Patent Publication Number: US-2018035278-A1

Title: Apparatus and method for proximity-based service communication

Description:
TECHNICAL FIELD 
     The present application relates to Proximity-based services (ProSe) and, in particular, to direct discovery and direct communication that are performed by using a direct interface between radio terminals. 
     BACKGROUND ART 
     The 3GPP Release 12 specifies Proximity-based services (ProSe) (see, for example. Non-patent Literature 1). ProSe includes ProSe discovery and ProSe direct communication. ProSe discovery makes it possible to detect proximity of radio terminals. ProSe discovery includes direct discovery (ProSe Direct Discovery) and network-level discovery (EPC-level ProSe Discovery). 
     ProSe Direct Discovery is performed through a procedure in which a radio terminal capable of performing ProSe (i.e., ProSe-enabled UE) detects another ProSe-enabled UE by using only the capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) possessed by these UEs. On the other hand, in EPC-level ProSe Discovery, a core network (i.e., Evolved Packet Core (EPC)) determines proximity of two ProSe-enabled UEs and notifies these UEs of detection of proximity. ProSe Direct Discovery may be performed by three or more ProSe-enabled UEs. 
     ProSe direct communication enables establishment of a communication path between two or more ProSe-enabled UEs existing in a direct communication range after the ProSe discovery procedure is performed. In other words, ProSe direct communication enables a ProSe-enabled UE to directly communicate with another ProSe-enabled UE, without communicating through a Public Land Mobile Network (PLMN) including a base station (eNodeB). ProSe direct communication may be performed by using a radio communication technology that is also used to access a base station (eNodeB) (i.e., E-UTRA technology) or by using a wireless radio access network (WLAN) radio technology (i.e., IEEE 802.11 radio technology). 
     According to 3GPP Release 12, a ProSe function communicates with a ProSe-enabled UE through a Public Land Mobile Network (PLMN) and assists ProSe discovery and ProSe direct communication. The ProSe function is a logical function that is used for 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 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). The ProSe function may be implemented in one or more network nodes or entities. In this specification, one or more network nodes or entities that implement the ProSe function are referred to as a “ProSe function entity” or a “ProSe function server”. 
     As described above, ProSe direct discovery and ProSe direct communication are performed on an inter-UE direct interface. This direct interface is referred to as a PC5 interface or a sidelink. Hereinafter, in this specification, communication including at least one of direct discovery and direct communication is referred to as “sidelink communication”. 
     A UE needs to communicate with a ProSe function before performing sidelink communication (see Non-patent Literature 1). In order to perform ProSe direct communication and ProSe direct discovery, the UE has to communicate with the ProSe function and acquire authentication information by the PLMN from the ProSe function in advance. Further, in the case of ProSe direct discovery, the UE has to transmit a discovery request to the ProSe function. Specifically, when the UE desires transmission (announcement) of discovery information on the sidelink, the UE transmits to the ProSe function a discovery request for the announcement. On the other hand, when the UE desires reception (monitoring) of discovery information on the sidelink, the UE transmits to the ProSe function a discovery request for the monitoring. Then, when the discovery request has succeeded, the UE is permitted to transmit or receive the discovery information on the inter-UE direct interface (e.g., sidelink or PC5 interface). 
     The allocation of radio resources for the sidelink communication to n UE is performed by a radio access network (e.g., Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) (see Non-patent Literatures 1 and 2). The UE which is permitted to perform the sidelink communication by the ProSe function performs ProSe direct discovery or ProSe direct communication by using radio resources configured by a radio access network node (e.g., eNodeB). Sections 23.10 and 23.11 of Non-patent Literature 2 describe details of the allocation of radio resources for the sidelink communication to a UE. 
     Regarding ProSe direct communication, two resource allocation modes, i.e., Scheduled resource allocation and Autonomous resource selection are specified. In the Scheduled resource allocation for ProSe direct communication, a UE requests an eNodeB to allocate resources and the eNB schedules resources for sidelink control and data for the UE. Specifically, the UE sends to the eNodeB a scheduling request together with a ProSe Buffer Status Report (BSR). 
     In the Autonomous resource selection of ProSe direct communication, a UE autonomously selects resources for sidelink control and data from a resource pool(s). An eNodeB may allocate a resource pool(s) for the Autonomous resource selection to a UE in a System Information Block (SIB)  18 . The eNodeB may allocate a resource pool for the Autonomous resource selection to a UE in Radio Resource Control (RRC)_CONNECTED via dedicated RRC signaling. This resource pool may be available when the UE is in RRC_IDLE. 
     Regarding ProSe direct discovery, two resource allocation modes, i.e., Scheduled resource allocation and Autonomous resource selection are also specified. In the Autonomous resource selection for ProSe direct discovery, a UE that desires transmission (announcement) of discovery information autonomously selects radio resources from a resource pool(s) for announcement. This resource pool is configured in UEs via broadcast (SIB  19 ) or dedicated signaling (RRC signaling). 
     In the Scheduled resource allocation for ProSe direct discovery, a UE requests an eNodeB to allocate resources for announcement via RRC signaling. The eNB allocates resources for announcement from a resource pool that is configured in UEs for monitoring. When the Scheduled resource allocation is used, the eNB indicates in SIB  19  that it provides resources for monitoring of ProSe direct discovery but does not provide resources for announcement. 
     Note that 3GPP Release 12 ProSe is one example of proximity-based services (ProSe) that are provided based on geographic proximity of a plurality of radio terminals. Similarly to 3GPP Release 12 ProSe, the proximity-based service in a public land mobile network (PLMN) includes discovery and direct-communication phases assisted by a function or a node (e.g., ProSe function) located in the network. In the discovery phase, geographic proximity of radio terminals is determined or detected. In the direct communication phase, the radio terminals perform direct communication. The direct communication is performed between radio terminals in proximity to each other, without communicating through a public land mobile network (PLMN). The direct communication is also referred to as “device-to-device (D2D) communication” or “peer-to-peer communication”. In this specification, the term “ProSe” is not limited to 3GPP Release 12 ProSe and refers to proximity-based service communication including at least one of discovery and direct communication. Further, each of the terms “proximity-based service communication” and “ProSe communication” in this specification refers to at least one of the discovery and the direct communication. 
