Method and device for controlling mobility

Abstract

The present disclosure relates to an L1 / L2 trigger-based mobility control technology and may provide a method and device, the method, by which a target base station controls performance of an LTM (L1 / L2 triggered mobility) operation, including the steps of: transmitting LTM configuration information to a source base station of a terminal; performing an early synchronization procedure with the terminal; transmitting timing advance information to the source base station; and receiving cell switch notification information from the source base station.

Description

Mobility control method and device

[0001] The present disclosure relates to a mobility control technology based on L1 / L2 triggers.

[0002] In mobile communication systems, handovers can occur frequently as terminals move. This is particularly true when users on high-speed vehicles, such as trains, automobiles, and airplanes, use mobile communication systems.

[0003] Conventional L3-based handovers suffer from the long processing time required for terminals to perform the necessary handover procedures to the target base station. This problem, coupled with the frequent occurrence of high-speed terminal movement and the increasing number of services requiring high latency, poses a risk of difficulty in meeting required service quality.

[0004] To address these issues, the need for lower-layer (L1 / L2)-triggered terminal mobility control is emerging. However, lower-layer-triggered terminal mobility control operations have focused on changes between distributed units (DUs) within the same base station.

[0005] Therefore, when the terminal's central unit (CU) or base station changes during high-speed movement, it has been difficult to apply the terminal's mobility control actions triggered at lower layers. This problem continues to cause delays in L3-based handovers.

[0006] In this situation, the need to perform mobility control operations of terminals triggered in lower layers even when the CU or base station is changed is emerging.

[0007] The present disclosure proposes a mobility control operation based on L1 / L2 triggers of a terminal.

[0008] In one aspect, the present embodiments may provide a method for controlling a target base station to perform an LTM (L1 / L2 triggered mobility) operation, the method including the steps of transmitting LTM configuration information to a source base station of a terminal, performing an early synchronization procedure with the terminal, transmitting timing advance information to the source base station, and receiving cell switch notification information from the source base station.

[0009] In another aspect, the present embodiments may provide a method for controlling a source base station to perform an LTM (L1 / L2 triggered mobility) operation, the method including the steps of receiving LTM configuration information from a target base station, receiving timing advance information from the target base station after the target base station performs an early synchronization procedure with a terminal, and transmitting cell switch notification information to the target base station.

[0010] In another aspect, the present embodiments may provide a target base station device that controls the performance of an LTM (L1 / L2 triggered mobility) operation, including a transmitter that transmits LTM configuration information to a source base station of a terminal, a control unit that controls the performance of an early synchronization procedure with the terminal, and a receiver that receives cell switch notification information from the source base station, wherein the transmitter unit transmits timing advance information to the source base station.

[0011] In another aspect, the present embodiments can provide a source base station device that includes a receiving unit that receives LTM configuration information from a target base station and receives timing advance information from the target base station after the target base station performs an early synchronization procedure with a terminal, and a transmitting unit that transmits cell switch notification information to the target base station, in a source base station that controls the performance of an LTM (L1 / L2 triggered mobility) operation.

[0012] The present disclosure can provide L1 / L2 trigger-based mobility control operation of a terminal.

[0013] FIG. 1 is a schematic diagram illustrating the structure of an NR wireless communication system to which the present embodiment can be applied.

[0014] FIG. 2 is a drawing for explaining a frame structure in an NR system to which the present embodiment can be applied.

[0015] FIG. 3 is a diagram for explaining a resource grid supported by a wireless access technology to which the present embodiment can be applied.

[0016] FIG. 4 is a diagram for explaining a bandwidth part supported by a wireless access technology to which the present embodiment can be applied.

[0017] FIG. 5 is a diagram illustrating an example of a synchronization signal block in a wireless access technology to which the present embodiment can be applied.

[0018] FIG. 6 is a diagram for explaining a random access procedure in a wireless access technology to which the present embodiment can be applied.

[0019] Figure 7 is a drawing for explaining CORESET.

[0020] FIG. 8 is a diagram for explaining the operation of a target base station according to one embodiment.

[0021] FIG. 9 is a diagram for explaining the operation of a source base station according to one embodiment.

[0022] FIG. 10 is a diagram for explaining the operation of an Inter-gNB-CU LTM procedure according to one embodiment.

[0023] Fig. 11 is a drawing showing the configuration of a target base station according to another embodiment.

[0024] Fig. 12 is a diagram showing the configuration of a source base station according to another embodiment.

[0025] Hereinafter, some embodiments of the present disclosure will be described in detail with reference to exemplary drawings. When adding reference numerals to components in each drawing, identical components may have the same numerals as much as possible even if they are shown in different drawings. In addition, when describing the present embodiments, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the technical idea of ​​the present invention, the detailed description may be omitted. When "includes," "has," "consists of," etc. are used in this specification, other parts may be added unless "only" is used. When a component is expressed in the singular, it may include a case in which the plural is included unless specifically stated otherwise.

[0026] Additionally, terms such as first, second, A, B, (a), (b), etc. may be used to describe components of the present disclosure. These terms are only intended to distinguish the components from other components, and the nature, order, sequence, or number of the components are not limited by the terms.

[0027] In a description of the positional relationship of components, when it is described that two or more components are "connected," "combined," or "connected," it should be understood that the two or more components may be directly "connected," "combined," or "connected," but that the two or more components may also be further "interposed" with another component to be "connected," "combined," or "connected." Here, the other component may be included in one or more of the two or more components that are "connected," "combined," or "connected" to each other.

[0028] In the description of the temporal flow relationship related to components, operation methods, or manufacturing methods, for example, when the temporal or flow relationship is described as “after”, “following”, “next to”, “before”, etc., it may also include cases where it is not continuous, unless “immediately” or “directly” is used.

[0029] Meanwhile, when numerical values ​​or corresponding information (e.g., levels, etc.) for components are mentioned, even without separate explicit description, the numerical values ​​or corresponding information may be interpreted as including an error range that may occur due to various factors (e.g., process factors, internal or external impact, noise, etc.).

[0030] The wireless communication system in this specification refers to a system for providing various communication services such as voice, data packets, etc. using wireless resources, and may include a terminal, a base station, or a core network.

[0031] The embodiments disclosed below can be applied to wireless communication systems using various wireless access technologies. For example, the embodiments can be applied to various wireless access technologies such as CDMA (code division multiple access), FDMA (frequency division multiple access), TDMA (time division multiple access), OFDMA (orthogonal frequency division multiple access), SC-FDMA (single carrier frequency division multiple access), or NOMA (non-orthogonal multiple access). In addition, the wireless access technology may not only refer to a specific access technology, but also to each generation of communication technologies established by various communication agreement organizations such as 3GPP, 3GPP2, WiFi, Bluetooth, IEEE, and ITU. For example, CDMA can be implemented with wireless technologies such as UTRA (universal terrestrial radio access) or CDMA2000. TDMA can be implemented with wireless technologies such as GSM (global system for mobile communications) / GPRS (general packet radio service) / EDGE (enhanced data rates for GSM evolution). OFDMA can be implemented in wireless technologies such as IEEE (Institute of Electrical and Electronics Engineers) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (evolved UTRA). IEEE 802.16m is an evolution of IEEE 802.16e and provides backward compatibility with systems based on IEEE 802.16e.UTRA is part of UMTS (universal mobile telecommunications system). 3GPP (3rd generation partnership project) LTE (long term evolution) is part of E-UMTS (evolved UMTS) that uses E-UTRA (evolved-UMTSterrestrial radio access), employing OFDMA in the downlink and SC-FDMA in the uplink. Thus, the present embodiments can be applied to currently disclosed or commercialized wireless access technologies, as well as wireless access technologies currently under development or to be developed in the future.

[0032] Meanwhile, the term "terminal" in this specification is a comprehensive concept that refers to a device that includes a wireless communication module that performs communication with a base station in a wireless communication system, and should be interpreted as a concept that includes not only UE (User Equipment) in WCDMA, LTE, NR, HSPA, and IMT-2020 (5G or New Radio), but also MS (Mobile Station), UT (User Terminal), SS (Subscriber Station), and wireless device in GSM. In addition, the terminal may be a user portable device such as a smartphone depending on the usage type, and in a V2X communication system, it may mean a vehicle, a device including a wireless communication module in the vehicle, etc. In addition, in the case of a Machine Type Communication system, it may mean an MTC terminal, M2M terminal, URLLC terminal, etc. that is equipped with a communication module to perform machine type communication.

[0033] The base station or cell in this specification refers to an end that communicates with a terminal in terms of a network, and includes various coverage areas such as Node-B, eNB (evolved Node-B), gNB (gNode-B), LPN (Low Power Node), Sector, Site, various types of antennas, BTS (Base Transceiver System), Access Point, Point (e.g., Transmission Point, Reception Point, Transmission / Reception Point), Relay Node, Mega Cell, Macro Cell, Micro Cell, Pico Cell, Femto Cell, RRH (Remote Radio Head), RU (Radio Unit), and Small Cell. In addition, a cell may mean including a BWP (Bandwidth Part) in the frequency domain. For example, a serving cell may mean an Activation BWP of a terminal.

[0034] Since the various cells listed above have a base station that controls one or more cells, the base station can be interpreted in two meanings. 1) It can be a device itself that provides a mega cell, macro cell, micro cell, pico cell, femto cell, or small cell in relation to a wireless area, or 2) it can indicate the wireless area itself. In 1), all devices that provide a given wireless area are controlled by the same entity or that interact to cooperatively configure the wireless area are all indicated as a base station. Depending on how the wireless area is configured, a point, a transceiver point, a transmission point, a reception point, etc. can be an embodiment of a base station. In 2), the wireless area itself that receives or transmits a signal from the perspective of a user terminal or a neighboring base station can also be indicated as a base station.

[0035] In this specification, a cell may mean a component carrier having coverage of a signal transmitted from a transmission / reception point or a transmission / reception point itself.

[0036] Uplink (UL, or uplink) refers to a method of transmitting and receiving data from a terminal to a base station, and downlink (DL, or downlink) refers to a method of transmitting and receiving data from a base station to a terminal. Downlink may refer to communication or a communication path from multiple transmission / reception points to a terminal, and uplink may refer to communication or a communication path from a terminal to multiple transmission / reception points. In this case, in the downlink, the transmitter may be part of the multiple transmission / reception points, and the receiver may be part of the terminal. In addition, in the uplink, the transmitter may be part of the terminal, and the receiver may be part of the multiple transmission / reception points.

[0037] Uplink and downlink transmit and receive control information through control channels such as PDCCH (Physical Downlink Control CHannel) and PUCCH (Physical Uplink Control CHannel), and transmit and receive data by configuring data channels such as PDSCH (Physical Downlink Shared CHannel) and PUSCH (Physical Uplink Shared CHannel). Hereinafter, the situation in which signals are transmitted and received through channels such as PUCCH, PUSCH, PDCCH, and PDSCH is also expressed in the form of 'transmitting and receiving PUCCH, PUSCH, PDCCH, and PDSCH.'

[0038] For clarity of explanation, the technical idea of ​​this invention is described below mainly with reference to the 3GPP LTE / LTE-A / NR (New RAT) communication system, but the technical features of this invention are not limited to the communication system.

[0039] After researching 4G (4th-Generation) communication technology, 3GPP develops 5G (5th-Generation) communication technology to meet the requirements of the next-generation wireless access technology of the ITU-R. Specifically, 3GPP develops LTE-A pro, which is an enhancement of LTE-Advanced technology to meet the requirements of the ITU-R, as a 5G communication technology, and NR, a new communication technology separate from 4G communication technology. Both LTE-A pro and NR refer to 5G communication technology, and in the following, 5G communication technology will be explained with NR as the center, unless a specific communication technology is specified.

[0040] The operating scenario in NR defines various operation scenarios by adding considerations for satellites, automobiles, and new verticals to the existing 4G LTE scenario, and in terms of service, it supports the eMBB (Enhanced Mobile Broadband) scenario, the mMTC (Massive Machine Communication) scenario that has high terminal density but is deployed over a wide area and requires low data rate and asynchronous access, and the URLLC (Ultra Reliability and Low Latency) scenario that requires high responsiveness and reliability and can support high-speed mobility.

