Apparatus and method for performing handover in a wireless communication system

Abstract

The present disclosure relates to a wireless communication system, in particular to a method for performing handover in a wireless communication system, the method comprising the steps of: receiving a measurement report message from a terminal; transmitting a handover request message to a target cell and a candidate cell based on the measurement report message; receiving a handover request acknowledgement message from the target cell and the candidate cell; transmitting a handover command message to the terminal; receiving a conditional handover indication message from the terminal in response to a conditional handover event detected by the terminal based on additional measurement of the target cell and the candidate cell; transmitting the conditional handover indication message to the target cell; and transmitting an uplink grant to the terminal in response to receiving the uplink grant from the target cell, wherein a time difference between the transmission of the conditional handover indication in the terminal and the reception of the uplink grant in the terminal can be below a set time threshold.

Description

Technical Field

[0001] This disclosure relates to wireless communication systems, and more particularly to devices and methods for performing handover in wireless communication systems. Background Technology

[0002] With the development of mobile communication services, non-terrestrial networks (NTNs) that are not limited by space have attracted attention, especially with the expectation that low-Earth orbit satellites will play a major role. Therefore, in 3GPP (3rd Generation Partnership Project), [the following text appears to be incomplete and requires further context: 3GPP, ... rd In the Generation Partnership Project, standardization is being implemented for non-terrestrial networks that are not limited by space in order to provide communication services efficiently.

[0003] This research was conducted in 2024 with funding from the Ministry of Science, ICT and Information, and with assistance from the Science and Technology Agency for Business Development (RS-2024-00423453, aimed at promoting the commercialization and industrialization of next-generation low-Earth orbit swarm satellite application technologies).

[0004] This research was conducted in 2024 with funding from the government (Ministry of Science, Technology and ICT) and assistance from the Information and Communications Planning and Evaluation Institute (NO.RS-2021-II210794, Development of 3D Space Mobile Communication Technology), (NO.RS-2021-II210704, Development of 3D-NET Core Technology for Ultra-High-Speed ​​Mobile Support). Summary of the Invention

[0005] Technical issues

[0006] This disclosure is intended to provide apparatus and methods for efficiently performing handover in wireless communication systems.

[0007] This disclosure is intended to provide apparatus and methods for performing RACH-less (Randon Access Channel-less) handover in a wireless communication system.

[0008] This disclosure is intended to provide an apparatus and method for performing a conditional handover (CHO) in a wireless communication system.

[0009] This disclosure provides apparatus and methods for performing handover between terrestrial base stations and satellite base stations in a wireless communication system.

[0010] This disclosure provides apparatus and methods for improving handover performance in non-terrestrial network environments within wireless communication systems.

[0011] This disclosure relates to devices and methods for providing conditional handover techniques and RACH-free handover techniques in wireless communication systems for use in non-terrestrial networks.

[0012] This disclosure provides apparatus and methods for combining conditional handover of non-terrestrial networks with RACH-free handover in wireless communication systems.

[0013] This disclosure provides apparatus and methods for supporting conditional RACH-handling in wireless communication systems, combining conditional handover with no RACH-handling, which includes signal strength, distance, and elevation angle based on timing.

[0014] This disclosure is intended to provide devices and methods for selecting a switching request message link in a wireless communication system.

[0015] Solution to the problem

[0016] According to an embodiment of this disclosure, a method for operating a base station in a wireless communication system is disclosed. The method may include: receiving a measurement report message from a terminal; sending a handover request message to a target cell and a candidate cell based on the measurement report message; receiving a handover request confirmation message from the target cell and the candidate cell; sending a handover command message to the terminal; receiving a conditional handover indication message from the terminal based on a conditional handover event detected by the terminal through additional measurements of the target cell and the candidate cell; sending the conditional handover indication message to the target cell; and if an uplink grant is received from the target cell, sending the uplink grant to the terminal, wherein the time difference between the sending of the conditional handover indication and the receiving of the uplink grant in the terminal may be below a set time threshold.

[0017] According to another embodiment of this disclosure, a base station in a wireless communication system is disclosed. The base station includes a transceiver and a processor connected to the transceiver. The processor is configured to: receive a measurement report message from a terminal; based on the measurement report message, send a handover request message to a target cell and a candidate cell; receive a handover request confirmation message from the target cell and the candidate cell; send a handover command message to the terminal; detect a conditional handover event based on additional measurements of the target cell and the candidate cell by the terminal; receive a conditional handover indication message from the terminal; and send the conditional handover indication message to the target cell; and if an uplink grant is received from the target cell, send the uplink grant to the terminal. The time difference between the transmission of the conditional handover indication and the reception of the uplink grant in the terminal can be below a set time threshold.

[0018] According to another embodiment of this disclosure, a terminal operation method in a wireless communication system is disclosed. The method may include the following steps: sending a measurement report message to a base station; performing measurements on a target cell and a candidate cell based on a handover command message received from the base station; sending a conditional handover indication message to the base station based on at least one of the conditional handover events in the target cell and the candidate cell; and, after receiving the conditional handover indication message, performing a no-RACH handover or a conditional handover with the target cell if an uplink grant is received before a time threshold has elapsed.

[0019] According to another embodiment of this disclosure, a terminal in a wireless communication system is disclosed. The terminal includes a transceiver and a processor connected to the transceiver. The processor is configured to: send a measurement report message to a base station; perform measurements on a target cell and a candidate cell based on a handover command message received from the base station; send a handover indication message to the base station based on a handover event satisfying at least one of the target cell and the candidate cell; and, after sending the conditional handover indication message, perform a no-RACH handover or a conditional handover with the target cell when an uplink grant is received before a time threshold has elapsed.

[0020] Invention Effects

[0021] According to embodiments of this disclosure, handover of terminals utilizing non-terrestrial networks can be performed efficiently. Attached Figure Description

[0022] Figure 1 An example of a satellite network according to an embodiment of the present disclosure is illustrated.

[0023] Figure 2 The illustration shows an example of handover in a satellite network.

[0024] Figure 3 Another example of a satellite network according to an embodiment of the present disclosure is illustrated.

[0025] Figure 4 The illustration shows the configuration of a device in a wireless communication system according to an embodiment of the present disclosure.

[0026] Figure 5 An example of a RACH-free handover step in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0027] Figure 6 An example of a condition switching step in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0028] Figure 7 An example of the operation steps of a terminal in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0029] Figure 8 An example of a switching step based on a time threshold is illustrated according to an embodiment of the present disclosure.

[0030] Figure 9 An example of the operating steps of a serving cell in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0031] Figure 10 An example of a RACH-free handover procedure in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0032] Figure 11 An example of a conditional handover operation step of a target cell in a wireless communication system according to an embodiment of the present disclosure is illustrated. Detailed Implementation

[0033] The terminology used in this embodiment takes into account the functionality of this embodiment and has selected commonly used terms as much as possible. However, these terms may vary depending on the intent of those skilled in the art, precedents, or the emergence of new technologies. In addition, in specific cases, there may be terms arbitrarily selected by the applicant, in which case their meanings will be described in detail in the corresponding sections. Therefore, the terminology used in this embodiment is not merely the name of the term, but is defined based on the meaning of the term and the overall context of the embodiment.

