Transmitting station, control circuit, and storage medium

A transmitting station includes an amplifier unit that amplifies a transmitting signal, a transmitting antenna that wirelessly transmits the transmitting signal that has been amplified, and a controller unit that regulates a frequency of the transmitting signal and an input power to the amplifier unit on the basis of an estimation value of reception quality that is a signal quality when the transmitting signal is received by a receiving station.

FIELD

The present invention relates to a transmitting station that transmits data as a radio signal, a receiving station, a control station, a data transmission system, and a control method for the data transmission system.

BACKGROUND

There has been developed a data transmission system intended to wirelessly transmit data acquired by an artificial satellite such as an observation satellite traveling in a low orbit around the Earth to a receiving station on the Earth. The low orbit is an orbit whose altitude is lower than a stationary orbit and generally corresponds to an orbit having an altitude less than or equal to 2000 km. With increasing precision of observation equipment mounted on the artificial satellite, an increasing amount of data is transmitted from the artificial satellite to the receiving station. Because of the increasing amount of data, a higher transmission rate is desired. The artificial satellite traveling in the low orbit is hereinafter abbreviated to “satellite”.

As a method of increasing a transmission rate, there has been a method of increasing the number of multiple values for modulation and a code rate to increase frequency usage efficiency. However, the higher the number of multiple values for modulation and the code rate are, the more likely it is that the receiving station has failure in data identification, and a required value for reception quality that is a signal quality of a reception signal to be received by the receiving station becomes higher. In the data transmission system using a satellite, a distance and an elevation angle between the satellite and the receiving station on the Earth vary as the satellite moves, and the amount of attenuation in a transmission signal varies with time in a communication channel between the satellite and the receiving station, so that the reception quality varies with time. For this reason, adaptive modulation is used to change the number of multiple values for modulation, the code rate, and the like of a transmitting signal according to temporal change in the reception quality.

Non Patent Literature 1 discloses a communication system using an adaptive modulation method called Adaptive Coding and Modulation (ACM) or an adaptive modulation method called Variable Coding and Modulation (VCM). The communication system described in Non Patent Literature 1 enables data transmission to satisfy a desired error rate by decreasing the number of multiple values for modulation as the reception quality becomes lower and increasing the number of multiple values for modulation as the reception quality becomes higher.

CITATION LIST

Non Patent Literature

Non Patent Literature 1: European Telecommunications Standards Institute European Standard (ETSI EN) 302 307-1 V1.4.1, November 2014

SUMMARY

Technical Problem

In the communication system described in the above-mentioned Non Patent Literature 1, a symbol rate needs to be increased for further improvement of the transmission rate. To increase the symbol rate, a transmitting station needs to increase transmission power of a signal with an amplifier part being used to regulate the transmission power. However, the amount of attenuation in the transmitting signal in the communication channel between the satellite and the receiving station varies with time in association with a positional relation between the satellite and the receiving station. Therefore, if the elevation angle between the satellite and the receiving station becomes higher after determination of input power to the amplifier part, the transmission power may become surplus even with the adaptive modulation being performed, and the frequency usage efficiency and the power utilization efficiency problematically decrease.

The present invention has been made in view of the above circumstances, and its object is to provide a transmitting station that is capable of improving frequency usage efficiency and power utilization efficiency when transmitting data as a radio signal.

Solution to Problem

In order to solve the problems mentioned above and achieve the object, the present invention provides a transmitting station comprising: an amplifier unit to amplify a transmitting signal; a transmitting antenna to wirelessly transmit the transmitting signal that has been amplified; and a controller unit to regulate a frequency of the transmitting signal and an input power to the amplifier unit on the basis of an estimation value of reception quality that is a signal quality when the transmitting signal is received by a receiving station.

Advantageous Effect of Invention

The transmitting station according to the present invention has an advantageous effect in that frequency usage efficiency and power utilization efficiency can be improved when data is transmitted as a radio signal.

DESCRIPTION OF EMBODIMENTS

With reference to the drawings, a detailed description is hereinafter provided of a transmitting station, a receiving station, a control station, a data transmission system, and a control method for the data transmission system according to embodiments of the present invention. It is to be noted that these embodiments are not restrictive of the present invention.

