Abstract:
A method for detecting inter-carrier-symbol interference (ICSI) in an OFDM system includes the steps of computing a first correlation value representing the correlation between at least one of first signals of a first symbol and at least one of second signals of a second symbol previous to the first symbol, wherein the first signal is transmitted via a first sub-carrier and the second signal is transmitted via a second sub-carrier adjacent to the first sub-carrier; computing a second correlation value representing the correlation between the at least one first signal and at least one of third signals of a third symbol next to the first symbol, wherein the signal is transmitted via the first sub-carrier and the third signal is transmitted via the second sub-carrier; comparing the first correlation value with the second correlation value; and adjusting the timing of the boundary according to the comparison result.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a co-pending application Ser. No. 10/710539, filed on the same day with the present patent application, entitled “METHOD AND APPARATUS OF DETECTING ISI/ICSI IN AN OFDM SYSTEM” and assigned to the same assignee, the contents of which are incorporated herein by reference. 
   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates to an apparatus for use in an OFDM system and a method thereof, and more particularly, to an apparatus for detecting ISI/ICSI in an OFDM system and a method thereof. 
   2. Description of the Prior Art 
   Most OFDM transceivers suffer from well-known problems of inter-symbol interference (ISI) and inter-carrier interference (ICI). An additional guard interval (GI) is added between two symbols to recover the ISI and the ICI. When receiving a packet including a plurality of symbols, a conventional OFDM receiver detects the boundary of each symbol, removes GI of each symbol according to the detected boundary of the symbol, and then demodulated the symbol through Fast Fourier Transform (FFT) operation. However the detected boundary may not be reliable owing to the influence of multi-path and other factors. 
   One conventional art applied to improve the precision of boundary detection is to estimate the time shift of the detected boundary according to the frequency domain linear phase shift of the demodulated data. Another conventional art disclosed is to estimate the time shift of the detected boundary according to the channel impulse response of the symbol. However, when the delay spread phenomenon is too severe, the ISI and ICI problem cannot be recovered by both of the two conventional techniques and the boundary detection may be imprecise which may cause divergence or even failure in receiving when receiving symbols. 
   SUMMARY OF INVENTION 
   It is therefore an objective of the claimed invention to provide a method and apparatus of detecting ISI/ICSI in an OFDM system to solve the above-mentioned problem. 
   According to the claimed invention, a method of detecting inter-carrier-symbol interference (ICSI) of a symbol for adjusting a boundary of the symbol utilized by an OFDM system is disclosed, wherein each symbol includes a plurality of signals respectively transmitting via a plurality of sub-carriers, the method comprising: computing a first correlation value representing the correlation between at least one of first signals of a first symbol and at least one of second signals of a second symbol previous to the first symbol, wherein the at least one first signal is transmitted via a first sub-carrier and the at least one second signal is transmitted via a second sub-carrier adjacent to the first sub-carrier; computing a second correlation value representing the correlation between the at least one first signal and at least one of third signals of a third symbol next to the first symbol, wherein the at least one first signal is transmitted via the first sub-carrier and the at least one third signal is transmitted via the second sub-carrier; comparing the first correlation value with the second correlation value; and adjusting the timing of the boundary according to the comparison result. 
   According to the claimed invention, an apparatus of detecting inter-carrier-symbol interference (ICSI) of a symbol for adjusting a boundary of the symbol utilized by an OFDM system is disclosed, wherein each symbol includes a plurality of signals respectively transmitting via a plurality of sub-carriers, the apparatus comprising: a first correlator for computing a first correlation value representing the correlation between at least one of first signals of a first symbol and at least one of second signals of a second symbol previous to the first symbol, wherein the at least one first signal is transmitted via a first sub-carrier and the at least one second signal is transmitted via a second sub-carrier adjacent to the first sub-carriers; a second correlator for computing a second correlation value representing the correlation between the at least one first signal and at least one of third signals of a third symbol next to the first symbol, wherein the at least one first signal is transmitted via the first sub-carrier and the at least one third signal is transmitted via the second sub-carrier; a comparator for comparing the first correlation value with the second correlation value; and a timing controller for adjusting the timing of the boundary according to the comparison result. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of an ISI detector according to one embodiment of the present invention. 
       FIG. 2  is a schematic diagram of an ISI detector according to another embodiment of the present invention. 
       FIG. 3  is a schematic diagram of an ICSI detector according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 1 , which is a schematic diagram of an ISI detector  20  according to one embodiment of the present invention. As shown in  FIG. 1 , the ISI detector  20  is coupled to a timing controller  62 , and the ISI detector  20  comprises two correlators  21 ,  41  for respectively generating a correlation value R pre  and a correlation value R nxt  and a comparator  60  to compare both correlation values. The correlation value R pre  represents the magnitude of the ISI caused by the previous symbol, and the correlation value R nxt  represents the magnitude of the ISI caused by the next symbol. The comparator  60  is used to compare the correlation value R pre  with the correlation value R nxt  and generate a control signal Sc according to the comparison result. The timing controller  62  is used to control the timing of a boundary of an OFDM system according to the control signal Sc. 
   As shown in  FIG. 1 , the correlator  21  of this embodiment comprises conjugating units  22 , . . . ,  32 , multipliers  24 , . . . ,  34 , low-pass filters  25 , . . . ,  35 , absolute value calculating units  26 , . . . ,  36 , and a summation unit  28 . The conjugating units  22 , . . . ,  32  are used for respectively generating conjugated pilot data P 1 (n)*, . . . , P k (n)* by conjugating corresponding pilot data P 1 (n), . . . , P k (n) that was transmitted using the current symbol. The multipliers  24 , . . . ,  34  are used for respectively generating product values by multiplying those conjugated pilot data P 1 (n)*, P k (n)* with a corresponding comparison data
 
