Startup method of communication device, wireless communication system, and wireless communication device

Abstract

Provided is a method for activating one or more communication devices installed on the ground, which is executed by a wireless communication system including the one or more communication devices and a moving wireless communication device, the method comprising: an activation signal generation step of generating an activation signal for activating the one or more communication devices; a frequency variation application step of applying, to the activation signal generated in the activation signal generation step, a frequency variation compensating for Doppler fluctuation indicating a temporal variation in Doppler shift occurring in the activation signal; and a transmission step of transmitting the activation signal, to which the frequency variation is applied in the frequency variation application step, from the wireless communication device to the one or more communication devices.

Description

Technical Field

[0001] The present invention relates to a method for starting a communication device, a wireless communication system, and a wireless communication device.

Background Art

[0002] With the development of IoT (Internet of Things) technology, it has been considered to install IoT terminals equipped with various sensors in various places. The IoT terminal may be installed in a place where it is difficult to install a base station, such as an offshore buoy, a ship, or a mountainous area. Therefore, a wireless communication system has been proposed in which data collected by IoT terminals installed in various places is relayed to a base station installed on the ground by a relay device mounted on a low-earth orbit satellite.

[0003] Since the IoT terminal is driven by electric power supplied from a battery, it is necessary to operate with low power consumption in order to extend the battery life. Therefore, in a satellite sensing platform, in order to realize the annual battery life of the IoT terminal, it is desirable that the IoT terminal starts up and transmits data uplink when it detects the arrival of a low-earth orbit satellite overhead. In order to detect the arrival of a low-earth orbit satellite in the IoT terminal, for example, a means for observing a downlink signal from the low-earth orbit satellite to the ground can be considered (see, for example, Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, since low-orbit satellites move at high speeds, time-varying Doppler shifts (hereinafter referred to as "Doppler variations") occur in the downlink signals transmitted from low-orbit satellites. Therefore, depending on the positional relationship between a low-orbit satellite and an IoT terminal at a certain point in time, Doppler variations with a variation amount exceeding the demodulation possible range of the downlink signal may occur. In such a case, there is a problem that the IoT terminal may not start up because it cannot demodulate the downlink signal even if the reception level of the downlink signal is sufficiently high. Such a problem is not limited to the signals transmitted from low-orbit satellites, but also occurs similarly in the signals transmitted from various wireless communication devices moving in the sky.

[0006] In view of the above circumstances, an object of the present invention is to provide a technology capable of starting a communication device installed on the ground even when a Doppler variation occurs in an activation signal transmitted from a wireless communication device moving in the sky.

Means for Solving the Problems

[0007] One aspect of the present invention is a method for starting up a communication device performed by a wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, the method including: a startup signal generation step of generating a startup signal for starting up the one or more communication devices; a frequency variation imparting step of imparting a frequency variation for compensating for a Doppler variation indicating a time variation of a Doppler shift occurring in the startup signal to the startup signal generated in the startup signal generation step; and a transmission step of transmitting the startup signal having the frequency variation imparted thereto in the frequency variation imparting step from the wireless communication device to the one or more communication devices.

[0008] One aspect of the present invention is a wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, wherein the wireless communication device includes: a startup signal generation unit that generates a startup signal for starting up the one or more communication devices; a frequency variation imparting unit that imparts a frequency variation for compensating for a time variation of a Doppler shift occurring in the startup signal to the startup signal generated by the startup signal generation unit; and a transmission unit that transmits the startup signal having the frequency variation imparted thereto by the frequency variation imparting unit, and the communication device includes: a reception unit that receives the startup signal transmitted from the wireless communication device; and a startup control unit that sets the own device to a startup state in response to the startup signal received by the reception unit.

[0009] One aspect of the present invention is a wireless communication device in a wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, the wireless communication device including: a startup signal generation unit that generates a startup signal for starting up the one or more communication devices; a frequency variation imparting unit that imparts a frequency variation for compensating for a time variation of a Doppler shift occurring in the startup signal to the startup signal generated by the startup signal generation unit; and a transmission unit that transmits the startup signal having the frequency variation imparted thereto by the frequency variation imparting unit.

Advantages of the Invention

[0010] According to the present invention, even if a Doppler fluctuation occurs in the activation signal transmitted from the wireless communication device moving in the air, it is possible to activate the communication device installed on the ground.

Brief Description of Drawings

[0011]

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[0012] Hereinafter, a method for activating a communication device, a wireless communication system, and a wireless communication device according to embodiments of the present invention will be described with reference to the drawings.

[0013] <First Embodiment> [Configuration of Wireless Communication System] Hereinafter, the configuration of the wireless communication system 1 in the first embodiment will be described. FIG. 1 is a schematic diagram for explaining the configuration of the wireless communication system 1 in the first embodiment of the present invention.

[0014] In the wireless communication system 1 in the first embodiment, as shown in FIG. 1, it has at least a mobile relay station 2 and one or more terminal stations 3. FIG. 1 shows, as an example, the case where two terminal stations 3-1 to 3-2 exist.

[0015] Since the mobile relay station 2 moves at high speed, Doppler fluctuations may occur when the activation signal transmitted from the mobile relay station 2 is received by the terminal station 3 located on the ground. Here, the activation signal refers to a downlink signal transmitted from the mobile relay station 2 moving in the sky toward the ground to activate the terminal station 3. Also, the Doppler fluctuations here refer to the time variation of the Doppler shift obtained by differentiating the Doppler shift. When Doppler fluctuations occur in the activation signal, the terminal station 3 located at a position outside the allowable range of the Doppler fluctuations may not be able to demodulate and decode the activation signal. The terminal station 3 located at a position outside the allowable range of the Doppler fluctuations is the terminal station 3 when the position of the mobile relay station 2 is at a high elevation angle as viewed from the position of the terminal station 3.

[0016] Therefore, as shown in FIG. 1, the ground area outside the allowable range of the Doppler fluctuations is a range centered on the position directly below the mobile relay station 2. FIG. 1 shows an area A1 that is an area where the terminal station 3 can receive the activation signal transmitted from the mobile relay station 2, and an area A2 within the area A1 that is outside the allowable range of the Doppler fluctuations.

[0017] For example, in FIG. 1, the terminal station 3-1 is located inside the area A1 and outside the area A2. Therefore, the terminal station 3-1 is located within the allowable range of the Doppler fluctuations and can demodulate and decode the activation signal transmitted from the mobile relay station 2. On the other hand, in FIG. 1, the terminal station 3-2 is located inside the area A2. Therefore, the terminal station 3-2 is located outside the allowable range of the Doppler fluctuations and may not be able to demodulate and decode the activation signal transmitted from the mobile relay station 2.

[0018] Therefore, the mobile relay station 2 in the first embodiment pre-emptively imparts a frequency variation of an amount that cancels (compensates for) such Doppler variations to the activation signal. Then, the mobile relay station 2 transmits the activation signal with the imparted frequency variation toward the ground. The amount of the frequency variation to be imparted is calculated based on the elevation angle of the mobile relay station 2 as seen from the position of the terminal station 3 and the like. The mobile relay station 2 calculates the amount of the generated Doppler variation (hereinafter referred to as the "Doppler variation amount") based on the elevation angle and the like, and imparts a frequency variation of an amount that cancels the calculated Doppler variation amount to the activation signal.

[0019] Hereinafter, as an example, a case where the allowable range of the amount of Doppler variation for the terminal station 3 to be able to demodulate and decode the activation signal is -15 to 15 [Hz / s] will be described. That is, it is assumed that the terminal station 3 can demodulate and decode the activation signal if the amount of Doppler variation generated in the activation signal transmitted from the mobile relay station 2 is within the range of -15 to 15 [Hz / s].

[0020] Also, in the following description, such a variation width of Doppler variation (here, a variation width of 15 [Hz / s]) for the terminal station 3 to be able to demodulate and decode the activation signal is referred to as "Doppler shift tolerance". The mobile relay station 2 imparts a frequency variation of an amount calculated by "maximum Doppler variation amount - Doppler shift tolerance" to the activation signal.

[0021] For example, when the altitude of the mobile relay station 2 is 570 [km] and the activation signal is in the 400 [MHz] band, the Doppler variation amount near directly below the mobile relay station 2, which is the point where the variation width of the Doppler variation amount is maximum, is approximately 134 [Hz / s]. As described above, the mobile relay station 2 imparts a frequency variation of "maximum Doppler variation amount - Doppler shift tolerance" to the activation signal. That is, in this case, the mobile relay station 2 imparts a frequency variation of 119 [Hz / s] (= 134 - 15) to the activation signal. By imparting and transmitting a frequency variation of 119 [Hz / s] to the activation signal, the terminal station 3 located directly below the mobile relay station 2 and in its vicinity can demodulate and decode the activation signal.

[0022] Figure 2 shows the relationship between the elevation angle and the Doppler shift amount when the mobile relay station 2 does not impart frequency fluctuations to the activation signal. Here, the elevation angle, as described above, is the elevation angle of the mobile relay station 2 as seen from the position of the terminal station 3.

