Digital pre-distortion nonlinear model forward synthesis method derived from circuit description

Abstract

The invention discloses a circuit description-derived forward synthesis method for a digital pre-distortion nonlinear model. The method mainly comprises the following steps of: establishing a frequency band continuous time domain expression, converting the frequency band expression to a baseband expression, converting the baseband continuous time domain expression to a baseband discrete time domain expression and the like. According to the method, the corresponding digital pre-distortion nonlinear model can be synthesized in the forward direction through the existing radio frequency power amplifier system equivalent circuit model, and an accurate model structure and initial parameter values are provided for establishment of a radio frequency power amplifier model in a power amplifier nonlinear pre-distortion technology. According to the method, priori knowledge of a radio frequency power amplifier system can be fully utilized, a nonlinear model structure most conforming to physical essence can be obtained, the minimum memory depth and nonlinear order can be directly obtained, and the method can be integrated in radio frequency power amplifier circuit simulation software.

Description

Technical Field

[0001] The invention belongs to the technical field of digital predistortion of radio frequency power amplifier systems, and in particular relates to a forward synthesis method of a digital predistortion nonlinear model derived from circuit description. Background Art

[0002] For RF power amplifier systems, high power added efficiency and high linearity are usually contradictory. In order to maximize the efficiency of RF power amplifier systems, the nonlinear problems faced by RF power amplifier systems must be solved, among which digital pre-distortion technology is one of the commonly used methods. In 5G systems, power density and operating bandwidth are further improved. At the same time, the GaN devices used also have thermal effects and trap effects, making the nonlinear problems of 5G systems more complicated. Traditional digital pre-distortion technology mainly uses the "black box model", that is, a pure mathematical model that generally describes nonlinear systems is used to model the RF power amplifier system. Due to the lack of physical prior knowledge, when faced with complex power amplifier circuit structures, the "black box model" is difficult to take into account the application requirements of high precision and low complexity.

[0003] Compared with the "black box model", nonlinear effects can also be expressed through physical models. In the circuit-level simulation process of RF power amplifier design, a large signal model containing nonlinear effects is adopted to more accurately evaluate parameters such as circuit efficiency, 1dB compression point and third-order intermodulation point. Therefore, based on the circuit description of the RF power amplifier system and using the full physical analysis process, an accurate nonlinear expression model, namely the "full physical model", can be obtained in theory. However, the current circuit-level simulation model cannot be directly used for digital pre-distortion technology.

[0004] First, the circuit-level model of the RF power amplifier is a frequency band model, while the digital pre-distortion technology requires a baseband model. Second, the circuit-level model of the RF power amplifier is a continuous time domain model, while the digital pre-distortion technology requires a discrete time domain model. Finally, the circuit-level model of the RF power amplifier is numerically solved in the form of a set of simultaneous calculus equations in the simulation, and the simulation does not require an analytical expression of the relationship between the input and output, while the digital pre-distortion technology requires an analytical expression of the relationship between the input and output. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a forward synthesis method for a digital predistortion nonlinear model derived from a circuit description.

[0006] The present invention is achieved by adopting the following technical solutions:

[0007] A method for forward synthesis of a digital predistortion nonlinear model from a circuit description comprises the following steps:

[0008] 1) Obtain an equivalent circuit model of the RF power amplifier system;

[0009] 2) Based on the equivalent circuit model, establish the frequency band continuous time domain calculus equation;

[0010] 3) simplifying and arranging the band continuous time domain calculus equation obtained in step 2) to transform it into a combination of a band continuous time domain memory linear subsystem calculus equation group and a band continuous time domain memoryless nonlinear subsystem equation group;

[0011] 4) using the harmonic balance method to convert the frequency band continuous time domain equations into baseband continuous time domain equations, including converting the frequency band continuous time domain memory linear subsystem calculus equations obtained in step 3) into baseband continuous time domain memory linear subsystem calculus equations, and converting the frequency band continuous time domain memoryless nonlinear subsystem equations obtained in step 3) into baseband continuous time domain memoryless nonlinear subsystem equations;

[0012] 5) Perform Laplace transform on the converted baseband continuous time domain memory linear subsystem calculus equations;

[0013] 6) using a bilinear z-transform to convert the Laplace transform expression obtained in step 5) into a discrete frequency domain system of equations;

[0014] 7) performing an inverse z-transform on the discrete frequency domain equations obtained in step 6) and arranging them to obtain a constant coefficient difference equation system;

[0015] 8) discretizing and arranging the baseband continuous-time domain memoryless nonlinear subsystem equations obtained in step 4) to obtain a baseband discrete-time domain memoryless nonlinear subsystem equations;

[0016] 9) Arrange the constant coefficient difference equations obtained in step 7) and the baseband discrete time domain memoryless nonlinear subsystem equations obtained in step 8) to obtain a baseband discrete time domain nonlinear model of the RF power amplifier system.

