Patent Publication Number: US-2022232402-A1

Title: Method and apparatus for providing communication in high-speed train environment

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Applications No. 10-2021-0006215, filed on Jan. 15, 2021, with the Korean Intellectual Property Office (KIPO), the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a communication technique in a high-speed train environment, and more specifically, to a technique for selective packet duplication transmission of data. 
     2. Description of Related Art 
     With the development of information and communication technology, various wireless communication technologies have been developed. Typical wireless communication technologies include long term evolution (LTE) and new radio (NR), which are defined in the 3rd generation partnership project (3GPP) standards. The LTE may be one of 4th generation (4G) wireless communication technologies, and the NR may be one of 5th generation (5G) wireless communication technologies. 
     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). 
     The 5G communication system may be working on specifications for extending mobile communication to various industrial fields. One of them may be to support broadband communication services in a high-speed mobile vehicle (e.g., transportation means) such as a high-speed train. Today&#39;s high-speed train supports a travel speed of over 500 km/h. 
     Meanwhile, the 5G communication system may use a very high frequency band such as millimeter wave (mmWave) in order to secure a broadband frequency. However, in the 5G communication system using such a very high frequency band, performance degradation may occur due to a Doppler frequency shift occurring in proportion to a movement speed and the frequency used. In addition, in the 5G communication system, a channel estimation error may occur due to channel characteristics that change rapidly due to the high movement speed. Due to the channel estimation error, the performance of the physical layer operating based on channel information may be degraded. Therefore, the 5G communication system may require technologies for improving mobile communication performance in such the high-speed train environment. 
     SUMMARY 
     Accordingly, exemplary embodiments of the present disclosure are directed to providing communication techniques for a high-speed train environment, which selectively apply a packet duplication transmission scheme. 
     According to a first exemplary embodiment of the present disclosure, an operation method of a terminal including a first relay and a second relay, in a communication system, may comprise: measuring, by the first relay, a first received signal quality of a first base station connected to the first relay; measuring, by the second relay, a second received signal quality of a second base station connected to the second relay; transmitting, by the first relay, a first protocol data unit (PDU) to the first base station based on the first received signal quality and the second received signal quality through a first bearer established in the first relay; and transmitting, by the second relay, a second PDU to the second base station based on the first received signal quality and the second received signal quality through a second bearer established in the second relay. 
     Each of the first relay and the second relay may perform functions of a radio link control (RLC) layer, a medium access control (MAC) layer, and a physical (PHY) layer, the first relay and the second relay may share functions of a packet data convergence protocol (PDCP) layer, and the first bearer and the second bearer may be split bearers. 
     When a packet duplication transmission scheme is used, the first PDU may be equal to the second PDU, and when a split transmission scheme is used, the first PDU may be different from the second PDU. 
     Each of the first received signal quality and the second received signal quality may be one of a reference signal received power (RSRP) and a reference signal received quality (RSRQ) for a reference signal, the first base station may be a master node, and the second base station may be a secondary node. 
     When the first received signal quality and the second received signal quality are less than or equal to a first threshold, the first PDU transmitted from the first relay to the first base station may be different from the second PDU transmitted from the second relay to the second base station. 
     When the first received signal quality and the second received signal quality exceed a first threshold, the first PDU transmitted from the first relay to the first base station may be equal to the second PDU transmitted from the second relay to the second base station. 
     When the first received signal quality and the second received signal quality exceed a first threshold and a value obtained by subtracting the first received signal quality from the second received signal quality is less than a second threshold, the first PDU transmitted from the first relay to the first base station may be equal to the second PDU transmitted from the second relay to the second base station. 
     According to a second exemplary embodiment of the present disclosure, an operation method of a first base station in a communication system may comprise: transmitting a first reference signal; receiving a first PDU from a first relay based on a first received signal quality of the first reference signal; and receiving, from a second base station, a second PDU received by the second base station from a second relay based on a second received signal quality of a second reference signal transmitted from the second base station, wherein when the first received signal quality at the first relay and the second received signal quality at the second relay are less than or equal to a first threshold, the first PDU and the second PDU are different, and when the first received signal quality and second received signal quality exceeds the first threshold, the first PDU and the second PDU are same. 
     The operation method may further comprise transmitting a message including the first threshold before transmitting the first reference signal. 
     The operation method may further comprise when the first PDU is same as the second PDU, selecting one among the first PDU and the second PDU, and transmitting the one to a core network, and when the first PDU is different from the second PDU, reassembling the first PDU and the second PDU, and transmitting the reassembled PDUs to the core network. 
     According to a third exemplary embodiment of the present disclosure, a terminal may comprise: a processor; a memory electronically communicating with the processor; and instructions stored in the memory, wherein when executed by the processor, the instructions cause the terminal to: measure, by the first relay, a first received signal quality of a first base station connected to the first relay; measure, by the second relay, a second received signal quality of a second base station connected to the second relay; transmit, by the first relay, a first PDU to the first base station based on the first received signal quality and the second received signal quality through a first bearer established in the first relay; and transmit, by the second relay, a second PDU to the second base station based on the first received signal quality and the second received signal quality through a second bearer established in the second relay. 
     When the first received signal quality and the second received signal quality exceed a first threshold, the first PDU transmitted from the first relay to the first base station may be equal to the second PDU transmitted from the second relay to the second base station. 
     When the first received signal quality and the second received signal quality exceed a first threshold and a value obtained by subtracting the first received signal quality from the second received signal quality is less than a second threshold, the first PDU transmitted from the first relay to the first base station may be equal to the second PDU transmitted from the second relay to the second base station. 
     According to the exemplary embodiments of the present disclosure, when a handover frequently occurs due to mobility of a high-speed train in a high-speed train communication system, it is made possible to transmit packets in duplicate at a time when a handover is highly likely to occur. In particular, according to the exemplary embodiments of the present disclosure, it is made possible to transmit packets in duplicate when necessary so that radio resources are not wasted in the high-speed train communication system. 
     To this end, the exemplary embodiments of the present disclosure propose a new condition for initiating and terminating the operation of transmitting packets in duplicate. As an example, according to the exemplary embodiments of the present disclosure, it is made possible to selectively apply the packet duplication transmission in an area where a handover occurs in a vehicle terminal or an area in which a signal quality of a radio link is low. Accordingly, the exemplary embodiments of the present disclosure may increase the reliability of wireless backhaul links in the high-speed train environment. In particular, according to the exemplary embodiments of the present disclosure, the reliability of the radio link in the high-speed train environment can be enhanced and a significant decrease in resource utilization efficiency can be prevented. 
    
    
     
       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 communication node in a communication system according to a first exemplary embodiment of the present disclosure. 
         FIG. 3  is a block diagram illustrating a control plane for EN-DC. 
         FIG. 4  is a block diagram illustrating a control plane for MR-DC. 
         FIG. 5  is a block diagram illustrating a data plane protocol for MR-DC on a vehicle terminal side. 
         FIG. 6  is a block diagram illustrating a data plane protocol for MR-DC on a network side. 
         FIG. 7  is a block diagram illustrating a network for packet duplication transmission. 
         FIG. 8  is a conceptual diagram illustrating an example in which PD is applied to CA. 
         FIG. 9  is a conceptual diagram illustrating an example in which PD is applied to DC. 
         FIG. 10  is a conceptual diagram illustrating a second exemplary embodiment of a communication system. 
         FIG. 11  is a conceptual diagram illustrating radio signal qualities for selectively applying packet duplication transmission in a dual relay-based split bearer scheme. 
         FIGS. 12A and 12B  are flowcharts for describing a first exemplary embodiment of a communication method for selectively applying packet duplication transmission in a dual relay-based split bearer scheme. 
         FIGS. 13A and 13B  are flowcharts for describing a second exemplary embodiment of a communication method for selectively applying packet duplication transmission in a dual relay-based split bearer scheme. 
     
