Wireless communication system, network device, satellite base station device, and control method and program thereof

Abstract

To reduce the time required for handover due to movement of a satellite base station device. [Solution] In a wireless communication system, a network device identifies a first time when a first satellite base station device will end providing service to a terminal device in a first location range and a second satellite base station device that will provide service after the first time, and switches a PDU session with the first satellite base station device associated with the terminal device to the second satellite base station device based on the first time without receiving a message transmitted in response to a Path Switch Request. When the second satellite base station device identifies the first time and receives an RRC Reconfiguration ACK message from the terminal device connected to the first satellite base station device, it transmits a Session Release message to the first satellite base station device without transmitting a Path Switch Request message.

Description

[Technical Field]

[0001] The present invention relates to a wireless communication system, a network device, a satellite base station device, and a control method and program for these. [Background technology]

[0002] For the fifth-generation mobile communication system (5G) and sixth-generation mobile communication system (6G), the integration of low-earth orbit (LEO) satellites and mobile networks is being considered. This integrated network is called the Non-Terrestrial Network (NTN). LEO satellites are now operating in a regenerative mode, where they function as base stations and communicate with other LEO satellites via Inter Satellite Links (ISLs), rather than in a transparent mode, where they act as repeaters that relay radio waves. In regenerative mode, LEO satellites act as base stations and accommodate terminal equipment (UE) within the cell area they provide service to.

[0003] Non-Patent Document 1 describes a technology for reducing the time required for handover in a network that integrates a satellite gNodeB (S-gNB), which is a LEO satellite, and a mobile network. In Non-Patent Document 1, the time of communication interruption during handover can be reduced by predicting the time when HO will occur and the LEO satellite to which HO will be performed for each UE. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Wu, Jiasheng, et al. “Accelerating Handover in Mobile Satellite Network.” arXiv preprint arXiv:2403.11502 (2024). Summary of the Invention [Problem to be solved by the invention]

[0005] Thus, in a wireless communication system including a satellite base station device, it has been an issue to reduce the communication interruption time during handover. [Means for solving the problem]

[0006] The present invention has been made in view of the above-mentioned problems, and has an object to provide a technique for reducing the time required for handover caused by movement of a satellite base station device.

[0007] A wireless communication system according to one aspect of the present invention comprises: A wireless communication system including a terminal device, first and second satellite base station devices, and a network device, The network device comprises: a first specifying means for specifying a first time when the first satellite base station device will end providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; a second identification means for identifying a terminal device located within the first location range; a switching means for switching a Protocol Data Unit (PDU) session with the first satellite base station device associated with the terminal device identified by the second identifying means to the second satellite base station device based on the first time identified by the first identifying means without receiving a message transmitted in response to a Path Switch Request; Equipped with the second satellite base station device, a third specifying means for specifying the first time when the first satellite base station device will end providing service within the first location range; a transmitting means for transmitting a Session Release message to the first satellite base station device without transmitting a Path Switch Request message when an RRC Reconfiguration ACK message is received from the terminal device connected to the first satellite base station device at the first time; providing means for providing a service to the terminal device within the first location range after the first time; The present invention is characterized by comprising: [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a technique for reducing the time required for handover due to movement of a satellite base station device. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration of a wireless communication system according to an embodiment of the present invention. [Figure 2] (A) Configuration diagram of satellite base station equipment, (B) Configuration diagram of network equipment, (C) Configuration diagram of terminal equipment [Figure 3] (A) Conceptual diagram of cell area layout, (B) Conceptual diagram of cell area layout in Mercator projection [Figure 4] Conceptual diagram of satellite base station equipment providing services in multiple cell areas over time [Figure 5] Sequence diagram of handover processing that occurs due to movement of a terminal device [Figure 6] Sequence diagram of handover processing that occurs due to movement of satellite base station equipment [Figure 7] 10 is a flowchart illustrating an example of processing executed by a terminal device. [Figure 8] 10 is a flowchart illustrating an example of processing executed by a satellite base station device. [Figure 9] A flowchart illustrating an example of processing executed by a network device. [Figure 10](A) A diagram showing the association between cell areas and satellite base station devices 10, (B) A diagram showing a data structure for managing the association between cell areas and satellite base station devices 10 DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. Furthermore, the same reference numerals are used to designate identical or similar components, and redundant descriptions will be omitted.

[0011] <System configuration> The configuration of a wireless communication system according to this embodiment is explained with reference to Fig. 1. The wireless communication system 1 shown in Fig. 1 comprises satellite base station devices 101 to 10 N (hereinafter, they may be referred to as satellite base station device 10 without distinction), network device 20, and terminal device 30.

[0012] The satellite base station device 10 can communicate with a core network 50 including a network device 20 via a ground station 40. In this embodiment, the satellite base station device 10 will be described as a low earth orbit (LEO) satellite that can operate as a base station device. The satellite base station device 10 functions as a base station device and provides services to terminal devices 30 located on the ground. In this embodiment, the satellite base station device 10 can communicate with the ground station 40 and other satellite base station devices 10.

