Abstract:
Provided are an uplink receiving device of a long term evolution (LTE)-based orthogonal frequency division multiplexing access (OFDMA) system, and a method of synchronizing a frequency thereof. The uplink receiving device includes a fast Fourier transform (FFT) processing unit eliminating a cyclic prefix (CP) from an OFDM uplink signal to perform a fast Fourier transform (FFT); a subcarrier extraction unit performing a subcarrier extraction operation with regard to an output of the FFT processing unit to thus only extract a signal of a specific terminal unit; and an inverse discrete Fourier transform (DFT) processing unit performing an inverse DFT on an output of the subcarrier extraction unit to generate a time domain signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application Nos. 10-2010-0049824 filed on May 27, 2010 and 10-2011-0017241 filed on Feb. 25, 2011, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an uplink receiving device, and more particularly, to an uplink receiving device applied to a long term evolution (LTE)-based orthogonal frequency division multiplexing access (OFDMA) system. 
         [0004]    2. Description of the Related Art 
         [0005]    In an LTE-based OFDMA system, a frequency interval between subcarriers is relatively small as compared to a transmission band, and since an orthogonality of respective subcarriers should be maintained at the time of a transmission, the LTE-based OFDMA system is very sensitive to a frequency offset as compared to a single carrier system. 
         [0006]    Thus, in a case in which a frequency offset is generated due to non-matching occurring in an oscillator for a transmission and receiving period or a doppler shift, since a receiving function may be greatly degraded, an accurate estimation for the frequency offset and a compensation process therefor are indispensably required in an uplink receiving device, a base station device. 
         [0007]      FIG. 1  is a block diagram of an uplink receiving device according to the related art. 
         [0008]    Referring to  FIG. 1 , each of a plurality of uplink receiving devices  100 - 1  to  100 -N may include a filter  110 , a frequency offset estimation unit  120 , an offset compensation unit  130 , a fast Fourier transform (FFT) processing unit  140 , and the like. 
         [0009]    All of a plurality of terminal units  11  to  1 N respectively include a frequency offset ε and a timing offset δ as well as each channel, and thus, only a signal of a corresponding terminal unit should be separately selected in order to demodulate a signal of a corresponding terminal unit in orthogonal frequency division multiplexing (OFDM) uplink signals. 
         [0010]    Thus, the filter  110  only extracts an OFDM signal of a corresponding terminal unit from an OFDM uplink signal in which all of a plurality of OFDM signals generated by the plurality of terminal units  11  to  1 N have been mixed. 
         [0011]    Then, the frequency offset estimation unit  120  and the offset compensation unit  130  estimate a frequency offset by using a cyclic prefix (CP) of the OFDM signal extracted through the filter  110  and continuously compensate threfor. 
         [0012]    Thereafter, the FFT processing unit  140  receives a signal frequency-synchronized by the frequency offset estimation unit  120  and the offset compensation unit  130  and removes the CP of the signal, to thus perform an FFT and demodulate the OFDM signal. 
         [0013]      FIG. 2  illustrates a frequency offset estimation process of an uplink receiving device according to the related art. 
         [0014]    As shown in  FIG. 2 , the frequency offset estimation unit  120  according to the related art estimates a frequency offset through an amount of change in a phase within two sections by using characteristics in which second halves of a protection section and an effective symbol section in a time domain are the same as each other, in a case of OFDM signal. 
         [0015]    That is, when the OFDM signal is y(n) in the time domain, a frequency offset {circumflex over (ε)} f  using the CP (or a protection section N GI ) is estimated according to the following mathematical expression 1. 
         [0000]    
       
         
           
             
                 
             
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         [0016]    Here, ‘n’ denotes a sample index of an OFDM signal, 0&lt;n&lt;N GI −1 a protection section, and N GI &lt;n&lt;N FFT +N GI  an effective symbol section. 
