KR100321937B1 - Frequency offset correcting system in orthogonal frequency division multiplex - Google Patents

Abstract

PURPOSE: A frequency offset correcting system in orthogonal frequency division multiplex is provided to reduce frequency offset estimation time and complexity of calculation to correct the frequency offset, thereby improving the performance of an orthogonal frequency division multiplex transmission system. CONSTITUTION: A frequency offset correcting system in orthogonal frequency division multiplex includes an analog/digital converter(100), an FFT unit(300), a frequency offset estimator(400), a frequency offset corrector(200), and an equalizer(500). The analog/digital converter converts received analog data that is transformed into time domain through IFFT into digital data. The FFT unit transforms the digital data into frequency domain. The frequency offset estimator estimates frequency offset of the received data, caused by a difference between characteristics of tuners of transmitting and receiving terminals, and outputs the maximum value among estimated values to the frequency offset corrector. The frequency offset corrector corrects the frequency offset according to the value output from the frequency offset estimator. The equalizer compensates for the quantity of attenuation of the received data.

Description

Frequency Offset Correction Device in Orthogonal Frequency Division Multiplexing

The present invention relates to a frequency offset correcting apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) scheme. More particularly, the present invention relates to a difference in the characteristics of a tuner of a transmitter and a receiver when restoring a frequency modulated by a transmitter. The present invention relates to a frequency offset correcting apparatus in an orthogonal frequency division multiplexing system that improves the performance of an orthogonal frequency division multiplexing system by correcting a frequency offset by reducing the estimation time of a frequency offset and the complexity of calculation.

In general, the orthogonal frequency division multiplexing is a type of multi-carrier digital modulation using a plurality of carriers, and is a modulation method used in mobile digital voice broadcasting or terrestrial digital television broadcasting.

This orthogonal frequency division multiplexing has a strong characteristic against noise and multipath.

In modern televisions, empty channels are installed to prevent interference. However, in the case of a single frequency channel using an orthogonal frequency division multiplexing scheme, the empty channel is not required, thereby increasing the number of channels.

The orthogonal frequency division multiplexing system described above is used in terrestrial digital television broadcasting used in Europe and terrestrial digital television broadcasting scheduled in Japan.

When receiving and converting a signal modulated and transmitted at the transmitter by the orthogonal frequency division multiplexing method into a baseband signal, synchronization between the transmission frequency and the reception frequency is caused by the difference in the characteristics of the transmitter and the receiver. Is often not correct, wherein the frequency difference as described above is called a frequency offset.

Since this frequency offset causes a decrease in signal size and interference between adjacent channels, the performance of the system is degraded. Therefore, the correction of the frequency offset is an important problem in determining the performance of a system using orthogonal frequency division multiplexing.

Therefore, several algorithms have been proposed to correct the frequency offset in orthogonal frequency division multiplexing, and these algorithms have two stages: a broad estimation that estimates a largely alternative frequency offset and a fine estimation that estimates a fine residual frequency offset. By estimating the frequency offset, the frequency offset generated at the receiver is corrected.

FIG. 1 is a schematic block diagram illustrating a frequency offset correction device according to the related art as described above, and FIG. 2 is a detailed block diagram of the frequency offset correction device of FIG.

As shown, the analog-to-digital converter 10 converts the received data r (t) in analog form, which is converted into a time domain through an inverse fast fourier transform (IFFT) at the transmitting end, to a digital signal. Convert.

The frequency offset correction unit 20 corrects the frequency offset of the received data generated due to the difference between the tuning characteristics of the transmitting end and the receiving end according to the width estimation value output from the frequency offset estimator 40 described later.

The Fast Fourier Transform (FFT) unit 30 converts the received data in the received time domain into the frequency domain.

The frequency offset estimator 40 estimates the frequency offset of the received data generated by the symbol unit due to the difference between the tuning characteristics of the transmitter and the receiver, in symbol units, and determines the frequency having the largest correlation among the estimated values as the width estimation value. Output to the government 20.

The frequency offset estimator 40 includes a wide estimator 40A estimating a general frequency offset and a fine estimator 40B estimating a fine residual frequency offset.

Here, the width estimation and the fine estimation performed by the frequency offset estimator 40 generally perform the fine estimation after the wide estimation is performed first. Alternatively, the wide estimation may be performed after the fine estimation is performed.

In addition, the fine estimation performed by the fine estimation unit 40B is continuously performed while data is received by the receiving end.