[0013] In this embodiment, the network device 20 will be described as a UPF (User Plane Function), but the network device 20 is not limited to this as long as it has a function of managing a session with the satellite base station device 10, which is associated with the terminal device 30 as described later. For example, the network device 20 may be an AMF (Access and Mobility Management Function) or an SMF (Session Management Function).

[0014] The terminal device 30 can connect to the satellite base station device 10 via a cell formed by one of the satellite base station devices 10. In this embodiment, the terminal device 30 will be described as user equipment (UE) such as a smartphone located on the ground.

[0015] It should be noted that the wireless communication system only needs to include two or more satellite base station devices 10 and one or more terminal devices 30, and the numbers of satellite base station devices 10 and terminal devices 30 are not limited.

[0016] <Hardware configuration> 2A shows a hardware configuration diagram of satellite base station device 10. Satellite base station device 10 includes wireless communication unit 101, control unit 102, beam control unit 103, and antenna 104.

[0017] The wireless communication unit 101 performs wireless communication with a terrestrial station or a terminal device 30 via an antenna 104. In this embodiment, the wireless communication unit 101 is capable of performing communication in accordance with the fifth generation (5G) standard defined by the Third Generation Partnership Project (3GPP (registered trademark)) or a successor standard to 5G, such as sixth generation (6G). The satellite base station device 10 also functions as a base station device (gNB) conforming to the 5G standard or a standard beyond 5G.

[0018] The control unit 102 includes one or more processors and controls the overall operation of the satellite base station device 10 by executing programs stored in a ROM (Read Only Memory) or a RAM (Random Access Memory). The beam control unit 103, as will be described later, controls the beam direction of the antenna 104 so as to provide service to terminal devices located within a predetermined terrestrial range (cell area). If the antenna 104 includes multiple antennas, the beam control unit 103 may control the beam direction by adjusting the phases of electromagnetic waves transmitted from the multiple antennas, or by controlling the attitude of the satellite base station device 10. The antenna 104 includes one or more antennas and communicates with terrestrial terminal devices 30. The beam control unit 103 is also capable of controlling the directivity of the antenna 104 or the attitude of the satellite base station device 10 to enable communication with a terrestrial station 40 or another satellite base station device 10. In one example, the antenna 104 may include multiple antennas and be capable of simultaneously communicating with multiple terminal devices 30, terrestrial stations 40, and another satellite base station device 10. Furthermore, the antenna 104 may include a plurality of antennas 104 so as to be capable of transmitting and receiving electromagnetic waves in a plurality of frequency bands.

[0019] 2B shows a hardware configuration diagram of the network device 20. The network device 20 includes a communication unit 201, a control unit 202, and a storage unit 203.

[0020] The communication unit 201 communicates with the satellite base station 10 via the core network 50 and the earth station 40 via at least one of a wired network and a wireless network.

[0021] The control unit 202 includes one or more processors, and executes programs stored in a ROM (Read Only Memory) or a RAM (Random Access Memory) to control the overall operation of the network device 20. The control unit 202 also switches user sessions associated with the terminal device 30, which will be described later.

[0022] The storage unit 203 stores the session information for each terminal device 30. In one example, the storage unit 203 may be an external device that operates as a storage device.

[0023] 2C shows a hardware configuration diagram of the terminal device 30. The terminal device 30 includes a wireless communication unit 301, a control unit 302, and an antenna 303.

[0024] The wireless communication unit 301 performs wireless communication with the satellite base station device 10 via an antenna 303. In this embodiment, the wireless communication unit 301 is assumed to be capable of performing communication in accordance with the fifth generation (5G) standard defined by the Third Generation Partnership Project (3GPP (registered trademark)) or a successor standard to 5G, such as sixth generation (6G). The terminal device 30 is assumed to function as user equipment (UE) of the 5G standard or a standard after 5G.

[0025] The control unit 302 includes one or more processors and one or more memories, and controls the overall operation of the terminal device 30. The antenna 303 includes one or more antennas and communicates with the satellite base station 10. The terminal device 30 may also be capable of communicating with a terrestrial base station device (not shown). The antenna 104 includes one or more antennas and communicates with the satellite base station device 10. In one example, the terminal device 30 may be capable of communicating with terrestrial cellular base stations and other terminal devices 30 in addition to the satellite base station device 10.

[0026] <Satellite placement example> 3 shows an example of the arrangement of satellite base station devices 10. The satellite base station devices 10 according to this embodiment are arranged at predetermined intervals so that the cells arranged on the ground are arranged at equal intervals. Note that the cells arranged on the ground may partially overlap, and after a given satellite base station device 10 finishes providing service in a given location range (cell area), another satellite base station device 10 starts providing service in that cell area. In this way, it becomes possible for a terminal device 30 to connect to any of the satellite base station devices 10 in multiple cell areas arranged on the ground.

