DISTORTION COMPENSATION DEVICE AND DISTORTION COMPENSATION METHOD

A distortion compensation device includes an address generating unit, an LUT, and a multiplier. The address generating unit calculates the magnitude of a first vector and the angle formed by the first vector and a second vector. The first vector is represented by using the origin in an IQ coordinate plane as the starting point and using a transmission signal point at a first time as the end point. The second vector is represented by using a transmission signal point at a second time that is predetermined time before the first time as the starting point and using the transmission signal point at the first time as the end point. The LUT specifies a distortion compensation coefficient by using the calculated magnitude and the calculated angle. The multiplier performs a predistortion process on a transmission signal input to an amplifier by using the specified distortion compensation coefficient.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-228051, filed on Nov. 20, 2015, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to a distortion compensation device and a distortion compensation method.

BACKGROUND

A radio transmitter is provided with an amplifier that amplifies power of transmission signals. In the radio transmitter, in general, in order to increase the power efficiency of the amplifier, the amplifier is operated in the vicinity of the saturation region of the amplifier. However, when the amplifier is operated in the vicinity of the saturation region, nonlinear distortion of the amplifier is increased. Thus, to meet the standard, such as an adjacent channel leakage ratio (ACLR), the spectrum emission mask (SEM), or the like, by reducing the nonlinear distortion, the radio transmitter is provided with a distortion compensation device that compensates nonlinear distortion.

A “predistortion (hereinafter, sometimes referred to as “PD”) method” is used as one of distortion compensation methods that is used in the distortion compensation device. In the distortion compensation device that uses the PD method, a distortion compensation coefficient that has the inverse properties of the nonlinear distortion of the amplifier is previously multiplied by the transmission signal that is input to the amplifier. Consequently, the linearity of an output from the amplifier is increased and distortion of an output from the amplifier is reduced.

Furthermore, in the amplifier with high power efficiency, it is known that a phenomenon called memory effect occurs. The memory effect is a phenomenon in which an output with respect to an input to an amplifier at a certain time is affected by an input in the past. In a distortion compensation method that compensates nonlinear distortion of an amplifier, there is a method that also compensates the memory effect in addition to the nonlinear distortion. With this method, in a transmission signal that has the I component and the Q component, a distortion compensation coefficient is determined by using information on a phase difference between the vector starting from the origin in the IQ coordinate plane to the current transmission signal point and the vector starting from the origin in the IQ coordinate plane to the transmission signal point at a predetermined time before. Related-art examples are described in Japanese Laid-open Patent Publication No. 2011-199428; Japanese Laid-open Patent Publication No. 2011-199429; U.S. Patent Application Publication No. 2011/0227643; and U.S. Patent Application Publication No. 2011/0227644.

However, in an amplifier, if the amplitude of an input signal is small, the nonlinear distortion included in the signal amplified by the amplifier is not so great, whereas if the amplitude of an input signal is great, the nonlinear distortion included in the signal amplified by the amplifier is large. Namely, the nonlinear distortion, when the amplitude of the signal that is input to the amplifier is great, is dominant. Thus, the improvement of the distortion compensation performance is more expected when the nonlinear distortion with large amplitude is compensated rather than the nonlinear distortion with small amplitude. Furthermore, in order to effectively use a limited circuit, the resolution of the distortion compensation coefficient is preferably increased when the amplitude of the input signal is large rather than when the amplitude of the input signal is small.

In a conventional distortion compensation method, a distortion compensation coefficient is determined on the basis of a phase difference between the current sample point of a transmission signal and a sample point of the transmission signal at a predetermined time before. Thus, the resolution in an area away from the origin in the IQ coordinate plane is lower than the resolution in the vicinity of the origin in the IQ coordinate plane.FIG. 10is a schematic diagram illustrating the resolution of a distortion compensation coefficient in a conventional distortion compensation method. InFIG. 10, the white circle represents the current sample point x(t) of the transmission signal and the black circles represent the sample points x(t−Δt) delayed by the predetermined time Δt from the current sample of the transmission signal. For example, as illustrated inFIG. 10, the case in which the current sample point x(t) of the transmission signal is changed from one of the samples x(t−Δt) at a predetermined time before is considered.FIG. 10illustrates a case in which the resolution Δθ0of the phase difference is 60 degrees.