In addition, the above-described wide estimator 40A includes a symbol delay unit 41 for generating a signal delayed by receiving the received data output from the FFT unit 30 and a signal delayed through the symbol delay unit 41. First and second continuous pilot signal extractors 42 and 43 for extracting a continuous pilot signal included in a symbol of S and a continuous pilot signal included in a signal output from the FFT unit 30, respectively; A continuous pilot signal index generator 44 for changing a shift index by one in order to extract the continuous pilot signal for each symbol by the first and second continuous pilot signal extractors 42 and 43, and an FFT The output signal of the unit 30, the signal delayed by the symbol delay unit 41, the delayed continuous pilot signal and the continuous pilot signal extracted by the first and second continuous pilot signal extractors 42 and 43; The magnitude of the correlation between the (correlation) according to Equation 1 to be described later A correlation index calculating section 45 and a correlation index calculating section 45 that finds a moving index having a maximum value among the magnitudes of the correlation values calculated by the correlation value calculating section 45, and uses the frequency as a wide estimated value for correcting the frequency offset. The moving index detection unit 46 outputs the offset correction unit 20.

The continuous pilot signal extracted by the first and second continuous pilot signal extractors 42 and 43 described above is carried at a specific position of a subcarrier in each symbol of the received data, and the transmitting end changes the input signal or It is a reference carrier signal used to stabilize a transmission signal against a gain difference.

The calculation performed by the correlation value calculator 45 to obtain the magnitude of the correlation value is as follows.

The equalizer unit 50 compensates and outputs the amount of attenuation of the received data converted into the frequency domain through the FFT unit 30.

Next, the operation of the frequency offset correction device according to the prior art configured as described above will be described in detail with reference to FIG.

3 is a diagram illustrating frequency synchronization timing when the frequency offset of FIG. 2 is estimated.

In general, due to the difference in the tuning characteristics of the transmitting end and the receiving end, a frequency offset occurs that is not synchronized with the transmission data and the reception data.

For example, according to the orthogonal frequency division multiplexing system of DVB-T (Digital Wave Broadcasting-Terrestrial), the frequency uncertainty caused by the difference in the tuning characteristics of the transmitter and receiver in the 8K format is approximately -70KHz to Since it is around 70KHz, the frequency recoverer should be able to correct the frequency offset up to ± 70KHz for accurate reception.

First, the received data in the time domain received by the receiver is converted into the frequency domain through the FFT unit 30.

The symbol delay unit 41 of the wideness estimation unit 40A receives the output signal of the FFT unit 30 to generate a signal by delaying one symbol, and the first continuous pilot signal extraction unit 42 generates the signal delay unit 41. Extracts the continuous pilot signal included in the output signal from.

The continuous pilot signal is extracted from the signal output from the FFT unit 30 through the second continuous pilot signal extracting unit 43.

In addition, the first and second continuous pilot signal extractors 42 and 43 extract the continuous pilot signals through the continuous pilot signal index generator 44 and change the moving indexes one by one.

The correlation value calculation unit 45 now outputs the output signal of the FFT unit 30, the signal delayed by the symbol delay unit 41, and the delayed signals extracted by the first and second continuous pilot signal extractors 42 and 43. The magnitude of the correlation value between the continuous pilot signal and the continuous pilot signal is calculated using Equation 1 described above.

In this case, the correlation value calculator 45 calculates the magnitude of the correlation value for each symbol by using Equation 1 described above.

As such, the correlation values calculated for each symbol in the correlation value calculator 45 are output to the movement index detector 46, and the movement index detector 46 detects the instantaneous movement index having the maximum value among the input correlation values. do.

The optimal index index obtained by the movement index detector 46 is fed back to the frequency offset correction unit 20 as a width estimation value to correct the frequency offset.

That is, when the wide estimation unit 40A of the conventional frequency correction apparatus performs the wide estimation as described above, as shown in FIG. 3, the wide index is calculated by increasing the moving index by one, and the wide estimation is performed. After completion, the frequency offset is corrected by outputting the optimum shift index to the frequency offset correction unit 20.

For example, according to the specification of the DVB-T orthogonal frequency division multiplexing system of Digital Video Broadcasting-Terrestrial (DVB-T), the variation of the moving index required for the estimation of the width in the orthogonal frequency division multiplexing in the 8k format is 140 And the number of symbols required for the width estimation is 140, and the number of subcarriers included in each symbol is 8448.

