Communication control method and user device

Abstract

A communication control method according to one aspect of the present invention is a communication control method for a mobile communication system. The communication control method comprises a step in which user equipment changes a frequency priority of a frequency used in a cell reselection procedure, depending on the altitude of the user equipment.

Description

Communication Control Method

[0001] The present disclosure relates to a communication control method in a mobile communication system.

[0002] The specifications of the Third Generation Partnership Project (3GPP), a standardization project for mobile communication systems, define an aerial UE (Aerial UE) (see, for example, Non-Patent Document 1 and Non-Patent Document 2). For example, an Aerial UE can report its altitude and its location information including its vertical and horizontal speeds. Through these specifications, 3GPP appropriately supports communication with an Aerial UE flying in the sky.

[0003] 3GPP TS 36.300 V17.1.0 (2022-6)3GPP TS 36.331 V17.1.0 (2022-6)

[0004] According to one aspect, there is provided a communication control method in a mobile communication system, the communication control method including a step of changing, by a user equipment, frequency priorities of frequencies used in a cell reselection procedure in accordance with an altitude of the user equipment.

[0005] According to one aspect, there is provided a communication control method in a mobile communication system, the communication control method including a step of transmitting, by a network node (or a network device), a tracking area code that identifies a tracking area formed at an altitude equal to or greater than an altitude threshold.

[0006] FIG. 1 is a diagram illustrating an example of the configuration of a mobile communication system according to the first embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a UE (user equipment) according to the first embodiment. FIG. 3 is a diagram illustrating an example of the configuration of a gNB (base station) according to the first embodiment. FIG. 4 is a diagram illustrating an example of the configuration of a protocol stack related to a user plane according to the first embodiment. FIG. 5 is a diagram illustrating an example of the configuration of a protocol stack related to a control plane according to the first embodiment. FIG. 6 is a diagram illustrating an example of a cell configuration according to the first embodiment. FIG. 7 is a diagram illustrating an example of an operation according to the first embodiment. FIG. 8 is a diagram illustrating an example of an operation according to the second embodiment. FIG. 9 is a diagram illustrating an example of an operation according to the third embodiment. FIG. 10 is a diagram illustrating an example of an operation according to the fourth embodiment.

[0007] A mobile communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0008] [First embodiment]

[0009] (Configuration of mobile communication system) Fig. 1 is a diagram showing the configuration of a mobile communication system according to the first embodiment. The mobile communication system 1 conforms to the 5th Generation System (5GS) of the 3GPP standard. Although the following description will be given using 5GS as an example, the mobile communication system may also be at least partially applied to an LTE (Long Term Evolution) system. The mobile communication system may also be at least partially applied to a 6th Generation (6G) system.

[0010] The mobile communication system 1 includes a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. Hereinafter, the NG-RAN 10 may be simply referred to as the RAN 10. Furthermore, the 5GC 20 may be simply referred to as the core network (CN) 20.

[0011] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user. For example, the UE 100 may be a mobile phone terminal (including a smartphone) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or a chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE).

[0012] The NG-RAN 10 includes a base station (called a "gNB" in a 5G system) 200. The gNBs 200 are connected to each other via an Xn interface, which is an interface between base stations. The gNB 200 manages one or more cells. The gNB 200 performs wireless communication with a UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, and the like. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource for wireless communication with the UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0013] In addition, the gNB 200 can also be connected to the EPC (Evolved Packet Core), which is the LTE core network. An LTE base station (eNB: evolved Node B) can also be connected to the 5GC 20. The LTE base station and the gNB 200 can also be connected via an inter-base station interface.

[0014] The 5GC20 includes an Access and Mobility Management Function (AMF) and a User Plane Function (UPF) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and the UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0015] 2 is a diagram illustrating an example of the configuration of a UE 100 (user equipment) according to the first embodiment. The UE 100 includes a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 constitute a wireless communication unit that performs wireless communication with the gNB 200.

[0016] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0017] The transmitting unit 120 performs various transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0018] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processes by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation, encoding / decoding, etc. of baseband signals. The CPU executes programs stored in the memory to perform various processes. Note that the control unit 130 may perform each process or operation in the UE 100 in each of the embodiments described below.

[0019] 3 is a diagram showing the configuration of a gNB 200 (base station) according to the first embodiment. The gNB 200 includes a transmitter 210, a receiver 220, a controller 230, and a backhaul communication unit 240. The transmitter 210 and the receiver 220 constitute a wireless communication unit that performs wireless communication with the UE 100. The backhaul communication unit 240 constitutes a network communication unit that communicates with the CN 20.

[0020] The transmitting unit 210 performs various transmissions under the control of the control unit 230. The transmitting unit 210 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

[0021] The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0022] The control unit 230 performs various controls and processes in the gNB 200. Such processes include processes for each layer described below. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processes by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation, encoding / decoding, etc. of baseband signals. The CPU executes programs stored in the memory to perform various processes. Note that the control unit 230 may perform each process or operation in the gNB 200 in each of the embodiments described below.

[0023] The backhaul communication unit 240 is connected to adjacent base stations via an Xn interface, which is an interface between base stations. The backhaul communication unit 240 is connected to the AMF / UPF 300 via an NG interface, which is an interface between a base station and a core network. Note that the gNB 200 is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected by an F1 interface, which is a fronthaul interface.

[0024] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0025] The user plane radio interface protocol includes a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a service data adaptation protocol (SDAP) layer.

[0026] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on a physical downlink control channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using a radio network temporary identifier (RNTI) and acquires the successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC parity bit scrambled by the RNTI added.

[0027] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of the UE 100 and the MAC layer of the gNB 200 via a transport channel. The MAC layer of the gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE 100.

[0028] The RLC layer transmits data to the receiving RLC layer using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via a logical channel.

[0029] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0030] The SDAP layer maps IP flows, which are units for Quality of Service (QoS) control by the core network, to radio bearers, which are units for QoS control by the Access Stratum (AS). Note that if the RAN is connected to the EPC, SDAP may not be required.

