Abstract:
The present disclosure relates to a pre-5 th -Generation (5G) or 5G communication system rt to be provided for supporting higher data rates Beyond 4 th -Generation (4G) communication system such as Long Term Evolution (LTE). An apparatus and method for acquiring synchronization information through the transmission and reception of signals between devices for device to device (D2D) communication is provided. A method for a user equipment (UE) in a wireless communication system includes detecting a synchronization signal of another UE in a predetermined measurement gap. The method also includes transmitting a detected result of the synchronization signal to an evolved Node B (eNB). A start time point of the measurement gap in a first measurement gap period and a star(time point of the measurement gap in a second measurement gap period are different from each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims priority under 35 U.S.C. §119(a) to Korean Application Serial No. 10-2014-0055915, which was filed in the Korean Intellectual Property Office on May 9, 2014, the entire content of which is hereby incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to device to device (D2D) communication. 
       BACKGROUND 
       [0003]    To meet the demand for wireless data traffic having increased since deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a. ‘Beyond 4G Network’ or a ‘Post LTE System’. 
         [0004]    The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. 
         [0005]    In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. 
         [0006]    In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed. 
         [0007]    Device to device (D2D) communication has recently received a lot of attention. In order to perform D2D communication, a process of setting synchronization between devices can be used. Synchronization between devices is set using time information provided from a synchronization base station or a Global Positioning System (GPS). However, there may be a case in which the time information is not provided from the synchronization base station or the GPS. 
       SUMMARY 
       [0008]    To address the above-discussed deficiencies, it is a primary object to provide an apparatus and a method for acquiring synchronization information through the transmission and reception of signals between devices for device to device (D2D) communication. 
         [0009]    The present disclosure is also directed to providing an apparatus and a method for acquiring synchronization information between devices for D2D communication even in a situation in which time information for synchronization is not provided. The present disclosure is also directed to providing an apparatus and a method for acquiring synchronization information between D2D user equipments (UEs) belonging to a specific evolved Node B (eNB) and D2D UEs located in an area in which they cannot communicate with an eNB. The present disclosure is also directed to providing an apparatus and a method for acquiring synchronization information between D2D UEs belonging to different eNBs. 
         [0010]    In a first example, a method for a UE in a wireless communication system is provided. The method includes measuring at least one neighboring UE at a device to device (D2D) operation frequency. The method also includes transmitting a measurement result to an eNB The measurement result includes information regarding at least one signal of neighboring UE satisfying a specific condition. 
         [0011]    In a second example, a method for an eNB in a wireless communication system is provided. The method includes receiving a measurement result from a UE. The measurement result includes information regarding at least one signal of at least one neighboring UE, which is measured at a device to device (D2D) operation frequency by the UE, the signal satisfying a specific condition. 
         [0012]    A user equipment (UE) in a wireless communication system is provided. The UE includes a processor configured to measure at least one neighboring UE at a device to device (D2D) operation frequency. The UE also includes transmitter configured to transmit a measurement result to an eNB. The measurement result includes information regarding at least one signal of neighboring UE satisfying a specific condition 
         [0013]    An apparatus of an eNB in a wireless communication system is provided. The apparatus includes a receiver configured to receive a measurement result from a UE. The the measurement result includes information regarding at least one signal of at least one neighboring UE, which is measured at a device to device (D2D) operation frequency by the UE, the signal satisfying a specific condition. 
         [0014]    Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as web as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0016]      FIG. 1  is a view showing an example operation of acquiring synchronization information between device to device (D2D) user equipments (UEs) according to this disclosure; 
           [0017]      FIGS. 2A ,  2 B,  2 C, and  2 D are views illustrating example processing flows between an evolved Node B (eNB) and a UE for an operation of acquiring synchronization information between D2D UEs according to this disclosure; 
           [0018]      FIGS. 3A and 3B  are views illustrating an example operation of transmitting a command for attempting to detect a D2D synchronization signal (D2DSS) and the related information from an eNB to a UE according to this disclosure; 
           [0019]      FIGS. 4A ,  4 B, and  4 C are views illustrating example patterns of a measurement gap for a D2DSS detection operation in a UE according to this disclosure; 
           [0020]      FIG. 5  is a view illustrating an example state transition operation for reporting a D2DSS detection result in a UE according to this disclosure; 
           [0021]      FIG. 6  is a view illustrating an example operation of transmitting a D2DSS by a UE selected in an eNB according to this disclosure; 
           [0022]      FIGS. 7A and 7B  are views illustrating example processing flows in an eNB for acquiring synchronization information between D2D UEs according to this disclosure; 
           [0023]      FIGS. 8A ,  8 B, and  8 C are views illustrating example processing flows of a D2DSS detection operation in a D2D UE according to this disclosure; 
           [0024]      FIGS. 9A and 9B  are views illustrating example processing flows of a D2DSS transmission operation in a D2D UE according to this disclosure; 
           [0025]      FIG. 10  is a view illustrating an example processing flow of an operation of acquiring synchronization information between D2D UEs according to this disclosure; 
           [0026]      FIG. 11  is a view illustrating an operation of acquiring synchronization information between D2D UEs according to this disclosure; 
           [0027]      FIGS. 12A. 12B ,  12 C, and  12 D are views illustrating example processing flows between an eNB and a UE for an operation of acquiring synchronization information between D2D UEs according to this disclosure; 
           [0028]      FIGS. 13A and 13B  are views illustrating an operation of transmitting a command for attempting to detect a D2DSS and the related information from an eNB to a UE according to this disclosure; 
           [0029]      FIGS. 14 and 15  are views illustrating an example D2DSS detection operation in a UE according to this disclosure; 
           [0030]      FIG. 16  is a view illustrating an operation of transmitting a D2DSS by a UE selected in an eNB according to this disclosure; 
           [0031]      FIGS. 17A and 17B  are views illustrating example processing flows in an eNB for acquiring synchronization information between D2D UEs according to this disclosure; 
           [0032]      FIGS. 18A ,  18 B, and  18 C are views illustrating example processing flows of a D2DSS detection operation in a D2D UE according to this disclosure; 
           [0033]      FIGS. 19A and 19B  are views illustrating example processing flows of a D2DSS transmission operation in a D2D UE according to this disclosure; 
           [0034]      FIG. 20  is a view illustrating an example processing flow of an operation of acquiring synchronization information between D2D UEs according to this disclosure; 
           [0035]      FIG. 21  is a view illustrated a structure of an example eNB for an operation of acquiring synchronization information between D2D UEs according to this disclosure; and 
           [0036]      FIG. 22  is a view illustrating an example structure of a D2D UE for an operation of acquiring synchronization information between D2D UEs according to this disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIGS. 1 through 22 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged electronic device and communication system. Hereinafter, the present disclosure will be described with reference to the accompanying drawings. The present disclosure may have various embodiments, and modifications and changes may be made therein. Therefore, the present disclosure will be described in conjunction with particular embodiments shown in the accompanying drawings. However, it should be understood that there is no intent to limit the present disclosure to the particular forms disclosed herein; rather, the present disclosure should be construed to cover all modifications, equivalents, and/or alternatives falling within the spirit and scope of the disclosure. In the description of the drawings, identical or similar reference numerals are used to designate identical or similar elements. 
         [0038]    A D2D terminal or D2D UE is an electronic device having a communication function. For example, the electronic device includes at least one of a smart phone, a tablet Personal Computer (PC), a mobile phone, a video phone, an e-book reader, a desktop PC, a laptop PC, a netbook computer, a Personal Digital Assistant (PDA), a Portable Multimedia Player (MP), an MP3 player, a mobile medical device, a camera, a wearable device (for example, a Head-Mounted-Device (HMD) such as electronic glasses, electronic clothes, an electronic bracelet, an electronic necklace, an electronic appcessory, an electronic tattoo, and a smart watch). 
         [0039]    According to some embodiments, the electronic device is a smart home appliance with a. communication function. The smart home appliance as an example of the electronic device includes at least one of, for example, a television, a Digital Video Disk (DVD) player, an audio, a refrigerator, an air conditioner, a vacuum cleaner, an oven, a microwave oven, a washing machine, an air cleaner, a set-top box, a TV box (such as, SAMSUNG HOMESYNC™, APPLETV®, or GOOGLETV®), a game console, an electronic dictionary, an electronic key, a camcorder, and an electronic picture frame. 
         [0040]    According to some embodiments, the electronic device includes at least one of various medical appliances (such as, magnetic resonance angiography (MRA), magnetic resonance imaging (MRI), computed tomography (CT), and ultrasonic machines), navigation equipment, a global positioning system (GPS) receiver, an event data recorder (EDR), a flight data recorder (FDR), automotive infotainment device, electronic equipment for ships (such as, ship navigation equipment and a gyrocompass), avionics, security equipment, a vehicle head unit, an industrial or home robot, an automatic teller machine (ATM) of a banking system, and a point of sales (POS) of a shop. 
         [0041]    According to some embodiments, the electronic device includes at least one of a part of furniture or a building/structure having a communication function, an electronic board, an electronic signature receiving device, a projector, and various kinds of measuring instruments (such as, a water meter, an electric meter, a gas meter, a radio wave meter, and the like). The electronic device according to the present disclosure can be a combination of one or more of the aforementioned various devices. Also, the electronic device according to the present disclosure can be a flexible device. Further, it is obvious to those skilled in the art that the electronic device according to the present disclosure is not limited to the aforementioned devices. 
         [0042]    Exemplary embodiments of the present disclosure which will be described below relate to an apparatus and a method which detects a D2D synchronization signal (D2DSS) transmitted from at least one neighboring device to device (D2D) user equipment (UE) so that D2D UEs mutually acquire synchronization information, and report the corresponding detected result to an evolved Node B (eNB). In the following description of the embodiments of the present disclosure, “eNB” and “cell” are used interchangeably. 
         [0043]    When D2D communication to which the embodiments of the present disclosure are applied fuses with cellular mobile communication, it is possible to increase the traffic accommodation capacity and reduce the overload. When terminals or UEs within the same cell or mutually adjacent cells mutually set a D2D link and then directly transmit and receive data through the D2D link without the intervention of the eNB, it is possible to reduce two links (such as, a link from any one UE to eNB and a link from eNB to another UE) into one link (such as, a link from any one UE to another UE), which results in gaining an advantage. D2D communication is efficiently used even in a situation in which there is no cellular network or the quality of a signal transmitted from the eNB is poor. 
         [0044]    The embodiments of the present disclosure include two embodiments.  FIGS. 1 to 10  are related to one embodiment of the present disclosure, and  FIGS. 11 to 20  are related to the other embodiment thereof. 
