Title of Invention

"A FREQUENCY OFFSET COMPENSATING APPARATUS IN AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING SYSTEM."

Abstract

The invention relates to a frequency offset compensating apparatus in an orthogonal frequency division multiplexing (OFDM) system, comprising a correlating portion (413) for demodulating input data by fast Fourier transformation, dalaying a demodulated symbol by a predetermined period and correlaing a symbol demodulated during the delay to the delayed symbol; a pilot position detecting portion (415) for comparing the correlation with a predetermined pilot position and extracting a correlation corresponding to the pilot position; and a frequency offset compensation portion (423) for estimating a frequency offset to the extracted correlation if the extracted correlation is a predetermined maximum and shifting the phase of pilot data by the estimated frequency offset.

Full Text

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) system, and in particular, to an apparatus and method for acquiring frequency synchronization using a correlation between successive symbols. 2 Description of the Related Art OFDM is widely applied to digital transmission technology including DAB (Digital Audio Broadcasting), the digital TV, WLAN (Wireless Local Area Network), and WATM (Wireless Asynchronous Transfer Mode). The OFDM is a multi-carrier scheme in which transmission data is divided into a plurality of data, modulated with a plurality of sub-earners, and transmitted in parallel. While the OFDM scheme was not used widely because of its hardware complexity, it becomes advantageous as digital signal processing techniques such as FFT (Fast Fourier Transform) and 1FFT (Inverse Fast Fourier Transform) have been developed. The OFDM scheme is different from conventional FDM (Frequency Division Multiplexing) m that data is transmitted with orthogonality maintained among a plurality of sub-carriers. Therefore, optimal transmission efficiency can be obtained during high rate transmission. Due to this advantage, the OFDM has been implemented in diverse forms like an OFDM/TDMA (Orthogonal Frequency Division Multiplexing/Time Division Multiple Access) system and an OFDM/CDMA (Orthogonal Frequency Division Multiplexing/Code Division Multiple Access) system - 1 - In the case that the Doppler effect occurs in view of channel characteristics or a tuner m a receiver is not stable, a transmission frequency may not be synchronized to a reception frequency in an OFDM transmission system. The unstable tuning between carriers produces a frequency offset which in turn changes the phase of an input signal Consequently, orthogonality is lost between sub-carriers and the decoding performance of the system is deteriorated. In this case, a slight frequency offset becomes a significant cause of the system performance deterioration Accordingly, frequency synchronization is essential to the OFDM transmission system to maintain orthogonality between sub-carriers. In general, a frequency offset in a receiver is eliminated on a sub-carrier interval basis and can be expressed as an integer part and a decimal part by dividing the frequency offset by a sub-carrier interval. Here, elimination of an initial frequency offset corresponding to the integer part is coarse frequency synchronization and elimination of a residual offset corresponding to the decimal part after the coarse frequency synchronization is fine frequency synchronization. The coarse frequency synchronization, however, takes a long time since the frequency offset of the integer part must repeatedly be compensated for to acquire frequency synchronization. Besides, if the decimal part is a 1/2 of the sub-carrier interval of, say, 1KHz, errors possibly generated during estimation of the frequency offset of the integer part must be corrected and the frequency offset of the decimal part must be compensated for Therefore, a long time is taken to acquire the fine frequency synchronization -2- It is, therefore, an object of the present invention to provide a frequency offset compensating apparatus and method for reducing time required to acquire frequency synchronization and simplifying a frequency offset compensation process. According to one aspect of the present invention, there is provided an apparatus and method for compensating for a frequency offset in an OFDM system. In a frequency offset compensating apparatus according to one aspect of the present invention, a correlating portion demodulates input data by fast Fourier transformation, delays a demodulated symbol by a predetermined period, and correlates a symbol demodulated during the delay to the delayed symbol. A pilot position detecting portion compares the correlation with a predetermined pilot position and extracts a correlation corresponding to the pilot position. A frequency offset