KR20110105319A - Apparatus and method for auto correction technique on linear/nonlinear distortion in wireless transmission - Google Patents

Abstract

The present invention relates to a distortion compensation device, and more particularly, to an apparatus and a method for compensating for a nonlinear distortion characteristic of a high power amplifier and a linear distortion characteristic of a channel filter. The automatic distortion compensator is an adaptive linear distortion for estimating linear distortion of the down-signaled down-signal adaptively based on the down-signaled down-signal output from the high power amplifier and the linear-distortion compensation signal outputted from the complex pre-equalizer. An estimator and an adaptive nonlinear distortion estimator for adaptively estimating nonlinear distortion of the estimated linear distortion signal based on the estimated linear distortion signal and the nonlinear distortion compensation signal output from the predistorter.

Description

Automatic Linear and Nonlinear Distortion Compensation Apparatus and Method {APPARATUS AND METHOD FOR AUTO CORRECTION TECHNIQUE ON LINEAR / NONLINEAR DISTORTION IN WIRELESS TRANSMISSION}

The present invention relates to a distortion compensator, and more particularly, to an apparatus and a method for compensating nonlinear distortion characteristics of a high power amplifier (hereinafter referred to as "HPA") and linear distortion characteristics of a channel filter.

The high power amplifier (HPA) is a device for amplifying an output before transmitting a communication signal, and has a characteristic of nonlinearly amplifying an input signal without linearly amplifying the input signal. Nonlinear effects distort the transmitted signal and widen the bandwidth, causing adjacent channel interference problems. One solution to this problem is predistortion, which linearizes the nonlinear amplification characteristics of high-power amplifiers. Therefore, the predistorter (PD) is located in front of the high output amplifier and serves to deform the signal in advance so that the final output value of the communication signal is linear.

Among the nonlinear distortion characteristic compensation techniques, the digital predistortion compensation technique (LDP) using a look-up table (hereinafter referred to as "LUT") compares a reference signal with a comparison signal. Construct a characteristic table corresponding to the inverse of the nonlinear distortion characteristic of the HPA. The digital predistortion compensation technique predistorts the transmission signal using the characteristic table, thereby maintaining the linearity of the output signal of the high output amplifier.

However, in the predistortion compensation method using a lookup table, it is difficult to estimate an inverse characteristic table of the nonlinear distortion characteristics of a high power amplifier when a residual distortion component other than the nonlinear distortion components of the HPA exists in the reference signal and the comparison signal. Conventional predistortion compensation techniques do not accurately compensate for the nonlinear distortion characteristics of high power amplifiers, leading to deterioration of the quality of the transmitted signal.

Therefore, techniques for distinguishing and removing residual distortion components other than the nonlinear distortion components of the high power amplifier to estimate accurate nonlinear distortion characteristics of the high power amplifier should be continuously studied.

The present invention provides an apparatus and method capable of compensating linear and nonlinear distortions regardless of the type of transmission signal by using a preequalizer and a predistorter at the stage after the modulator.

The present invention also provides an apparatus and method for efficiently extracting filter coefficients of a pre-equalizer by distinguishing and estimating equalization coefficients and symbol synchronization coefficients in a linear distortion compensation process.

In addition, the present invention provides an apparatus and method for efficiently compensating for nonlinear distortion by using a signal in which linear distortion is compensated in compensating for nonlinear distortion due to a high output amplifier.

In addition, the present invention provides an apparatus and method for adaptively estimating the nonlinear distortion characteristics of a high output amplifier that changes according to ambient environment characteristics or aging, and repeatedly updating the LUT in a table unit.

In addition, the present invention provides an apparatus and method for reducing the complexity of hardware while improving the low convergence rate in the process of updating the LUT by a table.

The automatic linear and nonlinear distortion compensator according to an embodiment of the present invention is adaptively downlinked on the basis of a downlink signal output from a high power amplifier and a linear distortion compensation signal output from a complex pre-equalizer. An adaptive linear distortion estimator for estimating a linear distortion of a downlink signal and an adaptive to estimate nonlinear distortion of the estimated linear distortion signal adaptively based on the estimated linear distortion signal and a nonlinear distortion compensation signal output from the predistorter It includes a nonlinear distortion estimator.

In addition, the automatic linear and nonlinear distortion compensation method according to an embodiment of the present invention adaptively downwards the frequency based on a downlink signal output from a high power amplifier and a downlink signal output from a complex pre-equalizer. Estimating the linear distortion of the estimated downlink signal and adaptively estimating the nonlinear distortion of the estimated linear distortion signal based on the estimated linear distortion signal and the nonlinear distortion compensation signal output from the predistorter. .

The present invention can provide an apparatus and method capable of compensating linear and nonlinear distortions regardless of the type of transmission signal by using a preequalizer and a predistorter at the stage after the modulator.

In addition, the present invention can provide an apparatus and method for efficiently extracting filter coefficients of the pre-equalizer by distinguishing and estimating equalization coefficients and symbol synchronization coefficients in a linear distortion compensation process.

In addition, the present invention can provide an apparatus and method for efficiently compensating for nonlinear distortion by using a signal whose linear distortion is compensated for in compensating for nonlinear distortion due to a high output amplifier.

In addition, the present invention can provide an apparatus and method for improving the equalizer performance by enabling a non-exhaustive frequency equalization by using a complex pre-equalization equalizer to compensate linear distortion characteristics.

In addition, the present invention can provide an apparatus and method that can reduce the hardware complexity and improve the performance by estimating the symbol synchronization coefficient by the adaptive estimation technique.

In addition, the present invention can provide an apparatus and method for efficiently removing residual distortion components scattered in a reference signal and a comparison signal to compensate for nonlinear distortion.

In addition, the present invention can provide an apparatus and method for adaptively estimating the nonlinear distortion characteristics of a high output amplifier that changes according to ambient environment characteristics or aging, and repeatedly updating the LUT in a table unit.

In addition, the present invention can provide an apparatus and method for reducing the complexity of the hardware while improving the low convergence rate in the process of updating the LUT in a table unit.