         [0045]    One embodiment of the present disclosure is based on a situation in which synchronization information should be acquired between D2D UEs belonging to a specific eNB (or cell) and D2D UEs located in an area in which they cannot communicate with an eNB such as a shadow area. In order for the D2D UEs located in the shadow area to communicate with the D2D UE belonging to the eNB, a synchronization process is required, and it is basically necessary that the D2D UEs belonging to the eNB should transmit synchronization signals. 
         [0046]    The other embodiment of the present disclosure is based on a situation in which D2D UEs belonging to different eNBs (or cells) should mutually acquire synchronization information. When the D2D UEs set synchronization according to downlink signals of individual eNBs, the D2D UEs belonging to the different eNBs are deviated in synchronization in a case of an asynchronous network in which time synchronization between the eNBs does not fit each other. Thus, in order to receive signals of the D2D UEs belonging to the different eNBs, it is necessary to detect a downlink signal timing of the corresponding eNB or a transmission signal timing of the D2D UEs belonging to the corresponding eNB. 
         [0047]      FIG. 1  is a view illustrating an example operation of acquiring synchronization information between D2D UEs according to this disclosure. Referring to  FIG. 1 , a D2D UE  110  located in an area in which it cannot communicate with an eNB such as a shadow area acquires synchronization information by receiving a D2DSS from a D2D UE  100  belonging to an eNB  200 . 
         [0048]    As to a basic flow according to one embodiment of the present disclosure as shown in  FIGS. 2   a  to  2   d , in operation S 210 , an eNB (such as the eNB  200 ) commands the D2D UEs (such as the UE  100 ) to attempt to detect the D2DSS, and transmits information required for the detection. The D2D UEs belonging to the eNB attempt to detect the corresponding D2DSS in operation S 220 , and report the corresponding detected result to the eNB in operation S 230 , as necessary. In operation S 240 , the eNB commands a specific D2D UE to transmit the D2DSS based on the detected result that has been reported by a large number of D2D UEs. Next, the specific D2D UE transmits the D2DSS to the D2D UEs located in the area in which they cannot communicate with the eNB such as a shadow area. Thus, the D2D UEs located in the area in which they cannot communicate with the acquire synchronization information of the D2D UEs belonging to the eNB, based on the corresponding D2DSS. 
         [0049]      FIGS. 2A ,  2 B,  2 C, and  2 D are views illustrating example processing flows between the eNB  200  and the D2D UE  100  for an operation of acquiring synchronization information between D2D UEs according to this disclosure. Here, a case in which only a single D2D UE  100  belonging to the eNB  200  performs a D2DSS detection operation is described. However, this is merely for convenience of description, and thus the D2DSS detection operation can be performed by one or more D2D UEs belonging to the eNB  200 . 
         [0050]    Referring to  FIG. 2A , in operation S 210 , the eNB  200  transmits a command for attempting to detect a D2DSS and the related information to the D2D UE  100 . In operation S 220 , the D2D UE  100  attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information which have been transmitted from the eNB  200 . In operation S 230 , the D2D UE  100  reports a D2DSS detection attempt result obtained by attempting to detect the D2DSS, to the eNB  200 . In operation S 240 , the eNB  200  receives the D2DSS detection attempt result from the D2D UE  100 , selects a UE to which the D2DSS is to be transmitted based on the received result, and commands the selected UE to transmit the D2DSS. In operation S 250 , the D2D UE  100  that has received a D2DSS transmission command from the eNB  200  transmits the D2DSS to the D2D UE  110  located in the area in which it cannot communicate with the eNB. 
         [0051]    Referring to  FIG. 2B , in operation S 210 , the eNB  200  transmits the command for attempting to detect the D2DSS and the related information to the D2D UE  100 . In operation S 220 , the D2D UE  100  attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information that has been transmitted from the eNB  200 . The D2D UE  100  such as in an RRC_Idle state switches the RRC_Idle state to an RRC_Connected state in operation S 225 , and reports the D2DSS detection attempt result to the eNB  200  in operation S 230 . In operation S 240 , the eNB  200  receives the D2DSS detection attempt result from the D2D UE  100 , selects a UE to which the D2DSS is to be transmitted based on the received result, and commands the selected UE to transmit the D2DSS. In operation S 250 , the D2D UE  100  that has received the D2DSS transmission command from the eNB  200  transmits the D2DSS to the D2D UE  110  located in the area in which it cannot communicate with the eNB. 
         [0052]    Referring to  FIG. 2C , in operation S 210 , the eNB  200  transmits the command for attempting to detect the D2DSS and the related information to the D2D UE  100 . In operation S 220 , the D2D UE  100  attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information which have been transmitted from the eNB  200 . In operation S 235 , the D2D UE  100  reports the D2DSS detection attempt result to the eNB  200  when the corresponding result satisfies a specific condition. In operation S 240 , the eNB  200  receives the D2DSS detection attempt result from the D2D UE  100 , selects a UE to which the D2DSS is to be transmitted based on the received result, and commands the selected UE to transmit the D2DSS. In operation S 250 , the D2D UE  100  that has received a D2DSS transmission command from the eNB  200  transmits the D2DSS to the D2D UE  110  located in the area in which it cannot communicate with the eNB. 
         [0053]    Referring to  FIG. 2D , in operation S 210 , the eNB  200  transmits the command for attempting to detect the D2DSS and the related information to the D2D UE  100 . In operation S 220 , the D2D UE  100  attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information which have been transmitted from the eNB  200 . The D2D UE  100  confirms a D2DSS detection result that satisfies the specific condition in operation S 245 , and transmits the D2DSS to the D2D UE  110  located in the area in which it cannot communicate with the eNB in operation S 250 . 
         [0054]    Hereinafter, the operations shown in  FIGS. 2A to 2D  will be specifically discussed. Here, a case in which the D2DSS detection operation is performed by a single D2D UE  100  is shown, but this is merely for convenience of description. For example, the D2DSS detection operation is performed by one or more D2D UEs  100 . 
         [0055]    In operation S 210 , the eNB  200  commands the D2D UEs  100  belonging to the corresponding eNB to attempt to detect the D2DSS. Information concerning that a shadow area is present in a place adjacent to the corresponding eNB or the adjacent eNB is not properly operated is transmitted to the corresponding eNB  200  via a network, and the eNB  200  issues a command for attempting to detect the D2DSS based on the transmitted information. According to an embodiment, the command for attempting to detect the D2DSS is issued to all of the D2D UEs which are in the RRC_Idle state and the RRC_Connected state in a broadcast manner. According to another embodiment, the command for attempting to detect the D2DSS is issued only to a specific D2D UE in a dedicated manner. According to still another embodiment, the command for attempting to detect the D2DSS is issued only to UEs that satisfy the specific condition. For example, the eNB  200  commands, to attempt to detect the D2DSS, only the D2D UEs whose signal strength such as RSRP (Reference Signal Received Power) of the serving cell is smaller than a specific threshold value. 
         [0056]      FIGS. 3A and 3B  are views illustrating an example operation of transmitting a command for attempting to detect a D2DSS and the related information from an eNB to a UE according to this disclosure. Referring to  FIG. 3A , the eNB  200  issues the command for attempting to detect the D2DSS to all of the D2D UEs  100  ( 100 - 1 ,  100 - 2 ,  100 - 3 , and  100 - 4 ) in a broadcast manner. Referring to  FIG. 3B , the eNB  200  issues the command for attempting to detect the D2DSS only to the specific D2D UE  100  ( 100 - 2 ). The specific D2D UE  100  ( 100 - 2 ) is a dedicatedly set UE or a UE that satisfies the specific condition. 
         [0057]    Referring again to operation S 210 , the eNB  200  provides the D2DSS detection-related information to the D2D UE  100 . A detection condition for the D2DSS is determined based on an absolute or relative signal strength (RSSI (Received Signal Strength Indicator), RSRP, or the like). According to an embodiment, when issuing the command for attempting to detect the D2DSS, the eNB  200  notifies the D2D UEs of the corresponding detection condition. According to another embodiment, the detection condition is promised in advance between the eNB 200 and the UE  100 . 
         [0058]    In operation S 220 , the D2D UEs  100  that have received the command for attempting to detect the D2DSS attempts to detect the D2DSS. For such detection operation, the eNB  200  notifies in advance the UEs of information about D2D communication frequencies or operation frequencies through a control message (such as an RRC (Radio Resource Control) message) or system information. In this instance, frequencies used by the D2D UEs belonging to the eNB and the D2D frequencies used by UEs located in the area in which they cannot communicate with the eNB such as the shadow area are different from each other, and the eNB notifies all of the UEs of the corresponding information. A UE in which one reception chain is provided and that currently communicates with a serving eNB is difficult to detect the D2DSS. In this case, the eNB  200  allocates a measurement gap to the UE through the RRC message or the like. For example, in the corresponding measurement gap, the UE is allowed to prevent transmission and reception of signals with the serving eNB. 
         [0059]      FIGS. 4A ,  4 B, and  4 C are views illustrating example patterns of a measurement gap for a D2DSS detection operation in a UE according to this disclosure, Referring to  FIG. 4A , a pattern for a measurement gap S 420  is given to be longer than a period of the D2DSS, For example, when the period of the D2DSS is 40 ms, the measurement gap S 420  is set as 41 ms, 42 ms, or the like such as larger than 40 ms. The eNB  200  can set a time when the measurement gap starts and a measurement gap period S 410 , and implicitly or explicitly notify the UE of the set information. For example, it is possible to define a gap pattern ID that sets an MGL (Measurement Gap Length) as 41 ms and an MGRP (Measurement Gap Repetition Period) T ms, and the eNB commands to set the measurement gap through the gap pattern ID and a gap offset. However, when the period of the D2DSS becomes longer, a problem occurs in that the measurement gap that does not communicate with the serving cell is increased. Such problem can be solved using patterns shown in  FIGS. 4B and 4C . 
         [0060]    Referring to  FIG. 4B , the pattern for the measurement gap is given to be shorter than the period of the D2DSS. For example, when the period of the D2DSS is 40 ms, a sum (A+B+C+D) of each short measurement gap MGL 1 is set to be larger than 40 ms. A time when each short measurement gap starts, such as, a gap offset vector such as a time offset vector for each of A, B, C. and D is [0, 2, 3] (corresponding unit is MGL1), and the eNB  200  sets the gap offset vector in advance or commands the UE to set the gap offset vector within a predetermined configuration. In addition, the eNB  200  can also set a time t 0  when the measurement gap with respect to the whole of A, B, C, and D starts and a measurement gap period MGRP 2, and notify the UE of the set information. 
         [0061]    As shown in  FIG. 4C , a pattern for the measurement gap is given, For example, when the period of the D2DSS is 40 ms, a sum (A+B+C+D) of each short measurement gap MGL is set to be larger than 40 ms. A gap offset vector such as a time offset for a time when each short measurement gap starts, such as, each of A, B, C, and D is [0, 1, 2, 3] (corresponding unit is MGL), and the eNB  200  sets the gap offset vector in advance or commands the UE to set the gap offset vector within a predetermined configuration. The eNB  200  can also set a time t 0  When the measurement gap A starts and a measurement gap period MGRP, and notify the UE of the set information. According to an embodiment, the MGRP is set in units of multiples of the period of the D2DSS of 40 ms. 
         [0062]    The method shown in  FIGS. 4B and 4C  can solve the problem in that the measurement gap that does not communicate with the cell is increased, by dispersing the measurement gap that does not communicate with the serving cell, but a time required for measurement is increased. Thus, it may be necessary to set a time of the short measurement gap according to requirements. 
         [0063]    In operation S 230 , the D2D UEs report the D2DSS detection result to the eNB  200 . The D2D UEs which are currently in the RRC_Idle state S 510  perform an RRC connection establishment process for the transition to the RRC_Connected state S 520 . For example, the UE enters a mode (or state) that reports the corresponding result to the eNB. In operation S 225 , the D2UEs such as in the RRC_Idle state S 510  is switched to the RRC_Connected state S 520  as illustrated in  FIG. 5 . 
         [0064]    In operation S 235 , only when the D2DSS detection result satisfies the specific condition even though the D2DSS is detected, the D2D UEs reports the D2DSS detection result to the eNB  200 . Such specific condition can be requested in advance by the eNB  200 , and include the contents of the following Table 1. 
         [0000]    
       
