Methods and apparatus for configuring fallback for each bearer in next-generation mobile communication systems when DAPS handover fails.

Abstract

This disclosure relates to a communication method and system for integrating fifth-generation (5G) communication systems to support higher data rates than fourth-generation (4G) systems using Internet of Things (IoT) technology. This disclosure can be applied to smart services based on 5G communication technology and IoT-related technologies, such as smart homes, smart buildings, smart cities, smart cars, connected cars, healthcare, digital education, smart retail, and security services. This disclosure also relates to a method and apparatus for performing a rollback in the event of a handover failure while performing an efficient handover that does not interrupt data transmission or reception during the handover process.

Description

TECHNICAL FIELD

[0001] The disclosure relates to a mobile communication system and a wireless communication system. More particularly, the disclosure relates to a method and apparatus for performing fallback at handover failure in case of performing an efficient handover method without data transmission / reception interruption during handover in a next-generation mobile communication system. BACKGROUND

[0002] To meet the demand for wireless data traffic having increased since deployment of 4th generation (4G) communication systems, efforts have been made to develop an improved 5th generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a 'Beyond 4G Network' or a 'Post long term evolution (LTE) System'.

[0003] The 5G communication system is considered to be implemented in a frequency band of 6 GHz or more, e.g., a 60 GHz band, so as to accomplish a higher data rate. To reduce propagation loss of the radio waves and increase a transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beam forming, large scale antenna techniques are discussed in 5G communication systems.

[0004] In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, a device to device (D2D) communication, a wireless backhaul, a mobile network, a cooperative communication, coordinated multi-points (CoMP), a reception-end interference cancellation, and the like.

[0005] In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), a non-orthogonal multiple access (NOMA), and a sparse code multiple access (SCMA) as an advanced access technology have been developed.

[0006] The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged as a new paradigm for the IoT. As the technology elements of the IoT, such as "sensing technology", "wired / wireless communication and network infrastructure", "service interface technology", and "security technology" have recently been developed, research on the sensor network, Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth has been conducted. The IoT has appeared as a new technological innovation for the existing information technology (IT) to provide a smart internet technological environment that is based on the interworking of the existing Internet technology and various industrial applications. The IoT is a combination of the technologies of the sensor network, M2M communication, MTC, and so forth, and the Big Data processing technology through the connection with the cloud server, and it has been applied to a variety of fields including smart home, smart building, smart city, smart car or connected car, smart grid, health care, smart home appliance, and advanced medical services.

[0007] In line with this, various attempts have been made to apply the 5G communication system to the IoT network. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication can be implemented by beamforming, MIMO, and array antennas. Application of a cloud Radio Access Network (RAN) as the above-described Big Data processing technology can also be considered as an example of convergence between the 5G technology and the IoT technology.

[0008] Meanwhile, recent development of communication systems has been accompanied by various researches on handover. Specifically, there has been extensive research on performing efficient handover without transmission / reception interruption.

[0009] The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. SUMMARY

[0010] TECHNICAL PROBLEM

[0011] There is a need for enhancing the current handover procedure for the next generation wireless communication system.

[0012] TECHNICAL SOLUTION

[0013] An efficient handover method is necessary in the next generation mobile communication system so that a service without data interruption and a low transmission latency can be supported. In addition, a method that can perform fallback while minimizing data interruption time in case of handover failure is needed.

[0014] Aspects of the present disclosure will at least solve the above-mentioned problems and / or disadvantages and provide at least the advantages described below. Accordingly, an aspect of the present disclosure is to provide various efficient handover methods in which, when handover is performed in a next-generation mobile communication system, a data interruption time does not occur due to handover, thereby supporting a service without data interruption.

[0015] Another aspect of the present disclosure is to provide an efficient method in which, if handover fails, a terminal or user equipment (UE) can quickly fall back to a source base station.

[0016] Additional aspects will be set forth in part in the description which follows, and, in part, will become apparent to those skilled in the art by reference to the description, or can be learned by practice of the presented embodiments.

[0017] According to an aspect of the present disclosure, a method performed by a terminal in a wireless communication system is provided. The method includes receiving, from a base station, information for reconfiguration with synchronization (sync), the information configuring at least one dual active protocol stack (DAPS) bearer, based on the information, establishing a signaling radio bearer (SRB) for a target cell group with a same configuration as a source cell group, wherein the SRB for the source cell group is suspended, and in a case where a security key is maintained for the SRB for the target cell group, configuring an initial value of a state variable of a packet data convergence protocol (PDCP) layer device for the target cell group with a value stored in the PDCP layer device for the source cell group.

[0018] According to another aspect of the present disclosure, a method performed by a base station in a wireless communication system is provided. The method includes generating, for a terminal, information for reconfiguration with synchronization, the information configuring at least one DAPS bearer, and transmitting the information to the terminal. Based on the information, a SRB for a target cell group is established with a same configuration as a source cell group, the SRB for the source cell group is suspended, and in a case where a security key is maintained for the SRB for the target cell group, an initial value of a state variable of a PDCP layer device for the target cell group is configured with a value of the PDCP layer device for the source cell group.

[0019] According to another aspect of the disclosure, a terminal in a wireless communication system is provided. The terminal includes a transceiver configured to transmit and receive signals and a controller. The controller is configured to receive, from a base station, information for reconfiguration with synchronization, the information configuring at least one DAPS bearer, based on the information, establish SRBs for a target cell group with a same configuration as a source cell group, wherein the SRBs for the source cell group are suspended, and in a case where security keys are maintained for the SRBs for the target cell group, configure initial values of state variables of PDCP layer devices for the target cell group with values stored in the PDCP layer devices for the source cell group.

[0020] According to another aspect of the disclosure, a base station in a wireless communication system is provided. The base station includes a transceiver configured to transmit and receive signals and a controller. The controller is configured to generate, for a terminal, information for reconfiguration with synchronization, the information configuring at least one DAPS bearer, and transmit the information to the terminal. Based on the information, SRBs for a target cell group are established with a same configuration as a source cell group, the SRBs for the source cell group are suspended, and in a case where security keys are maintained for the SRBs for the target cell group, initial values of state variables of PDCP layer devices for the target cell group are configured with values of the PDCP layer devices for the source cell group.

[0021] Various embodiments of the disclosure have been disclosed in detail in the following detailed description of the drawings, other aspects, advantages, and significant features of the disclosure will become apparent to those skilled in the art.

[0022] Technical effects

[0023] According to various embodiments of the disclosure, a handover procedure can be efficiently enhanced. BRIEF DESCRIPTION OF DRAWINGS

[0024] The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0025] Figure 1 A structure of a long term evolution (LTE) system according to an embodiment of the disclosure is illustrated;

[0026] Figure 2 A radio protocol structure in an LTE system according to an embodiment of the disclosure is illustrated;

[0027] Figure 3 A structure of a next-generation mobile communication system according to an embodiment of the disclosure is illustrated;

[0028] Figure 4 A radio protocol structure of a next-generation mobile communication system according to an embodiment of the disclosure is illustrated;

[0029] Figure 5 A procedure for establishing a connection with a network by switching a mode of a user equipment (UE) from a radio resource control (RRC) idle mode to an RRC connected mode according to an embodiment of the disclosure is illustrated;

[0030] Figure 6 A signaling procedure for performing a handover in a next-generation mobile communication system according to an embodiment of the disclosure is illustrated;

[0031] Figure 7 Particular stages of a first embodiment of an efficient handover method for minimizing a data interruption time due to a handover according to an embodiment of the disclosure are illustrated;

[0032] Figure 8 Particular stages of a second embodiment of an efficient handover method for minimizing a data interruption time due to a handover according to an embodiment of the disclosure are illustrated;

[0033] Figure 9A And Figure 9B A structure of an efficient packet data convergence protocol (PDCP) layer device applied in a dual active protocol stack (DAPS) handover method, which is a second embodiment of an efficient handover method, and a method of applying the structure according to various embodiments of the disclosure are illustrated;

[0034] Figure 10 A method of applying configuration information for each bearer when a DAPS handover method is configured according to an embodiment of the disclosure is illustrated;

[0035] Figure 11A And Figure 11B A method for driving different bearer-specific protocol layer devices for a signaling radio bearer (SRB), a bearer for which a DAPS handover method is configured, or a bearer for which a DAPS handover method is not configured by a UE that has received an RRCReconfiguration message or an RRCConnectionReconfiguration message indicating a DAPS handover method for each bearer to the UE according to various embodiments of the disclosure is illustrated;

[0036] Figure 12A , Figure 12B , Figure 13A , Figure 13B , Figure 14A And Figure 14BA method for driving different bearer-specific protocol layer devices by a UE that has received a message for a signaling radio bearer (SRB), a bearer configured with a DAPS handover method, or a bearer not configured with a DAPS handover method, when a DAPS handover method is indicated to the UE per bearer through an RRCReconfiguration message or an RRCConnectionReconfiguration message, according to various embodiments of the disclosure is shown;

[0037] Figure 15 A UE operation suitable for embodiments of the disclosure is shown;

[0038] Figure 16 A UE operation performing a fallback procedure when a handover fails in a DAPS handover method, according to embodiments of the disclosure is shown.

[0039] Figure 17 A structure of a UE according to embodiments of the disclosure is shown; and

[0040] Figure 18 A block diagram configuration of a transmission and reception point (TRP) in a wireless communication system according to embodiments of the disclosure is shown.

[0041] In all the drawings, the same reference numerals are used for the same elements throughout the several several views. DETAILED DESCRIPTION

[0042] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be taken as illustrative only. Accordingly, one of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the disclosure. In addition, descriptions of well-known functions and constructions can be omitted for clarity and conciseness.

[0043] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but are merely used to enable a clear and consistent understanding of the disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the disclosure as defined by the appended claims and their equivalents.

[0044] It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a component surface" includes reference to one or more of such surfaces.

[0045] Here, it will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart block or blocks. These computer program instructions can also be stored in a computer-usable or computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer- usable or computer-readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operations to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks.

[0046] Furthermore, each block of the flowchart illustrations can represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical functions. It should also be noted that in some alternative implementations, the functions noted in the blocks can occur out of the order noted in the flowcharts. For example, two blocks shown in succession can in fact be executed substantially concurrently or the blocks can sometimes be executed in the reverse order, depending upon the functionality involved.

[0047] As used herein, a "unit" refers to a software element or hardware element that performs a predetermined function, such as a Field Programmable Gate Array (FPGA) or an Application-Specific Integrated Circuit (ASIC). However, a "unit" does not always have + the meaning limited to software or hardware. The "unit" can be constructed so as to be stored in a computer-readable storage medium or execute one or more processors. Thus, the "unit" includes, for example, a software element, an object-oriented software element, a class element or a task element, a process, a function, an attribute, a program, a subroutine, a program code segment, a driver, firmware, a microcode, a circuit, data, a database, a data structure, a table, an array, and a parameter. Elements and functions provided by the "unit" can be combined with fewer elements and "units" or divided into more elements and "units." Also, the elements and "units" can be implemented to reproduce one or more CPUs within a device or a secure multimedia card.

[0048] In the following description, terms for identifying an access node, terms related to network entities, terms related to messages, terms related to interfaces between network entities, terms related to various identification information, and the like are illustratively used for convenience. Accordingly, the disclosure is not limited to the terms used below, and other terms related to the subject matter having equivalent technical meanings can be used.

[0049] In the following description, terms and names defined in the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) standard will be used to describe the disclosure for convenience of description. However, the disclosure is not limited to these terms and names, and can be applied in the same manner to systems conforming to other standards. In the disclosure, the term "eNB" can be used interchangeably with the term "gNB". That is, a base station described as an "eNB" can indicate a "gNB".

[0050] The disclosure proposes a seamless handover method capable of minimizing or reducing to 0 ms a data interruption time due to handover in a next-generation mobile communication system.

[0051] Specifically, the efficient handover method of the disclosure can include one or more of a plurality of features as follows. In addition, with respect to the efficient handover method of the disclosure, different handover methods can be applied to different bearers. In addition, the efficient handover method of the disclosure can include features derived by combining some or all of the following features with each other.

[0052] - When a UE performing data transmission to a source base station or data reception from the source base station (uplink data transmission or downlink data reception) through a plurality of first bearers of respective protocol layer devices (Physical (PHY) layer devices, Medium Access Control (MAC) layer devices, Radio Link Control (RLC) layer devices, or PDCP layer devices) can receive a handover command message (or an RRC reconfiguration message) from the source base station. The UE that has received the handover command message can configure a plurality of second bearers of new protocol layer devices (e.g., having the same bearer identifier as the protocol layer devices of the plurality of first bearers) corresponding to the protocol layer devices of the plurality of first bearers, and can maintain and perform data transmission to the source base station or data reception from the source base station (uplink data transmission or downlink data reception) without interruption through the plurality of first bearers.

[0053] - After receiving the handover command message as described above, based on the bearer configuration information or the protocol layer device information included in the handover command message, configuring a plurality of second bearers of newly configured protocol layer devices (PHY layer devices, MAC layer devices, RLC layer devices, or PDCP layer devices) for transmitting data to a target base station or receiving data from the target base station.

[0054] - In the above, the UE performs a random access procedure to the target base station through the protocol layer devices (e.g., MAC layer devices) of the multiple second bearers, while performing data transmission to or reception from the source base station through the protocol layer devices of the multiple first bearers (uplink data transmission or downlink data reception). In this case, the random access procedure can include transmission of a preamble, reception of a random access response, transmission of a message 3, or reception of a message 4 (e.g., reception of a contention resolution MAC control element (CE) or an uplink transmission resource).

[0055] - In the above, the UE completes a random access procedure to the target base station through the protocol layer devices (e.g., MAC layer devices) of the multiple second bearers, while performing data transmission to or reception from the source base station through the protocol layer devices of the multiple first bearers, and then transmits a handover complete message to the target base station through the protocol layer devices of the multiple second bearers.

[0056] - In the above, the UE completes a random access procedure to the target base station through the protocol layer devices (e.g., MAC layer devices) of the multiple second bearers, while performing data transmission to or reception from the source base station through the protocol layer devices of the multiple first bearers, and then transmits a handover complete message to the target base station through the protocol layer devices of the multiple second bearers, and performs data transmission or reception (uplink or downlink).

[0057] - In the above, when the UE successfully completes a random access procedure to the target base station, or when the UE first receives an uplink transmission resource from the target base station, the UE stops transmitting uplink data to the source base station through the protocol layer devices of the multiple first bearers, and switches uplink transmission so as to transmit uplink data to the target base station through the multiple second bearers.

[0058] - In the above, when the handover command message is received, the UE can continue to transmit data to or receive data from the source base station through the protocol layer devices of the multiple first bearers (uplink data transmission or downlink data reception), and can perform a random access procedure to the target base station through the protocol layer devices of the multiple second bearers. Further, when the UE successfully completes the random access procedure, or when the UE first receives an uplink transmission resource from the target base station, the UE can stop transmitting uplink data to the source base station through the protocol layer devices of the multiple first bearers, and transmit uplink data to the target base station through only the protocol layer devices of the multiple second bearers. Further, the UE can continuously receive downlink data from the source base station through only the protocol layer devices of the multiple first bearers, and continuously receive downlink data from the target base station through the protocol layer devices of the multiple second bearers.

[0059] - In the above, the first bearer and the second bearer can be configured as a structure of a second PDCP layer device, and the second PDCP layer device structure can be configured such that the first bearer (e.g., an RLC layer device, a MAC layer device, or a PHY layer device) for the source base station and the second bearer (e.g., an RLC layer device, a MAC layer device, or a PHY layer device) for the target base station are both connected to one PDCP layer device. Here, the uplink data can be transmitted through the PDCP layer device via one of the first bearer or the second bearer. That is, before the random access procedure to the target base station is completed and the random access procedure is successfully completed, or before the uplink transmission resource is initially received from the target base station, the UE transmits the uplink data through the first bearer; if the random access procedure to the target base station is performed, the random access procedure is successfully completed, and the uplink transmission resource is then first received from the target base station, the UE can stop data transmission through the first bearer and perform handover, thereby transmitting the uplink data to the target through the second bearer. However, in the second PDCP layer device structure, the UE can receive downlink data from the source base station or the target base station through the first bearer or the second bearer.

[0060] In the following, the present disclosure proposes an efficient handover procedure in which a data interruption time does not occur, based on the above-described features.

[0061] Further, in the following, in the case where the UE performs the efficient handover method of the present disclosure in which a data interruption time does not occur, if the UE fails to hand over, a method of quickly falling back to the source base station to configure connection again by using the features of the efficient handover method of the present disclosure is provided. As described above, the efficient handover method of the present disclosure specifically relates to maintaining connection to the source base station even when the handover procedure is performed, and performing fallback using the wireless connection connected to the existing source base station even if the handover fails.

[0062] Figure 1 A structure of a long term evolution (LTE) system according to an embodiment of the present disclosure is illustrated.

[0063] Reference Figure 1 The radio access network of the LTE system includes next generation base stations (also referred to as evolved Node Bs, hereinafter eNB, Node B, or base station) 1-05, 1-10, 1-15, and 1-20, a mobility management entity (MME) 1-25, and a serving gateway (S-GW) 1-30. A user equipment (hereinafter UE or terminal) 1-35 accesses an external network through the eNBs 1-05 to 1-20 and the S-GW 1-30.

[0064] In Figure 1In the middle, the eNBs 1-05 to 1-20 correspond to the existing Node B of the UMTS system. The eNB is connected to the UE 1-35 through a radio channel and performs a more complex task than the existing Node B. In the LTE system, since all user traffic related to real-time services, such as Voice over Internet Protocol (VoIP) via an Internet Protocol, is serviced through a shared channel, a device that performs scheduling by collecting state information such as a buffer state of the UE, an available transmission power state, and a channel state is required, and the eNBs 1-05 to 1-20 are responsible for this function of the device. Generally, one eNB controls multiple cells. For example, in order to implement a transmission rate of 100 Mbps, the LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in a 20 MHz bandwidth. In addition, the LTE system adopts an Adaptive Modulation and Coding (hereinafter, referred to as AMC) scheme for determining a modulation scheme and a channel coding rate based on a channel state of the UE. The S-GW 1-30 is a device for providing a data bearer under the control of the MME 1-25 and generating or removing the data bearer. The MME is responsible for various control functions in addition to the mobility management function of the UE and is connected to multiple base stations.

[0065] Figure 2 A radio protocol structure in an LTE system according to an embodiment of the disclosure is illustrated.

[0066] Reference Figure 2 The radio protocol of the LTE system includes PDCPs 2-05 and 2-40, RLCs 2-10 and 2-35, and MACs 2-15 and 2-30 in the UE and the eNB, respectively. The PDCPs 2-05 and 2-40 are used to perform operations such as IP header compression / restoration. The main functions of the PDCP are summarized as follows.

[0067] - Header compression and decompression: ROHC (Robust Header Compression) only

[0068] - Transfer of user data

[0069] - In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure in RLC-AM

[0070] - Sequence number reordering (for split bearers in DC (RLC-AM only): PDCP PDU routing for transmission and reordering of PDCP PDUs for reception)

[0071] - Duplicate detection of lower layer SDUs (Service Data Units) at PDCP re-establishment procedure in RLC-AM

[0072] - For RLC AM, retransmission of PDCP SDU at handover and for split bearer in DC, retransmission of PDCP PDU at PDCP data recovery procedure

[0073] - ciphering and deciphering

[0074] - timer-based SDU discard in uplink

[0075] RLC 2-10 and 2-35 can reconfigure PDCP protocol data units (PDUs) with a suitable size and perform ARQ operation. The main functions of RLC are summarized as follows.

[0076] - transfer of upper layer PDUs

[0077] - ARQ (error correction through ARQ (only for AM data transfer))

[0078] - concatenation, segmentation, and reassembly of RLC SDUs (only for unacknowledged mode (UM) and AM data transfer)

[0079] - re-segmentation of RLC data PDUs (only for AM data transfer)

[0080] - reordering of RLC data PDUs (only for UM and AM data transfer)

[0081] - duplicate detection (only for UM and AM data transfer)

[0082] - protocol error detection (only for AM data transfer)

[0083] - RLC SDU discard (only for UM and AM data transfer)

[0084] - RLC re-establishment

[0085] MAC 2-15 and 2-30 are connected to multiple RLC layer devices configured in one UE and can perform operations of multiplexing and demultiplexing RLC PDUs with MAC PDUs. The main functions of MAC are summarized as follows.

[0086] - mapping between logical channels and transport channels

[0087] - multiplexing of MAC SDUs belonging to one or different logical channels into transport channels, or demultiplexing of MAC SDUs from transport channels into transport blocks (TBs) delivered to / from the physical layer

[0088] - scheduling information reporting

[0089] - error correction through hybrid automatic repeat request (HARQ)

[0090] - priority handling between logical channels of one UE

[0091] - priority handling between UEs through dynamic scheduling

[0092] - multimedia broadcast multicast service (MBMS) service identification

[0093] - transport format selection

[0094] - padding

[0095] The physical layer 2-20 and 2-25 can perform operations of channel-coding and modulating higher layer data, forming the higher layer data into OFDM symbols, transmitting the OFDM symbols through a radio channel, or demodulating the OFDM symbols received through the radio channel, channel-decoding the OFDM symbols, and transmitting the OFDM symbols to a higher layer.

[0096] Figure 3 A structure of a next-generation mobile communication system according to an embodiment of the disclosure is shown.

[0097] Referring to Figure 3 , a radio access network of a next-generation mobile communication system (hereinafter, NR or 5G) includes a new radio node B (hereinafter, NR, gNB, or NR base station) 310 and a new radio core network (NR CN) 305. A user terminal (new radio user equipment, hereinafter, NR UE or terminal) 315 accesses an external network via the NR gNB 310 and the NR CN 305.

[0098] In Figure 3In the middle, the NR gNB 310 corresponds to an evolved Node B (eNB) of the existing LTE system. The NR gNB is connected to the NR UE 315 via a radio channel and can provide excellent services compared to the existing Node B. In the next-generation mobile communication system, since all types of user traffic are serviced through a shared channel, a device for performing scheduling by collecting state information such as a buffer state of a UE, an available transmission power state, and a channel state is required. In addition, the NR NB 310 is responsible for this function of the device. Generally, one NR gNB usually controls multiple cells. In order to implement ultra-high-speed data transmission compared to the existing LTE, the NR gNB can have the existing maximum bandwidth or a larger bandwidth, and can additionally employ a beamforming technique using orthogonal frequency division multiplexing (OFDM) as a radio access technology. In addition, the NR gNB employs an adaptive modulation and coding (AMC) scheme that determines a modulation scheme and a channel coding rate based on a channel state of a UE. The NR CN 305 performs functions such as mobility support, bearer configuration, QoS configuration, etc. The NR CN is a device responsible for various control functions in addition to the mobility management function of the UE, and is connected to multiple base stations. In addition, the next-generation mobile communication system can also operate in conjunction with the existing LTE system, and the NR CN can be connected to the MME 325 via a network interface. The MME is connected to the eNB 330, that is, to the existing base station.

[0099] Figure 4 A radio protocol structure of a next-generation mobile communication system according to an embodiment of the disclosure is shown.

[0100] Reference Figure 4 The radio protocol of the next-generation mobile communication system includes 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 in the UE and the NR base station, respectively.

[0101] The main functions of the NR SDAP 4-01 and 4-45 can include some of the following functions.

[0102] - Transfer of user plane data

[0103] - Mapping between QoS flows and DRBs for both downlink (DL) and uplink (UL)

[0104] - Marking QoS flow ID in both DL and UL packets

[0105] - Mapping of reflective QoS flow to DRB of UL SDAP PDU

[0106] For the SDAP layer device, the UE can be configured by an RRC message whether to use a header of the SDAP layer device or a function of the SDAP layer device for each PDCP layer device, for each bearer, and for each logical channel. When the SDAP header is configured, a NAS reflective QoS reflection 1-bit indicator (NAS reflective QoS) and an AS QoS reflection 1-bit indicator (AS reflective QoS) of the SDAP header are used to indicate that the UE enables to update or reconfigure mapping information of a QoS flow for uplink and downlink and mapping information for a data bearer. The SDAP header can include QoS flow ID information indicating QoS. The QoS information can be used as data processing priority, scheduling information, etc., to support a smooth service.

[0107] The main functions of the NR PDCP 4-05 and 4-40 can include some of the following functions.

[0108] - header compression and decompression (ROHC only)

[0109] - transfer of user data

[0110] - in-sequence delivery of upper layer PDUs

[0111] - out-of-sequence delivery of upper layer PDUs

[0112] - reordering of received PDCP PDUs

[0113] - duplicate detection of lower layer SDUs

[0114] - retransmission of PDCP SDUs

[0115] - ciphering and deciphering

[0116] - timer-based SDU discard in uplink

[0117] The reordering function of the NR PDCP device refers to a function of reordering PDCP PDUs received from a lower layer in order based on a PDCP sequence number (SN), and can include a function of transmitting data to an upper layer in a reordered order, a function of directly transmitting data to an upper layer without considering order, a function of reordering a sequence and recording a missing PDCP PDU, a function of providing a status report on a missing PDCP PDU to a transmission side, and a function of requesting retransmission of a missing PDCP PDU.

[0118] The main functions of the NR RLC 4-10 and 4-35 can include some of the following functions.

[0119] - transfer of upper layer PDUs

[0120] - In-sequence delivery of upper layer PDUs

[0121] - Out-of-sequence delivery of upper layer PDUs

[0122] - Error correction through ARQ

[0123] - Contatenation, segmentation, and reassembly of RLC SDUs

[0124] - Re-segmentation of RLC data PDUs

[0125] - Reordering of RLC data PDUs

[0126] - Duplicate detection

[0127] - Protocol error detection

[0128] - RLC SDU discard

[0129] - RLC re-establishment

[0130] The in-sequence delivery function of the NR RLC device refers to a function of transmitting the RLC SDU received from a lower layer to an upper layer in the order of reception, and if one RLC SDU is initially segmented into a plurality of RLC SDUs and received, can include a function of reassembling and transmitting the plurality of RLC SDUs. The in-sequence delivery function can include a function of reordering the received RLC PDUs based on the RLC SN or the PDCP SN, reordering the sequence and recording the missing RLC PDUs, providing a status report about the missing RLC PDUs to the transmission side, and requesting retransmission of the missing RLC PDUs. Alternatively, the in-sequence delivery function of the NR RLC device can include a function of sequentially transmitting only the RLC SDUs before the missing RLC SDU to the upper layer if the RLC SDU is missing, or sequentially transmitting all the RLC SDUs received before the timer starts to the upper layer even if there is a missing RLC SDU if the timer expires, or sequentially transmitting all the RLC SDUs received so far to the upper layer even if there is a missing RLC SDU if a predetermined timer expires. Further, the RLC PDU can be processed in the order in which the RLC PDU is received (in the order of arrival, regardless of the sequence number or the serial number), and can be transmitted to the PDCP device in a manner of out-of-sequence delivery. The in-sequence delivery function can include a function of receiving segments stored in a buffer or to be received later, reconfiguring the segments in one complete RLC PDU, processing the RLC PDU, and transmitting the RLC PDU to the PDCP device. The NR RLC layer can not include the concatenation function, and the concatenation function can be performed by the NR MAC layer, or can be replaced by the multiplexing function of the NR MAC layer.

