Method and apparatus for driving PDCP entity during DAPS handover in next generation wireless communication system

Abstract

The present disclosure relates to a communication method and system for integrating a fifth-generation (5G) communication system supporting higher data rates than fourth-generation (4G) systems and Internet of Things (IoT) technologies. The present disclosure can be applied to smart services based on 5G communication technologies and IoT-related technologies, such as smart homes, smart buildings, smart cities, smart cars, connected cars, healthcare, digital education, smart retail, and safety and security services. The present disclosure provides a method and apparatus for operating a PDCP entity during a handover without transmission / reception interruption.

Description

Technical Field

[0001] The present disclosure relates to a next-generation mobile communication system, and more particularly, to a method and apparatus for performing efficient handover (hereinafter referred to as dual active protocol stack (DAPS) handover) in the next-generation mobile communication system so that there is no interruption in data transmission / reception during handover. Background Art

[0002] To meet the increased demand for wireless data traffic since the deployment of 4G communication systems, efforts have been made to develop improved 5G or quasi-5G communication systems. Therefore, 5G or quasi-5G communication systems are also referred to as "beyond 4G networks" or "post-LTE systems." The 5G communication system defined by 3GPP is called the New Radio (NR) system.

[0003] 5G communication systems are expected to be implemented in higher frequency (millimeter wave) bands (e.g., 60 gigahertz (GHz) bands) to achieve higher data rates. To reduce radio wave propagation losses and increase transmission distances, beamforming, massive multiple-input multiple-output (MIMO), full-dimensional MIMO (FD-MIMO), array antennas, analog beamforming, and massive antenna technologies have been discussed and applied in 5G communication systems.

[0004] Furthermore, in 5G communication systems, system network improvements are being developed based on advanced small cells, cloud radio access networks (RAN), ultra-dense networks, device-to-device (D2D) communications, wireless backhaul, mobile networks, cooperative communications, coordinated multipoint (CoMP), and receiver-side interference cancellation. In 5G systems, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) have been developed as advanced coding modulation (ACM), as well as filter bank multicarrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as advanced access technologies.

[0005] The Internet, a human-centric network of connected devices where people generate and consume information, is now evolving into the Internet of Things (IoT), in which distributed entities (such as things) exchange and process information without human intervention. The Internet of Everything (IoE), a combination of IoT technology and big data processing technology via connectivity to cloud servers, has emerged. Because IoT implementation requires technological elements such as sensing technology, wired / wireless communication and network infrastructure, service interface technology, and security technology, sensor networks, machine-to-machine (M2M) communication, and machine-type communication (MTC) have recently been researched. Such an IoT environment can provide intelligent internet technology services that create new value for human life by collecting and analyzing data generated between connected things. IoT can be applied to a variety of fields, including smart homes, smart buildings, smart cities, smart cars (connected vehicles), smart grids, healthcare, smart appliances, and advanced medical services, through the integration and combination of existing information technology (IT) and various industrial applications.

[0006] In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as sensor networks, machine-type communications (MTC), and machine-to-machine (M2M) communications can be implemented through beamforming, MIMO, and array antennas. Cloud radio access networks (RAN), as an application of the aforementioned big data processing technologies, can also be considered an example of the convergence between 5G and IoT technologies.

[0007] Meanwhile, with the development of mobile communication systems, handover without interruption of data transmission / reception has been widely studied.

[0008] The above information is presented as background information only to assist in understanding the present disclosure. No determination has been made, and no assertion has been made, as to whether any of the above might be applicable as prior art to the present disclosure. Summary of the Invention

[0009] Technical issues

[0010] An efficient handover method is needed to support services without data interruption and with low levels of transmission delay in next generation mobile communication systems.

[0011] Solution to the problem

[0012] According to an embodiment of the present disclosure, a method performed by a terminal is provided. The method includes: performing a random access procedure for a data radio bearer (DRB) handover with a target base station based on a first message configuring the DRB as a dual active protocol stack (DAPS) bearer; identifying whether a packet data convergence protocol (PDCP) status report is triggered for the DRB; and, if the PDCP status report is triggered for the DRB, sending a second message including the PDCP status report to the target base station.

[0013] According to an embodiment of the present disclosure, a method performed by a base station is provided. The method includes: performing a random access procedure for a data radio bearer (DRB) handover with a terminal based on a first message configuring the DRB as a dual active protocol stack (DAPS) bearer; and receiving a second message including a packet data convergence protocol (PDCP) status report from the terminal when a PDCP status report is triggered for the DRB.

[0014] According to an embodiment of the present disclosure, a terminal is provided. The terminal includes: a transceiver configured to send and receive signals; and a controller configured to: based on a first message that configures a data radio bearer (DRB) as a dual active protocol stack (DAPS) bearer, perform a random access procedure for DAPS handover with a target base station, identify whether a packet data convergence protocol (PDCP) status report is triggered for the DRB, and, if the PDCP status report is triggered for the DRB, send a second message including the PDCP status report to the target base station.

[0015] According to an embodiment of the present disclosure, a base station is provided. The base station includes: a transceiver configured to transmit and receive signals; and a controller configured to: based on a first message configuring a data radio bearer (DRB) as a dual active protocol stack (DAPS) bearer, perform a random access procedure for DAPS handover with a terminal, and, if a PDCP status report is triggered for the DRB, receive a second message including a packet data convergence protocol (PDCP) status report from the terminal.

[0016] The present disclosure proposes various efficient handover methods for ensuring that no data interruption time occurs due to handover when handover is performed in a next generation mobile communication system, thereby supporting a service without data interruption.

[0017] Before proceeding to the following detailed description, it may be helpful to set forth definitions of certain words and phrases used throughout this patent document: the terms "include" and "comprising" and their derivatives mean inclusion without limitation; the term "or" is inclusive, meaning and / or; the phrases "associated with" and "associated therewith" and their derivatives may mean including, being included, interconnected with, containing, being contained within, connected to or connected with, coupled to or coupled with, communicable with, cooperating with, interwoven, juxtaposed, proximate to, bound to or bound with, having, having the property of, and the like; and the term "controller" means any device, system, or portion thereof that controls at least one operation, such device being implemented in hardware, firmware, or software, or some combination of at least two of hardware, firmware, or software. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely.

[0018] In addition, the various functions described below can be implemented or supported by one or more computer programs, each of which is formed of computer-readable program code and contained in a computer-readable medium. The terms "application" and "program" refer to one or more computer programs, software components, instruction sets, processes, functions, objects, classes, instances, related data, or parts thereof suitable for implementation with suitable computer-readable program code. The phrase "computer-readable program code" includes any type of computer code, including source code, object code, and executable code. The phrase "computer-readable medium" includes any type of medium that can be accessed by a computer, such as read-only memory (ROM), random access memory (RAM), hard drive, compact disk (CD), digital video disc (DVD), or any other type of memory. "Non-transitory" computer-readable media does not include wired, wireless, optical, or other communication links that transmit temporary electrical or other signals. Non-transitory computer-readable media include media that can permanently store data and media that can store data and overwrite it later, such as rewritable optical disks or erasable memory devices.

[0019] Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.

[0020] Advantageous Effects of the Invention

[0021] According to various embodiments of the present disclosure, DAPS handover can be efficiently enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts:

[0023] Figure 1 A diagram illustrating the structure of an LTE system to which the present disclosure is applicable is shown;

[0024] Figure 2 A diagram illustrating a radio protocol structure in an LTE system to which the present disclosure is applicable is shown;

[0025] Figure 3 A diagram illustrating a structure of a next-generation mobile communication system to which the present disclosure is applicable is shown;

[0026] Figure 4 A diagram illustrating a radio protocol structure of a next-generation mobile communication system to which the present disclosure is applicable is shown;

[0027] Figure 5 A diagram illustrating a process in which a UE switches from a Radio Resource Control (RRC) idle mode to an RRC connected mode and configures a connection with a network is shown;

[0028] Figure 6 A diagram illustrating a signaling procedure for performing handover proposed by the present disclosure in a next generation mobile communication system is shown;

[0029] Figure 7 Detailed steps of a first embodiment of an efficient handover method for minimizing data interruption time caused by handover in the present disclosure are shown;

[0030] Figure 8 Detailed steps of a second embodiment of an efficient handover method for minimizing data interruption time caused by handover in the present disclosure are shown;

[0031] Figure 9a and Figure 9b A schematic diagram illustrating the structure of an efficient packet data convergence protocol (PDCP) layer device applied to a dual-active protocol stack (DAPS) handover method and a method for applying the structure is shown. The dual-active protocol stack (DAPS) handover method is a second embodiment of the efficient handover method proposed in the present disclosure;

[0032] Figure 10 A diagram illustrating the operation of a UE applicable to an embodiment proposed in the present disclosure is shown;

[0033] Figure 11 The structure of the UE to which the embodiments of the present disclosure are applicable is shown; and

[0034] Figure 12 A block diagram configuration of a transmission reception point (TRP) in a wireless communication system to which embodiments of the present disclosure are applicable is shown. DETAILED DESCRIPTION

[0035] Discussed below Figures 1 to 12 The various embodiments used to describe the principles of the present disclosure in this patent document are merely exemplary and should not be construed in any way to limit the scope of the present disclosure. Those skilled in the art will understand that the principles of the present disclosure can be implemented in any suitably arranged system or device.

[0036] Hereinafter, the operating principle of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description of the present disclosure, detailed descriptions of known functions or configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure unnecessarily unclear. The terms described below are defined in consideration of the functions in the present disclosure and may vary depending on the user, the user's intention, or custom. Therefore, the definition of terms should be based on the content of the entire specification.

[0037] In the following description of the present disclosure, when a detailed description of related known functions or configurations may make the subject matter of the present disclosure unnecessarily unclear, the detailed description will be omitted. Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.

[0038] For the same reason, in the accompanying drawings, some elements may be enlarged, omitted or schematically shown. In addition, the size of each element does not fully reflect the actual size. In the accompanying drawings, the same or corresponding elements have the same reference numerals.

[0039] By referring to the embodiments described in detail below in conjunction with the accompanying drawings, the advantages and features of the present disclosure and the ways to achieve them will become clear. However, the present disclosure is not limited to the embodiments set forth below, but can be implemented in various different forms. The following embodiments are provided only to fully disclose the present disclosure and to inform those skilled in the art of the scope of the present disclosure, and the present disclosure is limited only by the scope of the appended claims. Throughout the specification, the same or similar reference numerals designate the same or similar elements.

[0040] Here, it will be understood that each block of the flowchart diagram and the combination of blocks in the flowchart diagram can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device create a component for implementing the function specified in one or more flowchart blocks. These computer program instructions can also be stored in a computer-usable or computer-readable memory, which can guide the computer or other programmable data processing device to operate in a specific manner so that the instructions stored in the computer-usable or computer-readable memory produce an article of manufacture including an instruction component that implements the function specified in one or more flowchart blocks. The computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are performed on the computer or other programmable device, thereby generating a computer-implemented process, so that the instructions executed on the computer or other programmable device provide steps for implementing the function specified in one or more flowchart blocks.

[0041] In addition, each block of the flowchart diagram may represent a module, code segment, or code portion, which includes one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions mentioned in the blocks may not appear in order. For example, two blocks shown in succession may actually be executed substantially simultaneously, or the blocks may sometimes be executed in reverse order, depending on the functions involved.

[0042] As used herein, "unit" refers to a software element or a hardware element that performs a predetermined function, such as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). However, "unit" does not always have the meaning that is limited to software or hardware. "Unit" can be constructed to be stored in an addressable storage medium or to run one or more processors. Therefore, "unit" includes, for example, software elements, object-oriented software elements, class elements or task elements, processes, functions, attributes, procedures, subroutines, program code segments, drivers, firmware, microcodes, circuits, data, databases, data structures, tables, arrays and parameters. The elements and functions provided by "unit" can be combined into smaller elements or "units" or divided into larger elements or "units". In addition, elements and "units" can be implemented to reproduce one or more CPUs in equipment or secure multimedia cards.

[0043] In the following description, for convenience, terms used to identify access nodes, terms related to network entities, terms related to messages, terms related to interfaces between network entities, terms related to various identification information, etc. are illustratively used. Therefore, the present disclosure is not limited to the terms used below, and other terms related to subjects with equivalent technical meanings may be used.

[0044] In the following description, for ease of description, the present disclosure will be described using the terms and names defined in the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) standard. However, the present disclosure is not limited to these terms and names and can be applied in the same manner to systems that comply with other standards. In the present disclosure, the term "ENB" can be used interchangeably with the term "gNB." That is, a base station described as an "ENB" can also indicate a "gNB."

[0045] The present disclosure proposes a seamless handover method capable of minimizing the data interruption time caused by handover (or making it 0 milliseconds (ms)) in a next generation mobile communication system.

[0046] Specifically, the efficient handover method proposed in the present disclosure may have one or more of the following features:

[0047] -If the UE is configured to send or receive data (send and receive uplink or downlink data) with the source base station through corresponding protocol layer devices (physical (PHY) layer devices, medium access control (MAC) layer devices, radio link control (RLC) layer devices or PDCP layer devices) of multiple first bearers, and if the UE receives a handover command message (or RRC reconfiguration message) from the source base station, the UE can configure multiple new second bearer protocol layer devices corresponding to the protocol layers of the multiple first bearers (for example, having the same bearer identifier), and can send / receive data (send and receive uplink or downlink data) while seamlessly maintaining continuous sending or reception of data (sending or receiving uplink or downlink data) with the source base station through the multiple first bearers.

[0048] -In combination with the above features, after the UE receives the handover command message, the protocol layer devices (PHY layer devices, MAC layer devices, RLC layer devices or PDCP layer devices) of the newly configured multiple second bearers can be configured to send / receive data with the target base station based on the bearer configuration information or protocol layer device information included in the handover command message.

[0049] In combination with the above features, when transmitting or receiving data (transmitting and receiving uplink or downlink data) with a source base station by using protocol layer devices of multiple first bearers, the UE can perform a random access procedure to a target base station by using protocol layer devices (e.g., MAC layer devices) of multiple second bearers. In this procedure, the random access procedure may include transmitting a preamble, receiving a random access response, transmitting Message 3, receiving Message 4 (e.g., a contention resolution MAC control element (CE) or receiving uplink transmission resources), etc.

[0050] -In combination with the above features, when sending or receiving data with the source base station by using multiple first-bearing protocol layer devices, the UE can complete the process of random access to the target base station by using multiple second-bearing protocol layer devices (for example, MAC layer devices), and can send a handover completion message to the target base station by using multiple second-bearing protocol layer devices.

[0051] -In combination with the above features, when sending or receiving data with the source base station by using multiple first-bearing protocol layer devices, the UE can complete the process of random access to the target base station through multiple second-bearing protocol layer devices (for example, MAC layer devices), and can send a handover completion message to the target base station by using multiple second-bearing protocol layer devices, and can send / receive data (uplink or downlink).

[0052] -In combination with the above features, when the UE has first received uplink transmission resources from the target base station after completing the process of random access to the target base station, the UE can stop sending data to the source base station through the protocol layer device using multiple first bearers, can switch to uplink transmission, and can send data to the target base station through multiple second bearers.

[0053] -In combination with the above features, when the UE has received the handover command message, the UE can continue to send / receive data (send and receive uplink and downlink data) with the source base station by using multiple first bearers. When the UE has performed a random access process to the target base station through the protocol layer devices of multiple second bearers, thereby successfully completing the random access process, and has first received uplink transmission resources from the target base station, the UE can stop sending uplink data to the source base station by using the protocol layer devices of multiple first bearers, and can send uplink data to the target base station by using only the protocol layer devices of multiple second bearers. In addition, the UE can continue to receive downlink data from the source base station through the protocol layer devices of multiple first bearers, and can also continue to receive downlink data from the target base station by using the protocol layer devices of multiple second bearers.

[0054] -In combination with the above features, the first bearer and the second bearer can be configured according to the second PDCP layer device structure. The second PDCP layer device structure may refer to a structure in which the first bearer for the source base station (e.g., an RLC layer device, a MAC layer device, or a PHY layer device) and the second bearer for the target base station (e.g., an RLC layer device, a MAC layer device, or a PHY layer device) are both connected to a single PDCP layer device, and uplink data is sent through the PDCP layer device, through one of the first bearer or the second bearer. That is, according to the second PDCP layer device structure, the UE can send uplink data through the first bearer until the UE performs a process of random access to the target base station, successfully completes the random access process, and first receives uplink transmission resources from the target base station. If the UE has performed a process of random access to the target base station, has successfully completed the random access process, and has first received uplink transmission resources from the target base station, the UE can stop sending data through the first bearer and can switch to sending uplink data to the target through the second bearer. However, in the case of the second PDCP layer device structure, the UE can receive downlink data from the source base station and / or the target base station through the first bearer and / or the second bearer.

[0055] In the following, an efficient handover process without data interruption time based on the above features will be proposed.

[0056] Figure 1 A diagram illustrating the structure of an LTE system to which the present disclosure is applied is shown.

[0057] refer to Figure 1 As shown in the figure, the radio access network of the LTE system includes evolved Node Bs (hereinafter referred to as ENBs, Node Bs, or base stations) 1-05, 1-10, 1-15, and 1-20, a mobility management entity (MME) 1-25, and a serving gateway (S-GW) 1-30. User equipment (hereinafter referred to as UE or terminal) 1-35 accesses external networks through the ENBs 1-05 to 1-20 and the S-GW 1-30.

[0058] Figure 1The ENBs 1-05 to 1-20 in the LTE system correspond to existing Node Bs in the Universal Mobile Telecommunications System (UMTS) system. The ENBs are connected to the UEs 1-35 via radio channels and play a more complex role than existing Node Bs. In the LTE system, user traffic, including real-time services such as Voice over Internet Protocol (VoIP), is provided via a shared channel associated with all services. Therefore, a device is required for aggregating and scheduling UE status information (such as buffer status, available transmission power, and channel status), and the ENBs 1-05 to 1-20 are responsible for this device. One ENB typically controls multiple cells. For example, to achieve a transmission rate of 100 Mbps, the LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) in a 20 MHz bandwidth as its radio access technology. Furthermore, adaptive modulation and coding (AMC) is applied, so that the modulation scheme and channel coding rate are determined based on the UE's channel status. A serving gateway (S-GW) 1-30 is a device configured to provide data bearers and generates or removes data bearers under the control of a mobility management entity (MME) 1-25. The MME is a device responsible for not only mobility management functions but also various control functions and is connected to multiple base stations.

[0059] Figure 2 A diagram illustrating a radio protocol structure in an LTE system to which the present disclosure is applicable is shown.

[0060] refer to Figure 2 The LTE system's radio protocols include PDCP 2-05 and 2-40, RLC 2-10 and 2-35, and MAC 2-15 and 2-30, respectively, for the UE and ENB. PDCP 2-05 and 2-40 are responsible for operations such as IP header compression / restoration. The main functions of PDCP are summarized as follows:

[0061] -Header compression and decompression: ROHC only

[0062] -Transmission of user data

[0063] - In-sequence delivery of upper layer PDUs during PDCP re-establishment for RLC Acknowledged Mode (AM)

[0064] - For split bearer in DC (supports RLC AM only): PDCP PDU routing for transmission and PDCP PDU reordering for reception

[0065] - For RLC AM, duplicate detection of lower layer service data units (SDUs) during PDCP re-establishment

[0066] - Retransmission of PDCP SDUs during handover for RLC AM and retransmission of PDCP PDUs during PDCP data recovery for split bearers in DC

[0067] -Encryption and decryption

[0068] - Timer-based SDU discard in uplink.

[0069] Radio Link Control (RLC) 2-10 and 2-35 reconfigures the PDCP protocol data unit (PDU) to an appropriate size and performs automatic repeat request (ARQ) operations, etc. The main functions of RLC are summarized as follows:

[0070] -Transmission of upper layer PDU

[0071] - Error correction through ARQ (for AM data transmission only)

[0072] - Concatenation, segmentation, and reassembly of RLC SDUs (only for Unacknowledged Mode (UM) and AM data transmission)

[0073] - Re-segmentation of RLC data PDUs (for AM data transmission only)

[0074] - Reordering of RLC data PDUs (only for UM and AM data transmission)

[0075] -Duplicate detection (for UM and AM data transmission only)

[0076] -Protocol error detection (for AM data transmission only)

[0077] -RLC SDU discard (only for UM and AM data transmission)

[0078] -RLC reconstruction.

[0079] MAC 2-15 and 2-30 are connected to various RLC layer devices configured in a single UE and perform operations of multiplexing RLC PDUs into MAC PDUs and demultiplexing RLC PDUs from MAC PDUs. The main functions of MAC are summarized as follows:

[0080] - Mapping between logical channels and transport channels

[0081] - Multiplexing MAC SDUs belonging to one or different logical channels into a transport block (TB) on a transport channel that is delivered to the physical layer / demultiplexing MAC SDUs belonging to one or different logical channels from a transport block (TB) on a transport channel that comes from the physical layer

[0082] -Dispatch information report

[0083] - Error correction through Hybrid ARQ (HARQ)

[0084] - Priority handling between logical channels of a UE

[0085] - Priority handling between UEs through dynamic scheduling

[0086] -Multimedia Broadcast Multicast Service (MBMS) service identifier

[0087] -Transmission format selection

[0088] -filling.

[0089] The physical layers 2-20 and 2-25 perform operations of channel coding and modulating upper layer data to obtain OFDM symbols, and transmit the OFDM symbols through a radio channel, or demodulate OFDM symbols received through a radio channel, perform channel decryption on them, and deliver them to an upper layer.

[0090] Figure 3 A diagram illustrating a structure of a next-generation mobile communication system to which the present disclosure is applicable is shown.

[0091] refer to Figure 3 As shown in the figure, the radio access network of the new radio (hereinafter referred to as NR or 5G) mobile communication system includes a new radio node B (hereinafter referred to as NR gNB or NR base station) 3-10 and a new radio core network (NR CN) 3-05. A new radio user equipment (hereinafter referred to as NR UE or terminal) 3-15 accesses an external network through the NR gNB 3-10 and NR CN 3-05.

