Communication system and communication method

Abstract

In this invention, a second communication device transmits notification information pertaining to the status of communication between a transmission device and a first communication device to the transmission device. The transmission device determines, on the basis of the notification information received from the second communication device, a transmission method for transmitting data to the first communication device, and transmits the data to the first communication device using the determined transmission method.

Description

[Technical Field]

[0001] The present invention relates to a communication system and a communication method. [Background technology]

[0002] With the development of IoT (Internet of Things) technology, the installation of IoT terminals equipped with various sensors in various locations is being considered. For example, it is expected that IoT will be used to collect data from offshore buoys, ships, mountainous areas, and other locations where it is difficult to install base stations. Meanwhile, there is a technology that uses LEO (Low Earth Orbit) satellites to wirelessly communicate with ground-based communication devices (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Boya Di, et al. "Ultra-dense LEO: Integrating Terrestrial-Satellite Networks into 5G and Beyond for Data Offloading", IEEE Transactions on Wireless Communications,Volume 18, No. 1,January 2019 Summary of the Invention [Problem to be solved by the invention]

[0004] Because LEO satellites are constantly moving as seen from Earth, opportunities for communication between terrestrial transmitters and LEO satellites are limited. Therefore, there is a need to increase the probability of communication between transmitters and LEO satellites during these limited opportunities.

[0005] In view of the above circumstances, the present invention aims to provide a communication system and a communication method that can increase the probability of communication between a communication device that communicates while moving and a terrestrial transmitting device when communication opportunities are limited. [Means for solving the problem]

[0006] One aspect of the present invention is a communication system comprising a first communication device that communicates while moving, a transmitting device that transmits data to the first communication device, and a second communication device that communicates with the transmitting device, wherein the second communication device transmits notification information to the transmitting device regarding the communication status between the transmitting device and the first communication device, and the transmitting device determines a method of transmitting the data to the first communication device based on the notification information received from the second communication device, and transmits the data to the first communication device using the determined transmission method.

[0007] One aspect of the present invention is a communication method involving a first communication device that communicates while moving, a transmitting device that transmits data to the first communication device, and a second communication device that communicates with the transmitting device, the communication method comprising the steps of: the second communication device transmitting notification information to the transmitting device regarding the communication status between the transmitting device and the first communication device; the transmitting device determining a method of transmitting the data to the first communication device based on the notification information received from the second communication device; and the transmitting device transmitting the data to the first communication device using the determined transmission method. [Effects of the Invention]

[0008] According to at least one of the above aspects, in communication between a communication device that communicates while moving and a terrestrial transmitter, it is possible to increase the probability of communication between the communication device and the transmitter in limited communication opportunities. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a configuration diagram of a wireless communication system according to an embodiment. [Figure 2] FIG. 2 is a block diagram showing the configuration of a LEO relay station provided in a LEO satellite according to the embodiment. [Figure 3] FIG. 2 is a block diagram showing the configuration of a GEO relay station provided in a GEO satellite according to the embodiment. [Figure 4] FIG. 2 is a block diagram showing a configuration of a terminal station according to the embodiment. [Figure 5] FIG. 2 is a block diagram showing a configuration of a base station according to the embodiment. [Figure 6] FIG. 10 is a flowchart showing processing of a terminal station according to the embodiment. [Figure 7] FIG. 10 is a flow chart showing processing of a LEO relay station according to the embodiment. [Figure 8] FIG. 10 is a flowchart showing processing of a base station according to the embodiment. [Figure 9] FIG. 10 is a flowchart showing processing of a GEO relay station according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0011] (First embodiment) 1 is a configuration diagram of a wireless communication system 1 according to an embodiment. The wireless communication system 1 includes a LEO satellite 2, a GEO (Geostationary Orbit) satellite 3, a terminal station 4, a gateway for LEO satellite (GWL) 5, a gateway for GEO satellite (GWG) 6, and a base station 7. The wireless communication system 1 includes one or more LEO satellites 2, one or more GEO satellites 3, one or more terminal stations 4, one or more GWLs 5, one or more GWGs 6, and one or more base stations 7. However, it is assumed that the number of terminal stations 4 is large.

[0012] The LEO satellite 2 relays communications between the terminal station 4 and the base station 7. Specifically, when the LEO satellite 2 passes over the terminal station 4, it receives a signal from the terminal station 4 and stores it as spectrum data. Furthermore, when the LEO satellite 2 passes over the GWL 5, it transmits a signal onto which the stored spectrum data is superimposed.

