Apparatus and method for performing connection re-establishment in wireless communication system

Abstract

The present disclosure relates to a method of operating a terminal in a communication system. The method comprises the steps of: receiving measurement control; measuring a signal so as to transmit a measurement report message to a serving cell; and, when a radio link failure is detected, performing cell reselection to a terrestrial cell other than the serving cell if the serving cell is a non-terrestrial network (NTN) cell and conditions for omitting radio resource control connection re-establishment (RRE) are satisfied.

Description

Device and method for performing connection reset in a wireless communication system

[0001] The present disclosure relates to a wireless communication system, and in particular to an apparatus and method for performing a connection reset in a wireless communication system.

[0002]

[0003] With the advancement of mobile communication services, Non-Terrestrial Networks (NTNs), which are free from spatial constraints, are attracting attention, and low-orbit satellites, in particular, are expected to play a major role. Accordingly, 3GPP (3 rd The Generation Partnership Project is proceeding with standardization for non-terrestrial networks without spatial constraints to provide effective communication services.

[0004] This research was conducted with funding from the Ministry of Science and ICT in 2024 and supported by the Korea Institute of Science and Technology Promotion (RS-2024-00423453, IP Advancement and Commercialization for the Promotion of Commercialization of Technology Utilizing Next-Generation Low Earth Orbit Satellite Constellation).

[0005] This research was conducted with funding from the government (Ministry of Science and ICT) in 2024 and supported by the Korea Institute of Information and Communications Technology Planning and Evaluation (No. RS-2021-II210794, Development of 3D Spatial Mobile Communication Technology), (No. RS-2022-II220704, Development of Core 3D-NET Technologies for Supporting Ultra-High-Speed ​​Mobile Vehicles).

[0006]

[0007] The present disclosure is intended to provide an apparatus and method for effectively performing connection reset in a wireless communication system.

[0008] The present disclosure is intended to provide an apparatus and method for performing conditional RRC (Radio Resource Control) connection resetting.

[0009] The present disclosure is intended to provide an apparatus and method for performing cell reselection or performing RRC connection resetting to a serving cell depending on whether conditions for performing RRC connection resetting are satisfied.

[0010] The present disclosure is intended to provide an apparatus and method for improving handover performance in a non-terrestrial network environment in a wireless communication system.

[0011] The present disclosure is intended to provide an apparatus and method for applying conditional RRC connection reset to a non-terrestrial network in a wireless communication system.

[0012]

[0013] According to one aspect of the present disclosure, a method of operation of a terminal in a system is disclosed. The method may include the steps of receiving measurement control, measuring a signal and transmitting a measurement report message to a serving cell, and, when a radio link failure is detected, if the serving cell is a non-terrestrial (NTN) cell and conditions for omitting a Radio Resource Control Connection re-establishment (RRE) are satisfied, performing cell reselection to a terrestrial cell other than the serving cell.

[0014] According to another aspect of the present disclosure, a terminal is disclosed in a system. The terminal includes a transceiver and a processor connected to the transceiver, and the processor receives measurement control, measures a signal and transmits a measurement report message to a serving cell, and if a wireless link failure is detected, the serving cell is a non-ground cell and satisfies conditions for omitting RRC connection reconfiguration, the processor may perform cell reselection to a ground cell other than the serving cell.

[0015]

[0016] According to embodiments of the present disclosure, RRC connection reconfiguration for terminals using a non-terrestrial network can be effectively performed.

[0017]

[0018] FIG. 1 illustrates an example of a network according to one embodiment of the present disclosure.

[0019] FIG. 2 illustrates another example of a network according to one embodiment of the present disclosure.

[0020] FIG. 3 illustrates the configuration of a device in a wireless communication system according to one embodiment of the present disclosure.

[0021] FIG. 4 illustrates an example of a procedure for omitting conditional RRC connection re-establishment (Radio Resource Control connection re-establishment, RRE) in a wireless communication system according to one embodiment of the present disclosure.

[0022] FIG. 5 illustrates an example of a conditional RRC connection reset procedure based on cell reselection condition determination in a wireless communication system according to one embodiment of the present disclosure.

[0023] FIG. 6 illustrates an example of a procedure for performing cell reselection when a radio link failure (RLF) is detected during a handover according to one embodiment of the present disclosure.

[0024] FIG. 7 illustrates an example of the operation procedure of a terminal in a wireless communication system according to one embodiment of the present disclosure.

[0025] FIG. 8 illustrates an example of a procedure for determining the priority of a cell for determining a cell re-selection condition according to one embodiment of the present disclosure.

[0026] FIG. 9 illustrates an example of a procedure for determining the strength of a signal for determining a cell reselection condition according to one embodiment of the present disclosure.

[0027] FIG. 10 illustrates an example of a procedure for determining the connection time of a cell re-selection condition determination according to one embodiment of the present disclosure.

[0028]

[0029] The terms used in these embodiments have been selected to be as widely used and general as possible, taking into account the functions within these embodiments; however, these terms may vary depending on the intent of those skilled in the art, case law, the emergence of new technologies, etc. Additionally, in specific cases, the applicant has arbitrarily selected terms, and in such cases, their meanings will be described in detail in the relevant sections. Therefore, the terms used in these embodiments should be defined not merely by their names, but based on their meanings and the content throughout these embodiments.

[0030] The embodiments are subject to various modifications and may take various forms; therefore, some embodiments are illustrated in the drawings and described in detail. However, this is not intended to limit the embodiments to the specific disclosed forms, and it should be understood that the embodiments include all modifications, equivalents, and substitutions that fall within the spirit and scope of the embodiments. The terms used herein are for the description of the embodiments only and are not intended to limit the embodiments.

[0031] Unless otherwise defined, the terms used in these embodiments have the same meaning as generally understood by those skilled in the art to which these embodiments pertain. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in these embodiments.

[0032]

[0033] The present disclosure relates to a technology for handover to a terminal using a non-terrestrial network (NTN). Alternatively, the present disclosure relates to a technology for cell reselection to a terminal using a non-terrestrial network. In the event that a Radio Link Failure (RLF) occurs in a source cell during a conditional handover or Dual Active Protocol Stack (DAPS) handover procedure, the terminal must perform a Radio Resource Control Connection Re-establishment (RRE) procedure with the source cell in general situations, except when information about the target is known. However, even if an RRE is performed due to a Radio Link Failure during a conditional handover or DAPS handover procedure, a handover may be performed again due to the characteristics of the non-terrestrial network environment. Frequent handovers may lead to resource waste or degrade communication performance due to interference. Consequently, in a non-terrestrial network environment, procedures related to RRE connection re-establishment may require additional procedures related to cell reselection. Accordingly, the present disclosure proposes a procedure for applying conditional RRC connection reconfiguration to non-terrestrial networks.

[0034]

[0035] FIG. 1 illustrates an example of a network according to one embodiment of the present disclosure.

[0036] Referring to FIG. 1, the network may include a non-terrestrial network and a terrestrial network (TN), and may include a terminal (110), satellites (120-1, 120-2), a gateway (130), and a base station. The terminal (110) is a user device equipped with hardware and software that receives cellular data from a satellite (120-1), and may be a mobile or fixed device. For example, the terminal (110) may include a mobile phone, a smartphone, a wearable device, or a UE (User Equipment). Furthermore, the terminal (110) is not limited to the examples described above, and may include any electronic device capable of cellular communication, such as a laptop or tablet PC. The terminal (110) is not limited to the examples described above. Although the network in FIG. 1 is depicted as including only a single terminal (110), this is merely an exemplary embodiment and is not limited thereto, and it is obvious that it may include multiple terminals (110).

[0037] Specifically, the terminal (110) can support communication protocols defined in 3GPP (3rd generation partnership project) standards (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) technology, WCDMA (wideband CDMA) technology, TDMA (time division multiple access) technology, FDMA (frequency division multiple access) technology, OFDM (orthogonal frequency division multiplexing) technology, Filtered OFDM technology, CP (cyclic prefix)-OFDM technology, DFT-s-OFDM (discrete Fourier transform-spread-OFDM) technology, OFDMA (orthogonal frequency division multiple access) technology, SC (single carrier)-FDMA technology, NOMA (non-orthogonal multiple access) technology, GFDM (generalized frequency division multiplexing) technology, FBMC (filter bank multi-carrier) technology, UFMC (universal filtered multi-carrier) technology, SDMA (space division multiple access) technology, etc.

