JP2017163276A - Transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transmitter that has satisfactory correction accuracy and, at the same time, is able to reduce a circuit scale.SOLUTION: A transmitter according to an embodiment comprises: a look-up table storing a distortion correction coefficient that corrects distortion of a power amplifier; a pre-distortion part that corrects distortion of a power amplifier by multiplying an input signal by a distortion correction coefficient read from a lookup table; and a loop back signal from a power amplifier. This transmitter further comprises a first counter and a control part. The first counter counts the number of times that a loop back signal from a power amplifier exceeds a predetermined threshold level. On the basis of the number of times counted by the first counter, the control part alters a reference position for the look-up table.SELECTED DRAWING: Figure 1

Description

  Embodiments relate to a transmitter.

  Conventionally, a transmitter for performing wireless communication includes an amplifier such as a power amplifier that amplifies a transmission signal. A power amplifier used for such communication generates nonlinear distortion in a signal waveform when a transmission signal is amplified. There is a digital predistortion method as a distortion compensation method for compensating for nonlinear distortion generated in a power amplifier. This digital predistortion method includes a fixed lookup table method and an adaptive digital predistortion method using a loopback signal from a power amplifier.

Japanese Patent No. 4801074

  The problem of the embodiment is to provide a transmitter that has good correction accuracy and can simultaneously reduce the circuit scale.

  The transmitter according to the embodiment corrects power amplifier distortion by multiplying an input signal by a distortion correction coefficient read out from the lookup table and a lookup table storing a distortion correction coefficient for correcting distortion of the power amplifier. A transmitter having a predistortion unit that performs a loopback signal from a power amplifier includes a first counter and a control unit. The first counter counts the number of times that the loopback signal from the power amplifier exceeds a predetermined threshold level. The control unit changes the reference position of the lookup table based on the number of times counted by the first counter.

It is a figure which shows the structure of the transmitter which concerns on 1st Embodiment. It is a figure for demonstrating the distortion which generate | occur | produces with a power amplifier. It is a figure which shows an example of the signal waveform of an input signal and a loopback signal. It is a figure for demonstrating the determination process of 1st Embodiment. It is a flowchart for demonstrating an example of the flow of the control process of the shift value which concerns on 1st Embodiment. It is a figure which shows the structure of the transmitter which concerns on 2nd Embodiment. It is a figure which shows an example of the signal waveform of a loopback signal. It is a figure for demonstrating the determination process of 2nd Embodiment. It is a flowchart for demonstrating an example of the flow of the control process of the shift value which concerns on 2nd Embodiment. It is a figure which shows the structure of the transmitter which concerns on 3rd Embodiment. It is a figure which shows the structure of the transmitter which concerns on 4th Embodiment.

  Hereinafter, embodiments will be described in detail with reference to the drawings.

  In the following embodiments, the inverse characteristic of the nonlinearity of the power amplifier is given to the input signal, and distortion compensation of a digital predistortion method is performed to obtain a signal with reduced nonlinearity as an output signal output from the power amplifier, A transmitter that compensates for non-linear distortion generated in a power amplifier will be described.

  As the digital predistortion method, there is a fixed lookup table method in which the distortion correction coefficient of the lookup table is a fixed value. In the fixed lookup table method, address shift can be performed uniformly in consideration of process variations, temperature characteristics, voltage characteristics, and the like.

  However, the fixed look-up table method that corrects non-linear distortion of the power amplifier using a fixed look-up table and uniform address control can reduce the circuit scale, but the power amplifier's non-linearity due to changes in temperature, voltage, etc. There is a problem that the correction accuracy is lowered when the distortion of the characteristic changes.

  As a digital predistortion method, there is an adaptive digital predistortion method that estimates a nonlinear distortion of a power amplifier using a loopback signal from the power amplifier and updates a distortion correction coefficient of a lookup table. In such an adaptive digital predistortion method, an input signal without distortion and a feedback signal having power amplifier distortion obtained by feedback are captured for a certain period, and the captured data are compared. As a result, the distortion of the power amplifier is estimated and the distortion correction coefficient in the lookup table is updated to improve the correction accuracy.

