Abstract:
A method and apparatus is disclosed for performing a synchronization of an incoming series of complex samples of an incoming signal with a known sequence of a plurality of coefficients which represents a pattern within the incoming signal, known to the synchronizer, where the synchronization is performed using a plurality of cross-correlations units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This is the first application filed for the present invention.  
         TECHNICAL FIELD  
         [0002]    This invention relates to the field of telecommunications. More precisely, this invention pertains to a method and apparatus for synchronizing a burst or a packet transmission signal.  
         BACKGROUND OF THE INVENTION  
         [0003]    In Orthogonal Frequency Division Multiplexing (OFDM), various methods are known and used for performing a synchronization of a signal with a known sequence.  
           [0004]    For packet transmission, such as in wireless LAN IEEE 802.11 systems or broadband wireless access IEEE 802.16a systems, an efficient method involves transmitting particular OFDM symbols which are known to a receiver.  
           [0005]    Now referring to FIG. 1, there is shown an embodiment of a prior art synchronizer. The synchronizer performs a synchronization of an incoming series of complex samples of an incoming signal with a known sequence of a plurality of coefficients Co . . . Cn which represent a pattern within the incoming signal, known to the synchronizer.  
           [0006]    The prior art synchronizer comprises a plurality of delay units  10 , a plurality of multipliers  12 , an adding unit  14  and a maximum finding unit  16 .  
           [0007]    Each delay unit  10  of the plurality of delay units  10  delays a corresponding incoming series of complex samples of the incoming signal by a fixed delay.  
           [0008]    Each multiplier  12  of the plurality of multipliers  12  multiplies a corresponding signal with a corresponding coefficient of the sequence of the plurality of coefficients to provide a corresponding multiplied signal.  
           [0009]    The adding unit  14  receives each of the multiplied signals and provides an added signal which is equal to the sum of each of the multiplied signals.  
           [0010]    The maximum finding unit  16  receives the added signal and provides a synchronization signal when the added signal satisfies criteria for identifying a maximum.  
           [0011]    Someone skilled in the art will appreciate that the maximum value signal is outputted in the case where the incoming input signal matches the sequence of the plurality of coefficients.  
           [0012]    Unfortunately, as will be appreciated by someone skilled in the art, such embodiments may be dependent on various parameters which may affect ultimately the maximum value signal provided by the maximum finding unit  16 . For instance, the signal may be affected by noise or the amplitude of the signal may be attenuated.  
           [0013]    It will also be appreciated by the skilled addressee that a multiplier of the plurality of multipliers  12  is costly to implement in hardware, especially in the case of a Field Programmable Gate Array (FPGA) or in the case of an Application Specific Integrated Circuit (ASIC).  
           [0014]    In view of the above, there is a need for a method and apparatus that will overcome the above-identified drawbacks and that will further be less complex to implement.  
         SUMMARY OF THE INVENTION  
         [0015]    It is an object of the invention to provide a method for synchronizing a burst with a known sequence of at least one coefficient.  
           [0016]    Yet another object of the invention is to provide an apparatus for synchronizing a burst with a known sequence of at least one coefficient.  
           [0017]    According to a first aspect of the invention, there is provided, in a communication device, a method of synchronizing at least one known coefficient to a transmitted signal having an arbitrary power value, the method comprising determining a quantization bit indicative of the arbitrary power value, correlating the quantization bit with the at least one known coefficient to obtain a correlated signal, selecting the correlated signal when the correlated signal matches a given criterion, synchronizing the transmitted signal with the at least one known coefficient using the selected correlated signal.  
