KR20120041627A - Apparatus and method for equalizing a signal using canceling interference in a wiless communication system - Google Patents

Abstract

PURPOSE: An apparatus and a method for equalizing a signal using canceling interference in a wireless communication system are provided to remove interference of a receiving signal which is generated by transmitting signals. CONSTITUTION: An equalizing unit(510) equalizes a receiving signal. The equalizing unit estimates a transmitting signal. An interference removing signal generating unit(520) generates a receiving signal which is removed from the receiving signal through the estimated transmitting signal. The equalizing unit combines a plurality of equalized receiving signals.

Description

Equalization device and method using interference cancellation in wireless communication system {APPARATUS AND METHOD FOR EQUALIZING A SIGNAL USING CANCELING INTERFERENCE IN A WILESS COMMUNICATION SYSTEM}

The present invention is directed to interference cancellation in a wireless communication system. More particularly, the present invention relates to a method and apparatus for equalizing a received signal using interference cancellation in a wireless communication system.

The High Speed Packet Access (HSPA) system of the 3rd Generation Partnership Project (3GPP) provides high-order modulation (HOM) with 64-QAM (Quadrature Amplitude Modulation) and 2x2 multiple-input and It is characterized by the use of multiple-output systems.

The HSPA system is characterized by the use of a data channel (HS-PDSCH) having a small spreading factor (SF) compared to a WCDMA mobile communication system. Symbols with a large SF could effectively suppress intersymbol interference (ISI) or inter-cell interference (ICI) through de-spreading of the receiver. In case of SF symbol, it is fundamentally difficult to protect the symbol by only reverse spreading. Therefore, a method of estimating a transmission signal using an equalizer instead of a commonly used rake reception method is used.

However, the equalizer has an advantage of effectively inversion of the channel matrix. However, when the interference signal is large, the inversion performance deteriorates and the reliability of the estimated transmission signal is lowered. For this reason, performing equalization while properly processing interference signals has been an important research issue for increasing reception performance. Meanwhile, techniques for performing signal detection while eliminating interference in a CDMA system or a MIMO system using interference cancellation have been studied in the past, but are not suitable for application to an HSPA system that suffers from frequency selective fading. Therefore, there is a request for an equalizer for interference cancellation.

The present invention provides an equalization method and apparatus for improving reception performance in a MIMO system.

The present invention provides a method and apparatus for removing interference in a received signal generated by mutually independent transmission signals in a MIMO system.

The present invention provides a method and apparatus for minimizing an approximation error when approximating an exponential function to a linear function in a process of cleaning equalized symbols in a receiver in a MIMO system.

A method of equalizing a received signal in a multiple input multiple output (MIMO) wireless communication system comprising at least two transmit antennas and at least two receive antennas, the method comprising: equalizing the received signal to estimate the transmitted signal Process of doing; Generating a plurality of received signals from which interference is removed from the received signal by using the estimated transmission signal; Independently equalizing the plurality of received signals; Estimating a transmission signal from which interference is removed by combining the plurality of independently equalized reception signals.

An apparatus for equalizing a received signal in a multiple input multiple output (MIMO) wireless communication system comprising at least two transmit antennas and at least two receive antennas, the apparatus comprising: equalizing the received signal to estimate the transmitted signal An equalizer; An interference cancellation signal generator configured to generate a plurality of received signals from which the interference has been removed from the received signal by using the estimated transmission signal, wherein the equalizer equalizes the plurality of received signals independently and equalizes the independently Combine the plurality of received signals to estimate the interference-free transmission signal.

Representative effects of the configuration of the present invention are as follows.

The present invention can generate a signal which eliminates interference in a received signal generated by mutually independent transmission signals in a MIMO system, and equalizes the signal from which the interference is removed to improve the reliability of equalization, thereby improving reception performance. In addition, the present invention can reduce the approximation error when approximating the exponential function to a linear function in the process of cleaning the equalized symbol.

