Patent Publication Number: US-10778324-B2

Title: D2D communication control apparatus, radio terminal, relay radio terminal candidate selection method, and non-transitory computer readable medium

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2016/000710, filed Feb. 10, 2016, claiming priority based on Japanese Patent Application No. 2015-127781, filed Jun. 25, 2015, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a D2D communication control apparatus, a radio terminal, a relay radio terminal candidate selection method, and a program, and relates to, for example, a D2D communication control apparatus, a radio terminal, a relay radio terminal candidate selection method, and a program regarding discovery of radio terminals in D2D communication. 
     BACKGROUND ART 
     In mobile communication systems, introduction of device-to-device (D2D) communication, in which a radio terminal directly communicates with another radio terminal, has been discussed. For example, 3rd Generation Partnership Project (3GPP), which defines standard specifications of mobile communication systems, specifies Proximity-based services (ProSe) as the D2D communication in Non-Patent Literature 1. ProSe includes ProSe discovery and ProSe direct communication. ProSe discovery makes it possible to detect proximity of radio terminals. ProSe direct communication enables establishment of a communication path between radio terminals discovered by the ProSe discovery. 
     Patent Literature 1 discloses a discovery procedure between radio terminals that perform D2D communication. Specifically, a User Equipment (UE)  100 - 1  transmits a discovery signal by broadcasting and a UE  100 - 2  performs processing for receiving the discovery signal that has been transmitted. The UE  100 - 2  performs processing for receiving the discovery signal, to thereby discover the UE  100 - 1  that has transmitted the discovery signal. Further, the UE  100 - 2  transmits a response signal to the UE  100 - 1 , whereby the UE  100 - 1  is able to determine that it has been discovered by the UE  100 - 2 . The UE  100 - 2  determines in advance regarding whether it is capable of performing D2D communication with the UE  100 - 1  based on the distance between the UE  100 - 2  and the UE  100 - 1 . Therefore, the UE  100 - 2  is able to perform processing for receiving the discovery signal that has been transmitted from the predetermined UE in advance. 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Literature 1] International Patent Publication No. WO 2015/045860 
       
    
     Non-Patent Literature 
     
         
         [Non-Patent Literature 1] 3GPP TS 23.303 V12.4.0 (March 2015), “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Proximity-based services (ProSe); Stage 2 (Release 12)”, March, 2015 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the discovery procedure disclosed in Patent Literature 1, it can be defined that the UE which is spaced apart from the UE  100 - 1  by a predetermined distance or smaller executes processing for receiving a transmission signal transmitted from the UE  100 - 1 . However, when there are a large number of UEs that are spaced apart from the UE  100 - 1  by the predetermined distance or smaller, the UE  100 - 1  receives response signals that have been transmitted from the large number of respective UEs. Accordingly, a problem occurs that interference occurs between the response signals and the UE  100 - 1  cannot normally receive the response signals. 
     One of the objects to be attained by the present invention is to provide a D2D communication control apparatus, a radio terminal, a relay radio terminal candidate selection method, and a program capable of suppressing interference that occurs between response signals transmitted from a large number of radio terminals. 
     Solution to Problem 
     A D2D communication control apparatus according to a first aspect of the present invention includes: a communication unit configured to receive determination information that can be used to determine whether each of a plurality of radio terminals is capable of performing direct communication (device-to-device (D2D) communication) with another radio terminal; and a selection unit configured to select, using the determination information between radio terminals that may operate as relay radio terminals, a candidate for a relay radio terminal that relays the communication between one of the plurality of radio terminals and a network, the relay radio terminal candidate transmitting a response signal in response to a first discovery signal transmitted from the one of the radio terminals by performing D2D communication with the one of the plurality of radio terminals and performing cellular communication with the network. 
     A radio terminal according to a second aspect of the present invention includes: a communication unit configured to receive determination information that can be used to determine whether each of a plurality of other radio terminals is capable of performing device-to-device (D2D) communication with another radio terminal; and a selection unit configured to select, using the determination information between radio terminals that may operate as relay radio terminals, a candidate for a relay radio terminal that relays the communication between one of the plurality of radio terminals and a network, the relay radio terminal candidate transmitting a response signal in response to a first discovery signal transmitted from the one of the radio terminals by performing D2D communication with the one of the plurality of radio terminals and performing cellular communication with the network. 
     A relay radio terminal candidate selection method according to a third aspect of the present invention includes: receiving determination information that can be used to determine whether each of a plurality of radio terminals is capable of performing device-to-device (D2D) communication with another radio terminal; and selecting, using the determination information between radio terminals that may operate as relay radio terminals, a candidate for a relay radio terminal that relays the communication between one of the plurality of radio terminals and a network, the relay radio terminal candidate transmitting a response signal in response to a first discovery signal transmitted from the one of the radio terminals by performing D2D communication with the one of the plurality of radio terminals and performing cellular communication with the network. 
     A program according to a fourth aspect of the present invention causes a computer to execute the following operations of: receiving determination information that can be used to determine whether each of a plurality of radio terminals is capable of performing device-to-device (D2D) communication with another radio terminal; and selecting, using the determination information between radio terminals that may operate as relay radio terminals, a candidate for a relay radio terminal that relays the communication between one of the plurality of radio terminals and a network, the relay radio terminal candidate transmitting a response signal in response to a first discovery signal transmitted from the one of the radio terminals by performing D2D communication with the one of the plurality of radio terminals and performing cellular communication with the network. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a D2D communication control apparatus, a radio terminal, a relay radio terminal candidate selection method, and a program capable of suppressing interference that occurs between response signals transmitted from a large number of radio terminals. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a D2D communication control apparatus according to a first embodiment; 
         FIG. 2  is a configuration diagram of a mobile communication system according to a second embodiment; 
         FIG. 3  is a configuration diagram of a D2D communication control apparatus according to the second embodiment; 
         FIG. 4  is a diagram showing a flow of selection processing in the D2D communication control apparatus according to the second embodiment; 
         FIG. 5  is a diagram showing a location in which radio terminals are present in a cell according to the second embodiment; 
         FIG. 6  is a diagram showing results of receiving discovery signals by radio terminals according to the second embodiment; 
         FIG. 7  is a diagram showing a flow of processing of transmitting determination information by a radio terminal according to the second embodiment; 
         FIG. 8  is a diagram showing a flow of processing of the radio terminal when it has received a relay terminal indication according to the second embodiment; 
         FIG. 9  is a diagram showing a flow of processing of the radio terminal when it has received a relay terminal request according to the second embodiment; 
         FIG. 10  is a diagram showing a sequence of processing for selecting a relay radio terminal candidate according to the second embodiment; 
         FIG. 11  is a configuration diagram of a D2D communication control apparatus according to a third embodiment; 
         FIG. 12  is a configuration diagram of a D2D communication control apparatus according to a fourth embodiment; 
         FIG. 13  is a block diagram showing a configuration example of a radio terminal according to several embodiments; 
         FIG. 14  is a block diagram showing a configuration example of a base station according to several embodiments; and 
         FIG. 15  is a block diagram showing a configuration example of a D2D communication control apparatus according to several embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Specific embodiments of the present invention will be explained hereinafter with reference to the drawings. First, with reference to  FIG. 1 , a configuration example of a D2D communication control apparatus  10  according to a first embodiment of the present invention will be described. The D2D communication control apparatus  10  may be a computer apparatus that is operated by a processor executing a program stored in a memory. Alternatively, the D2D communication control apparatus  10  may be a server apparatus. 
