KR20090052657A - Apparatus and method for eliminating frequency synchronization error in relay wireless commnication system - Google Patents

Abstract

The present invention relates to a wireless communication system of a relay method using Distributed Space Time Block Coding (DSTBC), which estimates a channel with each relay station and constructs an equivalent channel matrix in consideration of a space-time block coding code. An estimator to perform QR decomposition on the equivalent channel matrix, and a decomposer constituting an equalization matrix using the QR decomposition result, and equalizing the received signal by multiplying the received subcarrier-specific signal by the equalization matrix. Including an equalizer and a detector for detecting a subcarrier transmission signal using the equalized received signal and the R matrix obtained as a result of QR decomposition, it is possible to reduce overhead due to frequency offset estimation and improve frequency synchronization error cancellation performance. .

Relay station, Distributed Space Time Block Coding (DSTBC), QR decomposition, Decision Feedback Equalizer (DFE)

Description

Apparatus and method for eliminating frequency synchronization error in wireless communication system of relay method {APPARATUS AND METHOD FOR ELIMINATING FREQUENCY SYNCHRONIZATION ERROR IN RELAY WIRELESS COMMNICATION SYSTEM}

The present invention relates to a relay type wireless communication system, and more particularly, to an apparatus for removing frequency synchronization error in a relay type wireless communication system that performs distributed space time block coding (DSTBC) using relay stations. And to a method.

As demand for high-speed and high-capacity services using a wireless communication system increases, introduction of a relay station for relaying signals between a base station and a terminal is being considered. By using the relay station, an effect of increasing coverage of the base station and improving throughput is generated. That is, the system can improve the transmission rate by placing the relay station in a specific area having a poor channel environment, and can be located so that the terminal and the base station can communicate with the terminal outside the coverage of the base station by placing the relay station near the cell boundary.

Accordingly, various techniques for utilizing the relay station are currently being studied. One technique is distributed space time block coding (DSTBC) using a mobile relay station. 1 illustrates a communication scheme using a distributed space-time block coding technique using a mobile relay station. 1 illustrates a case of uplink transmission. Referring to FIG. 1, a terminal 110 transmits a signal to a plurality of mobile relay stations 120-1 to 120-4, and the plurality of mobile relay stations 120-1 to 120-4 are connected to the terminal. The signal from 110 is transmitted to the base station 130. However, the plurality of mobile relay stations 120-1 to 120-4 use a space-time block coding technique applied to a multiple input multiple output (MIMO) technique by serving as a virtual multiple antenna group. . In this case, the space-time block coding scheme used by the plurality of relay stations 120-1 through 120-4 depends on the total number of antennas of the plurality of mobile relay stations 120-1 through 120-4. For example, if a total of two antennas are used, an Alamouti code may be used.

In particular, when a system using a distributed space-time block coding scheme as shown in FIG. 1 is an Orthogonal Frequency Division Multiplexing (OFDM) -based system, perfect synchronization of time and frequency axes between a plurality of distributed RSs is essential. to be. Although a plurality of relay stations have different transmission distances from the transmitting end and the receiving end, a time synchronization error due to a transmission delay occurs, but the time synchronization error is simply solved due to characteristics of an OFDM based system. However, in the case of frequency synchronization error, the system must prepare a separate alternative to eliminate the frequency synchronization error. In a typical multiple input / output system, since one transmitter uses multiple antennas, the frequency offset between the antennas is the same. However, as described above, when a plurality of relay stations are gathered to form one multi-antenna group, frequency offsets of the plurality of relay stations are different from each other. An alternative to performing frequency synchronization with each of the plurality of relay stations at the receiving end is also valid, but this leads to very large overhead. Therefore, in order not to bear the overhead, there is a need for an alternative to allow the frequency offset difference of multiple relay stations at the receiving end and to overcome the frequency offset difference.

Accordingly, an object of the present invention is to provide an apparatus and method for removing frequency synchronization error in a relay type wireless communication system.

Another object of the present invention is to provide an apparatus and method for equalizing a received signal using QR decomposition in a relay type wireless communication system.