    
    
     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. 
     A communication system to which exemplary embodiments according to the present disclosure are applied will be described. The communication system to which the exemplary embodiments according to the present disclosure are applied is not limited to the contents described below, and the exemplary embodiments according to the present disclosure may be applied to various communication networks. Here, the communication system may be used in the same sense as a communication network. 
     Throughout the present specification, a terminal may refer to a mobile terminal (MT), mobile station (MS), advanced mobile station (AMS), high reliability mobile station (HR-MS), subscriber station (SS), portable subscriber station (PSS), access terminal (AT), user equipment (UE), and/or the like, and may include all or a part of functions of the MT, MS, AMS, HR-MS, SS, PSS, AT, UE, and/or the like. 
     In addition, throughout the present specification, a base station may refer to an advanced base station (ABS), high reliability base station (HR-BS), node B, evolved node B (eNodeB), gNodeB, access point (AP), radio access station (RAS), base transceiver station (BTS), mobile multihop relay (MMR)-BS, relay station performing a role of the base station, high reliability relay station (HR-RS) performing a role of the base station, small cell base station, and/or the like, and may include all or part of functions of BS, ABS, HR-BS, nodeB, eNodeB, gNodeB, AP, RAS, BTS, MMR-BS, RS, HR-RS, small cell base station, and/or the like. 
     On the other hand, in a communication system for a high-speed mobile vehicle such as a high-speed train (HST), there may be a direct communication scheme in which a user terminal in the mobile vehicle communicates directly with an external (e.g., terrestrial) base station and a relay communication scheme in which the user terminal communicates with the base station through a relay terminal installed in the mobile vehicle. 
     Here, in the direct communication scheme, a penetration loss may occur while a radio signal passes through a vehicle or carriage. Since the material of the carriages is usually metal, most signals may be transmitted through windows. However, since windows are coated with metallic components, the direct communication scheme may generally cause a penetration loss of 10-30 dB. Therefore, the direct communication scheme may have a relatively poor reception performance compared to the relay communication scheme. In addition, in the direct communication scheme, since the user terminal directly communicates with the external base station far away, power consumption of the terminal may be high. Further, in the direct communication scheme, all user terminals should individually perform signaling procedures according to their movements, which includes a handover that occurs when the base station is changed and a tracking area update (TAU) procedure that occurs when a tracking area is changed. In the direct communication scheme, user terminals existing in the same carriage have the same mobility, and thus control procedures related to the mobility may occur at almost the same time in all user terminals. Accordingly, the direct communication scheme may generate a large load on the network due to processing of such the control procedures. 
     In contrast, the relay communication scheme may be a scheme in which a relay terminal is installed outside a train (mainly, roof) to relay signals between the external base station and the user terminals. Such the relay communication scheme may have a two-step structure of a backhaul link between the external base station and the relay terminal and an access link between the relay terminal and each of the user terminals. 
     The backhaul link and the access link may be based on the same radio access technology (RAT) or different RATs. The biggest advantage of the relay communication scheme may be that signal attenuation by the train body does not occur. In addition, in the relay communication scheme, various performance degradation factors (e.g., inter-carrier interference due to Doppler Shift, channel estimation error, or the like) that occur depending on the high movement speed of the train may be solved by the relay terminal not the individual user terminals. 
     In general, as compared to the user terminal, the relay terminal may be expensive equipment in terms of hardware, and thus it may be advantageous to implement high transmission/reception performance. In addition, the relay communication scheme may reduce power consumption by allowing the user terminal to communicate with the relay terminal in a short distance instead of communicating with the external base station far away. Such the relay communication scheme may perform only a single control procedure through the relay terminal instead of performing control procedures (e.g., handover, location registration, location update, etc.) by the individual user terminals in terms of economy of control procedures. 
     The control procedures through the relay terminal may have the advantage of reducing signaling overhead, but when a radio link failure (RLF) occurs in the relay terminal, it affects a plurality of user terminals. In this reason, the reliability of the relay terminal may need to be guaranteed. In addition, not only the control procedures but also the wireless backhaul link may generally require higher transmission reliability than the access links. Therefore, in the high-speed train environment, the relay communication scheme may require methods for improving the reliability of the wireless backhaul link between the base station and the relay terminal. 
     As described above, when the user terminal located inside the high-speed train or high-speed mobile vehicle desires to receive mobile communication services, it may become difficult for the user terminal to directly communicate with the terrestrial base station due to a Doppler effect due to high mobility, the penetration loss caused by the body of the high-speed mobile vehicle, and the like. In order to solve the above-described problems, in the high-speed mobile communication system, the scheme in which the relay terminal is installed in the high-speed mobile vehicle and the relay terminal relays signals between the base station and the user terminals in the mobile vehicle may be introduced. In such the structure, the reliability may need to be high because a radio section between the base station and the relay terminal serves as the wireless backhaul. 
     In general, the high-speed mobile vehicle communication system may adopt a transmission scheme with low resource utilization efficiency in order to increase the reliability of the radio link. The present disclosure proposes a method for increasing the link reliability of the wireless backhaul in the high-speed train environment. In particular, the present disclosure proposes a method for increasing the link reliability and preventing a significant decrease in the resource utilization efficiency. 
     On the other hand, the standardization for the 3GPP radio access network (RAN) has proposed a high-speed train (HST) scenario as one of deployment scenarios of the 5G NR. In the HST scenario, relay terminals (or antennas) may be installed in the front and rear of the train, respectively, by using a structure in which the length of the entire high-speed trains is about 200-300 m, and each relay terminal may perform communication independently. 
     In the structure proposed by the HST scenario, since two relay terminals are located far enough away, the two relay terminals operate without interference with each other through beamforming, thereby obtaining twice the transmission performance compared to the case of using only a single relay terminal. In addition, in the structure proposed by the HST scenario, if different data is transmitted through two radio links of the two relay terminals, a double transmission speed may be obtained, and transmission reliability may be increased when the same data transmitted in duplicate. 
     The former case may correspond to a split transmission scheme using split bearers, and the latter case may correspond to a packet duplication (PD) transmission scheme using split bearers, which are specified in the 3GPP standard. When the structure proposed by the HST scenario operates in the PD transmission scheme, data transmission reliability may be improved, but resource utilization efficiency may be lowered as compared to the case of not using the PD transmission scheme. 
       FIG. 1  is a conceptual diagram illustrating a first exemplary embodiment of a communication system. 
     Referring to  FIG. 1 , a communication system  100  may include a gateway (GW)  111 , a plurality of cloud digital units (CDUs)  121  and  122 , a plurality of radio units (RUs) (or, remote radio heads (RRHs))  131  to  134 , a high speed vehicle  140 , and the like. Here, the high-speed vehicle  140  may be a high-speed train. 
     The GW  111  may be included in a core network of the communication system  100 . The GW  111  may be connected to the public Internet  112 . The GW  111  may be connected to the plurality of CDUs  121  and  122 . The GW  111  may control the plurality of CDUs  121  and  122 . 
     The plurality of CDUs  121  and  122  may be connected to the plurality of RUs  131  to  134 . For example, the first CDU  121  may be connected to the first RU  131  and the second RU  132  through optical fibers. The first CDU  121  may control the first RU  131  and the second RU  132 . In addition, the second CDU  122  may be connected to the third RU  133  and the fourth RU  134  through optical fibers. The second CDU  122  may control the third RU  133  and the fourth RU  134 . 
     A vehicle terminal (or, vehicle equipment)  141  may be disposed outside the high-speed train  140 . The vehicle terminal  141  may include a first relay terminal  141 - 1  and a second relay terminal  141 - 2 . For example, the second relay terminal  141 - 2  may be disposed on the top of the first vehicle of the high-speed train  140 . In addition, the first relay terminal  141 - 1  may be disposed on the top of the third vehicle of the high-speed train  140 . Here, the vehicle terminal may be referred to as a vehicle communication node. 
     The plurality of relay terminals  141 - 1  and  141 - 2  may be connected to the plurality of RUs  131  to  134  through a mobile wireless backhaul network. For example, the first relay terminal  141 - 1  may be connected to the first RU  131  and the second RU  134  through mobile wireless backhaul links. In addition, the second relay terminal  141 - 2  may be connected to the third RU  133  and the fourth RU  134  through mobile wireless backhaul links. 
     An access point (AP)  143  may be disposed inside the high-speed train  140 . For example, the AP  143  may be a femto cell or an access point for wireless fidelity (Wi-Fi). The AP  143  may be connected to the plurality of relay terminals  141 - 1  and  141 - 2 . The plurality of relay terminals  141 - 1  and  141 - 2  may provide high-speed mobile wireless backhaul links to the AP  143  by performing communication with the plurality of RUs  131  to  134 . 
     The AP  143  may provide an access link for a user terminal  145  carried by a passenger  144 . For example, the AP  143  may provide a high-speed mobile Internet service to the passenger  144  through the user terminal  145 . 
     Here, the user terminal  145  may not directly communicate with the plurality of RUs  131  to  134 . That is, the user terminal  145  may be indirectly connected to the plurality of RUs  131  to  134  through the AP  143  connected to the plurality of relay terminals  141 - 1  and  141 - 2 . Accordingly, the user terminal  145  may overcome radio wave attenuation that may occurs due to outer walls of the high-speed train  140 . 
     In addition, a plurality of user terminals located inside the high-speed train  140  may perform a group handover through the plurality of relay terminals  141 - 1  and  141 - 2  at cell edges of the plurality of RUs  131  to  134 . For example, the user terminal  145  may perform a group handover through the plurality of relay terminals  141 - 1  and  141 - 2 . Accordingly, a huge handover signaling overhead, which may occur when a plurality of user terminals individually and simultaneously perform handover procedures, may be prevented. 
     Here, the plurality of RUs  131  to  134  may have respective cell identifiers (IDs). Accordingly, not only a handover between RUs connected to different CDUs, but also a switching between RUs connected to the same CDU may be possible. For example, the handover between the second RU  132  connected to the first CDU  121  and the third RU  133  connected to the second CDU  122  as well as the switching between the first RU  131  and the second RU  132  connected to the first CDU  121  may be possible. 
     In addition, the plurality of relay terminals  141 - 1  and  141 - 2  may be easily implemented because there is no significant limitation in the implementation of hardware miniaturization, etc. compared to the user terminal  145 . In addition, since the user terminal  145  can receive services by using a commercialized communication technology through the AP  143 , an additional upgrade may be omitted. 