First Embodiment

FIG. 1is a diagram illustrating a configuration of a data transmission system1according to the first embodiment of the present invention. The data transmission system1includes a transmitting station100, a receiving station200, and a control station300. The transmitting station100is mounted to a satellite traveling in a low orbit around the Earth. The receiving station200is a radio communication device installed on the Earth and receives, from the transmitting station100, data collected by the satellite mounted with the transmitting station100. The control station300is a radio communication device installed on the Earth, generates control information for controlling radio communication between the transmitting station100and the receiving station200and informs the transmitting station100and the receiving station200of the generated control information.

The transmitting station100can transmit data to the receiving station200by use of radio communication. The data that the transmitting station100transmits to the receiving station200is, for example, observation data that is collected by the observation satellite mounted with the transmitting station100.

It is to be noted that a dotted line and a solid line connecting between blocks inFIG. 1represent forms of connection. The dotted line represents wireless connection, while the solid line represents wired connection. In other words, inFIG. 1, the transmitting station100and the receiving station200are wirelessly connected, the transmitting station100and the control station300are wirelessly connected, and the receiving station200and the control station300are connected by wire. It is to be noted, however, that the receiving station200and the control station300may be connected wirelessly.

FIG. 2is a diagram illustrating a functional configuration of the transmitting station100illustrated inFIG. 1. The transmitting station100includes a data generation unit101, a transmission buffer102, an encoder unit103, a modulator unit104, a waveform shaper unit105, an input power regulator unit106, a frequency converter unit107, an amplifier unit108, a transmitting antenna109, and a controller unit110.

The data generation unit101generates data to be transmitted, such as observation information, and stores the generated data in the transmission buffer102. The transmission buffer102outputs the stored data to the encoder unit103at a data transfer rate specified from the controller unit110. By changing the data transfer rate from the transmission buffer102, a symbol rate is regulated, and a frequency of a transmitting signal is regulated. Therefore, the transmission buffer102is an example of a frequency regulator unit. The encoder unit103encodes the data outputted from the transmission buffer102in an encoding method specified from the controller unit110. The encoder103outputs the encoded data to the modulator unit104. It is to be noted that the encoding method involves an encoding type and a code rate. The modulator unit104modulates the data outputted by the encoder unit103in a modulation method specified from the controller unit110. The modulator unit104outputs the modulated data to the waveform shaper unit105.

The waveform shaper unit105shapes a waveform of the data outputted by the modulator unit104to be band-limited with a roll-off specified from the controller unit110. The roll-off indicates a characteristic of a filter owned by the waveform shaper unit105. When the roll-off is α (0≤α≤1), a band is limited with 1+α times that of an ideal rectangular filter having the symbol rate. The larger the roll-off is, the more gradual an amplitude change of a signal is, but the smaller the roll-off is, the steeper the amplitude change of the signal is. The smaller the roll-off is, the higher the frequency usage efficiency is, but the higher a required value for reception quality is. By changing the roll-off, the frequency of the transmitting signal is regulated. Therefore, the waveform shaper unit105is an example of the frequency regulator unit. The waveform shaper unit105outputs the shaped data to the input power regulator unit106. The input power regulator unit106multiplies the data outputted by the waveform shaper unit105by a coefficient specified from the controller unit110. The input power regulator unit106outputs the data obtained after the multiplication of the coefficient to the frequency converter unit107. When the multiplication of the coefficient is performed by the input power regulator unit106, the power of a signal to be inputted to the amplifier unit108is regulated. Therefore, the input power regulator unit106regulates the input power to the amplifier unit108.

The frequency converter unit107converts the data outputted by the input power regulator unit106into a radio transmission frequency band signal using a frequency conversion amount specified from the controller unit110. The frequency converter unit107outputs the signal obtained by the conversion to the amplifier unit108. The amplifier unit108amplifies the power of the signal outputted by the frequency converter unit107. The amplifier unit108outputs the amplified signal to the transmitting antenna109. The transmitting antenna109transmits the signal outputted by the amplifier unit108as a radio signal.

