Abstract:
A method of receiving an orthogonal frequency division multiplexing (OFDM) signal. The method includes receiving sample signals from the OFDM signal, providing reference sample signals, calculating a first value representing a first correlation between the sample signals and different sample signals, calculating a second value representing a second correlation between the reference sample signals and different reference sample signals, the reference sample signals and the different reference sample signals corresponding to the sample signals and the different sample signals, calculating a third value from the first value and the second value, and detecting the OFDM signal in response to the third value.

Description:
BACKGROUND OF THE INVENTION 
   This application claims the benefit of Korean Patent Application No. 10-2005-0007223, filed on Jan. 26, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   1. Field of the Invention 
   This application relates to methods of receiving an Orthogonal Frequency Division Multiplexing (OFDM) signal, and more particularly, to methods of receiving an OFDM signal having a repetitive preamble signal, detecting the OFDM signal and performing a signal synchronization. 
   2. Description of the Related Art 
   An Orthogonal Frequency Division Multiplexing (OFDM) method is widely used to transmit and receive data through a channel. 
   In the OFDM method, a preamble signal having a repetitive characteristic is transmitted as part of an OFDM signal. Signal synchronization is performed in a receiving end using the transmitted preamble signal. 
   Conventionally, the OFDM signal may be received using a level detection method using a delay correlation, or a matched point detection method using a matched filter. 
   However, the receiving end using the level detection method usually malfunctions due to a large change in a correlation value in an additive white gausian noise (AWGN) environment. In addition, the level detection method requires computation for a signal synchronization. 
   In the matched point detection method, a matched filter peak point cannot be detected due to an existence of a frequency offset. 
   SUMMARY OF THE INVENTION 
   Embodiments include a method of receiving an orthogonal frequency division multiplexing (OFDM) signal includes receiving sample signals from the OFDM signal, providing reference sample signals, calculating a first value representing a first correlation between the sample signals and different sample signals, calculating a second value representing a second correlation between the reference sample signals and different reference sample signals, the reference sample signals and the different reference sample signals corresponding to the sample signals and the different sample signals, calculating a third value from the first value and the second value, and detecting the OFDM signal in response to the third value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  illustrates a frame format of an Orthogonal Frequency Division Multiplexing (OFDM) signal; 
       FIG. 2  illustrates the relationship between a correlation result using a delay correlation and the frame of the OFDM signal; 
       FIG. 3  illustrates the relationship between a correlation result using a matched filter method and the frame of the OFDM signal; 
       FIG. 4  is a block diagram illustrating an apparatus for receiving an OFDM signal using the matched filter method; 
       FIG. 5  is a block diagram illustrating an apparatus for receiving an OFDM signal according to an embodiment; 
       FIG. 6  illustrates the relationship between a correlation result using a differential correlation matching method according to an embodiment and the frame of the OFDM signal; 
       FIG. 7  illustrates an initial window field in a waveform of the correlation result according to an embodiment; 
       FIG. 8  illustrates an adjusted initial window field; and 
       FIG. 9  illustrates displacements of peak points in a multi-path environment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. Like reference numerals in the drawings denote like elements throughout the drawings. 
     FIG. 1  illustrates a frame format of an Orthogonal Frequency Division Multiplexing (OFDM) signal. Specifically,  FIG. 1  illustrates an OFDM frame format  10  having a repetitive preamble characteristic used for a wireless local area network (LAN) communication based on IEEE 802.11x standards. Referring to  FIG. 1 , the OFDM frame format  10  consists of a short preamble field  11 , a guard interval field  12 , a long preamble field  13 , and a valid data field  14 . 
   Each of preamble symbols t 1 ˜t 10  of the short preamble field  11  has sixteen (16) sample signals. Each of preamble symbols T 1  and T 2  of the long preamble field  13  has sixty-four (64) sample signals. 
   The short preamble field  11  having the repetitive preamble characteristic is used to detect and synchronize the OFDM signal. 
   An apparatus for receiving the OFMD signal checks whether the OFDM signal is received via a level detection method using a delay correlation, or a matched point detection method using a matched filter. 
   The delay relation method uses Equation 1 below 
                   ∑     k   =   0       k   =     m   -   1         ⁢       x   k     ⁢     x     k   -   m     *               (   1   )               
wherein, x k  is a k th  sample signal in a current preamble symbol of the received OFDM signal, x* k-m  is a complex conjugate of a sample signal in a previous preamble symbol corresponding to the x k  sample signal, and m is the number of samples in a preamble signal. For example, if k=3 and m=16, x 3  is a 3 rd  sample signal in the current preamble symbol, and x* 3-16  is a complex conjugate of a 3 rd  sample signal in a previous preamble symbol.
 
