KR101169880B1 - Digital predistortion method, system and computer-readable recording medium for compensating nonlinear power amplifier which receives ofdm signal - Google Patents

Abstract

PURPOSE: A digital predistortion method, a system, and a computer-readable recording medium for compensating nonlinear power amplifier which receives OFDM(Orthogonal Frequency Division Multiplexing) signal are provided to obtain the degree of frequency dependent phase shift of the OFDM input signal by using the AM-PM distortion of the power amplifier. CONSTITUTION: A power amplifier feature acquisition unit(210) obtains the frequency dependent phase shift degree of OFDM input signal frequency band by the AM-PM distortion feature of a power amplifier. By the obtained phase shift degree, a first compensation unit(220) compensates phase shift about a complex signal corresponding to one of sub input signal carrier frequencies. A second compensation unit(230) compensates the distorted wave length and phase of the output signal which is converted by the degree of the input signal about the compensated signal.

Description

DIGITAL PREDISTORTION METHOD, SYSTEM AND COMPUTER-READABLE RECORDING MEDIUM FOR COMPENSATING NONLINEAR POWER AMPLIFIER WHICH RECEIVES OFDM SIGNAL}

The present invention relates to a digital predistortion method, a system, and a computer-readable recording medium for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal. More particularly, an OFDM signal is input to a power amplifier having a nonlinear characteristic. In order to more effectively compensate for the distortion of the case, the power amplifier is subjected to a first compensation step of compensating for the phase shift characteristic of the power amplifier by extracting the degree of phase shift according to frequency with reference to the AM-PM distortion characteristic of the power amplifier. A method, system and computer readable recording medium for more fully compensating for the nonlinear characteristics of a power amplifier by undergoing a second compensation step of applying a predistortion technique with reference to AM-AM and AM-PM distortion characteristics of .

Recently, the mobile communication service has been demanded by a power amplifier capable of amplifying a wideband channel bandwidth and a digitized high frequency signal due to the development of a system capable of processing a large amount of data.

In order to achieve maximum power efficiency, the power amplifier may be used up to near the saturation region of the transistor, in which case the distortion signal may be generated by the nonlinear nature of the power amplifier. Distortion signals in nonlinear power amplifiers cause interference between adjacent channels, act as noise sources, and reduce the resources available within a given band. They must be removed, but because they occur close to the main signal, they can be removed using circuits such as filters. Difficult to do

In order to solve such a problem of the power amplifier, a linearization technique is used, and the linearization technique is fed back. Feedforward, predistortion, and the like are used. The most widely used methods are the feed forward method and the predistortion method. The feed forward method includes a signal cancellation loop for extracting an error signal and an error cancellation loop for removing an error signal generated by the main amplifier. The feedforward method has a large linearity improvement effect and a wide bandwidth but low efficiency due to delay line loss and additional circuitry.

The predistortion method extracts the nonlinear characteristics of the power amplifier to be linearized and applies a nonlinear algorithm having characteristics opposite to the nonlinearity of the power amplifier to convert the input signal to the power amplifier, thereby distorting the input signal of the power amplifier in advance. A method of linearizing the characteristics of a system.

The nonlinear characteristics of a power amplifier can be defined in many forms. First, the characteristic that the gain of the output voltage decreases as the magnitude of the input signal voltage increases is referred to as AM-AM distortion (Amplitude Modulation to Amplitude Modulation). In addition, the characteristic that the phase of the output signal changes as the magnitude of the signal voltage applied to the nonlinear power amplifier increases is referred to as AM-PM distortion (Amplitude Modulation to Phase Modulation) characteristic. Particularly important for the conversion modulation scheme.