[0023] The graph shown in the upper part of Figure 2 has the x-axis position (unit: km) of the mobile relay station 2 on the horizontal axis and the Doppler shift amount (unit: Hz / s) on the vertical axis. The x-axis position of the mobile relay station 2 here refers to the distance from the point directly below a specific position on the circular orbit of the mobile relay station 2 (for example, the point where the terminal station 3 exists) to the point directly below the actual position where the mobile relay station 2 exists. In the graph in the upper part of Figure 2, when the x-axis position of the mobile relay station 2 is 0 [km], it means that the position of the mobile relay station 2 is directly above the above-mentioned specific point (for example, the point where the terminal station 3 exists). That is, the elevation angle at this time is 90 [deg]. As shown in the graph in the upper part of Figure 2, when the x-axis position of the mobile relay station 2 is 0 [km], the fluctuation range of the Doppler shift amount is the largest, reaching approximately -134 [Hz / s].

[0024] Also, as shown in the graph in the upper part of Figure 2, even when the x-axis position of the mobile relay station 2 is -600 [km] or 600 [km], the Doppler shift amount is below -40 [Hz / s] and does not fall within the Doppler tolerance range (within -15 to 15 [Hz / s]). Therefore, at the point where the position relationship of the mobile relay station 2 is as described above (that is, the point where the x-axis position of the mobile relay station 2 is -600 [km] or 600 [km]), the terminal station 3 may not be able to demodulate and decode the activation signal. That is, it can be seen that if the position of the mobile relay station 2 is not further away from the position of the terminal station 3 (that is, when the position of the mobile relay station 2 is not at a lower elevation angle position), the terminal station 3 may not be able to demodulate and decode the activation signal.

[0025] The graph shown in the lower part of FIG. 2 has the x-axis position of the mobile relay station 2 (unit: km) on the horizontal axis and the elevation angle (unit: deg) on the vertical axis. As shown in the graph in the lower part of FIG. 2, when the x-axis position of the mobile relay station 2 is 0 [km], which means the position of the mobile relay station 2 is directly above the above-mentioned specific point, the elevation angle becomes 90 [deg].

[0026] For example, by comparing the upper graph and the lower graph in FIG. 2, it can be seen that when the elevation angle when viewing the mobile relay station 2 from the position of the terminal station 3 is approximately 40 [deg], the x-axis position of the mobile relay station 2 is approximately -700 [km] or 700 [km]. Therefore, even if the elevation angle when viewing the mobile relay station 2 from the position of the terminal station 3 is approximately 40 [deg], the Doppler shift amount does not fall within the Doppler tolerance range (within -15 to 15 [Hz / s]), and it can be understood that the terminal station 3 may not be able to demodulate and decode the start signal.

[0027] FIG. 3 is a diagram showing the relationship between the elevation angle and the Doppler shift amount when the mobile relay station 2 applies a frequency variation of the variation amount of "maximum Doppler shift amount - Doppler shift tolerance" to the start signal. That is, FIG. 3 represents the relationship between the elevation angle and the Doppler shift amount when the mobile relay station 2 applies a frequency variation of 119 [Hz / s] (= 134 - 15) to the start signal. Here, the elevation angle as mentioned above is the elevation angle of the mobile relay station 2 viewed from the position of the terminal station 3.

[0028] The graph shown in the upper part of FIG. 3 has the x-axis position of the mobile relay station 2 (unit: km) on the horizontal axis and the Doppler shift amount (unit: Hz / s) on the vertical axis. As shown in the graph in the upper part of FIG. 3, it can be seen that when the x-axis position of the mobile relay station 2 is within the range of approximately -200 to 200 [km], the Doppler shift amount falls within the range of -15 to 15 [Hz / s]. That is, it can be understood that the terminal station 3 existing at a point where the x-axis position of the mobile relay station 2 is within the range of approximately -200 to 200 [km] can demodulate and decode the start signal.

[0029] Also, when the start signal with the frequency variation of the variation amount of 119 [Hz / s] is used in this way, it can be seen that the ground range in which the x-axis position of the mobile relay station 2 is within the range of approximately -200 to 200 [km] becomes the activatable area of the terminal station 3. Note that the graph shown in the lower part of FIG. 3 is the same as the graph shown in the lower part of FIG. 2 and shows the relationship between the x-axis position and the elevation angle of the mobile relay station 2.

[0030] Furthermore, the mobile relay station 2 time-division transmits start signals with frequency variations of different variation amounts. For example, in addition to the start signal with the frequency variation of the variation amount of the above-mentioned "maximum Doppler variation amount - Doppler shift tolerance", the mobile relay station 2 generates and time-division transmits a start signal with the frequency variation of the variation amount of "maximum Doppler variation amount - Doppler shift tolerance × 3" and a start signal with the frequency variation of the variation amount of "maximum Doppler variation amount - Doppler shift tolerance × 5". In this way, the mobile relay station 2 performs time-division transmission in which a plurality of types of start signals with the frequency variation of the variation amount calculated by subtracting an odd multiple value of the Doppler shift tolerance from the maximum Doppler variation amount are switched and transmitted at a predetermined interval.

[0031] FIG. 4 is a diagram showing the relationship between the elevation angle and the Doppler variation amount when the mobile relay station 2 applies a frequency variation of the variation amount of "maximum Doppler variation amount - Doppler shift tolerance × 3" to the start signal. That is, FIG. 4 represents the relationship between the elevation angle and the Doppler variation amount when the mobile relay station 2 applies a frequency variation of 89 [Hz / s] (= 134 - 45) to the start signal. Note that the elevation angle here is, as described above, the elevation angle of the mobile relay station 2 as seen from the position of the terminal station 3.

[0032] The graph shown in the upper part of Figure 4 has the x-axis position of the mobile relay station 2 (unit: km) on the horizontal axis and the Doppler fluctuation amount (unit: Hz / s) on the vertical axis. As shown in the graph in the upper part of Figure 4, it can be seen that when the x-axis position of the mobile relay station 2 is within the range of approximately -400 to -200 [km] and within the range of approximately 200 to 400 [km], the Doppler fluctuation amount falls within -15 to 15 [Hz / s]. That is, it can be seen that the terminal station 3 existing at the points where the x-axis position of the mobile relay station 2 is within the range of approximately -400 to -200 [km] and within the range of approximately 200 to 400 [km] can demodulate and decode the start signal.

[0033] Also, in this way, when a start signal with a frequency fluctuation of a fluctuation amount of 89 [Hz / s] is used, it can be seen that the ground range where the x-axis position of the mobile relay station 2 is within the range of approximately -400 to -200 [km] and within the range of approximately 200 to 400 [km] becomes the startable area of the terminal station 3. Note that the graph shown in the lower part of Figure 4 is the same as the graphs shown in the lower part of Figure 2 and the lower part of Figure 3, and shows the relationship between the x-axis position and the elevation angle of the mobile relay station 2.

[0034] Figure 5 is a diagram showing the relationship between the elevation angle and the Doppler fluctuation amount when the mobile relay station 2 applies a frequency fluctuation with a fluctuation amount of "maximum Doppler fluctuation amount - Doppler shift tolerance × 5" to the start signal. That is, Figure 5 represents the relationship between the elevation angle and the Doppler fluctuation amount when the mobile relay station 2 applies a frequency fluctuation of 59 [Hz / s] (= 134 - 75) to the start signal. Here, the elevation angle mentioned here is, as described above, the elevation angle of the mobile relay station 2 as seen from the position of the terminal station 3.

[0035] The graph shown in the upper part of Fig. 5 has the x-axis position of the mobile relay station 2 (unit: km) on the horizontal axis and the Doppler fluctuation amount (unit: Hz / s) on the vertical axis. As shown in the graph in the upper part of Fig. 5, it can be seen that when the x-axis position of the mobile relay station 2 is within the range of approximately -600 to -400 [km] and within the range of approximately 400 to 600 [km], the Doppler fluctuation amount falls within -15 to 15 [Hz / s]. That is, it can be seen that the terminal station 3 existing at the point where the x-axis position of the mobile relay station 2 is within the range of approximately -600 to -400 [km] and within the range of approximately 400 to 600 [km] can demodulate and decode the start signal.

[0036] Also, when a start signal with a frequency fluctuation of a fluctuation amount of 59 [Hz / s] is used in this way, it can be seen that the ground range where the x-axis position of the mobile relay station 2 is within the range of approximately -600 to -400 [km] and within the range of approximately 400 to 600 [km] becomes the startable area of the terminal station 3. Note that the graph shown in the lower part of Fig. 5 is the same as the graphs shown in the lower parts of Fig. 2, Fig. 3, and Fig. 4, and shows the relationship between the x-axis position and the elevation angle of the mobile relay station 2.

