Patent Publication Number: US-10327199-B2

Title: Device-to-device (D2D) operation method performed by terminal in wireless communication system, and terminal using same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2015/004444, filed on Apr. 30, 2015, which claims the benefit of U.S. Provisional Application No. 61/986,907, filed on May 1, 2014, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to wireless communication, and more specifically, to a method for device-to-device (D2D) operation performed by a User Equipment (UE) in a wireless communication system, and a UE using the method. 
     Related Art 
     In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state. 
     For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced. 
     There is a growing interest in a Device-to-Device (D2D) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network. 
     The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D operation. 
     D2D operation may have various advantages in that it is communication between devices in proximity. For example, D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in D2D operation, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station. 
     Meanwhile, if a UE does not know a frequency at which it is not possible to receive a D2D signal, the UE has to monitor the D2D signal across the entire system band. Then, it may be difficult for the UE to receive the D2D signal properly and it may lead to unnecessary power consumption by the UE. In particular, this problem can happen to a UE in RRC idle state. 
     In addition, each UE may support a different PLMN based on capabilities thereof, and each cell may belong to a different PLMN. If the UE is informed of a list of frequencies at which it is not possible to transmit and receive a D2D signal, the UE does not need to receive a D2D signal at a frequency about a PLMN which is not supported by a corresponding cell. 
     SUMMARY OF THE INVENTION 
     The objective of the present disclosure is to provide a method for a device-to-device (D2D) operation performed by a UE in a wireless communication system, and a UE using the method. 
     In one aspect, provided is a method for device-to-device (D2D) operation performed by a User Equipment (UE) in a wireless communication system. The method includes receiving information indicating an allowed frequency at which reception of a D2D signal is allowed and a Public Land Mobile Network (PLMN) to which the allowed frequency is about and receiving the D2D signal based on the information. 
     The D2D signal may be a D2D discovery signal. 
     The information may be included in first system information. 
     The method may further include receiving second system that indicates a barring frequency at which reception of a D2D discovery signal is barred and a PLMN to which the barring frequency is about. 
     The first system information and the second system information may be broadcasted by a network. 
     The second system information may indicate multiple barring frequencies at which reception of a D2D signal is barred and a PLMN to which each of the multiple barring frequencies is about. 
     The information may indicate multiple allowed frequencies at which reception of a D2D signal is allowed and a PLMN to which each of the multiple allowed frequencies is about. 
     The UE may determine a frequency at which it is possible to perform D2D operation with respect to a PLMN that is determined by comparing the PLMN indicated by the information with a list of PLMNs in which it is possible to perform D2D operation, wherein the list of PLMNs is stored in the UE. 
     In another aspect, provided is a User Equipment (UE). The UE includes a Radio Frequency (RF) unit configured to transmit and receive a wireless signal and a processor configured to operate in conjunction with the RF unit. The processor is configured to receive information indicating an allowed frequency at which reception of a device-to-device (D2D) signal is allowed and a Public Land Mobile Network (PLMN) to which the allowed frequency is about, and to receive the D2D signal based on the information. 
     According to embodiments of the present disclosure, a UE is able to know a frequency at which a D2D signal is transmitted and received with respect to each PLMN. In addition, the UE is able to know a frequency at which a D2D signal is not received from each PLMN. Therefore, the UE may not need to perform unnecessary D2D signal monitoring and power consumption by the UE may be reduced. Furthermore, it may give rise to an improved success rate of D2D signal reception by the UE, thereby enhancing reliability of D2D operation for public safety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a wireless communication system to which the present invention is applied. 
         FIG. 2  is a diagram showing a wireless protocol architecture for a user plane. 
         FIG. 3  is a diagram showing a wireless protocol architecture for a control plane. 
         FIG. 4  is a flowchart illustrating the operation of UE in the RRC idle state. 
         FIG. 5  is a flowchart illustrating a process of establishing RRC connection. 
         FIG. 6  is a flowchart illustrating an RRC connection reconfiguration process. 
         FIG. 7  is a diagram illustrating an RRC connection re-establishment procedure. 
         FIG. 8  illustrates substates which may be owned by UE in the RRC_IDLE state and a substate transition process. 
         FIG. 9  shows a basic structure for ProSe. 
         FIG. 10  shows the deployment examples of types of UE performing ProSe direct communication and cell coverage. 
         FIG. 11  shows a user plane protocol stack for ProSe direct communication. 
         FIG. 12  shows the PC 5 interface for D2D direct discovery. 
         FIG. 13  is an embodiment of a ProSe discovery process. 
         FIG. 14  is another embodiment of a ProSe discovery process. 
         FIG. 15  is a method for D2D operation performed by a UE according to an embodiment of the present disclosure. 
         FIG. 16  is a diagram illustrating a method for D2D operation performed by a UE according to another embodiment of the present disclosure. 
         FIG. 17  is a diagram illustrating a method for D2D operation performed by a UE according to yet another embodiment of the present disclosure. 
         FIG. 18  is a block diagram illustrating a UE that implements an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a wireless communication system to which the present invention is applied. The wireless communication system may also be referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN) or a long term evolution (LTE)/LTE-A system. 
     The E-UTRAN includes at least one base station (BS)  20  which provides a control plane and a user plane to a user equipment (UE)  10 . The UE  10  may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT), a wireless device, etc. The BS  20  is generally a fixed station that communicates with the UE  10  and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc. 
     The BSs  20  are interconnected by means of an X2 interface. The BSs  20  are also connected by means of an S1 interface to an evolved packet core (EPC)  30 , more specifically, to a mobility management entity (MME) through S1-MME and to a serving gateway (S-GW) through S1-U. 
     The EPC  30  includes an MME, an S-GW, and a packet data network-gateway (P-GW). The MME has access information of the UE or capability information of the UE, and such information is generally used for mobility management of the UE. The S-GW is a gateway having an E-UTRAN as an end point. The P-GW is a gateway having a PDN as an end point. 
     Layers of a radio interface protocol between the UE and the network can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. Among them, a physical (PHY) layer belonging to the first layer provides an information transfer service by using a physical channel, and a radio resource control (RRC) layer belonging to the third layer serves to control a radio resource between the UE and the network. For this, the RRC layer exchanges an RRC message between the UE and the BS. 
       FIG. 2  is a diagram showing a wireless protocol architecture for a user plane.  FIG. 3  is a diagram showing a wireless protocol architecture for a control plane. The user plane is a protocol stack for user data transmission. The control plane is a protocol stack for control signal transmission. 
     Referring to  FIGS. 2 and 3 , a PHY layer provides an upper layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer which is an upper layer of the PHY layer through a transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. The transport channel is classified according to how and with what characteristics data is transferred through a radio interface. 
     Data is moved between different PHY layers, that is, the PHY layers of a transmitter and a receiver, through a physical channel. The physical channel may be modulated according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and use the time and frequency as radio resources. 
     The functions of the MAC layer include mapping between a logical channel and a transport channel and multiplexing and demultiplexing to a transport block that is provided through a physical channel on the transport channel of a MAC Service Data Unit (SDU) that belongs to a logical channel. The MAC layer provides service to a Radio Link Control (RLC) layer through the logical channel. 
     The functions of the RLC layer include the concatenation, segmentation, and reassembly of an RLC SDU. In order to guarantee various types of Quality of Service (QoS) required by a Radio Bearer (RB), the RLC layer provides three types of operation mode: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provides error correction through an Automatic Repeat Request (ARQ). 
     The RRC layer is defined only on the control plane. The RRC layer is related to the configuration, reconfiguration, and release of radio bearers, and is responsible for control of logical channels, transport channels, and PHY channels. An RB means a logical route that is provided by the first layer (PHY layer) and the second layers (MAC layer, the RLC layer, and the PDCP layer) in order to transfer data between UE and a network. 