     The term “public land mobile network (PLMN)” in this specification indicates a wide-area radio infrastructure network, and means a multiple-access mobile communication system. The multiple-access mobile communication system enables mobile terminals to perform radio communication substantially simultaneously by sharing radio resources including at least one of time resources, frequency resources, and transmission power resources among the mobile terminals. Typical examples of multiple-access technology Include Time Division Multiple Access (TDMA). Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and any combination thereof. The public land mobile network includes a radio access network and a core network. Examples of the public land mobile network include a 3GPP Universal Mobile Telecommunications System (UMTS), a 3GPP Evolved Packet System (EPS), a 3GPP2 CDMA2000 system, a Global System for Mobile communications (GSM (Registered Trademark))/General packet radio service (GPRS) system, a WiMAX system, and a mobile WiMAX system. The EPS includes a Long Term Evolution (LTE) system and an LTE-Advanced system. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2005-033536 
     Non Patent Literature 
     Non-patent Literature 1: 3GPP TS 23.303 V12.3.0 (2014-12), “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Proximity-based services (ProSe); Stage 2 (Release 12)”, December 2014
 
Non-patent Literature 2: 3GPP TS 36.300 V12.4.0 (2014-12), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 12)”, December 2014
 
     SUMMARY OF INVENTION 
     Technical Problem 
     Non-patent Literatures 1 and 2 describes that a base station (eNodeB) allocates radio resources for the sidelink communication including direct discovery and direct communication. However, Non-patent Literatures 1 and 2 do not describe that the location of a radio terminal (UE) is taken into account in a base station (eNodeB) when permitting the sidelink communication or allocating radio resources for the sidelink communication. 
     Patent Literature 1 describes that when a switch node receives calling information from a sender radio terminal through a base station, it inquires a subscriber database about the locations of the sender and receiver radio terminals. The switch node disclosed in Patent Literature 1 instructs the sender radio terminal to activate direct communication when the sender and receiver radio terminals are located in the same base station area (cell) or adjacent base station areas. However, Patent Literature 1 also does not disclose that a radio access network node (e.g., base station) located in a radio access network takes into account the location of a radio terminal when it permits sidelink communication or allocates radio resources for the sidelink communication. 
     If the location of a radio terminal is not taken into account in a base station when it permits the sidelink communication or allocates radio resources for the sidelink communication, the following problem or inconvenience may occur. For example, in the case where two radio terminals are distant from each other though they are located in the same cell (i.e., intra-cell) or adjacent cells (i.e., inter-cell), these two radio terminals cannot perform communication even if a radio access network node allocates radio resources for the sidelink communication. As a result, this would cause waste of radio resource and waste of battery power of the radio terminals. Further, for example, when the location of a radio terminal is not taken into account by a radio access network node, it may be impossible to make an appropriate radio setting (e.g., transmission power, a modulation scheme, or a coding rate) according to the inter-terminal distance between radio terminals that perform the sidelink communication. 
     One of the objects to be attained by embodiments disclosed herein is to provide an apparatus, a method, and a program that contribute to improving sidelink communication including direct discovery and direct communication. 
     Solution to Problem 
     In a first aspect, a radio access network node, located in a radio access network, includes a memory and at least one processor coupled to the memory. The at least one processor is configured to detect an event that triggers an occurrence of sidelink communication including at least one of direct discovery and direct communication. Further, the at least one processor is configured to acquire, in response to detection of the event, location information of at least one of a plurality of radio terminals that participate in the sidelink communication. 
     In a second aspect, a radio terminal includes a memory and at least one processor coupled to the memory. The at least one processor is configured to transmit to a radio access network node an indication regarding sidelink communication including at least one of direct discovery and direct communication. Further, the at least one processor is configured to transmit location information of the radio terminal to the radio access network node in response to a request from the radio access network node that has received the indication. Furthermore, the at least one processor is configured to receive from the radio access network node a message indicating whether the sidelink communication is permitted or not, or indicating a radio setting for the sidelink communication. 
     In a third aspect, a method performed by a radio access network node includes: (a) detecting an event that triggers an occurrence of sidelink communication including at least one of direct discovery and direct communication; and (b) acquiring, in response to detection of the event, location information of at least one of a plurality of radio terminals that participate in the sidelink communication. 
     In a fourth aspect, a method performed by a radio terminal includes: (a) transmitting to a radio access network node an indication regarding sidelink communication including at least one of direct discovery and direct communication to a radio access network node; (b) transmitting location information of the radio terminal to the radio access network node in response to a request from the radio access network node that has received the indication; and (c) receiving from the radio access network node a message indicating whether the sidelink communication is permitted or not, or indicating a radio setting for the sidelink communication. 
     In a fifth aspect, a radio access network node includes a memory and at least one processor coupled to the memory. The at least one processor is configured to take into account location information of at least one of a plurality of radio terminals that participate in sidelink communication including at least one of direct discovery and direct communication, when activating the sidelink communication, when permitting the sidelink communication, when allocating a radio resource for the sidelink communication, or when determining a radio setting for the sidelink communication. 
     In a sixth aspect, a method performed by a radio access network node includes taking into account location information of at least one of a plurality of radio terminals that participate in sidelink communication including at least one of direct discovery and direct communication, when activating a sidelink communication, when permitting the sidelink communication, when allocating a radio resource for the sidelink communication, or when determining a radio setting for the sidelink communication. 
