Method and apparatus for recovering terminal context in wireless communication system

Abstract

The present disclosure relates to a 5G or 6G communication system for supporting a higher data transmission rate than that of a 4G communication system such as LTE. A terminal and a method of a terminal in a wireless communication system can be provided. The method of a terminal may comprise the steps of: receiving a radio resource control (RRC) reconfiguration message from a first base station; and transmitting an RRC reestablishment request message to a second base station.

Description

Method and device for recovering terminal context in a wireless communication system

[0001] The present technology relates to the operation of a terminal or a base station in a mobile communication system. Specifically, it relates to a method and device for a terminal or base station to recover terminal context when moving between terminals.

[0002] Looking back at the evolution of wireless communication over successive generations, technologies have primarily been developed for human-facing services such as voice, multimedia, and data. With the commercialization of 5G (5th-generation) communication systems, an explosive increase in connected devices is expected to be connected to communication networks. Examples of networked objects include vehicles, robots, drones, home appliances, displays, smart sensors installed in various infrastructures, construction equipment, and factory equipment. Mobile devices are also expected to evolve into diverse form factors, such as augmented reality glasses, virtual reality headsets, and holographic devices. In the 6th-generation (6G) era, efforts are being made to develop improved 6G communication systems to connect hundreds of billions of devices and objects and provide diverse services. For this reason, 6G communication systems are often referred to as "beyond 5G."

[0003] The 6G communication system, expected to be realized in the future, will have a maximum transmission speed of terabytes per second (i.e., 1,000 gigabits per second) and a wireless latency of 100 microseconds (μsec). In other words, compared to 5G, the transmission speed in a 6G communication system will be 50 times faster, while the wireless latency will be reduced to one-tenth.

[0004] To achieve these high data rates and ultra-low latency, 6G communication systems are being considered for implementation in the terahertz band (e.g., from 95 gigahertz (GHz) to 3 terahertz (THz)). Compared to the millimeter wave (mmWave) band introduced in 5G, the terahertz band is expected to experience more severe path loss and atmospheric absorption, making it more crucial to ensure signal reach, or coverage, in this band. Key technologies to ensure coverage include radio frequency (RF) components, antennas, new waveforms that offer better coverage than OFDM (orthogonal frequency division multiplexing), beamforming, and multiple antenna transmission technologies such as massive multiple-input and multiple-output (MIMO), full-dimensional MIMO (FD-MIMO), array antennas, and large-scale antennas. In addition, new technologies such as metamaterial-based lenses and antennas, high-dimensional spatial multiplexing using orbital angular momentum (OAM), and reconfigurable intelligent surfaces (RIS) are being discussed to improve the coverage of terahertz band signals.

[0005] In addition, in order to improve frequency efficiency and system network, 6G communication systems are developing full duplex technology that utilizes the same frequency resources for uplink and downlink at the same time; network technology that integrates satellites and high-altitude platform stations (HAPS); network structure innovation technology that supports mobile base stations and enables optimization and automation of network operation; dynamic spectrum sharing technology through collision avoidance based on spectrum usage prediction; AI-based communication technology that utilizes artificial intelligence (AI) from the design stage and internalizes end-to-end AI support functions to realize system optimization; and next-generation distributed computing technology that realizes services with complexity that exceeds the limits of terminal computing capabilities by utilizing ultra-high-performance communication and computing resources (mobile edge computing (MEC), cloud, etc.). In addition, efforts are being made to further strengthen connectivity between devices, further optimize networks, promote softwareization of network entities, and increase the openness of wireless communications through the design of new protocols to be used in 6G communication systems, the implementation of hardware-based security environments, the development of mechanisms for the safe use of data, and the development of technologies for maintaining privacy.

[0006] Research and development of these 6G communication systems are expected to enable a new level of hyper-connected experience (the next hyper-connected experience) through the hyper-connectivity of 6G communication systems, which encompass not only connections between things but also connections between people and things. Specifically, 6G communication systems are expected to enable services such as truly immersive extended reality (Truly Immersive XR), high-fidelity mobile holograms, and digital replicas. Furthermore, services such as remote surgery, industrial automation, and emergency response, which are provided through enhanced security and reliability, will find application in diverse fields such as industry, medicine, automobiles, and home appliances.

[0007] According to an embodiment of the present disclosure, a method of a terminal in a wireless communication system may be provided. The method may include receiving a radio resource control (RRC) reconfiguration message related to a handover from a first base station. The method may include transmitting a random access (RA) preamble to a cell associated with a second base station. The method may include receiving a random access response (RAR) from a cell associated with the second base station. The method may include transmitting an RRC reconfiguration complete message to the cell associated with the second base station. The method may include performing a cell selection procedure if the RRC reconfiguration complete message is not delivered to the cell associated with the second base station. The method may include transmitting an RRC reestablishment request message to a base station of a cell selected through the cell selection procedure. The RRC reestablishment request message may include a physical cell ID (PCI) of a cell including a cell associated with the second base station.

[0008] According to an embodiment of the present disclosure, a terminal may be provided in a wireless communication system. The terminal may include a transceiver; and a processor connected to the transceiver. The processor may execute at least one command or program stored in a memory of the terminal, thereby allowing the terminal to receive a radio resource control (RRC) reconfiguration message related to a handover from a first base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the terminal to transmit a random access (RA) preamble to a cell associated with a second base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the terminal to receive a random access response (RAR) from a cell associated with the second base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the terminal to transmit an RRC reconfiguration complete message to a cell associated with the second base station. The processor may execute at least one command or program stored in the memory of the terminal, so that the terminal may perform a cell selection procedure when the RRC re-establishment completion message is not transmitted to a cell associated with the second base station. The processor may execute at least one command or program stored in the memory of the terminal, so that the terminal may transmit an RRC re-establishment request message to a base station of a cell selected through the cell selection procedure. The RRC re-establishment request message may include a PCI (physical cell ID) of a cell including a cell associated with the second base station.

[0009] According to an embodiment of the present disclosure, a method of a second base station in a wireless communication system may be provided. The method may include receiving a handover request message from a first base station. If the handover request is approved, the method may include configuring UE context information for a terminal. The method may include transmitting a handover request ACK (acknowledgement) message indicating a handover to the first base station based on the UE context information. The method may include receiving a random access (RA) preamble from the terminal. The method may include transmitting a random access response (RAR) to the terminal. If the RRC reconfiguration complete message is not transmitted from the terminal and a UE context release message is received from a base station of a cell selected through a cell selection procedure of the terminal, the method may include transmitting a UE context release message to the first base station.

[0010] According to an embodiment of the present disclosure, a second base station may be provided in a wireless communication system. The second base station may include a transceiver; and a processor connected to the transceiver. The processor may execute at least one command or program stored in a memory of the terminal, thereby allowing the second base station to receive a handover request message from the first base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the second base station to configure UE context information for the terminal when approving the handover request. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the second base station to transmit a handover request ACK (acknowledgement) message indicating a handover to the first base station based on the UE context information. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the second base station to receive a random access (RA) preamble from the terminal. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the second base station to transmit a random access response (RAR) to the terminal. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the second base station to transmit a UE context release message to the first base station when the RRC reconfiguration complete message is not transmitted from the terminal and a UE context release message is received from a base station of a cell selected through a cell selection procedure of the terminal.

[0011] FIG. 1 is a diagram illustrating the structure of an LTE system according to an embodiment of the present disclosure.

[0012] FIG. 2 is a diagram illustrating a wireless protocol structure of an LTE system according to an embodiment of the present disclosure.

[0013] FIG. 3 is a diagram illustrating the structure of a next-generation mobile communication system according to an embodiment of the present disclosure.

[0014] FIG. 4 is a diagram illustrating a wireless protocol structure of a next-generation mobile communication system according to an embodiment of the present disclosure.

[0015] FIG. 5 is a block diagram showing the configuration of a terminal according to an embodiment of the present disclosure.

[0016] FIG. 6 is a block diagram showing the configuration of a base station according to one embodiment of the present disclosure.

[0017] FIG. 7 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0018] FIG. 8 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0019] FIG. 9 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0020] FIG. 10 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0021] FIG. 11 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0022] FIG. 12 is a flowchart of a terminal, a base station, an AMF, and a UPF according to one embodiment of the present disclosure.

[0023] The operating principles of the present disclosure are described in detail below with reference to the attached drawings. In the following description of the present disclosure, detailed descriptions of related known functions or configurations will be omitted if they are deemed to unnecessarily obscure the gist of the present disclosure. Furthermore, the terms described below are defined based on the functions of the present disclosure and may vary depending on the intent or custom of the user or operator. Therefore, their definitions should be based on the overall content of this specification.