         [0017]    As such, in the plurality of uplink receiving devices  100 - 1  to  100 -N, the uplink receiving devices  100 - 1  to  100 -N may be prevented from deteriorating in reception performance in advance by extracting the OFDM signal of a corresponding terminal unit through the filter  110  that requires a very large hardware capacity to thus estimate and compensate for a frequency offset thereof. 
         [0018]    However, since the filter  110  provided with the plurality of uplink receiving devices  100 - 1  to  100 -N requires a very large capacity of a hardware, the number of terminal units to be supported is limited according to a capacitance of the hardware of the filter  110 . Therefore, when the number of the terminal units to be supported by the plurality of uplink receiving devices  100 - 1  to  100 -N is increased, the number of the filters  110  should be also increased. 
         [0019]    That is, the plurality of uplink receiving devices  100 - 1  to  100 -N according to the related art have a defect in which a relatively large amount of hardware capacity is required due to the use of the filter  110 . 
       SUMMARY OF INVENTION 
       [0020]    An aspect of the present invention provides an uplink receiving device capable of performing a newly applicable signal extraction scheme substituted for a filter signal extraction operation, by using a relatively reduced hardware capacity, thereby significantly reducing a magnitude of hardware in an uplink receiving device. 
         [0021]    According to an aspect of the present invention, there is provided an uplink receiving device including: an FFT processing unit eliminating a CP from an OFDM uplink signal to perform a FFT; a subcarrier extraction unit performing a subcarrier extraction operation with regard to an output of the FFT processing unit to thus only extract a signal of a specific terminal unit; and an inverse discrete fourier transform (DFT) processing unit performing an inverse DFT on an output of the subcarrier extraction unit to generate a time domain signal. 
         [0022]    The uplink receiving device may further include a frequency offset estimation unit estimating a frequency offset from a correlation between an output of the inverse DFT processing unit and an output of the subcarrier extraction unit; and an offset compensation unit compensating for the frequency offset contained in the OFDM uplink signal by using the frequency offset. 
         [0023]    The frequency offset estimation unit may estimate the frequency offset according to an expression 
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         [0000]    wherein r*(n) indicates a conjugate signal of an output from the inverse DFT processing unit, x(n) denotes an OFDM signal, M indicates a signal length of an output from the inverse DFT processing unit, xcor F  indicates a signal sum result from a multiplication-result signal pertaining to a 0&lt;n&lt;M/2−1 signal section, xcor B  designates a signal sum result from a multiplication-result signal pertaining to an M/2&lt;n&lt;M−1 signal section, and {circumflex over (ε)} f  denotes the frequency offset. 
         [0024]    The uplink receiving device may further include a loop filter bypassing the frequency offset of the frequency offset estimation unit to the offset compensation unit when the frequency offset has an initial value, and when the frequency offset is not the initial value, computing a difference of the frequency offset to thus inform the offset compensation unit of the computed difference of the frequency offset. 
         [0025]    According to another aspect of the present invention, there is provided a method of synchronizing a frequency of an uplink receiving device, the method including: eliminating a CP from an OFDM uplink signal to perform a fast fourier transform (FFT); performing a subcarrier extraction operation with regard to an FFT processing result to thus only extract a signal of a specific terminal unit; performing an inverse DFT for a subcarrier extraction output to generate a time domain signal; estimating a frequency offset from a correlation between an output of the inverse DFT execution output and the subcarrier extraction output; and compensating for the frequency offset contained in the OFDM uplink signal by using the frequency offset. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0027]      FIG. 1  is a block diagram of an uplink receiving device according to the related art; 
           [0028]      FIG. 2  illustrates a frequency offset estimation process of an uplink receiving device according to the related art; 
           [0029]      FIG. 3  is a block diagram of an uplink receiving device according to an embodiment of the present invention; 
           [0030]      FIG. 4  illustrates a frequency offset estimation process according to an embodiment of the present invention; and 
           [0031]      FIG. 5  is a flowchart illustrating a frequency synchronization method of an uplink receiving device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The present invention may be modified variably and may have various embodiments, particular examples of which will be illustrated in drawings and described in detail. 