On the other hand, in order to use the limited frequency more efficiently, the digital communication method uses a signal modulation technique. These modulation schemes pass the baseband signal before being up-converted by the carrier through a pulsed filter to have a high frequency band after the baseband signal is modulated. A typical modulation technique is orthogonal frequency-division multiplexing (OFDM). The OFDM scheme is a multi-carrier technology in which data to be transmitted is divided into a plurality of subcarriers, modulated, and then transmitted in parallel. The OFDM scheme is characterized in that optimal transmission efficiency can be obtained in high-speed data transmission by maintaining orthogonality between subcarriers. Due to these advantages, the OFDM scheme is applied to various broadband communications such as LTE, WiBro, WLAN, digital audio broadcasting (DAB), and digital video broadcasting (DVB).

Since OFDM signals generally have a wide frequency band, when the signal is applied to a nonlinear power amplifier, the degree of phase shift varies depending on the frequency. Therefore, the pre-distortion technique accurately corrects the AM-PM distortion and the AM-AM distortion. There was a problem that could not be compensated.

Accordingly, the present inventors have developed a digital predistortion method capable of more completely compensating for the nonlinearity of the power amplifier by compensating for the frequency dependent phase shift in advance when the OFDM signal is applied to the power amplifier having the nonlinear characteristic. .

It is an object of the present invention to solve all the problems described above.

Another object of the present invention is to provide a predistortion method for minimizing AM-AM distortion and AM-PM distortion by compensating for frequency dependent phase shift characteristics in advance when an OFDM signal is input to a power amplifier.

In order to accomplish the above object, a representative structure of the present invention is as follows.

According to an aspect of the present invention, in the digital predistortion method for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal, (a) the AM-PM distortion characteristic of the power amplifier with reference to the Obtaining a frequency dependent phase shift degree in a frequency band of an OFDM input signal to be input to a power amplifier, (b) corresponding to any one of the subcarrier frequencies of the input signal with reference to the obtained phase shift degree A first compensation step of pre-compensating and OFDM-modulating the phase shift with respect to the complex signal X (k), and (c) the phase shift-compensated signal based on the AM-AM and AM-PM distortion characteristics of the power amplifier The digital predistortion method includes a second compensation step of precompensating the distortion degree of the amplitude and phase of the output signal according to the change in the voltage magnitude of the input signal with respect to / RTI >

According to another aspect of the present invention, in the digital predistortion method for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal, (a) the center frequency of the OFDM input signal having the number of subcarriers Nc The frequency band of the input signal is referred to as A Hz, and the frequency band of the input signal is referred to as A + a Hz, and the phase according to the change of the voltage of the input signal at A + a Hz is due to the AM-PM distortion characteristic of the power amplifier. Of the subcarrier frequency of the input signal when lagging Φ _d above the phase at A Hz and leading to a change in voltage of the input signal at A a Hz leading Φ _d above the phase at A Hz A first compensation step of precompensating the phase shift by OFDM modulation by multiplying the complex signal X (k) corresponding to either by exp (jΦ (k)), where Φ (k) is (2kΦ _d / Nc); And (b) a distortion degree of an amplitude and a phase of an output signal according to a change in voltage magnitude of the input signal with respect to the signal whose phase shift is compensated based on AM-AM and AM-PM distortion characteristics of the power amplifier. There is provided a method comprising a second compensation step of compensating for in advance.

According to still another aspect of the present invention, in a digital predistortion system for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal, the AM-PM distortion characteristic of the power amplifier is referred to as follows. Obtain a frequency dependent phase shift degree in the frequency band of the OFDM input signal to be input to the power amplifier, and with reference to the obtained phase shift degree, a complex signal X (corresponding to any one of the subcarrier frequencies of the input signal) a first compensator for OFDM modulation by compensating for the phase shift with respect to k) in advance, and the AM-AM and AM-PM distortion characteristics of the power amplifier, and the phase shift compensated signal Digital predistortion system including a second compensation unit for compensating in advance the distortion degree of the amplitude and phase of the output signal according to the change in voltage magnitude It is a ball.

