Mobile communication system

Abstract

The present invention improves frequency utilization efficiency, prevents a decrease in the communication capacity of a terminal, and reduces interference in an upstream line and a downstream line of a service link of each base station in a mobile communication system provided with a ground base station and an airborne relay type base station having an antenna provided to a flying object or the like (such as a UAV, an HAPS, or a communication satellite) in the sky. The airborne relay type base station and the ground base station each carry out radio communication of the service link with the terminal by means of time division duplexing (TDD) using the same frequency. The airborne relay type base station and the ground base station employ opposite transmission timings and reception timings for the TDD radio communication in the service link. The ground base station estimates the direction of the service link antenna of a relay communication station and performs control to direct the null of the directional beam of the service link antenna of the ground base station to the estimated direction of the service link antenna of the relay communication station.

Description

Mobile communication systems

[0001] The present invention relates to a mobile communication system including a base station capable of wirelessly communicating with a terminal.

[0002] Conventionally, as flying objects or floating objects located in the sky, there are known UAVs that fly at altitudes of 18 km or less and HAPSs that fly in the stratosphere at altitudes of 18 km or more. Also known is a mobile communication system that includes an airborne relay base station as a first base station that wirelessly communicates with a terminal via a service link antenna of a relay communication station provided on the flying object or floating object such as a UAV or HAPS (airborne object) located in the sky, and a terrestrial base station as a second base station that wirelessly communicates with the terminal via an antenna located on land or sea, in which the same frequency is shared in the service link between the airborne relay base station and the terrestrial base station, enabling wireless communication between terminals of common specifications.

[0003] As a technology applicable to reducing interference between an airborne relay base station and a terrestrial base station in a mobile communication system, there is an interference control technology that adjusts and controls radio frames in the time domain (in subframe units) on the premise that the airborne relay base station and the terrestrial base station are time-synchronized with each other (see, for example, Patent Document 1 and Non-Patent Document 1). This interference control technology is a technology that complies with the LTE (Long Term Evolution)-Advanced standard, and is also called eICIC (enhanced Inter-Cell Interference Coordination).

[0004] JP 2012-129793 A

[0005] “Overview of 3GPP”, Release 10, V0.2.1 (2014-06).

[0006] When the above-mentioned conventional interference control technology is applied, the airborne relay base station and the terrestrial base station use the frequencies allocated to the service link in a time-division manner, so the radio signals used by the airborne relay base station and the terrestrial base station are orthogonal on the time axis and no mutual interference occurs. However, because of the time-division use of frequencies, neither the airborne relay base station nor the terrestrial base station can use all the frequencies allocated to the service link, so the communication capacity (maximum transmission rate, peak throughput) of the terminals on each service link decreases.

[0007] A mobile communication system according to one aspect of the present disclosure includes an airborne relay base station that wirelessly communicates with a terminal via a service link antenna of a relay communication station installed on an airborne or floating body located in the sky, and a terrestrial base station located on the ground or sea, where the airborne relay base station and the terrestrial base station are time-synchronized with each other. The airborne relay base station and the terrestrial base station each perform wireless service link communication with the terminal using a TDD (Time Division Duplex) method using the same frequency. The transmission timing and reception timing of the TDD wireless communication in the service link are reversed between the airborne relay base station and the terrestrial base station. The terrestrial base station estimates the direction of the service link antenna of the airborne relay base station and controls the null of the directional beam of the service link antenna of the terrestrial base station to point in the estimated direction of the service link antenna of the airborne relay base station.

[0008] In the mobile communication system, the overhead relay base station and the terrestrial base station may adjust their transmission timings to be opposite to each other using time information acquired by a GNSS (Global Navigation Satellite System) receiver.

[0009] In the mobile communication system, the relay communication station mounted on the flying body or the floating body may be configured as a repeater relay device (frequency conversion repeater relay device) that relays transmitted and received signals without regenerating them.

[0010] In the mobile communication system, the relay communication station mounted on the flying body or the floating body may be configured as a base station device that regenerates transmitted and received signals, re-modulates the regenerated signals, and relays them.

[0011] In the mobile communication system, the terrestrial base station may receive location information acquired by a GNSS (Global Navigation Satellite System) receiver mounted on the aircraft or the floating body via a gateway device, and estimate the direction of the service link antenna of the relay communication station based on the location information of its own base station.

[0012] In the mobile communication system, the terrestrial base station may have an array antenna as the service link antenna, and may use the array antenna to measure the direction of arrival of radio waves from the service link antenna of the relay communication station, and may estimate the direction of the service link antenna of the relay communication station from the measurement result.

[0013] The mobile communication system may further include an inter-base station cooperation control device that performs control between the overhead relay base station and the terrestrial base station, and the inter-base station cooperation control device may adjust the transmission timing by reversing the transmission and reception, using GNSS (Global Navigation Satellite System) time information received by the overhead relay base station and received via a gateway device, and the GNSS (Global Navigation Satellite System) time information received by the terrestrial base station.

[0014] The mobile communication system may further include an inter-base station cooperation control device that controls communication between the overhead relay base station and the terrestrial base station, and location information acquired by a GNSS (Global Navigation Satellite System) receiver mounted on the aircraft or the floating body may be transferred to the terrestrial base station via a gateway device and the inter-base station cooperation control device, and the terrestrial base station may estimate the direction of the service link antenna of the relay communication station based on the location information of its own terrestrial base station.

[0015] In the mobile communication system, the flying object or the floating object may be a communication satellite, a UAV (Unmanned Aerial Vehicle) that flies at an altitude of 18 km or less, or a HAPS that flies in the stratosphere at an altitude of 18 km or more.

[0016] The mobile communication system disclosed in this specification can reduce interference caused by downlink transmission signals from a service link antenna (airborne relay base station antenna) of a relay communication station installed on an aircraft or floating body located in the sky to reception signals on the uplink of a service link antenna (terrestrial base station antenna) of a terrestrial base station, as well as reduce interference caused by downlink transmission signals from a terrestrial base station antenna to reception signals on the uplink of an airborne relay base station antenna.

