KR101194817B1 - Digital pre-distortion apparatus and method for mimo system in the presence of crosstalk between signals - Google Patents

Abstract

The present invention relates to a predistortion apparatus and method for power amplifier linearization in a MIMO transmitter in which crosstalk exists in a transmission path. The present invention relates to a linearization of a power amplifier even when crosstalk between signals exists in a power amplifier input / output stage of a MIMO system. The purpose is to achieve.
The present invention for achieving this object, the M power amplifier non-linear compensator for compensating the non-linear characteristics of the M power amplifiers in the MIMO transmitter; An MxM matrix input stage crosstalk compensator for compensating crosstalk present in the M power amplifier input stages; An MxM matrix output stage compensator to compensate for crosstalk present in the M power amplifier output stages; And an adaptive counter for extracting coefficients of the power amplifier nonlinear compensator, the input crosstalk compensator, and the output crosstalk compensator (300) to compensate for the power amplifier input / output crosstalk and the nonlinear characteristics of the power amplifier. And a control unit.

Description

Predistortion apparatus and method for linearizing power amplifier in MIO transmitters where crosstalk exists in the transmission path TECHNICAL FIELD

The present invention relates to a predistortion apparatus and method for a multiple input multiple output (MIMO) transmitter, and more particularly to a transmission that enables linearization of power amplifiers in a MIMO transmitter where crosstalk exists on a multiple transmission signal path. A predistortion apparatus and method for power amplifier linearization in a MIMO transmitter where crosstalk exists in the path.

Predistortion is a technique that maximizes the efficiency of a power amplifier by compensating for the nonlinear characteristics of the power amplifier. The predistortion technology applicable to a single amplifier can be applied to MIMO systems using multiple amplifiers. However, if crosstalk exists between multiple transmission paths of a MIMO transmitter, the linearization of power amplifiers cannot be achieved by applying the existing technology as it is.

On the other hand, power amplifiers are essential devices in communication systems, and generally have nonlinear characteristics. Inband distortions that cause performance degradation, such as spectral regrowth and bit error rate (BER), which cause interference between adjacent channels, mainly come from nonlinearities in the power amplifier. To compensate for this nonlinearity, various techniques such as predistortion, feedforward, and feedback have been proposed. Among them, digital predistortion (DPD) has the advantages of simple structure, low component and low cost, which is widely used for linearizing power amplifiers.

Digital predistortion technology implements a digital predistorter at the baseband, which has the opposite characteristics as a power amplifier, so that the reversely distorted signal through the predistorter shows the linear characteristics as a whole through the nonlinear characteristics of the power amplifier. do.

Predistortion technology is directly applicable to MIMO communication systems because independent multiple radio frequency (RF) paths are integrated into one device for multiple antenna transmission. Recent advances in CMOS technology have resulted in the integration of MIMO transmitters into a single chip, as well as the integration of amplifiers and predistortion functions into one. However, crosstalk crosstalk problems have been reported in MIMO integrated chipsets. In the presence of such crosstalk, existing predistortion techniques do not operate normally. Recently, we have proposed a predistortion technique when crosstalk exists in MIMO systems. However, this technique only considers crosstalk at the power amplifier input stage (expressed as nonlinear crosstalk has occurred) and assumes that crosstalk at the output stage (expressed as linear crosstalk has occurred) is compensated at the receiver.

However, if there is crosstalk at the output stage of the power amplifier, the predistortion feedback signals interfere with each other, so that the correct predistortion coefficient cannot be obtained. Therefore, in the MIMO communication system, the predistortion algorithm should consider not only the crosstalk at the input but also the crosstalk at the output.

The present invention has been made in view of the above problems, and an object thereof is to achieve linearization of a power amplifier even when crosstalk between signals exists in a power amplifier input / output stage of a MIMO system.