According to yet another aspect of the present invention, in a digital predistortion system for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal, a center frequency of an OFDM input signal in which the number of subcarriers is Nc is A Hz. The frequency band of the input signal is Aa Hz to A + a Hz, and the phase according to the change of the voltage of the input signal at A + a Hz is A Hz due to the AM-PM distortion characteristic of the power amplifier. Corresponds to any one of the subcarrier frequencies of the input signal when the phase is delayed Φd and the phase according to the change in the voltage of the input signal at A Hz is Φd ahead of the phase at A Hz. A first compensator for compensating for the phase shift in advance by multiplying the complex signal X (k) by exp (jΦ (k)) by OFDM modulation, where Φ (k) is (2kΦd / Nc); And based on the AM-AM and AM-PM distortion characteristics of the power amplifier, the amplitude of the output signal and the degree of distortion of the phase according to the change in the voltage magnitude of the input signal are compensated in advance for the signal whose phase shift is compensated. A digital predistortion system is provided that includes a second compensator.

According to the present invention, a frequency dependent phase shift degree of an OFDM input signal is obtained with reference to AM-PM distortion characteristics of a power amplifier, and a complex signal X (k) corresponding to any one of subcarrier frequencies of the input signal is referred to as a reference. By precompensating for the phase shifts, the AM-AM distortion and AM-PM distortion of the nonlinear power amplifier can be minimized.

1 is a diagram illustrating a configuration of a system for a digital predistortion method for compensating for nonlinear characteristics of a power amplifier according to an embodiment of the present invention.
2 is a block diagram of a system for performing a digital predistortion method for compensating for nonlinear characteristics of a power amplifier in accordance with one embodiment of the present invention.
3 is a graph showing AM-PM distortion and AM-AM distortion characteristics when a simulation is applied to an OFDM signal to a power amplifier.
4 is a graph illustrating simulation results when an input signal for performing different compensation is applied to a power amplifier.
5 is a graph showing an output spectrum when an input signal before compensation and an input signal to which the present invention is applied are applied to a power amplifier.
FIG. 6 is a graph illustrating simulation results of an IQ variance when an input signal performing different compensation is applied to a power amplifier.

DETAILED DESCRIPTION The following detailed description of the invention refers to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It should be understood that the various embodiments of the present invention are different, but need not be mutually exclusive. For example, certain features, structures, and characteristics described herein may be implemented in other embodiments without departing from the spirit and scope of the invention in connection with an embodiment. It is also to be understood that the position or arrangement of the individual components within each disclosed embodiment may be varied without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is to be limited only by the appended claims, along with the full scope of equivalents to which such claims are entitled, if properly explained. Like reference numerals in the drawings refer to the same or similar functions throughout the several aspects.

DETAILED DESCRIPTION Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the present invention.

The present invention is characterized in that it is applied to an OFDM signal. For example, an OFDM signal may modulate a baseband signal into a signal of a high frequency band of several GHz, and the modulation band may vary from several tens of MHz depending on channel conditions.

In addition, the system as shown in FIG. 1 of the present invention is only one example, and the power amplifier characteristic acquisition unit 210 exists outside the system, and the first compensation unit 220 and the second compensation unit are present in the system. Various modifications may be envisioned, such that only 230 may be configured to be included.

In addition, in the present invention, as shown in FIG. 3, the phase values of the output signals at the center frequency (2.14 GHz in FIG. 3) and the boundary frequencies (2.13 GHz and 2.15 GHz) are different from each other. Different phase values of the output signal are referred to as frequency dependent phase shifts.

Hereinafter, a digital predistortion method, a system, and a computer-readable recording medium for compensating for nonlinear characteristics of a power amplifier with reference to a frequency dependent phase shift degree when an OFDM signal is applied will be described.

1 is a diagram illustrating an example of a system 200 for performing a digital predistortion method for compensating for nonlinear features of a power amplifier in accordance with one embodiment of the present invention.

As shown in FIG. 1, a system 200 for performing a digital predistortion method according to an embodiment of the present invention includes a power amplifier characteristic acquirer 210, a first compensator 220, and a second compensator. 230 and the controller 240.