     The function of a Packet Data Convergence Protocol (PDCP) layer on the user plane includes the transfer of user data and header compression and ciphering. The function of the PDCP layer on the user plane further includes the transfer and encryption/integrity protection of control plane data. 
     What an RB is configured means a process of defining the characteristics of a wireless protocol layer and channels in order to provide specific service and configuring each detailed parameter and operating method. An RB can be divided into two types of a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a passage through which an RRC message is transmitted on the control plane, and the DRB is used as a passage through which user data is transmitted on the user plane. 
     If RRC connection is established between the RRC layer of UE and the RRC layer of an E-UTRAN, the UE is in the RRC connected state. If not, the UE is in the RRC idle state. 
     A downlink transport channel through which data is transmitted from a network to UE includes a broadcast channel (BCH) through which system information is transmitted and a downlink shared channel (SCH) through which user traffic or control messages are transmitted. Traffic or a control message for downlink multicast or broadcast service may be transmitted through the downlink SCH, or may be transmitted through an additional downlink multicast channel (MCH). Meanwhile, an uplink transport channel through which data is transmitted from UE to a network includes a random access channel (RACH) through which an initial control message is transmitted and an uplink shared channel (SCH) through which user traffic or control messages are transmitted. 
     Logical channels that are placed over the transport channel and that are mapped to the transport channel include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH). 
     The physical channel includes several OFDM symbols in the time domain and several subcarriers in the frequency domain. One subframe includes a plurality of OFDM symbols in the time domain. An RB is a resources allocation unit, and includes a plurality of OFDM symbols and a plurality of subcarriers. Furthermore, each subframe may use specific subcarriers of specific OFDM symbols (e.g., the first OFDM symbol) of the corresponding subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval (TTI) is a unit time for subframe transmission. 
     The RRC state of UE and an RRC connection method are described below. 
     The RRC state means whether or not the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN. A case where the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN is referred to as an RRC connected state. A case where the RRC layer of UE is not logically connected to the RRC layer of the E-UTRAN is referred to as an RRC idle state. The E-UTRAN may check the existence of corresponding UE in the RRC connected state in each cell because the UE has RRC connection, so the UE may be effectively controlled. In contrast, the E-UTRAN is unable to check UE in the RRC idle state, and a Core Network (CN) manages UE in the RRC idle state in each tracking area, that is, the unit of an area greater than a cell. That is, the existence or non-existence of UE in the RRC idle state is checked only for each large area. Accordingly, the UE needs to shift to the RRC connected state in order to be provided with common mobile communication service, such as voice or data. 
     When a user first powers UE, the UE first searches for a proper cell and remains in the RRC idle state in the corresponding cell. The UE in the RRC idle state establishes RRC connection with an E-UTRAN through an RRC connection procedure when it is necessary to set up the RRC connection, and shifts to the RRC connected state. A case where UE in the RRC idle state needs to set up RRC connection includes several cases. For example, the cases may include a need to send uplink data for a reason, such as a call attempt by a user, and to send a response message as a response to a paging message received from an E-UTRAN. 
     A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management. 
     In the NAS layer, in order to manage the mobility of UE, two types of states: EPS Mobility Management-REGISTERED (EMM-REGISTERED) and EMM-DEREGISTERED are defined. The two states are applied to UE and the MME. UE is initially in the EMM-DEREGISTERED state. In order to access a network, the UE performs a process of registering it with the corresponding network through an initial attach procedure. If the attach procedure is successfully performed, the UE and the MME become the EMM-REGISTERED state. 
     In order to manage signaling connection between UE and the EPC, two types of states: an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The two states are applied to UE and the MME. When the UE in the ECM-IDLE state establishes RRC connection with the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in the ECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1 connection with the E-UTRAN. When the UE is in the ECM-IDLE state, the E-UTRAN does not have information about the context of the UE. Accordingly, the UE in the ECM-IDLE state performs procedures related to UE-based mobility, such as cell selection or cell reselection, without a need to receive a command from a network. In contrast, when the UE is in the ECM-CONNECTED state, the mobility of the UE is managed in response to a command from a network. If the location of the UE in the ECM-IDLE state is different from a location known to the network, the UE informs the network of its corresponding location through a tracking area update procedure. 
     System information is described below. 
     System information includes essential information that needs to be known by UE in order for the UE to access a BS. Accordingly, the UE needs to have received all pieces of system information before accessing the BS, and needs to always have the up-to-date system information. Furthermore, the BS periodically transmits the system information because the system information is information that needs to be known by all UEs within one cell. The system information is divided into a Master Information Block (MIB) and a plurality of System Information Blocks (SIBs). 
     The MIB may include the limited number of parameters which are the most essential and are most frequently transmitted in order to obtain other information from a cell. UE first discovers an MIB after downlink synchronization. The MIB may include information, such as a downlink channel bandwidth, a PHICH configuration, an SFN supporting synchronization and operating as a timing reference, and an eNB transmission antenna configuration. The MIB may be broadcasted on a BCH. 
     SystemInformationBlockType1 (SIB1 of included SIBs is included in a “SystemInformationBlockType1” message and transmitted. Other SIBs other than the SIB1 are included in a system information message and transmitted. The mapping of the SIBs to the system information message may be flexibly configured by a scheduling information list parameter included in the SIB1. In this case, each SIB is included in a single system information message. Only SIBs having the same scheduling required value (e.g. period) may be mapped to the same system information message. Furthermore, SystemInformationBlockType2 (SIB2) is always mapped to a system information message corresponding to the first entry within the system information message list of a scheduling information list. A plurality of system information messages may be transmitted within the same period. The SIB1 and all of the system information messages are transmitted on a DL-SCH. 
     In addition to broadcast transmission, in the E-UTRAN, the SIB1 may be channel-dedicated signaling including a parameter set to have the same value as an existing set value. In this case, the SIB1 may be included in an RRC connection re-establishment message and transmitted. 
     The SIB1 includes information related to UE cell access and defines the scheduling of other SIBs. The SIB1 may include information related to the PLMN identifiers, Tracking Area Code (TAC), and cell ID of a network, a cell barring state indicative of whether a cell is a cell on which UE can camp, a required minimum reception level within a cell which is used as a cell reselection reference, and the transmission time and period of other SIBs. 
     The SIB2 may include radio resource configuration information common to all types of UE. The SIB2 may include information related to an uplink carrier frequency and uplink channel bandwidth, an RACH configuration, a page configuration, an uplink power control configuration, a sounding reference signal configuration, a PUCCH configuration supporting ACK/NACK transmission, and a PUSCH configuration. 
     UE may apply a procedure for obtaining system information and for detecting a change of system information to only a PCell. In an SCell, when the corresponding SCell is added, the E-UTRAN may provide all types of system information related to an RRC connection state operation through dedicated signaling. When system information related to a configured SCell is changed, the E-UTRAN may release a considered SCell and add the considered SCell later. This may be performed along with a single RRC connection re-establishment message. The E-UTRAN may set a value broadcast within a considered SCell and other parameter value through dedicated signaling. 
     UE needs to guarantee the validity of a specific type of system information. Such system information is called required system information. The required system information may be defined as follows.
         If UE is in the RRC_IDLE state: the UE needs to have the valid version of the MIB and the SIB1 in addition to the SIB2 to SIB8. This may comply with the support of a considered RAT.   If UE is in the RRC connection state: the UE needs to have the valid version of the MIB, SIB1, and SIB2.       