1 is a block diagram of a transmission system used for general wireless broadcasting and communication.
2 is a view showing the operation principle of the predistorter.
3 is a block diagram of a transmission system according to an embodiment of the present invention.
4 is a block diagram of an automatic linear and nonlinear distortion compensator according to an embodiment of the present invention.
5 is a block diagram illustrating in more detail an automatic linear and nonlinear distortion compensation apparatus according to an embodiment of the present invention.
6 is a block diagram of a symbol synchronization compensation unit according to an embodiment of the present invention.
7 is a block diagram of an adaptive linear distortion estimation unit according to an embodiment of the present invention.
8 is a block diagram of an adaptive nonlinear distortion estimation unit according to an embodiment of the present invention.
9 is a flowchart of an automatic linear and nonlinear distortion compensation method according to an embodiment of the present invention.
10 is a detailed flowchart of a symbol synchronization distortion compensation step according to an embodiment of the present invention.
11 is a detailed flowchart of an adaptive linear distortion compensation step according to an embodiment of the present invention.
12 is a detailed flowchart of an adaptive nonlinear distortion compensation step according to an embodiment of the present invention.
FIG. 13 is a graph illustrating generation and display of AM / AM and AM / PM symbols using aligned reference signals and comparison signals stored in a memory buffer.
14 is a graph showing the distribution probability of AM / AM AM / PM symbols coming into each LUT interval.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram of a transmission system used for general wireless broadcasting and communication.

A general wireless broadcasting and communication transmission system modulates an input data signal into a wireless transmission system format supported by an encoder and a modulator. The modulated signal is up-modulated to the desired frequency band through the frequency uplinker. The up-modulated signal is passed through the high power amplifier to amplify the amplitude of the signal, and is transmitted through the antenna after passing through the channel filter so as not to affect the adjacent band. In general, HPA shows nonlinear distortion characteristics in the saturation region, and the channel filter shows linear distortion characteristics due to ripple and group delay in the passband. The transmission signal is inferior in SNR characteristics due to linear / nonlinear distortion, and linear / nonlinear distortion causes reduction in reception coverage or degradation of reception signal quality. Therefore, in order to compensate for the linear / nonlinear distortion, the transmitter uses a pre-equalization technique and a predistortion compensation technique.

The pre-equalization technique removes the linear distortion caused by the channel filter, and is implemented as an adaptive filter based on a least mean square algorithm (hereinafter, referred to as "LMS"). The transposition equalization technique has the advantage of simple implementation and good equalization performance. Also, the pre-equalization technique is generally used in front of a modulator operating at the same sampling rate as the symbol rate. The pre-equalization technique in front of the modulator can compensate for linear distortion in a band but cannot compensate for linear distortion outside a band. have. The predistortion compensation technique is described in FIG.

2 is a view showing the operation principle of the predistorter.

The predistortion compensation technique makes the HPA output signal linear by finally compensating for the inverse of the nonlinear characteristic generated in the HPA. The predistorter distorts the input symbol with a signal having an inverse function on the characteristics of the high output amplifier based on the characteristics of the high output amplifier. The distorted signal passes through the high power amplifier, resulting in non-linearity compensation.

A typical predistortion compensation technique known in the art includes a method using a LUT. In the method using the LUT, the nonlinear distortion characteristics are tabled and stored in a memory, and an index signal generates an output signal having an inverse function characteristic with the nonlinear distortion characteristics. The method using LUT is simple to implement and shows relatively good performance compared to techniques such as Polynomial Curve Fitting. However, there is a disadvantage in that the convergence speed is slow in estimating the HPA characteristics. A method of updating in units of tables to increase the convergence speed while using the LUT is described with reference to FIG. 8.

A reference signal and a comparison signal are required for adaptive filter coefficient estimation of the pre-equalization technique and LUT generation of the predistortion compensation technique. However, since the signal passing through the HPA and the channel filter is an analog signal, the digital signal passed through the frequency downlink by feeding back the signal from the rear end of the channel filter is used as a comparison signal. In the process of obtaining the comparison signal, the comparison signal includes nonlinear distortion components by HPA, linear distortion components by channel filter, symbol synchronization distortion by timing & phase offset, and jitter noise by frequency up / down modulation and filtering. And the like. Therefore, for efficient linear and nonlinear distortion compensation, it is necessary to efficiently remove residual distortion components except for linear and nonlinear distortion components.

In the present invention, (a) by using the linear distortion compensation signal and the frequency down-converted signal to estimate the linear distortion and symbol synchronization distortion at once and through the adaptive estimator and (b) the adaptive estimation apparatus through An adaptive estimation apparatus is provided that generates an LUT using a generated signal and a nonlinear distortion compensation signal, and updates the LUT repeatedly.

However, since the present invention uses a signal after the modulator, the present invention can be applied regardless of a modulation method and a standard, and is not limited to the embodiments described herein.

3 is a block diagram of a transmission system according to an embodiment of the present invention.

A transmission system according to an embodiment of the present invention includes a complex pre-equalizer 310, a predistorter 320, a frequency downlinker 330, and an automatic linear and nonlinear distortion compensator 340.

The complex pre-equalizer 310 compensates for the linear distortion component generated by the high power amplified signal passing through the channel filter. However, by using a complex pre-equalizer, the pre-equalizer can be positioned after the modulator. The complex pre-equalizer can compensate not only the linear distortion component in the band passing through the channel filter, but also the linear distortion component out of the band. This is because the complex pre-equalizer equalizes not only the real part but also the area of the complex part to compensate for the transpose. In other words, the complex pre-equalizer is capable of asymmetric frequency equalization. Also, since it is located after the modulator, it can be applied regardless of the modulation method and specification. The complex pre-equalizer 310 provides a reference signal to the automatic linear and nonlinear distortion compensator 340, and receives the equalization coefficient estimated from the automatic linear and nonlinear distortion compensator 340 to efficiently perform linear distortion compensation. do.

In addition, the predistorter 320 compensates for the nonlinear distortion component generated when the frequency-up signal passes through the high output amplifier. The input signal is distorted by a signal having an inverse characteristic with respect to the characteristics of the high output amplifier. The predistorter 320 provides a reference signal to the automatic linear and nonlinear distortion compensator 340, and receives the updated LUT from the automatic linear and nonlinear distortion compensator 340 to efficiently perform the nonlinear distortion compensation.

In addition, the frequency downlinker 330 is high-power amplified and downwards the signal passing through the channel filter to the same frequency band for comparison with reference signals provided by the complex pre-equalizer 310 and the predistorter 320. That is, the frequency downlinker 330 generates a comparison signal.