[0031] FIG. 5 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals).

[0032] The protocol stack of the radio interface of the control plane has a Radio Resource Control (RRC) layer and a Non-Access Stratum (NAS) instead of the SDAP layer shown in FIG.

[0033] RRC signaling for various settings is transmitted between the RRC layer of UE100 and the RRC layer of gNB200. The RRC layer controls logical channels, transport channels, and physical channels according to the establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC idle state. When the connection between the RRC of UE100 and the RRC of gNB200 is suspended, UE100 is in an RRC inactive state.

[0034] The NAS, which is located above the RRC layer, performs session management, mobility management, etc. NAS signaling is transmitted between the NAS of the UE 100 and the NAS of the AMF 300. Note that the UE 100 has an application layer and the like in addition to the radio interface protocol. Also, the layer below the NAS is called an Access Stratum (AS).

[0035] (UAV) Here, an unmanned aerial vehicle (UAV: Unmanned Aerial Vehicle or Uncrewed Aerial Vehicle; hereinafter, "unmanned aerial vehicle" may be referred to as "UAV") according to the first embodiment will be described.

[0036] A UAV generally refers to an unmanned aerial vehicle such as a drone. However, in the first embodiment, a UE located at an altitude equal to or greater than a predetermined threshold (or exceeding the predetermined threshold) is referred to as a UAV. A UAV may be a UE capable of wireless communication with a gNB 200 while flying unmanned in the sky, like an unmanned aerial vehicle. Alternatively, a UAV may be provided on an unmanned aerial vehicle. Alternatively, a UAV may be provided on a manned aerial vehicle. For example, when an airplane is flying at an altitude equal to or greater than a predetermined threshold, a UE owned by a user on board the airplane may also be a UAV. A UAV may be a UAV UE. Alternatively, a UAV may be an aerial UE (aerial UE). A UAV may be used to distinguish it from a UE used on the ground. However, when there is no particular distinction between the UE and the UAV, a UAV may be included in the UE as an example of a UE. In this case, a UAV and a UE may be collectively referred to as a UE. The configuration example of UE 100 shown in Figure 2 may represent a configuration example of a UAV.

[0037] In 3GPP, the following specifications are provided as functions to support an aerial UE:

[0038] First, the flying UE can report its altitude. For example, the flying UE can report its altitude when its altitude is above or below a threshold. At this time, the flying UE can also report its location information. The location information can include the horizontal and vertical speed of the flying UE.

[0039] Second, the LTE system network (E-UTRAN) can request the flying UE to report flight route information. The flight route information represents waypoints (passing point information or point information) on the route of the flying UE. The flight route information may include multiple waypoints. The waypoints are represented as three-dimensional position information. The flying UE may report time information (timestamp) for each waypoint by including it in the flight route information.

[0040] Third, whether or not a flying UE is supported (or whether or not it is permitted to function as a flying UE) is included in the subscription information for each user. A Home Subscriber Server (HSS) in an LTE system stores subscriber information for each user. Whether or not a flying UE is supported is included in the subscription information. The subscription information is transmitted from the HSS to an eNB, which is a base station in the LTE system, under the control of a Mobility Management Entity (MME). The eNB can determine whether or not the UE is permitted to function as a flying UE.

[0041] Fourth, the event H1 and the event H2 can be used as trigger conditions for a measurement report. The event H1 represents an event condition when the altitude of the flying UE exceeds a threshold. Meanwhile, the event H2 represents an event condition when the altitude of the flying UE falls below the threshold. These event conditions are determined to be satisfied using a hysteresis value, an offset value, and a threshold value in addition to the altitude.

[0042] The 3GPP specifications are based on the assumption that flying UEs (i.e., UAVs) will be used in the LTE system.

[0043] Meanwhile, 3GPP has begun discussions on introducing UAVs into NR (New Radio). Regarding UAVs, 3GPP has agreed to use the above-mentioned events H1 and H2, to report the altitude, position, and speed of the UAV, and to report the flight path plan.

[0044] (Terrestrial Cells and Aerial Cells) For example, assume that terrestrial cells and aerial cells coexist within a network. Fig. 6 is a diagram showing an example of a cell configuration in such a case.

[0045] As shown in Figure 6, the mobile communication system 1 includes a ground cell and an air cell. In the example shown in Figure 6, a ground cell is formed by gNB200-T1 and gNB200-T2, and an air cell is formed by gNB200-U. In Figure 6, in the ground cell, UE100-1 to 100-4 perform wireless communication with gNB200-T1 and 200-T2, and in the air cell, UAV150-1 and 150-2 perform wireless communication with gNB200-U.

[0046] Here, in order for UEs 100-1 to 100-4 to perform appropriate wireless communication in the ground cell and for UAVs 150-1 and 150-2 to perform appropriate wireless communication in the air cell, the following two scenarios are assumed.

[0047] In the first scenario, a dedicated frequency is assigned to the air cell, and different frequencies are used for the ground cell and the air cell. In the first scenario, for example, wireless communication by UAVs 150-1 and 150-2 and wireless communication by UEs 100-1 to 100-4 are performed using different frequencies, so that interference between the two wireless communications can be avoided.

[0048] On the other hand, the second scenario is a scenario in which the same frequency (or the same frequency range) is used in the terrestrial cell and the aerial cell. In the second scenario, since the terrestrial cell and the aerial cell share the same frequency, there is no need to increase frequency resources. Therefore, in the second scenario, frequency resources can be used more effectively.

[0049] (Communication Control Method According to First Embodiment) In the first embodiment, a case where the UE 100 performs cell reselection in the sky will be described. Here, general (or legacy) cell reselection will be described.

