CN106877825A - Digital predistortion device and method based on band-limited simplified nonlinear filter - Google Patents

Abstract

A digital predistortion device and method based on a band-limited simplified nonlinear filter, including a digital pre-distorter based on a band-limited simplified nonlinear filter pre-distortion model and a pre-distortion model training module, and the digital pre-distorter inputs The digital baseband signal is processed by digital predistortion, and the analog radio frequency signal output by the power amplifier is coupled by an attenuation coupler, quadrature demodulated by a broadband quadrature demodulator, low-pass filter, and low-sampling frequency analog-to-digital conversion to generate an output digital baseband The signal input digital baseband signal and the output digital baseband signal are synchronized and then input to the predistorter training module. The predistorter training module uses the least squares algorithm to perform parameter training and obtains the power amplifier model parameters based on the band-limited simplified nonlinear filter function, and sends the to the digital predistorter. The present invention uses a band-limited method on the nonlinear filter function model to reduce the sampling frequency of the digital pre-distortion analog-to-digital converter, thereby reducing the difficulty and cost of digital pre-distortion.

Description

Digital predistortion device and method based on band-limited simplified nonlinear filter

technical field

The invention relates to the technical field of wireless communication, in particular to a digital predistortion device and method based on a band-limited simplified nonlinear filter.

Background technique

With the wide application of the fourth-generation mobile communication system and the in-depth research on the fifth-generation mobile communication technology, the data traffic of the communication system is increasing day by day. The modulation method leads to a higher peak-to-average ratio of the signal. The peak-to-average ratio of the broadband baseband signal has high requirements on the linearity of the RF power amplifier.

High-speed DAC and ADC will increase system power consumption and cost, which has become the bottleneck of the current digital pre-distortion technology development. How to use low-speed DAC and ADC to realize digital pre-distortion with wider signal bandwidth is also becoming a research hotspot and development trend of current digital pre-distortion technology.

In 2012, Chao Yu published "Band-Limited Volterra Series-Based Digital Predistortion for Wideband RF Power Amplifiers" on IEEE Transactions on Microwave Theory and Techniques, introducing a band-limited method to reduce the sampling rate of the ADC, and using the second-order DDR The Volterra series model is simulated as the behavior model of the power amplifier, but this method has the disadvantage that the second-order DDR model contains high-order terms and is complex.

The output of the RF power amplifier generally uses an analog band-pass filter to limit the bandwidth of the output signal to a useful frequency range to reduce the sampling rate of the ADC. However, the incomplete feedback signal generated by the filter will reduce the linearization capability of traditional digital predistortion. In order to solve this problem, band-limited digital pre-distortion is an effective and practical method, and because digital pre-distortion processes the characteristics of baseband signals, RF band-pass filters are generally replaced by low-pass filters after analog-to-digital conversion.

The nonlinear filtering model is a nonlinear transfer function model describing the nonlinear system, which is a filtering network composed of a set of linear filters and a series of nonlinear parameters. Batruni R G. proposed a simplification of the nonlinear filter function in the patent Model Based Distortion for Power Amplifiers.US, 12/218032, 2008. The simplified function has no high-order terms and is suitable for FPGA implementation. But the function itself is not suitable for dealing with the linearization of the power amplifier under the broadband signal.

Contents of the invention

Purpose of the invention: Aiming at the problem that broadband signals cause the sampling frequency of analog-to-digital converters to be too high, and the Volterra series model behavior model is complex, the present invention provides a digital predistortion device and method based on a band-limited simplified nonlinear filter, The invention utilizes a band-limited method to effectively reduce the sampling frequency of the analog-to-digital converter. At the same time, the model composed of the low-pass FIR filter and the simplified nonlinear filter function has a simple structure and low complexity, and can well compensate the complex nonlinear characteristics and memory effects of the power amplifier.

Realize above-mentioned technical problem for solving, the technical scheme that the present invention adopts is:

A digital predistortion device based on a band-limited simplified nonlinear filter, which includes: a digital predistorter, a digital-to-analog converter, a wideband quadrature modulator, a power amplifier, an analog bandpass filter, an attenuation coupler, a wideband Orthogonal demodulator, analog-to-digital converter, digital low-pass filter and predistortion model training module are sequentially connected in series to form a loop; where:

The digital predistorter performs predistortion processing on the input digital baseband input signal and the model parameters output by the predistortion model training module based on the predistortion model of the band-limited simplified nonlinear filter function, and outputs the predistortion processing to the digital-to-analog converter. Signal; the signal after pre-distortion processing is sequentially processed by a digital-to-analog converter for digital-to-analog conversion, a broadband quadrature modulator for quadrature modulation, and a power amplifier for power amplification to obtain an analog radio frequency signal;

