Abstract:
An orthogonal frequency division multiplexing (OFDM) receiving apparatus, including a receiving unit, a subcarrier demodulation unit and a signal output processing unit, is provided. The receiving unit is for receiving an RF signal to generate a set of discrete signals. The subcarrier demodulation unit is coupled to the receiving unit, and used for demodulating a set of discrete signals to obtain a complex signal. The signal output processing unit is coupled to the subcarrier demodulation unit, and used for capturing and outputting real parts of the complex signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of and claims the priority benefit of an application Ser. No. 11/780,488, filed on Jul. 20, 2007, which claims the priority benefit of Taiwan application serial no. 96119576, filed on May 31, 2007, now allowed. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a data transmission and reception technologies. More particularly, the present disclosure relates to an orthogonal frequency division multiplexing (OFDM) transmitting and receiving method and a device thereof. 
         [0004]    2. Background 
         [0005]    OFDM technology using a concept of parallel data transmission and frequency division multiplexing has been in the 1960s, and has been widely applied to communication system and so on currently. Generally speaking, a communication system performs the transmission in the following two manners, namely single carrier and multi-carrier, under the limitation of constant bandwidth. The “multi-carrier transmission” means that a user can use a plurality of subcarriers at the same time to transmit and receive signals. The basic concept of the OFDM transmission uses a plurality of orthogonal subcarriers to transmit signals which are originally a single batch of high-speed data at a lower transmission speed. 
         [0006]    The OFDM technology, having a higher data transmission speed and a characteristic of effectively overcoming frequency selective fading channel, has been widely applied to various wireless communication systems at present. 
         [0007]    According to a common OFDM system as shown in  FIG. 1A , an inputted complex symbol signal d(k) has a real part d 0 (k) and an imaginary part d 1 (k), i.e. d(k)=d 0 (k)+j·d 1 (k), where d 0 (k) and d 1 (k) are real numbers and k=0, 1, 2, . . . , N−1 representing different subcarriers. Then, an inverse Fourier transform is adopted for modulation, and the data is carried on the subcarrier for transmission in the manner as shown in  FIG. 1B .  FIG. 1C  is a schematic view illustrating an orthogonality of the subcarriers. 
         [0008]    Currently, real parts and imaginary parts of signals are carried on cosine and sine subcarriers of a same frequency to transmit and receive data. However, under this architecture, the system performance is hardly to increase and to realize the frequency diversity. 
         [0009]    Therefore, for those skilled persons and researchers, how to improve the performance and the frequency diversity without increasing the complexity of the system is a key issue. 
       SUMMARY 
       [0010]    Accordingly, an exemplary embodiment of an OFDM receiving apparatus including a receiving unit, a subcarrier demodulation unit, and a signal output processing unit is provided by the present disclosure. The receiving unit is used to receive a radio frequency (RF) signal, for generating a set of discrete signals. The subcarrier demodulation unit is coupled to the receiving unit, and demodulates a set of discrete signals, which are carried by different subcarrier frequencies spaced by ½T where T is a symbol interval excluding a cyclic prefix (CP), to obtain a complex signal. The signal output processing unit is coupled to the subcarrier demodulation unit, for capturing and outputting real parts of the complex signal. 
         [0011]    Further, the present disclosure provides an exemplary embodiment of an OFDM transmitting and receiving method, which includes the following steps. First, the transmitter modulates a serious of complex signals having a plurality of real parts and a plurality of imaginary parts corresponding to the real parts by respectively carrying these real-valued real parts and imaginary parts on the subcarriers of different frequencies, which are orthogonal for real value signals. The receiver demodulates the real values, which are the real parts and imaginary parts of complex signals, transmitted on different subcarriers/frequencies. 
         [0012]    In order to make the aforementioned and other objects, features and advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below. 
         [0013]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments and, together with the description, serve to explain the principles of the invention. 
           [0015]      FIG. 1A  is a schematic view of a common OFDM system. 
           [0016]      FIG. 1B  is a schematic view illustrating a frequency allocation of subcarriers carrying data in the OFDM system. 
           [0017]      FIG. 1C  is a schematic view illustrating an orthogonality of subcarriers of the OFDM system. 
           [0018]      FIGS. 2A and 2B  are schematic views illustrating an allocation of subcarriers according to an exemplary embodiment, respectively. 
           [0019]      FIGS. 2C and 2D  are schematic views illustrating the orthogonality of the subcarriers and other subcarriers according to an exemplary embodiment, respectively. 
           [0020]      FIG. 3A  is a schematic view of a circuit architecture of an OFDM transmitting apparatus according to an exemplary embodiment, and  FIG. 3B  is a schematic view of a circuit architecture using a complete transmission end of  FIG. 3A . 
