Patent Publication Number: US-11659468-B2

Title: Method and apparatus for link configuration and routing of relay system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Korean Patent Applications No. 10-2020-0137886 filed on Oct. 22, 2020 and No. 10-2021-0087810 filed on Jul. 5, 2021 with the Korean Intellectual Property Office (KIPO), the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a technique for link configuration and routing of a relay system, and more particularly, to a technique for link configuration and routing of a relay system that continues communication service via a relay when communication with a base station is disconnected. 
     2. Description of Related Art 
     In order to process wireless data that increases rapidly after commercialization of the fourth generation (4G) communication system (e.g., long term evolution (LTE) communication system or LTE-Advanced (LTE-A) communication system), a fifth generation (5G) communication system (e.g., new radio (NR) communication system) using not only a frequency band (e.g., frequency band of 6 GHz or below) of the 4G communication system but also a frequency band (e.g., frequency band of 6 GHz or above) higher than the frequency band of the 4G communication system is being considered. The 5G communication system may support enhanced Mobile BroadBand (eMBB), Ultra-Reliable and Low Latency Communication (URLLC), and massive Machine Type Communication (mMTC). 
     Meanwhile, among the communication systems, a millimeter wave-based mobile communication system may provide a broadband service using beamforming. In the millimeter wave-based mobile communication system, a terminal may frequently experience communication service blockages due to obstacles such as trees, traffic signs, and automobiles according to millimeter wave frequency characteristics. In such the case, if an adjacent terminal performs a role of a relay, service continuity may be guaranteed to the terminal (i.e., remote terminal) whose communication service is disconnected. To this end, the adjacent terminal may perform forwarding of relay traffic of the terminal whose communication service is disconnected. As such, in order for the adjacent terminal to perform forwarding of relay traffic, a signaling procedure for link switching and a signaling procedure for registering, changing, and releasing routing information for the data forwarding may be required. 
     SUMMARY 
     Accordingly, exemplary embodiments of the present disclosure are directed to providing a method and an apparatus for link and routing configuration of a relay system, which can provide a procedure of link switching for an adjacent terminal to perform forwarding of relay traffic, and a procedure of registering, changing, and releasing routing information. 
     According to a first exemplary embodiment of the present disclosure, an operation method of a first terminal in a communication system may comprise: configuring a Uu access link with a base station, and receiving a first message from the base station, the first message including a relay radio network temporary identifier (RNTI) for identifying the first terminal, a node address of the base station, and a relay adaptation protocol (RAP) entity identifier (ID) for identifying a RAP entity configured for the first terminal; configuring a relay link with a second terminal when communication with the base station is disconnected; requesting link switching by transmitting to the base station a second message via the second terminal, the second message including the relay RNTI, the node address, and the RAP entity ID; and receiving a response to the second message from the base station via the second terminal, and switching a link with the base station from the Uu access link to the relay link. 
     The requesting of the link switching may comprise: generating a first message authentication code for integrity (MAC-I) by processing integrity protection based on the relay RNTI, the node address, and the RAP entity; generating the second message including the relay RNTI, the node address, the RAP entity, and the first MAC-I information; and requesting the link switching by transmitting the second message to the base station via the second terminal. 
     The receiving of the response and the switching of the link may comprise: receiving the response to the second message including the relay RNTI, the node address, and a second MAC-I from the base station via the second terminal; generating a third MAC-I by processing integrity protection based on the relay RNTI and the node address; and in response to determining that the third MAC-I and the second MAC-I match, switching the link with the base station from the Uu access link to the relay link, and transmitting uplink data through the relay link. 
     The operation method may further comprise, after the configuring of the relay link, configuring a routing table entry based on the node address, the RAP entity ID, and a first branch node address received from the second terminal in the configuring of the relay link; and in response to receiving the response to the second message from the base station, activating the routing table entry. 
     The operation method may further comprise: receiving, from the second terminal, a third message requesting update of the routing table entry, the third message including a second branch node address; and updating the routing table entry by reflecting the second branch node address. 
     The operation method may further comprise: starting a timer after transmitting the second message; and releasing the relay link if the response to the second message is not received within a waiting time defined by the timer. 
     According to a second exemplary embodiment of the present disclosure, an operation method of a second terminal in a communication system may comprise: configuring a relay link with a first terminal; receiving, from the first terminal, a second message requesting link switching, and transmitting the second message to a second base station, the second message including a root node address that is a node address of a first base station to which the first terminal is connected, and a relay adaptation protocol (RAP) entity ID for identifying a RAP entity for the first terminal configured in the first base station; configuring a routing table entry based on the root node address, the RAP entity ID, and a node address of the second base station; receiving a third message from the second base station in response to the second message, and transmitting the third message to the first terminal, the third message including the root node address and the RAP entity ID; and activating the routing table entry. 
     The configuring of the routing table entry may comprise: receiving the node address of the second base station from the second base station; detecting the root node address and the RAP entity ID in the second message; and configuring the routing table entry based on the root node address, the RAP entity ID, and the node address of the second base station. 
     The activating of the routing table entry may comprise: searching a routing table based on the root node address and the RAP entity ID of the third message; and activating the routing table entry when the routing table entry associated with the root node address and the RAP entity ID is retrieved in the routing table. 
     The operation method may further comprise: receiving, from the first terminal, a fourth message including the root node address and the RAP entity ID; searching a routing table based on the root node address and the RAP entity ID of the fourth message; and when the routing table entry associated with the root node address and the RAP entity ID is retrieved in the routing table, transmitting the fourth message based on the retrieved routing table entry. 
     The operation method may further comprise: receiving a node address of a third base station from the third base station by being connected to the third base station through inter-cell movement; and updating the routing table entry by reflecting the node address of the third base station received from the third base station. 
     According to a third exemplary embodiment of the present disclosure, an operation method of a base station in a communication system may comprise: transmitting a first message to a first terminal when the first terminal configures a Uu access link with the base station by being connected to the base station, the first message including a relay radio network temporary identifier (RNTI) for identifying the first terminal, a node address of the base station, and a relay adaptation protocol (RAP) entity identifier (ID) for identifying a RAP entity configured for the first terminal; receiving, from the first terminal, a second message requesting link switching via a second terminal, the second message including the relay RNTI, the root node address, and the RAP entity ID; identifying the Uu access link connected with the first terminal by using the relay RNTI and the RAP entity ID according to the second message requesting link switching, and switching a link with the first terminal from the Uu access link to a relay link via the second terminal; and transmitting, to the first terminal, a third message informing completion of the link switching via the second terminal, the third message including the relay RNTI, the node address, and the RAP entity ID. 
     The switching of the link may comprise: identifying the first terminal based on the relay RNTI of the second message; identifying the Uu access link corresponding to the first terminal by using the RAP entity ID; and switching the link with the first terminal from the identified Uu access link to the relay link via the second terminal. 
