Patent Publication Number: US-7590184-B2

Title: Blind preamble detection for an orthogonal frequency division multiplexed sample stream

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to communication methods and systems, and more particularly to blind preamble detection for an orthogonal frequency division multiplexed sample stream. 
     RELATED ART 
     Traditionally, in an OFDM receiver, preamble detection for a complex valued sample stream has been performed by using supervised techniques. Such supervised techniques require the knowledge of the pattern of the complex valued sample stream. Accordingly, these techniques do not work in an environment where there may be a large number of possible patterns of the complex valued sample stream. This is because it becomes computationally difficult to process the large number of possible patterns of the complex valued sample stream. 
     Thus, there is a need for methods and systems for preamble detection in a complex valued sample stream, which are blind and thus do not require the processing of large number of patterns of the complex valued sample stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limited by the accompanying figures, in which like references indicate similar elements, and in which: 
         FIG. 1  is a block diagram of an exemplary OFDM receiver, consistent with one embodiment of the invention; 
         FIG. 2  is a flow chart for an exemplary method for determining the presence of a preamble in an OFDM complex valued sample stream, consistent with one embodiment of the invention; 
         FIG. 3  is a time domain representation of a complex valued sample stream and an autoconvolved portion of the complex valued sample stream, consistent with one embodiment of the invention; 
         FIG. 4  is a frequency domain representation of a complex valued sample stream and a squared portion of the complex valued sample stream, consistent with one embodiment of the invention; 
         FIG. 5  is a diagram illustrating a peak in the frequency domain representation of a squared portion of the complex valued sample stream, consistent with one embodiment of the invention; 
         FIG. 6  is a partial flow chart of an exemplary method for determining a coarse timing of a complex valued sample stream, consistent with one embodiment of the invention; 
         FIG. 7  shows exemplary autoconvolved identical size portions for three time instants; 
         FIG. 8  is a partial flow chart of the exemplary method for determining a coarse timing of the complex valued sample stream, consistent with one embodiment of the invention; and 
         FIG. 9  shows exemplary autocorrelations between portions of the complex valued sample stream for three exemplary refined time instants. 
     
    
    
     Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In one aspect, a method for determining a presence of a preamble for an orthogonal frequency division multiplexed (OFDM) complex valued sample stream is provided. The exemplary method may include capturing a portion of the OFDM complex valued stream and autoconvolving the portion of the OFDM complex valued sample stream to generate an autoconvolved portion. The method may further include determining a presence of a preamble in the OFDM complex valued sample stream if a peak is detected in the autoconvolved portion. 
     In another aspect, a method for generating a coarse timing for an orthogonal frequency division multiplexed (OFDM) complex valued sample stream is provided. The method may include capturing a portion of the OFDM complex valued sample stream and autoconvolving the portion of the OFDM complex valued sample stream to generate an autoconvolved portion. The method may further include determining a presence of a preamble in the OFDM complex valued sample stream, if a peak is detected in the autoconvolved portion. The method may further include, if the presence of the preamble is determined, selecting one of a plurality of peaks in the autoconvolved portion having greatest energy among the plurality of peaks. The method may further include identifying at least three time instants relative to the selected one of the plurality of peaks and autoconvolving at least three identical size portions of the OFDM complex valued sample stream beginning at each of the at least three time instants. The method may further include selecting a time instant out of the at least three time instants that is earliest in time and has a peak in the autoconvolved identical size portion. The method may further include identifying at least three refined time instants located at predetermined displacements in time from the selected time instant. The method may further include, for each one of the at least three refined time instants, measuring autocorrelation between two portions of the OFDM complex valued sample stream. The method may further include selecting one of the at least three refined time instants having lowest phase variance of the autocorrelation as an end of a cyclic prefix of a preamble. 
     In yet another aspect, an orthogonal frequency division multiplexing (OFDM) receiver comprising an OFDM engine, wherein the OFDM engine may be configured to capture a portion of an OFDM complex valued sample stream, is provided. The OFDM receiver may further be configured to autoconvolve the portion of the OFDM complex valued sample stream to generate an autoconvolved portion. The OFDM receiver may further be configured to determine a presence of a preamble in the OFDM complex valued sample stream, if a peak is detected in the autoconvolved portion. 
       FIG. 1  is an exemplary block diagram of an OFDM receiver  10 , consistent with one embodiment of the invention. OFDM receiver  10  may include, among other components, an OFDM engine  12  and a RF/mixed signal processor  16 . By way of example, RF/mixed signal processor  16  may receive a RF signal  14  via an antenna. RF/mixed signal processor  16  may generate an OFDM complex valued sample stream  18 . OFDM engine  12  may capture the OFDM complex valued sample stream  18  and process it further in accordance with the embodiments of the invention. OFDM receiver  10  may be implemented using any combination of hardware, software, and/or firmware. Although  FIG. 1  shows only an OFDM engine  12  and a RF/mixed signal processor  16  as part of OFDM receiver  10 , the OFDM receiver may include additional or fewer components. 
       FIG. 2  is a flow chart for an exemplary method for determining the presence of a preamble in an OFDM sample stream, consistent with one embodiment of the invention. As used herein, the term “preamble” is not limited to the header or the beginning part of an OFDM frame, instead it covers similar structures that may be in the middle, end, or any other part of the OFDM frame. Thus, preamble, as used herein may cover structures referred to as mid-amble or post-amble. As part of the exemplary method, OFDM engine  12  may capture a portion of an OFDM complex valued sample stream (step  20 ). As part of this step, OFDM engine  12  ( FIG. 1 ) may store samples corresponding to the portion of the OFDM complex valued sample stream in one or more buffers. 
     Next, OFDM engine  12  may autoconvolve the portion of the OFDM complex valued sample stream to generate an autoconvolved portion (step  22 ). Autoconvolving, as used herein, means that the portion of the OFDM complex valued sample stream is convolved with itself. Thus, for example using the exemplary equation below an autoconvolved portion of the complex valued sample stream may be generated. 
               Autoconvolved   ⁢           ⁢   portion     =       ∑     i   =     1429   ⁢   n           1429   ⁢   n     +   682       ⁢       Sw   ⁡     (   ⅈ   )       ⁢     Sw   ⁡     (     mod   ⁡     (       (     k   -   ⅈ     )     ,   1024     )       )                 
where, S W  is the portion of the OFDM complex valued sample stream;
 
     i is the index of the portion of the OFDM complex valued sample stream; 
     n indexes the autoconvolved portions (thus n would increment for each autoconvolution performed); and 
     k is the index of the autoconvolved portion (thus k would increment for every sample in the resulting autoconvolved portion). 
     Although the above equation relates to circular autoconvolution, consistent with embodiments of the invention, linear or other types of autoconvolution may be used, as well. Additionally, although the above equation uses certain constant values, these values may be different for different OFDM applications, such as Digital Audio Broadcasting, Digital Video Broadcasting, Integrated Services Digital Broadcasting, Wireless LAN (IEEE 802.11(a/g), HiperLAN/2, MMAC), Wireless MAN, and IEEE 802.20, or other OFDM applications, standards, and/or platforms. The above example corresponds to the IEEE 802.16(e) standard. Autoconvolving the portion of the OFDM complex valued sample stream may further include transforming the portion of the OFDM complex valued sample stream from time domain into frequency domain to generate a frequency domain portion of the OFDM complex valued sample stream. Furthermore, as part of this step, the frequency domain portion of the OFDM complex valued sample stream may be squared to generate a squared frequency domain portion of the OFDM complex valued sample stream. Next, the squared frequency domain portion of the OFDM complex valued sample stream may be subjected to an inverse transform to generate a time domain portion of the OFDM complex valued sample stream. 
     The next step may include determining a presence of a preamble in the OFDM complex valued sample stream if a peak is detected in the autoconvolved portion (step  24 ). As used herein, the term “peak” denotes a power value that exceeds a predetermined threshold. However, the term peak is not so limited, and may mean comparisons of other attributes, such as amplitude or energy, of an OFDM complex valued sample stream to respective threshold values. By way of example, the peak in the autoconvolved portion may be detected by computing power values in each sample of the autoconvolved portion and comparing each power value to a predetermined threshold. 
       FIG. 3  is a time domain representation of an OFDM complex valued sample stream  32  and an autoconvolved portion  38  of the OFDM complex valued sample stream, consistent with one embodiment of the invention. OFDM complex valued sample stream may include an in-phase component  34  (identified as I) and a complex component  36  (identified as Q). Autoconvolved portion  38  may include peaks  40  and  42  in the autoconvolved IQ components. 
       FIG. 4  is a frequency domain representation of an OFDM complex valued sample stream  44  and frequency domain representation of a squared portion  46  of the OFDM complex valued sample stream, consistent with one embodiment of the invention.  FIG. 4  shows frequency representation of I and Q components ( 48  and  50 ) corresponding to the OFDM complex valued sample stream. The squared portion may include peaks  52  and  54  in the squared IQ components.  FIG. 5  shows a peak  56  in the frequency domain representation of a squared portion  46  of the OFDM complex valued sample stream. 
       FIG. 6  is a partial flow chart of an exemplary method for determining a coarse timing of a complex valued sample stream, consistent with one embodiment of the invention. As shown in  FIG. 6 , first OFDM engine  12  may determine whether a preamble has been determined (step  60 ). If no preamble is determined, then the OFDM engine  12  may repeat steps shown in the exemplary flow chart of  FIG. 1 . If the preamble is determined, then OFDM engine  12  may select one of a plurality of peaks in the autoconvolved portion having the greatest energy among the plurality of peaks (step  62 ). 
     Next, OFDM engine  12  may identify at least three time instants relative to the selected peak (step  64 ). By way of example, as shown in  FIG. 7 , the I component of the autoconvolved portion may have a peak  70  and the Q component of the autoconvolved portion may have a peak  72  (at the same time instant as peak  70 ). Peaks  70  and  72  may correspond to a particular sample index n pk  of the autoconvolved portion. A first time instant t 1    76  may be determined relative to the peaks  70  and  72  using the following equation:
 
