CN109617841B - Solid-state power amplifier OFDM signal digital predistortion module in airborne data chain and implementation method - Google Patents

Solid-state power amplifier OFDM signal digital predistortion module in airborne data chain and implementation method Download PDF

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CN109617841B
CN109617841B CN201811536037.0A CN201811536037A CN109617841B CN 109617841 B CN109617841 B CN 109617841B CN 201811536037 A CN201811536037 A CN 201811536037A CN 109617841 B CN109617841 B CN 109617841B
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CN109617841A (en
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闫朝星
付林罡
罗翔
彭浪
李辉
李响
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Beijing Research Institute of Telemetry
Aerospace Long March Launch Vehicle Technology Co Ltd
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Aerospace Long March Launch Vehicle Technology Co Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/38—Synchronous or start-stop systems, e.g. for Baudot code
    • H04L25/40—Transmitting circuits; Receiving circuits
    • H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2614—Peak power aspects
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
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Abstract

The invention discloses a solid-state power amplifier OFDM signal digital predistortion module in an airborne data chain, which comprises an iteration amplitude limiting filtering unit, a raised cosine filtering unit and an oversampling predistortion unit, wherein the iteration amplitude limiting filtering unit is used for reducing the peak-to-average ratio of an input signal; the raised cosine filtering unit reduces the out-of-band interference of the signal spectrum; and the oversampling predistortion unit performs nonlinear processing on the amplitude of the oversampling signal in the received signal to obtain and output the OFDM signal subjected to predistortion processing. The invention further discloses a realization method of the predistortion module, and by applying the module and the realization method, the peak-to-average ratio of the signal can be reduced, the in-band and out-of-band comprehensive optimization of the signal can be realized, and the complexity and the realization cost of the design of a transmitter can not be increased.

Description

Solid-state power amplifier OFDM signal digital predistortion module in airborne data chain and implementation method
Technical Field
The invention relates to the technical field of digital wireless communication transmission, in particular to a solid-state power amplifier OFDM signal digital predistortion module in an airborne data chain and an implementation method.
Background
In an unmanned aerial vehicle data chain system, an airborne data chain terminal needs to transmit high-speed image data to a ground station terminal, the airborne terminal usually adopts a power amplifier with output of several watts, the amplifier is generally a solid-state power amplifier, and the amplifier generally has better linearity. However, there is still poor non-linear behavior near the saturation point of solid state power amplifiers. The OFDM technology can achieve high-speed and reliable data transmission in a low elevation angle environment, but its high peak-to-average ratio also presents a more serious challenge to an onboard power amplifier with more limitations on weight, power consumption, and the like. For high peak-to-average ratio signals, the signals are often required to be pre-distorted through techniques such as power amplifier backoff, peak-to-average ratio suppression and nonlinear transformation.
Peak-to-Average Ratio suppression techniques for OFDM signals based on compression transformation and Clipping Filtering are designed in the documents WANG Y, Ge J H, WANG L, et al. The technology is suitable for the engineering field, but the technology is wide, and the characteristic that the unmanned aerial vehicle data chain airborne power amplifier usually adopts a solid-state power amplifier is not considered. From "Song ancestor; a Luo dynasty upper; zhangguan; the design problem also exists in electronic design engineering, 2012,20(23): 117-. In the literature, "lin qian, guo rienting, research on a peak-to-average ratio suppression and predistortion combined technology in an OFDM system", a digital television 2011,35(2):17-19 "aims at a TWTA traveling wave tube amplifier, reduces the peak-to-average ratio of an OFDM signal by using a limiting method, and then enlarges the linear range of a power amplifier by using a predistortion technology, but does not research on problems such as solid power amplification and out-of-band interference.