     Alternatively, the D2D communication control apparatus  10  may be an apparatus that executes a ProSe function. The ProSe function is a logical function that is used for public land mobile network (PLMN)-related operations required for ProSe. The functionality provided by the ProSe function includes, for example: (a) communication with third-party applications (a ProSe Application Server); (b) authentication of a radio terminal (UE) for ProSe discovery and ProSe direct communication; (c) transmission of configuration information for ProSe discovery and ProSe direct communication (e.g., EPC-ProSe-User ID) to a UE; and (d) provision of network-level discovery (i.e., EPC-level ProSe discovery). 
     In EPC-level ProSe discovery, a D2D communication control apparatus or a core network (Evolved Packet Core (EPC)) determines proximity of two radio terminals and notifies the two radio terminals of the result of the determination. 
     The apparatus that executes the ProSe function may be referred to as, for example, a ProSe function entity or a ProSe function server. 
     The D2D communication control apparatus  10  includes a communication unit  11  and a selection unit  12 . Each of the communication unit  11  and the selection unit  12  may be software, a module or the like whose processing is executed by a processor executing a program stored in a memory. Alternatively, each of the communication unit  11  and the selection unit  12  may be hardware such as a circuit or a chip. 
     The communication unit  11  receives determination information that can be used to determine whether each of radio terminals  21 - 24  is capable of performing the D2D communication with a radio terminal located nearby. 
     The radio terminals  21 - 24  may be, for example, a mobile telephone terminal, a smartphone terminal, or a Machine Type Communication (MTC) terminal that autonomously performs communication without requiring user manipulation. The D2D communication may be, for example, ProSe discovery and ProSe direct communication. 
     The determination information that can be used to determine whether the D2D communication can be performed with the radio terminal located nearby may be, for example, positional information generated by each of the radio terminals  21 - 24 . The positional information may be, for example, GNSS positional information obtained by a Global Navigation Satellite System (GNSS) receiver. The D2D communication control apparatus  10  may calculate the distance between radio terminals using, for example, the positional information on the radio terminals. The D2D communication control apparatus  10  may determine, when the calculated distance is shorter than a predetermined distance, that the radio terminals can perform D2D communication. 
     Further, the determination information may be information on a result of receiving a discovery signal that each of the radio terminals  21 - 24  has received from a nearby radio terminal. The discovery signal may be referred to as, for example, a Discovery signal. It is assumed here that the radio terminal that has received the discovery signal used for the determination information needs not transmit a response signal in response to the discovery signal. The D2D communication control apparatus  10  may determine that the radio terminal that has received the discovery signal and the radio terminal that has transmitted this discovery signal are able to perform D2D communication. 
     Each of the radio terminals  21 - 24  performs D2D communication with another radio terminal. Further, each of the radio terminals  21 - 24  performs cellular communication with a network. Accordingly, each of the radio terminals  21 - 24  serves as a relay radio terminal that relays communication between another radio terminal and the network. 
     The selection unit  12  selects, from among the radio terminals  21 - 24 , a candidate for a relay radio terminal, which is a relay radio terminal candidate that transmits the response signal in response to the discovery signal transmitted from one of the radio terminals  21 - 24  using the determination information between the radio terminals that may operate as the relay radio terminals. 
     The relay radio terminal performs cellular communication with the network using, for example, the cellular communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology). Further, while the radio terminal actually communicates with the network via one relay radio terminal, a plurality of relay radio terminal candidates may be present. The radio terminal transmits, when it communicates with the network via the relay radio terminal, the discovery signal to the nearby radio terminal. The plurality of relay radio terminal candidates transmit, to the radio terminal, the response signal in response to the discovery signal. The radio terminal that does not correspond to the relay radio terminal candidate does not transmit the response signal in response to the discovery signal. The radio terminal may select the relay radio terminal from among the relay radio terminal candidates in accordance with the result of receiving the response signal. 
     The determination information between the radio terminals that may operate as the relay radio terminals may be positional information on the radio terminal that may operate as the relay radio terminal or may be information on the result of receiving the discovery signal between the radio terminals that may operate as the relay radio terminals. 
     For example, all the radio terminals  21 - 24  may be the radio terminals that may operate as the relay radio terminals or the radio terminals  22 - 24  other than the radio terminal  21  may be the radio terminals that may operate as the relay radio terminals. 
     As described above, the D2D communication control apparatus  10  shown in  FIG. 1  is able to select the relay radio terminal candidate, which is the candidate for the relay radio terminal. Accordingly, even in the case in which the radio terminal has transmitted the discovery signal to the nearby radio terminal in order to search for the relay radio terminal, the radio terminal no longer receives the response signal from all the radio terminals that have received the discovery signal. It is therefore possible to suppress the interference that occurs between the response signals compared to the case in which all the radio terminals that have received the discovery signal transmit the response signal. 
     Further, the D2D communication control apparatus  10  is able to select the relay radio terminal candidate using the determination information between the radio terminals that may operate as the relay radio terminals. In other words, the D2D communication control apparatus  10  is able to select the relay radio terminal candidate in accordance with the positional relation of the radio terminals that may operate as the relay radio terminals, the discovery status between the radio terminals that may operate as the relay radio terminals or the like. 
     Further, while the example in which the D2D communication control apparatus  10  selects the relay radio terminal candidate has been described in  FIG. 1 , the base station, the core network apparatus, the radio terminal or the like arranged in the mobile communication network may execute the processing for selecting the relay radio terminal candidate. 
     Second Embodiment 
     Next, with reference to  FIG. 2 , a configuration example of a mobile communication system according to a second embodiment of the present invention will be described. The mobile communication system shown in  FIG. 2  includes the D2D communication control apparatus  10 , the radio terminals  21 - 24 , a core network  30 , and a base station  40 . 
     Since the D2D communication control apparatus  10  is similar to the D2D communication control apparatus  10  shown in  FIG. 1 , the detailed descriptions thereof will be omitted. Further, since the radio terminals  21 - 24  are also similar to the radio terminals  21 - 24  shown in  FIG. 1 , the detailed descriptions thereof will be omitted. 