[0027] Note that, as shown in FIG. 3 , the present embodiment will be described assuming that the cell areas are of equal size. However, this is not limited to this. It is sufficient that the timing at which service provision in a given cell area by one satellite base station device 10 will end can be determined by the satellite base station device 10, the network device 20, and the terminal device 30 that will provide service in that cell area after the satellite base station device 10. For example, the satellite base station device 10, the network device 20, and the terminal device 30 can determine the timing at which the service of the satellite base station device 10 will end by understanding the location information of the cell area and the locations and cruising speeds of the multiple satellite base station devices 10. However, if the cell areas are of equal size and the multiple satellite base station devices 10 cruise at the same cruising speed and at equal intervals, the satellite base station device 10, the network device 20, and the terminal device 30 can predict that the multiple satellite base station devices 10 will end service in the cell area at the same time intervals. Therefore, even without knowing the location of the satellite base station device 10, it is possible to predict the occurrence of a handover due to the movement of the satellite base station device 10 based only on the time elapsed since the previous handover occurred.

[0028] FIG. 4 shows a conceptual diagram of a satellite base station device 10 according to this embodiment providing services in a plurality of adjacent cells while moving.

[0029] At time t1, satellite base station device 10 begins providing service in first cell 401. Then, during the period up to time t1+Δt, beam control or attitude control of satellite base station device 10 is performed to move the directivity of antenna 104 so that it continues to cover cell area 402. Multiple satellite base station devices 10 are placed on a single orbit, and the spacing between satellite base station devices 10 on the same orbit, for example, the distance in the longitude direction, is equal to the length of the cell in the longitude direction. This makes it possible to provide continuous service in the cell area with a small number of satellite base station devices 10.

[0030] <Handover type> In this embodiment, the satellite base station device 10 orbits in a low earth orbit, and therefore the provision of service in the same cell area may be terminated due to the movement of the satellite base station device 10. Also, the terminal device 30 may move and leave the cell area. That is, handover of the terminal device 30 may occur due to the movement of the terminal device 30 itself or the movement of the satellite base station device 10.

[0031] The movement of the satellite base station device 10 may be caused by the rotation of the earth and the movement of the satellite base station device 10 relative to the ground, or by the satellite base station device 10 traveling around the earth.

[0032] For the purpose of explanation, assume that 11 satellite base station devices 10 are arranged in each of six orbital planes. In this case, a 6 × 11 cell arrangement is assumed, with six cells arranged in the longitude direction and 11 cells arranged in the latitude direction. It is also assumed that the satellite base station devices 10 travel in the latitude direction at a speed of 7.5 km / s, as shown in FIG. 3. In this case, assuming that the circumference of the Earth is 40,000 km, the cell served by each satellite base station device 10 is switched every (40,000 / 11) / 7.5 ≒ 485 seconds, i.e., approximately every 8 minutes in the latitude direction (north-south direction). On the other hand, assuming that the rotation speed is 0.4651 km / s, the cell served by each satellite base station device 10 is switched every (40,000 / 6) / 0.4651 ≒ 14,360 seconds in the longitude direction (east-west direction), i.e., approximately every 240 minutes in the longitude direction.

[0033] In this way, if the location range covered by a satellite base station device 10 is predetermined and the moving speed of the satellite base station device 10 is known, it is possible to determine the period during which the satellite base station device 10 will provide service in the cell area. Furthermore, by knowing the location and moving speed of each satellite base station device 10, it is possible to determine which other satellite base station device 10 will provide service in a given cell area after a given satellite base station device 10 has finished providing service in that cell area.

[0034] Therefore, in the wireless communication system 1 of this embodiment, when the satellite base station device 10 moves and ends service in the cell area where it is serving, causing a handover to occur for the terminal device 30, the wireless communication system 1 identifies a new satellite base station device 10 in the cell area that will provide service to the terminal device 30, enabling a fast handover.

[0035] In the following description, the time until the cell area provided by the satellite base station device 10 changes due to the rotation of the Earth is defined as Δtx, and the time until the cell area provided by the satellite base station device 10 changes due to the navigation of the satellite base station device 10 is defined as Δty. In the above example, Δtx = 14,360 seconds and Δty = 485 seconds. However, the above example is not limiting as long as it is possible to specify the period during which the satellite base station device 10 can provide service to a specific cell area by navigating it.

[0036] For example, cells located at higher latitudinal positions are expected to have smaller cell sizes than cells located near the equator, and therefore the satellite base station device 10, network device 20, and terminal device 30 may use different Δtx and Δty depending on the latitude of the cell in which the terminal device 30 is located.

[0037] <Message sequence diagram> Fig. 5 shows a conventional handover process executed in the wireless communication system 1. The process shown in Fig. 5 can be executed when the satellite base station device 10 providing service to the terminal device 30 changes due to movement of the terminal device 30, for example.