         
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 (1) a case in which the reception signal strength of the D2DSS 
               
             
          
           
               
                 transmitted from the D2D UE located in the area in which it cannot 
               
               
                 communicate with the eNB such as the shadow area is larger than a 
               
               
                 specific threshold value, or a case in which at least two of such 
               
               
                 signals are detected 
               
             
          
           
               
                   
                 (2) a case in which the reception signal strength of the D2DSS 
               
             
          
           
               
                 transmitted from the D2D UE that communicates with the eNB is larger 
               
               
                 than a specific threshold value, or a case in which at least two of 
               
               
                 such signals are detected 
               
             
          
           
               
                   
                 (3) a case in which the signal strength (such as RSRP (Reference 
               
             
          
           
               
                 Signal Received Power), RSRQ (Reference Signal Received Quality), 
               
               
                 SINR(Signal to Interference plus Noise Ratio), or the like) of the 
               
               
                 serving cell signal is smaller than a specific threshold value 
               
               
                   
               
             
          
         
       
     
         [0065]    In operation S 230  or operation S 235 , the D2D UEs report the D2DSS detection result to a serving cell, such as, the eNB  200 . The detection result reported to the eNB  200  includes the contents of the following Table 2. 
         [0000]    
       
         
               
               
             
               
             
               
               
             
               
             
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 (1) the number, a sequence index (identifier), a signal strength, 
               
             
          
           
               
                 or the like of the D2DSSs transmitted from the D2D UE located in the 
               
               
                 area in which it cannot communicate with the eNB such as a shadow area 
               
             
          
           
               
                   
                 (2) the number, a sequence index (identifier), a signal 
               
             
          
           
               
                 strength, or the like of the D2DSS transmitted from the D2D 
               
               
                 UE that communicates with the eNB 
               
             
          
           
               
                   
                 (3) signal strength of serving cell signal 
               
               
                   
                 (4) battery level of the UE 
               
               
                   
                   
               
             
          
         
       