{circumflex over (P)} 1 (n−1)
 
, . . . ,
 
{circumflex over (P)} k (n−1)
 
that was transmitted using the previous symbol. The low-pass filters  25 , . . . ,  35  are used for averaging the product values outputted from these multipliers  24 ,  34 , respectively. The absolute value calculating units  26 , . . . ,  36  are used for generating absolute values of the average values corresponding to the product values. The summation unit  28  is used for generating a correlation value R pre  by summing these absolute values.
 
   Similarly, the correlator  41  comprises conjugating units  42 , . . . ,  52 , multipliers  44 , . . . ,  54 , low-pass filters  45 , . . . ,  55 , absolute value calculating units  46 , . . . ,  56 , and a summation unit  48 . The conjugating units  42 , . . . ,  52  are used for respectively generating conjugated pilot data P 1 (n)*, . . . , P k (n)* by conjugating corresponding pilot data P 1 (n), . . . , P k (n) that was transmitted using a current symbol. The multipliers  44 , . . . ,  54  are used for respectively generating product values by multiplying those conjugated pilot data P 1 (n)*, . . . , P k (n)* with a corresponding comparison data
 
{circumflex over (P)} 1 (n+1)
 
, . . . ,
 
{circumflex over (P)} k (n+1)
 
that was transmitted using the next symbol. The low-pass filters  45 , . . . ,  55  are used for averaging the product values outputted from these multipliers  44 , . . . ,  54 , respectively. The absolute value calculating units  46 , . . . ,  56  are used for generating absolute values of the average values corresponding to the product values outputted from these multipliers  44 , . . . ,  54 . The summation unit  48  is used for generating a correlation value R nxt  by summing these absolute values.
 
   According to the well-known theorem of correlation, the following Equations (1) and (2) are used to better explain operations of the correlators  21 ,  41 . 
   
     
       
         
           
             
               
                 
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                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   1 
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                   R 
                   nxt 
                 
                 = 
                 
                   
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                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
   
   P k (n)* denotes the conjugated pilot data transmitted using an n th  symbol via the a k th  sub-carrier,
 
{circumflex over (P)} k (n−1)
 
denotes the comparison data transmitted using an (n−1) th  symbol via a k th  sub-carrier, and
 
{circumflex over (P)} k (n+1)
 
denotes another comparison data transmitted using an (n+1) th  symbol via a k th  sub-carrier. Please note that the more sub-carriers that are considered, the more reliable result will be generated.
 