Another object of the present invention in a relay wireless communication system

According to a first aspect of the present invention for achieving the above object, in a wireless communication system of a relay method using Distributed Space Time Block Coding (DSTBC), the receiving end apparatus estimates a channel with each relay station. An estimator constituting an equivalent channel matrix in consideration of a space-time block coding code, a QR decomposition of the equivalent channel matrix, and an equalizer constituting an equalization matrix using the QR decomposition result, and a received subcarrier signal And an equalizer for equalizing a received signal by multiplying by the equalization matrix, and a detector for detecting a subcarrier transmission signal using the equalized received signal and an R matrix obtained as a result of QR decomposition.

According to a second aspect of the present invention for achieving the above object, a signal detection method of a receiving end in a relay type wireless communication system using distributed space-time block coding, estimates a channel with each relay station, A process of constructing an equivalent channel matrix in consideration, QR decomposition of the equivalent channel matrix, constructing an equalization matrix using the QR decomposition result, and multiplying the equalization matrix by a received subcarrier signal, thereby receiving a received signal. And a step of detecting the transmission signal for each subcarrier by using the equalized received signal and the R matrix obtained as a result of QR decomposition.

By detecting signals using QR decomposition and Decision Feedback Equalizer (DFE) in a relay wireless communication system, overhead due to frequency offset estimation can be reduced and frequency synchronization error cancellation performance can be improved.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

Hereinafter, a description will be given of a technique for removing frequency synchronization error in a relay wireless communication system. In particular, a description will be given of a technique for eliminating frequency synchronization error in a wireless communication system of a relay method according to Orthogonal Frequency Division Multiplexing (hereinafter, referred to as 'OFDM'). Hereinafter, the present invention will be described using two relay stations having one antenna and using an Alamouti code as an example, and may be equally applicable to other cases.

According to the distributed space time block coding (DSTBC) considered in the present invention, the transmission signals of the transmitting end, the relay station A, and the relay station B are represented by Equation 1 below.

는 송신단에서 송신된 t번째 OFDM 심벌을 의미한다. In Equation 1,

Denotes the t-th OFDM symbol transmitted by the transmitter.

That is, relay station A and relay station B receive two OFDM symbols from a transmitter, and transmits received OFDM symbols by space-time block encoding according to an Alamouti code. If the frequency synchronization coincides, the equivalent channel matrix between the relay stations and the receiving end is expressed by Equation 2 below.

는 등가 채널 행렬, 상기

는 시공간 블록 부호화된 t번째 OFDM 심벌에서 n번째 부반송파에 대응되는 신호에 대한 채널, 상기

은 하나의 OFDM 심벌에 포함되는 신호 개수, 즉, 부반송파 개수를 의미한다.In Equation 2,

Is the equivalent channel matrix,

Is a channel for a signal corresponding to the n th subcarrier in the space time block coded t th OFDM symbol,

Denotes the number of signals included in one OFDM symbol, that is, the number of subcarriers.

However, if the frequency synchronization does not match, elements 0 in the equivalent channel matrix shown in Equation 2 become non-zero due to the frequency synchronization error. In addition, the diagonal components of each partial matrix of the equivalent channel matrix are also changed. As a result, the receiving end receives signals mapped to each subcarrier in a mixed form.

Accordingly, in order to separate signals for subcarriers mixed with each other due to frequency synchronization error, the receiver according to the present invention estimates an equivalent channel matrix, performs QR decomposition on the equivalent channel matrix for each partial matrix, and then sequentially Separates all subcarrier-specific signals. Here, the QR decomposition is an operation of decomposing one matrix into a product of a Q matrix and an R matrix, and the R matrix is an upper triangular matrix. The operation of the receiver will be described sequentially with reference to a formula.

First, the receiving end estimates a channel with each of the relay stations, and constructs an equivalent channel matrix having a form as shown in Equation 3 below.

