KR100867999B1 - Apparatus and method for receiving digital signal based on OFDM - Google Patents

Abstract

Disclosed are a digital signal receiver and method modulated by Orthogonal Frequency Division Multiplexing (OFDM). The device measures a signal receiver for receiving a digital signal modulated by an OFDM scheme including a preamble including a CAZAC sequence, cross-correlation between the received digital signal and the preamble, and a power value of the received digital signal. A time synchronization estimator for estimating the minimum time point and estimating it as a time synchronization time point, a channel estimator for estimating a channel compensation value through a fast Fourier transform using a time synchronization time point and a cross correlation, a time synchronization time point and a channel The data recovery unit recovers the data by using the compensation value. In the OFDM method, a time synchronization point for minimizing interference between adjacent symbols can be determined and accurate channel estimation can be performed simultaneously. It can be implemented effectively.

OFDM, CAZAC, ISI, ICI, FFT

Description

Apparatus and method for receiving digital signal based on OFDM

1 is a conceptual diagram illustrating a configuration of a digital signal receiving apparatus of the OFDM scheme according to an embodiment of the present invention.

2 is a diagram illustrating a preamble structure of an OFDM digital signal according to an embodiment of the present invention.

3A and 3B are diagrams illustrating a preamble structure of an OFDM digital signal according to an embodiment of the present invention.

4 is a diagram showing the structure of an OFDM digital signal receiving apparatus according to an embodiment of the present invention.

5 is a flowchart illustrating a flow of a digital signal receiving method of the OFDM scheme according to an embodiment of the present invention.

The present invention relates to an OFDM-based digital signal receiving apparatus and method, and more particularly, to a receiver for estimating time synchronization and channel for minimizing interference between adjacent symbols by using a preamble including a CAZAC sequence in an OFDM-based transmitting and receiving system. And to a method.

The OFDM transmission method is a method in which data having a long symbol period is loaded on several subcarriers in parallel and transmitted, using an Inverse Fast Fourier Transform (IFFT) and a Fast Fourier Transform (FFT). The modulation / demodulation process can be performed, and it is robust to inter-symbol interference (ISI), which is a major problem of high-speed communication, and has the advantage of making frequency selective fading appear as non-selective fading. These advantages allow high-speed data transmission systems such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Digital Terrestrial Television Broadcasting (DTTB), and Wireless LAN (Wireless). OFDM wireless communication systems, such as Local Area Network (WLAN) and IEEE 802.16 Broadband Wireless Access, have been developed. Recently, these OFDM wireless communication systems are considered as the core technology of the next generation of mobile communication. It is becoming.

In the OFDM transmission scheme, due to its structural characteristics, the transmitting side performs an IFFT to generate a transmission signal. At this time, the receiver sets a proper FFT interval by performing time synchronization during the preamble interval to recover the transmission signal. In addition, the channel compensation value is estimated to compensate for the distortion caused by the channel for the value after the FFT.

All of the advantages of the OFDM transmission scheme can be maintained when assuming accurate time synchronization for performing FFT and accurate channel estimation based on the same. However, in a multipath channel environment, the accuracy of time synchronization is reduced, and the time synchronization offset caused by this can sometimes result in severe inter-symbol interference (ISI) and inter-carrier interference (ICI). Can cause serious deterioration of system performance. The longer the maximum or average delay time of the multipath channel is, the more severe the performance deterioration becomes. In particular, even if the delay of the channel is longer than the guard interval, even if it is an accurate time synchronization point, there are many ISIs.

The technical problem to be achieved by the present invention is to find the FFT execution time of the location with the lowest ISI without detecting the accurate FFT execution time in a multipath channel environment, and to detect the time synchronization time, and to perform the time synchronization. The present invention provides an OFDM-based digital signal receiving apparatus and method having higher and more stable performance by calculating channel compensation values in a frequency domain in which ISI and ICI are minimized using the result value of.