           [0021]      FIG. 4A  is a schematic view of a circuit architecture of an OFDM receiving apparatus according to an exemplary embodiment, and  FIG. 4B  is a schematic view of a circuit architecture using a complete receiving end of  FIG. 4A . 
           [0022]      FIG. 5A  is a schematic view of a circuit architecture of an OFDM transmitting apparatus according to another exemplary embodiment, and  FIG. 5B  is a schematic view of a circuit architecture of a receiving end corresponding to  FIG. 5A . 
           [0023]      FIG. 6A  is a schematic view of a circuit architecture of an OFDM transmitting apparatus according to another exemplary embodiment, and  FIG. 6B  is a schematic view of a circuit architecture of a receiving end corresponding to  FIG. 6A . 
           [0024]      FIGS. 7A to 7D  illustrate a function of a complex number multiplier according to an exemplary embodiment. 
           [0025]      FIGS. 8A to 8B  illustrate a difference between the frequency diversity of the conventional art and the frequency diversity of the exemplary embodiments disclosed. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]      FIG. 2A  is a schematic view illustrating an allocation of subcarriers according to an embodiment. As shown in  FIG. 2A , the subcarriers carrying the real parts of the signal are indicated by a solid line, and the subcarriers carrying the imaginary parts of the signal are indicated by a dash line. In the embodiment, the subcarriers carrying the imaginary parts and the subcarriers carrying the real parts are alternately allocated, and are orthogonal to one another. The subcarriers carrying the imaginary parts can, for example, but not limited to, be allocated at a central position of two subcarriers indicated by the solid line, or approximately at the center. The allocation is not limited to that subcarriers carrying the real parts of the complex symbol and subcarriers carrying the corresponding imaginary parts are alternately allocated. For example, the subcarriers carrying the real parts and the subcarriers carrying the corresponding imaginary parts have several intervals, or the real parts and the imaginary parts are randomly allocated on the subcarriers. The allocation manner is determined depending on the design requirements of the system. 
         [0027]      FIG. 2B  is a schematic view illustrating another allocation of subcarriers according to an exemplary embodiment. As shown in  FIG. 2B , a subcarrier group corresponding to the real parts and a subcarrier group corresponding to the imaginary parts are separated from each other as long as they are orthogonal. In the above description, only two embodiments are illustrated, while the actual allocation manner is determined depending on the design requirements of the system, and is not limited herein. 
         [0028]      FIGS. 2C and 2D  are schematic views illustrating the orthogonality of the subcarriers and other subcarriers according to an exemplary embodiment, respectively. As shown in  FIG. 2C , the data is placed at peaks of the subcarriers, and other subcarriers can be placed at zero points as shown in the figure, such that they are orthogonal.  FIG. 2D  illustrates a difference between the number of the subcarriers of an exemplary embodiment and the conventional number and allocations. As shown in  FIG. 2D , the black dots indicate orthogonal positions of the subcarriers carrying the complex symbols according to the conventional OFDM, and the circles indicates newly added positions of subcarriers where the imaginary parts of the original complex symbols are placed according to an exemplary embodiment. Therefore, under the condition that the orthogonality is satisfied, by the use of the real and imaginary parts of the data are used according to an exemplary embodiment, the subcarriers carrying the data in the frequency domain becomes more, and the frequency diversity is enhanced, thus enhancing the system performance. 
         [0029]    A complex number multiplier can be used to achieve the above allocation of subcarriers. In addition, an inverse discrete Fourier transform (IDFT)/inverse fast Fourier transform (IFFT) unit or an discrete Fourier transform (DFT)/fast Fourier transform (FFT) unit can be used to process signals. Then, the circuit structure is further illustrated with exemplary embodiments. 
         [0030]      FIG. 3A  is a schematic view of a circuit architecture of an OFDM transmitting apparatus according to an exemplary embodiment, and  FIG. 3B  is a schematic view of a circuit architecture using a complete transmission end of  FIG. 3A . 
         [0031]    As shown in the example of  FIGS. 3A and 3B , the OFDM transmitting apparatus includes a signal processing unit, a first IFFT (or IDFT)  10  and a second IFFT (or IDFT)  12 , a complex number multiplier  14 , an adder  16 , and a transmitting unit. 
         [0032]    The signal processing unit is mainly used to divide the signal d(k) (d(k)=d 0 (k)+j·d 1 (k), where k=0, 1, 2, into real parts d 0 (k) and imaginary parts d 1 (k), where d 0 (k) and d 1 (k) are real values. The inner construction of the signal processing unit is not particularly limited as long as the objective can be achieved. In addition, the signal processing unit is replaced by d 0 (k) and d 1 (k) in  FIG. 3A  or subsequent drawings. 
         [0033]    N-point IFFT  10  receives the real parts d 0 (k) of the signal, and performs the inverse fast (or discrete) Fourier transform on it. After the Fourier transform, the N-point IFFT  10  outputs x 0 (n), expressed by the following formula: 
         [0000]    
       
