CN109474285A - A method of preprocessing DAC to cause in-band unevenness - Google Patents
A method of preprocessing DAC to cause in-band unevenness Download PDFInfo
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Abstract
一种预处理DAC引起带内不平坦处理的方法,属于数字通信技术领域。本发明针对数模转化器(Digital‑to‑Analog Converters,DAC)采用了零阶保持器,使其幅频特性上固有的存在衰落,导致带宽信号中幅度一致性很差的问题,提出本发明方法,本发明方法采用数字滤波器进行预失真处理的补偿方法来解决DAC器件固有的存在衰落,以达到改善宽带信号的带内平坦度。仿真结果显示,本发明有效改善了DAC衰落导致宽带信号的带内不平坦度问题。
A method for in-band uneven processing caused by preprocessing a DAC belongs to the technical field of digital communication. The present invention proposes the present invention aiming at the problem that the digital-to-Analog Converters (DAC) adopts a zero-order holder, which inherently has fading in its amplitude-frequency characteristics, resulting in poor amplitude consistency in the bandwidth signal. The method of the present invention adopts the compensation method of digital filter for pre-distortion processing to solve the inherent fading of the DAC device, so as to improve the in-band flatness of the broadband signal. Simulation results show that the present invention effectively improves the in-band unevenness problem of wideband signals caused by DAC fading.
Description
技术领域technical field
本发明涉及一种宽带数字通信系统带内不平坦抑制处理技术,特别是一种数字滤波器进行预失真处理DAC引起带内不平坦处理的方法,属于数字通信技术领域。The invention relates to an in-band unevenness suppression processing technology in a broadband digital communication system, in particular to a method for in-band unevenness processing caused by a digital filter performing predistortion processing DAC, and belongs to the technical field of digital communication.
背景技术Background technique
DAC(Digital-to-Analog Converters)器件作为发射机系统重要组成器件,广泛应用于移动通信以及卫星通信领域。由于DAC器件在实现上采用了零阶保持器,使其幅频特性上固有的存在衰落,导致带宽信号中幅度一致性很差,因而会影响卫星通信系统的性能。在窄带卫星通信系统中,该衰落的影响可忽略,但随着多载波宽带卫星通信技术的发展,在宽带高阶调制卫星通信系统应用中,DAC器件的固有衰落已经成为一个必须解决的问题。DAC (Digital-to-Analog Converters) device, as an important component of the transmitter system, is widely used in the field of mobile communication and satellite communication. Since the DAC device adopts a zero-order holder in its implementation, it inherently has fading in its amplitude-frequency characteristics, resulting in poor amplitude consistency in the bandwidth signal, which will affect the performance of the satellite communication system. In narrowband satellite communication systems, the influence of this fading can be ignored, but with the development of multi-carrier broadband satellite communication technology, the inherent fading of DAC devices has become a problem that must be solved in the application of broadband high-order modulation satellite communication systems.
近年来,随着多载波宽带通信技术发展的需求,器件生产厂商在DAC内部增加了内插、滤波等可配置的硬件单元,从而保证有效降低Sinc函数衰落的同时不增加DAC器件以外的处理,也不用提高器件间的数据传输速率。但是,采用数字内插技术提高转换速率的方法,只是将Sinc衰落的影响降低到一个可以接受的程度,并没有从产生机理上消除这一现象,而且PCB设计难度加大和设备功耗增加。In recent years, with the development of multi-carrier broadband communication technology, device manufacturers have added configurable hardware units such as interpolation and filtering inside the DAC to ensure that the fading of the Sinc function is effectively reduced without increasing the processing outside the DAC device. There is also no need to increase the data transfer rate between devices. However, the method of using digital interpolation technology to improve the conversion rate only reduces the influence of Sinc fading to an acceptable level, but does not eliminate this phenomenon from the mechanism of generation, and the difficulty of PCB design and equipment power consumption increase.
发明内容SUMMARY OF THE INVENTION
本发明解决的技术问题是:克服现有技术的不足,提供了一种采用数字滤波器进行预失真处理的补偿方法来解决DAC器件的Sinc函数衰落问题,以达到改善宽带信号的带内平坦度的方法。。The technical problem solved by the present invention is: to overcome the deficiencies of the prior art, a compensation method using a digital filter for pre-distortion processing is provided to solve the fading problem of the Sinc function of the DAC device, so as to improve the in-band flatness of the broadband signal. Methods. .