First, according to an embodiment of the present invention, the power amplifier characteristic acquisition unit 100 may perform a function of acquiring information regarding AM-PM and AM-AM distortion characteristics of the power amplifier.

In detail, the power amplifier characteristic obtaining unit 100 may include an AM-PM distortion characteristic of the power amplifier (that is, characteristics and input of the degree of change in the phase value of the output signal when the magnitude of the voltage of the input signal to be applied to the power amplifier changes). As the frequency value of the signal changes, it is possible to directly calculate the characteristic of the degree of change in the phase value of the output signal) or to indirectly obtain the characteristic information already calculated.

Here, the characteristic of the degree of change in the phase value of the output signal as the frequency value of the input signal changes is called a frequency dependent phase shift characteristic. As shown in the left graph of FIG. May be expressed as the difference between the phase value of the output signal at the center frequency of the frequency band of the OFDM input signal and the phase value of the output signal at any frequency within the frequency band.

In addition, when the magnitude of the voltage of the input signal to be applied to the power amplifier changes, the distortion characteristic of the degree of change in the phase value of the output signal is output as the magnitude of the voltage of the input signal increases as shown in the left graph of FIG. Indicates the distortion characteristic that the phase value of the signal gradually increases.

Meanwhile, the power amplifier characteristic acquisition unit 100 may perform a function of directly calculating AM-AM distortion characteristics of a power amplifier or indirectly acquiring already calculated characteristic information. Specifically, the right side of FIG. 3 will be described later. As in the graph, it indicates the characteristics of the degree of change in the magnitude value of the voltage of the output signal as the magnitude of the voltage of the input signal to be applied to the power amplifier changes.

Hereinafter, a method for obtaining a phase shift degree of a power amplifier having a nonlinear characteristic through simulation will be described.

3 is a graph showing AM-PM distortion and AM-AM distortion characteristics when a simulation is performed when an OFDM signal is applied to a power amplifier. Specifically, it is a graph showing a simulation result when an OFDM signal having a center frequency of 2.14 GHz and a modulation bandwidth of 20 MHz is applied to a power amplifier without linearization processing such as predistortion.

The power amplifier used in this simulation is a 150W-class-AB GaN power amplifier that is designed for W-CDMA base stations and can be applied to OFDM input signals. The power amplifier is designed to have a center frequency of 2.14 GHz and a drain bias voltage of 28V.

3 shows the AM-PM distortion characteristic of the output signal as the magnitude of the input voltage increases. Referring to the left graph of Fig. 3, the solid line is a line showing the phase value of the output signal with respect to the center frequency of 2.14 GHz, the dashed line above the solid line is 2.13 GHz, and the dotted line under the solid line is the phase of the output signal with 2.15 GHz. Line showing the value. For example, when comparing the phase value of the output signal at both boundary frequencies (2.13 GHz, 2.15 GHz) with that of the center frequency, the power amplifier to which the OFDM signal is applied has a frequency dependent phase shift characteristic of the output signal. It can be seen that.

3, when the input signal is 2.13 GHz, the phase value of the output signal is at an angle leading the phase value of the output signal at the center frequency, and when the input signal is 2.15 GHz, the output phase output value is the center. It can be seen that the lagging of the output phase of the frequency by an angle. Referring to the simulation result of FIG. 3, it can be seen that in this simulation, the difference between the phase value of the output signal at the center frequency and the boundary frequencies of both sides, that is, the phase shift value, is 4.3 °.

Accordingly, the degree of phase shift in the simulation as shown in the graph of FIG. 3 may be 4.3 °. Of course, this value can be changed depending on the type of power amplifier or the simulation environment.

Referring back to FIG. 1, the first compensator 220 may generate an OFDM signal compensated for the phase shift with reference to the frequency dependent phase shift degree obtained by the power amplifier characteristic acquirer 210. In detail, the first compensator 220 multiplies an input complex signal by a frequency dependent phase shift compensation function to be described later to generate an OFDM signal, and then performs a realization on different subcarriers having orthogonality in the frequency domain by fast Fourier transform. Can be.

