KR20050003663A - Apparatus of symbol timing recovery for OFDM receivers and method thereof - Google Patents

Abstract

PURPOSE: An apparatus and a method for recovering symbol timing of an OFDM receiver are provided to perform a recovery of the symbol timing irrespectively of a recovery of a carrier wave frequency by processing signals in a time domain. CONSTITUTION: An apparatus for recovering symbol timing of an OFDM(Orthogonal Frequency Division Multiplexing) receiver includes a first delay member(710), a complex conjugate member(720), a multiplier(730), a correlator(740), an absolute value calculator(750), a maximum region detector(760), a reference counter(780), and an intermediate value filter(770). The first delay member outputs a delay signal by delaying a received OFDM signal by a delay time corresponding to an effective symbol interval. The complex conjugate member calculates a conjugate of the delay signal. The multiplier multiplies the received signal with the conjugate. The correlator adds the output of the multiplier for a duration which is two times a protection period of the received signal to calculate a correlation value of the received signal and the delay signal. The maximum region detector detects a maximum region where an output of the absolute value calculator is the greatest. The intermediate value filter detects an intermediate value which is in the middle of the maximum region to recover the symbol timing.

Description

Apparatus of symbol timing recovery for OPDM receivers and method

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for recovering symbol timing of an Orthogonal Frequency Division Multiplexing (OFDM) receiver, and more particularly, to an OFDM receiver for recovering symbol timing by processing a received OFDM signal in a time domain. The present invention relates to a symbol timing recovery apparatus and a method thereof.

Orthogonal frequency division multiplexing (hereinafter referred to as "OFDM") is a method of converting a data stream input in serial form into parallel data in a predetermined block unit and then multiplexing the parallelized symbols to different orthogonal carrier frequencies. (Multiplexing) to convert a wideband transmission into a plurality of narrowband parallel transmissions. Therefore, OFDM has an advantage of maximizing frequency use efficiency.

In addition, since the symbol transmission time is increased by using a multi-carrier, it is strong to an interference signal (Ghost) by a multipath and has a strong characteristic against intersymbol interference (ISI) because it uses carrier frequencies orthogonal to each other.

Therefore, the configuration of a single frequency network (SFN), that is, it is possible to broadcast the whole country in one frequency, and can effectively use the limited frequency resources, which is suitable for terrestrial digital TV broadcasting.

In general, an OFDM transmitter that transmits an OFDM signal using time domain synchronization includes an inverse discrete Fourier transform that rearranges an OFDM signal formed on a frequency axis that provides one service allocated to a predetermined frequency band along a time axis. Do this.

An OFDM transmitter inserts a guard interval (GI) for suppressing interference between signals in front of an OFDM signal formed along a time axis by an inverse discrete Fourier transform.

1 is a diagram illustrating a frame structure of an OFDM signal transmitted by a general OFDM system.

Looking at the frame structure of an OFDM signal, an OFDM signal frame has a valid symbol interval (Ts) and a guard interval (GI) (Tg) including data for actual transmission.

The guard interval (GI) is used to reduce the inter-symbol interference that causes the signals transmitted in the radio channel to be delayed in a multipath channel environment so that the received symbols are superimposed on the next symbol transmitted continuously. In addition, the guard period is also used for symbol timing recovery to prevent timing drift due to symbol clock errors in the transmission and reception period in the OFDM receiver.

That is, the section corresponding to the guard interval (GI) inserted into the OFDM signal by the OFDM transmitter is represented by "a" in the figure, since it is configured to be the same as the "a '" section, which is the last part of the effective symbol section of the OFDM signal, The receiver can determine the position of the guard interval by restoring the symbol timing by taking a correlation between this guard interval of the received OFDM signal and the last part of the effective symbol interval and measuring the maximum value.

2 is a block diagram showing a part of an OFDM receiver including a conventional symbol timing recovery apparatus using a guard interval.

The OFDM receiver includes an analog-to-digital converter (ADC) 210, a serial-parallel converter 220, an initial acquisition circuit 230, a fine tracking circuit 240, an FFT unit 250, and a parallel-serial converter 260. Include. The symbol timing recovery apparatus includes an initial acquisition circuit 230 and a fine tracking circuit 240.

