System and method for envelope tracking and adaptive digital predistortion

Abstract

The application discloses a system based on envelope tracking and adaptive digital predistortion, comprising a predistorter, a digital-to-analog converter, a hybrid envelope amplifier and a power amplifier; a baseband signal is input into the hybrid envelope amplifier after being processed by the predistorter and the digital-to-analog converter in sequence, and the drain / collector of the power amplifier is powered by the dynamic modulation amplification of the hybrid envelope amplifier. The method comprises the following steps: 1, a baseband signal is input into the hybrid envelope amplifier after being processed by the predistorter and the digital-to-analog converter in sequence; 2, the signal is output as the input voltage of the power amplifier after being dynamically modulated by the linear voltage stabilizer and the switching stage after entering the hybrid envelope amplifier; 3, the input voltage of the power amplifier is sampled in a feedback path, and the predistorter can adaptively track the change of the amplifier characteristics in real time after being processed by an adaptive algorithm. The application improves the efficiency of the power amplifier and the accuracy of the digital predistortion technology based on the lookup table, and achieves better baseband linearization amplification effect.

Description

Technical Field

[0001] This invention belongs to the field of power amplifiers for radio frequency front-end chips, and particularly relates to a system and method based on envelope tracking and adaptive digital predistortion. Background Technology

[0002] With the development of science and technology and the economy, there are higher requirements for the transmission quality and capacity of modern communication systems. In radio communication systems, high-efficiency radio frequency power amplifiers are key components. Since the power amplifier accounts for a major portion of the transmitter's power consumption, existing technologies often employ non-constant envelope modulation to maximize the amplifier's efficiency and improve the spectral and data rates of the communication system. These non-constant envelope modulation methods generally have strict linearity requirements, typically necessitating power amplifiers with high linearity to prevent signal distortion. Therefore, the power amplifier must operate in the power back-off region most of the time, resulting in its average output power being far lower than its saturation output power, thus significantly reducing the amplifier's efficiency. Summary of the Invention

[0003] Objective of this invention: The objective of this invention is to provide a system and method based on envelope tracking and adaptive digital predistortion. The digital predistortion technology significantly improves the linearity of the power amplifier, while the envelope tracking technology dynamically provides power to the amplifier, enabling it to operate approximately in the saturation region, thereby improving its average efficiency across the entire power range. Overall, this achieves high linearity and high efficiency performance for the power amplifier.

[0004] Technical Solution: The present invention relates to a system based on envelope tracking and adaptive digital predistortion, applied to a power amplifier in an RF front-end. It inputs a baseband signal and outputs a highly linear and efficient RF signal. The system is characterized by comprising a predistorter, a digital-to-analog converter (DAC), a hybrid envelope amplifier, a feedback path, and a power amplifier. The baseband signal is processed sequentially by the predistorter and DAC to obtain a predistorted signal. This predistorted signal is divided into two outputs: one input to the power amplifier to output the RF signal, and the other input to the hybrid envelope amplifier. The predistorted signal is dynamically modulated by the hybrid envelope amplifier to power the power amplifier. The feedback path samples the RF signal and uses an adaptive algorithm to enable the predistorter to adaptively track the characteristic changes of the hybrid envelope amplifier in real time.

[0005] Furthermore, the hybrid envelope amplifier includes a linear regulator, a switching converter, a sensing resistor, and a comparator; a predistortion signal is input to the linear regulator of the hybrid envelope amplifier, and the output current of the linear regulator forms a positive voltage difference across the sensing resistor. The comparator senses the positive voltage difference and outputs a control voltage. The switching converter senses the direction of the output current of the linear regulator and turns on and off, thereby controlling the magnitude of the output current from the switching converter. When VIN is input, the voltage at node X rises, and the output current of the linear amplifier flows from node X to node Y. This current forms a positive voltage difference from node X to node Y across the sensing resistor RS. The comparator senses this voltage difference and outputs a control voltage VSW at a low level. After passing through the driver, it drives the switching transistors M3 to turn on and M4 to turn off. The current output by M3 charges the large inductor Lf, causing the output current ISW of the switching stage to gradually increase and raise the voltage at node Y. When the voltage at node Y is greater than G·VIN, the excess current from ISW flows from node Y to node X and through M2 to ground. At this time, the voltage difference across the sensing resistor is reversed, the comparator outputs a high level and controls M3 to turn off and M4 to turn on. M3 stops outputting current, and the excess current on inductor Lf is released to ground through M4, thereby reducing the voltage at node Y. This process repeats, causing the voltage at node Y to constantly fluctuate around G·VIN.

