CN113114122B - Improved RASCAL algorithm digital predistortion design method, system and application - Google Patents
Improved RASCAL algorithm digital predistortion design method, system and application Download PDFInfo
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Abstract
The invention belongs to the technical field of satellite communication, and discloses an improved RASCAL algorithm digital predistortion design method, an improved RASCAL algorithm digital predistortion design system and application, wherein the improved RASCAL algorithm digital predistortion design method comprises the following steps: dividing the TWTA characteristic into a linear section part and a nonlinear section part, wherein the linear section part only places the derivative value of the primary function of the partition in the partition range, and the nonlinear section part places the derivative coefficient values of the secondary term and the primary term of the secondary function; when the nonlinear amplification or the pre-distortion needs to be executed, a proper derivative coefficient is selected according to the amplitude value of the current transmission signal to be multiplied by a primary term or a secondary term of a corresponding interval, so that the output amplitude of the nonlinear model can be obtained, the execution mode of the output phase is similar, and the difference is that the output phase needs to be added with the calculated phase offset. The invention can effectively pre-compensate the memoryless nonlinear distortion in satellite communication, output signals more accurately and reduce the occupancy rate of hardware LUT resources.
Description
Technical Field
The invention belongs to the technical field of satellite communication, and particularly relates to a method, a system and application for designing improved RASCAL algorithm digital predistortion.
Background
Currently, a High Throughput communication Satellite (HTS), also called a High Throughput communication Satellite, has a communication capacity several times or even ten times higher than that of a conventional communication Satellite under the condition of using the same frequency resources, and a transmission bandwidth is large.
On the satellite transponder, the signal passes through an IMUX filter, then amplified by a TWTA, and finally passed through an OMUX filter to ensure spectral integrity during user link transmission. The frequency response of the IMUX and OMUX filters is not perfect, introducing linear distortion in the form of Inter Symbol Interference (ISI), while TWTA introduces nonlinear distortion.
As shown in FIG. 8, the AM-AM and AM-PM curves measured for the TWTA used in the DVB-S2X standard. The amplitude and phase values of the output of the satellite TWTA can be tested according to the amplitude of the input signal of the TWTA, and therefore the characteristic curve of the TWTA can be obtained.
The existing memoryless nonlinear HPA predistortion mode mainly has a digital predistortion scheme based on an RASCAL and a piecewise linear interpolation method, and the predistortion method needs to connect two adjacent discrete nodes by a straight line which is an interpolation function on a corresponding segment. Wright A S et al, in Experimental performance of an adaptive digital linear power amplifier [ C ]. International Microwave Symposium digest. IEEE, used a piecewise linear interpolation method to linearly fit known TWTA characteristics to quantify their characteristic curves; zhang Jia Lei improves the piecewise linear interpolation scheme in research and implementation of digital predistortion algorithm based on memory polynomial power amplifier model, in the scheme, a method for dividing curves by segments is obtained, and the dividing mode is changed into dividing curves by points. The algorithms are stored in the FPGA in a point form after being divided, so that the complexity of hardware implementation and the resource occupancy rate are high.
Through the above analysis, the problems and defects of the prior art are as follows:
(1) The frequency response of the IMUX and OMUX filters is not perfect, introducing linear distortion in the form of intersymbol interference, while TWTA introduces nonlinear distortion.
(2) In the existing memory-free nonlinear HPA predistortion mode, the division is stored in an FPGA in a point form, so that the complexity of hardware implementation and the resource occupancy rate are high.
The difficulty in solving the above problems and defects is: the TWTA of the satellite communication scene is a satellite-borne device, most of ground nonlinear algorithms can be completed through a self-adaptive compensation structure, while the satellite communication scene cannot be self-adaptive, so that the TWTA characteristics need to be analyzed in advance, and the transmitter part needs to be subjected to predistortion treatment to complete nonlinear pre-compensation.
The significance for solving the problems and the defects is as follows: most satellite communication predistortion adopts a lookup table mode, a linear fitting method can effectively solve the problems, but the problem of resource occupation caused by accuracy is obvious, so that the algorithm complexity and the resource occupation problem can be effectively balanced by the improved RASCAL algorithm digital predistortion design method provided by the invention.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a method, a system and an application for designing improved RASCAL algorithm digital predistortion, and particularly relates to a method, a system and an application for designing improved RASCAL algorithm digital predistortion based on an FPGA.