The ADC 210 converts the received OFDM signal into a digital signal through a process of sampling, quantization, and coding. The serial-parallel converter 220 and the FFT unit 250 convert the OFDM signals received in the serial form in the time domain into the parallel form in the frequency domain.

The initial acquisition circuit 230 uses the structural characteristics of the OFDM signal, that is, the guard interval ("a" in FIG. 1) and the last interval ("a '" in FIG. 1) of the effective symbol interval are the same. Detect. FIG. 3 is a detailed block diagram illustrating the initial acquisition circuit of FIG. 2.

The delayer 310 delays the received signal for a delay time Ts and outputs the delayed signal to the conjugate complex unit 320. The conjugate complex unit 320 calculates the conjugate complex number of the delayed signal and outputs it to the multiplier 330, and the correlator adds for the time Tg corresponding to the guard period, and calculates a correlation value between the received signal and the delayed signal. . The absolute value calculator 350 calculates an absolute value for the correlation value output from the correlator 340, and the peak detector 360 finds the maximum value of the calculated absolute value. The position of the guard interval is determined by detecting the timing at which the maximum value of the absolute value appears by the reference counter 380 performing the count during the OFDM signal interval Tg + Ts.

The fine tracking circuit 240 detects the symbol timing offset by using a relative phase rotation difference between the received data and the reference data in the frequency domain. 4 is a detailed block diagram illustrating the microtrace circuit of FIG. 2.

The multiplier 410 calculates a correlation value by multiplying input data by reference data. The reference data is the data known in advance in the OFDM transmission / reception period, and are actually multiplied by the conjugate complex number. As shown in FIG. 2, signal processing in the frequency domain is performed through the initial acquisition circuit 230, the FFT unit 250, and the parallel-serial converter 260 according to the detected position of the pilot subcarrier. This value corresponds to each pilot subcarrier extracted.

The phase estimator 420 calculates a relative phase rotation difference of the pilot subcarriers according to the calculated correlation value. The delay unit 430 delays the pilot subcarriers by the interval of different pilot subcarriers, and the adder 440 calculates the relative phase difference between the pilot subcarriers. The average value calculator 450 calculates an average value of the relative phase differences between the calculated pilot subcarriers to obtain a symbol timing offset. That is, the symbol timing offset is restored by taking advantage of the fact that the symbol timing offset is represented by the relative phase difference of the pilot subcarriers in the frequency domain.

The FFT unit 250 performs fast Fourier transform on the OFDM signal converted from the ADC 210 to the digital signal. The parallel-serial converter 260 converts the fast Fourier transformed parallel signal into a serial signal. The signal output from the parallel-serial converter 260 is input to an equalizer (not shown).

FIG. 5 is a diagram illustrating correlation characteristics when the symbol timing recovery apparatus of FIG. 2 is used. In the figure, a lock-in region is indicated in the last section of the protection section. The lock-in region is a portion of the protection section that is not contaminated by multipath signal components.

The symbol timing recovery apparatus of the OFDM receiver should allow the reception FFT window to start within the lock-in region. However, the symbol timing recovery apparatus of the conventional OFDM receiver has a small change in symbol timing offset value detected such as a channel having a low environmental change in time, for example, an additive white Gaussian noise (AWGN) channel. As a hypothetical development, it shows correlation characteristics as shown in FIG. 5 under a multipath channel. That is, since the correlation value has the highest value out of the lock-in area, the position of the reception FFT window is biased, which causes a problem of performance deterioration.

In addition, the microtracking circuit of the conventional symbol timing recovery apparatus as shown in FIGS. 2 and 4 uses signal processing for a training sequence in the frequency domain. Therefore, the OFDM transmitter must transmit the training sequence, and the OFDM receiver must accurately detect the position of the subcarrier, that is, the pilot subcarrier, to which the successive sequence is transmitted. Accordingly, there is a problem that the frequency utilization efficiency is lowered and an accurate carrier recovery process is required in advance.

An object of the present invention for solving the above problems is, the symbol timing recovery apparatus of the OFDM receiver that does not require a carrier frequency recovery process by restoring the symbol timing through the signal processing in the time domain for the received OFDM signal and To provide that method.