In the example illustrated inFIG. 10, the phase difference Δθ between the vector of the current sample point x(t) of the transmission signal and the vector of each of the sample points x(t−Δt) of the transmission signal at the predetermined time before is included in an area70of the angular range of Δθ0(for example, 60 degrees) in the IQ coordinate plane. Thus, in the conventional distortion compensation method, even if the current sample point x(t) of the transmission signal is changed from one of the sample points x(t−Δt) illustrated inFIG. 10, regarding the phase difference Δθ between x(t) and x(t−Δt), the same distortion compensation coefficient is determined. Thus, in the conventional distortion compensation method, it is difficult to distinguish each of the changes between the sample points in the transmission signal illustrated inFIG. 10and determine a distortion compensation coefficient in accordance with the change in each of the transmission signals.

Furthermore, in also the conventional distortion compensation method, by increasing the resolution of an angle, it is also possible to distinguish each of the changes between the sample points in the transmission signal illustrated inFIG. 10and associate each of the changes with different distortion compensation coefficients. However, if the resolution of an angle is increased in the conventional distortion compensation method, the number of coefficients is increased in the distortion compensation that uses power series and the number of tables is accordingly increased in the distortion compensation that uses a Look Up Table (LUT); therefore, the size of a circuit becomes large and it takes time for convergence of the distortion compensation coefficients.

SUMMARY

According to an aspect of an embodiment, a distortion compensation device includes a calculating unit, a specifying unit, and a distortion compensation unit. The calculating unit calculates a magnitude of a first vector and an angle formed by the first vector and a second vector. The first vector is a vector represented by using an origin in an IQ coordinate plane as a starting point and using a transmission signal point at a first time as an end point. The second vector is a vector represented by using a transmission signal point at a second time that is predetermined time before the first time as a starting point and using the transmission signal point at the first time as an end point. The specifying unit specifies a distortion compensation coefficient by using the magnitude of the first vector and the angle formed by the first vector and the second vector calculated by the calculating unit. The distortion compensation unit performs, by using the distortion compensation coefficient specified by the specifying unit, a predistortion process on a transmission signal that is input to an amplifier.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained with reference to accompanying drawings. The distortion compensation device and the distortion compensation method disclosed in the present application are not limited to the embodiments described below. Furthermore, the embodiments can be used in any appropriate combination as long as the processes do not conflict with each other.

[a] First Embodiment

FIG. 1is a block diagram illustrating an example of the distortion compensation device10. The distortion compensation device10includes a multiplier11, an LUT12, a digital-to-analog converter (DAC)13, an up converter14, an oscillator15, an amplifier16, a coupler17, and an antenna18. Furthermore, the distortion compensation device10according to the embodiment includes a down converter19, an analog-to-digital converter (ADC)20, a subtracter21, a complex conjugate calculating unit22, an updating unit23, and an address generating unit30. In a radio communication system that has, for example, a base station and a terminal, the distortion compensation device10is mounted on the base station or the terminal or, alternatively, mounted on both the base station and the terminal.

In the embodiment, the transmission signal is, for example, a digital baseband signal that includes therein an in-phase component signal (I signal) and a quadrature component signal (Q signal). The address generating unit30calculates the amplitude of a first vector that indicates the current sample point of the transmission signal. The first vector is, for example, the vector, in the IQ coordinate plane, represented by using the origin in the IQ coordinate plane as the starting point and using the sample point of the transmission signal at the first time as the end point. In the embodiment, the first time is, for example, the current time. Furthermore, the address generating unit30calculates the input angle formed by the first vector and a second vector. The second vector is, for example, the vector, in the IQ coordinate plane, represented by using the sample point of the transmission signal at the second time that is a predetermined time before the first time as the starting point and using the sample point of the transmission signal at the first time as the end point.

Then, the address generating unit30outputs the value of the amplitude of the first vector to the LUT12as a first address. Furthermore, the address generating unit30outputs the value of the input angle to the LUT12as a second address. The address generating unit30is an example of a calculating unit.

The LUT12specifies a distortion compensation coefficient by using the input angle calculated by the address generating unit30. Specifically, the LUT12holds, for example, a distortion compensation coefficient by associating the distortion compensation coefficient with the combination of the first address and the second address. When the first address and the second address are output from the address generating unit30, the LUT12specifies the distortion compensation coefficient that is associated with the combination of these addresses. Then, the LUT12outputs the specified distortion compensation coefficient to the multiplier11. The LUT12is an example of a specifying unit.

