CN106169985B - Envelope tracking amplifier digital predistortion method based on improved model - Google Patents

Envelope tracking amplifier digital predistortion method based on improved model Download PDF

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CN106169985B
CN106169985B CN201610757405.9A CN201610757405A CN106169985B CN 106169985 B CN106169985 B CN 106169985B CN 201610757405 A CN201610757405 A CN 201610757405A CN 106169985 B CN106169985 B CN 106169985B
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input signal
envelope
envelope tracking
power
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李德隆
邱梦婷
葛佳
俞晖
弋朝伟
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Shanghai Jiaotong University
Leadcore 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
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
    • H03F1/3241—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits
    • H03F1/3258—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits based on polynomial terms
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers

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Abstract

The invention provides an envelope tracking amplifier digital predistortion method based on an improved model, which comprises the following steps: the improved memory polynomial model is used for dynamically adjusting predistortion parameters, and input data of the predistorter is added from a single baseband signal to be a baseband signal and controlled by an envelope signal together. The method overcomes the defect that the traditional memory polynomial model predistortion method aiming at the power amplifier with fixed bias voltage is not suitable for the envelope tracking amplifier which dynamically changes the bias voltage of the power amplifier, achieves better linearization effect under the condition of the same complexity, and improves the linearity of the envelope tracking amplifier.

