Abstract:
The present disclosure relates to a pre-5th-generation (5G) or 5G communication system to be provided for supporting higher data rates beyond 4th-generation (4G) communication system such as a long term evolution (LTE). 
     A method for providing a device-to-device (D2D) communication-based service for an isolated user equipment (IUE) by a relay user equipment (RUE) in a mobile communication system is provided. The method includes establishing a D2D link with an IUE; and supporting establishment of a relay cellular link for the IUE based on the established D2D link, wherein the RUE is within a service coverage of an enhanced node B (eNB).

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a U.S. National Stage application under 35 U.S.C. §371 of an International application filed on Aug. 22, 2014 and assigned application number PCT/KR2014/007833, which claimed the benefit of a Korean patent application filed on Aug. 22, 2013 in the Korean Intellectual Property Office and assigned Serial number 10-2013-0099814, the entire disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to an apparatus and method for providing a device-to-device (D2D) communication-based service for an isolated user equipment (IUE) in a mobile communication system. 
       BACKGROUND 
       [0003]    To meet the demand for wireless data traffic, which has increased since deployment of 4th-generation (4G) communication systems, efforts have been made to develop an improved 5th-generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post long-term evolution (LTE) system’. 
         [0004]    It is considered that the 5G communication system will be implemented in millimeter wave (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To reduce propagation loss of radio waves and increase a transmission distance, a beam forming technique, a massive multiple-input multiple-output (MIMO) technique, a full dimensional MIMO (FD-MIMO) technique, an array antenna technique, an analog beam forming technique, and a large scale antenna technique 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, a D2D communication, a wireless backhaul, a moving network, a cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation, and the like. 
         [0006]    In the 5G system, a hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and a sliding window superposition coding (SWSC) as an advanced coding modulation (ACM) scheme, and a filter bank multi carrier (FBMC) scheme, a non-orthogonal multiple Access (NOMA) scheme, and a sparse code multiple access (SCMA) scheme as an advanced access technology have been developed. 
         [0007]    Generally, in a D2D communication system, a device identifies neighbor devices which are proximate to the device itself, and transmits and receives data by establishing a radio link with a specific neighbor device if necessary. This process for establishing a radio link and transmitting and receiving data is locally performed among devices without assistance of additional apparatus, so a D2D communication system has an advantage that rapid market introduction is possible without additional infrastructure compared to other wireless communication system. Further, a D2D communication system may solve a traffic overload problem which is concentrated on a base station or an access point (AP) by locally accepting data traffic. 
         [0008]    So, a standard organization such as a 3rd generation partnership project (3GPP), and a institute of electrical and electronics engineers (IEEE) has standardized a D2D communication standard based on long-term evolution advanced (LTE-A), wireless-fidelity (Wi-Fi), and/or the like, and various D2D communication schemes have been developed. Recently, a 3GPP has actively progressed a standardization study for supporting a proximity service among devices. Specially, a D2D communication scheme which may increase a data rate between neighbor devices and may decrease transmission delay between the neighbor devices has been considered as a scheme which is appropriated for supporting the proximity service among the devices. 
         [0009]    Meanwhile, a 3GPP uses a relay node (RN) in order to increase network capacity of a long term evolution (LTE) system and extend a service coverage. 
         [0010]    A structure of a general LTE mobile communication system including an RN will be described with reference to  FIG. 1 . 
         [0011]      FIG. 1  schematically illustrates a structure of a general LTE mobile communication system including an RN. 
         [0012]    Referring to  FIG. 1 , the LTE mobile communication system includes an evolved packet core (EPC)  110  and an enhanced-universal terrestrial radio access network (E-UTRAN)  120 . The EPC  110  includes an mobile management entity (MME)/serving-gateway (S-GW)  111 , and an MME/S-GW  113 , and the E-UTRAN  120  includes an enhanced node B (eNB)  115 , a donor eNB (DeNB)  117 , and an RN  119 . 
         [0013]    The RN  119  relays a control signal and data traffic between the LTE mobile communication system and a user equipment (UE) by connecting a backhaul link with the LTE mobile communication system using a Un interface as a radio interface. 
         [0014]    The DeNB  117  is an eNB which supports relay communication of the RN  119 , supports a network access of the RN  119 , and modifies a communication parameter of the RN  119  by periodically or a periodically transmitting a control signal. 
         [0015]    The EPC  110  performs an authenticating operation for the RN  119 , and performs an operation for providing parameters used for communication of the RN  119 , and the like. 
         [0016]    In an LTE mobile system, an RN is deployed within a service coverage of a cell or at a boundary of a service coverage of a cell, and supports communication for a UE which exists at a hot-spot area, a shadow area, and a cell boundary area. However, an RN is generally deployed at a fixed location, so the RN is difficult to support communication for a shadow area which may fluidly occur. 
         [0017]    Further, an RN is additionally deployed and operated by a service provider, so deployment and operating cost is relatively expensive. Specially, if an eNB or an RN at a specific area does not perform a normal function due to occurrence of a disaster, UEs which exist at the specific area may not perform normal communication. 
         [0018]    So, in an LTE mobile communication system, there is a need for a scheme for providing normal communication for UEs which exist at a disaster area, a shadow area, a cell boundary area, and the like. 
         [0019]    The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
       SUMMARY 
       [0020]    An embodiment of the present disclosure proposes an apparatus and method for providing a D2D communication-based service for an IUE in a mobile communication system. 
         [0021]    An embodiment of the present disclosure proposes an apparatus and method for providing a D2D communication-based service to an IUE which exists at outside of a service coverage of an eNB in a mobile communication system by establishing a link appropriate for a situation. In accordance with an aspect of the present disclosure, a method for providing a device-to-device (D2D) communication-based service for an isolated user equipment (IUE) by a relay user equipment (RUE) in a mobile communication system is provided. The method includes establishing a D2D link with an IUE; and supporting establishment of a relay cellular link for the IUE based on the established D2D link, wherein the RUE is within a service coverage of an enhanced node B (eNB). 
         [0022]    In accordance with another aspect of the present disclosure, a method for receiving a device-to-device (D2D) communication-based service by an isolated user equipment (IUE) in a mobile communication system is provided. The method includes establishing a D2D link with a relay user equipment (RUE) which is within a service coverage of an enhanced node B (eNB); and transmitting a D2D discovery message to candidate RUEs which exist around the IUE if authentication for the D2D communication is requested in the mobile communication system. 
         [0023]    In accordance with another aspect of the present disclosure, a method for providing a device-to-device (D2D) communication-based service for an isolated user equipment (IUE) by an enhanced node B (eNB) in a mobile communication system is provided. The method includes receiving a message indicating request for communication support for an IUE from each of candidate relay user equipments (RUEs) which determine to support relay communication for the IUE among candidate RUEs which exist around the IUE; and determining whether to support communication for the IUE based on capacity of the eNB and system configuration information. 
         [0024]    In accordance with another aspect of the present disclosure, a method for receiving a device-to-device (D2D) communication-based service by an isolated user equipment (IUE) in a mobile communication system is provided. The method includes performing link change to a serving enhanced node B (SeNB) which is at the same cell or a relay user equipment (RUE) which is at the same cell according to change of a point of attachment (PoA); and performing a handover and link change to a target eNB (TeNB) upon moving into a service coverage of the TeNB different from the SeNB. 
         [0025]    In accordance with another aspect of the present disclosure, a method for providing a device-to-device (D2D) communication-based service for an isolated user equipment (IUE) by a target enhanced node B (TeNB) in a mobile communication system is provided. The method includes detecting that a point of attachment (PoA) for an IUE is changed from a serving enhanced node B (SeNB) to a TeNB; and performing a handover and link change for the IUE. 
         [0026]    In accordance with another aspect of the present disclosure, a relay user equipment (RUE) in a mobile communication system is provided. The RUE includes a transmitting unit; a receiving unit; and a control unit, wherein the control unit controls the transmitting unit and the receiving unit to establish a device-to-device (D2D) link with an isolated user equipment (IUE), and supports establishment of a relay cellular link for the IUE based on the established D2D link, and wherein the RUE is within a service coverage of an enhanced node B (eNB). 
         [0027]    In accordance with another aspect of the present disclosure, an isolated user equipment (IUE) in a mobile communication system is provided. The IUE includes a transmitting unit; a receiving unit; and a control unit, wherein the control unit controls the transmitting unit and the receiving unit to establish a D2D link with a relay user equipment (RUE) which is within a service coverage of an enhanced node B (eNB), and the transmitting unit transmits a D2D discovery message to candidate RUEs which exist around the IUE if authentication for the D2D communication is requested in the mobile communication system. 
         [0028]    In accordance with another aspect of the present disclosure, an enhanced node B (eNB) in a mobile communication system is provided. The eNB includes a receiving unit configured to receive a message indicating request for communication support for an IUE from each of candidate relay user equipments (RUEs) which determine to support relay communication for the IUE among candidate RUEs which exist around the IUE; and a control unit configured to determine whether to support communication for the IUE based on capacity of the eNB and system configuration information. 
         [0029]    In accordance with another aspect of the present disclosure, an isolated user equipment (IUE) in a mobile communication system is provided. The IUE includes a transmitting unit; a receiving unit; and a control unit, wherein the control unit controls the transmitting unit and the receiving unit to perform link change to a serving enhanced node B (SeNB) which is at the same cell or a relay user equipment (RUE) which is at the same cell according to change of a point of attachment (PoA), and to perform a handover and link change to a target eNB (TeNB) when the IUE moves into a service coverage of the TeNB different from the SeNB. 
