# Patent

## Title
Apparatus and method for estimating timing offset of OFDM symbol, and method of recovering symbol timing of OFDM symbol

## Abstract

An apparatus for estimating a timing offset of each of orthogonal frequency division multiplexing (OFDM) symbols that are fast Fourier transform (FFT)-processed. A first correlator calculates correlation values and outputs a first correlation signal. A second correlator calculates correlation values and outputs a second correlation signal. A third correlator calculates a correlation value and outputs a third correlation signal. A delay correlator calculates a correlation value and outputs a timing offset information signal. A phase estimator detects phase components of the timing offset information signal and a sliding integrator accumulates the detected phase components of the timing offset information signal during a symbol interval and outputs an average value of the accumulated phase components during the symbol interval as the timing offset of each OFDM symbol.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present disclosure relates to orthogonal frequency division multiplexing (OFDM), and more particularly, to an apparatus and method for estimating timing offset of OFDM symbols and a method of recovering symbol timing of OFDM symbols.
2. Discussion of the Related Art
Many digital audio broadcasting (DAB) systems, digital multimedia broadcasting (DMB) systems, wireless local area network (WLAN) systems and the like use an orthogonal frequency division multiplexing (OFDM) communication method wherein, data is transmitted using a plurality of orthogonal subcarriers.
FIG. 1 shows a conventional frame structure of OFDM symbols. Referring to FIG. 1, each frame comprises a plurality of OFDM symbols S1, S2, S3, . . . . A NULL symbol is placed at the beginning of each frame. A first OFDM symbol S1 of each frame is a preamble symbol. The preamble symbol includes reference information on the OFDM symbols of each frame. In particular, DAB systems use a phase reference symbol (PRS) as a preamble symbol.
Each symbol interval is divided into a guard interval (GI) and a valid symbol interval (VSI). A copy of a final predetermined portion SF of the VSI is inserted into the GI to prevent inter-symbol interference (ISI).
FIG. 2 is a block diagram of a conventional receiver 200 used by the OFDM communication method. Referring to FIG. 2, a tuner 211 converts a signal received through an antenna (not shown) into an analog intermediate frequency (IF) band signal. An analog-to-digital converter (ADC) 212 converts the analog IF band signal into a digital signal. An interpolator 213 performs interpolation processing on symbol data received from the ADC 212 based on an interpolation frequency clock. A GI remover 214 removes a GI from each OFDM symbol and sends only valid symbols to a fast Fourier transform (FFT) block 215. A differential demodulator 216 performs differential demodulation on symbols which have been FFT-processed by the FFT block 215. A frequency deinterleaver 217 deinterleaves signals that have been output by the differential demodulator 216 and transfers the deinterleaved signals to a subsequent processing block (not shown).
The receiver 200 used by the OFDM communication method performs carrier frequency synchronization and symbol synchronization in order to exactly recover received OFDM symbols.
A carrier frequency error (carrier frequency offset) occurs due to a difference between the frequency of an oscillator used by a transmitter and the frequency of an oscillator used by the receiver 200. Each OFDM symbol comprises a plurality of adjacent subcarriers. Therefore, a small carrier frequency error can greatly influence the recovery of each OFDM symbol. To remove the carrier frequency error and perform the carrier frequency synchronization, the receiver 200 comprises a carrier recovery circuit 220. The carrier recovery circuit 220 comprises a coarse carrier recovery block 221 for compensating for an integral multiple frequency error and a fine carrier recovery block 222 for compensating for a decimal multiple frequency error.
A symbol timing recovery circuit 230 for carrying out the symbol synchronization finds an exact starting point of each OFDM symbol to perform an exact FFT process. The symbol timing recovery circuit 230 comprises a coarse symbol timing recovery block 231 for compensating for an integral multiple timing error and a fine symbol timing recovery block 240 for compensating for a decimal multiple timing error.
The OFDM communication method uses carrier recovery technology for performing the carrier frequency synchronization and symbol timing recovery technology for performing the symbol synchronization.
SUMMARY OF THE INVENTION
Exemplary embodiments of the present invention provide an apparatus and method for precisely estimating a timing offset of each of a plurality of orthogonal frequency division multiplexing (OFDM) symbols based on correlation values between received OFDM symbols and a reference OFDM symbol.