     In a seventh aspect, a program includes a set of Instructions (software codes) that, when loaded into a computer, causes the computer to perform a method according to the above-described third, fourth or sixth aspect, 
     Advantageous Effects of Invention 
     According to the above-described aspects, it is possible to provide an apparatus, a method, and a program that contribute to improving sidelink communication including direct discovery and direct communication. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a configuration example of a public land mobile network according to several embodiments; 
         FIG. 2  shows a configuration example of a public land mobile network according to several embodiments; 
         FIG. 3  is a flowchart showing an example of an operation of a radio access network node (e.g., eNodeB) according to a first embodiment; 
         FIG. 4  is a sequence diagram showing an example of a control procedure regarding sidelink communication according to the first embodiment; 
         FIG. 5  is a sequence diagram showing an example of a control procedure regarding sidelink communication according to the first embodiment; 
         FIG. 6  is a sequence diagram showing an example of a control procedure regarding sidelink communication according to the first embodiment; 
         FIG. 7  is a sequence diagram showing an example of a control procedure regarding sidelink communication according to the first embodiment; 
         FIG. 8  is a flowchart showing an example of an operation of a radio access network node (e.g., eNodeB) according to a second embodiment; 
         FIG. 9  is a sequence diagram showing an example of a control procedure regarding sidelink communication according to the second embodiment; 
         FIG. 10  is a flowchart showing an example of an operation of a radio terminal (e.g., UE) according to the second embodiment; 
         FIG. 11  is a flowchart showing an example of an operation of a radio access network node (e.g., eNodeB) according to a third embodiment; 
         FIG. 12  is a sequence diagram showing an example of a control procedure regarding sidelink communication according to the third embodiment; 
         FIG. 13  is a flowchart showing an example of an operation of a radio terminal (e.g., UE) according to the third embodiment; 
         FIG. 14  is a block diagram showing a configuration example of a radio access network node (e.g., eNodeB) according to several embodiments; and 
         FIG. 15  is a block diagram showing a configuration example of a radio terminal (e.g., UE) according to several embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Specific embodiments are described hereinafter in detail with reference to the drawings. The same or corresponding elements are denoted by the same symbols throughout the drawings, and duplicated explanations are omitted as necessary for the sake of clarity. 
     Embodiments described below will be explained mainly using specific examples with regard to an Evolved Packet System (EPS). However, these embodiments are not limited to being applied to the EPS and may also be applied to other mobile communication networks or systems such as a 3GPP (UMTS), a 3GPP2 CDMA2000 system, a GSM/GPRS system, and a WiMAX system. 
     First Embodiment 
       FIG. 1  shows a configuration example of a PLMN  100  according to this embodiment. Both a UE  1 A and a UE  1 B are radio terminals capable of performing ProSe (i.e., ProSe-enabled UEs), and they can perform sidelink communication on an inter-terminal direct Interface (i.e., PCS inter face or sidelink)  103 . This sidelink communication includes at least one of ProSe direct discovery and ProSe direct communication. The sidelink communication is performed by using a radio communication technology that is also used to access a base station (eNodeB)  21  (e.g., E-UTR A technology). 
     The eNodeB  21  is an entity located in a radio access network (i.e., E-UTRAN)  2 , manages a cell  22 , and is able to perform communication ( 101  and  102 ) with the UEs  1 A and  1 B by using the E-UTRA technology. While  FIG. 1  shows a situation where both the UE  1 A and UE  1 B are located in the identical cell  22  for the sake of clarity, such a UE arrangement is merely an example. For example, the UE  1 A may be located within one of adjacent cells managed by different eNodeBs  21  and the UE  1 B may be located within the other of the adjacent cells. 
     A core network (i.e., EPC)  3  includes a plurality of user-plane entities (e.g., Serving Gateway (S-GW) and a Packet Data Network Gateway (P-GW)) and a plurality of control-plane entities (e.g., Mobility Management Entity (MME) and a Home Subscriber Server (HSS)). The user-plane entities relay user data of the UEs  1 A and  1 B between the E-UTRAN  2  and an external network (Packet Data Network (PDN)). The control-plane entitles perform various types of control for the UEs  1 A and  1 B including mobility management, session management (bearer management), subscriber information management, and billing management. 
     In order to use a ProSe service (e.g., EPC-level ProSe Discovery, ProSe Direct Discovery, or ProSe Direct Communication), each of the UEs  1 A and  1 B attaches to the EPC  3  through the E-UTRAN  2 , establishes a Packet Data Network (PDN) connection for communicating with a ProSe function entity  4 , and transmits and receives ProSe control signaling to and from the ProSe function entity  4  through the E-UTRAN  2  and the EPC  3 . The UEs  1 A and  1 B may use EPC-level ProSe Discovery provided by the ProSe function entity  4 . The UEs  1 A and  1 B may receive, from the ProSe function entity  4 , a message indicating permission for the UEs  1 A and  1 B to activate (enable) ProSe Direct Discovery or ProSe Direct Communication. The UEs  1 A and  1 B may receive, from the ProSe function entity  4 , configuration information for ProSe Direct Discovery or ProSe Direct Communication in the cell  22 . 
       FIG. 2  shows reference points used for ProSe. Each reference point may also be referred to as an “interface”.  FIG. 2  shows a non-roaming architecture in which the UEs  1 A and  1 B use a subscription of the same PLMN  100 . 
     A PC 1  reference point is a reference point between a ProSe application in each UE  1  (UEs  1 A and  1 B) and a ProSe application server  5 . The PC 1  reference point is used to define application-level signaling requirements. 
     A PC 2  reference point is a reference point between the ProSe application server  5  and the ProSe function entity  4 . The PC 2  reference point is used to define interactions between the ProSe application server  5  and the ProSe functionality provided by the 3GPP EPS through the ProSe function entity  4 . 