本发明的技术解决方案是:一种预处理DAC引起带内不平坦处理的方法,在输入宽带信号进入DAC器件进行数模转换前,采用与DAC器件中使用的保持器对宽带信号衰落特性函数的幅度特性互补的补偿数字滤波器对输入宽带信号进行预处理,再将预处理后的补偿宽带信号输出至DAC器件。The technical solution of the present invention is: a method for preprocessing DAC to cause in-band unevenness processing. Before the input wideband signal enters the DAC device for digital-to-analog conversion, the fading characteristic function of the wideband signal is adjusted by the holder used in the DAC device. The compensated digital filter with complementary amplitude characteristics preprocesses the input wideband signal, and then outputs the preprocessed compensated wideband signal to the DAC device.
进一步地,所述保持器为零阶保持器。Further, the retainer is a zero-order retainer.
进一步地,所述DAC器件中使用的保持器对宽带信号衰落特性函数的幅度特性为其中,Hzoh(f)为所述零阶保持器的频率特性响应函数。Further, the amplitude characteristic of the holder used in the DAC device to the broadband signal fading characteristic function is: Wherein, H z o h (f) is the frequency characteristic response function of the zero-order retainer.
进一步地,所述其中,为零阶保持器的时域特性响应函数,rect(*)为矩形函数。Further, the in, Time domain characteristic response function of zero-order retainer, rect(*) is a rectangular function.
进一步地,所述输入宽带信号经过补偿数字滤波器的时域响应为其中,x(k-n)为输入宽带信号,为补偿数字滤波器系数归一化值。Further, the time domain response of the input broadband signal through the compensation digital filter is Among them, x(kn) is the input broadband signal, Normalized value for compensated digital filter coefficients.
进一步地,所述其中,hcomp(n)为所述补偿数字滤波器的系数,round(*)为取整函数,max(*)为取最大值函数,D为所述DAC器件的量化位数。Further, the Wherein, h comp (n) is the coefficient of the compensation digital filter, round(*) is the rounding function, max(*) is the maximum value function, and D is the quantization number of the DAC device.
进一步地,所述补偿数字滤波器的系数为其中,为补偿数字滤波器频率特性响应的逆傅里叶变换系数,C0为n=0时的逆傅里叶变换系数值,n=0,1,...,N-1,且N为滤波器阶数,k=1,2,...,M,且M=(N-1)/2,fs为DAC采样频率,f为输入宽带信号频率,Ts为DAC采样周期。Further, the coefficient of the compensation digital filter is in, In order to compensate the inverse Fourier transform coefficient of the frequency characteristic response of the digital filter, C 0 is the inverse Fourier transform coefficient value when n=0, n=0,1,...,N-1, and N is the filter The order of the device, k=1,2,...,M, and M=(N-1)/2, f s is the DAC sampling frequency, f is the input wideband signal frequency, and T s is the DAC sampling period.
本发明与现有技术相比的优点在于:The advantages of the present invention compared with the prior art are:
(1)本发明采用补偿数字滤波器,通过设计幅频特性与之互补的数字滤波器对宽带信号进行数字预失真处理,从而保证DAC器件转化后的最终输出宽带信号具有良好的带内平坦度,而且是软件实现,没有增加PCB板设计的难度;(1) The present invention adopts a compensating digital filter, and performs digital pre-distortion processing on the broadband signal by designing a digital filter with complementary amplitude-frequency characteristics, so as to ensure that the final output broadband signal converted by the DAC device has good in-band flatness , and it is implemented by software, without increasing the difficulty of PCB board design;
(2)本发明采用补偿数字滤波器,通过频域采样的数值方法设计数字滤波器,即对反sinc函数先进性采样,然后进行逆傅里叶变化,从而可以简便快速获得补偿数字滤波器的系数,可编程实现,同时不需要增加相应的硬件,降低了硬件资源开销;(2) The present invention adopts the compensation digital filter, and designs the digital filter by the numerical method of frequency domain sampling, that is, the advanced sampling of the inverse sinc function, and then the inverse Fourier transformation is performed, so that the compensation digital filter can be easily and quickly obtained. Coefficient, programmable implementation, and no need to increase the corresponding hardware, reducing the hardware resource overhead;
(3)本发明采用补偿数字滤波器进行补偿处理的方法降低DAC器件衰落,由于补偿数字滤波器系数的变化范围较大,通过对补偿数字滤波器系数进行归一化处理,降低了由于系数变化导致宽带信号包络的变化,从而减小宽带信号经过非线性器件(如功率放大器)导致的信号失真,另一方面便于FPGA实现;(3) The present invention adopts the method of compensating digital filter for compensation processing to reduce the fading of DAC devices. Since the variation range of the compensating digital filter coefficients is relatively large, by normalizing the compensating digital filter coefficients, the coefficient variation due to the coefficient variation is reduced. It leads to the change of the envelope of the broadband signal, thereby reducing the signal distortion caused by the broadband signal passing through nonlinear devices (such as power amplifiers), and on the other hand, it is convenient for FPGA implementation;
(4)本发明采用补偿数字滤波器,降低了DAC器件对高阶调制信号的非线性影响,从而提高宽带卫星通信系统的频谱效率。(4) The present invention adopts the compensation digital filter, which reduces the nonlinear influence of the DAC device on the high-order modulation signal, thereby improving the spectral efficiency of the broadband satellite communication system.
