Mobile relay station

Abstract

A mobile relay station that moves in the air, the mobile relay station comprising: a waveform sampling unit that receives a signal transmitted from each of one or more transmission devices located on the ground and samples the received signal to thereby generate waveform data of the signal; a control unit that determines a down-sampling rate on the basis of predetermined criteria; a waveform data compression unit that down-samples, at the down-sampling rate determined by the control unit, the waveform data generated by the waveform sampling unit or waveform data obtained by frequency-shifting the waveform data; and a communication unit for a base station that transmits the waveform data down-sampled by the waveform data compression unit to the base station.　

Description

Mobile relay station

[0001] The present invention relates to a mobile relay station.

[0002] In recent years, there has been much research into satellite IoT (Internet of Things) platforms using low-earth orbit satellites. A satellite IoT platform is a system that uses low-earth orbit satellites to collect sensor data from IoT terminals installed anywhere on Earth, including areas that are difficult to cover with terrestrial communication networks, such as on the ocean or in mountainous regions.

[0003] In order to receive weak radio waves transmitted by a desired satellite IoT terminal on a low-orbit satellite, in the face of interference between satellite IoT terminals and the arrival of numerous interfering signals from terrestrial IoT terminals that are widely used on the ground, it is effective to install multiple receiving antennas on the low-orbit satellite and perform receiving beam control (see, for example, non-patent document 1).

[0004] To accommodate multiple terminals and multiple methods, a system configuration is being considered in which multiple receiving antennas are mounted on a low-orbit satellite, and the received waveform data of each receiving antenna sampled within the low-orbit satellite is transmitted to the ground, and receiving beam control is performed by offline signal processing on the ground (see, for example, non-patent document 2).

[0005] J. Chu, X. Chen, C. Zhong and Z. Zhang, “Robust Design for NOMA-Based Multibeam LEO Satellite Internet of Things”, IEEE Internet of Things Journal, vol. 8, no. 3, pp. 1959-1970, 2021.F. Yamashita, D. Goto, Y. Kojima, M. Matsui, K. Itokawa, K. Yoshizawa, K. Sakamoto, Y. Fujino, C. Kato, and M. Nakadai, “920-MHz IoT platform via LEO satellite employing feeder-link MIMO technology”, Proc. 2020 International Conference on Emerging Technologies for Communications (ICETC2020), A1-2, Dec. 2020.

[0006] If a low-orbit satellite captures waveforms over the entire frequency band used by IoT terminals, the amount of waveform data will be enormous, which will cause congestion on the downlink lines from the mobile relay stations on the low-orbit satellites to terrestrial base stations and increase the cost of the downlink lines.One possible method for reducing the amount of waveform data is to concentrate the channels used by IoT terminals in a specific band by some means depending on the uplink traffic generation status of the IoT terminals, thereby reducing the sampling rate required for the mobile relay stations to capture waveforms.

[0007] However, to realize the above method, it is necessary to specify the channel to be used by each of the many IoT terminals, and it may be necessary to add functions to the IoT terminals to realize the exchange of control signals, which increases the power consumption of the IoT terminals. Therefore, in the past, there was a problem that it was not possible to reduce the amount of waveform data when a mobile relay station relays the waveform data of a signal received without requiring additional functions to the IoT terminals. This problem is not limited to IoT terminals, but occurs in all transmitting devices that transmit signals containing data to mobile relay stations moving in the sky.

[0008] In view of the above circumstances, the present invention aims to provide a technology that can reduce the amount of waveform data when relaying the waveform data of a signal received at a mobile relay station, without requiring any additional functionality to a transmitting device that transmits a signal containing data to a mobile relay station moving in the sky.

[0009] One aspect of the present invention is a mobile relay station that moves in the sky and includes: a waveform sampling unit that receives signals transmitted from one or more transmitting devices located on the ground and generates waveform data of the signals by sampling the received signals; a control unit that determines a downsampling rate based on predetermined criteria; a waveform data compression unit that downsamples the waveform data generated by the waveform sampling unit or waveform data obtained by frequency-shifting the waveform data at the downsampling rate determined by the control unit; and a base station communication unit that transmits the waveform data downsampled by the waveform data compression unit to a base station.

[0010] The present invention makes it possible to reduce the amount of waveform data when relaying the waveform data of a signal received at a mobile relay station without requiring any additional functionality to a transmitting device that transmits a signal containing data to a mobile relay station moving in the sky.

[0011] FIG. 1 is a diagram illustrating an example of a configuration of a wireless communication system in a first embodiment. FIG. 2 is a diagram illustrating an example of downsampling in the first embodiment. FIG. 3 is a sequence diagram illustrating a flow of processing performed by the wireless communication system in the first embodiment. FIG. 4 is a diagram illustrating an example of a configuration of a wireless communication system in a second embodiment. FIG. 5 is a sequence diagram illustrating a flow of processing performed by the wireless communication system in the second embodiment. FIG. 6 is a diagram illustrating an example of a configuration of a wireless communication system in a third embodiment. FIG. 7 is a sequence diagram illustrating a flow of processing performed by the wireless communication system in the third embodiment. FIG. 8 is a diagram illustrating an example of a configuration of a wireless communication system in a fourth embodiment. FIG. 9 is a diagram illustrating an example of downsampling in the fourth embodiment. FIG. 10 is a diagram illustrating an advantage arising from frequency shift in the fourth embodiment. FIG. 11 is a diagram illustrating a disadvantage arising from frequency shift in the fourth embodiment. FIG. 12 is a diagram illustrating a demerit arising from frequency shift in the fourth embodiment. FIG. 13 is a diagram illustrating an example of a configuration of a wireless communication system in a fifth embodiment. FIG. 14 is a diagram illustrating an example of a configuration of a wireless communication system in a sixth embodiment.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] (Summary) Before describing the specific contents of the present invention, an overview of the present invention will be described. A wireless communication system in the present invention includes one or more terminal stations and one or more base stations installed on the ground, and a mobile relay station moving in the sky. The mobile relay station receives signals transmitted from each of the one or more terminal stations. The mobile relay station samples each received signal to generate and store waveform data for each signal. The mobile relay station transmits the stored waveform data for each signal to the base station. Note that the mobile relay station may transmit waveform data obtained by shifting the frequency of the stored waveform data for each signal to the base station.

[0014] Before transmitting the stored waveform data of each signal to the base station, the mobile relay station downsamples the waveform data or frequency-shifted waveform data at a downsampling rate (e.g., 1 / 2, 1 / 3, 1 / 4, etc.) determined based on a predetermined criterion. This allows the mobile relay station to reduce the amount of data. Note that downsampling can cause interference due to aliasing, so the mobile relay station determines the downsampling rate within a range where interference is not a problem.

[0015] The following three criteria can be used to determine the downsampling rate. The mobile relay station determines the downsampling rate based on one of the following three criteria, and downsamples the waveform data or frequency-shifted waveform data at the determined downsampling rate. The mobile relay station determines the downsampling rate for each data collection area (hereinafter referred to as "data collection area").

[0016] (Decision Criterion 1) The downsampling rate is determined for each data collection area based on the demodulation results from past orbits (the demodulation success rate when passing over each area and the sampling rate at that time). (Decision Criterion 2) The downsampling rate is determined for each data collection area based on the number of terminal stations within the data collection area, i.e., the density of terminal stations (population density). (Decision Criterion 3) A spectrum analysis of the waveform data is performed inside the mobile relay station, and the downsampling rate is determined for each data collection area based on the number of signals at each frequency.

[0017] The above-described configuration makes it possible to reduce the amount of waveform data when relaying the waveform data of a signal received at a mobile relay station without requiring any additional configuration at the terminal station. Specific configurations for realizing the above processing will be described below using each embodiment.