Various feedback type pre-distortion algorithms and implementation structures based on memory or non-memory are designed in documents of Huang Zheng Peng, Roxiong, Wireless communication radio frequency power amplifier nonlinear distortion optimization design, computer simulation, 2017. "," Zuoming ", Power amplifier nonlinear characteristic and correction technology research, Ningbo university, 2015.", "Hu Li, Ebali &. Power amplifier memory effect analysis and memory pre-distortion technology software, 2011.", "Wu Jing. pre-distortion technology research in OFDM system, Sigan electronics university, 2008.", "Ai B, Yang Z-x, Pan C-y, et al analysis on LUT based prediction method for HPA with memory. IEEE Transactions on Broadcasting 2007,53(1): 127-. However, the implementation of such an algorithm needs to add a hardware channel, which greatly affects the instability and implementation cost of the system.
With the magnitude order rise of the high-speed data transmission code rate of the airborne data link of the unmanned aerial vehicle, the OFDM signal shows the advantage of high-speed and reliable data transmission in a low elevation angle environment, but the characteristic of high peak-to-average power ratio of the OFDM signal provides great challenges for airborne solid-state power amplification. On one hand, the method in the above document only considers reducing the peak-to-average ratio of the signal, and cannot realize the in-band and out-of-band comprehensive optimization of the signal; on one hand, the designed feedback predistortion method increases the design complexity and the realization cost of the transmitter.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, provides a solid-state power amplifier OFDM signal digital predistortion module in an airborne data chain and an implementation method thereof, reduces the peak-to-average ratio of signals, realizes in-band and out-of-band comprehensive optimization of the signals, and does not increase the complexity and implementation cost of transmitter design.
The above purpose of the invention is realized by the following technical scheme:
the solid-state power amplifier OFDM signal digital predistortion module in the airborne data chain comprises an iteration amplitude limiting filtering unit, a raised cosine filtering unit and an oversampling predistortion unit, wherein the iteration amplitude limiting filtering unit is used for reducing the peak-to-average ratio of an input signal and outputting the peak-to-average ratio to the raised cosine filtering unit; the raised cosine filtering unit reduces the out-of-band interference of a signal spectrum and outputs the out-of-band interference to the oversampling predistortion unit; and the oversampling predistortion unit performs nonlinear processing on the amplitude of the oversampling signal in the received signal to obtain and output the OFDM signal subjected to predistortion processing.
The predistortion realization method based on the digital predistortion module comprises the following steps:
step one, configuring a digital predistortion module according to the parameter characteristics of an airborne data link terminal solid-state power amplifier, wherein the digital predistortion module comprises a target amplitude limiting amplitude A of an iterative amplitude limiting filtering unitMOut-of-band rejection gamma of raised cosine filter unitCOversampling digital predistortion power amplifier saturation amplitude DS；
Performing primary pre-distortion processing on the OFDM signal of the airborne terminal by an iterative amplitude limiting and filtering unit to obtain the OFDM signal with the reduced peak-to-average ratio;
step three, the raised cosine filtering unit reduces the ith OFDM symbol x after the peak-to-average ratio is reducedi(n) performing time domain windowing to obtain windowed out-of-band rejection gammaCOf an OFDM signal xi,w(n) for the windowed OFDM signal xi,w(n) performing time domain upsampling and filtering by adopting a sinc interpolation function and a Kaiser window filter to obtain an OFDM oversampling signal y (n);
and step four, the oversampling predistortion unit carries out amplitude distortion processing on the OFDM oversampling signal y (n) according to a nonlinear function to obtain a predistortion processed OFDM signal s (n).
In the step (one), the parameter characteristic of the airborne data link terminal solid-state power amplifier is the amplitude relation characteristic of the input signal s (t) and the output signal z (t), the phase relation characteristic of the airborne data link terminal solid-state power amplifier is kept unchanged, and the general amplitude relation characteristic F (A) of the solid-state power amplifier is setI(t),AO(t)) is:
Figure BDA0001906893260000031
wherein A isI(t) is the amplitude level of the solid-state power amplifier input signal s (t), and is expressed as: a. theI(t)＝|s(t)|，AO(t) amplitude level of solid-state power amplifier output signal z (t), AO(t)＝|z(t)|，ASIs the saturation level of the solid state power amplifier.
