KR20090122826A - Method and apparatus on frame synchronization for ofdm system - Google Patents

Abstract

PURPOSE: A method and a device for synchronizing a frame for an OFDM system are provided to obtain synchronization by matching the frame synchronization in an OFDM using ternary sequence. CONSTITUTION: In a transmission terminal, a cyclic prefix is inserted to a serial data frame of a time domain(S310). The ternary sequence is inserted into the cyclic prefix(S311). The data frame into which the ternary sequence is inserted is transmitted to a reception terminal through digital to analog conversion(S314). In the reception terminal, the analog to digital conversion for a reception signal is performed(S316). The frame synchronization is obtained by correlating the digitally converted signal using the same ternary sequence as the transmission terminal(S317). The cyclic prefix is removed in the data frame(S318).

Description

Method and apparatus on frame synchronization for OFDM system

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM system, and more particularly, to a method and apparatus for frame synchronization, which is one of important points for a system to operate normally in an OFDM-based system.

Orthogonal Frequency Division Multiplexing (OFDM) modulation has a high data rate using multiple carriers, and has a strong characteristic against selective fading of frequency and impact noise in the time domain as compared with a single carrier. Since the received signal is distorted to the same magnitude within a narrow carrier spacing, only a relatively simple channel equalizer can compensate for the effects of multipath. Orthogonal frequency division multiplexing is used for wireless LAN systems due to these advantages.

1 is a block diagram illustrating a structure of a transmitter / receiver in a physical layer of a general OFDM system.

Referring to FIG. 1, an input bit stream to be transmitted is transferred from a source 100 to a serial to parallel conversion 102. The N-symbols are then collected by the serial-to-parallel converter 102 and passed through an encoder 104 to an Inverse Fast Fourier Transform (IFFT) transformer 106 to convert the transmitted symbols from frequency domain to time domain symbols. do. Then, parallel-to-serial converter 108 converts the parallel time-domain symbols into serial symbols. The collected N data symbols are referred to as 'OFDM symbols'. Symbols in the time domain obtained through the parallel-to-serial converter 108 are added with a guard interval in the Cyclic Prefix Inserter 110 in units of OFDM symbols to remove the influence of the multipath channel, and digital / analog. The converter 112 converts the signal in the digital domain into the analog domain and then transmits the signal to the receiver.

When the signal transmitted from the transmitting end is received through the channel 114, the receiving end converts the received signal from the analog domain to the digital domain signal through the analog-to-digital converter 116. The guard interval remover 118 then removes the guard interval from the OFDM symbol contaminated by the effects of multiple paths. The received signal from which the guard interval is removed is passed through the serial-to-parallel converter 120 and then converted into a signal in the frequency domain through a fast Fourier transform (FFT) converter 122. The received symbols transformed into the frequency domain are decoded by the decoder 124 and then finally output to the output bit stream via the parallel-to-serial conversion 126.

However, since the OFDM system is transmitted in block units, frame synchronization is necessary for the system to operate normally. Conventional frame synchronization method adds a specific synchronization sequence such as m-sequence or Walsh sequence to a specific position of OFDM data on the time axis, and the receiver uses the correlation between the received signal and the sequence added at the transmitter to adjust the frame synchronization position. Find.

This method recognizes the part with the largest correlation value as the synchronization position. When the correlation is actually calculated, an error may occur because the difference between the part with the largest correlation value and the other part is not clear. When this error occurs, frame synchronization is not adjusted when the receiving end undergoes the FFT process after the OFDM signal is received, which has a problem that greatly affects the demodulation performance.

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and an object of the present invention is to provide a frame synchronization method and apparatus capable of effectively frame synchronization in an OFDM system using a ternary sequence.

One aspect of the present invention for achieving the above object is (a) inserting a guard interval in the serial data frame of the time domain at the transmitting end, (b) inserting a ternary sequence into the guard interval, (c) 3 A data frame having a binary sequence inserted is transmitted to a receiving end through digital / analog conversion, (d) performing analog / digital conversion on the received signal at the receiving end, and (e) 3 same as the transmitting end on the digital converted signal. There is provided a frame synchronization method for an OFDM system comprising obtaining frame synchronization by correlating using a binary sequence, and (f) removing a guard interval from a data frame.