The controller unit110receives the control information from the control station300and controls an operation of each of the units of the transmitting station100on the basis of the received control information. The control information includes one or more of the following: a data transfer amount to be specified to the transmission buffer102, an encoding method to be specified to the encoder unit103, a modulation method to be specified to the modulator unit104, a filter roll-off to be specified to the waveform shaper unit105, a coefficient to be specified to the input power regulator unit106, and a frequency conversion amount to be specified to the frequency converter unit107. The control information is instructional information on the frequency of the transmitting signal, the input power to the amplifier unit108, and others for the transmitting station100.

Upon receiving the control information from the control station300, the controller110checks whether or not the control information includes information elements to be communicated respectively to the transmission buffer102, the encoder unit103, the modulator unit104, the waveform shaper unit105, the input power regulator unit106, and the frequency converter unit107. If the control information includes the information element to be communicated, the controller unit110controls, in accordance with the control information, at least one of the transmission buffer102, the encoder unit103, the modulator unit104, the waveform shaper unit105, the input power regulator unit106, and the frequency converter unit107. Such a configuration allows the controller110to adjust, for example, the data transfer rate of the transmission buffer102, the encoding method to be used by the encoder unit103, the modulation method to be used by the modulator unit104, the roll-off of the waveform shaper unit105, the coefficient to be used by the input power regulator unit106, and the frequency conversion amount to be used by the frequency converter unit107.

FIG. 3is a diagram illustrating a functional configuration of the receiving station200illustrated inFIG. 1. The receiving station200includes a receiving antenna201, a frequency converter unit202, a waveform shaper unit203, a demodulator unit204, a decoder unit205, a reception buffer206, and a controller unit207.

The receiving antenna201receives a radio signal transmitted from the transmitting antenna109of the transmitting station100. The receiving antenna201outputs the received signal to the frequency converter unit202. The frequency converter unit202converts the signal outputted by the receiving antenna201into a frequency specified from the controller unit207, and then performs conversion into an electrical signal and outputs it. The waveform shaper unit203shapes a waveform of the electrical signal outputted by the frequency converter unit202with a roll-off specified from the controller unit207. The waveform shaper unit203outputs the waveform-shaped data to the demodulator unit204.

The demodulator unit204demodulates the data outputted by the waveform shaper unit203in a demodulation method specified from the controller unit207. The demodulator unit204outputs the data obtained after the demodulation to the decoder unit205. The decoder unit205decodes the data outputted by the demodulator unit204in a decoding method specified from the controller207, causes the reception buffer206to store the resultant decoded data therein.

The controller unit207receives the control information from the control station300and controls an operation of each of the units of the receiving station200on the basis of the received control information. The control information includes one or more of the following: a frequency to be specified to the frequency converter unit202, a roll-off to be specified to the waveform shaper unit203, a demodulation method to be specified to the demodulator unit204, and a decoding method to be specified to the decoder unit205.

Upon receiving the control information from the control station300, the controller unit207checks whether or not the control information includes information elements to be communicated respectively to the frequency converter unit202, the waveform shaper unit203, the demodulator unit204, and the decoder unit205. If the control information includes the information element to be communicated, the controller207controls, in accordance with the control information, at least one of the frequency converter unit202, the waveform shaper unit203, the demodulator unit204, and the decoder unit205.

The control information includes information elements corresponding to the information elements to be communicated to the transmitting station100. Therefore, the receiving station200can correctly receive the transmitting signal. The corresponding information element means an information element for enabling the receiving station200to normally handle the signal processed by the transmitting station100. When an encoding method with a code rate of 0.5 is specified to the transmitting station100, the corresponding information element corresponds to, for example, a decoding method in which the receiving station200can decode data having the code rate of 0.5. Moreover, the roll-off to be specified to the waveform shaper unit203of the receiving station200is adjusted to be identical with the roll-off specified to the waveform shaper unit105of the transmitting station100.

FIG. 4is a diagram illustrating a functional configuration of the control station300illustrated inFIG. 1. The control station300includes a location management unit301, a reception quality estimator unit302, a transmission controller unit303, and a control information transmit and receive unit304.

