30  may determine one of the BSC  41 , the RNC  42 , the eNB  43  and the eeNB  44  to which the NAS: Service Request message or a message corresponding to the NAS: Service Request message is transmitted, by using other decision criteria. 
     In view of  FIG. 8 , the MME  60  transmits the Service Notification Request message to which the information related to the service type has been set, to the UE  30  via the eNB  43  (S 19 ). Further, the MME  60  may set, to the Service Notification Request message, the information related to the service type, and information indicating a frequency band which is suitable to execute service indicated by the service type or information which relates to a RAT which is suitable to execute the service indicated by the service type. 
     Next, when receiving the Service Notification Request message, the UE  30  selects a RAT used for communication and a frequency band by using the information related to the service type (S 20 ). In this regard, the UE  30  may select the RAT used for communication by taking into account the information related to the service type, and RAT radio wave environment, service importance or a packet unit price of each RAT. 
     Next, when selecting the RAT and the frequency, the UE  30  transmits the NAS: Service Request message to the apparatus related to the selected RAT (S 21 ).  FIG. 8  illustrates an example where the UE  30  transmits the NAS: Service Request message to the SGSN  50  via the RNC  42 . That is, the UE  30  changes a NAS: Service Request message transmission destination from the MME  60  to the SGSN  50  in step S 18 . That is, the UE  30  causes migration of the RAT used for communication from 4G to 3G. 
     The UE  30  may set, to the NAS: Service Request message, information related to migratable RATs, information indicating that the RAT has been changed and information related to the number of changes of the RAT. 
     When, for example, the number of changes of the RAT performed by the UE  30  is smaller than a predetermined number, the SGSN  50 , the MME  60  and the eMME  70  may transmit a Service Notification Request message again and encourage the change of the RAT. Alternatively, when the information indicating that the RAT has been changed is set to the NAS: Service Request message, the SGSN  50 , the MME  60  and the eMME  70  may determine to execute call control by using the RAT selected by the UE  30  without transmitting the Service Notification Request message to the UE  30 . Alternatively, by taking into a congestion situation or an available resource situation of the RAT selected by the UE  30 , the SGSN  50 , the MME  60  and the eMME  70  may encourage the UE  30  to change the RAT. 
     In step S 21  or a subsequent step, the mobile communication network  20  executes call control to communicate with the UE  30  via the RNC  42 . 
     As described above, by using the communication system according to the second embodiment, the UE  30  can select an appropriate frequency band and RAT according to service executed during an incoming call. Consequently, it is possible to prevent a plurality of UEs from intensively using a new RAT and a frequency band which enables high speed communication. 
     Third Embodiment 
     Next, a configuration example of a MME  60  according to the third embodiment of the present invention will be described with reference to  FIG. 9 . In this regard, a SGSN  50  and an eMME  70  are assumed to employ the same configuration as that of the MME  60  and will not be described in detail. 
     The MME  60  includes a RAT selecting unit  61 , a communication unit  62  and a communication unit  63 . The RAT selecting unit  61 , the communication unit  62  and the communication unit  63  may be software or modules which execute processing when a processor executes programs stored in a memory. Alternatively, the RAT selecting unit  61 , the communication unit  62  and the communication unit  63  may be configured as circuits. 
     The communication unit  62  communicates with an eNB  43  by using a predetermined protocol of a S1 interface set between the communication unit  62  and the eNB  43 . The communication unit  63  communicates with a SGW  80  by using a predetermined protocol of a S11 interface set between the communication unit  63  and the SGW  80 . 
     The RAT selecting unit  61  receives information related to a service type of service executed by a UE  30  when the SGW  80  performs incoming call processing on the UE  30  via the communication unit  63 . The RAT selecting unit  61  selects a RAT and a frequency band used by the UE  30  for communication by using the received information related to the service type. The RAT selecting unit  61  transmits the selected RAT and frequency band to the UE  30  via the communication unit  62 . 
     Next, a flow of RAT selection processing according to the third embodiment of the present invention will be described with reference to  FIG. 10 . In this regard, it is assumed that, before step S 30  in  FIG. 10  is executed, steps  11  to S 18  in  FIG. 7  are executed. 
     The MME  60  receives a NAS: Service Request message transmitted from the UE  30  in step S 18  in  FIG. 7 . Further, information related to migratable RATs and a radio wave situation of each RAT are assumed to be set to the NAS: Service Request message. 
