Method for Supporting Data Forwarding during Conditional Handover and Dual-Stack Protocol Handover in Next Generation Mobile Communication Systems

Abstract

The present invention relates to a communication method and system for integrating a fifth-generation (5G) communication system with Internet of Things (IoT) technology to support higher data rates beyond fourth-generation (4G) systems. The present invention can be applied to intelligent services based on 5G communication technology and IoT-related technologies, such as smart home, smart building, smart city, smart car, connected car, healthcare, digital education, smart retail, security and safety services. According to the present invention, data forwarding can be effectively supported during conditional handover and dual-stack protocol handover.

Description

Technical Field

[0001] The present invention relates to the operation of a base station in a mobile communication system. Background Art

[0002] In order to meet the ever-growing demand for wireless data traffic since the deployment of 4G communication systems, efforts have been made to develop improved 5G or pre-5G communication systems. Therefore, 5G or pre-5G communication systems are also referred to as "Beyond 4G Networks" or "Post-LTE Systems". The 5G communication system is considered to be implemented in a higher frequency (mmWave) band, such as the 60 GHz band, in order to achieve higher data rates. In order to reduce the propagation loss of radio waves and increase the transmission distance, beamforming, massive multiple-input multiple-output (MIMO), full-dimensional MIMO (FD-MIMO), array antennas, analog beamforming, and massive antenna technologies have been discussed in 5G communication systems. In addition, in 5G communication systems, system network improvements based on advanced small cells, cloud radio access network (RAN), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, mobile networks, cooperative communication, coordinated multi-point (CoMP), receive-end interference cancellation, etc. are being developed. In 5G systems, hybrid FSK and QAM modulation (FQAM) and sliding window superimposed coding (SWSC) have been developed as advanced coding modulation (ACM), and filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) have been developed as advanced access technologies.

[0003] The Internet is a human-centered connected network in which humans generate and consume information, and is now evolving into the Internet of Things (IoT), in which distributed entities (such as things) exchange and process information without human intervention. Through connecting to cloud servers, the Internet of Everything (IoE) that combines IoT technology and big data processing technology has emerged. Since IoT implementation requires technical elements such as "sensing technology", "wired / wireless communication and network infrastructure", "service interface technology", and "security technology", sensor networks, machine-to-machine (M2M) communication, machine type communication (MTC), etc. have been recently studied. Such an IoT environment can provide intelligent Internet technology services, creating new values for human life by collecting and analyzing data generated between connected things. Through the integration and combination of existing information technology (IT) and various industrial applications, the Internet of Things can be applied to various fields including smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, healthcare, smart home appliances, and advanced medical services.

[0004] In view of this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as sensor networks, machine type communication (MTC), and machine-to-machine (M2M) communication can be implemented through beamforming, MIMO, and array antennas. The application of cloud radio access network (RAN) as the above-mentioned big data processing technology can also be regarded as an example of the integration between 5G technology and IoT technology.

[0005] The above information is provided only as background information to assist in understanding the present disclosure. No decision has been made and no assertion has been made as to whether any of the above can be used as prior art for the present disclosure. Summary of the Invention

[0006] [Technical Problem]

[0007] The disclosed embodiments provide a signaling method and a base station node processing method for supporting efficient data forwarding when performing data forwarding between base stations to effectively support conditional handover (CHO) and dual-stack protocol handover (DAPS handover) to improve handover performance in a mobile communication system.

[0008] [Solution to the Problem]

[0009] According to an embodiment of the present invention, a method performed by a first base station in a wireless communication system may include: during a specific handover process, transmitting a first message for a bearer context modification request from a central unit control plane (CU-CP) to a central unit user plane (CU-UP), the first message including information about an early forwarding count request; transmitting a second message for a bearer context modification response from the CU-UP to the CU-CP, the second message including information about a first downlink (DL) count; and transmitting a third message including information about the first DL count from the CU-CP to a second base station.

[0010] According to another embodiment of the present invention, a first base station in a wireless communication system may include: a transceiver and at least one processor configured to transmit a first message for a bearer context modification request from a central unit control plane (CU-CP) to a central unit user plane (CU-UP) during a specific handover process, the first message including information about an early forwarding count request; transmit a second message for a bearer context modification response from the CU-UP to the CU-CP, the second message including information about a first downlink (DL) count; and control the transceiver to transmit a third message including information about the first DL count from the CU-CP to a second base station.

[0011] [Advantageous Effects of the Invention]

[0012] According to the present invention, data forwarding can be effectively supported during condition switching and dual-stack protocol switching.

[0013] Before proceeding with the following detailed description, it may be advantageous to set forth definitions of certain words and phrases used in this patent document: "include" and "comprise" and their derivatives mean including but not limited to; the term "or" includes inclusive, meaning and / or; the phrases "associated with" and "associated therewith" and their derivatives may mean including, being included therein, being interconnected with, containing, being contained therein, being connected to or coupled with, being communicable with, cooperating with, interleaving, juxtaposing, being adjacent to, being bound to or combined with, having, having the attribute of, etc.; and the term "controller" means any device, system, or part thereof that controls at least one operation, such device may be implemented in hardware, firmware, software, or a combination of at least two of the same. It should be noted that the functions associated with any particular controller may be centralized or distributed, whether local or remote.

[0014] In addition, the various functions described below can be implemented or supported by one or more computer programs, each computer program being formed of computer-readable program code and embodied in a computer-readable medium. The terms "application" and "program" mean one or more computer programs, software components, instruction sets, processes, functions, objects, classes, instances, associated data, or a part thereof, adapted to be implemented in appropriate computer-readable program code. The phrase "computer-readable program code" includes any type of computer code, including source code, object code, and executable code. The phrase "computer-readable medium" includes any type of medium that can be accessed by a computer, such as read-only memory (ROM), random access memory (RAM), hard disk drive, optical disc (CD), digital video disc (DVD), or any other type of memory. A "non-transitory" computer-readable medium does not include wired, wireless, optical, or other communication links that transmit transient electrical or other signals. Non-transitory computer-readable mediums include media that can permanently store data and media that can store data and subsequently overwrite the data, such as rewritable optical discs or erasable storage devices.

[0015] Definitions of specific words and phrases are provided in this patent document, and those of ordinary skill in the art should understand that in many cases (if not most cases), such definitions apply to both the previous and future uses of such defined words and phrases. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] To more fully understand the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:

[0017] Figure 1A diagram showing an example of an applicable structure of a next-generation mobile communication system;

[0018] Figure 2 A diagram showing an embodiment for determining whether to perform early data forwarding in the CU-CP of a source RAN node to support early data forwarding when performing conditional handover (CHO) or dual-active protocol stack handover (DAPS-HO) between RAN nodes, where each node is divided into CU-CP and CU-UP;

[0019] Figure 3 A diagram showing an embodiment for determining whether to perform early data forwarding in the CU-UP of a source RAN node to support early data forwarding when performing conditional handover (CHO) or dual-active protocol stack handover (DAPS-HO) between RAN nodes, where each node is divided into CU-CP and CU-UP;

[0020] Figure 4 A diagram showing an example of the configuration of signaling messages exchanged between CU-CP and CU-UP when performing conditional handover (CHO) or dual-active protocol stack handover (DAPS-HO) between RAN nodes, where each RAN node is divided into CU-CP and CU-UP;

[0021] Figure 5 A diagram showing an example of the configuration of signaling messages exchanged between CU-CP and CU-UP in the case of determining whether to perform early data forwarding in the CU-UP of a source RAN node to support early data forwarding when performing conditional handover (CHO) or dual-active protocol stack handover (DAPS HO) between RAN nodes, where each node is divided into CU-CP and CU-UP;

[0022] Figure 6 A diagram showing an embodiment for efficiently performing early data forwarding using the same data forwarding information when configuring conditional handover for multiple target cells of a single target RAN node;

[0023] Figure 7 A diagram showing an embodiment for effectively performing early data forwarding using indication information indicating the use of the same data forwarding information when configuring conditional handover for multiple target cells of a single target RAN node;

[0024] Figure 8 A diagram showing an embodiment for processing a handover request message received by a target RAN node to effectively perform early data forwarding when configuring conditional handover for multiple target cells of a single target RAN node;

[0025] Figure 9is an example showing the configuration of signaling messages exchanged between a source RAN node and a target RAN node when conditional handover is configured for multiple target cells of a single target RAN node, so as to effectively perform early data forwarding;

[0026] Figure 10 is a diagram showing the structure of a terminal according to an embodiment; and

[0027] Figure 11 is a diagram showing the structure of a base station according to an embodiment. Detailed Description of the Invention

[0028] Figures 1 to 11 , and various embodiments for describing the principles of the present invention in this patent document are only for illustrative purposes and should not be construed as limiting the scope of the present invention in any way. Those skilled in the art will understand that the principles of the present invention can be implemented in any appropriately arranged system or device.

