KR20030063859A - Encoding and decoding method using ofdm symbol structure - Google Patents

Abstract

PURPOSE: An encoding and decoding method using an OFDM(Orthogonal Frequency Division Multiplexing) symbol structure is provided to reduce the time required for estimating start positions of window symbols, and use another guard intervals and useful intervals even under a channel environment having a burst error as much as the length of guard interval. CONSTITUTION: Output samples of an IFFT(Inverse Fast Fourier Transform) are divided into N/2 sample values from X1 to X_N/2, and N/2 sample values from X_N/2+1 to X_N for inserting guard intervals(G) respectively. Among first useful interval blocks of X1 to X_N/2, G/2 second half samples are used as the first guard interval. Among second useful interval blocks of X_N/2+1 to X_N, G/2 second half samples are used as the second guard interval. To divide a point of time of first or second guard interval in recovering a position of an FFT(Fast Fourier Transform) window, G2 second half samples are multiplied by -1 to be used as the guard interval.

Description

ENCODED AND DECODING METHOD USING OFDM SYMBOL STRUCTURE}

The present invention provides a new method for inserting symbol guard intervals to exhibit robust characteristics against burst errors in orthogonal frequency division multiplexing (OFDM) modulation and to quickly estimate the starting position of the FFT window. The present invention relates to a technique for defining and performing code / decoding using OFDM symbols having such a structure. In particular, the present invention relates to shortening the estimation time of the start position of a window symbol, and to using the guard interval and the effective interval in a channel environment with congestion error. The present invention relates to a coding / decoding method using an OFDM symbol structure capable of accurately determining a window position.

OFDM is a special form of multicarrier transmission, in which one data string is transmitted on a subcarrier with a lower data rate. OFDM has a strong characteristic against frequency selective fading and narrow interference, and is widely applied in various fields. Particularly, OFDM is also used in DVB-T, which is a European digital terrestrial broadcasting standard. FIG. 1 shows the basic structure of the DVB-T system.

The signal passing through the channel code part is transmitted through the IFFT to the channel in the form of a symbol as shown in FIG. 2 shows the structure of an OFDM frame. DVB-T defines the size of FFT in 2K mode (N = 2048) and 8K mode (N = 8192). One symbol is divided into a valid interval and a guard interval region. A section copied from the last part of the valid interval is used as the guard interval.

In a DVB-T broadcasting system, the transmitter's IFFT output is realigned to the receiver's FTT after it passes through the channel. In this case, since the guard interval is inserted between the symbols and transmitted to prevent the interference between symbols, the receiver should detect the start point of the symbol in the received sample string, remove the guard interval, and input only the valid sample string into the FFT. . 3 shows a part in which FFT window position recovery is performed.

4 is a timing diagram for FFT window position recovery in the time domain. An OFDM symbol consists of a guard interval of G samples and a valid data interval of N samples, and uses the second G sample values of the N samples as a guard interval. Accordingly, it can be seen that the values of the latter part of the guard period and the valid period are the same, and the distance between corresponding samples corresponds to N samples. G and G 'are the second half of the guard period and the valid period that respectively constitute an OFDM symbol.

In the initialization process, since the receiver does not know the starting point of the OFDM symbol, a random point "0" is set to N + G input sample sequences ( Save). Also, N + G input sample strings that are input by N sample intervals later than the "0" time point ( Save). The first stored sample sequence contains the second half of the valid period of the j th symbol, and the second stored sample sequence stores the guard interval of the j th symbol.

When the start points of the two sample strings overlap each other, the second half of the guard period of the j th symbol and the valid period of the j th symbol coincide, and the start point of the OFDM symbol is estimated using this characteristic. In addition, it can be seen that 2N + G samples are required to estimate the starting point of the OFDM symbol for one window position recovery.

In general, window position recovery uses an approximate window restoration algorithm to estimate the range from 0 to 15 from the window start position, and then finds the exact position using a fine window algorithm using the output value of the FFT. This process is performed repeatedly over tens of symbols. Algorithms for restoring the window position are to use the absolute value of the difference of the sample, the method of using the correlation value of the sample, etc. In such a method, the following formula [1], [2] To locate the window.

In Equation 1, the first input sample string And the second input sample string Accumulates the difference in size between two sample values corresponding to the interval between N sample strings during the protection period (G), calculates the cumulative value from n = 0 to N + G-1, and indicates the point where the minimum accumulated value is obtained. It determines with a restoration time. [Equation 2] is a method of determining the point of time when the correlation value of the real part between the two sample values is the largest as the window position restoration point.

In order to restore the conventional FFT window position in DVB-T, a total of 2N + G sample values are required, and N + G accumulated values as long as G lengths are stored. In addition, if distortion occurs in the second half of the effective period by the guard interval or the G length when the symbol is transmitted through the channel, it is difficult to use the corresponding two samples when restoring the window position.

