Abstract:
A technology is provided capable of improving the efficiency of an OFDM system by obtaining the performance in Bit Error Rate (BER) in a wireless communication using OFDM and determining the minimum FFT input bit that produces a SNR difference of 0.1 dB or below with respect to a theoretical BER graph at a desired performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2010-0111011, filed on Nov. 9, 2010, the disclosure of which is incorporated by reference in its entirety for all purposes. 
     BACKGROUND 
     1. Field 
     The following description relates to a terminal that is used in an Orthogonal Frequency Division Multiplexing (OFDM) system, and an apparatus and a method for minimizing the number of input bits of a Fast Fourier Transformer (FFT) in performing a fast fourier transformation at a receiving end of a OFDM based Wibro terminal. 
     2. Description of the Related Art 
     When a received symbol is modulated in an Orthogonal Frequency Division Multiplexing (OFDM) system, the modulation is performed using Fast Fourier Transform (FFT). Since a FFT engine is highly regarded in implementing the overall OFDM system, many studies have been undertaken to enhance the implementation efficiency of the FFT engine. 
     Various methods have been proposed to effective implement the inner logic of the FFT engine. According to a technology related to the input bits providing the optimum performance of the FFT, at least 6 bits of input bits are required to obtain Signal to Noise Ratio (SNR) loss of 0.1 dB or below, that is, 6 bits are the minimum number of input bits without affecting the FFT performance. 
     The FFT engine is highly regarded in implementing the overall OFDM system. Accordingly, as the number of input bits of the FFT engine increases, the complexity of the interior design of the FFT engine is significantly increased. 
     Accordingly, there is a need to obtain the performance in Bit Error Rate (BER) in a wireless communication using OFDM and determine the minimum FFT input bit that produces a SNR difference of 0.1 dB or below with respect to a theoretical graph at a desired performance. In addition, there is a need to know the change of the minimum input bit of the FFT engine according to the modulation scheme and adaptively determine the number of the FFT input bits. 
     SUMMARY 
     In one aspect, there is provided a technology capable of obtaining the performance in Bit Error Rate (BER) in a wireless communication using OFDM and determining the minimum FFT input bit that produces a SNR difference of 0.1 dB or below with respect to a theoretical BER graph at a desired performance. 
     In addition, there is provided a technology capable of knowing the change of the minimum input bit of the FFT engine according to the modulation scheme and adaptively determining the number of the FFT input bits. 
     In one general aspect, there is provided a receiving apparatus for providing input bits for a Fast Fourier Transformer (FFT) according to a modulation scheme on an Orthogonal Frequency Division Multiplexing (OFDM) system using Fourier Transform (FT), the receiving apparatus including: a modulation scheme determiner configured to determine a modulation scheme, which is performed on an OFDM signal received on the OFDM system; and an input bit determiner configured to provide upper 7 bits of the OFDM signal as input bits for a Fast Fourier Transformer, if the modulation scheme determiner determines that the OFDM signal is a Quadrature Phase Shift Keying (QPSK) signal. 
     The input bit determiner may provide upper 8 bits of the OFDM signal as input bits for the Fast Fourier Transformer if the modulation scheme determiner determines that the OFDM signal is a 16 Quadrature Amplitude Modulation (16 QAM) signal. 
     In another general aspect, there is provided An Orthogonal Frequency Division Multiplexing (OFDM) signal receiving method for providing input bits for a Fast Fourier Transformer (FFT) according to a modulation scheme, the method including: receiving an OFDM signal that is modulated by a predetermined modulation scheme in an OFDM system; determining a modulation scheme that is performed on the received OFDM signal; and providing upper 7 bits of the OFDM signal as input bits for the FFT, which is included in a receiving apparatus on the OFDM system, if the modulation scheme is determined to a Quadrature Phase Shift Keying (QPSK). 