Compensating and modulating the frequency dependent phase shift of the input signal by the first compensator 220 may be expressed as in Equation 1 below.

[Equation 1]

In Equation 1, X (k) represents a complex signal corresponding to any one of subcarrier frequencies of the input signal, Nc represents the number of subcarriers in OFDM signal modulation, and NFFT represents the Fast Fourier Transform magnitude. Unlike the general OFDM modulation process, the first compensator 220 compensates the frequency dependent phase shift in advance by multiplying the exp (jΦ (k)) term by the input signal X (k) expressed as a complex number, and then performing the OFDM modulation signal. Can be generated. The phase shift compensation function exp (jΦ (k)) is a function generated by referring to the degree of phase shift described with reference to FIG. 3 to compensate for the frequency dependent phase shift. According to this, Φ (k) can be represented by the following equation (2).

&Quot; (2) "

Φ _d in Equation 2 represents the difference between the phase value of the output signal at the center frequency of the frequency band of the OFDM input signal and the phase value of the output signal at both boundary frequencies, that is, the degree of phase shift (ie, 4.3 °).

Since the complex signal X (k) is a complex signal corresponding to any one of the subcarrier frequencies according to an embodiment of the present invention, the frequency dependent phase shift for each subcarrier frequency is compensated by multiplying the phase shift compensation function. Can be. Here, it is assumed that the power amplifier has a linear phase shift characteristic according to the change of frequency in the frequency band.

However, Φ (k) in Equation 2 assumes a linear phase shift, but is not necessarily limited thereto, and for example, Φ (k) determined by a lookup table may be determined. Various modifications can be envisioned.

Meanwhile, the second compensator 230 refers to the AM-AM and AM-PM distortion characteristics of the power amplifier, and the amplitude of the output signal according to the change in the voltage magnitude of the input signal with respect to the signal whose frequency dependent phase shift is compensated for. And a function of compensating for the degree of distortion of the phase in advance.

More specifically, the second compensator 230 may calculate a function f (| x |) and an AM-PM distortion degree that express AM-AM distortion characteristics in the output signal x (n) of the first compensator 220. With reference to the function g (| x (n) |), the signal x _d (n) that compensates for the AM-AM and AM-PM distortion of the power amplifier may be generated.

At this time, the process of calculating the amplitude of the signal x _d (n) can be described by Equation 3 as follows.

&Quot; (3) "

Here, f ^{- 1} (| x |) is an inverse function of f (| x |), Av is a target voltage gain, and x _c is a value satisfying f (x _c ) = Av * x _c . Referring to Equation 3, when the value of the input signal x (n) is less than or equal to x _c , the second compensation unit 230 linearly increases the magnitude of the output signal at an Av ratio and maintains the output signal at a constant value greater than or equal to x _c. To compensate. Av can be set to be less than the original gain of the power amplifier when the input power is below a predetermined value to widen the interval of the input voltage to maintain linearity. In addition, the signal x _d (n) may be expressed by Equation 4 below with reference to the amplitude of x _d (n) obtained from Equation 3 above.

&Quot; (4) "

Referring to Equation 4, the second compensator 230 compensates for g (| x (n) |) corresponding to AM-PM distortion (that is, phase distortion of the output signal according to the magnitude of the input voltage). It can be seen that the signal x _d (n) is generated.

Next, the control unit 240 according to an embodiment of the present invention, the function of controlling the flow of data between the power amplifier characteristic acquisition unit 210, the first compensation unit 220 and the second compensation unit 230. To perform. That is, the controller 240 according to the present invention controls the flow of data from / to an external signal or between components of the predistortion system 200, thereby obtaining the power amplifier characteristic obtaining unit 210 and the first compensating unit 220. ) And the second compensation unit 230 control to perform a unique function. In addition, the controller 240 may perform a function of providing a signal output from the second compensator 230 to the power amplifier as an OFDM input signal.