[0092] Figure 3The NR gNB 3-10 in the NR system corresponds to the ENB in ​​the existing LTE system. The NR gNB is connected to the NR UE 3-15 via a radio channel and can provide services superior to those of the existing Node B. In an NR mobile communication system, all user traffic is provided via shared channels. Therefore, equipment is required to aggregate and schedule UE status information (such as buffer status, available transmission power status, and channel status), and the NR NB 3-10 is responsible for this equipment. One NR gNB typically controls multiple cells. It can have a bandwidth equal to or greater than the existing maximum bandwidth to achieve ultra-fast data transmission compared to the current speed of LTE, and OFDM can be used as the radio access technology, allowing beamforming techniques to be additionally combined. In addition, an AMC scheme is applied, so that the modulation scheme and channel coding rate are determined according to the UE's channel status. The NR CN 3-05 performs functions such as mobility support, bearer configuration, and Quality of Service (QoS) configuration. The NR CN is a device that is responsible not only for UE-related mobility management functions but also for various control functions and is connected to multiple base stations. In addition, the NR mobile communication system can interoperate with the existing LTE system, and the NR CN is connected to the MME 3-25 through a network interface. The MME is connected to the ENB 3-30 which is an existing base station.

[0093] Figure 4 A diagram illustrating a wireless protocol structure of an NR mobile communication system to which the present disclosure is applicable is shown.

[0094] refer to Figure 4 The wireless protocols of the NR mobile communication system include NR Service Data Adaptation Protocol (SDAP) 4-01 and 4-45, NR PDCP 4-05 and 4-40, NR RLC 4-10 and 4-35, and NR MAC 4-15 and 4-30 related to UE and NR base station.

[0095] The main functions of NR SDAP 4-01 and 4-45 may include some of the following:

[0096] -Transmission of user plane data

[0097] - Mapping between QoS flows and DRBs for both downlink (DL) and uplink (UL)

[0098] - Marking QoS flow ID in both DL and UL packets

[0099] - Reflective QoS flow to DRB mapping for UL SDAP PDU.

[0100] In conjunction with the SDAP layer device, the RRC message can be used to configure the UE whether to use the header of the SDAP layer device for each PDCP layer device, for each bearer or for each logical channel, or whether to use the function of the SDAP layer device. If the SDAP header has been configured, the NAS QoS reflective configuration one-bit indicator (NASreflective QoS) of the SDAP header and its AS QoS reflective configuration one-bit indicator (AS reflective QoS) can be used to instruct the UE to update or reconfigure the uplink and downlink QoS flows and mapping information about data bearers. The SDAP header may include QoS flow ID information indicating QoS. QoS information can be used as data processing priority, scheduling information, etc. for efficient service support.

[0101] The main functions of NR PDCP 4-05 and 4-40 may include some of the following functions:

[0102] -Header compression and decompression: ROHC only

[0103] -Transmission of user data

[0104] - In-sequence delivery of upper layer PDUs

[0105] - Out-of-order delivery of upper layer PDUs

[0106] - Reordering of received PDCP PDUs

[0107] - Duplicate detection of lower layer SDUs

[0108] -Retransmission of PDCP SDU

[0109] -Encryption and decryption

[0110] - Timer-based SDU discard in uplink.

[0111] The above-mentioned reordering function of the NR PDCP device refers to a function of reordering PDCP PDUs received from the lower layer according to the PDCP sequence number (SN), and may include a function of delivering the reordered data to the upper layer. Alternatively, the reordering function may include a function of directly delivering data without considering the order, a function of recording PDCP PDUs lost due to reordering, a function of reporting the status of the lost PDCP PDUs to the transmitting side, and a function of requesting retransmission of the lost PDCP PDUs.

[0112] The main functions of NR RLC 4-10 and 4-35 may include some of the following functions:

[0113] -Transmission of upper layer PDU

[0114] - In-sequence delivery of upper layer PDUs

[0115] - Out-of-order delivery of upper layer PDUs

[0116] - Error correction via ARQ

[0117] -RLC SDU concatenation, segmentation and reassembly

[0118] - Re-segmentation of RLC data PDUs

[0119] -Reordering of RLC data PDUs

[0120] -Duplicate detection

[0121] -Protocol error detection

[0122] -RLC SDU discarded

[0123] -RLC reconstruction

[0124] The above-mentioned in-sequence delivery function of the NR RLC device refers to a function of delivering the RLC SDU received from the lower layer to the upper layer in sequence. If the original single RLC SDU has been segmented into multiple RLC SDUs (to be received later), the in-sequence delivery function may include a function of reassembling and delivering these RLC SDUs. The in-sequence delivery function may include a function of reordering the received RLC PDUs with reference to the RLC SN or PDCP SN, may include a function of recording the RLC PDUs lost due to reordering, may include a function of reporting the status of the lost RLC PDUs to the transmitting side, and may include a function of requesting retransmission of the lost RLC PDUs. If there is a lost RLC SDU, the in-sequence delivery function may include a function of delivering only the RLC SDUs preceding the lost RLC SDU in sequence to the upper layer. Alternatively, if a predetermined timer has expired even when there is a lost RLC SDU, the in-sequence delivery function may include a function of delivering all RLC SDUs received before the timer has started to the upper layer in sequence. Alternatively, if a predetermined timer has expired even when there are lost RLC SDUs, the in-sequence delivery function may include a function of delivering all RLC SDUs received so far to the upper layer in sequence. In addition, the RLC PDUs can be processed in the order in which they are received (regardless of the order of the sequence numbers, as long as they arrive) and can be delivered to the PDCP device regardless of the order (out-of-order delivery). In the case of segmentation, segments stored in a buffer or to be received later may be received, reconfigured into a single complete RLC PDU, processed, and delivered to the PDCP device. The NR MAC layer may not include a concatenation function, and this function may be performed by the NR MAC layer or replaced by a multiplexing function of the NR MAC layer.

[0125] The out-of-order delivery function of the NR RLC device is a function of immediately delivering the RLC SDU received from the lower layer to the upper layer regardless of the order. If the original single RLC SDU has been segmented into multiple RLC SDUs (to be received later), the out-of-order delivery function may include the function of reassembling and delivering these RLC SDUs. The out-of-order delivery function may include the function of storing the RLC SN or PDCP SN of the received RLC PDU, reordering them, and recording lost RLC PDUs.

[0126] NR MAC 4-15 and 4-30 can be connected to multiple NR RLC layer devices configured in a single UE, and the main functions of NR MAC may include some of the following functions:

[0127] - Mapping between logical channels and transport channels

[0128] -Multiplexing / demultiplexing of MAC SDU

[0129] -Dispatch information report

[0130] - Error correction through HARQ

[0131] - Priority handling between logical channels of a UE

[0132] - Priority handling between UEs through dynamic scheduling

[0133] -MBMS service identifier

[0134] -Transmission format selection

[0135] -filling.

[0136] The NR PHY layers 4-20 and 4-25 may perform operations of channel coding and modulating upper layer data to obtain OFDM symbols and transmit the OFDM symbols through a radio channel, or demodulate OFDM symbols received through a radio channel, perform channel decryption on the OFDM symbols, and deliver them to an upper layer.

[0137] Figure 5 A schematic diagram showing a process of a UE switching from an RRC idle mode to an RRC connected mode and configuring a connection with a network in the present disclosure is shown.

[0138] exist Figure 5 In the RRC Connected Mode, if a UE that is transmitting / receiving data in RRC Connected Mode fails to transmit / receive data for some reason or within a predetermined time, the gNB may send an RRC Connection Release message to the UE, thereby instructing the UE to switch to RRC Idle Mode (5-01). If a UE without a configured connection (hereinafter referred to as an Idle Mode UE) has data to transmit later, the UE performs an RRC Connection Establishment procedure with the gNB. The UE establishes reverse transmission synchronization with the gNB through a random access procedure and sends an RRC Connection Request to the gNB (5-05). This message includes the UE identifier, the reason for configuring the connection (EstablishmentCause), and other information. The gNB sends an RRC Connection Setup message to cause the UE to configure the RRC connection (5-10).

[0139] This message includes configuration information for each service / bearer / RLC device, each logical channel, or each bearer, and may include at least one of the following: whether robust header compression (ROHC) is used for each bearer / logical channel; ROHC configuration information (e.g., ROHC version, initial information, etc.); required status report (statusReportRequired) information (information used by the gNB to instruct the UE to send a PDCP status report); drb-ContinueROHC information (configuration information indicating that ROHC configuration information is maintained and used as is, and this configuration information may be sent when included in the PDCP layer device configuration information (PDCP-config)); etc. In addition, this message contains RRC connection configuration information, etc. A bearer used for an RRC connection is also called a signaling radio bearer (SRB) and is used to transmit / receive RRC messages, which are control messages between the UE and the gNB.

[0140] After configuring the RRC connection, the UE sends an RRC Connection Setup Complete message to the gNB (5-15). This message includes a control message called a SERVICE REQUEST, which the UE uses to request the MME to establish a bearer configuration for the intended service. The gNB sends the SERVICE REQUEST message included in the RRC Connection Setup Complete message to the MME or Access and Mobility Management Function (AMF) (5-20), and the MME or AMF determines whether to provide the service requested by the UE. If it is determined that the service requested by the UE is provided, the MME or AMF sends an INITIAL CONTEXT SETUP REQUEST message to the gNB (5-25). This message includes information such as QoS information to be applied when a data radio bearer (DRB) is configured and security-related information to be applied to the DRB (e.g., security keys, security algorithms).

[0141] In addition, if the gNB fails to receive UE capability information from the MME or AMF, the gNB may send a UE capability information request message to the UE to identify the UE capability information (5-26). Upon receiving the UE capability information request message, the UE may configure and generate a UE capability information message and report it to the gNB (5-27). The UE capability information may include the type of handover method supported by the UE. For example, the UE may report the UE capability to the gNB using a predetermined indicator indicating whether the UE supports the efficient handover method (or Dual Active Protocol Stack (DAPS)) proposed in the present disclosure.

[0142] After identifying UE capability information, when instructing the UE on a handover, the gNB can define an indicator for each handover method, thereby indicating which handover method is being indicated in the Handover Command message. For example, the gNB can indicate the efficient handover method (DAPS handover method) proposed in the present disclosure to the UE. As another method, the gNB can configure the DAPS handover method for each bearer (DRB or SRB) of the UE. When the gNB configures the DPAS handover method for the UE, it can indicate the DPAS handover method along with other handover methods, such as a conditional handover method (multiple target cells are configured; multiple conditions are configured for the UE; and if the conditions are met during the UE's cell selection or reselection process, the UE performs a handover to one of the target cells) or a handover method without a random access procedure (RACH-less), thereby preventing data loss or transmission delays that may occur during the handover. The UE can then perform a handover procedure to the target gNB according to the handover method indicated in the Handover Command message.

[0143] The gNB exchanges a SecurityModeCommand message (5-30) and a SecurityModeComplete message (5-35) to configure security with the UE. After security configuration is complete, the gNB sends an RRCConnectionReconfiguration message (5-40) to the UE.

[0144] This message includes configuration information for each service / bearer / RLC device, each logical channel, or each bearer, and may include at least one of the following: whether ROHC is used for each bearer / logical channel; ROHC configuration information (e.g., ROHC version, initial information, etc.); required status report (statusReportRequired) information (information used by the gNB to instruct the UE to send a PDCP status report); drb-ContinueROHC information (configuration information indicating that ROHC configuration information is maintained and used as is, and this configuration information may be sent when included in the PDCP layer device configuration information (PDCP-config)); etc. In addition, this message contains RRC connection configuration information, etc. A bearer used for an RRC connection is also called an SRB and is used to transmit / receive RRC messages, which are control messages between the UE and the gNB.

[0145] In addition, this message includes configuration information for the DRBs used to handle user data. The UE uses this information to configure the DRBs and sends an RRCConnectionReconfigurationComplete message to the gNB (5-45). After completing DRB configuration with the UE, the gNB sends an Initial Context Setup Complete message to the MME or AMF (5-50). After receiving the Initial Context Setup Complete message, the MME or AMF exchanges an S1 Bearer Setup message and an S1 Bearer Setup Response message to configure the S1 bearer with the S-GW (5-55, 5-60). The S1 bearer establishes a data transmission connection between the S-GW and the gNB and corresponds one-to-one with the DRBs. After the above procedures are completed, the UE sends / receives data with the gNB via the S-GW (5-65, 5-70). Therefore, the normal data transmission process includes the following three steps: RRC connection configuration, security configuration, and DRB configuration. In addition, for some reasons, the gNB may send an RRC Connection Reconfiguration message to the UE to update, add or modify the UE's existing configuration (5-75).

[0146] As used herein, the term "bearer" can be used to encompass both SRBs and DRBs. SRBs refer to signaling radio bearers, while DRBs refer to data radio bearers. SRBs are typically used to transmit and receive RRC messages between RRC layer devices, while DRBs are typically used to transmit and receive user layer data. Furthermore, a UM DRB refers to a DRB used with an RLC layer device operating in unacknowledged mode (UM), while an AM DRB refers to a DRB used with an RLC layer device operating in acknowledged mode (AM).

[0147] Figure 6 A diagram illustrating a signaling procedure for performing handover proposed by the present disclosure in a next generation mobile communication system is shown.

[0148] The UE 6-01 in RRC connected mode reports cell measurement information (measurement report) to the source eNB 6-02 periodically or if a specific event is met (6-05). The source eNB determines whether the UE is to perform handover to a neighboring cell based on the measurement information. Handover refers to a technique of replacing the source eNB that provides service to the UE in connected mode with another eNB (or another cell of the same eNB). After determining the handover, the source eNB sends a handover (HO) request message (e.g., a handover preparation information message) to the new eNB (i.e., the target eNB 6-03) that is assumed to provide service to the UE, thereby requesting the handover (6-10). If the target eNB accepts the handover request, the target eNB sends a HO request confirmation message (e.g., a handover command message) to the source eNB (6-15). After receiving the message, the source eNB sends a HO command message (or an RRC reconfiguration message included in the dedicated control channel (DCCH) of the HO request confirmation message) (6-20). The source eNB extracts the HO command message from the message received from the target eNB, and the HO command message is delivered to the UE by using an RRC Connection Reconfiguration message (6-20).

[0149] The present disclosure proposes a method in which, when the source eNB sends the handover preparation information message 6-10 to the target eNB as described above, and when the target eNB sends the handover command message 6-15 to the source eNB in ​​response thereto, these two messages are used to determine an efficient DAPS handover method.

[0150] The first embodiment of the method for determining an efficient DAPS handover method proposed in the present disclosure is as follows:

[0151] The first embodiment is characterized in that the entity that determines the DAPS handover method is the source eNB. The first embodiment is also characterized in that if the source eNB has requested the target eNB to perform the DAPS handover method, the target eNB always instructs or performs the DAPS handover method.

[0152] -The source eNB may define a new indicator in the Handover Preparation Information message to indicate / request the target eNB that the source eNB will perform the DAPS handover method proposed in this disclosure. The Handover Preparation Information message may include the UE's current bearer configuration information, security key information, cell group configuration information, UE capability information, etc. The source eNB may pre-share the capabilities of the target eNB to pre-identify whether the target eNB supports the DAPS handover method. Alternatively, the source eNB may indicate to the target eNB that the source eNB will perform the DAPS handover method, thereby notifying the target eNB that the source eNB can perform data forwarding quickly or in advance (i.e., advance data forwarding) and may instruct the target eNB to receive the data forwarding and be prepared to process it quickly. The source eNB may send a request for the DAPS handover method to each bearer (DRB or SRB).

[0153] If the target eNB has received a handover preparation information message from the source eNB and has identified an indicator requesting the DAPS handover method, the target eNB may configure an RRC reconfiguration message to be used to indicate handover to the UE, such that the RRC reconfiguration message includes an indicator indicating the DAPS handover method and includes bearer configuration information, security key information, cell group configuration information, or system information necessary for the UE to perform the DAPS handover method. The target eNB may include the configured RRC reconfiguration message in the DL-DCCH message of the handover command message and may deliver it to the source eNB. The target eNB may indicate the DAPS handover method separately for each bearer (DRB or SRB).

[0154] If the source eNB receives the handover command message, it may extract the RRC reconfiguration message included in the handover command message or send an RRC reconfiguration message to the UE to instruct the handover. The source eNB may identify the indicated DAPS handover method for each bearer and perform the DAPS handover method for each bearer (DRB or SRB).

[0155] The second embodiment of the method for determining an efficient DAPS handover method proposed in the present disclosure is as follows:

[0156] The second embodiment is characterized in that the entity that determines the DAPS handover method is the target eNB. The second embodiment is also characterized in that if the source eNB has requested the target eNB to perform the DAPS handover method by using an indicator, the target eNB may reject the request, may accept the request, or may indicate another handover method to the source eNB through a handover command message.

[0157] -The source eNB may define a new indicator in the Handover Preparation Information message to indicate / request the target eNB that the source eNB will perform the DAPS handover method proposed in this disclosure. The Handover Preparation Information message may include the UE's current bearer configuration information, security key information, cell group configuration information, UE capability information, etc. The source eNB may pre-share the capabilities of the target eNB to pre-identify whether the target eNB supports the DAPS handover method. Alternatively, the source eNB may indicate to the target eNB that the source eNB will perform the DAPS handover method, thereby notifying the target eNB that the source eNB can perform data forwarding quickly or in advance (i.e., advance data forwarding) and instructing the target eNB to receive the data forwarding and be ready to process it quickly. The source eNB may send a request for the DAPS handover method to each bearer (DRB or SRB).

[0158] If the target eNB has received a Handover Preparation Information message from the source eNB and has identified an indicator requesting the DAPS handover method, the target eNB may reject or accept the DAPS handover request based on whether the target eNB can support the DAPS handover method, the current transmission resource amount, or the scheduling situation. Alternatively, the target eNB may indicate another handover method. The target eNB may include an indicator rejecting the request for the DAPS handover method, an indicator accepting the request, or an indicator indicating a different type of handover method in the Handover Command message and send it to the source eNB. The target eNB may configure an RRC Reconfiguration message, which will be used to indicate the handover to the UE, such that the RRC Reconfiguration message includes an indicator indicating the DAPS handover method if the DAPS handover method has been accepted, an indicator indicating another handover method if the DAPS handover method has been rejected, and includes bearer configuration information, security key information, cell group configuration information, or system information required for the UE to perform the DAPS handover method or the other handover method. The target eNB may include the configured RRC reconfiguration message in the DL-DCCH message of the handover command message and may pass it to the source eNB. The target eNB may indicate the DAPS handover method separately for each bearer (DRB or SRB).

[0159] If the source eNB receives the Handover Command message, it can identify the indicator included in the Handover Command message to confirm whether the request for the DAPS handover method was accepted or rejected. If the request was accepted, the source eNB can also perform the DAPS handover method and extract the RRC reconfiguration message included in the Handover Command message, or send the RRC reconfiguration message to the UE, thereby instructing the handover. However, if, as a result of identifying the indicator included in the Handover Command message, it is determined that the request for the DAPS handover method was rejected, or if another handover method was indicated, the source eNB can also perform the other handover method indicated by the target eNB. Furthermore, the source eNB can extract the RRC reconfiguration message included in the Handover Command message, or send the RRC reconfiguration message to the UE, thereby instructing the handover. Alternatively, even if the Handover Command message does not include a separate indicator, the source eNB can read the RRC reconfiguration message included in the Handover Command message to identify the handover method indicated by the target eNB and confirm whether the request for the DAPS handover method was accepted or rejected. The source eNB may also perform a handover method (e.g., a DAPS handover method or another handover method) indicated in the RRC reconfiguration message. The source eNB may identify the indicated DAPS handover method for each bearer and thereby perform the DAPS handover method for each bearer (DRB or SRB).

[0160] The third embodiment of the method for determining an efficient DAPS handover method proposed in the present disclosure is as follows:

[0161] The third embodiment is characterized in that the entity that determines the DAPS handover method is the target eNB. The third embodiment is also characterized in that the target eNB identifies the capabilities of the UE and determines the handover method (e.g., the DAPS handover method) based on whether the target eNB can support the DAPS handover method, the current transmission resource amount, or the scheduling situation.

[0162] The source eNB may include the UE's current bearer configuration information, security key information, cell group configuration information, UE capability information, etc. in a Handover Preparation Information message and may send this message to request the target eNB to perform a handover. The source eNB may pre-share the target eNB's capabilities to pre-identify whether the target eNB supports the DAPS handover method. If the target eNB has indicated that it will perform the DAPS handover method, the source eNB may quickly or preemptively forward data (i.e., early data forwarding).

[0163] -The target eNB may receive the handover preparation information message and may determine a handover method (e.g., a DAPS handover method) based on UE capability information, whether the target eNB can support the DAPS handover method, the current transmission resource amount, or scheduling conditions. If the DAPS handover method has been determined, the target eNB may include an indicator indicating the DAPS handover method in the handover command message and may send the handover command. The target eNB may configure an RRC reconfiguration message to indicate handover to the UE if the DAPS handover method has been determined, such that the RRC reconfiguration message includes an indicator indicating the DAPS handover method if another handover method different from the DAPS handover method has been determined, and includes an indicator indicating the other handover method, and includes bearer configuration information, security key information, cell group configuration information, or system information required for the UE to perform the DAPS handover method or the other handover method. The target eNB may include the configured RRC reconfiguration message in a DL-DCCH message of the handover command message and may transmit it to the source eNB. The target eNB can indicate the DAPS handover method individually for each bearer (DRB or SRB).

[0164] If the source eNB receives the handover command message, it may identify the indicator included in the handover command message to confirm whether the DAPS handover method has been determined. If the DAPS handover method has been indicated, the source eNB may also implement the DAPS handover method and may extract the RRC reconfiguration message included in the handover command message or may send the RRC reconfiguration message to the UE to indicate the handover. However, if, as a result of identifying the indicator included in the handover command message, it is determined that the DAPS handover method has not been determined, or if another handover method has been indicated, the source eNB may also implement another handover method indicated by the target eNB. Furthermore, the source eNB may extract the RRC reconfiguration message included in the handover command message or may send the RRC reconfiguration message to the UE to indicate the handover. Alternatively, even if the handover command message does not include a separate indicator, the source eNB may read the RRC reconfiguration message included in the handover command message to identify the handover method indicated by the target eNB and confirm whether the DAPS handover method has been determined. If another handover method has been indicated, the source eNB may also perform the indicated handover method.The source eNB may identify the indicated DAPS handover method for each bearer and thereby perform the DAPS handover method for each bearer (DRB or SRB).

[0165] One or more methods of the first embodiment, the second embodiment, or the third embodiment for determining an efficient DAPS handover method proposed in the present disclosure may be combined and extended to a new embodiment.