Description

Envelope tracking amplifier digital predistortion method based on improved model
Technical Field
The invention relates to the field of wireless communication, in particular to an envelope tracking amplifier digital predistortion method based on an improved memory polynomial model.
Background
With the continuous progress and development of wireless systems, the increase of signal bandwidth and signal peak-to-average power ratio is brought by the increase of signal rate, and in order to meet the linearity requirement of communication systems, a power amplifier needs to be operated in a power back-off region, which seriously reduces the efficiency of power amplification. In order to solve the contradiction, researchers propose that an Envelope tracking technology is combined with the current Power Amplifier, and an Envelope-tracking Power Amplifier (ETPA) can dynamically adjust the voltage of a direct current Power supply according to the Envelope of an input signal of the Power Amplifier, so that the Power Amplifier works in a close saturation region within a wide Power range, and the Power Amplifier efficiency is improved.
The technical difficulty of envelope tracking power amplification is that the variable bias voltage of the envelope tracking power amplifier enables the power amplifier to work in a saturation region more, so that the memory effect of the power amplifier is obviously enhanced, and the nonlinear distortion is serious. Aiming at the contradiction between the power amplification efficiency and the linearity, the digital predistortion technology can be combined, so that the nonlinear distortion is reduced while the power amplification efficiency of the terminal is improved. The traditional mathematical model of the static power amplifier cannot well express the characteristics of the envelope tracking amplifier, so that the predistortion device is not ideal in improving nonlinearity.
Disclosure of Invention
In view of the shortcomings in the prior art, it is an object of the present invention to provide an improved model-based digital predistortion method for an envelope tracking amplifier.
The invention provides a digital predistortion method of an envelope tracking amplifier based on an improved model, which comprises the following steps:
step A: calculating a corresponding envelope signal V of a baseband input signal x (n) of an envelope tracking amplifierenvAnd taking the baseband input signal and the adjusted envelope signal as the input of a predistorter;
and B: using input envelope signal VenvDynamically controlling memory polynomial model parameters and outputting a predistortion signal;
and C: and (3) self-adaptively obtaining parameters of the predistorter by utilizing an indirect learning method, and sending the predistorted signal to the envelope tracking amplifier to improve the linearity of the envelope tracking amplifier.
Preferably, the step a includes:
step A1: the baseband input signal x (n) of the envelope tracking amplifier is represented in complex form and the input power is calculated such that:
Pin_W＝0.5*real(Vin*conj(Vin/Rin))
converting the power signal to dBm:
Pin_dBm＝10*log(Pin_W)+30
in the formula: pin_WRepresenting the power of the baseband input signal, VinRepresenting the voltage value, R, of the baseband input signalinRepresenting the resistance value of an input link of the power amplifier, real (phi) representing the operation of taking the real part of the complex signal, conj (phi) representing the transposition operation of the complex signal, and Pin_dBmRepresents the baseband input signal power, unit: dBm;
step A2: adjusting according to the power of the input signal and the bias voltage range of the power amplifier controlled by the envelope modulator to obtain the input signal V required by the predistorterenv：
Figure BDA0001098903290000021
In the formula: pminRepresenting the power of the input signal, P, corresponding to the minimum bias voltagemaxIndicating that the maximum bias voltage corresponds to the input signal power.
Preferably, the step B includes:
step B1: the mathematical formula for a conventional memory polynomial model is given as follows:
Figure BDA0001098903290000022
step B2: step B1 is modified to be a functional form taking the envelope of the input signal as a variable, that is, the parameters of the memory polynomial model are dynamically changed, and the specific mathematical formula is as follows:
Figure BDA0001098903290000023
in the formula: y (n) represents the output signal of the power amplifier, cpmThe method comprises the steps of representing a memory polynomial model parameter, wherein n represents an nth digit digital signal, M represents a current memory depth, P represents a current polynomial order, x (n-M) represents a power amplifier input signal, the value range of a maximum polynomial order P is P ≥ 1, and the value range of a maximum memory depth M is M ≥ 1; bpmPolynomial model parameters used to represent input signal envelope calculation memory polynomial model parameters,n denotes the maximum order of the function.
Preferably, the step C includes:
step C1: the method comprises the following steps of establishing a digital predistorter by taking an improved memory polynomial mathematical model as a core, carrying out predistortion treatment on a baseband input signal x (n), and taking an obtained predistorted signal z (n) as an input signal of an envelope tracking amplifier, wherein the specific mathematical expression is as follows:
Figure BDA0001098903290000031
step C2: adaptive processing by indirect learning structure, fitting and memorizing polynomial parameter expression f (V) by polynomial modelenv) And f (V)env) Substituting into a memory polynomial mathematical model to obtain a polynomial fitting parameter b by an RLS algorithmpm。
Compared with the prior art, the invention has the following beneficial effects:
proved by a large number of computer simulations and practical experiments, the invention can better improve the linearity of the envelope tracking amplifier by the envelope tracking amplifier digital predistortion method based on the improved memory polynomial model. The digital predistortion method can effectively improve the signal linearity in the application of the envelope tracking amplifier, thereby achieving the purpose of improving the energy efficiency.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 is a block diagram of an envelope tracking amplifier digital predistortion system based on an improved memory polynomial model;
FIG. 2 is a schematic diagram of an improved memory polynomial model;
fig. 3 is a schematic diagram of the output signal power spectral density results using the improved predistorter.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that it would be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. All falling within the scope of the present invention.
