KR20040107831A - Carrier synchronization device for TDS-OFDM system using FFT processing of the sync segment signal and method thereof - Google Patents

Abstract

PURPOSE: A carrier frequency recovery device using a synchronous segment signal and an FFT in TDS-OFDM system and a recovering method thereof are provided to correct a carrier frequency offset by multiplying a received synchronous segment signal and a locally oscillating synchronous signal together and by performing an FFT operation therefor. CONSTITUTION: A multiplier(410) is used for multiplying a synchronous segment signal and a locally oscillating segment signal together. A decimator(415) is used for extracting an output value of the multiplier in a sample period k. An N/k-point FFT unit(420) multiplies an output value of the decimator by carriers having a predetermined interval and it integrates the multiplied values. A maximum value detector(425) is used for producing an absolute value of the N/k-point FFT unit so as to estimate a carrier frequency of a received OFDM signal.

Description

Carrier synchronization device for TDS-OPEM system using FFT processing of the sync segment signal and method

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carrier frequency recovery apparatus and method of a TDS-OFDM system, and more particularly, to a synchronization segment and an FFT embedded in an OFDM signal in an OFDM system. The present invention relates to a carrier frequency restoring apparatus and a method for improving a carrier frequency offset correction range using a fast fourier transform.

Orthogonal frequency division multiplexing (OFDM) scheme converts a data stream input in serial form into parallel data in a predetermined block unit, and then multiplexes the parallelized symbols to different orthogonal carrier frequencies to each other. It is a method of converting broadband transmission into multiple 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 the interference signal (Ghost) by the multipath, and since the carrier frequencies are orthogonal to each other, it has a strong characteristic to Intersymbol Interference (ISI).

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.

OFDM transmitter is formed by inserting a guard interval (GI) to suppress interference between signals in front of the OFDM signal formed along the time axis by an inverse discrete Fourier transform, and inserting synchronization information such as a sync segment before the guard period. Send a frame.

1 illustrates an example of a frame structure of an OFDM signal formed when an OFDM signal is transmitted using a general TDS-OFDM transmitter. Referring to the figure, an OFDM signal frame has a valid symbol interval, a guard interval (GI) and a sync segment including data for actually transmitting.

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 interval is used for carrier recovery to effectively find frequency and phase synchronization in the receiver of the OFDM wireless modem. That is, the section corresponding to the guard interval GI inserted into the OFDM signal by the transmitter is represented by "a" in the figure, which is configured to be the same as the section "a '" which is the last part of the effective symbol section of the OFDM signal. Can recover the carrier frequency by measuring the relative phase rotation difference of the last portion of this guard interval and the effective symbol interval of the received OFDM signal. The sync segment Tss includes a Pseudo Noise (PN) sequence and is transmitted every time interval "Tss + Tg + Ts", and the OFDM system performs sync acquisition and channel estimation by using the same.

Figure 2a is a block diagram showing a carrier frequency recovery device of a conventional OFDM system. Referring to the drawings, the carrier frequency restoring apparatus 200 includes a delay unit 205, a conjugate complex unit 210, a multiplier 215, a correlator 220, an absolute value calculator 225, a peak detector 230, And a phase estimator 235.

As described above, the carrier frequency restoration apparatus measures the relative phase rotation difference between the guard period and the effective symbol period by using the frame structure of the OFDM signal. That is, delay 205 delays the received OFDM signal by the time Ts, and multiplier 215 multiplies the conjugate complex by the delayed OFDM signal. The peak detector 230 locates the guard interval, and the correlator 220 separates the guard interval (“a” in FIG. 1) from the guard interval and the last interval of the effective symbol interval separated by a predetermined time Ts (FIG. The correlation value with "a '" of 1 is calculated. The phase estimator 235 calculates the phase of the calculated correlation value.

FIG. 3 is a diagram illustrating a characteristic curve showing a region of a carrier frequency offset reconstructible by the carrier frequency restoring apparatus of FIG. 2A.

The conventional carrier frequency restoration apparatus has a characteristic curve as shown in FIG. 3A since it is a method of measuring phase rotation between a guard interval and an effective symbol interval separated by a time Ts. Referring to the drawings, according to the conventional carrier frequency restoring apparatus, the area of the recoverable frequency offset is limited to " ± 1/2 Ts "

FIG. 2B is a block diagram showing a carrier frequency recovery apparatus of a conventional code division multiple access (CDMA) system. The carrier frequency restoring apparatus includes a first multiplier 310, a delay unit 320, a sliding integrator 315, a second multiplier 325, a correlator 327, and a phase estimator 330.

