Satellite receiver and satellite communication system

A satellite receiver includes: demultiplexing units each demultiplexing, into subchannel signals of a predetermined band, a digital reception signal obtained by converting a calibration signal received by a corresponding one of receiving antenna elements into a digital signal; excitation coefficient multiplication units multiplying the subchannel signals by an excitation coefficient; a complex adder adding the subchannel signals multiplied by the excitation coefficient together for each subchannel signal of the same band; a correlation detection unit calculating, with the use of one demultiplexing unit as a reference demultiplexing unit, a cross-correlation value for each subchannel signal output from each demultiplexing unit different from the reference demultiplexing unit with respect to a subchannel signal of a same band output from the reference demultiplexing unit; and an excitation coefficient generation unit generating a corrected excitation coefficient based on a cross-correlation value and an excitation coefficient created in advance.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on PCT filing PCT/JP2020/002765, filed Jan. 27, 2020, the entire contents of which is incorporated herein by reference.

FIELD

The present disclosure relates to a satellite receiver and a satellite communication system that receive signals using an array antenna.

BACKGROUND

With an increase in communication capacity, a relay satellite system that flexibly controls communication traffic is required. Regarding control of area flexibility of communication traffic in a relay satellite system, there is a technology called beam forming for forming transmission and reception beams by controlling excitation coefficients of a plurality of antennas, that is, amplitudes and phases thereof. Examples of a beam forming method include microwave beam forming (MBF) using a phase shifter for microwaves, and digital beam forming (DBF) that controls excitation coefficients by digital signal processes. Since the digital beam forming enables integration as compared with the microwave beam forming, the number of beams can be increased.

For example, in a satellite relay device described in Patent Literature 1, the following means is described as a method of digital beam forming; each of signals received by receiving antenna elements is subjected to frequency division and multiplied by a weighting coefficient in an excitation coefficient multiplication unit, and reception signals are combined to form a beam. With respect to a plurality of signal transmitters on the ground, the satellite relay device automatically calculates, on the relay device side, excitation coefficients that maximize reception efficiency, and multiplies reception signals by the excitation coefficients, thereby forming a beam.

In such digital beam forming, it is important to reduce a gain difference, a delay difference, and a phase difference between receiving antenna element systems.

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

In general, when establishing communication by beam forming using a plurality of feed elements, it is necessary to reduce an amplitude difference, a phase error, and a delay difference between the feed elements caused by individual differences between components and temperature fluctuations. The amplitude difference and the phase error between the feed elements cause a shape change from an ideal beam shape when an excitation coefficient is determined and antenna gain degradation. In addition, the delay difference between the feed elements causes intersymbol interference when combining reception signals in the elements. In particular, in a case of achieving a beam for establishing broadband communication, the influence of the delay difference is increased, which results in deterioration in communication quality.

In the invention described in Patent Literature 1, optimum excitation coefficients can be automatically set for a plurality of ground transmission stations (ground terminals) by the satellite relay device, but on the other hand, it is not possible to separate a gain difference, a delay difference, and a phase difference between element systems based on a signal arrival direction of each ground terminal from the above-described errors between element systems, that is, an amplitude difference, a phase error, and a delay difference between feed elements generated inside the satellite relay device. That is, in the invention described in Patent Literature 1, a beam formed by beam forming can be optimized for each of the ground terminals, but any intended beam cannot be formed independently of the positions of the ground terminals. In particular, when the number of ground terminals is large, there may be a case where a plurality of ground terminals are covered by the same beam. In such a case, there arises a need to make, by a communication control station on the ground or the like, a plan for an area to be covered from information regarding, for example, arrangement of the ground terminals, and to form a beam in accordance with the plan.

The present disclosure has been made in view of the above, and an object thereof is to obtain a satellite receiver capable of improving communication efficiency by calibrating errors between element systems generated inside a device, specifically, a gain error, a delay error, and a phase error.