The invention is realized in such a way that an improved RASCAL algorithm digital predistortion design method comprises the following steps:
dividing the TWTA characteristic into a linear section and a nonlinear section, wherein the linear section only places the derivative value of the primary function of the partition in the partition range, and the nonlinear section places the second-order term and the derivative coefficient value of the first-order term of the second-order function; this has the advantage that the quadratic function can better fit the non-linear behavior before and after the saturation region, with a higher accuracy than linear fitting.
When the nonlinear amplification or the pre-distortion needs to be executed, a proper derivative coefficient is selected according to the amplitude value of the current transmission signal to be multiplied by a primary term or a secondary term of a corresponding interval, so that the output amplitude of the nonlinear model can be obtained, the execution mode of the output phase is similar, and the difference is that the output phase needs to be added with the calculated phase offset. Although this step is implemented using the previous lookup table scheme, the length of the lookup table is reduced, thereby reducing the resource usage. (after modification)
Further, the improved RASCAL algorithm digital predistortion design method further comprises a process of calculating the TWTA output signal, wherein the process of calculating the TWTA output signal comprises two cases that the input signal amplitude is in a linear region and the input signal amplitude is in a nonlinear region.
Further, when the input signal amplitude is in the linear region, e.g. between a, b, i.e. a < | v in I < b, the calculation of its output signal is as follows:
(1) Extracting corresponding amplitude coefficient values w at the amplitude points a and b a ；
(2) Will w a Multiplying the amplitude of the input signal at the current moment;
(3) Extracting the corresponding phase coefficient values f at the amplitude points a and b ia ；
(4) Will f is mixed ia Corresponding to the current time to outputAdding the phases of the incoming signals;
(5) Using the formula y (n) = a (r) e j(θ+φ(r)) An output signal value is calculated.
Further, when the input signal amplitude is in the non-linear region, i.e. between e and f, i.e. e < | v in If is less than f, the calculation process of the output signal is as follows:
(1) Extracting the corresponding amplitude coefficient values w at the amplitude points e and f e1 And w e2 ；
(2) Will w e1 And w e2 Amplitude multiplication with the first and second order terms of a quadratic function;
(3) Extracting the corresponding amplitude coefficient values w at the amplitude points e and f e1 And w e2 ；
(4) Will f is ie Adding the phase of the input signal corresponding to the current moment;
(5) Using the formula y (n) = A (r) e j(θ+φ(r)) An output signal value is calculated.
Further, the TWTA model input-output principle includes:
the input signal of the power amplifier can be represented by the following formula:
x(n)＝re jθ ；
where r represents the amplitude of the signal and θ represents the phase of the signal.
After TWTA, the amplitude and phase of the output signal change due to its AM-AM and AM-PM characteristics, as shown in the following equation:
y(n)＝A(r)e j(θ+φ(r)) ；
wherein A (r) represents the AM-AM effect of TWTA and φ (r) represents the AM-PM effect of TWTA.
Further, the interpolation means that a continuous function is interpolated on the basis of discrete data such that this continuous curve passes through all given discrete data points. Interpolation is to estimate approximate values at other points according to values of the complementary continuous function at finite points, and is an important method for solving the discrete function approximation problem; the interpolation method comprises a piecewise linear interpolation method, a spline interpolation method, a Lagrange interpolation method and a Newton interpolation method;
segmentationThe linear interpolation is to connect two adjacent discrete nodes by a straight line, and the straight line is an interpolation function on a corresponding section; in which two discrete points (x) are directly connected 1 ,y 1 ) And (x) 2 ,y 2 ) The linear interpolation function between the two points is obtained, and the expression of the linear interpolation function can be easily given according to the knowledge of the quadratic function:
thus, the values of the discrete point data between these two points are approximated according to the linear function.
Another object of the present invention is to provide an improved RASCAL algorithm digital predistortion design system applying the improved RASCAL algorithm digital predistortion design method, the improved RASCAL algorithm digital predistortion design system comprising: the device comprises a predistortion module, a predistortion parameter calculation module and a TWTA module.