The carrier frequency restorer corrects the carrier frequency offset using the PN sequence included in the received sync segment signal. First, the first multiplier 310 multiplies the received sync segment signal by the local oscillation sync segment signal and outputs the result to the sliding integrator 315. The sliding integrator 315 varies the integration period according to the size of the carrier frequency offset to reduce the influence of various sleep components. In this case, the sync segment section "Tss" is divided into "Tm" sections, and integration is performed for the section corresponding to each "Tss / Tm".

The delay unit 320 delays the integral value of each section of the sync segment in which the integration is performed by the sliding integrator 315 by the length "Tm" of one section. The second multiplier 325 multiplies the integral value of each delayed interval with the integral value of the interval separated by "Tm" therefrom. The correlator 327 obtains a correlation value of the delayed integral value and the integral value of the interval separated by "Tm".

The phase estimator 330 finally estimates the carrier frequency offset by measuring the relative phase change of the integral value of the adjacent section of the sync segment by the integrator with respect to the correlation value which is the output value of the correlator 327. In this case, for the system parameter value "Tm", the output values of the sliding integrators 315 which do not overlap each other in the sync segment signal section Tss are "Tss / Tm", so "Tss / Tm-1". By calculating the average of the phase change value, the final phase change value is obtained and the carrier frequency offset is estimated accordingly.

Since the method of using the sync segment signal measures phase rotation at "Tss / Tm" time interval, the reconstructible area is "± Tm / 2Tss", which is wider than the method using the guard interval. However, since the system parameter value "Tm" should be smaller as the carrier frequency offset is larger, the performance is degraded in a low signal-to-noise ratio (SNR) environment.

SUMMARY OF THE INVENTION An object of the present invention for solving the above problems is to provide a carrier frequency restoring apparatus and method that can improve a carrier frequency offset correction range by using a sync segment and an FFT inserted into a received OFDM signal. It is.

1 illustrates a frame structure of an OFDM signal transmitted by a general quadrature frequency division multiplexing (TDS-OFDM) transmitter;

Figure 2a is a block diagram showing a carrier frequency recovery device of a conventional OFDM system,

2B is a block diagram showing a carrier frequency recovery apparatus of a conventional CDMA system;

FIG. 3 is a view showing a characteristic curve showing an area of a carrier frequency offset recoverable by the carrier frequency restoration device of FIG. 2A;

4 is a view showing a carrier frequency recovery apparatus using the FFT of the TDS-OFDM system according to the present invention,

5 is a block diagram showing the function of a carrier frequency recovery apparatus for explaining a carrier frequency recovery method according to the present invention;

6 is a flowchart showing the operation of the carrier frequency recovery apparatus according to the present invention.

Explanation of symbols on the main parts of the drawings

410: multiplier 415: decimator unit

420: N / k-point FFT unit 425: highest value detector

510: first multiplier 520: second multiplier

530: integrator 540: absolute value calculation unit

550: peak detector

According to the present invention, in the TDS-OFDM system for receiving an OFDM signal in which a sync segment signal is inserted, a multiplier for multiplying the sync segment signal by a local oscillating sync segment signal, and outputting the multiplier by a sample period k. An decimator unit for extracting at intervals, an N / k-point FFT unit for multiplying the output values of the decimator unit at predetermined intervals, integrating and calculating an absolute value, and estimating a carrier frequency of the received OFDM signal Is achieved by a carrier frequency restoring apparatus comprising a peak detector for detecting a maximum value among the calculated absolute values.

Preferably, when the number of symbols constituting the sync segment signal is "N", the decimator "k" is a divisor of "N" greater than or equal to " ¹ " and "N ^1/2 ". It calculates among smaller integers.

Preferably, the N / k-point FFT unit has an interval of "e ^{j2πt / Tss} " when the number of symbols constituting the sync segment signal is "N" and Tss is a section corresponding to the sync segment signal. The "N / k" carriers apart are multiplied by the output of the multiplier.

In addition, an object of the present invention, in the carrier frequency recovery method of the TDS-OFDM system for receiving an OFDM signal in which a synchronization segment signal is inserted, multiplying the synchronization segment signal by a local oscillation synchronization segment signal, the multiplication value is sampled A decimation step of extracting at interval k intervals, multiplying the extracted multiplied carriers at predetermined intervals, calculating an absolute value after integrating, and estimating a carrier frequency of the received OFDM signal; Achieved by a carrier frequency recovery method comprising detecting a maximum value among the absolute values.