[0034] This embodiment can be modified in various ways and can have multiple forms. Some embodiments are illustrated in the accompanying drawings for detailed description. However, this is not intended to limit this embodiment to a specific form of disclosure, and should be understood to include all modifications, equivalents, or substitutions encompassing the ideas and techniques of this embodiment. The terminology used in this specification is for illustrative purposes only and is not intended to limit this embodiment.

[0035] Unless otherwise defined, the terms used in this embodiment have the same meaning as commonly understood by those skilled in the art. Terms identical to their definitions in commonly used dictionaries should be interpreted as having the meaning consistent with their meaning in the relevant technical context, and should not be interpreted as having an idealized or overly formalized meaning unless explicitly defined in this embodiment.

[0036] This disclosure relates to terminal handover techniques utilizing non-terrestrial networks (NTNs). In terrestrial networks, Conditional Handover (CHO) technology has been developed to reduce handover failure rates, and RACH-less (Random Access Channel-less) handover technology has been developed to reduce handover downtime and signaling overhead. These various handover techniques can also be similarly applied to non-terrestrial network environments. However, without intermediate steps to effectively combine conditional handover and RACH-less handover, communication performance may actually degrade due to resource waste or interference. In particular, the problem of communication performance may be more pronounced in non-terrestrial network environments where frequent handovers occur. As a result, new steps are required in non-terrestrial network environments to simultaneously apply conditional handover and RACH-less handover. Therefore, this disclosure proposes steps for applying conditional handover and RACH-less handover to non-terrestrial networks.

[0037] Figure 1 An example of a satellite network according to an embodiment of the present disclosure is illustrated.

[0038] refer to Figure 1 The satellite network includes terminal 110, satellite 120-1, satellite 120-2, and gateway 130. Terminal 110 is a user equipment equipped with hardware and software that receives cellular data from satellite 120-1, and can be a mobile or fixed device. For example, terminal 110 may include mobile phones, smartphones, wearable devices, and UEs (user equipment). Furthermore, terminal 110 is not limited to the examples described above, and includes all electronic devices capable of cellular communication, such as laptops and tablet PCs. Figure 1 The satellite network diagram in the image shows only a single terminal 110, but it is only an illustrative embodiment and is not limited thereto. Of course, it may include multiple terminals 110.

[0039] Specifically, terminal 110 can support 3GPP (3rd Generation Partnership Project). rdThe communication protocols specified in the Generation Partnership Project (GPP) standard (e.g., LTE communication protocol, LTE-A communication protocol, NR communication protocol, etc.). Multiple communication nodes 110 to 130 can support CDMA (Code Division Multiple Access), WCDMA (Wideband CDMA), TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), OFDM (Orthogonal Frequency Division Multiplexing), Filtered OFDM, CP (Cyclic Prefix)-OFDM, DFT-s-OFDM (Discrete Fourier Transform-Spread-OFDM), OFDMA (Orthogonal Frequency Division Multiple Access), SC (Single Carrier)-FDMA, NOMA (Non-Orthogonal Multiple Access), GFDM (Generalized Frequency Division Multiplexing), and FBMC (Filter Bank Multicarrier). Multi-carrier technology, UFMC (universal filtered multi-carrier) technology, SDMA (space division multiple access) technology, etc.

[0040] Satellites 120-1 and 120-2 fly in designated orbits, providing coverage areas of a certain size by forming beams pointing towards the ground. In connection with this disclosure, satellite 120-1 may refer to a serving satellite, and satellite 120-2 may refer to a target satellite. Gateway 130 provides a link to satellites 120-1 and 120-2 for network connectivity.

[0041] The link between terminal 110 and satellite 120-1 is called a service link and can be based on the NR standard defined in 3GPP. The link between satellite 120-1, satellite 120-2, and gateway 130 is called a feeder link and can be based on a 3GPP or non-3GPP radio interface. Inter-satellite links (ISL) are mainly used for regenerative satellite scenarios.

[0042] In the case of transparent satellites based on the NR-RAN architecture, the satellite radio interface for the feeder link and service link can be NR-Uu. In the case of transparent satellites, the satellite performs radio frequency filtering, frequency conversion, and amplification functions. In the case of regenerative satellites, the satellite has onboard functions built-in, thus enabling it to perform not only radio frequency filtering, frequency conversion, and amplification, but also some or all of the base station functions such as switching and routing, encoding and modulation, and decoding and demodulation.

[0043] Figure 2 The illustration shows an example of handover in a satellite network.

[0044] refer to Figure 2 According to each satellite of this disclosure ( Figure 2 The serving satellite, candidate satellite, and target satellite can provide a cell with a certain size of coverage to the terminal. Furthermore, each satellite can be connected to the gateway 130 via a feeder link. This link can be a link based on the NR standard. Alternatively, it can be adapted to newly defined links in evolving next-generation wireless communication systems, or links applicable to various interfaces of communication systems introduced based on industry needs rather than the NR standard. In the following text, in connection with this disclosure, the terms serving cell, target cell, and candidate cell may be used, corresponding to serving satellite, target satellite, and candidate satellite respectively, and may be used interchangeably.

[0045] A service satellite can refer to a satellite that provides current communication with a terminal in a satellite network. For example, a service satellite may include geostationary satellites, low Earth orbit satellites, medium Earth orbit satellites, polar orbit satellites, elliptical orbit satellites, etc., and is not limited to these types. In addition, the satellites mentioned in this disclosure are not limited to a specific satellite configuration, and may include any satellite or combination of satellites that can provide functional connectivity between gateway 130 and terminal 110.

[0046] The target satellite can refer to the satellite that is replaced by the serving satellite when the terminal 110 is outside the beam coverage range of the serving satellite or is located near the boundary, and the strength of the signal received by the terminal meets the handover conditions.

[0047] Candidate satellites can refer to satellites within a satellite network that can be selected as target satellites. That is, candidate satellites can be determined based at least on factors such as satellite visibility, signal strength, connection stability, latency, and network load. On the other hand, in the following text, neighboring cells may be mentioned, which can refer to satellites adjacent to the target satellite and the candidate satellite, respectively referred to as the target cell and the candidate cell.

[0048] In connection with this disclosure, terminal 110 periodically measures the signal strength of satellite base stations adjacent to the serving satellite and reports the signal strength and the duration of its own communication service to the serving satellite. Then, when the signal strength received by terminal 110 from the serving satellite meets predetermined handover conditions, it attempts to hand over to the target satellite.

[0049] Figure 3 Another example of a satellite network according to an embodiment of the present disclosure is illustrated. Figure 3 The illustration shows an example of NTN providing non-terrestrial access to UE210 using NTN payload 220 and NTN gateway 230. UE210 can be connected to... Figure 1 The terminal 110 described herein has the same configuration. (See reference...) Figure 2 The link between NTN payload 220 and UE 210 is a serving link, which can be based on the Uu interface. The link between NTN payload 220 and NTN gateway 230 is a feeder link. The link between NTN gateway 230 and AMF / UPF 240 can be based on the NG interface. NTN payload 220 can transparently forward radio protocols received from UE 210 to NTN gateway 230 via the serving link. Similarly, NTN payload 220 can transparently forward radio protocols received from NTN gateway 230 to UE 210 via the feeder link.