[0013] The GEO satellite 3 receives broadcast information indicating the communication status between the LEO satellite 2 and the terminal station 4 from the base station 7, and transmits the broadcast information to the terminal station 4.

[0014] The terminal station 4, GWL 5, GWG 6, and base station 7 are installed on the Earth, such as on land or sea. The terminal station 4 is, for example, an IoT terminal. The GWL 5 and GWG 6 are earth stations. The terminal station 4 stores and transmits sensor data to the LEO satellite 2. The base station 7 receives spectrum data from the LEO satellite 2 via the GWL 5 and recovers the sensor data. The base station 7 also analyzes the communication status between the LEO satellite 2 and the terminal station 4 based on the spectrum data received from the LEO satellite 2, and generates broadcast information, which it then transmits to the GEO satellite 3 via the GWG 6. The GWL 5 is an antenna station provided to realize MIMO communication between the base station 7 and the LEO satellite 2. The multiple GWLs 5 are positioned apart from each other so that the difference in the angles of arrival of signals from the multiple antennas provided on the LEO satellite 2 is large. Each GWL 5 converts a signal received from the LEO satellite 2 into an electrical signal and outputs it to the base station 7. The GWG 6 is an antenna station provided to realize communication between the base station 7 and the GEO satellite 3. The GWG 6 converts the electrical signal transmitted from the base station into a radio signal and transmits it to the GEO satellite 3 .

[0015] Hereinafter, the LEO satellite 2 and the GEO satellite 3 will also be collectively referred to as satellites, and the GWL 5 and the GWG 6 will also be collectively referred to as earth stations. Furthermore, a radio signal from the terminal station 4 to the satellite will be referred to as a terminal uplink signal, and a radio signal from the satellite to the terminal station 4 will be referred to as a terminal downlink signal. A radio signal from the earth station to the satellite will be referred to as an earth station uplink signal, and a radio signal from the satellite to the earth station will be referred to as an earth station downlink signal.

[0016] LEO satellite 2 is constantly moving as seen from Earth. Its altitude is below 2,000 km, and it orbits the Earth once every 1.5 hours. Terminal station 4 and LEO satellite 2 can communicate when terminal station 4 is within the footprint of LEO satellite 2. GEO satellite 3 has an altitude of approximately 36,000 km, and it orbits the Earth once every day. GEO satellite 3 is always located in the same place in the sky as seen from Earth. Terminal station 4 and GEO satellite 3 can always communicate. In other words, the period during which communication is possible between terminal station 4 and GEO satellite 3 is longer than the period during which communication is possible between terminal station 4 and LEO satellite 2. In areas with many terminal stations 4, multiple terminal stations 4 attempt to communicate with LEO satellite 2 simultaneously during limited communication opportunities, which can easily cause collisions between terminal uplink signals and reduce the probability of communication.

[0017] Therefore, in the wireless communication system 1 of this embodiment, the GEO satellite 3, which has a relatively high probability of communication with the terminal station 4, transmits broadcast information related to the communication status with the LEO satellite 2 to the terminal station 4. Then, the terminal station 4 determines a transmission method that will increase the probability of communication with the LEO satellite 2 based on the broadcast signal. The GEO satellite 3, which is always in the same position as seen from the Earth, is always able to communicate with the terminal station 4. Therefore, the LEO satellite 2 can reliably transmit the latest broadcast information to the terminal station 4.

[0018] The configuration of each device will now be described. Fig. 2 is a block diagram showing the configuration of the LEO relay station 20 provided in the LEO satellite 2 according to the embodiment. The LEO relay station 20 includes a plurality of first antennas 21, a terminal communication unit 22, a base station communication unit 23, and a plurality of second antennas 24. The first antenna 21 is used for communication with the terminal station 4. The second antenna 24 is used for communication with the base station 7. The terminal communication unit 22 includes a storage unit 221, a reception schedule determination unit 222, a receiver 223, a combiner 224, and a spectrum converter 225.

[0019] The storage unit 221 stores position data of the terminal station 4 and orbit data of the LEO satellite 2. The position data of the terminal station 4 is represented by, for example, latitude and longitude. The orbit data of the LEO satellite 2 is data that makes it possible to obtain the position, speed, movement direction, etc. of the LEO satellite 2 at any time. The storage unit 221 has a storage area for storing spectrum data of the terminal uplink signal received from the terminal station 4.

[0020] The reception schedule determination unit 222 identifies the timing for receiving a signal from each terminal station 4 based on the position data and trajectory data of the terminal station 4 stored in the storage unit 221. The reception unit 223 receives signals via the multiple first antennas 21.