By using the distortion compensation coefficient specified by the LUT12, the multiplier11performs a predistortion process on the transmission signal that is input to the amplifier16. Specifically, the multiplier11performs the predistortion process on the transmission signal by multiplying the distortion compensation coefficient output from the LUT12by the transmission signal. Then, the multiplier11outputs the transmission signal that has been subjected to the predistortion process (hereinafter, referred to as a PD signal) to the DAC13. The multiplier11is an example of a distortion compensation unit.

The DAC13converts the PD signal that has been output from the multiplier11from a digital signal to an analog signal. Then, the DAC13outputs the PD signal converted to the analog signal to the up converter14.

The up converter14up-converts, by using the local oscillator signal output from the oscillator15, the PD signal that has been converted to the analog signal. A quadrature modulator, a mixer, or the like is included in the up converter14. Then, the up converter14outputs the up-converted PD signal to the amplifier16.

The amplifier16amplifies the power of the PD signal that has been up-converted by the up converter14. Then, the amplifier16outputs, to the coupler17, the signal in which the power has been amplified.

The coupler17outputs, to the antenna18, the signal in which the power has been amplified by the amplifier16and feeds back a part of the signal to the down converter19. The signal that has been output to the antenna18is emitted to space from the antenna18.

The down converter19down-converts, by using the local oscillator signal output from the oscillator15, the signal that has been fed back from the coupler17. A quadrature demodulator, a mixer, or the like is included in the down converter19. Then, the down converter19outputs the down-converted signal to the ADC20.

The ADC20converts the signal that has been down-converted by the down converter19from the analog signal to the digital signal. Then, the ADC20outputs the signal converted to the digital signal to both the subtracter21and the complex conjugate calculating unit22.

The subtracter21calculates a difference between the transmission signal and the signal output from the ADC20. The subtracter21outputs the calculated differential signal to the updating unit23.

The complex conjugate calculating unit22calculates the complex conjugate of the signal output from the ADC20. Then, the complex conjugate calculating unit22outputs the calculated complex conjugate signal to the updating unit23.

The updating unit23calculates an updated distortion compensation coefficient on the basis of the distortion compensation coefficient output from the LUT12, the differential signal output from the subtracter21, and the complex conjugate signal output from the complex conjugate calculating unit22. The updating unit23calculates the updated distortion compensation coefficient by using, for example, the algorithm, such as least mean square (LMS), recursive least squares (RLS), or the like. Then, the updating unit23updates the distortion compensation coefficient in the LUT12by using the calculated distortion compensation coefficient.

Details of the Address Generating Unit30

In the following, details of the address generating unit30will be described.FIG. 2is a block diagram illustrating an example of the address generating unit30according to a first embodiment. The address generating unit30according to the embodiment includes an amplitude calculating unit31, a first address generating unit32, a delay unit33, a subtracting unit34, a phase calculating unit35, a phase calculating unit36, a subtracting unit37, and a second address generating unit38.

The amplitude calculating unit31calculates the amplitude of the first vector. The first vector is the vector, in the IQ coordinate plane, represented by using the origin of the IQ coordinate plane as the starting point and using the current sample point x(t) of the transmission signal as the end point. The amplitude calculating unit31calculates, as the amplitude of the first vector, for example, the square root of the sum of the square of the I component of the first vector and the square of the Q component of the first vector. The amplitude of the first vector is an example of the index indicating the magnitude of the first vector.

The first address generating unit32normalizes the value of the amplitude of the first vector calculated by the amplitude calculating unit31to the first address with a predetermined number of bits. Then, the first address generating unit32outputs the first address to the LUT12.

The delay unit33delays the current sample point x(t) of the transmission signal by the predetermined time Δt. The subtracting unit34calculates the second vector by subtracting the sample point x(t−Δt) of the transmission signal delayed by Δt by the delay unit33from the current sample point x(t) of the transmission signal. In the embodiment, the second vector is represented by x(t)−x(t−Δt).

The phase calculating unit35calculates the phase θ1(t) of the first vector represented by using the origin in the IQ coordinate plane as the starting point and using the current sample point x(t) of the transmission signal as the end point. The phase θ1(t) calculated by the phase calculating unit35is represented by, for example, Equation (1) below.