     In general, the validity of system information may be guaranteed up to a maximum of 3 hours after being obtained. 
     In general, service that is provided to UE by a network may be classified into three types as follows. Furthermore, the UE differently recognizes the type of cell depending on what service may be provided to the UE. In the following description, a service type is first described, and the type of cell is described. 
     1) Limited service: this service provides emergency calls and an Earthquake and Tsunami Warning System (ETWS), and may be provided by an acceptable cell. 
     2) Suitable service: this service means public service for common uses, and may be provided by a suitable cell (or a normal cell). 
     3) Operator service: this service means service for communication network operators. This cell may be used by only communication network operators, but may not be used by common users. 
     In relation to a service type provided by a cell, the type of cell may be classified as follows. 
     1) An acceptable cell: this cell is a cell from which UE may be provided with limited service. This cell is a cell that has not been barred from a viewpoint of corresponding UE and that satisfies the cell selection criterion of the UE. 
     2) A suitable cell: this cell is a cell from which UE may be provided with suitable service. This cell satisfies the conditions of an acceptable cell and also satisfies additional conditions. The additional conditions include that the suitable cell needs to belong to a Public Land Mobile Network (PLMN) to which corresponding UE may access and that the suitable cell is a cell on which the execution of a tracking area update procedure by the UE is not barred. If a corresponding cell is a CSG cell, the cell needs to be a cell to which UE may access as a member of the CSG. 
     3) A barred cell: this cell is a cell that broadcasts information indicative of a barred cell through system information. 
     4) A reserved cell: this cell is a cell that broadcasts information indicative of a reserved cell through system information. 
       FIG. 4  is a flowchart illustrating the operation of UE in the RRC idle state.  FIG. 4  illustrates a procedure in which UE that is initially powered on experiences a cell selection process, registers it with a network, and then performs cell reselection if necessary. 
     Referring to  FIG. 4 , the UE selects Radio Access Technology (RAT) in which the UE communicates with a Public Land Mobile Network (PLMN), that is, a network from which the UE is provided with service (S 410 ). Information about the PLMN and the RAT may be selected by the user of the UE, and the information stored in a Universal Subscriber Identity Module (USIM) may be used. 
     The UE selects a cell that has the greatest value and that belongs to cells having measured BS and signal intensity or quality greater than a specific value (cell selection) (S 420 ). In this case, the UE that is powered off performs cell selection, which may be called initial cell selection. A cell selection procedure is described later in detail. After the cell selection, the UE receives system information periodically by the BS. The specific value refers to a value that is defined in a system in order for the quality of a physical signal in data transmission/reception to be guaranteed. Accordingly, the specific value may differ depending on applied RAT. 
     If network registration is necessary, the UE performs a network registration procedure (S 430 ). The UE registers its information (e.g., an IMSI) with the network in order to receive service (e.g., paging) from the network. The UE does not register it with a network whenever it selects a cell, but registers it with a network when information about the network (e.g., a Tracking Area Identity (TAI)) included in system information is different from information about the network that is known to the UE. 
     The UE performs cell reselection based on a service environment provided by the cell or the environment of the UE (S 440 ). If the value of the intensity or quality of a signal measured based on a BS from which the UE is provided with service is lower than that measured based on a BS of a neighboring cell, the UE selects a cell that belongs to other cells and that provides better signal characteristics than the cell of the BS that is accessed by the UE. This process is called cell reselection differently from the initial cell selection of the No. 2 process. In this case, temporal restriction conditions are placed in order for a cell to be frequently reselected in response to a change of signal characteristic. A cell reselection procedure is described later in detail. 
       FIG. 5  is a flowchart illustrating a process of establishing RRC connection. 
     UE sends an RRC connection request message that requests RRC connection to a network (S 510 ). The network sends an RRC connection establishment message as a response to the RRC connection request (S 520 ). After receiving the RRC connection establishment message, the UE enters RRC connected mode. 
     The UE sends an RRC connection establishment complete message used to check the successful completion of the RRC connection to the network (S 530 ). 
       FIG. 6  is a flowchart illustrating an RRC connection reconfiguration process. An RRC connection reconfiguration is used to modify RRC connection. This is used to establish/modify/release RBs, perform handover, and set up/modify/release measurements. 
     A network sends an RRC connection reconfiguration message for modifying RRC connection to UE (S 610 ). As a response to the RRC connection reconfiguration message, the UE sends an RRC connection reconfiguration complete message used to check the successful completion of the RRC connection reconfiguration to the network (S 620 ). 
     Hereinafter, a public land mobile network (PLMN) is described. 
     The PLMN is a network which is disposed and operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by a Mobile Country Code (MCC) and a Mobile Network Code (MNC). PLMN information of a cell is included in system information and broadcasted. 
     In PLMN selection, cell selection, and cell reselection, various types of PLMNs may be considered by the terminal. 
     Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC of a terminal IMSI. 
     Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN. 
     Registered PLMN (RPLMN): PLMN successfully finishing location registration. 
     Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN. 
     Each mobile service consumer subscribes in the HPLMN. When a general service is provided to the terminal through the HPLMN or the EHPLMN, the terminal is not in a roaming state. Meanwhile, when the service is provided to the terminal through a PLMN except for the HPLMN/EHPLMN, the terminal is in the roaming state. In this case, the PLMN refers to a Visited PLMN (VPLMN). 
     When UE is initially powered on, the UE searches for available Public Land Mobile Networks (PLMNs) and selects a proper PLMN from which the UE is able to be provided with service. The PLMN is a network that is deployed or operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by Mobile Country Code (MCC) and Mobile Network Code (MNC). Information about the PLMN of a cell is included in system information and broadcasted. The UE attempts to register it with the selected PLMN. If registration is successful, the selected PLMN becomes a Registered PLMN (RPLMN). The network may signalize a PLMN list to the UE. In this case, PLMNs included in the PLMN list may be considered to be PLMNs, such as RPLMNs. The UE registered with the network needs to be able to be always reachable by the network. If the UE is in the ECM-CONNECTED state (identically the RRC connection state), the network recognizes that the UE is being provided with service. If the UE is in the ECM-IDLE state (identically the RRC idle state), however, the situation of the UE is not valid in an eNB, but is stored in the MME. In such a case, only the MME is informed of the location of the UE in the ECM-IDLE state through the granularity of the list of Tracking Areas (TAs). A single TA is identified by a Tracking Area Identity (TAI) formed of the identifier of a PLMN to which the TA belongs and Tracking Area Code (TAC) that uniquely expresses the TA within the PLMN. 
     Thereafter, the UE selects a cell that belongs to cells provided by the selected PLMN and that has signal quality and characteristics on which the UE is able to be provided with proper service. 
     The following is a detailed description of a procedure of selecting a cell by a terminal. 
     When power is turned-on or the terminal is located in a cell, the terminal performs procedures for receiving a service by selecting/reselecting a suitable quality cell. 
     A terminal in an RRC idle state should prepare to receive a service through the cell by always selecting a suitable quality cell. For example, a terminal where power is turned-on just before should select the suitable quality cell to be registered in a network. If the terminal in an RRC connection state enters in an RRC idle state, the terminal should selects a cell for stay in the RRC idle state. In this way, a procedure of selecting a cell satisfying a certain condition by the terminal in order to be in a service idle state such as the RRC idle state refers to cell selection. Since the cell selection is performed in a state that a cell in the RRC idle state is not currently determined, it is important to select the cell as rapid as possible. Accordingly, if the cell provides a wireless signal quality of a predetermined level or greater, although the cell does not provide the best wireless signal quality, the cell may be selected during a cell selection procedure of the terminal. 
     A method and a procedure of selecting a cell by a terminal in a 3GPP LTE is described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “User Equipment (UE) procedures in idle mode (Release 8)”. 
     A cell selection process is basically divided into two types. 
     The first is an initial cell selection process. In this process, UE does not have preliminary information about a wireless channel. Accordingly, the UE searches for all wireless channels in order to find out a proper cell. The UE searches for the strongest cell in each channel. Thereafter, if the UE has only to search for a suitable cell that satisfies a cell selection criterion, the UE selects the corresponding cell. 
     Next, the UE may select the cell using stored information or using information broadcasted by the cell. Accordingly, cell selection may be fast compared to an initial cell selection process. If the UE has only to search for a cell that satisfies the cell selection criterion, the UE selects the corresponding cell. If a suitable cell that satisfies the cell selection criterion is not retrieved though such a process, the UE performs an initial cell selection process. 
     The cell selection criterion may be defined as below equation 1.
 