[0050] Therefore, the NTN payload 220 can support the following connectivity: A base station can serve multiple NTN payloads. An NTN payload can be served by multiple base stations.

[0051] The NTN payload 220 can change its carrier frequency before retransmitting data in the serving link. That is, the NTN payload 220 can use different carrier frequencies in the serving link and the feeder link. For NTN, at least one of the following can be used as the network identifier: AMF name, NCGI (NR cellglobal identifier), CgNB ID, global gNB ID, TAI (tracking area identity), S-NSSAI (Single Network Slice Selection Assistance information), NSAG (Network Slice ASGroup), NID (Network Identifier), CAG (Closed Access Group) ID, and Local NG-RAN Node ID. Additionally, a Mapped Cell ID can also be used. The tracking area can correspond to a fixed geographical area.

[0052] Non-Geosynchronous orbit (NGSO) includes low Earth orbit with an altitude of approximately 300 km to 1500 km and medium Earth orbit with an altitude of approximately 7000 km to 25000 km.

[0053] Service links can be categorized into three types: earth-fixed, quasi-earth-fixed, and earth-moving. Earth-fixed links provide a beam that continuously covers the same geographic area at all times. For example, a satellite in geosynchronous orbit (GSO) can provide an earth-fixed service link. Quasi-earth-fixed links provide a beam that continuously covers the same geographic area for a defined period of time, or a beam that continuously covers different geographic areas for different periods of time. For example, a satellite in a non-geosynchronous orbit using a steerable beam can provide a quasi-earth-fixed service link. Earth-moving links provide a beam whose coverage area slides across the Earth's surface. For example, a satellite in a non-geosynchronous orbit using a fixed or steerable beam can provide an earth-moving service link.

[0054] Using satellites in non-geosynchronous orbit, base stations can provide quasi-geo-fixed cell coverage or geo-mobile cell coverage. Using satellites in geosynchronous orbit, base stations can provide geo-fixed cell coverage. In the case of non-geosynchronous orbit, changes to the serving link can be related to changes in the serving satellite 120-1.

[0055] In recent years, 3GPP has been standardizing handover-related aspects in non-terrestrial networks. In Rel-17, to reduce handover failure rates, it was decided to support conditional handover. In Rel-18, in non-terrestrial network environments, to reduce signal overhead and handover interruption time caused by RACH, it was decided to support RACH-free handover. Furthermore, discussions are ongoing regarding methods combining conditional handover and RACH-free handover in non-terrestrial network environments. When combining these two handover technologies, since non-terrestrial base stations cannot know the handover execution time of terrestrial terminals, uplink grants may not be easily transmitted via PDCCH. Therefore, this disclosure proposes a technique that combines time-based conditional handover with RACH-free handover to address this problem.

[0056] Figure 4 The illustration shows the configuration of a device in a wireless communication system according to an embodiment of the present disclosure. Figure 4 The equipment can be understood as a reference. Figure 1The device described is a part of the structure of any one of terminal 110, satellite 120-1, satellite 120-2, and gateway 130.

[0057] refer to Figure 4 The device may include a processor 310, a communication unit 320, and a storage unit 330.

[0058] Processor 310 can control the overall functions and operations of the device. Processor 310 may include ASIC (application-specific integrated circuit), other chipsets, logic circuits, and / or data processing devices.

[0059] The communication unit 320 is connected to the processor 310 to transmit and receive wireless signals. The communication unit 320 may include baseband circuitry for processing wireless signals. For example, the communication unit 320 may include a short-range communication unit, a mobile communication unit, or a broadcast receiving unit. In one embodiment, the communication unit 320 can transmit and receive data with other devices, such as base stations, satellites, etc.

[0060] Storage 330 is hardware that stores various data processed by processor 310. For example, storage 330 can store the SIR value of the transmitting terminal for the transmission target terminal, information about the transmission target terminal group for each transmitting terminal, etc. Furthermore, storage 330 can store applications, drivers, etc., driven by processor 310. Storage 330 may include RAM (random access memory) such as DRAM (dynamic random access memory), SRAM (static random access memory); ROM (read-only memory); EEPROM (electrically erasable programmable read-only memory); CD-ROM; Blu-ray or other optical disc storage; HDD (hard disk drive); SSD (solid state drive); or flash memory.

[0061] Figure 4 The structure can be understood as at least a part of a terminal, base station, satellite, and gateway. Figure 4 When the structure is part of a satellite, the satellite, in addition to Figure 4 In addition to the components illustrated herein, other hardware equipment required for orbital flight may also be included. Figure 4When the structure is part of a gateway or base station, the gateway or base station may also include components that support wired communication, etc.

[0062] Figure 5 The illustration shows an example of a RACH-free handover process in a wireless communication system according to an embodiment of the present disclosure. In the following description, for ease of explanation, the serving satellite (or serving cell) will be referred to as the first base station 520-1. The target satellite (or target cell) and candidate satellite (or candidate cell) may correspond to any one of the second base station 520-2, the third base station 520-3, and the fourth base station (520-2), respectively, and may be modified according to the steps described below. Furthermore, base stations 520-2, 520-3, and 520-4 may be collectively referred to as neighboring cells or neighboring base stations, etc.

[0063] refer to Figure 5 In step S501, terminal 510 measures the signal. Specifically, in order to reserve resources for handover, terminal 510 measures the signal strength of base stations 520-1, 520-2, 530-3, and 530-4. Terminal 510 can measure the signal strength according to a predetermined period, which may be referred to as the measurement interval in this disclosure.

[0064] In detail, terminal 510 can measure the signal strength of base stations 520-1, 520-2, 530-3, and 530-4 to detect a handover preparation event. As an example, if the measured signal strength or quality is below a predefined threshold or within a predetermined range, terminal 510 can determine that a handover preparation event has been detected. Furthermore, as an example, if the signal strength or quality provided by neighboring base stations 520-2, 530-3, and 530-4 is above a specific threshold compared to the signal strength or quality provided by the first base station 520-1 of the serving cell, terminal 510 can determine that a handover preparation event has been detected.

[0065] In step S503, terminal 510 sends a measurement report to the first base station 520-1. Specifically, terminal 510 periodically measures the signal strength of the first base station 520-1 and base stations 520-2, 530-3, and 530-4, and sends a measurement report message. The measurement report may include at least a portion of the set elements of a measurement report. For example, the measurement configuration of the measurement report may include a measurement object, a reporting configuration, a measurement ID (measurement identities), a quantity configuration, and a measurement gap. As an example, terminal 510 may measure at least one of SINR (Signal to Interference and Noise Ratio), RSRQ (Reference Signal Received Quality), and RSRP (Reference Signal Received Power) of the first base station 520-1 and base stations 520-2, 530-3, and 530-4. In addition, the terminal 510 can measure the power strength of the received signal (RSSI, Received Signal Strength Indicator) and the signal-to-noise ratio (SNR), but is not limited to these.

[0066] In step S505, the first base station 520-1 decides to hand over. Specifically, the first base station 520-1 confirms at least one of RSRP, RSRQ, or SINR through the received measurement report. At this time, the first base station 520-1 may decide to hand over when the measured signal strength or quality is below a predefined threshold or within a predetermined range. Furthermore, as an example, the first base station 520-1 may decide to hand over when the signal strength or quality provided by neighboring base stations 520-2, 530-3, and 530-4 is above a specific threshold compared to the signal strength or quality provided by the first base station 520-1, which is the serving cell.