附图说明Description of drawings
图1为本发明预处理DAC引起带内不平坦框图;1 is a block diagram of in-band unevenness caused by a preprocessing DAC of the present invention;
图2为DAC器件Sinc函数衰落特性图;Fig. 2 is the fading characteristic diagram of the Sinc function of the DAC device;
图3为本发明方法补偿数字滤波器特性图;Fig. 3 is the characteristic diagram of the compensation digital filter of the method of the present invention;
图4为本发明方法补偿数字滤波器的补偿效果图;Fig. 4 is the compensation effect diagram of the compensation digital filter of the method of the present invention;
图5为成型滤波后的信号频谱图;Fig. 5 is the signal spectrogram after shaping filtering;
图6为经过DAC器件且未经补偿的信号频谱图;Fig. 6 is the signal spectrum diagram that passes through the DAC device and is not compensated;
图7为本发明方法中的补偿数字滤波器对信号频谱的影响;Fig. 7 is the influence of the compensation digital filter in the method of the present invention on the signal spectrum;
图8为本发明方法补偿后的最终信号频谱图。FIG. 8 is a final signal spectrum diagram after compensation by the method of the present invention.
具体实施方式Detailed ways
针对DAC器件在实现上采用了零阶保持器,使其幅频特性上固有的存在衰落,导致带宽信号中幅度一致性很差,因而会影响卫星通信系统的性能。在窄带卫星通信系统中,该衰落的影响可忽略,但随着多载波宽带卫星通信技术的发展,在宽带高阶调制卫星通信系统应用中,DAC器件的固有衰落已经成为一个必须解决的问题。A zero-order holder is used in the implementation of the DAC device, which inherently has fading in its amplitude-frequency characteristics, resulting in poor amplitude consistency in the bandwidth signal, which will affect the performance of the satellite communication system. In narrowband satellite communication systems, the influence of this fading can be ignored, but with the development of multi-carrier broadband satellite communication technology, the inherent fading of DAC devices has become a problem that must be solved in the application of broadband high-order modulation satellite communication systems.
为了实现上述目的,本发明技术方案是这样实现的:In order to achieve the above-mentioned purpose, the technical scheme of the present invention is achieved in this way:
如图1,一种预处理DAC引起带内不平坦处理的方法,采用数字滤波器进行预失真处理的补偿方法来解决DAC器件固有的存在衰落问题,以达到改善宽带信号带内平坦度的信号处理过程;As shown in Figure 1, a method of pre-processing DAC to cause in-band unevenness processing uses a digital filter to perform pre-distortion processing compensation method to solve the inherent fading problem of DAC devices, so as to improve the in-band flatness of broadband signals. processing;
所述预处理DAC引起带内不平坦处理方法仿真验证平台由输入模块,FEC编码模块,交织模块,星座映射模块,成型滤波模块,补偿数字滤波器模块,模拟DAC衰落模块组成。The simulation verification platform for the in-band unevenness processing method caused by the preprocessing DAC is composed of an input module, an FEC encoding module, an interleaving module, a constellation mapping module, a shaping filter module, a compensation digital filter module, and an analog DAC fading module.