     The operation method may further comprise: configuring a routing table entry based on the node address and the RAP entity ID upon receiving the first message; performing an integrity check on the second message; and activating the routing table entry if the integrity check is successful. 
     According to the exemplary embodiments of the present disclosure, the terminal relay system may provide a procedure of link switching from a Uu access link to a relay link for a terminal whose communication service is disconnected. In addition, according to the exemplary embodiments of the present disclosure, the terminal relay system may provide a procedure of registering, changing, and releasing routing information for data forwarding of a terminal whose communication service is disconnected. In addition, according to the exemplary embodiments of the present disclosure, by providing a terminal relay signaling procedure, a terminal whose communication service is disconnected due to an obstacle or the like may transmit and receive data using a relay function of an adjacent terminal. In addition, according to the exemplary embodiments of the present disclosure, by providing a terminal relay signaling procedure, a terminal not capable of transmitting and receiving data outside a cell coverage may transmit and receive data using a relay function of an adjacent terminal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a conceptual diagram illustrating a first exemplary embodiment of a communication system. 
         FIG.  2    is a block diagram illustrating a first exemplary embodiment of a communication node constituting a communication system. 
         FIG.  3    is a conceptual diagram illustrating a first exemplary embodiment of a terminal relay system. 
         FIG.  4    is a conceptual diagram illustrating a second exemplary embodiment of a terminal relay system. 
         FIG.  5    is a diagram of a protocol stack structure of a terminal relay system according to a first exemplary embodiment. 
         FIG.  6    is a block diagram illustrating a first exemplary embodiment of entities of a terminal relay system. 
         FIG.  7    is a conceptual diagram illustrating a method of assigning addresses and identifiers to a base station and entities of the base station. 
         FIG.  8    is a sequence chart illustrating a first exemplary embodiment of a method for configuring a link between a remote terminal and a base station. 
         FIG.  9    is a sequence chart illustrating a first exemplary embodiment of a method for configuring a link between a remote terminal and a relay terminal. 
         FIG.  10    is a sequence chart illustrating a first exemplary embodiment of a method for switching a link with a remote terminal from a Uu access link to a relay link. 
         FIG.  11    is a flowchart illustrating a first exemplary embodiment of an operation method of an RRC entity of a remote terminal. 
         FIG.  12    is a flowchart illustrating a first exemplary embodiment of a link switching method of an RRC entity of a base station. 
         FIG.  13    is a flowchart illustrating a first exemplary embodiment of a link switching method of an RRC entity of a remote terminal. 
         FIG.  14    is a flowchart illustrating a first exemplary embodiment of a method for configuring routing information performed by a remote terminal. 
         FIG.  15    is a flowchart illustrating a first exemplary embodiment of a method for configuring routing information performed by a RAP entity of a relay terminal. 
         FIG.  16    is a flowchart illustrating a first exemplary embodiment of a method for data relaying performed by a relay terminal. 
         FIG.  17    is a conceptual diagram illustrating a third exemplary embodiment of a terminal relay system. 
         FIG.  18    is a flowchart illustrating a first exemplary embodiment of a method of changing a branch node address performed by a relay terminal. 
         FIG.  19    is a flowchart illustrating a first exemplary embodiment of a method for changing a branch node address performed by a remote terminal. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present disclosure are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing embodiments of the present disclosure. Thus, embodiments of the present disclosure may be embodied in many alternate forms and should not be construed as limited to embodiments of the present disclosure set forth herein. 
     Accordingly, while the present disclosure is capable of various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the present disclosure to the particular forms disclosed, but on the contrary, the present disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this present disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Hereinafter, exemplary embodiments of the present disclosure will be described in greater detail with reference to the accompanying drawings. In order to facilitate general understanding in describing the present disclosure, the same components in the drawings are denoted with the same reference signs, and repeated description thereof will be omitted. 
       FIG.  1    is a conceptual diagram illustrating a first exemplary embodiment of a communication system. 
     Referring to  FIG.  1   , a communication system  100  may comprise a plurality of communication nodes  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 ,  120 - 2 ,  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , and  130 - 6 . Here, the communication system may be referred to as a ‘communication network’. Each of the plurality of communication nodes may support at least one communication protocol. For example, each of the plurality of communication nodes  110  to  130  may support communication protocols defined in the 3rd generation partnership project (3GPP) technical specifications (e.g., LTE communication protocol, LTE-A communication protocol, NR communication protocol, or the like). The plurality of communication nodes  110  to  130  may support code division multiple access (CDMA) based communication protocol, wideband CDMA (WCDMA) based communication protocol, time division multiple access (TDMA) based communication protocol, frequency division multiple access (FDMA) based communication protocol, orthogonal frequency division multiplexing (OFDM) based communication protocol, filtered OFDM based communication protocol, cyclic prefix OFDM (CP-OFDM) based communication protocol, discrete Fourier transform-spread-OFDM (DFT-s-OFDM) based communication protocol, orthogonal frequency division multiple access (OFDMA) based communication protocol, single carrier FDMA (SC-FDMA) based communication protocol, non-orthogonal multiple access (NOMA) based communication protocol, generalized frequency division multiplexing (GFDM) based communication protocol, filter band multi-carrier (FBMC) based communication protocol, universal filtered multi-carrier (UFMC) based communication protocol, space division multiple access (SDMA) based communication protocol, or the like. Each of the plurality of communication nodes may have the following structure. 
       FIG.  2    is a block diagram illustrating a first exemplary embodiment of a communication node constituting a communication system. 
     Referring to  FIG.  2   , a communication node  200  may comprise at least one processor  210 , a memory  220 , and a transceiver  230  connected to the network for performing communications. Also, the communication node  200  may further comprise an input interface device  240 , an output interface device  250 , a storage device  260 , and the like. Each component included in the communication node  200  may communicate with each other as connected through a bus  270 . However, each of the components included in the communication node  200  may be connected not to the common bus  270  but to the processor  210  through an individual interface or an individual bus. For example, the processor  210  may be connected to at least one of the memory  220 , the transceiver  230 , the input interface device  240 , the output interface device  250 , and the storage device  260  through dedicated interfaces. 
     The processor  210  may execute a program stored in at least one of the memory  220  and the storage device  260 . The processor  210  may refer to a central processing unit (CPU), a graphics processing unit (GPU), or a dedicated processor on which methods in accordance with embodiments of the present disclosure are performed. Each of the memory  220  and the storage device  260  may be constituted by at least one of a volatile storage medium and a non-volatile storage medium. For example, the memory  220  may comprise at least one of read-only memory (ROM) and random access memory (RAM). 
