Apparatus and method for performing connection reestablishment in a wireless communication system

Abstract

The disclosure relates to a method for a terminal to act in a wireless communication system, the method comprising the steps of: receiving a measurement control; measuring a signal; transmitting a measurement report message to a serving cell; and upon detecting a radio link failure, the serving cell being a Non-Terrestrial Networks (NTN) cell, performing cell reselection to a terrestrial cell that is not the serving cell when a condition for omitting Radio Resource Control Connection re-establishment (RRE) is met.

Description

Technical Field

[0001] This disclosure relates to wireless communication systems, and more particularly to apparatus and methods for performing connection reconstruction 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 re-establishing a connection in a wireless communication system.

[0007] This disclosure is intended to provide apparatus and methods for performing the reconstruction of a conditional RRC (Radio Resource Control) connection.

[0008] This disclosure is intended to provide apparatus and methods for performing cell reselection based on whether conditions for performing RRC connection re-establishment are met, or for performing RRC connection re-establishment to the serving cell.

[0009] This disclosure is intended to provide apparatus and methods for improving handover performance in non-terrestrial network environments within wireless communication systems.

[0010] This disclosure is intended to provide apparatus and methods for reconstructing conditional RRC connections in a wireless communication system for use in a non-terrestrial network.

[0011] Solution to the problem

[0012] According to an embodiment of this disclosure, a method for operating a terminal in a system is disclosed. The method may include the following steps: receiving measurement control; measuring a signal and sending a measurement report message to the serving cell; and when a radio link failure is detected, and the serving cell is a non-terrestrial network (NTN) cell, and the conditions for omitting Radio Resource Control Connection re-establishment (RRE) are met, performing cell reselection to a terrestrial cell that is not the serving cell.

[0013] 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 receives measurement control and measurement signals, sends measurement report messages to the serving cell, and when a wireless link failure is detected, if the serving cell is a non-terrestrial cell and the conditions for omitting RRC connection reconstruction are met, performs cell reselection to a terrestrial cell that is not the serving cell.

[0014] Invention Effects

[0015] According to embodiments of this disclosure, RRC connection reconstruction for terminals utilizing non-terrestrial networks can be performed efficiently. Attached Figure Description

[0016] Figure 1 An example of a network according to an embodiment of the present disclosure is illustrated.

[0017] Figure 2 Another example of a network according to an embodiment of the present disclosure is illustrated.

[0018] Figure 3 The illustration shows the configuration of a device in a wireless communication system according to an embodiment of the present disclosure.

[0019] Figure 4 The illustration shows an example of a wireless communication system according to an embodiment of the present disclosure where the step of RRC connection re-establishment (Radio Resource Control connection re-establishment, RRE) is omitted.

[0020] Figure 5The illustration shows an example of a conditional RRC connection reconstruction step based on cell reselection conditions in a wireless communication system according to an embodiment of the present disclosure.

[0021] Figure 6 The illustration shows an example of the steps for performing cell reselection when a radiolink failure (RLF) is detected during a handover, according to an embodiment of the present disclosure.

[0022] 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.

[0023] Figure 8 An example of the steps for determining the priority of a cell based on cell reselection conditions according to an embodiment of the present disclosure is illustrated.

[0024] Figure 9 The illustration shows an example of the steps for determining the signal strength in accordance with an embodiment of the present disclosure to confirm the cell reselection condition.

[0025] Figure 10 An example of the steps for determining the connectability time according to an embodiment of the present disclosure for confirming cell reselection conditions is illustrated. Detailed Implementation

[0026] 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.

[0027] 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.

[0028] Unless otherwise defined, the terms used in this embodiment have the same meaning as commonly understood by those skilled in the art. Terms that are 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.

[0029] This disclosure relates to techniques for handover of terminals using non-terrestrial networks (NTNs). Furthermore, this disclosure relates to techniques for cell reselection for terminals using NTNs. During conditional handover or DAPS (Dual Active Protocol Stack) handover, when a radio link failure (RLF) occurs in the source cell, the terminal performs an RRC (Radio Resource Control Connection Re-establishment) step with the source cell, except when information about the target is known. However, even if an RRC re-establishment is performed during conditional handover or DAPS handover, due to the characteristics of the NTN environment, a second handover may occur. Frequent handovers can degrade communication performance due to resource waste or interference. Ultimately, in a NTN environment, the steps related to RRC re-establishment may further require steps related to cell reselection. Therefore, this disclosure proposes a procedure for applying RRC re-establishment to NTNs.

[0030] Figure 1 An example of a network according to an embodiment of the present disclosure is illustrated.

[0031] refer to Figure 1 The network can include non-terrestrial networks and terrestrial networks (TN), and can include terminal 110, satellite 120-1, satellite 120-2, gateway 130, and base stations. 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 can 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 network diagram in the figure 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.

[0032] 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.

[0033] Satellites 120-1 and 120-2 fly in designated orbits, providing a cell with a certain size of coverage by forming beams pointing towards the ground. In connection with this disclosure, satellite 120-1 can refer to a serving satellite, and satellite 120-2 can refer to a target satellite. Furthermore, the serving satellite and target satellite can be referred to as a serving cell and a target cell, respectively, and these terms can be used interchangeably. Additionally, in this disclosure, satellites 120-1 and 120-2 can be collectively referred to as a non-terrestrial cell. Gateway 130 provides satellites 120-1 and 120-2 with a link that allows them to access the network. Base station 140 can refer to a wireless communication device fixed at a terrestrial base station or in a specific location, capable of providing a cell with a predetermined size of coverage. In connection with this disclosure, satellites 120-1, 120-2, and base station 140 can be referred to as a gNB (gNodeB) or a cell. In addition, the last cell previously connected to by terminal 110 may be mentioned. This last cell can be referred to as the last gNB, the last serving gNB, the source cell, etc. As an example, the last cell can refer to the serving cell previously connected before a handover occurs and a new cell is connected.

[0034] The link between terminal 110 and satellite 120-1 is called the serving 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 the feeder link and can be based on a 3GPP or non-3GPP radio interface. Inter-satellite links (ISL) are mainly used for regenerative satellite applications.

[0035] 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.

[0036] Figure 2 Another example of a network according to an embodiment of the present disclosure is illustrated. Figure 3The illustration shows an example of providing non-terrestrial NTN to UE210 using NTN payload 220 and NTN gateway 230. UE210 can actually 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 and can be based on the Uu interface. The link between NTN payload 220 and UE 210 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 via the serving link to NTN gateway 230. Similarly, NTN payload 220 can transparently forward radio protocols received from NTN gateway 230 via the feeder link to UE 210.

[0037] Therefore, the NTN payload 220 can support the connectivity described below. A base station can serve multiple NTN payloads. An NTN payload can be served by multiple base stations.

[0038] Before retransmitting data to the serving link, the NTN payload 220 can change the carrier frequency. That is, the NTN payload 220 can use different carrier frequencies for 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 cell global identifier), CgNB ID, global gNB ID, TAI (tracking area identity), S-NSSAI (Single Network Slice Selection Assistance information), NSAG (Network Slice AS Group), 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.

[0039] 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.

[0040] 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.

[0041] 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 (switches) can be related to changes in the serving satellite 120-1.

[0042] In recent years, 3GPP has been implementing handover-related standardization 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, uplink grants are difficult to transmit via PDCCH because non-terrestrial base stations cannot know the handover execution time of terrestrial terminals. Therefore, this disclosure proposes a technique that combines time-based conditional handover with RACH-free handover to address this problem.

