Patent Publication Number: US-9888511-B2

Title: Mobile communication system, base station, user terminal and processor

Description:
TECHNICAL FIELD 
     The present disclosure relates to a mobile communication system that supports D2D communication, a base station, a user terminal, and a processor. 
     BACKGROUND ART 
     In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release (see Non Patent Document 1). 
     In the D2D communication, a plurality of neighboring user terminals (a user terminal group) perform direct communication without passing through a core network. That is, a data path of the D2D communication does not pass through the core network. On the other hand, a data path of normal communication (cellular communication) of a mobile communication system passes through the core network. 
     Further, in order to perform the D2D communication, a user terminal performs a process of discovering a partner terminal in the D2D communication. The user terminal transmits a discovery-use signal for discovering the partner terminal (or for being discovered by the partner terminal) so as to discover the partner terminal. The user terminal having discovered the partner terminal starts the D2D communication with the partner terminal. 
     PRIOR ART DOCUMENT 
     Non-Patent Document 
     
         
         [Non Patent Document 1] 3GPP technical report “TR 22.803 V12.0.0” December 2012 
       
    
     SUMMARY 
     Currently, there is no mechanism for appropriately controlling transmission power used for the D2D communication. Thus, when a user terminal freely sets the transmission power, the user terminal that performs the D2D communication may apply interference to another user terminal existing in surroundings thereof or a base station. 
     Further, also with respect to transmission power in the process of discovering the partner terminal in the D2D communication, as in the case of the transmission power used for the D2D communication, there is no mechanism for appropriately controlling the transmission power used for the process of discovering. Therefore, the user terminal that transmits the discovery-use signal may apply interference. 
     Therefore, the present disclosure provides a mobile communication system capable of appropriately controlling transmission power used for a process of discovering a partner terminal in D2D communication or for the D2D communication, a base station thereof, a user terminal thereof, and a processor thereof. 
     According to an embodiment, a user terminal comprises: a receiver configured to receive a System Information Brock (SIB) from a base station; and a transmitter configured to transmit a discovery signal for discovering another user terminal in a discovery procedure, wherein the SIB includes first information and second information. The first information is information for controlling transmission power of the discovery signal. The first information is information associated with each of a plurality of radio resources including time-frequency region available for the discovery procedure. The second information is information for specifying each of the plurality of radio resources, and the transmitter is configured to directly transmit the discovery signal to another user terminal with transmission power based on the first information. 
     According to an embodiment, a processor for controlling a user terminal is configured to: perform a process of receiving a System Information Brock (SIB) from a base station; and perform a process of transmitting a discovery signal for discovering another user terminal in a discovery procedure, wherein the SIB includes first information and second information. The first information is information for controlling transmission power of the discovery signal. The first information is information associated with each of a plurality of radio resources including time-frequency region available for the discovery procedure. The second information is information for specifying each of the plurality of radio resources, and the processor is configured to perform a process of directly transmitting the discovery signal to another user terminal with transmission power based on the first information. 
     According to an embodiment, a base station comprises: a controller configured to include first information and second information into the SIB; and a transmitter configured to transmit a System Information Brock (SIB), wherein the first information is information for controlling transmission power of the discovery signal. The first information is information associated with each of a plurality of radio resources including time-frequency region available for the discovery procedure, and the second information is information for specifying each of the plurality of radio resources. 
     According to an embodiment, a processor configured for controlling a base station is configured to: perform a process of including first information and second information into the SIB; and perform a process of transmitting a System Information Brock (SIB), wherein the first information is information for controlling transmission power of the discovery signal. The first information is information associated with each of a plurality of radio resources including time-frequency region available for the discovery procedure, and the second information is information for specifying each of the plurality of radio resources. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an LTE system. 
         FIG. 2  is a block diagram of UE. 
         FIG. 3  is a block diagram of eNB. 
         FIG. 4  is a protocol stack diagram of a radio interface in the LTE system. 
         FIG. 5  is a configuration diagram of a radio frame used in the LTE system. 
         FIG. 6  is a diagram illustrating a data path in cellular communication. 
         FIG. 7  is a diagram illustrating a data path in D2D communication. 
         FIG. 8  is a diagram for explaining transmission and reception of transmission power control information. 
         FIG. 9  is a transmission power control table showing association of a subcarrier used for the D2D communication with transmission power used for the D2D communication. 
         FIG. 10  is a sequence diagram showing an operation example of a mobile communication system according to a first embodiment. 
         FIG. 11  is a flowchart for explaining an example of selecting a subcarrier according to the first embodiment. 
         FIG. 12  is a diagram for explaining a case where the subcarrier is changed. 
         FIG. 13  is a diagram for explaining a case of shifting to communication that passes through eNB  200 . 
         FIG. 14  is a transmission power control table showing association of a subcarrier used for the D2D communication with transmission power used for the D2D communication according to the present modification of the first embodiment. 
         FIG. 15  is a flowchart for explaining an example of selecting a subcarrier according to a modification of the first embodiment. 
         FIG. 16  is a transmission power control table showing association of a subcarrier used for the D2D communication with transmission power used for the D2D communication according to a second embodiment. 
         FIG. 17  is a flowchart for explaining an example of selecting a subcarrier according to the second embodiment. 
         FIG. 18  is a diagram for explaining a distance between a base station and a user terminal and magnitude of transmission power according to the second embodiment. 
         FIG. 19  is a diagram for explaining association of a subcarrier used for the D2D communication with transmission power used for the D2D communication according to a third embodiment. 
         FIG. 20  is a diagram for explaining a relation between a subcarrier used for the D2D communication and an area of a cell. 
     
    
    
     MODE FOR CARRYING OUT THE DISCLOSURE 
     [Overview of Embodiment] 
     A mobile communication system according to an embodiment supports cellular communication in which a data path passes through a core network and D2D communication that is direct device-to-device communication in which a data path does not pass through the core network. The mobile communication system has a user terminal that performs a process of discovering a partner terminal in the D2D communication or performs the D2D communication. A plurality of radio resources that are the time-frequency resources that can be used for the process of discovering or the D2D communication consist of radio resources associated with transmission power used for the process of discovering or the D2D communication. The user terminal performs the process of discovering or the D2D communication on the basis of the transmission power associated with the radio resource used by the user terminal. As a result, the transmission power used for process of discovering or D2D communication is limited due to the radio resources used by user terminal, thereby making it possible to appropriately control the transmission power. 
     A mobile communication system according to a first embodiment and a second embodiment has a base station that control the cell in which the user terminal exists. The base station transmits, to the user terminal, transmission power control information indicating the plurality of radio resources and the transmission power associated with each of the plurality of radio resources. The user terminal selects the radio resource used for the process of discovering or the D2D communication from the plurality of radio resources on the basis of the transmission power control information. As a result, base station can control transmission power selected by user terminal on the basis of transmission power control information. 
     In the first embodiment and the second embodiment, the transmission power associated with each of the plurality of radio resource indicates a range of the transmission power. The user terminal, on the basis of the transmission power control information, selects the radio resource from the plurality of radio resources so that the used transmission power to be used for process of discovering or the D2D communication is within the range of the transmission power. As a result, user terminal can selects an appropriate radio resource on the basis of transmission power used for process of discovering or D2D communication. 
     In the first embodiment, the user terminal reselects the radio resource when changing the used transmission power. As a result, user terminal can perform process of discovering or D2D communication with an appropriate transmission power. 