The present invention for achieving the technical problem relates to a predistortion device for linearizing the power amplifier in the MIMO transmitter with crosstalk in the transmission path, M power to compensate for the non-linear characteristics of the M power amplifier in the MIMO transmitter Amplified nonlinear compensators; An MxM matrix input stage crosstalk compensator for compensating crosstalk present in the M power amplifier input stages; An MxM matrix output stage compensator to compensate for crosstalk present in the M power amplifier output stages; And an adaptive counter for extracting coefficients of the power amplifier nonlinear compensator, the input crosstalk compensator, and the output crosstalk compensator to compensate for the power amplifier input / output crosstalk and the nonlinear characteristics of the power amplifier. Characterized in that it comprises a.

The present invention relates to a predistortion method for power amplifier linearization in a MIMO transmitter in which crosstalk exists in a transmission path, the method comprising: extracting coefficients of a power amplifying nonlinear compensator, an input crosstalk compensator, and an output crosstalk compensator; (b) applying the coefficients extracted through step (a) to the power amplifier nonlinear compensator; And (c) the output of the power amplifying nonlinear compensator is amplified by a power amplifier; Characterized in that it comprises a.

According to the present invention as described above, even if crosstalk occurs at both the power amplifier input terminal and the output terminal in the MIMO transmission path, the non-linear characteristics of the power amplifiers are compensated for, so that the power amplifiers have a linear characteristic.

1 is an overall configuration diagram conceptually showing a predistortion apparatus for linearizing a power amplifier in a MIMO transmitter in which crosstalk exists in a transmission path according to the present invention.
2 is a diagram illustrating a baseband equivalent model when crosstalk exists at an input terminal and an output terminal of a power amplifier in a multi-antenna transmitter.
3 is an exemplary view showing a predistortion structure according to the present invention.
4 is an exemplary view showing an indirect learning structure according to the present invention.
5 is a graph for comparing the performance of a predistortion apparatus for power amplifier linearization in a MIMO transmitter in which crosstalk exists in a transmission path according to the present invention.

Specific features and advantages of the present invention will become more apparent from the following detailed description based on the accompanying drawings. It is to be noted that the detailed description of known functions and constructions related to the present invention is omitted when it is determined that the gist of the present invention may be unnecessarily blurred.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings.

A predistortion apparatus for power amplifier linearization in a MIMO transmitter in which crosstalk exists in a transmission path according to the present invention will be described with reference to FIGS. 1 to 5.

1 is a schematic diagram illustrating a predistortion device for power amplifier linearization in a MIMO transmitter in which crosstalk exists in a transmission path according to the present invention. As illustrated, nonlinear characteristics of M power amplifiers in a MIMO transmitter are illustrated. M power amplification nonlinear compensator 100 to compensate for the error, MxM matrix input crosstalk compensator 200 to compensate for crosstalk present in the M power amplifier input stages, and compensate for crosstalk present in the M power amplifier output stages. In order to compensate the nonlinear characteristics of the MxM matrix output stage crosstalk compensator 300 and the power amplifier input / output stage crosstalk and the power amplifier, the power amplifier nonlinear compensator 100, the input crosstalk compensator 200, and the output crosstalk compensator 300 may be used. And an adaptive counter 400 for extracting the coefficients.

Meanwhile, in the predistortion method for power amplifier linearization in a MIMO transmitter in which crosstalk exists in a transmission path according to the present invention, (a) a power amplifying nonlinear compensator 100, an input crosstalk compensator 200, and an output crosstalk compensator ( Extracting coefficients of (300), (b) applying the coefficients extracted through step (a) to the power amplifying nonlinear compensator 100, and (c) outputting the power amplifying nonlinear compensator 100 This step is amplified by the power amplifier.

Hereinafter, the predistortion device according to the present invention will be described in more detail.

2 is a baseband equivalent model when crosstalk exists at the input and output ends of a power amplifier in a multi-antenna transmitter.

The M transmit antenna systems each have a power amplifier in the M RF paths. The input value of the power amplifier having crosstalk of the j-th path is as shown in Equation 1 below.