[0018] (First embodiment) Fig. 1 is a diagram showing an example of the configuration of a wireless communication system 100 in the first embodiment. The wireless communication system 100 includes a terminal station 10, a mobile relay station 20, and a base station 30. The wireless communication system 100 may include any number of terminal stations 10, mobile relay stations 20, and base stations 30. N or more terminal stations 10 (N is an integer equal to or greater than 1) are included. In the following description, when distinguishing among the N terminal stations 10, they are distinguished by adding a sub-number, such as terminal station 10-1.

[0019] The terminal station 10 and the base station 30 are installed at specific locations on the earth, such as on land or sea. Hereinafter, communication from the terminal station 10 and the base station 30 to the mobile relay station 20 will be referred to as uplink, and communication from the mobile relay station 20 to the terminal station 10 and the base station 30 will be referred to as downlink.

[0020] The terminal station 10 collects data such as environmental data detected by a sensor and transmits it wirelessly to the mobile relay station 20. The terminal station 10 can transmit data to the mobile relay station 20 only if the mobile relay station 20 is located within the communication area of ​​the terminal station 10. When the mobile relay station 20 instructs the terminal station 10 on transmission timing, the terminal station 10 wirelessly transmits the collected data to the mobile relay station 20 at the instructed transmission timing. The terminal station 10 is, for example, an IoT (Internet of Things) terminal. The terminal station 10 is an example of a transmission device.

[0021] The mobile relay station 20 is mounted on a mobile body, and its communication area moves over time. The mobile relay station 20 is provided, for example, on a low earth orbit (LEO) satellite. The altitude of a LEO satellite is 2000 km or less, and at an altitude of about 350 km, for example, it orbits the Earth once in about 1.5 hours.

[0022] Because the mobile relay station 20 mounted on the LEO satellite communicates while moving at high speed, the time during which each terminal station 10 and base station 30 can communicate with the mobile relay station 20 is limited. Specifically, from the perspective of the ground, the mobile relay station 20 passes overhead every few minutes. Therefore, the terminal station 10 collects data such as environmental data detected by sensors and stores the collected data. The terminal station 10 transmits a terminal uplink signal containing the collected data at a timing when communication with the mobile relay station 20 is possible.

[0023] While moving above the Earth, the mobile relay station 20 receives terminal uplink signals transmitted from each of the multiple terminal stations 10. The mobile relay station 20 accumulates data received from each terminal station 10 via the terminal uplink signals, downsamples the accumulated data, and wirelessly transmits it to the base station 30 via a downlink signal at a timing when communication with the base station 30 is possible. The base station 30 acquires the data collected by the terminal stations 10 from the received downlink signals.

[0024] Before determining the downsampling rate, the mobile relay station 20 wirelessly transmits the accumulated data to the base station 30 without downsampling. Then, after acquiring information indicating the downsampling rate for each data collection area from the base station 30, the mobile relay station 20 downsamples the accumulated data based on the downsampling rate indicated in the acquired information indicating the downsampling rate for each data collection area. Here, "after acquiring the information indicating the downsampling rate for each data collection area from the base station 30" refers to the next rotation or later, based on the timing (e.g., rotation) at which the information indicating the downsampling rate for each data collection area was acquired from the base station 30.

[0025] The mobile relay station 20 has a plurality of antennas used for wireless communication with the terminal station 10 and an antenna used for wireless communication with the base station 30. Therefore, the mobile relay station 20 can perform wireless communication with the terminal station 10 and wireless communication with the base station 30 in parallel.

[0026] Mobile relay stations can be implemented using geostationary satellites or relay stations mounted on unmanned aerial vehicles such as drones and HAPS (High Altitude Platform Stations). However, relay stations mounted on geostationary satellites have a wide ground coverage area (footprint), but due to their high altitude, the link budget for IoT terminals installed on the ground is very small. On the other hand, relay stations mounted on drones or HAPS have a high link budget but a narrow coverage area.

[0027] Furthermore, batteries are required for drones and solar panels for HAPS. In this embodiment, for example, a mobile repeater station 20 is mounted on a LEO satellite. Therefore, the link budget is kept within the limits, and since LEO satellites orbit outside the atmosphere, there is no air resistance and fuel consumption is low. In addition, the footprint is larger than when a repeater station is mounted on a drone or HAPS.

[0028] The base station 30 receives waveform data transmitted as a downlink signal from the mobile relay station 20. The base station 30 demodulates the received waveform data to restore the data collected by the terminal station 10. The base station 30 also calculates a downsampling rate for each data collection area based on the waveform data transmitted from the mobile relay station 20. The base station 30 transmits to the mobile relay station 20 an uplink signal including information indicating the calculated downsampling rate for each data collection area.

[0029] Next, the configurations of the terminal station 10, the mobile relay station 20, and the base station 30 in the first embodiment will be described.

[0030] [Configuration of Terminal Station] The terminal station 10 includes a transmission data storage unit 11 and a transmission unit 12. The transmission data storage unit 11 stores data to be transmitted (hereinafter referred to as "transmission data"). The transmission data is, for example, environmental data detected by a sensor. The transmission unit 12 communicates with the mobile relay station 20. The transmission unit 12 reads the transmission data from the transmission data storage unit 11 and wirelessly transmits a terminal uplink signal containing the read transmission data from an antenna.

[0031] The transmitter 12 transmits signals, for example, using 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 12 may transmit signals to other terminal stations 10 using time division multiplexing, OFDM (Orthogonal Frequency Division Multiplexing), etc. The transmitter 12 may perform beamforming of signals transmitted from multiple antennas using a method predetermined for the wireless communication method used.

[0032] [Configuration of Mobile Relay Station] The mobile relay station 20 includes a receiving unit 21, a waveform sampling unit 22, an information storage unit 23, a waveform data compression unit 24, a control unit 25, and a base station communication unit 26.

[0033] The receiver 21 includes N antennas and receives terminal uplink signals transmitted from each terminal station 10. The waveform sampler 22 samples transmission data included in each of the N terminal uplink signals received by each of the N antennas included in the receiver 21. The waveform sampler 22 generates waveform data indicating values ​​obtained by sampling for each piece of transmission data. The waveform sampler 22 associates the reception time of the terminal uplink signal at the antenna, the generated waveform data, and area information, and stores them in the information storage unit 23.

[0034] Here, the area information is information for identifying a data collection area from which a terminal uplink signal is received. As described above, the mobile relay station 20 moves at high speed in the sky. Therefore, the mobile relay station 20 communicates not only with terminal stations 10 located in a specific data collection area, but also with terminal stations 10 located in all data collection areas through which the mobile relay station 20 moves. Therefore, it is desirable for the mobile relay station 20 to add data collection area information to identify which data collection area the terminal uplink signal is acquired from.

[0035] The mobile relay station 20 orbits the Earth's sky by moving along a predetermined path at basically the same speed. Therefore, the timing at which the mobile relay station 20 is located above each data collection area is predetermined. For example, the mobile relay station 20 is located above data collection area A in a certain time period A, and above data collection area B in a certain time period B. Therefore, the waveform sampling unit 22 can identify in which data collection area the terminal uplink signal was acquired, based on the time at which the terminal uplink signal was acquired.

[0036] The information storage unit 23 stores the information saved by the waveform sampling unit 22 for each piece of transmission data. For example, waveform data for each data collection area is stored in the information storage unit 23. The information storage unit 23 is configured using a storage device such as a magnetic storage device or a semiconductor storage device.

[0037] The waveform data compression unit 24 downsamples each waveform data for each data collection area stored in the information storage unit 23 in accordance with the downsampling rate for each data collection area instructed by the control unit 25. The waveform data compression unit 24 outputs each downsampled waveform data to the base station communication unit 26. Note that, unless instructed to do so by the control unit 25, the waveform data compression unit 24 outputs each waveform data for each data collection area stored in the information storage unit 23 to the base station communication unit 26 without downsampling.