     In the first embodiment, when the user terminal determining that the used transmission power is larger than any upper limit value of the transmission power associated with each of the plurality of radio resources on the basis of the transmission power control information, the user terminal transmits, to the base station, information requesting that the cellular communication is performed. As a result, when the transmission power of the D2D communication is so large as to apply interference, communication that passes through the base station is performed, and thus, occurrence of interference can be prevented. 
     In the second embodiment, the transmission power control information further includes information indicating a distance from the base station. The transmission power associated with each of the plurality of radio sources is associated with the distance from the base station so that the upper limit value of the transmission power becomes larger according to the distance from the base station. The user terminal selects the radio resource from the plurality of radio resources on the basis of the distance from the base station to the user terminal. As a result, user terminal selects radio resource so that the upper limit value of transmission power becomes larger according to the distance from base station, and thus, the user terminal can perform an appropriate process of discovering or D2D communication by taking the influence of interference to base station into consideration. 
     In the second embodiment, the information indicating the distance from the base station is information indicating the pass loss between the base station and the user terminal. As a result, user terminal can perform an appropriate process of discovering or D2D communication even if the user terminal does not know the direct distance between the user terminal and the base station. 
     In the third embodiment, the plurality of radio resources are the plurality of frequency bands divided in the frequency direction. The plurality of frequency bands are divided into high power frequency band associated with a transmission power in which an upper limit value of the transmission power is larger than a predetermined value and low power frequency band associated with a transmission power in which an upper limit value of the transmission power is equal to or less than the predetermined value. The low power frequency band is provided adjacent to cellular frequency band used only for the cellular communication. The high power frequency band is provided distant from the cellular frequency band. As a result, even in the case that D2D communication was performed by radio resource that is adjacent to (near) the frequency band used for cellular communication, since the transmission power of the D2D communication is small, it is possible to reduce the interference applied to the user terminal which performing D2D communication with the radio resource in the frequency band used only for cellular communication. 
     In other embodiment, the mobile communication system further comprising a base station that manages the cell in which the user terminal exists and a neighbor base station that manages a neighbor cell adjacent to the cell. Out of the plurality of radio resources, the radio resource that is available at the edge of the cell is different from the radio resource that is available at the edge of the neighbor cell. As a result, it is possible to prevent the occurrence of interference between the terminal group that performing D2D communication in cell and the terminal group that performing D2D communication in the neighbor cell. 
     A base station according to an embodiment is a base station in a mobile communication system that supports cellular communication in which a data path passes through a core network and D2D communication that is direct device-to-device communication in which a data path does not pass through the core network. The base station has a transmission unit configured to transmit transmission power control information including information on a plurality of radio resources that are the time-frequency resources that can be used for a process of discovering a partner terminal in the D2D communication or for the D2D communication and the information on transmission power associated with each of the plurality of radio resources. 
     A user terminal according to an embodiment is a user terminal in a mobile communication system that supports cellular communication in which a data path passes through a core network and D2D communication that is direct device-to-device communication in which a data path does not pass through the core network. The user terminal has a control unit configured to perform a process of discovering a partner terminal in the D2D communication or to perform the D2D communication. A plurality of radio resources that are the time-frequency resources that can be used for the process of discovering or the D2D communication consist of radio resources associated with transmission power used for the process of discovering or the D2D communication. The control unit performs the process of discovering or the D2D communication on the basis of transmission power associated with radio resource used by the user terminal. 
     A processor according to an embodiment is a processor provided in a base station in a mobile communication system that supports cellular communication in which a data path passes through a core network and D2D communication that is direct device-to-device communication in which a data path does not pass through the core network. The processor executes a process of transmitting, to a user terminal, transmission power control information including information on a plurality of radio resources that are the time-frequency resources that can be used for a process of discovering a partner terminal in the D2D communication or for the D2D communication and the information on transmission power associated with each of the plurality of radio resources. 
     A processor according to an embodiment is a processor provided in a user terminal in a mobile communication system that supports cellular communication in which a data path passes through a core network and D2D communication that is direct device-to-device communication in which a data path does not pass through the core network. The processor executes a process of performing a process of discovering a partner terminal in the D2D communication or performing the D2D communication. A plurality of radio resources that are the time-frequency resources that can be used for the process of discovering or the D2D communication consist of radio resources associated with transmission power used for the process of discovering or the D2D communication. The processor executes a process of performing the process of discovering or the D2D communication on the basis of transmission power associated with radio resource used by the user terminal. 
     Hereinafter, with reference to the accompanying drawings, the description will be provided for each embodiment when D2D communication is introduced to a cellular mobile communication system (hereinafter, an “LTE system”) configured according to the 3GPP standards. 
     [First Embodiment] 
     Hereinafter, the first embodiment will be described. 
     (LTE system) 
       FIG. 1  is a configuration diagram of an LTE system according to the present embodiment. 
     As illustrated in  FIG. 1 , the LTE system includes a plurality of UEs (User Equipments)  100 , E-UTRAN (Evolved-Universal Terrestrial Radio Access Network)  10 , and EPC (Evolved Packet Core)  20 . The E-UTRAN  10  and the EPC  20  constitute a network. 
     The UE  100  is a mobile radio communication device and performs radio communication with a cell (a serving cell) with which a connection is established. The UE  100  corresponds to the user terminal. 
     The E-UTRAN  10  includes a plurality of eNBs  200  (evolved Node-Bs). Each eNB  200  corresponds to a base station. The eNB  200  manages a cell and performs radio communication with the UE  100  that establishes a connection with the cell. 
     It is noted that the “cell” is used as a term indicating a minimum unit of a radio communication area, and is also used as a term indicating a function of performing radio communication with the UE  100 . 
     The eNB  200 , for example, has a radio resource management (RRM) function, a function of routing user data, and a measurement control function for mobility control and scheduling. 
     The EPC  20  includes MME (Mobility Management Entity)/S-GWs (Serving-Gateways)  300 , and OAM  400  (Operation and Maintenance). Further, the EPC  20  corresponds to a core network. 
     The MME is a network node that performs various mobility controls and the like, for the UE  100  and corresponds to a controller. The S-GW is a network node that performs control to transfer user data and corresponds to a mobile switching center. 
     The eNBs  200  are connected mutually via an X2 interface. Furthermore, the eNB  200  is connected to the MME/S-GW  300  via an S1 interface. 
     The OAM  400  is a server device managed by an operator and performs maintenance and monitoring of the E-UTRAN  10 . 
     Next, the configurations of the UE  100  and the eNB  200  will be described. 
       FIG. 2  is a block diagram of the UE  100 . As illustrated in  FIG. 2 , the UE  100  includes an antenna  101 , a radio transceiver  110 , a user interface  120 , a GNSS (Global Navigation Satellite System) receiver  130 , a battery  140 , a memory  150 , and a processor  160 . The memory  150  and the processor  160  constitute a control unit. 
     The UE  100  may not have the GNSS receiver  130 . Furthermore, the memory  150  may be integrally formed with the processor  160 , and this set (that is, a chip set) may be called a processor  160 ′. 
     The antenna  101  and the radio transceiver  110  are used to transmit and receive a radio signal. The antenna  101  includes a plurality of antenna elements. The radio transceiver  110  converts a baseband signal output from the processor  160  into the radio signal, and transmits the radio signal from the antenna  101 . Furthermore, the radio transceiver  110  converts the radio signal received by the antenna  101  into the baseband signal, and outputs the baseband signal to the processor  160 . 
