Abstract:
Methods and apparatus for FEC frame header detection are provided, suitable for use in a DVB-C2 receiver. A first method comprises demodulating with a quadrature phase shift keying (QPSK) demapper, followed by correlation and symmetry measures for detection of a robust FEC header. A second method comprises demodulating with a 16 quadrature amplitude modulation (QAM) demapper, followed by correlation and symmetry measures for detection of a high efficiency FEC leader. Another embodiment comprises using both the first and second methods to find the FEC header because the first symbol of the FEC frame header may be sent either in robust mode or in high efficiency mode. There is also provided a method and apparatus for generating an alternate decision statistic for determining detection of the FEC frame header.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/269,753, entitled “FEC FRAME HEADER DETECTION ALGORITHM FOR DVB-C2,” filed Jun. 29, 2009, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present principles relate to a method for recovering a Forward Error Correction (FEC) frame header, such as used in the DVB-C2 transmission standard. 
       BACKGROUND OF THE INVENTION 
       [0003]    Digital transmission techniques often use either quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) to modulate data. The modulated data sometimes has undergone additional encryption methods prior to modulation to make its recovery accurate and robust. The Digital Video Broadcasting (DVB) standards use QPSK and QAM techniques for data transmission. 
         [0004]    The Digital Video Broadcasting standards consortium is a group that defines the standards for various transmission systems. One of those standards is used for digital cable transmission, the DVB-C standard. A second generation digital cable standard, the DVB-C2 standard, uses Reed-Muller encoding of information followed by mixing with a pseudo-noise (PN) sequence. A receiver needs to remove the effect of the PN sequence prior to recovery of the information bits. 
         [0005]    A frame header of Forward Error Correction (FEC) is used in the DVB-C2 transmission Standard to support Adaptive Coding and Modulation (ACM) or Variable Coding and Modulation (VCM) in each FEC block. The FEC frame header (FECFrame) is attached in front of each FECFrame or two consecutive FECFrames to indicate the coding rate, modulation type and physical layer pipe identifier to a receiver. Thus, FECFrame detection is required in a DVB-C2 receiver. Two types of FEC frame headers can be generated in an encoder. First, in robust mode, the FEC frame header is generated using quadrature phase shift keying (QPSK). Second, in high efficiency mode, the FEC frame header is generated using 16 quadrature amplitude modulation (16-QAM). Based on the type of the FECFrame header, FECFrame detection methods and apparatus are described in this invention, suitable for use with the DVB-C2 standard. 
       SUMMARY OF THE INVENTION 
       [0006]    The present principles are directed to a method and apparatus for FEC frame header detection, suitable for use in the DVB-C2 transmission standard. 
         [0007]    According to an aspect of the present principles, there is provided a method for recovery of an FEC frame header. The method comprises the steps of demodulating a received complex data symbol, followed by computing an estimated 32-bit pseudo-noise sequence, and computing a binary correlation. The correlation output is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method continues on the current symbol, an estimated 32-bit Reed-Muller codeword is decoded, followed by majority logic decoding on the last 10 bits of the codeword, and computation of a Reed-Muller (RM) symmetry measurement. The symmetry measurement is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method continues on the current symbol, majority logic decoding is performed to recover the 16 information bits of the FEC header. 
         [0008]    According to another aspect of the present principles, there is provided an apparatus. The apparatus comprises a demodulator for demodulating a received complex data symbol, for example with a demapper, circuitry for computing an estimated pseudo-noise sequence using the demodulated complex data symbol, a processor for computing a binary correlation of the estimated pseudo-noise sequence with a Reed-Muller codeword, a first comparator for comparing the binary correlation with a first threshold, a decoder for generating an estimated 32-bit RM codeword if the binary correlation is greater than or equal to the first threshold, majority logic decoding circuitry for operating on the estimated 32-bit RM codeword, circuitry for computing a RM symmetry measure, a second comparator for comparing the symmetry measure with a second threshold, and circuitry for majority logic decoding that recovers the 16 information bits of the FEC frame header. 
         [0009]    According to another aspect of the present principles, there is provided another method for recovery of an FEC frame header. The method includes the steps of demodulating a received complex data symbol using quadrature phase shift keying (QPSK), for example with a QPSK demapper, followed by computing an estimated 32-bit pseudo-noise sequence, and computing a binary correlation. The correlation output is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method of recovery continues on the current symbol, an estimated 32-bit Reed-Muller codeword is decoded, followed by majority logic decoding on the last 10 bits of the codeword, and computation of a Reed-Muller (RM) symmetry measurement. The symmetry measurement is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method of recovery continues on the current symbol, majority logic decoding is performed to recover the 16 information bits of the FEC header. The method also comprises, performing in parallel with the previous steps, the steps of demodulating a received complex data symbol by 16 quadrature amplitude modulation, followed by computing an estimated 32-bit pseudo-noise sequence, and computing a binary correlation. The correlation output is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method of recovery continues on the current symbol, an estimated 32-bit Reed-Muller codeword is decoded, followed by majority logic decoding on the last 10 bits of the codeword, and computing of a Reed-Muller (RM) symmetry measurement. The symmetry measurement is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method of recovery continues on the current symbol, majority logic decoding is performed to recover the 16 information bits of the FEC header. Depending on whether the FEC frame header was sent in robust or high efficiency mode, the corresponding path of the two methods performed concurrently will give a decision statistic indicative of FEC frame detection. According to another aspect of the present principles, there is provided an apparatus to implement to aforementioned method. 