[0038] In this way, the waveform data compression unit 24 does not downsample all of the waveform data at the same downsampling rate, but downsamples the waveform data according to the downsampling rate determined for each data collection area. In this way, the waveform data compression unit 24 compresses each waveform data by downsampling.

[0039] The waveform data compression unit 24 includes a waveform data reading unit 241 and a downsampling unit 242. The waveform data reading unit 241 sequentially reads out each piece of waveform data stored in the information storage unit 23. For example, the waveform data reading unit 241 reads out the waveform data in the order in which it was stored in the information storage unit 23. The downsampling unit 242 uses the waveform data read out by the waveform data reading unit 241 to downsample the data at a predetermined sampling rate (e.g., 1 / 2 (50%)) instructed by the control unit 25.

[0040] The control unit 25 determines the downsampling rate at which each waveform data item is to be downsampled in the waveform data compression unit 24, based on the information indicating the downsampling rate for each data collection area acquired from the base station 30. For example, the control unit 25 determines downsampling rate A corresponding to data collection area A indicated in the information indicating the downsampling rate for each data collection area as the downsampling rate at which each waveform data item based on the terminal uplink signal acquired in data collection area A is to be downsampled. For example, the control unit 25 determines downsampling rate B corresponding to data collection area B indicated in the information indicating the downsampling rate for each data collection area as the downsampling rate at which each waveform data item based on the terminal uplink signal acquired in data collection area B is to be downsampled. In this way, the control unit 25 determines the downsampling rate at which waveform data is to be sampled.

[0041] Then, based on the determined sampling rate for each data collection area, the control unit 25 controls the waveform data compression unit 24 to perform downsampling in the next revolution of the mobile relay station 20. Specifically, the mobile relay station 20 repeatedly revolves around the same orbit. Therefore, the control unit 25 controls the waveform data compression unit 24 to perform downsampling in the next revolution, using the revolution for which the downsampling rate for each data collection area was obtained from the base station 30 as a reference.

[0042] For example, the control unit 25 instructs the downsampling unit 242 to downsample waveform data based on a terminal uplink signal acquired in data collection area A at a downsampling rate corresponding to data collection area A. Furthermore, the control unit 25 instructs the downsampling unit 242 to downsample waveform data based on a terminal uplink signal acquired in data collection area B at a downsampling rate corresponding to data collection area B.

[0043] The control unit 25 controls the waveform data compression unit 24 based on the already acquired downsampling rate information for each data collection area until the downsampling rate for each data collection area is again obtained from the base station 30. Furthermore, in the initial state (for example, when the downsampling rate for each data collection area has not yet been obtained from the base station 30), the control unit 25 does not instruct the waveform data compression unit 24 to perform downsampling.

[0044] The base station communication unit 26 communicates with the base station 30. The base station communication unit 26 transmits, for example, a downlink signal including waveform data for each data collection area to the base station 30. In the first embodiment, the base station communication unit 26 transmits downsampled waveform data and non-downsampled waveform data to the base station 30. The non-downsampled waveform data is used by the base station 30 to obtain a downsampling rate for each data collection area. After information indicating the downsampling rate for each data collection area is obtained from the base station 30, the downsampled waveform data is used by the base station 30 to obtain a downsampling rate for each data collection area.

[0045] The base station communication unit 26 also receives information indicating the downsampling rate for each data collection area transmitted via uplink from the base station 30. The base station communication unit 26 outputs the received information indicating the downsampling rate for each data collection area to the control unit 25.

[0046] [Configuration of Base Station] The base station 30 includes a base station communication unit 31 and a signal processing unit 32. The base station communication unit 31 communicates with the mobile relay station 20. The base station communication unit 31 receives, for example, a downlink signal including waveform data for each data collection area transmitted from the mobile relay station 20. The base station communication unit 31 transmits, for example, information indicating the downsampling rate for each data collection area calculated by the signal processing unit 32 to the mobile relay station 20.

[0047] The signal processing unit 32 calculates the downsampling rate for each data collection area using waveform data for each data collection area included in the downlink signal received by the base station communication unit 31. The signal processing unit 32 determines the downsampling rate for each data collection area based on, for example, demodulation results from past orbits (the demodulation success rate and the sampling rate at that time when passing over each data collection area). The signal processing unit 32 may sequentially derive, for example, a downsampling rate that maximizes the expected value of the demodulation success rate when passing over each data collection area in the next orbit.

[0048] FIG. 2 is a diagram illustrating an example of downsampling in the first embodiment. The left diagram in FIG. 2 shows the frequency spectrum of a signal represented by waveform data S1 to S3. The downsampling unit 242 downsamples the waveform data S1 to S3 by, for example, 1 / 2. As a result, the waveform data S1 to S3 become as shown in the right diagram in FIG. 2. In the right diagram in FIG. 2, the positions of waveform data S2 and waveform data S3 are changed. This is because aliasing occurs in each of the waveform data S2 and S3, with the frequencies at both ends of the reception band after downsampling being the Nyquist frequency. For waveform data S2, aliasing occurs with the frequency at the left end of the reception band, and for waveform data S3, aliasing occurs with the frequency at the right end of the reception band being the Nyquist frequency, and the frequencies are shifted in opposite directions by the difference between the frequency of the original signal and the Nyquist frequency. As shown in the left diagram in FIG. 2, if the state before downsampling is sparse, there may be no interference even if aliasing occurs. Note that even if interference occurs, it can be separated to a certain extent by beam control.

[0049] [Operation of wireless communication system 100] Fig. 3 is a sequence diagram showing the flow of processing performed by the wireless communication system 100 in the first embodiment. It is assumed that, at the start of the processing in Fig. 3, the mobile relay station 20 has not yet acquired information indicating the downsampling rate for each data collection area from the base station 30.

[0050] The base station communication unit 31 of the base station 30 receives a downlink signal transmitted from the mobile relay station 20 (step S101). The base station communication unit 31 outputs the received downlink signal to the signal processing unit 32. The signal processing unit 32 demodulates an uplink signal from each of one or more waveform data included in the downlink signal (step S102). That is, the signal processing unit 32 demodulates the transmission data transmitted by each terminal station 10 from each of one or more waveform data included in the downlink signal.

[0051] The signal processing unit 32 determines a downsampling rate for each data collection area based on the demodulation result (step S103). The signal processing unit 32 outputs information indicating the determined downsampling rate for each data collection area to the base station communication unit 31. The base station communication unit 31 wirelessly transmits the information indicating the downsampling rate for each data collection area output from the signal processing unit 32 to the mobile relay station 20 at a timing when communication with the mobile relay station 20 is possible (step S104).

[0052] The base station communication unit 26 of the mobile relay station 20 receives the information indicating the downsampling rate for each data collection area transmitted from the base station 30. The base station communication unit 26 outputs the received information indicating the downsampling rate for each data collection area to the control unit 25. The control unit 25 stores the information indicating the downsampling rate for each data collection area output from the base station communication unit 26 in a memory (not shown) (step S105).

[0053] Furthermore, based on the information indicating the downsampling rate for each data collection area stored in the memory, the control unit 25 controls the waveform data compression unit 24 to perform downsampling in the next rotation of the mobile relay station 20. In response to the instruction from the control unit 25, the waveform data compression unit 24 performs downsampling in the next rotation of the mobile relay station 20 (step S106). Specifically, the waveform data reading unit 241 sequentially reads out the waveform data stored in the information storage unit 23.

[0054] The downsampling unit 242 downsamples the waveform data read by the waveform data reading unit 241 at a downsampling rate indicated by information indicating the downsampling rate for each data collection area. Area information is associated with each waveform data. Therefore, the downsampling unit 242 references the information indicating the downsampling rate for each data collection area and determines the downsampling rate corresponding to the data collection area specified by the area information.