     Referring again to  FIG.  1   , the communication system  100  may comprise a plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2 , and a plurality of terminals  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , and  130 - 6 . Each of the first base station  110 - 1 , the second base station  110 - 2 , and the third base station  110 - 3  may form a macro cell, and each of the fourth base station  120 - 1  and the fifth base station  120 - 2  may form a small cell. The fourth base station  120 - 1 , the third terminal  130 - 3 , and the fourth terminal  130 - 4  may belong to the cell coverage of the first base station  110 - 1 . Also, the second terminal  130 - 2 , the fourth terminal  130 - 4 , and the fifth terminal  130 - 5  may belong to the cell coverage of the second base station  110 - 2 . Also, the fifth base station  120 - 2 , the fourth terminal  130 - 4 , the fifth terminal  130 - 5 , and the sixth terminal  130 - 6  may belong to the cell coverage of the third base station  110 - 3 . Also, the first terminal  130 - 1  may belong to the cell coverage of the fourth base station  120 - 1 , and the sixth terminal  130 - 6  may belong to the cell coverage of the fifth base station  120 - 2 . 
     Here, each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may be referred to as NodeB (NB), evolved NodeB (eNB), gNB, advanced base station (ABS), high reliability-base station (HR-BS), base transceiver station (BTS), radio base station, radio transceiver, access point (AP), access node, radio access station (RAS), mobile multihop relay-base station (MMR-BS), relay station (RS), advanced relay station (ARS), high reliability-relay station (HR-RS), home NodeB (HNB), home eNodeB (HeNB), road side unit (RSU), radio remote head (RRH), transmission point (TP), transmission and reception point (TRP), relay node, or the like. Each of the plurality of terminals  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , and  130 - 6  may be referred to as user equipment (UE), terminal equipment (TE), advanced mobile station (AMS), high reliability-mobile station (HR-MS), terminal, access terminal, mobile terminal, station, subscriber station, mobile station, portable subscriber station, node, device, on-board unit (OBU), or the like. 
     Each of the plurality of communication nodes  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 ,  120 - 2 ,  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , and  130 - 6  may support cellular communication (e.g., LTE, LTE-Advanced (LTE-A), etc.) defined in the 3rd generation partnership project (3GPP) specification. Each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may operate in the same frequency band or in different frequency bands. The plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may be connected to each other via an ideal backhaul link or a non-ideal backhaul link, and exchange information with each other via the ideal or non-ideal backhaul. Also, each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may be connected to the core network through the ideal backhaul link or non-ideal backhaul link. Each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may transmit a signal received from the core network to the corresponding terminal  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , or  130 - 6 , and transmit a signal received from the corresponding terminal  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , or  130 - 6  to the core network. 
     Each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may support OFDMA-based downlink (DL) transmission, and SC-FDMA-based uplink (UL) transmission. In addition, each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may support a multi-input multi-output (MIMO) transmission (e.g., single-user MIMO (SU-MIMO), multi-user MIMO (MU-MIMO), massive MIMO, or the like), a coordinated multipoint (CoMP) transmission, a carrier aggregation (CA) transmission, a transmission in unlicensed band, a device-to-device (D2D) communication (or, proximity services (ProSe)), an Internet of Things (IoT) communication, a dual connectivity (DC), or the like. Here, each of the plurality of terminals  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , and  130 - 6  may perform operations corresponding to the operations of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  (i.e., the operations supported by the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2 ). 
     For example, the second base station  110 - 2  may transmit a signal to the fourth terminal  130 - 4  in the SU-MIMO manner, and the fourth terminal  130 - 4  may receive the signal from the second base station  110 - 2  in the SU-MIMO manner. Alternatively, the second base station  110 - 2  may transmit a signal to the fourth terminal  130 - 4  and fifth terminal  130 - 5  in the MU-MIMO manner, and the fourth terminal  130 - 4  and fifth terminal  130 - 5  may receive the signal from the second base station  110 - 2  in the MU-MIMO manner. Each of the first base station  110 - 1 , the second base station  110 - 2 , and the third base station  110 - 3  may transmit a signal to the fourth terminal  130 - 4  in the CoMP transmission manner, and the fourth terminal  130 - 4  may receive the signal from the first base station  110 - 1 , the second base station  110 - 2 , and the third base station  110 - 3  in the CoMP manner. Also, each of the plurality of base stations  110 - 1 ,  110 - 2 ,  110 - 3 ,  120 - 1 , and  120 - 2  may exchange signals with the corresponding terminals  130 - 1 ,  130 - 2 ,  130 - 3 ,  130 - 4 ,  130 - 5 , or  130 - 6  which belongs to its cell coverage in the CA manner. Each of the base stations  110 - 1 ,  110 - 2 , and  110 - 3  may control D2D communications between the fourth terminal  130 - 4  and the fifth terminal  130 - 5 , and thus the fourth terminal  130 - 4  and the fifth terminal  130 - 5  may perform the D2D communications under control of the second base station  110 - 2  and the third base station  110 - 3 . 
     Meanwhile, a millimeter wave-based mobile communication system may provide a broadband service using beamforming. In the millimeter wave-based mobile communication system, a terminal may frequently experience communication service blockages due to obstacles such as trees, traffic signs, and automobiles according to millimeter wave frequency characteristics. In such the case, if an adjacent terminal performs a role of a relay, service continuity may be guaranteed to the terminal whose communication service is disconnected. 
       FIG.  3    is a conceptual diagram illustrating a first exemplary embodiment of a terminal relay system. 
     Referring to  FIG.  3   , a terminal relay system may include a base station  310 , a first terminal  321 , a second terminal  322 , and a third terminal  323 . The first terminal  321  and the second terminal  322  may be located in a communication service coverage of the base station  310 , and the third terminal  323  may be located out of the communication service coverage of the base station  310 . Here, the third terminal  323  may move out of the communication service coverage of the base station  310  due to a movement therefrom. As such, when the third terminal  323  is located out of the communication service coverage of the base station  310 , the third terminal  323  may access the base station  310  via the second terminal  322  to perform data transmission and reception with the base station  310 . In this case, the second terminal  322  may relay communication between the base station  310  and the third terminal  323 . On the other hand, since the first terminal  321  is located in the communication service coverage of the base station  310 , the first terminal  321  may directly access the base station  310  to transmit and receive data to and from the base station  310 . In this case, the first terminal  321  may enter a communication disconnected (e.g., blockage) state in which data cannot be transmitted/received to and from the base station  310  due to an obstacle or the like. In this case, the first terminal  321  may access the base station  310  via the second terminal  322  to transmit and receive data to and from the base station  310 . Accordingly, the second terminal  322  may relay communication between the base station  310  and the first terminal  321 . When a communication disconnected state occurs in which data cannot be directly transmitted/received to and from the base station  310  due to an obstacle or the like as in the case of the first terminal  321 , the terminal relay system may use the adjacent second terminal  322  as a relay so that the first terminal  321  continues to perform data transmission and reception with the first base station  310 . In addition, when a terminal is unable to transmit and receive data to and from the base station  310  outside the cell coverage as in the case of the third terminal  323 , the terminal relay system may use the adjacent second terminal  322  as a relay so that the third terminal  323  continues to perform data transmission and reception with the first base station  310 . 
