JP6547621B2 - Transmitter and distortion correction method - Google Patents

Description

  The present invention relates to a transmitter that performs distortion correction processing for suppressing distortion generated in an amplifier, and a distortion correction method for suppressing distortion generated in an amplifier.

  In the transmitter, DPD (Digital Pre-Distortion) processing is performed in which the transmission signal is corrected using a distortion correction vector in order to suppress non-linear distortion caused by the amplifier. The distortion correction vector is determined based on an error between a feedback signal, which is feedback of the transmission signal amplified by the amplifier, and the transmission signal. Since a delay occurs in the feedback signal, a transmission signal delayed according to the delay occurring in the feedback signal is used when determining the distortion correction vector. The distortion compensation apparatus disclosed in Patent Document 1 calculates the correlation value while changing the phase between the transmission signal and the feedback signal, and determines the delay time occurring in the circuit including the transmission power amplifier and the feedback loop based on the maximum correlation. Then, the distortion compensation coefficient is calculated using the transmission signal delayed by the determined delay time and the feedback signal.

JP 2001-189685 A

  In order to improve the accuracy of the calculation of the distortion correction vector, it is necessary to make the input timing of the transmission signal and the feedback signal to the circuit for calculating the distortion correction vector be in time units shorter than the sampling time. By oversampling the transmission signal and the feedback signal, it is possible to make the input timing be a unit of time shorter than the sampling time, but there is a problem that circuit size and parts cost increase when performing oversampling. .

  The present invention has been made in view of the above-described circumstances, and has an object of simplifying the configuration of a circuit for distortion correction processing without reducing the accuracy of the distortion correction processing.

In order to achieve the above object, a transmitter according to a first aspect of the present invention is:
A correlation value is detected between a test signal which is a frequency division multiplexed signal having an autocorrelation characteristic and which is input to an amplifier as a transmission signal, and a feedback signal which is feedback of the test signal amplified by the amplifier, and the correlation value is detected. A first delay detection unit that detects a first delay time, which is a delay time that is an integral multiple of the sampling time, according to the maximum value of
A result of performing a Fourier transform on the test signal delayed by the first delay time taken out by a determined interval, and a result of performing a Fourier transform on the feedback signal extracted by the determined interval A second delay detection unit for detecting a second delay time which is a delay time shorter than the sampling time according to a phase characteristic obtained from the ratio of
A delay unit that delays the transmission signal by a total time of the first delay time and the second delay time;
A distortion analysis unit that calculates a distortion correction vector for correcting distortion in the amplifier according to an error between the transmission signal delayed by the delay unit and the feedback signal;
A pre-distortion unit that corrects the transmission signal input to the amplifier according to the distortion correction vector;
Equipped with

Preferably, the processing of the second delay detection unit is repeated until the second delay time becomes equal to or less than a threshold.
The delay unit delays the transmission signal by a total time of the first delay time and the second delay time which is equal to or less than the threshold.

Preferably, the delay unit is
A buffer memory for delaying the transmission signal according to the first delay time;
A finite impulse response filter that delays the transmission signal delayed by the buffer memory according to the second delay time;
Equipped with
The processing of the second delay detection unit is repeated, and the tap coefficient for each tap of the finite impulse response filter is updated each time the second delay time is detected.

  Preferably, the first data which is continuous data including the first data is extracted from the orthogonal frequency division multiplexed signal based on the modulated pseudo random noise signal, and the last data of the orthogonal frequency division multiplexed signal is extracted And a second data, which is continuous data including the second data, the second data, the orthogonal frequency division multiplexed signal, and a signal generated by sequentially combining the first data are used as the test signal.

