WIRELESS COMMUNICATION SYSTEM, RELAY DEVICE, COMMUNICATION DEVICE, AND WIRELESS COMMUNICATION METHOD

A first offset compensator configured to compensate for frequency offsets occurring during communications between a plurality of communication devices and a relay device, wherein when the first offset compensator is provided on the relay device, the first offset compensator gives a statistical frequency offset obtained from a statistic of a plurality of frequency offsets occurring during communications between respective ones of the plurality of communication devices and the relay device to a receiver configured to receive wireless signals transmitted from respective ones of the plurality of communication devices, and when the first offset compensator is provided on each of the plurality of communication devices, the first offset compensator gives a frequency offset occurring during communications between the communication device provided with the first offset compensator and the relay device to a transmitter configured to transmit wireless signals to the relay device.

TECHNICAL FIELD

The present invention relates to a wireless communication system, a relay device, a communication device, and a wireless communication method.

BACKGROUND ART

With the development of IoT (Internet of Things) technology, installation of IoT terminals equipped with various sensors in various locations is being considered. For example, IoT terminals are expected to be utilized to collect data in places, such as on buoys or ships on the ocean or in mountainous regions, where base stations are difficult to install.

Besides, a technique for wirelessly communicating between IoT terminals and UAVs (Unmanned Aerial Vehicles) or geostationary satellites has also been proposed.

When a moving body such as a UAV or a geostationary satellite moves, communications conducted between the moving body and a communication device, such as an IoT terminal or a base station, installed on earth are impacted by Doppler shift. Conventionally, a technique has been proposed to reduce the impact of the Doppler shift by causing a communication device on the receiving side to compensate for frequency offsets of uplink signals (see, for example, Non-patent Literature 1).

CITATION LIST

SUMMARY OF THE INVENTION

Technical Problem

With the technique described in Non-patent Literature 1, the communication device on the receiving side compensates for frequency offsets of uplink signals transmitted from respective ones of plural terminal devices. That is, the technique described in Non-patent Literature 1 compensates for frequency offsets so as to reduce the frequency offset for each terminal device.

Application of this technique to communications conducted between communication devices installed on earth and a moving body is effective when the number of communication devices that transmit uplink signals to the moving body is small. However, there is a problem in that amounts of computation of the moving body receiving the uplink signals increase with increases in the number of communication devices transmitting the uplink signals to the moving body.

In view of the above circumstances, an object of the present invention is to provide a technique capable of reducing an impact of Doppler shift while suppressing amounts of computation.

Means for Solving the Problem

According to one aspect of the present invention, there is provided a wireless communication system that includes a plurality of communication devices and a relay device, which is mobile, the wireless communication system comprising a first offset compensation unit adapted to compensate for frequency offsets occurring during communications between the plurality of communication devices and the relay device, wherein when the first offset compensation unit is provided on the relay device, the first offset compensation unit gives a statistical frequency offset obtained from a statistic of a plurality of frequency offsets occurring during communications between respective ones of the plurality of communication devices and the relay device to a reception unit adapted to receive wireless signals transmitted from respective ones of the plurality of communication devices, and when the first offset compensation unit is provided on each of the plurality of communication devices, the first offset compensation unit gives a frequency offset occurring during communications between the communication device provided with the first offset compensation unit and the relay device to a transmission unit adapted to transmit wireless signals to the relay device.

According to another aspect of the present invention, there is provided a relay device in a wireless communication system that includes a plurality of communication devices and the relay device, which is mobile, the relay device comprising: a reception unit adapted to receive wireless signals transmitted from respective ones of the plurality of communication devices; and a first offset compensation unit adapted to compensate for frequency offsets occurring during communications between the plurality of communication devices and the relay device, wherein the first offset compensation unit gives a statistical frequency offset obtained from a statistic of a plurality of frequency offsets occurring during communications between respective ones of the plurality of communication devices and the relay device to the reception unit.

According to another aspect of the present invention, there is provided a communication device in a wireless communication system that includes a plurality of communication devices and a relay device, which is mobile, the communication device comprising: a transmission unit adapted to transmit wireless signals to the relay device; and a first offset compensation unit adapted to compensate for frequency offsets occurring during communications between the plurality of communication devices and the relay device, wherein the first offset compensation unit gives a frequency offset occurring during communications between the communication device and the relay device to the transmission unit.

According to another aspect of the present invention, there is provided a wireless communication method for a wireless communication system that includes a plurality of communication devices and a relay device, which is mobile, the wireless communication method comprising an offset compensation step whereby the plurality of communication devices or the relay device compensates for frequency offsets occurring during communications between the plurality of communication devices and the relay device, wherein when the relay device has the offset compensation step, the offset compensation step gives a statistical frequency offset obtained from a statistic of a plurality of frequency offsets occurring during communications between respective ones of the plurality of communication devices and the relay device to a reception unit adapted to receive wireless signals transmitted from respective ones of the plurality of communication devices, and when each of the plurality of communication devices has the offset compensation step, the offset compensation step gives frequency offsets occurring during communications between the communication devices having the offset compensation step and the relay device to a transmission unit adapted to transmit wireless signals to the relay device.