     The user interface  120  is an interface with a user carrying the UE  100 , and includes, for example, a display, a microphone, a speaker, various buttons and the like. The user interface  120  receives an operation from a user and outputs a signal indicating the content of the operation to the processor  160 . 
     The GNSS receiver  130  receives a GNSS signal in order to obtain location information indicating a geographical location of the UE  100 , and outputs the received signal to the processor  160 . 
     The battery  140  accumulates a power to be supplied to each block of the UE  100 . 
     The memory  150  stores a program to be executed by the processor  160  and information to be used for a process by the processor  160 . 
     The processor  160  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal, and a CPU (Central Processing Unit) that performs various processes by executing the program stored in the memory  150 . The processor  160  may further include a codec that performs encoding and decoding on sound and video signals. The processor  160  executes various processes and various communication protocols described later. 
       FIG. 3  is a block diagram of the eNB  200 . As illustrated in  FIG. 3 , the eNB  200  includes an antenna  201 , a radio transceiver  210 , a network interface  220 , a memory  230 , and a processor  240 . The memory  230  and the processor  240  constitute a control unit. In addition, the memory  230  is integrated with the processor  240 , and this set (that is, a chipset) may be called a processor  240 ′. 
     The antenna  201  and the radio transceiver  210  are used to transmit and receive a radio signal. The antenna  201  includes a plurality of antenna elements. The radio transceiver  210  converts the baseband signal output from the processor  240  into the radio signal, and transmits the radio signal from the antenna  201 . Furthermore, the radio transceiver  210  converts the radio signal received by the antenna  201  into the baseband signal, and outputs the baseband signal to the processor  240 . 
     The network interface  220  is connected to the neighboring eNB  200  via the X2 interface and is connected to the MME/S-GW  300  via the S1 interface. The network interface  220  is used in communication performed on the X2 interface and communication performed on the S1 interface. 
     The memory  230  stores a program to be executed by the processor  240  and information to be used for a process by the processor  240 . 
     The processor  240  includes the baseband processor that performs modulation and demodulation, encoding and decoding and the like on the baseband signal and a CPU that performs various processes by executing the program stored in the memory  230 . The processor  240  executes various processes and various communication protocols described later. 
       FIG. 4  is a protocol stack diagram of a radio interface in the LTE system. 
     As illustrated in  FIG. 4 , the radio interface protocol is classified into a layer  1  to a layer  3  of an OSI reference model, wherein the layer  1  is a physical (PHY) layer. The layer  2  includes a MAC (Media Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The layer  3  includes an RRC (Radio Resource Control) layer. 
     The PHY layer performs encoding and decoding, modulation and demodulation, antenna mapping and demapping, and resource mapping and demapping. The PHY layer provides a transmission service to an upper layer by using a physical channel. Between the PHY layer of the UE  100  and the PHY layer of the eNB  200 , data is transmitted through the physical channel. 
     The MAC layer performs priority control of data, and a retransmission process and the like by hybrid ARQ (HARQ). Between the MAC layer of the UE  100  and the MAC layer of the eNB  200 , data is transmitted via a transport channel. The MAC layer of the eNB  200  includes a transport format of an uplink and a downlink (a transport block size, a modulation and coding scheme and the like) and a MAC scheduler to decide a resource block to be assigned. 
     The RLC layer transmits data to an RLC layer of a reception side by using the functions of the MAC layer and the PHY layer. Between the RLC layer of the UE  100  and the RLC layer of the eNB  200 , data is transmitted via a logical channel. 
     The PDCP layer performs header compression and decompression, and encryption and decryption. 
     The RRC layer is defined only in a control plane. Between the RRC layer of the UE  100  and the RRC layer of the eNB  200 , a control signal (an RRC message) for various types of setting is transmitted. The RRC layer controls the logical channel, the transport channel, and the physical channel in response to establishment, re-establishment, and release of a radio bearer. When an RRC connection is established between the RRC of the UE  100  and the RRC of the eNB  200 , the UE  100  is in a connected state, and when the RRC connection is not established, the UE  100  is in an idle state. 
     A NAS (Non-Access Stratum) layer positioned above the RRC layer performs session management and mobility management, for example. 
       FIG. 5  is a configuration diagram of a radio frame used in the LTE system. In the LTE system, OFDMA (Orthogonal Frequency Division Multiplexing Access) is employed in a downlink, and SC-FDMA (Single Carrier Frequency Division Multiple Access) is employed in an uplink, respectively. 
     As illustrated in  FIG. 5 , the radio frame is configured by 10 subframes arranged in a time direction, wherein each subframe is configured by two slots arranged in the time direction. Each subframe has a length of 1 ms and each slot has a length of 0.5 ms. Each subframe includes a plurality of resource blocks (RBs) in a frequency direction, and a plurality of symbols in the time direction. Each symbol is provided at a head thereof with a guard interval called a cyclic prefix (CP). The resource block includes a plurality of subcarriers in the frequency direction. A radio resource unit configured by one subcarrier and one symbol is called a resource element (RE). 
     Among radio resources assigned to the UE  100 , a frequency resource can be designated by a resource block and a time resource can be designated by a subframe (or slot). 
     In the downlink, an interval of several symbols at the head of each subframe is a control region mainly used as a physical downlink control channel (PDCCH). Furthermore, the remaining interval of each subframe is a region that can be mainly used as a physical downlink shared channel (PDSCH). Moreover, in each subframe, cell-specific reference signals (CRSs) are distributed and arranged. 
     In the uplink, both ends in the frequency direction of each subframe are control regions mainly used as a physical uplink control channel (PUCCH). Furthermore, the center portion in the frequency direction of each subframe is a region that can be mainly used as a physical uplink shared channel (PUSCH). Moreover, in each subframe, a demodulation reference signal (DMRS) and a sounding reference signal (SRS) are arranged. 
     (D2D Communication) 
     Next, description will be provided by comparing the D2D communication with the normal communication (the cellular communication) in the LTE system. 
       FIG. 6  is a diagram illustrating a data path in the cellular communication. In this case,  FIG. 6  illustrates the case in which the cellular communication is performed between UE  100 - 1  that establishes a connection with eNB  200 - 1  and UE  100 - 2  that establishes a connection with eNB  200 - 2 . It is noted that the data path indicates a transfer path of user data (a user plane). 
     As illustrated in  FIG. 6 , the data path of the cellular communication passes through the core network. Specifically, the data path is set to pass through the eNB  200 - 1 , the S-GW  300 , and the eNB  200 - 2 . 
       FIG. 7  is a diagram illustrating a data path in the D2D communication. In this case,  FIG. 7  illustrates the case in which the D2D communication is performed between the UE  100 - 1  that establishes a connection with the eNB  200 - 1  and the UE  100 - 2  that establishes a connection with the eNB  200 - 2 . 
     As illustrated in  FIG. 7 , the data path of the D2D communication does not pass through the core network. That is, direct radio communication is performed between the UEs. As described above, when the UE  100 - 2  exists in the vicinity of the UE  100 - 1 , the D2D communication is performed between the UE  100 - 1  and the UE  100 - 2 , thereby obtaining an effect that a traffic load of the network and a battery consumption amount of the UE  100  are reduced, for example. In addition, in a mode called Locally Routed (locally routed mode), a data path passes through the eNB  200  without passing through the S-GW  300 . 