Solution to Problem

In order to solve the above-described problems and achieve the object, a satellite receiver includes: N demultiplexing units to each demultiplex, into a plurality of subchannel signals of a predetermined band, a digital reception signal obtained by converting a calibration signal received by a corresponding one of N receiving antenna elements into a digital signal; and N excitation coefficient multiplication units to multiply each of the plurality of subchannel signals by an excitation coefficient. In addition, the satellite receiver includes: a complex adder to add the plurality of subchannel signals that are multiplied by the excitation coefficient together for each subchannel signal of the same band; and a correlation detection unit to, with the use of one demultiplexing unit among the N demultiplexing units as a reference demultiplexing unit, calculate a cross-correlation value for each subchannel signal output from each demultiplexing unit different from the reference demultiplexing unit with respect to a subchannel signal of the same band output from the reference demultiplexing unit. Furthermore, the satellite receiver includes an excitation coefficient generation unit to generate a corrected excitation coefficient that is an excitation coefficient by which the excitation coefficient multiplication units multiply the subchannel signals on the basis of a cross-correlation value calculated by the correlation detection unit and an excitation coefficient created in advance to form a desired reception beam, and the N is an integer of 2 or more.

Advantageous Effects of Invention

A satellite receiver according to the present disclosure achieves an effect that it is possible to improve communication efficiency by calibrating errors between element systems generated inside a device.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a satellite receiver and a satellite communication system according to each embodiment of the present disclosure will be described in detail with reference to the drawings. The disclosure is not limited to the embodiments.

First Embodiment

FIG.1is a diagram illustrating an example configuration of a satellite receiver according to a first embodiment. A satellite receiver1according to the first embodiment includes receiving antenna elements11ato11d, analog-to-digital converters (hereinafter referred to as ADCs)12ato12d, demultiplexing units13ato13d, a DBF reception unit20, a tracking telemetry & command (TTC) transmitting/receiving antenna54, and a TTC processing unit55. The DBF reception unit20includes excitation coefficient multiplication units21ato21d, a correlation detection unit22, a complex adder23, an inter-element-system error detection unit31, a correction coefficient storage unit32, an excitation coefficient storage unit33, and complex multipliers34ato34d. The satellite receiver1is installed, for example, on a satellite to constitute a satellite communication system. Note that, in the satellite receiver1illustrated inFIG.1, the number of element systems including the receiving antenna elements11x, the ADCs12x, and the demultiplexing units13x(x=a to d) is four, but the number of element systems is not limited to four.

An object of the satellite receiver1is to correct gain, delay, and phase errors between element systems of N elements (N=4 in the example illustrated inFIG.1), and to obtain desired reception characteristics regardless of errors between the element systems with respect to an actual communication signal3received from a ground station2. The satellite receiver1corrects the gain, delay, and phase errors between the element systems using a calibration signal10of a band Δfcal transmitted from a calibration ground station51.

In the satellite receiver1, the receiving antenna elements11xeach receive the actual communication signal3from the ground station2and the calibration signal10from the calibration ground station51. The ADCs12xdigitally convert reception signals input from the receiving antenna elements11x. The demultiplexing units13xdemultiplex digital reception signals, which are the digitally-converted signals, for each predetermined frequency band. The excitation coefficient multiplication units21xmultiply the reception signals that have been demultiplexed by the demultiplexing units13xby corrected excitation coefficients generated by the complex multipliers34xto be described later. The complex adder23performs vector-synthesis of the reception signals after multiplication by the corrected excitation coefficients each output from one of the plurality of excitation coefficient multiplication units21xto form a reception beam. The correlation detection unit22performs correlation detection of reception signals between the element systems. The inter-element-system error detection unit31detects errors between the element systems on the basis of results of the correlation detection performed by the correlation detection unit22and calculates a correction coefficient for correcting the errors between the element systems. The correction coefficient storage unit32stores the correction coefficient calculated by the inter-element-system error detection unit31. The excitation coefficient storage unit33stores an excitation coefficient for forming a desired reception beam. The complex multipliers34xeach multiply the correction coefficient for the errors between the element systems stored in the correction coefficient storage unit32and the reception beam-formation excitation coefficient stored in the excitation coefficient storage unit33together to generate a corrected excitation coefficient. The TTC transmitting/receiving antenna54receives a command signal53from a ground satellite control station52. The TTC processing unit55delivers the command signal53input from the TTC transmitting/receiving antenna54to the inter-element-system error detection unit31and the excitation coefficient storage unit33.