     
         [0066]    In operation S 240 , the eNB  200  selects the specific UE to which the D2DSS is to be transmitted based on the detected result that has been reported in operation S 230  or operation S 235 , and commands the selected specific UE to transmit the D2DSS. In operation S 250 , the specific UE that has received the D2DSS transmission command from the eNB  200  transmits the D2DSS. The transmission of the D2DSS is performed without the command from the eNB  200 . For example, in operation S 245 , the UE that has confirmed the detected result that satisfies the condition requested by the eNB  200  transmits the D2DSS without an additional command of the eNB in operation S 250 . 
         [0067]    The eNB  200  commands the UEs selected for D2DSS transmission to periodically report the detected result, and thereby continuously update and manage a pool of the D2D UE that transmits the D2DSS. The eNB  200  preferentially commands, to transmit the D2DSS, the UE (such as  100 ) that has detected that the strength and the number of the D2DSSs transmitted from the D2D UE that communicates with the eNB  200  are larger than those of the D2DSSs transmitted from the D2D UE (such as  110 ) located in the area in which it cannot communicate with the eNB  200  such as the shadow area. The eNB  200  commands, to transmit the D2DSS, user located in a cell boundary in which no D2DSS is detected. When there are a plurality of UEs that have detected that the strength of the D2DSS transmitted from the D2D UE that communicates with the eNB  200  is larger than that of the D2DSS transmitted from the D2D UE located in the area in which it cannot communicate with the eNB  200 , the eNB  200  commands some of the plurality of UEs to interrupt the D2DSS transmission. In addition, the eNB  200  commands, to interrupt the D2DSS transmission, some of the D2D UEs that have reported that sequence indexes of the D2DSSs transmitted from the D2D UEs located in the area in which they cannot communicate with the eNB  200  such as the shadow area are the same. 
         [0068]    The D2D UE located in the area in which it cannot communicate with the eNB such as the shadow area sets synchronization based on the corresponding D2DSS when detecting the D2DSS transmitted from the UE that communicates with the eNB. Next, the D2D UE located in the area in which it cannot communicate with the eNB acquires transmittable resource pool configuration information and receivable resource pool configuration information based on D2D resource pool information signal or D2D Synchronization Channel (hereinafter, referred to as “D2DSCH”) transmitted from the UE that communicates with the eNB. Transmission and reception resource pools can be equal to each other, or the reception resource pool can include the transmission resource pool. For example, When the D2D resource pool that is currently operated within the eNB is a type of the D2D resource pool which the eNB allocates to the D2D UE in a dedicated manner, the eNB additionally allocates a contention-based D2D resource pool. The resource pools of two types are subjected to Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM). The UE that cannot communicate with the eNB acquires the corresponding information through the D2DSCH transmitted from the UE that communicates with the eNB, transmits the corresponding information only to the contention-based D2D resource pool, and receives the corresponding information from both the resource pools of two types. 
         [0069]      FIG. 6  is a view illustrating an example operation of transmitting a D2DSS by a UE selected in an eNB according to this disclosure. Referring to  FIG. 6 , the UE  100  transmits the D2DSS to the UE  110 . According to an embodiment, the eNB  200  selects the UE  100  to which the D2DSS is to be transmitted based on the reported detection result, and commands the selected UE  100  to transmit the D2DSS. Next, the UE  100  transmits the D2DSS to the UE  110 . According to another embodiment, the UE  100  that has confirmed the detection result that satisfies the condition requested by the eNB  200  transmits the D2DSS to the UE  110  without a D2DSS transmission command from the eNB  200 . 
         [0070]      FIGS. 7A and 7B  are views showing processing flows in an eNB for acquiring synchronization information between D2D UEs according to this disclosure. These processing flows can be performed by the eNB  200  shown in  FIG. 1 . Referring to  FIG. 7A , in operation S 710 , the eNB  200  receives information concerning that a shadow area is present in a place adjacent to the corresponding eNB or the adjacent eNB is not properly operated, via a network, In operation S 720 , the eNB  200  issues a command for attempting to detect the D2DSS to the D2D UEs belonging to the eNB  200 . According to an embodiment, the command for attempting to detect the D2DSS is issued to all of the D2D UEs belonging to the eNB  200  in a broadcast manner. According to another embodiment, the command for attempting to detect the D2DSS is issued only to a dedicated UE among the D2D UEs belonging to the eNB  200 . According to still another embodiment, the command for attempting to detect the D2DSS is issued only to a UE that satisfies a specific condition among the D2D UEs belonging to the eNB  200 . For example, the specific condition is a case in which the signal strength of the serving cell is smaller than a specific threshold value. 
         [0071]    In addition, in operation S 720 , the eNB  200  transmits D2DSS detection-related information to the D2D UEs belonging to the eNB  200 . According to an embodiment, the D2DSS detection-related information includes information about a D2D communication frequency or an operation frequency. According to another embodiment, the D2DSS detection-related information includes information about a measurement gap for D2DSS detection. The D2DSS detection-related information is provided through an RRC message or system information. Such D2DSS detection-related information is provided to the D2D UEs in advance before the command for attempting to detect the D2DSS is provided. 
         [0072]    Referring to  FIG. 7B , in operation S 730 , the eNB  200  receives a D2DSS detection result. According to an embodiment, the D2DSS detection result includes information about the number, a sequence index, and strength of the D2DSSs transmitted from the D2D located in the area in which it cannot communicate with the eNB, and information about the number, a sequence index, and strength of the D2DSSs transmitted from the D2D UE that communicates with the eNB. According to another embodiment, the D2DSS detection result further includes information about the strength of servicing cell signals. According to still another embodiment, the D2DSS detection result further includes information about a battery level of the UE. 
         [0073]    In operation S 740 , the eNB  200  selects a UE to which the D2DSS is to be transmitted based on the received D2DSS detection result. According to an embodiment, the eNB  200  selects a UE proximity to the UE located in the area in which it cannot communicate with the eNB such as the shadow area while being located in a cell boundary, among the UEs belonging to the area of the eNB  200 . In operation S 750 , the eNB  200  commands the selected UE to transmit the D2DSS. 
         [0074]      FIGS. 8A ,  8 B, and  8 C are views illustrating example processing flows of a D2DSS detection operation in a D2D UE according to this disclosure. These processing flows can be performed by the D2D UE  100  shown in  FIG. 1 . 
         [0075]    Referring to  FIG. 8A , in operation S 810 , the D2D UE  100  receives the command for attempting to detect the D2DSS from the eNB  200 . According to an embodiment, the command for attempting to detect the D2DSS is issued to all of the D2D UEs belonging to the eNB  200  in a broadcast manner. According to another embodiment, the command for attempting to detect the D2DSS is issued only to a dedicated UE among the D2D UEs belonging to the eNB  200 . According to still another embodiment, the command for attempting to detect the D2DSS is issued only to a UE that satisfies a specific condition among the D2D UEs belonging to the eNB  200 . For example, the specific condition is a case in which the signal strength of the serving cell is smaller than a specific threshold value. 
         [0076]    In addition, in operation S 810 , the D2D UE  100  receives D2DSS detection-related information from the eNB  200 . According to an embodiment, the D2DSS detection-related information includes information about a. D2D communication frequency or an operation frequency. According to another embodiment, the D2DSS detection-related information includes information about a measurement gap for D2DSS detection. The D2DSS detection-related information is provided through an RRC message or system information. Such D2DSS detection-related information is received in advance before the command for attempting to detect the D2DSS is received. 
         [0077]    In operation S 820 , the D2D UE  100  attempts to detect the D2DSS in response to the received command for attempting to detect the D2DSS. In operation S 830 , the D2D UE  100  reports a D2DSS detection result to the eNB  200 . According to an embodiment, the D2DSS detection result includes information about the number, a sequence index, and strength of the D2DSSs transmitted from the D2D UE located in the area in which it cannot communicate with the eNB, and information about the number, a sequence index, and strength of the D2DSSs transmitted from the D2D UE that communicates with the eNB. According to another embodiment, the D2DSS detection result further includes information about the strength of servicing cell signals. According to still another embodiment, the D2DSS detection result further includes information about a battery level of the UE. 
         [0078]    Referring to  FIG. 8B , in operation S 810 , the D2D UE  100  receives, from the eNB  200 , the command for attempting to detect the D2DSS and D2DSS detection attempt-related information. In operation S 820 , the D2D UE  100  attempts to detect the D2DSS in response to the received command for attempting to detect the D2DSS. In operation S 825 , the D2D UE  100  that is currently in an RRC_Idle state switches from the RRC_Idle state to an RRC_Connected state in which the corresponding detection result is reported to the eNB, by performing an RRC connection establishment process. In operation S 830 , the D2D UE  100  reports a D2DSS detection result to the eNB  200 . 