The phase calculating unit36calculates the phase θ2(t) of the second vector calculated by the subtracting unit34. The phase θ2(t) to be calculated by the phase calculating unit36is represented by, for example, Equation (2) below.

The subtracting unit37calculates the input angle Δθin(t) on the basis of both the phase θ1(t) of the first vector calculated by the phase calculating unit35and the phase θ2(t) of the second vector calculated by the phase calculating unit36. Specifically, the subtracting unit37calculates the input angle Δθin(t) by using, for example, Equation (3) below.

The second address generating unit38normalizes the value of the input angle Δθin(t) calculated by the subtracting unit37to the second address with a predetermined number of bits. Then, the second address generating unit38outputs the second address to the LUT12.

In the following, the direction of the second vector will be described.FIG. 3is a schematic diagram illustrating the direction of the second vector. In the IQ coordinate plane illustrated inFIG. 3, when a positive frequency is included in a change in the sample point of the transmission signal, the sample point of the transmission signal is rotated counterclockwise around the origin in the IQ coordinate plane. In contrast, when a negative frequency is included in a change in the sample point of the transmission signal, the sample point of the transmission signal is rotated clockwise around the origin in the IQ coordinate plane. Furthermore, inFIG. 3, the white circle represents the current sample point x(t) of the transmission signal and the black circle represents the sample point x(t−Δt) delayed by Δt from the current sample point of the transmission signal.

Reference numerals40and41illustrated inFIG. 3indicate the second vector represented by using the sample point x(t−Δt) as the starting point and using the sample point x(t) as the end point. The phase θ2(t) of the second vector40is calculated by Equation (2) above. However, the rotational component (frequency) is not represented only by the phase θ2(t) of the second vector40. For example, as illustrated inFIG. 3, it is assumed of a case in which the second vector40is located in the first quadrant in the IQ coordinate plane, the second vector41is located in the third quadrant in the IQ coordinate plane, and the phase θ2(t) of the second vector40and the phase θ2(t) of the second vector41are the same.

In this case, the second vector40located in the first quadrant indicates the positive frequency component and the second vector41located in the third quadrant indicates the negative frequency component. However, if a distortion compensation coefficient is specified by only using the phase θ2(t), the same distortion compensation coefficient is specified for both the second vectors40and41.

Thus, the distortion compensation device10according to the embodiment calculates, by using a first vector as the reference, the input angle Δθin(t) that is the phase difference between the first vector and a second vector and then specifies the distortion compensation coefficient by using the calculated input angle Δθin(t).FIG. 4is a schematic diagram illustrating an example of an input angle. When the second vector43is translated in the IQ coordinate plane such that the starting point falls on the origin in the IQ coordinate plane, the angle formed by the translated second vector43′ and the first vector42is the angle, as illustrated inFIG. 4, obtained by subtracting the phase θ1(t) of the first vector42from the phase θ2(t) of the second vector43′. Furthermore, the relationship between the angle formed by the second vector43′ and the first vector42and the input angle Δθin(t) is, as illustrated inFIG. 4, alternate-interior angles. Thus, the input angle Δθin(t) is θ2(t)−θ1(t).

By specifying the distortion compensation coefficient using the input angle Δθin(t), for example, as illustrated inFIG. 3, the angular range of 360 degrees is divided into, for example, equal six parts and, if a distortion compensation coefficient is associated with each of the angular ranges, regarding the second vector40located in the first quadrant, the distortion compensation coefficient associated with an area53is used. In contrast, regarding the second vector41located in the third quadrant, the distortion compensation coefficient associated with an area50that is different from the area53is used. Thus, the distortion compensation device10can distinguish the second vectors including the rotational components and can allocate different distortion compensation coefficients to the respective second vectors. Furthermore, in the example illustrated inFIG. 3, each of the areas50to55of the angular ranges indicated in the first quadrant and each of the areas50to55of the angular ranges indicated in the third quadrant has the relationship in which the areas in one of the quadrants are rotated around the origin in the IQ coordinate plane by 180 degrees.