         [0010]    According to another aspect of the present invention, there is provided a method for FEC frame header recovery. The method comprises the steps of demodulating a received complex data symbol using QPSK, for example using a demapper, followed by computing a soft correlation to measure the pseudo-noise sequence, followed by scaling of the soft correlation output. The scaled soft correlation output is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method of recovery continues on the current symbol, an estimated 32-bit Reed-Muller codeword is decoded, followed by majority logic decoding on the last 10 bits of the codeword, and computing of a Reed-Muller (RM) symmetry measurement. The symmetry measurement is compared to a threshold to determine whether to continue the recovery method on the current symbol or whether to demodulate the next symbol. If the method of recovery continues on the current symbol, majority logic decoding is performed to recovery the 16 information bits of the FEC header. 
         [0011]    According to another aspect of the present principles, there is provided an apparatus. The apparatus comprises a demodulator for demodulating a received complex data symbol, such as with a QPSK demapper, circuitry for computing a soft correlation to measure pseudo-noise sequence, a scaler for scaling the soft correlation output, a first comparator for comparing the scaled soft correlation with a first threshold, a decoder for generating an estimated 32-bit RM codeword if the scaled soft correlation is greater than or equal to the first threshold, majority logic decoding circuitry for operating on the estimated 32-bit RM codeword, circuitry for computing a RM symmetry measure, a second comparator for comparing the symmetry measure with a second threshold, and circuitry for majority logic decoding that recovers the 16 information bits of the FEC frame header. 
         [0012]    According to another aspect of the present invention, there is provided a method of FEC frame header detection. The method comprises forming a first scaled correlation of a received data symbol with a pseudo-noise sequence, forming a second scaled correlation of a Reed-Muller codeword with a version of the received data symbol, summing the first and second scaled correlations to generate a decision statistic, and comparing the decision statistic with a threshold to determine if an FEC frame header has been detected. 
         [0013]    According to another aspect of the present invention, there is provided an apparatus for FEC frame header detection. The apparatus comprises first and second circuits for forming scaled correlations of a received data symbol with a PN sequence and a RM codeword with a version of the received data symbol, respectively. The apparatus also comprises an adder for summing the two scaled correlations and a comparator for comparing the sum to a threshold to determine if an FEC frame header has been detected. 
         [0014]    These and other aspects, features and advantages of the present principles will become apparent from the following detailed description of exemplary embodiments, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1(   a ) shows an embodiment of a robust FECFrame header;  1 ( b ) shows an embodiment of a high efficiency FECFrame header;  1 ( c ) shows the position of the FEC frame header within a Data Slice Packet. 
           [0016]      FIG. 2  shows a generator matrix of RM(32,16) code. 
           [0017]      FIG. 3  shows a flow diagram of one embodiment of a method for FEC header recovery under the present principles. 
           [0018]      FIG. 4  shows an apparatus to recover an FEC header under the present principles. 
           [0019]      FIG. 5  shows a method for FEC header recovery under the present principles for either robust or high efficiency mode. 
           [0020]      FIG. 6  shows an apparatus to recover an FEC header under the present principles using either robust or high efficiency mode. 
           [0021]      FIG. 7  shows a method for FEC header recovery using the robust mode and a soft correlation. 
           [0022]      FIG. 8  shows an apparatus for FEC header recovery using the robust mode and a soft correlation. 
           [0023]      FIG. 9  shows a method for determining an FEC header decision statistic under the present principles. 
           [0024]      FIG. 10  shows an apparatus for determining an FEC header decision statistic under the present principles. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    An approach for FEC frame header processing, suitable for use in the Digital Video Broadcasting C2 broadcast standard, is described herein. 
         [0026]    Two encoding schemes for generating an FECFrame Header are shown in the  FIGS. 1(   a ) and  1 ( b ). Initially the 16 bits of the L1 signaling part  1  are FEC encoded by a Reed-Muller (32,16) encoder. Subsequently each bit of the 32 bit Reed-Muller codeword is split to form an upper and a lower branch. The lower branch applies a cyclic shift within each Reed-Muller codeword and scrambles the resulting data using a specific PN sequence, called an MPS sequence. The difference in the two encoding schemes is that a QPSK constellation is used for the robust FECFrame header and a 16QAM constellation is used for the high efficiency FECFrame header. The structure of the information bits within the FEC frame header and the position of the header within a Data Slice Packet is shown in  FIG. 1(   c ). Thus, in a receiver, from received QPSK symbols in the robust mode or received 16-QAM symbols in the high efficiency mode, to generate an estimated 32-bit RM codeword, you will need to remove the effect of PN sequence. 
         [0027]    The robust FECFrame header detection can be performed by the following steps:
   1. Assume that the 32-symbol complex sequence (s 0 , s 1 , . . . , s 31 )=(r i , r i+1 , . . . , r i+31 ) is the robust FECFrame header and demodulate them into a 64-bit sequence (a 0 , a 1 , . . . , a 63 ) by a QPSK demapper. The complex symbol, r i  is a received data symbol after removing its corresponding channel gain.   2. Compute the estimated 32-bit PN sequence {tilde over (w)} RM =({tilde over (w)} 0   TM ,{tilde over (w)} 1   RM , . . . , {tilde over (w)} 31   RM ) by w (k+2)     31      RM =α 2k ⊕a (2k+5)   64 , k=0, 1, . . . , 31 where (x) y  is the result of x modulo y and an exclusive or operation is used.   3. Compute the binary correlation of {tilde over (w)} RM  and w RM  by C p =Σ k=0   31 (2{tilde over (w)} k   RM −1)(2w k   RM −1). The variable w RM  represents the 32 bit scrambling sequence in the lower branch of the FEC frame header encoder that is a Reed-Muller code and {tilde over (w)} RM  is the received codeword.   
 
