Method and apparatus for controlling interruption time to be reduced for terminal in which l3 handover and l1 / l2 triggered mobility (LTM) are configured, in wireless communication system

Abstract

The present disclosure relates to a 5G or 6G communication system for supporting a higher data transmission rate than a 4G communication system, such as LTE. According to the present disclosure, when L3 handover and LTM are configured for an arbitrary terminal, L3 HO or LTM can be performed in consideration of LTM preparation, and thus, an interruption time may be reduced.

Description

Method and device for controlling the reduction of interruption time for a terminal configured with L3 handover and L1 / L2 triggered mobility (LTM) in a wireless communication system

[0001] The present disclosure relates to the operation of a terminal and a base station in a wireless communication or mobile communication system. In particular, it relates to the operation of a terminal and a base station to resolve problems that may arise when L3 handover and LTM are simultaneously configured in a terminal.

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

[0003] The 6G communication system, expected to be realized around 2030, will have a maximum transmission speed of terabytes per second (i.e., 1,000 gigabits per second) and a wireless latency of 100 microseconds (μsec). In other words, compared to 5G, the transmission speed in a 6G communication system will be 50 times faster, while the wireless latency will be reduced to one-tenth.

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

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

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

[0007] Meanwhile, the need for a solution to solve problems that may arise when layer 3 (L3) handover and LTM (L1 / L2 triggered mobility) handover are established simultaneously has arisen.

[0008] The purpose of the present disclosure is to propose a method and device for solving a problem in which L3 handover (HO) is performed without considering LTM preparation when L3 handover (HO) and LTM are set for an arbitrary terminal.

[0009] In a wireless communication system according to one embodiment of the present disclosure, a method performed by a source distributed unit (DU) entity may include the steps of: receiving, from a terminal, a measurement report message including information indicating whether at least one neighboring cell measured by the terminal is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell; transmitting the measurement report message to a centralized unit (CU) entity; receiving an LTM trigger request message from the CU entity based on the measurement report message; determining, based on reception of the LRM trigger request message, whether an LTM cell switch command medium access control (MAC) control element (CE) can be transmitted to the terminal; and transmitting an LTM trigger response message to the CU entity if it is determined that the LTM cell switch command MAC CE transmission is possible.

[0010] Meanwhile, in a wireless communication system according to another embodiment of the present disclosure, a method performed by a centralized unit (CU) entity may include the steps of: receiving a measurement report message from a source distributed unit (DU) entity; determining, based on the measurement report message, whether a target DU entity is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell; transmitting, to the source DU entity, an LTM trigger request message if the target DU entity is determined to be the LTM candidate cell; and receiving, based on the LRM trigger request message, an LTM trigger response message from the source DU entity.

[0011] Meanwhile, in a wireless communication system according to another embodiment of the present disclosure, a source distributed unit (DU) entity may include a control unit configured to receive, from a transceiver and a terminal, a measurement report message including information indicating whether at least one neighboring cell measured by the terminal is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell, through the transceiver, transmit the measurement report message to a centralized unit (CU) entity, receive an LTM trigger request message from the CU entity based on the measurement report message, determine whether an LTM cell switch command medium access control (MAC) control element (CE) can be transmitted to the terminal based on reception of the LRM trigger request message, and control to transmit an LTM trigger response message to the CU entity if it is determined that the LTM cell switch command MAC CE transmission is possible.

[0012] Meanwhile, in a wireless communication system according to another embodiment of the present disclosure, a centralized unit (CU) entity may include a control unit configured to receive a measurement report message from a source distributed unit (DU) entity through a transceiver and the transceiver, determine based on the measurement report message whether a target DU entity is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell, and, if the target DU entity is determined to be the LTM candidate cell, transmit an LTM trigger request message to the source DU entity, and control to receive an LTM trigger response message from the source DU entity based on the LRM trigger request message.

[0013] According to the present disclosure, when L3 handover and LTM are set for any terminal, L3 HO or LTM can be performed by taking LTM preparation into consideration, so that interruption time can be reduced.

[0014] Figure 1 is a diagram illustrating the interruption time of a handover process that may generally occur.

[0015] Figure 2 is a diagram showing the procedure of L3 handover according to the start of work.

[0016] Figure 3 is a diagram showing the procedure of LTM according to the start of work.

[0017] FIG. 4 is a diagram for explaining a problem that may occur in an environment where L3 HO and LTM are set simultaneously by the proposed priority method according to one embodiment of the present disclosure;

[0018] FIG. 5 is a diagram illustrating an LTM cell switch request procedure according to an embodiment of the present disclosure;

[0019] FIG. 6 is a diagram showing information indicating whether a neighboring cell measured by a terminal is an LTM candidate cell or not, which may be included in an L3 measurement report, according to one embodiment of the present disclosure;

[0020] Figure 7 is a drawing showing the specific contents of messages proposed in the present disclosure;

[0021] FIG. 8 is a diagram showing a successfully performed LTM trigger request procedure according to one embodiment of the present disclosure;

[0022] FIG. 9 is a diagram illustrating a case where an LTM trigger request procedure fails according to an embodiment of the present disclosure;

[0023] FIG. 10 is a diagram illustrating a message structure in which 'LTMCandidateCellFlag' is transmitted according to one embodiment of the present disclosure;

[0024] FIG. 11 is a diagram illustrating an example of a successful LTM trigger procedure performed when a terminal transmits an L3 measurement report including an 'LTMCandidateCellFlag' IE as disclosed in FIG. 10 according to an embodiment of the present disclosure;

[0025] FIG. 12 is a diagram illustrating an embodiment in which a successful LTM trigger procedure is performed when a terminal transmits an L3 measurement report including an 'LTMCandidateCellFlag' IE as disclosed in FIG. 10 according to another embodiment of the present disclosure;

[0026] FIG. 13 is a diagram illustrating an example in which an LTM trigger procedure fails (e.g., is not successful) when a terminal transmits an L3 measurement report including an 'LTMCandidateCellFlag' IE as disclosed in FIG. 10 according to an embodiment of the present disclosure;

[0027] FIG. 14 is a block diagram illustrating the structure of a terminal according to an embodiment of the present disclosure, and

[0028] FIG. 15 is a block diagram illustrating the structure of a base station according to an embodiment of the present disclosure.