A method of recovering timing of each OFDM symbol based on an exactly estimated timing offset of each OFDM symbol is also provided.
According to an aspect of an exemplary embodiment of the present invention, an apparatus for estimating a timing offset of each of a plurality of orthogonal frequency division multiplexing (OFDM) symbols that are fast Fourier transform (FFT)-processed is provided. The apparatus includes a first correlator that calculates correlation values between a first OFDM symbol and subsequent OFDM symbols and outputs a first correlation signal. A second correlator calculates correlation values between a reference OFDM symbol and subsequent OFDM symbols and outputs a second correlation signal. A third correlator calculates a correlation value between the first correlation signal and the second correlation signal and outputs a third correlation signal. A delay correlator calculates a correlation value between a delay correlation signal obtained by delaying the third correlation signal by a predetermined subcarrier interval and the third correlation signal. The delay correlator outputs a timing offset information signal. A phase estimator detects phase components of the timing offset information signal. A sliding integrator accumulates the detected phase components of the timing offset information signal during a symbol interval and outputs an average value of the accumulated phase components during the symbol interval as the timing offset of each OFDM symbol.
The timing offset of each OFDM symbol is output.
The first OFDM symbol is a preamble symbol including reference information on OFDM symbols of a frame.
The preamble symbol is a phase reference symbol (PRS) when the apparatus for estimating the timing offset is applied to a receiver of a digital audio broadcasting (DAB) system.
The reference OFDM symbol is a PRS previously determined by a transmitter and a receiver.
The predetermined subcarrier interval is determined according to the range of estimation of the timing offset.
The apparatus further includes a buffer that stores the first OFDM symbol and sends the first OFDM symbol to the first correlator. A reference OFDM symbol generator generates the reference OFDM symbol and sends the reference OFDM symbol to the second correlator.
The apparatus further includes a demultiplexer that sends the first OFDM symbol among received OFDM symbols of a frame to the buffer and sends subsequent OFDM symbols to the first correlator and the second correlator.
According to another exemplary embodiment of the present invention, a method of estimating a timing offset of each of a plurality of FFT-processed OFDM symbols of a frame is provided. The method includes calculating correlation values between a first OFDM symbol and subsequent OFDM symbols and a first correlation signal is generated. Correlation values are calculated between a reference OFDM symbol and the subsequent OFDM symbols and a second correlation signal is generated. A correlation value is calculated between the first correlation signal and the second correlation signal and a third correlation signal is generated. A correlation value is calculated between a delay correlation signal obtained by delaying the third correlation signal by a predetermined subcarrier interval and the third correlation signal, and a timing offset information signal is generated. Phase components of the timing offset information signal are accumulated during a symbol interval and an average value of the accumulated phase components are output during the symbol interval as the timing offset of each OFDM symbol.
According to another exemplary embodiment of the present invention, a symbol timing recovering method includes outputting a timing offset of each OFDM symbol using the method of estimating the timing offset. The timing offset of each OFDM symbol is input into a numerically controlled oscillator (NCO), and an interpolation frequency clock corresponding to the timing offset of each OFDM symbol is generated. Interpolation-processing is performed on input symbol data based on the interpolation frequency clock.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features of the present disclosure will become more apparent by describing in detail exemplary embodiments of the present invention with reference to the attached drawings in which:
FIG. 1 shows a conventional frame structure having orthogonal frequency division multiplexing (OFDM) symbols;
FIG. 2 is a block diagram of a conventional receiver used by an OFDM communication method;
FIG. 3 is a block diagram of a receiver used by an OFDM communication method to which a timing offset estimating apparatus, according to an embodiment of the present invention, is applicable;
FIG. 4 is a block diagram of a timing offset estimating apparatus according to an embodiment of the present invention; and
FIG. 5 is a detailed block diagram of the timing offset estimating apparatus illustrated in FIG. 4.



DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
Exemplary embodiments of the present invention will now be described more fully with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.
FIG. 3 is a block diagram of a receiver used by an orthogonal frequency division multiplexing (OFDM) communication method according to an exemplary embodiment of the present invention. The method is applied to a timing offset estimating apparatus. The timing offset estimating apparatus, according to an exemplary embodiment of the present invention, can be included in a line symbol timing recovery block 340 illustrated in FIG. 3 and will be described with reference to FIGS. 4 and 5.
Referring to FIG. 3, the receiver comprises an analog-to-digital converter (ADC) 312, an interpolator 313, a guard interval (GI) remover 314, a fast Fourier transform (FFT) block 315, and a differential demodulator 316, which respectively correspond to the ADC 212, the interpolator 213, the GI remover 214, the FFT block 215, and the differential demodulator 216 illustrated in FIG. 2. Signals output by the ADC 312 are divided into an in-phase component (Ic) and a quadrature component (Qc).
A coarse carrier recovery block 321, a fine carrier recovery block 322, an adder 323, a numerically controlled oscillator (NCO) 324, an in-phase multiplier 325, and a quadrature multiplier 326 serve as a carrier recovery circuit. A coarse symbol timing recovery block 331, the fine symbol timing recovery block 340, and an NCO 333 serve as a symbol timing recovery circuit.
A carrier recovery signal that is a combination of a signal output by the coarse carrier recovery block 321 and a signal output by the fine carrier recovery block 322 is input into the NCO 324. The NCO 324 sends a carrier frequency clock, corresponding to the carrier recovery signal, to the in-phase multiplier 325 and the quadrature multiplier 326 to remove a carrier frequency error. An in-phase low pass filter (LPF) 301 and a quadrature LPF 302 are used as an image rejection filter to reject an image frequency signal included in the output signal of the ADC 312.
The coarse symbol timing recovery block 331 receives OFDM symbols that are FFT-processed by the FFT block 315 and finds an exact starting point of each OFDM symbol in order to perform an exact FFT process. When an inverse fast Fourier transform (IFFT) is required to perform a coarse symbol timing recovery, the FFT block 315 can serve as an IFFT block as illustrated. A fine coarse symbol timing recovery performs a more line and exact symbol timing recovery than the coarse symbol timing recovery alone. The fine symbol timing recovery block 340 comprises the timing offset estimating apparatus that estimates a timing offset of each OFDM symbol based on the FFT-processed OFDM symbols. The NCO 333 receives a timing offset εn of each OFDM symbol from the fine symbol timing recovery block 340 and generates an interpolation frequency clock corresponding to the timing offset εn of each OFDM symbol. The interpolator 313 interpolation processes symbol data received from the in-phase LPF 301 and the quadrature LPF 302 based on the interpolation frequency clock received from the NCO 333.
The timing offset estimating apparatus according to an exemplary embodiment of the present invention will be described with reference to FIGS. 4 and 5.
FIG. 4 is a block diagram of the timing offset estimating apparatus according to an exemplary embodiment of the present invention. Referring to FIG. 4, the timing offset estimating apparatus comprises a FFT block 415, a first correlator 4410, a second correlator 4420, a third correlator 4430, a delay correlator 4440, a phase estimator 4450, and a sliding integrator 4460. As used herein, “n” denotes a symbol index and “m” denotes a subcarrier index.
The first correlator 4410 calculates correlation values between a first OFDM symbol r1,m and subsequent OFDM symbols rn,m and outputs a first correlation signal C1(n,m). For example, the first correlator 4410 calculates a correlation value between the first OFDM symbol r1,m and a second OFDM symbol r2,m and outputs the first correlation signal C1(2,m). A correlation value between the first OFDM symbol and a third OFDM symbol r3,m is calculated. The first correlation signal C1(3,m) is output. A correlation value between the first OFDM symbol r1,m and a fourth OFDM symbol r4,m is calculated. The first correlation signal C1(4,m) is output.
The first OFDM symbol r1,m is a preamble symbol including reference information on each of the OFDM symbols of a frame. The timing offset estimating apparatus according to the present exemplary embodiment of the present invention can be employed in a digital audio broadcasting (DAB) system, a digital multimedia broadcasting (DMB) system, a wireless local area network (WLAN) system and the like. A phase reference symbol (PRS) is the preamble symbol in the DAB system that employs the timing offset estimating apparatus according to the present exemplary embodiment.
The second correlator 4420 calculates correlation values between a reference OFDM symbol rPRS and the subsequent OFDM symbols rn,m and outputs a second correlation signal CPRS(n,m). For example, the second correlator 4420 calculates a correlation value between the reference OFDM symbol rPRS and the second OFDM symbol r2,m and outputs the second correlation signal CPRS(2,m). The second correlator 4420 calculates a correlation value between the reference OFDM symbol rPRS and the third OFDM symbol r3,m and outputs the second correlation signal CPRS(3,m). The second correlator 4420 calculates a correlation value between the reference OFDM symbol rPRS and the fourth OFDM symbol r4,m and outputs the second correlation signal CPRS(4,m).
The reference OFDM symbol rPRS is a PRS previously determined by a transmitter and a receiver. The second correlator 4420 can receive the reference phase symbol rPRS from a reference OFDM symbol generator (see 570 in FIG. 5) included in the receiver. As used herein, “r1,m” denotes a PRS received by the receiver and “rPRS” denotes a PRS generated by the receiver.
The third correlator 4430 calculates a correlation value between the first correlation signal C1(n,m) and the second correlation signal CPRS(n,m) and outputs a third correlation signal CCOR(n,m).
The delay correlator 4440 calculates a correlation value between a delay correlation signal (see CCOR(n,m−W) in FIG. 5) obtained by delaying the third correlation signal CCOR(n,m) by a predetermined subcarrier interval W and the third correlation signal CCOR(n,m), and outputs a timing offset information signal Fn. The predetermined subcarrier interval W is determined according to the range of estimation of the timing offset. In detail, the range of estimation of the timing offset can be varied according to the predetermined subcarrier interval W.
The phase estimator 4450 detects phase components φn of the timing offset information signal Fn received from the delay correlator 4440.
The sliding integrator 4460 accumulates the detected phase components φn of the timing offset information signal Fn during a symbol interval, and outputs an average value of the accumulated phase components during the symbol interval as the timing offset εn of each OFDM symbol. The timing offset εn of each OFDM symbol output by the sliding integrator 4460 can he input into the NCO 333 and is a basis for the fine symbol timing recovery.
FIG. 5 is a detailed block diagram of the timing offset estimating apparatus illustrated in FIG. 4. Referring to FIG. 5, the timing offset estimating apparatus comprises a first correlator 5410, a second correlator 5420, a third correlator 5430, a delay correlator 5440, a phase estimator 5450, a sliding integrator 5460, a FFT block 515, a differential demodulator 516, a demultiplexer 550, a buffer 560, and a reference OFDM symbol generator 570.
The demultiplexer 550 receives OFDM symbols that are FFT-processed by the FFT block 515. The demultiplexer 550 sends the first OFDM symbol r1,m to the buffer 560 among the OFDM symbols of the frame and sends the subsequent OFDM symbols rn,m to the first correlator 5410 and the second correlator 5420. The buffer 560 stores the first OFDM symbol r1,m received from the demultiplexer 550 and sends the first OFDM symbol r1,m to the first correlator 5410. The reference OFDM symbol generator 570 generates the reference OFDM symbol RPRS and sends the reference OFDM symbol rPRS to the second correlator 5420.
The mth subcarrier data sn,m of an FFT-processed nth OFDM symbol is expressed in Equation 1 below. A transfer function Hn,m of a transmission channel including a multiple path is expressed in Equation 2 below The OFDM symbols rn,m output from the FFT block 515 to the demultiplexer 550 is expressed in Equation 3 below and can be derived from equations 1 and 2.