[0038] Satellites (120-1, 120-2) fly in a fixed orbit and can provide a cell with coverage of a certain size by forming a beam toward the ground. In relation to the present disclosure, satellite (120-1) may refer to a serving satellite and satellite (120-2) may refer to a target satellite. Additionally, the serving satellite and the target satellite may be referred to as a serving cell and a target cell, respectively, and may be used interchangeably. Furthermore, in the present disclosure, satellites (120-1, 120-2) may be collectively referred to as non-ground cells. A gateway (130) provides the satellites (120-1, 120-2) with a link to connect to a network. A base station (140) may refer to a terrestrial base station or a wireless communication device fixed at a specific location and can provide a cell with coverage of a certain size. In connection with the present disclosure, satellites (120-1, 120-2) and base station (140) may be referred to as gNB (gNodeB) or cell. Additionally, the last cell to which the terminal (110) was previously connected may be mentioned in the present disclosure, and said last cell may be referred to as last gNB, Last Serving gNB, source cell, etc. For example, the last cell may refer to the serving cell to which the terminal was previously connected before a handover occurred and the terminal was connected to a new cell.

[0039] The link between the terminal (110) and the satellite (120-1) is called a service link and may be based on NR standards defined by 3GPP. The link between the satellites (120-1, 120-2) and the gateway (130) is called a feeder link and may be based on a 3GPP or non-3GPP wireless interface. An inter-satellite link (ISL) may be used mainly for regenerative satellites.

[0040] For transparent satellites based on an NR-RAN architecture, the satellite radio interfaces of the feeder link and service link may be NR-Uu. For transparent satellites, the satellite performs radio frequency filtering, frequency conversion, and amplification functions. For regenerative satellites, onboard functions are built into the satellite, and accordingly, the satellite can perform radio frequency filtering, frequency conversion, and amplification, as well as some or all of the base station functions such as switching and routing, coding and modulation, and decoding and demodulation.

[0041]

[0042] FIG. 2 illustrates another example of a network according to one embodiment of the present disclosure. FIG. 3 illustrates an example of an NTN providing non-terrestrial access to a UE (210) using an NTN payload (220) and an NTN gateway (230). Here, the UE (210) may be substantially the same configuration as the terminal (110) described in FIG. 1. Referring to FIG. 2, the link between the NTN payload (220) and the UE (210) is a service link and may be based on a Uu interface. The link between the NTN payload (220) and the NTN gateway (230) is a feeder link. The link between the NTN gateway (230) and the AMF / UPF (240) may be based on an NG interface. The NTN payload (220) can transparently forward wireless protocols received from the UE (210) to the NTN gateway (230) via the service link. Similarly, the NTN payload (220) can transparently forward wireless protocols received from the NTN gateway (230) via a feeder link to the UE (210).

[0043] To this end, the following connectivity may be supported by the NTN payload (220). A base station may service multiple NTN payloads. An NTN payload may be serviced by multiple base stations.

[0044] The NTN payload (220) can change the carrier frequency before retransmitting data on the service link. That is, the NTN payload (220) can use different carrier frequencies on the service link and the feed link. For the NTN, at least one of the following may be used as a network identifier: AMF name, NCGI (NR cell global identifier), CgNB ID (identifier), 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 (identifier). Additionally, a mapped cell ID may be used. Here, the tracking area may correspond to a fixed geographical area.

[0045] Non-geosynchronous orbits (NGSO) include a low Earth orbit at an altitude of about 300 km to 1500 km and a medium Earth orbit at an altitude of about 7000 km to 25000 km.

[0046] Service links can be classified into the following three types: earth-fixed, quasi-earth-fixed, and earth-moving. The earth-fixed type provides beam(s) that continuously cover the same geographical area at all times. For example, a satellite in a geosynchronous orbit (GSO) can provide an earth-fixed type service link. The quasi-earth-fixed type provides beam(s) that continuously cover the same geographical area for a limited period and beams that cover different geographical areas during different periods. For example, a satellite in a non-earth-synchronous orbit can provide a quasi-earth-fixed type service link using steerable beams. The earth-moving type provides beams where the coverage area slides across the Earth's surface. For example, a satellite with a non-Earth-synchronous orbit can provide an Earth-moving type service link using fixed or steerable beams.

[0047] By using a satellite with a non-Earth-synchronous orbit, the base station can provide quasi-Earth-fixed cell coverage or Earth-mobile cell coverage. By using a satellite with an Earth-synchronous orbit, the base station can provide Earth-fixed cell coverage. In the case of a non-Earth-synchronous orbit, a switch of the service link may be referred to a change of the serving satellite (120-1).

[0048] Recently, 3GPP has been standardizing handovers in non-terrestrial networks. In Rel-17, it was decided to support conditional handover to reduce handover failure rates, and in Rel-18, it was decided to support RACH-free handover to reduce signaling overhead caused by RACH and handover interruption time in non-terrestrial network environments. Furthermore, discussions are underway regarding methods to combine conditional handover and RACH-free handover in non-terrestrial network environments. When combining the two handover technologies, it may be difficult to transmit uplink grants via PDCCH because the non-terrestrial base station does not know the handover execution timing of the terrestrial terminal. Therefore, this disclosure proposes a technology that combines timing-based conditional handover and RACH-free handover to solve this problem.

[0049]

[0050] FIG. 3 illustrates the configuration of a device in a wireless communication system according to one embodiment of the present disclosure. The device of FIG. 3 may be understood as a part of the structure of any one of the devices described with reference to FIG. 1, for example, a terminal (110), satellites (120-1, 120-2), a gateway (130), and a base station (140).

[0051] Referring to FIG. 3, the device may include a processor (310), a communication unit (320), and a memory (330).

[0052] The processor (310) can control the overall function and operation of the device. The processor (310) may include an application-specific integrated circuit (ASIC), other chipsets, logic circuits, and / or data processing devices.

[0053] The communication unit (320) is connected to the processor (310) to transmit and receive wireless signals. The communication unit (320) may include a baseband circuit for processing wireless signals. For example, the communication unit (320) may include a short-range communication unit, a mobile communication unit, and a broadcast reception unit. In one embodiment, the communication unit (320) may transmit and receive data to and from other devices, such as a base station, a satellite, etc.

[0054] Memory (330) is hardware that stores various data processed by the processor (310). For example, the memory (330) may store SIR values ​​for the transmission target terminals of the transmitting terminals, information regarding transmission target terminal groups for each transmitting terminal, etc. Additionally, the memory (330) may store applications, drivers, etc. to be driven by the processor (310). The memory (330) may include RAM (random access memory) such as DRAM (dynamic random access memory) and 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.

[0055] The structure of FIG. 3 can be understood as at least part of a terminal, base station, satellite, or gateway. If the structure of FIG. 3 is part of a satellite, the satellite may further include other hardware devices necessary for orbiting in addition to the components exemplified in FIG. 3. If the structure of FIG. 3 is part of a gateway or base station, the gateway or base station may further include components that support wired communication, etc.

[0056]

[0057] In a wireless communication system, a radio link failure may occur when the quality of the radio link degrades or is disconnected. Radio link failures can be detected and handled at various layers of the network. For example, a terminal may declare a radio link failure if a radio problem timer expires after radio problem detection from the physical layer, if the RACH procedure fails, if a Radio Link Control Failure (RLC) occurs, or if an Uplink Listen-Before-Talk (LBT) failure is detected.

[0058] Meanwhile, a network where terrestrial and non-terrestrial networks coexist can maximize the flexibility and efficiency of the communication system, thereby providing better service quality to users. For example, in a situation where the serving cell is a non-terrestrial network, if a radio link failure (RLF) occurs while the terminal is performing a handover to a terrestrial cell, the terminal can perform an RRC connection reset.

[0059] Specifically, after a wireless link failure is declared, the terminal searches for a suitable cell and performs an RRC connection reset. However, if it fails to find a suitable cell for a predetermined period of time, the terminal transitions to the RRC IDLE state.

[0060] For example, if a source cell wireless link failure occurs during a DAPS handover, the terminal stops transmitting or receiving data over the source link and releases the source link, but maintains the source RRC configuration value so that necessary configuration information can be reused later. If a handover failure to the target cell is declared, the terminal selects an appropriate cell and attempts to reconfigure the RRC connection; if it fails to find an appropriate cell for a predetermined period of time, it switches to the RRC IDLE state and declares a handover failure.

[0061] Additionally, as an example, if a source cell wireless link failure occurs during a conditional handover, the terminal searches for a suitable cell. Furthermore, if the selected cell is a candidate cell for the conditional handover, or if the network has configured the terminal to attempt a conditional handover after a wireless link failure, the terminal retryes the conditional handover or performs an RRC connection reset. If the terminal fails to find a suitable cell within a certain period after the wireless link failure, the terminal transitions to the RRC IDLE state.

