Abstract:
An apparatus and a method for reducing an error vector magnitude in an orthogonal frequency division multiplexing (OFDM) receiver. The method includes the steps of inputting a receiving symbol including a guard interval and an effective symbol interval following the guard interval, in which a front portion of the guard interval and a rear portion of the effective symbol interval have windowing intervals corresponding to windowing of a transmitter, and replacing a signal of the rear windowing interval with a signal of an interval between the front windowing interval and the effective symbol interval, thereby outputting a signal of the effective symbol interval, which substitutes for a signal of the rear windowing interval, to a fast Fourier transform (FFT) section.

Description:
PRIORITY  
       [0001]     This application claims priority to an application entitled “Apparatus And Method For Reducing Error Vector Magnitude In Orthogonal Frequency Division Multiplexing receiver” filed with the Korean Intellectual Property Office on Jan. 17, 2005 and assigned Serial No. 2005-4338, the content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to an orthogonal frequency division multiplexing (OFDM) system, and more particularly, to an apparatus and a method for reducing an error vector magnitude (EVM) in an OFDM receiver.  
         [0004]     2. Description of the Related Art  
         [0005]     In an OFDM scheme, a data stream having a high transmission rate is divided into various data streams having a low transmission rate. The data streams are simultaneously transmitted in parallel to each other using a plurality of sub-carriers. Such an OFDM scheme has a high data rate, superior frequency efficiency and superior characteristics against a frequency fading channel.  
         [0006]     In order to prevent orthogonality between sub-carriers from being broken due to channel characteristics, a guard interval, which is longer than a delay spread of a channel, is inserted between OFDM symbols (hereinafter, referred to as “symbols”), thereby removing inter-symbol interference (ISI).  
         [0007]     In addition, in order to ensure continuity over all symbol intervals including the guard interval, a cyclic prefix (CP) is inserted into the guard interval. That is, a part of the symbol is copied as a CP and the CP is insert into the guard interval.  
         [0008]     In addition, the guard interval having the CP is located at a front part of the symbols such that the symbols are cyclically extended, thereby preventing an inter-carrier interference (ICI).  
         [0009]     Further, the OFDM scheme achieves a parallel transmission of sub-carriers by using an inverse fast Fourier transform (IFFT) of a transmitter and a fast Fourier transform (FFT) of a receiver. Accordingly, each sub-carrier of an OFDM signal includes a sinc function to overlapped the sub carriers with each other while maintaining the orthogonality therebetween. Because of the sinc function characteristics, the OFDM signal is not band limited. That is, the OFDM signal causes interference between adjacent bands.  
         [0010]     In order to reduce interference between adjacent bands, a transmission scheme has been suggested in which data is transmitted to sub-carriers with the exception of some of sub-carriers of a frequency band. That is, signals are not transmitted to some sub-carriers located at both ends of a corresponding frequency band. However, because the sinc function has a relatively high side lobe, the number of sub-carriers, which do not receive the data, must be increased in order to remove the interference between the adjacent bands through the above transmission scheme. In this case, frequency efficiency is significantly degraded.  
         [0011]     Consequently, a time windowing scheme has been suggested to reduce the interference between adjacent bands while properly maintaining frequency efficiency. The side lobe of the sinc function can be effectively reduced through the time windowing scheme. A raised cosine window is commonly used as a window of the time windowing scheme.  
         [0012]     A raised cosine windowing scheme using the raised cosine window includes a 1 symbol interval raised cosine windowing scheme (hereinafter, referred to as “1 symbol interval windowing”) and a 3 symbol interval raised cosine windowing scheme (hereinafter, referred to as “3 symbol interval windowing”).  
         [0013]      FIG. 1  illustrates the 1 symbol interval windowing. Referring to  FIG. 1 , T s  is a symbol period representing 1 symbol interval, T g  is a guard interval, and T b  is an effective symbol interval. In  FIG. 1 , one symbol includes the guard interval T g  and the effective symbol interval T b  following the guard interval T g . As described above, the guard interval T g  is inserted into the symbol as the CP by copying a rear portion of the effective symbol interval T b .  
         [0014]     If a time domain OFDM signal is x(n), a transmission signal s(n), which is obtained by applying the 1 symbol interval windowing to the x(n), is represented as shown in Equation (1). 
 
