Abstract:
An apparatus for controlling PAPR in an OFDM communication system and method thereof are disclosed, by which implementation is simplified and enhanced PAPR characteristics are provided. The present application includes outputting GN b  data symbols by oversampling N b  parallel data symbols, spreading the oversampled data symbols using DFT, and mapping the spread signal to subcarriers.

Description:
[0001]     This application claims the benefit of the Korean Patent Application No. 10-2006-0023113, filed on Mar. 13, 2006, which is hereby incorporated by reference as if filly set forth herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present disclosure relates to controlling PAPR. Although the present disclosure is suitable for a wide scope of applications, it is particularly suitable for decreasing PAPR characteristics in an orthogonal frequency division multiplexing (hereinafter abbreviated OFDM) system.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Generally, in the OFDM system, signal processing is performed on a channel tending to have frequency-selective fading in a frequency domain to bring about flat fading. So, the OFDM system enables more efficient communications. Due to this advantage, the OFDM is widely adopted by wireless communication systems.  
         [0006]     Meanwhile, the OFDM system has a problem of a peak to average power ratio (hereinafter abbreviated PAPR). If the PAPR is big, a power amplifier having a large linear interval to amplify a signal corresponding to a peak power is needed. Yet, a product cost for manufacturing the power amplifier having the large linear interval is too high. In case that a power amplifier has a small linear interval, a signal amplified in a non-linear interval gets distorted.  
         [0007]     To decreasing the PAPR, various methods have been proposed. As an example of the methods, variants of OFDM like SC-FDMA, offset DFT-SOFDM and precoded DFT-S-OFDM are provided. In these methods of modifying OFDM signal generation, PAPR characteristics of a transmission signal is enhanced in a manner of spreading a transmission data vector by DFT before mapping data to subcarriers in a frequency domain and then mapping the data to the subcarriers. In the methods of modifying OFDM signal generation, it is in common that a signal is spread by DFT. In particular, since the data signal mapped by the subcarriers is performed by IDFT at a final transmission step, powers of signals having peak powers are cancelled out to reduce a power variation of the final transmission signal.  
         [0008]      FIG. 1  is a block diagram of an example of an OFDM signal generator supporting SC-FDMA,  
         [0009]     Referring to  FIG. 1 , a serial-to-parallel converting unit  11  converts a data symbol inputted in series to a parallel signal. A signal spreading unit  12  performs a dispreading on the paralleled data symbol in a frequency domain using Discrete Fourier Transform (hereinafter abbreviated DFT) before generating an OFDM signal. Equation 1 indicates a method of dispreading a parallel signal using N b sN b  DFT matrix. 
 
 s   F   =F   N     b     ×N     b     s   x   [Equation 1]
 
         [0010]     In Equation 1, ‘N’ indicates the number of subcarriers provided to an OFDM signal, ‘s x ’ indicates a data symbol vector, ‘s F ’ indicates a vector of which data is spread in a frequency domain, and ‘s Tx ’ indicates an OFDM symbol vector transmitted in a time domain. Moreover, ‘F N     b     ×N     b   ’ is a DFD matrix of ‘N b sN b ’ used in dispreading a data symbol.  
         [0011]     A subcarrier mapping unit  13  maps the spread vector s F  to subcarrier using a subcarrier allocating pattern. An Inverse Discrete Fourier Transform (hereinafter abbreviated IDFT) unit  14  transforms the signal mapped to the subcarrier into a signal in a time domain. Equation 2 represents Inverse Discrete Fourier Transform. 
 
 s   Tx   =F   N×N   −1   s   F    [Equation 2]
 