   Values obtained by multiplying preamble sample signals in the current preamble symbol by corresponding complex conjugates of preamble sample signals in the previous preamble symbol are summed up, resulting in a correlation result shown in  FIG. 2 . 
     FIG. 2  illustrates the relationship between the correlation result using the delay correlation and the frame of the OFDM signal. Referring to  FIG. 2 , when the OFDM signal is received, a value of the correlation result is greater than a predetermined threshold value in the short preamble field and less than the predetermined threshold value in other fields. 
   Specifically, the level detection method using the delay correlation calculates a delay correlation value using Equation 1, compares the calculated delay correlation value with the predetermined threshold value, and determines whether the OFDM signal is received. 
   The method requires a relatively simple hardware structure having a small size, and partly shares computations with a frequency offset algorithm. However, the method requires an additional computation to perform signal synchronization and makes it difficult to properly determine the threshold value due to a great change in the correlation value in a degradation of a signal-to-noise ratio (SNR) margin, causing an erroneous detection of the OFDM signal. 
   The matched filter method uses Equation 2 below 
   
     
       
         
           
             
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     k 
                     = 
                     
                       m 
                       - 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     x 
                     k 
                   
                   ⁢ 
                   
                     r 
                     k 
                     * 
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   wherein, x k  is a k th  sample signal in a preamble symbol in a receiving end, r k  is a reference signal corresponding to the x k  sample signal in a transmitting end, and m is the number of samples in a preamble signal. 
   Values obtained by multiplying preamble sample signals by complex conjugates of reference signals corresponding to the preamble sample signals are summed up, resulting in a correlation result shown in  FIG. 3 . 
     FIG. 3  illustrates the relationship between a correlation result using the matched filter method and the frame of the OFDM signal. Referring to  FIG. 3 , when the OFDM signal is received, a value of the correlation result has a peak shape in the short preamble field. Specifically, the matched point detection method detects periodic peaks of the correlation result using Equation 2, detects the OFDM signal, and performs the signal synchronization. 
     FIG. 4  is a block diagram illustrating an apparatus for receiving an OFDM signal using the matched filter method. Referring to  FIG. 4 , a computation unit  41  multiplies real number components and imaginary number components of a received preamble signal Xin by real number components R 0 _re through R 15 _re and imaginary number components R 0 _im through R 15 _im of each reference signal, sums up the multiplied values, and outputs a real number component Corr_re of the correlation result obtained by Equation 2. A controller  42  receives the real number component Corr_re and performs a detection of the OFDM signal Mode_detect and a signal synchronization Timing_acq. 
   Even though such a matched filter method must perform multiplication, it can stably perform the OFDM signal detection Mode_detect and the signal synchronization Timing_acq in an additive white gausian noise (AWGN) environment. However, when the received preamble signal Xin includes a frequency offset, the periodic peaks of the correlation result shown in  FIG. 3  disappear. 
   Differential correlation matching is calculated using Equation 3 below 
   
     
       
         
           
             
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     k 
                     = 
                     
                       m 
                       - 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     x 
                     k 
                   
                   ⁢ 
                   
                     x 
                     
                       k 
                       - 
                       1 
                     
                     * 
                   
                   ⁢ 
                   
                     r 
                     k 
                   
                   ⁢ 
                   
                     r 
                     
                       k 
                       - 
                       1 
                     
                     * 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   wherein, x k  is a k th  sample signal of a received preamble signal, x k−1  is a k−1 th  sample signal of the received preamble signal, r k  is a k th  sample signal of a reference signal, r k−1  is a k−1 th  sample signal of the reference signal, m is the number of samples in the preamble signal in symbol units, and * denotes a complex conjugate. 
   The differential correlation matching is calculated by multiplying correlation values of the two adjacent reference signals r k  and r k−1  by correlation values of the input signals x k  and x k−1  and summing up the multiplied values. 
   For example, when the received preamble signal includes the frequency offset Δω and phase offset Δθ, x k =r k e jΔωk  and, x k−1 =r k−1 e jΔθ . Δθ may denote a common phase noise at the beginning of a correlation computation. 
   Equation 4 is as follows. 
   
     
       
         
           
             
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     k 
                     = 
                     
                       m 
                       - 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     r 
                     k 
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       j 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       d 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       k 
                     
                   
                   ⁢ 
                   
                     r 
                     
                       k 
                       - 
                       1 
                     
                     * 
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       
                         - 
                         jΔω 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       k 
                     
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       - 
                       jΔθ 
                     
                   
                   ⁢ 
                   
                     r 
                     k 
                   
                   ⁢ 
                   
                     r 
                     
                       k 
                       - 
                       1 
                     
                     * 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   After canceling terms in Equation 5, the summed value is given by Equation 5, 
   
     
       
         
           
             
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     k 
                     = 
                     
                       m 
                       - 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     r 
                     k 
                   