     It is noted that cases in which the D2D communication is started include (a) a case in which the D2D communication is started after a partner terminal (proximal terminal) is discovered by performing an operation for discovering a partner terminal, and (b) a case in which the D2D communication is started without performing an operation for discovering a partner terminal. 
     For example, in the above-described case (a), one UE  100  of the UE  100 - 1  and the UE  100 - 2  discovers the other UE  100  existing in the vicinity of the one UE  100 , so that the D2D communication is started. 
     In such a case, in order to discover the partner terminal, the UE  100  has a (Discover) function of discovering another UE  100  (proximal terminal) existing in the vicinity of the UE  100 , and/or a (Discoverable) function of being discovered by another UE  100 . 
     Specifically, the UE 100 - 1  transmits a discovery signal (Discovery signal/Discoverable signal) indicates a signal for discovering the partner terminal (proximal terminal) or a signal that is used to be discovered from the partner terminal (proximal terminal). The UE  100 - 2  which received the discovery signal discovers the UE  100 - 1 . When the UE  100 - 2  transmits a response to the discovery signal, the UE  100 - 1  that has transmitted the discovery signal discovers the UE  100 - 2 , which is the partner terminal. 
     It is noted that the UE  100  need not necessarily perform the D2D communication even upon discovering a partner terminal. For example, after mutually discovering each other, the UE  100 - 1  and the UE  100 - 2  may perform a negotiation, and determine whether or not to perform the D2D communication. When each of the UE  100 - 1  and the UE  100 - 2  agrees to perform the D2D communication, the D2D communication starts. Additionally, the UE  100 - 1  may report the discovery of the proximal UE  100  (that is, the UE  100 - 2 ) to an upper layer (e.g. application, etc.), if the UE  100 - 1  did not perform the D2D communication after discovering the partner terminal. For example, the application executes the process (e.g. process of plotting the location of the UE  100 - 2  to the map information) based on the report. 
     Furthermore, the UE  100  may report the eNB  200  that the partner terminal has been discovered and may receive an instruction from the eNB  200  indicate the commutation with the partner terminal is performed in cellular communication or in the D2D communication. 
     On the other hand, in the above-described case (b), for example, the UE  100 - 1  starts the transmission (such as a notification through broadcasting) of a signal for the D2D communication without specifying a partner terminal. Thus, the UE  100  is capable of starting the D2D communication regardless of the existence of the discovery of a partner terminal. It is noted that the UE  100 - 2  that is performing the standby operation for the signal for the D2D communication performs synchronization or/and demodulation on the basis of the signal from the UE  100 - 1 . 
     (Transmission Power Control Information) 
     Next, transmission power control information according to the present embodiment will be described by using  FIG. 8  and  FIG. 9 . 
       FIG. 8  is a diagram for explaining transmission and reception of transmission power control information according to the first embodiment.  FIG. 9  is a transmission power control table showing association of a subcarrier used for the D2D communication with transmission power used for the D2D communication. 
     The plurality of radio resources which are the time-frequency resources that can be used for the discovery process or the D2D communication consist of the radio resources associated with the transmission power used for the discovery process or the D2D communication. The transmission power control information is information indicating the plurality of radio resources and the transmission power associated with each of the plurality of radio resources, and specifically, is as described in the transmission power control table shown in  FIG. 9 . 
     As shown in  FIG. 8 , the eNB  200  transmits, to the UE  100 - 1  and the UE  100 - 2 , a transmission power control message including the transmission power control information. The UE  100 - 1  and the UE  100 - 2  receive the transmission power control message. Each of the UE  100 - 1  and the UE  100 - 2  performs the discovery process on the basis of the transmission power associated with the radio resource used for the discovery process. Further, each of the UE  100 - 1  and the UE  100 - 2  performs the D2D communication on the basis of the transmission power associated with the radio resource used for the D2D communication. 
     In the present embodiment, a frequency band used for the D2D communication is divided into a plurality of subcarriers (plurality of radio resource). Each of the plurality of subcarriers used for the D2D communication is associated with the transmission power used for the discovery process or the D2D communication. 
     Specifically, as shown in  FIG. 8  and  FIG. 9 , each of the plurality of subcarriers (f 0 , f 1 , . . . fn) is associated with a maximum value (TxPwrMax 0 , TxPwrMax 1 , . . . , TxPwrMaxn) of the transmission power used for the D2D communication, an upper limit value (TxPwrUpThresh 0 , TxPwrUpThresh 1 , . . . , TxPwrUpThreshn) of the transmission power used for the D2D communication, a lower limit value (TxPwrDnThresh 0 , TxPwrDnThresh 1 , . . . , TxPwrDnThreshn) of the transmission power used for the D2D communication, a subcarrier (fup 0 , fup 1 , . . . , fupn) of a change destination when the transmission power is equal to or larger than the upper limit value, and a subcarrier (fdown 0 , fdown 1 , . . . , fdownn) of a change destination when the transmission power is equal to or less than the lower limit value. It is noted that the upper limit value of the transmission power may be equal to the maximum value of the transmission power or larger than the maximum value of the transmission power. 
     Each of the plurality of subcarriers is associated with each of different upper limit values of the transmission power. Accordingly, for example, the upper limit value TxPwrUpThresh 0  of the transmission power associated with the subcarrier f 0  is smaller than the upper limit value TxPwrUpThresh 1  of the transmission power associated with the subcarrier f 1 . 
     Further, each of the plurality of subcarriers is also associated with each of different lower limit values of the transmission power. Accordingly, for example, the lower limit value TxPwrDnThresh 0  of the transmission power associated with the subcarrier f 0  is smaller than the upper limit value TxPwrDnThresh 1  of the transmission power associated with the subcarrier f 1 . 
     Each of the plurality of subcarriers is associated with the upper limit value and the lower limit value of the transmission power, and thus, each of the plurality of subcarriers is associated with the range of the transmission power. 
     As shown in  FIG. 8 , the eNB  200  transmits, to the UE  100 - 1  and the UE  100 - 2 , the plurality of subcarriers (f) and information (TxPwrMax, TxPwrUpThresh, TxPwrDnThresh, fup, and fdown) associated with each of the plurality of subcarriers as shown in  FIG. 9  as a transmission power control message of the D2D communication. The eNB  200  may transmit the transmission power control message of the D2D communication in a unicast manner by D2DPowerControl, or may transmit the transmission power control message in a broadcast manner by a master information block (MIB) or a system information block (SIB). 
     (Operation of Mobile Communication System According to First Embodiment) 
     Next, by using  FIG. 10  to  FIG. 13 , an operation of a mobile communication system according to the first embodiment will be described. 
       FIG. 10  is a sequence diagram showing an operation example of the mobile communication system according to the first embodiment.  FIG. 11  is a flowchart for explaining an example of selecting a subcarrier according to the first embodiment.  FIG. 12  is a diagram for explaining a case where the subcarrier is changed.  FIG. 13  is a diagram for explaining a case of shifting to communication that passes through the eNB  200 . 
     As shown in  FIG. 10 , in step  101 , the UE  100 - 1  decides transmission power used for the D2D communication and selects a subcarrier. 