[0029] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0030] When describing the embodiments of the present invention, descriptions related to technical content that is well known in the art and not directly associated with the present invention will be omitted. The omission of such unnecessary descriptions is intended to prevent the main idea of the present invention from being obscured and to more clearly convey the main idea.

[0031] For the same reason, in the drawings, some elements may be exaggerated, omitted, or shown schematically. In addition, the size of each element does not fully reflect the actual size. In the drawings, the same or corresponding elements have the same reference numerals.

[0032] Advantages and features of the present invention and methods for achieving them will be apparent from the embodiments described in detail below in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments described below, but can be implemented in various different forms. The following embodiments are provided only to fully disclose the present invention and to inform those skilled in the art of the scope of the present invention, and the present invention is only defined by the scope of the appended claims. Throughout the specification, the same or similar reference numerals denote the same or similar elements.

[0033] Here, it should 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, a special-purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions executed via the processor of the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart block. These computer program instructions can also be stored in a computer-usable or computer-readable memory, which can direct a computer or other programmable data processing apparatus to operate in a particular manner, such that the instructions stored in the computer-usable memory or computer-readable memory produce an article of manufacture including instruction means for implementing the functions specified in the flowchart block. The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, such that the instructions executed on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block.

[0034] In addition, each block of the flowchart illustrations can represent a module, a segment, or a portion of code, which includes one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions recorded in the blocks may occur out of order. For example, two blocks shown in succession may in fact be executed substantially concurrently, or may sometimes be executed in the reverse order, depending on the functions involved.

[0035] In the following description, for convenience, terms for identifying access nodes, terms for referring to network entities, terms for referring to messages, terms for referring to interfaces between network entities, terms for referring to various identification information, etc. are used illustratively. Therefore, the present invention is not limited by the terms used hereinafter, and other terms referring to the subject matter having the same technical meaning can be used.

[0036] In the following description, for ease of description, the present invention uses terms and names defined in the standards of 5G, NR, or LTE systems. However, the present invention is not limited by these terms and names, and can be applied to systems conforming to other standards in the same manner.

[0037] That is to say, the detailed description of the embodiments of the present invention will be directed to the communication standards defined by 3GPP. However, based on the determination of those skilled in the art, with some modifications, the main idea of the present invention can be applied to other communication systems with a similar technical background without significantly deviating from the scope of the present invention.

[0038] Figure 1FIG. is a diagram showing an example of the structure of a next-generation mobile communication system to which the technology of the present invention is applicable. The radio access network (RAN) nodes 1-100 and 1-200 specified in the structure may be mobile communication base stations, such as LTE eNBs and NR gNBs, which are connected to a mobile communication core network (CN), such as an evolved packet core network (EPC) or a 5G core network (5GC) 1-400. The RAN nodes 1-100 and 1-200 can be divided into a central unit (CU) and a distributed unit (DU), where the CU is further divided into a control plane (CU-CP) and a user plane (CU-UP). Each RAN node may include one or more CU-CPs, one or more CU-UPs, and one or more DUs. The CU-CP, CU-UP, and DU constituting one RAN node may be configured together. For example, the CU and the DU (where the CU-CP and the CU-UP are implemented together) may be included in one RAN node; the CU-CP and the DU may be implemented together, while the CU-UP may be separately configured in another RAN node; another RAN node may be configured in the form of an integrated base station, where the CU-CP, CU-UP, and DU are implemented together; and the RAN nodes may be configured by other combinations. The CU and the DU support each function of the base station respectively. For example, the CU may support the RRC / PDCP layer, and the DU may support the RLC / MAC / PHY / RF layer. The CU and the DU may be connected via an interface between the internal functions of the base station, such as interface W1 or interface F1. The CU may be further divided into a CU-CP and a CU-UP. For example, the CU-CP may support the RRC / PDCP (for RRC) layer, and the CU-UP may support the PDCP (for user data transmission) layer. The CU-CP and the CU-UP may be connected via an interface between the internal functions of the base station, such as interface E1. Here, the base station is manufactured in an integrated structure or an independent type structure, and connections between integrated structure base stations, connections between independent type base stations, and connections between integrated structure base stations and independent structure base stations are possible. The RAN nodes may be connected via an inter-base station interface, such as interface X2 or interface Xn. The RAN nodes may be connected to the core network via an inter-base station core network interface, such as interface S1 or interface NG. The technology provided in the present invention is operable when a handover is performed between base stations when the terminal 1-300 moves to a target RAN node while being connected to a source RAN node, regardless of whether it is an integrated base station or a separate type base station.

[0039] Figure 2 and Figure 3Illustrates various embodiments for supporting early data forwarding when a terminal performs conditional handover (CHO) or dual-stack protocol handover (DAPS handover) between RAN nodes, where each RAN node is divided into a CU-CP and a CU-UP. Early data forwarding is a method of transmitting (data forwarding) the data stored in the source RAN node for the terminal service to the target RAN node before the terminal accesses the target RAN node to perform a handover. In Figure 2 and Figure 3 the RAN node can be divided into a CU-CP, a CU-UP, and a DU, and Figure 2 and Figure 3 illustrate an example where the CU-CP and the DU are configured as one entity. However, the CU-CP and the DU can be further divided, and regardless of the content of the present invention, the CU-CP and the DU can operate whether they are integrated or independent. Figure 2 and Figure 3 include embodiments of the case using conditional handover (CHO), but the content included in the present invention can be applied to the case using dual protocol stack handover (DAPS-HO).

[0040] Specifically, Figure 2 illustrates an embodiment where the CU-CP of the source RAN node determines whether to perform early data forwarding and supports early data forwarding. When the CU-CP receives a measurement report sent by a terminal (UE) via the DU of the source RAN node in Figure 2 operation 2-100, according to the signal measurement results of the RAN node of the terminal and other determination conditions, the source RAN node can determine to perform conditional handover (CHO), and can determine potential target RAN nodes, as shown in operation 2-110. In the case of conditional handover, one or more potential target RAN nodes can be determined, and a process for each potential target RAN node and CHO can be performed. Figure 2 only includes the process for CHO with one potential target RAN node. However, this is only one embodiment, and the process associated with Figure 2the same procedures disclosed in. When performing DAPS HO, the procedures for DAPS HO with one target RAN node can be executed. After determining to perform CHO, the CU-CP of the source RAN node can send a handover request message to the potential target RAN node as in operation 2-120, and in the case of CHO, the handover request message can include a CHO initiation indication, target cell ID information, and content for proposing data forwarding for each data radio bearer or QoS flow. In the case of DAPS HO, it can include an indication of DAPS HO instead of a CHO indication. The CU-CP of the target RAN node that has received the handover request message for CHO can determine the CU-UP of the target RAN node to serve the terminal, can send a bearer context establishment request as in operation 2-130, and can request data forwarding information for each data radio bearer or QoS flow. One or more CU-UPs can be used for one terminal, and Figure 2 only includes procedures with one CU-UP, and when using one or more CU-UPs, the same procedures can be executed for each CU-UP. The CU-UP of the target RAN node that has received the bearer context establishment request message can perform DRB configuration for the corresponding service, etc., and can send a bearer context establishment response message including data forwarding information for each data radio bearer or QoS flow to the CU-CP of the target RAN node, as shown in operation 2-140. The CU-CP of the target RAN node determines whether to accept the handover, and then can send a handover request confirmation message to the source RAN node, as shown in operation 2-150. The information required for CHO can be included in the message. The CU-CP of the source RAN node can send an RRC reconfiguration message to the terminal via the DU as shown in operation 2-160, and can receive an RRC reconfiguration complete message from the terminal as shown in operation 2-170, and complete the CHO configuration.