 s ( n )= w ( n )× x ( n ), for 0 ≦n≦N   s −1  (1) 
 
         [0015]     In addition, a time window coefficient w(n) is defined as Equation 2.  
               w   ⁡     (   n   )       =     {             0.5   ×     (     1   +     cos   ⁡     (     π   ×     (     1   +     n   m       )       )         )       ,               0.5   ×     (     1   +     cos   ⁡     (     π   ×       n   -     (       (       N   s     -   1     )     -   m     )       m       )         )                       (   2   )             
 
         [0016]     wherein, 0≦n&lt;m, m≦n&lt;N s −m, and N s −m≦n≦N s −1.  
         [0017]     In Equations (1) and (2), N s  is a number of time samples for the symbol period T s  and m is a window size.  
         [0018]     Referring to Equations (1) and (2), in a transmitter of an OFDM system, with respect to a signal having the symbol period T s  as illustrated in  FIG. 1 , an interval between a start point of the symbol and the window size m is multiplied by  
         0.5   ×     (     1   +     cos   ⁡     (     π   ×     (     1   +     n   m       )       )         )       ,       
 
 an interval between the window size m and N s −m is multiplied by 1, and an interval between N s −m and an end of the symbol is multiplied by  
         0.5   ×     (     1   +     cos   ⁡     (     π   ×       n   -     (       (       N   s     -   1     )     -   m     )       m       )         )       ,       
 
 thereby achieving the 1 symbol interval windowing. 
 
         [0019]     Because the interval between the window size m and N s −m is multiplied by 1, the signal is identical to the original signal. An interval between the front window size m and the rear window size m is a windowing interval that causes distortion to the original signal through the actual windowing.  
         [0020]     In the following description, the interval of the front window size is called a “front windowing interval” and the interval of the rear window size is called an “rear windowing interval” for one symbol.  
         [0021]      FIG. 2  illustrates a 3 symbol interval windowing scheme. Referring to  FIG. 2 , T s  is a symbol period representing 1 symbol interval, T g  is a guard interval, and T b  is an effective symbol interval. According to the 3 symbol interval windowing scheme, a previous symbol signal is overlapped with a prefix portion T prefix  of a present symbol and a post symbol signal is overlapped with a postfix portion T postfix  of the present symbol.  
         [0022]     If a time domain OFDM signal is x(n), a transmission signal s(n), which is obtained by applying the 3 symbol interval windowing to the x(n), is represented as shown Equation (3).  
                 s   ⁡     (   n   )       =       w   ⁡     (   n   )       ×       ∑       k   =       -     N   used       /   2         k   ≠   0           N   used     /   2       ⁢       b   k     ⁢     exp   ⁡     (       (     j   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢   Δ   ⁢           ⁢   f     )     ⁢     (     n   -     N   g       )       )               ,           ⁢       for   ⁢           -   m     ≤   n   ≤       N   s     +   m               (   3   )             
 
         [0023]     In addition, a time window coefficient w(n) is defined as Equation 4.  
               w   ⁡     (   n   )       =     {             0.5   ×     (     1   +     cos   ⁡     (     π   ×     (     1   +       n   +   m       2   ⁢   m         )       )         )       ,               0.5   ×     (     1   +     cos   ⁡     (     π   ×       n   -     (       N   s     -   m     )       m       )         )                       (   4   )             
 
         [0024]     wherein, −m≦n&lt;m, m≦n&lt;N s −m, and N s −m&lt;n≦N s +m.  
         [0025]     In Equations (3) and (4), N s  is a number of time samples for the symbol period T s  and m is a window size. In addition, b k  is a frequency domain signal transmitted from a k th  sub-carrier, N g  is a number of time samples for the guard interval T g , and N used  is a number of sub-carriers corresponding to ang IFFT size, except for virtual carriers, which do not transmit the signal. That is, N used  identifies the number of sub-carriers that can receive the pilot or data among the sub-carriers corresponding to the IFFT size.  
         [0026]     Because the windowing scheme intentionally causes distortion to the signal, an error vector magnitude of a system may be degraded and hardware complexity may be increased due to the windowing scheme. In particular, although the 1 symbol interval windowing scheme can be easily achieved while significantly reducing the side lobe, it may exert a bad influence upon EVM performance of the system.  
         [0027]     The EVM is defined as shown Equation 5 and is used to measure a preciseness of modulation of the transmitter. In addition, the EVM, together with a spectrum mask, is an important parameter for achieving a transmission system and must satisfy conditions defined by the standard of the OFDM system.  
             EVM   =           1   N     ⁢       ∑   1   N     ⁢     (       Δ   ⁢           ⁢     I   2       +     Δ   ⁢           ⁢     Q   2         )           S   max   2                 (   5   )             
 