         
           
             
               
                 x 
                 0 
               
                
               
                 ( 
                 n 
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             = 
             
               
                 
                   1 
                   
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                  
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       0 
                     
                     
                       N 
                       - 
                       1 
                     
                   
                    
                   
                       
                   
                    
                   
                     
                       
                         d 
                         0 
                       
                        
                       
                         ( 
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                           j2π 
                            
                           
                               
                           
                            
                           kn 
                         
                         N 
                       
                     
                   
                 
               
               = 
               
                 IFFT 
                  
                 
                   { 
                   
                     
                       d 
                       0 
                     
                      
                     
                       ( 
                       k 
                       ) 
                     
                   
                   } 
                 
               
             
           
         
       
     
         [0034]    In addition, the imaginary parts d 1 (k) are converted with the carrier frequency different from that of d 0 (k), and the output is x 1 (n), expressed by the following formula: 
         [0000]    
       
         
           
             
               
                 x 
                 1 
               
                
               
                 ( 
                 n 
                 ) 
               
             
             = 
             
               
                 
                   1 
                   
                     N 
                   
                 
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                     ∑ 
                     
                       k 
                       = 
                       0 
                     
                     
                       N 
                       - 
                       1 
                     
                   
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                         ( 
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                             j2π 
                              
                             
                               ( 
                               
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                                   1 
                                   / 
                                   2 
                                 
                               
                               ) 
                             
                           
                            
                           n 
                         
                         N 
                       
                     
                   
                 
               
               = 
               
                 
                   
                      
                     
                       
                         jπ 
                          
                         
                             
                         
                          
                         n 
                       
                       N 
                     
                   
                    
                   
                     1 
                     
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                         = 
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                           d 
                           1 
                         
                          
                         
                           ( 
                           k 
                           ) 
                         
                       
                        
                       
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                             j2π 
                              
                             
                                 
                             
                              
                             kn 
                           
                           N 
                         
                       
                     
                   
                 
                 = 
                 
                   
                      
                     
                       
                         jπ 
                          
                         
                             
                         
                          