     The core network  30  may be, for example, an EPC, and includes a plurality of user-plane entities and a plurality of control-plane entities. The user-plane entity may be, for example, Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW). Further, the control-plane entity may be Mobility Management Entity (MME) and Home Subscriber Server (HSS). Each of the user-plane entity and the control-plane entity may be referred to as a core network apparatus. Further, the core network  30  may include a ProSe function entity, a ProSe function server or the like. Further, the user-plane entity or the control-plane entity may execute the ProSe function as the ProSe function entity. 
     The plurality of user-plane entities relay user data of the radio terminals  21 - 24  between the radio access network including the base station  40  and an external network. The plurality of control-plane entities perform various kinds of control for the radio terminals  21 - 24  including mobility management, session management (bearer management), subscriber information management, and billing management. 
     The base station  40  forms a cell  41 . The cell  41  is an area where radio terminals are able to perform cellular communication with the base station  40 . Further, a coverage hole  42  is an area in the cell  41  and is an area in which radio terminals cannot perform cellular communication with the base station  40  or an area in which a desired cellular communication (whose communication rate is equal to or larger than a predetermined value) cannot be performed. For example, the coverage hole  42  is generated in a building located in the cell  41 , an area surrounded by a plurality of buildings and the like. The base station  40  may be, for example, an evolved NodeB (eNB) defined by the 3GPP. 
       FIG. 2  shows that the radio terminal  22  and the radio terminal  23  are the relay radio terminal candidates. Specifically, the radio terminal  21  transmits the discovery signal to the nearby radio terminals  22 - 24  in order to communicate with the base station  40  via the relay radio terminal. The discovery signal that the radio terminal  21  transmits to the nearby radio terminals  22 - 24  in order to communicate with the base station  40  via the relay radio terminal will be explained as a relay terminal request in the following description. 
     The radio terminal  21  is located close to the coverage hole  42 , and when the radio terminal  21  estimates that it will not be able to communicate with the base station  40  any more or will not be able to perform a desired cellular communication with the base station  40 , the radio terminal  21  may transmit the relay terminal request to the nearby radio terminals  22 - 24 . The radio terminal  21  may periodically measure, for example, Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ) and may estimate that it will not be able to communicate with the base station  40  when the communication quality indicated by the RSRP or the RSRQ is lower than a predetermined communication quality. In other words, the radio terminal  21  may determine the timing when the relay terminal request is transmitted based on the value of RSRP or RSRQ. Further, the index for measuring, by the radio terminal  21 , the communication quality is not limited to RSRP or RSRQ. Alternatively, the radio terminal  21  may periodically measure the communication rate (throughput) of the data transmitted to and received from the base station  40  and estimate that it will not be able to perform a desired cellular communication with the base station  40  when the value of the communication rate that has been measured is below a predetermined value. 
     The radio terminal  22  and the radio terminal  23 , which are the relay radio terminal candidates, each transmit the response signal to the radio terminal  21  in response to the relay terminal request, and the radio terminal  24 , which is not the relay radio terminal candidate, does not transmit the response signal to the radio terminal  21 . 
     The radio terminal  21  communicates with the base station  40  via one of the radio terminal  22  and the radio terminal  23 , which are the relay radio terminal candidates. 
     While a state in which the radio terminals  21 - 24  are located in the cell  41  is shown in  FIG. 2 , some of the radio terminals  21 - 24  may be located in a cell formed by another base station. 
     Next, with reference to  FIG. 3 , a configuration example of the D2D communication control apparatus  10  according to the second embodiment of the present invention will be described. The D2D communication control apparatus  10  includes the communication unit  11 , the selection unit  12 , a transmission data processing unit  13 , and a reception data processing unit  14 . Since the communication unit  11  and the selection unit  12  are similar to the communication unit  11  and the selection unit  12  in  FIG. 1 , the detailed descriptions thereof will be omitted. 
     The reception data processing unit  14  receives the determination information via the communication unit  11 . Further, the reception data processing unit  14  outputs the determination information that it has received to the selection unit  12 . The selection unit  12  selects the relay radio terminal candidate using the determination information. The selection unit  12  outputs the information regarding the relay radio terminal candidate that it has selected to the transmission data processing unit  13 . 
     The transmission data processing unit  13  transmits, to the core network apparatus, an indication signal used to notify the relay radio terminal candidate selected by the selection unit  12  that it is the relay radio terminal candidate via the communication unit  11 . The core network apparatus is arranged in the core network  30 . In the following description, the indication signal is described as a relay terminal indication. The transmission data processing unit  13  may configure, as the destination of the relay terminal indication, address information on the radio terminal that has been selected as the relay radio terminal candidate by the selection unit  12 . The relay terminal indication may be generated in the selection unit  12  or may be generated in the transmission data processing unit  13 . 
     Next, with reference to  FIG. 4 , a flow of the selection processing in the D2D communication control apparatus  10  according to the second embodiment of the present invention will be explained. First, the selection unit  12  determines whether it holds the determination information (S 11 ). The selection unit  12  may store or record, upon receiving the determination information output from the reception data processing unit  14 , the determination information in a memory or the like in the D2D communication control apparatus  10 . 
     When the selection unit  12  determines that it does not hold the determination information, the process in Step S 11  is repeated. 
     When the selection unit  12  determines that it holds the determination information, the selection unit  12  selects the relay radio terminal candidate using the determination information (S 12 ). Next, the transmission data processing unit  13  transmits the relay terminal indication to the relay radio terminal candidate selected in the communication unit  11  (S 13 ). 
     When the transmission data processing unit  13  transmits the relay terminal indication to the radio terminal that has been selected as the relay radio terminal candidate in Step S 13 , it may also transmit information for indicating the timing when the response signal in response to the relay terminal request is transmitted as well. Specifically, the transmission data processing unit  13  or the selection unit  12  may control the relay radio terminal candidates in such a way that the relay radio terminal candidates transmit the response signals at timings different from one another. By making the timings when the plurality of relay radio terminal candidates transmit the response signals in response to the relay terminal request different from one another, it is possible to reduce the interference that occurs between the response signals compared to the case in which the plurality of relay radio terminal candidates transmit the response signals at the same timing. 
     The information regarding the timing when the response signal is transmitted may include at least one of an offset value from the timing when the relay terminal request has been received from the radio terminal  21  to the timing when the response signal is transmitted and information regarding the subframe number at which the transmission of the response signal is permitted. 
     The details of the processing for selecting the relay radio terminal candidate in Step S 12  will now be explained. First, a case in which the D2D communication control apparatus  10  receives the positional information on each of the radio terminals as the determination information will be explained. The selection unit  12  calculates, from the positional information on each of the radio terminals, the distance X between the radio terminals. The selection unit  12  may select, for example, the radio terminals having a distance X between them which is equal to or larger than a distance a (a&gt;0) as the relay radio terminal candidates. By selecting the radio terminals that are spaced apart from each other by a predetermined distance a or larger as the relay radio terminal candidates, it is possible to select the radio terminals whose radio environments are different from each other. 