     A PC 3  reference point is a reference point between each UE  1  (UEs  1 A and  1 B) and the ProSe function entity  4 . The PC 3  reference point is used to define interactions between the UE  1  and the ProSe function entity  4  (e.g., UE registration, application registration, and authorizations for ProSe Direct Discovery and EPC-level ProSe Discovery requests). The PC 3  reference point depends on the user plane of the EPC  3  and, accordingly, ProSe control signaling between each UE  1  and the ProSe function entity  4  is transferred on this user plane. 
     A PC 4   a  reference point is a reference point between the ProSe function entity  4  and an HSS  33 . This reference point is used by the ProSe function entity  4 , for example, to acquire subscriber information related to ProSe services. 
     A PC 4   b  reference point is a reference point between the ProSe function entity  4  and a Secure User Plane Location (SUPL) Location Platform (SLP)  34 . This reference point is used by the ProSe function entity  4 , for example, to acquire location information of each UE  1  (UEs  1 A and  1 B). The SLP assists the UEs  1  in GPS positioning and receives measurement results from the UEs  1 , thereby intermittently acquiring, from the UE  1 , location reporting by which the current locations of the UEs  1  can be estimated. 
     A PC 5  reference point is a reference point between UEs  1  (ProSe-enabled UEs), and is used for the control and user planes of ProSe Direct Discovery, ProSe Direct Communication, and ProSe UE-to-Network Relay. 
     Next, a control procedure regarding the sidelink communication according to this embodiment is described.  FIG. 3  is a flowchart showing an example (process  300 ) of an operation of an eNodeB  21  regarding the sidelink communication. In block  301 , the eNodeB  21  detects an event that triggers an occurrence of sidelink communication. 
     For example, the event that triggers sidelink communication may be reception by the eNodeB  21  of an indication regarding the sidelink communication transmitted from one of the UEs  1  that participate in the sidelink communication. The indication regarding the sidelink communication may indicate that the UE  1  wants to perform the sidelink communication or that the UE  1  has an interest in the sidelink communication. Specifically, the indication regarding the sidelink communication may be a ProSe Direct indication indicating an interest in ProSe direct communication or may be a Discovery Indication indicating that the UE  1  wants to perform a ProSe direct discovery announcement. Alternatively, the indication regarding the sidelink communication may indicate a radio resource allocation request for the sidelink communication. Specifically, the indication regarding the sidelink communication may be a scheduling request (e.g., scheduling request with a ProSe BSR) for the sidelink communication transmitted from the UE  1  to the eNodeB  21 . 
     Alternatively, the event that triggers sidelink communication may be reception by the eNodeB  21  of a predetermined message transmitted from a control entity (e.g., ProSe function entity  4  or MME  31 ) that relates to the sidelink communication and is located in a higher-level network (e.g., EPC  3 ). 
     In block  302 , in response to the detection of the event in block  301 , the eNodeB  21  acquires location information of at least one of the plurality of UEs  1  (e.g., UE  1 A or  1 B) that participate in the sidelink communication. The acquisition of the location information of the UE(s)  1  by the eNodeB  21  is performed before the start of the sidelink communication. This location information is acquired by the UE  1  and indicates the current or latest location of the UE  1 . The eNodeB  21  may receive the location information of the UE  1  directly from the UE  1  (i.e., via RRC signaling on an LTE-Uu interface). Alternatively, the eNodeB  21  may receive the location information of the UE  1  through a server. 
     The location information of the UE  1  preferably indicates a more detailed location than a cell-level location. For example, the location information of the UE  1  may include Global Navigation Satellite System (GNSS) location information that is obtained by a GNSS receiver possessed by the UE  1 . The GNSS location information indicates latitude and longitude. Additionally or alternatively, the location information of the UE  1  may include Radio Frequency (RF) fingerprints. The RF fingerprints include information about peripheral cell measurement (e.g., cell ID (ECG1 or Cell-Id) and Reference Signal Received Power (RSRP)) measured by the UE  1 . 
     The eNodeB  21  may acquire the location information of some or all of the plurality of UEs  1  that participate in the sidelink communication. For example, when the eNodeB  21  has already acquired detailed locations of some of the plurality of UEs  1  that participate in the sidelink communication by an RRC measurement report or the like, the eNodeB  21  may acquire the location information of one or more of the remaining UEs  1  regarding which the eNodeB  21  still has not obtained detailed locations. 
     By the operation shown in  FIG. 3 , the eNodeB  21  can take into account the location information of the UE(s)  1  when the eNodeB  21  performs various processes regarding the sidelink communication. In some implementations, the eNodeB  21  may take into account the location information of the UE(s)  1  when the eNodeB  21  determines whether to activate the sidelink communication, whether to permit the sidelink communication, or whether to allocate radio resources for the sidelink communication. For example, when the inter-terminal distance between the UEs  1 A and  1 B estimated from their location information is equal to or shorter than a threshold, the eNodeB  21  may allocate radio resources for the sidelink communication between the UEs  1 A and  1 B. Conversely, when the inter-terminal distance between the UEs  1 A and  1 B exceeds the threshold, the eNodeB  21  may reject a radio resource allocation request for the sidelink communication sent from the UE  1 A or  1 B. 
     In some implementations, the eNodeB  21  may take into account the location information of the UE(s)  1  when determining a radio setting for the sidelink communication. For example, the eNodeB  21  may determine a radio setting for the sidelink communication between the UEs  1 A and  1 B according to an inter-terminal distance between the UEs  1 A and  1 B estimated from their location information. This radio setting may designate at least one of a frequency resource, a time resource, transmission power, a modulation scheme, and a coding rate. 
