KR20050043290A - Apparatus for synchronizing symbols of ofdm signal using training signals and method thereof - Google Patents

Abstract

Symbol synchronization device and method for OFDM signal using the training signal according to the present invention is a symbol synchronization in the OFDM input signal to which the training signal sequence of the same pattern is repeated n times (n is an integer) and the cyclic prefix is added to the data An apparatus for searching for, comprising: a phase correlation unit for calculating and outputting a correlation value of the training signal sequence; A peak detector which determines whether a peak is generated by comparing the cross-correlation value with a predetermined threshold value; An effective peak detector for determining the validity of the peak detected by the peak detector; And a final peak determining unit for determining whether the peak determined by the effective peak detection unit is the last peak occurring in the training signal sequence and transmitting the input signal as an output signal when the final peak is the final peak. By finding the last peak point of the cross-correlation value irrespective of the signal detection, gain control, and modem initial operation of the initial frequency offset compensation of the system receiving the OFDM signal, it is used to find the exact symbol synchronization and transfer it to the FFT stage. The performance of the modem receiver can be improved, and by observing the peak generated inside the symbol synchronizer, it is possible to quickly determine a symbol sync non-acquisition error and a false acquisition error, thereby preventing unnecessary operation of the modem receiver.

Description

Apparatus for synchronizing symbols of OFDM signal using training signals and method etc.

The present invention relates to an apparatus and method for finding symbol synchronization in an Orthogonal FDM (Orthogonal Frequency Division Multiplexing) receiving system. More particularly, the present invention relates to a delay of a signal using cross-correlation with a training signal. And apparatus for finding the exact starting point of a symbol without increasing the amount of computation.

In general, an OFDM receiving system employs a symbol synchronizer to find the starting point of a symbol from a received signal. After the start point of the symbol is found, the number of subchannels of the OFDM signal is transmitted to the FFT block to find a transmission value corresponding to each subchannel. However, in a wireless channel environment, the transmitted signal undergoes multi-path fading. In order to compensate for the frequency and phase distortion from the distorted input signal and find the starting point of the symbol, a training signal is added to the front end of the wireless OFDM frame and transmitted. The wireless OFDM system currently used indoors, IEEE802.11a and HiperLAN / 2, transmits using a repeated pattern as a training signal. The OFDM receiving system receives a training signal sequence to detect a signal, adjust gain, and compensate for an initial frequency offset. It is recommended to perform the function of symbol synchronization acquisition.

In this case, in order to find the synchronization of a symbol, an auto-correlation method for checking a correlation between a received signal and a delayed reception signal and a correlation for calculating a correlation between the received signal and a training signal known in advance are used. There is a cross-correlation method. The autocorrelation method uses a correlation of the cyclic prefix of an OFDM frame without the aid of a training signal. Various methods have been reported. However, this method is difficult to make an accurate estimation because the result of the correlation value is not output as a peak but is kept above the threshold. However, the latter method uses the cross-correlation of the predetermined training signal and the received signal, so that a peak value is generated, which is an accurate estimation, and thus is suitable for a frame using the training signal. However, if the peak value is obtained in order to implement the hardware, it is difficult to implement. Therefore, if the peak value exceeds a certain value (threshold value), the peak value is determined.

A general symbol synchronization device using the cross-correlation method is shown in FIG. In FIG. 1, the phase correlator 110 calculates the cross-correlation value, and the comparison determiner 120 determines whether the peak value is compared with the peak determination threshold value, and the peak counter 130 counts the number of generated peaks. If the number of peaks is the same as the desired value, the switch 140 is turned on and a synchronized symbol is output.

However, since the received signal varies according to the channel condition of the signal passing through the wireless channel, the signal that has completed the signal detection, gain control, and initial frequency offset compensation may have a different correlation value from frame to frame. In addition, if the signal detection, gain control, and initial frequency offset compensation methods are different, the number of peaks may be different.

The technical problem to be achieved by the present invention is an apparatus for finding a symbol synchronization simply, irrespective of a method of performing signal detection, gain adjustment, and initial phase compensation using a training signal when an OFDM signal to which a training signal is added is received, and To provide that method.

In order to achieve the above technical problem, a symbol synchronization apparatus for an OFDM signal using a training signal according to the present invention has an OFDM input in which a training signal sequence in which a training signal of the same pattern is repeated n times (n is an integer) and a cyclic prefix are added to data. An apparatus for finding symbol synchronization in a signal, comprising: a phase correlation unit for calculating and outputting a correlation value of the training signal sequence; A peak detector which determines whether a peak is generated by comparing the cross-correlation value with a predetermined threshold value; An effective peak detector for determining the validity of the peak detected by the peak detector; And a final peak determining unit for determining whether the peak determined by the effective peak detection unit is the last peak occurring in the training signal sequence and transmitting the input signal as an output signal when the final peak is the final peak.