[0166] The eNB can indicate the efficient handover method (DAPS handover method) proposed in the present disclosure to the UE through an RRC reconfiguration message. As another method, the eNB can configure the DAPS handover method for each bearer (DRB or SRB) of the UE. For example, the source eNB or the target eNB can define a new indicator indicating the efficient handover method (DAPS handover method) for each bearer identifier or logical channel identifier (LCID) in the bearer configuration information, PDCP configuration information, or RLC configuration information through an RRC message, and can use this indicator to indicate the efficient handover method to the UE for each bearer or each LCID. When the eNB configures the DPAS handover method for the UE, it can indicate the DPAS handover method together with other handover methods (for example, a conditional handover method (multiple target cells are configured; multiple conditions are configured for the UE; and if these conditions are met during the UE's cell selection or reselection process, the UE performs a handover process to one target cell) or a handover method without a random access process), thereby preventing data loss or transmission delay that may occur during the handover.

[0167] If this message is received, the UE stops data transmission / reception with the source eNB or continues data transmission / reception with the source eNB according to the configured handover method, and starts the T304 timer. T304 refers to a timer configured so that if the UE fails to hand over to the target eNB within a predetermined time (for example, if the T304 timer has expired), the original configuration of the UE is restored and the UE switches to the RRC idle state. In addition, the RRC connection reestablishment process can be triggered based on the T304 timer. As another method, if an efficient handover method has been configured and if the connection with the source eNB is valid, the UE can fall back, report the handover failure to the source eNB, and reconfigure the connection. The source eNB transmits the sequence number (SN) status of the uplink / downlink data to each bearer (for example, an RLC UM bearer or an RLC AM bearer), and if downlink data or uplink exists, the sequence number (SN) of the uplink / downlink data is transmitted to the target eNB (6-30, 6-35).

[0168] The UE attempts random access to the target cell indicated by the source eNB (6-40). Random access is both to inform the target cell that the UE is moving to the target cell through handover and to perform uplink synchronization with the target cell. For random access, the UE sends a preamble corresponding to the preamble ID received from the source eNB or a randomly selected preamble ID to the target cell. After sending the preamble, and after a specific number of subframes have passed, the UE monitors whether a random access response (RAR) message is sent from the target cell. The monitoring time interval is called the random access response window (RAR window). If the RAR is received during a specific time period (6-45), the UE sends a HO complete message to the target eNB as an RRC reconfiguration complete message (6-55). If the RAR is successfully received from the target eNB in ​​this way, the UE ends the T304 timer (6-50).

[0169] The target eNB requests the source eNB to perform path modification to modify the path of the configured bearer (6-60, 6-65), and notifies the source eNB to delete the UE context of the UE (6-70). In addition, the target eNB may send an RRC message (e.g., an RRC reconfiguration message) to the UE (6-71), thereby sending an indicator indicating disconnection from the source eNB. As another method, the target eNB may send / indicate MAC control information, RLC control information, or PDCP control information to indicate disconnection from the source eNB. Therefore, the UE attempts to receive data from the target eNB from the start time of the RAR window. After receiving the RAR, the UE sends an RRC reconfiguration complete message and receives downlink transmission resources or uplink transmission resources, thereby starting to send / receive data with the target eNB.

[0170] The present disclosure proposes a seamless handover method, in which data interruption time caused by handover can be minimized or reduced to 0 ms in a next generation mobile communication system.

[0171] The UE can configure multiple first bearers with the source eNB and can send / receive data (send and receive uplink or downlink data) through the corresponding protocol layer device (PHY layer device, MAC layer device, RLC layer device, or PDPC layer device) of each bearer. However, in the following description and drawings, for ease of description, it will be assumed that the UE has one bearer. Obviously, despite such an assumption in the drawings and descriptions, the following description is equally applicable to the case where the UE configures multiple bearers and operates accordingly.

[0172] Figure 7 Detailed steps of a first embodiment of an efficient handover method for minimizing data interruption time caused by handover in the present disclosure are shown.

[0173] according to Figure 7 In a first embodiment of an efficient handover method, in a first step 7-01, UE 7-20 may transmit and receive data with source eNB 7-05. If a handover command message is received from the source eNB, UE 7-20 may disconnect from source eNB 7-05 according to the handover method indicated by the handover command message (e.g., an RRC reconfiguration message), perform a random access procedure to target eNB 7-10, and perform a handover procedure. Alternatively, UE 7-20 may continuously transmit and receive data with source eNB 7-05 to minimize data interruption time during handover according to the handover method indicated by the source eNB.

[0174] Figure 7 A first embodiment of the efficient handover method in is characterized in that when the UE performs a process of random accessing the target eNB 7-10 according to the handover method indicated by the handover command message in the second step 7-02, when the UE sends a preamble, or when the UE first sends data with uplink transmission resources by using physical uplink control channel (PUCCH) or physical uplink shared channel (PUSCH) transmission resources, the UE 7-20 stops sending / receiving data (sending uplink data and receiving downlink data) with the source eNB 7-05.

[0175] Figure 7 The first embodiment of the efficient handover method is characterized in that the UE 7-20 completes the process of random access to the target eNB 7-10 in the third step 7-03, sends a handover completion message to the target eNB 7-10, and starts sending / receiving data (sending uplink data and receiving downlink data) with the target eNB7-10.

[0176] Figure 8 Detailed steps of a second embodiment of an efficient handover method for minimizing data interruption time caused by handover in the present disclosure are shown.

[0177] according to Figure 8 In the second embodiment of the efficient handover method in the present disclosure, in a first step 8-01, UE 8-20 can transmit / receive data with source eNB 8-05. If a handover command message is received from the source eNB 8-05, and if the handover command message has indicated the second embodiment of the efficient handover method proposed in the present disclosure (e.g., the DAPS handover method) or has indicated the second embodiment of the efficient handover method proposed in the present disclosure (e.g., the DAPS handover method) for each bearer, then despite having received the handover command message, UE 8-20 can still continuously transmit / receive data with source eNB 8-05 through protocol layer device 8-22 of the first bearer, so as to minimize the data interruption time occurring during the handover.

[0178] In addition, if the UE has identified an indication of the second embodiment of the efficient handover method proposed in the present disclosure (e.g., the DAPS handover method) from a handover command message included in the RRC layer device, or if the UE has identified an indicator regarding the DAPS handover method for each bearer, the RRC layer device may pass the indicator to the PDCP layer device corresponding to each bearer or bearer for which the DAPS handover method has been indicated. After receiving the indicator, the PDCP layer device switches from the first PDCP layer device structure 9-11 or 9-12 to the second PDCP layer device structure 9-20. The first step 8-01 may refer to a step in which the UE 8-20 receives a handover command message (RRC reconfiguration message) from the source eNB 8-05. In addition, according to the configuration included in the handover command message received by UE 8-20, when switching to the second PDCP layer device structure 9-20, UE 8-20 can pre-configure or establish a protocol layer device (PHY layer device, MAC layer device, RLC layer device or PDCP layer device) 8-21 for the second bearer of the target eNB 8-10, can generate (induce) and update the security key for the target eNB8-10, and can configure the header (or data) compression context for the target eNB 8-10.

[0179] In addition, it will be assumed that UE 8-20 has received a handover command message, and the handover command message has indicated the DAPS handover method proposed by the present disclosure, or has indicated the DAPS handover method for a specific bearer, or has newly configured a PDCP reordering timer value. Then, when switching from the first PDCP layer device structure or function 9-11 or 9-12 to the second PDCP layer device structure or function 9-20 proposed by the present disclosure for each bearer or for a bearer for which the DAPS handover method has been indicated, UE 8-20 can update the parameter for reordering to the "expected next received PDCP sequence number or count value", stop the reordering timer, and restart the timer. In addition, when UE 8-20 has received a handover command message (e.g., an RRC reconfiguration message), the UE's RRC layer device can start a first timer (e.g., T304). If UE 8-20 performs a random access procedure to the target eNB 8-10 to perform handover, and if the random access procedure is successfully completed (e.g., the first condition proposed in the present disclosure is satisfied), the first timer may be stopped. If the handover fails and if the first timer expires, if the connection with the source eNB 8-05 is valid, the UE 8-20 may fall back, report the handover failure to the source eNB 8-05, and attempt connection recovery. If the connection with the source eNB 8-05 is invalid, the UE 8-20 may perform an RRC connection reestablishment procedure.

[0180] The handover command message can be configured and established so that the second bearer has the same bearer identifier as the first bearer, thereby ensuring that no data interruption time occurs for each bearer. In addition, the second embodiment is characterized in that the PDCP layer device of the first bearer and the PDCP layer device of the second bearer operate as a logically single PDCP layer device. Figure 9a and Figure 9b Describe the operation method in more detail.

[0181] Furthermore, in conjunction with the second embodiment, if the UE 8-20 is configured to be able to transmit uplink data to both the source eNB 8-05 and the target eNB 8-10, then in the second embodiment, the uplink data may be transmitted to only one of the source eNB 8-05 and the target eNB 8-10, thereby avoiding the problem of reduced coverage due to a lack of transmission power of the UE 8-20, or the problem of having to determine to which eNB (link selection) a transmission resource request for uplink data will be sent and then the uplink data will be transmitted. Specifically, if the UE 8-20 in the second embodiment cannot simultaneously transmit uplink data to different eNBs at different frequencies or the same frequency (dual uplink transmission is not possible), then the uplink data may be transmitted to only one of the source eNB 8-05 and the target eNB 8-10 in a single time unit. Therefore, UE 8-20 can send a scheduling request to only one of the source eNB 8-05 and the target eNB 8-10, can send a report on the size of the data to be sent by the PDCP layer device (e.g., a buffer status report) to only one of the source eNB 8-05 and the target eNB 8-10, can receive uplink transmission resources, and can send uplink data to only one eNB. In addition, even if a handover command message is received from the source eNB 8-05, UE 8-20 may not initialize the MAC layer device of the first bearer to prevent data loss caused by continued transmission and reception of data due to HARQ retransmission. The second embodiment is also characterized in that, in the case of an AM mode RLC layer device, RLC retransmission can be performed continuously.

[0182] As another method, if the handover command message indicates the second embodiment of the efficient handover method proposed by the present disclosure (DAPS handover method) for each bearer, the UE 8-20 can continuously transmit or receive data with the source eNB 8-05 only for the bearers for which the second embodiment (DAPS handover method) has been indicated by the handover command message, only for the PDCP layer device corresponding to the LCID, only for the RLC layer device or the MAC layer device, or only for the data corresponding to the bearer or the LCID. In addition, even if the first condition proposed by the present disclosure is met (for example, uplink data transmission is switched to the target eNB 8-10), the UE 8-20 can continuously transmit or receive RLC control data (RLC status report), PDCP control data (ROHC feedback or PDCP status report), or HARQ retransmission with the source eNB 8-05 only for the bearers for which the second embodiment (DAPS handover method) has been indicated by the handover command message, only for the PDCP layer device corresponding to the LCID, or only for the RLC layer device or the MAC layer device. In addition, the handover command message indicates or has indicated the second embodiment of the efficient handover method proposed by the present disclosure (DAPS handover method) for each bearer. UE 8-20 stops sending or receiving data with the source eNB 8-05 for the bearer for which the second embodiment (DAPS handover method) is not indicated by the handover command message, for the PDCP layer device corresponding to the LCID, or for the RLC layer device or MAC layer device.

[0183] Combine Figure 8 In a second embodiment of the efficient handover method, even when UE 8-20 performs a random access procedure to the target eNB 8-10 indicated by the handover command message through the protocol layer device of the second bearer in the second step 8-02, UE 8-20 can continuously transmit or receive data (send uplink data or receive downlink data) with the source eNB 8-05 through the protocol layer device of the first bearer. The second step 8-02 may refer to a step in which UE 8-20 performs a cell selection or reselection procedure and performs a random access procedure for the target cell indicated by the handover command message (RRC reconfiguration message) received from the source eNB 8-05.

[0184] Figure 8The second embodiment of the efficient handover method in the embodiment is characterized in that, if the first condition is satisfied in the third step 8-03, the UE 8-20 stops sending uplink data to the source eNB 8-05 via the protocol layer device 8-22 of the first bearer, and sends uplink data to the target eNB 8-10 via the protocol layer device 8-21 of the second bearer. In addition, the UE 8-20 can continuously receive downlink data from the source eNB 8-05 and the target eNB 8-10 via the protocol layer devices of the first bearer and the second bearer.

[0185] The third step 8-03 is a step in which the first condition is met and UE 8-20 switches uplink transmission from source eNB 8-05 to target eNB 8-10. Specifically, until the first condition is met, UE 8-20 transmits uplink data to source eNB 8-05 via the first bearer. If the first condition is met, UE 8-20 stops transmitting uplink data to source eNB 8-05 via the first bearer and begins transmitting uplink data to target eNB 8-10 via the second bearer.

[0186] Specifically, in combination with the second PDCP layer device structure 9-20 proposed in the present disclosure, if the first condition is met, the PDCP layer device that has sent uplink data through the first bearer receives an indicator from the lower layer device (if the random access process from the MAC layer device to the target eNB has been successful) or from the upper layer device (if the first timer in the RRC layer device has expired). Then, the PDCP layer device can stop sending uplink data through the first bearer and can switch to start sending uplink data through the second bearer. In addition, as in Figure 9a and Figure 9b In the PDCP layer device structure proposed in

[15] , the receiving PDCP layer device 8-21 of the second bearer is driven integrally with the receiving PDCP layer device 8-22 of the first bearer, and seamless data reception from the source eNB 8-05 or the target eNB 8-10 can be continuously performed by using information such as stored transmit / receive data, sequence number information, and header compression and decompression context. The first condition can be one of the following conditions. The first condition proposed below provides an uplink data transmission switching time point configured to most efficiently use transmission resources and minimize data interruption time.

[0187] -If UE 8-20 has successfully completed the process of random access to the target eNB 8-10 through the layer device of the second bearer (e.g., MAC layer device), if UE 8-20 has successfully completed the process of random access to the target eNB 8-10 through the layer device of the second bearer (e.g., MAC layer device) and has received the first uplink transmission resource allocated to UE 8-20 from the target eNB 8-10, or if the uplink transmission resource has been first indicated to UE 8-20, it can be determined that the first condition is met.

[0188] *For example, more specifically, if the UE 8-20 has received a handover command message from the source eNB 8-05, if the UE 8-20 has been instructed to randomly access the target eNB 8-10, and if the instructed random access is a contention-free random access (CFRA) (for example, if a pre-designated preamble or UE cell identifier (for example, a cell radio network temporary identifier (C-RNTI)) has been assigned to the UE 8-20)

[0189] **When UE 8-20 has sent a pre-designated preamble to the cell of target eNB 8-10 and has received a RAR message, the random access procedure can be considered to have been successfully completed. Therefore, if UE 8-20 has received the first uplink transmission resource allocated, included, or indicated by the RAR message, it can be determined that the first condition is met. Alternatively, if the uplink transmission resource is first received after receiving the RAR message, it can be determined that the first condition is met.

[0190] *If the UE 8-20 has received a Handover Command message from the source eNB 8-05, if the UE 8-20 has been instructed to randomly access the target eNB 8-10, and if the instructed random access is Contention Based Random Access (CBRA) (e.g., if a pre-designated preamble or UE cell identifier (e.g., C-RNTI) has not been assigned to the UE 8-20)

[0191] **If the UE 8-20 has transmitted a preamble (e.g., any preamble) to the cell of the target eNB 8-10, has received a RAR message, has transmitted message 3 (e.g., a handover complete message) by using the uplink transmission resources allocated, included, or indicated by the RAR message, and has received a MAC CE indicating that contention has been resolved (i.e., a contention resolution MAC CE) from the target eNB through message 4, or has received uplink transmission resources through a PDCCH corresponding to the C-RNTI of the UE 8-20, then it can be considered that the UE 8-20 has successfully completed the procedure for random access to the target eNB 8-10. Therefore, if the UE 8-20 first monitors the PDCCH thereafter and first receives uplink transmission resources (or first receives an indication of uplink transmission resources) through the PDCCH corresponding to the C-RNTI of the UE, it can be determined that the first condition is met. Alternatively, if the uplink transmission resource allocated by the RAR message is large enough so that UE 8-20 can transmit uplink data after sending message 3, it can be determined that the first uplink transmission resource has been received and the first condition has been met. That is, if the RAR is received, it can be determined that the first uplink transmission resource has been received and the first condition has been met.

[0192] - If the handover command message received by the UE 8-20 also indicates a handover method that does not require a random access procedure (RACH-less handover),

[0193] *If the handover command message includes uplink transmission resources about the target eNB,

[0194] **If UE 8-20 transmits message 3 (e.g., a handover complete message or an RRC reconfiguration complete message) by using the uplink transmission resources of the target eNB, and if UE 8-20 receives a UE identity confirmation MACCE from the eNB through message 4, or if UE 8-20 receives uplink transmission resources through a PDCCH corresponding to the UE's C-RNTI, UE 8-20 may determine that the random access procedure has been successfully completed and the first condition is satisfied. As another method, if, after the random access procedure is successfully completed, a first uplink transmission resource is received through a PDCCH corresponding to the UE's C-RNTI as a result of PDCCH monitoring, UE 8-20 may determine that the first condition is satisfied.

[0195] *If the handover command message does not include uplink transmission resources regarding the target eNB 8-10,

[0196] **If, as a result of PDCCH monitoring regarding the target eNB (or cell), uplink transmission resources are received through the PDCCH corresponding to the UE's C-RNTI, if the UE 8-20 sends Message 3 (e.g., a Handover Complete message or an RRC Reconfiguration Complete message) by using the uplink transmission resources and receives a UE Identity Confirmation MAC CE from the eNB, or if the UE 8-20 receives uplink transmission resources through the PDCCH corresponding to the UE's C-RNTI, the UE 8-20 may determine that the random access procedure has been successfully completed and the first condition is satisfied. As another method, if, after the random access procedure is successfully completed, as a result of PDCCH monitoring, a first uplink transmission resource is received through the PDCCH corresponding to the UE's C-RNTI, the UE 8-20 may determine that the first condition is satisfied.

[0197] Hereinafter, in conjunction with the DAPS handover method proposed by the present disclosure, a method for efficiently switching uplink data from the source eNB 8-05 to the target eNB 8-10 will be proposed. Whether the first condition is satisfied can be confirmed or detected by the MAC layer device (or RRC layer device) of the target eNB corresponding to the second bearer according to one of the following methods, which can be combined and expanded into a new method.

[0198] -First method: For example, if the RRC reconfiguration message received by the UE 8-20 indicates DAPS handover, the UE 8-20 may configure a MAC layer device for the target eNB 8-10 corresponding to the second bearer, and the MAC layer device may perform a random access procedure and may confirm whether the first condition is met. If the first condition is met, in combination with the DAPS handover method proposed in the present disclosure, the MAC layer device may send an indicator to an upper layer device (e.g., a PDCP layer device) to instruct it to switch uplink data transmission via the first bearer from the source eNB to the target eNB via the second bearer.

[0199] -Second method: As another method, for example, if the RRC reconfiguration message received by UE 8-20 indicates DAPS handover, UE 8-20 may configure a MAC layer device for the target eNB 8-10 corresponding to the second bearer, and the MAC layer device may perform a random access procedure and confirm whether the first condition is satisfied. If the first condition is satisfied, the MAC layer device may indicate to the upper layer device (e.g., RRC layer device) that the first condition is satisfied. In addition, in conjunction with the DAPS handover method proposed in the present disclosure, the upper layer device (e.g., RRC layer device) may send an indicator to the lower layer device (e.g., PDCP layer device) to instruct it to switch uplink data transmission over the first bearer from the source eNB 8-05 to the target eNB 8-10 over the second bearer. If the first condition is satisfied, or if the random access procedure to the target eNB 8-10 is successfully performed, the upper layer device (e.g., RRC layer device) stops the first timer. To this end, if the first timer is stopped, the RRC layer device may send an indicator to the PDCP layer device to instruct it to switch uplink data transmission.

[0200] -Third method: If the RRC reconfiguration message received by UE 8-20 indicates DAPS handover, UE 8-20 may configure a MAC layer device for the target eNB 8-10 corresponding to the second bearer. If the RRC layer device of UE 8-20 has sent an indicator to the lower layer device (e.g., MAC layer device) to indicate that it will perform DAPS handover, the MAC layer device may perform a random access procedure and may confirm whether the first condition is met. In conjunction with the DAPS handover method proposed in the present disclosure, if the first condition is met, the MAC layer device may send an indicator to the upper layer device (e.g., PDCP layer device) to indicate that it will switch uplink data transmission via the first bearer from the source eNB 8-05 to the target eNB 8-10 via the second bearer.

[0201] - Fourth method: As another method, if the RRC reconfiguration message received by UE 8-20 indicates DAPS handover, UE 8-20 may configure a MAC layer device for the target eNB 8-10 corresponding to the second bearer. If the RRC layer device of UE 8-20 has sent an indicator to a lower layer device (e.g., a MAC layer device) to indicate that it will perform a DAPS handover, the MAC layer device may perform a random access procedure and may confirm whether a first condition is satisfied. If the first condition is satisfied, the MAC layer device may indicate to the upper layer device (e.g., an RRC layer device) that the first condition is satisfied. After recognizing the indicator, the upper layer device (e.g., an RRC layer device) may stop the first timer, as the first timer is stopped if the first condition proposed in the present disclosure is satisfied or if the random access procedure to the target eNB is successfully performed. In addition, in conjunction with the DAPS handover method proposed in the present disclosure, the upper layer device (e.g., an RRC layer device) may send an indicator to the lower layer device (e.g., a PDCP layer device) to indicate that it will switch uplink data transmission via the first bearer from the source to the target via the second bearer.