[0041] To meet these scenarios, NR introduces a wireless communication system that incorporates new waveform and frame structure technologies, low latency technologies, support for ultra-high frequency bands (mmWave), and forward compatibility technologies. In particular, NR systems offer various technological changes in terms of flexibility to ensure forward compatibility. The key technical features of NR are described below with reference to the drawings.

[0042] <NR 시스템 일반>

[0043] Figure 1 is a schematic diagram illustrating the structure of an NR system to which the present embodiment can be applied.

[0044] Referring to Fig. 1, the NR system is divided into 5GC (5G Core Network) and NR-RAN parts, and the NG-RAN is composed of gNBs and ng-eNBs that provide user plane (SDAP / PDCP / RLC / MAC / PHY) and control plane (RRC) protocol termination for UE (User Equipment). gNBs or gNBs and ng-eNBs are interconnected via the Xn interface. gNBs and ng-eNBs are each connected to the 5GC via the NG interface. The 5GC can be configured to include an AMF (Access and Mobility Management Function) that is responsible for the control plane such as terminal access and mobility control functions, and an UPF (User Plane Function) that is responsible for the control function for user data. NR includes support for both frequency bands below 6 GHz (FR1, Frequency Range 1) and frequency bands above 6 GHz (FR2, Frequency Range 2).

[0045] gNB refers to a base station that provides NR user plane and control plane protocol termination to terminals, and ng-eNB refers to a base station that provides E-UTRA user plane and control plane protocol termination to terminals. The base station described in this specification should be understood to encompass both gNB and ng-eNB, and may also be used to refer to gNB or ng-eNB separately as needed.

[0046] <NR 웨이브 폼,뉴머롤러지 및 프레임 구조>

[0047] NR uses the CP-OFDM waveform with a cyclic prefix for downlink transmission, and CP-OFDM or DFT-s-OFDM for uplink transmission. OFDM technology is easily combined with MIMO (Multiple Input Multiple Output) and offers the advantages of high spectral efficiency and low-complexity receivers.

[0048] Meanwhile, in NR, the requirements for data rates, latency, and coverage differ across the three scenarios mentioned above. Therefore, it is necessary to efficiently satisfy these requirements across the frequency bands that comprise any NR system. To this end, technologies have been proposed to efficiently multiplex radio resources based on multiple different numerologies.

[0049] Specifically, the NR transmission numerator is determined based on the sub-carrier spacing and the cyclic prefix (CP), and is changed exponentially using the μ value as an exponent value of 2 based on 15 kHz, as shown in Table 1 below.

[0050] μsubcarrier intervalCyclic prefixSupported for dataSupported for synch015NormalYesYes130NormalYesYes260Normal, ExtendedYesNo3120NormalYesYes4240NormalNoYes

[0051] As shown in Table 1 above, the numerology of NR can be divided into five types according to the subcarrier spacing. This is different from the fixed 15 kHz subcarrier spacing of LTE, one of the 4G communication technologies. Specifically, the subcarrier spacing used for data transmission in NR is 15, 30, 60, and 120 kHz, and the subcarrier spacing used for synchronization signal transmission is 15, 30, 120, and 240 kHz. In addition, the extended CP is applied only to the 60 kHz subcarrier spacing. Meanwhile, the frame structure in NR is defined as a 10 ms frame consisting of 10 subframes of the same length of 1 ms. One frame can be divided into 5 ms half frames, and each half frame contains 5 subframes. In the case of a 15 kHz subcarrier spacing, one subframe consists of one slot, and each slot consists of 14 OFDM symbols. FIG. 2 is a diagram for explaining the frame structure in an NR system to which the present embodiment can be applied.

[0052] Referring to Fig. 2, a slot is fixedly composed of 14 OFDM symbols in the case of a normal CP, but the length of the slot in the time domain may vary depending on the subcarrier spacing. For example, in the case of a numerology with a 15 kHz subcarrier spacing, a slot is composed of 1 ms in length, which is the same length as a subframe. In contrast, in the case of a numerology with a 30 kHz subcarrier spacing, a slot is composed of 14 OFDM symbols, but two slots may be included in one subframe with a length of 0.5 ms. In other words, subframes and frames are defined with fixed time lengths, while slots are defined by the number of symbols, and their time lengths may vary depending on the subcarrier spacing.

[0053] Meanwhile, NR defines slots as the basic scheduling unit and also introduces mini-slots (or sub-slots, or non-slot-based scheduling) to reduce transmission delay in the wireless section. Using wider subcarrier spacing reduces transmission delay in the wireless section by shortening the length of each slot inversely. Mini-slots (or sub-slots) are designed to efficiently support URLLC scenarios and allow scheduling in units of 2, 4, or 7 symbols.

[0054] Furthermore, unlike LTE, NR defines uplink and downlink resource allocation at the symbol level within a single slot. To reduce HARQ delay, a slot structure was defined that allows HARQ ACK / NACKs to be transmitted directly within the transmission slot. This slot structure is referred to as a self-contained structure and will be described in detail.

[0055] NR is designed to support a total of 256 slot formats, of which 62 are used in 3GPP Rel-15. It also supports a common frame structure that configures FDD or TDD frames through various combinations of slots. For example, it supports a slot structure in which all symbols in a slot are set to downlink, a slot structure in which all symbols are set to uplink, and a slot structure in which downlink and uplink symbols are combined. NR also supports data transmission being distributed and scheduled across one or more slots. Therefore, a base station can use a slot format indicator (SFI) to inform a UE whether a slot is a downlink slot, an uplink slot, or a flexible slot. The base station can indicate the slot format by indicating an index of a table configured through UE-specific RRC signaling using the SFI, and can also indicate it dynamically through DCI (Downlink Control Information) or statically or semi-statically through RRC.

[0056] <NR 물리 자원 >

[0057] In relation to physical resources in NR, antenna ports, resource grids, resource elements, resource blocks, and bandwidth parts are considered.

[0058] Antenna ports are defined such that the channel through which a symbol on an antenna port is carried can be inferred from the channel through which another symbol on the same antenna port is carried. Two antenna ports are said to be quasi co-located (QC / QCL) if the large-scale properties of the channel through which a symbol on one antenna port is carried can be inferred from the channel through which a symbol on the other antenna port is carried. Here, the large-scale properties include one or more of delay spread, Doppler spread, frequency shift, average received power, and received timing.

[0059] FIG. 3 is a diagram for explaining a resource grid supported by a wireless access technology to which the present embodiment can be applied.

[0060] Referring to Figure 3, a resource grid may exist for each numeral, as NR supports multiple numerals on the same carrier. Furthermore, resource grids may exist based on antenna ports, subcarrier spacing, and transmission direction.

[0061] A resource block (RB) consists of 12 subcarriers and is defined solely in the frequency domain. Furthermore, a resource element (RE) consists of one OFDM symbol and one subcarrier. Therefore, as shown in Figure 3, the size of a single RB can vary depending on the subcarrier spacing. NR also defines "Point A," which serves as a common reference point for the RB grid, as well as common RBs and virtual RBs.

[0062] FIG. 4 is a diagram for explaining a bandwidth part supported by a wireless access technology to which the present embodiment can be applied.

[0063] Unlike LTE, where the carrier bandwidth is fixed at 20 MHz, NR sets the maximum carrier bandwidth from 50 MHz to 400 MHz for each subcarrier interval. Therefore, it is not assumed that all terminals will use the entire carrier bandwidth. Accordingly, NR allows terminals to designate bandwidth parts (BWPs) within the carrier bandwidth, as illustrated in Figure 4. Furthermore, bandwidth parts are associated with a single numerology, consist of a subset of consecutive common resource blocks, and can be dynamically activated over time. Each terminal is configured with up to four bandwidth parts for both the uplink and downlink, and data is transmitted and received using the bandwidth parts activated at a given time.

[0064] In the case of a paired spectrum, the uplink and downlink bandwidth parts are set independently, and in the case of an unpaired spectrum, the downlink and uplink bandwidth parts are set in pairs so that they can share a center frequency to prevent unnecessary frequency re-tuning between downlink and uplink operations.

[0065] <NR 초기 접속>

[0066] In NR, a terminal performs cell search and random access procedures to connect to a base station and perform communication.

[0067] Cell search is a procedure in which a terminal synchronizes to the cell of a corresponding base station, obtains a physical layer cell ID, and obtains system information using a synchronization signal block (SSB) transmitted by the base station.

[0068] FIG. 5 is a diagram illustrating an example of a synchronization signal block in a wireless access technology to which the present embodiment can be applied.

[0069] Referring to FIG. 5, SSB is composed of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS), each occupying 1 symbol and 127 subcarriers, and a PBCH spanning 3 OFDM symbols and 240 subcarriers.

[0070] The terminal receives SSB by monitoring SSB in the time and frequency domain.

[0071] SSB can be transmitted up to 64 times in 5ms. Multiple SSBs are transmitted in different transmission beams within 5ms, and the terminal performs detection assuming that SSBs are transmitted every 20ms based on a specific beam used for transmission. The number of beams that can be used for SSB transmission within 5ms can increase as the frequency band increases. For example, up to 4 SSB beams can be transmitted below 3GHz, up to 8 in the frequency band between 3GHz and 6GHz, and up to 64 different beams can be used for SSB transmission in the frequency band above 6GHz.

[0072] SSB contains two symbols in one slot, and the starting symbol and number of repetitions within the slot are determined as follows depending on the subcarrier spacing.

[0073] Meanwhile, unlike SS in conventional LTE, SSB is not transmitted at the center frequency of the carrier bandwidth. This means that SSB can be transmitted even in locations other than the center of the system bandwidth, and when supporting wideband operation, multiple SSBs can be transmitted in the frequency domain. Accordingly, the terminal monitors SSB using the synchronization raster, which is a candidate frequency location for monitoring SSB. The carrier raster, which is the center frequency location information of the channel for initial access, and the synchronization raster are newly defined in NR. The synchronization raster has a wider frequency interval than the carrier raster, which can support the terminal's fast SSB search.

[0074] A UE can obtain the MIB through the PBCH of the SSB. The MIB (Master Information Block) includes the minimum information required for the UE to receive the remaining system information (RMSI, Remaining Minimum System Information) broadcast by the network. In addition, the PBCH may include information on the position of the first DM-RS symbol in the time domain, information for the UE to monitor SIB1 (e.g., SIB1 numerology information, information related to SIB1 CORESET, search space information, PDCCH-related parameter information, etc.), offset information between the common resource block and the SSB (the absolute position of the SSB within the carrier is transmitted through SIB1), etc. Here, the SIB1 numerology information is also applied equally to some messages used in the random access procedure for the UE to access the base station after completing the cell search procedure. For example, the numerology information of SIB1 may be applied to at least one of messages 1 to 4 for the random access procedure.

[0075] The aforementioned RMSI may refer to SIB1 (System Information Block 1), and SIB1 is broadcast periodically (e.g., every 160 ms) in the cell. SIB1 contains information necessary for the UE to perform the initial random access procedure and is periodically transmitted via PDSCH. In order for the UE to receive SIB1, it must receive numerology information used for SIB1 transmission and CORESET (Control Resource Set) information used for SIB1 scheduling via PBCH. The UE checks scheduling information for SIB1 using SI-RNTI in CORESET and acquires SIB1 on PDSCH according to the scheduling information. The remaining SIBs, excluding SIB1, may be transmitted periodically or upon request of the UE.

[0076] FIG. 6 is a diagram for explaining a random access procedure in a wireless access technology to which the present embodiment can be applied.

[0077] Referring to FIG. 6, once cell search is complete, the terminal transmits a random access preamble for random access to the base station. The random access preamble is transmitted via the PRACH. Specifically, the random access preamble is transmitted to the base station via the PRACH, which consists of consecutive radio resources in a specific slot that is periodically repeated. Generally, when a terminal initially accesses a cell, a contention-based random access procedure is performed, and when performing random access for beam failure recovery (BFR), a non-contention-based random access procedure is performed.