A distortion correction method according to a second aspect of the present invention is a distortion correction method performed by a transmitter that amplifies an input transmission signal with an amplifier and outputs the amplified transmission signal from an antenna.
Detecting a correlation value between a test signal which is a frequency division multiplexed signal having an autocorrelation characteristic and which is input to the amplifier as the transmission signal, and a feedback signal which is feedback of the test signal amplified by the amplifier; A first delay detection step of detecting a first delay time which is an integral multiple of the sampling time according to the maximum value of the correlation value;
A result of performing a Fourier transform on the test signal delayed by the first delay time taken out by a determined interval, and a result of performing a Fourier transform on the feedback signal extracted by the determined interval A second delay detection step of detecting a second delay time which is a delay time shorter than the sampling time according to a phase characteristic obtained from the ratio of
Distortion analysis for calculating a distortion correction vector for correcting distortion in the amplifier according to an error between the transmission signal delayed by the total of the first delay time and the second delay time and the feedback signal Step and
A pre-distortion step of correcting the transmission signal input to the amplifier according to the distortion correction vector;
Equipped with

Preferably, the transmitter repeats the processing of the second delay detection step until the second delay time becomes equal to or less than a threshold.
In the distortion analysis step, the distortion correction vector is calculated according to the transmission signal delayed by the total time of the first delay time and the second delay time which is equal to or less than the threshold, and the feedback signal. Ru.

  Preferably, the transmitter extracts first data, which is continuous data including data at the beginning, of an orthogonal frequency division multiplexed signal based on a modulated pseudo random signal, and the orthogonal data of the orthogonal frequency division multiplexed signal Extracting the second data which is continuous data including the last data, and testing the signal generated by sequentially combining the second data, the orthogonal frequency division multiplexed signal, and the first data Used as a signal.

  According to the present invention, in addition to the first delay time, which is a delay time that is a multiple of the sampling time, it is obtained from the ratio of the transmission signal delayed by the first delay time and the result of the Fourier transform of the feedback signal. A second delay time which is a delay time shorter than the sampling time according to the phase characteristic to be detected, and a distortion correction vector from the transmission signal and the feedback signal delayed by the total time of the first delay time and the second delay time It is possible to further simplify the configuration of the circuit for the distortion correction process without reducing the accuracy of the distortion correction process.

It is a block diagram showing an example of composition of a transmitter concerning an embodiment of the invention. It is a block diagram showing an example of composition of a distortion amendment vector calculation part concerning an embodiment. It is a figure which shows the example of the test signal in embodiment. It is a figure which shows the example of the correlation characteristic in embodiment. It is a figure which shows the example of the phase characteristic in embodiment. It is a flowchart which shows an example of operation | movement of the delay time detection which the transmitter which concerns on embodiment performs.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the figures, the same or equivalent parts are denoted by the same reference numerals.

  FIG. 1 is a block diagram showing a configuration example of a transmitter according to an embodiment of the present invention. The transmitter 1 amplifies the input transmission signal and outputs the amplified transmission signal from the antenna 10. The transmitter 1 performs pre-distortion processing for correcting in advance distortion of the transmission signal generated at the time of amplification. Each part of the transmitter 1 is controlled by the controller 30. The controller 30 includes a central processing unit (CPU) 31, a random access memory (RAM) 33, and a read-only memory (ROM) 34. Although signal lines from the controller 30 to each part are omitted to avoid complication and to facilitate understanding, the controller 30 is connected to each part of the transmitter 1 via I / O (Input / Output) 32. Control the start and end of those processes, and the contents of the process. The ROM 34 stores a control program for the controller 30 to control the operation of the transmitter 1. The controller 30 controls the transmitter 1 based on the control program. Each part of the transmitter 1 will be described.

A transmission signal input from a transmission source (not shown) to the transmitter 1 is input to a digital pre-distortion (DPD) processing unit 2, a power calculation unit 3 and a distortion correction vector calculation unit 20. The transmission signal is an I (In-phase) Q (Quadrature) signal such as a two-signal or an SSB (Single Side Band) voice signal. The power calculation unit 3 calculates the power of the transmission signal and sends it to the address detection unit 4 as expressed by the following equation (1). In the following equation (1), s (t) is a complex transmission signal, s ^* (t) is a complex conjugate signal that is a complex conjugate of s (t), and s _i (t) is s (t) , And s _q (t) is a Q component of s (t), and j is an imaginary unit.