At this point, in the conventional distortion compensation, for example, as described with reference toFIG. 10, the angular ranges associated with the distortion compensation coefficients are set by using the origin in the IQ coordinate plane as the center. Thus, in the IQ coordinate plane, in an area away from the origin, i.e., in an area in the vicinity of the current sample point x(t) of the transmission signal when the amplitude is large, the resolution of the angle is low. Therefore, for example, in the example illustrated inFIG. 10, even when the current sample point x(t) of the transmission signal is shifted from one of the sample points x(t−Δt), the same distortion compensation coefficient is used. Thus, it is difficult to improve the distortion compensation performance.

In contrast, with the distortion compensation device10according to the embodiment, the input angle Δθin(t) is calculated by using the first vector as the reference and the distortion compensation coefficient is specified by using the calculated input angle Δθin(t). Thus, with the distortion compensation device10according to the embodiment, for example, as illustrated inFIG. 5, in the IQ coordinate plane, in an area away from the origin, i.e., in an area in the vicinity of the current sample point x(t) of the transmission signal when the amplitude is large, a predetermined resolution can be obtained.FIG. 5is a schematic diagram illustrating an example of the resolution of a distortion compensation coefficient according to the first embodiment.FIG. 5exemplifies the resolution obtained by dividing 360 degrees into equal six parts with its center at the current sample point x(t) of the transmission signal.

For example, when compared with a case in which 360 degrees are divided into equal six parts, in the distortion compensation method according to the embodiment illustrated inFIG. 5, the resolution of the angle is higher in an area in which the amplitude of the transmission signal is large than that in the conventional distortion compensation method illustrated inFIG. 10. Thus, the distortion compensation device10according to the embodiment can more precisely identify the frequency properties in the area in which the amplitude of the transmission signal is large and use different distortion compensation coefficients in accordance with the respective frequency properties. Therefore, the distortion compensation device10according to the embodiment can reduce nonlinear distortion affected by the memory effect of the amplifier16more than before.

Furthermore, even if the same resolution as that used in the conventional device is used, the distortion compensation device10according to the embodiment can increase the resolution in the area in which the amplitude of the transmission signal is large. Thus, even if the number of addresses set in the LUT12is about the same number of addresses used in the past, the distortion compensation device10according to the embodiment can efficiently reduce nonlinear distortion of the amplifier16due to the memory effect or the like. Therefore, the distortion compensation device10according to the embodiment can improve the distortion compensation performance while reducing an increase in the size of a circuit.

Operation of the Distortion Compensation Device10

In the following, the operation of the distortion compensation device10according to the first embodiment will be described.FIG. 6is a flowchart illustrating an operation of a distortion compensation device10according to the first embodiment. The distortion compensation device10performs the operation indicated by the flowchart for each, for example, sample timing of the transmission signal.

First, when the transmission signal is input, the amplitude calculating unit31calculates the amplitude of the first vector, in the IQ coordinate plane, represented by using the origin of the IQ coordinate plane as the starting point and using the current sample point x(t) of the transmission signal as the end point (Step S100). Then, the first address generating unit32normalizes the value of the amplitude of the first vector calculated by the amplitude calculating unit31to the first address with a predetermined number of bits and then outputs the normalized value to the LUT12.

Then, the address generating unit30calculates the input angle that is the angle formed by the first vector and the second vector (Step S101). Specifically, the delay unit33delays the current sample point x(t) of the transmission signal by the predetermined time Δt. The subtracting unit34calculates the second vector by subtracting the sample point x(t−Δt) from the sample point x(t). The phase calculating unit35calculates the phase θ1(t) of the first vector. The phase calculating unit36calculates the phase θ2(t) of the second vector. The subtracting unit37calculates the input angle Δθin(t) by subtracting the phase θ1(t) of the first vector from the phase θ2(t) of the second vector. Then, the second address generating unit38normalizes the value of the input angle Δθin(t) to the second address with the predetermined number of bits and then outputs the second address to the LUT12.

Then, the LUT12specifies the distortion compensation coefficient associated with the first address and the second address output from the address generating unit30(Step S102). Then, the LUT12outputs the specified distortion compensation coefficient to the multiplier11. The multiplier11performs the predistortion process on the transmission signal that is input to the amplifier16by multiplying the distortion compensation coefficient specified by the LUT12by the transmission signal (Step S103).