[0043] Figure 3 The illustration shows the configuration of a device in a wireless communication system according to an embodiment of the present disclosure. Figure 3 The equipment can be understood as a reference. Figure 1 The described device is a part of the structure of any one of terminal 110, satellite 120-1, satellite 120-2, gateway 130, and base station 140.

[0044] refer to Figure 3 The device may include a processor 310, a communication unit 320, and a storage unit 330.

[0045] refer to Figure 3 It can control the overall functions and actions of the device. The processor 310 may include an ASIC (application-specific integrated circuit), other chips, logic circuits, and / or data processing devices.

[0046] 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.

[0047] Storage 330 is hardware that stores various data processed in processor 310. For example, storage 330 can store the SIR value of the sending terminal for the receiving terminal, information about the receiving terminal group for each sending 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.

[0048] Figure 3 The structure can be understood as at least a part of a terminal, base station, satellite, and gateway. Figure 3 When the structure is part of a satellite, the satellite, in addition to Figure 3 In addition to the components illustrated herein, it may also include other hardware equipment required for orbital flight. Figure 3 When the structure is part of a gateway or base station, the gateway or base station may also include components that support wired communication, etc.

[0049] In wireless communication systems, a radio link failure may occur when the quality of the radio link deteriorates or breaks. Radio link failures can be detected and handled at various layers of the network. As an example, after a radio problem is detected at the physical layer, a terminal can declare a radio link failure when the radio problem timer expires, the RACH procedure fails, an RLC (Radio Link Control Failure) occurs, or an uplink LBT (Uplink Listen-Before-Talk) failure is detected.

[0050] On the other hand, networks where terrestrial and non-terrestrial networks coexist can maximize the flexibility and efficiency of communication systems, providing users with higher quality services. For example, if a terminal experiences a radio link failure (RLF) while handing over to a terrestrial cell in a non-terrestrial network environment, the terminal can perform RRC connection reconstruction.

[0051] Specifically, after declaring a radio link failure, the terminal searches for a suitable cell and performs RRC connection reconstruction. However, if no suitable cell is found within the predetermined time, the terminal transitions to the RRC IDLE state.

[0052] As an example, during DAPS handover, when a source cell radio link failure occurs, the terminal interrupts sending or receiving data through the source link, either by disabling the source link or by maintaining the source RRC configuration value, which can be reused later. If the handover to the target cell fails, the terminal selects an appropriate cell and attempts to rebuild the RRC connection. If no suitable cell is found within a predetermined time, the terminal transitions to the RRC IDLE state and declares the handover failed.

[0053] Furthermore, as an example, during conditional handover, when a source cell radio link failure occurs, the terminal explores suitable cells. Additionally, if the selected cell is a candidate cell for conditional handover, when the network attempts to configure the terminal for conditional handover after the radio link failure, the terminal either retryes the conditional handover or performs RRC connection reconstruction. If the terminal cannot find a suitable cell within a certain time after a radio link failure, the terminal transitions to the RRC IDLE state.

[0054] As mentioned above, in the cases of DAPS handover and conditional handover, even if a radio link failure occurs in the source cell, there are situations where RRC connection reconstruction is not performed. For example, if a radio link failure occurs after a handover command is sent from the target cell, the terminal will not perform RRC connection reconstruction but will instead perform a handover to the target cell. In most other cases, when a radio link failure occurs, the terminal will perform RRC connection reconstruction to the serving cell. Conversely, if the serving cell is a non-terrestrial cell (or a non-terrestrial serving cell, NTN serving cell), and a radio link failure occurs at the edge of a non-terrestrial cell, even if the terminal performs RRC connection reconstruction, the likelihood of RRC connection reconstruction failing is high due to the high probability of another handover within a short period of time given the mobility of non-terrestrial cells. Therefore, in the following text, the target cell is prioritized as a terrestrial cell. When the terminal is located at the edge of a non-terrestrial cell or near the edge of its coverage area, the terminal will not perform RRC connection reconstruction but will switch to the RRC IDLE state, prompting the terminal to reselect a terrestrial cell.

[0055] On the other hand, as a standard procedure related to this disclosure, Radio Link Monitoring (RLM) in RRC connected state can be considered. In RRC connected state, the terminal can perform Radio Link Monitoring (RLM) based on reference signals (e.g., SSB, CSI-RS) in the active BWP (Bandwidth Part) and signal quality thresholds set by the network. SSB-based Radio Link Monitoring is performed based on a CD-SSB (Cell Defining SSB) connected to the initial DL BWP and can be configured in a DL BWP including the initial DL BWP or the corresponding CD-SSB. Furthermore, the terminal can perform Radio Link Monitoring based on a non-cell defining SSB. Thus, CSI-RS-based Radio Link Monitoring can also be performed if configured in the active DL BWP. During DASP handover, the terminal can continuously monitor for radio link failures in the source cell until the random access procedure in the target cell is successfully completed. At the physical layer, the terminal can detect wireless link failures when a timer started after detecting a wireless failure expires (e.g., when the timer stops when the wireless failure is resolved), when a timer set when a measurement report is triggered expires, when another wireless failure timer is running, when a RACH procedure failure occurs, and when an RLC failure occurs.

[0056] When using a shared spectrum channel, if a persistent uplink Listen-Before-Talk (LBT) fault is detected, the IAB-MT can declare a radio link failure upon receiving a backhaul radio link failure.

[0057] After a radio link failure, the terminal can maintain its RRC settings. For example, during DAPS handover, when a radio link failure occurs in the source cell, the terminal stops sending / receiving data to the source cell and terminates the source cell, but can maintain the source RRC configuration.

[0058] On the other hand, when a cell announces a handover failure, the terminal can select an appropriate cell and initiate RRC connection reconstruction. Furthermore, if no suitable cell is found within a certain time, the terminal transitions to RRC IDLE state.

[0059] On the other hand, when a radio link failure occurs in the source cell during conditional handover, the terminal selects an appropriate cell, which becomes a candidate cell for conditional handover. When the network is ready for a conditional handover attempt, the terminal attempts to perform the handover. If no conditional handover attempt is made, the terminal performs RRC connection reconstruction. Furthermore, if no suitable cell is found within a certain time, the terminal transitions to the RRCIDLE state.

[0060] On the other hand, when a radio link failure occurs in the source cell of MCG LTM (Local Triggered Mobility), the terminal selects an appropriate cell. If the selected cell is an LTM candidate cell, it attempts RACH-based LTM. If the terminal does not perform RACH-based LTM, it performs RRC connection reconstruction. Specifically, if the terminal does not find an appropriate cell, it transitions to the RRC IDLE state; otherwise, it selects an appropriate cell and attempts RRC connection reconstruction.

[0061] On the other hand, when an RLF occurs in the IAB backhaul link, the terminal can apply the same mechanisms and procedures as the access link. As an example, this could include backhaul radio link fault detection and radio link fault recovery.

[0062] In the following situations, the IAB-DU can send a backhaul radio link failure detection indication to the child node. For example, when the IAB-MT starts RRC connection reconstruction, if the IAB-MT is in dual-connected state and detects a radio link failure in the backhaul link, making UL rerouting impossible, the IAB-MT receives a backhaul radio link failure detection indication from the parent node. If there is no backhaul link unaffected by the radio link failure condition, the IAB-DU can send the backhaul radio link failure detection indication to the child node.

[0063] When a child node receives a backhaul radio link failure detection indication, it can perform local rerouting of upstream traffic using an available backhaul link. Upon recovery from a radio link failure, the IAB-DU can send a backhaul radio link failure recovery indication to the child node. The child node receiving the radio link failure recovery indication can undo previously executed actions. If radio link failure recovery fails within the IAB node, for example, if the RRC connection re-establishment process fails, the IAB node can send a backhaul radio link failure indication to the child node. Backhaul radio link failure detection indications, recovery indications, and radio link failure indications can be transmitted via the BAP (Control PDU).