[0067] In step S507, the first base station 520-1 sends a handover request. Specifically, the first base station 520-1 sends a handover request message to the target cell. The target cell can be determined based on measurement reports for base stations 520-2, 530-3, and 530-4. As an example, the target cell can be determined based on RSRP, RSSI, RSRQ, SINR, traffic load of each base station, priority order, speed of terminal 510, frequency band, call state, and terminal capabilities (UE capabilities) for base stations 520-2, 530-3, and 530-4. In the following explanation, for ease of explanation, it is assumed that the second base station 520-2 is determined to be the target cell, and note that the target cell may change as the steps are executed. Similarly, it is assumed that the third base stations 520-3 and 520-4 are determined to be candidate cells, and these may change as the steps are executed.

[0068] The handover request message includes information for terminal 510, which may include UE identity, source cell ID, target cell ID, terminal context, target cell information, E-RAB (E-UTRAN Radio Access Bearer), etc., but is not limited to these. On the other hand, the first base station 520-1 can send the handover request message to the third base station 520-3 and the fourth base station 520-4, which are candidate cells. For example, the first base station 520-1 can directly send the handover request message to the third base station 520-3, or it can send it indirectly through the core network 530. Similarly, the first base station 520-1 can also send the handover request message to the second base station 520-2, which is the target cell, through the core network 530. That is, the first base station 520-1 sends the handover request message to candidate satellites that meet the handover event. The handover request message can be sent via an inter-satellite link (ISL) or a feeder link. Serving cell 520-1 determines the link by comparing the delay time generated by the link between satellites with the delay time generated by the feeder link, and can use this link to send the handover request message.

[0069] In step S509, the second base station 520-2, acting as the target cell, and the third and fourth base stations 520-3 and 520-4, acting as candidate cells, respectively perform admission control. Specifically, the second base station 520-2 decides whether to accept the handover request based on the handover request message. If there is a risk of insufficient handover resources or degraded handover quality, it can reject the handover request. As an example, the second base station 520-2 can calculate the available bandwidth, frequency resources, cell capacity, and QoS requirements to determine whether to accept the handover.

[0070] In step S511, the second base station 520-2, acting as the target cell, and the third and fourth base stations 520-3 and 520-4, acting as candidate cells, respectively send handover request acknowledgements. Specifically, when the second base station 520-2 determines that handover is possible based on the admission control execution result, it sends a handover request acknowledgement message to the first base station 520-1. Conversely, the third and fourth base stations 520-3 and 520-4 can send handover request acknowledgement messages to the first base station 520-1. For example, the third and fourth base stations 520-3 and 520-4 can send the handover request acknowledgement messages directly to the first base station 520-1, or they can send them indirectly through the core network 530. Similarly, the second base station 520-2 can also send the handover request message to the first base station 520-1 through the core network 530.

[0071] In step S513, the first base station 520-1, serving as the first base station, sends a handover command. Specifically, the first base station 520-1 can send a handover command message to the terminal 510. The handover command message can include information about the second base station 520-2 as the target cell and the third base station 520-3 and the fourth base station 520-4 as candidate cells. This information can include C-RNTI, Timing Advance (TA), and location information. Thus, the first base station 520-1 can allocate RNTIs (e.g., G(group)-RNTI, C-RNTI) for broadcast or multicast to the terminal 510. The corresponding RNTIs can be provided in advance through system information or provided along with measurement settings. Steps S501 to S513 described above can be called handover preparation steps, which can be understood as the process of reserving resources for performing the handover.

[0072] In step S515, terminal 510 performs additional signal measurements. This can be understood as terminal 510 detecting the execution event for handover. Specifically, in order to perform a RACH-free handover or conditional handover, terminal 510 performs additional measurements on the signal strength of base stations 520-2, 530-3, and 530-4, which are neighboring cells. As an example, terminal 510 can measure at least one of SINR, RSRQ, and RSRP of the first base station 520-1, which is the serving cell, and the base stations 520-2, 530-3, and 530-4, which are neighboring cells. In the following explanation, it is assumed that although terminal 510 performs additional signal measurements, the second base station 520-2 is maintained as the target cell, and the third and fourth cells 520-3 and 520-4 are candidate cells. Of course, the target cell and candidate cells can be changed due to the additional signal measurements performed by terminal 510.

[0073] In step S517, terminal 510 sends a conditional handover indication. Specifically, when at least one of the second base station 520-2 (target cell) and the third base station 520-3 and fourth base station 520-4 (candidate cells) satisfies a conditional handover event, terminal 510 sends a conditional handover indication message to the cell that satisfies the conditional handover event. As an example, terminal 510 sends the conditional handover indication message to first base station 520-1, and first base station 520-1 can send the conditional handover indication message to second base station 520-2, third base station 520-3, and fourth base station 520-4 that satisfies the conditional handover event. As another example, terminal 510 sends the conditional handover indication message to first base station 520-1, and first base station 520-1 can send the message to second base station 520-2, third base station 520-3, and fourth base station 520-4 that satisfies the conditional handover event through core network 530.

[0074] After sending a conditional handover instruction, terminal 510 waits for a time threshold until it receives a PDCCH signal. Then, when the time threshold expires, terminal 510 sends a conditional handover instruction again and waits for another time threshold until it receives a PDCCH. The time threshold is a timer set by terminal 510 after receiving the handover command message to prevent RACH-based handover waiting; it can be set based on the physical time of data packet transmission from terminal 510 to the receiving end. The receiving end may include a second base station 520-2 as the target cell and a third base station 520-3 and a fourth base station 520-4 as candidate cells, and can be set differently according to user definition or system settings.

[0075] In step S519, the second base station 520-2, acting as the target cell, sends a PDCCH. Specifically, the second base station 520-2 sends an uplink grant to the first base station 520-1 via the PDCCH. At this time, the third base station 520-3 and the fourth base station 520-4, acting as other candidate satellites, can release the resources reserved for handover. Specifically, the conditional handover indication message may include information about the target cell or candidate cells. As an example, the third base station 520-3 and the fourth base station 520-4, having received the conditional handover indication message, are identified as candidate cells in their relationship with the terminal 510, and therefore the resources reserved for handover can be released.

[0076] exist Figure 5 The diagram illustrates the transmission of a PDCCH by the second base station 520-2, but it is not limited to this. When the target cell for handover changes, the PDCCH can be transmitted by either candidate cell 520-3 or candidate cell 520-4, which has been changed to the target cell. Furthermore, in... Figure 5The diagram illustrates the transmission of PDCCH to the first base station 520-1, but it is not limited to this and may include the direct transmission of PDCCH to the terminal 510.

[0077] In step S521, when sending the PDCCH to the first base station 520-1, the terminal 510 sends an RRC connection completion (Radio Resource Control connection reconfiguration complete) message. Specifically, when the terminal 510 receives the PDCCH, it transmits the applicable radio resource settings completed according to the RRC connection to the second base station 520-2, which is the target cell. As an example, the terminal 510 sends a handover confirmation message to the target cell 520-2 using the uplink grant information based on the received PDCCH.

