PATENT ABSTRACT
A circuit for detecting a guard interval in an orthogonal frequency division multiplication (OFDM) communication system, using two successive autocorrelation calculations for a received sample train. The circuit determines a guard interval by calculating the maximum correlation value first autocorrelation calculation step (delaying an OFDM signal by the length of valid data N, the useful data duration); and a second autocorrelation calculation step (delaying the OFDM signal by a guard interval detection period (N+G 1 ), the sum of the valid data length (N) and the minimum guard interval (GI)). Performing two autocorrelation calculation steps provides improved detection of the guard interval, being free from noises and distortions and producing the maximum correlation value and position more distinctly.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 of Korean Patent Application 2005-00273 filed on Jan. 3, 2005, the entirety of which is hereby incorporated by reference. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to orthogonal frequency division multiplication (OFDM) data transmission systems, and more particularly to methods and circuits for detecting and deciding the length of guard intervals included in received signals, and receivers including the circuits. 
   2. Description of the Related Art 
   OFDM systems are widely used and regarded as being advantageous for high-frequency data transmission through wired and wireless communication channels such digital audio broadcasts, digital television (DTV), and wireless local area networks (WLAN). OFDM is already part of WLAN, DVB (Digital Video Broadcast), and BWA (broadband wireless access) standards, and is a strong candidate for some of the fourth generation (4G) wireless telephone technologies. While a conventional (non-OFDM) communication system performs high-frequency data transmission using a single carrier (frequency), OFDM systems use multiple carriers (a multi-carrier scheme) having mutual orthogonality, so that it is possible to reduce the data rate on each carrier (e.g., by increasing the symbol period of each carrier wave) by the number of sub-carrier waves while retaining current data transmission speed, and to mitigate symbol interference due to the multi-path phenomena. 
   Synchronization for demodulating OFDM signals needs symbol timing sync, carrier wave sync, and sampling frequency sync. Correct synchronization is an important factor to determine the reliability of the system and the efficiency of data transmission. Since the OFDM scheme demodulates a received signal (including a succession of OFDM symbols) using a form of fast Fourier transform (FFT), it is necessary to complete a correct symbol timing sync operation to define a (symbol) period to operate the FFT. 
     FIG. 1  shows the structure of an OFDM symbol used in an OFDM system. The “OFDM symbol period” is composed of the useful data duration (N samples) not including the guard interval duration (e.g., G 1 ). Useful information (e.g., WLAN transmission data) is positioned within the useful data duration (N samples), and the guard interval contains a copy of a portion of the useful data that is used as a cyclic prefix for preventing symbol interference caused in the environment of transmission. 
   Cyclic prefix is a crucial feature of OFDM used to combat the inter-symbol-interference (ISI) and inter-channel-interference (ICI) introduced by the multi-path channel through which the signal is propagated. The basic idea is to replicate part of the OFDM time-domain waveform from the back to the front to create a guard interval. The duration of the guard interval (the guard interval duration) T g  should be longer than the worst-case delay spread of the target multi-path environment. At the receiver side, data in the guard interval (where symbols are expected to interfere with each other by multi-path) is ignored, and the rest of the duration is demodulated by OFDM. The inserted waveform is called the “Guard Interval”, and its length is called “Guard Interval Duration”. 
   The data of the guard interval generally copies the (data) value of a portion of the symbol (in the useful data duration). The length of the guard interval (guard interval duration) is varied by the transmitter in accordance with the length of (expected) symbol interference (SI) generated in the environment of transmission. In order to absorb multi-path interference, the copied waveform of the OFDM symbols is inserted (in the guard interval) to effectively separate consecutive OFDM symbols by a length (guard interval) based on the expected delay of the multi-path interference. The effect of the inter-symbol interference can be minimized to assure reliable communication if the guard interval is long enough. Hereinafter, it is assumed that the useful data duration has the length of N samples and the guard interval has the length of G 1 , G 2 , G 3 , or G 4  samples. The length of the guard interval is generally N/32(G 1 ), N/16(G 2 ), N/8(G 3 ), and N/4(G 4 ) ( i.e., a selected fraction of the length of the symbol, the useful data duration, N). 
   In the standard of European digital television broadcasting system, there are 2048 samples in a useful data period (one symbol), thus N=2048. Thus, it can be seen that G 1 =2048/32=64 samples, G 2 =120 samples, G 3 =256 samples, and G 4 =512 samples. The guard interval data is defined by copying a part of the useful data (symbol) and thus has a large correlation with the copied portion of the delayed useful data. 
     FIG. 2  is a block diagram of a conventional guard interval detection circuit. The guard interval detection circuit includes a correlator (autocorrelator)  210 , a peak detector  240 , a peak-to-peak interval detector  250 , and a guard interval decider  260 . 
   The correlator  210  includes a delay circuit delaying a received signal by the useful data interval size (N samples), a complex conjugating circuit  212  converting a delayed complex signal into a complex conjugate signal, a multiplier  213  performing a complex-multiplication operation on the delayed complex signals, and a sliding adder  249  adding the multiplied complex sample trains output from the multiplier within a window having the length of the minimum guard interval G 1 . With this construction, the conventional guard interval detection circuit obtains correlation (autocorrelation) values using the complex multiplication of received sampled data trains with the complex conjugate of N-sample delayed values of the received sampled data trains. 
   The delay circuit  211  is a kind of memory (e.g., shift register) delaying a sample train by a predetermined time (e.g., by N samples). The delay time is variable based on symbol length (N) for various OFDM systems. 
   The complex conjugator  212  converts the delayed received complex sample train into its conjugate complex value train. 
   The multiplier  213  performs multiplication of the non-delayed received sample train and its delayed complex conjugate train. 
   The sliding adder  214  adds (and accumulates) the product of the multiplication (of the received sample train and its delayed complex conjugate sample train) output from the multiplier  213 , within the window having the width of the shortest guard interval length (G 1 ), and stores the result (sums) in a memory that shifts (delays) the operation by one sample interval. If the non-delayed received sample train and the delayed complex conjugate of the received sample train are the same (high correlation), a complex sum train having a relatively large absolute value is generated from the sliding adder indicating a high correlation. Conversely, the multiplication of completely non-correlative sample trains generates a relatively small complex value. 
   The output of the sliding adder  214  is defined in Equation 1 as follows: 
   