[0029] The operating principles of the present invention will be described in detail below with reference to the attached drawings. In the following description of the present invention, detailed descriptions of known functions or components will be omitted if they are deemed to unnecessarily obscure the gist of the invention. Furthermore, the terms described below are defined based on their functions in the present invention and may vary depending on the intentions or practices of the user or operator. Therefore, their definitions should be based on the overall content of this specification.

[0030] In the following description of the present invention, detailed descriptions of known functions or configurations will be omitted if they are deemed to unnecessarily obscure the gist of the present invention. Hereinafter, embodiments of the present invention will be described with reference to the attached drawings.

[0031] The terms used in the following description to identify connection nodes, terms referring to messages, terms referring to interfaces between connection nodes, and terms referring to various identification information are provided for convenience of explanation. Therefore, the present invention is not limited to the terms described below, and other terms that refer to objects with equivalent technical meanings may be used.

[0032] For convenience of explanation, the present invention uses terms and names defined in the 3rd Generation Partnership Project Long Term Evolution (3GPP) LTE standard. However, the present invention is not limited to these terms and names and can be equally applied to systems conforming to other standards. In the present invention, the term "eNB" may be used interchangeably with "gNB" for convenience of explanation. For example, a base station described as an eNB may also represent a gNB.

[0033] To support terminal mobility in wireless communication systems, various mobility management methods, including L3 handover, are supported. However, interruption times may occur during the handover process to support terminal mobility.

[0034] FIG. 1 is a diagram illustrating an interruption time of a handover process that may generally occur. For example, FIG. 1 illustrates an interruption time in a handover process that a terminal may experience. The interruption time may be defined as the time from the time the terminal receives a handover command message from a base station to the time the terminal first transmits uplink (UL) data to a target cell. In this case, the interruption time may include UE reconfiguration, downlink (DL) synchronization, and UL synchronization (e.g., RACH procedure), as illustrated in FIG. 1.

[0035] In wireless communication systems, various methods have been proposed to reduce the interruption time that can occur during mobility management, including handover. For example, there is a method called conditional handover (CHO). However, because DL synchronization and UL synchronization account for a significant portion of the interruption time, proposed methods, such as CHO, may be minimally effective.

[0036] Accordingly, L1 / L2 triggered mobility (LTM) was designed to reduce interruption time.

[0037] Specifically, FIG. 2 is a diagram showing a procedure of L3 handover according to work initiation, and FIG. 3 is a diagram showing a procedure of LTM according to work initiation.

[0038] To summarize the L3 handover procedure according to one embodiment, as illustrated in FIG. 2, a terminal (200) existing in an RRC (radio resource control) connected state at S200 can transmit an L3 measurement report to a source DU (distributed unit) (210) at S210. For example, the terminal (200) can perform measurements on at least one neighboring cell and report the measurement results.

[0039] In step S220, the L3 measurement report can be transmitted from the source DU (210) to the CU (centralized unit) (230).

[0040] In step S230, if L3 handover is determined by the CU (230) based on the L3 measurement report, the CU (230) can transmit an L3 HO request to the target DU (220) in S240. And, in S245, the target DU (220) can transmit an L3 HO acknowledge to the CU (230).

[0041] In step S250, the CU (230) may transmit a UE context modification request to the source DU (210), and in step S255, the source DU (210) may transmit a UE context modification acknowledge to the CU (230).

[0042] And when the source DU (210) transmits an RRC reset message to the terminal (200) in step S260 and the terminal (200) transmits an RRC reset complete message to the source DU (210) in step S265, the RACH procedure may be performed in step S270. The RACH procedure of S270 may include steps such as the terminal (200) transmitting a PRACH preamble to the target DU (220), the terminal (200) receiving MSG2 from the target DU (220), and the terminal (200) transmitting MSG 3 to the target DU (220).

[0043] In the L3 HO of FIG. 2, an interruption time as described above may occur during the process of performing DL synchronization and UL synchronization procedures with the target DU after the terminal receives the HO command based on the RRC reset message.

[0044] FIG. 3 is a diagram illustrating a proposed LTM procedure for reducing the interruption time. The LTM procedure is summarized based on FIG. 3 . At S300, a terminal (300) in an RRC connected state can transmit a measurement report to a base station (310) at S305. The base station (310) may be an nr base station, such as a gNB. According to an embodiment, the base station (310) may be configured with one CU and multiple DUs (e.g., a source DU and a target DU).

[0045] In step S310, the base station (310) may perform an LTM candidate preparation procedure. Furthermore, the base station (310) may transmit information about LTM candidate settings via an RRC reset message in step S315. For example, the base station (310) may generate information about the LTM candidate settings through the LTM candidate preparation procedure and transmit the generated information about the LTM candidate settings to the terminal (300).

[0046] The terminal (300) that has received the RRC reset message can transmit an RRC reset completion message to the base station (310). At this time, S305 to S320 may be LTM preparation procedures.