[0078] The terminal receives a random access response to the transmitted random access preamble. The random access response may include a random access preamble identifier (ID), an UL Grant (uplink radio resource), a temporary C-RNTI (Temporary Cell - Radio Network Temporary Identifier), and a TAC (Time Alignment Command). Since one random access response may include random access response information for one or more terminals, the random access preamble identifier may be included to indicate which terminal the included UL Grant, temporary C-RNTI, and TAC are valid for. The random access preamble identifier may be an identifier for the random access preamble received by the base station. The TAC may be included as information for the terminal to adjust uplink synchronization. The random access response may be indicated by a random access identifier on the PDCCH, i.e., an RA-RNTI (Random Access - Radio Network Temporary Identifier).

[0079] Upon receiving a valid random access response, the terminal processes the information contained in the random access response and performs scheduled transmission to the base station. For example, the terminal applies TAC and stores a temporary C-RNTI. Furthermore, using the UL Grant, the terminal transmits data stored in its buffer or newly generated data to the base station. In this case, information that identifies the terminal must be included.

[0080] Finally, the terminal receives a downlink message for contention resolution.

[0081] <NR CORESET>

[0082] The downlink control channel in NR is transmitted in a CORESET (Control Resource Set) with a length of 1 to 3 symbols, and transmits uplink / downlink scheduling information, SFI (Slot format Index), and TPC (Transmit Power Control) information.

[0083] To ensure system flexibility, NR introduced the CORESET concept. CORESET (Control Resource Set) refers to time-frequency resources for downlink control signals. A terminal can decode control channel candidates using one or more search spaces within the CORESET time-frequency resources. A QCL (Quasi CoLocation) assumption is established for each CORESET, which is used to inform the characteristics of analog beam direction in addition to the delay spread, Doppler spread, Doppler shift, and average delay assumed by the conventional QCL.

[0084] Figure 7 is a drawing for explaining CORESET.

[0085] Referring to Figure 7, a CORESET can exist in various forms within the carrier bandwidth within a single slot, and in the time domain, a CORESET can consist of up to three OFDM symbols. In addition, a CORESET is defined as a multiple of six resource blocks up to the carrier bandwidth in the frequency domain.

[0086] The first CORESET is indicated via the MIB as part of the initial bandwidth part configuration, allowing the terminal to receive additional configuration and system information from the network. After establishing a connection with the base station, the terminal can receive and configure one or more CORESET information via RRC signaling.

[0087] In this specification, the terms frequency, frame, subframe, resource, resource block, region, band, subband, control channel, data channel, synchronization signal, various reference signals, various signals or various messages related to NR (New Radio) may be interpreted in the past or present meaning or in various meanings used in the future.

[0088] The present embodiments relate to L1 / L2 trigger-based mobility (LTM) control technology for mobile communication terminals. In particular, a method and device for supporting LTM between different base stations / gNB-CUs are proposed.

[0089] LTM (L1 / L2 Triggered Mobility)

[0090] Technology for L1 / L2 Triggered Mobility (LTM: L1 / L2 Triggered Mobility, hereinafter referred to as LTM for convenience of explanation. However, this is for convenience of explanation and can be replaced with any name such as Layer 1 Mobility, Layer 2 Mobility, or lower layer mobility) function is being researched. LTM is a procedure in which the gNB receives an L1 measurement report from the UE and, based on the L1 measurement report, transmits a cell switching command signaled through MAC CE to change the serving cell (e.g. PCell / PSCell) of the UE. The cell switching (switch or change) command indicates an LTM candidate cell configuration (or LTM candidate cell identification information) that the gNB prepares in advance and provides to the UE through RRC signaling. Then, the UE switches to the target cell according to the cell switching command. The network can request the UE to acquire early timing advance (TA) for the candidate cell before the cell switch. Early timing advance acquisition (ETA) is triggered by the PDCCH order. The network indicates via a cell switch command whether the UE should access the target cell via an RA procedure if no TA value is provided, or via a PUSCH transmission using the indicated TA value. For RACH-less LTM, the UE accesses the target cell via a configured grant provided in RRC signaling and selects a configured grant opportunity associated with the beam indicated in the cell switch command. If the UE does not receive a configured grant in RRC signaling, the UE monitors the PDCCH for dynamic scheduling from the target cell during LTM cell switching.

[0091] LTM supports both intra-gNB-DU mobility and intra-gNB-CU mobility. Thus, prior art LTM only supports mobility within an intra-gNB-CU. Therefore, to support inter-gNB-CU mobility, an L3-based handover method (e.g., a network-based handover in which a source base station triggers a handover by sending an RRC reconfiguration message containing information necessary for accessing a target cell, or a conditional handover executed by a UE when a handover execution condition is met) must be used, rather than LTM. This can increase interruption times during cell changes compared to LTM.

[0092] Here, gNB-CU represents a logical node hosting the RRC, SDAP, and PDCP protocols of the base station (or a logical node hosting one or more of the radio access upper layer protocols). NR provides a structure that separates the base station (gNB) into a centralized node (hereinafter referred to as CU / gNB-CU for convenience of explanation) and a distributed node (hereinafter referred to as DU / gNB-DU for convenience) to support efficient network construction. The wireless network consists of a set of base stations connected to the 5GC (5G Core network) via the NG interface. The base stations are interconnected via the Xn interface. One base station can consist of one gNB-CU and one or more gNB-DUs. The gNB-CU and gNB-DU are connected via the F1 interface. One gNB-DU can be connected to only one gNB-CU. The NG interface and the Xn-C interface for one base station consisting of the gNB-CU and the gNB-DU are terminated at the gNB-CU. The gNB-DUs connected to a gNB-CU appear as a single base station to other base stations and the 5GC. A gNB-DU is a logical node that hosts the RLC, MAC, and PHY layers of the base station (or one or more radio access protocols not included in the gNB-CU). A gNB-DU supports one or more cells. A cell is supported by only one gNB-DU.

[0093] Thus, the current LTM only supports mobility within intra-gNB-CUs. Providing inter-gNB-CU mobility requires an L3-based handover method, not LTM. This increases interruption times during cell changes compared to LTM.

[0094] To address these issues, the present disclosure proposes an inter-gNB-CU-based LTM control procedure and device. This provides lower interruption times compared to conventional handovers, enabling faster service delivery.

[0095] Hereinafter, a method for providing mobility based on 5G NR radio access technology will be described. However, this is for convenience of explanation, and the present embodiments can also be applied to cell change / switch based on any radio access technology (e.g., 6G). The embodiments described in this specification include information elements, procedures, and operation contents specified in any NR standard (e.g., TS 38.321, which is an NR MAC standard, TS 38.331, which is an NR RRC standard). Even if the definition of the corresponding information element, related procedure, and related terminal operation contents are not described in this specification, the corresponding contents specified in the standard specification may be included in the present disclosure. It is obvious that the embodiments provided below can be implemented individually or by arbitrarily combining / combining each embodiment, and this also falls within the scope of the present disclosure.

[0096] Any of the functions described below may be defined as individual terminal capabilities (UE radio capabilities or UE Core network capabilities) and transmitted by the terminal to the base station / core network entity (e.g., AMF / SMF) via corresponding signaling. Alternatively, any of the functions may be combined / combined and defined as corresponding terminal capabilities and transmitted to the base station / core network entity via corresponding signaling at the request of the terminal.

[0097] The base station may transmit / instruct the terminal via an RRC message information indicating whether to allow / support / configure any function or any combination of functions described below. For example, the terminal may be instructed to do so before or after the configuration / application of the function / combination of functions, or simultaneously with the configuration / application of the function / combination of functions. The RRC message may be broadcast via system information. Alternatively, the terminal may be instructed via a dedicated RRC message.

[0098] Currently, 3GPP Rel-18 LTM indicates cell switch commands based on MAC CE. However, the present embodiments may also include an LTM method that indicates cell switch commands via DCI in addition to MAC CE. Furthermore, in this specification, the term "cell switch" is used interchangeably with "cell change," and the terms may be used interchangeably as needed.

[0099] FIG. 8 is a diagram for explaining the operation of a target base station according to one embodiment.

[0100] Referring to FIG. 8, in a method for controlling the performance of LTM (L1 / L2 triggered mobility) operation, a target base station can perform a step of transmitting LTM configuration information to a source base station of a terminal (S810).

[0101] For example, the target base station can transmit LTM configuration information to the source base station on which the terminal is camped in order to perform communication. For example, the LTM configuration information can be included in a response message to a handover request message transmitted from the source base station to the target base station. For example, the LTM configuration information can be included in a handover request confirmation message. Alternatively, the LTM configuration information can be included in an LTM configuration request response or confirmation message for an LTM configuration request. Here, the handover request confirmation message, the LTM configuration request response message, and the LTM configuration request confirmation message mean the same message. That is, the aforementioned LTM configuration information can be included in a response message to any message that the source base station requests the target base station to understand mobility control of the terminal.

[0102] Meanwhile, LTM configuration information may include at least one of Transmission Configuration Indicator (TCI) status information and Random Access Channel (RACH) configuration information. The TCI status information and RACH configuration information may be used for early synchronization between the terminal and the target base station. Therefore, the TCI status information and RACH configuration information may be included in the early synchronization configuration information.

[0103] In addition, the LTM configuration information may further include at least one of a reference signal configuration for a target candidate cell, a target candidate cell ID, a target candidate cell global ID, a target base station ID, a target NG-RAN node ID, and a target gNB-DU ID.

[0104] LTM configuration information can be transmitted through the Xn interface between the source base station and the target base station.

[0105] The target base station may perform a step of performing an early synchronization procedure with the terminal (S820).

[0106] For example, if the LTM operation of the terminal is instructed to the terminal by the source base station, the target base station can perform an early synchronization procedure with the terminal. That is, the terminal can perform an early synchronization operation toward the target candidate cell. For example, if a random access (contention-free random access) to the target base station (or target candidate cell) is triggered from the source base station, the terminal can initiate the early synchronization procedure by transmitting a random access preamble to the instructed target candidate cell. Through this, the terminal can perform early synchronization with the target candidate cell before receiving the cell switch command, thereby performing fast mobility control operations. In this procedure, the target base station acquires a Timing Advanced (TA), and therefore, the early synchronization procedure is also described as an early TA acquisition procedure in the present disclosure.

[0107] The target base station can perform a step of transmitting timing advance information to the source base station (S830).

[0108] For example, the target base station can transmit timing advance information to the source gNB-DU via the target gNB-CU and the source gNB-CU. For example, the timing advance information can include at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and a timing advance value. For example, the timing advance information can be transmitted to the source base station via a CU-CU TA information transfer message.

[0109] Thereafter, the terminal receives an LTM cell switch command from the source base station. The LTM cell switch command can be indicated to the terminal via MAC CE or DCI.

[0110] The target base station can perform a step of receiving cell switch notification information from the source base station (S840).

[0111] For example, cell switch notification information can be transmitted to the target gNB-DU through the source gNB-CU and the target gNB-CU. For example, the cell switch notification information can include at least one of target cell identifier information and TCI state identifier (TCI state ID) information. In addition, the cell switch notification information can be received by the target base station through a CU-CU cell switch notification message.

[0112] Meanwhile, after the step of receiving cell switch notification information, the target base station may further perform a step of transmitting a handover success message indicating that the terminal has successfully accessed the target base station to the source base station (S850).

[0113] For example, after receiving cell switch notification information, if the terminal completes a cell switch to the target base station, the target base station can transmit a handover success message to the source base station to notify the terminal of the termination of the LTM operation. This completes the terminal's LTM operation between base stations, allowing the terminal to communicate via the target base station.

[0114] Through the above operations, LTM operations can be performed not only within conventional intra-base station (or CU) but also between inter-base station (or CU). Therefore, terminal interruption time can be reduced, providing fast and uninterrupted data communication services.

[0115] Below, the operation according to the present disclosure from the source base station side is described.

[0116] FIG. 9 is a diagram for explaining the operation of a source base station according to one embodiment.

[0117] Referring to FIG. 9, a source base station can perform a step of receiving LTM configuration information from a target base station in a method of controlling the performance of an LTM (L1 / L2 triggered mobility) operation (S910).

[0118] For example, a source base station may transmit a handover request message to a target base station. The source base station may receive LTM configuration information in response to the handover request message. For example, the LTM configuration information may be included in a response message to the handover request message transmitted from the source base station to the target base station. For example, the LTM configuration information may be included in a handover request confirmation message. Alternatively, the LTM configuration information may be included in an LTM configuration request response or confirmation message for an LTM configuration request. Here, the handover request confirmation message, the LTM configuration request response message, and the LTM configuration request confirmation message mean the same message. That is, the LTM configuration information described above may be included in a response message to any message that the source base station requests the target base station to understand mobility control of the terminal.