[0064] Figure 4 The illustration shows an example where the conditional RRC connection reconstruction step in a wireless communication system according to an embodiment of the present disclosure is omitted. The first base station 420-1 is a serving cell, which may also be a non-terrestrial cell. Furthermore, the second base station 420-2 is a target cell, which may correspond to either a non-terrestrial cell or a terrestrial cell. The first base station 420-1 may be referred to as a source gNB or source eNB, and similarly, the second base station 420-2 may be referred to as a target gNB or target eNB.

[0065] In addition, not in Figure 4 As illustrated in the diagram, the wireless communication system according to one embodiment of this disclosure may further include a third base station, which may be the target of cell reselection. As an example, the third base station may be a base station of a different type from the second base station; for instance, if the second base station is a non-terrestrial cell, the third base station may be a terrestrial cell.

[0066] refer to Figure 4 In step S401, the MME (Mobility Management Entity) sets area restrictions. The MME430, as a control node, is responsible for signal processing, mobility management, and session establishment between the terminal 410 and the network within the control plane. Specifically, the MME430 performs area restriction provision to implement area-based connection control, restricting or allowing the terminal 410's connection in a specific area. As an example, the MME430 restricts specific services through area restrictions; for instance, it can restrict high-speed data services in a specific area, allowing only voice calls or low-speed data. Furthermore, the MME430 can set area restrictions to limit roaming, refuse handover to a specific area, or prevent the terminal (410) from connecting to secure areas such as military bases or government agencies.

[0067] In step S403, the first base station 420-1 sends measurement control information to the terminal 410. Specifically, the first base station 420 sends a measurement control message to the terminal 410 requesting measurements of frequency, cell, signal quality, etc. As an example, the measurement control message may include information on measurement items such as SINR (Signal to Interference and Noise Ratio), RSRQ (Reference Signal Received Quality), and RSRP (Reference Signal Received Power).

[0068] In step S405, terminal 410 measures the signal. Specifically, terminal 410 measures the signal strength of base stations 420-1 and 420-2 in order to reserve resources for handover. Terminal 410 can measure the signal strength according to a predetermined period, which may be referred to as the measurement interval in this disclosure.

[0069] Specifically, terminal 410 can measure the signal strength of base stations 420-1 and 420-2 to detect a handover preparation event. For example, if the measured signal strength or quality is below a predefined threshold or within a predetermined range, terminal 410 can determine that a handover preparation event has been detected. Furthermore, as an example, if the signal strength or quality provided by the second base station 420-2 is above a specific threshold compared to the signal strength or quality provided by the first base station 420-1 (serving cell), terminal 410 can determine that a handover preparation event has been detected.

[0070] On the other hand, terminal 410 and the first base station 420-1, as well as the first base station 420-1 and the gateway 440, can exchange data packets (or user data). Gateway 440 manages user data traffic and can perform handover, data buffering, and data traffic management during roaming.

[0071] Furthermore, the first base station 420-1 can allocate an uplink (UL) to the terminal 410. Uplink allocation is the process by which the terminal 410 allocates the necessary resources to transmit data to the first base station 420-1, which serves as the cell. The uplink refers to the path from the terminal 410 to the first base station 420-1 for data transmission. Uplink allocation can be performed using L1 signaling or L2 signaling. L1 signaling, as the physical layer, can include all actions related to the actual transmission and reception of wireless signals. L2 signaling, as the data link layer, can include actions related to direct data transmission and connection maintenance.

[0072] In step S407, terminal 40 sends a measurement report to the first base station 420-1. Specifically, terminal 410 periodically measures the signal strength of the first base station 420-1 and other base stations (not shown) 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 410 may measure at least one of SINR, RSRQ, and RSRP of the first base station 420-1 and other base stations. In addition, terminal 410 may measure RSSI (Received Signal Strength Indicator) of received signal power strength and SNR (Signal to Noise Ratio) of signal-to-noise ratio, but is not limited to these.

[0073] In step S409, the first base station 420-1 decides to handover. Specifically, the first base station 420-1 confirms at least one of RSRP, RSRQ, or SINR through the 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 420-1 may decide to handover. Furthermore, as an example, if the signal strength or quality provided by other base stations is above a specific threshold compared to the signal strength or quality provided by the first base station 420-1, which is the serving cell, the first base station 420-1 may decide to handover. The above process can be understood as the process by which the first base station 420-1 detects a triggering event from the measurement report.

[0074] In step S411, the first base station 420-1 sends a handover request. Specifically, the first base station 420-1 sends a handover request message to the target cell. The target cell can be determined based on measurement reports from other base stations. As an example, the target cell's RSRP, RSSI, RSRQ, and SINR for other base stations can be determined based on the traffic load, priority, terminal 410 speed, frequency band, call state, and terminal performance (UE capabilities), etc. In the following explanation, for the sake of clarity, it is assumed that the second base station 420-2 is determined to be the target cell; the second base station 420-2 can be a non-terrestrial cell.

[0075] The handover request message includes information for terminal 410, which may include, but is not limited to, UE identity, source cell ID, target cell ID, terminal context, target cell information, E-RAB (E-UTRAN Radio Access Bearer), etc. The handover request message can be sent via an inter-satellite link (ISL) or a feeder link.

[0076] In step S413, the second base station 420-2, acting as the target cell, performs admission control. Specifically, the second base station 420-2 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 420-2 can calculate available bandwidth, frequency resources, cell capacity, and QoS requirements to determine whether to accept the handover.

[0077] In step S415, the second base station 420-2 sends a handover request acknowledge message to the first base station 420-1. Specifically, when the second base station 420-2 determines that a handover is possible after performing admission control, it sends a handover request acknowledge message to the first base station 420-1.

[0078] On the other hand, the first base station 420-1 can allocate downlink (DL) to the terminal. Downlink refers to the path through which the network (for example, a non-terrestrial cell or a terrestrial cell) sends data to the terminal 410, and downlink allocation refers to the process by which the network allocates radio resources so that the terminal 410 can receive data.

[0079] Furthermore, the first base station 420-1 can send a handover command. Specifically, the first base station 420-1 can send a handover command message to the terminal 410. The handover command message can include information about the second base station 420-2, which is the target cell. This information can include C-RNTI, Timing Advance (TA), and location information. Thus, the first base station 420-1 can allocate RNTIs for broadcast or multicast (e.g., G-RNTI, C-RNTI) to the terminal 410. The corresponding RNTIs are provided in advance through system information or along with the settings for measurement. Steps S401 to S415 described above can be called handover preparation steps, which can be understood as the process of reserving resources for performing the handover.

[0080] In step S417, terminal 410 performs RACH. Terminal 410 obtains uplink grant information from the first base station 420-1 through the RACH step, and uses this information to send a handover confirmation message to the second base station 420-2. Specifically, terminal 410 sends a preamble to the second base station 420-2, and the second base station 420-2 can send its response to RA (Random Access) to terminal 410. Then, terminal 410 sends RRC connection complete (Radio Resource Control connection reconfiguration complete) to the second base station 420-2, which can transmit the applicable radio resource settings based on the RRC connection complete.