[0024] The terms used in the following description to identify connection nodes, terms referring to network entities, terms referring to messages, terms referring to interfaces between network entities, and terms referring to various identification information are provided for convenience of explanation. Therefore, the present invention is not limited to the terms described below, and other terms referring to objects with equivalent technical meanings may be used.

[0025] Hereinafter, a base station is an entity that performs resource allocation of a terminal, and may be at least one of a gNode B (gNB), an eNode B (eNB), a Node B, a BS (Base Station), a wireless access unit, a base station controller, or a node on a network. The terminal may include a UE (User Equipment), an MS (Mobile Station), a cellular phone, a smartphone, a computer, or a multimedia system capable of performing a communication function.

[0026] In the present disclosure, downlink (DL) refers to a wireless transmission path for a signal transmitted from a base station to a terminal, and uplink (UL) refers to a wireless transmission path for a signal transmitted from a terminal to a base station. In addition, although LTE (Long Term Evolution), LTE-A (LTE-Advanced), 5G (5th-generation) system, or 6G (6th-generation) system may be described below as an example, embodiments of the present disclosure may also be applied to other communication systems having similar technical backgrounds or channel types. For example, 5th generation mobile communication technology (5G, new radio, NR) developed after LTE-A may be included in a system to which embodiments of the present disclosure may be applied, and 5G or 6G below may also be a concept that includes existing LTE, LTE-A, and other similar services. In addition, the present disclosure may be applied to other communication systems with some modifications within a scope that does not significantly deviate from the scope of the present disclosure at the discretion of a person having skilled technical knowledge. At this time, it will be understood that each block of the processing flow diagrams and combinations of the flow diagrams can be performed by computer program instructions.

[0027] These computer program instructions may be installed in a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing apparatus, so that the instructions executed by the processor of the computer or other programmable data processing apparatus create means for performing the functions described in the flowchart block(s). These computer program instructions may also be stored in a computer-available or computer-readable memory that can be directed to a computer or other programmable data processing apparatus to implement functions in a particular manner, so that the instructions stored in the computer-available or computer-readable memory can produce an article of manufacture that includes instruction means for performing the functions described in the flowchart block(s). The computer program instructions may also be installed on a computer or other programmable data processing apparatus, so that a series of operational steps are performed on the computer or other programmable data processing apparatus to create a computer-implemented process, so that the instructions executing on the computer or other programmable data processing apparatus can provide steps for performing the functions described in the flowchart block(s).

[0028] Additionally, each block may represent a module, segment, or portion of code that contains one or more executable instructions for executing a specific logical function(s). It should also be noted that in some alternative implementation examples, the functions mentioned in the blocks may occur out of order. For example, two blocks shown in succession may in fact be executed substantially simultaneously, or the blocks may sometimes be executed in reverse order depending on the corresponding function. In this case, the term '~unit' used in the present embodiment means software or a hardware component such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit), and the '~unit' may perform certain roles. However, the '~unit' is not limited to software or hardware. The '~unit' may be configured to be on an addressable storage medium and may be configured to execute one or more processors. Thus, as an example, the '~ unit' includes components such as software components, object-oriented software components, class components, and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuits, data, databases, data structures, tables, arrays, and variables. The functionality provided within the components and '~ units' may be combined into a smaller number of components and '~ units' or further separated into additional components and '~ units'. In addition, the components and '~ units' may be implemented to reproduce one or more CPUs within a device or a secure multimedia card. Also, in an embodiment, the '~ unit' may include one or more processors.

[0029] All functions or operations described in this document may be performed by a single processor or a combination of processors. A single processor or a combination of processors is a circuitry that performs processing, and may include circuitry such as an Application Processor (AP), a Communication Processor (CP), a Graphical Processing Unit (GPU), a Neural Processing Unit (NPU), a Microprocessor Unit (MPU), a System on Chip (SoC), an Integrated Chip (IC), or an AI accelerator.

[0030] The expression “configured to” used in the present disclosure can be used interchangeably with, for example, “suitable for”, “having the capacity to”, “designed to”, “adapted to”, “made to”, or “capable of”, depending on the context.

[0031] The term "configured (or set up) to" may not necessarily mean "specifically designed to" hardware. Instead, in some contexts, the phrase "a system configured to" may mean that the system, in conjunction with other devices or components, is "capable of" doing something.

[0032] For example, the phrase "a processor configured (or set) to perform A, B, and C" may include a dedicated processor (e.g., an embedded processor) for performing those operations, or a generic-purpose processor (e.g., a CPU or application processor) that enables the device to perform those operations by executing one or more program codes, instructions, or software stored in memory.

[0033] Singular expressions may include plural expressions unless the context clearly indicates otherwise. Terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the art described herein.

[0034] For convenience of explanation, the present invention uses terms and names defined in the 5GS and NR standards, which are standards defined by the 3rd Generation Partnership Project (3GPP) among the existing communication standards. However, the present invention is not limited to the above terms and names and can be equally applied to wireless communication networks that follow other standards. For example, the present invention can be applied to the 3GPP 5GS / NR (5th generation mobile communication standard).

[0035] 5G mobile communication technology defines a wide frequency band to enable fast transmission speeds and new services, and can be implemented not only in the sub-6GHz frequency band such as 3.5 gigahertz (3.5GHz), but also in the ultra-high frequency band called millimeter wave (mmWave) such as 28GHz and 39GHz ('Above 6GHz'). In addition, for 6G mobile communication technology, which is called the system after 5G communication (Beyond 5G), implementation in the terahertz band (for example, the 3 terahertz (3THz) band at 95GHz) is being considered to achieve a transmission speed that is 50 times faster than 5G mobile communication technology and an ultra-low latency time that is reduced to one-tenth.

[0036] In the early stages of 5G mobile communication technology, the goal is to support services and satisfy performance requirements for enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), and massive Machine-Type Communications (mMTC). These include beamforming and massive MIMO to mitigate path loss of radio waves in ultra-high frequency bands and increase the transmission distance of radio waves, support for various numerologies (such as operation of multiple subcarrier intervals) and dynamic operation of slot formats for efficient use of ultra-high frequency resources, initial access technology to support multi-beam transmission and wideband, definition and operation of BWP (Bidth Part), new channel coding methods such as LDPC (Low Density Parity Check) codes for large-capacity data transmission and Polar Code for reliable transmission of control information, and L2 pre-processing (L2). Standardization has been made for network slicing, which provides dedicated networks specialized for specific services, and pre-processing.

[0037] Currently, discussions are underway to improve and enhance the initial 5G mobile communication technology in consideration of the services that 5G mobile communication technology was intended to support, and physical layer standardization is in progress for technologies such as V2X (Vehicle-to-Everything) to help autonomous vehicles make driving decisions and increase user convenience based on their own location and status information transmitted by vehicles, NR-U (New Radio Unlicensed) for the purpose of system operation that complies with various regulatory requirements in unlicensed bands, NR terminal low power consumption technology (UE Power Saving), Non-Terrestrial Network (NTN), which is direct terminal-satellite communication to secure coverage in areas where communication with terrestrial networks is impossible, and Positioning.

[0038] In addition, standardization of wireless interface architecture / protocols is in progress for technologies such as intelligent factories (Industrial Internet of Things, IIoT) to support new services through linkage and convergence with other industries, Integrated Access and Backhaul (IAB) that provides nodes for expanding network service areas by integrating wireless backhaul links and access links, Mobility Enhancement technology including Conditional Handover and Dual Active Protocol Stack (DAPS) handover, and 2-step random access (2-step RACH for NR) that simplifies random access procedures. Standardization is also in progress for system architecture / services such as 5G baseline architecture (e.g., Service-based Architecture, Service-based Interface) for grafting Network Functions Virtualization (NFV) and Software-Defined Networking (SDN) technologies, and Mobile Edge Computing (MEC) that provides services based on the location of the terminal.

[0039] Once these 5G mobile communication systems are commercialized, an explosive increase in connected devices will be connected to the communication network, necessitating enhanced functionality and performance of 5G mobile communication systems and integrated operation of these connected devices. To this end, new research will be conducted on improving 5G performance and reducing complexity, supporting AI services, supporting metaverse services, and drone communications by utilizing eXtended Reality (XR), Artificial Intelligence (AI), and Machine Learning (ML) to efficiently support Augmented Reality (AR), Virtual Reality (VR), and Mixed Reality (MR).