     Accordingly, when, for example, the radio terminal that communicates with the network via the relay radio terminal selects a relay radio terminal from among the relay radio terminal candidates, the relay radio terminal candidate that is using a radio channel with a high radio quality can be selected as the relay radio terminal. 
     Alternatively, the selection unit  12  may define a distance b (b&gt;a) as the upper-limit value of the distance X. The selection unit  12  may specify, for example, the longest distance in the cell  41  such as the length of the diameter or the length of the long diameter of the cell  41  as the distance b. 
     Alternatively, the selection unit  12  may select a relay radio terminal candidate for each radio communication terminal. Specifically, the selection unit  12  may allocate a relay radio terminal candidate to the radio terminal  21  different from the relay radio terminal candidate allocated to the radio terminal  22  or the like. When, for example, the selection unit  12  is to select a relay radio terminal candidate to be allocated to the radio terminal  21 , the selection unit  12  may select the radio terminals other than the radio terminal  21  having a distance X between them which is from a (inclusive) to b (inclusive), the radio terminal being spaced apart from the radio terminal  21  by c (c&gt;0) or smaller as the relay radio terminal candidate. The distance c may be, for example, a distance in which the D2D communication can be performed with the radio terminal  21 . Further, the selection unit  12  may select a radio terminal which is spaced apart from the radio terminal  21  by d (0&lt;d&lt;c) or larger as the relay radio terminal candidate. By setting the radio terminal which is spaced apart from the radio terminal  21  by a predetermined distance d as the relay radio terminal candidate, the selection unit  12  is able to select the radio terminal which is spaced apart from the coverage hole by a predetermined distance as the relay radio terminal candidate. When the radio terminal  21  is located near the coverage hole, the radio terminal which is located substantially in the same place as the radio terminal  21  is also located near the coverage hole. Therefore, the distance d may be defined in order to exclude the radio terminal that is located substantially in the same place as the radio terminal  21  from the relay radio terminal candidates as the relay radio terminal candidate of the radio terminal  21 . 
     Next, a case in which the D2D communication control apparatus  10  has received information on the result of receiving the discovery signal in each of the radio terminals as the determination information will be described. The information on the result of receiving the discovery signal may include, for example, at least one of identification information on the radio terminal that has transmitted the discovery signal, the reception power of the discovery signal that has been received, and the number of times that the discovery signal transmitted from the radio terminal is received. In the following description, a case in which the identification information on the radio terminal that has transmitted the discovery signal is included in the determination information will be mainly described. 
     Each of the radio terminals receives the discovery signal that has been transmitted from one or more radio terminals. It is assumed that, when each of the radio terminals receives the discovery signal, it has discovered the radio terminal that has transmitted this discovery signal. In this example, the radio terminal that the radio terminal has discovered is referred to as a discovery terminal element. The selection unit  12  may select the relay radio terminal candidate by, for example, the following procedure.
     (a) The radio terminals in which the number of discovery terminal elements is equal to or smaller than a threshold in the determination information are selected as the relay radio terminal candidates. Alternatively, the radio terminals in which the number of discovery terminal elements is the smallest in the determination information may be selected as the relay radio terminal candidates.   (b) Among the radio terminals that do not include the discovery terminal elements of the relay radio terminal candidates selected in (a), the radio terminals in which the number of discovery terminal elements is equal to or smaller than a threshold are selected as the relay radio terminal candidates. Alternatively, among the radio terminals that do not include the discovery terminal elements of the relay radio terminal candidate selected in (a), the radio terminals in which the number of discovery terminal elements is the smallest may be selected as the relay radio terminal candidates.   (c) The procedure (b) is repeated until all the radio terminals in the cell are included in the discovery terminal elements of the relay radio terminal candidates.   

     The procedures (b) and (c) may be repeated until all the radio terminals are counted n times (n is an integer equal to or larger than 1) as the discovery terminal elements of the relay radio terminal candidates. 
     While the radio terminal in which the number of discovery terminal elements is small has been selected as the relay radio terminal candidates in the aforementioned procedures (a)-(c), the radio terminal in which the number of discovery terminal elements is large may be selected as the relay radio terminal candidates. For example, the radio terminal in which the number of discovery terminal elements is equal to or larger than a threshold or the radio terminal in which the number of discovery terminal elements is the largest may be selected as the relay radio terminal candidates. 
     With reference to  FIGS. 5 and 6 , the procedures (a)-(c) will be explained in detail.  FIG. 5  shows a location where radio terminals are present in the cell  41 . The black circles indicate the radio terminals. Further, the numerals attached next to the black circles are identification information on the radio terminals. 
       FIG. 6  shows a report radio terminal in association with a discovery terminal element in the report radio terminal, the report radio terminal being the radio terminal that has transmitted the determination information including the information on the result of receiving the discovery signal. 
     For example,  FIG. 6  shows that the radio terminal  1  has discovered the radio terminal  2 . Further, it shows that the radio terminal  2  has discovered the radio terminals  1  and  7 . The same is applicable to the radio terminal  3  and the subsequent radio terminals. 
     First, by the procedure (a), the radio terminals  1 ,  3 ,  7 ,  8 ,  9 ,  16 , and  17  in which the number of discovery terminal elements is the smallest, that is, 1, are selected as the relay radio terminal candidates. The selection order shown in  FIG. 6  indicates the order of selection as the relay radio terminal candidates.  FIG. 6  shows that the radio terminals  1 ,  3 ,  7 ,  8 ,  9 ,  16 , and  17  have been selected first as the relay radio terminal candidates. 
     Next, by the procedure (b), from among the radio terminals that do not include the discovery terminal elements of the radio terminals  1 ,  3 ,  7 ,  8 ,  9 ,  16 , and  17 , the radio terminals  2 ,  5 ,  6 ,  18 , and  20  in which the number of discovery terminal elements is the smallest, that is, 2, are selected as the relay radio terminal candidates. That is, the radio terminals  2 ,  5 ,  6 ,  18 , and  20  are selected second as the relay radio terminal candidates. 
     Next, by the procedure (c), from among the radio terminals that do not include the discovery terminal elements of the relay radio terminal candidates selected in the procedures (a) and (b), the radio terminals  4  and  12  in which the number of discovery terminal elements is the smallest, that is, 3, are selected as the relay radio terminal candidates. Further, from among the radio terminals that do not include the discovery terminal elements of the relay radio terminal candidates selected in the aforementioned procedure, the radio terminals  10 ,  13 ,  14 , and  15  in which the number of discovery terminal elements is the smallest, that is, 4, are selected as the relay radio terminal candidates. From the aforementioned procedure, the radio terminals  1 - 20  are included in the discovery terminal elements of any one of the relay radio terminal candidates. 