The location management unit301manages a location of the satellite mounted with the transmitting station100and a location of the receiving station200. The location of the satellite changes with time, while the location of the receiving station200is fixed.

The reception quality estimator unit302estimates the reception quality when the receiving station200receives a signal transmitted by the transmitting station100. The reception quality estimator unit302can estimate the reception quality using the locations of the transmitting station100and the receiving station200that are managed by the location management unit301. An estimation value of the reception quality only has to be a value indicative of the reception quality and, for example, corresponds to an amount of attenuation or a signal level of a received signal. The estimation value of the reception quality is usually composed of two or more factors. The reception quality varies depending on, for example, an elevation angle of the transmitting station100as viewed from the receiving station200, a rainfall condition in a place where the receiving station200is disposed, a directivity and a beam pattern of the transmitting antenna109equipped in the transmitting station100, a transmission power of the transmitting station100, a noise level specific to the receiving station200, and others.

The reception quality estimator unit302calculates a distance between the transmitting station100and the receiving station200and uses the calculated distance to estimate distance attenuation of a signal, which occurs between the transmitting station100and the receiving station200. Moreover, the reception quality estimator unit302uses the location of the receiving station200, managed by the location management unit301, to estimate the amount of rainfall attenuation of a signal. The reception quality estimator unit302can calculate the estimation value of the reception quality by taking into consideration the two or more factors. When the directivity and beam pattern of the transmitting antenna109, the transmission power of the transmitting station100, the noise level specific to the receiving station200, and others are fixed, the reception quality estimator unit302can also use the elevation angle of the transmitting station100as viewed from the receiving station200as an estimation value of the reception quality. The reception quality estimator unit302calculates the estimation value of the reception quality and outputs the calculated estimation value to the transmission controller unit303.

Alternatively, if the transmitting station100transmits a known signal on a regular basis, the reception quality estimator unit302can estimate the reception quality using a signal level of the known signal received by the receiving station200.

The transmission controller unit303generates the control information to be communicated to the transmitting station100and the receiving station200. The transmission controller unit303uses the reception quality estimation value outputted by the reception quality estimator unit302to select the information elements such as an appropriate modulation method and an appropriate code rate that satisfy a desired error rate. Details of a method in which the transmission controller unit303generates the control information are described later. The transmission controller unit303generates the control information including the selected information elements and outputs the generated control information to the control information transmit and receive unit304.

The control information transmit and receive unit304transmits the control information outputted by the transmission controller unit303to each of the transmitting station100and the receiving station200.

Now, description is provided for a hardware configuration intended to achieve functions of the transmitting station100, the receiving station200, and the control station300.FIG. 5is a diagram illustrating a control circuit10for realizing the functions the transmitting station100, receiving station200, and control station300illustrated inFIG. 1.

The control circuit10includes an input unit11, a processor12, a memory13, and an output unit14. The input unit11is an interface circuit that receives data inputted from outside of the control circuit10and outputs the data to the processor12. The processor12is a central processing unit (CPU), which is also referred to as a central processor, a processing unit, an arithmetic unit, a microprocessor, a microcomputer, a digital signal processor (DSP), or the like. The memory13is, for example, a magnetic disk, or a nonvolatile or volatile semiconductor memory such as a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable programmable ROM (EPROM), or an electrically EPROM (EEPROM). The output unit14is an interface circuit that outputs data from the processor12or the memory13to the outside of the control circuit10.

The processor12can implement each of the functions of the transmitting station100, the receiving station200, and the control station300by reading and executing a computer program stored in the memory13. The memory13is also used as a temporary memory in each process that is executed by the processor12.

Not only the above-described control circuit10but also dedicated hardware can be used to achieve the functions of the transmitting station100, the receiving station200, and the control station300. The dedicated hardware is, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or any combination of these.

Each of the units of the transmitting station100, the receiving station200, and the control station300may be functionally configured by use of their respective separate circuits or devices, or a plurality of functional units may be configured by use of a single circuit or device.

Next, the method in which the transmission controller unit303generates the control information is described in detail.FIG. 6is a flowchart illustrating an operation of the control station300illustrated inFIG. 4. In this example, an elevation angle is used as the estimation value of the reception quality.