[0040] In addition, the development of these 5G mobile communication systems includes new waveforms to ensure coverage in the terahertz band of 6G mobile communication technology, multi-antenna transmission technologies such as Full Dimensional MIMO (FD-MIMO), Array Antenna, and Large Scale Antenna, metamaterial-based lenses and antennas to improve the coverage of terahertz band signals, high-dimensional spatial multiplexing technology using Orbital Angular Momentum (OAM), Reconfigurable Intelligent Surface (RIS) technology, as well as full duplex technology to improve the frequency efficiency and system network of 6G mobile communication technology, satellite, AI (Artificial Intelligence) from the design stage and AI-based communication technology that realizes system optimization by internalizing end-to-end AI support functions, and ultra-high-performance communication and computing resources to provide services with complexity that exceeds the limits of terminal computing capabilities. It can serve as a basis for the development of next-generation distributed computing technologies that can be realized by utilizing them.

[0041] As described above and with the development of mobile communication systems, various services have become available, and methods for effectively providing these services are required.

[0042] When a failure occurs during the movement of a terminal, it may become unclear from which cell the terminal's context will be moved in the network. The present disclosure may provide a method for this purpose.

[0043] According to an embodiment of the present invention, when a terminal fails to move, it selects a clue about the failure and provides it to a newly connected cell, thereby allowing the terminal to acquire information about a reference cell from which the network obtains terminal context.

[0044] FIG. 1 is a diagram illustrating the structure of an LTE system according to an embodiment of the present disclosure.

[0045] Referring to FIG. 1, as illustrated, a wireless access network of an LTE system may be composed of next-generation base stations (Evolved Node Bs, hereinafter referred to as ENBs, Node Bs or base stations) (1-05, 1-10, 1-15, 1-20), a mobility management entity (MME) (1-25) and an S-GW (1-30, Serving-Gateway). A user equipment (UE or terminal) (1-35) may access an external network through the ENBs (1-05 to 1-20) and the S-GW (1-30).

[0046] In Fig. 1, ENBs (1-05 to 1-20) may correspond to existing Node Bs of a UMTS system. ENBs are connected to UEs (1-35) via a wireless channel and may perform more complex roles than existing Node Bs. In an LTE system, all user traffic, including real-time services such as VoIP (Voice over IP) via the Internet Protocol, can be serviced through a shared channel. Therefore, a device that collects status information such as the buffer status, available transmission power status, and channel status of UEs and performs scheduling is required, and ENBs (1-05 to 1-20) may be responsible for this. One ENB can typically control multiple cells. For example, in order to achieve a transmission rate of 100 Mbps, an LTE system may use, for example, Orthogonal Frequency Division Multiplexing (OFDM) as a wireless access technology in a 20 MHz bandwidth. Additionally, an adaptive modulation and coding (AMC) method can be applied, which determines the modulation scheme and channel coding rate according to the channel condition of the terminal. The S-GW (1-30) is a device that provides a data bearer and can create or remove a data bearer according to the control of the MME (1-25). The MME is a device that is responsible for various control functions as well as mobility management functions for the terminal and can be connected to multiple base stations.

[0047] FIG. 2 is a diagram illustrating a wireless protocol structure of an LTE system according to an embodiment of the present disclosure.

[0048] Referring to Figure 2, the wireless protocol of the LTE system may be composed of Packet Data Convergence Protocol (PDCP) (2-05, 2-40), Radio Link Control (RLC) (2-10, 2-35), and Medium Access Control (MAC) (2-15, 2-30) in the terminal and ENB, respectively. PDCP may be responsible for operations such as IP header compression / decompression. The main functions of PDCP can be summarized as follows.

[0049] - Header compression and decompression (ROHC only)

[0050] - User data transfer function

[0051] - In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC AM

[0052] - Order reordering function (For split bearers in DC (only support for RLC AM): PDCP PDU routing for transmission and PDCP PDU reordering for reception)

[0053] - Duplicate detection function (Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM)

[0054] - Retransmission function (Retransmission of PDCP SDUs at handover and, for split bearers in DC, of ​​PDCP PDUs at PDCP data-recovery procedure, for RLC AM)

[0055] - Encryption and decryption functions (Ciphering and deciphering)

[0056] - Timer-based SDU discard in uplink.

[0057] Radio Link Control (RLC) (2-10, 2-35) can perform ARQ operations, etc. by reconfiguring PDCP packet data units (PDUs) to an appropriate size. The main functions of RLC can be summarized as follows.

[0058] - Data transfer function (Transfer of upper layer PDUs)

[0059] - ARQ function (Error Correction through ARQ (only for AM data transfer))

[0060] - Concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer)

[0061] - Re-segmentation of RLC data PDUs (only for AM data transfer)

[0062] - Reordering of RLC data PDUs (only for UM and AM data transfer)

[0063] - Duplicate detection (only for UM and AM data transfer)

[0064] - Error detection function (Protocol error detection (only for AM data transfer))

[0065] - RLC SDU discard function (RLC SDU discard (only for UM and AM data transfer))

[0066] - RLC re-establishment function

[0067] MAC (2-15, 2-30) connects to multiple RLC layer devices configured in a single terminal and can perform operations of multiplexing RLC PDUs into MAC PDUs and demultiplexing RLC PDUs from MAC PDUs. The main functions of MAC can be summarized as follows.

[0068] - Mapping function (Mapping between logical channels and transport channels)

[0069] - Multiplexing and demultiplexing function (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)

[0070] - Scheduling information reporting function

[0071] - HARQ function (Error correction through HARQ)

[0072] - Priority handling between logical channels of one UE

[0073] - Priority handling between UEs by means of dynamic scheduling

[0074] - MBMS service identification function

[0075] - Transport format selection function

[0076] - Padding function

[0077] The physical layer (2-20, 2-25) can perform the operation of channel coding and modulating upper layer data, converting it into OFDM symbols and transmitting it through a wireless channel, or demodulating and channel decoding OFDM symbols received through a wireless channel and transmitting them to a higher layer.

[0078] FIG. 3 is a diagram illustrating the structure of a next-generation mobile communication system according to an embodiment of the present disclosure.

[0079] Referring to FIG. 3, a wireless access network of a next-generation mobile communication system (hereinafter referred to as NR or 5g) may be composed of a next-generation base station (New Radio Node B, hereinafter referred to as NR gNB or NR base station) (3-10) and a next-generation radio core network (New Radio Core Network, NR CN) (3-05). A next-generation radio user equipment (New Radio User Equipment, NR UE or terminal) (3-15) may access an external network through the NR gNB (3-10) and the NR CN (3-05).

[0080] In Fig. 3, the NR gNB (3-10) may correspond to the eNB (Evolved Node B) of the existing LTE system. The NR gNB is connected to the NR UE (3-15) via a wireless channel and can provide superior services than the existing Node B. In the next-generation mobile communication system, all user traffic can be serviced through a shared channel. Therefore, a device that collects status information such as the buffer status of the UEs, the available transmission power status, and the channel status and performs scheduling is required, and the NR NB (3-10) may be responsible for the scheduling. One NR gNB may control multiple cells. In the next-generation mobile communication system, in order to implement ultra-high-speed data transmission compared to the general LTE, a bandwidth exceeding the general maximum bandwidth may be applied. In addition, beamforming technology may be additionally grafted using Orthogonal Frequency Division Multiplexing (OFDM) as a wireless access technology. In addition, an adaptive modulation and coding (AMC) method that determines a modulation scheme and a channel coding rate according to the channel status of the terminal may be applied. NR CN (3-05) can perform functions such as mobility support, bearer setup, and QoS setup. NR CN is a device that is responsible for various control functions as well as mobility management functions for the terminal and can be connected to multiple base stations. In addition, the next-generation mobile communication system can also be linked with the LTE system, and NR CN can be connected to MME (3-25) through a network interface. MME can be connected to eNB (3-30), which is an LTE base station.

[0081] FIG. 4 is a diagram illustrating a wireless protocol structure of a next-generation mobile communication system according to an embodiment of the present disclosure.

[0082] Referring to FIG. 4, the wireless protocol of the next-generation mobile communication system consists of NR Service Data Adaptation Protocol (SDAP) (4-01, 4-45), NR PDCP (4-05, 4-40), NR RLC (4-10, 4-35), NR MAC (4-15, 4-30), and NR PHY (4-20, 4-25) in the terminal and NR base station, respectively.