In addition, the automatic linear and nonlinear distortion compensator 340 utilizes the complex pre-equalizer output signal, the predistorter output signal, and the frequency downlink output signal as reference signals and comparison signals. The automatic linear and nonlinear distortion compensator receives three signals and outputs the equalization coefficient applied to the complex pre-equal equalizer and the LUT value applied to the predistorter, and iteratively updates the linear and nonlinear distortion compensation components by repeating the update. Will be removed. Details of the automatic linear and nonlinear distortion compensator 340 will be described with reference to FIGS. 4 and 5.

4 is a block diagram of an automatic linear and nonlinear distortion compensator according to an embodiment of the present invention.

Referring to FIG. 4, the automatic linear and nonlinear distortion compensator 340 according to an embodiment of the present invention includes a symbol synchronous distortion compensator 410, an adaptive linear distortion estimator 420, and an adaptive nonlinear distortion estimator 430. ).

The symbol synchronous distortion compensator 410 compensates for the symbol synchronous distortion based on the downlink signal output from the high output amplifier and whose frequency is lowered. In this case, the symbol synchronization distortion may be generated by timing and phase offset. The symbol synchronous distortion compensator 410 compensates for the symbol synchronous distortion due to the timing and phase offset of the comparison signal generated while passing through the high output amplifier and the channel filter. The symbol synchronization distortion compensator 410 will be described in detail with reference to FIG. 6.

In addition, the adaptive linear distortion estimator 420 adaptively lowers the frequency of the downlink signal having the frequency lowered based on the downlink signal output from the high output amplifier and the linear distortion compensation signal output from the complex pre-equalizer. Estimate Linear distortion may be adaptively estimated by using a frequency downlink signal having no symbol synchronization distortion as a comparison signal. When the symbol synchronization distortion is not compensated and the linear distortion is estimated, the receiver receiving the signal transmitted through the antenna may compensate the linear distortion through separate symbol synchronization distortion compensation. In addition, the adaptive linear distortion estimator 420 may adaptively estimate the linear distortion based on the signal whose symbol synchronization distortion is compensated for and the linear distortion compensation signal output from the complex pre-equalizer. The adaptive linear distortion estimator 420 updates LMS (Least) to minimize the residual error of the reference signal output in order to compensate for the linear distortion caused by the channel filter in the comparison signal and the complex pre-equalizer with symbol synchronization distortion compensation. Mean square coefficient is applied to estimate equalization coefficient and symbol synchronization coefficient. The estimated equalization coefficient is applied again to the complex pre-equalizer to compensate for the linear distortion due to the channel filter, and the estimated symbol synchronization coefficient is applied at the next compensation in the symbol synchronization distortion compensator. The adaptive linear distortion estimator 420 will be described in detail with reference to FIG. 7.

In addition, the adaptive nonlinear distortion estimator 430 adaptively estimates the nonlinear distortion based on the estimated linear distortion signal and the nonlinear distortion compensation signal output from the predistorter. The adaptive nonlinear distortion estimator 430 generates and updates the LUT using a comparison signal compensated for linear distortion and a reference signal output from the predistorter to compensate for the nonlinear distortion caused by the high output amplifier. The generated and updated LUTs are newly applied to predistorters to efficiently compensate for nonlinear distortions caused by high power amplifiers. The adaptive nonlinear distortion estimator 430 will be described in detail with reference to FIG. 8.

5 is a block diagram illustrating in more detail an automatic linear and nonlinear distortion compensation apparatus according to an embodiment of the present invention.

Referring to FIG. 5, the automatic linear and nonlinear distortion compensator according to an embodiment of the present invention captures a predetermined period of a reference signal output from the complex pre-equalizer and a frequency down signal passing through the frequency down signal. Since the captured reference signal and the comparison signal do not match the symbol delay or the symbol power, it is necessary to first align the symbol delay and the symbol power. The symbol delay aligner 510 calculates a symbol delay using a correlation function, aligns the symbol delays of the reference signal and the comparison signal, and the symbol power normalizer 520 adjusts the power of the frequency downlink signal. Normalize equally to power of linear distortion compensation signal. The linear distortion compensation signal and the frequency downlink signal in which the symbol delay and the symbol power are aligned are stored in the memory buffer 530. The signal stored in the memory buffer 530 is used when the adaptive linear distortion estimator 420 adaptively calculates the equalization coefficient and the symbol synchronization coefficient.

The frequency down-signal in which the symbol delay and the symbol power are aligned is a symbol synchronous distortion compensated by the symbol synchronous distortion compensator 410 as a comparison signal for estimating linear and nonlinear distortion. The aligned linear distortion compensation signal, which is stored in the memory buffer 530 and the signal whose symbol synchronization distortion is compensated for, is used as a comparison signal and a reference signal by the adaptive linear distortion estimation unit 420 to estimate the equalization coefficient and the symbol synchronization coefficient. The adaptive linear distortion estimator 420 adaptively estimates the equalization coefficient and the symbol synchronization coefficient as much as the signal stored in the memory buffer 530. The equalized coefficients that are adaptively calculated are stored in the equalization coefficient memory 540 and the symbol synchronization coefficients are stored in the symbol synchronization coefficient memory 550. The equalization coefficients and symbol synchronization coefficients stored in the memory are later applied to the complex pre-equalizer and symbol synchronization compensator.

The symbol delay alignment unit 560 aligns the symbol delays of the symbol synchronization distortion output from the adaptive linear distortion estimation unit 420 and the linear distortion estimated signal and the nonlinear distortion compensation signal output from the predistorter. The signal in which the symbol delay is aligned is normalized by the symbol power normalization unit 570 equal to the power of the nonlinear distortion compensation signal. A nonlinear distortion compensation signal in which symbol delay and symbol power are aligned and a signal in which linear distortion is estimated adaptively are stored in a memory buffer.

Based on the reference signal and the comparison signal stored in the memory buffer, the adaptive nonlinear distortion estimator 430 generates a LUT and updates the LUT by the number of signals stored in the memory buffer to estimate the nonlinear distortion characteristic of the high output amplifier. The generated LUT is stored in the nonlinear distortion compensation table memory, and the adaptive nonlinear distortion estimator 430 updates the generated LUT in a table unit to increase the convergence speed in estimating the characteristics of the high output amplifier. The values stored in the equalization coefficient memory 540, the symbol synchronization coefficient memory 550, and the nonlinear distortion compensation table memory 580 are applied to the complex pre-equal equalizer, the symbol synchronization distortion compensator, and the predistorter after estimated repetition.