[0021] The combining unit 224 combines the multiple signals received by the receiving unit 223 via the multiple first antennas 21 in accordance with a predetermined combining parameter. The combining parameter is represented, for example, by a phase and amplitude offset of each first antenna 21. The combining parameter is calculated based on the signal reception timing determined by the reception schedule determination unit 222 and the positional relationship between the LEO satellite 2 and the communication partner terminal station 4 at that reception timing. In other embodiments, the combining parameter may always be a constant value. The combining unit 224 regenerates the terminal uplink signal by combining the signals.

[0022] The spectrum conversion unit 225 converts the signal synthesized by the synthesis unit 224 into a frequency spectrum. The spectrum conversion unit 225 obtains the frequency spectrum of the received signal by, for example, FFT (Fast Fourier Transform). The spectrum conversion unit 225 records spectrum data representing the generated frequency spectrum in the storage unit 221. The spectrum data is represented by a combination of frequency and power of the frequency.

[0023] The base station communication unit 23 transmits spectrum data representing the waveform of the terminal uplink signal received by the terminal communication unit 22 to the base station 7 by MIMO. The base station communication unit 23 includes a storage unit 231, a transmission schedule determination unit 232, a control unit 233, a MIMO communication unit 234, a data generation unit 235, and a transmission data modulation unit 236.

[0024] The storage unit 231 stores a communication time period with the base station 7 that is determined in advance from the position of the base station 7 and the orbit of the LEO satellite 2. The storage unit 231 also stores in advance a weight of the base station downlink signal to be transmitted from each second antenna 24 for each transmission time during the communication time period. The transmission time may be expressed, for example, as the elapsed time from the start of transmission. The weight for each transmission time is calculated based on the orbit data of the LEO satellite 2 and the position of each GWL 5.

[0025] The transmission schedule determination unit 232 determines a transmission time period for each spectrum data item based on the communication time period and the number of spectrum data items stored in the storage unit 221. For example, the transmission schedule determination unit 232 determines the transmission time period for each spectrum data item by dividing the length of the communication time period by the number of spectrum data items, and determines the transmission time period for each spectrum data item by dividing the communication time period by the transmission time.

[0026] The control unit 233 instructs the MIMO communication unit 234 on the weight for each transmission time read from the storage unit 231. The MIMO communication unit 234 establishes MIMO communication with the base station 7 using a predetermined protocol.

[0027] The data generation unit 235 converts the spectrum data stored in the storage unit 221 into parallel signals and modulates them. The modulated parallel signals are weighted by weights specified by the control unit 233 and transmitted from each second antenna 24 as a base station downlink signal.

[0028] 3 is a block diagram showing the configuration of a GEO relay station 30 provided in a GEO satellite 3 according to an embodiment. The GEO relay station 30 includes an antenna 31 and an earth station communication unit 32. The antenna 31 is used for communication with the terminal station 4 and the base station 7. The earth station communication unit 32 includes a receiving unit 321, a storage unit 322, and a transmitting unit 323.

[0029] The receiver 321 receives a base station uplink signal from the GWG 6 via the antenna 31. The base station uplink signal includes broadcast information generated by the base station 7. The broadcast information includes orbital information of the LEO satellite 2, as well as the transmission method and transmission frequency for each terrestrial area. The orbital information is represented by a time series of the latitude and longitude of the LEO satellite 2. Each area is, for example, a mesh of the earth divided into multiple areas, and is represented by the latitude and longitude of the four corners of the mesh. The multilevel number, the number of bits of the error correction code, and the transmission power are specified as the transmission method. Note that in other embodiments, an error detection code may be used instead of the error correction code. The transmission frequency is the frequency at which the terminal station 4 transmits a terminal uplink signal during a communication time period. The memory unit 322 stores the broadcast information received by the receiver 321. The transmitter 323 transmits the broadcast information stored in the memory unit 322 to the terminal station 4 via the antenna 31 using a terminal downlink signal. This allows the GEO relay station 30 to reliably transmit to the terminal station 4 the notification information that indicates the analysis result of the communication status between the LEO satellite 2 and the terminal station 4 by the base station 7.

[0030] FIG. 4 is a block diagram showing the configuration of the terminal station 4 according to the embodiment. The terminal station 4 includes a data storage unit 41, a receiving unit 42, a positioning unit 43, a condition determination unit 44, a transmitting unit 45, and one or more antennas 46. The terminal station 4 is an example of a transmitting device that transmits data to a communication device. The data storage unit 41 stores sensor data. The receiving unit 42 receives terminal downlink signals from GEO satellites 3 via the multiple antennas 46 and reads out broadcast information. The positioning unit 43 identifies the terrestrial position of the terminal station 4 using the Global Navigation Satellite System (GNSS) and identifies the area in which the terminal station is located.