[0041] Thereafter, as in operation 2-200, it can be determined to perform early data forwarding in the CU-CP of the source RAN node. When conditional handover is configured for one or more target RANs or target cells, the CU-CP of the source RAN node can determine the target RAN node or target cell for which early data forwarding is to be performed. As in operation 2-210, the CU-CP of the source RAN node can send a bearer context modification request message to the CU-UP of the source RAN node. The message can include an indication for requesting early data forwarding from the CU-UP, or count information for the first downlink (DL) PDCP packets to be forwarded during early data forwarding, or can include both of the above information. The CU-CP of the source RAN node can perform transmission, including a list of information required for data forwarding received from the target RAN node. The CU-UP that has received the bearer context modification request message from the CU-CP can perform data forwarding configuration for each target RAN node or target cell included in the list of information for data forwarding, and can send a bearer context modification response message to the CU-CP, as shown in operation 2-220. The message can include count information for the first downlink (DL) PDCP packets forwarded. As shown in operation 2-230, the CU-CP of the source RAN node can send an early forwarding transmission message including the first DL count information received from the CU-UP to the target RAN node configured with CHO or DAPS. The CU-CP of the target RAN node can send a bearer context modification request message including the first DL count information received from the source RAN node to the CU-UP of the target RAN node, as shown in operation 2-240. The CU-UP of the target RAN node can respond by sending a bearer context modification response message to the CU-CP of the target RAN node, as shown in operation 2-250. In the CU-UP, after the terminal performs a handover to the target RAN node, when the target RAN node sends data to the terminal, the first DL count information can be used for encryption and the like. The CU-UP of the source RAN node can start forwarding the downlink (DL) data of the terminal to the target RAN node, as shown in operation 2-300, while sending a bearer context modification response message including the first DL count to the CU-CP of the source RAN node, as shown in operation 2-220.

[0042] Thereafter, if the source RAN node has completed the transmission of data in the DL data forwarded from the source RAN node to the target RAN node to the terminal, the source RAN node may, if necessary, notify the target RAN node of the data information of the successfully transmitted data, and the target RAN node stores unnecessary data to reduce resource waste. To this end, the CU-CP or CU-UP of the source RAN node may determine whether to transmit the discard information of the early data forwarding data. Operations 2-400 to 2-450 represent the case where the CU-CP of the source RAN node determines to transmit the data discard information. Operations 2-500 to 2-550 represent the case where the CU-UP of the source RAN node determines to transmit the data discard information.

[0043] Specifically, when the CU-CP of the source RAN node determines the transmission of the data discard information as shown in Operation 2-400, it may send a bearer context modification request message to the CU-UP of the source RAN node as shown in Operation 2-410. The bearer context modification request message includes information requesting DL discard related to the data information that has been successfully transmitted to the terminal from the data forwarded to the target RAN node. The CU-UP of the source RAN node may send a bearer context modification response message including the DL discard information to the CU-CP of the source RAN node as shown in Operation 2-420. As shown in Operation 2-430, the CU-CP of the source RAN node that has received the message may send an early forwarding transmission message including the DL discard information to the target RAN node configured with CHO or DAPS. The CU-CP of the target RAN node may send a bearer context modification request message including the DL discard information to the CU-UP of the target RAN node as shown in Operation 2-440. The CU-UP may respond with a bearer context modification response message as shown in Operation 2-450.

[0044] When the CU-UP of the source RAN node determines the transmission of data discard information as in operation 2-500, the CU-UP of the source RAN node may send a message required for bearer context modification to the CU-CP of the source RAN node, including DL discard information related to the data information that has been successfully transmitted to the terminal from the data forwarded to the target RAN node, as shown in operation 2-510. The CU-CP of the source RAN node may respond by sending a bearer context modification confirmation message to the CU-CP of the source RAN node, as shown in operation 2-520. As shown in operation 2-530, the CU-CP of the source RAN node that has received the message required for bearer context modification may send an early forwarding transmission message including DL discard information to the target RAN node configured with CHO or DAPS. As shown in operation 2-540, the CU-CP of the target RAN node may send a bearer context modification request message including DL discard information to the CU-UP of the target RAN node, and the CU-UP of the target RAN node may respond to the CU-CP of the target RAN node via a bearer context modification response message, as shown in operation 2-550.

[0045] Thereafter, according to the handover trigger conditions configured for the terminal by the source RAN node via the RRC reconfiguration message in operation 2-160, when the terminal is to perform a handover as shown in operation 2-600, the terminal may perform a random access to the target cell of the target RAN node as shown in operation 2-610. After the random access is successful, the terminal may send an RRC reconfiguration complete message to the target RAN node, as shown in operation 2-620. As shown in operation 2-630, the CU-CP of the target RAN node that has received the RRC reconfiguration complete message from the terminal may send a handover success message indicating that the terminal has performed the handover to the source RAN node. As shown in operation 2-800, a process of updating the core network node and the data path between the RAN node for data transmission of the UE (terminal) and the core network may be performed. In operation 2-640, the CU-CP of the source RAN node that has received the handover success message may send a bearer context modification request message including PDCP SN status request information, etc., as in a conventional handover process, and in operation 2-650, the CU-UP may send a bearer context modification response message including PDCP SN status information to the CU-CP. The CU-CP of the source RAN node that has received the PDCP DL / UL count information may send an SN status transfer message including DL / UL SN status information to the target RAN node, as shown in operation 2-660. The CU-CP of the target RAN node that has received the SN status transfer message may send a bearer context modification request message including PDCP SN status information to the CU-UP, as shown in operation 2-670. The CU-UP of the target RAN node may respond by sending a bearer context modification response message to the CU-CP, as shown in operation 2-680. The CU-UP of the source RAN node may send a bearer context modification response message including PDCP SN status information to the CU-CP as shown in operation 2-650, while continuing to forward DL data to the target RAN node as shown in operation 2-700 and performing uplink (UL) data forwarding when a configuration request is made.

[0046] Thereafter, the CU-CP of the target RAN node may send a UE context release message to the source RAN node, as shown in operation 2-900. The CU-CP of the source RAN node that has received the message may send a bearer context release command message to the CU-UP of the source RAN node, as shown in operation 2-910. The CU-UP of the source RAN node may respond by sending a bearer context release complete message, as shown in operation 2-920. In addition, even if the CU-UP of the source RAN node has deleted the relevant information and configuration of the terminal (UE) and the terminal has not performed a handover, if there is another target RAN node for which conditional handover has been configured, a handover cancellation message is sent to allow cancellation of the pre-configured conditional handover.