[0050] Cell reselection is performed when a UE 100 in an RRC idle state or an RRC inactive state moves from a serving cell to a neighboring cell. Specifically, the UE 100 identifies a neighboring cell on which the UE 100 should camp by a cell reselection procedure, and reselects the identified neighboring cell. The serving cell and the neighboring cell may be managed by the same gNB 200. The UE 100 may be managed by different gNBs 200. The cell reselection procedure is, for example, as follows.

[0051] First, the UE 100 performs frequency prioritization processing based on the priority for each frequency specified by the gNB 200, for example, by an RRC release message. The lower the frequency priority, the higher the priority. However, conversely, the lower the number, the lower the priority.

[0052] Second, UE 100 measures the radio quality of each of the serving cell and the neighboring cell. Specifically, the following applies. That is, UE 100 measures the received power and reception quality (i.e., radio quality) of reference signals (e.g., CD-SSB (Cell Defining-Synchronization Signal and PBCH block)) transmitted by each of the serving cell and the neighboring cell. UE 100 always measures the radio quality for frequencies having a higher priority than the priority of the frequency of the current serving cell. On the other hand, for frequencies having a priority equal to or lower than the priority of the frequency of the current serving cell, UE 100 measures the radio quality of the frequency having the same priority or a lower priority when the radio quality of the current serving cell falls below a predetermined quality.

[0053] Third, UE 100 reselects a cell on which it camps based on the measurement result. Specifically, the following applies. That is, when the frequency priority of a neighboring cell is higher than the priority of a current serving cell, and the neighboring cell satisfies a predetermined quality standard (i.e., a minimum required quality standard) for a predetermined period, UE 100 may perform cell reselection to the neighboring cell. When the frequency priority of the neighboring cell is the same as the priority of the current serving cell, UE 100 may rank the radio qualities of the neighboring cells and perform cell reselection to a neighboring cell having a higher rank than the rank of the current serving cell for a predetermined period. When the frequency priority of the neighboring cell is lower than the priority of the current serving cell, and when a state in which the radio quality of the current serving cell is lower than a certain threshold and the radio quality of the neighboring cell is higher than another threshold continues for a predetermined period, UE 100 may perform cell reselection to the neighboring cell.

[0054] This is an overview of the general cell reselection procedure.

[0055] Here, consider a case where air cells and ground cells coexist in a network, as shown in Fig. 6. In such a case, when UE 100 is located in the air (i.e., UAV 150), it is desirable to reselect an air cell in the cell reselection procedure. On the other hand, when UE 100 is located on the ground (i.e., when UE 100 is located on the ground as a normal UE 100), it is desirable to reselect a ground cell in the cell reselection procedure.

[0056] However, there is currently no mechanism in 3GPP for the UE 100 (or UAV 150) to reselect between a ground cell and an air cell.

[0057] Therefore, in the first embodiment, an object is to enable the UE 100 to reselect an air cell and a ground cell separately, so that the UE 100 can appropriately perform communication even in the air.

[0058] It should be noted that 3GPP specifies a high speed dedicated network (HSDN) (for example, 3GPP TS 38.304 V17.1.0 (2022-06)). In the HSDN, a cell called an HSDN cell exists. A UE 100 in a high-mobility state can regard the HSDN cell as the highest priority cell. On the other hand, a UE 100 that is not in a high-mobility state can regard the HSDN cell as the lowest priority cell. For example, a UE 100 moving along a high-speed train can easily reselect an HSDN cell installed along the tracks in preference to other cells. Therefore, the HSDN can appropriately support communication for a UE in a high-mobility state.

[0059] However, the HSDN is designed to accommodate the UE 100 moving along the ground, and is therefore difficult to apply to the UAV 150, which moves in the altitude direction (i.e., moves up and down).

[0060] In the first embodiment, a user equipment (e.g., UE 100) changes the frequency priority of a frequency used in a cell reselection procedure according to the altitude of the user equipment. Therefore, for example, when the altitude of the UE 100 is equal to or greater than a predetermined threshold (or an altitude threshold) (i.e., when the UE 100 is located in the sky), the frequency priority of the airspace frequency can be set to the highest priority. Furthermore, for example, when the altitude of the UE 100 is less than the predetermined threshold (i.e., when the UE 100 is located on the ground), the frequency priority of the airspace frequency can be set to the lowest priority. Here, the airspace frequency is, for example, a frequency that the UE 100 can use at an altitude equal to or greater than the predetermined threshold. As a result, for example, when the UE 100 is located in the sky, the UE 100 is more likely to reselect a neighboring cell (airspace cell) that supports the airspace frequency in the cell reselection procedure than other cells. Conversely, for a UE 100 located on the ground, the frequency has the lowest priority, making it more difficult for the UE 100 to reselect the airspace cell compared to other cells. That is, the UE 100 located in the sky is more likely to reselect an air cell than a ground cell, and the UE 100 located on the ground is more likely to reselect a ground cell than an air cell. Therefore, the UE 100 can appropriately communicate in the sky.

[0061] (Example of operation according to the first embodiment) Fig. 7 is a diagram illustrating an example of operation according to the first embodiment. It is assumed that the UE 100 is in an RRC connected state with respect to the serving cell before starting the operation.

[0062] In step S10, the gNB 200 transmits altitude threshold information indicating an altitude threshold (hereinafter, sometimes referred to as "altitude threshold"). The altitude threshold information may be transmitted by broadcast using system information (SIB). The altitude threshold information may be transmitted to the UE 100 using a dedicated message (e.g., an RRC Release message). The altitude threshold may be a predetermined threshold.