[0017] FIG. 1 is a diagram illustrating an example of the overall configuration of a mobile communication system according to an embodiment. FIG. 2A is a diagram illustrating an example of the configuration of a relay communication station constituting an example of a HAPS base station constituting the mobile communication system according to the embodiment. FIG. 2B is a diagram illustrating an example of the configuration of a HAPS-GW corresponding to FIG. 2A. FIG. 3A is a diagram illustrating an example of the configuration of a relay communication station constituting another example of a HAPS base station constituting the mobile communication system according to the embodiment. FIG. 3B is a diagram illustrating an example of the configuration of a HAPS-GW corresponding to FIG. 3A. FIG. 4A is a diagram illustrating an example of interference from a HAPS downlink to a terrestrial base station terminal when a HAPS base station is applied to a system configuration of a reference example. FIG. 4B is a diagram illustrating an example of interference from a terrestrial base station terminal to an uplink when a HAPS base station is applied to a current configuration. FIG. 5 is a diagram illustrating an example of the arrangement of time slots of radio resources set in a HAPS base station and a terrestrial base station to which inter-cell interference control technology (eICIC) is applied when a HAPS base station is applied to the system configuration of a reference example. Fig. 6A is a diagram showing uplinks and downlinks between a terrestrial base station and a terrestrial base station terminal when a TDD (time division multiplexing) scheme is used as the transmission scheme for the service link in the mobile communication system according to the embodiment. Fig. 6B is a diagram showing an example of a frequency band in which the same frequency is shared in uplinks and downlinks between the terrestrial base station and the terrestrial base station terminal in the TDD scheme. Fig. 6C is a diagram showing an example of an arrangement of time slots in uplinks and downlinks between the terrestrial base station and the terrestrial base station terminal in the TDD scheme. Fig. 7 is a diagram showing an example of an arrangement of time slots in uplinks and downlinks when synchronization control is performed to match transmission timings and reception timings among multiple terrestrial base stations in a system configuration using a conventional TDD scheme. Fig. 8A is a diagram showing an example of downlink interference when transmission timings and reception timings among multiple terrestrial base stations are matched by synchronization control in a system configuration using a conventional TDD scheme. Fig. 8B is a diagram showing an example of uplink interference when transmission timings and reception timings among multiple terrestrial base stations are matched by synchronization control in a system configuration using a conventional TDD scheme.Fig. 9 is a diagram showing an example of time slot arrangement for uplink and downlink links when synchronization control is performed to reverse the transmission timing and reception timing between a terrestrial base station and a HAPS base station using a TDD scheme in a mobile communication system according to an embodiment. Fig. 10A is a diagram showing an example of interference from a terrestrial base station to the uplink link of a HAPS base station when synchronization control is performed to reverse the transmission timing and reception timing between the terrestrial base station and the HAPS base station. Fig. 10B is a diagram showing an example of interference from a HAPS base station to the uplink link of a terrestrial base station when synchronization control is performed to reverse the transmission timing and reception timing between the terrestrial base station and the HAPS base station. Fig. 10C is a diagram collectively showing the interference in Figs. 10A and 10B. Fig. 11A is a diagram showing a reduction in interference from a terrestrial base station to the uplink link of a HAPS base station when null forming is applied to beamforming in the antenna of the terrestrial base station according to an embodiment. Fig. 11B is a diagram showing a reduction in interference from a HAPS base station to the uplink link of a terrestrial base station when null forming is applied to beamforming in the antenna of the terrestrial base station according to an embodiment. FIG. 11C is a diagram summarizing the interference reductions in FIGS. 11A and 11B. FIG. 12A is a diagram illustrating an example of an antenna and a base station device of a terrestrial base station according to an embodiment. FIG. 12B is a diagram illustrating an example of beamforming in the terrestrial base station. FIG. 13 is a diagram illustrating an example of beamforming control in a terrestrial base station using a GPS receiver equipped with HAPS via an inter-base station cooperation control device in a mobile communication system according to an embodiment. FIG. 14 is a diagram illustrating an example of the configuration of main parts of an antenna and a base station device when beamforming control is performed using a GPS receiver equipped with HAPS in a terrestrial base station according to an embodiment. FIG. 15 is a diagram illustrating an example of the configuration of main parts of an antenna and a base station device when beamforming control is performed using HAPS radio wave arrival direction estimation in a terrestrial base station according to an embodiment. FIG. 16 is a diagram illustrating another example of the configuration of main parts of an antenna and a base station device when beamforming control is performed using HAPS radio wave arrival direction estimation in a terrestrial base station according to an embodiment. FIG. 17 is a diagram illustrating an example of control of transmission and reception timing between a terrestrial base station and a HAPS base station according to an embodiment.FIG. 18 is a diagram illustrating an example of control of transmission and reception timing between a terrestrial base station and a HAPS base station according to the embodiment.

[0018] Various embodiments will be described below with reference to the drawings. Note that each drawing merely shows a schematic representation of the shape, size, and positional relationship to the extent that the contents of the present invention can be understood, and therefore the present invention is not limited to the shape, size, and positional relationship exemplified in each drawing. Furthermore, the numerical values ​​exemplified below are merely preferred examples of the present invention, and therefore the present invention is not limited to the exemplified numerical values.

[0019] In this embodiment, an example of a mobile communication system in which the same frequency is shared between a HAPS base station (HAPS cellular system) as an airborne relay base station and a terrestrial base station (terrestrial cellular system) will be mainly described, but the present invention can also be applied to a mobile communication system in which the relay communication station of the airborne relay base station is installed on an aircraft or floating object other than a HAPS.

[0020] Fig. 1 is a diagram showing an example of the configuration of a mobile communication system (mobile phone system) according to an embodiment of the present invention. In Fig. 1, the mobile communication system of this embodiment includes a HAPS base station (HAPS cellular system) 10 as a first base station (air relay base station) and a terrestrial base station (terrestrial cellular system) 20 as a second base station, as a plurality of base stations capable of wireless communication with a terminal 30.