compensating portion estimates a frequency offset to the extracted correlation if the extracted correlation is a predetermined maximum and shifts the phase of pilot data by the estimated frequency offset. In a frequency offset compensating apparatus according to another aspect of the present invention, a correlating portion demodulates input data by fast Fourier transformation, delays a demodulated symbol by a predetermined period, and correlates a symbol demodulated during the delay to the delayed symbol. A pilot position detecting portion compares the correlation with a predetermined pilot position and extracts a correlation corresponding to the pilot position. A frequency offset compensating portion detects the maximum of extracted correlations, estimates a frequency offset by comparing the maximum correlation and adjacent correlations with a predetermined threshold, and shifts the phase of pilot data by the estimated frequency offset. -3- In a frequency offset compensating method according to a third aspect of the present invention, input data is demodulated by fast Fourier transformation and delayed a demodulated symbol by a predetermined period. A symbol demodulated during the delay is correlated to the delayed symbol. The correlation is compared with a predetermined pilot position and a correlation corresponding to the pilot position is extracted. A frequency offset is estimated to the extracted con elation if the extracted correlation is a predetermined maximum and the phase of pilot data is shifted by the estimated frequency offset. In a frequency offset compensating method according to a fourth aspect of the present invention, input data is demodulated by fast Fourier transformation and delayed a demodulated symbol by a predetermined period A symbol demodulated during the delay is correlated to the delayed symbol. The correlation is compared with a predetermined pilot position and a correlation corresponding to the pilot position is extracted. The maximum is detected from extracted correlations, a frequency offset is estimated by comparing the maximum correlation and adjacent correlations with a predetermined threshold, and the phase of pilot data is shifted by the estimated frequency offset. BRIEF DESCRIPTION OF THE ACCOMPANYING DRA WINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG 1 is a block diagram of a transmitter in an OFDM system to which the present invention is applied, FIG. 2 is a block diagram of a receiver in the OFDM system; FIG 3 illustrates the structure of a signal frame in the OFDM system; FIG 4 is a block diagram of a frequency offset compensating apparatus -4- according to an embodiment of the present invention; FIGs. 5 A and 5B illustrate outputs of an FFT (Fast Fourier Transformer) shown in FIG. 4; FIG. 6 is a graph showing pilot correlations for use in acquiring coarse frequency synchronization according to the embodiment of the present invention; FIG 7 is a graph showing pilot correlations for use m acquiring fine frequency synchronization according to the embodiment of the present invention; and FIG 8 is a graph showing ratios of the greater of adjacent correlations to a maximum correlation corresponding to the decimal part of a frequency offset. DETAILED DESCRIPTION OE THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described herembelow with reference to the accompanying drawings In the following description, well-known functions or constructions are not described in detail since they would obscure the invention m unnecessary detail. FIG. 1 is a block diagram of a transmitter man OFDM system to which the present invention is applied The OFDM system is incorporated into a CDMA system. Referring to FIG. 1, a spreading portion 111 spreads transmission data (Tx data) with orthogonal codes and PN (Pseudorandom Noise) sequences. Both the transmitter and a receiver know orthogonal codes and PN sequences. An adder 113 sums the spread data received from the spreading portion 111 and outputs a serial signal d(k). A senal-to-parallel converter (SPC) 115 converts the serial signal d(k) to a predetermined number of, say, N parallel data dm (k). The symbol length of the parallel data dm (k) is increased to N times the symbol - 5 - length of the serial data d(k) An IFFT (Inverse Fast Fourier Transformer) 117 modulates the parallel data din(k) with N sub-carriers and outputs modulation data don(n) That is, the parallel data dm(k) is converted to a complex number corresponding to the phase and amplitude of each sub-earner, assigned to the respective sub-earners on a frequency spectrum, and inverse fast Fourier-transformed on a time spectrum. Thus, the parallel data dln(k) are modulated with the N sub-carriers to the modulation data don(n) which can be expressed as A parailel-to-senal converter (PSC) 119 converts the parallel data don(n) to serial data ds(n). A guard