1 is a view showing a frame structure of a signal transmitted by a general OFDM system,

2 is a block diagram showing a part of an OFDM receiver including a conventional symbol timing restoring apparatus using a guard interval;

3 is a block diagram showing in detail the initial acquisition circuit of FIG.

4 is a block diagram illustrating the microtrace circuit of FIG. 2 in detail;

5 is a view showing a correlation characteristic when using the symbol timing restoration apparatus of FIG.

6 is a block diagram showing a part of an OFDM receiver including a symbol timing restoration apparatus using signal processing in a time domain according to the present invention;

7 is a block diagram showing in detail the initial acquisition circuit of FIG.

8 is a block diagram illustrating in detail the microtrace circuit of FIG. 6;

9 is a view showing a correlation characteristic when using the symbol timing restoration apparatus of FIG.

10A is a flowchart illustrating a symbol timing restoration method using an initial acquisition circuit of a symbol timing restoration apparatus according to the present invention; and

10B is a flowchart illustrating a symbol timing restoration method using a microtrace circuit of a symbol timing restoration apparatus according to the present invention.

Explanation of symbols on the main parts of the drawings

610: ADC 620: serial-to-parallel converter

630: initial acquisition circuit 640: fine tracking circuit

650: FFT unit 660: parallel series converter

710: delayer 720: conjugate complex number

730: multiplier 740: correlator

750: absolute value calculation unit 760: peak area detector

770: median filter 780: reference counter

830: delay 840: adder

850: average value calculator

The above object is a first delay for outputting a delay signal by delaying a received OFDM signal for a delay time Ts corresponding to an effective symbol interval, and a conjugate complex number for calculating a conjugate complex number of the delay signal. A multiplier multiplying the conjugate signal of the received signal and the delayed signal, and adds the output value of the multiplier for a time (2xTg) corresponding to twice the time period corresponding to the guard interval of the received signal, thereby performing the addition. A correlator that calculates a correlation value between the signal and the delay signal, an absolute value calculator that calculates an absolute value of the correlation value, a maximum area detector that detects the highest area where the absolute value is maximum, and an entire symbol interval of the received signal The reference counter to detect the position of the reference period performing the counting and the guard interval of the received signal for a time corresponding to (Tg + Ts) to restore the symbol timing, Depending on the coefficient can be achieved by the symbol timing recovery apparatus comprising a median filter for detecting an intermediate value in the middle of the top area.

Preferably, the symbol timing restoring apparatus comprises: a second delayer for delaying the intermediate value for a predetermined delay time and calculating a delay value; an adder and a symbol timing offset for subtracting the delay value from the intermediate value to calculate a subtraction value; The apparatus further includes an average value calculator for calculating an average value of the subtracted values to obtain.

The above object is also according to the present invention, outputting a delay signal by delaying the received OFDM signal for a delay time (Ts) corresponding to an effective symbol interval, calculating a conjugate complex number of the delay signal to receive the received signal. Calculating a correlation value between the received signal and the delayed signal by performing addition for a value multiplied by 2 for a time (2xTg) corresponding to twice the time corresponding to the guard interval of the received signal. Calculating an absolute value of the maximum area, detecting the highest area at which the absolute value is maximum, and a reference counter performed during a time Tg + Ts corresponding to the entire symbol interval of the received signal. By detecting the intermediate value located in the middle, the symbol timing recovery method comprising the step of detecting the position of the guard interval of the received signal and recovering the symbol timing.

Preferably, the method for restoring a symbol timing includes receiving the intermediate value and delaying it for a predetermined delay time, calculating a delay value, subtracting the delay value from the intermediate value, calculating a subtracted value, and performing a symbol timing offset. Calculating a mean value of the subtracted values to obtain.

Hereinafter, the present invention will be described in detail with reference to the drawings.

6 is a block diagram showing a part of an OFDM receiver including a symbol timing recovery apparatus according to the present invention. Referring to the drawings, an OFDM receiver includes an analog to digital converter (ADC) 610, a serial and parallel converter 620, an initial acquisition circuit 630, a fine tracking circuit 640, and a fast fourier transform (FFT) unit 650, And a parallel series converter 660. Meanwhile, the symbol timing restoration apparatus according to the present invention includes an initial acquisition circuit 630 and a fine tracking circuit 640.