     The structures of the GW  111 , the plurality of CDUs  121  and  122 , the plurality of RUs  131  to  134 , the plurality of relay terminals  141 - 1  and  141 - 2 , the AP  143 , and the user terminal  145  will be described with reference to  FIG. 2  below. 
       FIG. 2  is a block diagram illustrating a communication node in a communication system according to a first exemplary embodiment of the present disclosure. 
     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 component included in the communication node  200  may not be connected to the common bus  270  but may be connected to the processor  210  via an individual interface or a separate 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  via a dedicated interface. 
     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). 
     Meanwhile, in order to satisfy both the robustness required in a control plane and the capacity increase required in a user plane, the 3GPP specifications have defined a dual-connectivity (DC) structure wherein a vehicle terminal is connected to two base stations (eNB or gNB) at the same time. The DC may also be referred to as multi-connectivity (MC) in consideration of the form of being connected to two or more base stations. In addition, the DC may be referred to as ‘multi-radio dual connectivity (MR-DC)’ by being extended to interworking between the vehicle terminal and RATs (e.g., WiFi) other than the 3GPP RAT (e.g., LTE or NR). 
     The plurality of base stations supporting DC functions for the vehicle terminal (e.g., the plurality of base stations connected to the vehicle terminal) may be classified into a master base station and a secondary base station according to the performed function(s) thereof. The master base station may be referred to as a master node (MN), and the secondary base station may be referred to as a secondary node (SN). 
     In addition, the 3GPP standard has proposed a non-standalone (NSA) structure for operator desiring rapid commercialization, in which the NR technology is used together with the conventional LTE system, and a standalone (SA) structure in which the NR technology is used together with a new network structure. 
     In the NSA structure, an evolved packet core (EPC) may be used as a core network (CN), an LTE base station (i.e., eNB) may be used as the MN, and an NR base station (i.e., gNB) may be used as the SN. The DC for the above-described NSA structure may be referred to as an evolved universal mobile telecommunications system (UMTS) terrestrial radio access (E-UTRA) new-radio dual connectivity (EN-DC). The MN may be defined as a radio access network (RAN) node that provides a connection for accessing the control plane to the CN, and the SN may be defined as a RAN node for providing additional user plane resources to the vehicle terminal without a connection for accessing the control plane. 
       FIG. 3  is a block diagram illustrating a control plane for EN-DC. 
     Referring to  FIG. 3 , the control plane for EN-DC may include an EPC  310 , a MeNB  320  that is a master eNodeB, an SgNB  330  that is a secondary gNodeB, and a UE  340 . 
     Here, the EPC  310  may be a core network of an Internet Protocol (IP) mobile communication system for the 3GPP LTE system, and may support packet-based real-time and non-real-time services. The EPC  310  may include a serving gateway (SGW or S-GW), a packet data network (PDN) gateway (PGW or P-GW), a mobility management entity (MME), a serving general packet radio service (GPRS) supporting node (SGSN), and an enhanced packet data gateway (ePDG). 
     In addition, the MeNB  320  may be a device that provides a radio interface to the UE  340 , and may provide radio resource management functions such as radio bearer control, radio admission control, dynamic radio resource allocation, load balancing, and inter-cell interference control. The user plane of the SgNB  330  may be associated with the core network, and the control plane of the SgNB  330  may be associated with the core network through the MeNB  320 . The MeNB  320  may be associated with the MME of the EPC  310  via an S1-C interface. 
     The UE  340  may be connected to the MeNB  320  and the SgNB  330  via Uu interfaces. Here, in the Uu interface as a radio interface, the control plane for transmitting and receiving control messages and the user plane for providing user data may be defined. 
     In the above-described configuration, the MeNB  320  may create an S1-MME control connection with the control entity MME of the EPC  310  that is the core of the LTE system, and relay control message transmission and reception between the MME and the UE  340 . In addition, the MeNB  320  may create an RRC connection with the UE  340  by using the LTE radio technology, and may manage an RRC state based on the connection. 
     Meanwhile, the UE  340  may establish bearers by being connected to the EPC  310  via the MeNB  320 . The MeNB  320  may establish bearers for the UE  340 , and the UE  340  may be in an RRC connection state with the MeNB  320 . In this state, the MeNB  320  may determine whether to use DC for the UE  340  in consideration of a current congestion state, a data transmission/reception state of the UE  340 , existence of a gNB that will serve as an SN around the eNB, a congestion state of the gNB, and/or the like. 
     When the MeNB  320  determines to use DC for the UE  340 , the MeNB  320  may transmit and receive X2-C control messages with the SgNB  330  via an X2 interface. In addition, the MeNB  320  may execute a procedure of changing some of the bearers being serviced to the UE  340  through LTE radio resources, to be serviced via the SgNB  330 . 
     In the above-described EN-DC control plane structure, the RRC protocol may exist in both the MeNB  320  and the SgNB  330 , but the UE  340  may follow the RRC state of the MeNB  320 . Also, there may be only one control plane connection of the CN for the UE  340 . 
       FIG. 4  is a block diagram illustrating a control plane for MR-DC. 
     Referring to  FIG. 4 , the control plane for MR-DC may include a new radio core (NGC)  410 , an MN  420  that is a master node, and an SN  430  that is a secondary node, and a UE  440 . 
     Here, the NGC  410  may manage 5G communication of the UE  440 , and may support packet-based real-time and non-real-time services. In addition, the MN  420  may be a device that provides a radio interface to the UE  440 , and may provide radio resource management functions such as radio bearer control, radio admission control, dynamic radio resource allocation, load balancing, and inter-cell interference control. The user plane of the SN  430  may be associated with the NGC  410 , and the control plane of the SN  430  may be associated with the NGC  410  via the MN  420 . The MN  420  may be associated with the NGC  410  via an NG-C interface. 
     The UE  440  may be connected to the MN  420  and the SN  430  via Uu interfaces. Here, the Uu interface may be a radio interface defining the control plane for transmitting and receiving control messages and the user plane for providing user data. 
     In the above-described configuration, the MN  420  may relay transmission and reception of messages between the NGC  410  and the UE  440 . In addition, the MN  420  may create an RRC connection with the UE  440  by using the 5G communication technology, and may manage an RRC state based on the connection. 
     In the above-described MR-DC control plane structure, the RRC protocol may exist in both the MN  420  and the SN  430 , but the UE  440  may follow the RRC state of the MN  420 . Also, there may be only one control plane connection of the CN for the UE  440 . 
     In the MR-DC structure, when viewed from the vehicle terminal, three bearer types (i.e., master cell group (MCG) bearer, secondary cell group (SCG) bearer, and split bearer) may exist. On the other hand, when viewed from the network, each bearer type may be further classified into two types depending on whether a termination point of the bearer is the MN or the SN, and thus a total of six bearer types may exist. 
       FIG. 5  is a block diagram illustrating a data plane protocol for MR-DC on a vehicle terminal side. 
     Referring to  FIG. 5 , the data plane protocol structure for MR-DC on the vehicle terminal side may include a SDAP layer  510 , a first packet data convergence protocol (PDCP) layer  521  for supporting MCG bearer(s), a second PDCP layer  522  for supporting split bearers, a third PDCP layer  523  for supporting SCG bearer(s), a first MN radio link control (RLC) layer  531  for supporting the MCG bearer(s), a second MN RLC layer  532  for supporting the split bearers, a first SN RLC layer  533  for supporting the split bearers, a second RLC layer  534  for supporting the SCG bearer(s), an MN medium access control (MAC) layer  541  for supporting the MCG bearer(s) and a part of the split bearers, and an SN MAC layer  542  for supporting the SCG bearer(s) and a part of the split bearers. 
     In the vehicle terminal having such the MR-DC structure, the second PDCP layer  522  may generate PDCP protocol data units (PDUs) for data to be transmitted in order to transmit the data in the split transmission scheme by using the split bearers. Then, the second PDCP layer  522  may split the PDCP PDUs into PDCP PDUs to be transmitted to the MN and PDCP PDUs to be transmitted to the SN, deliver the PDCP PDUs to be transmitted to the MN to the second MN RLC layer  532 , and deliver the PDCP PDUs to be transmitted to the SN to the first SN RLC layer  533 . Then, the second MN RLC layer  532  may transmit the PDCP PDUs to the MN through the MN MAC layer, and the first SN RLC layer  533  may transmit the PDCP PDUs to the SN through the SN MAC layer. 
     On the other hand, in order for the vehicle terminal having the above-described data plane protocol structure for MR-DC to transmit data in the PD transmission scheme by using the split bearers, the second PDCP layer  522  may generate PDCP PDUs for the data to be transmitted. In addition, the second PDCP layer  522  may deliver the PDCP PDUs to the second MN RLC layer  532 , and may deliver the same PDCP PDUs to the first SN RLC layer  533 . Then, the second MN RLC layer  532  may transmit the PDCP PDUs to the MN through the MN MAC layer, and the first SN RLC layer  533  may transmit the same PDCP PDUs to the SN through the SN MAC layer. 
       FIG. 6  is a block diagram illustrating a data plane protocol for MR-DC on a network side. 
     Referring to  FIG. 6 , the data plane protocol structure for MR-DC on the network side may include a data plane protocol structure  600  of an MN or MeNB and a data plane protocol structure  650  of an SN or SgNB. 
     Here, the data plane protocol structure  600  of the MN or MeNB may include a SDAP layer  610  for supporting MCG bearer(s), split bearers, and SCG bearer(s), a first PDCP layer  621  for supporting the MCG bearer(s), a second PDCP layer  622  for supporting the split bearers, a third PDCP layer  623  for supporting the SCG bearer(s), a first RLC layer  631  for supporting the MCG bearer(s), second and third RLC layers  632  and  633  for supporting the split bearers, a fourth RLC layer  634  for supporting the SCG bearer(s), and an MN MAC layer  641  for supporting the MCG bearer(s), the split bearers, and the SCG bearer(s). 
     In addition, the data plane protocol structure  650  of the SN or SgNB may include a SDAP layer  660  for support MCG bearer(s), split bearers, and SCG bearer(s), a first PDCP layer  671  for supporting the MCG bearer(s), a second PDCP layer  672  for supporting the split bearers, a third PDCP layer  673  for supporting the SCG bearer(s), a first RLC layer  681  for supporting the MCG bearer(s), second and third RLC layers  682  and  683  for supporting the split bearers, a fourth RLC layer  684  for supporting the SCG bearer(s), and an MN MAC layer  691  for supporting the MCG bearer(s), the split bearers, and the SCG bearer(s). 
     In the above-described structure, the second PDCP layer  622  of the data plane protocol structure  600  of the MN or MeNB may be connected to the second RLC layer  682  of the data plane protocol structure  650  of the SN or SgNB via an X2/Xn interface. In addition, in the above-described structure, the third PDCP layer  623  of the data plane protocol structure  600  of the MN or MeNB may be connected to the first RLC layer  681  of the data plane protocol structure  650  of the SN or SgNB via an X2/Xn interface. 
     Similarly, the first PDCP layer  671  of the data plane protocol structure  650  of the SN or SeNB may be connected to the fourth RLC layer  634  of the data plane protocol structure  600  of the MN or MeNB via an X2/Xn interface. In addition, in the above-described structure, the second PDCP layer  672  of the data plane protocol structure  650  of the SN or SgNB may be connected to the third RLC layer  633  of the data plane protocol structure  600  of the MN or MeNB via an the X2/Xn interface. 
     Meanwhile, the main functions of the SDAP layers  510 ,  610 , and  650  may include some of the following functions. Of course, they may not be limited to the following examples.
         Transfer of user plane data   Mapping between a QoS flow and a data radio bearer (DRB) for both DL and UL   Marking QoS flow ID in both DL and UL packets   Reflective QoS flow to DRB mapping for the UL SDAP PDUs       