Preferably, in the decimation step, when the number of symbols constituting the sync segment signal is "N", the "k" is a divisor of "N", which is greater than or equal to "1" and greater than "N ^1/2 ". Choose from small integers.

Preferably, in the calculating of the absolute value, when the number of symbols constituting the sync segment signal is "N" and Tss is a section corresponding to the sync segment signal, at intervals of "e ^{j2πt / Tss} ". The "n / k" carriers apart are multiplied by the output of the multiplier.

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

4 is a view showing a carrier frequency recovery apparatus using a fast fourier transform (FFT) of the TDS-OFDM system according to the present invention, Figure 5 is a carrier frequency recovery to explain the concept of the carrier frequency recovery method according to the present invention A block diagram showing the device in detail.

First, the concept of the carrier frequency recovery method according to the present invention will be described in detail with reference to the carrier frequency recovery apparatus of FIG. The carrier frequency restoring apparatus includes a first multiplier 510, a second multiplier 520, an integrator 530, an absolute value calculator 540, and a maximum value detector 550.

The first multiplier 510 multiplies the received sync segment signal by the locally oscillated sync segment signal. Since the sync segment signal is a signal known in advance in the transmission and reception period, the local oscillation sync segment signal is the same as the sync segment signal transmitted by the transmitter. However, in this case, for convenience of calculation, it is preferable to set the local oscillating sync segment signal to a conjugate complex number of the sync segment signal transmitted by the transmitter.

On the other hand, the received sync segment signal includes a carrier frequency offset generated in the process of reaching the receiver. Therefore, multiplying the local oscillating sync segment signal cancels the sync segment signal component transmitted by the transmitter, leaving only the component corresponding to the carrier frequency offset.

The second multiplier 530 preferably includes a plurality of multipliers with " N ". Each multiplier of the second multiplier 530 multiplies the output value of the first multiplier 510 by "N / k" carriers spaced at intervals of "e ^{j2πt / Tss} ". Here, "Tss" is a length of a section corresponding to the sync segment signal as shown in FIG.

The integrator 530 preferably includes a plurality of integrators that are " N / k ", each integrating an output value of each multiplier of the second multiplier 520. Also, the absolute value calculator 540 preferably includes a plurality of absolute value calculators of " N ", each of which calculates an absolute value of the output value of each integrator of the integrator 530.

The peak detector 550 finds the maximum value among the output values of the integrator 540. The frequency of the carrier having the detected maximum value is estimated as the actual carrier frequency offset. This value is obtained by multiplying and integrating the respective carriers separated by a predetermined interval with the component corresponding to the carrier frequency offset, which is the output of the first multiplier 510, for the carrier closest to the conjugate complex number of the component corresponding to the carrier frequency offset. The absolute value is the maximum.

As such, a technique of estimating carrier frequency offset is called a maximum-likelihood estimation method. The maximum likelihood estimation method has the advantage of wider correction range and shorter convergence time than other methods.

The maximum likelihood estimation method is not generally used because the hardware requirements are large and complicated in the implementation of the algorithm. However, in the present invention, an FFT (Fast Fourier Transform) that is conventionally provided in a TDS-OFDM system may be used for a carrier frequency restoration device that implements a maximum likelihood estimation method.

The carrier frequency restoration apparatus of FIG. 4 implements such a maximum likelihood estimation method and uses the FFT of the TDS-OFDM system for this purpose. The carrier frequency restoring apparatus includes a multiplier 410, a decimator 415, an N / k-point FFT unit 420, and a maximum value detector 425.

The multiplier 410 multiplies the received sync segment signal with the locally oscillated sync segment signal. The decimator 415 extracts the output of the multiplier 410 at sample interval k intervals. "N" is the number of symbols constituting the segment sync signal, "k" is calculated from the "1" or equal to "N ^1/2" smaller than a divisor of the integer "N".

A general TDS-OFDM system includes an FFT unit. Since the FFT unit is used for data demodulation, and the data demodulation is performed during the effective symbol interval of the OFDM signal, the FFT unit is not used for data demodulation during the synchronous segment signal interval that is different in time. Therefore, the present invention can be used for the carrier frequency recovery using the sync segment signal without affecting the original function of the FFT unit provided in the TDS-OFDM system.

TDS-OFDM receiver generally uses 3780-point FFT unit. In the present invention, the existing FFT unit may be used as it is, or a part of the existing FFT unit may be used. The N / k-point FFT unit 420 is a part of the FFT unit used for data demodulation. The N / k-point FFT unit 420 is used for carrier frequency recovery in the synchronous segment signal section and used for data demodulation in the effective symbol section. Since the length of the sync segment signal is about "1/10" of the effective symbol, the size of the N / k-point FFT unit 420 corresponds to "1 / (10 X k)" of the size of the entire FFT unit.