[0006] Furthermore, the linear regulator includes an operational amplifier and a push-pull output stage. The predistortion signal is processed by the operational amplifier and then output through the push-pull output stage. The push-pull output is a push-pull circuit composed of two transistors or field-effect transistors.

[0007] Furthermore, the switching converter includes a driver and an output pair of transistors; the output current of the linear regulator forms a positive voltage difference across the sensing resistor. The comparator senses this voltage difference and outputs a low-level control voltage. After passing through the driver, the output pair of transistors is turned on, causing the output current of the switching converter to gradually increase. When the current exceeds the threshold, the output voltage of the hybrid envelope amplifier increases, causing the voltage across the sensing resistor to reverse. After passing through the driver, the output pair of transistors is turned off, thereby reducing the current.

[0008] This invention also discloses a method based on envelope tracking and adaptive digital predistortion systems, characterized by comprising the following steps:

[0009] Step 1: The baseband signal is processed by a predistorter and a digital-to-analog converter in sequence to obtain a predistorted signal. The predistorted signal is divided into two outputs: one is input to the power amplifier to output the RF signal, and the other is input to the hybrid envelope amplifier.

[0010] Step 2: After a predistorted signal enters the hybrid envelope amplifier, it passes through a linear regulator, a switching converter, an inductive resistor, and a comparator for dynamic modulation output to power the power amplifier;

[0011] Step 3: After sampling the RF signal through the feedback path, the predistorter is able to adaptively track the characteristic changes of the hybrid envelope amplifier in real time through an adaptive algorithm.

[0012] Furthermore, in step 1, the predistortion signal is based on the baseband signal x. in The baseband signal x is processed as follows. in Calculate the power ρ in , ρ in After quantization, a one-dimensional index value X is obtained from the LUT table. Then, the LUT table is queried based on the X value. The amplitude table and phase table of the LUT table are used to approximate the nonlinear inverse function of the power amplifier. The nonlinear inverse function compensates for distortion at its points to ensure that the overall output is linear. The amplitude table and phase table store the amplitude correction factor and phase correction factor, respectively. Adaptive algorithms are used in both tables to update the amplitude and phase values ​​of the predistortion coefficients in the lookup table. The predistortion coefficients are then multiplied by the baseband signal x. in Then, by performing an r / p conversion on the multiplication result, the output of the predistorter can be obtained.

[0013] Furthermore, the adaptive algorithm includes the LMS algorithm, the RASCAL algorithm, or the RLS algorithm.

[0014] Furthermore, the r / p conversion satisfies:

[0015] R n =βρ n +e n

[0016] ψn=θ n +u n

[0017] e n and μ n For measurement error, R n For amplitude, ψ n Let β be the phase, β be the adjustment coefficient, and the goal of linearization is...

[0018] ψ n =θ n A0 serves as the criterion for judging the magnitude of the actual adaptive algorithm output. If the output is less than this criterion, the magnitude of the original output is multiplied by a parameter. If the output is greater than this criterion, it means that the output meets the expected target and no changes are made.

[0019] Therefore, the adaptive algorithm is adopted here:

[0020]

[0021] H θ (ρn ) n+1 =H θ (ρ n ) n -β(ψ n -θn)

[0022] Where H θ and H R These represent the amplitude and phase information of the predistorter after the adaptive algorithm adjustment, respectively. The adjustment coefficients α and β are used to adjust the convergence speed and stability of the algorithm.

[0023] Output and Input ρ in The corresponding multiplicative predistortion factor δ = γexp(jσ) means that the input signal of the hybrid envelope amplifier becomes:

[0024] x pd =x in ×δ=ρ in ×exp(jθ in )×γexp(jσ)=ρ in ×γexp(j(σ+θ in ))

[0025] In the formula, X Pd X is the input signal of the mixed envelope amplifier. in Let ρ be the input baseband signal, δ be the multiplication predistortion factor, and ρ be the input baseband signal. in θ is the power of the input signal. in Let γ be the phase of the input signal, and σ be the amplitude and phase of the multiplication predistortion factor, respectively.