  However, in the adaptive digital predistortion method, the correction accuracy of the nonlinear distortion can be increased by performing the loopback control, but the distortion estimation unit for estimating the timing adjustment and the distortion for performing the loopback control. Such a circuit is required. In particular, a delay adjustment circuit that performs timing adjustment needs to accurately match the timing of an input signal and a loopback signal, so that a matched filter, an FFT circuit, and the like are required, resulting in an increase in circuit scale. Therefore, the adaptive digital predistortion method has a problem that the scale of hardware increases.

Therefore, in the following embodiments, a transmitter that has a higher correction accuracy than the fixed lookup table method and can have a smaller circuit scale than the adaptive digital predistortion method will be described.
(First embodiment)
First, the configuration of the transmitter according to the first embodiment will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of a transmitter according to the first embodiment.

  The transmitter 1 of the embodiment includes a predistortion unit 11, a D / A converter 12, an orthogonal modulation unit 13, a power amplifier 14, an antenna 15, an arithmetic circuit 16, a delay adjustment circuit 17, and a comparator. 18, a counter 19, an orthogonal demodulator 20, an A / D converter 21, an average power calculator 22, a gain adjuster 23, an arithmetic circuit 24, a comparator 25, a counter 26, and a difference circuit 27, a shift value control unit 28, an adder circuit 29, and a lookup table 30.

  The transmitter 1 is supplied with an input signal (transmission signal) of complex data composed of an I value and a Q value. This input signal is supplied to the predistortion unit 11 and the arithmetic circuit 16. This input signal is a digital signal in which serial digital data is alternately distributed bit by bit by a serial / parallel converter, for example, and converted into an in-phase component (I value) and a quadrature component (Q value).

  The predistortion unit 11 generates a signal obtained by multiplying the input transmission signal by the distortion correction coefficient read from the lookup table 30, and outputs the signal to the D / A converter 12. That is, the predistortion unit 11 gives in advance an inverse characteristic of distortion generated in the power amplifier 14 to the input signal by digital signal processing.

  Here, the distortion generated in the power amplifier 14 will be described. FIG. 2 is a diagram for explaining the distortion generated in the power amplifier.

  The distortion of the nonlinearity of the power amplifier 14 is known in advance as to what kind of distortion occurs due to process variations and the like. In general, as shown in FIG. 2, in the power amplifier 14, as the amplitude of the input signal (symbol 40) increases, distortion of nonlinearity (symbol 42) in which the amplitude of the output signal (symbol 41) decreases. To do. Therefore, in the look-up table 30, the distortion inverse characteristic (reference numeral 43) of the power amplifier 14 is such that the distortion correction coefficient is 1 when the amplitude is small and the distortion correction coefficient is greater than 1 when the amplitude is large. A correction coefficient is stored. In the predistortion unit 11, the high linearity of the power amplifier 14 is realized by multiplying the input signal by such an inverse characteristic.

  Returning to FIG. 1, the D / A converter 12 converts the signal output from the predistortion unit 11 from a digital signal to an analog signal and outputs the analog signal to the quadrature modulation unit 13. The quadrature modulation unit 13 performs quadrature modulation on the signal output from the D / A converter 12 and outputs an input signal subjected to quadrature modulation to the power amplifier 14. More specifically, the quadrature modulation unit 13 multiplies the I-value input signal output from the D / A converter 12 by the reference carrier, and shifts the reference carrier by 90 ° in phase to the Q-value input signal. And adding these two multiplication results to obtain an orthogonally modulated input signal.

  The power amplifier 14 amplifies the input signal from the quadrature modulation unit 13 and sends it out via the antenna 15. The input signal amplified by the power amplifier 14 is input to the quadrature demodulator 20 as a loopback signal.

  Here, the control of the shift value of the lookup table 30 will be described with reference to FIGS. FIG. 3 is a diagram illustrating an example of signal waveforms of the input signal and the loopback signal, and FIG. 4 is a diagram for describing the determination processing according to the first embodiment.

  The arithmetic circuit 16 calculates the square of the absolute value of the input signal that has been input, and obtains the power amount of the input signal. Specifically, the arithmetic circuit 16 obtains the coordinates of the polar coordinate system from the input signals of the I value and the Q value. The arithmetic circuit 16 obtains the amplitude of the input signal by calculating the absolute value of the distance from the origin of the coordinates. The arithmetic circuit 16 obtains the electric energy (power value) by squaring the obtained amplitude. This electric energy is input to the adder circuit 29 as an address for reading the distortion correction coefficient of the lookup table 30. Further, this electric energy is input to the comparator 18. The delay adjustment circuit 17 will be described later.