The reception quality estimator unit302of the control station300estimates the reception quality that is a signal quality when the transmitting signal transmitted by the transmitting station100is received by the receiving station200(step S101). The transmission controller unit303selects a modulation method and a code rate on the basis of the estimated reception quality (step S102).

FIG. 7is a chart illustrating a relationship between the elevation angle between the transmitting station100and the receiving station200that are illustrated inFIG. 1, and a line margin corresponding to a combination of the modulation method and the code rate. A horizontal axis ofFIG. 7represents the elevation angle between the transmitting station100and the receiving station200, while a vertical axis ofFIG. 7represents the line margin. The line margin is a difference between the required value of the reception quality that is required to satisfy the desired error rate and the estimation value of the reception quality at each elevation angle. This means that the desired error rate is satisfied if the line margin is positive, and the desired error rate is not satisfied if the line margin is negative. The higher the elevation angle is, the lower the distance attenuation and the rainfall attenuation are. Therefore, when the transmission power of the transmitting station100is constant, the higher the elevation angle is, the more increased the line margin is.

InFIG. 7, MODCOD denotes a combination of the modulation method and the code rate, and the larger a suffixed number is, the higher the number of multiple values for modulation and the code rate are. In other words, the larger the number is, the higher the required value of the reception quality is. The transmission controller unit303selects a combination of the modulation method and the code rate which brings about a positive line margin and high frequency usage efficiency.

For example, when the elevation angle is between a1 and a2, the transmission controller unit303selects MODCOD (1). When the elevation angle is between a2 and a3, the transmission controller unit303selects MODCOD (2). When the elevation angle is more than or equal to a3, the transmission controller unit303selects MODCOD (3).

A return is made to the explanation ofFIG. 6. After selecting the modulation method and the code rate, the transmission controller unit303regulates a value of the input power to the amplifier unit108of the transmitting station100on the basis of the reception quality (step S103).

FIG. 8is a chart illustrating a relationship between the input power to the amplifier unit108of the transmitting station100illustrated inFIG. 2, and an output power21from the amplifier unit108and a modulation accuracy22of a radio signal outputted from the transmitting antenna109.

A horizontal axis ofFIG. 8represents the input power to the amplifier unit108, while a vertical axis ofFIG. 8represents the output power21from the amplifier unit108or the modulation accuracy22of the radio signal outputted from the transmitting antenna109. The amplifier unit108shows a linear amplifying characteristic or a non-linear amplifying characteristic, depending on the input power value. In the example ofFIG. 8, the relationship between the input power to the amplifier unit108and the output power from the amplifier unit108is linear until the input power value is i1, and the input power is amplified by a constant gain α. When the value of the input power to the amplifier unit108is i1, a value of the output power is o1. If the input power value is larger than i1, the relationship between the input power and the output power becomes non-linear. When the input power value is i3, the output is saturated, and the input power is amplified with a gain smaller than α, whereby the output power value becomes o3.

Therefore, when the estimation value of the reception quality is more than or equal to a predetermined threshold, the line margin becomes excessive as illustrated inFIG. 7, so that the transmission controller unit303selects an input power value at which the amplifying characteristic of the amplifier unit108becomes linear. When the estimation value of the reception quality is less than the predetermined threshold, high output is required, and so the transmission controller unit303selects an input power value at which the amplifying characteristic of the amplifier unit108becomes non-linear.

A return is made to the explanation ofFIG. 6. After regulating the input power value, the transmission controller unit303seamlessly adjusts a value of the roll-off of the waveform shaper unit105and a value of the symbol rate on the basis of the regulated input power (step S104).

When the input power value is selected such that the amplifying characteristic of the amplifier unit108is non-linear, harmonics are caused by signal distortion. In response, the transmission controller unit303increases the roll-off, and in order to prevent interference to the outside of the band assigned to the system1, the transmission controller unit303decreases the symbol rate. When the input power value is selected such that the amplifying characteristic of the amplifier unit108is linear, the signal distortion is reduced. In response, the transmission controller unit303makes the roll-off smaller than when the amplifying characteristic is non-linear, and increases the symbol rate.