[0026] Furthermore, in step 2, the hybrid envelope amplifier uses hysteresis current feedback control to achieve smooth power separation between the linear stage regulator and the switching converter.

[0027] Furthermore, in step 3, the actual output of the power amplifier is sampled in the feedback path, and V... o(t) The signal is attenuated by coupling, passed through a down-conversion channel, and transformed into a digital baseband signal by an orthogonal demodulator, filter, and ADC module to obtain a feedback signal. This feedback signal is compared with the original input signal after a certain delay. An adaptive algorithm is then used to refresh the amplitude γ and phase σ of the multiplication predistortion factor δ, enabling the predistorter to adaptively track changes in amplifier characteristics in real time.

[0028] Invention Principle: The system and method based on envelope tracking and adaptive digital predistortion in this design effectively solve the problems of efficiency and linearity. By adding a predistorter at the front end of the power amplifier, whose characteristics are inversely related to the nonlinear characteristics of the power amplifier, the distortion of the nonlinear signal is compensated, achieving linear amplification. This linear optimization technique is called predistortion technology. A schematic diagram of the digital predistortion technology principle can be found below. Figure 1 As shown, digital predistortion technology, as one of the most promising technologies in power amplifier linearization, can be applied to baseband, intermediate frequency (IF), and radio frequency (RF). Currently, digital predistortion schemes mainly include two categories: lookup table (LUT) based digital predistortion technology and polynomial model based digital predistortion technology. Compared with the memory polynomial method, the lookup table method is simpler to implement and less complex, requires fewer resources, and has a faster computation speed. For memoryless power amplifiers, predistortion methods using polar coordinates or complex gain lookup tables can effectively improve the amplifier's nonlinear distortion, effectively suppress out-of-band spectral spread, and reduce adjacent channel interference. For power amplifiers with memory effects, linear optimization can be performed using a two-dimensional lookup table digital predistortion method, or a linearization method combining lookup tables and polynomials can be used. Envelope tracking technology, as a dynamic power supply modulation technique, can significantly improve the back-off efficiency of power amplifiers. The envelope amplifier is a key module in the envelope tracking power amplifier system. It provides dynamically modulated power to the power amplifier, and its efficiency directly determines the performance of the envelope tracking power amplifier system, keeping the power amplifier in a saturated state and greatly improving the efficiency of the power amplifier.

[0029] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0030] 1. Envelope tracking technology, as a dynamic power modulation technique, can significantly improve the back-off efficiency of power amplifiers. The envelope amplifier is a key module in an envelope tracking power amplifier system, providing dynamically modulated power to the power amplifier. Its efficiency directly determines the performance of the envelope tracking power amplifier system, keeping the power amplifier in saturation and greatly improving the efficiency of the power amplifier.

[0031] 2. During system operation, as the power amplifier characteristics change, an appropriate adaptive algorithm can automatically update the predistortion values ​​in the table. This allows the predistorter to adaptively track the changes in power amplifier characteristics in real time, thereby effectively improving the linearity of the power amplifier.

[0032] 3. This invention achieves a performance balance between high linearity and high efficiency in power amplifiers. Attached Figure Description

[0033] Figure 1 This is a block diagram illustrating the principle of the digital predistortion technology of the present invention.

[0034] Figure 2 This is a system block diagram of the adaptive digital predistortion technology based on radio frequency power amplifier of the present invention;

[0035] Figure 3 This is a structural diagram of the adaptive algorithm for finding table entry values ​​in the predistorter of the present invention;

[0036] Figure 4 This is a block diagram of the adaptive digital predistortion technology based on radio frequency power amplifier of the present invention;

[0037] Figure 5 The structural block diagram of the adaptive digital predistortion technology based on radio frequency power amplifier of the present invention is shown below;

[0038] Figure 6 This is a block diagram of the hybrid envelope amplifier principle of the present invention;

[0039] Figure 7 This is a schematic diagram of the hybrid envelope amplifier of the present invention. Detailed Implementation

[0040] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0041] The system disclosed in this invention, based on envelope tracking and adaptive digital predistortion, includes a predistorter, a digital-to-analog converter, a hybrid envelope amplifier, and a power amplifier. The baseband signal is processed sequentially by the predistorter and the digital-to-analog converter and then input into the hybrid envelope amplifier. The hybrid envelope amplifier dynamically modulates and amplifies the signal to supply power to the drain / collector of the power amplifier.