[0078] In step S523, the second base station 520-2, acting as the target cell, sends a handover completion message to the first base station 520-1, acting as the serving cell. Upon receiving the handover completion message, the first base station 520-1 terminates its connection with the terminal 510 and releases the resources allocated to the terminal 510. Furthermore, the first base station 520-1 sends its SN status (Sequence Number Status) and data to the second base station 520-2 to prevent duplicate data transmission and data loss.

[0079] In the following text, through Figure 6 The process of performing conditional handover steps when no RACH handover fails is explained. Figure 6 An example of a conditional handover step in a wireless communication system according to an embodiment of the present disclosure is illustrated. Hereinafter, for ease of description, the serving satellite (or serving cell) will be referred to as the first base station 620-1. The target satellite (or target cell) and candidate satellite (or candidate cell) may correspond to any one of the second base station 620-2, the third base station 620-3, and the fourth base station 620-2, respectively, and may be changed according to the steps described below. Furthermore, base stations 620-2, 620-3, and 620-4 may be collectively referred to as neighboring cells or neighboring base stations, etc.

[0080] refer to Figure 6 In step S601, terminal 610 measures the signal. Specifically, in order to reserve resources for handover, terminal 610 measures the signal strength of base stations 620-1, 620-2, 630-3, and 630-4. Terminal 610 can measure the signal strength according to a predetermined period, which is referred to as the measurement interval in this disclosure.

[0081] In detail, terminal 610 measures the signal strength of base stations 620-1, 620-2, 630-3, and 630-4, and can detect a handover preparation event. As an example, when the measured signal strength or quality is below a predefined threshold or within a predetermined range, terminal 610 can determine that a handover preparation event has been detected. Furthermore, as an example, when the signal strength or quality provided by neighboring base stations 520-2, 520-3, and 520-4 is above a specific threshold compared to the signal strength or quality provided by the first base station 520-1, which is the serving cell, terminal 510 can determine that a handover preparation event has been detected.

[0082] In step S603, terminal 610 sends a measurement report to the first base station 620-1. Specifically, terminal 610 periodically measures the signal strength of the first base station 620-1 and base stations 620-2, 630-3, and 630-4 and sends a measurement report message. The measurement report may include at least a portion of the set elements of a measurement report. For example, the measurement configuration of the measurement report may include a measurement object, a reporting configuration, measurement identities, a quantity configuration, and a measurement gap. As an example, terminal 610 may measure at least one of SINR (Signal to Interference and Noise Ratio), RSRQ (Reference Signal Received Quality), and RSRP (Reference Signal Received Power) of the first base station 620-1 and base stations 620-2, 630-3, and 630-4. Therefore, the terminal 610 can measure RSSI (Received Signal Received Quality), which is the strength of the received signal, and SNR (Signal to Noise Ratio), which is the ratio between signal and noise, but is not limited to these.

[0083] In step S605, the first base station 620-1 decides to hand over. Specifically, the first base station 620-1 confirms at least one of RSRP, RSRQ, or SINR through a received measurement report. At this time, if the measured signal strength or quality is below a predefined threshold or within a predetermined range, the first base station 620-1 may decide to hand over. Furthermore, as an example, if the signal strength or quality provided by neighboring base stations 620-2, 630-3, and 630-4 is above a specific threshold compared to the signal strength or quality provided by the first base station 620-1, which is the serving cell, the terminal 610 may decide to hand over. Furthermore, as an example, if the signal strength or quality provided by neighboring base stations 620-2, 630-3, and 630-4 is above a specific threshold compared to the signal strength or quality provided by the first base station 620-1, which is the serving cell, the first base station 620-1 may decide to hand over.

[0084] In step S607, the first base station 620-1 sends a handover request. Specifically, the first base station 620-1 sends a handover request message to the target cell. The target cell can be determined based on measurement reports for base stations 620-2, 630-3, and 630-4. As an example, the target cell is determined based on RSRP, RSSI, RSRQ, SINR, traffic load, and priority of base stations 620-2, 630-3, and 630-4; and the speed, frequency band, call state, and UE capabilities of terminal 610. In the following explanation, for ease of illustration, the second base station 620-2 will be described as the target cell. Note that the target cell may change depending on the execution of the steps. Similarly, the third base station 620-3 and the fourth base station 620-4 will be described as candidate cells, and these may also change depending on the execution of the steps.

[0085] The handover request message includes information for terminal 610, which may include UE identity, source cell ID, target cell ID, terminal context, target cell information, E-RAB (E-UTRAN Radio Access Bearer), etc., but is not limited to these. On the other hand, the first base station 620-1 can send the handover request message to the third base station 620-3 and the fourth base station 620-4, which are candidate cells. For example, the first base station 620-1 can send the handover request message directly to the third base station 620-3, or indirectly through the core network 630. Similarly, the first base station 620-1 can also send the handover request message to the second base station 620-2, which is the target cell, through the core network 630. That is, the first base station 620-1 sends the handover request message to candidate satellites that meet the handover event. The handover request message can be sent via an inter-satellite link (ISL) or a feeder link. The first base station 620-1 compares the delay time generated by the link between satellites with the delay time generated by the feeder link to determine the link, and uses the link to send the handover request message.

[0086] In step S609, the second base station 620-2, acting as the target cell, and the third and fourth base stations 620-3 and 620-4, acting as candidate cells, respectively perform admission control. Specifically, the second base station 620-2 decides whether to accept the handover request based on the handover request message. If there is a risk of insufficient network resources or quality degradation, the handover request can be rejected. As an example, the second base station 620-2 calculates available bandwidth, frequency resources, cell capacity, and QoS requirements to determine whether to accept the handover.

[0087] In step S611, the second base station 620-2, acting as the target cell, and the third base station 620-3 and fourth base station 620-4, acting as candidate cells, each send a handover request acknowledgement. Specifically, when the second base station 620-2 determines that a handover is possible based on the admission control results, it sends a handover request acknowledgement message to the first base station 620-1. Alternatively, the third base station 620-3 and fourth base station 620-4 can send the handover request acknowledgement message directly to the first base station 620-1. For example, the third base station 620-3 and fourth base station 620-4 can send the handover request acknowledgement message directly to the first base station 620-1, or they can send it indirectly through the core network 630. Similarly, the second base station 620-2 can also send the handover request message to the first base station 620-1 through the core network 630.

[0088] In step S613, the first base station 620-1, serving as the first base station, sends a handover command. Specifically, the first base station 620-1 can send a handover command message to the terminal 610. The handover command message can include information about the second base station 620-2 as the target cell and the third base station 620-3 and the fourth base station 620-4 as candidate cells. This information can include C-RNTI, Timing Advance (TA), and location information. Thus, the first base station 620-1 can allocate RNTIs for broadcast or multicast (e.g., G-RNTI, C-RNTI) to the terminal 610. The corresponding RNTIs can be provided in advance through system information or provided along with measurement settings. Steps S601 to S613 described above can be called handover preparation steps, which can be understood as the process of reserving resources for performing the handover.