The analog radio frequency signal is sequentially passed through the analog bandpass filter for bandpass filtering, the attenuation coupler for power coupling, the broadband quadrature demodulator for quadrature demodulation, the analog-to-digital converter for low-sampling frequency analog-to-digital conversion, and low-pass filtering After low-pass filtering by the filter, the band-limited digital baseband signal is obtained and passed to the pre-distortion model training module;

The pre-distortion model training module trains the parameters of the pre-distortion model according to the digital baseband signal and the band-limited digital baseband signal, and passes the model parameters obtained from the training to the digital predistorter;

The output signal of the attenuation coupler is the output signal of the digital predistortion device based on the simplified nonlinear filter of the band limit;

The predistortion model is:

in, Represented as the complex envelope data of the digital baseband input signal at sampling time n; Represents the band-limited complex envelope data at sampling time n after the output signal of the attenuating coupler passes through the broadband quadrature demodulator, the analog-to-digital converter and the digital low-pass filter; Represents the coefficient vector of the digital low-pass filter; M represents the memory depth of the pre-distortion model; J represents the threshold number of the pre-distortion model; K represents the order of the digital low-pass filter; β _j represents the segmentation point of the jth threshold value; c _m,j represent the parameters of the predistortion model, m∈[0,1,...,M].

The present invention also proposes a digital predistortion method based on a band-limited simplified nonlinear filter, the method comprising steps:

(1) build the digital predistortion device based on the simplified nonlinear filter of band limit as claimed in claim 1;

(2) At the current sampling time n, the digital baseband input signal is processed by the digital predistorter based on the predistortion model of the band-limited simplified nonlinear filter function Perform digital pre-distortion processing to obtain the processed signal

(3) Through the digital-to-analog converter, broadband quadrature modulator and power amplifier pair After performing digital-to-analog conversion, quadrature modulation and power amplification in sequence, an analog radio frequency signal is obtained;

(4) Perform analog band-pass filtering, power coupling, quadrature demodulation, After analog-to-digital conversion and low-pass filtering at low sampling frequency, a band-limited digital baseband signal is obtained

(5) The pre- _distortion model training module uses the least squares algorithm to process the digital baseband input signal and band-limited digital baseband signals Perform parameter training, calculate the predistortion model parameters of n+1 at the next sampling moment, and upload the calculated predistortion model parameters of n+1 to the digital predistorter;

(6) Determine whether n≥n _max is satisfied, and n _max is the preset maximum number of iterations; if n≥n _max is satisfied, then stop iteration; otherwise, set n=n+1, and return to step (2).

Further, the segmentation point values of each threshold are evenly distributed between 0 and 1, β _j =(j-1)/J.

Further, the memory depth M of the predistortion model and the threshold number J of the predistortion model satisfy: M≤3, J≤10.

Beneficial effect: compared with the prior art, the present invention has the following advantages:

1) By passing the output signal through a low-pass filter to realize the band-limiting function, it can reduce the sampling frequency of the analog-to-digital converter and reduce the difficulty of system implementation.

2) The complicated high-order operation of the Volterra series model is avoided, the number of multipliers used is reduced, and the complexity of digital signal processing is reduced.

3) Since the sampling frequency of the analog-to-digital converter is reduced by using the band-limiting method, the amount of data processing when implementing pre-distortion is also reduced, and the modeling time is reduced.

Description of drawings

Fig. 1 is the structural representation of the digital predistortion device based on the simplified nonlinear filter of band limit of the present invention;

Fig. 2 is the flowchart of digital predistortion model modeling of the present invention;

Fig. 3 is the power spectrum after digital predistortion under the sampling frequency of the analog-to-digital converter of 200 MHz and the band-limited simplified nonlinear filtering method according to the present invention.

detailed description

The present invention will be further described below in conjunction with accompanying drawing.

Fig. 1 shows the structure schematic diagram of the digital predistortion device based on the simplified nonlinear filter of band limit that the present invention proposes, comprises the digital predistorter of the predistortion model based on the simplified nonlinear filter function of band limit, analog bandpass Filter, low-pass filter, low sampling frequency analog-to-digital converter, digital-to-analog converter, wideband quadrature modulator, power amplifier, attenuation coupler, wideband quadrature demodulator and predistortion model training module, where:

The input terminal of the digital predistorter is externally connected with a digital baseband input signal and the model parameters obtained by the training of the predistortion model training module; The distortion model obtains the output signal after predistortion processing Signal The input baseband I/Q signal is generated after passing through the digital-to-analog converter in turn. The input baseband I/Q signal is quadrature modulated by a broadband quadrature modulator and then input to the power amplifier. The output power amplifier of the power amplifier outputs an analog radio frequency signal through an analog bandpass filter, Attenuation coupler power coupling and broadband quadrature demodulator quadrature demodulation into output baseband I/Q signal, output baseband I/Q signal passes through analog-to-digital converter with low sampling frequency and low-pass filter to generate band-limited digital baseband output signal.