[0083] Key features of NR SDAP (4-01, 4-45) may include some of the following:

[0084] - Transfer of user plane data

[0085] - Mapping function between QoS flow and data bearer for both DL and UL

[0086] - QoS flow ID marking function for uplink and downlink (marking QoS flow ID in both DL and UL packets)

[0087] - Ability to map relective QoS flow to data bearer for uplink SDAP PDUs (reflective QoS flow to DRB mapping for the UL SDAP PDUs).

[0088] For an SDAP layer device, a terminal can be configured by a Radio Resource Control (RRC) message for each PDCP layer device, per bearer, or per logical channel, whether to use the header of the SDAP layer device or whether to use the function of the SDAP layer device. When the SDAP header is configured, the terminal can instruct the terminal to update or reset the mapping information for the QoS flow and data bearer of the uplink and downlink by using a 1-bit indicator for reflecting the Non-Access Stratum (NAS) Quality of Service (QoS) in the SDAP header (NAS reflective QoS) and a 1-bit indicator for reflecting the Access Stratum (AS) QoS in the SDAP header. The SDAP header can include QoS flow ID information indicating QoS. The QoS information can be used as data processing priority, scheduling information, etc. to support a smooth service.

[0089] The main functions of NR PDCP (4-05, 4-40) may include some of the following functions:

[0090] - Header compression and decompression (ROHC only)

[0091] - User data transfer function

[0092] - In-sequence delivery of upper layer PDUs

[0093] - Out-of-sequence delivery of upper layer PDUs

[0094] - PDCP PDU reordering for reception

[0095] - Duplicate detection of lower layer SDUs

[0096] - Retransmission function (Retransmission of PDCP SDUs)

[0097] - Encryption and decryption functions (Ciphering and deciphering)

[0098] - Timer-based SDU discard in uplink.

[0099] In the above, the reordering function of the NR PDCP device may refer to a function of reordering PDCP PDUs received from a lower layer in order based on a PDCP SN (sequence number). The reordering function of the NR PDCP device may include a function of transmitting data to an upper layer in the reordered order, or a function of transmitting data directly without considering the order, a function of recording lost PDCP PDUs by reordering the order, a function of reporting a status of lost PDCP PDUs to the transmitting side, and a function of requesting retransmission of lost PDCP PDUs.

[0100] The main functions of NR RLC(4-10, 4-35) may include some of the following functions:

[0101] - Data transfer function (Transfer of upper layer PDUs)

[0102] - In-sequence delivery of upper layer PDUs

[0103] - Out-of-sequence delivery of upper layer PDUs

[0104] - ARQ function (Error Correction through ARQ)

[0105] - Concatenation, segmentation and reassembly of RLC SDUs

[0106] - Re-segmentation of RLC data PDUs

[0107] - Reordering of RLC data PDUs

[0108] - Duplicate detection function

[0109] - Protocol error detection

[0110] - RLC SDU discard function

[0111] - RLC re-establishment function

[0112] In the above, the in-sequence delivery function of the NR RLC device may refer to the function of sequentially delivering RLC SDUs received from the lower layer to the upper layer. If a single RLC SDU is originally received divided into multiple RLC SDUs, the in-sequence delivery function of the NR RLC device may include the function of reassembling and delivering them.

[0113] The in-sequence delivery function of the NR RLC device may include a function to reorder received RLC PDUs based on the RLC SN (sequence number) or PDCP SN (sequence number), a function to record lost RLC PDUs by reordering them, a function to report status of lost RLC PDUs to the transmitter, and a function to request retransmission of lost RLC PDUs.

[0114] The in-sequence delivery function of the NR RLC device may include a function to sequentially deliver only the RLC SDUs up to the lost RLC SDU to the upper layer when there is a lost RLC SDU.

[0115] The in-sequence delivery function of the NR RLC device may include a function to deliver to the upper layer in sequence all RLC SDUs received before a predetermined timer starts if there are lost RLC SDUs and a predetermined timer has expired.

[0116] The in-sequence delivery function of the NR RLC device may include a function to deliver all RLC SDUs received so far to the upper layer in order if a predetermined timer has expired, even if there are lost RLC SDUs.

[0117] An NR RLC device can process RLC PDUs in the order they are received and deliver them to an NR PDCP device, regardless of the order of the sequence number (out-of-sequence delivery).

[0118] When an NR RLC device receives a segment, it can receive segments that are stored in a buffer or will be received later, reconstruct them into a complete RLC PDU, and then transmit them to the NR PDCP device.

[0119] The NR RLC layer may not include concatenation functionality, and the functionality may be performed by the NR MAC layer or replaced by the multiplexing functionality of the NR MAC layer.

[0120] In the above, the out-of-sequence delivery function of the NR RLC device may refer to the function of directly delivering RLC SDUs received from a lower layer to an upper layer regardless of the order. The out-of-sequence delivery function of the NR RLC device may include the function of reassembling and delivering the RLC SDUs when an original RLC SDU is received fragmented into multiple RLC SDUs. The out-of-sequence delivery function of the NR RLC device may include the function of storing and arranging the RLC SN or PDCP SN of the received RLC PDUs to record the lost RLC PDUs.

[0121] NR MAC (4-15, 4-30) can be connected to multiple NR RLC layer devices configured in one terminal, and the main functions of NR MAC can include some of the following functions.

[0122] - Mapping function (Mapping between logical channels and transport channels)

[0123] - Multiplexing / demultiplexing of MAC SDUs

[0124] - Scheduling information reporting function

[0125] - HARQ function (Error correction through HARQ)

[0126] - Priority handling between logical channels of one UE

[0127] - Priority handling between UEs by means of dynamic scheduling

[0128] - MBMS service identification function

[0129] - Transport format selection function

[0130] - Padding function

[0131] The NR PHY layer (4-20, 4-25) can perform operations such as channel coding and modulating upper layer data, converting it into OFDM symbols and transmitting it through a wireless channel, or demodulating and channel decoding OFDM symbols received through a wireless channel and transmitting them to a higher layer.

[0132] FIG. 5 is a block diagram showing the configuration of a terminal according to an embodiment of the present disclosure.

[0133] Referring to the above drawing, the terminal includes an RF (Radio Frequency) processing unit (5-10), a baseband processing unit (5-20), a storage unit (5-30), and a control unit (5-40).

[0134] The RF processing unit (5-10) performs functions for transmitting and receiving signals through a wireless channel, such as signal band conversion and amplification. The RF processing unit (5-10) up-converts the baseband signal provided from the baseband processing unit (5-20) into an RF band signal and transmits it through an antenna, and down-converts the RF band signal received through the antenna into a baseband signal. For example, the RF processing unit (5-10) may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a digital to analog convertor (DAC), an analog to digital convertor (ADC), etc. In the drawing, only one antenna is shown, but the terminal may be equipped with multiple antennas. In addition, the RF processing unit (5-10) may include multiple RF chains. Furthermore, the RF processing unit (5-10) may perform beamforming. For the above beamforming, the RF processing unit (5-10) can adjust the phase and size of each signal transmitted and received through multiple antennas or antenna elements. In addition, the RF processing unit can perform MIMO and can receive multiple layers when performing the MIMO operation.

[0135] The baseband processing unit (5-20) above performs a conversion function between a baseband signal and a bit stream according to the physical layer specifications of the system. For example, when transmitting data, the baseband processing unit (5-20) generates complex symbols by encoding and modulating a transmission bit stream. In addition, when receiving data, the baseband processing unit (5-20) restores the reception bit stream by demodulating and decoding the baseband signal provided from the RF processing unit (5-10). For example, in the case of following the OFDM (orthogonal frequency division multiplexing) method, when transmitting data, the baseband processing unit (5-20) generates complex symbols by encoding and modulating a transmission bit stream, maps the complex symbols to subcarriers, and then configures OFDM symbols through an inverse fast Fourier transform (IFFT) operation and a cyclic prefix (CP) insertion. In addition, when receiving data, the baseband processing unit (5-20) divides the baseband signal provided from the RF processing unit (5-10) into OFDM symbol units, restores signals mapped to subcarriers through FFT (fast Fourier transform), and then restores the received bit string through demodulation and decoding.

