Abstract:
Provided are an apparatus for estimating frequency offset from received signal and method for the same. The apparatus and method estimates frequency offset precisely without increment of autocorrelator by performing moving average filtering on a noised signal to thereby alleviate jitter. The frequency offset estimating apparatus includes: moving average filter for alleviating jitter of received signal; multiplier for multiplying a filtered signal by conjugate complex operanded pilot signal; phase-rotation value calculator for calculating a phase-rotation value from multiplicand operanded signal by using of symbol delay and an autocorrelation function; frequency offset estimator for estimating frequency offset from the phase-rotation value based on a smoothing function multiplication.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a mobile broadcast interactive satellite multimedia access technology; and, more particularly, to an apparatus and a method for estimating the carrier frequency offsets between transmitted (Tx) signal and Received (Rx) signal in Time division multiple access (TDMA) system.  
       DESCRIPTION OF RELATED ART  
       [0002]     In general, a receiving terminal of a wired/wireless communication system performs a process of reproducing the original signal after a received RF signal through a radio frequency(RF) module and an intermediate frequency(IF) module. In this procedure, the RF signal is down converted into a baseband signal based on a reference frequency created by an analog local oscillator.  
         [0003]     Nonetheless, since distortion occurs in the process of converting the RF signal to the baseband signal, it is impossible to perform normal recovery of the carrier frequency due to the imperfection of the analog local oscillator.  
         [0004]     Furthermore, if the receiving terminal would be on move, the carrier frequency of the receiving RF signal would be shifted due to the Doppler effect and thus the original signal cannot be recovered normally.  
         [0005]     A burst modem based on a TDMA uses feedforward frequency estimation circuit to estimate the offset of the carrier frequency and Doppler frequency. Herein, due to a problem of initial capture time for frequency offset, the feedforward frequency estimation circuit is used instead of a feedbackward frequency estimation circuit.  
         [0006]     For example, a frequency estimating method autocorrelator using and an arc tangent lookup table is disclosed in an article entitled “Data-Aided Freq. Estimation for Burst Digital Transmission”, by U. Mengali and M. Morelli (hereinafter which will be referred to as M&amp;M) IEEE Trans. Commun. Vol. 45, No. 1 January 1997.  
         [0007]     Also, a frequency estimating method using a similar autocorrelator to ‘M&amp;M’ is disclosed in an article entitled “Planna filtered techniques for burst mode carrier synchronization” (which will be referred to as Fitz), by IEEE Glebecom &#39;91, Phoenix, Ariz. M. P. Fitz, December 1991 and in an article entitled “Carrier Frequency Recovery in All Digital Modems for Burst Mode Transmission” by M. Luise and R. Reggiannini(which will be referred to as L&amp;R), IEEE Trans. Commun. Vol. 43, No. 3 March 1995.  
         [0008]     Generally, frequency offset lock range normalized to symbol rate, should be about 0.1 to remove the noise of transmitted(Tx) signal by matched filter, but the ‘Fitz’ and ‘L&amp;R’ cannot satisfy this requirement.  
         [0009]     Furthermore, the frequency offset estimating method suggested by the ‘M&amp;M’, ‘Fitz’ and ‘L&amp;R’, can make nearly perfect estimation by using an autocorrelator which is half as long as a half length of Data-Aided symbol. However, the autocorrelator has a problem that it increases the number of multipliers and memories to hereby increase the complexity.  
         [0010]     Therefore, it is required that a frequency offset estimation apparatus is developed to perform accurate frequency offset estimation with a simple hardware structure.  
       SUMMARY OF THE INVENTION  
       [0011]     It is, therefore, an object of the present invention to provide an apparatus and a method for estimating the frequency offset through a received(Rx) signal in TDMA system. An apparatus for estimating a frequency offset from a received(Rx) signal, comprising: a filter for alleviating a jitter of received signal thereby obtaining a filtered signal; a multiplier for multiplying the filtering signal by a complex conjugate pilot signal thereby obtaining a multiplied signal; a phase-rotation value calculator for calculating a phase-rotation value from the multiplied signal by using a symbol delay and an autocorrelation function; and a frequency offset estimator for estimating the frequency offset based on the phase-rotation value through a multiplication of a smoothing function.  