According to an aspect of the present invention, there is provided an apparatus for receiving a digital signal, the signal receiving unit receiving a digital signal modulated by an orthogonal frequency division multiplexing (OFDM) method using a preamble including a CAZAC sequence; By measuring the degree of cross-correlation between the received digital signal and the preamble and the power value of the received digital signal, the time point at which interference between adjacent symbols is minimized is calculated, and the time point when the interference between adjacent symbols is minimum is a time synchronization time point. A time synchronization estimator; A channel estimator estimating a channel compensation value through a first fast Fourier transform using the time synchronization point and the degree of cross correlation; And a data recovery unit for restoring data by using the time synchronization point and the channel compensation value.

According to an embodiment of the present invention, there is provided a method for receiving a digital signal, the method comprising the steps of: (a) receiving a digital signal modulated by an orthogonal frequency division multiplexing (OFDM) method using a preamble including a CAZAC sequence ; (b) calculating a time point when interference between adjacent symbols is minimized by measuring a degree of cross correlation between the received digital signal and the preamble and a power value of the received digital signal, and determining a time point when the interference between adjacent symbols is minimum Estimating at the time of synchronization; (c) estimating a channel compensation value through a first fast Fourier transform using the time synchronization point and the degree of cross correlation; (d) restoring data by using the time synchronization time point and the channel compensation value.

Hereinafter, a method and an apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

1 is a view showing the structure of a digital signal receiving apparatus of the OFDM method according to an embodiment of the present invention.

Referring to FIG. 1, the receiver includes a signal receiver 100, a time synchronization estimation unit 101, a channel estimation unit 102, and a data restoration unit 103.

The signal receiver 100 receives a digital signal modulated by an orthogonal frequency division multiplexing (OFDM) method using a preamble including a sequence having an autocorrelation degree of 1.

The time synchronization estimator 101 calculates a point of time when interference between adjacent symbols is minimized by measuring a degree of cross correlation between the digital signal received by the signal receiver 100 and a preamble included therein and a power value of the received digital signal. The time point when the interference between adjacent symbols is minimized is estimated as a time synchronization time point.

The channel estimator 102 estimates a channel compensation value through a first fast Fourier transform using the time synchronization point estimated by the time synchronization estimator 101 and the measured degree of cross-correlation.

The data recovery unit 103 restores the data using the time synchronization time estimated by the time synchronization estimation unit 101 and the channel compensation value estimated by the channel estimation unit 102.

2 is a diagram illustrating a preamble structure of an OFDM digital signal.

Referring to FIG. 2, the preamble of the OFDM-based digital signal has a constant amplitude zero auto correlation (CAZAC) sequence in the time domain, and structurally zero auto-correlation (ZAC) at both ends of the CAZAC sequence. It is characterized by having a repeated value to maintain the characteristic. That is, as shown in Fig. 2, sequences A and B of lengths G1 and G2, respectively, are located at both ends of a CAZAC sequence of length N.

The constant amplitude zero auto-correlation (CAZAC) sequence is characterized by a constant amplitude of each sample, which has a value of 1 for accurate autocorrelation but a mathematical value of 0 for autocyclic correlation. . When the receiver receives a correlation between the CAZAC sequence and the received signal, the characteristic is represented as a noise power value or an interference value between adjacent symbols in a section other than the correct autocorrelation.

In other words, the autocorrelation value of the CAZAC sequence has an impulse shape, and because of this property, the CAZAC sequence is frequently considered for signal detection and synchronization in a communication system.

When the CAZAC sequence is called C, the characteristics of the preamble can be summarized as in Equation 1.

3A and 3B are diagrams illustrating a preamble structure of an OFDM digital signal according to an embodiment of the present invention. Referring to FIG. 3A, a preamble of a digital signal applied to the present invention has a preamble structure in which a CAZAC sequence having the same value is repeated three times. Referring to FIG. 3B, a preamble of a digital signal applied to the present invention has a preamble structure having a cyclic prefix and a cyclic suffix.

4 is a diagram illustrating a configuration of an OFDM digital signal receiver according to an embodiment of the present invention. The time synchronization estimator 110, the channel estimator 120, and the data reconstruction unit ( 130 is shown in detail.