         [0012]     In Equation 2, F N×N  is N b sN b  DFT matrix used to transform a signal in a frequency domain to a signal in a time domain and s Tx  is a signal generated in a time domain by IDFT, A parallel-to-serial converting unit  15  converts the parallel signal converted in a time domain to a serial signal. A cyclic prefix inserting unit  16  inserts a cyclic prefix in a signal to avoid interference between OFDM symbols and then transmits the signal.  
         [0013]     The offset DFT-S-OFDM system improves the PAPR performance of the SC-FDMA. Yet, the offset DFT-S-OFDM system requires a considerable load of calculations to make its implementation complicated. In particular, in the process of performing DFT dispreading, DFT matrix is applied in a manner of separating an input symbol into a real part and an imaginary part. So, the calculation load increases to make the corresponding implementation more complicated. Hence, the demand for a system, of which implementation is simpler than the offset DFT-S-OFDM system with PAPR performance better than that of the offset DFT-S-OFDM system, rises.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present application is directed to control PAPR and method thereof that substantially obviates one or more problems due to limitations and disadvantages of the related art.  
         [0015]     An object of the present application is to provide an apparatus for controlling PAPR in an OFDM system and method thereof, by which implementation is simplified and by which enhanced PAPR characteristics are provided.  
         [0016]     Additional advantages, objects, and features of the application will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the application. The objectives and other advantages of the application may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0017]     To achieve these objects and other advantages and in accordance with the purpose of the application, as embodied and broadly described herein, a method of controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, according to the present application includes the steps of outputting GN b  data symbols by oversampling N b  parallel data symbols, spreading the oversampled data symbols using DFT (discrete Fourier transform), and mapping the spread signal to subcarriers.  
         [0018]     In another aspect of the present application, a method of controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, includes the steps of spreading N b  parallel data symbols using DFT (discrete Fourier transform), shifting a phase of each of the spread data symbols by a phase value corresponding to a case of oversampling the corresponding data symbol, summing the spread data symbol and the phase-shifted data symbol, and mapping the summed symbols to subcarriers.  
         [0019]     In another aspect of the present application, an apparatus for controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, includes an oversampling unit outputting GN b  data symbols by oversampling N b  parallel data symbols, a discrete Fourier transform unit spreading the oversampled data symbols using DFT (discrete Fourier transform), and a subcarrier mapping unit mapping the spread signal to subcarriers.  
         [0020]     In another aspect of the present application, an apparatus for controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, includes a discrete Fourier transform unit spreading N b  parallel data symbols using DFT (discrete Fourier transform), a phase shift unit shifting a phase of each of the spread data symbols by a phase value corresponding to a case of oversampling the corresponding data symbol, a signal summing unit summing the spread data symbol and the phase-shifted data symbol, and a subcarrier mapping unit mapping the summed symbols to subcarriers.  
         [0021]     It is to be understood that both the foregoing general description and the following detailed description of the present application are exemplary and explanatory and are intended to provide farther explanation of the application as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the application and together with the description serve to explain the principle of the application. In the drawings:  
         [0023]      FIG. 1  is a block diagram of an OFDM signal generator according to SC-FDMA;  
         [0024]      FIG. 2  is block diagram of an OFDM system according to a first embodiment;  
         [0025]      FIG. 3  is block diagram of an OFDM system according to a second embodiment;  
         [0026]      FIG. 4  is a graph of PAPR performance of an OFDM system according to one embodiment; and  
         [0027]      FIG. 5  is a graph of PAPR performance of an OFDM system according to another embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Reference will now be made in detail to the preferred embodiments of the present application, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0029]     In case that a signal is spread in a frequency domain based on DFT, if a length of a data vector is N b , a maximum number N b  of data vectors appear in a transmission signal. Hence, it is able to decrease PAPR in a manner that input data appear in the transmission signal as many as possible. And, oversampling can be performed to increase a size of the input data with maintaining a size of the transmission data. In particular, after oversampling has been performed on the data vectors, the oversampled vectors are transformed to be transmitted.  
         [0030]      FIG. 2  is block diagram of an OFDM system according to a first embodiment.  
         [0031]     Referring to  FIG. 2 , a serial-to-parallel converting unit  21  converts a data symbol inputted in series to a parallel signal. And, an oversampling unit  22  oversamples N b  data symbols G-times (oversampling coefficient) to extend a number of input symbols by G times with maintaining a number of independent data symbols, In the following description, an example of ‘G=2’ is explained.  
         [0032]     First of all, assuming that a signal s x  which is inputted to the oversampling unit  22  after being converted in parallel by the serial-to-parallel converting unit  21  is [s x (0), s x (1) 0 s x (N b −1)] T , Equation 3 indicates a method of oversampling data symbols. 
 