2 is a block diagram of a system for performing a digital predistortion method for compensating for nonlinear characteristics of a power amplifier in accordance with one embodiment of the present invention. Referring to FIG. 2, there are two “comparison” blocks. Each “comparison” block compares the input signal of the first compensator 220 or the second compensator 230 with the output signal of the power amplifier. The function of acquiring the characteristic may be performed by the power amplifier characteristic acquiring unit 210 described above.

Referring to FIG. 2, an OFDM input signal and a frequency dependent phase shift value of a power amplifier to which the input signal is input are respectively input to the first compensator 220. In this case, the frequency dependent phase shift value input to the first compensator 220 may be a value obtained by comparing a phase of an original OFDM input signal with a phase of a signal down-converting the output signal of the power amplifier.

In addition, a value obtained by referring to the output signal x (n) of the first compensator 220 and the AM-AM and AM-PM distortion characteristics of the power amplifier may be input to the second compensator 230. The AM-AM and AM-PM distortion characteristics of the power amplifier may be obtained by comparing the signal obtained by the power amplifier characteristic obtaining unit 210 with the down-converted output signal of the power amplifier and x (n).

In addition, the output signal x _d (n) of the second compensator 230 is converted into a radio frequency (RF) signal using an up-converter, and the converted RF signal is converted into an RF PA, that is, a power amplifier. Can be applied. The signal output from the power amplifier may be transmitted to the load, or may be converted to a baseband signal by a down-converter and transmitted to each “comparison” block, for example, the power amplifier characteristic acquirer 210, as described above.

Hereinafter, the effects of the present invention will be described in detail using simulation results when the input signal to which the digital predistortion method of the present invention is applied is applied to a power amplifier.

First, FIG. 4 is a graph illustrating simulation results when an input signal for performing different compensation is applied to a power amplifier.

(A) of FIG. 4 applies an input signal that does not perform any compensation, and (b) applies an input signal that compensates only general AM-AM and AM-PM distortions without compensating for frequency dependent phase shift. , (c) shows the amplitude output and the phase output when the input signal is subjected to the first compensation step of compensating for frequency dependent phase shift and then to the second compensation step of compensating for AM-AM and AM-PM distortion. It is a graph. For accurate comparison, the same average output voltage and average efficiency were applied to the simulations in cases (a) to (c).

Referring to FIG. 4, when comparing (a) and (b), a distortion characteristic of a power amplifier having a non-linear characteristic when an input signal that compensates for only general AM-AM and AM-PM distortion is applied to the power amplifier It can be seen that is effectively corrected to some extent. Specifically, the point group showing the amplitude output and the phase output shows a tendency to bend down or up, i.e., distortion, as the input voltage increases in the graph (a), but (b) the degree of bending decreases considerably in the graph. Can be. However, in the case of (b), there was a disadvantage that scattering of the point group is considerable, and this disadvantage can be improved as in (c) by applying the present invention. That is, referring to (c), the scattering of the point group is much reduced than in the case of (b). Therefore, when both the first compensation step for compensating for frequency dependent phase shift and the second compensation step for compensating for AM-AM and AM-PM distortion, the phase shift and the degree of distortion of the power amplifier output signal can be reduced the most. It can be seen that.

5 is a graph showing an output spectrum when an input signal before compensation and an input signal to which the present invention is applied are applied to a power amplifier.

Referring to FIG. 5, when (a) applies an uncompensated input signal, (b) shows an output spectrum when the input signal according to the present invention is applied to a power amplifier, and (b) shows an output spectrum. It can be seen that the channel power is lowered and the enlargement of the spectral regrowth is reduced.

FIG. 6 is a graph illustrating simulation results of an IQ variance when an input signal performing different compensation is applied to a power amplifier.