         [0033]    However, it should be understood that the following exemplifying description of the invention is not intended to restrict the invention to specific forms of the present invention but rather the present invention is meant to cover all modifications, similarities and alternatives which are included in the spirit and scope of the present invention. 
         [0034]      FIG. 3  is a block diagram of an uplink receiving device according to an embodiment of the present invention. 
         [0035]    With reference to  FIG. 3 , each of uplink receiving devices  200 - 1  to  200 -N may include an FFT processing unit  210 , a subcarrier extraction unit  220 , and an inverse discrete fourier transform (DFT) processing unit  230 , a frequency offset estimation unit  240 , a loop filter  250 , an offset compensation unit  260 , and the like. 
         [0036]    That is, the uplink receiving device  200  may be provided with the omission the filter  110  according to the related art, and may perform a signal extraction operation using a relatively simple arithmetic operation in the FFT processing unit  210 , the subcarrier extraction unit  220 , and the inverse DFT processing unit  230 , thereby significantly decreasing a hardware capacity required for a signal extraction operation. 
         [0037]    Meanwhile, in the above-described configuration, an FFT operation may be first performed and then a frequency offset estimation operation may be performed. In this case, a CP is removed during a process in which the FFT operation is performed such that the frequency offset estimation operation using the CP according to the related art cannot be executed. Therefore, according to an embodiment of the present invention, a frequency offset estimation method based on a time domain in which the CP is not used, may additionally be proposed. 
         [0038]    Hereinafter, functions of respective configurative elements will be described. 
         [0039]    The FFT processing unit  210  may eliminate a CP of an OFDM uplink signal to then perform an FFT operation and thus demodulate the OFDM signal. 
         [0040]    The subcarrier extraction unit  220  may execute a subcarrier extraction operation based on a frequency band used by a corresponding terminal unit to selectively only extract an OFDM signal of the corresponding terminal unit among output signals from the FFT processing unit  210 . 
         [0041]    The inverse DFT processing unit  230  may perform an inverse DFT with regard to the signal extracted through the subcarrier extraction unit  220  to convert the signal into a signal r(n) of a time domain. Here, the time domain signal is a pilot signal previously recognized by the uplink receiving device  200 . In addition, the inverse DFT processing unit  230  may convert a signal of a frequency domain into a time domain-based signal, whereby the frequency offset estimation unit  240  may perform a frequency offset estimation operation using the converted time domain-based signal. 
         [0042]    The frequency offset estimation unit  240  may estimate a frequency offset of a specific terminal unit from a correlation between an output signal of the inverse DFT processing unit  230  and an input signal, an OFDM signal of a corresponding terminal unit, of the inverse DFT processing unit  230 . 
         [0043]    In more detail, as shown in  FIG. 4 , the frequency offset estimation unit  240  may generate a conjugate signal r*(n) with regard to the signal r(n) of the time domain, and after that, may remove the pilot signal by multiplying the conjugate signal r*(n) by an OFDM signal x(n) such that only a frequency offset component is left. Subsequently, a multiplication-result signal r*(n)·x(n) may be equivalently divided into two sections, and then, each section signal summing operation may be performed, and respective section signal sums may be multiplied by each other. Thereafter, a frequency offset of a corresponding terminal unit may be estimated from a phase provided with respect to the multiplied result, which may appear as the following mathematical expression 2. 
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         [0044]    Here, M denotes a signal length of an output of the inverse DFT processing unit  230 , a length of a signal r(n) of a time domain, and xcor F  denotes a signal sum result of multiplication-result signals pertaining to a 0&lt;n&lt;M/2−1 signal section, and xcor B  indicates a signal sum result of multiplication-result signals pertaining to an M/2&lt;n&lt;M−1 signal section. 