     Next, the MME  60  determines a RAT and a frequency band which the UE  30  needs to use for communication by using the information received in step S 13  in  FIG. 7  and related to the service type indicating service executed by the UE  30 , and information received in step S 18  in  FIG. 7  and related to the migratable RATs of the UE  30  (S 30 ). RAT selection processing executed by the RAT selecting unit  61  of the MME  60  is the same as RAT selection processing executed by a RAT selecting unit  32  of the UE  30  in  FIG. 3 . 
     Next, the MME  60  transmits the Service Notification Request message to which the RAT and the frequency band which the UE  30  needs to use for communication have been set, to the UE  30  via the eNB  43  (S 31 ). In this case, the UE  30  is assumed to set 3G as a RAT which the UE  30  needs to use for communication, and set 800 MHz as the frequency band. 
     Next, the UE  30  transmits the NAS: Service Request message to connect to a mobile communication network  20  by using the RAT and the frequency band set to the Service Notification Request message (S 32 ). The UE  30  decides that 3G is set as the RAT which needs to be selected and 800 MHz is set as the frequency band which needs to be selected in the Service Notification Request message in step S 31 , and therefore transmits the NAS: Service Request message to the SGSN  50  via the RNC  42 . 
     In step S 32 , the UE  30  changes a NAS: Service Request message transmission destination from the MME  60  in step S 18  to the SGSN  50 . That is, the UE  30  migrates the RAT which is used for communication from 4G to 3G according to a migration instruction from the MME  60 . 
     Similar to step S 21  in  FIG. 8 , the UE  30  may set, to the NAS: Service Request message, information related to migratable RATs, information indicating that a RAT has been changed, and information related to the number of changes of the RAT. 
     Further, the MME  60  decides in step S 30  that the RAT which the UE  30  needs to use for communication is 4G, and the frequency band used by the UE  30  is also the same as a frequency band used to transmit the NAS: Service Request message in step S 18  in  FIG. 7 . In this case, the MME  60  may execute call control processing related to the UE  30  without performing processing in step S 31 . 
     Further, when receiving the NAS: Service Request message in step S 32 , the SGSN  50  may perform the RAT selection processing again similar to the processing in step S 30 . 
     As described above, by using the communication system according to the third embodiment, the MME  60  can select an appropriate frequency band and RAT which the UE  30  needs to use for communication according to service executed by the UE  30  during an incoming call to the UE  30 . Consequently, it is possible to prevent a plurality of UEs from intensively using a RAT of a new communication scheme and a frequency band which enables high speed communication. 
     Fourth Embodiment 
     Next, a flow of carrier aggregation processing according to the fourth embodiment of the present invention will be described with reference to  FIGS. 11 and 12 . Steps S 41  and S 42  are the same as steps S 11  and S 12  in  FIG. 7  and therefore will not be described in detail. 
     Next, a SGW  80  transmits a DDN message to which a service type has been set, to a MME  60  in which subscriber information of a UE  30  has been registered (S 43 ). 
     Next, the MME  60  transmits a Page message to an eNB which supports 4G as a RAT and uses an 800 MHz frequency band, an eNB which uses a 1.5 GHz frequency band and an eNB which uses a 3 GHz frequency band (S 44 ). That is, the MME  60  transmits a Page message to a plurality of eNBs which use different frequency bands. 
     Next, when receiving the Page message, the eNB which uses the 800 MHz frequency band, the eNB which uses the 1.5 GHz frequency band and the eNB which uses the 3 GHz frequency band perform paging processing on UEs located in communication areas of the respective eNBs (S 45 ). 
     Next, the UE  30  transmits a NAS: Service Request message as a response message to the paging processing in step S 45  to the MME  60  via one of the eNB which uses the 800 MHz frequency band, the eNB which uses the 1.5 GHz frequency band and the eNB which uses the 3 GHz frequency band (S 46 ).  FIG. 11  illustrates an example where the UE  30  transmits the NAS: Service Request message to the eNB which uses the 1.5 GHz frequency band. The UE  30  may set information related to a radio wave situation of each frequency band and information related to combinations which enable carrier aggregation to the NAS: Service Request message. 
     In step S 46 , for example, the UE  30  may decide that radio wave environment between the UE  30  and the eNB which uses the 1.5 GHz frequency band is the best, and transmit the NAS: Service Request message to the eNB which uses the 1.5 GHz frequency band. Alternatively, when the eNB which uses the 800 MHz frequency band, the eNB which uses the 1.5 GHz frequency band and the eNB which uses the 3 GHz frequency band execute paging processing, the UE  30  may transmit the NAS: Service Request message to the eNB which executes the paging processing which the UE  30  has recognized the earliest. The UE  30  may determine one of the eNBs to which the NAS: Service Request message or a message corresponding to the NAS: Service Request message is transmitted, by using other decision criteria. 