[0119] Meanwhile, LTM configuration information may include at least one of Transmission Configuration Indicator (TCI) status information and Random Access Channel (RACH) configuration information. The TCI status information and RACH configuration information may be used for early synchronization between the terminal and the target base station. Therefore, the TCI status information and RACH configuration information may be included in the early synchronization configuration information.

[0120] In addition, the LTM configuration information may further include at least one of a reference signal configuration for a target candidate cell, a target candidate cell ID, a target candidate cell global ID, a target base station ID, a target NG-RAN node ID, and a target gNB-DU ID.

[0121] LTM configuration information can be transmitted through the Xn interface between the source base station and the target base station.

[0122] For example, a source base station can transmit LTM configuration information to a terminal for LTM operation of the terminal. When the LTM operation of the terminal is instructed by the source base station to the terminal, the target base station can perform an early synchronization procedure with the terminal. That is, the terminal can perform an early synchronization operation toward the target candidate cell. For example, when a random access (contention-free random access) to the target base station (or target candidate cell) is triggered from the source base station, the terminal can initiate an early synchronization procedure by transmitting a random access preamble to the instructed target candidate cell. Through this, the terminal can perform early synchronization with the target candidate cell before receiving a cell switch command, thereby performing fast mobility control operations. In this procedure, the target base station acquires a Timing Advanced (TA), and therefore, the early synchronization procedure is also described as an early TA acquisition procedure in the present disclosure.

[0123] Additionally, the source base station may perform a step of receiving timing advance information from the target base station after the target base station performs an early synchronization procedure with the terminal (S920).

[0124] For example, timing advance information can be transmitted to the source gNB-DU via the target gNB-CU and the source gNB-CU. For example, the timing advance information can include at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and a timing advance value. For example, the timing advance information can be received by the source base station via a CU-CU TA information transfer message.

[0125] Afterwards, the source base station transmits an LTM cell switch command to the terminal. The LTM cell switch command can be indicated to the terminal via MAC CE or DCI.

[0126] The source base station can perform a step of transmitting cell switch notification information to the target base station (S930).

[0127] For example, cell switch notification information can be transmitted to the target gNB-DU via the source gNB-CU and the target gNB-CU. For example, the cell switch notification information can include at least one of target cell identifier information and TCI state identifier (TCI state ID) information. In addition, the cell switch notification information can be transmitted to the target base station via a CU-CU cell switch notification message.

[0128] Meanwhile, the source base station may further perform a step of receiving a handover success message from the target base station indicating that the terminal has successfully accessed the target base station after the step of transmitting cell switch notification information (S940).

[0129] For example, after receiving cell switch notification information, if the terminal completes a cell switch to the target base station, the target base station can transmit a handover success message to the source base station to notify the terminal of the termination of the LTM operation. This completes the terminal's LTM operation between base stations, allowing the terminal to communicate via the target base station.

[0130] Through the above operations, LTM operations can be performed not only within conventional intra-base station (or CU) but also between inter-base station (or CU). Therefore, terminal interruption time can be reduced, providing fast and uninterrupted data communication services.

[0131] Below, more specific and diverse embodiments of the operations of the aforementioned target base station and source base station are described. The operations of each embodiment and device described below may be performed in any combination of any action or step.

[0132] Inter-gNB-CU LTM signaling procedure

[0133] To support inter-gNB-CU LTM mobility, signaling procedures for this need to be specifically defined.

[0134] FIG. 10 is a diagram illustrating the operation of an Inter-gNB-CU LTM procedure according to one embodiment. Each procedure will be described with reference to FIG. 10.

[0135] 0. For example, based on the (inter-CU) neighbor cell measurement results via L3 Measurement Report message, the source base station / gNB-CU decides to initiate the addition of (inter-CU) LTM configuration.

[0136] 1. The source base station / gNB-CU sends a message to the target base station / gNB-CU to request inter-CU LTM configuration.

[0137] For example, the message may be a handover request message. This allows for utilizing information elements used in the existing handover request process. For example, the message may include information indicating (inter-CU) LTM (or an information element indicating a request for any information element included in the inter-CU LTM configuration). The source base station / gNB-CU may indicate the indication information so that the target base station / gNB-CU can consider / distinguish it and process (inter-CU) LTM.

[0138] For another example, the message may be a newly defined message that is distinct from the handover request message. For example, a message type (Message Type IE) may be defined that is distinct from the handover request message.

[0139] A message requesting Inter-CU LTM configuration may include LTM indication information, indicating that the message is requesting a candidate LTM configuration to be set up. As another example, the message may include lower layer configuration request information (e.g., Request for Lower Layer Configuration). As another example, the message may include request information for a candidate LTM to be set up (requested LTM Information to be Setup IE).

[0140] Candidate LTM request information may include one or more of LTM indication information, lower layer configuration request information, CSI resource configuration request information, early synchronization (or early TA acquisition) request information, information for requesting a TCI state, and LTM SSB configuration request information. Early synchronization (or early TA acquisition) request information may include information for requesting RACH configuration as information for requesting information required to perform an early uplink synchronization procedure (e.g., frequencyInfoUL, rach-ConfigGeneric, ssb-PerRACH-Occasion). CSI resource configuration request information indicates information for requesting LTM CSI resource configuration information (ltm-CSI-ResourceConfigId: identify an instance of LTM-CSI-ResourceConfig IE, ltm-CSI-SSB-ResourceSet: one SS / PBCH block resource set from one or more LTM candidate cells) that defines one or more CSI resources / resource sets for the LTM candidate configuration.

[0141] The LTM SSB configuration request information indicates information for requesting the configuration of an SS / PBCH block used for L1 measurement. As another example, the target base station / gNB-CU may transmit LTM information to be setup (LTM Information to be Setup IE) when transmitting a UE context setup request message to the target gNB-DU in step 2 below. As another example, the target base station / gNB-CU may consider the target candidate cell ID (target cell global ID or target cell ID or target PCI) included in the request as being related to LTM. In addition, the target candidate cell ID (target cell global ID or target cell ID or target PCI) and one or more of the generated lower layer configuration information may be included in a response message to the message.

[0142] A message for requesting Inter-CU LTM configuration may include one or more of: a target candidate cell ID (or a list / set of multiple target candidate cell IDs associated with the corresponding target base station / gNB-CU, or PCI(s)), one target cell global ID (or a list / set of multiple target cell global IDs associated with the corresponding target base station / gNB-CU), the target cell ID, (global) source gNB ID, source NG-RAN node ID, source gNB-DU ID, PRACH resource request information and lower layer configuration request information used to generate the reference configuration. One target candidate cell ID and one target cell global ID (E-UTRA CGI or an NR CGI) may be associated.

[0143] A message requesting an Inter-CU LTM configuration may include information for an LTM reference configuration (e.g., an LTM Reference Configuration IE), information for indicating an LTM reference configuration, information for indicating whether an LTM reference configuration is present, or information for indicating an LTM reference configuration request. If the information is included, the target eNB / gNB-CU may (instruct) the associated / connected target gNB-DU to consider it for generating an LTM lower layer configuration. As another example, the message requesting an Inter-CU LTM configuration may be transmitted including an LTM Reference Configuration Information Element / Container. This allows the target eNB / gNB-CU to consider the LTM Reference Configuration Information Element for generating an LTM candidate configuration. Note that an LTM candidate configuration represents a configuration associated with one LTM candidate cell. The LTM candidate cell configuration may be a complete LTM candidate cell configuration or a delta / difference configuration with respect to the LTM reference configuration. The LTM reference configuration represents a common configuration for all configured (incomplete) LTM candidate configurations within the same cell group. The LTM reference configuration is used by the UE to generate a complete LTM candidate cell configuration by applying the LTM candidate cell configuration on top of the LTM reference configuration.

[0144] A message for requesting inter-CU LTM configuration may include LTM configuration information (LTM-Config IE) of a source base station. The information may include one or more of attemptLTM-Switch (If present, the UE shall execute an LTM cell switch if the selected cell is an LTM candidate cell and it is the first cell selection after failure), LTM reference configuration information, LTM candidate cell configuration information to be additionally modified (ltm-CandidateToAddModList), ltm-ServingCellNoResetID (This field is used by the UE to determine on whether L2 reset should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.), and ltm-ServingCellUE-MeasuredTA-ID (This field is used by the UE to determine on whether UE-based TA measurements should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.).

[0145]

[0146] 2. The target base station / gNB-CU sends a terminal context setup request message including (one) target candidate cell ID received from the source base station / gNB-CU to the target candidate gNB-DU. The message may include one or more of the following information: source base station / gNB-CU ID, source gNB ID, source NG-RAN node ID, source gNB-DU ID, early synchronization information request information, PRACH resource request information, and lower layer configuration request information used to generate a reference configuration.

[0147]

[0148] 3. If the target candidate gNB-DU accepts the LTM configuration request, the target candidate gNB-DU may respond with a UE context setup response message containing the generated lower layer RRC configuration (e.g. TCI state configuration, RACH configuration) and the RS (reference signal) configuration for the accepted target candidate cell.

[0149]

[0150] 4. The target base station / gNB-CU sends a message to the source base station / gNB-CU to respond / confirm the inter-CU LTM configuration request.

[0151] For example, the message could be a handover response / confirmation message, allowing for the use of information elements used in the existing handover response process.

[0152] For another example, the message may be a newly defined message that is distinct from the handover response / confirmation message. For example, a message type (Message Type IE) may be defined that is distinct from the handover response / confirmation message.

[0153]

[0154] A message responding to an Inter-CU LTM configuration request may include one or more of RS configuration, TCI configuration, RACH configuration, target candidate cell ID (e.g. PCI), target cell global ID, (global) target gNB ID, target NG-RAN node ID, and target gNB-DU ID for the accepted target candidate cell. To avoid random access collisions between UEs accessing the gNB-DU (or from the gNB-DU to another gNB-DU) and UEs accessing the gNB-DU from another gNB-DU, it is desirable that Contention Free Random Access (CFRA) resources be shared only between UEs within the gNB-DU or between the gNB-DU and another gNB-DU. For this purpose, it may be desirable for a source eNB / gNB-CU to indicate the source gNB-CU ID and / or the source gNB-DU ID when requesting / responding to the target eNB / gNB-CU for CFRA resources.

[0155]

[0156] A message responding to an inter-CU LTM configuration request may include one or more of LTM reference configuration information, LTM candidate configuration ID, LTM candidate configuration information, LTM candidate PCI, complete LTM configuration indication information, LTM TCI state information, LTM early uplink sync configuration information, LTM SSB configuration information, and LTM terminal measurement TA-ID. Here, the LTM reference configuration information includes an RRC reconfiguration message used to configure a reference configuration for LTM. The LTM candidate configuration ID indicates information indicating / identifying the LTM candidate configuration. The LTM candidate PCI indicates the PCI of the special cell of the configuration included in the LTM candidate configuration information. The complete LTM configuration indication information indicates whether the LTM candidate configuration in the LTM candidate configuration information is a complete configuration. The LTM TCI state information indicates a list of TCI states to be added / modified / released. LTM Early Uplink Sync Configuration Information indicates the information required to perform the Early Uplink Synchronization Procedure (e.g. frequencyInfoUL, rach-ConfigGeneric, ssb-PerRACH-Occasion). LTM SSB Configuration Information indicates the configuration of the SS / PBCH block used for L1 measurements. LTM Terminal Measurement TA-ID indicates information indicating whether to perform terminal-based TA measurements when the LTM cell switch procedure is executed for an LTM candidate cell.

[0157] As another example, the message may include a container containing an LTM-Candidate IE (or an RRC internode message containing sub-information of the LTM-Candidate IE). The container may include one or more of the following information: LTM reference configuration information, LTM candidate configuration ID, LTM candidate configuration information, LTM candidate PCI, complete LTM configuration indication information, LTM TCI state information, LTM early uplink sink configuration information, LTM SSB configuration information, and LTM terminal measurement TA-ID.