[0021] The HAPS base station 10 wirelessly communicates with a terminal (hereinafter also referred to as a "HAPS base station terminal") 30(1) via a service link antenna (also referred to as a "HAPS base station antenna") 112 of a relay communication station 11 provided on a HAPS ("High Altitude Pseudolite" or "High Altitude Platform Station") 100, which serves as an air vehicle or floating body located in the sky. The HAPS 100 is equipped with at least one of a battery and a solar power generation system, for example, and can fly using electric power. The HAPS 100 may be an airship-type HAPS as shown in the figure, or a solar plane-type HAPS. Furthermore, the air vehicle or floating body on which the relay communication station 11 is provided may be a HAPS, or may be an artificial satellite (e.g., a communication satellite), a balloon, or an unmanned aerial vehicle (UAV) such as a drone or UAS (Unmanned Aircraft Systems). Furthermore, the HAPS 100, the UAV, or other flying object or floating object may fly using at least one of a battery and an engine as a power source. The UAV may be, for example, an unmanned aircraft that runs on fuel or a drone that runs on a battery or the like.

[0022] The HAPS 100 equipped with the relay communication station 11 may be controlled, for example, by autonomous control or external control, to float or fly in an airspace (floating airspace) at an altitude H of 100 km or less above the ground G (or sea level). The airspace in which the HAPS 100 is located may be, for example, a stratospheric airspace at an altitude H of 18 km or more and 50 km or less. This airspace may be an airspace at an altitude of 15 km or more and 25 km or less where meteorological conditions are relatively stable, and may particularly be an airspace at an altitude of approximately 20 km. Furthermore, the airspace may be an airspace at an altitude of 0.1 km or more and 18 km or less for UAVs. Furthermore, the airspace may be an airspace at an altitude of 0.05 km or more for drones.

[0023] The relay communication station 11 includes a feeder link antenna unit (hereinafter also referred to as "FL antenna") 111 and a service link antenna unit (HAPS base station antenna) 112. The relay communication station 11 can perform feeder link FL communication with a HAPS gateway device (hereinafter referred to as "HAPS-GW") 12 installed on the ground (or sea) via the FL antenna 111. The FL antenna 111 is, for example, an array antenna whose directivity can be controlled, as described below, and may be a massive antenna in which a large number of antenna elements are arranged two-dimensionally and whose directivity can be controlled in the horizontal and vertical directions.

[0024] Furthermore, the relay communication station 11 can communicate with the HAPS base station terminal 30(1) over a service link SL(1) via a HAPS base station antenna 112. The HAPS base station antenna 112 is, for example, an array antenna whose directivity can be controlled, and may be a massive antenna in which many antenna elements are arranged two-dimensionally and whose directivity can be controlled in the horizontal and vertical directions.

[0025] The HAPS-GW 12 is connected to the core network 40 of the mobile communication network via a wired or wireless communication line, and includes a feeder link antenna unit (hereinafter also referred to as "FL antenna") 121 consisting of a parabolic antenna, a massive antenna capable of controlling horizontal and vertical directivity, etc. The HAPS-GW 12 can perform feeder link FL communication with the HAPS-equipped relay communication station 11 via the FL antenna 121.

[0026] The terrestrial base station 20 includes an antenna unit (hereinafter also referred to as "terrestrial base station antenna") 21 and a base station device 22 connected to a core network 40 of a mobile communication network by a wired or wireless communication line such as optical fiber. The base station device 22 can communicate over a service link SL(2) with a terminal (hereinafter also referred to as "terrestrial base station terminal") 30(2) via the terrestrial base station antenna 21. The terrestrial base station antenna 21 is, for example, an array antenna whose direction of directivity can be controlled, and may be a massive antenna in which a large number of antenna elements are arranged two-dimensionally and whose directivity can be controlled in the horizontal and vertical directions.

[0027] The HAPS-GW 12 of the HAPS base station 10 and the base station device 22 of the terrestrial base station 20 are each connected to an inter-base station cooperation control device (also referred to as an "inter-base station network cooperation control device" or "inter-system cooperation control device") 50 by a wired or wireless communication line such as optical fiber. The HAPS base station 10 and the terrestrial base station 20 are time-synchronized with each other via the inter-base station cooperation control device 50, and timing control is performed such that the transmission and reception timing of the HAPS base station 10 is opposite to that of the terrestrial base station 20.

[0028] The HAPS base station 10 and the terrestrial base station 20 are each configured using hardware such as a computer device having a CPU, memory, etc., an external communication interface unit for the core network 40 and the base station cooperation control device 50, and a wireless communication unit, and by executing a predetermined program, wireless communication can be performed between the HAPS base station terminal 30(1) and the terrestrial base station terminal 30(2), and communication can be performed with the core network 40 and the base station cooperation control device 50.

[0029] In the mobile communication system of this embodiment, the same wireless transmission method is used for communication of the service link SL(1) of the HAPS base station 10 and communication of the service link SL(2) of the terrestrial base station 20, and the same frequency is shared to improve frequency efficiency of the service link. The wireless transmission method is a TDD (Time Division Duplex) method that transmits and receives signals using the same frequency in a time-division manner, and can be, for example, a TDD method such as a communication method of LTE (Long Term Evolution) or LTE-Advanced, a communication method of a fourth-generation mobile phone, a communication method of a fifth-generation mobile phone, or a subsequent next-generation mobile phone.

[0030] A HAPS base station terminal 30(1) capable of connecting to and communicating with a HAPS base station 10 and a terrestrial base station terminal 30(2) capable of connecting to and communicating with a terrestrial base station 20 are mobile communication terminals 30 with the same specifications. The terminal 30 is a mobile phone, a smartphone, a portable personal computer with mobile communication capabilities, or the like, and is also called a mobile terminal, a user equipment (UE), a mobile station, a mobile device, or a portable communication terminal. The terminal 30 may be a modular mobile station incorporated into a vehicle such as an automobile or a mobile object such as a drone, or may be a terminal device for a device for the Internet of Things (IoT).

[0031] The terminal 30 is configured using hardware such as a computer device having a CPU, memory, etc., and a wireless communication unit, and by executing a predetermined program, it is possible to perform wireless communication between the HAPS base station 10 and the terrestrial base station 20.

[0032] The HAPS base station 10 of this embodiment is composed of a relay communication station 11 mounted on a HAPS 100 in the sky, or is composed of a relay communication station 11 and a HAPS-GW 12 installed on the ground (or at sea).