[0131] The out-of-sequence delivery function of the NR RLC device refers to a function of directly transmitting the RLC SDU received from a lower layer to an upper layer regardless of the order thereof, and if one RLC SDU has been initially segmented into a plurality of RLC SDUs and received, can include a function of reassembling the plurality of RLC SDUs and transmitting them, and a function of storing the RLC SN or the PDCP SN of the received RLC PDU, reordering the sequence, and recording the missing RLC PDU.

[0132] The NR MACs 4-15 and 4-30 can be connected to a plurality of NR RLC layer devices configured in one UE, and the main functions of the NR MAC can include some of the following functions.

[0133] - Mapping between logical channels and transport channels

[0134] - Multiplexing / demultiplexing of MAC SDUs

[0135] - Scheduling information reporting

[0136] - Error correction through HARQ

[0137] - Priority handling between logical channels of one UE

[0138] - Priority handling between UEs through dynamic scheduling

[0139] - MBMS service identity

[0140] - Transport format selection

[0141] - Padding

[0142] The NR PHY layers 4-20 and 4-25 can perform the following operations: channel coding and modulation on higher layer data, forming the higher layer data into OFDM symbols, transmitting the OFDM symbols via a radio channel, or demodulating and channel decoding the OFDM symbols received via a radio channel, and delivering the OFDM symbols to a higher layer.

[0143] Figure 5 A procedure for establishing a connection with a network by switching a mode of a UE from an RRC idle mode to an RRC connected mode is shown according to an embodiment of the disclosure.

[0144] Reference Figure 5 When a UE that transmits and receives data in an RRC connected mode does not transmit or receive data for a predetermined reason or for a predetermined period of time, the base station can transmit an RRCConnectionRelease message to the UE to switch the UE to an RRC idle mode (indicated by reference numeral 5-01). In the future, when data to be transmitted is generated, the UE that does not currently have a connection established (hereinafter, an idle mode UE) performs an RRC connection establishment procedure with the base station. The UE establishes reverse transmission synchronization with the base station through a random access procedure, and transmits an RRCConnectionRequest message to the base station (indicated by reference numeral 5-05). The message includes a reason for establishing a connection (establishmentCause) and an identifier of the UE. The base station transmits an RRCConnectionSetup message so that the UE establishes an RRC connection (indicated by reference numeral 5-10).

[0145] The message includes configuration information for each service, bearer, RLC device, or logical channel, information indicating whether to use ROHC for each bearer or logical channel, ROHC configuration information (e.g., ROHC version, initial information, etc.), statusReportRequired information by which the base station instructs the UE to perform a PDCP status report, and drb-ContinueROHC information indicating configuration information to maintain and use ROHC configuration information as is, and the configuration information can be included in PDCP layer device configuration information (pdcp-config) and transmitted. In addition, the message includes RRC connection configuration information, etc. A bearer for an RRC connection is also referred to as a signaling radio bearer (SRB), and is used for transmission or reception of an RRC message, which is a control message between the UE and the base station.

[0146] The UE that has established the RRC connection transmits an RRCConnectionSetupComplete message to the base station (indicated by reference numeral 5-15). The message includes a control message called "SERVICE REQUEST" by which the UE requests the MME to establish a bearer for a predetermined service. The base station transmits the "SERVICE REQUEST" message included in the RRCConnectionSetupComplete message to the MME or an access management function (AMF) (indicated by reference numeral 5-20), and the MME or AMF determines whether to provide the service requested by the UE. As a result of the determination, if the MME or AMF determines to provide the service requested by the UE, the MME or AMF transmits a message called "INITIAL CONTEXT SETUP REQUEST" (or "INI CONTEXT SETUP") to the base station (indicated by reference numeral 5-25). The message includes information such as QoS information to be applied when establishing a data radio bearer (DRB) and security-related information (e.g., security key, security algorithm) to be applied to the DRB.

[0147] Further, when the base station does not receive the capability information of the UE from the MME or the AMF, the base station can transmit a UE capability information query message to the UE in order to identify the capability information of the UE (indicated by reference numeral 5-26). Upon receiving the UE capability information query message, the UE can configure and generate a UE capability information message, and report the UE capability information message to the base station (indicated by reference numeral 5-27). The UE capability information message can include the type of handover method supported by the UE. For example, the UE capability can be reported to the base station by an indicator about whether the UE supports the efficient handover method (Dual Active Protocol Stack, DAPS) of the present disclosure. When the base station identifies the UE capability information, the base station can instruct the UE to perform handover through a handover command message by defining an indicator according to each handover method in the handover command message, and transmit the indicator to the UE. For example, the base station can instruct the UE to perform the efficient handover method (DAPS handover method) of the present disclosure, and can configure the DAPS handover method for each bearer (DRB or SRB) of the UE by using another method. When the base station configures the DAPS handover method as well as other handover methods (for example, a conditional handover method (a method in which the UE performs a procedure of performing handover to one target cell when a plurality of target cells and a plurality of conditions are configured for the UE and the UE satisfies the above conditions in a cell selection or reselection procedure) or a random access channel (RACH)-free handover method) for the UE, data loss or transmission delay that can occur during handover can be prevented. The UE can perform a handover procedure to a target base station according to the handover method instructed through the handover command message.

[0148] The base station and the UE exchange a security mode command message 5-30 and a security mode complete message 5-35 in order to configure security. When the security configuration is completed, the base station transmits an RRCConnectionReconfiguration message to the UE (indicated by reference numeral 5-40).

[0149] The message includes configuration information for each service, bearer, RLC device, or logical channel, information indicating whether to use ROHC for each bearer or logical channel, ROHC configuration information (for example, ROHC version, initial information, etc.), statusReportRequired information (by which the base station instructs the UE to perform a PDCP status report), and drb-ContinueROHC information (configuration information indicating to maintain and use the ROHC configuration information as it is, and the configuration information can be included in PDCP layer device configuration information (pdcp-config) and transmitted). Further, the message includes RRC connection configuration information, etc. The bearer for the RRC connection is also referred to as a signaling radio bearer (SRB), and is used for transmission or reception of an RRC message, which is a control message between the UE and the base station.

[0150] Further, the message includes configuration information of a DRB in which user data is to be processed, and the UE configures the DRB by applying the information and transmits an RRCConnectionReconfigurationComplete message to the base station (indicated by reference numeral 5-45). The base station, which has completed the DRB setup with the UE, transmits an "INITIAL CONTEXT SETUP COMPLETE" ("INI CONTEXT RESPONSE") message to the MME or AMF (indicated by reference numeral 5-50), and the MME or AMF, which has received the "INITIAL CONTEXT SETUP COMPLETE" message, exchanges an S1 Bearer Setup message and an S1 Bearer Setup Response message in order to establish an S1 bearer for an S-GW (indicated by reference numerals 5-55 and 5-60). The S1 bearer is a data transmission connection established between the S-GW and the base station, and corresponds to the DRB one-to-one. When all the above-described procedures are completed, the UE transmits or receives data via the base station through the S-GW (indicated by reference numerals 5-65 and 5-70). This general data transmission procedure is mainly configured by the three phases of RRC connection setup, security setup, and DRB setup. Further, the base station can transmit an RRC connection reconfiguration message in order to newly perform, add, or change the configuration for the UE for a predetermined reason (indicated by reference numeral 5-75).

[0151] In the disclosure, a bearer can include an SRB and a DRB, the SRB denotes a signaling radio bearer, and the DRB denotes a data radio bearer. The SRB is mainly used to transmit or receive an RRC message of an RRC layer device, and the DRB is mainly used to transmit or receive user layer data. Further, the UM DRB denotes a DRB using an RLC layer device operating in a UM mode, and the AM DRB denotes a DRB using an RLC layer device operating in an AM mode.

[0152] In the disclosure, a bearer configured with a DAPS handover method can be configured such that an identifier of a bearer configured in an RRC message is included in a bearer list configured with a DAPS handover method, or is not included in a bearer list without a DAPS handover method. Alternatively, a bearer configured with a DAPS handover method can denote or indicate a bearer in which a DAPS handover method configuration indicator is present in configuration information of each bearer, or a bearer in which a DAPS handover method configuration indicator is configured in PDCP layer device configuration information.

[0153] In the disclosure, a bearer not configured with the DAPS handover method can be configured such that an identifier of a bearer configured in an RRC message is not included in a list of bearers configured with the DAPS handover method or is included in a list of bearers not configured with the DAPS handover method. Alternatively, a bearer not configured with the DAPS handover method can mean or indicate a bearer in which a DAPS handover method configuration indicator is not present in configuration information of each bearer or a bearer in which the DAPS handover method configuration indicator is not configured in PDCP layer device configuration information.

[0154] In the disclosure, a source base station can be understood as a source cell (which can be a primary cell (PCell), a special cell (Spcell), or a secondary cell (SCell)) or a source cell group (which can be a source cell group or a master cell group). A target base station can be understood as a target cell (which can be a PCell, a Spcell, or a SCell) or a target cell group (which can be a target cell group or a master cell group).

[0155] Figure 6 A signaling procedure for performing a handover in a next-generation mobile communication system according to an embodiment of the disclosure is illustrated.

[0156] Reference Figure 6 When a periodic event or a specific event is satisfied, the UE 601 in an RRC connected mode state reports cell measurement information (measurement report) to a current source base station (source eNB) 602 (indicated by reference numeral 605). The source base station determines whether to perform a handover to a neighboring cell based on the measurement information. The handover is a technique for changing a source base station that serves a UE in a connected mode state to another base station (or another cell of the same base station). If the source base station determines the handover, the source base station transmits an HO request message (e.g., a handover preparation information message) to a new base station (i.e., a target base station 603) that will serve the UE 601, thereby requesting the handover (indicated by reference numeral 610). If the target base station 603 accepts the handover request, the target base station 603 transmits an HO request acknowledgement (ACK) message (e.g., a handover command message) to the source base station 602 (indicated by reference numeral 615). Upon receipt of the message, the source base station 602 transmits a handover command message (HO command message, or an RRCReconfiguration message included in a DCCH of the HO request ACK message) to the UE 601 (indicated by reference numeral 620). The source base station 602 extracts the handover command (HO command) message from the message received from the target base station 603 and transmits it to the UE 601 by using the RRCConnectionReconfiguration message (indicated by reference numeral 625).

[0157] In the disclosure, when the source base station 602 transmits a handover preparation information message (indicated by reference numeral 610) and the target base station 603 transmits a handover command message (indicated by reference numeral 615) to the source base station 602 in response thereto, a method of determining an efficient DAPS handover method by using the two messages is provided.

[0158] A first embodiment of determining the efficient DAPS handover method of the disclosure is as follows.

[0159] In the first embodiment, the subject of determining the DAPS handover method can be the source base station. In addition, in the first embodiment, when the source base station requests the DAPS handover method, the target base station can always indicate or perform the DAPS handover method.

[0160] - The source base station can indicate to the target base station that the source base station will perform the DAPS handover method of the disclosure by defining a new indicator in the handover preparation information message, and can request the DAPS handover method. The handover preparation information message can include current bearer configuration information of the UE, security key information, cell group configuration information, or UE capability information. In the above, the source base station can know in advance whether the target base station supports the DAPS handover method by pre-sharing the capability of the target base station. In addition, the source base station transmits an indication to the target base station that the source base station will perform the DAPS handover method, thereby can inform the target base station that the source base station can perform fast or early data forwarding, and instruct the target base station to receive data forwarding and prepare for fast processing of the data forwarding. In the above, the source base station can perform a request for the DAPS handover method for each bearer (DRB or SRB).

[0161] - In the above, the target base station receives the handover preparation information message, and recognizes that an indicator for requesting the DAPS handover method is included therein. Then, in the case of configuring an RRCReconfiguration message for instructing the UE to perform handover, the target base station can configure the RRCReconfiguration message by including an indicator indicating the DAPS handover method, and by including bearer configuration information, security key information, cell group configuration information, or system information that are needed when the UE performs the DAPS handover method. In addition, the target base station can include the configured RRCReconfiguration message in a DL-DCCH message of the handover command message to transmit it to the source base station. In the above, the target base station can indicate the DAPS handover method for each bearer (DRB or SRB).

[0162] - In the above, when the source base station receives the handover command message, the source base station can extract the RRCReconfiguration message included in the handover command message or transmit the RRCReconfiguration message to the UE to instruct to perform the handover. In the above, the source base station can identify the indicated DAPS handover method for each bearer and perform the DAPS handover method for each bearer (DRB or SRB).

[0163] The second embodiment of determining the efficient DAPS handover method of the disclosure is as follows.

[0164] In the second embodiment, the subject of determining the DAPS handover method can be the target base station. In addition, in the second embodiment, when the source base station requests the DAPS handover method to the target base station by using the indicator, the target base station can reject or accept the request or can instruct the source base station through the handover command message indicating another handover method.

[0165] - The source base station can indicate to the target base station that the source base station will perform the DAPS handover method of the disclosure by defining a new indicator in the handover preparation information message and can request the DAPS handover method to the target base station. The handover preparation information message can include the current bearer configuration information of the UE, the security key information, the cell group configuration information, or the UE capability information. In the above, the source base station can know in advance whether the target base station supports the DAPS handover method by sharing the capability of the target base station in advance. In the above, the source base station transmits the indication to the target base station that the source base station will perform the DAPS handover method, thereby can inform the target base station that the source base station can perform fast or early data forwarding and instruct the target base station to receive the data forwarding and prepare for fast processing of the data forwarding. In the above, the source base station can perform the request for the DAPS handover method for each bearer (DRB or SRB).

[0166] - In the above, the target base station receives the handover preparation information message, and identifies that the indicator for requesting the DAPS handover method is included therein. Then, the target base station can reject or accept the request for the DAPS handover according to whether the target base station can support the DAPS handover method, the amount of current transmission resources, or scheduling, or can indicate another handover method. In the above, the target base station can include an indicator for rejecting the request for the DAPS handover request, an indicator for accepting the request, or an indicator indicating another type of handover method in the handover command message, and transmit the handover command message. In the above, when the target base station configures the RRCReconfiguration message for instructing the UE to perform the handover, the target base station can configure the RRCReconfiguration message to include an indicator indicating the DAPS handover method when the DAPS handover request is accepted, and an indicator indicating another handover method when the DAPS handover request is rejected, and the target base station can configure the RRCReconfiguration message to include bearer configuration information, security key information, cell group configuration information, or system information that are needed when the UE performs the DAPS handover method or the other handover method. Furthermore, the target base station can include the configured RRCReconfiguration message in the DL-DCCH message of the handover command message, to transmit the DL-DCCH message to the source base station. In the above, the target base station can perform the indication of the DAPS handover method for each bearer (DRB or SRB).

[0167] - In the above, when the source base station receives the handover command message, the source base station can identify whether the request for the DAPS handover method is accepted or rejected by identifying the indicator included in the handover command message. If the request is accepted, the source base station can also perform the DAPS handover method, and the source base station can extract the RRCReconfiguration message included in the handover command message or transmit the RRCReconfiguration message to the UE in order to instruct to perform the handover. However, when the indicator included in the handover command message is identified, if the request for the DAPS handover method is rejected, or if another handover method is indicated, the source base station can also perform the other handover method indicated by the target base station. Further, the source base station can extract the RRCReconfiguration message included in the handover command message or transmit the RRCReconfiguration message to the UE in order to instruct to perform the handover. As another method, in the above, even if there is no separate indicator in the handover command message, the source base station can read the RRCReconfiguration message included in the handover command message to identify the handover method indicated by the target base station, and identify whether the request for the DAPS handover method is accepted or rejected; and the source base station can also perform the handover method (e.g., the DAPS handover method or the other handover method) indicated in the RRCReconfiguration message. In the above, the source base station can identify the indicated DAPS handover method for each bearer, and perform the DAPS handover method for each bearer (DRB or SRB).

[0168] The third embodiment of determining the efficient DAPS handover method of the above disclosure is as follows.

[0169] In the third embodiment, the subject of determining the DAPS handover method can be the target base station. Further, in the third embodiment, the target base station identifies the capability of the UE, and determines the handover method (e.g., the DAPS handover method) according to whether the target base station can support the DAPS handover method, the amount of current transmission resources, or scheduling.

[0170] - The source base station can include the current bearer configuration information, security key information, cell group configuration information, or UE capability information of the UE in the handover preparation information message, and can transmit the handover preparation information message to the target base station in order to request the handover. In the above, the source base station can know in advance whether the target base station supports the DAPS handover method by sharing the capability of the target base station in advance. If the target base station indicates to perform the DAPS handover method, the source base station can perform the fast or early data forwarding.

[0171] - In the above, the target base station receives the handover preparation information message, and the target base station can determine a handover method (e.g., DAPS handover) according to the capability information of the UE or whether the target base station can support the DAPS handover method, the amount or scheduling of the current transmission resource. As described above, when the DAPS handover method is determined based on the handover command message, the target base station can include an indicator indicating the DAPS handover method in the message and transmit the message. As described above, in the case of determining the DAPS handover method, the target base station can include an indicator indicating the DAPS handover method in the RRCReconfiguration message configuring the UE to perform the handover; in the case of determining another DAPS handover method other than the DAPS handover method, the target base station can include an indicator indicating the other DAPS handover method in the RRCReconfiguration message; and the target base station can configure the RRCReconfiguration message by including bearer configuration information, security key information, cell group configuration information, or system information required when the UE performs the DAPS handover method or the other handover method. Further, the target base station can include the configured RRCReconfiguration message in the DL-DCCH message of the handover command message to transmit the DL-DCCH message to the source base station. In the above, the target base station can perform the indication of the DAPS handover method for each bearer (DRB or SRB).

[0172] - In the above, when the source base station receives the handover command message, the source base station can identify whether the DAPS handover is determined by identifying the indicator included in the handover command message. If the DAPS handover method is indicated, the source base station can also perform the DAPS handover method, and can indicate the handover by extracting the RRCReconfiguration message included in the handover command message or by transmitting the RRCReconfiguration message to the UE. However, if the DAPS handover method is not determined when the indicator included in the handover command message is identified, or if another handover method is indicated, the source base station can also perform the other handover method indicated by the target base station. In addition, the source base station can extract the RRCReconfiguration message included in the handover command message or transmit the RRCReconfiguration message to the UE to indicate the performance of the handover. As another method, even if there is no separate indicator in the handover command message, the source base station can read the RRCReconfiguration message included in the handover command message to identify the handover method indicated by the target base station, and identify whether the determination of the DAPS handover method is established. If another handover method is indicated, the source base station can also perform the indicated handover method. The source base station can identify the indicated DAPS handover method for each bearer, and perform the DAPS handover method for each bearer (DRB or SRB).

[0173] An extended new embodiment can be provided by combining the methods of determining the first, second, or third embodiment of the efficient DAPS handover method of the present disclosure.

[0174] By the RRCReconfiguration message, the base station can indicate the efficient handover method (DAPS handover method) of the present disclosure to the UE, and can configure the DAPS handover method for each bearer (DRB or SRB) of the UE by using another method. For example, the base station can define a new indicator indicating the efficient handover method (DAPS handover method) in the RRC message based on bearer configuration information, PDCP configuration information, or RLC configuration information of each bearer identifier or logical channel identifier. Further, the base station can indicate the efficient handover method for each bearer or each logical channel identifier to the UE by using the indicator. As described above, when the base station configures the DAPS handover method for the UE, other handover methods (for example, a conditional handover method (a method in which the UE is configured with a plurality of target cells and a plurality of conditions and performs a procedure of moving to one target cell if the UE satisfies the above conditions in a cell selection procedure or a reselection procedure) or a handover method without a random access procedure) are indicated together with the configuration, so that the UE can prevent data loss or transmission delay that can occur during handover. Upon receiving the message, the UE stops or continues to transmit or receive data to or from the source base station according to the configured handover method, and starts a T304 timer. When the UE fails to perform handover to the target base station within a predetermined time (for example, when the T304 timer expires), the T304 timer causes the UE to return to its original configuration and switch to an RRC idle state. Further, the UE can trigger an RRC connection reestablishment procedure, and if the efficient handover method is configured using another method and the connection with the source base station is valid, the UE can fall back to report handover failure to the source base station. The source base station transmits a sequence number (SN) state for uplink or downlink data to each bearer (for example, an RLC UM bearer or an RLC AM bearer), and if there is downlink data or uplink data, the source base station transmits the downlink data or the uplink data to the target base station (indicated by reference numerals 630 and 635). The UE attempts to perform random access to the target cell indicated by the source base station (indicated by reference numeral 640). The random access is performed in order to inform the target cell that the UE is moving and to match uplink synchronization with the target cell. For the random access, the UE transmits a preamble ID provided from the source base station or a preamble corresponding to a randomly selected preamble ID to the target cell. After transmitting the preamble, the UE monitors whether a random access response (RAR) message is transmitted from the target cell when a certain number of subframes elapses. The time period of monitoring is referred to as a random access response window (RAR window). If the random access response (RAR) is received during a certain period (indicated by reference numeral 645), the UE transmits a handover (HO) complete message to the target base station by using an RRC reconfiguration complete message (indicated by reference numeral 655).Upon successful reception of the random access response from the target base station, the UE interrupts or terminates the T304 timer (indicated by reference numeral 650). The target base station requests a path modification to the source base station in order to modify the configured path of the bearer (indicated by reference numerals 660 and 665), and transmits a request for deletion of the UE context of the UE to the source base station (indicated by reference numeral 670). In addition, the target base station can transmit an RRC message (for example, an RRCReconfiguration message) 671 to the UE to instruct the UE to release the connection with the source base station by using an indicator. Alternatively, the target base station can transmit MAC control information, RLC control information, or PDCP control information to the UE to instruct the UE to release the connection with the source base station. Accordingly, the UE attempts to receive data from the target base station from the start time of the RAR window, transmits an RRC reconfiguration complete message after reception of the RAR, and receives a downlink transmission resource or an uplink transmission resource, thereby starting to transmit data to or receive data from the target base station 675.

[0175] When the base station configures or instructs the UE for the second embodiment (DAPS handover method) of the present disclosure by using a handover command message or an RRC message (for example, an RRCReconfiguration message), the applicable method is as follows. In the present disclosure, when the base station configures the UE for the DAPS handover method, or when the UE receives a handover command message for configuring the DAPS handover method from the base station, the base station can perform the adopted method by applying one or several of the following methods.

[0176] - Method 1-1: When the handover is indicated or configured to the UE, the base station (source base station, target base station, LTE base station, or NR base station) can include MobilityControlInfo or ReconfigurationWithSync configuration information in the RRCReconfiguration message and can define an indicator in SRB-ToAddMod or DRB-ToAddMod of SRB-ToAddModList or DRB-ToAddModList in the bearer configuration information (RadioResourceConfigDedicated or RadioBearerConfig) of the RRCReconfiguration message to indicate the DAPS handover method for each bearer (SRB or DRB). As another method, the base station can define an indicator in pdcp-config in SRB-ToAddMod or DRB-ToAddMod to indicate the DAPS handover method for each bearer. As another method, when the LTE base station indicates or configures the handover to the UE through the LTE RRCRecofiguration message, in the case of LTE, pdcp-config is not defined in SRB-ToAddMod and a default PDCP layer device configuration is used; thus, for SRB, an indicator is defined in SRB-ToAddMod to configure the DAPS handover method for each bearer, and for DRB, an indicator is defined in pdcp-config in DRB-ToAddMod to configure the DAPS handover method for each bearer. Upon receiving the handover command message (RRCReconfiguration message) configured above, the UE can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration, and can perform the general handover method for the bearer for which the DAPS handover method is not configured. Furthermore, if the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message includes an indicator indicating the type of the handover method (e.g., MakeBeforeBreak handover, Rach-skip handover, Rach-less handover, or conditional handover (CHO)) indicated or configured for the UE and the DAPS handover method is configured for each bearer, the UE can apply the DAPS handover method before the type of the other handover method when performing the handover procedure for each bearer as described above.For example, the UE can perform the DAPS handover method for a bearer configured with the DAPS handover method according to the configuration, and can perform the handover method configured according to the indicator for a bearer for which the DAPS handover method is not configured, which indicates or configures the type of handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or Conditional Handover (CHO)) for the UE in the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message. As another method, when the DAPS handover method is configured for at least one bearer or a predetermined bearer in order to reduce the complexity of the UE implementation, the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message can not indicate or configure the type of handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or Conditional Handover (CHO)) for the UE. Furthermore, upon receiving the above-configured handover command message (RRCReconfiguration message), when the DAPS handover method is configured for at least one bearer or a predetermined bearer for each bearer, the UE can apply an RRC layer device, an SDAP layer device, a MAC layer device, a PHY layer device, or an SRB processing method, etc. to perform the DAPS handover method of the present disclosure. Furthermore, for a bearer for which the DAPS handover method is configured for each bearer, the UE can apply a method for processing a PDCP layer device, an RLC layer device, etc. to perform the DAPS handover method of the present disclosure.