[0202] According to the first, second, third, or fourth methods, if a PDCP layer device receives an indicator indicating that a first condition is satisfied, or an indicator indicating uplink data transmission from a source eNB to a target eNB (e.g., if a DAPS handover method is indicated), from an upper layer device (e.g., an RRC layer device) or a lower layer device (e.g., a MAC layer device), the PDCP layer device may perform the following protocol layer device operations to effectively switch uplink data transmission, and may perform one or more of the following operations to prevent data loss caused by uplink data transmission. The following operations may be applied to a PDCP layer device connected to an AM DRB or a UM DRB (an RLC layer device operating in AM mode or an RLC layer device operating in UM mode). If data to be transmitted exists in its buffer before the first condition is satisfied or before receiving the indicator indicating that the first condition is satisfied, the PDCP layer device may indicate the size or amount of data to be transmitted (e.g., the PDCP data amount) to the MAC layer device of the first bearer for the source eNB, thereby notifying the MAC layer device of the presence of data to be transmitted, and may transmit the uplink data to the source eNB. The MAC layer device for the first bearer of the source eNB may then perform a scheduling request or buffer status report procedure to receive the uplink transmission resources allocated to it from the source eNB. However, if the first condition is met, or if an indicator indicating that the first condition is met is received, the PDCP layer device may switch the uplink data transmission to the target eNB 8-10 in the following manner:

[0203] -The PDCP layer device may indicate that the size or amount of data to be transmitted to the MAC layer device for the first bearer of the source eNB 8-05 is zero (or that there is no data to be transmitted to the MAC layer device for the first bearer of the source eNB 8-05), in order to switch uplink data transmission from the first bearer for the source eNB 8-05 to the second bearer for the target eNB 8-10. That is, the PDCP layer device may indicate to the MAC layer device for the first bearer that the PDCP data amount is zero, thereby notifying that there is no more data to be transmitted (even if there is actually data to be transmitted in the buffer, the PDCP layer device may indicate that there is no data to be transmitted to the MAC layer device for the first bearer of the source eNB, in order to switch uplink data transmission). However, if the handover method (DAPS handover method) of the second embodiment of the present disclosure is indicated as proposed in the present disclosure, or in the case of a bearer for which the handover method (DAPS handover method) of the second embodiment of the present disclosure is indicated, and if RLC control data (RLC status report) or PDCP control data (PDCP status report or ROCH feedback) is generated, the UE 8-20 can indicate the amount of data corresponding to the RLC control data or PDCP control data to the MAC layer device, and can send data to the source eNB.

[0204] - The PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode) discards all stored PDCP PDUs (e.g., PDCP SDUs are not discarded to prevent loss of original data) and, starting with the first data (e.g., PDCP SDU) for which its successful delivery is not confirmed from the lower layer (e.g., the RLC layer device corresponding to the first bearer for the source eNB 8-05), performs a new header compression procedure based on the header context for the target eNB 8-10 for data (PDCP SDUs in the buffer) arranged in ascending order according to the COUNT value (or PDCP sequence number) assigned before the first condition is satisfied or before the indicator indicating that the first condition is satisfied is received. The PDCP layer device applies the security key for the target eNB 8-10, thereby re-performing the integrity procedure or ciphering procedure, configuring the PDCP header, and delivering it to the lower layer device (RLC layer device for the second bearer of the target eNB), thereby performing retransmission or transmission. In other words, the PDCP layer device performs cumulative retransmission starting with the first data for which its successful delivery is not confirmed. As another method, when performing retransmission, the PDCP layer device may perform retransmission only for data from a lower layer (eg, an RLC layer device for the first bearer of the source eNB 8-05) whose successful delivery has not been confirmed.

[0205] More specifically, the PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode) discards all stored PDCP PDUs to be transmitted to the source eNB 8-05 via the first protocol layer device already connected to the PDCP layer device (e.g., to prevent loss of original data, the PDCP SDU is not discarded). The PDCP layer device performs a new header or data compression process by applying a header compression (or data compression) protocol context (or security key) corresponding to the target eNB 8-10 based on a COUNT value (or PDCP sequence number) assigned before the first condition is satisfied or before the indicator indicating that the first condition is satisfied is received, only for data (e.g., PDCP SDUs) from the lower layer (which is the first protocol layer device for the source eNB 8-05, such as the RLC layer device) whose successful delivery has not been confirmed. The PDCP layer device re-performs an integrity process or a ciphering process, configures a PDCP header, and transmits it to the lower layer device (which is the second protocol layer device for transmitting the PDCP header to the target eNB 8-10), thereby performing retransmission or transmission. That is, in order to prevent transmission resources from being wasted, the PDCP layer device may perform selective retransmission only for data whose successful delivery has not been confirmed.

[0206] As another method, a transmission or retransmission operation may be performed after releasing the lower layer (e.g., a transmitting or receiving RLC layer device or a MAC layer device) which is the first protocol layer device for sending data to the source eNB 8-05. If the transmission or retransmission operation is extended to the UM DRB, the PDCP layer device connected to the RLC layer device operating in UM mode may consider data that has not yet been delivered to the lower layer device, data for which the PDCP discard timer has not expired, or data to which a PDCP sequence number (or COUNT value) has been assigned as data received from the upper layer device or newly received data. Without restarting the PDCP discard timer for each piece of data, the PDCP layer device may perform header (or data) compression on the data by using the header (or data) compression context (or security key) for the target eNB 8-10. Alternatively, the PDCP layer device may perform a ciphering or integrity protection process, may generate a PDCP header, may concatenate the header, and may perform transmission or retransmission. The PDCP layer device may process data in ascending order of the COUNT value assigned before this procedure is triggered, and may perform transmission or retransmission. In addition, the window state parameter of the PDCP layer device connected to the UM DRB or AM DRB is not initialized but used without change.

[0207] If there is data to be transmitted in the buffer, the PDCP layer device may indicate the size or amount of data to be transmitted (e.g., PDCP data amount) to the MAC layer device of the second bearer for the target eNB 8-10, thereby notifying the presence of data to be transmitted and performing uplink data transmission switching to the target eNB 8-10. The MAC layer device of the second bearer for the target eNB 8-10 may then perform a scheduling request or buffer status report procedure to receive uplink transmission resources allocated from the target eNB 8-10.

[0208] -If an indicator indicating that the first condition is satisfied is received from an upper layer device (e.g., an RRC layer device) or from a lower layer device (e.g., a MAC layer device), the LTE or NR PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode) may switch uplink data transmission to a second bearer (e.g., an RLC layer device or a MAC layer device) for the target eNB 8-10, and may send an indicator to the lower layer device (e.g., an RLC layer device or a MAC layer device) of the first bearer for the source eNB 8-05 to instruct it to discard data (e.g., PDCP data (PDCP PDU)). This is because for AM DRB switching uplink data transmission, data whose successful delivery is not confirmed is retransmitted through the second bearer for the target eNB 8-10, and for this reason, data transmission to the source eNB 8-05 through the first bearer is unnecessary if continued, and thus transmission resources will be wasted. When sending an indicator to the lower layer device to instruct it to discard data, the PDCP layer device may send the discard indicator only for PDCP user data (PDCP data PDU) and not for PDCP control data (PDCP control PDU, such as PDCP status report or ROHC feedback). This is because PDCP control data (PDCP control PDU, such as PDCP status report or ROHC feedback) is information that needs to be sent in order to continuously receive downlink data from the source eNB 8-05 even after the first condition is met, and if this data is lost, downlink data transmission may be erroneous.

[0209] An LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode) may switch uplink data transmission to a second bearer (e.g., an RLC layer device or a MAC layer device) for the target eNB 8-10 and may send an indicator to a lower layer device (e.g., an RLC layer device or a MAC layer device) to instruct it to discard all PDCP user data (PDCP data PDUs) stored or saved by it except for PDCP control data (PDCP control PDUs, e.g., PDCP status reports or ROHC feedback).

[0210] As another method, an LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode) may switch uplink data transmission to a second bearer (e.g., an RLC layer device or a MAC layer device) for the target eNB 8-10, and may send an indicator (e.g., an RLC layer device or a MAC layer device) to a lower layer device to instruct it to discard PDCP user data (PDCP data PDU) other than PDCP control data (PDCP control PDU, e.g., PDCP status report or ROHC feedback) that has not been successfully delivered or that has not been stored or retained by it. The NR PDCP layer device may be connected to an LTE RLC layer device or an NR RLC layer device to transmit or receive data, and the LTE PDCP layer device may be connected only to an LTE RLC layer device to transmit or receive data. Therefore, specific operations are proposed as follows:

[0211] *If an indicator indicating discard of data (e.g., PDCP user data) has been received from an LTE or NR PDCP layer device, and if the RLC layer device that has received the discard indicator is an LTE RLC layer device,

[0212] **If part of the user data (PDCP PDU, PDCP Data PDU, or RLC SDU) received from an upper layer device (PDCP layer device) has not been mapped to RLC user data (RLC Data PDU) or has not been generated as RLC user data, the LTE PDCP layer device discards the user data. Therefore, if part of the user data has been mapped to RLC user data (RLC Data PDU) or has been generated as RLC user data, the LTE PDCP layer device can transmit the data to the source eNB without discarding the user data.

[0213] *If an indicator indicating discard of data (e.g., PDCP user data) has been received from an LTE or NR PDCP layer device, and if the RLC layer device that has received the discard indicator is an NR RLC layer device,

[0214] **If the user data (PDCP PDU, PDCP data PDU or RLC SDU) received from the upper layer device (PDCP layer device) or part of the user data is neither delivered nor sent to the lower layer device, the NR RLC layer device discards the user data. Therefore, if the user data or part of the user data has been delivered or sent to the lower layer device, the NR RLC layer device can send the data to the source eNB without discarding the user data. Therefore, unlike the LTE RLC layer device, the NR RLC layer device can discard more data because even if the user data has been generated as RLC user data, it can be discarded as long as it has not been delivered to the lower layer device. In addition, unnecessary data transmission can be prevented more efficiently.

[0215] - The process of sending a data discard indicator from an LTE or PDCP layer device connected to an AM DRB to a lower layer device may not be applied to a UM DRB to prevent transmission delay or data loss. This is because a UM DRB may not perform a retransmission process.

[0216] -However, as another method, the process in which the LTE or PDCP layer device connected to the AM DRB sends a data discard indicator to the lower layer device can be extended and applied to the UM DRB to minimize uplink data transmission to the source eNB after uplink data transmission switching. For example, if an indicator indicating that the first condition is satisfied is received from an upper layer device (RRC layer device) or a lower layer device (MAC layer device), the LTE or NR PDCP layer device connected to the UM DRB (RLC layer device operating in UM mode) can switch uplink data transmission to the second bearer (e.g., RLC layer device or MAC layer device) for the target eNB 8-10, and can send an indicator to the lower layer device (e.g., RLC layer device or MAC layer device) of the first bearer for the source eNB 8-05 to instruct it to discard data (e.g., PDCP data (PDCP PDU)). This is because, if uplink data transmission is switched to the UM DRB and if data transmission to the source eNB 8-05 through the first bearer is minimized, transmission delay caused by forwarding data from the source eNB 8-05 to the target eNB 8-10 can be prevented, and the source eNB 8-05 can be prevented from wasting transmission resources. When an indicator is sent to a lower layer device to instruct it to discard data, the PDCP layer device can send a discard indicator only for PDCP user data (PDCP data PDU) and not for PDCP control data (PDCP control PDU, for example, PDCP status report or ROHC feedback). This is because PDCP control data (PDCP control PDU, for example, PDCP status report or ROHC feedback) is information that needs to be sent even after the first condition is met in order for the UE 8-20 to continuously receive downlink data from the source eNB 8-05, and if it is lost, downlink data transmission may be erroneous.

[0217] An LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode) may switch uplink data transmission to a second bearer (e.g., an RLC layer device or a MAC layer device) for the target eNB 8-10, and may send an indicator to a lower layer device (e.g., an RLC layer device or a MAC layer device) of a first bearer for the source eNB 8-05 to instruct it to discard all PDCP user data (PDCP data PDUs) stored or saved by it except for PDCP control data (PDCP control PDUs, e.g., PDCP status reports or ROHC feedback).

[0218] As another method, an LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode) may switch uplink data transmission to a second bearer (e.g., an RLC layer device or a MAC layer device) for the target eNB 8-10, and may send an indicator to a lower layer device (e.g., an RLC layer device or a MAC layer device) of a first bearer for the source eNB 8-05 to instruct it to discard PDCP user data (PDCP data PDUs) other than PDCP control data (PDCP control PDUs, e.g., PDCP status reports or ROHC feedback) that has been transferred from the PDCP layer device to the lower layer device, is not actually sent by the lower layer device, has a PDCP discard timer for which has not expired, or is stored or retained. The NR PDCP layer device may be connected to an LTE RLC layer device or an NR RLC layer device to send or receive data, and the LTE PDCP layer device may be connected only to an LTE RLC layer device to send or receive data. Therefore, specific operations are proposed as follows:

[0219] *If an indicator indicating discard of data (e.g., PDCP user data) has been received from an LTE or NR PDCP layer device, and if the RLC layer device that has received the discard indicator is an LTE RLC layer device,

[0220] **If a portion of user data (PDCP PDU, PDCP Data PDU, or RLC SDU) received from an upper layer device (PDCP layer device) has not been mapped to RLC user data (RLC Data PDU) or has not been generated as RLC user data, the LTE RLC layer device discards the user data. Therefore, if a portion of user data has been mapped to RLC user data (RLC Data PDU) or has been generated as RLC user data, the LTE RLC layer device can transmit the data to the source eNB without discarding the user data.

[0221] *If an indicator indicating discard of data (e.g., PDCP user data) has been received from an LTE or NR PDCP layer device, and if the RLC layer device that has received the discard indicator is an NR RLC layer device,

[0222] **If the user data (PDCP PDU, PDCP data PDU or RLC SDU) received from the upper layer device (PDCP layer device) or part of the user data is neither delivered nor sent to the lower layer device, the NR RLC layer device discards the user data. Therefore, if the user data or part of the user data has been delivered or sent to the lower layer device, the NR RLC layer device can send the data to the source eNB without discarding the user data. Therefore, unlike the LTE RLC layer device, the NR RLC layer device can discard more data because even if the user data has been generated as RLC user data, it can be discarded as long as it has not been delivered to the lower layer device. In addition, unnecessary data transmission can be prevented more efficiently.

[0223] In conjunction with the second embodiment of the efficient handover method proposed in the present disclosure (e.g., the DAPS handover method), even after receiving a handover command message (e.g., an RRC reconfiguration message), the UE 8-20 can continuously receive downlink data from the source eNB 8-05 or the target eNB 8-10 through the protocol layer device of the first bearer for the source eNB 8-05 or the second bearer for the target eNB 8-10. In addition, for the AM bearer, transmission can be allowed so that the RLC status report (which is not data) can be continuously sent uplink to the source eNB (or target eNB) through the protocol layer device of the first bearer (or second bearer) to ensure that downlink data can be efficiently received from the source eNB (or target eNB) or that the source eNB (or target eNB) can efficiently send downlink data. That is, even if the UE has already handed over uplink data transmission to the target eNB based on the first condition being satisfied, data transmission may be allowed through the first bearer for the source eNB if an RLC status report, HARQ ACK, NACK, or PDCP control data (PDCP ROHC feedback or PDCP status report) needs to be sent to the source eNB. This is because, in the case of an AM bearer, if successful delivery is not indicated by an RLC status report after data is sent to the transmitting end (i.e., if no RLC status report is received), data cannot be sent thereafter.

[0224] Specifically, combined Figure 8In the second embodiment of the efficient handover method, it is assumed that, because the first condition is satisfied, in the third step (8-03), UE 8-20 has stopped sending uplink data to the source eNB through the protocol layer device 8-22 of the first bearer and has been switched to start sending uplink data to the target eNB through the protocol layer device 8-21 of the second bearer. Even in this case, UE 8-20 can continuously send HARQ ACK or HARQ NACK information, RLC status report (ACK or NACK information), or PDCP control data (e.g., PDCP status report or PDCP ROHC feedback information) through the protocol layer device of the first bearer (or the second bearer), so that downlink data can be efficiently received from the source eNB (or the target eNB), or the source eNB (or the target eNB) can efficiently send downlink data.

[0225] In addition, combined Figure 8 In the second embodiment of the efficient handover method, it is assumed that since the first condition is met, in the third step (8-03), the UE 8-20 has stopped sending uplink data to the source eNB through the protocol layer device 8-22 of the first bearer, and has switched to start sending uplink data to the target eNB through the protocol layer device 8-21 of the second bearer. Even in this case, the UE 8-20 can continue the data transmission caused by the HARQ retransmission of the MAC device or the retransmission caused by the AM mode RLC layer device to ensure that no data is lost to the source eNB 8-05. In addition, in combination with Figure 8In the second embodiment of the efficient handover method in FIG, it is assumed that, because the first condition is satisfied, in the third step (8-03), UE 8-20 has stopped transmitting uplink data to source eNB 8-05 via protocol layer device 8-22 of the first bearer and has switched to start transmitting uplink data to target eNB 8-10 via protocol layer device 8-21 of the second bearer. In this case, source eNB 8-05 or target eNB 8-10 can divide time and allocate transmission resources to UE 8-20 so that the uplink transmission resources to target eNB 8-10 and the uplink transmission resources to source eNB 8-05 do not conflict. If the uplink transmission resources to the target eNB 8-10 and the uplink transmission resources to the source eNB 8-05 conflict and overlap, the UE 8-20 may prioritize the uplink transmission resources to the source eNB 8-05, thereby transmitting data to the source eNB, in order to maintain downlink data transmission from the source eNB 8-05 or to continuously receive downlink data transmission without any problems. Alternatively, if the uplink transmission resources to the target eNB 8-10 and the uplink transmission resources to the source eNB 8-05 conflict and overlap, the UE 8-20 may prioritize the uplink transmission resources to the target eNB 8-10, thereby transmitting data to the target eNB 8-10, in order to maintain downlink data transmission from the target eNB 8-10.

[0226] Specifically, if UE 8-20 has received a handover command message and a handover corresponding to the second embodiment of the present disclosure (e.g., the DAPS handover method) has been indicated, or a handover corresponding to the second embodiment of the present disclosure (e.g., the DAPS handover method) has been indicated for each bearer, UE 8-20 or the bearer for which the DAPS handover method has been indicated may perform a scheduling request through a first protocol layer device, may send a buffer status report to the source eNB 8-05, may receive uplink transmission resources, may send uplink data, and may receive downlink data from the source eNB 8-05 until the first condition is met. However, if the first condition is met, UE 8-20 may no longer send data to the source eNB 8-05, may switch the uplink, may perform a scheduling request through a second protocol layer device, may send a buffer status report to the target eNB 8-10, may receive uplink transmission resources, and may send uplink data to the target eNB 8-10. However, UE 8-20 can continuously receive downlink data from source eNB 8-05, and even after uplink transmission switching, UE 8-20 can continuously send HARQ ACK or HARQ NACK corresponding to the downlink data, RLC status report or PDCP control data (e.g., PDCP status report or ROHC feedback information). In addition, even if the first condition is met, UE 8-20 can continuously receive downlink data from source eNB 8-05 or target eNB 8-10.

[0227] Figure 8 A second embodiment of the efficient handover method in

[0065] is characterized in that, if a second condition is satisfied, then in fourth step 8-04, UE 8-20 stops receiving downlink data from source eNB 8-05 via protocol layer device 8-22 of the first bearer or disconnects from source eNB 8-05. The second condition can be one of the following conditions. Furthermore, PDCP layer device 8-21 of the second bearer can continue seamless data transmission or reception with target eNB 8-10 by using information stored in PDCP layer device 8-22 of the first bearer, such as transmitted or received data, sequence number information, and header compression and decompression context.

[0228] If the UE has performed a procedure of randomly accessing the target eNB 8-10 through the layer device 8-21 of the second bearer and has received the RAR, it may be determined that the second condition is satisfied.

[0229] -If the UE has performed a random access procedure to the target eNB 8-10 through the layer device of the second bearer, has received the RAR, has configured a handover complete message, and has sent the handover complete message to the target eNB 8-10, it can be determined that the second condition is met.

[0230] -If the UE has completed the process of random accessing the target eNB 8-10 through the layer device of the second bearer, and has first sent data by using PUCCH or PUSCH uplink transmission resources, or has first received PUCCH or PUSCH uplink transmission resources, it can be determined that the second condition is met.

[0231] - If the eNB has configured a separate timer for the UE 8-20 through an RRC message, and if the timer has expired, it can be determined that the second condition is met.

[0232] *The timer may be started if the UE has received a handover command message from the source eNB 8-05, if the UE has started random access to the target eNB 8-10 (having sent a preamble), if the UE has received a RAR from the target eNB 8-10, if the UE has sent a handover complete message to the target eNB 8-10, or if the UE has first sent data by using PUCCH or PUSCH uplink transmission resources.

[0233] -If UE 8-20 has performed a random access procedure to the target eNB 8-10 through the layer device of the second bearer, has received the RAR, has configured a handover completion message, and has sent the handover completion message to the target eNB 8-10, and if successful delivery of the handover completion message has been confirmed by the MAC layer device (HARQ ACK) or the RLC layer device (RLCACK), it can be determined that the second condition is met.

[0234] -If UE 8-20 has performed a random access process to the target eNB 8-10 through the layer device of the second bearer and has received an RAR, or has configured a handover completion message, has sent the handover completion message to the target eNB 8-10, and has first received uplink transmission resources allocated from the target eNB 8-10, or if uplink transmission resources have first been indicated, it can be determined that the second condition is met.

[0235] When the source eNB performs the efficient handover proposed by the present disclosure, the source eNB may determine when to stop sending downlink data to the UE 8-20 or when to disconnect from the UE 8-20. For example, the source eNB may determine when to stop sending downlink data to the UE 8-20 or when to disconnect from the UE 8-20 in a predetermined manner (e.g., when a predetermined timer has expired (the timer may be started after the handover is instructed), or when the source eNB 8-05 has received an indication from the target eNB 8-10 that the UE 8-20 has been successfully handed over to the target eNB 8-10). In addition, if no downlink data is received from the source eNB 8-05 within a predetermined time, the UE 8-20 may determine that the second condition is satisfied, may determine that the UE 8-20 has been disconnected from the source eNB 8-05, and may disconnect from it.

[0236] -If the UE has received an instruction (e.g., an RRC message (e.g., RRC configuration message), MAC CE, RLC control PDU, or PDCP control PDU) from the target eNB 8-10 to disconnect from the source eNB 8-05, it can be determined that the second condition is met.

[0237] -If the UE 8-20 has received an instruction to disconnect from the source eNB 8-05 (e.g., an RRC message (e.g., an RRC configuration message), a MAC CE, an RLC control PDU, or a PDCP control PDU) from the source eNB 8-05, it can be determined that the second condition is met.

[0238] -If the UE 8-20 fails to receive downlink data from the source eNB 8-05 within a predetermined time, it can be determined that the second condition is met.

[0239] -If UE 8-20 has successfully completed the process of random access to the target eNB 8-10 through the layer device of the second bearer (for example, the MAC layer device), if UE 8-20 has successfully completed the process of random access to the target eNB 8-10 through the layer device of the second bearer and has received the first uplink transmission resource allocated from the target eNB 8-10, or if the uplink transmission resource has been first indicated to UE 8-20, it can be determined that the second condition is met.