     With respect to the SDAP layer, the terminal may be instructed through an RRC message from the base station whether to use a header of the SDAP layer or whether to use the function of the SDAP layer for each PDCP layer, each bearer, or each logical channel. When the SDAP header is configured, a 1-bit indicator (e.g., non-access stratum (NAS) reflective quality of service (QoS)) and a 1-bit indicator (e.g., access stratum (AS) reflective QoS) in the SDAP header may indicate to the terminal whether to update or reconfigure mapping information for uplink and downlink QoS flows and data bearers. The SDAP header may include QoS flow ID information indicating a QoS. The QoS information may be used as data processing priority, scheduling information, or the like to support smooth service provisioning. 
     In addition, the PDCP layers  521  to  523 ,  621  to  623 , and  671  to  673  may be in charge of operations such as IP header compression/decompression. The main functions of the PDCP layer may be summarized as follows, and may not be limited to the following examples.
         Header compression and decompression: robust header compression (ROHC) only   Transfer of user data   In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC acknowledge mode (AM)   For split bearers in DC (only support for RLC AM): PDCP PDU routing for transmission and PDCP PDU reordering for reception   Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM   Retransmission of PDCP SDUs at handover and, for split bearers in DC, of PDCP PDUs at PDCP data-recovery procedure, for RLC AM   Ciphering and deciphering   Timer-based SDU discard in uplink       