[0047] Step S325 is an early synchronization procedure, in which the terminal (300) can perform DL / UL synchronization with at least one candidate cell. For example, the terminal (300) can identify at least one candidate cell based on the information about the LTM candidate configuration received from the base station (310) in S315, and perform DL / UL synchronization with the at least one candidate cell.

[0048] In S330, the terminal (300) can transmit an L1 measurement report to the base station (310). If the LTM decision is performed by the base station (310) in S335, the base station (310) can transmit a cell switch command to the terminal (300) using a medium access control (MAC) control element (CE). At this time, the terminal (300) that receives the MAC CE can be separated from the source and apply target settings in step S345. In step S350, the terminal (300) can perform a RACH procedure. The steps S330 to S350 may be LTM execution procedures.

[0049] And LTM can be completed in S355 as an LTM completion procedure.

[0050] As illustrated in Fig. 3, in LTM, the early synchronization procedure allows the terminal (300) to be synchronized in advance before receiving the cell switch command, so that the interruption time can be reduced.

[0051] In addition, LTM has the advantage of not requiring L3 layer processing compared to L3 HO because such cell switch commands are transmitted through MAC CE based on L1 measurement reports (e.g., by generating a command message in the MAC layer).

[0052] Meanwhile, the network can simultaneously configure LTM and L3 handovers for any terminal. Since LTM and L3 handovers are performed at the network level, terminals must manage the priority between LTM and L3 handovers. Typically, the priority between LTM and L3 handovers can be set in the following three scenarios.

[0053] 1. If L3 handover is performed before LTM (e.g., if the UE receives the L3 handover command message before the gNB-DU transmits the LTM cell switch command MAC CE), L3 handover has higher priority.

[0054] 2. If LTM is performed before L3 handover (e.g., if the UE receives the LTM notify message before the gNB-CU sends the L3 handover command message), LTM has a higher priority.

[0055] 3. If LTM and L3 handovers are performed almost simultaneously (e.g., after the gNB-DU transmits the LTM cell switch command MAC CE, the UE receives the L3 handover command message), LTM has a higher priority.

[0056] The proposed priority scheme above basically gives higher priority to the handover message transmitted earlier, but if the L3 HO command and the LTM cell switch MAC CE are transmitted simultaneously, the LTM is given higher priority. However, the above priority management scheme can only cover the priority handling method between handover messages after the HO command message (e.g., L3 handover and LTM cell switch MAC CE) is generated. In addition, the handover decision may be performed depending on the implementation at the network level. In this case, even if the network is ready to support LTM to the UE, the L3 handover may proceed, which may nullify the benefits of LTM (e.g., significantly reduced interruption time compared to L3 handover).

[0057] Specifically, FIG. 4 is a diagram illustrating a problem that may occur in an environment where L3 HO and LTM are set simultaneously by the proposed priority method according to one embodiment of the present disclosure.

[0058] For example, a terminal (400) existing in an RRC connected state in S400 may be a terminal in which both L3 HO and LTM are set.

[0059] At S405, the terminal (400) can transmit an L3 measurement report to the source DU (410). And at S410, the source DU (410) can transmit the L3 measurement report to the CU (430).

[0060] In S415, an LTM candidate preparation procedure may be performed between the source DU (410) and CU (430).

[0061] At S420, the source DU (410) can transmit LTM configuration information to the terminal (400) via an RRC reset message. And at S425, the terminal (400) can transmit an RRC reset complete message to the source DU (410).

[0062] As described above in FIG. 3, as an early synchronization procedure in S430, the terminal (400) can perform DL / UL synchronization with at least one candidate cell. For example, the terminal (400) can identify at least one candidate cell based on the LTM configuration information received from the source DU (410) in S420, and perform DL / UL synchronization with the at least one candidate cell.

[0063] At S435, the terminal (400) can transmit an L3 measurement report triggered by event A3 to the source DU (410). And at S440, the L3 measurement report triggered by event A3 can be transmitted from the source DU (410) to the CU (430).

[0064] Based on the L3 measurement report triggered by the above event A3 in S445, the CU (430) can make an L3 HO decision.

[0065] In step S450, the CU (430) may transmit a UE context modification request to the source DU (410), and in step S455, the source DU (410) may transmit a UE context modification acknowledge to the CU (430).

[0066] And in step S460, the CU (430) can transmit an RRC reset message including an L3 HO command to the terminal (400).

[0067] Thereafter, a RACH procedure may be performed at step S465. The RACH procedure of S465 may include steps such as the terminal (400) transmitting a PRACH preamble to the target DU (420), the terminal (400) receiving MSG2 from the target DU (420), and the terminal (400) transmitting MSG 3 to the target DU (420).

[0068] As described in the above procedure, the network and the terminal can complete preparation for LTM support through the LTM preparation phase. (For example, steps S415 to S430) After that, when the terminal transmits an L3 measurement report to the base station (for example, the source DU), the source DU can forward the received L3 measurement report to the CU. And the CU generates an L3 handover command based on the received L3 measurement report and forwards it to the terminal, so the terminal can perform an L3 handover rather than an LTM based on the above-described priority method.

[0069] Accordingly, even though LTM preparation is performed, the terminal must perform the RACH procedure unnecessarily due to the performance of L3 HO, so the interruption time is not reduced.

[0070] Therefore, the present disclosure proposes a method in which, when a CU receives an L3 measurement report and determines that a handover is necessary, it can consider whether an LTM trigger is required instead of immediately transmitting an L3 handover. According to one embodiment, the CU can request the DU to transmit an LTM cell switch command MAC CE, taking into account the state of the DU (LTM ready state).

[0071] FIG. 5 is a diagram illustrating an LTM cell switch request procedure according to one embodiment of the present disclosure.