는 등가 채널 행렬, 상기

는 중계국A로부터 수신된 첫 번째 OFDM 심벌에 대한 등가 채널 부분 행렬, 상기

는 중계국B로부터 수신된 첫 번째 OFDM 심벌에 대한 등가 채널 부분 행렬, 상기

는 중계국B로부터 수신된 두 번째 OFDM 심벌에 대한 등가 채널 부분 행렬, 상기

는 중계국A로부터 수신된 두 번째 OFDM 심벌에 대한 등가 채널 부분 행렬을 의미한다.In Equation 3,

Is the equivalent channel matrix,

Is an equivalent channel partial matrix for the first OFDM symbol received from RS A,

Is an equivalent channel partial matrix for the first OFDM symbol received from RS B,

Equivalent channel submatrix for the second OFDM symbol received from RS B,

Denotes an equivalent channel partial matrix for the second OFDM symbol received from RS A.

The receiver generates an equalization channel as shown in Equation 4 below by performing a QR equalization matrix on each partial matrix of the equivalent channel matrix.

은 등화 채널, 즉, 등가 채널 행렬의 각 부분 행렬을 QR 분해한 결과 얻어지는 R행렬들을 부분 채널로 갖는 행렬, 상기

는 등가 채널 행렬의 부분 행렬 X를 QR 분해한 결과 얻어지는 R행렬의 i행j열 원소를 의미한다.In Equation 4,

Is an equalization channel, i.e., a matrix having R matrices as partial channels obtained by QR decomposition of each partial matrix of the equivalent channel matrix,

Denotes an i-row j-column element of the R matrix obtained by QR decomposition of the partial matrix X of the equivalent channel matrix.

The receiver generates an equalization matrix for equalizing the received signal using the equalization channel. The equalization matrix is generated using a property such as Equation 5 below, and is a matrix having an inverse of the Q matrix for each partial matrix as a partial matrix.

는 채널 행렬, 상기

은 QR 분해 결과 얻 어지는 Q행렬, 상기

은 QR 분해 결과 얻어지는 R행렬을 의미한다. In Equation 5,

Is the channel matrix,

Q matrix obtained as a result of QR decomposition,

Denotes the R matrix obtained as a result of QR decomposition.

Subsequently, the receiving end converts OFDM symbols received from two relay stations into two subcarrier signals by performing Fast Fourier Transform (FFT) operation, and multiplies the equalization matrix to multiply the received signal by Equation 6 below. Equalize together.

은 등화 행렬, 상기

는 2회에 걸쳐 2개의 중계국들로부터 수신된 신호 벡터, 상기

는 등가 채널 행렬, 상기

는 송신 신호 벡터, 상기

는 잡음, 상기

은 등화 채널을 의미한다.In Equation 6,

Silver equalization matrix, the

Is a signal vector received from two relay stations in two times,

Is the equivalent channel matrix,

Is the transmit signal vector,

Is noise, remind

Means equalization channel.

The received signal corresponding to the Nth subcarrier in the equalized received signal is not affected by the interference and includes only a transmission signal component corresponding to the Nth subcarrier. Accordingly, the receiving end detects a transmission signal corresponding to the Nth subcarrier in each submatrix, and decodes four signals detected for each submatrix according to an Alamouti code to detect two transmission signals corresponding to the Nth subcarrier.

The receiver removes interference of two transmission signals corresponding to the Nth subcarrier from the received signal corresponding to the N−1th subcarrier. If this is expressed as an equation, Equation 7 is obtained.

는 등화된 t번째 OFDM 심벌의 k번째 부반송파에 대응되는 신호에서 다른 부반송파의 신호들로부터의 간섭을 제거한 신호, 상기

는 등화된 t번째 OFDM 심벌의 k번째 부반송파에 대응되는 신호, 상기

은 부반송파 개수, 상기

은 중계국A에 대응되는 부분 행렬의 등화 채널의 k행n열 원소, 상기

은 중계국B에 대응되는 부분 행렬의 등화 채널의 k행n열 원소, 상기

는 t번째 OFDM 심벌의 n번째 부반송파에 대응되는 송신 신호의 검출 값을 의미한다.In Equation 7,

Is a signal which removes interference from signals of other subcarriers in a signal corresponding to a kth subcarrier of an equalized t-th OFDM symbol,

Is a signal corresponding to k th subcarrier of an equalized t th OFDM symbol,

Is the number of subcarriers,

Is the k-row n-column element of the equalization channel of the partial matrix corresponding to relay station A,

Is the k-row n-column element of the equalization channel of the partial matrix corresponding to relay station B,

Denotes a detection value of a transmission signal corresponding to the n th subcarrier of the t th OFDM symbol.