The satellite receiver1having the above configuration receives the actual communication signal3from the ground station2on the basis of reception characteristics for a reception beam determined by the excitation coefficient and the reception characteristics of the receiving antenna elements11ato11d, and outputs the actual communication signal3as a signal24. In the satellite receiver1, the inter-element-system error detection unit31and the complex multipliers34ato34dconstitute an excitation coefficient generation unit.

Next, an operation of the satellite receiver1will be described in detail. Note that, in the present embodiment, the number of receiving antenna elements may be described as N for convenience. N is an integer of 2 or more.

First, as a basic operation of the satellite receiver1, the actual communication signals3received by the receiving antenna elements11ato11dare digitally converted by the ADCs12ato12d. A sampling frequency fs of the ADCs is set to a value that satisfies the sampling theorem for the reception signals. In a case where this is not satisfied due to the original frequency, the condition is satisfied by performing intermediate frequency (IF) conversion or the like. Furthermore, the ADCs12ato12deach perform a filtering process as necessary to remove aliasing.

The demultiplexing units13ato13deach demultiplex the digitally-converted signal into subchannels of a band Δfch to generate subchannel signals. The demultiplexing units13ato13deach have a function of performing quadrature detection of the digitally-converted reception signal and analyzing a spectrum thereof. The demultiplexing units13ato13deach perform spectral resolution on a signal with resolution corresponding to the band width Δfch out of the total band Δf. Therefore, the number K of subchannels after spectral resolution is K=Δf/Δfch. In addition, by the quadrature detection, decomposition into orthogonal components of an I signal (complex number real part) and a Q signal (complex number imaginary part) is performed, and the subchannel signals are output. Hereinafter, each subchannel signal is treated as a complex number.

All subchannel (I,Q) signals output from the demultiplexing units13ato13dare input to the DBF reception unit20. Here, in the example illustrated inFIG.1, one DBF reception unit20is provided, and this assumes a case where one reception beam for communication is formed. In a case where the number of reception beams for communication increases, it is only required to prepare another DBF reception unit20depending on the number of reception beams to be formed. If frequency bands allocated to reception beams do not overlap, one DBF reception unit20may be used for a plurality of reception beams.

A basic function of the DBF reception unit20is to control reception characteristics of the entire array antenna by multiplying the signals demultiplexed into the subchannels of the respective elements by excitation coefficients of complex numbers by the excitation coefficient multiplication units21ato21dand then adding the signals of all the elements together for each of the subchannels by the complex adder23. The signal24output from the complex adder23is transmitted as an actual communication signal to another ground station (not illustrated) different from the ground station2via a satellite transmitter (not illustrated). Alternatively, the signal24is transmitted to the ground station via the satellite transmitter after frequency switching between the subchannels, that is, a channelization operation is performed.

The excitation coefficients multiplied in the excitation coefficient multiplication units21ato21dare transmitted to the satellite on which the satellite receiver1is installed from the ground satellite control station52via the command signal53, for example. The excitation coefficients transmitted from the ground satellite control station52are stored in the excitation coefficient storage unit33via the TTC transmitting/receiving antenna54and the TTC processing unit55in the satellite receiver1, and are delivered to the excitation coefficient multiplication units21ato21d. The TTC transmitting/receiving antenna54transmits and receives the command signal53and a telemetry signal (not illustrated), and the TTC processing unit55performs deliver control of information on the excitation coefficients and the like transmitted with the command signal53to each functional unit in the DBF reception unit20, and collects the telemetry signal from each functional unit.

If there is no error between the element systems inside the satellite receiver1, desired reception characteristics depending on an excitation coefficient set via the ground satellite control station52are obtained for the entire array antenna. However, in practice, a gain difference, a delay difference, and a phase difference occur for each of the element systems from the receiving antenna elements11ato11dto the demultiplexing units13ato13d. The differences for each element system are caused by temperature fluctuations in a transmission path such as a waveguide and a coaxial cable, a difference in path length at the time of design, a clock shift of the ADCs, and the like. Furthermore, in the satellite receiver1, an amplifier for amplifying a signal level and a frequency converter for performing frequency conversion may be used, and it is difficult to completely match characteristics thereof with each other between the element systems. As described above, between the plurality of element systems included in the satellite receiver1, there is an error between element systems which is an error based on characteristics of hardware constituting the element systems.