6 is a block diagram of a symbol synchronization compensation unit according to an embodiment of the present invention.

Conventionally known symbol synchronization acquisition techniques include techniques using Gardner and Citta Loop. However, the present invention can accurately compensate for the linear distortion due to the channel filter applied to the complex pre-equalizer by distinguishing and compensating the symbol sync distortion among the linear distortion compensation.

Referring to FIG. 6, the symbol synchronization distortion compensator 410 may include an interpolator 610 and a phase compensator 620.

The interpolator 610 compensates the timing offset by receiving the frequency down signal in which the symbol delay and the symbol power stored in the memory buffer are aligned. The phase compensator 620 compensates for the phase offset by receiving a signal whose timing offset is compensated. An operation performed by each interpolator 610 and the phase compensator 620 may be expressed by the following equation.

[ Equation  One]

[ Equation  2]

i l = 0 l = 1 l = 2 l = 3 2 0 -1/6 0 1/6 One 0 One 1/2 -1/2 0 One -1/2 -One 1/2 -One 0 -1/3 1/2 -1/6

및

는 타이밍 오프셋 업데이트 계수과 위상 오프셋 업데이트 계수를 나타내며,

는 표1에 표시된 것처럼, Cubic 인터폴레이터의 계수를 나타내며, N 은 인터폴레이터의 차수를 나타낸다. I₁, I₂는 표1에서 보면, i 열에서 해당하는 계수를 가리키며, i 에서 2,1,0,-1 중 임의의 경우가 될 수 있다. 예를 들면, I₁이 -1이고, I₂가 2이면, i 는 -1에서 2인 경우까지 -1, 0, 1, 2에 해당하는 4번의 연산을 필요로 한다. 표1은 임의의 계수에 대한 표이므로 고정된 것이 아니다. 정렬된 주파수 하향신호는 수학식 1과 2를 거쳐 최종적으로 심볼동기 왜곡을 보상한다. 이 때, 와

값은 적응 선형 왜곡 추정부를 통해 계산하게 된다.

,

의 초기값은 0이다. x(i)는 정렬된 주파수 하향신호로 입력신호이며, y(t)는 타이밍 오프셋이 보상된 신호, z(t)는 위상 오프셋이 보상되어 최종적으로 심볼 동기 왜곡이 보상된 신호이다.Equation 1 is an equation applied to the interpolator 610, and Equation 2 is an equation applied to the phase compensator 620. At this time,

And

Denotes a timing offset update coefficient and a phase offset update coefficient,

Denotes the coefficient of the Cubic interpolator as shown in Table 1, and N denotes the order of the interpolator. In Table ₁ , I ₁ and I ₂ indicate corresponding coefficients in column i, and may be any case of 2,1,0, -1 in i. For example, if I ₁ is -1 and I ₂ is 2, i requires four operations corresponding to -1, 0, 1, 2 from -1 to 2. Table 1 is not fixed because it is a table of arbitrary coefficients. The aligned frequency downlink signal finally compensates for symbol synchronization distortion through Equations 1 and 2. At this time, Wow

The value is calculated by the adaptive linear distortion estimator.

,

The initial value of is 0. x (i) is an input frequency downlink signal, y (t) is a timing offset compensated signal, and z (t) is a phase offset compensated signal.

7 is a block diagram of an adaptive linear distortion estimation unit according to an embodiment of the present invention.

Referring to FIG. 7, the adaptive linear distortion estimator 420 adaptively uses the aligned frequency downlink signal z (t) and the linear distortion compensation signal r (t) aligned with the symbol synchronization distortion compensation. It may include an adaptive processor 710 for updating the equalization coefficient and the symbol synchronization coefficient. The operation of the adaptive processor 710 may be expressed by the following equation.

[ Equation  3]

는 길이 K인 전치 등화기 필터계수를 나타내며,

이다. 생성된 전치 등화기 필터 계수는 메모리에 저장되었다가 추후 전치 등화기에 적용 된다. 적응적 처리부(710)는 적응적으로 등화 계수 및 심볼 동기 계수를 업데이트하기 위해 기준 신호와 비교 신호의 잔차 오류를 최소화하는 방향으로 업데이트하는 LMS 기법을 적용하였으며, 필터 업데이트 수식은 다음과 같다.

Denotes the pre-equalizer filter coefficient of length K,

to be. The generated pre-equalizer filter coefficients are stored in memory and later applied to the pre-equalizer. The adaptive processor 710 applies an LMS technique for adaptively updating the equalization coefficient and the symbol synchronization coefficient in a direction of minimizing the residual error of the reference signal and the comparison signal, and the filter update equation is as follows.

[ Equation  4]

은 필터 계수의 스텝 사이즈(step-size) 값을 나타낸다. 스텝 사이즈 값은 임의로 설정할 수 있으며, 추정되는 계수의 정확도에 따라 설정할 수 있다. err 은 오차신호를 의미하며, 정렬된 선형 왜곡 보상신호(r(t))와 적응적 처리부(710)에서 처리된 정렬된 주파수 하향 변환 신호(f(t))의 차이를 의미한다. 즉, 기준 신호와 비교 신호의 차이를 의미한다. Z^*는 정렬된 주파수 하향 변환 신호

의 공액 복소수를 의미한다. 적응적 처리부(710)는 적응적인 계산을 통해 전치 등화기의 필터계수, 즉, 등화계수를 계산할 수 있다.At this time,

Denotes a step-size value of the filter coefficient. The step size value can be set arbitrarily and can be set according to the accuracy of the estimated coefficient. err means an error signal and means a difference between the aligned linear distortion compensation signal r (t) and the aligned frequency down-converted signal f (t) processed by the adaptive processor 710. That is, the difference between the reference signal and the comparison signal. Z ^* is an aligned frequency downconversion signal

Means the conjugate complex number. The adaptive processor 710 may calculate a filter coefficient of the pre-equalizer, that is, the equalization coefficient through adaptive calculation.