         [0030]    In accordance with another aspect of the present disclosure, a target enhanced node B (TeNB) in a mobile communication system is provided. The TeNB includes a control unit configured to detect that a point of attachment (PoA) for an IUE is changed from a serving enhanced node B (SeNB) to a TeNB; and a transmitting unit and a receiving unit configured to perform a handover and link change for the IUE. 
         [0031]    Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the disclosure. 
         [0032]    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 well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The above and other aspects, features and advantages of certain exemplary embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0034]      FIG. 1  schematically illustrates a structure of a general LTE mobile communication system including an RN; 
           [0035]      FIG. 2  schematically illustrates a basic network environment of an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0036]      FIGS. 3A and 3B  schematically illustrate a process for establishing a relay cellular link for an IUE in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0037]      FIG. 4  schematically illustrates a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0038]      FIGS. 5A to 5C  schematically illustrate a process for establishing a D2D communication link and a relay cellular communication link in a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0039]      FIG. 6  schematically illustrates an environment that an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure; 
           [0040]      FIG. 7  schematically illustrates a process in which an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure; 
           [0041]      FIG. 8  schematically illustrates an environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0042]      FIGS. 9A and 9B  schematically illustrate a process in which an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0043]      FIG. 10  schematically illustrates an environment that an IUE performs link change to another RUE which is at the same cell due to degradation of link quality with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0044]      FIGS. 11A and 11B  schematically illustrate a process in which an IUE performs link change to another RUE which is at the same cell due to degradation of quality of a link with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0045]      FIG. 12  schematically illustrates an environment that an RUE which supports communication for an IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0046]      FIGS. 13A and 13B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that an RUE which supports communication of the IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0047]      FIG. 14  schematically illustrates an environment that an IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0048]      FIGS. 15A and 15B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that the IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0049]      FIG. 16  schematically illustrates an environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0050]      FIGS. 17A and 17B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that the IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0051]      FIG. 18  schematically illustrates an environment that an IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0052]      FIGS. 19A and 19B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that the IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure; 
           [0053]      FIG. 20  schematically illustrates an inner structure of a UE in an LTE mobile communication system according to an embodiment of the present disclosure; and 
           [0054]      FIG. 21  schematically illustrates an inner structure of en eNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
       
    
    
       [0055]    Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures. 
       DETAILED DESCRIPTION 
       [0056]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
         [0057]    The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
         [0058]    It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
         [0059]    Although ordinal numbers such as “first,” “second,” and so forth will be used to describe various components, those components are not limited herein. The terms are used only for distinguishing one component from another component. For example, a first component may be referred to as a second component and likewise, a second component may also be referred to as a first component, without departing from the teaching of the inventive concept. The term “and/or” used herein includes any and all combinations of one or more of the associated listed items. 
         [0060]    The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “has,” when used in this specification, specify the presence of a stated feature, number, step, operation, component, element, or combination thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, components, elements, or combinations thereof. 
         [0061]    The terms used herein, including technical and scientific terms, have the same meanings as terms that are generally understood by those skilled in the art, as long as the terms are not differently defined. It should be understood that terms defined in a generally-used dictionary have meanings coinciding with those of terms in the related technology. 
         [0062]    According to various embodiments of the present disclosure, an electronic device may include communication functionality. For example, an electronic device may be 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 PC, a personal digital assistant (PDA), a portable multimedia player (PMP), an mp3 player, a mobile medical device, a camera, a wearable device (e.g., a head-mounted device (HMD), electronic clothes, electronic braces, an electronic necklace, an electronic appcessory, an electronic tattoo, or a smart watch), and/or the like. 
         [0063]    According to various embodiments of the present disclosure, an electronic device may be a smart home appliance with communication functionality. A smart home appliance may be, 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 washer, a dryer, an air purifier, a set-top box, a TV box (e.g., Samsung HomeSync™, Apple TV™, or Google TV™), a gaming console, an electronic dictionary, an electronic key, a camcorder, an electronic picture frame, and/or the like. 
         [0064]    According to various embodiments of the present disclosure, an electronic device may be a medical device (e.g., magnetic resonance angiography (MRA) device, a magnetic resonance imaging (MRI) device, computed tomography (CT) device, an imaging device, or an ultrasonic device), a navigation device, a global positioning system (GPS) receiver, an event data recorder (EDR), a flight data recorder (FDR), an automotive infotainment device, a naval electronic device (e.g., naval navigation device, gyroscope, or compass), an avionic electronic device, a security device, an industrial or consumer robot, and/or the like. 
         [0065]    According to various embodiments of the present disclosure, an electronic device may be furniture, part of a building/structure, an electronic board, electronic signature receiving device, a projector, various measuring devices (e.g., water, electricity, gas or electro-magnetic wave measuring devices), and/or the like that include communication functionality. 
         [0066]    According to various embodiments of the present disclosure, an electronic device may be any combination of the foregoing devices. In addition, it will be apparent to one having ordinary skill in the art that an electronic device according to various embodiments of the present disclosure is not limited to the foregoing devices. 
         [0067]    According to various embodiments of the present disclosure, for example, a device may be an electronic device. 
         [0068]    An embodiment of the present disclosure proposes an apparatus and method for providing a device-to-device (D2D) communication-based service for an isolated user equipment (IUE) in a mobile communication system. Here, the IUE is a terminal which is out of a service coverage of an enhanced node B (eNB) in a mobile communication system. 
         [0069]    An embodiment of the present disclosure proposes an apparatus and method for providing a D2D communication-based service to an IUE which exists at outside of a service coverage of an eNB in a mobile communication system by establishing a link appropriate for a situation. 
         [0070]    A method and apparatus proposed in various embodiments of the present disclosure may be applied to various communication systems such as a digital video broadcast system such as a mobile broadcast service such as a digital multimedia broadcasting (DMB) service, a digital video broadcasting-handheld (DVP-H), an advanced television systems committee-mobile/handheld (ATSC-M/H) service, and the like, and an internet protocol television (IPTV), a moving picture experts group (MPEG) media transport (MMT) system, an evolved packet system (EPS), a long term evolution (LTE) mobile communication system, an LTE-advanced (LTE-A) mobile communication system, a high speed downlink packet access (HSDPA) mobile communication system, a high speed uplink packet access (HSUPA) mobile communication system, a high rate packet data (HRPD) mobile communication system proposed in a 3 rd  generation partnership project 2 (3GPP2), a wideband code division multiple access (WCDMA) mobile communication system proposed in the 3GPP2, a code division multiple access (CDMA) mobile communication system proposed in the 3GPP2, an institute of electrical and electronics engineers (IEEE) 802.16m communication system, and a mobile internet protocol (Mobile IP) system, and/or the like. 
         [0071]    For convenience, in an embodiment of the present disclosure, it will be assumed that the mobile communication system is an LTE mobile communication system. 
         [0072]    An embodiment of the present disclosure includes a procedure for establishing a D2D communication link and a relay cellular communication link for initial network registration of an IUE and a procedure for maintaining and managing, and changing a communication mode for maintaining and managing, and changing a communication link of an IUE. 
         [0073]    Firstly, a procedure for establishing a D2D communication link and a relay cellular communication link for initial network registration of an IUE according to an embodiment of the present disclosure will be described below. 
         [0074]    Prior to a description of a procedure for establishing a D2D communication link and a relay cellular communication link for initial network registration of an IUE, the following will be assumed. 
         [0075]    Firstly, it will be assumed that user equipments (UEs) in which D2D communication is possible may dispersively perform a discovery operation and a pairing operation each other. 
         [0076]    Secondly, it will be assumed that a radio resource used for performing a discovery operation and a pairing operation of UEs is a D2D communication dedicated resource in an LTE mobile communication system and uses a predefined radio frequency band. 
         [0077]    Thirdly, an embodiment of the present disclosure assumes an LTE mobile communication system which has an environment in which there are an IUE which has no accessible enhanced node B (eNB) around and a relay UE (RUE) which accesses a serving eNB (SeNB) around the IUE, and this will be described with reference to  FIG. 2 . 
         [0078]      FIG. 2  schematically illustrates a basic network environment of an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0079]    Referring to  FIG. 2 , the LTE mobile communication system includes an IUE  211 , an RUE  213  and an eNB  215 . Here, it will be assumed that a link between the IUE  211  and the RUE  213  is a D2D link, and a link between the RUE  213  and the eNB  215  is a relay cellular link. 
         [0080]    A basic network environment of an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 2 , and a process for establishing a relay cellular link for an IUE in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 3A and 3B . 
         [0081]      FIGS. 3A and 3B  schematically illustrate a process for establishing a relay cellular link for an IUE in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0082]    Referring to  FIGS. 3A and 3B , the LTE mobile communication system includes an IUE  310 , an RUE  320 , an SeNB  330 , and an MME  340 . A process for establishing a relay cellular link for an IUE described in  FIGS. 3A and 3B  includes a D2D link establishing and terminal relay communication requesting procedure  350  in which the IUE  310  establishes a D2D link with the RUE  320  which is around the IUE  310 , and requests relay cellular communication support to the RUE  320 , a radio resource control (RRC) connection establishing procedure  360  in which the RUE  320  requests to establish an RRC connection between the RUE  320  and the SeNB  330  in order to support relay communication for the IUE  310 , an authenticating procedure  370  in which the SeNB  330  performs operations such as authentication, security setup, location information update, and the like for the IUE  310  with the MME  340 , and a relay cellular communication link establishing and managing procedure  380  in which the SeNB  330  reconfigures a parameter for an RRC connection between the IUE  310  and the RUE  320 , or between the SeNB  330  and the RUE  320 . 