     The RLC layers  531  to  534 ,  631  to  634 , and  681  to  684  may perform automatic repeat request (ARQ) operations by reconfiguring the PDCP PDU to an appropriate size. The main functions of the RLC layer may be summarized as follows, and may not be limited to the following examples.
         Transfer of upper layer PDUs   Error Correction through ARQ (only for AM data transfer)   Concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer)   Re-segmentation of RLC data PDUs (only for AM data transfer)   Reordering of RLC data PDUs (only for UM and AM data transfer)   Duplicate detection (only for UM and AM data transfer)   Protocol error detection (only for AM data transfer)   RLC SDU discard (only for UM and AM data transfer)   RLC re-establishment       

     In addition, the MAC layers  541 ,  542 ,  641 , and  691  may be connected to several RLC layers, may multiplex RLC PDUs in a MAC PDU, and may perform an operation of demultiplexing RLC PDUs from a MAC PDU. The main functions of the MAC layers may be summarized as follows, and may not be limited to the following examples.
         Mapping between logical channels and transport channels   Multiplexing/demultiplexing of MAC SDUs belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels   Scheduling information reporting   Error correction through HARQ   Priority handling between logical channels of one UE   Priority handling between UEs by means of dynamic scheduling   MBMS service identification   Transport format selection   Padding       

     Meanwhile, the 3GPP 5G standard Release-15 includes various features to support the ultra-reliable low-latency communication (URLLC) services. The various features may include a packet duplication transmission function considered in the layer 2 (L2). This may be a type of selection diversity based URLLC technique, in which two independent RBs are established in the PDCP layer, and the same PDCP PDU is transmitted through the two RBs. In this case, even when a packet loss occurs in one RB, the packet may be successfully received in the other RB. From a theoretical point of view, the packet duplication transmission may be based on the reliability theory. 
       FIG. 7  is a block diagram illustrating a network for packet duplication transmission. 
     Referring to  FIG. 7 , a network for packet duplication transmission may have N radio links (i.e., R 1  to R n )  701 - 1  to  701 - n  each having an independent channel environment. Here, n and N are natural numbers, and 1≤n≤N may be established. 
     Assuming that the same data is transmitted through the N radio links  701 - 1  to  701 - n  each having an independent channel environment, the overall reliability R of the radio links  701 - 1  to  701 - n  may be calculated as shown in Equation 1 below. 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     1 
                     - 
                     
                       
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                           i 
                           = 
                           1 
                         