와

계수 역시 다음의 적응적 필터 업데이트 수식을 통해 계산할 수 있다. 심볼 동기 계수는 타이밍 오프셋 업데이트 계수(

) 및 위상 오프셋 업데이트 계수(

)를 포함할 수 있다.In addition, the adaptive processor 710 may compensate for symbol synchronization distortion.

Wow

Coefficients can also be calculated using the following adaptive filter update formula: The symbol synchronization coefficient is the timing offset update coefficient (

) And phase offset update coefficient (

) May be included.

[ Equation  5]

[ Equation  6]

와

은 심볼동기 계수의 step-size 를 나타내며, err_re는 정렬된선형 왜곡 보상 신호의 제곱 값과 등화 계수의 공액 복소수에 심볼동기왜곡 보상된 신호를 곱한 값의 제곱 값의 차이, Zu_k는 심볼동기왜곡 보상된 신호를 u_k로 편미분한 값, Z_θ는 상기 심볼동기왜곡 보상된 신호를

로 편미분한 값을 나타낸다. 즉, err_re는 오차신호의 실수부를 의미한다. 스텝 사이즈 값은 임의로 설정할 수 있으며, 추정되는 계수의 정확도에 따라 설정할 수 있다.

는

의 실수부(Ich) 값과 허수부(Qch) 값을 나타낸다. 따라서, 수학식5,6을 이용하면 적응적으로

와

값을 계산할 수 있으며, 추후 심볼 동기 왜곡 보상부에서 적용된다. 따라서, 적응 선형 왜곡 추정부를 이용하여 선형왜곡을 제거할 수 있으며, 채널필터에 의한 선형왜곡특성과 심볼동기 왜곡에 의한 선형왜곡특성을 구별하여 추정함으로써 전치등화기 필터 계수를 효율적으로 추정이 가능하다. 적응적 처리부(710)는 메모리 버퍼에 저장된 정렬된 주파수 하향 변환 신호와 정렬된 선형 왜곡 보상 신호를 계속적으로 입력받아 등화 계수, 타이밍 오프셋 업데이트 계수, 위상 오프셋 업데이트 계수를 적응적으로 추정한다.At this time,

Wow

Is the step-size of the symbol synchronization coefficient, err _re is the difference between the square value of the aligned linear distortion compensation signal and the conjugate complex number of the equalization coefficient multiplied by the symbol synchronization distortion compensated signal, and Zu _k is symbol synchronization. A value obtained by differentially dividing a distortion compensated signal by u _k , Z _θ represents the symbol synchronous distortion compensated signal.

Denotes the partial derivative. In other words, err _re means the real part of the error signal. The step size value can be set arbitrarily and can be set according to the accuracy of the estimated coefficient.

Is

The real part (Ich) value and the imaginary part (Qch) value of are shown. Therefore, by using Equations 5 and 6,

Wow

The value can be calculated and applied later in the symbol sync distortion compensator. Accordingly, the linear distortion can be removed using the adaptive linear distortion estimator, and the pre-equalizer filter coefficients can be estimated efficiently by distinguishing and estimating the linear distortion characteristic due to the channel filter and the linear distortion characteristic due to the symbol synchronous distortion. . The adaptive processor 710 continuously receives the aligned frequency down-conversion signal and the aligned linear distortion compensation signal stored in the memory buffer to adaptively estimate the equalization coefficient, the timing offset update coefficient, and the phase offset update coefficient.

8 is a block diagram of an adaptive nonlinear distortion estimation unit according to an embodiment of the present invention.

Referring to FIG. 8, the adaptive nonlinear distortion estimator 430 may include a lookup table generator 810, a memory 820, and an adaptive updater 830.

The lookup table generator 810 generates a lookup table (LUT) based on the nonlinear distortion compensation signal and the estimated linear distortion signal. The lookup table generator 810 generates a LUT using the nonlinear distortion compensation signal k (t) as a reference signal and the estimated linear distortion signal f (t) as a comparison signal. The estimated linear distortion signal can be used as an accurate comparison signal because the symbol synchronization distortion is compensated and the linear distortion is compensated to have only the nonlinear characteristics of the high output amplifier. The lookup table generator 810 generates AM / AM and AM / PM symbols using the aligned reference signals and the comparison signals stored in the memory buffer, and takes the average value of the symbols that enter the same LUT interval as shown in the following equation. Select the interval representative value. The lookup table generator 810 may improve the nonlinear distortion compensation performance by minimizing the residual error component when constructing the nonlinear distortion compensation table by taking the average value of the symbols entering the same LUT section as the section representative value. Accordingly, the lookup table generator 810 may use the same random distribution of other noise values, such as jitter noise and white Gaussian noise (AWGN) scattered in each AM / AM AM / PM symbol. ) Characteristics can be minimized by calculating the average value of the interval unit.

[ Equation  7]

In this case, N represents the number of symbols coming into a specific LUT interval. The method using the above equation has the advantage of easy implementation, but when only one LUT is generated, the nonlinear distortion compensation performance is relatively lower than the polynomial method using the curve fitting. Therefore, it is necessary to update the LUT repeatedly to improve the nonlinear distortion compensation performance.

Also, the memory 820 stores a lookup table generated by the lookup table generator 810 and a lookup table updated by the adaptive updater. The memory 820 provides the predistorter with the last updated lookup table when a predetermined number of lookup table updates are completed through signals stored in the memory buffer.

In addition, the adaptive updater 830 adaptively updates the lookup table based on the input of the estimated linear distortion signal stored in the memory buffer based on the generated lookup table. The adaptive updater 830 updates the existing lookup table stored in the memory by the table unit as much as the nonlinear distortion compensation signal and the estimated linear distortion signal stored in the memory buffer. If the newly created LUT is defined as LUT_new and the LUT currently stored in memory is defined as LUT_old, the LUT is updated by using the following table-based update formula.