     More specifically, the eNodeB  21  may increase transmission power permitted for the sidelink communication as the inter-terminal distance increases. Additionally or alternatively, when the inter-terminal distance exceeds a threshold, the eNodeB  21  may apply, to the sidelink communication, a modulation scheme having a smaller required carrier-to-noise ratio (CNR) than a modulation scheme used when the inter-terminal distance is shorter than the threshold. To put it differently, the adoption of a modulation scheme having a smaller required CNR means the adoption of a modulation scheme having a larger inter-point distance on a constellation (usually means a lower transmission speed). Additionally or alternatively, when the inter-terminal distance exceeds a threshold, the eNodeB  21  may apply, to the sidelink communication, a lower coding rate than when the inter-terminal distance is shorter than the threshold. 
       FIG. 4  is a sequence diagram showing an example (process  400 ) of an operation for acquiring the location information of the UE(s)  1  performed by the eNodeB  21 . In block  401 , the eNodeB  21  receives from the UE  1 A an indication regarding the sidelink communication (e.g., ProSe Direct indication, Discovery Indication, or scheduling request with ProSe BSR). In block  402 , in response to the reception of the indication regarding the sidelink communication sent from the UE  1 A, the eNodeB  21  requests the UE  1 A to send its location information. In block  403 , the eNodeB  21  receives the location information from the UE  1 A. 
       FIG. 5  is a sequence diagram showing an example (process  500 ) of an operation for acquiring location information of the UE(s)  1  performed by the eNodeB  21 . The process  500  shown in  FIG. 5  is a specific example of the process  400  shown in  FIG. 4 . In the example shown in  FIG. 5 , the eNodeB  21  acquires location information of the UE  1  by using an existing RRC measurement procedure. Note that, an RRC measurement that is extended so as to include location information can be used for Minimization of Drive Tests (MDT) and is also referred to as an Immediate MDT measurement report (measurement information). 
     In block  501 , the eNodeB  21  receives an indication regarding the sidelink communication from the UE  1 A. In block  502 , in response to the reception of the indication regarding the sidelink communication sent from the UE  1 A (block  501 ), the eNodeB  21  transmits an RRC CONNECTION RECONFIGURATION message to the UE  1 A. This RRC CONNECTION RECONFIGURATION message contains an “INCLUDE LOCATION INFO (includeLocationInfo)” information element (IE), The “INCLUDE LOCATION INFO” IE is designated in an RRC Measurement Configuration by the eNodeB  21  to request the UE  1  to include its location information in an RRC measurement report. 
     In block  503 , the UE  1 A transmits a response message (i.e., RRC CONNECTION RECONFIGURATION COMPLETE) to the RRC CONNECTION RECONFIGURATION message (block  502 ). In block  504 , the UE  1 A transmits to the eNodeB  21  an RRC measurement report including its location information. 
       FIG. 6  is a sequence diagram showing an example (process  600 ) of an operation for acquiring location information of the UE(s)  1  performed by the eNodeB  21 . The process  500  shown in  FIG. 5  is a specific example of the process  400  shown in  FIG. 4 . In the example shown in  FIG. 6 , the eNodeB  21  acquires location information of the UE  1  by using a UE information procedure, which is one of existing RRC procedures. 
     In block  601 , the eNodeB  21  receives an indication regarding the sidelink communication from the UE  1 A. In block  602 , in response to the reception of the indication regarding the sidelink communication sent from the UE  1 A (block  601 ), the eNodeB  21  transmits a UE INFORMATION REQUEST message to the UE  1 A. This UE INFORMATION REQUEST message includes a “LOGGED MEASUREMENT REPORT REQUEST (logMeasReportReq)” IE. The “LOGGED MEASUREMENT REPORT REQUEST” IE is used to request the UE  1  to report logged measurement information stored in the UE  1  to the eNodeB  21 . The logged measurement information can be used for the MDT and is also referred to as a Logged MDT measurement report (measurement information). The logged measurement information includes location information (e.g., GNSS location information) of the UE  1  at the time when radio measurement is performed. 
     In block  603 , in response to the UE INFORMATION REQUEST message (block  602 ), the UE  1 A transmits a UE INFORMATION RESPONSE message. This UE INFORMATION RESPONSE message includes logged measurement information including the location information of the UE  1 A. 
     According to the procedure shown in  FIG. 5 or 6 , the eNodeB  21  can use an ordinary RRC procedure specified in the current 3GPP specifications to acquire the location information of the UE  1 , thereby reducing the impacts on the existing specifications regarding the UE  1 . 
       FIG. 7  is a sequence diagram showing an example (process  700 ) of an operation for acquiring location information of the UE  1  performed by the eNodeB  21 . As shown in  FIG. 7 , the eNodeB  21  may receive location information of the UE(s)  1  through a server. In the example shown in  FIG. 7 , the eNodeB  21  receives an indication regarding the sidelink communication from the UE  1 A (block  701 ). Then, the eNodeB  21  requests location information of the UE(s)  1  (UE  1 A or  1 B, or both of them) from a Trace Collection Entity (TCE)  71  (block  702 ) and acquires this location information from the TCE  71  (block  703 ). The Trace Collection Entity (TCE) is a node that collects Logged MDT measurement information or immediate MDT measurement information. The eNodeB  21  may use the latest location information of the UE  1  included in the latest MDT measurement information collected by the TCE  71 . The server, which the eNodeB  21  accesses to acquire the location information of the UE(s)  1 , may be a server (e.g., SLP  34 ) different from the TCE. 
     According to the procedure shown in  FIG. 7 , since the eNodeB  21  acquires the location information of the UE(s)  1  through a server (e.g., TCE  71  or SLP  34 ) different from the UE  1 , the signaling between the UE(s)  1  and the eNodeB  21  can be reduced. As a result of this, the load on the UE(s)  1  can be reduced. 
     In the above-described procedures shown in  FIGS. 4 to 7 , for example, when the indication regarding the sidelink communication (block  401 ,  501 ,  601  or  701 ) indicates the UE  1 B, the eNodeB  21  may acquire location information from the UE  1 B as well as (or instead of) from the UE  1 A. 