         [0083]    Each of the procedures will be described below. 
         [0084]    Firstly, the D2D link establishing and terminal relay communication requesting procedure  350  in which the IUE  310  establishes the D2D link with the RUE  320  which is around the IUE  310 , and requests the relay cellular communication support to the RUE  320  will be described below. 
         [0085]    The IUE  310  performs a cell searching operation in order to search a neighbor eNB that the IUE  310  may preferentially access. If there is no neighbor eNB that the IUE  310  may access, the IUE  310  detects that the IUE  310  is on an isolated state, performs a D2D discovery operation to discover UEs which are around the IUE  310  at operation  311 , and performs a D2D link pairing operation with a specific terminal, i.e., the RUE  320  at operation  313 . An embodiment of the present disclosure proposes a relaying capable (RC) field with a preset number of bits-size, e.g., 1-bit size for a discovery message which is periodically transmitted by each UE for performing a discovery operation in order for the IUE  310  to select an RUE. That is, an arbitrary UE may set a field value of an RC field included in the discovery message to a preset value, e.g., ‘1’ in order to indicate that the arbitrary UE normally accesses an eNB and the arbitrary UE may operate as an RUE at the same time. On the contrary to this, the arbitrary UE may set the field value of the RC field included in the discovery message to a preset value, e.g., ‘0’ in order to indicate that the arbitrary UE may not operate as an RUE even though the arbitrary UE normally accesses an eNB. The IUE  310  receives discovery messages which are broadcasted by neighbor UEs, measures received signal strength of the received discovery messages, and detects a bit value of an RC bit included in each of the discovery messages. The IUE  310  selects an UE which is determined as the most appropriate for D2D communication or relay cellular communication as an RUE based on the bit value of the RC bit. The IUE  310  establishes a D2D link with the selected RUE, i.e., the RUE  320  based on a D2D communication protocol which is applied in a corresponding system, and performs D2D communication with the RUE  320  through the D2D link at operation  315 . 
         [0086]    After establishing the D2D link with the RUE  320 , the IUE  310  transmits a D2D_RRC connection request message to the RUE  320  through a D2D control channel (D2DCCH) if there is a need for performing a relay cellular communication at operation  317 . The D2DCCH is a D2D channel for transmitting and receiving a control message between terminals, and the D2CCH is a D2D channel which is newly proposed in an embodiment of the present disclosure. Of course, the D2DCCH may implemented by modifying a general D2D channel. The D2D_RRC connection request message is a message which is newly proposed for the IUE  310  to request relay cellular communication support to the RUE  320  in an embodiment of the present disclosure, and includes an identifier (ID) field between the IUE  310  and the RUE  320  and a D2D_establishment cause field. An ID between the IUE  310  and the RUE  320  is used for identifying messages which are received while the RUE  320  performs D2D communication with one or more IUEs. The D2D_establishment cause field is a field which is newly defined for indicating a reason for the request for the relay cellular communication support of the IUE  310  in an embodiment of the present disclosure. The IUE  310  sets a field value of the D2D_establishment cause field to a preset value, e.g., ‘1’ upon requesting communication in an abnormal communication environment such as a disaster environment, and sets a field value of the D2D_establishment cause field to a preset value, e.g., ‘0’ upon requesting communication in a normal communication environment. Upon receiving D2D_RRC connection request messages from one or more IUEs, the RUE  320  detects a bit value of a D2D_establishment cause field in each of the received D2D_RRC connection request messages, and preferentially supports relay cellular communication for an IUD which transmits a D2D_RRC connection request message of which a corresponding bit value is set to 1. 
         [0087]    Next, the RRC connection establishing procedure  360  in which the RUE  320  requests to establish the RRC connection between the RUE  320  and the SeNB  330  in order to support the relay communication for the IUE  310  will be described below. 
         [0088]    Upon receiving the D2D_RRC connection request message from the IUE  310 , the RUE  320  determines whether relay communication support for the IUE  310  is possible based on the D2D_RRC connection request message at operation  319 . If the relay communication support for the IUE  310  is possible, according to a current communication state of the RUE  320 , the RUE  320  transmits, to the SeNB  330 , and receives, from the SeNB  330 , an RRC connection request message, an RRC connection setup message, and an RRC connection setup complete message, or an RRC connection re-establishment request message, an RRC connection re-establishment, and an RRC connection re-establishment complete message. A communication state of the RUE  320  may be one of an RRC-idle state and an RRC-connected state. 
         [0089]    In a general LTE mobile communication system, RRC connection request related messages are transmitted, to an eNB, and received, from the eNB, in a case that a UE does not perform cellular communication, i.e., in an RRC-idle state case, and UEs request a radio resource for control message transmission and reception with an eNB through a procedure for transmitting and receiving RRC connection request related messages, and a corresponding resource is allocated to the UEs. In a general LTE mobile communication system, RRC connection re-establishment related messages are transmitted, to an eNB, and received, from the eNB, in a case that a UE performs cellular communication, i.e., in an RRC-connected state case, and a UE reports to an eNB that there is a need for reestablishing an RRC connection due to degradation of channel quality of an existing RRC link by transmitting, to an eNB, and receiving, from the eNB, RRC connection re-establishment related messages, and receives a parameter for a new RRC connection. An RRC connection request message and an RRC connection re-establishment request message which are firstly transmitted by a UE among the messages described above include an information element (IE) such as an ID which may identify a UE which transmits a corresponding message, an establishment cause indicating a reason for RRC connection request, and the like. The establishment cause IE is used for a UE to indicate a reason for an RRC connection of the UE to an eNB in a general LTE mobile communication system, and includes an RRC connection reason field indicating one of six reasons, and two spare fields. Here, the RRC connection reason field indicates one of emergency, high priority access, mobile terminating access, mobile originating signaling, mobile originating data, and delay tolerant access. 
         [0090]    So, an embodiment of the present disclosure includes a D2D_relaying field into the establishment cause IE thereby the RUE  320  uses the D2D_relaying field in order to request relay cellular communication for the IUE  310  to the SeNB  330 . 
         [0091]    In an embodiment of the present disclosure, it will be assumed that the RUE  320  is on an RRC-Idle state. So, the RUE  320  transmits an RRC connection request message to the SeNB  330  through a common control channel (CCCH) at operation  321 . The RRC connection request message includes an ID between an RUE and an IUE, and an establishment cause IE. After receiving the RRC connection request message from the RUE  320 , the SeNB  330  determines whether to support relay communication for the IUE  310  based on the RRC connection request message at operation  323 , and transmits an RRC connection setup message as a response message to the RRC connection request message to the RUE  320  corresponding to the determined result to inform whether to support the relay communication for the IUE  310  and allocate a dedicated control channel (DCCH) as a dedicated resource for an RRC connection between the SeNB  330  and the RUE  320  at operation  325 . After receiving information on the DCCH, the RUE  320  informs that radio resource allocation has been normally completed by transmitting an RRC connection setup complete message as a response message to the RRC connection setup message to the SeNB  330  through the DCCH at operation  327 . 
         [0092]    Thirdly, the authenticating procedure  370  in which the SeNB  330  performs the operations such as the authentication, the security setup, the location information update, and the like for the IUE  310  with the MIME  340  will be described below. 
         [0093]    The SeNB  330  requests authentication for the IUE  310  by transmitting an initial UE message to the MME  430  at operation  329 . An RRC establishment cause IE included in the Initial UE message is a field for indicating a reason why the SeNB  330  transmits the Initial UE message to the MIME  340  in a general LTE mobile communication system, and includes a field identical to an establishment cause IE included in the RRC connection request message. In an embodiment of the present disclosure, the RRC establishment cause IE is extended like the establishment cause IE, and used for the SeNB  330  to request relay cellular communication for the IUE  310  to the MIME  340 . After receiving the initial UE message, the MME  340  detects a D2D_relying field included in the initial UE message to perform an authentication operation for the IUE  310  at operation  331 . If the authentication for the IUE  310  has been successfully completed, the MIME  340  performs a cipher key generating and location information updating operation for the IUE  310  with the SeNB  330  based on relay communication of the RUE  320  at operation  331 . 
         [0094]    Lastly, the relay cellular communication link establishing and managing procedure  380  in which the SeNB  330  reconfigures the parameter for the RRC connection between the IUE  310  and the RUE  320 , or between the SeNB  330  and the RUE  320  will be described below. 
         [0095]    The SeNB  330  includes a parameter for an RRC connection to be used for data transmission and reception between the SeNB  330  and the RUE  320  or between the RUE  320  and the IUE  310  into an RRC connection reconfiguration message to transmit the RRC connection reconfiguration message to the RUE  320  at operation  333 . The RUE  320  receives the RRC connection reconfiguration message to reconfigure an RRC connection between the RUE  320  and the SeNB  330 , and transmits the RRC connection reconfiguration message to the IUE  310  through a D2DCCH at operation  335 . 