The invention provides a digital predistortion method of an envelope tracking amplifier based on an improved model, which comprises the following steps:
step A: calculating a corresponding envelope signal V of a baseband input signal x (n) of an envelope tracking amplifierenvAnd taking the baseband input signal and the adjusted envelope signal as the input of a predistorter;
and B: improved upon the conventional memory polynomial model suitable for use in a fixed bias power amplifier, using an input envelope signal VenvDynamically controlling memory polynomial model parameters and outputting a predistortion signal;
and C: and self-adaptively obtaining predistortion parameters by using a brief introduction learning method, and sending the predistorted signal to the envelope tracking amplifier to improve the linearity of the envelope tracking amplifier.
Said step A calculating a corresponding envelope signal V of the baseband signal x (n)envAnd processing the envelope signal to be used as a predistorter input signal, which comprises the following steps:
step A1: the baseband input signal x (n) of the envelope tracking amplifier is expressed in complex form and the input power is calculated therefrom, such that:
Pin_W＝0.5*real(Vin*conj(Vin/Rin))
converting the power signal to dBm:
Pin_dBm＝10*log(Pin_W)+30
step A2: obtaining an input signal V required by the predistorter according to the correspondence between the input signal power and the bias voltage range of the power amplifier controlled by the envelope modulatorenv：
Figure BDA0001098903290000041
Since the bias voltage of the RF power amplifier cannot be 0 and cannot exceed the maximum voltage value that can be output by the envelope modulator, the adjustable bias voltage of the envelope tracking amplifier has a fixed range, where P isminIs the input signal power, P, corresponding to the minimum bias voltagemaxThe maximum bias voltage corresponds to the input signal power.
Step B, the traditional memory polynomial model is improved, and the method specifically comprises the following steps:
step B1: the traditional memory polynomial model is suitable for a fixed bias voltage power amplifier, and aiming at the characteristic of the variable bias voltage of the envelope tracking amplifier, the characteristic of the power amplifier cannot be well expressed, and the mathematical expression is as follows:
Figure BDA0001098903290000042
step B2: the parameters of the above traditional memory polynomial are modified into a functional form taking the envelope of the input signal as a variable, and the model parameters of the memory polynomial are dynamically changed, wherein polynomial fitting is adopted, and the mathematical expression of the polynomial fitting is as follows:
Figure BDA0001098903290000043
and step C, taking the improved memory polynomial model as a core, and adaptively adjusting the predistorter by using an indirect learning structure, wherein the step C specifically comprises the following steps:
step C1: the method comprises the following steps of taking an improved memory polynomial mathematical model as a core to build a digital predistorter, carrying out predistortion treatment on a baseband signal x (n), and taking an obtained predistorted signal z (n) as an input signal of the ETPA, wherein the mathematical expression is as follows:
Figure BDA0001098903290000051
step C2: adaptive processing by indirect learning structure, fitting and memorizing polynomial parameter expression f (V) by polynomial modelenv) Substituting the parameters into a memory polynomial mathematical model to obtain a polynomial fitting parameter b by an RLS algorithmpm。
Fig. 1 illustrates a digital predistortion system structure of an envelope tracking amplifier based on an improved memory polynomial model, and the digital predistortion method of the envelope tracking amplifier based on the improved memory polynomial model mainly comprises three levels: firstly, obtaining an envelope signal of an input signal as another input parameter of a predistorter to control a memory polynomial model parameter; secondly, constructing a predistorter based on an improved memory polynomial model according to the size of an input envelope signal; and finally, self-adaptively obtaining parameters of the predistorter by utilizing an indirect learning structure RLS algorithm and realizing the linearity optimization of the envelope tracking amplifier.
Fig. 2 shows an illustration of an improved memory polynomial model. The improved memory polynomial model aims at the characteristics of the envelope tracking amplifier, the envelope of an input signal is processed to serve as another input signal of a mathematical model, and the defect that a traditional memory polynomial model predistortion method aiming at a power amplifier with fixed bias voltage is not suitable for the envelope tracking amplifier with dynamically changed power amplifier bias voltage is overcome by combining a traditional memory polynomial model.
In the experimental process, the digital predistortion method of the envelope tracking amplifier shown in the invention is explained by using an ADS simulation circuit-based case, and fig. 3 is a power spectral density graph of an output signal under three conditions of using no predistorter, using a traditional memory polynomial model predistorter and using an improved memory polynomial predistorter. According to the method provided by the invention, an input signal in an experiment is a 10MHz bandwidth LTE signal, the improvement of the linearity of an output signal by different predistorters is judged by NMSE and ACPR, and the result is shown in Table 1. As can be seen from table 1, the predistortion system based on the improved memory polynomial model provides the best nonlinear improvement of the output signal under two different predistortion processes, particularly in that the sideband power leakage ratio (ACPR) of the output signal is improved by 5-6dB compared with the conventional memory polynomial predistorter.
TABLE 1
Figure BDA0001098903290000052
More specifically, in a preferred embodiment of the present invention, the present invention is realized by the following technical solution, and the present invention includes the following steps:
the first step is as follows: corresponding envelope signal V of baseband LTE input signal x (n)envAs an input to a predistorter;
the second step is that: constructing an improved memory polynomial model in a predistorter;
the third step: and self-adaptively obtaining parameters of the predistorter by utilizing an indirect learning structure RLS algorithm and realizing the linearity optimization of the envelope tracking amplifier.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The embodiments and features of the embodiments of the present application may be combined with each other arbitrarily without conflict.