[0081] In step S419, terminal 410 detects a radio link failure. As an example, after detecting a radio problem at the physical layer, terminal 410 may declare a radio link failure if the radio problem timer expires, the RACH procedure fails, an RLC (Radio Link Control Failure) occurs, or an uplink Listen-Before-Talk (Uplink Listen-Before-Talk) failure is detected. Thus, when the signal measurements of RSRP, RSRQ, SINR, etc., of terminal 410 drop below a predetermined threshold, a radio link failure can be detected or declared. This disclosure focuses on terminal 410 declaring a radio link failure during handover, but is not limited to this. As long as it does not conflict with this disclosure, it can include declaring a radio link failure at various layers such as packet transmission and channel connection.

[0082] In step S421, terminal 410 performs conditional RRC connection reconstruction. Specifically, terminal 410 does not directly perform RRC connection reconstruction, but performs conditional RRC connection reconstruction. When performing cell reselection, it can switch to RRC IDLE state. On the other hand, when a radio link failure is detected, if terminal 410 immediately performs RRC connection reconstruction, a handover may be performed again within a short period of time, and the possibility of RRC connection reconstruction failing again is high. For example, when terminal 410 is located near the coverage boundary of the first base station 420-1, which is a non-terrestrial cell, even if RRC connection reconstruction is performed, due to the mobility of the non-fixed first base station 420-1, an immediate handover may be required. Therefore, according to this disclosure, terminal 410 can perform cell reselection based on whether the conditions for RRC connection reconstruction with the existing serving cell and cell reselection with other cells that are not serving cells are met, or perform RRC connection reconstruction with the first base station 420-1, which is the serving cell. When certain conditions are met, cell reselection is performed. Therefore, the conditions proposed in this disclosure can be referred to as "conditions for omitting RRC connection reconstruction," "conditions for performing cell reselection," or technical terms with equivalent meanings. For the convenience of the following explanation, the proposed conditions are referred to as "cell reselection conditions."

[0083] Terminal 410, as a cell reselection condition, can consider cell priority, signal measurement values, and / or connectivity time. As an example, terminal 410 can study the priority of target candidate ground cells for performing cell reselection. Furthermore, as an example, terminal 410 can determine whether signal measurement values, such as the RSRP and RSRQ values ​​of the first base station 420-1 serving as the cell, are less than predetermined thresholds. Furthermore, as an example, terminal 410 can confirm the connectivity time with the first base station 420-1 serving as the cell. That is, terminal 410 can study the above conditions to confirm the probability of successful RRC connection reconstruction and the probability of requesting a handover again. On the other hand, terminal 410 can maintain RRC settings in the event of a radio link failure.

[0084] Figure 5 The illustration shows an example of a conditional RRC connection reconstruction step based on cell reselection conditions in a wireless communication system according to an embodiment of the present disclosure. A first base station 420-1 is a serving cell, which may be a non-terrestrial cell. Furthermore, a second base station 420-2 is a target cell, which may be equivalent to either a non-terrestrial cell or a terrestrial cell. The first base station 420-1 may be referred to as a source gNB or source eNB, and similarly, the second base station 420-2 may be referred to as a target gNB or target eNB.

[0085] In addition, not in Figure 5As illustrated in the diagram, the wireless communication system according to one embodiment of this disclosure may further include a third base station, which can be the target of cell reselection. As an example, the third base station may be a base station of a different type from the second base station; for instance, if the second base station is a non-terrestrial cell, the third base station may be a terrestrial cell.

[0086] refer to Figure 5 In step S501, MME530 sets area restrictions. MME530 is a control node and can be an object responsible for handling signals between terminal 510 and the network, managing mobility, and setting sessions in the control plane. Specifically, MME530 implements area restrictions to control area-based connections, allowing or restricting connections of terminal 510 in specific areas. As an example, MME530 restricts specific services through area restrictions, such as limiting high-speed data services in a specific area, or allowing only voice calls or low-speed data. Furthermore, MME530 can set area restrictions to limit roaming, deny handover to specific areas, or restrict terminal (410) from connecting to secure areas such as military bases or government agencies.

[0087] In step S503, the first base station 520-1 sends a measurement control message to the terminal 510. Specifically, the first base station 520 sends a measurement control message to the terminal 510 requesting measurements of frequency, cell, signal quality, etc. As an example, the measurement control message may include information on measurement items such as SINR, RSRQ, and RSRP.

[0088] In step S505, terminal 510 measures the signal. Specifically, terminal 510 measures the signal strength of base stations 520-1 and 520-2 in order to reserve resources for handover. Terminal 510 can measure the signal strength according to a predetermined period, which may be referred to as the measurement interval in this disclosure.

[0089] In detail, terminal 510 can measure the signal strength of base stations 520-1 and 520-2 to 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 510 can determine that a handover preparation event has been detected. Furthermore, as an example, when the signal strength or quality provided by the second base station 520-2 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.

[0090] In step S507, terminal 510 sends a measurement report to the first base station. Specifically, terminal 510 periodically measures the signal strength of the first base station 520-1 and other base stations (not shown) and sends a measurement report message. The measurement report may include at least a portion of the elements of a predefined measurement report. For example, the measurement structure of the measurement report may include the measurement object, report configuration, measurement ID, quantity configuration, and measurement interval. As an example, terminal 510 may measure at least one of SINR, RSRQ, and RSRP of the first base station 520-1 and other base stations. Thus, terminal 510 may measure RSSI as the power strength of the received signal and SNR as the ratio between signal and noise, but is not limited to these.

[0091] In step S509, 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, if the measured signal strength or quality is below a predefined threshold or within a predetermined range, the first base station 520-1 may decide to hand over. Furthermore, as an example, if the signal strength or quality provided by other base stations 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, the first base station 520-1 may decide to hand over. The above process can be understood as the process by which the first base station 520-1 detects a triggering event from the measurement report.

[0092] In step S511, the first base station 520-1 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 from other base stations. As an example, the target cell is determined based on RSRP, RSSI, RSRQ, SINR of other base stations; traffic load and priority of each base station; speed, frequency band, connection status, and terminal capabilities of terminal 510, etc. In the following explanation, for ease of illustration, the second base station 520-2 will be set as the target cell. The second base station 520-2 can be a non-terrestrial cell.

[0093] The handover request message includes information for terminal 510, which may include, but is not limited to, UE Identity, source cell ID, target cell ID, terminal context, target cell information, E-RAB, etc. The handover request message can be sent via inter-satellite links or feeder links.

[0094] In step S513, the second base station 520-2, acting as the target cell, performs 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 network resources or quality degradation, the handover request can be rejected. As an example, the second base station 520-2 calculates available bandwidth, frequency resources, cell capacity, and QoS requirements to determine whether to accept the request.

[0095] In step S515, the second base station 520-2 sends a handover request confirmation to the first base station 520-1. Specifically, if the second base station 520-2 determines that a handover is possible based on the result of the admission control, it sends a handover request confirmation message to the first base station 520-1.

[0096] On the other hand, the first base station 520-1 can send 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, which is the target cell. This information can include C-RNTI, timing advance, and location information. Thus, the first base station 520-1 can allocate RNTIs for broadcast or multicast (e.g., G (group)-RNTI, C-RNTI) to the terminal 510. The corresponding RNTIs can be provided in advance through system information or provided together with the settings for measurement. Steps S501 to S515 described above can be called handover preparation steps, which can be understood as the process of reserving resources for performing handover.

[0097] In step S517, terminal 510 executes RACH. Terminal 510 obtains uplink grant information from the first base station 520-1 through the RACH procedure, and uses this information to send a handover confirmation message to the second base station 520-2. Specifically, terminal 510 sends a preamble to the second base station 520-2, and the second base station 520-2 can send a response to the RA to terminal 510. Then, terminal 510 sends an RRC connection completion message to the second base station 520-2, indicating that the radio resource settings have been correctly applied according to the RRC connection completion.