  
    
      
        
          s
          
            n
            ,
            m
          
        
        =
        
          
            
            
              s
              
                n
                ,
                m
              
            
            
          
          ·
          
            e
            
              -
              
                jφ
                
                  n
                  ,
                  m
                
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  
  
    
      
        
          H
          
            n
            ,
            m
          
        
        =
        
          
            
            
              H
              
                n
                ,
                m
              
            
            
          
          ·
          
            e
            
              -
              
                jϛ
                
                  n
                  ,
                  m
                
              
            
          
        
      
    
    
      
        (
        2
        )
      
    
  
  
    
      
        
          r
          
            n
            ,
            m
          
        
        =
        
          
            
              
              
                s
                
                  n
                  ,
                  m
                
              
              
            
            ·
            
              
              
                H
                
                  n
                  ,
                  m
                
              
              
            
            ·
            
              e
              
                -
                
                  j
                  ⁡
                  
                    (
                    
                      
                        φ
                        
                          n
                          ,
                          m
                        
                      
                      +
                      
                        ϛ
                        
                          n
                          ,
                          m
                        
                      
                      +
                      
                        
                          2
                          ⁢
                          π
                          ⁢
                          
                            
                          
                          ⁢
                          m
                          ⁢
                          
                            
                          
                          ⁢
                          ɛ
                        
                        N
                      
                    
                    )
                  
                
              
            
          
          +
          
            N
            
              n
              ,
              m
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




wherein, φn,m denotes phase information on the mth subcarrier data sn,m of the FFT-processed nth OFDM symbol, ζn,m denotes delay information on the transmission channel, (2πmε/N) denotes information on the timing offset ε, and Nn,m denotes noise. N of the (2πmε/N) denotes the number of all FFT points.
Referring to Equation 3, the first OFDM symbol r1,m stored in the buffer 560 is expressed in Equation 4 below.




  
    
      
        
          r
          
            1
            ,
            m
          
        
        =
        
          
            
              
              
                s
                
                  1
                  ,
                  m
                
              
              
            
            ·
            
              
              
                H
                
                  1
                  ,
                  m
                
              
              
            
            ·
            
              e
              
                -
                
                  j
                  ⁡
                  
                    (
                    
                      
                        φ
                        
                          1
                          ,
                          m
                        
                      
                      +
                      
                        ϛ
                        
                          1
                          ,
                          m
                        
                      
                      +
                      
                        
                          2
                          ⁢
                          π
                          ⁢
                          
                            
                          
                          ⁢
                          m
                          ⁢
                          
                            
                          
                          ⁢
                          ɛ
                        
                        N
                      
                    
                    )
                  
                
              
            
          
          +
          
            N
            
              1
              ,
              m
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




The reference OFDM symbol rPRS generated by the reference OFDM symbol generator 570 is expressed in Equation 5 below.

rPRS=|s1,m|·e−j·φ1,m   (5)

The reference OFDM symbol rPRS is generated by the reference OFDM symbol generator 570 and thus is not related to the transmission channel. Therefore, the reference OFDM symbol rPRS includes size information |S1,m| and the phase information φn,m.
The first correlator 5410 comprises a conjugation unit 5411 and a multiplication unit 5412. The conjugation unit 5411 receives the first OFDM symbol r1,m from the buffer 560 and outputs a conjugation complex signal of the first OFDM symbol r1,m. The multiplication unit 5412 multiplies the conjugation complex signal of the first OFDM symbol r1,m and subsequent OFDM symbols rn,m and outputs the first correlation signal C1(n,m). Referring to Equations 3 and 4, the first correlation signal C1(n,m) can be expressed in Equation 6 below.




  
    
      
        
          
            C
            1
          
          ⁡
          
            (
            
              n
              ,
              m
            
            )
          
        
        =
        
          
            
              r
              
                n
                ,
                m
              
            
            ⊗
            
              r
              
                1
                ,
                m
              
              *
            
          
          =
          
            
              
                
                
                  
                    s
                    
                      1
                      ,
                      m
                    
                  
                  ·
                  
                    s
                    
                      n
                      ,
                      m
                    
                  
                
                
              
              ·
              
                
                
                  
                    H
                    
                      1
                      ,
                      m
                    
                  
                  ·
                  
                    H
                    
                      n
                      ,
                      m
                    
                  
                
                
              
              ·
              
                e
                
                  -
                  
                    j
                    ⁡
                    
                      (
                      
                        
                          φ
                          
                            n
                            ,
                            m
                          
                        
                        -
                        
                          φ
                          
                            1
                            ,
                            m
                          
                        
                        +
                        
                          ϛ
                          
                            n
                            ,
                            m
                          
                        
                        -
                        
                          ϛ
                          
                            1
                            ,
                            m
                          
                        
                      
                      )
                    
                  
                
              
            
            +
            
              N
              
                n
                ,
                m
              
              ′
            
          
        
      
    
    
      
        (
        6
        )
      
    
  




wherein, N′n,m denotes a coarse component including noise.
The second correlator 5420 comprises a conjugation unit 5421 and a multiplication unit 5422. The conjugation unit 5421 receives the reference OFDM symbol rPRS and outputs a conjugation complex signal of the reference OFDM symbol rPRS. The multiplication unit 5422 multiplies the conjugation complex signal of the reference OFDM symbol rPRS and subsequent OFDM symbols rn,m and outputs the second correlation signal CPRS(n,m). Referring to Equations 3 and 5, the second correlation signal CPRS(n,m) can be expressed in Equation 7 below.




