KR100492359B1 - Symbol timing detection apparatus of ofdm system - Google Patents

Abstract

The present invention relates to an apparatus for detecting a symbol synchronization in an OFDM system. The present invention relates to a symbol synchronization device for generating a difference signal by delaying an OFDM signal having a periodic structure of a frame structure by a symbol period, and obtaining instantaneous power for the difference signal. Means for reducing the effect of noise by moving averaging; means for estimating the moving averaged average noise power for a preceding training symbol; setting an optimal threshold; candidate timing using a moving subtraction method for the difference signal discriminated by the optimal threshold; Means for detecting the signal, reference timing determining means for identifying and determining accurate and reliable reference timing in real time from the detected candidate timing information, and symbol timing generating means for generating symbol timing from the reference timing information. Using the estimated average noise power, always obtain the optimal noise power threshold. In this case, a constant timing detection performance is provided for any channel regardless of the channel environment, and a frequency offset exists by applying a new detection method, the moving subtraction method, to a signal determined by an optimal threshold value. Even in the presence of unwanted noise, it effectively reduces the probability of occurrence of '-' timing (timing position shorter than zero, that is, timing position of 0-n), resulting in favorable synchronization performance and real-time processing for correct timing. By using the timing determining unit for identifying and determining, symbol timing can be detected at high speed, and symbol timing detection is performed by subtraction operation, so that signal processing can be simplified and hardware of low complexity can be constructed.

Description

SYMBOL TIMING DETECTION APPARATUS OF OFDM SYSTEM}

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an apparatus for detecting symbol synchronization in an orthogonal frequency division multiplexing system (hereinafter referred to as 'OFDM'). Providing a threshold provides consistent timing detection for any channel, effectively reducing the probability of '-' timing in the presence of frequency offset or unwanted noise, resulting in excellent synchronization performance. The symbol timing detection is performed by a subtraction operation, so that the signal processing can be simplified and the hardware configuration of low complexity can be achieved, and the symbol synchronization detection apparatus of the OFDM system can achieve symbol synchronization in the OFDM system in a real-time processing manner. will be.

In general, OFDM processes data blocks in parallel and transmits them in parallel on a plurality of subcarriers orthogonal to each other. The generation of such an OFDM signal is obtained by inverse fast Fourier transform (IFFT) processing after serially converting serial data. The OFDM signal is constructed in units of frames by data symbols and training symbols that are created using the IFFT processing section of the data as the data sample section.

In reception, data is recovered by data demodulation after symbol recovery using a fast Fourier transform (FFT) method after analog-to-digital conversion on a baseband (baseband) signal of transmission.

In order to recover useful data in a receiving system, it is very important to match the start timing of data symbols in an OFDM signal having a periodic structure in units of frames, and to match the start timing of these frames is called frame synchronization. The symbol synchronization timing for performing the FFT is generated.

The symbol of the OFDM signal is composed of N_G` + `N` sample intervals by the guard interval of N_G` samples and the data sample interval of N samples as shown in FIG. The protection interval set to prevent the interference between symbols is created by copying the N_G` samples at the end of the data sample and attaching it to the beginning of the data sample. Use a value that is twice as long and the data sample interval is greater than the guard interval. It's tummy long.

In general, a receiver of an OFDM system, as shown in FIG. 1, an analog-to-digital converter (ADC) 1 for converting digitally to a received baseband (baseband) signal and a reference symbol for detecting reference symbol timing. The timing detection unit 2, the FFT window control unit 3, the guard interval, that is, the guard interval elimination unit 4, the fast Fourier transform (FFT) unit 5 and the data demodulation unit 6 are formed.

Conventionally, in order to generate timing pulses for FFT window control, a method using correlation characteristics between received signals is used. In this method, a signal having a periodic structure is delayed by a symbol period to correlate, and then a reference symbol timing is estimated by comparing a threshold to a correlation value of periodicity. This method, which is an autocorrelation method, has the advantage of being simple to calculate, but has a small difference from the correlation value in the case of a deviation of one sample, resulting in inferior accuracy of the detected timing.