Srxlev&gt;0 AND Squal&gt;0
 
where:
 
 Srxlev=Q   rxlevmeas −( Q   rxlevmin   +Q   rxlevminoffset )− P compensation
 
 Squal=Q   qualmeas −( Q   qualmin   +Q   qualminoffset )  [Equation 1]
 
     Here, the variables in the equation 1 may be defined as below table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Srxlev 
                 Cell selection RX level value (dB) 
               
               
                 Squal 
                 Cell selection quality value (dB) 
               
               
                 Q rxlevmeas   
                 Measured cell RX level value (RSRP) 
               
               
                 Q qualmeas   
                 Measured cell quality value (RSRQ) 
               
               
                 Q rxlevmin   
                 Minimum required RX level in the cell (dBm) 
               
               
                 Q qualmin   
                 Minimum required quality level in the cell (dB) 
               
               
                 Q rxlevminoffset   
                 Offset to the signalled Q rxlevmin  taken into account in the 
               
               
                   
                 Srxlev evaluation as a result of a periodic search for a 
               
               
                   
                 higher priority PLMN while camped normally in a 
               
               
                   
                 VPLMN 
               
               
                 Q qualminoffset   
                 Offset to the signalled Q qualmin  taken into account in the 
               
               
                   
                 Squal evaluation as a result of a periodic search for a 
               
               
                   
                 higher priority PLMN while camped normally in a 
               
               
                   
                 VPLMN 
               
               
                 Pcompensation 
                 max(P EMAX  − P PowerClass , 0) (dB) 
               
               
                 P EMAX   
                 Maximum TX power level an UE may use when 
               
               
                   
                 transmitting on the uplink in the cell (dBm) defined as 
               
               
                   
                 P EMAX  in [TS 36.101] 
               
               
                 P PowerClass   
                 Maximum RF output power of the UE (dBm) according 
               
               
                   
                 to the UE power class as defined in [TS 36.101] 
               
               
                   
               
            
           
         
       
     
     Signalled values, i.e., Q rxlevminoffset  and Q qualminoffset , may be applied to a case where cell selection is evaluated as a result of periodic search for a higher priority PLMN during a UE camps on a normal cell in a VPLMN. During the periodic search for the higher priority PLMN as described above, the UE may perform the cell selection evaluation by using parameter values stored in other cells of the higher priority PLMN. 
     After the UE selects a specific cell through the cell selection process, the intensity or quality of a signal between the UE and a BS may be changed due to a change in the mobility or wireless environment of the UE. Accordingly, if the quality of the selected cell is deteriorated, the UE may select another cell that provides better quality. If a cell is reselected as described above, the UE selects a cell that provides better signal quality than the currently selected cell. Such a process is called cell reselection. In general, a basic object of the cell reselection process is to select a cell that provides UE with the best quality from a viewpoint of the quality of a radio signal. 
     In addition to the viewpoint of the quality of a radio signal, a network may determine priority corresponding to each frequency, and may inform the UE of the determined priorities. The UE that has received the priorities preferentially takes into consideration the priorities in a cell reselection process compared to a radio signal quality criterion. 
     As described above, there is a method of selecting or reselecting a cell according to the signal characteristics of a wireless environment. In selecting a cell for reselection when a cell is reselected, the following cell reselection methods may be present according to the RAT and frequency characteristics of the cell.
         Intra-frequency cell reselection: UE reselects a cell having the same center frequency as that of RAT, such as a cell on which the UE camps on.   Inter-frequency cell reselection: UE reselects a cell having a different center frequency from that of RAT, such as a cell on which the UE camps on   Inter-RAT cell reselection: UE reselects a cell that uses RAT different from RAT on which the UE camps       

     The principle of a cell reselection process is as follows. 
     First, UE measures the quality of a serving cell and neighbor cells for cell reselection. 
     Second, cell reselection is performed based on a cell reselection criterion. The cell reselection criterion has the following characteristics in relation to the measurements of a serving cell and neighbor cells. 
     Intra-frequency cell reselection is basically based on ranking. Ranking is a task for defining a criterion value for evaluating cell reselection and numbering cells using criterion values according to the size of the criterion values. A cell having the best criterion is commonly called the best-ranked cell. The cell criterion value is based on the value of a corresponding cell measured by UE, and may be a value to which a frequency offset or cell offset has been applied, if necessary. 
     Inter-frequency cell reselection is based on frequency priority provided by a network. UE attempts to camp on a frequency having the highest frequency priority. A network may provide frequency priority that will be applied by UEs within a cell in common through broadcasting signaling, or may provide frequency-specific priority to each UE through UE-dedicated signaling. A cell reselection priority provided through broadcast signaling may refer to a common priority. A cell reselection priority for each terminal set by a network may refer to a dedicated priority. If receiving the dedicated priority, the terminal may receive a valid time associated with the dedicated priority together. If receiving the dedicated priority, the terminal starts a validity timer set as the received valid time together therewith. While the valid timer is operated, the terminal applies the dedicated priority in the RRC idle mode. If the valid timer is expired, the terminal discards the dedicated priority and again applies the common priority. 
     For the inter-frequency cell reselection, a network may provide UE with a parameter (e.g., a frequency-specific offset) used in cell reselection for each frequency. 
     For the intra-frequency cell reselection or the inter-frequency cell reselection, a network may provide UE with a Neighboring Cell List (NCL) used in cell reselection. The NCL includes a cell-specific parameter (e.g., a cell-specific offset) used in cell reselection. 
     For the intra-frequency or inter-frequency cell reselection, a network may provide UE with a cell reselection black list used in cell reselection. The UE does not perform cell reselection on a cell included in the black list. 
     Ranking performed in a cell reselection evaluation process is described below. 
     A ranking criterion used to apply priority to a cell is defined as in Equation 1.
 
 Rs=Qmeas,s+Qhyst, Rn=Qmeas,s−Qoffset   [Equation 2]
 
     In this case, Rs is the ranking criterion of a serving cell, Rn is the ranking criterion of a neighbor cell, Qmeas,s is the quality value of the serving cell measured by UE, Qmeas,n is the quality value of the neighbor cell measured by UE, Qhyst is the hysteresis value for ranking, and Qoffset is an offset between the two cells. 
     In Intra-frequency, if UE receives an offset “Qoffsets,n” between a serving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does not Qoffsets,n, Qoffset=0. 
     In Inter-frequency, if UE receives an offset “Qoffsets,n” for a corresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does not receive “Qoffsets,n”, Qoffset=Qfrequency. 
     If the ranking criterion Rs of a serving cell and the ranking criterion Rn of a neighbor cell are changed in a similar state, ranking priority is frequency changed as a result of the change, and UE may alternately reselect the twos. Qhyst is a parameter that gives hysteresis to cell reselection so that UE is prevented from to alternately reselecting two cells. 
     UE measures RS of a serving cell and Rn of a neighbor cell according to the above equation, considers a cell having the greatest ranking criterion value to be the best-ranked cell, and reselects the cell. 
     In accordance with the criterion, it may be checked that the quality of a cell is the most important criterion in cell reselection. If a reselected cell is not a suitable cell, UE excludes a corresponding frequency or a corresponding cell from the subject of cell reselection. 
     A Radio Link Failure (RLF) is described below. 
     UE continues to perform measurements in order to maintain the quality of a radio link with a serving cell from which the UE receives service. The UE determines whether or not communication is impossible in a current situation due to the deterioration of the quality of the radio link with the serving cell. If communication is almost impossible because the quality of the serving cell is too low, the UE determines the current situation to be an RLF. 
     If the RLF is determined, the UE abandons maintaining communication with the current serving cell, selects a new cell through cell selection (or cell reselection) procedure, and attempts RRC connection re-establishment with the new cell. 
     In the specification of 3GPP LTE, the following examples are taken as cases where normal communication is impossible.
         A case where UE determines that there is a serious problem in the quality of a downlink communication link (a case where the quality of a PCell is determined to be low while performing RLM) based on the radio quality measured results of the PHY layer of the UE   A case where uplink transmission is problematic because a random access procedure continues to fail in the MAC sublayer.   A case where uplink transmission is problematic because uplink data transmission continues to fail in the RLC sublayer.   A case where handover is determined to have failed.   A case where a message received by UE does not pass through an integrity check.       