[0177] - Method 1-2: When the handover is indicated or configured to the UE, the base station (source base station, target base station, LTE base station, or NR base station) can include the MobilityControlInfo or ReconfigurationWithSync configuration information in the RRCReconfiguration message and can define an indicator in the DRB-ToAddMod of the DRB-ToAddModList in the bearer configuration information (radio resource configuration dedicated or radio bearer configuration) of the RRCReconfiguration message to indicate the DAPS handover method for each bearer (DRB), and for the SRB, the base station can not introduce a separate indicator for configuring the DAPS handover method. That is, when the UE receives the handover command message (RRCReconfiguration message) configured as described above, when the DAPS handover method is configured for at least one bearer (DRB) or a predetermined bearer (DRB) for each bearer, the UE can apply the SRB processing method to perform the DAPS handover method of the present disclosure. Alternatively, by defining an indicator in the pdcp-config in the DRB-ToAddMod, the base station can indicate the DAPS handover method for each bearer. When the UE receives the configured handover command message (RRCReconfiguration message), the UE can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration, and can perform the general handover method for the bearer for which the DAPS handover method is not configured. Furthermore, if the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message includes an indicator indicating or configuring the type of handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or conditional handover (CHO)) for the UE, and the DAPS handover method is configured for each bearer, the UE can apply the DAPS handover method before the type of other handover methods when performing the handover procedure for each bearer as described above. For example, the UE can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration, and can perform the handover method configured according to the indicator indicating or configuring the type of handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or conditional handover (CHO)) for the UE in the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message for the bearer for which the DAPS handover method is not configured.As another method, when the DAPS handover method is configured for at least one bearer or a predetermined bearer in order to reduce the complexity of the UE implementation, the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message can not indicate or configure the type of the handover method (e.g., MakeBeforeBreak handover, skipRach handover, noRach handover, or conditional handover (CHO)) for the UE. In addition, when the UE receives the handover command message (RRCReconfiguration message) configured as described above, if the DAPS handover method is configured for at least one bearer or a predetermined bearer for each bearer, the RRC layer device, SDAP layer device, MAC layer device, PHY layer device, or SRB processing method for performing the DAPS handover method of the present disclosure, etc. can be applied; and for the bearer for which the DAPS handover method is configured for each bearer, the UE can apply a method for processing the PDCP layer device, RLC layer device, etc. to perform the DAPS handover method of the present disclosure.

[0178] - Method 2-1: When the handover is indicated or configured to the UE, the base station (source base station, target base station, LTE base station, or NR base station) can include MobilityControlInfo or ReconfigurationWithSync configuration information in the RRCReconfiguration message and define and include an indicator for indicating or configuring the DAPS handover method in the MobilityControlInfo or ReconfigurationWithSync, thereby indicating that the DAPS handover method has been configured for at least one bearer or predetermined bearer. In addition, the base station can define the indicator in SRB-ToAddMod or DRB-ToAddMod of SRB-ToAddModList or DRB-ToAddModList in the bearer configuration information (radio resource configuration dedicated or radio bearer configuration) of the RRCReconfiguration message to indicate the DAPS handover method for each bearer (SRB or DRB). As another method, the base station can define the indicator in pdcp-config in SRB-ToAddMod or DRB-ToAddMod to indicate the DAPS handover method for each bearer. As another method, when the LTE base station indicates or configures the handover to the UE through the LTE RRCRecofiguration message, in the case of LTE, pdcp-config is not defined in SRB-ToAddMod and a default PDCP layer device configuration is used; therefore, for SRB, the indicator is defined in SRB-ToAddMod to configure the DAPS handover method for each bearer, and for DRB, the indicator is defined in pdcp-config in DRB-ToAddMod to configure the DAPS handover method for each bearer. The UE that has received the above-configured handover command message (RRCReconfiguration message) can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration, and can perform the general handover method for the bearer for which the DAPS handover method is not configured. In addition, if the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message includes an indicator indicating or configuring the type of the handover method (e.g., MakeBeforeBreak handover, skipRach handover, noRach handover, or conditional handover CHO) for the UE and the DAPS handover method is configured for each bearer, the UE can apply the DAPS handover method before the other handover method types when performing the handover procedure for each bearer as described above.For example, the DAPS handover method can be performed for a bearer configured with the DAPS handover method according to the configuration, and the handover method configured according to the indicator indicating or configuring the type of the handover method (e.g., Make Before Break handover, SkipRach handover, NoRach handover, or Conditional Handover (CHO)) for the UE in the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message can be performed for a bearer not configured with the DAPS handover method. As another method, when the DAPS handover method is configured for at least one bearer or a predetermined bearer in order to reduce the complexity implemented by the UE, the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message can not indicate or configure the type of the handover method (e.g., Make Before Break handover, SkipRach handover, NoRach handover, or Conditional Handover (CHO)) for the UE. Further, upon receiving the handover command message (RRCReconfiguration message) configured as described above, if the indicator for indicating or configuring the DAPS handover method is included or configured in the MobilityControlInfo or ReconfigurationWithSync, the UE can apply the RRC layer device, SDAP layer device, MAC layer device, PHY layer device, or SRB processing method, etc. to perform the DAPS handover method of the present disclosure. Further, the UE can apply a method for processing the PDCP layer device, RLC layer device, etc. to the bearer for which the DAPS handover method is configured for each bearer to perform the DAPS handover method of the present disclosure.

[0179] - Method 2-2: When the handover is indicated or configured to the UE, the base station (source base station, target base station, LTE base station, or NR base station) can include MobilityControlInfo or ReconfigurationWithSync configuration information in the RRCReconfiguration message and define and include an indicator for indicating or configuring the DAPS handover method in the MobilityControlInfo or ReconfigurationWithSync, thereby indicating that the DAPS handover method has been configured for at least one bearer or predetermined bearer. In addition, the base station can define the indicator in the DRB-ToAddMod of the DRB-ToAddModList in the bearer configuration information (radio resource configuration dedicated or radio bearer configuration) of the RRCReconfiguration message to indicate the DAPS handover method for each bearer (DRB); and for the SRB, the base station can not introduce a separate indicator for configuring the DAPS handover method. That is, upon receiving the handover command message (RRCReconfiguration message) configured as described above, when the DAPS handover method is configured for at least one bearer (DRB) or predetermined bearer (DRB) for each bearer, the UE can apply the SRB processing method to perform the DAPS handover method of the present disclosure. As another method, by defining the indicator in the pdcp-config in the DRB-ToAddMod, the base station can indicate the DAPS handover method for each bearer. The UE that has received the configured handover command message (RRCReconfiguration message) can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration for each bearer, and can perform the general handover method for the bearer for which the DAPS handover method is not configured. In addition, if the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message includes an indicator indicating or configuring the type of handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or conditional handover CHO) for the UE, and the DAPS handover method is configured for each bearer, the UE can apply the DAPS handover method before the type of other handover methods when performing the handover procedure for each bearer as described above.For example, the DAPS handover method can be performed for a bearer configured with the DAPS handover method according to the configuration, and the UE can perform a handover method configured according to an indicator indicating or configuring a type of a handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or Conditional Handover (CHO)) for the UE in the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message for a bearer not configured with the DAPS handover method. As another method, when the DAPS handover method is configured for at least one bearer or a predetermined bearer in order to reduce the complexity of the UE implementation, the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message can not indicate or configure the type of the handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or Conditional Handover (CHO)) for the UE. In addition, upon receiving the handover command message (RRCReconfiguration message) configured as described above, if an indicator for indicating or configuring the DAPS handover method is included or configured in the MobilityControlInfo or ReconfigurationWithSync, the UE can apply an RRC layer device, an SDAP layer device, a MAC layer device, a PHY layer device, or an SRB processing method, etc. to perform the DAPS handover method of the present disclosure. In addition, the UE can apply a method for processing a PDCP layer device, an RLC layer device, etc. to a bearer for which the DAPS handover method is configured for each bearer to perform the DAPS handover method of the present disclosure.

[0180] - Method 3-1: When the handover is indicated or configured to the UE, the base station (source base station, target base station, LTE base station, or NR base station) can include the MobilityControlInfo or ReconfigurationWithSync configuration information in the RRCReconfiguration message and define and include an indicator for indicating or configuring the DAPS handover method in the MobilityControlInfo or ReconfigurationWithSync, thereby indicating that the DAPS handover method has been configured for at least one bearer or predetermined bearer. In addition, the base station can configure and include a bearer list for which the DAPS handover method is configured, and include identifiers (SRB or DRB) of the bearers for which the DAPS handover method is configured in the bearer list, in order to indicate whether the DAPS handover method is configured for each bearer. As another method, the base station can configure and include a bearer list for which the DAPS handover method is not configured, and include identifiers (SRB or DRB) of the bearers for which the DAPS handover method is not configured in the bearer list, in order to indicate whether the DAPS handover method is configured for each bearer. The UE that has received the above-mentioned handover command message (RRCReconfiguration message) configured can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration, the bearer of which the identifier is included in the bearer list for which the DAPS handover method is configured, or the bearer of which the identifier is not included in the bearer list for which the DAPS handover method is not configured. In addition, the UE can perform the general handover method for the bearer for which the DAPS handover method is not configured, the bearer of which the identifier is not included in the bearer list for which the DAPS handover method is configured, or the bearer of which the identifier is included in the bearer list for which the DAPS handover method is not configured. In addition, if the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message includes an indicator indicating or configuring the type of the handover method (e.g., MakeBeforeBreak handover, SkipRach handover, NoRach handover, or conditional handover CHO) for the UE, and the DAPS handover method is configured for each bearer, the UE can apply the DAPS handover method before the type of the other handover method when performing the handover procedure for each bearer as described above. For example, the UE can perform the DAPS handover method for the bearer for which the DAPS handover method is configured according to the configuration, the bearer of which the identifier is included in the bearer list for which the DAPS handover method is configured, or the bearer of which the identifier is not included in the bearer list for which the DAPS handover method is not configured.Furthermore, the UE can execute a handover method configured according to an indicator for bearers that are not configured with a DAPS handover method, bearers whose identifiers are not included in the list of bearers configured with a DAPS handover method, or bearers whose identifiers are included in the list of bearers without a configured DAPS handover method. This indicator specifies or configures the type of handover method (e.g., MakeBeforeBreak handover, skipped Rach handover, no Rach handover, or conditional handover (CHO)) for the UE in the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message. Alternatively, when configuring a DAPS handover method for at least one bearer or a predetermined bearer to reduce the complexity of UE implementation, the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message may not specify or configure the type of handover method (e.g., MakeBeforeBreak handover, skipped Rach handover, no Rach handover, or conditional handover (CHO)) for the UE. Furthermore, upon receiving the handover command message (RRCReconfiguration message) configured as described above, if the indicator used to indicate or configure the DAPS handover method is included or configured in MobilityControlInfo or ReconfigurationWithSync, the UE can apply RRC layer device, SDAP layer device, MAC layer device, PHY layer device, or SRB processing method, etc., to execute the DAPS handover method of this disclosure. Furthermore, regarding bearers for which a DAPS handover method is configured for each bearer, bearers whose identifiers are included in the list of bearers configured with a DAPS handover method, or bearers whose identifiers are not included in the list of bearers without a configured DAPS handover method, the UE can apply methods used for processing PDCP layer device, RLC layer device, etc., to execute the DAPS handover method of this disclosure.

[0181] Method 3-2: When instructing or configuring handover to the UE, the base station (source base station, target base station, LTE base station, or NR base station) may include MobilityControlInfo or ReconfigurationWithSync configuration information in the RRCReconfiguration message, and define and include an indicator in the MobilityControlInfo or ReconfigurationWithSync for instructing or configuring the DAPS handover method, thereby indicating that the DAPS handover method has been configured for at least one bear or a predetermined bear. Furthermore, the UE may configure and include a list of bears with configured DAPS handover methods, and include the identifier (SRB or DRB) of the bears with configured DAPS handover methods in the bear list to indicate whether the DAPS handover method is configured for each bear. Alternatively, the base station may configure and include a list of bears without configured DAPS handover methods, and include the identifier (SRB or DRB) of the bears without configured DAPS handover methods in the bear list to indicate whether the DAPS handover method is configured for each bear. Furthermore, for SRBs, the UE may not introduce a separate indicator for configuring the DAPS handover method. In other words, upon receiving the handover command message (RRCReconfiguration message) configured above, when a DAPS handover method is configured for each bearer as at least one bearer (DRB) or a predetermined bearer (DRB), the UE can apply the SRB processing method to execute the DAPS handover method of this disclosure. The UE, having received the configured handover command message (RRCReconfiguration message), can execute the DAPS handover method for bearers configured with a DAPS handover method, bearers whose identifiers are included in the list of bearers configured with a DAPS handover method, or bearers whose identifiers are not included in the list of bearers without a configured DAPS handover method. Furthermore, the UE can execute a general handover method for bearers without a configured DAPS handover method, bearers whose identifiers are not included in the list of bearers configured with a DAPS handover method, or bearers whose identifiers are included in the list of bearers without a configured DAPS handover method. Furthermore, if the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message includes an indicator for indicating or configuring the type of handover method for the UE (e.g., MakeBeforeBreak handover, skip Rach handover, no Rach handover, or conditional handover (CHO)), and the DAPS handover method is configured for each bearer, then when the handover procedure is performed for each bearer as described above, the UE can apply the DAPS handover method before other handover method types.For example, a UE can execute a DAPS handover method for a bearer configured with a DAPS handover method, whose identifier is included in the list of bearers configured with a DAPS handover method, or whose identifier is not included in the list of bearers without a DAPS handover method. Furthermore, a UE can execute a handover method configured according to an indicator for a bearer that is not configured with a DAPS handover method, whose identifier is not included in the list of bearers configured with a DAPS handover method, or whose identifier is included in the list of bearers without a DAPS handover method. This indicator indicates or configures the type of handover method (e.g., MakeBeforeBreak handover, skip Rach handover, no Rach handover, or conditional handover (CHO)) for the UE in the MobilityControlInfo or ReconfigurationWithSync configuration information of the handover command message. As another approach, when configuring a DAPS handover method for at least one bearer or a predetermined bearer to reduce the complexity of UE implementation, the MobilityControlInfo or ReconfigurationWithSync configuration information in the handover command message may not indicate or configure the type of handover method for the UE (e.g., MakeBeforeBreak handover, skipped RACH handover, no RACH handover, or conditional handover (CHO)). Furthermore, upon receiving the handover command message (RRCReconfiguration message) configured as described above, if the indicator used to indicate or configure the DAPS handover method is included or configured in the MobilityControlInfo or ReconfigurationWithSync, the UE can apply an RRC layer device, SDAP layer device, MAC layer device, PHY layer device, or SRB processing method to execute the DAPS handover method of this disclosure. Furthermore, regarding bearers for which a DAPS handover method is configured for each bearer, bearers whose identifiers are included in the list of bearers configured with a DAPS handover method, or bearers whose identifiers are not included in the list of bearers without a configured DAPS handover method, the UE may apply the methods used for handling PDCP layer devices, RLC layer devices, etc., to perform the DAPS handover method of this disclosure.

[0182] This disclosure presents a seamless handover method in next-generation mobile communication systems that can minimize or reduce data interruption time caused by handover to 0 ms.

[0183] The UE is configured with multiple first bearers and transmits data to or receives data from the source base station (uplink data transmission or downlink data reception) through the corresponding protocol layer devices (PHY layer devices, MAC layer devices, RLC layer devices, or PDCP layer devices) of the bearers. However, for ease of explanation, the following description will be presented as if the UE included only one bearer in the figures and description. Needless to say, the content described below with reference to one bearer can be applied to multiple bearers.

[0184] Figure 7 A specific stage of a first embodiment of an efficient handover method for minimizing data interruption time due to handover, according to embodiments of the present disclosure, is shown.

[0185] refer to Figure 7 According to a first embodiment of the efficient handover method, in the first stage 7-01, if UE 7-20 receives a handover command message from source base station 7-05 while simultaneously sending data to or receiving data from the source base station, UE 7-20 can release its connection with the source base station according to the handover method indicated by the handover command message (e.g., an RRCReconfiguration message), execute a procedure for randomly accessing the target base station, and perform a handover procedure. Alternatively, the UE can continue sending or receiving data from the source base station to minimize the data interruption time that occurs during the handover according to the indicated handover method.

[0186] according to Figure 7 In the first embodiment of the efficient handover method, in the second phase 7-02, when UE 7-20 performs a random access procedure to the target base station 7-10, sends a preamble, or initially sends data to the uplink transmission resources by using the Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH) transmission resources according to the handover method indicated by the handover command message, UE 7-20 may stop sending data to the source base station or receiving data from the source base station (uplink data transmission and downlink data reception).

[0187] according to Figure 7 In the first embodiment of the efficient handover method, in the third stage 7-03, the UE 7-20 can complete the random access procedure to the target base station, can send a handover completion message, and can start sending data to or receiving data from the target base station (uplink data transmission and downlink data reception).

[0188] A first embodiment of the efficient handover method disclosed herein can describe the handover method executed when the DAPS handover method is not configured.

[0189] Figure 8A specific stage of a second embodiment of an efficient handover method for minimizing data interruption time due to handover, according to embodiments of the present disclosure, is shown.

[0190] refer to Figure 8 ,according to Figure 8 In a second embodiment of the efficient handover method, during the first stage 8-01, UE 8-20 can receive a handover command message from source base station 8-05 and simultaneously send data to or receive data from the source base station. If the UE receives the handover command message, and the second embodiment of the efficient handover method of this disclosure (e.g., the DAPS handover method) is indicated by the handover command message or indicated for each bearer, even if the UE has already received the handover command message, the UE can continue to send data to or receive data from the source base station through the protocol layer device 8-22 of the first bearer, in order to minimize the data interruption time that occurs during the handover. Furthermore, when the UE's RRC layer device identifies the indication of the second embodiment of the efficient handover method of this disclosure (e.g., the DAPS handover method) through the handover command message, or identifies the indicator of the DAPS handover method for each bearer, the RRC layer device sends the indicator to each bearer or the PDCP layer device corresponding to the bearer indicating the DAPS handover method. Then, upon receiving the indicator, the PDCP layer device retrieves data from the first PDCP layer device (by...). Figure 9A and Figure 9BThe UE switches from the structure indicated by reference numerals 9-11 or 9-12 to the structure of the second PDCP layer device (indicated by reference numeral 9-20). The first phase can describe the phase in which the UE receives the handover command message (RRCReconfiguration message) from the base station. Furthermore, when switching to the second PDCP layer device structure, the UE can pre-configure or establish the protocol layer device (PHY layer device, MAC layer device, RLC layer device, or PDCP layer device) 8-21 of the second bearer for the target base station 8-10 according to the configuration included in the received handover command message. In addition, the UE can derive and update the security key of the target base station 8-10, and can configure the header (or data) compression context of the target base station 8-10. Furthermore, when a handover command message received by the UE indicates the DAPS handover method of this disclosure or indicates a DAPS handover method for a specific bearer, or a new PDCP reordering timer value is configured, for each bearer or bearer indicating the DAPS handover method, the UE can switch from the structure or function of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) to the structure or function of the second PDCP layer device (indicated by reference numerals 9-20) of this disclosure. When performing the handover as described above, the UE updates the variables used for reordering using the expected PDCP sequence number or count value to be received next, stops the reordering timer, and restarts it. Additionally, when a handover command message (e.g., an RRC reconfiguration message) is received, the UE's RRC layer device can start a first timer (e.g., T304). Furthermore, when a random access procedure is executed for the target base station 9-03 to perform the handover and the random access procedure completes successfully (e.g., when the first condition described in this disclosure is met), the first timer can be interrupted. If the handover fails and the first timer expires, the UE will perform a fallback when the connection to the source base station is valid, in order to report the handover failure to the source base station and attempt to restore the connection; and when the connection to the source base station is invalid, the UE can execute the RRC connection reconstruction procedure.

[0191] Through the handover command message, the second bearer can be configured and established to have the same bearer identifier as the first bearer, ensuring that no data interruption occurs for each bearer. Furthermore, in the second embodiment, the PDCP layer devices of the first and second bearers can logically operate as a single PDCP layer device, and more detailed operating methods will be provided in [the following section / document / etc.]. Figure 9A and Figure 9BAs described in the description. Furthermore, in the second embodiment, when the UE enables uplink data 9-03 to be transmitted to both the source base station and the target base station, it is necessary to prevent coverage reduction problems caused by insufficient UE transmission power, or the problem of requesting transmission resources from the base station and determining which base station to send uplink data to (i.e., link selection) when transmitting uplink data. Accordingly, in the second embodiment, the uplink data can be sent to only one of the source base station and the target base station 9-03. Specifically, in the second embodiment, if the UE does not have the ability to simultaneously transmit uplink data to different base stations at different frequencies or the same frequency (i.e., dual uplink transmission), the UE can send uplink data to only one of the source base station and the target base station 9-03 in one time unit. Therefore, the UE can request scheduling from only one of the source base station and the target base station 9-03, and can send a report (e.g., a buffer status report (BSR)) on the size of the data to be transmitted from the PDCP layer device to one of the source base station and the target base station 9-03 and receive uplink transmission resources, thereby sending uplink data to only one base station. Furthermore, even if the UE receives a handover command message from the source base station, the UE does not initialize the MAC layer device of the first bearer to prevent data loss caused by continued data transmission or reception due to HARQ retransmission. Additionally, the RLC layer device in AM mode can continuously perform RLC retransmission. As another method, when the second embodiment of the efficient handover method disclosed herein (DAPS handover method) is indicated for each bearer by the handover command message, the UE can continuously transmit data to or continuously receive data from the source base station only for the PDCP layer device, RLC layer device, or MAC layer device corresponding to the bearer or logical channel identifier for which the second embodiment (DAPS handover method) is indicated by the handover command message, or only for the data corresponding to that bearer or logical channel identifier. Furthermore, even when the first condition described in this disclosure is met (e.g., when uplink data transmission is switched to target base station 9-03), the UE may continuously send or receive RLC control data (e.g., RLC status report), PDCP control data (ROHC feedback or PDCP status report), or HARQ retransmissions only from the source base station for the PDCP layer device, RLC layer device, or MAC layer device corresponding to the bearer or logical channel identifier for which the second embodiment (DAPS handover method) is indicated via the handover command message. Additionally, when the second embodiment (DAPS handover method) of the efficient handover method of this disclosure is indicated or has been indicated for each bearer via the handover command message, the UE stops sending data to or receiving data from the source base station for the PDCP layer device, RLC layer device, or MAC layer device corresponding to the bearer or logical channel identifier for which the second embodiment (DAPS handover method) is not indicated via the handover command message.Furthermore, when the UE receives a handover command message, which indicates the DAPS handover method of this disclosure or indicates a DAPS handover method for a specific bearer, configures a DAPS handover method for at least one bearer, configures a DAPS handover method for a predetermined bearer, or newly configures QoS flow and bearer mapping information, the UE can switch from the structure or function of the first SDAP layer device to the structure or function of the second SDAP layer device of this disclosure for each bearer or for the bear for which a DAPS handover method has been indicated. Furthermore, according to the structure of the second SDAP layer device, the UE maintains existing first QoS flow and bearer mapping information for the source base station to process uplink data to be sent to the source base station and downlink data to be received from the source base station. Additionally, the UE configures the newly configured second QoS flow and bearer mapping information in the handover command message and uses this configuration to process uplink data to be sent to the target base station 9-03 and downlink data to be received from the target base station 9-03. In other words, in the structure of the second SDAP layer device disclosed herein, data from the source base station and the target base station 9-03 is classified and processed by maintaining first QoS flow and bearer mapping information or second QoS flow and bearer mapping information for the source base station. In the structure of the second SDAP layer device, the SDAP layer device can identify whether the data received from the lower layer is from the source base station or the target base station 9-03 by using a 1-bit indicator in the SDAP header, a 1-bit indicator in the PDCP header, or information indicated by the PDCP layer device. Furthermore, if the base station instructs the UE to perform a DAPS handover method for each bearer via a handover command message, the UE is allowed to always instruct the DAPS handover method for the default bearer (default DRB), so that when data is generated in a new QoS flow that does not correspond to the bearer mapping information and QoS flow during the DAPS handover procedure, uplink data is always sent to the default bearer. If the DAPS handover method is not configured for the default bearer, data interruption may occur because uplink data transmission in a new QoS flow during the handover is impossible. In another method, when a handover command message (e.g., an RRCReconfiguration message) is received, indicating the second embodiment (DAPS handover method), and the SDAP layer device configuration information or the second QoS flow and bearer mapping information of the target base station is configured via an RRC message, the UE may apply the SDAP layer device configuration information or the second QoS flow and bearer mapping information when the first condition described in this disclosure is met.Furthermore, if the second embodiment (DAPS handover method) is indicated for each bearer via the handover command message, then when maintaining the first QoS flow and bearer mapping information of the source base station, the UE can maintain and apply only the first QoS flow and bearer mapping information corresponding to the bearers indicating the second embodiment, and can release or not apply the first QoS flow and bearer mapping information corresponding to bearers not indicating the second embodiment. Additionally, by configuring the SDAP layer device configuration information or the second QoS flow and bearer mapping information of the target base station via RRC messages, when the first condition described in this disclosure is met, the UE applies the SDAP layer device configuration information or the second QoS flow and bearer mapping information to send data to or receive data from the target base station.

[0192] refer to Figure 8 According to a second embodiment of the efficient handover method, in the second phase 8-02, even when UE 8-20 executes a random access procedure for the target base station 8-20 indicated by the handover command message through the protocol layer device of the second bearer, UE 8-20 can continue to send data to or receive data from the source base station (uplink data transmission or downlink data reception) through the protocol layer device of the first bearer. In the second phase, the UE executes a cell selection or reselection procedure and executes a random access procedure for the target cell indicated by the handover command message (i.e., RRCReconfiguration message) received from the source base station.

[0193] according to Figure 8In a second embodiment of the efficient handover method, when UE 8-20 meets the first condition in the third phase 8-03, for a bearer configured with the DAPS handover method, UE 8-20 can stop sending uplink data to the source base station through the protocol layer device 8-22 of the first bearer, and can send uplink data to the target base station through the protocol layer device 8-21 of the second bearer. Furthermore, the UE can continuously receive downlink data from both the source and target base stations through the protocol layer devices of the first and second bearers. The third phase describes the stage where the UE meets the first condition and switches uplink transmission from the source base station to the target base station. Specifically, the third phase describes the stage where the UE sends uplink data to the source base station through the first bearer until the first condition is met, and when the first condition is met, stops sending uplink data to the source base station through the first bearer and starts sending uplink data to the target base station through the second bearer. Specifically, when a PDCP layer device of the second PDCP layer device structure of this disclosure, configured with the DAPS handover method, satisfies a first condition and receives an indicator from a lower-layer device (when the MAC layer device successfully completes the random access procedure to the target base station) or a higher-layer device (when the first timer in the RRC layer device expires) while transmitting uplink data through the first bearer for a bearer configured with the DAPS handover method, the PDCP layer device can stop and switch the transmission of uplink data through the first bearer, thereby starting uplink data transmission through the second bearer. Furthermore, as... Figure 9A and Figure 9B As shown in the structure of the PDCP layer device, the receiving PDCP layer device 8-21 of the second bearer and the receiving PDCP layer device 8-22 of the first bearer are driven together, and can continuously perform data reception from the source base station or the target base station by using stored transmitted or received data, sequence number information, or information such as header compression and decompression context. The first condition can be one of the following conditions. The first condition described below is the uplink data transmission switching time in which transmission resources are used most efficiently and data interruption time is minimized.

