KR100966054B1 - Method for Rate Adjustment per Subcarrier in MIMO System - Google Patents

Abstract

The present invention is a method of determining and transmitting a modulation method for each subcarrier by using a condition number (CN), which is channel information obtained through a process of changing a role of a transmitting end and a receiving end in an orthogonal frequency division multiplexing (OFDM) MIMO system. A variable transmission method for each subcarrier in a multiple input multiple output system, the method comprising: (a) calculating a condition number (CN) which is channel information; And (b) determining a modulation method for each subcarrier according to the value of the state number.

MIMO, multiple, input, output, antenna, detection, modulation, CN, channel, information, number, variable

Description

Variable transmission method according to subcarriers in a multi-input multiple output system {Method for Rate Adjustment per Subcarrier in MIMO System}

The present invention relates to a variable transmission method for each subcarrier in a multiple input multiple output (MIMO) system and a method of calculating a state number used therein. More specifically, Orthogonal Frequency Division Multiple (OFDM) In the MIMO system, a method of determining and transmitting a modulation method for each subcarrier using a condition number (CN), which is channel information obtained through a process of changing a role of a transmitter and a receiver, and a method of calculating a state number used therein. It is about.

The present invention is derived from the research conducted as part of the IT new growth engine core technology development project of the Ministry of Information and Communication and the Ministry of Information and Communication Research and Development. [Task Management Number: 2006-S-014-02, Title: 200Mbps IEEE 802.11n Modem] And RF chipset development].

Currently, wireless communication systems have an increasing demand for transmitting high quality and large capacity multimedia data using limited frequencies. A multiple input multiple output (MIMO) system is a method for transmitting a large amount of data using a limited frequency. The MIMO system can form a plurality of independent fading channels using multiple antennas at the transmitting and receiving ends, and transmits different signals for each transmitting antenna, thereby greatly improving the data transmission rate. Accordingly, the MIMO system can transmit a larger amount of data without further increasing the frequency.

However, the MIMO system has a disadvantage in that it is weak in interference and frequency selective fading between symbols generated during high speed transmission. To overcome this disadvantage, Orthogonal Frequency Division Multiplexing (OFDM) is used together. The OFDM scheme is currently the most suitable modulation scheme for high speed data transmission, in which one data string is transmitted on a subcarrier having a lower data rate.

Channel environment for wireless communication has multipath due to obstacles such as buildings. In a multi-path wireless channel, delay spread by multipath occurs, and inter-symbol interference (ISI) occurs when the delay spread time is larger than the time when the next symbol is transmitted. In this case, frequency selective fading occurs in the frequency domain. When one carrier is used, an equalizer is used to remove inter-symbol interference components. However, as the speed of data increases, so does the complexity of the equalizer.

Eventually, if the MIMO system and the OFDM system are combined, the advantages of the MIMO system can be used as they are, and the disadvantages can be offset by the OFDM system. A form having N transmit antennas and N receive antennas is a general MIMO system, and a structure in which OFDM technology is combined with this system is a MIMO-OFDM system.

1A and lb are block diagrams schematically showing the configuration of a MIMO-OFDM system. 1A is a block diagram of a transmitter of a MIMO-OFDM system, and FIG. 1B is a block diagram of a receiver of a MIMO-OFDM system.

Referring to FIG. 1A, the serial / parallel conversion unit (S / P) 101 separates transmission data into a plurality of data strings before encoding the transmission data, and each encoder 102 converts the data converted in parallel. Encode. After encoding, the data is modulated by the QAM mapper 103 according to a modulation scheme (eg, BPSK, QPSK, 16QAM, 64QAM). The modulated symbol is converted into a signal on the time axis through an inverse fast fourier transform (IFFT) 104. The CP adder 105 inserts a Cyclic Prefix (CP) code for the guard interval into the symbol converted to the time axis. The digital-to-analog conversion and radio frequency (D / A & RF) unit 106 then converts the digital signal with the cyclic prefix into an analog signal, converts the analog signal into a radio frequency signal (RF), Send through.

Referring to FIG. 1B, the analog-to-digital conversion and radio frequency (A / D & RF) unit 107 converts an RF signal into an analog signal and then converts the analog signal into a digital signal. The CP remover 108 removes the cyclic prefix (CP) code inserted for the guard interval, and transmits the signal from which the cyclic prefix is removed, to the Fast Fourier Transform (FFT) unit 109. The fast Fourier transform (FFT) unit 109 performs fast Fourier transform on the input parallel signal. The MIMO receiver 110 performs estimation on the transmission data symbols generated by the fast Fourier transform. The MIMO receiver 110 calculates a log likelihood ratio (LLR) from the estimated symbols. The decoder 111 demodulates each data string transmitted from the MIMO receiver 110 to estimate transmission data. The parallel / serial converter (P / S) 112 converts the parallel data demodulated by each decoder 111 into serial data.