     An RRC connection re-establishment procedure is described in more detail below. 
       FIG. 7  is a diagram illustrating an RRC connection re-establishment procedure. 
     Referring to  FIG. 7 , UE stops using all the radio bearers that have been configured other than a Signaling Radio Bearer (SRB) # 0 , and initializes a variety of kinds of sublayers of an Access Stratum (AS) (S 710 ). Furthermore, the UE configures each sublayer and the PHY layer as a default configuration. In this process, the UE maintains the RRC connection state. 
     The UE performs a cell selection procedure for performing an RRC connection reconfiguration procedure (S 720 ). The cell selection procedure of the RRC connection re-establishment procedure may be performed in the same manner as the cell selection procedure that is performed by the UE in the RRC idle state, although the UE maintains the RRC connection state. 
     After performing the cell selection procedure, the UE determines whether or not a corresponding cell is a suitable cell by checking the system information of the corresponding cell (S 730 ). If the selected cell is determined to be a suitable E-UTRAN cell, the UE sends an RRC connection re-establishment request message to the corresponding cell (S 740 ). 
     Meanwhile, if the selected cell is determined to be a cell that uses RAT different from that of the E-UTRAN through the cell selection procedure for performing the RRC connection re-establishment procedure, the UE stops the RRC connection re-establishment procedure and enters the RRC idle state (S 750 ). 
     The UE may be implemented to finish checking whether the selected cell is a suitable cell through the cell selection procedure and the reception of the system information of the selected cell. To this end, the UE may drive a timer when the RRC connection re-establishment procedure is started. The timer may be stopped if it is determined that the UE has selected a suitable cell. If the timer expires, the UE may consider that the RRC connection re-establishment procedure has failed, and may enter the RRC idle state. Such a timer is hereinafter called an RLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as an RLF timer. The UE may obtain the set value of the timer from the system information of the serving cell. 
     If an RRC connection re-establishment request message is received from the UE and the request is accepted, a cell sends an RRC connection re-establishment message to the UE. 
     The UE that has received the RRC connection re-establishment message from the cell reconfigures a PDCP sublayer and an RLC sublayer with an SRB 1 . Furthermore, the UE calculates various key values related to security setting, and reconfigures a PDCP sublayer responsible for security as the newly calculated security key values. Accordingly, the SRB  1  between the UE and the cell is open, and the UE and the cell may exchange RRC control messages. The UE completes the restart of the SRB 1 , and sends an RRC connection re-establishment complete message indicative of that the RRC connection re-establishment procedure has been completed to the cell (S 760 ). 
     In contrast, if the RRC connection re-establishment request message is received from the UE and the request is not accepted, the cell sends an RRC connection re-establishment reject message to the UE. 
     If the RRC connection re-establishment procedure is successfully performed, the cell and the UE perform an RRC connection reconfiguration procedure. Accordingly, the UE recovers the state prior to the execution of the RRC connection re-establishment procedure, and the continuity of service is guaranteed to the upmost. 
       FIG. 8  illustrates substates which may be owned by UE in the RRC_IDLE state and a substate transition process. 
     Referring to  FIG. 8 , UE performs an initial cell selection process (S 801 ). The initial cell selection process may be performed when there is no cell information stored with respect to a PLMN or if a suitable cell is not discovered. 
     If a suitable cell is unable to be discovered in the initial cell selection process, the UE transits to any cell selection state (S 802 ). The any cell selection state is the state in which the UE has not camped on a suitable cell and an acceptable cell and is the state in which the UE attempts to discover an acceptable cell of a specific PLMN on which the UE may camp. If the UE has not discovered any cell on which it may camp, the UE continues to stay in the any cell selection state until it discovers an acceptable cell. 
     If a suitable cell is discovered in the initial cell selection process, the UE transits to a normal camp state (S 803 ). The normal camp state refers to the state in which the UE has camped on the suitable cell. In this state, the UE may select and monitor a paging channel based on information provided through system information and may perform an evaluation process for cell reselection. 
     If a cell reselection evaluation process (S 804 ) is caused in the normal camp state (S 803 ), the UE performs a cell reselection evaluation process (S 804 ). If a suitable cell is discovered in the cell reselection evaluation process (S 804 ), the UE transits to the normal camp state (S 803 ) again. 
     If an acceptable cell is discovered in the any cell selection state (S 802 ), the UE transmits to any cell camp state (S 805 ). The any cell camp state is the state in which the UE has camped on the acceptable cell. 
     In the any cell camp state (S 805 ), the UE may select and monitor a paging channel based on information provided through system information and may perform the evaluation process (S 806 ) for cell reselection. If an acceptable cell is not discovered in the evaluation process (S 806 ) for cell reselection, the UE transits to the any cell selection state (S 802 ). 
     Now, a device-to-device (D2D) operation is described. In 3GPP LTE-A, a service related to the D2D operation is called a proximity service (ProSe). Now, the ProSe is described. Hereinafter, the ProSe is the same concept as the D2D operation, and the ProSe and the D2D operation may be used without distinction. 
     The ProSe includes ProSe direction communication and ProSe direct discovery. The ProSe direct communication is communication performed between two or more proximate UEs. The UEs may perform communication by using a protocol of a user plane. A ProSe-enabled UE implies a UE supporting a procedure related to a requirement of the ProSe. Unless otherwise specified, the ProSe-enabled UE includes both of a public safety UE and a non-public safety UE. The public safety UE is a UE supporting both of a function specified for a public safety and a ProSe procedure, and the non-public safety UE is a UE supporting the ProSe procedure and not supporting the function specified for the public safety. 
     ProSe direct discovery is a process for discovering another ProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only the capabilities of the two types of ProSe-enabled UE are used. EPC-level ProSe discovery means a process for determining, by an EPC, whether the two types of ProSe-enabled UE are in proximity and notifying the two types of ProSe-enabled UE of the proximity. 
     Hereinafter, for convenience, the ProSe direct communication may be referred to as D2D communication, and the ProSe direct discovery may be referred to as D2D discovery. 
       FIG. 9  shows a basic structure for ProSe. 
     Referring to  FIG. 9 , the basic structure for ProSe includes an E-UTRAN, an EPC, a plurality of types of UE including a ProSe application program, a ProSe application server (a ProSe APP server), and a ProSe function. 
     The EPC represents an E-UTRAN core network configuration. The EPC may include an MME, an S-GW, a P-GW, a policy and charging rules function (PCRF), a home subscriber server (HSS) and so on. 
     The ProSe APP server is a user of a ProSe capability for producing an application function. The ProSe APP server may communicate with an application program within UE. The application program within UE may use a ProSe capability for producing an application function. 
     The ProSe function may include at least one of the followings, but is not necessarily limited thereto.
         Interworking via a reference point toward the 3rd party applications   Authorization and configuration of UE for discovery and direct communication   Enable the functionality of EPC level ProSe discovery   ProSe related new subscriber data and handling of data storage, and also handling of the ProSe identities   Security related functionality   Provide control towards the EPC for policy related functionality   Provide functionality for charging (via or outside of the EPC, e.g., offline charging)       

     A reference point and a reference interface in the basic structure for ProSe are described below.
         PC 1 : a reference point between the ProSe application program within the UE and the ProSe application program within the ProSe APP server. This is used to define signaling requirements in an application dimension.   PC 2 : a reference point between the ProSe APP server and the ProSe function. This is used to define an interaction between the ProSe APP server and the ProSe function. The update of application data in the ProSe database of the ProSe function may be an example of the interaction.   PC 3 : a reference point between the UE and the ProSe function. This is used to define an interaction between the UE and the ProSe function. A configuration for ProSe discovery and communication may be an example of the interaction.   PC 4 : a reference point between the EPC and the ProSe function. This is used to define an interaction between the EPC and the ProSe function. The interaction may illustrate the time when a path for 1:1 communication between types of UE is set up or the time when ProSe service for real-time session management or mobility management is authenticated.   PC 5 : a reference point used for using control/user plane for discovery and communication, relay, and 1:1 communication between types of UE.   PC 6 : a reference point for using a function, such as ProSe discovery, between users belonging to different PLMNs.   SGi: this may be used to exchange application data and types of application dimension control information.       