[0089] In step S615, terminal 610 performs additional signal measurements. This can be understood as terminal 610 detecting the execution event for handover. Specifically, in order to perform a RACH-free handover or conditional handover, terminal 610 additionally measures the signal strength of base stations 620-2, 630-3, and 630-4, which are neighboring cells. As an example, terminal 610 can measure at least one of SINR, RSRQ, and RSRP of the first base station 620-1, which is the serving cell, and the base stations 620-2, 630-3, and 630-4, which are neighboring cells. In the following description, although terminal 610 performs additional signal measurements, the description continues with the premise that the second base station 620-2 is the target cell and the third and fourth cells 620-3 and 620-4 are candidate cells. However, due to the additional signal measurements performed by terminal 610, the target and candidate cells can of course be changed.

[0090] In step S617, terminal 610 sends a conditional handover indication. Specifically, when at least one of the second base station 620-2, the third base station 620-3, and the fourth base station 620-4 in terminal 610 satisfies the conditional handover event, terminal 610 sends a conditional handover indication message to the cell that satisfies the conditional handover event. After sending the conditional handover indication message, terminal 610 waits again for a time threshold until receiving the PDCCH. Then, when the time threshold is reached, terminal 610 sends the conditional handover indication again and waits again for the time threshold until receiving the PDCCH. Terminal 610 repeats the above process up to the specified number of conditions. The number of conditions can refer to the number of times the S617 step is repeated, which can be understood as the maximum number of times it can be repeated. As an example, the number of conditions can be determined based on the handover interruption time according to the RACH, the physical time of data packet transmission from terminal 610 to the receiving end, and the handover failure timer.

[0091] In step S619, during the conditional count, if no PDCCH is received, the terminal performs RACH. Terminal 610 obtains the uplink grant information from the first base station 620-1 through the RACH step and uses this information to send a handover confirmation message to the second base station 620-2. Specifically, terminal 610 can send a preamble to the second base station 620-2, and the second base station 620-2 can send an acknowledgment to terminal 610 for RA (Random Access). Then, terminal 610 sends an RRC connection complete (Radio Resource Control connection reconfiguration complete) message to the second base station 620-2, indicating that the radio resource settings have been successfully applied according to the RRC connection completion.

[0092] In step S621, the second base station 620-2, acting as the target cell, sends a handover completion message to the first base station 620-1, acting as the serving cell. Upon receiving the handover completion message, the first base station 620-1 disconnects from the terminal 610 and releases the resources allocated to the terminal 610. Furthermore, the first base station 620-1 sends SN status (Sequence Number Status) and data to the second base station 620-2 to prevent duplicate data transmission and data loss.

[0093] Figure 7 An example of the operation steps of a terminal in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0094] refer to Figure 7 In step S701, the terminal sends a measurement report. Specifically, the terminal periodically measures the signal strength of the serving cell and neighboring cells and sends a measurement report message. As an example, the terminal can measure at least one of the SINR, RSRQ, RSRP, RSSI, and SNR of the serving cell and neighboring cells.

[0095] In step S703, the terminal receives a handover command. Specifically, the terminal receives a handover command message from the serving cell. This message may include information about the target cell and candidate cells, including C-RNTI, Timing Advance (TA), and location information.

[0096] In step S705, the terminal performs a handover based on a time threshold. Before performing the handover, the terminal may perform additional signal measurements. This can be understood as the terminal detecting the handover execution event. Specifically, in order to perform a RACH-free handover or a conditional handover, the terminal additionally measures the signal strength of the base stations of neighboring cells. As an example, the terminal may measure at least one of the SINR, RSRQ, and RSRP of the serving cell and neighboring cells.

[0097] Specifically, the terminal performs RACH-free handover and conditional handover based on a time threshold and a condition count. More specifically, after sending a conditional handover indication, the terminal waits for a time threshold until it receives the PDCCH, and then repeats the above process up to the condition count.

[0098] In step S707, the terminal sends an RRC connection completion message. Specifically, after attempting a handover without RACH, if the terminal 110 does not receive a PDCCH within the time threshold and conditional count, it sends an RRC connection completion message to the target cell upon successful handover, based on the conditional handover processing steps. As an example, when the terminal receives the PDCCH, it transmits to the target cell the radio resource settings that have been correctly applied according to the RRC connection completion message.

[0099] Figure 8 An example of a switching step based on a time threshold is illustrated according to an embodiment of the present disclosure.

[0100] refer to Figure 8 In step S801, the terminal performs additional signal measurements. Specifically, in order to perform a RACH-free handover or a conditional handover, the terminal further measures the signal strength of neighboring cells. As an example, the terminal can measure at least one of the SINR, RSRQ, and RSRP of the serving cell and neighboring cells. The steps following the terminal's additional signal measurements can be called the handover execution step.

[0101] In step S803, the terminal determines whether the conditional handover event is met. The terminal actually adds measurement signals to perform the handover, and executes the handover when the conditional handover event is met. The terminal can check whether the conditional handover event is met in each predetermined cycle; if not, the terminal executes step S801 again.

[0102] A conditional handover event can be defined as a condition used to perform a handover. For example, a conditional handover event can be satisfied when factors such as RSSI (Resonance Score Indicator) of power intensity, SNR (Signal-to-Noise Ratio), and signal quality drop below a predefined threshold or enter a predetermined range. The setting of a conditional handover event can be more stringent than the conditions for signal strength or quality studied and measured by the serving cell to reserve resources during the handover preparation process.

[0103] In step S805, when a conditional handover event is met, the terminal sends a conditional handover indication based on a time threshold. Specifically, when at least one of the target cell and candidate cells meets the conditional handover event, the terminal sends a conditional handover indication message to the cell that meets the conditional handover event.

[0104] As one example, the terminal sends a handover indication message to the serving cell, which can then send the same message to target and candidate cells that meet the handover event criteria. As another example, the terminal sends a handover indication message to the serving cell, which can then forward the message to target and candidate cells that meet the handover event criteria via the core network.

[0105] After sending a conditional handover indication, the terminal waits for a time threshold until it receives the PDCCH. Then, when the time threshold expires, the terminal sends a conditional handover indication again and waits for another time threshold until it receives the PDCCH. The time threshold is a timer that the terminal uses to wait for a RACH-based handover after receiving the handover command message; it can be set based on the physical time of data packet transmission from the terminal to the receiving end. The receiving end can include a target cell and candidate cells, which can be specified differently based on user definition or system settings.

[0106] In step S807, the terminal determines whether to receive the PDCCH before the time threshold expires. First, after sending a condition switching indication, the terminal waits for the time threshold until it receives the PDCCH. Specifically, the PDCCH can be sent from the target cell to the serving cell, but it is not limited to this; the PDCCH can be directly transmitted from the target cell to the terminal.

[0107] The time threshold refers to the specific time after sending the condition switching indication message, until the PDCCH is received. This specific time can be called... The term "time threshold" can be used interchangeably in the following context. Specifically, it refers to the timer during which terminal 110 waits for a RACH-to-handover transition after receiving the handover command message; it can be defined as the physical time for transmitting data packets from the terminal to the receiving end. As an example, the time threshold can be defined as the maximum round-trip time of data packets from the terminal to the receiving end. For instance, the maximum round-trip time can be determined by dividing the distance from the terminal to the receiving end by twice the speed of light, and can be compensated for by factors such as inter-satellite routing, network processing delays, signal interference, and physical obstacles.