The predistortion model training module trains the parameters of the band-limited simplified nonlinear filtering model according to the digital baseband input signal and the band-limited digital baseband output signal, and pushes the model parameters obtained from the training to the digital predistorter.

The present invention also proposes a digital predistortion method based on a band-limited simplified nonlinear filter, the method comprising steps:

(1) build the digital predistortion device based on the simplified nonlinear filter of band limit as claimed in claim 1;

(2) At the current sampling time n, the digital baseband input signal is processed by the digital predistorter based on the predistortion model of the band-limited simplified nonlinear filter function Perform digital pre-distortion processing to obtain the processed signal

(3) Through the digital-to-analog converter, broadband quadrature modulator and power amplifier pair After performing digital-to-analog conversion, quadrature modulation and power amplification in sequence, an analog radio frequency signal is obtained;

(4) Perform analog band-pass filtering, power coupling, quadrature demodulation, After analog-to-digital conversion and low-pass filtering at low sampling frequency, a band-limited digital baseband signal is obtained

(5) The pre- _distortion model training module uses the least squares algorithm to process the digital baseband input signal and band-limited digital baseband signals Perform parameter training, calculate the predistortion model parameters of n+1 at the next sampling moment, and upload the calculated predistortion model parameters of n+1 to the digital predistorter;

(6) Determine whether n≥n _max is satisfied, and n _max is the preset maximum number of iterations; if n≥n _max is satisfied, then stop iteration; otherwise, set n=n+1, and return to step (2).

The specific process of modeling the model based on the band-limited simplified nonlinear filter function in the above technical solution is shown in Figure 2, and the specific steps are as follows:

1) Obtain the input and output data of model training, usually for normalization processing;

2) Select the threshold number J, each point β _i of the threshold value, the memory depth M of the model, and the order K of the digital low-pass filter. The threshold value β _j can be evenly distributed between 0 and 1, and β _j =(j-1)/J is taken in this embodiment.

3) The unknown model parameter c _m,j is obtained by using the least square method.

The following takes an LTE-A signal with a bandwidth of 100MHz input to a 2.55GHz RF power amplifier as an example, and the input and output data of the power amplifier are collected synchronously and then normalized.

The sampling frequency of the analog-to-digital converter is 200MHz, and the parameters of the simplified nonlinear filter function are J=6, M=3, and when β _j =(j-1)/J, the simplified nonlinear filter function of the band limit is the number of the model Figure 3 shows the frequency spectrum of the predistortion signal after passing through the power amplifier. It can be seen that the linearity and memory effect of the power amplifier are still well compensated when the sampling frequency is only twice the signal bandwidth.

As can be seen from the above, the power amplifier digital pre-distortion method of the present invention applies the band-limited method to a simplified nonlinear filter model, reduces the sampling frequency of the analog-to-digital converter, and reduces the cost and difficulty of implementing digital pre-distortion; simultaneously avoids It eliminates complex modeling in models such as Volterra series, reduces the difficulty and complexity of digital signal processing, and can well compensate the complex nonlinear characteristics of power amplifiers.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications are also possible. It should be regarded as the protection scope of the present invention.

Claims (4)

1. A digital predistortion apparatus based on a band-limited simplified nonlinear filter, comprising: a loop formed by sequentially connecting a digital predistorter, a digital-to-analog converter, a broadband quadrature modulator, a power amplifier, an analog band-pass filter, an attenuation coupler, a broadband quadrature demodulator, an analog-to-digital converter, a digital low-pass filter and a predistortion model training module in series; wherein:

the digital predistorter carries out predistortion processing on an input digital baseband input signal and a model parameter output by a predistortion model training module based on a predistortion model of a band-limited simplified nonlinear filter function, and outputs a signal after the predistortion processing to a digital-to-analog converter; carrying out digital-to-analog conversion on the signal subjected to the pre-distortion treatment through a digital-to-analog converter, carrying out quadrature modulation on a broadband quadrature modulator, and carrying out power amplification on a power amplifier to obtain an analog radio frequency signal;

the analog radio frequency signal is subjected to band-pass filtering by an analog band-pass filter, power coupling by an attenuation coupler, quadrature demodulation by a broadband quadrature demodulator, analog-to-digital conversion of low sampling frequency by an analog-to-digital converter and low-pass filtering by a low-pass filter in sequence to obtain a band-limited digital baseband signal and transmit the band-limited digital baseband signal to a pre-distortion model training module;

the predistortion model training module trains parameters of a predistortion model according to the digital baseband signal and the band-limited digital baseband signal, and transmits the model parameters obtained by training to the digital predistorter;

the output signal of the attenuation coupler is the output signal of the digital predistortion device based on the band-limited simplified nonlinear filter;

the predistortion model is as follows:

$\begin{array}{c}\tilde{u}\left(n\right)=\underset{k=0}{\overset{K}{\Σ}}{f}_0\left(V\right(n-k\left)\right){\tilde{x}}_{bl}(n-k)\widetilde{\mathrm{\ω}}\left(k\right)+\mathrm{...}\\ \underset{k=0}{\overset{K}{\Σ}}{f}_M\left(V\right(n-k\left)\right){\tilde{x}}_{bl}(n-M-k)\widetilde{\mathrm{\ω}}\left(k\right)\end{array}$

$\begin{array}{c}{f}_0\left(V\right(n\left)\right)=\underset{j=1}{\overset{J}{\Σ}}{c}_{0,j}|v\left(n\right)-{\mathrm{\β}}_j|+\underset{j=1}{\overset{J}{\Σ}}{c}_{0,j}|v(n-1)-{\mathrm{\β}}_j|+\mathrm{...}\\ \underset{j=1}{\overset{J}{\Σ}}{c}_{0,j}|v(n-M)-{\mathrm{\β}}_j|\\ f_1\left(V\right(n\left)\right)=\underset{j=1}{\overset{J}{\Σ}}{c}_{1,j}|v(n-1)-{\mathrm{\β}}_j|+\underset{j=1}{\overset{J}{\Σ}}{c}_{1,j}|v(n-1-1)-{\mathrm{\β}}_j|+\mathrm{...}\\ \underset{j=1}{\overset{J}{\Σ}}{c}_{1,j}|v(n-1-M)-{\mathrm{\β}}_j|\\ \mathrm{......}\end{array}$

$\begin{array}{c}{f}_M\left(V\right(n\left)\right)=\underset{j=1}{\overset{J}{\Σ}}{c}_{M,j}|v(n-M)-{\mathrm{\β}}_j|+\underset{j=1}{\overset{J}{\Σ}}{c}_{M,j}|v(n-M-1)-{\mathrm{\β}}_j|+\mathrm{...}\\ \underset{j=1}{\overset{J}{\Σ}}{c}_{M,j}|v(n-M-M)-{\mathrm{\β}}_j|\end{array}$

$v\left(n\right)=\left|{\tilde{x}}_{bl}\left(n\right)\right|;v(n-1)=\left|{\tilde{x}}_{bl}(n-1)\right|;...;v(n-M)=\left|{\tilde{x}}_{bl}(n-M)\right|$

wherein,complex envelope data represented as a digital baseband input signal at a sampling instant n;complex envelope data representing the band limit of an output signal of the attenuation coupler at a sampling time n after passing through a broadband quadrature demodulator, an analog-to-digital converter and a digital low-pass filter;representing coefficient vectors of a digital low-pass filter, M representing a memory depth of the predistortion model, J representing a threshold number of the predistortion model, K representing an order of the digital low-pass filter, β_jA segmentation point value representing a jth threshold; c. C_m，jParameters representing the predistortion model, M ∈ [0, 1, …, M]。

2. A method for digital predistortion based on a band-limited simplified nonlinear filter, the method comprising the steps of:

(1) constructing a digital predistortion apparatus based on a band-limited simplified nonlinear filter as claimed in claim 1;

(2) at the current sampling time n, the digital baseband input signal is subjected to the predistortion model of the band-limited simplified nonlinear filter function through the digital predistorterPerforming digital pre-distortion processing to obtain processed signal

(3) Through digital-to-analog converter, wideband quadrature modulator and power amplifier pairAfter digital-to-analog conversion, quadrature modulation and power amplification are sequentially carried out, an analog radio frequency signal is obtained;

(4) analog band-pass filtering, power coupling, quadrature demodulation, analog-to-digital conversion with low sampling frequency and low-pass filtering are sequentially carried out on the analog radio-frequency signal through an analog band-pass filter, an attenuation coupler, a broadband quadrature demodulator, an analog-to-digital converter and a digital low-pass filter, and then a band-limited digital baseband signal is obtained

(5) Predistortion model training module threshold value β_jMemory depth M of model, order K of digital low-pass filter, and digital baseband input signal using least square algorithmSum band limited digital baseband signalPerforming parameter training, calculating a predistortion model parameter at the next sampling time n +1, and uploading the calculated predistortion model parameter of n +1 to a digital predistorter;

(6) judging whether n is more than or equal to n_max，n_maxIs a preset maximum iteration number; if n is more than or equal to n_maxThen stopping iteration; otherwise, let n be n +1, return to step (2).

3. According to claim 2The method for digital predistortion based on a simplified non-linear filter with limits is characterized in that the segmentation point values of the thresholds are evenly distributed between 0 and 1, β_j＝(j-1)/J。

4. The method of claim 3, wherein the memory depth M of the predistortion model and the threshold number J of the predistortion model satisfy: m is less than or equal to 3, and J is less than or equal to 10.