CN109474285A - A method of pretreatment DAC causes with interior uneven processing - Google Patents
A method of pretreatment DAC causes with interior uneven processing Download PDFInfo
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- CN109474285A CN109474285A CN201811556426.XA CN201811556426A CN109474285A CN 109474285 A CN109474285 A CN 109474285A CN 201811556426 A CN201811556426 A CN 201811556426A CN 109474285 A CN109474285 A CN 109474285A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/02—Transmitters
- H04B1/04—Circuits
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/02—Transmitters
- H04B1/04—Circuits
- H04B2001/0408—Circuits with power amplifiers
- H04B2001/0425—Circuits with power amplifiers with linearisation using predistortion
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Abstract
A method of pretreatment DAC causes to belong to digital communication technology field with interior uneven processing.The present invention is directed to digital-to-analogue converter (Digital-to-Analog Converters, DAC zero-order holder) is used, make presence decline intrinsic in its amplitude-frequency characteristic, lead to the problem that amplitude coincidence is very poor in bandwidth signal, it is proposed the method for the present invention, the method of the present invention carries out the compensation method of pre-distortion to solve the intrinsic presence of DAC device and decline, to reach the inband flatness for improving broadband signal using digital filter.Simulation result shows, the present invention effectively improve DAC decline cause broadband signal with interior unevenness degree problem.
Description
Technical Field
The invention relates to an in-band unevenness suppression processing technology of a broadband digital communication system, in particular to a method for carrying out in-band unevenness processing caused by DAC (digital-to-analog converter) predistortion processing on a digital filter, belonging to the technical field of digital communication.
Background
DAC (Digital-to-Analog Converters) devices are used as important component devices of a transmitter system and widely applied to the fields of mobile communication and satellite communication. Because the DAC device is implemented by using a zero-order keeper, the DAC device inherently has fading in amplitude-frequency characteristics, which results in poor amplitude consistency in a bandwidth signal, and thus, the performance of a satellite communication system is affected. In a narrow band satellite communication system, the influence of the fading is negligible, but with the development of a multi-carrier broadband satellite communication technology, the inherent fading of the DAC device has become a problem to be solved in a broadband high-order modulation satellite communication system application.
In recent years, with the development requirement of a multi-carrier broadband communication technology, device manufacturers add configurable hardware units such as interpolation, filtering and the like in a DAC, so that the Sinc function fading is effectively reduced, processing except for the DAC device is not added, and the data transmission rate between devices is not increased. However, the method of increasing the slew rate by using the digital interpolation technique only reduces the influence of Sinc fading to an acceptable level, and does not eliminate the phenomenon from the generation mechanism, and the PCB design difficulty is increased and the device power consumption is increased.
Disclosure of Invention
The technical problem solved by the invention is as follows: the method overcomes the defects of the prior art, and provides a compensation method for carrying out predistortion processing by adopting a digital filter to solve the problem of Sinc function fading of a DAC device so as to achieve the purpose of improving the in-band flatness of a broadband signal. .
The technical solution of the invention is as follows: before an input broadband signal enters a DAC device for digital-to-analog conversion, a compensation digital filter which is complementary with the amplitude characteristic of a retainer used in the DAC device to the broadband signal fading characteristic function is adopted to preprocess the input broadband signal, and then the preprocessed compensation broadband signal is output to the DAC device.
Further, the holder is a zero order holder.
Further, the amplitude characteristic of the keeper used in the DAC device to the broadband signal fading characteristic function isWherein Hzoh(f) Is a frequency characteristic response function of the zero order keeper.
Further, theWherein,for the time domain characteristic response function of the zeroth order keeper, rect (×) is a rectangular function.
Further, the time domain response of the input broadband signal passing through the compensation digital filter isWherein x (k-n) is an input broadband signal,to compensate for the digital filter coefficient normalization value.
Further, theWherein h iscompAnd (n) is the coefficient of the compensation digital filter, round (×) is a rounding function, max (×) is a maximum function, and D is the quantization digit of the DAC device.
Further, the compensation digital filter has coefficients ofWherein,inverse Fourier transform coefficient, C, for compensating frequency characteristic response of digital filter0An inverse fourier transform coefficient value when N is 0,1,., N-1, and N is a filter order, k is 1, 2.., M, and M is (N-1)/2, fsFor DAC sampling frequency, f for input broadband signal frequency, TsIs the DAC sampling period.