In order to achieve the above technical problem, in the symbol synchronization method of an OFDM signal using a training signal according to the present invention, a training signal sequence in which a training signal of the same pattern is repeated n times (n is an integer) and an OFDM input having a cyclic prefix added to the data A method for finding symbol synchronization in a signal, the method comprising: calculating and outputting a correlation value of the training signal sequence; Comparing the cross-correlation value with a predetermined threshold to determine whether a peak has occurred; Determining the validity of the detected peaks; And determining whether the peak determined to be valid is the last peak occurring in the training signal sequence, and transmitting the input signal as an output signal when the peak is the final peak.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 2 is a diagram showing the configuration of a wireless OFDM frame input to the OFDM symbol synchronization apparatus according to the present invention, Figure 3 is a block diagram showing the configuration of an OFDM symbol synchronization apparatus using a training signal according to the present invention. 4 is a block diagram illustrating a detailed configuration of the peak detector of FIG. 3. 5 is a view showing the size of the observation time of the OFDM symbol synchronization apparatus using a training signal according to the present invention.

The signal transmitted over the air has a form in which a training signal is added to the front of the actual data transmitted to the receiver. The frame structure of the IEEE 802.11a of the OFDM scheme input to the symbol synchronization apparatus according to the present invention is shown in FIG. The training signal section includes a short training signal section (t1 to t10; 210) in which a short length signal pattern is repeated 10 times, a cyclic prefix 220 of a long training signal, and a long training in which a signal pattern of a relatively long length is repeated twice. It consists of a signal section (T1 and T2) 225, the data to be transmitted is composed of a cyclic prefix 230 and the user data section 235 of the user data section. The cyclic prefixes 220 and 230 copy a part of the second half of the signal passing through the OFDM modulation. The cyclic prefixes 220 and 230 are added to prevent orthogonality between subchannels when data is received through multiple paths. The OFDM receiving system uses the short training signal 210 to perform signal detection, antenna selection, signal gain adjustment, symbol timing synchronization acquisition, and initial frequency offset correction. The long training signal 225 is used for subchannel equalization and precision frequency offset correction. Signal detection, diversity selection, and scaling are performed using the first n training signals in the short length training signal 210 section, and coarse frequency offset correction and symbol synchronization are restored using the remaining (10-n) training signals. do. The number of peaks may vary depending on the operation of the initial receiving system in signal sensing, gain adjustment, and initial frequency offset compensation.

A symbol synchronization acquisition apparatus according to the present invention will be described with reference to FIG. 3. The symbol synchronization apparatus of the OFDM signal using the training signal according to the present invention includes a phase correlation unit 310, a peak detection unit 320, an effective peak determination unit 330, a final peak determination unit 340, and a switch unit 350. It is composed.

The phase correlation unit 310 has a short training signal section therein and calculates a cross-correlation with a sequentially input signal.

The peak detector 320 determines whether the calculated value input with the comparator is larger than the peak determination threshold. In other words, the peak detector 320 detects occurrence of the peak value and notifies that the peak value is not captured when the peak value does not occur. The detailed operation will be described with reference to FIG. 4. The peak detector 320 includes a first comparator 410, a second comparator 420, and a first counter 430. The first comparing unit 410 compares the correlation calculation value with a preset peak determination threshold and notifies a later stage that a peak has occurred. The first counter 430 counts the number of correlation values from the moment the output of the phase correlation unit 310 to the first peak occurs. The second comparison unit 420 notifies that the symbol synchronization is missed when the number of correlation values is greater than the first observation time shown in FIG. 5. The first observation time 1 is a time corresponding to the end of the short training signal, as shown in FIG. If the peak does not occur even after this time has elapsed, an OFDM error may be sent to a higher level, and the OFDM system may stop receiving based on the captured error signal.

The effective peak determination unit 330 calculates the number of generated peaks. The second observation time required by the effective peak determination unit 330 may be arbitrarily set by the user. In the case of the second observation time as in the example of FIG. 5, three peak values appear. However, if the occurrence value of the peak is much larger than the expected value, it is determined as an invalid symbol synchronization. In this case, a false acquisition error signal is generated. The false catch state is propagated up the modem and quickly shuts down the modem receiver. Therefore, it is possible to prevent time and power consumption caused by continuing the operation of the receiver with wrong symbol synchronization.