     Before starting the D2D communication, the UE  100 - 1  decides the transmission power used for the D2D communication. For example, the UE  100 - 1  decides the transmission power on the basis of a previously set defined value. Alternatively, the UE  100 - 1  decides used transmission power after change when determining to change the used transmission power to be used for the D2D communication with the UE  100 - 2 . 
     A case where the UE  100 - 1  which performed the D2D communication with the UE  100 - 2  using the subcarrier f 1  changes the subcarrier will be described as an example, below. 
     The UE  100 - 1  reselects a subcarrier. Specifically, the UE  100 - 1  selects a subcarrier used for the D2D communication from the plurality of subcarriers on the basis of the transmission power control information received from the eNB  200 . By using  FIG. 11 , selection of a subcarrier will be described. 
     As shown in  FIG. 11 , in step  201 , the UE  100 - 1  determines whether or not the used transmission power (TxPwr) is equal to or larger than the upper limit value (TxPwrUpThresh(fi)) associated with the subcarrier (fi). The UE  100 - 1  performs a process in step  202  when the used transmission power is equal to or larger than the upper limit value. On the other hand, the UE  100 - 1  performs a process in step  205  when the used transmission power is less than the upper limit value. 
     In step  202 , the UE  100 - 1  determines whether or not a subcarrier (fup(fi)) of a change destination exists. The UE  100 - 1  performs a process in step  203  when the subcarrier (fup(fi)) of the change destination exists. On the other hand, the UE  100 - 1  performs a process in step  204  when the subcarrier (fup(fi)) of the change destination does not exist, that is, when the used transmission power is larger than any upper limit value associated with the subcarrier (fup(fi)) of the change destination. 
     In step  203 , the UE  100 - 1  selects the subcarrier (fup(fi)) of the change destination, and performs a process in step  201  again. 
     In step  204 , the UE  100 - 1  decides to perform communication that passes through the eNB  200 . 
     In step  205 , the UE  100 - 1  determines whether or not the used transmission power (TxPwr) is equal to or less than the lower limit value (TxPwrDnThresh(fi)). When the used transmission power is equal to or less than the lower limit value, the UE  100 - 1  performs a process in step  206 . On the other hand, when the used transmission power is larger than the lower limit value, the UE  100 - 1  performs a process in step  208 . 
     In step  206 , the UE  100 - 1  determines whether or not a subcarrier (fdown(fi)) of a change destination exists. The UE  100 - 1  performs a process in step  207  when the subcarrier (fdown(fi)) of the change destination exists. On the other hand, when the subcarrier (fdown(fi)) of the change destination does not exist, the UE  100 - 1  performs a process in step  208 . 
     In step  207 , the UE  100 - 1  selects the subcarrier (fdown(fi)) of the change destination, and performs a process in step  201  again. 
     In step  208 , the UE  100 - 1  selects the subcarrier (fi) as a subcarrier used for the D2D communication. 
     According to steps described above, a subcarrier in which used transmission power is included in a predetermined range of transmission power is selected. That is, the UE  100 - 1  selects a subcarrier from the plurality of subcarriers so that the used transmission power is within a range of the transmission power associated with the selected subcarrier. 
     When returning to  FIG. 10 , for example, the selected subcarrier is the same as a subcarrier that is being used (that is, the selected subcarrier is the subcarrier f 1 ), the UE  100 - 1  performs the D2D communication as is. Accordingly, the UE  100 - 1  and the UE  100 - 2  perform the D2D communication by using the subcarrier that is being used so that the used transmission power does not exceed the maximum value of the transmission power associated with the subcarrier that is being used. 
     Further, in the present embodiment, the subcarrier is associated with the upper limit value and the lower limit value. Accordingly, the UE  100 - 1  and the UE  100 - 2  perform the D2D communication in a range of the transmission power associated with the f 1  (a range not exceeding the upper limit value or the lower limit value). 
     On the other hand, when the selected subcarrier is different from the subcarrier that is being used, the UE  100 - 1  performs a process in step  102 . It is noted that, in the present embodiment, description will be provided on the assumption that a subcarrier f 2  is selected. 
     As shown in  FIG. 10  and  FIG. 12 , in step  102 , the UE  100 - 1  requests the UE  100 - 2  to change the subcarrier. The UE  100 - 2  receives the request to change the subcarrier. 
     The request to change the subcarrier includes information indicating an identifier indicating the UE  100 - 2 , an application identifier indicating an application used in the D2D communication, and the selected subcarrier f 2 . 
     In step  103 , the UE  100 - 2  sends a response of the subcarrier change to the UE  100 - 1 . The UE  100 - 1  receives the response of the subcarrier change. 
     The response of the subcarrier change includes information indicating the identifier indicating the UE  100 - 2 , the application identifier indicating an application used in the D2D communication, and the selected subcarrier f 2 . 
     In step  104 , each of the UE  100 - 1  and the UE  100 - 2  changes the subcarrier to the subcarrier f 2 . Then, the D2D communication is performed with the used transmission power by using the subcarrier f 2 . Accordingly, the UE  100 - 1  and the UE  100 - 2  perform the D2D communication by using the subcarrier f 2  so that the used transmission power does not exceed the maximum value of the transmission power associated with the subcarrier f 2 . 
     Next, description will be provided by using  FIG. 10  and  FIG. 13  for a case where the UE  100 - 1  decides to perform communication that passes through the eNB  200 . 
     In step  105 , the UE  100 - 1  transmits, to the UE  100 - 2  and the eNB  200 , a request for the communication that passes through the eNB  200 . The UE  100 - 2  and the eNB  200  receive the request for the communication that passes through the eNB  200 . 
     The request for the communication that passes through the eNB  200  includes information indicating the identifier indicating the UE  100 - 2  and the application identifier indicating an application used in the D2D communication. It is noted that the request for the communication that passes through the eNB  200  includes information indicating the identifier indicating the UE  100 - 1 . 
     In step  106 , the eNB  200  transmits, to the network  300 , the request for the communication that passes through the eNB  200 . The network  300  receives the request for the communication that passes through the eNB  200 . The network  300  is, for example, a core network of, such as an upper station (e.g. MME) of the eNB  200 . 
     The request for the communication that passes through the eNB  200  transmitted by the eNB  200  includes information indicating the identifier indicating the UE  100 - 1 , the identifier indicating the UE  100 - 2 , and the application identifier. 
     In step  107 , the network  300  updates states of the UE  100 - 1  and the UE  100 - 2 . That is, the network  300  performs a process for the UE  100 - 1  and the UE  100 - 2  to perform the communication that passes through the eNB  200 . 
     In step  108 , the network  300  applies an instruction of the communication that passes through the eNB  200  to each of the UE  100 - 1  and the UE  100 - 2  via the eNB 200 . Each of the UE  100 - 1  and the UE  100 - 2  receives the instruction of the communication that passes through the eNB  200 . 
     The instruction of the communication that passes through the eNB  200  includes information indicating the identifier indicating the UE  100 - 1 , the identifier indicating the UE  100 - 2 , and the application identifier. 
     After that, the UE  100 - 1  and the UE  100 - 2  perform the communication that passes through the eNB  200 . 
     (Modification of First Embodiment) 
     Next, by using  FIG. 14  and  FIG. 15 , a modification of the first embodiment will be explained. It is noted that description will be provided while focusing a portion different from the above-described embodiment, and description of a similar portion will be omitted, where necessary. 