Therefore, the satellite receiver1includes a functional unit for detecting and correcting an error between the element systems inside the DBF reception unit20. The functional unit performs a signal process on the basis of the calibration signal10transmitted from the calibration ground station51, and performs detection of an error and calculation of a correction coefficient.

The band of the calibration signal10transmitted from the calibration ground station51is defined as Δfcal. The band Δfcal is desirably the same as the band Δf that can be processed by the demultiplexing units13ato13dand the excitation coefficient multiplication units21ato21d. This is because the band Δfcal of the calibration signal10becomes a band in which calibration can be guaranteed as the satellite receiver1after calibration. As the calibration ground station51, a portable very small aperture terminal (VSAT) and the like are exemplified. In order to avoid interference with a communication signal, calibration using the calibration signal10is performed at a time when communication via the actual communication signal3is not performed.

The calibration signal10is to be received by the receiving antenna elements11ato11d. Since the calibration signal10is spatially distributed, the same signal is to be received by all the receiving antenna elements11ato11d. However, the power and the phase of the calibration signal received by each of the receiving antenna elements11ato11dat that time vary depending on the reception characteristics of each of the receiving antenna elements11ato11d. For example, when a reception gain of a receiving antenna element is small with respect to a location of the calibration ground station51, a signal is hardly received by the receiving antenna element. In calibration between the element systems, it is necessary to input a calibration signal to at least two elements. Therefore, the calibration ground station51is arranged, among the ground locations covered by radiation patterns of the receiving antenna elements11ato11d, at a location where a sufficient gain can be obtained, that is, a location where a gain equal to or larger than a predetermined value can be obtained, with respect to at least two elements.

The calibration signal10is converted into demultiplexed subchannel signals similarly to the actual communication signal3via the ADCs12ato12dand the demultiplexing units13ato13d.

The subchannel signals of the respective element systems demultiplexed by the demultiplexing units13ato13dare extracted and input to the correlation detection unit22. The correlation detection unit22calculates a correlation vector regarding the reception signals of the respective element systems for each of the subchannels. In the calculation of the correlation vector, by correlating a reception signal of an element system with the highest reception power with a reception signal of another element system, a correlation vector using the element system with the highest reception power as reference can be obtained. That is, one reference is determined, and correlation vectors of the reception signals of all the other element systems are obtained with respect to the reception signal of the element system as reference. On the basis of the correlation vectors obtained by the correlation detection unit22, the inter-element-system error detection unit31calculates a correction coefficient of each of the subchannels of the respective element systems.

The correction coefficients calculated by the inter-element-system error detection unit31are stored in the correction coefficient storage unit32, and are multiplied by the beam-formation excitation coefficient stored in the excitation coefficient storage unit33by the complex multipliers34ato34d. Consequently, corrected excitation coefficients in which errors between the element systems generated inside the satellite receiver1have been corrected are calculated and set for the respective element systems and the respective subchannels by the excitation coefficient multiplication units21ato21d. As a result, it becomes possible to perform beam formation without being affected by errors between the element systems generated inside the satellite receiver1. Note that the complex multipliers34ato34dconstitute a multiplication unit that generates the corrected excitation coefficients.

Here, calculation of a correction coefficient based on a cross-correlation value of complex numbers performed by the inter-element-system error detection unit31will be described. First, a signal of a subchannel k of a receiving antenna element n after demultiplexing can be described as the following formula (1). Note that, in the following description, the receiving antenna element may be simply referred to as an “element” for convenience of description.
[Formula 1]
Xn[k]=In[k]+jQn[k]=Anejθn(1)

In formula (1), Inand Qnare a real part and an imaginary part of a complex number Xn, respectively, Anis an amplitude of the complex number Xn, and θnis an argument of the complex number Xn. Now, with the use of the element n as reference, an element m having a gain difference, a delay difference, and a phase difference is considered, and a signal of each subchannel k can be described as the following formula (2).
[Formula 2]
Xm[k]=Im[k]+jQm[k]=AmΔmnej(θm+δmn)(2)

Here, with the use of the element n as reference, an amplitude error due to a gain difference between the element systems inside the satellite receiver1is denoted by Δmn, and a phase error due to a delay difference and a phase difference between the element systems is denoted by δmn.