           [0018]    According to another aspect of the invention, there is provided, in a communication device, a synchronizer for synchronizing a known sequence of coefficients, the sequence of coefficients having a real part and an imaginary part, to an incoming input signal, the incoming input signal having a real part and an imaginary part, comprising a first correlation unit correlating the imaginary part of the incoming input signal with the imaginary part of the sequence of at least one coefficient to provide a first correlated signal, a second correlation unit correlating the real part of the incoming input signal with the real part of the sequence of at least one coefficient to provide a second correlated signal, a third correlation unit correlating the imaginary part of the incoming input signal with the real part of the sequence of at least one coefficient to provide a third correlated signal, a fourth correlation unit correlating the real part of the incoming input signal with the imaginary part of the sequence of at least one coefficient to provide a fourth correlated signal, a first adding unit subtracting the first correlated signal and the second correlated signal to “4” to provide a first added signal, a second adding unit subtracting the third correlated signal and adding the fourth correlated signal to provide a second added signal and a criterion matching unit determining which of the first added signal and the second added signal matches a given criterion.  
           [0019]    According to another aspect of the invention, there is provided, in a communication system, a synchronizer for determining when a transmitted signal, having an arbitrary power value, matches at least one known coefficient, the synchronizer comprising a quantizer determining a quantization bit indicative of the arbitrary power value, a correlator to correlate the quantization bit with the at least one known coefficient to obtain a correlated signal, a selector to select the correlated signal when the correlated signal matches a given criterion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:  
         [0021]    [0021]FIG. 1 is a schematic which shows a prior art synchronizer;  
         [0022]    [0022]FIG. 2 is a schematic which shows a synchronizer which performs a synchronization of an incoming input signal with a sequence of at least one coefficient according to the preferred embodiment of the invention;  
         [0023]    [0023]FIG. 3 is a flowchart which shows the preferred embodiment of the invention; according to a first step, an incoming input signal is provided, a sequence of at least one coefficient is then provided and the incoming input signal provided is synchronized with the sequence of at least one coefficient;  
         [0024]    [0024]FIG. 4 is a flowchart which shows how the incoming input signal is provided;  
         [0025]    [0025]FIG. 5 is a flowchart which shows how the sequence of at least one coefficient is provided;  
         [0026]    [0026]FIG. 6 is a flowchart which shows how synchronization is achieved between the incoming input signal and the sequence of at least one coefficient in the preferred embodiment of the invention; and  
         [0027]    [0027]FIG. 7 is a schematic which shows a synchronizer which performs a synchronization of an incoming input signal with a sequence of a plurality of coefficients according to the preferred embodiment of the invention. 
     
    
       [0028]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    Now referring to FIG. 3, there is shown how the synchronizer  17  operates in the preferred embodiment of the invention.  
         [0030]    According to step  20 , an incoming input signal to synchronize with a sequence of at least one coefficient is provided to the synchronizer.  
         [0031]    According to step  22 , the sequence of at least one coefficient is provided to the synchronizer  17 .  
         [0032]    According to step  24 , the provided incoming input signal is synchronized with the provided sequence of at least one coefficient.  
         [0033]    Now referring to FIG. 4, there is shown how the signal to synchronize, according to step  20 , is provided in one embodiment of the invention.  
         [0034]    According to step  30 , the incoming input signal is received by the synchronizer. In the preferred embodiment of the invention, the incoming input signal comprises a real part and an imaginary part.  
         [0035]    According to step  32 , the real part of the incoming input signal is extracted.  
         [0036]    According to step  34 , the imaginary part of the incoming input signal is extracted.  
         [0037]    According to step  36 , signum of the extracted real part of the incoming input signal is computed.  
         [0038]    While someone skilled in the art will appreciate that signum (Sgn( )) function is usually defined as follows for a function f:  
           Sgn ( f )=1 if  f&gt; 0;  
           Sgn ( f )=0 if  f= 0; and  
         [0039]    Sgn(f)=−1 if f&lt;0, it will be appreciated in the following that the signum function used in the preferred embodiment described herein is defined as follows:  
           Sgn ( f )=1 if ƒ≧0;  
           Sgn ( f )=−1 if ƒ&lt;0.  
         [0040]    According to step  37 , signum of the extracted imaginary part of the incoming input signal is computed.  
         [0041]    Now referring to FIG. 5, there is shown how the sequence of at least one coefficient is provided according to step  22 .  