Effect of the First Embodiment

As is clear from the description above, in the distortion compensation device10according to the embodiment, the address generating unit30calculates the magnitude of the first vector and calculates the angle formed by the first vector and the second vector. The first vector is the vector represented by using the origin in the IQ coordinate plane as the starting point and using the transmission signal point at the first time as the end point. The second vector is the vector represented by using the transmission signal point at the second time that is a predetermined time before the first time as the starting point and using the transmission signal point at the first time as the end point. Furthermore, in the distortion compensation device10according to the embodiment, the LUT12specifies distortion compensation coefficient by using the magnitude of the first vector and the angle formed by the first vector and the second vector calculated by the address generating unit30. Furthermore, in the distortion compensation device10according to the embodiment, by using the distortion compensation coefficient specified by the LUT12, the multiplier11performs the predistortion process on the transmission signal input to the amplifier16. Consequently, the distortion compensation device10according to the embodiment can improve the distortion compensation performance while reducing an increase in the size of a circuit.

[b] Second Embodiment

The distortion compensation device10according to a second embodiment differs from the distortion compensation device10according to the first embodiment in that distortion compensation coefficient is specified by further using the magnitude of the second vector. Furthermore, the overall configuration of the distortion compensation device10is the same as that in the first embodiment except for the address generating unit30. Thus, in a description below, the configuration of the address generating unit30is mainly described.

FIG. 7is a block diagram illustrating an example of the address generating unit30according to a second embodiment. The address generating unit30according to the second embodiment includes the amplitude calculating unit31, the first address generating unit32, the delay unit33, the subtracting unit34, the phase calculating unit35, the phase calculating unit36, the subtracting unit37, the second address generating unit38, and an amplitude calculating unit39. Furthermore, the blocks illustrated inFIG. 7having the same reference numerals as those illustrated inFIG. 2have the same configuration as the blocks illustrated inFIG. 2except for the following points described below; therefore, descriptions thereof will be omitted.

The amplitude calculating unit39calculates the amplitude of the second vector calculated by the subtracting unit34. The amplitude of the second vector is an example of the index that indicates the magnitude of the second vector. The amplitude calculating unit39calculates, as the amplitude of the second vector, for example, the square root of the sum of the square of the I component of the second vector and the square of the Q component of the second vector. The amplitude A(t) of the second vector calculated by the amplitude calculating unit39is represented by, for example, Equation (4) below.

Furthermore, the amplitude calculating unit39may also output the logarithmic value of the amplitude of the second vector calculated by the subtracting unit34to the second address generating unit38as the amplitude A(t) of the second vector.

The second address generating unit38normalizes the value of the input angle Δθin(t) calculated by the subtracting unit37to the value with a predetermined number of bits and normalizes the value of the amplitude A(t) of the second vector calculated by the amplitude calculating unit39to the value with a predetermined number of bits. Then, the second address generating unit38generates the second address on the basis of the normalized values and then output the generated second address to the LUT12. The second address generating unit38normalizes the value of the input angle Δθin(t) calculated by the subtracting unit37to the value with the number of bits of, for example, M1and normalizes the value of the amplitude A(t) of the second vector calculated by the amplitude calculating unit39to the value with the number of bits of, for example, M2. Then, the second address generating unit38generates the second address with the number of bits of (M1+M2) bits.

Effect of the Second Embodiment

In this way, in the distortion compensation device10according to the second embodiment, the address generating unit30further calculates the magnitude of the second vector and the LUT12specifies the distortion compensation coefficient by further using the magnitude of the second vector. Consequently, in addition to the input angle, the LUT12can specify the distortion compensation coefficient, in accordance with an amount of change in the second vector, i.e., the distance between the current sample point x(t) of the transmission signal and the sample point x(t−Δt) of the transmission signal at the predetermined time before. Thus, for example, as illustrated inFIG. 8, in addition to the areas50to55indicating the respective angular ranges of the input angle Δθin(t), areas60to63indicating the ranges of different amplitudes of the second vectors are generated with its center at the current sample point x(t) of the transmission signal.

Consequently, the distortion compensation device10can increase the resolution of the angle and the amplitude of the second vector in the area in which the amplitude of the transmission signal is large. Thus, the distortion compensation device10can further finely identify the frequency properties in the area in which the amplitude of the transmission signal is large and use different distortion compensation coefficients in accordance with each of the frequency properties. Therefore, the distortion compensation device10according to the embodiment can reduce the nonlinear distortion affected by the memory effect of the amplifier16more than before.