     In the providing of input bits, upper 8 bits of the OFDM signal may be provided as input bits for the FFT if the modulation scheme is determined to 16 Quadrature Amplitude Modulation (16 QAM). 
     As described above, the performance in Bit Error Rate (BER) is obtained in a wireless communication using OFDM and the minimum FFT input bit producing a SNR difference of 0.1 dB or below with respect to a theoretical graph at a desired performance is determined In addition, the change of the minimum input bit of the FFT engine according to the modulation scheme is obtained and the number of the FFT input bits is determined based on the change of the number of the minimum input bit. 
     In addition, according to a computer simulation on a QPSK symbol the FFT performance is maintained when the number of input bits is 7 or above, and in the case of 16 QAM symbols, the FFT performance is maintained when the number of input bits is 8 or above. 
     In addition, the performance is maintained to be equivalent to the theoretical FFT performance. The number of FFT input bits is adaptively determined according to the modulation scheme, so that the complexity of the interior of the FFT engine is decreased and the power consumption is reduced. 
     Other features will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the attached drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a transmitting device and a receiving device on an Orthogonal Frequency Division Multiplexing (OFDM) system. 
         FIG. 2  is a graph illustrating the BER performance when a QPSK modulation is used in the OFDM system. 
         FIG. 3  is a graph illustrating the BER performance when a 16 QAM modulation is used in the OFDM system. 
         FIG. 4  is a block diagram illustrating the configuration of an example of a receiving apparatus in the OFDM system. 
         FIG. 5  is a block diagram illustrating the control flow of an example of a reception process in the OFDM system. 
     
    
    
     Elements, features, and structures are denoted by the same reference numerals throughout the drawings and the detailed description, and the size and proportions of some elements may be exaggerated in the drawings for clarity and convenience. 
     DETAILED DESCRIPTION 
     The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses and/or systems described herein. Various changes, modifications, and equivalents of the systems, apparatuses and/or methods described herein will suggest themselves to those of ordinary skill in the art. Descriptions of well-known functions and structures are omitted to enhance clarity and conciseness. 
     Hereinafter, an example will be described with reference to accompanying drawings. 
       FIG. 1  is a diagram illustrating an example of a transmitting device and a receiving device on an Orthogonal Frequency Division Multiplexing (OFDM) system. As shown in  FIG. 1 , a transmitting device  100  and a receiving device  110  on an Orthogonal Frequency Division Multiplexing (OFDM) have the following configuration. 
     The transmitting device  100  includes a preamble  101 , a random number generator  102 , a modulator  103 , a subcarrier allocator  104 , a pilot inserter  105 , a Mux  106 , an Inverse Fast Fourier Transformer (IFFT)  107  and a cyclic prefix (CP) inserter  108 . 
     The receiving device  110  includes an amplifier  111 , an analog-to-digital converter  112 , a cyclic prefix (CP) remover  113 , a Fast Fourier Transformer (FFT)  114 , a subcarrier re-allocator  115  and a demodulator  116 . 
     When a OFDM signal, which is converted through the above described components of the transmitting device  100  on the OFDM system, is transmitted to the receiving device  110 , noise may be added to the OFDM signal. Such noise is assumed as Additive White Gaussian Noise (AWGN). 
     The AWGN is an implementation of noise coming from all of the transmission medium/communication equipment and many natural sources, such as thermal vibrations of electrons of a conductor. Since free electrons of solid elements forming a communication equipment vibrates due to heat, the voltage of noise randomly varies with time. The distribution of noise complies with a normal distribution, that is, Gaussian distribution. 
     A bit error rate (BET) is calculated by comparing an original signal, which is desired to transmit from the transmitting device  100 , with a signal, which is obtained by regenerating a received signal including noise. The BET calculation  120  is shown as a graph in  FIGS. 2 and 3 . 
       FIG. 2  shows the BER performance when QPSK modulation is used on the OFDM system. 