 t   1 =1429 n/f   s +( n   pk −1)/2 f   s  
 
     where, n indexes the autoconvolved portions (thus n would increment for each autoconvolution performed); and 
     f s  is the sampling rate of the OFDM complex valued sample stream. 
     Next, a second time instant t 2    78  may be determined by subtracting a value of ⅙f ?  from the value of t 1 , where f ?  is the sub-carrier spacing. A third time instant t 3    80  may be determined by subtracting a value of ⅓f ?  from the value of t 1 , where f ?  is the sub-carrier spacing. Other values of t 1 , t 2 , and t 3  may be computed for OFDM systems that do not comply with IEEE 802.16(e) standard. Although the above exemplary equation uses certain number of time instants and certain constant values, these may be different for different OFDM applications, such as Digital Audio Broadcasting, Digital Video Broadcasting, Integrated Services Digital Broadcasting, Wireless LAN (IEEE 802.11(a/g), HiperLAN/2, MMAC), Wireless MAN, and IEEE 802.20, or other OFDM applications, standards, and/or platforms. The above example corresponds to the IEEE 802.16(e) standard. Although  FIG. 7  shows three time instants, fewer or additional time instants may be determined. 
     Next, OFDM engine  12  may autoconvolve three identical size portions of the OFDM complex valued sample stream beginning at each of the at least three time instants (step  66 ). Exemplary values of the size of the autoconvolved three identical size portions include T fft  and T fft /2, where T fft  is 1/f ? . Using the exemplary equation below three identical size portions of the OFDM complex valued stream beginning at each of the at least three time instants may be autoconvolved. 
               Autoconvolved   ⁢           ⁢   identical   ⁢           ⁢   size   ⁢           ⁢   portion     =       ∑     i   =   tiXfs       tiXfs   +   682       ⁢       Sw   ⁡     (   ⅈ   )       ⁢     Sw   ⁡     (     mod   ⁡     (       (     k   -   ⅈ     )     ,   2048     )       )                 
where, S W  is the portion of the OFDM complex valued sample stream;
 