Effects of the Invention

The present invention makes it possible to reduce effects of Doppler shift while suppressing amounts of computation.

DESCRIPTION OF EMBODIMENTS

First Embodiment

FIG.1is a configuration diagram of a wireless communication system1according to a first embodiment. The wireless communication system1includes a mobile relay station2, plural terminal stations3, and a base station4. The number of each of the mobile relay stations2, the terminal stations3, and the base stations4included in the wireless communication system1is arbitrary It is assumed that there are a large number of terminal stations3.

The mobile relay station2is an example of a relay device mounted on a moving body, and an area in which the mobile relay station2can perform communication moves with the passage of time. The mobile relay station2is provided in, for example, a low Earth orbit (LEO) satellite. The altitude of the LEO satellite is 2000 km or less, and the LEO satellite orbits over the Earth in about 1.5 hours. While moving over the Earth, the mobile relay station2receives data transmitted from the terminal station3, as a wireless signal. The mobile relay station2transmits the received data to the base station4by wireless.

The terminal station3collects data such as environmental data detected by a sensor and wirelessly transmits the data to the mobile relay station2. The terminal station3is, for example, an IoT terminal. Only two terminal stations3are shown inFIG.1. Hereinafter signals transmitted from the terminal stations3to the mobile relay station2will be referred to as terminal uplink signals.

The base station4receives the data collected by the terminal station3from the mobile relay station2.

The terminal stations3and the base station4are installed at specific positions on earth such as on the ground or on the ocean.

It is conceivable to use a relay device mounted on an unmanned aerial vehicle such as a geostationary satellite, a drone or a high altitude platform station (HAPS) as a mobile relay station. However, in the case of a relay station mounted on a geostationary satellite, the coverage area (footprint) on the ground is wide, but a link budget for IoT terminals installed on the ground is considerably small due to a high altitude. On the other hand, in the case of a relay station mounted on a drone or a HAPS, the link budget is high, but the coverage area is narrow.

Furthermore, drones require batteries and HAPS require solar panels. In the present embodiment, the mobile relay station2is mounted on a LEO satellite. Therefore, in addition to keeping the link budget within a limit, the LEO satellite has no air resistance and consumes less fuel because it orbits outside the atmosphere. In addition, the footprint is also large as compared to the case where a relay station is mounted on a drone or a HAPS.

The mobile relay station2mounted on the LEO satellite performs communication while moving at a high speed. Consequently, signals exchanged between the mobile relay station2and the terminal stations3undergo Doppler shift. For example, the frequency of a terminal uplink signal received by the mobile relay station2shifts from the frequency existing at the time of transmission from the terminal station3, in a predetermined range. The larger the impact of Doppler shift, the larger the frequency shift. Similarly, signals (hereinafter referred to as “downlink signals”) transmitted from the mobile relay station2to the terminal stations3and the base station4also undergo Doppler shift.

If Doppler shift has a large impact, communications conducted between the mobile relay station2and the terminal stations3as well as communications conducted between the mobile relay station2and the base station4will be impacted. For example, if the frequency of a terminal uplink signal shifts greatly due to Doppler shift, it is likely that the mobile relay station2will no longer be able to receive the terminal uplink signal. For example, it is likely that the terminal stations3and the base station4will no longer be able to receive the downlink signal transmitted from the mobile relay station2.

Thus, the mobile relay station2according to the first embodiment performs control to reduce the impact of Doppler shift on the communications conducted between the mobile relay station2and the terminal stations3. Specifically, the mobile relay station2according to the first embodiment calculates statistical values of plural frequency offsets occurring during communications between respective ones of the plural terminal stations3and the mobile relay station2. Then, the mobile relay station2changes a receivable frequency band based on the calculated statistical values of the plural frequency offsets.

Configurations of individual devices will be described.

The mobile relay station2includes an antenna21, a terminal communication unit22, a base station communication unit24, and an antenna25.

The terminal communication unit22includes a transmission/reception unit221, an offset compensation unit222(first offset compensation unit), and a terminal signal demodulation unit223.

The transmission/reception unit221conducts communications with the terminal stations3. For example, the transmission/reception unit221receives terminal uplink signals via the antenna21.

The offset compensation unit222compensates for the frequency offsets occurring during communications between the plural terminal stations3and the mobile relay station2. More specifically, the offset compensation unit222gives a statistical frequency offset obtained from a statistic of plural frequency offsets occurring during communications between respective ones of the plural terminal stations3and the mobile relay station2to the transmission/reception unit221. The statistic may be, for example, a mean value or a median.

Giving a statistical frequency offset to the transmission/reception unit221means changing the frequency band receivable by the transmission/reception unit221in such a way as to eliminate the statistical frequency offset. In this way, by giving a statistical frequency offset to the transmission/reception unit221, the mobile relay station2corrects for a shift corresponding to the statistical frequency offset.