[ Equation  8]

는 LUT 업데이트의 스텝 사이즈(step-size)를 나타낸다. 적응적 업데이트부(830)는 LUT의 업데이트를 통해, 최종적으로 LUT이 HPA 특성에 적응적으로 따라가도록 생성이 가능하다. 기존의 LUT 업데이트는 Symbol 단위의 입력을 받아 LUT의 특정구간에 해당하는 값으로 Index mapping하고, symbol 단위로 LUT 구간 마다 따로 업데이트하였다. 그런데, 적응적 업데이트부(830)는 테이블단위의 업데이트를 통해 Jitter noise에 더 강인한 특성을 보이며, 나아가 비선형왜곡보상 테이블을 구성할 때 잔류 오류성분을 최소화 하여 비선형 왜곡 보상 성능을 향상시킬 수 있다. 왜냐하면, 적응적 업데이트부(830)는 각각의 AM/AM AM/PM심볼에 산재되어 있는 지터 노이즈(Jitter noise)값들이 가지는 동일 확률분포(Uniform Random Distribution)특성을 구간단위의 평균값 계산을 통해 최소화 할 수 있기 때문이다. 또한, 적응적 업데이트부(830)는 HPA의 주변온도나 노후화로 인한 특성변화에 적응적으로 대체할 수 있다.At this time,

Represents the step-size of the LUT update. The adaptive updater 830 may generate the LUT to adaptively follow the HPA characteristics through the update of the LUT. Existing LUT update receives input in symbol unit and index mapping to the value corresponding to a specific section of LUT, and updates each LUT section in symbol unit. However, the adaptive updater 830 exhibits more robust characteristics to the jitter noise through the table unit update, and further improves the nonlinear distortion compensation performance by minimizing residual error components when constructing the nonlinear distortion compensation table. Because the adaptive updater 830 minimizes the uniform random distribution characteristic of the jitter noise values scattered in each AM / AM AM / PM symbol by calculating the average value of each unit Because you can. In addition, the adaptive update unit 830 may adaptively replace the characteristic change caused by the ambient temperature or aging of the HPA.

The automatic linear and nonlinear distortion compensator according to an embodiment of the present invention adaptively updates symbol synchronization coefficients, equalization coefficients, and LUTs by using signals from the complex pre-equalizer and the predistorter after the modulator. Can be updated based on the value before the update. The symbol synchronization coefficients and equalization coefficients are adaptively updated through iteration based on the coefficients of the previous stage as much as the signals stored in the memory buffer in the adaptive linear distortion compensation apparatus. The LUT is adaptively updated through iteration based on the LUT of the previous stage as much as the signal stored in the memory buffer in the adaptive nonlinear distortion compensator.

9 is a flowchart of an automatic linear and nonlinear distortion compensation method according to an embodiment of the present invention.

The transmission system according to an embodiment of the present invention initializes (910) the estimated count. Coefficients and LUTs applied to the complex pre-equalizer and the predistorter are determined when the predetermined number of estimates is satisfied. Therefore, when the estimation starts, the estimation count is initialized to count the estimation number.

In addition, the transmission system according to an embodiment of the present invention compensates for symbol synchronization distortion due to timing and phase offset based on the downlink signal output from the high output amplifier and whose frequency is down (920). The transmission system according to an embodiment of the present invention compensates for the symbol synchronization distortion caused by the timing and phase offset of the comparison signal generated while passing through the high output amplifier and the channel filter. The difference from the prior art is that the symbol synchronization distortion is compensated by distinguishing it from the portion which estimates the linear distortion.

Further, the transmission system according to an embodiment of the present invention adaptively estimates the linear distortion based on the signal whose symbol synchronization distortion is compensated for and the linear distortion compensation signal output from the complex pre-equalizer (930). In addition, the transmission system according to an embodiment of the present invention can adaptively estimate linear distortion based on a downlink signal whose frequency is not compensated for symbol synchronization distortion and a linear distortion compensation signal output from a complex pre-equalizer. have. The transmission system according to the embodiment of the present invention updates the LMS to minimize the residual error of the reference signal output to compensate for the linear distortion caused by the channel filter in the comparison signal and the complex pre-equalizer with symbol synchronization distortion compensation. The equalization coefficient and the symbol synchronization coefficient are estimated by applying the (Least Mean Square) technique.

In addition, the transmission system according to the embodiment of the present invention adaptively estimates the nonlinear distortion based on the estimated linear distortion signal and the nonlinear distortion compensation signal output from the predistorter (940). A transmission system according to an embodiment of the present invention generates and updates a LUT using a comparison signal compensated for linear distortion and a reference signal output to compensate for nonlinear distortion caused by a high output amplifier in a predistorter.

In addition, the transmission system according to an embodiment of the present invention increases the estimated count by one (950). The equalization coefficient and the symbol synchronization coefficient required for the linear distortion compensation are estimated, and the LUT required for the nonlinear distortion compensation is updated, and the estimated count is increased by one.

In addition, the transmission system according to an embodiment of the present invention applies the estimated equalization coefficient to the complex preequal equalizer (960). By applying the estimated equalization coefficients, the complex pre-equalizer equalizes the complex pre-equalization to the input signal corresponding to the linear distortion due to the channel filter. The signal output from the complex pre-equalizer is provided as a linear distortion compensation signal and provided as a reference signal to the automatic linear and nonlinear distortion compensation device.

In addition, the transmission system according to an embodiment of the present invention applies the LUT generated in the predistorter (970). The predistorter uses an updated LUT to generate a nonlinear distortion compensation signal corresponding to the nonlinear distortion caused by the high output amplifier. The signal output from the predistorter is provided as a reference signal to the automatic linear and nonlinear distortion compensation device as a nonlinear distortion compensation signal.

In addition, the transmission system according to an embodiment of the present invention determines whether the estimated count is greater than or equal to the preset number a (980). If the estimation count is less than the preset estimation number a, the estimation is repeated again. If the estimation count is greater than or equal to a, the estimation is terminated and the transmission signal is transmitted through the frequency uplink, high power amplification, channel filter, and antenna by applying the determined coefficient and LUT. do.

10 is a detailed flowchart of a symbol synchronization distortion compensation step according to an embodiment of the present invention.

The transmission system according to an embodiment of the present invention may compensate for the timing offset (1010). The transmission system according to an embodiment of the present invention can compensate for the timing offset by receiving a frequency down signal in which the symbol delay and the symbol power stored in the memory buffer are aligned.

In addition, the transmission system according to an embodiment of the present invention may compensate for the phase offset (1020). A transmission system according to an embodiment of the present invention may compensate for a phase offset of a signal whose timing offset is compensated. Details of symbol synchronization distortion compensation are the same as those described with reference to FIG. 6.

11 is a detailed flowchart of an adaptive linear distortion compensation step according to an embodiment of the present invention.