As described above, in the conventional OFDM modulation technique, when distortion is collected at the second half of the effective period by the guard interval or the G length when a symbol is transmitted through a channel, it is difficult to use two corresponding samples when restoring the window position.

Accordingly, an object of the present invention is to shorten the start position estimation time of a window symbol, and to use a different guard interval and a valid interval even in a channel environment in which there is a concatenation error by the length of the guard interval in the latter part of the guard interval or the effective interval. A code / decoding method using an OFDM symbol structure is provided.

1 is a block diagram of a typical DVB-T transmitter.

2 is a format diagram of an OFDM frame.

3 is a block diagram of an OFDM system.

4 is an explanatory diagram showing an example of FFT window position restoration;

5 is a block diagram of an OFDM system having two guard intervals according to the present invention.

6A-6C are diagrams illustrating the position restoration of FFT windows according to the present invention.

*** Explanation of symbols for the main parts of the drawing ***

51,55: Parallel / serial converter 52: IFFT

53,57: parallel / serial converter 54: adder

56: FFT

In the encoding method using the OFDM symbol structure according to the present invention, an IFFT output sample of length N may be used. from With N / 2 sample values, from A first step of dividing the N / 2 sample values into and inserting the guard intervals respectively; G / 2 latter samples of the first validity block up to the first guard interval To use, Second guard interval by copying the second half of G / 2 samples A second process used as; When the receiver performs the position recovery of the FFT window, in order to be able to distinguish between the first guard interval or the second guard interval, the second valid block may be identified. The second half of the dog sample is multiplied by -1, and is used as a protection period. The operation of the present invention will be described in detail with reference to FIGS. 5 and 6 as follows.

In the present invention, the effective interval N output from the IFFT is divided into two and a guard interval is inserted. That is, the output sample of the IFFT having the length N as shown in FIG. from With N / 2 sample values, from Up to N / 2 sample values are used to obtain a symbol. Of course, all of the valid interval blocks of length N / 2 have one protection interval. In this way, the interval between the guard intervals is increased by N / 2 from each other, and even if a G-length continuous error occurs in the guard interval or the corresponding validity interval, the symbol position can be estimated using the other guard interval and the effective interval. do.

In the existing VSB-T, the latter half of the N-length sample was used as a guard interval. However, the OFDM symbol according to the present invention has two guard intervals of G / 2 size. That is, the form of the protective section according to the present invention is G / 2 latter samples of the first validity block up to the first guard interval It can be seen that used. Similarly, Second guard interval by copying the second half of G / 2 samples Make Therefore, a total of two guard intervals for one symbol Each valid section block has one guard section. Of course, the size of the guard interval of the conventional OFDM symbol G and the two guard intervals proposed in the present invention ( ) Is the same size.

5, the second valid block The latter sample is multiplied by -1 to show the protection interval. This is to distinguish whether it is time of the first guard interval or the second guard interval when performing the position restoration of the FFT window. If you do not multiply by -1, even if the window relocation algorithm determines the guard interval within a symbol, Recognition This is because it cannot be determined.

With respect to the OFDM symbol proposed in the present invention, the receiver can recover with only N + G samples during FFT recovery. There are two modes that can be restored with the sample.

First, the restoration process when the total sample observation interval is (N + G) is described as follows.

FIG. 6A illustrates a process of restoring the FFT window position at the receiving end when transmitting an OFDM symbol including a guard interval by the method proposed by the present invention. Is the second half of G / 2 of the valid samples 0 to N / 2-1. Is the same protection interval as Is the second half of G / 2 samples from the effective samples N / 2 to N-1. Multiplied by -1, the guard intervals are the same size. Therefore, there are two guard zones in one symbol and the total guard zone size. to be.

In the restoration initialization process of the window, since the receiver does not know the starting point of the OFDM symbol, it sets an arbitrary point "0". Input sample columns ( Save). Also, it is input by N / 2 sample intervals later than the "0" time point. Input sample columns ( Save).

First stored sample string Contains the first half of the first validity interval block of the jth symbol, and the second stored sample sequence stores the second half of the first validity interval block of the jth symbol. Comparing the samples of the two sample strings by the amount of G / 2 accumulates, this process is performed up to N / 2 + G while moving by one sample.

Protection section And validity interval When the sample position at which is matched is known, the starting point of the OFDM symbol can be known. In the case of Figure 6A And validity interval Since is over, the starting position of the jth symbol can be estimated. Also, it can be seen that a total of N + G / 2 samples should be observed in estimating the start time of an OFDM symbol for one window position recovery.