  The address detection unit 4 detects the address a (s) corresponding to the power of the transmission signal and sends it to the LUT (Look Up Table) 5 as expressed by the following equation (2). In the following equation (2), Pmax is the maximum power of the transmission signal s (t) before being processed by the DPD processing unit 2, and N is a bit of D-A (Digital-to-Analogue) converter 6 It is a number.

The LUT 5 outputs the distortion correction vector corresponding to the address sent from the address detection unit 4 to the DPD processing unit 2. The DPD processing unit 2 performs distortion correction processing to correct the transmission signal according to the distortion correction vector by subjecting the transmission signal s (t) to complex multiplication with the distortion correction vector as represented by the following equation (3). The transmission signal s _PD (t) on which the distortion correction processing has been performed is sent to the DA converter 6. In the following equation (3), L (a) is a distortion correction vector output from the LUT 5 in accordance with the address a (s). L _i (a) is the I component of L (a), and L _q (a) is the Q component of L (a).

The DA converter 6 converts the transmission signal s _PD (t) subjected to the distortion correction processing from a digital signal into an analog signal, and sends the analog signal to the mixer 7. The mixer 7 quadrature modulates the transmission signal converted into analog according to the reference signal which is a signal output from the local oscillator 8, and sends it to the amplifier 9. The amplifier 9 amplifies the orthogonally modulated transmission signal and outputs the amplified transmission signal to the antenna 10, and the antenna 10 outputs the amplified transmission signal. A feedback signal that is feedback of the transmission signal output from the amplifier 9 is input to the gain adjustment unit 11. The gain adjustment unit 11 is configured by at least one of an amplifier and an attenuator. The gain adjustment unit 11 adjusts the gain of the feedback signal and sends it to the mixer 12. The mixer 12 quadrature demodulates the feedback signal according to the reference signal output from the local oscillator 8 and sends it to an analog-to-digital (AD) converter 13. The A-D converter 13 converts the quadrature-demodulated feedback signal from an analog signal into a digital signal and sends it to the distortion correction vector calculation unit 20.

  By passing through the D-A converter 6, the analog circuit from the D-A converter 6 to the A-D converter 13, and the A-D converter 13, a delay occurs in the feedback signal. Therefore, the distortion correction vector calculation unit 20 calculates a distortion correction vector for correcting the distortion in the amplifier 9 according to the error between the transmission signal delayed according to the delay generated in the feedback signal and the feedback signal. The distortion correction vector calculation unit 20 includes a delay unit 21 for delaying the transmission signal, a distortion analysis unit 22 for calculating a distortion correction vector according to an error between the delayed transmission signal and the feedback signal, and a delayed transmission. The delay detection unit 23 detects a delay time set by the delay unit 21 according to the signal and the feedback signal. The distortion analysis unit 22 updates the distortion correction vector stored in the LUT 5 with the calculated distortion correction vector. The distortion analysis unit 22 calculates distortion correction vectors at predetermined intervals, and updates the distortion correction vectors stored in the LUT 5.

  FIG. 2 is a block diagram showing an example of the configuration of a distortion correction vector calculation unit according to the embodiment. The distortion correction vector calculation unit 20 detects a delay time using a test signal which is a frequency division multiplexed signal having autocorrelation characteristics and is input as a transmission signal, and the transmission signal and feedback signal delayed by the delay time The distortion correction vector is calculated in accordance with the difference between them and output to the LUT 5. Each part of the distortion correction vector calculation unit 20 will be described. The delay unit 21 delays a signal input for a first delay time which is a multiple of the sampling time detected by the first delay detection unit 26 as described later, and a buffer memory 24 which will be described later. A FIR (Finite Impulse Response) filter 25 is provided to delay an input signal by a second delay time that is shorter than the sampling time detected by the second delay detection unit 27 as described above.

Assuming that the sampling time is Ts, the first delay time is n × Ts, and the second delay time is Tf, the delayed transmission signal s _d (t) output by the buffer memory 24 is represented by the following equation (4) The delayed transmission signal s _d (t) output from the FIR filter 25 is expressed by the following equation (5).

