Abstract:
A frequency recovery device and method, the method including a) dividing first data units and corresponding second data units delayed by time intervals, passing only parts of the data units from among the divided data units in the divided intervals, dividing the first and the second data units into four parts each having a 2k-mode interval size, passing each part and the data units contained in a predetermined-sized pull-in range interval positioned at front and rear parts of the data interval, allowing the remaining data to be padded to zero, and generating the padding result; b) generating pilot patterns in the divided data units; c) performing correlation between the padding results of the first and second data units at individual positions of the generated pilot patterns, and generating correlation values; and d) estimating a maximum correlation value from among the correlation values as a frequency offset.

Description:
This application claims the benefit of Korean Patent Application No. P04-0074475, filed on Sep. 17, 2004, which is hereby incorporated by reference as if fully set forth herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a frequency recovery apparatus and method for use in a digital broadcast receiver, and more particularly to a coarse frequency recovery apparatus and method which has very strong resistance to a sampling offset in a Digital Video Broadcasting-Terrestrial (DVB-T) broadcast receiver. 
   2. Discussion of the Related Art 
   Generally, a DVB-T broadcast system can perform data transmission/reception using an Orthogonal Frequency Division Multiplexing (OFDM) scheme. 
   According to the basic principles of the above-mentioned OFDM scheme, a data stream having a high transfer rate is divided into a plurality of data streams, each of which has a low transfer rate, and the divided data streams are simultaneously transmitted using a plurality of sub-carriers. 
   The above-mentioned OFDM scheme has been widely used to transmit/receive broadcast signals due to a variety of advantages, the most important of which is that broadcast signals based on the OFDM scheme have very strong resistance to either frequency selective fading or narrowband interference. 
   A plurality of carrier systems can be efficiently implemented using an Inverse Fast Fourier Transform (IFFT) based on orthogonality among a plurality of carriers. 
   An OFDM receiver basically performs a carrier recovery operation to synchronize a desired signal. A frequency synchronization operation allows a Radio Frequency (RF) carrier frequency of a transmitter to coincide with that of a receiver. 
   The difference between carrier frequencies of the transmitter and the receiver is referred to as “frequency offset”. The OFDM scheme has very weak resistance to the frequency offset. 
   The frequency offset incurs two serious problems in a signal transmitted according to the OFDM scheme. One problem is that the magnitude of a signal transmitted via individual sub-carriers decreases when the signal is demodulated using a Fast Fourier Transform (FFT) process. The other problem is that orthogonality between sub-carriers is no longer maintained due to the occurrence of Inter-Carrier Interference (ICI). 
   In other words, the OFDM scheme has a relatively narrow frequency interval between sub-carriers as compared to a transmission band, such that the sub-carriers may be greatly affected by a small frequency offset. 
   Therefore, when transmitting/receiving a desired signal according to the OFDM scheme, the frequency synchronization technology can be considered to be one of important technologies capable of improving performance of the OFDM receiver. 
   The OFDM scheme controls the frequency synchronization action to be operated in two modes. The above-mentioned two modes are a coarse frequency synchronization mode and a fine frequency synchronization mode. 
   The coarse frequency synchronization mode estimates an integer multiple of a sub-carrier interval closest to an initial frequency offset, and compensates for the estimated result. 
   The fine frequency synchronization mode estimates a frequency offset lower than half of an interval between neighboring sub-carriers, and compensates for a frequency on the basis of the estimated frequency offset. 
   In other words, the coarse frequency synchronization method can allow an initial frequency offset of higher than the sub-carrier interval to be reduced to half of the sub-carrier interval or less. The frequency offset reduced to half of the sub-carrier interval or less can be compensated for using the fine frequency synchronization method. 
   As described above, the coarse frequency synchronization method must estimate an integer multiple of the sub-carrier interval closest to the initial frequency offset. A basic configuration of the above-mentioned coarse frequency offset estimator is shown in  FIG. 1 . 
     FIG. 1  is a block diagram illustrating a conventional coarse frequency offset estimator. 
   The coarse frequency estimator shown in  FIG. 1  performs correlation between FFT output signals and the delayed results of the FFT output signals, such that it determines a changed value of a predetermined pilot pattern, at which the maximum corresponding value is generated, to be a coarse frequency offset. 
   In order to determine the above-mentioned coarse frequency offset, a pilot pattern generator  121  generates 45 pilot patterns at a 2k mode, and generates 175 pilot patterns at an 8k mode. The pilot pattern generator  121  generates the remaining values other than the pilot patterns with padding values of zeros at the 2k mode or at the 8k mode. 