 s   xo   =s   xo   1   +s   xo   2   [Equation 3]
 
         [0033]     In Equation 3, s xo  is a result from oversampling of the input signal s x , and s xo   1  and s xo   2  are shown in  FIG. 4  and  FIG. 5  respectively. 
 
 s   xo   1   =[s   x (0) 0  s   x (1) 0 . . .  s   x ( N   b −1) 0] T   [Equation 4]
 
 s   xo   2 =[0  s   x (0) 0  s   x (1) . . . 0  s   x ( N   b −1)] T   [Equation 5]
 
         [0034]     In Equation 5, s xo   2  is exemplarily generated by shifting s xo   1  by one by one. Yet, s xo   2  can be generated by shifting s xo   1  by an arbitrary k.  
         [0035]     A discrete Fourier transform unit  23  performs DFT on the oversampled signal to bring about a dispreading effect in a frequency domain. And, the DFT can be expressed as Equation 6. 
 
 s   Fo   =s   Fo   1   +s   Fo   2   [Equation 6]
 
         [0036]     In Equation 6, s Fo   1  and s Fo   2  can be represented as Equation 7 and Equation 8, respectively. 
 
 s   Fo   1   =F   2N     b     ×N     b     s   xo   1   [Equation 7]
 
 s   Fo   2   =F   2N     b     ×N     b     s   xo   2   [Equation 8]
 
         [0037]     In this case, F 2N     b     sN     b    is a transform matrix consisting of upper N b  of F 2N     b     s2N     b   . A subcarrier mapping &amp; frequency window unit  24  maps the DFT-performed signal to subcarrier and then executes a frequency window.  
         [0038]     The frequency-domain signal mapped to the subcarrier is inputted to an inverse Fourier transform unit  25 . The inverse Fourier transform unit  25  performs Inverse Fast Fourier Transform (hereinafter abbreviated IFFT) on the input signal to transform it into a time-domain signal. And, a parallel-to-serial converting unit  26  converts the time-domain transformed parallel signal to a serial signal.  
         [0039]     A cyclic prefix (hereinafter abbreviated CP) inserting &amp; time window unit  24  to eliminate interference between OFDM symbols adds a CP to the serial signal and then executes a time window.  
         [0040]      FIG. 3  is block diagram of an OFDM system according to a second embodiment of the present application.  
         [0041]     Referring to  FIG. 3 , the respective components of s Fo   1  and s Fo   2  in Equation 7 and Equation 8 can be represented as Equation 9.  
                         s   Fo   1     ⁡     (   p   )       =       ∑     k   =   0         2   ⁢     N   b       -   1       ⁢       exp   ⁡     (       j   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   pk       2   ⁢     N   b         )       ⁢     s   xo   1                     =       ∑       k   ′     =   0         N   b     -   1       ⁢       exp   ⁡     (       j2π   ⁢           ⁢     p   ⁡     (     2   ⁢     k   ′       )           2   ⁢     N   b         )       ⁢     s   xo   1                     =       s   F     ⁡     (   p   )               ⁢     
     ⁢               s   Fo   2     ⁡     (   p   )       =       ∑     k   =   0         2   ⁢     N   b       -   1       ⁢       exp   ⁡     (       j   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   pk       2   ⁢     N   b         )       ⁢     s   xo   2                     =       ∑       k   ′     =   0         N   b     -   1       ⁢       exp   ⁡     (       j2π   ⁢           ⁢     p   ⁡     (       2   ⁢     k   ′       +   1     )           2   ⁢     N   b         )       ⁢     s   xo   2                     =     exp   ⁢           ⁢     (       j2π   ⁢           ⁢   p       2   ⁢     N   b         )     ⁢       s   F     ⁡     (   p   )                         [     Equation   ⁢           ⁢   9     ]             
 