[Equation 1]

, 는 i번째 경로에서 j번째 경로의 누화 계수이며

= 1이다. 이 신호들은 전력 증폭기에 의해 증폭된다. 만약 전력 증폭기가 memoryless 특성을 갖는다고 가정하면 전력 증폭기는 다음의 [수식 2] 와 같이 표현될 수 있다.here

, Is the crosstalk coefficient for the j th path in the i th path

= 1 These signals are amplified by the power amplifier. If it is assumed that the power amplifier has a memoryless characteristic, the power amplifier may be expressed as Equation 2 below.

[Equation 2]

는 j번째 전력 증폭기의 특성 계수이다. 증폭 후, 증폭된 출력 값들은 출력단에서의 누화에 의해 다음의 [수식 3] 과 같이 서로 간섭한다.Where 2L-1 is a nonlinear order,

Is the characteristic coefficient of the j th power amplifier. After amplification, the amplified output values interfere with each other as shown in Equation 3 by crosstalk at the output stage.

[Equation 3]

는 i번째 경로에서 j번째 경로로의 누화 계수이며,

= 1이다. At this time,

Is the crosstalk coefficient from the i th path to the j th path,

= 1

By combining [Equation 1], [Equation 2] and [Equation 3], the input / output relational expression of the power amplifier with which crosstalk is generated can be obtained as shown in [Equation 4] below.

[Equation 4]

In the predistortion algorithm, the coefficients for compensating a crosstalk generated power amplifier are calculated by using the amplifier output signal including the above crosstalk. However, in the existing predistortion algorithm, the coefficients of the predistorter cannot be obtained without compensating crosstalk at the power amplifier output stage because the feedback signal is deteriorated by the crosstalk of the output stage. Therefore, the present invention proposes a predistortion structure and algorithm for linearizing a power amplifier when crosstalk is present at the input and output ends.

A compensation block for compensating for nonlinearity when crosstalk occurs may be configured as shown in FIG. 3. The proposed compensation block consists of three parts: output crosstalk compensation block, power amplifier nonlinear compensation block, and input crosstalk compensation block in order. Each block compensates for crosstalk and nonlinear characteristics that occur in the power amplifier section. If you rewrite Equation 1 in matrix form,

[Equation 4]

이고, A는 성분이

인 M×M 행렬이다. At this time,

Where A is the component

Is an M × M matrix.

로 디자인 되는데, F는 입력 누화 보상을 위한 M×M 행렬이다. 그러므로, 입력 누화 보상기에서 입력값과 출력값의 관계는

과 같고, In Equation 4, the compensator for nonlinear crosstalk

Where F is the M × M matrix for input crosstalk compensation. Therefore, the relationship between input value and output value in input crosstalk compensator

Is the same as

이다. At this time,

to be.

In the same manner, it is represented by the following [formula 5] in [formula 3].

[Equation 5]

이때,

,

이고, B는 성분이

인 M×M 행렬이다.

At this time,

,

Where B is the component

Is an M × M matrix.

이고, G는 출력 누화 보상기에 대한 M×M 행렬이다. Therefore, linear crosstalk compensators

And G is the M × M matrix for the output crosstalk compensator.

이고,

,

이다. At this time,

ego,

,

to be.

Assuming the input and output crosstalks are compensated, the nonlinearity of the power amplifier can be compensated independently in each path.

Since it is difficult to find the exact nonlinear characteristics of the nonlinear model of Equation 2, the approximate inverse function model is generally used. The inverse characteristic can be expressed as Equation 6 below.

[Equation 6]

을 만족하도록 설계한다. 보상 블록들의 계수를 얻기 위해서는 누화가 발생된 전력 증폭기의 특성 정보를 미리 알아야 한다. Where 2Q-1 is a nonlinear order, Is the coefficient of the j th PD to compensate for the nonlinearity of the power amplifier.

Design to satisfy. In order to obtain the coefficients of the compensation blocks, it is necessary to know in advance the characteristic information of the power amplifier from which the crosstalk is generated.