     The modification is different from the above-described first embodiment in terms of the transmission power control information and a way of selecting a subcarrier. 
     (Transmission Power Control Information) 
       FIG. 14  is a transmission power control table showing association of a subcarrier used for the D2D communication with transmission power used for the D2D communication according to the present modification. 
     As shown in  FIG. 14 , in the present modification, each of subcarriers (f 0 , f 1 , . . . , fn) used for the D2D communication is associated with a maximum value (TxPwrMax 0 , TxPwrMax 1 , . . . , TxPwrMaxn) of the transmission power used for the D2D communication, and a minimum value (TxPwrMin 0 , TxPwrmin 1 , . . . , TxPwrminn) of the transmission power used for the D2D communication. 
     In the present modification, the eNB  200  transmits, to the UE  100 - 1  and the UE  100 - 2 , a plurality of subcarriers (f) and information (TxPwrMax and TxPwrMin) associated with each of the plurality of subcarriers as shown in  FIG. 14  as a transmission power control message of the D2D communication. It is noted that an amount of information of the transmission power control message transmitted by the eNB  200  is reduced compared to that of the above-described embodiment. 
     (Operation of Mobile Communication System According to Modification of First Embodiment) 
     Next, by using  FIG. 15 , an operation of a mobile communication system according to the modification of the first embodiment will be described. Other than deciding the transmission power used for the D2D communication and selecting a subcarrier in step  101  of the first embodiment, the operation is the same as that of the first embodiment, therefore, description of the same operation will be omitted. 
       FIG. 15  is a flowchart for explaining an example of selecting a subcarrier according to the modification of the first embodiment. 
     First, when determining to change the transmission power used for the D2D communication, the UE  100 - 1  decides used transmission power that is actually used for the D2D communication. 
     As shown in  FIG. 15 , in step  301 , the UE  100 - 1  regards a subcarrier fenb used for communication that passes through the eNB  200  as a selected subcarrier fselect (a subcarrier candidate). 
     In step  302 , the UE  100 - 1  selects the subcarrier fi for the D2D communication on the basis of the transmission power control information. Specifically, the UE  100 - 1  selects the subcarrier fi from the plurality of subcarriers (f 0  to fn) indicated by the transmission power control information. 
     In step  303 , the UE  100 - 1  determines whether the used transmission power (TxPwr) is equal to or larger than a lower limit value (TxPwrMin(fi)) associated with the selected subcarrier fi and smaller than an upper limit value (TxPwrMax(fi)) associated with the selected subcarrier fi. When the used transmission power is equal to or larger than the lower limit value and smaller than the upper limit value, the UE  100 - 1  performs a process in step  304 . On the other hand, when the used transmission power is smaller than the lower limit value or the used transmission power is equal to or larger than the upper limit value, the UE  100 - 1  performs a process in step  302 . 
     Specifically, the UE  100 - 1  selects another subcarrier different from the already selected subcarrier, then performs the process in step  303 . The UE  100 - 1  repeats the process in step S 302  until each of the plurality of subcarriers is all selected. 
     In step  304 , the UE  100 - 1  regards the subcarrier fi as a selected subcarrier fselect (a subcarrier candidate), and performs the process in step  302 . The UE  100 - 1  performs a process in step  305  after performing the process in step  302  for all subcarriers indicated by the transmission power control information. 
     In step  305 , the UE  100 - 1  determines whether or not the selected subcarrier fselect (a subcarrier candidate) corresponds to a subcarrier fcurrent that is currently being used in the D2D communication. When the subcarrier fselect corresponds to the fcurrent, the UE  100 - 1  and the UE  100 - 2  perform the D2D communication with the subcarrier fcurrent as is. On the other hand, when the subcarrier fselect (a subcarrier candidate) does not correspond to the fcurrent, a process in step  306  is performed. 
     In step  306 , the UE  100 - 1  determines whether the subcarrier fselect (a subcarrier candidate) selected in step  303  corresponds to the subcarrier fenb selected in step  301 . That is, the UE  100 - 1  determines whether or not the subcarrier candidate is only the subcarrier fenb. When the subcarrier fi does not correspond to the subcarrier fenb (that is, the subcarrier candidate includes subcarrier candidate other than the subcarrier fenb), the UE  100 - 1  selects the subcarrier fi as a subcarrier used for the D2D communication (s 307 ). On the other hand, when the subcarrier fi corresponds to the subcarrier fenb (that is, the subcarrier candidate is only the subcarrier fenb), it is decided to perform the communication that passes through the eNB  200  (S 308 ). 
     (Summary of First Embodiment) 
     In the present embodiment, each of the UE  100 - 1  and the UE  100 - 2  performs the D2D communication on the basis of the transmission power associated with the subcarriers used by the UE  100 - 1  and the UE  100 - 2 . Specifically, the subcarrier associated with the upper limit value that is smaller than the used transmission power to be used for the D2D communication by the UE  100 - 1 , is selected, and the UE  100 - 1  performs the D2D communication by using the selected subcarrier so that the used transmission power does not exceed the upper limit value. As a result, the subcarrier associated with the upper limit value of the transmission power smaller than the used transmission power is appropriately selected, thereby making it possible to appropriately control the transmission power. 
     In the present embodiment, the eNB  200  transmits the transmission power control information to the UE  100 - 1 , and the UE  100 - 1  selects a subcarrier to be used for the D2D communication from a plurality of subcarrier on the basis of the transmission power control information. As a result, the UE  100 - 1  selects the subcarrier, and thus, the eNB  200  does not need to select the subcarrier, and the load on the eNB  200  can be reduced. 
     In the present embodiment, the transmission power associated with each of the plurality of subcarriers indicates the range of the transmission power. The UE  100 - 1 , on the basis of the transmission power control information, selects a radio resource used for the D2D communication from the plurality of subcarriers so that the used transmission power to be used for the D2D communication is within a range of the transmission power. Specifically, the UE  100 - 1  performs the D2D communication by using a predetermined subcarrier, when the used transmission power is equal to or larger than the lower limit value and smaller than the upper limit value, the UE  100 - 1  selects the subcarrier fi on the basis of the transmission power control information, and the UE  100 - 1  performs the D2D communication by using the subcarrier fi so that the used transmission power does not exceed a subcarrier upper limit value. As a result, even when the used transmission power is changed, it becomes possible to appropriately control the transmission power by appropriately selecting the subcarrier associated with the upper limit value of the transmission power that is smaller than the used transmission power. 
     In the present embodiment, when changing the used transmission power, the UE  100 - 1  reselects a subcarrier used for the D2D communication. As a result, the UE  100 - 1  is possible to select a subcarrier in accordance with the used transmission power after change; therefore, the UE  100 - 1  is capable of performing the D2D communication with an appropriate transmission power. 
     In the present embodiment, when determining that the used transmission power is larger than any upper limit value associated with each of subcarriers of the change destination, the UE  100 - 1  transmits, to the eNB  200 , information requesting to perform the communication that passes through the eNB  200 . As a result, when the transmission power of the D2D communication is so large as to apply interference, the communication that passes through the eNB  200  is performed, and thus, occurrence of interference can be prevented. 
     [Second Embodiment] 
     Next, by using  FIG. 16  to  FIG. 18 , the second embodiment will be explained. It is noted that description will be provided while focusing a portion different from the above-described embodiment, and description of a similar portion will be omitted, where necessary. 