The terminal signal demodulation unit223demodulates the terminal uplink signals received by the transmission/reception unit221and outputs the demodulation results as demodulation information to the base station communication unit24.

The base station communication unit24transmits receive waveform information to the base station4using a downlink signal of a desired wireless communication scheme. The base station communication unit24includes a storage241, a control unit242, a transmission data modulation unit243, and a transmission unit244.

The storage241stores precalculated transmission start timing based on orbital information about the LEO satellite carrying the mobile relay station2and the position of the base station4. The orbital information about the LEO satellite provides information about the position, velocity, moving direction, and the like of the LEO satellite at any desired time. Send time may be expressed, for example, by elapsed time with respect to the transmission start timing.

The control unit242controls the transmission data modulation unit243and the transmission unit244such that the receive waveform information will be transmitted to the base station4at the transmission start timing that is stored in the storage241.

The transmission data modulation unit243reads the receive waveform information as transmit data out of a data storage unit23, modulates the read transmit data, and thereby generates a base station downlink signal.

The transmission unit244converts the base station downlink signal from an electric signal into a wireless signal and transmits the resulting signal from the antenna25.

The terminal station3includes a data storage31, a transmission/reception unit32, an estimation unit33, and one or more antennas34.

The data storage31stores sensor data and the like.

The transmission/reception unit32conducts communications with the mobile relay station2. For example, the transmission/reception unit32reads sensor data as terminal transmit data out of the data storage31. The transmission/reception unit32wirelessly transmits a terminal uplink signal containing the read terminal transmit data from the antennas34. For example, the transmission/reception unit32receives the downlink signal transmitted from the mobile relay station2and outputs the downlink signal to the estimation unit33.

The transmission/reception unit32transmits and receives signals using, for example, LPWA (Low Power Wide Area). Examples of LPWA include LoRaWAN (R), Sigfox (R), LTE-M (Long Term Evolution for Machines), and NB (Narrow Band)-IoT, and any desired wireless communication scheme can be used. The transmission/reception unit32may exchange data with other terminal stations3using time-division multiplexing, OFDM (Orthogonal Frequency Division Multiplexing), or the like.

By a predetermined method in the wireless communication scheme to be used, the transmission/reception unit32determines the channel and transmission timing to be used by the local station to transmit a terminal uplink signal. By a predetermined method in the wireless communication scheme to be used, the transmission/reception unit32may also form beams of signals to be transmitted from the plural antennas34.

The estimation unit33estimates Doppler frequency based on a downlink signal.

The base station4includes an antenna41, a reception unit42, a base station signal reception processing unit43, and a terminal signal reception processing unit44. The reception unit42converts a terminal downlink signal received via the antenna41into an electric signal. The base station signal reception processing unit43demodulates and decodes the receive signal converted into an electric signal by the reception unit42and thereby obtains receive waveform information. The base station signal reception processing unit43outputs the receive waveform information to the terminal signal reception processing unit44.

The terminal signal reception processing unit44performs a reception process for the terminal uplink signal indicated by the receive waveform information. In so doing, the terminal signal reception processing unit44acquires terminal transmit data by performing a reception process based on the wireless communication scheme used for transmission by the terminal station3. The terminal signal reception processing unit44includes a terminal signal demodulation unit441and a terminal signal decode unit442.

The terminal signal demodulation unit441demodulates waveform data and outputs a symbol obtained by the demodulation to the terminal signal decode unit442. The terminal signal demodulation unit441may perform demodulation after compensating the signal indicated by the waveform data for the Doppler shift of the terminal uplink signal received by the antenna21of the mobile relay station2. The Doppler shift that impacts the terminal uplink signal received by the antenna21is calculated in advance based on the position of the terminal station3and the orbital information about the LEO satellite carrying the mobile relay station2. The terminal signal decode unit442decodes the symbol demodulated by the terminal signal demodulation unit441and thereby obtains the terminal transmit data transmitted from the terminal station3.

Operation of the wireless communication system1will be described.

FIG.2is a sequence diagram showing a flow of a frequency offset process of the wireless communication system1according to the first embodiment. InFIG.2, description will be given by taking as an example a case in which two terminal stations3(terminal station3-1and terminal station3-2) are provided in the wireless communication system1. To distinguish functional parts of the two terminal stations3-1and3-2from each other, branch numbers “−1” and “−2” are assigned to the respective functional parts. The process shown inFIG.2is performed either periodically or when a predetermined timing elapses. The predetermined timing may coincide with a preset time or with when the mobile relay station2has just moved a preset distance.

The mobile relay station2transmits downlink signals to the terminal stations3-1and3-2via the transmission/reception unit221(step S101).

The terminal stations3-1and3-2receive the downlink signals transmitted from the mobile relay station2via respective antennas34-1and34-2. The received downlink signals are inputted to estimation units33-1and33-2.

The estimation unit33-1of the terminal station3-1estimates Doppler frequency based on the inputted downlink signal (step S102). Regarding the Doppler frequency, specifically, the estimation unit33-1estimates the Doppler frequency using the inputted downlink signal, based on expression (1) below.