                         n 
                       
                       N 
                     
                   
                   × 
                   IFFT 
                    
                   
                     { 
                     
                       
                         d 
                         1 
                       
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                         ( 
                         k 
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         [0000]    It can be seen from the above formula that the conversion from d 1 (k) to x 1 (n) can be equivalently achieved by using an N-point IFFT and multiplying a complex coefficient e jπn/N , e.g. the N-point IFFT  12  and the complex number multiplier  14  in the figure. The complex number multiplier  14  functions to make the N-point IFFT  12  to generate a shift on frequency, so that the N-point IFFT  12  has a different carrier frequency from the N-point IFFT  10 . 
         [0035]    Then, the output of the complex number multiplier  14  and the output of the N-point IFFT  10  are inputted into the adder  16 , and combined into an output signal x(n)=x 0 (n)+x 1 (n). That is, the distribution of the subcarriers carrying the data becomes the example as shown in  FIG. 2A  or  2 B. 
         [0036]    Afterwards, the signal after processed by the adder  16  is transmitted to the transmitting unit for processing, so as to be transmitted to a receiving side through an antenna and the like. The general architecture of the transmitting unit processing is as shown in  FIG. 3B , which includes a cyclic prefix (CP) adding unit  80 , a digital to analog converter (DAC)  82 , an RF module  84 , and an antenna  86 . The CP adding unit  80  mainly add the CP to the discrete signal (or symbol) x(n). The subcarriers remains orthogonal under multi-path channel to avoid the interference between the carriers by adding the CP. For example, taking the OFDM as an example, one OFDM symbol interval may include a protection interval (cyclic prefix) and a data symbol interval (i.e. FFT integration interval). The signal added with the added CP is transmitted to the DAC  82  for conversion, and then is processed by the RF module  84 , such that the signal can be transmitted by the antenna  86 . 
         [0037]    Then, the present disclosure is illustrated to be applied to the receiving end.  FIG. 4A  is a schematic view of a circuit architecture of an OFDM receiving apparatus according to an exemplary embodiment, and  FIG. 4B  is a schematic view of a circuit architecture using a complete receiving end of  FIG. 4A . 
         [0038]    The circuit of the receiving end is substantially similar to that of the transmission end, except that the procedure is reversed. As shown in  FIG. 4A , the OFDM receiving apparatus includes a receiving unit, a complex number multiplier  24 , a first N-point FFT (or DFT)  20  and a second N-point FFT (or DFT)  22 , and signal output processing units (real part retrieving units)  26 ,  28 . 
         [0039]    As shown in  FIG. 4B , the signal after received by an antenna  96  passes through an RF module  94 , an analog to digital converter (ADC)  92 , and a CP removal unit  90  to generate a discrete signal x(n). Thereafter, the signal x(n) is transmitted to the N-point FFT (or DFT)  20  to be output, and passes through the complex number multiplier  24  and is processed by the N-point FFT (or DFT)  22  to be output. Finally, the signals output by the N-point FFTs (or DFTs)  20 ,  22  are processed by the signal output processing units  26 ,  28  for taking out the real parts, and the originally transmitted data d 0 (k) and d 1 (k) are resumed. The processing method is substantially reverse to that of  FIG. 3A . 
         [0040]    In the general structure, the complex data (composed of the real and the imaginary parts) is inputted into FFT (or DFT), so for the N-point complex data, the FFT (or DFT) is complex in terms of requiring Nlog(N) multipliers. However, in this embodiment, the real and the imaginary parts of the data are transmitted by different subcarriers, so it is possible to use two FFTs (or DFTs) to achieve the purpose with halved calculation complexity. For the N-point data, the complexity of the FFT (or DFT) and the complex number multiplier  14  is that Nlog(N)+4N−4 multipliers are used. Here, the amount (complexity) will not be increased too much as compared with the conventional art, but the diversity of the frequency is increased, such that the signal processing becomes more perfect. Particularly, for the current communication standard, the N value is great, and the difference becomes smaller. 