                         N 
                       
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                         ( 
                         
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                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, N may be the number of the independent radio links  701 - 1  to  701 - n , and R i  may be a transmission reliability of the radio link i. The independent radio links  701 - 1  to  701 - n  may use different frequencies. Alternatively, the independent radio links  701 - 1  to  701 - n  may be obtained by generating the radio links  701 - 1  to  701 - n  through different base stations. 
     That is, in order to implement the packet duplication transmission, it is necessary to establish two independent RBs. In the 3GPP standard, the dual connectivity (DC) and carrier aggregation (CA) schemes may be considered. 
       FIG. 8  is a conceptual diagram illustrating an example in which PD is applied to CA. 
     Referring to  FIG. 8 , in order to apply PD to CA, a protocol structure may include a PDCP layer  810 , RLC layers  821  and  822 , a MAC layer  830 , and HARQ entities  841  and  842 . 
     In the present exemplary embodiment, a packet duplication entity may be defined in the MAC layer  830  of the vehicle terminal side, and the packet duplication entity may generate a plurality of duplicated MAC PDUs by using one MAC PDU delivered from a multiplexing entity of the MAC layer  830 , and deliver them to the HARQ entities  841  and  842  each of which corresponds to a component carrier (CC). In addition, each of the HARQ entities  841  and  842  may transmit the MAC PDU to the receiving side by using an independent redundancy version (RV). 
       FIG. 9  is a conceptual diagram illustrating an example in which PD is applied to DC. 
     Referring to  FIG. 9 , a protocol structure for applying PD to DC may include a PDCP layer  910 , RLC layers  921  and  922 , MAC layers  931  and  932  and HARQ entities  941  and  942 . 
     In the present exemplary embodiment, data may be duplicated by the PDCP layer  910 . Then, one may be transmitted to the receiving side through the RLC layer  921  and the MAC layer  931  of the MCG, and the other may be delivered to the receiving side through the RLC layer  922  and the MAC layer  932  of the SCG. 
     As described above, in the case of DC, two paths may be configured through the MCG and the SCG, respectively, and in the case of CA, two paths may be configured for component carriers, respectively. 
     In the high-speed train environment, relay terminals may be installed at the front and at the rear of the train, respectively. The DC-based PD scheme may be applied because the structure in which each relay terminal is connected to a different base station through beamforming is considered. 
       FIG. 10  is a conceptual diagram illustrating a second exemplary embodiment of a communication system. 
     Referring to  FIG. 10 , a communication system may include a core network  1010 , a plurality of base stations  1020  and  1030 , a plurality of relay terminals  1040  and  1050 , and a high-speed mobile vehicle  1060 . Here, the high-speed mobile vehicle  1060  may be a high-speed train. In addition, the first relay terminal  1040  and the second relay terminal  1050  may constitute a vehicle terminal. 
     The core network  1010  may be a 5G core network or an EPC, and may be connected to the plurality of base stations  1020  and  1030 . Among the plurality of base stations  1020  and  1030 , the first base station  1020  may be connected to the second relay terminal  1050 , and the second base station  1030  may be connected to the first relay terminal  1040 . The first and second base stations  1020  and  1030  may be implemented as gNBs. The first base station  1020  positioned at the rear of the high-speed mobile vehicle  1060  may be an MN, and the second base station  1030  positioned at the front of the high-speed mobile vehicle  1060  may be an SN. Alternatively, the first base station  1020  positioned at the rear of the high-speed mobile vehicle  1060  may be an SN, and the second base station  1030  positioned at the front of the high-speed mobile vehicle  1060  may be an MN. 
     The first base station  1020  serving as the MN among the base stations  1020  and  1030  may include a physical (PHY) layer  1021 , a MAC layer  1022 , an RLC layer  1023 , and a PDCP layer  1024 . The second base station  1030  serving as the SN among the base stations  1020  and  1030  may include a PHY layer  1031 , a MAC layer  1032 , and an RLC layer  1033 . As such, looking at the protocol structure in terms of the network, the first base station  1020  performing MN functions may support a radio access protocol including the PDCP layer  1024  (or, SDAP when connected to 5GC), and the second bae station  1030  performing SN functions may support a radio access protocol including the RLC layer  1033 . 
     The relay terminals  1040  and  1050  may be disposed on the outside of the high-speed train  1060 . For example, the first relay terminal  1040  may be disposed on the top of the first vehicle of the high-speed train  1060 . In addition, the second relay terminal  1050  may be disposed on the top of the third vehicle of the high-speed train  1060 . 
     The relay terminals  1040  and  1050  may be connected to the base stations  1020  and  1030  through a mobile wireless backhaul network. In this case, the first relay terminal  1040  may be connected to the second base station  1030  serving as the SN through a mobile wireless backhaul link. In addition, the second relay terminal  1050  may be connected to the first base station  1020  serving as the MN through a mobile wireless backhaul link. As such, the second relay terminal  1050  installed in the high-speed train  1070  may be connected to the first base station  1020  performing MN functions, the second relay terminal  1040  may be connected to the second base station  1030  performing SN functions, and radio bearers having the split bearer type may be configured. As such, the bearer type may be a split bearer, and termination points of the bearers may be the first base station  1040  performing MN functions or the second base station  1050  performing SN functions. 
     Here, the first relay terminal  1040  may support a PHY layer  1041 , a MAC layer  1042 , an RLC layer  1043 , and a PDCP layer  1044 . The second relay terminal  1050  may support a PHY layer  1051 , a MAC layer  1053 , an RLC layer  1053 , and the PDCP layer  1044 . In this case, the first relay terminal  1040  and the second relay terminal  1050  may share the PDCP layer  1044 . 
     As described above, the radio access protocols (i.e., radio protocol connected with the MN, radio protocol connected with the SN) of the relay terminals  1040  and  1050  may be configured, and the configured radio access protocols may be combined at the PDCP layer  1044 . Such the protocol structures may be the same as those defined in MR-DC of 3GPP. 
     The split bearers may transmit packets in duplicate, if necessary. That is, the split bearers may use the packet duplication transmission scheme in which the same data is transmitted in duplicate, if necessary. This may be a scheme for increasing transmission reliability in the radio section. 
     In a general mobile communication system, since a quality of a radio link is deteriorated when a handover occurs, the transmission reliability may be deteriorated. The same may be true in the high-speed train environment, and handover occurs frequently due to the mobility of the high-speed train  1060 . In the present disclosure, a method of transmitting data using the packet duplication transmission scheme based on split bearers at a time when a handover is highly likely to occur may be considered. 
     However, if the packet duplication transmission scheme is used, the transmission reliability may be increased, but since the same data is transmitted twice, there may be a disadvantage in that radio resources are wasted. Therefore, according to the present disclosure, the communication system may use the packet duplication transmission scheme only when necessary. To this end, the present disclosure proposes a new condition(s) for initiating and terminating a handover. In addition, the present disclosure proposes a method of applying the packet duplication transmission scheme to a duration in which the condition is satisfied. 
     In the above-described dual relay-based high-speed train system structure, the second relay terminal  1050  among the two relay terminals  1040  and  1050  may be connected to the first base station  1020  performing MN functions, and the first relay terminal  1040  may be connected to the second base station  1030  performing SN functions. Accordingly, as the high-speed train  1060  moves, the SN and MN may be changed. The 3GPP standard defines a procedure for changing the SN and MN. The change of the MN may be defined as an MN handover, and the change of the SN may be defined as an SN change. 
     In the dual relay structure, among the first relay terminal  1040  and the second relay terminal  1050 , a relay terminal connected to the base station first may establish MN bearers, and a relay terminal connected to the base station later may establish SN bearers. When the first relay terminal  1040  establishes MN bearers and the second relay terminal  1050  establishes SN bearers, the MN handover and the SN change may be repeated as the high-speed train  1060  moves. 
     On the other hand, when the first relay terminal  1040  establishes SN bearers and the second relay terminal  1050  establishes MN bearers, the SN change and MN handover may be repeated as the high-speed train  1060  moves. That is, according to the structure proposed in the present disclosure, there is no need to define a separate handover procedure, and the procedures defined in the 3GPP standard may be applied as they are. 
     A method for improving the reliability of wireless backhaul by selectively applying the split transmission scheme and the packet duplication scheme using split bearers in the double relay-based high-speed train system structure may be proposed. The basic concept of this scheme may be described as a scheme in which, in a period where the reliability of the wireless backhaul is low, the relay terminals  1040  and  1050  may transmit data by applying the packet duplication transmission scheme using split bearers, and in other periods, the relay terminals  1040  and  1050  may transmit data by applying the split transmission scheme using split bearers. 
     For example, the first relay terminal  1040  may measure a quality of a signal received from the second base station  1030 , and the second relay terminal  1050  may measure a quality of a signal received from the first base station  1020 . Then, the first relay terminal  1040  may share the measured received signal quality of the second base station  1030  by informing it to the second relay terminal  1050 , and the second relay terminal  1050  may share the measured received signal quality of the first base station  1020  by informing it to the first relay terminal  1040 . 
     Here, each of the received signal qualities of the first base station  1020  and the second base station  1030  may be at least one of a reference signal received power (RSRP) and a reference signal received quality (RSRQ) thereof. 
     In the process of splitting and transmitting data using the split bearers, each of the first relay terminal  1040  and the second relay terminal  1050  may compare the received signal qualities of the first base station  1030  and the second base station  1040  with a first threshold value T 1 . Here, the first relay terminal  1040  and the second relay terminal  1050  may receive information on the first threshold value from the first base station  1020  or the second base station  1030 . 
     When the received signal qualities of the first base station  1020  and the second base station  1030  are less than or equal to the first threshold value T 1 , the PDCP layer  1044  may generate PDCP PDUs for data to be transmitted when the data to be transmitted exits. In addition, the PDCP layer  1044  may split the PDCP PDUs into first PDCP PDUs to be transmitted to the first base station  1020  and second PDCP PDUs to be transmitted to the second base station  1030 . Thereafter, the PDCP layer  1044  may deliver the second PDCP PDUs to be transmitted to the second base station  1030  to the first RLC layer  1043  for a split bearer connected to the second base station  1030 . In addition, the PDCP layer  1044  may deliver the first PDCP PDUs to be transmitted to the first base station  1020  to the second RLC layer  1053  for a split bearer connected to the first base station  1020 . 
     Then, the first RLC layer  1043  may transmit the second PDCP PDU to the second base station  1030  through the first MAC layer  1042  and the first PHY layer  1041  for the split bearer connected to the second base station  1030 . The second RLC layer  1053  may transmit the first PDCP PDUs to the first base station  1020  through the second MAC layer  1052  and the second PHY layer  1051  for the split bearer connected to the first base station  1020 . 
     In the above-described situation, the PDCP layer  1024  of the first base station  1020  may receive the first PDCP PDUs transmitted by the second relay terminal  1050  through the PHY layer  1021 , the MAC layer  1022 , and the RLC layer  1023 . In addition, the PDCP layer  1024  of the first base station  1020  may receive, from the second base station  1030 , the second PDCP PDUs transmitted from the first relay terminal  1040 . Then, the PDCP layer  1024  of the first base station  1020  may identify sequence numbers (SNs) of the first PDCP PDUs and the second PDCP PDUs, reassemble them by using the sequence numbers, and deliver the reassembled PDUs to the core network. 
     On the other hand, in the process of transmitting the data in duplicate by using the split bearers, each of the first relay terminal  1040  and the second relay terminal  1050  may compare the received signal qualities of the first base station  1030  and the second base station  1040  with the first threshold T 1 . As a result of the comparison, when the received signal qualities of the first relay terminal  1040  and the second relay terminal  1050  exceeds the first threshold T 1 , the PDCP layer  14044  of the first relay terminal  1040  and the second relay terminal  1050  may generate PDCP PDUs for data to be transmitted. In addition, the PDCP layer  1044  may deliver the PDCP PDUs to the second RLC layer  1053  for the split bearer connected to the first base station  1020 , and deliver the same PDCP PDUs to the first RLC layer  1043  for the split bearer connected to the second base station  1030 . 
     Then, the first RLC layer  1043  may duplicate the PDCP PDUs, and deliver the PDCP PDUs to the second base station  1030  through the first MAC layer  1042  and the first PHY layer  1041  for the split bearer connected to the second base station  1030 . The second RLC layer  1053  may duplicate the PDCP PDU, and deliver the PDCP PDUs to the first base station  1020  through the second MAC layer  1052  and the second PHY layer  1051  for the split bearer connected to the first base station  1020 . 
     In the above-described situation, the PDCP layer  1024  of the first base station  1020  may receive first PDCP PDUs transmitted by the second relay terminal  1050  through the PHY layer  1021 , the MAC layer  1022 , and the RLC layer  1023 . In addition, the PDCP layer  1024  of the first base station  1020  may receive, from the second base station  1030 , the second PDCP PDUs transmitted from the first relay terminal  1040 . In this case, the PDCP layer  1024  of the first base station  1020  may identify sequence numbers (SNs) of the first PDCP PDU and the second PDCP PDU. If they are confirmed as the same PDCP PDU, either one may be discarded, and the other PDCP PDU may be delivered to the core network. 
     As described above, the packet duplication transmission scheme using split bearers is a method of transmitting the same data twice in duplicate, so resources may be wasted. Therefore, it is necessary to minimize execution of the packet duplication transmission scheme only when necessary. Since a period in which the reliability of the wireless backhaul is low may mainly correspond to an area (i.e., handover area) in which a handover is likely to occur, it may be necessary to apply the packet duplication transmission scheme to such the handover area. In addition, in the dual relay structure, the two relay terminals  1040  and  1050  may be disposed apart by a distance corresponding to the length of the train. This may mean that the two relay terminals  1040  and  1050  have different handover timings. 
     In general, the length of the high-speed train may be regarded as an average of 200 m. When the train speed is 100 km/h, 300 km/h, or 500 km/h, handover occurrence timings of the two relay terminals  1040  and  1050  may have a time difference of 7 seconds, 2.5 seconds, or 1.5 seconds. 
     Therefore, in the dual relay structure, when a handover occurs in one radio link, a handover may not occur in the other radio link within at least 7 seconds, 2.5 seconds, or 1.5 seconds. In the dual relay structure, a data loss that may occur due to the handover may be prevented and reliability may be improved by performing packet duplication transmission through these two radio links. 
     Meanwhile, even when the handover is not in progress, it is necessary to apply the packet duplication transmission scheme in advance because the reliability of the radio link is lowered in the vicinity of the position at which the handover occurs. Therefore, the relay terminals  1040  and  1050  may need to determine when to apply the packet duplication transmission scheme. Determining the time point may be regarded as determining a trade-off between the reliability of the radio link and the resource utilization efficiency. That is, if the relay terminals  1040  and  1050  apply the packet duplication transmission scheme in advance, the system may be operated in a direction to improve the reliability, and if the packet duplication transmission scheme is applied later, the system may be operated in a direction to increase the resource utilization efficiency. 
       FIG. 11  is a conceptual diagram illustrating radio signal qualities for selectively applying packet duplication transmission in a dual relay-based split bearer scheme. 
     Referring to  FIG. 11 , the radio signal qualities for selectively applying the packet duplication transmission scheme to the dual relay-based split bearers may include a received signal quality R(f,t) measured by a first relay terminal  1130  on a reference signal of a target base station  1110  (the same as the second base station  1030  of  FIG. 10 ), a received signal quality R(f,s) measured by the first relay terminal  1130  on a reference signal of a source base station  1120  (the same as the first base station  1020  of  FIG. 10 ), a received signal quality R(r,t) measured by the second relay terminal  1140  on the reference signal of the target base station  1110 , and a received signal quality R(r,s) measured by the second relay terminal  1140  on the reference signal of the source base station  1120 . 
     Here, each of the received signal qualities may be at least one of RSRP and RSRQ for the radio signal of the target base station  1110  or the source base station  1120 . 
     These may be summarized as follows. 
     R(f,t): received signal quality measured by the first relay terminal  1130  on the reference signal transmitted from the target base station  1110   
     R(f,s): received signal quality measured by the first relay terminal  1130  on to the reference signal transmitted from the source base station  1120   
     R(r,t): received signal quality measured by the second relay terminal  1140  on the reference signal transmitted from the target base station  1110   
     R(r,s): received signal quality measured by the second relay terminal  1140  on the reference signal transmitted from the source base station  1120   
     Using the four received signal quality parameters defined above, the application time of the packet duplication transmission scheme may be derived as follows. When the high-speed train enters the handover area, a handover may occur first in the first relay terminal  1130 , and the access base station of the first relay terminal  1130  may be changed from the source base station  1120  to the target base station  1110 . Thereafter, as the first relay terminal  1130  exits the handover area, the second relay terminal  1140  may perform a handover, so that the access base station of the second relay terminal  1140  may be changed from the source base station  1120  to the target base station  1110 . 
     Meanwhile, in order to apply the packet duplication transmission scheme, the first relay terminal  1130  and the second relay terminal  1140  may need to be able to create two independent radio links. Therefore, in order to simultaneously configure two radio links in the handover area, the first relay terminal  1130  and the second relay terminal  1140  may have to satisfy the following conditions Equation 2 and Equation 3. 
     