  The delay detection unit 23 is delayed by the first delay detection unit 26 that detects a first delay time according to the correlation value between the test signal delayed by the delay unit 21 and the feedback signal, and by the delay unit 21. Second delay detection for detecting a second delay time according to a phase characteristic obtained from a ratio of a result of FFT (Fast Fourier Transformation) on a test signal and a result of FFT on a feedback signal A unit 27 is provided.

  FIG. 3 is a diagram showing an example of a test signal in the embodiment. The test signal used to detect the delay time is a frequency division multiplexed signal having an autocorrelation characteristic. In the present embodiment, an orthogonal frequency-division multiplexing (OFDM) signal is used as the test signal. Since the OFDM signal is a signal close to noise, the delay time can be detected by detecting the correlation of the signals. In the example of FIG. 3, the first data which is continuous data including the data at the beginning is extracted from the OFDM signal of one symbol, and the second data which is the continuous data including the data at the tail end of the OFDM signal. The second data, the OFDM signal, and a signal generated by sequentially combining the first data are used as a test signal. Although distortion occurs in the signal when it passes through the delay filter, by using the test signal shown in FIG. 3, it is possible to secure a signal of one symbol section without distortion. The PAPR (Peak to Average Power Ratio) of the test signal is preferably as small as possible. In the present embodiment, an OFDM signal generated based on data obtained by QN (Quadrature Phase Shift Keying) modulation of a PN (Pseudo-random Noise) signal is used as a test signal.

The first delay time detection process will be described. Even when the first delay time is not set, the transmission signal s _d (t) input to the first delay detection unit 26 is strictly s because there is a delay due to the FIR filter 25. Different from (t). However, if the frequency characteristic of the signal passband is flat, there is no error that affects the accuracy of the delay time, so the first delay detection unit 26 generates s _d (t) and feedback signal r (t). Based on the first delay time is detected. The correlation function c (t) between the delayed transmission signal s _d (t) and the feedback signal r (t) is expressed by the following equation (6), and the square c _s of the absolute value of the correlation function c (t) (T) is expressed by the following equation (7). s (t) is a complex transmission signal, s _d ^* (t) is a complex conjugate of s _d (t), and c ^* (t) is a complex conjugate of c (t).

  FIG. 4 is a diagram showing an example of the correlation characteristic in the embodiment. FIG. 4 shows the equation (7), where the horizontal axis is the coefficient n used to represent the first delay time, and the vertical axis is the correlation value. In the example of FIG. 4, since the peak of the correlation value is at the position of n = 6, the first delay time is calculated to be 6Ts, and the delay time of the buffer memory 24 is set to 6Ts.

Next, the second delay time detection process will be described. After 6 Ts is set to the delay time of the buffer memory 24, the second delay detection unit 27 receives the delay, which is input through the first delay detection unit 26, as expressed by the following equation (8). The ratio of the result of FFT of the transmission signal s _d (t) delayed by the unit 21 and the result of FFT of the feedback signal r (t) is calculated. In the following equation (8), ΔT is a second delay time.

FIG. 5 is a diagram showing an example of phase characteristics in the embodiment. The horizontal axis is the normalized frequency (in thousands), and the vertical axis is the result of FFT of the transmission signal s _d (t) represented by the above equation (8) and the result of FFT of the feedback signal r (t) It is a difference. The phase characteristic of the transfer function of the above equation (8) in the section where the transmission subcarrier shown by the dotted circle in FIG. 5 is present is a waveform proportional to the delay time. The second delay detection unit 27 calculates the slope θ of the phase characteristic of the section in which the transmission subcarrier is present, based on the slope of the straight line connecting the values of the two points or the least squares method. The second delay time ΔT is expressed by the following equation (9) based on the gradient θ of the phase characteristic shown in the above equation (8) and FIG. The second delay detection unit 27 detects the second delay time ΔT from the slope θ of the phase characteristic using the following equation (9). The tap coefficients for each tap of the FIR filter 25 are set such that the delay time of the FIR filter 25 becomes the second delay time ΔT.

