Abstract:
A synchronization apparatus and method for an OFDM system includes four portions: delay conjugate multiplication, phase processing, delay moving sum and minimum value detection. The invention performs the correlation phase operation and finds out the location of abnormal change for the phase difference of the output signal. The location is used for the reference of symbol synchronization. It overcomes the problems of incorrect judge about synchronization location due to the channel fading or noise. The invention can be applied to wire/wireless communications or digital video broadcasting-terrestrial systems, specialized in symbol synchronization at a receiving end.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to the transmission of an orthogonal frequency division multiplexing (OFDM) system, and more specifically to a method and an apparatus for the synchronization of the OFDM systems using guard interval. 
   BACKGROUND OF THE INVENTION 
   In digital communication or broadcasting, the transmission end transmits information in the form of electromagnetic wave through a physical channel, such as air, to the receiving end. Due to the non-ideal channel effect, such as multi-path reflection and signal fading, there is usually distortion at the receiving end. Because the OFDM based on the multi-carrier modulation is effective in handling the multi-path reflection problem, it is becoming a mainstream technology in the wire/wireless communications and digital broadcasting-terrestrial systems. 
   The OFDM applications include: asymmetric digital subscriber line (ADSL), very-high-speed digital subscriber line (VDSL), digital audio broadcasting (DAB), wireless local area network (WLAN) IEEE802.11a/g/n, Ultra Wideband (UWB), dedicated short range communication (DSRC), integrated service digital broadcasting-terrestrial (ISDB-T), digital video broadcasting-terrestrial broadcasting (DVB-T), and digital video broadcasting-handheld (DVB-H), and so on. 
   The output signals from the OFDM transmission end will have a guard interval with cyclic prefix placed in front of each symbol to fight against the inter-symbol interference (ISI) caused by the multi-path effect.  FIG. 1  shows a complete OFDM symbol, in which the guard interval includes Ng sample points, and the original symbol includes N sample points; therefore the length of the symbol is N+Ng. The European TV transmission defines the 2K mode and the 8K mode of the original symbol length, combined with four different guard interval lengths, including 1/32, 1/16, ⅛, and ¼ of the original symbol length for protection in different transmission environments. 
   Before processing the data, the OFDM receiving end must remove the data of the guard interval interfered by the ISI in order to extract correct effective data range to perform the Fast Fourier Transform (FFT) of the corresponding length (2K or 8K mode). If the extracted data range is interfered by ISI, the following data will all be affected by a phase difference. This may prolong the time required for decoding the symbol, or even corrupt the following data beyond decoding. It is therefore important to provide an effective symbol synchronization method. 
   The current symbol synchronization methods are divided into three categories. The first category includes methods, based on the repetition characteristics of the guard interval data, using correlation computation to analyze the correlation magnitude for processing. The second category includes methods, based on the repetition characteristics of the guard interval data, using correlation computation to analyze the correlation phase for processing. The third category includes methods combining the above two characteristics. 
   U.S. Pat. No. 6,421,401 disclosed a method based on cyclic prefix of the guard interval, combing delay conjugate multiplication and moving average techniques to process the data with two passes of delay correlation computation and observe the magnitude of the generated characteristics signal to find the maximum position as the output. This method analyzes the correlation magnitude for processing. 
   U.S. Pat. No. 5,991,289 disclosed a method based on cyclic prefix of the guard interval, combing delay conjugate multiplication and moving average techniques to process the data with delay multiplication to extract the phase difference and a moving sum to generate the characteristics signal, and then observe the transition position of the phase difference as the output. This method has the drawback of being easily affected by the channel decay or noise to misjudge the synchronization position. 
   US. Patent publication No. 2004/0,208,269 disclosed a method using correlation magnitude and correlation phase. After extracting, the correlation magnitude is multiplied with the correlation phase, passed through a self-parameter filter, and computed with a delay correlation computation, the transition position of the characteristics signal is observed as output. This method has the drawback of high complexity. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the disadvantages of the aforementioned conventional OFDM synchronization systems. The first object of the present invention is to provide a synchronization method for an OFDM system. The present invention uses the cyclic prefix characteristics of the guard intervals as the basis, and uses the delay conjugate multiplication, phase processing, delay moving sum and minimum value detection techniques for correlation phase computation to be used by the receiving end for symbol synchronization coarse tuning. 
   The synchronization apparatus of the present invention includes a delay conjugate multiplication module, a phase processing module, a delay moving sum module and a minimum value detector. 
   Based on the apparatus, the synchronization method includes the following steps. First, the delay conjugate multiplication operation is performed on the received signal to generate a delay conjugate multiplication signal. Then, the phase computation is performed on the delay conjugate multiplication signal to obtain a differential phase absolute value signal. The delay moving sum operation crossing at least a symbol is performed on the differential phase signal to obtain a delay moving sum signal. Finally, the transition of the phase difference of the delay moving sum signal is found for using in symbol synchronization. 
   The method and apparatus of the present invention have the advantages of anti-noise, anti-interference caused by channel effect and multi-path, and high precision. The present invention not only prevents the conventional misjudge of synchronization caused by fading and noise effect, the present invention also improves the overall effectiveness, and is applicable to digital audio and video broadcasting systems. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic view of the cyclic prefix characteristics of the guard interval of a typical OFDM system. 
       FIG. 2  shows a block diagram of an embodiment of the OFDM synchronization apparatus of the present invention. 
       FIG. 3  shows a waveform of the related signals of  FIG. 2 . 
       FIG. 4  shows the elements and flowchart of the delay conjugate multiplication module. 
       FIG. 5  shows the elements and flowchart of the phase computation module. 
       FIG. 6  shows the elements and flowchart of the delay moving sum module. 
       FIG. 7  shows a schematic view of the simulated results of the present invention and the conventional technologies. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  is a block diagram of the synchronization apparatus for the OFDM system. As shown in  FIG. 2 , the apparatus includes a delay conjugate multiplication module  201 , a phase computation module  202 , a delay moving sum module  203  and a minimum value detector  204 . 
   Delay conjugate multiplication module  201  performs the time delay on the received signal r(n), extracts the conjugate value, and multiply with the received signal r(n) to obtain a delay conjugate multiplication signal α(n). Delay conjugate multiplication signal α(n) passes through phase computation module  202  to obtain a differential phase absolute value signal γ(n). Differential phase absolute value signal γ(n−i) passes through delay moving sum module  203  for a delay moving sum computation crossing at least a symbol to obtain a delay moving sum signal ρ(n), with i as an index. Delay moving sum signal ρ(n) passes through minimum value detector  204  to find a transition location Γ of the phase difference for using in the synchronization of OFDM symbols. 
     FIG. 3  shows a waveform of the related signals in  FIG. 2 . As shown in  FIG. 3 , the locations of the relative minimum value of delay moving sum signal ρ(n) is the transition location Γ of the phase difference, which is also the reference location Γ for the synchronization of OFDM symbols. 
   The following describes the elements and the flowchart of the modules of  FIG. 2 . 
     FIG. 4  shows the elements and the operation flowchart of the delay conjugate multiplication module. As shown in  FIG. 4 , delay conjugate multiplication module  201  includes a delay unit  201   a , a conjugate computation unit  201   b , and a multiplier  201   c . The operation delay conjugate multiplication module  201  is described as follows. 
   Delay unit  201   a  performs a time delay on received signal r(n) to generate a time delay signal. The duration of the delay is the original data length N of the symbol. The time delay signal passes through conjugate computation unit  201   b  to extract a conjugate value and a time delay conjugate signal is obtained. The delay conjugate signal is multiplied with received signal r(n) by multiplier  201   c  to generate a delay conjugate multiplication signal α(n), where
 