     Second Embodiment 
     This embodiment provides a modified example of the control procedure regarding the sidelink communication described in the first embodiment. A configuration example of a public land mobile network according to this embodiment is the same as that shown in  FIGS. 1 and 2 . 
       FIG. 8  is a flowchart showing an example (process  800 ) of an operation of an eNodeB  21  according to this embodiment. Processes in blocks  801  and  802  are similar to those in blocks  301  and  302  shown in  FIG. 3 . In block  803 , the eNodeB  21  takes into account the location information of the UE  1  when it determines whether to activate sidelink communication, whether to permit the sidelink communication, or whether to allocate radio resources for the sidelink communication. 
     For example, as already described, when the inter-terminal distance between the UEs  1 A and  1 B estimated from their location information is equal to or shorter than a threshold, the eNodeB  21  may allocate radio resources for the sidelink communication between the UEs  1 A and  1 B. Conversely, when the inter-terminal distance between the UEs  1 A and  1 B exceeds the threshold, the eNodeB  21  may reject a radio resource allocation request for the sidelink communication sent from the UE  1 A or  1 B. In this way, it is possible to prevent waste of radio resources and waste of battery power of the UE(s)  1 , which would otherwise occur when the inter-terminal distance is too long and hence the UEs  1 A and  1 B cannot perform the sidelink communication. 
       FIG. 9  is a sequence diagram showing an example (process  900 ) of a sidelink control procedure according to this embodiment. Processes in blocks  901  to  903  are similar to those in blocks  401  to  403  shown in  FIG. 4 . In block  904 , the eNodeB  21  determines, based on the location information of the UE  1 A, whether to permit the sidelink communication requested by the UE  1 A or to permit allocation of radio resources to this sidelink communication. The eNodeB  21  performs either block  905 A or block  905 B according to a result of the determination in block  904 . 
     When the eNodeB  21  permits the sidelink communication or the resource allocation, the eNodeB  21  configures radio resources for the sidelink communication in the UE  1 A (block  905 A). For example, the eNodeB  21  may schedule resources for sidelink control and data for the UE  1 A in accordance with the Scheduled resource allocation of ProSe direct communication. Alternatively, the eNodeB  21  may allocate to the UE  1 A, via dedicated RRC signaling, a resource pool(s) for the Autonomous resource selection of ProSe direct communication. Alternatively, the eNodeB  21  may allocate, to the UE  1 A, resources for announcement from a resource pool that Is configured in UEs for monitoring in accordance with the Scheduled resource allocation of ProSe direct discovery. 
     On the other hand, when the eNodeB  21  rejects the sidelink communication or the resource allocation, the eNodeB  21  transmits to the UE  1 A a message indicating that the sidelink communication is rejected (block  905 B). 
     It should be noted that the procedure shown in  FIG. 9  is merely an example. As described in the first embodiment, the eNodeB  21  may acquire the location information of the UE  1 B instead of the location information of the UE  1 A. Further, the eNodeB  21  may acquire the location information of one or both of the UE  1 A and  1 B from a server. 
       FIG. 10  is a flowchart showing an example (process  1000 ) of an operation of the UE  1  according to this embodiment. In block  1001 , the UE  1  transmits an indication regarding the sidelink communication to the eNodeB  21 . In block  1002 , In response to a request from the eNodeB  21 , the UE  1  transmits Its location information to the eNodeB  21 . In block  1003 , the UE  1  receives from the eNodeB  21  a message indicating whether the sidelink communication is permitted or not. 
     In this embodiment, the eNodeB  21  takes into account the location information of the UE(s)  1  when it determines whether to activate the sidelink communication, whether to permit the sidelink communication, or whether to allocate radio resources for the sidelink communication. As a result of this, the eNodeB  21  can perform efficient control for sidelink communication (e.g., radio resource allocation) in which the location of the UE(s)  1  is taken into account. 
     Third Embodiment 
     This embodiment provides a modified example of the control procedure regarding the sidelink communication described in the first embodiment. A configuration example of a public land mobile network according to this embodiment is the same as that shown in  FIGS. 1 and 2 . 
       FIG. 11  is a flowchart showing an example (process  1100 ) of an operation of an eNodeB  21  according to this embodiment. Processes in blocks  1101  and  1102  are similar to those In blocks  301  and  302  shown in  FIG. 3 . In block  1103 , the eNodeB  21  takes into account the location Information of the UE  1  when it determines a radio setting for the sidelink communication. 
     For example, as already described, the eNodeB  21  may determine a radio setting for the sidelink communication between the UEs  1 A and  1 B according to an inter-terminal distance between the UEs  1 A and  1 B estimated from their location information. This radio setting may designate at least one of a frequency resource, a time resource, transmission power, a modulation scheme, and a coding rate. The eNodeB  21  may increase transmission power permitted for the sidelink communication as the inter-terminal distance increases. Additionally or alternatively, when the inter-terminal distance exceeds a threshold, the eNodeB  21  may apply, to the sidelink communication, a modulation scheme having a smaller required CNR than a modulation scheme used when the inter-terminal distance is shorter than the threshold. Additionally or alternatively, when the inter-terminal distance exceeds a threshold, the eNodeB  21  may apply, to the sidelink communication, a lower coding rate than when the inter-terminal distance is shorter than the threshold. 
       FIG. 12  is a sequence diagram showing an example (process  1200 ) of a sidelink control procedure according to this embodiment. Processes in blocks  1201  to  1   203  are similar to those in blocks  401  to  403  shown in  FIG. 4 . In block  1204 , the eNodeB  21  determines a radio setting for the sidelink communication for the UE  1 A based on the location information of the UE  1 A. In block  1205 , the eNodeB  21  transmits the determined radio setting to the UE  1 A. 