[0158]

[0159] 5. The source base station / gNB-CU sends a terminal context modification request message to the source gNB-DU. The message may include one or more of the following information: RS configuration, TCI configuration, RACH configuration, target candidate cell ID, target cell global ID, (global) target gNB ID, target NG-RAN node ID, and target gNB-DU ID for the accepted target candidate cell.

[0160]

[0161] 6. The source gNB-DU sends a UE context modification response message to the source base station / gNB-CU. The message may include one or more of the following information: RS configuration for the source cell, prepared target candidate cells, and generated CSI report configuration.

[0162]

[0163] 7. The source base station / gNB-CU sends a downlink RRC message transfer message to the source gNB-DU. The message includes a generated RRC reconfiguration message with an LTM configuration. The message may include a container including an LTM-Candidate IE (or an RRC internode message including sub-information of the LTM-Candidate IE). The container may include one or more of the following information: LTM reference configuration information, LTM candidate configuration ID, LTM candidate configuration information, LTM candidate PCI, complete LTM configuration indication information, LTM TCI state information, LTM early uplink sync configuration information, LTM SSB configuration information, and LTM terminal measurement TA-ID.

[0164]

[0165] 8. The source gNB-DU forwards the received RRC reconfiguration message to the terminal.

[0166]

[0167] 9. The terminal responds to the source gNB-DU with an RRC reconfiguration complete message.

[0168]

[0169] 10. The source gNB-DU forwards the RRC reconfiguration complete message to the gNB-CU via an uplink RRC message transfer message.

[0170]

[0171] 11. An early synchronization procedure can be performed between the terminal and the target gNB.

[0172]

[0173] 12. The target candidate DU sends TA (timing advance), associated CFRA resource information, candidate cell ID, and source gNB-DU ID to the source gNB-DU through the target gNB-CU and source gNB-CU.

[0174]

[0175] 13. The terminal transmits the lower layer measurement results (e.g. L1 measurement) to the source gNB-DU.

[0176]

[0177] 14. The source gNB-DU can decide to run LTM on the target candidate cell.

[0178]

[0179] 15. The source gNB-DU sends an LTM command (e.g. LTM MAC CE) to the terminal.

[0180]

[0181] 16. The source gNB-DU sends an LTM cell change (switch) notification message to the target gNB-DU via the source gNB-CU and the target gNB-CU. The message may include one or more of the following information: target cell ID, target cell global ID, (global) source gNB ID, source NG-RAN node ID, source gNB-DU ID, and TCI state ID.

[0182]

[0183] 17. The target gNB-DU can detect terminal access.

[0184]

[0185] 18. The target gNB-DU sends an access success message, including the target cell ID, to the target base station / gNB-CU. The access success message may also be forwarded from the target base station to the source base station. For example, the access success message may be a handover success message.

[0186] Some steps in the LTM operation procedures between base stations described above may be omitted. Furthermore, the names of the messages and procedures described above are exemplary and may be changed.

[0187] Below, more diverse embodiments and contents required for performing LTM operations between base stations are described.

[0188] Initiation or release of candidate cell modification by target base station / gNB-CU

[0189] LTM, which enables faster cell switching compared to L3-based handover, supports subsequent LTM. A subsequent LTM cell switch means that the UE performs an LTM cell switch without the network needing to reconfigure the candidate cell configuration. Subsequent LTMs can be performed by repeating the early synchronization, LTM execution, and LTM completion steps after each LTM completion without releasing other LTM candidate cell configurations. This subsequent LTM has enabled effective LTM within the same base station / gNB-CU (Intra-gNB-CU LTM). However, in the case of LTM between base stations / gNB-CUs (Inter-gNB-CU LTM), since the master base station / cell group changes depending on the LTM cell switch, radio resource changes / reconfigurations controlled by each base station are required before subsequent LTMs are performed compared to LTM within the same base station. This may increase the possibility of radio resource changes / reconfigurations occurring through coordination / signaling between base stations. Therefore, it may be desirable to allow the target base station / gNB-CU to initiate modification or release of the candidate configuration associated with that base station at any point during the execution (preparation / early synchronization / execution / completion) of the LTM (Inter-gNB-CU LTM) between the base station / gNB-CU.

[0190] For example, a target eNB / gNB-CU may request a candidate cell configuration modification to a source eNB / gNB-CU. The target eNB / gNB-CU may transmit a candidate cell modification request (modification required) message to the source eNB / gNB-CU via an Xn interface. The source eNB / gNB-CU (inter-CU) decides to initiate the corresponding LTM configuration modification. Upon receiving the message (or upon instructing the UE to send an RRC reconfiguration message indicating the corresponding candidate cell modification, or upon receiving an RRC reconfiguration complete message for the corresponding RRC reconfiguration message), the source eNB / gNB-CU may transmit a candidate cell modification confirm message to the target eNB / gNB-CU.

[0191] The above-described candidate cell modification request (modification required) message may include an LTM candidate configuration (or an LTM candidate configuration to be modified). For example, the message may include one or more of LTM reference configuration information, an LTM candidate configuration ID, an LTM candidate configuration information, an LTM candidate PCI, complete LTM configuration indication information, an LTM TCI state information, an LTM early uplink sync configuration information, an LTM SSB configuration information, and an LTM terminal measurement TA-ID. Here, the LTM reference configuration information includes an RRC reconfiguration message used to configure a reference configuration for LTM. The LTM candidate configuration ID indicates information indicating / identifying the LTM candidate configuration. The LTM candidate PCI indicates the PCI of the special cell of the configuration included in the LTM candidate configuration information. The complete LTM configuration indication information indicates whether the LTM candidate configuration in the LTM candidate configuration information is a complete configuration. The LTM TCI state information indicates a list of TCI states to be added / modified / released. LTM Early Uplink Sync Configuration Information indicates the information required to perform the Early Uplink Synchronization Procedure (e.g. frequencyInfoUL, rach-ConfigGeneric, ssb-PerRACH-Occasion). LTM SSB Configuration Information indicates the configuration of the SS / PBCH block used for L1 measurements. LTM Terminal Measurement TA-ID indicates information indicating whether to perform terminal-based TA measurements when the LTM cell switch procedure is executed for an LTM candidate cell.

[0192] As another example, the message may include a container including an LTM-Candidate IE (or an RRC internode message including sub-information of the LTM-Candidate IE). The container may include one or more of LTM reference configuration information, an LTM candidate configuration ID, an LTM candidate configuration information, an LTM candidate PCI, a complete LTM configuration indication information, an LTM TCI state information, an LTM early uplink sink configuration information, an LTM SSB configuration information, and an LTM terminal measurement TA-ID. The source base station / gNB-CU may instruct the terminal to send an RRC reconfiguration message including the LTM candidate configuration (or the LTM candidate configuration to be modified).

[0193] For another example, a target eNB / gNB-CU may request a source eNB / gNB-CU to release a candidate cell configuration. The target eNB / gNB-CU may transmit a candidate cell release request (release required) message to the source eNB / gNB-CU via the Xn interface. The message may include a candidate configuration ID to be released (e.g., LTM-CandidateId). The source eNB / gNB-CU (inter-CU) decides to initiate the LTM configuration release / modification. Upon receiving the message (or upon instructing the UE to send an RRC reconfiguration message indicating the candidate cell release, or upon receiving an RRC reconfiguration complete message for the RRC reconfiguration message), the source eNB / gNB-CU may transmit a candidate cell release confirm message to the target eNB / gNB-CU.

[0194] When a candidate cell is modified by the source base station / gNB-CU (or when the target base station / gNB-CU changes to the source base station / gNB-CU according to LTM), the target base station / gNB-CU is notified of the corresponding information.

[0195] As mentioned above, in the case of LTM between base stations / gNB-CUs (Inter-gNB-CU LTM), since the master base station / cell group changes depending on the LTM cell switch, compared to LTM within the same base station, the radio resources controlled by each base station may need to be changed / reconfigured before the subsequent LTM is performed, or this may increase the possibility of radio resource changes / reconfigurations occurring through coordination / signaling between base stations. Therefore, it may be desirable to take this into consideration when LTM between base stations / gNB-CUs (Inter-gNB-CU LTM) is performed (prepared / early synchronized / executed / completed) to effectively support the subsequent LTM.

[0196] For example, after an LTM cell switch is performed to support subsequent LTM, the current terminal configuration (or current serving cell / cell group configuration) of the terminal and the terminal configuration (or target candidate cell / cell group configuration) for the target candidate cell can be shared between the current base station / gNB-CU of the terminal and the target candidate base station / gNB-CU.

[0197] For example, after an LTM cell switch is performed to support subsequent LTM, if the current base station / gNB-CU of the corresponding terminal is the target base station / gNB-CU before the previous LTM, and the source base station / gNB-CU before the previous LTM becomes the current target candidate base station / gNB-CU, the current terminal configuration (or current serving cell / cell group configuration) of the corresponding terminal and the terminal configuration (or target candidate cell / cell group configuration) for the target candidate cell can be shared. For example, after the LTM procedure between base stations in FIG. 8 is completed (e.g., after step 18), the target base station in FIG. 10 becomes the current serving base station from the terminal's perspective. The corresponding base station can transmit LTM configuration information (LTM-Config IE) to the previous source base station. The information includes attemptLTM-Switch(If present, the UE shall execute an LTM cell switch if selected cell is a LTM candidate cell and it is the first cell selection after failure), LTM reference configuration information, LTM candidate cell configuration information to be additionally modified (ltm-CandidateToAddModList), ltm-ServingCellNoResetID(This field is used by the UE to determine whether L2 reset should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.) and ltm-ServingCellUE-MeasuredTA-ID(This field is used by the UE to determine on whether UE-based TA measurements should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.) may contain one or more pieces of information.

[0198] For example, when candidate cell modification / release is performed by a source base station / gNB-CU, the current terminal configuration (or current serving cell / cell group configuration) of the terminal and the terminal configuration (or target candidate cell / cell group configuration) for the target candidate cell need to be shared between the current base station / gNB-CU of the terminal and the target candidate base station / gNB-CU.

[0199] For another example, an LTM procedure within the source base station may be performed. Alternatively, the LTM configuration (modification / change / reconfiguration of one or more information elements included in the corresponding lower information elements) for a cell associated with the source base station (source gNB-CU / gNB-DU) may be changed by the source base station. For example, the source base station may transmit an RRC reconfiguration message to the terminal to modify / change / reconfigure the corresponding LTM configuration. The terminal may receive an RRC reconfiguration complete message for the corresponding RRC reconfiguration message. The source base station / gNB-CU may transmit a message to the target base station / gNB-CU to instruct modification / change of the corresponding LTM configuration.

[0200] The message may include LTM configuration information (LTM-Config IE) of the source base station. The information may include one or more of attemptLTM-Switch (If present, the UE shall execute an LTM cell switch if the selected cell is an LTM candidate cell and it is the first cell selection after failure), LTM reference configuration information, LTM candidate cell configuration information to be additionally modified (ltm-CandidateToAddModList), ltm-ServingCellNoResetID (This field is used by the UE to determine on whether L2 reset should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.), and ltm-ServingCellUE-MeasuredTA-ID (This field is used by the UE to determine on whether UE-based TA measurements should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.).

[0201] For another example, similar to the aforementioned Inter-gNB-CU LTM signaling, the source base station may transmit a message to indicate modification / change of the corresponding LTM configuration. The source base station / gNB-CU may transmit a message to the target base station / gNB-CU to indicate modification / change of the corresponding LTM configuration.

[0202] The message may include LTM configuration information (LTM-Config IE) of the source base station. The information may include one or more of attemptLTM-Switch (If present, the UE shall execute an LTM cell switch if the selected cell is an LTM candidate cell and it is the first cell selection after failure), LTM reference configuration information, LTM candidate cell configuration information to be additionally modified (ltm-CandidateToAddModList), ltm-ServingCellNoResetID (This field is used by the UE to determine on whether L2 reset should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.), and ltm-ServingCellUE-MeasuredTA-ID (This field is used by the UE to determine on whether UE-based TA measurements should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell.). The LTM configuration (modification / change / reconfiguration of one or more information elements included in the corresponding sub-information elements) for the cell associated with the source base station (source gNB-CU / gNB-DU) may be changed by the source base station. For example, the source base station may transmit an RRC reconfiguration message to the terminal to modify / change / reconfigure the corresponding LTM configuration.