         [0012]     A method for estimating a frequency offset from a received signal, comprising the steps of: filtering a jitter by moving average filtering on the received signal; multiplying the filtered signal by complex a conjugate pilot signal; calculating a phase rotation value from the multiplied signal by using a symbol delay and an autocorrelation; and estimating the frequency offset by multiplying the calculated phase rotation value with a smoothing function. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  is a block diagram showing an apparatus for estimating a frequency offset from an in accordance with one embodiment of the present invention;  
         [0015]      FIG. 2  is a detailed schematic diagram describing a moving average filtering unit of  FIG. 1 ;  
         [0016]      FIG. 3  is a detailed schematic diagram describing a phase rotation measuring unit of  FIG. 1 ;  
         [0017]      FIG. 4  is a detailed schematic diagram describing a frequency offset estimating unit of  FIG. 1 ;  
         [0018]      FIG. 5  is a flowchart illustrating a method for estimating a frequency offset from a received signal in accordance with another embodiment of the present invention;  
         [0019]      FIG. 6  is a diagram illustrating a burst transmission structure for the frequency estimation method in accordance with the present invention;  
         [0020]      FIG. 7  is a graph showing a frequency offset capture area of the frequency estimating apparatus in accordance with the present invention; and  
         [0021]      FIG. 8  is a graph showing the accuracy of the frequency estimating apparatus in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Hereafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.  
         [0023]      FIG. 1  is a block diagram showing an apparatus estimating a frequency offset from an in accordance with one embodiment of the present invention;  
         [0024]     As shown, the apparatus include a moving average filtering unit  110 , a multiplier  120 , a phase rotation measuring unit  130 , a frequency offset estimating unit  140  and a frequency offset correcting unit  150 .  
         [0025]     The moving average filtering unit  110  receives a received signal ri(k) including a noise and alleviates the jitter of the received signal ri(k). Namely, the moving average filtering unit  110  enhances the accuracy of frequency offset estimation without increasing the number of autocorrelators by decreasing the distribution of the noise added received signal ri(k).  
         [0026]     The multiplier  120  multiplies a filtered signal by a complex conjugate pilot signal, and the phase rotation measuring unit  130  measures a phase rotation value between a preceding symbol and a current symbol by using a symbol delay and an autocorrelation function.  
         [0027]     The frequency offset estimating unit  140  estimates a frequency offset by using the measured phase rotation value and a smoothing function. The frequency offset correcting unit  150  corrects the frequency offset of the received signal by multiplying the noise added received signal by the estimated frequency offset, thereby outputting an output signal ro(k)  
         [0028]      FIG. 2  is a detailed diagram describing a moving average filtering unit  1 ;  FIG. 3  is a detailed diagram describing phase rotation measuring unit and  FIG. 4  is a detailed schematic describing a frequency offset estimating unit of  FIG. 1 .  
         [0029]     The moving average filtering unit  110  includes a plurality of delay taps  210 , a plurality of multiplier  220 , and a summing block  230 .  
         [0030]     In the drawing, ‘α’ denotes the number of delay taps  210 , ‘1/α’ means a filter coefficient as a weight value, and the initial delay tap is set to ‘0’. At a random point, when a received signal ri(k) is putted into the moving average filtering unit  110 , it is stored in a first delay tap  210  and coincidentally transmitted for an output of the moving average filtering unit  110 . The received signal ri(k) is expressed as the following equation 1.
 