         [0042]    According to step  40 , the sequence of at least one coefficient is received by the synchronizer. In the preferred embodiment of the invention, the sequence of at least one coefficient comprises a real part and an imaginary part.  
         [0043]    According to step  42 , the real part of the sequence of at least one coefficient is extracted.  
         [0044]    According to step  44 , the imaginary part of the sequence of at least one coefficient is extracted.  
         [0045]    According to step  46 , signum of the extracted real part of the sequence of at least one coefficient is computed.  
         [0046]    According to step  47 , signum of the extracted imaginary part of the sequence of at least one coefficient is computed.  
         [0047]    Now referring to FIG. 6, there is shown how the synchronization is achieved between the provided incoming input signal and the sequence of at least one coefficient according to step  24  in the preferred embodiment of the invention.  
         [0048]    According to step  50 , a mapped signum of the real part of the incoming input signal is provided. In fact, the signum of the real part of the incoming input signal, which is computed according to step  36 , is mapped. Preferably, the signum of the real part of the incoming input signal is mapped in order to be located at 0 and 1 rather than to be located at −1 and 1.  
         [0049]    According to step  52 , a mapped signum of the imaginary part of the incoming input signal is provided. The signum of the imaginary part of the incoming input signal, which is computed according to step  37 , is mapped. Preferably, the signum of the imaginary part of the incoming input signal is mapped in order to be located at 0 and 1 rather than to be located at −1 and 1.  
         [0050]    According to step  54 , a mapped signum of the real part of the sequence of at least one coefficient is provided. The signum of the real part of the sequence of at least one coefficient, which is computed according to step  46 , is mapped. Preferably, the signum of the real part of the sequence of at least one coefficient is mapped in order to be located at 0 and 1 rather than to be located at −1 and 1.  
         [0051]    According to step  56 , a mapped signum of the imaginary part of the sequence of at least one coefficient is provided. The signum of the imaginary part of the sequence of at least one coefficient, which is computed according to step  47 , is mapped. Preferably, the signum of the imaginary part of the sequence of at least one coefficient is mapped in order to be located at 0 and 1 rather than to be located at −1 and 1.  
         [0052]    According to step  58 , an exclusive or (XOR) operation is performed using the mapped signum of the real part of the signal, the mapped signum of the imaginary part of the signal, the mapped signum of the real part of the sequence of at least one coefficient and the mapped signum of the imaginary part of the sequence of at least one coefficient. A cross-correlated result signal is provided upon completion of the exclusive or (XOR) operation.  
         [0053]    Someone skilled in the art will appreciate that such exclusive or (XOR) operation is of great advantage as it is very easy to implement in hardware.  
         [0054]    According to step  60 , a sum is computed using the result of the exclusive or (XOR) operation performed in step  58 .  
         [0055]    According to step  62 , a maximum in the cross-correlated signal is located using the result of the sum computed according to step  60 . In the preferred embodiment, the maximum is located by detecting the maximum of p=y*(n)·y(n), where y(n) is the cross-correlated signal.  
         [0056]    It will be appreciated that detecting the maximum in the cross correlated signal is a specific case of a more general embodiment where step  62  is achieved when the cross-correlated signal matches a given criterion. Such given criterion may be provided depending on an application. In fact, it will be appreciated that in the case where some noise is added to the signal, finding the maximum may not be reliable. Alternatively, the given criterion may be defined using statistical parameters for instance.  
         [0057]    Now referring to FIG. 2, there is shown a synchronizer  17  in one embodiment of the invention.  
         [0058]    In this embodiment, an incoming input signal G, having an arbitrary power value, is provided to the synchronizer. The incoming input signal G comprises a real part Re(G) and an imaginary part Im(G).  
         [0059]    The synchronizer  17  synchronizes the incoming input signal G with a sequence of at least one coefficient C. In this embodiment, the sequence comprises one coefficient. The sequence of at least one coefficient comprises a real part Re(C) and an imaginary part Im(C).  