[0033] 2A is a diagram showing an example of the configuration of a relay communication station 11 constituting an example of a HAPS base station 10 in the mobile communication system of the embodiment. In this example configuration, the relay communication station 11 is a wireless relay device (hereinafter referred to as a "frequency conversion repeater") that converts the frequency of a feeder link, which has a different frequency from that of a service link. In the downlink, the relay communication station 11 converts the frequency of the feeder link transmitted from the HAPS-GW 12 to the frequency of the service link and transmits it to the HAPS base station terminal 30(1). Meanwhile, in the uplink, the relay communication station 11 converts the frequency of the service link transmitted from the HAPS base station terminal 30(1) to the frequency of the feeder link and transmits it to the HAPS-GW 12.

[0034] In this example, the relay communication station 11 of the HAPS base station 10 is configured as a repeater relay device (frequency converting repeater) having a repeater 113 and a frequency converting device 114. The repeater 113 has a low-noise amplifier that amplifies a reception signal of the service link SL(1) received via the HAPS base station antenna 112, a power amplifier that amplifies a transmission signal of the service link SL(1) transmitted via the HAPS base station antenna (SL antenna) 12, and the like. The frequency converting device 114 converts between the frequency of the service link SL(1) and the frequency of the feeder link FL.

[0035] FIG. 2B is a diagram showing an example of the configuration of the HAPS-GW 12 corresponding to FIG. 2A . The HAPS-GW 12 in this example has a base station device 122 and a frequency conversion device 123. The base station device 122 has a baseband processing device that processes baseband signals of the service link, a communication interface unit for communicating with the core network 40 via a backhaul line, and the like. The frequency conversion device 123 converts between the frequency of the service link signal input / output to / from the base station device 122 and the frequency of the feeder link signal transmitted / received via the FL antenna 121. The HAPS-GW 12 also has a function of communicating with the inter-base station cooperation control device 50 in order to adjust the transmission / reception timing of the service link SL with the terrestrial base station 20 and to transfer location information of the HAPS 100.

[0036] 3A is a diagram showing an example of the configuration of a relay communication station 11 constituting another example of the HAPS base station 10 in the mobile communication system of the embodiment. In this example configuration, the relay communication station 11 of the HAPS base station 10 is composed of a base station device similar to that of a normal terrestrial base station, and a feeder link transceiver in which the feeder link between the relay communication station 11 of the HAPS base station 10 and the HAPS-GW uses a frequency different from that of the service link. Unlike the service link, the feeder link allows the selection of an optimal wireless transmission method as appropriate.

[0037] In this example, the relay communication station 11 of the HAPS base station 10 has a base station device 115 equivalent to the terrestrial base station device 22, and a feeder link transceiver 116. The base station device 115 has a low-noise amplifier that amplifies a reception signal of the service link SL(1) received via the HAPS base station antenna 112, a power amplifier that amplifies a transmission signal of the service link SL(1) transmitted via the HAPS base station antenna 112, a baseband processing device that processes baseband signals of the service link, etc. The feeder link transceiver 116 transmits and receives backhaul line signals transmitted and received via the FL antenna 111 to and from the HAPS-GW 12.

[0038] 3B is a diagram showing an example of the configuration of the HAPS-GW 12 corresponding to FIG. 3A. The HAPS-GW 12 in this example has a feeder link transceiver 124. The feeder link transceiver 124 transmits and receives backhaul line signals transmitted and received via an FL antenna 121 to and from an airborne relay communication station 11 mounted on the HAPS 100. The feeder link transceiver 124 also communicates with the core network 40 via the backhaul line. The HAPS-GW 12 also has a function of communicating with the inter-base station cooperation control device 50 to adjust the transmission and reception timing of the service link SL with the terrestrial base station 20 and to transfer location information of the HAPS 100.

[0039] In a configuration including a HAPS base station 10 and a terrestrial base station 20 as shown in Figure 1, if the HAPS base station 10 and the terrestrial base station 20 transmit radio waves at the same time in the FDD system or the TDD system, or if the HAPS base station terminal 30(1) and the terrestrial base station terminal 30(2) transmit signals at the same time, interference may occur between the HAPS cellular system in the sky and the terrestrial cellular system. For example, as shown in Figure 4A, when a HAPS downlink signal is transmitted from a relay communication station 11 mounted on the HAPS 100 in the sky to the HAPS base station terminal 30(1), the transmitted signal on the HAPS downlink may reach the terrestrial base station terminal 30(2) which is receiving the terrestrial downlink signal from the terrestrial base station 20, causing interference from the HAPS downlink to the terrestrial base station terminal 30(2). Furthermore, as shown in FIG. 4B , when a terrestrial base station terminal 30(2) is transmitting a terrestrial uplink signal to a terrestrial base station 20, the terrestrial uplink transmission signal may reach a relay communication station 11 of the HAPS 100 in the sky that is receiving a HAPS uplink signal from the HAPS base station terminal 30(1), and interference from the terrestrial base station terminal 30(2) may occur in the HAPS uplink of the relay communication station 11 mounted on the HAPS 100.

[0040] Conventionally, an inter-cell interference control technology called eICIC, which complies with the above-mentioned LTE-Advanced standard, has been known as a technology for controlling interference between a plurality of base stations.

[0041] 5 is a diagram showing an example of the allocation of time slots of radio resources set in each of the HAPS base station 10 and the terrestrial base station 20 when the inter-cell interference control technique (eICIC) is applied to a HAPS base station in the system configuration of the reference example. As shown in FIG. 5 , in the conventional inter-cell interference control technique (eICIC), radio resources of the same frequency are time-divided to assign different time slots to each of the HAPS base station 10 and the terrestrial base station 20. This makes the radio resources orthogonal on the time axis, thereby enabling mutual avoidance of interference at the same frequency between the HAPS base station 10 and the terrestrial base station 20. However, in the conventional inter-cell interference control technique (eICIC), the radio resources (time slots) are time-divided and used in each of the HAPS base station 10 and the terrestrial base station 20. Therefore, all frequencies (all time) allocated to the mobile communication system of the embodiment cannot be used, and the communication capacity (maximum transmission rate, peak throughput) of the service links of each of the HAPS base station 10 and the terrestrial base station 20 decreases. In particular, the communication capacity of terrestrial base station terminals connected to a terrestrial base station 20 with a large number of terminals in its cell decreases.