[0055] The downsampling unit 242 downsamples the waveform data read by the waveform data reading unit 241 to the determined downsampling rate. For example, the downsampling unit 242 downsamples the waveform data associated with the area information of the data collection area A at the downsampling rate A to achieve the downsampling rate A corresponding to the data collection area A specified by the area information. The downsampling unit 242 outputs the downsampled waveform data to the base station communication unit 26.

[0056] The base station communication unit 26 accumulates the downsampled waveform data output from the downsampling unit 242 until it becomes possible to communicate with the base station 30. Because the mobile relay station 20 moves at high speed, it is not always able to communicate with the base station 30. Therefore, the downsampled waveform data is accumulated in the base station communication unit 26 until the mobile relay station 20 becomes able to communicate with the base station 30.

[0057] When communication with the base station 30 becomes possible, the base station communication unit 26 transmits a downlink signal including the accumulated downsampled waveform data to the base station 30 (step S107). The base station communication unit 31 of the base station 30 receives the downlink signal transmitted from the mobile relay station 20. The base station communication unit 31 outputs the received downlink signal to the signal processing unit 32. The signal processing unit 32 demodulates the uplink signal from each of the one or more pieces of downsampled waveform data included in the downlink signal (step S108). If demodulation fails, the signal processing unit 32 notifies the mobile relay station 20 via the base station communication unit 31 of information indicating the time and frequency ranges including the downsampled waveform data that failed to be demodulated. The mobile relay station 20 retransmits only the downsampled waveform data instructed by the mobile relay station 20.

[0058] According to the wireless communication system 100 configured as described above, the mobile relay station 20 includes a waveform sampling unit 22 that receives terminal uplink signals transmitted from each terminal station 10 and generates waveform data of the terminal uplink signals by sampling the received terminal uplink signals; a control unit 25 that determines a downsampling rate based on a predetermined criterion; a waveform data compression unit 24 that downsamples the waveform data generated by the waveform sampling unit 22 at the downsampling rate determined by the control unit 25; and a base station communication unit 26 that transmits the waveform data downsampled by the waveform data compression unit 24 to the base station 30.

[0059] In this way, each terminal station 10 only needs to transmit transmission data to the mobile relay station 20 at a timing when it is available for transmission, as in the conventional case. The mobile relay station 20 downsamples the waveform data at a downsampling rate determined by the control unit 25 and transmits the data to the base station 30. Therefore, there is no need to provide an additional component in the terminal station 10, and it is possible to reduce the amount of waveform data when relaying the waveform data of a signal received at the mobile relay station 20.

[0060] Furthermore, in the wireless communication system 100, the base station 30 calculates a downlink rate obtained based on the demodulation results of past rotations for each data collection area of ​​the mobile relay station 20 as a downsampling rate for each data collection area, and notifies the mobile relay station 20. The control unit 25 of the mobile relay station 20 determines the downsampling rate for each area to be the downsampling rate specified by the information indicating the downsampling rate for each data collection area notified by the base station 30. This makes it possible to determine the downsampling rate for each area taking into account the past communication status of the mobile relay station 20.

[0061] Second Embodiment In the second embodiment, a configuration will be described in which the downsampling rate is determined by a method different from that in the first embodiment. For example, in the second embodiment, a configuration will be described in which the downsampling rate is determined according to the number of terminal stations located in a data collection area, i.e., the density of terminal stations (population density). The distribution of terminal station locations tends to be similar to the population distribution, and it is thought that the more populated an area is, the more densely terminal stations tend to be concentrated.

[0062] 4 is a diagram showing an example of the configuration of a wireless communication system 100a according to the second embodiment. The wireless communication system 100a includes a terminal station 10, a mobile relay station 20a, and a base station 30a. The wireless communication system 100a differs in configuration from the wireless communication system 100 in that the wireless communication system 100a includes a mobile relay station 20a and a base station 30a instead of the mobile relay station 20 and the base station 30. The other configurations of the wireless communication system 100a are the same as those of the wireless communication system 100. The following description will focus on the differences from the wireless communication system 100.

[0063] Except for the process for determining the downsampling rate for each data collection area, the mobile relay station 20a performs the same processes as the mobile relay station 20. For example, the mobile relay station 20a determines the downsampling rate for each data collection area within the mobile relay station 20a without acquiring information indicating the downsampling rate for each data collection area from the base station 30a.

[0064] The base station 30a performs the same processing as the base station 30, except that it does not calculate the downsampling rate for each data collection area. That is, the base station 30a receives waveform data transmitted as a downlink signal from the mobile relay station 20a. The base station 30a demodulates the received waveform data to restore the data collected by the terminal station 10.

[0065] [Configuration of Mobile Relay Station] The mobile relay station 20 a includes a receiving unit 21 , a waveform sampling unit 22 , an information storage unit 23 a , a waveform data compression unit 24 , a control unit 25 a , and a base station communication unit 26 .

[0066] The information storage unit 23a stores information saved by the waveform sampling unit 22 for each piece of transmission data. For example, the information storage unit 23 stores waveform data for each data collection area. The information storage unit 23 is configured using a storage device such as a magnetic storage device or a semiconductor storage device. Furthermore, the information storage unit 23a stores information indicating the density of terminal stations 10 located in each data collection area (hereinafter referred to as "terminal density information"). The terminal density information may be calculated in advance for each data collection area and stored in the information storage unit 23a, or may be calculated based on acquired transmission data and stored in the information storage unit 23a.

[0067] Based on the terminal density information stored in the information storage unit 23a, the control unit 25a determines a downsampling rate for downsampling each piece of waveform data in the waveform data compression unit 24. Based on the density of terminal stations 10 located in data collection area A indicated by the terminal density information, for example, the control unit 25a determines a downsampling rate for downsampling each piece of waveform data based on terminal uplink signals acquired in data collection area A. Based on the density of terminal stations 10 located in data collection area B indicated by the terminal density information, the control unit 25a determines a downsampling rate for downsampling each piece of waveform data based on terminal uplink signals acquired in data collection area B.

[0068] As an example, areas with a large number (high density) of terminal stations 10 located within a data collection area tend to have a large amount of waveform data. Therefore, if waveform data collected in a data collection area where the number of terminal stations 10 is equal to or greater than a certain threshold is downsampled at a downsampling rate of 1 / 3 or 1 / 4, there is a high possibility that interference caused by aliasing will become a problem. Therefore, in data collection areas where the number of terminal stations 10 is equal to or greater than a certain threshold, the control unit 25a determines the downsampling rate to be 1 / 2 or the like within a range in which interference caused by aliasing will not become a problem.

[0069] On the other hand, there tends to be less waveform data in areas where the number of terminal stations 10 located within the data collection area is small (low density). Therefore, even if waveform data collected in a data collection area where the number of terminal stations 10 is less than a certain threshold is downsampled at a downsampling rate of 1 / 4, interference caused by aliasing is unlikely to become a problem. Therefore, in data collection areas where the number of terminal stations 10 is less than a certain threshold, the control unit 25ab determines the downsampling rate to be 1 / 2, etc., within a range where interference caused by aliasing does not become a problem. In this way, the control unit 25a can sequentially derive a downsampling rate that maximizes the expected demodulation success rate when passing over each data collection area in the next orbit.

[0070] In the above example, the control unit 25a determines the downsampling rate based on whether the number of terminal stations 10 exceeds a certain threshold. However, the control unit 25a may determine the downsampling rate using multiple thresholds. For example, if a first threshold and a second threshold (where the first threshold is greater than the second threshold) are used as the multiple thresholds, the control unit 25a determines the downsampling rate to be 1 / 2 in a data collection area where the number of terminal stations 10 is equal to or greater than the first threshold. Furthermore, the control unit 25a determines the downsampling rate to be 1 / 3 in a data collection area where the number of terminal stations 10 is less than the first threshold and equal to or greater than the second threshold. Furthermore, the control unit 25a determines the downsampling rate to be 1 / 4 in a data collection area where the number of terminal stations 10 is less than the second threshold.