         [0096]    The IUE  310  performs a reconfiguration operation for a D2D link and an RRC connection based on the RRC connection reconfiguration message, and transmits an RRC connection reconfiguration complete message as a response message to the RRC connection reconfiguration message to the RUE  320  through a D2DCH at operation  337 . After receiving the RRC connection reconfiguration complete message from the IUE  310 , the RUE  320  performs a reconfiguration operation for a D2D link and an RRC connection based on the RRC connection reconfiguration message, and transmits an RRC connection reconfiguration complete message to the SeNB  330  through a DCCH at operation  339 . 
         [0097]    So, data transmission and reception through a relay cellular link become possible among the IUE  310 , the RUE  320 , and the SeNB  330  at operation  341 . 
         [0098]    Although  FIGS. 3A and 3B  illustrate a process for establishing a relay cellular link for an IUE in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 3A and 3B . For example, although shown as a series of operations, various operations in  FIGS. 3A and 3B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0099]    A process for establishing a relay cellular link for an IUE in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIGS. 3A and 3B . In a network environment which does not supports distribute D2D link establishment for UEs, or a network environment in which there is a need for authenticating all terminals on performing communication, i.e., a network environment which there is a need for D2D communication which is based on eNB control, there may be difficult to establish a relay cellular link for an IUE as described in  FIGS. 3A and 3B . 
         [0100]    So, a process for establishing a D2D communication link and a relay cellular communication link in a network environment that a D2D communication which is based on eNB control is requested will be described below. Firstly, a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 4 . 
         [0101]      FIG. 4  schematically illustrates a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0102]    Referring to  FIG. 4 , the LTE mobile communication system includes an eNB B  411 , UEs  413 ,  415 , and  417 , an IUE  419 , an eNB A  421 , and UEs  423 ,  425 , and  427 . 
         [0103]    A network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 4 , and a process for establishing a D2D communication link and a relay cellular communication link in a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 5A to 5C . 
         [0104]      FIGS. 5A to 5C  schematically illustrate a process for establishing a D2D communication link and a relay cellular communication link in a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0105]    Referring to  FIGS. 5A to 5C , the LTE mobile communication system includes an IUE  510 , an RUE  520 , an SeNB  530 , and an MME  540 . Prior to a description of a process for establishing a D2D communication link and a relay cellular communication link described in  FIGS. 5A to 5C , the following will be assumed. 
         [0106]    Firstly, it will be assumed that UEs need to perform D2D link pairing with a specific UE after D2D communication support and authentication from a network. 
         [0107]    Secondly, it will be assumed that there is one or more candidate RUEs around an IUE, and each candidate RUE accesses a specific eNB (SeNB). 
         [0108]    A process for establishing a D2D communication link and a relay cellular communication link described in  FIGS. 5A to 5C  includes a D2D discovery and D2D communication requesting procedure  550 , an RRC connection establishing procedure  560 , an authenticating procedure  570 , a D2D communication link establishing and managing procedure  580 , and a relay cellular communication link establishing and managing procedure  590 , and each of the D2D discovery and D2D communication requesting procedure  550 , the RRC connection establishing procedure  560 , the authenticating procedure  570 , the D2D communication link establishing and managing procedure  580 , and the relay cellular communication link establishing and managing procedure  590  will be described below. 
         [0109]    Firstly, the D2D discovery and D2D communication requesting procedure  550  will be described below. 
         [0110]    The IUE  510  detects that the IUE  510  is on an isolated state through a cell discovery operation, and performs a D2D discovery operation for neighbor terminals at operation  511 . Upon detecting that there is one or more UEs around the IUE  510  according to the discovery operation result, the IUE  510  broadcasts a D2D_RRC connection request message to the one or more UEs through a D2DCCH at operation  513 . The D2D_RRC connection request message includes an ID of the IUE  510 , and an establishment cause IE indicating a reason why the D2D_RRC connection request message is transmitted. After receiving the D2D_RRC connection request message, UEs determine whether to support communication for the IUE  510  based on received signal strength of the D2D_RRC connection request message, a state of the UEs which receive the D2D_RRC connection request message, and the like at operation  515 . 
         [0111]    Secondly, the RRC connection establishing procedure  560  will be described below. 
         [0112]    Candidate RUEs which determine that communication support for the IUE  510  is possible according to the determined result for whether to support the communication for the IUE  510  transmits an RRC connection request message to eNBs (SeNBs) that the candidate RUEs access to request communication support for the IUE  510  at operation  517 . After receiving the RRC connection request message, the eNBs (SeNBs) determine whether to support relay communication for the IUE  510 , and select an RUE which will support communication of the IUE  510  based on received signal strength (link quality) received from corresponding RUEs. The eNBs (SeNBs) select an SeNB which will support the communication of the IUE  510  based on link quality between an RUE which is selected in each eNB (SeNB) and the IUE  510 , or traffic load of each cell with neighbor eNBs (SeNBs) of the eNBs (SeNBs) through an X2 interface at operation  519 . After performing the described process, the SeNB  530  allocates a resource for an RRC connection between the SeNB  530  and the RUE  520 , and transmits an RRC connection setup message including information on this to the RUE  520  through a CCCH at operation  521 . After receiving the RRC connection setup message from the SeNB  530 , the RUE  520  transmits an RRC connection setup complete message as a response message to the RRC connection setup message to the SeNB  530  through a DCCH at operation  523 . 
         [0113]    Thirdly, the authenticating procedure  570  will be described below. 
         [0114]    In a general LTE mobile communication system, authentication for a UE is performed by acquiring an authentication vector for a specific UE through a home subscriber server (HSS) and performing a mutual authentication operation with the UE using the acquired authentication vector in an MME. 
         [0115]    An embodiment of the present disclosure defines a minimum authenticating procedure as an authenticating procedure for establishing a D2D link of the IUE  510 . In the minimum authenticating procedure, the MME  540  receives an ID of the IUE  510  from the RUE  520  in order to perform authentication for the IUE  510 , and transmits the ID of the IUE  510  to an HSS (not shown in  FIGS. 5A to 5C ). The HSS determines whether the IUD  510  is a UE which is registered at a corresponding cellular system based on the received ID of the IUE  510 , and transmits the determined result for this to the MME  540 . If the minimum authenticating procedure has been normally completed, the MME  540  informs this to the SeNB  530 , and the SeNB  530  transmits resource information for D2D communication of the IUE  510  and the RUE  520  at operation  527 . 
         [0116]    Fourthly, the D2D communication link establishing and managing procedure  580  will be described below. 
         [0117]    The SeNB  530  transmits an RRC connection reconfiguration message to the RUE  520  at operation  529  to allocate a resource for an RRC connection between the SeNB  530  and the RUE  520  or D2D communication between the RUE  520  and the IUE  510 . After receiving the RRC connection reconfiguration message from the SeNB  530 , the RUE  520  performs a D2D link pairing operation with the IUE  510  at operation  531 . After performing the D2D link pairing operation with the IUE  510 , the RUE  520  transmits resource information for communication between the RUE  520  and the IUE  510  using an RRC connection reconfiguration message through a D2DCCH at operation  533 . 
         [0118]    After receiving the RRC connection reconfiguration message from the RUE  520 , the IUE  510  selects a communication mode type at operation  535 , and transmits an RRC connection reconfiguration complete message including information indicating the selected communication mode type to the RUE  520  at operation  537 . The communication mode type indicates a type of a communication mode that a corresponding UE intends to perform. In  FIGS. 5A to 5C , it will be assumed that the IUE  510  selects the type of the communication mode as relay cellular type communication. After receiving the RRC connection reconfiguration complete message from the IUE  510 , the RUE  520  transmits the RRC connection reconfiguration complete message to the SeNB  530  at operation  539 . The RRC connection reconfiguration complete message transmitted at operations  537  and  539  includes a non-critical extensions future field, and the non-critical extensions future field is defined as a D2D_relayed cellular link request field which is a field indicating whether to select relay cellular communication in an IUE in an embodiment of the present disclosure. That is, if there is a need for relay cellular communication, the IUE  510  sets a field value of the D2D_relayed cellular link request field to a preset value, e.g., ‘1’. On the contrary to this, if there is no need for relay cellular communication, the IUE  510  sets a field value of the D2D_relayed cellular link request field to a preset value, e.g., ‘0’. After the RRC connection reconfiguration complete message through the RUE  520 , the SeNB  530  determines whether to support relay cellular communication for the IUE  510  based on a field value of a D2D_relayed cellular link request field included in the RRC connection reconfiguration complete message. 
         [0119]    Fifthly, the relay cellular communication link establishing and managing procedure  590 , i.e., the relay cellular communication link establishing and managing procedure  590  which corresponds to operations  541  to  557  is performed like relay cellular communication link establishing and managing procedure described in  FIGS. 3A and 3B , and a description thereof will be omitted herein. 
         [0120]    Although  FIGS. 5A to 5C  illustrate a process for establishing a D2D communication link and a relay cellular communication link in a network environment that a D2D communication which is based on eNB control is requested in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 5 a  to 5 c   . For example, although shown as a series of operations, various operations in  FIGS. 5 a  to 5 c    could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0121]    A procedure for establishing a D2D communication link and a relay cellular communication link for initial network registration of an IUE according to an embodiment of the present disclosure has been described with reference to  FIGS. 2 to 5   c , and a procedure for maintaining, managing, and changing a communication mode for maintaining, managing, and changing a communication link of an IUE according to an embodiment of the present disclosure will be described below. 