The transmission system according to an embodiment of the present invention may generate a transposition equalization coefficient (1110). The transmission system according to an embodiment of the present invention may adaptively update the filter coefficients by using the aligned frequency downlink signal with symbol synchronization distortion compensation and the linear distortion compensation signal aligned with each other. The transmission system according to an embodiment of the present invention applies an LMS technique for adaptively updating the filter coefficients in a direction to minimize the residual error of the reference signal and the comparison signal. The generated pre-equalization coefficient is adaptively updated as much as the signal stored in the memory buffer, and later applied to the complex pre-equalizer.

In addition, the transmission system according to an embodiment of the present invention may generate a symbol synchronization coefficient (1120). The generated symbol synchronization coefficients are updated as adaptively as the signals stored in the memory buffer, and later applied to compensate for symbol synchronization distortion. Details of the adaptive linear distortion compensation are the same as those described with reference to FIG. 7.

12 is a detailed flowchart of an adaptive nonlinear distortion compensation step according to an embodiment of the present invention.

The transmission system according to an embodiment of the present invention may generate a lookup table (1210). The transmission system according to an embodiment of the present invention may generate a lookup table (LUT) based on the nonlinear distortion compensation signal and the estimated linear distortion signal. In the adaptive nonlinear distortion compensation step, the LUT may be generated using the nonlinear distortion compensation signal as a reference signal and the estimated linear distortion signal as a comparison signal.

In addition, the transmission system according to an embodiment of the present invention may update the lookup table (1220). The transmission system according to an embodiment of the present invention may adaptively update the lookup table according to the input of the estimated linear distortion signal stored in the memory buffer based on the generated lookup table. The adaptive nonlinear distortion compensation step may update the existing lookup table stored in the memory by the table unit as much as the nonlinear distortion compensation signal and the estimated linear distortion signal stored in the memory buffer. Details of the adaptive nonlinear distortion compensation are the same as those described with reference to FIG. 8.

FIG. 13 is a graph illustrating generation and display of AM / AM AM / PM symbols using aligned reference signals and comparison signals stored in a memory buffer. The method of generating the LUT generates AM / AM and AM / PM symbols using the aligned reference signals and the comparison signals stored in the memory buffer, and selects the average value of the symbols coming into the same LUT interval as the interval representative value.

14 is a graph showing the distribution probability of AM / AM AM / PM symbols coming into each LUT interval.

Referring to FIG. 14, it can be seen that samples entering each LUT interval do not enter all LUTs with the same probability but have different symbol generation probabilities according to LUT intervals. Therefore, the LUT update increases the step size in a section with a large number of samples in proportion to the probability distribution, and increases the step size in a section with a small number of samples, rather than having a fixed step size. By applying a variable step-size technique to reduce the step size, the convergence speed of the LUT can be improved. The following equation shows how to calculate the LUT step-size value.

[ Equation  9]

는 최소 업데이트 스텝 사이즈(step-size)를 나타내고,

은 구간별 샘플 빈도 확률을 나타내며, M은 LUT을 만들기 위해 사용된 전체 AM, PM 샘플 수를 나타내고,

는 n번째 LUT구간에서 구간 평균값에 적용된 AM, PM 샘플 수를 나타낸다.At this time,

Represents the minimum update step size,

Denotes the probability of sample frequency per interval, M denotes the total number of AM and PM samples used to create the LUT,

Denotes the number of AM and PM samples applied to the mean value of the interval in the nth LUT interval.

Finally, the updated LUT value is stored in memory and applied to the predistorter later, and the automatic linear and nonlinear distortion compensation device recaptures the feedback signal to which linear and nonlinear distortion compensation are applied to generate equalization coefficient, symbol synchronization coefficient, and LUT. Repeat.

The present invention relates to an apparatus and method for compensating the nonlinear distortion characteristics of a high output amplifier and the linear distortion characteristics of a channel filter, which are problems of a transmission system used for wireless broadcasting and communication such as DMB, DVB-T, ISDB-T, and ATSC. will be.

Automatic linear and nonlinear distortion compensation apparatus according to an embodiment of the present invention removes the linear distortion components present in the reference signal and the comparison signal and estimates the HPA nonlinear distortion characteristics using the LUT. It retains the advantages of using a conventional LUT and shows better nonlinear distortion compensation by eliminating linear distortion components. In addition, the automatic linear and nonlinear distortion compensation apparatus according to an embodiment of the present invention proposes a table-based iterative LUT update technique for estimating the characteristics of the HPA that changes with time, and the HPA nonlinear distortion characteristics as a result of the iterative LUT update Demonstrates adaptive estimation. In addition, the automatic linear and nonlinear distortion compensation apparatus according to an embodiment of the present invention calculates an equalization coefficient to compensate for the linear distortion component caused by the channel filter in the residual distortion component, and calculates a complex pre-equal equalizer (Complex). Applied to pre-equalizer).

The complex pre-equalizer according to the embodiment of the present invention may be located after the modulator.

, 이때,등화기탭계수(K=등화기탭계수)

,

를 나타낸다. 전치 등화기 탭이 실수부만 있을 때 위 식은 다음과 같이 쓸 수 있다.If the pre-equalizer input is x (t) and the output is y (t), y (t) is obtained as

Equalizer tap coefficient (K = equal tap coefficient)

,

Indicates. When the pre-equalizer tab has only a real part, the above expression can be written as

Therefore, it can be represented by K real multiplication and K real addition.

When the pre-equalizer tab has a complex part, the equation can be written as

Therefore, it can be represented by 4K real multiplication and 4K real addition.

In the complex pre-equalizer, real multiplication and real addition operations increase by four times. However, existing methods must pass through additional VSB demodulators to obtain the preequalizer coefficients. Therefore, the amount of computation required as an additional VSB demodulator is required, which requires much more multiply and add operations than the computation in the pre-equalizer. As a result, using a complex pre-equalizer can reduce the complexity in hardware implementation.

In addition, complex pre-equalizers are applicable regardless of the particular modulation and demodulation scheme (eg, VSB / QAM / PSK, etc.). This is because the complex pre equalizer is applied to the rear end of the modulator.

In addition, existing real pre-equalizers are only applicable to certain modulation schemes (PAM series or BPSK) that have data only on real numbers. However, the QAM-based modulation scheme recently used has data in the imaginary part as well as data in the imaginary part, so the realization equalizer has a limitation in its equalization performance.