1 is a diagram illustrating the structure of a 2x2 MIMO transmitter in an HSPA system;
2 is a diagram illustrating an equalization process of a channel and a receiver between a transmitter and a receiver in a 2x2 MIMO transmitter in an HSPA system;
3 is a scatter plot showing the output of an LMMSE equalizer when there is interference between transmission signals;
4 is a scatter plot showing the output of the LMMSE equalizer when there is no interference between the transmitted signals;
5 is a diagram illustrating a configuration of an LMMSE equalizer in a receiver of a 2X2 MIMO transceiving system according to an embodiment of the present invention;
6 is a view for explaining the configuration of the interference cancellation signal generator 520 of FIG.
7 is a diagram for explaining an equalization method in a receiver according to an embodiment of the present invention;
8 is a view for explaining step 705 of FIG. 7 in detail;
9 is a diagram illustrating the contribution of each constellation point to the nonlinear MMSE estimate in the constellation of 16-QAM modulation scheme;
FIG. 10 is a diagram illustrating an approximation result using a Taylor series of an exponential function. FIG.
FIG. 11 is a view for explaining a method of dividing an exponential function by sections in order to approximate an exponential function to a linear function according to an embodiment of the present invention; FIG.
12 is a diagram illustrating a process of determining an approximated linear function of an exponential function in a corresponding interval according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is to be noted that the same elements among the drawings are denoted by the same reference numerals whenever possible. Also, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted.

In the following description, the signal processing will be described based on the signal in the frequency domain unless otherwise specified as a time domain signal. However, this is merely for convenience of description, and the signal processing of the present invention may be performed in the frequency domain or the time domain.

1 is a diagram illustrating the structure of a 2x2 MIMO transmitter in an HSPA system.

Referring to FIG. 1, each data that has undergone two independent transport channel processing (101, 103) is subjected to precoding matrix after spreading and scrambling (103, 104) The weight according to the matrix is multiplied. The weight is generated in the weight generator 120, and the precoding matrix is defined by Equation 1.

For reference, in Equation 1, w may have four kinds of values, and a precoding matrix having the largest energy among four signals composed of a combination of a channel and all possible precoding matrices at the receiver may be used. And the receiver is defined in the HSPA standard to send the selected precoding matrix to the transmitter.

The precoded signals are summed alternately in summers 109 and 110 and transmitted to the receiver through a total of four wireless channels through transmit antennas 113 and 114. Hereinafter, an equalization process of a channel and a receiver between a transmitter and a receiver will be described.

2 is a diagram illustrating an equalization process of a channel and a receiver between a transmitter and a receiver in a 2x2 MIMO transmitter in an HSPA system.

Referring to FIG. 2, X1 and X2, which are independent transmission signals output from the transmitter 200, are output through the transmission antenna 1 201 and the transmission antenna 2 202 and are over a wireless channel represented by a channel matrix H. Transmitted to receiver 210.

The channel matrix H is composed of H ₁ , H ₂ , H ₃ , and H ₄ , where H ₁ represents a channel between transmit antenna 1 201 and receive antenna 1 214, and H ₂ represents transmit antenna 1 201. ) Means a channel between transmit antenna 2 (215) and H ₃ denotes a channel between transmit antenna 2 (202) and receive antenna 1 (214), and H ₄ denotes a transmit antenna transmit antenna 2 (202) and receive antenna It means a channel between two (215).

The receiver 210 receives signals Y ₁ and Y ₂ through the receiving antenna 1 214 and the receiving antenna 2 215, respectively, and the received signals Y ₁ and Y ₂ are the LMMSE equalizer 111 and the MIMO decoder 113. Is input to the receiver.

In FIG. 2, the reception signals Y ₁ and Y ₂ may be represented by Equation 2 below.

N ₁ denotes a noise signal in the received signal Y ₁ , and N ₂ denotes a noise signal in the received signal Y ₂ .

When Equation 2 is expressed as a discrete-time signal, Equation 3 is expressed.

Meanwhile, the LMMSE equalizer 211 of the receiver 210 estimates a transmission signal using cross-correlation between the received signal and the desired signal and auto-correlation between the received signals. Output

Specifically, the LMMSE equalizer 111 is designed to minimize the mean square error (MSE) between the transmission signal and the equalizer output signal, where MSE is defined as in Equation 4. .

The MMSE weight that satisfies Equation 4 is expressed as a product of cross-correlation and auto-correlation inverse matrix as shown in Equation 5 below.

Therefore, the output of the equalizer in the 2x2 MIMO system is represented by Equation 6 below.