Therefore, the variance of the timing error becomes large, and in particular, the variance of the timing error greatly increases when it is not an optimal threshold value. In this method, the correlation value varies according to the channel environment (power delay profile, etc.), and the optimal threshold value actually varies according to the channel environment. Therefore, if the channel environment is not fixed, the desired timing detection performance can be guaranteed. The result will be impossible.

The method of using cross-correlation detects start timing information of an OFDM symbol from a result of taking a cross-correlation between a reference signal such as a training symbol received through a channel and this signal stored in a receiver. Compared with autocorrelation, this method is more computational and more complicated in hardware, but the correlation is more precise because the difference in correlation between adjacent samples is large.

However, if there is a frequency error, it leads to a sudden decrease in performance. In addition, since the correlation peak of this method follows the power profile of the channel as it is, it may be difficult to detect the timing because the position of the maximum correlation peak is changed, especially for a non_minimum phase channel in which the indirect wave is larger than the direct wave. In this method, since the magnitude of the correlation peak varies depending on the channel environment, the optimum threshold value varies depending on the channel, and thus, the desired timing detection performance cannot be guaranteed unless the channel environment is fixed.

In addition, there is a method of finding the minimum position of the absolute value of the difference between the received signals, but there is a problem that it is difficult to detect the timing of the symbol start position when a frequency offset occurs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and its object is to reduce the influence of noise by instantaneous power signaling and moving average method for the difference signal between received signals and to optimize the estimated noise power by using the estimated average noise power. Noise power threshold value (effective noise power value that affects the discrimination of actual signal and noise, including the effects of additional noise or frequency offset included in the transmitted signal, or additional noise at the receiving front end) It is to provide a symbol synchronization detection device of an OFDM system to provide an optimal threshold value for any channel environment to ensure a constant timing detection performance and system performance.

It is another object of the present invention to apply a moving subtraction method as a new timing information detection method, effectively applying a '-' timing error even when frequency offset or unwanted noise exist by simple subtraction. In addition, the present invention provides a symbol synchronization detection apparatus for an OFDM system that not only improves the timing synchronization performance but also provides a reliable timing detection method through a timing determiner that identifies and determines accurate timing. .

Yet another object of the present invention is to perform symbol subtraction by shift subtraction operation, thereby simplifying signal processing and low complexity hardware configuration, and enabling OFDM symbol synchronization in an OFDM system in real time. It is to provide a symbol synchronization detection device of.

In order to achieve the above object, the symbol synchronization detecting apparatus of the OFDM system of the present invention generates a difference signal by delaying a received signal by a symbol period using a periodic structure of an OFDM signal having a frame structure. The instantaneous power for this difference signal is obtained by this method. In this case, during the delay time period after the delay time by the difference signalization process, the added noise period is canceled and the noise power in this period is calculated by the effective noise power calculation and After the average noise power is estimated, the optimum threshold value regardless of the channel environment (power profile or maximum delay, etc.) is set and the symbol timing is performed in real time through the moving subtraction method and the timing decision unit. It is characterized by detecting.

Hereinafter, with reference to the accompanying drawings will be described in detail the configuration and operation of the symbol synchronization detection apparatus of the OFDM system according to the present invention.

3 is a block diagram of an apparatus for detecting a symbol synchronization in an OFDM system according to the present invention, and includes a reference symbol period delay unit 11 for delaying a received signal of an OFDM signal having a periodic structure by a symbol period T according to a frame structure; A subtractor 12 for generating a difference signal between the received signal of the OFDM signal and two signals delayed through the reference symbol period delay unit 11, and an absolute value of the difference signal generated by the subtractor 12; The square calculator 13 to calculate, the moving averager 14 which calculates and averages the sum of the squares of the absolute values calculated by the square calculator 13 with respect to the symbol period, and the signal are canceled by the difference signalization process. It is the noise set by the effective noise power calculation and control unit 15 and the effective noise power calculation and control unit 15 which obtains the average noise power for the lost noise section and sets the noise threshold level as the effective noise power. The level discriminator 16 outputs the signal level of the moving averager 14 larger than the system level to " 1 " and the small signal level " 0 ", and outputs the level discriminator 16 to the sample of the symbol. A moving subtractor 17 for finding a difference for each interval T_s` (symbol period T` = `N``T_s`, N; number of samples) and a position of a value greater than 0 in the output of the moving subtractor 17 Among the candidate timing extractors 18 to be extracted and the candidate timings extracted by the candidate timing extractor 18, 0 ′ + n ′ set by the length n (n is 2 to 3) of the shift subtractor 17 ( A reference symbol timing determiner 19 that determines a candidate timing pulse position point, which is indicated first within a timing error range of 0), and the reference symbol timing determiner 19 determined by the reference symbol timing determiner 19; And a symbol timing generator 20 for outputting symbol timing from the reference symbol timing.