     In view of  FIG. 12 , the MME  60  transmits the Service Notification Request message to which the information related to the service type has been set, to the UE  30  via the eNB which uses the 1.5 GHz frequency band (S 47 ). Further, when carrier aggregation is performed, the MME  60  may set information, too, which relates to combinations of frequency bands suitable to service indicated by a service type, to the Service Notification Request message. 
     Next, when receiving the Service Notification Request message, the UE  30  selects frequency bands to combine when performing carrier aggregation by using the information related to the service type (S 48 ). When, for example, the service indicated by the service type is moving image distribution of low resolution, the UE  30  may determine to combine the 800 MHz frequency band including a 5 MHz frequency band and the 1.5 GHz frequency band including a 10 MHz frequency band and perform carrier aggregation. Alternatively, when the service indicated by the service type is moving image distribution of high resolution such as  4 K or  8 K, the UE  30  may determine to combine a plurality of frequency bands such as 20 MHz×2 in a 3 GHz frequency band including 20 MHz frequency band and perform carrier aggregation. 
     In this regard, by taking into account the information related to the service type, radio wave environment of each frequency band, service importance or a packet unit price of each frequency band, the UE  30  may select a combination of frequency bands used for carrier aggregation. 
     Next, when selecting the frequency bands to combine to perform carrier aggregation, the UE  30  transmits the NAS: Service Request message to the MME  60  via the eNBs which use the selected frequency bands (S 49 ). The UE  30  sets the information related to the selected frequency bands to the NAS: Service Request message. 
     As described above, by using the communication system according to the fourth embodiment of the present invention, the UE  30  can select a combination of frequency bands which are used for carrier aggregation according to a service type. Consequently, it is possible to select different frequency bands to perform carrier aggregation for service which needs a wide frequency band and service which needs a narrow frequency band. Consequently, it is possible to prevent a plurality of UEs from concentrating on a frequency band which can secure a wide frequency band when carrier aggregation is performed. 
     Fifth Embodiment 
     Next, a flow of carrier aggregation processing according to the fifth embodiment of the present invention will be described with reference to  FIG. 13 . In this regard, it is assumed that, before step S 51  in  FIG. 13  is executed, steps S 41  to S 46  in  FIG. 11  are executed. 
     In step S 46  in  FIG. 11 , a MME  60  receives a NAS: Service Request message transmitted from a UE  30 . Further, information related to a radio wave situation of each frequency band and information related to combinations which enable carrier aggregation are assumed to be set to the NAS: Service Request message. 
     Next, the MME  60  determines a combination of frequency bands which the UE  30  needs to use for carrier aggregation by using the information received in step S 43  in  FIG. 11  and related to a service type indicating service executed by the UE  30 , and information received in step S 46  in  FIG. 11  and related to combinations of frequency bands which enable carrier aggregation of the UE  30  (S 51 ). Processing related to selection of a combination of frequency bands which are used for carrier aggregation executed by the MME  60  is the same as processing executed by the UE  30  in step S 48  in  FIG. 12 . 
     Next, the MME  60  transmits to the UE  30  a Service Notification Request message to which the information related to the combination of the frequency bands which the UE  30  needs to use for carrier aggregation has been set (S 52 ). 
     Next, the UE  30  transmits the NAS: Service Request message to the MME  60  via an eNB which uses a frequency band instructed by the MME  60  (S 53 ). 
     As described above, by using the communication system according to the fifth embodiment of the present invention, the MME  60  can select a combination of frequency bands which are used for carrier aggregation according to a service type. Consequently, it is possible to select different frequency bands to perform carrier aggregation for service which needs a wide frequency band and service which needs a narrow frequency band, for example. Consequently, it is possible to prevent a plurality of UEs from concentrating on a frequency band which can secure a wide frequency band when carrier aggregation is performed. 
     Sixth Embodiment 
     Next, a flow of network connection processing according to the fifth embodiment of the present invention will be described with reference to  FIGS. 14 and 15 . Steps S 61  and S 62  in  FIG. 13  are the same as steps S 11  and S 12  in  FIG. 7 , and therefore will not be described in detail. 
     Next, a SGW  80  transmits a DDN message to which a service type has been set, to a MME  60  and an eMME  70  in which subscriber information of a UE  30  has been registered (S 63 ). 
     Next, the MME  60  transmits a Page message to an eNB which supports 4G as a RAT and uses an 800 MHz frequency band and an eNB which uses a 1.5 GHz frequency band (S 64 ). That is, the MME  60  transmits a Page message to a plurality of eNBs which use different frequency bands. Further, the eMME  70  transmits the Page message to an eeNB which supports 5G as a RAT and uses a 3 GHz frequency band (S 65 ). 