  
    
      
        
          
            C
            PRS
          
          ⁡
          
            (
            
              n
              ,
              m
            
            )
          
        
        =
        
          
            
              r
              
                n
                ,
                m
              
            
            ⊗
            
              r
              PRS
              *
            
          
          =
          
            
              
                
                
                  
                    s
                    
                      1
                      ,
                      m
                    
                  
                  ·
                  
                    s
                    
                      n
                      ,
                      m
                    
                  
                
                
              
              ·
              
                
                
                  H
                  
                    n
                    ,
                    m
                  
                
                
              
              ·
              
                e
                
                  -
                  
                    j
                    ⁡
                    
                      (
                      
                        
                          φ
                          
                            n
                            ,
                            m
                          
                        
                        -
                        
                          φ
                          
                            1
                            ,
                            m
                          
                        
                        +
                        
                          ϛ
                          
                            n
                            ,
                            m
                          
                        
                        +
                        
                          
                            2
                            ⁢
                            π
                            ⁢
                            
                              
                            
                            ⁢
                            m
                            ⁢
                            
                              
                            
                            ⁢
                            ɛ
                          
                          N
                        
                      
                      )
                    
                  
                
              
            
            +
            
              N
              
                n
                ,
                m
              
              ′
            
          
        
      
    
    
      
        (
        7
        )
      
    
  




wherein, N′n,m denotes a coarse component including noise.
The third correlator 5430 comprises a conjugation unit 5431 and a multiplication unit 5432. The conjugation unit 5431 receives the first correlation signal C1(n,m) and outputs a conjugation complex signal of the first correlation signal C1(n,m). The multiplication unit 5432 multiplies the conjugation complex signal of the first correlation signal C1(n,m) and the second correlation signal CPRS(n,m) and outputs the third correlation signal CCOR(n,m). Referring to Equations 6 and 7, the third correlation signal CCOR(n,m) can be expressed in Equation 8 below.




  
    
      
        
          
            C
            COR
          
          ⁡
          
            (
            
              n
              ,
              m
            
            )
          
        
        =
        
          
            
              
                C
                PRS
              
              ⁡
              
                (
                
                  n
                  ,
                  m
                
                )
              
            
            ⊗
            
              
                C
                1
                *
              
              ⁡
              
                (
                
                  n
                  ,
                  m
                
                )
              
            
          
          =
          
            
              
                
                  
                  
                    
                      s
                      
                        1
                        ,
                        m
                      
                    
                    ·
                    
                      s
                      
                        n
                        ,
                        m
                      
                    
                  
                  
                
                2
              
              ·
              
                
                
                  H
                  
                    1
                    ,
                    m
                  
                
                
              
              ·
              
                
                  
                  
                    H
                    
                      n
                      ,
                      m
                    
                  
                  
                
                2
              
              ·
              
                e
                
                  -
                  
                    j
                    ⁡
                    
                      (
                      
                        
                          ϛ
                          
                            1
                            ,
                            m
                          
                        
                        +
                        
                          
                            2
                            ⁢
                            π
                            ⁢
                            
                              
                            
                            ⁢
                            m
                            ⁢
                            
                              
                            
                            ⁢
                            ɛ
                          
                          N
                        
                      
                      )
                    
                  
                
              
            
            +
            
              N
              
                n
                ,
                m
              
              ′
            
          
        
      
    
    
      
        (
        8
        )
      
    
  




wherein N′n,m denotes a coarse component including noise.
In Equation 8, a phase component of third correlation signal CCOR(n,m) is a combination of a component ζ1,m of a delayed transmission channel and a component 2πmε/N of the timing offset ε. The component ζ1,m of the delayed transmission channel is removed to detect only the component 2πmε/N of the timing offset ε. The delay correlator 5440 removes the component ζ1,m of the delayed transmission channel.
The delay correlator 5440 comprises a delay unit 5441, a conjugation unit 5442, and a multiplication unit 5443. The delay unit 5441 outputs the delay correlation signal CCOR(n,m−W) obtained by delaying the third correlation signal CCOR(n,m) by the predetermined subcarrier interval W. The conjugation unit 5442 outputs a conjugation complex signal of the delay correlation signal CCOR(n,m−W). The multiplication unit 5443 multiplies the conjugation complex signal of the delay correlation signal CCOR(n,m−W) and the third correlation signal CCOR(n,m), and outputs the timing offset information signal Fn.
The delay correlation signal CCOR(n,m−W) can be expressed as follows:




  
    
      
        
          
            C
            COR
          
          ⁡
          
            (
            
              n
              ,
              
                m
                -
                w
              
            
            )
          
        
        =
        
          
            
              
                
                
                  
                    s
                    
                      1
                      ,
                      
                        m
                        -
                        w
                      
                    
                  
                  ·
                  
                    s
                    
                      n
                      ,
                      
                        m
                        -
                        w
                      
                    
                  
                
                
              
              2
            
            ·
            
              
              
                H
                
                  1
                  ,
                  
                    m
                    -
                    w
                  
                
              
              
            
            ·
            
              
                
                
                  H
                  
                    n
                    ,
                    
                      m
                      -
                      w
                    
                  
                
                
              
              2
            
            ·
            
              e
              
                -
                
                  j
                  ⁡
                  
                    (
                    
                      
                        ϛ
                        
                          1
                          ,
                          
                            m
                            -
                            w
                          
                        
                      
                      +
                      
                        
                          2
                          ⁢
                          
                            π
                            ⁡
                            
                              (
                              
                                m
                                -
                                w
                              
                              )
                            