                   ⁢ 
                   
                     r 
                     
                       k 
                       - 
                       1 
                     
                     * 
                   
                   ⁢ 
                   
                     r 
                     k 
                   
                   ⁢ 
                   
                     r 
                     
                       k 
                       - 
                       1 
                     
                     * 
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       - 
                       jΔθ 
                     
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   The real number component is separated from the result of Equation 5, and is given by Equation 6, 
   
     
       
         
           
             
               
                 
                   ∑ 
                   
                     k 
                     ⁢ 
                     
                         
                     
                     = 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   
                     k 
                     ⁢ 
                     
                         
                     
                     = 
                     
                         
                     
                     ⁢ 
                     
                       m 
                       ⁢ 
                       
                           
                       
                       - 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     ( 
                     
                       
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     
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                                           ( 
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                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         re 
                                       
                                     
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                                       r 
                                       
                                         
                                           ( 
                                           
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                                         ⁢ 
                                         
                                             
                                         
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                                         re 
                                       
                                     
                                   
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                                           ( 
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                             ⁢ 
                             
                                 
                             
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                               ( 
                               
                                 
                                   
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                                       ⁢ 
                                       
                                           
                                       
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                                       im 
                                     
                                   
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                                     r 
                                     
                                       
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                                           ⁢ 
                                           
                                               
                                           
                                           - 
                                           
                                               
                                           
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                                       ⁢ 
                                       
                                           
                                       
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                                       re 
                                     
                                   
                                 
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                                     r 
                                     
                                       
                                         ( 
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                                         ) 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       re 
                                     
                                   
                                   ⁢ 
                                   
                                       
                                   
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                                     r 
                                     
                                       
                                         ( 
                                         
                                           k 
                                           ⁢ 
                                           
                                               
                                           
                                           - 
                                           
                                               
                                           
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                                           1 
                                         
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                                       im 
                                     
                                   
                                 
                               
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                             2 
                           
                         
                       
                     
                     ⁢ 
                     
                         
                     
                     ) 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Δθ 
                 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
     FIG. 5  illustrates the constitution of an apparatus  50  for receiving an OFDM signal according to an embodiment of the present invention. Referring to  FIG. 5 , the apparatus  50  for receiving the OFDM signal includes a computation unit  51  and a controller  52 . The computation unit  51  receives the preamble signal Xin having a repetitive characteristic and obtains the computation results Corr_re using Equation 6. The controller  52  performs the detection of the OFDM signal Mode_detect and the signal synchronization Timing_acq using the apparatus  50  for receiving an OFDM signal. 
   The computation unit  51  includes delay units  53 , multiplication units  54 , summation units  55 ,  56  and  57 , and a correlation computation unit  58 . The multiplication units  54  multiply the preamble signal Xin by a reference signal. The first summation unit  55  calculates a first computation result Corr_re_re and the second summation unit  56  calculates a second computation result Corr_im_im. The third summation unit  57  sums up the first computation result Corr_re_re and the second computation result Corr_im_im and calculates a third computation result Corr_re. The third computation result Corr_re is given in Equation 6. The first computation result Corr_re_re and the second computation result Corr_im_im are given by Equations 7 and 8, respectively. 
   
     
       
         
           
             
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     k 
                     = 
                     
                       m 
                       - 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     
                       ( 
                       
                         
                           
                             r 
                             
                               
                                 ( 
                                 k 
                                 ) 
                               
                               ⁢ 
                               re 
                             
                           
                           ⁢ 
                           
                             r 
                             
                               
                                 ( 
                                 
                                   k 
                                   - 
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                                 ) 
                               
                               ⁢ 
                               re 
                             
                           
                         
                         + 
                         
                           
                             r 
                             
                               
                                 ( 
                                 k 
                                 ) 
                               
                               ⁢ 
                               im 
                             
                           
                           ⁢ 
                           
                             r 
                             
                               
                                 ( 
                                 
                                   k 
                                   - 
                                   1 
                                 
                                 ) 
                               
                               ⁢ 
                               im 
                             
                           
                         
                       
                       ) 
                     
                     2 
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Δθ 
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
           
             
               
                 
                   ∑ 
                   
                     k 
                     = 
                     0 
                   
                   
                     k 
                     = 
                     
                       m 
                       - 
                       1 
                     
                   
                 
                 ⁢ 
                 
                   
                     
                       ( 
                       
                         
                           
                             r 
                             
                               
                                 ( 
                                 k 
                                 ) 
                               
                               ⁢ 
                               im 
                             
                           
                           ⁢ 
                           
                             r 
                             
                               
                                 ( 
                                 
                                   k 
                                   - 
                                   1 
                                 
                                 ) 
                               
                               ⁢ 
                               re 
                             
                           
                         
                         - 
                         
                           
                             r 
                             
                               
                                 ( 
                                 k 
                                 ) 
                               