The implementation method of the step (II) is as follows:
(2.1) generating an airborne terminal time domain equivalent digital baseband OFDM signal x (n) by an iterative amplitude limiting and filtering unit:
(2.2) performing time domain amplitude limiting on the time domain equivalent digital baseband OFDM signal x (n), wherein the specific method comprises the following steps:
according to the target amplitude AMConducting the routing of light according to criterion | x (n)>AMOr | x (n) | < AMAnd judging the amplitude, wherein | | represents the operation of solving the signal amplitude, and the following processing is carried out after the judgment:
Figure BDA0001906893260000041
wherein x' (n) is a time-domain limited signal; θ (n) is the phase of signal x (n), and c (n) is the clipped component;
(2.3) FFT transforming the amplitude limited component c (n) to frequency domain data C (k), and filtering the frequency domain data C (k) to obtain updated OFDM data xi+1(n)；
(2.4) updating OFDM data x obtained in the step (2.3)i+1(n), carrying out iterative updating according to the step (2.2), and if the iterative times reach the designed maximum iterative times M0If yes, terminating iteration and outputting a signal, otherwise, iteratively executing the steps (2.2) - (2.4) until the designed maximum iteration number M is reached0。
The implementation method of the step (2.1) is as follows:
for source data X (k) over a length N in the frequency domainsThe inverse Fourier transform IFFT of the method obtains a time domain equivalent digital baseband OFDM signal x (n), and the expression is
Figure BDA0001906893260000042
Wherein: n is a positive integer, and N is not less than 0 and not more than Ns-1; k is a positive integer, and k is more than or equal to 0 and less than or equal to Ns-1; length NsData X (k) of (a) includes NuA data subcarrier and Ns-NuZero-valued virtual subcarriers; esIs the transmit signal power.
In the step (2.2), c (n) is represented as follows:
Figure BDA0001906893260000043
in the step (2.3), the frequency domain data c (k) is filtered to obtain the updated OFDM data xi+1(n) the method is as follows:
will Ns-NuSetting the virtual subcarriers C (k) to be zero to obtain frequency domain data C' (k); performing IFFT on the frequency domain data C '(k) to obtain time domain data C' (n), and performing the i-th update:
xi+1(n)＝xi(n)+μci(n)
wherein x isi(n) is the time domain signal at the ith iteration, xi+1And (n) is a signal updated by the (i + 1) th iteration, and mu is an updating step parameter.
In the step (III), xi,w(n) is represented by
Figure BDA0001906893260000051
Wherein N isgThe length of the cyclic prefix of the OFDM signal is set to Ng＝Ns/8；xi,w(n) is the ith time-domain windowed OFDM symbol, cpAnd cs are respectively cyclic prefix windowing coefficient and cyclic suffix windowing coefficient, and the prefix windowing coefficient is cp＝[0 0.0125 0.0495 0.1091 0.1883 0.2831 0.3887 0.5 0.6113 0.7169 0.8117 0.8909 0.9505 0.9875 1]The suffix windowing coefficient and the prefix windowing coefficient are symmetrically equal.
The implementation manner of the step (IV) is as follows:
according to the relation between the input signal power and the output signal power of the selected solid-state power amplifier, the solid-state power is obtained by analyzing through a mathematical regression methodAmplitude dependence characteristic F (A) of amplifierI(t),AO(t)), and then the inverse function is performed to obtain the nonlinear function F (D)I(n),DO(n))：
F(DI(n),DO(n))＝F-1(AI(t),AO(t))
The nonlinear function expression of the amplitude relation of the solid-state power amplifier is as follows:
Figure BDA0001906893260000052
wherein D isI(n) is the amplitude value of the input signal y (n): i (n) | DI(n)；DO(n) is the amplitude value of the output signal s (n): s (n) | ═ DO(n)；DSTo saturation level A of the corresponding solid state power amplifierSThe power amplifier saturation amplitude of (2).