FIG. 9is a chart illustrating a first example of the filter characteristic of the waveform shaper unit105illustrated inFIG. 2. When the amplifying characteristic of the amplifier108is non-linear, the transmission controller unit303can specify a roll-off such that the waveform shaper unit105has a filter characteristic as illustrated inFIG. 9.FIG. 10is a chart illustrating a second example of the filter characteristic of the waveform shaper unit105illustrated inFIG. 2. When the amplifying characteristic of the amplifier unit108is linear, the transmission controller unit303can specify a roll-off such that the waveform shaper unit105has a filter characteristic as illustrated inFIG. 10. It is to be noted here that even if the roll-off changes, values for stopband fm are equal.

A return is made to the explanation ofFIG. 6. The transmission controller unit303reselects a modulation method and a code rate on the basis of the adjusted input power (step S105). If the distortion components are reduced after the adjustment of the input power, that is to say, if the amplifying characteristic of the amplifier unit108becomes linear, the transmission controller unit303can also reselect the modulation method and the code rate such that the number of multiple values for modulation and the code rate are even higher to satisfy the desired error rate. It is to be noted that step S105may be omitted.

The transmission controller unit303generates control information including the above selected or regulated/adjusted information elements such as the modulation method, the code rate, the input power, the roll-off, and the symbol rate (step S106). The control information transmit and receive unit304informs the transmitting station100and the receiving station200of the generated control information (step S107).

In the above example, the control information is transmitted from the control station300to the transmitting station100and the receiving station200. However, the control information to be communicated to the receiving station200may be transmitted via the transmitting station100in order for a timing of changing the information element specified to each of the units of the transmitting station100to match with a timing of changing the information element specified to each of the units of the receiving station200. In this case, the receiving station200demodulates a data part of the transmitted data, corresponding to the control information in advance to obtain the control information, and thereafter demodulates a data part other than the control information, namely, a data part outputted from the transmission buffer102. With such a configuration, transmission processing in the transmitting station100and reception processing in the receiving station200can be synchronized.

As described above, the frequency of the transmitting signal and the input power to the amplifier unit108are regulated based on the estimation value of the reception quality in the first embodiment of the present invention. With such a configuration, on the basis of the estimation value of the reception quality, the transmitting station100can achieve the appropriate frequency usage efficiency to obtain the desired error rate and can reduce the signal distortion by preventing the transmission power from being excessive, and furthermore, even when the signal distortion is caused, the transmitting station100can minimize deterioration in reception quality. Therefore, the frequency usage efficiency and power utilization efficiency can both be improved, and high-speed data transmission can be achieved.

According to the first embodiment described above, the symbol rate and the roll-off are adjusted based on the estimation value of the reception quality. Specifically, in the case where the input power determined based on the estimation value of the reception quality is used, when the amplifying characteristic of the amplifier unit108becomes non-linear to cause distortion in a signal, the roll-off is increased so that the waveform shaper unit105can cut off the harmonics that are caused by the signal distortion thereby minimizing the deterioration in reception quality. At this time, if the symbol rate is decreased, the interference to the outside of the band assigned to the data transmission system1can be prevented.

In the above embodiment, in cases where the transmitting station100performs polarization multiplexing or frequency multiplexing, contents of data to be multiplexed may be changed based on the elevation angle, that is to say, based on the estimation value of the reception quality. When the estimation value of the reception quality is less than the predetermined threshold, identical data sets may be multiplexed by use of the polarization multiplexing or the frequency multiplexing, or identical data sets may be multiplexed using space time block coding (STBC). Such a configuration enables data to be reliably transmitted. When the estimation value of the reception quality is higher than or equal to the predetermined threshold, different data sets are multiplexed. With such a configuration, data transmission efficiency can be improved.

Second Embodiment

In the second embodiment of the present invention, description is provided for a configuration in which two or more radio signals are simultaneously transmitted in order to achieve a wider bandwidth of radio communication. In the second embodiment, the overall configuration of the data transmission system1is similar to the configuration of the first embodiment except for use of a transmitting station400, which is described below, in place of the transmitting station100illustrated inFIG. 1and some of the operations of the control station300. Therefore, the overall configuration of the data transmission system1is not described here. The following description is mainly of differences from the first embodiment.