         [0031]    If C p &lt;T 1 , go to step 1 and advance symbol index by 1, e.g., i=i+1. 
         [0032]    If C p ≧T 1 , perform step 4.
   4. Each bit of the 32-bit RM codeword is decoded by combining log-likelihood ratios of upper branch bit and its corresponding lower branch bit. After some straightforward simplifications, the estimated 32-bit RM codeword {tilde over (λ)}=({tilde over (λ)} 0 , {tilde over (λ)} 1 , . . . , {tilde over (λ)} − ) is decoded by   
 
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         5. The estimated 32-bit RM codeword {tilde over (λ)}=({tilde over (λ)} 0 , {tilde over (λ)} 1 , . . . , {tilde over (λ)} 31 ) can be decoded by a 3-stage majority-logic decoding. The last 10 bits, ({tilde over (b)} 6 , {tilde over (b)} 7 , . . . , {tilde over (b)} 15 ), are decoded from the received code vector {tilde over (λ)}=({tilde over (λ)} 0 , {tilde over (λ)} 1 , . . . , {tilde over (λ)} 31 ) in the first stage. These 10 bits are removed from {tilde over (λ)} to form a modified code vector {tilde over (λ)} (1) ={tilde over (λ)}−(0, 0, . . . , 0, {tilde over (b)} 6 , {tilde over (b)} 7 , . . . , {tilde over (b)} 15 )·G. 
         6. The modified code vector {tilde over (λ)} (1)  has a symmetric structure and it can be used to double confirm if the 32-symbol complex sequence (s 0 , s 1 , . . . , s 31 ) is the FECFrame header. The RM autocorrelation of the received modified code vector is computed by 
       
     
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         [0000]    The RM symmetry measure is then computed by C RM =Σ k=0   4 |R RM (k)|. 
         [0036]    If C RM &lt;T 2 , go to step 1 and advance symbol index by 1, e.g., i=i+1. 
         [0037]    If C RM ≧T 2 , which means FECFrame header is detected, perform step 7.
   7. The first stage of majority-logic decoding is carried out in step 6. Perform the remaining two stages of majority-logic decoding procedure to obtain the 16 information bit.   
 