[0038] When the terminal device 30 transmits a measurement report indicating that the signal strength of the signal transmitted from the satellite base station device 10 has fallen below a threshold to the satellite base station device (source node) of the handover origin, the terminal device 30 and the satellite base station device 10 determine to execute handover (S501). Note that the terminal device 30 may receive a Conditional Handover (CHO) command in advance from the satellite base station device 10 to which it is connected, and execute autonomous handover of the terminal device 30 to the satellite base station device 10 of the handover destination when predetermined conditions included in the CHO command are satisfied.

[0039] Next, the source satellite base station identifies the destination satellite base station (target node) based on the measurement results of the communication quality with other satellite base stations included in the measurement report and transmits a handover request (S502). The target node performs admission control (S503), and if it determines to accept the handover, transmits an Ack message in response to the handover request (S504).

[0040] Next, the source node sends a handover command to the terminal device 30 using an RRC Reconfiguration message (S505), and sends a sequence number (SN) status transfer (SN Status Transfer) message to the target node to transfer uplink and downlink PDCP (Packet Data Convergence Protocol) SN, hyperframe number (HFN) status, etc. (S506).

[0041] Then, a random access procedure is executed between the terminal device 30 and the target node (S507), and when an RRC Reconfiguration ACK message is transmitted from the terminal device 30 to the target node (S508), the target node transmits a Path Switch Request to an AMF (Access and Mobility Management Function) via the ground station (S509). The AMF transmits a Path Switch Request Transfer message to an SMF (Session Management Function) (S510), and the SMF transmits a Modify Bearer Request to the network device 20 (S511). This enables the network device 20 to determine which terminal device 30 will be handed over to which satellite base station device 10, i.e., to which satellite base station device 10 the session associated with the terminal device 30 to be handed over should be switched.

[0042] When the network device 20 determines whether to switch the session (S512), it transmits a Modify Bearer Response to the SMF (S513), the SMF transmits a Path Switch Request Transfer ACK to the AMF (S514), and the AMF transmits a Path Switch Request ACK to the target node (S515). This allows the target node to know that the settings related to the handover have been completed on the core network 50 side.

[0043] When the target node receives the Path Switch Request ACK, it determines that the configuration on the core network 50 side is complete, and sends a Session Release message to the source node (S516), which completes the handover process by releasing the release for the terminal device 30 being handed over by the source node.

[0044] In conventional handover processing, a delay of about 300 ms occurs in processing on the core network 50 side, and in an environment where handover processing occurs frequently, one of the problems is that the overhead of the processing delay is large. For this reason, the wireless communication system 1 according to this embodiment realizes handover processing with reduced processing delay.

[0045] 6 shows the handover process according to this embodiment. The processes of S602 to S608 are the same as those of S501 to S508, and therefore their explanation will be omitted. Instead of the processes of S509 to S515, the network device 20 according to this embodiment estimates the time when handover will occur in advance, and switches the session associated with the terminal device 30 from the source node to the target node at the estimated time.

[0046] In S601, the network device 20 determines in advance the timing of handover and the target node based on the movement route of the source node and information on the range of locations where the source node provides service. Thereafter, in S610, the session is switched from the source node to the target node at the timing determined in S601. Furthermore, the target node 10 transmits a Session Release message to the source node without transmitting a Path Switch Request at the switching timing (S611). This makes it possible to suppress the processing delay of S509 to S515, thereby reducing the processing delay when executing the handover process.

[0047] The Path Switch Request sent at S509 in Figure 5 includes the RAN-UE-NGAP-ID, AMF-UE-NGAP-ID, user location information, UE security functions, and information indicating the destination of the PDU (Protocol Data Unit) session switching.

[0048] Here, RAN-UE-NGAP-ID and AMF-UE-NGAP-ID are metadata indicating the source and destination. The UE security function does not need to be transmitted if the terminal device inherits the security context during handover. Furthermore, the user location information is information that enables the AMF to determine in which cell area the terminal device 30 is located after handover. If the handover occurs due to the movement of the satellite base station device 10 rather than the movement of the terminal device 30, the cell area in which the terminal device 30 is located remains unchanged, and therefore the location information of the terminal device 30 stored in the AMF does not need to be updated. Furthermore, the PDU session switching destination is included so that the network device 20 can determine with which satellite base station device a PDU session is to be established for a specific terminal device 30. Therefore, if the handover occurs due to the movement of the satellite base station device 10 rather than the movement of the terminal device 30, and the network device 20 can determine with which satellite base station device a PDU session is to be established for a specific terminal device 30, the Path Switch Request can be omitted.

[0049] The following describes the processes executed by the terminal device 30, satellite base station device 10, and network device 20 according to this embodiment.

[0050] <Processing example> Fig. 7 shows an example of processing executed by the terminal device 30 according to this embodiment. The processing shown in Fig. 7 is realized by the processor of the terminal device 30 executing a program stored in memory. Note that the processing shown in Fig. 7 will be described as being executed at predetermined time intervals.