[0063] In step S11, the gNB 200 may transmit airspace frequency information indicating an airspace frequency. The airspace frequency may be a frequency available to the UE 100 (i.e., the UAV 150) flying at an altitude equal to or higher than the altitude threshold. The airspace frequency may be an adjacent frequency available in an adjacent cell (airspace cell). The airspace frequency information may include a list of multiple airspace frequencies. In this case, each entry in the list may include an identifier indicating that the frequency is for airspace use. The gNB 200 may also transmit airspace cell information indicating a cell (i.e., an airspace cell) that supports the airspace frequency (or an adjacent cell that supports the airspace frequency). The airspace cell may be a cell formed at an altitude equal to or higher than the altitude threshold. Alternatively, the airspace cell may be a cell that provides coverage formed at an altitude equal to or higher than the altitude threshold. Alternatively, the airspace cell may be a cell that can communicate with the UE 100 (i.e., the UAV 150) flying at an altitude equal to or higher than the altitude threshold. Alternatively, the airspace cell may be a cell optimized for communication with the UE 100 (i.e., the UAV 150) flying at an altitude equal to or higher than an altitude threshold. The airspace cell information may include a cell ID of the airspace cell. Similarly, the airspace cell information may include a list of multiple airspace cells and an identifier indicating that the cell is for airspace use. The gNB 200 may transmit at least one of airspace frequency information and airspace cell information. The airspace frequency information and the airspace cell information may be transmitted separately or may be transmitted as a single piece of information. The airspace frequency information and the airspace cell information may be transmitted by broadcast using system information (SIB), or may be transmitted to the UE 100 using a dedicated message (e.g., an RRC release message).

[0064] In step S12, the gNB 200 may notify that it is capable of communication in the sky. Specifically, the gNB 200 may transmit sky coverage communication availability information indicating that communication is possible for the UE 100 having an altitude equal to or greater than the altitude threshold. Alternatively, the gNB 200 may transmit sky coverage communication availability information indicating whether communication is possible for the UE 100 having an altitude equal to or greater than the altitude threshold. Alternatively, the gNB 200 may transmit sky coverage communication availability information indicating whether coverage is provided for the UE 100 having an altitude equal to or greater than the altitude threshold. The sky coverage communication availability information (and sky coverage communication availability information) may be broadcast using system information (SIB). The information may also be transmitted to the UE 100 using a dedicated message (e.g., an RRC release message). When receiving the aerial coverage communication availability information from the serving cell, the UE 100 may recognize that the frequency used in communication with the serving cell is an aerial frequency.

[0065] The UE 100 may grasp the air frequency (and / or the air cell) and the terrestrial frequency (and / or the terrestrial cell) through steps S11 and S12.

[0066] In step S13, the UE 100 transitions from the RRC connected state to the RRC idle state or the RRC inactive state.

[0067] In step S14, the UE 100 executes a cell reselection procedure. For example, the UE 100 performs the following process.

[0068] First, the UE 100 determines the altitude of the UE 100 based on an altitude threshold (step S10). For example, when the UE 100's altitude is equal to or greater than the altitude threshold (or exceeds the altitude threshold), the UE 100 determines that the UE 100 is in the "sky (or high altitude)", and when the UE 100's altitude is less than the altitude threshold (or is equal to or less than the altitude threshold), the UE 100 determines that the UE 100 is on the "ground (or low altitude)". The altitude of the UE 100 may be measured by an altitude sensor provided in the UE 100. The altitude of the UE 100 may be measured by a distance sensor (such as a radar or a lidar) provided in the UE 100. Note that the altitude may be expressed in terms of above sea level. The altitude may be expressed in terms of altitude. The altitude may be expressed in terms of height above ground.

[0069] Second, the UE 100 changes the frequency priority of the frequency used in cell reselection according to the altitude. For example, when the UE 100's altitude is "in the sky", the UE 100 changes the air frequency (step S11) to the highest priority. Alternatively, when the UE 100's altitude is "in the sky", the UE 100 may change the air cell (step S11) that supports the air frequency to the highest priority. On the other hand, when the UE 100's altitude is "on the ground", the UE 100 changes the air frequency (step S11) to the lowest priority. Alternatively, when the UE 100's altitude is "on the ground", the UE 100 may change the air cell (step S11) that supports the air frequency to the lowest priority.

[0070] Third, the UE 100 performs a cell reselection procedure using a frequency (or cell) whose frequency priority has been changed according to the altitude.

[0071] As a result, when the UE 100 is located in the "sky", it can execute a cell reselection procedure by regarding the sky frequency as the highest priority. The UE 100 located in the "sky" may execute a cell reselection procedure by regarding the sky cell as the highest priority. That is, the UE 100 can execute a cell reselection procedure by regarding at least one of the sky frequency and the sky cell as the highest priority. Therefore, in the cell reselection procedure, the UE 100 can more easily reselect an sky cell that supports the sky frequency than other neighboring cells (terrestrial cells), and can more easily camp on the sky cell.

[0072] On the other hand, when the UE 100 is located "on the ground," it can execute a cell reselection procedure by regarding the aerial frequency as the lowest priority. The UE 100 located "on the ground" may execute a cell reselection procedure by regarding the aerial cell as the lowest priority. That is, the UE 100 can execute a cell reselection procedure by regarding at least one of the aerial frequency and the aerial cell as the lowest priority. Therefore, in the cell reselection procedure, the UE 100 can more easily reselect another neighboring cell (a terrestrial cell) than the aerial cell, and can more easily camp on the terrestrial cell.

[0073] Second Embodiment Next, a second embodiment will be described, focusing mainly on the differences from the first embodiment.

[0074] In the first embodiment, a case where two layers, "ground" and "altitude", exist for altitude has been described, but this is not limiting. In the second embodiment, a case where altitude exists in three or more layers will be described. Even in such a case, when the UE 100 is located in each of the three or more layers (or each altitude), the frequency available in each layer can be regarded as the highest frequency priority, and a cell reselection procedure can be performed.

[0075] As a result, when the UE 100 executes a cell reselection procedure while located in each layer, the UE 100 can easily reselect a cell that supports a frequency corresponding to each altitude and camp on the cell. Therefore, the UE 100 can appropriately communicate with the airspace cell using the airspace frequency.

[0076] (Operation example according to second embodiment) Fig. 8 is a diagram illustrating an operation example according to the second embodiment. In Fig. 8 as well, it is assumed that the UE 100 is in an RRC connected state with respect to the serving cell before the start of the operation.