The predistortion module is divided into analog predistortion and digital predistortion;
the piecewise linear interpolation part is executed in MATLAB simulation software, and the calculated partial derivative functions are respectively input into the predistortion module and the TWTA module;
the characteristics of the digital predistortion module and the power amplifier are both nonlinear, the characteristics of the digital predistortion module and the power amplifier are just opposite, and the signal is subjected to regression linear amplification after passing through two nonlinear models.
It is a further object of the invention to provide a computer device comprising a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to perform the steps of:
dividing the TWTA characteristic into a linear section and a nonlinear section, wherein the linear section only places the derivative value of the primary function of the partition in the partition range, and the nonlinear section places the second-order term and the derivative coefficient value of the first-order term of the second-order function; when the nonlinear amplification or the pre-distortion needs to be executed, a proper derivative coefficient is selected according to the amplitude value of the current transmission signal to be multiplied by a primary term or a secondary term of a corresponding interval, so that the output amplitude of the nonlinear model can be obtained, the execution mode of the output phase is similar, and the difference is that the output phase needs to be added with the calculated phase offset.
It is another object of the present invention to provide a computer-readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of:
dividing the TWTA characteristic into a linear section part and a nonlinear section part, wherein the linear section part only places the derivative value of the primary function of the partition in the partition range, and the nonlinear section part places the derivative coefficient values of the secondary term and the primary term of the secondary function; when the nonlinear amplification or the pre-distortion needs to be executed, a proper derivative coefficient is selected according to the amplitude value of the current transmission signal to be multiplied by a primary term or a secondary term of a corresponding interval, so that the output amplitude of the nonlinear model can be obtained, the execution mode of the output phase is similar, and the difference is that the output phase needs to be added with the calculated phase offset.
Another object of the present invention is to provide an information data processing terminal for implementing the improved RASCAL algorithm digital predistortion design system.
By combining all the technical schemes, the invention has the advantages and positive effects that: the invention provides an improved RASCAL algorithm digital predistortion design method, which provides an improved scheme of segmented linear interpolation aiming at the complexity of realizing a predistortion algorithm by an FPGA (field programmable gate array).
The invention is oriented to DVB-S2X and other broadband satellite communication systems, and mainly researches a nonlinear pre-compensation algorithm of a satellite-borne TWTA (travelling wave tube amplifier). Specifically, the given TWTA characteristic cannot be represented by a determined function, so that TWTA model equation fitting needs to be carried out on the given AM-AM characteristic and AM-PM characteristic curve.
Compared with the traditional digital predistorter based on the RASCAL algorithm, the digital predistorter based on the improved RASCAL algorithm provided by the invention is improved in a segmentation mode, the amplitude predistortion coefficient and the phase predistortion coefficient of each amplitude section of an input signal are the same in the traditional digital predistorter based on the RASCAL algorithm, and the digital predistorter based on the improved RASCAL algorithm ensures that different input signal amplitudes have specific amplitude predistortion coefficients and phase predistortion coefficients. Compared with the prior art, the accuracy of the improved algorithm in the aspects of simulating TWTA nonlinear distortion and predistortion output is better improved, the output signal is more accurate, and the occupancy rate of hardware LUT resources is reduced.
Compared with the traditional RASCAL algorithm, the improved RASCAL algorithm provided by the invention improves the model estimation precision and reduces the occupation of LUT resources for a single carrier satellite system. Comparing the simulated data in the FPGA with the MATLAB model output, wherein the difference of the results is 10 -5 In the following, the accuracy and effectiveness of the improved method is illustrated. In addition, the method is suitable for realizing the non-adaptive predistortion algorithm of the broadband satellite communication system, and can also be applied to adaptive filter coefficient estimation in a memory nonlinear precompensation scheme.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments of the present invention will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flowchart of a digital predistortion design method for an improved RASCAL algorithm according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of a predistortion principle provided by an embodiment of the present invention.
Fig. 3 is a schematic diagram of a predistortion simulation structure provided in an embodiment of the present invention.