[0051] In S701, the terminal device 30 determines whether a change in cell area has occurred due to movement of the terminal device 30. The determination in S701 can be made based on location information acquired from a GPS (Global Positioning System) sensor (not shown) of the terminal device 30.

[0052] If it is determined that the terminal device 30 has entered a different cell area due to movement of the terminal device 30 (Yes in S701), the terminal device 30 proceeds to the process of S702 and executes handover to the satellite base station device 10 that provides service in the cell area of ​​the destination. The process of S702 is similar to the conventional handover process caused by movement of the terminal device 30, and therefore a description thereof will be omitted.

[0053] If it is determined that the cell area has not changed due to the movement of the terminal device 30 (No in S701), the terminal device 30 proceeds to S703 and determines whether a predetermined time has elapsed since the handover. Specifically, in S703, if the satellite base station device 10 last moved due to the rotation of the Earth and the first time t_lastHO_x that has elapsed since the time the handover occurred exceeds a first threshold (Δtx), it is determined that a first handover will occur due to the movement of the satellite base station device 10 again due to the rotation of the Earth. Furthermore, if the cell area covered by the satellite base station device 10 last changed due to the cruising of the satellite base station device 10 and the second time t_lastHO_y that has elapsed since the time the handover occurred exceeds a second threshold (Δty), it is determined that a second handover will occur due to the cruising of the satellite base station device 10 again.

[0054] If it is determined that the first time has exceeded the first threshold ("first time has elapsed" in S703), the terminal device 30 proceeds to processing at S704, determines to hand over to a satellite base station device 10 located to the east, and identifies the satellite base station device 10 located to the east of the satellite base station device 10 to which it is currently connected.

[0055] If it is determined that the second time has exceeded the second threshold ("second time has elapsed" in S703), the terminal device 30 proceeds to the process at S705 and identifies the direction in which the connected satellite base station device 10 is cruising. If the satellite base station device 10 is a satellite base station device 10 moving north ("northward" in S705), the terminal device 30 determines that the satellite base station device 10 on the same orbit as the connected satellite base station device 10 and cruising south is the target base station device for handover (S706). If the satellite base station device 10 is a satellite base station device 10 moving south ("southward" in S705), the terminal device 30 determines that the satellite base station device 10 on the same orbit as the connected satellite base station device 10 and cruising north is the target satellite base station device for handover (S707).

[0056] Following S704, S706, and S707, the terminal device 30 performs handover to the identified satellite base station device 10 as the handover destination, and updates the neighboring cell information (S708). Then, it updates t_lastHO_x or t_lastHO_y, which was the factor that determined that handover would occur (S709). Then, it updates the timer t_lastHO_x or t_lastHO_y, which was determined that handover would occur, to 0 (S710).

[0057] If it is determined that neither the first nor the second time exceeds the threshold value (No in S703), the terminal device 30 proceeds to S710, adds the elapsed time to t_lastHO_x and t_lastHO_y, and ends the processing shown in FIG. 7.

[0058] Figure 8 shows an example of processing executed by satellite base station device 10. The processing shown in Figure 8 is realized by the processor of satellite base station device 10 executing a program stored in memory. Note that the processing shown in Figure 8 will be explained assuming that it is executed by satellite base station device 10 at predetermined time intervals.

[0059] First, the satellite base station device 10 makes a handover decision for the terminal device 30 connected to itself (S801). As in conventional handover processing, this decision is made to hand over the terminal device 30 connected to itself when certain conditions are met, such as the signal strength of the measurement report of the terminal device 30 becoming less than a threshold value, or a certain time has passed since the terminal device 30 connected to the satellite base station device 10. When the satellite base station device 10 determines that a handover due to movement of the terminal device 30 has occurred for the terminal device 30 connected to itself (Yes in S801), the satellite base station device 10 proceeds to processing in S802, identifies a handover destination for the terminal device 30, and executes conventional handover processing. In S802, as shown in FIG. 5, a Path Switch Request is transmitted to notify the network device 20 that the terminal device 30 is about to perform a handover.

[0060] If it is determined that a handover of the terminal device 30 connected to the satellite base station device 10 will not occur (No in S801), the satellite base station device 10 determines whether a predetermined time has elapsed since the satellite base station device 10 last started providing service (S803). Specifically, if the satellite base station device 10 last moved due to the rotation of the Earth and the first time t_lastHO_x that has elapsed since the time the handover occurred exceeds a first threshold (Δtx), it is determined that a first handover will occur due to the satellite base station device 10 moving again due to the rotation of the Earth. Furthermore, if the cell area covered by the satellite base station device 10 last changed due to the navigation of the satellite base station device 10 and the second time t_lastHO_y that has elapsed since the time the handover occurred exceeds a second threshold (Δty), it is determined that a second handover will occur due to the navigation of the satellite base station device 10.