     It should be noted that the procedure shown in  FIG. 12  is merely an example. As described in the first embodiment, the eNodeB  21  may acquire the location information of the UE  1 B instead of the location information of the UE  1 A. Further, the eNodeB  21  may acquire the location information of one or both of the UE  1 A and  1 B from a server. 
       FIG. 13  is a flowchart showing an example (process  1300 ) of an operation of the UE  1  according to this embodiment. Processes in blocks  1301  and  1302  are similar to those In blocks  1001  and  1002  shown in  FIG. 10 . In block  1303 , the UE  1  receives a radio setting for the sidelink communication from the eNodeB  21 . 
     In this embodiment, the eNodeB  21  takes into account the location information of the UE  1  when the eNodeB  21  determines the radio setting (e.g., transmission power, modulation scheme, coding rate, or any combination of them) for the sidelink communication. As a result of this, the eNodeB  21  can make an efficient radio setting in which the location of the UE(s)  1  is taken into account. 
     Lastly, configuration examples of the base station (e.g., eNodeB  21 ) and the radio terminal (e.g., UE  1 ) according to the above-described embodiments are described. The base station (eNodeB  21 ) described in the above-described embodiments may include a wireless transceiver for communicating with radio terminals (UEs  1 ) and a controller coupled to the wireless transceiver. The controller performs processes of the base station (eNodeB  21 ) described in the above-described embodiments. 
     The radio terminal (UE  1 ) described in the above-described embodiments may include a wireless transceiver for communicating with a base station (eNodeB  21 ) and a controller coupled to the wireless transceiver. The controller performs processes of the radio terminal (UE  1 ) described in the above-described embodiments. 
       FIG. 14  is a block diagram showing a configuration example of the eNodeB  21  according to the above-described embodiments. Referring to  FIG. 14 , the eNodeB  21  includes a wireless transceiver  1401 , a network interface  1402 , a processor  1403 , and a memory  1404 . The wireless transceiver  1401  is configured to communicate with UEs  1 . The network interface  1402  is used to communicate with a network node (e.g., MME  31 , an S/P-GW  32 , and a TCE  71 ), The network interface  1402  may include, for example, a Network Interface Card (NIC) conforming to the IEEE 802.3 series. 
     The processor  1403  loads software codes (computer programs) from the memory  1404  and executes the loaded software codes, and thereby performs processes of the eNodeB  21  described in the above-described embodiments. The processor  1403  may be, for example, a microprocessor, a Micro Processing Unit (MPU), or a Central Processing Unit (CPU). The processor  1403  may include a plurality of processors. The processor  1403  may include a baseband processor and an application processor. The baseband processor performs digital baseband signal processing (i.e., data-plane processing) and control-plane processing for wireless communication. The baseband processor may include a modem processor (e.g.. Digital Signal Processor (DSP)) that performs the digital baseband signal processing and a protocol-stack-processor (e.g., CPU or an MPU) that performs the control-plane processing. Meanwhile, the application processor executes a system software program (Operating System (OS)) and various application programs (e.g., a call application, a WEB browser, a mailer, a camera operation application, and a music player application) from a memory  406  or from other memories (not shown), thereby providing various functions of the UE 1 . 
     The memory  1404  consists of a volatile memory and a nonvolatile memory. The volatile memory is, for example, a Static Random Access Memory (SRAM), a Dynamic RAM (DRAM), or a combination of them. The nonvolatile memory is, for example, a Mask Read Only Memory (MROM), a Programmable ROM (PROM), a flash memory, a hard disk drive, or any combination of them. The memory  1404  may include a storage that is remotely arranged from the processor  1403 . In this case, the processor  1403  may access the memory  1404  through the network interface  1402  or an I/O interface (not shown). 
     In the example shown in  FIG. 14 , the memory  1404  is used to store software modules including a ProSe module  1405 . The ProSe module  1405  includes instructions and data necessary for performing processes of the eNodeB  21  regarding the sidelink communication described in the above-described embodiments. The ProSe module  1405  may include a plurality of software modules. The processor  1403  loads software modules including the ProSe module  1405  from the memory  1404  and executes these loaded modules, and thereby performing the processes of the eNodeB  21  described in the above-described embodiments. 
       FIG. 15  shows a configuration example of the UE  1 . Referring to  FIG. 15 , the UE  1  includes a wireless transceiver  1501 , a processor  1502 , and a memory  1503 . The wireless transceiver  1501 , the processor  1502 , the memory  1503  or any combination thereof can be referred to as circuits or circuitry. The wireless transceiver  1501  is used for communication ( 101  or  102  in  FIG. 1 ) with the E-UTRAN  2  (eNodeB  21 ) and for ProSe direct communication ( 103  in  FIG. 1 ). The wireless transceiver  1501  may include a plurality of transceivers, for example, an E-UTRA (Long Term Evolution (LTE)) transceiver and a WLAN transceiver. 
     The processor  1502  loads software (computer program) from the memory  1503  and executes these loaded software, and thereby performs processes of the UE  1 , i.e., the processes described with reference to the sequence diagrams and the flowchart in the above-described embodiments (e.g., process  400 ,  500 ,  600 ,  700 ,  800 ,  820 ,  900  or  920 ). The processor  1502  may be, for example, a microprocessor, an MPU, or a CPU. The processor  1502  may include a plurality of processors. 
     The memory  1503  consists of a volatile memory and a nonvolatile memory. The volatile memory is, for example, an SRAM, a DRAM, or a combination of them. The nonvolatile memory is, for example, an MROM, a PROM, a flash memory, a hard disk drive, or any combination of them. The memory  1503  may include a storage that is located apart from the processor  1502 . In this case, the processor  1502  may access the memory  1503  through an I/O interface (not shown). 
     In the example shown in  FIG. 15 , the memory  1503  is used to store software modules including a ProSe module  1504 . The ProSe module  1504  includes instructions and data necessary for performing processes of the UE  1  described in the above-described embodiments. The ProSe module  1504  may include a plurality of software modules. The processor  1502  loads software modules including the ProSe module  1504  from the memory  1503  and executes these loaded modules, and thereby performing the processes of the UE  1  described in the above-described embodiments. 