interval inserter 121 inserts guard intervals into the serial data ds(n) When a transmission channel causes mter-symbol interference (ISI), a guard interval is inserted between transmission symbols to absorb the inter-symbol interference. Consequently, orthogonality between sub-carriers can be maintained in the OFDM system, A digital-to-analog converter (DAC) 123 converts the data received from the guard interval inserter 121 to an analog baseband signal ds(t). A mixer 125 mixes the signal ds(t) with a synthetic earner frequency received from a frequency synthesizer 127 and outputs real transmission data P(t), given by where We is the synthetic carrier frequency A transmission filter 129 filters out the transmission data P(t) to a baseband signal and transmits the baseband signal. -6- Additive White Gaussian noise (AWGN) is added to the transmission data P(t) m the air during transmission to a receiver. The structure of the receiver will be described with reference to FIG. 2. FIG 2 is a block diagram of the receiver in the OFDM system to which the present invention is applied. A carrier signal including the AWGN is applied to the input of a multiplier 211. The multiplier 211 down-converts the frequency of the input carrier signal to an intermediate frequency (IF) by mixing the earner signal with a predetermined local oscillation frequency synthesized in a frequency synthesizer 213. A low pass filter (LPF) 215 low-pass-filters the IF signal received from the multiplier 211 and an ADC 217 converts an IF signal received from the LPF 215 to a digital signal by sampling the IF signal at predetermined intervals. A guard interval remover 219 removes guard intervals from the digital signal. An SPC 221 converts the guard intervals-free signal received from the guard interval remover 219 to parallel data. An FFT 223 OFDM-demodulates the parallel data with a plurality of sub-carriers and a PSC 225 converts the demodulated parallel data to serial data by adding symbol data received in parallel. A despreader 227 despreads the serial symbol data, thereby recovering original data. It is important to acquire frequency synchronization between the sub-carriers in the above receiver The frequency synchronization makes it possible to maintain orthogonality between the sub-carriers and thus to decode a signal accurately A frequency offset compensating apparatus for the frequency synchronization according to an embodiment of the present invention will be described below -7- FIG. 3 illustrates the format of a signal frame used m the OFDM system shown in FIGs 1 and 2. Referring to FIG 3, a frame 300 used in the OFDM system includes 12 symbols. A symbol NULL indicates the start of the frame 300 and symbols Sync #1 and Sync #2 are sync symbols, and data symbol #1 301 to data symbol #9 are real transmission data A data symbol, say, data symbol #1 301 has a predetermined number of, for example, 256 data A predetermined number of, for example, 10 pilot data are inserted m the 256 data so that the receiver can compensate for a frequency offset between sub-carriers by phase estimation of the pilot data. FIG 4 is a block diagram of the frequency offset compensating apparatus according to the embodiment of the present invention. As stated above, the FFT 223 OFDM-demodulates an input signal with a plurality of sub-carriers The OFDM demodulation signals are in the format shown in FIG 3. Let the signals output from the FFT 223 be Snk (n is a symbol number, k is a sub-carrier number, and if K sub-camers are used, 1 A correlation between an nth symbol and an (n-H)th symbol output from the correlator 413 can be expressed as -8- and information about the positions of pilot data is where Pm is a set of the numbers of sub-carriers with pilot data, m is an integer part of a frequency offset and -M When M = 20, the integer part of the frequency offset can be estimated to ±20 by where the estimated integer part of the frequency offset, fois m when the correlation Cm is a maximum, that is, when the correlation of pilot data is a maximum, as output from the pilot locator 415. A pilot position shifter 417 shifts a pilot position so that the pilot position detector 415 detects a pilot with ) the maximum correlation. The pilot correlation detected in the pilot position detector 415 is accumulated m a pilot correlation accumulator 419 The accumulated correlation is Cm. A frequency offset estimator 421 outputs an estimated integer part of a frequency offset, fa to a frequency offset compensator 423. The frequency offset compensator 423 compensates for the integer part fo. Compensation for the integer part of a frequency offset will be described -9- referring to FIGs. 5A, 5B, and 6. FIGs. 5A and 5B illustrate outputs of the FFT 223 shown m FIG. 4. FIG. 5 illustrates a signal with a frequency offset - 0 output from the FFT 223 in a frequency