)을 살펴보면,

가 포함되어 있다. 한편,

에서 상기 <수학식 2>와 같이 송신 스트림 X₁의 관점에서는 송신 스트림 X₂의 신호가 간섭으로 작용하고, 송신 스트림 X₂의 관점에서는 송신 스트림 X₁의 신호가 간섭으로 작용하게 된다.On the other hand, the output of the conventional LMMSE equalizer (

),

Is included. Meanwhile,

In view of the <Equation 2> X ₁ and the transmission stream transmitted stream of the transmission signal X is X ₂ stream acts as interference from the viewpoint, and the _second as a signal of a transmission stream, X ₁ is acting as an interference in.

In general, however, the distance between the MIMO transmitting antennas is much smaller than the distance between the transceivers in a mobile communication environment. In this case, according to the plane wave theory, the channel experienced by the signal output from each transmitting antenna is known to have the same phase difference only according to the incident angle of the plane wave. The same channel experienced by the output signal of the transmitting antenna may occur even when the correlation between the receiving antennas is high.

As a result, in this environment, the H ₁ and H ₂ channels are almost identical, and the H ₃ and H ₄ channel values are almost identical. Therefore, Equation 2 may be expressed by Equation 7 below.

In this environment where the H ₁ and H ₂ channels are almost the same and the H ₃ and H ₄ channels are almost the same, the inversion of the autocorrelation matrix of the LMMSE has a large error. The error rate of the output signal of the equalization is also very large. Therefore, the LMMSE equalizer output signal in the case where there is interference between the transmission signals is much larger in the estimation of the transmission signal than the LMMSE equalizer output signal in the absence of mutual interference.

3 is a scatter plot showing the output of the LMMSE equalizer when there is interference between transmission signals, and FIG. 4 is a scatter plot showing the output of the LMMSE equalizer when there is no interference between transmission signals. plot). 4 shows that there is less error in the estimation of the transmission signal than in FIG. Thus, there is a need to remove or reduce inter-stream interference to reduce the error rate of the LMMSE equalizer output.

According to this need, the present invention generates a received signal from which mutual interference has been removed and uses it for equalization. The basic concept of the present invention will be briefly described.

The present invention estimates a transmission signal by equalizing the received signal in Equation (7), generates a plurality of received signals whose interference is removed from the received signal using the estimated transmission signal, and removes the interference. Equalize the plurality of received signals independently and combine them to estimate the transmission signal from which interference has been removed. That is, a diversity gain can be obtained at the same time as estimating a transmission signal having high reliability by generating received signals from which interference has been removed, and adding and equalizing them. On the other hand, by repeatedly performing this process it is possible to further improve the quality of the received signal.

Although the following description of the present invention has been described using a 2X2 MIMO transmission / reception system as an example, this is merely for convenience of description and may be equally applied to other multiple transmission / reception systems other than 2X2.

5 is a diagram illustrating a configuration of an LMMSE equalizer in a receiver of a 2X2 MIMO transceiving system according to an embodiment of the present invention.

The equalizer 500 of the present invention includes an equalizer 510 and an interference cancellation signal generator 520.

',

''은 각각 (

₁',

₂')과 (

₁'',

₂'')로 구성된다.In FIG. 5, the input signal Y is composed of (Y ₁ , Y ₂ ) received by two antennas. In addition, the outputs Y 'and Y''of the interference cancellation signal generator 520 are composed of (Y ₁ ', Y ₂ ') and (Y ₁ '', Y ₂ ''), respectively. In addition, the output of the first LMMSE equalizer 511

',

'' Is each (

₁ ',

₂ ') and (

₁ ``,

₂ '').

The received signal Y is input to the equalizer 510 and the interference cancellation signal generator 520.

)를 간섭 제거 신호 생성부(520)로 전달한다.The equalizer 510 equalizes the received signal Y using one of the first LMMSE equalizer 511 or the second LMMSE equalizer 513 to estimate a transmission signal, and estimates the estimated transmission signal (

) Is transmitted to the interference cancellation signal generator 520.

)를 이용하여 서로 다른 안테나에서 생성된 송신 신호들 상호 간 간섭을 제거한 신호들(Y', Y'')를 생성한다. 이후, 상기 Y'과 Y''은 등화부(510)로 입력되며, 제1 LMMSE 등화기(511)와 제2 LMMSE 등화기(513) 각각에서 독립적으로 등화되어

',

''이 출력된다. The interference cancellation signal generator 520 may transmit the estimated transmission signal in the received signal Y (

) To generate the signals (Y ', Y'') to remove the interference between the transmission signals generated from different antennas. Then, Y 'and Y''are input to the equalizer 510 and are independently equalized in each of the first LMMSE equalizer 511 and the second LMMSE equalizer 513.