[0077] As shown in FIG. 8, in step S20, the gNB 200 may transmit multiple pieces of altitude threshold information. For example, if the gNB 200 transmits three pieces of altitude threshold information (altitude threshold A1, altitude threshold A2, and altitude threshold A3), four layers can be configured: altitudes below the altitude threshold A1, altitudes from above the altitude threshold A1 to below the altitude threshold A2, altitudes from above the altitude threshold A2 to below the altitude threshold A3, and altitudes above the altitude threshold A3. If the gNB 200 transmits two pieces of altitude threshold information, three layers of altitude can be configured, and if the gNB 200 transmits four pieces of altitude threshold information, five layers of altitude can be configured. The gNB 200 may transmit multiple pieces of altitude threshold information according to the number of altitude layers. The altitude threshold information may represent an altitude range. For example, in the case of four layers, "-∞ to A1," "A1 to A2," "A2 to A3," and "A3 to ∞" may be represented as multiple pieces of altitude threshold information. Note that multiple altitude thresholds may be included in one piece of altitude threshold information in list format.

[0078] In step S21, the gNB 200 may transmit information linking each altitude threshold to a frequency (or a cell supporting the frequency) available at each altitude. That is, the gNB 200 may transmit information linking each of a plurality of altitude thresholds to a frequency available at each altitude and a cell supporting the frequency. For example, information linking the altitude threshold A1 to a frequency f1 (or a cell #1 supporting the frequency f1) available at an altitude below the altitude threshold A1 is transmitted, and information linking the altitude threshold A2 to a frequency f2 (or a cell #2 supporting the frequency f2) available at an altitude above the altitude threshold A1 and below the altitude threshold A2 is transmitted. In order for the gNB 200 to transmit such linking information, the gNB 200 may transmit aerial frequency information (step S11 in FIG. 7) as in the first embodiment. In this case, the aerial frequency information may be linked to an altitude threshold in the entry for each aerial frequency (and / or aerial cell). The aerial frequency information may include frequency information on terrestrial frequencies (or terrestrial cells) other than the aerial frequencies (or aerial cells) as adjacent frequency information, and in this case, may also be linked to altitude threshold information. The aerial frequency information may include linking information for linking the frequency (and / or cell) with the altitude threshold information. The gNB200 may transmit the linking information included in the aerial frequency information. Note that the linking information may be transmitted using system information (SIB). The linking information may also be transmitted using dedicated signaling (for example, an RRC release message).

[0079] In step S22, the gNB200 may transmit altitude range information regarding the altitude range that can be supported by itself (or the serving cell). The gNB200 may transmit the altitude range information using system information (SIB). The gNB200 may transmit the altitude range information using dedicated signaling (e.g., an RRC release message).

[0080] In step S23, the UE 100 transitions to an RRC idle state or an RRC inactive state.

[0081] In step S24, the UE 100 determines its own altitude, determines frequency priority based on the determined altitude, and executes a cell reselection procedure. For example, the UE 100 performs the following process.

[0082] First, the UE 100 determines the altitude of the UE 100 based on a plurality of pieces of altitude threshold information (step S20). The UE 100 determines at which altitude of each of the three or more layers the UE 100 is located based on its own altitude measured by a sensor and each piece of altitude threshold information. In this case, the UE 100 may represent its own altitude as an altitude state. For example, when the UE 100's altitude is less than the altitude threshold A1, the UE 100 may determine the altitude state as "ground," when the UE 100's altitude is equal to or greater than the altitude threshold A1 but less than the altitude threshold A2, the UE 100 may determine the altitude state as "low altitude," when the UE 100's altitude is equal to or greater than the altitude threshold A2 but less than the altitude threshold A3, the UE 100 may determine the altitude state as "medium altitude," and when the UE 100's altitude is equal to or greater than the altitude threshold A3, the UE 100 may determine the altitude state as "high altitude."

[0083] Second, the UE 100 changes the corresponding frequency (or cell) to the highest priority according to its own altitude. For example, when the UE 100's altitude is less than the altitude threshold A1, the UE 100 changes the frequency available below the altitude threshold A1 to the highest frequency priority, and when the UE 100's altitude is greater than or equal to the altitude threshold A1 but less than the altitude threshold A2, the UE 100 changes the frequency available between the altitude threshold A1 and less than the altitude threshold A2 to the highest frequency priority. When the relationship between the altitude threshold and the frequency available at each altitude is transmitted to the UE 100 as association information (step S21), the UE 100 can use the association information to grasp the altitude threshold used when determining its own altitude and the frequency associated with the altitude threshold. Furthermore, when the air frequency information is transmitted, the relationship between each frequency and the altitude threshold is indicated in each list, so the UE 100 can use the air frequency information to grasp the altitude threshold used for altitude determination and the frequency corresponding to the altitude threshold. Then, the UE 100 changes the frequency to the highest priority.

[0084] Third, the UE 100 performs a cell reselection procedure using the frequency (or cell) whose frequency priority has been changed.

[0085] As a result, the UE 100 can perform a cell reselection procedure by regarding a frequency available at the altitude at which the UE 100 is located as the highest priority, depending on the altitude at which the UE 100 is located. The UE 100 can reselect a cell according to each altitude and communicate with the cell.

[0086] Third Embodiment Next, a third embodiment will be described, focusing on the differences from the first embodiment.

[0087] In the second embodiment, an example in which linking information between altitude (or altitude threshold) and frequency is transmitted is described, but in the third embodiment, an example in which linking information between altitude (or altitude threshold) and frequency priority is transmitted is described.

[0088] Specifically, first, a base station (e.g., gNB200) transmits a frequency priority according to the altitude of a user equipment (e.g., UE100). Second, the user equipment applies the frequency priority according to the altitude of the user equipment to a frequency (e.g., a frequency used in a cell reselection procedure). This allows, for example, the gNB200 to set the frequency priority used for an airspace frequency to the highest priority. Therefore, the UE100 can more easily reselect an airspace cell that supports an airspace frequency in the cell reselection procedure, and can appropriately communicate with the airspace cell in the airspace.