[0037] Hereinafter, a start signal with a frequency fluctuation of "maximum Doppler fluctuation amount - Doppler shift tolerance" will be referred to as "start signal A", a start signal with a frequency fluctuation of "maximum Doppler fluctuation amount - Doppler shift tolerance × 3" will be referred to as "start signal B", and a start signal with a frequency fluctuation of "maximum Doppler fluctuation amount - Doppler shift tolerance × 5" will be referred to as "start signal C".

[0038] In this way, the mobile relay station 2 can perform time-division transmission in which the start signal A, the start signal B, and the start signal C are sequentially switched at a predetermined interval, and the range in which the startable areas by these three start signals are combined can be set as the startable area of the terminal station 3. That is, the mobile relay station 2 can set the ground range where the x-axis position of the mobile relay station 2 is within the range of approximately -600 to 600 [km] as the startable area of the terminal station 3.

[0039] Also, in the above example, the activatable area of the terminal station 3 by the activation signal A, the activatable area of the terminal station 3 by the activation signal B, and the activatable area of the terminal station 3 by the activation signal C do not overlap with each other. However, the present invention is not limited to such a configuration, and a configuration may be adopted in which the activatable areas of the terminal station 3 by each activation signal are partially overlapped (with a margin) with each other little by little.

[0040] For example, a value for a predetermined margin is determined in advance. An activation signal with a frequency variation of "maximum Doppler shift amount - Doppler shift tolerance" is set as the activation signal A, an activation signal with a frequency variation of "maximum Doppler shift amount - Doppler shift tolerance × 3 - margin × 1" is set as the activation signal B, and an activation signal with a frequency variation of "maximum Doppler shift amount - Doppler shift tolerance × 5 - margin × 2" is set as the activation signal C. Also, the number of types of activation signals used is not limited to three, and any number of activation signals may be used. By adopting such a configuration, for example, it is possible to prevent a gap from occurring between activatable areas at the timing when the activation signal is switched due to the high-speed movement of the mobile relay station 2.

[0041] [Functional Configuration of Wireless Communication System] Hereinafter, the functional configuration of the wireless communication system 1 will be described. FIG. 6 is a block diagram showing the functional configuration of the wireless communication system 1 in the first embodiment of the present invention. As shown in FIG. 6, the wireless communication system 1 includes a mobile relay station 2, one or more terminal stations 3, and a base station 4.

[0042] The number of the mobile relay station 2, the terminal stations 3, and the base station 4 included in the wireless communication system 1 is arbitrary. It is assumed that the number of the terminal stations 3 is large. FIG. 6 shows a case where the wireless communication system 1 has two terminal stations 3-1 and 3-2. In the following description, when there is no particular distinction between the terminal stations 3-1 and 3-2, they are simply referred to as the terminal station 3.

[0043] The mobile relay station 2 is an example of a wireless communication device mounted on a moving body, and the communicable area moves over time. When the mobile relay station 2 reaches above the data collection area, it transmits an activation signal for activating the ground terminal station 3. The mobile relay station 2 transmits the activation signal periodically, for example, at intervals of several seconds. The data collection area is an area for collecting data acquired by the terminal station 3. The mobile relay station 2 determines, for example, whether it has reached above the data collection area based on the orbit information and time information of the mobile relay station 2.

[0044] The mobile relay station 2 of the present embodiment is provided on a LEO (Low Earth Orbit) satellite. The altitude of the LEO satellite is 2000 [km] or less, and it orbits around the earth in about 1.5 hours. The terminal station 3 and the base station 4 are installed on the earth such as on the ground or at sea. Hereinafter, the wireless signal transmitted from the terminal station 3 to the mobile relay station 2 is referred to as a terminal uplink signal, and the signal transmitted from the mobile relay station 2 to the terminal station 3 and the base station 4 is referred to as a downlink signal.

[0045] Since the mobile relay station 2 mounted on the LEO satellite communicates while moving at high speed, the time during which each individual terminal station 3 or base station 4 can communicate with the mobile relay station 2 is limited. Specifically, when viewed from the ground, the mobile relay station 2 passes over the sky in about several minutes. Therefore, the terminal station 3 collects and stores data such as environmental data detected by the sensor. The terminal station 3 transmits the set terminal uplink signal with the collected data at a timing when communication with the mobile relay station 2 is possible.

[0046] The mobile relay station 2 receives terminal uplink signals transmitted from each of the plurality of terminal stations 3 while moving over the earth's sky. The mobile relay station 2 accumulates the data received by the terminal uplink signal from the terminal station 3, and wirelessly transmits the accumulated data to the base station 4 by a downlink signal at a timing when communication with the base station 4 is possible. The base station 4 acquires the data collected by the terminal station 3 from the received downlink signal.

[0047] The mobile relay station 2 has an antenna for use in wireless communication with the terminal station 3 and an antenna for use in wireless communication with the base station 4. Therefore, the mobile relay station 2 can perform wireless communication with the terminal station 3 and wireless communication with the base station 4 in parallel.

[0048] As the mobile relay station 2, it is conceivable to use a relay station mounted on a geostationary satellite, a drone, an unmanned aircraft such as a HAPS (High Altitude Platform Station), etc. However, in the case of a relay station mounted on a geostationary satellite, although the ground coverage area (footprint) is large, due to its high altitude, the link budget for IoT terminals installed on the ground is very small. On the other hand, in the case of a relay station mounted on a drone or HAPS, although the link budget is high, the coverage area is narrow. Furthermore, a drone requires a battery and a HAPS requires a solar panel.

[0049] In this embodiment, the mobile relay station 2 is mounted on a LEO satellite. Therefore, in addition to the link budget being within limits, since the LEO satellite orbits outside the atmosphere, there is no air resistance and fuel consumption is also low. Also, when the mobile relay station 2 is mounted on a LEO satellite, the footprint is larger compared to the case where a relay station is mounted on a drone or HAPS.

[0050] The terminal station 3 collects data such as environmental data detected by a sensor, for example. The terminal station 3 is activated based on an activation signal transmitted from the mobile relay station 2 and wirelessly transmits the collected data to the mobile relay station 2. For example, when the transmission timing is instructed from the mobile relay station 2, the terminal station 3 wirelessly transmits the collected data to the mobile relay station 2 at the instructed transmission timing. The terminal station 3 is an aspect of a communication device.

[0051] The base station 4 receives the data collected by the terminal station 3 from the mobile relay station 2.

[0052] The terminal station 3 and the base station 4 are installed at specific locations on the earth such as on the ground or at sea.

[0053] The configurations of the respective devices will be described in more detail below. First, the configuration of the mobile relay station 2 will be described. As shown in FIG. 6, the mobile relay station 2 includes one antenna 21, a terminal communication unit 22, a storage unit 23, a control unit 24, a base station communication unit 25, and one antenna 26. Note that the mobile relay station 2 may include a plurality of antennas 21. In such a case, the mobile relay station 2 performs reception processing by MIMO (multiple-input and multiple-output).

[0054] The terminal communication unit 22 includes a transceiver unit 221, an activation signal generation unit 223, a frequency fluctuation imparting unit 225, a frequency conversion unit 227, and a reception waveform recording unit 228.

[0055] The transceiver unit 221 receives a terminal uplink signal via the antenna 21. In this way, the transceiver unit 221 communicates with one or more terminal stations 3 via the antenna 21.

[0056] The frequency conversion unit 227 converts an RF (Radio Frequency) signal, which is the terminal uplink signal received by the transceiver unit 221, into a baseband signal using a quadrature demodulator or the like. The frequency conversion unit 227 outputs the baseband signal to the reception waveform recording unit 228.

[0057] The reception waveform recording unit 228 acquires the baseband signal output from the frequency conversion unit 227. The reception waveform recording unit 228 samples the baseband signal and records the waveform data obtained by the sampling. The reception waveform recording unit 228 stores the waveform data in the storage unit 23 as reception data 232.

[0058] The activation signal generation unit 223 generates an activation signal for activating the terminal station 3. Note that the activation signal may include information for identifying a specific terminal station 3 and information indicating the transmission timing of the terminal uplink signal, etc.

[0059] The frequency variation imparting unit 225 imparts a frequency variation to the activation signal generated by the activation signal generation unit 223 under the control of the variation imparting control unit 242.

[0060] The storage unit 23 stores at least the orbit information 231, the received data 232, and the imparting table 233. The orbit information 231 is information regarding the orbit of the LEO satellite carrying the mobile relay station 2, and is information that can obtain, for example, the position, velocity, moving direction, etc. of the LEO satellite at any given time. The received data 232 is data collected by the terminal station 3 and is data to be transmitted to the base station 4. The imparting table 233 is a table in which the values of the variation amounts of the frequency variations imparted to the activation signal by the frequency variation imparting unit 225 are registered for each of a plurality of types of activation signals.

[0061] FIG. 7 is a diagram showing an example of the imparting table 233 in the first embodiment of the present invention. The imparting table 233 has a plurality of records representing information regarding the variation amounts of the frequency variations imparted to the activation signal. The records have values of the activation signal type and the imparted variation amount.