Hardware

In the following, hardware of the distortion compensation device10described in the first and the second embodiments will be described.FIG. 9is a block diagram illustrating an example of hardware of the distortion compensation device10. The distortion compensation device10includes, for example, as illustrated inFIG. 9, a memory100, a processor101, a radio circuit102, and the antenna18.

The radio circuit102performs a process, such as up conversion or the like, on the signal output from the processor101and transmits the processed signal via the antenna18. Furthermore, the radio circuit102includes the amplifier16. The radio circuit102performs a process, such as down conversion or the like, on the signal output from the amplifier16and feeds back the processed signal to the processor101. The radio circuit102implements the function performed by, for example, the multiplier11, the DAC13, the up converter14, the oscillator15, the amplifier16, the coupler17, the down converter19, an ADC20and the like.

The memory100stores therein various kinds of programs or the like that implement the function of, for example, the LUT12, the subtracter21, the complex conjugate calculating unit22, the updating unit23, and the address generating unit30. By executing the programs read from the memory100, the processor101implements the function of, for example, the LUT12, the subtracter21, the complex conjugate calculating unit22, the updating unit23, and the address generating unit30. Furthermore, in the distortion compensation device10illustrated inFIG. 9, a single number of the memory100, the processor101, the radio circuit102, and the antenna18are provided; however, regarding the memory100, the processor101, the radio circuit102, and the antenna18, multiple number of devices may also be provided.

Others

The technology disclosed in the present application is not limited to the embodiments described above and various modifications are possible as long as they do not depart from the spirit of the present application.

For example, in the first and the second embodiments described above, the address generating unit30generates the first address on the basis of the amplitude that is one of the indices of the magnitude of the first vector; however, the disclosed technology is not limited to this. For example, the address generating unit30may also generate the first address on the basis of the power that is one of the indices of the magnitude of the first vector. Furthermore, in the second embodiment described above, the address generating unit30generates the second address on the basis of the input angle and the amplitude that is one of the indices of the magnitude of the second vector; however, the disclosed technology is not limited to this. For example, the address generating unit30may also generate the second address on the basis of the input angle and the power that is one of the indices of the magnitude of the second vector.

Furthermore, in the second embodiment described above, the amplitude calculating unit39calculates the amplitude of the second vector; however, the disclosed technology is not limited to this. For example, regarding the sample points of a plurality of transmission signals at different time, the amplitude calculating unit39may also average the amplitude of the first vectors and calculate the ratio of the averaged amplitude of the first vector to the amplitude of the second vector. Specifically, for example, on the basis of Equation (5) below, the amplitude calculating unit39calculates the ratio R(t) of the average amplitude of the first vector to the amplitude of the second vector.

In Equation (5) above, N represents the number of the first vectors at different time.

The second address generating unit38generates the second address on the basis of the value of the input angle Δθin(t) calculated by the subtracting unit37and the value of the ratio R(t) calculated by the amplitude calculating unit39.

In this way, by specifying the distortion compensation coefficient on the basis of the ratio of the average amplitude of the first vector to the amplitude of the second vector, the distortion compensation device10can increase the resolution of the distortion compensation coefficient when the amplitude of the transmission signal is large. Consequently, the distortion compensation device10can reduce the nonlinear distortion of the amplifier16due to the memory effect or the like more than before.

Furthermore, in Equation (5) above, the logarithmic value obtained by dividing the amplitude of the second vector by the average amplitude of the first vector is calculated as the ratio R(t); however, as another example, the value obtained by dividing the amplitude of the second vector by the average amplitude of the first vector may also be calculated as the ratio R(t). Furthermore, in Equation (5) above, the logarithmic value of a value obtained by dividing the amplitude that is one of the indices of the magnitude of the second vector by the average amplitude that is one of the indices of the magnitude of the first vector is calculated as the ratio R(t); however, the disclosed technology is not limited to this. For example, the logarithmic value of a value obtained by dividing the power that is one of the indices of the magnitude of the second vector by the average power that is one of the indices of the magnitude of the first vector may also be calculated as the ratio R(t).

According to an aspect of an embodiment, it is possible to improve the distortion compensation performance while reducing an increase in the size of a circuit.