CN109474285A - A method of preprocessing DAC to cause in-band unevenness - Google Patents

Abstract

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

A method of preprocessing DAC to cause in-band unevenness

technical field

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.

Background technique

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.

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.

SUMMARY OF THE INVENTION

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. .

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.

Further, the retainer is a zero-order retainer.

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.

Further, the in, Time domain characteristic response function of zero-order retainer, rect(*) is a rectangular function.

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.

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.

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.

The advantages of the present invention compared with the prior art are:

(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) 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) 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) 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.

Description of drawings

1 is a block diagram of in-band unevenness caused by a preprocessing DAC of the present invention;

Fig. 2 is the fading characteristic diagram of the Sinc function of the DAC device;

Fig. 3 is the characteristic diagram of the compensation digital filter of the method of the present invention;

Fig. 4 is the compensation effect diagram of the compensation digital filter of the method of the present invention;

Fig. 5 is the signal spectrogram after shaping filtering;

Fig. 6 is the signal spectrum diagram that passes through the DAC device and is not compensated;

Fig. 7 is the influence of the compensation digital filter in the method of the present invention on the signal spectrum;

FIG. 8 is a final signal spectrum diagram after compensation by the method of the present invention.

Detailed ways

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.

In order to achieve the above-mentioned purpose, the technical scheme of the present invention is achieved in this way:

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;

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.

The input data module is an interface for inputting data.

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.

Said interleaving is a device for realizing maximum change of information structure without changing information content.

The constellation mapping described is to improve transmission efficiency.

The shaping filtering described is the process of bringing the baseband signal into a form suitable for transmission.

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.

The DAC performs digital-to-analog conversion on the shaped and filtered signal and outputs an analog signal.

The compensation method for performing predistortion processing by the digital filter includes the following steps:

Step A. The input signal is interleaved, constellation mapping, and shaping filtering forms Signal_modu;

Step B. Take the conversion rate f _s (f _s =1GHz) of the DAC device, and the filter order N=33;

Step C. Using frequency domain sampling method to design compensation digital filter, the formula is as follows:

in,

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;

The broadband compensation digital filter coefficients h _comp (n) are normalized to:

The broadband compensation digital filter coefficients h _comp (n) are quantized as:

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.

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:

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;

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)

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)

The zero-order holder ZOH (Zero Order Hold) model of the DAC device is:

where rect() is a rectangle function. Its corresponding frequency response is:

in,

In the time domain:

In the frequency domain: Y'(f)=Y(f)×H _zoh (f).

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.

In the formula, f is the signal frequency point, and f _s is the conversion rate.

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.

Describe the specific implementation of the present invention according to the accompanying drawings:

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. .

The compensation method for performing predistortion processing by the digital filter includes the following steps:

Step A. The input signal is interleaved, constellation mapping, and shaping filtering forms Signal_modu;

Step B. Take the conversion rate f _s (f _s =1GHz) of the DAC device, and the filter order N=33;

Step C. Using frequency domain sampling method to design compensation digital filter, the formula is as follows:

in,

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;

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:

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;

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)

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)

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.

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.

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.

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.

Claims (7)

1. a kind of method that pretreatment DAC causes in-band uneven processing, it is characterized in that: before input broadband signal enters DAC device and carries out digital-to-analog conversion, adopt and the holder used in DAC device to the amplitude of broadband signal fading characteristic function The compensated digital filter with complementary characteristics preprocesses the input wideband signal, and then outputs the preprocessed compensated wideband signal to the DAC device. 2 . The method for in-band unevenness processing caused by a preprocessing DAC according to claim 1 , wherein the holder is a zero-order holder. 3 . 3. The method for in-band unevenness processing caused by a preprocessing DAC according to claim 2, wherein the amplitude characteristic of the holder used in the DAC device to the wideband signal fading characteristic function is: Wherein, H _zoh (f) is the frequency characteristic response function of the zero-order retainer. 4. The method for in-band unevenness processing caused by a preprocessing DAC according to claim 3, wherein: the in, Time domain characteristic response function of zero-order retainer, rect(*) is a rectangular function. 5. The method for in-band unevenness processing caused by a preprocessing DAC according to claim 4, wherein 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. 6. The method for in-band unevenness processing caused by a preprocessing DAC according to claim 5, wherein: 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. 7. The method for in-band unevenness processing caused by a preprocessing DAC according to claim 6, wherein 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.