Abstract:
Provided is a method of processing a WCDMA signal timing offset for a signal analyzer. The method includes estimating an integer multiple timing offset of WCDMA baseband sample data corresponding to an amount of at least one frame; generating a frequency domain signal which is time delayed corresponding to a fractional timing offset estimation resolution after generating the frequency domain signal by performing an FFT calculation on an already-known reference signal; converting each time-delayed frequency domain signal into a time domain signal by performing an IFFT calculation on each time-delayed frequency domain signal and calculating a correlation between an input signal from a position of the integer multiple timing offset and the time domain signal; and estimating a delay time leading to a maximum correlation as a fractional timing offset.

Description:
BACKGROUND 
       [0001]    The embodiment relates to a method of processing a WCDMA signal timing offset for a signal analyzer, and more particularly, to a method of processing a WCDMA signal timing offset for a signal analyzer, which is capable of estimating and compensating for a timing offset at a high accuracy within 1% EVM (0.1 sample) by using FFT/IFFT and a fractional RRC filter even while using a low sampling frequency of twice the WCDMA chip rate. 
         [0002]    As well known in the art, WCDMA (Wideband Code Division Multiple Access) defined in 3GPP TS.25 standard, which is one of 3 rd  generation wireless communication systems, is an asynchronous mobile communication scheme having a chip rate of Fc=3.84 Mcps. 
         [0003]      FIG. 1  is a view illustrating a radio frame structure of the WCDMA system including a synchronous channel. As shown in  FIG. 1 , according to the radio frame structure of the WCDMA system, chips are transmitted within a TTI (Transmission Time Interval) and each TTI includes one radio frame to 8 radio frames. Each radio frame is a duration of 10 ms and is divided into 15 slots. The slot includes 2560 chips. Thus, the TTI may include 15 to 120 slots. 
         [0004]    Meanwhile, the primary synchronization channel includes a modulation code of 256 chips. The same modulation code is transmitted through primary synchronization channel at the same location in each slot. Since the secondary synchronization channel is a code sequence formed by combining modulation codes each having 256 chips, which correspond to 15 slots of one radio frame, the modulation codes of the secondary synchronization channel each is transmitted in parallel with the codes of the first synchronization channel every slot and are configured to be different from each other every slot. 
         [0005]    The WCDMA system having the frame structure described above performs the cell search and physical layer synchronization to detect a downlink WCDMA signal through following three steps. 
         [0006]    First step: The slot synchronization is obtained by synchronizing a sample/chip boundary through PSCH. 
         [0007]    Second step: The frame synchronization and scrambling code are obtained through SSCH. 
         [0008]    Third step: Fractional sample/chip boundary synchronization, scrambling code index synchronization and channel estimation through CPICH (Common Pilot Channel). 
         [0009]    Until the CPICH is decoded, it is impossible to estimate the channel and exactly detect a remaining DPxCH signal such as s DPDCH (Dedicated Physical Data Channel) signal or a DPCCH (Dedicated Physical Control Channel) signal. The frequency, timing and phase offsets are estimated and compensated by using the detected CPICH signal so that the channel is estimated. 
         [0010]    The WCDMA communication technique uses a symbol modulation scheme of a chip rate very higher than a data rate. In this case, since the sampling timing offset must be very smaller that the data sampling timing offset, the demodulation performance is very sensitive to the timing offset (error). 
         [0011]    According to the related art, a maximum correlation value search scheme has been mainly used to estimate a timing offset. Thus, a signal analyzer according to the related art is required to perform an over sampling, which is 40 to 80 times the chip rate of 3.84 MHz of the WCDMA system, to maintain the condition of 1% EVM at the sampling frequency of 30.72 (=3.84*8) MHz, that is, the timing synchronization within 0.1 sample, so that too much resource is inefficiently consumed. 
       DOCUMENT OF RELATED ART 
     Patent Document 
       [0012]    Patent document 1: Korean Unexamined Patent Publication No. 10-2004-0070693 (Title: Apparatus for acquiring synchronization in asynchronous next-generation mobile communication system and method thereof) 
         [0013]    Patent document 2: Korean Unexamined Patent Publication No. 10-2010-0006111 (Title: Method of analyzing a signal for a mobile communication system) 
         [0014]    Patent document 3: Korean Unexamined Patent Publication No. 10-2010-0130659 (Title: method and apparatus for Time tracking of mobile communication receiver for channel estimation) 
       SUMMARY 
       [0015]    To satisfy the requirement described above, an object of the embodiment is to provide a method of processing a WCDMA signal timing offset for a signal analyzer, which is capable of estimating and compensating a timing offset at a high accuracy within 1% EVM (0.1 sample) by using FFT/IFFT and a fractional RRC filter even while using a low sampling frequency of twice the WCDMA chip rate. 
         [0016]    According to an embodiment, there is provided a method of processing a WCDMA signal timing offset for a signal analyzer, which includes: estimating an integer multiple timing offset of WCDMA baseband sample data corresponding to an amount of at least one frame; generating a frequency domain signal which is time delayed corresponding to a fractional timing offset estimation resolution after generating the frequency domain signal by performing an FFT calculation on an already-known reference signal; converting each time-delayed frequency domain signal into a time domain signal by performing an IFFT calculation on each time-delayed frequency domain signal and calculating a correlation between an input signal from a position of the integer multiple timing offset and the time domain signal; and estimating a delay time leading to a maximum correlation as a fractional timing offset. 
         [0017]    The frequency domain signal R(ω) is obtained by performing the FFT calculation on a PSCH r(t) through a following equation, 
         [0000]        R (ω)=FFT[ r ( t ), N   ft ]
       wherein N ft  is a number of FFT samples.       
 