At this time, in step (b), it is preferable that two identical ternary sequences are inserted at the beginning of the guard period.

And in step (b), the ternary sequence is preferably (+ 1,0, -1).

In the step (e), it is preferable to obtain frame synchronization based on the portion where the correlation value is maximum by integrating the same ternary sequence as the transmitting end by one chip and multiplying the received signal.

A frame synchronization device for a transmitting end of an OFDM system according to another aspect of the present invention includes a serial-to-parallel converter for converting an input bit stream to be transmitted from serial data to parallel data, an encoder for encoding parallel data in a QPSK scheme, encoded Inverse Fourier transformer for converting data from frequency domain to time domain, Parallel-to-serial converter for converting parallel data in time domain to serial data, Guard interleaver inserting guard interval in frame of serial data, ternary in guard interval A ternary sequence inserter for inserting a sequence and a DA converter for converting digital data into which an ternary sequence is inserted are converted into analog signals.

In this case, the ternary sequence inserter inserts two identical ternary sequences at the beginning of the guard period.

The ternary sequence inserter inserts (+ 1,0, -1) as a ternary sequence.

A frame synchronization device for a receiving end of an OFDM system according to still another aspect of the present invention is an AD converter for converting a received analog signal into digital data, and a frame by correlating the digitally converted signal by using the same ternary sequence as the transmitting end. Frame synchronizer to synchronize, guard interval remover to remove guard interval from data frame, serial-to-parallel converter to convert serial data without guard interval to parallel data, parallel data signal in time domain to frequency domain signal A Fourier transformer, a decoder for decoding a signal in the frequency domain, and a parallel-to-serial converter for converting the decoded parallel data into serial data.

In this case, the frame synchronizer moves the same ternary sequence by the chip one by one, multiplies by the received signal, and integrates the frame synchronizer to obtain frame synchronization based on the portion where the correlation value is maximum.

As described above, according to the present invention, since the correlation characteristic of the ternary sequence is better than that of other sequences, synchronization can be more effectively obtained than the conventional OFDM frame synchronization method. In addition, since frame synchronization is achieved without additional synchronization symbols, the data rate is efficient.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the following embodiments are provided to those skilled in the art to fully understand the present invention, and may be modified in various forms, and the scope of the present invention is limited to the embodiments described below. It doesn't happen.

2 is a block diagram illustrating a frame synchronization device for an OFDM system according to a preferred embodiment of the present invention, and FIG. 3 is a flowchart illustrating a frame synchronization method for an OFDM system according to a preferred embodiment of the present invention. Indicates. Frame synchronization refers to a process in which a receiver finds a starting point of a new frame in the process of transmitting and receiving an OFDM signal block in an OFDM system.

2 and 3, an input bit stream to be transmitted is transferred from a source 200 to a serial to parallel converter 202 and converted into parallel data (S302). Then, the signal mapped from the encoder 204 through quadrature phase shift keying (QPSK) is modulated into Nc subcarriers by using an inverse fast Fourier transform (S306) in an inverse fast fourier transform (IFFT) converter 206. .

The modulated data undergoes parallel-to-serial conversion in the parallel-to-serial converter 208 (S308), and a guard interval is inserted into the frame in the guard interval inserter 210 (S310). Subsequently, in the ternary sequence inserter 211, a ternary sequence is added to the beginning of the guard interval of the OFDM data on the time axis (S311). As can be seen in Figure 4, two identical ternary sequences are preferably added to the start of the guard interval of the OFDM frame. A ternary sequence is a sequence with three signal levels. For example, a binary sequence has two levels of +1 and -1, and a ternary sequence has three levels of +1, 0 and -1.

When inserting a ternary sequence into the guard interval, it is desirable to insert the ternary sequence with a sufficiently low energy so as not to affect the normal operation of the OFDM system. Therefore, in order to lower the energy of the signal, the amplitude of the signal must be reduced. In other words, a sequence with a level of + 1,0, -1 is less energy than a sequence with a level of + 5,0, -5. In addition, the length of the sequence is also preferably shorter in order to reduce the energy.