     &lt;ProSe Direct Communication&gt; 
     ProSe direct communication is communication mode in which two types of public safety UE can perform direct communication through a PC 5 interface. Such communication mode may be supported when UE is supplied with services within coverage of an E-UTRAN or when UE deviates from coverage of an E-UTRAN. 
       FIG. 10  shows the deployment examples of types of UE performing ProSe direct communication and cell coverage. 
     Referring to  FIG. 10( a ) , types of UE A and B may be placed outside cell coverage. Referring to  FIG. 10( b ) , UE A may be placed within cell coverage, and UE B may be placed outside cell coverage. Referring to  FIG. 10( c ) , types of UE A and B may be placed within single cell coverage. Referring to  FIG. 10( d ) , UE A may be placed within coverage of a first cell, and UE B may be placed within coverage of a second cell. 
     ProSe direct communication may be performed between types of UE placed at various positions as in  FIG. 10 . 
     Meanwhile, the following IDs may be used in ProSe direct communication. 
     A source layer-2 ID: this ID identifies the sender of a packet in the PC 5 interface. 
     A destination layer-2 ID: this ID identifies the target of a packet in the PC 5 interface. 
     An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5 interface. 
       FIG. 11  shows a user plane protocol stack for ProSe direct communication. 
     Referring to  FIG. 11 , the PC 5 interface includes a PDCH, RLC, MAC, and PHY layers. 
     In ProSe direct communication, HARQ feedback may not be present. An MAC header may include a source layer-2 ID and a destination layer-2 ID. 
     &lt;Radio Resource Assignment for ProSe Direct Communication&gt; 
     ProSe-enabled UE may use the following two types of mode for resource assignment for ProSe direct communication. 
     1. Mode  1   
     Mode  1  is mode in which resources for ProSe direct communication are scheduled by an eNB. UE needs to be in the RRC_CONNECTED state in order to send data in accordance with mode  1 . The UE requests a transmission resource from an eNB. The eNB performs scheduling assignment and schedules resources for sending data. The UE may send a scheduling request to the eNB and send a ProSe Buffer Status Report (BSR). The eNB has data to be subjected to ProSe direct communication by the UE based on the ProSe BSR and determines that a resource for transmission is required. 
     2. Mode  2   
     Mode  2  is mode in which UE directly selects a resource. UE directly selects a resource for ProSe direct communication in a resource pool. The resource pool may be configured by a network or may have been previously determined. 
     Meanwhile, if UE has a serving cell, that is, if the UE is in the RRC_CONNECTED state with an eNB or is placed in a specific cell in the RRC_IDLE state, the UE is considered to be placed within coverage of the eNB. 
     If UE is placed outside coverage, only mode  2  may be applied. If the UE is placed within the coverage, the UE may use mode  1  or mode  2  depending on the configuration of an eNB. 
     If another exception condition is not present, only when an eNB performs a configuration, UE may change mode from mode  1  to mode  2  or from mode  2  to mode  1 . 
     &lt;ProSe Direct Discovery&gt; 
     ProSe direct discovery refers to a procedure that is used for ProSe-enabled UE to discover another ProSe-enabled UE in proximity and is also called D2D direct discovery. In this case, E-UTRA radio signals through the PC 5 interface may be used. Information used in ProSe direct discovery is hereinafter called discovery information. 
       FIG. 12  shows the PC 5 interface for D2D direct discovery. 
     Referring to  FIG. 12 , the PC 5 interface includes an MAC layer, a PHY layer, and a ProSe Protocol layer, that is, a higher layer. The higher layer (the ProSe Protocol) handles the permission of the announcement and monitoring of discovery information. The contents of the discovery information are transparent to an access stratum (AS). The ProSe Protocol transfers only valid discovery information to the AS for announcement. 
     The MAC layer receives discovery information from the higher layer (the ProSe Protocol). An IP layer is not used to send discovery information. The MAC layer determines a resource used to announce discovery information received from the higher layer. The MAC layer produces an MAC protocol data unit (PDU) for carrying discovery information and sends the MAC PDU to the physical layer. An MAC header is not added. 
     In order to announce discovery information, there are two types of resource assignment. 
     1. Type  1   
     As a method in which resources for announcement of discovered information are allocated not specifically to a terminal, a base station provides a resource pool configuration for announcement of the discovered information to terminals. The configuration is included in a system information block (SIB) to be signaled by a broadcast scheme. Alternatively, the configuration may be provided while being included in a terminal specific RRC message. Alternatively, the configuration may be broadcast signaling of another layer except for an RRC message or terminal specific signaling. 
     The terminal autonomously selects the resource from an indicated resource pool and announces the discovery information by using the selected resource. The terminal may announce the discovery information through an arbitrarily selected resource during each discovery period. 
     2. Type  2   
     The type  2  is a method for assigning a resource for announcing discovery information in a UE-specific manner. UE in the RRC_CONNECTED state may request a resource for discovery signal announcement from an eNB through an RRC signal. The eNB may announce a resource for discovery signal announcement through an RRC signal. A resource for discovery signal monitoring may be assigned within a resource pool configured for types of UE. 
     An eNB  1 ) may announce a type  1  resource pool for discovery signal announcement to UE in the RRC_IDLE state through the SIB. Types of UE whose ProSe direct discovery has been permitted use the type  1  resource pool for discovery information announcement in the RRC_IDLE state. Alternatively, the eNB  2 ) announces that the eNB supports ProSe direct discovery through the SIB, but may not provide a resource for discovery information announcement. In this case, UE needs to enter the RRC_CONNECTED state for discovery information announcement. 
     An eNB may configure that UE has to use a type  1  resource pool for discovery information announcement or has to use a type  2  resource through an RRC signal in relation to UE in the RRC_CONNECTED state. 
       FIG. 13  is an embodiment of a ProSe discovery process. 
     Referring to  FIG. 13 , it is assumed that UE A and UE B have ProSe-enabled application programs managed therein and have been configured to have a ‘friend’ relation between them in the application programs, that is, a relationship in which D2D communication may be permitted between them. Hereinafter, the UE B may be represented as a ‘friend’ of the UE A. The application program may be, for example, a social networking program. ‘3GPP Layers’ correspond to the functions of an application program for using ProSe discovery service, which have been defined by 3GPP. 
     Direct discovery between the types of UE A and B may experience the following process. 
     1. First, the UE A performs regular application layer communication with the APP server. The communication is based on an Application Program Interface (API). 
     2. The ProSe-enabled application program of the UE A receives a list of application layer IDs having a ‘friend’ relation. In general, the application layer ID may have a network access ID form. For example, the application layer ID of the UE A may have a form, such as “adam@example.com.” 
     3. The UE A requests private expressions code for the user of the UE A and private representation code for a friend of the user. 
     4. The 3GPP layers send a representation code request to the ProSe server. 
     5. The ProSe server maps the application layer IDs, provided by an operator or a third party APP server, to the private representation code. For example, an application layer ID, such as adam@example.com, may be mapped to private representation code, such as “GTER543$#2FSJ67DFSF.” Such mapping may be performed based on parameters (e.g., a mapping algorithm, a key value and so on) received from the APP server of a network. 
     6. The ProSe server sends the types of derived representation code to the 3GPP layers. The 3GPP layers announce the successful reception of the types of representation code for the requested application layer ID to the ProSe-enabled application program. Furthermore, the 3GPP layers generate a mapping table between the application layer ID and the types of representation code. 
     7. The ProSe-enabled application program requests the 3GPP layers to start a discovery procedure. That is, the ProSe-enabled application program requests the 3GPP layers to start discovery when one of provided ‘friends’ is placed in proximity to the UE A and direct communication is possible. The 3GPP layers announces the private representation code (i.e., in the above example, “GTER543$#2FSJ67DFSF”, that is, the private representation code of adam@example.com) of the UE A. This is hereinafter called ‘announcement’. Mapping between the application layer ID of the corresponding application program and the private representation code may be known to only ‘friends’ which have previously received such a mapping relation, and the ‘friends’ may perform such mapping. 
     8. It is assumed that the UE B operates the same ProSe-enabled application program as the UE A and has executed the aforementioned 3 to 6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery. 
     9. When the UE B receives the aforementioned ‘announce’ from the UE A, the UE B determines whether the private representation code included in the ‘announce’ is known to the UE B and whether the private representation code is mapped to the application layer ID. As described the 8 step, since the UE B has also executed the 3 to 6 steps, it is aware of the private representation code, mapping between the private representation code and the application layer ID, and corresponding application program of the UE A. Accordingly, the UE B may discover the UE A from the ‘announce’ of the UE A. The 3GPP layers announce that adam@example.com has been discovered to the ProSe-enabled application program within the UE B. 
     In  FIG. 13 , the discovery procedure has been described by taking into consideration all of the types of UE A and B, the ProSe server, the APP server and so on. From the viewpoint of the operation between the types of UE A and B, the UE A sends (this process may be called announcement) a signal called announcement, and the UE B receives the announce and discovers the UE A. That is, from the aspect that an operation that belongs to operations performed by types of UE and that is directly related to another UE is only step, the discovery process of  FIG. 13  may also be called a single step discovery procedure. 
       FIG. 14  is another embodiment of a ProSe discovery process. 
     In  FIG. 14 , types of UE  1  to  4  are assumed to types of UE included in specific group communication system enablers (GCSE) group. It is assumed that the UE  1  is a discoverer and the types of UE  2 ,  3 , and  4  are discoveree. UE  5  is UE not related to the discovery process. 
     The UE  1  and the UE  2 - 4  may perform a next operation in the discovery process. 
     First, the UE  1  broadcasts a target discovery request message (may be hereinafter abbreviated as a discovery request message or M 1 ) in order to discover whether specific UE included in the GCSE group is in proximity. The target discovery request message may include the unique application program group ID or layer-2 group ID of the specific GCSE group. Furthermore, the target discovery request message may include the unique ID, that is, application program private ID of the UE  1 . The target discovery request message may be received by the types of UE  2 ,  3 ,  4 , and  5 . 
     The UE  5  sends no response message. In contrast, the types of UE  2 ,  3 , and  4  included in the GCSE group send a target discovery response message (may be hereinafter abbreviated as a discovery response message or M 2 ) as a response to the target discovery request message. The target discovery response message may include the unique application program private ID of UE sending the message. 
     An operation between types of UE in the ProSe discovery process described with reference to  FIG. 14  is described below. The discoverer (the UE  1 ) sends a target discovery request message and receives a target discovery response message, that is, a response to the target discovery request message. Furthermore, when the discoveree (e.g., the UE  2 ) receives the target discovery request message, it sends a target discovery response message, that is, a response to the target discovery request message. Accordingly, each of the types of UE performs the operation of the 2 step. In this aspect, the ProSe discovery process of  FIG. 14  may be called a 2-step discovery procedure. 
     In addition to the discovery procedure described in  FIG. 14 , if the UE  1  (the discoverer) sends a discovery conform message (may be hereinafter abbreviated as an M 3 ), that is, a response to the target discovery response message, this may be called a 3-step discovery procedure. 
     Following is description about the present disclosure. 
     If a UE does know a frequency at which it is not possible to receive a D2D signal, the UE has to monitor the D2D signal across the entire system band. Then, it may be difficult for the UE to receive the D2D signal properly and it may result in increased power consumption by the UE. This problem is more serious to an RRC idle-state UE which performs D2D operation based on system information. 
     In the present disclosure, for operation efficiency of a UE in RRC idle state and power saving, a network, for example, a serving cell, may provide a UE with system information including a list of frequencies at which it is not possible to transmit and receive a D2D signal. The system information may be broadcasted by the serving cell 
     Meanwhile, each UE may be able to perform D2D operation in a different PLMN, and each cell may belong to a different PLMN. In addition, a specific D2D signal of interest may be transmitted from a specific PLMN. If only the list of frequencies at which a D2D signal is able to be transmitted and received is provided to a UE, the UE may make an unnecessary attempt to receive a D2D signal at a frequency about a PLMN not supporting D2D operation or at a frequency about a PLMN which does not transmit the D2D signal of interest, so that the UE may unnecessarily waste the power. 
       FIG. 15  is a method for D2D operation performed by a UE according to an embodiment of the present disclosure. 
     Referring to  FIG. 15 , in S 210 , a UE receives system information that indicates a frequency at which reception of a D2D discovery signal is permitted and a PLMN to which the frequency is about. The UE may be a UE in RRC idle state. 
     The following table shows an example of system information including a list of frequencies (permitted frequency) at which it is not possible to transmit and receive a signal related to D2D discovery. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 -- ASN1START 
               