         [0019]    The PSCH r(t) is a signal RRC-filtered at a sampling rate which is twice a WCDMA chip rate. 
         [0020]    The frequency domain signal R(w) is obtained by performing the FFT calculation on a CPICH r(t) through a following equation, 
         [0000]        R (ω)=FFT[ r ( t ), N   ft ],
       wherein N ft  is a number of FFT samples.       
 
         [0022]    The CPICH r(t) is a signal RRC-filtered at a sampling rate which is twice a WCDMA chip rate. 
         [0023]    Each delayed time domain signal is obtained by a following equation, 
         [0000]        r ( t−τ   1   ̂*i )=IFFT[ R (ω) e   j*ω*τ     1     ̂*i) ]
       wherein τ 1 ̂=T c /N 1 , I=0, . . . , N 1 , r′(t−τ 1 ̂*i) is a conjugate complex number of r(t−τ 1 ̂*i), and N 1  is a fractional timing offset estimation resolution.       
 
         [0025]    The correlation y(t) is obtained by a following equation, 
         [0000]        y ( t )=Σ[ x ( t )* r ′( t−τ   1   ̂*i )].
 
         [0026]    The method further includes compensating for the fractional timing offset by applying the estimated fractional timing offset to a fractional RRC filter as a compensation coefficient. 
         [0027]    The RRC filter is operated according to a following equation, 
         [0000]    
       
         
           
             
               