[0031] The condition determination unit 44 reads out the transmission method associated with the area in which the terminal station 4 is located from the broadcast information read by the receiver 42, and determines the transmission method of the transmitter 45. The condition determination unit 44 also identifies a transmission time period for the terminal uplink signal based on the orbital data of the LEO satellite 2 included in the broadcast information. That is, the condition determination unit 44 identifies the time period in which the location of the terminal station 4 is within the coverage of the first antenna 21 provided on the LEO satellite 2 as the transmission time period for the terminal uplink signal. The condition determination unit 44 randomly determines the transmission start timing based on the identified transmission time period and the transmission frequency indicated by the broadcast information. For example, the condition determination unit 44 determines the number of times to transmit the terminal uplink signal from the identified transmission time period and transmission frequency, and determines the transmission start timing based on a random number so that the transmission periods of the terminal uplink signals do not overlap. This allows the communication timings of the terminal stations 4 to be shifted, thereby improving communication efficiency.

[0032] The transmitter 45 wirelessly transmits a terminal uplink signal from the antenna 46, in which the sensor data stored in the data storage unit 41 is set as terminal transmission data, in accordance with the transmission start timing and transmission method determined by the condition determination unit 44. That is, the transmitter 45 transmits a signal using the multi-level number, error correction code, and transmission power stored in the broadcast information from the LEO satellite 2. The transmitter 45 transmits the signal using, for example, LPWA (Low Power Wide Area). LPWA includes 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 transmitter 45 may also transmit to other terminal stations 4 using time division multiplexing, OFDM (Orthogonal Frequency Division Multiplexing), MIMO, etc. The transmitter 45 determines the channel and transmission timing to be used by its own station to transmit the terminal uplink signal using a method predetermined for the wireless communication method used. Furthermore, the transmitter 45 may perform beamforming of signals to be transmitted from the multiple antennas 46 using a method predetermined for the wireless communication system being used.

[0033] 5 is a block diagram showing the configuration of a base station 7 according to an embodiment. The base station 7 includes a MIMO receiver 71, a base station signal reception processor 72, a terminal signal reception processor 73, an analyzer 74, a notification information generator 75, and a transmitter 76.

[0034] The MIMO receiver 71 aggregates base station downlink signals received from multiple GWLs 5. The MIMO receiver 71 stores a weight for each reception time of the base station downlink signal received by each GWL 5 based on the orbital data of the LEO satellite 2 and the position of each GWL 5. For example, the reception time may be expressed as the elapsed time from the timing of reception start. The MIMO receiver 71 multiplies the base station downlink signal input from each GWL 5 by a weight corresponding to the reception time of that base station downlink signal and combines the weight-multiplied received signals. Note that the same weight may be used regardless of the reception time. The base station signal reception processor 72 demodulates and decodes the combined received signal to obtain demodulated information. The base station signal reception processor 72 outputs the demodulated information to the terminal signal reception processor 73.

[0035] The terminal signal reception processing unit 73 performs reception processing of the terminal uplink signal. The terminal signal reception processing unit 73 decodes the symbols of the terminal uplink signal from the spectrum data indicated by the demodulation information, and obtains the terminal transmission data transmitted from the terminal station 4. In other words, the terminal signal reception processing unit 73 decodes the symbols of the terminal uplink signal by converting the frequency domain waveform indicated by the spectrum data into a time domain waveform.

[0036] The analysis unit 74 analyzes the communication status between the terminal station 4 and the LEO satellite 2 based on the terminal uplink signal received by the terminal signal reception processing unit 73. The analysis unit 74 calculates, for example, a decoding success rate of the terminal uplink signal for each area. The decoding success rate of the terminal uplink signal is an example of a statistical value of communication probability. If the decoding success rate of the terminal uplink signal is lower than a predetermined threshold, the analysis unit 74 adjusts the transmission method and communication timing between the terminal station 4 and the LEO satellite 2.