     
       
         
           
             
               
                 
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                 [ 
                 
                   Equation 
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   In Equation 1, the parameter y denotes a complex input sample, and the parameter R is a result of sliding-addition of the products (of multiplication of the non-delayed input sample by the complex conjugate of the input signal delayed by N samples) within a window having the minimum guard interval length (GI=G 1 ). The parameter n represents an order of input samples. As the result (sum) of the sliding addition is also represented as a complex sample train, it is difficult to detect the sample size by the conventional guard interval detection circuit. 
   In the convention guard interval detection circuit, the length of the current guard intervals is obtained, after taking the absolute value of Rn and detecting the location of the maximum value, from operating the detected point throughout several OFDM symbols and utilizing a displacement value of the maximum value location. The guard interval&#39;s length is estimated by detecting a starting point accompanying a significant amplitude supposed to correspond to a guard interval with large correlation. 
   The peak detector  240  stores data, obtained from the correlator  210  in a sliding window (data frame), (in a memory) and identifies the largest stored value as a peak value. 
   The peak-to-peak interval detector  250  finds the maximum value position stored in the peak detector  240  and stores the detected position value in a memory. 
   The guard interval decider  260  detects the guard interval length by using displacement values of the maximum value position stored in the peak-to-peak interval detector  250 . 
   Thus, the guard interval length is determined from finding a peak point having the largest correlation value by performing a self-correlation (autocorrelation) operation (correlating the input sample train with a copy of the input sample train delayed by the useful data duration of the input sample train). This utilizes the characteristic of the input signal train that the guard interval&#39;s data is set by copying a portion of the useful data and thus has large correlation with the delayed input signal train (itself, delayed). 
   In the conventional guard interval detector configuration of  FIG. 2 , detecting the guard interval length using one autocorrelation calculation and the maximum value of the result of the autocorrelation calculation, it has been difficult to assure the performance of position estimation for the maximum value, due to noises and distortions normally existing in the transmission channel. Further, as the interval including the maximum value is typically not a distinct sharp point but rather a lengthened time period (planar, platau) during the correlation calculation, it has been insufficient to estimate a guard interval length correctly. Therefore, it is necessary to increase the performance of a guard interval length estimation in an OFDM system using a guard interval affected by a channel condition, and to enhance the reliability of a timing sync in a receiver using correct detection of the guard interval. 
   SUMMARY OF THE INVENTION 
   Exemplary embodiments of the present invention provide a method and an apparatus for correctly and reliably detecting a guard interval by including a correlator operable with a regularizing step in guard interval detecting circuit. 
   An aspect of the invention is a guard interval detection circuit of an orthogonal frequency division multiplication (OFDM) system, comprising: a first (auto)correlator (delaying a received sample train by a first time and obtaining a correlation value between the received sample train and the delayed sample train); and a second (auto)correlator (delaying the output of the first (auto)correlator by a second time and obtaining a correlation value between the output of the first correlator and the delayed output of the first correlator). The guard interval detection circuit may further comprise a detector finding the maximum value of the output of the second (auto)correlator and a point (e.g., precise position) of the maximum value and deciding therefrom the type of the current guard interval. 