[0071] Then, the control unit 25a controls the waveform data compression unit 24 to perform downsampling in the next rotation of the mobile relay station 20a based on the determined sampling rate for each data collection area. The instructions given by the control unit 25a to the waveform data compression unit 24 are the same as those given by the control unit 25.

[0072] When the terminal density information is updated, the control unit 25a may re-determine the sampling rate for each data collection area based on the updated terminal density information. Also, in the initial state (e.g., when the terminal density information is not stored in the information storage unit 23a), the control unit 25a does not instruct the waveform data compression unit 24 to perform downsampling.

[0073] [Operation of wireless communication system 100a] Fig. 5 is a sequence diagram showing the flow of processing performed by the wireless communication system 100a in the second embodiment. It is assumed that terminal density information is stored in the mobile relay station 20a at the start of the processing in Fig. 5.

[0074] The control unit 25a of the mobile relay station 20a reads out the terminal density information stored in the information storage unit 23a (step S201). The control unit 25a determines the downsampling rate for each data collection area based on the read out terminal density information (step S202). The control unit 25a controls the waveform data compression unit 24 to perform downsampling in the next rotation of the mobile relay station 20a based on the information indicating the determined downsampling rate for each data collection area.

[0075] In response to an instruction from the control unit 25a, the waveform data compression unit 24 performs downsampling in the next rotation of the mobile relay station 20a (step S203). Specifically, the waveform data reading unit 241 sequentially reads out the waveform data stored in the information storage unit 23a. The downsampling unit 242 downsamples the waveform data read out by the waveform data reading unit 241 at a downsampling rate indicated by information indicating the downsampling rate for each data collection area. Area information is associated with each waveform data. Therefore, the downsampling unit 242 references the information indicating the downsampling rate for each data collection area and determines the downsampling rate corresponding to the data collection area identified by the area information.

[0076] The downsampling unit 242 downsamples the waveform data read by the waveform data reading unit 241 to the determined downsampling rate. For example, the downsampling unit 242 downsamples the waveform data associated with the area information of the data collection area A at the downsampling rate A to achieve the downsampling rate A corresponding to the data collection area A specified by the area information. The downsampling unit 242 outputs the downsampled waveform data to the base station communication unit 26.

[0077] The base station communication unit 26 accumulates the downsampled waveform data output from the downsampling unit 242 until it becomes possible to communicate with the base station 30a. Because the mobile relay station 20a moves at high speed, it is not always able to communicate with the base station 30a. Therefore, the downsampled waveform data is accumulated in the base station communication unit 26 until the mobile relay station 20a becomes able to communicate with the base station 30a.

[0078] When communication with the base station 30a becomes possible, the base station communication unit 26 transmits a downlink signal including the accumulated downsampled waveform data to the base station 30a (step S204). The base station communication unit 31 of the base station 30a receives the downlink signal transmitted from the mobile relay station 20a. The base station communication unit 31 outputs the received downlink signal to the signal processing unit 32. The signal processing unit 32a demodulates the uplink signal from each of the one or more downsampled waveform data included in the downlink signal (step S205). If demodulation fails, the signal processing unit 32a notifies the mobile relay station 20a via the base station communication unit 31 of information indicating the time and frequency ranges including the downsampled waveform data that failed to be demodulated. The mobile relay station 20a retransmits only the downsampled waveform data instructed by the mobile relay station 20a.

[0079] According to the wireless communication system 100a configured as above, it is possible to obtain the same effects as those of the first embodiment.

[0080] Furthermore, in the wireless communication system 100a, the downsampling rate is determined by a method different from that of the first embodiment. Specifically, the control unit 25a determines the downsampling rate for each data collection area where data is collected by the mobile relay station 20a based on the number of terminal stations 10 located within the data collection area. For example, since there tends to be more waveform data in areas with a large number of terminal stations 10 located within the data collection area (high density), the control unit 25a determines the downsampling rate to be 1 / 4 or the like within a range in which interference does not pose a problem. Furthermore, since there tends to be less waveform data in areas with a small number of terminal stations 10 located within the data collection area (low density), the control unit 25a determines the downsampling rate to be 1 / 2 or the like within a range in which interference does not pose a problem. In this way, the control unit 25a determines the downsampling rate for each data collection area according to the number (density) of terminal stations 10 located within the data collection area. Therefore, downsampling makes it possible to reduce the amount of waveform data while suppressing interference caused by aliasing.

[0081] (Third Embodiment) In the third embodiment, a configuration will be described in which the downsampling rate is determined by a method different from that in the first embodiment. For example, in the third embodiment, a configuration will be described in which spectrum analysis of waveform data is performed inside a mobile relay station and the downsampling rate is determined based on the number of signals at each frequency.

[0082] 6 is a diagram showing an example of the configuration of a wireless communication system 100b according to the third embodiment. The wireless communication system 100b includes a terminal station 10, a mobile relay station 20b, and a base station 30a. The wireless communication system 100b differs in configuration from the wireless communication system 100a in that the wireless communication system 100b includes a mobile relay station 20b instead of the mobile relay station 20a. The other configurations of the wireless communication system 100b are the same as those of the wireless communication system 100a. The following description will focus on the differences from the wireless communication system 100a.

[0083] The mobile relay station 20b performs the same processing as the mobile relay station 20a, except for the processing for determining the downsampling rate for each data collection area. For example, the mobile relay station 20b performs spectrum analysis on the waveform data and determines the downsampling rate for each data collection area based on the number of signals at each frequency.

[0084] [Configuration of Mobile Relay Station] The mobile relay station 20b includes a receiving unit 21, a waveform sampling unit 22, an information storage unit 23, a waveform data compression unit 24, a control unit 25b, a base station communication unit 26, and an analysis unit 27b.

[0085] The analysis unit 27b performs a spectrum analysis of each waveform data for each data acquisition area stored in the information storage unit 23. Specifically, the analysis unit 27b performs an FFT (Fast Fourier Transform) on each waveform data for each data acquisition area stored in the information storage unit 23, thereby converting the waveform data into data in the frequency domain.

[0086] The analyzer 27b then calculates the number of signals at each frequency based on the frequency domain data of each waveform data for each data collection area. This allows the analyzer 27b to acquire information on the number of signals at each frequency for each waveform data for each data collection area. The analyzer 27b acquires information on the number of signals at each frequency for data collection area A, for example, by performing an FFT on each waveform data based on a terminal uplink signal acquired in data collection area A. The analyzer 27b acquires information on the number of signals at each frequency for data collection area B, for example, by performing an FFT on each waveform data based on a terminal uplink signal acquired in data collection area B.

[0087] Based on information about the number of signals at each frequency acquired by the analyzer 27b for each waveform data item in each data collection area, the controller 25b determines a downsampling rate for downsampling each waveform data item in the waveform data compressor 24. Based on information about the number of signals at each frequency in the data collection area A, for example, the controller 25b determines a downsampling rate for downsampling each waveform data item based on the terminal uplink signal acquired in the data collection area A. Based on information about the number of signals at each frequency in the data collection area B, the controller 25b determines a downsampling rate for downsampling each waveform data item based on the terminal uplink signal acquired in the data collection area B.

[0088] For example, areas where a large number of frequencies exist tend to have a large amount of waveform data. Therefore, if waveform data collected in a data collection area where the number of frequencies existing in signals is equal to or greater than a certain threshold is downsampled at a downsampling rate of 1 / 3 or 1 / 4, there is a high possibility that interference caused by aliasing will become a problem. Therefore, in data collection areas where the number of frequencies existing in signals is equal to or greater than a certain threshold, the control unit 25b determines the downsampling rate to be 1 / 2 or the like within a range where interference caused by aliasing will not become a problem.

