KR100313860B1 - Fine Frequency Reconstruction Device and Method in OFDM Transmission Method - Google Patents

Abstract

The present invention relates to a fine frequency recovery apparatus and method therefor in an OFDM transmission scheme, and the present invention is to extract a pilot signal from the OFDM signal received in a fast-free conversion, add a pilot signal for one symbol, the odd number symbol And multiplying the conjugate complex number of the pilot signal addition value by the pilot signal addition value corresponding to the even-numbered symbol, and correcting the frequency error by extracting a phase error estimation value between the symbols by the multiplied value. The present invention has the advantage of reducing the complexity of the calculation and eliminating the need for additional memory.

Description

Micro frequency recovery apparatus and method thereof in ODF transmission system

The present invention relates to an orthogonal frequency division multiplexing (OFDM) transmission scheme, and more particularly, to an apparatus and method for fine frequency recovery in an OFDM transmission scheme.

OFDM is a type of multicarrier modulation in digital signal transmission, and uses a frequency band used in single carrier data transmission and transmits data in multiple subcarriers in parallel in a long symbol period.

In an OFDM system, when a received signal is converted into a baseband signal, a frequency offset occurs when the transmission frequency is not synchronized with the reception frequency due to a difference in characteristics of a tuner of a transmitter and a receiver. This frequency offset causes a reduction in signal size and interference between adjacent channels, which degrades the performance of the system. Therefore, frequency offset correction is an important issue in determining the performance of an OFDM system. The process for frequency error correction in an OFDM system consists of a wide estimation step of estimating a general frequency error corresponding to an integer multiple of a subchannel interval and a fine estimation step of estimating a residual frequency error.

1 is a block diagram showing a frequency recovery apparatus in a general OFDM transmission scheme.

First, the input OFDM signal r (t) is converted into a digital complex sample by the ADC unit 110 and converted into a signal in the frequency domain through the frequency error correcting unit 120 and the FFT unit 130. The FFT-converted signal is extracted from the frequency error estimator 140 and input to the frequency error corrector 120. In this case, the frequency error estimator 140 generally includes a wide error estimator 142 and a fine error estimator 144, and performs wide error estimation first and fine error estimation later, or fine error estimation first. You can do this later and perform a wide error estimation. The received signal is corrected by the frequency error estimated by the frequency error correction unit 120, and the channel distortion is compensated by the equalizer 150 through the FFT unit 130.

2 is a block diagram of a frequency recovery apparatus including a detailed block diagram of the fine error estimation unit 144 of FIG. 1.

The fine error estimator 140 corrects the frequency error by extracting only a pilot signal in units of 2 symbols from the input symbol signals. That is, the first symbol (odd number) is stored in the symbol delay unit 242 when the contact point c of the switch SW1 is switched to the contact point a. The pilot signal is extracted by the second pilot extractor 243 from the first symbol stored at the time when the contact point c of the next switch SW1 is switched to the contact point b, and then the pilot signal is a conjugate complex conversion part. Is converted to a conjugate complex number using (244). The transformed conjugate complex pilot signal is then multiplied by a pilot signal corresponding to the second symbol extracted from the first pilot extractor 241 in the multiplier 245. The pilot signal of the symbol multiplied by the multiplier 245 is added by the adder 246 for two symbol periods. The phase error estimator 248 divides the pilot signal sum during the two symbol periods added by the adder 246 into the real part Im and the imaginary part Re using the MLE (Maximum Likelihood Estimate) proposed by Moose. As shown in Equation 1, the phase error estimation value is detected.

here Y _1k = K-th pilot signal of odd (1,3,5, ...) received symbol, Y _2k = K-th pilot signal of even-numbered (2, 4, 6, ...) received symbols, P = number of pilots per symbol.

The DPLL unit 247 inputs a phase estimation value as shown in Equation 1 to drive a digital phase lock loop (PLL) and outputs an error voltage thereof. The VCO unit 249 oscillates a frequency according to the error voltage input from the DPLL unit 247 to control the frequency error correction unit 120. The frequency error correcting unit 120 corrects a frequency error of an input OFDM signal according to the voltage controlled oscillation frequency generated by the VCO unit 249. At this time, the phase error estimation value estimated by the phase error estimator 248 corrects the third and fourth symbols.

Next, while the third symbol is stored in the symbol delay unit 240 and the fourth symbol is input, the frequency error is estimated in the same manner as described above.