     In the example shown in  FIG. 6 , the radio terminals  11  and  19  are not selected as the relay radio terminal candidates. Therefore, even when the radio terminals  11  and  19  receive the relay terminal request, they do not transmit a response signal to the radio terminal that has transmitted the relay terminal request. Further, with respect to  FIG. 6 , the case in which, when one of the two radio terminals has received the discovery signal, the other radio terminal also receives the discovery signal has been described. However, there is also a case in which while one radio terminal has received the discovery signal, the other radio terminal cannot receive the discovery signal due to the difference between the timing when the discovery signal is transmitted to the one radio terminal and the timing when it is transmitted to the other radio terminal. In this case as well, the aforementioned procedures (a)-(c) can be executed. 
     In the aforementioned description, the procedure for selecting a common relay radio terminal candidate in the cell  41  has been described. That is, no matter which one of the radio terminals located in the cell  41  has transmitted the relay terminal request, the common relay radio terminal candidates that have received the relay terminal request transmit the response signal. In other words, any one of the radio terminals located in the cell  41  is able to transmit the relay terminal request to at least one of the common relay radio terminal candidates. 
     On the other hand, the relay radio terminal candidate may be selected for each radio terminal in the cell  41 . When, for example, the relay radio terminal candidate is selected for each radio terminal, the selection unit  12  may select, from among the radio terminals including the target radio terminal as the discovery terminal element, the radio terminal in which the number of discovery terminal elements is the smallest as the relay radio terminal candidate. 
     For example, with reference to  FIG. 6 , the case in which the relay radio terminal candidate of the radio terminal  4  is selected will be explained. The radio terminals  3 ,  5 , and  6  include the radio terminal  4  as the discovery terminal element. Among them, the radio terminal  3  has the least number of discovery terminal elements. Accordingly, the radio terminal  3  is selected as the relay radio terminal candidate of the radio terminal  4 . 
     Further, when the selection unit  12  selects the relay radio terminal candidate for each radio terminal, the selection unit  12  may select, as the relay radio terminal candidate, the radio terminals that include the target radio terminal as the discovery terminal element and may become the relay radio terminals having not discovered each other. 
     For example, a case in which the relay radio terminal candidate of the radio terminal  20  is selected in  FIG. 6  will be described. The radio terminals  16  and  19  include the radio terminal  20  as the discovery terminal element. Among them, the radio terminal  16  and the radio terminal  19  have not discovered each other. Accordingly, the radio terminals  16  and  19  are selected as the relay radio terminal candidates of the radio terminal  20 . 
     In the aforementioned description, the procedure for selecting the relay radio terminal candidate when the identification information on the radio terminal is used has been mainly described above. Alternatively, the selection unit  12  may select the relay radio terminal candidate using information regarding the reception power of the discovery signal or the number of times that the discovery signal has been received. 
     When, for example, the selection unit  12  selects the relay radio terminal candidate with respect to one radio terminal, the selection unit  12  may select the radio terminal that has transmitted the discovery signal whose reception power in the one radio terminal is larger than a predetermined value as the relay radio terminal candidate. In this case, it is possible to maintain a high communication quality between the one radio terminal and the relay radio terminal candidate. Alternatively, the selection unit  12  may select, when selecting the relay radio terminal candidate with respect to one radio terminal, the radio terminal that has transmitted the discovery signal whose reception power in the one radio terminal is smaller than a predetermined value as the relay radio terminal candidate. In this case, the reception power is small, whereby it is possible to reduce the interference that occurs between the response signals transmitted to the one radio terminal. 
     Alternatively, when the number of times that the discovery signal transmitted from one radio terminal has been received is larger than a predetermined value, this radio terminal may be selected as the relay radio terminal candidate. In this case, it is possible to improve the probability that the D2D communication can be normally executed. 
     Besides the aforementioned information, when, for example, each of the radio terminals has already operated as the relay radio terminal, the selection unit  12  may select the relay radio terminal candidate in accordance with the number of radio terminals that are executing D2D communication. For example, the selection unit  12  may select the radio terminal in which the number of radio terminals that are executing D2D communication is smaller than a predetermined value as the relay radio terminal. It is therefore possible to reduce the processing load of the relay radio terminal candidate. 
     Besides the aforementioned information, the selection unit  12  may select the relay radio terminal candidate in accordance with, for example, the communication quality or the radio quality of the cellular communication line in each of the radio terminals. For example, the radio terminal in which the communication quality or the radio quality of a cellular communication line is higher than a predetermined value may be selected as the relay radio terminal candidate. Accordingly, when the relay radio terminal candidate is operated as the relay radio terminal, communication with excellent throughput and the like can be achieved. 
     Besides the aforementioned information, the selection unit  12  may select, for example, the relay radio terminal candidate in accordance with a residual capacity of a battery in each of the radio terminals. The selection unit  12  may select, for example, the radio terminal whose residual capacity of the battery is larger than a predetermined capacity as the relay radio terminal candidate. 
     Further, the selection unit  12  may select the relay radio terminal candidate by combining positional information, determination information, information on the number of radio terminals that are executing D2D communication, information on the communication quality of the cellular communication line, information on the residual capacity of the battery and the like. 
     Next, with reference to  FIG. 7 , a flow of the processing of transmitting, by the radio terminal, the determination information according to the second embodiment of the present invention will be explained. In this example, a flow of the processing in the radio terminal  21  will be explained. Since the processing of the other radio terminals is similar to the processing of the radio terminal  21 , the detailed descriptions thereof will be omitted. 
     First, the radio terminal  21  determines whether the determination information has been generated (S 21 ). The radio terminal  21  generates the determination information at the timing when, for example, the GNSS receiver has acquired the positional information, at the timing when the discovery signal transmitted from another radio terminal has been received or the like. 
     When it is determined that the determination information has not been generated, the radio terminal  21  repeats the processing of Step S 21 . When it is determined that the determination information has been generated, the radio terminal  21  transmits the determination information that has been generated to the D2D communication control apparatus  10  (S 22 ). 
     Next, with reference to  FIG. 8 , a flow of the processing of the radio terminal when it has received the relay terminal indication will be explained. In this example, a flow of processing in the radio terminal  22  will be explained. Since the processing in the other radio terminals is similar to that in the radio terminal  22 , the detailed descriptions thereof will be omitted. First, the radio terminal  22  determines whether it has received the relay terminal indication transmitted from the D2D communication control apparatus  10  (S 31 ). When it is determined that the relay terminal indication has not been received, the radio terminal  22  repeats the processing of Step S 31 . 