         [0045]    The loop filter  250  may operate so that a frequency offset {circumflex over (ε)} f  initially estimated by the frequency offset estimation unit  240  is bypassed to the offset compensation unit  260 , and from a next estimation value, the loop filter  250  may inform the offset compensation unit  260  of a difference of the frequency offset {circumflex over (ε)} f . As such, the operation in which the loop filter  250  directly bypasses the initial frequency offset {circumflex over (ε)} f  to the offset compensation unit  260  may be performed to remove a settling delay caused due to a closed loop feedback of the loop filter  250 . 
         [0046]    The offset compensation unit  260  may compensate for a frequency offset of the OFDM uplink signal input to each of the uplink receiving devices  200 - 1  to  200 -N by using a frequency offset {circumflex over (ε)} f  provided through the loop filter  250  or a difference of the frequency offset {circumflex over (ε)} f  to thus provide a frequency-synchronized signal to the FFT processing unit  210 . 
         [0047]      FIG. 5  is a flowchart illustrating a frequency synchronization method of an uplink receiving device according to an embodiment of the present invention. 
         [0048]    First, when an OFDM uplink signal is input to each of the uplink receiving devices  200 - 1  to  200 -N, the FFT processing unit  210  may remove the CP of the OFDM uplink signal thereto to perform an FFT and demodulate the OFDM signal in operation S 1 . 
         [0049]    Thereafter, the subcarrier extraction unit  220  may perform a subcarrier extraction operation according to a preset frequency band to thus selectively only extract an OFDM signal of a corresponding terminal unit. 
         [0050]    Then, the inverse DFT processing unit  230  may perform an inverse DFT with regard to the signal extracted through the subcarrier extraction unit  220  to thus convert the OFDM signal of a corresponding terminal unit into the signal r(n) of a time domain in operation S 3 . 
         [0051]    Subsequently, the frequency offset estimation unit  240  may multiply the conjugate signal r*(n) of an output signal r(n) from the inverse DFT processing unit  230  by the OFDM signal x(n), and then, a multiplication-result signal r*(n)·x(n) may be equivalently divided into two sections, and summing operation may be then performed for each section signal. That is, a summing operation 
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         [0000]    for the multiplication-result signals corresponding to the 0&lt;n&lt;M/2−1 signal section, and a summing operation 
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         [0000]    for the multiplication-result signals corresponding to the M/2&lt;n&lt;M−1 signal section may be respectively performed in operation S 4 . 
         [0052]    Thereafter, respective section signal sum results 
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         [0000]    may be multiplied by each other, and then, a frequency offset of a corresponding terminal unit may be estimated as 
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         [0000]    from a phase provided from the multiplied result xcor F *·xcor B , in operation S 5 . 
         [0053]    When the frequency offset {circumflex over (ε)} f  estimated through operation S 5  is an initial value, the loop filter  250  may bypass the offset to the offset compensation unit  260 , and the offset compensation unit  260  may perform a frequency compensation operation according to the initial frequency offset in operation S 7 . 
         [0054]    Meanwhile, when the frequency offset estimated through operation S 5  is not an initial value, the loop filter  250  may compare a previous frequency offset {circumflex over (ε)} f  with a current frequency offset {circumflex over (ε)} f  to calculate a frequency difference, in operation S 8 , and the offset compensation unit  260  may perform a frequency compensation operation using the of the frequency offset in operation S 9 . 
         [0055]    While the present invention has been shown and described in connection with the embodiments in the, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims. 
         [0056]    As set forth above, according to embodiments of the present invention, an uplink receiving device may perform a signal extraction operation, using a relatively simple arithmetic operation scheme in an FFT processing unit, a subcarrier extraction unit and an inverse DFT processing unit, by omitting a filter according to the related art, thereby greatly reducing a hardware capacity required for a signal extraction operation. Accordingly, an uplink receiving device according to the embodiments of the present invention may have an overall reduction in hardware. 
         [0057]    While the present invention has been shown and described in connection with the embodiments described herein, it will be apparent to those skilled in the art that modifications and variations can be made to the present invention without departing from the spirit and scope thereof, as defined by the appended claims.