',

Is printed.

'과

''는 가산기(515)에서 선형 가산 되어

'''이 출력된다. 상기

'''는 최종적으로 추정된 송신 신호가 된다. 이렇게 본 발명의 실시예에서는 송신 신호들 상호 간에 의한 간섭이 제거된 복수 개의 수신 신호들을 생성하고 이를 독립적으로 등화한 이후 등화된 신호들을 선형 가산하여

'''을 출력하여, 등화기의 출력, 즉, 추정된 송신 신호의 신뢰성을 높일 수 있다. 한편, 상기

'''를 다시 간섭 제거 신호 생성부(520)으로 입력하여 상기 동작들을 반복하면 추정된 송신 신호의 신뢰성은 더 높아진다.After that

'and

'' Is linearly added in adder 515

'''Is printed. remind

'''Becomes the finally estimated transmission signal. As described above, according to the embodiment of the present invention, a plurality of received signals from which interference between transmission signals are removed are generated and equalized independently, and then linearized addition of the equalized signals.

By outputting ''', the output of the equalizer, that is, the reliability of the estimated transmission signal can be improved. Meanwhile, above

Inputting '''back to the interference cancellation signal generator 520 and repeating the above operations increases the reliability of the estimated transmission signal.

Y 'is represented by Equation (8), and Y' 'is represented by Equation (9).

On the other hand, since the transmission signal estimated by the LMMSE appears in the form of attenuation of the original transmission signal, Equation 8 and Equation 9 are represented by Equation 10 when the Equation 8 is expressed again as a matrix-vector model.

In Equation 10, the attenuation of the transmission signal is expressed by substituting the attenuation of the channel matrix. By redesigning the filter coefficients of the equalizer after the interference is removed according to Equation 10, the performance of the equalization can be improved.

을

'으로 치환하면 되고, 하기 <수학식 11>에서 Y를 Y''으로, Y₁을 Y₁'', Y₂를 Y₂''으로 치환하고,

을

''으로 치환하면 된다.On the other hand, the result of equalizing Y 'and the result of equalizing Y''are expressed based on Equation 11 below. That is, substituted by the following <Equation 11> Y to Y 'as, for Y ₁ Y _1', a Y ₂ Y ₂ 'in,

of

And Y, Y ₁ to Y ₁ '', Y ₂ to Y ₂ '' in Equation 11 below,

of

Replace with ''.

FIG. 6 is a diagram illustrating a configuration of the interference cancellation signal generator 520 of FIG. 5.

The interference cancellation processor 520 includes a symbol generator 610, a symbol cleaner 620, a virtual transmitter 630, a virtual channel unit 640, and an interference cancellation processor 650.

로부터 심볼을 생성한다. 상기 심볼 생성부(620)는 심볼을 생성하기 위하여

에 상기 도 1에서 설명한 송신기의 신호 처리 절차의 역 절차를 적용한다. 즉,

에 역 프리코딩 행렬을 곱한 이후, 역 스크램블링을 수행하고, 역 확산을 수행하여 심볼을 생성한다. 상기 역 확산은 역 하다마드(Hadamard) 변환을 통하여 수행될 수 있고, 상기 역 스크램블링은 PN 코드를 곱하여 수행되는 것이 일반적이다.The symbol generator 610 estimates the transmitted signal transmitted from the interference cancellation signal generator 520.

Create a symbol from. The symbol generator 620 generates a symbol

The reverse procedure of the signal processing procedure of the transmitter described above with reference to FIG. In other words,

After multiplying the inverse precoding matrix by, inverse scrambling is performed and inverse spreading is performed to generate a symbol. The inverse spreading may be performed through an inverse Hadamard transformation, and the inverse scrambling is generally performed by multiplying the PN code.

The symbols generated by the symbol generator 610 are multiplied by the gain g of the code channel and the symbol s, and are represented by Equation 12 below.

The symbol cleaner 620 may perform symbol cleaning on the generated symbols to generate more reliable symbols than the symbols generated by the symbol generator 620. The present invention proposes a method for efficiently performing symbol cleaning, which will be described later with reference to FIG. 9.