FIG. 11is a diagram illustrating a functional configuration of the transmitting station400according to the second embodiment of the present invention. InFIG. 11and in the following description, where the transmitting station400includes two or more constituent elements having the same name, the two or more constituent elements each have a hyphen and a number suffixed after a symbol in order for the constituent elements to be distinguished. When the constituent elements need not be distinguished, the hyphen and the number suffixed after the symbol are omitted for representation thereof. For example, when an encoder unit403-1and an encoder unit403-2need not be distinguished, each of these units is simply referred to as encoder unit403.

The transmitting station400includes a data generation unit401, a transmission buffer402, the encoder unit403-1and encoder unit403-2, modulator unit404-1and modulator unit404-2, waveform shaper unit405-1and waveform shaper unit405-2, and phase regulator unit406-1and phase regulator unit406-2. The transmitting station400further includes input power regulator unit407-1and input power regulator unit407-2, frequency converter unit408-1and frequency converter unit408-2, amplifier unit409-1and amplifier unit409-2, a multiplexer unit410, a transmitting antenna411, a phase difference detector unit412, and a controller unit413.

InFIG. 11, the number of the constituent elements having the same name is two, but the present invention is not limited to this example. The transmitting station400can include three or more constituent elements having the same name. Of the above-mentioned constituent elements, constituent elements other than the phase regulator units406, the phase difference detector unit412, and the multiplexer unit410having the same functions as the constituent elements having identical names inFIG. 2, and so their detailed description will be omitted.

The data generation unit401generates data to be transmitted, such as observation data, and causes the transmission buffer402to store the generated data therein. The transmission buffer402outputs the stored data to each of the encoder units403at a transfer rate specified from the controller unit413. Each of the encoder units403encodes the data outputted from the transmission buffer402using an encoding method specified from the controller unit413. Each of the encoder units403outputs the encoded data to each of the modulator units404.

The modulator units404modulate the data sets outputted from the encoder units403using modulation methods specified from the controller unit413, respectively. The modulator units404output the modulated data sets to the waveform shaper units405, respectively. The waveform shaper units405shape waveforms of the data sets outputted from the modulator units404, respectively so that the data sets are band-limited with roll-offs specified from the controller unit413. The waveform shaper units405output the shaped signals to the phase regulator units406, respectively.

The phase regulator units406rotates the phase of the signals outputted from the waveform shaper units405, respectively on the basis of phase adjustment information pieces sent from the phase difference detector412for notification. In such a case, the phase regulator unit406performs the phase rotation of the signal so that a phase difference becomes zero between signals outputted respectively by the amplifier units409. The phase regulator units406output the phase-rotated signals to the input power regulator units407, respectively.

The input power regulator units407multiply the signals outputted by the phase regulator units406, respectively by coefficients specified from the controller unit413. When the input power regulator units407perform multiplication of the coefficients, respectively, powers of signals inputted to the amplifier units409, respectively, are adjusted. Therefore, each input power regulator unit407serves to regulate the input power to the amplifier unit409. The input power regulator units407output the signals obtained after the input power regulation, to the frequency converter units408, respectively.

The frequency converter units408convert the signals outputted by the input power regulator units407, respectively into signals in a radio transmission frequency band using frequency conversion amounts specified from the controller unit413, respectively. The frequency converter units408output the signals obtained by the conversion to the amplifier units409, respectively.

The amplifier units409amplify the signals outputted by the frequency converter units408, respectively. The amplifier units409output the amplified signals to each of the multiplexer unit410and the phase difference detector unit412.

The multiplexer unit410multiplexes the signals outputted respectively from the amplifier units409to obtain a radio signal and transmits the radio signal to the receiving station200via the transmitting antenna411. The phase difference detector unit412detects a phase difference between the signals outputted by the amplifier units409and generates phase adjustment information reflecting the detected phase difference. The phase difference detector unit412informs each of the phase regulator units406of the generated phase adjustment information. Here the phase adjustment information is set such that the phase difference becomes zero. The controller unit413specifies the information elements respectively to the transmission buffer402, the encoder units403, the modulator units404, the waveform shaper units405, the input power regulator units407, and the frequency converter units408on the basis of the control information to control operations of these units.