Compared with the prior art, the invention has the advantages that:
(1) the invention adopts the compensation digital filter, carries out digital pre-distortion treatment on the broadband signal by designing the digital filter with complementary amplitude-frequency characteristics, thereby ensuring that the final output broadband signal converted by the DAC device has good in-band flatness, and is realized by software without increasing the difficulty of PCB design;
(2) the invention adopts the compensation digital filter, designs the digital filter by a numerical method of frequency domain sampling, namely, performs advanced sampling on an inverse sinc function, and then performs inverse Fourier transform, thereby simply, conveniently and quickly obtaining the coefficient of the compensation digital filter, realizing programmability, simultaneously, not needing to increase corresponding hardware, and reducing the hardware resource overhead;
(3) the invention reduces the DAC device fading by adopting the method of compensation processing by the compensation digital filter, because the change range of the compensation digital filter coefficient is larger, the change of the broadband signal envelope caused by the coefficient change is reduced by carrying out normalization processing on the compensation digital filter coefficient, thereby reducing the signal distortion caused by the broadband signal passing through a nonlinear device (such as a power amplifier), and being convenient for FPGA realization on the other hand;
(4) the invention adopts the compensation digital filter, reduces the nonlinear influence of the DAC device on the high-order modulation signal, thereby improving the spectrum efficiency of the broadband satellite communication system.
Drawings
FIG. 1 is a block diagram of the pre-processing DAC of the present invention causing in-band non-planarity;
FIG. 2 is a graph of fading characteristics of a Sinc function of a DAC device;
FIG. 3 is a diagram of a compensated digital filter characteristic according to the present invention;
FIG. 4 is a diagram illustrating the compensation effect of the digital filter compensated by the method of the present invention;
FIG. 5 is a graph of a shaped filtered signal spectrum;
FIG. 6 is a graph of the spectrum of an uncompensated signal passing through a DAC device;
FIG. 7 illustrates the effect of a compensating digital filter on the signal spectrum in the method of the present invention;
FIG. 8 is a diagram of the final signal spectrum after compensation by the method of the present invention.
Detailed Description
A zero-order retainer is adopted for the DAC device in implementation, so that inherent fading exists in amplitude-frequency characteristics of the DAC device, amplitude consistency in a bandwidth signal is poor, and performance of a satellite communication system is affected. In a narrow band satellite communication system, the influence of the fading is negligible, but with the development of a multi-carrier broadband satellite communication technology, the inherent fading of the DAC device has become a problem to be solved in a broadband high-order modulation satellite communication system application.
In order to achieve the purpose, the technical scheme of the invention is realized as follows:
referring to fig. 1, a method for preprocessing in-band unevenness caused by DAC, which adopts a digital filter to perform a compensation method of predistortion processing to solve the inherent fading problem of DAC devices, so as to achieve a signal processing procedure for improving in-band flatness of wideband signals;
the simulation verification platform for the in-band unevenness processing method caused by the pre-processing of the DAC is composed of an input module, an FEC coding module, an interleaving module, a constellation mapping module, a forming filtering module, a compensation digital filter module and an analog DAC fading module.
The input data module is an interface for inputting data.
The FEC encoding is to correct errors of code elements, and by adding error control code elements into the sending code elements, a receiving end can find the error codes by using the code element supplementary lists and can restore the error codes to correct values.
The interleaving is a device for realizing maximum change of information structure without changing information content.
The constellation mapping is to improve transmission efficiency.
The shaping filtering is a process of making the baseband signal into a form suitable for transmission.
The compensating digital filter solves the inherent fading problem of the DAC device so as to achieve the signal processing process of improving the in-band flatness of the broadband signal.
And the DAC carries out digital-to-analog conversion on the signals subjected to the molding filtering and outputs analog lower signals.
The compensation method for the digital filter to carry out the predistortion processing comprises the following steps:
a, interweaving an input Signal, performing constellation mapping, and forming and filtering to form Signal _ modu;
step B, taking the conversion rate f of the DAC devices(fs1GHz), filter order N33;
and C, designing a compensation digital filter by adopting a frequency domain sampling method, wherein the formula is as follows:
wherein,
step D, for the obtained hcomp(n) normalizing and quantizing, wherein the result is the coefficient of the compensation digital filter;
compensating the digital filter coefficients h for the wide bandcomp(n) normalized to:
compensating the digital filter coefficients h for the wide bandcomp(n) quantified as:
where round (×) is an integer function, D is the number of quantization bits of the DAC, which are used to digitize the amplitude axis of the analog audio signal and determine the dynamic range of the digitized analog signal.