When an excitation coefficient for the subchannel k of the element m is denoted by Gmejψm, an output after multiplexing of N elements by the complex adder23can be described by the following formula (3).
[Formula 3]
Yerr=ΣmNGmAmΔmnejψmej(θm+δmn)(3)

In an ideal case where there is no error between the element systems, the following formula (4) is obtained, and it can be seen that the errors between the element systems affect the reception characteristics.
[Formula 4]
Ynom=ΣmNGmAmejψmejθm(4)

Now, when the cross-correlation of these complex numbers is calculated on the basis of formulas (1) and (2), the cross-correlation can be described as the following formula (5).

In a case where the number of elements is N, n takes any one of values of 1 to N, and m takes all values of 1 to N. In a case of m=n, formula (5) is autocorrelated. Here, Amand An, τmand τn, as well as φmand φnare amplitude, delay, and phase determined by the reception characteristics of respective receiving antenna elements m with respect to the position of the calibration ground station51, respectively, and are set as in the following formula (6).

An effect that a time delay in a time domain appears as a phase term in a frequency domain after demultiplexing is based on the relationship represented by the following formula (7). In formula (7), reference character “F” written above the arrow represents Fourier transform.

In addition, the delay difference and the phase difference can be converted via a carrier wavelength, but are separately described here for organization, phase rotation of 2π or more is regarded as the delay difference, and the phase difference is assumed to have a value in a range of 0 to 2π.

Furthermore, in formula (5), the second term includes a complex number based on errors between the element systems generated inside the satellite receiver1, which is represented by the following formula (8).
[Formula 8]
Δmnejδmn=Δmnexp(2πjτmnkΔfch+jϕmn)  (8)

An amplitude error due to a gain difference between the element systems inside the satellite receiver1is denoted by Δmn, a delay error between the element systems is denoted by τmn, and a phase error between the element systems is denoted by φmn. Therefore, if the complex number represented by formula (8) can be calculated, errors between the element systems generated inside the satellite receiver1can be detected, and a reciprocal thereof serves as a correction coefficient.

On the basis of the above discussion, detection of an error between the element systems and calculation of a correction coefficient will be described.

First, the correlation detection unit22calculates cross-correlation and autocorrelation using the element n as reference for all subchannels of all elements. In a case where the number of elements is N and the number of subchannels is K, the correlation detection unit22calculates N×K correlation values (cross-correlation and autocorrelation), and the N×K correlation values are output to the inter-element-system error detection unit31.

Next, the inter-element-system error detection unit31selects a specific element m from the correlation values input from the correlation detection unit22, and performs detection of errors and calculation of correction values for all K subchannels corresponding to the element m.

First, regarding the estimation of the amplitude error Δmn, when a ratio is calculated from the autocorrelation of the element n and the cross-correlation of the element m and the element n, the following formula (9) is obtained.

Amand Ancan be estimated in advance as the reception characteristics of the receiving antenna elements m and n with respect to the position of the calibration ground station51, and, by removing the term Δ0mnindicated in formula (10) including these from formula (9) by division, Δmncan be calculated as indicated in formula (11).

Regarding the delay error and the phase error, when a phase of a cross-correlation value is extracted, the following formula (12) is obtained.
[Formula 12]
Arg(XmXn*)=2π(τm−τn+τmn)kαfch+(ϕm−ϕn+ϕmn)  (12)

τm, τn, φm, and φncan be estimated as the reception characteristics of the receiving antenna elements m and n with respect to the position of the calibration ground station51, and, by removing the term τ0mnindicated in formula (13) including these from formula (12) by subtraction, formula (14) is obtained.
[Formula 13]
ϕ0mn=2π(τm−τn)kΔfch+(ϕm−ϕn)  (13)
[Formula 14]
Arg(XmXn*)−ϕ0mn=2πτmnkΔfch+ϕmn(14)

A correlation vector phase after removal of φ0mncan be expressed as inFIG.2.FIG.2is a diagram illustrating an example of a correlation vector phase. InFIG.2, the horizontal axis represents the frequency of a received signal, that is, subchannels. The vertical axis represents the correlation vector phase indicated by formula (14), that is, the correlation vector phase after removal of φ0mn. As can be seen fromFIG.2and formula (14), the delay error τmnappears as a constant inclination amount (phase rotation) with respect to the subchannel k, and the phase error φmnappears as an offset amount.