[0037] When the decoding success rate falls below a threshold, the analysis unit 74 changes the transmission method to improve the transmission quality. Specifically, when the decoding success rate falls below a threshold, the analysis unit 74 reduces the number of signal levels, increases the number of bits of the error correction code, and increases the transmission power. The smaller the number of signal levels, the lower the symbol error rate and the higher the transmission quality. Furthermore, the longer the error correction code, the larger the number of bits that can correct errors and the higher the transmission quality. Furthermore, the higher the transmission power, the lower the signal-to-noise ratio and the lower the probability of an error occurring. Note that even when an error detection code is used instead of an error correction code, the analysis unit 74 increases the number of bits of the error detection code when the decoding success rate falls below a threshold.

[0038] When the decoding success rate becomes lower than the threshold, the analysis unit 74 reduces the transmission frequency of the terminal uplink signal. Since the terminal station 4 randomly determines the timing to start transmitting the terminal uplink signal within the transmission time slot, the lower the transmission frequency of the terminal uplink signal, the lower the probability of collision of the terminal uplink signals.

[0039] The broadcast information generator 75 generates broadcast information indicating the analysis result of the analyzer 74. Specifically, the broadcast information generator 75 generates broadcast information including information associating an area with a transmission method and communication timing related to the area, and orbital information of the LEO satellite 2. The transmitter 76 transmits the broadcast information generated by the broadcast information generator 75 to the GEO satellite 3 via the GWG 6 as a base station uplink signal.

[0040] The following describes the operation of the wireless communication system 1. Fig. 6 is a flow chart showing the processing of the terminal station 4 according to the embodiment.

[0041] The terminal station 4 acquires data detected by an external or internal sensor (not shown) and writes the acquired data to the data storage unit 41 (step S101). The receiver 42 of the terminal station 4 receives a terminal downlink signal transmitted from a GEO satellite 3 (step S102). The positioning unit 43 identifies the terrestrial position of the terminal station 4 using the Global Navigation Satellite System (GNSS) and identifies the area in which the terminal station is located (step S103).

[0042] The condition determination unit 44 reads out the transmission method associated with the area to which the local station belongs from the broadcast information included in the terminal downlink signal received in step S102, and determines the transmission method of the transmitter 45 (step S104).The condition determination unit 44 also identifies the transmission time period of the terminal uplink signal based on the orbital data of the LEO satellite (step S105).

[0043] The condition determination unit 44 randomly determines the transmission start timing of the terminal uplink signal based on the identified transmission time period and the transmission frequency associated with the area to which the terminal station 4 belongs in the broadcast information (step S106). The transmission unit 45 determines whether the current time is the transmission start timing (step S107). If the current time is not the transmission start timing (step S107: NO), the terminal station 4 returns the process to step S101.

[0044] On the other hand, if the transmitter 45 determines that the current time is the timing to start transmitting the uplink signal (step S107: YES), it reads out the sensor data from the data storage unit 41, sets the read sensor data as terminal transmission data, and sets the terminal uplink signal of the transmission method determined in step S104. The transmitter 45 wirelessly transmits the terminal uplink signal in which the terminal transmission data has been set from the antenna 46 (step S108).

[0045] The transmitter 45 determines whether a predetermined transmission time has elapsed since the transmission start timing (step S109). If the transmission time has not elapsed (step S109: NO), the terminal station 4 returns the process to step S107. As a result, the terminal station 4 continues transmitting the uplink signal during the transmission time period.

[0046] 7 is a flow diagram showing the processing of the LEO relay station 20 according to the embodiment. The reception schedule determination unit 222 of the LEO satellite 2 determines the terminal station 4 that is to receive the terminal uplink signal for each time based on the position data and orbit data of the terminal station 4 stored in the storage unit 221 (step S121). The multiple reception units 223 receive the terminal uplink signals transmitted from the terminal station 4 (step S122). Depending on the wireless communication method of the transmitting terminal station 4, there are cases where the terminal uplink signal is received from only one terminal station 4 in a time-division manner for the same frequency, and cases where the terminal uplink signals are received from multiple terminal stations 4 simultaneously at the same frequency. The synthesis unit 224 synthesizes the terminal uplink signals received by the multiple reception units 223 in accordance with predetermined synthesis parameters (step S123). The spectrum conversion unit 225 converts the signal synthesized by the synthesis unit 224 into spectrum data and records it in the storage unit 221 (step S124).

[0047] The transmission schedule determination unit 232 refers to the storage unit 231 and determines whether the current time is included in the communication time zone with the base station 7 (step S125). If the current time is not included in the communication time zone with the base station 7 (step S125: NO), the process returns to step S121. On the other hand, if the current time is included in the communication time zone with the base station 7 (step S125: YES), the transmission schedule determination unit 232 determines the transmission time for each spectrum data item based on the number of spectrum data items stored in the storage unit 221 and the length of the communication time zone with the base station 7 (step S126).