     When it is determined that the radio terminal  22  has received the relay terminal indication, the radio terminal  22  determines whether the radio terminal  22  satisfies a condition of the relay radio terminal candidate (S 32 ). The condition of the relay radio terminal candidate may be, for example, the residual capacity of the battery is larger than a predetermined capacity, the number of radio terminals to be relayed is smaller than a predetermined number, or the cellular communication quality is better than a predetermined quality. 
     When it is determined that the radio terminal  22  does not satisfy the condition of the relay radio terminal candidate, the radio terminal  22  ends the processing. That is, the radio terminal  22  does not execute the operation as the relay radio terminal candidate. 
     When it is determined that the radio terminal  22  satisfies the condition of the relay radio terminal candidate, the radio terminal  22  configures the information for operating as the relay radio terminal candidate (S 33 ). The information for operating as the relay radio terminal candidate may be, for example, information that defines operations and the like when the relay terminal request is received. 
     Next, with reference to  FIG. 9 , a flow of the processing of the radio terminal when it has received the relay terminal request will be explained. In this example, a flow of processing in the radio terminal  22  will be explained. First, the radio terminal  22  determines whether it has received the relay terminal request that has been transmitted from another radio terminal (S 41 ). 
     When it is determined that the radio terminal  22  has not received the relay terminal request, the radio terminal  22  repeats the processing of Step S 41 . When it is determined that the radio terminal  22  has received the relay terminal request, the radio terminal  22  determines whether it is the relay radio terminal candidate (S 42 ). In other words, the radio terminal  22  receives the relay terminal indication and determines whether it satisfies the condition of the relay radio terminal candidate. 
     When it is determined that the radio terminal  22  is the relay radio terminal candidate, the radio terminal  22  transmits the response signal to the radio terminal that has transmitted the relay terminal request (S 43 ). When it is determined in Step S 42  that the radio terminal  22  is not the relay radio terminal candidate, it ends the processing. In other words, when it is determined that the radio terminal  22  is not the relay radio terminal candidate, the radio terminal  22  does not transmit the response signal to the radio terminal that has transmitted the relay terminal request. 
     Next, with reference to  FIG. 10 , a sequence of processing for selecting the relay radio terminal candidate according to the second embodiment of the present invention will be explained. First, each of the radio terminals  21 - 24  transmits the discovery signal to the nearby radio terminal. Specifically, the radio terminal  22  transmits, in Steps S 51 -S 53 , the discovery signal to the radio terminal  21 , the radio terminal  23 , and the radio terminal  24 . In a similar way, in Steps S 54 -S 56 , the radio terminal  23  transmits the discovery signal to the radio terminal  22 , the radio terminal  21 , and the radio terminal  24 . In a similar way, in Steps S 57 -S 59 , the radio terminal  21  transmits the discovery signal to the radio terminal  22 , the radio terminal  23 , and the radio terminal  24 . In a similar way, in Steps S 60 -S 62 , the radio terminal  24  transmits the discovery signal to the radio terminal  23 , the radio terminal  22 , and the radio terminal  21 . 
     While the discovery signal is transmitted to the radio terminal  22 , the radio terminal  23 , the radio terminal  21 , and the radio terminal  24  in this order in Steps S 51 -S 62 , the order of transmitting the discovery signal is not limited to this order. Further, while the processing of specifying, by each of the radio terminals, the destination radio terminal and transmitting the discovery signal to this destination radio terminal has been described in Steps S 51 -S 62 , each of the radio terminals may collectively transmit the discovery signal to the nearby radio terminals by broadcasting. 
     Next, upon receiving the discovery signal, each of the radio terminals  21 - 24  transmits the determination information including the results of receiving the discovery signal to the D2D communication control apparatus  10  (S 63 -S 66 ). Next, the D2D communication control apparatus  10  selects the relay radio terminal candidate using the determination information transmitted from each of the radio terminals  21 - 24  (S 67 ). Next, the D2D communication control apparatus  10  transmits the relay terminal indication to the radio terminal that has been selected as the relay radio terminal candidate. In this example, the D2D communication control apparatus  10  selects the radio terminal  22  and the radio terminal  23  as the relay radio terminal candidates and transmits the relay terminal indication to the radio terminal  23  and the radio terminal  22  in Steps S 68  and S 69 . 
     While data transmission and data reception between the radio terminals  21 - 24  and the D2D communication control apparatus  10  are performed via the base station  40  and the core network  30 , the base station  40  and the core network  30  are not shown in  FIG. 10 . 
     As described above, by using the communication system according to the second embodiment of the present invention, in the D2D communication control apparatus  10 , the relay terminal indication is transmitted to the radio terminal that has been selected as the relay radio terminal candidate via the core network  30  and the base station  40 . Accordingly, the radio terminal that has received the relay terminal indication recognizes that it is the relay radio terminal candidate and transmits the response signal in response to the relay terminal request transmitted from another radio terminal. On the other hand, the radio terminal that has not received the relay terminal indication recognizes that it is not the relay radio terminal candidate and does not transmit the response signal in response to the relay terminal request transmitted from another radio terminal. Accordingly, the number of response signals transmitted to the radio terminal that has transmitted the relay terminal request becomes smaller than that in the case in which all the radio terminals that have received the relay terminal request transmit the response signals. It is therefore possible to reduce the interference that occurs between the response signals. 
     Further, the D2D communication control apparatus  10  may select the common relay radio terminal candidate in the cell  41  or may select the relay radio terminal candidate for each radio terminal located in the cell  41 . Accordingly, the radio terminals located in the cell  41  are each able to reliably transmit the discovery signal to one of the relay radio terminal candidates. Further, when the relay radio terminal candidate is selected for each radio terminal, the number of relay radio terminal candidates can be reduced more than that in the case in which the common relay radio terminal candidate is selected in the cell  41 . Accordingly, when the relay radio terminal candidate is selected for each radio terminal, it is possible to further reduce the occurrence of the interference between the response signals. 
     Third Embodiment 
     Next, with reference to  FIG. 11 , a configuration example of a base station  50  according to a third embodiment of the present invention will be explained. The base station  50  includes the selection unit  12 , the transmission data processing unit  13 , the reception data processing unit  14 , a communication unit  51 , and a radio communication unit  52 . Since the selection unit  12 , the transmission data processing unit  13 , and the reception data processing unit  14  execute the functions or the processing similar to those of the selection unit  12 , the transmission data processing unit  13 , and the reception data processing unit  14  in  FIG. 3 , the detailed descriptions thereof will be omitted. 
     The communication unit  51  communicates with the core network apparatus arranged in the core network  30 . The radio communication unit  52  performs radio communication with the radio terminals  21 - 24  and the like located in the cell that the base station  50  forms. The reception data processing unit  14  receives the determination information transmitted from the radio terminals  21 - 24  via the radio communication unit  52 . The reception data processing unit  14  outputs the determination information that has been received to the selection unit  12 . 