The virtual transmitter 630 applies the signal processing procedure of the transmitter of FIG. 1 to the symbol on which the cleaning is performed. That is, after spreading and scrambling are performed on the symbols, the precoding matrix is multiplied. The spreading may be performed through a Hadamard transform, and the scrambling is generally performed by multiplying a PN code.

On the other hand, the virtual channel unit 640 is a channel to the output signal of the virtual transmitter 630 in order for the output signal of the virtual transmitter 630 to experience the channel (H) between the transmitter 200 and the receiver 210. Multiply the function. If the signal is processed in the time domain, convolution of the channel function will be performed.

'''을 출력한다.The interference cancellation processor 650 outputs Y 'of Equation 8 and Y''of Equation 9 by using the output signal of the virtual channel unit 640 in the received signal Y. It generates reliable received signals with interference between transmission signals removed. Since Y 'and Y''are input to the equalizer 510 as described in FIG. 5 and independently equalized, linear addition is performed.

Print '''.

)의 신뢰성을 더욱 향상시키기 위한 구성들이다. 따라서 시스템의 복잡성을 감소시키고자 하는 경우에 상기 801 내지 807 단계는 생략될 수도 있다. 이 경우 가상 채널부(640)에 입력되는 신호는 수신 신호(Y)가 등화부(510)에서 등화되어 출력된 신호(

)가 될 것이다.Meanwhile, the symbol generator 610, the symbol cleaner 620, and the virtual transmitter 630 are equalized and estimated by the equalizer 510.

) To further improve the reliability. Therefore, steps 801 to 807 may be omitted in order to reduce the complexity of the system. In this case, the signal input to the virtual channel unit 640 is a signal output after the received signal Y is equalized by the equalizer 510.

Will be

7 is a view for explaining an equalization method in a receiver according to an embodiment of the present invention.

) 출력한다.If the receiver receives the signal (Y) transmitted from the transmitter in step 701, the equalizer 510 equalizes the received signal to estimate the transmission signal in step 703 (

)

In step 705, the interference cancellation signal generator 520 generates received signals Y ′ and Y ″ from which interference between transmission signals is removed using the estimated transmission signal.

',

''을 생성하고, 이를 선형 가산하여

'''을 출력하여 최종적으로 추정된 송신 신호를 생성한다.In step 707, the equalizer 510 independently equalizes Y 'and Y''.

',

'' And add it linearly

'''Is output to generate the finally estimated transmission signal.

8 is a view for explaining operation 705 of FIG.

)로부터 심볼을 생성한다. 상기 심볼 생성부(610)는 심볼을 생성하기 위하여

에 상기 도 1에서 설명한 송신기의 신호 처리 절차의 역 절차를 적용한다. 즉,

에 역 프리코딩 행렬을 곱한 이후, 역 스크램블링을 수행하고, 역 확산을 수행하여 심볼을 생성한다. 상기 역 확산은 역 하다마드(Hadamard) 변환을 통하여 수행될 수 있고, 상기 역 스크램블링은 PN 코드를 곱하여 수행되는 것이 일반적이다.In operation 801, the symbol generator 610 may estimate the transmitted signal (sent from the interference cancellation signal generator 520).

Create a symbol from. The symbol generator 610 generates a symbol

The reverse procedure of the signal processing procedure of the transmitter described above with reference to FIG. In other words,

After multiplying the inverse precoding matrix by, inverse scrambling is performed and inverse spreading is performed to generate a symbol. The inverse spreading may be performed through an inverse Hadamard transformation, and the inverse scrambling is generally performed by multiplying the PN code.

In operation 803, the symbol cleaner 620 may perform symbol cleaning on the generated symbols to generate more reliable symbols than the symbols generated by the symbol generator 620. The present invention proposes a method for efficiently performing symbol cleaning, which will be described later with reference to FIG. 9.

In operation 805, the virtual transmitter 630 applies the signal processing procedure of the transmitter of FIG. 1 to the symbol on which the cleaning is performed. That is, after spreading and scrambling are performed on the symbols, the precoding matrix is multiplied. The spreading may be performed through a Hadamard transform, and the scrambling is generally performed by multiplying a PN code.

In operation 807, the virtual channel unit 640 outputs the output signal of the virtual transmitter 630 to the output signal of the virtual transmitter 630 in order to experience the channel H between the transmitter 200 and the receiver 210. Apply the channel by multiplying the channel function.