   This embodiment of ISI detector is for use in the OFDM system that the pilot of different symbols transmitted via the same sub-carrier have known but different predetermined values. As the result,
 
{circumflex over (P)} k (n−1)
 
and
 
{circumflex over (P)} k (n+1)
 
denote those known predetermined values of pilot in this embodiment. Since the pilots of two different symbols are different, the correlation between pilots of different symbols is due to the interference between these two symbols. Therefore, if the correlation value R pre  is greater than the correlation value R nxt , it means that the interference is mainly introduced from using the previous symbol, which is due to the timing of the detected boundary is ahead of that of the ideal boundary. In this manner, the timing controller  62  delays the timing of the boundary according to the control signal Sc outputted from the comparator  60 . On the otherhand, if the correlation value R pre  is less than the correlation value R nxt , it means that the interference is mainly introduced from the following symbol, which is due to the timing of the detected boundary lags behind that of the ideal boundary. In this manner, the comparator  60  outputs the control signal Sc to the timing controller  62  for advancing the timing of the boundary. As a result, the ISI effect is alleviated.
 
   Please refer to  FIG. 2 , which is a schematic diagram of an ISI detector  80  according to another embodiment of the present invention. As show in  FIG. 2 , the ISI detector  80  comprises two correlators  90 ,  110  and a comparator  120 . The correlators  90 ,  110  are used for generating correlation values R pre  and R nxt , respectively. The comparator  120  compares the correlation value R pre  with the correlation value R nxt  for outputting a control signal Sc to control the timing controller  129 . 
   In this embodiment, the correlator  90  has 1 st  delay circuits 91 a , . . . ,  101   a,  2 nd  delay circuits  91   b , . . . ,  101   b , conjugating units  92 , . . . ,  102 , multipliers  93 , . . . ,  103 , equalizers  94   a , . . . ,  104   a , slicers  94   b , . . . ,  104   b , low-pass filters  95 , . . . ,  105 , absolute value calculating units  96 , . . . ,  106 , and a summation unit  98 . Concerning the other correlator  110 , it has 1 st  delay circuits  111 , . . . ,  121 , conjugating units  112 , . . . ,  122 , multipliers  113 , . . . ,  123 , equalizers  114   a , . . . ,  124   a , slicers  114   b , . . . ,  124   b , low-pass filters  115 , . . . ,  125 , absolute value calculating units  116 , . . . ,  126 , and a summation unit  128 . Please note that the components shown in  FIGS. 1 and 2  that have the same name have substantially the same functionality and operation. The related description, therefore, is not repeated for simplicity. 
   For an OFDM system having pilot transmitted via the same pilot sub-carrier using different symbols corresponding to the same value, the ISI detector  80  is preferably utilized. As shown in  FIG. 2 , the comparison data
 
{circumflex over (Q)} 1 (n−1)
 
, . . . ,
 
{circumflex over (Q)} k (n−1)
 
are the decision results from received data signals Q 1 (n−1), . . . , Q k (n−1) through the corresponding equalizers  94   a , . . . ,  104   a , and the slicers  94   b , . . . ,  104   b , wherein the data signals Q 1 (n−1), . . . , Q k (n−1) are delayed by the corresponding 1 st  delay circuits  91   a , . . . ,  101   a,  2 nd  delay circuits  91   b , . . . ,  101   b  and then transmitted to the equalizers  94   a , . . . ,  104   a . Regarding the comparison data signals
 
{circumflex over (Q)} 1 (n+1)
 
, . . . ,
 
{circumflex over (Q)} k (n+1),
 
they are generated by directly equalizing and slicing the data signals Q 1 (n+1), . . . , Q k (n+1) with the corresponding equalizers  114   a , . . . ,  124   a  and slicers  94   b , . . . ,  104   b .  
 
   It should be noted that the symbol Q(.) represents the received data signal of the corresponding sub-carrier and the symbol
 
{circumflex over (Q)}(.)
 
represents the result of equalizing and slicing of the data signal of Q(.).
 
   With the circuit configuration shown in  FIG. 2 , the correlation values R pre  and R nxt  are computed according to the following equations (3) and (4). 
   
     
       
         
           
             
               
                 
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                   pre 
                 
                 = 
                 
                   
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                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   3 
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                   R 
                   nxt 
                 
                 = 
                 
                   
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                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
   
   Q k (n)* denotes the conjugated data signal transmitted using an n th  symbol via the a k th  sub-carrier,
 
{circumflex over (Q)} k (n−1)
 
denotes the equalized comparison data signal transmitted using an (n−1) th  symbol via a k th  sub-carrier, and
 
{circumflex over (Q)} k (n+1)
 
denotes another equalized comparison data signal transmitted using an (n+1) th  symbol via a k th  sub-carrier.
 