[0042] In this embodiment, in order to avoid interference between the HAPS base station 10 and the terrestrial base station 20 and to prevent a decrease in the communication capacity (maximum transmission rate, peak throughput) of the service links of the HAPS base station 10 and the terrestrial base station 20, the TDD (Time Division Duplex) method adopted in standards such as the fifth generation of mobile communications is used as the transmission and reception method for the service links of each base station 10, 20.

[0043] In the TDD (Time Division Duplex) system, the same frequency is used for the uplink and downlink, and time slots are time-division multiplexed for use on the uplink and downlink. For example, in a terrestrial base station 20, as shown in Figures 6A and 6B, the same frequency is used for the uplink and downlink between the terrestrial base station 20 and the terrestrial base station terminal 30(2). Then, as shown in Figure 6C, radio resources (time slots) on the time axis are time-division multiplexed for use between downlink transmission (base station transmission) from the terrestrial base station 20 to the terrestrial base station terminal 30(2) and uplink transmission (terminal transmission) from the terrestrial base station terminal 30(2) to the terrestrial base station 20.

[0044] When the HAPS base station 10 and the terrestrial base station 20 of this embodiment use a TDD (Time Division Duplex) system, the HAPS base station 10 and the terrestrial base station 20 can completely share the same frequency by applying an interference reduction technique described below that avoids interference between the HAPS base station 10 and the terrestrial base station 20. Furthermore, the HAPS base station 10 and the terrestrial base station 20 can use all frequencies (all times) allocated to the mobile communication system, preventing a decrease in the communication capacity of the service link of each base station 10, 20. In particular, a decrease in the communication capacity of terrestrial base station terminals connected to a terrestrial base station 20 with a large number of terminals in its cell can be prevented.

[0045] In a system in which the same frequency is shared by multiple base stations using the conventional TDD method, in order to avoid interference between the uplink and downlink lines of each base station, synchronization control is performed between multiple base stations 20(1) and 20(2), for example, as shown in FIG. 7, so that the transmission timing of base station transmissions on the downlink lines from the base station to the terminal is synchronized, and the transmission timing of terminal transmissions on the uplink lines from the terminal to the base station is synchronized.

[0046] When a communication method of a terrestrial cellular system using a terrestrial base station 20 (hereinafter referred to as the "terrestrial cellular system") and a communication method of a HAPS cellular system using a HAPS base station 10 (hereinafter referred to as the "HAPS cellular system") are used simultaneously, as shown in Figure 8A, interference waves from the HAPS base station 10 that transmits signals over a wide area, particularly on the downlink of the service link, may degrade the communication quality of many terrestrial base station terminals 30(2) located in the terrestrial cell 20C, and as shown in Figure 8B, signals transmitted by a very large number of terrestrial base station terminals 30(2) may degrade the communication quality of the HAPS base station terminal 30(1) located in the HAPS cell 10C. Therefore, in order to use the terrestrial cellular system and the HAPS cellular system simultaneously, it is necessary to reduce interference.

[0047] In the mobile communication system of this embodiment, the transmission timing and reception timing between the HAPS base station 10 and the terrestrial base station 20 are reversed.

[0048] 9 is a diagram showing an example of the arrangement of time slots for uplink and downlink links when synchronization control is performed to reverse the transmission timing and reception timing between a terrestrial base station 20 and a HAPS base station 10 using the TDD scheme in a mobile communication system according to the embodiment. In each of a plurality of consecutive transmission / reception frames (radio frames) shown in Fig. 9, the transmission timing and transmission period of the downlink link from the terrestrial base station 20 to the terrestrial base station terminal 30(2) match the transmission timing and transmission period of the uplink link from the HAPS base station terminal 30(1) to the HAPS base station 10. On the other hand, the transmission timing and transmission period of the uplink link from the terrestrial base station terminal 30(2) to the terrestrial base station 20 match the transmission timing and transmission period of the downlink link from the HAPS base station 10 to the HAPS base station terminal 30(1).

[0049] Here, the terrestrial base station 20 and the HAPS base station 10 may adjust their transmission timings to be opposite to each other using time information acquired by a GNSS (Global Navigation Satellite System) receiver.

[0050] As shown in Figure 9, by reversing the transmission timing and reception timing of the HAPS base station 10 and the terrestrial base station 20 for the uplink and downlink, the way in which interference is received changes significantly compared to when the transmission timing and reception timing of the HAPS base station 10 and the terrestrial base station 20 are the same for the uplink and downlink as shown in Figure 7.

[0051] 10A, the downlink of terrestrial base station 20 becomes the uplink of the service link of HAPS base station 10. Therefore, the radio waves of the downlink transmission sent from terrestrial base station antenna 21 reach HAPS base station antenna 112 as shown in the figure, and the downlink of terrestrial base station 20 causes interference with the uplink of the service link of HAPS base station 10.

[0052] In addition, the terrestrial base station terminal 30(2) and the HAPS base station terminal 30(1) are located at a relatively large distance, and since they are both located on the ground, there are many obstructions, such as buildings, between the terminals. Therefore, the interference signal power received when the uplink transmission signal of the HAPS base station terminal 30(1) reaches the terrestrial base station terminal 30(2) is generally negligibly small.

[0053] 10B, the uplink of the terrestrial base station 20 becomes the downlink of the service link of the HAPS base station 10. Therefore, the radio waves of the downlink transmission of the service link transmitted from the HAPS base station antenna 112 reach the terrestrial base station antenna 21 as shown in the figure, causing interference with the uplink of the terrestrial base station 20.

[0054] In addition, the HAPS base station terminal 30(1) and the terrestrial base station terminal 30(2) are located at a relatively large distance, and since they are both located on the ground, there are many obstructions, such as buildings, between the terminals. Therefore, the interference signal power received by the HAPS base station terminal 30(1) when the uplink transmission signal from the terrestrial base station terminal 30(2) reaches the HAPS base station terminal 30(1) is generally negligibly small.

[0055] 10A and 10B, when the terrestrial cellular system and the HAPS cellular system share the same frequency as shown in Fig. 10C, the problem can be summarized as interference between the terrestrial base station antenna 21 and the HAPS base station service link antenna 112. In other words, if the interference between the terrestrial base station antenna 21 and the HAPS base station service link antenna 112 is reduced, the terrestrial cellular system and the HAPS cellular system can share the same frequency.