Since the errors between the element systems generated inside the satellite receiver1can be described by formula (8), the correction coefficients thereof are indicated by the following formula (15) using values that can be calculated from formulas (11) and (14). By using the correction coefficient indicated by formula (15), the amplitude error, the delay error, and the phase error can be corrected using the element n as reference for each subchannel of the element m.

Formula (15) is the reciprocal of formula (8). The correction of the delay error and the phase error can be expressed as inFIG.2.

The inter-element-system error detection unit31calculates the correction coefficient indicated by formula (15) for each of N−1 elements other than the element n as reference, and stores the correction coefficients in the correction coefficient storage unit32.

Corrected excitation coefficients are generated by multiplying the correction coefficients stored in the correction coefficient storage unit32and the beam-formation excitation coefficient stored in the excitation coefficient storage unit33together by the complex multipliers34ato34d. By the excitation coefficient multiplication units21ato21dmultiplying input signals (subchannels) by the corrected excitation coefficients, desired reception characteristics that are not affected by errors between the element systems of each of the subchannels are achieved.

As described above, Δ0mnand φ0mnused in the calculation indicated in the above formula (11) and the calculation indicated in the above formula (14) are amounts determined by the reception characteristics of the receiving antenna elements m and n with respect to the position of the calibration ground station51. These amounts are calculated by the ground satellite control station52on the basis of position information on the calibration ground station51and reception characteristic information on the receiving antenna elements11ato11d, transmitted by the command signal53, received by the TTC transmitting/receiving antenna54, and then can be given to the inter-element-system error detection unit31via the TTC processing unit55.

Furthermore, in calculating the correction coefficients, it is also possible that correction with respect to Δ0mnand φ0mnare added in advance to the beam-formation excitation coefficient set by the command signal53from the ground satellite control station52without performing, by the inter-element-system error detection unit31, division by Δ0mnand subtraction of φ0mnas indicated by formulas (11) and (14). In that case, the correction coefficients are indicated by formula (16), and the excitation coefficients stored in the excitation coefficient storage unit33are indicated by formula (17).

Alternatively, in calculating the correction coefficients, it is also possible to calculate the correction coefficients in the ground satellite control station52instead of calculating the correction coefficients inside the satellite receiver1. That is, a satellite receiver1A configured as illustrated inFIG.3can be employed.FIG.3is a diagram illustrating another example configuration of a satellite receiver according to the first embodiment.

The satellite receiver1A illustrated inFIG.3has a configuration obtained by replacing the DBF reception unit20of the satellite receiver1illustrated inFIG.1with a DBF reception unit20A. The DBF reception unit20A includes the excitation coefficient multiplication units21ato21d, the correlation detection unit22, the complex adder23, and the excitation coefficient storage unit33. These components are the same as the components denoted by the same reference numerals of the DBF reception unit20included in the satellite receiver1.

The satellite receiver1A transmits the cross-correlation values detected by the correlation detection unit22to the ground satellite control station52through the TTC processing unit55and the TTC transmitting/receiving antenna54via a telemetry signal59. The ground satellite control station52calculates correction coefficients by performing a process similar to that of the inter-element-system error detection unit31of the DBF reception unit20included in the satellite receiver1, and calculates corrected excitation coefficients by multiplying the beam-formation excitation coefficient by the correction coefficients. The ground satellite control station52transmits the calculated corrected excitation coefficients to the satellite receiver1A via the command signal53. After receiving the command signal53from the ground satellite control station52by the TTC transmitting/receiving antenna54, the satellite receiver1A stores the corrected excitation coefficients included in the command signal53in the excitation coefficient storage unit33via the TTC processing unit55. The excitation coefficient multiplication units21ato21dmultiply the input signals by the corrected excitation coefficients stored in the excitation coefficient storage unit33.