                               ⁢ 
                               re 
                             
                           
                           ⁢ 
                           
                             r 
                             
                               
                                 ( 
                                 
                                   k 
                                   - 
                                   1 
                                 
                                 ) 
                               
                               ⁢ 
                               im 
                             
                           
                         
                       
                       ) 
                     
                     2 
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Δθ 
                 
               
             
             
               
                 ( 
                 8 
                 ) 
               
             
           
         
       
     
   
   In Equation 7, values obtained by multiplying the real number components and multiplying the imaginary number components of two adjacent signals in Equation 6 are summed up. In Equation 8, values obtained by multiplying the real number components and the imaginary number components of two adjacent signals in Equation 6 are summed up. 
     FIG. 6  illustrates the relationship between a correlation result using the differential correlation matching method and the frame of the OFDM signal. The differential correlation matching method may be used to generate peak points or lowest points in a waveform of the correlation result even though the preamble signal includes the frequency offset. 
   Referring to  FIG. 6 , the first computation result Cor_re_re, the second computation result _im _im, and the third computation result Corr_re are given in Equations 7, 8 and 6, respectively. The correlation result has peak points in the preamble field of the frame of the OFDM signal. The correlation between the preamble signal and the reference signal disappears during the guard interval field after the preamble field and accordingly the peak points disappear. 
   The method of receiving the OFDM signal detects and synchronizes the OFMD signal using the peak points shown in  FIG. 6 . For example, if the number of peak points of the second computation result Corr_im_im that match the lowest points of the third computation result Corr_re is greater than a predetermined threshold value, it is determined that the OFDM signal is received. Alternatively, if the number of peak points of the third computation result Corr_re is greater than a predetermined threshold value, it is determined that the OFDM signal is received. 
   Each of final peak points of the second and third computation results, _im _im and Corr_re, is used as a signal synchronization point to perform the signal synchronization. To detect and synchronize the OFDM signal, the waveform of the correlation result is divided into windows on the basis of the peak points. 
     FIG. 7  illustrates an initial window field in the waveform of the correlation result according to an embodiment. Referring to  FIG. 7 , the initial window field in symbol units is established at a point having a stable automatic gain control (AGC) in the preamble signal. Window fields are adjusted so that the lowest points of the second computation result Corr_im_im and the peak points of the third computation result Corr_re correspond to a center point of a window field. 
     FIG. 8  illustrates an adjusted initial window field. Referring to  FIG. 8 , the initial window is adjusted and the peak points are continuously detected. If the number of peak points of the second computation result Corr_im_im that match the lowest points of the third computation result Corr_re is greater than a predetermined threshold value, it is determined that the OFDM signal is received. Alternatively, if the number of window fields where the lowest points of the second computation result _im _im correspond to the peak points of the third computation result Corr_re is greater than a predetermined number, it is determined that the OFDM signal is received. The peak points of the third computation result Corr_re or the lowest points of the second computation result Corr_im_im of a final window are used as a signal synchronization point to synchronize the OFMD signal. 
   A differential correlation matching method can stably detect and synchronize the OFDM signal even with a frequency offset or degradation from AWGN. A symbol timing synchronization can be quickly performed without additional hardware. 
   The peak points of the correlation computation result may be displaced to +1 or −1 samples due to multi-path fading in a multi-path environment. 
     FIG. 9  illustrates displacements of peak points in the multi-path environment. Referring to  FIG. 9 , the lowest points of the second computation result Corr_im_im do not correspond to the peak points of the third computation result Corr_re. The peak points and the lowest points are separated by a few samples due to the multi-path fading. In this case, the method of receiving the OFDM signal determines that the lowest points of the second computation result Corr_im_im correspond to the peak points of the third computation result Corr_re in a predetermined window field when the lowest points of the second computation result _im _im and the peak points of the third computation result are included with a 3 sample period in the predetermined window field. That is, the lowest points of the second computation result and the peak points of the third computation result can be displaced to +1 or −1 samples from a center point of the predetermined window field. In this case, if the number of window fields where the peak points and the lowest points included in a predetermined sample range is greater than a predetermined number, it is determined that the OFDM signal is detected. 
   In view of the OFDM signal synchronization, since a frequency domain equalizer can compensate for a change in a FFT window in the characteristics of the OFDM signal, a point before the predetermined sample can be determined as the OFDM signal synchronization point rather than the earliest point among the peak points of the third computation result Corr_re and the lowest points of the second computation result Corr_im_im. 
   A method of receiving an OFDM signal according can detect and synchronize the OFDM signal quickly and accurately even with a frequency offset or degradation from AWGN, thereby improving throughput in a high-speed wireless communication. 
   While this invention has been particularly shown and described with reference to an 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. Thus, the scope of the present invention is limited not by the foregoing but by the following claims, and all differences within the range of equivalents thereof should be interpreted as being within the scope of the present invention.