   Using continual pilot informations generated from the pilot pattern generator  121 , current symbols generated from the FFT unit  107 , and one-symbol-delayed symbols generated from the delay  117  are correlated in a cross-correlator  119 , such that their correlation results are generated from the cross-correlator  119 . 
   An estimator  115  receives output values of the correlator  119 , tracks a maximum value from among a plurality of correlation values contained in a data interval, and estimates a coarse frequency offset using position information of the maximum value. 
   The above-mentioned frequency offset is generated because a frequency of a received signal passing through the FFT unit  107  moves on a frequency domain. If there is no frequency offset, the maximum value from among the correlation values is generated at a specific position at which a data interval is terminated. 
   Therefore, the coarse frequency offset can be estimated according to variations of the maximum value position. 
   An accumulator  113  shown in  FIG. 1  compensates for the coarse frequency offset acquired by the above-mentioned estimation process. 
   If a sampling clock offset is generated before the above-mentioned coarse frequency offset estimation method, and the generated sampling clock offset passes through the FFT unit  107 , the FFT unit  107  converts the sampling clock offset into a phase variation value, such that the phase variation value has a negative influence upon the coarse frequency offset estimation result. 
   In other words, generally, if the sampling clock offset is 100 ppm, a single sample is inserted or omitted at intervals of a predetermined distance corresponding to four symbols in the 2k mode. In the case of the 8k mode, a single sample is inserted or omitted at intervals of a predetermined distance corresponding to a single symbol. 
   Therefore, a frequency offset is slightly robust to the sampling clock offset at the 2k mode, but it has very weak resistance to the sampling clock offset at the 2k mode. 
   In other words, the apparatus of the above-mentioned conventional coarse frequency offset estimation and the method thereof has more difficulty in estimating the frequency offset in the 8k mode than in the 2k mode under a predetermined sampling clock offset environment (e.g., if the sampling clock offset is 100 ppm). 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a frequency recovery apparatus and method for use in a digital broadcast receiver that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide an apparatus and method for estimating a coarse frequency offset, which can more correctly estimate a frequency offset regardless of a mode. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a frequency recovery apparatus for use in a digital broadcast receiver for receiving a digital broadcast signal transmitted according to an orthogonal frequency division multiplexing (OFDM) scheme, comprises: a division window for dividing first data units, which are transformed from OFDM symbols into frequency-domain values and the second data units which the first data units are delayed by some intervals respectively, generating divided data units, and passing only some data units from among the divided data units; a pilot pattern generator for generating a pilot pattern associated with data units contained in each of the divided windows; and a cross-correlator for performing correlation between the first data units and the second data units at a individual position of the pilot pattern generated from the pilot pattern generator, and generating the correlation resultant values. 
   Preferably, the OFDM symbols are the symbols in an 8k mode. 
   Preferably, the frequency-domain values are performed by a Fast Fouier Transform operation. 
   Preferably, the second data units are the data, which the first data units are delayed by a single OFDM-symbol interval 
   Preferably, the division window passes only first data units divided into a predetermined-sized data interval, and second data units of a predetermined-sized pull-in range interval positioned at front and rear parts of the divided data interval, allows data of the remaining intervals other than the first and second data units to be padded to zero, and generates the padding results. 
   Preferably, the predetermined-sized data interval has a 2k-mode interval size. 
   Preferably, the size of the pull-in range interval is indicative of a predetermined size corresponding to 170 data units. 
   Preferably, the division window comprises four windows, each of which divides a data interval of the 8k mode into data intervals each having a 2k-mode data interval size. 
   Preferably, the pilot pattern generator comprises four pattern generators, each of which generates pilot patterns associated with the 2k-mode-interval-sized data interval from among the 8k-mode data units, respectively. 
   Preferably, the cross-correlator comprises four correlator, each of which performs cross-correlation between output data of a single window capable of generating the 2k-mode-interval-sized data interval from among the 8k-mode data units and delayed data of the output data of the single window. 
   Preferably, the frequency recovery apparatus for use in the digital broadcast receiver further comprises: an estimator for generating a maximum cross-correlation value from among cross-correlation values generated from the four cross-correlators. 