[0194] - When the UE successfully completes the random access procedure to the target base station through the second bearer layer device (e.g., MAC layer device), when the UE successfully completes the random access procedure to the target base station through the second bearer layer device (e.g., MAC layer device) and receives the first uplink transmission resource assigned from the target base station, or when the uplink transmission resource is first indicated to the UE, it can be determined that the first condition is met.

[0195] For example, more specifically, when the UE receives a handover command message from the source base station and receives an instruction to perform random access to the target base station, if the received random access is a contention-free random access procedure (CFRA) (e.g., if a pre-specified preamble or UE cell identifier (e.g., Cell Radio Network Temporary Identifier (C-RNTI)) is assigned),

[0196] Since the random access procedure can be considered successfully completed when the UE sends a pre-specified preamble to the target base station's cell and receives a Random Access Response (RAR) message, the first condition can be determined to be met when the first uplink transmission resource assigned, included, or indicated by the Random Access Response message is received. Alternatively, the first condition can be determined to be met when the first uplink transmission resource is received after receiving the RAR.

[0197] If the UE receives a handover command message from the source base station and receives an instruction to perform random access to the target base station, and if the commanded random access is based on a contention-based random access procedure (CBRA) (e.g., if no pre-assigned preamble or UE cell identifier (e.g., C-RNTI) is assigned),

[0198] When the UE sends a preamble (e.g., a random preamble) to the target base station's cell, receives a Random Access Response (RAR) message, and sends message 3 (e.g., a handover completion message) using uplink transmission resources allocated, included, or indicated by the RAR message, receives a MACCE (e.g., a contention-resolved MAC CE) indicating that contention has been resolved by message 4 from the target base station, or receives uplink transmission resources via the physical downlink control channel (PDCCH) corresponding to the UE's CRNTI, the UE can recognize that the random access procedure to the target base station has been successfully completed. Therefore, the UE can monitor the PDCCH and determine that the first condition is met upon the first receipt of uplink transmission resources or upon the first receipt of an indication via the PDCCH corresponding to the UE's CRNTI. Alternatively, if the size of the uplink transmission resources allocated by the RAR message is sufficient to send message 3, and the UE can additionally send uplink data, it can be determined that the first uplink transmission resources have been received and the first condition is met. In other words, after receiving the RAR, it can be determined that the first uplink transmission resources have been received and the first condition is met.

[0199] - If a handover method without random access procedures is not required (RACH handover), it is also indicated by the handover command message received by the UE.

[0200] *If the handover command message includes uplink transmission resources for the target base station,

[0201] When the UE sends message 3 (e.g., a handover completion message or an RRCReconfigurationComplete message) to the uplink transmission resources of the target base station, receives a UE authentication MAC CE from the base station via message 4, or receives uplink transmission resources via the PDCCH corresponding to the UE's C-RNTI, it can be determined that the random access procedure has been successfully completed and the first condition is met. Alternatively, it can be determined that the first condition is met by monitoring the PDCCH to receive the first uplink transmission resource via the PDCCH corresponding to the UE's C-RNTI after the random access procedure has been successfully completed.

[0202] *If the handover command message does not contain uplink transmission resources for the target base station,

[0203] **When a UE receives uplink transmission resources via the PDCCH corresponding to its C-RNTI by monitoring the PDCCH of the target base station (or cell), sends message 3 (e.g., a handover completion message or an RRCReconfigurationComplete message) to the uplink transmission resources, receives a UE authentication MAC CE from the base station, or receives uplink transmission resources via the PDCCH corresponding to its C-RNTI, it can be determined that the random access procedure has been successfully completed and the first condition has been met.** Alternatively, it can be determined that the first condition has been met when the first uplink transmission resource is received via the PDCCH corresponding to the UE's C-RNTI after the random access procedure has been successfully completed.

[0204] -1> When the DAPS handover method is indicated to the UE via a handover command message, and a two-step random access procedure is configured or indicated via a handover command message (e.g., RRCReconfiguration message),

[0205] -1> Alternatively, in the absence of a handover command message (e.g., an RRCReconfiguration message) configuring or instructing a 2-step random access procedure, the UE supports the 2-step random access procedure through its UE capabilities, through the system information of the target cell, and broadcasts 2-step random access procedure information (e.g., random access resources or thresholds for determining whether to perform 2-step random access); or in the case where the UE receives system information and performs a 2-step random access procedure for the target cell because the signal strength is higher than or greater than the threshold broadcast through the system information.

[0206] *2> When the two-step random access procedure is successfully completed, the UE can determine that the first condition is met.

[0207] *2> The two-step random access procedure can be specifically performed using either the CBRA method or the CFRA method.

[0208] **3> If the UE executes the above CBRA-based 2-step random access procedure,

[0209] ***4> The UE transmits a preamble via transport resources used for two-step random access (e.g., transport resources configured by the PRACH timing or by the base station via RRC messages or transport resources broadcast via system information), and may transmit data (e.g., MsgA MAC PDU) via transport resources used for data transmission (e.g., PUSCH timing). This data may include MAC control information (C-RNTI MAC CE), which includes the UE identifier (C-RNTI) or an RRC message (RRCReconfigurationComplete message or handover completion message).

[0210] ***4> The UE can monitor the PDCCH scrambled by the UE identifier (C-RNTI) or the first identifier (MsgB-RNTI), the first identifier being derived from the time or frequency of the preamble transmission.

[0211] ***4> If the UE receives a PDCCH scrambled with the UE identifier, if the PDCCH allocates downlink transmission resources, or if the downlink transmission resources receive MAC control information (Timing Advance Command MACCE) for timing adjustment,

[0212] ****5> The UE can confirm that the two-step random access procedure has been successfully completed and the first condition has been met.

[0213] ***4> If the UE receives a PDCCH scrambled with the first identifier (MsgB-RNTI), the PDCCH allocates downlink transmission resources, or the downlink transmission resources receive a back-to-back random access response to the preamble sent by the UE (i.e., a back-to-back RAR used to send MsgA to another transmission resource when the base station receives the preamble but does not receive MsgA).

[0214] ****5> The UE can send data (MsgA MAC PDU) to the transport resources indicated by the fallback random access response.

[0215] ****5> The UE can monitor the PDCCH scrambled by the UE identifier (C-RNTI).

[0216] ****5> If the UE receives a PDCCH scrambled with the UE identifier, or if the PDCCH allocates uplink transmission resources, the UE determines that the two-step random access procedure has been successfully completed and that the first condition is met.

[0217] **3> If the UE executes the above CFRA-based 2-step random access procedure,

[0218] ***4> The UE transmits a preamble via transport resources for two-step random access (e.g., transport resources specified by the PRACH timing or by the base station via an RRC message), and may transmit data (e.g., MsgA MAC PDU) via transport resources for data transmission (e.g., PUSCH timing). This data may include MAC control information (C-RNTI MAC CE), which includes the UE identifier (C-RNTI) or an RRC message (RRCReconfigurationComplete message or handover completion message).

[0219] ***4> The UE can monitor the PDCCH scrambled by the UE identifier (C-RNTI) or the first identifier (MsgB-RNTI), the first identifier being derived from the time or frequency of the preamble transmission.

[0220] ***4> If the UE receives a PDCCH scrambled with the UE identifier, or the PDCCH allocates downlink transmission resources, or the downlink transmission resources receive MAC control information for timing adjustment (Timing Advance Command MAC CE),

[0221] ****5> The UE can confirm that the two-step random access procedure has been successfully completed and that the first condition has been met.

[0222] ***4> If the UE receives a PDCCH scrambled with the first identifier (MsgB-RNTI), the PDCCH allocates downlink transmission resources, or the downlink transmission resources receive a back-to-back random access response to the preamble sent by the UE (i.e., a back-to-back RAR used to send MsgA to another transmission resource when the base station receives the preamble but does not receive MsgA).

[0223] ****5> The UE can confirm that the two-step random access procedure has been successfully completed and the first condition has been met.

[0224] ****5> The UE can send data to the transport resources indicated by the fallback random access response (MsgA MACPDU).

[0225] -1> As another method, if the DAPS handover method is indicated to the UE via a handover command message, and a two-step random access procedure is configured or indicated via a handover command message (e.g., an RRCReconfiguration message), the UE can determine that the first condition is met. For example, in the above case, the UE can determine that the first condition is met before starting the two-step random access procedure.

[0226] -1> As another method, if the DAPS handover method is indicated to the UE via a handover command message, or a two-step random access procedure is configured or indicated via a handover command message (e.g., an RRCReconfiguration message), the UE can determine that a first condition is met if the transport resources (e.g., PUSCH) configured for data transmission in the two-step random access procedure have a value greater than a first threshold, or if a timing advance value for time timing adjustment is included in the RRC message. The first threshold can be configured by the base station via an RRC message (e.g., RRCReconfiguration), can be broadcast via system information, or can be configured as the size of the data to be transmitted from the UE. For example, in the above case, the UE can determine that the first condition is met before starting the two-step random access procedure. As another method, if the timing advance value for time timing adjustment is included in the RRC message, or if a two-step random access procedure is configured, the UE can directly transmit data via the configured transport resources (e.g., transport resources configured via an RRC message or transport resources indicated by the PDCCH in which the UE monitors the target base station) without transmitting a preamble. Accordingly, in the above-described case, the UE can determine that the first condition has been met before starting the two-step random access procedure, or when transmitting data, or before transmitting data. Alternatively, if the timing advance value for time timing adjustment is included in the RRC message, or if the two-step random access procedure is configured in the RRC message, the UE can directly transmit data via the configured transmission resources (e.g., transmission resources configured via the RRC message or transmission resources indicated by the PDCCH of the target base station monitored by the UE) without transmitting a preamble. In the above-described case, if the configured transmission resources (PUSCH) (e.g., transmission resources configured via the RRC message or transmission resources indicated by the PDCCH of the target base station monitored by the UE) have a value greater than the first threshold, or if the RRC message includes a timing advance value for time timing adjustment, the UE can determine that the first condition has been met before starting the two-step random access procedure, or when transmitting data, or before transmitting data.

[0227] The following provides an efficient method for switching uplink data from a source base station to a target base station in the DAPS handover method of this disclosure. As described above, one of the following methods can be used in the MAC layer device or RRC layer device of the target base station corresponding to the second bearer to identify or detect whether the first condition is met, and the following methods can be combined and extended into new methods.

[0228] - First method: For example, in the case where DAPS handover is indicated by an RRCReconfiguration message received by the UE, the UE configures a MAC layer device for the target base station corresponding to the second bearer, and the MAC layer device can execute a random access procedure and identify whether a first condition is met. Furthermore, if the first condition is met, the MAC layer device can send an indicator to a higher-layer device (e.g., a PDCP layer device) of the bearer configured with the DAPS handover method, in the DAPS handover method of this disclosure, the indicator indicating that uplink data transmission will be switched from the source base station via the first bearer to the target base station via the second bearer.

[0229] - Second method: As another method, for example, when DAPS handover is indicated via an RRCReconfiguration message received by the UE, the UE configures a MAC layer device for the target base station corresponding to the second bearer, and the MAC layer device can execute a random access procedure and identify whether a first condition is met. Furthermore, if the first condition is met, the MAC layer device can send an indication to a higher-layer device (e.g., an RRC layer device) that the first condition has been met. Additionally, in the DAPS handover method of this disclosure, the higher-layer device (e.g., the RRC layer device) can send an indicator to a lower-layer device (e.g., a PDCP layer device) of the bearer configured with the DAPS handover method, indicating that uplink data transmission should be switched from the source base station via the first bearer to the target base station via the second bearer. Since the higher-layer device (e.g., the RRC layer device) stops a first timer when the first condition described in this disclosure is met or when a random access procedure to the target base station is successfully executed, the RRC layer device can instruct the PDCP layer device of the bearer configured with the DAPS handover method to perform a handover when the first timer is interrupted.

[0230] - A third method: For example, in the case where DAPS handover is indicated via an RRCReconfiguration message received by the UE, the UE configures a MAC layer device for the target base station corresponding to the second bearer. If the UE's RRC layer device sends an indicator to a lower-layer device (e.g., the MAC layer device) instructing the RRC layer device to perform DAPS handover, the MAC layer device can execute a random access procedure and identify whether the first condition is met. Furthermore, if the first condition is met, in the DAPS handover method of this disclosure, the MAC layer device can send an indicator to a higher-layer device (e.g., a PDCP layer device) of the bearer configured with the DAPS handover method, indicating that uplink data transmission will be switched from the source base station via the first bearer to the target base station via the second bearer.

[0231] - Fourth method: In another method, when a DAPS handover is indicated in the RRCReconfiguration message received by the UE, the UE configures a MAC layer device for the target base station corresponding to the second bearer. If the UE's RRC layer device sends an indicator to a lower-layer device (e.g., the MAC layer device) instructing the RRC layer device to perform a DAPS handover, the MAC layer device can execute a random access procedure and identify whether a first condition is met. Furthermore, if the first condition is met, the MAC layer device can send an indication that the first condition is met to a higher-layer device (e.g., the RRC layer device). When the indicator is identified, the higher-layer device (e.g., the RRC layer device) stops a first timer when the first condition described in this disclosure is met or when the random access procedure to the target base station is successfully executed, making the first timer interruptible. Furthermore, in the DAPS handover method disclosed herein, a higher-layer device (e.g., an RRC layer device) may send an indicator to a lower-layer device (e.g., a PDCP layer device) of a bearer configured with the DAPS handover method, the indicator indicating that uplink data transmission shall be switched from the source base station via the first bearer to the target base station via the second bearer.

[0232] According to the first, second, third, or fourth method, if the PDCP layer device receives an indicator from a higher-layer device (e.g., an RRC layer device) or a lower-layer device (e.g., a MAC layer device) indicating that a first condition has been met, or an indicator to switch uplink data transmission from the source base station to the target base station (e.g., when indicating a DAPS handover method), the PDCP layer device may perform the operations of the protocol layer device described below to effectively perform the handover of uplink data transmission, and perform one or more of the following operations to prevent data loss due to uplink data transmission. The following operations can be applied to PDCP layer devices connected to an AM DRB or UM DRB (an RLC layer device operating in AM mode or an RLC layer device operating in UM mode). As described above, if data to be transmitted exists in the buffer before the first condition is met or before receiving the indicator indicating that the first condition has been met, the PDCP layer device indicates the size or amount of data to be transmitted (e.g., PDCP data amount) to the MAC layer device for the first bearer of the source base station to notify of the existence of data to be transmitted, and performs uplink data transmission to the source base station. Then, the MAC layer device for the first bearer of the source base station can execute a procedure to request a scheduling request or buffer status report to the source base station in order to receive the allocation of uplink transmission resources. However, when the first condition is met or an indicator indicating that the first condition is met is received, for the bearer configured with the DAPS handover method, the uplink data transmission handover to the target base station is performed as follows.

[0233] - The uplink or downlink ROHC context of the source base station is not initialized and is used as is, while the uplink or downlink ROHC context of the target base station is initialized and can start in an initial state (e.g., IR state in U mode).

[0234] To switch uplink data transmission from the first bearer used by the source base station to the second bearer used by the target base station, the PDCP layer device can send an indication to the MAC layer device used by the source base station that the size and amount of data to be transmitted are zero (or none). In other words, the PDCP layer device can send an indication to the MAC layer device used by the source base station that the amount of data in the PDCP layer device is zero, thus indicating that no more data needs to be transmitted (even if there is actual data to be transmitted in the buffer, an indication that no data needs to be transmitted can be sent to the MAC layer device used by the source base station to switch uplink data transmission).

[0235] However, as described in this disclosure, when the handover method (DAPS handover method) of the second embodiment of this disclosure is indicated, if the first condition is met, or if RLC control data (RLC status report) or PDCP control data (PDCP status report or ROHC feedback) of the source base station is generated, the PDCP layer device of the bearer can indicate the amount of data corresponding to the RLC control data or PDCP control data to the MAC layer device of the source base station, and can perform data transmission to the source base station or the RLC layer device of the source base station. However, as described in this disclosure, when the handover method (DAPS handover method) of the second embodiment of this disclosure is indicated, or when the first condition is met for a bearer indicated by the handover method (DAPS handover method) of the second embodiment of this disclosure, if RLC control data (RLC status report) or PDCP control data (PDCP status report or ROHC feedback) for the target base station is generated, the PDCP layer device of the bearer can indicate the amount of data corresponding to the RLC control data or PDCP control data to the MAC layer device of the target base station, and can perform data transmission to the target base station or the RLC layer device of the target base station. If the first condition is not met, the PDCP layer device can indicate the generated data (PDCP data PDU or PDCP control PDU) and the amount of data corresponding to the data to the MAC layer device of the source base station, and can perform data transmission to the source base station or the RLC layer device of the source base station. Therefore, after receiving an indicator indicating that the first condition has been met in the structure of the second PDCP layer device described for the bearer configured with the DAPS handover method, when indicating the amount of data to the MAC layer device of the target base station, the second PDCP layer device can indicate PDCP control data, RLC control data, or the amount of data other than the data size to the MAC layer device of the target base station, which will be sent to the bearer of the source base station or the MAC layer device of the source base station.

[0236] - A PDCP layer device connected to an AM DRB (an RLC layer device operating in AM mode) (where all previously stored PDCP PDUs are discarded (e.g., PDCP SDUs are not discarded to prevent loss of original data)) can perform a new header compression procedure on data (PDCP SDUs in the buffer) starting from the first data (e.g., a PDCP SDU) that was not successfully transmitted from a lower layer (e.g., the RLC layer device corresponding to the first bearer for the source base station), in ascending order of a count value (or PDCP sequence number) based on the header context of the target base station. This count value is assigned before the first condition is met or before receiving an indicator indicating that the first condition is met. Furthermore, the PDCP layer device connected to the AM DRB can re-execute an integrity procedure or encryption procedure by applying the target base station's security key, configure the PDCP header, and send the PDCP header to the lower layer device (the RLC layer device for the second bearer of the target base station) to perform retransmission or transmission. In other words, the PDCP layer device performs cumulative retransmission starting from the first data that was not successfully transmitted. As another approach, when performing the above retransmissions, the PDCP layer device can only perform retransmissions for data that has not yet been identified as successfully transmitted from a lower layer (e.g., the RLC layer device for the first bearer of the source base station). More specifically, the PDCP layer device connected to the AMDRB (an RLC layer device operating in AM mode) (where all stored PDCP PDUs are discarded so they can be sent to the source base station via a first protocol layer device connected to the PDCP layer device (e.g., PDCP SDUs are not discarded to prevent loss of the original data)) performs selective retransmission only for data (e.g., PDCP SDUs) that has not yet been successfully transmitted from the first protocol layer device (e.g., an RLC layer device) acting as the source base station. This is done by applying a header compression (or data compression) protocol context or security key corresponding to the target base station based on a count value (or PDCP sequence number) assigned before or before receiving an indicator indicating that the first condition is met. The device then re-executes the header or data compression procedure, re-executes the integrity procedure or encryption procedure, configures the PDCP header, and sends the PDCP header to the lower layer device (the RLC layer device serving as the second bearer for the target base station) to perform retransmission or transmission. In other words, to prevent waste of transmission resources, the PDCP layer device can perform selective retransmission only for data that has not yet been successfully transmitted. As another method, transmission or retransmission operations can be performed by releasing a lower layer (e.g., a transmitting or receiving RLC layer device or a MAC layer device) that is the first protocol layer device used to send data to the source base station.If the transmission or retransmission procedure is extended to UM DRB, a PDCP layer device connected to an RLC layer device operating in UM mode can: treat data that has not yet been sent to a lower-layer device, data whose PDCP discard timer has not yet expired, or data fragments that have already been assigned a PDCP sequence number (or count value) as data received from a higher-layer device or newly received data, to perform header (or data) compression of the data fragments using the header (or data) compression context or security key of the target base station without restarting the PDCP discard timer for each data fragment; perform encryption or integrity protection procedures for this purpose; generate and conjugate a PDCP header, and perform its transmission or retransmission; process the data in ascending order of the count values ​​assigned before triggering the procedure; and perform its transmission or retransmission. Furthermore, the window state variables of PDCP layer devices connected to UM DRB or AMDRB are not initialized, but are maintained and used as is.

[0237] - As described above, if data to be transmitted exists in the buffer, the PDCP layer device indicates the size or amount of data to be transmitted (e.g., PDCP data volume) to the MAC layer device of the second bearer for the target base station to notify that data to be transmitted exists, and performs uplink data transmission handover to the target base station. The MAC layer device of the second bearer for the target base station can then execute procedures to request scheduling or report buffer status to the target base station in order to receive allocation of uplink transmission resources.

[0238] - For a bearer that indicates (or configures) the second embodiment (or DAPS handover method), when the first condition is met, the UE may release the configuration information or context of the data compression protocol (e.g., uplink data compression protocol) of the source base station. Alternatively, when the first condition is met, a higher-layer device of the UE (e.g., an RRC layer device) may instruct a PDCP layer device to release or reconfigure the configuration information or context of the data compression protocol (e.g., uplink data compression protocol). However, for a bearer that does not indicate (or configure) the second embodiment (or DAPS handover method), when a handover command message is received, the UE may release the configuration information or context of the data compression protocol (e.g., uplink data compression (UDC) protocol). Alternatively, when a handover command message is received, a higher-layer device of the UE (e.g., an RRC layer device) may instruct a PDCP layer device (for the source base station) to release or reconfigure the configuration information or context of the data compression protocol (e.g., uplink data compression protocol). This is because, for bearers configured with the DAPS handover method, data needs to be compressed using the context or configuration information of the source base station's data compression protocol until the first condition is met, and then sent to the source base station.

[0239] When a DAPS handover method is configured for at least one of the bearers configured for the UE, or a DAPS handover method is configured for a predetermined bearer, if a first condition is met or if an indicator indicating that the first condition is met is received, the UE may perform one of the following methods for a bearer for which a DAPS handover method is not configured.

[0240] - First method: If the first condition is met, a higher-layer device of the UE (e.g., an RRC layer device) can trigger or request a PDCP reconstruction procedure for one or more bearers that are not configured with the DAPS handover method (even if the target base station has configured a PDCP reconstruction procedure for the bearer in the handover command message, the procedure can still be executed when the first condition is met). A PDCP layer device that has received a request for a PDCP reconstruction procedure can execute different PDCP reconstruction procedures for the corresponding bearer. For example, for a UM DRB, the PDCP layer device can initialize window state variables and, based on the target base station's header (or data) compression context or security key, perform compression, encryption, or integrity protection on data that has not yet been sent to a lower-layer device or data whose PDCP drop timer has not expired, in ascending order of count values, to perform its transmission or retransmission. Furthermore, if a reordering timer is running, the PDCP layer device can interrupt and initialize the reordering timer and can sequentially process the received data (PDCP SDU or PDCP PDU) and send it to a higher-layer device. For AM DRB, PDCP layer devices may not initialize window state variables and may perform compression, encryption, or integrity protection on the first data (PDCP SDU or PDCP PDU) that has not yet been successfully transmitted from lower-layer devices, based on the header (or data) compression context or security key of the target base station, in ascending order of counter value or PDCP sequence number, to perform its transmission or retransmission. For bearers(s) without a configured DAPS handover method, the reason for executing the PDCP reconstruction procedure when the first condition is met, rather than when a handover command message is received, is that if the handover procedure to the target base station fails, the UE can perform a fallback to the source base station. During the fallback, data segments compressed based on the header (or data) compression context of the target base station and encrypted or protected for integrity based on the target base station's security key become useless and need to be discarded. Furthermore, when a fallback is required, the PDCP reconstruction procedure is executed again, and data segments to be sent to the bearer need to be compressed based on the header (or data) compression context of the source base station and encrypted or protected for integrity again based on the source base station's security key. Therefore, unnecessary processing occurs. Thus, when the UE executes the DAPS handover method, for bearers without a configured DAPS handover method, the PDCP reconstruction procedure is not triggered or executed upon receiving the handover command message. The target base station can configure the PDCP reconstruction procedure for the bearer without using the handover command message, and can trigger or execute the PDCP reconstruction procedure when the first condition is met. Furthermore, for bearers with a configured DAPS handover method, the PDCP reconstruction procedure is not executed.

[0241] - The second method: Upon receiving a handover command message, a higher-layer device of the UE (e.g., an RRC layer device) can trigger or request a PDCP reconstruction procedure for (multiple) bearers that are not configured with the DAPS handover method. A PDCP layer device that has received a request for a PDCP reconstruction procedure can execute different PDCP reconstruction procedures for the corresponding bearer. For example, for a UM DRB, the PDCP layer device can initialize window state variables and, based on the header (or data) compression context or security key of the target base station, perform compression, encryption, or integrity protection on data that has not yet been sent to a lower-layer device or data whose PDCP drop timer has not expired, in ascending order of count values, to perform its transmission or retransmission. Furthermore, if a reordering timer is running, the PDCP layer device can interrupt and initialize the reordering timer, and can sequentially process the received data (PDCP SDU or PDCP PDU) and send it to a higher-layer device. For AM DRB, PDCP layer devices may not initialize window state variables and may perform compression, encryption, or integrity protection on the first data (PDCP SDU or PDCP PDU) that has not yet been successfully transmitted from lower-layer devices, based on the header (or data) compression context or security key of the target base station, in ascending order of count value or PDCP sequence number, to perform its transmission or retransmission. When a handover command message is received for bearer(s) without a configured DAPS handover method, the data is compressed based on the header (or data) compression context of the target base station and encrypted and protected for integrity based on the security key of the target base station through a PDCP reconstruction procedure. Accordingly, if the UE fails to hand over to the target base station (e.g., if the first timer expires or the radio connection to the target base station fails), and a backoff to the source base station is possible and a backoff is performed, the base station needs to support the UE discarding data segments (e.g., PDCP PDUs) generated or processed in the bearer for transmission to the target base station, and reprocessing data segments (e.g., PDCP SDUs) based on the header (or data) compression context or security key of the source base station. Accordingly, the base station may request or instruct higher-layer devices of the UE (e.g., RRC layer devices) to reconfigure instructions for discarded data segments (e.g., PDCP PDUs) for bearers that do not indicate a DAPS handover method, these data segments being processed based on the target base station's configuration information (security key or header (or data) compression context), PDCP reconstruction procedure, or the source base station's configuration information (security key or header (or data) compression context); and regenerate or process the data based on the source base station's configuration information.