In expression (1), x denotes a propagated distance between the mobile relay station2and the terminal stations3, t denotes elapsed time, f denotes the center frequency of the downlink signal, c denotes the velocity of light, and v denotes the travel velocity of the mobile relay station2. The travel velocity v of the mobile relay station2is the change in the propagated distance per unit time and can be found, for example, from the orbital information about the mobile relay station2. For example, the travel velocity v can be found using (x2-x1)/(t2-t1).

The estimation unit33-1converts the estimated Doppler frequency into Doppler frequency experienced by uplink frequency (step S103). The estimation unit33-1transmits a frequency offset value, as the Doppler frequency resulting from the conversion, to the mobile relay station2via a transmission/reception unit32-1(step S104).

The estimation unit33-2of the terminal station3-2estimates Doppler frequency based on the inputted downlink signal (step S105). The estimation unit33-2converts the estimated Doppler frequency into Doppler frequency experienced by uplink frequency (step S106). The estimation unit33-2transmits the frequency offset value resulting from the conversion, to the mobile relay station2via a transmission/reception unit32-2(step S107).

The mobile relay station2receives the post-conversion frequency offset values transmitted from the terminal stations3-1and3-2, via the antenna21. The received post-conversion frequency offset values are inputted to the offset compensation unit222. The offset compensation unit222performs statistical calculations on the inputted post-conversion frequency offset values and thereby calculates a statistical frequency offset (step S108). For example, the offset compensation unit222calculates the statistical frequency offset by calculating a mean value of the plural inputted post-conversion frequency offset values.

The offset compensation unit222gives the calculated statistical frequency offset to the transmission/reception unit221, and thereby makes offset compensation (step S109). Specifically, by giving the statistical frequency offset to the transmission/reception unit221, the offset compensation unit222changes the frequency band to a value receivable by the transmission/reception unit221. That is, by giving the statistical frequency offset to the transmission/reception unit221, the offset compensation unit222corrects the range of the frequency band receivable by the transmission/reception unit221. The transmission/reception unit221receives a signal in the frequency band corrected by the offset compensation unit222(step S110).

In the wireless communication system1configured as described above, the offset compensation unit222compensates for the frequency offsets occurring during communications between the plural terminal stations3and the mobile relay station2. Specifically, the offset compensation unit222gives a statistical frequency offset obtained from a statistic of plural frequency offsets occurring during communications between respective ones of the plural terminal stations3and the mobile relay station2to the transmission/reception unit221. That is, before performing a reception process, the mobile relay station2preestimates Doppler frequency occurring during communications with each of the terminal stations3and compensates for the frequency offsets such that the offsets will be smoothed among all the terminal stations3. Consequently, the mobile relay station2can reduce the Doppler frequency of the entire system. Furthermore, rather than compensating for the frequency offset of each terminal station3individually as with conventional techniques, the mobile relay station2takes statistics of frequency offset results obtained from the respective terminal stations3and determines a compensation range for the frequency offsets. Therefore, the wireless communication system1can reduce amounts of computation needed to compensate for the frequency offsets. This allows the wireless communication system1to reduce the impact of Doppler shift while suppressing amounts of computation.

Variation of First Embodiment

The mobile relay station2may be configured to receive terminal uplink signals via plural antennas and use MIMO (Multiple Input Multiple Output) to transmit base station downlink signals.

FIG.3is a configuration diagram of a wireless communication system Ta according to a variation of the first embodiment. InFIG.1, the same components as those of the wireless communication system1according to the first embodiment shown inFIG.1are denoted by the same reference signs as the corresponding components inFIG.1, and description thereof will be omitted. The wireless communication system1aincludes a mobile relay station2a, terminal stations3, and a base station4a.

The mobile relay station2aincludes N antennas21(N is an integer equal to or larger than 2), a terminal communication unit22a, a base station communication unit24a, and plural antennas25. Individual ones of the N antennas21will be referred to as antennas21-1to21-N.

The terminal communication unit22aincludes N transmission/reception units221, an offset compensation unit222a(first offset compensation unit), N terminal signal demodulation units223, and a synthesis unit224. The N transmission/reception units221will be referred to as transmission/reception units221-1to221-N. The N terminal signal demodulation units223will be referred to as terminal signal demodulation units223-1to223-N.

The transmission/reception unit221-n(n is an integer not smaller than 1 but not larger than N) receives a terminal uplink signal via the antenna21-n.

The offset compensation unit222acompensates for the frequency offsets occurring during communications between the plural terminal stations3and the mobile relay station2a. More specifically, the offset compensation unit222agives a statistical frequency offset obtained from a statistic of plural frequency offsets occurring during communications between respective ones of the plural terminal stations3and the mobile relay station2ato the transmission/reception unit221-n. A method for calculating the statistical frequency offset is similar to the method according to the first embodiment. A difference from the first embodiment is that the offset compensation unit222agives the statistical frequency offset to the transmission/reception unit221-n.