Fig. 4 is a schematic diagram of a predistortion principle provided by an embodiment of the present invention.
Fig. 5 is a schematic diagram of linear interpolation provided by the embodiment of the present invention.
Fig. 6 (a) is a schematic diagram of a conventional RASCAL structure provided in an embodiment of the present invention.
FIG. 6 (b) is a schematic diagram of an improved RASCAL structure provided by the embodiment of the present invention
Fig. 7 is a schematic diagram of an FPGA implementation structure according to an embodiment of the present invention.
FIG. 8 is a schematic diagram of AM-AM and AM-PM characteristic curves of the TWTA in Ka band according to the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the invention.
Aiming at the problems in the prior art, the invention provides a method, a system and an application for improving the RASCAL algorithm digital predistortion design, and the invention is described in detail below with reference to the accompanying drawings.
As shown in fig. 1, the method for designing digital predistortion of an improved RASCAL algorithm provided by the embodiment of the present invention includes the following steps:
s101, dividing the TWTA characteristic into a linear section and a nonlinear section, wherein the linear section only places the derivative value of the primary function of the section in the range of the section, and the nonlinear section places the second-order term and the derivative coefficient value of the first-order term of the second-order function.
S102, when the nonlinear amplification or the pre-distortion is required to be executed, a proper derivative coefficient is selected according to the amplitude value of the current transmission signal to be multiplied by the primary term or the secondary term of the corresponding interval, and then the output amplitude of the nonlinear model can be obtained.
S103, the output phase is performed similarly, except that the output phase needs to be added to the calculated phase offset.
And S104, after the TWTA characteristic fitting method is realized, fitting is carried out on the predistorter, and the characteristic function of the predistorter is similar to that of the TWTA, except that the AM-AM characteristic and the AM-PM characteristic are opposite, so that the operation method is the same as that of the method.
And S105, inputting the output signal of the predistorter into a TWTA module, sampling the output signal, and comparing the sampled output signal with a corresponding signal obtained by software such as MATLAB (matrix laboratory), thereby completing the verification of the correctness of the method.
The modified RASCAL algorithm digital predistortion design method provided by the present invention may be implemented by other steps, and the modified RASCAL algorithm digital predistortion design method provided by the present invention in fig. 1 is only a specific embodiment.
The technical solution of the present invention is further described with reference to the following examples.
The invention aims to provide an improved scheme of segmented linear interpolation aiming at the defects of the prior art and the complexity of realizing a predistortion algorithm by an FPGA (field programmable gate array). The strategy combines the characteristics of offline training data of a satellite environment and uses a lookup table mode to realize the predistortion scheme, so that a low-complexity predistortion compensation method based on the improved RASCAL algorithm is provided.
The design scheme of the digital predistorter based on the improved RASCAL algorithm is still consistent with the design scheme of the traditional digital predistorter based on the RASCAL algorithm on the whole, and only the improvement is made on the aspects of fitting the TWTA characteristic curve and generating and updating the predistortion coefficient.
The predistortion technology is that a nonlinear module is added at the input end of a power amplifier, and the characteristic of the module is just opposite to the characteristic of the power amplifier, so that the characteristic and the characteristic can be mutually offset. When the signal passes through the nonlinear module, the signal can be linearly amplified through the power amplifier. The principle is shown in fig. 2.
First, a predistortion simulation structure is discussed, and the present invention is divided into three parts, namely a predistortion module, a predistortion parameter calculation module and a TWTA module, as shown in fig. 3.
The predistortion technique is classified into analog predistortion and digital predistortion. Digital predistortion is the mainstream of current wireless communication base stations due to the advantages of high operability, good improvement effect and the like.
In fig. 3, the piecewise linear interpolation part will be executed in MATLAB simulation software, and the calculated partial derivative functions are input into the predistortion module and the TWTA module, respectively.
As shown in fig. 4, the characteristics of the digital predistortion module and the power amplifier are both nonlinear, and the characteristics of the digital predistortion module and the power amplifier are just opposite, and the signal returns to linear amplification after passing through two nonlinear models.
TWTA model input-output principle:
the input signal of the power amplifier can be represented by the following formula:
x(n)＝re jθ ；
where r represents the amplitude of the signal and θ represents the phase of the signal.