     Next, when receiving the Page message, the eNB which uses the 800 MHz frequency band, the eNB which uses the 1.5 GHz frequency band and the eeNB which uses the 3 GHz frequency band perform paging processing on UEs located in communication areas of the eNBs and the eeNB (S 66 ). 
     Next, the UE  30  transmits a NAS: Service Request message as a response message to the paging processing in step S 66  to the MME  60  or the eMME  70  via one of the eNB which uses the 800 MHz frequency band, the eNB which uses the 1.5 GHz frequency band and the eeNB which uses the 3 GHz frequency band (S 67 ).  FIG. 11  illustrates an example where the UE  30  transmits the NAS: Service Request message to the eNB which uses the 1.5 GHz frequency band. The UE  30  may set information related to a radio wave situation of each frequency band and information related to combinations which enable carrier aggregation to the NAS: Service Request message. 
     In step S 66 , for example, the UE  30  may decide that radio wave environment between the UE  30  and the eNB which uses the 1.5 GHz frequency band is the best, and transmit the NAS: Service Request message to the eNB which uses the 1.5 GHz frequency band. Alternatively, when the eNB which uses the 800 MHz frequency band, the eNB which uses the 1.5 GHz frequency band and the eeNB which uses the 3 GHz frequency band execute paging processing, the UE  30  may transmit the NAS: Service Request message to the one of the eNBs and the eeNB which executes the paging processing which the UE  30  has recognized the earliest. The UE  30  may determine one of the eNBs or the eeNB to which the NAS: Service Request message or a message corresponding to the NAS: Service Request message is transmitted, by using other decision criteria. 
     In view of  FIG. 15 , the MME  60  transmits the Service Notification Request message to the UE  30  via the eNB which uses the 1.5 GHz frequency band (S 68 ). The MME  60  sets, to the Service Notification Request message, the information related to the service type indicating service executed by the UE  30 , and information about combinations of frequency bands which are suitable to service indicated by the service type when the UE  30  performs carrier aggregation. 
     Next, when receiving the Service Notification Request message, the UE  30  decides whether or not to perform carrier aggregation or to connect to another RAT which enables high speed communication (S 69 ). In this regard, the UE  30  may perform decision processing in step S 69  by using information related to a service type and a movement speed of the UE  30 . 
     When, for example, the service type indicates moving image distribution of high resolution, the UE  30  decides whether to secure a wide band of 40 MHz by communicating with an eeNB which uses a 3 GHz frequency band or to secure a wide band of 20 MHz×2 by performing carrier aggregation with an eNB which uses a 1.5 GHz frequency band. 
     In this regard, when moving at a low movement speed or stopping, the UE  30  may decide to secure the wide band of 40 MHz by communicating with the eeNB which uses the 3 GHz frequency band. Alternatively, when moving at a high movement speed, the UE  30  may decide to secure a wide band of 20 MHz×2 by performing carrier aggregation with the eNB which uses the 1.5 GHz frequency band. When the UE  30  moves at the high movement speed, it is possible to realize communication of stable communication quality by performing carrier aggregation by using a low frequency band which hardly causes disconnection. Further, when the UE  30  stops or moves at the low movement speed, it is possible to realize communication of stable communication quality by using a high frequency band of radio wave straightness. 
     In step S 69 , the UE  30  decides to secure the wide band of 40 MHz by communicating with the eeNB which uses the 3 GHz frequency band. Next, the UE  30  transmits a NAS: Service request message to the eMMB  70  via the eeNB which uses the 3 GHz frequency band (S 70 ). 
     Further, in step S 69 , the UE  30  may decide to secure the wide band of 20 MHz×2 by performing carrier aggregation with the eNB which uses the 1.5 GHz frequency band. In other words, in step S 69 , the UE  30  may determine to perform carrier aggregation and determine a frequency band which is used for carrier aggregation. 
     As described above, by using the communication system according to the sixth embodiment of the present invention, the UE  30  can determine to perform carrier aggregation or to migrate to another RAT which enables high speed communication according to information related to a service type and a movement speed of the UE  30 . Further, the UE  30  can determine a frequency band to use when performing carrier aggregation. 
     The UE  30  can determine whether to perform carrier aggregation or to secure a wide frequency band by using one RAT according to the movement speed of the UE  30 , so that it is possible to perform high speed communication of stable communication quality. 