[0098] In step S519, terminal 510 detects a radio link failure. As an example, after detecting a radio failure at the physical layer, terminal 510 can detect or declare a radio link failure when a radio failure timer expires, a RACH procedure failure occurs, an RLC failure occurs, or an uplink LBT failure is detected. Consequently, when signal measurements such as RSRP, RSRQ, and SINR drop below a predetermined threshold, terminal 510 can detect or declare a radio link failure. In this disclosure, the terminal is described primarily in the context of declaring a radio link failure during handover, but this is not a limitation. Unless otherwise specified in this disclosure, declaring a radio link failure may also be included at various layers such as those involved in packet transmission and channel access.

[0099] In step S521, terminal 510 determines the cell reselection conditions. Specifically, terminal 510 does not immediately perform RRC connection reconstruction, but instead determines the cell reselection conditions, and can switch to RRC IDLE state during cell reselection.

[0100] In detail, the terminal 510 can consider cell priority, signal measurement values, and connectivity time as conditions for cell reselection. As an example, the terminal 510 can study the priority of target candidate ground cells for cell reselection. Specifically, the terminal 510 can determine whether a ground cell is assigned a priority for cell reselection. In an area where the first base station 520-1 (a non-ground cell) coexists with a ground cell, in the case of a ground network, services requiring high data transmission rates and low latency can be applied. Therefore, considering the different characteristics of ground and non-ground networks, to improve the user experience, the terminal 510 selects a cell from either ground or non-ground cells for cell camping or RRC connection settings based on potential services, mobility status, or operator plans. That is, the priority of ground and non-ground cells can be pre-determined during cell reselection for RRC connection reconstruction of the terminal 510. As an example, ground cells can be assigned absolute priority during cell reselection. Therefore, the terminal 510 can study whether a ground cell has been assigned a priority for cell reselection.

[0101] Additionally, as an example, terminal 510 can determine whether signal measurement values, such as the RSRP and RSRQ values ​​of the first base station 520-1 serving the cell, are less than a predetermined threshold. Specifically, when at least one of the RSRP and RSRQ values ​​of the first base station 520-1 is low, the signal quality is poor, which may increase the likelihood of RRC connection reconstruction failure. Therefore, terminal 510 compares the value of at least one of the RSRP and RSRQ values ​​of the first base station 520-1 with the threshold to investigate the possibility of failure.

[0102] Furthermore, for example, terminal 510 can confirm the available connection time with the first base station 520-1. Specifically, if terminal 510 is located near the coverage boundary of the first base station 520-1, signal quality may degrade, and even if RRC reconnection is performed, the mobility of the non-fixed first base station 520-1 may necessitate a handover within a short period. This could degrade network performance. Therefore, terminal 510 confirms the available connection time considering its relative position to the first base station 520-1. In other words, terminal 510 can determine the likelihood of successful RRC connection reconstruction and the possibility of requesting a handover again by studying the above conditions.

[0103] In step S523, terminal 510 performs cell reselection or RRC connection reconstruction based on whether the cell reselection conditions are met. Specifically, if a ground cell is assigned priority, terminal 510 switches to RRC IDLE and then performs ground cell reselection. Simultaneously, if terminal 510 studies the priority of the target candidate ground cell for cell reselection and a non-ground cell is assigned priority, terminal 510 can switch back to the second base station 520-2 as the target cell, or perform RRC connection reconstruction with the existing serving cell, i.e., the first base station 520-1.

[0104] Furthermore, if signal measurements, such as the RSRP and RSRQ values ​​of the first base station 520-1 serving as the cell, are lower than a predetermined threshold, the terminal switches to RRC IDLE and then performs a ground cell reselection. Conversely, if the RSRP and RSRQ values ​​of the first base station 520-1 serving as the cell are higher than the predetermined threshold, the terminal 510 can perform another handover or re-establish an RRC connection with the first base station 520-1.

[0105] Furthermore, if the connectivity time between the first base station 520-1 (serving cell) and the terminal 510 is less than a predetermined threshold, the terminal 510 switches to RRC IDLE and then performs a ground cell reselection. Conversely, if the connectivity time between the first base station 520-1 and the terminal 510 is greater than the predetermined threshold, the terminal 510 can perform another handover or re-establish an RRC connection with the first base station 520-1.

[0106] Figure 6 illustrates an example of the steps for performing cell reselection when a radio link failure is detected during a handover according to an embodiment of the present disclosure.

[0107] As shown in Figure 6, in step S601, the terminal detects a radio link failure for the target cell during handover. The terminal can receive measurement control from the serving cell, measure the signals of surrounding base stations, and send a measurement report message to the serving cell. If the serving cell decides to handover and the terminal receives a handover command from the serving cell, the terminal performs a handover to the target cell. Simultaneously, if a radio link failure is detected at the physical layer, such as when a radio link failure timer expires, a RACH procedure failure occurs during a handover attempt, an RLC failure occurs, or an uplink LBT failure is detected, the terminal can declare a radio link failure. Furthermore, if signal measurements such as RSRP, RSRQ, and SINR are below predetermined thresholds, the terminal can also declare a radio link failure.

[0108] In step S603, the terminal determines the cell reselection conditions. Specifically, even if a radio link failure is detected, the terminal will not immediately perform RRC connection reconstruction. Instead, the terminal will study the cell reselection conditions used for cell reselection and switch to RRC IDLE during cell reselection.

[0109] For example, a terminal can use cell priority as a cell reselection condition. Specifically, a terminal can determine whether a terrestrial cell has been assigned a priority for cell reselection. Considering the different characteristics of terrestrial and non-terrestrial networks, to improve user experience, a terminal can select a cell in a terrestrial or non-terrestrial network for cell camping or RRC connection settings based on potential services, mobility status, or operator plans. For example, a terminal can investigate whether a terrestrial cell has been assigned a priority for cell reselection.

[0110] Furthermore, for example, the terminal can use signal strength as a condition for cell reselection. For instance, the terminal can determine whether its signal measurements, such as the serving cell's RSRP and RSRQ, are below a predetermined threshold. Specifically, if at least one of the serving cell's RSRP and RSRQ values ​​is low, the signal quality may be poor, increasing the likelihood of RRC connection re-establishment failure. Therefore, if at least one of the serving cell's RSRP and RSRQ values ​​is below the predetermined threshold, the terminal can reselect a ground cell.

[0111] Furthermore, for example, the terminal can confirm the available connection time with the serving cell as a condition for cell reselection. Specifically, if the terminal is located near the coverage boundary of the serving cell (a non-terrestrial cell), signal quality may be degraded, and even if RRC reconnection is performed, handover may be required within a short period due to the mobility of non-fixed non-terrestrial cells. This may degrade network performance. Therefore, the terminal confirms the available connection time to consider its relative position to the serving cell. For example, if the available connection time is less than a predetermined threshold, the terminal can reselect a terrestrial cell. In other words, the terminal can determine the likelihood of successful RRC connection reconstruction and the possibility of needing to handover again by studying the above conditions.

[0112] In step S605, the terminal performs cell reselection without RRC connection re-establishment. Specifically, the terminal performs cell reselection based on whether cell reselection conditions are met. Specifically, if a ground cell is assigned priority, the terminal switches to RRC IDLE and then performs ground cell reselection. Furthermore, if signal measurements, such as the serving cell's RSRP and RSRQ values, are below predetermined thresholds, the terminal switches to RRC IDLE and then performs ground cell reselection. Additionally, if the connectivity time between the serving cell and the terminal is below a predetermined threshold, the terminal switches to RRC IDLE and then performs ground cell reselection.