The implementation manner of the step (IV) is as follows:
the airborne power amplifier is set to be F (A) according to different flying heights of aircrafts and different temperature environments, so that the input signal power-output signal power characteristic of the power amplifier is influencedI(t),AO(t),Ta(T)), wherein Ta(T) is the temperature of the airborne solid-state power amplifier, and the parameter can be generally acquired by an airborne sensor and transmitted to an airborne data link terminal and recorded as Ta(n); thereby the temperature TaThe (t) parameter can be used to assist the non-linear function F (D)I(n),DO(n)) digital predistortion, the mathematical relationship of which is expressed as:
F(DI(n),DO(n)，Ta(n))＝F-1(AI(t),AO(t),Ta(t))；
the nonlinear function expression of the amplitude relation of the solid-state power amplifier is as follows:
Figure BDA0001906893260000061
wherein D isI(n) is the amplitude value of the input signal y (n): i (n) | DI(n)；DO(n) is the amplitude value of the output signal s (n): s (n) | ═ DO(n)；DSTo saturation level A of the corresponding solid state power amplifierSThe power amplifier saturation amplitude of (2).
Compared with the prior art, the invention has the following beneficial effects:
(1) the invention relates to a digital predistortion method of a solid-state power amplifier OFDM signal in an airborne data chain, which comprehensively optimizes the in-band peak-to-average power ratio inhibition and out-band interference inhibition indexes of the OFDM signal in practical engineering by comprehensively configuring an iterative amplitude limiting filter unit, a raised cosine filter unit and an oversampling predistortion unit in a digital predistortion module;
(2) the invention relates to a digital predistortion method for a solid-state power amplifier OFDM signal in an airborne data chain, which comprises the steps of reducing the peak-to-average ratio of the signal through an iterative amplitude limiting filtering unit, and reducing the out-of-band interference of the signal spectrum through raised cosine filtering to realize the predistortion processing of the airborne OFDM data chain signal at a sampling rate of one time;
(3) according to the digital predistortion method for the solid-state power amplifier OFDM signal in the airborne data chain, the amplitude relation characteristic of the solid-state power amplifier is obtained through the analysis of a mathematical regression method according to the relation between the input signal power and the output signal power of the selected solid-state power amplifier, so that the amplitude predistortion of the over-sampling rate signal is completed, a hardware feedback channel is not needed, and the complexity of the design of a transmitter and the realization cost are not increased.
Drawings
FIG. 1 is a block diagram of the digital predistortion method module of the solid state power amplifier OFDM signal in the airborne data chain of the invention;
fig. 2 is an input/output characteristic curve of the solid-state power amplifier RFPA2026 of the solid-state power amplifier OFDM signal digital predistortion method in the airborne data chain of the present invention at 2.14 GHz;
FIG. 3 shows the solid state power amplification characteristic and nonlinear function characteristic of the solid state power amplification OFDM signal digital predistortion method in the airborne data link according to the present invention;
fig. 4 shows the power spectrum performance of the predistortion OFDM signal of the digital predistortion method for the solid-state power amplifier OFDM signal in the airborne data chain of the present invention.
Detailed Description
The invention is described in further detail below with reference to the following figures and specific examples:
as shown in fig. 1, three modules of joint predistortion processing are configured: the device comprises an iterative amplitude limiting filtering unit, a raised cosine filtering unit and an oversampling predistortion unit, and predistortion indexes are distributed according to needs. The iterative amplitude limiting filtering unit is used for reducing the peak-to-average ratio of the input signal, the raised cosine filtering unit is used for reducing the out-of-band interference of the signal spectrum, and the oversampling pre-distortion unit is used for carrying out nonlinear processing on the amplitude of the oversampling signal in the received signal. And realizing digital predistortion of the solid-state power amplifier OFDM signal in the airborne data chain based on nonlinear function digital predistortion of the inverse function of the solid-state power amplifier characteristic.