α( n ) =r ( n−i ) ·r *( n−i−N )  (1)
 
n is time index and i is index.
 
     FIG. 5  shows the elements and the flowchart of the phase computation module. As shown in  FIG. 5 , phase computation module  202  includes a phase angular computation unit  202   a , a differential computation unit  202   b , an absolute value computation unit  202   c , and a threshold value slicer  202   d . Differential computation unit  202   b  further includes a differential delay unit  202   b   1  and a differential subtractor  202   b   2 . The operation of phase computation module  202  is described as follows. 
   Delay conjugate multiplication signal α(n) passes through phase angular computation unit  202   a  of phase computation module  202  for extracting phase angle to obtain a delay conjugate multiplication phase angle β(n).
 
β( n )= ARG [α( n )]  (2)
 
where ARG is the phase extraction computation.
 
   Delay conjugate multiplication phase angle β(n) passes through differential computation unit  202   b  of phase computation module  202  for pre-post sample differential computation to obtain a differential phase signal. Differential computation unit  202   b  can be implemented with differential delay unit  202   b   1  and differential subtractor  202   b   2 . Delay conjugate multiplication phase angle β(n) passes through differential delay unit  202   b   1  for time delay to obtain a delay conjugate multiplication phase angle β(n−1) of the previous sampling point. The duration of the delay is the length of a sampling interval. Subtractor  202   b   2  subtracts delay conjugate multiplication phase angle β(n−1) of the previous sampling point from delay conjugate multiplication phase angle β(n) to obtain a differential phase signal. The differential phase signal passes through absolute value computation unit  202   c  to obtain a differential phase absolute value signal γ(n).
 