In operation 809, the signals Y 'and Y ″ are generated from the received signal Y by removing the interference between the transmission signals using the symbols to which the channel is applied.

)의 신뢰성을 더욱 향상시키기 위한 구성들이다. 따라서 시스템의 복잡성을 감소시키고자 하는 경우에 상기 801 내지 807 단계는 생략될 수 도 있다. 이 경우 807단계에서 가상 채널부(640)는 수신 신호(Y)가 등화부(510)에서 등화되어 추정된 송신 신호(

)에 채널 함수를 곱할 것이다. 만일 신호가 시간 영역에서 처리되는 경우라면 채널 함수의 컨벌루션이 수행될 것이다.Meanwhile, in steps 801 to 805, the transmission signal estimated by being equalized by the equalizer 510 (

) To further improve the reliability. Therefore, steps 801 to 807 may be omitted in order to reduce the complexity of the system. In this case, in operation 807, the virtual channel unit 640 receives the estimated transmission signal (E) after the received signal Y is equalized by the equalizer 510.

) Will be multiplied by the channel function. If the signal is processed in the time domain, convolution of the channel function will be performed.

Hereinafter, the symbol cleaning scheme proposed by the present invention mentioned in FIG. 6 will be described.

Symbol cleaning is commonly referred to as soft slicing, and may be used interchangeably in the following. The soft slicing scheme is generally used as a linear MMSE scheme and a general, that is, a non-linear MMSE scheme.

The result of soft slicing using the linear MMSE method is known to be represented by Equation (13), and the result of soft slicing using the nonlinear MMSE method is known to be represented by Equation (14).

Meanwhile, if Equation 14 is rearranged, Equation 15 may be expressed.

 Referring to Equation 15, it is necessary to implement an exponential function in order to implement soft slicing of the nonlinear MMSE method. However, there is a problem that it is very complicated to implement the exponential function in hardware. Therefore, the present invention proposes a method of approximating an exponential function to a piecewise linear function that is easy to implement in hardware.

The reason for performing symbol cleaning, that is, soft slicing, is to increase the reliability of the symbol generated by the symbol generator 610. On the other hand, the non-linear MMSE method may be expressed as a linear combination of all constellation points on the constellation. In this case, contributions to the result value of Equation 15 are different from each other according to positions on the constellations of the received symbols.

For example, in the constellation of the 16-QAM modulation scheme as shown in FIG. 9, the contribution of each constellation point to the nonlinear MMSE estimate is higher as it is closer to the received signal y 900 in the time domain, and as it is farther away. In FIG. 9, the contribution of the constellation 901 is highest, the constellation 903 is next highest, and the contribution of the farthest constellations 905 is low.

On the other hand, the easiest way to think about when approximating an exponential function is to first approximate the exponential function using a Taylor series, as shown in Fig. 10, or to consider the constellation point that is farthest from the received signal. There is a way not. Reference numerals 1010, 1020, and 1030 of FIG. 10 denote actual exponential functions, approximation results using a first order Taylor series, and approximation results using a second order Taylor series.

As can be seen in FIG. 10, the simulation results show that the schemes do not work as desired or exhibit significantly worse results when using the actual nonlinear MMSE function.

Therefore, in order to approximate the exponential function more precisely, the present invention proposes a method of dividing the exponential function according to its importance and expressing the exponential function as an optimal linear function for each interval.

In the present invention, in order to minimize the error when approximating the exponential function to a linear function, the size of the interval of the exponential function is set differently according to the magnitude of the function value of the exponential function. This will be explained with reference to FIG.

In addition, when the linear function is determined from the exponential function of each of the differently set intervals, the linear function determined by the start and end points of the actual exponential function in the corresponding interval is appropriately parallelized to determine the finally approximated linear function. The parallel movement is set so as to satisfy a condition that the mean square error (MSE) between the exponential function and the finally approximated linear function is minimized. That is, the parallel movement is determined to satisfy the MMSE. This will be explained with reference to FIG. 12.

FIG. 11 is a diagram illustrating a method of dividing an exponential function by sections in order to approximate an exponential function as a linear function according to an embodiment of the present invention.

Referring to FIG. 11, it can be seen that an exponential function is divided into four sections, that is, D1 1101, D2 1103, D3 1105, and D4 1107.