Therefore, initially, it is necessary to estimate the characteristics of the power amplifier from which crosstalk has occurred. However, even without this estimation process, the coefficients of the proposed predistorter can be directly obtained by using indirect learning. Now we derive the algorithm for obtaining the coefficients of the proposed block as follows.

값에 의해 감쇄된다고 가정하면, 4 shows an indirect learning structure in which an adaptive compensation block is used in a feedback path to obtain an inverse function characteristic of a crosstalk generated power amplifier, and coefficient values calculated in the proposed compensation block are used as a feedforward path. applied to the compensator of path). The final output of the crosstalk generated power amplifier is the target gain.

Assuming it is attenuated by a value,

과

은

,

and

silver

,

으로 표현된다.

.

In the block of the jth feedback path, the output value is

이다.

to be.

In order to calculate the coefficient of the compensation block, a cost function is defined as in Equation 7 below.

[Formula 7]

,

이고, N 은 관찰된 데이터의 개수이다. 여기서는 feedforward 경로의 블록이 인지 과정에서 bypass 되었다고 가정한다. At this time,

,

And N is the number of observed data. In this case, it is assumed that the block of the feedforward path has been bypassed in the recognition process.

,

이 된다.then,

,

.

,

,

를 갱신하기 위한 적응 알고리즘은 stochastic gradient method를 적용하여 얻을 수 있고, N=1인 경우 최종 알고리즘은 전통적인 LMS(least mean squares) 알고리즘이다.

의 j번째 경로에서 갱신 알고리즘은 다음의 [수식 8] 과 같다.Coefficient

,

,

The adaptive algorithm for updating is obtained by applying stochastic gradient method, and when N = 1, the final algorithm is the traditional least mean squares (LMS) algorithm.

The update algorithm in the j-th path of is shown in [Equation 8].

[Equation 8]

,

는 스텝 사이즈 이다. 이와 같이 j번째 경로의 역 비선형 특성

에 대한 적응 알고리즘은 다음의 [수식 9] 와 같다.At this time,

,

Is the step size. Thus the inverse nonlinear nature of the j-th path

The adaptive algorithm for is given by Equation 9 below.

[Equation 9]

At this time, [Formula 10]

이고,

는 스텝 사이즈 이다.

ego,

Is the step size.

에 대한 적응 알고리즘은 [수식 11] 과 같다. Finally,

The adaptive algorithm for is given by Equation 11.

[Equation 11]

, At this time,

,

이고,

는 스텝 사이즈 이다.

ego,

Is the step size.

The coefficient update algorithms [Equation 8], [Equation 9] and [Equation 11] are calculated at the same time, and after convergence, the coefficients finally obtained are applied to the proposed predistortion block of the feedforward path.

The performance of the predistortion algorithm proposed in the present invention was verified by computer simulation. The simulation environment is as follows. For simplicity, we consider two transmit antenna systems (M = 2), the transmit data are generated independently and the transmit symbols are modulated by Quadrature Phase Shift Keying (QPSK).

), 위상은 임의로 선택되었다. 전력 증폭기의 비선형성을 모델링함에 있어서 [수식 13] 과 같은 Saleh model의 memoryless 비선형 모델이 쓰였다. The symbols of each path are transferred to an orthogonal frequency division multiplexing (OFDM) modulator having a total of 1024 subcarriers. Of the 1024 subcarriers, 840 subcarriers are used for data transmission and the rest are used for subcarrier protection. The amplitude of crosstalk at the front and back of the power amplifier is assumed to be 20 dB, ie,

), The phase was randomly selected. In modeling the nonlinearity of the power amplifier, a memoryless nonlinear model of Saleh's model is used.

[Equation 13]

5 compares power spectrum density of power amplifier output values when various PD algorithms are applied. Existing predistortion schemes do not account for crosstalk at the amplifier output stage, thus showing that the power amplifier is not linearized. However, the proposed method shows good linearization performance by compensating crosstalk at both the input and output stages of the amplifier.