         [0079]    Referring to  FIG. 8C , in operation S 810 , the D2D UE  100  receives, from the eNB  200 , the command for attempting to detect the D2DSS and the D2DSS detection attempt-related information. In operation S 820 , the D2D UE  100  attempts to detect the D2DSS in response to the received command for attempting to detect the D2DSS. In operation S 835 , the D2D UE  100  reports the D2DSS detection result to the eNB  200  only when the D2DSS detection result satisfies the specific condition. Such specific condition is provided by the eNB  200  in advance. According to an embodiment, the specific condition includes a case in which the reception signal strength of the D2DSS transmitted from the D2D UE located in the area in which it cannot communicate with the eNB such as the shadow area is larger than a specific threshold value, or a case in which at least two of such signals are detected. According to another embodiment, the specific condition includes a case in which the reception signal strength of the D2DSS transmitted from the D2D UE that communicates with the eNB is larger than a specific threshold value, or a case in which at least two of such signals are detected. According to still another embodiment, the specific condition includes a case in which the signal strength (such as Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), Signal to interference plus Noise Ratio (SINR), or the like) of the serving cell signals is smaller than a specific threshold value. According to yet another embodiment, the specific condition includes a combination of the above-mentioned conditions. 
         [0080]      FIGS. 9A and 9B  are views illustrating example processing flows of a D2DSS transmission operation in a D2D UE according to this disclosure. These processing flows can be performed by the D2D UE  100  shown in  FIG. 1 . In this instance, the D2D UE  100  is a UE that is selected by the eNB  200  based on the D2DSS detection result. Referring to  FIG. 9A , in operation S 910 , the D2D UE  100  receives a D2DSS transmission command from the eNB  200 . In operation S 920 , the D2D UE  100  transmits the D2DSS to the UE  110  in response to the received D2DSS transmission command. Referring to  FIG. 9B , in operation S 915 , the D2D UE  100  determines whether the D2DSS detection result satisfies a specific condition. In operation S 920 , when it is determined that the D2DSS detection result satisfies the specific condition, the D2D UE  100  transmits the D2DSS to the UE  110 . 
         [0081]      FIG. 10  is a view illustrating an example processing flow of an operation of acquiring synchronization information between D2D UEs according to this disclosure. This processing flow can be performed by the D2D UE  110  shown in  FIG. 1 . Referring to  FIG. 10 , in operation S 1010 , the D2D UE  110  receives the D2DSS transmitted from the D2D UE  100  belonging to the eNB. In operation S 1020 , the D2D UE  110  acquires synchronization information from the received D2DSS. In operation S 1030 , the D2D UE  110  performs D2D communication with the D2D UE  100  belonging to the eNB using the acquired synchronization information. 
         [0082]      FIG. 11  is a view showing an operation of acquiring synchronization information between D2D UEs according to another exemplary embodiment of the present disclosure. Referring to  FIG. 11 , D2D UEs  100 A and  100 B which belong to asynchronous eNBs (or cells) different from each other mutually acquire synchronization information. For example, the D2D UE  100 A belonging to an eNB  200 A receives the D2DSS such as a D2D synchronization signal from the D2D UE  100 B belonging to an eNB  200 B adjacent to the eNB  200 A, thereby acquiring the synchronization information. By way of another example, the D2D UE  100 B belonging to the eNB  200 B receives the D2DSS such as the D2D synchronization signal from the D2D UE  100 A belonging to the adjacent eNB  200 A, thereby acquiring the synchronization information. 
         [0083]    As to a basic flow according to this disclosure as shown in  FIGS. 12A ,  12 B,  12 C, and  12 D, in operation S 1210 , an eNB (such as the eNB  200 A) commands D2D UEs (such as the UE  100 A) to attempt to detect the D2DSS, and transmits information required for the detection. The D2D UEs belonging to the eNB attempt to detect the corresponding D2DSS in operation S 1220 , and report the corresponding detected result to the eNB in operation S 1230 , as necessary. In operation S 1240 , the eNB commands a specific D2D UE to transmit the D2DSS based on the detected result that has been reported by a large number of D2D UEs. When failing to detect a cellular synchronization signal (SS) transmitted from the adjacent eNB, the D2D UEs acquires synchronization information of the D2D UEs belonging to the adjacent eNB based on the D2DSS transmitted from the UE belonging to the adjacent eNB. 
         [0084]      FIGS. 12A ,  12 B,  12 C, and  12 D are views illustrating example processing flows between the eNB  200 A and the D2D UE  100 A for an operation of acquiring synchronization information between D2D UEs according to this disclosure. Here, a case in which only a single D2D UE  100 A belonging to the eNB  200 A performs the D2DSS detection operation is described. However, this is merely for convenience of description, and thus the D2DSS detection operation is performed by one or more D2D UEs belonging to the eNB  200 A. 
         [0085]    Referring to  FIG. 12A , in operation S 1210 , the eNB  200 A transmits a command for attempting to detect the D2DSS and the related information to the D2D UE  100 A. In operation S 1220 , the D2D UE  100 A attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information which have been transmitted from the eNB  200 A. In operation S 1230 , the D2D UE  100 A reports a D2DSS detection attempt result to the eNB  200 A. In operation S 1240 , the eNB  200 A receives the D2DSS detection attempt result from the D2D UE  100 A, selects a UE to which the D2DSS is to be transmitted based on the received result, and commands the selected UE to transmit the D2DSS. In operation S 1250 , the D2D UE  100 A that has received a D2DSS transmission command from the eNB  200 A transmits the D2DSS to the D2D UE  100 B that belongs to the eNB  200 B. 
         [0086]    Referring to  FIG. 12B , in operation S 1210 , the eNB  200 A transmits the command for attempting to detect the D2DSS and the related information to the D2D UE  100 A. In operation S 1220 , the D2D UE  100 A attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information that has been transmitted from the eNB  200 A. The D2D UE  100 A that is in an RRC_Idle state switches from the RRC_Idle state to an RRC_Connected state in operation S 1225 , and reports the D2DSS detection attempt result to the eNB  200 A in operation S 1230 . In operation S 1240 , the eNB  200 A receives the D2DSS detection attempt result from the D2D UE  100 A, selects a UE to which the D2DSS is to be transmitted based on the received result, and commands the selected UE to transmit the D2DSS. In operation S 1250 , the D2D UE  100 A that has received the D2DSS transmission command from the eNB  200 A transmits the D2DSS to the D2D UE  110 B that belongs to the eNB  200 B. 
         [0087]    Referring to  FIG. 12C , in operation S 1210 , the eNB  200 A transmits the command for attempting to detect the D2DSS and the related information to the D2D UE  100 A. In operation S 1220 , the D2D UE  100 A attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information which have been transmitted from the eNB  200 A. In operation S 1235 , the D2D UE  100 A reports the D2DSS detection attempt result to the eNB  200 A when the corresponding result satisfies a specific condition. In operation S 1240 , the eNB  200 A receives the D2DSS detection attempt result from the D2D UE  100 A, selects a UE to which the D2DSS is to be transmitted based on the received result, and commands the selected UE to transmit the D2DSS. In operation S 1250 , the D2D UE  100 A that has received a D2DSS transmission command from the eNB  200 A transmits the D2DSS to the D2D UE  100 B that belongs to the eNB  200 B. 
         [0088]    Referring to  FIG. 12D , in operation S 1210 , the eNB  200 A transmits the command for attempting to detect the D2DSS and the related information to the D2D UE  100 A. In operation S 1220 , the D2D UE  100 A attempts to detect the D2DSS in response to the command for attempting to detect the D2DSS and the related information which have been transmitted from the eNB  200 A. The D2D UE  100 A confirms a D2DSS detection result that satisfies the specific condition in operation S 1245 , and transmits the D2DSS to the D2D UE  100 B that belongs to the eNB  200 B in operation S 1250 . 
         [0089]    Hereinafter, the operations shown in  FIGS. 12A to 12D  will be specifically discussed. Here, a case in which the D2DSS detection operation is performed by a single D2D UE  100 A is shown, but this is merely for convenience of description. For example, the D2DSS detection operation is performed by one or more D2D UEs. 
         [0090]    In operation S 1210 , the eNB  200 A commands the D2D UE  100 A belonging to the corresponding eNB to attempt to detect the D2DSS. Here, the D2DSS is a signal transmitted from the D2D UE belonging to a serving eNB. According to an embodiment, the command for attempting to detect the D2DSS is issued to all of the D2D UEs which are in the RRC_Idle state and the RRC_Connected state in a broadcast manner. According to another embodiment, the command for attempting to detect the D2DSS is issued only to a specific D2D UE in a dedicated manner. According to still another embodiment, the command for attempting to detect the D2DSS is issued only to UEs that satisfy the specific condition. For example, the eNB  200 A commands, to attempt to detect the D2DSS, only the D2D UEs whose signal strength such as RSRP of the serving cell is smaller than a specific threshold value. 
         [0091]      FIGS. 13A and 13B  are views illustrating an example operation of transmitting a command for attempting to detect a D2DSS and the related information from an eNB to a UE according to this disclosure. Referring to  FIG. 13A , the eNB  200 A issues the command for attempting to detect the D2DSS to all of the D2D UEs  100 A ( 100 A- 1 ,  100 A- 2 ,  100 A- 3 , and  100 A- 4 ) in a broadcast manner. 
         [0092]    Referring to  FIG. 13B , the eNB  200 A issues the command for attempting to detect the D2DSS only to the specific D2D UE  100 A ( 100 A- 2 ). The specific D2D UE  100 A ( 100 A- 2 ) is a dedicatedly set UE or a UE that satisfies the specific condition. Referring again to operation S 1210 . the eNB  200 A provides the D2DSS detection-related information to the D2D UE  100 A. A detection condition for the D2DSS is determined based on an absolute or relative signal strength (such as RSSI, RSRP, or the like). According to an embodiment, when issuing the command for attempting to detect the D2DSS, the eNB  200 A notifies the D2D UEs of the corresponding detection condition. According to another embodiment, the detection condition is promised in advance between the eNB  200 A and the UE  100 A. 
         [0093]    In operation S 1220 , the D2D UEs  100 A that have received the command for attempting to detect the D2DSS attempts to detect the D2DSS. In operation S 1230 , the D2D UEs report the D2DSS detection result to the eNB  200 A. The D2D UEs which are currently in the RRC_Idle state performs an RRC connection establishment process for the transition to the RRC_Connected state. For example, the UE enters a mode (or state) that reports the corresponding result to the eNB. In operation S 1235 , only when the D2DSS detection result satisfies the specific condition even though the D2DSS is detected, the D2D UEs reports the D2DSS detection result to the eNB  200 A, Such specific condition is requested in advance by the eNB  200 A, and includes contents of the following Table 3. 
         [0000]    
       