In the first section D1 (1101), because the size of the function value is large, a more accurate approximation can be obtained by setting the interval interval narrower, and in the second section D2 (1103), the section value is smaller than the D1 (1101) section. The interval of is made larger than D1 (1101). Therefore, when the value of the exponential function is large, a more accurate function of the result of soft slicing according to Equation 15 can be obtained, and as a result, the nonlinear MMSE value is more accurate.

12 is a diagram illustrating a process of determining an approximated linear function of an exponential function in a corresponding interval according to an embodiment of the present invention.

Reference numeral 1201 denotes a point mapping for determining a linear function by using a start point and an end point of an actual exponential function in a corresponding section. When the linear function is determined according to the method of point mapping 1201, it can be seen that the error becomes very large in the remaining portions except for the start point and the end point of the linear function. This results in a large error between the linear and exponential functions. Therefore, in order to minimize this error, the present invention allows the linear function determined by the point mapping to be a linear function satisfying the MMSE condition. Reference numeral 1203 denotes the determination of the linear function such that the MMSE condition is satisfied according to an embodiment of the present invention. Hereinafter, a process of determining a linear function to satisfy the MMSE condition will be described.

When the coordinates of the start point and the end point of the exponential function are [x ₀ , y ₀ ], [x ₁ , y ₁ ] in an arbitrary interval, [x ₀ , y ₀ ], [x ₁ , y ₁ ] The linear function as the start point and the end point is determined by Equation 16 below.

In order to minimize the error between the linear function and the exponential function in the linear function of Equation 16, the slope value α and the y intercept value β may be determined to minimize the error between the linear function and the exponential function. In the present invention, the gradient value α and the y intercept value β are determined to minimize Mean Square Error (MSE).

MSBs according to the α and β values are represented by the following Equations 17 and 18.

According to Equation 18, Equation 17 is rearranged as in Equation 19.

In Equation 19, MSE (α, β) of Equation 17 is expressed as J (α, β) for convenience. On the other hand, in order to find α and β in which J (α, β) of Equation 19 is minimized, that is, MSE (α, β) is minimized, the values of partial derivatives of Equation 19 with respect to α and β are It is necessary to find α and β values that become zero. This process is represented by Equation 20 below.

To rearrange Equation 20, Equation 21 is represented.

According to Equation 21, J and α, ie, α and β may be determined to minimize MSE (α, β). Therefore, the linear function determined according to the determined α and β becomes the final approximated linear function of the exponential function of the interval.

An example of approximating an exponential function according to the above-described exponential function approximation method of the present invention will be described. If the interval of the exponential function is divided into six stages, the linear function approximated in each interval is expressed by Equation 22 below.

That is, referring to Equation 24, since the value of the exponential function decreases as x increases, the length of the corresponding section becomes longer as x increases. The α and β values for determining the linear function for each exponential function in the divided sections are determined by Equation 21.

Hereinafter, the result of simulating the equalization performance of the present invention as described above.

Simulation was performed in the HSPA 2x2 MIMO system environment, and simulation was performed for QPSK, 16-QAM, and 64-QAM, and soft slicing is used for linear slicing, in order to identify equalization performance according to the symbol cleaning, that is, symbol slicing. Nonlinear Slicing An approximate nonlinear (ALN) slicing scheme was used. The pseudo nonlinear slicing method divides the exponential function into 16 sections.

Simulation results show that using the equalizer proposed in the present invention, a reception gain of about 2 to 3 dB can be obtained compared to the conventional LMMSE equalizer.

Specifically, in the case of using the QPSK modulation method, the received signal from which the interference signal has been removed according to the present invention is equalized, and when the linear function approximation method is used in soft slicing, the performance improvement of about 2.5 dB was confirmed. An additional 2dB gain was found when using the method or similar nonlinear slicing method.

On the other hand, when 16-QAM modulation is used, the received signal from which the interference signal is removed according to the present invention is equalized, and when the linear function approximation method is used for soft slicing, the performance improvement of about 2 dB was confirmed. The gain of 3dB was confirmed when using the slicing method or the pseudo nonlinear slicing method.

In addition, when the 64-QAM modulation is used, the received signal from which the interference signal is removed is equalized according to the present invention, and when the linear function approximation method is used during soft slicing, the performance improvement of about 2 dB was confirmed. When using the slicing method or the pseudo nonlinear slicing method, an additional 2dB gain was confirmed.