     The selection unit  12  selects the relay radio terminal candidate using the determination information received from the reception data processing unit  14 . The transmission data processing unit  13  transmits, to the radio terminal that has been selected by the selection unit  12  to be the relay radio terminal candidate, the relay terminal indication that notifies that this radio terminal is the relay radio terminal candidate. 
     As described above, the base station  50  includes the selection unit  12  included in the D2D communication control apparatus  10  in  FIG. 3 . Therefore, the base station  50  is able to select the relay radio terminal candidate using the determination information transmitted from the radio terminals  21 - 24 . Accordingly, the determination information and the relay terminal indication are not communicated between the base station  50  and the D2D communication control apparatus  10  any more or the amount of the determination information and the number of relay terminal indications to be communicated are reduced. It is therefore possible to reduce the amount of traffic in the core network  30 . 
     Fourth Embodiment 
     Next, with reference to  FIG. 12 , a configuration example of a radio terminal  60  according to a fourth embodiment of the present invention will be explained. The radio terminal  60  includes the selection unit  12 , the transmission data processing unit  13 , the reception data processing unit  14 , and a radio communication unit  61 . Since the selection unit  12 , the transmission data processing unit  13 , and the reception data processing unit  14  execute functions or processing similar to those of the selection unit  12 , the transmission data processing unit  13 , and the reception data processing unit  14  in  FIG. 3 , the detailed descriptions thereof will be omitted. 
     The radio communication unit  61  performs radio communication with a base station  70  and performs D2D communication with a nearby radio terminal  65  and the like. The reception data processing unit  14  may receive the determination information from another radio terminal by performing D2D communication. Alternatively, the reception data processing unit  14  may receive the determination information from another radio terminal via the base station  70 . The reception data processing unit  14  outputs the determination information that has been received to the selection unit  12 . 
     The selection unit  12  selects the relay radio terminal candidate using the determination information received from the reception data processing unit  14 . The transmission data processing unit  13  transmits, to the radio terminal that has been selected by the selection unit  12  to be the relay radio terminal candidate, the relay terminal indication for notifying that this radio terminal is the relay radio terminal candidate. The transmission data processing unit  13  may transmit the relay terminal indication to the radio terminal that has been selected as the relay radio terminal candidate by performing D2D communication using the radio communication unit  61 . Alternatively, the transmission data processing unit  13  may transmit the relay terminal indication to the radio terminal that has been selected as the relay radio terminal candidate via the base station  70  using the radio communication unit  61 . 
     As described above, the radio terminal  60  includes the selection unit  12  included in the D2D communication control apparatus  10  shown in  FIG. 3 . Therefore, the radio terminal  60  is able to select the relay radio terminal candidate using the determination information transmitted from another radio terminal. In this case, the radio terminal  60  may receive the determination information via the base station  70  or may receive the determination information by another radio terminal by performing D2D communication. Further, the radio terminal  60  that collects the determination information may be arbitrarily determined from among the plurality of radio terminals located in the cell. Alternatively, the radio terminal  60  that collects the determination information may be defined, for example, using a specific criterion such as the radio terminal in which the number of discovery terminal elements is the largest. The radio terminal  60  that collects the determination information may be defined by the D2D communication control apparatus  10  or may be specified by an administrator or the like. 
     Lastly, configuration examples of the radio terminal  21 , the base station  40 , and the D2D communication control apparatus  10  according to the aforementioned embodiments will be described.  FIG. 13  is a block diagram showing a configuration example of the radio terminal  21 . A Radio Frequency (RF) transceiver  1101  performs analog RF signal processing to communicate with the base station  40 . The analog RF signal processing performed by the RF transceiver  1101  includes frequency up-conversion, frequency down-conversion, and amplification. The RF transceiver  1101  is coupled to an antenna  1102  and a baseband processor  1103 . That is, the RF transceiver  1101  receives modulated symbol data (or OFDM symbol data) from the baseband processor  1103 , generates a transmission RF signal, and supplies the transmission RF signal to the antenna  1102 . Further, the RF transceiver  1101  generates a baseband reception signal based on a reception RF signal received by the antenna  1102 , and supplies the baseband reception signal to the baseband processor  1103 . 
     The baseband processor  1103  performs digital baseband signal processing (i.e., data plane processing) and control plane processing for radio communication. The digital baseband signal processing includes (a) data compression/decompression, (b) data segmentation/concatenation, (c) composition/decomposition of a transmission format (i.e., transmission frame), (d) channel coding/decoding, (e) modulation (i.e., symbol mapping)/demodulation, and (f) generation of OFDM symbol data (i.e., baseband OFDM signal) by Inverse Fast Fourier Transform (IFFT). On the other hand, the control plane processing includes communication management of layer  1  (e.g., transmission power control), layer  2  (e.g., radio resource management and hybrid automatic repeat request (HARQ) processing), and layer  3  (e.g., signalling regarding attach, mobility, and call management). 
     In the case of LTE and LTE-Advanced, the digital baseband signal processing performed by the baseband processor  1103  may include signal processing of a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, a MAC layer, and a PHY layer. Further, the control plane processing performed by the baseband processor  1103  may include processing of a Non-Access Stratum (NAS) protocol, an RRC protocol, and MAC CE. 
     The baseband processor  1103  may include a modem processor (e.g., a Digital Signal Processor (DSP)) that performs the digital baseband signal processing and a protocol stack processor (e.g., a Central Processing Unit (CPU) or a Micro Processing Unit (MPU)) that performs the control plane processing. In this case, the protocol stack processor, which performs control plane processing, may be integrated with an application processor  1104  described below. 
     The application processor  1104  is also referred to as a CPU, an MPU, a microprocessor, or a processor core. The application processor  1104  may include a plurality of processors (processor cores). The application processor  1104  loads a system software program (Operating System (OS)) and various application programs (e.g., a voice call application, a WEB browser, a mailer, a camera operation application, and a music player application) from a memory  1106  or from another memory (not shown) and executes these programs, thereby providing various functions of the radio terminal  21 . 
     In some implementations, as represented by a dashed line ( 1105 ) in  FIG. 13 , the baseband processor  1103  and the application processor  1104  may be integrated on a single chip. In other words, the baseband processor  1103  and the application processor  1104  may be implemented in a single System on Chip (SoC) device  1105 . An SoC device may be referred to as a system Large Scale Integration (LSI) or a chipset. 