[0062] As mentioned above, in the case of DAPS handover and conditional handover, there are instances where an RRC connection reset is not performed even if a wireless link failure occurs at the source cell. For example, even if a wireless link failure occurs after transmitting a handover command from the target cell, the terminal performs a handover to the target cell without performing an RRC connection reset. In the majority of cases, excluding these instances, the terminal performs an RRC connection reset to the serving cell upon the occurrence of a wireless link failure. On the other hand, if the serving cell is a non-terrestrial cell (or non-terrestrial serving cell, NTN serving cell) and the wireless link failure occurs at the edge of the non-terrestrial cell, even if the terminal performs an RRC connection reset, there is a high probability that the handover will be performed again within a short period due to the mobility of the non-terrestrial cell, and the RRC connection reset is also likely to fail. Therefore, the following proposes a method in which, if the priority of the target cell is a ground cell and the terminal is located near the edge of a non-ground cell or the boundary of the coverage of a non-ground cell, the terminal switches to the RRC IDLE state without performing an RRC connection reset and re-selects the ground cell.

[0063] Meanwhile, as a standard procedure related to the present disclosure, Radio Link Monitoring (RLM) in an RRC connection state may be considered. The terminal may perform Radio Link Monitoring (RLM) in an active Bandwidth Part (BWP) in an RRC connection state based on a reference signal (e.g., SSB, CSI-RS) and a signal quality threshold set by the network. SSB-based Radio Link Monitoring is performed based on a Cell Defining SSB (CD-SSB) connected to an initial DL BWP and may be configured in the initial DL BWP or in a DL BWP containing the said CD-SSB. Additionally, the terminal may perform Radio Link Monitoring based on a non-cell defining SSB. Furthermore, CSI-RS-based Radio Link Monitoring may also be performed if configured in an active DL BWP. During a DAPS handover, the terminal may continue to detect Radio Link failures in the source cell until the random access procedure in the target cell is successfully completed. The terminal can detect a wireless link failure when a timer started after detecting a wireless problem at the physical layer expires (e.g., when the timer is stopped when the wireless problem is resolved), when a timer set at the time of a measurement report trigger expires, when another wireless problem timer is running, or when a RACH procedure fails and an RLC fails.

[0064] When using a shared spectrum channel, upon detecting a consistent uplink Listen-Before-Talk (LBT) failure, the IAB-MT can declare a radio link failure when it receives a backhaul radio link failure.

[0065] After a wireless link failure, the terminal can maintain the RRC configuration values. For example, when a wireless link failure occurs in the source cell during a DAPS handover, the terminal stops transmitting / receiving data on the source link and releases the source link, but can maintain the source RRC configuration.

[0066] Meanwhile, if a handover failure is declared at the target cell, the terminal may select an appropriate cell and initiate an RRC connection reset. Additionally, if an appropriate cell is not found within a certain period of time, the terminal transitions to the RRC IDLE state.

[0067] Meanwhile, if a wireless link failure occurs at the source cell during a conditional handover, the terminal selects an appropriate cell; if that cell is a candidate cell for the conditional handover and the network has configured a conditional handover attempt, the terminal attempts to execute the conditional handover. If the conditional handover is not attempted, the terminal performs an RRC connection reset. Additionally, if a suitable cell is not found within a certain period of time, the terminal transitions to the RRC IDLE state.

[0068] Meanwhile, when a wireless link failure occurs in the source cell of MCG LTM (Local Triggered Mobility), the terminal selects an appropriate cell, and if the selected cell is an LTM candidate cell, attempts to execute RACH-based LTM. If the terminal does not execute RACH-based LTM, the terminal performs an RRC connection reset. Specifically, if no appropriate cell is found, the terminal switches to the RRC IDLE state, and otherwise selects an appropriate cell and attempts to reset the RRC connection.

[0069] Meanwhile, when an RLF occurs on the IAB backhaul link, the same mechanisms and procedures as those on the access link may be applied to the terminal. For example, this may include backhaul wireless link failure detection and wireless link failure recovery.

[0070] IAB-DU may transmit a backhaul wireless link failure detection indicator to a child node in the following cases. For example, when IAB-MT starts an RRC connection reset, when IAB-MT detects a wireless link failure on a backhaul link while in a dual-connected state and UL rerouting is not possible, when IAB-MT receives a backhaul wireless link failure detection indicator from a parent node and there is no backhaul link unaffected by the wireless link failure condition, IAB-DU may transmit a backhaul wireless link failure detection indicator to a child node.

[0071] When a child node receives a backhaul wireless link failure detection indicator, it can perform local rerouting for upstream traffic over available backhaul links. Upon wireless link failure recovery, the IAB-DU can transmit a backhaul wireless link failure recovery indicator to the child node. Upon receiving the wireless link failure recovery indicator, the child node can revert previously performed actions. If wireless link failure recovery fails at the IAB node, for example, if the RRC connection reset procedure fails, the IAB node can transmit a backhaul wireless link failure indicator to the child node. Backhaul wireless link failure detection indicators, recovery indicators, and wireless link failure indicators can be transmitted to the BAP (Control PDU).

[0072]

[0073] FIG. 4 illustrates an example of a procedure for omitting conditional RRC connection reconfiguration in a wireless communication system according to one embodiment of the present disclosure. The first base station (420-1) is a serving cell and may be a non-ground cell. Additionally, the second base station (420-2) is a target cell, and the second base station (420-2) may correspond to either a non-ground cell or a ground cell. The first base station (420-1) may be referred to as a source gNB or a source eNB, and similarly, the second base station (420-2) may be referred to as a target gNB or a target eNB.

[0074] Additionally, although not illustrated in FIG. 4, a wireless communication system according to one embodiment of the present disclosure may additionally include a third base station, and the third base station may be subject to cell re-selection. For example, the third base station may be a base station of a different type from the second base station, and for instance, if the second base station is a non-ground cell, the third base station may be a ground cell.

[0075] Referring to FIG. 4, in step S401, the Mobility Management Entity (MME) (430) sets an area restriction. The MME (430) is a control node and may be an object responsible for signal processing, mobility management, and session setup between the terminal (410) and the network in the control plane. Specifically, the MME (430) can restrict or allow the terminal (410) to access in a specific area by providing an area restriction to enable area-based access control. For example, the MME (430) can set restrictions on specific services through the area restriction, such as restricting high-speed data services in a specific area or allowing only voice calls or low-speed data. Additionally, the MME (430) can set an area restriction to restrict roaming, refuse handovers to specific areas, or prevent the terminal (410) from accessing in secure areas such as military bases or government agencies.

[0076] In step S403, the first base station (420-1) transmits information regarding measurement control to the terminal (410). Specifically, the first base station (420) transmits a measurement control message to the terminal (410) requesting measurements regarding frequency, cell, signal quality, etc. As an example, the measurement control message may include information regarding measurement items such as SINR (Signal to Interference and Noise Ratio), RSRQ (reference signal received quality), and RSRP (reference signals received power).

[0077] In step S405, the terminal (410) measures the signal. Specifically, the terminal (410) measures the signal strength of base stations (420-1, 420-2) to reserve resources for handover. The terminal (410) may measure the signal strength according to a predetermined period, which may be referred to as a measurement interval in this disclosure.

[0078] In detail, the terminal (410) can detect a preparation event for a handover by measuring the signal strength of base stations (420-1, 420-2). For example, the terminal (410) may determine that a preparation event for a handover has been detected if the measured signal strength or quality is below a predefined threshold or within a predetermined range. Additionally, for example, the terminal (410) may determine that a preparation event for a handover has been detected 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), which is the serving cell.

[0079] Meanwhile, the terminal (410) and the first base station (420-1), and the first base station (420-1) and the gateway (440) can exchange packet data (or user data). The gateway (440) manages user data traffic and can perform handover, data buffering, and data traffic management during roaming.

[0080] Additionally, the first base station (420-1) may allocate an uplink (UL) to the terminal (410). The allocation of the uplink is a process of allocating the necessary resources for the terminal (410) to transmit data to the first base station (420-1), which is the serving cell, and the uplink refers to the path through which the terminal (410) transmits data to the first base station (420-1). Uplink allocation may be performed through L1 signaling or L2 signaling, where L1 signaling is the physical layer and may include all operations related to the transmission and reception of actual wireless signals. L2 signaling is the data link layer and may include operations related to direct data transmission and connection maintenance.

[0081] In step S407, the terminal (410) transmits a measurement report to the first base station (420-1). Specifically, the terminal (410) periodically measures the signal strength of the first base station (420-1) and other base stations (not shown) and transmits a measurement report message. The measurement report may include at least some of the elements of a set 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, the terminal (410) may measure at least one of the SINR, RSRQ, and RSRP of the first base station (420-1) and other base stations. In addition, the terminal (410) can measure the power strength of the received signal, such as RSSI (Received Signal Strength Indicator), and the ratio between the signal and noise, such as SNR (Signal to Noise Ratio), but is not limited thereto.

[0082] In step S409, the first base station (420-1) determines a handover. Specifically, the first base station (420-1) checks at least one of RSRP, RSRQ, or SINR through the received measurement report. At this time, the first base station (420-1) may determine a handover if the strength or quality of the measured signal is below a predefined threshold or is within a predetermined range. Additionally, as an example, the first base station (420-1) may determine a handover if the strength or quality of the signal provided by other base stations is above a specific threshold than the strength or quality of the signal provided by the first base station (420-1), which is the serving cell. The above process can be understood as the first base station (420-1) detecting a triggering event from the measurement report.

[0083] In step S411, the first base station (420-1) transmits a handover request. Specifically, the first base station (420-1) transmits a handover request message to the target cell. The target cell may be determined based on measurement reports regarding other base stations. For example, the target cell may be determined based on RSRP, RSSI, RSRQ, SINR for other base stations, traffic load of each base station, priority, speed of the terminal (410), frequency band, call state, and UE capabilities. For convenience of explanation, the following description assumes that the second base station (420-2) is determined as the target cell, but the second base station (420-2) may be a non-terrestrial cell.

[0084] The handover request message includes information about the terminal (410), and 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. Here, the handover request message may be transmitted via an inter-satellite link (ISL) or a feeder link.

[0085] In step S413, the second base station (420-2), which is the target cell, performs admission control. Specifically, the second base station (420-2) determines whether to accept a handover request based on a handover request message, and may reject the handover request if network resources are insufficient or if there is a concern about quality degradation. For example, the second base station (420-2) may determine whether to accept by calculating available bandwidth, frequency resources, cell capacity, and Quality of Service requirements.

[0086] In step S415, the second base station (420-2) transmits a handover request acknowledge to the first base station (420-1). Specifically, if the second base station (420-2) determines, as a result of performing acceptance control, that a handover is possible, it transmits a handover request acknowledge message to the first base station (420-1).

[0087] Meanwhile, the first base station (420-1) can allocate a downlink (DL) to the terminal (410). A downlink refers to a path through which a network (e.g., a non-ground cell, a ground cell) transmits data to the terminal (410), and downlink allocation refers to the process of the network allocating wireless resources so that the terminal (410) can receive data.

[0088] Additionally, the first base station (420-1) may transmit a handover command. Specifically, the first base station (420-1) may transmit a handover command message to the terminal (410). The handover command message may include information about the second base station (420-2), which is the target cell, and said information may include C-RNTI, Timing Advance (TA), and location information. Through this, the first base station (420-1) may assign an RNTI (e.g., G(group)-RNTI, C-RNTI) for broadcast or multicast to the terminal (410). The RNTI may be provided in advance through system information or provided along with the settings for measurement. The aforementioned steps S401 through S415 may be referred to as handover preparation steps and may be understood as a process of reserving resources for executing a handover.

[0089] In step S417, the terminal (410) performs RACH. Through the RACH procedure, the terminal (410) obtains uplink grant information of the first base station (420-1) and can use it to transmit a handover confirmation message to the second base station (420-2). Specifically, the terminal (410) transmits a preamble to the second base station (420-2), and the second base station (420-2) can transmit a response to the RA (Random Access) to the terminal (410). Next, the terminal (410) can transmit a Radio Resource Control connection reconfiguration complete message to the second base station (420-2) to convey that the radio resource settings following the RRC connection completion have been successfully applied.

[0090] In step S419, the terminal (410) detects a wireless link failure. For example, the terminal (410) may declare a wireless link failure when the radio problem timer expires after radio problem detection from the physical layer, when the RACH procedure fails, when a Radio Link Control Failure (RLC) occurs, or when an Uplink Listen-Before-Talk (LBT) failure is detected. In addition, the terminal (410) may detect or declare a wireless link failure when signal measurement values ​​such as RSRP, RSRQ, and SINR fall below a predetermined threshold value. In this disclosure, the description focuses on the terminal (410) declaring a wireless link failure during a handover, but is not limited thereto, and may include declaring a wireless link failure at various layers where data packet transmission, channel connection, etc., take place, provided that it does not conflict with this disclosure.

[0091] In step S421, the terminal (410) performs a conditional RRC connection reset. Specifically, the terminal (410) does not perform an RRC connection reset immediately, but performs a conditional RRC connection reset and can switch to an RRC IDLE state when performing cell reselection. Meanwhile, if the terminal (410) performs an RRC connection reset immediately upon detection of a wireless link failure, a handover may be performed again within a short period, and there is a high probability that the RRC connection reset will also fail. For example, if the terminal (410) is located near the coverage boundary of the first base station (420-1), which is a non-ground cell, even if an RRC connection reset is performed, a handover may be required immediately due to the mobility of the first base station (420-1), which is not fixed. Accordingly, the terminal (410) according to the present disclosure may perform cell reselection or perform RRC connection resetting to the first base station (420-1), which is the serving cell, depending on whether the conditions for resetting the RRC connection to the existing serving cell and reselecting the cell to a cell other than the serving cell are satisfied. Here, since cell reselection is performed when the conditions are satisfied, the conditions proposed in the present disclosure may be referred to as 'conditions for omitting RRC connection resetting,' 'conditions for performing cell reselection,' or terms having an equivalent technical meaning. For convenience of explanation below, the proposed conditions are referred to as 'cell reselection conditions.'

[0092] The terminal (410) may consider the priority of the cell, signal measurement values, and / or connection availability time, etc., as cell reselection conditions. For example, the terminal (410) may review the priority of the target candidate ground cell for performing cell reselection. Specifically, the terminal (410) may determine whether the ground cell is set as a priority for performing cell reselection. In addition, for example, the terminal (410) may determine whether the signal measurement values, such as the RSRP and RSRQ values ​​of the first base station (420-1) which is the serving cell, are below a predetermined threshold. In addition, for example, the terminal (410) may check the connection availability time with the first base station (420-1) which is the serving cell. That is, the terminal (410) may review the above conditions to determine the possibility of success in RRC connection resetting and the possibility that a handover will be required again. Meanwhile, the terminal (410) may maintain the RRC setting value in the event of a wireless link failure.

[0093] FIG. 5 illustrates an example of a conditional RRC connection reset procedure based on cell reselection condition determination in a wireless communication system according to one embodiment of the present disclosure. The first base station (420-1) is a serving cell and may be a non-ground cell. Additionally, the second base station (420-2) is a target cell, and the second base station (420-2) may correspond to either a non-ground cell or a ground cell. The first base station (420-1) may be referred to as a source gNB or a source eNB, and similarly, the second base station (420-2) may be referred to as a target gNB or a target eNB.

[0094] Additionally, although not illustrated in FIG. 5, a wireless communication system according to one embodiment of the present disclosure may additionally include a third base station, and the third base station may be subject to cell re-selection. For example, the third base station may be a base station of a different type from the second base station, and for instance, if the second base station is a non-ground cell, the third base station may be a ground cell.

[0095] Referring to FIG. 5, in step S501, the MME (530) sets a regional regulation. The MME (530) is a control node and may be an object responsible for signal processing, mobility management, and session setup between the terminal (510) and the network in the control plane. Specifically, the MME (530) can provide a regional regulation to enable region-based access control, thereby restricting or allowing the terminal (510) to access in a specific region. For example, the MME (530) can set a restriction on specific services through the regional regulation, such as restricting high-speed data services in a specific region or allowing only voice calls or low-speed data. Additionally, the MME (530) can set a regional regulation to restrict roaming, refuse handover to a specific region, or restrict the terminal (510) from accessing in secure areas such as military bases or government agencies.

[0096] In step S503, the first base station (520-1) transmits measurement control to the terminal (510). Specifically, the first base station (520) transmits a measurement control message to the terminal (510) requesting measurements regarding frequency, cell, signal quality, etc. As an example, the measurement control message may include information on measurement items such as SINR, RSRQ, and RSRP.

[0097] In step S505, the terminal (510) measures the signal. Specifically, the terminal (510) measures the signal strength of base stations (520-1, 520-2) to reserve resources for handover. The terminal (510) may measure the signal strength according to a predetermined period, which may be referred to as a measurement interval in this disclosure.

[0098] In detail, the terminal (510) can detect a handover preparation event by measuring the signal strength of base stations (520-1, 520-2). For example, the terminal (510) may determine that a handover preparation event has been detected if the measured signal strength or quality is below a predefined threshold or within a predetermined range. Additionally, for example, the terminal (510) may determine that a handover preparation event has been detected if the signal strength or quality provided by the second base station (520-2) is above a specific threshold than the signal strength or quality provided by the first base station (520-1), which is the serving cell.

[0099] In step S507, the terminal (510) transmits a measurement report to the first base station (520-1). Specifically, the terminal (510) periodically measures the signal strength of the first base station (520-1) and other base stations (not shown) and transmits a measurement report message. The measurement report may include at least a portion of the elements of a set measurement report. For example, the measurement configuration of the measurement report may include a measurement object, a report configuration, a measurement ID, a quantity configuration, and a measurement interval. As an example, the terminal (510) may measure at least one of the SINR, RSRQ, and RSRP of the first base station (520-1) and other base stations. In addition, the terminal (510) may measure the RSSI, which is the power strength of the received signal, and the SNR, which is the ratio between the signal and noise, but is not limited thereto.

[0100] In step S509, the first base station (520-1) determines a handover. Specifically, the first base station (520-1) checks at least one of RSRP, RSRQ, or SINR through the received measurement report. At this time, the first base station (520-1) may determine a handover if the strength or quality of the measured signal is below a predefined threshold or is within a predetermined range. Additionally, as an example, the first base station (520-1) may determine a handover if the strength or quality of the signal provided by other base stations is above a specific threshold than the strength or quality of the signal provided by the first base station (520-1), which is the serving cell. The above process can be understood as the first base station (520-1) detecting a triggering event from the measurement report.

[0101] In step S511, the first base station (520-1) transmits a handover request. Specifically, the first base station (520-1) transmits a handover request message to the target cell. The target cell may be determined based on measurement reports regarding other base stations. For example, the target cell may be determined based on RSRP, RSSI, RSRQ, SINR regarding other base stations, traffic load of each base station, priority, speed of the terminal (510), frequency band, connection status, and terminal performance. For convenience of explanation, the following description assumes that the second base station (520-2) is determined as the target cell, but the second base station (520-2) may be a non-terrestrial cell.

[0102] The handover request message includes information about the terminal (510), and may include, but is not limited to, UE Identity, source cell ID, target cell ID, terminal context, target cell information, E-RAB, etc. Here, the handover request message may be transmitted via a link between satellites or a feeder link.

[0103] In step S513, the second base station (520-2), which is the target cell, performs acceptance control. Specifically, the second base station (520-2) determines whether to accept a handover request based on a handover request message, and may reject the handover request if network resources are insufficient or if there is a concern about quality degradation. For example, the second base station (520-2) may determine whether to accept by calculating available bandwidth, frequency resources, cell capacity, and QoS requirements.

[0104] In step S515, the second base station (520-2) transmits a handover request confirmation to the first base station (520-1). Specifically, if the second base station (520-2) determines, as a result of performing acceptance control, that a handover is possible, it transmits a handover request confirmation message to the first base station (520-1).

[0105] Meanwhile, the first base station (520-1) can transmit a handover command. Specifically, the first base station (520-1) can transmit a handover command message to the terminal (510). The handover command message may include information about the second base station (520-2), which is the target cell, and said information may include C-RNTI, timing advance, and location information. Through this, the first base station (520-1) can assign an RNTI (e.g., G(group)-RNTI, C-RNTI) for broadcast or multicast to the terminal (510). The RNTI may be provided in advance through system information or provided along with the settings for measurement. The aforementioned steps S501 through S515 may be referred to as handover preparation steps and can be understood as a process of reserving resources for executing a handover.

[0106] In step S517, the terminal (510) performs RACH. The terminal (510) obtains uplink grant information of the first base station (520-1) through the RACH procedure and can use this to transmit a handover confirmation message to the second base station (520-2). Specifically, the terminal (510) transmits a preamble to the second base station (520-2), and the second base station (520-2) can transmit a response to the RA to the terminal (510). Next, the terminal (510) can transmit RRC connection completion to the second base station (520-2) to convey that the wireless resource settings following the RRC connection completion have been successfully applied.

[0107] In step S519, the terminal (510) detects a wireless link failure. For example, the terminal (510) may detect or declare a wireless link failure when a wireless problem timer expires after detecting a wireless problem from the physical layer, when a RACH procedure fails, when an RLC failure occurs, or when an uplink LBT failure is detected. In addition, the terminal (510) may detect or declare a wireless link failure when signal measurement values ​​such as RSRP, RSRQ, SINR, etc. fall below a predetermined threshold value. In the present disclosure, the description focuses on the terminal (510) declaring a wireless link failure during a handover, but is not limited thereto, and may include declaring a wireless link failure at various layers where data packet transmission, channel connection, etc., take place, provided that it does not conflict with the present disclosure.

[0108] In step S521, the terminal (510) determines the cell reselection condition. Specifically, the terminal (510) does not immediately perform an RRC connection reset, but determines the cell reselection condition and can switch to the RRC IDLE state when reselecting the cell.

[0109] In detail, the terminal (510) may consider the priority of the cell, signal measurement values, and connection availability time as cell reselection conditions. For example, the terminal (510) may review the priority of target candidate ground cells for performing cell reselection. Specifically, the terminal (510) may determine whether the ground cell is set as a priority for performing cell reselection. In an area where the first base station (520-1), which is a non-ground cell, and the ground cell coexist, the ground network may be suitable for services requiring high data transmission rates and low latency. Accordingly, to consider the different characteristics of the ground network and the non-ground network and to improve the user experience, the terminal (510) selects a cell from among the ground cell or the non-ground cell for cell camping or RRC connection setup according to potential services, mobility status, or the operator's plan. That is, the priority of the ground cell and the non-ground cell in performing cell reselection for the RRC connection reset of the terminal (510) may be predetermined. For example, in cell reselection, a ground cell may be assigned absolute priority. Accordingly, the terminal (510) can examine whether a ground cell has been assigned priority in order to perform cell reselection.

[0110] Additionally, as an example, the terminal (510) can determine whether signal measurement values, such as RSRP and RSRQ values ​​of the first base station (520-1) which is a serving cell, are below a predetermined threshold. Specifically, if at least one of the RSRP and RSRQ of the first base station (520-1) is low, the signal quality is poor, and the possibility of RRC connection resetting failure may increase. Accordingly, the terminal (510) examines whether there is a possibility of failure by comparing at least one of the RSRP and RSRQ values ​​of the first base station (520-1) with the threshold.

[0111] Additionally, as an example, the terminal (510) can check the connection availability time with the first base station (520-1), which is the serving cell. Specifically, if the terminal (510) is located near the coverage boundary of the first base station (520-1), signal quality may be degraded, and even if an RRC reconnection is performed, there is a high probability that a handover must be performed within a short time due to the mobility of the first base station (520-1), which is not fixed. This can be a factor that reduces the performance of the network. Therefore, the terminal (510) checks the connection availability time to consider the relative position in relation to the first base station (520-1). That is, the terminal (510) can review the conditions described above to check the possibility of success in resetting the RRC connection and the possibility that a handover will be required again.

[0112] In step S523, the terminal (510) performs cell reselection or RRC connection reconfiguration based on whether the cell reselection condition is satisfied. Specifically, if a ground cell is assigned priority, the terminal (510) performs ground cell reselection after switching to RRC IDLE. Meanwhile, if the terminal (510) examines the priority of target candidate ground cells for performing cell reselection and finds that a non-ground cell is assigned priority, the terminal (510) may perform a handover again to the target cell, the second base station (520-2), or perform RRC connection reconfiguration to the existing serving cell, the first base station (520-1).

[0113] Additionally, if the signal measurement values, such as the RSRP and RSRQ values ​​of the first base station (520-1) which is the serving cell, are below a predetermined threshold value, the terminal (510) switches to RRC IDLE and performs ground cell re-selection. Meanwhile, if the RSRP and RSRQ values ​​of the first base station (520-1) which is the serving cell are above a predetermined threshold value, the terminal (510) may perform a handover again or perform an RRC connection reset to the first base station (520-1).

[0114] Additionally, if the connection time between the first base station (520-1), which is the serving cell, and the terminal (510) is less than a predetermined threshold, the terminal (510) switches to RRC IDLE and performs ground cell re-selection. Meanwhile, if the connection time between the first base station (520-1), which is the serving cell, and the terminal (510) is greater than or equal to a predetermined threshold, the terminal (510) may perform a handover again or perform an RRC connection reset to the first base station (520-1).

[0115]

[0116] FIG. 6 illustrates an example of a procedure for performing cell reselection when a wireless link failure is detected during handover according to one embodiment of the present disclosure.

[0117] Referring to FIG. 6, in step S601, the terminal detects a wireless link failure for the target cell while performing a handover. The terminal receives measurement control from the serving cell, measures signals for surrounding base stations, and can transmit a measurement report message to the serving cell. When a handover is determined in the serving cell and a handover command is received from the serving cell, the terminal performs a handover to the target cell. Meanwhile, the terminal may declare a wireless link failure if the wireless problem timer expires after detecting a wireless problem from the physical layer, if the RACH procedure fails while attempting a handover, if an RLC failure occurs, or if an uplink LBT failure is detected. In addition, the terminal may declare a wireless link failure if signal measurement values ​​such as RSRP, RSRQ, and SINR fall below a predetermined threshold value.

[0118] In step S603, the terminal determines the cell reselection conditions. Specifically, even if a wireless link failure is detected, the terminal does not immediately perform an RRC connection reset, but reviews the cell reselection conditions for cell reselection and can switch to the RRC IDLE state when reselecting a cell.

[0119] For example, the terminal may consider the priority of a cell as a cell reselection condition. For instance, the terminal may review the priority of target candidate ground cells as a cell reselection condition. Specifically, the terminal may determine whether a ground cell is set as a priority to perform cell reselection. To consider the different characteristics of ground and non-ground networks and to improve the user experience, the terminal may select a ground cell or a non-ground cell for cell camping or RRC connection setup based on potential services, mobility status, or the operator's plan. For instance, the terminal may review whether a ground cell has been assigned a priority to perform cell reselection.

[0120] In addition, as an example, the terminal may consider signal strength as a cell reselection condition. For instance, the terminal may determine whether signal measurement values, such as RSRP and RSRQ of the serving cell, are below a predetermined threshold. Specifically, if at least one of the RSRP and RSRQ of the serving cell is low, the signal quality is poor, which may increase the likelihood of failure in RRC connection resetting. Accordingly, the terminal may reselect the ground cell if at least one of the RSRP and RSRQ of the serving cell is below a predetermined threshold.

[0121] Additionally, as an example, the terminal can check the connection availability time with the serving cell as a cell re-selection condition. Specifically, if the terminal is located near the coverage boundary of the serving cell, which is a non-ground cell, signal quality may degrade, and even if RRC reconnection is performed, there is a high probability that a handover must be performed within a short time due to the mobility of the non-ground cell. This can be a factor that reduces network performance. Therefore, the terminal checks the connection availability time to consider the relative position in relation to the serving cell. For example, if the connection availability time is below a predetermined threshold, the terminal can re-select the ground cell. That is, the terminal can review the conditions described above to determine the probability of success in RRC connection re-establishment and the probability that a handover will be required again.

[0122] In step S605, the terminal performs cell reselection without resetting the RRC connection. Specifically, the terminal performs cell reselection based on whether the cell reselection conditions are satisfied. Specifically, if a ground cell is assigned priority, the terminal switches to RRC IDLE and performs ground cell reselection. Additionally, if signal measurement values, such as the RSRP and RSRQ values ​​of the serving cell, are below a predetermined threshold, the terminal switches to RRC IDLE and performs ground cell reselection. Additionally, if the connection availability time between the serving cell and the terminal is below a predetermined threshold, the terminal switches to RRC IDLE and performs ground cell reselection.

[0123]

[0124] FIG. 7 illustrates an example of the operation procedure of a terminal in a wireless communication system according to one embodiment of the present disclosure.

[0125] Referring to FIG. 7, in step S701, the terminal maintains an RRC connection state and a connection management state. Specifically, the terminal can detect a wireless link failure while maintaining communication with a serving cell. For example, the terminal may declare a wireless link failure if, after detecting a wireless problem from the physical layer, the wireless problem timer expires, the RACH procedure fails, an RLC failure occurs, or an uplink LBT failure is detected. In addition, a wireless link failure may occur if signal measurement values ​​such as RSRP, RSRQ, and SINR fall below a predetermined threshold. Not limited to this, the terminal's declaration of a wireless link failure may also include declarations made at various layers where data packet transmission, channel access, etc., take place.

[0126] In step S703, the terminal checks the priority of the cell. Specifically, upon detecting a wireless link failure, the terminal checks the priority of the RRC cell as a condition for cell reselection without immediately performing an RRC connection reset. For example, the terminal may review the priority of target candidate ground cells for performing cell reselection. That is, the terminal can determine whether a ground cell is set as a priority for performing cell reselection. In areas where ground and non-ground cells coexist, ground networks may be suitable for services requiring high data transmission rates and low latency. Accordingly, to consider the different characteristics of ground and non-ground networks and to improve the user experience, the terminal selects a ground cell or a non-ground cell for cell camping or RRC connection setup based on potential services, mobility status, or the operator's plan.

[0127] The priority for ground cells and non-ground cells in performing cell reselection for the terminal's RRC connection reset may be predetermined. For example, in cell reselection, ground cells may be assigned absolute priority. Accordingly, the terminal may examine whether a ground cell has been assigned priority in order to perform cell reselection. For example, if a ground cell has been assigned priority, the terminal may consider performing ground cell reselection after switching to RRC IDLE, but may additionally examine the signal strength. Furthermore, for example, if a wireless link failure occurs while both the serving cell and the target cell are non-ground cells, and the terminal examines the cell priority to perform cell reselection and finds that a non-ground cell has been assigned priority, the terminal may perform a handover again to the non-ground cell that is the target cell, or perform an RRC connection reset to the existing serving cell.

[0128] In step S705, the terminal checks the signal strength. Specifically, the terminal can determine whether signal measurement values, such as the RSRP and RSRQ of the serving cell, are below a predetermined threshold as a cell reselection condition. Specifically, if at least one of the RSRP and RSRQ of the serving cell is low, the signal quality is poor, which may increase the likelihood of RRC connection resetting failure. Therefore, the terminal examines the possibility of RRC connection resetting failure by comparing at least one of the RSRP and RSRQ of the serving cell with the threshold. It goes without saying that the terminal may additionally examine SINR, RSSI, SNR, etc., as cell reselection conditions.

[0129] For example, if the RSRP and RSRQ values ​​of a non-ground cell serving as a serving cell are below a predetermined threshold, the terminal may switch to RRC IDLE and then consider re-selecting a ground cell, but may further review the connection availability time. Meanwhile, if the RSRP and RSRQ values ​​of the serving cell are above a predetermined threshold, the terminal may perform a handover again or re-establish the RRC connection to the existing serving cell.

[0130] In step S707, the terminal checks the connection availability time. Specifically, if the terminal is located near the coverage boundary of the serving cell, signal quality may degrade, and even if RRC reconnection is performed, there is a high probability that a handover must be performed within a short time due to the mobility of the serving cell, which is a non-fixed, non-ground cell. This can be a factor that reduces network performance. Therefore, the terminal checks the connection availability time as a cell reselection condition to consider the relative position in relation to the serving cell.

[0131] For example, if the connection time between the serving cell and the terminal is less than a predetermined threshold, the terminal switches to RRC IDLE and performs ground cell re-selection. Meanwhile, if the connection time between the serving cell and the terminal is greater than or equal to a predetermined threshold, the terminal may perform a handover again or re-establish the RRC connection with the serving cell.

[0132] Meanwhile, if the serving cell is a non-terrestrial cell and a wireless link failure occurs at the edge of the non-terrestrial cell, even if the terminal performs an RRC connection reset to the non-terrestrial cell, there is a high probability that a handover will be performed again within a short time due to the mobility of the non-terrestrial cell, and the RRC connection reset is also highly likely to fail. In other words, steps S703 to S707 described above can be understood as a process of considering cell reselection to increase the probability of success for the RRC connection reset and to minimize communication interruption and resource waste by reviewing cell reselection conditions, rather than the terminal immediately performing an RRC connection reset to the serving cell.

[0133] In step S709, the terminal performs cell reselection or RRC connection reconfiguration based on whether the cell reselection conditions are satisfied. For example, if all cell reselection conditions in steps S703 through S707 are satisfied, the terminal performs ground cell reselection. Additionally, for example, if at least one of the cell reselection conditions is satisfied, the terminal may reselect a ground cell. The order and verification details of the cell reselection conditions may be changed in user settings or system settings. For example, the terminal may check only the cell priority, and if a ground cell has priority, switch to the RRC IDLE state and reselect a ground cell.

[0134] Additionally, some of the cell reselection conditions may be omitted. That is, if at least one of the cell reselection conditions is not satisfied in steps S703 to S707, the terminal may perform an RRC connection reset with the serving cell or perform a handover to the target cell. Additionally, as an example, if all of the cell reselection conditions are not satisfied, the terminal may perform an RRC connection reset with the serving cell or perform a handover to the target cell.

[0135]

[0136] FIG. 8 illustrates an example of a procedure for determining the priority of a cell for determining a cell re-selection condition according to one embodiment of the present disclosure.

[0137] Referring to FIG. 8, in step S801, the terminal checks whether the ground cell is prioritized. Specifically, when a wireless link failure is detected, the terminal does not immediately perform an RRC connection reset but checks the priority of the RRC cell as a cell reselection condition. For example, the terminal can review the priority of the target candidate ground cell for performing cell reselection. That is, the terminal can determine whether the ground cell is set as a priority for performing cell reselection.

[0138] Since cell reselection is a procedure in which a terminal moves to a cell providing a higher quality signal, the terminal can utilize frequency priority as the cell priority. For example, the terminal can determine frequency priority by using system information transmitted via broadcast from the base station. That is, the terminal can utilize the received system information during cell reselection while in the RRC IDLE state. For example, the terminal can determine whether the frequency band of a terrestrial cell has been determined as the priority by checking the frequency priority information included in the system information.

[0139] Additionally, the terminal can check frequency priority or cell reselection criteria through an RRC release message. The RRC release message is used to disconnect the RRC connection and, for example, can be used when switching the terminal's RRC connection status to the RRC IDLE state. Furthermore, the RRC release message may include information regarding cell reselection criteria as it includes instructions for RRC reconfiguration to reset the RRC connection. That is, the terminal can check the frequency priority information or cell reselection criteria included in the RRC release message to determine whether the frequency band of a ground cell has been determined as the priority or whether the ground cell has been determined as the criterion for cell reselection.

[0140] Additionally, the terminal may refer to cell reselection criteria of other radio access technologies (RAT). For example, the terminal may refer to cell reselection criteria applied to other radio access technologies (for example, switching from LTE to 5G).

[0141] In step S803, the terminal performs ground cell reselection if the ground cell has priority. For example, the terminal can determine the priority of the ground cell by checking system information transmitted from the base station via broadcast or dedicated signaling. For example, information regarding the priority of the ground cell may be transmitted periodically or non-periodically through the base station's system information (e.g., SIB19). Additionally, for example, the terminal can check the priority of the ground cell by checking system information transmitted from the base station and stored in advance. Specifically, information regarding the priority of the ground cell may be indicated by at least one parameter included in the system information. For example, the terminal can check the priority of the ground cell by checking information regarding the frequency of the ground cell included in the system information. If the ground cell's frequency band is identified as having a relatively higher priority, the terminal switches to RRC IDLE and performs ground cell reselection.

[0142] In addition, the terminal checks the frequency priority information or cell re-selection criteria included in the RRC release message, and if the frequency band of the ground cell is determined as the priority or the ground cell is determined as the criterion for cell re-selection, it switches to RRC IDLE and performs ground cell re-selection.

[0143] Additionally, if the terminal refers to the cell re-selection criteria of another wireless access technology and determines that a ground cell is the criterion for cell re-selection, it switches to RRC IDLE and performs ground cell re-selection.

[0144] In step S805, if the ground cell does not take precedence, the terminal performs an RRC connection reset to the non-ground cell. Specifically, the terminal checks system information, RRC release messages, cell re-selection criteria of other RATs, etc., and if the non-ground cell takes precedence, it may perform an RRC connection reset with the non-ground cell or perform a handover to the target cell.

[0145] Meanwhile, regarding the evaluation process for a terminal to perform cell reselection, the following standards may be considered. The absolute priority of various NR frequencies or inter-RAT frequencies may be provided to the terminal by inheriting from other RATs during system information, RRC release messages, or inter-RAT cell (re)selection. In the case of system information, NR frequencies or inter-RAT frequencies may be listed without priority (i.e., no cell reselection priority field is provided for the corresponding frequencies). Cell reselection priority may refer to the cell reselection criteria of the present disclosure. If the cellreselectionpriority or nsag-cellreselectionpriority field is provided in a dedicated signal, the terminal must ignore the cellreselectionpriority and nsag-cellreselectionpriority fields provided in the system information. When the terminal is normally camped, if the terminal supports slice-based cell reselection and receives network slice and NSAG information to be used for cell reselection from the NAS, the terminal must derive the cellreselectionpriority in accordance with the provisions of 5.2.4.11.

[0146]

[0147] FIG. 9 illustrates an example of a procedure for determining the strength of a signal for determining a cell reselection condition according to one embodiment of the present disclosure.

[0148] Referring to FIG. 9, the terminal checks in step S901 whether the signal measurement value of the serving cell is less than a first threshold. Specifically, the terminal can determine whether the signal measurement value, e.g., the RSRP and RSRQ values ​​of the serving cell, is less than the first threshold as a cell reselection condition. Hereinafter, the signal measurement value is It can be referred to as.

[0149] More specifically, the terminal can check whether the result of reflecting the hysteresis value in the signal measurement value is less than a first threshold. The hysteresis value may be pre-configured during network design, and the higher the hysteresis value, the more intermittently handovers occur, but the latency may be longer. Therefore, the hysteresis value according to the present disclosure may be set to a high value in an environment with frequent signal fluctuations by analyzing the environment in which the network operates. Additionally, the hysteresis value may be set based on QoS requirements. Likewise, the higher the value of the first threshold, the more intermittently handovers occur and the latency may be longer. Therefore, the first threshold according to the present disclosure may be set to a high value in an environment with frequent signal fluctuations by analyzing the environment in which the network operates, and may be determined based on QoS requirements. Meanwhile, the first threshold is as follows: It may be referred to as. The result of reflecting the hysteresis value in the signal measurement value and whether it is below the first threshold value can be determined by the following Equation 1. Meanwhile, the hysteresis value is as follows: It can be referred to as.

[0150] [Mathematical Formula 1]

[0151]

[0152] In step S903, the terminal performs ground cell reselection. Specifically, if the result of the calculation of Equation 1 described above, reflecting the hysteresis value in the signal measurement value of the serving cell, is less than the first threshold value, the terminal determines that there is a high probability that the RRC connection reconfiguration will fail, switches to the RRC IDLE state, and performs ground cell reselection.

[0153] In step S905, the terminal performs an RRC connection reset to a non-ground cell. Specifically, if the result of calculating the above-described mathematical formula 1, and the result of reflecting the hysteresis value to the signal measurement value of the serving cell is greater than or equal to the first threshold value, the terminal determines that there is a high probability that the RRC connection reset will succeed and performs the RRC connection reset.

[0154]

[0155] FIG. 10 illustrates an example of a procedure for determining the connection time of a cell re-selection condition determination according to one embodiment of the present disclosure.

[0156] Referring to FIG. 10, in step S1001, the terminal checks whether the connection availability time is less than a second threshold. The connection availability time may refer to the time range during which the terminal can communicate directly with the non-ground cell, depending on the mobility of the non-ground cell. Specifically, if the terminal is located near the coverage boundary of the serving cell, signal quality may degrade, and even if RRC reconnection is performed, there is a high probability that a handover must be performed within a short time due to the mobility of the serving cell, which is a non-fixed non-ground cell. This can be a factor that reduces network performance. Therefore, the terminal checks the connection availability time as a cell reselection condition to consider the relative position in relation to the serving cell. Hereinafter, the connection availability time It can be referred to as.

[0157] More specifically, the terminal can determine the connection availability time based on information regarding the connection availability time received from the base station. For example, information regarding the connection availability time may be transmitted periodically or non-periodically through the base station's system information (e.g., SIB19). Specifically, information regarding the connection availability time may be indicated by at least one parameter (e.g., t-Service) included in the system information. Here, at least one parameter may indicate information regarding the time when service for the area currently covered by the base station is interrupted. For example, at least one parameter may include location information of non-ground cells, the speed of non-ground cells, etc., and may include information regarding the distance between the terminal and the non-ground cells based on the location information. Meanwhile, the terminal can derive the connection availability time based on the received information regarding the connection availability time. Additionally, the connection availability time may be included in and transmitted within the information regarding the connection availability time received from the base station.

[0158] Whether the connection availability time induced or received through the process described above is less than the second threshold can be determined by the following Equation 2. Meanwhile, the second threshold is as follows: It can be referred to as.

[0159] [Mathematical Formula 2]

[0160]

[0161] In step S1003, the terminal performs ground cell reselection. Specifically, if the connection availability time is less than the second threshold value as a result of calculating the mathematical formula 2 described above, the terminal does not perform RRC connection resetting, switches to the RRC IDLE state, and performs ground cell reselection. That is, if the connection availability time is less than a predetermined value, it can be understood as a process in which the terminal determines that it must perform a handover again within a short period even if it performs RRC connection resetting, and therefore does not perform RRC connection resetting.

[0162] Meanwhile, regarding cell re-selection, the following standard procedure may be considered. The terminal may 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 to SIB19 and the terminal supports the initiation of location-based measurements for a (quasi) Earth fixed cell of a non-terrestrial network and has acquired location information, or if the distance between the terminal and the serving cell referenceLocation is shorter than distanceThresh, the terminal may not perform measurements within the same frequency. Conversely, if the situation contrary to the above is not met, the terminal must perform measurements within the same frequency.

[0163] Additionally, if distanceThresh and movingReferenceLocation are broadcast to SIB19 and the terminal supports the initiation of a location-based measurement for a ground moving cell of a non-terrestrial network and obtains location information, or if the distance between the terminal's location and the serving cell reference location based on movingReferenceLocation is shorter than distanceThresh, the terminal may not perform a measurement within the same frequency. Conversely, in cases contrary to the above-described situation, the terminal must perform a measurement within the same frequency.

[0164] The terminal must apply the following rules to the NR inter-RAT frequencies and inter-RAT frequencies displayed in the system information, which may apply where priority is provided as defined in 5.2.4.1.

[0165] For inter-NR frequencies or inter-RAT frequencies that are higher than the reselection priority of the current NR frequency, the terminal must perform measurements of the inter-NR frequency or inter-RAT frequency of higher priority in accordance with TS 38.133 [8]. For inter-NR frequencies that are equal to or lower than the reselection priority of the current NR frequency and inter-RAT frequencies of lower priority, for example, if the serving cell satisfies the conditions Srxlev > SnonIntraSearchP and Squal > SnonIntraSearchQ, or if distanceThresh and referenceLocation are broadcast to SIB19, or if the terminal supports the initiation of location-based measurements for non-terrestrial network (quasi) earth fixed cells and obtains location information, or if the distance between the terminal and the serving cell reference location is shorter than distanceThresh, the terminal may not perform measurements of inter-NR frequency cells of the same or lower priority or inter-RAT frequency cells of lower priority. In cases contrary to the above, the terminal must perform measurements of NR inter-RAT frequency cells of the same or lower priority or lower priority inter-RAT frequency cells in accordance with TS 38.133 [8].

[0166] Meanwhile, if distanceThresh and movingReferenceLocation are broadcast to SIB19 and the terminal supports the initiation of location-based measurements for non-terrestrial network earth moving cells and obtains location information, or if the distance between the terminal's location and the serving cell reference location based on movingReferenceLocation is shorter than distanceThresh, the terminal may not perform measurements of NR inter-RAT frequency cells of the same or lower priority or lower priority inter-RAT frequency cells. Conversely to the situation described above, the terminal must perform measurements of NR inter-RAT frequency cells of the same or lower priority or lower priority inter-RAT frequency cells in accordance with TS 38.133 [8]. If the terminal supports relaxed measurements and relaxedMeasurement is present in SIB2, the terminal may further relax the necessary measurements as specified in Section 5.2.4.9.

[0167] In the case of a terminal camped in a cell of a non-terrestrial network, if the terminal supports TN measurement omission and has acquired location information, and if coverageAreaInfoList and tn-AreaIdList are broadcast from system information, the terminal may not perform TN frequency measurement regardless of priority if it is not within the range of the corresponding frequency provided by tn-AreaIdList.

[0168] If t-Service exists in SIB19 for the serving cell and the terminal supports time-based measurement start, the terminal must perform intra-frequency, inter-frequency, or inter-RAT measurements before t-Service, regardless of the distance between the terminal and the serving cell reference location or whether the serving cell satisfies the conditions Srxlev > SIntraSearchP and Squal > SIntraSearchQ, or Srxlev > SnonIntraSearchP and Squal > SnonIntraSearchQ. The exact time to start measurements before t-Service may vary depending on the terminal implementation. The terminal must perform higher-priority inter-NR frequency or inter-RAT frequency measurements in accordance with TS 38.133 [8], regardless of the remaining service time of the serving cell (i.e., regardless of the time remaining until t-Service).

[0169] When evaluating the distance between the terminal and the serving cell reference location, the terminal obtaining location information may vary depending on the terminal implementation. Additionally, in the case of a moving Earth cell, the terminal maintaining a valid serving cell reference location may vary depending on the terminal implementation and can be derived based on serving satellite orbit information, epochTime, and movingReferenceLocation.

[0170]

[0171] Meanwhile, those skilled in the art related to the present embodiment will understand that it may be implemented in modified forms without departing from the essential characteristics of the above description. Therefore, the disclosed methods should be considered in an illustrative rather than a restrictive sense. The scope of the present disclosure is defined by the claims, not by the foregoing description, and all variations within the scope of equivalence should be interpreted as being included in the present disclosure.

[0172]

[0173] The present invention can be used in a device for performing a handover in a wireless communication system.

Claims

1. In a method of operation of a terminal in a wireless communication system, A step of receiving measurement control; A step of measuring a signal and transmitting a measurement report message to a serving cell; and A method comprising the step of performing cell reselection to a terrestrial cell other than the serving cell when a radio link failure is detected, provided that the serving cell is a non-terrestrial (NTN) cell and conditions for omitting Radio Resource Control Connection re-establishment (RRE) are satisfied.

2. In Paragraph 1, A method in which the above radio link failure is determined by at least one of the following cases: when the radio problem timer expires after radio problem detection, when the RACH procedure fails, when a Radio Link Control Failure (RLC) occurs, or when an Uplink Listen-Before-Talk (LBT) failure is detected.

3. In Paragraph 1, The above conditions are, A method comprising at least one of the following: the priority of the ground cell is higher than that of the non-ground cell, the signal strength of the non-ground cell is less than a threshold, or the connection time with the non-ground cell is less than a threshold.

4. In Paragraph 1, A method further comprising the step of performing cell re-selection to the ground cell if the serving cell is the non-ground cell and the condition is satisfied.

5. In Paragraph 4, The step of performing cell reselection with the above ground cell is, A step of checking whether the above ground cell is set to a higher priority than the above non-ground cell; and A method comprising the step of performing cell reselection to the ground cell when the ground cell is set to a high priority.

6. In Paragraph 4, The step of performing cell reselection with the above ground cell is, A step of comparing the signal measurement value of the above-mentioned non-ground cell with a first threshold value; and A method comprising the step of performing cell reselection to the ground cell when the signal measurement value is less than the first threshold value.

7. In Paragraph 6, The step of checking whether it is less than the first threshold value is, A method comprising the step of checking whether the result of reflecting a hysteresis value in the above signal measurement value is less than the first threshold value.

8. In Paragraph 4, The step of performing cell reselection with the above ground cell is, A step of determining the possible connection time between the terminal and the non-ground cell based on the moving reference location of the non-ground cell; A method comprising the step of performing cell re-selection to the ground cell when the above connection possible time is less than a second threshold.

9. In a terminal of a wireless communication system, Transmitter / receiver; and It includes a processor connected to the above-mentioned transmitter and receiver, The above processor is, Receive measurement control, Measure the signal and send a measurement report message to the serving cell, A terminal that controls cell reselection to a ground cell other than the serving cell when a wireless link failure is detected, provided that the serving cell is a non-ground cell and the conditions for omitting RRC connection resetting are satisfied.

10. In Paragraph 9, The above wireless link failure is determined by at least one of the cases where the wireless problem timer expires after wireless problem detection, the RACH procedure fails, an RLC failure occurs, or an uplink LBT failure is detected.

11. In Paragraph 9, The above conditions are, A terminal comprising at least one of the following: the priority of the ground cell is higher than that of the non-ground cell, the signal strength of the non-ground cell is less than a threshold, or the connection time with the non-ground cell is less than a threshold.

12. In Paragraph 9, The above processor is, A terminal that controls the cell re-selection to the ground cell when the above serving cell is the above non-ground cell and the above condition is satisfied.

13. In Paragraph 12, The above processor is, To perform cell reselection with the above ground cell, check whether the above ground cell is set with a higher priority than the above non-ground cell, and A terminal that controls the cell re-selection to the ground cell when the ground cell is set to a high priority.

14. In Paragraph 12, The above processor is, To perform cell reselection with the above ground cell, compare the signal measurement value of the above non-ground cell with a first threshold value, and A terminal that controls the cell re-selection to the ground cell when the signal measurement value is less than the first threshold value.

15. In Paragraph 14, The above processor is, A terminal that controls whether the result of reflecting a hysteresis value in the above signal measurement value is less than the above first threshold value.

16. In Paragraph 12, The above processor is, To perform cell re-selection to the ground cell, the connection possible time between the terminal and the non-ground cell is determined based on the movement reference position of the non-ground cell, and A terminal that controls the cell re-selection to the ground cell when the above connection availability time is less than a second threshold.

Images