As shown in Equation 1, the fine error estimator 144 multiplies the conjugate complex number of the first symbol signal by the second symbol signal and divides them by the real coded imaginary part to obtain a sum. Therefore, the conventional fine error estimator 144 requires a large amount of complex multiplication by the number of FFTs included in one symbol. For example, in the case of 8k format of the European terrestrial broadcasting standard DVB-T, the number of consecutive pilots is 177, and the calculation required for each block requires 177 complex multiplications and 176 complex additions. Converting this to a real number calculation, complex multiplication consists of four real multiplications and two real additions, and complex addition consists of two real additions. And 706 (177 x 2 + 176 x 2) real additions are required. The fine error estimation differs from system to system, but requires dozens of symbols, resulting in high computational complexity and the need for multiple multipliers and adders. In addition, there is a disadvantage in that an additional memory is required while delaying one symbol signal.

The technical problem to be achieved by the present invention is to first add the signals for one symbol at the time of fine frequency restoration of the OFDM system, and then add the conjugate complex number and the pilot signal of the even symbol. To reduce the complexity of the calculation by estimating the phase error between symbols by multiplying by and to provide a fine frequency recovery apparatus and method that does not require additional memory.

1 is a block diagram showing a frequency recovery apparatus in a general OFDM transmission scheme.

FIG. 2 is a block diagram of a frequency recovery apparatus including a detailed block diagram of the fine error estimation unit 144 of FIG. 1.

3 is a block diagram showing a frequency recovery apparatus in an OFDM transmission method according to the present invention.

In order to solve the above technical problem, the present invention provides a method for correcting a frequency error by converting an OFDM received signal configured in symbol units to a fast free signal, and extracting a pilot signal from the fast free converted OFDM received signal for one symbol. Adding a pilot signal of; Multiplying the conjugate complex number of the pilot signal addition value corresponding to the odd symbol by the pilot signal addition value corresponding to the even symbol; The fine frequency recovery method in the OFDM transmission method comprising the step of correcting the frequency error by extracting the phase error estimation value between symbols by the value multiplied in the above process.

In order to solve the above other technical problem, the present invention provides an apparatus for correcting a frequency error by converting an OFDM received signal configured in units of symbols in a fast free order, the FFT unit for fast Fourier transforming the OFDM received signal: fast in the FFT Extract the pilot from the pre-converted signal, add it for one symbol, multiply the conjugate complex number of the pilot signal addition value for the odd symbol by the pilot signal addition value for the even symbol, and phase error between symbols from the multiplied value A phase error estimator for estimating a; And a frequency error correcting unit for correcting a frequency error of the OFDM received signal inputted by the phase error estimation value extracted from the phase error estimating unit.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

3 is a block diagram showing a frequency recovery apparatus in the OFDM transmission method according to the present invention, the ADC unit 310, frequency error correction 320, FFT unit 330, frequency error estimation The frequency error estimator 340 includes a pilot extractor 341, an adder 342, a switch SW2, a conjugate complex converter 343, and a multiplication. The unit 344 includes a phase error estimator 345, a DPLL unit 346, and a VCO unit 347.

The OFDM signal r (t) input in units of symbols is converted into a digital complex sample by the ADC unit 310, and is converted into a signal in the frequency domain through the frequency error correcting unit 320 and the FFT unit 330. The FFT transformed signal extracts a fine frequency error estimate from the frequency error estimator 340 and inputs it to the frequency error corrector 320. At this time, the symbol input to the fine error estimation unit 340 is calculated in units of two symbols. That is, the error estimate calculated from the first (odd) and second (even) symbols input to the fine error estimation unit 140 is compensated for the third and fourth symbols, and the error estimate calculated for the third and fourth symbols is Compensated in two symbols. The channel distortion is compensated for by the equalizer 350 in the signal output after frequency error correction by the FFT unit 330.

As shown in FIG. 3, the OFDM received signal in the symbol unit output from the FFT unit 330 is input to the pilot extractor 341 to extract a pilot signal. The pilot extractor 341 extracts a plurality of pilot signals from an OFDM signal in symbol units input from the FFT unit 330. Here, a pilot signal is a specific subcarrier used to transmit a value promised by a transmitter for synchronization between a transmitter and a receiver in an OFDM system. For example, in the 8K format of DVB-T, the European terrestrial broadcasting standard, the number of continuous pilot signals per symbol is 177. Next, the adder 342 adds a plurality of pilot signals for one symbol. At this time, the pilot signal sum corresponding to the first symbol is obtained. When the contact point c of the next switch SW2 is switched to the contact point a, the sum of the pilots corresponding to the first symbol is converted into a conjugate complex number by the conjugate complex number conversion unit 343. Subsequently, the multiplier 344 multiplies the sum of the conjugate complex of the pilot signal corresponding to the first symbol and the pilot signal corresponding to the second symbol generated when the contact c of the switch SW2 is switched to the contact b. Become. The phase error estimator 345 has a result value multiplied by the multiplier 344 and divides the real part Im and the imaginary part Re to detect a phase error estimate value corresponding to the phase difference between two symbols as shown in Equation 1 below. do.

here Y _1k = K-th pilot signal of odd (1,3,5, ...) symbols, Y _2k = K-th pilot signal of even-numbered (2, 4, 6, ...) symbols, P = number of pilots per symbol.