Step E, because the direct construction of the zero-order retainer model of the DAC device in the MATLAB is difficult, when the fading characteristics of the Sinc function of the DAC device are designed in a simulation mode, the frequency domain sampling method which is the same as that in the step B is adopted, and the specific formula is as follows:
step F. for the obtained h'comp(n) normalizing and quantizing, wherein the result is the coefficient of the fading of the Sinc function of the DAC device;
g, interleaving, constellation mapping, forming and filtering to form Signal _ modu, outputting the Signal _ modu as Signal _ DAC through a DAC (convolution operation), and observing the frequency spectrum of the Signal at the moment by using an MATLAB self-contained tool;
Signal_dac＝Signal_modu*h′comp(n)
step H, the method is the same as the above, the Signal _ modu Signal is output as Signal _ final after passing through the compensation digital filter and the DAC device, and then the frequency spectrum of the final output Signal is observed;
Signal_final＝Signal_modu*hcomp(n)*h′comp(n)
the zero-Order retainer ZOH (zero Order hold) model of the DAC device is as follows:
in the formula, rect () is a rectangular function. It corresponds to a frequency response of:
wherein,
in the time domain:
in the frequency domain: y' (f) ═ Y (f) xhzoh(f)。
From the above analysis it can be seen that: due to the zero-order keeper adopted in the DAC device, when the digital signal is converted into the analog signal, the frequency spectrum of the digital signal is subjected to Sinc function fading. 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 by referring to the following formula.
Wherein f is a signal frequency point, fsIs the slew rate.
The present invention is further illustrated by the following examples, but it should not be construed that the scope of the above-described subject matter is limited to the following examples. Various substitutions and alterations can be made without departing from the technical idea of the invention and the scope of the invention is covered by the present invention according to the common technical knowledge and the conventional means in the field.
The specific implementation of the invention is illustrated with reference to the accompanying drawings:
as shown in fig. 1, a method for preprocessing DAC induced in-band unevenness comprises an input module, an FEC encoding module, an interleaving module, a constellation mapping module, a shaping filtering module, a compensation digital filter module, and an analog DAC fading module.
The compensation method for the digital filter to carry out the predistortion processing comprises the following steps:
a, interweaving an input Signal, performing constellation mapping, and forming and filtering to form Signal _ modu;
step B, taking the conversion rate f of the DAC devices(fs1GHz), filter order N33;
and C, designing a compensation digital filter by adopting a frequency domain sampling method, wherein the formula is as follows:
wherein,
step D, for the obtained hcomp(n) normalizing and quantizing, wherein the result is the coefficient of the compensation digital filter;
step E, because the direct construction of the zero-order retainer model of the DAC device in the MATLAB is difficult, when the fading characteristics of the Sinc function of the DAC device are designed in a simulation mode, the frequency domain sampling method which is the same as that in the step B is adopted, and the specific formula is as follows:
step F. for the obtained h'comp(n) normalizing and quantizing, wherein the result is the coefficient of the fading of the Sinc function of the DAC device;
g, interleaving, constellation mapping, forming and filtering to form Signal _ modu, outputting the Signal _ modu as Signal _ DAC through a DAC (convolution operation), and observing the frequency spectrum of the Signal at the moment by using an MATLAB self-contained tool;
Signal_dac＝Signal_modu*h′comp(n)
step H, the method is the same as the above, the Signal _ modu Signal is output as Signal _ final after passing through the compensation digital filter and the DAC device, and then the frequency spectrum of the final output Signal is observed;
Signal_final＝Signal_modu*hcomp(n)*h′comp(n)
fig. 2 to 4 show graphs (digital domain) of simulated signal spectra. Comparing fig. 2 and 3, it can be seen that: the designed compensating digital filter has amplitude-frequency characteristics complementary to the fading of the Sinc function of the DAC device. Fig. 4 shows the final result after compensation for Sinc fading of the DAC device, and it can be seen from comparison with fig. 2 that: after compensation by the compensation digital filter, the in-band fluctuation of the signal is reduced to be within 0.1 dB. This shows that the designed compensating digital filter can compensate the Sinc function fading well, and is in line with the expected guess.