domain. Pilot data exists m sub-carriers #8, 22, .. (p1 = 8, p2 = 22, ...) and real data exists on the other sub-earners in the symbol Po - [8+0, 22+0, ...] correspond to pilot positions and thus Cm is a maximum when m = 0. For a frequency offset m = 1 as shown m FIG. 5B, Po = [8+0, 22+0, . ] does not correspond to the pilot positions. In view of m = 1, P1 = [8+1, 22+1, ... ] correspond to the pilot positions and thus Cm is a maxim when m - 1. Thus, the integer part of a frequency offset is estimated to be 1. FIG. 6 is a graph showing pilot correlations output from the pilot correlation accumulator 419 in the case of coarse frequency synchronization acquisition according to the embodiment of the present invention. In FIG. 6, the maximum correlation of data maiked by circles is identical to that of data marked by triangles. Let two successive symbols circle-marked data and triangle-marked data be an nth symbol and an (n+l)th symbol, respectively. They are correlated, the correlation is compared with predetermined pilot data, and pilot positions are detected according to the comparison result. The pilot correlation is accumulated m the pilot correlation accumulator 419 and when the accumulated correlation is a maximum, a frequency offset can be estimated. Hence, when a frequency offset is 0, the maximum correlations of the successive "symbols are identical. Thus, it is possible to accurately estimate the frequency offset. So far the coarse frequency synchronization, that is, estimation and compensation of the integer part of a frequency offset has been described. Now, - 10- there will be given a description of fine frequency synchronization, that is, estimation and compensation of the decimal part of the frequency offset. After the coarse frequency synchronization is acquired by estimating and ? compensating for a frequency offset being an integer multiple of a sub-carrier interval, the fine frequency synchronization is performed. FIG. 7 is a graph showing pilot correlations for the fine frequency synchronization acquisition m a frequency domain where a frequency offset is a 1/2 of the sub-carrier interval. Referring to FIG. 7, when the frequency offset is a 1/2 of the sub-carrier interval, the maximum correlations of the successive data marked by circles and triangles are different because it is difficult to estimate the frequency offset being a 1/2 of the sub-carrier interval accurately. Returning to FIG 4, the pilot correlation accumulator 419 outputs correlations as many as the number of Cm (-M as CMAX =MAC(Cm). Assuming that CMax``````````` is Cm, a final frequency offset can be estimated - 11 - utilizing Cm and its adjacent correlations Cm_, and Cm+1 by where to determine the threshold value THRESHOLD, detected by simulation while the decimal part of the estimated frequency offset is increased by O.lHz from -0.5 to +0.5Hz FIG. 8 is a graph showing ratios of the greater of adjacent correlations to a maximum correlation corresponding to the decimal part of the frequency offset in the simulation In FIG. 8, triangles at the same offset denote ratios of the greater of adjacent correlations to the maximum correlation and circles on a solid line 'denote the averages of the ratios when the decimal parts of frequency offsets exist in the simulation. If for THRESHOLD of 0.5, an estimated decimal part of the frequency offset is given as 0 4 Hz is within +0.1 Hz after compensation. In accordance with the present invention as described above, it is possible to acquire coarse frequency synchronization and fine frequency synchronization, that is, to estimate the integer and decimal parts of a frequency offset by correlating two successive symbols. Therefore, the time required for acquiring the coarse and fine frequency synchronization is reduced even when the decimal part of the frequency offset is a 1/2 of a sub-carrier interval. While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. - 12- WE CLAIM 1. A frequency offset compensating apparatus in an orthogonal frequency division multiplexing (OFDM) system, comprising: a correlating portion (413) for demodulating input data by fast Fourier transformation, delaying a demodulated symbol by a predetermined period and correlating a symbol demodulated during the delay to the delayed symbol; a pilot position detecting portion (415) for comparing the correlation with a predetermined pilot position and extracting a correlation corresponding to the pilot position; and a frequency offset compensating portion (423) for estimating a frequency offset to the extracted correlation if the extracted correlation is a predetermined maximum and shifting the phase of pilot data by the estimated frequency offset. 2. The frequency offset compensating apparatus as claimed in claim 1, wherein the correlating portion (413) comprises: a delay (411) for delaying the demodulated symbol by the predetermined period; and a correlator (413) for correlating the current demodulated symbol to the delayed symbol. 