  As shown in FIG. 3, a predetermined threshold level TH is set in the comparator 18 (second comparison unit), and the amplitude of the input signal (the amount of power) indicated by reference numeral 40 exceeds the threshold level TH. It is determined whether or not it is, and the determination result is output to the counter 19.

  The counter 19 (second counter) counts the number of times that the input signal exceeds the threshold level TH during a predetermined time based on the determination result from the comparator 18, and counts the number (for example, X times). Is output to the difference circuit 27.

  On the other hand, the quadrature demodulator 20 performs quadrature demodulation on the loopback signal output from the power amplifier 14 and outputs it to the A / D converter 21. The A / D converter 21 converts the loopback signal quadrature demodulated by the quadrature demodulator 20 from an analog signal to a digital signal, and outputs it to the average power calculator 22 and the gain adjuster 23.

  The average power calculation unit 22 calculates the average power of the input loopback signal and outputs the average power information to the gain adjustment unit 23. Average power information of a known input signal (transmission signal) is input to the gain adjustment unit 23. The gain adjustment unit 23 calculates a difference between the average power information of the loopback signal from the average power calculation unit 22 and the average power information of a known input signal (transmission signal). Then, the gain adjusting unit 23 adjusts the gain of the loopback signal from the A / D converter 21 according to the difference between the two average power information, and outputs it to the arithmetic circuit 24. As a result, processing is performed to make the average power of the loopback signal equal to the average power of the input signal.

  The arithmetic circuit 24 calculates the absolute square of the gain-adjusted loopback signal, calculates the electric energy of the loopback signal, and outputs it to the comparator 25. As shown in FIG. 3, the comparator 25 (first comparison unit) is set with the same predetermined threshold level TH as that of the comparator 18, and the amplitude of the loopback signal (the amount of power) indicated by reference numeral 41. Determines whether or not the threshold level TH exceeds the threshold level TH, and the determination result is output to the counter 26.

  Based on the determination result from the comparator 25, the counter 26 (first counter) counts the number of times that the loopback signal exceeds the threshold level TH during a predetermined time, and counts the number (for example, Y times). ) Is output to the difference circuit 27.

  Thus, the difference circuit 27 receives the number of times that the input signal exceeds the threshold level TH during a predetermined time and the number of times that the loopback signal exceeds the threshold level TH during a predetermined time. Is done. The input signal may be delayed by the delay adjustment circuit 17 as a delay adjustment unit so that the phases of the input signal and the loopback signal are substantially the same. Thereby, the predetermined time counted by the counters 19 and 26 can be shortened. Even in this case, since it is not necessary to completely match the phases of the input signal and the loopback signal, timing adjustment is easy, and the circuit scale of the delay adjustment circuit 17 does not increase.

  The difference circuit 27 receives the number of times that the input signal from the counter 19 exceeds the threshold level TH (X times) and the number of times that the distorted loopback signal from the counter 26 exceeds the threshold level TH (Y times). Is done. The difference circuit 27 calculates the difference between these numbers, here, XY times, and outputs it to the shift value control unit 28.

  If the difference value (XY times) calculated by the difference circuit 27 is within the reference range, the shift value control unit 28 ends the process without adjusting the shift value of the lookup table 30. Specifically, as shown in FIG. 4, when the difference value (XY times) calculated by the difference circuit 27 is between −A times and A times, the shift value control unit 28 falls within the reference range. judge.

  On the other hand, when the difference value (XY times) calculated by the difference circuit 27 is outside the reference range, the shift value control unit 28 looks up the look-up table 30 so that the difference value (XY times) falls within the reference range. Adjust the shift value. Specifically, as shown in FIG. 4, when the difference value (XY times) calculated by the difference circuit 27 is less than -A times or more than A times, the shift value control unit 28 determines the reference value as shown in FIG. Judged out of range.