[0242] According to a second embodiment of the efficient handover method of this disclosure (e.g., the DAPS handover method), even after the UE receives a handover command message (e.g., an RRCReconfiguration message), the UE can continuously receive downlink data from the source base station or the target base station via a protocol layer device for the first bearer of the source base station or for the second bearer of the target base station. Furthermore, to smoothly receive downlink data from or transmit downlink data from the source base station (or the target base station), for the AM bearer, the UE can allow RLC status reports (excluding data) to be continuously transmitted to the source base station (or the target base station) via uplink transmission through the protocol layer device of the first bearer (or the second bearer). That is, even if the UE switches uplink data transmission to the target base station by satisfying the first condition described above, when it is necessary to send RLC status reports, HARQ ACK, NACK, or PDCP control data (PDCP ROHC feedback or PDCP status reports) to the source base station, data transmission can still be allowed via the first bearer for the source base station. This is because, in the case of AM bearer, after sending data to the sending terminal, if there is no successful transmission of an indication of an RLC status report (i.e., if no RLC status report is received), data transmission cannot continue. Specifically, in Figure 8 In the second embodiment of the efficient handover method, even if UE 8-20 stops sending uplink data to the source base station through the protocol layer device 8-22 of the first bearer because the first condition is met in the third stage 8-03, and switches to start sending uplink data to the target base station through the protocol layer device 8-21 of the second bearer, UE can still enable continuous transmission of HARQ ACK or HARQ NACK information, RLC status reports (ACK or NACK information), or PDCP control data (e.g., PDCP status reports or ROHC feedback information) through the protocol layer device of the first bearer (or second bearer) to smoothly receive downlink data from the source base station (or target base station), or enable the source base station (or target base station) to smoothly transmit downlink data. Furthermore, in Figure 8 In the second embodiment of the efficient handover method, even if UE 8-20 stops sending uplink data to the source base station through the protocol layer device 8-22 of the first bearer because the first condition is met in the third stage 8-03, and switches to start sending uplink data to the target base station through the protocol layer device 8-21 of the second bearer, the UE can still continuously perform data transmission caused by HARQ retransmission of the MAC layer device or data transmission caused by retransmission of the RLC layer device in AM mode, so as to prevent data loss to the source base station. In the above, Figure 8In the second embodiment of the efficient handover method, if UE 8-20 stops sending uplink data to the source base station through the protocol layer device 8-22 of the first bearer and switches to start sending uplink data to the target base station through the protocol layer device 8-21 of the second bearer because the first condition is met in the third stage 8-03, then the source base station or the target base station can allocate time and allocate transmission resources to the UE accordingly, so that the uplink transmission resources for the target base station and the uplink transmission resources for the source base station do not conflict. If the uplink transmission resources for the target base station and the uplink transmission resources for the source base station conflict and overlap, then the UE can give priority to the uplink transmission resources for the source base station and perform data transmission to the source base station in order to maintain downlink data transmission from the source base station without problems, or continuously receive uplink transmission resources from the source base station. As another approach, if the uplink transmission resources for the target base station and the uplink transmission resources for the source base station conflict and overlap, the UE may prioritize the uplink transmission resources for the target base station and perform data transmission to the target base station in order to maintain downlink data transmission from the target base station.

[0243] Specifically, when the UE receives a handover command message or when a handover corresponding to the second embodiment of this disclosure (e.g., DAPS handover) is indicated for each bearer handover, the UE or bearer indicating the DAPS handover can execute a scheduling request through the first protocol layer device and send a buffer status report to the source base station before the first condition is met, in order to receive uplink transmission resources and send uplink data, and receive downlink data from the source base station. However, if the first condition is met, the UE no longer sends data to the source base station and switches the uplink to execute the scheduling request through the second protocol layer device, and can send a buffer status report to the target base station, receive uplink transmission resources, and send uplink data to the target base station. However, the UE can continuously receive downlink data from the source base station, and even after the uplink transmission switch, can continuously send HARQ ACK, HARQ NACK, RLC status report, or PDCP control data (e.g., PDCP status report or ROHC feedback information) corresponding to the downlink data. Furthermore, even if the first condition is met, the UE can continue to receive downlink data from the source base station or the target base station.

[0244] exist Figure 8In the second embodiment of the efficient handover method, in the fourth state 8-04, if UE8-20 meets the second condition, the UE can stop receiving downlink data from the source base station 8-05 through the protocol layer device 8-22 of the first bearer, or release the connection to the source base station. The second condition can be one of the following conditions. Furthermore, the PDCP layer device 8-21 of the second bearer can continuously perform data transmission to the target base station or data reception from the target base station by using the transmit or receive data, sequence number information, or information such as header compression and decompression context stored in the PDCP layer device 8-22 of the first bearer.

[0245] - When the UE executes the random access procedure to the target base station through the second bearer layer device 8-21 and receives the random access response, it can be determined that the second condition is met.

[0246] - When the UE executes the random access procedure to the target base station, receives the random access response, configures the handover completion message and sends it to the target base station through the layer device of the second bearer, it can be determined that the second condition is met.

[0247] - When the UE completes the random access procedure to the target base station through the second bearer layer device and sends data to the PUCCH or PUSCH uplink transmission resources for the first time, or when it initially receives the PUCCH or PUSCH uplink transmission resources, it can be determined that the second condition is met.

[0248] - The base station can set a separate timer for the UE via RRC messages, and when the timer expires, it can determine that the second condition is met.

[0249] *A timer can be started when the UE has received a handover command message from the source base station, started random access to the target base station (when the UE sends a preamble), received a random access response from the target base station, sent a handover completion message to the target base station, or initially sent data to the PUCCH or PUSCH uplink transmission resources.

[0250] - When the UE executes the random access procedure to the target base station through the second bearer layer device, receives the random access response, configures the handover completion message and sends it to the target base station, and then the successful transmission of the handover completion message is recognized by the MAC layer device (HARQ ACK) or RLC layer device (RLC ACK), it can be determined that the second condition has been met.

[0251] - When the UE performs a random access procedure to the target base station through the layer device of the second bearer, receives a random access response and configures a handover completion message and sends it to the target base station, and then initially receives an allocation of uplink transmission resources or an indication of uplink transmission resources from the target base station, it can be determined that the second condition has been met.

[0252] When the source base station performs the efficient handover of this disclosure, the time for ceasing downlink data transmission to the UE or the time for releasing the connection with the UE can be determined. For example, this time can be determined using a predetermined method (e.g., when a predetermined timer expires (which can be started after the handover instruction is executed), or when the source base station receives an instruction from the target base station indicating that the UE has successfully performed the handover to the target base station). Furthermore, if no downlink data is received from the source base station within a predetermined time period, the UE can determine that a second condition is met and can determine to release the connection with the source base station and release the connection.

[0253] - When the UE receives an indicator (e.g., an RRC message (e.g., an RRCReconfiguration message) indicating the release of the connection with the source base station from the target base station, MAC CE, RLC control PDU, or PDCP control PDU, it can be determined that the second condition is met.

[0254] - If the UE does not receive downlink data from the source base station within the predetermined time, then the second condition can be determined to be met.

[0255] - The second condition can be determined to be satisfied when the UE successfully completes the random access procedure to the target base station through the second bearer layer device (e.g., MAC layer device), or when the UE successfully completes the random access procedure to the target base station through the second bearer layer device and receives the allocation of the first uplink transmission resource from the target base station, or when the uplink transmission resource is first indicated to the UE.

[0256] For example, more specifically, when the UE receives a handover command message from the source base station and receives an indication to perform random access to the target base station, if the received random access is CFRA (e.g., if a pre-specified preamble or UE cell identifier (e.g., C-RNTI) is assigned),

[0257] Since the random access procedure can be determined to have been successfully completed when the UE sends a pre-specified preamble to the target base station's cell and receives a Random Access Response (RAR) message, the second condition can be determined to be met when the first uplink transmission resource assigned, included, or indicated by the Random Access Response message is received. Alternatively, the second condition can be determined to be met when the first uplink transmission resource is received after receiving the RAR.

[0258] *If the UE receives a handover command message from the source base station and receives an instruction to perform random access to the target base station, and if the commanded random access is CBRA (e.g., if no pre-assigned preamble or UE cell identifier (e.g., C-RNTI) is assigned),

[0259] When the UE sends a preamble (e.g., a random preamble) to the target base station's cell, receives a Random Access Response (RAR) message, sends message 3 (e.g., a handover completion message) using uplink transmission resources allocated, included, or indicated by the RAR message, receives a Contention Resolution MAC CE indicating that contention has been resolved from the target base station via message 4, or receives uplink transmission resources via the PDCCH corresponding to the RNTI, the UE can identify that the random access procedure to the target base station has been successfully completed. Accordingly, when the UE monitors the PDCCH and receives uplink transmission resources for the first time via the PDCCH corresponding to the UE's C-RNTI, or receives its indication, the second condition is determined to be met. Alternatively, if the size of the uplink transmission resources allocated via the RAR message is sufficient to send message 3, and the UE can additionally send uplink data, it can be determined that uplink transmission resources have been received for the first time and the second condition is met. That is, when the RAR is received, it can be determined that uplink transmission resources have been received for the first time and the second condition is met.

[0260] - If a handover method without random access procedures is not required (RACH handover), it is also indicated by the handover command message received by the UE.

[0261] *If the handover command message includes uplink transmission resources for the target base station,

[0262] When the UE sends message 3 (e.g., a handover completion message or an RRCReconfigurationComplete message) to the uplink transmission resources of the target base station, receives a UE authentication MAC CE from the base station via message 4, or receives uplink transmission resources via the PDCCH corresponding to the UE's C-RNTI, it can be determined that the random access procedure has been successfully completed and the second condition is met. Alternatively, it can be determined that the second condition is met when the PDCCH is monitored after the random access procedure has been successfully completed and the first uplink transmission resource is received via the PDCCH corresponding to the UE's C-RNTI.

[0263] *If the handover command message does not contain uplink transmission resources for the target base station,

[0264] - When the UE receives uplink transmission resources by monitoring the PDCCH of the target base station (or cell) via the PDCCH corresponding to the UE's C-RNTI, sends message 3 (e.g., a handover completion message or an RRCReconfigurationComplete message) to the uplink transmission resources, receives a UE authentication MAC CE from the base station, or receives uplink transmission resources via the PDCCH corresponding to the UE's C-RNTI, it can be determined that the random access procedure has been successfully completed and the second condition is met. Alternatively, it can be determined that the second condition is met when the UE receives the first uplink transmission resource via the PDCCH corresponding to the UE's C-RNTI after the random access procedure has been successfully completed.

[0265] In the foregoing, when the UE executes the second embodiment of the efficient handover method of this disclosure (e.g., the DAPS handover method), if it is identified that the UE's RRC layer device, MAC layer device, or RLC layer device for the first bearer of the source base station, or the RRC layer device, MAC layer device, or RLC layer device for the second bearer of the target base station, satisfies the second condition described in this disclosure, then a higher-layer or lower-layer device may send an indicator indicating that the second condition is satisfied to the UE or the bearer's PDCP layer device to execute the DAPS handover method. If the UE's PDCP layer device receives an indicator indicating that the second condition is satisfied from a lower-layer or higher-layer device, or if the second condition is satisfied, one or more procedures, as described below, may be executed for the bearer or UE configured with the DAPS handover method to successfully complete the second embodiment of the efficient handover method of this disclosure.

[0266] - The UE can release the first bearer used by the source base station and release the connection with the source base station. Furthermore, before releasing the first bearer used by the source base station, the UE can perform an RLC reconstruction procedure on the RLC layer device corresponding to the first bearer used by the source base station (e.g., if a reordering timer is running, the timer is paused or initialized; if received data is stored in a buffer, the stored data can be processed and sent to a higher-layer device; and if data to be sent is in a buffer, the data can be discarded), or it can initialize the MAC layer device.

[0267] - When the UE disconnects from the source base station, the UE can trigger the PDCP status reporting procedure and configure the PDCP status reporting to send a PDCP status report to the target base station in order to report the reception status of the downlink data received from the source base station.

[0268] - If the second condition is met, then for each bearer or for the bearer indicating the DAPS handover method, the UE can transfer the second PDCP layer device (by...) Figure 9A and Figure 9BWhen the structure or function of the device (indicated by reference numerals 9-20) is switched to that of the first PDCP layer device of this disclosure (indicated by reference numerals 9-11 or 9-12), variables used for realignment can be initialized, and a reordering timer can be interrupted and initialized. Furthermore, the UE can perform a decryption procedure or header (or data) decompression on data segments stored in the buffer (e.g., data segments received from the source base station) for realignment by applying the source base station's security key or header decompression context, and then discard the source base station's security key or header decompression context. Additionally, the UE can send the processed data to higher layers in ascending order. That is, in the above case, when the second condition is met, the UE can perform a decryption procedure or header (or data) decompression on data segments stored in the buffer (e.g., data segments received from the source base station) for realignment by applying the source base station's security key or header decompression context, and then discard the source base station's security key or header decompression context. In another method, if the second condition is met, for each bearer or for a bearer indicating the DAPS handover method, the UE can switch the structure or function of the second PDCP layer device (indicated by reference numerals 9-20) to the structure or function of the third PDCP layer device of this disclosure (indicated by reference numerals 9-30), and can use the variables and reordering timers for realignment as is without stopping and initialization. However, the UE can perform a decryption procedure or header (or data) decompression on the data segments stored in the buffer (e.g., data segments received from the source base station) by applying the source base station's security key or header decompression context for realignment, and then discard the source base station's security key or header decompression context. Furthermore, the UE can send the processed data to higher layers in ascending order. That is, in the above case, when the second condition is met, the UE can perform a decryption procedure or header (or data) decompression on the data segments stored in the buffer (e.g., data segments received from the source base station) by applying the source base station's security key or header decompression context for realignment, and then discard the source base station's security key or header decompression context. In the foregoing, the UE may release the QoS mapping information of the SDAP layer device of the source base station, the security key information of the source base station of the PDCP layer device, the header (or data) compression context information of the source base station, or the RLC layer device or MAC layer device of the source base station. In the foregoing, for each bearer or for a bearer indicating the DAPS handover method, switching the structure or function of the second PDCP layer device (indicated by reference numerals 9-20) to the structure or function of the first PDCP layer device of this disclosure may indicate that the PDCP layer device is reconfigured, and may be performed upon receiving a reconfiguration indicator for the PDCP layer device from a higher-layer device (e.g., an RRC layer device).For example, the UE receives a handover command message, and a higher-layer device (e.g., an RRC layer device) sends a PDCP layer device reconfiguration indicator to the PDCP layer device of the bearer configured with the DAPS handover method, thereby reconfiguring the structure or function of the first PDCP layer device to the structure or function of the second PDCP layer device. Furthermore, if a second condition is met, the higher-layer device (e.g., the RRC layer device) can send a PDCP layer device reconfiguration indicator to the PDCP layer device of the bearer to reconfigure the structure or function of the second PDCP layer device to the structure or function of the first PDCP layer device. For example, whenever a PDCP layer device reconfiguration indicator is received from a higher-layer device (e.g., an RRC layer device) using a toggle scheme, the PDCP layer device of the bearer can reconfigure the structure or function of the first PDCP layer device to the structure or function of the second PDCP layer device, or vice versa.

[0269] When the second condition described in this disclosure is met while executing the DAPS handover method of this disclosure, the UE can release the first bearer for the source base station and switch the structure and function of the second SDAP layer device applied to each bearer or the bearer indicating the DAPS handover method to the structure and function of the first SDAP layer device, and apply that structure and function. Furthermore, if the second condition is met, for each bearer or for the bearer indicating the DAPS handover method, the UE can switch the structure or function of the second PDCP layer device to the structure or function of the first PDCP layer device of this disclosure, and can maintain the mapping information between the second QoS stream and the bearer of the target base station and the second bearer. Furthermore, after completing data processing by applying the mapping information of the first QoS stream and the bearer to data segments received from the source base station (e.g., all data segments received from the source base station) before releasing the mapping information of the first QoS stream and the bearer or the first bearer of the source base station, the UE can release the first QoS stream and the bearer or the mapping information of the first bearer of the source base station. Furthermore, the UE can send the processed data to higher layers in ascending order. In other words, as described above, when the second condition is met, the UE can process data fragments (e.g., all data fragments received from the source base station) stored in the buffer by applying the mapping information between the source base station's first QoS stream and bearer (e.g., procedures for reading SDAP header information and updating mapping information, configuring SDAP headers, or routing or sending data to appropriate higher-layer or lower-layer devices based on the mapping information between the first QoS stream and bearer). Then, it can discard the mapping information between the source base station's first QoS stream and bearer. As described above, the SDAP layer device can define and apply a 1-bit indicator for the new SDAP header, a 1-bit indicator for the PDCP header, SDAP control data (e.g., downlink end marker), or information indicated by the PDCP layer device, and can identify the last data received from the source base station based on the defined information. Therefore, after performing data processing by applying the mapping information between the source base station's first QoS stream and bearer to the last data received from the source base station, the UE can discard the mapping information between the source base station's first QoS stream and bearer. Furthermore, as mentioned above, the SDAP layer device can continuously maintain the mapping information between the second QoS flow and the bearer, and process uplink data or downlink data transmission to the target base station based on this mapping information.

[0270] - The UE can initialize the MAC layer equipment of the source base station, and when the second PDCP layer equipment structure of the bearer configured with the DAPS handover method is converted to the first PDCP layer equipment structure, the UE can perform the RLC layer equipment reconstruction procedure or release procedure for the RLC layer equipment of the source base station in the second PDCP layer equipment structure.

[0271] *In this disclosure, when the second condition is met or an indicator indicating that the second condition is met is received from a higher-layer device (e.g., an RRC layer device) or a lower-layer device (e.g., a MAC layer device), the procedure for the UE to rebuild or release the RLC layer device of the first bearer for the source base station may specifically follow one of the following methods.

[0272] **First Method:** If the RLC layer device used for the first bearer of the source base station is an LTE RLC layer device with the DAPS handover method configured above, then a higher-layer device (e.g., an RRC layer device) can instruct the LTE RLC layer device to perform a reconstruction procedure. Specifically, if a reordering timer is running, the LTE RLC layer device can stop or initialize the timer, and if stored data exists, the LTE RLC layer device can process the stored data and send the processed data to the higher-layer device to reduce transmission delays caused by the reordering timer. Furthermore, the RLC layer device can initialize variables and discard data segments used for transmission. Then, the higher-layer device (e.g., an RRC layer device) can instruct the release of the LTE RLC layer device. However, if the RLC layer device used for the first bearer of the source base station is an NR RLC layer device, then the higher-layer device (e.g., an RRC layer device) can instruct the direct release of the NR RLC layer device without reconstruction. This is because NR RLC layer devices always perform out-of-order transmission, so there is no stored data, and even if stored data exists, it is fragmented, so it is acceptable to discard it as is. As mentioned above, different procedures can be applied depending on the RLC layer device configured for each bearer.

[0273] **Second method:** If the RLC layer device used for the first bearer of the source base station is an LTE RLC layer device with the DAPS handover method configured above, then a higher-layer device (e.g., an RRC layer device) can instruct the LTE RLC layer device to perform a reconstruction procedure. Specifically, if a reordering timer is running, the LTE RLC layer device can stop or initialize the timer, and if stored data exists, the LTE RLC layer device can process the stored data and send the processed data to the higher-layer device to reduce transmission delays caused by the reordering timer. Alternatively, the higher-layer device (e.g., an RRC layer device) can release the LTE RLC layer device. However, if the RLC layer device used for the first bearer of the source base station is an NR RLC layer device, then the higher-layer device (e.g., an RRC layer device) can instruct the NR RLC layer device to be released directly without reconstruction. This is because NR RLC layer devices always perform out-of-order transmission, so there is no stored data, and even if stored data exists, it is fragmented data, so it is not a problem if it is discarded as is. As mentioned above, different procedures can be applied depending on the RLC layer device configured for each bearer.

[0274] **Third method:** If the RLC layer device used for the first bearer of the source base station is an LTE RLC layer device of the bearer configured with the DAPS handover method as described above, the target base station may, when sending an RRC message (e.g., RRCReconfiguration) to the UE including an indicator for releasing the connection with the source base station or the first bearer of the base station, include an indicator for re-establishing the LTE RLC layer device (e.g., reestablishRLC) in the configuration information (e.g., RLC-config) of the LTE RLC layer device of the bearer in the RRC message, and send the message (or may include an indicator for releasing the LTE RLC layer device). Therefore, upon receiving an indication for releasing the connection with the source base station or after the second condition is met, the higher-layer device (e.g., the RRC layer device) can read the RRC message and may instruct the LTE RLC layer device to perform a reconstruction procedure according to the indication for the LTE RLC reconstruction procedure (or release). Specifically, if a reordering timer is running, the LTE RLC layer device can stop or initialize the timer, and if stored data exists, the LTE RLC layer device can process the stored data and send the processed data to a higher-layer device to reduce transmission latency caused by the reordering timer. Furthermore, the LTE RLC layer device can initialize variables and discard data segments intended for transmission. Then, the higher-layer device (e.g., the RRC layer device) can release the LTE RLC layer device. If the RLC layer device for the first bearer of the source base station is an LTE RLC layer device for a bearer configured with the DAPS handover method as described above, the target base station can include an indicator indicating the release of the NR RLC layer device in the configuration information (e.g., RLC-config) of the bearer in the RRC message when sending an RRC message (e.g., RRCReconfiguration) to the UE that includes an indicator for releasing the connection with the source base station or for the first bearer of the source base station. As described above, the target base station can instruct different procedures based on the RLC layer device configured for each bearer via RRC messages, and accordingly, the UE can apply the procedures to each bearer.

[0275] **Fourth method:** If the RLC layer device used for the first bearer of the source base station is an LTE RLC layer device of the bearer configured with the DAPS handover method as described above, the target base station may, when sending an RRC message (e.g., RRCReconfiguration) to the UE including an indicator for releasing the connection with the source base station or the first bearer of the base station, include an indicator for re-establishing the LTE RLC layer device (e.g., reestablishRLC) in the configuration information (e.g., RLC-config) of the LTE RLC layer device of the bearer in the RRC message, and send the message (or may include an indicator for releasing the LTE RLC layer device). Therefore, upon receiving an indication for releasing the connection with the source base station or after the second condition is met, the higher-layer device (e.g., the RRC layer device) can read the RRC message and may instruct the LTE RLC layer device to perform a reconstruction procedure according to the indication for the LTE RLC reconstruction procedure (or release). Specifically, if a reordering timer is running, the LTE RLC layer device can stop or initialize the timer, and if stored data exists, the LTE RLC layer device can process the stored data and send the processed data to a higher-layer device to reduce transmission latency caused by the reordering timer. Furthermore, the RLC layer device can initialize variables and discard data segments intended for transmission. Alternatively, a higher-layer device (e.g., an RRC layer device) can release the LTE RLC layer device. If the RLC layer device for the first bearer of the source base station is an NR RLC layer device for a bearer configured with the DAPS handover method described above, the target base station can include an indicator indicating the release of the NR RLC layer device or the reconstruction of the NR RLC layer device in the RRC message (e.g., RRCReconfiguration) when sending an RRC message to the UE that includes an indicator for releasing the connection with the source base station or for the first bearer of the source base station. As described above, the target base station can instruct different procedures through RRC messages based on the RLC layer device configured for each bearer, and accordingly, the UE can apply the procedures to each bearer.

[0276] - The receiving PDCP layer device can process or store data received due to the reconstruction procedure of lower layer devices (e.g., RLC layer devices), and for UM DRB, it can perform a header decompression procedure on the stored data (received from the source base station) or all stored data based on the header compression context (ROHC or Ethernet header compression (EHC)) of the source base station.

[0277] - The receiving PDCP layer device can process or store data received due to the reconstruction procedure of lower layer devices (e.g., RLC layer devices), and for AM DRB, it can perform a header decompression procedure on the stored data (received from the source base station) or all stored data based on the header compression context (ROHC or Ethernet header compression (EHC)) of the source base station.

[0278] - As another method, the receiving PDCP layer device can process or store data received due to the reconstruction procedure of lower layer devices (e.g., RLC layer devices), and for UM DRB or AM DRB, if no indicator (drb-Continue ROHC or drb-Continue Ethernet Header Compression (EHC)) is configured to continue using the header compression context, the header decompression procedure can be performed on the stored data (received from the source base station) or all stored data based on the header compression context (ROHC or Ethernet Header Compression (EHC)).

[0279] - After executing the above procedure, the sending PDCP layer device or the receiving PDCP layer device can discard or release the security key or header compression context of the source base station.

[0280] Refer to this disclosure Figure 6 When sending a handover command message (indicated by reference numerals 6-20) to the UE, the base station can define an indicator for embodiments of this disclosure in the handover command message (e.g., an RRCReconfiguration message) and can indicate to the UE whether to trigger a handover procedure corresponding to the embodiment. Furthermore, the UE can execute the handover procedure according to the handover method indicated by the handover command message, and for example, the UE can execute a second embodiment of the efficient handover method (DAPS handover method) of this disclosure to perform handover to the target base station while minimizing data interruption time. As another approach, for embodiments of this disclosure, the base station can define an indicator for each bearer through the handover command message and can more specifically indicate the bearer to which the embodiment is applied during handover. For example, the base station can indicate that the second embodiment of this disclosure is applied only to AM bearers operated by RLC layer devices operating in AM mode, or the indication can be extended to apply to UM bearers operated by RLC layer devices operating in UM mode. Furthermore, it is assumed that embodiments of this disclosure are applied to DRBs. However, if necessary (e.g., when the UE maintains the SRB for the source base station and fails to hand over to the target base station, thus allowing the SRB for the source base station to report or resume the handover failure message), the embodiments of this disclosure can be extended and applied to the SRB.