Referring to FIG. 4, the reception apparatus includes a preliminary time synchronization estimation unit 410, a cross correlation unit 420, a timing metric calculation unit 430, a moving total power calculation unit 440, and a final time synchronization estimation unit 450. And an impulse response output unit 460, a first fast Fourier transform unit 470, a second fast Fourier transform unit 480, and an equalizer 490.

The preliminary time synchronization estimation unit 410 estimates, as a preliminary time synchronization time, a position index (i) for performing an approximate FFT that can be obtained through a simple operation. As a representative method of estimating the preliminary time synchronization time point, there is a method of using correlation characteristics with a preamble or a cyclic prefix. For example, there is a method of normalizing P (i) calculated as in Equation 2 to the power value of the received signal.

The cross-correlator 420 cross-correlates the received signal with the preamble C during an interval based on the position index i, which is a preliminary time synchronization time estimated by the preliminary time synchronization estimation unit, as shown in Equation 3 ( Measure the degree of cross-correlation. At this time, the length (V = a + b) of the arbitrary intervals that cross each other can be arbitrarily set in consideration of the performance of the method for obtaining the preliminary time synchronization, the lengths of the guard sections G1 and G2, and the like.

Where C (m) represents the value of the m-th CAZAC sequence and N represents the length of the CAZAC sequence.

The timing metric calculator 430 calculates the timing metric M (i) using the degree of cross-correlation measured by the cross correlator 420, that is, P (i). The timing metric is a standardized information of the correlation between the received signal and the preamble by the power value of the received signal. One method of obtaining the timing metric is shown in Equation 4.

대신 P(i)의 절대값을 사용할 수도 있다. 분모항의 R(i)는 타이밍 메트릭 M(i)을 표준화(normalize)하기 위한 것으로 추정 구간의 전력 값을 사용한다.

을 구하기 위한 하나의 방법은 수학식 5가 될 수 있다.At this point, we need to

You can also use the absolute value of P (i) instead. R (i) of the denominator is for normalizing the timing metric M (i) and uses the power value of the estimation interval.

One method for obtaining the equation may be Equation 5.

The moving total power calculation unit 440 calculates a moving total power value or a moving average value during the moving section arbitrarily set for the timing metric M (i) calculated by the timing metric calculation unit 430. This process aims to obtain the total power value of the received signal, which can be expressed as Equation 6 or Equation 7.

In this case, the size W of the moving interval for obtaining the average power or the total power can be arbitrarily determined in consideration of the characteristics of the system, and in consideration of the ISI and ICI, the length of the cyclic prefix interval of OFDM is most desirable. May be an implementation value of.

The final time synchronization estimation unit 450 detects an i _fft which is an index value when the AP (i) is maximum with respect to the AP (i), which is the mobile total power value calculated by the mobile total power calculation unit 440, and determines the final time. It is estimated at the point of synchronization. This can be expressed as Equation 8.

시점으로부터 N개의 샘플을 취해(즉,

) 제2고속푸리에변환을 수행하여 주파수 영역의 신호를 복원한다. 그러나, 이렇게 복원된 주파수 영역의 신호는 채널에 의해 왜곡된 신호이기 때문에 이를 보상하기 위한 주파수 영역의 채널 보상값 이 필요하다. The second fast Fourier transform unit 480 estimates the time synchronization index estimated by the final time synchronization estimation unit 450 among the received signals r (i).

Take N samples from the point in time (i.e.

The second fast Fourier transform is performed to restore the signal in the frequency domain. However, since the reconstructed signal in the frequency domain is a signal distorted by the channel, a channel compensation value in the frequency domain is required to compensate for this.

부터 L개의 값을 취한다. The impulse response output unit 460 can simply obtain a channel compensation value in the frequency domain using an operation value performed in a time synchronization process, and the process is as follows. As shown in Equation 9, the output value of the negative final time synchronization estimation unit 450 among the output values P (i) of the cross-correlator 420.

Take L values from.

Where L is ideally the maximum delay tap of the channel and can be set equal to the CP length in general. H defined in Equation (9) is the impulse response of the channel.

The first fast Fourier transform unit 470 performs a first fast Fourier transform on the impulse response output from the impulse response output unit 460. In this case, the channel compensation value estimated through the first fast Fourier transform is a channel compensation value that minimizes ISI and minimizes ICI generated by ISI.