[0089] (Operation example according to third embodiment) Fig. 9 is a diagram illustrating an operation example according to the third embodiment. In Fig. 9 as well, it is assumed that the UE 100 is in an RRC connected state with respect to the serving cell before the start of the operation.

[0090] As shown in Figure 9, in step S30, the gNB200 transmits altitude threshold information. The altitude threshold information may be an altitude threshold that distinguishes between "ground" and "sky", as in the first embodiment. The altitude threshold information may represent each range of "ground" and "sky". Alternatively, as in the second embodiment, multiple altitude threshold information may be transmitted.

[0091] In step S31, the gNB 200 transmits advanced frequency priority information representing frequency priority according to altitude (i.e., frequency priority for each altitude) for the frequency used in cell reselection. The advanced frequency priority information may be broadcast using system information (SIB). The advanced frequency priority information may be transmitted to the UE 100 using an individual message (e.g., an RRC release message). The frequency priority for each altitude may include a frequency priority according to altitude for each frequency as follows:

[0092] (X1) Frequency f1: If it is "ground", the frequency priority is "7", and if it is "sky", the frequency priority is "0".

[0093] (X2) Frequency f2: For "ground", the frequency priority is "1", and for "sky", the frequency priority is "6". Alternatively, the frequency priority for each altitude may include the frequency priority of each frequency for each altitude, as follows:

[0094] (Y1) "Terrestrial": The frequency priority of frequency f1 is "7", and the frequency priority of frequency f2 is "1"

[0095] (Y2) "Above the sky": The frequency priority of frequency f1 is "0", and the frequency priority of frequency f2 is "6".

[0096] Although the example of the altitude state has been described using two layers, "on the ground" and "in the sky," three layers (e.g., "on the ground," "low altitude," or "high altitude") or more may be used as in the second embodiment. In this case, as in the case of two layers, frequency priority for each layer (or for each altitude) may be included in the advanced frequency priority information for each frequency (or the frequency priority of each frequency for each layer may be included in the advanced frequency priority information). In the above example, an example in which the altitude is represented by the altitude state of the UE 100 (e.g., "on the ground" or "in the sky") has been described, but this is not limiting. The altitude may be represented by an altitude range (e.g., "-∞ to A" or "A to ∞"). Alternatively, the altitude may be represented by an altitude threshold (e.g., "less than altitude threshold A" corresponds to "on the ground" in the above example, and "equal to or greater than altitude threshold A" corresponds to "in the sky" in the above example). The advanced frequency priority information may include information indicating that the conventional (or legacy) frequency priority is applied as the frequency priority in the case of "on the ground," and that the new frequency priority is applied as the frequency priority in the case of "in the sky." The advanced frequency priority information may be linking information that links the altitude (or altitude threshold) with the frequency priority.

[0097] In step S32, the UE 100 transitions to an RRC idle state or an RRC inactive state.

[0098] In step S33, the UE 100 executes a cell reselection procedure. In the cell reselection procedure, the UE 100 applies, to each frequency, a frequency priority according to the altitude of the UE 100. For example, the UE 100 performs the following process.

[0099] That is, UE 100 measures its own altitude and compares it with altitude threshold information (step S30) to determine its own altitude (or altitude state). Then, UE 100 applies frequency priority according to its own altitude to each frequency based on altitude frequency priority information (step S31). For example, when UE 100's altitude is "in the sky", UE 100 applies frequency priority for "in the sky" to each frequency. Also, for example, when UE 100's altitude is "on the ground", UE 100 applies frequency priority for "on the ground" to each frequency. UE 100 executes a cell reselection procedure using the frequency to which the frequency priority is applied.

[0100] Fourth Embodiment Next, a fourth embodiment will be described.

[0101] The fourth embodiment will describe an example in which a tracking area code (TAC) for the sky is transmitted. Specifically, a base station (e.g., gNB 200) transmits a tracking area code that identifies a tracking area formed at an altitude equal to or higher than an altitude threshold.

[0102] As a result, for example, the UE 100 can perform a cell reselection procedure by regarding a frequency belonging to the TAC for airspace as the highest frequency priority. Therefore, even in the fourth embodiment, the UE 100 can reselect an airspace cell that supports the frequency and appropriately communicate with the cell.

[0103] Furthermore, when the UE 100 moves within a tracking area indicated by an air TAC, the UE 100 may not need to transmit a location registration request (Tracking Area Update Request) message. Alternatively, when the UE 100 moves within a registration area indicated by one or more air TACs, the UE 100 may not need to transmit a registration request (Registration Request) message. Therefore, the UE 100 can reduce power consumption compared to a case where the UE 100 transmits each message every time the UE 100 moves. Furthermore, when the UE 100 enters or leaves a tracking area indicated by an air TAC (or a registration area indicated by an air TAC) (i.e., when the UE 100 enters a terrestrial tracking area or a terrestrial registration area), the UE 100 transmits a location registration request (or registration request) message to the network (e.g., the AMF 300). Therefore, the network (e.g., AMF 300) can determine whether UE 100 is in the air or on the ground.

[0104] In this way, by providing a TAC for the sky in addition to a TAC for the ground, a UE 100 (i.e., a UAV 150) flying in the sky can communicate properly with a gNB 200.

[0105] (Example of Operation According to Fourth Embodiment) Next, an example of operation according to the fourth embodiment will be described.

[0106] Fig. 10 is a diagram illustrating an example of operation according to the fourth embodiment. Before the operation illustrated in Fig. 10 is started, it is assumed that the UE 100 is in an RRC connected state with respect to the serving cell.