After the interference is removed as shown in Equation 7, the receiving end detects the transmission signal from the interference canceled signal. The receiver detects transmission signals corresponding to all subcarriers by repeating the aforementioned interference cancellation and transmission signal detection by the number of subcarriers.

Hereinafter, the present invention will be described in detail with reference to the drawings the configuration and operation of the receiving end for detecting a signal in the above-described manner.

2 is a block diagram of a receiver in a relay wireless communication system according to an exemplary embodiment of the present invention.

As shown in FIG. 2, the receiver includes a radio frequency (RF) receiver 202, an OFDM demodulator 204, a subcarrier demapper 206, a channel estimator 208, a QR decomposer 210, and an equalizer. 212, a Decision Feedback Equalizer (DFE) detector 214, and a demodulator and decoder 216.

The RF receiver 202 converts an RF band signal received through an antenna into a baseband signal. Here, the RF band signal received through the antenna is a sum of OFDM symbols transmitted after distributed space-time block coding in a plurality of relay stations. The OFDM demodulator 204 divides the signal from the RF receiver 202 in OFDM symbol units, removes a cyclic prefix (CP), and converts the signals into subcarrier signals through an FFT operation. The subcarrier demapper 206 classifies the subcarrier signals according to a processing procedure. That is, the subcarrier mapper 206 extracts a signal to be used for channel estimation and provides it to the channel estimator 208, and extracts a signal to be equalized and demodulated and provides it to the equalizer 212.

The channel estimator 208 estimates a channel for each subcarrier with each RS using a predetermined signal, i.e., a pilot signal, for channel estimation provided from the subcarrier demapper 206. In particular, according to the present invention, the channel estimator 208 constructs an equivalent channel matrix considering the space-time block coding code. Here, the equivalent channel matrix is composed of as many partial matrices as the number of OFDM symbols transmitted from each RS. For example, in the case of the Alamouti code, the equivalent channel matrix is composed of four partial matrices, as shown in Equation 3 above.

The QR decomposer 210 performs QR decomposition on each partial matrix of the equivalent channel matrix, and generates an equalization matrix composed of inverse matrices of the Q matrixes of each partial matrix. The QR decomposer 210 provides the equalization matrix to the equalizer 212, and provides the DFE detector 214 with a matrix composed of R matrices of each sub-matrix. The equalizer 212 equalizes the received signal by multiplying the equalization matrix by the received signal for each subcarrier.

The DFE detector 214 detects a transmission signal for each subcarrier by using R matrixes obtained by QR decomposition of each partial matrix of the equalized received signal and the equivalent channel matrix. In detail, the DFE detector 214 first detects a signal corresponding to the Nth subcarrier in each partial matrix, that is, a signal multiplied by an element of the last column of the last row of the R matrix, and stores the detected signals by the partial matrix. Decoded according to the coding code to detect the transmission signals corresponding to the N-th subcarrier. The DFE detector 214 removes the interference of two transmission signals corresponding to the Nth subcarrier from the received signal corresponding to the N-1th subcarrier as shown in Equation 7 and then removes the interference signal. The transmission signal is detected from the The DFE detector 214 repeats the aforementioned interference cancellation and transmission signal detection by the number of subcarriers, thereby detecting transmission signals corresponding to all subcarriers.

The demodulator and decoder 216 demodulates the signals detected by the DFE detector 214 into an encoded bit string, and decodes the encoded bit string into an information bit string.

3 illustrates a signal detection procedure of a receiver in a relay wireless communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 3, the receiving end converts the OFDM symbols received in step 301, that is, a signal obtained by combining the OFDM symbols transmitted after distributed space-time block coding in a plurality of RSs into subcarrier signals.