Claims (3)

1. An improved model based digital predistortion method for an envelope tracking amplifier, comprising the steps of:
step A: calculating a corresponding envelope signal V of a baseband input signal x (n) of an envelope tracking amplifierenvAnd taking the baseband input signal and the adjusted envelope signal as the input of a predistorter;
and B: using input envelope signal VenvDynamically controlling memory polynomial model parameters and outputting a predistortion signal;
and C: self-adaptively obtaining parameters of a predistorter by utilizing an indirect learning method, and sending the predistorted signal to an envelope tracking amplifier to improve the linearity of the envelope tracking amplifier;
the step B comprises the following steps:
step B1: the mathematical formula for a conventional memory polynomial model is given as follows:
Figure FDA0002370121650000011
step B2: step B1 is modified to be a functional form taking the envelope of the input signal as a variable, that is, the parameters of the memory polynomial model are dynamically changed, and the specific mathematical formula is as follows:
Figure FDA0002370121650000012
in the formula: y (n) represents the output signal of the power amplifier, cpmThe method comprises the steps of representing a memory polynomial model parameter, wherein n represents an nth digit digital signal, M represents a current memory depth, P represents a current polynomial order, x (n-M) represents a power amplifier input signal, the value range of a maximum polynomial order P is P ≥ 1, and the value range of a maximum memory depth M is M ≥ 1; bpmPolynomial model parameters used for computing memory polynomial model parameters representing the envelope of the input signal, and N represents the maximum order of the function.
2. The improved model based digital predistortion method for an envelope tracking amplifier according to claim 1, wherein said step a comprises:
step A1: the baseband input signal x (n) of the envelope tracking amplifier is represented in complex form and the input power is calculated such that:
Pin_W＝0.5*real(Vin*conj(Vin/Rin))
converting the power signal to dBm:
Pin_dBm＝10*log(Pin_W)+30
in the formula: pin_WRepresenting the power of the baseband input signal, VinRepresenting the voltage value, R, of the baseband input signalinRepresenting the resistance value of an input link of the power amplifier, real (phi) representing the operation of taking the real part of the complex signal, conj (phi) representing the transposition operation of the complex signal, and Pin_dBmRepresents the baseband input signal power, unit: dBm;
step A2: according to input signal workAdjusting the power amplifier bias voltage range controlled by the envelope modulator to obtain the input signal V required by the predistorterenv：
Figure FDA0002370121650000021
In the formula: pminRepresenting the power of the input signal, P, corresponding to the minimum bias voltagemaxIndicating that the maximum bias voltage corresponds to the input signal power.
3. The improved model based digital predistortion method for an envelope tracking amplifier according to claim 1, wherein said step C comprises:
step C1: the method comprises the following steps of establishing a digital predistorter by taking an improved memory polynomial mathematical model as a core, carrying out predistortion treatment on a baseband input signal x (n), and taking an obtained predistorted signal z (n) as an input signal of an envelope tracking amplifier, wherein the specific mathematical expression is as follows:
Figure FDA0002370121650000022
step C2: adaptive processing by indirect learning structure, fitting and memorizing polynomial parameter expression f (V) by polynomial modelenv) And f (V)env) Substituting into a memory polynomial mathematical model to obtain a polynomial fitting parameter b by an RLS algorithmpm。
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