After TWTA, the amplitude and phase of the output signal change due to its AM-AM and AM-PM characteristics, as shown in the following equation:
y(n)＝A(r)e j(θ+φ(r)) ；
wherein A (r) represents the AM-AM effect of TWTA and φ (r) represents the AM-PM effect of TWTA.
As shown in fig. 8, when the amplitude of the input signal is small, the distortion of the signal is not large, so the AM/AM is almost a straight line, and the AM/PM is also close to 0; as the amplitude of the input signal increases, the AM/AM curve becomes significantly bowed downward, indicating that the gain is decreasing and the power is approaching saturation, while the AM/PM curve becomes further from the abscissa, indicating that the phase distortion is also increasing.
In a satellite environment, because the characteristics of a satellite transponder cannot give a definite expression quantitatively, curve plotting can be performed only by transmitting and receiving data, and the problem needs to perform characteristic fitting by means of interpolation fitting, namely a piecewise linear interpolation method which is the most common method. The improved piecewise linear interpolation theory proposed by the present invention is given below.
Interpolation means the interpolation of a continuous function on the basis of discrete data, so that this continuous curve passes through all given discrete data points. Interpolation is to estimate the approximate value at other points according to the value of the continuous function at the finite points, and is an important method for solving the approximation problem of the discrete function. There are many specific interpolation methods, such as the common piecewise linear interpolation method, spline interpolation method, lagrange interpolation method, newton interpolation method, etc., and different interpolation methods have their own advantages and disadvantages. Among the interpolation methods, piecewise linear interpolation is the simplest and widely used one. Piecewise linear interpolation is to connect two adjacent discrete nodes together by a straight line, which is an interpolation function on the corresponding segment.
As shown in FIG. 5, two discrete points (x) are directly connected 1 ,y 1 ) And (x) 2 ,y 2 ) The result is a linear interpolation function between the two points. The expression of the linear interpolation function can be easily given according to the knowledge of the quadratic function:
so that the value of discrete point data between these two points, such as point (x) in fig. 5, can be approximated from the linear function 3 ,y 3 ). Although the smoothness is poor due to different derivatives at segmentation points, the piecewise linear interpolation has good convergence and stability, is simple in algorithm and convenient to implement in practical engineering, and is widely applied.
The conventional RASCAL-based algorithm divides the amplitude of the input signal in segments, each amplitude corresponding to only one amplitude output and one phase output. As shown in FIG. 6 (a), the input signal amplitudes are respectively associated with amplitude outputs w in a to b, b to c, and c to d 1 、w 2 、w 3 . The amplitude of the input signal is within a-b, when the amplitude of the input signal is within a-b, the amplitude | v of the input signal at the moment in | multiplied by a corresponding amplitude coefficient w within the range 1 I.e. calculating the pre-compensation corresponding to the time signalCompensated or TWTA amplified. Similarly, the phase of the input signal at this time is added with the corresponding phase coefficient, and then is the pre-compensation value or the value amplified by the TWTA on the phase of the input signal. The improved RASCAL algorithm no longer uses the amplitude segment as the division unit of the amplitude and phase coefficients, but divides the TWTA characteristic into two parts, a linear piecewise part and a non-linear piecewise part, wherein the linear piecewise part only places the derivative values of the first-order function of the partition within the partition range, and the non-linear piecewise part places the second-order term of the second-order function and the derivative coefficient values of the first-order term. When the nonlinear amplification or the pre-distortion needs to be executed, a proper derivative coefficient is selected according to the amplitude value of the current transmission signal to be multiplied by a primary term or a secondary term of a corresponding interval, so that the output amplitude of the nonlinear model can be obtained, the execution mode of the output phase is similar, and the difference is that the output phase needs to be added with the calculated phase offset. As shown in FIG. 6 (b), the input signal amplitude corresponds to one or two amplitude coefficient values w at points a, b, c, d, e, and f, respectively a 、w b 、w c 、w d 、
The FPGA calculation steps for TWTA are given below:
when the input signal amplitude lies in a linear region, e.g. between a, b, i.e. a < | v in If b, the output signal is calculated as follows:
1. extracting corresponding amplitude coefficient values w at the amplitude points a and b a ；
2. Will w a Multiplying the amplitude of the input signal at the current moment;
3. extracting the corresponding phase coefficient values f at the amplitude points a and b ia ；
4. Will f is mixed ia Adding the phase of the input signal corresponding to the current moment;
5. using the formula y (n) = A (r) e j(θ+φ(r)) An output signal value is calculated.