       
         
           
             
               
                 
                   
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                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     R 
                     ⁡ 
                     
                       ( 
                       
                         r 
                         , 
                         s 
                       
                       ) 
                     
                   
                   &gt; 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     These conditions Equation 2 and Equation 3 may be conditions in which a signal outage does not occur in the two radio links in order to apply the packet duplication transmission scheme in the handover area. The duration satisfying the above-described condition may be a duration indicated as a PD transmission candidate duration in  FIG. 11 . T 1 , as the first threshold value, and may be appropriately determined by the target base station  1110  or the source base station  1120 . The target base station  1110  or the source base station  1120  transmits a message including the first threshold value before transmitting the reference signal to the first relay terminal  1130  and the second relay terminal  1140 , thereby delivering configuration information to the first relay terminal  1130  and the second relay terminal  1140 . Then, the first relay terminal  1130  and the second relay terminal  1140  may receive and configure the first threshold value from the target base station  1110  or the source base station  1120 . 
     Additionally, the first relay terminal  1130  and the second relay terminal  1140  may transmit data in duplicate by the applying the packet duplication transmission scheme when a condition of Equation 4 is satisfied in addition to the conditions of Equation 2 and Equation 3. That is, when the conditions of Equation 2 to Equation 4 are satisfied, the first relay terminal  1130  and the second relay terminal  1140  may transmit the data in duplicate by applying the packet duplication transmission scheme. 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       ⁡ 
                       
                         ( 
                         
                           r 
                           , 
                           s 
                         
                         ) 
                       
                     
                     - 
                     
                       R 
                       ⁡ 
                       
                         ( 
                         
                           f 
                           , 
                           t 
                         
                         ) 
                       
                     
                   
                   &lt; 
                   