In step 303, the receiving end estimates a channel for each subcarrier with each RS using a pre-defined signal for channel estimation, that is, a pilot signal. In addition, the receiver configures an equivalent channel matrix considering the space-time block coding code. Here, the equivalent channel matrix is composed of partial matrices corresponding to the number of OFDM symbols transmitted from each RS. For example, in the case of the Alamouti code, the equivalent channel matrix is composed of four partial matrices, as shown in Equation 3 above.

After constructing the equivalent channel matrix, the receiver proceeds to step 305 to QR decompose each partial matrix of the equivalent channel matrix. That is, the receiving end obtains the Q matrix and the R matrix of each partial matrix by QR decomposition of each partial matrix of the equivalent channel matrix.

In operation 307, the receiving end generates an equalization matrix composed of inverse matrices of the Q matrixes of each sub-matrix. In other words, the receiving end generates an equalization matrix having the inverses of the Q matrices as partial matrices.

After generating the equalization matrix, the receiver proceeds to step 309 to equalize the received signal by multiplying the equalized matrix by the received signal for each subcarrier.

After equalizing the received signal, the receiver proceeds to step 311 and detects a transmission signal corresponding to the nth subcarrier. In this case, n is initialized to N, and the Nth subcarrier is the last subcarrier, and a received signal corresponding to the Nth subcarrier in an equalized received signal is not affected by interference from a transmission signal corresponding to another subcarrier.

After detecting the transmission signal corresponding to the nth subcarrier, the receiver proceeds to step 313 to remove interference due to the signal corresponding to the nth subcarrier from the received signal corresponding to the n-1th subcarrier.

In operation 315, the receiving end detects a transmission signal corresponding to the n−1 th subcarrier.

After detecting the transmission signal corresponding to the n-th subcarrier, the receiver proceeds to step 317 and checks whether n-1 is 1. In other words, the receiver checks whether signals corresponding to each of all subcarriers have been detected.

If n-1 is not 1, that is, if there is an undetected signal, the receiver proceeds to step 319 to decrease n by 1 and then proceeds to step 313. On the other hand, if n-1 is 1, i.e., all signals are detected, the receiving end terminates this procedure.

Each sub-matrix of the equalized received signal and the equivalent channel matrix is detected using sub-carrier transmission signals using R matrices obtained by QR decomposition. That is, the DFE detector 214 detects a signal corresponding to the Nth subcarrier in each partial matrix, decodes the signals detected for each partial matrix according to the space-time block coding code, and detects transmission signals corresponding to the Nth subcarrier. . The DFE detector 214 removes the interference of two transmission signals corresponding to the Nth subcarrier from the received signal corresponding to the N-1th subcarrier as shown in Equation 7 and then removes the interference signal. The transmission signal is detected from the The DFE detector 214 repeats the aforementioned interference cancellation and transmission signal detection by the number of subcarriers, thereby detecting transmission signals corresponding to all subcarriers.

4 shows the performance of a detection scheme according to the invention. 4 illustrates a simulation result of the system to which the detection method according to the present invention is applied, and the communication environment assumed in the simulation is as shown in Table 1 below.

 parameter  value channel model Rayleigh fading channel power delay profile exponential decaying (β = 0.2) length of channel taps 6 number of subcarriers 64 length of cyclic prefix 16 modulation order QPSK number of relay node 2

In FIG. 4, the horizontal axis represents bit energy to noise power (Eb / No), and the vertical axis represents bit error rate (BER). In addition, in the legend of FIG. 4, Δε is a simulation variable representing the degree of frequency synchronization error, and means a frequency offset to subcarrier frequency interval ratio Δf / f _s . And ε _k means the frequency offset between the relay station k and the receiving end. When Δε is 0, frequency synchronization between two relay stations and a receiver is exactly matched. In other words, the performance when Δε is 0 is the maximum limit of the frequency error cancellation performance. Referring to FIG. 4, when Δε is 0.1 or less, the detection scheme according to the present invention is close to the performance of a perfectly synchronized system, and the performance loss in this case is about 0.5 dB or less. And, in the case of Δε of 0.2, the bit error rate is ^10-4 when, the performance loss of the detection method according to the invention is about 3dB. This is because the influence of interference increases as the frequency offset increases, and the detection scheme according to the present invention exhibits an interference cancellation effect when Δε is 0.2 or less. In general, the transmit diversity order is evaluated as the slope of the bit error rate graph. When Δε is 0.2 or less in FIG. 4, it can be confirmed that the same transmit diversity order as that of a perfectly synchronized system is obtained.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