[0056] Therefore, in this embodiment, the terrestrial base station 20 estimates the direction of the service link antenna 112 of the HAPS base station above and applies null forming, which directs the null of the directional beam of the terrestrial base station antenna 21 in that direction, thereby simultaneously achieving interference reduction, as shown in Figures 11A and 11B. Figures 11A and 11B can be combined to form Figure 11C, which shows that the proposed null forming can simultaneously reduce interference between the terrestrial base station antenna 21 and the service link antenna 112 of the HAPS base station, allowing the terrestrial cellular system and the HAPS cellular system to share the same frequency. The null forming is null forming that directs the directional null of the terrestrial base station antenna 21 in the direction of the HAPS base station antenna 112, thereby significantly suppressing the transmission power and reception power in that direction.

[0057] Fig. 12A is a diagram showing an example of an antenna 21 and a base station device 22 of a terrestrial base station 20 according to an embodiment. Fig. 12B is a diagram showing an example of beamforming in the terrestrial base station 20. In this embodiment, as shown in Fig. 12A, a massive antenna, which is an array antenna in which a large number of antenna elements 210 are arranged two-dimensionally, is used as the terrestrial base station antenna 21. The base station device 22 includes a transmission / reception device 221 and a null forming unit 222.

[0058] The null-forming unit 222 performs transmission signal processing to generate amplitudes and phases for performing predetermined null-forming on each signal to be transmitted via the multiple antenna elements of the terrestrial base station antenna 21 (hereinafter referred to as transmission null-forming weights), and to generate transmission signals by superimposing the transmission null-forming weights on the transmission signals of each antenna element output from the transmission / reception device 221, and performs reception signal processing to generate amplitudes and phases for performing predetermined null-forming on each received signal received via the multiple antenna elements of the terrestrial base station antenna 21 (hereinafter referred to as reception null-forming weights), and to superimpose the reception null-forming weights on each received signal to generate reception signals to the transmission / reception device 221. Note that in the TDD system, the transmission null-forming weights and reception null-forming weights may be the same.

[0059] The transmission and reception null-forming weights are calculated based on the direction of the HAPS base station (horizontal angle θ in the horizontal plane, elevation angle φ in the vertical plane) observed from the terrestrial base station, for example, using position information of the terrestrial base station antenna 21 of the own station and position information of the HAPS base station antenna 112 (position information of the HAPS 100).

[0060] If the terrestrial base station antenna 21 has a large number of antenna elements, for example, if it is configured as a massive antenna, by appropriately controlling the transmission and reception null-forming weights generated by the null-forming unit 222, it is possible to direct the directional null in the direction of the HAPS base station antenna 112 while directing the directional beam Bnf of the terrestrial base station antenna 21 in the direction of the terrestrial base station terminal 30(2).

[0061] Examples of the beamforming control involving null forming described above include a method of creating a null beamforming weight using position information acquired by a GNSS (Global Navigation Satellite System) receiver equipped with HAPS, and a control of creating a null beamforming weight by scanning a beam at regular angles (Δθ, Δφ) in the horizontal and vertical planes using the beamforming function of the terrestrial base station antenna, measuring the received power of the downlink signal transmitted by the service link antenna of the HAPS base station, and estimating the direction in which the received power is maximum as the direction of HAPS.

[0062] 13 is a diagram showing an example of beamforming control of a terrestrial base station 20 using a HAPS-equipped GPS receiver 15 via an inter-base station cooperation control device 50 in a mobile communication system according to the embodiment. Note that this example is an example in which the GNSS receiver is a GPS receiver 15 that receives signals from GPS satellites, but a GNSS receiver that receives signals from GNSS satellites other than GPS may also be used.

[0063] 13 , a HAPS 100 equipped with a relay communication station 11 in the sky is equipped with a GPS receiver 15. Location information of the HAPS 100 acquired by the GPS receiver is transmitted from the relay communication station 11 to a HAPS-GW 12 via a feeder link FL. The HAPS-GW 12 transfers the location information of the HAPS 100 received from the relay communication station 11 to each ground base station 20 via an inter-base station cooperation control device 50.

[0064] 14 is a diagram showing an example of the configuration of the main parts of the antenna 21 and the base station device 22 when performing antenna null forming by using a GPS receiver equipped with HAPS in the terrestrial base station 20 according to the embodiment. In FIG. 14, parts that are common to those in FIG. 12A are given the same reference numerals and descriptions thereof will be omitted.

[0065] 14 , the base station device 22 includes a radio wave arrival direction estimator 223 and a weight calculator 224. The radio wave arrival direction estimator 223 calculates the horizontal angle θ and elevation angle φ of the direction of the HAPS 100 (the direction of the service antenna of the HAPS base station) relative to the position of the base station, based on the position information of the base station itself and the position information of the HAPS 100 transferred via the inter-base station cooperation control device 50. The weight calculator 224 calculates a null-forming weight W using a null-forming technique, such as the directionally constrained minimization of power (DCMP), with the radio wave arrival direction estimated by the radio wave arrival direction estimator 223 as a known value. The weight W calculated by the weight calculation unit 224 is set in the null forming unit 222 and superimposed on the transmission signal transmitted via each antenna element of the terrestrial base station antenna 21 and the received signal received via each antenna element of the terrestrial base station antenna 21.

[0066] FIG. 15 is a diagram showing an example of the configuration of the main parts of the antenna 21 of the terrestrial base station 20 and the base station device 22 when estimating the direction of arrival of HAPS radio waves in the terrestrial base station 20 according to the embodiment by beam scanning. This example illustrates estimation of the direction of arrival of HAPS radio waves by beam scanning in the elevation angle direction, which is the vertical plane. In practice, scanning is performed at regular angles (Δθ, Δφ) in the horizontal and vertical planes to search for the direction (θ, φ) = (nΔθ, mΔφ) where the received power is maximum. In general, the direction where the received power is maximum is the direction of the service antenna of the HAPS base station. Note that in FIG. 15, parts common to those in the above-mentioned FIGS. 12A and 14 are designated by the same reference numerals, and their description will be omitted.