[0107] As shown in FIG. 10 , in step S40, the gNB 200 transmits a TAC (hereinafter, sometimes referred to as an "air TAC") that identifies a tracking area formed at an altitude equal to or higher than the altitude threshold (i.e., "air") . The air TAC is distinguished from a TAC used for ground use. There may be multiple air TACs. In this case, each air TAC can identify each of the multiple air tracking areas. A registration area for air may be formed at an altitude equal to or higher than the altitude threshold. In this case, the air registration area may be configured by one or more air TACs. The air TAC may be broadcast using system information (SIB). The air TAC may be transmitted to the UE 100 using a dedicated message (e.g., an RRC release message). Alternatively, the AMF 300 may transmit the air TAC to the UE 100 using a NAS message. The gNB200 may transmit a public land mobile network (PLMN) ID for the air. In this case, the PLMN ID for the air is distinguished from the PLMN ID for the ground.

[0108] In step S41, the gNB 200 transmits altitude threshold information. The altitude threshold information may be the same as in the first embodiment, or may be transmitted as multiple altitude threshold information as in the second embodiment.

[0109] In step S42, the gNB 200 transmits system information (SIB1). SIB1 includes TACs of a serving cell and neighboring cells used for terrestrial use. SIB1 may include information about an aerial frequency (and / or an aerial cell supporting the aerial frequency) belonging to an aerial tracking area. The information may include an aerial TAC that identifies the aerial tracking area, along with the aerial frequency (and / or aerial cell) belonging to the aerial tracking area. The information may be transmitted to the UE 100 by a dedicated message (e.g., an RRC release message). Alternatively, SIB1 may include information about an aerial frequency (and / or aerial cell) belonging to an aerial PLMN. The information may include an aerial PLMN ID that identifies the aerial PLMN, along with the aerial frequency (and / or aerial cell) belonging to the aerial PLMN. This information may also be sent to UE 100 by a dedicated message (e.g., an RRC release message).

[0110] In step S43, the UE 100 transitions to an RRC idle state or an RRC inactive state.

[0111] In step S44, the UE 100 determines whether it is in the sky or on the ground. As in the first embodiment, the UE 100 may determine whether it is located in the “sky” or on the “ground” based on the altitude measured using the sensor and the altitude threshold information (step S41).

[0112] In step S45, UE 100 executes a cell reselection procedure. When UE 100's altitude is equal to or greater than an altitude threshold (i.e., when UE 100 is located "in the sky"), UE 100 may execute the cell reselection procedure by regarding at least one of the airspace frequency and the airspace cell as the highest priority. Specifically, UE 100 identifies the airspace frequency (or the airspace cell) based on, for example, the airspace TAC (step S40) and the airspace frequency (or the airspace cell) belonging to the airspace TAC (step S42). Then, when UE 100 is located "in the sky", UE 100 may execute the cell reselection procedure by regarding the airspace frequency (or the airspace cell) as the highest priority.

[0113] On the other hand, when the UE 100's altitude is less than the altitude threshold, i.e., when the UE 100 is located "on the ground," the UE 100 may execute a cell reselection procedure by regarding at least one of the airspace frequency and the airspace cell as the lowest priority. Specifically, the UE 100 grasps the airspace frequency (or the airspace cell) based on, for example, the airspace TAC (step S40) and the airspace frequency (or the airspace cell) belonging to the airspace TAC (step S42). Then, when the UE 100 is located "on the ground," the UE 100 may execute a cell reselection procedure by regarding the airspace frequency (or the airspace cell) as the lowest priority.

[0114] The air-based PLMN ID may be used in a PLMN selection procedure. That is, when the UE 100 recognizes that it is located in the "air" (or when the altitude of the UE 100 is equal to or greater than the altitude threshold), the UE 100 may select the air-based PLMN indicated by the air-based PLMN ID using the PLMN selection procedure. On the other hand, when the UE 100 recognizes that it is located on the "ground" (or when the altitude of the UE 100 is less than the altitude threshold), the UE 100 may select the ground-based PLMN indicated by the ground-based PLMN ID using the PLMN selection procedure.

[0115] [Other embodiments]

[0116] The above-described operational flows are not limited to being implemented independently, but can also be implemented by combining two or more operational flows. For example, some steps of one operational flow may be added to another operational flow, or some steps of one operational flow may be replaced with some steps of another operational flow. In each flow, it is not necessary to execute all steps, and only some steps may be executed.

[0117] In the above-described embodiments and examples, an example in which the base station is an NR base station (gNB) has been described, but the base station may be an LTE base station (eNB) or a 6G base station. The base station may also be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may also be a DU of the IAB node. The UE 100 may also be an MT (Mobile Termination) of the IAB node.

[0118] Also, the term "network node" primarily refers to a base station, but may also refer to a device in the core network or part of a base station (CU, DU, or RU).

[0119] A program may be provided that causes a computer to execute each process performed by the UE 100 or the gNB 200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, the program can be installed on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM. Furthermore, circuits that execute each process performed by the UE 100 or the gNB 200 may be integrated, and at least a portion of the UE 100 or the gNB 200 may be configured as a semiconductor integrated circuit (chip set, SoC: System on a chip).

[0120] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, the term "or," as used in this disclosure, is not intended to mean an exclusive or. Furthermore, any reference to elements using designations such as "first," "second," etc., as used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, a reference to a first and a second element does not imply that only two elements may be employed therein or that the first element must precede the second element in some way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall include the plural unless the context clearly indicates otherwise.

[0121] Although one embodiment has been described in detail above with reference to the drawings, the specific configuration is not limited to the above, and various design changes can be made within the scope of the gist. Furthermore, it is also possible to combine all or part of each embodiment, each operation, each process, and each step within the scope of consistent combinations.

[0122] This application claims priority to U.S. Provisional Application No. 63 / 413,317 (filed October 5, 2022), the entire contents of which are incorporated herein by reference.

[0123] (Supplementary Note) (Supplementary Note 1) A communication control method in a mobile communication system, comprising: a step in which a user equipment changes frequency priorities of frequencies used in a cell reselection procedure according to an altitude of the user equipment.