The digital predistortion method of the solid-state power amplifier OFDM signal in the airborne data link mainly comprises the following steps:
step one, configuring a digital predistortion module according to the parameter characteristics of an airborne data link terminal solid-state power amplifier, wherein the digital predistortion module comprises a target amplitude limiting amplitude A of an iterative amplitude limiting filtering unitMOut-of-band rejection gamma of raised cosine filter unitCOversampling digital predistortion power amplifier saturation amplitude DS。
The parameter characteristic of the airborne data link terminal solid-state power amplifier is mainly the amplitude relation characteristic of an input signal s (t) and an output signal z (t), the phase relation characteristic of the airborne data link terminal solid-state power amplifier is kept unchanged, and the general amplitude relation characteristic F (A) of the solid-state power amplifier is setI(t),AO(t)) is:
Figure BDA0001906893260000081
wherein A isI(t) is the amplitude level of the solid-state power amplifier input signal s (t), and is expressed as: a. theI(t)＝|s(t)|，AO(t) amplitude level of solid-state power amplifier output signal z (t), AO(t)＝|z(t)|，ASIs the saturation level of the solid state power amplifier.
And (II) carrying out preliminary pre-distortion treatment on the OFDM signal of the airborne terminal by the iterative amplitude limiting and filtering unit to obtain the OFDM signal with the reduced peak-to-average ratio.
(1) Generating an equivalent digital baseband OFDM signal of the airborne terminal: information source data X (k) subjected to linear modulation on frequency domain and having the length of NsThe inverse Fourier transform IFFT of the method obtains a time domain OFDM signal x (n), and the expression is
Figure BDA0001906893260000082
Wherein: n is a positive integer, and N is not less than 0 and not more than Ns-1; k is a positive integer, and k is more than or equal to 0 and less than or equal to Ns-1; length NsData X (k) of (a) includes NuA data subcarrier and Ns-NuZero-valued virtual subcarriers; esIs the transmit signal power.
(2) And performing time domain amplitude limiting on the time domain OFDM signal x (n), wherein the specific method comprises the following steps:
according to the target amplitude AMConducting criterion | x (n) on the amplitude>AMOr | x (n) | < AMWherein | | represents the operation of solving the signal amplitude, and the following processing is carried out after the judgment:
Figure BDA0001906893260000083
wherein x' (n) is the clipped signal; θ (n) is the phase of the signal x (n). The clipped component c (n) is represented as follows:
Figure BDA0001906893260000084
(3) FFT conversion of the clipped signal c (n) to frequency domain data c (k), filtering of the frequency domain data c (k): will Ns-NuSetting the virtual subcarriers C (k) to be zero to obtain frequency domain data C' (k); performing IFFT on the frequency domain data C '(k) to obtain time domain data C' (n), and updating for the ith time:
xi+1(n)＝xi(n)+μci(n)
wherein x isi(n) is the time domain signal at the ith iteration，xi+1And (n) is a signal updated by the (i + 1) th iteration, and mu is an updating step parameter.
(4) And (4) updating the OFDM time domain data x obtained in the step (3)i+1(n), carrying out iterative updating according to the step (2), and if the iteration number reaches the designed maximum iteration number M0If yes, terminating iteration and outputting a signal, otherwise, iteratively executing the steps (2) to (4) until the designed maximum iteration number M is reached0。
Step (III), the raised cosine filtering unit iteratively amplitude-limiting and filtering the OFDM signal of the airborne terminal obtained in the step (II), and a raised cosine window filter is adopted to carry out filtering on the ith OFDM symbol xi(n) performing time domain windowing to obtain windowed out-of-band rejection gammaCOf an OFDM signal xi,w(n) is represented by
Figure BDA0001906893260000091
Wherein N isgThe length of the cyclic prefix of the OFDM signal is set to Ng＝Ns/8；xi,wAnd (n) is the OFDM symbol after the ith time domain windowing. c. CpAnd csRespectively a cyclic prefix windowing coefficient and a cyclic suffix windowing coefficient, wherein the prefix windowing coefficient is cp＝[0 0.0125 0.0495 0.1091 0.1883 0.2831 0.3887 0.5 0.6113 0.7169 0.8117 0.8909 0.9505 0.9875 1]The suffix windowing coefficient and the prefix windowing coefficient are symmetrically equal.
OFDM signal x after raised cosine windowingi,wAnd (n), performing time domain upsampling and filtering by adopting a sinc interpolation function and a Kaiser window filter to obtain an OFDM oversampled signal y (n).