In such an OFDM-based multi-antenna system, in order to double the data rate, the transmitting end simultaneously transmits data in parallel in the same frequency band using two antennas of the transmitting end. The transmitted signal is detected at the receiving end. In order for the signal received at the receiving end to be received at the same data rate as that transmitted at the transmitting end, the mixed signal in the wireless channel must be separated in parallel through a detector at the receiving end.

The performance of such a detector is determined by the detection algorithm. In general, a detector capable of detecting at least a signal-to-noise ratio has a high complexity. In addition, a detector using a low complexity algorithm requires an input signal having a relatively high signal-to-noise ratio at the receiving end. In addition, in the case of binary representation of the signal input and processed by the detector, the number of binary representations of the signal must be increased to increase the accuracy of the signal and the detection decoding performance. Increasing the base size of a signal increases hardware complexity. Therefore, the performance and complexity of the detector is generally inversely related.

Accordingly, a technique for improving a reception performance by adjusting a signal transmitted from a transmitter, regardless of the reception performance of the receiver, has been proposed, and a representative technique thereof is a beam-forming technique. Beamforming technology is a technique for artificially improving the environment of a wireless channel by changing the power size of each antenna of the transmission signal with the information of the wireless channel. Here, if there is one limitation, in order to transmit in the MIMO scheme, there should always be more transmit antennas than receive antennas.

Accordingly, the present invention has been proposed to solve the above problems of the prior art, using a subcarrier modulation scheme using a state number (CN), which is channel information obtained through a process of changing the roles of a transmitter and a receiver in a MIMO system. It is an object of the present invention to provide a variable transmission method for each subcarrier and a state number calculation method used in the multi-input multiple output system for determining and transmitting.

Other objects and advantages of the present invention can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. Also, it will be readily appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

According to an aspect of the present invention, there is provided a variable transmission method for each subcarrier in a multi-input multiple output system, the method comprising: (a) calculating a condition number (CN) which is channel information; And (b) determining a modulation method for each subcarrier according to the value of the state number.

Preferably, the state number is obtained by changing the roles of the transmitting end and the receiving end, and is a value obtained by dividing the maximum value of a single value by the minimum value in a diagonal matrix.

Preferably, the state number calculating means calculates a first matrix by multiplying a single (Q) matrix and an upper triangular (R) matrix obtained as a result of QR decomposition of a channel matrix, The second matrix is calculated by multiplying the obtained single (Q) matrix and the upper triangular (R) matrix, and multiplying the single (Q) matrix obtained by the QR decomposition of the second matrix and the upper triangular (R) matrix. Repeating until the nth matrix is calculated, a single value is calculated from the diagonal matrix using the absolute values of the diagonal elements of the obtained R matrix.

In addition, according to the present invention, a method for calculating a state number in order to variably apply a modulation scheme for each subcarrier according to a state number which is channel information in a multi-input multiple output system, comprising: (a) a channel; Calculating a first matrix by multiplying a single (Q) matrix and an upper triangular (R) matrix obtained as a result of QR decomposition of the matrix; (b) QR decomposing the first matrix; (c) calculating a second matrix by multiplying a single (Q) matrix and an upper triangular (R) matrix obtained as a result of QR decomposition of the first matrix; (d) performing QR decomposition on the calculated matrix and multiplying the QR decomposition result until the nth matrix is calculated; And (e) calculating a single value in the diagonal matrix using the absolute value of the diagonal element of the R matrix obtained as a result of step (d).

The present invention as described above, one device is performed by changing the role of the transmitting end and the receiving end, by applying a different modulation scheme for each sub-carrier using the state number (CN) which is the channel information obtained accordingly, to the radio channel environment It is possible to flexibly determine the subcarrier modulation scheme, and the receiver can stably receive data.

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, in which: There will be. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In a MIMO system using multiple transmit / receive antennas, a receiver needs a MIMO detector. The present invention will be described taking the case of four transmitting antennas and four receiving antennas as an example.

When an MIMO detector is to be implemented, various algorithms can be applied. In the present invention, a zero forcing (ZF) nulling method is used. First, the signal vector x transmitted through the two transmitting antennas passes the wireless channel transfer function H, and then is added to the noise vector n generated at the receiving end, and finally the signal matrix r inputted to the detector through the four receiving antennas at the receiving end is It is expressed as Equation 1 below.