                          
                          ⁢
                          ɛ
                        
                        N
                      
                    
                    )
                  
                
              
            
          
          +
          
            N
            
              n
              ,
              
                m
                -
                w
              
            
            ′
          
        
      
    
    
      
        (
        9
        )
      
    
  




The timing offset information signal Fn can be expressed as follows:




  
    
      
        
          
            
              Fn
              =
              
                
                  
                    C
                    COR
                  
                  ⁡
                  
                    (
                    
                      n
                      ,
                      m
                    
                    )
                  
                
                ⊗
                
                  
                    C
                    COR
                    *
                  
                  ⁡
                  
                    (
                    
                      n
                      ,
                      
                        m
                        -
                        w
                      
                    
                    )
                  
                
              
            
          
        
        
          
            
              =
              
                
                  
                    
                    
                      Fn
                      -
                      
                        N
                        ⁢
                                              
                    
                    
                  
                  ·
                  
                    e
                    
                      -
                      
                        j
                        ⁡
                        
                          (
                          
                            
                              ϛ
                              
                                1
                                ,
                                m
                              
                            
                            +
                            
                              
                                2
                                ⁢
                                
                                  
                                
                                ⁢
                                π
                                ⁢
                                
                                  
                                
                                ⁢
                                m
                                ⁢
                                
                                  
                                
                                ⁢
                                ɛ
                              
                              N
                            
                            -
                            
                              ϛ
                              
                                1
                                ,
                                
                                  m
                                  -
                                  w
                                
                              
                            
                            -
                            
                              
                                2
                                ⁢
                                
                                  π
                                  ⁡
                                  
                                    (
                                    
                                      m
                                      -
                                      w
                                    
                                    )
                                  
                                
                                ⁢
                                ɛ
                              
                              N
                            
                          
                          )
                        
                      
                    
                  
                
                +
                
                  N
                  
                    n
                    ,
                    m
                  
                  ′′′′
                
              
            
          
        
        
          
            
              =
              
                
                  
                    
                    
                      Fn
                      -
                      
                        N
                        
                          n
                          ,
                          m
                        
                        ′′′′
                      
                    
                    
                  
                  ·
                  
                    e
                    
                      -
                      
                        j
                        ⁡
                        
                          (
                          
                            
                              ϛ
                              
                                1
                                ,
                                m
                              
                            
                            -
                            
                              ϛ
                              
                                1
                                ,
                                
                                  m
                                  -
                                  w
                                
                              
                            
                            +
                            
                              
                                2
                                ⁢
                                π
                                ⁢
                                
                                  
                                
                                ⁢
                                w
                                ⁢
                                
                                  
                                
                                ⁢
                                ɛ
                              
                              N
                            
                          
                          )
                        
                      
                    
                  
                
                +
                
                  N
                  
                    n
                    ,
                    m
                  
                  ′′′′
                
              
            
          
        
      
    
    
      
        (
        10
        )
      
    
  




Referring to Equations 8 and 9, Equation 10 satisfies that

|Pn|=|s1,m·sn,m·s1,m−W·sn,m−W|2·|H1,m·H1,m−W|·|Hn,m·Hn,m−W|2 

The delay characteristics of the transmission channel during a symbol interval remain almost constant. For example, the delay characteristics of the transmission channel during a symbol interval remain substantially invariable to influence the estimation of the timing offset. Therefore, in Equation 10, ζ1,m−ζ1,m−W≈0. Therefore, the delay correlator 5440 removes the component of the delayed transmission channel from the phase component of the third correlation signal CCOR(n,m).
The phase estimator 5450 detects phase components φn of the timing offset information signal Fn. Since the noise component N′′′′n,m of Equation 10 has random characteristics, its average value is close to 0. The phase components φn of the timing offset information signal Fn are expressed as follows:




  
    
      
        
          φ
          n
        
        =
        
          
            2
            ⁢
            π
            ⁢
            
              
            
            ⁢
            w
            ⁢
            
              
            
            ⁢
            E
          
          N
        
      
    
    
      
        (
        11
        )
      
    
  




However, when the phase estimator 5450 estimates the phase components φn of the timing offset information signal Fn, an instant estimation error can occur according to the environment of the transmission channel. To prevent the instant estimation error, the sliding integrator 5460 accumulates the phase components φn output by the phase estimator 5450 during a symbol interval, and outputs an average value of the accumulated phase components as the timing offset εn.
The sliding integrator 5460 comprises an integrator 5461 and a multiplier 5462. The integrator 5461 accumulates the phase components φn of the timing offset information signal Fn during a symbol interval. The multiplier 5462 outputs the average value obtained by multiplying the accumulated phase components by an average constant N/2πW as the timing offset εn of each OFDM symbol.
The operation of the phase estimator 5450 and the sliding integrator 5460 can be explained as follows:




  
    
      
        
          
            
              Yn
              =
              
              ⁢
              
                
                  
                    N
                    
                      2
                      ⁢
                      π
                      ⁢
                      
                        
                      
                      ⁢
                      W
                    
                  
                  ·
                  
                    1
                    
                      K
                      -
                      W
                    
                  
                
                ⁢
                
                  
                    ∑
                    
                      
                        m
                        =
                        
                          
                            -
                            K
                          
                          /
                          2
                        
                      
                      ,
                      
                        m
                        ≠
                        0
                      
                    
                    