The terminal signal demodulation unit223-n(n is an integer not smaller than 1 but not larger than N) demodulates the terminal uplink signal received by the transmission/reception unit221-nand outputs demodulation results to the synthesis unit224.

The synthesis unit224combines demodulation results received as input from the respective terminal signal demodulation units223-1to223-N and outputs resulting demodulation information to the base station communication unit24a.

The base station communication unit24arelays terminal uplink signals to the base station4ausing MIMO. The base station communication unit24aincludes a storage241, a control unit242, a transmission data modulation unit243, and a MIMO transmission/reception unit245.

The storage241prestores weights for the base station downlink signals transmitted from the respective antennas25, where the weights are classified by send time. The send time may be expressed, for example, by elapsed time with respect to the transmission start timing. The weights classified by send time are calculated based on orbital information about the LEO satellite and positions of the respective antennas41. The orbital information about the LEO satellite provides information about the position, velocity, moving direction, and the like of the LEO satellite at any desired time. Note that a fixed weight may be used regardless of the send time.

The control unit242instructs the MIMO transmission/reception unit245to use the weights classified by send time and read out of the storage241.

When demodulation information is outputted by the synthesis unit224, the transmission data modulation unit243accepts input of the demodulation information as transmit data, converts the transmit data into parallel signals, and modulates the parallel signals.

The MIMO transmission/reception unit245assigns weights to the modulated parallel signals based on instructions from the control unit242and thereby generates base station downlink signals to be transmitted from the respective antennas25. The MIMO transmission/reception unit245transmits the generated base station downlink signals from the respective antennas25using MIMO.

The base station4aincludes plural antennas41, a MIMO transmission/reception unit45, a base station signal reception processing unit43a, and a terminal signal reception processing unit44a.

The antennas41are placed at positions away from one another so as to increase an angle difference of arrival of the signals from the plural antennas25of the mobile relay station2. The antennas41output the base station downlink signals received from the mobile relay station2to the MIMO transmission/reception unit45by converting the signals into electric signals.

The MIMO transmission/reception unit45puts together the base station downlink signals received via the plural antennas41. Based on orbital information about the LEO satellite and positions of the respective antennas41, the MIMO transmission/reception unit45stores weights for the base station downlink signals received by the respective antennas41by classifying the weights by receive time. The receive time may be expressed, for example, by elapsed time with respect to reception start timing. The MIMO transmission/reception unit45multiplies the base station downlink signals inputted through the respective antennas41by the weights corresponding to the receive times of the base station downlink signals and combines receive signals multiplied by the weights. Note that the same weight may be used regardless of the receive time.

The base station signal reception processing unit43ademodulates and decodes the receive signals resulting from the combination and thereby obtains demodulation information. The base station signal reception processing unit43aoutputs the demodulation information to the terminal signal reception processing unit44a.

The terminal signal reception processing unit44aperforms a reception process for the terminal uplink signals indicated by the receive waveform information. In so doing, the terminal signal reception processing unit44aacquires terminal transmit data by performing a reception process based on the wireless communication scheme used for transmission by the terminal station3. The terminal signal reception processing unit44aincludes N terminal signal demodulation units441, a terminal signal decode unit442, a distribution unit443, and a synthesis unit444. The N terminal signal demodulation units441will be referred to as terminal signal demodulation units441-1to441-N.

The distribution unit443reads waveform data received at the same receive time out of the receive waveform information and outputs the read waveform data to the terminal signal demodulation units441-1to441-N according to antenna identifiers associated with the waveform data. That is, the distribution unit443outputs the waveform data associated with the antenna identifier of the antenna21-nto the terminal signal demodulation unit441-n.

The terminal signal demodulation units441-1to441-N demodulate respective signals represented by waveform data, and output symbols obtained by the demodulation to the synthesis unit444. The terminal signal demodulation unit441-nmay perform demodulation after compensating the signal represented by the waveform data for the Doppler shift of the terminal uplink signal received by the antenna21-nof the mobile relay station2. The Doppler shift that impacts the terminal uplink signal received by each antenna21-nis calculated in advance based on the position of the terminal station3and the orbital information about the LEO satellite carrying the mobile relay station2b. The synthesis unit444additively synthesizes the symbols received as input from the terminal signal demodulation units441-1to441-N, respectively, and outputs the result of additive synthesis to the terminal signal decode unit442. The terminal signal decode unit442decodes the additively synthesized symbols and thereby obtains the terminal transmit data transmitted from the terminal station3.

With the above configuration, even when MIMO communications are conducted between the mobile relay station2aand the base station4a, amounts of computation needed to compensate for frequency offsets can be reduced. This makes it possible to reduce the impact of Doppler shift while suppressing amounts of computation.

Second Embodiment

In a second embodiment, description will be given of a configuration in which a statistical frequency offset is calculated by a technique different from the first embodiment. More specifically, according to the second embodiment, a mobile relay station estimates Doppler frequency based on surrounding information such as position information about terminal stations and position information, altitude information, orbital information about the mobile relay station. Then, based on the estimated Doppler frequency, the mobile relay station changes a receivable frequency band.