[0048] The data generation unit 235 performs parallel conversion on the spectrum data stored in the storage unit 221, and the transmission data modulation unit 236 modulates the parallel-converted spectrum data. The MIMO communication unit 234 weights the transmission data modulated by the transmission data modulation unit 236 with the weight instructed by the control unit 233, and generates a base station downlink signal to be transmitted from each second antenna 24. The MIMO communication unit 234 transmits each generated base station downlink signal from the second antenna 24 by MIMO (step S127). When the LEO satellite 2 has transmitted all the spectrum data stored in the storage unit 221, the process returns to step S121.

[0049] 8 is a flow diagram showing the processing of the base station 7 according to the embodiment. Each GWL 5 of the base station 7 receives a base station downlink signal from the LEO satellite 2 (step S141). Each GWL 5 converts the received base station downlink signal into an electrical signal and outputs the resulting received signal to the MIMO receiving unit 71. The MIMO receiving unit 71 synchronizes the timing of the received signals received from each GWL 5. The MIMO receiving unit 71 combines the received signals received by each GWL 5 based on a weight. The base station signal receiving processing unit 72 demodulates the combined received signal (step S142). The base station signal receiving processing unit 72 outputs waveform data obtained by decoding the demodulated received signal to the terminal signal receiving processing unit 73.

[0050] The terminal signal receiving processor 73 decodes the symbols of the terminal uplink signal indicated by the waveform data to obtain the terminal transmission data transmitted from the terminal station 4 (step S143). Note that the terminal signal receiving processor 73 can also use a decoding method with a large calculation load, such as SIC (Successive Interference Cancellation).

[0051] The analysis unit 74 selects a plurality of terrestrial areas one by one (step S144), and executes the following processes from step S145 to step S148 for the selected area. First, the analysis unit 74 calculates the decoding success rate of the terminal uplink signal for the area selected in step S144 based on the terminal uplink signal received by the terminal signal reception processing unit 73 (step S145). The analysis unit 74 determines whether the decoding success rate is equal to or greater than a predetermined threshold (step S146). If the decoding success rate is equal to or greater than the threshold (step S146: YES), the analysis unit 74 maintains the transmission method and transmission frequency for the area selected in step S144.

[0052] On the other hand, if the decoding success rate is less than the threshold (step S146: NO), the analysis unit 74 decreases the number of signal levels of the transmission method for the area selected in step S144 by a unit amount, increases the number of bits of the error correction code by a unit amount, and increases the transmission power by a unit amount (step S147). The number of signal levels, the number of bits, and the unit amount of power are determined in advance. The analysis unit 74 also multiplies the transmission frequency of the terminal uplink signal by a predetermined decrease rate (a value greater than 0 and less than 1) to decrease it (step S148).

[0053] The broadcast information generator 75 generates broadcast information indicating the orbit information of the LEO satellite 2 and the transmission method and transmission frequency adjusted in steps S144 to S148 (step S149). The transmitter 76 transmits the broadcast information generated by the broadcast information generator 75 to the GEO satellite 3 via the GWG 6 as a base station uplink signal (step S150). The base station 7 then repeats the processes from step S141.

[0054] 9 is a flow diagram showing processing of the GEO relay station 30 according to the embodiment. The receiver 321 of the GEO relay station 30 receives a base station uplink signal from the GWG 6 via the antenna 31 (step S161). The receiver 321 stores broadcast information included in the received base station uplink signal in the memory 322 (step S162). The transmitter 323 transmits the broadcast information stored in the memory 322 to the terminal station 4 via the antenna 31 by using a terminal downlink signal (step S163).

[0055] According to the first embodiment, the terminal station 4 receives, from the GEO satellite 3, broadcast information generated based on the communication status between the LEO satellite 2 and the terminal station 4, and transmits data to the LEO satellite 2 using a transmission method specified in the broadcast information. This allows the terminal station 4 to transmit data to the LEO satellite 2 using a transmission method according to the communication status. As a result, the wireless communication system 1 can avoid collisions of terminal uplink signals between the terminal stations 4 and improve communication efficiency. The improved communication efficiency makes it possible to reduce the transmission power of the terminal station 4. Since the GEO satellites 3 have more opportunities to communicate with the terminal stations 4 than the LEO satellites 2, the terminal stations 4 can reliably receive the broadcast information and determine a transmission method according to the communication situation.