4 is a block diagram of the moving subtractor 17, showing a moving subtraction window of length N. The moving subtractor 17 delays an output signal of the level discriminator 16. A delayer 17-1, an adder 17-2 for adding the delayed signals in the plurality of delayers 17-1, an output signal of the level discriminator 16, and the adder 17-2. And a subtractor 17-3 for obtaining the difference between the output signals.

The operation of the symbol synchronization detection apparatus of the OFDM system according to the present invention configured as described above will be described in detail with reference to FIGS. 5 to 6.

5 is a view illustrating an operation principle of the moving subtractor of FIG. 4, and FIG. 6 is an output waveform diagram of each component block of the symbol synchronization detection apparatus of the OFDM system according to the present invention.

Prior to the detailed description of the present invention, a brief description of the frame configuration of the OFDM signal is as follows.

OFDM uses 10 short training symbols and 2 or 3 long training symbols at the head of the frame as shown in FIG. 2 for frame synchronization. In this frame configuration, the periods of ten short training symbols, which are two OFDM symbol periods, are used to estimate average noise power, and are used to detect symbol timing and channel equalization during two or three long training symbols. Lose.

6 (a) is a waveform diagram illustrating an OFDM baseband (baseband) received signal in a frequency offset and a multipath channel environment. The beginning of an OFDM signal having a periodic structure includes a training symbol to be used for timing estimation. Added.

6B is a signal obtained by delaying the OFDM baseband (baseband) received signal by one period (N_p` = `N_G` +` N``) of a reference symbol, and FIG. The difference signal of the output of the subtractor 12, which obtains the difference between the OFDM baseband (baseband) received signal and the delayed received signal, and a portion of which the level is weak is a noise generated by canceling the signal by the difference signalization process. The interval appears after the first 1 OFDM symbol period (N_p '=' N_G '+' N ') of the short training symbol of each frame.

FIG. 6 (d) shows the output of the square calculator 13 of the absolute value of the difference signal of the subtractor 12, which is formed of the instantaneous power of the difference signal at every sample time T_s (T ′ = `N′T_s). 6E is an output signal of the moving averager 14 for moving average instantaneous power.

The calculation of the noise power and the setting of the optimum value, that is, the effective noise power calculation and the control unit 15 for setting the optimum threshold value mean noise power during the noise power level period in which the level is weak in the output of the moving averager 14. Obtain the noise threshold level by the effective noise power.

Then, the output signal of the moving averager 14 is input to the level discriminator 16 which outputs a signal level larger than the threshold level as "1" and a small signal level as "0" compared to the noise threshold level.

On the other hand, the output signal of the level discriminator 16 outputting the waveform as shown in FIG. 6 (f) is the sample interval T_s` (symbol period T` = `N``T_s` of symbols) as shown in FIG. , N; is inputted to the moving subtractor 17 for obtaining a difference for each sample number, and the discriminator output signal is converted into three kinds of signals having values of +,-, and 0, and the waveform as shown in FIG. Make

5 is a view showing the operation principle of the moving subtractor 17 having a length of 3, where (a) is the original signal, (b) and (c) are the one-sample and two-sample delayed signals, respectively. , (d) is the original signal minus (b) and (c).