[0061] If it is determined that the first time period has exceeded the first threshold value ("first time period has elapsed" in S803), the satellite base station device 10 proceeds to processing in S804, determines that it will terminate service in the cell area in which it is currently providing service, and provide service in a cell area located to the west, and specifies that it will move to a cell area located to the west of the satellite base station device 10 to which it is currently connected.

[0062] If it is determined that the second time has exceeded the second threshold ("second time has elapsed" in S803), the satellite base station device 10 proceeds to the process at S805 and determines to terminate the provision of service in the cell area in which it currently provides service. Then, if the satellite base station device 10 is moving north at S805 ("northward" in S805), it determines to move to the cell area served by a satellite base station device 10 on the same orbit that is traveling south (S806). On the other hand, if the satellite base station device 10 is moving south at S805 ("southward" in S805), it determines to move to the cell area served by a satellite base station device 10 on the same orbit that is traveling north (S807). That is, in S804, S806, and S807, the cell area to which the satellite base station device 10 will move and provide service is identified.

[0063] Following S804, S806, and S807, the satellite base station device 10 determines to end service in the cell area it currently provides and to start service in the cell area identified in S804, S806, and S807, and identifies the satellite base station device 10 that provided service in the identified cell area until recently (S808). Then, when an HO Request is received from the satellite base station device 10, a random access is made from the terminal device 30 connected to the satellite base station device 10, and when an RRC Reconfiguration ACK is received, the satellite base station device 10 transmits a Session Release signal to the handover source satellite base station device 10 without transmitting a Path Switch Request (S809). Then, the satellite base station device 10 updates t_lastHO_x or t_lastHO_y, which was the factor that determined that a handover would occur, to 0 (S810).

[0064] If it is determined that neither the first nor the second time exceeds the threshold value (No in S803), the terminal device 30 proceeds to S8101, adds the elapsed time to t_lastHO_x and t_lastHO_y, and ends the processing shown in FIG. 8.

[0065] Fig. 9 shows an example of processing executed by the network device 20. The processing shown in Fig. 9 is realized by the processor of the network device 20 executing a program stored in memory. Note that the processing shown in Fig. 9 will be described as being executed by the network device 20 at predetermined time intervals.

[0066] In S901, the network device 20 determines whether or not a Modify Bearer Request has been received from the SMF, i.e., whether or not a Path Switch Request has been sent by the satellite base station device 10. If a Modify Bearer Request has been received (Yes in S901), the network device 20 proceeds to S902, determines that a handover has occurred due to cell movement of the terminal device 30, and performs downlink path determination and session switching in the same manner as in conventional handover processing.

[0067] If a Modify Bearer Request has not been received (No in S901), the network device 20 determines whether a predetermined time has elapsed since the start of service provision for each satellite base station device 10 (S903). Specifically, for each satellite base station device 10, if the first time t_lastHO_x that has elapsed since the last time a handover occurred when the satellite base station device 10 moved due to the rotation of the Earth exceeds a first threshold (Δtx), the network device 20 determines that a first handover will occur due to the satellite base station device 10 moving again due to the rotation of the Earth. Furthermore, if the cell area covered by the satellite base station device 10 last changes due to the navigation of the satellite base station device 10 and the second time t_lastHO_y that has elapsed since the last time a handover occurred exceeds a second threshold (Δty), the network device 20 determines that a second handover will occur due to the navigation of the satellite base station device 10.

[0068] If it is determined that the first time period has exceeded the first threshold value ("first time period has elapsed" in S903), the network device 20 proceeds to S904, where it determines that the provision of service by the satellite base station device 10 for which the first time period since the start of service provision has exceeded the first threshold value has ended, and that handover will be performed to a satellite base station device 10 located east of the satellite base station device 10. Therefore, the satellite base station device 10 located east of the satellite base station device 10 that is ending service provision is identified as the target node of the handover destination.

[0069] If it is determined that the second time has exceeded the second threshold ("second time has elapsed" in S903), the network device 20 proceeds to S905, determines that the provision of service by the satellite base station device 10 for which the second time since the start of service provision has exceeded the second threshold will end, and identifies the cruising direction of that satellite base station device 10. If the satellite base station device 10 is heading north ("northward" in S905), the network device 20 determines that the satellite base station device 10 on the same orbit as the satellite base station device 10 for which the second time has elapsed since the start of service provision and cruising south is the target base station device for handover (S906). If the satellite base station device 10 is a satellite base station device 10 moving south ("southbound" in S905), the network device 20 determines that the satellite base station device 10 to be handed over is the satellite base station device 10 traveling north and on the same orbit as the satellite base station device 10 for which the second time has elapsed since the start of service provision (S907).