[0072] For example, a terminal (500) existing in an RRC connected state in S500 may be a terminal in which both L3 HO and LTM are set.

[0073] In S505, the terminal (500) can transmit an L3 measurement report to the source DU (510). And in S510, the source DU (510) can transmit the L3 measurement report to the CU (530).

[0074] In S515, an LTM candidate preparation procedure may be performed between the source DU (510) and CU (530).

[0075] In S520, the source DU (510) can transmit LTM configuration information to the terminal (500) via an RRC reset message. In addition, in S525, the terminal (500) can transmit an RRC reset complete message to the source DU (510).

[0076] As an early synchronization procedure in S530, the terminal (500) can perform DL / UL synchronization with at least one candidate cell. For example, the terminal (500) can identify at least one candidate cell based on the LTM configuration information received from the source DU (510) in S520, and perform DL / UL synchronization with the at least one candidate cell.

[0077] In S535, the terminal (500) may transmit an L3 measurement report triggered by event A3 to the source DU (510). At this time, the terminal (500) may include information (e.g., an information element (IE)) indicating whether the measured DU is an LTM candidate cell or not. The specific contents of the information indicating whether the measured DU is an LTM candidate cell will be described later.

[0078] And in S540, the L3 measurement report triggered by the above event A3 can be transmitted from the source DU (510) to the CU (530).

[0079] In S545, the CU (530) can parse the L3 measurement report (MR). For example, the CU (530) can determine whether a handover is required for the terminal (500) based on the received L3 measurement report. In addition, the CU (530) can determine whether the target DU (520) is an LTM candidate cell. For example, the CU (530) can determine whether the target DU (520) is an LTM candidate cell based on information indicating whether the DU measured by the terminal (500) included in the L3 measurement report is an LTM candidate cell.

[0080] In step S550, the CU (530) may transmit an LTM trigger request. For example, if the target DU (520) is determined to be an LTM candidate cell, the CU (530) may transmit an LTM trigger request to the source DU (510).

[0081] In step S555, the CU (530) may receive an LTM trigger response from the source DU (510). For example, it may be determined that the source DU (510) can transmit an LTM cell switch command MAC CE after receiving an LTM TRIGGER REQUEST message. At this time, the source DU (510) may transmit an LTM TRIGGER RESPONSE to the CU (530). At this time, the possibility of the source DU (510) transmitting the LTM cell switch command MAC CE may be determined based on whether the source DU (510) can include a field value in the MAC CE (for example, at least one of target configuration ID, TA command, TCI state ID, and BWP ID).

[0082] And in S560, the source DU (510) can transmit an LTM cell switch command MAC CE to the terminal (500).

[0083] Meanwhile, FIG. 6 is a diagram showing information indicating whether a neighboring cell measured by a terminal that can be included in an L3 measurement report is an LTM candidate cell according to one embodiment of the present disclosure.

[0084] According to one embodiment of the present disclosure, information indicating whether the neighboring cell measured by the terminal is an LTM candidate cell may be 'LTMCandidateCellFlag', an IE that may be included in a UL-DCCH-Message. The IE may be Boolean type data. For example, if 'LTMCandidateCellFlag' is true, it indicates that the neighboring cell measured by the terminal is an LTM candidate cell, and the opposite case (e.g., false) may indicate that the measured neighboring cell is not an LTM candidate cell. The IE allows the CU to avoid a separate search operation to determine whether the neighboring cell measured by the terminal is an LTM candidate cell while parsing the L3 measurement report. For example, the CU can determine whether the neighboring cell is an LTM candidate cell based on the 'LTMCandidateCellFlag' IE without performing a separate search operation.

[0085] Meanwhile, the present disclosure proposes signaling between CU and DU.

[0086] For example, a CU may transmit a message requesting a DU to transmit an LTM cell switch command MAC CE. The message may be an LTM TRIGGER REQUEST message. However, the LTM TRIGGER REQUEST message is only an example, and the specific message name may vary. The LTM TRIGGER REQUEST message is a message that a CU transmits to a DU (e.g., a source DU), and its purpose is to request that the DU transmit an LTM cell switch command MAC CE to a terminal.

[0087] Additionally, the LTM TRIGGER RESPONSE message may be a message transmitted from the DU to the CU. The LTM TRIGGER RESPONSE message may be a message indicating that LTM triggering has been successfully performed. The LTM TRIGGER RESPONSE message is merely an example, and the specific message name may vary.

[0088] Meanwhile, the LTM TRIGGER FAILURE message may be a message transmitted from the DU to the CU. The LTM TRIGGER FAILURE message may be a message indicating that LTM triggering has failed. The LTM TRIGGER FAILURE message is merely an example, and the specific message name may vary.

[0089] FIG. 7 is a diagram illustrating specific contents of messages proposed in the present disclosure. According to one embodiment, the LTM TRIGGER REQUEST message, the LTM TRIGGER RESPONSE message, and the LTM TRIGGER FAILURE message may include at least one of a message type, a gNB-CU UE F1AP ID, a gNB-DU UE F1AP ID, and NR PCI. The message type is an IE that may indicate whether the transmitted message is an LTM TRIGGER REQUEST or an LTM TRIGGER RESPONSE. The gNB-CU UE F1AP ID and the gNB-DU UE F1AP ID may be identifiers (IDs) of terminals used in the CU and the DU, respectively. In addition, the NR PCI may indicate a PCI (physical cell ID) of a target cell.

[0090] Additionally, the LTM TRIGGER FAILURE message may include a CAUSE IE. For example, the CAUSE IE may include a purpose of informing the CU of the reason for which LTM triggering failed.