The final peak decision unit 340 observes during the length of the cyclic prefix 220 after the effective peak is input to observe whether the peak occurs. If a peak occurs using the last signal t10 of the short training signal, the next cyclic prefix 220 which is connected next cannot generate a peak value. Therefore, the final peak value is determined. However, if it is not the last short training signal, it is determined that it is not the last peak value because the peak value is generated by the next short signal interval within the period. The observation time of the final peak determination unit 340 is the third observation time shown in FIG. 5, and corresponds to the cyclic prefix 230 of the long training signal 225. Therefore, since the long training signal can be used immediately after the short training signal, the long training signal can be transmitted to an FFT (Fast Fourier Transform) stage that performs input signal processing without time delay.

Finally, when it is determined that the final peak value is determined through the final peak determination unit 340, the switch 350 outputs an input signal to the next stage by connecting the switch. When the switch is turned on, it is output from the long training signal 225.

An embodiment of a symbol synchronization method of an OFDM signal using a training signal according to the present invention will now be described with reference to FIG. 6. 6 is a flowchart illustrating a flow of an OFDM symbol synchronization method using a training signal according to the present invention.

First, a training signal sequence in which the training signal of the same pattern is repeated n times (n is an integer) and an OFDM input signal added to the data are calculated to calculate a cross-correlation value. The calculated cross-correlation value is compared with a previously set peak determination threshold value to determine whether a peak occurs (step 610), and if the value is larger than the threshold value, it is notified later that a peak has occurred. The number of correlation values from the moment of occurrence of the cross-correlation value to the occurrence of the first peak is counted. Counting indicates that the symbol synchronization is missed when the number of correlation values is greater than the first observation time shown in FIG. 5. If the peak does not occur even after the first observation time has elapsed, an OFDM error is generated and sent to a higher level, and the OFDM system may stop reception based on the non-captured error signal (step 620).

The number of peaks generated in the next step is calculated during the second observation time. If the occurrence value is much larger than the expected value, an invalid symbol synchronization is determined. In this case, a false acquisition error signal can be generated and used to stop the operation of the modem receiver. If the occurrence value is less than or equal to the expected value, it is determined as a valid peak and the process proceeds to the next step (step 630).

When the peak determined as a valid peak is input in step 630, the peak is observed for the length of the cyclic prefix 220 to observe whether the peak occurs. Specifically, when the peak is input, the counter is initialized and the elapsed time is observed, and when the peak is equal to the third observation time, the peak is determined. That is, if a peak is generated by using the last signal t10 of the short training signal, the next cyclic prefix 220 which is connected next may not determine the peak value, and thus may be determined as the last peak value. However, if it is not the last short training signal, it is determined that it is not the last peak value because the peak value is generated by the next short signal interval within the period.

If it is determined as the last peak value, a signal for driving a switch for connecting the input signal to the next stage for processing the input signal is generated to enable normal signal processing (step 640).

The symbol synchronization method of the OFDM signal using the training signal according to the present invention can also be embodied as computer readable codes on a computer readable recording medium. The computer-readable recording medium includes all kinds of recording devices in which data that can be read by a computer system is stored. Examples of computer-readable recording media include ROM, RAM, CD-ROM, magnetic tape, hard disk, floppy disk, flash memory, optical data storage device, and also carrier wave (e.g. transmission over the Internet). It is also included to be implemented in the form of. The computer readable recording medium can also be distributed over computer systems connected over a computer network so that the computer readable code is stored and executed in a distributed fashion. Also, the font ROM data structure according to the present invention can be read by a computer on a recording medium such as a computer readable ROM, RAM, CD-ROM, magnetic tape, hard disk, floppy disk, flash memory, optical data storage device, and the like. It can be implemented as code.

As described above, according to the symbol synchronization device and the method of the OFDM signal using the training signal according to the present invention, regardless of the signal detection, gain control, initial frequency offset compensation modem initial operation of the system receiving the OFDM signal By finding the last peak point of the cross-correlation value, we use it to find the correct symbol synchronization and transfer it to the FFT stage. Accordingly, the performance of the OFDM modem receiver can be improved, and by observing the peak generated inside the symbol synchronizer, it is possible to quickly determine the symbol synchronization non-acquisition error and the false acquisition error, thereby preventing unnecessary operation of the modem receiver.

1 is a block diagram showing a configuration of a symbol synchronization apparatus using a conventional cross correlation.

2 is a diagram illustrating a configuration of a wireless OFDM frame input to an OFDM symbol synchronization device according to the present invention.

3 is a block diagram showing the configuration of an OFDM symbol synchronization apparatus using a training signal according to the present invention.

4 is a block diagram illustrating a detailed configuration of the peak detector of FIG. 3.

5 is a view showing the size of the observation time of the OFDM symbol synchronization apparatus using a training signal according to the present invention.