[0062] The value of the activation signal type represents identification information for distinguishing a plurality of types of activation signals that are sequentially switched and transmitted at predetermined intervals by the frequency variation imparting unit 225. The value of the imparted variation amount is the value of the variation amount of the frequency variation imparted to each of the above-described plurality of types of activation signals. For example, the unit of the value of the imparted variation amount is Hz / s.

[0063] In the example shown in FIG. 7, for example, at the uppermost row of the imparting table 233, the activation signal type "activation signal A" and the imparted variation amount "119" are associated with each other. This indicates that during the period when the activation signal A is transmitted, a frequency variation with a variation amount of 119 [Hz / s] is imparted to the symbol signal generated by the activation signal generation unit 223 by the frequency variation imparting unit 225.

[0064] The control unit 24 is configured using a processor such as a CPU (Central Processing Unit) and a storage medium such as a memory. By executing a program, the control unit 24 realizes the functions of the operation control unit 241 and the variation imparting control unit 242. Some or all of these functional units may be realized by hardware (including circuitry) such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array), or may be realized by the cooperation of software and hardware. Some of these functions do not necessarily need to be pre-installed in the mobile relay station 2 and may be realized by installing an additional application program in the mobile relay station 2.

[0065] The operation control unit 241 refers to the orbit information 231 and the time information, and determines whether the location where the LEO satellite equipped with the mobile relay station 2 is currently located is above the data collection area. If it is the data collection area, the operation control unit 241 instructs the variation imparting control unit 242 to acquire the value of the amount of frequency variation, and instructs the terminal communication unit 22 to transmit an activation signal. On the other hand, if it is not the data collection area, the operation control unit 241 does nothing in particular.

[0066] The variation imparting control unit 242 acquires, according to the instruction from the operation control unit 241, information indicating the amount of frequency variation to be imparted to the activation signal from the imparting table 233 stored in the storage unit 23. The variation imparting control unit 242 controls the frequency variation imparting unit 225 so that the acquired amount of frequency variation is imparted to the activation signal. The variation imparting control unit 242 switches the amount of frequency variation imparted to the activation signal by the frequency variation imparting unit 225 at a predetermined interval.

[0067] The base station communication unit 25 reads out the received data 232 (waveform data) stored in the storage unit 23 from the storage unit 23 as transmission data to the base station 4. The base station communication unit 25 performs encoding and modulation of the transmission data to generate a downlink signal. The base station communication unit 25 transmits the downlink signal from the antenna 26 at a timing when communication with the base station 4 is possible.

[0068] In this embodiment, the fluctuation application control unit 242 is configured to switch the fluctuation amount of the frequency fluctuation applied to the activation signal by the frequency fluctuation application unit 225 at predetermined intervals. However, the present invention is not limited to this. For example, the activation signal generated by the activation signal generation unit 223 is distributed to a plurality of frequency fluctuation application units (not shown), and each of the plurality of frequency fluctuation application units applies frequency fluctuations with different fluctuation amounts to the activation signal, and the fluctuation application control unit 242 may be configured to switch one frequency fluctuation application unit to be in an active state at predetermined intervals.

[0069] Next, the configuration of the terminal station 3 will be described. As shown in FIG. 6, the terminal station 3 includes a data storage unit 31, a transmission / reception unit 32, a demodulation unit 33, an activation control unit 34, and an antenna 35. Note that the terminal station 3 is in a sleep state except for some functions until it receives an activation signal from the mobile relay station 2 in order to suppress power consumption. Here, some functions are, for example, the data storage unit 31, the transmission / reception unit 32, the demodulation unit 33, and the activation control unit 34 shown in FIG. 6. The terminal station 3 may include a plurality of antennas 35.

[0070] Environmental data detected by the sensor is stored in the data storage unit 31. The transmission / reception unit 32 communicates with the mobile relay station 2. For example, the transmission / reception unit 32 receives a downlink signal transmitted from the mobile relay station 2. For example, the transmission / reception unit 32 reads out environmental data from the data storage unit 31 as terminal transmission data. The transmission / reception unit 32 wirelessly transmits the set terminal uplink signal of the read terminal transmission data from the antenna 35.

[0071] The transceiver unit 32 transmits and receives signals, for example, by LPWA (Low Power Wide Area). Examples of LPWA include LoRaWAN (registered trademark), Sigfox (registered trademark), LTE-M (Long Term Evolution for Machines), NB (Narrow Band)-IoT, etc., but any wireless communication method can be used. The transceiver unit 32 may transmit and receive with other terminal stations 3 by time-division multiplexing, OFDM (Orthogonal Frequency Division Multiplexing), etc. The transceiver unit 32 may perform beamforming of the signals transmitted from the plurality of antennas 35 by a method determined in advance in the wireless communication method used.

[0072] The demodulation unit 33 demodulates and decodes the downlink signal received by the transceiver unit 32. A Doppler shift occurs in the downlink signal according to the distance between the mobile relay station 2 and the terminal station 3.

[0073] The startup control unit 34 controls to shift the terminal station 3 from the sleep state to the startup state according to the startup signal included in the downlink signal demodulated and decoded by the demodulation unit 33.

[0074] Next, the configuration of the base station 4 will be described. As shown in FIG. 6, the base station 4 includes an antenna 41. The base station 4 converts the downlink signal received by the antenna 41 into an electrical signal and then performs demodulation and decoding to obtain waveform data. The base station 4 performs reception processing of the terminal uplink signal included in the waveform data. At this time, the base station 4 performs reception processing by the wireless communication method used by the terminal station 3 to acquire the terminal transmission data.

[0075] As described above, in this embodiment, the mobile relay station 2 is configured to convert the RF signal, which is the terminal uplink signal, into a baseband signal, record the waveform data obtained by sampling the baseband signal, and transmit the recorded data to the base station 4. Therefore, in this embodiment, it is assumed that the mobile relay station 2 does not perform reception processing for obtaining terminal transmission data.

[0076] However, the present invention is not limited to such a configuration. For example, the mobile relay station 2 may sample the terminal uplink signal as an RF signal and transmit the obtained waveform data to the base station 4. Then, the base station 4 may convert the RF signal representing the waveform data into a baseband signal and perform reception processing.

[0077] Alternatively, for example, the mobile relay station 2 may convert the RF signal, which is the terminal uplink signal, into a baseband signal and perform reception processing up to demodulation and decoding of the terminal uplink signal. Then, the mobile relay station 2 may transmit the terminal transmission data obtained by the reception processing to the base station 4.

[0078] [Operation of Mobile Relay Station] Hereinafter, an example of the operation of the mobile relay station 2 will be described. FIG. 8 is a flowchart showing the flow of the activation process of the terminal station 3 performed by the mobile relay station 2 in the first embodiment of the present invention.

[0079] The operation control unit 241 determines that the current position of the mobile relay station 2 is above the data collection area (step S101). The operation control unit 241 instructs the fluctuation imparting control unit 242 to acquire the value of the fluctuation amount of the frequency fluctuation, and instructs the terminal communication unit 22 to transmit an activation signal. In response to the instruction from the operation control unit 241, the fluctuation imparting control unit 242 refers to the imparting table 233 and acquires the value of the fluctuation amount of the frequency fluctuation for each type of activation signal (for example, the above activation signal A, activation signal B, and activation signal C) (step S102).

[0080] The start signal generation unit 223 generates a start signal in response to an instruction from the operation control unit 241 (step S103). The start signal generation unit 223 outputs the generated start signal to the frequency fluctuation imparting unit 225. The fluctuation imparting control unit 242 controls the frequency fluctuation imparting unit 225 so that the amount of frequency fluctuation acquired from the imparting table 233 is imparted to the start signal. The frequency fluctuation imparting unit 225 imparts a frequency fluctuation to the start signal generated by the start signal generation unit 223 under the control of the fluctuation imparting control unit 242 (step S104).

[0081] The frequency fluctuation imparting unit 225 outputs the start signal with the frequency fluctuation imparted thereto to the transceiver unit 221. The transceiver unit 221 transmits the start signal output from the frequency fluctuation imparting unit 225 as a downlink signal to the ground via the antenna 21 (step S105).

[0082] When a predetermined time has elapsed (step S106·YES), the fluctuation imparting control unit 242 controls the frequency fluctuation imparting unit 225 to change the amount of frequency fluctuation imparted to the start signal (step S107).

[0083] When the operation control unit 241 determines that the current position of the mobile relay station 2 has passed over the data collection area (step S108·YES), the operation control unit 241 terminates the start-up process of the terminal station 3 performed by the mobile relay station 2. That is, while the mobile relay station 2 is over the data collection area, the processes from step S103 to step S108 are repeatedly executed.

[0084] [Operation of the terminal station] Hereinafter, an example of the operation of the terminal station 3 will be described. FIG. 9 is a flowchart showing the flow of the start-up process of the terminal station 3 in the first embodiment of the present invention.