And step (IV) the oversampling predistortion unit performs amplitude distortion on the windowed filtered OFDM signal y (n) obtained in the step (III) according to a nonlinear function to obtain s (n), and the amplitude relation characteristic F (A) of the solid-state power amplifier can be obtained through analysis of a mathematical regression method according to the relation between the input signal power and the output signal power of the selected solid-state power amplifierI(t),AO(t)), and then inverting the function to obtain the nonlinear function
F(DI(n),DO(n))＝F-1(AI(t),AO(t))
In general, the nonlinear function expression of the amplitude relationship of a solid-state power amplifier is:
Figure BDA0001906893260000101
wherein D isI(n) is the amplitude value of the input signal y (n): i (n) | DI(n)；DO(n) is the amplitude value of the output signal s (n): s (n) | ═ DO(n)；DSTo saturation level A of the corresponding solid state power amplifierSThe power amplifier saturation amplitude of (2).
In particular, the onboard power amplifier is subjected to different temperature environments according to the flight altitude of the aircraft, so that the input signal power-output signal power characteristic of the power amplifier is influenced, and is set to be F (A)I(t),AO(t),Ta(T)), wherein Ta(T) is the temperature of the onboard solid-state power amplifier, and the parameter can be generally acquired by an onboard sensor and transmitted to an onboard data link terminal, which is recorded as Ta(n); thereby the temperature TaThe (t) parameter can be used to assist the non-linear function F (D)I(n),DO(n)) digital predistortion, the mathematical relationship of which is expressed as:
F(DI(n),DO(n)，Ta(n))＝F-1(AI(t),AO(t),Ta(t))
after the pre-distortion treatment, the analog OFDM signal s (t) which is obtained by the DA digital-to-analog conversion of the output OFDM signal s (n) is sent to a solid-state power amplifier to be amplified and sent out.
Example 1
The performance simulation result of the solid-state power amplifier OFDM signal digital predistortion method in the airborne data chain is analyzed.
A simulation system is constructed according to FIG. 1, and the parameter of the OFDM system is Ns＝1024，Nu＝720，NgAnd 128, the iterative amplitude limiting and filtering unit restrains 3dB according to the peak-to-average ratio, and configures a target amplitude limiting amplitude A.The prefix windowing coefficient of the raised cosine filter unit is cp＝[0 0.0125 0.0495 0.1091 0.1883 0.2831 0.3887 0.5 0.6113 0.7169 0.8117 0.8909 0.9505 0.9875 1]The suffix windowing coefficient and the prefix windowing coefficient are symmetrically equal. The solid-state power amplifier refers to a 3-state and 2W output power amplifier RFPA2026 of rfmd corporation, and the frequency range is 700MHz-2700 MHz.
As shown in FIG. 2, when the power amplifier module RFPA2026 is at 2.17GHz, the corresponding output signal power is 18dBm, 28dBm, 32dBm, 33dBm, 34dBm and 34.5dB when the input signal power is-20 dBm, -10dB, -6dBm, -4dBm, -2dBm and 0dBm at the temperature of 40 ℃, and the output signal power is reduced by 1 dBm-0.5 dBm when the temperature is raised to 85 ℃, and the nonlinear function is obtained by regression fitting of the data characteristic. The solid-state power amplifier characteristic and the nonlinear function characteristic in the airborne data chain are shown in figure 3.
The power spectrum of the solid-state power amplifier output signal obtained after the nonlinear function is pre-distorted according to the method of the invention is shown in fig. 4. Analysis shows that when raised cosine filtering and oversampling predistortion are not available, the out-of-band normalized frequency spectrum of the OFDM signal is reduced by about 25 dB; when oversampling predistortion is performed and raised cosine windowing filtering is not performed, the out-of-band normalized spectrum of the OFDM signal is expanded by about 60%; when an iteration amplitude limiting filtering unit, a raised cosine filtering unit and an oversampling predistortion unit are added for joint predistortion treatment, the out-of-band normalized frequency spectrum is reduced by about more than 50 dB. Therefore, the method can effectively realize the OFDM signal predistortion of the airborne data link solid-state power amplifier.