[0281] In embodiments of this disclosure, when the UE performs data transmission to or data reception from a source base station via a protocol layer device of the first bearer, and performs data transmission to or data reception from a target base station via a protocol layer device of the second bearer, the MAC layer devices of the first and second bearers operate separate discontinuous reception (DRX) cycles to reduce UE battery consumption. That is, even after receiving a handover command message, the UE can continuously apply the DRX cycle of the MAC layer device used when transmitting or receiving data via the protocol layer device of the first bearer, and can stop the DRX cycle according to the first or second condition of this disclosure. Furthermore, the UE can apply the DRX cycle separately to the MAC layer device of the second bearer according to the instructions of the target base station.

[0282] Furthermore, in this disclosure, the UE stops uplink transmission to the source base station through the protocol layer device of the first bearer and stops receiving downlink data from the source base station, and indicates that the UE rebuilds, initializes or releases the protocol layer device (PHY layer device, MAC layer device, RLC layer device or PDCP layer device) of the first bearer.

[0283] In the embodiments of this disclosure, for ease of description, it has been described that the UE is configured with a first bearer for a source base station or a second bearer for a target base station. This can be easily extended and similarly applied to cases where the UE is configured with multiple first bearers for a source base station or multiple second bearers for a target base station. As another method, it can be easily extended and similarly applied to cases where multiple bearers for multiple target base stations are configured. For example, the UE can perform a handover procedure to a first target base station and configure the second bearer. If the handover fails, the UE can perform a handover procedure to a second target base station and configure the second bearer, thereby allowing the UE to search and determine cells among multiple target base stations that meet predetermined conditions (e.g., have a value equal to or greater than a predetermined signal strength), and to determine a cell to perform the handover procedure.

[0284] Figure 9A and Figure 9B The structure of a high-efficiency PDCP layer device used in the DAPS handover method as a second embodiment of the high-efficiency handover method is shown, as well as methods of applying the structure according to various embodiments of the present disclosure.

[0285] refer to Figure 9A and Figure 9B They propose detailed structures and functions of efficient PDCP layer devices used in the DAPS handover method, which is a second embodiment of the efficient handover method of this disclosure, and allow different structures of the PDCP layer devices to be applied to each bearer at different times while executing the DAPS handover procedure.

[0286] For example, before receiving a handover command message from a base station, the UE can process data and send or receive the data (indicated by reference numeral 9-01) by using the structure and function of the first PDCP layer device of this disclosure for each bearer (indicated by reference numeral 9-11 or 9-12).

[0287] However, if the UE receives a handover command message from the base station that indicates the DAPS handover method of this disclosure, or indicates a DAPS handover method for a specific bearer, then for each bearer or for the bearer that indicates the DAPS handover method, the UE can process data and send or receive data (indicated by reference numeral 9-02) by applying the structure and function of the second PDCP layer device of this disclosure (indicated by reference numeral 9-20). That is, when a handover command message is received and the handover command message indicates the DAPS handover method of this disclosure, or indicates a DAPS handover method for a specific bearer, for each bearer or for the bear that indicates the DAPS handover method, the UE can switch from the structure or function of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) for each bearer to the structure or function of the second PDCP layer device of this disclosure (indicated by reference numeral 9-20). In another method, when the first condition described in this disclosure is met, for each bearer or bearer indicating a DAPS handover method, the UE can switch from the structure or function of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) for each bearer to the structure or function of the second PDCP layer device (indicated by reference numeral 9-02) of this disclosure (indicated by reference numeral 9-20). Furthermore, when the UE receives a handover command message indicating the DAPS handover method of this disclosure, or indicating the DAPS handover method for a specific bearer, or when a new PDCP reordering timer value is configured, the UE updates the reordering parameters to the expected PDCP sequence number or count value to be received next, and stops and restarts the reordering timer if the UE may switch from the structure or function of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) to the structure or function of the second PDCP layer device (indicated by reference numeral 9-20) of this disclosure for each bearer or bearer indicating a DAPS handover method.

[0288] Furthermore, when the UE executes the DAPS handover method of this disclosure, and if the second condition described in this disclosure is met, the UE can switch back to the structure or function of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) by releasing the first bearer for the source base station from the structure or function of the second PDCP layer device (indicated by reference numerals 9-20), which is applied for each bearer or bearer indicating the DAPS handover method. Additionally, when the second condition described in this disclosure is met, and for each bearer or bearer indicating the DAPS handover method, when the UE switches from the structure or function of the second PDCP layer device (indicated by reference numerals 9-20) to the structure or function of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) of this disclosure, the UE can initialize the variables used for reordering, interrupt and initialize the reordering timer. Furthermore, the UE can perform a decryption procedure or header (or data) decompression by applying the source base station's security key or header decompression context to a data segment (e.g., data received from the source base station) stored in a buffer for reordering, and then discard the source base station's security key or header decompression context. Additionally, the UE can send the processed data to a higher layer in ascending order. That is, in the above scenario, when the second condition is met, the UE can perform a decryption procedure or header (or data) decompression by applying the source base station's security key or header decompression context to a data segment (e.g., data received from the source base station) stored in a buffer for reordering, and then discard the source base station's security key or header decompression context.

[0289] As another method, the UE executes the DAPS handover method of this disclosure, and if the second condition described in this disclosure is met, the UE can switch again to the structure or function of the third PDCP layer device (indicated by reference numerals 9-30) by releasing the bearer for the source base station from the structure or function of the second PDCP layer device (indicated by reference numerals 9-20), which is applied for each bearer or bearer indicating the DAPS handover method. Furthermore, when the second condition described in this disclosure is met, and when the UE switches from the structure or function of the second PDCP layer device (indicated by reference numerals 9-20) to the structure or function of the third PDCP layer device (indicated by reference numerals 9-30) of this disclosure for each bearer or for a bearer indicating the DAPS handover method, the UE can use the variables and reordering timers for reordering as is without stopping or initializing them. However, the UE can perform a decryption procedure or header (or data) decompression by applying the source base station's security key or header decompression context to a data segment (e.g., data received from the source base station) stored in a buffer for reordering, and then discard the source base station's security key or header decompression context. Furthermore, the processed data can be sent to a higher layer in ascending order. That is, in the above scenario, when the second condition is met, the UE can perform a decryption procedure or header (or data) decompression by applying the source base station's security key or header decompression context to a data segment (e.g., data received from the source base station) stored in a buffer for reordering, and then discard the source base station's security key or header decompression context.

[0290] As disclosed above Figure 9A and Figure 9B As described above, when the UE performs handover by applying different structures and functions of a first PDCP layer device (indicated by reference numerals 9-11 or 9-12), a second PDCP layer device (indicated by reference numerals 9-20), or a third PDCP layer device (indicated by reference numerals 9-30) to each bearer at different times, there is no data loss and data interruption time can be minimized.

[0291] Figure 9A and Figure 9B The structure of the first PDCP layer device (indicated by reference numerals 9-11 or 9-12) may have a (1-1) PDCP layer device structure, a (1-2) PDCP layer device structure, a (1-3) PDCP layer device structure or a (1-4) PDCP layer device structure, which will be described later, and may have the following features.

[0292] -1>(In the case of the (1-1) PDCP layer device structure) For example, if the UE applies the structure and function of the first PDCP layer device (indicated by reference numerals 9-11) 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), then the structure may have the following characteristics.

[0293] *2> The receiving PDCP layer device can first detect data outside the window, or detect duplicate data received. (Due to retransmissions for RLC AM, and the possible differences between the LTE RLC SN and PDCP SN sizes, duplicate data or data outside the window may be received. The window indicates the range of PDCP sequence numbers or count values ​​within which valid data is received.)

[0294] **3> Before discarding out-of-window or duplicate data, the UE performs a decryption and header decompression procedure. (Because the data may contain useful information for the header decompression procedure (e.g., IR packets or header compression information), the UE can identify and then discard the data.)

[0295] *2> Received data that was not discarded was directly decrypted without data reordering, and a header decompression procedure was performed on it. This is because the E-UTRA AM RLC layer device arranges the data sequentially and sends it to the PDCP layer device.

[0296] *2> In addition, when sending data to higher layers, the UE sends the data in ascending order of the count value.

[0297] -1>(In the case of the structure of the (1-2) PDCP layer device) For example, if the UE applies the structure and function of the first PDCP layer device (indicated by reference numerals 9-11) 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), then the structure may have the following characteristics.

[0298] *2> The procedure for detecting data outside the window or detecting duplicate data is not executed. This is because the UM E-UTRA RLC layer device does not perform the retransmission procedure.

[0299] *2> In addition, the received data is directly decrypted, and its header is decompressed.

[0300] *2> Then, after the reordering process, the data can be sent directly to a higher level (e.g., in ascending order).

[0301] -1> (In the case of the structure of the (1-3) PDCP layer devices) For example, if the UE applies the structure and functions of the first PDCP layer device (indicated by reference numerals 9-11) to a PDCP layer device (e.g., an E-UTRA PDCP layer device or an LTE PDCP layer device) in which split bearers, packet duplicate bearers, or LTE radio LAN (WLAN) aggregation (LWA) bearers are configured, a reordering procedure and a reordering timer are always applied to it, and the structure may have the following characteristics:

[0302] *2> Its characteristic lies in that it can detect data outside the received data window or duplicate data. (This is because received data can be retransmitted from different RLC layer devices or RLC AM at different times, and because the sizes of the LTERLC SN and PDCP SN may be different, it is possible to receive data outside the window or duplicate data.)

[0303] **3> Execute the decryption procedure. However, do not execute the header compression / decompression procedure. (Because E-UTRAPDCP cannot configure header compression protocols for split bearers or LWQ bearers.**

[0304] **3> If an integrity protection or verification procedure has already been performed, the data that has undergone that procedure is discarded. If the integrity verification procedure fails, the data can be discarded and reported to a higher-level device.

[0305] **3> Discard data outside the window or duplicate data.

[0306] *2> If the data is not discarded, its characteristic is that the decryption process can be executed directly without reordering the received data. Furthermore, when integrity protection or verification procedures are configured, integrity verification is performed. If data integrity protection or verification procedures have already been performed, the data is discarded. If the integrity verification procedure fails, the data can be discarded and reported to higher-level devices.

[0307] *2> Furthermore, if the received data is arranged in order, and if the data is arranged in ascending order without gaps in the PDCP sequence number or count value, and a header compression procedure is performed (if a header compression or decompression procedure is configured), the characteristic is that the data can be sent to higher layers in ascending order.

[0308] *2> If the reordering timer is running

[0309] **3> If the data corresponding to the count value (which is the same as the value obtained by subtracting 1 from the value maintained by the variable used for reordering) is sent to a higher-level device, or if all data is sent to a higher level without gaps in the PDCP sequence number (or count value).

[0310] ***4> Interrupt and initialize the reordering timer.

[0311] *2> If the reordering timer is not running

[0312] **3> If there is data stored in the buffer but not sent to a higher-level device, or if there is a gap in the PDCP sequence number (or count value).

[0313] ***4>Start the reordering timer.

[0314] ***4> In addition, update the variables used for reordering with the expected PDCP sequence number or count value to be received next.

[0315] *2> If the reordering timer has expired

[0316] **3> If a header decompression procedure is configured for the stored data in ascending order of PDCP sequence number or count value for values ​​smaller than the reorder variable value, then the header decompression procedure is executed on the stored data, and the data is sent to a higher-level device.

[0317] **3> If a header decompression program is configured for the stored data in ascending order of PDCP sequence number or count value for values ​​equal to or greater than the reordered variable value, then the header decompression program is executed on the stored data, and the data is sent to a higher-level device.

[0318] **3> Then, use the PDCP sequence number or count value of the last data sent to the higher layer to update the variable value.

[0319] **3> If data is stored in the buffer but not sent to a higher-level device, or if there are gaps in the PDCP sequence number (or count value).

[0320] ***4>Start the reordering timer.

[0321] ***4> In addition, update the variables used for reordering with the expected PDCP sequence number or count value to be received next.

[0322] -1>(In the case of the structure of the (1-4) PDCP layer device) For example, if the UE applies the structure and function of the first PDCP layer device (indicated by reference numerals 9-12) to the NR PDCP layer device, a reordering procedure and a reordering timer are always applied, and the following characteristics may be present.

[0323] *2> Its characteristic is that it first performs a decryption procedure on the received data.

[0324] *2> When an integrity protection or verification procedure is configured, the received data is subjected to the integrity protection or verification procedure, and if the integrity verification procedure fails, the data can be discarded and reported to a higher-level device.

[0325] *2> It can detect data outside the data window or duplicate data relative to the received data. (Its characteristic is that out-of-window data or redundancy detection is performed after the above decryption procedure is executed. According to another method, out-of-window data detection or duplicate data detection is performed only after the decryption procedure is executed if integrity protection or verification procedures are configured. Furthermore, if integrity protection or verification procedures are not configured, the decryption procedure is only performed on data that has not been discarded after performing out-of-window data detection or duplicate data detection.)

[0326] **3> Discard data outside the window or duplicate data.

[0327] *2> If the above data is not discarded, the received data is sorted, and if the data is arranged in ascending order without gaps in the PDCP sequence number or count value, the header compression procedure is performed (if a header compression or decompression procedure is configured), and the data can be sent to higher layers in ascending order.

[0328] *2> In addition, when sending data to higher layers, the count values ​​are sent in ascending order.

[0329] *2> If the reordering timer is running

[0330] **3> If data corresponding to a count value (which is the same as the value obtained by subtracting 1 from the value maintained by the variable used for reordering) is sent to a higher-level device, if all data is sent to the higher level without gaps in the PDCP sequence number (or count value), or if the value of the variable storing the PDCP sequence number or count value of the data to be sent to the higher level is greater than or equal to the value of the variable used for reordering.

[0331] ***4> Interrupt and initialize the reordering timer.

[0332] *2> If the reordering timer is not running

[0333] **3> If data is stored in the buffer but not sent to a higher-level device, if there is a gap in the PDCP sequence number (or count value), or if the value of the variable storing the count value of the first data not sent to a higher level is less than the value of the variable used for reordering.

[0334] ***4> In addition, update the variables used for reordering with the expected PDCP sequence number or count value to be received next.

[0335] ***4>Start the reordering timer.

[0336] *2> If the reordering timer has expired

[0337] **3> If a header compression / decompression procedure is configured for the stored data in ascending order of PDCP sequence number or count value for values ​​smaller than the reorder variable value, then the header decompression procedure is executed on the stored data, and the data is sent to a higher-level device.

[0338] **3> If the header decompression program is configured sequentially for stored data in ascending order of PDCP sequence number or count value for values ​​equal to or greater than the reordered variable value, then the header decompression program is executed on the stored data, and the data is sent to a higher-level device.

[0339] **3> Then, update the variable value of the first data that was not sent to the higher layer with the PDCP sequence number or count value of the first data that was not sent to the higher layer.

[0340] **3> If data is stored in the buffer but not sent to a higher-level device, if there is a gap in the PDCP sequence number (or count value), or if the value of the variable storing the count value of the first data that has not yet been sent to a higher level is less than the value of the variable used for reordering.

[0341] ***4> In addition, update the variables used for reordering with the expected PDCP sequence number or count value to be received next.

[0342] ***4>Start the reordering timer.

[0343] Figure 9A and Figure 9B The structure of the second PDCP layer device (indicated by reference numerals 9-20) may have the following (2-1) PDCP layer device structure or (2-2) PDCP layer device structure 2-2 of this disclosure, and may have the following characteristics.

[0344] This disclosure presents an efficient second PDCP layer device structure for handover, as shown by reference numerals 9-20 in the figures. The structure of the second PDCP layer device can be applied to a second embodiment of the efficient handover method of this disclosure for minimizing data interruption time.

[0345] In the second PDCP layer device structure, the UE can send data to or receive data from the source base station 9-21 through the protocol layer device of the first bearer (e.g., SDAP layer device, PDCP layer device, RLC layer device, or MAC layer device), and can send data to or receive data from the target base station 9-22 through the protocol layer device of the second bearer (e.g., SDAP layer device, PDCP layer device, RLC layer device, or MAC layer device).

[0346] The PDCP layer devices of the first and second bearers can be configured separately in the UE, but logically operate as a single PDCP layer device (such as reference numerals 9-20). Specifically, a PDCP layer device can be implemented using functions, where the functions of the PDCP layer device are divided into functions of higher PDCP layer devices (e.g., sequence number assignment, reordering, sequence delivery, or duplicate detection) and functions of two lower PDCP layer devices for the source and target base stations (e.g., decryption or encryption, header (or data) compression or decompression, integrity protection or verification, or duplicate detection). Furthermore, in the DAPS handover method described above, when the first condition is met, the UE sends uplink data transmission to the source base station and switches to the target base station, and can subsequently receive downlink data from both the source and target base stations. Therefore, regarding the header (or data) compression protocol context, only one context is maintained and applied to the source or target base station for the uplink, and two contexts are maintained and applied to the source or target base station for the downlink.

[0347] Based on the second PDCP layer structure as described above, the (2-1) PDCP layer structure of this disclosure (e.g., an E-UTRA PDCP layer device for the DAPS handover method) may have the following characteristics.

[0348] The higher-level PDCP layer devices can perform the task of assigning PDCP sequence numbers to data received from higher-level devices. Furthermore, based on the functions of the lower-level PDCP layer devices 9-21 and 9-22 for each source base station and each target base station, header (or data) compression procedures are applied using separate security keys configured for each source base station and each target base station. This allows the header (or data) compression context or security key configured for the source base station to be applied to the data to be sent to the source base station, and the header (or data) compression context or security key configured for the target base station to be applied to the data to be sent to the target base station. Additionally, if integrity protection is configured, the two lower-level PDCP layer devices 9-21 and 9-22 can apply integrity protection procedures to the PDCP header and data (PDCP SDU) and apply encryption to them. They can also perform transmissions such that data to be sent to the source base station is sent to the first bearer's RLC layer device, and data to be sent to the target base station is sent to the second bearer's RLC layer device. The two lower-level transmit PDCP layer devices 9-21 and 9-22 can perform parallel data processing, handling header compression, integrity protection, or encryption procedures in parallel to accelerate data processing. Different security keys are used to perform integrity protection or encryption procedures within the two lower-level transmit PDCP layer devices. Furthermore, their feature allows for the logical execution of compression, integrity protection, or encryption procedures on different data segments by applying different compression contexts, security keys, or security algorithms within a single transmit PDCP layer device.

[0349] The functions of the PDCP receiving layer devices can be executed for data segments received from corresponding lower-layer devices, specifically for data received from the two RLC layer devices used for each source base station and each target base station. That is, the functions of the lower-layer receiving PDCP devices 9-21 and 9-22 used for the source and target base stations can independently execute procedures for detecting out-of-window data or duplicate data based on the PDCP sequence number or count value for data received from the corresponding RLC layer devices. Alternatively, for ease of implementation, the lower-layer receiving PDCP devices 9-21 and 9-22 can execute procedures for detecting out-of-window data or duplicate data based on the PDCP sequence number or count value of all received data, without distinguishing between the corresponding RLC layer devices. As another method, for more accurate duplicate detection, the lower-layer receiving PDCP devices 9-21 and 9-22 can execute procedures for detecting out-of-window data based on the PDCP sequence number or count value of all received data, without distinguishing between the corresponding RLC layer devices, and can independently execute duplicate data detection procedures for data segments received from the corresponding RLC layer devices. As another method, when data received from different base stations overlap, in order to prevent data loss in the header compression protocol, the lower receiving PDCP layer devices 9-21 and 9-22 can perform a procedure to detect out-of-window data based on the PDCP sequence number or count value of all received data, without distinguishing the corresponding RLC layer device. Furthermore, the duplicate data detection procedure can be performed on all data segments after performing a decryption procedure, a duplicate data detection procedure, an integrity protection procedure, or a header (or data) decompression procedure on the corresponding data segments received from the RLC layer device.

[0350] By using separate header (or data) compression contexts or security keys configured for each source base station and each target base station, lower-receiver PDCP layer devices can apply decryption procedures directly to received data segments, and when integrity protection is configured, these functions can apply integrity verification procedures to PDCP headers and data (PDCP SDUs).

[0351] In the (2-1) PDCP layer device structure, the header (or data) decompression procedure is executed directly without sorting the data segments received from the RLC layer device for the first bearer of each source base station, and the header (or data) decompression procedure is also executed directly without sorting the data received from the RLC layer device for the second bearer of each target base station. Furthermore, to distinguish between data received from the RLC layer device for the first bearer of each source base station and data received from the RLC layer device for the second bearer of each target base station, the (2-1) PDCP layer device structure can define an indicator for each data segment to distinguish whether the data was received from the source base station or the target base station. Alternatively, the (2-1) PDCP layer device structure can define a 1-bit indicator in the PDCP header, SDAP header, or RLC header to distinguish whether the data was received from the source base station or the target base station. Furthermore, in the (2-1) PDCP layer device structure, for all data segments received from the RLC layer device of the first bearer for the source base station and the data segments received from the RLC layer device of the second bearer for the target base station (where the header (or data) compression procedure of the data segments has been completed), a duplicate detection procedure based on the PDCP sequence number or count value is performed (in this procedure, only one data segment (including previously received data or data sent to higher layers) is retained for each PDCP sequence number or count value, and the remaining data segments are discarded). Additionally, in the (2-1) PDCP layer device structure, a reordering procedure is performed on all data segments received from the RLC layer device of the first bearer for the source base station and the data segments received from the RLC layer device of the second bearer for the target base station in ascending order based on the PDCP sequence number or count value, and the data can be sent to higher layer devices in sequence. As described above, since a PDCP layer device can receive data from different base stations, i.e., receive data from the first bearer or the second bearer in any order, its characteristic is that a reordering procedure must always be performed.

[0352] Each function of the two lower-receiver PDCP layer devices can perform parallel data processing, i.e., execute header compression, integrity protection, or encryption procedures in parallel to accelerate data processing based on PDCP sequence numbers or count values. Integrity protection, encryption, or decompression procedures can be performed using different header (or data) compression contexts or security keys. Furthermore, these functions can perform integrity protection, encryption, or decompression procedures on different data segments by logically applying different header (or data) compression contexts, security keys, or security algorithms within a single transmitting PDCP layer device. Additionally, depending on the function of the lower-receiver PDCP layer device, out-of-order decryption or integrity verification procedures can be performed on each received data segment, regardless of the order of the PDCP sequence numbers or count values.

[0353] When distinguishing between a first bearer layer device and a second bearer layer device, a PDCP layer device can differentiate itself by considering that the first bearer layer device and the second bearer layer device are connected to different MAC layer devices or have different logical channel identifiers, or that the first bearer layer device and the second bearer layer device are different RLC layer devices connected to different MAC layer devices or using different encryption keys. Furthermore, PDCP layer devices perform encryption or decryption procedures for uplink and downlink data using different security keys and perform compression or decompression using different compression protocol contexts.

[0354] Based on the second PDCP layer structure as described above, the (2-2) PDCP layer structure of this disclosure (e.g., an NR PDCP layer device for the DAPS handover method) may have the following characteristics.

[0355] The higher-level PDCP layer devices can perform the task of assigning PDCP sequence numbers to data segments received from higher-level devices. Furthermore, according to the functions of the two lower-level PDCP layer devices 9-21 and 9-22 for the source and target base stations, header (or data) compression procedures are applied using separate security keys configured for each source and target base station. This allows the header (or data) compression context or security key configured for the source base station to be applied to the data to be sent to the source base station, and the header (or data) compression context or security key configured for the target base station to be applied to the data to be sent to the target base station. Additionally, according to the functions of the two lower-level PDCP layer devices 9-21 and 9-22, if integrity protection is configured, an integrity protection procedure is applied to the PDCP header and data (PDCP SDU), an encryption procedure is applied to them, and transmission can be performed such that data to be sent to the source base station is sent to the first bearer's transmitting RLC layer device, and data to be sent to the target base station is sent to the second bearer's transmitting RLC layer device. The two lower-level transmit PDCP layer devices 9-21 and 9-22 can perform parallel data processing, handling header compression, integrity protection, or encryption procedures in parallel to accelerate data processing. Different security keys are used to perform integrity protection or encryption procedures within the two lower-level transmit PDCP layer devices. Furthermore, their feature allows for the logical execution of compression, integrity protection, or encryption procedures on different data segments by applying different compression contexts, security keys, or security algorithms within a single transmit PDCP layer device.

[0356] The functions of the receiving PDCP layer devices (specifically, the functions of the lower receiving PDCP layer devices 9-21 and 9-22 for the source and target base stations) can independently perform procedures for detecting out-of-window data or duplicate data for each lower layer device (specifically, for data received from the two RLC layer devices for each source and target base station) based on PDCP sequence numbers or count values. Alternatively, for ease of implementation, the lower receiving PDCP layer devices 9-21 and 9-22 can perform procedures for detecting out-of-window data or duplicate data based on the PDCP sequence numbers or count values ​​of all received data, without distinguishing between the corresponding RLC layer devices. Furthermore, for more accurate duplicate detection, the lower receiving PDCP layer devices 9-21 and 9-22 can perform procedures for detecting out-of-window data based on the PDCP sequence numbers or count values ​​of all received data, without distinguishing between the corresponding RLC layer devices, and can independently perform duplicate data detection procedures for data segments received from the corresponding RLC layer devices. As another method, when data received from different base stations overlap, the lower receiving PDCP layer devices 9-21 and 9-22 can perform a procedure to detect out-of-window data based on the PDCP sequence number or count value of all received data, without distinguishing the corresponding RLC layer device, in order to prevent data loss of the header compression protocol. After performing a decryption procedure, integrity protection procedure, or header (or data) decompression procedure on the data segments received from the RLC layer device, a duplicate data detection procedure can be performed on all data segments.

[0357] By using separate header (or data) compression contexts or security keys configured for each source base station and each target base station, lower-receiver PDCP layer devices can apply decryption procedures directly to received data segments, and when integrity protection is configured, these functions can apply integrity verification procedures to PDCP headers and data (PDCP SDUs).