     As described above with reference to  FIGS. 14 and 15 , each of the processors included in the eNodeB  21  and the UE  1  according to the above-described embodiments executes one or more programs including instructions to cause a computer to perform an algorithm described with reference to the drawings. These programs may be stored in various types of non-transitory computer readable media and thereby supplied to computers. The non-transitory computer readable media includes various types of tangible storage media. Examples of the non-transitory computer readable media include a magnetic recording medium (such as a flexible disk, a magnetic tape, and a hard disk drive), a magneto-optic recording medium (such as a magneto-optic disk), a Compact Disc Read Only Memory (CD-ROM), CD-R, CD-R/W, and a semiconductor memory (such as a mask ROM, a Programmable ROM (PROM), an Erasable PROM (EPROM), a flash ROM, and a Random Access Memory (RAM)). These programs may be supplied to computers by using various types of transitory computer readable media. Examples of the transitory computer readable media include an electrical signal, an optical signal, and an electromagnetic wave. The transitory computer readable media can be used to supply programs to a computer through a wire communication path such as an electrical wire and an optical fiber, or wireless communication path. 
     Other Embodiments 
     Each of the above-described embodiments may be used individually, or two or more of the embodiments may be appropriately combined with one another. 
     The processes of taking into account the location information of the UE(s)  1  at the time of (a) the start of sidelink communication, (b) the permission of the sidelink communication, (c) the allocation of radio resources for the sidelink communication, and (d) the configuration of a radio setting for the sidelink communication described above in the second and third embodiments may be performed by using location information of the UE  1  acquired in advance by the eNodeB  21 , before the eNodeB  21  detects an event that triggers an occurrence of the sidelink communication. That is, when the eNodeB  21  activates a sidelink communication including at least one of direct discovery and direct communication, permits the sidelink communication, allocates radio resources for the sidelink communication, or determines a radio setting for the sidelink communication, the eNodeB  21  may take into account location information of at least one of a plurality of UEs  1  that participate in the sidelink communication. 
     Further, a part or the whole of the radio setting for the sidelink communication may be performed by the UE  1  instead of the eNodeB  21 . In this case, for example, the eNodeB  21  provides information about the estimated inter-terminal distance to the UE  1 A and then the UE  1 A determines transmission power, a modulation scheme, a coding rate, or the like based on this information. Note that the information provided to the UE  1 A by the eNodeB  21  may be location information of the UE  1 B instead of the information about the estimated inter-terminal distance. 
     In the second and third embodiments, the eNodeB  21  may further take into account measurement information of an uplink radio resource acquired by any one of the plurality of UEs  1 . In some implementations, the eNodeB  21  may receive measurement information of an uplink radio resource during the procedure for acquiring the RRC measurement information (or Immediate MDT measurement report) or the Logged measurement information (or Logged MDT measurement report) described above with reference to  FIGS. 5 and 6 . In the 3GPP ProSe, a subset of uplink resources is used for sidelink communication in an in-coverage state. For example, the eNodeB  21  may take into account the uplink radio resource measurement information acquired by the UE  1  to allocate, to the sidelink communication, uplink radio resources that provide good quality in a place where the UE  1  is located. In this way, it is possible to contribute to improving the quality of the sidelink communication. 
     Further, the uplink radio resource measurement information acquired by the UE  1  may be taken into account by the eNodeB  21  independently of the location information of the UE  1 . That is, when the eNodeB  21  activates the sidelink communication including at least one of direct discovery and direct communication, permits the sidelink communication, allocates radio resources for the sidelink communication, or determines a radio setting for the sidelink communication, the eNodeB  21  may take into account the uplink radio resource measurement information acquired by a plurality of UEs  1  that participate in the sidelink communication. 
     In the above-described embodiments, cases where the UEs  1 A and  1 B that perform sidelink communication are located in the same cell (i.e., intra-cell) are described based on  FIG. 1 . However, the UEs  1 A and  1 B may be located in different cells (e.g., adjacent cells) from each other (i.e., inter-cell). In this case, the eNodeB  21  may perform radio resource allocation and a radio setting for the Inter-cell sidelink communication based on location information of a UE  1  located in its own cell  22 , or location information of a UE  1  located in a cell managed by another eNodeB, or both of them. 
     The above-described embodiments are described by using specific examples mainly related to the EPS. However, these embodiments may be applied to other mobile communication systems such as a Universal Mobile Telecommunications System (UMTS), a 3GPP2 CDMA2000 system (1×RTT, High Rate Packet Data (HRPD)), a Global System for Mobile communications (GSM)/General packet radio service (GPRS) system, and a mobile WiMAX system. In this case, the processes or the procedures regarding sidelink communication performed by the eNodeB  21  described in the above-described embodiments may be performed by a radio access network node having a radio resource management function (e.g., Radio Network Controller (RNC) in a UMTS or Base Station Controller (BSC) in a GSM system). 
     Further, the above-described embodiments are merely examples of applications of the technical ideas obtained by the inventor. Needless to say, these technical ideas are not limited to the above-described embodiments and various modifications can be made thereto. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-036287, filed on Feb. 26, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
       1 A,  1 B User Equipment (UE) 
       2  Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 
       3  Evolved Packet Core (EPC) 
       4  Proximity-based Services (ProSe) function entity 
       5  ProSe application server 
       21  evolved NodeB (eNodeB) 
       22  cell 
       33  Home Subscriber Server (HSS) 
       34  Secure User Plane Location (SUPL) Location Platform (SLP) 
       71  Trace Collection Entity (TCE) 
       100  Public Land Mobile Network (PLMN) 
       103  inter-UE direct interface (sidelink)