[0047] Figure 3 An embodiment is shown in which the CU-UP of the source RAN node determines whether to perform early data forwarding and supports early data forwarding. When the CU-CP receives a measurement report sent by the terminal (UE) via the DU of the source RAN node in Figure 3 operation 3-100, the source RAN node may determine to perform a conditional handover (CHO) in operation 3-110 based on the signal measurement results of the terminal RAN node and other determination conditions. The source RAN node may determine potential target RAN nodes. In the case of a conditional handover, one or more potential target RAN nodes may be determined, and a process for each potential target RAN node and CHO may be executed. On the other hand, Figure 3 only includes a process for a CHO with one potential target RAN node, and the same process may be executed using another potential target RAN node. When performing DAPS HO, a process for DAPS HO with one target RAN node may be executed. After determining to perform a CHO, the CU-CP of the source RAN node may send a handover request message to the potential target RAN node, as shown in operation 3-120. In the case of a CHO, the handover request message may include an indication of the CHO initiation, target cell ID information, and content proposing data forwarding for each data radio bearer or QoS flow. For DAPS HO, an indication of DAPS HO may be included instead of the CHO indication. The CU-CP of the target RAN node that has received the CHO handover request message from the source RAN node may determine the CU-UP of the target RAN node to serve the terminal, may send a bearer context establishment request message to the CU-UP of the target RAN node, as shown in operation 3-130, and may request data forwarding information for each data radio bearer or QoS flow. One or more CU-UPs may be used for one terminal, and Figure 3It only includes procedures with one CU-UP, and when one or more CU-UPs are used, the same procedure is performed for each CU-UP. The CU-UP of the target RAN node that receives the bearer context establishment request message can perform DRB configuration for the corresponding service, etc., and can send a bearer context establishment response message including data forwarding information for each data radio bearer or QoS flow to the CU-CP of the target RAN node, as shown in operation 3-140. The CU-CP of the target RAN node determines whether to accept the handover, and then can send a handover request confirmation message to the source RAN node, as shown in operation 3-150. The message can include the information required for CHO. The CU-CP of the source RAN node can send an RRC reconfiguration message to the terminal through the DU, as shown in operation 3-160. As shown in operation 3-170, the terminal can respond by sending an RRC reconfiguration complete message. The CU-CP that has received the RRC reconfiguration complete message via the DU of the source RAN node can send a bearer context modification message to the CU-UP as shown in operation 3-180, can indicate that CHO has been configured in the message, and can include information for forwarding data transmitted from the target RAN node. When conditional handover is configured for one or more target RAN nodes or target cells, the CU-CP can determine the target RAN node or target cell for which early data forwarding is to be performed, and then can include a list of information required for data forwarding for each target RAN node or target cell. When a list of information for data forwarding is received when configuring CHO, different from the case of traditional handover, the CU-UP does not immediately start data forwarding, and can start data forwarding after the CU-UP determines whether to perform data forwarding.

[0048] Thereafter, if the CU-UP of the source RAN node determines to start early data forwarding in operation 3-200, the CU-UP of the source RAN node may send a message required for bearer context modification to the CU-CP of the source RAN node, as shown in operation 3-210. This message may include an indication to start forwarding early data to the CU-CP, count information of the first downlink (DL) PDCP packets forwarded, or both. The CU-CP of the source RAN node that has received the message required for bearer context modification from the CU-UP of the source RAN node may respond by sending a bearer context modification confirmation message to the CU-CP, as shown in operation 3-220. As shown in operation 3-230, the CU-CP of the source RAN node may send an early forwarding transmission message including the first DL count information received from the CU-UP of the source RAN node to the target RAN node configured with CHO or DAPS. The CU-CP of the target RAN node may send a bearer context modification request message including the first DL count information received from the source RAN node to the CU-UP of the target RAN node, as shown in operation 3-240. In addition, the CU-UP of the target RAN node may respond by sending a bearer context modification response message to the CU-CP of the target RAN node, as shown in operation 3-250. In the CU-UP of the target RAN node, after the terminal performs a handover to the target RAN node, when data is transmitted from the target RAN node to the terminal, the first DL count information is used for encryption. In operation 3-300, the CU-UP of the source RAN node may start forwarding the downlink (DL) data of the terminal to the target RAN node while sending a message required for bearer context modification including the first DL count to the CU-CP of the source RAN node in operation 3-210.

[0049] After that, if the source RAN node has completed the transmission of data in the DL data forwarded from the source RAN node to the target RAN node to the terminal, the source RAN node can notify the target RAN node of the data information that has been successfully transmitted when necessary, and the target RAN node stores unnecessary data to reduce resource waste. When the CU-UP of the source RAN node determines the transmission of data discard information as shown in operation 3-400, as shown in operation 3-410, the CU-UP of the source RAN node can send a message required for bearer context modification to the CU-CP of the source RAN node, which includes DL discard information related to the data information that has been successfully transmitted to the terminal from the data forwarded to the target RAN node. The CU-CP of the source RAN node can respond by sending a bearer context modification confirmation message to the CU-UP of the source RAN node, as shown in operation 3-420. As shown in operation 3-430, the CU-CP of the source RAN node that has received the message required for bearer context modification can send an early forwarding transmission message including DL discard information to the target RAN node configured with CHO or DAPS. As shown in operation 3-440, the CU-CP of the target RAN node can send a bearer context modification request message including DL discard information to the CU-UP of the target RAN node, and the CU-UP of the target RAN node can respond via a bearer context modification response message, as shown in operation 3-450.

[0050] Thereafter, according to the handover trigger conditions configured for the terminal by the source RAN node via the RRC reconfiguration message in operation 3-160, when the terminal is to perform a handover as shown in operation 3-500, the terminal can perform random access to the target cell of the target RAN node as shown in operation 3-510. After successful random access, the terminal can send an RRC reconfiguration complete message to the target RAN node, as shown in operation 3-520. As shown in operation 3-530, the CU-CP of the target RAN node that has received the RRC reconfiguration complete message from the terminal can send a handover success message indicating that the terminal has performed the handover to the source RAN node. As shown in operation 3-700, a process of updating the core network node and the data path between the RAN node for data transmission of the UE (terminal) and the core network can be performed. The CU-UP of the source RAN node can send a bearer context modification response message including PDCP SN status information to the CU-CP of the source RAN node, as shown in operation 3-550. As shown in operation 3-560, the CU-CP of the source RAN node that has received the PDCP DL / UL count information can send an SN status transfer message including DL / UL SN status information to the target RAN node. As shown in operation 3-570, the CU-CP of the target RAN node that has received the SN status transfer message can send a bearer context modification request message including PDCP SN status information to the CU-UP of the target RAN node. The CU-UP of the target RAN node can respond by sending a bearer context modification response message to the CU-CP of the target RAN node, as shown in operation 3-580. The CU-UP of the source RAN node can send a bearer context modification response message including PDCP SN status information to the CU-CP of the source RAN node, as shown in operation 3-550. As shown in operation 3-600, while continuing to forward DL data to the target RAN node, when a request is configured, uplink (UL) data forwarding can be performed simultaneously.

[0051] Thereafter, the CU-CP of the target RAN node can send a UE context release message to the source RAN node, as shown in operation 3-800. The CU-CP of the source RAN node that has received the message can send a bearer context release command message to the CU-UP of the source RAN node, as shown in operation 3-810. In addition, even if the CU-UP of the source RAN node has responded by sending a bearer context release complete message as shown in operation 3-820, and the relevant information and configuration of the terminal (UE) have been deleted, and the terminal has not performed a handover, if there is another target RAN node configured for conditional handover, a handover cancellation message is sent to allow cancellation of the pre-configured conditional handover.

[0052] Figure 4 is shown to supportFigure 2 and Figure 3 A diagram of examples of additional information required for signaling messages exchanged between CU-CP and CU-UP according to embodiments of. Figure 4 Examples of information to be additionally included in previously used bearer context modification request messages, bearer context modification response messages, and bearer context modification required messages. However, if necessary, new signaling messages can be defined and included, or Figure 4 the information of can be included in another existing signaling message.

[0053] Figure 4 (a) of is an example of information to be included in a bearer context modification request message. In Figure 4 (a) of, early data forwarding request information can be included for each data radio bearer (DRB) provided for the corresponding terminal, for use by the CU-CP of the source RAN node to request early data forwarding from the CU-UP. The CU-UP of the source RAN node can include a list of data forwarding information for each target RAN node or target cell required for data forwarding to the target RAN node. The data forwarding information is GPRS tunneling protocol (GTP) tunnel information and can include transport layer addresses and tunnel endpoint identifier (TEID) information for GRP tunnel configuration, e.g., including IP addresses and port numbers. When early data forwarding starts, first PDCP DL count request information and PDCP DL discard information request information can be included, where the first PDCP DL count request information includes whether to include the hyperframe number (HFN) and PDCP sequence number (SN) of the first downlink (DL) SDU packet forwarded by the CU-UP of the source RAN node, and the PDCP DL discard information request information enables the request for information required for data discard in the target RAN node after early data forwarding starts. The bearer context modification request message sent from the CU-CP of the target RAN node to the CU-UP can include PDCP DL discard information and the first PDCP DL count information received from the CU-CP in the source RAN node for the CU-UP to process the packets early forwarded by the CU-UP.