FIG.4is a configuration diagram of a wireless communication system1baccording to the second embodiment. InFIG.4, the same components as those of the wireless communication system1according to the first embodiment shown inFIG.1are denoted by the same reference signs as the corresponding components inFIG.1, and description thereof will be omitted. The wireless communication system1bincludes a mobile relay station2b, plural terminal stations3b, and a base station4. The numbers of mobile relay stations2b, terminal stations3b, and base stations4of the wireless communication system1bare optional, but it is assumed that the number of terminal stations3bis large.

The mobile relay station2bincludes an antenna21, a terminal communication unit22b, a base station communication unit24, and an antenna25.

The terminal communication unit22bincludes a transmission/reception unit221, an offset compensation unit222b(first offset compensation unit), and a terminal signal demodulation unit223.

The offset compensation unit222bcompensates for the frequency offsets occurring during communications between the plural terminal stations3and the mobile relay station2. More specifically, based on the altitude and orbit of the mobile relay station2, the offset compensation unit222bestimates the frequency offsets occurring during communications between the plural terminal stations3and the mobile relay station2and calculates a statistical frequency offset using the values of the plural estimated frequency offsets.

The terminal station3bincludes a data storage31, a transmission/reception unit32, and one or more antennas34. The terminal station3bdiffers in configuration from the terminal station3in that no estimation unit33is provided. Otherwise, the terminal station3bis similar in configuration to the first embodiment.

In this way, unlike the first embodiment, the terminal station3baccording to the second embodiment does not estimate the frequency offsets occurring between the terminal stations3band the mobile relay station2b.

Operation of the wireless communication system1bwill be described.

FIG.5is a flowchart showing a flow of a frequency offset process performed by the mobile relay station2baccording to the second embodiment. The process shown inFIG.5is performed either periodically or when a predetermined timing elapses.

The offset compensation unit222bestimates Doppler frequency based on surrounding information (step S201). Specifically, the offset compensation unit222bholds information such as shown inFIG.6and estimates Doppler frequency based on surrounding information such as position information about the terminal stations3band position information, altitude information, and orbital information about the mobile relay station2b. For example, the offset compensation unit222bestimates the Doppler frequency of each terminal station3b.

FIG.6is a diagram showing time variation of Doppler shift occurring according to altitude of the mobile relay station2b. InFIG.6, the abscissa represents elapsed time and the ordinate represents Doppler shift of an uplink signal. When the elapsed time is 0 sec., the mobile relay station2bis located right above a specific terminal station3b. InFIG.6, it can be seen that when the mobile relay station2bis located right above the specific terminal station3b, no Doppler shift occurs.

The offset compensation unit222bcalculates a statistical frequency offset based on plural estimated Doppler frequencies. The offset compensation unit222bgives the calculated statistical frequency offset to the transmission/reception unit221, and thereby makes offset compensation (step S202). Specifically, by giving the statistical frequency offset to the transmission/reception unit221, the offset compensation unit222bcorrects the range of the frequency band receivable by the transmission/reception unit221. The transmission/reception unit221receives a signal in the frequency band corrected by the offset compensation unit222b(step S203).

The wireless communication system1bconfigured as described above can reduce amounts of computation needed to compensate for the frequency offsets by a method different from the first embodiment. Furthermore, as shown inFIG.6, the time variation of the Doppler shift occurring according to the altitude of the mobile relay station2bis fixed to some extent. The use of such information allows the amounts of computation to be reduced. This makes it possible to reduce the impact of Doppler shift while suppressing the amounts of computation.

Variation of Second Embodiment

As with the first embodiment, the wireless communication system1bmay be configured to receive terminal uplink signals via plural antennas and use MIMO to transmit base station downlink signals. When the wireless communication system1bis configured in this way, it is sufficient to replace the configuration of the base station communication unit24of the mobile relay station2band the configuration of the base stations4with the configuration of the base station communication unit24aand the configuration of the base station4a, respectively, shown inFIG.3.

Third Embodiment

In a third embodiment, description will be given of a configuration in which offset compensation is made by a terminal station. Specifically, Doppler frequency occurring between a mobile relay station and a terminal station is estimated by the terminal station. Then, the terminal station generates an uplink signal shifted by an amount equivalent to the estimated Doppler frequency and transmits the uplink signal to the mobile relay station2.

FIG.7is a configuration diagram of a wireless communication system1caccording to the third embodiment. InFIG.7, the same components as those of the wireless communication system1according to the first embodiment shown inFIG.1are denoted by the same reference signs as the corresponding components inFIG.1, and description thereof will be omitted. The wireless communication system1cincludes a mobile relay station2c, plural terminal stations3c, and a base station4. The numbers of mobile relay stations2c, terminal stations3c, and base stations4of the wireless communication system1care optional, but it is assumed that the number of terminal stations3cis large.

The mobile relay station2cincludes an antenna21, a terminal communication unit22c, a base station communication unit24, and an antenna25.