   Therefore, if the correlation value R pre  is greater than the correlation value R nxt , it means that the interference is mainly caused by the previous symbol, in this manner, the timing of the boundary is delayed by the timing controller  129 . If the correlation value R pre  is smaller than the correlation value R nxt , it means that the interference is mainly caused by the next symbol, in this manner, the timing is advanced by the timing controller  129 . In the end, the ISI effect is alleviated. 
   It is well-known that the ISI might be introduced by adjacent sub-carriers as well. That is, inter-carrier-symbol-interference (ICSI) occurs. Please refer to  FIG. 3 , which is a schematic diagram of an ICSI detector  160  according to the third embodiment of the present invention. In this embodiment, k sub-carriers of the different symbols for transmitting data are chosen through decision directed method for determining ISI. Since the data of two different sub-carriers are different, the correlation between the data of different sub-carriers is due to the interference between these two sub-carriers. 
   The ICSI detector  160  has two correlators  130 ,  150  and a comparator  170 . The correlator  130  includes conjugating units  131 , . . . ,  141 , equalizers  132   a ,  132   b , . . . ,  142   a ,  142   b , slicers  132   c ,  132   d , . . . ,  142   c ,  142   d , multipliers  133   a ,  133   b , . . . ,  143   a ,  143   b , low-pass filters  134   a ,  134   b , . . . ,  144   a ,  144   b , absolute value calculating units  136   a ,  136   b , . . . ,  146   a ,  146   b , and a summation unit  138 . Similarly, the correlator  150  includes conjugating units  151 , . . . ,  161 , equalizers  152   a ,  152   b , . . . ,  162   a ,  162   b , slicers  152   c ,  152   d , . . . ,  162   c ,  162   d , multipliers  153   a ,  153   b , . . . ,  163   a ,  163   b , low-pass filters  154   a ,  154   b , . . . ,  164   a ,  164   b , absolute value calculating units  156   a ,  156   b , . . . ,  166   a ,  166   b , and a summation unit  158 . 
   It is obvious that the correlators  130 ,  150  have substantially the same circuit architecture. However, the data inputted into the correlators  130 ,  140  are different. Please note that the components shown in  FIGS. 1 ,  2 , and  3  that have the same name have substantially the same functionality and operation. The related description, therefore, is not repeated for simplicity. The following equations (5) and (6) are used to better explain operations of the correlators  130  and  150 . 
   
     
       
         
           
             
               
                 
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                 Equation 
                 ⁢ 
                 
                     
                 
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                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
   
   D k (n)* represents the conjugate of data D k (n) transmitted using an n th  symbol via a k th  sub-carrier,
 
{circumflex over (D)} k−1 (n−1)
 
denotes a decision result of data D k−1 (n−1) transmitted using an (n−1) th  symbol via a (k−1) th  sub-carrier, and
 
{circumflex over (D)} k+1 (n−1)
 
denotes a decision result of data D k+1 (n−1) transmitted using an (n−1) th  symbol via a (k+1) th  sub-carrier. As a result, the correlation value R pre  is computed to estimate the magnitude of ICSI imposed upon the data D k (n). That is, the ICSI generated from the adjacent (k−1) th  sub-carrier and (k+1) th  sub-carrier using a previous symbol is calculated according to the above Equation (5).
 
   
     
       
         
           
             
               
                 
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                 Equation 
                 ⁢ 
                 
                     
                 
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                   ( 
                   6 
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   D k (n)* represents the conjugate of data D k (n) transmitted using an n th  symbol via a k th  sub-carrier,
 
{circumflex over (D)} k−1 (n+1)
 
denotes a decision result of data D k−1 (n+1) transmitted using an (n+1) th  symbol via a (k−1) th  sub-carrier, and
 