[0070] Following S904, S906, and S907, the network device 20 determines that a handover will occur from the satellite base station device 10 for which a predetermined time has elapsed since the start of service provision to the identified satellite base station device 10 as the handover destination (S908). In S908, the network device 20 identifies the terminal device 30 connected to the satellite base station device 10 for which a predetermined time has elapsed since the start of service provision. Next, the network device 20 switches the session between the network device 20 and the satellite base station device 10 to be identified, associated with the terminal device 30 to be identified, to the identified satellite base station device 10. Then, the network device 20 updates the timer t_lastHO_x or t_lastHO_y for which it has determined that a handover will occur to 0 (S909).

[0071] If it is determined that neither the first nor the second time has elapsed (No in S903), the network device 20 proceeds to S910, adds the elapsed time to t_lastHO_x and t_lastHO_y, and ends the processing shown in FIG.

[0072] In this way, in this embodiment, the satellite base station device 10 determines the timing to end the provision of service, and determines to hand over the terminal device 30 connected to that satellite base station device 10 to another satellite base station device 10 that will provide service after the satellite base station device 10. Therefore, the handover process can be executed with a smaller calculation load than if it were to determine for each terminal device 30 whether or not a handover will occur.

[0073] 10(A) and 10(B), a data structure for managing the association between cell areas and satellite base station devices 10 is shown.

[0074] As shown in Fig. 10(A), an identifier is assigned to each cell area and track plane. Cell areas 1011-1013, 1021-1023, and 1031-1033 are arranged at equal intervals on track planes 1001-1003. Note that in the example of Fig. 10, each of the three track planes will be described as having three cell areas, but the number of track planes and the number of cell areas are not limited to this.

[0075] 10(B), the network device 20 holds a Key-Value Store (KVS) table for each orbit. In the KVS table, an identifier of the satellite base station device 10, such as an IP address, is stored in Value so that the satellite base station device 10 providing service in a specific cell area can be identified using the following position- and time-based function:

[0076] Key ={Loc ID-(Time%T) / Δt}%N Here, N is the number of satellite base station devices per orbital plane, T is the time it takes for a satellite to orbit the Earth, and Loc ID is an identifier assigned to each cell area and is defined so that 1≦Loc ID≦N. Δt=T / N, i.e., the length of time required for a satellite to move over a cell area.

[0077] The KVS table is set so that the satellite base station device 10 providing service at that time and location for each of the above keys is stored as a value. For example, for a cell area with Loc ID=1, it is determined that the satellite base station device 10 with Key=1, i.e., Sat1, is providing service at time 0. Similarly, it can be determined that the satellite base station device 10 with Key=0, i.e., Sat0, is providing service at time Δt. Similarly, it can be determined that the satellite base station device 10 with Key=N-1, i.e., Sat N-1, is providing service at time 2Δt. In this way, by having a KVS table that stores in advance the identifiers of satellite base station devices 10 located in each orbital plane, it is possible to quickly determine the satellite base station device 10 providing service at a given time.

[0078] As described above, in the wireless communication system according to this embodiment, the satellite base station device and the network device individually determine the time when a handover due to movement of the satellite base station device will occur. As a result, the satellite base station device at the handover destination transmits a Session Release message to the satellite base station device at the handover source without transmitting a Path Switch Request. This reduces the processing delay from transmitting a Path Switch Request until receiving a Path Switch Request ACK, enabling high-speed handover.

[0079] Alternatively, in the wireless communication system 1 according to this embodiment, the satellite base station device 10 and the network device 20 can simultaneously determine the handover timing. This prevents a PDU session from remaining on the network device 20 side after transmitting a Session Release message, and prevents downlink information addressed to the terminal device 30 from being transmitted to the source node after handover.

[0080] <Other embodiments> The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0081] 1: wireless communication system, 10: satellite base station equipment, 20: network equipment, 30: terminal equipment, 40: earth station, 50: core network

Claims

1. A wireless communication system including a terminal device, first and second satellite base station devices, and a network device, The network device comprises: a first specifying means for specifying a first time when the first satellite base station device will end providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; a second identification means for identifying a terminal device located within the first location range; a switching means for switching a Protocol Data Unit (PDU) session with the first satellite base station device associated with the terminal device identified by the second identifying means to the second satellite base station device based on the first time identified by the first identifying means without receiving a message transmitted in response to a Path Switch Request; Equipped with the second satellite base station device, a third specifying means for specifying the first time when the first satellite base station device will end providing services within the first location range; a transmitting means for transmitting a Session Release message to the first satellite base station device without transmitting a Path Switch Request message when an RRC Reconfiguration ACK message is received from the terminal device connected to the first satellite base station device at the first time; providing means for providing a service to the terminal device within the first location range after the first time; A wireless communication system comprising:

2. the network device has information capable of identifying the first location range and information capable of identifying the orbits of the first and second satellite base station devices; 2. The wireless communication system according to claim 1, wherein said first specifying means specifies said first time and said second satellite base station device based on said information.