[0108] For example, when the altitude of the regenerating satellite serving as the receiver is 600 km, the maximum round-trip time can be determined by doubling the one-way time (the straight-line distance between the terminal and the satellite divided by the speed of light), which is approximately 12.89 ms. Furthermore, the maximum round-trip time can be compensated for by obstacle factors; for instance, the maximum round-trip time can be determined by increasing the maximum round-trip time by a certain percentage.

[0109] At this time, the receiving end can be set as the target cell of the handover object. When any of the candidate cells is changed to the handover object, the candidate cell that is changed to the target cell can be set as the receiving end.

[0110] In step S809, if a PDCCH is received before the time threshold expires, the terminal will perform a RACH-free handover with the target cell. Specifically, when the target cell sends an uplink grant via PDCCH, the terminal will complete the handover to the target cell without a RACH procedure.

[0111] In step S811, if the PDCCH is not received before the time threshold expires, the terminal determines whether the condition count has been exceeded. The condition count can refer to a specific number of times the terminal sends a condition switching indication message to execute RACH. This specific number can be called... The following terms can be used interchangeably. That is, the condition count can refer to the number of times a conditional handover indication is sent, which can be understood as the maximum number of times it can be repeated. As an example, the condition count can be determined based on the handover interruption time according to RACH, the physical time for transmitting data packets from the terminal to the receiver, and the handover failure timer. Furthermore, the condition count can be determined by considering a time threshold.

[0112] For example, given that the altitude of the regenerating satellite serving as the receiver is 600km, the time threshold is set to 12.89ms, and the handover interruption time can be set to 8.5ms. Furthermore, the sum of the time threshold for the number of condition repetitions and the handover interruption time must be less than the handover failure timer T304 (50ms). That is, the sum of the time threshold for the number of condition repetitions and the handover interruption time needs to be less than the value of 50ms for the handover failure timer T304; therefore, the maximum number of condition repetitions can be set to 3 (50ms > 12.89ms). 3+8.5ms).

[0113] In step S813, if the number of times the terminal sends a condition switching indication exceeds the condition count, the terminal executes RACH. On the other hand, if the number of times the terminal sends a condition switching indication does not exceed the condition count, the terminal repeats the steps after S805.

[0114] Figure 9 An example of the operating steps of a serving cell in a wireless access system according to an embodiment of the present disclosure is illustrated.

[0115] refer to Figure 9 In step S901, the serving cell receives a measurement report and determines handover conditions. Specifically, the serving cell confirms RSRP, RSRQ, or SINR, etc., through the measurement report sent from the terminal. Then, when the measured signal strength or quality drops below a predefined threshold or enters a predetermined range, the serving cell can decide to handover. Furthermore, as an example, when the signal strength or quality provided by a neighboring base station is above a certain threshold compared to the signal strength or quality provided by the serving cell, the serving cell can decide to handover.

[0116] In step S903, the serving cell sends a handover request and receives a handover request confirmation at the same time as sending a handover command.

[0117] Specifically, the serving cell sends a handover request message to the target cell. The target cell can be determined based on measurement reports of the base stations that are neighboring cells. As an example, the target cell can be determined based on the base station's RSRP, RSSI, RSRQ, SINR, traffic load of each base station, priority, terminal speed, frequency band, connection status, and terminal performance.

[0118] The handover request message includes information about the terminal, which may include, but is not limited to, the UE identity, source cell ID, target cell ID, terminal context, target cell information, and E-RAB (E-UTRAN Radio Access Bearer). The serving cell can send the handover request message directly to the candidate cell, or it can send it indirectly through the core network. Similarly, the serving cell can also send the handover request message to the target cell through the core network.

[0119] When the target cell determines that a handover is possible after performing admission control, it sends a handover request message to the serving cell. Conversely, the candidate cell can send a handover request confirmation message to the serving cell. For example, the candidate cell can send the handover request message directly to the serving cell, or it can send it indirectly through the core network. Similarly, the target cell can also send a handover request message to the serving cell through the core network.

[0120] The serving cell can send a handover command message to the terminal. This message can include information about the target cell and candidate cells, such as the C-RNTI, Timing Advance (TA), and location information. The serving cell can then assign the terminal an RNTI for broadcast or multicast (e.g., G-RNTI, C-RNTI). The corresponding RNTI can be provided in advance via system information or along with measurement settings.

[0121] The serving cell can send and receive handover request messages, handover request confirmation messages, and handover command messages using either a feeder link or an inter-satellite link. Specifically, the serving cell compares the latency generated by the feeder link with the latency generated by the inter-satellite link to determine the link to use. For example, to ensure communication stability, the serving cell can utilize the link with the longer latency among the aforementioned links.

[0122] In step S905, the serving cell receives and sends conditional handover indications. Specifically, the serving cell receives conditional handover indications from the terminal and sends conditional handover indication messages to cells that meet the conditional handover event. As an example, the serving cell can directly send the message to the target cell and candidate cells that meet the conditional handover event. As another example, the serving cell can send conditional handover indication messages to the target cell and candidate cells through the core network.

[0123] In step S907, the serving cell receives the handover completion message. Specifically, after sending a conditional handover indication, the serving cell transmits and receives PDCCH before the time threshold expires, completing the conditional handover steps of no RACH handover or RACH handover, and then receives the handover completion message from the target cell. Based on the received handover completion message, the serving cell terminates its connection with the terminal and releases the resources allocated to the terminal. Furthermore, the serving cell sends its SN status and data to the target cell to prevent data duplication and loss.

[0124] Figure 10 An example of a RACH-free handover procedure in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0125] refer to Figure 10 In step S1001, neighboring cells receive handover requests and send confirmation of handover requests based on admission control. A neighboring cell can be a cell adjacent to the serving cell, which can be determined using the serving cell's neighbor cell list.

[0126] Specifically, the serving cell sends a handover request message to the target cell among its neighboring cells. The target cell can be determined based on measurement reports of neighboring cells. As an example, the target cell can be determined based on the RSRP, RSSI, RSRQ, SINR, traffic load of each base station, priority, terminal speed, frequency band, connection status, and terminal performance of neighboring cells.

[0127] The handover request message includes information about the terminal, which may include, but is not limited to, the UE identity, source cell ID, target cell ID, terminal context, target cell information, and E-RAB (E-UTRAN Radio Access Bearer). On the other hand, the serving cell can directly send the handover request message to candidate cells among neighboring cells. For example, the serving cell can directly send the handover request message to a candidate cell, or indirectly through the core network. Similarly, the serving cell can also send the handover request message to the target cell through the core network.

[0128] The target cell and candidate cells each perform admission control separately. Specifically, the second base station 520-2 decides whether to accept the handover request based on the handover request message. If there is a risk of insufficient network resources or degraded quality, the handover request can be rejected.

[0129] Next, both the target cell and the candidate cell send handover request confirmation messages. Specifically, if the target cell determines that a handover is possible based on the admission control results, it sends a handover request confirmation message to the serving cell. On the other hand, the candidate cell can send a handover request message to the serving cell. For example, the candidate cell can send the handover request confirmation message directly to the serving cell, or it can send it indirectly through the core network. Similarly, the target cell can also initiate a handover request message to the serving cell through the core network.

[0130] In step S1003, neighboring cells determine whether they are the target cell based on the received condition handover indication message. The condition handover indication message may include information about the target cell or candidate cells.