 r ( k )= c   k   ·e   j(2πf     d     kT+θ)   +n ( k )  Eq. 1
 
         [0031]     Wherein, k means a symbol/sample unit that activated by 1 sample per symbol; Ck means a predetermined pilot signal; n(k) means a noise signal; fd means a frequency offset; and θmeans a phase rotation value.  
         [0032]     The signal stored in the first delay tap  210   1 , is transmitted to a second delay tap  210   2  and it is transmitted coincidentally for the moving average filtering unit  110 . The transmission process is performed serially, and the last delay tap signal is transmitted for the output of the moving average filtering unit  110 . Herein, the signal, transmitted for the output of moving average filtering unit  110 , is multiplied by the filter coefficient ‘1/ 60  ’ in the multiplier  220 , and a multiplied signal is outputted and add up to one signal as Equation 2 by summer  230 .  
                 r   ′     ⁡     (   k   )       =       1   α     ⁢       ∑     m   =   0       α   -   1       ⁢           ⁢     r   ⁢           ⁢     (   m   )                   Eq   .           ⁢   2             
 
         [0033]     The multiplier  120  multiplies the filtered signal r′(k) by complex conjugate pilot signal(C*k) and executes the operation based on Equation 3 thereby obtaining a multiplied signal z(k). Here, the pattern of complex conjugate pilot signal is 1, 1, 1 . . . .
 
 z ( k )= e   j(2πf     d     kT+θ)   +n ( k ) C   k *  Eq. 3
 
         [0034]     The phase rotation measuring unit  130  includes a symbol delay unit  310 , an autocorrelation calculating unit  320 , and a phase rotation value calculating unit  330 .  
         [0035]     In detail, after the multiplied signal z(k) is d-symbol delayed in the symbol delay unit  310 , the complex conjugate operation is executed. And, the delayed and complex conjugated signal z*(k−d) is multiplied by the multiplied signal z(k). Thereafter, the autocorrelation calculating unit  320  calculates an autocorrelation coefficient R(d) by using the operated signal z(k)z*(k−d) based on equation 4.  
                 R   ⁢           ⁢     (   d   )       =       1       L   0     -   d       ⁢       ∑     k   =   d         L   0     -   1       ⁢           ⁢     z   ⁢           ⁢     (   k   )     ⁢       z   *     ⁡     (     k   -   d     )               ,           ⁢     1   ≤   d   ≤   N     ,           ⁢     N   ≤       L   0     2               Eq   .           ⁢   4             
 
         [0036]     Wherein, R(d) is a calculating result of phase rotation value from symbol of d point to L 0 −1 symbol and L 0  is a estimation length of symbol and the number of autocorrelator included in the autocorrelation calculating unit  320  can be extended up to N, where N is a positive number. Herein, the multiply operand coefficient is N(2L 0 −N−1)/2.  
         [0037]     The phase rotation value calculating unit  330  calculates phase rotation value between a previous symbol and a present symbol by calculating an argument of the autocorrelation coefficient.
 
φ=[arg R ( d )−arg R ( d− 1)] 2π   Eq. 5
 
         [0038]     Subsequently, the frequency offset estimating unit  140  multiplies the phase rotation value by a coefficient w(d) defined by Equation 6. Herein, the coefficient defined in Equation 6 is a value of a smoothing function decided by the number of delay symbols ‘d’ of the symbol delay unit  310 .  
               ω   ⁢           ⁢     (   d   )       ⁢     =   Δ     ⁢       3   ⁡     [         (       L   0     -   d     )     ⁢     (       L   0     -   d   +   1     )       -     N   ⁡     (       L   0     -   N     )         ]         N   ⁡     (       4   ⁢     N   2       -     6   ⁢     NL   0       +     3   ⁢     L   0   2       -   1     )                 Eq   .           ⁢   6                     f   ^     d     ⁢   T     =       1     2   ⁢   π       ⁢       ∑     d   =   1     N     ⁢           ⁢       ω   ⁡     (   d   )       ×       [       arg   ⁢     {     R   ⁡     (   d   )       }       -     arg   ⁢     {     R   ⁡     (     d   -   1     )       }         ]       2   ⁢   π                     Eq   .           ⁢   7             
 