[0358] In the (2-2) PDCP layer device architecture, data segments received from the RLC layer device for the first bearer of each source base station and data segments received from the RLC layer device for the second bearer of each target base station are reordered. Then, a header (or data) decompression procedure is performed by applying the header (or data) compression context of each base station (source or target base station) to each data received from each base station (source or target base station) in ascending order of PDCP sequence number or count value. Furthermore, to distinguish between data received from the RLC layer device for the first bearer of each source base station and data received from the RLC layer device for the second bearer of each target base station, the (2-2) PDCP layer device architecture can define an indicator for each data segment to distinguish whether the data was received from the source base station or the target base station. Alternatively, the (2-2) PDCP layer device architecture can define a 1-bit indicator in the PDCP header, SDAP header, or RLC header to distinguish whether the data was received from the source base station or the target base station. Furthermore, the (2-2) PDCP layer device structure can perform a duplicate detection procedure based on PDCP sequence numbers or count values ​​for all data segments received from the RLC layer device of the first bearer for the source base station and data segments received from the RLC layer device of the second bearer for the target base station (where header (or data) compression of the data segments has been completed). In this procedure, only one data segment (including previously received data or data sent to higher layers) is retained for each PDCP sequence number or count value, and the remaining data segments are discarded. Additionally, all data segments received from the RLC layer device of the first bearer for the source base station and data segments received from the RLC layer device of the second bearer for the target base station are sent to higher-layer devices in ascending order based on PDCP sequence numbers or count values. As mentioned above, since a PDCP layer device can receive data from different base stations, i.e., from the first bearer or the second bearer regardless of the order, a reordering procedure is always required.

[0359] Each function of the two lower-receiver PDCP layer devices can perform parallel data processing based on PDCP sequence numbers or count values, i.e., performing header compression, integrity protection, or encryption procedures in parallel to accelerate data processing. Integrity protection, encryption, or decompression procedures can be performed using different header (or data) compression contexts or security keys. Furthermore, the functions of the two lower-receiver PDCP layer devices can logically apply different header (or data) compression contexts, security keys, or security algorithms within a single transmitting PDCP layer device to perform integrity protection, encryption, or decompression procedures on different data segments. Additionally, depending on the functions of the lower-receiver PDCP layer devices, out-of-order decryption or integrity verification procedures can be performed on each received data segment, regardless of the order of the PDCP sequence numbers or count values.

[0360] When distinguishing between a first bearer layer device and a second bearer layer device, a PDCP layer device can differentiate between the first bearer layer device (or the first RLC layer device) and the second bearer layer device (or the second RLC layer device) by considering that the first bearer layer device and the second bearer layer device are connected to different MAC layer devices or have different logical channel identifiers, or that the first bearer layer device and the second bearer layer device are different RLC layer devices connected to different MAC layer devices or using different encryption keys. Furthermore, a PDCP layer device can perform encryption or decryption procedures for uplink and downlink data using different security keys and compression or decompression of that data using different compression protocol contexts.

[0361] This disclosure proposes an efficient third PDCP layer device structure for handover, as shown by reference numerals 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 disclosed herein. Furthermore, in the structure of the third PDCP layer device of this disclosure, the function of the PDCP layer device can be the same as that of the structure of the second PDCP layer device of this disclosure. However, the third PDCP layer device structure can be obtained by releasing the first bearer for the source base station from the structure of the second PDCP layer device. Specifically, the structure of the third PDCP layer device of this disclosure has the same function as the structure of the second PDCP layer device as described above, but has a structure obtained by releasing the first bearer for the source base station (e.g., SDAP layer device, PDCP layer device, RLC layer device, or MAC layer device). Accordingly, the structure of the third PDCP layer device is obtained by releasing the QoS mapping information of the SDAP layer device of the source base station, the security key information of the source base station of the PDCP layer device, or the header (or data) compression context information of the source base station, the RLC layer device of the source base station, or the MAC layer device.

[0362] In the following text, Figure 6 In this disclosure, a feature is proposed that when a UE receives a handover command message and applies the bearer configuration information included in the handover command message, it applies the bearer configuration information based on different methods according to the handover type indicated by the handover command message.

[0363] - When the UE receives a handover command message, if the first handover method (e.g., the first embodiment of this disclosure or a general handover method) is indicated by the handover command message, ReconfigWithSync information, or MobilityControlInfo information, if the second embodiment of this disclosure (DAPS handover method) is not configured above, or if the second embodiment of this disclosure (DAPS handover method) is not configured for any bearer by bearer configuration information,

[0364] When a default bearer is configured in the SDAP layer device configuration information configured via the handover command message, the default bearer for the source base station can be configured as the default bearer for the target base station indicated in the configuration information.

[0365] When the second QoS flow and bearer mapping information is configured in the SDAP layer device configuration information configured via the handover command message, the first QoS flow and bearer mapping information applied to the source base station is released, and the second QoS flow and bearer mapping information can be applied. Alternatively, the first QoS flow and bearer mapping information applied to the source base station can be replaced with the second QoS flow and bearer mapping information.

[0366] When a data discard timer value is configured in the PDCP layer device configuration information configured via the handover command message, the discard timer value can be directly applied to the PDCP layer device corresponding to the bearer identifier in the configuration information.

[0367] If the drb-ContinueROHC indicator is configured to false in the PDCP layer device configuration information configured via the handover command message, the PDCP layer device corresponding to the bearer identifier in the configuration information can initialize the context of the header compression or decompression protocol. If the drb-ContinueROHC indicator is configured to true, the PDCP layer device corresponding to the bearer identifier in the configuration information does not initialize the context of the header compression or decompression protocol.

[0368] When a reordering timer value is configured in the PDCP layer device configuration information configured via the handover command message, the reordering timer value can be directly applied to the PDCP layer device corresponding to the bearer identifier of the configuration information.

[0369] *Upon receiving the handover command message, the PDCP layer device can be rebuilt. For example, for SRB, window state variables can be initialized, and stored data (PDCP SDUs or PDCP PDUs) can be discarded; and for UM DRB, window state variables can be initialized, and data that has not yet been sent to lower-layer devices or data whose PDCP cancellation timer has not expired can be compressed or encrypted based on the target base station's header (or data) compression context or security key, in ascending order of count values, or the data can undergo integrity protection for transmission or retransmission. Furthermore, if a reordering timer is running, it can be interrupted and initialized, and received data (PDCP SDUs or PDCP PDUs) can be processed in order and sent to higher-layer devices. For AM DRB, window state variables are not initialized, and compression or encryption is performed based on the target base station's header (or data) compression context or security key, in ascending order of PDCP sequence number or count value, starting with the first data (PDCP SDU or PDCP PDU) that was not successfully sent from a lower-layer device, or its integrity protection is performed for transmission or retransmission.

[0370] When security key-related configuration information or security algorithms are configured in the security configuration information configured through the handover command message, the configuration information is used to derive new security keys or security configuration information, release existing security keys or security configuration information, or replace existing security keys or security configuration information with new security keys or security configuration information.

[0371] When a new logical channel identifier is configured in the RLC layer device configuration information configured via the handover command message, the new logical channel identifier is released from the existing logical channel identifier corresponding to the bearer identifier indicated in the RLC layer device configuration information, or the existing logical channel identifier can be replaced and configured as the new logical channel identifier.

[0372] When an RLC reconstruction procedure is configured in the RLC layer device configuration information configured via the handover command message, the RLC reconstruction procedure can be executed for the RLC layer device corresponding to the bearer identifier indicated in the RLC layer device configuration information. Specifically, the transmitting RLC layer device can execute the RLC reconstruction procedure to discard all stored data. Furthermore, when the reordering timer is running, the receiving RLC layer device can interrupt and initialize the reordering timer, process all stored data, and send the processed data to higher-layer devices. Additionally, the receiving RLC layer device can start the MAC layer device. Furthermore, the receiving RLC layer device can start the MAC layer device of the source base station and use the MAC layer device of the target base station.

[0373] * The MAC layer device can initialize and stop data transmission or reception for the source base station and each bearer. Furthermore, the MAC layer device can stop monitoring the PDCCH for the first UE identifier (C-RNTI) assigned from the source base station. Additionally, the MAC layer device can stop the procedure for requesting scheduling from the source base station, or release transmission resources used for scheduling. Furthermore, the PHY or MAC layer device can execute a random access procedure to the target base station. If the handover procedure to the target base station is successfully completed, the PHY or MAC layer device can resume data transmission to or from the target base station, and can begin monitoring the PDCCH for the second UE identifier (C-RNTI) assigned from the target base station. Furthermore, the PHY or MAC layer device can receive system frame numbers from the target base station and perform synchronization. Additionally, the PHY or MAC layer device can initiate or execute a procedure for requesting scheduling from the target base station.

[0374] The PHY layer device can perform channel measurements on the source base station, generate channel measurement reports, or stop sending HARQ ACK or NACK messages. Then, the PHY layer device can perform downlink synchronization procedures on the target base station. Furthermore, the PHY layer device can configure configuration information for the target base station (or Spcell or PCell) received via handover command messages in lower-layer devices or within the PHY layer device itself. If the handover procedure to the target base station is successfully completed, the PHY layer device can begin sending HARQ ACK or NACK messages to the target base station, or it can send HARQ ACK or NACK messages to the target base station. Additionally, the PHY or MAC layer device can receive system frame numbers from the target base station and perform synchronization. Furthermore, the PHY or MAC layer device can initiate or execute procedures requesting scheduling from the target base station.

[0375] *When the RLC layer device configuration information configured via the handover command message is newly configured, an RLC reconstruction procedure can be performed on the RLC layer device corresponding to the bearer identifier indicated in the RLC layer device configuration information.

[0376] When a second priority for a logical channel is newly configured in the MAC layer device configuration information configured via a handover command message, the first priority corresponding to the logical channel identifier indicated in the configuration information is released, or the first priority corresponding to the logical channel identifier can be replaced and configured as the newly configured second priority.

[0377] When a new prioritized bit rate (PBR) for a logical channel is configured in the MAC layer device configuration information configured via a handover command message, the first PBR corresponding to the logical channel identifier indicated in the configuration information can be released, or the first PBR corresponding to the logical channel identifier can be replaced and configured as the newly configured second PBR. The priority bit rate is a value that increases for each logical channel over a predetermined time period (e.g., each TTI). When uplink transmission resources are received, a logical channel prioritization (LCP) procedure is performed. Data for logical channels can be transmitted by considering priority and priority bit rate; the higher the priority or the larger the priority bit rate, the more data can be transmitted.

[0378] When a second bucket size (bucketSizeDuration) for a logical channel is newly configured in the MAC layer device configuration information configured via a handover command message, the first bucket size (bucketSizeDuration) corresponding to the logical channel identifier indicated in the configuration information can be released, or the first bucket size (bucketSizeDuration) corresponding to the logical channel identifier can be replaced and configured as the newly configured second bucket size (bucketSizeDuration). The bucket size indicates the maximum value that the priority bit rate can have when the priority bit rate is accumulated.

[0379] If the MAC layer device configuration information configured via the handover command message includes the second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information, then the previously configured first allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information can be released. Alternatively, the previously configured first allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information can be replaced and configured as the newly configured second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information.

[0380] When the UE receives a handover command message, if the handover command message or the ReconfigWithSync information and MobilityControlInfo information indicate or configure a second handover method (e.g., the second embodiment of this disclosure or the DAPS handover method), if the DAPS handover method is indicated or configured for each bearer identifier, or if the second embodiment of this disclosure (DAPS handover method) is configured for a predetermined bearer through bearer configuration information, or if the second embodiment of this disclosure (DAPS handover method) is configured for at least one bearer through bearer configuration information,

[0381] When a default bearer is configured in the SDAP layer device configuration information configured via the handover command message, the DAPS handover method of this disclosure is executed. By applying a second SDAP layer device structure, the default bearer used for the existing source base station can be maintained, and the default bearer information indicated in the configuration information can be configured as the default bearer for the target base station. Alternatively, when the first condition described in this disclosure is met, the default bearer used for the existing source base station can be switched to the default bearer for the target base station indicated in the configuration information.

[0382] When the second QoS flow and bearer mapping information are configured in the SDAP layer device configuration information configured via the handover command message, the DAPS handover method of this disclosure is executed. By applying the second SDAP layer device structure, the first QoS flow and bearer mapping information already applied to the source base station can be maintained, and the second QoS flow and bearer mapping information can be applied to the data of the target base station. Alternatively, when the first condition described in this disclosure is met, the second QoS flow and bearer mapping information of the target base station can be applied.

[0383] When a data discard timer value is configured in the PDCP layer device configuration information configured via the handover command message, the DAPS handover method described above in this disclosure is executed, and by applying the second PDCP layer device structure, the discard timer value can be directly applied to the PDCP layer device corresponding to the bearer identifier of the configuration information.

[0384] * It is not necessary to rebuild the PDCP layer device that has been instructed or configured with the DAPS handover method via the handover command message, and the following procedures can be performed. For example, for SRB, window state variables can be initialized (the initialization of variables can be omitted so that a rollback can be performed in case of DAPS handover failure). Specifically, when initializing the counter value or window state variable value, in order to address the security problem caused by reusing the same counter value from the beginning when performing the DAPS handover rollback procedure (the risk of exposing the security key due to sending different data fragments using the same security key and the same counter value when performing the DAPS handover rollback procedure), the counter value can be left uninitialized, the window state variable (TX_NEXT) can be sent, or the window state variable (RX_NEXT and RX_DELIV) can be received, and the existing variable values ​​can be continuously used or maintained; or the stored data (PDCP SDU) can be discarded. In addition, for UM DRB, the window state variable can be left uninitialized, and data that has not yet been sent to lower-layer devices or data whose PDCP discard timer has not expired can continue to be sent to or received from the source base station. Alternatively, for AM DRBs, window state variables may not be initialized, and data can continue to be sent to or received from the source base station. Furthermore, regarding procedures for SRBs, in the case of suspending the SRB for the source base station, or in the case of the SRB for the target base station, to address security issues arising from the reuse of the same count value from the beginning when performing DAPS handover rollback procedures, the existing count value or the value of the transmit window variable or receive window variable for the source base station's SRB is applied to the SRB established for the target base station and maintained (or by configuring the count value or the value of the transmit window variable or receive window variable of the source base station's SRB to the count value or the value of the transmit window variable or receive window variable of the target base station's SRB). Additionally, for the SRB for the target base station, the target base station's security key can be derived or applied, and the SRB's PDCP layer device can perform encryption, decryption, integrity protection, or authentication procedures by applying the target base station's security key. Furthermore, for the SRB for the source base station, old data (e.g., RRC messages from the source base station) can be discarded. The procedure for the SRB of the target base station can be defined as a new procedure (e.g., DAPSRB establishment or PDCP layer device reconstruction procedure) and instructed, triggered or executed, and the procedure for the SRB can be extended and applied to UM DRB or AM DRB that is not configured with the DAPS handover method.

[0385] *PDCP layer devices that have not been instructed or configured with a DAPS handover method via a handover command message can be rebuilt. For example, for SRBs, window state variables can be initialized, and stored data (PDCP SDUs or PDCP PDUs) can be discarded. For UM DRBs, window state variables are initialized, and data that has not yet been sent to lower-layer devices or whose PDCP cancellation timers have not expired can be compressed or encrypted in ascending order of count values ​​based on the target base station's header (or data) compression context or security key, or integrity protection can be performed for transmission or retransmission. Furthermore, if a reordering timer is running, it is stopped and initialized, and received data (PDCP SDUs or PDCP PDUs) can be processed sequentially and sent to higher-layer devices. For AM DRBs, window state variables are not initialized, and compression or encryption is performed based on the target base station's header (or data) compression context or security key, starting with the first data (PDCP SDU or PDCP PDU) that has not been successfully sent from a lower-layer device in ascending order of PDCP sequence number or count value, or integrity protection can be performed for transmission or retransmission. Additionally, RLC layer devices can execute a reconstruction procedure.

[0386] *The DAPS handover method of this disclosure is executed when the drb-ContinueROHC indicator in the configuration information of a PDCP layer device that has been instructed or configured with the DAPS handover method via a handover command message is configured to "false". Here, by applying the second PDCP layer device structure, the PDCP layer device corresponding to the bearer identifier in the configuration information can use the header compression or decompression protocol context of the source base station as is, and can initialize the header compression or decompression protocol context of the target base station, starting from an initial state (e.g., IR state). If the drb-ContinueROHC indicator is configured to "true", the DAPS handover method of this disclosure is executed, and by applying the second PDCP layer device structure, the PDCP layer device corresponding to the bearer identifier in the configuration information can use the header compression or decompression protocol context of the source base station as is, and can apply the header compression or decompression protocol context of the target base station in the same form as the header compression or decompression protocol context of the source base station. For example, the header compression or decompression protocol context of the source base station can be copied to the header compression or decompression protocol context of the target base station and applied as is. Alternatively, the same header compression or decompression protocol context can be applied to either the source or target base station.

[0387] When a reordering timer value is configured via configuration information of a PDCP layer device that is configured with a DAPS handover method indicated by a handover command message, the DAPS handover method of this disclosure is executed, and by applying a second PDCP layer device structure, the reordering timer value can be directly applied to the PDCP layer device corresponding to the bearer identifier of the configuration information.

[0388] *When security key-related configuration information or security algorithms are configured through security configuration information indicating or configuring the DAPS handover method via a handover command message, or when an indicator indicating a new procedure exists in the PDCP layer device configuration information, the configuration information is used to derive a new security key or security configuration information, and the DAPS handover method described above in this disclosure can be executed. Furthermore, by applying a second PDCP layer device structure, the existing security key or security configuration information of the source base station is maintained, and the security key or security configuration information of the target base station can be configured as a new security key or security configuration information.

[0389] *The DAPS handover method of this disclosure can be executed when a new logical channel identifier is configured in the RLC layer device configuration information of a bearer that has been indicated or configured with the DAPS handover method via a handover command message. Furthermore, by applying a second PDCP layer device structure, existing logical channel identifiers are maintained for the RLC layer device or MAC layer device for the first bearer of the source base station that corresponds to the bearer identifier indicated in the RLC layer device configuration information, and the RLC layer device or MAC layer device for the second bearer of the target base station can be configured with the new logical channel identifier indicated in the configuration information.

[0390] *For the RLC layer device of the source base station in the RLC layer device configuration information of the bearer that is indicated by the handover command message or configured with the DAPS handover method, the RLC reconstruction procedure may not be executed. Specifically, without executing the RLC reconstruction procedure, the sending RLC layer device continues to transmit the stored data, and the receiving RLC layer device continues to process the stored data and the received data to avoid data interruption. However, when the first condition described in this disclosure is met, according to the method of this disclosure, regarding the AM or UM bearer, the PDCP layer device configured with the DAPS handover method can send a data discard indicator for PDCP user data (PDCP data PDU) to indicate the discarding of data (PDCP data PDU) to the RLC layer device of the source base station. Accordingly, the RLC layer device of the source base station discards the PDCP data PDU, but can perform transmission without discarding the PDCP control PDU.

[0391] *For RLC layer devices that do not have an instruction or the DAPS handover method configured in the handover command message, an RLC reconstruction procedure can be executed, or if an RLC reconstruction procedure is configured, it can be executed. Specifically, by executing the RLC reconstruction procedure, the sending RLC layer device can perform a procedure to discard all stored data (PDCP data PDUs or PDCP control PDUs). Furthermore, when the reordering timer is running, the receiving RLC layer device can interrupt and initialize the reordering timer, process all stored data, and send the processed data to higher-level devices.

[0392] When a new RLC layer device configuration information is configured for a bearer indicated by a handover command message or configured with the DAPS handover method, the DAPS handover method of this disclosure is executed. By applying the second PDCP layer device structure, the existing RLC configuration information is maintained for the RLC layer device for the first bearer of the source base station corresponding to the bearer identifier indicated in the RLC layer device configuration information. The RLC layer device for the second bearer of the target base station can be configured via the new RLC layer device configuration information indicated in the configuration information.

[0393] * Can be executed for configuring this disclosure. Figure 10 Method 1, Method 2, or Method 3 for MAC layer devices.

[0394] * Can be executed for configuration Figure 10 Method 1, Method 2, or Method 3 of the MAC layer device, and can continuously perform data transmission or reception for bearers for which the DAPS handover method is indicated or configured in the MAC layer device, without initializing the MAC layer device, and can stop data transmission or reception for bearers for which the DAPS handover method is not indicated or configured.

[0395] *In the above, the MAC layer device of the source base station is not initialized, and it can continuously monitor the PDCCH for the first UE identifier (C-RNTI) assigned from the source base station. Furthermore, the MAC layer device of the source base station can continue to execute the procedure for requesting scheduling from the source base station. Additionally, the PHY or MAC layer device of the target base station can apply configuration using the configuration information received via the handover command message and execute the random access procedure to the target base station. If the handover procedure to the target base station is successfully completed, the PHY or MAC layer device of the target base station can begin sending data to or receiving data from the target base station, and can begin monitoring the PDCCH for the second UE identifier (C-RNTI) assigned from the target base station. Furthermore, the PHY or MAC layer device can receive the system frame number from the target base station and perform synchronization. Additionally, the PHY or MAC layer device of the target base station can initiate or execute the procedure for requesting scheduling from the target base station. The UE can monitor the PDCCH in the PHY or MAC layer device of the source base station for a first UE identifier assigned from the source base station until the connection with the source base station is released or the second condition described in this disclosure is met. The UE can also monitor the PDCCH in the PHY or MAC layer device of the target base station for a second UE identifier assigned from the target base station. As described above, if the connection with the source base station is released or the second condition described in this disclosure is met, the UE can stop monitoring the PDCCH in the PHY or MAC layer device of the source base station for the first UE identifier assigned from the source base station, or can release the transmission resources used for the scheduling request.

[0396] The source base station's PHY layer equipment can maintain configuration information, perform channel measurements for the source base station, execute channel measurement reports, or continue executing procedures for sending HARQ ACK or NACK. Furthermore, the target base station's PHY or MAC layer equipment executes the target base station's downlink synchronization procedures. Additionally, the configuration information for the target base station (or Spcell or PCell) received via handover command messages can be configured in lower-layer equipment or the PHY layer equipment of the target base station. If the handover procedure to the target base station is successfully completed, the target base station's PHY or MAC layer equipment can begin sending HARQ ACK or NACK information to the target base station, or can execute its transmission. Furthermore, the target base station's PHY or MAC layer equipment can receive system frame numbers from the target base station and perform synchronization. Additionally, the target base station's PHY or MAC layer equipment can initiate or execute procedures for requesting scheduling from the target base station, performing channel measurements, or reporting channel measurement results. The UE may monitor the PDCCH in the PHY or MAC layer device of the source base station for a first UE identifier assigned from the source base station until the connection with the source base station is released or the second condition described in this disclosure is met. The UE may also monitor the PDCCH in the PHY or MAC layer device of the target base station for a second UE identifier assigned from the target base station. As described above, if the connection with the source base station is released or the second condition described in this disclosure is met, the UE stops monitoring the PDCCH in the PHY or MAC layer device of the source base station for the first UE identifier assigned from the source base station, or may release the transmission resources used for the scheduling request.

[0397] * Can execute commands for configuring or processing this disclosure. Figure 10 Method 1, Method 2 or Method 3 of SRB.

[0398] *The DAPS handover method of this disclosure can be executed when a new logical channel priority is configured in the MAC layer device configuration information configured via the handover command message. Furthermore, by applying a second PDCP layer device structure, existing configuration information is maintained for the MAC layer device for the first bearer of the source base station corresponding to the bearer identifier indicated above. A new logical channel identifier, as indicated in the configuration information, can be configured for the MAC layer device for the second bearer of the target base station, and a newly configured second priority bit rate corresponding to the logical channel identifier indicated in the configuration information can be applied to it. Alternatively, when the first condition described in this disclosure is met, a priority can be applied to the MAC layer device for the second bearer of the target base station for each logical channel identifier.

[0399] *The DAPS handover method of this disclosure can be executed when a second PBR for a logical channel is newly configured in the MAC layer device configuration information configured via the handover command message. Furthermore, by applying a second PDCP layer device structure, existing configuration information is maintained for the MAC layer device for the first bearer of the source base station corresponding to the bearer identifier indicated above, and a new logical channel identifier indicated in the configuration information can be configured for the MAC layer device for the second bearer of the target base station, and a newly configured second priority bit rate corresponding to the logical channel identifier indicated in the configuration information can be applied to it. Alternatively, the second priority bit rate can be applied to the logical channel identifier in the MAC layer device for the second bearer of the target base station starting from the time when the first condition described in this disclosure is met (correspondingly, uplink transmission resources can be fairly distributed when a different handover method is indicated for each bearer). When applied to each logical channel identifier, the priority bit rate is a value that increases for each logical channel over a predetermined time (e.g., each TTI). When uplink transmission resources are received, the Logical Channel Prioritization (LCP) procedure is performed. Data for logical channels can be sent by taking priority and priority bit rate into account, and the higher the priority or the larger the priority bit rate, the more data can be sent.

[0400] Furthermore, when applying the DAPS handover method described above, if the UE needs to transmit uplink data via the first bearer used by the source base station because the first condition described in this disclosure has not yet been met, the MAC layer device of the first bearer will only select the bearer or logical channel identifier that indicated the DAPS handover method (or the handover method used to continue data transmission to the source base station even after receiving the handover command message) during the LCP procedure as the target of the LCP procedure and execute the LCP procedure on it. Bearer or logical channel identifiers that do not apply the DAPS handover method should not be selected as the target of the LCP procedure because uplink data cannot be transmitted to the source base station after receiving the handover command message.

[0401] When a new second bucket size (bucketSizeDuration) for a logical channel is configured in the MAC layer device configuration information configured via the handover command message, the DAPS handover method of this disclosure is executed. A second PDCP layer device structure is applied, and existing configuration information is maintained for the MAC layer device of the first bearer for the source base station corresponding to the bearer identifier indicated above. A new logical channel identifier, indicated by the configuration information, can be configured for the MAC layer device of the second bearer for the target base station, and a newly configured second bucket size corresponding to the logical channel identifier indicated by the configuration information can be configured. Alternatively, the second bucket size can be applied to the logical channel identifier in the MAC layer device of the second bearer for the target base station starting from the time when the first condition described in this disclosure is met (correspondingly, uplink transmission resources can be fairly distributed when a different handover method is indicated for each bearer). The bucket size indicates the maximum value that the priority bit rate may have when the priority bit rate is accumulated.

[0402] *If the MAC layer device configuration information configured via the handover command message includes second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information, the DAPS handover method described above in this disclosure is executed. Furthermore, by applying a second PDCP layer device structure, existing configuration information is maintained for the MAC layer device corresponding to the bearer identifier indicated above for the first bearer of the source base station, and the second allowed SCell information, allowed subcarrier spacing information, maximum PUSCH period, or logical channel group configuration information indicated in the configuration information can be configured for the MAC layer device for the second bearer of the target base station.

[0403] Figure 10 A method for applying configuration information to each bearer when the DAPS handover method is configured, according to an embodiment of the present disclosure, is illustrated.