[0017] A further improvement of the present invention is that the equivalent circuit model in step 1) includes the thermal effect and trap effect of the transistor.

[0018] A further improvement of the present invention is that the transistor adopts a GaN device.

[0019] A further improvement of the present invention is that the frequency band continuous time domain calculus equation in step 2) is obtained by converting the equivalent circuit model in step 1) using Kirchhoff's law and network parameter expression.

[0020] A further improvement of the present invention is that the network parameter expression conversion is used to process the distributed parameter elements in the equivalent circuit model of step 1).

[0021] A further improvement of the present invention is that step 3) is achieved by decoupling calculus operations and nonlinear operations on the nonlinear equations in the band continuous time domain calculus equation group obtained in step 2).

[0022] A further improvement of the present invention is that the simplification process described in step 3) is achieved by selecting state variables as unknown quantities through the state space method to eliminate other unknown quantities, so as to reduce the number of unknown quantities and the number of equations in the calculus equation, thereby achieving the purpose of simplifying the expression of the equation.

[0023] A further improvement of the present invention is that step 4) converts the frequency band continuous time domain equation into the baseband continuous time domain equation by bringing the carrier modulation time domain expressions of the I and Q signals of the input signal into the equation group obtained in step 3), and then using the harmonic balance method to sort out the fundamental wave terms.

[0024] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0025] The present invention provides a forward synthesis method of a digital predistortion nonlinear model derived from a circuit description, which can forward synthesize a corresponding digital predistortion nonlinear model through an existing radio frequency power amplifier system equivalent circuit model, and can provide an accurate model structure and initial parameter values ​​for establishing a radio frequency power amplifier model in the power amplifier nonlinear predistortion technology. The present invention has the following advantages:

[0026] 1. The present invention provides a forward synthesis method of a digital predistortion nonlinear model derived from a circuit description, which can make full use of the prior knowledge of the radio frequency power amplifier system to obtain a nonlinear model structure that best conforms to the physical essence.

[0027] 2. The present invention provides a forward synthesis method of a digital predistortion nonlinear model derived from a circuit description, which can directly obtain the minimum memory depth and nonlinear order.

[0028] 3. The present invention provides a forward synthesis method of a digital predistortion nonlinear model derived from a circuit description, which can be integrated into a radio frequency power amplifier circuit simulation software. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A forward synthesis flow chart of a digital predistortion nonlinear model derived from a circuit description according to the present invention;

[0030] Figure 2 is an equivalent circuit diagram of a radio frequency power amplifier system according to an embodiment of the present invention;

[0031] Figure 3 It is a structural diagram corresponding to the nonlinear model of the radio frequency power amplifier according to an embodiment of the present invention;

[0032] Figure 4 This is a comparison diagram of the output signal spectrum calculated by the nonlinear model of an embodiment of the present invention and the output signal spectrum simulated based on commercial circuit-level simulation software. DETAILED DESCRIPTION

[0033] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to be able to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0034] The present invention provides a forward synthesis method for a digital predistortion nonlinear model based on a circuit description, and its modeling flow chart is as follows: Figure 1 shown.

[0035] Below Figure 2 The radio frequency power amplifier system shown is taken as an example to illustrate the beneficial effects of a digital predistortion nonlinear model forward synthesis method based on circuit description provided by the present invention.

[0036] Figure 2 The example of the RF power amplifier system shown in the figure uses a gate capacitor C GS and drain-source current I DS Equivalently, the bias network consists of inductors and capacitors, and the input and output matching networks only retain the coupling capacitors.

[0037] Based on a forward synthesis method of a digital predistortion nonlinear model derived from a circuit description provided by the present invention, Figure 2 The nonlinear model of the RF power amplifier system shown is extracted:

[0038] 1) Obtain the equivalent circuit model of the RF power amplifier system, such as Figure 2 shown.