     i is the index of the portion of the OFDM complex valued sample stream; 
     f s  is the sampling rate of the OFDM complex valued sample stream; and 
     k is the index of the autoconvolved portion (thus k would increment for every sample in the resulting autoconvolved identical size portion). 
     Although the above equation uses certain constant values, these values may be different for different OFDM applications, such as Digital Audio Broadcasting, Digital Video Broadcasting, Integrated Services Digital Broadcasting, Wireless LAN (IEEE 802.11(a/g), HiperLAN/2, MMAC), Wireless MAN, and IEEE 802.20, or other OFDM applications, standards, and/or platforms. The above example corresponds to the IEEE 802.16(e) standard. Referring to  FIG. 7  now, exemplary autoconvolved identical size portions  82 ,  84 , and  86  for the three time instants t 1 , t 2 , and t 3  are shown. 
       FIG. 8  is a partial flow chart of the exemplary method for determining a coarse timing of the complex valued sample stream, consistent with one embodiment of the invention. As part of determining the coarse timing of the complex valued sample stream, OFDM engine  12  may select a time instant that is earliest in time and has a peak in the autoconvolved identical size portion (step  90 ). Thus, for example, OFDM engine  12  may select one of time instants t 1 , t 2 , and t 3 , which satisfies two conditions: first it has a peak in the corresponding autoconvolved identical size portion and second that it is the earliest in time. Referring to  FIG. 7 , by way of example, t 2  may be selected, since it has a peak and is the earliest in time. 
     Next, OFDM engine  12  may identify at least three refined time instants located at predetermined displacements in time from the selected time instant (step  92 ). By way of example, a first refined time instant may be located at a displacement of T fft /3 from the selected time instant (earlier in time than the selected time instant), the second refined time instant may be located at a displacement of T fft /3 from the selected time instant (later in time than the selected time instant), and the third refined time instant may be located at a zero displacement from the selected time instant. As part of this step, additional refined time instants may be identified. For example, additional refined time instants may be located at displacements of T fft /6, T fft /2, and 2 T fft /3 (both earlier in time and later in time, for example). 
     Next, OFDM engine  12  may, for each one of the three refined time instants, measure autocorrelation between two portions of the OFDM complex valued sample stream (step  94 ). Referring to  FIG. 9 , the three refined time instants t 4 , t 5 , and t 6    100  are shown. As part of this step, an autocorrelation between a portion  102  (starting at t 6 ) and a portion  104  (starting at t 6 +T fft ) may be determined. In addition, an autocorrelation between a portion  106  (starting at t 5 ) and a portion  108  (starting at t 5 +T fft ) may be determined. In addition, an autocorrelation between a portion  110  (starting at t 4 ) and a portion  112  (starting at t 4 +T fft ) may be determined. By way of example, each one of these portions ( 102 ,  104 ,  106 ,  108 ,  110 , and  112 ) may be T fft /8 wide. For example, autocorrelation between the two relevant portions may be determined using the following equation: 
     
       
         
           
             
               
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     where, k is a frequency index of the autocorrelation function; 
     T fft  is 1/f ? ; 
     f s  is the sampling rate of the OFDM complex valued sample stream; 
     N fft  is equal to T fft *f s ; and 
     ? is the incremental delay relative to N fft . 
     Although the above equation uses certain constant values, these values may be different for different OFDM applications, such as Digital Audio Broadcasting, Digital Video Broadcasting, Integrated Services Digital Broadcasting, Wireless LAN (IEEE 802.11(a/g), HiperLAN/2, MMAC), Wireless MAN, and IEEE 802.20, or other OFDM applications, standards, and/or platforms. The above example corresponds to the IEEE 802.16(e) standard. 
     Next, OFDM engine  12  may select one of the refined time instants having the lowest phase variance of the autocorrelation as an end of a cyclic prefix of a preamble (step  96 ). By way of example, as part of this step, a histogram of the phase of the previously computed autocorrelation may be generated. By way of example, the histogram may be generated using the following equation:
 
 f ( b ( n ))= hist (∠ R   rr ( k, Nfft ))
 
     Next, as part of this step, the phase variance of the autocorrelation may be computed. By way of example, the phase variance of the autocorrelation may be computed using the following equation: 
     
       
         
           
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     where f(b(n)) is the histogram of the angle of the autocorrelation; and 
     b(n) are the bin centers of the histogram of the autocorrelation. 
     In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.