[0136] The baseband processing unit (5-20) and the RF processing unit (5-10) transmit and receive signals as described above. Accordingly, the baseband processing unit (5-20) and the RF processing unit (5-10) may be referred to as a transmitter, a receiver, a transceiver, or a communication unit. Furthermore, at least one of the baseband processing unit (5-20) and the RF processing unit (5-10) may include a plurality of communication modules to support a plurality of different wireless access technologies. In addition, at least one of the baseband processing unit (5-20) and the RF processing unit (5-10) may include different communication modules to process signals of different frequency bands. For example, the different wireless access technologies may include wireless LAN (e.g., IEEE 802.11), a cellular network (e.g., LTE), etc. Additionally, the different frequency bands may include a super high frequency (SHF) (e.g., 2.NRHz, NRhz) band and a millimeter wave (mm wave) (e.g., 60GHz) band.

[0137] The storage unit (5-30) stores data such as basic programs, application programs, and setting information for the operation of the terminal. In particular, the storage unit (5-30) can store information related to a second access node that performs wireless communication using a second wireless access technology. In addition, the storage unit (5-30) provides the stored data at the request of the control unit (5-40).

[0138] The above control unit (5-40) controls the overall operations of the terminal. For example, the control unit (5-40) transmits and receives signals through the baseband processing unit (5-20) and the RF processing unit (5-10). In addition, the control unit (5-40) records and reads data in the storage unit (5-40). For this purpose, the control unit (5-40) may include at least one processor. For example, the control unit (5-40) may include a communication processor (CP) that performs control for communication and an application processor (AP) that controls upper layers such as application programs.

[0139] In an embodiment of the present disclosure, the processor may be implemented to cause the terminal to perform a predetermined function disclosed in FIGS. 1 to 12 by executing instructions or program codes stored in the storage unit (4-40).

[0140] FIG. 6 is a block diagram showing the configuration of a base station according to one embodiment of the present disclosure.

[0141] As shown in the above drawing, the base station is configured to include an RF processing unit (6-10), a baseband processing unit (6-20), a backhaul communication unit (6-30), a storage unit (6-40), and a control unit (6-50).

[0142] The RF processing unit (6-10) performs functions for transmitting and receiving signals through a wireless channel, such as signal band conversion and amplification. The RF processing unit (6-10) up-converts the baseband signal provided from the baseband processing unit (6-20) into an RF band signal and transmits it through an antenna, and down-converts the RF band signal received through the antenna into a baseband signal. For example, the RF processing unit (6-10) may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc. In the drawing, only one antenna is shown, but the first access node may have multiple antennas. In addition, the RF processing unit (6-10) may include multiple RF chains. Furthermore, the RF processing unit (6-10) may perform beamforming. For the beamforming, the RF processing unit (6-10) may adjust the phase and magnitude of each signal transmitted and received through multiple antennas or antenna elements. The above RF processing unit can perform a downlink MIMO operation by transmitting one or more layers.

[0143] The baseband processing unit (6-20) above performs a conversion function between a baseband signal and a bit stream according to the physical layer specifications of the first wireless access technology. For example, when transmitting data, the baseband processing unit (6-20) generates complex symbols by encoding and modulating a transmission bit stream. In addition, when receiving data, the baseband processing unit (6-20) restores the reception bit stream by demodulating and decoding the baseband signal provided from the RF processing unit (6-10). For example, in the case of OFDM, when transmitting data, the baseband processing unit (6-20) generates complex symbols by encoding and modulating a transmission bit stream, maps the complex symbols to subcarriers, and then configures OFDM symbols through IFFT operation and CP insertion. In addition, when receiving data, the baseband processing unit (6-20) divides the baseband signal provided from the RF processing unit (6-10) into OFDM symbol units, restores the signals mapped to subcarriers through FFT operation, and then restores the received bit string through demodulation and decoding. The baseband processing unit (6-20) and the RF processing unit (6-10) transmit and receive signals as described above. Accordingly, the baseband processing unit (6-20) and the RF processing unit (6-10) may be referred to as a transmitter, a receiver, a transceiver, a communication unit, or a wireless communication unit.

[0144] The above backhaul communication unit (6-30) provides an interface for communicating with other nodes within the network. The above backhaul communication unit (6-30) converts a bit string transmitted from the main base station to other nodes, such as auxiliary base stations and core networks, into a physical signal, and converts a physical signal received from the other nodes into a bit string.

[0145] The storage unit (6-40) stores data such as basic programs, application programs, and configuration information for the operation of the main base station. In particular, the storage unit (6-40) can store information on bearers assigned to connected terminals, measurement results reported from connected terminals, and the like. In addition, the storage unit (6-40) can store information that serves as a judgment criterion for whether to provide or terminate multiple connections to a terminal. In addition, the storage unit (6-40) provides the stored data at the request of the control unit (6-50).

[0146] The control unit (6-50) controls the overall operations of the base station. For example, the control unit (6-50) transmits and receives signals through the baseband processing unit (6-20) and the RF processing unit (6-10) or through the backhaul communication unit (6-30). In addition, the control unit (6-50) records and reads data in the storage unit (6-40). For this purpose, the control unit (6-50) may include at least one processor.

[0147] In an embodiment of the present disclosure, the processor may be implemented to cause the base station to perform a predetermined function disclosed in FIGS. 1 to 12 by executing instructions or program codes stored in the storage unit (6-40).

[0148] FIG. 7 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0149] Referring to FIG. 7, when performing HO (handover) confirmed in the field, RREReq (RRC Re-Establishment request) transmission to the HO target cell (target cell, trg) of the terminal may be performed.

[0150] FIG. 8 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0151] Referring to FIG. 8, when performing HO (handover), if the cell selected by the terminal during RRE (RRC Re-Establishment) operation is a new serving cell rather than the HO target cell (target cell, trg), RREReq transmission may be performed.

[0152] Referring to FIGS. 7 and 8, a terminal may receive an RRC reconfiguration message including an HO command from an HO source cell. The terminal may start a timer (e.g., T304). The terminal may perform a HO procedure. As a result, the terminal may perform a random access (RA). For example, if a contention-free RA (CFRA) is configured in the HO command, the terminal may perform a CFRA. After a successful RA (e.g., CFRA), a radio link failure (RLF) may occur in the HO target cell, and the terminal may not be able to transmit an RRCReconfigurationComplete to the HO target cell. As a result, an RRE (RRC Re-Establishment) procedure may be performed by the terminal and the base station.

[0153] Referring to Fig. 7, the UE may determine that the cell selected in the RRE is again the HO target cell. Accordingly, the UE transmits an RREReq to the HO target cell. At this time, the UE may send UE Identity information to the HO target cell based on the target cell information. Based on the UE Identity information, the target gNB may configure the UE using the UE context existing in the target cell. In this case, the UE context of the HO source gNB is maintained within the HO source gNB, and when an internal timer expires, the source gNB may request the UE context from the AMF. However, if the UE configuration is already completed by the target gNB, the target gNB may request a path switch of user data through the AMF. If the path switch request is accepted by the AMF, the UE context within the AMF may be deleted. Accordingly, the UE context release requested by the HO source gNB to the AMF may be regarded as an error.

[0154] Referring to FIG. 8, if the cell selected by the UE in the RRE is a cell of a new gNB (e.g., cell 2), the UE can transmit an RREReq to the corresponding gNB (e.g., gNB2). At this time, the UE Identity information can be sent to the corresponding gNB based on the target cell information. Based on the UE Identity information, the new gNB can obtain the UE context from the target gNB, and can configure the UE in the new gNB using the UE context. In this case, the UE context of the HO source gNB is maintained as it is in the HO source gNB, and when an internal timer (e.g., T304) expires, the HO source gNB can request the UE context from the AMF. If the configuration of the UE is already completed by the target GNB, the target gNB can request a path switch of user data through the AMF, and if the path switch request is accepted, the UE context in the AMF can be deleted. Accordingly, the UE context release requested by the HO source gNB to the AMF may be considered an error.

[0155] Referring to FIG. 7, there may be a case where a terminal transmits an RREReq message to a target cell while performing a HO instruction (command).

[0156] If a terminal receives a handover instruction and a failure occurs during the handover, the RRC re-establishment procedure is performed. During this process, failures occurring during the handover are currently defined in the specification to indicate only handover failures or other types of failures.

[0157] However, depending on the type of RA performed by the UE, there may be a situation where the failure occurring in the middle is not only a handover failure or another type of failure, but both. According to an embodiment of the present disclosure, in this case, it can be suggested how to perform a re-establishment procedure. In addition, in this case, it can be suggested which cell's UE context the network will use. For reference, the types of RA performed by the UE include CFRA (contention-free random access) 4 steps and 2 steps, and CBRA (contention-based random access) 4 steps and 2 steps. Among these procedures, the 4-step CFRA only includes a procedure up to which the UE receives an RAR from the base station, and does not include a procedure for transmitting the UE's data to UL. Here, when the UE transmits UL data, it means a UL transmission scheduled by receiving a UL grant, and may include a case where the UE transmits an RRCReconfigurationComplete message according to the HO procedure.