[0091] FIG. 8 is a diagram illustrating a successfully performed LTM trigger request procedure according to one embodiment of the present disclosure.

[0092] In step S820, the CU (800) may transmit an LTM TRIGGER REQUEST message to the DU (810). At this time, the DU (810) that receives the LTM TRIGGER REQUEST message may determine whether it can transmit an LTM cell switch command MAC CE. For example, the DU (810) may determine whether it can include (e.g., fill in) a field value (e.g., at least one of target configuration ID, TA command, TCI state ID, and BWP ID) in the LTM cell switch command MAC CE. If it can be included, the DU (810) may determine that it can transmit the LTM cell switch command MAC CE.

[0093] And in step S830, the DU (810) can transmit an LTM TRIGGER RESPONSE message to the CU (800). For example, if it is determined as a result of the judgment that the DU (810) can transmit an LTM cell switch command MAC CE, the DU (810) can transmit an LTM TRIGGER RESPONSE message to the CU (800).

[0094] Meanwhile, FIG. 9 is a diagram showing a case where the LTM trigger request procedure fails according to one embodiment of the present disclosure.

[0095] In step S920, the CU (900) may transmit an LTM TRIGGER REQUEST message to the DU (910). At this time, the DU (910) that receives the LTM TRIGGER REQUEST message may determine whether it can transmit the LTM cell switch command MAC CE. For example, the DU (910) may determine whether it can include (e.g., fill in) a field value (e.g., at least one of target configuration ID, TA command, TCI state ID, and BWP ID) in the LTM cell switch command MAC CE. If it cannot be included, the DU (910) may determine that it cannot transmit the LTM cell switch command MAC CE.

[0096] If DU (910) determines that it cannot transmit LTM cell switch command MAC CE in step S930, it can generate an LTM TRIGGER FAILURE message including CAUSE information and transmit it to CU (900).

[0097] In one embodiment, if an abnormal condition such as a syntax error occurs, the DU (910) may operate as an unsuccessful operation.

[0098] Meanwhile, FIG. 10 is a diagram illustrating a message structure in which 'LTMCandidateCellFlag' is transmitted according to one embodiment of the present disclosure.

[0099] As described above, 'LTMCandidateCellFlag' may be an IE indicating whether at least one neighboring cell measured by the UE is an LTM candidate cell or not. The 'LTMCandidateCellFlag' IE may be included in an RRC reconfiguration message. For example, 'LTMCandidateCellFlag' may be included as a BOOLEAN in the MeasResultNR of the RRC reconfiguration message. If 'LTMCandidateCellFlag' is true, it indicates that the neighboring cell measured by the UE is an LTM candidate cell, and the opposite case (e.g., false) may indicate that the measured neighboring cell is not an LTM candidate cell. The IE allows the CU to avoid a separate search operation to determine whether the neighboring cell measured by the UE is an LTM candidate cell while parsing the L3 measurement report.

[0100] FIG. 11 is a diagram illustrating an example of a successful LTM trigger procedure performed when a terminal transmits an L3 measurement report including an 'LTMCandidateCellFlag' IE as disclosed in FIG. 10 according to an embodiment of the present disclosure.

[0101] Any terminal (1100) that has performed DL / UL synchronization with at least one candidate cell as in the above-described embodiment can transmit an L3 measurement report triggered by event A3 to the source DU (1110) in step S1140. At this time, the terminal (1100) can include information (e.g., information element (IE)) indicating whether the measured DU is an LTM candidate cell or not. For example, as illustrated in FIG. 11, MeasResultNR included in an RRC message transmitted by the terminal (1100) can include information on the PCI of the target cell (e.g., 'physcellid': 205 in FIG. 11) and 'LTMCandidateCellFlag' indicated as 'true'. In the present disclosure, an L3 measurement report including 'LTMCandidateCellFlag' is referred to as a modified L3 measurement report according to one embodiment.

[0102] And at S1145, the modified L3 measurement report triggered by the above event A3 can be transmitted from the source DU (1110) to the CU (1130).

[0103] In S1150, the CU (1130) can parse the modified L3 measurement report (MR). For example, the CU (1130) can determine whether a handover is required for the terminal (1100) based on the received modified L3 measurement report. In addition, the CU (1130) can determine whether the target DU (1120) is an LTM candidate cell. For example, in the embodiment illustrated in FIG. 11, the CU (1130) can determine the target DU (1120) with a 'physcellid' of 205 as an LTM candidate cell based on the 'LTMCandidateCellFlag' in which the DU with a 'physcellid' of 205 measured by the terminal (1100) included in the modified L3 measurement report is marked as 'true'.

[0104] In step S1155, the CU (1130) may transmit an LTM trigger request. For example, if it is determined that the target DU (1120) is an LTM candidate cell, the CU (1130) may transmit an LTM trigger request message to the source DU (1110). The LTM trigger request message may include at least one of a message type, a gNB-CU UE F1AP ID, a gNB-DU UE F1AP ID, and an NR PCI (e.g., 205).

[0105] In step S1160, the CU (1130) may receive an LTM trigger response from the source DU (1110). For example, it may be determined that the source DU (1110) can transmit an LTM cell switch command MAC CE after receiving an LTM TRIGGER REQUEST message. At this time, the source DU (1110) may transmit an LTM TRIGGER RESPONSE to the CU (1130). At this time, the possibility of the source DU (1110) transmitting the LTM cell switch command MAC CE may be determined based on whether the source DU (1110) can include (fill in) a field value in the MAC CE (for example, at least one of target configuration ID, TA command, TCI state ID, and BWP ID).

[0106] And in S1165, the source DU (1110) can transmit an LTM cell switch command MAC CE to the terminal (1100).