[0113] Figure 7 The illustration shows an example of terminal operation steps in a wireless communication system according to an embodiment of the present disclosure.

[0114] like Figure 7 As shown, in step S701, the terminal maintains both the RRC connection state and the connection management state. Specifically, while maintaining communication with the serving cell, the terminal can detect radio link failures. For example, when a radio failure is detected at the physical layer, such as when the radio failure timer expires, the RACH procedure fails, an RLC failure occurs, or an uplink LBT failure is detected, the terminal may announce a radio link failure. Furthermore, a radio link failure may occur if signal measurements such as RSRP, RSRQ, and SINR fall below predetermined thresholds. This is not limited to these scenarios; it may also include the terminal announcing radio link failures at various layers, such as during packet transmission and channel connection.

[0115] In step S703, the terminal confirms the cell priority. Specifically, when a radio link failure is detected, the terminal does not immediately perform RRC connection reconstruction, but instead confirms the RRC cell priority as a cell reselection condition. For example, the terminal can study the priority of target candidate ground cells to perform cell reselection. That is, the terminal can determine whether a ground cell is set as the priority for performing cell reselection. In areas where ground and non-ground cells coexist, the ground network may be more suitable for services requiring high data rates and low latency. Therefore, to consider the different characteristics of ground and non-ground networks and improve user experience, the terminal will select a cell from ground or non-ground cells for cell camping or RRC connection reconstruction based on potential services, mobility status, or operator plans.

[0116] When performing cell reselection for RRC connection re-establishment of a terminal, the priorities of terrestrial and non-terrestrial cells can be preset. For example, terrestrial cells can be assigned absolute priority during cell reselection. Therefore, in order to perform cell reselection, the terminal can examine whether a terrestrial cell has already been assigned priority. For example, if a terrestrial cell has been assigned priority, the terminal can consider performing terrestrial cell reselection after switching to RRC IDLE, but may also examine signal strength at the same time. Furthermore, for example, if a radio link failure occurs when both the serving cell and the target cell are non-terrestrial cells, and the terminal examines cell priorities to perform cell reselection, and if a non-terrestrial cell has been assigned priority, the terminal can either switch back to the non-terrestrial cell as the target cell or perform RRC connection re-establishment with the existing serving cell.

[0117] In step S705, the terminal confirms the signal strength. Specifically, the terminal can determine whether signal measurements, such as the serving cell's RSRP and RSRQ values, are below a predetermined threshold, using this as a cell reselection condition. Specifically, if at least one of the serving cell's RSRP and RSRQ values ​​is low, the signal quality may be poor, increasing the likelihood of RRC connection re-establishment failure. Therefore, the terminal compares at least one of the serving cell's RSRP and RSRQ values ​​with the threshold to investigate the possibility of RRC connection re-establishment failure. Of course, the terminal can also use SINR, RSSI, and SNR as cell reselection conditions for further investigation.

[0118] For example, if the RSRP and RSRQ values ​​of the non-terrestrial cell serving as the terminal are lower than a predetermined threshold, the terminal may consider performing a terrestrial cell reselection after switching to RRC IDLE, but the connectivity time can be further investigated. Meanwhile, if the RSRP and RSRQ values ​​of the serving cell are above the predetermined threshold, the terminal may perform another handover or re-establish an RRC connection with the existing serving cell.

[0119] In step S707, the terminal confirms the available connection time. Specifically, if the terminal is located near the edge of the serving cell's coverage area, signal quality may degrade. Even if RRC reconnection is performed, the mobility of the serving cell (a non-fixed, non-terrestrial cell) increases the likelihood of handover within a short period, thus reducing network performance. Therefore, the terminal uses the available connection time as a cell reselection criterion, taking into account its relative position to the serving cell.

[0120] For example, if the connectivity time between the serving cell and the terminal is less than a predetermined threshold, the terminal switches to RRC IDLE and then performs a ground cell reselection. Conversely, if the connectivity time between the serving cell and the terminal is greater than the predetermined threshold, the terminal can perform another handover or rebuild the RRC connection with the serving cell.

[0121] Meanwhile, if the serving cell is a non-terrestrial cell and the radio link failure occurs at the edge of the non-terrestrial cell, even if the terminal performs RRC connection reconstruction on the non-terrestrial cell, due to the mobility of the non-terrestrial cell, a handover is likely to occur again within a short period of time, and the RRC connection reconstruction may also fail. In other words, the above steps S703 to S707 can be understood as the terminal considering cell reselection, studying cell reselection conditions to improve the success rate of RRC connection reconstruction, and minimizing communication interruptions and resource waste, rather than immediately performing RRC connection reconstruction on the serving cell.

[0122] In step S709, the terminal performs cell reselection or RRC connection reconstruction based on whether the cell reselection conditions are met. For example, if all cell reselection conditions in steps S703 to S707 are met, the terminal performs terrestrial cell reselection. Alternatively, for example, if at least one cell reselection condition is met, the terminal can also reselect a terrestrial cell. The order and confirmation items of the cell reselection conditions can be changed in user settings or system settings. For example, the terminal can only confirm cell priority; if the terrestrial cell has priority, it can switch to RRC IDLE and reselect a terrestrial cell.

[0123] Furthermore, some cell reselection conditions can be omitted. That is, if at least one cell reselection condition in steps S703 to S707 is not met, the terminal can either re-establish an RRC connection with the serving cell or switch to the target cell. Alternatively, for example, if none of the cell reselection conditions are met, the terminal can also re-establish an RRC connection with the serving cell or switch to the target cell.

[0124] Figure 8 illustrates an example of the steps for determining cell priority when determining cell reselection conditions according to an embodiment of the present disclosure.

[0125] As shown in Figure 8, in step S801, the terminal determines whether the ground cell has a priority. Specifically, after detecting a radio link failure, the terminal does not immediately perform RRC connection reconstruction, but instead confirms the priority of the RRC cell as a cell reselection condition. For example, the terminal can study the priority of the target candidate ground cell to perform cell reselection. In other words, the terminal can determine whether the ground cell has been set as the priority for performing cell reselection.

[0126] Since cell reselection is the process of a terminal moving to a cell with higher signal quality, the terminal can use frequency priority as cell priority. For example, the terminal can use system information broadcast by the base station to determine frequency priority. That is, the terminal can use the received system information during cell reselection in RRC IDLE state. For example, the terminal can check the frequency priority information contained in the system information to determine whether the frequency band of the ground cell has priority.

[0127] Furthermore, the terminal can confirm frequency priority or cell reselection criteria through the RRC release message. The RRC release message is used to release the RRC connection, for example, when the terminal's RRC connection state switches to the RRC IDLE state. Moreover, since the RRC release message contains RRC reconfiguration instructions for RRC connection reconstruction, it can contain cell reselection criterion information. In other words, the terminal can confirm the frequency priority information or cell reselection criteria contained in the RRC release message to determine whether the frequency band of the terrestrial cell has been assigned priority, or whether the terrestrial cell has been selected as the cell reselection criterion.

[0128] In addition, the terminal can also refer to cell reselection standards for other radio access technologies (RATs). For example, the terminal can refer to cell reselection standards applied to other radio access technologies (e.g., the transition from LTE to 5G).