[0203] LTM configuration coordination by the source base station / gNB-CU

[0204] A source base station / gNB-CU can generate an LTM reference configuration for LTM configuration. The source base station / gNB-CU can determine which base station / node / entity to retrieve / query / receive a low layer configuration from to generate the reference configuration.

[0205] For example, if a source eNB / gNB-CU decides to search for a lower layer configuration through a target candidate gNB-DU, the source eNB / gNB-CU may request the target candidate gNB-DU to generate a lower layer configuration through the target eNB / gNB-CU. The source eNB / gNB-CU may include indication information for requesting an LTM reference configuration (or an LTM lower layer configuration) to the target eNB / gNB-CU (or information indicating whether to include an LTM reference configuration). The target candidate gNB-DU may generate a lower layer configuration based on the request and transmit the lower layer configuration to the source eNB / gNB-CU through the target eNB / gNB-CU. The target candidate gNB-DU may provide a full lower layer configuration if it has not received a reference configuration from the source / target eNB / gNB-CU.

[0206] As another example, if the source BS / gNB-CU generated the reference configuration (or if the source BS / gNB-CU retrieved and received the reference configuration via the source gNB-DU (or any gNB-DU connected to the source BS / gNB-CU)), the source BS / gNB-CU may transmit the reference configuration to the target candidate gNB-DU via the target BS / gNB-CU. The source BS / gNB-CU may include LTM reference configuration (or LTM lower layer configuration) information to the target BS / gNB-CU. The target candidate gNB-DU may provide either a delta lower layer configuration or a full lower layer configuration for the reference configuration, if it has received the reference configuration from the source / target BS / gNB-CU.

[0207] For another example, for Inter-gNB-CU LTM, the RS configuration must be transmitted from the target candidate gNB-DU to the source gNB-DU. Therefore, the source base station / gNB-CU can request the target candidate gNB-DU to generate an RS configuration through the target base station / gNB-CU. The source base station / gNB-CU can include instructions for requesting an RS configuration (or LTM RS configuration) to the target base station / gNB-CU. The target candidate gNB-DU can generate an RS configuration based on the request and transmit it to the source base station / gNB-CU through the target base station / gNB-CU.

[0208] For another example, for inter-gNB-CU LTM, the RS configuration must be transmitted from the source gNB-DU to the target candidate gNB-DU. Therefore, the source base station / gNB-CU can transmit the source gNB-DU RS configuration to the target candidate gNB-DU through the target base station / gNB-CU. The source base station / gNB-CU can include the corresponding indication information to the target base station / gNB-CU. The target candidate gNB-DU can generate an RS configuration based on this and transmit it to the source base station / gNB-CU through the target base station / gNB-CU.

[0209] For example, early synchronization of cells associated with different base stations needs to be supported for inter-gNB-CU LTM. The source gNB-DU can request the UE to perform early synchronization / TA acquisition of the target candidate cell before the cell switch (or before transmitting the cell switch command to the UE). The early synchronization / TA acquisition can be triggered by a PDCCH order. For this purpose, the LTM configuration generated by the target base station / gNB-CU and transmitted to the UE via the source base station / gNB-CU can include CFRA resources for early synchronization / TA acquisition.

[0210] When CFRA is triggered by a PDCCH order from a source cell / gNB-DU, the UE may transmit a random access preamble to the indicated target candidate cell. The target candidate cell / gNB-DU, which receives the random access preamble, may transmit the corresponding TA information to the source cell / gNB-DU. Through this, the source cell / gNB-DU may consider (or recognize) that the UE has (successfully) performed / completed an early synchronization / TA acquisition procedure with the target candidate cell / gNB-DU. To this end, the target candidate cell / gNB-DU may transmit the TA information to the target base station / gNB-CU upon receiving the random access preamble. The target base station / gNB-CU may transmit the TA information to the source base station / gNB-CU. The source base station / gNB-CU may transmit the TA information to the source gNB-DU.

[0211] The source gNB-DU may consider TA information from a target candidate cell indicated by its cell identifier (or its NR CGI or its target candidate configuration identifier or its target candidate cell identifier) ​​containing the received information. The TA information may include one or more of the following information: cell identifier, NR CGI, target candidate configuration identifier, target candidate cell identifier, target gNB-DU ID, target gNB ID, source gNB-DU ID, source gNB ID, TA Value (e.g. TA value defined in 3GPP TS 38.213) and associated UE information (e.g. RACH-ConfigDedicated IE).

[0212] As another example, for an inter-gNB-CU LTM (or if the target candidate cell is a target cell associated with an inter-gNB-CU), the network (e.g., the source gNB-DU) may include information in the cell switch command to instruct the UE to access the target cell using a random access procedure. This may restrict the inter-gNB-CU LTM to perform uplink synchronization using a random access procedure.

[0213] For another example, after the source gNB-DU sends an LTM cell change command to the UE, the source gNB-DU may send an LTM cell change notification message / information to the target gNB-DU via the source gNB-CU and the target gNB-CU. The message may include one or more of the following information: a target cell ID, a target cell global ID (e.g., NR CGI), a (global) source gNB ID, a source NG-RAN node ID, a source gNB-DU ID, and a TCI state ID.

[0214] The source base station / gNB-CU that received the message / information from the source gNB-DU may consider that the LTM cell switch command was transmitted to the terminal and that the corresponding target candidate cell is the cell indicated by the received information (e.g., NR CGI). The target base station / gNB-CU that received the message / information from the source base station / gNB-CU may consider that the LTM cell switch command was transmitted to the terminal and that the corresponding target candidate cell is the cell indicated by the received information (e.g., NR CGI). Furthermore, the information may be considered to indicate a selected TCI state. The target gNB-DU that received the message / information from the target base station / gNB-CU may consider that the LTM cell switch command was transmitted to the terminal and that the corresponding target candidate cell is the cell indicated by the received information (e.g., NR CGI). Furthermore, the information may be considered to indicate a selected TCI state.

[0215] For another example, the cell switch command may include information for instructing the UE to access the target cell with a random access procedure. When one or more of the following occurs: receiving (or after receiving) an LTM cell change notification message / information of the target gNB-DU from the source-gNB-DU / target-gNB-CU, receiving a random access preamble from the UE, successfully completing the random access procedure to the UE, receiving any PUSCH (e.g., first uplink data) from the UE, receiving a PUSCH including an RRC reconfiguration complete message from the UE, or receiving an RRC reconfiguration complete message from the UE, the target gNB-DU may consider / detect that the UE access is successful. The target gNB-DU may send an access success message including a target cell ID to the target base station / gNB-CU. The target base station / gNB-CU may send an access success message to the source base station / gNB-CU. The access success message may include one or more of the following information: the cell ID, the cell global ID, the (global) gNB ID, the NG-RAN node ID, and the target gNB-DU ID.

[0216] If the LTM candidate cell configuration includes a configured grant, the UE may receive an LTM cell switch command including beam indication information and access the target cell via the configured grant. The UE may select a configured grant occasion associated with the beam. The target gNB-DU may consider / detect that the UE access is successful if at least one of: receiving (or after receiving) an LTM cell change notification message / information of the target gNB-DU from the source-gNB-DU / target-gNB-CU; receiving uplink data from the UE for the configured grant occasion; receiving any PUSCH (e.g., first uplink data) from the UE; receiving a PUSCH including an RRC reconfiguration complete message from the UE; and receiving an RRC reconfiguration complete message from the UE.

[0217] The target gNB-DU can send an access success message including the target cell ID to the target base station / gNB-CU. If the LTM candidate cell configuration does not include a configured grant, the UE can receive an LTM cell switch command and monitor the PDCCH to receive dynamic scheduling (e.g., scheduling grant) from the target cell. When (or after) the target gNB-DU receives the LTM cell change notification message / information, the target gNB-DU can instruct the UE to perform dynamic scheduling via the PDCCH. As another example, if the UE is configured for RACH-less LTM but does not have a configured grant, it can configure PUCCH resources for SR. When one or more of the following occurs: receiving (or after receiving) an LTM cell change notification message / information of the target gNB-DU from the source-gNB-DU / target-gNB-CU; receiving a PUCCH / SR from the UE; receiving a random access preamble from the UE; receiving any PUSCH (e.g., first uplink data) from the UE; receiving a PUSCH including an RRC reconfiguration complete message from the UE; or receiving an RRC reconfiguration complete message from the UE, the target gNB-DU may consider / detect that the UE access is successful. The target gNB-DU may send an access success message including a target cell ID to the target base station / gNB-CU.

[0218] Below, another embodiment according to the present disclosure is described. The following embodiment can be applied in combination or in combination with the above-described operation.

[0219] For LTM within the same eNB / gNB-CU (Intra-gNB-CU LTM), LTM cell switch can be performed (prepared / early synced / executed / completed) while maintaining the PDCP entities of the UE and eNB for the SRB / DRB. Accordingly, when the LTM switch is triggered, at least one PDCP entity, its state variables, buffers, and timers can be maintained for each SRB / DRB in the current UE configuration. For LTM within the same gNB-DU within the same eNB / gNB-CU, no separate additional action is required for the PDCP entity. For LTM between different gNB-DUs within the same eNB / gNB-CU, the PDCP data recovery procedure can be triggered depending on the air interface change.

[0220] To support LTM between eNBs / gNB-CUs (Inter-gNB-CU LTM), it may be difficult to maintain PDCP entities because LTM is executed through different eNBs / gNB-CUs. Reconfiguration of PDCP entities may be required to service a PDCP entity served by a source eNB to be serviced by a target eNB (or, to service a PDCP entity served by a source eNB to be serviced by a target eNB, release of the PDCP entity served by the source eNB and establishment of a PDCP entity served by the target eNB may be required). The base station may enable the terminal to control the PDCP protocol by distinguishing between intra-base station LTM and inter-base station LTM by instructing the terminal with information indicating whether PDCP entity re-establishment will be performed when the LTM cell switch procedure is triggered for the LTM candidate cell (or information that the terminal will use to determine whether PDCP entity re-establishment will be performed, or information indicating whether PDCP entity termination and establishment will be performed, or information that the terminal will use to determine whether PDCP entity termination and establishment will be performed; hereinafter, re-establishment is described for convenience of explanation, but it is obvious that termination and establishment are also included in the scope of the present disclosure). Alternatively, the base station may enable the terminal to control the PDCP protocol by distinguishing between intra-base station LTM and inter-base station LTM by instructing the terminal with any information indicating the (User-plane or PDCP) protocol processing operation of the terminal, including a determination of whether PDCP entity re-establishment will be performed when the LTM cell switch procedure is triggered for the LTM candidate cell. For convenience of explanation, this is referred to as LTM-PDCP Reset Indication Information hereinafter. This is for convenience of explanation and may be changed to any other name. Here, the PDCP Reset Indication Information may be information indicating PDCP re-establishment or information used to determine whether to re-establish PDCP.Accordingly, the PDCP reset indication information may be renamed as identification information used to determine whether to reset the PDCP. Accordingly, the present disclosure may be equally applicable even when the PDCP reset indication information is renamed as identification information used to determine whether to reset the PDCP.

[0221] PDCP re-establishment indication information may be included in the LTM configuration (LTM-Config) information element indicated through the RRC reconfiguration message. Here, the LTM configuration information element is used to provide LTM candidate configurations and is linked to the MCG / SCG. The LTM configuration information element may include, as sub-information elements, an LTM reference configuration information element including an RRC reconfiguration message / information element, and an LTM candidate list information element to be additionally modified. The LTM candidate list information element to be additionally modified may include, as sub-information elements, an LTM candidate configuration (ltm-CandidateConfig) information element, an information element for identifying an LTM candidate configuration (ltm-CandidateId), a PCI information element of a special cell included in the LTM candidate sub-information element, and an LTM candidate (LTM-Candidate) information element list / set including, as sub-information elements, a configuration information element used to perform an early synchronization procedure.