In the above description, the calibration signal10of one calibration ground station51is input to all the N elements, but depending on the design of the receiving antenna elements11ato11d, the calibration signal10cannot be input from one calibration ground station51to all the N elements in some cases. In that case, it is only required to provide a plurality of calibration ground stations. For example, each calibration ground station is arranged as illustrated inFIG.4.FIG.4is a diagram illustrating an example arrangement of each calibration ground station in a case where a plurality of calibration ground stations are provided. In the example illustrated inFIG.4, the calibration ground stations51P and51Q are prepared in order to calibrate five elements of element #1to element #5. The calibration ground station51P is a calibration ground station for calibrating element #2and element #4using element #1as reference, and is arranged at a point of overlapping of main beams of element #1, element #2, and element #4having high gain as reception characteristics so that a sufficient signal/noise (S/N) ratio can be obtained in correlation calculation. Similarly, the calibration ground station51Q is a calibration ground station for calibrating element #3and element #5using element #2as reference, and is arranged at a point of overlapping of main beams of element #2, element #3, and element #5having high gain as reception characteristics so that a sufficient S/N ratio can be obtained in correlation calculation.

In that case, the DBF reception unit20of the satellite receiver1first uses the calibration ground station51P to calibrate element #2and element #4using element #1as reference, and then uses the calibration ground station51Q to calibrate element #3and element #5using element #2as reference. Although the element used as reference is different between the calibration using the calibration ground station51P and the calibration using the calibration ground station51Q, it is satisfactory if at least one element is shared as the arrangement of the calibration ground stations. In the example illustrated inFIG.4, element #2is shared by calibration by the calibration ground station51P and calibration by the calibration ground station51Q. Therefore, although element #3and element #5are calibrated using element #2as reference, the correction coefficient of element #2with respect to element #1has been calculated, and consequently, the correction coefficients of element #3and element #5using element #1as reference can be calculated by adding the correction coefficients of element #3and element #5using element #2as reference and the correction coefficient of element #2using element #1as reference together. Even in a case where the number of elements increases, calibration using the same reference element can be performed for all the elements by arranging a calibration ground station by a similar method.

According to the present calibration method and configuration, the phase correction value can be given to all the subchannel signals band-divided in the frequency domain, and the delay error and the phase error can be corrected over the entire band regardless of the total band Δf. Regarding the correction of the delay error, in a case of real-time correction by delay line, since the residual delay error after the delay correction appears as continuous phase rotation over the total band Δf, a large phase error occurs at a frequency away from the frequency as reference for correction, which causes intersymbol interference. According to the present calibration method, this phase rotation is confined within a band of one subchannel, and interference can be reduced. Consequently, it is desirable that the bandwidth Δfchof one subchannel can be narrowed as much as possible.

The satellite receiver1according to the first embodiment described above achieves the following effects as compared with a conventional satellite communication system such as the satellite communication system described in Patent Literature 1.

The satellite receiver1achieves an effect that it is possible to detect and correct errors between the element systems inside the satellite receiver1separately from the gain, delay, and phase differences based on the position of the calibration ground station51and the reception characteristics of the receiving antenna elements11ato11d, and to improve communication efficiency. In addition, the satellite receiver1achieves an effect that it is possible to reduce interference due to a delay between the element systems with respect to a broadband communication signal, and to further improve the communication efficiency. Furthermore, the satellite receiver1achieves an effect that it is possible to minimize devices related to calibration among devices to be installed on the satellite.

Second Embodiment

FIG.5is a diagram illustrating an example configuration of a satellite receiver1B according to a second embodiment. The satellite receiver1B has a configuration obtained by adding, to the satellite receiver1according to the first embodiment illustrated inFIG.1, a calibration signal generation unit56that generates a calibration signal, a calibration signal transmitting antenna57, and a look up table (LUT)58that stores coupling characteristics between the calibration signal transmitting antenna57and the receiving antenna elements11ato11d. Since the other components of the satellite receiver1B are the same as the components denoted by the same reference numerals of the satellite receiver1, detailed description thereof will be omitted.

While the satellite receiver1according to the first embodiment detects errors between the element systems using the calibration signal10transmitted by the calibration ground station51and calculates a correction coefficient for correcting a beam-formation excitation coefficient, the satellite receiver1B according to the second embodiment performs a similar process using the calibration signal10generated by the calibration signal generation unit56provided inside thereof. In order to avoid interference with a communication signal, calibration using the calibration signal generated by the calibration signal generation unit56is performed at a time when communication via the actual communication signal3is not performed.

A calibration process performed by the satellite receiver1B will be described. The calibration signal generated by the calibration signal generation unit56and transmitted from the calibration signal transmitting antenna57is input to the receiving antenna elements11ato11d. The reception characteristics (gain and phase) at that time are based on pass characteristics (gain and phase) from the calibration signal transmitting antenna57to the receiving antenna elements11ato11d, and are different for each element.