Subsequently, the candidate timing extractor 18 extracts a position having a value greater than 0 from the output of the shift subtractor 17 as 1 for the length of time T_s' as shown in FIG. Send to

Then, the reference symbol timing determining unit 19 receives the signal detection information (enable pulse) of the candidate timing extractor 18 as shown in FIG. Counts the number of samples [2 (N_G + N) + 2N_G + 1)-+ n] within the n error range set before and after 2 (N_G + N) + 2N_G + 1 sample position point) as the FFT starting point. The candidate timing pulse position point, which appears first, is output as a critical timing signal.

In other words, the candidate timing pulse position point counted first within the timing error range of 0` +-n` (zero indicates an accurate timing point) among the candidate timings with respect to the error n set so as not to affect the system performance. Determined by symbol timing.

Subsequently, the symbol timing generator 20 generates symbol timing by receiving the reference symbol timing information.

The shift subtraction method of the present invention is a method of subtracting itself from a delayed one, and serves to eliminate the occurrence of pseudo timing around a desired timing due to frequency offset or unexpected noise. This helps to improve timing synchronization performance.

In addition, the functions of moving subtraction and candidate timing extraction can be performed at the same time, and the circuit can be easily implemented through two logic gates and one delayer, which can be simplified in hardware.

As described above, the present invention relates to an apparatus for estimating a symbol timing of an OFDM system, and since an optimal threshold is always set using an estimated average noise power, a constant timing estimation performance is performed for any channel regardless of the channel environment. Will be provided. By applying the moving subtraction method to the signal discriminated by the optimal threshold, the device can reduce the probability of occurrence of '-' timing effectively even in the presence of frequency offset or unwanted noise. Ensure accurate synchronization performance. In addition, symbol timing can be detected at high speed by using a timing determiner that identifies and determines correct timing in a real-time processing manner. In the above method, symbol timing detection is performed by a subtraction operation, thereby simplifying signal processing and configuring hardware of low complexity.

Accordingly, the present invention provides an excellent and reliable OFDM symbol timing detection performance and guarantees the performance of the system in applying an OFDM system to a real channel environment whose channel characteristics are not known in advance.

1 is a block diagram of a receiver of a general OFDM system;

2 is a diagram illustrating an example of a configuration of a preamble symbol;

3 is a block diagram of an apparatus for detecting a symbol synchronization in an OFDM system according to the present invention;

4 is a block diagram illustrating a moving subtractor of FIG. 3;

5 is a view illustrating an operation principle of the moving subtractor of FIG. 3;

Fig. 6 is an output waveform diagram of each component block of the symbol synchronization detection device of the OFDM system according to the present invention.

<Description of the symbols for the main parts of the drawings>

11: reference symbol period delay 12: subtractor

13: square calculator 14: moving average

15: effective noise power calculation and control unit

16: level discriminator 17: moving subtractor

18: candidate timing extractor 19: reference symbol timing determination unit

20: symbol timing generator

Claims (1)

A reference symbol period delay unit for delaying a received signal of an OFDM signal having a periodic structure by a frame structure by a symbol period T; A subtractor for generating a difference signal between the received signal of the OFDM signal and two signals delayed through a reference symbol period delayer; A square calculator for calculating instantaneous power by calculating a square of an absolute value of the difference signal made by the subtractor; A moving average that calculates and averages a sum of squares of absolute values calculated by the square calculator for a symbol period; An effective noise power calculation and control unit for obtaining an average noise power for a noise section generated by canceling a signal by the difference signalization process and setting a noise threshold level as an effective noise power; A level discriminator for outputting a signal level of the moving averager greater than the noise threshold level set by the effective noise power calculation and control unit to "1" and a small signal level to "0"; A moving subtractor for calculating a difference of the output of the level discriminator for each symbol interval T_s`; A candidate timing extractor extracting a candidate timing by extracting a position having a value greater than 0 from an output of the shift subtractor; A reference symbol timing determining unit configured to determine, as reference symbol timing, a candidate timing of a position point counted first within a timing error range of 0` + n` among candidate timings extracted by the candidate timing extractor; And a symbol timing generator for outputting symbol timing from the reference symbol timing determined by the reference symbol timing determining unit.