[0085] In step S105 of the flowchart in FIG. 8, the downlink signal transmitted from the mobile relay station 2 is received by the terminal station 3 located within the range where the radio wave transmitted from the mobile relay station 2 reaches. The transceiver unit 32 of the terminal station 3 receives the downlink signal transmitted from the mobile relay station 2 (step S110).

[0086] The transceiver unit 32 of the terminal station 3 outputs the received downlink signal to the demodulation unit 33. The demodulation unit 33 of the terminal station 3 demodulates and decodes the downlink signal (step S111). The activation control unit 34 of the terminal station 3 controls to shift the terminal station 3 from the sleep state to the activation state based on the activation signal demodulated and decoded by the demodulation unit 33 (step S112).

[0087] The transceiver unit 32 of the terminal station 3 transmits a terminal uplink signal based on the environmental data stored in the data storage unit 31 to the mobile relay station 2 (step S113). Thereby, the mobile relay station 2 can receive the terminal uplink signals transmitted from each terminal station 3.

[0088] According to the wireless communication system 1 in the first embodiment configured as described above, the mobile relay station 2 pre - assigns a frequency variation that cancels (compensates for) the Doppler variation to the activation signal and transmits the activation signal with the frequency variation towards the ground. Thus, even if a Doppler variation occurs in the activation signal received at the terminal station 3 installed on the ground, it is canceled by the frequency variation pre - assigned to the activation signal, so that the terminal station 3 can demodulate the activation signal. In this way, in the wireless communication system 1, even when a Doppler variation occurs in the activation signal transmitted from the mobile relay station 2 moving in the air, it is possible to activate the terminal station 3 installed on the ground.

[0089] Furthermore, in the wireless communication system 1 according to the first embodiment, since the mobile relay station 2 sequentially transmits a plurality of types of activation signals each having a frequency variation with a different variation amount toward the ground while switching at a predetermined interval, it is possible to cancel (compensate for) Doppler variations with a wider variation amount, and thus a wider activatable area of the terminal station 3 can be secured.

[0090] In the first embodiment, a configuration is shown in which the mobile relay station 2 determines whether it is above the data collection area based on the orbit information 231 and the time information. However, the present invention is not limited to such a configuration. For example, the mobile relay station 2 may be configured to determine whether it is above the data collection area by other methods. Specifically, for example, the mobile relay station 2 may grasp the start time and the end time for data collection from the terminal station 3 by an uplink communication from the base station 4, and determine that it is above the data collection area between the start time and the end time.

[0091] In the first embodiment, the case where the mobile body on which the mobile relay station 2 is mounted is a LEO satellite has been described. However, the mobile body may be a geostationary satellite, a drone, or other flying bodies flying in the sky such as HAPS.

[0092] <Modification Example of the First Embodiment> Hereinafter, a modification example of the first embodiment will be described. In the above-described first embodiment, the mobile relay station 2 is configured to perform time-division transmission in which the activation signal A, the activation signal B, and the activation signal C are sequentially switched at a predetermined interval and transmitted. In this case, as will be described below, the activatable area of the terminal station 3 by the activation signal A and the activatable area of the terminal station 3 by the activation signal B are adjacent to each other, and the activatable area of the terminal station 3 by the activation signal B and the activatable area of the terminal station 3 by the activation signal C are adjacent to each other.

[0093] As shown in the upper graph of FIG. 3, the activatable area of the terminal station 3 by the activation signal A is a ground area where the x-axis position of the mobile relay station 2 is within a range of approximately -200 to 200 [km]. Also, as shown in the upper graph of FIG. 4, the activatable area of the terminal station 3 by the activation signal B is a ground area where the x-axis position of the mobile relay station 2 is within a range of approximately -400 to -200 [km] and within a range of approximately 200 to 400 [km]. Further, as shown in the upper graph of FIG. 5, the activatable area of the terminal station 3 by the activation signal C is a ground area where the x-axis position of the mobile relay station 2 is within a range of approximately -600 to -400 [km] and within a range of approximately 400 to 600 [km].

[0094] FIG. 10 is a schematic diagram for explaining the relationship between the transmission timing of the activation signal and the activatable area by the wireless communication system 1 in the first embodiment of the present invention described above. As shown in FIG. 10, according to the wireless communication system 1 in the first embodiment described above, each time the amount of frequency variation applied to the activation signal is switched at a predetermined interval, the activatable area shifts so that adjacent ground areas become the activatable areas of the terminal station 3 in order.

[0095] However, in this case, it is considered that the chance for a plurality of terminal stations 3 in adjacent areas to transmit the terminal uplink signal to the mobile relay station 2 at the same timing increases. FIG. 11 is a schematic diagram for explaining the relationship between the transmission timing of the activation signal and the transmission timing of the terminal uplink signal by the wireless communication system 1 in the first embodiment of the present invention described above.

[0096] In FIG. 11, the horizontal axis represents time. As shown in FIG. 11, the mobile relay station 2 in the first embodiment described above transmits the activation signal toward the ground while switching in order of the activation signal A ("Activation A" in FIG. 11), the activation signal B ("Activation B" in FIG. 11), and the activation signal C ("Activation C" in FIG. 11) at a predetermined interval.

[0097] In this case, for example, there is a possibility that the terminal uplink signal transmitted from the terminal station 3 activated by the activation signal A ("Up A" in FIG. 11), the terminal uplink signal transmitted from the terminal station 3 activated by the activation signal B ("Up B" in FIG. 11), and the terminal uplink signal transmitted from the terminal station 3 activated by the activation signal C ("Up C" in FIG. 11) are transmitted at the same timing. Thus, when terminal uplink signals are transmitted at the same timing from a plurality of terminal stations 3 existing in adjacent areas to each other, there is a higher possibility that the terminal uplink signals collide with each other and the mobile relay station 2 cannot perform signal separation.

[0098] On the other hand, the mobile relay station 2 in the modification of the first embodiment described below controls the transmission timing and transmission order of each activation signal so that terminal uplink signals are not transmitted at the same timing from a plurality of terminal stations 3 existing in adjacent areas to each other.

[0099] FIG. 12 is a schematic diagram for explaining the relationship between the transmission timing of the activation signal and the activatable area by the wireless communication system 1 in the modification of the first embodiment of the present invention. When the wireless communication system 1 in the modification of the first embodiment switches the amount of frequency variation given to the activation signal at a predetermined interval, it controls so that the activatable area after the switching does not become an area adjacent to the activatable area before the switching. For example, as shown in FIG. 12, the wireless communication system 1 in the modification of the first embodiment controls so that the timing of transmitting the activation signal A and the activation signal C and the timing of transmitting the activation signal B do not become close to each other.

[0100] FIG. 13 is a schematic diagram for explaining the relationship between the transmission timing of the activation signal and the transmission timing of the terminal uplink signal by the wireless communication system 1 in a modified example of the first embodiment of the present invention. In FIG. 13, the horizontal axis represents time. As shown in FIG. 13, the mobile relay station 2 in the modified example of the first embodiment transmits the activation signal A ("Activation A" in FIG. 13) toward the ground, and then continues to transmit the activation signal C ("Activation C" in FIG. 13) toward the ground after a predetermined period has elapsed.

[0101] In this case, for example, there is a possibility that the terminal uplink signal ("Up A" in FIG. 13) transmitted from the terminal station 3 activated by the activation signal A and the terminal uplink signal ("Up C" in FIG. 13) transmitted from the terminal station 3 activated by the activation signal C are transmitted at the same timing. Thus, if terminal uplink signals are transmitted from a plurality of terminal stations 3 at the same timing, the terminal uplink signals may collide with each other.

[0102] However, as described above, the activation-enabled area by the activation signal A and the activation-enabled area by the activation signal C are not adjacent to each other. Thus, even if the terminal uplink signals transmitted at the same timing from a plurality of terminal stations 3 existing in non-adjacent areas collide, it is easy to separate the signals by reception beam control. Therefore, the mobile relay station 2 can demodulate and decode both the terminal uplink signal ("Up A" in FIG. 13) transmitted from the terminal station 3 activated by the activation signal A and the terminal uplink signal ("Up C" in FIG. 13) transmitted from the terminal station 3 activated by the activation signal C.

[0103] Then, the mobile relay station 2 transmits another activation signal that is an activatable area adjacent to the activatable area by the previously transmitted activation signal, with a slight delay. For example, as shown in FIG. 13, the mobile relay station 2 transmits the activation signal A and the activation signal C toward the ground, and then transmits the activation signal B toward the ground after a slight delay. Specifically, for example, the mobile relay station 2 transmits the activation signal A and the activation signal C within the same downlink transmission period, and transmits the activation signal B in a downlink transmission period different from the above downlink transmission period.