         [0039]    The high efficiency FECFrame header detection can be performed by the following steps:
   1. Assume that the 16-symbol complex sequence (s 0 , s 1 , . . . , s 15 )=(r i , r i+1 , . . . , r i+15 ) is the robust FECFrame header and demodulate them into a 64-bit sequence (a 0 , a 1 , . . . , a 63 ) by a 16QAM demapper. The complex symbol, r i  is a received data symbol after removing its corresponding channel gain.   2. Compute the estimated 32-bit PN sequence {tilde over (w)} RM =({tilde over (w)} 0   RM ,{tilde over (w)} 1   RM , . . . , {tilde over (w)} 31   RM ) by w (k+2)     31     RM =a 2k ⊕a (2k+5)   64 , k=0,1, . . . , 31 where (x) y  is the result of x modulo y.   3. Compute the binary correlation of {tilde over (w)} RM  and w RM  by C p =Σ k=0   31 (2{tilde over (w)} k   RM −1)(2w k   RM −1).   
 
         [0043]    If C p &lt;T 1 , go to step 1 and advance symbol index by 1, e.g., i=i+1. 
         [0044]    If C p ≧T 1 , perform step 4.
   4. Each bit of the 32-bit RM codeword is decoded by combining log-likelihood ratios of upper branch bit and its corresponding lower branch bit. After some straightforward simplifications, the estimated 32-bit RM codeword {tilde over (λ)}=({tilde over (λ)} 0 , {tilde over (λ)} 1 , . . . , {tilde over (λ)} 31 ) is decoded by computing k=0,1, . . . , 15:   
 
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         [0000]    where σ 2  is the estimated variance of noise.
   5. The estimated 32-bit RM codeword {tilde over (λ)}=({tilde over (λ)} 0 , {tilde over (λ)} 1 , . . . , {tilde over (λ)} 31 ) can be decoded by a 3-stage majority-logic decoding. The last 10 bits, ({tilde over (b)} 6 , {tilde over (b)} 7 , . . . , {tilde over (b)} 15 ), are decoded from the received code vector {tilde over (λ)}=({tilde over (λ)} 0 , {tilde over (λ)} 1 , . . . , {tilde over (λ)} 31 ) in the first stage. These 10 bits are removed from {tilde over (λ)} to form a modified code vector {tilde over (λ)} (1) ={tilde over (λ)}−(0, 0, . . . , 0, {tilde over (b)} 6 , {tilde over (b)} 7 , . . . , {tilde over (b)} 15 )·G.   6. The modified code vector {tilde over (λ)} (1)  has a symmetric structure and it can be used to double confirm if the 32-symbol complex sequence (s 0 , s 1 , . . . , s 31 ) is the FECFrame header. The RM autocorrelation of the received modified code vector is computed by   
 
         [0000]    
       
         
           
             
               
                 R 
                 RM 
               
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         [0000]    The RM symmetry measure is then computed by C RM =Σ k=0   4 |R RM (k)|. 
         [0048]    If C RM &lt;T 2 , go to step 1 and advance symbol index by 1, e.g., i=i+1. 
         [0049]    If C m ≧T 2 , which means FECFrame header is detected, perform step 7.
   7. The first stage of majority-logic decoding is carried out in step 6. Perform the remaining 2 stages of majority-logic decoding procedure to obtain the 16 information bit.   
 