3. The frequency offset compensating apparatus as claimed in claim 1, wherein the predetermined period is one symbol period. 4. The frequency offset compensating apparatus as claimed in claim 1, wherein the pilot position detecting portion (415) comprises: a pilot position shifter (417) for storing pilot data transmitted by a transmitter and information about the positions of the pilot data and shifting the pilot data positions under predetermined control; and a pilot position detector (415) for detecting a correlation among the outputs of the correlator according to a pilot position received from the pilot position shifter (417). 5. The frequency offset compensating apparatus as claimed in claim 1, wherein the frequency offset compensating portion (423) comprises: an accumulator (419) for accumulating the extracted correlation; a frequency offset estimator (421) for estimating a maximum accumulated correlation to be the frequency offset if the accumulated correlation reaches a predetermined value; and a frequency offset compensator (423) for shifting the phase of the pilot data according to the estimated frequency offset. 6. The frequency offset compensating apparatus as claimed in claim 1, wherein the frequency offset can be estimated up to as many times as twice the number of sub-carries. 7. A frequency offset compensating apparatus in an OFDM system, comprising: a correlating portion for demodulating input data by fast Fourier transformation, delaying a demodulated symbol by a predetermined period, and correlating a symbol demodulated during the delay to the delayed symbol; a pilot position detecting portion for comparing the correlation with a predetermined pilot position and extracting a correlation corresponding to the pilot position; and a frequency offset compensating portion for detecting the maximum of extracted correlations, estimating a frequency offset by comparing the maximum correlation and adjacent correlations with a predetermined threshold, and shifting the phase of pilot data by the estimated frequency offset, 8. The frequency offset compensating apparatus as claimed in claim 7, wherein the correlating portion comprises: 14. a delay for delaying the demodulated symbol by the predetermined period; and a correlator for correlating the current demodulated symbol to the delayed symbol, 9. The frequency offset compensating apparatus as claimed in claim 7, wherein the predetermined period is one symbol period. 10.The frequency offset compensating apparatus as claimed in claim 7, wherein the pilot position detecting portion comprises: a pilot position shifter for storing pilot data transmitted by a transmitter and information about the positions of the pilot data and shifting the pilot data positions under predetermined control; and a pilot position detector for detecting a correlation among the outputs of the correlator according to a pilot position received from the pilot position shifter, II.The frequency offset compensating apparatus as claimed in claim 7, wherein the frequency offset compensating portion comprises: an accumulator for accumulating the extracted correlation; a frequency offset estimator for detecting the maximum of extracted correlations and estimating the frequency offset by comparing the maximum correlation and the adjacent correlations with the predetermined threshold; and a frequency offset compensator for shifting the phase of pilot data by the estimated frequency offset. 12.The frequency offset compensating apparatus as claimed in claim 11, wherein the frequency offset estimator estimates the frequency offset by THRESHOLD) THRESHOLD) 15 ait the cases except for Cm-1> Cm+1 > THRESHOLD, andCm-1> Cm+i > THRESHOLD) where CMAX is the maximum correlation and f0 is the estimated frequency offset. 13.The frequency offset compensating apparatus as claimed in claim 12, wherein the THRESHOLD is 0.5. 14. The frequency offset compensating apparatus as claimed in claim 7, wherein the frequency offset estimation is performed after coarse frequency synchronization is acquired. 15. A frequency offset compensating method in an OFDM system, comprising the steps of: demodulating input data by fast Fourier transformation, delaying a demodulated symbol by a predetermined period, and correlating a symbol demodulated during the delay to the delayed symbol; comparing the correlation with a predetermined pilot position and extracting a correlation corresponding to the pilot position; and estimating a frequency offset to the extracted correlation if the extracted correlation is a predetermined maximum and shifting the phase of pilot data by the estimate frequency offset. 