         
               
               
             
               
             
               
               
             
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 (1) in which the reception signal strength of the D2DSS transmitted 
               
             
          
           
               
                 from the D2D UE within the same eNB is larger than a specific threshold 
               
               
                 value, or a case in which at least two of such signals are detected 
               
             
          
           
               
                   
                 (2) a case in which the signal strength (such as RSRP, RSRQ, SINR, 
               
             
          
           
               
                 or the like) of the serving cell signal is smaller than a specific 
               
               
                 threshold value 
               
               
                   
               
             
          
         
       
     
         [0094]    In operation S 1230  or operation S 1235 , the DD UEs report the D2DSS detection result to a serving cell, such as, the eNB  200 A. The detection result reported to the eNB  200 A includes the contents of the following Table 4. 
         [0000]    
       
         
               
               
             
               
             
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 (1) the number, a sequence index (identifier), a signal strength, 
               
             
          
           
               
                 or the like of the D2DSSs transmitted from the D2D UE within the 
               
               
                 same eNB 
               
             
          
           
               
                   
                 (2) signal strength of serving cell signal 
               
               
                   
                 (3) battery level of the UE 
               
               
                   
                   
               
             
          
         
       
     
         [0095]    In operation S 1240 , the eNB  200 A selects the specific UE to which the D2DSS is to be transmitted based on the detected result that has been reported in operation S 1230  or operation S 1235 , and commands the selected specific UE to transmit the D2DSS. In operation S 1250 , the specific UE that has received the D2DSS transmission command from the eNB  200 A transmits the D2DSS. The transmission of the D2DSS is performed without the command from the eNB  200 A. The transmission of the D2DSS is performed without the command from the eNB  200 A. For example, in operation S 1245 , the UE that has confirmed the detected result that satisfies the condition requested by the eNB  200 A transmits the D2DSS without an additional command of the eNB in operation S 1250 . The eNB  200 A activates the UE which remains in the RRC_Idle state until operation S 1230  is completed, through paging or the like, so that the corresponding UE becomes in the RRC_Connected state. The eNB  200 A commands the UEs selected for D2DSS transmission to periodically report the detected result, and thereby continuously update and manage a pool of the D2D UE that transmits the D2DSS. 
         [0096]      FIG. 14  is a view illustrating an example communication frame structure in a D2D UE to which this disclosure is applied. Referring to  FIG. 14 , a communication frame includes a cellular communication gap S 1410  and a D2D communication gap S 1420 . When failing to detect a cellular synchronization signal (SS) transmitted from the adjacent eNB, the D2D UEs acquires synchronization information of the D2D UEs belonging to the adjacent eNB based on the D2DSS transmitted from the UE belonging to the adjacent eNB. First, when a D2D communication frequency of the adjacent eNB is different from that of the eNB, the eNB notifies the UEs of information about the D2D communication frequency in advance through an RRC message or system information. 
         [0097]    The UE in which one reception chain is provided and that currently communicates with a serving eNB is difficult to detect the D2DSS. In this case, the eNB allocates a measurement gap to the UE through the RRC message or the like. For example, in the corresponding measurement gap, the UE does not transmit and receive signals with the serving eNB. However, the D2D signal such as a discovery signal is transmitted in a significantly long period such as in units of several hundreds of milliseconds (ms) to several seconds, as shown in  FIG. 14 . In this case, when the measurement gap is set according to the signal period of the D2DSS transmitted in the D2D communication gap S 1420 , a measurement gap that does not communicate with the serving eNB is significantly increased, which causes a problem in cellular communication. Thus, if the D2D UE is aware of an approximate time difference between the eNBs using SFN (System Frame Number) or the like, it is possible to more easily detect the D2DSS. According to an embodiment, time difference information such as an SFN difference between the eNBs is mutually shared through an interface between the eNBs. According to another embodiment, a high-order entity of the eNB transmits the time difference information such as the SFN difference between the eNBs, to each eNB. According to still another embodiment, in order to acquire the time difference information such as the SFN difference between the eNBs, the eNB commands a UE located in a cell boundary to measure and report SFN difference information with the adjacent cell. The eNB that has acquired the SFN difference information in this manner notifies the D2D LIB of the corresponding information, and set the measurement gap to be small using the corresponding information. 
         [0098]      FIG. 15  is a view illustrating example patterns of measurement gaps by a D2DSS detection operation in a UE according to this disclosure. Referring to  FIG. 15 , an adjacent cell and a serving cell are deviated in synchronization. In this case, the D2D communication gap S 1420  is 1 frame, and a D2DSS transmission gap S 1425  is included in the D2D communication gap S 1420 ) and thereby smaller than 1 frame. In order to detect the D2DSS, in one embodiment, a measurement gap S 1430  is used, and in another embodiment, a D2D Rx GAP S 1440  is used. 
         [0099]    In one embodiment, in a case in which the serving eNB is aware of the SFN difference with the adjacent eNB, when 1 frame or 1.1 frame slightly larger than 1 frame is set as the measurement gap S 1430  and allocated to the UE, a communication interruption time with the serving cell is significantly reduced. In the other embodiment, when 2 frame or 2.1 frame slightly larger than 2 frame is allocated to the UE as the D2D Rx GAP S 1440 , the UE measures the D2DSS and receive a D2D signal. For this, the serving eNB should be aware of D2D resource configuration information of the adjacent eNB. According to an embodiment, the D2D resource configuration information is mutually shared through the interface between the eNBs. According to another embodiment, the high-order entity of the eNB transmits the D2D resource configuration information to each eNB. According to still another embodiment, the eNB commands a UE located in a cell boundary to acquire and report system information with the adjacent cell. 
         [0100]      FIG. 16  is a view illustrating an example operation of transmitting a D2DSS by a UE selected in an eNB according to this disclosure. Referring to  FIG. 16 , the D2D UE  100 A transmits the D2DSS to the UE  100 B belonging to the adjacent eNB  200 B. According to an embodiment, the D2D UE  100 A transmits the D2DSS to the UE  100 B belonging to the adjacent eNB  200 B in response to a D2DSS transmission command received from the eNB  200 A. According to another embodiment, the D2D UE  100 A determines whether a D2DSS detection result satisfies a specific condition, and transmits the D2DSS to the UE  100 B belonging to the adjacent eNB  200 B when it is determined that the D2DSS detection result satisfies the specific condition. 
         [0101]      FIGS. 17A and 17B  are views showing processing flows in an eNB for acquiring synchronization information between D2D UEs according to another exemplary embodiment of the present disclosure, These processing flows are performed by the eNB  200 A shown in  FIG. 11 . Referring to  FIG. 17A , in operation S 1710 , the eNB  200 A issues a command for attempting to detect the D2DSS to D2D UEs belonging to the eNB  200 A. According to an embodiment, the command for attempting to detect the D2DSS is issued to all of the D2D UEs belonging to the eNB  200 A in a broadcast manner. According to another embodiment, the command for attempting to detect the D2DSS is issued only to a dedicated UE among the D2D UEs belonging to the eNB  200 A. According to still another embodiment, the command for attempting to detect the D2DSS is issued only to a UE that satisfies a specific condition among the D2D UEs belonging to the eNB  200 A. For example, the specific condition is a case in which the signal strength of the serving cell is smaller than a specific threshold value. 
         [0102]    In addition, in operation S 1710 , the eNB  200 A transmits D2DSS detection-related information to the D2D UEs belonging to the eNB  200 A. According to an embodiment, the D2DSS detection-related information includes information about a D2D communication frequency or an operation frequency. According to another embodiment, the D2DSS detection-related information includes information about a measurement gap (S 1430  or S 1440  of  FIG. 15 ) for D2DSS detection. The D2DSS detection-related information is provided through an RRC message or system information. Such D2DSS detection-related information is provided to the D2D UEs in advance before the command for attempting to detect the D2DSS is provided. 
         [0103]    Referring to  FIG. 17B , in operation S 1730 , the eNB  200 A receives a D2DSS detection result. According to an embodiment, the D2DSS detection result includes information about the number, a sequence index, and strength of the D2DSSs transmitted from the D2D UE within the same eNB. According to another embodiment, the D2DSS detection result further includes information about the strength of servicing cell signals. According to still another embodiment, the D2DSS detection result further includes information about a battery level of the UE. In operation S 1740 , the eNB  200 A selects a UE to which the D2DSS is to be transmitted based on the received D2DSS detection result. According to an embodiment, the eNB  200 A selects a UE proximity to the UE  100 B belonging to the adjacent eNB  200 B while being located in a cell boundary, among the UEs belonging to the area of the eNB In operation S 1750 , the eNB  200 A commands the selected UE to transmit the D2DSS. 