Data in which the ternary sequence is inserted in the guard interval is transmitted from the DA converter 212 to the transmitter via the channel 214 via the digital-to-analog conversion S312 (S314). When the transmitted signal is received, frame synchronization is performed at the receiving end as follows.

That is, after the analog-to-digital conversion (S316) is performed in the AD converter 216 for the received data, the frame synchronizer 217 receives the same ternary sequence as the transmitter and correlates with the received OFDM signal. Frame synchronization is obtained (S317). Therefore, the receiving end does not know the starting position of the frame, but it is necessary to know in advance information about the ternary sequence sent by the transmitting end in addition to the OFDM data. In order to estimate the start position of the OFDM frame, the frame synchronizer moves the same ternary sequence by the chip by one chip, multiplies the received signal, and integrates the received signal. In this case, the portion where the correlation value is maximum is the position where the guard band starts in the OFDM frame.

이며, 불연속 신호의 경우

이다. 여기서, x(t)는 수신된 신호이고, y(t)는 송신단에서 OFDM 데이터에 더해진 3진 시퀀스이자 수신단이 가지고 있는 동일한 3진 시퀀스이다. 따라서 프레임 동기화부는 3진 시퀀스를 입력받아 이를 계속 이동해가면서 수신된 신호와 3진 시퀀스와의 상관값을 계산한다. 그러면 수신 신호의 3진 시퀀스 부분과 프레임 동기화부로 입력된 3진 시퀀스가 정확히 일치하는 순간 최대 상관값이 나타나게 되고, 이를 이용하여 보호구간의 시작위치 및 OFDM 프레임의 시작 위치를 찾을 수 있다.Specifically, the correlation is for continuous signals

For discrete signals

to be. Here, x (t) is a received signal, and y (t) is a ternary sequence added to OFDM data at the transmitting end and the same ternary sequence possessed by the receiving end. Therefore, the frame synchronization unit receives the ternary sequence and continuously moves it to calculate the correlation value between the received signal and the ternary sequence. Then, the maximum correlation value appears at the instant when the ternary sequence portion of the received signal and the ternary sequence inputted to the frame synchronizer are exactly matched, and the start position of the guard interval and the start position of the OFDM frame can be found using this.

After the frame synchronization is secured, the guard interval is removed by the guard interval remover 218 (S318), and is converted into a signal in the frequency domain through the fast Fourier transformer 222 via the serial-parallel converter 220. Received symbols transformed into the frequency domain are decoded by the decoder 224 and then finally output 228 to the output bit stream via the parallel-to-serial converter 226.

According to the present invention, it shows better performance than the conventional OFDM frame synchronization method. That is, since the correlation characteristic of the ternary sequence is better than that of other sequences, synchronization can be effectively obtained. The periodicity of two identical ternary sequences makes it possible to obtain timing synchronization robustly to the frequency offset. It is also efficient in data rate because frame synchronization is achieved without additional synchronization symbols.

This effect is explained using Figs. 5A and 5B.

FIG. 5A shows a case where a 64-sample m-sequence is used twice for frame synchronization, and FIG. 5B shows a correlation result graph when a ternary sequence of 73 samples is used twice. 5A and 5B show results of the AWGN channel.

As can be seen in Figures 5a and 5b, it can be seen that the correlation characteristics are significantly better when using a ternary sequence than when using an m-sequence. In other words, when the ternary sequence is used, the position of the frame where the maximum correlation appears can be reliably found, so that the synchronization performance is good. In this experiment, we experimented assuming the AWGN channel environment, but the actual situation may have different noises and interference signals from different users, so it may be difficult to find the maximum correlation when using m-sequence. This is because the sidelobe generated next to the maximum correlation is larger when using the m-sequence.

According to a preferred embodiment of the present invention, it is preferable to use two identical ternary sequences. This will be described with reference to FIGS. 6 and 7.

6 (a) to 6 (d) show correlation results for acquiring synchronization using a ternary sequence having a length of 73 samples in an AWGN channel. 6 (a) is a case where the ternary sequence is used once, FIG. 6 (b) is used twice, and FIG. 6 (c) is used three times, FIG. 6 (d) Is the case of repeated use four times. As can be seen from (a) to (d) of Figure 6, it can be seen that the maximum correlation value can be found most clearly when the ternary sequence is used twice. Since the part where the maximum correlation occurs is the part where the sequence matches exactly, the frame position can be found accurately.