         [0039]     Because the argument operated phase rotation value and the phase offset estimation value are not linear, the argument operated phase rotation value and the frequency offset are lineared by performing the normalization with multiplying the coefficient defined in Equation 6.  
         [0040]     Subsequently, the frequency offset correcting unit  150  corrects the frequency offset of received signal by multiplying the frequency offset estimation value by the received signal expressed in Equation 1.  
         [0041]      FIG. 5  is a flowchart illustrating a method for estimating a frequency offset from a received signal in accordance another preferred embodiment of the present invention.  
         [0042]     As shown, first, the moving average filtering of noised received signal alleviates the jitter.  
         [0043]     Subsequently, at step s 520 , the filtered signal is multiplied by complex conjugate pilot signal. Then at step s 530 , a phase rotation value is calculated from multiplied signal by using the symbol delay and autocorrelation.  
         [0044]     Then, at step s 540 , the frequency offset is calculated by multiplying the calculated phase rotation value by smoothing function. Then at step s 550 , frequency offset is corrected by multiplying estimated frequency offset by formal received signal.  
         [0045]      FIG. 6  is a diagram illustrating a burst transmission structure for the frequency offset estimation method in accordance with one preferred embodiment of the present invention.  
         [0046]     As shown, a type 1 burst transmitting structure includes a preamble and a data field. The apparatus for estimating a frequency offset from a received signal of the present invention, estimates frequency offset in preamble of received burst, corrects frequency offset of the proper data field in the direction of reference numeral  610 .  
         [0047]     Furthermore, the burst transmitting structure of a type 2 is a distributed pilot structure that a pilot and a data field is separated. As shown, the apparatus for estimating a frequency estimates the frequency offset in pilot # 1 . Then corrects frequency offset of the forepart data field in pilot # 2  in the direction of a reference numeral  620 .  
         [0048]     Also, a frequency offset in pilot # 2  is corrected, corrects frequency offset of the forepart data field in pilot # 3  is corrected in the direction of reference numeral  630 .  
         [0049]     The burst transmit structure of a type 3 includes a preamble, data field and a postamble. The apparatus for estimating a frequency offset simultaneously corrects the frequency offset of the data field in the direction of reference numerals  640 ,  650 .  
         [0050]     Moreover, the burst transmitting a data field of type 4 includes a data field, a midamble and a data field, and the apparatus for estimating a frequency offset of the present invention corrects the frequency offset of the data field in the direction of reference numerals  660  and  670  by frequency estimating of each preamble and postamble.  
         [0051]      FIG. 7  is a graph showing a frequency offset capture area of the apparatus for estimating a frequency offset in accordance with one preferred embodiment of the present invention.  
         [0052]     Referring to  FIG. 7 , a reference numeral  710  showing a frequency offset capture area of existing frequency offset estimating method, reference numerals  720 ,  730  and  740  showing the frequency offset capture area of the present invention each having 2, 6, 10 taps in a moving average filter. The abscissa axis of  FIG. 7  is a frequency offset that normalized to symbol rate, and the ordinate axis is an average value of frequency offset by frequency offset estimating apparatus.  
         [0053]     As shown, the frequency offset lock range of reference numeral  710  is about 0.4. The more taps the moving average filter has, the narrower the offset lock range of the apparatus for estimating a frequency offset  720  and  730 ,  740  each having 2, 6, 10 taps become. For example, if the number of the taps of the moving average filter is 10, the frequency offset lock range of the apparatus for estimating a frequency offset is about 0.05. As the number of moving average filter is increased, the accuracy of apparatus for estimating a frequency offset is increased but the frequency offset lock range is decreased.  
         [0054]      FIG. 8  is a graph showing the accuracy of the apparatus for estimating a frequency offset in accordance with one preferred embodiment of the present invention.  
         [0055]     The abscissa axis is a Eb/No, the ordinate axis is a normalized variance of the frequency offset. The burst transmitting structure of preferred embodiment is a type 1 of  FIG. 4 , and the length of the preamble is 64.  
         [0056]     The reference numeral  810  illustrated in  FIG. 8  is a marginal value of frequency offset variance theoretically, and the reference numerals  820  and  830  are frequency offset distribution of the conventional frequency offset estimating method of having 4 and 32, autocorrelators, and the reference numerals  840 ,  850  and  860  are a frequency offset distribution of the frequency offset estimating having autocorrelators and having 2, 6 and 10 taps in present invention.  
         [0057]     As shown, if the number of autocorrelators is 32, the conventional method has a frequency offset distributing value that is close to a theoretic value. However it has a problem complexity in calculation and hardware that is increased, if the number of autocorrelators is 4, the complexity in calculation and hardware is decreased, but here is a problem that the performance is lower than the theoretic value.  
         [0058]     However, the apparatus for estimating a frequency offset  840 ,  850  and  860 , having autocorrelators, which is suggested in present invention, can acquire a value close to the offset variance value as the number of moving average filter is increased. That is, the apparatus for estimating a frequency offset of the present invention, shows performance approximated theoretic value without increasing the complexity of calculation and hardware, by not increasing the number of autocorrelators and by using the moving average filter.  
         [0059]     The present application contains subject matter related to Korean patent application no. 2003-79003, filed in the Korean Patent Office on Nov. 10, 2003, the entire contents of which being incorporated herein by reference.  
         [0060]     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention.