Apparatus and method for performing handover in a communication system

Abstract

The present disclosure relates to a handover in a communication system, and an operating method of a terminal may include the steps of: transmitting a measurement report to a first base station; receiving from the first base station an RRC (radio resource control) reconfiguration message including at least one of information on a radio network temporary identifier (RNTI) of a second base station, information on a timing difference between the first base station and the second base station, and information on uplink resources allocated by the second base station as information on candidate base stations for handover; and performing handover to the second base station using the information included in the RRC reconfiguration message.

Description

[Technical Field]

[0001] The present disclosure relates to communication systems, and more particularly to an apparatus and method for performing handover in a communication system. [Background technology]

[0002] Mobile communication systems are gradually evolving with each generation. Following the successful commercialization of the LTE (long term evolution) system, 5G (5G) is th Standardization and commercialization of 6G (6 generation) systems is currently underway. th There has been active discussion about the (internal) handover generation (IHV) system. Meanwhile, with the development of mobile communication systems, user requirements for various services and industry service requirements are increasing day by day, and various solutions are required to meet these requirements. Therefore, there is a need for an efficient handover solution that can meet the reliability of service data transmission and service requirements. This invention is the result of a Commercialization Promotion Agency for R&D Outcome (COMPA) project on intellectual property advancement and commercialization aimed at promoting the commercialization of next-generation low-earth orbit satellite constellation technology. Summary of the Invention [Problem to be solved by the invention]

[0003] The present disclosure provides an apparatus and method for performing a handover in a communication system.

[0004] The present disclosure provides an apparatus and method for performing handover without a random access procedure in a communication system.

[0005] The present disclosure provides an apparatus and method for performing a conditional handover (CHO) in a communication system.

[0006] The present disclosure provides an apparatus and method for performing handover between a terrestrial base station and a satellite base station in a communication system.

[0007] The present disclosure provides an apparatus and method for performing handover based on the position and altitude angle of a satellite base station in a communication system. [Means for solving the problem]

[0008] According to one embodiment of the present disclosure, a method for operating a terminal in a communication system may include the steps of: transmitting a measurement report to a first base station; receiving an RRC (radio resource control) reconfiguration message from the first base station, the RRC reconfiguration message including at least one of information on a radio network temporary identifier (RNTI) of a second base station, information on a timing difference between the first base station and the second base station, and information on uplink resources allocated by the second base station, as information on a candidate base station for handover; and performing handover to the second base station using the information included in the RRC reconfiguration message.

[0009] According to one embodiment of the present disclosure, the method may further include determining the second base station as a target base station based on an execution condition for handover indicated by the RRC reconfiguration message.

[0010] According to one embodiment of the present disclosure, the execution condition can be defined based on at least one of a received signal strength for a base station, a distance between a terminal and the base station, and an altitude angle of the base station.

[0011] According to one embodiment of the present disclosure, the method may further include the steps of sending a handover command message to the first base station and the second base station, receiving an uplink grant from the second base station, and sending a handover complete message to the second base station.

[0012] According to one embodiment of the present disclosure, the handover command message may be broadcast to the first base station and the second base station.

[0013] According to one embodiment of the present disclosure, at least one of the candidate base stations may include a satellite base station.

[0014] According to an embodiment of the present disclosure, among the candidate base stations, a terrestrial base station may have a higher priority as a target base station than a satellite base station.

[0015] According to one embodiment of the present disclosure, the method may further include a step in which the second base station is a satellite base station and a step in which a measurement configuration message is received from the first base station, the measurement configuration message including at least one of altitude information and orbit information of the second base station.

[0016] According to one embodiment of the present disclosure, the measurement report may be transmitted if a condition related to the elevation angle of the first base station and the elevation angle of the second base station is satisfied.

[0017] According to one embodiment of the present disclosure, a method for operating a first base station in a communication system may include the steps of receiving a measurement report from a terminal, determining to perform handover of the terminal, transmitting a handover request message to at least one neighboring base station, receiving a handover request confirmation message from the at least one neighboring base station, and transmitting to the terminal an RRC (radio resource control) reconfiguration message including at least one of information regarding a candidate base station for handover, information regarding an RNTI (radio network temporary identifier) ​​of a second base station, information regarding a timing difference between the first base station and the second base station, and information regarding uplink resources allocated by the second base station.

[0018] According to one embodiment of the present disclosure, the method may further include the steps of receiving a handover command message from the terminal and receiving a handover complete message from a second base station among the at least one neighboring base station.

[0019] According to one embodiment of the present disclosure, the handover request confirmation message may include information about the RNTI, information about the timing difference, and information about the uplink resource.

[0020] According to one embodiment of the present disclosure, at least one of the at least one neighboring base station may include a satellite base station.

[0021] According to one embodiment of the present disclosure, the method may further include a step in which the second base station is a satellite base station and a measurement configuration message is sent to the terminal, the measurement configuration message including at least one of altitude information and orbit information of the second base station. [Effects of the Invention]

[0022] According to an embodiment of the present disclosure, a handover to an optimal cell or base station for a terminal can be performed in a next-generation mobile communication system in which various types of base stations coexist, which enables a handover to an optimal cell for efficiently supporting multiple services that can be configured for one terminal.

[0023] Furthermore, the present disclosure provides an advantage of performing a faster handover by previously recognizing information about a target base station when performing a handover between a terrestrial base station and at least one satellite base station, or between satellite base stations, in a next-generation communication system including a terrestrial base station and a satellite base station. The present disclosure also provides an advantage of supporting different service coverage according to the characteristics of a target base station selected for each service. This allows a terminal to effectively perform a handover without delay due to a random access procedure by previously obtaining timing information about the target base station before determining whether to perform a handover between multiple base stations. The present disclosure also provides an advantage of allowing a terminal to select an optimal base station for its location by considering information about surrounding satellite base stations. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 illustrates an example of a satellite network according to one embodiment of the present disclosure.

[0025] [Figure 2] FIG. 1 illustrates another example of a satellite network according to an embodiment of the present disclosure.

[0026] [Figure 3] FIG. 1 is a diagram illustrating a configuration of devices in a communication system according to an embodiment of the present disclosure.

[0027] [Figure 4] FIG. 1 is a diagram illustrating an example of a random access procedure in a communication system.

[0028] [Figure 5] FIG. 1 illustrates an example of a conditional handover (CHO) procedure in a communication system.

[0029] [Figure 6] FIG. 1 illustrates an example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0030] [Figure 7] FIG. 10 is a diagram illustrating another example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0031] [Figure 8] FIG. 10 illustrates another example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0032] [Figure 9] FIG. 10 illustrates an operation of acquiring timing information for a target cell in a communication system according to one embodiment of the present disclosure.

[0033] [Figure 10] FIG. 10 illustrates another example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0034] [Figure 11] FIG. 1 is a diagram illustrating an example of a situation in which a terrestrial base station and a satellite base station provide services in a communication system according to an embodiment of the present disclosure.

[0035] [Figure 12] FIG. 10 illustrates an example of a handover procedure that takes into account a satellite base station in an apparatus applicable to the present disclosure.

[0036] [Figure 13]FIG. 10 is a diagram illustrating an example of a procedure in which a terminal performs measurements on a base station in a communication system according to an embodiment of the present disclosure.

[0037] [Figure 14] FIG. 10 illustrates satellite parameter values ​​that can be measured by an apparatus according to one embodiment of the present disclosure.

[0038] [Figure 15] FIG. 1 illustrates an example procedure for determining whether to perform a handover based on various parameters according to one embodiment of the present disclosure.

[0039] [Figure 16] 10A and 10B are diagrams illustrating examples of changes in signal strength of a terrestrial base station and a satellite base station depending on the location of a terminal.

[0040] [Figure 17] FIG. 1 illustrates an example of a radio resource control reestablishment (RRE) procedure according to one embodiment of the present disclosure.

[0041] [Figure 18] FIG. 1 illustrates an example of a modified radio resource control reestablishment (RRE) procedure according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0042] The terms used in this embodiment are currently widely used and general terms that have been selected as much as possible while taking into consideration the functions of this embodiment, but these terms may change depending on the intentions of engineers in the relevant field, precedents, the emergence of new technologies, etc. In addition, in certain cases, the applicant may arbitrarily select terms, and in such cases, their meanings will be described in detail in the relevant section. Therefore, the terms used in this embodiment should be defined based on the meanings of the terms and the overall content of this embodiment, rather than simply the names of the terms.

[0043] The present embodiment may be modified in various ways and may have various forms, and some embodiments will be illustrated in the drawings and described in detail. However, this is not intended to limit the present embodiment to the particular disclosed form, and it should be understood that the present embodiment includes all modifications, equivalents, and alternatives included within the spirit and technical scope of the present embodiment. The terms used in this specification are used merely to describe the embodiments and are not intended to limit the present embodiment.

[0044] Unless otherwise defined, the terms used in this embodiment have the same meaning as commonly understood by a person of ordinary skill in the art to which this embodiment belongs. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the contextual meaning of the related art, and should not be interpreted in an idealized or overly formal sense unless explicitly defined in this embodiment.

[0045]

[0046] FIG. 1 is a diagram illustrating an example of a satellite network according to one embodiment of the present disclosure.

[0047] Referring to Figure 1, the satellite network includes terminal 110, satellites 120-1 and 120-2, and gateway 130. Terminal 110 is a user device and may be mobile or fixed. Satellites 120-1 and 120-2 fly in defined orbits and form beams toward the ground to provide cells of a certain size with coverage. Gateway 130 provides satellites 120-1 and 120-2 with links that allow them to connect to the network.

[0048] The link between the terminal 110 and the satellite 120-1 is called a service link and can be based on the NR standard. The link between the satellites 120-1, 120-2 and the gateway 130 is called a feeder link and can be based on various air interfaces. An inter-satellite link (ISL) can be used mainly in the case of regenerative satellites.

[0049] In the case of a transparent satellite based on the NR RAN architecture, the satellite radio interface for the feeder link and service link may be NR-Uu. In the case of a transparent satellite, the satellite performs radio frequency filtering, frequency conversion, and amplification functions. In the case of a regenerative satellite, on-board functions are built into the satellite, allowing the satellite to perform some or all of the base station functions, such as switching and routing, coding and modulation, and decoding and demodulation, as well as radio frequency filtering, frequency conversion, and amplification.

[0050]

[0051] FIG. 2 illustrates another example of a satellite network according to an embodiment of the present disclosure. FIG. 2 illustrates an example of an NTN that provides non-terrestrial connectivity to a UE 210 using an NTN payload 220 and an NTN gateway 230. 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 radio protocols received from the UE 210 via the service link to the NTN gateway 230. Similarly, the NTN payload 220 can transparently forward radio protocols received from the NTN gateway 230 via the feeder link to the UE 210.

[0052] To this end, the following connectivity can be supported by the NTN payload 220: A base station can serve multiple NTN payloads. An NTN payload can be served by multiple base stations.

[0053] 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 for the service link and the feed link. For the NTN, at least one of an AMF name, an NR cell global identifier (NCGI), a C base station ID, a global base station ID, a tracking area identity (TAI), a Single Network Slice Selection Assistance information (S-NSSAI), a Network Slice As Group (NSAG), a Network Identifier (NID), a Closed Access Group (CAG) ID, and a local NG-RAN node ID may be used as a network identifier, and a mapped cell ID may also be used. Here, the tracking area may correspond to a fixed geographical region.

[0054] Non-geosynchronous orbits (NGSOs) include low Earth orbits with altitudes between about 300 km and 1500 km, and medium Earth orbits with altitudes between about 7000 km and 25000 km.

[0055] Service links can be classified into three types: earth-fixed, quasi-earth-fixed, and earth-moving. The earth-fixed type provides a beam that continuously covers the same geographic area at all times. For example, a satellite having a geosynchronous orbit (GSO) can provide an earth-fixed type service link. The quasi-earth-fixed type provides a beam that continuously covers the same geographic area for a limited time period and a beam that covers a different geographic area for another time period. For example, a satellite having a non-geosynchronous orbit can provide a quasi-earth-fixed type service link using a steerable beam. The earth-moving type provides a beam whose coverage area moves across the Earth's surface. For example, a satellite having a non-geosynchronous orbit can provide an earth-moving type service link using a fixed or steerable beam.

[0056] With satellites having non-geosynchronous orbits, base stations can provide quasi-geofixed cell coverage or geomobile cell coverage. With satellites having geosynchronous orbits, base stations can provide geofixed cell coverage. In the case of non-geosynchronous orbits, a switch in the service link can refer to a change in the serving satellite.

[0057]

[0058] 3 shows a configuration of devices in a communication system according to an embodiment of the present disclosure. The devices in FIG. 3 can be understood as a partial structure of any one of the devices described with reference to FIG. 1, such as the terminal 110, satellites 120-1 and 120-2, and gateway 130.

[0059] Referring to FIG. 3, the device may include a processor 210 , a communication unit 220 and a memory 330 .

[0060] The processor 310 may control the overall functionality and operation of the device and may include an application-specific integrated circuit (ASIC), other chipset, logic circuitry, and / or data processing device.

[0061] The communication unit 320 is coupled to the processor 310 and transmits and receives 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 receiving unit. In one embodiment, the communication unit 320 may transmit and receive data to and from other devices, such as base stations and satellites.

[0062] The memory 330 is hardware that stores various data processed by the processor 310. For example, the memory 330 may store an SIR value for a transmission target terminal of a transmitting terminal, information on a transmission target terminal group for each transmitting terminal, etc. The memory 330 may also store applications, drivers, etc. that are operated by the processor 310. The memory 330 may include random access memory (RAM) such as dynamic random access memory (DRAM) or static random access memory (SRAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), CD-ROM, Blu-ray or other optical disk storage, hard disk drive (HDD), solid state drive (SSD), or flash memory.

[0063] The structure in Fig. 3 can be understood as at least a part of a terminal, a base station, a satellite, or a gateway. When the structure in Fig. 3 is part of a satellite, the satellite may further include other hardware devices necessary for flying in orbit in addition to the components illustrated in Fig. 3. When the structure in Fig. 3 is part of a gateway or a base station, the gateway or base station may further include components that support wired communication, etc.

[0064]

[0065] Fig. 4 is a diagram illustrating a random access procedure in a communication system, Fig. 4 illustrates a four-step random access procedure in a communication system.

[0066] 4, in step S401, a UE 410 that is not registered with a base station 420 transmits a random access preamble (RAP) to the base station 420 via a physical random access channel (PRACH). The UE 410 selects a random access preamble in a PRACH slot and transmits it in the PRACH slot to initiate a random access procedure.

[0067] In step S403, the base station 420 that has received the random access preamble transmits a random access response. To this end, the base station 420 monitors the random access channel, receives the random access preamble, confirms the request from the UE 410, and then transmits the random access response. The random access response may include the random access preamble, a timing advance (TA), and a cell-radio network temporary identifier (C-RNTI). The UE 410 receives the random access response, decodes the random access response, and obtains the temporary identifier and initial radio resource allocation information.

[0068] In step S405, if a contention occurs based on the assigned preamble ID, the UE 410 transmits a contention request message. If the contention is not resolved, the UE 410 retransmits the random access preamble and retries the random access procedure.

[0069] In step S407, the base station 420 completes the random access procedure by allocating radio resources to the UE 410 and sending a contention resolution message to the UE 410. After that, a logical connection for data transmission between the UE 410 and the base station 420 is established.

[0070] The four-step random access procedure shown in Figure 4 helps to ensure stable and rapid initial connection between the UE 410 and the base station 420. The following information is provided to the UE 410 through the random access procedure shown in Figure 4:

[0071] The TA value is the initial uplink timing alignment. To solve the timing problem, the timing is continuously measured using the uplink signal. The TA is used to adjust the time at which a subframe arrives. The C-RNTI is UE identification information. The C-RNTI is defined as shown in Table 1.

[0072] [Table 1]

[0073] An uplink grant is an uplink resource allocation and has a format that indicates the timing of data transmission.

[0074] 5 is a diagram illustrating an example of a conditional handover (CHO) procedure in a communication system. A conditional handover is a handover performed based on a handover execution condition determined by a terminal. Here, the execution condition is defined based on the signal strength of the serving cell and at least one neighboring cell. For example, the absolute value of the signal strength, the difference in signal strength, etc. may be predefined or may be defined as exceeding or being below a predetermined threshold.

[0075] Referring to FIG. 5, in step S501, a terminal 510 measures and reports signal strengths of a source base station 520, a target base station 530, and another base station 540. If the measurement values ​​satisfy a given condition, a measurement event is detected, and the terminal 510 reports the measurement values ​​to the source base station 520. For example, a measurement event may be defined as a signal strength for a neighboring cell being greater than the signal strength for the serving cell. In this case, the signal strength may be determined by adding the measurement value and at least one offset. A hysteresis value may also be applied. The at least one offset may include at least one of an offset for the measurement target and an offset for the cell.

[0076] In step S503, the source base station 520 determines whether to perform a conditional handover. To determine whether a handover is necessary, thresholds for parameters such as reference signal received power (RSRP) and reference signal received quality (RSRQ) can be set. The thresholds can be dynamically adjusted according to network quality, user requirements, mobility, etc.

[0077] In steps S505 and S507, the source base station 520 transmits a handover request to the target base station 530 and another base station 540. At this time, information about the terminal 510 may be transmitted together. The information about the terminal 510 may include at least one of the capabilities of the terminal 510, a measurement report of the terminal 510, and Quality of Service (QoS).

[0078] In steps S509 and S511, the target base station 530 and the other base station 840 perform admission control. In other words, the target base station 530 and the other base station 540 determine whether to allow handover based on information about the terminal 510. Whether to allow handover can be determined based on at least one of a frequency band, a transmission power, a QoS, a network capacity, and a load balance.

[0079] In steps S513 and S515, the target base station 530 and the other base station 540 send a Handover Request Acknowledge message to the source base station 520. The Handover Request Acknowledge signal may include whether to allow the handover, the C-RNTI, and transport layer information.

[0080] In step S517, the source base station 520 transmits a radio resource control (RRC) reconfiguration (RRC configuration) message to the terminal 510. That is, the source base station 520 instructs the terminal 510 to perform a handover. The RRC reconfiguration message may include information on candidate cells for the conditional handover and information on an execution condition for the conditional handover. For example, the execution condition may include information on a measurement object and information on a measurement value threshold. In addition, according to various embodiments, the RRC reconfiguration message may include at least one of new cell information, a new C-RNTI that the terminal 510 uses with the target base station 530, transmission layer information configured for the terminal 510 by the base station, RRC context information, and security information.

[0081] In step S519, the terminal 510 transmits an RRC reconfiguration complete message to the source base station 520. The terminal 510 may notify the source base station 520 that it has successfully received the RRC reconfiguration message and established a connection with the target base station 530 based on information provided in the message.

[0082] In step S521, the terminal 510 evaluates a conditional handover condition. The terminal 510 determines whether the execution condition for the conditional handover confirmed via the RRC reconfiguration message is met and checks which cell satisfies the execution condition. For example, the conditional handover condition can be defined based on received signal strength. For example, the execution condition can be defined as the signal strength for the neighboring cell being greater than a threshold, the sum of the signal strength for the neighboring cell and an offset being greater than a threshold, or the signal strength for the neighboring cell being greater than the signal strength for the serving cell.

[0083] In steps S523 and S525, the terminal 510 performs synchronization and RA through cooperation with the target base station 530 and other base stations 540 based on the received RRC reconfiguration message. The synchronization procedure is a procedure in which the terminal 510 adjusts frame timing based on the PSS (primary synchronization signal) and SSS (secondary synchronization signal) of the target base station 530. After synchronization is completed, the terminal 510 performs the RA procedure.

[0084] In step S527, the terminal 510 transmits a handover confirmation message to the target base station 530. In step S529, the target base station 530 transmits a handover completion message to the source base station 520. When the handover is completed, the terminal 510 releases the connection with the source base station 520, and the source base station 520 releases the resources allocated to the terminal 510.

[0085] FIG. 6 is a diagram illustrating an example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0086] Referring to FIG. 6, in step S601, a terminal 610 measures and reports signal strengths of a source base station 620, a target base station 630, and another base station 640. If the measurement values ​​satisfy a given condition, a measurement event is detected, and the terminal 610 reports the measurement values ​​to the source base station 620. For example, a measurement event may be defined as a signal strength for a neighboring cell being greater than the signal strength for the serving cell. In this case, the signal strength may be determined by adding the measurement value and at least one offset. A hysteresis value may also be applied. The at least one offset may include at least one of an offset for the measurement target and an offset for the cell.

[0087] Here, the offset for the measurement object (e.g., Ofn) is the measurement object specific offset of the reference signal of the neighbor cell, and the offset for the cell (e.g., Ocn) is the cell specific offset of the neighbor cell and can be set to zero if not configured for the neighbor cell. Here, according to one embodiment of the present disclosure, the neighbor cell may include a satellite cell. Therefore, the offset for the measurement object or the offset for the cell can be set or defined differently depending on whether the base station is a terrestrial base station or a satellite base station.

[0088] Furthermore, at least one timer can be used for measurement reporting. For example, a time to trigger (TTT) can be used as a condition for the length of time that satisfies an event condition. Timer T321 or T322 starts upon receiving configuration information (e.g., meaConfig) for measurement and expires when the measurement result set by the configuration information is obtained. When timer T321 or T322 expires, the terminal 810 can initiate a measurement reporting procedure and abort the associated measurement. At least one value of the TTT and timers can be provided to the terminal 810 via configuration information (e.g., ReportConfig, etc.) for measurement. Here, the TTT value and / or timer value can be set or defined differently depending on whether the neighbor cell being measured is a terrestrial cell or a satellite cell.

[0089] In step S603, the source base station 620 determines whether to perform a conditional handover, that is, the source base station 620 determines that the terminal 610 will perform a conditional handover based on the measurement report.

[0090] In steps S605 and S607, the source base station 620 transmits a handover request to the target base station 630 and another base station 640. At this time, information about the terminal 610 may be transmitted together. The information about the terminal 610 may include at least one of the capabilities of the terminal 610, a measurement report of the terminal 610, and QoS.

[0091] In steps S609 and S611, the target base station 630 and the other base stations 640 perform admission control. Based on the received information about the terminal 610, the target base station 630 and the other base stations 640 determine whether to allow a handover. Whether to allow a handover can be determined based on at least one of frequency band, transmission power, QoS, network capacity, and load balance. In other words, the target base station 630 and the other base stations 640 determine whether they can accommodate the terminal 610 based on available resources, the number of currently connected terminals, etc.

[0092] In steps S613 and S615, the target base station 630 and the other base station 640 transmit handover request confirmation signals to the source base station 620. The handover request confirmation signal includes at least one of whether to allow handover, a C-RNTI, a timing difference, and an uplink grant. The timing difference indicates a difference in signal arrival time that occurs depending on the distance between the terminal 610 and each base station 630 and 640, and is used to create an accurate synchronization condition between the terminal 610 and the base stations.

[0093] In step S617, the source base station 620 transmits an RRC reconfiguration message to the terminal 610. That is, the source base station 620 instructs the terminal 610 to perform a handover. The RRC reconfiguration message may include information on candidate cells for the conditional handover and information on an execution condition for the conditional handover. For example, the execution condition may include information on a measurement object and information on a measurement value threshold. In addition, according to various embodiments, the RRC reconfiguration message may include at least one of new cell information, a new C-RNTI that the terminal 610 will use with the target base station 630, a timing difference, and an uplink grant.

[0094] In step S619, the terminal 610 transmits an RRC reconfiguration complete message to the source base station 620. The terminal 610 notifies the source base station 620 that it has successfully received the RRC reconfiguration message and established a connection with the target base station 630 based on information provided in the message.

[0095] In step S621, the terminal 610 evaluates a conditional handover condition. The terminal 610 monitors channels with multiple candidate base stations including the target base station 630 and can determine whether the execution condition for the conditional handover is met based on the measurement results. For example, the execution condition can be defined as the signal strength for the neighboring cell being greater than a threshold, the sum of the signal strength for the neighboring cell and an offset being greater than a threshold, or the signal strength for the neighboring cell being greater than the signal strength for the serving cell. That is, the terminal 610 can determine whether the execution condition is met based on whether the measurement value exceeds a threshold. In this embodiment, the fulfillment of the condition for the target base station 630 is determined.

[0096] In step S623, the terminal 610 synchronizes with the target base station 630 and releases the connection with the source base station 620.

[0097] In step S625, the terminal 610 that has successfully completed the handover transmits a handover confirmation message to the target base station 630. In step S627, the target base station 630 transmits a handover completion message to the source base station 620. When the handover is completed, the terminal 610 releases the connection with the source base station 620, and the source base station 620 releases the resources allocated to the terminal 610.

[0098] According to the embodiment of Fig. 6, the terminal 610 does not need to use a separate RA procedure when performing synchronization, thereby reducing the time required for the handover process and the time during which communication is interrupted when performing handover.

[0099]

[0100] FIG. 7 is a diagram illustrating another example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0101] Referring to FIG. 7, in step S701, a terminal 710 measures and reports signal strengths of a source base station 720, a target base station 730, and another base station 740. If the measurement values ​​satisfy a given condition, a measurement event is detected, and the terminal 710 reports the measurement values ​​to the source base station 720. For example, a measurement event may be defined as a signal strength for a neighboring cell being greater than the signal strength for the serving cell. In this case, the signal strength may be determined by adding the measurement value and at least one offset. A hysteresis value may also be applied. The at least one offset may include at least one of an offset for the measurement target and an offset for the cell.

[0102] Here, the offset for the measurement object (e.g., Ofn) is the measurement object specific offset of the reference signal of the neighbor cell, and the offset for the cell (e.g., Ocn) is the cell specific offset of the neighbor cell and can be set to zero if not configured for the neighbor cell. Here, according to one embodiment of the present disclosure, the neighbor cell may include a satellite cell. Therefore, the offset for the measurement object or the offset for the cell can be set or defined differently depending on whether the base station is a terrestrial base station or a satellite base station.

[0103] In addition, at least one timer can be used for measurement reporting. For example, a time to trigger (TTT) can be used as a condition for the length of time that satisfies an event condition. In addition, timer T321 or timer T322 starts upon receiving configuration information (e.g., meaConfig) for measurement and expires when the measurement result set by the configuration information is obtained. When timer T321 or timer T322 expires, the terminal 810 can start a measurement reporting procedure and abort the associated measurement. At least one value of the TTT and timers can be provided to the terminal 810 via configuration information for measurement (e.g., ReportConfig, etc.). In this case, the TTT bit value and / or timer value can be set or defined differently depending on whether the neighbor cell to be measured is a terrestrial cell or a satellite cell.

[0104] In step S703, the source base station 720 determines whether to perform a conditional handover. To determine whether a handover is necessary, thresholds for parameters such as RSRP and RSRQ can be set. The thresholds can be dynamically adjusted according to network quality, user requirements, mobility, etc.

[0105] In steps S705 and S707, the source base station 720 transmits a handover request to the target base station 730 and another base station 740. At this time, information about the terminal 710 may be transmitted together. The information about the terminal 710 may include at least one of the capabilities of the terminal 710, a measurement report of the terminal 710, and QoS.

[0106] In steps S709 and S711, the target base station 730 and the other base station 740 perform admission control. The target base station 730 and the other base station 740 determine whether to allow handover based on the received information about the terminal 710. Whether to allow handover can be determined based on at least one of frequency band, transmission power, QoS, network capacity, and load balance.

[0107] In steps S713 and S715, the target base station 730 and the other base station 740 transmit handover request confirmation signals to the source base station 720. The handover request confirmation signal may include at least one of whether to allow handover, a C-RNTI, and a timing difference. The timing difference indicates a difference in signal arrival time that occurs depending on the distance between the terminal 710 and each of the base stations 730 and 740, and is used to create an accurate synchronization condition between the terminal 710 and the base stations.

[0108] In step S717, the source base station 720 transmits an RRC reconfiguration message to the terminal 710. That is, the source base station 720 instructs the terminal 710 to perform a handover. The RRC reconfiguration message may include information on candidate cells for the conditional handover and information on an execution condition for the conditional handover. For example, the execution condition may include information on a measurement object and information on a measurement value threshold. In addition, according to various embodiments, the RRC reconfiguration message may include at least one of new cell information, a new C-RNTI that the terminal 710 will use with the target base station 730, and a timing difference.

[0109] In steps S719 and S721, the target base station 730 and the other base stations 740 transmit uplink grants to the terminal 710 via a physical downlink control channel (PDCCH). The uplink grants include resource information for data transmission. For example, PDCCH downlink control information (DCI) format 0_0 or format 0_1 ​​can be used. That is, the terminal 710 continuously monitors the downlink of the target base station 730 according to the make-before-break (MBB) rule described below, so that the target base station 730 and the other base stations 740 can directly transmit uplink grants to the terminal 710.

[0110] In step S723, the terminal 710 transmits an RRC reconfiguration complete message to the source base station 720. The terminal 710 may notify that it has successfully received the RRC reconfiguration message and established a connection with the target base station 730 based on information provided in the message.

[0111] In step S725, the terminal 710 evaluates a conditional handover condition. The terminal 710 monitors channels with multiple candidate base stations, including the target base station 730, and can determine whether the execution condition for the conditional handover is met based on the measurement results. For example, the execution condition can be defined as the signal strength for the neighboring cell being greater than a threshold, the sum of the signal strength for the neighboring cell and an offset being greater than a threshold, or the signal strength for the neighboring cell being greater than the signal strength for the serving cell. That is, the terminal 710 can determine whether the execution condition is met based on whether the measurement value exceeds a threshold. In this embodiment, the fulfillment of the condition for the target base station 730 is determined.

[0112] In step S727, the terminal 710 synchronizes with the target base station 730 and releases its connection with the source base station 720. In step S729, the terminal 710 transmits a handover confirmation message to the target base station 730. In step S731, the target base station 730 transmits a handover complete message to the source base station 720. When the handover is completed, the terminal 710 releases its connection with the source base station 720, and the source base station 720 releases the resources allocated to the terminal 710.

[0113] According to the embodiment of Figure 7, the terminal 710 does not need to use a separate RA procedure when performing synchronization. This reduces the time required for the handover process and the time during which communication is interrupted when performing handover. Also, since the target base station 730 and the other base station 740 directly transmit the uplink grant to the terminal 710, there are advantages in that the burden on the source base station 720 is reduced and other information can be added to the RRC reconfiguration message.

[0114] MBB handover is a handover method in which the terminal 710 simultaneously receives all channels of the source base station 720 and the target base station 730 until the handover is completed, and then disconnects one channel after the handover is completed. To utilize the procedure of Figure 7, the conditions for MBB handover can also be taken into consideration. Assuming that the MBB handover method is applied, the terminal 710 continuously monitors the downlink of the target base station 730, so that the target base station 730 can directly allocate an uplink grant to the terminal 710.

[0115] Furthermore, the timing difference information of the target base station 730 will be described as follows. The timing difference can be calculated based on the downlink propagation delay (DL propagation delay) received from the source base station 720 and the target base station 730. Therefore, the terminal 710 can calculate the timing difference based on the timing value of the target base station 730 received from the target base station 730 and the timing value of the source base station 720 received from the source base station 720.

[0116] According to the embodiment of Figure 7, the terminal 710 does not need to use a separate RA procedure when performing synchronization. This reduces the time required for the handover process and the time during which communication is interrupted when performing handover. In addition, since the target base station 730 and the other base station 740 directly transmit the uplink grant to the terminal 710, the burden on the source base station 720 is reduced and other information can be added to the RRC reconfiguration message.

[0117]

[0118] FIG. 8 is a diagram illustrating another example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0119] Referring to FIG. 8, in step S801, a terminal 810 measures and reports signal strengths of a source base station 820, a target base station 830, and another base station 840. If the measurement values ​​satisfy a given condition, a measurement event is detected, and the terminal 810 reports the measurement values ​​to the source base station 820. For example, a measurement event may be defined as a signal strength for a neighboring cell being greater than the signal strength for the serving cell. In this case, the signal strength may be compared to the sum of the measured signal strength and at least one offset. A hysteresis value may also be applied. The at least one offset may include at least one of an offset for the measurement target and an offset for the cell.

[0120] Here, the offset for the measurement object (e.g., Ofn) is the measurement object specific offset of the reference signal of the neighbor cell, and the offset for the cell (e.g., Ocn) is the cell specific offset of the neighbor cell and can be set to zero if not configured for the neighbor cell. Here, according to one embodiment of the present disclosure, the neighbor cell may include a satellite cell. Therefore, the offset for the measurement object or the offset for the cell can be set or defined differently depending on whether the base station is a terrestrial base station or a satellite base station.

[0121] Furthermore, at least one timer can be used for measurement reporting. For example, a time to trigger (TTT) can be used as a condition for the length of time that satisfies an event condition. Timer T321 or T322 starts upon receiving configuration information (e.g., meaConfig) related to measurement and expires upon obtaining a measurement result configured by the configuration information. When timer T321 or T322 expires, the terminal 810 can initiate a measurement reporting procedure and abort the associated measurement. At least one value of the TTT and timers can be provided to the terminal 810 via configuration information (e.g., ReportConfig, etc.) for measurement. Here, the TTT value and / or timer value can be set or defined differently depending on whether the neighbor cell being measured is a terrestrial cell or a satellite cell.

[0122] In step S803, the source base station 820 determines whether to perform a conditional handover. That is, the source base station 820 determines that the terminal 810 will perform a conditional handover based on the measurement report.

[0123] In steps S805 and S807, the source base station 820 transmits a handover request to the target base station 830 and another base station 840. At this time, information about the terminal 810 may be transmitted together. The information about the terminal 810 may include at least one of the capabilities of the terminal 810, a measurement report of the terminal 810, and QoS.

[0124] In steps S809 and S811, the target base station 830 and the other base station 840 perform admission control. In other words, the target base station 830 and the other base station 840 determine whether to allow handover based on the received information about the terminal 810. Whether to allow handover can be determined based on at least one of frequency band, transmission power, QoS, network capacity, and load balance. In other words, the target base station 830 and the other base station 840 determine whether they can accommodate the terminal 810 based on available resources, the number of currently connected terminals, etc.

[0125] In steps S813 and S815, the target base station 830 and the other base station 840 transmit handover request confirmation signals to the source base station 820. The handover request confirmation signal includes at least one of whether to allow handover, a C-RNTI, a timing difference, and uplink resource information. The timing difference indicates a difference in signal arrival time that occurs depending on the distance between the terminal 810 and each base station 830 and 840, and is used to create an accurate synchronization condition between the terminal 810 and the base station. The uplink resource information includes information regarding resources on which the terminal 810 can transmit a handover indication in step S823.

[0126] In step S817, the source base station 820 transmits an RRC reconfiguration message to the terminal 810. That is, the source base station 820 instructs the terminal 810 to perform a handover. The RRC reconfiguration message may include information on candidate cells for the conditional handover and information on an execution condition for the conditional handover. For example, the execution condition may include information on a measurement object and information on a measurement value threshold. In addition, according to various embodiments, the RRC reconfiguration message may include at least one of new cell information, a new C-RNTI that the terminal 810 uses with the target base station 830, a timing difference, and uplink resource information.

[0127] In step S819, the terminal 810 transmits an RRC reconfiguration complete message to the source base station 820. The terminal 810 notifies the source base station 820 that it has successfully received the RRC reconfiguration message and established a connection with the target base station 830 based on information provided in the message.

[0128] In step S821, the terminal 810 evaluates a conditional handover condition. The terminal 810 monitors channels with multiple candidate base stations including the target base station 830 and determines whether the execution condition for the conditional handover is met based on the measurement results. For example, the execution condition may be defined as the signal strength for the neighboring cell being greater than a threshold, the sum of the signal strength for the neighboring cell and an offset being greater than a threshold, or the signal strength for the neighboring cell being greater than the signal strength for the serving cell. That is, the terminal 810 may determine whether the execution condition is met based on whether the measurement value exceeds a threshold. In this embodiment, the fulfillment of the condition for the target base station 830 is determined.

[0129] In step S823, the terminal 810 transmits a handover command message to the source base station 820 and the target base station 830. The target base station 830 may not need to allocate an uplink grant to the terminal 810 until the conditional handover condition is met. Therefore, in step S821, if the conditional handover condition is met, the terminal 810 transmits a handover command message to the source base station 820 and the target base station 830.

[0130] According to various embodiments, the terminal 810 may transmit the handover command message using the resources allocated by the source base station 820 and the target base station 830, respectively, or may simultaneously transmit the handover command message via broadcast. The resources for transmitting the handover command message are determined based on the uplink resource information received in step S817. When the terminal 810 broadcasts the handover command message, the target base station 830 may allocate the same resources for receiving the broadcast message as the resources allocated by the source base station 820 for receiving the broadcast message. To this end, the target base station 830 and the source base station 820 may share information regarding resources for receiving the broadcast message in advance.

[0131] In step S825, the target base station 830 that has received the handover command message transmits uplink grant information to the terminal 810. The terminal 810 can receive the uplink grant information from the target base station 830 using the C-RNTI and timing difference value included in the RRC reconfiguration message received in step S817.

[0132] In step S827, the terminal 810 synchronizes with the target base station 830 and releases the connection with the source base station 820. In step S829, the terminal 810 sends a handover confirmation message to the target base station 830. In step S831, the target base station 830 sends a handover complete message to the source base station 820.

[0133]

[0134] FIG. 9 illustrates an operation of acquiring timing information for a target cell 930 in a communication system according to an embodiment of the present disclosure. FIG. 9 illustrates an operation of acquiring TA information of a target cell 930 without a RACH procedure. One of the main purposes of the RACH procedure during handover is to acquire the target cell 930 TA. In the absence of a RACH procedure, if the source cell 920 and the target cell 930 are synchronized in real time, the terminal 910 can acquire the target cell 930 TA without an explicit TA command. As shown in FIG. 9, the terminal 910 first acquires the downlink propagation delay difference between the source cell 920 and the target cell 930. The uplink propagation delay can be considered to be the same as the downlink propagation delay, and the terminal 910 acquires the TA of the target cell 930 by TA. target =TA source -2(T1-T2).

[0135]

[0136] FIG. 10 is a diagram illustrating another example of a handover procedure in a communication system according to an embodiment of the present disclosure.

[0137] Referring to FIG. 10, in step S1001, a terminal 1010 measures and reports the signal strengths of a source base station 1020, a target base station 1030, and another base station 1040. If the measurement values ​​satisfy a given condition, a measurement event is detected, and the terminal 1010 reports the measurement values ​​to the source base station 1020. For example, a measurement event can be defined as a signal strength for a neighboring cell being greater than the signal strength for the serving cell. In this case, the signal strength can be determined by adding the measurement value and at least one offset. A hysteresis value can also be applied. The at least one offset can include at least one of an offset for the measurement target and an offset for the cell.

[0138] Here, the offset for the measurement object (e.g., Ofn) is the measurement object specific offset of the reference signal of the neighbor cell, and the offset for the cell (e.g., Ocn) is the cell specific offset of the neighbor cell and can be set to zero if not configured for the neighbor cell. Here, according to one embodiment of the present disclosure, the neighbor cell may include a satellite cell. Therefore, the offset for the measurement object or the offset for the cell can be set or defined differently depending on whether the base station is a terrestrial base station or a satellite base station.

[0139] Furthermore, at least one timer can be used for measurement reporting. For example, a time to trigger (TTT) can be used as a condition for the length of time that satisfies an event condition. Timer T321 or T322 starts upon receiving configuration information (e.g., meaConfig) related to measurement and expires upon obtaining a measurement result configured by the configuration information. When timer T321 or T322 expires, the terminal 810 can initiate a measurement reporting procedure and abort the associated measurement. At least one value of the TTT and timers can be provided to the terminal 810 via configuration information (e.g., ReportConfig, etc.) for measurement. Here, the TTT value and / or timer value can be set or defined differently depending on whether the neighbor cell being measured is a terrestrial cell or a satellite cell.

[0140] In step S1003, the source base station 1020 determines whether to perform a conditional handover, that is, the source base station 1020 determines that the terminal 1010 will perform a conditional handover based on the measurement report.

[0141] In steps S1005 and S1007, the source base station 1020 transmits a handover request to the target base station 1030 and another base station 1040. At this time, information about the terminal 1010 can be transmitted together. The information about the terminal 1010 can include at least one of the capabilities of the terminal 1010, a measurement report of the terminal 1010, and QoS. When the source base station 1020 requests a handover, it also requests timing information.

[0142] In steps S1009 and S1011, the target base station 730 and the other base stations 740 perform admission control. The target base station 1030 and the other base stations 1040 determine whether to allow handover based on the received information about the terminal 1010. Whether to allow handover can be determined based on at least one of frequency band, transmission power, QoS, network capacity, and load balance. In other words, the target base station 1030 and the other base stations 1040 determine whether they can accommodate the terminal 1010 based on available resources, the number of connected terminals, etc.

[0143] In steps S1013 and S1015, the target base station 1030 and the other base station 1040 transmit handover request confirmation signals to the source base station 1020. The handover request confirmation signal may include at least one of whether to allow handover, a C-RNTI, and timing information. The timing information may be used for synchronization between the terminal 1010 and the base station. If the base station is a satellite, the timing information may include not only the timing difference but also at least one of the satellite velocity, satellite orbit, satellite altitude, measured time, etc. Therefore, even if the base station is moving, it is possible to calculate a precise timing difference.

[0144] In step S1017, the source base station 1020 transmits an RRC reconfiguration message to the terminal 1010. That is, the source base station 1020 instructs the terminal 1010 to perform a handover. The RRC reconfiguration message may include information on candidate cells for the conditional handover and information on an execution condition for the conditional handover. For example, the execution condition may include information on a measurement object and information on a measurement value threshold. In addition, according to various embodiments, the RRC reconfiguration message may include at least one of new cell information, a new C-RNTI that the terminal 1010 uses with the target base station 1030, and timing information.

[0145] In steps S1019 and S1021, the target base station 1030 and the other base station 1040 transmit an uplink grant to the terminal 1010 via a PDCCH. The uplink grant indicates a time when data transmission is possible. For example, PDCCH DCI format 0_0 or format 0_1 ​​can be used. For example, the target base station 1030 performs a PDCCH transmission procedure (e.g., Msg in RACH), thereby transmitting the uplink grant. That is, the terminal 1010 continuously monitors the downlink of the target base station via the MBB, so that the target base station 1030 can directly transmit the uplink grant to the terminal 1010.

[0146] In step S1023, the terminal 1010 transmits an RRC reconfiguration complete message to the source base station 1020. The terminal 1010 notifies the source base station 1020 that it has successfully received the RRC reconfiguration message and established a connection with the target base station 1030 based on information provided in the message.

[0147] In step S1025, the terminal 1010 evaluates the conditional handover condition. The terminal 1010 monitors channels with multiple candidate base stations including the target base station 1030 and can determine whether the execution condition for the conditional handover is met based on the measurement results. For example, the execution condition can be defined as the signal strength for the neighboring cell being greater than a threshold, the sum of the signal strength for the neighboring cell and an offset being greater than a threshold, or the signal strength for the neighboring cell being greater than the signal strength for the serving cell. That is, the terminal 1010 can determine whether the execution condition is met based on whether the measurement value exceeds a threshold. In this embodiment, it is determined that the condition for the target base station 1030 is met when the measurement value exceeds a threshold.

[0148] In step S1027, the terminal 1010 synchronizes with the target base station 1030 and releases its connection with the source base station 1020. In step S1029, the terminal 1010, which has successfully completed the handover, sends a handover confirmation message to the target base station 1030. In step S1031, the target base station 1030 sends a handover completion message to the source base station 1020. When the handover is completed, the terminal 1010 releases its connection with the source base station 1020, and the source base station 1020 releases the resources allocated to the terminal 1010.

[0149] According to the embodiment of Fig. 10, the terminal 1010 does not need to use a separate RA procedure when performing synchronization, thereby reducing the time required for the handover process and the time during which communication is interrupted when performing the handover.

[0150] MBB handover is a handover method in which the terminal 1010 simultaneously receives all channels of the source base station 1020 and the target base station 1030 until the handover is completed, and then disconnects one channel after the handover is completed. To utilize the procedure of Figure 10, the conditions for MBB handover can also be taken into consideration. Assuming that the MBB handover method is applied, the terminal 1010 continuously monitors the downlink of the target base station, so that the target base station can directly allocate an uplink grant to the terminal 1010.

[0151] The method for receiving the timing value is as follows: When the source base station 1020 transmits a handover request to a candidate base station (e.g., a target base station 1030 and another base station 1040) to determine whether admission is possible, the candidate base station notifies its timing value via an ACK in response to the handover request. The serving cell that receives the handover request ACK transmits the value to the terminal 1010 via an RRC reconfiguration message.

[0152] There may be a situation where the target base station 1030 and the terminal 1010 are not directly connected. However, since the target base station 1030 and the terminal 1010 do not communicate directly with each other and the serving base station notifies the timing information via an RRC reconfiguration message, a connection between the terminal 1010 and the target base station does not need to be established. The terminal 1010 can calculate the timing difference based on the received information.

[0153]

[0154] FIG. 11 is a diagram illustrating an example of a situation in which a terrestrial base station and a satellite base station provide services in a communication system according to an embodiment of the present disclosure.

[0155] Referring to FIG. 11, a first satellite base station 1110, a second satellite base station 1120, a third satellite base station 1130, a first terrestrial base station 1140, a second terrestrial base station 1150, and a third terrestrial base station 1160 each have their own coverage area. The satellite base stations 1110-1130 can communicate using inter-satellite link (ISL) technology. The inter-satellite link technology provides a more stable and secure data transmission path because it does not go through a terrestrial station. By applying the inter-satellite link technology, each satellite base station 1110-1130 can quickly process high-volume data. The inter-satellite communication technology can be applied to various types of satellites, including low earth orbits (LEO) and geostationary orbits (GEO). For example, the inter-satellite link can be designed based on one of various types of signals, such as laser signals or RF signals.

[0156] The satellite base stations 1110-1130 and the terrestrial base stations 1140-1160 can communicate with each other via ground-satellite link (GSL) technology. The GSL technology is a technology for communication between the satellite base stations 1110-1130 and the terrestrial base stations 1140-1160, and can be used to transmit data collected by the satellite base stations to the terrestrial stations or for the terrestrial base stations to control or monitor the satellite base stations. Through the GSL technology and the terrestrial-satellite link technology, the terminal can select a target base station for a handover procedure, either a terrestrial base station or a satellite base station.

[0157] The satellite base stations 1110-1130 operate at higher altitudes than the terrestrial base stations 1140-1160. Therefore, the satellite base stations 1110-1130 can provide wider coverage than the terrestrial base stations 1140-1160. However, because the satellite base stations 1110-1130 are located farther from terminals located on the ground than the terrestrial base stations 1140-1160, signals transmitted from the satellite base stations 1110-1130 may experience relatively greater path loss. According to various embodiments, these differences between the satellite base stations 1110-1130 and the terrestrial base stations 1140-1160 can be taken into consideration in the handover procedure.

[0158]

[0159] 12 shows an example of a handover procedure that takes into account a satellite base station in an apparatus applicable to the present disclosure. FIG. 12 is a diagram illustrating an operation method of a terminal performing a handover.

[0160] 12, in step S1201, a terminal connects to and communicates with a first terrestrial base station. The terminal performs a random access procedure to connect to the first terrestrial base station. In the random access procedure, the terminal transmits a random access preamble to the terrestrial base station, and the terrestrial base station transmits a random access response to the terminal.

[0161] In step S1203, the terminal determines whether a handover is necessary. The terminal measures the strength of signals received from the first terrestrial base station and neighboring base stations, and determines whether a handover is necessary based on the measured signal strength. If a handover is determined to be necessary, a target base station may also be selected.

[0162] In step S1205, the terminal checks whether a second terrestrial base station is found. That is, the terminal checks whether another neighboring terrestrial base station is selected as the target base station. If another terrestrial base station is found, the terminal performs handover to the found second terrestrial base station in step S1207.

[0163] On the other hand, if no other terrestrial base stations are found, the terminal acquires information about the satellite base station from the first base station in step S1209. If the terminal already has information about the satellite base station, step S1209 can be omitted. The information about the satellite base station can include the CID (cell ID) of the cell provided by the satellite base station, orbit information, resource status information, altitude information, etc.

[0164] In step S1211, the terminal performs handover to the satellite base station. Specifically, the terminal may select the satellite base station as a target base station, synchronize with the satellite base station, and request connection establishment.

[0165] In the embodiment described with reference to Figure 12, the terrestrial base station has priority over the satellite base station in selecting a target base station for handover. However, in other embodiments, the satellite base station may have higher priority. In this case, the terminal may search for the satellite base station first, rather than the terrestrial base station first. In another embodiment, the terminal may treat the terrestrial base station and the satellite base station with the same priority.