[0089] On the other hand, there tends to be less waveform data in areas where the number of frequencies where signals exist is small. Therefore, even if waveform data collected in a data collection area where the number of frequencies where signals exist is less than a certain threshold is downsampled at a downsampling rate of 1 / 4, there is a low possibility that interference caused by aliasing will become a problem. Therefore, in data collection areas where the number of frequencies where signals exist is less than a certain threshold, the control unit 25b determines a downsampling rate such as 1 / 2 within a range where interference caused by aliasing will not become a problem. In this way, the control unit 25b can sequentially derive a downsampling rate that maximizes the expected demodulation success rate when passing over each data collection area in the next orbit.

[0090] In the above example, the control unit 25b determines the downsampling rate based on whether the number of frequencies at which signals are present exceeds a certain threshold. However, the control unit 25b may determine the downsampling rate using multiple thresholds. For example, if a first threshold and a second threshold (the first threshold > the second threshold) are used as the multiple thresholds, the control unit 25b determines the downsampling rate to be 1 / 2 in a data collection area where the number of frequencies at which signals are present is equal to or greater than the first threshold. Furthermore, the control unit 25b determines the downsampling rate to be 1 / 3 in a data collection area where the number of frequencies at which signals are present is less than the first threshold and equal to or greater than the second threshold. Furthermore, the control unit 25b determines the downsampling rate to be 1 / 4 in a data collection area where the number of frequencies at which signals are present is less than the second threshold.

[0091] Then, the control unit 25b controls the waveform data compression unit 24 to perform downsampling in the next rotation of the mobile relay station 20b based on the determined sampling rate for each data collection area. The instructions given by the control unit 25b to the waveform data compression unit 24 are the same as those given by the control unit 25.

[0092] Note that the control unit 25b controls the waveform data compression unit 24 based on the already acquired information on the number of signals at each frequency until the analysis unit 27b obtains the information on the number of signals at each frequency again. Furthermore, in the initial state (for example, a state in which the information on the number of signals at each frequency has not been obtained), the control unit 25b does not instruct the waveform data compression unit 24 to perform downsampling.

[0093] [Operation of wireless communication system 100b] Fig. 7 is a sequence diagram showing the flow of processing performed by wireless communication system 100b in the third embodiment. It is assumed that waveform data is stored in information storage unit 23 at the start of the processing in Fig. 7.

[0094] The analyzer 27b of the mobile relay station 20b reads out each waveform data for each data collection area stored in the information storage unit 23. The analyzer 27b performs a spectrum analysis of the read out waveform data for each data collection area (step S301). As a result, the analyzer 27b converts each waveform data for each data collection area into frequency domain data. The analyzer 27b then acquires information on the number of signals at each frequency for each waveform data for each data collection area (step S302).

[0095] The analyzer 27b determines the downsampling rate for each data collection area based on the acquired information on the number of signals at each frequency (step S303). The controller 25b controls the waveform data compressor 24 to perform downsampling in the next rotation of the mobile relay station 20b based on the information indicating the determined downsampling rate for each data collection area.

[0096] In response to an instruction from the control unit 25b, the waveform data compression unit 24 performs downsampling in the next rotation of the mobile relay station 20b (step S304). Specifically, the waveform data reading unit 241 sequentially reads out the waveform data stored in the information storage unit 23. The downsampling unit 242 downsamples the waveform data read out by the waveform data reading unit 241 at a downsampling rate indicated by information indicating the downsampling rate for each data collection area. Area information is associated with each waveform data. Therefore, the downsampling unit 242 references the information indicating the downsampling rate for each data collection area and determines the downsampling rate corresponding to the data collection area identified by the area information.

[0097] The downsampling unit 242 downsamples the waveform data read by the waveform data reading unit 241 to the determined downsampling rate. For example, the downsampling unit 242 downsamples the waveform data associated with the area information of the data collection area A at the downsampling rate A to achieve the downsampling rate A corresponding to the data collection area A specified by the area information. The downsampling unit 242 outputs the downsampled waveform data to the base station communication unit 26.

[0098] The base station communication unit 26 accumulates the downsampled waveform data output from the downsampling unit 242 until it becomes possible to communicate with the base station 30a. Because the mobile relay station 20b moves at high speed, it is not always able to communicate with the base station 30a. Therefore, the downsampled waveform data is accumulated in the base station communication unit 26 until the mobile relay station 20b becomes able to communicate with the base station 30a.

[0099] When communication with the base station 30a becomes possible, the base station communication unit 26 transmits a downlink signal including the accumulated downsampled waveform data to the base station 30a (step S305). The base station communication unit 31 of the base station 30a receives the downlink signal transmitted from the mobile relay station 20b. The base station communication unit 31 outputs the received downlink signal to the signal processing unit 32. The signal processing unit 32b demodulates the uplink signal from each of the one or more downsampled waveform data included in the downlink signal (step S306). If demodulation fails, the signal processing unit 32a notifies the mobile relay station 20b via the base station communication unit 31 of information indicating the time and frequency ranges including the downsampled waveform data that failed to be demodulated. The mobile relay station 20b retransmits only the downsampled waveform data instructed by the mobile relay station 20b.

[0100] According to the wireless communication system 100b configured as above, it is possible to obtain the same effects as those of the first embodiment.

[0101] Furthermore, in the wireless communication system 100b, the downsampling rate is determined by a method different from that of the first embodiment. Specifically, the control unit 25b determines the downsampling rate for each data collection area based on the number of signals at each frequency obtained by spectral analysis of waveform data obtained for each area where data is collected in the mobile relay station 20b. For example, in an area where there are many frequencies where the number of signals is equal to or greater than a threshold, the control unit 25b determines the downsampling rate to be 1 / 4 or the like within a range where interference is not a problem. Furthermore, in an area where there are few frequencies where the number of signals is equal to or greater than a threshold, the control unit 25b determines the downsampling rate to be 1 / 2 or the like within a range where interference is not a problem. In this way, the control unit 25b determines the downsampling rate for each data collection area according to the number of signals at each frequency. Therefore, downsampling makes it possible to reduce the amount of waveform data while suppressing interference caused by aliasing.

[0102] (Fourth Embodiment) In the first to third embodiments, a configuration was described in which waveform data obtained based on transmission data from each terminal station is downsampled without any processing. In contrast, in the fourth embodiment, a configuration is described in which waveform data is shifted to a positive frequency to extract only the I component on the real axis and then downsampled. Note that in the fourth embodiment, the same method as in the first embodiment is used to determine the downsampling rate.

[0103] 8 is a diagram showing an example of the configuration of a wireless communication system 100c according to the fourth embodiment. The wireless communication system 100c includes a terminal station 10, a mobile relay station 20c, and a base station 30. The wireless communication system 100c differs in configuration from the wireless communication system 100 in that it includes a mobile relay station 20c instead of the mobile relay station 20. Other configurations of the wireless communication system 100c are similar to those of the wireless communication system 100. The following description will focus on the differences from the wireless communication system 100.

[0104] The mobile relay station 20c performs the same processing as the mobile relay station 20, except that it performs frequency shifting on the waveform data and then downsampling.

[0105] [Configuration of Mobile Relay Station] Mobile relay station 20c includes a receiving unit 21, a waveform sampling unit 22, an information storage unit 23, a waveform data compression unit 24c, a control unit 25, and a base station communication unit 26. In mobile relay station 20c, the operation performed by waveform data compression unit 24c differs from that in the first embodiment, but the processing of the other functional units is the same as that of mobile relay station 20.

[0106] The waveform data compressor 24c frequency-shifts each piece of waveform data for each data collection area stored in the information storage unit 23, and then downsamples the data in accordance with the downsampling rate for each data collection area instructed by the control unit 25. The waveform data compressor 24c outputs the downsampled waveform data to the base station communication unit 26. Note that, when not instructed by the control unit 25, the waveform data compressor 24c frequency-shifts each piece of waveform data for each data collection area stored in the information storage unit 23, and then outputs the data to the base station communication unit 26 without downsampling.