                      
                        K
                        /
                        2
                      
                      -
                      W
                    
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    arg
                    (
                    
                      
                        
                          
                          Pn
                          
                        
                        ·
                        
                          e
                          
                            -
                            
                              j
                              ⁡
                              
                                (
                                
                                  
                                    Ϛ
                                    
                                      1
                                      ,
                                      m
                                    
                                  
                                  -
                                  
                                    Ϛ
                                    
                                      1
                                      ,
                                      
                                        m
                                        -
                                        w
                                      
                                    
                                  
                                  +
                                  
                                    
                                      
                                        2
                                        ⁢
                                        π
                                        ⁢
                                        
                                          
                                        
                                        ⁢
                                        Wɛ
                                      
                                      N
                                    
                                    ⁢
                                     
                                
                                )
                              
                            
                          
                        
                      
                      +
                      
                        N
                        
                          n
                          ,
                          m
                        
                        ′′′′
                      
                    
                    )
                  
                
              
            
          
        
        
          
            
              ≈
              
              ⁢
              
                
                  
                    N
                    
                      2
                      ⁢
                      
                        
                      
                      ⁢
                      π
                      ⁢
                      
                        
                      
                      ⁢
                      W
                    
                  
                  ·
                  
                    1
                    
                      K
                      -
                      W
                    
                  
                
                ⁢
                
                  
                    ∑
                    
                      
                        m
                        =
                        
                          
                            -
                            K
                          
                          /
                          2
                        
                      
                      ,
                      
                        m
                        ≠
                        0
                      
                    
                    
                      
                        K
                        /
                        2
                      
                      -
                      W
                    
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    arg
                    ⁡
                    
                      (
                      
                        
                          
                            
                            Pn
                            
                          
                          ·
                          
                            e
                            
                              
                                -
                                j
                              
                              ⁢
                              
                          
                        
                        +
                        
                          N
                          
                            n
                            ,
                            m
                          
                          ′′′′
                        
                      
                      )
                    
                  
                
              
            
          
        
        
          
            
              ≈
              
              ⁢
              
                ɛ
                n
              
            
          
        
      
    
    
      
        (
        12
        )
      
    
  




As described above,

|Pn|=|s1,m·sn,m·s1,m−W·sn,m−W|2·|H1,m·H1,m−W|·|Hn,m·Hn,m−W|2 

In Equation 12, Yn denotes a signal output by the phase estimator 5450 and the sliding integrator 5460. In Equation 12, K denotes the number of transmitted subcarriers. In the DAB system, K=1536. However, K may also have other values according to the specifications of the DAB system.
A timing offset estimating method according to an exemplary embodiment of the present invention will now be described.
Among FFT-processed OFDM symbols of a frame, correlation values between the first OFDM symbol r1,m and subsequent OFDM symbols rn,m are calculated, and a first correlation signal C1(n,m) is generated. The first OFDM symbol r1,m is a preamble symbol including reference information on each of the OFDM symbols of a frame. In particular, a PRS is the preamble symbol in the DAB system.
Correlation values between a reference OFDM symbol rPRS and subsequent OFDM symbols rn,m are calculated and a second correlation signal CPRS(n,m) is generated. The reference OFDM symbol rPRS is a PRS previously determined by a transmitter and a receiver. The the first OFDM symbol r1,m denotes a PRS received by the receiver from the transmitter. The reference OFDM symbol rPRS denotes a PRS generated by the receiver.
A correlation value between the first correlation signal C1(n,m) and the second correlation signal CPRS(n,m) is calculated, and a third correlation signal CCOR(n,m) is generated.
A correlation value between a delay correlation signal CCOR(n,m−W) obtained by delaying the third correlation signal CCOR(n,m) by a predetermined subcarrier interval W and the third correlation signal CCOR(n,m) is calculated and a timing offset information signal Fn is generated. The predetermined subcarrier interval W is determined according to the range of estimation of the timing offset.
Phase components φn of the timing offset information signal Fn are accumulated during a symbol interval and an average value of the accumulated phased components is output as the timing offset εn of each OFDM symbol. According to the timing offset estimating method of the present exemplary embodiment, the timing offset εn of each OFDM symbol can be output by symbols rather than by frames.
A symbol liming recovering method including the timing offset estimating method according to the present exemplary embodiment will now be described with reference to FIG. 3.
The timing offset εn of each OFDM symbol is output using the timing offset estimating method, which is performed by the fine symbol timing recovery block 340.
The timing offset εn of each OFDM symbol is input into the NCO 333, and an interpolation frequency clock corresponding to the timing offset εn of each OFDM symbol is generated.
Input symbol data is interpolation processed based on the interpolation frequency clock, which is performed by the interpolator 313. Other operations of the symbol timing recovering method can be understood by one of ordinary skill in the pertinent art.
According to the exemplary embodiment of the present invention, a timing offset of each OFDM symbol can be precisely estimated. Further, fine and exact timing of each OFDM symbol can be recovered based on the estimated timing offset of each OFDM symbol.
While exemplary embodiments of the present invention have been particularly shown and described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention.