         [0042]     In Equation 9, if the oversampled vector is transformed, the result is related to an original signal S F . And, s Fo   1  and s Fo   2  differ from each other in a phase component. Hence, the OFDM system shown in  FIG. 2  can be implemented by the method shown in  FIG. 3 .  
         [0043]     Similar to the former embodiment shown in  FIG. 2 , a serial-to-parallel converting unit  31  converts a data symbol inputted in series to a parallel signal. And, a discrete Fourier transform unit  32  performs spreading on the parallel-converted data symbol using DFT matrix of N b ×N b .  
         [0044]     As shown in Equation 9, if phases of the respective spread input symbols are shifted and summed, it brings about the same effect of oversampling.  
         [0045]     So, a phase shifting unit  33  shifts a phase of the input symbol in a manner shown in Equation 9. And, a summing unit  34  generates a signal resulting from summing an input phase shifted symbol and an non-shifted symbol.  FIG. 3  shows a case that a value of G (oversampling coefficient) is 2. Yet, the number of input signals inputted to the summing unit  34  is normally ‘G’. For instance, if G is 3, a signal outputted from the discrete Fourier transform unit  32 , a signal resulting from performing a first phase shift on the signal outputted from the discrete Fourier transform unit  32 , and a signal resulting from performing a second phase shift on the signal outputted from the discrete Fourier transform unit  32  are summed up.  
         [0046]     A subcarrier mapping &amp; frequency window unit  35  maps the DFT-performed signal to subcarrier and then executes a frequency window. The frequency-domain signal mapped to the subcarrier is inputted to an inverse Fourier transform unit  36 . The inverse Fourier transform unit  36  transforms the inputted signal into a time-domain signal by performing inverse fast Fourier transform (hereinafter abbreviated IFFT) on the inputted signal. A parallel-to-serial converting unit  37  converts the parallel signal transformed into the time-domain signal to a serial signal. And, a cyclic prefix (hereinafter abbreviated CP) inserting &amp; time window unit  38  to avoid interference between OFDM symbols adds CP to the serially converted signal and then executes a time window.  
         [0047]      FIG. 4  is a graph of PAPR performance of an OFDM system according to one embodiment.  
         [0048]     First of all, in  FIG. 4 , if N=512, N b =128 and if subcarriers are allocated to a continuous subcarrier group, PAPR performance of each system is represented as CCDF (complementary cumulative distribution function) over PAPR.  
         [0049]     Referring to  FIG. 4 , a curve  41  indicates PAPR performance of an OFDM system in case of not using a system for PAPR performance enhancement, and a curve  42  indicates PAPR performance in case of applying SC-FDMA system.  
         [0050]     A curve  43  indicates PAPR performance in case of applying Offset-DFT-S-OFDM SC-FDMA, and a curve  44  indicates PAPR performance of an OFDM system according to the present application.  
         [0051]      FIG. 5  is a graph of PAPR performance of an OFDM system according to another embodiment.  
         [0052]     In  FIG. 5 , if N=512, N b =64 and if subcarriers are allocated to a continuous subcarrier group, PAPR performance of each system is represented as CCDF (complementary cumulative distribution function) over PAPR.  
         [0053]     Referring to  FIG. 5 , a curve  51  indicates PAPR performance of an OFDM system in case of not using a system for PAPR performance enhancement, and a curve  52  indicates PAPR performance in case of applying SC-FDMA system.  
         [0054]     A curve  53  indicates PAPR performance in case of applying Offset-DFT-S-OFDM SC-FDMA, and a curve  54  indicates PAPR performance of an OFDM system according to the present application.  
         [0055]     Accordingly, the present application provides the following effect or advantage.  
         [0056]     Most of all, the present application enables PAPR to be efficiently controlled with a simple structure for implementation.  
         [0057]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present application without departing from the spirit or scope of the applications. Thus, it is intended that the present application covers the modifications and variations of this application provided they come within the scope of the appended claims and their equivalents.