[0158] Currently, the success or failure of HO of a terminal is defined as the same as the success of RA. Accordingly, in the case of the above 4-step CFRA, the RA itself may succeed, but the subsequent transmission of the RRCReconfigurationComplete message to the target cell is not actually included in the HO procedure. Due to this discrepancy in the standard, if the RA succeeds, but the subsequent transmission of the RRCReconfigurationComplete message to the target cell fails, and if the cause is an RLF, the terminal will perform the RRE (RRCReestablishment) procedure, and during the RRE procedure, it can perform cell selection and transmit the RREReq (RRCReestablishmentRequest) message to the selected cell.

[0159] In this process, the UE can forward the RREReq message to the selected cell by including the PCI (physical cell ID) and the C-RNTI value used in the previous serving cell as information to notify the previous serving gNB for UE context retrieval. However, in the case of the mismatch, that is, when the completion of HO and the completion of RA do not match, it is unclear whether the serving cell is the HO source cell or the HO target cell, and accordingly, the target from which the context is to be retrieved may be unclear in the network.

[0160] At least some of the following could be suggested as ways to address this issue:

[0161] FIG. 9 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0162] According to one embodiment of the present disclosure, the definition of a successful handover of a terminal may be defined as follows. A successful handover of a terminal is defined as the terminal successfully transmitting an RRCReconfigurationcomplete message to a target cell after the terminal receives an RRCReconfiguration message including a reconfigurationWithSync field, i.e., an HO command.

[0163] The UE receives an RRCReconfiguration message (HO command) and starts the T304 timer. If the UE performs the RA procedure with the target cell and successfully transmits the RRCReconfigurationComplete message to the target cell, the T304 timer is stopped.

[0164] If the terminal receives the HO command, starts the T304 timer, performs RA, and after the RA is successfully completed, the terminal cannot transmit the RRCReconfigurationComplete message to the target cell due to a certain problem in the target cell (e.g., RLF occurrence, inability to receive UL grant, etc.), and in particular, if the terminal cannot transmit the RRCReconfigurationComplete message to the target cell until the T304 timer expires, the terminal may consider it a handover failure.

[0165] Accordingly, the terminal can enter the RRE procedure. Accordingly, the terminal performs cell selection, and the terminal can transmit an RREReq message to the target cell (the selected cell) after performing the RA procedure (or without the RA procedure).

[0166] At this time, the RREReq message may include a UE identity value as a method for the network to recognize the last serving cell or the last serving gNB for UE context retrieval and may be transmitted from the terminal to the target cell. This UE identity value may include the PCI of the source cell of the HO (e.g., the PCI of cell 0), the AFRCN, and the C-RNTI value given for use in the source cell (e.g., the C-RNTI received from cell 0). The cause value of the RRE may include handover failure or reconfigurationWithSync failure. For example, referring to FIG. 9, the terminal may transmit a cause value indicating reconfigurationWithSync failure.

[0167] The gNB of the serving cell that received the RREReq can recognize the corresponding terminal and check whether the terminal's UE context exists in the serving cell. If the terminal's UE context exists, the gNB of the serving cell that received the RREReq uses the corresponding UE context. If the terminal's UE context does not exist, the gNB of the serving cell that received the RREReq can request and obtain the UE context from the source gNB using the PCI of the source cell included in the RREReq. Thereafter, the gNB of the serving cell that received the RREReq can transmit an RRE message to the terminal and, if necessary, can transmit additional settings to the terminal through an RRCReconfiguration message. If the terminal accepts all of the settings and transmits an RREcomplete message and an RRCReconfigurationComplete message to the cell, the serving cell that received the RREReq can instruct the source gNB to delete the UE context.

[0168] In an embodiment of the present disclosure, regardless of the presence or absence of a local UE context of the serving cell, the gNB of the serving cell that received the RREReq message can request UE context retrieval from the gNB (i.e., the HO source gNB) based on the cell information included in the RREReq message. In this case, the gNB of the serving cell that received the RREReq can ignore or perform an operation of deleting the UE context included in the local.

[0169] FIG. 10 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0170] FIG. 11 is a flowchart of a terminal, a base station, and an AMF according to one embodiment of the present disclosure.

[0171] Referring to FIGS. 10 and 11, when a terminal transmits an RREReq message, the terminal may include at least one of the following information in the RREReq message and transmit it to a cell selected during the RRE operation, specifically in a situation where an RLF occurs during HO operation, rather than an event of either HOF (handover failure) or RLF.

[0172] - As UE identity information, PCI and / or HO source cell

[0173] - C-RNTI set for use in HO source cell,

[0174] - and / or as an RRE cause indicator: otherFailure, meaning RLF, or as a new indicator, an indicator indicating the occurrence of RLF in the HO target cell after successful RA during HO execution.

[0175] The transmission condition of the above RREReq message is that when the terminal receives the HO command and performs RA to the target cell successfully and fails to transmit the HO complete message (RRCReconfigurationComplete) to the target cell, or when RLF occurs, the terminal and base station perform the RRE procedure, and the terminal can transmit the RREReq message to the cell selected during the RRE procedure.

[0176] The gNB that receives the above message can use the context to provide additional settings to the terminal if it has the UE context of the terminal.

[0177] In one embodiment, regardless of the presence or absence of a local UE context of the serving cell, a UE context retrieval can be requested from a gNB (i.e., an HO source gNB) based on the cell information included in the RREReq message. In this case, the UE context included in the local can be ignored or deleted.

[0178] Referring to Figure 10, a cell selected among RREs may be an HO target cell.

[0179] Referring to Figure 11, the cell selected among the RREs may be another cell.

[0180] FIG. 12 is a flowchart of a terminal, a base station, an AMF, and a UPF according to one embodiment of the present disclosure.

[0181] The base station of the source cell can transmit an RRCReconfiguration message (HO command) to the UE. The UE can receive the RRCReconfiguration message (HO command). In this case, the UE can start the T304 timer and perform the RA procedure. The types of RA performed by the UE include CFRA (contention-free random access) 4 steps and 2 steps, and CBRA (contention-based random access) 4 steps and 2 steps.

[0182] In the RA procedure, the terminal can transmit an RA preamble to the base station of the target cell. In the RA procedure, the terminal can receive a random access response (RAR) from the base station of the target cell. In one embodiment, if it is a CFRA, the terminal that receives the RAR can determine that the RA has been successfully completed.

[0183] When the terminal performs the RA procedure with the target cell and successfully transmits the RRCReconfigurationComplete message to the target cell, the T304 timer is stopped.

[0184] When a terminal receives an HO command, it can start a T304 timer and perform an RA procedure. After the RA procedure is successfully completed, if an RRCReconfigurationComplete message cannot be delivered to the target cell due to a certain problem in the target cell (e.g., RLF occurrence, inability to receive UL grant, etc.), or if the RRCReconfigurationComplete message cannot be delivered to the target cell before the T304 timer expires, at least one of the following cases applies: the terminal can enter an RRE procedure. Accordingly, the terminal can perform cell selection, and the terminal can transmit an RREReq message to the selected cell through an RA procedure (or without the RA procedure).

[0185] Referring to FIG. 12, when a terminal transmits an RREReq message, the terminal may include at least one of the following information in the RREReq message and transmit it to a cell selected during the RRE procedure, in a situation where an RLF occurs during HO execution, rather than an event of either HOF or RLF.

[0186] - As UE identity information, PCI of HO target cell (e.g. PCI of cell 1)

[0187] - C-RNTI configured for use in the HO target cell (e.g., C-RNTI received from cell 1),

[0188] - RRE cause indicator

[0189] At least one of the following can be possible as an RRE cause directive:

[0190] > HO failure indicator: In this case, the gNB of the selected cell that received the RREReq message (e.g., gNB2 Cell2, gNB1 Cell1) can perform UE context retrieval with itself and the target gNB included in the message.

[0191] > New indicator: This indicator is a new indicator and may be an indicator indicating the occurrence of RLF in the HO target cell after RA success when performing HO.

[0192] > otherFailure: As before, it can indicate RLF in the serving cell, and the receiving gNB can perform UE context retrieval to the target gNB.