[0240] *For example, more specifically, if the UE 8-20 has received a handover command message from the source eNB 8-05, if the UE 8-20 has been instructed to randomly access the target eNB 8-10, and if the instructed random access is a contention-free random access (CFRA) (for example, if a pre-designated preamble or UE cell identifier (e.g., C-RNTI) has been allocated thereto)

[0241] **When UE 8-20 has sent a pre-assigned preamble to the cell of target eNB 8-10 and has received a RAR message, the random access procedure can be considered to have been successfully completed. Therefore, if UE 8-20 has received the first uplink transmission resource allocated, included, or indicated by the RAR message, it can be determined that the second condition is met. Alternatively, if the uplink transmission resource is first received after receiving the RAR message, it can be determined that the second condition is met.

[0242] * If the UE 8-20 has received a Handover Command message from the source eNB 8-05, if the UE 8-20 has been instructed to randomly access the target eNB 8-10, and if the instructed random access is Contention Based Random Access (CBRA) (e.g., if a pre-designated preamble or UE cell identifier (e.g., C-RNTI) has not been allocated to it)

[0243] **If the UE 8-20 has transmitted a preamble (e.g., any preamble) to the cell of the target eNB 8-10, has received a RAR message, has transmitted message 3 (e.g., a handover complete message) by using the uplink transmission resources allocated, included, or indicated by the RAR message, and has received a MAC CE (contention resolution MAC CE) indicating that contention has been resolved from the target eNB 8-10 through message 4, or has received uplink transmission resources through a PDCCH corresponding to the C-RNTI of the UE 8-20, then it can be considered that the UE 8-20 has successfully completed the procedure for random access to the target eNB 8-10. Therefore, if the UE 8-20 thereafter first monitors the PDCCH and first receives uplink transmission resources (or first receives an indication of uplink transmission resources) through the PDCCH corresponding to the C-RNTI of the UE, it can be determined that the second condition is satisfied. Alternatively, if the uplink transmission resource allocated by the RAR message is large enough so that UE 8-20 can transmit uplink data after sending message 3, it can be determined that the first uplink transmission resource has been received and the second condition has been satisfied. That is, if the RAR message is received, it can be determined that the first uplink transmission resource has been received and the second condition has been satisfied.

[0244] - If the handover command message received by the UE 8-20 also indicates a handover method that does not require a random access procedure (RACH-less handover),

[0245] *If the handover command message includes uplink transmission resources about the target eNB 8-10,

[0246] **If UE 8-20 transmits message 3 (e.g., a handover complete message or an RRC reconfiguration complete message) by using the uplink transmission resources of the target eNB 8-10, and if UE 8-20 receives a UE identity confirmation MAC CE from the target eNB 8-10 through message 4, or if UE 8-20 receives uplink transmission resources through a PDCCH corresponding to the C-RNTI of UE 8-20, UE 8-20 may determine that the random access procedure has been successfully completed and the second condition is satisfied. As another method, if, after the random access procedure is successfully completed, as a result of PDCCH monitoring, a first uplink transmission resource is received through a PDCCH corresponding to the C-RNTI of the UE, UE 8-20 may determine that the second condition is satisfied.

[0247] *If the handover command message does not include uplink transmission resources regarding the target eNB 8-10,

[0248] -If, as a result of PDCCH monitoring of the target eNB (or cell), uplink transmission resources are received through the PDCCH corresponding to the UE's C-RNTI, if UE 8-20 sends message 3 (e.g., a handover complete message or an RRC reconfiguration complete message) by using the uplink transmission resources and receives a UE identity confirmation MAC CE from the target eNB 8-10, or if UE 8-20 receives uplink transmission resources through the PDCCH corresponding to the UE's C-RNTI, then UE 8-20 determines that the random access procedure has been successfully completed and the second condition is satisfied. As another method, if, after the random access procedure is successfully completed, as a result of PDCCH monitoring, a first uplink transmission resource is received through the PDCCH corresponding to the UE's C-RNTI, then UE 8-20 may determine that the second condition is satisfied.

[0249] When UE 8-20 executes the second embodiment of the efficient handover method proposed in the present disclosure (DAPS handover method), if the RRC layer device, MAC layer device, or RLC layer device of the UE 8-20 for the first bearer of the source eNB 8-05 or the RRC layer device, MAC layer device, or RLC layer device of the second bearer of the target eNB confirms that the second condition proposed in the present disclosure is met, an indicator can be sent to the PDCP layer device of the UE or bearer executing the DAPS handover method to indicate that the second condition is met. If the PDCP layer device of UE 8-20 receives an indicator indicating that the second condition is met from a lower layer or upper layer device, the indicator can perform one or more of the following processes, thereby successfully completing the efficient handover method proposed in the present disclosure.

[0250] -UE 8-20 may release the first bearer for the source eNB 8-05 and may disconnect from the source eNB 8-05.

[0251] -After disconnecting from the source eNB 8-05, the UE 8-20 may trigger a PDCP status reporting procedure to report the reception status of downlink data received from the source eNB 8-05 to the target eNB 8-10, may configure a PDCP status report, and may send a PDCP status report to the target eNB 8-10.

[0252] -If the second condition is met, the UE 8-20 may switch from the structure or function 9-20 of the second PDCP layer device to the structure or function 9-11 or 9-12 of the first PDCP layer device for each bearer or for a bearer for which the DAPS handover method has been indicated. The UE 8-20 may initialize parameters for reordering, may stop a reordering timer, and may initialize the timer. The UE 8-20 may apply the security key or header decompression context for the source eNB 8-05 to the data for reordering (e.g., data received from the source eNB) stored in the buffer, thereby performing a decryption process or header (or data) decompression, and may then discard the security key or header decompression context for the source eNB 8-05. In addition, the UE 8-20 may pass the processed data to the upper layer in ascending order. That is, if the second condition is met, the UE 8-20 may apply the security key or header decompression context for the source eNB 8-05 to the data for reordering stored in the buffer (e.g., data received from the source eNB), thereby performing a decryption process or header (or data) decompression, and may then discard the security key or header decompression context for the source eNB 8-05.

[0253] As another method, if the second condition is met, the UE 8-20 may switch from the structure or function 9-20 of the second PDCP layer device to the structure or function 9-30 of the third PDCP layer device proposed in the present disclosure for each bearer or for a bearer to which the DAPS handover method has been indicated. The UE 8-20 may continue to use the parameters and reordering timers for reordering as they are without stopping or initializing them. However, the UE 8-20 may apply the security key or header decompression context for the source eNB 8-05 to the data for reordering (e.g., data received from the source eNB) stored in the buffer, thereby performing a decryption process or header (or data) decompression, and may then discard the security key or header decompression context for the source eNB 8-05. In addition, the UE 8-20 may pass the processed data to the upper layer in ascending order. That is, if the second condition is met, the UE 8-20 may apply the security key or header decompression context for the source eNB 8-05 to the data for reordering stored in the buffer (e.g., data received from the source eNB), thereby performing a decryption process or header (or data) decompression, and may then discard the security key or header decompression context for the source eNB 8-05.

[0254] UE 8-20 may release QoS mapping information of the SDAP layer device for the source eNB 8-05, security key information of the PDCP layer device for the source eNB 8-05, header (or data) compression context information of the source eNB 8-05, and RLC layer device or MAC layer device for the source eNB 8-05. If the second condition is met, when the bearer for which the second embodiment of the present disclosure (DAPS handover method) has been configured, the structure of the second PDCP layer device is changed, reconfigured, or switched to the structure of the first or third PDCP layer device, or when the structure of the first PDCP layer device of the present disclosure is changed, reconfigured, or switched to the structure of the second PDCP layer device, the proposed window parameter (e.g., the (1-1)th window parameter, the (1-2)th window parameter, the (1-3)th window parameter, the (1-4)th window parameter, the (2-1)th window parameter, the (2-2)th window parameter, the (2-3)th window parameter, or the (2-4)th window parameter) update procedure may be applied. As another method, if the second condition is met, when the structure of the second PDCP layer device is changed, reconfigured or switched to the structure of the first or third PDCP layer device when the bearer for which the second embodiment of the present disclosure (DAPS handover method) has been configured is used, the window parameters already used (for example, the (1-1)th window parameter, the (1-2)th window parameter, the (1-3)th window parameter, the (1-4)th window parameter, the (2-1)th window parameter, the (2-2)th window parameter, the (2-3)th window parameter or the (2-4)th window parameter) are maintained and used as is.

[0255] In this disclosure Figure 6In the present invention, when the eNB sends a handover command message 6-20 to the UE, the handover command message (e.g., an RRC reconfiguration message) can be used to define an indicator regarding an embodiment proposed in the present disclosure, and can be used to indicate to the UE which handover process corresponding to which embodiment is to be triggered. The UE can perform the handover process according to the handover method indicated by the handover command message. For example, the UE can perform the second embodiment of the efficient handover method proposed in the present disclosure (DAPS handover method), thereby performing handover to the target eNB while minimizing the data interruption time. As another method, the handover command message can be used to define an indicator regarding an embodiment proposed in the present disclosure for each bearer, and can be used to specifically indicate which embodiment will be applied to which bearer during the handover. For example, it can be indicated that the second embodiment of the present disclosure is applied only to AM bearers driving RLC layer devices in AM mode, or that the second embodiment of the present disclosure can be widely applied to UM bearers driving RLC layer devices in UM mode. In addition, it will be assumed that the embodiments proposed in the present disclosure will be applied to DRBs. However, if necessary (for example, if a UE maintaining an SRB on a source eNB fails to hand over to a target eNB and thus a handover failure message can be reported or recovered by using the SRB on the source eNB), the embodiments proposed in the present disclosure can also be widely applied to SRBs.

[0256] According to an embodiment of the present disclosure, when a UE sends / receives data with a source eNB through a protocol layer device of a first bearer and sends / receives data with a target eNB through a protocol layer device of a second bearer, the MAC layer device of the first bearer and the MAC layer device of the second bearer can respectively apply separate discontinuous reception (DRX) cycles, thereby reducing UE battery consumption. That is, assuming that the UE has already applied the DRX cycle of the MAC layer device when sending / receiving data through the protocol layer device of the first bearer, the UE can still apply the same cycle even after receiving the handover command message, and can also suspend DRX according to the first condition or the second condition of the present disclosure. In addition, the UE can follow instructions from the target eNB to apply a separate DRX cycle to the MAC layer device of the second bearer.

[0257] In addition, the description in the present disclosure that the UE stops uplink transmission to the source eNB through the protocol layer device of the first bearer and stops receiving downlink data from the source eNB means that the UE reconstructs, initializes or releases the protocol layer device (PHY layer device, MAC layer device, RLC layer device or PDCP layer device) of the first bearer.

[0258] In the above description of the embodiments of the present disclosure, for ease of description, it has been assumed that a first bearer for a source eNB or a second bearer for a target eNB is configured for the UE, but the description can be easily extended and equally applied to a case where multiple first bearers for a source eNB or multiple second bearers for a target eNB are configured for the UE. As another method, the description can be easily extended and equally applied to a case where multiple bearers are configured for multiple target eNBs. For example, the second bearer can be configured when performing a handover process to the first target eNB. If the handover fails, the second bearer can be configured when performing a handover process to the second target eNB. Therefore, the UE can independently search for a cell that meets a predetermined condition (e.g., a predetermined signal strength or higher) among multiple target eNBs, can determine a single cell, and can perform a handover process.

[0259] Figure 9a and Figure 9b A schematic diagram showing the structure of an efficient PDCP layer device applied in a DAPS handover method (which is the second embodiment of the efficient handover method proposed in the present disclosure) and a method for applying the structure is shown.

[0260] Figure 9a and Figure 9b The detailed structure and functions of the efficient PDCP layer device applied in the DAPS handover method (which is the second embodiment of the efficient handover method proposed in the present disclosure) are proposed, and the method is characterized in that when performing the DAPS handover process, the structure of the PDCP layer device proposed below (different PDCP layer structures) can be applied to each bearer at different time points.

[0261] For example, before receiving the handover command message from the eNB, the UE may apply the structure and function 9-11 or 9-12 of the first PDCP layer device proposed in the present disclosure for each bearer, thereby processing data and sending or receiving data (9-01).

[0262] However, if the UE has received a handover command message from the eNB, and if the handover command message indicates the DAPS handover method proposed by the present disclosure, or if the handover command message indicates the DAPS handover method for a specific bearer, the UE can apply the structure and function 9-20 of the second PDCP layer device proposed by the present disclosure to the corresponding bearer or to the bearer for which the DAPS handover method has been indicated, thereby processing data and sending or receiving the data (9-02). That is, if the UE has received a handover command message, and if the handover command message indicates the DAPS handover method proposed by the present disclosure, or if the handover command message indicates the DAPS handover method for a specific bearer, the UE can switch from the structure or function 9-11 or 9-12 of the first PDCP layer device already used for each bearer to the structure or function 9-20 of the second PDCP layer device proposed by the present disclosure for each bearer or for the bearer for which the DAPS handover method has been indicated. As another method, if the first condition proposed by the present disclosure is met, the UE can switch from the structure or function 9-11 or 9-12 of the first PDCP layer device that has been used for each bearer to the structure or function 9-20 (9-02) of the second PDCP layer device proposed by the present disclosure for each bearer or for a bearer for which the DAPS handover method has been indicated. In addition, it will be assumed that the UE has received a handover command message, and the handover command message indicates the DAPS handover method proposed by the present disclosure, or the handover command message indicates the DAPS handover method for a specific bearer, or the PDCP reordering timer value is newly configured. When switching from the structure or function 9-11 or 9-12 of the first PDCP layer device to the structure or function 9-20 of the second PDCP layer device proposed by the present disclosure for each bearer or for a bearer for which the DAPS handover method has been indicated, the UE can update the parameter for reordering to the "expected next received PDCP sequence number or COUNT value", stop the reordering timer, and restart the timer.

[0263] In addition, if the second condition proposed in the present disclosure is met when the UE executes the DAPS handover method proposed in the present disclosure, the UE may release the structure and function 9-20 of the second PDCP layer device that has been applied for each bearer or for a bearer for which the DAPS handover method has been indicated from the first bearer used for the source eNB, may switch back to the structure and function 9-11 or 9-12 of the first PDCP layer device, and may apply the structure and function 9-11 or 9-12 of the first PDCP layer device. In addition, if the second condition is met, and when the UE switches from the structure or function 9-20 of the second PDCP layer device to the structure and function 9-11 or 9-12 of the first PDCP layer device proposed in the present disclosure for each bearer or for a bearer for which the DAPS handover method has been indicated, the UE may initialize parameters for reordering, may stop the reordering timer, and may initialize the timer. In addition, the UE may apply the security key or header decompression context for the source eNB to the data for reordering stored in the buffer (e.g., data received from the source eNB), thereby performing a decryption process or header (or data) decompression, and then may discard the security key or header decompression context for the source eNB. In addition, the UE may pass the processed data to the upper layer in ascending order. That is, if the second condition is met, the UE may apply the security key or header decompression context for the source eNB to the data for reordering stored in the buffer (e.g., data received from the source eNB), thereby performing a decryption process or header (or data) decompression, and then may discard the security key or header decompression context for the source eNB.

[0264] Alternatively, if the second condition proposed in the present disclosure is satisfied when the UE performs the DAPS handover method proposed in the present disclosure, the UE may release the second PDCP layer device structure and function 9-20, which has been applied for each bearer or for a bearer for which the DAPS handover method has been indicated, from the bearer used for the source eNB, and may switch to the third PDCP layer device structure and function 9-30, and may apply the third PDCP layer device structure and function 9-30. Furthermore, if the second condition is satisfied, and when the UE switches from the second PDCP layer device structure or function 9-20 to the third PDCP layer device structure and function 9-30 proposed in the present disclosure for each bearer or for which the DAPS handover method has been indicated, the UE may not stop or initialize the parameters and reordering timer for reordering and may continue to use them as they are. However, the UE may apply the security key or header decompression context for the source eNB to the data for reordering stored in the buffer (e.g., data received from the source eNB), thereby performing a decryption process or header (or data) decompression, and may then discard the security key or header decompression context for the source eNB. In addition, the UE may deliver the processed data to the upper layer in ascending order. That is, if the second condition is met, the UE may apply the security key or header decompression context for the source eNB to the data for reordering stored in the buffer (e.g., data received from the source eNB), thereby performing a decryption process or header (or data) decompression, and then may discard the security key or header decompression context for the source eNB.

[0265] As mentioned above with reference to the present disclosure Figure 9a and Figure 9b As proposed, if handover is performed by applying the structure and function 9-11 or 9-12 of the first PDCP layer device, the structure and function 9-20 of the second PDCP layer device, or the structure and function 9-30 of the third PDCP layer device, which are different from each other, for each bearer at different time points, the UE can avoid data loss and minimize data interruption time.

[0266] The receiving operation of the receiving PDCP layer device in the structure of the first PDCP layer device or the structure of the second PDCP layer device proposed in the present disclosure may be specified by using the following window parameters or constants:

[0267] - The (1-1) window parameter (Next_PDCP_TX_SN): This is a window parameter used by the LTE PDCP layer device and indicates the PDCP sequence number of the next data (PDCP SDU), the next data to be processed (PDCP SDU), or the next data to be transmitted (PDCP SDU) in the transmitting PDCP layer device. When the PDCP layer device is established, the initial value of the window parameter is set to zero.

[0268] - The (1-2) window parameter (Next_PDCP_RX_SN): is a window parameter used by the LTE PDCP layer device and indicates the PDCP sequence number that the receiving PDCP layer device expects to receive next. When the PDCP layer device is established, the initial value of the window parameter is set to zero.

[0269] - The (1-3) window parameter (Last_Submitted_PDCP_RX_SN): is a window parameter used by the LTE PDCP layer device and indicates the PDCP sequence number corresponding to the last data (PDCP SDU) delivered by the receiving PDCP layer device to the upper layer device. When the PDCP layer device is established, the window parameter is set to the first constant value (Maximum_PDCP_SN).

[0270] - Window parameter (1-4) (Reordering_PDCP_RX_COUNT): This is a window parameter used by the LTE PDCP layer device. This window parameter can be used when the receiving PDCP layer device performs the reordering function. This window parameter indicates, stores, or has the next COUNT value corresponding to the data (PDCP PDU) for which the reordering timer has been triggered.

[0271] - A first constant value (Maximum_PDCP_SN): is a constant value used by the LTE PDCP layer device, and a value of 2^(length of the PDCP sequence number)-1 is stored as the constant value.

[0272] -Second constant value (Reordering_Window or Window_Size): is a constant value used by the LTE PDCP layer device or the NR PDCP layer device, and the value of 2^(length of the PDCP sequence number - 1) is stored as the constant value.

[0273] -(2-1) Window parameter (TX_NEXT): This is a window parameter used by the NR PDCP layer device and indicates the COUNT value of the next data (PDCP SDU), the next data to be processed (PDCP SDU), or the next data to be transmitted (PDCP SDU) in the transmitting PDCP layer device. When the PDCP layer device is established, the initial value of the window parameter is set to zero.

[0274] -(2-2) Window parameter (RX_NEXT): is a window parameter used by the NR PDCP layer device and indicates the COUNT value that the receiving PDCP layer device expects to receive. When the PDCP layer device is established, the initial value of the window parameter is set to zero.

[0275] - The (2-3) window parameter (RX_DELIV): is a window parameter used by the NR PDCP layer device and indicates the COUNT value corresponding to the first data (PDCP SDU) that is not delivered from the receiving PDCP layer device to the upper layer device. When the PDCP layer device is established, the initial value of the window parameter is set to zero.

[0276] - The (2-4) window parameter (RX_REORD): is a window parameter used by the NR PDCP layer device. This window parameter may be used when the receiving PDCP layer device performs a reordering function. This window parameter indicates, stores, or has the next COUNT value corresponding to the data (PDCP PDU) for which the reordering timer has been triggered.

[0277] Figure 9a and Figure 9b The structure 9-11 or 9-12 of the first PDCP layer device proposed in the present disclosure may have the following (1-1)th PDCP layer device, (1-2)th PDCP layer device, (1-3)th PDCP layer device, or (1-4)th PDCP layer device proposed in the present disclosure, and may have the following features:

[0278] 1>(In the case of the structure of the (1-1)th PDCP layer device), for example, if the UE applies the structure and functions 9-11 of the first PDCP layer device to a PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) connected to an AM RLC layer device (e.g., an E-UTRA AM RLC layer device), the following process may be performed based on the (1-2)th window parameter, the (1-3)th window parameter, or the second constant value.

[0279] 2> The receiving PDCP layer device may first perform out-of-window data detection or duplicate data detection on the received data based on the (1-2)th window parameter, the (1-3)th window parameter, or the second constant value. (RLC AM may have retransmissions, and the LTE RLC SN and the PDCP SN may have different sizes. Therefore, duplicate data or out-of-window data may be received. The window refers to the area of ​​the PDCP sequence number in which the COUNT value of valid data is received.)

[0280] 3> The UE performs a decryption process and a header decompression process, and then discards out-of-window data or duplicate data based on the (1-2)th window parameter, the (1-3)th window parameter, or the second constant value. (This is because the out-of-window data or duplicate data may include useful information for the header decompression process (e.g., IR packet or header compression information). Therefore, after inspection, the out-of-window data or duplicate data can be discarded.)

[0281] 2> The UE drives the PUSH window based on the (1-2)th window parameter, the (1-3)th window parameter, or the second constant value. Furthermore, the UE immediately decrypts the received, non-discarded data segments without reordering them, and then performs the header decompression process. This is because the E-UTRA AM RLC layer device sorts the data segments in sequence and delivers them to the PDCP layer device.

[0282] 2> In addition, the UE delivers multiple pieces of data to the upper layer in ascending order of COUNT values.

[0283] 2> If the UE has received a handover command message, and if the handover command message (e.g., an RRC reconfiguration message) indicates the second embodiment (DAPS handover method) proposed in the present disclosure for an AM DRB having a structure of a (1-1)th PDCP layer device performing the above-mentioned procedure (e.g., a PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) connected to an AM RLC layer device (e.g., an E-UTRA AM RLC layer device), the UE may change, reconfigure, or switch from the structure of the (1-1)th PDCP layer device to the second PDCP layer device, and may perform the following window parameter update procedure:

[0284] 3> In the structure of the (1-1)th PDCP layer device, the UE performs a received data processing operation based on the (1-2)th window parameter, the (1-3)th window parameter, or the second constant value, but in the structure of the second PDCP layer device, the UE performs a received data processing operation based not only on the (1-2)th window parameter, the (1-3)th window parameter, or the second constant value, but also on the (1-4)th window parameter for the reordering function. Therefore, when the structure of the second PDCP layer device is changed, reconfigured, or switched to, the UE may configure, update, or initialize the (1-4)th window parameter to include the (1-2)th window parameter and a COUNT value of a corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, if the handover command message has configured a reordering timer value or if the reordering timer is being driven, the UE may stop the reordering timer, restart the reordering timer with the newly configured timer value, and configure, update, or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. If the reordering timer is not being driven, the UE may update the reordering timer value to the new reordering timer value, and configure, update, or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, if the PDCP sequence number of the data (PDCP SDU) first received after the structure of the second PDCP layer device is changed or reconfigured is x (or if the value of x is not zero), the (1-4)th window parameter can be configured, updated or initialized to include the calculation result value of [(x+1) modulo (first constant value+1)] and the corresponding received hyperframe number (HFN) COUNT value or a COUNT value related thereto.