         [0060]    The synchronizer  17  comprises a first correlation unit  18  which correlates the imaginary part of the incoming input signal Im(G) with the imaginary part of the sequence of at least one coefficient Im(C) to provide a first correlated signal.  
         [0061]    The synchronizer  17  comprises a second correlation unit  18  which correlates the real part of the incoming input signal Re(G) with the real part of the sequence of at least one coefficient Re(C) to provide a second correlated signal.  
         [0062]    The synchronizer  17  comprises a third correlation unit  18  which correlates the imaginary part of the incoming input signal Im(G) with the real part of the sequence of at least one coefficient Re(C) to provide a third correlated signal.  
         [0063]    The synchronizer  17  comprises a fourth correlation unit  18  which correlates the real part of the incoming input signal Re(G) with the imaginary part of the sequence of at least one coefficient Im(C) to provide a fourth correlated signal.  
         [0064]    The synchronizer  17  further comprises a first adding unit  14 . The first adding unit  14  subtracts the first correlated signal and the second correlated signal to 4 to provide a first added signal.  
         [0065]    The synchronizer  17  further comprises a second adding unit  14 . The second adding unit  14  subtracts the third correlated signal and adds the fourth correlated signal to provide a second added signal.  
         [0066]    The synchronizer  17  further comprises the maximum finding unit  16  which determines which of the first added signal and the second added signal provides a maximum value. Someone skilled in the art will appreciate that it is then possible to synchronize the incoming input signal G with the sequence of at least one coefficient C when the maximum value is located according to step  62  of FIG. 6. It will further be appreciated that the maximum finding unit  16  is a specific case of a criterion matching unit.  
         [0067]    Now referring to FIG. 7, there is shown a schematic of a synchronizer which performs a synchronization of an incoming input signal with a sequence of a plurality of coefficients according to the preferred embodiment of the invention.  
         [0068]    The synchronizer comprises a plurality of correlation units  18 , a plurality of delay units  15 , a plurality of adding units  14  and a maximum finding unit  16 .  
         [0069]    The plurality of correlation units  18 , the plurality of adding units  14  and the maximum finding unit  16  function in the same manner as described with FIG. 2.  
         [0070]    Each delay unit of the plurality of delay units  15  is used to delay an incoming signal by a predetermined delay.  
         [0071]    The result of a cross-correlation of the sequence of a plurality of coefficients with the incoming input signal is  
               y        (   n   )       =              ∑     i   =   0       N   -   1                c        (   i   )       *     *     g        (     i   +   n     )                       =                ∑     i   =   0       N   -   1            [       Re        {     c        (   i   )       }        Re        {     g        (     i   +   n     )       }       +     Im        {     c        (   i   )       }        Im        {     g        (     i   +   n     )       }         ]       +                            j          ∑     i   =   0       N   -   1            [       Re        {     c        (   i   )       }        Im        {     g        (     i   +   n     )       }       -     Im        {     c        (   i   )       }        Re        {     g        (     i   +   n     )       }         ]         ,                                 
 
         [0072]    where c(i) are the plurality of complex coefficients, g(i) are complex samples of the incoming input signal, N is the number of coefficients of the sequence, i is the coefficient index and n is the sample index.  