Claims (14)

10. A method of equalizing a received signal in a multiple input multiple output (MIMO) wireless communication system comprising at least two transmit antennas and at least two receive antennas.
Estimating the transmission signal by equalizing the reception signal;
Generating a plurality of received signals from which interference is removed from the received signal by using the estimated transmission signal;
Independently equalizing the plurality of received signals;
Estimating a transmission signal from which interference is removed by combining the plurality of independently equalized reception signals. The method of claim 1, wherein the generating of the plurality of received signals from which interference has been removed includes:
Equalizing the received signal to generate a symbol by applying an inverse procedure of the transmitter signal processing to the estimated transmission signal;
Cleaning the generated symbol;
Applying a virtual transmission procedure to the cleaned symbols;
Applying a channel between the transmitter and the receiver to a symbol to which the virtual transmission procedure is applied;
And generating a plurality of received signals from which the interference is removed from the received signal using the symbol to which the channel is applied. The method of claim 2, wherein the cleaning of the symbol comprises:
The symbol is cleaned using a nonlinear minimize mean square error (MMSE) method.
The exponential function used in the non-linear MMSE scheme is approximated by a linear linear function. 4. The method of claim 3, wherein when the exponential function is approximated to a linear linear function,
The exponential function is divided into a plurality of sections by setting a length of a section having a large value of the exponential function to be shorter than a length of a section having a small value of the exponential function,
In each of the divided sections, a linear linear function for minimizing an average squared error (MSE) value between the exponential function and the linear linear function is determined as the approximated linear linear function in the MIMO wireless communication system. How to equalize received signal. The method of claim 2, wherein the generating of the symbol by applying an inverse procedure of the transmitter signal processing comprises:
And equalizing the received signal by multiplying an inverse precoding matrix, performing inverse scrambling, and inverse spreading, to equalize the received signal in the MIMO wireless communication system. The method of claim 2, wherein the applying of the virtual transmission procedure comprises:
And spreading the cleaned symbols, performing scrambling, and multiplying a precoding matrix. The method of claim 1, wherein the generating of the plurality of received signals from which the interference has been removed comprises:
And generating a plurality of received signals from which the interference has been eliminated by the same number as the number of the received antennas. An apparatus for equalizing a received signal in a multiple input multiple output (MIMO) wireless communication system comprising at least two transmit antennas and at least two receive antennas.
An equalizer which equalizes the received signal and estimates the transmitted signal;
An interference cancellation signal generator configured to generate a plurality of received signals from which interference is removed from the received signal by using the estimated transmission signal,
The equalizer equalizes the plurality of received signals independently, and combines the plurality of independently equalized received signals to estimate the transmitted signal from which interference has been removed. Device. The method of claim 8, wherein the interference cancellation signal generator,
A symbol generation unit for generating a symbol by applying the inverse procedure of the transmitter signal processing to the estimated transmission signal by equalizing the received signal;
A symbol cleaning unit cleaning the generated symbols;
A virtual transmitter for applying a virtual transmission procedure to the cleaned symbol;
A virtual channel unit applying a channel between the transmitter and the receiver to a symbol to which the virtual transmission procedure is applied;
And an interference cancellation processor configured to generate a plurality of received signals from which the interference is removed from the received signal by using the symbol to which the channel is applied. The method of claim 9, wherein the symbol cleaning unit,
The symbol is cleaned using a nonlinear minimize mean square error (MMSE) method.
And an exponential function used in the nonlinear MMSE scheme to approximate a linear function as a linear linear function. The method of claim 9, wherein the symbol cleaning unit,
When we approximate the exponential function as a linear linear function,
The exponential function is divided into a plurality of sections by setting a length of a section having a large value of the exponential function to be shorter than a length of a section having a small value of the exponential function,
In each of the divided sections, a linear linear function for determining an average squared error (MSE) value between the exponential function and the linear linear function is determined as the approximated linear linear function. Equalizing the received signal at. The method of claim 9, wherein the symbol generation unit,
And equalizing the received signal by multiplying an inverse precoding matrix, performing inverse scrambling, and inverse spreading, and equalizing the received signal in the MIMO wireless communication system. The method of claim 9, wherein the virtual transmission unit,
Spreading the cleaned symbols, performing scrambling, and multiplying a precoding matrix. The method of claim 8, wherein the interference signal generation unit,
And equalizing the number of the receiving antennas, and generating a plurality of received signals from which the interference has been removed.

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