In the satellite receiver1B, the ADCs12ato12dand the demultiplexing units13ato13ddigitally convert the calibration signal transmitted from the calibration signal transmitting antenna57and demultiplex the calibration signal into subchannels, and then the correlation detection unit22calculates a cross-correlation value of each subchannel. The correlation detection unit22outputs the calculated cross-correlation values to the inter-element-system error detection unit31.

Similarly to the inter-element-system error detection unit31of the satellite receiver1according to the first embodiment, the inter-element-system error detection unit31calculates a gain error, a delay error, and a phase error between the element systems on the basis of the input cross-correlation values. At that time, it is necessary to separate the pass characteristics (gain and phase) from the calibration signal transmitting antenna57to the receiving antenna elements11ato11dfrom the gain error, the delay error, and the phase error between the element systems generated inside the satellite receiver1B.

The pass characteristics from the calibration signal transmitting antenna57to the receiving antenna elements11ato11dare determined by the arrangement of the calibration signal transmitting antenna57and the receiving antenna elements11ato11d, and can be estimated in advance at design. Alternatively, the pass characteristics can be obtained by measurement before the satellite is launched. The amount thereof is obtained in advance for each subchannel of each element and stored in the LUT58.

The inter-element-system error detection unit31of the satellite receiver1according to the first embodiment calculates the correction coefficient by removing Δ0mnand φ0mnestimated from the reception characteristics of the respective receiving antenna elements m and n with respect to the position of the calibration ground station51from the correlation values, and the inter-element-system error detection unit31of the satellite receiver1B also calculates the correction coefficient by a similar method. At that time, the inter-element-system error detection unit31of the satellite receiver1B removes Δ0mnand φ0mnestimated from the pass characteristics from the calibration signal transmitting antenna57to the receiving antenna elements11ato11dfrom the correlation values.

If the pass characteristics from the calibration signal transmitting antenna57to the receiving antenna element m can be described as formula (18), the inter-element-system error detection unit31can calculate the correction coefficient by setting Δ0mnand φ0mnas indicated in formulas (19) and (20).
[Formula 18]
Sm=Zmejξm(18)
[Formula 19]
ϕ0mn=ξm(19)
[Formula 20]
Δ0mn=Zm(20)

After calculating the correction coefficients for the gain difference, the delay difference, and the phase difference between the element systems, the inter-element-system error detection unit31stores the correction coefficients in the correction coefficient storage unit32.

In general, the pass characteristics from the calibration signal transmitting antenna57to the receiving antenna elements11ato11dhave a small gain, and become more noticeable as a distance between two antennas increases. Therefore, in a case where the cross-correlation value cannot be obtained with a sufficient S/N ratio, a method of providing a plurality of calibration signal transmitting antennas57is considered. In that case, as described in the first embodiment with reference toFIG.4, calibration can be performed for all elements by making it possible to share at least one receiving antenna element between different calibration signal transmitting antennas. Alternatively, by directly estimating and using pass characteristics between the plurality of calibration signal transmitting antennas, pass characteristics using the same calibration signal transmitting antenna as reference may be calculated for all the receiving antenna elements.

The satellite receiver1B according to the second embodiment achieves the following effects as compared with a conventional satellite communication system such as the satellite communication system described in Patent Literature 1.

The satellite receiver1B achieves an effect similar to that of the satellite receiver1according to the first embodiment, specifically, an effect that it is possible to detect and correct errors between the element systems inside the satellite receiver1B separately from the gain, delay, and phase differences based on the position of the calibration ground station51and the reception characteristics of the receiving antenna elements11ato11d, and to improve communication efficiency. In addition, the satellite receiver1B achieves an effect that it is possible to reduce interference due to a delay between the element systems with respect to a broadband communication signal, and to further improve the communication efficiency. Furthermore, since the satellite receiver1B can perform closed calibration in the satellite and does not require a calibration ground station, the operation thereof is facilitated.

The configurations described in the embodiments above are merely examples of the content of the present disclosure and can be combined with other known technology and part thereof can be omitted or modified without departing from the gist of the present disclosure.

REFERENCE SIGNS LIST