Here, the row of the channel transfer function matrix H indicates the order of receive antennas, and the column indicates the order of transmit antennas. As shown in Equation 1, the signal input to each receiving antenna is composed of the sum of the signal after receiving each transmission signal x multiplied by the channel transfer function H on the wireless channel and the noise n of the receiving end. Therefore, in order to effectively detect and isolate a desired transmission signal at the receiver, a nulling matrix W must be obtained.

The nulling matrix W can be obtained using the components of the radio channel transfer function H. This nulling matrix is called zero forcing because it minimizes the signal size to zero, which is not the signal to be detected (first or second). The W matrix may be expressed as Equation 2 below.

W = (H ^H H) ^-1 H ^H

Where H ^H is the transpose of the H matrix. Δ represents a determinant. Therefore, the first column of the W matrix corresponds to the vector W ₀ for detecting the first transmission signal, and the second column corresponds to the vector W ₁ for detecting the second transmission signal. Detection of the transmission signal may be expressed as Equation 3 below.

Here, the detected signal vector x has the noise vector n amplified by (H ^H H) ⁻¹ H ^H. This amplified noise results in poor reception performance, especially due to the characteristics of the channel matrix H. Covariance of the noise is given by Equation 4 below.

Referring to Equation 4, elements of the noise vector n are not correlated with each other by Gaussian distribution but are correlated with each other by a W matrix. And the degree of correlation depends on the components of the MIMO channel matrix H, whereby the reception performance depends a lot.

In this case, if a singular value decomposition of the channel matrix H is performed, it may be expressed as Equation 5 below.

In the diagonal matrix λ _max denotes the maximum value of a single value, and λ _min denotes the minimum value of a single value. The condition number (CN) is a value obtained by dividing the maximum value of a single value by the minimum value. The higher the value of this state number (CN), the greater the correlation between the elements of the noise vector n, and the determinant needed to find the inverse matrix when the W matrix is obtained becomes close to zero. H ^H H exhibits a singular characteristic. As a result, the performance of the receiver becomes worse. Due to the reception performance dependent on the channel matrix, a technique such as beamforming is applied to an existing system, and the transmission power is changed for each antenna, so that the effect of artificially adjusting the channel matrix H value is obtained.

However, the present invention calculates and determines a transmission rate of a transmission signal per subcarrier of an OFDM signal in advance using a state number (CN) value known to the transmitting end. In a typical wireless system, the transmitting end and the receiving end frequently change their roles. For example, when the base station transmits data, it operates as a transmitting end, but when the base station receives data, it operates as a receiving end. As such, when a device changes its role to a transmitting end and a receiving end. This status number CN can be obtained.

FIG. 2 illustrates a case in which a state number CN is piloted with channel matrix information when 114 valid subcarriers are used in an OFDM system having a fast Fourier transform (FFT) size of 128. Referring to FIG. 2, it can be seen that a status number (CN) value is different for each subcarrier. In the present invention, data is transmitted by using a different modulation scheme for each subcarrier using this state number (CN).

In the IEEE 802.11n WLAN system, there are four modulation schemes, BSPK, QPSK, 16-QAM, and 64-QAM, respectively. Therefore, according to the status number (CN), the modulation scheme is divided into four as shown in FIG. 3, and when the status number (CN) is 0 to 4, the BPSK modulation scheme is used, and the status number (CN) is 12 In the case of up to 16, 64 QAM is applied.

In other words, as shown in FIG. 4, the apparatus according to the present invention includes a CN calculation 401 for calculating a state number (CN), which is channel information obtained through a process of changing a role of a transmitter and a receiver, and the CN calculation. And a modulation scheme determination 402 for determining a modulation scheme for each subcarrier according to the state number CN calculated by 401. Here, the state number CN is a numerical value obtained by dividing the maximum value of a single value by the minimum value.

However, there is a problem in that the complexity is too large to calculate single value decomposition for every subcarrier as shown in Equation 5. Therefore, the present invention calculates a single value as shown in FIG. 5 in order to reduce the amount of calculation resulting from the single value decomposition. The single value calculation shown in FIG. 5 may be expressed as Equation 6 below.

Referring to FIG. 5, the channel matrix H is QR-decomposed (501), and the upper triangular matrix R and the single matrix Q obtained as a result of the QR decomposition are multiplied with each other to generate a matrix A ₀ (502). The matrix A ₀ is further QR decomposed (503), and the matrix A ₁ is generated by multiplying the upper triangular matrix R and the single matrix Q obtained as a result of the QR decomposition of A ₀ (504). Repeating this process until the matrix A _n is generated (505), the absolute value of the diagonal elements of the upper triangular matrix R converges to the single values of the diagonal matrix Λ in Equation 5. This convergence rate is so fast that the absolute value of the diagonal of R has a value very close to a single value if the above process is repeated about five times. Repeating five times does not increase the hardware by five times, but rather reduces the resources required for the implementation to convert the output to an input.