[0054] Figure 4 (b) of is an example of information to be included in a bearer context modification response message or a bearer context modification required message. In Figure 4In (b) of , according to the content included in the PDCP DL discard information request information and the first PDCP DL count information request information included in the bearer context modification request message sent by the CU-UP, the CU-UP of the source RAN node may include the first PDCP DL count request information and the PDCP DL discard information request information, wherein the first PDCP DL count request information includes whether the hyperframe number (HFN) and the PDCP sequence number (SN) of the first downlink (DL) SDU packets forwarded by the CU-UP for each data radio bearer (DRB) provided to the terminal are included, and the PDCP DL discard information request information enables the request for the information required for data discard in the target RAN node after the start of early data forwarding. When the CU-UP of the source RAN node determines early data forwarding, this information may be included in the bearer context modification required message sent from the CU-UP of the source RAN node to the CU-CP.

[0055] Figure 5 Shows the additional information required for the signaling messages exchanged between the CU-CP and the CU-UP to support Figure 3 Examples of embodiments of . In addition, in the additional required information, Figure 5 Shows in addition to Figure 4 The information included in. Figure 5 Is an example of the information to be additionally included in the previously used bearer context modification request message and the bearer context modification required message. However, if necessary, new signaling messages can be defined and included, or Figure 5 The information of can be included in another existing signaling message.

[0056] Figure 5 (a) of is an example of the information to be included in the bearer context modification request message. In Figure 5 In (a) of , if it is determined that conditional handover or DAPS handover is to be performed in the CU-CP of the source RAN node, CHO trigger information or DAPS indicator information may be included, wherein the CHO trigger information indicates whether conditional handover is being performed when sending the bearer context modification request message to the CU-UP, and the DAPS indicator information indicates whether DAPS handover is being performed. As shown in Figure 4 In (a) of , together with this information, the bearer context modification request message may include one or more data forwarding related information of the target RAN node for data forwarding, and the CU-UP of the source RAN node may determine the time point of early data forwarding based on the information including the CHO trigger or DAPS indicator, or may perform late data forwarding according to the subsequent determination of the CU-UP, rather than performing data forwarding in the same manner as when performing a normal handover.

[0057] Figure 5 in (b) and Figure 5 in (c) are examples of information to be included in the bearer context modification required message. If it is determined to perform early data forwarding in the CU-UP of the source RAN node, a notification that early data forwarding has started can be provided to the CU-CP by using the bearer context modification required message. The determination to start early data forwarding can be indicated by using the terminal level as shown in Figure 5 (b) or the data radio bearer (DRB) level as shown in Figure 5 (c). When early data forwarding is performed in the CU-UP of the source RAN node, the first PDCP DL count information can be sent as indicated in Figure 4 (b).

[0058] When conditional handover is configured, a RAN node can support one or more cells, so the source RAN node can request conditional handover configuration from one or more of the cells supported by a target RAN node. However, the traditional handover process supports configuring only one target cell in a one-time handover configuration process, and even when performing conditional handover, conditional handover can be configured for only one target cell in the same way during the handover process for conditional handover configuration. If it is determined to configure conditional handover for one or more target cells supported by a target RAN node, the source RAN node continues with the handover process to perform separate conditional handover configuration for each target cell constrained by the target RAN node. In the case of the same terminal, in the source RAN node, the identifier capable of identifying the terminal (e.g., source RAN UE APID) is configured to have the same value, and the target cell ID information constrained by the target is configured to be different for the handover request message transmitted to the target RAN node, so it is determined whether the handover processes with different target RAN nodes correspond to multiple CHO configurations of the same terminal.

[0059] Figure 6 and Figure 7 illustrate an embodiment for efficiently performing early data forwarding when conditional handover is configured for multiple target cells of a target RAN node of a terminal. When conditional handover is configured for multiple target cells of a target RAN node, according to the existing conditional handover configuration process, data forwarding should be performed for each target cell. In this case, the same data of the same terminal should be redundantly forwarded to the same target RAN node unnecessarily, and signaling messages (such as related early forwarding transmission messages) including the same information should be sent to the target RAN node redundantly. According to Figure 6 and Figure 7In the embodiments disclosed in [reference], when multiple target cells of a target RAN node are configured by CHO, unnecessary redundant data can be prevented from being forwarded from the source RAN node to the target RAN node, and unnecessary redundant transmission of signaling messages can be prevented. In Figure 6 and Figure 7 a RAN node can be divided into CU-CP, CU-UP, and DU, or can be configured as a single RAN entity. Figure 6 and Figure 7 show that CU-CP and DU are configured as a single entity, while CU-UP is configured as a separate entity only in the target RAN node. However, CU-CP and DU can be separated again, and the internal configuration of the source RAN node can operate regardless of whether CU-CP and DU in the target RAN node are integrated or separated, regardless of the content of the present invention.

[0060] Figure 6 shows an embodiment in which, in the case of conditional handover configuration of the same terminal, the target RAN node includes the same information in the data forwarding information included in the handover request confirmation message to respond to the source RAN node, thereby preventing the source RAN node from forwarding redundant data and redundant transmission signaling messages to the target RAN node. In Figure 6 operation 6-100 in [reference], the source RAN node can receive a measurement report sent by the terminal. Based on the signal measurement results of the RAN node of the terminal and other determination conditions, the source RAN node can determine to perform conditional handover (CHO) and determine potential target RAN nodes, as shown in operation 6-110. In the case of conditional handover, one or more potential target RAN nodes can be determined, and a process for each potential target RAN node and CHO can be executed. However, Figure 6 only includes a process for CHO with one potential target RAN node, and the same process can be executed using another potential target RAN node. In Figure 6 operation 6-110 in [reference], one or more cells in the cells supported by a target RAN node can be determined to be configured for conditional handover. After determining to perform CHO, the source RAN node can send a handover request message to the potential target RAN node as shown in operation 6-120, and in the case of CHO, the handover request message can include an indication of the initiation of CHO, target cell ID information, and content for proposing data forwarding for each data radio bearer or QoS flow. The CU-CP of the target RAN node that has received the handover request message for CHO from the source RAN node can determine the CU-UP of the target RAN node to serve the terminal, can send a bearer context establishment request as shown in operation 6-130, and can request data forwarding information for each data radio bearer or QoS flow. One terminal can use one or more CU-UPs. InFigure 6Among them, it only includes a process with one CU-UP, and if one or more CU-UPs are used, the same process can be performed for each CU-UP. The CU-UP of the target RAN node that receives the bearer context establishment request message can perform DRB configuration for the corresponding service, etc., and can send a bearer context establishment response message to the CU-CP of the target RAN node, which includes data forwarding information for each data radio bearer or QoS flow, as shown in operation 6-140. The CU-CP of the target RAN node determines whether to accept the handover, and then can send a handover request confirmation message to the source RAN node, as shown in operation 6-150. The message can include information required for CHO, such as information for data forwarding. Since it is determined in operation 6-110 to configure CHO for multiple cells of the same target RAN node, the source RAN node can send a handover request message for one target cell to the target RAN node as shown in operation 6-120, and then can send a handover request message for another target cell to the same target RAN node as in operation 6-160. Here, in the two handover request messages, the identifier used by the source RAN node to identify the terminal (e.g., the source RAN node UEAP ID) can be configured to have the same value to indicate that the target RAN node corresponds to the CHO configuration of multiple target cells of the same terminal. In the two handover request messages, the requested target cell ID information including each target cell information can be configured differently to have values corresponding to each target cell. The CU-CP of the target RAN node that has received the handover request message from the source RAN node in operation 6-160 can determine whether conditional handover has been configured for the terminal for another target cell by using the included identifier for identifying the terminal, and if conditional handover configuration has been performed for the terminal, the CU-CP of the target RAN node may not send an additional bearer context request message to the CU-UP of the target RAN node as shown in operation 6-130, and can send a handover request confirmation message to the source RAN node as shown in operation 6-170. The message can include information for the same data forwarding that has been performed for the same source RAN node in operation 6-150 for data forwarding of the terminal. The source RAN node that receives the handover request confirmation message in operation 6-170 can confirm that the same data forwarding information for the same terminal has been received from the same target RAN node, and can continue with internal configuration to enable only one data forwarding. If the CU-CP and CU-UP are separated in the source RAN node, the CU-CP of the source RAN node can perform a signaling process to transmit the data forwarding information of the same target RAN node required for one terminal to the CU-UP only once.When receiving a handover request confirmation message from the target RAN node, the source RAN node may send an RRC reconfiguration message to the terminal as shown in operation 6-180, receive an RRC reconfiguration complete message from the terminal, and complete the CHO configuration as shown in operation 6-190.