## Claims

1. An apparatus for estimating a timing offset of each of a plurality of orthogonal frequency division multiplexing (OFDM) symbols that are fast Fourier transform (FFT)-processed, the apparatus comprising:
a first correlator calculating correlation values between a first OFDM symbol of the plurality of OFDM symbols and subsequent OFDM symbols of the plurality of OFDM symbols and outputting a first correlation signal;
a second correlator calculating correlation values between a reference OFDM symbol and the subsequent OFDM symbols and outputting a second correlation signal;
a third correlator calculating a correlation value between the first correlation signal and the second correlation signal and outputting a third correlation signal;
a delay correlator calculating a correlation value between a delay correlation signal obtained by delaying the third correlation signal by a predetermined subcarrier interval and the third correlation signal, and outputting a timing offset information signal;
a phase estimator detecting phase components of the timing offset information signal; and
a sliding integrator accumulating the detected phase components of the timing offset information signal during a symbol interval, and outputting an average value of the accumulated phase components during the symbol interval as the timing offset of each OFDM symbol of the plurality of OFDM symbols.

2. The apparatus of claim 1, wherein the timing offset of each OFDM symbol is output.

3. The apparatus of claim 1, wherein the first OFDM symbol is a preamble symbol including reference information on OFDM symbols of a frame.

4. The apparatus of claim 3, wherein the apparatus for estimating the timing offset is applied to a receiver of a digital audio broadcasting (DAB) system and the preamble symbol is a phase reference symbol (PRS).

5. The apparatus of claim 4, wherein the reference OFDM symbol is a PRS previously determined by a transmitter and a receiver.

6. The apparatus of claim 1, wherein the predetermined subcarrier interval is determined according to a range of estimation of the timing offset.

7. The apparatus of claim 1, further comprising:
a buffer storing the first OFDM symbol and sending the first OFDM symbol to the first correlator; and
a reference OFDM symbol generator generating the reference OFDM symbol and sending the reference OFDM symbol to the second correlator.

8. The apparatus of claim 7, further comprising:
a demultiplexer sending the first OFDM symbol among received OFDM symbols of a frame to the buffer, and sending the subsequent OFDM symbols to the first correlator and the second correlator.

9. The apparatus of claim 1, wherein the first correlator comprises:
a conjugation unit receiving the first OFDM symbol and outputting a conjugation complex signal of the first OFDM symbol; and
a multiplication unit multiplying the conjugation complex signal of the first OFDM symbol and the subsequent OFDM symbols and outputting the first correlation signal.

10. The apparatus of claim 1, wherein the second correlator comprises:
a conjugation unit receiving the reference OFDM symbol and outputting a conjugation complex signal of the reference OFDM symbol; and
a multiplication unit multiplying the conjugation complex signal of the reference OFDM symbol and the subsequent OFDM symbols and outputting the second correlation signal.

11. The apparatus of claim 1, wherein the third correlator comprises:
a conjugation unit receiving the first correlation signal and outputting a conjugation complex signal of the first correlation signal; and
a multiplication unit multiplying the conjugation complex signal of the first correlation signal and the second correlation signal and outputting the third correlation signal.

12. The apparatus of claim 1, wherein the delay correlator comprises:
a delay unit, receiving the third correlation signal and outputting the delay correlation signal obtained by delaying the third correlation signal by the predetermined subcarrier interval;
a conjugation unit receiving the delay correlation signal and outputting a conjugation complex signal of the delay correlation signal; and
a multiplication unit multiplying the conjugation complex signal of the delay correlation signal and the third correlation signal, and outputting the timing offset information signal.

13. The apparatus of claim 1, wherein the sliding integrator comprises:
an integrator accumulating the phase components of the timing offset information signal during the symbol interval; and
a multiplier outputting the average value obtained by multiplying the accumulated phase components by an average constant as the timing offset of each OFDM symbol.

14. A method of estimating a timing offset of each of a plurality of OFDM symbols of a frame that are FFT-processed, the method comprising:
calculating correlation values between a first OFDM symbol of the plurality of OFDM symbols and subsequent OFDM symbols of the plurality of OFDM symbols and generating a first correlation signal;
calculating correlation values between a reference OFDM symbol and the subsequent OFDM symbols and generating a second correlation signal;
calculating a correlation value between the first correlation signal and the second correlation signal and generating a third correlation signal;
calculating a correlation value between a delay correlation signal obtained by delaying the third correlation signal by a predetermined subcarrier interval and the third correlation signal, and generating a timing offset information signal; and
accumulating phase components of the timing offset information signal during a symbol interval, and outputting an average value of the accumulated phase components during the symbol interval as the timing offset of each OFDM symbol of the plurality of OFDM symbols.

15. The method of claim 14, wherein the first OFDM symbol is a preamble symbol including reference information on the plurality of OFDM symbols of the frame.

16. The method of claim 15, wherein, the method of estimating the timing offset is applied to a DAB system, the preamble symbol is a phase reference symbol (PRS), and the reference OFDM symbol is a PRS previously determined by a transmitter and a receiver.

17. The method of claim 16, wherein the preamble symbol is a PRS received by the receiver from the transmitter, and the reference OFDM symbol is a PRS generated in the receiver.

18. The method of claim 14, wherein the predetermined subcarrier interval is determined according to a range of estimation of the timing offset.

19. The method of claim 14, wherein the timing offset of each OFDM symbol is output.

20. A method for recovering symbol timing, comprising:
outputting a timing offset of each of a plurality of OFDM symbols using the method of estimating the timing offset of claim 14;
inputting the timing offset of each OFDM symbol into a numerically controlled oscillator (NCO), and generating an interpolation frequency clock corresponding to the timing offset of each OFDM symbol; and
interpolation-processing input symbol data based on the interpolation frequency clock.