[0104] According to the wireless communication system 1 in the modification of the first embodiment configured as described above, the mobile relay station 2 imparts a frequency variation that cancels (compensates) the Doppler variation to the activation signal, and transmits the activation signal with the imparted frequency variation toward the ground. As a result, even if a Doppler variation occurs in the activation signal received at the terminal station 3 installed on the ground, it is canceled by the frequency variation previously imparted to the activation signal, so that the terminal station 3 can demodulate and decode the activation signal. Thus, in the wireless communication system 1, even when a Doppler variation occurs in the activation signal transmitted from the mobile relay station 2 moving in the sky, it is possible to activate the terminal station 3 installed on the ground.

[0105] Furthermore, in the wireless communication system 1 in the modification of the first embodiment, the transmission timing of each activation signal is controlled so that terminal uplink signals are not transmitted from a plurality of terminal stations 3 existing in adjacent areas at the same timing. With such a configuration, the wireless communication system 1 can suppress the situation where the terminal uplink signals cannot be demodulated and decoded due to a collision of a plurality of terminal uplink signals. As described above, even if terminal uplink signals transmitted at the same timing from a plurality of terminal stations 3 existing in non-adjacent areas collide, the mobile relay station 2 can perform signal separation by reception beam control, so that each terminal uplink signal can be demodulated and decoded.

[0106] <Second Embodiment> Next, a second embodiment will be described. The above-described first embodiment is an embodiment assuming a case where the signal bandwidth of the start signal is relatively wide, for example, several tens of kHz. In this case, the influence of the Doppler shift of the start signal can be compensated by the frequency offset compensation function generally provided on the terminal station 3 side. Therefore, in the above-described first embodiment, only the influence of the Doppler fluctuation is a problem, and the influence of the Doppler shift is not considered.

[0107] However, the narrower the signal bandwidth of the start signal becomes, the narrower the range of the Doppler shift that can be compensated. The influence of the Doppler shift on the order of kHz caused by the high-speed movement of the low-earth orbit satellite cannot be compensated only by the frequency offset compensation function generally provided on the terminal station 3 side. Therefore, the wireless communication system 1a in the second embodiment described below compensates not only the influence of the Doppler fluctuation but also the influence of the Doppler shift. That is, the moving relay station 2a in the second embodiment pre-applies a frequency shift for canceling the assumed Doppler shift to the start signal and then transmits the start signal to the terminal station 3.

[0108] Also, according to the wireless communication system 1 in the above-described first embodiment, depending on the amount of variation in the frequency variation applied to the start signal, for example, as shown in FIGS. 1, 4, and 5 described above, startable areas are formed in the ground range in front of and behind the moving relay station 2, respectively.

[0109] For example, as shown in FIG. 4, under the conditions of the first embodiment, when the mobile relay station 2 imparts a frequency variation with a variation amount of "maximum Doppler variation amount - Doppler shift tolerance × 3" to the activation signal, an activatable area is formed in the range 200 to 400 [km] in front of the mobile relay station 2 and in the range 200 to 400 [km] behind the mobile relay station 2. Further, for example, as shown in FIG. 5, under the conditions of the first embodiment, when the mobile relay station 2 imparts a frequency variation with a variation amount of "maximum Doppler variation amount - Doppler shift tolerance × 5" to the activation signal, an activatable area is formed in the range 400 to 600 [km] in front of the mobile relay station 2 and in the range 400 to 600 [km] behind the mobile relay station 2.

[0110] However, the area in front of the mobile relay station 2 is generally the area that the mobile relay station 2 is about to head to, and it is an area where it is desired to actively collect data from the terminal station 3 existing in the area. On the other hand, the area behind the mobile relay station 2 is generally an area where the mobile relay station 2 is moving away over time, and the communication success rate of communication between the terminal station 3 existing in the area and the mobile relay station 2 is gradually decreasing. Therefore, it is not an area where it is desired to actively collect data from the terminal station 3 existing in the area. For such a ground area behind the mobile relay station 2, it is possible to expect a higher communication success rate by entrusting the data collection from the terminal station 3 existing in the area to another mobile relay station 2 that will come later.

[0111] Note that, as described above, the wireless communication system 1a in the second embodiment described below is configured to compensate not only for the influence of Doppler variation but also for the influence of Doppler shift. Thereby, the wireless communication system 1a can also perform activation control of the terminal station 3, such as activating only the terminal station 3 existing in the area in front of the mobile relay station 2a in the second embodiment.

[0112] Specifically, in the terminal station 3 existing in the area in front of the mobile relay station 2a, due to the influence of Doppler shift, the reception frequency of the start signal shifts in a higher direction. The mobile relay station 2a pre-shifts the transmission frequency of the start signal in a lower direction in order to compensate for such an influence of Doppler shift. At this time, the mobile relay station 2a also performs, in addition, the imparting of frequency fluctuation to the start signal for compensating for the influence of Doppler fluctuation, which the mobile relay station 2 in the first embodiment described above performs.

[0113] Also, in the terminal station 3 existing in the area behind the mobile relay station 2a in the second embodiment, due to the influence of Doppler shift, the reception frequency of the start signal shifts in a lower direction. And if the mobile relay station 2a does not impart a frequency shift for canceling the Doppler shift to the start signal at this time, the terminal station 3 on the side of the area behind the mobile relay station 2a cannot compensate for the influence of Doppler shift.

[0114] By having such a configuration, the wireless communication system 1a in the second embodiment can activate only the terminal station 3 existing in the ground area in front of the mobile relay station 2a. Conversely, of course, it is also possible to configure the wireless communication system 1a in the second embodiment to activate only the terminal station 3 existing in the area behind the mobile relay station 2a.

[0115] [Configuration of Wireless Communication System] Hereinafter, the configuration of the wireless communication system 1a in the second embodiment will be described in more detail. FIG. 14 is a schematic diagram for explaining the configuration of the wireless communication system 1a in the second embodiment of the present invention.

[0116] In the wireless communication system 1a in the second embodiment, as shown in FIG. 14, it has at least a mobile relay station 2a and one or more terminal stations 3. FIG. 14 shows, as an example, the case where two terminal stations 3-1 and 3-3 exist.

[0117] Since the mobile relay station 2a moves at high speed, when the start signal transmitted by the mobile relay station 2a is received by the terminal stations 3 arranged in each area, a Doppler shift on the order of KHz occurs. FIG. 15 is a diagram showing an example of the relationship between the position of the mobile relay station 2a relative to the terminal station 3 and the Doppler shift. When a Doppler shift occurs in the start signal, there are cases where the terminal station 3 cannot demodulate and decode the start signal.

[0118] Therefore, when transmitting the start signal, the mobile relay station 2a in the second embodiment transmits a start signal with a frequency shift applied thereto.

[0119] It is assumed that the shift amount of the frequency shift is determined in advance based on the altitude of the mobile relay station 2a (more specifically, the moving speed of the mobile relay station 2a determined by the altitude), the downlink transmission frequency, and the position of the mobile relay station 2a and the area. For example, when it is desired to activate the terminal station 3 in an area approximately 300 [km] ahead from directly below the mobile relay station 2a towards the front of the mobile relay station 2a, the mobile relay station 2a can cancel the influence of the Doppler shift by applying a frequency shift with a shift amount of approximately -5 [kHz] to the start signal, enabling the terminal station 3 to demodulate and decode the start signal.

[0120] [Functional Configuration of Wireless Communication System] Hereinafter, the functional configuration of the wireless communication system 1a will be described. FIG. 16 is a block diagram showing the functional configuration of the wireless communication system 1a in the second embodiment of the present invention. In the functional configuration of the wireless communication system 1a, functional units having the same configuration as the functional units provided in the wireless communication system 1 in the aforementioned first embodiment shown in FIG. 6 may be denoted by the same reference numerals and the description thereof may be omitted.

[0121] While the wireless communication system 1 in the aforementioned first embodiment shown in FIG. 6 has the mobile relay station 2, the wireless communication system 1a in the second embodiment shown in FIG. 16 has a mobile relay station 2a instead of the mobile relay station 2. Further, while the mobile relay station 2 in the aforementioned first embodiment shown in FIG. 6 has the terminal communication unit 22 and the control unit 24, the mobile relay station 2a in the second embodiment shown in FIG. 16 has the terminal communication unit 22a and the control unit 24a instead of the terminal communication unit 22 and the control unit 24.

[0122] The terminal communication unit 22a in the second embodiment shown in FIG. 16 further has a frequency shift imparting unit 225a in addition to the functional configuration of the terminal communication unit 22 in the aforementioned first embodiment shown in FIG. 6. Also, the control unit 24a in the second embodiment shown in FIG. 16 further has a shift imparting control unit 242a in addition to the functional configuration of the control unit 24 in the aforementioned first embodiment shown in FIG. 6.

[0123] Hereinafter, the configuration of the mobile relay station 2a will be described. As shown in FIG. 16, the mobile relay station 2 includes one antenna 21, a terminal communication unit 22a, a storage unit 23, a control unit 24a, a base station communication unit 25, and one antenna 26. Note that the mobile relay station 2 may include a plurality of antennas 21. When configured in this way, the mobile relay station 2 performs reception processing by MIMO.