A description is provided next of the operation of the control station300.FIG. 12is a flowchart illustrating the operation of the control station300according to the second embodiment of the present invention. Operations similar to those in the first embodiment have the same reference characters and are not described. The following description is mainly of operations different from the operations in the first embodiment illustrated inFIG. 6.

The transmission controller unit303of the control station300selects a modulation method, a code rate and the number of channels on the basis of the estimated reception quality (step S202). The number of channels is the number of signals to be multiplexed in a frequency domain by the multiplexer unit410. When the multiplexer unit410multiplexes only an output of the amplifier unit409-1, the number of channels is one. When the signals outputted by the amplifier units409-1and409-2are signals obtained by converting different data sets into different frequencies, the number of channels is two. The transmission controller unit303reselects a modulation method, a code rate, and the number of channels on the basis of the regulated input power (step S205). The transmission controller unit303generates control information including the modulation method, the code rate, the number of channels, the input power, the roll-off, and the symbol rate (S206).

FIG. 13is a chart illustrating a relationship between an elevation angle between the transmitting station400illustrated inFIG. 11and the receiving station200illustrated inFIG. 1, and a line margin corresponding to a combination of the modulation method, the code rate, and the number of channels. A horizontal axis ofFIG. 13represents the elevation angle, while a vertical axis thereof represents the line margin. The transmission controller unit303can set the number of channels to two when the elevation angle is more than or equal to a4.

In the above description, the multiplexer unit410multiplexes the different data sets with different frequencies, but the present invention is not limited to this example. If the estimation value of the reception quality is less than a threshold, the multiplexer unit410may multiplex identical data sets. In such a case, the multiplexer unit410can multiplex the identical data sets with different frequencies. In this case, the receiving station200can improve the reception quality by adding up the identical data sets received at the different frequencies. The multiplexer unit410may convert the identical data sets to the same frequency and then multiplex its conversion result. In this case, since the signals outputted respectively by the amplifier units409-1and409-2are in phase with each other because of the phase regulator units406, a signal power obtained by the multiplex can be improved as compared with a case where only the output of the amplifier unit409-1is transmitted. When the estimation value of the reception quality is higher than or equal to the threshold, the multiplexer unit410can multiplex data sets that differ from each other.

Instead of multiplexing the two channels with different frequencies, the multiplexer unit410may separate the two channels into different polarized waves and then multiplex them. The multiplexer unit410can perform multiplexing with the output of the amplifier unit409-1being regarded as a left-handed circularly polarized wave and with the output of the amplifier unit409-2being regarded as a right-handed circularly polarized wave, respectively.

As described above, according to the second embodiment of the invention, the radio communication with a wider bandwidth can be achieved in addition to the effects of the first embodiment.

The above configurations illustrated in the embodiments are illustrative of contents of the present invention, and can be combined with other publicly known techniques and partly omitted and/or modified without departing from the gist of the present invention.

In the above embodiment, the transmitting station100is mounted on the satellite, but the present invention is not limited to this example. The technique of the present invention can be applied to a case where the transmitting station100is mounted on a mobile object such as an aircraft, whereby the reception quality varies with time.

In the above embodiment, the information element to be specified to each of the units of the transmitting station100and the receiving station200is included in the control information, but the present invention is not limited to this example. The control information includes the estimation value of the reception quality, and the transmitting station100and the receiving station200may each determine the information element to be specified to each of the units on the basis of the estimation value of the reception quality. In this case, the transmitting station100and the receiving station200each prestore correspondence relations between values of the information elements and estimation values of the reception quality and select the value of the information element based on the correspondence relations to specify it to each of the units. Alternatively, the information element may be changed by means of a predetermined change pattern. The transmitting station100and the receiving station200each prestore the change pattern and can use this change pattern to change, on the basis of a time elapsed since reception of the control information, the information element to be specified to each of the units.

REFERENCE SIGNS LIST