16.The frequency offset compensating method as claimed in claim 15, wherein the correlation step comprises the steps of: delaying the demodulated symbol by the predetermined period; and correlating the current demodulated symbol to the delayed symbol, 17.The frequency offset compensating method as claimed in claim 15, wherein the predetermined period is one symbol period. 18. The frequency offset compensating method as claimed in claim 15, wherein the pilot position detection step comprises the steps of: storing pilot data transmitted by a transmitter and information about the position of the pilot data and shifting the pilot data positions under predetermined control; and detecting a correlation corresponding to a pilot position shift among correlations. 19.The frequency offset compensating method as claimed in claim 15, wherein the frequency offset compensation step comprises the steps of: accumulating the extracted correlation; estimating a maximum accumulated correlation to be the frequency offset if the accumulated correlation reaches a predetermined value; and shifting the phase of the pilot data according to the estimated frequency offset. 20.The frequency offset compensating method as claimed in claim 15, wherein the frequency offset can be estimated up to as many times as twice the number of sub-carriers. 21.A frequency offset compensating method in an OFDM system, comprising the steps of: demodulating input data by fast Fourier transformation, delaying a demodulated symbol by a predetermined period, and correlating a symbol demodulated during the delay to the delayed symbol; comparing the correlation with a predetermined pilot position and extracting a correlation corresponding to the pilot position; and detecting the maximum of extracted correlations, estimating a frequency offset by comparing the maximum correlation and adjacent correlations with a predetermined threshold and shifting the phase of pilot data by the estimated frequency offset. 22. The frequency offset compensating method as claimed in claim 21, wherein the correlation step comprises the steps of: delaying the demodulated symbol by the predetermined period; and correlating the current demodulated symbol to the delayed symbol. (7. 23.The frequency offset compensating method as claimed in claim 21, wherein the predetermined period is one symbol period. 24,The frequency offset compensating method as claimed in claim 21, wherein the pilot position detection step comprises the steps of: storing pilot data transmitted by a transmitter and information about the position of the pilot data and shifting the pilot data positions under predetermined control; and detecting a correlation corresponding to a pilot position shift among correlations. 25. The frequency offset compensating method as claimed in claim 21, wherein the frequency offset compensation step comprises the steps of : accumulating the extracted correlation; estimating the maximum of extracted correlations and estimating the frequency offset by comparing the maximum correlation and the adjacent correlations with the predetermined threshold; and shifting die phase of the pilot data by the estimated frequency offset. 26.The frequency offset compensating method of claim 25, wherein the frequency offset estimator estimates the frequency offset by THRESHOLD, where CMAX is the maximum correlation and fo is the estimated frequency offset. 19 27. The frequency offset compensating method as claimed in claim 26, wherein the THRESHOLD is 0.5 28. The frequency offset compensating method as claimed in claim 21, wherein the frequency offset estimation is performed after coarse frequency synchronization is acquired. The invention relates to a frequency offset compensating apparatus in an orthogonal frequency division multiplexing (OFDM) system, comprising a correlating portion (413) for demodulating input data by fast Fourier transformation, dalaying a demodulated symbol by a predetermined period and correlaing a symbol demodulated during the delay to the delayed symbol; a pilot position detecting portion (415) for comparing the correlation with a predetermined pilot position and extracting a correlation corresponding to the pilot position; and a frequency offset compensation portion (423) for estimating a frequency offset to the extracted correlation if the extracted correlation is a predetermined maximum and shifting the phase of pilot data by the estimated frequency offset.

Documents:

00555-cal-2001 abstract.pdf

00555-cal-2001 claims.pdf

00555-cal-2001 correspondence.pdf

00555-cal-2001 description(complete).pdf

00555-cal-2001 drawings.pdf

00555-cal-2001 form-1.pdf

00555-cal-2001 form-18.pdf

00555-cal-2001 form-2.pdf

00555-cal-2001 form-3.pdf

00555-cal-2001 form-5.pdf

00555-cal-2001 g.p.a.pdf

00555-cal-2001 letters patent.pdf

00555-cal-2001 priority document others.pdf

00555-cal-2001 priority document.pdf

00555-cal-2001 reply f.e.r.pdf

555-CAL-2001-FORM-27.pdf