  At this time, the shift value control unit 28 adjusts the shift value by a value proportional to the magnitude of the difference value (XY times). Note that the shift value control unit 28 may adjust the shift value by a certain value (predetermined value) when the difference value is outside the reference range. The shift value control unit 28 outputs the shift value determined in this way to the adder circuit 29.

  The adder circuit 29 outputs an address obtained by adding the shift value output from the shift value control unit 28 to the address output from the arithmetic circuit 16 to the lookup table 30. The lookup table 30 reads out the distortion correction coefficient in accordance with the address output from the adder circuit 29 and outputs it to the predistortion unit 11. Thereby, the predistortion unit 11 can generate a signal obtained by multiplying the input signal by the distortion correction coefficient read from the lookup table 30.

  Next, the shift value control processing of the transmitter configured as described above will be described. FIG. 5 is a flowchart for explaining an example of the flow of the shift value control process according to the first embodiment.

  First, the counter 19 counts the number of input signals exceeding the threshold level TH during a predetermined time (step S1). Here, the number of input signals exceeding the threshold level TH is X times. Next, the counter 26 counts the number of times that the loopback signal exceeds the threshold level TH during a predetermined time (step S2). Here, the number of times that the loopback signal exceeds the threshold level TH is Y times.

  Next, the shift value control unit 28 determines whether or not XY times are within the reference range (step S3). If the shift value control unit 28 determines that XY times are within the reference range (S3: YES), the process ends. On the other hand, when the shift value control unit 28 determines that XY times are not within the reference range (S3: NO), the shift value control unit 28 looks up to a predetermined value or a value proportional to the size of XY times. The shift value of the table is adjusted (step S4), and the process returns to step S1 and the same processing is repeated. And the shift value control part 28 will complete | finish a process, if XY times enter in the reference | standard range (step S3: YES).

  In the process of FIG. 5, the process is ended when XY times are within the reference range. However, for example, even when XY times are within the reference range, the process returns to step S1 and the same process is repeated. You may do it. Thereby, even when the XY times are out of the reference range due to a temperature change or the like, the shift value of the lookup table 30 can be immediately changed so that the XY times are within the reference range.

  Further, when there is a predetermined change such as a temperature change or a voltage change, the process of FIG. 5 may be performed. For example, a temperature sensor or the like is provided, and when it is detected that the temperature has changed by a predetermined value or more, the process of FIG. 5 is performed.

  As described above, the transmitter 1 counts the number of times that the input signal and the loopback signal exceed the threshold level TH during a predetermined time, and determines the optimum value of the look-up table 30 according to the difference between the numbers. The reference position is determined. Therefore, the transmitter 1 does not require a high-performance delay adjustment circuit unlike the conventional applied digital predistortion method that updates the distortion correction coefficient of the lookup table, and is higher than the conventional applied digital predistortion method. The circuit scale can be reduced.

  Further, the transmitter 1 determines the optimum reference position of the lookup table 30 using the input signal and the loopback signal even when there are variations in process, voltage, temperature, etc., with respect to the fixed lookup table method. Therefore, the correction accuracy is improved.

Therefore, according to the transmitter of this embodiment, the correction accuracy is better than that of the fixed lookup table method, and the circuit scale can be made smaller than that of the adaptive digital predistortion method.
(Second Embodiment)
Next, a second embodiment will be described.

  In the first embodiment, the transmitter for adjusting the shift value of the lookup table using the input signal and the loopback signal has been described. However, in the second embodiment, the lookup is performed using only the loopback signal. A transmitter for adjusting the shift value of the table will be described.

  FIG. 6 is a diagram illustrating a configuration of a transmitter according to the second embodiment. FIG. 7 is a diagram illustrating an example of a signal waveform of the loopback signal, and FIG. 8 is a diagram for describing a determination process according to the second embodiment. In FIG. 6, the same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In FIG. 7, the same components as those in FIG.

  As shown in FIG. 6, the transmitter 1a is configured by deleting the delay adjustment circuit 17, the comparator 18, the counter 19, and the difference circuit 27 from the transmitter 1 of FIG. The transmitter 1a is configured using a shift value control unit 28a instead of the shift value control unit 28 of FIG.