The ADC 610 converts the received OFDM signal into a digital signal. The serial-parallel converter 620 converts an OFDM signal received in a serial form in the time domain into a parallel form in the frequency domain. The FFT unit 650 performs fast Fourier transform on the OFDM signal based on the position of the FFT window detected by the initial acquisition circuit 630. The parallel-serial converter 660 converts the fast Fourier transformed parallel signal into a serial signal. The signal output from the parallel-serial converter 660 is input to an equalizer (not shown).

Each component of the OFDM receiver including the symbol timing restoring device according to the present invention is the same as in the prior art except for the initial acquisition circuit 630 and the fine tracking circuit 640. Since the output has a configuration in which the fine tracking circuit 640 is directly input, the configuration is different from that of the prior art.

FIG. 7 is a detailed block diagram of the initial acquisition circuit of FIG. 6, and FIG. 8 is a detailed block diagram of the microtrace circuit of FIG. 6. 7 and 8, a symbol timing restoration apparatus according to the present invention will be described in detail.

The initial acquisition circuit 630 includes a delay unit 710, a conjugate complex unit 720, a multiplier 730, a correlator 740, an absolute value calculating unit 750, a maximum area detector 760, and an intermediate value filter 770. ) And a reference counter 780. The initial acquisition circuit 630 uses the fact that the guard section of the OFDM signal and the last part of the effective symbol section are the same. That is, the correlation between received samples spaced apart by the effective symbol interval Ts is taken and observed during the OFDM signal interval Tg + Ts, and then the guard interval is found from the position where the maximum value is obtained.

The delay unit 710 delays the received signal for a delay time Ts and outputs the delay signal to the conjugate complex unit 720. The conjugate complex unit 720 calculates the conjugate complex number of the delayed signal and outputs it to the multiplier 730, and the correlator 740 adds the signal for two times the protection interval (2xTg), thereby receiving the received signal and the delayed signal. Calculate the correlation value of.

In the initial acquisition circuit 630 of the symbol timing restoration apparatus according to the present invention, the correlation interval of the correlator 740 is increased to "2Tg", unlike the prior art, and uses the highest region detection method rather than the peak detection method.

FIG. 9 is a diagram illustrating correlation characteristics when the symbol timing recovery apparatus of FIG. 6 is used. Referring to the figure, unlike the case of FIG. 5 in which the peak occurs outside the lock-in area, the maximum correlation value appears over the entire lock-in area. Therefore, if the intermediate value located in the middle of the region where the maximum correlation value appears is detected, the position of the reception FFT window is not biased even under a multipath environment, and thus performance degradation may be avoided.

The absolute value calculator 750 calculates an absolute value for the correlation value output from the correlator 740, and the highest area detector 760 selects the highest area that is the area where the absolute value of the correlation value is maximum as described above. Detect.

The median value filter 770 detects the median value in the middle of the highest region according to the coefficient of the reference counter 780 which performs the coefficient for the time Tg + Ts corresponding to the entire symbol interval of the OFDM signal. Identify the location of the guard zone.

The microtracking circuit 640 includes a delayer 830, an adder 840, and an average value calculator 850. The fine tracking circuit 640 detects the symbol timing offset by detecting a timing difference between the intermediate values using the intermediate value that is the output value of the initial acquisition circuit 630. This takes advantage of the fact that the symbol timing offset is represented by the change in the median value. That is, when timing drift occurs due to a symbol clock error between the transceivers, since the symbol timing estimation value changes with time, the symbol clock error can be compensated by measuring the amount of change.

The delay unit 830 calculates a delay value by delaying the intermediate value output from the initial acquisition circuit 630 for a predetermined delay time. The delay time may be, for example, a time corresponding to a difference between adjacent intermediate values input to the delay unit 830. The adder 840 subtracts the delay value from the input intermediate value to calculate a subtracted value. The average value calculator 850 calculates an average value of a subtracted value that is an output value of the adder 840 to obtain a symbol timing offset.

10A is a flowchart illustrating a symbol timing recovery method using an initial acquisition circuit of a symbol timing recovery apparatus according to the present invention, and FIG. 10B is a flowchart illustrating a symbol timing recovery method using a fine tracking circuit of a symbol timing recovery apparatus according to the present invention. to be.