         [0073]    A signum of the last equation provides that  
               y        (   n   )       =              ∑     i   =   0       N   -   1            [       sign        {     Re        {     c        (   i   )       }       }        sign        {     Re        {     g        (     i   +   n     )       }       }       +                                sign        {     Im        {     c        (   i   )       }       }        sign        {     Im        {     g        (     i   +   n     )       }       }       ]     +                        j          ∑     i   =   0       N   -   1            [       sign        {     Re        {     c        (   i   )       }       }        sign        {     Im        {     g        (     i   +   n     )       }       }       -                                sign        {     Im        {     c        (   i   )       }       }        sign        {     Re        {     g        (     i   +   n     )       }       }       ]                               
 
         [0074]    The last equation may be expressed as:  
           y        (   n   )       =         ∑     i   =   0       N   -   1            [         m   RR          (     i   ,   n     )       +       m   II          (     i   ,   n     )         ]       +     j          ∑     i   =   0       N   -   1            [         m   RI          (     i   ,   n     )       -       m   IR          (     i   ,   n     )         ]             ,                         
 
         [0075]    where  
           m   RR  ( i, n )≡sign{ Re{c ( i )}}sign{ Re{g ( i+n )}} 
           m   II ( i, n )≡sign{ Im{c ( i )}}sign{ Im{g ( i+n )}} 
           m   RI ( i, n )≡sign{ Re{c ( i )}}sign{ Im{g ( i+n )}} 
           m   IR ( i, n )≡sign{ Im{c ( i )}}sign{ Re{g ( i+n )}} 
         [0076]    A mapping is then performed in order to have a result of a signum being one of 0 and 1, rather than being one of −1 and 1. To achieve such result, it is necessary that:  
           x   0 =1−2 x   n .  
         [0077]    With the expressions:  
           b   RR          (     i   ,   n     )       ≡       1   -       m   RR          (     i   ,   n     )         2                 b   II          (     i   ,   n     )       ≡       1   -       m   II          (     i   ,   n     )         2                   b   RI          (     i   ,   n     )       ≡       1   -       m   RI          (     i   ,   n     )         2       ,     it                 is                 possible                 to                 show                 that                 b   IR          (     i   ,   n     )       ≡       1   -       m   IR          (     i   ,   n     )         2                     y        (   n   )       =         ∑     i   =   0       N   -   1            [     1   -     2        b   RR       +   1   -     2        b   II         ]       +     j          ∑     i   =   0       N   -   1            [     1   -     2        b   RI       -     (     1   -     2        b   IR         )       ]                       =       2          ∑     i   =   0       N   -   1            [     1   -     b   RR     -     b   II       ]         +     2      j          ∑     i   =   0       N   -   1            [       b   IR     -     b   RI       ]                       =       2      N     -     2          ∑     i   =   0       N   -   1            b   RR         -     2          ∑     i   =   0       N   -   1            b   II         +     2        j        [         ∑     i   =   0       N   -   1            b   IR       -       ∑     i   =   0       N   -   1            b   RI         ]                                       
 
         [0078]    which may be simplified as  
         y        (   n   )       =     N   -       ∑     i   =   0       N   -   1            b   RR       -       ∑     i   =   0       N   -   1            b   II       +     j        [         ∑     i   =   0       N   -   1            b   IR       -       ∑     i   =   0       N   -   1            b   RI         ]                               
 
         [0079]    The above-identified formula is therefore implemented in FIG. 7.  
         [0080]    Someone skilled in the art will appreciate that such embodiment enables performing exclusive-OR which is of great advantage as the exclusive-OR, performed in units  18  of FIGS. 2 and 7, is easy to implement in hardware. It will further be appreciated that only one bit is used for quantization. Such one-bit quantization is of great advantage as signum function is not dependent on the amplitude of the signal but rather on the sign value of the input power of the signal.  
         [0081]    The maximum finding unit  16  operates by finding a peak in the amplitude or amplitude-squared component of y(n).  
         [0082]    More precisely, the peak is equal to:  
           p ( n )= y ( n )· y ( n )*  
         [0083]    In one embodiment, the maximum finding unit  16  operates by finding a maximum which is located when the sequence of the plurality of coefficients C 0  . . . Cn matches the coefficients in an input signal.  
         [0084]    Alternatively, the maximum finding unit  16  operates using a given criterion. The given criterion may be a threshold for instance. The threshold is chosen such that the probability of providing a false maximum is minimized while at the same time the probability of missing a valid trigger is sufficiently low. These conditions require estimating the probability distribution functions for the false synchronization and for a valid synchronization where the signal is distorted due to various impairments.  
         [0085]    The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.