[0129] In step S803, if a ground cell is assigned a priority, the terminal performs ground cell reselection. For example, the terminal can determine the priority of a ground cell by confirming system information sent by the base station via broadcast or dedicated signaling. For example, the priority information of a ground cell can be sent periodically or aperiodically via the base station's system information (e.g., SIB19). Furthermore, for example, the terminal can confirm the priority of a ground cell by confirming pre-stored system information sent from the base station. Specifically, the priority information of a ground cell can be indicated by at least one parameter included in the system information. For example, the terminal can confirm the frequency information of the ground cell included in the system information to confirm the priority of the ground cell. If it is confirmed that the frequency band of the ground cell has a relatively high priority, the terminal switches to the RRC IDLE state and performs ground cell reselection.

[0130] In addition, the terminal will also confirm the frequency priority information or cell reselection criteria contained in the RRC release message. If the frequency band of the ground cell is determined to be the priority or the ground cell is determined to be the cell reselection criterion, the terminal will switch to the RRC IDLE state and perform ground cell reselection.

[0131] In addition, when the terrestrial cell is determined as the cell reselection standard by referring to the cell reselection standards of other wireless access technologies, the terminal switches to RRC IDLE state and performs terrestrial cell reselection.

[0132] In step S805, if the terrestrial cell does not have priority, the terminal will perform RRC connection reconstruction with the non-terrestrial cell. Specifically, if the terminal confirms the results of system information, RRC release messages, and other RAT cell reselection criteria, and the non-terrestrial cell has priority, it can either perform RRC connection reconstruction with that non-terrestrial cell or switch to the target cell.

[0133] Meanwhile, the following criteria can be considered during the terminal's cell reselection evaluation process. The absolute priority of various NR frequencies or inter-RAT frequencies can be provided to the terminal through system information, RRC release messages, or by inheritance from other RATs during inter-RAT cell (re)selection. If it is system information, NR frequencies or inter-RAT frequencies can be listed without specifying priorities (i.e., no cell reselection priority field is provided for the corresponding frequencies). Cell reselection priorities can refer to the cell reselection criteria of this disclosure. If the cellreselectionpriority or nsag-cellreselectionpriority fields are provided in the dedicated signal, the terminal should ignore the cellreselectionpriority and nsag-cellreselectionpriority fields provided in the system information. If the terminal is typically in a camped state, supports slice-based cell reselection, and receives network slice and NSAG information for cell reselection from the NAS, the terminal should derive the cellreselectionpriority according to section 5.2.4.11.

[0134] Figure 9 illustrates an example of the steps for confirming the signal strength for determining cell reselection conditions according to an embodiment of the present disclosure.

[0135] like Figure 9 As shown, in step S901, the terminal determines whether the signal measurement value of the serving cell is lower than a first threshold. Specifically, the terminal can determine whether the signal measurement value, such as the RSRP or RSRQ value of the serving cell, is lower than the first threshold, and use this as a cell reselection condition. Hereinafter, the signal measurement value can be simply referred to as... .

[0136] More specifically, the terminal can confirm whether the signal measurement value reflecting the hysteresis value is less than a first threshold. The hysteresis value can be preset during network design; a higher hysteresis value results in more intermittent handovers, but may also lead to longer delays. Therefore, according to this disclosure, by analyzing the network operating environment, a higher hysteresis value can be set in environments with frequent signal fluctuations. Furthermore, the hysteresis value can be set according to Quality of Service (QoS) requirements. Similarly, a higher first threshold results in more intermittent handovers, but may also lead to longer delays. Therefore, according to this disclosure, by analyzing the network operating environment, a higher first threshold can be set in environments with frequent signal fluctuations, and the first threshold can be determined according to QoS requirements. The first threshold will be referred to below as... The signal measurement value reflecting the hysteresis value and whether it is less than the first threshold can be calculated using the following mathematical formula 1. The hysteresis value will be referred to below as... .

[0137] [Mathematical Expression 1]

[0138]

[0139] In step S903, the terminal performs cell reselection. Specifically, if the result of the calculation of the above mathematical formula 1, where the hysteresis value is reflected in the signal measurement value of the serving cell, is less than the first threshold, the terminal determines that the possibility of RRC connection reconstruction failure is high, switches to the RRC IDLE state, and performs cell reselection.

[0140] In step S905, the terminal performs RRC connection reconstruction with the non-terrestrial cell. Specifically, if the result reflecting the hysteresis value in the serving cell signal measurement calculated according to the above mathematical formula 1 is above the first threshold, the terminal determines that the probability of successful RRC connection reconstruction is high and performs RRC connection reconstruction.

[0141] Figure 10 illustrates an example of the steps for determining the connectable time according to an embodiment of the present disclosure, based on the criteria for confirming cell reselection.

[0142] As shown in Figure 10, in step S1001, the terminal confirms whether the connectable time is less than a second threshold. Connectable time refers to the time range within which the terminal can directly communicate with a non-terrestrial cell due to its mobility. Specifically, if the terminal is located near the edge of the serving cell's coverage area, signal quality may degrade, and even if RRC reconnection is performed, handover may occur within a short time due to the mobility of the serving cell, which is a non-fixed non-terrestrial cell. This may reduce network performance. Therefore, the terminal uses connectable time as a cell reselection condition, taking into account its relative position to the serving cell. Connectable time will be abbreviated as [missing term]. .

[0143] More specifically, the terminal can determine the connectability time based on the connectability time information received from the base station. For example, the base station can periodically or non-periodically transmit connectability time information through system information (e.g., SIB19). Specifically, at least one parameter (e.g., t-Service) included in the system information can indicate the connectability time information. Here, at least one parameter can indicate the time information of service interruption in the area currently covered by the base station. For example, at least one parameter can include the location information of the non-terrestrial cell, the speed of the non-terrestrial cell, etc., and can include information about the distance between the terminal and the non-terrestrial cell based on the location information. Simultaneously, the terminal can deduce the connectability time based on the received connectability time information. Furthermore, the connectability time can also be included in the connectability time information received from the base station during transmission.

[0144] Whether the connectable time derived or received through the above process is less than the second threshold can be determined by the following mathematical formula 2. The second threshold will be referred to below as... .

[0145] [Mathematical Expression 2]

[0146]

[0147] In step S1003, the terminal performs ground cell reselection. Specifically, if the calculation result of the above mathematical formula 2 indicates that the available connection time is less than the second threshold, then RRC connection reconstruction is not performed; instead, the terminal switches to the RRC IDLE state and performs ground cell reselection. In other words, if the available connection time is less than a predetermined value, it can be understood that even if the terminal performs RRC connection reconstruction, it is determined that another handover needs to be performed within a short period of time, therefore RRC connection reconstruction is not performed.

[0148] Simultaneously, the following standard procedure can be considered for cell reselection. The terminal can use the following rules to limit needed measurements. If the serving cell satisfies the conditions Srxlev > SIntraSearchP and Squal > SIntraSearchQ, or if distanceThresh and referenceLocation are broadcast in SIB19, and the terminal supports initiating location-based measurements for (quasi-)earth fixed cells in non-terrestrial networks and has acquired location information, or if the distance between the terminal and the serving cell's referenceLocation is less than distanceThresh, then the terminal may not perform co-frequency measurements. On the other hand, in the opposite case, the terminal should perform co-frequency measurements.

[0149] Furthermore, if distanceThresh and movingReferenceLocation are broadcast in SIB19, and the terminal supports initiating location-based measurements for non-terrestrial mobile cells and has acquired location information, or if the distance between the terminal's location and the serving cell reference location based on movingReferenceLocation is less than distanceThresh, then the terminal may not perform intra-frequency measurements. Conversely, in the opposite case, the terminal should perform intra-frequency measurements.