[0107] In this way, the waveform data compression unit 24c does not downsample all of the frequency-shifted waveform data at the same downsampling rate, but downsamples them according to the downsampling rate determined for each data acquisition area. In this way, the waveform data compression unit 24c compresses each frequency-shifted waveform data by downsampling.

[0108] The waveform data compression unit 24c includes a waveform data reading unit 241, a downsampling unit 242c, and a frequency shifting unit 243c. The frequency shifting unit 243c performs a frequency shift on the waveform data read by the waveform data reading unit 241. Specifically, the frequency shifting unit 243c frequency shifts the waveform data so that the reception band becomes a positive frequency, thereby acquiring real part data (I component data) of the waveform data.

[0109] The downsampling unit 242c downsamples the real part data of the waveform data that has been frequency-shifted by the frequency shift unit 243c at a predetermined sampling rate (e.g., 1 / 2 (50%)) instructed by the control unit 25.

[0110] [Operation of Wireless Communication System 100c] The processing performed by the wireless communication system 100c is substantially the same as the processing shown in Fig. 3. The difference is that in the processing of step S106 in Fig. 3, the frequency shifter 243c performs frequency shifting on the waveform data before the downsampling unit 242c performs downsampling. Therefore, the downsampling unit 242c downsamples the waveform data that has been frequency shifted by the frequency shifter 243c at the downsampling rate indicated by the information indicating the downsampling rate for each data collection area. The subsequent processing is the same as in Fig. 3.

[0111] FIG. 9 is a diagram illustrating an example of downsampling in the fourth embodiment. The left diagram of FIG. 9 shows the frequency spectrum of a signal represented by waveform data S1 to S3. The downsampling unit 242c downsamples the waveform data S1 to S3 by half after the IQ waveform has been shifted to a positive frequency by the frequency shift unit 243c, for example. As a result, the waveform data S1 to S3 become as shown in the right diagram of FIG. 9. In the right diagram of FIG. 9, the positions of waveform data S2 and waveform data S3 are reversed. This is because aliasing occurs in each of waveform data S2 and S3, with the frequency at the right end of the reception band after downsampling being the Nyquist frequency, and the frequency is shifted in the negative direction by the difference between the frequency of the original signal and the Nyquist frequency. In the right diagram of FIG. 9, the frequency spectra of waveform data S1 and S3 are horizontally inverted due to frequency shifting. As shown in the left diagram of FIG. 9, if the state before downsampling is sparse, there may be no interference even if aliasing occurs. Even if interference occurs, it can be separated to a certain extent by beam control.

[0112] 10 to 12 will be used to explain cases where benefits are obtained by left-right inverting the frequency spectrum as shown in FIG. 9 and cases where disadvantages are obtained by left-right inverting the frequency spectrum. FIG. 10 is a diagram for explaining benefits obtained by frequency shifting in the fourth embodiment. FIG. 11 is a diagram for explaining disadvantages obtained by frequency shifting in the fourth embodiment. FIG. 12 is a diagram for explaining demerits obtained by frequency shifting in the fourth embodiment.

[0113] FIG. 10A shows the frequency spectra of waveform data S1 and S2. The downsampling unit 242c downsamples the waveform data S1 and S2 by 1 / 2 after the IQ waveforms have been shifted to positive frequencies by the frequency shift unit 243c, for example. As a result, interference occurs between the waveform data S1 and S2 due to aliasing, as shown in FIG. 10B. However, as shown in FIG. 10C, assume that signals Sg1 and Sg2 arrive at the receiving array (N antennas) of the mobile relay station 20c from the same direction. In this case, as shown in FIG. 10D, signals Sg1 and Sg2 arrive at the receiving array (N antennas) of the mobile relay station 20c from opposite directions, making them possible to separate in the spatial domain.

[0114] FIG. 11A shows the frequency spectra of waveform data S1 and S2. The downsampling unit 242c downsamples the waveform data S1 and S2 by 1 / 2 after the IQ waveforms have been shifted to positive frequencies by the frequency shift unit 243c, for example. As a result, interference occurs between the waveform data S1 and S2 due to aliasing, as shown in FIG. 11B. However, as shown in FIG. 11C, assume that signals Sg1 and Sg2 arrive at the receiving array (N antennas) of the mobile relay station 20c from different directions. In this case, as shown in FIG. 11D, signals Sg1 and Sg2 arrive at the receiving array (N antennas) of the mobile relay station 20c from the same direction, making separation in the spatial domain impossible.

[0115] FIG. 12 illustrates the benefits of horizontally inverting the frequency spectrum when a narrowband signal such as Sigfox is transmitted from the terminal station 10. (A) in FIG. 12 shows the frequency spectra of waveform data S1 and S2. The downsampling unit 242c downsamples the waveform data S1 and S2 by 1 / 2 after the IQ waveform has been shifted to a positive frequency by the frequency shift unit 243c, for example. As a result, interference occurs due to aliasing between the waveform data S1 and S2, as shown in (B) in FIG. 12. Compensating for Doppler fluctuations in signal 1 corresponding to waveform data S1 results in the result shown in (C) in FIG. 11. By horizontally inverting the frequency spectrum, the Doppler fluctuations are also inverted. Compensating for Doppler fluctuations results in narrowband and spreading, which can improve the SIR (Signal to Interference Ratio). Even if signal 1 corresponding to waveform data S1 and signal 2 corresponding to waveform data S2 arrive at mobile relay station 20c from the same direction, they may be able to be demodulated due to the SIR improvement effect of Doppler fluctuation compensation.

[0116] According to the wireless communication system 100c configured as above, it is possible to obtain the same effects as those of the first embodiment.

[0117] Furthermore, in the wireless communication system 100c, the waveform data is frequency-shifted, and the IQ waveform is shifted to a positive frequency, thereby downsampling the real part of the waveform data. This allows for separation and demodulation in the spatial domain even when interference occurs due to aliasing. As a result, it is possible to improve demodulation accuracy.

[0118] Fifth Embodiment In the fifth embodiment, a configuration will be described in which a downsampling rate is determined by a method different from that of the fourth embodiment. For example, in the fifth embodiment, the same method as that of the second embodiment is used as a method for determining the downsampling rate.

[0119] 13 is a diagram showing an example of the configuration of a wireless communication system 100d according to the fifth embodiment. The wireless communication system 100d includes a terminal station 10, a mobile relay station 20d, and a base station 30d. The wireless communication system 100d differs in configuration from the wireless communication system 100c in that the wireless communication system 100d includes a mobile relay station 20d and a base station 30d instead of the mobile relay station 20c and the base station 30. The other configurations of the wireless communication system 100d are the same as those of the wireless communication system 100c. The following description will focus on the differences from the wireless communication system 100c.

[0120] The mobile relay station 20d performs the same processing as the mobile relay station 20c except for the processing for determining the downsampling rate for each data collection area. For example, the mobile relay station 20d determines the downsampling rate for each data collection area by itself without acquiring information indicating the downsampling rate for each data collection area from the base station 30d.

[0121] The base station 30d performs the same processing as the base station 30a.

[0122] [Configuration of Mobile Relay Station] The mobile relay station 20d includes a receiving unit 21, a waveform sampling unit 22, an information storage unit 23d, a waveform data compression unit 24c, a control unit 25d, and a base station communication unit 26.

[0123] The information storage unit 23d stores information saved by the waveform sampling unit 22 for each piece of transmission data. For example, the information storage unit 23d stores waveform data for each data collection area. The information storage unit 23d is configured using a storage device such as a magnetic storage device or a semiconductor storage device. Furthermore, the information storage unit 23d stores terminal density information. The terminal density information may be calculated in advance for each data collection area and stored in the information storage unit 23d, or may be calculated based on the acquired transmission data and stored in the information storage unit 23d.