[0285] 3> After the structure of the second PDCP layer device is changed or reconfigured, the (1-2)th window parameter or the (1-3)th window parameter is used as is without changing the parameter value.

[0286] 1>(In the case of the structure of the (1-2)th PDCP layer device), for example, if the UE applies the structure and functions 9-11 of the first PDCP layer device to a PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) connected to a UM RLC layer device (e.g., an E-UTRA UM RLC layer device), the following process may be performed based on the (1-2)th window parameter or the second constant value.

[0287] 2> The UE may not perform the process of detecting out-of-window data or duplicate data because the UM E-UTRA RLC layer device does not have a retransmission process.

[0288] 2> The UE may determine the HFN value or the COUNT value based on the (1-2)th window parameter or the second constant value, may drive the PULL window process, may immediately perform a decryption process on the received data, and may perform a header decompression process.

[0289] 2> Additionally, the UE may deliver (eg, in ascending order) to the layer immediately above the reordering process.

[0290] 2> If the UE has received a handover command message, and if the handover command message (e.g., an RRC reconfiguration message) indicates the second embodiment (DAPS handover method) proposed in the present disclosure for a UM DRB having a structure of a (1-2)th PDCP layer device performing the above-mentioned procedure (e.g., a PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) connected to a UM RLC layer device (e.g., an E-UTRA UM RLC layer device), the UE may change, reconfigure, or switch from the structure of the (1-2)th PDCP layer device to the second PDCP layer device, and may perform the following window parameter update procedure:

[0291] 3> In the structure of the (1-2)th PDCP layer device, the UE performs a received data processing operation based on the (1-2)th window parameter or the second constant value, but in the structure of the second PDCP layer device, the UE performs a received data processing operation based not only on the (1-2)th window parameter or the second constant value, but also on the (1-3)th window parameter and the (1-4)th window parameter for the reordering function. Therefore, when the structure of the second PDCP layer device is changed, reconfigured, or switched, the (1-3)th window parameter or the (1-4)th window parameter may be updated by the following method:

[0292] 4> The UE may configure, update or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, if the handover command message has configured a reordering timer value, or if the reordering timer is being driven, the UE may stop the reordering timer, may restart the reordering timer with the newly configured timer value, and may configure, update or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. If the reordering timer is not being driven, the UE may update the reordering timer value to the new reordering timer value, and may configure, update or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, if the PDCP sequence number of the data (PDCP SDU) first received after the structure of the second PDCP layer device is changed or reconfigured is x (or if the value of x is not zero), the (1-4)th window parameter can be configured, updated or initialized to include the calculation result value of [(x+1) modulo (first constant value+1)] and the corresponding received hyperframe number (HFN) COUNT value or a COUNT value related thereto.

[0293] 4>The UE may configure, update or initialize the (1-3)th window parameter to the PDCP sequence number corresponding to the last data (PDCP SDU) delivered to the upper layer device. As another method, the UE may configure, update or initialize the (1-3)th window parameter to the calculation result value of [((1-2)th window parameter value-1) modulo (first constant value+1)]. As another method, if the PDCP sequence number of the data (PDCP SDU) first received after the structure of the second PDCP layer device is changed or reconfigured is x (or if the x value is not zero), the (1-3)th window parameter may be configured, updated or initialized to the calculation result value of [x modulo (first constant value+1)].

[0294] 4> After changing or reconfiguring the structure of the second PDCP layer device, the UE can use the (1-2)th window parameter as is without changing the parameter value.

[0295] 1> (In the case of the structure of the (1-3)th PDCP layer device), if the UE applies the structure and functions 9-11 of the first PDCP layer device to a PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) configured as, for example, a split bearer, a packet repetition bearer, or an LTE wireless local area network (WLAN) aggregation (LWA) bearer, the UE may always apply the reordering procedure and the reordering timer, and may perform the following process based on the (1-2)th window parameter, the (1-3)th window parameter, and the (1-4)th window parameter.

[0296] 2> The UE may first detect out-of-window data or duplicate data for the received data based on the (1-2)th window parameter, the (1-3)th window parameter, the (1-4)th window parameter, or the first constant value. (This is because the RLC AM may retransmit data or may receive data from different RLC layer devices at different time points; the LTE RLC SN and PDCP SN may have different sizes; therefore, out-of-window data or duplicate data may be received.)

[0297] 3> The UE performs the decryption process but does not perform the header decompression process. (This is because E-UTRA PDCP cannot configure the header compression protocol for split bearers or LWA bearers.)

[0298] 3> If the integrity protection or verification process is performed, the UE discards the data after performing the process. If the integrity verification process fails, the data may be discarded and reported to the upper layer device.

[0299] 3>UE discards data outside the window or duplicate data.

[0300] 2> The UE may perform a PUSH window operation based on the (1-2)th window parameter, the (1-3)th window parameter, the (1-4)th window parameter, or the first constant value. If not discarded, the UE may immediately perform a decryption process for the received multiple pieces of data without reordering them. In addition, if integrity protection or verification is configured, the UE performs integrity verification. If the integrity protection or verification process is performed, the UE discards the data after performing the process. If the integrity verification process fails, the UE may discard the data and report to the upper layer device.

[0301] 2> In addition, the UE may reorder the received multiple pieces of data. If they are sequentially ordered in ascending order without gaps in PDCP sequence numbers or COUNT values, the UE may perform header compression (if configured) and pass the data to upper layers in ascending order.

[0302] 2> If you are driving the reorder timer

[0303] 3> If data corresponding to the COUNT value having the same value as the value obtained by subtracting 1 from the value maintained by the parameter for reordering has been delivered to the upper layer device, or if all data has been delivered to the upper layer and there is no gap in the PDCP sequence number (or COUNT value)

[0304] 4>UE stops and initializes the reordering timer.

[0305] 2> If the reorder timer is not driven

[0306] 3> If there is data that has not yet been delivered to the upper layer device and is stored in the buffer, or if there is a gap in the PDCP sequence number (or COUNT value)

[0307] 4>UE starts the reordering timer.

[0308] 4> In addition, the UE updates the parameter used for reordering to the expected next received PDCP sequence number or COUNT value.

[0309] 2> If the reordering timer has expired

[0310] 3> If the header decompression process is configured for multiple stored data in ascending order of PDCP sequence number or COUNT value for a value less than the reordering parameter value, the UE performs the header decompression process and passes it to the upper layer device.

[0311] 3> If the header decompression process is configured continuously for multiple stored data in ascending order of PDCP sequence number or COUNT value for a value equal to or greater than the reordering parameter value, the UE performs the header decompression process and passes it to the upper layer device.

[0312] 3> In addition, the UE updates the parameter value regarding the last data delivered to the upper layer to the PDCP sequence number or CONT value of the last delivered data.

[0313] 3> If there is data that has not yet been delivered to the upper layer device and is stored in the buffer, or if there is a gap in the PDCP sequence number (or COUNT value)

[0314] 4>UE starts the reordering timer.

[0315] 4> In addition, the UE updates the parameter used for reordering to the expected next received PDCP sequence number or COUNT value.

[0316] 2> If the UE has received a handover command message, and if the handover command message (e.g., an RRC reconfiguration message) indicates the second embodiment (DAPS handover method) proposed in the present disclosure for a DRB having a structure connected to a (1-3)th PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) performing the above-mentioned process, the UE may change, reconfigure, or switch from the structure of the (1-3)th PDCP layer device to the second PDCP layer device, and may perform the following window parameter update process:

[0317] 3> The UE may configure, update or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, if the handover command message has configured a reordering timer value, or if the reordering timer is being driven, the UE may stop the reordering timer, may restart the reordering timer with the newly configured timer value, and may configure, update or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. If the reordering timer is not being driven, the UE may update the reordering timer value to the new reordering timer value, and may configure, update or initialize the (1-4)th window parameter to a COUNT value including the (1-2)th window parameter and the corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, if the PDCP sequence number of the data (PDCP SDU) first received after the structure of the second PDCP layer device is changed or reconfigured is x (or if the value of x is not zero), the (1-4)th window parameter may be configured, updated, or initialized to include a calculation result value of [(x+1) modulo (first constant value+1)] and a COUNT value of the corresponding received hyperframe number (HFN) or a COUNT value related thereto. As another method, the UE may maintain the existing (1-4)th window parameter value and may apply the value as it is.

[0318] 3> After changing or reconfiguring the structure of the second PDCP layer device, the UE can maintain the parameter values ​​of the (1-2)th window parameter, the (1-3)th window parameter or the (1-4)th window parameter and can use them as they are.

[0319] 1>(In the case of the structure of the (1-4)th PDCP layer device), if the UE applies the structure and functions 9-12 of the first PDCP layer device to the NR PDCP layer device, the UE can always apply the reordering procedure and reordering timer, and can perform the following process based on the (2-2)th window parameter, the (2-3)th window parameter, the (2-4)th window parameter or the first constant value.

[0320] 2> The UE may first perform a decryption process on the received data.

[0321] 2> If integrity protection or verification procedures are configured, the UE may perform integrity protection or verification procedures on the received data. If the integrity verification procedure fails, the data may be discarded and reported to the upper layer device.

[0322] 2> The UE detects out-of-window data or duplicate data based on the (2-2)th window parameter, the (2-3)th window parameter, the (2-4)th window parameter, or the first constant value for the received data. (The UE may detect out-of-window data or duplicate data after performing the decryption process. As another method, the UE may detect out-of-window data or duplicate data after performing the decryption process only if an integrity protection or verification process has been configured. If no integrity protection or verification process is configured, the UE may perform the decryption process only on data that has not been discarded after detecting out-of-window data or duplicate data.)

[0323] 3>UE discards data outside the window or duplicate data.

[0324] 2> The UE may drive the PUSH window based on the (2-2)th window parameter, the (2-3)th window parameter, the (2-4)th window parameter, or the first constant value. If there is no discard, the UE may sort the received multiple pieces of data in sequence, and if they are sorted continuously in ascending order without gaps in the PDCP sequence number or COUNT value, the UE may perform a header compression process (if a header compression process or decompression process has been configured) and may deliver the data to the upper layer in ascending order.

[0325] 2> In addition, the UE passes data to the upper layer in ascending order of COUNT values.

[0326] 2> If you are driving the reorder timer

[0327] 3> If data corresponding to the COUNT value having the same value as the value obtained by subtracting 1 from the value maintained by the parameter for reordering has been delivered to the upper layer device, if all data has been delivered to the upper layer and there is no gap in the PDCP sequence number (or COUNT value), or if the value of the parameter storing the value of the PDCP sequence number (or COUNT value) of data to be delivered to the upper layer is greater than or equal to the value of the parameter for reordering

[0328] 4>UE stops and initializes the reordering timer.

[0329] 2> If the reorder timer is not driven

[0330] 3> If there is data that has not been delivered to the upper layer device and is stored in the buffer, if there is a gap in the PDCP sequence number (or COUNT value), or if the value of the parameter storing the COUNT value of the first data that has not been delivered to the upper layer is less than the value of the parameter for reordering

[0331] 4> In addition, the UE updates the parameter used for reordering to the expected next received PDCP sequence number or COUNT value.

[0332] 4>UE starts the reordering timer.

[0333] 2> If the reordering timer has expired

[0334] 3> If the header decompression process is configured for multiple stored data in ascending order of PDCP sequence number or COUNT value for a value less than the reordering parameter value, the UE performs the header decompression process and passes it to the upper layer device.

[0335] 3> If the header decompression process is configured continuously for multiple stored data in ascending order of PDCP sequence number or COUNT value for a value equal to or greater than the reordering parameter value, the UE performs the header decompression process and passes it to the upper layer device.

[0336] 3> In addition, the UE updates the parameter value regarding the first data that has not been delivered to the upper layer to the PDCP sequence number or COUNT value of the first data that has not been delivered to the upper layer.

[0337] 3> If there is data that has not been delivered to the upper layer device and is stored in the buffer, if there is a gap in the PDCP sequence number (or COUNT value), or if the value of the parameter storing the COUNT value of the first data that has not been delivered to the upper layer is less than the value of the parameter for reordering 4> In addition, the UE updates the parameter for reordering to the expected next received PDCP sequence number or COUNT value.

[0338] 4>UE starts the reordering timer.

[0339] 2> If the UE has received a handover command message, and if the handover command message (e.g., an RRC reconfiguration message) indicates the second embodiment (DAPS handover method) proposed in the present disclosure for a DRB having a structure connected to a (1-4)th PDCP layer device (e.g., an NRPDCP layer device) performing the above-mentioned procedure, the UE may change, reconfigure, or switch from the structure of the (1-4)th PDCP layer device to the second PDCP layer device, and may perform the following window parameter update procedure:

[0340] 3>The UE may configure, update or initialize the (2-4)th window parameter to the COUNT value of the (2-2)th window parameter. As another method, if the handover command message has configured the reordering timer value, or if the reordering timer is being driven, the UE may stop the reordering timer, may restart the reordering timer with the newly configured timer value, and may configure, update or initialize the (2-4)th window parameter to the COUNT value of the (2-2)th window parameter. If the reordering timer is not driven, the UE may update the reordering timer value to the new reordering timer value, and may configure, update or initialize the (2-4)th window parameter to the COUNT value of the (2-2)th window parameter. As another method, if the COUNT value of the data (PDCP SDU) first received after the structure of the second PDCP layer device is changed or reconfigured is x (or if the x value is not zero), the UE may configure, update or initialize the (2-4)th window parameter to the COUNT value of x+1. As another method, the UE may maintain the existing (2-4)th window parameter value and may apply the value as it is.

[0341] 3> After changing or reconfiguring the structure of the second PDCP layer device, the UE can maintain the parameter values ​​of the (2-2)th window parameter, the (2-3)th window parameter or the (2-4)th window parameter and can use them as they are.

[0342] Figure 9a and Figure 9b The structure of the second PDCP layer device proposed in 9-20 may have the following (2-1) PDCP layer device structure or (2-2) PDCP layer device structure, and may have the following features:

[0343] The present disclosure proposes a structure of a second PDCP layer device with efficient handover, as shown in Figures 9 to 20. The structure of the second PDCP layer device can be applied to the second embodiment of the efficient handover method for minimizing data interruption time proposed in the present disclosure.

[0344] In the second PDCP layer device structure, the UE can send or receive data with the source eNB 9-21 through the protocol layer device of the first bearer (for example, an SDAP layer device, a PDCP layer device, an RLC layer device, or a MAC layer device), and can send or receive data with the target eNB 9-22 through the protocol layer device of the second bearer (for example, an SDAP layer device, a PDCP layer device, an RLC layer device, or a MAC layer device).

[0345] The PDCP layer device of the first bearer and the PDCP layer device of the second bearer can be configured separately for the UE, but can be logically operated as a single PDCP layer device, as shown in 9-20. Specifically, a single PDCP layer device can be implemented so that for each source eNB and each target eNB, the functions of the PDCP layer device are divided into the functions of the upper PDCP layer device (e.g., sequence number allocation function, reordering function, in-order delivery function, or duplicate detection function) and the functions of the two lower PDCP layer devices (e.g., encryption or decryption function, header (or data) compression or header (or data) decompression function, integrity protection or verification function, or duplicate detection function). In addition, in combination with the DAPS handover method proposed above, the UE can send an uplink to the source eNB and can be handed over to the target eNB if the first condition is met. The UE can continuously receive downlink data from the source eNB and the target eNB. Therefore, in conjunction with the header (or data) compression protocol context, for the uplink, only one type of context for the source eNB or target eNB may be maintained and applied, while for the downlink, two types of context for the source eNB or target eNB may be maintained and applied.

[0346] Based on the second PDCP layer structure proposed above, the (2-1) PDCP layer structure proposed in the present disclosure (e.g., an E-TRAN PDCP layer device for a DAPS handover method) may have the following features. Furthermore, the (2-1) PDCP layer structure proposed in the present disclosure is characterized in that, in the (2-1) PDCP layer structure, a PUSH window is driven based on a (1-2) window parameter, a (1-3) window parameter, or a (1-4) window parameter, and received data is processed.

[0347] The upper transmitting PDCP layer device function can play the role (or function) of assigning PDCP sequence numbers to multiple data received from the upper layer device. In addition, in combination with the two lower transmitting PDCP layer device functions 9-21 and 9-22 for each source eNB and each target eNB, separate security keys configured for each source eNB and each target eNB can be used so that the header (or data) compression context or security key configured for the source eNB is applied to the data to be sent to the source eNB, and the header (or data) compression context or security key configured for the target eNB is applied to the data to be sent to the target eNB. If the header (or data) compression process has been configured, the header (or data) compression process can be applied. If integrity protection has been configured, the integrity protection process is applied to the PDCP header and data (PDCP SDU), and the encryption process is applied thereto. The data to be sent to the source eNB can be delivered to the transmitting RLC layer device of the first bearer, and the data to be sent to the target eNB can be delivered to the transmitting RLC layer device of the second bearer, thereby performing transmission. The two lower transmitting PDCP layer device functions 9-21 and 9-22 are characterized in that, to accelerate data processing rates, header compression, integrity protection, or encryption processes can be performed in parallel (parallel data processing), and the two lower transmitting PDCP layer device functions can use different security keys to perform integrity protection or encryption processes. In addition, different types of compression contexts, security keys, or security algorithms can be applied within a logically single transmitting PDCP layer device, thereby performing compression, integrity protection, or encryption processes on different pieces of data.

[0348] In conjunction with the receiving PDCP layer device, and for data received from the corresponding lower layer devices (specifically, for data received from two RLC layer devices for each source eNB and each target eNB), the lower receiving PDCP layer device functions 9-21 and 9-22 for the source eNB or target eNB can refer to the PDCP sequence number or COUNT value to independently perform a process of detecting out-of-window data or detecting duplicates for the data received from the corresponding RLC layer device. As another method, the UE can refer to the PDCP sequence number or COUNT value to perform a process of detecting out-of-window data or detecting duplicates for the entire received data (without distinguishing between the corresponding RLC layer devices). As another method, for the purpose of more accurate duplicate detection, the UE can refer to the PDCP sequence number or COUNT value to detect out-of-window data for the entire received data (without distinguishing between the corresponding RLC layer devices), and can independently perform a duplicate detection process for each piece of data received from each RLC layer device. As another method, if multiple pieces of data received from different eNBs overlap, the UE can refer to the PDCP sequence number or COUNT value and detect out-of-window data for the entire received data (regardless of the corresponding RLC layer device) to prevent data loss for the header compression protocol. In conjunction with the duplicate detection process, the UE can perform a decryption process, an integrity protection process, or a header (or data) decompression process for each piece of data received from each RLC layer device, and then perform a duplicate detection process for the entire data.

[0349] The lower layer functions of the receiving PDCP layer device are characterized in that the decryption process is immediately applied to the received data by using a separate header (or data) compression context or security key configured for each source eNB and each target eNB, and if integrity protection has been configured, the integrity protection process can be applied to the PDCP header and data (PDCP SDU).

[0350] The (2-1) PDCP layer structure is characterized in that the UE can immediately perform a header (or data) decompression process on the data received from the RLC layer device of the first bearer for each source eNB without reordering it, and can immediately perform a header (or data) decompression process on the data received from the RLC layer device of the second bearer for each target eNB without reordering it. In addition, in order to distinguish between the data received from the RLC layer device of the first bearer for each source eNB and the data received from the RLC layer device of the second bearer for each target eNB, the UE can define an indicator for each piece of data to distinguish between the data received from the source eNB and the data received from the target eNB. As another method, the UE can define a one-bit indicator in the PDCP header, SDAP header, or RLC header to distinguish between the data received from the source eNB and the data received from the target eNB. In addition, the UE can perform a duplicate detection process (a process of discarding all data except one piece of data for each PDCP sequence number or COUNT value (which can be applied to previously received data or data delivered to an upper layer)) for all data received from the RLC layer device of the first bearer for the source eNB and the data received from the RLC layer device of the second bearer for the target eNB based on the PDCP sequence number or COUNT value, which has completed the header (or data) compression process. In addition, the UE can perform a reordering process in ascending order based on the PDCP sequence number or COUNT value for all data received from the RLC layer device of the first bearer for the source eNB and the data received from the RLC layer device of the second bearer for the target eNB, and then deliver the data to the upper layer device in this order. A single PDCP layer device can receive data from different eNBs, that is, receive data from the first bearer or the second bearer, regardless of the order, and the reordering process must always be performed.

[0351] The two lower receiving PDCP layer device functions are characterized in that, in order to speed up the data processing rate, header compression, integrity protection or encryption processes (parallel data processing) are performed in parallel based on each PDCP sequence number or COUNT value, and different types of header (or data) compression contexts or security keys are used to perform integrity protection, encryption or decompression processes. In addition, the UE can apply different types of header (or data) compression contexts, security keys or security algorithms within a logically single transmitting PDCP layer device, thereby performing integrity protection, encryption or decompression processes for different pieces of data. In addition, the lower receiving PDCP layer device function is characterized in that, regardless of the order of the PDCP sequence number or COUNT value, an out-of-order decryption or integrity verification process is performed for each piece of received data.

[0352] When a single PDCP layer device distinguishes between a layer device of a first bearer and a layer device of a second bearer, consideration is given to the fact that they are connected to different MAC layer devices or are assigned different logical channel identifiers, or to the fact that they are different RLC layer devices connected to different MAC layer devices, or to the fact that they use different encryption keys. In this way, the layer device of the first bearer (or the first RLC layer device) and the layer device of the second bearer (or the second RLC layer device) are distinguished, different security keys are used to perform encryption or decryption processes for uplink data and downlink data, and different types of compression protocol texts are used to compress or decompress uplink data and downlink data.