         [0122]    An embodiment of the present disclosure defines seven scenarios which may occur according to movement of an IUE or an RUE, and proposes a procedure for maintaining, managing, and changing a communication mode which may be applied to each of the seven scenarios. The seven scenarios defined in an embodiment of the present disclosure are classified into a case that a communication link is changed at the same cell and a case that a communication link is changed to another cell. Here, communication link change at the same cell denotes communication link change to an eNB which is at the same cell or another RUE which belongs to the same cell according to change of a point of attachment (PoA) of an IUE because of movement of the IUE or an RUE. Alternatively, link change to another cell denotes link change according that there is a need for changing an eNB which supports communication of an IUE because of movement of UEs. 
         [0123]    A procedure for maintaining, managing, and changing a communication mode for maintaining, managing, and changing a communication link of an IUE according to an embodiment of the present disclosure in a case that link change at the same cell will be assumed will be described below. 
         [0124]    An environment that an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure will be described with reference to  FIG. 6 . 
         [0125]      FIG. 6  schematically illustrates an environment that an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure. 
         [0126]    Referring to  FIG. 6 , the LTE mobile communication system includes an SeNB  611 , an RUE  613 , and an IUE  615 . Upon moving into a service coverage of a specific cell while performing relay cellular communication through the RUE  613  at outside of the service coverage of the specific cell, the IUE  615  performs direct communication with the SeNB  611 . The IUE  615  generates and updates a list for a candidate RUE for communication link change by periodically measuring and comparing communication link quality of the IUE  615 , and received signal strength from the SeNB  611  or received signal strength from neighbor UEs which may support relay cellular communication. 
         [0127]    An environment that an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure has been described with reference to  FIG. 6 , and a process in which an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure will be described with reference to  FIG. 7 . 
         [0128]      FIG. 7  schematically illustrates a process in which an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure. 
         [0129]    Referring to  FIG. 7 , the LTE mobile communication system includes an IUE  710 , an RUE  720 , and an SeNB  730 . 
         [0130]    A process in which the IUE  710  changes a communication link from a relay communication link (a relay cellular communication link) with the RUE  720  to a direct communication link with an SeNB illustrated in  FIG. 7  includes a measuring and reporting procedure  740  in which the IUE  710  performs a neighbor cell searching operation and a measuring operation form the SeNB  730  through relay of the RUE  720 , and reports a measured result according to the measuring operation, an RRC connection re-establishment request and RRC connection re-establishment response procedure  750  in which the IUE  710  determines that communication with the SeNB  730  is possible, and a radio resource for direct communication is allocated from the SeNB  730  to the IUE  710 , and an RRC connection re-configuration procedure  760  in which the IUE  710  receives information for reconfiguring an existing RRC connection from the SeNB  730  according to change of a network environment. Each of the measuring and reporting procedure  740  in which the IUE  710  performs the neighbor cell searching operation and the measuring operation from the SeNB  730  through the relay of the RUE  720 , and reports the measured result according to the measuring operation, the RRC connection re-establishment request and RRC connection re-establishment response procedure  750  in which the IUE  710  determines that the communication with the SeNB  730  is possible, and the radio resource for the direct communication is allocated from the SeNB  730  to the IUE  710 , and the RRC connection re-configuration procedure  760  in which the IUE  710  receives the information for reconfiguring the existing RRC connection from the SeNB  730  according to the change of the network environment will be described below. 
         [0131]    Firstly, the measuring and reporting procedure  740  in which the IUE  710  performs the neighbor cell searching operation and the measuring operation from the SeNB  730  through the relay of the RUE  720 , and reports the measured result according to the measuring operation will be described below. 
         [0132]    While data transmission and reception is performed among the IUE  710 , the RUE  720 , and the SeNB  730  through a relay cellular link at operation  711 , the SeNB  730  transmits a logged measurement configuration message to the IUE  710  through the RUE  720  at operation  713 . After receiving the logged measurement configuration message from the SeNB  730  through the RUE  20 , the IUE  710  performs a neighbor cell searching operation, a quality measuring operation for a link in which communication is ongoing, a D2D discovery operation, and the like based on measurement configuration information included in the logged measurement configuration message at operation  715 , and reports a measured report to the SeNB  730  using a measurement report message at operation  717 . In an embodiment of the present disclosure, the IUE  710  performs the D2D discovery operation to detect presence of neighbor terminals of the IUE  710 , and generates and updates a list for candidate RUEs for which link change is considered to be possible in the IUE  710  at operation  715 . 
         [0133]    If it is determined that there is a need for the link change, the IUE  710  performs link change to a corresponding UE by transmitting a communication request message to candidate RUEs included in a list of the IUE  710 . After receiving the measurement report message from the IUE  710 , the SeNB  730  determines whether to perform link change of the IUE  710  based on the measurement report, transmits an RRC connection release message to the IUE  710  upon determining that there is a need for the link change at operation  719 , and allocates a uplink (UL) resource to the IUE  710  thereby the IUE  710  may transmit an RRC connection re-establishment request message at operation  721 . The RRC Connection Release message is a message that an eNB transmits to an IUE in order to release an RRC connection, and includes a release cause IE indicating a reason for RRC connection release. The release cause IE indicates one of three reasons, i.e., load balancing, cs-fallback, and other. An embodiment of the present disclosure additionally defines a D2D_mode switch reason indicating a reason for link change of an IUE to use the D2D_mode switch reason in order for an eNB to control a communication mode of an IUE. In the measuring and reporting procedure  740 , messages transmitted and received between the IUE  710  and the SeNB  730  are transmitted and received through a D2DCCH which is newly defined between the IUE  710  and the RUE  720 , and a DCCH between the RUE  720  and the SeNB  730 . 
         [0134]    Secondly, the RRC connection re-establishment request and RRC connection re-establishment response procedure  750  in which the IUE  710  determines that the communication with the SeNB  730  is possible, and the radio resource for the direct communication is allocated from the SeNB  730  to the IUE  710  will be described below. 
         [0135]    After a resource for uplink transmission is allocated from the SeNB  730  to the IUE  710 , in the RRC connection re-establishment request and RRC connection re-establishment response procedure  750 , the IUE  710  transmits, to the SeNB  730 , and receives, from the SeNB  730 , an RRC connection re-establishment request message, an RRC connection re-establishment message, and an RRC connection re-establishment complete message, so a resource for a new RRC connection is allocated to the IUE  710  at operations  723 ,  725 , and  727 . 
         [0136]    Thirdly, the RRC connection re-configuration procedure  760  in which the IUE  710  receives the information for reconfiguring the existing RRC connection from the SeNB  730  according to the change of the network environment will be described below. 
         [0137]    In the RRC connection re-configuration procedure  760 , the IUE  710  transmits, to the SeNB  730 , and receives, from the SeNB  730 , an RRC connection reconfiguration message and an RRC connection reconfiguration complete message to reconfigure an RRC connection between the IUE  710  and the SeNB  730  at operations  727  and  729 . According that the RRC connection is reconfigured, the IUE  710  performs direct cellular communication with the SeNB  730  through the allocated resource at operation  733 . 
         [0138]    Although  FIG. 7  illustrates a process in which an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure, various changes could be made to  FIG. 7 . For example, although shown as a series of operations, various operations in  FIG. 7  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0139]    A process in which an IUE changes a communication link from a relay communication link (a relay cellular communication link) with an RUE to a direct communication link with an SeNB in an LTE mobile communication link according to an embodiment of the present disclosure has been described with reference to  FIG. 7 , and an environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 8 . 
         [0140]      FIG. 8  schematically illustrates an environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0141]    Referring to  FIG. 8 , the LTE mobile communication system includes an SeNB  811 , an RUE  813 , and an IUE  815 . The IUE  815  performs change to a relay cellular link with another terminal in a general communication link (direct communication or relay cellular communication), so the IUE  815  may use a D2D link and relay cellular link establishing procedure proposed in an embodiment of the present disclosure. 
         [0142]    An environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 8 , and a process in which an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 9A and 9B . 
         [0143]      FIGS. 9A and 9B  schematically illustrate a process in which an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0144]    Referring to  FIGS. 9A and 9B , the LTE mobile communication system includes an IUE  910 , an RUE  920 , and an SeNB  930 . 
         [0145]    A process in which the IUE  910  moves out of a service coverage of the SeNB  930  while performing direct communication with the SeNB  930 , and establishes a relay cellular communication link with a specific RUE, i.e., the RUE  920  illustrated in  FIGS. 9A and 9B  includes a measuring and reporting procedure  940  of the IUE  910  and a D2D communication link and relay cellular communication link establishing procedure  950  for the IUE  910 . The measuring and reporting procedure  940  of the IUE  910  is similar to a measuring and reporting procedure  740  as described in  FIG. 7 , and a description thereof will be omitted herein. Here, the IUE  910  selects the RUE  920  as a candidate RUE using a D2D discovery operation, and performs a D2D link pairing operation with the RUE  920  upon receiving an RRC connection release message. The D2D communication link and relay cellular communication link establishing procedure  950  illustrated in  FIGS. 9A and 9B  is similar to a relay cellular communication link establishing and managing procedure  380  as described in  FIGS. 3A and 3B , and a description thereof will be omitted herein. 
         [0146]    Meanwhile, the IUE  910  has performed an authenticating procedure with a network already, so the IUE  910  does not perform additionally an authenticating procedure with the network. 