γ( n )=|β( n )−β( n −1)|  (3)
 
   After the absolute value computation, threshold value slicer  202   d  can be used to change the differential phase signal γ(n) higher than a threshold to a high value, and change the differential phase signal γ(n) lower than a threshold to a low value. Hence, the differential phase signal γ(n) can be expressed with a single bit to simplify the design complexity. 
     FIG. 6  shows the elements and flowchart of the delay moving sum module. As shown in  FIG. 6 , delay moving sum module  203  includes a cross-symbol delay adder  203   a , and a moving sum unit  203   b . Cross-symbol delay sum unit  203   a  further includes a cross-symbol delay unit  203   a   1  and a cross-symbol adder  203   a   2 . Moving sum unit  203   b  further includes a first moving sum delay unit  203   b   1 , a second moving sum delay unit  203   b   2  and a moving sum adder-subtractor  203   b   3 . The operation of delay moving sum module  203  is described as follows. 
   Differential phase signal γ(n−i) passes through delay moving sum module  203  for cross S-symbols delay sum computation to obtain a delay moving sum signal ρ(n), with S being at least 1 and 
                   ρ   ⁡     (   n   )       =       ∑     i   =   1     Ng     ⁢       ∑     s   =   1     S     ⁢     {     γ   ⁡     (     n   -   i   -   sL     )       }                 (   4   )               
where S=1, 2, 3, 4 . . . , K and S is a natural number, L=N+Ng, L is the length of symbol data, which is the sum of length of guard interval Ng and the original data length N.
 
   When S is 1, the meaning of equation (4) is that the differential phase absolute value signal γ(n) of the current symbol must be summed with the differential phase absolute value signal γ(n−L) of the previous symbol by cross-symbol delay sum unit  203   a  to obtain a cross-symbol delay sum signal. 
   Cross-symbol delay sum unit  203   a  can be implemented with a cross-symbol delay unit  203   a   1  and a cross-symbol adder  203   a   2 . Differential phase absolute value signal γ(n) passes through S cross-symbol delay units  203   a   1  for the time delay of the duration of the total length of one to S symbols to generate S cross-symbol delay signals. Differential phase absolute value signal γ(n) and the S cross-symbol delay signals are summed by cross-symbol adder  203   a   2  to obtain a cross-symbol delay sum signal. 
   The cross-symbol delay sum signal passes through moving sum unit  203   b  for moving sum computation to obtain a delay moving sum signal ρ(n). Moving sum unit  203   b  can be implemented with a first moving sum delay unit  203   b   1 , a second moving sum delay unit  203   b   2  and a moving sum adder-subtractor  203   b   3 . First moving sum delay unit  203   b   1  performs the time delay on cross-symbol delay sum signal to obtain a moving delay sum signal. The delay duration is the length of a guard interval. Second moving sum delay unit  203   b   2  performs time delay on the moving delay sum signal to obtain a delay moving sum signal of the previous sample point. The delay duration is the length of a sampling. Moving sum adder-subtractor  203   b   3  adds the cross-symbol delay sum signal to the delay moving sum signal of the previous sampling point and subtracts the moving delay sum signal to obtain a delay moving sum signal ρ(n). 
   As shown in  FIG. 3 , the location of relative minimum value of delay moving sum signal ρ(n) is the phase difference transition location  64  of delay moving sum signal ρ(n) found by minimum value detector  204 . Γ is also the reference location for the synchronization of OFDM symbols. Minimum value detector  204  can be implemented with a local minimum value detecting circuit or a peak detecting circuit. The former finds the minimum value of delay moving sum signal ρ(n) of the interval regularly for a fixed interval. The latter detects delay moving sum signal ρ(n), all the time, for a high-to-low or a low-to-high transition to find the relative minimum value of delay moving sum signal ρ(n). 
     FIG. 7  shows the simulated results of the present invention and the conventional techniques in an environment of a rural channel. The rural channel has the bandwidth of 6 MHz, with the 2K mode for symbol length, the guard interval being 1/32 of the symbol length. The sampling frequency shift is 40 ppm and the carrier frequency shift is 12.36 sub-carrier spacings. As shown in  FIG. 7 , the present invention, in comparison with the conventional techniques, improves the efficiency by 1.5-1.8 dB. 
   In summary, the present invention provides a synchronization apparatus and method for an OFDM system, including four portions: delay conjugate multiplication, phase processing, delay moving sum and minimum value detection. The invention performs the correlation phase operation and finds out the location of abnormal change for the phase difference of the output signal. The location is used for the reference of symbol synchronization. It overcomes the problems of incorrect judge about synchronization location due to the channel fading or noise, and improves the efficiency. 
   Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.