[0353] Based on the second PDCP layer structure proposed above, the (2-2)th PDCP layer structure proposed in the present disclosure (e.g., an NR PDCP layer device for the DAPS handover method) may have the following features. Furthermore, the (2-2)th PDCP layer structure proposed in the present disclosure is characterized in that, in the (2-2)th PDCP layer structure, a PUSH window is driven based on a (2-2)th window parameter, a (2-3)th window parameter, or a (2-4)th window parameter, and received data is processed.

[0354] The upper transmitting PDCP layer device function can play the role of assigning PDCP sequence numbers to multiple data received from the upper layer device. In addition, in combination with the two lower transmitting PDCP layer device functions 9-21 and 9-22 for each source eNB and each target eNB, separate security keys configured for each source eNB and each target eNB can be used so that the header (or data) compression context or security key configured for the source eNB is applied to the data to be sent to the source eNB, and the header (or data) compression context or security key configured for the target eNB should be used for the data to be sent to the target eNB. If the header (or data) compression process has been configured, the header (or data) compression process can be applied. If integrity protection has been configured, the integrity protection process is applied to the PDCP header and data (PDCP SDU), and the encryption process is applied thereto. The data to be sent to the source eNB can be delivered to the transmitting RLC layer device of the first bearer, and the data to be sent to the target eNB can be delivered to the transmitting RLC layer device of the second bearer, thereby performing transmission. The two lower transmitting PDCP layer device functions 9-21 and 9-22 are characterized in that, to accelerate data processing rates, header compression, integrity protection, or encryption processes can be performed in parallel (parallel data processing), and the two lower transmitting PDCP layer device functions can use different security keys to perform integrity protection or encryption processes. In addition, different types of compression contexts, security keys, or security algorithms can be applied within a logically single transmitting PDCP layer device, thereby performing compression, integrity protection, or encryption processes on different pieces of data.

[0355] In conjunction with a receiving PDCP layer device, and for data received from corresponding lower layer devices (specifically, for data received from two RLC layer devices for each source eNB and each target eNB), the lower receiving PDCP layer device functions 9-21 and 9-22 for the source eNB or target eNB can refer to the PDCP sequence number or COUNT value and independently perform a process of detecting out-of-window data or detecting duplicates for the data received from the corresponding RLC layer device. As another method, for ease of implementation, the UE can refer to the PDCP sequence number or COUNT value and perform a process of detecting out-of-window data or detecting duplicates for the entire received data (without distinguishing between the corresponding RLC layer devices). As another method, for the purpose of more accurate duplicate detection, the UE can refer to the PDCP sequence number or COUNT value and detect out-of-window data for the entire received data (without distinguishing between the corresponding RLC layer devices), and can independently perform a duplicate detection process for each piece of data received from each RLC layer device. As another method, if multiple pieces of data received from different eNBs overlap, the UE can refer to the PDCP sequence number or COUNT value and detect out-of-window data for the entire received data (regardless of the corresponding RLC layer device) to prevent data loss for the header compression protocol. In conjunction with the duplicate detection process, the UE can perform a decryption process, an integrity protection process, or a header (or data) decompression process for each piece of data received from each RLC layer device, and then perform a duplicate detection process for the entire data.

[0356] The lower layer functions of the receiving PDCP layer device are characterized in that the decryption process is immediately applied to the received data by using a separate header (or data) compression context or security key configured for each source eNB and each target eNB, and if integrity protection has been configured, the integrity protection process can be applied to the PDCP header and data (PDCP SDU).

[0357] The (2-2) PDCP layer structure is characterized in that a reordering process is performed on all of the data received from the RLC layer device of the first bearer for each source eNB and the data received from the RLC layer device of the second bearer for each target eNB, and a header (or data) decompression process is performed by applying the header (or data) compression context of each eNB (source eNB or target eNB) to each piece of data received from each eNB (source eNB or target eNB) in ascending order of the PDCP sequence number or COUNT value. In addition, in order to distinguish between the data received from the RLC layer device of the first bearer for each source eNB and the data received from the RLC layer device of the second bearer for each target eNB, the UE can define an indicator for each piece of data to distinguish between the data received from the source eNB and the data received from the target eNB. As another method, the UE can define a one-bit indicator in the PDCP header, SDAP header, or RLC header to distinguish between the data received from the source eNB and the data received from the target eNB. In addition, the UE can perform a duplicate detection process (a process of discarding all data except one piece of data for each PDCP sequence number or COUNT value (which can be applied to previously received data or data delivered to an upper layer)) for all data received from the RLC layer device of the first bearer for the source eNB and the data received from the RLC layer device of the second bearer for the target eNB, which has completed the header (or data) compression process, based on the PDCP sequence number or COUNT value. In addition, the UE can deliver data to the upper layer device in ascending order based on the PDCP sequence number or COUNT value for all data received from the RLC layer device of the first bearer for the source eNB and the data received from the RLC layer device of the second bearer for the target eNB. A single PDCP layer device can receive data from different eNBs, that is, receive data from the first bearer or the second bearer, regardless of the order, and a reordering process must always be performed.

[0358] The two lower receiving PDCP layer device functions are characterized in that, in order to speed up the data processing rate, header compression, integrity protection or encryption processes are performed in parallel (parallel data processing) based on each PDCP sequence number or COUNT value, and different types of header (or data) compression contexts or security keys are used to perform integrity protection, encryption or decompression processes. In addition, the UE can apply different types of header (or data) compression contexts, security keys or security algorithms within a logically single transmitting PDCP layer device, thereby performing integrity protection, encryption or decompression processes for different pieces of data. In addition, the lower receiving PDCP layer device function is characterized in that, regardless of the order of the PDCP sequence number or COUNT value, an out-of-order decryption or integrity verification process is performed for each piece of received data.

[0359] When a single PDCP layer device distinguishes between a layer device of a first bearer and a layer device of a second bearer, consideration is given to the fact that they are connected to different MAC layer devices or are assigned different logical channel identifiers, or to the fact that they are different RLC layer devices connected to different MAC layer devices, or to the fact that they use different encryption keys. In this way, the layer device of the first bearer (or the first RLC layer device) and the layer device of the second bearer (or the second RLC layer device) are distinguished, different security keys are used to perform encryption or decryption processes for uplink data and downlink data, and different types of compression protocol texts are used to compress or decompress uplink data and downlink data.

[0360] The present disclosure proposes a structure of a third PDCP layer device for efficient handover, as shown in 9-30. The structure of the third PDCP layer device can be applied to the second embodiment of the efficient handover method for minimizing data interruption time proposed in the present disclosure. In addition, the structure of the third PDCP layer device proposed in the present disclosure is characterized in that the function of the PDCP layer device is the same as the structure of the second PDCP layer device proposed in the present disclosure. However, the structure of the third PDCP layer device is characterized in that the first bearer for the source eNB is released from the second PDCP layer device structure. Specifically, the structure of the third PDCP layer device proposed in the present disclosure has the same function as the structure of the second PDCP layer device proposed in the present disclosure, but is characterized in that the first bearer for the source eNB (e.g., SDAP layer device, PDCP layer device, RLC layer device or MAC layer device) is released. Therefore, the structure of the third PDCP layer device proposed in the present disclosure is characterized in that the QoS mapping information of the SDAP layer device for the source eNB, the security key information of the PDCP layer device for the source eNB, the header (or data) compression context information for the source eNB, or the RLC layer device or MAC layer device for the source eNB are released.

[0361] If the second condition of the present disclosure is met, or if the UE is to release the radio connection with the source eNB (9-03), the UE may change, reconfigure or switch the structure of the second PDCP layer device to the structure of the first PDCP layer device or the structure of the third PDCP layer device for the bearer for which the second embodiment of the present disclosure (DAPS handover method) is configured. In addition, when the UE changes, reconfigures or switches the structure of the second PDCP layer device to the structure of the first PDCP layer device or the structure of the third PDCP layer device for the bearer for which the second embodiment of the present disclosure (DAPS handover method) is configured, the UE may apply the window parameter (e.g., the (1-1)th window parameter, the (1-2)th window parameter, the (1-3)th window parameter, the (1-4)th window parameter, the (2-1)th window parameter, the (2-2)th window parameter, the (2-3)th window parameter or the (2-4)th window parameter) update process proposed in the present disclosure when changing, reconfiguring or switching from the structure of the first PDCP layer device to the structure of the second PDCP layer device. As another method, if the second condition is met, or if the UE wants to release the wireless connection with the source eNB, when the UE changes, reconfigures or switches the structure of the second PDCP layer device to the structure of the first PDCP layer device or the structure of the third PDCP layer device for the bearer for which the second embodiment of the present disclosure (DAPS handover method) is configured, the values ​​of the window parameters already used (for example, the (1-1)th window parameter, the (1-2)th window parameter, the (1-3)th window parameter, the (1-4)th window parameter, the (2-1)th window parameter, the (2-2)th window parameter, the (2-3)th window parameter or the (2-4)th window parameter) can be maintained and used as is.

[0362] Figure 10 A diagram illustrating UE operations applicable to embodiments proposed in this disclosure is shown.

[0363] exist Figure 10In the embodiment, for each bearer, UE 10-05 can send or receive data with the source eNB through the first PDCP layer device structure. However, if the UE receives a handover command message, and if the handover command message indicates the DAPS handover method of the second embodiment proposed in the present disclosure, or if the handover command message indicates the DAPS handover method for each bearer, the UE switches to the structure of the second PDCP layer device related to the target eNB indicated by the message for each bearer or for the bearer for which the DAPS handover method is indicated. The UE configures / establishes the protocol layer device of the second bearer and performs the process of random access to the target eNB through the established protocol layer device (10-10, 10-15). During this process, the UE can continuously send or receive data (send uplink data and receive downlink data) with the source eNB through the protocol layer device of the first bearer (10-20).

[0364] If the first condition (10-25) is met, the UE may stop sending uplink data to the source eNB through the protocol layer device of the first bearer, may switch uplink data transmission, may send uplink data to the target eNB through the protocol layer device of the second bearer, and may continuously receive downlink data from the source eNB and the target eNB through the protocol layer devices of the first bearer and the second bearer (10-30). In addition, the PDCP layer device of the second bearer may continue to seamlessly send or receive data with the target eNB by using information stored in the PDCP layer device of the first bearer (e.g., sent data, received data, sequence number information, or header compression and decompression context). If the first condition is not met, the UE may continue to check the first condition (10-35) while continuing to perform the already executed process.

[0365] If the second condition (10-40) is met, the UE may stop receiving downlink data from the source eNB through the protocol layer device of the first bearer (10-45). In addition, the PDCP layer device of the second bearer may continue to seamlessly transmit or receive data with the target eNB by using information stored in the PDCP layer device of the first bearer (e.g., transmitted data, received data, sequence number information, or header compression and decompression context) (10-45).

[0366] If the second condition is not satisfied, the UE may continue to check the second condition while continuing to perform the already performed procedure (10-50).

[0367] The specific embodiment of the PDCP layer device proposed in the present disclosure may execute different processes according to the handover type indicated by the handover command message received by the UE, as follows:

[0368] - If the handover type indicated by the handover command message received by the UE from the source eNB corresponds to the handover of the first embodiment (e.g., a normal handover procedure),

[0369] *The UE may perform a PDCP reestablishment procedure for the PDCP layer device for each bearer. For example, the UE may initialize the window state parameters for the SRB and discard the stored data (PDCP SDU or PDCP PDU), and may initialize the window state parameters for the UM DRB. In addition, for data that has not yet been sent to the lower layer device, or for data for which the PDCP discard timer has not expired, the UE may compress it in ascending order of the COUNT value based on the header (or data) compression context or security key of the target eNB, or may encrypt it, or may perform integrity protection, and may perform transmission or retransmission. In addition, if the reordering timer is being driven, the UE may stop and initialize the timer, may continuously process multiple pieces of received data (PDCP SDU or PDCP PDU), and may pass the data to the upper layer device. With respect to AMDRB, the UE may not initialize the window state parameters. The UE can compress or encrypt the first piece of data (PDCP SDU or PDCP PDU) whose successful delivery has not been confirmed from the lower layer device based on the header (or data) compression context or security key of the target eNB, in ascending order of the PDCP sequence number or COUNT value, or perform its integrity protection, and can perform transmission or retransmission.

[0370] - If the handover type indicated by the handover command message received from the source eNB corresponds to the handover of the second embodiment (or the handover indicated for each bearer),

[0371] *After receiving the handover command message indicating the DAPS handover method, the PDCP layer device may perform the following procedures without performing the PDCP re-establishment procedure. For example, for SRBs, the UE may initialize the window state parameters (parameter initialization may be omitted to allow for fallback in the event of a DAPS handover failure), or may discard stored data (PDCP SDUs or PDCP PDUs). In addition, for UM DRBs, the UE may not initialize the window state parameters and may continue to send or receive data with the source eNB for data that has not yet been sent to the lower layer device or for data for which the PDCP discard timer has not expired. In addition, for AM DRBs, the UE may not initialize the window state parameters and may continue to send or receive data with the source eNB.

[0372] *If the first condition of the present disclosure is met, the UE may perform the proposed procedure for each bearer (or for a bearer to which the second embodiment is indicated).

[0373] *If the second condition of the present disclosure is met, the UE may perform the proposed procedure for each bearer (or for a bearer to which the second embodiment is indicated).

[0374] In addition, if the source eNB indicates to the UE that the handover proposed in the embodiment of the present disclosure is applied, the source eNB may start data forwarding if the following third condition is met: The third condition may mean that one or more of the following conditions are met.

[0375] - If an indication is received from the target eNB that the UE has successfully completed the handover

[0376] - If a Handover Command message is sent to the UE

[0377] - If a Handover Command message is sent to the UE and if successful delivery (HARQ ACK, NACK, RLC ACK or NACK) of the Handover Command message is recognized

[0378] - If the source eNB receives an indication from the UE that it is disconnected from the source eNB (e.g., an RRC message (e.g., an RRC reconfiguration message), a MAC CE, an RLC control PDU, or a PDCP control PDU).

[0379] - If the Handover Command message is sent to the UE, if a predetermined timer is driven, and if the timer expires

[0380] - If information regarding confirmation of successful delivery of downlink data (eg, HARQ ACK or NACK, or RLC ACK or NACK) is not received from the UE within a predetermined time.

[0381] Next, the present disclosure proposes a method, wherein, if the eNB indicates the first embodiment (normal handover method) or the second embodiment (DAPS handover method) proposed in the present disclosure to the UE through an RRC message (for example, a handover command message), or if the first embodiment (normal handover method) or the second embodiment (DAPS handover method) proposed in the present disclosure is indicated for each bearer (or each logical channel) of the UE, the UE performs the handover process according to the first embodiment or the second embodiment proposed in the present disclosure, and the LTE or NR PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode) or the LTE or NR PDCP layer device connected to the UM DRB (RLC layer device operating in UM mode) generates and configures conditions for triggering a PDCP status report and the triggered PDCP status report.

[0382] If one of the following conditions is met, each bearer-specific PDCP layer device may trigger, generate, and configure a PDCP status report, and may pass the PDCP status report to the lower layer device for transmission. The operations proposed below may be applied to the PDCP layer device of the UE or eNB.

[0383] - If the upper layer device (RRC layer device) configures the LTE or NR PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode) to trigger or send a PDCP status report (or an indicator (e.g., status report required) in the RRC message configures whether to trigger a PDCP status report)

[0384] * If the UE receives an RRC message (e.g., a handover message) from the eNB, if the message indicates the handover method corresponding to the first embodiment to the UE, and if the PDCP re-establishment procedure is indicated to the LTE or NR PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode), or if the UE's upper layer device (e.g., RRC layer device) indicates the PDCP re-establishment procedure to the LTE or NR PDCP layer device connected to the UE's AM DRB (RLC layer device operating in AM mode)

[0385] **An LTE or NR PDCP layer device connected to an AM DRB (RLC layer device operating in AM mode) can trigger and configure a PDCP status report and can send this PDCP report to the target or source eNB.

[0386] *If the UE receives an RRC message (e.g., a handover message) from the eNB, if the RRC message indicates the handover method corresponding to the first embodiment to the UE, and if the PDCP data resumption procedure is indicated to the LTE or NR PDCP layer device connected to the AM DRB (RLC layer device operating in AM mode), or if the upper layer device (e.g., RRC layer device) indicates the PDCP data resumption procedure to the LTE or NR PDCP layer device connected to the UE's AM DRB (RLC layer device operating in AM mode)

[0387] **An LTE or NR PDCP layer device connected to an AM DRB (RLC layer device operating in AM mode) can trigger and configure a PDCP status report and can send this PDCP report to the target or source eNB.

[0388] *If an RRC message (e.g., a handover message) received by the UE from the eNB indicates a handover method corresponding to the second embodiment (DAPS handover method) to the UE or for each bearer of the UE, and if the DAPS handover method (or the procedure proposed by the present disclosure through an indicator) is indicated to an LTE or NR PDCP layer device connected to an AM DRB (RLC layer device operating in AM mode), or if an upper layer device of the UE (e.g., an RRC layer device) indicates a PDCP handover method (or the procedure proposed by the present disclosure through an indicator) to an LTE or NR PDCP layer device connected to an AM DRB (RLC layer device operating in AM mode) of the UE through an indicator.

[0389] **An LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode) can trigger and configure a PDCP status report and can send the PDCP report to the target or source eNB. If the DAPS handover method is indicated, the source eNB can send a handover command message to the UE for the data of the bearer for which the DAPS handover method is indicated, and can immediately start downlink or uplink data forwarding to the target eNB. Therefore, if the UE sends a PDCP status report on downlink data to the source eNB (indicating whether the downlink data was successfully received), unnecessary data forwarding can be prevented.

[0390] *If an RRC message (e.g., a handover message) received by the UE from the eNB indicates a handover method corresponding to the second embodiment (DAPS handover method) to the UE or for each bearer of the UE, if the DAPS handover method (or the procedure proposed by the indicator in the present disclosure) is configured for an LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode), and if a lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer, if the first timer has stopped) sends an indicator (e.g., an indicator indicating uplink data transmission switching) to the PDCP layer device, because the first condition proposed by the present disclosure is satisfied, the PDCP layer device receives the indicator, or if the lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer, if the first timer has stopped) sends an indicator (e.g., an indicator indicating uplink data transmission switching) to the PDCP layer device, because the first condition proposed by the present disclosure is satisfied, the PDCP layer device receives the indicator.

[0391] **An LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode) may trigger and configure a PDCP status report and may send the PDCP report to the target or source eNB. When the PDCP layer device switches uplink data transmission, the UE may send a PDCP status report (or new PDCP control data) to the source or target eNB, thereby indicating to the source or target eNB that the PDCP layer device corresponding to the UE's bearer has switched uplink data transmission. The source eNB or target eNB may identify the point in time at which the UE meets the first condition and may therefore reflect it in the transmission resources used to schedule and manage the UE.

[0392] *If an RRC message (e.g., a handover message) received by the UE from the eNB indicates a handover method corresponding to the second embodiment (DAPS handover method) to the UE or for each bearer of the UE, if the DAPS handover method (or the procedure proposed by the indicator in the present disclosure) is configured for an LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode), and if a lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer) sends an indicator (e.g., an indicator indicating that it is disconnected from the source eNB) to the PDCP layer device, because the second condition proposed by the present disclosure is satisfied, the PDCP layer device receives the indicator, or if a lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer) sends an indicator (e.g., an indicator indicating that it is disconnected from the source eNB) to the PDCP layer device, because the second condition proposed by the present disclosure is satisfied, the PDCP layer device receives the indicator.

[0393] **An LTE or NR PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode) may trigger and configure a PDCP status report and may send the PDCP report to the target or source eNB. When the PDCP layer device is disconnected from the source eNB, the UE may send a PDCP status report (or new PDCP control data) to the source or target eNB, thereby indicating to the source or target eNB that it has been disconnected from the source eNB. The source or target eNB may identify the point in time at which the UE meets the second condition and may therefore reflect this in scheduling and managing the UE's transmission resources.

[0394] - If the upper layer device (RRC layer device) configures the LTE or NR PDCP layer device connected to the UM DRB (RLC layer device operating in UM mode) to trigger or send a PDCP status report (or an indicator (e.g., status report required) in the RRC message configures whether to trigger a PDCP status report)

[0395] *If an RRC message (e.g., a handover message) received by the UE from the eNB indicates a handover method corresponding to the second embodiment (DAPS handover method) to the UE or for each bearer of the UE, and if the DAPS handover method (or the procedure proposed by the indicator in the present disclosure) is indicated to the LTE or NR PDCP layer device connected to the UM DRB (RLC layer device operating in UM mode), or if an upper layer device of the UE (e.g., an RRC layer device) indicates the DAPS handover method (or the procedure proposed by the indicator in the present disclosure) to the LTE or NR PDCP layer device connected to the UM DRB (RLC layer device operating in UM mode) of the UE

[0396] **An LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode) can trigger and configure a PDCP status report and can send the PDCP report to the target or source eNB. If the DAPS handover method is indicated, the source eNB can send a handover command message to the UE for the data of the bearer for which the DAPS handover method is indicated, and can immediately start forwarding downlink or uplink data to the target eNB. Therefore, if the UE sends a PDCP status report on downlink data to the source eNB (indicating whether the downlink data was successfully received), unnecessary data forwarding can be prevented.

[0397] *If an RRC message (e.g., a handover message) received by the UE from the eNB indicates a handover method corresponding to the second embodiment (DAPS handover method) to the UE or for each bearer of the UE, if the DAPS handover method (or the procedure proposed by the indicator in the present disclosure) is configured for an LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode), if a lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer, if the first timer has stopped) sends an indicator (e.g., an indicator indicating uplink data transmission switching) to the PDCP layer device, because the first condition proposed by the present disclosure is satisfied, such that the PDCP layer device receives the indicator, or if the lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer, if the first timer has stopped) sends an indicator (e.g., an indicator indicating uplink data transmission switching) to the PDCP layer device, because the first condition proposed by the present disclosure is satisfied, such that the PDCP layer device receives the indicator

[0398] **An LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode) may trigger and configure a PDCP status report and may send the PDCP report to the target or source eNB. When the PDCP layer device switches uplink data transmission, the UE may send a PDCP status report (or new PDCP control data) to the source or target eNB, thereby indicating to the source or target eNB that the PDCP layer device corresponding to the UE's bearer has switched uplink data transmission. The source or target eNB may identify the point in time at which the UE meets the first condition and may therefore reflect this in scheduling and managing the UE's transmission resources.