               
                 SystemInformationBlockType19-r12 ::= SEQUENCE { 
               
            
           
           
               
               
            
               
                  discConfig-r12 
                   SEQUENCE { 
               
               
                   discRxPool-r12 
                   SL-DiscRxPoolList-r12, 
               
            
           
           
               
               
               
            
               
                   discTxPoolCommon-r12 
                   SL-DiscTxPoolList-r12 
                    OPTIONAL,--  Need 
               
            
           
           
               
            
               
                 OR 
               
            
           
           
               
               
            
               
                   discTxPowerInfo-r12 
                   SL-DiscTxPowerInfoList-r12 OPTIONAL,--Cond Tx 
               
            
           
           
               
               
               
            
               
                   discSyncConfig-r12 
                  SL-SyncConfigList-r12 
                 OPTIONAL-- Need OR 
               
               
                  } 
                   
                   
               
               
                   
                   
                 OPTIONAL,  -- Need OR 
               
               
                  discInterFreqList-r12 
                  SL-CarrierFreqInfoList-r12 
                 OPTIONAL,-- Need OR 
               
               
                  lateNonCriticalExtension 
                 OCTET STRING 
                 OPTIONAL, 
               
            
           
           
               
            
               
                  ... 
               
               
                 } 
               
               
                 SL-CarrierFreqInfoList-r12 ::= SEQUENCE (SIZE (1..maxFreq)) OF SL-CarrierFreqInfo-r12 
               
            
           
           
               
               
               
            
               
                 SL-CarrierFreqInfo-r12::= 
                 SEQUENCE { 
                   
               
               
                  carrierFreq-r12 
                  ARFCN-ValueEUTRA-r9, 
                   
               
               
                  plmn-IdentityList-r12 
                   PLMN-IdentityList4-r12 
                    OPTIONAL--  Need 
               
               
                 OP 
                   
                   
               
               
                 } 
                   
                   
               
            
           
           
               
            
               
                 PLMN-IdentityList4-r12 ::=SEQUENCE (SIZE (1..maxPLMN-r11)) OF PLMN-IdentityInfo2-r12 
               
            
           
           
               
               
               
            
               
                 PLMN-IdentityInfo2-r12 ::= 
                 CHOICE { 
                   
               
               
                  plmn-Index-r12 
                   
                 INTEGER (1..maxPLMN-r11), 
               
            
           
           
               
               
            
               
                  plmnIdentity-r12 
                         PLMN-Identity 
               
            
           
           
               
               
               
            
               
                 } 
                   
                   
               
            
           
           
               
            
               
                 -- ASN1STOP 
               
               
                   
               
            
           
         
       
     