   In another aspect of the present invention, there is provided a frequency recovery method for a digital broadcast signal transmitted according to an orthogonal frequency division multiplexing (OFDM) scheme, comprising the steps of: a) dividing first data units, which are transformed from OFDM symbols into frequency-domain values and the second data units which the first data units are delays by some intervals respectively, passing only some data units from among the divided data units, allowing the remaining data to be padded to zero, and generating the padding result; b) generating pilot patterns associated with the divided data intervals; c) performing correlation between the padding result of the first data units and the padding result of the second data units at individual positions of the generated pilot patterns associated with the divided data intervals, and generating correlation values; and d) estimating a maximum correlation value from among the correlation values as a frequency offset 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a block diagram illustrating a conventional coarse frequency offset; 
       FIG. 2  shows general continual pilot positions; 
       FIG. 3  shows output signals of a correlator associated with a frequency offset; 
       FIG. 4  is a block diagram illustrating a coarse frequency offset estimator in accordance with a preferred embodiment of the present invention; and 
       FIG. 5  shows exemplary division windows according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 2  shows sub-carrier positions corresponding to continual pilots within a single symbol, which is defined as 1705 samples at the 2k mode and is defined as 6817 samples at the 8k mode. 
   As shown in  FIG. 2 , 45 pilot patterns are generated in the 2k mode, and 175 pilot patterns are generated in the 8k mode. 
   However, in fact, zeroes are inserted into a part of the symbols, such that the data can be FFT/IFFT-processed. The inserted data interval is referred to as a guard band. 
   Therefore, the pilot patterns are contained in 2048 samples extended in the 2k mode, and in 8192 samples extended in the 8k mode. 
     FIG. 3  shows output values of the above-mentioned cross-correlator associated with the frequency offset. 
   As can be seen from  FIG. 3 , if a relative frequency ε is zero, i.e., if there is no frequency offset, a correlation value reaches the maximum correlation value at a specific position at which a symbol is terminated. 
   However, if the frequency offset occurs, the position of the maximum correlation value is changed according to the frequency offset. 
   The relative frequency offset is indicative of a specific value calculated when an actual frequency offset is divided by a sub-carrier interval. 
     FIG. 4  is a block diagram illustrating a coarse frequency offset estimator for use in a frequency recovery apparatus of a digital broadcast receiver in accordance with a preferred embodiment of the present invention. 
   The embodiment of a coarse frequency offset estimator will hereinafter be described with reference to  FIG. 4 . 
   As shown in  FIG. 4 , the embodiment of a coarse frequency offset estimator includes an FFT unit  401 , a delay  403 , four division windows  405 ,  407 ,  409 , and  411 , four division pilot pattern generators  413 ,  415 ,  417 , and  419 , four cross-correlators  421 ,  423 ,  425 , and  427 , and an estimator  429 . The FFT unit  401  receives digital OFDM symbols, performs an FFT process on the received OFDM symbols, and converts the symbols into frequency-domain values. The delay  403  receives current data converted into frequency-domain values, and delays the received data by a single symbol interval. Each of the four division windows  405 ,  407 ,  409 , and  411  receives the current data and the previous data having passed through the delay  403 . In the 8k mode, each of the four division windows  405 ,  407 ,  409 , and  411  passes 1705 data units (or 1704 data units) corresponding to a 2k-mode-data-interval and data units corresponding to a pull-in range, and the remaining data values to be padded to zero. In the 8k mode, each of the division pilot pattern generators  413 ,  415 ,  417 , and  419  generates 45 pilot patterns (or 44 pilot patterns) corresponding to the 2k mode, and allows the remaining pilot patterns to be padded to zero. The cross-correlators  421 ,  423 ,  425 , and  427  allow output values of the division windows  405 ,  407 ,  409 , and  411  to be correlated with output values of the delay  403  at the positions of pilot patterns from the division pilot pattern generators  413 ,  415 ,  417 , and  419 , and finally generate correlation values. The estimator  429  estimates a coarse frequency offset using the maximum correlation value from among a plurality of division correlation values generated from the cross-correlators. 
   Operations of the above-mentioned embodiment of coarse frequency offset estimator according to the present invention will hereinafter be described. 
   The FFT unit  401  receives a plurality of 8k-mode OFDM symbols converted into digital values, performs a Fourier transform on the received OFDM symbols, and generates the resultant values. 
   In this case, if a frequency offset is present in the received OFDM symbols, the OFDM symbols incur a variation in frequency while passing through the FFT unit  401 . 
   If data in a 8k-mode data interval, which are changed to frequency domain, and other data delayed by a single symbol interval passes through the division windows  405 ,  407 ,  409 , and  411 , 1705 data units (or 1704 data units) corresponding to the 2k mode from among the above-mentioned data contained in the intervals and data corresponding to the pull-in range pass through the above-mentioned windows, and the remaining data is filtered. 