         [0147]    Although  FIGS. 9A and 9B  illustrate a process in which an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 9A and 9B . For example, although shown as a series of operations, various operations in  FIGS. 9A and 9B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0148]    A process in which an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB, and establishes a relay cellular communication link with a specific RUE in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIGS. 9A and 9B , and an environment that an IUE performs link change to another RUE which is at the same cell due to degradation of link quality with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 10 . 
         [0149]      FIG. 10  schematically illustrates an environment that an IUE performs link change to another RUE which is at the same cell due to degradation of link quality with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0150]    Referring to  FIG. 10 , the LTE mobile communication system includes an SeNB  1011 , an RUE A  1013 , an IUE  1015 , and an RUE B  1017 . In  FIG. 10 , the IUE  1015  performs link change to a new RUE which is at the same cell, i.e., the RUE A  103  according to degradation of quality of a link with an existing RUE, i.e., the RUE B  1017 . In this case, the IUE  1015  performs change to a relay cellular link with another terminal in a general communication link (direct communication or relay cellular communication), so the IUE  1015  may use a D2D link and relay cellular link establishing procedure proposed in an embodiment of the present disclosure. 
         [0151]    An environment that an IUE performs link change to another RUE which is at the same cell due to degradation of link quality with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 10 , and a process in which an IUE performs link change to another RUE which is at the same cell due to degradation of quality of a link with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 11A and 11B . 
         [0152]      FIGS. 11A and 11B  schematically illustrate a process in which an IUE performs link change to another RUE which is at the same cell due to degradation of quality of a link with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0153]    Referring to  FIGS. 11A and 11B , the LTE mobile communication system includes an IUE  1110 , an RUE A  1120 , an RUE B  1130 , and an SeNB  1140 . 
         [0154]    A process in which the IUE  1110  performs link change to another RUE which is at the same cell, i.e., the RUE A  1120  due to degradation of quality of a link with an existing RUE, i.e., the RUE B  1130  illustrated in  FIGS. 11A and 11B  includes a measuring and reporting procedure  1150  of the IUE  1110  and a D2D communication link and relay cellular communication link establishing procedure  1160  for the IUE  1110 . The measuring and reporting procedure  1150  of the IUE  1110  is similar to a measuring and reporting procedure  740  as described in  FIG. 7 , and a description thereof will be omitted herein. Here, the IUE  1110  selects the RUE A  1120  as a candidate RUE using a D2D discovery operation, and performs a D2D link pairing operation with the RUE A  1120  upon receiving an RRC connection release message. The D2D communication link and relay cellular communication link establishing procedure  1160  illustrated in  FIGS. 11A and 11B  is similar to a relay cellular communication link establishing and managing procedure  380  as described in  FIGS. 3A and 3B , and a description thereof will be omitted herein. 
         [0155]    Meanwhile, the IUE  1110  has performed an authenticating procedure with a network already, so the IUE  1110  does not perform additionally an authenticating procedure with the network. 
         [0156]    Although  FIGS. 11A and 11B  illustrate a process in which an IUE performs link change to another RUE which is at the same cell due to degradation of quality of a link with an existing RUE in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 11A and 11B . For example, although shown as a series of operations, various operations in  FIGS. 11A and 11B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0157]    A procedure for maintaining, managing, and changing a communication mode for maintaining, managing, and changing a communication link of an IUE according to an embodiment of the present disclosure in a case of assuming link change at the same cell has been described with reference to  FIGS. 6 to 11   b , and a procedure for maintaining, managing, and changing a communication mode for maintaining, managing, and changing a communication link of an IUE according to an embodiment of the present disclosure in a case of assuming link change to another cell will be described below. 
         [0158]    An environment that an RUE which supports communication for an IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 12 . 
         [0159]      FIG. 12  schematically illustrates an environment that an RUE which supports communication for an IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0160]    Referring to  FIG. 12 , the LTE mobile communication system includes an SeNB  1211 , an IUE  1213 , an RUE  1215 , and a target eNB (TeNB)  1217 . The RUE  1215  of the IUE  1213  moves into a service coverage of an eNB other than the SeNB  1211 , i.e., the TeNB  1217 , and performs a handover to the TeNB  1217 , and the IUE  1213  maintains a cellular relay communication link with the RUE  1215 . 
         [0161]    Further, a handover procedure used in a general LTE mobile communication system may be extended and applied as a procedure in which the IUE  1213  maintains an existing link with the RUE  1215  or directly accesses a new eNB, i.e., the TeNB  1217 . The RUE  1215  performs a handover to the TeNB  1217 , and the IUE  1213  maintains a relay cellular link with the RUE  1215 , and receives information for a new RRC connection reconfiguration from the TeNB  1217  to which the RUE  1215  performs the handover to reconfigure an RRC connection. 
         [0162]    An environment that an RUE which supports communication for an IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 12 , and a process in which an IUE performs link change to another cell in an environment that an RUE which supports communication of the IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 13A and 13B . 
         [0163]      FIGS. 13A and 13B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that an RUE which supports communication of the IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0164]    Referring to  FIGS. 13A and 13B , the LTE mobile communication system includes an IUE  1310 , an RUE  1320 , an SeNB  1330 , a TeNB  1340 , and an MME  1350 . 
         [0165]    The RUE  1320  of the IUE  1310  moves into a service coverage of another eNB, i.e., the TeNB  1340 , and performs a handover to the TeNB  1340 , and the IUE  1310  maintains a cellular relay communication link with the RUE  1320 . Further, a handover procedure used in a general LTE mobile communication system may be extended and applied as a procedure in which the IUE  1310  maintains an existing link with the RUE  1320  or directly accesses the TeNB  1340 . The RUE  1320  performs a handover to the TeNB  1340 , and the IUE  1310  maintains a relay cellular link with the RUE  1320 , and receives information for a new RRC connection reconfiguration from the TeNB  1340  to which the RUE  1320  performs the handover to reconfigure an RRC connection. 
         [0166]    A process in which an IUE performs link change to another cell in an environment that an RUE which supports communication of the IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure as illustrated in  FIGS. 13A and 13B  includes a measuring and reporting procedure  1360  of the IUE  1310 , a handover request and response, and RRC connection reconfiguration procedure  1370  through the SeNB  1330  and the TeNB  1340 , and a packet synchronization and handover completion procedure  1380  after link change of the RUE  1320  or the IUE  1310 . 
         [0167]    Firstly, the measuring and reporting procedure  1360  is similar to a measuring and reporting procedure  740  as described in  FIG. 7 , and a description thereof will be omitted herein. 
         [0168]    Secondly, the handover request and response, and RRC connection reconfiguration procedure  1370  through the SeNB  1330  and the TeNB  1340  will be described below. 
         [0169]    The SeNB  1330  determines that there is a need for a handover for the IUE  1310  or the RUE  1320  based on a measured result included in a measurement report message received from the RUE  1320  at operation  1317 . So, the SeNB  1330  transmits a handover request message to the TeNB  1340  at operation  1319 , and the TeNB  1340  controls to accept the handover for the IUE  1310  or the RUE  1320  based on the handover request message at operation  1321 , and transmits a handover request ack message as a response message to the handover request message to the SeNB  1330  at operation  1323 . 
         [0170]    The handover request message and the handover request ack message are transmitted and received between an SeNB and a TeNB for determining whether to perform a handover of a UE in a general LTE mobile communication system. An embodiment of the present disclosure includes a D2D_handover field with a preset bit-size, e.g., a 2 bit-size into a cause IE included in a handover request message used in a general LTE mobile communication system to additionally indicate a reason for handover request for an IUE. If a field value of the D2D_handover field is set to ‘00’, the D2D_handover field indicates that it is not for handover request for an IUE. If a field value of the D2D_handover field is set to ‘01’, the D2D_handover field indicates that it is for handover request for an IUE in a normal environment. If a field value of the D2D_handover field is set to ‘10’, the D2D_handover field indicates that it is for handover request for an IUE in a disaster environment. The TeNB  1340  includes information on an RRC connection which will be used by the IUE  1310  or the RUE  1320  in a service coverage of the TeNB  1340  into the handover request ack message to transmit the handover request ack message. After receiving the handover request ack message, the SeNB  1330  transmits an RRC connection reconfiguration message to the IUE  1310  or the RUE  1320  to inform resource information for an RRC connection received from the TeNB  1340  to a corresponding UE at operation  1325 . 
         [0171]    Thirdly, the packet synchronization and handover completion procedure  1380  after link change of the RUE  1320  or the IUE  1310  will be described below. 
         [0172]    After receiving the RRC connection reconfiguration message from the SeNB  1330 , the IUE  1310  or the RUE  1320  releases an existing RRC connection with the SeNB  1330 , and performs a synchronization operation with the TeNB  1340  at operation  1327 . The SeNB  1330  transmits packet information which is being transmitted to the IUE  1310  or the RUE  1320  to the TeNB  1340  to support packet synchronization for the TeNB  1340  at operations  1329 ,  1331 , and  1333 . After the packet synchronization process, the IUE  1310  or the RUE  1320  informs to the TeNB  1340  that a handover has been completed by transmitting an RRC connection reconfiguration complete message to the TeNB  1340  at operation  1337 , and performs relay cellular communication with the TeNB  1340  through the RUE  1320  or direct cellular communication with the TeNB  1340  at operations  1339  and  1341 . The TeNB  1340  transmits a UE context release message to the SeNB  1330  at operation  1343 , so the SeNB  1330  releases a radio resource which has been allocated to the IUE  1310  or the RUE  1320  which has performed the handover at operation  1345 . 