[0039] 2) Establish the frequency band continuous time domain calculus equation expression.

[0040] V GS =V GQ +v GS (1)

[0041] V DS =V DQ +v DS (2)

[0042]

[0043]

[0044]

[0045]

[0046]

[0047] I DS =β n (V GS -V TH ) 2 (1+λV DS ) (8)

[0048] Where V GS and V DS are the gate-source voltage and drain-source voltage respectively; V GQ and V DQ are the gate and drain DC bias voltages, v GS and v DS are the gate and drain AC voltages, L 1 and L 2 is the inductance, i 1 and i 2 is the current, t is the time, C 1 and C 4 is the capacitance, v in and v out are the input and output AC voltages, β n is the magnification, V TH is the threshold voltage, λ is the channel length modulation coefficient, R out is the load resistance.

[0049] 3) Simplify and organize the calculus equations into a combination of the calculus equations of the linear subsystem with memory and the non-linear subsystem without memory.

[0050] Calculus equations for linear subsystems with memory:

[0051]

[0052]

[0053]

[0054]

[0055] Where: x 1 、x 2 、x 3 and x 4are the unknown state variables, and y is the output.

[0056] Memoryless nonlinear subsystem equations:

[0057] y=[-(x 2 +I DQ )-β n ((x 3 +V GQ )-V TH ) 2 (1+λ(x 4 +V DQ ))]R out (13)

[0058] Among them I DQ is the drain bias current.

[0059] 4) Use the harmonic balance method to convert the frequency band continuous time domain equation into the baseband continuous time domain equation.

[0060] Let x in =S i cos(ω c t)+S q sin(ω c t) or x in =S i cos(ω c t)-S q sin(ωc c t), where S i and S q Represent the input I and Q signals respectively, ω c is the carrier angular frequency. Substitute it into the equation in step 3) and use the harmonic balance method to extract cos(ω c t) and sin(ω c t) term to obtain the baseband continuous time domain equation:

[0061]

[0062]

[0063]

[0064] Where X in represents the input baseband signal, Y represents the baseband output signal, X 2 and X 3 is the state variable, i is the imaginary unit, and Y* represents the conjugate of Y.

[0065] 5) Perform Laplace transform on the converted baseband linear subsystem calculus equations with memory.

[0066]

[0067]

[0068] The subscript s represents the Laplace transform of the corresponding variable.

[0069] 6) Use the bilinear z transform to convert the Laplace transform expression into a discrete frequency domain equation system.

[0070] make Where T is the baseband sampling period and z is a parameter. Substitute it into the equation obtained in step 5) to get the discrete frequency domain equation group:

[0071]

[0072]

[0073] 7) Perform an inverse z-transform on the equation obtained in step 6) and sort it out to obtain a system of differential equations with constant coefficients.

[0074]

[0075]

[0076] Where n is the discrete time sequence number.

[0077] 8) The baseband continuous-time domain memoryless nonlinear subsystem equation group obtained in step 4) is discretized and sorted to obtain the baseband discrete-time domain memoryless nonlinear subsystem equation group.

[0078]

[0079] 9) Arrange the expressions obtained in step 7) and step 8) to obtain the baseband discrete time domain nonlinear model of the RF power amplifier system.

[0080]

[0081] X 21 (n) = -a 21,1 X 21 (n-1)+b y,1 Y(n-1) (25)

[0082] X 3 (n) = -a 3,1 X 3 (n-1)-a 3,2 X 3 (n-2)+[b in,0 X in(n)+b in,1 X in (n-1)+b in,2 X in (n-2)] (26)

[0083] Among them, X 21 is a state variable.

[0084] k 3 =2β n (V GQ -V TH )(1+λV DQ ) (27)

[0085]

[0086]

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093] like Figure 3 As shown, a system block diagram corresponding to the nonlinear model obtained in step 9) of this embodiment is given. Figure 4 The comparison between the output signal spectrum calculated by the nonlinear model obtained in step 9) of this embodiment and the output signal spectrum obtained based on commercial circuit-level simulation software is given. It can be seen that the results obtained by the forward synthesis method of a digital predistortion nonlinear model derived from a circuit description provided by the present invention are consistent with the circuit-level simulation results, which proves the effectiveness of the present invention. Figure 3 and Figure 4 , indicating that the forward synthesis method of the digital pre-distortion nonlinear model derived from the circuit description provided by the present invention can provide an accurate model structure and initial parameter values ​​for the establishment of the RF power amplifier model in the power amplifier nonlinear pre-distortion technology. If further optimization is performed on this basis, a high-precision and low-complexity RF power amplifier pre-distortion model can be obtained.