When the input signal amplitude is in the non-linear region, i.e. between e and f, i.e. e < | v in If, | < f, the calculation process for its output signal is as follows:
1. extracting the corresponding amplitude coefficient values w at the amplitude points e and f e1 And w e2 ；
2. Will w e1 And w e2 Multiplying the first order and second order magnitudes of the quadratic function;
3. extracting the corresponding amplitude coefficient values w at the amplitude points e and f e1 And w e2 ；
4. Will f is ie Adding the phase of the input signal corresponding to the current moment;
5. using the formula y (n) = A (r) e j(θ+φ(r)) An output signal value is calculated.
The above is the process of calculating the TWTA output signal, and the predistorter output signal is the same. Compared with the traditional digital predistorter based on the RASCAL algorithm, the digital predistorter based on the improved RASCAL algorithm is improved in a segmentation mode, the amplitude predistortion coefficient and the phase predistortion coefficient of each amplitude section of an input signal are the same in the traditional digital predistorter based on the RASCAL algorithm, and the digital predistorter based on the improved RASCAL algorithm ensures that different input signal amplitudes have specific amplitude predistortion coefficients and phase predistortion coefficients. Compared with the prior art, the improved algorithm has the advantages that the precision of the TWTA nonlinear distortion and predistortion output simulation is improved better, the output signal is more accurate, and the occupancy rate of hardware LUT resources is reduced.
The structure of the FPGA based on the modified RASCAL predistortion algorithm is shown in fig. 7.
In summary, for a single carrier satellite system, the improved RASCAL algorithm improves the model estimation accuracy compared with the conventional RASCAL algorithm, and additionally reduces the occupation of LUT resources. Comparing the simulated data in the FPGA with the MATLAB model output, wherein the difference of the results is 10 -5 In the following, the accuracy and effectiveness of the improved method is illustrated. In addition, the method is suitable for realizing the non-adaptive predistortion algorithm of the broadband satellite communication system, and can also be applied to adaptive filter coefficient estimation in a memory nonlinear precompensation scheme.
In the above embodiments, all or part of the implementation may be realized by software, hardware, firmware, or any combination thereof. When used in whole or in part, can be implemented in a computer program product that includes one or more computer instructions. When loaded or executed on a computer, cause the flow or functions according to embodiments of the invention to occur, in whole or in part. The computer may be a general purpose computer, a special purpose computer, a network of computers, or other programmable device. The computer instructions may be stored in a computer readable storage medium or transmitted from one computer readable storage medium to another, for example, the computer instructions may be transmitted from one website site, computer, server, or data center to another website site, computer, server, or data center via wire (e.g., coaxial cable, fiber optic, digital Subscriber Line (DSL), or wireless (e.g., infrared, wireless, microwave, etc.)). The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device, such as a server, a data center, etc., that includes one or more of the available media. The usable medium may be a magnetic medium (e.g., floppy Disk, hard Disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid State Disk (SSD)), among others.
The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.