   A “correlator” herein may have the same architecture as the (autocorrelator) circuit  210  aforementioned in operation and structure, but its input, output, and delay is variable. 
   In a preferred embodiment, the first (auto)correlator performs conventionally in the same manner as defined in the Equation 1 (including a step of complex multiplication with an input sample train and a delayed complex conjugate of the input sample train delayed by one symbol length N, and a step of sliding addition within a window having the length of the minimum guard interval). This operation is carried out in the same as defined in the Equation 1. The output of such a conventional autocorrelator may then be regularized by multiplication with the output of a regularizer, which may function to filter Additive White Gaussian Noise (AWGN). 
   The regularizer obtains the power value for the input complex sample train, performs sliding addition within a window having the length of the minimum guard interval, and inverts the sum and outputs the result as a fraction between 0 and 1. 
   The second (auto)correlator performs a self-correlation (autocorrelation) calculation with the regularized sample train and the regularized sample train delayed by the guard interval detection period. The guard interval detection period is the sum of the OFDM symbol period (useful data duration, N samples) and the minimum guard interval duration (G 1 ). 
   Next, power value is obtained by a squaring circuit generating the magnitude of the output of the second (auto)correlator. 
   With the exemplary configuration described above, the two steps of (auto)correlation calculation effectively removes the influence of less correlative AWGN (Additive zero-mean (White) Gaussian Noise) or distortions, and obtains a more distinct peak pattern for precise guard interval detection. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention defined in the claims to those skilled in the art. Like numerals refer to like elements throughout the specification. In the drawings: 
       FIG. 1  is a schematic diagram illustrating the structure of OFDM symbol and the types of guard intervals; 
       FIG. 2  is a block diagram of a conventional guard interval detection circuit; 
       FIG. 3  is a block diagram of a guard interval detection circuit in accordance with an embodiment of the invention; 
       FIG. 4  is a block diagram of the regularizer  380  shown in  FIG. 3 ; and 
       FIG. 5  is a timing diagram illustrating signal flow and operation of the circuits shown in  FIGS. 3 and 4 . 
     In  FIG. 5 , the numeric labels (e.g., 64, 128, 192, 384, 448, 576, 896, 1344, and 2048) are dimensions (measured in sampling period T) and not distinct element reference numerals. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 3  is a functional block diagram of a guard interval detection circuit in accordance with an embodiment of the invention. The guard interval detection circuit is comprised of a first (auto)correlator  310 , a regularizer  380 , a second (auto)correlator  320 , a power evaluator  330 , a peak detector  340  (e.g., the same as conventional peak detector  240 ), a peak-to-peak detector  350  (e.g., the same as conventional peak-to-peak detector  250 ), a guard interval decider  360 , and a guard interval table  370 . 
   The first (auto)correlator  310  performs a conventional correlation step (including the substeps of: complex multiplication of an input sample train and a the complex conjugate of a delayed copy of the input sample train (delayed by one symbol length, N samples), and the substep of performing sliding addition within a window having the length of the minimum guard interval (G 1 ). This correlation operation is carried out in the same manner as defined in the former Equation 1. 
   The regularizer  380  obtains and squares the absolute value of the input sample train, representing the result as a fraction. The operation and structure of the regularizer  380  will be further described in conjunction with  FIG. 4  below. 
   The multiplier  390  makes the signal processing operation easier by regularizing sample trains to a magnitude of between 0˜1 as a result of the correlation calculation (by multiplying the outputs of the first correlator  310  and regularizer  380 ). The resultant value output from the multiplier  390  is defined by Equation 2. 
   