The terminal communication unit22cincludes a transmission/reception unit221and a terminal signal demodulation unit223. The mobile relay station2cdiffers in configuration from the mobile relay station2in that no offset compensation unit222is provided. Otherwise, the terminal communication unit22cis similar in configuration to the first embodiment.

In this way, unlike the first embodiment, the mobile relay station2caccording to the third embodiment does not give any frequency offset to the transmission/reception unit221. That is, the mobile relay station2cdoes not compensate for the frequency offsets unlike the mobile relay station2.

The terminal station3cincludes a data storage31, a transmission/reception unit32, an estimation unit33, one or more antennas34, and an offset compensation unit35.

The offset compensation unit35compensates for frequency offsets occurring during communications between the plural terminal stations3cand the mobile relay station2c. More specifically, the offset compensation unit35gives the frequency offsets occurring during communications between the terminal station3c(local device) and the mobile relay station2cto the transmission/reception unit32.

Giving a frequency offset to the transmission/reception unit32means making a change so as to shift the frequency band available to the transmission/reception unit32for use in transmission by an amount equivalent to the frequency offset. If the frequency offset is “20 MHz,” the uplink signal transmitted from the terminal station3cwill be received by the mobile relay station2cwith the frequency of the uplink signal being shifted by “20 MHz.” Thus, when the frequency offset is “20 MHz,” the offset compensation unit35changes the frequency of the uplink signal transmitted from the terminal station3csuch that the frequency will be shifted by “−20 MHz.” Consequently, the impact of the offset can be reduced when the uplink signal is received by the mobile relay station2c.

Operation of the wireless communication system1cwill be described.

FIG.8is a sequence diagram showing a flow of a frequency offset process of the wireless communication system1caccording to the third embodiment. InFIG.7, description will be given by taking as an example a case in which two terminal stations3c(terminal station3c-1and terminal station3c-2) are provided in the wireless communication system1c. To distinguish functional parts of the two terminal stations3c-1and3c-2from each other, branch numbers “−1” and “−2” are assigned to the respective functional parts. The process shown inFIG.8is performed either periodically or when a predetermined timing elapses.

The mobile relay station2ctransmits downlink signals to the respective terminal stations3c-1and3c-2via the transmission/reception unit221(step S301).

The terminal stations3c-1and3c-2receive the downlink signals transmitted from the mobile relay station2cvia the antennas34-1and34-2, respectively. The received downlink signals are inputted to the estimation units33-1and33-2, respectively.

The estimation unit33-1of the terminal station3c-1estimates Doppler frequency based on the inputted downlink signal (step S302). The estimation unit33-1converts the estimated Doppler frequency into Doppler frequency experienced by uplink frequency (step S303). The estimation unit33-1outputs the frequency offset value resulting from the conversion to an offset compensation unit35-1.

The offset compensation unit35-1of the terminal station3c-1gives the transmission/reception unit32-1the value of the frequency offset resulting from the conversion and thereby compensates for the offset (step S304). Specifically, the offset compensation unit35-1corrects the range of the frequency band to be used for transmission by the transmission/reception unit32-1. The transmission/reception unit32-1of the terminal station3c-1transmits the terminal uplink signal to the mobile relay station2cusing the frequency band corrected by the offset compensation unit35-1(step S305).

The estimation unit33-2of the terminal station3c-2estimates Doppler frequency based on the inputted downlink signal (step S306). The estimation unit33-2converts the estimated Doppler frequency into Doppler frequency to be experienced by uplink frequency (step S307). The estimation unit33-2outputs the frequency offset value resulting from the conversion to an offset compensation unit35-2.

The offset compensation unit35-2of the terminal station3c-2gives the transmission/reception unit32-2the value of the frequency offset resulting from the conversion and thereby compensates for the offset (step S308). Specifically, the offset compensation unit35-2corrects the range of the frequency band to be used for transmission by the transmission/reception unit32-2. The transmission/reception unit32-2of the terminal station3c-2transmits the terminal uplink signal to the mobile relay station2cusing the frequency band corrected by the offset compensation unit35-2(step S309).

The mobile relay station2creceives the terminal uplink signals transmitted from the terminal stations3c-1and3c-2via the antenna21(step S310).

The wireless communication system1cconfigured as described above compensates for a frequency offset in each of the terminal stations3cbefore conducting communications with the mobile relay station2c. Specifically, when the Doppler frequency to be experienced by uplink frequency is known, the terminal station3ccompensates for the frequency offset by giving an error corresponding to the Doppler frequency to a signal during transmission. This makes it possible to reduce the impact of Doppler shift. Furthermore, with the wireless communication system1c, since the computation needed to compensate for the frequency offsets are performed by the individual terminal stations3c, there is no need for the mobile relay station2cto perform computation needed to compensate for the frequency offsets. This makes it possible to suppress the amounts of computation. Thus, the impact of Doppler shift can be reduced while suppressing the amounts of computation.