[0067] 15 , the base station device 22 includes a radio wave arrival direction estimator 223, a beam scanner 225, and a received power measurer 226. The beam scanner 225 scans a vertical plane in the sky with beam B at a constant angle, while the received power measurer 226 measures the received power received via the antenna 21. The radio wave arrival direction estimator 223 determines the horizontal angle θ and elevation angle φ of the direction of beam B at which the received power is maximum as the radio wave arrival direction (horizontal angle θ and elevation angle φ) of the HAPS.

[0068] Fig. 16 is a diagram showing another example of the configuration of the main parts of the antenna 21 of the terrestrial base station 20 and the base station device 22 when beamforming control is performed using HAPS radio wave arrival direction estimation in the terrestrial base station according to the embodiment. The example in Fig. 16 is an example of HAPS radio wave arrival direction estimation using radio wave arrival angle estimation. Note that in Fig. 16, parts that are common to Figs. 12A and 14 described above are assigned the same reference numerals, and descriptions thereof will be omitted.

[0069] 16 , the base station device 22 includes a radio wave arrival direction estimation unit 223 and a radio wave arrival angle measurement unit 227. When the HAPS base station 10 transmits a downlink signal, the radio wave arrival angle measurement unit 227 estimates the radio wave arrival angle (horizontal angle θ, elevation angle φ) of the downlink signal by signal processing using an array antenna (massive antenna) 21. An example of a technology for estimating the radio wave arrival angle is the radio wave arrival angle measurement technology known as "MUSIC technology." The radio wave arrival direction estimation unit 223 determines the radio wave arrival angle (horizontal angle θ and elevation angle φ) estimated by the radio wave arrival angle measurement unit 227 as the radio wave arrival direction (horizontal angle θ and elevation angle φ) of the HAPS.

[0070] With the arrival direction of the radio waves from the HAPS (horizontal angle θ and elevation angle φ) estimated in each of FIGS. 15 and 16 known, the weight calculation unit 224 described above calculates the weight W by, for example, the directionally constrained minimization of power (DCMP), which is a null-forming technique.

[0071] 17 and 18 are diagrams showing an example of control of transmission and reception timing between the terrestrial base station 20 and the HAPS base station 10 according to the embodiment. In the mobile communication system of the present embodiment, the transmission and reception timing of the terrestrial base station 20 and the HAPS base station 10 must be reversed, so the inter-system (inter-base station) cooperation control device 50 adjusts the transmission and reception timing of the terrestrial base station 20 and the HAPS base station 10 so that the transmission and reception frame times of the terrestrial base station 20 and the HAPS base station 10 are reversed. In the example of FIG. 17 , the adjustment of the transmission and reception timing is performed by the HAPS-GW 12. In the example of FIG. 18 , the adjustment of the transmission and reception timing is performed by the relay communication station 11 mounted on the HAPS 100.

[0072] As described above, according to this embodiment, which uses the TDD method as the transmission and reception method, it is possible to share the same frequency for each service link between the terminal and the service link antenna (HAPS base station antenna) of the HAPS base station (first base station) and the service link antenna (terrestrial base station antenna) of the terrestrial base station (second base station), and it is possible to overcome the problem that the inter-cell interference control technology (eICIC) shown in FIG. 5 enables sharing of the same frequency but results in a decrease in the communication capacity of the terminal.

[0073] Furthermore, in this embodiment, the same frequency is shared in each service link between the terminal and the service link antenna (HAPS base station antenna) of the HAPS base station 10 and the terminal and the service link antenna (terrestrial base station antenna) of the terrestrial base station 20, and the TDD system is used as the transmission and reception system, with the transmission timing and reception timing of the TDD system wireless communication in the service link being reversed between the HAPS base station 10 and the terrestrial base station 20. As a result, the downlink of the service link of the terrestrial base station 20 has the same timing as the uplink of the service link of the HAPS base station 10, and there is no interference whatsoever from the terminal (terrestrial base station terminal) 30(2) communicating with the terrestrial base station 20 on the service link uplink of the HAPS base station 10, and only downlink interference of the service link transmitted from the terrestrial base station 20 occurs. On the other hand, the downlink of the service link of the HAPS base station 10 has the same timing as the uplink of the service link of the terrestrial base station 20, so the downlink of the service link of the HAPS base station 10 does not interfere at all with the downlink of the terrestrial base station terminal 30(2), but does interfere with the uplink of the service link of the terrestrial base station 20. From the above, the interference of the uplink and downlink of the service link between the HAPS base station 10 and the terrestrial base station 20 does not interfere at all with the terrestrial base station terminal 30(2) and the HAPS base station terminal 30(1) communicating with the respective base stations 10, 20, but only between the HAPS base station 10 and the terrestrial base station 20. Therefore, in order to suppress interference between the cell (HAPS cell) 10C formed by the HAPS base station 10 and the cell (terrestrial cell) 20C formed by the terrestrial base station 20, it is sufficient to suppress the interference between the HAPS base station 10 and the terrestrial base station 20. Therefore, in this embodiment, by directing the null of the directional beam of the terrestrial base station antenna in the direction of the HAPS base station antenna, it is possible to reduce the interference that the downlink transmission signal from the HAPS base station antenna causes to the reception signal on the uplink of the terrestrial base station antenna, and it is also possible to reduce the interference that the downlink transmission signal from the terrestrial base station antenna causes to the reception signal on the uplink of the HAPS base station antenna.

[0074] Furthermore, according to this embodiment, the frequency utilization rate can be doubled by sharing the frequency between the HAPS service link and the terrestrial base station.

[0075] Furthermore, according to this embodiment, this can be achieved by using a normal TDD system (5G, etc.) and changing the transmission and reception timing of the terrestrial base station 20 and the HAPS base station 10 (without any special changes).

[0076] Furthermore, according to this embodiment, the terrestrial base station 20 can achieve this by using a base station array antenna, which is commonly used in 5G, to control the beam to point a null toward the HAPS located above. No special equipment needs to be added. In particular, since the HAPS direction can be easily estimated using information such as GPS installed in the HAPS, null control to point the null in that direction is easy.

[0077] Furthermore, according to this embodiment, the HAPS base station 10 does not require any special device or control for sharing frequencies.