7A to 7D show correlation results for acquiring synchronization using a ternary sequence of 73 samples in a general communication channel. As shown in (a) to (d) of FIG. 7, it can be seen that the synchronous performance is best when the ternary sequence is repeatedly used twice as shown in (b) of FIG. 7.

1 is a block diagram illustrating a structure of a transmitter / receiver in a physical layer of a general OFDM system.

2 is a block diagram illustrating a frame synchronization device for an OFDM system according to a preferred embodiment of the present invention.

3 is a flowchart illustrating a frame synchronization method for an OFDM system according to an embodiment of the present invention.

4 is an exemplary diagram for explaining that two identical ternary sequences are added to the start of the guard interval of an OFDM frame.

FIG. 5A is a correlation result graph when a 64-sample m-sequence is used twice for frame synchronization, and FIG. 5B is a correlation result graph when a ternary sequence of 73 samples is used twice.

6 is a correlation result graph for obtaining synchronization using one to four ternary sequences in an AWGN channel.

7 is a correlation result graph for acquiring synchronization using one to four ternary sequences in a general communication channel.

Claims (11)

In the frame synchronization method for an OFDM system, (a) inserting a guard interval into a serial data frame in a time domain at a transmitting end; (b) inserting a ternary sequence into the guard interval; (c) transmitting the data frame into which the ternary sequence is inserted, to the receiving end through digital / analog conversion; (d) performing analog / digital conversion on the received signal at the receiving end; (e) correlating the digitally converted signal using the same ternary sequence as the transmitting end to obtain frame synchronization; (f) removing the guard interval from the data frame. The method of claim 1, wherein in step (b), a ternary sequence is inserted at the beginning of the guard period. 2. The frame synchronization method according to claim 1, wherein in step (b), two identical ternary sequences are inserted in the guard interval. 2. The method of claim 1, wherein in step (b), the ternary sequence is (+ 1,0, -1). The method of claim 1, wherein in the step (e), frame synchronization is obtained based on a portion having a maximum correlation value by integrating the same ternary sequence as the transmitting end by one chip and multiplying the received signal. Frame synchronization method. In the frame synchronization apparatus for the transmitting end of the OFDM system, A serial-to-parallel converter for converting an input bit stream to be transmitted from serial data to parallel data; An encoder for encoding parallel data in a QPSK scheme; An inverse Fourier transformer for converting the encoded data from the frequency domain to the time domain; A parallel-to-serial converter for converting parallel data in the time domain into serial data; A guard interval inserter for inserting a guard interval into a frame of serial data; A ternary sequence inserter for inserting a ternary sequence into the guard interval; And a DA converter for converting digital data inserted with a ternary sequence into an analog signal. 7. The frame synchronizer of claim 6, wherein the ternary sequence inserter inserts a ternary sequence at the beginning of the guard period. 7. The frame synchronization device as claimed in claim 6, wherein the ternary sequence inserter inserts two identical ternary sequences in the guard interval. 7. The frame synchronization device according to claim 6, wherein the ternary sequence inserter inserts (+ 1,0, -1) as a ternary sequence. In the frame synchronization apparatus for the receiving end of the OFDM system, An AD converter for converting the received analog signal into digital data; A frame synchronizer for obtaining frame synchronization by correlating the digitally converted signal using the same ternary sequence as the transmitting end; A guard interval remover for removing the guard interval from the data frame; A serial-to-parallel converter for converting serial data from which the guard interval is removed to parallel data; A Fourier transformer for converting the parallel data signal in the time domain into the signal in the frequency domain; A decoder for decoding the signal in the frequency domain; And a parallel-to-serial converter for converting the decoded parallel data into serial data. The frame synchronizer of claim 10, wherein the frame synchronizer acquires frame synchronization based on a portion having a maximum correlation value by multiplying the same ternary sequence as the transmitting end by one chip and multiplying the received signal. Device.

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