1 is a diagram illustrating a communication scheme using Distributed Space Time Block Coding (DSTBC) technique using a mobile relay station in a relay communication system;

2 is a block diagram of a receiver in a relay wireless communication system according to an embodiment of the present invention;

3 is a diagram illustrating a signal detection procedure of a receiver in a relay wireless communication system according to an embodiment of the present invention;

4 shows the performance of a detection scheme according to the invention.

Claims (14)

In a receiving end device in a relay type wireless communication system to which Distributed Space Time Block Coding (DSTBC) is applied, An estimator for estimating a channel with each relay station and configuring an equivalent channel matrix in consideration of a space-time block coding code; A QR decomposition of the equivalent channel matrix, and a decomposer constituting an equalization matrix using the QR decomposition result; An equalizer for equalizing a received signal by multiplying the equalization matrix by a received subcarrier signal; And a detector for detecting a subcarrier transmission signal using the equalized received signal and the R matrix obtained as a result of QR decomposition. The method of claim 1, And said equivalent channel matrix comprises submatrices as many as the number of signals transmitted from each relay station. The method of claim 2, And the decomposer QR decomposes each partial matrix of the equivalent channel matrix, and constructs an equalization matrix composed of inverse matrices of the Q matrices obtained as a result of the QR decomposition. The method of claim 2, And the detector detects the subcarrier transmission signal using the equalized received signal and the R matrices obtained by QR decomposition of each partial matrix. The method of claim 4, wherein And the detector detects a signal multiplied by an element of the last column of the last row of each of the first R matrices. The method of claim 5, And the detector detects a transmission signal by decoding signals detected for each partial matrix according to the space-time block coding code. The method of claim 6, The detector detects transmission signals corresponding to the nth to Nth subcarriers when N subcarriers are used, and transmits the transmission signals corresponding to the nth to Nth subcarriers from a reception signal corresponding to the Nth subcarrier. Apparatus for removing the interference caused. In the method of detecting a signal at the receiving end in a wireless communication system of a relay method using Distributed Space Time Block Coding (DSTBC), Estimating a channel with each relay station and constructing an equivalent channel matrix in consideration of a space-time block coding code; QR decomposition of the equivalent channel matrix and constructing an equalization matrix using the QR decomposition result; Equalizing a received signal by multiplying the equalization matrix by a received subcarrier signal; And detecting the transmission signal for each subcarrier using the equalized received signal and the R matrix obtained as a result of QR decomposition. The method of claim 8, Wherein the equivalent channel matrix is composed of as many sub- matrices as the number of signals transmitted from each relay station. The method of claim 9, The process of constructing the equalization matrix, QR decomposing each partial matrix of the equivalent channel matrix, and constructing an equalization matrix composed of inverse matrices of the Q matrices obtained as a result of the QR decomposition. The method of claim 9, Detecting the transmission signal for each subcarrier, The equalized received signal and each sub-matrix are performed using R matrices obtained as a result of QR decomposition. The method of claim 11, The process of detecting the transmission signal for each subcarrier, And detecting a signal multiplied by an element of a last column of a last row of each of the R matrices. The method of claim 12, The process of detecting the transmission signal for each subcarrier, And detecting the transmission signal by decoding the detected signals for each partial matrix according to the space-time block coding code. The method of claim 13, The process of detecting the transmission signal for each subcarrier, When N subcarriers are used, detecting transmission signals corresponding to n th to N th subcarriers; And removing interference caused by transmission signals corresponding to the nth to Nth subcarriers from the received signal corresponding to the Nth subcarrier.

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