[0078] Furthermore, according to this embodiment, the transmission timing adjustment between the terrestrial base station 20 and the HAPS base station 10 can be easily achieved by the inter-system (base station) cooperation control device 50 .

[0079] The present invention can improve the frequency utilization efficiency at HAPS base stations and terrestrial base stations, prevent a decrease in terminal communication capacity, and provide a system that enables frequency sharing between HAPS base stations and terrestrial base stations without adding special equipment to the HAPS base stations or terrestrial base stations, thereby contributing to the achievement of Goal 9 of the Sustainable Development Goals (SDGs), which is to "build resilient infrastructure, promote industry, innovation and foster innovation."

[0080] The processing steps described in this specification and the components of the mobile communication system, the airborne relay base station, the HAPS cellular system, the terrestrial base station, the terrestrial cellular system, the relay communication station, the HAPS-GW, the terminal (user equipment, mobile station, mobile machine), and the base station cooperation control device can be implemented by various means. For example, these processing steps and components may be implemented by hardware, firmware, software, or a combination thereof.

[0081] For hardware implementation, the processing units and other means used to implement the above steps and components in an entity (e.g., various wireless communication devices, wireless relay devices, Node Bs, servers, gateways, switches, computers, hard disk drive devices, or optical disk drive devices) may be implemented in one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processors (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, electronic devices, other electronic units designed to perform the functions described herein, computers, or combinations thereof.

[0082] Furthermore, with regard to firmware and / or software implementations, the means used to realize the above components may be implemented as programs (e.g., code, such as procedures, functions, modules, instructions, etc.) that perform the functions described herein. In general, any computer / processor-readable medium tangibly embodying firmware and / or software code may be used to implement means, such as a processing unit, used to realize the above steps and components described herein. For example, the firmware and / or software code may be stored in memory and executed by a computer or processor, such as in a controller. The memory may be implemented within the computer or processor, or external to the processor. The firmware and / or software code may also be stored on a computer or processor readable medium, such as, for example, random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), programmable read-only memory (PROM), electrically erasable programmable read-only memory (EEPROM), flash memory, floppy disk, compact disk (CD), digital versatile disk (DVD), magnetic or optical data storage device, etc. The code may be executed by one or more computers or processors and may cause the computers or processors to perform certain aspects of the functionality described herein.

[0083] Moreover, the description of the embodiments disclosed herein is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to the present disclosure will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other variations without departing from the spirit or scope of the present disclosure. Thus, the present disclosure is not intended to be limited to the examples and designs described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0084] 10: HAPS base station (airborne relay base station) 10C: HAPS cell 11: Relay communication station 12: HAPS-GW 15: GPS receiver 20: Terrestrial base station 20C: Terrestrial cell 21: Terrestrial base station antenna (service link antenna) 22: Base station device 30: Terminal 30(1): HAPS base station terminal 30(2): Terrestrial base station terminal 40: Core network 50: Base station cooperation control device 100: HAPS 111: FL antenna 112: HAPS base station antenna (service link antenna) 113: Repeater 114: Frequency conversion device 115: Base station device 116: Feeder link transceiver 121: FL antenna 122: Base station device 123: Frequency conversion device 124: Feeder link transceiver 210 : Antenna element 221 : Transmitter / receiver device 222 : Null forming unit 223 : Radio wave arrival direction estimation unit 224 : Weight calculation unit 225 : Beam scanning unit 226 : Received power measurement unit 227 : Radio wave arrival angle measurement unit

Claims

1. A mobile communications system comprising an airborne relay-type base station which wirelessly communicates with a terminal via a service link antenna of a relay communications station installed on an aircraft or floating body located in the sky, and one or more terrestrial base stations which wirelessly communicate with the terminal via a service link antenna installed on the ground or sea, the airborne relay-type base station and the terrestrial base station being time-synchronized with each other, the airborne relay-type base station and the terrestrial base station each perform wireless service link communication with the terminal using a TDD (Time Division Duplex) method using the same frequency, the transmission timing and reception timing of the TDD method wireless communication in the service link are reversed between the airborne relay-type base station and the terrestrial base station, and the terrestrial base station estimates the direction of the service link antenna of the airborne relay-type base station and controls the null of the directional beam of the service link antenna of the terrestrial base station to be directed in the estimated direction of the service link antenna of the airborne relay-type base station.

2. A mobile communication system according to claim 1, characterized in that the relay communication station mounted on the flying object or the floating object is composed of a repeater relay device which relays transmitted and received signals without regenerating them.

3. A mobile communication system according to claim 1, characterized in that the relay communication station mounted on the flying body or the floating body is composed of a base station device which regenerates transmitted and received signals and re-modulates and relays the regenerated signals.

4. A mobile communication system according to any one of claims 1 to 3, further comprising an inter-base station control device for controlling communication between the overhead relay base station and the terrestrial base station, wherein the inter-base station control device adjusts transmission timing by reversing transmission and reception, using GNSS (Global Navigation Satellite System) time information received by the overhead relay base station and received via a gateway device, and GNSS (Global Navigation Satellite System) time information received by the terrestrial base station.

5. A mobile communications system according to any one of claims 1 to 3, further comprising an inter-base station control device for controlling communication between the above-mentioned overhead relay base station and the above-mentioned terrestrial base station, wherein position information acquired by a GNSS (Global Navigation Satellite System) receiver mounted on the above-mentioned flying body or the above-mentioned floating body is transferred to the above-mentioned terrestrial base station via a gateway device and the inter-base station control device, and wherein the above-mentioned terrestrial base station estimates the direction of the service link antenna of the above-mentioned relay communications station based on the position information of its own terrestrial base station.

6. A mobile communication system according to any one of claims 1 to 3, characterized in that the terrestrial base station has an array antenna as the service link antenna, measures the direction of arrival of radio waves from the service link antenna of the relay communication station using the array antenna, and estimates the direction of the service link antenna of the relay communication station from the measurement results.

7. A mobile communication system according to any one of claims 1 to 3, characterized in that the flying object or the floating object is a communication satellite, a UAV (Unmanned Aerial Vehicle) flying at an altitude of 18 km or less, or a HAPS flying in the stratosphere at an altitude of 18 km or more.

Images