6 is a flowchart illustrating a flow of an OFDM symbol synchronization method using a training signal according to the present invention.

Claims (12)

In the apparatus for finding the symbol synchronization in the OFDM signal inputted to the training signal sequence and the cyclic prefix is repeated n times (n is an integer) of the training signal of the same pattern, A phase correlation unit for calculating and outputting a correlation value of the training signal sequence; A peak detector which determines whether a peak is generated by comparing the cross-correlation value with a predetermined threshold value; An effective peak detector for determining the validity of the peak detected by the peak detector; And And a final peak determining unit for determining whether the peak determined by the effective peak detection unit is valid is the last peak occurring in the training signal sequence and transmitting the input signal as an output signal when the final peak is the final peak. A symbol synchronization device for an OFDM input signal using a. The method of claim 1, wherein the peak detection unit A first comparison unit comparing the magnitude of the cross-correlation value and a threshold to determine a peak when the cross-correlation value is greater than the threshold value; A first counter for counting the number of cross-correlation values inputted from an input point of an initial cross-correlation value until a first peak occurs; And And a second comparator for determining that a peak has not been captured if the count value of the first counter is greater than a predetermined first observation time. 2. The method of claim 2, wherein the first observation time Symbol synchronization device of the OFDM signal using the training signal, characterized in that corresponding to the time occupied by the training signal sequence. The method of claim 1, wherein the effective peak determination unit A second counter for counting the number of peaks occurring during the second predetermined observation time; And And comparing the count value with a predetermined probable value, when the count value is large, an error signal is generated, and when the count value is smaller than or equal to, the third comparison decision unit outputting the valid peak after determining that it is a valid peak. A symbol synchronization device for an OFDM signal using a signal. The method of claim 4, wherein the second observation time The symbol synchronization apparatus of the OFDM signal using the training signal, characterized in that the time occupied by the signal sequence smaller than n of the training signal sequence. The method of claim 1, wherein the final peak determination portion A third counter that is reset when the valid peak is input and counts a delay time until a second valid peak is input; And And a fourth comparison decision unit for comparing the count value with a predetermined third observation time and determining the valid peak as a final peak and transmitting the input signal as an output signal. Symbol synchronization device for OFDM signals. The method of claim 6, wherein the third observation time The symbol synchronization device of the OFDM signal using the training signal, characterized in that the time occupied by the cyclic prefix. In a method of finding a symbol synchronization in an OFDM input signal in which a training signal sequence in which a training signal of the same pattern is repeated n times (n is an integer) and a cyclic prefix is added to data, (a) calculating and outputting a correlation value of the training signal sequence; (b) comparing the cross-correlation value with a predetermined threshold to determine whether a peak has occurred; (c) determining the validity of the peak detected in step (b); And (d) determining whether the peak determined to be valid is the last peak occurring in the training signal sequence, and transmitting the input signal as an output signal if the peak is final, and transmitting the input signal as an output signal. Symbol synchronization method of signal. The method of claim 8, wherein step (a) (a1) comparing the cross-correlation value with a magnitude of a threshold and determining a peak when the cross-correlation value is greater than the threshold value; (a2) counting the number of the cross-correlation values inputted from the time of input of the first cross-correlation value until the first peak occurs; And (a3) if the count value is greater than a first observation time corresponding to the time occupied by the training signal sequence, determining that a peak has not been captured; and a symbol synchronization method of an OFDM signal using a training signal . The method of claim 8, wherein step (b) (b1) counting the number of peaks occurring during the second observation time which is a time occupied by a signal sequence smaller than n of the training signal sequence; And (b2) comparing the count value with a predetermined possible value and generating an error signal if the count value is large, and determining the valid peak if the count value is small, and outputting the valid peak; The symbol synchronization method of the OFDM signal using the. The method of claim 8, wherein step (c) (c1) counting the delay time until the valid peak is input and a second valid peak is input; And (c2) comparing the count value with a third observation time, which is a time occupied by the cyclic prefix, and determining the valid peak as a final peak, and transmitting the input signal as an output signal. A symbol synchronization method of an OFDM signal using a training signal. In a method of finding a symbol synchronization in an OFDM input signal in which a training signal sequence in which a training signal of the same pattern is repeated n times (n is an integer) and a cyclic prefix is added to data, (a) calculating and outputting a correlation value of the training signal sequence; (b) comparing the cross-correlation value with a predetermined threshold to determine whether a peak has occurred; (c) determining the validity of the peak detected in step (b); And (d) determining whether the peak determined to be valid is the last peak occurring in the training signal sequence, and if the final peak is the final peak, transferring the input signal as an output signal; Recording media.