[0107] Meanwhile, FIG. 12 is a diagram showing an example in which a successful LTM trigger procedure is performed when a terminal transmits an L3 measurement report including an 'LTMCandidateCellFlag' IE as disclosed in FIG. 10 according to another embodiment of the present disclosure.

[0108] For example, a terminal (1200) existing in an RRC connected state in S1200 may be a terminal in which both L3 HO and LTM are set.

[0109] At S1205, the terminal (1200) can transmit an L3 measurement report to the source DU (1210). And at S1210, the source DU (1210) can transmit the L3 measurement report to the CU (1230).

[0110] In S1215, an LTM candidate preparation procedure may be performed between the source DU (1210) and the CU (1230).

[0111] In S1220, the source DU (1210) can transmit LTM configuration information to the terminal (1200) via an RRC reset message. In addition, in S1225, the terminal (1200) can transmit an RRC reset complete message to the source DU (1210).

[0112] As an early synchronization procedure in S1230, the terminal (1200) can perform DL / UL synchronization with at least one candidate cell. For example, the terminal (1200) can identify at least one candidate cell based on the LTM configuration information received from the source DU (1210) in S1220, and perform DL / UL synchronization with the at least one candidate cell.

[0113] In step S1235, the terminal (1200) may transmit an L3 measurement report triggered by event A3 to the source DU (1210). At this time, the terminal (1200) may include information (e.g., information element (IE)) indicating whether the measured DU is an LTM candidate cell or not. For example, as illustrated in FIG. 12, the MeasResultNR included in the RRC message transmitted by the terminal (1200) may include information on the PCI of the target cell (e.g., 'physcellid': 205 in FIG. 12) and 'LTMCandidateCellFlag' indicated as 'true'. In the present disclosure, an L3 measurement report including 'LTMCandidateCellFlag' is referred to as a modified L3 measurement report according to one embodiment.

[0114] And at S1240, the modified L3 measurement report triggered by the above event A3 can be transmitted from the source DU (1210) to the CU (1230).

[0115] In S1245, the CU (1230) can parse the modified L3 measurement report (MR). For example, the CU (1230) can determine whether a handover is required for the terminal (1200) based on the received modified L3 measurement report. In addition, the CU (1230) can determine whether the target DU (1220) is an LTM candidate cell. For example, in the embodiment illustrated in FIG. 12, the CU (1230) can determine the target DU (1220) with a 'physcellid' of 205 as an LTM candidate cell based on the 'LTMCandidateCellFlag' in which the DU with a 'physcellid' of 205 measured by the terminal (1200) included in the modified L3 measurement report is marked as 'true'.

[0116] In step S1250, the CU (1230) may transmit an LTM trigger request. For example, if it is determined that the target DU (1220) is an LTM candidate cell, the CU (1230) may transmit an LTM trigger request message to the source DU (1210). The LTM trigger request message may include at least one of a message type, a gNB-CU UE F1AP ID, a gNB-DU UE F1AP ID, and an NR PCI (e.g., 205).

[0117] In step S1255, the CU (1230) may receive an LTM trigger response from the source DU (1210). For example, it may be determined that the source DU (1210) can transmit an LTM cell switch command MAC CE after receiving an LTM TRIGGER REQUEST message. At this time, the source DU (1210) may transmit an LTM TRIGGER RESPONSE to the CU (1230). At this time, the possibility of the source DU (1210) transmitting the LTM cell switch command MAC CE may be determined based on whether the source DU (1210) can include (fill in) a field value in the MAC CE (for example, at least one of target configuration ID, TA command, TCI state ID, and BWP ID).

[0118] And in S1260, the source DU (1210) can transmit an LTM cell switch command MAC CE to the terminal (1200).

[0119] Additionally, in S1265, an LTM cell switch notification procedure may be performed between the source DU (1210), the target DU (1220), and the CU (1230).

[0120] Based on the above procedure, if the target DU (1220) transmits a UL grant to the terminal (1200) at S1270, the terminal (1200) can transmit a PUSCH to the target DU (1220) at S1275.

[0121] Meanwhile, FIG. 13 is a diagram illustrating an example in which an LTM trigger procedure fails (e.g., is not successful) when a terminal transmits an L3 measurement report including an 'LTMCandidateCellFlag' IE as disclosed in FIG. 10 according to an embodiment of the present disclosure.

[0122] For example, a terminal (1300) existing in an RRC connected state in S1300 may be a terminal in which both L3 HO and LTM are set.

[0123] At S1305, the terminal (1300) can transmit an L3 measurement report to the source DU (1310). And at S1310, the source DU (1310) can transmit the L3 measurement report to the CU (1330).

[0124] In S1315, an LTM candidate preparation procedure may be performed between the source DU (1310) and the CU (1330).

[0125] At S1320, the source DU (1310) can transmit LTM configuration information to the terminal (1300) via an RRC reset message. And at S1325, the terminal (1300) can transmit an RRC reset complete message to the source DU (1310).

[0126] As an early synchronization procedure in S1330, the terminal (1300) can perform DL / UL synchronization with at least one candidate cell. For example, the terminal (1300) can identify at least one candidate cell based on the LTM configuration information received from the source DU (1310) in S1320, and perform DL / UL synchronization with the at least one candidate cell.

[0127] In step S1335, the terminal (1300) may transmit an L3 measurement report triggered by event A3 to the source DU (1310). At this time, the terminal (1300) may include information (e.g., information element (IE)) indicating whether the measured DU is an LTM candidate cell or not. For example, as illustrated in FIG. 13, the MeasResultNR included in the RRC message transmitted by the terminal (1300) may include information on the PCI of the target cell (e.g., 'physcellid': 205 in FIG. 13) and 'LTMCandidateCellFlag' indicated as 'true'. In the present disclosure, an L3 measurement report including 'LTMCandidateCellFlag' is referred to as a modified L3 measurement report according to one embodiment.