         [0051]    Because the FECFrame header mode is unknown, two possible modes should be tried in each symbol index i, i.e., assuming r i  is the first symbol of the FECFrame header in robust mode or in high efficiency mode. 
         [0052]    In the robust FECFrame header mode, it is possible to use a soft correlation to replace steps 2 and 3. We can use C p,soft =Σ k=0   31 Re(s k )·Im(s (k+2)     32   )·(2·w (k+2)     32   −1) which is a soft correlation between {tilde over (w)} RM  and w RM , as a measure of the PN sequence. When the soft correlation is used in combination with the RM symmetry measure, a scaling is required. 
         [0053]    The modified code vector {tilde over (λ)} (1) ={tilde over (λ)}−(0, 0, . . . , {tilde over (b)} 6 , {tilde over (b)} 7 , . . . , {tilde over (b)} 15 )·G has a symmetric structure because for the transmitted 32-bit RM code vector λ, λ (1)  is a linear combination of the first 6 rows of the generator matrix and these 6 rows have a symmetric structure. 
         [0054]    The decision of the FECFrame header detection is made by two stages in steps 3 and 7. The decision statistic can also be formed by C=α 1 C p +α 2 C RM , where α 1  and α 2  are combining coefficients. That means, in step 3 of the detection, no matter what value of C p , the detection procedures are performed to step 6 and use C and a threshold T 3  to determine if it is a FECFrame header. If α 1 =0, it means that steps 2 and 3 are skipped and only the RM symmetry measure is used to make decision. If α 2 =0, it means that step 6 is skipped and only the PN correlation is used to make decision. 
         [0055]    The RM symmetry measure can be computed by C RM =Σ k=0   K |R RM (k)|, K=0, 1, 2, 3, or 4. The larger the number of K, the more robust the RM symmetry measure. In addition, any combination of |R RM (k)| can be used as a RM symmetry measure. 
         [0056]    One embodiment of the present principles is illustrated in  FIG. 3 , which shows a method for FEC frame header detection. A received complex data symbol is demodulated with a demapper in step  310 . The demodulated output is used to compute an estimated 32-bit PN sequence in step  320 . The binary correlation of the estimated 32-bit PN sequence is taken in step  330  and the correlation output is compared to a first threshold in step  334 . If the correlation is less than the first threshold value, the symbol index is increased by 1 in step  338  and the process goes back to demodulation of the next symbol in step  310 . If the correlation is greater than or equal to the first threshold, each bit of the 32-bit RM codeword is decoded by combining log-likelihood ratios of the upper branch bits with their corresponding lower branch bits to produce an estimated 32-bit RM codeword in step  340 . The first stage of majority logic decoding is performed in step  350  to produce 10 bits of a modified code vector. This modified code vector is used in step  360  to double confirm if the 32-symbol complex data symbol is the FEC frame header by performing an autocorrelation. If the autocorrelation value is less than a second threshold as checked in step  364 , the symbol index is increased in step  368  and demodulation of the next symbol is performed by proceeding back to step  310 . If the correlation is greater than or equal to the second threshold, the remaining two stages of majority logic decoding is performed in step  370  to obtain the 16 information bits of the FEC frame header. 
         [0057]    Another embodiment of the present principles is illustrated in  FIG. 4 , which shows an apparatus for FEC frame header detection. A received complex data symbol is demodulated with a demodulator  410 . The demodulated output is in signal communication with processor  430  which is used to compute an estimated 32-bit PN sequence and also to compute the binary correlation of the estimated 32-bit PN sequence. The correlation output is compared to a first threshold by processor  430 . If the correlation is less than the first threshold value, the symbol index is increased and demodulation of the next symbol is performed by demodulator  410 . If the correlation is greater than or equal to the first threshold, each bit of the 32-bit RM codeword is decoded by combining log-likelihood ratios of the upper branch bits with their corresponding lower branch bits to produce an estimated 32-bit RM codeword by decoder  440 , which is in signal communication with processor  430  output and its comparator signal output. The first stage of majority logic decoding is performed by decoder  440  which produces 10 bits of a modified code vector. This modified code vector is used by decoder  440  to calculate a RM symmetry measure to double confirm if the 32-symbol complex data symbol is the FEC frame header by performing an autocorrelation. If the autocorrelation value is less than a second threshold as checked by decoder  440 , the symbol index is increased and demodulation of the next symbol is performed by demodulator  410 . If the correlation is greater than or equal to the second threshold, the remaining two stages of majority logic decoding is performed by decoder  440  to obtain the 16 information bits of the FEC frame header. 
         [0058]    Another embodiment of the present principles is illustrated in  FIG. 5 , which shows a method for FEC frame header detection. The method comprises using the aforementioned detection method that uses QPSK demodulation in step  510  concurrently with the method of detection using 16-QAM demodulation in step  520 . A determination is made in step  530  to decide whether the FEC frame header has been detected in either robust or in high efficiency mode. 
         [0059]    Another embodiment of the present principles is illustrated in  FIG. 6 , which shows an apparatus. The apparatus is comprised of the detection circuitry  610  that uses QPSK demodulation and the detection circuitry  620  that uses 16-QAM demodulation. The outputs of these circuits are in signal communication with circuitry  630  that detects the FEC frame header in robust or in high efficiency mode. 