       FIG.  4    is a conceptual diagram illustrating a second exemplary embodiment of a terminal relay system. 
     Referring to  FIG.  4   , a terminal relay system may include a first base station  411 , a second base station  412 , a first terminal  421 , and a second terminal  422 . The first terminal  421  may be located in a communication service coverage of the first base station  412 , and the second terminal  422  may be located in a communication service coverage of the second base station  412 . As such, since the first terminal  421  is located in the communication service coverage of the first base station  411 , the first terminal  421  may directly access the first base station  411  to transmit and receive data to and from the first base station  411 . In addition, since the second terminal  422  is located in the communication service coverage of the second base station  412 , the second terminal  422  may directly access the second base station  412  to transmit and receive data to and from the second base station  412 . From such the state, the first terminal  421  may enter a communication disconnected state in which data cannot be transmitted and received to and from the first base station  411  due to an obstacle or the like. In this case, the first terminal  421  may access the first base station  411  via the second terminal  422  and the second base station  412  to transmit and receive data to and from the first base station  411 . In this case, the second terminal  422  may relay communication between the second base station  412  and the first terminal  421 . In addition, the second base station  412  may relay communication between the first base station  411  and the second terminal  422 . As such, even when the second terminal  422  adjacent to the first terminal  421  is located out of the communication service coverage of the first base station  411  providing a communication service to the first terminal  421 , the terminal relay system may enable continuous data transmission/reception between the first base station  411  and the first terminal  421  via the second base station  412  providing a communication service to the second terminal  422 . 
       FIG.  5    is a diagram of a protocol stack structure of a terminal relay system according to a first exemplary embodiment. 
     Referring to  FIG.  5   , a protocol stack structure of the terminal relay system may include a protocol stack structure of a base station  510 , a protocol stack structure of a relay terminal  520 , and a protocol stack structure of a remote terminal  530 . 
     Here, the protocol stack structure of the base station  510  may include a first service data adaptation protocol (SDAP) layer  511 , a first packet data convergence protocol (PDCP) layer  512 , a first relay adaptation protocol (RAP) layer  513 , a first radio link control (RLC) layer  514 - 1 , a second RLC layer  514 - 2 , a first medium access control (MAC) layer  515 - 1 , a second MAC layer  515 - 2 , a first physical (PHY) layer  516 - 1 , and a second PHY layer  516 - 2 . In the protocol stack structure of the base station  510 , the first SDAP layer  511 , the first PDCP layer  512 , the first RLC layer  514 - 1 , the first MAC layer  515 - 1 , and the first PHY layer  516 - 1  may support direct transmission and reception of data through a Uu access link of a Uu interface before the remote terminal  530  enters a communication disconnected state. In addition, when the remote terminal  530  enters a communication disconnected state, the first SDAP layer  511 , the first PDCP layer  512 , the first RAP layer  513 , the second RLC layer  514 - 2 , the second MAC layer  515 - 2 , and the second PHY layer  516 - 2  in the protocol stack structure of the base station  510  may support communication through a Uu access link  540  of a Uu interface with the relay terminal  520 , thereby transmitting and receiving data to and from the remote terminal  530  via the relay terminal  520 . 
     Meanwhile, the protocol stack structure of the relay terminal  520  may include a second RAP layer  521 - 1 , a third RAP layer  521 - 2 , a third RLC layer  522 - 1 , a fourth RLC layer  522 - 2 , a third MAC layer  523 - 1 , a fourth MAC layer  523 - 2 , a third PHY layer  524 - 1 , and a fourth PHY layer  524 - 2 . In the protocol stack structure of the relay terminal  520 , the second RAP layer  521 - 1 , the third RLC layer  522 - 1 , the third MAC layer  523 - 1 , and the first PHY layer  524 - 1  may support communication with the base station  510  through the Uu access link  540  of the Uu interface. In contrast, in the protocol stack structure of the relay terminal  520 , the third RAP layer  521 - 2 , the fourth RLC layer  522 - 2 , the fourth MAC layer  523 - 2 , and the fourth PHY layer  524 - 2  may support communication with the remote terminal  530  through a sidelink or a relay link  550 . 
     In addition, the protocol stack structure of the remote terminal  530  may include a second SDAP layer  531 , a second PDCP layer  532 , a fourth RAP layer  533 , a fifth RLC layer  534 - 1 , a sixth RLC layer  534 - 2 , a fifth MAC layer  535 - 1 , a sixth MAC layer  535 - 2 , a fifth PHY layer  536 - 1 , and a sixth PHY layer  536 - 2 . In the protocol stack structure of the remote terminal  530 , the second SDAP layer  531 , the second PDCP layer  532 , the fifth RLC layer  534 - 1 , the fifth MAC layer  535 - 1 , and the fifth PHY layer  536 - 1  may support direct data transmission and reception with the base station  510  through a Uu access link before the remote terminal enters a communication disconnected state. In contrast, in the protocol stack structure of the remote terminal  530 , the second SDAP layer  531 , the second PDCP layer  532 , the fourth RAP layer  533 , the sixth RLC layer  534 - 2 , the sixth MAC The layer  535 - 2 , and the sixth PHY layer  536 - 2  may support communication with the relay terminal  520  through the sidelink or a relay link  550  when the remote terminal  530  enters a communication disconnected state. 
     As described above, in the terminal relay system, the first RAP layer  513  may be added in the protocol stack structure of the base station  510  as a lower layer of the first PDCP layer  512  so that the base station  510  communicates with the relay terminal  520  to transmit and receive data to and from the remote terminal  530 , and the second RLC layer  514 - 2 , the second MAC layer  515 - 2 , the second PHY layer  516 - 2  may be provided to support the added first RAP layer  513 . In addition, in the terminal relay system, the second RAP layer  521 - 1  may be added in the protocol stack structure of the relay terminal  520  so that the relay terminal  520  communicates with the base station  510  to transmit and receive data to and from the base station  510 , and the third RLC layer  522 - 1 , the third MAC layer  523 - 1 , and the third PHY layer  524 - 1  may be provided to support the added second RAP layer  521 - 1 . 