Therefore, since the correlation value calculating unit 45 calculates the correlation value for each symbol, as shown in Equation 1 above, when the width estimation is performed, the number of symbols required for the 3 × number of subcarriers per symbol × width estimation is calculated. ) And multiplication by ((number of subcarriers per symbol-1) x (number of symbols required for wideband estimation)).

That is, in the 8k format of the DVB-T orthogonal frequency division multiplexing system, a complex multiplication of about 3.35 million times (3 × 8448 × 140) and a complex addition of about 1.18 million (8447 × 140) are performed.

However, in the conventional frequency offset correction device as described above, since the moving index is increased by one symbol for each symbol as described in Equation 1, the estimation of the width takes a lot of time. Since the computational computation is very large, the performance of a system using orthogonal frequency division multiplexing is greatly degraded.

Accordingly, an object of the present invention is to reduce orthogonal frequency division multiplexing by reducing the frequency estimation time and the complexity of the calculation of the frequency offset caused by the difference in the characteristics of the tuner of the transmitter and the receiver when restoring the transmitted frequency. An object of the present invention is to provide a frequency offset correcting apparatus in an orthogonal frequency division multiplexing scheme capable of improving the performance of a transmission system.

1 is a schematic block diagram illustrating a frequency offset correction apparatus according to the prior art;

2 is a block diagram showing in detail the frequency offset correction device according to the prior art of FIG.

3 is a diagram illustrating frequency synchronization timing when the frequency offset of FIG. 2 is estimated.

4 is a schematic block diagram illustrating an example of a frequency offset correction device according to the present invention;

FIG. 5 is a diagram illustrating frequency synchronization timing when the frequency offset of FIG. 4 is estimated.

Explanation of symbols on the main parts of the drawings

100: analog to digital converter 200: frequency offset correction unit

300: FFT unit 400: frequency offset width estimation unit

410: switch unit 420: first memory unit

421: Second memory unit 430: First circular moving unit

431: Second cyclic moving unit 440: First continuous pilot signal extracting unit

441: second continuous pilot signal extraction unit

450: Continuous pilot signal index memory section

460: correlation value calculation unit 470: moving index detection unit

500: equalizer part

In the orthogonal frequency division multiplexing apparatus according to the present invention for achieving the above object, in the frequency offset correction apparatus for correcting the frequency offset of the received data caused by the difference in the tuning characteristics of the transmitting end and the receiving end, An analog / digital converter for converting received data in the time domain through the IFFT into a digital signal and an FFT unit for converting the received data in the time domain output from the analog / digital converter into the frequency domain; A frequency offset is estimated every time of an integer multiple of the subcarriers included in the symbol of the signal output from the FFT unit, and the widest estimated value for correcting the frequency offset of the received data is detected by detecting the largest correlation among the estimated values. Frequency offset width estimation part to output, frequency And a frequency offset correction unit for correcting the frequency offset of the data received at the receiving end according to the width estimation value output from the number offset width estimation unit, and an equalizer unit for compensating and outputting the attenuation amount of the received data converted into the frequency domain through the FFT unit. It features.

Hereinafter, a frequency offset correcting apparatus in an orthogonal frequency division multiplexing system of the present invention will be described in detail with reference to the accompanying drawings.

4 is a schematic block diagram illustrating an example of a frequency offset correction apparatus in an orthogonal frequency division multiplexing method according to the present invention.

As shown in the drawing, the analog-to-digital converter 100 converts the received data r (t) in analog form, which is converted into a time domain through an IFFT at a transmitter, into a digital signal.

The frequency offset correction unit 200 corrects the frequency offset of the received data generated due to the difference between the tuning characteristics of the transmitter and the receiver according to the width estimation value output from the frequency offset width estimation unit 400 to be described later.

The FFT unit 300 converts the received data in the time domain output from the analog-to-digital converter 100 into the frequency domain.

The frequency offset width estimator 400 estimates the frequency offset of the received data generated by the difference between the tuning characteristics of the transmitter and the receiver at every integer times of the subcarriers included in the symbol, and uses the maximum value among the estimated values as the width estimation value. Output to the frequency correction unit 200.

Here, in addition to the width estimation performed by the frequency offset width estimation unit 400, the fine estimation (not shown) is continuously performed while data is received by the receiver as in the above-described conventional technology.