         [0028]     In Equation (5), S max   2  is an outermost portion of a constellation point, i.e., a constellation point having a greatest magnitude, ΔI 2 and ΔQ 2  are error vectors of a real number axis and an imaginary number axis, i.e., error vectors of an in-phase axis and a quadrature phase axis, and N is a number of sub-carriers.  
         [0029]      FIG. 3  illustrates an EVM simulation result according to the window size of the 1 symbol interval windowing scheme, in which the window size m is represented as 4, 8, 12, 16, 24, and 32, with respect to transmission signals of a quadrature phase shift keying (QPSK) scheme and a 16-quadrature amplitude modulation (QAM) scheme. Referring to  FIG. 3 , the EVM becomes degraded as the window size m increases. Accordingly, the 1 symbol interval windowing scheme does not satisfy the condition of the EVM.  
         [0030]     In contrast, although the 3 symbol interval windowing scheme does not significantly degrade the EVM as compared with the 1 symbol interval windowing scheme, it is difficult to realize the 3 symbol interval windowing scheme. That is, according to the 3 symbol interval windowing scheme, it is necessary to know the signal of the next symbol in order to transmit the present symbol. Consequently, a processing delay of a 1 symbol interval may occur, increasing a control logic and a buffer size. Accordingly, the 3 symbol interval windowing scheme may increase the necessary hardware complexity.  
       SUMMARY OF THE INVENTION  
       [0031]     Accordingly, the present invention has been designed to provide an apparatus and a method for reducing an EVM with a relatively simple structure.  
         [0032]     According to an aspect of the present invention, there is provided a method of reducing an error vector magnitude in an orthogonal frequency division multiplexing (OFDM) receiver. The method includes the steps of: inputting a receiving symbol including a guard interval and an effective symbol interval following the guard interval, in which a front portion of the guard interval and a rear portion of the effective symbol interval have windowing intervals corresponding to windowing of a transmitter; and replacing a signal of the rear windowing interval with a signal of an interval between the front windowing interval and the effective symbol interval, thereby outputting a signal of the effective symbol interval, which substitutes for a signal of the rear windowing interval, to a fast Fourier transform (FFT) section. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     The above and other features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0034]      FIG. 1  is a schematic view illustrating a conventional 1 symbol interval raised cosine windowing scheme;  
         [0035]      FIG. 2 a  schematic view illustrating a conventional 3 symbol interval raised cosine windowing scheme;  
         [0036]      FIG. 3  is a graph illustrating an EVM simulation result of a signal according to the 1 symbol interval raised cosine windowing scheme;  
         [0037]      FIG. 4  is a graph illustrating a spectrum simulation result of a signal according to the 1 symbol interval raised cosine windowing scheme;  
         [0038]      FIG. 5  is a block view illustrating an OFDM receiver according to an embodiment of the present invention;  
         [0039]      FIG. 6  is a schematic view illustrating a symbol realignment process according to an embodiment of the present invention;  
         [0040]      FIG. 7  is a flowchart illustrating a symbol realignment process according to an embodiment of the present invention; and  
         [0041]      FIG. 8  is a graph illustrating bit error rate (BER) performance according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0042]     Preferred embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings. In the following detailed description, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.  
         [0043]     According to the present invention, the 1 symbol interval windowing scheme is applied to a transmitted symbol. Thus, the windowing scheme and the window coefficient in the transmitter are identical to those of the above 1 symbol interval windowing scheme. Accordingly, the structure of the transmitter is identical to the system using the 1 symbol interval windowing scheme.  
         [0044]     Accordingly, a spectrum of the transmitter is identical to a spectrum according to the 1 symbol interval windowing scheme.  
         [0045]      FIG. 4  illustrates a spectrum of a signal according to the 1 symbol interval windowing scheme, in which spectrums according to a window size of a 16-QAM signal having 1024 sub-carriers are illustrated. Among 1024 sub-carriers, data is transmitted to 864 sub-carriers and remaining sub-carriers are not used in order to reduce adjacent band interference.  
         [0046]     In addition, the windowing scheme is not applied to an ideal filter, in which a half band lowpass filter having a stop band attenuation gain of 80 dB is used as an interpolation filter in order to interpolate a transmission signal by two times.  
         [0047]     As illustrated in  FIG. 4 , the stop band attenuation gain increases and an inclination degree of the side lobe steepens as the window size increases.  
         [0048]     According to the present invention, the EVM can be completely removed by changing a structure of a receiver while obtaining such a windowing effect at a transmitter.  
         [0049]     In  FIG. 1 , portions causing distortion to the signal due to the windowing of the 1 symbol interval windowing scheme are the front windowing interval, which is the guard interval T g  in which n=0 to m, and the rear windowing interval, which is the effective symbol interval T b  in which n=N s −m to N s −1. However, the signal of the guard interval T g  is removed at an input terminal of an FFT section of the OFDM receiver and only the signal of the effective symbol interval T b  is restored, such that the signal of the guard interval T g  does not exert an influence upon the EVM. Therefore, the signal, which undergoes the windowing in the effective symbol interval T b  may exert an influence upon the EVM.  
         [0050]     In addition, the time domain signal is a basically continuous signal and the windowing signal of the effective symbol interval T b  is identical to the signal located at a rear portion of the CP inserted into the guard interval T g . Accordingly, if the signal is realigned according to Equation (6) when removing the CP before the signal is input into the FFT section, an influence caused by the windowing can be completely removed.  
               y   ⁡     (   l   )       =     {             r   ⁡     (       N   g     +   l     )       ,             for   ⁢           ⁢   0     ≤   1   &lt;       N   FFT     -   m                   r   ⁡     (       N   g     +   l   -     N   FFT       )       ,               for   ⁢           ⁢     N   FFT       -   m     ≤   1   &lt;       N   FFT     -   1                       (   6   )             
 