[0150] The terminal should apply the following rules to the NR inter-frequency and inter-RAT frequencies indicated in the system information, which can be allocated according to the priorities defined in 5.2.4.1.

[0151] For inter-NR frequencies or inter-RAT frequencies with a reselection priority higher than the current NR frequency, the terminal shall perform measurements on the higher-priority inter-NR frequency or inter-RAT frequency as specified in TS 38.133[8]. For inter-NR frequencies with a reselection priority equal to or lower than the current NR frequency and inter-RAT frequencies with a lower priority, for example, if the serving cell satisfies the conditions Srxlev > SnonIntraSearchP and Squal > SnonIntraSearchQ, or distanceThresh and referenceLocation are broadcast in SIB19, and the terminal supports location-based measurement initiation for non-terrestrial (quasi-)earth fixed cells and has acquired location information, or the distance between the terminal and the reference location of the serving cell is less than distanceThresh, the terminal may not perform measurements on inter-NR frequency cells with the same or lower priority or inter-RAT frequency cells with a lower priority. Conversely, the terminal shall perform measurements on inter-NR frequency cells with the same or lower priority or inter-RAT frequency cells with a lower priority as specified in TS 38.133[8].

[0152] Meanwhile, if distanceThresh and movingReferenceLocation are broadcast in SIB19, and the terminal supports location-based measurement initiation for non-terrestrial network earth-moving cells and has acquired location information, or if the distance between the terminal's location and the serving cell reference location based on movingReferenceLocation is less than distanceThresh, the terminal may not perform measurements for NR inter-frequency cells of the same or lower priority or inter-RAT frequency cells of lower priority. Conversely, the terminal shall perform measurements for NR inter-frequency cells of the same or lower priority or inter-RAT frequency cells of lower priority according to TS 38.133 [8]. If the terminal supports relaxed measurement and relaxationMeasurement exists in SIB2, the terminal may further relax the required measurements in accordance with the provisions of Section 5.2.4.9.

[0153] For terminals camped on non-terrestrial network cells, if the terminal supports TN measurement omission, has obtained location information, and coverageAreaInfoList and tn-AreaIdList have been broadcast in the system information, then if the terminal is not within the corresponding frequency range provided by tn-AreaIdList, then regardless of priority, the terminal may not perform TN frequency measurements.

[0154] If a t-Service exists in the SIB19 of the serving cell and the terminal supports time-based measurement initiation, the terminal should perform intra-frequency, inter-frequency, or inter-RAT measurements before the t-Service, regardless of the distance between the terminal and the serving cell reference location, and regardless of whether the serving cell satisfies the conditions Srxlev > SIntraSearchP and Squal > SIntraSearchQ or Srxlev > SnonIntraSearchP and Squal > SnonIntraSearchQ. The specific time to start measurements before the t-Service may depend on the terminal's implementation. The terminal should perform higher-priority inter-NR frequency or inter-RAT frequency measurements according to TS 38.133 [8], without considering the remaining service time of the serving cell (i.e., this may be unrelated to the remaining time of the t-Service).

[0155] When assessing the distance between a terminal and the serving cell reference location, the location information acquired by the terminal may vary depending on the terminal implementation. Furthermore, for moving Earth cells, the terminal's ability to maintain a valid serving cell reference location depends on the terminal implementation, which can be derived based on serving satellite orbit information, epoch time, and movingReferenceLocation.

[0156] 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 features described above. Therefore, the disclosed method is not intended to be limiting but rather illustrative. The scope of this disclosure is shown in the claims rather than in the foregoing description, and all differences within its equivalent scope are to be interpreted as included in this disclosure.

[0157] Potential for commercial applications

[0158] This invention can be used in wireless communication systems for devices that perform handover.

Claims

1. A method for operating a terminal in a wireless communication system, comprising: The steps for receiving measurement control; The steps for measuring signals and sending measurement report messages to the serving cell; as well as When a radio link failure is detected, if the serving cell is a non-terrestrial cell and the conditions for omitting RRC connection reconstruction are met, the cell is reselected to a terrestrial cell that is not the serving cell.

2. The method according to claim 1, wherein, The wireless link failure is determined based on at least one of the following: wireless fault detection, wireless fault timer timeout, RACH procedure failure, RLC failure, or uplink LBT failure detection.

3. The method according to claim 1, wherein, The conditions include at least one of the following: the terrestrial cell has a higher priority than the non-terrestrial cell; the signal strength of the non-terrestrial cell is less than a threshold; or the connection time with the non-terrestrial cell is less than a threshold.

4. The method according to claim 1, wherein, Also includes: When the serving cell is a non-terrestrial cell and the condition is met, the step of reselecting the cell as a terrestrial cell is executed.

5. The method according to claim 4, wherein, The steps of reselecting the cell as the ground cell include: The step of confirming whether the terrestrial cell is set to a higher priority compared to the non-terrestrial cell; and When the ground cell is set to a high priority, the step of reselecting the ground cell is executed.

6. The method according to claim 4, wherein, The steps of reselecting the cell as the ground cell include: The step of comparing the signal measurement value of the non-terrestrial cell with a first threshold; and When the signal measurement value is less than the first threshold, the step of reselecting the cell as the ground cell is executed.

7. The method according to claim 6, wherein, The step of confirming whether it is less than the first threshold includes: The step of determining whether the result reflecting the hysteresis value in the signal measurement is less than the first threshold.

8. The method according to claim 4, wherein, The steps of reselecting the cell as the ground cell include: The steps of determining the connectivity time between the terminal and the non-terrestrial cell based on the mobile reference location of the non-terrestrial cell; and When the available connection time is less than the second threshold, the step of reselecting the cell as the ground cell is executed.

9. A terminal in a wireless communication system, comprising: transceiver; as well as The processor connected to the transceiver The processor control is as follows: Receive measurement control, The measurement signal is sent to the serving cell as a measurement report message. When a radio link failure is detected, if the serving cell is a non-terrestrial cell and the condition for omitting RRC connection reconstruction is met, cell reselection is performed to a terrestrial cell that is not the serving cell.

10. The terminal according to claim 9, wherein, The wireless link failure is determined based on at least one of the following: wireless fault detection, wireless fault timer timeout, RACH procedure failure, RLC failure, or uplink LBT failure detection.

11. The terminal according to claim 9, wherein, The conditions include at least one of the following: the terrestrial cell has a higher priority than the non-terrestrial cell; the signal strength of the non-terrestrial cell is less than a threshold; or the connection time with the non-terrestrial cell is less than a threshold.

12. The terminal according to claim 9, wherein, The processor control is as follows: When the serving cell is a non-terrestrial cell, and the condition is met, the cell is reselected as a terrestrial cell.

13. The terminal according to claim 12, wherein, The processor control is as follows: In order to perform the cell reselection as the ground cell, it is confirmed whether the ground cell is set to a higher priority compared to the non-ground cell. When the ground cell is set to a high priority, the cell is reselected as the ground cell.

14. The terminal according to claim 12, wherein, The processor control is as follows: To perform the cell reselection as the terrestrial cell, the signal measurement value of the non-terrestrial cell is compared with a first threshold. When the signal measurement value is less than the first threshold, the cell is reselected as the ground cell.

15. The terminal according to claim 14, wherein, The processor control is as follows: Confirm whether the result reflecting the hysteresis value in the signal measurement is less than the first threshold.

16. The terminal according to claim 12, wherein, The processor control is as follows: In order to perform the cell reselection to the ground cell, the connectivity time between the terminal and the non-ground cell is determined based on the mobility reference position of the non-ground cell. When the available connection time is less than the second threshold, the cell is reselected as the ground cell.

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