[0399] *If an RRC message (e.g., a handover message) received by the UE from the eNB indicates a handover method corresponding to the second embodiment (DAPS handover method) to the UE or for each bearer of the UE, if the DAPS handover method (or the procedure proposed by the indicator in the present disclosure) is configured for an LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode), and if a lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer) sends an indicator (e.g., an indicator indicating that it is disconnected from the source eNB) to the PDCP layer device, because the second condition proposed by the present disclosure is satisfied, the PDCP layer device receives the indicator, or if a lower layer device (a MAC layer device) or an upper layer device (e.g., an RRC layer) sends an indicator (e.g., an indicator indicating that it is disconnected from the source eNB) to the PDCP layer device, because the second condition proposed by the present disclosure is satisfied, the PDCP layer device receives the indicator.

[0400] **An LTE or NR PDCP layer device connected to a UM DRB (an RLC layer device operating in UM mode) may trigger and configure a PDCP status report and may send the PDCP report to the target or source eNB. When the PDCP layer device is disconnected from the source eNB, the UE may send a PDCP status report (or new PDCP control data) to the source or target eNB, thereby indicating to the source or target eNB that it has been disconnected from the source eNB. The source or target eNB may identify the point in time at which the UE meets the second condition and may therefore reflect this in scheduling and managing the UE's transmission resources.

[0401] If a PDCP status report is triggered in an LTE or NR PDCP layer device connected to an AM DRB (RLC layer device operating in AM mode) or an LTE or NR PDCP layer device connected to a UM DRB (RLC layer device operating in UM mode) according to the PDCP status report triggering condition proposed in the present disclosure, the PDCP status report may be constructed as follows:

[0402] - If a PDCP status report is triggered in the NR PDCP layer device connected to the AM DRB or UM DRB, or if a PDCP status report is triggered

[0403] *The UE may construct a PDCP status report as follows:

[0404] **The UE configures the first missing COUNT (FMC) value of the PDCP status report as the value of the RX_DELIV parameter (the COUNT value of the first data not delivered to the upper layer device).

[0405] **If the RX_DELIV value (the COUNT value of the first data that has not been passed to the upper device) is less than the RX_NEXT value (the COUNT value of the next data to be received)

[0406] ***The UE may configure the length of the bitmap field to include a length corresponding to a multiple of 8, ranging from the COUNT value of the PDCP SDU excluding the first lost PDCP SDU to the COUNT value of the last out-of-sequence data, or may configure the length of the bitmap field to include a length ranging from the COUNT value of the PDCP SDU excluding the first lost PDCP SDU to the COUNT value of the PDCP SDU (such that the PDCP control data (PDCP status report) has a size of 9000 bytes). The length may be configured based on whichever of these two conditions is satisfied first.

[0407] ***If the PDCP SDU corresponding to the bitmap field is not successfully received, or if header decompression failure occurs, the UE may configure the bitmap field corresponding to the PDCP SDU to 0.

[0408] ***If the PDCP SDU corresponding to the bitmap field is successfully received, the UE may configure the bitmap field corresponding to the PDCP SDU to 1.

[0409] When sending a PDCP status report configured as above to a lower layer device, the UE may use the first PDCP PDU of the sending PDCP layer device to send it to the lower layer device. In other words, the UE may assign the highest priority to the PDCP status report and, if a PDCP status report is generated, may first deliver the PDCP report to the lower layer device, thereby ensuring fast transmission.

[0410] - If a PDCP status report is triggered in the LTE PDCP layer device connected to the AM DRB, or if a PDCP status report is triggered

[0411] *If there is data received due to re-establishment of lower layer devices (RLC layer devices), the UE may first process the data and then construct the PDCP status report as follows:

[0412] **The UE configures the first missing PDCP sequence number (FMS) value to a value obtained by adding 1 to the sequence number of the first lost PDCP SDU, the PDCP sequence number of the first data not delivered to the upper layer device, or the PDCP sequence number of the last data delivered to the upper layer device.

[0413] **If at least one out-of-sequence PDCP SDU or more than one of them is stored in the buffer

[0414] ***The UE may configure the length of the bitmap field to include a length corresponding to a multiple of 8, from the PDCP sequence value excluding the first lost PDCP SDU to the PDCP sequence value of the last out-of-sequence data, or may configure the length of the bitmap field to include a length from the PDCP sequence value excluding the first lost PDCP SDU to the PDCP sequence value (such that the PDCP control data (PDCP status report) PDCP SDU has a size of 8188 bytes). The length may be configured based on whichever of these two conditions is satisfied first.

[0415] ***If the PDCP SDU corresponding to the bitmap field is not successfully received, or if header decompression fails, the UE may configure the bitmap field corresponding to the PDCP SDU to 0.

[0416] ***If the PDCP SDU corresponding to the bitmap field is successfully received, the UE may configure the bitmap field corresponding to the PDCP SDU to 1.

[0417] When sending a PDCP status report configured as above to a lower layer device, the UE may use the first PDCP PDU of the sending PDCP layer device to send it to the lower layer device. In other words, the UE may assign the highest priority to the PDCP status report and, if a PDCP status report is generated, may first deliver it to the lower layer device, thereby ensuring fast transmission.

[0418] - If a PDCP status report is triggered in the LTE PDCP layer device connected to the UM DRB, or if a PDCP status report is triggered

[0419] *If there is data received due to re-establishment of lower layer devices (RLC layer devices), the UE may first process the data and then compose a PDCP status report as follows:

[0420] **The UE configures the first missing PDCP sequence number (FMS) value or a new field value to a value obtained by adding 1 to the sequence number of the first lost PDCP SDU, the PDCP sequence number of the first data not delivered to the upper layer device, or the PDCP sequence number of the last data delivered to the upper layer device.

[0421] **If at least one out-of-sequence PDCP SDU or more than one of them is stored in the buffer

[0422] ***The UE may configure the length of the bitmap field to include a length corresponding to a multiple of 8, ranging from the PDCP sequence value excluding the first lost PDCP SDU (or the first PDCP SDU not delivered to the upper layer device or the PDCP SDU of the last data delivered to the upper layer device) to the PDCP sequence value of the last out-of-sequence data, or may configure the length of the bitmap field to include a length ranging from the PDCP sequence value excluding the first lost PDCP SDU to the PDCP sequence value (such that the PDCP control data (PDCP status report) PDCP SDU has a size of 8188 bytes). The length may be configured based on whichever of these two conditions is satisfied first.

[0423] ***If the PDCP SDU corresponding to the bitmap field is not successfully received, or if header decompression fails, the UE may configure the bitmap field corresponding to the PDCP SDU to 0.

[0424] ***If the PDCP SDU corresponding to the bitmap field is successfully received, the UE may configure the bitmap field corresponding to the PDCP SDU to 1.

[0425] When sending a PDCP status report configured as above to a lower layer device, the UE may use the first PDCP PDU of the sending PDCP layer device to send it to the lower layer device. In other words, the UE may assign the highest priority to the PDCP status report and, if a PDCP status report is generated, may first deliver it to the lower layer device, thereby ensuring fast transmission.

[0426] *As another method, if there is data received due to the reestablishment of the lower layer device (RLC layer device), the UE may first process the data. Thereafter, as another method, the UE may construct a PDCP status report as follows:

[0427] **The UE configures the First Missing PDCP Sequence Number (FMS) value of the PDCP Status Report, its Next Received PDCP Sequence Number (NRS) field value, or its new field value as the expected next received PDCP sequence number.

[0428] **If at least one out-of-sequence PDCP SDU or more than one of them is stored in the buffer

[0429] ***The UE may configure the length of the bitmap field to a length corresponding to a multiple of 8, including the PDCP sequence value in descending order from the PDCP sequence value excluding the next expected PDCP SDU to the PDCP sequence value of the last data delivered to the upper layer, or the length of the bitmap field may be configured to a length in descending order from the PDCP sequence value excluding the next expected PDCP SDU to the PDCP sequence value of the PDCP SDU (such that the PDCP control data (PDCP status report) has a size of 8188 bytes). The length may be configured based on whichever of these two conditions is satisfied first.

[0430] ***If the PDCP SDU corresponding to the bitmap field is not successfully received, or if header decompression fails, the UE may configure the bitmap field corresponding to the PDCP SDU to 0.

[0431] ***If the PDCP SDU corresponding to the bitmap field is successfully received, the UE may configure the bitmap field corresponding to the PDCP SDU to 1.

[0432] When sending a PDCP status report configured as above to a lower layer device, the UE may use the first PDCP PDU of the sending PDCP layer device to send it to the lower layer device. In other words, the UE may assign the highest priority to the PDCP status report and, if a PDCP status report is generated, may first deliver it to the lower layer device, thereby ensuring fast transmission.

[0433] If the NR PDCP layer device connected to the UM DRB or AM DRB receives the PDCP status report constructed as above, and if the bitmap field is configured to 1, or if successful delivery of each data having a COUNT value less than the FMC field is confirmed, the PDCP layer device can perform a process of discarding data.

[0434] If the LTE PDCP layer device connected to the AM DRB receives the PDCP status report constructed as above, and if the bitmap field is configured to 1, or if successful delivery of each data having a COUNT value less than the FMS field is confirmed, the PDCP layer device can perform a process of discarding data.

[0435] If the LTE PDCP layer device connected to the UM DRB receives the PDCP status report constructed as above, and if the bitmap field is configured to 1, or if the successful delivery of each piece of data having a COUNT value less than the FMS field (or NRS field or new field) or each piece of data having a COUNT value greater than the FMS field (or NRS field or new field) is confirmed, the PDCP layer device can perform the process of discarding data.

[0436] If the PDCP status report has been received as described above, and if data whose successful delivery is confirmed by the PDCP status report has been delivered to a lower layer device (e.g., an RLC layer device), the PDCP layer device may send an indicator to the lower layer device to instruct it to discard the data. The lower layer device that has received the discard indicator operates as follows:

[0437] *If an indicator indicating discard of data (e.g., PDCP user data) is received from an LTE or NR PDCP layer device, and if the RLC layer device that has received the discard indicator is an LTE RLC layer device

[0438] **If part of the user data (PDCP PDU, PDCP data PDU, or RLC SDU) received from an upper layer device (PDCP layer device) has not been mapped to RLC user data (RLC data PDU) or has not been generated as RLC user data, the LTE PDCP layer device discards the user data. Therefore, if part of the user data has been mapped to RLC user data (RLC data PDU) or has been generated as RLC user data, the LTE PDCP layer device can transmit the data to the source eNB without discarding the user data.

[0439] *If an indicator indicating discard of data (e.g., PDCP user data) has been received from an LTE or NR PDCP layer device, and if the RLC layer device that has received the discard indicator is an NR RLC layer device,

[0440] **If the user data (PDCP PDU, PDCP data PDU or RLC SDU) received from the upper layer device (PDCP layer device) or part of the user data is neither delivered nor sent to the lower layer device, the NR RLC layer device discards the user data. Therefore, if the user data or part of the user data has been delivered or sent to the lower layer device, the NR RLC layer device can send the data to the source eNB without discarding the user data. Therefore, unlike the LTE RLC layer device, the NR RLC layer device can discard more data because even if the user data has been generated as RLC user data, it can be discarded as long as it has not been delivered to the lower layer device. In addition, unnecessary data transmission can be prevented more efficiently.

[0441] Figure 11 The structure of a UE to which the embodiments of the present disclosure are applicable is shown.

[0442] 1 , the UE includes a radio frequency (RF) processing unit 11 - 10 , a baseband processing unit 11 - 20 , a storage unit 11 - 30 , and a control unit 11 - 40 .

[0443] The RF processing unit 11-10 is configured to perform functions for transmitting / receiving signals through a radio channel, such as signal band conversion and amplification. That is, the RF processing unit 11-10 up-converts the baseband signal provided by the baseband processing unit 11-20 into an RF band signal, transmits the RF band signal through an antenna, and down-converts the RF band signal received by the antenna into a baseband signal. For example, the RF processing unit 11-10 may include a transmit filter, a receive filter, an amplifier, a mixer, an oscillator, a digital-to-analog converter (DAC), an analog-to-digital converter (ADC), etc. Although only one antenna is shown in the figure, the UE may include multiple antennas. In addition, the RF processing unit 11-10 may include multiple RF chains. In addition, the RF processing unit 11-10 may perform beamforming. For the purpose of beamforming, the RF processing unit 11-10 may adjust the phase and amplitude of the corresponding signals transmitted / received by multiple antennas or antenna elements. In addition, the RF processing unit may perform MIMO and may receive multiple layers when performing MIMO operations. The RF processing unit 11 - 10 may appropriately configure multiple antennas or antenna elements under the control of the control unit to perform receive beam scanning, or may adjust the direction and beam width of the receive beam so that the receive beam is coordinated with the transmit beam.

[0444] The baseband processing unit 11-20 is configured to perform conversion between baseband signals and bit streams according to the system's physical layer specifications. For example, during data transmission, the baseband processing unit 11-20 generates complex symbols by encoding and modulating the transmitted bit stream. Furthermore, during data reception, the baseband processing unit 11-20 recovers the received bit stream by demodulating and decoding the baseband signal provided by the RF processing unit 11-10. For example, if an orthogonal frequency division multiplexing (OFDM) scheme is used, during data transmission, the baseband processing unit 11-20 generates complex symbols by encoding and modulating the transmitted bit stream, maps the complex symbols to subcarriers, and then configures OFDM symbols through an inverse fast Fourier transform (IFFT) operation and cyclic prefix (CP) insertion. Furthermore, during data reception, the baseband processing unit 11-20 divides the baseband signal provided by the RF processing unit 11-10 into OFDM symbol units, recovers the signals mapped to the subcarriers through a fast Fourier transform (FFT) operation, and then recovers the received bit stream through demodulation and decoding.

[0445] The baseband processing unit 11-20 and the RF processing unit 11-10 send and receive signals as described above. Therefore, the baseband processing unit 11-20 and the RF processing unit 11-10 can be referred to as a sending unit, a receiving unit, a sending / receiving unit, or a communication unit. In addition, at least one of the baseband processing unit 11-20 and the RF processing unit 11-10 may include multiple communication modules to support a variety of different radio access technologies. In addition, at least one of the baseband processing unit 11-20 and the RF processing unit 11-10 may include different communication modules to process signals in different frequency bands. For example, different radio access technologies may include LTE networks, NR networks, etc. In addition, different frequency bands may include ultra-high frequency (SHF) (e.g., 2.5 GHz or 5 GHz) bands and millimeter wave (e.g., 60 GHz) bands.

[0446] The storage unit 11-30 is configured to store data used for UE operation, such as basic programs, application programs, and configuration information. The storage unit 11-30 provides the stored data at the request of the control unit 11-40.

[0447] The control unit 11-40 is configured to control the overall operation of the UE. For example, the control unit 11-40 sends / receives signals through the baseband processing unit 11-20 and the RF processing unit 11-10. In addition, the control unit 11-40 records and reads data in the storage unit 11-40. To this end, the control unit 11-40 may include at least one processor. For example, the control unit 11-40 may include a communication processor (CP) configured to perform communication control and an application processor (AP) configured to control an upper layer (such as an application). The control unit 11-40 may also include a multi-connection processing unit 11-42 that supports multi-connection.

[0448] Figure 12 A block diagram configuration of a TRP in a wireless communication system to which an embodiment of the present disclosure is applicable is shown.

[0449] As shown in the figure, the eNB includes an RF processing unit 12-10, a baseband processing unit 12-20, a backhaul communication unit 12-30, a storage unit 12-40, and a control unit 12-50.

[0450] The RF processing unit 12-10 is configured to perform functions for transmitting / receiving signals via a radio channel, such as signal band conversion and amplification. That is, the RF processing unit 12-10 up-converts the baseband signal provided by the baseband processing unit 12-20 into an RF band signal, transmits the RF band signal via an antenna, and down-converts the RF band signal received via the antenna into a baseband signal. For example, the RF processing unit 12-10 may include a transmit filter, a receive filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc. Although only one antenna is shown in the figure, the first access node may include multiple antennas. In addition, the RF processing unit 12-10 may include multiple RF chains. In addition, the RF processing unit 12-10 may perform beamforming. For the purpose of beamforming, the RF processing unit 12-10 may adjust the phase and amplitude of the corresponding signals transmitted / received via multiple antennas or antenna elements. The RF processing unit may perform downward MIMO operations by transmitting at least one layer.

[0451] The baseband processing unit 12-20 is configured to perform conversion between baseband signals and bit streams according to the physical layer specifications of the first radio access technology. For example, during data transmission, the baseband processing unit 12-20 generates complex symbols by encoding and modulating the transmitted bit stream. Furthermore, during data reception, the baseband processing unit 12-20 recovers the received bit stream by demodulating and decoding the baseband signal provided by the RF processing unit 12-10. For example, if an OFDM scheme is used, during data transmission, the baseband processing unit 12-20 generates complex symbols by encoding and modulating the transmitted bit stream, maps the complex symbols to subcarriers, and then configures OFDM symbols through an IFFT operation and CP insertion. Furthermore, during data reception, the baseband processing unit 12-20 divides the baseband signal provided by the RF processing unit 12-10 into OFDM symbol units, recovers the signals mapped to the subcarriers through an FFT operation, and then recovers the received bit stream through demodulation and decoding. The baseband processing unit 12-20 and the RF processing unit 12-10 transmit and receive signals as described above. Therefore, the baseband processing unit 12-20 and the RF processing unit 12-10 may be referred to as a transmitting unit, a receiving unit, a transmitting / receiving unit, or a communication unit.

[0452] The communication unit 12-30 is configured to provide an interface for communicating with other nodes in the network.

[0453] The storage unit 12-40 is configured to store data used for master eNB operations, such as basic programs, applications, and configuration information. Specifically, the storage unit 12-40 may store information regarding bearers assigned to connected UEs, measurement results reported by connected UEs, and the like. Furthermore, the storage unit 12-40 may store information used as a reference for determining whether to provide or terminate multiple connections to a UE. Furthermore, the storage unit 12-40 provides stored data upon request from the control unit 12-50.

[0454] The control unit 12-50 is configured to control the overall operation of the master eNB. For example, the control unit 12-50 transmits / receives signals via the baseband processing unit 12-20 and the RF processing unit 12-10, or via the backhaul communication unit 12-30. Furthermore, the control unit 12-50 records and reads data from the storage unit 12-40. To this end, the control unit 12-50 may include at least one processor. The control unit 12-50 may also include a multi-connection processing unit 12-52 that supports multi-connection.

[0455] The embodiments of the present disclosure described and shown in the specification and drawings have been presented to easily explain the technical content of the present disclosure and help understand the present disclosure, and are not intended to limit the scope of the present disclosure. Therefore, in addition to the embodiments disclosed herein, the scope of the present disclosure should be interpreted as including all changes and modifications derived based on the technical ideas of the present disclosure.

[0456] Although the present disclosure has been described with various embodiments, various changes and modifications may occur to those skilled in the art. The present disclosure is intended to encompass such changes and modifications as fall within the scope of the appended claims.

Claims

1. A method performed by a terminal in a wireless communication system, the method comprising: receiving, from a source base station, a first radio resource control (RRC) message including first information about a dual-active protocol stack (DAPS) and second information about a need for a status report, wherein the first information indicates that a data radio bearer (DRB) is associated with the DAPS, and the second information indicates that the DRB is configured to send a packet data convergence protocol (PDCP) status report; performing, based on the first RRC message, a random access procedure for DAPS handover with the target base station; When the medium access control (MAC) entity recognizes that the random access procedure is successfully completed, it indicates to the upper layer that the random access procedure is successfully completed; If the upper layer recognizes the successful completion of the random access procedure, it requests the PDCP entity to switch the uplink data for the DRB; In case of uplink data switching requested by upper layers, the first PDCP status report for the DRB is triggered by the PDCP entity; Sending a first PDCP status report to the target base station; receiving, from the target base station, a second RRC message including an indication to release the connection with the source base station; triggering a second PDCP status report for the DRB based on the indication; as well as Send a second PDCP status report to the target base station.

2. The method according to claim 1, in, The DRB associated with the first PDCP status report is an acknowledged mode AM DRB.

3. A method performed by a base station in a wireless communication system, the method comprising: performing, with the terminal, a random access procedure for DAPS handover by using a data radio bearer (DRB) associated with a dual-activity protocol stack (DAPS); receiving, from the terminal, a first Packet Data Convergence Protocol (PDCP) status report for the DRB if the random access procedure is successfully completed, wherein the first PDCP status report is associated with an uplink data handover from the source base station to the base station; sending a radio resource control (RRC) message including an instruction to release the connection with the source base station to the terminal; and A second PDCP status report for the DRB is received from the terminal.

4. The method according to claim 3, wherein: The DRB associated with the first PDCP status report is an acknowledged mode AMDRB.

5. A terminal in a wireless communication system, the terminal comprising: a transceiver configured to transmit and receive signals; as well as The controller is configured as: receiving, from a source base station, a first radio resource control (RRC) message including first information about a dual-active protocol stack (DAPS) and second information about a need for a status report, wherein the first information indicates that a data radio bearer (DRB) is associated with the DAPS, and the second information indicates that the DRB is configured to send a packet data convergence protocol (PDCP) status report, Based on the first RRC message, performing a random access procedure for DAPS handover with the target base station, When the MAC entity recognizes that the random access procedure is successfully completed, it indicates to the upper layer that the random access procedure is successfully completed. When the upper layer recognizes the successful completion of the random access procedure, it requests the PDCP entity to switch the uplink data for the DRB. In case of uplink data switching requested by the upper layer, the first PDCP status report for the DRB is triggered by the PDCP entity. Sending a first PDCP status report to the target base station; receiving, from the target base station, a second RRC message including an indication to release the connection with the source base station, triggering a second PDCP status report for the DRB based on the indication, and Send a second PDCP status report to the target base station. The terminal according to claim 5 , in, The DRB associated with the first PDCP status report is an acknowledged mode AM DRB.

7. A base station in a wireless communication system, the base station comprising: a transceiver configured to transmit and receive signals; as well as The controller is configured as: performing a random access procedure for DAPS handover with the terminal by using a data radio bearer (DRB) associated with the dual active protocol stack (DAPS), receiving, from the terminal, a first Packet Data Convergence Protocol (PDCP) status report for the DRB in a case where the random access procedure is successfully completed, wherein the first PDCP status report is associated with an uplink data handover from a source base station to the base station, sending a radio resource control (RRC) message to the terminal including an instruction to release the connection with the source base station, and A second PDCP status report for the DRB is received from the terminal. The base station according to claim 7 , wherein: The DRB associated with the first PDCP status report is an acknowledged mode AMDRB.

Images