         [0051]     wherein, r(n) is a receiving signal, N g  is a number of time samples during the guard interval T g , N FFT  is an FFT size, and y(l) is a realigned FFT input signal.  
         [0052]      FIG. 5  is a block view illustrating an OFDM receiver according to an embodiment of the present invention. Referring to  FIG. 5 , the OFDM receiver includes a symbol realigner  104  aligned between a synchronization section  102  and an FFT section  106 . A baseband digital receiving signal obtained from a received signal is transmitted to the synchronization section  102  through a receiving filter  100  such that the baseband digital receiving signal is frequency-synchronized with a symbol timing.  
         [0053]     Conventionally, an output signal of the synchronization section  102  is input into a guard interval extracting section (not shown) in which the CP of the output signal is removed. Thereafter, the signal is input into the FFT section.  
         [0054]     However, in the OFDM receiver illustrated in  FIG. 5 , the output signal of the synchronization section  102  is input into the FFT section  106  through the symbol realigner  104 . The symbol realigner  104  realigns the signal according to Equation (6) and outputs a signal to be input into the FFT section  106 . That is, as illustrated in  FIG. 6 , the symbol realigner  104  replaces the signal of a rear windowing interval  204  with a signal of an interval  202  between a front winding interval  200  of the guard interval T g  and the effective symbol interval T b , such that the windowing signal of the effective symbol interval T b , which substitutes for the signal of the rear windowing interval  204  is input into the FFT section  106 .  
         [0055]     As described above, the windowing signal of the effective symbol interval T b , i.e., the signal of the rear windowing interval  204  is identical to the signal located at the interval  202  formed at a rear portion of the CP inserted into the guard interval T g . Therefore, instead of the signal of the rear windowing interval  204 , which is distorted due to the windowing at the transmitter, the signal of the interval  202 , which is not influenced by the windowing, is transmitted to the FFT section  106 , such that the bad influence caused by the windowing can be completely removed at the transmitter.  
         [0056]      FIG. 7  is a flowchart illustrating a symbol realignment process of the symbol realigner  104 , in which each symbol is processed according to Equation (6). In step  300 , the symbol realigner  104  receives a signal having a receiving symbol from the synchronization section  102  and stores the signal therein. If the receiving signal is r(n), as illustrated in  FIG. 6 , the signal relates to the symbol period T s , so n is in the range of 0 to (N FFT +N g −1).  
         [0057]     In step  302 , l is initialized to 0. If it is determined in step  304  that l is smaller than NFFT-m, step  306  is performed. However, if it is determined in step  304  that l is not smaller than NFFT-m, step  310  is performed.  
         [0058]     In step  306 , y(l)=r(N g +l) is input into the FFT section  106 . l is increased by 1 in step  308 . The procedure returns to step  304 .  
         [0059]     If it is determined in step  310  that l is smaller than N FFT , step  312  is performed. However, if it is determined in step  310  that l is not smaller than NFFT, the procedure may end.  
         [0060]     In step  312 , y(l)=r(N g +l−N FFT ) is input into the FFT section  106 . l is increased by 1 in step  308 . Thereafter, the procedure returns to step  304 .  