[0131] In step S1005, when the target cell receives the condition handover indication message, the target cell sends an uplink grant via PDCCH. Specifically, the target cell sends an uplink grant to the serving cell via PDCCH.

[0132] In step S1007, if the recipient of the conditional handover indication message is a candidate cell, the handover is cancelled. That is, the candidate cell releases the resources reserved for handover. The conditional handover indication message may include information about the target cell or the candidate cell. As an example, if the candidate cell receiving the conditional handover indication message can be identified as a candidate cell in its relationship with the terminal, then the resources reserved for handover can be released. In this way, the target cell can perform a RACH-free handover with the terminal.

[0133] In step S1009, the target cell sends a handover completion message. Specifically, the target cell sends a handover completion message to the serving cell and receives the SN status and data from the serving cell to prevent data duplication and loss.

[0134] Figure 11 An example of a conditional handover operation step of a target cell in a wireless communication system according to an embodiment of the present disclosure is illustrated.

[0135] refer to Figure 11 In step S1101, the target cell receives the handover request and sends a handover request confirmation based on the admission control. Specifically, after receiving the handover request message from the serving cell, the target cell executes the admission control and, if it determines that a handover is possible, sends a handover request confirmation message to the serving cell.

[0136] In step S1103, if the target cell does not receive a conditional handover indication during the conditional count period, it performs a RACH with the terminal. The conditional count can refer to the specific number of times the terminal sends a conditional handover indication message to perform the RACH. This specific number can be called... The following terms can be used interchangeably. That is, the number of conditional handover attempts can refer to the number of times a conditional handover indication is sent, which can be understood as the maximum number of repetitions. Specifically, the terminal obtains the uplink grant information of the serving cell through the RACH procedure and uses this information to send a handover confirmation message to the target cell. More specifically, the terminal sends a preamble to the target cell, and the target cell can send a response to the RA to the terminal. Then, the terminal sends an RRC connection completion message to the target cell, conveying the radio resource settings that have been correctly applied according to the RRC connection completion. If the target cell is unable to send uplink grants through the PDCCH, the target cell also performs RACH with the terminal.

[0137] In step S1105, the target cell sends a handover completion message. Specifically, the target cell sends a handover completion message to the serving cell and receives the SN status and data from the serving cell to prevent data duplication and loss.

[0138] According to the various embodiments described above, a RACH-free handover can be performed for the target cell. Furthermore, according to the various embodiments, when a RACH-free handover cannot be performed, a RACH handover can be performed for the target cell according to the steps described above. The RACH-free handover according to the various embodiments can be performed in various scenarios. For example, when the target cell is a satellite base station, the above-described RACH-free handover can be performed. As an example, when there are multiple target cells, including at least one satellite base station and at least one terrestrial base station, the RACH-free conditional handover applies only to the satellite base station, while the RACH-equipped conditional handover can be performed for the terrestrial base station. In this case, the aforementioned time-related information and uplink authorization are only provided to the target cell that is the satellite base station.

[0139] On the other hand, in the technical field related to this embodiment, it can be understood that those skilled in the art will implement variations without departing from the essential characteristics described above. Therefore, the disclosed method should be considered from an interpretative rather than a limiting perspective. The scope of this disclosure is shown in the claims rather than in the foregoing description, and all differences within the equivalent scope are to be interpreted as included in this disclosure.

[0140] Potential for commercial applications

[0141] This invention can be used in wireless communication systems for devices that perform handover.

Claims

1. A method for operating a base station in a wireless communication system, comprising: The steps for receiving measurement report messages from the terminal; The step of sending a handover request message to the target cell and candidate cells based on the measurement report message; The step of receiving a handover request confirmation message from the target cell and the candidate cell; The step of sending a handover command message to the terminal; The step of receiving a conditional handover indication message from the terminal based on the additional measurements of the target cell and candidate cells by the terminal to detect a conditional handover event; The step of sending the condition handover indication message to the target cell; as well as If an uplink grant is received from the target cell, then the step of sending the uplink grant to the terminal is as follows: The time difference between the transmission of the condition switching indication in the terminal and the reception of the uplink grant in the terminal is below a set time threshold.

2. The method according to claim 1, wherein, The switching request message is sent using at least one of the inter-satellite link and the feeder link.

3. The method according to claim 2, wherein, The handover request message is sent based on a link determined by a first delay time occurring through the inter-satellite link and a second delay time occurring through the feeder link.

4. The method according to claim 1, wherein, The condition handover indication message is sent to the target cell and the candidate cell that meet the above-mentioned condition handover event.

5. The method according to claim 1, wherein, The time threshold is set based on the physical time of the data packets transmitted from the terminal to the receiving end. The receiving end is either the target cell or the candidate cell.

6. The method according to claim 5, wherein, The time threshold is set based on the maximum round-trip time of the data packet from the terminal to the receiving end.

7. The method according to claim 1, wherein, The condition switching indication message is received by the base station a maximum number of times for each of the time thresholds.

8. The method according to claim 7, wherein, The number of conditions is determined based on the RACH handover interruption time, round-trip time, handover failure timer, and the time threshold.

9. The method according to claim 7, wherein, The steps for receiving the handover completion message include: If the uplink grant is not received from the target cell within the specified number of conditions, the terminal performs RACH with the target cell and receives the handover completion message.

10. A base station in a wireless communication system, comprising: transceiver; as well as The processor connected to the transceiver The processor is configured as follows: Receive measurement report messages from the terminal. Based on the measurement report message, a handover request message is sent to the target cell and the candidate cell. Receive handover request confirmation messages from the target cell and the candidate cell. Send a handover command message to the terminal. Based on the additional measurements of the target cell and candidate cells by the terminal, a conditional handover event is detected, and a conditional handover indication message is received from the terminal. Send the condition handover indication message to the target cell, and If an uplink grant is received from the target cell, then the uplink grant is sent to the terminal. The time difference between the transmission of the condition switching indication in the terminal and the reception of the uplink grant in the terminal is below a set time threshold.

11. A terminal operation method in a wireless communication system, comprising: The steps to send the measurement report message to the base station; The steps of performing measurements on the target cell and candidate cells based on the handover command message received from the base station; The step of sending a condition handover indication message to the base station based on at least one of the condition handover events of the target cell and the candidate cell; as well as After sending the conditional handover indication message, if an uplink grant is received before the time threshold has elapsed, the step of performing a no-RACH handover or conditional handover with the target cell is performed.

12. The method according to claim 11, wherein, If no uplink authorization is received before the time threshold is reached after sending the condition switching indication message, the method further includes the step of sending the condition switching indication message again.

13. The method according to claim 12, wherein, The condition switching indication message is sent again with the maximum set number of conditions.

14. The method according to claim 12, wherein, The step of resending the condition switching indication message includes: resending the condition switching indication message when the number of times the condition switching indication message is sent is less than the set number of conditions.

15. A terminal of a wireless communication system, comprising: transceiver; as well as The processor connected to the transceiver The processor is configured as follows: Send measurement report message to base station Based on the handover command message received from the base station, measurements are performed on the target cell and candidate cells. Based on a handover event that satisfies at least one of the target cell and the candidate cell, a handover indication message is sent to the base station. After sending the handover indication message, if an uplink grant is received before a time threshold has elapsed, a no-RACH handover or a conditional handover is performed with the target cell.

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