The design of the FIR filter 25 will be described. If the impulse response of the filter is b (t), the impulse response of the filter further delayed by T _f from this filter is b (t−T _f ). If an impulse response b (t−T _f ) is easily obtained, that value can be used to further delay T _f for a filter whose impulse response is b (t). if b _{(t-T f)} is not obtained, by utilizing the fact that the system transfer function of the delay _{T f} is exp (-j2πfT _f), from b (t), the following equation (10) B (t−T _f ) can be obtained as B (t−ΔT) is calculated by substituting ΔT calculated by the above equation (9) into T _f of the following equation (10). The tap coefficients for each tap of the FIR filter 25 are set based on b (t−ΔT). The value obtained by the following equation (10) is a complex number, and a small imaginary value is generated due to the truncation error of the tap of the FIR filter 25, but the small value causes no problem in practical operation. Do. Although the following equation (10) is a logical equation, in actual digital signal processing, instead of Fourier transform and inverse Fourier transform, calculation is performed by FFT, IFFT (Inverse Fast Fourier Transformation: Inverse Fast Fourier Transform).

The transmitter 1 repeats the process of the second delay detection unit 27 and the setting of the tap coefficient for each tap of the FIR filter 25 until the second delay time ΔT becomes equal to or less than the threshold. The threshold is determined by the accuracy of the input timings of the delayed transmission signal s _d (t) and the feedback signal r (t), which are obtained by the distortion analysis unit 22. Although the number of taps of the FIR filter 25 has an ideal characteristic as the number thereof increases, the amount of calculation becomes enormous. In the present embodiment, by repeating the process of the second delay detection unit 27 and the setting of the tap coefficient for each tap of the FIR filter 25, even the filter cut off with a relatively small number of taps can accurately perform the second delay time. ΔT can be made equal to or less than a threshold, and the accuracy of the distortion correction vector calculated by the distortion analysis unit 22 can be improved.

FIG. 6 is a flowchart showing an example of an operation of delay time detection performed by the transmitter according to the embodiment. The first delay detection unit 26 calculates a correlation value between the delayed transmission signal s _d (t) and the feedback signal r (t), and detects a peak of the correlation value (step S110). The first delay detection unit 26 detects a first delay time from the peak of the correlation value (step S120). The delay time of the buffer memory 24 is set according to the detected first delay time (step S130).

The second delay detection unit 27 calculates phase characteristics from the ratio of the result of FFT of the transmission signal s _d (t) delayed by the delay unit 21 and the result of FFT of the feedback signal r (t), and the phase The second delay time is detected from the characteristics (step S140). The tap coefficient for each tap of the FIR filter 25 is set according to the second delay time (step S150). If the second delay time ΔT is equal to or less than the threshold (step S160; Y), the transmitter 1 ends the process of delay time detection. If the second delay time ΔT is larger than the threshold (step S160; N), the process returns to step S140, and the process of detecting the second delay time and setting the tap coefficient for each tap of the FIR filter 25 is repeated. .

  As described above, according to the transmitter 1 of the embodiment, in addition to the first delay time, the second delay time is detected, and only the total time of the first delay time and the second delay time is detected. By delaying the transmission signal, the timing of the transmission signal and the feedback signal input to the distortion analysis unit 22 can be made to be in time units shorter than the sampling time. The transmitter 1 has a simpler circuit configuration than a circuit that performs oversampling processing. That is, the transmitter 1 can further simplify the configuration of the circuit for the distortion correction process without reducing the accuracy of the distortion correction process.

  Embodiments of the present invention are not limited to the above-described embodiments. The circuit configuration in the above-described embodiment is an example.