[0056] (Second embodiment) The base station 7 according to the first embodiment determines the transmission method and transmission frequency of the terminal station 4 for each area. In contrast, the base station 7 according to the second embodiment determines the transmission method and transmission start timing for each terminal station 4. The configuration of the wireless communication system 1 is the same as that of the first embodiment.

[0057] An analysis unit 74 of a base station 7 according to the second embodiment reduces the allowable number of simultaneous transmissions of terminal uplink signals for an area where the decoding success rate falls below a threshold. The base station 7 determines the transmission start timings of multiple terminal stations 4 belonging to that area so as to satisfy the changed allowable number of simultaneous transmissions. The transmission method of the terminal stations may be determined in the same manner as in the first embodiment. A broadcast information generation unit 75 of the base station 7 generates broadcast information that stores the transmission method and transmission start timing of the base station 7 in association with the ID of the base station 7.

[0058] The terminal station 4 according to the second embodiment does not randomly determine the transmission start timing, but transmits the terminal uplink signal at the transmission start timing indicated by the broadcast information.

[0059] According to the second embodiment, the terminal station 4 receives, from the GEO satellite 3, broadcast information generated based on the communication status between the LEO satellite 2 and the terminal station 4, and transmits data to the LEO satellite 2 at the transmission start timing specified in the broadcast information. This allows the wireless communication system 1 to avoid collisions of terminal uplink signals between the terminal stations 4 and improve communication efficiency. The improved communication efficiency makes it possible to reduce the transmission power of the terminal station 4.

[0060] The terminal station 4 includes a processor, a memory, an auxiliary storage device, and the like, all connected via a bus, and functions as a device including a receiving unit 42, a positioning unit 43, a condition determining unit 44, and a transmitting unit 45 by executing a program. The base station 7 includes a processor, a memory, an auxiliary storage device, and the like, all connected via a bus, and functions as a device including an analyzing unit 74, a broadcast information generating unit 75, and a transmitting unit 76 by executing a program. Examples of the processor include a CPU (Central Processing Unit), a GPU (Graphic Processing Unit), and a microprocessor. The program may be recorded on a computer-readable recording medium. Examples of the computer-readable recording medium include a storage device such as a magnetic disk, a magneto-optical disk, an optical disk, or a semiconductor memory. The program may be transmitted via a telecommunications line. All or part of the functions of the terminal station 4 or the base station 7 may be implemented using a custom LSI (Large Scale Integrated Circuit) such as an ASIC (Application Specific Integrated Circuit) or a PLD (Programmable Logic Device). Examples of PLDs include PAL (Programmable Array Logic), GAL (Generic Array Logic), CPLD (Complex Programmable Logic Device), and FPGA (Field Programmable Gate Array). Such integrated circuits are also included in the scope of processors.

[0061] (Other embodiments) Although one embodiment has been described in detail above with reference to the drawings, the specific configuration is not limited to the above, and various design modifications are possible. That is, in other embodiments, the order of the above-described processes may be changed as appropriate. Furthermore, some processes may be executed in parallel.

[0062] The LEO relay station 20, GEO relay station 30, terminal station 4 and base station 7 according to the above-described embodiments may be configured by a single computer, or the functional configuration may be distributed among multiple computers, with the multiple computers cooperating with each other.

[0063] According to the above-described embodiment, the GEO satellite 3 is an example of a second communication device that transmits broadcast information to the terminal station 4, but this is not limiting. For example, in other embodiments, a quasi-zenith satellite or a medium earth orbit satellite may transmit broadcast information to the terminal station 4. In still other embodiments, a high altitude platform station (HAPS) or a drone at a lower altitude than the LEO satellite 2 may transmit broadcast information. In still other embodiments, each terminal station 4 and a base station 7 may be connected via a network, and the base station 7 may transmit broadcast information directly to the terminal station 4. In still other embodiments, the broadcast information may be transmitted to the terminal station 4 via a terrestrial wireless communication network. That is, the second communication device may be a terrestrial base station that constitutes a mobile communication network or a fixed wireless communication network (e.g., FWA: Fixed Wireless Access). In these cases, it is preferable that the communication device that transmits broadcast information has more opportunities to communicate with the terminal station 4 than the LEO satellite 2.

[0064] According to the above-described embodiment, the transmission method and transmission frequency are adjusted when the communication conditions between the LEO satellite 2 and the terminal station 4 are poor, and the transmission method and transmission frequency are maintained when the communication conditions are good, but this is not limiting. For example, in another embodiment, the transmission method and transmission frequency may be adjusted to improve communication quality when the communication conditions between the LEO satellite 2 and the terminal station 4 are poor, and the transmission method and transmission frequency may be adjusted to improve communication efficiency when the communication conditions are good.