     
       
         
           
             
               
                 
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   In equation 2, the parameter C n  is the value obtained from regularizing complex sample trains of R n . 
   The second correlator  320  operates in the same manner as the first correlator  310  except that the delay is the sum of the useful data duration (N) and the minimum guard interval (G 1 ). 
   The power evaluator  330  squares the absolute value of the output sample train of the second correlator  320 , and squaring the absolute value results in a train of real numbers. 
   The peak detector  340  inputs the real-number train from the power evaluator  330 , and stores the maximum peak values every N+G 1  time and stores the peak times (times at which a maximum peak value is present). 
   The peak-to-peak detector  350  calculates intervals from the starting points a periodic reference time of N+G 1  to the peak points (peak times, from detected peak points), which are stored by peak detector  340 , every N+G 1  time. 
   The guard interval decider  360  outputs a signal indicating the type of the guard interval based on comparing differential values of the time intervals between every reference time N+G 1  and the peak points, which are output from the peak-to-peak detector, with the index of the guard interval table  370 . 
   The guard interval table  370  corresponds to a memory storing the (expected) time intervals of the peak times every guard interval detection period. The stored time interval to be generated every symbol will be referred with Table 1 described later. 
   The guard interval detection circuit according to an exemplary embodiment of the invention performs the conventional self-correlation (autocorrelation) calculation by way of conjugate complex multiplication with an original sample train and a delayed sample train (delayed by the useful data duration (N samples)) in the first correlator  310 , and regularizes the result of the conventional correlation calculation. And then, the regularized correlation result is put into the second correlator  320  which performs a secondary correlation calculation with a delay of N+G 1  (the sum of the useful data duration and the minimum guard interval). The secondary correlation result is used by the power evaluator  330  to output the real-number train of power values. The real-number train of power values is used in the peak-to-peak detector  350  for detecting the maximum (peak) points in the peak detector  340 , and for obtaining the intervals from detection starting points of guard intervals (by calculating intervals between the maximum (peak) points within every (guard interval) detection period N+G 1 . The guard interval decider  360  outputs information of the guard interval based on identifying an indexing value of the guard interval length table 30, which matches with the resultant value of the peak-to-peak detector  350 , as the corresponding guard interval, 
     FIG. 4  is a block diagram of the regularizer  380  shown in  FIG. 3 . Referring to  FIG. 4 , the regularizer  380  is comprised of a power evaluator  381  (that obtains a real-number power value for each input sample), a sliding adder  382  (performing addition with each sample within the window of the minimum guard interval G 1 ), and a fraction operating circuit (divider)  383  (dividing the sum of the output power values from the sliding adder  382  by the complex sample train output from the first correlator  310 ). 
     FIG. 5  is a timing diagram illustrating sequential steps of a method of processing received input signals and the method of operation of the circuits shown in  FIGS. 3 and 4 , for 4 types of guard interval (1) 1/32, 2) 1/16, 3) ⅛, 4) ¼. 
   Referring to  FIG. 5 , the method of processing signals is performed by way of the sequential steps A(AStep), B(BStep), C(CStep), D(DStep), E(EStep), F(FStep), and G(GStep), wherein: 
   Step A represents a received sample train; Step B represents a sample train delayed from the sample train of the step B by the useful data duration of N samples; Step C represents complex a sample train as the result of correlation calculation with the sample train of the steps A and B and sliding addition; Step D shows a signal form of the absolute value for the complex sample train produced in step C; Step E represents a complex sample train (a delayed copy of the complex sample train of the step C, delayed by the interval N+G 1  that corresponds to a sum of the useful data duration and the minimum guard interval); Step F represents the sample train resulting from a second correlation calculation with the complex sample train of the step C and the (delayed) complex sample train of the step E (delayed version of the sample trains of the step C,); and Step G represents distribution of the real-number power data obtained from evaluation of the power of the resultant sample train of the correlation calculation by the step F. 
   As illustrated in  FIG. 5 , it can be seen that the result of the correlation calculation depends upon the mode of the guard intervals. For the various types of the guard intervals, (i.e., 1/32, 1/16, ⅛, and ¼ modes), the step D represents the distribution profile of peaks when performing the first correlation calculation. And, the peak distribution after the second correlation calculation is shown in the step G for the various types of the guard intervals. The magnitude of peak appearing in the (guard interval) detection period N+G 1  after the received sample train passes through the circuits of the  FIG. 3  is configured with a distinct topological pattern as shown in the step D. The conventional peak pattern in the step D has been insufficient to be detected because the peaks are configured flat in the 1/32 mode. However, as the pattern of the sample train after the two correlation calculation processes has a more distinct (e.g., pointed) peak value, this distinction makes the detection more precise and easy. This feature of the embodiment of the invention is helpful for improving the performance of estimating the guard interval because detecting the guard interval through the two steps of correlation calculation greatly reduces the effects of noises and distortions. 
   The numbers shown in  FIG. 5  represent dimensions such as detection periods, delay intervals, and the number of samples in the standard of the European digital broadcasting system. These data are summarized as indexed and shown in Table 1 as follows: 
   