[0124] (Supplementary Note 2) The communication control method according to Supplementary Note 1, further comprising: a step of a network node transmitting altitude threshold information indicating an altitude threshold; and a step of the user equipment determining an altitude of the user equipment based on the altitude threshold.

[0125] (Supplementary Note 3) The communication control method according to Supplementary Note 1 or Supplementary Note 2, further comprising a step in which the network node transmits at least one of airspace frequency information indicating airspace frequencies available at altitudes equal to or higher than the altitude threshold and airspace cell information indicating airspace cells supporting the airspace frequencies.

[0126] (Supplementary Note 4) The communication control method according to any one of Supplementary Notes 1 to 3, wherein the changing step includes a step in which, when the altitude of the user equipment is equal to or greater than the altitude threshold, the user equipment executes the cell reselection procedure by regarding at least one of the airspace frequency and the airspace cell as having the highest priority, and when the altitude of the user equipment is less than the altitude threshold, the user equipment executes the cell reselection procedure by regarding at least one of the airspace frequency and the airspace cell as having the lowest priority.

[0127] (Supplementary Note 5) The communication control method according to any one of Supplementary Notes 1 to 4, further comprising a step of transmitting, by a network node, sky coverage communication availability information indicating that communication is possible to the user equipment having an altitude equal to or higher than a predetermined threshold.

[0128] (Supplementary Note 6) The communication control method according to any one of Supplementary Notes 1 to 5, wherein the step of transmitting altitude threshold information includes a step in which the network node transmits a plurality of pieces of altitude threshold information, and the step of determining includes a step in which the user equipment determines each altitude of the user equipment based on each of the altitude thresholds.

[0129] (Supplementary Note 7) The communication control method according to any one of Supplementary Notes 1 to 6, further comprising a step in which the network node transmits information linking each of the plurality of altitude thresholds with either of the available frequencies and cells supporting the frequencies according to each altitude, and the changing step includes a step in which the user equipment changes the highest priority to either of the frequency and the cell supporting the frequency according to the altitude of the user equipment.

[0130] (Supplementary Note 8) The communication control method according to any one of Supplementary Notes 1 to 7, further comprising a step of the network node transmitting the frequency priority corresponding to an altitude of the user equipment for the frequency, and the changing step includes a step of the user equipment applying the frequency priority corresponding to the altitude of the user equipment to the frequency.

[0131] (Supplementary Note 9) A communication control method in a mobile communication system, comprising: a step in which a network node transmits a tracking area code that identifies a tracking area formed at an altitude equal to or greater than an altitude threshold.

[0132] (Supplementary Note 10) The communication control method according to Supplementary Note 9 further comprises the steps of: the network node transmitting one of a frequency belonging to the tracking area and a cell supporting the frequency; and the user equipment, when an altitude of the user equipment is equal to or greater than the altitude threshold, performing a cell reselection procedure by regarding at least one of the frequency belonging to the tracking area and the cell supporting the frequency as the highest priority; and when an altitude of the user equipment is less than the altitude threshold, performing the cell reselection procedure by regarding at least one of the frequency belonging to the tracking area and the cell supporting the frequency as the lowest priority.

Claims

1. A communication control method in a mobile communication system, comprising: a user device changing a frequency priority of a frequency used in a cell reselection procedure according to an altitude of the user device. The communication control method.

2. A network node transmitting altitude threshold information representing an altitude threshold; and the user device determining the altitude of the user device based on the altitude threshold. The communication control method according to claim 1.

3. The network node further transmitting at least one of airspace frequency information representing an airspace frequency available at an altitude equal to or higher than the altitude threshold and airspace cell information representing an airspace cell supporting the airspace frequency. The communication control method according to claim 2.

4. The changing includes: when the altitude of the user device is equal to or higher than the altitude threshold, the user device executing the cell reselection procedure considering at least one of the airspace frequency and the airspace cell as having the highest priority; and when the altitude of the user device is lower than the altitude threshold, the user device executing the cell reselection procedure considering at least one of the airspace frequency and the airspace cell as having the lowest priority. The communication control method according to claim 3.

5. The network node further transmitting airspace coverage communication availability information indicating that communication with the user device having an altitude equal to or higher than a predetermined threshold is possible. The communication control method according to claim 1.

6. The transmitting of the altitude threshold information includes the network node transmitting a plurality of pieces of altitude threshold information; and the determining includes the user device determining each altitude of the user device based on each altitude threshold. The communication control method according to claim 2.

7. The network node further transmitting association information associating each of the plurality of altitude thresholds with one of the frequency available according to each altitude and a cell supporting the frequency; and the changing includes the user device changing one of the frequency according to the altitude of the user device and the cell supporting the frequency to have the highest priority. The communication control method according to claim 6.

8. The network node further transmitting the frequency priority according to the altitude of the user device for the frequency. Said changing includes the user device applying the frequency priority corresponding to the altitude of the user device to the frequency. The communication control method according to claim 2.

9. A communication control method in a mobile communication system, wherein the user device receives a tracking area code for identifying a tracking area formed at an altitude equal to or higher than an altitude threshold from a network node. Communication control method.

10. Furthermore, the network node transmits either the frequency belonging to the tracking area or a cell supporting the frequency, and when the altitude of the user device is equal to or higher than the altitude threshold, the user device regards at least one of the frequency belonging to the tracking area and the cell supporting the frequency as having the highest priority and performs a cell reselection procedure; when the altitude of the user device is lower than the altitude threshold, the user device regards at least one of the frequency belonging to the tracking area and the cell supporting the frequency as having the lowest priority and performs the cell reselection procedure. The communication control method according to claim 9.

11. A user device in a mobile communication system, comprising a control unit that changes the frequency priority of a frequency used in a cell reselection procedure according to the altitude of the user device. User device.

12. A user device in a mobile communication system, comprising a receiving unit that receives a tracking area code for identifying a tracking area formed at an altitude equal to or higher than an altitude threshold from a network node. User device.