所述的输入数据模块是输入数据的接口。The input data module is an interface for inputting data.
所述的FEC编码是对码元进行纠错,通过在发送码元中加入差错控制码元,接收端利用这些码元补单可以发现错码,还可以将错码恢复到正确的取值。The FEC coding is to perform error correction on the symbols. By adding error control symbols to the transmitted symbols, the receiving end can use these symbols to supplement the list to find the error code and restore the error code to the correct value.
所述的交织是一种实现最大限度的改变信息结构而不改变信息内容的器件。Said interleaving is a device for realizing maximum change of information structure without changing information content.
所述的星座映射是提高传输效率。The constellation mapping described is to improve transmission efficiency.
所述的成型滤波是让基带信号变为适合传输形式的过程。The shaping filtering described is the process of bringing the baseband signal into a form suitable for transmission.
所述的补偿数字滤波器是解决DAC器件固有的存在衰落问题,以达到改善宽带信号带内平坦度的信号处理过程。The compensating digital filter is a signal processing process for solving the inherent fading problem of the DAC device, so as to improve the in-band flatness of the broadband signal.
所述的DAC对成型滤波后的信号进行数模转换并输出模拟下信号。The DAC performs digital-to-analog conversion on the shaped and filtered signal and outputs an analog signal.
所述数字滤波器进行预失真处理的补偿方法包括如下步骤:The compensation method for performing predistortion processing by the digital filter includes the following steps:
步骤A.对输入信号进行交织,星座映射,成型滤波形成Signal_modu;Step A. The input signal is interleaved, constellation mapping, and shaping filtering forms Signal_modu;
步骤B.取DAC器件的转换速率fs(fs=1GHz),滤波器阶数N=33;Step B. Take the conversion rate f s (f s =1GHz) of the DAC device, and the filter order N=33;
步骤C.采用频域采样法设计补偿数字滤波器,公式如下:Step C. Using frequency domain sampling method to design compensation digital filter, the formula is as follows:
其中,in,
步骤D.对得到的hcomp(n)进行归一化及量化,结果即为补偿数字滤波器的系数;Step D. normalization and quantization are carried out to the obtained h comp (n), and the result is the coefficient of the compensation digital filter;
对宽带补偿数字滤波器系数hcomp(n)归一化为:The broadband compensation digital filter coefficients h comp (n) are normalized to:
对宽带补偿数字滤波器系数hcomp(n)量化为:The broadband compensation digital filter coefficients h comp (n) are quantized as:
其中,round(*)为取整函数,D为DAC的量化位数,量化位是对模拟音频信号的幅度轴进行数字化,它决定了模拟信号数字化以后的动态范围。Among them, round(*) is the rounding function, D is the quantization bit of the DAC, and the quantization bit is to digitize the amplitude axis of the analog audio signal, which determines the dynamic range of the analog signal after digitization.
步骤E.由于MATLAB中直接构造DAC器件的零阶保持器模型比较困难,因此在模拟设计DAC器件Sinc函数衰落特性时,采用和步骤B相同的频域采样法,具体公式如下:Step E. Since it is difficult to directly construct the zero-order keeper model of the DAC device in MATLAB, when simulating the design of the sinc function fading characteristics of the DAC device, the same frequency domain sampling method as in step B is adopted, and the specific formula is as follows:
步骤F.对得到的h′comp(n)进行归一化及量化,结果即为DAC器件Sinc函数衰落的系数;Step F. Normalize and quantize the obtained h′ comp (n), and the result is the coefficient of fading of the Sinc function of the DAC device;
步骤G.对交织,星座映射,成型滤波形成Signal_modu经过DAC器件(卷积运算)输出为Signal_dac,用MATLAB自带工具观察此时信号的频谱;Step G. to interleaving, constellation mapping, shaping filtering to form Signal_modu through DAC device (convolution operation) and output as Signal_dac, observe the frequency spectrum of signal at this moment with MATLAB self-carrying tool;
Signal_dac=Signal_modu*h′comp(n)Signal_dac=Signal_modu* h'comp (n)
步骤H.方法同上,将Signal_modu信号先后经过补偿数字滤波器和DAC器件输出为Signal_final,然后观察最终输出信号的频谱;Step H. The method is the same as above, the Signal_modu signal is output as Signal_final through the compensation digital filter and the DAC device successively, and then the frequency spectrum of the final output signal is observed;
Signal_final=Signal_modu*hcomp(n)*h′comp(n)Signal_final=Signal_modu* hcomp (n)* h′comp (n)
所述DAC器件的零阶保持器ZOH(Zero Order Hold)模型为:The zero-order holder ZOH (Zero Order Hold) model of the DAC device is:
式中,rect()为矩形函数。它对应的频率响应为:where rect() is a rectangle function. Its corresponding frequency response is:
其中, in,
在时域: In the time domain:
在频域:Y'(f)=Y(f)×Hzoh(f)。In the frequency domain: Y'(f)=Y(f)×H zoh (f).