[0042] The workflow is as follows:

[0043] 1. First, from the baseband signal x in Calculate the power ρ in , ρ inAfter quantization, a one-dimensional index value X is obtained from the LUT table, and then the LUT table is looked up based on the X value. Two one-dimensional lookup tables (amplitude table and phase table) are used to approximate the inverse function of the amplifier's nonlinearity. The output of the predistorter is obtained by multiplying the amplitude gain by the input signal and then rotating its phase. This method is based on multiplication-based predistortion, with two one-dimensional lookup tables R and S, which store the phase correction factor and amplitude correction factor, respectively, which are combined into a complex gain. In practical applications, the index address is determined by amplitude indexing technology, and then the corresponding complex gain is looked up to predistort the input signal. Although this method requires r / p transformation (converting rectangular coordinates to polar coordinates) of the input signal of the predistorter and the output signal of the quadrature demodulation, increasing the computational complexity, the two tables can be respectively processed by simple and easy-to-control adaptive algorithms, generally recursive or gradient methods, such as the LMS algorithm, RASCAL algorithm, RLS algorithm, etc. The initial value setting of the lookup table is simple and easy to converge. The block diagram of the predistorter structure based on the lookup table design is shown in [link to block diagram]. Figure 4 , Figure 4 The content in the box can be accessed through Figure 3 The adaptive algorithm structure is shown below. The block diagram of the adaptive algorithm for looking up table entries in the predistorter is shown below. Figure 3 As shown in the figure, in order to achieve linearization, the following conditions must be met through r / p transformation (Cartesian coordinates / polar coordinates):

[0044]

[0045] ψn=θ n +u n

[0046] e n and μ n For measurement error, R n For amplitude, ψ n Let β be the phase, β be the adjustment coefficient, and the goal of linearization is...

[0047] ψ n =θ n A0 serves as the criterion for judging the magnitude of the actual adaptive algorithm output. If the output is less than this criterion, the magnitude of the original output is multiplied by a parameter. If the output is greater than this criterion, it means that the output meets the expected target and no changes are made.

[0048] Therefore, the adaptive algorithm is adopted here:

[0049]

[0050] H θ (ρ n ) n+1 =Hθ (ρ n ) n -β(ψ n -θn)

[0051] Where H θ and H R These represent the amplitude and phase information of the predistorter after the adaptive algorithm adjustment, respectively. The adjustment coefficients α and β are used to adjust the convergence speed and stability of the algorithm.

[0052] 2. Output and Input ρ in The corresponding multiplicative predistortion factor δ = γexp(jσ) means that the input signal of the hybrid envelope amplifier becomes:

[0053] x pd =x in ×δ=ρ in ×exp(jθ in )×γexp(jσ)=ρ in ×γexp(j(σ+θ in ))

[0054] In the formula, X Pd X is the input signal of the mixed envelope amplifier. in Let ρ be the input baseband signal, δ be the multiplication predistortion factor, and ρ be the input baseband signal. in θ is the power of the input signal. in Let γ be the phase of the input signal, and σ be the amplitude and phase of the multiplication predistortion factor, respectively.