         [0041]      FIG. 5A  is a schematic view of a circuit architecture of an OFDM transmitting apparatus according to another exemplary embodiment, and  FIG. 5B  is a schematic view of a circuit architecture of a receiving end corresponding to  FIG. 5A . The architectures of  FIGS. 5A and 5B  are varied examples of  FIGS. 3A and 4A . Here, the IFFT/FFT (or IDFT/DFT) for processing the real and the imaginary parts is shared. 
         [0042]    The example of the circuit architecture of the transmission end is as shown in  FIG. 5A , which mainly includes a signal processing unit, an input switching device SW 1 , an N-point IFFT (or IDFT)  30 , an output switching device SW 2 , a buffer  32 , a complex number multiplier  34 , an adder  36 , and a transmitting unit. The basic structure and operation of the transmitting unit can be referred to the corresponding descriptions of  FIG. 3B , so the details will not be described herein again. 
         [0043]    The signal processing unit divides the signal d(k)=d 0 (k)+j·d 1 (k) into the real parts d 0 (k) and the imaginary parts d 1 (k). In this embodiment, the signal processing unit can firstly arrange the real and the imaginary parts of the signal in order, and then transmits them to the N-point IFFT (IDFT)  30  in sequence. For example, the real and the imaginary parts of the signal are transmitted to the N-point IFFT (IDFT)  30  for the Fourier transform through the input switching device SW 1  in the sequence of d 0 ( 0 ), d 1 ( 0 ), d 0 ( 1 ), ·d 1 ( 1 ) . . . . Definitely, the above sequence is only an example, and the designer can change the sequence to be input into the N-point IFFT (IDFT)  30  according to practical requirements. After the processing, the real and the imaginary parts are respectively transmitted to the buffer  32  and the complex number multiplier  34  through the output switching device SW 2 . For example, the real part data d 0 ( 0 ) after being processed by the N-point IFFT (IDFT) 30 is firstly registered in the buffer  32 . After the imaginary part data d 1 ( 0 ) is processed by the N-point IFFT (IDFT)  30  and is multiplied by the complex coefficient, the output signals of the buffer  32  and the complex number multiplier  34  are added by the adder  36 . The result of addition is transmitted to the transmitting unit. Therefore, the real and the imaginary parts of the data can also be respectively transmitted by different subcarriers through the exemplary architecture of  FIG. 5A , and the orthogonality between the subcarriers can also be maintained. 
         [0044]    In addition, the receiving end receives data by the procedure which is inverse to that of  FIG. 5A . The receiving unit is partially the same as that of the  FIG. 3B . As shown in the example of  FIG. 5B , the data after being received by the antenna of the receiving unit passes through the RF module, the ADC, and the CP removal unit etc. Then, the switching device SW 1  is used to input the signal x(n) into the N-point FFT (DFT)  40  for the Fourier transform. Then, the switching unit SW 2  is used to take out the real parts of the signal, so that the real part retrieving unit  44  and the real part retrieving unit  46  output d 0 (k) and d 1 (k) respectively, where k=0, 1, 2, . . . , N−1. 
         [0045]      FIG. 6A  is a schematic view of a circuit architecture of an OFDM transmitting apparatus according to another exemplary embodiment, and  FIG. 6B  is a schematic view of a circuit architecture of a receiving end corresponding to  FIG. 6A . In  FIG. 6B , x′(n) denotes the complex conjugate of x(n). The difference between this embodiment and the other above embodiments that only one IFFT/FFT (or IDFT/DFT) is used. In the above embodiment, one IFFT/FFT (IDFT/DFT) is used to perform the Fourier transform on the real and the imaginary parts of the data signal respectively, such that the real and the imaginary parts are carried on different and orthogonal subcarriers. In this embodiment, a larger IFFT/FFT (IDFT/DFT) is used to perform the Fourier transform. Generally speaking, the real and the imaginary parts of the data respectively have N point, so the 2N-point IFFT/FFT (IDFT.DFT) having larger processing capability is used to perform the Fourier transform. In this manner, it is possible that only one IFFT/FFT (IDFT/DFT) is used. 