Referring to FIG. 6, the graph on the left side applies an input signal in which no preprocessing is performed, and the graph on the middle side applies an input signal that compensates for only general AM-AM and AM-PM distortion. For analyzing an error vector magnitude when applying an input signal after a first compensation step that compensates for frequency dependent phase shift and after a second compensation step that compensates for AM-AM and AM-PM distortion. A graph showing IQ dispersion. When the error vector magnitude is -24.7 dB for the left graph, -26.1 dB for the middle graph, and -32.9 dB for the right graph, when the input signal through the first and second compensation steps is applied as in the present invention. It can be seen that the error of is the smallest.

The embodiments of the present invention described above can be implemented in the form of program instructions that can be executed through various computer components and recorded on a computer-readable recording medium. The computer-readable recording medium may include program commands, data files, data structures, and the like, alone or in combination. Program instructions recorded on the computer-readable recording medium may be those specially designed and configured for the present invention, or may be known and available to those skilled in the computer software arts. Examples of computer-readable recording media include magnetic media such as hard disks, floppy disks and magnetic tape, optical recording media such as CD-ROMs, DVDs, and magneto-optical media such as floptical disks. media), and hardware devices specifically configured to store and execute program instructions, such as ROM, RAM, flash memory, and the like. Examples of program instructions include not only machine code generated by a compiler, but also high-level language code that can be executed by a computer using an interpreter or the like. The hardware device may be configured to operate as one or more software modules to perform the process according to the invention, and vice versa.

Although the present invention has been described by specific embodiments such as specific components and the like, but the embodiments and the drawings are provided to assist in a more general understanding of the present invention, the present invention is not limited to the above embodiments. For those skilled in the art, various modifications and variations can be made from these descriptions.

Accordingly, the spirit of the present invention should not be limited to the above-described embodiments, and all of the equivalents or equivalents of the claims, as well as the appended claims, fall within the scope of the spirit of the present invention. I will say.

200: system for performing the digital predistortion method
210: power amplifier characteristic acquisition unit
220: first compensation unit
230: second compensation unit
240:

Claims (16)

A digital predistortion method for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal,
(a) obtaining a frequency dependent phase shift degree in a frequency band of an OFDM input signal to be input to the power amplifier with reference to the AM-PM distortion characteristic of the power amplifier,
(b) a first compensation step of pre-compensating and OFDM-modulating a phase shift with respect to a complex signal X (k) corresponding to any one of subcarrier frequencies of the input signal with reference to the obtained degree of phase shift, and
(c) based on the AM-AM and AM-PM distortion characteristics of the power amplifier, for the signal whose phase shift is compensated, the degree of distortion of the amplitude and phase of the output signal according to the change of the voltage magnitude of the input signal; Second compensation step to compensate in advance
Digital predistortion method comprising a. The method of claim 1,
and (d) providing the signal subjected to the second compensation step as the OFDM input signal to the power amplifier. The method of claim 1,
In the step (b)
The signal x (n) whose phase shift is compensated for

(exp (jΦ (k)) is the phase shift compensation function, Nc is the number of subcarriers, and NFFT is the Fast Fourier Transform magnitude.
Digital predistortion method characterized in that the expression. The method of claim 3,
When the frequency dependent phase shift degree is Φ _d ,
Φ (k) is

Digital predistortion method characterized in that the expression. The method of claim 4, wherein
And φ _d is a difference between the phase value of the output signal at the center frequency of the frequency band of the OFDM input signal and the phase value of the output signal at both boundary frequencies. The method of claim 1,
When the signal compensated by the second compensation step is x _d (n),

(x (n) is a signal whose phase shift is compensated by the first compensation step, and g (| x (n) |) is a function representing the AM-PM distortion characteristic of the power amplifier)
Digital predistortion method characterized in that represented by. The method of claim 6,
Remind | x _d (n) |