[0166] In the embodiment described with reference to Fig. 12, the terminal connects to the first terrestrial base station. However, in other embodiments, steps S1203 to S1211 in Fig. 12 may be performed even when the terminal connects to the first satellite base station.

[0167]

[0168] In the various embodiments described above, the terminal may perform measurements on a satellite base station. According to one embodiment, when the terminal measures a signal from a satellite base station, altitude angle information of the satellite base station may be used. The altitude angle may be obtained in various ways. For example, the altitude angle may be calculated based on the satellite's nadir point, the satellite's orbit, and the satellite's altitude. In this case, the source base station may transmit necessary information to the terminal by including information on the satellite's orbit and altitude in a measurement configuration message. As another example, when the satellite base station moves along a fixed orbit, the altitude angle of the satellite base station may be calculated based on the terminal's position and the time of reception of a reference signal.

[0169]

[0170] 13 illustrates an example of a procedure in which a terminal performs measurements on a base station in a communication system according to an embodiment of the present disclosure, where the terminal receives a reference signal of the base station and acquires information about the base station.

[0171] 13, in steps S1301 to S1305, the base stations 1320 to 1340 transmit reference signals to the terminal 1310. The reference signals may be transmitted periodically or non-periodically. Although not shown in FIG. 13, prior to this step, the source base station 1320 may transmit configuration information for measurement to the terminal 1310.

[0172] In step S1307, the terminal 1310 detects a reference signal. The terminal 1310 can acquire information about the channels with each of the base stations 1320 to 1340 based on the reference signals transmitted from the base stations 1320 to 1340. That is, the terminal can receive reference signals from the source base station 1320 and neighboring base stations (e.g., the target base station 1330 and another base station 1340) based on the configuration information for measurement and perform measurements.

[0173] In step S1309, the terminal 1310 detects the positions of the base stations 1320 to 1340. In other words, the terminal 1310 can detect the positions of the base stations 1320 to 1340 based on measurement results based on the reference signal. For example, the distance to the base stations 1320 to 1340 can be detected based on the received strength of the reference signal, and the direction to the base stations 1320 to 1340 can be detected based on the received beam used when receiving the reference signal.

[0174] In step S1311, the terminal 1310 detects the altitude angle of a satellite base station among the base stations 1320 to 1340. Whether each of the base stations 1320 to 1340 is a satellite base station or a terrestrial base station can be determined based on configuration information for measurement provided from the source base station 1320. For example, the configuration information for measurement includes a neighbor base station list, and the neighbor base station list can include at least one of information on the type of each neighbor base station (e.g., terrestrial, satellite), an identifier of each neighbor base station (e.g., physCellId), and a dedicated offset for the cell (e.g., CellIndividualOffset). Here, the neighbor base station list can be included in configuration information including a measurement item (e.g., measObject). That is, the configuration information for a measurement item (e.g., MeasObject IE) may include a neighbor base station list (e.g., CellsToRemoveList, cellsToAddModList), and thresholds associated with the measurement (e.g., thresholdSRP, thresholdRSRQ, thresholdSINR, thresholdRSRP, thresholdRSRQ, thresholdSINR).

[0175] Here, the cell identifier may explicitly or implicitly indicate that the base station is a satellite base station. Alternatively, a measurement item (e.g., measObject) providing a list of neighboring base stations for the satellite base station may be provided as separate configuration information. Furthermore, according to one embodiment, the configuration information specific to the satellite base station may include epoch information, synchronization-related information, information related to the satellite's movement trajectory, cell characteristic (e.g., fixed cell, moving cell) information, cell center position information (e.g., reference position), and information required to determine the altitude angle (e.g., maximum altitude angle, minimum altitude angle, etc.).

[0176] According to one embodiment, the terminal 1310 can determine the altitude angle of the satellite base station based on the vertical point of the satellite base station, the orbit of the satellite base station, and the altitude of the satellite base station. The terminal 1310 can obtain information necessary to calculate the altitude angle, such as orbit information and altitude information of the satellite base station, using the configuration information, and can therefore calculate the altitude angle of the satellite base station.

[0177] In step S1313, the terminal 1310 transmits a measurement report to the source base station 1320. The measurement report includes measurement results for each of the base stations 1320 to 1340. For example, the measurement report may include received signal strength for each of the base stations 1320 to 1340. Here, the measurement report can be used as a request for handover. Therefore, the measurement report can be transmitted when certain requirements are met. For example, whether to transmit the measurement report can be determined based on the measured altitude angle of the satellite base station. According to one embodiment, if the measured altitude angle of the satellite base station exceeds a threshold, the terminal 1310 can transmit the measurement report.

[0178]

[0179] FIG. 14 illustrates satellite parameter values ​​that an apparatus can measure according to an embodiment of the present disclosure. Referring to FIG. 14, a terminal 1430 can measure RSRP and RSRQ, which are indicators of the signal strength of a first satellite base station 1410 and a second satellite base station 1420. The terminal 1430 can also measure additional parameters, such as the distance between the terminal 1430 and the satellite base stations 1410 and 1420, and the altitude angles (e.g., θ1 and θ2) of the satellite base stations 1410 and 1420. The various parameters measured as shown in FIG. 14 can be used to determine whether to perform a handover. A handover procedure based on various parameters can be performed as shown in FIG. 15.

[0180] 15 illustrates an example procedure for determining whether to perform a handover based on various parameters according to one embodiment of the present disclosure. FIG. 15 illustrates a method of operation of a terminal when performing a handover between satellite base stations.

[0181] 15, in step S1501, the terminal measures the signal strengths of the source satellite base station and the neighbor satellite base station, the distance difference between the two base stations, and the altitude angle of each base station, and reports the measurement results to the source satellite base station. To do this, the terminal can use reference signals transmitted from the source satellite base station and the neighbor satellite base station.

[0182] In step S1503, the terminal determines whether the signal strength of the neighbor satellite base station is superior to that of the source satellite base station. The terminal can determine whether the signal strength of the neighbor satellite base station is superior to that of the source satellite base station by comparing the signal strengths of the base stations measured in step S1501.

[0183] If the signal strength of the neighbor satellite base station is superior to that of the source satellite base station, the terminal checks whether the distance to the source satellite base station is greater than the distance to the neighbor satellite base station in step S1505. The terminal can check whether the distance to the source satellite base station is greater than the distance to the neighbor satellite base station by comparing the distances to the base stations measured in step S1501.

[0184] If the distance to the source satellite base station is greater than the distance to the adjacent satellite base station, in step S1507, the terminal determines whether the altitude angle of the source satellite base station is greater than the altitude angle of the adjacent satellite base station. The terminal can determine whether the altitude angle of the source satellite base station is greater than the altitude angle of the adjacent satellite base station by comparing the altitude angles of the base stations measured in step S1501.

[0185] If the altitude angle of the source satellite base station is greater than that of the neighboring satellite base station, the terminal determines to perform a handover to the neighboring satellite base station in step S1509 and performs the handover. That is, if the signal strength of the neighboring satellite base station is greater, the neighboring satellite base station is closer than the source satellite base station, and the altitude angle of the neighboring satellite base station is smaller than that of the source satellite base station, the terminal performs a handover to the neighboring satellite base station. Here, when comparing signal strengths, if the difference between the measured values ​​is greater than or equal to a threshold, it can be determined that one signal strength is greater than the other signal strength. Also, when comparing distances, if the difference between the measured values ​​is greater than or equal to a threshold, it can be determined that one distance is greater than the other distance. Also, when comparing altitude angles, if the difference between the measured values ​​is greater than or equal to a threshold, it can be determined that one altitude angle is greater than the other altitude angle.

[0186] In the embodiment described with reference to Figure 15, whether to perform a handover procedure is determined based on all of the signal strength, the distance to the satellite base station, and the altitude angle of the satellite base station. However, the source satellite base station does not necessarily have to use all three indicators to determine whether to perform a handover. That is, in other embodiments, the terminal can determine whether to perform a handover based on at least one of the signal strength, the distance to the satellite base station, and the altitude angle of the satellite base station.

[0187]

[0188] Figure 16 shows an example of changes in signal strength of a terrestrial base station and a satellite base station depending on the location of a terminal. Referring to Figure 16, when a terminal moves the same distance, the change in received signal strength is greater when using a terrestrial base station than when using a satellite base station. The distance between the satellite base station and the terminal is significantly greater than the distance between the terrestrial base station and the terminal. Therefore, if the terminal moves the same distance, the rate of change in the distance between the satellite base station and the terminal before and after the movement is smaller than the rate of change in the distance between the terrestrial base station and the terminal before and after the movement. Furthermore, the change in angle from the satellite base station to the terminal due to the movement is smaller than the change in angle from the terrestrial base station to the terminal due to the movement.