{circumflex over (D)} k+1 (n+1)
 
denotes a decision result of data D k+1 (n+1) transmitted using an (n+1) th  symbol via a (k+1) th  sub-carrier. It is clear that the correlation value R nxt  is also computed to estimate the magnitude of ICSI imposed upon the data D k (n). In other words, the ICSI generated from the adjacent (k−1)th sub-carrier and (k+1) th  sub-carrier using a following symbol is calculated according to the above Equation (6). Please note that data processed by correlators  130  and  150  are transmitted via data sub-carriers not pilot sub-carriers. Finally, the comparator  170  shown in  FIG. 3  compares the correlation value R pre  with the correlation value R nxt  for searching a greater one. If the correlation value R pre  is greater than the correlation value R nxt , it means that the interference is mainly caused by the previous symbol, in this manner, the timing of the boundary would be delayed by the timing controller  172 . If the correlation value R pre  is smaller than the correlation value R nxt , it means that the interference is mainly caused by the next symbol, in this manner, the timing of the boundary of the OFDM system would be advanced by the timing controller  172 . Therefore, the ICSI effect is alleviated.]
 
   In the above embodiments, please note the absolute values are directly summed to generate the wanted correlation values R pre  and R nxt . However, the correlation values R pre  and R nxt  can be generated by using square values instead of the absolute values. For instance, each of the product values is squared before the summation value is calculated. That is, the above Equations (1)-(6) are replaced with the following equations, respectively. 
   
     
       
         
           
             
               
                 
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                     2 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   1.1 
                   ) 
                 
               
             
           
           
             
               
                 
                   R 
                   nxt 
                 
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                     2 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   2.1 
                   ) 
                 
               
             
           
           
             
               
                 
                   R 
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                                   n 
                                   - 
                                   1 
                                 
                                 ) 
                               
                             
                             · 
                             
                               
                                 
                                   Q 
                                   k 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   n 
                                   ) 
                                 
                               
                               * 
                             
                           
                           ] 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   3.1 
                   ) 
                 
               
             
           
           
             
               
                 
                   R 
                   nxt 
                 
                 = 
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       1 
                     
                     K 
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         E 
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   Q 
                                   ^ 
                                 
                                 k 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   n 
                                   + 
                                   1 
                                 
                                 ) 
                               
                             
                             · 
                             
                               
                                 
                                   Q 
                                   k 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   n 
                                   ) 
                                 
                               
                               * 
                             
                           
                           ] 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   4.1 
                   ) 
                 
               
             
           
           
             
               
                 
                   R 
                   pre 
                 
                 = 
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       1 
                     
                     K 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             E 
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       D 
                                       ^ 
                                     
                                     
                                       k 
                                       - 
                                       1 
                                     
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       - 
                                       1 
                                     
                                     ) 
                                   
                                 
                                 · 
                                 
                                   
                                     
                                       D 
                                       k 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       n 
                                       ) 
                                     
                                   
                                   * 
                                 
                               
                               ] 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             E 
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       D 
                                       ^ 
                                     
                                     
                                       k 
                                       + 
                                       1 
                                     
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       - 
                                       1 
                                     
                                     ) 
                                   
                                 
                                 · 
                                 
                                   
                                     
                                       D 
                                       k 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       n 
                                       ) 
                                     
                                   
                                   * 
                                 
                               
                               ] 
                             
                           
                           ) 
                         
                         2 
                       
                     
                     ) 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   5.1 
                   ) 
                 
               
             
           
           
             
               
                 
                   R 
                   nxt 
                 
                 = 
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       1 
                     
                     K 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             E 
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       D 
                                       ^ 
                                     
                                     
                                       k 
                                       - 
                                       1 
                                     
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                 
                                 · 
                                 
                                   
                                     
                                       D 
                                       k 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       n 
                                       ) 
                                     
                                   
                                   * 
                                 
                               
                               ] 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             E 
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       D 
                                       ^ 
                                     
                                     
                                       k 
                                       + 
                                       1 
                                     
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                 
                                 · 
                                 
                                   
                                     
                                       D 
                                       k 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       n 
                                       ) 
                                     
                                   
                                   * 
                                 
                               
                               ] 
                             
                           
                           ) 
                         
                         2 
                       
                     
                     ) 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   6.1 
                   ) 
                 
               
             
           
         
       
     
   
   The method and related device disclosed in the embodiments of the present invention for detecting ISI/ICSI in an OFDM system for adjusting a boundary of the OFDM system first computes correlation values to predict the source of the ISI/ICSI and then adjusting the boundary after the source of the ISI/ICSI is determined. Therefore, the performance of tracking the boundary of the OFDM system is greatly improved. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.