     The second embodiment is different from the above-described first embodiment in that each of the plurality of subcarriers is associated with the upper limit value of the transmission power so that the maximum value (TxPwrMax) of the transmission power becomes larger according to the distance from the eNB  200 . 
     (Transmission Power Control Information) 
       FIG. 16  is a transmission power control table (transmission power control information) showing association of a subcarrier used for the D2D communication with transmission power used for the D2D communication according to the second embodiment. 
     As shown in  FIG. 16 , the transmission power control information includes the information indicating the distance from the eNB  200 , in addition to the information of the plurality of radio resources (the plurality of subcarriers) used for the D2D communication and of the transmission power associated with each of the plurality of radio resources. In the present embodiment, the information indicating the distance from the eNB  200  is the information indicating the bath loss between the eNB  200  and the UE  100 . Therefore, the transmission power is associated with the distance from the base station. 
     Specifically, each of the subcarriers (f 0 , f 1 , . . . , fn) used for the D2D communication is, plus the transmission power (TxPwrMax, TxPwrUpThresh, TxPwrDnThresh), associated with an upper limit value (PLup 0 , PLup 1 , . . . , PLupn) of path loss and a lower limit value (PLdn 0 , PLdn 1 , . . . , PLdnn) of the path loss. Respective upper limit values (PLup) of the path loss are values different from each other. Further, respective lower limit values (PLdn) of the path loss are values different from each other. 
     Further, regarding each of the subcarriers (f 0 , f 1 , . . . , fn) used for the D2D communication, the subcarrier is associated with the upper limit value of the transmission power so that the upper limit value of the transmission power becomes larger according to a distance from the eNB  200  (that is, the distance between the eNB  200  and the UE  100 ). 
     The eNB  200  transmits, to the UE  100 - 1  and the UE  100 - 2 , the plurality of subcarriers (f) and information (TxPwrMax, TxPwrUpThresh, TxPwrDnThresh, fup, fdown, PLup, and PLdn) associated with each of the plurality of subcarriers as shown in  FIG. 16  as a transmission power control message of the D2D communication. That is, the eNB  200  transmits, to the UE  100 - 1  and the UE  100 - 2 , information indicating the plurality of subcarriers, upper limit values of the transmission power associated with the plurality of subcarriers, and path loss associated with the plurality of subcarriers as a transmission power control message of the D2D communication. 
     (Operation of Mobile Communication System According to Second Embodiment) 
     Next, by using  FIG. 17 , an operation of a mobile communication system according to the second embodiment will be described. Other than deciding the transmission power used for the D2D communication and selecting a subcarrier in step  101  of the first embodiment, the operation is the same as that of the first embodiment, therefore, description of the same operation will be omitted.  FIG. 17  is a flowchart for explaining an example of selecting a subcarrier according to the second embodiment. 
     First, as shown in  FIG. 17 , in step  401 , the UE  100 - 1  estimates path loss of a reference signal transmitted from the eNB  200 . For example, the UE  100 - 1  can estimate the path loss by difference between information on transmission power of the reference signal of the eNB  200  included in the reference signal transmitted from the eNB  200  and received power of the reference signal received by the UE  100 - 1 . 
     In step  402 , the UE  100 - 1  regards the subcarrier fenb used for the communication that passes through the eNB  200  as a selected subcarrier fselect (a subcarrier candidate). 
     In step  403 , the UE  100 - 1  selects the subcarrier fi for the D2D communication on the basis of the transmission power control information. Specifically, the UE  100 - 1  selects the subcarrier fi from the plurality of subcarriers (f 0  to fn) indicated by the transmission power control information. 
     In step  404 , the UE  100 - 1  determines whether the estimated path loss (PL) estimated in step  401  is equal to or larger than a lower limit value (PLdn(fi)) of the path loss associated with the selected subcarrier fi and smaller than an upper limit value (PLup(fi)) of the path loss associated with the selected subcarrier fi. When the estimated path loss is equal to or larger than the lower limit value and smaller than the upper limit value, the UE  100 - 1  performs a process in step  405 . On the other hand, when the used transmission power is smaller than the lower limit value or the used transmission power is equal to or larger than the upper limit value, the UE  100 - 1  performs a process in step  403 . Specifically, the UE  100 - 1  selects another subcarrier different from the already selected subcarrier, then performs the process in step  404 . The UE  100 - 1  repeats the process in step S 403  until each of the plurality of subcarriers is all selected. 
     In step S 405 , the UE  100 - 1  regards the subcarrier fi as a selected subcarrier fselect (a subcarrier candidate), and performs the process in step  403 . The UE  100 - 1  performs a process in step  406  after performing the process in step  403  for all subcarriers indicated by the transmission power control information. 
     Steps  406  to  409  correspond to steps  305  to  308  of the modification in the first embodiment in which the transmission power is replaced with the path loss. 
     (Summary of Second Embodiment) 
     In the present embodiment, the subcarrier is associated with the upper limit value of the transmission power so that the upper limit value of the transmission power becomes larger according to the distance from the eNB  200 . The UE  100  selects a subcarrier from the plurality of subcarriers in accordance with the distance from the eNB  200  to the UE  100 . As a result, when the distance between the eNB  200  and the UE  100  is long, (for example, in  FIG. 18 , when the UE  100 - 1  exists in an area  2 ), a subcarrier with a large upper limit value of the transmission power (a subcarrier  2  in  FIG. 18 ) is used, and the D2D communication can be performed in a large area. On the other hand, when the distance between the eNB  200  and the UE  100  is short, (for example, in  FIG. 18 , when the UE  100 - 1  exists in an area  1  or the area  2 ), a subcarrier with a small upper limit value of the transmission power (a subcarrier  1  in  FIG. 18 ) is used, and the interference to the eNB  200  can be prevented. 
     In the present embodiment, the information indicating the distance from the eNB  200  is the information indicating the path loss between the eNB  200  and the UE  100 . Thus, each of the plurality of subcarriers is associated with the path loss of the reference signal transmitted from the eNB  200 , the eNB  200  transmits, to the UE  100 - 1  and the UE  100 - 2 , information indicating the plurality of subcarriers, the upper limit values of the transmission power associated with the plurality of subcarriers, and path loss associated with the plurality of subcarriers, the UE  100 - 1  calculates the path loss between the eNB  200  and the UE  100 - 1 , and the UE  100 - 1  selects a subcarrier on the basis of the power control information and the path loss. As a result, the UE  100 - 1  selects the subcarrier associated with the upper limit value of the transmission power while considering the positional relation between the UE  100 - 1  and the eNB  200 , and thus, the eNB  200  does not need to select the subcarrier and the interference to the eNB  200  can be prevented. 
     [Third Embodiment] 
     Next, by using  FIG. 19 , the third embodiment will be explained. It is noted that description will be provided while focusing a portion different from the above-described embodiment, and description of a similar portion will be omitted, where necessary. 
     In the above-described second embodiment, each of the plurality of subcarriers is associated with the upper limit value of the transmission power according to the distance from the eNB  200 , however, in the present embodiment, each of the plurality of subcarriers is associated with the upper limit value of the transmission power according to a relation between the D2D frequency band used for the D2D communication and a cellular frequency band used for the cellular communication. 