         [0104]      FIGS. 18A ,  18 B, and  18 C are views illustrating example processing flows of a D2DSS detection operation in a D2D UE according to this disclosure. These processing flows are performed by the D2D UE  100 A shown in  FIG. 11 . Referring to  FIG. 18A , in operation S 1810 , the D2D UE  100 A receives the command for attempting to detect the D2DSS from the eNB  200 A. According to an embodiment, the command for attempting to detect the D2DSS is issued to all of the D2D UEs belonging to the eNB  200 A in a broadcast manner. According to another embodiment, the command for attempting to detect the D2DSS is issued only to a dedicated UE among the D2D UEs belonging to the eNB  200 A. According to still another embodiment, the command for attempting to detect the D2DSS is issued only to a UE that satisfies a specific condition among the D2D UEs belonging to the eNB  200 A. For example, the specific condition is a case in which the signal strength of the serving cell is smaller than a specific threshold value. 
         [0105]    In addition, in operation S 1810 , the D2D UE  100 A receives D2DSS detection-related information from the eNB  200 A. According to an embodiment, the D2DSS detection-related information includes information about a D2D communication frequency or an operation frequency. According to another embodiment, the D2DSS detection-related information includes information about a measurement gap for D2DSS detection. The D2DSS detection-related information is provided through an RRC message or system information. Such D2DSS detection-related information is received in advance before the command for attempting to detect the D2DSS is received. 
         [0106]    In operation S 1820 , the D2D UE  100 A attempts to detect the D2DSS in response to the received command for attempting to detect the D2DSS. In operation S 1830 , the D2D UE  100 A reports a D2DSS detection result to the eNB  200 A. According to an embodiment, the D2DSS detection result includes information about the number, a sequence index, and strength of the D2DSSs transmitted from the D2D UE within the same eNB. According to another embodiment, the D2DSS detection result further includes information about the strength of servicing cell signals. According to still another embodiment, the D2DSS detection result further includes information about a battery level of the UE. 
         [0107]    Referring to  FIG. 18B , in operation S 1810 , the D2D UE  100 A receives, from the eNB  200 A, the command for attempting to detect the D2DSS and D2DSS detection attempt-related information. In operation S 1820 , the D2D UE  100 A attempts to detect the D2DSS in response to the received command for attempting to detect the D2DSS. In operation S 1825 , the D2D UE  100 A that is currently in an RRC_Idle state switches from the RRC_Idle state to an RRC_Connected state in which the corresponding detection result is reported to the eNB, by performing an RRC connection establishment process. In operation S 1830 , the D2D UE  100 A reports a D2DSS detection result to the eNB  200 A. 
         [0108]    Referring to  FIG. 18C , in operation S 1810 , the D2D UE  100 A receives, from the eNB  200 A, the command for attempting to detect the D2DSS and the D2DSS detection attempt-related information. In operation S 1820 . the D2D UE  100 A attempts to detect the D2DSS in response to the received command for attempting to detect the D2DSS. In operation S 1835 , the D2D UE  100 A reports the D2DSS detection result to the eNB  200 A only when the D2DSS detection result satisfies the specific condition. Such a specific condition is provided by the eNB  200 A in advance. According to an embodiment, the specific condition includes a case in which the reception signal strength of the D2DSS transmitted from the D2D UE within the same eNB is larger than a specific threshold value, or a case in which at least two of such signals are detected. According to another embodiment, the specific condition includes a case in which the signal strength (such as RSRP, RSRQ, SINR, or the like) of the serving cell signals is smaller than a specific threshold value. According to still another embodiment, the specific condition includes a combination of the case in which the reception signal strength of the D2DSS transmitted from the D2D UE within the same eNB is larger than the specific threshold value or the case in which at least two of such signals are detected, and the case in which the signal strength (such as RSRP, RSRQ, SINR, or the like) of the serving cell signals is smaller than a specific threshold value. 
         [0109]      FIGS. 19A and 19B  are views illustrating example processing flows of a D2DSS transmission operation in a D2D UE according to this disclosure. These processing flows are performed by the D2D UE  100 A shown in  FIG. 11 . In this instance, the D2D  100 A is a UE that is selected by the eNB  200 A based on the D2DSS detection result. Referring to  FIG. 19A , in operation S 1910 , the D2D UE  100 A receives a D2DSS transmission command from the eNB  200 A. In operation S 1920 , the D2D UE  100 A transmits the D2DSS to the UE  100 B belonging to the adjacent eNB  200 B in response to the received D2DSS transmission command. Referring to  FIG. 19B , in operation S 1915 , the D2D UE  100 A determines whether the D2DSS detection result satisfies a specific condition. In operation S 1920 , when it is determined that the D2DSS detection result satisfies the specific condition, the D2D UE  100 A transmits the D2DSS to the UE  100 B belonging to the adjacent eNB  200 B. 
         [0110]      FIG. 20  is a view showing a processing flow of an operation of acquiring synchronization information between D2D UEs according to another exemplary embodiment of the present disclosure. This processing flow are performed by the D2D UE  110 B shown in  FIG. 11 . Referring to  FIG. 20 , in operation S 2010 , the D2D UE  1003  belonging to the eNB  200 B receives the D2DSS transmitted from the D2D UE  100 A belonging to the adjacent eNB  200 A. In operation S 2020 , the D2D UE  100 B acquires synchronization information from the received D2DSS. In operation S 2030 , the D2D UE  110 B performs D2D communication with the D2D UE  100 A belonging to the eNB  200 A using the acquired synchronization information. 
         [0111]      FIG. 21  is a view showing the structure of an eNB device for an operation of acquiring synchronization information between D2D UEs according to exemplary embodiments of the present disclosure. Such an eNB device is included in the eNB  200  shown in  FIG. 1  or the eNB  200 A shown in  FIG. 11 . Referring to  FIG. 21 , an eNB includes a wireless transmission and reception unit  210 , a control unit  220 , a storage unit  230 , and a network communication unit  240 . The wireless transmission and reception unit  210  (or transceiver) performs a function for transmitting and receiving signals through a wireless channel, such as band conversion or amplification of signals. The wireless transmission and reception unit  210  up-converts a baseband signal for transmission into an RF band signal, and then transmits the up-converted signal via an antenna. In addition, the wireless transmission and reception unit  210  down-converts the RE band signal received via the antenna into the baseband signal. For example, the wireless transmission and reception unit  210  includes a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, and the like. In  FIG. 21 , a case in which only a single antenna is included in the eNB is shown, but the eNB includes a plurality of antennas. 
         [0112]    The network communication unit  240  provides an interface for performing communication with other nodes within a network. For example, the network communication unit  240  processes signals transmitted to other eNBs or a high-order (such as core network) entity, and processes signals received from the other eNBs or the high-order entity. According to an embodiment, the network communication unit  240  receives, from the high-order entity, information concerning that a shadow area is present in a place adjacent to the corresponding eNB or the adjacent eNB is not properly operated. 
         [0113]    The storage unit  230  stores basic programs for operating the eNB, application programs, and data such as setting information. In addition, the storage unit  230  provides the stored data in response to the request of the control unit  220 . The control unit  220  controls overall operations of the eNB. The control unit  220  includes at least one processor. According to the embodiments of the present disclosure, the control unit  220  performs control operations so that the D2D UEs in which no synchronization is taken place mutually acquires synchronization information. According to an embodiment, the control unit  220  performs operations according to the processing flow shown in  FIGS. 2A to 2D  and the processing flow shown in  FIGS. 7A and 7B . According to another embodiment, the control unit  220  performs operations according to the processing flow shown in  FIGS. 12A to 12D  and the processing flow shown in  FIGS. 17A and 17B . 
         [0114]    According to an embodiment of the present disclosure, an eNB device that serves a second UE inside a cell for D2D communication with a first UE outside the cell includes a transmission and reception unit and a control unit. The transmission and reception unit commands a UE inside at least one cell including the second UE to detect a D2DSS. The control unit selects a specific UE among at least one UEs based on a D2DSS detection result that has been reported by the at least one UE. The transmission and reception unit commands the selected specific UE to transmit the D2DSS to the first UE. 
         [0115]    According to an embodiment of the present disclosure, an eNB device includes a receiver that receives the D2DSS detection result from the UE. The D2DSS detection result includes a result obtained by detecting a synchronization signal of another UE in a predetermined measurement gap by the UE. A start time point of the measurement gap in a first measurement gap period and a start time point of the measurement gap in a second measurement gap period are different from each other. According to an embodiment, an offset pattern of the start time point of the measurement gap in the first measurement gap period and an offset pattern of the start time point of the measurement gap in the second measurement gap period are different from each other. 
         [0116]    According to an embodiment, a sum of the measurement gaps in each of predetermined measurement gap periods is larger than or equal to a transmission period of the synchronization signal between the UE and the other UE. According to an embodiment, the measurement gap is a gap in which the UE does not perform communication with the eNB. According to an embodiment, the eNB device further includes a transmitter that transmits information about the measurement gap and the first and second measurement gap periods to the UE. According to an embodiment, the detection result includes the number, the strength, and an identifier of the synchronization signals transmitted from the other UE. 
         [0117]      FIG. 22  is a view illustrating an example structure of a D2D UE device for an operation of acquiring synchronization information between D2D UEs according to this disclosure. Such a UE device can be included in the UE  100  shown in  FIG. 1 . The UE  100 A shown in  FIG. 11  can be configured in the same manner. 
         [0118]    Referring to  FIG. 22 , the UE device  100  includes a bus  10 , a processor  20 , a memory  30 , an I/O (input/output) interface  40 , a display  50 , and a communication interface  60 . 