[0128] And at S1340, the modified L3 measurement report triggered by the above event A3 can be transmitted from the source DU (1310) to the CU (1330).

[0129] In S1345, the CU (1330) can parse the modified L3 measurement report (MR). For example, the CU (1330) can determine whether a handover is required for the terminal (1300) based on the received modified L3 measurement report. In addition, the CU (1330) can determine whether the target DU (1320) is an LTM candidate cell. For example, in the embodiment illustrated in FIG. 13, the CU (1330) can determine the target DU (1320) with a 'physcellid' of 205 as an LTM candidate cell based on the 'LTMCandidateCellFlag' in which the DU with a 'physcellid' of 205 measured by the terminal (1300) included in the modified L3 measurement report is marked as 'true'.

[0130] In step S1350, the CU (1330) may transmit an LTM trigger request. For example, if it is determined that the target DU (1320) is an LTM candidate cell, the CU (1330) may transmit an LTM trigger request message to the source DU (1310). The LTM trigger request message may include at least one of a message type, a gNB-CU UE F1AP ID, a gNB-DU UE F1AP ID, and an NR PCI (e.g., 205).

[0131] After the source DU (1310) receives the LTM TRIGGER REQUEST message, it can determine whether it can transmit the LTM cell switch command MAC CE. For example, the source DU (1310) can determine whether to transmit based on whether the field values ​​(e.g., at least one of target configuration ID, TA command, TCI state ID, and BWP ID) in the MAC CE can be included (filled in). If it is determined that the source DU (1310) cannot transmit the LTM cell switch command MAC CE to the terminal (1300), it can transmit an LTM TRIGGER FAILURE message to the CU (1330). For example, if it is determined that the source DU (1310) cannot include (fill in) the field values ​​of the LTM cell switch command MAC CE, it can be determined that the LTM cell switch command MAC CE transmission is not possible. At this time, the LTM TRIGGER FAILURE message can include a CAUSE IE. For example, the CAUSE IE may include a purpose of informing the CU that LTM triggering failed for some reason.

[0132] According to an embodiment of the present disclosure, when L3 handover and LTM are set simultaneously, LTM can be performed opportunistically depending on the network situation, so that the interruption time for the terminal can be reduced.

[0133] Meanwhile, FIG. 14 is a block diagram illustrating the structure of a terminal according to an embodiment of the present disclosure.

[0134] Referring to FIG. 14, the terminal may include a transceiver (1410), a control unit (1420), and a storage unit (1430). In the present invention, the control unit may be defined as a circuit or an application-specific integrated circuit or at least one processor.

[0135] The transceiver (1410) can transmit and receive signals with other network entities. For example, the transceiver (1410) can transmit measurement reports to a base station and transmit and receive RRC messages.

[0136] The control unit (1420) can control the overall operation of the terminal according to the embodiment proposed in the present invention. For example, the control unit (1420) can control the signal flow between each block so that the operation according to the flowchart described above is performed. According to one embodiment, the control unit (1420) can generate a measurement report including the 'LTMcandidateCellFlag' IE. In addition, the control unit (1420) can control the transmission of the measurement report including the 'LTMcandidateCellFlag' IE to the base station.

[0137] The storage unit (1430) can store at least one of the information transmitted and received through the transmission and reception unit (1410) and the information generated through the control unit (1420).

[0138] FIG. 15 is a diagram illustrating the structure of a base station according to an embodiment of the present disclosure. Meanwhile, according to an embodiment, FIG. 15 may be a CU or DU entity.

[0139] Referring to FIG. 15, the base station may include a transceiver (1510), a control unit (1520), and a storage unit (1530). In the present invention, the control unit may be defined as a circuit or an application-specific integrated circuit or at least one processor.

[0140] The transceiver (1510) can transmit and receive signals with other network entities. For example, the transceiver (1510) can transmit and receive SR and UL grants with a terminal.

[0141] The control unit (1520) can control the overall operation of the base station according to the embodiment proposed in the present invention. For example, the control unit (1520) can control the signal flow between each block to perform the operation according to the flowchart described above. Specifically, in the case of a CU entity, the control unit (1520) can control to generate and transmit an LTM TRIGGER REQUEST message according to the embodiment of the present invention. In addition, in the case of a DU entity, the control unit (1520) can control to generate and transmit an LTM TRIGGER RESPONSE message according to the embodiment of the present invention.

[0142] The storage unit (1530) can store at least one of the information transmitted and received through the transmission and reception unit (1510) and the information generated through the control unit (1520).

[0143] In the specific embodiments of the present disclosure described above, components included in the invention are expressed in the singular or plural form, depending on the specific embodiment presented. However, the singular or plural expressions are selected to suit the presented situation for convenience of explanation, and the present disclosure is not limited to singular or plural components. Components expressed in the plural form may be composed of singular elements, or components expressed in the singular form may be composed of plural elements.

[0144] Meanwhile, the embodiments of the present disclosure disclosed in this specification and drawings are only specific examples to easily explain the technical contents of the present disclosure and help understand the present disclosure, and are not intended to limit the scope of the present disclosure. In other words, it will be apparent to those skilled in the art that other modified examples based on the technical idea of ​​the present disclosure are possible. In addition, each of the above embodiments can be combined and operated with each other as needed. For example, parts of one embodiment of the present disclosure and another embodiment can be combined with each other to operate a base station and a terminal. In addition, other modified examples based on the technical idea of ​​the above embodiments can be implemented in various systems such as an FDD LTE system, a TDD LTE system, a 5G or NR system.