The above description is only for the best mode of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Those skilled in the art will appreciate that the invention may be practiced without these specific details.

Claims (9)

1. A predistortion realization method of a solid-state power amplifier OFDM signal digital predistortion module in an airborne data chain is characterized in that: the digital predistortion module comprises an iteration amplitude limiting filtering unit, a raised cosine filtering unit and an oversampling predistortion unit, wherein the iteration amplitude limiting filtering unit is used for reducing the peak-to-average ratio of an input signal and outputting the reduced peak-to-average ratio to the raised cosine filtering unit; the raised cosine filtering unit reduces the out-of-band interference of a signal spectrum and outputs the out-of-band interference to the oversampling predistortion unit; the over-sampling pre-distortion unit carries out non-linear processing on the amplitude of the over-sampling signal in the received signal to obtain and output an OFDM signal subjected to pre-distortion processing;
the predistortion implementation method comprises the following steps:
step one, configuring a digital predistortion module according to the parameter characteristics of an airborne data link terminal solid-state power amplifier, wherein the digital predistortion module comprises a target amplitude limiting amplitude A of an iterative amplitude limiting filtering unitMOut-of-band rejection gamma of raised cosine filter unitCOversampling digital predistortion power amplifier saturation amplitude DS；
Performing primary pre-distortion processing on the OFDM signal of the airborne terminal by an iterative amplitude limiting and filtering unit to obtain the OFDM signal with the reduced peak-to-average ratio;
step three, the raised cosine filtering unit reduces the ith OFDM symbol x after the peak-to-average ratio is reducedi(n) performing time domain windowing to obtain windowed out-of-band rejection gammaCOf an OFDM signal xi,w(n) for the windowed OFDM signal xi,w(n) performing time domain upsampling and filtering by adopting a sinc interpolation function and a Kaiser window filter to obtain an OFDM oversampling signal y (n);
and step four, the oversampling predistortion unit carries out amplitude distortion processing on the OFDM oversampling signal y (n) according to a nonlinear function to obtain a predistortion processed OFDM signal s (n).
2. The predistortion realization method of claim 1, wherein: in the step (one), the parameter characteristic of the airborne data link terminal solid-state power amplifier is the amplitude relation characteristic of the input signal s (t) and the output signal z (t), the phase relation characteristic of the airborne data link terminal solid-state power amplifier is kept unchanged, and the general amplitude relation characteristic F (A) of the solid-state power amplifier is setI(t),AO(t)) is:
Figure FDA0003022464750000011
wherein A isI(t) is the amplitude level of the solid-state power amplifier input signal s (t), and is expressed as: a. theI(t)＝|s(t)|，AO(t) amplitude level of solid-state power amplifier output signal z (t), AO(t)＝|z(t)|，ASIs the saturation level of the solid state power amplifier.
3. The predistortion realization method of claim 1, wherein: the implementation method of the step (II) is as follows:
(2.1) generating an airborne terminal time domain equivalent digital baseband OFDM signal x (n) by an iterative amplitude limiting and filtering unit:
(2.2) performing time domain amplitude limiting on the time domain equivalent digital baseband OFDM signal x (n), wherein the specific method comprises the following steps:
according to the target amplitude AMConducting the routing of light according to criterion | x (n)>AMOr | x (n) | < AMAnd judging the amplitude, wherein | | represents the operation of solving the signal amplitude, and the following processing is carried out after the judgment:
Figure FDA0003022464750000021
wherein x' (n) is a time-domain limited signal; θ (n) is the phase of signal x (n), and c (n) is the clipped component;
(2.3) FFT transforming the amplitude limited component c (n) to frequency domain data C (k), and filtering the frequency domain data C (k) to obtain updated OFDM data xi+1(n)；
(2.4) updating OFDM data x obtained in the step (2.3)i+1(n), carrying out iterative updating according to the step (2.2), and if the iterative times reach the designed maximum iterative times M0If yes, terminating iteration and outputting a signal, otherwise, iteratively executing the steps (2.2) - (2.4) until the designed maximum iteration number M is reached0。
4. A predistortion realization method according to claim 3, characterized in that: the implementation method of the step (2.1) is as follows:
for source data X (k) over a length N in the frequency domainsThe inverse Fourier transform IFFT of the method obtains a time domain equivalent digital baseband OFDM signal x (n), and the expression is
Figure FDA0003022464750000031
Wherein: n is a positive integer, and N is not less than 0 and not more than Ns-1; k is a positive integer, and k is more than or equal to 0 and less than or equal to Ns-1; length NsData X (k) of (a) includes NuA data subcarrier and Ns-NuZero-valued virtual subcarriers; esIs the transmit signal power.