The equalizer 490 finally restores data by compensating for the signal of the frequency domain, which is the output value of the second fast Fourier transform unit 480, with the channel value of the frequency domain, which is the output value of the first fast Fourier transform unit 470. do.

5 is a flowchart illustrating a flow of a digital signal receiving method of the OFDM scheme according to an embodiment of the present invention.

Referring to FIG. 5, first, a digital signal modulated by an orthogonal frequency division multiplexing (OFDM) method including a preamble including a CAZAC sequence is received (S510).

Next, the cross-correlation degree between the received digital signal and the preamble included therein and the power value of the received digital signal are measured to calculate the point of time when the interference between adjacent symbols is minimized and the time point when the interference between adjacent symbols is minimum. The time synchronization is estimated at step S520.

Thereafter, a channel compensation value is estimated through a first fast Fourier transform using the estimated time synchronization point and the measured degree of cross-correlation (S530).

Finally, the data is restored using the estimated time synchronization time point and the estimated channel compensation value (S540).

The invention can also be embodied as computer readable code on a computer readable recording medium. Computer-readable recording media include all kinds of recording devices that store data that can be read by a computer system. Examples of computer-readable recording media include ROM, RAM, CD-ROM, magnetic tape, floppy disks, optical data storage devices, and the like, which are also implemented in the form of carrier waves (for example, transmission over the Internet). Include. The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. In addition, functional programs, codes, and code segments for implementing the present invention can be easily inferred by programmers in the art to which the present invention belongs.

Such a method and apparatus of the present invention have been described with reference to the embodiments shown in the drawings for clarity, but these are merely exemplary, and various modifications and equivalent other embodiments are possible to those skilled in the art. Will understand. Therefore, the true technical protection scope of the present invention will be defined by the appended claims.

As described above, the present invention sets the point where the minimum ISI is the FFT performing point in an orthogonal frequency multiplexing (OFDM) system, thereby minimizing the ISI, thereby improving and stabilizing the performance of the OFDM receiver. In addition, it is possible to implement a high performance, low complexity receiver by simultaneously performing accurate channel estimation that minimizes ISI and minimizes ICI generated by ISI using the results of operations performed for time synchronization. According to the present invention, since the minimum ISI is set as the FFT execution point in an orthogonal frequency multiplexing (OFDM) system, the performance of the OFDM receiver can be improved and stabilized by minimizing the ISI. In addition, it is possible to realize high performance and low complexity receivers by simultaneously performing accurate channel estimation to minimize ISI and minimizing ICI generated by ISI using the results of operations performed for time synchronization.

Claims (21)