     In addition to these, in the terminal relay system, the third RAP layer  521 - 2  may be added in the protocol stack structure of the relay terminal  520  so that the relay terminal  520  communicates with the remote terminal  530  to transmit and receive data to and from the remote terminal  530 , and the fourth RLC layer  522 - 2 , the fourth MAC layer  523 - 2 , and the fourth PHY layer  524 - 2  may be provided to support the added third RAP layer  521 - 2 . Further, in the terminal relay system, the fourth RAP layer  533  may be added in the protocol stack structure of the remote terminal  530  as a lower layer of the second PDCP layer  532  so that the remote terminal  530  can communicate with the relay terminal  520  to transmit and receive data to and from the relay terminal  520 , and the sixth RLC layer  534 - 2 , the sixth MAC layer  535 - 2 , and the sixth PHY layer  536 - 2  may be provided to support the added fourth RAP layer  533 . As described above, in order to support the relay function, the first RAP layer  513  and the fourth RAP layer  533  may be located between the PDCP layers  512  and  532  and the RLC layers  514 - 1 ,  514 - 2 ,  534 - 1 , and  534 - 2  according to the general protocol stack structure including the SDAP layers  511  and  531 , PDCP layers  512  and  532 , RLC layers  514 - 1 ,  514 - 2 ,  534 - 1 , and  534 - 2 , and MAC layer  515 - 1 ,  515 - 2 ,  535 - 1 , and  535 - 2 . 
     As described above, the terminal relay system may use a plurality of RAP layers  513 ,  521 - 1 ,  521 - 2 , and  533  to provide relay to the base station  510  through the relay terminal  520  adjacent to the remote terminal  530 . The base station  510  and the remote terminal  530  may perform data transmission and reception with the relay terminal  520  through data relaying using the first and fourth RAP layers  513  and  533  when normal data transmission and reception is impossible through a Uu access link. Then, data transmission/reception between the base station  510  and the remote terminal  530  may be relayed using the second RAP layer  521 - 1  and the third RAP layer  521 - 2  of the relay terminal  520 . To this end, the plurality of RAP layers  513 ,  521 - 1 ,  521 - 2 , and  533  may perform functions of registering, changing, and releasing routing information for data transmission and reception between the base station  510  and the remote terminal  530 . 
     In such the situation, when the remote terminal  530  enters a communication disconnected state and direct data communication with the base station  510  using a Uu access link is impossible, the remote terminal  530  may perform data communication with the base station  510  through the relay terminal  520  capable of communicating with the base station  510  using the Uu access link  540 . 
       FIG.  6    is a block diagram illustrating a first exemplary embodiment of entities of a terminal relay system. 
     Referring to  FIG.  6   , a structure of entities of the terminal relay system may include an entity structure  610  for remote terminals and an entity structure  620  for relay terminals in a base station  600 . Here, a remote terminal  640  may also support a relay function. Here, the entity structure  610  for remote terminals in the base station  600  may include a plurality of remote terminal (rUE) PDCP entities  611 , a rUE RAP entity  612 , a plurality of rUE RLC entities  613  not associated with the RAP entity  612 , and a plurality of rUE RLC entities  614  associated with the RAP entity  612 . In addition, the entity structure  620  for relay terminals in the base station  600  may include a plurality of relay terminal (RUE) PDCP entities  621 , a RUE RAP entity  622 , a plurality of RUE RLC entities  623  not associated with the RAP entity  622 , and a plurality of RUE RLC entities  624  associated with the RAP entity  622 . 
     As described above, the base station  600  may configure the RAP entities  612  and  622  when initial connection configuration with the terminals  640  and  650  is completed. That is, the base station  600  may configure the RAP entities  612  and  622  for the respective terminals  640  and  650  when the initial connection configuration with all the terminals  640  and  650  supporting the relay function is completed. In this case, the base station  600  may control the terminals  640  and  650  so that the terminals  640  and  650  may configure the RAP entities and the RLC entities associated with the RAP entities. Then, when the remote terminal  640  enters a communication disconnected state and a relay connection is configured using the relay terminal  650 , the base station  600  may connect the rUE RAP entity  612  and the RUE RAP entity  622  to perform relay data transmission and reception. 
     Meanwhile, the RAP entities of the RAP layer may perform routing to transmit relay data to a next node or terminal. As such, in order for the RAP entity to perform routing, relay-related addresses and identifiers (IDs) for data transmission may be required. 
       FIG.  7    is a conceptual diagram illustrating a method of assigning addresses and identifiers to a base station and entities of the base station. 
     Referring to  FIG.  7   , a structure of entities of the base station may include an entity structure  710  for remote terminals and an entity structure  720  for relay terminals. Here, the entity structure  710  for remote terminals of the base station  700  may include a plurality of rUE PDCP entities  711 , a rUE RAP entity  712 , a plurality of rUE RLC entities  713  not associated with the rUE RAP entity  712 , and a plurality of rUE RLC entities  714  associated with rUE RAP entity  712 . In addition, the entity structure  720  for relay terminals of the base station  700  may include a plurality of RUE PDCP entities  721 , a RUE RAP entity  722 , a plurality of RUE RLC entities  723  not associated with the RUE RAP entity  722 , and a plurality of RUE RLC entities  724  associated with the RUE RAP entity  722 . 
     The base station  700  may have a node address. The node address may be classified into a root node address and a branch node address according to a type of a terminal to which the base station provides a communication service. Here, a root node address may refer to an address of a base station in which an RRC and PDCP for a remote terminal are configured to provide a communication service to the remote terminal. The remote terminal may receive a node address from the corresponding base station and configure it as a root node address. In addition, the remote terminal may transmit the root node address to a relay terminal. Meanwhile, a branch node address may refer to a node address of a base station that provides a communication service to a relay terminal supporting a Uu access link among relay terminals. The relay terminal may access the corresponding base station to receive the node address and configure it as a branch node address. In addition, the relay terminal may transmit the branch node address to the remote terminal. Accordingly, the remote terminal and the relay terminal may determine whether the root node address and the branch node address are the same. When the root node address and the branch node address are the same, the remote terminal and the relay terminal may determine that the base station to which the remote terminal was connected before a disconnection of communication is the same as the base station to which the relay terminal is connected. On the other hand, when the root node address and the branch node address are different, the remote terminal and the relay terminal may determine that the base station to which the remote terminal was connected before a disconnection of communication is different from the base station to which the relay terminal is connected. 
     The base station  700  may assign a RAP entity ID to each of the RAP entities  712  and  722 . That is, the base station  700  may assign a rUE RAP entity ID to the rUE RAP entity  712 , and may assign a RUE RAP entity ID to the RUE RAP entity  722 . As such, the base station  700  may assign different RAP entity IDs to the rUE RAP entity  712  and the RUE RAP entity  722 . Accordingly, the base station  700  may identify the remote terminal using the root node address and the rUE RAP entity ID. Similarly, the base station may identify the relay terminal using the branch node address and the RUE RAP entity ID. 
     The base station  710  may configure the RUE PDCP entity  721  and the RUE RLC entity  724  when the relay terminal completes connection configuration through the Uu access link. The base station  710  may deliver root node address information, RAP entity ID information, and the like to the relay terminal through a control message, etc. The relay terminal receiving the information may configure the RAP entity and the RLC entity, and store the information on the root node address, RAP entity ID, and the like. 