[0189] Therefore, as described with reference to Fig. 15, handover can be performed based on not only signal strength but also altitude angle, distance, etc. According to one embodiment, in a next-generation communication system including a terrestrial base station and a satellite base station, when a handover is performed between a terrestrial base station and at least one satellite base station, or between a satellite base station and another satellite base station, the terminal can perform the handover more quickly by knowing information about the target base station in advance.

[0190] In addition, the coverage of a satellite base station may be wider than that of a terrestrial base station. As a result, the terminal can selectively perform handover taking into account the characteristics of the service being used. For example, in the case of a service requiring relatively high stability, the terminal can perform handover to a satellite base station providing relatively wider coverage. On the other hand, in the case of a service requiring relatively low delay or relatively high data reliability, the terminal can perform handover to a satellite base station providing relatively narrower coverage. As a result, handover can be performed to a cell supporting different service coverage depending on the characteristics of the target base station selected for each service. This is because the terminal can obtain timing information of the target base station in advance before determining whether to perform handover among multiple base stations, thereby allowing handover to be performed effectively without delay due to a random access procedure. In addition, the terminal can select the optimal base station for its location taking into account information about surrounding satellite base stations.

[0191]

[0192] FIG. 17 is a diagram illustrating an example of an RRC reestablishment (RRE) procedure according to one embodiment of the present disclosure.

[0193] 17, in step S1701, a terminal 1710 transmits an RRC reestablishment request message to a connected base station 1720. That is, the terminal 1710 initiates an RRE procedure in response to detecting RLF (radio link failure). If synchronization between the terminal 1710 and the base station 1720 is lost in the physical layer or if the terminal 1710 cannot receive a signal from the base station 1720, the terminal 1710 can determine RLF.

[0194] In step S1703, the base station 1720 transmits an RRC Reestablishment message to the terminal. The base station 1720 is the immediately preceding serving cell of the terminal 1710 and has prior knowledge of the terminal 1710's information (e.g., UE context information) via a random access procedure or a previously received handover request message, and therefore can quickly reestablish RRC.

[0195] In step S1705, the terminal 1710 transmits an RRC reconfiguration procedure completion message to the base station 1720. This allows the connection between the terminal 1710 and the base station 1720 to be restored.

[0196] The RRE procedure described with reference to FIG. 17 may have the following problems when applied to a satellite base station. When a terminal performs the RRE procedure with a satellite base station, the procedure may not be performed smoothly due to a long propagation delay, which increases the time required for the handover procedure. In addition, if a satellite uses a moving beam, the cell may move a long distance during the RLF. As a result, even if the terminal successfully performs the RRE procedure, the terminal may still experience RLF. Therefore, the present disclosure proposes a modified RRE (modified RRC reestablishment) procedure as shown in FIG. 18 below.

[0197]

[0198] 18 illustrates an example of a modified RRE procedure according to an embodiment of the present disclosure. 18 illustrates an operation method of a terminal.

[0199] 18, in step S1801, a terminal connects to and communicates with a satellite base station. The terminal can connect to and communicate with the satellite base station via an initial connection or handover.

[0200] In step S1803, the terminal determines whether RLF occurs. If the terminal loses synchronization with the base station due to degradation of channel quality, or if the terminal cannot receive the signal from the base station, or if a certain rate or more of decoding errors occur, the terminal can determine RLF. If RLF does not occur, the terminal returns to step S1801 and continues to communicate with the satellite base station.

[0201] On the other hand, when an RLF occurs, the terminal searches for a terrestrial base station in step S1805. That is, instead of performing RRE with the satellite base station that was the previous serving cell in response to the RLF, the terminal preferentially searches for a terrestrial base station as a candidate for a new serving cell.

[0202] If a terrestrial base station is found, the terminal performs a modified RRE procedure with the terrestrial base station in step S1807. Specifically, the terminal transmits an RRC re-establishment request message to the terrestrial base station and receives an RRC re-establishment message from the terrestrial base station. To perform the RRE procedure with the terrestrial base station, the terrestrial base station is required to have information about the terminal. To this end, according to one embodiment, the satellite base station can share information about the terminal with surrounding terrestrial base stations.

[0203] On the other hand, if no terrestrial base station is found, in step S1809, the terminal performs the RRE procedure with the satellite base station, which is the previous serving cell. Since the satellite base station holds information about the terminal, it can perform the RRE procedure. Specifically, the terminal transmits an RRC re-establishment request message to the satellite base station and receives an RRC re-establishment message from the satellite base station.

[0204]

[0205] According to the various embodiments described above, a conditional handover can be performed without a RACH for a target cell. According to various embodiments, a conditional handover without a RACH can be performed in various scenarios. For example, when the target cell is a satellite base station, the above-described conditional handover without a RACH can be performed. As an example, when there are multiple target cells, including at least one satellite base station and at least one terrestrial base station, a conditional handover without a RACH can be applied only to the satellite base station, and a conditional handover with a RACH can be performed to the terrestrial base station. In this case, the above-described timing-related information and uplink grant information can be provided only to the target cell, which is a satellite base station.

[0206]

[0207] However, those skilled in the art will understand that the present invention can be realized in various modified forms without departing from the essential characteristics described above. Therefore, the disclosed method should be considered from an illustrative perspective, not a restrictive perspective. The scope of the present disclosure is defined by the claims, not the foregoing description, and all differences within the scope of the claims should be construed as being within the scope of the present disclosure. [Industrial Applicability]

[0208] The above is also applicable to other communication systems.

Claims

1. 1. A method of operating a terminal in a communication system, comprising: sending a measurement report to the first base station; receiving, from the first base station, a radio resource control (RRC) reconfiguration message including at least one of information on a radio network temporary identifier (RNTI) of a second base station, information on a timing difference between the first base station and the second base station, and information on uplink resources allocated by the second base station as information on a candidate base station for handover; performing a handover to a second base station using information included in the RRC reconfiguration message.

2. The method of claim 1 , further comprising: determining the second base station as a target base station based on an execution condition for handover indicated by the RRC reconfiguration message.

3. The method of claim 2 , wherein the execution condition is defined based on at least one of a received signal strength for a base station, a distance between the terminal and the base station, and an elevation angle of the base station.

4. sending a handover command message to the first base station and the second base station; receiving an uplink grant from the second base station; The method of claim 1 , further comprising: sending a handover complete message to the second base station.

5. The method of claim 4 , wherein the handover direction message is broadcast to the first base station and the second base station.

6. The method of claim 1 , wherein at least one of the candidate base stations comprises a satellite base station.

7. The method of claim 6 , wherein among the candidate base stations, a terrestrial base station has a higher priority as a target base station than a satellite base station.

8. the second base station is a satellite base station; The method of claim 7, further comprising receiving a measurement configuration message from the first base station, the measurement configuration message including at least one of altitude information and orbit information of the second base station.

9. The method of claim 1 , wherein the measurement report is transmitted if a condition related to the elevation angle of the first base station and the elevation angle of the second base station is satisfied.

10. 1. A method of operating a first base station in a communications system, comprising: receiving a measurement report from the terminal; determining to perform a handover of the terminal; sending a handover request message to at least one neighboring base station; receiving a handover request confirmation message from the at least one neighbor base station; transmitting, to the terminal, a radio resource control (RRC) reconfiguration message including at least one of information on a radio network temporary identifier (RNTI) of a second base station, information on a timing difference between the first base station and the second base station, and information on uplink resources allocated by the second base station as information on candidate base stations for handover.

11. receiving a handover command message from the terminal; The method of claim 10, further comprising: receiving a handover complete message from a second base station of the at least one neighbor base station.

12. The method of claim 10 , wherein the handover request confirmation message includes information about the RNTI, information about the timing difference, and information about the uplink resource.

13. The method of claim 10 , wherein at least one of the at least one neighboring base station comprises a satellite base station.

14. the second base station is a satellite base station; The method of claim 10, further comprising: transmitting a measurement configuration message to the terminal, the measurement configuration message including at least one of altitude information and orbit information of the second base station.

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