[0124] The terminal communication unit 22a includes a transceiver unit 221, an activation signal generation unit 223, a frequency variation imparting unit 225, a frequency shift imparting unit 225a, a frequency conversion unit 227, and a received waveform recording unit 228.

[0125] The transceiver unit 221 receives a terminal uplink signal through the antenna 21. In this way, the transceiver unit 221 communicates with one or more terminal stations 3 through the antenna 21.

[0126] The frequency conversion unit 227 converts the RF signal, which is the terminal uplink signal received by the transceiver unit 221, into a baseband signal using a quadrature demodulator or the like. The frequency conversion unit 227 outputs the baseband signal to the received waveform recording unit 228.

[0127] The received waveform recording unit 228 acquires the baseband signal output from the frequency conversion unit 227. The received waveform recording unit 228 samples the baseband signal and records the waveform data obtained by the sampling. The received waveform recording unit 228 stores the waveform data in the storage unit 23 as received data 232.

[0128] The start signal generation unit 223 generates a start signal for starting the terminal station 3.

[0129] The frequency variation applying unit 225 applies a frequency variation to the start signal generated by the start signal generation unit 223 under the control of the variation applying control unit 242.

[0130] The frequency shift applying unit 225a applies a frequency shift to the start signal to which a frequency variation has been applied by the frequency variation applying unit 225 under the control of the shift applying control unit 242a.

[0131] The storage unit 23 stores at least orbit information 231, received data 232, and an application table 233a. The orbit information 231 is information regarding the orbit of the LEO satellite mounted on the mobile relay station 2, and is information that can obtain, for example, the position, speed, and moving direction of the LEO satellite at an arbitrary time. The received data 232 is data collected by the terminal station 3 and is data to be transmitted to the base station 4. The application table 233a is a table in which the value of the variation amount of the frequency variation applied to the start signal by the frequency variation applying unit 225 and the value of the shift amount of the frequency shift applied to the start signal by the frequency shift applying unit 225a are registered for each start signal.

[0132] The value of the shift amount of the frequency shift included in the assignment table 233a is a value determined in advance based on the altitude of the mobile relay station 2a (more specifically, the moving speed of the mobile relay station 2a determined by the altitude), the downlink transmission frequency, and the position of the mobile relay station 2a and the area, as described above.

[0133] The control unit 24a is configured using a processor such as a CPU and a memory. The control unit 24a realizes the functions of the operation control unit 241, the variation assignment control unit 242, and the shift assignment control unit 242a by executing a program. Some or all of these functional units may be realized by hardware such as an ASIC, a PLD, or an FPGA, or may be realized by cooperation between software and hardware. Some of these functions do not necessarily need to be pre-mounted on the mobile relay station 2a, and may be realized by installing an additional application program on the mobile relay station 2a.

[0134] The operation control unit 241 refers to the orbit information 231 and the time information, and determines whether the location where the LEO satellite carrying the mobile relay station 2a is currently located is above the data collection area. If it is the data collection area, the operation control unit 241 instructs the variation assignment control unit 242 to acquire the value of the variation amount of the frequency variation, instructs the shift assignment control unit 242a to acquire the value of the shift amount of the frequency shift, and instructs the terminal communication unit 22a to transmit an activation signal. On the other hand, if it is not the data collection area, the operation control unit 241 does nothing in particular.

[0135] The variation assignment control unit 242 acquires the value of the variation amount of the frequency variation to be applied to the activation signal from the assignment table 233a stored in the storage unit 23 in response to an instruction from the operation control unit 241. The variation assignment control unit 242 controls the frequency variation application unit 225 so that the acquired variation amount of the frequency variation is applied to the activation signal. The variation assignment control unit 242 switches the variation amount of the frequency variation applied to the activation signal by the frequency variation application unit 225 at predetermined intervals.

[0136] The shift application control unit 242a acquires, in response to an instruction from the operation control unit 241, the value of the shift amount of the frequency shift applied to the activation signal from the application table 233a stored in the storage unit 23. The shift application control unit 242a controls the frequency shift application unit 225a so that the acquired shift amount of the frequency shift is applied to the activation signal. The shift application control unit 242a switches the shift amount of the frequency shift applied to the activation signal by the frequency shift application unit 225a at a predetermined interval.

[0137] [Operation of Mobile Relay Station] Hereinafter, an example of the operation of the mobile relay station 2a will be described. FIG. 17 is a flowchart showing the flow of the activation process of the terminal station 3 performed by the mobile relay station 2a in the second embodiment of the present invention.

[0138] The operation control unit 241 determines that the current position of the mobile relay station 2a is above the data collection area (step S201). The operation control unit 241 instructs the fluctuation application control unit 242 to acquire the value of the fluctuation amount of the frequency fluctuation, instructs the shift application control unit 242a to acquire the value of the shift amount of the frequency shift, and instructs the terminal communication unit 22a to transmit an activation signal.

[0139] The fluctuation application control unit 242, in response to an instruction from the operation control unit 241, refers to the application table 233a and acquires the value of the fluctuation amount of the frequency fluctuation for each type of activation signal (for example, activation signal A, activation signal B, and activation signal C) (step S202). The shift application control unit 242a, in response to an instruction from the operation control unit 241, refers to the application table 233a and acquires the value of the shift amount of the predetermined frequency shift to be applied to the activation signal (step S203).

[0140] The activation signal generation unit 223 generates an activation signal in response to an instruction from the operation control unit 241 (step S204). The activation signal generation unit 223 outputs the generated activation signal to the frequency fluctuation application unit 225.

[0141] The variation application control unit 242 controls the frequency variation application unit 225 so that the frequency variation of the variation amount acquired from the application table 233 is applied to the activation signal. The frequency variation application unit 225 applies a frequency variation to the activation signal generated by the activation signal generation unit 223 under the control of the variation application control unit 242 (step S205).

[0142] The shift application control unit 242a controls the frequency shift application unit 225a so that the frequency shift of the shift amount acquired from the storage unit 23 is applied to the activation signal. The frequency shift application unit 225a further applies a frequency shift to the activation signal to which a frequency variation has been applied by the frequency variation application unit 225 under the control of the shift application control unit 242a (step S206).

[0143] The frequency shift application unit 225a outputs the activation signal to which the frequency shift has been applied to the transceiver unit 221. The transceiver unit 221 transmits the activation signal output from the frequency shift application unit 225a as a downlink signal to the ground via the antenna 21 (step S207).

[0144] When a predetermined time has elapsed (step S208·YES), the variation application control unit 242 changes the variation amount of the frequency variation to be applied to the activation signal by the frequency variation application unit 225, and the shift application control unit 242a changes the shift amount of the frequency shift to be applied to the activation signal by the frequency shift application unit 225a (step S209).

[0145] When the operation control unit 241 determines that the current position of the mobile relay station 2a has passed over the data collection area (step S210·YES), the operation control unit 241 terminates the activation process of the terminal station 3 performed by the mobile relay station 2a. That is, while the mobile relay station 2a is over the data collection area, the mobile relay station 2a repeatedly executes the processes from step S204 to step S210.

[0146] Note that since the operation of the terminal station 3 in the second embodiment is the same as the operation of the terminal station 3 in the first embodiment shown in FIG. 9 described above, the description thereof is omitted.

[0147] According to the wireless communication system 1a in the second embodiment configured as described above, the mobile relay station 2a pre - assigns a frequency shift that cancels (compensates for) the Doppler shift to the start signal and transmits the start signal with the frequency shift applied. As a result, even if a Doppler shift occurs in the start signal received at the terminal station 3 installed on the ground, since a frequency shift with an appropriate shift amount for the area is pre - given, the start signal is emphasized by the Doppler shift. Therefore, the start signal can be demodulated and decoded at the terminal station 3. As a result, the terminal station 3 can be started. In this way, in the wireless communication system 1a, even when a Doppler shift occurs in the start signal transmitted from the mobile relay station 2a moving in the air, it is possible to start the terminal station 3 installed on the ground.

[0148] Furthermore, according to the wireless communication system 1a in the second embodiment, the mobile relay station 2a pre - assigns a frequency variation that cancels (compensates for) the Doppler variation to the start signal and transmits the start signal with the frequency variation applied toward the ground. As a result, even if a Doppler variation occurs in the start signal received at the terminal station 3 installed on the ground, it is canceled by the frequency variation pre - given to the start signal, so the terminal station 3 can demodulate the start signal. In this way, in the wireless communication system 1a, even when a Doppler variation occurs in the start signal transmitted from the mobile relay station 2a moving in the air, it is possible to start the terminal station 3 installed on the ground.

[0149] Furthermore, in the wireless communication system 1a in the second embodiment, since the mobile relay station 2a transmits a plurality of start signals with different amounts of frequency variation applied in sequence while switching at a predetermined interval toward the ground, it is possible to cancel (compensate for) a wider range of Doppler variations, so a wider start - up area for the terminal station 3 can be secured.