     The memory  1106  is a volatile memory, a non-volatile memory, or a combination thereof. The memory  1106  may include a plurality of memory devices that are physically independent from each other. The volatile memory is, for example, a Static Random Access Memory (SRAM), a Dynamic RAM (DRAM), or a combination thereof. The non-volatile memory is, for example, a mask Read Only Memory (MROM), an Electrically Erasable Programmable ROM (EEPROM), a flash memory, a hard disc drive, or any combination thereof. The memory  1106  may include, for example, an external memory device that can be accessed from the baseband processor  1103 , the application processor  1104 , and the SoC  1105 . The memory  1106  may include, for example, an internal memory device that is integrated in the baseband processor  1103 , the application processor  1104 , or the SoC  1105 . Further, the memory  1106  may include a memory in a Universal Integrated Circuit Card (UICC). 
     The memory  1106  may store software modules (computer programs) including instructions and data for performing the processing by the radio terminal  21  described in the aforementioned embodiments. In some implementations, the baseband processor  1103  or the application processor  1104  may load the software modules from the memory  1106  and execute these loaded software modules, thereby performing the processing of the radio terminal  21  described with reference to the sequence diagrams and the flowcharts in the aforementioned embodiments. 
       FIG. 14  is a block diagram showing a configuration example of the base station  40  according to the aforementioned embodiments. Referring to  FIG. 12 , the base station  40  includes an RF transceiver  1201 , a network interface  1203 , a processor  1204 , and a memory  1205 . The RF transceiver  1201  performs analog RF signal processing in order to communicate with the radio terminal  21 . The RF transceiver  1201  may include a plurality of transceivers. The RF transceiver  1201  is coupled to an antenna  1202  and the processor  1204 . The RF transceiver  1201  receives modulated symbol data (or OFDM symbol data) from the processor  1204 , generates a transmission RF signal, and supplies the transmission RF signal to the antenna  1202 . Further, the RF transceiver  1201  generates a baseband reception signal based on a reception RF signal received by the antenna  1202 , and supplies the baseband reception signal to the processor  1204 . 
     The network interface  1203  is used to communicate with a network node (e.g., Mobility Management Entity (MME) and a Serving Gateway (S-GW)). The network interface  1203  may include, for example, a network interface card (NIC) conforming to the IEEE 802.3 series. 
     The processor  1204  performs digital baseband signal processing (data plane processing) and control plane processing for radio communication. In the case of LTE and LTE-Advanced, for example, the digital baseband signal processing performed by the processor  1204  may include signal processing of the PDCP layer, the RLC layer, the MAC layer, and the PHY layer. Further, the control plane processing performed by the processor  1204  may include processing of the  51  protocol, the RRC protocol, and MAC CE. 
     The processor  1204  may include a plurality of processors. The processor  1204  may include, for example, a modem processor (e.g., a DSP) that performs the digital baseband signal processing and a protocol stack processor (e.g., a CPU or an MPU) that performs the control plane processing. 
     The memory  1205  is composed of a combination of a volatile memory and a non-volatile memory. The volatile memory is, for example, an SRAM, a DRAM, or a combination thereof. The non-volatile memory is, for example, an MROM, a PROM, a flash memory, a hard disc drive, or a combination thereof. The memory  1205  may include a storage that is spaced apart from the processor  1204 . In this case, the processor  1204  may access the memory  1205  via the network interface  1203  or an I/O interface (not shown). 
     The memory  1205  may store software modules (computer programs) including instructions and data for performing processing by the base station  40  described in the aforementioned embodiments. In some implementations, the processor  1204  may load these software modules from the memory  1205  and execute these loaded software modules, thereby performing processing of the base station  40  described with reference to the sequence diagrams and the flowcharts in the aforementioned embodiments. 
       FIG. 15  is a block diagram showing a configuration example of the D2D communication control apparatus  10  according to the aforementioned embodiments. Referring to  FIG. 15 , the D2D communication control apparatus  10  includes a network interface  1301 , a processor  1302 , and a memory  1303 . The network interface  1301  is used to communicate with the radio terminal  21 . The network interface  1301  may include, for example, a network interface card (NIC) conforming to the IEEE 802.3 series. 
     The processor  1302  loads software (computer program) from the memory  1303  and executes the loaded software, thereby performing the processing of the D2D communication control apparatus  10  described with reference to the sequence diagrams and flowcharts in the aforementioned embodiments. The processor  1302  may include, for example, a microprocessor, an MPU, or a CPU. The processor  1302  may include a plurality of processors. 
     The memory  1303  is composed of a combination of a volatile memory and a non-volatile memory. The memory  1303  may include a storage spaced apart from the processor  1302 . In this case, the processor  1302  may access the memory  1303  via an I/O interface (not shown). 
     In the example shown in  FIG. 15 , the memory  1303  is used to store software modules including a control module for D2D communication. The processor  1302  loads these software modules from the memory  1303  and executes these loaded software modules, thereby performing the processing of the D2D communication control apparatus  10  described in the aforementioned embodiments. 
     As described above with reference to  FIGS. 13 to 15 , each of the processors included in the radio terminal  21 , the base station  40 , and the D2D communication control apparatus  10  according to the aforementioned embodiments executes one or more programs including instructions to cause a computer to perform an algorithm described with reference to the drawings. The program(s) can be stored and provided to a computer using any type of non-transitory computer readable media. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as flexible disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g., magneto-optical disks), Compact Disc Read Only Memory (CD-ROM), CD-R, CD-R/W, and semiconductor memories (such as mask ROM, Programmable ROM (PROM), Erasable PROM (EPROM), flash ROM, Random Access Memory (RAM), etc.). The program(s) may be provided to a computer using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g., electric wires, and optical fibers) or a wireless communication line. 
     Further, the aforementioned embodiments may be executed independently from each other or may be combined with each other as appropriate. 
     Note that the present invention is not limited to the aforementioned embodiments and may be changed as appropriate without departing from the spirit of the present invention. 
     While the present invention has been described above with reference to the embodiments, the present invention is not limited to the aforementioned embodiments. Various changes that may be understood by those skilled in the art within the scope of the invention may be made to the configurations and the details of the present invention. 
     REFERENCE SIGNS LIST 
     
         
           10  D2D COMMUNICATION CONTROL APPARATUS 
           11  COMMUNICATION UNIT 
           12  SELECTION UNIT 
           13  TRANSMISSION DATA PROCESSING UNIT 
           14  RECEPTION DATA PROCESSING UNIT 
           21  RADIO TERMINAL 
           22  RADIO TERMINAL 
           23  RADIO TERMINAL 
           24  RADIO TERMINAL 
           30  CORE NETWORK 
           40  BASE STATION 
           41  CELL 
           42  COVERAGE HOLE 
           50  BASE STATION 
           51  COMMUNICATION UNIT 
           52  RADIO COMMUNICATION UNIT 
           60  RADIO TERMINAL 
           61  RADIO COMMUNICATION UNIT 
           65  RADIO TERMINAL 
           70  BASE STATION