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         [0028]    wherein t=t−τ 1 *i maxc . 
         [0029]    According to the method of processing a WCDMA signal timing offset for a signal analyzer of the embodiment, a timing offset may be estimated and compensated at a high accuracy within 1% EVM (0.1 sample) by using FFT/IFFT and a fractional RRC filter even while using a low sampling frequency of twice the WCDMA chip rate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a view illustrating a structure of a wireless frame of a WCDMA system including a synchronization channel. 
           [0031]      FIG. 2  is a flowchart illustrating a method of processing a WCDMA signal timing offset for a signal analyzer according to an embodiment. 
           [0032]      FIG. 3  is a graph illustrating a correlation obtained with a fractional timing offset estimation resolution of N 1 =128 according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0033]    Hereinafter, a method of processing a WCDMA signal timing offset for a signal analyzer according to a preferable embodiment will be described in detail with reference to accompanying drawings. 
         [0034]    First, to receive and analyze a WCDMA signal, timing, frequency and phase synchronizations are required and specifically, at the sampling frequency of 30.72 MHz (the LTE clock frequency is used to compatible with an LTE signal analyzer), the timing, frequency and phase synchronizations are required to be satisfied within 0.1 sample, 1 Hz and 1°. 
         [0035]    To this end, according to an embodiment, there is proposed a scheme capable of rapidly securing the timing synchronization by using FFT/IFFT while the sampling rate (2*Fc=7.68 MHz), which is about twice the chip rate Fc of the WCDMA system, is used. 
         [0036]      FIG. 2  is a flowchart illustrating a method of processing a WCDMA signal timing offset for a signal analyzer according to an embodiment. 
         [0037]    First, a signal analyzer according to an embodiment receives a WCDMA signal. The signal analyzer down-converts the WCDMA signal into an analog baseband I/Q signal and converts the analog baseband I/Q signal into digital data again. For example, the signal analyzer converts the analog baseband I/Q signal into I/Q sample data of a sampling rate of 30.72 MHz and a resolution of 16-bit and captures the I/Q sample data. In the following description, the I/Q sample data may not be separately explained. In this case, in step S 10 , the captured sample data are matched-filtered. In this case, an RRC (Root Raised Cosine) filtering operation is performed. 
         [0038]    As well known in the art, since the RRC filtering demodulates a signal by utilizing spectrum aliasing characteristics in digital domain, a receiver is required to perform over sampling. In this case, since the minimum over sampling rate is 2, after the original sampling rate is decimated to 4:1, the RRC filtering is performed to use the minimum over rate. 
         [0039]    In this case, the first step described above, that is, the PSC synchronization is performed, and at the same time, the timing and frequency offset compensations are performed. To this end, in the step S 10 , the frequency offset F off  is set into 0, and then, in step S 20 , a sampling value in units of slots is calculated. Preferably, to more stably perform PSC detection, after the sample values in units of slots in one wireless frame, in which total 15 slots exist, are calculated, the average value of them is used. 
         [0040]    Next, in step S 30 , the correlation between the sample values in units of slots obtained in step S 20  and a reference PSC sample (sampled at twice of the chip rate) defined in the standard is calculated to search for the maximum correlation value C max  and the location thereof. Then, in step S 40 , the maximum correlation value C max  and the location thereof are matched with the corresponding frequency offset F off  to be stored. 
         [0041]    Next, in steps S 50  and S 60 , in a state that a new frequency offset is set by increasing or decreasing a current frequency offset by a predetermined increment Δf (increased in once, or decreased in another time), the steps S 20  to S 60  are repeated until the new frequency offset reaches the predetermined maximum frequency offset ±F max . In this case, preferably, the increment Δf of the frequency offset is set as 50 Hz which is half of 100 Hz in consideration of the time length 10 ms of the wireless frame and the maximum frequency offset F max  is set as 750 Hz (15*100 Hz*½) in consideration of the number of PSCs of 15 included in one wireless frame. 
         [0042]    As described above, in the state that the maximum correlation value C max  with respect to all frequency offsets and the location thereof are calculated and stored, in step S 70 , after confirming the maximum correlation value C max  and the location, a frequency offset matched with the maximum correlation value C max  and the location is estimated as the frequency offset of a received signal. In addition, the integer multiple timing offset of the PSC is estimated based on the sample location of the maximum correlation value C max . 
         [0043]    Meanwhile, as expressed as the following Equation 1 according to an embodiment, the correlation calculation of step S 30  may be performed in frequency domain by using the Fourier transform characteristics. When the correlation calculation is performed in frequency domain, an FFT algorithm may be applied thereto. As a result, as the number of samples is increased, the calculation may be effectively performed with a small amount of computation. 
         [0044]    The correlation calculation for estimating a timing offset may be performed by following Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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         [0045]    In the Equation 1, R(τ) is a correlation value, x(t+τ) is an input signal delayed for time τ (corresponding to one sample when the embodiment is based on 512 samples), s(t)′ is a reference signal, that is, a conjugate complex number of PSC having 512 samples, F represents an FFT calculation, and F −1  represents an IFFT calculation. 
         [0046]    The following Table 1 is a table of comparing an amount of computation according to the correlation calculation scheme of the embodiment with that of the related art. According to the embodiment, as shown in Table 1, when based on 512 samples, the amount of computation is reduced to 57 times. When based on 4096 samples, the amount of computation is reduced to 341 times 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Related art 
                 Embodiment 
                 Remarks 
               