       FIG.  8    is a sequence chart illustrating a first exemplary embodiment of a method for configuring a link between a remote terminal and a base station. 
     Referring to  FIG.  8   , in the method for configuring a link between a remote terminal and a base station, if the remote terminal is connected to the base station before a communication service with the base station is disconnected, the base station may transmit to the remote terminal an RRC reconfiguration message including a relay radio network temporary identifier (RNTI), a node address, a RAP entity ID, and the like (S 810 ). As such, the remote terminal may receive the RRC reconfiguration message including the relay RNTI, node address, RAP entity ID, and the like from the base station, store the information, and transmit an RRC reconfiguration complete message to the base station in response thereto (S 820 ). In this case, the remote terminal may configure the node address received from the base station as a root node address. 
     Meanwhile, when the remote terminal enters a communication disconnected state in which data transmission and reception cannot be performed through a Uu access link due to an obstacle or the like, the remote terminal may configure a sidelink or relay link with an adjacent terminal. 
       FIG.  9    is a sequence chart illustrating a first exemplary embodiment of a method for configuring a link between a remote terminal and a relay terminal. 
     Referring to  FIG.  9   , in the method for configuring a link between a remote terminal and a relay terminal, if the remote terminal enters a communication disconnected state in which data transmission and reception cannot be performed with the base station through the Uu access link due to an obstacle or the like, the remote terminal may transmit a link configuration request message to the relay terminal in order to configure a side link or a relay link with the relay terminal (S 910 ). Then, in response to the link configuration request message, the relay terminal may transmit a link configuration response message including a node address of a currently connected base station to the remote terminal (S 920 ). Accordingly, the remote terminal may configure the node address included in the link configuration response message received from the relay terminal as a branch node address, and may transmit a link configuration complete message to the relay terminal (S 930 ). Through this process, an RRC entity of the remote terminal may perform link switching from the Uu access link to the relay link when the configuration of the side link or the relay link is completed. 
       FIG.  10    is a sequence chart illustrating a first exemplary embodiment of a method for switching a link with a remote terminal from a Uu access link to a relay link. 
     Referring to  FIG.  10   , in the method for switching a link with a remote terminal from a Uu access link to a relay link, an RRC entity of the remote terminal may configure a link switching request message and deliver it to a RAP entity when configuration of the relay link is completed (S 1010 ). A process in which the RRC entity of the remote terminal configures the link switching request message and transmits it to the relay terminal may be described in detail with reference to  FIG.  11    as follows. 
       FIG.  11    is a flowchart illustrating a first exemplary embodiment of an operation method of an RRC entity of a remote terminal. 
     Referring to  FIG.  11   , in the operation method of an RRC entity of a remote terminal, the RRC entity of the remote terminal may generate a message authentication code for integrity (MAC-I) by processing integrity protection based on the relay RNTI, root node address, RAP entity ID, and the like when the configuration of the relay link is completed (S 1011 ). In addition, the RRC entity may configure the link switching request message including the relay RNTI, root node address, RAP entity ID, signaling radio bearer 1 (SRB1) uplink (UL) count (COUNT), and MAC-I information (S 1012 ). Then, the RRC entity of the remote terminal may transmit the configured link switching request message to the RAP entity of the remote terminal (S 1013 ). In addition, the RRC entity of the remote terminal may start a link switching timer (e.g., timer Txx) (S 1014 ). 
     Referring again to  FIG.  10   , upon receiving the link switching request message from the RRC entity, the RAP entity of the remote terminal may configure a relay information setup request message by forming a relay information setup request control PDU using the relay RNTI, root node address, RAP entity ID, SRB1 UL count, and MAC-I information of the link switching request message. Then, the RAP entity of the remote terminal may transmit the relay information setup request message to the RAP entity of the relay terminal (S 1020 - 1 ). Then, the RAP entity of the relay terminal may register the routing information in the relay information setup request control PDU included in the relay information setup request message to a routing table. Then, the RAP entity of the relay terminal may transmit the received relay information setup request message to a RUE RAP entity of the base station (S 1020 - 2 ). Meanwhile, the RUE RAP entity of the base station receiving the relay information setup request message from the RAP entity of the relay terminal may register the routing information in the relay information setup request control PDU included in the relay information setup request message to a routing table. The RUE RAP entity of the base station may transmit the received relay information setup request message to a rUE RAP entity of the base station (S 1020 - 3 ). Accordingly, the rUE RAP entity of the base station may configure a link switching request message using the relay RNTI, root node address, RAP entity ID, SRB1 UL count, and MAC-I information included in the received relay information setup request message, and deliver it to an RRC entity of the base station (S 1030 ). Then, the RRC entity of the base station may perform integrity protection on the link switching request message, and perform link switching according to the link switching request of the link switching request message when an integrity check on the message is successful (S 1040 ). Then, the RRC entity may generate a link switching message according to the link switching and deliver it to the rUE RAP entity of the base station (S 1050 ). 
       FIG.  12    is a flowchart illustrating a first exemplary embodiment of a link switching method of an RRC entity of a base station. 
     Referring to  FIG.  12   , in the link switching method of the RRC entity of the base station, upon receiving the link switching request message, the RRC entity of the base station may identify the remote terminal based on the relay RNTI included in the received link switching request message (S 1041 ). Then, the RRC entity of the base station may generate a MAC-I based on the relay RNTI, root node address, RAP entity ID, and SRB1 UL count included in the link switching request message (S 1042 ). Then, the RRC entity of the base station may perform integrity check based on the generated MAC-I to authenticate validity of the terminal (S 1043 ). Accordingly, the RRC entity of the base station may determine whether the integrity check is successful (S 1044 ), and if the integrity check fails, the RRC entity of the base station may terminate the link switching. On the other hand, if the integrity check is successful, the RRC entity of the base station may generate a MAC-I for integrity protection based on the relay RNTI, root node address, and RAP entity ID (S 1045 ). 
     In addition, the RRC entity of the base station may generate a link switching message including the generated MAC-I, relay RNTI, root node address, RAP entity ID, and SRB1 downlink (DL) count, and deliver it to the rUE RAP entity of the base station (S 1046 ). In this case, the RRC entity of the base station may instruct a PDCP entity of the base station to switch an uplink path from the Uu access link to the relay link (S 1047 ). According to the request of the RRC entity of the base station, the PDCP entity may generate a link switching complete message after switching the uplink path from the Uu access link to the relay link and transmit it to the RRC entity of the base station. Accordingly, the RRC entity of the base station may receive the link switching complete message from the PDCP entity of the base station (S 1048 ). As such, when the RRC entity of the base station receives the link switching complete message from the PDCP entity of the base station, it may instruct the PDCP entity of the base station to switch a downlink path from the Uu access link to the relay link (S 1049 ). Accordingly, the PDCP entity of the base station may switch the downlink path from the Uu access link to the relay link. 