The frequency offset width estimator 400 passes through only the first and second symbols of the signal output from the FFT unit 300 and blocks other signals, and the switch unit 410. First and second memory units 420 and 421 for storing the first and second symbols of the signal output from the FFT unit 300 that has passed, and the first and second memory units 420 and 421, respectively. The first and the second cyclic shift unit 430, 431 and the first and second cyclic shift unit 430, 431 for cyclically moving the sub-carriers included in the symbol output from First and second continuous pilot signal extracting units 440 and 441 for extracting continuous pilot signals carried on a subcarrier output from the subcarrier, a continuous pilot signal index memory unit 450 storing positions of continuous pilots, The magnitudes of the correlation values of the continuous pilot signals extracted by the first and second continuous pilot signal extractors 440 and 441 will be described later. Is a correlation value calculating unit 460 calculated according to Equation 2 and a moving index of the instant having the maximum value among the magnitudes of the correlation values calculated by the correlation value calculating unit 460. The moving index detection unit 470 outputs to the frequency offset correction unit 200 as a width estimation value to be corrected.

In the above-described first and second circular movement units 430 and 431, the circular movement is performed by each subcarrier moving one by one to the right and the subcarrier of the rearmost N is the front of the first subcarrier 1. It can also be configured as a right cyclic shift (right cyclic shift) that is moved to the left side, on the contrary, the left cyclic shift (left cyclic) in which each subcarrier moves to the left one by one, and the front subcarrier moves to the rear of the rear subcarrier shift).

The calculation performed by the correlation value calculator 460 to obtain the magnitude of the correlation value is as follows.

The equalizer 500 compensates and outputs the amount of attenuation of the received data converted into the frequency domain through the FFT unit 300.

Next, the operation of the frequency offset correction device in the orthogonal frequency division multiplexing system according to the present invention configured as described above will be described in detail with reference to FIG.

FIG. 5 is a diagram illustrating frequency synchronization timing when the frequency offset of FIG. 4 is estimated.

First, the received data in the time domain received by the receiver is converted into the frequency domain through the FFT unit 300.

In addition, the signal passing through the FFT unit 300 is continuously input in series to the frequency offset width estimation unit 400. In the switch unit 410, only the first and second symbols of the signal output from the FFT unit 300 are received. Pass through and block other signals.

The first symbol that has passed through the switch unit 410 is stored in the first memory unit 420, and the second symbol is stored in the second memory unit 421.

In this case, the frequency offset width estimation unit 400 does not perform frequency offset estimation while a total of two symbols are input.

Now, when two symbols are stored in the first and second memory units 420 and 421, the subcarriers in each symbol are cyclically moved by the first and second circular movement units 430 and 431.

Thereafter, the first and second continuous pilot signal extractors 440 and 441 transmit a signal corresponding to the continuous pilot position in the subcarriers cyclically moved through the first and second circular shifters 430 and 431. Extract.

In other words, two symbols stored in the first and second memory units 420 and 421 are cyclically moved by the subcarrier interval at each time corresponding to an integer multiple of the subcarrier interval, and the continuous pilot signal is extracted.

In the continuous pilot signal index memory unit 450, the continuous pilot position is stored.

The correlation value calculator 460 now calculates the magnitude of the correlation value between the delayed continuous pilot signal and the continuous pilot signal extracted by the first and second continuous pilot signal extractors 440 and 441 using Equation 2 described above. Calculate

The moving index detection unit 470 finds the largest value among the correlation values repeatedly obtained by the correlation value calculating unit 460, obtains a width estimation value that is a moving index value at that time, and uses the obtained frequency estimation correction unit 200 ), The frequency offset is corrected.

In this case, the frequency offset estimation and correction of the above-described frequency offset width estimation unit 400 starts from the time when the first symbol and the second symbol of the signal output from the FFT unit 300 are input, and then the third symbol is input to the receiver. Since the frequency offset is calculated every time by an integer multiple of the subcarrier interval, the frequency offset can be estimated and corrected within a maximum of 3 symbols.

Therefore, as shown in FIG. 5, the width estimation time in the present invention is greatly shortened as compared with the width estimation time mentioned in the prior art, thereby improving performance of the system.

For example, according to the standard of the orthogonal frequency division multiplexing system of DVB-T as in the above-described prior art, the frequency offset is considered to be -70KHz to 70KHz in the orthogonal frequency division multiplexing in the case of 8k format, The amount of change of the moving index is 140, the number of subcarriers included in each symbol is 8448, and the number of elements of the continuous pilot index set P in Equation 2 is 177.