               
                   
               
             
             
               
                 512 * 512 
                 512 * log 2 (512) 
                  57 times 
               
               
                 4096 * 4096 
                 4096 * log 2 (4096) 
                 341 times 
               
               
                   
               
             
          
         
       
     
         [0047]    As described above, when the frequency offset estimation and the integer multiple timing offset estimation are completed, in step S 90 , a fractional timing offset is estimated based on the location of the estimated integer multiple timing offset. To this end, FFT characteristics expressed as following Equations 2 and 3 are used. 
         [0000]      FFT[ r ( t−t   0 )]=FFT[ r ( t )] e   j*ω*(−t     0     )   =R (ω) e   j*ω*(−t     0     )   [Equation 2]
 
         [0000]        r ( t−t   0 )=IFFT[ R (ω) e   j*ω*(−t     0     ) ]  [Equation 3]
 
         [0048]    When using Equations 2 and 3, if a signal R(w) is previously generated and the sample interval to is set as T c (=1/F c )/N 1 , only a required time delay signal may be rapidly generated through the IFFT calculation, where N 1  is a fractional timing offset estimation resolution which may be predetermined as 128 in the embodiment. When the location of the maximum correlation value is determined by calculating the correlation between the time-delayed signal and the input signal, the location may be immediately estimated as the fractional timing offset. This relation may be expressed as follows with PSCH as a reference signal. 
         [0049]    First, a frequency domain signal R(ω) for PSCH r(t) previously defined in the standard is generated according to following Equation 4. 
         [0000]        R (ω)=FFT[ r ( t ), N   ft ]  [Equation 4]
 
         [0050]    In the Equation 4, N ff  is the number of samples of FFT. For example, N ff  may be set as 4096. In addition, a PSCH signal, which is RRC filtered at the sampling rate of twice the chip rate, is used as r(t). 
         [0051]    Next, the correlation between the input signal x(t) RRC-filtered at the sampling rate of twice the chip rate and r(t−τ 1 ̂*i) is calculated. r(t−τ 1 ̂*i) and the correlation may be obtained through following Equations 5 and 6. 
         [0000]        r ( t−τ   1   ̂*i )=IFFT[ R (ω) e   j*ω*τ     1     ̂*i) ],         τ 1   ̂=T   c   /N   1   ,i= 0, . . . , N   1   [Equation 5]
 
         [0000]        y ( t )=Σ[ x ( t )* r ′( t−τ   1   ̂*i )]  [Equation 6]
 
         [0052]    In Equation 6, r′(t−τ 1 ̂*i) is the conjugate complex number of r(t−τ 1 ̂*i). The location i(i maxc ) at which the correlation obtained through Equation 6 is maximized is obtained. This location is the fractional timing offset. 
         [0053]      FIG. 3  is a graph illustrating a correlation obtained with a fractional timing offset estimation resolution of N 1 =128 through Equation 6 according to an embodiment. It may be understood through  FIG. 3  that the fractional timing offset is about 60τ 1 . 
         [0054]    When the fractional timing offset is estimated as described above, the fractional timing offset is applied to the RRC filter as a compensation coefficient, so that the fractional timing offset may be exactly compensated. 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0055]    The Equation 7 is a mathematical expression of the RRC filtering defined in 3GPP TS.25.101/104. In the Equation 7, when t is replaced with t−τ 1 *i maxc , the fractional timing offset may be exactly compensated. 
         [0056]    Meanwhile, since the fractional timing offset may not be exactly detected through the PSCH due to the phase offset, if CPICH is used, the fractional timing offset may be more exactly detected. Even in this case, only the reference signal is replaced with the CPICH and the fractional timing offset may be estimated through the same method as that described above. 
         [0057]    The method of processing a WCDMA signal timing offset for a signal analyzer described with reference to accompanying drawings in this disclosure is for an illustrative purpose only, and the embodiment is not limited thereto. Thus, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art within the spirit and scope of the embodiment and they will fall within the scope of the embodiment.