Claims

1. In a wireless communication system, for a source distributed unit (DU) entity, Transmitter and receiver; and Receive, from the terminal, a measurement report message including information indicating whether at least one neighboring cell measured by the terminal is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell through the transceiver, The above measurement report message is transmitted to the centralized unit (CU) entity, Based on the above measurement report message, an LTM trigger request message is received from the CU entity, Based on the reception of the above LRM trigger request message, it is determined whether an LTM cell switch command medium access control (MAC) control element (CE) can be transmitted to the terminal. A source DU entity including a control unit that controls transmission of an LTM trigger response message to the CU entity when it is determined that the above LTM cell switch command MAC CE transmission is possible.

2. In paragraph 1, If it is determined that the above LTM cell switch command MAC CE transmission is not possible, the step of transmitting an LTM trigger failure message to the CU entity is further included; The above LTM trigger failure message is, A source DU entity characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, a physical cell ID (PCI) of a target DU entity, and information about a cause of LTM triggering failure.

3. In paragraph 1, The above control unit, A source DU entity characterized in that it is determined that transmission of the LTM cell switch command MAC CE is possible when at least one of a target configuration identifier (ID), a timing advance (TA) command, a transmission configuration indicator (TCI) state ID, and a bandwidth part (BWP) ID included in the LTM cell switch command MAC CE can be set.

4. In paragraph 1, The above LRM trigger request message and LTM trigger response message are, A source DU entity characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, and a physical cell ID (PCI) of a target DU entity.

5. In a wireless communication system, in a centralized unit (CU) entity, Transmitter and receiver; and Receives a measurement report message from a source distributed unit (DU) entity through the above transceiver, Based on the above measurement report message, determine whether the target DU entity is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell, If the target DU entity is determined to be the LTM candidate cell, an LTM trigger request message is transmitted to the source DU entity, A CU entity including a control unit that controls receiving an LTM trigger response message from the source DU entity based on the LRM trigger request message.

6. In paragraph 5, The above control unit, Based on the above LRM trigger request message, control to receive an LTM trigger failure message from the source DU entity, The above LTM trigger failure message is, A CU entity characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, a physical cell ID (PCI) of the target DU entity, and information about a cause of LTM triggering failure.

7. In paragraph 5, The above LTM trigger response message is, A CU entity characterized in that at least one of a target configuration identifier (ID), a timing advance (TA) command, a transmission configuration indicator (TCI) state ID, and a bandwidth part (BWP) ID included in an LTM cell switch command medium access control (MAC) control element (CE) can be set.

8. In paragraph 5, The above LRM trigger request message and LTM trigger response message are, A CU entity characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, and a physical cell ID (PCI) of the target DU entity.

9. A method performed by a source distributed unit (DU) entity in a wireless communication system, A step of receiving, from a terminal, a measurement report message including information indicating whether at least one neighboring cell measured by the terminal is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell; A step of transmitting the above measurement report message to a centralized unit (CU) entity; A step of receiving an LTM trigger request message from the CU entity based on the above measurement report message; A step of determining whether an LTM cell switch command medium access control (MAC) control element (CE) can be transmitted to the terminal based on reception of the LRM trigger request message; and A method comprising: a step of transmitting an LTM trigger response message to the CU entity when it is determined that the LTM cell switch command MAC CE transmission is possible; 10. In paragraph 9, If it is determined that the above LTM cell switch command MAC CE transmission is not possible, the step of transmitting an LTM trigger failure message to the CU entity is further included; The above LTM trigger failure message is, A method characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, a physical cell ID (PCI) of a target DU entity, and information about a cause of LTM triggering failure.

11. In paragraph 9, The above judging step is, If at least one of a target configuration identifier (ID), a timing advance (TA) command, a transmission configuration indicator (TCI) state ID, and a bandwidth part (BWP) ID included in the LTM cell switch command MAC CE can be set, it is determined that the LTM cell switch command MAC CE transmission is possible, The above LRM trigger request message and LTM trigger response message are, A method characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, and a physical cell ID (PCI) of a target DU entity.

12. A method performed by a centralized unit (CU) entity in a wireless communication system, A step of receiving a measurement report message from a source distributed unit (DU) entity; A step of determining whether a target DU entity is a layer 1 (L1) / L2 triggered mobility (LTM) candidate cell based on the above measurement report message; If the target DU entity is determined to be the LTM candidate cell, a step of transmitting an LTM trigger request message to the source DU entity; and A method comprising: receiving an LTM trigger response message from the source DU entity based on the LRM trigger request message; 13. In paragraph 12, Further comprising: a step of receiving an LTM trigger failure message from the source DU entity based on the LRM trigger request message; The above LTM trigger failure message is, A method characterized by including at least one of message type information, the terminal ID used in the CU entity, the terminal ID used in the source DU, the physical cell ID (PCI) of the target DU entity, and information about the cause of LTM triggering failure.

14. In paragraph 12, The above LTM trigger response message is, A method characterized in that at least one of a target configuration identifier (ID), a timing advance (TA) command, a transmission configuration indicator (TCI) state ID, and a bandwidth part (BWP) ID included in an LTM cell switch command medium access control (MAC) control element (CE) can be set.

15. In paragraph 12, The above LRM trigger request message and LTM trigger response message are, A method characterized by including at least one of message type information, an ID of the terminal used in the CU entity, an ID of the terminal used in the source DU, and a physical cell ID (PCI) of the target DU entity.

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