A signal receiver configured to receive a digital signal modulated by an orthogonal frequency division multiplexing (OFDM) scheme using a preamble including a CAZAC sequence; By measuring the degree of cross-correlation between the received digital signal and the preamble and the power value of the received digital signal, the time point at which interference between adjacent symbols is minimized is calculated, and the time point when the interference between adjacent symbols is minimum is a time synchronization time point. A time synchronization estimator; A channel estimator estimating a channel compensation value through a first fast Fourier transform using the time synchronization point and the degree of cross correlation; And a data recovery unit for restoring data using the time synchronization point and the channel compensation value. The method of claim 1, The preamble includes a left sequence located to the left of the CAZAC sequence and a right sequence located to the right of the CAZAC sequence, The left sequence has a value equal to a value of a predetermined section corresponding to the end of the CAZAC sequence, and the right sequence has a value equal to a value of a predetermined section corresponding to the front of the CAZAC sequence. Receiver. The method of claim 2, The preamble has a structure in which the CAZAC sequence has a structure repeatedly arranged three times with the same value. The method of claim 2, The preamble has a prefix of a cyclic structure and a suffix of the cyclic structure at both ends of the CAZAC sequence. The method of claim 1, And a preliminary time synchronization time calculating unit for calculating a preliminary time synchronization time that is an approximate time synchronization time through a simple operation from the received digital signal. The degree of cross-correlation is a digital signal receiver, characterized in that the degree of cross-correlation between the signal and the preamble corresponding to a predetermined section based on the preliminary time synchronization time point calculated. The method of claim 1, The estimation of the time synchronization time is obtained by calculating the total power value of the moving section or the average power value of the moving section through the calculation using the measured degree of cross-correlation and the power value of the section taking the cross correlation. And a time synchronization time point for estimating the total power value or the average power value between the moving sections as a maximum. The method of claim 6 The preamble has a prefix of a cyclic structure and a suffix of a cyclic structure at both ends of the CAZAC sequence, And the moving section has the same size as the size of the prefix of the cyclic structure. The method of claim 1, And the first fast Fourier transform is performed with respect to the degree of cross-correlation of a predetermined size interval starting from the estimated time synchronization point. The method of claim 8, The preamble has a prefix of a cyclic structure and a suffix of a cyclic structure at both ends of the CAZAC sequence, The interval of the degree of cross-correlation for performing the first fast Fourier transform has a size equal to the size of the prefix of the cyclic structure. The method of claim 1, Restoring the data includes performing a second fast Fourier transform on the received digital signal in the range of the CAZAC sequence size starting from the estimated time synchronization point, restoring a signal in a frequency domain, and restoring a signal in the frequency domain. The digital signal receiving device, characterized in that for compensating through the estimated channel compensation value. (a) receiving a digital signal modulated in an orthogonal frequency division multiplexing (OFDM) scheme using a preamble comprising a CAZAC sequence; (b) calculating a time point when interference between adjacent symbols is minimized by measuring a degree of cross-correlation between the received digital signal and the preamble and a power value of the received digital signal, and determining when the interference between adjacent symbols is minimum Estimating the time synchronization point; (c) estimating a channel compensation value through a first fast Fourier transform using the time synchronization point and the degree of cross correlation; (d) restoring data using the time synchronization time point and the channel compensation value. The method of claim 11, The preamble of step (a) includes a left sequence located to the left of the CAZAC sequence and a right sequence located to the right of the CAZAC sequence, The left sequence has a value equal to a value of a predetermined section corresponding to the end of the CAZAC sequence, and the right sequence has a value equal to a value of a predetermined section corresponding to the front of the CAZAC sequence. Receive method. The method of claim 11, The preamble of step (a) has a structure in which the CAZAC sequence has a structure repeatedly arranged three times with the same value. The method of claim 11, The preamble of step (a) has a prefix of a cyclic structure and a suffix of the cyclic structure at both ends of the CAZAC sequence. The method of claim 11, Step (b) is (b-1) calculating a preliminary time synchronization time that is an approximate time synchronization time from the received digital signal; (b-2) calculating a degree of cross-correlation between a signal corresponding to a predetermined period and the preamble based on the calculated preliminary time synchronization time point; Digital signal receiving method further comprises. The method of claim 11, In the estimation of the time synchronization point in step (b), the total power value of the moving section or the average power value of the moving section is obtained by calculation using the measured degree of cross correlation and the power value of the section taking the cross correlation. And estimating a time point at which the total power value of the moving section or the average power value of the moving section is maximized as a time synchronization point. The method of claim 16 The preamble of step (a) has a prefix of the cyclic structure and a suffix of the cyclic structure at both ends of the CAZAC sequence, The moving section of step (b) has a size equal to the size of the prefix of the cyclic structure. The method of claim 11, The first fast Fourier transform of the step (c) is performed for the degree of cross-correlation of the interval of a predetermined size starting from the estimated time synchronization time. The method of claim 18, The preamble of step (a) has a prefix of the cyclic structure and a suffix of the cyclic structure at both ends of the CAZAC sequence, The interval of the degree of cross-correlation for performing the first fast Fourier transform of step (c) has the same size as the size of the prefix of the cyclic structure. The method of claim 11, In step (d), (d-1) restoring a signal in a frequency domain by performing a second fast Fourier transform on the received digital signal in a range of CAZAC sequence sizes starting from the estimated time synchronization point; (d-2) compensating for the restored frequency domain signal through the estimated channel compensation value. A computer-readable recording medium containing a program for performing the method according to any one of claims 11 to 20.

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