由以上分析可以看出:由于DAC器件中采用了零阶保持器,当数字信号转换为模拟信号时会对其频谱产生Sinc函数衰落。衰落的具体值与设计中采用的DAC器件转换速率、信号频点、带宽有关,可以参照以下公式计算。It can be seen from the above analysis that due to the zero-order holder used in the DAC device, when the digital signal is converted into an analog signal, the Sinc function fading will occur to its spectrum. The specific value of fading is related to the conversion rate, signal frequency point, and bandwidth of the DAC device used in the design, and can be calculated with reference to the following formula.
式中,f为信号频点,fs为转换速率。In the formula, f is the signal frequency point, and f s is the conversion rate.
下面结合实施例对本发明作进一步说明,但不应该理解为本发明上述主题范围仅限于下述实施例。在不脱离本发明上述技术思想的情况下,根据本领域普通技术知识和惯用手段,做出各种替换和变更,均应包括在本发明的保护范围内。The present invention will be further described below in conjunction with the examples, but it should not be understood that the scope of the above-mentioned subject matter of the present invention is limited to the following examples. Without departing from the above-mentioned technical idea of the present invention, various substitutions and changes can be made according to common technical knowledge and conventional means in the field, which shall be included in the protection scope of the present invention.
根据附图来说明本发明的具体实施:Describe the specific implementation of the present invention according to the accompanying drawings:
如图1所示,一种预处理DAC引起带内不平坦处理的方法,由输入模块,FEC编码模块,交织模块,星座映射模块,成型滤波模块,补偿数字滤波器模块,模拟DAC衰落模块组成。As shown in Figure 1, a method for in-band unevenness processing caused by preprocessing DAC is composed of an input module, an FEC encoding module, an interleaving module, a constellation mapping module, a shaping filter module, a compensation digital filter module, and an analog DAC fading module. .
所述数字滤波器进行预失真处理的补偿方法包括如下步骤:The compensation method for performing predistortion processing by the digital filter includes the following steps:
步骤A.对输入信号进行交织,星座映射,成型滤波形成Signal_modu;Step A. The input signal is interleaved, constellation mapping, and shaping filtering forms Signal_modu;
步骤B.取DAC器件的转换速率fs(fs=1GHz),滤波器阶数N=33;Step B. Take the conversion rate f s (f s =1GHz) of the DAC device, and the filter order N=33;
步骤C.采用频域采样法设计补偿数字滤波器,公式如下:Step C. Using frequency domain sampling method to design compensation digital filter, the formula is as follows:
其中,in,
步骤D.对得到的hcomp(n)进行归一化及量化,结果即为补偿数字滤波器的系数;Step D. normalization and quantization are carried out to the obtained h comp (n), and the result is the coefficient of the compensation digital filter;
步骤E.由于MATLAB中直接构造DAC器件的零阶保持器模型比较困难,因此在模拟设计DAC器件Sinc函数衰落特性时,采用和步骤B相同的频域采样法,具体公式如下:Step E. Since it is difficult to directly construct the zero-order keeper model of the DAC device in MATLAB, when simulating the design of the sinc function fading characteristics of the DAC device, the same frequency domain sampling method as in step B is adopted, and the specific formula is as follows:
步骤F.对得到的h′comp(n)进行归一化及量化,结果即为DAC器件Sinc函数衰落的系数;Step F. Normalize and quantize the obtained h′ comp (n), and the result is the coefficient of fading of the Sinc function of the DAC device;
步骤G.对交织,星座映射,成型滤波形成Signal_modu经过DAC器件(卷积运算)输出为Signal_dac,用MATLAB自带工具观察此时信号的频谱;Step G. to interleaving, constellation mapping, shaping filtering to form Signal_modu through DAC device (convolution operation) and output as Signal_dac, observe the frequency spectrum of signal at this moment with MATLAB self-carrying tool;