  As shown in FIG. 7, a predetermined threshold level TH is set in the comparator 25 as in the first embodiment. The comparator 25 (first comparison unit) determines whether or not the loopback signal exceeds the threshold level TH and outputs the determination result to the counter 26. Based on the determination result, the counter 26 (first counter) counts the number of times the loopback signal exceeds the threshold level TH during a predetermined time, and outputs it to the shift value control unit 28a.

  When the count value (Y times) counted by the counter 26 is within the reference range, the shift value control unit 28a ends the process without adjusting the shift value of the lookup table 30. Specifically, as shown in FIG. 8, when the count value (Y times) counted by the counter 26 is between B times and C times, the shift value control unit 28a determines that it is within the reference range.

  Here, by using CCDF (complementary cumulative distribution function), the probability that a signal larger than the average power is generated can be obtained. Therefore, the number of signals that exceed the threshold level TH during a predetermined time can be obtained. That is, since the number of signals that exceed the threshold level TH during a predetermined time is known, the number obtained in this way is used as the center of the reference range, and a predetermined number of times (B to C times) above and below that number. ) Is within the reference range.

  On the other hand, when the count value (Y times) counted by the counter 26 is outside the reference range, the shift value control unit 28a adjusts the shift value of the lookup table 30 so that the count value (Y times) falls within the reference range. To do. Specifically, as shown in FIG. 8, when the count value (Y times) counted by the counter 26 is less than B times or more than C times, the shift value control unit 28a determines that it is out of the reference range. To do.

  At this time, the shift value control unit 28a adjusts the shift by a value proportional to the magnitude of the count value (Y times). Note that the shift value control unit 28a may adjust the shift value by a predetermined value when the count value (Y times) is out of the reference range. The shift value control unit 28 a outputs the shift value determined in this way to the adder circuit 29. Other configurations are the same as those of the first embodiment.

  Next, the shift value control processing of the transmitter configured as described above will be described. FIG. 9 is a flowchart for explaining an example of the flow of the shift value control process according to the second embodiment.

  First, the counter 26 counts the number of times the loopback signal exceeds the threshold level TH during a predetermined time (step S11). Here, the number of loopback signals exceeding the threshold level TH is Y times.

  Next, the shift value control unit 28a determines whether or not Y times are within the reference range (step S12). If the shift value control unit 28a determines that Y times are within the reference range (step S12: YES), the process ends. On the other hand, when the shift value control unit 28a determines that Y times are not within the reference range (step S12: NO), the shift of the lookup table to a predetermined value or a value proportional to the magnitude of Y times is performed. The value is adjusted (step S13), the process returns to step S11, and the same processing is repeated. And the shift value control part 28a will complete | finish a process, if Y times enter in the reference | standard range (step S12: YES).

  As in the first embodiment, in the process of FIG. 9, even if Y times are within the reference range, the process may return to step S11 and the same process may be repeated, or a temperature change, a voltage change, etc. When there is a predetermined change, the process of FIG. 9 may be performed.

As described above, the transmitter 1a determines the optimum reference position of the lookup table 30 only from the amplitude information of the loopback signal. As a result, the transmitter 1a of the second embodiment obtains the same effect as the first embodiment and shifts the address of the lookup table 30 using only the loopback signal. The circuit scale can be made smaller than that of the transmitter 1 of the embodiment.
(Third embodiment)
Next, a third embodiment will be described.

  In the first embodiment, the transmitter that performs quadrature demodulation of the loopback signal has been described. In the third embodiment, a transmitter that demodulates the loopback signal by envelope detection will be described.

  FIG. 10 is a diagram illustrating a configuration of a transmitter according to the third embodiment. In FIG. 10, the same components as those in FIG.

  As shown in FIG. 10, the transmitter 1b is configured by deleting the arithmetic circuit 24 from the transmitter 1 of FIG. Further, the transmitter 1b is configured using an envelope detector 50 instead of the quadrature demodulator 20 of FIG.

  The envelope detector 50 demodulates the loopback signal by detecting (detecting) the envelope of the loopback signal output from the power amplifier 14 and outputs the demodulated signal to the A / D converter 21. The A / D converter 21 converts the loopback signal demodulated by the envelope detector 50 from an analog signal to a digital signal, and outputs it to the average power calculation unit 22 and the gain adjustment unit 23.