[0061] Thereafter, if it is determined in operation 6-200 to perform early data forwarding in the source RAN node and when conditional handover is configured for one or more target RAN nodes, the CU-CP may determine the target RAN node for which early data forwarding is to be performed. As shown in operation 6-210, the source RAN node may send an early forwarding transmission message including first DL count information for forwarding data to the target RAN node configured with CHO. Regardless of whether multiple target cells of one target RAN node are configured, for one terminal, only one early forwarding transmission message may be sent to one target RAN node. The CU-CP of the target RAN node may send a bearer context modification request message including the first DL count information received from the source RAN node to the CU-UP of the target RAN node, as shown in operation 6-220. The CU-UP of the target RAN node may respond by sending a bearer context modification response message to the CU-CP, as shown in operation 6-230. In the CU-UP of the target RAN node, after the terminal performs a handover to the target RAN node, when data is sent from the target RAN node to the terminal, the first DL count information may be used to perform encryption. As shown in operation 6-300, the source RAN node may start forwarding the downlink (DL) data of the terminal to the target RAN node, and at the same time, as shown in operation 6-210, send an early forwarding transmission message including the first DL count to the target RAN node.

[0062] After that, if the source RAN node has completed the transmission of data in the DL data forwarded from the source RAN node to the target RAN node to the terminal, the source RAN node can notify the target RAN node of the data information that has been successfully sent when necessary, and the target RAN node stores unnecessary data to reduce resource waste. To this end, when the source RAN node determines the transmission of data discard information in the CU-CP of the source RAN node as shown in operation 6-400, the source RAN node can send an early forwarding transmission message including DL discard information to the target RAN node configured with CHO as shown in operation 6-410. Among them, for one terminal, regardless of whether multiple target cells of a target RAN node are configured, only one early forwarding transmission message can be sent to one target RAN node. As shown in operation 6-420, the CU-CP of the target RAN node can send a bearer context modification request message including DL discard information to the CU-UP of the target RAN node, and the CU-UP of the target RAN node can respond via a bearer context modification response message as shown in operation 6-430.

[0063] Thereafter, according to the handover trigger conditions configured for the terminal by the source RAN node via the RRC reconfiguration message in operation 6-180, when the terminal is to perform a handover as shown in operation 6-500, the terminal can perform random access to the target cell of the target RAN node as shown in operation 6-510. After successful random access, the terminal can send an RRC reconfiguration complete message to the target RAN node, as shown in operation 6-520. The CU-CP of the target RAN node that has received the RRC reconfiguration complete message from the terminal can send a handover success message to the source RAN node, indicating that the terminal has performed the handover, as shown in operation 6-530, and can perform an update process of the data path between the core network and the RAN node for data transmission between the terminal (UE) and the core network node, as shown in operation 6-700. In operation 6-540, the source RAN node that has received the handover success message can send an SN status transmission message including DL / UL SN status information, etc., to the target RAN node as shown in a regular handover process. As shown in operation 6-550, the CU-CP of the target RAN node that has received the SN status transmission message can send a bearer context modification request message including PDCP SN status information to the CU-UP of the target RAN node. The CU-UP of the target RAN node can respond by sending a bearer context modification response message to the CU-CP, as shown in operation 6-560. The source RAN node can send an SN status transmission message including DL / UL SN status information to the target RAN node as shown in operation 6-540, while continuing to forward DL data to the target RAN node as shown in operation 6-600 and performing uplink (UL) data forwarding simultaneously when a configuration request is made.

[0064] Thereafter, the target RAN node sends a UE context release message to the source RAN node, as shown in operation 6-800. The source RAN node that has received the same message deletes the relevant information and configuration of the terminal (UE), and the terminal does not perform a handover. However, if there is another target RAN node for which conditional handover has been configured, a handover cancellation message is sent so that the pre-configured conditional handover can be cancelled.

[0065] Figure 7 is an embodiment in which, in the case of conditional handover configuration of the same terminal, the target RAN node includes information indicating the same data forwarding information for the handover request confirmation message in response to the source RAN node, thereby preventing the source RAN node from forwarding redundant data and redundant transmission signaling messages to the target RAN node. When the source RAN node is in Figure 7When receiving a measurement report sent by a terminal (UE) in operation 7-100, the source RAN node may determine to perform a conditional handover (CHO), and may determine potential target RAN nodes, based on the signal measurement results of the terminal RAN node and other determination conditions, as shown in operation 7-110. In the case of a conditional handover, one or more potential target RAN nodes may be determined, and a process for each potential target RAN node and CHO may be performed. However, Figure 7 only includes the process for a CHO with one potential target RAN node. However, this is merely an embodiment, and the same process may be performed for another potential target RAN node. In Figure 7 operation 7-110, one or more cell configuration conditional handovers may be determined for a cell supported by a target RAN node. After determining to perform a CHO, the source RAN node may send a handover request message to the potential target RAN node, as shown in operation 7-120. In the case of a CHO, the handover request message may include an indication of the initiation of the CHO, target cell ID information, and content for proposing data forwarding for each data radio bearer or QoS flow. The CU-CP of the target RAN node that has received the handover request message for the CHO from the source RAN node may determine the CU-UP of the target RAN node to serve the terminal, may send a bearer context establishment request message as shown in operation 7-130, and may request data forwarding information for each data radio bearer or QoS flow. One or more CU-UPs may be used for one terminal, and Figure 6Only includes a procedure with one CU-UP, and when one or more CU-UPs are used, the same procedure can be executed for each CU-UP. The CU-UP of the target RAN node that receives the bearer context establishment request message can perform DRB configuration for the corresponding service, etc., and can send a bearer context establishment response message to the CU-CP of the target RAN node, which includes data forwarding information for each data radio bearer or QoS flow, as shown in operation 7-140. The CU-CP of the target RAN node determines whether to accept the handover, and then can send a handover request confirmation message to the source RAN node, as shown in operation 7-150, where the message includes information required for CHO, such as information for data forwarding. In operation 7-110, the source RAN node has determined to configure CHO for multiple cells of the same target RAN node. Therefore, as shown in operation 7-120, the source RAN node can send a handover request message for one target cell to the target RAN node, and then, as shown in operation 7-160, can send a handover request message for another target cell to the same target RAN node. Here, in the two handover request messages, the identifier used by the source RAN node to identify the terminal (e.g., source RAN node UEAP ID) can be configured to have the same value to indicate that the target RAN node corresponds to the CHO configuration of multiple target cells of the same terminal. In the two handover request messages, the requested target cell ID information including each target cell information can be configured differently to have values corresponding to each target cell. The CU-CP of the target RAN node that has received the handover request message in operation 7-160 can determine whether conditional handover for another target cell has been configured for the terminal by using the included identifier for identifying the terminal. If conditional handover configuration has been performed for the terminal, then, as shown in operation 7-130, no additional bearer context request message is sent to the CU-UP of the target RAN node, and, as shown in operation 7-170, a handover request confirmation message can be sent to the source RAN node. The message may already include data forwarding information indication information for reuse, indicating that the data forwarding information of the terminal is used in the same way, i.e., the data forwarding information included in another handover request confirmation message, where CHO has been configured for the terminal. The source RAN node that has received the handover request confirmation message in operation 7-170 can confirm that the same data forwarding information of the same terminal has been used from the same target RAN node, and can continue with internal configuration to enable only one data forwarding. If the CU-CP and CU-UP are separated in the source RAN node, the CU-CP of the source RAN node can perform a signaling process to transmit the data forwarding information of the same target RAN node required for one terminal to the CU-UP only once.When the source RAN node receives a handover request confirmation message from the target RAN node, the source RAN node may send an RRC reconfiguration message to the terminal as shown in operation 7-180. The source RAN node may receive an RRC reconfiguration complete message from the terminal and complete the CHO configuration, as shown in operation 7-190.