1 transmitter 2 DPD processor 3 power calculator 4 address detector 5 LUT
DESCRIPTION OF SYMBOLS 6 DA converter 7 12 mixer 8 local oscillator 9 amplifier 10 antenna 11 gain adjustment part 13 AD conversion part 20 distortion correction vector calculation part 21 delay part 22 distortion analysis part 23 delay detection part 24 buffer memory 25 FIR filter 26 first delay detection unit 27 second delay detection unit 30 controller 31 CPU
32 I / O
33 RAM
34 ROM

Claims (7)

A correlation value is detected between a test signal which is a frequency division multiplexed signal having an autocorrelation characteristic and which is input to an amplifier as a transmission signal, and a feedback signal which is feedback of the test signal amplified by the amplifier, and the correlation value is detected. A first delay detection unit that detects a first delay time, which is a delay time that is an integral multiple of the sampling time, according to the maximum value of
A result of performing a Fourier transform on the test signal delayed by the first delay time taken out by a determined interval, and a result of performing a Fourier transform on the feedback signal extracted by the determined interval A second delay detection unit for detecting a second delay time which is a delay time shorter than the sampling time according to a phase characteristic obtained from the ratio of
A delay unit that delays the transmission signal by a total time of the first delay time and the second delay time;
A distortion analysis unit that calculates a distortion correction vector for correcting distortion in the amplifier according to an error between the transmission signal delayed by the delay unit and the feedback signal;
A pre-distortion unit that corrects the transmission signal input to the amplifier according to the distortion correction vector;
Transmitter comprising: The processing of the second delay detection unit is repeated until the second delay time becomes equal to or less than a threshold value,
The delay unit delays the transmission signal by a total time of the first delay time and the second delay time equal to or less than the threshold.
The transmitter according to claim 1. The delay unit is
A buffer memory for delaying the transmission signal according to the first delay time;
A finite impulse response filter that delays the transmission signal delayed by the buffer memory according to the second delay time;
Equipped with
The processing of the second delay detection unit is repeated, and the tap coefficient for each tap of the finite impulse response filter is updated each time the second delay time is detected.
The transmitter according to claim 2.   The first data, which is continuous data including the first data, is extracted from the orthogonal frequency division multiplexed signal based on the modulated pseudo random noise signal, and the continuous data including the last data of the orthogonal frequency division multiplexed signal. The second data, which is data, is extracted, and a signal generated by sequentially combining the second data, the orthogonal frequency division multiplexed signal, and the first data is used as the test signal. The transmitter according to any one of 3. A distortion correction method performed by a transmitter that amplifies an input transmission signal by an amplifier and outputs the amplified transmission signal from an antenna,
Detecting a correlation value between a test signal which is a frequency division multiplexed signal having an autocorrelation characteristic and which is input to the amplifier as the transmission signal, and a feedback signal which is feedback of the test signal amplified by the amplifier; A first delay detection step of detecting a first delay time which is an integral multiple of the sampling time according to the maximum value of the correlation value;
A result of performing a Fourier transform on the test signal delayed by the first delay time taken out by a determined interval, and a result of performing a Fourier transform on the feedback signal extracted by the determined interval A second delay detection step of detecting a second delay time which is a delay time shorter than the sampling time according to a phase characteristic obtained from the ratio of
Distortion analysis for calculating a distortion correction vector for correcting distortion in the amplifier according to an error between the transmission signal delayed by the total of the first delay time and the second delay time and the feedback signal Step and
A pre-distortion step of correcting the transmission signal input to the amplifier according to the distortion correction vector;
A distortion correction method comprising: The transmitter repeats the processing of the second delay detection step until the second delay time becomes equal to or less than a threshold value,
In the distortion analysis step, the distortion correction vector is calculated according to the transmission signal delayed by the total time of the first delay time and the second delay time which is equal to or less than the threshold, and the feedback signal. The
The distortion correction method according to claim 5.   The transmitter extracts first data, which is continuous data including data at the beginning, of an orthogonal frequency division multiplexed signal based on a modulated pseudo random signal, and the last one of the orthogonal frequency division multiplexed signals. A second data which is continuous data including data is extracted, and a signal generated by sequentially combining the second data, the orthogonal frequency division multiplex signal, and the first data is used as the test signal. A distortion correction method according to claim 5 or 6.

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