[0193] The transmission condition of the above RREReq message is that when the terminal receives the RRCReconfiguration message (HO command) and successfully performs the RA procedure to the target cell and fails to transmit the HO complete message (RRCReconfigurationComplete) to the target cell, or when an RLF occurs, the terminal performs the RRE procedure and can transmit the RREReq message to the cell selected during the RRE procedure.

[0194] Regardless of the indications suggested in the RREReq message, the gNB that receives the RREReq message can use the context of the UE of the corresponding terminal to provide additional settings to the terminal if it has the context.

[0195] In one embodiment of the present disclosure, regardless of the indicator proposed in the RREReq message, the gNB receiving the RREReq message may request UE context retrieval from a gNB (i.e., HO source gNB) (e.g., gNB0 Cell0) based on cell information included in the RREReq message, regardless of whether a local UE context of the serving cell exists. In this case, the gNB receiving the RREReq message may ignore or erase the UE context included in the local.

[0196] In all cases where UE context retrieval is performed from the target gNB, including FIG. 12, at least some or all of the following Sol procedures may be additionally performed.

[0197] Sol: From the HO source gNB (e.g., gNB0 Cell0 (src)), the HO target gNB (e.g., gNB1 Cell1 (trg)) can receive the HO request. The HO target gNB can admit the HO request, configure the context of the corresponding UE, and configure the target cell configuration or HO command based on the context of the corresponding UE.

[0198] When the HO target gNB sends a HO request ACK message to the HO source gNB including the target cell configuration or HO command (i.e., the HO request is received and accepted), and the HO target gNB does not receive the HO complete message, i.e., the RRCReconfigurationComplete message sent by the UE,

[0199] i) If the HO target gNB receives a RREReq message from the UE, or ii) if the HO target gNB receives a UE context retrieve request message from another gNB (of a cell selected by the UE), the HO target gNB may provide the UE context it has stored for retrieval, and additionally, the HO target gNB may transmit a UE context release message to the HO source gNB.

[0200] For example, in case i), the HO target gNB can provide the UE with settings appropriate for the UE context through an RRE message and then an RRCReestablishment and RRCReconfiguration message based on the corresponding UE context. After this, the HO target gNB can recognize that the RRC establishment is complete and request a path switch (HO source gNB to target gNB) of UP data through AMF. After the HO target gNB receives this response, it can request a UE context release from the HO source gNB through an Xn message.

[0201] In case ii), the HO target gNB can transmit UE context information to the gNB that transmitted the received Retrieve UE context request through the Retrieve UE context response. Afterwards, the gNB of the selected cell (selected cell or new serving cell) can provide the UE with settings (via RRE and RRCReconfiguration messages) appropriate for the UE context through the RRCReestablishment and RRCReconfiguration messages. Afterwards, the gNB of the selected cell recognizes that the RRC establishment is complete and can request a path switch (HO target gNB to the gNB of the selected cell (or serving gNB)) of the UP data through the AMF. After the gNB of the selected cell receives a response to the path switch request, if the gNB of the selected cell requests UE context release to the HO target gNB, the HO target gNB sends the above information, i.e.,

[0202] - Confirm that the UE to be released is a UE of the context previously established during the HO preparation process.

[0203] - Requesting UE context retrieval of the terminal from another gNB (in this example, the gNB of the cell selected by the terminal),

[0204] - Finally, when a UE context release request is received from another gNB (gNB of the cell selected by the terminal),

[0205] If satisfied, the HO target gNB can request UE context release of the Xn message from the HO source gNB.

[0206] Referring to FIG. 12, when the last HO target gNB receives a context release request from the gNB of the selected cell, the HO target gNB can delete its stored UE context, and at the same time, the HO target gNB can request UE context release from the HO source gNB, and the HO source gNB can perform an operation to delete the corresponding UE context.

[0207] According to an embodiment of the present disclosure, a UE context for context retrieval can be determined by the terminal and instructed to the NW.

[0208] The information included in RREReq is UE Identity, and PCI and C-RNTI can be included by the terminal as one of the source cell or target cell related information.

[0209] RREReq may contain an indicator indicating a new cause value, which may be an indicator indicating that the UE has encountered an RLF while performing HO. Alternatively, it may indicate that RRCReconfigurationComplete (HO complete) has not been transmitted during HO, or that RA has succeeded during HO, but an RLF has occurred in the target cell.

[0210] The transmission condition of the above RREReq message is that if the terminal receives the HO command and performs RA to the target cell successfully and fails to transmit the HO complete message (RRCReconfigurationComplete) to the target cell, or if RLF occurs, the terminal performs RRE and can transmit the RREReq message to the cell selected during the performance.

[0211] The serving gNB can retrieve UE context based on the cell information included in the received message.

[0212] Alternatively, if the local UE context of the serving gNB itself is checked and exists, the local UE context can be used, and if there is no local UE context, retrieval based on the indicated cell can be performed.

[0213] In this case, if the context of the HO target gNB is used, the above Sol procedure can be additionally considered.

[0214] According to an embodiment of the present disclosure, a terminal may include both HO source cell-based UE Identity information and HO target cell-based UE Identity information in an RREReq message.

[0215] In this case, indicators indicating HO source cell and target cell, or an order within a specific field can be used. Additionally, for the source cell, an indicator indicating HOF or RA completion can be linked and included as a cause value, and for the target cell, an indicator indicating other or RLF can be linked and included as a cause value.

[0216] Alternatively, instead of the above cause value pair, a single indicator may be included in conjunction with it. This indicator may be an indicator indicating that RA was successful during HO execution but transmission of the HO complete message to the target cell failed, or that an RLF occurred.

[0217] The transmission condition of the above RREReq message is that when the terminal receives the HO command and performs RA to the target cell successfully and fails to transmit the HO complete message (RRCReconfigurationComplete) to the target cell or when RLF occurs, the terminal performs RRE, and the terminal can transmit the RREReq message to the cell selected during RRE performance.

[0218] In this case, the serving gNB that received the message can target at least one of the following UE contexts:

[0219] Opt 1. either source cell or target cell: it's up to the network

[0220] Opt 2. sourcell

[0221] Opt 3. target cell

[0222] Opt 4. as legacy, first check local context, and above opt 1~3

[0223] According to an embodiment of the present disclosure, a method of a terminal in a wireless communication system may be provided. The method may include receiving a radio resource control (RRC) reconfiguration message related to a handover from a first base station. The method may include transmitting a random access (RA) preamble to a cell associated with a second base station. The method may include receiving a random access response (RAR) from a cell associated with the second base station. The method may include transmitting an RRC reconfiguration complete message to the cell associated with the second base station. The method may include performing a cell selection procedure if the RRC reconfiguration complete message is not delivered to the cell associated with the second base station. The method may include transmitting an RRC reestablishment request message to a base station of a cell selected through the cell selection procedure. The RRC reestablishment request message may include a physical cell ID (PCI) of a cell including a cell associated with the second base station.

[0224] The RRC re-establishment request message may include a cell-radio network temporary identifier (C-RNTI) of a cell including a cell associated with the second base station.

[0225] The above RRC re-establishment request message may include a cause indicator indicating another failure.

[0226] The method may include receiving, from a base station of the selected cell, an RRC reset message including UE context information of a cell associated with the second base station.

[0227] According to an embodiment of the present disclosure, a terminal may be provided in a wireless communication system. The terminal may include a transceiver; and a processor connected to the transceiver. The processor may execute at least one command or program stored in a memory of the terminal, thereby allowing the terminal to receive a radio resource control (RRC) reconfiguration message related to a handover from a first base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the terminal to transmit a random access (RA) preamble to a cell associated with a second base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the terminal to receive a random access response (RAR) from a cell associated with the second base station. The processor may execute at least one command or program stored in the memory of the terminal, thereby allowing the terminal to transmit an RRC reconfiguration complete message to a cell associated with the second base station. The processor may execute at least one command or program stored in the memory of the terminal, so that the terminal may perform a cell selection procedure when the RRC re-establishment completion message is not transmitted to a cell associated with the second base station. The processor may execute at least one command or program stored in the memory of the terminal, so that the terminal may transmit an RRC re-establishment request message to a base station of a cell selected through the cell selection procedure. The RRC re-establishment request message may include a PCI (physical cell ID) of a cell including a cell associated with the second base station.

[0228] The above RRC re-establishment request message may include the C-RNTI of a cell including a cell associated with the second base station.

[0229] The above RRC re-establishment request message may include a cause indicator indicating another failure.