[0404] refer to Figure 10When the UE receives a handover command message (indicated by reference numeral 10-01), if a second handover method (e.g., the second embodiment of this disclosure or the DAPS handover method) is indicated by the ReconfigWithSync information, if the DAPS handover method is indicated for each bearer identifier or logical channel identifier, if the DAPS handover method is configured for at least one bearer, or if the DAPS handover method is configured for a predetermined bearer, the UE can generate or establish MAC layer equipment for the target base station (or target cell) at the time of receiving the handover command message. The UE can then continue to send data to or receive data from the source base station through the MAC layer equipment of the source base station (or source cell), only for the bearers (AM bearers or UM bearers) for which the DAPS handover method is indicated, until the first condition described in this disclosure is met from the time the handover command message is received. Furthermore, when the first condition is met, the UE can switch uplink data to the target base station and can receive downlink data from the source base station until the connection with the source base station is released. However, for bearers that do not specify a DAPS handover method, from the time the handover command message is received until the first condition described in this disclosure is met, the UE may not send data to or receive data from the source base station, or may cease sending or receiving data from the source base station. Accordingly, the following methods can be applied to the UE to perform the operations of this disclosure, and can be modeled as shown in reference numerals 10-21 or 10-22. Furthermore, if the second condition described in this disclosure is met, thereby releasing the source base station, the method can be modeled as shown in reference numerals 10-31 or 10-32.

[0405] - As suggested in this disclosure, when the UE receives a handover command message, if a second handover method (e.g., the second embodiment of this disclosure or the DAPS handover method) is indicated via ReconfigWithSync information, if the DAPS handover method is indicated for each bearer identifier or logical channel identifier, if the DAPS handover method is configured for at least one bearer, or if the DAPS handover method is configured for a predetermined bearer, the UE can suspend the SRB configured for the MAC layer device of the source base station. Furthermore, the UE's higher-layer device (e.g., RRC layer device) can instruct the MAC layer device of the source base station to perform a reconfiguration (MAC reconfiguration) using configuration information obtained by excluding configuration information related to bearers for which the DAPS handover method was not indicated via the handover command message from the current MAC layer device's configuration information. As another method, higher-layer devices of the UE (e.g., RRC layer devices) can instruct the MAC layer device of the source base station to perform a reconfiguration (MAC reconfiguration) using configuration information obtained by including only the configuration information related to the bearers that indicate the DAPS handover method via the handover command message in the current MAC layer device's configuration information. In the above context, if the UE reconfigures the MAC layer device for the source base station, the MAC layer device of the UE's source base station can maintain a logical channel identifier indicating the DAPS handover method, or the corresponding priority bit rate or bucket size, and can release or de-use or apply logical channel identifiers or the corresponding priority bit rate or bucket size that do not indicate a DAPS handover method. Furthermore, higher-layer devices of the UE (e.g., RRC layer devices) can perform PDCP reconstruction procedures or RLC reconstruction procedures for bearers that do not indicate a DAPS handover method; can apply PDCP configuration information or RLC configuration information configured through the handover message or suspend the bearer; can suspend the bearer in the MAC layer device of the originating base station; can configure PDCP configuration information or RLC configuration information in the MAC layer device of the target base station (when a first condition is met, PDCP configuration information or RLC configuration information can be configured in the MAC layer device of the target base station); can instruct the MAC layer device of the target base station to configure or apply bearer configuration information, such as logical channel identifiers or priority bit rates or bucket sizes configured for the target base station, for bearers that do not indicate a DAPS handover method through the handover command message; and can switch the connection between the PDCP layer device or RLC layer device and the MAC layer device of the source base station (the PDCP layer device or RLC layer device corresponds to the bearer that does not indicate a DAPS handover method) to the connection between the PDCP layer device or RLC layer device and the MAC layer device of the target base station.Then, for example, from that point in time, when executing the Logical Channel Prioritization (LCP) procedure for data transmission, the UE's MAC layer device for the source base station can select only the logical channel identifiers corresponding to the bearers indicating the DAPS handover method as candidate groups and execute the LCP procedure on them. As described above, the procedure by which a higher-layer device (e.g., an RRC layer device) reconfigures the source base station's MAC layer device enables the source base station's MAC layer device to perform partial initialization (partial MAC reset) to execute the procedure. For example, configuration information for MAC layer devices used for bearers not indicating the DAPS handover method can be initialized, released, or suspended. Furthermore, data transmission to or from the source base station can be performed until the first condition described in this disclosure is met. For logical channel identifiers corresponding to bearers not indicating the DAPS handover method, the source base station's MAC layer device can initialize the priority bit rate and release or suspend the bearer without further applying the procedure for calculating the priority bit rate accumulation. Furthermore, for logical channel identifiers corresponding to bearers indicating DAPS handover methods, the MAC layer device of the source base station can continue to maintain the priority bit rate and perform an accumulation calculation procedure. If the first condition is met, data transmission is switched to the target base station, and the MAC layer device of the target base station can initialize the priority bit rate for newly configured logical channel identifiers (logical channel identifiers corresponding to bearers indicating DAPS handover methods or bearers not indicating DAPS handover methods), or can begin accumulation calculation (in other words, when the MAC layer device for the target base station receives a handover command message, it initializes the priority bit rate and can begin accumulation calculation). If bearers not indicating DAPS handover methods are configured or suspended as described above, the MAC layer device of the target base station can configure or resume the bearer to perform data transmission to or from the target base station, and can initialize the priority bit rate or begin accumulation calculation. Furthermore, before the second condition described in this disclosure is met, data reception from the source base station or the target base station is performed, and when the second condition is met (indicated by reference numeral 10-03), the MAC layer device of the source base station is initialized, and the RLC layer device, PDCP layer device, or bearer configuration information corresponding to the bearer that does not indicate the DAPS handover method can be released from the MAC layer device of the source base station, and the RLC layer device or bearer configuration information corresponding to the bearer that indicates the DAPS handover method can be released from the structure of the second PDCP layer device or MAC layer device of the source base station (indicated by reference numerals 10-31 or 10-32).If the handover procedure fails and the connection with the source base station is valid, as described below in this disclosure, the UE can execute a fallback procedure to the source base station (indicated by reference numeral 10-02), restore the SRB configured in the MAC layer device of the source base station and report the handover failure, apply the existing bearer configuration information of the source base station and the original MAC layer device configuration information before receiving the handover command message again (e.g., the RRC layer device can reconfigure the MAC layer device configuration information used before receiving the handover command message), and restore data transmission to or from the source base station for each bearer (indicated by reference numeral 10-10). Alternatively, higher-layer devices of the UE (e.g., the RRC layer device) can instruct an RLC reconstruction procedure for bearers indicating a DAPS handover method or bearers not indicating a DAPS handover method. Alternatively, as described above, the configuration information of the source base station's MAC layer device to be applied when the source base station executes the DAPS handover method can be configured via RRC messages.

[0406] In the method of this disclosure, when the UE receives a handover command message, if a second handover method (e.g., the second embodiment of this disclosure or the DAPS handover method) is indicated by the ReconfigWithSync information, if the DAPS handover method is indicated for each bearer identifier or logical channel identifier, if the DAPS handover method is configured for at least one bearer, or if the DAPS handover method is configured for a predetermined bearer, the UE can apply the SRB configured in the MAC layer device of the source base station by using one or more of the following methods.

[0407] When the UE receives a handover command message, if a second handover method (e.g., the second embodiment of this disclosure or the DAPS handover method) is indicated via ReconfigWithSync information, if a DAPS handover method is indicated for each bearer identifier or logical channel identifier, if a DAPS handover method is configured for at least one bearer, or if a DAPS handover method is configured for a predetermined bearer,

[0408] *Method 1: The SRB can be suspended in the MAC layer device of the source base station. Alternatively, the window state variables can be initialized by executing the reconstruction procedure of the RLC layer device or the PDCP layer device of the SRB, and the stored data (PDCP SDU, PDCP PDU, RLC SDU, or RLC PDU) can be discarded. Alternatively, the SRB can be configured in the MAC layer device of the target base station according to the configuration received via the handover message. Alternatively, the SRB of the MAC layer device of the source base station can be configured according to the configuration received via the handover message and can be switched and connected to the MAC layer device of the target base station. If the UE fails in the handover procedure and executes the fallback procedure, the SRB suspended in the MAC layer device of the source base station can be restored, or the SRB configured for the target base station can be reconfigured using the existing configuration of the source base station and switched and connected to the MAC layer device of the source base station for recovery.

[0409] *Method 2: The SRB can be suspended in the MAC layer device of the source base station. Alternatively, the reconstruction procedure of the RLC layer device or the PDCP layer device of the SRB can be suspended without execution. Alternatively, the SRB can be configured in the MAC layer device of the target base station according to the configuration received via the handover message. As another method, the SRB of the MAC layer device of the source base station can be configured according to the configuration received via the handover message and can be switched and connected to the MAC layer device of the target base station. If the UE fails in the handover procedure and executes the fallback procedure, the SRB suspended in the MAC layer device of the source base station can be restored, or the existing configuration of the source base station can be used to reconfigure the SRB configured for the target base station and switch and connect it to the MAC layer device of the source base station for recovery.

[0410] *Method 3: The SRB can be suspended in the MAC layer device of the source base station. Alternatively, the reconstruction procedure for the RLC layer device or PDCP layer device of the SRB can be omitted, and the window state variables can be left uninitialized, but stored data (PDCP SDU, PDCP PDU, RLC SDU, or RLC PDU) can be discarded (to prevent unsent RRC messages from being unnecessarily sent later). Alternatively, the SRB can be configured in the MAC layer device of the target base station according to the configuration received via the handover message. As another method, the SRB of the MAC layer device of the source base station can be configured according to the configuration received via the handover message and can be switched and connected to the MAC layer device of the target base station. If the UE fails in the handover procedure and executes the fallback procedure, the SRB suspended in the MAC layer device of the source base station can be restored, or the existing configuration of the source base station can be used to reconfigure the SRB configured for the target base station, and it can be switched and connected to the MAC layer device of the source base station for restoration. Furthermore, according to the procedure for SRBs, specifically, it is permissible to suspend the SRB for the source base station or enable the SRB configured for the target base station. This is to address the security issue arising from reusing the same count value from the beginning when the DAPS handover rollback procedure is executed as described above. For the SRB established for the target base station, the existing count value or its transmit or receive window variable value for the source base station's SRB is applied or maintained (or the count value or its transmit or receive window variable value for the source base station's SRB can be configured as the count value or its transmit or receive window variable value for the target base station's SRB). Furthermore, for the SRB for the target base station, a security key for the target base station can be derived or applied, and the PDCP layer device of the SRB can apply the security key to perform encryption or decryption or integrity protection or verification procedures. Additionally, for the SRB for the source base station, old data (e.g., RRC messages from the source base station) can be discarded. Procedures for the SRB used by the target base station can be defined as new procedures (e.g., DAPS SRB establishment or PDCP layer device reconstruction procedures) and instructed, triggered, or executed. Furthermore, procedures for the SRB can be extended and applied in the same manner as procedures for UM DRBs or AMDRBs without a configured DAPS handover method. More specifically, if the UE fails in the handover procedure and executes a fallback procedure, the SRB used by the source base station can be restored, or the SRB used by the target base station can be released.As another method, the target base station is enabled so that, to address security issues arising from using the same count value from the beginning, the existing count value or its transmit or receive window variable value for the SRB used by the source base station is applied or maintained (alternatively, the count value or its transmit or receive window variable value for the target base station's SRB can be configured to be the same as the count value or its transmit or receive window variable value for the source base station's SRB), and the SRB used by the target base station can be released. Furthermore, the source base station's security key can be applied to the source base station's SRB, and the PDCP layer device of the SRB applies the source base station's security key to perform encryption or decryption or integrity protection or verification procedures. Additionally, old data (e.g., RRC messages from the source base station) can be discarded for the SRB used by the source base station. The procedures for the SRB can be extended and applied in the same manner as the procedures for UM DRBs or AM DRBs without configured DAPS handover methods.

[0411] In the following sections of this disclosure, when a UE executes the second embodiment of the efficient handover method of this disclosure (DAPS handover method), if the UE fails during handover, a method is provided for quickly falling back to the source base station and re-establishing the connection by utilizing the characteristics of the aforementioned DAPS handover method. Specifically, the characteristics of the DAPS handover method of this disclosure refer to performing data transmission or reception by maintaining the connection with the source base station even during the handover procedure. Furthermore, in the following sections of this disclosure, a handover method is provided for performing fallback by using the existing radio connection established with the source base station even if the handover fails.

[0412] In this disclosure Figure 8 In the second embodiment of the efficient handover method described herein (DAPS handover method), even when a handover command message is received from the source base station, the handover procedure to the target base station is executed while maintaining data transmission to or from the source base station (reference numeral 8-02). Furthermore, this disclosure proposes a procedure for backtracking to the source base station when the handover procedure to the target base station described above fails.

[0413] If, as described above, the UE fails during the handover procedure to the target base station, a method is needed to identify whether the radio connection between the UE and the source base station is valid in order to fall back to the source base station. This is because when the radio connection between the UE and the source base station is invalid, if the UE handover fails and a fallback to the source base station is executed, the fallback procedure to the source base station will also fail, resulting in a very long data interruption time and significant data loss. Furthermore, when the radio connection between the UE and the source base station is valid, the SRB configured in both the UE and the source base station needs to be maintained.

[0414] First, this disclosure proposes new timers applicable to handover methods and provides specific operations for each timer. Furthermore, the specific operations of the timers may include performing different operations based on the type of handover method indicated by the base station via a handover command message. Additionally, a method for releasing or maintaining a connection with the source base station or an SRB configuration based on the handover method is provided.

[0415] To efficiently execute the handover procedure, this disclosure introduces a first timer (e.g., T304), a second timer (e.g., T310), a third timer (e.g., T312), or a fourth timer (e.g., a timer for rollback), and these timers are run and applied during the handover procedure. The first timer (e.g., T304), second timer (e.g., T310), third timer (e.g., T312), or fourth timer (e.g., a timer for rollback) of this disclosure performs different operations depending on the type of handover method indicated by the handover command message. In the foregoing, the first timer (e.g., T304) is a timer for determining whether the handover has been successfully executed, the second timer (e.g., T310) is a timer for determining whether the wireless connection is valid, the third timer (e.g., T312) is an auxiliary timer for determining whether the wireless connection is valid, and is also a timer for triggering the frequency measurement procedure and reporting the frequency measurement results. Furthermore, the fourth timer (e.g., a timer for rollback) is used to send a message indicating that the handover has failed to the source base station by executing a rollback procedure to the source base station when the handover fails during the execution of the second embodiment of the efficient handover method of this disclosure (DAPS handover method), and then to determine whether the rollback procedure has been successfully executed or has failed.

[0416] The following description illustrates the specific operation of a first timer (e.g., T304), a second timer (e.g., T310), a third timer (e.g., T312), or a fourth timer (e.g., a timer for rollback) that supports the efficient handover method of this disclosure, according to the indicated handover method.

[0417] -1> If the UE receives an out-of-sync indicator (out-of-sync indicator) a predetermined number of times (e.g., configured by the base station) from a lower-layer device (e.g., a MAC layer device or a PHY layer device), thus detecting a problem in the physical layer device, the UE can start a second timer (e.g., T310) when the first timer has not run. Furthermore, when the UE receives an in-sync indicator (in-sync indicator) a predetermined number of times (e.g., configured by the base station) from a lower-layer device, the second timer is interrupted when a handover procedure is triggered (initiated) or when an RRC connection reconstruction procedure is initiated. If the second timer expires, the UE triggers or initiates the RRC connection reconstruction procedure. Alternatively, the UE switches to RRC idle mode and triggers or initiates the RRC connection reconstruction procedure.

[0418] -1> While the second timer is running, the UE starts the third timer when it triggers a frequency measurement procedure for a frequency measurement identifier configured with the third timer. Furthermore, the UE can interrupt the third timer when it receives a predetermined number of radio connectivity signal synchronization indicators (synchronization indicators) from a lower-layer device (e.g., which the base station can configure), when a handover procedure is triggered (when the handover procedure is initiated), or when an RRC connection re-establishment procedure is initiated. If the third timer expires, the UE triggers or initiates the RRC connection re-establishment procedure. Alternatively, the UE switches to RRC idle mode and triggers or initiates the RRC connection re-establishment procedure.

[0419] -1> If the UE indicates a first handover method (e.g., the first embodiment of this disclosure or a general handover method) via a handover command message received from the base station (obtained by including a mobility indication (MobilityControlInfo or ReconfigurationWithSync) or handover indication in the RRCReconfiguration message), when the UE receives the handover command message, if the second embodiment of this disclosure (DAPS handover method) is not configured, or the second embodiment of this disclosure (DAPS handover method) is not configured for a predetermined bearer via bearer configuration information, then the first handover method (e.g., the first embodiment of this disclosure or a general handover method) is indicated via the handover command message, ReconfigWithSync information, or MobilityControlInfo information.

[0420] *2> In this disclosure, upon receiving the handover command message (a message obtained by including a mobility indication (MobilityControlInfo or ReconfigWithSync) or a handover indication in an RRCReconfiguration message), the UE triggers the handover procedure and starts the first timer.

[0421] *2> When the above handover procedure is triggered, the UE releases the SRB (e.g., SRB1) configured for the source base station and configures the SRB (e.g., SRB1) for the target base station based on the configuration information configured in the handover command message.

[0422] *2> When the above handover procedure is triggered, if the second timer is running, the UE can interrupt the second timer. Furthermore, even if the conditions for starting the second timer are met (when a predetermined number of asynchronous indicators of the radio connection signal are received from a lower layer) while the first timer is running, the second timer will not be started. In other words, the second timer is not used while the first timer is running.

[0423] *2> When the above handover procedure is triggered, if the third timer is running, the UE can interrupt the third timer. Furthermore, the third timer is started only when the condition that the third timer is running is met (when a frequency measurement procedure is triggered for a frequency measurement identifier configured with the third timer). That is, since the second timer is not used when the first timer is running, the third timer is also not used.

[0424] *2> In the above text, if the handover procedure or random access procedure to the target base station is successfully completed, the UE interrupts the first timer.

[0425] *2> If the first timer expires (for example, if the handover procedure to the target base station fails), the UE executes the RRC connection reconstruction procedure (which may release the connection with the base station and start the RRC connection procedure again from scratch, i.e., execute the cell selection or reselection procedure, execute the random access procedure, and send the RRC connection reconstruction request message).

[0426] -1> In cases where the UE indicates a second handover method (e.g., the second embodiment or the DAPS handover method) via a handover command message received from the base station (obtained by including a mobility indication (MobilityControlInfo or ReconfigurationWithSync) or handover indication in the RRCReconfiguration message) (or can be extended and applied to cases where the UE also indicates a conditional handover method); if the UE receives the handover command message, the second embodiment of this disclosure (DAPS handover method) is configured via the handover command message, ReconfigWithSync information, or MobilityControlInfo information; if the second embodiment of this disclosure (DAPS handover method) is configured for a predetermined bearer via bearer configuration information; or if the second embodiment of this disclosure (DAPS handover method) is configured for at least one bearer via bearer configuration information.

[0427] *2> In this disclosure, upon receiving a handover command message (a message that includes a mobility indication (MobilityControlInfo or ReconfigurationWithSync) or handover indication in the RRCReconfiguration message), the UE triggers a handover procedure and starts a first timer. If a conditional handover method is indicated together, the UE may start the first timer when selecting a cell from multiple target cells and initiating a handover procedure or executing a random access procedure.

[0428] *2> If the DAPS handover method is initiated when the above handover procedure is triggered, the UE maintains or suspends the SRB (e.g., SRB1) configured for the source base station, and configures the SRB (e.g., SRB1) for the target base station based on the configuration information configured in the handover command message. Alternatively, if the DAPS handover method is initiated when the above handover procedure is triggered, the UE: may maintain or suspend the SRB (e.g., SRB1) configured for the source base station, and rebuild the PDCP layer device or RLC layer device for the SRB used for the source base station to initialize window state variables or interrupt timers; may instruct the discarding of stored data (PDCPSDU or PDCP PDU) (this may be performed when the fallback procedure of this disclosure is triggered); and may configure the SRB (e.g., SRB1) for the target base station based on the configuration information configured via the handover command message. Alternatively, the UE may configure the first bearer for the source base station and the second bearer for the target base station by applying the second PDCP layer device structure of this disclosure to the SRB. As another method, when the second PDCP layer device structure is applied to the SRB, the UE can rebuild the PDCP layer device or the RLC layer device for the first bearer to initialize window state variables, interrupt timers, and indicate the discarding of stored data (PDCP SDU or PDCP PDU) (this can be performed when the fallback procedure of this disclosure is triggered). That is, as another method, when the fallback procedure of this disclosure is triggered, the UE can execute a procedure to discard data remaining in the SRB used by the source base station or stored in the buffer (e.g., old RRC messages), or can trigger or instruct the UE's higher-layer device (e.g., the RRC layer device) to discard data remaining in the PDCP layer device of the SRB or stored in the buffer (e.g., old RRC messages). This is because old RRC messages should be prevented from being sent to the source base station.

[0429] *2> If the DAPS handover method is triggered when the above handover procedure is initiated, the UE can avoid interrupting the source base station's second timer even if the source base station's second timer is running. Furthermore, the UE starts the second timer when the condition for starting the second timer even when the first timer is running (or when the first timer is not running) is met (when a predetermined number of asynchronous indicators of the radio connection signal are received from a lower layer). The second timer can operate for the radio connection between the UE and the source base station. Alternatively, two second timers can be operated: one for the radio connection between the UE and the source base station, and the other for the radio connection between the UE and the target base station. That is, the second timer can be used for the radio connection with either the source or target base station even when the first timer is running. However, even when the second timer expires, if the first timer has not expired and is running, the UE can avoid triggering the RRC connection reconstruction procedure. In other words, specifically, even if the second timer of the source base station expires or a radio connection failure (RLF) occurs, if the first timer has not expired and is running, and a random access procedure to the target base station or a handover procedure to the target base station is being executed, the UE can release the radio connection with the source base station without triggering the RRC connection reconstruction procedure. Furthermore, the UE may not release the RRC configuration information (e.g., bearer configuration information) configured by the source base station, and can reuse this RRC configuration information if the RRC connection reconstruction procedure is triggered later. Additionally, even if the second timer expires, if the first timer has not expired and is running, the UE will not trigger the RRC connection reconstruction procedure and can send a report to the source or target base station indicating that the radio connection with the source base station has failed, or the UE can release the connection with the source base station (e.g., release the first bearer used by the source base station) or suspend the first bearer used by the source base station. However, when the second timer expires, if the first timer has expired or has been interrupted, or if the first timer has not been activated because it was not started, the UE can trigger the RRC connection reconstruction procedure. The reason for operating the second timer even during the handover procedure is that, if a handover failure occurs as a result of monitoring the radio connection between the UE and the source base station, a fallback procedure can be executed if the radio connection with the source or target base station is valid. Furthermore, when the second timer for the target base station expires, or when the radio connection with the target base station fails, if the first timer has expired, been interrupted, or has not yet started and is not running, or if the random access procedure to the target base station has been successfully executed, the UE can trigger the RRC connection reconstruction procedure.

[0430] *2> If the DAPS handover method is triggered when the above handover procedure is triggered, the UE will not interrupt the source base station's third timer even if the third timer is running. Furthermore, the third timer is started when the condition that the third timer is started only when the second timer is running is met (when a frequency measurement procedure is triggered for a frequency measurement identifier configured with the third timer). That is, since the second timer is used even when the first timer is running, the third timer can also be used. The third timer can operate for the radio connection between the UE and the source base station. In another method, two third ...

Claims

1. A method performed by a terminal in a wireless communication system, the method comprising: receiving, from a base station, radio resource control (RRC) information including information for a reconfiguration with synchronization (sync) and information configuring at least one dual active protocol stack (DAPS) bearer; if the at least one DAPS bearer is configured, establishing, for a signaling radio bearer (SRB), a radio link control (RLC) layer and a logical channel for a target cell group with a same configuration as for a source cell group, and suspending the SRB for the source cell group; configuring initial values of state variables of a packet data convergence protocol (PDCP) layer for the target cell group with values stored in the PDCP layer for the source cell group; and if a T304 timer expires, the at least one DAPS bearer is configured, and no radio link failure is detected in a source primary cell (PCell), performing PDCP service data unit (SDU) discard in the PDCP layer of the source PCell and re-establishing an RLC layer for the source PCell. 2.The method of claim 1, further comprising: if the at least one DAPS bearer is configured, configuring initial values of state variables of a PDCP layer for a source PCell with values stored in the PDCP layer for a target PCell, for the SRB. identifying the reconfiguration with sync as having failed based on an expiration of the T304 timer configured by the information, and 3. The method of claim 1, wherein, wherein the state variables include at least one of TX_NEXT, RX_NEXT, RX_DELIV, and a state variable indicating a COUNT value following a COUNT value associated with a PDCP data protocol data unit (PDU) that triggered a reordering timer. 4.A terminal in a wireless communication system, the terminal comprising: a transceiver configured to transmit and receive signals; and a controller configured to: receive, from a base station, radio resource control (RRC) information including information for a reconfiguration with synchronization (sync) and information configuring at least one dual active protocol stack (DAPS) bearer; if the at least one DAPS bearer is configured, establish, for a signaling radio bearer (SRB), a radio link control (RLC) layer and a logical channel for a target cell group with a same configuration as for a source cell group, and suspend the SRB for the source cell group; configure initial values of state variables of a packet data convergence protocol (PDCP) layer for the target cell group with values stored in the PDCP layer for the source cell group; and if a T304 timer expires, the at least one DAPS bearer is configured, and no radio link failure is detected in a source primary cell (PCell), perform PDCP service data unit (SDU) discard in the PDCP layer of the source PCell and re-establish an RLC layer for the source PCell. the controller is further configured to: ​ 5. The terminal according to claim 4, wherein ​ If the at least one DAPS bearer is configured, for the SRB, initial values of state variables of a PDCP layer for the source PCell are configured with values stored in the PDCP layer for the target PCell.

6. The terminal according to claim 4, wherein based on expiry of the T304 timer configured by the information, the reconfiguration with sync is identified as having failed, and wherein the state variables include at least one of TX_NEXT, RX_NEXT, RX_DELIV, and a state variable indicating a COUNT value following a COUNT value associated with a PDCP Data Protocol Data Unit, PDU, that triggered a reordering timer.

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