Signal_dac=Signal_modu*h′comp(n)Signal_dac=Signal_modu* h'comp (n)
步骤H.方法同上,将Signal_modu信号先后经过补偿数字滤波器和DAC器件输出为Signal_final,然后观察最终输出信号的频谱;Step H. The method is the same as above, the Signal_modu signal is output as Signal_final through the compensation digital filter and the DAC device successively, and then the frequency spectrum of the final output signal is observed;
Signal_final=Signal_modu*hcomp(n)*h′comp(n)Signal_final=Signal_modu* hcomp (n)* h′comp (n)
图2至图4所示即为仿真得到的信号频谱图(数字域)。对比图2和图3,可以看出:设计的补偿数字滤波器具有与DAC器件Sinc函数衰落互补的幅频特性。图4给出了对DAC器件Sinc衰落进行补偿后的最终结果,对比图2可以看出:经过补偿数字滤波器补偿后,信号的带内波动减小到0.1dB以内。这说明设计的补偿数字滤波器可以很好地补偿Sinc函数衰落,符合预期猜想。Figure 2 to Figure 4 show the simulated signal spectrum (digital domain). Comparing Figure 2 and Figure 3, it can be seen that the designed compensation digital filter has amplitude-frequency characteristics complementary to the fading of the Sinc function of the DAC device. Figure 4 shows the final result after compensating the sinc fading of the DAC device. Compared with Figure 2, it can be seen that the in-band fluctuation of the signal is reduced to less than 0.1dB after compensation by the compensating digital filter. This shows that the designed compensation digital filter can compensate the fading of the Sinc function well, which is in line with the expected conjecture.
图5至图8是本发明具体实施例,采用本发明所述方法得到的频谱图,其中图5为成型滤波后的信号频谱图,可以看出信号具有平滑的带内平坦度;图6为信号经过DAC器件且未经补偿时的频谱图,由于DAC器件的Sinc函数衰落特性使得信号带内平坦度受到很大影响,产生了明显的非线性失真;图7所示为补偿数字滤波器对信号频谱的影响,对比图6可以看出,设计的补偿数字滤波器具有与DAC器件Sinc衰落相互补的幅频特性;图8为对DAC器件Sinc衰落进行补偿后的最终输出结果,可以看出补偿数字滤波器很好地补偿了DAC器件Sinc函数衰落所引起的信号带内非线性失真,信号带内平坦度得到了明显改善,系统性能得到提升。Fig. 5 to Fig. 8 are spectrograms obtained by adopting the method of the present invention according to specific embodiments of the present invention, wherein Fig. 5 is the spectrogram of the signal after shaping and filtering, and it can be seen that the signal has smooth in-band flatness; Fig. 6 shows The spectrum diagram of the signal when the signal passes through the DAC device without compensation, due to the fading characteristics of the Sinc function of the DAC device, the in-band flatness of the signal is greatly affected, resulting in obvious nonlinear distortion; Figure 7 shows the compensation digital filter. The influence of the signal spectrum, compared with Figure 6, it can be seen that the designed compensation digital filter has amplitude-frequency characteristics that complement the sinc fading of the DAC device; Figure 8 is the final output result after compensating the sinc fading of the DAC device, it can be seen that The compensation digital filter can well compensate the nonlinear distortion in the signal band caused by the fading of the Sinc function of the DAC device, and the in-band flatness of the signal is obviously improved, and the system performance is improved.
以上所述,仅为本发明最佳的具体实施方式,但本发明的保护范围并不局限于此,任何熟悉本技术领域的技术人员在本发明揭露的技术范围内,可轻易想到的变化或替换,都应涵盖在本发明的保护范围之内。The above is only the best specific embodiment of the present invention, but the protection scope of the present invention is not limited to this. Substitutions should be covered within the protection scope of the present invention.
本发明说明书中未作详细描述的内容属本领域技术人员的公知技术。The content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.
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