First, a signal is received (S1010) and the received signal is delayed for a delay time Ts to output a delay signal (S1020). The conjugate complex number of the delayed signal is calculated and added to the value multiplied by the received signal for 2 timesTg corresponding to twice the protection interval, thereby calculating a correlation value between the received signal and the delayed signal (S1030).

The absolute value of the correlation value is calculated, and the highest region that is the region where the absolute value of the correlation value is maximum is detected (S1040). The intermediate value located in the middle of the highest region is detected according to a reference counter performed for a time period Tg + Ts corresponding to the symbol interval of the OFDM signal (S1050) to determine the position of the guard interval and restore symbol timing.

Then, for fine tracking of symbol timing, the symbol timing offset is detected by detecting a timing difference between the intermediate values using the intermediate value calculated in FIG. 10A.

First, an intermediate value is input (S1060), and the delayed value is output by delaying the input intermediate value for a predetermined delay time (S1070). The subtracted value is obtained by subtracting the delay value from the input intermediate value (S1080). The average value of the subtracted values is calculated to obtain a symbol timing offset (S1090).

Accordingly, by restoring the symbol timing with respect to the OFDM signal through the symbol timing offset value, the OFDM signal can be restored more stably and accurately. In addition, not only a single channel environment, but also a multipath channel environment can more efficiently restore an OFDM signal.

According to the present invention, symbol timing recovery may be performed through signal processing in the time domain, so that symbol timing may be restored regardless of carrier frequency recovery. Therefore, the design process of the synchronization algorithm of the OFDM receiver is very simple, and the symbol timing recovery performance does not depend on the carrier frequency recovery result.

In addition, the present invention exhibits excellent symbol timing recovery performance not only in a single channel, such as an additive white Gaussian noise channel, but also in a multipath channel. Therefore, the present invention can be applied to various systems such as DTV (Digital Television), DAB (Digital Audio Broadcasting), WLAN, and DMT (Discrete Multi-Tone) for the US.

In the above described and illustrated with respect to the preferred embodiment of the present invention, the present invention is not limited to the specific preferred embodiment described above, without departing from the gist of the invention claimed in the claims in the art Various modifications can be made by those skilled in the art, and such changes are within the scope of the claims.

Claims (4)

A first delayer for delaying the received OFDM signal for a delay time (Ts) corresponding to an effective symbol interval to output a delay signal; A conjugate complex unit calculating a conjugate complex number of the delay signal; A multiplier that multiplies the conjugate complex number of the received signal with the delay signal; A correlator configured to add an output value of the multiplier for a time (2xTg) corresponding to twice the time corresponding to a guard interval of the received signal to calculate a correlation value between the received signal and the delayed signal; An absolute value calculator for calculating an absolute value of the correlation value; A maximum area detector for detecting a maximum area where the absolute value is maximum; A reference counter for performing a count for a time Tg + Ts corresponding to the entire symbol period of the received signal; And And a median filter for detecting a median value located in the middle of the highest region according to the coefficient of the reference counter to detect the position of the guard interval of the received signal and restore the symbol timing. Restoration device. The method of claim 1, A second delayer for delaying the intermediate value for a predetermined delay time to calculate a delay value; An adder which subtracts the delay value from the intermediate value to calculate a subtracted value; And And a mean value calculator for calculating an average value of the subtracted values to obtain a symbol timing offset. Outputting a delayed signal by delaying the received OFDM signal for a delay time (Ts) corresponding to an effective symbol interval; By calculating the conjugate complex number of the delayed signal and multiplying the received signal, an addition is performed for a time (2xTg) corresponding to twice the time corresponding to the guard interval of the received signal, thereby adding the received signal and the delay. Calculating a correlation value of the signal; Calculating an absolute value of the correlation value and detecting a maximum region where the absolute value is maximum; According to a reference counter performed during a time Tg + Ts corresponding to the entire symbol interval of the received signal, the intermediate value located in the middle of the highest region is detected to detect the position of the guard interval of the received signal and symbol timing. Restoring; symbol timing restoration method comprising a. The method of claim 3, wherein Calculating a delay value by receiving the intermediate value and delaying the intermediate value for a predetermined delay time; Calculating a subtracted value by subtracting the delay value from the intermediate value; And And calculating a mean value of the subtracted values to obtain a symbol timing offset.