[0222] Thus, an LTM candidate configuration represents a configuration associated with a single LTM candidate cell. An LTM candidate cell configuration may be a complete LTM candidate cell configuration or a delta / difference configuration relative to an LTM reference configuration. An LTM reference configuration represents a common configuration within the same cell group for all configured (incomplete) LTM candidate configurations. An LTM reference configuration is used by a terminal to generate a complete LTM candidate cell configuration by applying an LTM candidate cell configuration on top of the LTM reference configuration.

[0223] For example, the LTM configuration (LTM-Config) information may include LTM-PDCP re-establishment indication information for the serving cell to indicate identification / differentiation information of the current serving cell / cell group (or identification / differentiation information of the PDCP entity re-establishment configuration of the current serving cell / cell group, or information for identifying / differentiating the current serving cell / special cell / cell group generated based on the reference configuration and / or candidate configuration applied to the current serving cell / cell group, or information for identifying / differentiating the PDCP entity re-establishment configuration of the current serving cell / special cell / cell group generated based on the reference configuration and / or candidate configuration applied to the current serving cell / cell group) that serves as a basis for determining whether the terminal requires (or will perform) a PDCP entity re-establishment (or a basis for determining the operation of the terminal including whether the PDCP entity re-establishment is required / performed).

[0224] For example, when a base station instructs a terminal with a first RRC reconfiguration message containing an LTM configuration (LTM-Config) having at least one LTM candidate configuration, the LTM configuration must include LTM-PDCP reconfiguration indication information for the serving cell (mandatory present). Otherwise, the LTM configuration must not include LTM-PDCP reconfiguration indication information for the serving cell (otherwise, the field is absent). For example, when a terminal is instructed with a first RRC reconfiguration message containing an LTM configuration, the LTM configuration stored for the terminal can determine whether to reconfigure PDCP through the LTM-PDCP reconfiguration indication information configuration for the current serving cell and the candidate cell configuration when LTM is performed, through the LTM-PDCP reconfiguration indication information configuration for the current serving cell and the LTM-PDCP reconfiguration indication information for the indicated candidate cell.

[0225] For example, the LTM candidate configuration (ltm-CandidateConfig) information, which includes the RRC reconfiguration message / information element used to configure one LTM candidate cell, can include LTM-PDCP reconfiguration indication information in the radio bearer configuration information element included below it. This allows the terminal to determine whether to reconfigure PDCP through the LTM-PDCP reconfiguration indication information for each radio bearer included in the candidate configuration when an LTM cell switch including candidate cell configuration identification information linked to the candidate configuration is performed.

[0226] For another example, the LTM candidate (LTM-Candidate) information element, which includes the aforementioned LTM candidate configuration (ltm-CandidateConfig) information and information for identifying the LTM candidate configuration (ltm-CandidateId) as sub-information elements, may include LTM-PDCP reset indication information. This allows the terminal to determine whether to reset LTM-PDCP for the radio bearer associated with the candidate cell when an LTM cell switch including the information identified by the candidate cell configuration is performed.

[0227] For another example, the LTM-PDCP reset indication information included in the LTM-Candidate information element may be indicated as a different information element / parameter from the LTM-PDCP reset indication information included in the LTM-Config information element. For example, the LTM-PDCP reset indication information included in the LTM-Config information element may indicate identification / identification information of the current serving cell / cell group for which the UE determines whether to reset the PDCP entity (or identification / identification information of the PDCP entity reset configuration of the current serving (special) cell / cell group), and the LTM-PDCP reset indication information included in the LTM-Candidate information element may indicate identification / identification information of the target candidate (special) cell / cell group for which the UE determines whether to reset the PDCP entity (or identification / identification information of the PDCP entity reset configuration of the target candidate (special) cell / cell group).

[0228] LTM, which enables faster cell switching compared to L3-based handover, supports subsequent LTM. A subsequent LTM cell switch means that the UE performs an LTM cell switch without the network needing to reconfigure the candidate cell configuration. Subsequent LTMs can be performed by repeating the early synchronization, LTM execution, and LTM completion steps after each LTM completion without releasing other LTM candidate cell configurations. This subsequent LTM has enabled effective LTM within the same base station / gNB-CU (Intra-gNB-CU LTM). However, in the case of LTM between base stations / gNB-CUs (Inter-gNB-CU LTM), since the master base station / cell group changes depending on the LTM cell switch, radio resource changes / reconfigurations controlled by each base station are required before subsequent LTMs are performed compared to LTM within the same base station. This may increase the possibility of radio resource changes / reconfigurations occurring through coordination / signaling between base stations. Therefore, it may be desirable to effectively support subsequent LTMs by taking this into consideration when LTM (Inter-gNB-CU LTM) is performed (prepared / early synchronized / executed / completed) between base stations / gNB-CUs.

[0229] For example, after an LTM cell switch is performed to support subsequent LTM, the current terminal configuration (or current serving cell / cell group configuration) of the terminal and the terminal configuration (or target candidate cell / cell group configuration) for the target candidate cell need to be shared between the current base station / gNB-CU of the terminal and the target candidate base station / gNB-CU. When LTM cell switch operation is triggered, the identification / differentiation information of the current serving cell / cell group (or the identification / differentiation information of the current serving cell / cell group configuration, or the identification / differentiation information of the PDCP reset indication information of the current serving cell / cell group, or the information for identifying / differentiating the current serving cell / special cell based on the reference configuration and / or candidate configuration applied to the current serving cell / cell group or identifying / differentiating the PDCP reset indication information of the current serving cell / special cell) that serves as the basis for determining whether the terminal needs (or will) reset the PDCP entity (or the basis for determining the operation of the terminal including whether the PDCP entity reset is needed / will be performed) may be stored / managed as terminal information. For convenience of explanation, this is hereinafter referred to as the serving cell LTM-PDCP reset terminal variable. This is for convenience of explanation and may be changed to any other name.

[0230] The UE can maintain the serving cell LTM-PDCP reset terminal variable associated with the LTM configuration (LTM-Config) (associated with MCG / SCG). When the UE receives the LTM configuration (LTM-Config), if the corresponding LTM configuration (LTM-Config) includes LTM-PDCP reset indication information (LTM-PDCP reset indication information for the serving cell), and if the current serving cell LTM-PDCP reset terminal variable includes LTM-PDCP reset indication information (LTM-PDCP reset indication information for the serving cell), the LTM-PDCP reset indication information (LTM-PDCP reset indication information for the serving cell) value in the serving cell LTM-PDCP reset terminal variable can be replaced with the received LTM-PDCP reset indication information (LTM-PDCP reset indication information for the serving cell). Otherwise, the LTM-PDCP reset indication information (LTM-PDCP reset indication information for the serving cell) can be stored in the serving cell LTM-PDCP reset terminal variable.

[0231] When the LTM cell switch is triggered by receiving the LTM MAC CE, the RRC of the terminal that received the instruction by the MAC layer can maintain the serving cell LTM-PDCP reset terminal variable when releasing / removing the current dedicated radio configuration associated with the cell group for which the LTM cell switch procedure was triggered.

[0232] For example, if the value of the LTM-PDCP Reset Indication Information (LTM-PDCP Reset Indication Information for the candidate cell) included in the LTM Candidate Information Element (LTM-Candidate IE) in the LTM Configuration Terminal Variable indicated by the lower layer is equal to the value of the LTM-PDCP Reset Indication Information for the serving cell in the Serving Cell LTM-PDCP Reset Terminal Variable, the current PDCP entity in the LTM candidate configuration indicated by the lower layer may continue to be used (or the current PDCP entity may continue to be used when applying the LTM candidate configuration indicated by the lower layer. Or the PDCP entity of the current source serving cell may continue to be used as the PDCP entity of the target candidate cell when applying the LTM candidate configuration indicated by the lower layer).

[0233] Otherwise, for each drb-Identity value that is part of the current UE configuration, the PDCP entity of this DRB may be triggered to perform PDCP re-establishment. The value of the serving cell LTM-PDCP reset indication information in the serving cell LTM-PDCP reset terminal variable may be replaced with the value of the LTM-PDCP reset indication information contained in the LTM candidate information element in the LTM configuration terminal variable indicated by the lower layer. Here, the LTM configuration terminal variable is a terminal variable used to store the reference configuration and the LTM candidate configuration, and includes the LTM reference configuration and the LTM candidate configurations (LTM candidate list to be further modified) information elements.

[0234] For another example, if the value of the LTM-PDCP Reset Indication Information (LTM-PDCP Reset Indication Information for the candidate cell) contained in the LTM Candidate Information Element (LTM-Candidate IE) in the LTM Configuration UE Variable indicated by the lower layer is not equal to the value of the LTM-PDCP Reset Indication Information for the serving cell in the Serving Cell LTM-PDCP Reset UE Variable, for each drb-Identity value that is part of the current UE configuration, the PDCP entity of this DRB may be triggered to perform PDCP re-establishment.

[0235] The value of the serving cell LTM-PDCP reset indication information in the serving cell LTM-PDCP reset terminal variable may be replaced with the value of the LTM-PDCP reset indication information included in the LTM candidate information element in the LTM configuration terminal variable indicated by the lower layer. Otherwise, the current PDCP entity in the LTM candidate configuration indicated by the lower layer may continue to be used (or the current PDCP entity may continue to be used when applying the LTM candidate configuration indicated by the lower layer; or the PDCP entity of the current source serving cell may continue to be used as the PDCP entity of the target candidate cell when applying the LTM candidate configuration indicated by the lower layer).

[0236] Hereinafter, the aforementioned embodiment will be described as an example. The base station can instruct the terminal connected to the current source serving cell an LTM configuration having four candidate cell configurations. For example, the candidate cell configurations are as follows. For candidate configuration ID 1, it has a PCell (PCI 1) linked to gNB-CU1 and gNB-DU1, and the LTM-PDCP reset indication information of the corresponding candidate cell is 1. For candidate configuration ID 2, it has a PCell (PCI 2) linked to gNB-CU1 and gNB-DU1, and the LTM-PDCP reset indication information of the corresponding candidate cell is 1. For candidate configuration ID 3, it has a PCell (PCI 3) linked to gNB-CU1 and gNB-DU2, and the LTM-PDCP reset indication information of the corresponding candidate cell is 1. For candidate configuration ID 4, there is a PCell (PCI 4) linked to gNB-CU2 and gNB-DU3, and the LTM-PDCP reset indication information of the corresponding candidate cell is 2. And the serving cell has the same PCell (PCI 1) as candidate configuration 1, and the LTM-PDCP reset indication information of the serving cell is 1. When the UE receives an LTM cell switch command that triggers a cell change from the serving cell to candidate configuration 4, since the LTM-PDCP reset indication information (1) of the serving cell and the LTM-PDCP reset indication information (2) of the candidate cell are not the same, if an SRB (SRB-ID) / DRB (DRB-ID) that is the same as an SRB (SRB-ID) / DRB (DRB-ID) that exists in the terminal configuration of the current serving cell exists in the candidate cell configuration, the UE can instruct to reset the PDCP entity for the corresponding SRB (SRB-ID) / DRB (DRB-ID).

[0237] As described above, the present embodiments can effectively perform cell switching between base stations based on LTM by providing a procedure for LTM between base stations and related signaling information to a terminal.

[0238] Fig. 11 is a drawing showing the configuration of a target base station according to another embodiment.

[0239] Referring to FIG. 11, a target base station (1100) that controls the performance of an LTM (L1 / L2 triggered mobility) operation may include a transmitter (1120) that transmits LTM configuration information to a source base station of a terminal, a control unit (1110) that controls the performance of an early synchronization procedure with the terminal, and a receiver (1130) that receives cell switch notification information from the source base station. The transmitter (1120) may transmit timing advance information to the source base station.

[0240] For example, the transmitter (1120) may transmit LTM configuration information to the source base station on which the terminal is camped in order to perform communication. For example, the LTM configuration information may be included in a response message to a handover request message transmitted from the source base station to the target base station. For example, the LTM configuration information may be included in a handover request confirmation message. Alternatively, the LTM configuration information may be included in an LTM configuration request response or confirmation message for an LTM configuration request. That is, the aforementioned LTM configuration information may be included in a response message to any message that the source base station requests the target base station to understand mobility control of the terminal.