[0150] In the second embodiment, a configuration was shown in which the mobile relay station 2a determines whether or not it is above the data collection area based on the orbit information 231 and the time information. However, the configuration is not limited to this. For example, the mobile relay station 2a may obtain the start time and end time for data collection from the terminal station 3 through an uplink communication from the base station 4, and determine that it is above the data collection area during the period from the start time to the end time.

[0151] In the second embodiment, the case where the mobile body on which the mobile relay station 2a is mounted is a LEO satellite was described. However, the mobile body may be other flying bodies flying in the air such as a geostationary satellite, a drone, or a HAPS.

[0152] It is also possible to combine the configuration of the wireless communication system 1 described as a modification of the first embodiment with the configuration of the wireless communication system 1a in the second embodiment.

[0153] According to the above-described embodiments, the wireless communication system includes one or more communication devices installed on the ground and a mobile wireless communication device. For example, the wireless communication system is the wireless communication system 1 or the wireless communication system 1a in the embodiment, the communication device is the terminal station 3 in the embodiment, and the wireless communication device is the mobile relay station 2 or the mobile relay station 2a in the embodiment.

[0154] The above wireless communication device includes a start signal generation unit, a frequency variation imparting unit, and a transmission unit. For example, the start signal generation unit is the start signal generation unit 223 in the embodiment, the frequency variation imparting unit is the frequency variation imparting unit 225 in the embodiment, and the transmission unit is the transceiver unit 221 in the embodiment.

[0155] The above startup signal generation unit generates a startup signal for starting one or more communication devices. The above frequency variation imparting unit imparts a frequency variation for compensating for the time variation of the Doppler shift occurring in the startup signal to the startup signal generated by the startup signal generation unit. The above transmission unit transmits the startup signal to which the frequency variation has been imparted by the frequency variation imparting unit.

[0156] The above communication device includes a reception unit and a startup control unit. For example, the reception unit is the transmission / reception unit 32 in the embodiment, and the startup control unit is the startup control unit 34 in the embodiment. The above reception unit receives the startup signal transmitted from the wireless communication device. The startup control unit activates the own device in accordance with the startup signal received by the reception unit.

[0157] Note that in the above wireless communication system, at least one of the variation amounts of the frequency variation imparted to the startup signal may be calculated by subtracting the Doppler tolerance indicating the variation range of the allowable variation amount for the communication device to demodulate and decode the startup signal from the maximum value of the variation amount of the Doppler variation.

[0158] Note that the above wireless communication system may further include a variation imparting control unit. For example, the variation imparting control unit is the variation imparting control unit 242 in the embodiment. The above variation imparting control unit may change the variation amount of the frequency variation imparted to the startup signal by the frequency variation imparting unit at predetermined intervals.

[0159] Note that in the above wireless communication system, the variation amount of the frequency variation changed at predetermined intervals may be calculated by subtracting an odd multiple value of the Doppler tolerance from the maximum value of the variation amount of the Doppler variation.

[0160] Note that the above-described variation imparting control unit may change the amount of variation of the imparted frequency variation so that the ground area where the communication device can be started by the start signal immediately before the amount of variation of the imparted frequency variation is changed and the ground area where the communication device can be started by the start signal immediately after the amount of variation of the imparted frequency variation is changed do not become adjacent areas to each other.

[0161] Note that the above wireless communication system may further include a frequency shift imparting unit. For example, the wireless communication system is the wireless communication system 1a in the embodiment, and the frequency shift imparting unit is the frequency shift imparting unit 225a in the embodiment. The above frequency shift imparting unit may impart a frequency shift for compensating the Doppler shift generated in the start signal to the start signal generated by the start signal generating unit.

[0162] Some or all of the processes performed by the mobile relay station 2 and the mobile relay station 2a in the above-described embodiment may be realized by a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed. Here, the "computer system" is assumed to include hardware such as an OS and peripheral devices. Further, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, or a storage device such as a hard disk incorporated in a computer system.

[0163] Furthermore, the "computer-readable recording medium" may include those that dynamically hold a program for a short time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, and those that hold a program for a certain period of time, such as a volatile memory inside a computer system serving as a server or client in that case. Also, the above program may be for realizing a part of the aforementioned functions, and furthermore, it may be capable of realizing the aforementioned functions in combination with a program already recorded in a computer system, or it may be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0164] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Industrial Applicability

[0165] The present invention can be applied to a technique for communicating with a moving body equipped with a mobile relay station.

Explanation of Reference Numerals

[0166] 1, 1a... wireless communication system, 2, 2a... mobile relay station, 3... terminal station, 4... base station, 21... antenna, 22, 22a... terminal communication unit, 23... storage unit, 24, 24a... control unit, 25... base station communication unit, 26... antenna, 31... data storage unit, 32... transceiver, 33... demodulation unit, 34... startup control unit, 35... antenna, 41... antenna, 221... transceiver, 223... startup signal generation unit, 225... frequency variation imparting unit, 225a... frequency shift imparting unit, 227... frequency conversion unit, 228... received waveform recording unit, 241... operation control unit, 242... variation imparting control unit, 242a... shift imparting control unit

Claims

1. A method for starting up the communication device in a wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, comprising: a startup signal generation step of generating a startup signal for starting up the one or more communication devices; a frequency variation imparting step of imparting a frequency variation for compensating for a Doppler variation indicating a time variation of a Doppler shift occurring in the startup signal to the startup signal generated in the startup signal generation step; a transmission step of transmitting the startup signal to which the frequency variation has been imparted in the frequency variation imparting step from the wireless communication device to the one or more communication devices; a variation imparting control step of changing the amount of variation of the frequency variation imparted to the startup signal in the frequency variation imparting step at predetermined intervals; A method for starting up a communication device having the above steps.

2. A method for starting up the communication device in a wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, comprising: a startup signal generation step of generating a startup signal for starting up the one or more communication devices; a frequency variation imparting step of imparting a frequency variation for compensating for a Doppler variation indicating a time variation of a Doppler shift occurring in the startup signal to the startup signal generated in the startup signal generation step; a transmission step of transmitting the startup signal to which the frequency variation has been imparted in the frequency variation imparting step from the wireless communication device to the one or more communication devices; having at least one of the amounts of variation of the frequency variation imparted to the startup signal is calculated by subtracting a Doppler tolerance indicating a variation range of the amount of variation allowable for the communication device to demodulate and decode the startup signal from the maximum value of the amount of variation of the Doppler variation; A method for starting up a communication device.

3. a variation imparting control step of changing the amount of variation of the frequency variation imparted to the startup signal in the frequency variation imparting step at predetermined intervals The method for starting up a communication device according to claim 2, further comprising the above step.

4. The amount of variation of the frequency variation changed at the predetermined intervals is calculated by subtracting a value that is an odd multiple of the Doppler tolerance from the maximum value of the amount of variation of the Doppler variation. The method for starting up a communication device according to claim 3.

5. In the frequency variation imparting control step, the amount of variation of the frequency variation to be imparted is changed so that the on-ground area where the communication device can be activated by the activation signal immediately before the amount of variation of the imparted frequency variation is changed and the on-ground area where the communication device can be activated by the activation signal immediately after the amount of variation of the imparted frequency variation is changed do not become adjacent areas to each other. The method for activating a communication device according to claim 3.

6. A frequency shift imparting step of imparting a frequency shift for compensating for the Doppler shift occurring in the activation signal to the activation signal generated by the activation signal generating step The method for activating a communication device according to any one of claims 1 to 5, further comprising the above.

7. A wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, wherein the wireless communication device has an activation signal generating unit that generates an activation signal for activating the one or more communication devices, a frequency variation imparting unit that imparts a frequency variation for compensating for the time variation of the Doppler shift occurring in the activation signal to the activation signal generated by the activation signal generating unit, a transmission unit that transmits the activation signal to which the frequency variation has been imparted by the frequency variation imparting unit, and a variation imparting control unit that changes the amount of variation of the frequency variation imparted to the activation signal by the frequency variation imparting unit at predetermined intervals. The wireless communication system is provided with the above. The communication device has a reception unit that receives the activation signal transmitted from the wireless communication device, and an activation control unit that sets the own device to an activated state in response to the activation signal received by the reception unit. The wireless communication system is provided with the above.

8. The wireless communication device in a wireless communication system having one or more communication devices installed on the ground and a moving wireless communication device, wherein the wireless communication device has an activation signal generating unit that generates an activation signal for activating the one or more communication devices, a frequency variation imparting unit that imparts a frequency variation for compensating for the time variation of the Doppler shift occurring in the activation signal to the activation signal generated by the activation signal generating unit, a transmission unit that transmits the activation signal to which the frequency variation has been imparted by the frequency variation imparting unit, and a variation imparting control unit that changes the amount of variation of the frequency variation imparted to the activation signal by the frequency variation imparting unit at predetermined intervals.

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