  The gain adjusting unit 23 gains the loopback signal from the A / D converter 21 according to the difference between the average power information of the loopback signal from the average power calculation unit 22 and the average power information of the known input signal. Are adjusted and output to the comparator 25. Other configurations and shift value control processing are the same as those in the first embodiment.

With such a configuration, the transmitter 1b of the third embodiment obtains the same effect as that of the first embodiment, and uses the envelope detector 50 to demodulate the loopback signal. The circuit scale can be made smaller than that of the transmitter 1 of the first embodiment having a transmitter (orthogonal demodulator).
(Fourth embodiment)
Next, a fourth embodiment will be described.

  In the second embodiment, the transmitter that quadrature-demodulates the loopback signal has been described. In the fourth embodiment, a transmitter that demodulates the loopback signal by envelope detection will be described.

  FIG. 11 is a diagram illustrating a configuration of a transmitter according to the fourth embodiment. In FIG. 11, the same components as those in FIG. 6 are denoted by the same reference numerals and description thereof is omitted.

  As shown in FIG. 11, the transmitter 1c is configured by deleting the arithmetic circuit 24 from the transmitter 1a of FIG. Further, the transmitter 1c is configured using an envelope detector 51 instead of the quadrature demodulator 20 of FIG.

  The envelope detector 51 demodulates the loopback signal by detecting (detecting) the envelope of the loopback signal output from the power amplifier 14 and outputs the demodulated signal to the A / D converter 21. The A / D converter 21 converts the loopback signal demodulated by the envelope detector 51 from an analog signal to a digital signal, and outputs it to the average power calculation unit 22 and the gain adjustment unit 23.

  The gain adjusting unit 23 gains the loopback signal from the A / D converter 21 according to the difference between the average power information of the loopback signal from the average power calculation unit 22 and the average power information of the known input signal. Are adjusted and output to the comparator 25. Other configurations and shift value control processing are the same as those in the second embodiment.

  With such a configuration, the transmitter 1c of the fourth embodiment obtains the same effects as those of the second embodiment, and uses the envelope detector 51 to demodulate the loopback signal. The circuit scale can be made smaller than that of the transmitter 1a of the second embodiment having a transmitter (orthogonal demodulator).

  It should be noted that each step in the flowchart in the specification may be executed in a different order for each execution by changing the execution order and executing a plurality of steps at the same time, as long as it does not contradict its nature.

  Although several embodiments of the invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1,1a ... Transmitter, 11 ... Predistortion part, 12 ... D / A converter, 13 ... Quadrature modulation part, 14 ... Power amplifier, 15 ... Antenna, 16, 24 ... Arithmetic circuit, 17 ... Delay adjustment circuit, 18 , 25... Comparator, 19, 26... Counter, 20... Quadrature demodulator, 21... A / D converter, 22... Average power calculator, 23 ... Gain adjuster, 27 ... Difference circuit, 28 ... Shift value controller , 29 ... adder circuit, 30 ... look-up table.

Claims (5)

A lookup table for storing distortion correction coefficients for correcting distortion of the power amplifier; and a predistortion unit for correcting distortion of the power amplifier by multiplying an input signal by a distortion correction coefficient read from the lookup table; A transmitter having a loopback signal from the power amplifier,
A first counter that counts the number of times a loopback signal from the power amplifier exceeds a predetermined threshold level;
A control unit that changes a reference position of the lookup table based on the number of times counted by the first counter;
A transmitter comprising: A second counter that counts the number of times the input signal exceeds the predetermined threshold level;
A difference circuit that calculates a difference between the number of times counted by the second counter and the number of times counted by the first counter;
The transmitter according to claim 1, wherein the control unit changes a reference position of the lookup table based on a difference value calculated by the difference circuit. An average power calculator for calculating an average power of the loopback signal;
Based on the calculated average power of the loopback signal and the average power of the input signal, a gain adjustment unit that performs gain adjustment to make the average power of the loopback signal equal to the average power of the input signal; The transmitter according to claim 1, further comprising:   The transmitter according to claim 2, further comprising a delay adjustment unit that delays the input signal for a predetermined time. An envelope detector for detecting an envelope of the power amplifier;
The transmitter according to any one of claims 1 to 4, wherein the loopback signal is an output signal of the envelope detector.

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