                     T 
                     ⁢ 
                     2 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, T 2  may be referred to as a second threshold value. T 2  may be appropriately determined by the target base station  1110  or the source base station  1120 . The target base station  1110  or the source base station  1120  may transmit a message including the second threshold value before transmitting the reference signal to the first relay terminal  1130  and the second relay terminal  1140 , thereby delivering configuration information to the first relay terminal  1130  and the second relay terminal  1140 . Then, the first relay terminal  1130  and the second relay terminal  1140  may receive and configure the second threshold value T 2  from the target base station  1110  or the source base station  1120 . 
     As shown in  FIG. 11 , as T 2  increases, the duration to which the packet duplication transmission scheme is applied may increase, and as T 2  decreases, the duration to which the packet duplication transmission scheme is applied may decrease. Accordingly, the target base station  1110  or the source base station  1120  may configure the second threshold value T 2  in consideration of a trade-off between communication reliability and data transmission efficiency according to the wireless backhaul requirements. That is, the minimum value may be selected among T 2  values required to secure the reliability to satisfy the wireless backhaul requirements. 
       FIGS. 12A and 12B  are flowcharts for describing a first exemplary embodiment of a communication method for selectively applying packet duplication transmission in a dual relay-based split bearer scheme. 
     Referring to  FIGS. 12A and 12B , in a communication method for selectively applying the packet duplication transmission in the dual relay-based split bearer scheme, the first relay terminal may receive a reference signal from the target base station (S 1201 ), and measure a received signal quality R(f,t) (S 1202 ). In addition, the first relay terminal may receive a reference signal from the source base station (S 1204 ), and measure a received signal quality R(f,s) (S 1205 ). Similarly, the second relay terminal may receive a reference signal from the target base station (S 1201 ), and measure a received signal quality R(r,t) (S 1203 ). In addition, the second relay terminal may receive a reference signal from the source base station (S 1204 ), and measure a received signal quality R(r,s) (S 1206 ). 
     Here, each of the received signal qualities may be at least one of RSRP, RSRQ, and a combination thereof with respect to the reference signal of the target base station or the source base station. 
     The first relay terminal may transmit, to the second relay terminal, information on the received signal quality R(f,t) measured by receiving the reference signal from the target base station and the received signal quality R(f,s) measured by receiving the reference signal from the source base station, thereby sharing the received signal qualities R(f,t) and R(f,s) with the second relay terminal (S 1207 ). In addition, similarly, the second relay terminal may transmit, to the first relay terminal, information on the received signal quality R(r,t) measured by receiving the reference signal from the target base station and the received signal quality R(r,s) measured by receiving the reference signal from the source base station, thereby sharing the received signal qualities R(r,t) and R(r,s) with the first relay terminal (S 1207 ). 
     Accordingly, in the first relay terminal, the received signal quality R(f,t) may satisfy Equation 2, and the received signal quality R(r,s) may satisfy Equation 3, so that it may be determined whether to perform the packet duplication transmission scheme (S 1208 ). That is, it may be determined whether the current time corresponds to a PD transmission candidate duration. Similarly, in the second relay terminal, the received signal quality R(f,t) may satisfy Equation 2, and the received signal quality R(r,s) may satisfy Equation 3, so that it may be determined whether to perform the packet duplication transmission scheme (S 1209 ). That is, it may be determined whether the current time corresponds to a PD transmission candidate duration. 
     As a result of the determination of the first relay terminal, if there is transmission data to be transmitted and the current time corresponds to the PD transmission candidate duration, the PDCP layer common to the first relay terminal and the second relay terminal may generate PDCP PDUs of the corresponding transmission data (S 1210 ). In addition, the PDCP layer may deliver the PDCP PDUs to the RLC layer of the first relay terminal for the split bearer connected to the target base station, and deliver the same PDCP PDUs to the RLC layer of the second relay terminal for the split bearer connected to the source base station (S 1212 ). 
     Then, the RLC layer of the first relay terminal may duplicate the PDCP PDU, and may transmit the duplicated PDCP PDU to the target base station through the MAC layer and the PHY layer of the first relay terminal for the split bearer connected to the target base station (S 1213 ). Similarly, the RLC layer of the second relay terminal may duplicate the PDCP PDU, and may transmit the duplicated PDCP PDU to the source base station through the MAC layer and the PHY layer of the second relay terminal for the split bearer connected to the source base station (S 1214 ). 
     On the other hand, as a result of the determination of the second relay terminal, if there is transmission data to be transmitted and the current time corresponds to the PD transmission candidate duration, the PDCP layer common to the first relay terminal and the second relay terminal may generate PDCP PDUs of the corresponding transmission data (S 1211 ). In addition, the PDCP layer may deliver the PDCP PDUs to the RLC layer of the first relay terminal for the split bearer connected to the target base station (S 1212 ), and deliver the same PDCP PDUs to the RLC layer of the second relay terminal for the split bearer connected to the source base station. 
     Then, the RLC layer of the first relay terminal may duplicate the PDCP PDU, and may transmit the PDCP PDUs to the target base station through the MAC layer and the PHY layer of the first relay terminal for the split bearer connected to the target base station (S 1213 ). Similarly, the RLC layer of the second relay terminal may duplicate the PDCP PDU, and may transmit the PDCP PDUs to the source base station through the MAC layer and the PHY layer of the second relay terminal for the split bearer connected to the source base station (S 1214 ). 
       FIGS. 13A and 13B  are flowcharts for describing a second exemplary embodiment of a communication method for selectively applying packet duplication transmission in a dual relay-based split bearer scheme. 
     Referring to  FIGS. 13A and 13B , in a communication method for selectively applying the packet duplication transmission scheme in the dual relay-based split bearer scheme, the first relay terminal may receive a reference signal from a target base station (S 1301 ), and measure a received signal quality R(f,t) (S 1302 ). In addition, the first relay terminal may receive a reference signal from a source base station (S 1304 ), and measure a received signal quality R(f,s) (S 1305 ). Similarly, the second relay terminal may receive the reference signal from the target base station (S 1301 ), and measure a received signal quality R(r,t) (S 1303 ). In addition, the second relay terminal may receive the reference signal from the source base station (S 1304 ), and measure a received signal quality R(r,s) (S 1306 ). 
     Here, each of the received signal qualities may be at least one of RSRP, RSRQ, and a combination thereof with respect to the reference signal of the target base station or the source base station. 
     The first relay terminal may transmit, to the second relay terminal, information on the received signal quality R(f,t) measured by receiving the reference signal from the target base station and the received signal quality R(f,s) measured by receiving the reference signal from the source base station, thereby sharing the received signal qualities R(f,t) and R(f,s) with the second relay terminal (S 1307 ). In addition, similarly, the second relay terminal may transmit, to the first relay terminal, information on the received signal quality R(r,t) measured by receiving the reference signal from the target base station and the received signal quality R(r,s) measured by receiving the reference signal from the source base station, thereby sharing the received signal qualities R(r,t) and R(r,s) with the first relay terminal (S 1307 ). 
     Accordingly, in the first relay terminal, the received signal quality R(f,t) may satisfy Equation 2, and the received signal quality R(r,s) may satisfy Equation 3, so that it may be determined whether to perform the packet duplication transmission scheme (S 1308 ). That is, it may be determined whether the current time corresponds to a PD transmission candidate duration. Similarly, in the second relay terminal, the received signal quality R(f,t) may satisfy Equation 2, and the received signal quality R(r,s) may satisfy Equation 3, so that it may be determined whether to perform the packet duplication transmission scheme (S 1309 ). That is, it may be determined whether the current time corresponds to a PD transmission candidate duration. 
     As a result of the determination, if the current time corresponds to the PD transmission candidate duration, the first relay terminal may identify whether the received signal quality R(f,t) and the received signal quality R(r,s) satisfy the condition of Equation 4 to determine whether the current time corresponds to a packet duplication transmission timing (S 1310 ). As a result of the determination, if there is transmission data to be transmitted and the current time corresponds to the packet duplication transmission timing, the PDCP layer common to the first relay terminal and the second relay terminal may generate PDCP PDUs of the corresponding transmission data (S 1312 ). In addition, the PDCP layer may deliver the PDCP PDUs to the RLC layer of the first relay terminal for the split bearer connected to the target base station, and deliver the same PDCP PDUs to the RLC layer of the second relay terminal for the split bearer connected to the source base station (S 1314 ). 
     Then, the RLC layer of the first relay terminal may duplicate the PDCP PDU, and may transmit the PDCP PDUs to the target base station through the MAC layer and the physical layer of the first relay terminal for the split bearer connected to the target base station (S 1315 ). Similarly, the RLC layer of the second relay terminal may duplicate the PDCP PDU, and may transmit the PDCP PDUs to the source base station through the MAC layer and the PHY layer of the second relay terminal for the split bearer connected to the source base station (S 1316 ). 
     On the other hand, as a result of the determination of the second relay terminal, it may be identified whether the received signal quality R(f,t) and the received signal quality R(r,s) satisfy the condition of Equation 4 to determine whether the current time corresponds to a packet duplication transmission timing (S 1311 ). As a result of the determination, if there is transmission data to be transmitted and the current time corresponds to the packet duplication transmission timing, the PDCP layer common to the first relay terminal and the second relay terminal may generate PDCP PDUs of the corresponding transmission data (S 1313 ). In addition, the PDCP layer may deliver the PDCP PDUs to the RLC layer of the first relay terminal for the split bearer connected to the target base station, and deliver the same PDCP PDUs to the RLC layer of the second relay terminal for the split bearer connected to the source base station (S 1314 ). 
     Then, the RLC layer of the first relay terminal may duplicate the PDCP PDU, and may transmit the PDCP PDUs to the target base station through the MAC layer and the PHY layer of the first relay terminal for the split bearer connected to the target base station (S 1315 ). Similarly, the RLC layer of the second relay terminal may duplicate the PDCP PDU, and may transmit the PDCP PDUs to the source base station through the MAC layer and the PHY layer of the second relay terminal for the split bearer connected to the source base station (S 1316 ). 
     Meanwhile, although only the uplink has been described herein, the same procedure may be applied to the downlink. To this end, the relay terminals may measure received signal strengths of signals received from the base stations, and report the measured values to the base stations. Then, the PDCP of the base station (MN) may perform packet duplication according to the condition of the measured values. 
     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.