[0094] In summary, the present invention provides a forward synthesis method of a digital predistortion nonlinear model derived from a circuit description, which forward synthesizes a corresponding digital predistortion nonlinear model through an existing RF power amplifier system equivalent circuit model, and provides an accurate model structure and initial parameter values ​​for the establishment of a RF power amplifier model in the power amplifier nonlinear predistortion technology. The present invention can make full use of the prior knowledge of the RF power amplifier system, obtain the nonlinear model structure that best conforms to the physical essence, can directly obtain the minimum memory depth and nonlinear order, and can be integrated in the RF power amplifier circuit simulation software.

[0095] The above content is a further detailed description of the present invention in combination with specific implementation methods. It cannot be determined that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of patent protection determined by the claims submitted by the present invention.

Claims

1. A forward synthesis method for a digital predistortion nonlinear model derived from a circuit description, characterized in that: The following steps are involved: 1) Obtain an equivalent circuit model of the RF power amplifier system; 2) Based on the equivalent circuit model, establish the frequency band continuous time domain calculus equation; 3) simplifying and arranging the band continuous time domain calculus equation obtained in step 2) to transform it into a combination of a band continuous time domain memory linear subsystem calculus equation group and a band continuous time domain memoryless nonlinear subsystem equation group; 4) using the harmonic balance method to convert the frequency band continuous time domain equations into baseband continuous time domain equations, including converting the frequency band continuous time domain memory linear subsystem calculus equations obtained in step 3) into baseband continuous time domain memory linear subsystem calculus equations, and converting the frequency band continuous time domain memoryless nonlinear subsystem equations obtained in step 3) into baseband continuous time domain memoryless nonlinear subsystem equations; 5) Perform Laplace transform on the converted baseband continuous time domain memory linear subsystem calculus equations; 6) using a bilinear z-transform to convert the Laplace transform expression obtained in step 5) into a discrete frequency domain system of equations; 7) performing an inverse z-transform on the discrete frequency domain equations obtained in step 6) and arranging them to obtain a constant coefficient difference equation system; 8) discretizing and arranging the baseband continuous-time domain memoryless nonlinear subsystem equations obtained in step 4) to obtain a baseband discrete-time domain memoryless nonlinear subsystem equations; 9) Arrange the constant coefficient difference equations obtained in step 7) and the baseband discrete time domain memoryless nonlinear subsystem equations obtained in step 8) to obtain a baseband discrete time domain nonlinear model of the RF power amplifier system.

2. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 1, characterized in that: Step 1) The equivalent circuit model includes the thermal effect and trap effect of the transistor.

3. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 2, characterized in that: The transistor uses GaN devices.

4. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 1, characterized in that: Step 2) The band continuous time domain calculus equation is obtained by converting the equivalent circuit model in step 1) using Kirchhoff's law and network parameter expression.

5. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 4, characterized in that: The network parameter expression conversion is used to process the distributed parameter elements in the equivalent circuit model in step 1).

6. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 1, characterized in that: Step 3) is achieved by decoupling the calculus operation and the nonlinear operation of the nonlinear equations in the band continuous time domain calculus equation group obtained in step 2).

7. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 1, characterized in that: The simplification process described in step 3) is achieved by selecting state variables as unknown quantities through the state space method to eliminate other unknown quantities, so as to reduce the number of unknown quantities and the number of equations in the calculus equation, thereby achieving the purpose of simplifying the expression of the equation.

8. The method for forward synthesis of a digital predistortion nonlinear model from circuit description according to claim 1, characterized in that: Step 4) converting the frequency band continuous time domain equation into the baseband continuous time domain equation is obtained by substituting the carrier modulation time domain expressions of the I and Q signals of the input signal into the equation group obtained in step 3), and then using the harmonic balance method to sort out the fundamental wave terms.

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