     (Transmission Power Control Information) 
       FIG. 19  is a diagram for explaining association of a subcarrier used for the D2D communication with transmission power used for the D2D communication according to the third embodiment. 
     In an example shown in  FIG. 19  ( 1 ), a frequency band is divided into a frequency band used for the cellular communication and a frequency band used for the D2D communication. The frequency band used for the D2D communication is divided into a low D2D frequency band associated with low transmission power in which an upper limit value of the transmission power is equal to or less than a predetermined value and a high D2D frequency band associated with high transmission power in which a upper limit value of the transmission power is larger than the predetermined value. The low D2D frequency band is adjacent to the frequency band used for the cellular communication. On the other hand, the high D2D frequency band is distant from the cellular frequency band. In an example shown in  FIG. 19  ( 1 ), the frequency band used for the cellular communication is not overlapped with the frequency band used for the D2D communication. 
     In an example shown in  FIG. 19  ( 2 ), a frequency band used for the cellular communication is partially overlapped with a frequency band used for the D2D communication. A frequency band used for the cellular communication and the D2D communication is associated with the low transmission power in which an upper limit value of the transmission power is equal to or less than the predetermined value. A frequency band used for only the D2D communication is associated with the high transmission power in which an upper limit value of the transmission power is larger than the predetermined value. A frequency band used for only the cellular communication is adjacent to the frequency band used for the cellular communication and the D2D communication (the low D2D frequency band). 
     In an example shown in  FIG. 19  ( 3 ), a frequency band used for the cellular communication is partially overlapped with a frequency band used for the D2D communication. A frequency band used for the cellular communication and the D2D communication is associated with the low transmission power in which transmission power is equal to or less than the predetermined value. A frequency band used for only the D2D communication is associated with the high transmission power in which transmission power is larger than the predetermined value and the low transmission power in which transmission power is smaller than the predetermined value. A frequency band used for only the cellular communication is adjacent to the frequency band used for the cellular communication and the D2D communication (the low D2D frequency band). 
     (Summary of Third Embodiment) 
     According to the present embodiment, the frequency band for the D2D communication that is adjacent to the frequency band used only for the cellular communication is associated with the upper limit value of the transmission power that is equal to or less than the predetermined value. That is, the low D2D frequency band is provided adjacent to the frequency band used only for the cellular communication. On the other hand, the high D2D frequency band is provided distant from the frequency band used only for the cellular communication. As a result, the neighboring frequency band for the D2D communication that is adjacent to the frequency band used for only the cellular communication is associated with the low transmission power for the D2D communication. Accordingly, it is possible to utilize the frequency band for the cellular communication and the frequency band for the D2D communication not only in a completely separated state, but also in a partially shared state. That is, by considering the transmission power, the interference can be prevented even in a state where the frequency band for the cellular communication and the frequency band for the D2D communication are partially shared, and thus, it is possible to realize a system operation with good frequency use efficiency. 
     [Other Embodiments] 
     Thus, the present disclosure has been described with the embodiments. However, it should not be understood that those descriptions and drawings constituting a part of this disclosure limit the present disclosure. From this disclosure, a variety of alternate embodiments, examples, and applicable techniques will become apparent to one skilled in the art. 
     In the above-described embodiment, the used transmission power to be used for the D2D communication is described, however, the used transmission power to be used for the process of discovering is also applicable. 
     Further, in the above-described first embodiment, a case where the subcarrier is changed is described as an example, however, the present disclosure is not limited thereto. The UE  100 - 1  is capable of selecting a subcarrier in much the same way as the above-described first and second embodiments, when selecting a subcarrier used for the D2D communication from the plurality of subcarriers before starting the D2D communication. 
     Further, in the above-described embodiment, the UE  100  selects a subcarrier, however, the present disclosure is not limited thereto. For example, the network  300  may select the subcarrier used for the D2D communication. 
     For example, the UE  100 - 1  transmits information of the used transmission power that is used to the network  300  via the eNB  200 . The network  300  can transmit, to the UE  100 - 1  via the eNB  200 , information indicating a subcarrier in which the used transmission power does not exceed the upper limit value on the basis of received information. 
     Further, in the above-described second embodiment, regarding each of the subcarriers used for the D2D communication, the subcarrier is associated with the upper limit value of the transmission power so that the upper limit value of the transmission power becomes larger according to the distance from the eNB  200 , however, the present disclosure is not limited thereto. 
     For example, in  FIG. 20 , in a first base station  200 - 1  that manages a first cell  250 - 1 , a frequency band f 1  for the D2D communication is used at the periphery side of the first cell  250 - 1 , and a frequency band f 2  for the D2D communication is used at the center side of the first cell  250 - 1 . On the other hand, in a second base station  200 - 2 , adjacent to the first base station  250 - 1 , that manages a second cell  250 - 2 , the frequency band f 2  for the D2D communication is used at the periphery side of the second cell  250 - 2 , and the frequency band f 1  for the D2D communication is used at the center side of the second cell  250 - 2 . That is, the frequency band for the D2D communication used in the periphery of the first cell is different from the frequency band for the D2D communication used in the periphery of the second cell. Therefore, out of the plurality of radio resources (the plurality of frequency bands), the radio resource (the frequency band f 1 ) that is available at the edge of the first cell  250 - 1  is different from the radio resource (the frequency band f 2 ) that is available at the edge of a neighboring cell, the second cell  250 - 2 . As a result, it is possible to prevent occurrence of interference between a user terminal group that is performing the D2D communication in the first cell  250 - 1  and a user terminal group that is performing the D2D communication in the second cell  250 - 2 . 
     Further, in each of the above-described embodiments, the plurality of radio resources are the plurality of frequency bands (specifically, subcarriers) divided in the frequency direction, however, for example, the plurality of radio resources may be the plurality of radio resources divided in a time axis direction. 
     Moreover, in the above-described second embodiment, the information indicating the distance from the eNB  200  is the path loss between the eNB  200  and the UE  100 , however, the present disclosure is not limited thereto. The information indicating the distance from the eNB  200 , for example, may be information indicating the physical distance such as [m] and [km] from the eNB  200 . In this case, on the basis of the location information of the UE  100 - 1  and the location information of the eNB  200 , the UE  100 - 1  is capable of selecting a subcarrier from the plurality of subcarriers according to the calculated distance from the eNB  200 . 
     In addition, in the above-described first and second embodiments, the UE  100 - 1  selects a subcarrier on the basis of the transmission power control information from the eNB  200 , however, the present disclosure is not limited thereto. The UE  100 - 1  may select a subcarrier on the basis of setting information that is previously set in the UE  100 - 1 . For example, the UE  100 - 1  is capable of selecting a subcarrier on the basis of the setting information when the UE  100 - 1  does not exist in a cell managed by the eNB  200 . 
     Needless to say, each of the above-described embodiments and modifications may be combined, where necessary. 
     The above-described embodiment has described an example in which the present disclosure is applied to the LTE system. However, the present disclosure may also be applied to systems other than the LTE system, as well as the LTE system. 
     In addition, the entire content of U.S. Provisional Application No. 61/766,505 (filed on Feb. 19, 2013) is incorporated in the present specification by reference. 
     INDUSTRIAL APPLICABILITY 
     As described above, the mobile communication system, the base station, the user terminal according to the present disclosure are able to appropriately controlling transmission power used for a process of discovering a partner terminal in D2D communication or for the D2D communication, thus they are useful for a mobile communication field.