[0066] Thereafter, as shown in operation 7-200, it may be determined to perform early data forwarding in the source RAN node. When conditional handover is configured for one or more target RAN nodes, the CU-CP of the source RAN node may determine the target RAN node for which early data forwarding is to be performed. As shown in operation 7-210, the source RAN node may send an early forwarding transmission message including the first DL count information of the forwarded data to the target RAN node configured with CHO. Regardless of whether multiple target cells of one target RAN node are configured, for one terminal, only one early forwarding transmission message may be sent to one target RAN node. The CU-CP of the target RAN node may send a bearer context modification request message including the first DL count information received from the source RAN node to the CU-UP of the target RAN node, as shown in operation 7-220. The CU-UP of the target RAN node may respond by sending a bearer context modification response message to the CU-CP, as shown in operation 7-230. In the CU-UP, after the terminal performs a handover to the target RAN node, when the target RAN node sends data to the terminal, the first DL count information may be used for encryption and the like. As shown in operation 7-300, the source RAN node may start forwarding the downlink (DL) data of the terminal to the target RAN node, and at the same time, send an early forwarding transmission message including the first DL count to the target RAN node, as shown in operation 7-210.

[0067] After that, if the source RAN node has completed the transmission of data in the DL data forwarded from the source RAN node to the target RAN node to the terminal, the source RAN node can, if necessary, notify the target RAN node of the data information that has been successfully sent, and the target RAN node stores unnecessary data to reduce resource waste. To this end, when the source RAN node determines the transmission of data discard information in the CU-CP of the source RAN node as in operation 7-400, the source RAN node can send an early forwarding transmission message including DL discard information to the target RAN node configured with CHO as shown in operation 7-410. At this time, for one terminal, regardless of whether multiple target cells of a target RAN node are configured, only one early forwarding transmission message can be sent to one target RAN node. As shown in operation 7-420, the CU-CP of the target RAN node can send a bearer context modification request message including DL discard information to the CU-UP, and the CU-UP can respond via a bearer context modification response message as shown in operation 7-430.

[0068] After that, according to the handover trigger condition configured for the terminal by the source RAN node via the RRC reconfiguration message in operation 7-180, the terminal can determine to perform a handover as shown in operation 7-500. As shown in operation 7-510, the terminal can perform random access to the target cell of the target RAN node. After successful random access, the terminal can send an RRC reconfiguration complete message to the target RAN node as shown in operation 7-520. The CU-CP of the target RAN node that has received the RRC reconfiguration complete message from the terminal can send a handover success message to the source RAN node indicating that the terminal has performed a handover as shown in operation 7-530, and can perform an update process of the data path between the core network and the RAN node for data transmission between the terminal (UE) and the core network node as shown in operation 7-700. In operation 7-540, the source RAN node that has received the handover success message can send an SN status transfer message including DL / UL SN status information, etc. to the target RAN node as shown in a conventional handover process. The CU-CP of the target RAN node that has received the SN status transfer message can send a bearer context modification request message including PDCP SN status information to the CU-UP as shown in operation 7-550. The CU-UP of the target RAN node can respond by sending a bearer context modification response message to the CU-CP as shown in operation 7-560. The source RAN node can send an SN status transmission message including DL / UL SN status information to the target RAN node as shown in operation 7-540, while continuing to forward DL data to the target RAN node as shown in operation 7-600 and performing uplink (UL) data forwarding when a configuration request is made.

[0069] Thereafter, as shown in operation 7-800, the target RAN node may send a UE context release message to the source RAN node. In addition, even if the source RAN node that has received the message has deleted the relevant information and configuration of the terminal (UE) and the terminal does not perform a handover, if there is another target RAN node for which conditional handover has been configured, a handover cancellation message may be sent to allow cancellation of the pre-configured conditional handover.

[0070] Figure 8 An embodiment of internal processing is shown in the case where conditional handover is configured for multiple target cells of a target RAN node of a terminal, in order to effectively perform early data forwarding when the target RAN node or the CU-CP of the target RAN node receives a handover request message from the source RAN node. In Figure 8 it, only the processing required to effectively support early data forwarding in the target RAN node is included, and other processes that may have been processed previously are not included, but these processes are required regardless of Figure 8 the process.

[0071] When the target RAN node is in Figure 8When receiving a handover request message from a source RAN node during operation 8-100, it is possible to determine whether the message is a handover request message sent for conditional handover, as shown in operation 8-200. If the message is a handover request message that only includes information for supporting existing handovers, the normal handover preparation process can be executed as shown in operation 8-700. If the message is a handover request message for conditional handover configuration in operation 8-200, it is possible to identify the identifier used by the source RAN node to identify the terminal, for example, the source RAN node UEAP ID, to determine whether conditional handover has been configured for the terminal, as shown in operation 8-300. In the case of the first handover request message for terminal conditional handover configuration, it is possible to determine whether each DRB or PDU session supports data forwarding. If data forwarding is supported, data forwarding information for receiving forwarded data can be configured, as shown in operation 8-400. As in operation 8-450, data forwarding information can be included for each DRB or PDU session, which will be included in the handover request confirmation message to be sent to the source RAN node, and the relevant terminal information and data forwarding information are stored. The remaining steps of conditional handover preparation are as shown in operation 8-600. If there is a case where conditional handover is configured for the terminal in operation 8-300, first, a handover request confirmation message is configured for the terminal, which will be sent to the source RAN node. Then, transmission can be executed, including data forwarding information for each DRB or PDU session, which is the same as the information transmitted to the source RAN node, or including indication information indicating the use of previously transmitted data forwarding information in the same manner. As shown in operation 8-600, the remaining process of conditional handover preparation can be executed.

[0072] Figure 9 Shows an example of the additional information required for the signaling messages exchanged between the source RAN node and the target RAN node to support Figure 7 the embodiments of. Figure 9 Is an example of the information to be additionally included in the previously used handover request confirmation message. As Figure 7 shown, if the target RAN node configures multiple target cells to the source RAN node for conditional handover for the same terminal, the data forwarding information for each DRB or PDU session can be configured as before and included in the handover request confirmation message for the first response, and the subsequent handover request confirmation message to be sent can include data forwarding information reuse indication information instead of data forwarding information, and this indication information indicates the use of data forwarding information in the same manner so as to be sent as Figure 9 shown. The indication information for data forwarding information reuse can be classified as UE level or DRB and PDU session level for inclusion.

[0073] Figure 10It is a diagram showing the structure of a terminal according to an embodiment.

[0074] Refer to Figure 10 , the terminal may include a radio transceiver 1010, a controller 1020, and a storage 1030. In the present invention, the controller may be defined as a circuit, an application-specific integrated circuit, or at least one processor.

[0075] The wireless transceiver 1010 may send a signal to another network entity or receive a signal from another network entity. For example, the radio transceiver 1010 may receive a signal from a base station and may send a signal including a message (such as a measurement report) to the base station.

[0076] According to the embodiment provided by the present invention, the controller 1020 may control the overall operation of the terminal. For example, the controller 1020 may control the signal flow between each block to perform operations according to the above flowchart. Specifically, according to one embodiment, the controller 1020 may control the operations provided in the present invention so as to be able to send a measurement report to the source RAN node based on a reconfiguration message received from the source RAN node, and determine whether to perform a handover according to the configured handover trigger condition.

[0077] The storage 1030 may store at least one of the information sent or received via the transceiver 1010 and the information generated via the controller 1020. For example, the storage 1030 may store information related to the measurement report, information related to the handover, etc.

[0078] Figure 11 It is a diagram showing the structure of a base station according to an embodiment.