Equation 3 below is the input sample string And the second input sample string Accumulates the size difference between two sample values corresponding to N / 2 sample intervals during the protection interval (G / 2), calculates the cumulative value from n = 0 to N / 2 + G, and then selects the time point with the minimum cumulative value. It is determined at the time of restoring the FFT window position. For reference, compared to the conventional method, the cumulative sample range is reduced from G to G / 2, and the process of finding the smallest value by comparing the cumulative value is also reduced from N to N / 2.

6B shows the location of the stored sample stream. And validity interval As a result, the guard interval of the j symbol is consequently This is the case where the position of. If using the Windows algorithm If the position of is estimated, the actual starting position of the jth symbol is It can be seen that the sample is preceded by (N / 2 + G / 2).

Note that in the case of FIG. 6A And validity interval Since Equation 3 is the same, Equation 3 shows that the absolute value of the difference between the samples is the smallest. And validity interval Since the sign of is reversed, it is necessary to estimate the maximum point where the absolute value of the difference of the sample is largest as shown in [Equation 4].

On the other hand, the recovery process when the sample observation interval is 3 (N + G) / 2 will be described as follows.

FIG. 6C illustrates a restoration method when the observation range of the sample for window position restoration is increased to 3 (N + G) / 2. During window restore initialization, random point "0" is set to N + 3G / 2 input sample strings ( Save). Also, N + 3G / 2 input sample strings inputted N / 2 sample intervals later than "0" Save).

Since the sample viewing interval is wide, the guard interval and It is a structure that can determine each. Guard interval when using an algorithm that uses the absolute value of the sample At the point of time, the cumulative value shows the largest value and vice versa. When the cumulative value is calculated at the point of time, the smallest value appears. Therefore, it can be seen that the point of time j having the smallest value is the point of time of the symbol window.

If you use this method, or When channel distortion occurs where the cumulative distinction is not apparent at or If any part of the channel has a lot of distortion By using one of the more obvious values of cumulative values, the position of the symbol can be restored.

As described in detail above, the present invention uses two guard intervals having a guard interval (G) / 2 size in an OFDM symbol structure, so that when the observation interval is 3 (N + G) / 2 samples, the guard interval is used. Even if the length error occurs in the guard interval or the corresponding validity interval, other guard intervals and validity intervals separated by N / 2 can be used, so that the window position of the symbol can be accurately determined.

In addition, the cumulative calculation amount of the sample and the cumulative value to be stored are reduced, thereby improving the window position restoration speed of the symbol. In general, the reconstruction of the FFT window is performed over several tens of degrees, and thus, the method of the present invention can save a considerable amount of time.

Claims (6)

About output sample of IFFT from With N / 2 sample values, from A first step of dividing the N / 2 sample values into and inserting the guard intervals (G), respectively; Of the first validity interval blocks up to, G / 2 latter samples are taken from the first guard interval ( ), Copy the second half of G / 2 samples from the second validity block until The second process used as; The second valid block is the second valid block so that the receiver can distinguish the time point of the first or second guard interval when restoring the position of the FFT window. And a third process of multiplying the second half samples by -1 and using the guard interval as a guard interval. If the total sample interval is (N + G), when restoring the FFT window position at the receiver, set an arbitrary point "0" Input sample columns ( ) And entered N / 2 sample intervals later than the "0" time point. Input sample columns ( A first process of storing); In estimating the start time of an OFDM symbol by comparing the samples of the two sample sequences, the guard interval ( ) And validity interval ( If the value of) is the same, the second process of finding a point of time where the absolute value of the difference of the sample is the smallest and determining it as the FFT window restoration point; The protective section ( ) And validity interval ( And a third step of determining the FFT window reconstruction time when the absolute value of the sample difference is the largest when the sign of () is reversed. The method of claim 2, wherein the second process comprises: an input sample sequence And the second input sample string Accumulates the size difference between two sample values corresponding to N / 2 sample intervals during the protection interval (G / 2), calculates the cumulative value from n = 0 to N / 2 + G, and then selects the time point with the minimum cumulative value. The following [Equation] is used to determine the FFT window position recovery time. The decoding method according to claim 2, wherein the third process uses the following Equation to find the maximum time point at which the absolute value of the sample difference is largest. The method according to claim 2, wherein when the sample observation interval is 3 (N + G) / 2, N + 3G / 2 input sample strings are set by setting an arbitrary point “0” during the window restoration initialization process. ), And enter N + 3G / 2 input sample strings (N / 2 sample intervals later than "0"). The method for decoding using an OFDM symbol structure, further comprising the step of storing a). The guard interval according to claim 2, wherein the sample observation interval is 3 (N + G) / 2 or If the channel distortion is not clear in the cumulative value at any one of the decoding method using the OFDM symbol structure, characterized in that further comprising the step of restoring the position of the symbol using a more obvious value of the cumulative value .