As such, the present invention does not perform a single value decomposition that requires complex computation such as beamforming, quickly calculates a status number (CN) through several simple QR decompositions, and adjusts a transmission rate using the calculated status number. . In the present invention, since the data rate is variable for each subcarrier, in case of a WLAN packet format, the information on the data rate for each subcarrier should be included in a signal field corresponding to header information.

On the other hand, the method of the present invention as described above can be written in a computer program. And the code and code segments constituting the program can be easily inferred by a computer programmer in the art. In addition, the written program is stored in a computer-readable recording medium (information storage medium), and read and executed by a computer to implement the method of the present invention. The recording medium may include any type of computer readable recording medium.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the technical spirit of the present invention for those skilled in the art to which the present invention pertains. It is not limited by the drawings.

1A and 1B are schematic configuration diagrams of an OFDM-based MIMO system to which the present invention is applied;

2 and 3 are graphs showing channel numbers (CNs) for subcarriers;

4 is a view for explaining a variable transmission method for each subcarrier according to the present invention;

5 is a flowchart illustrating a channel number (CN) calculation process according to the present invention.

Claims (7)

A method for determining a modulation scheme by varying transmission data for each subcarrier in a data transmission / reception apparatus of a multiple input multiple output system using an orthogonal division multiple access scheme, (a) Singular value decomposition of a matrix of radio channel transfer functions obtained by using the result of receiving the signals transmitted for each subcarrier from each transmit antenna of the multiple input multiple output system for each subcarrier Condition number (CN), which is channel information calculated as a ratio of a minimum value of a single value to a maximum value of a single value for each subcarrier in the decomposed diagonal matrix for each subcarrier. Calculating; And (b) determining a modulation scheme corresponding to the state number value for each subcarrier by using the range of the state number value set corresponding to each modulation scheme. Transmission method comprising a. The method of claim 1, The state number is, And when the role is changed from the receiving end to the transmitting end, it is obtained when operating as the receiving end. delete The method of claim 1 or 2, wherein the calculation of the single value, A first QR decomposition using the matrix of the radio channel transfer function as a target matrix, and obtaining a first result matrix by multiplying an upper triangular matrix obtained as a result of the first QR decomposition and a single matrix; Using the first result matrix as the target matrix, second QR decomposition is performed in the same manner as the first QR decomposition, and the first result matrix is updated by the product of the upper triangular matrix obtained as a result of the second QR decomposition and the single matrix. A second step of doing; A third step of performing the second step a predetermined number of times; A fourth step of taking an absolute value of values located diagonally of an upper triangular matrix for obtaining a first result matrix updated at the last iteration of the third step; And A fifth step of calculating a ratio of a minimum value to a maximum value of an element located diagonally of the upper triangular matrix taking the absolute value as the single value; Transmission method comprising a. The method of claim 4, wherein the predetermined number of times, Determine the number of times the absolute value of the element located on the diagonal of the upper triangular matrix converges to a single value of the diagonal matrix Characterized in that the transmission method. In a method for calculating the state number in order to variably apply a modulation scheme for each subcarrier according to a state number which is channel information in a data transmission and reception apparatus of a multiple input multiple output system using an orthogonal multiplexing multiple access scheme, Acquiring a wireless channel transfer function matrix using a result of receiving signals transmitted for each subcarrier from each transmit antenna of the multiple input multiple output system for each subcarrier; Performing a first QR decomposition using the matrix of the radio channel transfer function as a target matrix, and obtaining a first result matrix by multiplying an upper triangular matrix obtained as a result of the first QR decomposition and a single matrix; A second QR decomposition of the first result matrix in the same manner as the first QR decomposition, and a second step of updating the first result matrix by a product of an upper triangular matrix obtained as a result of the second QR decomposition and a single matrix; ; A third step of performing the second step a predetermined number of times; A fourth step of taking absolute values of values located diagonally of an upper triangular matrix for obtaining the first result matrix obtained in the last iteration of the third step; And A fifth step of calculating a ratio of a minimum value to a maximum value of an element located diagonally of an upper triangular matrix having the absolute value as the state number for each subcarrier; State number calculation method comprising a. The method of claim 6, wherein the predetermined number of times, Determine the number of times the absolute value of the element located on the diagonal of the upper triangular matrix converges to a single value of the diagonal matrix Method for calculating a state number, characterized in that.

Images