     Referring again to  FIG.  10   , upon receiving the link switching message from the RRC entity of the base station, the rUE RAP entity of the base station may configure a relay information setup control PDU including the MAC-I, relay RNTI, root node address, RAP entity ID, and SRB1 DL count included in the link switching message, generate a relay information setup message including the relay information setup control PDU, and transmit it to the RUE RAP entity of the base station (S 1060 - 1 ). Accordingly, the RUE RAP entity of the base station may transmit the relay information setup message received from the rUE RAP entity of the base station to the RAP entity of the relay terminal (S 1060 - 2 ). 
     Accordingly, the RAP entity of the relay terminal receiving the relay information setup message may activate a routing table entry in which the routing information included in the relay information setup control PDU is stored. Then, the RAP entity of the relay terminal may transmit the relay information setup message including the relay information setup control PDU (S 1060 - 3 ). Accordingly, the RAP entity of the remote terminal may receive the relay information setup message from the RAP entity of the relay terminal, generate a link switching message including the MAC-I, relay RNTI, root node address, RAP entity ID, and SRB1 DL count included in the received relay information setup message, and transmit it to the RRC entity of the remote terminal (S 1070 ). As such, upon receiving the link switching message, the RRC entity of the remote terminal may perform integrity protection, and if the integrity check is successful, the RRC entity of the remote terminal may perform link switching by transmitting uplink data through the relay link (S 1080 ). 
       FIG.  13    is a flowchart illustrating a first exemplary embodiment of a link switching method of an RRC entity of a remote terminal. 
     Referring to  FIG.  13   , in the link switching method of the RRC entity of the remote terminal, the RRC entity of the remote terminal may determine whether the link switching message is received within a waiting time by referring to the running link switching timer (S 1071 ). As a result of the determination, the RRC entity of the remote terminal may stop the link switching timer if the link switching message is received within the waiting time (S 1072 ). In addition, the RRC entity of the remote terminal may generate a MAC-I based on the relay RNTI, root node address, RAP entity ID, and SRB1 DL count of the link switching message (S 1073 ). Then, the RRC entity of the remote terminal may perform integrity check by comparing the generated MAC-I with the MAC-I included in the received link switching message (S 1074 ). 
     The RRC entity of the remote terminal may determine whether the integrity check is successful (S 1075 ), and if the integrity check fails, the RRC entity of the remote terminal may terminate the link switching. On the other hand, if the integrity check is successful, the RRC entity of the remote terminal may instruct the PDCP entity of the remote terminal to switch a path from the Uu access link to the relay link (S 1076 ). Then, the PDCP entity of the remote terminal may switch the path from the Uu access link to the relay link. On the other hand, if the link switching message is not received within the waiting time as a result of determining whether the link switching message is received within the waiting time by referring to the running link switching timer, the RRC entity of the remote terminal may release the configured relay link (S 1077 ), and may perform a process of configuring a new relay link (S 1078 ). 
     Referring again to  FIG.  10   , after indicating the link switching to the PDCP entity of the remote terminal, the RRC entity of the remote terminal may generate a link switching complete message, and deliver it to the RRC entity of the base station through the PDCP entity of the remote terminal, RRC entity of the remote terminal, RAP entity of the relay terminal, RUE RAP entity of the base station, rUE RAP entity of the base station, and PDCP entity of the base station (S 1090 ). 
     Meanwhile, when the configuration of the sidelink or the relay link is completed, the remote terminal may perform a procedure for configuring the routing information while performing the link switching process. 
       FIG.  14    is a flowchart illustrating a first exemplary embodiment of a method for configuring routing information performed by a remote terminal. 
     Referring to  FIG.  14   , in the method of configuring routing information performed by the remote terminal, the RRC entity of the remote terminal may deliver to the RAP entity of the remote terminal relay link configuration information including a source layer 2 ID, destination layer 2 ID, and the like and relay routing information including the root node address, the branch node address, the RAP entity ID, and the like (S 1410 ). Then, the RAP entity of the remote terminal may receive the root node address, branch node address, RAP entity ID, and the like from the RRC entity of the remote terminal, and register them to the routing table (S 1420 ). Thereafter, the RRC entity of the remote terminal may generate the link switching request message and deliver it to the RAP entity of the remote terminal (S 1430 ). Then, the RAP entity of the remote terminal may form a relay information setup control PDU including the link switching request message (S 1440 ), and transmit it to the relay terminal as the relay information setup request message (S 1450 ). Here, the relay information setup request control PDU may include the root node address, branch node address, RAP entity ID, and the like as relay link routing information. Thereafter, when the RAP entity of the remote terminal receives the relay information setup control PDU including the link switching message from the relay terminal through the relay information setup message (S 1460 ), the RAP entity of the remote terminal may activate the registered relay link routing information (S 1470 ). In addition, the RAP entity of the remote terminal may deliver the link switching message included in the relay information setup message to the RRC entity of the remote terminal (S 1480 ). Accordingly, the RRC entity of the remote terminal may perform the link switching. 
       FIG.  15    is a flowchart illustrating a first exemplary embodiment of a method for configuring routing information performed by a RAP entity of a relay terminal. 
     Referring to  FIG.  15   , in the method for the RAP entity of the relay terminal to configure routing information, the RRC entity of the relay terminal may transmit information on the source layer 2 ID and destination layer 2 ID of the relay link to the RAP entity of the relay terminal when the configuration of the relay link is completed. Then, the RAP entity of the relay terminal may receive the information from the RRC entity of the relay terminal, and store the source layer 2 ID and the destination layer 2 ID of the relay link (S 1501 ). In addition, upon receiving the relay information setup request control PDU including the root node address, branch node address, RAP entity ID, and the like from the RAP entity of the remote terminal by using the stored source layer 2 ID and destination layer 2 ID through the relay information setup request message (S 1502 ), the RAP entity of the relay terminal may configure a routing table entry based on the routing information such as the root node address, branch node address, RAP entity ID, source layer 2 ID, destination layer 2 ID, and the like (S 1503 ). Then, the RAP entity of the relay terminal may start a relay information setup timer (e.g., timer T_s) (S 1505 ) while maintaining the configured routing table entry in an inactive state (S 1504 ). In addition, the RAP entity of the relay terminal may forward the relay information setup request control PDU to the base station through the relay information setup request message (S 1506 ). 