   In this case, the pull-in range is indicative of a data interval arbitrarily set to track a position variation of the maximum correlation value at front and rear parts of data interval in the divided window. Generally, the pull-in range is set in the range from −170 to +170. 
     FIG. 5  shows exemplary division windows for use in a frequency recovery apparatus of a digital broadcast receiver according to the present invention. The range of data passing through the above-mentioned windows will hereinafter be described with reference to  FIG. 5 . 
   A single data interval defined as 8192 extended samples simultaneously with including a guard interval in the 8k mode is divided into a plurality of data intervals corresponding to four 2k modes and data intervals corresponding to the pull-in range. 
   A data interval (i.e., data from 0 to 1704) composed of 1705 samples corresponding to the 2k mode, and other data corresponding to the pull-in range present in the direction (±) of the data interval pass through a ¼ window denoted by  405  in  FIG. 3 . The data of the remaining intervals is padded to zero. The data padded to zero is denoted by a character “x”. 
   A data interval (i.e., data from 1705 to 3408) composed of 1704 samples corresponding to the 2k mode, and other data corresponding to the pull-in range present in the direction (±) of the data interval pass through a 2/4 window denoted by  407  in  FIG. 3 . The data of the remaining intervals is padded to zero. 
   In this way, a data interval from 4784 to 6487 and other data corresponding to a data interval present in the direction (±) of the data interval pass through a ¾ window denoted by  409  in  FIG. 3 . A data interval from 6488 to 8191 and other data corresponding to a data interval present in the direction (±) of the data interval pass through a 4/4 window denoted by  411  in  FIG. 3 . 
   The data of the remaining intervals in the data interval from 4874 to 6487 and in the data interval from 6488 to 8191 is padded to zero. 
   Each of the division pilot pattern generators  413 ,  415 ,  417 , and  419  generates 45 pilot patterns (or 44 pilot patterns) corresponding to the 2k mode in the 8k mode. 
   As shown in  FIG. 2 , the ¼ pilot pattern generator  413  generates 45 pilot patterns (i.e., patterns from 0 to 1704) corresponding to the 2k mode from among 177 continual pilot patterns generated in the 8k mode. The remaining parts other than the 45 pilot patterns are padded to zero. 
   The 2/4 pilot pattern generator  415  generates 44 pilot patterns corresponding to the 2k mode, i.e., pilot patterns from 1752 to 3408, and the remaining parts other than the 44 pilot patterns are padded to zero. 
   In this way, the ¾ pilot pattern generator  417  generates only pilot patterns from 3456 to 5112, the 4/4 pilot pattern generator  419  generates only pilot patterns from 5160 to 6816, and the remaining pilot patterns other than the above-mentioned pilot patterns are padded to zero. 
   The cross-correlators  421 ,  423 ,  425 , and  427  recognize position information of pilot patterns generated from the division pilot pattern generators  413 ,  415 ,  417 , and  419 , performs correlation between output values of the division windows  405 ,  407 ,  409 , and  411  and values calculated when output values of the division windows are delayed by a single symbol, and outputs the correlation result. 
   In other words, each of the cross-correlators  421 ,  423 ,  425 , and  427  performs cross-correlation between first data units and second data units at a predetermined position at which pilots having patterns corresponding to the 2k mode are generated, and outputs the cross-correlation result. In this case, the first data units are contained in one of four parts of the data interval having 8192 samples of the 8k mode, and the second data units are generated when the first data units are delayed by a single symbol. 
   Individual correlation calculation values generated from the four cross-correlators  421 ,  423 ,  425 , and  427  are applied to the estimator  429 . 
   The estimator  429  estimates a frequency offset using a single maximum value from among four division correlation maximum values. 
   The reason why the frequency offset is estimated using the above-mentioned method is that the method for estimating the frequency offset using the maximum value of the division correlation values has the highest reliability. 
   In this way, the coarse frequency offset estimation method according to the present invention divides a correlation interval of the 8k mode into 2k-mode intervals, and performs a frequency offset estimation process, such that individual phase variations are reduced, resulting in reduction of a negative influence caused by a sampling clock offset. 
   Therefore, the influence of the sampling clock offset is also reduced, resulting in reduction of frequency offset estimation errors. 
   As apparent from the above description, an apparatus and method for estimating a frequency offset according to the present invention can more correctly estimate a frequency offset even in the 8k mode, regardless of a mode. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.