In FIG. 5, (a) is an output of the power amplifier PA to which the predistorter PD is not applied, and (b) is an output of the power amplifier PA to which the conventional predistorter PD is applied. (c) is the output of the power amplifier PA applying the predistorter PD considering the conventional nonlinear crosstalk, and (d) is the output of the power amplifier PA using the proposed predistorter PD. e) is the output of the power amplifier PA to which the existing predistorter PD is applied when there is no crosstalk, and (f) is an input signal.

While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. It will be appreciated by those skilled in the art that numerous changes and modifications may be made without departing from the invention. Accordingly, all such appropriate modifications and changes, and equivalents thereof, should be regarded as within the scope of the present invention.

100: power amplifier nonlinear compensator 200: input crosstalk compensator
300: output crosstalk compensator 400: adaptive counter

Claims (6)

M power amplification nonlinear compensators 100 for compensating the nonlinear characteristics of the M power amplifiers in a MIMO transmitter;
An MxM matrix input stage crosstalk compensator (200) for compensating crosstalk present in the M power amplifier input stages;
An MxM matrix output stage compensator (300) for compensating crosstalk present in the M power amplifier output stages; And
An adaptive counter 400 for extracting coefficients of the power amplifier nonlinear compensator 100, the input crosstalk compensator 200, and the output stage crosstalk compensator 300 to compensate for crosstalk of the power amplifier input / output stage and nonlinear characteristics of the power amplifier; Including,
The adaptive counter 400,
The coefficients of the power amplifier nonlinear compensator 100, the input crosstalk compensator 200, and the output crosstalk compensator 300 for compensating the power amplifier input / output stage crosstalk and nonlinear characteristics are expressed by the following Equations 8, 9, and 9 A predistortion device for linearizing a power amplifier in a MIMO transmitter having crosstalk in a transmission path, characterized in that it is simultaneously extracted through Equation 11.
Equation 8

At this time,

,

Is the step size,
[Equation 9]

At this time,

ego,

Is the step size,
[Equation 11]

At this time,

,

ego,

Is the step size.
delete The method of claim 1,
In order to obtain an inverse function characteristic of a crosstalk generated power amplifier, the power amplifying nonlinear compensator 100, the input crosstalk compensator 200, and the output crosstalk compensator 300 are used in a feedback path.
Crosstalk is applied to the transmission path, characterized in that the values of the coefficients calculated from the power amplification nonlinear compensator 100, the input crosstalk compensator 200 and the output crosstalk compensator 300 are applied to the compensator of the feedforward path. Predistortion device for power amplifier linearization in existing MIMO transmitters.
(a) extracting coefficients of the power amplification nonlinear compensator 100, the input crosstalk compensator 200, and the output crosstalk compensator 300;
(b) applying the coefficients extracted through step (a) to the power amplifier nonlinear compensator (100); And
(c) amplifying the output of the power amplifying nonlinear compensator (100) by a power amplifier; Including,
In the step (a)
The coefficients of the power amplifier nonlinear compensator 100, the input crosstalk compensator 200, and the output crosstalk compensator 300 for compensating the power amplifier input / output stage crosstalk and nonlinear characteristics are expressed by the following Equations 8, 9, and 9 A predistortion method for linearizing a power amplifier in a MIMO transmitter in which crosstalk exists in a transmission path, which is simultaneously extracted through Equation 11.
Equation 8

At this time,

,

Is the step size,
[Equation 9]

At this time,

ego,

Is the step size,
[Equation 11]

At this time,

,

ego,

Is the step size.
delete The method of claim 4, wherein
In order to obtain an inverse function characteristic of a crosstalk generated power amplifier, the power amplifying nonlinear compensator 100, the input crosstalk compensator 200, and the output crosstalk compensator 300 are used in a feedback path.
Crosstalk is applied to the transmission path, characterized in that the values of the coefficients calculated from the power amplification nonlinear compensator 100, the input crosstalk compensator 200 and the output crosstalk compensator 300 are applied to the compensator of the feedforward path. Predistortion method for power amplifier linearization in existing MIMO transmitters.

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