     Thereafter, the RAP entity of the relay terminal may determine whether the relay information setup control PDU is received from the base station through the relay information setup message within a relay setup time by referring to the relay information setup timer (S 1507 ). As a result of the determination, if the RAP entity of the relay terminal receives the relay information setup control PDU from the base station within the relay setup time, the RAP entity of the relay terminal may search for a routing table entry based on the root node address, branch node address, and RAP entity ID included in the relay information setup control PDU (S 1508 ). When a routing table entry is retrieved, the RAP entity of the relay terminal may switch the retrieved routing table entry to an active state (S 1510 ), and may stop the relay information setup timer (S 1511 ). Then, the RAP entity of the relay terminal may transmit the received relay information setup control PDU to the remote terminal through the relay information setup message (S 1512 ). If a routing table entry is not retrieved in the routing table, the RAP entity of the relay terminal may delete the received relay information setup control PDU (S 1513 ). On the other hand, the RAP entity of the relay terminal may delete the associated routing table entry if the relay information setup control PDU is not received within the relay setup time by referring to the relay information setup timer (S 1514 ). 
       FIG.  16    is a flowchart illustrating a first exemplary embodiment of a method for data relaying performed by a relay terminal. 
     Referring to  FIG.  16   , in the method for data relaying performed by the relay terminal, the RAP entity of the relay terminal may activate the routing table entry (S 1601 ) and may start a data reception timer (e.g., T_a) (S 1602 ). The RAP entity of the relay terminal may determine whether data is received within a data reception time by referring to the data reception timer (S 1603 ). If data is received within the data reception time, the RAP entity of the relay terminal may search for a routing table entry based on the root node address, branch node address, and RAP entity ID included in the received data (S 1604 ). The RAP entity of the relay terminal may determine whether a matched routing table entry is retrieved (S 1605 ), and if a matched routing table entry is retrieved, the RAP entity of the relay terminal may transmit the data to the corresponding base station based on the retrieved routing table entry (S 1606 ). In addition, the RAP entity of the relay terminal may restart the data reception timer (S 1607 ). If a matched routing table entry is not found in the routing table, the RAP entity of the relay terminal may delete the received data (S 1608 ). On the other hand, the RAP entity of the relay terminal may delete the routing table entry associated with the data reception timer if data is not received within the data reception time by referring to the data reception timer (S 1609 ). In addition, the RAP entity of the relay terminal may release the routing information associated with the relay link if there is no routing table entry associated with the relay link (S 1610 ), and may notify the RRC entity of the relay terminal of such the release (S 1611 ). 
       FIG.  17    is a conceptual diagram illustrating a third exemplary embodiment of a terminal relay system. 
     Referring to  FIG.  17   , a terminal relay system may include a first base station  1711 , a second base station  1712 , a first terminal  1721 , and a second terminal  1722 . The first terminal  1721  and the second terminal  1722  may be located in a communication service coverage of the first base station  1711 . As such, since the first terminal  1721  is located in the communication service coverage of the first base station  1711 , the first terminal  1721  may directly access the first base station  1711  to transmit and receive data to and from the first base station  1711 . In addition, since the second terminal  1722  is located in the communication service coverage of the second base station  1712 , the second terminal  1722  may directly access the second base station  1712  to transmit and receive data to and from the second base station  1712 . From such the state, the first terminal  1721  may enter a communication disconnected state in which data cannot be transmitted and received to and from the first base station  1711  due to an obstacle or the like. In this case, the first terminal  1721  may access the first base station  1711  via the second terminal  1722  to transmit and receive data to and from the first base station  1711 . In this case, the second terminal  1722  may relay communication between the first base station  1711  and the first terminal  1721 . 
     Meanwhile, in such the situation, the second terminal  1722  may move to a communication service coverage of the second base station  1712  by handover or the like. In this case, since the second terminal  1722  is located in the communication service coverage of the second base station  1712 , the second terminal  1722  may access the second base station  1712  to transmit and receive data to and from the second base station  1712 . In such the state, the first terminal  1721  may transmit and receive data to and from the first base station  1711  via the second base station  1712  through the second terminal  1722 . In this case, the second terminal  1722  may relay communication between the second base station  1712  and the first terminal  1721 . In addition, the second base station  1712  may relay communication between the first base station  1711  and the second terminal  1722 . 
     As described above, when the second terminal  1712  moves from the communication service coverage of the first base station  1711  to the communication service coverage of the second base station  1712  due to a handover or the like, a change in the branch node address may be required. In this case, the second terminal  1722  may transmit a relay information change indication message to the first terminal  1721  to inform the first terminal  1721  that the branch node address has been changed. Here, the relay information change indication message may include the changed new branch node address. 
       FIG.  18    is a flowchart illustrating a first exemplary embodiment of a method of changing a branch node address performed by a relay terminal. 
     Referring to  FIG.  18   , in the method for the relay terminal to change the branch node address, the RAP entity of the relay terminal may identify whether the branch node address is changed according to inter-cell movement (S 1801 ). As a result of identifying whether the branch node address is changed, if the branch node address is changed, the RAP entity of the relay terminal may configure a new routing table entry by reflecting the new branch node address to the root node address and RAP entity ID of the previous routing table entry (S 1802 ). Then, the RAP entity of the relay terminal may start a routing information update timer (e.g., timer T_a) associated with the new routing table entry (S 1803 ). 
     Thereafter, the RAP entity of the relay terminal may generate and transmit a relay information change indication message through the relay link in order to notify remote terminals connected to the relay link of the change in the branch node address (S 1804 ). Then, the RAP entity of the relay terminal may delete the previous routing table entry if the routing information update timer expires by referring to the routing information update timer (S 1805 ). 
       FIG.  19    is a flowchart illustrating a first exemplary embodiment of a method for changing a branch node address performed by a remote terminal. 
     Referring to  FIG.  19   , in the method of changing the branch node address performed by the remote terminal, upon receiving the relay information change indication message from the relay terminal (S 1901 ), the RAP entity of the remote terminal may generate and add a new routing table entry reflecting the new branch node address to the root node address and the RAP entity ID of the previous routing table entry to the routing table (S 1902 ). In addition, the RAP entity of the remote terminal may start a routing information update timer (e.g., timer T_a) associated with the routing table entry added to the routing table (S 1903 ). Then, the RAP entity of the remote terminal may transmit a PDU using the new branch node address (S 1904 ). In addition, the RAP entity of the remote terminal may delete the previous routing table entry when the routing information update timer expires by referring to the routing information update timer (S 1905 ). 
     The exemplary embodiments of the present disclosure may be implemented as program instructions executable by a variety of computers and recorded on a computer readable medium. The computer readable medium may include a program instruction, a data file, a data structure, or a combination thereof. The program instructions recorded on the computer readable medium may be designed and configured specifically for the present disclosure or can be publicly known and available to those who are skilled in the field of computer software. 
     Examples of the computer readable medium may include a hardware device such as ROM, RAM, and flash memory, which are specifically configured to store and execute the program instructions. Examples of the program instructions include machine codes made by, for example, a compiler, as well as high-level language codes executable by a computer, using an interpreter. The above exemplary hardware device can be configured to operate as at least one software module in order to perform the embodiments of the present disclosure, and vice versa. 
     While the embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations may be made herein without departing from the scope of the present disclosure.