The DPLL unit 346 inputs a phase estimation value as shown in Equation 1 to drive a digital phase lock loop (PLL) and outputs an error voltage thereof. The VCO unit 347 oscillates in frequency according to the error voltage output from the DPLL unit 346 and is input to the frequency error correcting unit 320. Therefore, the frequency error correction unit 320 corrects the frequency error of the input OFDM signal according to the frequency generated by the VCO unit 347.

The phase error estimated by the first and second symbols is corrected in the third and fourth symbols. Next, the frequency error is corrected in the same manner for the third and fourth symbols.

The present invention is not limited to the above-described embodiment, and of course, modifications may be made by those skilled in the art within the spirit of the present invention. That is, the algorithm proposed in the present invention can be implemented in any system using OFDM. For example, European and Japanese digital TVs employing OFDM as the modulation scheme, Digital Audio Broadcasting (DAB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATH) Multi Carrier Code Division Multiple Access Algorithm applicable to frequency synchronization circuit in

Meanwhile, the above-described embodiments of the present invention can be written as a program that can be executed on a computer. And it can be implemented in a general-purpose digital computer for operating the program from a medium used in the computer. The media includes storage media such as magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical reading media (e.g., CD-ROM, DVD, etc.) and carrier waves (e.g., transmitted over the Internet). . A method of correcting a frequency error by converting an OFDM received signal configured in units of symbols into a fast free signal, the method comprising: extracting a pilot signal from the OFDM received signal converted into the fast free signal and adding a pilot signal for one symbol; Multiplying the conjugate complex number of the pilot signal addition value corresponding to the odd number symbol by the pilot signal addition value corresponding to the even number symbol, and extracting a phase error estimate value between the symbols using the product multiplied in the above step to correct the frequency error. Stores program code that can be run on a computer.

And functional program code and code segments for implementing the present invention can be easily inferred by programmers in the art to which the present invention belongs.

As described above, according to the present invention, a complex multiplication must be calculated for each signal of a symbol, but the algorithm according to the present invention eliminates this complex multiplication and simply corrects an average error with the result of adding a signal for one symbol interval. For example, in the case of the 8k standard of DVB-T, the number of consecutive pilots is 177, and the amount of computation required in each step requires one complex multiplication and 352 (176 × 2) complex additions. Converting this to a real number calculation, complex multiplication consists of four real multiplications and two real additions, and complex addition consists of two real additions, so four real multiplications and 706 (1 × 2 + 352 ×) 2) Add a real number. The algorithm according to the present invention does not change the number of real multiplications compared with the conventional algorithm, but the number of real multiplications is significantly reduced, and the use of additional memory is also unnecessary.

Claims (4)

A method of correcting a frequency error by converting an OFDM received signal configured in symbol units at high speed free, Extracting a pilot signal from the OFDM signal which has been converted into the fast free signal and adding a pilot signal for one symbol; Multiplying the conjugate complex number of the pilot signal addition value corresponding to the odd symbol by the pilot signal addition value corresponding to the even symbol; And extracting a phase error estimation value between symbols by a value multiplied by the process to correct the frequency error. The method of claim 1, wherein the phase error estimate is Where Im = real part, Re = imaginary part, Y _1k = K-th pilot signal of odd (1,3,5, ...) symbols, Y _2k = K-th pilot signal of even (2, 4, 6, ...) symbols, P = fine frequency recovery method in the OFDM transmission method characterized in that the number of pilots per symbol. Claims [1] An apparatus for correcting a frequency error by converting an OFDM received signal configured in symbol units at high speed freely, An FFT unit for fast Fourier transforming the OFDM received signal; The pilot is extracted from the signal transformed by the FFT into the high-speed free-preset, and added for one symbol. A phase error estimator for estimating a phase error between symbols from the phase error signal; And a frequency error correction unit for correcting a frequency error of the OFDM received signal input by the phase error estimation value extracted from the phase error estimation unit. The method of claim 3, wherein the error estimating unit A pilot extracting unit extracting a pilot signal from a signal in a symbol unit FFT by the FFT unit; An adder which adds the pilot signal extracted by the pilot extractor for one symbol; A multiplier for multiplying a conjugate complex number of a pilot signal addition value corresponding to an odd number symbol output from the adder by a pilot signal addition value corresponding to an even number symbol; And a phase error estimator detecting a phase error estimation value between symbols by a multiplication value multiplied by the multiplier.