[0065] In the above-described embodiment, the quality of the communication conditions is determined based on the decoding success rate of the terminal uplink signal, but this is not limiting. For example, in other embodiments, the quality of the communication conditions may be determined using other values ​​related to the statistics of the communication probability between the LEO satellite 2 and the terminal station 4, such as the average value of the channel capacity. The channel capacity can be calculated, for example, from the error rate of the terminal uplink signal.

[0066] In the above-described embodiment, the base station 7 analyzes the communication status between the LEO satellite 2 and the terminal station 4 and adjusts the transmission method and transmission frequency, but this is not limited to this. That is, in other embodiments, the broadcast information may not include the transmission method and transmission frequency. For example, in other embodiments, the base station 7 may transmit broadcast information indicating the decoding success rate for each area to the GEO satellite 3, and the GEO satellite 3 or the terminal station 4 that receives the broadcast information from the GEO satellite 3 may adjust the transmission method and transmission frequency based on the decoding success rate. That is, in the wireless communication system 1, the capacity specifying unit 223 and the method determining unit 224 may be provided in the LEO satellite 2 or in the terminal station 4.

[0067] In the above-described embodiment, the LEO satellite 2 moves above the Earth, and the terminal station 4 and the base station 7 are located on the Earth, but the wireless communication system 1 according to other embodiments may target a celestial body other than the Earth, such as the Moon. [Explanation of symbols]

[0068] 1...wireless communication system 2...LEO satellite 20...LEO relay station 21...first antenna 22...terminal communication unit 221...storage unit 222...reception schedule determination unit 223...receiving unit 224...combining unit 225...spectrum conversion unit 23...base station communication unit 231...storage unit 232...transmission schedule determination unit 233...control unit 234...MIMO communication unit 235...data generation unit 236...transmission data modulation unit 24...second antenna 3...GEO satellite 30...GEO relay station 31...antenna 32...earth station communication unit 321...receiving unit 322...storage unit 323...transmitting unit 4...terminal station 41...data storage unit 42...receiving unit 43...positioning unit 44...condition determination unit 45...transmitting unit 46...antenna 5...GWL 6...GWG 7...base station 71...MIMO receiving unit 72... Base station signal receiving processing unit 73... Terminal signal receiving processing unit 74... Analysis unit 75... Notification information generating unit 76... Transmission unit

Claims

1. a first communication device that communicates while moving; a transmitting device for transmitting data to the first communication device; a second communication device that communicates with the transmitting device and is located at a higher altitude than the first communication device; Equipped with the second communication device transmits, to the transmitting device, notification information regarding a communication status between the transmitting device and the first communication device; The transmitting device determines a method for transmitting the data to the first communication device based on the notification information received from the second communication device, and transmits the data to the first communication device using the determined transmission method. Communication system.

2. A base station receiving the data from the first communication device Equipped with the first communication device transmits the data received from the transmitting device to the base station; the base station generates the broadcast information based on the data received from the first communication device and transmits the broadcast information to the second communication device; The second communication device transmits the broadcast information received from the base station to the transmitting device. The communication system of claim 1 .

3. the transmitting device and the base station are provided on a celestial body, the first communication device is installed on a low-earth orbit satellite moving above the celestial body, The second communication device is provided on a geostationary satellite located above the celestial body. The communication system according to claim 2 .

4. a plurality of transmitting devices including the transmitting device; The base station generates the broadcast information based on a statistical value of communication probabilities of the plurality of transmitting devices.

4. The communication system according to claim 2 or 3.

5. The base station generates the notification information for causing the transmitting device to change the transmission method when a statistical value of communication probabilities of the plurality of transmitting devices is equal to or less than a predetermined threshold. The communication system according to claim 4.

6. A period during which communication is possible between the transmitting device and the second communication device is longer than a period during which communication is possible between the transmitting device and the first communication device. A communication system according to any one of claims 1 to 5.

7. 1. A communication method for a first communication device that communicates while moving, a transmitting device that transmits data to the first communication device, and a second communication device that communicates with the transmitting device and is located at a higher altitude than the first communication device, comprising: a step of transmitting, by the second communication device, notification information regarding a communication status between the transmission device and the first communication device to the transmission device; a step of determining a method of transmitting the data to the first communication device based on the notification information received from the second communication device by the transmitting device; the transmitting device transmitting the data to the first communication device using the determined transmission method; A communication method comprising:

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