5. The predistortion realization method of claim 4, wherein: in the step (2.2), c (n) is represented as follows:
Figure FDA0003022464750000032
6. the predistortion realization method of claim 4, wherein: in the step (2.3), the frequency domain data c (k) is filtered to obtain the updated OFDM data xi+1(n) the method is as follows:
will Ns-NuSetting the virtual subcarriers C (k) to be zero to obtain frequency domain data C' (k); performing IFFT on the frequency domain data C '(k) to obtain time domain data C' (n), and performing the i-th update:
xi+1(n)＝xi(n)+μci(n)
wherein x isi(n) is the time domain signal at the ith iteration, xi+1And (n) is a signal updated by the (i + 1) th iteration, and mu is an updating step parameter.
7. According to claimThe predistortion realization method of claim 4, characterized by: in the step (III), xi,w(n) is represented by
Figure FDA0003022464750000033
Wherein N isgThe length of the cyclic prefix of the OFDM signal is set to Ng＝Ns/8；xi,w(n) is the ith time-domain windowed OFDM symbol, cpAnd csRespectively a cyclic prefix windowing coefficient and a cyclic suffix windowing coefficient, wherein the prefix windowing coefficient is cp＝[0 0.0125 0.0495 0.1091 0.1883 0.2831 0.3887 0.5 0.6113 0.7169 0.8117 0.8909 0.9505 0.9875 1]The suffix windowing coefficient and the prefix windowing coefficient are symmetrically equal.
8. The predistortion realization method of claim 1, wherein: the implementation manner of the step (IV) is as follows:
according to the relation between the input signal power and the output signal power of the selected solid-state power amplifier, the amplitude relation characteristic F (A) of the solid-state power amplifier is obtained by analyzing through a mathematical regression methodI(t),AO(t)), and then the inverse function is performed to obtain the nonlinear function F (D)I(n),DO(n))：
F(DI(n),DO(n))＝F-1(AI(t),AO(t))
The nonlinear function expression of the amplitude relation of the solid-state power amplifier is as follows:
Figure FDA0003022464750000041
wherein D isI(n) is the amplitude value of the input signal y (n): i (n) | DI(n)；DO(n) is the amplitude value of the output signal s (n): s (n) | ═ DO(n)；DSTo saturation level A of the corresponding solid state power amplifierSThe power amplifier saturation amplitude of (2).
9. The predistortion realization method of claim 1, wherein: the implementation manner of the step (IV) is as follows:
the airborne power amplifier is set to be F (A) according to different flying heights of aircrafts and different temperature environments, so that the input signal power-output signal power characteristic of the power amplifier is influencedI(t),AO(t),Ta(T)), wherein Ta(T) is the temperature of the airborne solid-state power amplifier, and the parameter can be generally acquired by an airborne sensor and transmitted to an airborne data link terminal and recorded as Ta(n); thereby the temperature TaThe (t) parameter can be used to assist the non-linear function F (D)I(n),DO(n)) digital predistortion, the mathematical relationship of which is expressed as:
F(DI(n),DO(n)，Ta(n))＝F-1(AI(t),AO(t),Ta(t))；
the nonlinear function expression of the amplitude relation of the solid-state power amplifier is as follows:
Figure FDA0003022464750000042
wherein D isI(n) is the amplitude value of the input signal y (n): i (n) | DI(n)；DO(n) is the amplitude value of the output signal s (n): s (n) | ═ DO(n)；DSTo saturation level A of the corresponding solid state power amplifierSThe power amplifier saturation amplitude of (2).
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