Variation of Third Embodiment

As with the first embodiment, the wireless communication system1cmay be configured to receive terminal uplink signals via plural antennas and use MIMO to transmit base station downlink signals. When the wireless communication system1cis configured in this way, it is sufficient to replace the configuration of the base station communication unit24of the mobile relay station2cand the configuration of the base stations4with the configuration of the base station communication unit24aand the configuration of the base station4a, respectively, shown inFIG.3.

The embodiments described above involves reducing the impact of Doppler shift occurring on the uplink from terminal stations to a mobile relay station. As described above, the impact of Doppler shift occurs on the downlink as well. Thus, each of the above embodiments may be configured to reduce Doppler shift occurring between the mobile relay station and the base station as well. This configuration will be described by taking as an example the first embodiment. As methods for reducing Doppler shift occurring between the mobile relay station and the base station, a first method and a second method are available.

According to the first method, the mobile relay station2or the base station4calculates frequency offsets based on orbital information about the mobile relay station2.

According to the second method, the frequency offsets occurring between the mobile relay station2and the base station4are calculated by the base station4and fed back to the mobile relay station2.

First, cases in which the first method is used will be described.

(When the mobile relay station2calculates frequency offsets based on orbital information about the mobile relay station2)

In this configuration, the base station communication unit24of the mobile relay station2further includes an offset compensation unit (second offset compensation unit). The offset compensation unit of the base station communication unit24estimates the frequency offsets occurring during communications between the base station4and the mobile relay station2based on the altitude and orbit of the mobile relay station2and position information about the base station4. A specific process is similar to that of the offset compensation unit222b. The offset compensation unit of the base station communication unit24gives the values of the estimated frequency offsets to a transmission/reception unit244and thereby corrects the range of the frequency band available to the transmission/reception unit244for use in transmission.

If the frequency offset is “20 MHz,” the downlink signal transmitted from the mobile relay station2will be received by the base station4with the frequency of the downlink signal being shifted by “20 MHz.” Thus, the offset compensation unit of the base station communication unit24gives a frequency offset during transmission such that the frequency offset that will impact the downlink signal received by the base station4will be reduced. Consequently, the impact of the offset can be reduced when the downlink signal is received by the base station4.

(When the base station4calculates frequency offsets based on orbital information about the mobile relay station2)

In this configuration, the base station communication unit24of the mobile relay station2further includes an offset compensation unit (second offset compensation unit) and the base station4further includes an estimation unit. The estimation unit estimates the frequency offsets occurring during communications between the base station4and the mobile relay station2based on the altitude and orbit of the mobile relay station2and position information about the base station4. A specific process is similar to that of the offset compensation unit222b. The base station4notifies the mobile relay station2of the values of the estimated frequency offsets. The offset compensation unit of the base station communication unit24gives the frequency offset values notified of by the base station4to the transmission/reception unit244and thereby corrects the range of the frequency band available to the transmission/reception unit244for use in transmission.

First, a case in which the second method is used will be described.

In this configuration, the base station communication unit24of the mobile relay station2further includes an offset compensation unit (second offset compensation unit) and the base station4further includes an estimation unit. The mobile relay station2transmits a known signal such as a beacon signal to the base station4. The estimation unit of the base station4estimates Doppler frequency based on the received known signal. A specific process is similar to that of the estimation unit33. Subsequently, estimation unit of the base station4notifies the mobile relay station2of the estimated Doppler frequency as a frequency offset value. The mobile relay station2receives the frequency offset value notified of. The offset compensation unit of the base station communication unit24gives the frequency offset value notified of by the base station4to the transmission/reception unit244and thereby corrects the range of the frequency band available to the transmission/reception unit244for use in transmission.

Although a case where a moving body on which the mobile relay station is mounted is a LEO satellite has been described in the above embodiments, it may be a geostationary satellite, a drone, a HAPS, or another aircraft flying over the sky.

The statistical calculations performed by the offset compensation unit222and the estimation process performed by the estimation unit33in the embodiments described above may be implemented by a computer. In that case, programs that implement these functions may be recorded on a computer-readable recording medium and read and executed by a computer system to implement the functions. It is assumed that the “computer system” referred to herein includes an OS and hardware such as peripheral devices. Also, the “computer-readable recording medium” refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, or a CD-ROM, or a storage device such as a hard disk built into the computer system.

Furthermore, the “computer-readable recording medium” may include a recording medium such as a communications wire that dynamically holds a program for a short time when the program is transmitted through a network such as the Internet or a communications line such as a telephone line, and a recording medium such as a volatile memory that holds the program in the computer system for a set amount of time when the computer system is acting as a server or a client during the transmission. The above program may be designed to implement only part of the functions described above or implement the functions described above in conjunction with a program prestored in the computer system. Alternatively, the above functions may be implemented using a programmable logic device such as a FPGA (Field Programmable Gate Array).

Embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but specific configurations of the present invention are not limited to the embodiments described above and include designs and the like without departing from the gist of the invention.

Industrial Applicability

The present invention is applicable to techniques for conducting communications with mobile bodies carrying mobile relay stations.

REFERENCE SIGNS LIST