[0079] Refer to Figure 11 , the base station may include a radio transceiver 1110, another base station / core network transceiver 1120, a controller 1130, and a storage 1140. In the present invention, the controller may be defined as a circuit, an application-specific integrated circuit, or at least one processor.

[0080] Figure 11 The base station shown in

[0081] Figure 11Base stations can be divided into a Centralized Unit (CU) and a Distributed Unit (DU), where the CU is further divided into a Control Plane (CU-CP) and a User Plane (CU-UP). A base station can include one or more CU-CPs, one or more CU-UPs, and one or more DUs. The CU-CP, CU-UP, and DU that make up a base station can be configured together. For example, the CU where the CU-CP and CU-UP are implemented together and the DU can be included in one base station; the CU-CP and DU can be implemented together in another base station, while the CU-UP is configured separately; another base station can be configured in the form of an integrated base station where the CU-CP, CU-UP, and DU are implemented together; and a base station can be configured through other combinations.

[0082] The radio transceiver 1110 can send signals to or receive signals from another network entity. The transceiver 1110 can send signals to or receive signals from a terminal, or can send signals including messages (such as RRE reconfiguration for controlling the operation of the terminal).

[0083] Another base station / core network transceiver 1120 can send signals to or receive signals from another network entity. For example, a source RAN node and a target RAN node can send and receive signals including messages (such as a handover request message for requesting a handover and a handover request acknowledgment message for responding to the handover request), and the source RAN node and the target RAN node can send and receive user data for data forwarding.

[0084] According to the embodiments provided by the present invention, the controller 1130 can control the overall operation of the base station. For example, the controller 1130 can control the signal flow between the respective blocks to perform operations according to the above flowcharts. Specifically, according to an embodiment, when the source RAN node configures conditional handovers for multiple target cells of the target RAN node, the controller 1130 can control the operations provided in the present invention to effectively perform early data forwarding in the target RAN node.

[0085] If the base station is divided into a CU and a DU, and the CU is divided into a CU-CP and a CU-UP, then the controller 1130 can control the transmission of messages and / or information between the CU-CP, CU-UP, and DU.

[0086] Specifically, during a specific handover process, the controller 1130 transmits a first message for a bearer context modification request (the first message includes information about an early forwarding count request and is transmitted from the CU-UP to the CU-CP) and a second message for a bearer context modification response (the second message includes information about a first Downlink (DL) count) from the Centralized Unit Control Plane (CU-CP) to the Centralized Unit User Plane (CU-UP).

[0087] The controller 1130 controls the transceiver to send a third message including information about the first DL count from the CU-CP to the second base station.

[0088] In this case, the information about the first DL count is sent to the CU-CP of the second base station, and the information about the first DL count is transmitted from the CU-CP of the second base station to the CU-UP of the second base station using a fourth message for carrying the context modification request.

[0089] The controller 1130 uses the CU-CP to determine a request for information about DL discard, transmits a fifth message for carrying the context modification request from the CU-CP to the CU-UP, the fifth message being used to request information about DL discard, and transmits a sixth message for carrying the context modification response from the CU-UP to the CU-CP, the sixth message including information about DL discard.

[0090] The controller 1130 controls the transceiver to send a seventh message including information about DL discard from the CU-CP to the CU-CP of the second base station, wherein the information about DL discard is transmitted from the CU-CP of the second base station to the CU-UP of the second base station.

[0091] In this case, the information about DL discard indicates the DL count value of the last data successfully transmitted to the terminal.

[0092] The controller 1130 uses the CU-UP to initiate a process for transmitting information about DL discard and transmits an eighth message including information about DL discard from the CU-UP to the CU-CP, wherein the information about DL discard indicates the DL count of the last data successfully transmitted to the terminal.

[0093] Here, the specific handover process includes conditional handover (CHO) or dual active protocol stack (DAPS) handover.

[0094] The storage 1140 can store at least one of the information sent or received via the radio transceiver 1110 and other base station / core network transceivers 1120, and the information generated via the controller 1130. For example, the storage 1140 can store information for conditional handover and information for effectively performing early data forwarding.

[0095] In the above detailed embodiments of the present invention, according to the proposed detailed embodiments, the elements included in the present invention are represented in singular or plural. However, for ease of description, the singular form or the plural form is appropriately selected according to the presented situation, and the present invention is not limited by the elements represented in singular or plural. Therefore, the elements represented in plural may also include a single element, or the elements represented in singular may also include multiple elements.

[0096] Although the present invention has been described with various embodiments, various changes and modifications can be suggested to those skilled in the art. The present invention is intended to embrace changes and modifications that fall within the scope of the appended claims.

Claims

1. A method for a first base station in a wireless communication system, the method comprises: During a handover process, transmitting, from a Centralized Unit Control Plane (CU-CP) of the first base station to a Centralized Unit User Plane (CU-UP) of the first base station, a first message for carrying a bearer context modification request, the first message including information about an early forwarding count request; Transmitting, from the CU-UP to the CU-CP, a second message for carrying a bearer context modification response, the second message including information about a Downlink (DL) discard, the information about the DL discard indicating a DL count value associated with data successfully transmitted to a terminal based on the information about the early forwarding count request; and Transmitting, from the CU-CP to a second base station, a third message including the information about the DL discard.

2. The method according to claim 1, wherein, the information about the DL discard indicates the DL count value of the last data successfully transmitted to the terminal, and is transmitted, using a fourth message for carrying a bearer context modification request, from a CU-CP of the second base station to a CU-UP of the second base station.

3. The method according to claim 1, wherein, the second message further includes information about a first DL count, and wherein the third message further includes the information about the first DL count.

4. The method according to claim 3, wherein, the information about the first DL count is transmitted, using a fifth message for carrying a bearer context modification request, from a CU-CP of the second base station to a CU-UP of the second base station.

5. The method according to claim 1, further comprises: Initiating, by the CU-UP, a process for transmitting the information about the DL discard, the information about the DL discard indicating the DL count value of the last data successfully transmitted to the terminal; and Transmitting, from the CU-UP to the CU-CP, a sixth message including the information about the DL discard, the information about the DL discard indicating the DL count value of the last data successfully transmitted to the terminal.

6. The method according to claim 1, wherein, the handover process includes a Conditional Handover (CHO) or a Dual Active Protocol Stack (DAPS) handover.

7. A first base station for a wireless communication system, the first base station comprises: a transceiver; and at least one processor operatively connected to the transceiver, the at least one processor being configured to: During a handover process, transmit, from a Centralized Unit Control Plane (CU-CP) of the first base station to a Centralized Unit User Plane (CU-UP) of the first base station, a first message for carrying a bearer context modification request, the first message including information about an early forwarding count request, Transmit, from the CU-UP to the CU-CP, a second message for carrying a bearer context modification response, the second message including information about a Downlink (DL) discard, the information about the DL discard indicating a DL count value associated with data successfully transmitted to a terminal based on the information about the early forwarding count request, and Control the transceiver to send a third message including information about the DL discard from the CU-CP to the second base station.

8. The first base station according to claim 7, wherein, The information about the DL discard indicates the DL count value of the last data successfully transmitted to the terminal, and is transmitted from the CU-CP of the second base station to the CU-UP of the second base station using a fourth message for carrying a context modification request.

9. The first base station according to claim 7, wherein, The second message further includes information about a first DL count, and wherein the third message further includes information about the first DL count.

10. The first base station according to claim 9, wherein, The information about the first DL count is transmitted from the CU-CP of the second base station to the CU-UP of the second base station using a fifth message for carrying a context modification request.

11. The first base station according to claim 7, wherein, The at least one processor is further configured to: Initiate a process for sending information about the DL discard by the CU-UP, the information about the DL discard indicating the DL count value of the last data successfully transmitted to the terminal, and Transmit a sixth message including information about the DL discard from the CU-UP to the CU-CP, the information about the DL discard indicating the DL count value of the last data successfully transmitted to the terminal.

12. The first base station according to claim 7, wherein, The handover process includes a conditional handover CHO or a dual active protocol stack DAPS handover.

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