[0124] The control unit 25d determines a downsampling rate for downsampling each piece of waveform data in the waveform data compression unit 24c based on the terminal density information stored in the information storage unit 23d. For example, the control unit 25d determines a downsampling rate for downsampling each piece of waveform data based on terminal uplink signals acquired in data collection area A based on the density of terminal stations 10 located in data collection area A indicated by the terminal density information. The control unit 25d determines a downsampling rate for downsampling each piece of waveform data based on terminal uplink signals acquired in data collection area B based on the density of terminal stations 10 located in data collection area B indicated by the terminal density information.

[0125] Then, the control unit 25d controls the waveform data compression unit 24c to perform downsampling in the next rotation of the mobile relay station 20d based on the determined sampling rate for each data collection area. The instructions given by the control unit 25d to the waveform data compression unit 24c are the same as those given by the control unit 25c.

[0126] When the terminal density information is updated, the control unit 25d may re-determine the sampling rate for each data collection area based on the updated terminal density information. Furthermore, in the initial state (e.g., when the terminal density information is not stored in the information storage unit 23d), the control unit 25d does not instruct the waveform data compression unit 24c to perform downsampling.

[0127] [Operation of Wireless Communication System 100d] The processing performed by the wireless communication system 100d is substantially the same as the processing shown in Fig. 5. The difference is that in the processing of step S203 in Fig. 5, the frequency shifter 243c performs frequency shifting on the waveform data before the downsampling unit 242c performs downsampling. Therefore, the downsampling unit 242c downsamples the waveform data that has been frequency shifted by the frequency shifter 243c at the downsampling rate indicated by the information indicating the downsampling rate for each data collection area. The subsequent processing is the same as in Fig. 5.

[0128] According to the wireless communication system 100d configured as above, it is possible to obtain the same effects as those of the fourth embodiment.

[0129] Sixth Embodiment In the sixth embodiment, a configuration will be described in which the downsampling rate is determined by a method different from that of the fourth embodiment. For example, in the sixth embodiment, the same method as that of the third embodiment is used as the method for determining the downsampling rate.

[0130] 14 is a diagram showing an example of the configuration of a wireless communication system 100e according to the sixth embodiment. The wireless communication system 100e includes a terminal station 10, a mobile relay station 20e, and a base station 30d. The wireless communication system 100e differs in configuration from the wireless communication system 100d in that the wireless communication system 100e includes a mobile relay station 20e instead of the mobile relay station 20d. The remaining configuration of the wireless communication system 100e is the same as that of the wireless communication system 100d. The following description will focus on the differences from the wireless communication system 100d.

[0131] The mobile relay station 20e performs the same processing as the mobile relay station 20c, except for the processing for determining the downsampling rate for each data collection area. For example, the mobile relay station 20e performs a spectrum analysis on the waveform data and determines the downsampling rate for each data collection area based on the number of signals at each frequency.

[0132] [Configuration of Mobile Relay Station] The mobile relay station 20e includes a receiving unit 21, a waveform sampling unit 22, an information storage unit 23, a waveform data compression unit 24c, a control unit 25e, a base station communication unit 26, and an analysis unit 27e.

[0133] The analysis unit 27e performs a spectrum analysis of each waveform data for each data acquisition area stored in the information storage unit 23. Specifically, the analysis unit 27e performs an FFT on each waveform data for each data acquisition area stored in the information storage unit 23, thereby converting the waveform data into data in the frequency domain.

[0134] As a result, the analyzer 27e acquires information on the number of signals at each frequency for each piece of waveform data in each data collection area. For example, the analyzer 27e performs an FFT on each piece of waveform data based on terminal uplink signals acquired in data collection area A to acquire information on the number of signals at each frequency in data collection area A. For example, the analyzer 27e performs an FFT on each piece of waveform data based on terminal uplink signals acquired in data collection area B to acquire information on the number of signals at each frequency in data collection area B.

[0135] The control unit 25e determines a downsampling rate for downsampling each waveform data in the waveform data compression unit 24c, based on the information on the number of signals at each frequency acquired for each waveform data of each data collection area by the analysis unit 27e. Then, the control unit 25e controls the waveform data compression unit 24c to perform downsampling in the next rotation of the mobile relay station 20e, based on the determined sampling rate for each data collection area. The instructions given by the control unit 25b to the waveform data compression unit 24c are the same as those given by the control unit 25c.

[0136] Note that the control unit 25e controls the waveform data compression unit 24c based on the already acquired information on the number of signals at each frequency until the analysis unit 27e acquires the information on the number of signals at each frequency again. Furthermore, in the initial state (e.g., a state in which the information on the number of signals at each frequency has not been acquired), the control unit 25e does not instruct the waveform data compression unit 24c to perform downsampling.

[0137] [Operation of Wireless Communication System 100e] The processing performed by the wireless communication system 100e is substantially the same as the processing shown in Fig. 7. The difference is that in the processing of step S304 in Fig. 7, the frequency shifter 243c performs frequency shifting on the waveform data before the downsampling unit 242c performs downsampling. Therefore, the downsampling unit 242c downsamples the waveform data that has been frequency shifted by the frequency shifter 243c at the downsampling rate indicated by the information indicating the downsampling rate for each data collection area. The subsequent processing is the same as in Fig. 7.

[0138] According to the wireless communication system 100e configured as above, it is possible to obtain the same effects as those of the fourth embodiment.

[0139] Some or all of the processing performed by the mobile relay stations 20, 20a, 20b, 20c, 20d, and 20e or the base stations 30, 30a, and 30d in the above-described embodiments may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed. Note that the term "computer system" here includes hardware such as an operating system (OS) and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as a flexible disk, a magneto-optical disk, a read-only memory (ROM), and a compact disc-ROM (CD-ROM), as well as storage devices such as a hard disk built into a computer system.

[0140] Furthermore, the term "computer-readable recording medium" may include a medium that dynamically stores a program for a short period of 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, or a medium that stores a program for a certain period of time, such as a volatile memory within a computer system that serves as a server or client in such a case. The program may also be one that realizes some of the above-mentioned functions, or one that can realize the above-mentioned functions in combination with a program already stored in the computer system, or one that can be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0141] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0142] The present invention can be applied to a wireless communication system having a mobile body equipped with a mobile relay station.

[0143] 10, 10-1 to 10-N... terminal station, 11... transmission data storage unit, 12... transmission unit, 20, 20a, 20b, 20c, 20d, 20e... mobile relay station, 21... reception unit, 22... waveform sampling unit, 23, 23a, 23d... information storage unit, 24, 24c... waveform data compression unit, 25, 25a, 25b, 25d, 25e... control unit, 26... base station communication unit, 27b, 27e... analysis unit, 30, 30a, 30d... base station, 31... base station communication unit, 32, 32a, 32d... signal processing unit, 100, 100a, 100b, 100c, 100d, 100e... wireless communication system, 241... waveform data reading unit, 242, 242c... downsampling unit 243c...frequency shift unit

Claims

1. A mobile relay station moving in the sky, comprising: a waveform sampling unit that receives signals transmitted from one or more transmitting devices located on the ground and generates waveform data of the signals by sampling the received signals; a control unit that determines a downsampling rate based on predetermined criteria; a waveform data compression unit that downsamples the waveform data generated by the waveform sampling unit or waveform data obtained by frequency-shifting the waveform data at the downsampling rate determined by the control unit; and a base station communication unit that transmits the waveform data downsampled by the waveform data compression unit to a base station.

2. The mobile relay station according to claim 1, wherein the control unit determines a downlink rate obtained based on demodulation results from past rotations for each area where data is collected in the mobile relay station as a downsampling rate for each area.

3. The mobile relay station according to claim 1, wherein the control unit determines a downsampling rate for each area from which data is collected in the mobile relay station based on the number of one or more transmitting devices located within the area.

4. The mobile relay station according to claim 1, wherein the control unit determines the downsampling rate for each area based on the number of signals at each frequency obtained by spectral analysis of the waveform data generated by the waveform sampling unit or waveform data obtained by frequency shifting the waveform data.

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