[0241] Meanwhile, LTM configuration information may include at least one of Transmission Configuration Indicator (TCI) status information and Random Access Channel (RACH) configuration information. The TCI status information and RACH configuration information may be used for early synchronization between the terminal and the target base station. Therefore, the TCI status information and RACH configuration information may be included in the early synchronization configuration information.

[0242] In addition, the LTM configuration information may further include at least one of a reference signal configuration for a target candidate cell, a target candidate cell ID, a target candidate cell global ID, a target base station ID, a target NG-RAN node ID, and a target gNB-DU ID.

[0243] LTM configuration information can be transmitted through the Xn interface between the source base station and the target base station.

[0244] In addition, when the LTM operation of the terminal is instructed by the source base station to the terminal, the control unit (1110) can perform an early synchronization procedure with the terminal. That is, the terminal can perform an early synchronization operation toward the target candidate cell. For example, when a random access (contention-free random access) to the target base station (or target candidate cell) is triggered from the source base station, the terminal can initiate an early synchronization procedure by transmitting a random access preamble to the instructed target candidate cell. Through this, the terminal can perform early synchronization with the target candidate cell before receiving the cell switch command, thereby performing a fast mobility control operation. In this procedure, the control unit (1110) acquires a Timing Advanced (TA), and therefore, the early synchronization procedure is also described as an early TA acquisition procedure in the present disclosure.

[0245] The transmitter (110) can transmit timing advance information to the source gNB-DU via the target gNB-CU and the source gNB-CU. For example, the timing advance information can include at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and a timing advance value. For example, the timing advance information can be transmitted to the source base station via a CU-CU TA information transfer message.

[0246] Thereafter, the terminal receives an LTM cell switch command from the source base station. The LTM cell switch command can be indicated to the terminal via MAC CE or DCI.

[0247] The receiving unit (1130) can receive cell switch notification information from the source base station. For example, the cell switch notification information can be transmitted to the target gNB-DU through the source gNB-CU and the target gNB-CU. For example, the cell switch notification information can include at least one of target cell identifier information and TCI state identifier (TCI state ID) information. In addition, the cell switch notification information can be received by the target base station through a CU-CU cell switch notification message.

[0248] Meanwhile, after receiving cell switch notification information, the transmitter (1120) can transmit a handover success message to the source base station indicating that the terminal has successfully accessed the target base station.

[0249] For example, after the receiving unit (1120) receives cell switch notification information and the terminal completes cell switching to the target base station, the transmitting unit (1120) can transmit a handover success message to the source base station to notify the terminal of the end of the LTM operation. Through this, the LTM operation between the base stations of the terminal is completed, and the terminal can perform communication through the target base station.

[0250] Through the above operations, LTM operations can be performed not only within conventional intra-base station (or CU) but also between inter-base station (or CU). Therefore, terminal interruption time can be reduced, providing fast and uninterrupted data communication services.

[0251] In addition, the control unit (1110) controls the overall operation of the target base station (1100) to support the LTM operation between base stations required to perform the aforementioned embodiments.

[0252] The transmitter (1120) and receiver (1130) are used to transmit and receive signals, messages, and data necessary to perform the aforementioned embodiments with the terminal and the source base station.

[0253]

[0254] Fig. 12 is a diagram showing the configuration of a source base station according to another embodiment.

[0255] Referring to FIG. 12, a source base station (1200) that controls the performance of an LTM (L1 / L2 triggered mobility) operation may include a receiving unit (1220) that receives LTM configuration information from a target base station, receives timing advance information from the target base station after the target base station performs an early synchronization procedure with a terminal, and a transmitting unit (1230) that transmits cell switch notification information to the target base station.

[0256] For example, the transmitter (1230) may transmit a handover request message to the target base station. The receiver (1220) may receive LTM configuration information in response to the handover request message. For example, the LTM configuration information may be included in a response message to the handover request message transmitted by the source base station to the target base station. For example, the LTM configuration information may be included in a handover request confirmation message. Alternatively, the LTM configuration information may be included in an LTM configuration request response or confirmation message for the LTM configuration request.

[0257] Meanwhile, LTM configuration information may include at least one of Transmission Configuration Indicator (TCI) status information and Random Access Channel (RACH) configuration information. The TCI status information and RACH configuration information may be used for early synchronization between the terminal and the target base station. Therefore, the TCI status information and RACH configuration information may be included in the early synchronization configuration information.

[0258] In addition, the LTM configuration information may further include at least one of a reference signal configuration for a target candidate cell, a target candidate cell ID, a target candidate cell global ID, a target base station ID, a target NG-RAN node ID, and a target gNB-DU ID.

[0259] LTM configuration information can be transmitted through the Xn interface between the source base station and the target base station.

[0260] For example, the transmitter (1220) can transmit LTM configuration information to the terminal for the LTM operation of the terminal. When the LTM operation of the terminal is instructed to the terminal by the source base station, the target base station can perform an early synchronization procedure with the terminal. That is, the terminal can perform an early synchronization operation toward the target candidate cell. For example, when a random access (contention-free random access) to the target base station (or target candidate cell) is triggered from the source base station, the terminal can initiate the early synchronization procedure by transmitting a random access preamble to the instructed target candidate cell. Through this, the terminal can perform early synchronization with the target candidate cell before receiving the cell switch command, thereby performing a fast mobility control operation.

[0261] Timing advance information can be transmitted to the source gNB-DU via the target gNB-CU and the source gNB-CU. For example, the timing advance information can include at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and a timing advance value. For example, the timing advance information can be received by the receiver (1230) via a CU-CU TA information transmission message.

[0262] Thereafter, the transmitter (1220) transmits an LTM cell switch command to the terminal. The LTM cell switch command can be directed to the terminal via MAC CE or DCI.

[0263] Meanwhile, the cell switch notification information may be transmitted to the target gNB-DU via the source gNB-CU and the target gNB-CU. For example, the cell switch notification information may include at least one of target cell identifier information and TCI state identifier (TCI state ID) information. In addition, the cell switch notification information may be transmitted to the target base station via a CU-CU cell switch notification message.

[0264] Additionally, the receiving unit (1230) can receive a handover success message from the target base station indicating that the terminal has successfully accessed the target base station after transmitting the cell switch notification information.

[0265] For example, after receiving cell switch notification information, if the terminal completes a cell switch to the target base station, the target base station can transmit a handover success message to the source base station to notify the terminal of the termination of the LTM operation. This completes the terminal's LTM operation between base stations, allowing the terminal to communicate via the target base station.

[0266] In addition, the control unit (1210) controls the overall operation of the source base station (1200) to support the LTM operation between base stations required to perform the aforementioned embodiments.

[0267] The transmitter (1220) and receiver (1230) are used to transmit and receive signals, messages, and data necessary to perform the aforementioned embodiments with the terminal and target base station.

[0268] The above-described embodiments may be supported by standard documents disclosed in at least one of the wireless access systems, IEEE 802, 3GPP, and 3GPP2. That is, steps, components, and parts not described in the present embodiments to clearly illustrate the technical concepts herein may be supported by the above-described standard documents. Furthermore, all terms disclosed in this specification may be explained by the above-described standard documents.

[0269] The embodiments described above may be implemented through various means. For example, the embodiments may be implemented through hardware, firmware, software, or a combination thereof.

[0270] In the case of hardware implementation, the method according to the present embodiments may be implemented by one or more ASICs (Application Specific Integrated Circuits), DSPs (Digital Signal Processors), DSPDs (Digital Signal Processing Devices), PLDs (Programmable Logic Devices), FPGAs (Field Programmable Gate Arrays), processors, controllers, microcontrollers, or microprocessors.

[0271] When implemented using firmware or software, the methods according to the present embodiments may be implemented in the form of devices, procedures, or functions that perform the functions or operations described above. The software code may be stored in a memory unit and executed by a processor. The memory unit may be located within or outside the processor and may exchange data with the processor via various known means.

[0272] Additionally, terms such as "system," "processor," "controller," "component," "module," "interface," "model," or "unit" as described above may generally refer to a computer-related entity, such as hardware, a combination of hardware and software, software, or software in execution. For example, the aforementioned components may be, but are not limited to, a process driven by a processor, a processor, a controller, a control processor, an object, a thread of execution, a program, and / or a computer. For example, both an application running on a controller or a processor and the controller or the processor may be components. One or more components may be within a process and / or thread of execution, and the components may be located on a single device (e.g., a system, a computing device, etc.) or distributed across two or more devices.

[0273] The above description is merely an illustrative example of the technical idea of ​​the present disclosure, and those skilled in the art to which the present disclosure pertains will appreciate that various modifications and variations can be made without departing from the essential characteristics of the technical idea of ​​the present disclosure. In addition, the present embodiments are not intended to limit the technical idea of ​​the present disclosure but rather to explain it, and therefore the scope of the technical idea of ​​the present disclosure is not limited by these embodiments. The scope of protection of the present disclosure should be interpreted by the claims below, and all technical ideas within a scope equivalent thereto should be interpreted as being included within the scope of the rights of the present disclosure.

[0274]

[0275] CROSS-REFERENCE TO RELATED APPLICATION

[0276] This patent application claims priority under 35 USC §119(a) to Korean Patent Application No. 10-2023-0144200, filed in Korea on October 25, 2023, and Korean Patent Application No. 10-2024-0139563, filed in Korea on October 14, 2024, the entire contents of which are incorporated herein by reference. In addition, this patent application claims priority in countries other than the United States for the same reasons, the entire contents of which are incorporated herein by reference.

Claims

1. A method for controlling a target base station to perform LTM (L1 / L2 triggered mobility) operation, A step of transmitting LTM configuration information to the source base station of the terminal; A step of performing an early synchronization procedure with the above terminal; a step of transmitting timing advance information to the source base station; and A method comprising the step of receiving cell switch notification information from the source base station.

2. In paragraph 1, The above LTM configuration information is: A method for including at least one of TCI (Transmission Configuration Indicator) status information and RACH (Random Access CHannel) configuration information in a handover request confirmation message for a handover request message received from the source base station.

3. In paragraph 1, The above timing advance information is, A method comprising at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and timing advance value.

4. In paragraph 1, The above cell switch notification information is: A method comprising at least one of target cell identifier information and TCI state identifier (TCI state ID) information.

5. In paragraph 1, A method further comprising the step of transmitting a handover success message indicating that the terminal has successfully accessed the target base station to the source base station after the step of receiving the cell switch notification information.

6. In a method for controlling a source base station to perform LTM (L1 / L2 triggered mobility) operation, A step of receiving LTM configuration information from a target base station; A step of receiving timing advance information from the target base station after the target base station performs an early synchronization procedure with the terminal; and A method comprising the step of transmitting cell switch notification information to the target base station.

7. In paragraph 6, The above LTM configuration information is: A method that is included in a handover request confirmation message and includes at least one of TCI (Transmission Configuration Indicator) status information and RACH (Random Access CHannel) configuration information.

8. In paragraph 6, The above timing advance information is, A method comprising at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and timing advance value.

9. In paragraph 6, The above cell switch notification information is: A method comprising at least one of target cell identifier information and TCI state identifier (TCI state ID) information.

10. In paragraph 6, A method further comprising, after the step of transmitting the cell switch notification information, a step of receiving a handover success message from the target base station indicating that the terminal has successfully accessed the target base station.

11. In a target base station that controls the performance of LTM (L1 / L2 triggered mobility) operation, A transmitter that transmits LTM configuration information to the source base station of the terminal; A control unit for controlling the performance of the above terminal and early synchronization procedure; and Including a receiving unit that receives cell switch notification information from the source base station, The above transmitter, A target base station that transmits timing advance information to the source base station.

12. In paragraph 11, The above LTM configuration information is: A target base station that is included in a handover request confirmation message for a handover request message received from the source base station, and includes at least one of TCI (Transmission Configuration Indicator) status information and RACH (Random Access CHannel) configuration information.

13. In paragraph 11, The above timing advance information is, A target base station including at least one of candidate cell identifier information, associated Contention Free Random Access (CFRA) resource information, and timing advance value.

14. In paragraph 11, The above cell switch notification information is: A target base station including at least one of target cell identifier information and TCI state identifier (TCI state ID) information.

15. In paragraph 11, The above transmitter, A target base station that further transmits a handover success message to the source base station after receiving the cell switch notification information, indicating that the terminal has successfully accessed the target base station.

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