         [0046]    As shown in the example of  FIG. 6A , similar to the example of  FIG. 3A  or  5 A, the signal processing unit (not shown) divides the signal d(k) into the real parts d 0 (k) and the imaginary parts d 1 (k), signal d(k)=d 0 (k)+j·d 1 (k), where k=0, 1, 2, . . . , N−1. The real parts d 0 (k) and the imaginary parts d 1 (k) are input into the 2N-point IFFT (or IDFT)  50 . After the Fourier transform, the first N outputs, x(0), . . . , x(N−1), are transmitted by the antenna through the transmitting unit (referring to  FIG. 3B ), and are received by the receiving end. In this embodiment, one single 2N-point IFFT (or IDFT)  50  is used, so the subcarriers carrying the real and the imaginary parts of the data can maintain orthogonal to one another. 
         [0047]    In addition, on the design of the data input, for example the real parts d 0 (k) are inputted to odd pins of the 2N-point IFFT (or IDFT)  50 , and the imaginary parts d 1 (k) are inputted to even pins of the 2N-point IFFFT (or IDFT)  50 . In addition, the real parts d 0 (k) and imaginary parts d 1 (k) in pairs are input into the 2N-point IFFT (or IDFT)  50 , i.e. the exemplary situation as shown in  FIG. 6A . Definitely, the input configuration of the real parts d 0 (k) and the imaginary parts d 1 (k) is not particularly limited, and is made according to the requirements of the designer. Next, as shown in  FIG. 3B , the signals x(0), . . . , x(N−1) are processed by the CP adding unit, the DAC, and the RF module etc, such that the signal can be transmitted by the antenna. 
         [0048]    Otherwise, the construction of the receiving end is as shown in the example of  FIG. 6B . The receiving unit as shown in  FIG. 4B  receives the discrete signals x(0), . . . , x(N−1). The received signals, x(0), . . . , x(N−1), are fed to the first N inputs to the 2N-point DFT/FFT unit. And x(1), . . . , x(N−1) are inverse-ordered, complex conjugated and fed to the last N−1 inputs to the 2N-point DFT/FFT unit. After the Fourier transform, the signal output processing unit  54  takes out the real parts of the input signal, so as to output d 0 (k) and d 1 (k). The complexity of  FIGS. 6A and 6B  is 2Nlog(2N)/2=Nlog(2N). 
         [0049]    In the embodiment, for the purpose of convenience, the combination of the units such as the inverse Fourier transform unit, the complex number multiplier, and the adder of the transmitting end is referred to as a subcarrier orthogonalization unit. The combination of the units such as the Fourier transform unit, the complex number multiplier, and the adder of the receiving end is referred to as a subcarrier demodulation unit. 
         [0050]      FIGS. 7A to 7D  illustrate a function of a complex number multiplier according to an exemplary embodiment. The circuit diagram of  FIG. 3A  is used for the illustration, the views of subcarriers in  FIGS. 7A to 7D  respectively correspond to the parts marked by A to D in  FIG. 3A . The subcarriers carrying the imaginary parts of the signal processed by IFFT  12  is as shown in  FIG. 7A , and the subcarriers carrying the real parts processed by IFFT  10  is as shown in  FIG. 7C . Next, after being processed by the complex number multiplier  14 , i.e, after multiplied by the complex coefficient e jπn/N , the view as shown in  FIG. 7A  generates a shift to obtain the view as shown by the dash line of  FIG. 7B . That is, the complex number multiplier performs the shift process on the subcarriers carrying the imaginary parts of the signal. 
         [0051]    Then, by the use of the adder of the example of  FIG. 3A , the signal output by the IFFT  10  and the signal output by the complex number multiplier  14  are added, and thus the waveform diagram as shown in  FIG. 7D  is obtained. In other words, the real and the imaginary parts of the signal are carried on different and orthogonal subcarriers. Then, the procedure such as the CP processing, the digital to analog converting DAC, and the RF signal processing is performed on the signal as shown in  FIG. 7D , and the signal is transmitted by the antenna and is received by the receiving end. In addition, the complex number multiplier at the receiving end performs the inverse processing, which can refer to the illustration of  FIGS. 7A to 7D . 
         [0052]      FIGS. 8A to 8B  illustrate a difference between the frequency diversity of the conventional art and the frequency diversity of some exemplary embodiments. As shown in  FIG. 8A , there are only four QPSK subcarriers, the data carried on the subcarrier is the complex signal. On the contrary, in  FIG. 8B , there are eight BPSK carriers, and the data carried on the carriers is the real and the imaginary parts of the QPSK signal respectively. The frequency diversity are increased apparently, thus improving the system performance. 
         [0053]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the invention cover modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.