3. the first satellite base station device provides service in a second location range that is at least partially different from the first location range after the first time; The network device comprises: a fourth identification means for identifying a terminal device located within the range of the second location; a second switching means for switching a Protocol Data Unit (PDU) session with the first satellite base station device associated with the terminal device identified by the fourth identifying means to the first satellite base station device based on the first time identified by the first identifying means without receiving a message transmitted in response to a Path Switch Request; 2. The wireless communication system of claim 1, further comprising:

4. the first and second satellite base station devices travel on the same orbit; The wireless communication system according to claim 1, characterized in that the first identification means identifies the second satellite base station device based on a database that stores identifiers of multiple location ranges including the first location range and identifiers of satellite base station devices that provide services in each location range in association with each other.

5. A network device of a wireless communication system including a terminal device and first and second satellite base station devices, a first specifying means for specifying a first time when the first satellite base station device will end providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; a second identification means for identifying a terminal device located within the first location range; a switching means for switching a Protocol Data Unit (PDU) session with the first satellite base station device associated with the terminal device identified by the second identifying means to the second satellite base station device based on the first time identified by the first identifying means without receiving a message transmitted in response to a Path Switch Request; A network device comprising:

6. A satellite base station device, a specifying means for specifying a first time when the other satellite base station device will end providing services within the first location range; a transmitting means for transmitting a Session Release message to the other satellite base station device without transmitting a Path Switch Request message when an RRC Reconfiguration ACK message is received from a terminal device connected to the other satellite base station device at the first time; providing means for providing a service to the terminal device within the first location range after the first time; A satellite base station device comprising:

7. A control method executed by a wireless communication system including a terminal device, first and second satellite base station devices, and a network device, comprising: the network device identifying a first time when the first satellite base station device will cease providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; the network device identifying a terminal device located within the first location; The network device switches a Protocol Data Unit (PDU) session with the first satellite base station device associated with the identified terminal device to the second satellite base station device based on the identified first time without receiving a message transmitted in response to a Path Switch Request; the second satellite base station device identifying the first time when the first satellite base station device will end providing service within the first location range; transmitting a Session Release message to the first satellite base station device without transmitting a Path Switch Request message when the second satellite base station device receives an RRC Reconfiguration ACK message from the terminal device connected to the first satellite base station device at the first time; A control method comprising:

8. A control method executed by a network device in a wireless communication system including a terminal device and first and second satellite base station devices, comprising: Identifying a first time when the first satellite base station device will stop providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; identifying a terminal device located within the first location range; Switching a Protocol Data Unit (PDU) session with the first satellite base station device associated with the identified terminal device to the second satellite base station device based on the identified first time without receiving a message transmitted in response to a Path Switch Request; A control method comprising:

9. A control method executed by a satellite base station device, comprising: Identifying a first time when the other satellite base station device will end providing service within the first location range; transmitting a Session Release message to the other satellite base station device without transmitting a Path Switch Request message when receiving an RRC Reconfiguration ACK message from the terminal device connected to the other satellite base station device at the first time; providing service to the terminal device within the first location range after the first time; A control method comprising:

10. A computer of a wireless communication system including a terminal device, first and second satellite base station devices, and a network device, a first identification step in which the network device identifies a first time when the first satellite base station device will terminate providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; a second identification step in which the network device identifies a terminal device located within the first location range; a switching step in which the network device switches a Protocol Data Unit (PDU) session with the first satellite base station device associated with the terminal device identified in the second identification step to the second satellite base station device based on the first time identified in the first identification step without receiving a message transmitted in response to a Path Switch Request; a third specifying step in which the second satellite base station device specifies the first time when the first satellite base station device will end providing service within the first location range; a transmission step of transmitting a Session Release message to the first satellite base station device without transmitting a Path Switch Request message when the second satellite base station device receives an RRC Reconfiguration ACK message from the terminal device connected to the first satellite base station device at the first time; a providing step of the second satellite base station device providing a service to the terminal device within the first location range after the first time; A program for causing a control method including:

11. A computer of a network device of a wireless communication system including a terminal device and first and second satellite base station devices, a first identifying step of identifying a first time when the first satellite base station device will end providing service in a first location range, and a second satellite base station device that will provide service in the first location range after the first time; a second identification step of identifying a terminal device located within the range of the first location; a switching step of switching a Protocol Data Unit (PDU) session with the first satellite base station device associated with the terminal device identified in the second identification step to the second satellite base station device based on the first time identified in the first identification step without receiving a message transmitted in response to a Path Switch Request; A program for causing a control method including:

12. The satellite base station equipment computer a step of identifying a first time when the other satellite base station device will end providing service within the first location range; a transmitting step of transmitting a Session Release message to the other satellite base station device without transmitting a Path Switch Request message when an RRC Reconfiguration ACK message is received from the terminal device connected to the other satellite base station device at the first time; providing a service to the terminal device within the first location range after the first time; A program for causing a control method including:

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