     
       
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Interval between peak and every periodic detection starting point (N + G1) 
             
           
        
         
             
               Guard interval 
               Symbol 
               Symbol 
               Symbol 
               Symbol 
               Symbol 
             
             
               type 
               period 1 
               period 2 
               period 3 
               period 4 
               period 5 
             
             
                 
             
             
                1/32 mode 
               0 
               0 
               0 
               0 
               0 
             
             
               (G1 = 64 samples) 
             
             
                1/16 mode 
               G1 (64) 
               2G1 (128) 
               3G1 (192) 
               4G1 (256) 
               5G1 (320) 
             
             
               (G2 = 128 samples) 
             
             
               ⅛ mode 
               3G1 (192) 
               6G1 (384) 
               9G1 (576) 
               12G1 (768)  
               15G1 (960)  
             
             
               (G3 = 256 samples) 
             
             
               ¼ mode 
               7G1 (448) 
               14G1 (896) 
               21G1 (1344) 
               28G1 (1792) 
               35G1 (2240) 
             
             
               (G4 = 512 samples) 
             
             
                 
             
           
        
       
     
   
   The guard interval decider  360  ( FIG. 3 ) determines the current type of the guard intervals by comparing the peak intervals, measured from peak detection (reference period N+G 1 ) starting points, with reference to the expected time intervals stored in Table 1. 
   Thus, a method of the invention is comprised performing the first self-correlation (autocorrelation) calculation of the original OFDM sample train and a delayed OFDM sample train (delayed by one of the useful data duration (i.e., delayed one OFDM symbol, N), and then performing a second self-correlation (autocorrelation) calculation of the (regularized, averaged, filtered) first autocorrelation train and a first autocorrelation train delayed by one sample length (N). A step of regularizing (averaging) the result of the first self-correlation (autocorrelation) calculation may be performed before performing the second self-correlation (autocorrelation) calculation of the (regularized, averaged, filtered) first autocorrelation train and the (regularized, averaged, filtered) first autocorrelation train delayed by one sample length. And, the method detects the type of the current guard intervals based on calculating the time intervals between the peak energy of the second autocorrelation calculation, (e.g., by comparison with the detection starting points (and to a periodic reference interval having a period of N+G 1 ). This improves the precision and reliability of timing synchronization. Performing the successive two steps of autocorrelation calculations provides an improvement of performance in detecting guard intervals for a OFDM receiver that effectively eliminates the influences of less correlative AWGN or distortions flowing into the system, and greatly increases the magnitude of the peak for detection, and generates the peak values to be easily detected rather than the conventional plane (flat-topped) results. 
   Although the present invention has been described in connection with the exemplary embodiments of the present invention illustrated in the accompanying drawings, it is not limited thereto. It will be apparent to those skilled in the art that various substitutions, modifications and changes may be thereto without departing from the scope and spirit of the invention. 
   According to the aforementioned description about the invention, the guard interval detection circuit of the OFDM receiver includes a signal processing procedure performing the two steps of (auto)correlation calculations for a received sample train, which enables obtaining a more correct result. Performing two steps of correlation calculations improves the performance of the OFDM system by correctly detecting guard intervals for the system and effectively blocking the influence of less correlative AWGN or distortions flowing into the system and providing a more distinct peak pattern.