         [0173]    Although  FIGS. 13A and 13B  illustrate a process in which an IUE performs link change to another cell in an environment that an RUE which supports communication of the IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 13A and 13B . For example, although shown as a series of operations, various operations in  FIGS. 13A and 13B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0174]    A process in which an IUE performs link change to another cell in an environment that an RUE which supports communication of the IUE moves out of a service coverage of another eNB in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIGS. 13A and 13B , and an environment that an IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 14 . 
         [0175]      FIG. 14  schematically illustrates an environment that an IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0176]    Referring to  FIG. 14 , the LTE mobile communication system includes an SeNB  1411 , an RUE  1413 , an IUE  1415 , and a TeNB  1417 . The IUE  1415  moves into a service coverage of the TeNB  1417  while performing relay cellular communication through the RUE  1413  which is in a service coverage of the SeNB  1411  at outside of a service coverage of a cellular system, and performs a handover to the TeNB  1417 . That is, the IUE  1415  releases an existing D2D link and an existing relay cellular link with the RUE  1413 , and performs link change to the TeNB  1417 . 
         [0177]    An environment that an IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 14 , and a process in which an IUE performs link change to another cell in an environment that the IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 15A and 15B . 
         [0178]      FIGS. 15A and 15B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that the IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0179]    Referring to  FIGS. 15A and 15B , the LTE mobile communication system includes an IUE  1510 , an RUE  1520 , an SeNB  1530 , a TeNB  1540 , and an MME  1550 . 
         [0180]    The IUE  1510  moves into a service coverage of the TeNB  1540  while performing relay cellular communication through the RUE  1520  which is in a service coverage of the SeNB  1530  at outside of a service coverage of a cellular system, and performs a handover to the TeNB  1540 . Further, a handover procedure used in a general LTE mobile communication system may be extended and applied as a procedure in which the IUE  1510  maintains an existing link with the RUE  1520  or directly accesses the TeNB  1540 . The IUE  1510  releases an existing D2D link and an existing relay cellular link with the RUE  1520 , and performs link change to the TeNB  1540 . 
         [0181]    A process in which an IUE performs link change to another cell in an environment that the IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure as illustrated in  FIGS. 15A and 15B  includes a measuring and reporting procedure  1560  of the IUE  1510 , a handover request and response, and RRC connection reconfiguration procedure  1570  through the SeNB  1530  and the TeNB  1540 , and a packet synchronization and handover completion procedure  1580  after link change of the RUE  1520  or the IUE  1510 . 
         [0182]    Firstly, the measuring and reporting procedure  1560  is similar to a measuring and reporting procedure  740  as described in  FIG. 7 , and a description thereof will be omitted herein. 
         [0183]    Secondly, the handover request and response, and RRC connection reconfiguration procedure  1570  through the SeNB  1530  and the TeNB  1540  will be described below. 
         [0184]    The SeNB  1330  determines that there is a need for a handover for the IUE  1510  or the RUE  1520  based on a measured result included in a measurement report message received from the RUE  1520  at operation  1517 . So, the SeNB  1530  transmits a handover request message to the TeNB  1540  at operation  1519 , and the TeNB  1540  controls to accept the handover for the IUE  1510  or the RUE  1520  based on the handover request message at operation  1521 , and transmits a handover request ack message as a response message to the handover request message to the SeNB  1530  at operation  1523 . 
         [0185]    The handover request message and the handover request ack message are transmitted and received between an SeNB and a TeNB for determining whether to perform a handover of a UE in a general LTE mobile communication system. An embodiment of the present disclosure includes a D2D_handover field with a preset bit-size, e.g., a 2 bit-size into a cause IE included in a handover request message used in a general LTE mobile communication system to additionally indicate a reason for handover request for an IUE. If a field value of the D2D_handover field is set to ‘00’, the D2D_handover field indicates that it is not for handover request for an IUE. If a field value of the D2D_handover field is set to ‘01’, the D2D_handover field indicates that it is for handover request for an IUE in a normal environment. If a field value of the D2D_handover field is set to ‘10’, the D2D_handover field indicates that it is for handover request for an IUE in a disaster environment. The TeNB  1540  includes information on an RRC connection which will be used by the IUE  1510  or the RUE  1520  in a service coverage of the TeNB  1540  into the handover request ack message to transmit the handover request ack message. After receiving the handover request ack message, the SeNB  1530  transmits an RRC connection reconfiguration message to the IUE  1510  or the RUE  1520  to inform resource information for an RRC connection received from the TeNB  1540  to a corresponding UE at operation  1525 . 
         [0186]    Thirdly, the packet synchronization and handover completion procedure  1580  after link change of the RUE  1520  or the IUE  1510  will be described below. 
         [0187]    After receiving the RRC connection reconfiguration message from the SeNB  1530 , the IUE  1510  or the RUE  1520  releases an existing RRC connection with the SeNB  1530 , and performs a synchronization operation with the TeNB  1540  at operation  1527 . The SeNB  1530  transmits packet information which is being transmitted to the IUE  1510  or the RUE  1520  to the TeNB  1540  to support packet synchronization for the TeNB  1540  at operations  1529 ,  1531 , and  1533 . After the packet synchronization process, the IUE  1510  or the RUE  1520  informs to the TeNB  1540  that a handover has been completed by transmitting an RRC connection reconfiguration complete message to the TeNB  1540  at operation  1537 , and performs relay cellular communication with the TeNB  1540  through the RUE  1520  or direct cellular communication with the TeNB  1540  at operations  1539  and  1541 . The TeNB  1540  transmits a UE context release message to the SeNB  1530  at operation  1543 , so the SeNB  1530  releases a radio resource which has been allocated to the IUE  1510  or the RUE  1520  which has performed the handover at operation  1545 . 
         [0188]    Although  FIGS. 15A and 15B  illustrate a process in which an IUE performs link change to another cell in an environment that the IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 15A and 15B . For example, although shown as a series of operations, various operations in  FIGS. 15A and 15B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0189]    A process in which an IUE performs link change to another cell in an environment that the IUE moves into a service coverage of a TeNB while performing relay cellular communication through an RUE which is in a service coverage of an SeNB at outside of a service coverage of a cellular system in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIGS. 15A and 15B , and an environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 16 . 
         [0190]      FIG. 16  schematically illustrates an environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0191]    Referring to  FIG. 16 , the LTE mobile communication system includes an SeNB  1611 , an IUE  1613 , an RUE  1615 , and a TeNB  1617 . The IUE  1613  moves out of a service coverage of the SeNB  1611  while performing direct communication with the SeNB  1611 , so the IUE  1613  establishes a D2D link and a relay cellular link with the RUE  1615  which is within a service coverage of the TeNB  1617 . That is, the IUE  1613  performs relay link change to the RUE  1615  which is within the service coverage of the TeNB  1617  due to degradation of quality of a link with the SeNB  1611 . After a cellular communication link with the SeNB  1611  is terminated, the IUE  1613  performs link change to the TeNB  1617  by performing a procedure for establishing a D2D link and a relay cellular link for the RUE  1615  with the TeNB  1617 . 
         [0192]    An environment that an IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 16 , and a process in which an IUE performs link change to another cell in an environment that the IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 17A and 17B . 
         [0193]      FIGS. 17A and 17B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that the IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0194]    Referring to  FIGS. 17A and 17B , the LTE mobile communication system includes an IUE  1710 , an RUE  1720 , an SeNB  1730 , and a TeNB  1740 . 
         [0195]    The IUE  1710  moves out of a service coverage of the SeNB  1730  while performing direct communication with the SeNB  1730 , so the IUE  1710  establishes a D2D link and a relay cellular link with the RUE  1720  which is within a service coverage of the TeNB  1740 . That is, the IUE  1710  performs relay link change to the RUE  1720  which is within the service coverage of the TeNB  1740  due to degradation of quality of a link with the SeNB  1730 . After a cellular communication link with the SeNB  1730  is terminated, the IUE  1710  performs link change to the TeNB  1740  by performing a procedure for establishing a D2D link and a relay cellular link for the RUE  1720  with the TeNB  1740 . 
         [0196]    A process in which an IUE performs link change to another cell in an environment that the IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure as illustrated in  FIGS. 17A and 17B  includes a measuring and reporting procedure  1750 , and a D2D link and relay cellular link establishing procedure  1760  The measuring and reporting procedure  1750  is similar to a measuring and reporting procedure  1150  as described in  FIGS. 11A and 11B , the D2D link and relay cellular link establishing procedure  1760  is similar to a D2D communication link and relay cellular communication link establishing procedure  1160  as described in  FIGS. 11A and 11B , and a description thereof will be omitted herein. 
         [0197]    Although  FIGS. 17A and 17B  illustrate a process in which an IUE performs link change to another cell in an environment that the IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 17A and 17B . For example, although shown as a series of operations, various operations in  FIGS. 17A and 17B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0198]    A process in which an IUE performs link change to another cell in an environment that the IUE moves out of a service coverage of an SeNB while performing direct communication with the SeNB in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIGS. 17A and 17B , and an environment that an IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 18 . 