[0055] 3. The baseband signal processed by the predistorter is shaped into an envelope signal, which, after digital-to-analog conversion, serves as the input signal to the power modulator. This signal is then amplified by the envelope amplifier and supplied to the drain / collector of the power amplifier. The hybrid envelope amplifier structure is as follows: Figure 7As shown, it consists of a wide-bandwidth but low-efficiency linear regulator (hereinafter referred to as the linear stage) and a high-efficiency but narrow-bandwidth switching converter (hereinafter referred to as the switching stage). The high-efficiency switching stage is connected in parallel with the linear stage, which is based on an operational amplifier. The total efficiency of the envelope amplifier is the combination of the two. This structure uses hysteresis current feedback control (HCFC) to achieve smooth power separation between the linear stage and the switching stage. The linear stage consists of an operational amplifier and a push-pull output stage to achieve linear following of the input and obtain a certain current output capability. The switching stage consists of a hysteresis comparator, a switching driver stage, and an output pair of transistors. Resistor RS is a sensing resistor, and its resistance value is much smaller than that of the load resistor RL, so that the voltage drop across it is minimized. The switching stage turns on and off by sensing the direction of the output current of the linear amplifier, always outputting DC and low-frequency components of current, while the linear stage outputs high-frequency current components that the switching stage cannot respond to quickly enough. Node X is the output terminal of the linear stage, and node Y is the output terminal of the entire envelope amplifier.

[0056] Assume the gain of the linear stage is G. The circuit's operation can be briefly described as follows: When VIN is input, the voltage at node X rises, and the output current of the linear amplifier flows from node X to node Y. This current forms a positive voltage difference from node X to node Y across the sensing resistor RS. The comparator senses this voltage difference and outputs a control voltage VSW at a low level. After passing through the driver, this voltage difference drives the switching transistors M3 to turn on and M4 to turn off. The current output by M3 charges the large inductor Lf, causing the output current ISW of the switching stage to gradually increase and raise the voltage at node Y. When the voltage at node Y is greater than G·VIN, the excess current from ISW flows from node Y to node X and through M2 to ground. At this time, the voltage difference across the sensing resistor is reversed, the comparator outputs a high level and controls M3 to turn off and M4 to turn on. M3 stops outputting current, and the excess current on inductor Lf is released to ground through M4, thus lowering the voltage at node Y. This process repeats, causing the voltage at node Y to constantly fluctuate around G·VIN.

[0057] The radio frequency (RF) signal in the RF signal path is amplified and output by a power amplifier that has undergone dynamic power modulation. The supply voltage of the power amplifier's drain / collector dynamically changes with the amplitude of the input RF signal envelope; its magnitude follows the change in signal envelope size, hence the term envelope tracking. This causes the power amplifier to receive power that varies with the output power, receiving higher power at high power levels and lower power at low power levels, thus mitigating the efficiency degradation issue in the back-off region to some extent. Time synchronization and digital predistortion algorithms are used to improve system efficiency and power utilization.

[0058] 4. The actual output of the power amplifier is sampled in the feedback path, and V... o(t) The signal is attenuated through coupling, then passes through a down-conversion channel, and is converted into a digital baseband signal mainly by a quadrature demodulator, filter, and ADC module. This results in a feedback signal, which is compared with the original input signal after a certain delay. An adaptive algorithm is then used to refresh the amplitude γ and phase σ of the multiplication predistortion factor δ. This allows the predistorter to adaptively track changes in amplifier characteristics in real time.