     In the above table, ‘discRxPool’ indicates a resource using which a UE in RRC idle state or an RRC connected UE is allowed to receive D2D discovery announcement; ‘discTxPoolCommon’ indicates a resource using which a UE in RRC idle state is allowed to transmit the D2D discovery announcement; and ‘discInterFreqList’ indicates a list of neighboring frequencies at which D2D discovery announcement is supported and may include a ‘maxFreq’ number of ‘SL-CarrierFreqInfo’. ‘SL-CarrierFreqInfo’ uses ‘carrierFreq’ to indicate a specific frequency which represents, for example, absolute radio frequency channel number (ARFCN), and uses ‘plmn-IdentityList’ to indicate a PLMN to which the specific frequency is about. 
     The UE receives a D2D discovery signal based on the system information in S 220 . 
     Based on frequency and PLMN information included in the system information, the UE may determine a frequency and a PLMN where it is possible to perform D2D operation desired by the UE. For example, based on the received PLMN information and information stored in the UE about a list of PLMNs in which it is possible to perform D2D operation, the UE may determine a frequency at which it is possible to perform D2D operation. 
     When it comes to receiving a D2D discovery signal based on the system information, the UE may receive a D2D discovery signal at a random frequency among frequencies included in the frequency list included in the system information. At this point, the UE may receive the D2D discovery signal at a frequency of a PLMN other than a PLMN to which the UE belongs. 
     In another example, the UE may be provided with priorities among frequencies included in the list, that is, priorities among frequencies at which D2D discovery signals are received. In this case, at first, the UE may attempt to receive a D2D discovery signal (e.g., D2D discovery announcement) at the highest priority frequency. 
     In yet another example, at first, the UE may attempt to receive a D2D discovery signal from a frequency about a PLMN preferred by the UE. Priorities among PLMNs may be set by the UE in advance or may be set by a user of the UE. 
     Meanwhile, a network may provide a UE with a list of frequencies (barring frequencies) at which reception of a D2D discovery signal is barred. The list of barring frequencies may be included in system information broadcasted by a serving cell. 
       FIG. 16  is a diagram illustrating a method for D2D operation performed by a UE according to another embodiment of the present disclosure. 
     Referring to  FIG. 16 , in S 310 , a UE receives system information indicating a frequency at which reception of a D2D discovery signal is barred (a barring frequency) and a PLMN to which the frequency (the barring frequency) is about. The UE may be a UE in RRC idle state. Alternatively, the UE may be an RRC connected UE. 
     A network may provide a UE with D2D setting information which includes a list of PLMNs supporting D2D operation and a list of frequencies at which discovery of a D2D signal is barred. Accordingly, when performing D2D operation based on the D2D settings that are usable in a cell belonging to a PLMN in which D2D operation is determined to be performed, the UE may verify a frequency which does not attempt to receive a D2D discovery signal. 
     The UE does not attempt to receive a D2D discovery signal at the barring frequencies in S 320 . 
     For example, the UE may not attempt to receive a D2D discovery signal at any frequency which is included in the list of barring frequencies. 
     In another example, even though the UE receives a D2D discovery signal at a frequency included in the list of barring frequencies, the UE may not report the network that a suitable D2D discovery signal is received. More specifically, the UE may not send a ProSe App code included in the D2D discovery signal to a ProSe function. 
     Meanwhile, the network may inform the UE of a list of PLMNs from which reception of a D2D discovery signal is barred. The list of barring PLMNs may be included in the system information to be transmitted to the UE or may be transmitted to the UE separately from the system information. 
     Once the UE receives a list of PLMNs from which reception of a D2D discovery signal is barred, even though a D2D discovery signal is received at a frequency about one of the barring PLMNs, the UE may not send a ProSe App code included in the received D2D discovery signal to a ProSe function. In other words, in such a case, the UE may not report the network that a suitable D2D discovery signal is received. 
     In another example, if the UE receives a list of PLMNs from which reception of a D2D discovery signal is barred, the UE may not attempt to receive a D2D discovery signal at a frequency about one of the PLMNs. 
     The UE may determine whether PLMN list included in the system information received from the cell includes a PLMN included in a list of barring PLMNs. If all PLMNs of the PLMN list included in the system information received from the cell correspond to the list of barring PLMNs, the UE may not at all receive a D2D discovery signal at a corresponding frequency nor perform a subsequent operation. 
     Meanwhile, the network may inform the UE of a list of PLMNs from which reception of a D2D discovery is permitted. The list of permitted PLMNs may be included in the system information to be transmitted to the UE or may be transmitted to the UE separately from the system information. 
     The network may provide the UE with D2D setting information which includes a list of PLMNs supporting D2D operation and a list of frequencies at which D2D signal discovery is possible with respect to the respective PLMNs. Accordingly, when performing D2D operation based on D2D settings able to be used in a cell belonging to a PLMN determined to support D2D, the UE may verify a frequency which is able to attempt to receive a D2D discovery signal 
     If the list of PLMNs from which reception of a D2D discovery signal is permitted, is received and a discovery signal is received at a frequency about one of the PLMNs, the UE may send a ProSe App code included in the D2D discovery signal to a ProSe function. In other words, in such a case, the UE may report the network that a suitable D2D discovery signal is received. 
     The UE may determine whether a PLMN list included in system information received from a cell includes a PLMN included in the list of permitted PLMNs. If the PLMN list contained in the system information received from the cell includes at least one PLMN belonging to the list of permitted PLMNs, the UE may receive a D2D discovery signal at a corresponding frequency nor perform a subsequent operation. 
     Meanwhile, the methods described with reference to  FIGS. 15 and 16  may be applied in combination. 
       FIG. 17  is a diagram illustrating a method for D2D operation performed by a UE according to yet another embodiment of the present disclosure. 
     Referring to  FIG. 17  a network transmits first system information to UE  1  in S 401 , and then transmits second system information to the UE in S 402 . The first system information may be system information indicating a frequency at which reception of a D2D discovery signal is permitted and a PLMN to which the frequency is about (the system information may be, for example, the system information described in Table 2). The second system information may be system information indicating a frequency at which reception of a D2D discovery signal is barred (that is, a barring frequency) and a PLMN to which the frequency is about (that is, a barring PLMN). 
     UE  1  may perform settings for receiving a D2D discovery signal based on the system information in S 403 . Then, UE  1  may perform D2D operation with UE  2  based on the settings in S 404 : in other words, the UE receives a D2D discovery signal from UE  2 . 
     Meanwhile, a network, that is, a serving cell, may provide a UE with an inter-frequency discovery gap. The inter-frequency discovery gap may be a time during which an attempt is allowed to receive a D2D discovery signal at a frequency different from the current frequency. The UE may be a RRC connected UE. 
     The UE may inform the serving cell of a list of frequencies at which the UE wants to receive a D2D discovery signal. At this point, the UE may inform the serving cell of information on priorities among frequencies with regard to reception. Considering the above list and the priorities, the serving cell may set an inter-frequency discovery gap in which the UE is permitted to receive a D2D discovery signal at at least one frequency. 
     Once the UE receives the inter-frequency discovery gap from the serving cell, the UE may attempt to receive a D2D discovery signal within the inter-frequency discovery gap. In addition, monitoring the serving cell (that is, monitoring a Physical Downlink Control Channel (PDCCH) which is a control channel for receiving a downlink signal of the serving cell) may not be performed within the inter-frequency discovery gap. 
     If it is not possible to receive a D2D discovery signal within the set inter-frequency discovery gap, the UE may request that the serving cell set an additional inter-frequency, discovery gap. Alternatively, the UE may request, from the serving cell, a new inter-frequency discovery gap different from the already-set inter-frequency discovery gap. 
       FIG. 18  is a block diagram illustrating a UE that implements an embodiment of the present disclosure. 
     Referring to  FIG. 18 , a UE  1100  includes a processor  1110 , a memory  1120 , and a Radio Frequency (RF) unit  1130 . The processor  1110  implements the proposed function, procedure, and/or method. For example, the processor  1110  may receive information, indicating a permitted frequency at which reception of a D2D signal is permitted and a PLMN to which the permitted frequency is about, and may receive a D2D signal based on the information. In addition, the UE may receive information indicating a frequency at which reception of a D2D signal is barred and a PLMN to which the barring frequency is about, and may perform D2D operation based on the information. 
     The RF unit  1130  is connected to the processor  1110  and sends and receives radio signals. 
     The processor may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, and/or data processors. The memory may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The RF unit may include a baseband circuit for processing a radio signal. When the above-described embodiment is implemented in software, the above-described scheme may be implemented using a module (process or function) which performs the above function. The module may be stored in the memory and executed by the processor. The memory may be disposed to the processor internally or externally and connected to the processor using a variety of well-known means.