         [0119]    The bus  10  is a circuit that mutually connects the above-described components and transmits communication (such as a control message) among the above-described components. 
         [0120]    The processor  20  receives instructions from the above-described other components (such as the memory  30 , the I/O interface  40 , the display  50 , the communication interface  60 , or the like) through the bus  10 , decode the received instructions, and perform operations or data processing according to the decoded instructions. 
         [0121]    The memory  30  stores an instruction or data received from the processor  20  or the other components (such as the I/O interface  40 , the display  50 , the communication interface  60 , or the like) or generated by the processor  20  or the other components. The memory  30  includes programming modules, for example, a kernel  31 , middleware  32 , an Application Programming Interface (API)  33 , applications  34 , and the like. In addition, the memory  30  includes a synchronization processing module  35  for a synchronization operation (or synchronization function) according to various embodiments of the present disclosure. The above-described individual programming modules are formed with hardware, or a combination of hardware and at least one of software or firmware. 
         [0122]    The kernel  31  controls or manage system resources (such as the bus  10 , the processor  20 , or the memory  30 ) used to execute operations or functions which have been implemented in the remaining other programming modules, for example, the middleware  32 , the API  33 , the applications  34 , and the synchronization processing module  35 . In addition, the kernel  31  provides an interface that accesses individual components of the UE device  100  in the middleware  32 , the API  33 , the applications  34 , or the synchronization processing module  35  to thereby control or manage the individual components thereof. 
         [0123]    The middleware  32  acts as an intermediary so that the API  33 , the applications  34 , or the synchronization module  35  communicates with the kernel  31  to transmit and receive data. In addition, with regard to operation requests received from the applications  34 , the middleware  32  performs control (such as scheduling or load balancing) for the operation requests, using a method of assigning a priority capable of using the system resources (such as the bus  10 , the processor  20 , or the memory  30 ) of the UR device  100  to, for example, at least one application of the applications  34 , or the like. 
         [0124]    The API  33  is an interface for controlling a function provided from the kernel  31  or the middle  32  by the applications  34 , and includes, for example, at least one interface or function (such as a command) for file control, window control, image processing, or character control, and the like. 
         [0125]    The applications  34  include a variety of applications. For example, the applications  34  include an SMS/MMS application, an e-mail application, a calendar application, an alarm application, a health care application (such as an application for measuring the quantity of exercise or blood glucose, or the like), an environmental information application (such as an application for providing air pressure, humidity, temperature information, and the like), and the like. 
         [0126]    The synchronization processing module  35  performs a control operation so that the D2D UEs in which no synchronization is taken place mutually acquires synchronization information. According to an embodiment, the synchronization processing module  35  performs operations according to the processing flows shown in  FIGS. 2A to 2D , the processing flows shown in  FIGS. 8   a  to  8   c,  the processing flows shown in  FIGS. 9A and 9B , and the processing flow shown in  FIG. 10 . According to another embodiment, the synchronization processing module  35  performs operations according to the processing flows shown in  FIGS. 12A to 12D , the processing flows shown in  FIGS. 18A to 18C , the processing flows shown in  FIGS. 19A and 19B , and the processing flow shown in  FIG. 20 . 
         [0127]    The I/O interface  40  transmits an instruction or data input from a user through an input and output device (such as a sensor, a keyboard, or a touch screen), to the processor  20 , the memory  30 , the communication interface  60  through, for example, the bus  10 . For example, the I/O interface  40  provides data about a user&#39;s touch input through a touch screen to the processor  20 . In addition, the I/O interface  40  outputs the instruction or data received from the processor  20 , the memory  30 , and the communication interface  60  via, for example, the bus  10 , through the input and output device (such as a speaker or a display). For example, the I/O interface  40  outputs voice data that has been processed through the processor  20  to a user through the speaker. 
         [0128]    The display  50  displays a variety of information (such as multimedia data or text data) to a user. The communication interface  60  (or the transceiver) connects communication between the UE  100  and the eNB  200  and communication between the UE  100  and another UE  110 . For example, the communication interface  60  is connected to a network  140  through wireless communication to support communication between the UE  100  and the eNB  200 . By way of another example, the communication interface  60  is connected to the network  140  through wireless communication to support D2D communication between the UE and the other UE  110 . The wireless communication includes at least one of, for example, Wireless Fidelity (Wi Fi), BLUETOOTH® (BT), Near Field Communication (NFC), Global Positioning System (GPS), and cellular communication (such as LTE, LTE-A, CDMA, WCDMA, UMTS, WiBro, GSM, or the like). 
         [0129]    The network  140  is a telecommunications network. The telecommunications network includes at least one of a computer network, the Internet, the Internet of things, and a telephone network. According to an embodiment, a protocol for D2D communication between the UE  100  and the external UE  110  is supported from at least one of the applications  34 , the application programming interface  33 , the middleware  32 , the kernel  31 , and the communication interface  60 . 
         [0130]    According to an embodiment of the present disclosure, a second UE device inside a cell for D2D communication with a first UE outside the cell includes a communication interface and a synchronization processing module. The synchronization processing module detects a D2DSS in response to a command for detecting the D2DSS transmitted from an eNB that serves the second UE, reports the D2DSS detection result to the eNB through the communication interface, and transmits the D2DSS to the first UE in response to a D2DSS transmission command received from the eNB after reporting the D2DSS detection result. 
         [0131]    A UE device according to an embodiment of the present disclosure includes a processor that detects a synchronization signal of another UE in a predetermined measurement gap, and a transmitter that transmits the detection result to an eNB. A start time point of the measurement gap in a first measurement gap period and a start time point of the measurement gap in a second measurement gap period are different from each other. According to an embodiment, an offset pattern of the start time point of the measurement gap in the first measurement gap period and an offset pattern of the start time point of the measurement gap in the second measurement gap period are different from each other. According to an embodiment, a sum of the measurement gaps in each of predetermined measurement gap periods is larger than or equal to a transmission period of the synchronization signal between the UE and the other UE. According to an embodiment, the measurement gap is a gap in which the UE does not perform communication with the eNB. 
         [0132]    According to an embodiment, the UE device further includes a receiver that receives information about the measurement gap and the first and second measurement gap periods from the eNB. According to an embodiment of the present disclosure, the transmitter transmits the detected result to the eNB when the detected result satisfies a specific condition. The specific condition includes one of a case in which a reception signal strength of the synchronization signal transmitted from the other UE is larger than a specific threshold value and a case in which a reception signal strength of a signal transmitted from the eNB is smaller than a specific threshold value. According to an embodiment, the detected result includes the number, strength, and an identifier of the synchronization signals transmitted from the other UE. According to an embodiment, the transmitter further transmits the synchronization signal to the other UE. 
         [0133]    As described herein, according to the embodiments of the present disclosure, there are provided the method and apparatus that mutually acquires synchronization information by the D2D UEs in which no synchronization is taken place, thereby improving the efficiency of synchronization signal searching performed by the neighboring D2D UEs. 
         [0134]    Although the present disclosure has been described by the restricted embodiments and the drawings as described above, the present disclosure is not limited to the aforementioned embodiments, and various modifications and alterations is made from the descriptions by those skilled in the art to which the present disclosure pertains. 
         [0135]    Operations according to this disclosure can be implemented by a single controller. In this case, program commands for executing operations, which are implemented by various computers, can be recorded in a computer-readable recording medium. The computer readable medium includes a program command, a data file, a data structure, and the like independently or in combination. The program commands can be those designed and configured especially for the present disclosure or those known to and usable by a person skilled in the art. For example, the computer readable recoding medium includes magnetic media such as a hard disk, a floppy disk, and a magnetic tape, optical media such as a CD-ROM and a DVD, magneto-optical media such as a floptical disk, and hardware devices such as a ROM, RAM, and a flash memory, which are specially constructed in such a manner that they stores and execute a program command. Examples of the program command include a machine language code generated by a compiler and a high-level language code executable by a computer through an interpreter and the like. When all or some of the eNBs or the relays described in the present disclosure are implemented by a computer program, a computer-readable recording medium that stores the computer program is also included in the present disclosure. Therefore, the scope of the present disclosure is not limited to the embodiment described above, and should be defined by the accompanying claims and the equivalents of the claims. 
         [0136]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.