         [0061]     That is, y(l)=r(N g +l) is input into the FFT section  106  while continuously increasing l by 1 until l becomes N FFT -m, such that the signal between the effective symbol interval T b  and the rear windowing interval  204 , not including the signal of the guard interval T g , as illustrated in  FIG. 6 , is input into the FFT section  106 . If all of the signals between the effective symbol interval T b  and the rear windowing interval  204  have been output, y(l)=r(N g +l−N FFT ) is input into the FFT section  106  while continuously increasing  1  by 1 until l becomes N FFT , such that the signal of the interval  200  between the front windowing interval  200  and the effective symbol interval T b  is input into the FFT section  106 , instead of the signal of the rear windowing interval  204 .  
         [0062]     Similarly to the conventional OFDM receiver, the signal input into the FFT section  106  may undergo the FFT. Thereafter, the signal is output to a channel estimator/compensator  108  to perform channel estimation and channel distortion compensation according to the estimated channel value. The data is restored by a modulator/forward error correction (FFC) decoder  110 .  
         [0063]      FIG. 8  is a graph illustrating bit error rate (BER) performance depending on a signal to noise ratio (SNR) of a receiver under an additive white Gaussian noise (AWGN) channel environment according to an embodiment of the present invention. The simulation environment includes an FFT size of 1024, a modulation scheme of QPSK, a convolutional turbo code (CTC) of ½ code, and a window size of 32.  
         [0064]     As illustrated in  FIG. 8 , the receiver of the present invention can improve the BER performance, as compared with the receiver, which does not perform the symbol realignment, because the symbol realigner  104  can remove the signal distortion, which is intentionally caused by the windowing.  
         [0065]     Therefore, the receiver according to the present invention prevents EVM performance from being degraded by the windowing in the ideal receiver in which the channel does not exert an influence upon the EVM.  
         [0066]     In addition, as illustrated in  FIG. 8 , the BER performance, which may be degraded in the AWGN channel caused by the windowing, can be relatively improved.  
         [0067]     Further, because the present invention uses characteristics of the OFDM signal, the present invention can be easily realized by realigning an order of the receiving signals, thereby lowering hardware complexity.  
         [0068]     Although the present invention may reduce the guard interval of the OFDM, because the 1 symbol interval windowing scheme represents superior windowing performance, as illustrated in  FIG. 4 , it is sufficient to set the window size to 4 or 8. Therefore, there is no ban influence even if the guard interval of the OFDM is reduced according to the present invention.  
         [0069]     In addition, the channel estimation can also be properly performed without limitation.  
         [0070]     While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled 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 as defined by the appended claims.  
         [0071]     For example, although the present invention has been described in relation to the OFDM system employing the 1 symbol interval windowing scheme, the present invention is also applicable for the OFDM system employing the 3 symbol interval windowing scheme. In this case, the signal of the postfix portion T postfix  is replaced with the signal of the rear portion of the guard interval T g  illustrated in  FIG. 2 , i.e., replaced with the signal of the prefix portion T prefix .  
         [0072]     In addition, the present invention is not only applicable for an orthogonal frequency division multiple access (OFDMA) system based on the OFDM, but also for systems employing the OFDM time windowing scheme in order to reduce the EVM.