Claims (7)
1. An improved RASCAL algorithm digital predistortion design method, characterized in that the improved RASCAL algorithm digital predistortion design method comprises: dividing the TWTA characteristic into a linear section part and a nonlinear section part, wherein the linear section part only places the derivative value of the primary function of the partition in the partition range, and the nonlinear section part places the derivative coefficient values of the secondary term and the primary term of the secondary function; when nonlinear amplification or pre-distortion needs to be executed, selecting a proper derivative coefficient according to the amplitude value of the current transmission signal to multiply with a primary term or a secondary term of a corresponding interval, so as to obtain the output amplitude of the nonlinear model, wherein the execution mode of the output phase is similar, and the difference lies in that the output phase needs to be added with the calculated phase offset;
the improved RASCAL algorithm digital predistortion design method further comprises a process of calculating the TWTA output signal, wherein the process of calculating the TWTA output signal comprises two conditions that the amplitude of the input signal is in a linear region and the amplitude of the input signal is in a non-linear region;
when the input signal amplitude lies in a linear region, e.g. between a, b, i.e. a < | v in If b, the output signal is calculated as follows:
(1) Extracting corresponding amplitude coefficient values w at the amplitude points a and b a ；
(2) Will w a Multiplying the amplitude of the input signal at the current moment;
(3) Extracting the corresponding phase coefficient values f at the amplitude points a and b ia ；
(4) Will f is mixed ia Adding the phase of the input signal corresponding to the current moment;
(5) Using the formula y (n) = a (r) e j(θ+φ(r)) Calculating an output signal value;
when the input signal amplitude is in the non-linear region, i.e. between e and f, i.e. e < | v in If is less than f, the calculation process of the output signal is as follows:
(1) Extracting the corresponding amplitude coefficient values w at the amplitude points e and f e1 And w e2 ；
(2) Will w e1 And w e2 Amplitude multiplication with the first and second order terms of a quadratic function;
(3) Extracting the corresponding amplitude coefficient values w at the amplitude points e and f e1 And w e2 ；
(4) Will f is ie Adding the phase of the input signal corresponding to the current moment;
(5) Using the formula y (n) = A (r) e j(θ+φ(r)) An output signal value is calculated.
2. The method for improving the digital predistortion design of RASCAL algorithm according to claim 1, wherein the TWTA model input-output principle comprises:
the input signal of the power amplifier can be represented by the following formula:
x(n)＝re jθ ；
where r represents the amplitude of the signal and θ represents the phase of the signal;
after TWTA, the amplitude and phase of the output signal change due to its AM-AM and AM-PM characteristics, as shown in the following equation:
y(n)＝A(r)e j(θ+φ(r)) ；
wherein A (r) represents the AM-AM effect of TWTA and φ (r) represents the AM-PM effect of TWTA.
3. The improved RASCAL algorithm digital predistortion design method of claim 1 wherein interpolating means interpolating a continuous function on the basis of discrete data such that the continuous curve passes through all given discrete data points; interpolation is to estimate the approximate values of other points according to the values of the complementary interpolation continuous function at the finite points, and is an important method for solving the approximation problem of the discrete function; the interpolation comprises a piecewise linear interpolation method, a spline interpolation method, a Lagrange interpolation method and a Newton interpolation method;
piecewise linear interpolation is to connect two adjacent discrete nodes by a straight line, and the straight line is an interpolation function on a corresponding segment; in which two discrete points (x) are directly connected 1 ,y 1 ) And (x) 2 ,y 2 ) The linear interpolation function between the two points is obtained, and the expression of the linear interpolation function can be easily given according to the knowledge of the quadratic function:
thus, the values of the discrete point data between these two points are approximated according to a linear function.
4. An improved RASCAL algorithm digital predistortion design system implementing the improved RASCAL algorithm digital predistortion design method of any of claims 1-3, characterized in that the improved RASCAL algorithm digital predistortion design system comprises: the system comprises a predistortion module, a predistortion parameter calculation module and a TWTA module;
the predistortion module is divided into analog predistortion and digital predistortion;
the piecewise linear interpolation part is executed in MATLAB simulation software, and the calculated partial derivative functions are respectively input into a predistortion module and a TWTA module;
the characteristics of the digital predistortion module and the power amplifier are both nonlinear, the characteristics of the digital predistortion module and the power amplifier are just opposite, and the signal is subjected to regression linear amplification after passing through two nonlinear models.
5. A computer arrangement, characterized in that the computer arrangement comprises a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to carry out the method of digital predistortion design of an improved RASCAL algorithm according to any one of claims 1 to 3.
6. A computer readable storage medium storing a computer program which, when executed by a processor, causes the processor to carry out the method of improving the RASCAL algorithm digital predistortion design of any of claims 1 to 3.
7. An information data processing terminal, characterized in that the information data processing terminal is adapted to implement the improved RASCAL algorithm digital predistortion design system as claimed in claim 4.
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