Methods according to an embodiment of the present invention can be implemented in the form of program instructions that can be executed by various computer means and recorded in a computer readable medium. The computer readable medium may include program instructions, data files, data structures, etc. alone or in combination. Program instructions recorded on the media may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well-known and available to those having skill in the computer software arts.

As described above, the present invention has been described by way of limited embodiments and drawings, but the present invention is not limited to the above embodiments, and those skilled in the art to which the present invention pertains various modifications and variations from such descriptions. This is possible.

Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the claims below but also by the equivalents of the claims.

Claims (17)

An adaptive linear distortion estimator for adaptively estimating the linear distortion of the downlink signal with the frequency down based on the downlink signal output from the high power amplifier and downlink frequency down and the linear distortion compensation signal output from the complex pre-equalizer; And
Adaptive nonlinear distortion estimator for adaptively estimating nonlinear distortion of the estimated linear distortion signal based on the estimated linear distortion signal and the nonlinear distortion compensation signal output from the predistorter The method of claim 1,
A symbol synchronous distortion compensator for compensating symbol synchronous distortion of a downlink signal output from the high output amplifier with a frequency down
Automatic linear and nonlinear distortion compensation device further comprising. The method of claim 2,
The symbol synchronization distortion compensation unit,
An interpolator for compensating for a timing offset in the downlink signal having the frequency down; And
A phase compensator for compensating a phase offset in the signal in which the timing offset is compensated
Automatic linear and nonlinear distortion compensation device comprising a. The method of claim 2,
The adaptive linear distortion estimator,
An adaptive processor for adaptively updating equalization coefficients and symbol synchronization coefficients based on the signal for which the symbol synchronization distortion is compensated and the linear distortion compensation signal.
Automatic linear and nonlinear distortion compensation device comprising a. The method of claim 4, wherein
The equalization coefficient is adaptively calculated through Equation 1,
Where H _n is the equalization coefficient of the complex pre-equalizer, gain1 is the step size of the equalization coefficient, err is the difference between the linear distortion compensation signal and the aligned frequency down-converted signal adaptively processed, and Z ^* is symbol synchronization distortion compensation. Conjugate complex value of the signal,
[Equation 1]

The symbol synchronization coefficient includes a timing offset update coefficient and a phase offset update coefficient,
The timing offset update coefficient is adaptively calculated through Equation 2,
Where u _k is a timing offset update coefficient, gain2 is a step size of the timing offset update coefficient, and err _re is a square value of a product of a symbol synchronization distortion-compensated signal multiplied by a conjugate complex number of the linear distortion compensation signal and an equalization coefficient. Zu _k is a value obtained by differentially dividing the symbol synchronous distortion-compensated signal by u _k ,
[Equation 2]

The phase offset update coefficient is adaptively calculated through Equation 3,
here,

Is the phase offset update coefficient _, gain3 is the step size of the phase offset update coefficient, and Z _θ is the symbol synchronous distortion compensated signal.

The partial derivative of
[Equation 3]

Automatic linear and nonlinear distortion compensation device, characterized in that. The method of claim 1,
The adaptive nonlinear distortion estimator,
A lookup table generator configured to generate a lookup table (LUT) based on the nonlinear distortion compensation signal and the estimated linear distortion signal; And
An adaptive updater adaptively updating the lookup table according to the input of the estimated linear distortion signal stored in a memory buffer based on the generated lookup table
Automatic linear and nonlinear distortion compensation device comprising a. The method of claim 6,
The adaptive update unit,
Here, when LUT_old is a lookup table currently stored in memory, LUT_new is a newly created lookup table, and μ is a step size, automatic linear and nonlinear distortion compensation is performed by using the following formula. Device.

The method of claim 7, wherein
When updating by the table unit, the automatic linear and nonlinear distortion compensation device, characterized in that for applying the step size variably according to the symbol occurrence probability according to the interval of the lookup table. The method of claim 1,
And the signals output from the complex pre-equalizer and the predistorter are signals after the modulator. The method of claim 1,
A symbol delay alignment unit for aligning a symbol delay between the linear distortion compensation signal and the frequency downlink signal; And
Symbol power normalization unit for normalizing the symbol power of each signal in which the symbol delay is aligned
Automatic linear and nonlinear distortion compensation device further comprising. The method of claim 1,
A symbol delay alignment unit for aligning a symbol delay between the estimated linear distortion signal and the nonlinear distortion compensation signal; And
Symbol power normalization unit for normalizing the symbol power of each signal in which the symbol delay is aligned
Automatic linear and nonlinear distortion compensation device further comprising. The method of claim 1,
The signals output from the complex pre-equalizer and the predistorter are updated based on the values before the update when adaptively updating the symbol synchronization coefficients, the equalization coefficients, and the lookup table by using the signals of the modulator rear ends. Automatic linear and nonlinear distortion compensation device. Adaptively estimating a linear distortion of the downlinked downlink signal based on a downlink signal output from a high output amplifier downlinked in frequency and a linear distortion compensation signal output from a complex pre-equalizer; And
Adaptively estimating nonlinear distortion of the estimated linear distortion signal based on the estimated linear distortion signal and the nonlinear distortion compensation signal output from the predistorter
Automatic linear and nonlinear distortion compensation method comprising a. The method of claim 13,
Compensating for the symbol synchronization distortion of the downlink signal output from the high power amplifier and downlinked in frequency
Automatic linear and nonlinear distortion compensation method further comprising. The method of claim 14,
Compensating the symbol synchronization distortion,
Compensating for a timing offset in the downlink signal from which the frequency is down; And
Compensating for a phase offset in the signal with the timing offset compensated
Automatic linear and nonlinear distortion compensation method comprising a. The method of claim 14,
The adaptively estimating linear distortion,
Adaptively updating equalization coefficients and symbol synchronization coefficients based on the signal whose symbol synchronization distortions are compensated for
Automatic linear and nonlinear distortion compensation step comprising. The method of claim 13,
The adaptively estimating nonlinear distortion may include:
Generating a lookup table (LUT) based on the nonlinear distortion compensation signal and the estimated linear distortion signal; And
Adaptively updating a lookup table according to an input of the estimated linear distortion signal stored in a memory buffer based on the generated lookup table
Automatic linear and nonlinear distortion compensation step comprising.

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