The N / k-point FFT unit 420 performs an FFT operation, which corresponds to a function performed by the second multiplier 520 and the integrator 530 of the carrier frequency restoration device of FIG. 5. That is, the N / k-point FFT unit 420 multiplies and integrates the output values of the multiplier 415 by carriers spaced apart at intervals of "e ^{j2πt / Tss} ". The maximum value detector 425 calculates the absolute value of the output value of the N / k-point FFT unit 420 and detects the maximum value among the calculated absolute values. The frequency of the carrier having the maximum value detected in this way corresponds to the actual carrier frequency offset.

In general, the length of a sync segment signal used in a TDS-OFDM system is about "1/10" of "Ts". Therefore, according to the present invention, the correction region is about 10 * N / k times larger than the conventional method using the protection interval.

6 is a flowchart showing the operation of the carrier frequency recovery apparatus according to the present invention. Referring to the figure, the carrier frequency recovery apparatus receives an OFDM signal including a sync segment signal (S610). Subsequently, the multiplier 410 multiplies the received sync segment signal with the local oscillation sync segment signal (S615), and the decimator unit 415 extracts the multiplier output at sample period k intervals.

The N / k-point FFT unit 420 multiplies the output values of the multiplier 410 by the carriers spaced at intervals of " e ^{j2πt / Tss &} quot ;, and calculates a correlation value with each carrier by integrating (S620). To calculate the power of each (S625). The highest value detector 425 detects the maximum value among the calculated absolute values (S630), and the frequency of the carrier having the detected maximum value is estimated as the actual carrier frequency offset.

According to the present invention, the received sync segment signal is multiplied by a local oscillating sync segment signal, an FFT operation is performed by using an existing FFT unit, and a maximum likelihood estimation method of selecting a maximum value of a power value resulting from performing the FFT is used. To correct the carrier frequency offset. Accordingly, the carrier frequency range that can be restored is wider and the convergence time is shorter than that of the scheme using the guard interval.

In addition, instead of using a separate signal or circuit for carrier frequency recovery, a synchronization segment signal inserted and transmitted in an OFDM signal is used and a conventional FFT unit can be used to save time and cost.

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 (6)

A carrier frequency restoration apparatus of a TDS-OFDM system for receiving an OFDM signal in which a sync segment signal is inserted, A multiplier for multiplying the sync segment signal with a local oscillating sync segment signal; A decimator unit for extracting an output value of the multiplier with a sample period k; An N / k-point FFT unit for multiplying and integrating the output values of the decimator unit at predetermined intervals, respectively; And And a maximum value detector for calculating an absolute value of an output value of the N / k-point FFT unit to estimate a carrier frequency of the received OFDM signal, and detecting a maximum value among the calculated absolute values. Carrier frequency recovery device. The method of claim 1, The decimator unit, when the number of symbols constituting the synchronization segment signal is N, the carrier frequency restoring device, characterized in that k is selected from integers greater than or equal to 1 and less than N ^1/2 as a divisor of N . The method of claim 2, The N / k-point FFT unit, when the number of symbols constituting the sync segment signal is N, and Tss is the length of the section corresponding to the sync segment signal, N / n ^dropped at intervals of e ^{j2πt / Tss} A carrier frequency restoring apparatus characterized by multiplying k carriers by an output value of the multiplier. In the carrier frequency recovery method of a TDS-OFDM system for receiving an OFDM signal with a synchronization segment signal, Multiplying the sync segment signal by a local oscillating sync segment signal; Decimating the multiplication value by a sample period k; Multiplying and integrating the output values of the multiplier with the carriers spaced apart at predetermined intervals, respectively; And Calculating an absolute value of the integrated value and detecting a maximum value among the calculated absolute values in order to estimate a carrier frequency of the received OFDM signal. The method of claim 4, wherein In the decimation step, And when the number of symbols constituting the sync segment signal is N, calculating k from an integer greater than or equal to ¹ and smaller than N ^1/2 as a divisor of N. The method of claim 5, In the step of calculating the absolute value, When the number of symbols constituting the sync segment signal is N, and Tss is the length of a section corresponding to the sync segment signal, N / k carriers spaced at intervals of e ^{j2πt / Tss} are output to the output value of the multiplier. Carrier frequency recovery method characterized in that the multiplication.