(f (| x |) is a function expressing AM-AM distortion characteristic of the power amplifier, f ^{− 1} (| x |) is an inverse function of f (| x |), Av is a target voltage gain after linearization, x _c Is a value satisfying f (x _c ) = Av * x _c )
Digital predistortion method characterized in that represented by. The method of claim 7, wherein
And wherein Av is set to be less than the original gain of the power amplifier when the input power is below a predetermined value. A digital predistortion method for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal,
(a) A center frequency of an OFDM input signal having the number of subcarriers Nc is called A Hz, and a frequency band of the input signal is called Aa Hz to A + a Hz, and according to AM-PM distortion characteristics of the power amplifier, A The phase according to the change of the voltage of the input signal at + a Hz is lagging Φ _d than the phase at A Hz and the phase according to the change of voltage of the input signal at Aa Hz is higher than the phase at A Hz A first compensation step of precompensating the phase shift by leading OFDM modulation by multiplying the complex signal X (k) corresponding to any one of the subcarrier frequencies of the input signal by exp (jΦ (k)) when Φ _{d is} leading. Φ (k) is (2kΦ _d / Nc); And
(b) based on the AM-AM and AM-PM distortion characteristics of the power amplifier, for the signal whose phase shift is compensated, the degree of distortion of the amplitude and phase of the output signal according to the change of the voltage magnitude of the input signal; Second compensation step to compensate in advance
≪ / RTI > The method of claim 8,
When the signal compensated by the second compensation step is x _d (n),

(x (n) is a signal whose phase shift is compensated by the first compensation step, and g (| x (n) |) is a function representing the AM-PM distortion characteristic of the power amplifier)
Digital predistortion method characterized in that represented by. The method of claim 10,
Remind | x _d (n) |

(f (| x |) is a function expressing AM-AM distortion characteristic of the power amplifier, f ^{− 1} (| x |) is an inverse function of f (| x |), Av is a target voltage gain after linearization, x _c Is a value satisfying f (x _c ) = Av * x _c )
Digital predistortion method characterized in that represented by. A digital predistortion system for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal,
With reference to the AM-PM distortion characteristic of the power amplifier, a frequency dependent phase shift degree in a frequency band of the OFDM input signal to be input to the power amplifier is obtained, and with reference to the obtained phase shift degree, the input is obtained. A first compensator for OFDM-modulating the phase shift with respect to the complex signal X (k) corresponding to any one of the subcarrier frequencies of the signal in advance, and
With reference to the AM-AM and AM-PM distortion characteristics of the power amplifier, the amplitude of the output signal and the degree of distortion of the phase according to the change in the voltage magnitude of the input signal are compensated in advance for the signal whose phase shift is compensated. Second compensation part to say
Digital predistortion system comprising a. The method of claim 12,
And a power amplifier characteristic acquisition unit for acquiring AM-AM and AM-PM distortion characteristics of the power amplifier. The method of claim 12,
And a power amplifier receiving the OFDM input signal output from the second compensator. A digital predistortion system for compensating for nonlinear characteristics of a power amplifier receiving an OFDM signal,
The center frequency of the OFDM input signal having the number of subcarriers Nc is called A Hz, the frequency band of the input signal is called Aa Hz to A + a Hz, and A + a Hz due to the AM-PM distortion characteristic of the power amplifier. delay than Φ _d phase at the phase of change in the voltage of the input signal a Hz in and (lagging) Aa Hz phase than Φ _d ahead in the a Hz phase according to a change in voltage on the input signal at A first compensator for compensating for phase shift in advance by OFDM modulation by multiplying the complex signal X (k) corresponding to any one of the subcarrier frequencies of the input signal by exp (jΦ (k)). k) is (2kΦ _d / Nc); And
On the basis of the AM-AM and AM-PM distortion characteristics of the power amplifier, for the signal compensated for the phase shift in advance to compensate for the distortion of the amplitude and phase of the output signal according to the change in the voltage magnitude of the input signal Second Compensation Unit
Digital predistortion system comprising a. A computer-readable medium for recording a computer program for executing the method according to any one of claims 1 to 11.

Images