Claims

1. A system based on envelope tracking and adaptive digital predistortion, applied to a power amplifier in an RF front-end, which takes a baseband signal as input and outputs a highly linear and efficient RF signal, characterized in that... It includes a predistorter, a digital-to-analog converter, a hybrid envelope amplifier, a feedback path, and a power amplifier. The baseband signal is processed sequentially by the predistorter and the digital-to-analog converter to obtain a predistorted signal. The predistorted signal is divided into two outputs: one is input to the power amplifier to output the RF signal, and the other is input to the hybrid envelope amplifier. The predistorted signal is dynamically modulated by the hybrid envelope amplifier to power the power amplifier. The feedback path samples the RF signal output by the power amplifier and uses an adaptive algorithm to enable the predistorter to adaptively track the characteristic changes of the hybrid envelope amplifier in real time. Predistortion signal based on baseband signal The baseband signal is processed as follows. Calculate the power , After quantization, a one-dimensional index value X is obtained from the LUT table. Then, the LUT table is queried based on the X value. The amplitude table and phase table of the LUT table are used to approximate the nonlinear inverse function of the power amplifier. The nonlinear inverse function compensates for distortion at its points to ensure that the overall output is linear. The amplitude table and phase table store the amplitude correction factor and phase correction factor, respectively. Adaptive algorithms are used in both tables to update the amplitude and phase values ​​of the predistortion coefficients in the lookup table. The predistortion coefficients are then multiplied by the baseband signal. Then, by performing an r / p conversion on the multiplication result, the output of the predistorter can be obtained; The adaptive algorithm includes the LMS algorithm, the RASCAL algorithm, or the RLS algorithm; The r / p conversion satisfies: ； ； and To account for measurement error, For amplitude, For phase, The adjustment coefficient is used, and the goal of linearization is... , , As a criterion for judging the magnitude of the actual adaptive algorithm output, if the output is less than this standard, the magnitude of the original output is multiplied by a parameter; if the output is greater than this standard, it means that the output meets the expected target and no changes are made. Therefore, the adaptive algorithm is adopted here: ； ； in and These represent the amplitude and phase information of the predistorter after adjustment by the adaptive algorithm, respectively, and the adjustment coefficient. To adjust the convergence speed and stability of the algorithm; Output and Input Corresponding multiplicative predistortion factor = At this point, the input signal of the hybrid envelope amplifier becomes: ； In the formula, The input signal for the mixed envelope amplifier, The input baseband signal, The multiplication predistortion factor, The power of the input signal. The phase of the input signal, and These represent the amplitude and phase of the multiplicative predistortion factor, respectively.

2. The system based on envelope tracking and adaptive digital predistortion according to claim 1, characterized in that, The hybrid envelope amplifier includes a linear regulator, a switching converter, a sensing resistor, and a comparator; The predistorted signal is input to the linear regulator of the hybrid envelope amplifier. The output current of the linear regulator forms a positive voltage difference across the sensing resistor. The comparator senses the positive voltage difference and outputs a control voltage, which supplies power to the power amplifier. The switching converter senses the direction of the output current of the linear regulator and turns on and off, thereby controlling the magnitude of the output current from the switching converter.

3. The system based on envelope tracking and adaptive digital predistortion according to claim 2, characterized in that, The linear regulator includes an operational amplifier and a push-pull output stage. The predistortion signal is processed by the operational amplifier and then output through the push-pull output stage. The push-pull output is a push-pull circuit composed of two transistors or field-effect transistors.

4. The system based on envelope tracking and adaptive digital predistortion according to claim 2, characterized in that, The switching converter includes a driver and an output pair of transistors. The output current of the linear regulator forms a positive voltage difference across the sensing resistor. The comparator senses this voltage difference and outputs a low-level control voltage. After passing through the driver, the output pair of transistors is turned on, causing the output current of the switching converter to gradually increase. When the current exceeds the threshold, the output voltage of the hybrid envelope amplifier increases, causing the voltage across the sensing resistor to reverse. After passing through the driver, the output pair of transistors is turned off, thereby reducing the current.

5. A method based on the envelope tracking and adaptive digital predistortion system as described in claim 2, characterized in that, Includes the following steps: Step 1: The baseband signal is processed by a predistorter and a digital-to-analog converter in sequence to obtain a predistorted signal. The predistorted signal is divided into two outputs: one is input to the power amplifier to output the RF signal, and the other is input to the hybrid envelope amplifier. Step 2: After a predistorted signal enters the hybrid envelope amplifier, it passes through a linear regulator, a switching converter, an inductive resistor, and a comparator for dynamic modulation output to power the power amplifier; Step 3: After sampling the RF signal through the feedback path, the predistorter is able to adaptively track the characteristic changes of the hybrid envelope amplifier in real time through an adaptive algorithm.

6. The method according to claim 5, characterized in that, In step 2, the hybrid envelope amplifier uses hysteresis current feedback control to achieve smooth power separation between the linear stage regulator and the switching converter.

7. The method according to claim 5, characterized in that, In step 3, the actual output of the power amplifier is sampled in the feedback path. The signal is attenuated through coupling, passed through a down-conversion channel, and then converted into a digital baseband signal by an orthogonal demodulator, filter, and ADC module. This feedback signal is compared with the original input signal after a certain delay, and then an adaptive algorithm is used to adjust the multiplication predistortion factor. amplitude and phase The predistorter is refreshed so that it can adaptively track changes in amplifier characteristics in real time.

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