     In  FIG. 2 , Uncoded BER performance is obtained by simulating the transmitting device  100  and the receiving device  110  of the OFDM system shown in  FIG. 1  using a Quadrature Phase Shift Keying (QPSK). 
     As shown in  FIG. 2 , at  1 E- 4  of BER corresponding to a desired performance for input of QPSK modulated signals, the miniumum input bit for the FFT producing a difference of Eb/No of 0.1 dB or below with respect to the theoretical graph is 7 bits or above. 
       FIG. 3  illustrates the BER performance when 16 QAM modulation is used on the OFDM system. 
     In  FIG. 3 , Uncoded BER performance is obtained by simulating the transmitting device  100  and the receiving device  110  of the OFDM system shown in  FIG. 1  using 16 Quadrature Amplitude Modulation (QAM) 
     As shown in  FIG. 3 , at  1 E- 5  of BER corresponding to a desired performance for input of 16 QAM modulated signals, the minimum input bit of the FFT producing a difference of Eb/No of 0.1 dB or below with respect to the theoretical graph is 8 bits or above. 
       FIG. 4  is a diagram illustrating the configuration of an example of a receiving apparatus of the OFDM system. 
     The configuration of a receiving apparatus shown in  FIG. 4  is identical to that of the receiving device  110  shown in  FIG. 2  except for a modulation scheme determiner  400  and an input bit determiner  410 . 
     The modulation scheme determiner  400  and an input bit determiner  410  are disposed between the CP remover  113  and the FFT  114 . 
     The modulation scheme determiner  400  determines a modulation scheme of the received signal. In general, it is determined whether the modulation scheme is a QPSK or a QAM. 
     If the modulation determiner  400  determines that the OFDM signal is a Quadrature Phase Shift Keying (QPSK) signal, the input bit determiner  410  controls input bits such that only the upper most 7 bits are used among all bits input to the FFT  114 . In addition, the input bit determiner  410  controls input bits such that only the upper most 8 bits are used among all bits input to the FFT  114 , if the modulation determiner  400  determines that the OFDM signal is a 16 Quadrature Amplitude Modulation (16 QAM) signal. 
     The FFT  114  receives the input bits, which are controlled by the input bit determiner  410 , and performs a signal transformation. Thereafter, the respective components  115  and  116  operate to recover the original signal. 
       FIG. 5  is a flowchart illustrating the control flow of an example of a reception process of the OFDM system. 
     A receiving apparatus on an OFDM system receives an OFDM signal that is modulated according to a predetermined modulation scheme ( 500 ). The OFDM system includes a transmitting apparatus, which performs a modulation through an Inverse Fast Fourier Transformer (IFFT) and transmits the modulated signal, and the receiving apparatus which demodulates the received OFDM signal through an Fast Fourier Transformer (FFT). 
     The modulation scheme performed on the received OFDM signal is determined ( 510 ). In general, the modulation scheme performed in the OFDM system is QPSK or QAM modulation scheme. 
     If the determined modulation scheme is a Quadrature Phase Shift Keying (QPSK), upper 7 bits of the OFDM signal are provided as input bits for the FFT that is included in the receiving apparatus ( 520 ). 
     If the determined modulation scheme is a 16 Quadrature Amplitude Modulation (16 QAM), upper 8 bits of the OFDM signal are provided as input bits for the FFT ( 530 ). 
     The OFDM system may receive an OFDM signal including a signal, which is modulated by the transmitting apparatus, and noise and process the received OFDM signal. In the determining of the modulation scheme in operation  510 , a modulation scheme may be determined on a signal that is obtained by converting a received OFDM signal to a digital signal and removing Cyclic Prifix from the digital signal. 
     The disclosure can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. 
     Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves such as data transmission through the Internet. The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     Also, functional programs, codes, and code segments for accomplishing the present invention can be easily construed by programmers skilled in the art to which the present invention pertains. A number of exemplary embodiments have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.