     Seventh Embodiment 
     Next, a flow of network connection processing according to the seventh embodiment of the present invention will be described with reference to  FIG. 16 . In this regard, it is assumed that, before step S 81  in  FIG. 16  is executed, steps S 61  to S 67  in  FIG. 14  are executed. Further, in step S 67  in  FIG. 14 , a UE  30  may set information related to a movement speed of the UE  30  to a NAS: Service Request message. 
     In step S 67  in  FIG. 14 , a MME  60  receives the NAS: Service Request message transmitted from the UE  30 . Further, information related to a radio wave situation of each frequency band, and information related to combinations which enable carrier aggregation are assumed to be set to the NAS: Service Request message. 
     Next, the MME  60  decides whether the UE  30  needs to perform carrier aggregation or to connect to another RAT which enables high speed communication by using the information received in step S 63  in  FIG. 14  and related to a service type for communication of the UE  30 , and information received in step S 67  in  FIG. 14  and related to combinations of frequency bands which enable the UE  30  to perform carrier aggregation (S 81 ). Processing executed by the MME  60  in step S 81  is the same as processing executed by the UE  30  in step S 69  in  FIG. 15 . 
     Next, the MME  60  transmits to the UE  30  the Service Notification Request message to which a decision result in step S 81  has been set (S 82 ). In this regard, the MME  60  is assumed to decide that the UE  30  secures a wide band of 20 MHz×2 by performing carrier aggregation with an eNB which uses the 1.5 GHz frequency band. 
     Next, the UE  30  transmits the NAS: Service Request message to the MME  60  via an eNB which uses a frequency band instructed by the MME  60  (S 83 ). 
     As described above, by using the communication system according to the seventh embodiment of the present invention, the MME  60  can determine whether to perform carrier aggregation or to migrate to another RAT which enables high speed communication according to information related to the service type and the movement speed of the MME  60 . Further, the UE  30  can determine a frequency band to use when performing carrier aggregation. 
     The UE  30  can determine whether to perform carrier aggregation or to secure a wide frequency band by using one RAT according to the movement speed of the UE  30 , so that it is possible to perform high speed communication of stable communication quality. 
     The present invention has been described as a hardware configuration in the above embodiments. However, the present invention is not limited to this. The present invention can also be realized by causing a CPU (Central Processing Unit) to execute processing of the UE, the eNB and the MME. 
     In the above example, the program can be stored by using various types of non-transitory computer readable media, and be supplied to a computer. The non-transitory computer readable media include various types of tangible storage media. The non-transitory computer readable media include, for example, magnetic recording media (e.g., flexible disks, magnetic tapes and hard disk drives), magnetooptical recording media (e.g., optical magnetic disks), CD-ROMs (Read Only Memory), CD-Rs, CD-R/Ws and semiconductor memories (e.g., mask ROMs, PROMs (Programmable ROM), EPROMs (Erasable PROM), flash ROMs and RAMs (Random Access Memory)). Further, the programs may be supplied to the computers by various types of transitory computer readable media. The transitory computer readable media include, for example, electrical signals, optical signals and electromagnetic waves. The transitory computer readable media can supply the programs to the computers via wired communication channels such as electrical cables or optical wires or wireless communication channels. 
     In addition, the present invention is not limited to the above embodiments, and can be optionally changed without departing from the spirit of the invention. 
     The present invention has been described above with reference to the embodiments. However, the present invention is not limited to the above. The configurations and the details of the present invention can be variously changed within the scope of the invention as long as one of ordinary skill in the art can understand the changes. 
     This application claims priority to Japanese Patent Application No. 2015-054536 filed on May 18, 2015, the entire contents of which are incorporated by reference herein. 
     REFERENCE SIGNS LIST 
     
         
           10  MOBILE COMMUNICATION TERMINAL 
         COMMUNICATION UNIT 
           12  COMMUNICATION UNIT 
           13  DETERMINING UNIT 
           20  MOBILE COMMUNICATION NETWORK 
           30  UE 
           31  APPLICATION 
           32  RAT SELECTING UNIT 
           33  COMMUNICATION UNIT 
           34  COMMUNICATION UNIT 
           35  COMMUNICATION UNIT 
           36  COMMUNICATION UNIT 
           41  BSC 
           42  RNC 
           43  eNB 
           44  eeNB 
           50  SGSN 
           60  MME 
           61  RAT SELECTING UNIT 
           62  COMMUNICATION UNIT 
           63  COMMUNICATION UNIT 
           70  eMME 
           80  SGW 
           81  PGW 
           82  PCRF 
           83  SERVICE SERVER 
           84  EXTERNAL NETWORK