[0230] The processor executes at least one command or program stored in the memory of the terminal, so that the terminal can receive an RRC reset message including UE context information of a cell including a cell linked to the second base station from the base station of the selected cell.

[0231] According to an embodiment of the present disclosure, a method of a second base station in a wireless communication system may be provided. The method may include receiving a handover request message from a first base station. If the handover request is approved, the method may include configuring UE context information for a terminal. The method may include transmitting a handover request ACK (acknowledgement) message indicating a handover to the first base station based on the UE context information. The method may include receiving a random access (RA) preamble from the terminal. The method may include transmitting a random access response (RAR) to the terminal. If the RRC reconfiguration complete message is not transmitted from the terminal and a UE context release message is received from a base station of a cell selected through a cell selection procedure of the terminal, the method may include transmitting a UE context release message to the first base station.

[0232] According to an embodiment of the present disclosure, a second base station may be provided in a wireless communication system. The second base station may include a transceiver; and a processor connected to the transceiver.

[0233] The processor executes at least one command or program stored in the memory of the terminal, thereby allowing the second base station to receive a handover request message from the first base station.

[0234] The processor can configure UE context information for the terminal when the second base station approves the handover request by executing at least one command or program stored in the memory of the terminal.

[0235] The processor can execute at least one command or program stored in the memory of the terminal, thereby causing the second base station to transmit a handover request ACK (acknowledgement) message indicating a handover to the first base station based on the UE context information.

[0236] The processor executes at least one command or program stored in the memory of the terminal, thereby allowing the second base station to receive a random access (RA) preamble from the terminal.

[0237] The processor can execute at least one command or program stored in the memory of the terminal, thereby causing the second base station to transmit a random access response (RAR) to the terminal.

[0238] The processor may transmit a UE context release message to the first base station when the second base station receives a UE context release message from a base station of a cell selected through a cell selection procedure of the terminal without transmitting the RRC reconfiguration completion message from the terminal by executing at least one command or program stored in the memory of the terminal.

[0239] The methods according to the embodiments described in the claims or specification of the present invention may be implemented in the form of hardware, software, or a combination of hardware and software.

[0240] When implemented in software, a computer-readable storage medium storing one or more programs (software modules) may be provided. The one or more programs stored in the computer-readable storage medium are configured for execution by one or more processors within an electronic device. The one or more programs include instructions that cause the electronic device to execute methods according to the embodiments described in the claims or specification of the present invention.

[0241] These programs (software modules, software) may be stored in random access memory, non-volatile memory including flash memory, read only memory (ROM), electrically erasable programmable read only memory (EEPROM), magnetic disc storage device, compact disc ROM (CD-ROM), digital versatile discs (DVDs) or other forms of optical storage device, magnetic cassette. Or, they may be stored in a memory configured as a combination of some or all of these. In addition, each configuration memory may be included in multiple numbers.

[0242] Additionally, the program may be stored in an attachable storage device that is accessible via a communication network such as the Internet, an intranet, a local area network (LAN), a wide local area network (WLAN), a storage area network (SAN), or a combination thereof. Such a storage device may be connected to a device implementing an embodiment of the present invention via an external port. Additionally, a separate storage device on the communication network may be connected to a device implementing an embodiment of the present invention.

[0243] In the specific embodiments of the present invention described above, components included in the invention are expressed in the singular or plural form depending on the specific embodiment presented. However, the singular or plural expressions are selected to suit the presented situation for convenience of explanation, and the present invention is not limited to singular or plural components. Even components expressed in the plural form may be composed of singular elements, or even components expressed in the singular form may be composed of plural elements.

[0244] While the detailed description of the present invention has described specific embodiments, it is clear that various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the scope of the following claims but also by equivalents thereof.

[0245] A specific example for explaining an embodiment according to the present disclosure is only one combination of each criterion, method, detailed method, and operation, and through a combination of at least two or more of the various techniques described, a terminal or base station can perform an operation for restoring a UE context in a wireless mobile communication system. Furthermore, at this time, the operation may be performed according to a method determined through one or a combination of at least two or more of the aforementioned techniques. For example, it may be possible to perform a portion of the operation of one embodiment in combination with a portion of the operation of another embodiment.

[0246] A device-readable storage medium may be provided in the form of a non-transitory storage medium. Here, the term "non-transitory storage medium" simply means a tangible device that does not contain signals (e.g., electromagnetic waves). This term does not distinguish between cases where data is permanently stored in the storage medium and cases where data is temporarily stored. For example, a "non-transitory storage medium" may include a buffer in which data is temporarily stored.

[0247] According to one embodiment, the method according to various embodiments disclosed in the present document may be provided as included in a computer program product. The computer program product may be traded as a product between a seller and a buyer. The computer program product may be distributed in the form of a machine-readable storage medium (e.g., compact disc read-only memory (CD-ROM)), or may be distributed online (e.g., downloaded or uploaded) through an application store or directly between two user devices (e.g., smartphones). In the case of online distribution, at least a portion of the computer program product (e.g., a downloadable app) may be temporarily stored or temporarily generated in a machine-readable storage medium, such as the memory of a manufacturer's server, an application store's server, or an intermediary server.

Claims

1. A method of a terminal in a wireless communication system, comprising: a step of receiving an RRC (radio resource control) reconfiguration message related to handover from a first base station; A step of transmitting a random access (RA) preamble to a cell linked to a second base station; A step of receiving a random access response (RAR) from a cell linked to the second base station; A step of transmitting an RRC reconfiguration complete message to a cell linked to the second base station; and A step of performing a cell selection procedure when the RRC reset completion message is not transmitted to a cell linked to the second base station; A step of transmitting an RRC reestablishment request message to a base station of a cell selected through a cell selection procedure, A method in which the RRC re-establishment request message includes a PCI (physical cell ID) of a cell including a cell linked to the second base station.

2. In paragraph 1, A method wherein the RRC re-establishment request message further includes a cell-radio network temporary identifier (C-RNTI) of a cell including a cell associated with the second base station.

3. In paragraph 1 or 2, A method wherein the RRC re-establishment request message further includes a cause indicator indicating another failure.

4. In any one of paragraphs 1 to 3, A method further comprising the step of receiving an RRC reset message including UE context information of a cell associated with the second base station from a base station of the selected cell.

5. In a wireless communication system, the terminal, transceiver; and Including a processor connected to the above transceiver, The above processor, Receive a radio resource control (RRC) reconfiguration message related to handover from the first base station, Transmit a RA (random access) preamble to the cell linked to the second base station, Receive a random access response (RAR) from a cell linked to the second base station, Transmit an RRC reconfiguration complete message to a cell linked to the second base station, If the RRC reset completion message is not delivered to the cell linked to the second base station, a cell selection procedure is performed, Transmit an RRC reestablishment request message to the base station of the selected cell through the cell selection procedure, The terminal, wherein the RRC re-establishment request message includes a PCI (physical cell ID) of a cell including a cell linked to the second base station.

6. In paragraph 5, The terminal, wherein the RRC re-establishment request message further includes a C-RNTI of a cell including a cell linked to the second base station.

7. In paragraph 5 or 6, The terminal, wherein the RRC re-establishment request message further includes a cause indicator indicating another failure.

8. In any one of paragraphs 5 to 7, the processor A terminal that receives an RRC reset message including UE context information of a cell including a cell linked to the second base station from a base station of the selected cell.

9. In a method of a second base station in a wireless communication system, A step of receiving a handover request message from a first base station; When approving a handover request, a step of configuring UE context information for the terminal; A step of transmitting a handover request ACK (acknowledgement) message indicating handover to the first base station based on the UE context information; A step of receiving a RA (random access) preamble from the terminal; A step of transmitting a random access response (RAR) to the terminal; and A method comprising the step of transmitting a UE context release message to the first base station when the RRC reconfiguration completion message is not transmitted from the terminal and a UE context release message is received from the base station of the cell selected through the cell selection procedure of the terminal.

10. In a second base station in a wireless communication system, transceiver; and Including a processor connected to the above transceiver, The above processor, Receive a handover request message from the first base station, When approving a handover request, configure UE context information for the terminal, Transmitting a handover request ACK (acknowledgement) message indicating handover to the first base station based on the UE context information, Receive a RA (random access) preamble from the terminal, Transmit a RAR (random access response) to the above terminal, A second base station that transmits a UE context release message to the first base station when the RRC reconfiguration completion message is not transmitted from the terminal and a UE context release message is received from the base station of the cell selected through the cell selection procedure of the terminal.

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