Fig. 5 to 8 are frequency spectrums obtained by the method of the present invention according to an embodiment of the present invention, wherein fig. 5 is a frequency spectrum of a shaped and filtered signal, and it can be seen that the signal has smooth in-band flatness; FIG. 6 is a frequency spectrum diagram of a signal passing through a DAC device without compensation, wherein flatness in a signal band is greatly affected due to the Sinc function fading characteristics of the DAC device, and obvious nonlinear distortion is generated; fig. 7 shows the influence of the compensation digital filter on the signal spectrum, and comparing with fig. 6, it can be seen that the designed compensation digital filter has amplitude-frequency characteristics complementary to Sinc fading of the DAC device; fig. 8 is a final output result after compensation is performed on the Sinc fading of the DAC device, and it can be seen that the compensation digital filter well compensates the nonlinear distortion in the signal band caused by the Sinc function fading of the DAC device, the flatness in the signal band is obviously improved, and the system performance is improved.
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 those matters not described in detail in the present specification are well known in the art.
Claims (7)
1. A method of pre-processing a DAC to cause in-band non-planarity, comprising: before the input broadband signal enters the DAC device for digital-to-analog conversion, the input broadband signal is preprocessed by adopting a compensation digital filter which is complementary to the amplitude characteristic of the broadband signal fading characteristic function by a retainer used in the DAC device, and then the preprocessed compensation broadband signal is output to the DAC device.
2. The method of claim 1 for preprocessing DAC caused in-band unflatness, wherein: the holder is a zero order holder.
3. A method of pre-processing a DAC to cause in-band unflatness, as claimed in claim 2, wherein: the amplitude characteristic of the keeper used in the DAC device to the broadband signal fading characteristic function isWherein Hzoh(f) Is a frequency characteristic response function of the zero order keeper.
4. A method of pre-processing a DAC to cause in-band unflatness, as claimed in claim 3, wherein: the above-mentionedWherein, for the time domain characteristic response function of the zeroth order keeper, rect (×) is a rectangular function.
5. The method of claim 4 for preprocessing DAC caused in-band unflatness, wherein: the time domain response of the input broadband signal after passing through the compensation digital filter isWherein x (k-n) is an input broadband signal,to compensate for the digital filter coefficient normalization value.
6. The method of claim 5 for preprocessing DAC caused in-band unflatness, wherein: the above-mentionedWherein h iscompAnd (n) is the coefficient of the compensation digital filter, round (×) is a rounding function, max (×) is a maximum function, and D is the quantization digit of the DAC device.
7. The method of claim 6 for preprocessing DAC caused in-band unflatness, wherein: the compensation digital filter has coefficients ofWherein,inverse Fourier transform coefficient, C, for compensating frequency characteristic response of digital filter0An inverse fourier transform coefficient value when N is 0,1,., N-1, and N is a filter order, k is 1, 2.., M, and M is (N-1)/2, fsFor DAC sampling frequency, f for input broadband signal frequency, TsIs the DAC sampling period.
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103178779A (en) * | 2011-12-21 | 2013-06-26 | 北京普源精电科技有限公司 | Signal generator with amplitude compensation function and method thereof |
| CN103414486A (en) * | 2013-08-19 | 2013-11-27 | 电子科技大学 | Zero intermediate frequency I/Q mismatch compensation circuit of 60-GHz communication system |
| CN105242242A (en) * | 2015-08-27 | 2016-01-13 | 西安空间无线电技术研究所 | Super broadband signal pre-distortion compensation method based on parameter fitting |
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103178779A (en) * | 2011-12-21 | 2013-06-26 | 北京普源精电科技有限公司 | Signal generator with amplitude compensation function and method thereof |
| CN103414486A (en) * | 2013-08-19 | 2013-11-27 | 电子科技大学 | Zero intermediate frequency I/Q mismatch compensation circuit of 60-GHz communication system |
| CN105242242A (en) * | 2015-08-27 | 2016-01-13 | 西安空间无线电技术研究所 | Super broadband signal pre-distortion compensation method based on parameter fitting |
Non-Patent Citations (1)
| Title |
|---|
| 翟盛华等: ""DAC器件Sinc函数衰落的补偿方法研究"", 《DAC器件SINC函数衰落的补偿方法研究》 * |
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Application publication date: 20190315 |