In the prior art as described above, the amount of calculation is increased by multiplying the received signal, the delayed signal, and the corresponding continuous pilot signals, respectively, but in the present invention, the amount of calculation is significantly reduced by using only the signal corresponding to the continuous pilot position in the received data.

That is, as shown in Equation 2, since the multiplication is performed only in the portion corresponding to the position of the continuous pilot signal in the process of obtaining the correlation value, the total calculation amount required for the width estimation is required for the number of continuous pilot signals per symbol x the width. Complex multiplication by the amount of change in the moving index) and complex addition by ((number of continuous pilot signals per symbol-1) x amount of change in the moving index required for the wide bandwidth estimation).

Therefore, in the case of the 8k format of the orthogonal frequency division multiplexing system of the DVB-T described above, complex number multiplication of No. 24780 (177 × 140) and complex addition of No. 24640 (176 × 140) are performed.

The complex multiplication consists of four real multiplications and two real additions, and the complex additions consist of two real additions. Real multiplication and real addition of number 98840 (24780 × 2 + 24640 × 2) are performed.

Here, it is a matter of course that the width estimation time can be performed for several symbols in consideration of the calculation speed in addition to the above-described maximum three symbols.

In addition, the frequency offset correction device according to the present invention as described above can be implemented in any system using the orthogonal frequency division multiplexing method.

In other words, it is applied to European and Japanese digital television, Digital Audio Broadcasting (DAB), Wireless Local Area Network (WLAN), Multi Carrier Code Division Multiple Access (MC-CDMA), etc. It can be used.

As described above, according to the frequency offset correcting apparatus in the orthogonal frequency division multiplexing method of the present invention, the amount of calculation required for the estimation of the width is reduced by reducing the width estimation time, which is performed to correct the frequency offset generated at the receiving end, within a maximum of 3 symbols. There is an advantage that can be greatly reduced to significantly reduce the time required to restore the received frequency.

In addition, the width estimation time and the complexity of the calculation are greatly improved, and as a result, the performance of the orthogonal frequency division multiplexing system is greatly improved.

Claims (5)

In the frequency offset correction device for correcting the frequency offset of the received data caused by the difference in the tuning characteristics of the transmitter and the receiver, An analog / digital converter for converting the received data in analog form, which is converted into a time domain through an IFFT at the transmitting end, into a digital signal; An FFT unit converting received data in the time domain output from the analog / digital converter into a frequency domain; A switch unit for passing only the first and second symbols output from the FFT unit and blocking other signals, and storing first and second symbols of signals output from the FFT unit passing through the switch unit, respectively; Outputs from the first and second circular shifters and the first and second circular shifters to circulate and move the subcarriers included in the symbols output from the first and second memory units and the first and second memory units one by one. Continuous pilot positions for extracting the continuous pilot signal of each symbol from the first and second continuous pilot signal extracting unit for extracting the continuous pilot signal carried on the subcarrier to be used and the first and second continuous pilot signal extracting unit Magnitude of the correlation value between the continuous pilot signal index memory unit and the continuous pilot signal extracted by the first and second continuous pilot signal extractors The frequency offset correction unit finds a moving index at the instant having a maximum value among the magnitudes of the correlation values calculated by the correlation value calculating unit and the correlation value calculating unit, and uses the wide range estimated value to correct the frequency offset. A frequency offset wide width estimator configured to include a moving index detector for outputting to A frequency offset correction unit for correcting a frequency offset of data received at a receiving end according to a width estimation value output from the frequency offset width estimation unit; And And an equalizer unit for compensating and outputting an attenuation amount of the received data converted into the frequency domain through the FFT unit. The method of claim 1, wherein the frequency offset estimation is not performed until the first symbol of the signal output from the FFT unit is stored in the first memory unit and the second symbol is stored in the second memory unit. Frequency offset correction apparatus in orthogonal frequency division multiplexing. 2. The apparatus of claim 1, wherein the calculation of the correlation value performed by the correlation value calculator is performed by the following equation. [Equation] The frequency offset estimation method of claim 1, wherein the frequency offset estimation in the frequency offset width estimation unit estimates using only a signal corresponding to a position of a continuous pilot signal among data received at a receiver. Offset correction device The frequency offset estimation unit of claim 1, wherein the frequency offset estimation is performed from a time at which the third symbol is input to the receiver after the first symbol and the second symbol of the signal output from the FFT unit are input. An apparatus for correcting frequency offset in orthogonal frequency division multiplexing, characterized in that it is made within 3 symbols.