         [0060]    One embodiment of the present principles is illustrated in  FIG. 7 , which shows a method for FEC frame header detection  700 . A received complex data symbol is demodulated with a QPSK demapper in step  710 . The demodulated output is used to compute a soft correlation in step  720 . The soft correlation is scaled in step  730  and the scaled soft correlation output is compared to a first threshold in step  734 . If the correlation is less than the first threshold value, the symbol index is increased by 1 in step  338  and the process goes back to demodulation of the next symbol in step  710 . If the correlation is greater than or equal to the first threshold, each bit of the 32-bit RM codeword is decoded by combining log-likelihood ratios of the upper branch bits with their corresponding lower branch bits to produce an estimated 32-bit RM codeword in step  740 . The first stage of majority logic decoding is performed in step  750  to produce 10 bits of a modified code vector. This modified code vector is used in step  760  to double confirm if the 32-symbol complex data symbol is the FEC frame header by performing an autocorrelation. If the autocorrelation value is less than a second threshold as checked in step  764 , the symbol index is increased in step  768  and demodulation of the next symbol is performed by proceeding back to step  710 . If the correlation is greater than or equal to the second threshold, the remaining two stages of majority logic decoding is performed in step  770  to obtain the 16 information bits of the FEC frame header. 
         [0061]    Another embodiment of the present principles is illustrated in  FIG. 8 , which shows an apparatus for FEC frame header detection  800 . A received complex data symbol is demodulated with a demodulator  810 . The demodulated output is in signal communication with circuitry  830  and is used to compute a soft correlation. The soft correlation is scaled by scaler within circuitry  830  and the correlation output is compared to a first threshold within circuitry  830 . If the correlation is less than the first threshold value, the symbol index is increased and demodulation of the next symbol is performed by demodulator  810 . If the correlation is greater than or equal to the first threshold, each bit of the 32-bit RM codeword is decoded by combining log-likelihood ratios of the upper branch bits with their corresponding lower branch bits to produce an estimated 32-bit RM codeword by decoder  840 , which is in signal communication with circuitry  830  output and its comparison output. The first stage of majority logic decoding is performed by circuitry  840  which produces 10 bits of a modified code vector. This modified code vector is used by a RM symmetry circuit within decoder  840  to double confirm if the 32-symbol complex data symbol is the FEC frame header by performing an autocorrelation. If the autocorrelation value is less than a second threshold as checked by a second comparator within decoder  840 , the symbol index is increased and demodulation of the next symbol is performed by demodulator  810 . If the correlation is greater than or equal to the second threshold, the remaining two stages of majority logic decoding is performed by decoder  840  to obtain the 16 information bits of the FEC frame header. 
         [0062]    Another aspect of the present principles is illustrated in  FIG. 9 , which shows a method  900  for forming a decision statistic for FEC frame header detection. A first scaled correlation is formed in step  910 , followed by forming a second scaled correlation in step  920 . The scaled correlation outputs are next summed in step  930  and compared with a threshold to determine if an FEC frame header has been detected in step  940 . 
         [0063]    Another aspect of the present principles is illustrated in  FIG. 10 , which shows an apparatus  1000  for forming a decision statistic for FEC frame header detection. Circuitry  1010  for forming a first scaled correlation and circuitry  1020  for forming a second scaled correlation are in signal communication with an adder  1030  which sums the scaled correlation circuitry outputs. The adder output is in signal communication with a comparator  1040  that compares the adder output with a threshold to determine if an FEC frame header has been detected. 
         [0064]    The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (“DSP”) hardware, read-only memory (“ROM”) for storing software, random access memory (“RAM”), and non-volatile storage. 
         [0065]    Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
         [0066]    The present description illustrates the present principles. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the present principles and are included within its spirit and scope. 
         [0067]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the present principles and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0068]    Moreover, all statements herein reciting principles, aspects, and embodiments of the present principles, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0069]    Thus, for example, it will be appreciated by those skilled in the art that the block diagrams presented herein represent conceptual views of illustrative circuitry embodying the present principles. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
         [0070]    In the claims hereof, any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements that performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The present principles as defined by such claims reside in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. It is thus regarded that any means that can provide those functionalities are equivalent to those shown herein. 
         [0071]    Reference in the specification to “one embodiment” or “an embodiment” of the present principles, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment of the present principles. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
         [0072]    In conclusion, methods and apparatus for FEC frame header detection have been provided which detect the FEC frame header in both robust and high efficiency modes. Under the principles described herein, both methods may employed to determine if the FEC frame header has been sent in robust mode or in high efficiency mode. There is also provided a method and apparatus for FEC frame header detection in the robust mode using a soft correlation. There is also provided a method and apparatus for generating a decision statistic for FEC frame header detection.