         [0199]      FIG. 18  schematically illustrates an environment that an IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0200]    Referring to  FIG. 18 , the LTE mobile communication system includes an SeNB  1811 , an IUE  1813 , an RUE A  1815 , a TeNB  1817 , and an RUE B  1819 . The IUE  1813  establishes a D2D link and a relay cellular link with the RUE A  1815  which is within a service coverage of the TeNB  1817  due to degradation of quality of a corresponding link while performing relay communication through the RUE B  1819  which is within a service coverage of the SeNB  1811 . That is, the IUE  1813  performs relay link change to the RUE A  1815  which is within the service coverage of the TeNB  1817  due to degradation of quality of a link with the SeNB  1811 . After a cellular communication link with the SeNB  1811  is terminated, the IUE  1813  performs link change to the TeNB  1817  by performing a procedure for establishing a D2D link and a relay cellular link for the RUE A  1815  with the TeNB  1817 . 
         [0201]    An environment that an IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 18 , and a process in which an IUE performs link change to another cell in an environment that the IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIGS. 19A and 19B . 
         [0202]      FIGS. 19A and 19B  schematically illustrate a process in which an IUE performs link change to another cell in an environment that the IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0203]    Referring to  FIGS. 19A and 19B , the LTE mobile communication system includes an IUE  1910 , an RUE A  1920 , an RUE B  1930 , an SeNB  1940 , and a TeNB  1950 . 
         [0204]    The IUE  1920  establishes a D2D link and a relay cellular link with the RUE A  1920  which is within a service coverage of the TeNB  1950  due to degradation of quality of a corresponding link while performing relay communication through the RUE B  1930  which is within a service coverage of the SeNB  1940 . That is, the IUE  1910  performs relay link change to the RUE A  1920  which is within the service coverage of the TeNB  1950  due to degradation of quality of a link with the SeNB  1940 . After a cellular communication link with the SeNB  1940  is terminated, the IUE  1910  performs link change to the TeNB  1950  by performing a procedure for establishing a D2D link and a relay cellular link for the RUE A  1920  with the TeNB  1950 . 
         [0205]    A process in which an IUE performs link change to another cell in an environment that the IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure as illustrated in  FIGS. 19A and 19B  includes a measuring and reporting procedure  1960 , and a D2D link and relay cellular link establishing procedure  1970 . The measuring and reporting procedure  1960  is similar to a measuring and reporting procedure  1150  as described in  FIGS. 11A and 11B , the D2D link and relay cellular link establishing procedure  1970  is similar to a D2D communication link and relay cellular communication link establishing procedure  1160  as described in  FIGS. 11A and 11B , and a description thereof will be omitted herein. 
         [0206]    Although  FIGS. 19A and 19B  illustrate a process in which an IUE performs link change to another cell in an environment that the IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure, various changes could be made to  FIGS. 19A and 19B . For example, although shown as a series of operations, various operations in  FIGS. 19A and 19B  could overlap, occur in parallel, occur in a different order, or occur multiple times. 
         [0207]    A process in which an IUE performs link change to another cell in an environment that the IUE establishes a D2D link and a relay cellular link with an RUE which is within a service coverage of a TeNB due to degradation of quality of a corresponding link while performing relay communication through an RUE which is within a service coverage of an SeNB in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIGS. 19A and 19B , and an inner structure of a UE in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 20 . 
         [0208]      FIG. 20  schematically illustrates an inner structure of a UE in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0209]    Referring to  FIG. 20 , a UE  2000  includes a transmitting unit  2011 , a control unit  2013 , a receiving unit  2015 , and a storage unit  2017 . Here, the UE  2000  may be an IUE or an RUE. 
         [0210]    The control unit  2013  controls the overall operation of the UE  2000 . More particularly, the control unit  2013  controls the UE  2000  to perform an operation for providing a service to an IUE in an LTE mobile communication system as described  FIGS. 2 to 19   b . The operation for providing the service to the IUE in the LTE mobile communication system has been described with reference to  FIGS. 2 to 19   b , and a description thereof will be omitted herein. 
         [0211]    The transmitting unit  2011  transmits various messages to other devices or an eNB under a control of the control unit  2013 . The various messages transmitted in the transmitting unit  2011  have been described in  FIGS. 2 to 19   b  and a description thereof will be omitted herein. 
         [0212]    The receiving unit  2015  receives various messages from other devices or an eNB under a control of the control unit  2013 . The various messages received in the receiving unit  2015  have been described in  FIGS. 2 to 19   b  and a description thereof will be omitted herein. 
         [0213]    The storage unit  2017  stores a program related to an operation for the UE  2000  to provide a service to an IUE in an LTE mobile communication system, various data, and the like. The storage unit  2017  stores the various messages which are received by the receiving unit  2015  from the other devices or the eNB. 
         [0214]    While the transmitting unit  2011 , the control unit  2013 , the receiving unit  2015 , and the storage unit  2017  are described in the UE  2000  as separate units, it is to be understood that this is merely for convenience of description. In other words, two or more of the transmitting unit  2011 , the control unit  2013 , the receiving unit  2015 , and the storage unit  2017  may be incorporated into a single unit. 
         [0215]    An inner structure of a UE in an LTE mobile communication system according to an embodiment of the present disclosure has been described with reference to  FIG. 20 , and an inner structure of an eNB in an LTE mobile communication system according to an embodiment of the present disclosure will be described with reference to  FIG. 21 . 
         [0216]      FIG. 21  schematically illustrates an inner structure of en eNB in an LTE mobile communication system according to an embodiment of the present disclosure. 
         [0217]    Referring to  FIG. 21 , an eNB  2100  includes a transmitting unit  2111 , a control unit  2113 , a receiving unit  2115 , and a storage unit  2117 . Here, the eNB  2100  may be an SeNB or a TeNB. 
         [0218]    The control unit  2113  controls the overall operation of the eNB  2100 . More particularly, the control unit  2113  controls the eNB  2100  to perform an operation for providing a service to an IUE in an LTE mobile communication system as described  FIGS. 2 to 19   b . The operation for the eNB  2100  to provide the service to the IUE in the LTE mobile communication system has been described with reference to  FIGS. 2 to 19   b , and a description thereof will be omitted herein. 
         [0219]    The transmitting unit  2111  transmits various messages to an MIME or devices under a control of the control unit  2113 . The various messages transmitted in the transmitting unit  2111  have been described in  FIGS. 2 to 19   b  and a description thereof will be omitted herein. 
         [0220]    The receiving unit  2115  receives various messages from an MME or devices under a control of the control unit  2113 . The various messages received in the receiving unit  2115  have been described in  FIGS. 2 to 19   b  and a description thereof will be omitted herein. 
         [0221]    The storage unit  2117  stores a program related to an operation for the eNB  2100  to provide a service to an IUE in an LTE mobile communication system, various data, and the like. The storage unit  2117  stores the various messages which are received by the receiving unit  2115  from the MME or the devices. 
         [0222]    While the transmitting unit  2111 , the control unit  2113 , the receiving unit  2115 , and the storage unit  2117  are described in the eNB  2100  as separate units, it is to be understood that this is merely for convenience of description. In other words, two or more of the transmitting unit  2111 , the control unit  2113 , the receiving unit  2115 , and the storage unit  2117  may be incorporated into a single unit. 
         [0223]    An embodiment of the present disclosure enables to provide a D2D communication-based service for an IUE in a mobile communication system. 
         [0224]    An embodiment of the present disclosure enables to provide a D2D communication-based service to an IUE which exists at outside of a service coverage of an eNB in a mobile communication system by establishing a link appropriate for a situation. 
         [0225]    An embodiment of the present disclosure enables to support cellular communication for IUEs without additional device deployment and management in a mobile communication system thereby providing a D2D communication-based service. 
         [0226]    Certain aspects of the present disclosure may also be embodied as computer readable code on a non-transitory computer readable recording medium. A non-transitory computer readable recording medium is any data storage device that can store data, which can be thereafter read by a computer system. Examples of the non-transitory computer readable recording medium include read only memory (ROM), random access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The non-transitory computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. In addition, functional programs, code, and code segments for accomplishing the present disclosure can be easily construed by programmers skilled in the art to which the present disclosure pertains. 
         [0227]    It can be appreciated that a method and apparatus according to an embodiment of the present disclosure may be implemented by hardware, software and/or a combination thereof. The software may be stored in a non-volatile storage, for example, an erasable or re-writable ROM, a memory, for example, a RAM, a memory chip, a memory device, or a memory integrated circuit (IC), or an optically or magnetically recordable non-transitory machine-readable (e.g., computer-readable), storage medium (e.g., a compact disk (CD), a digital versatile disk (DVD), a magnetic disk, a magnetic tape, and/or the like). A method and apparatus according to an embodiment of the present disclosure may be implemented by a computer or a mobile terminal that includes a controller and a memory, and the memory may be an example of a non-transitory machine-readable (e.g., computer-readable), storage medium suitable to store a program or programs including instructions for implementing various embodiments of the present disclosure. 
         [0228]    The present disclosure may include a program including code for implementing the apparatus and method as defined by the appended claims, and a non-transitory machine-readable (e.g., computer-readable), storage medium storing the program. The program may be electronically transferred via any media, such as communication signals, which are transmitted through wired and/or wireless connections, and the present disclosure may include their equivalents. 
         [0229]    An apparatus according to an embodiment of the present disclosure may receive the program from a program providing device which is connected to the apparatus via a wire or a wireless and store the program. The program providing device may include a memory for storing instructions which instruct to perform a content protect method which has been already installed, information necessary for the content protect method, and the like, a communication unit for performing a wired or a wireless communication with a graphic processing device, and a controller for transmitting a related program to a transmitting/receiving device based on a request of the graphic processing device or automatically transmitting the related program to the transmitting/receiving device. 
         [0230]    While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.