Abstract:
The present invention relates to a high efficiency Multi Carrier wireless transceiver and a method thereof. The object of the present invention is to provide the high efficiency wireless transceiver and a method thereof which reduces the transmission bandwidth to one half of required bandwidth in the wireless transceiver system using the Multi Carrier Code Division Multiple Access method. The present invention particularly relates to a wireless transceiving system using the Multi Carrier modulation method among high speed radio transmission technique, and has an effect of doubling the transmission efficiency in a system using a wired or wireless transmission channel by reducing the transmission bandwidth to one half of required bandwidth by using the symmetry of a Discrete Fourier Transformed signal. The present invention is used in the Multi Carrier wireless transceiver.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless transceiver system using Multi Carrier modulation and, in particular, to a Multi Carrier transceiver which can reduce a transmission bandwidth to one half of required bandwidth by using a symmetry of discrete Fourier transformed signal and a method thereof. 
     2. Information Disclosure Statement 
     In general, a high speed wireless transmission mainly uses a Direct-Sequence Spectrum Spread Method, a frequency hopping method or a Multi Carrier transmission method. 
     With reference to FIGS. 1 and 2, a basic concept of multi carrier modulation method used in the multi carrier transmission method is described. 
     FIG. 1 illustrates a basic transmission structure of the multi carrier modulation method having N subcarriers. 
     The basic concept of the multi carrier modulation method is to transmit by dividing an available frequency into several subchannels. These subchannels orthogonally overlap each other. 
     When a symbol interval is taken as T, the interval Δf between the subcarriers which can assure the orthogonality is a multiple of k/T (k is an integer). 
     FIG. 2 shows a signal spectrum of multi carrier modulation method. 
     The system of FIGS. 1 and 2 can be analyzed by an equation 1.                      s        (   t   )       =                  ∑     k   =   0       n   -   1                         X   k          cos        (       2      π                   f   c        t     +     2      π                 k                 Δ                 ft       )                       =                    ∑     k   =   0       N   -   1                         X   k        cos                 2                 π                   f   c        t                 cos                 2      π        k   T        t       -       ∑     k   =   0       N   -   1                         X   k        sin                 2      π                   f   c        t                 sin                 2                 π        k   T        t                     =                      m   l          (   t   )          cos                 2                 π                   f   c        t     -         m   q          (   t   )          sin                 2                 π                   f   c        t                       m   l          (   t   )       =                  ∑     k   =   0       N   -   1                         X   k        cos                 2                 π        k   T        t                       m   q          (   t   )       =                  ∑     k   =   0       N   -   1                         X   k        sin                 2                 π        k   T        t                     [     Equation                 1     ]                                
     In equation 1, it can be known that s(t) is in form obtained by modulating baseband signals m I (t) and m g (t) with a frequency fc. Defining t=nT/N in the baseband signals m I (t) and m g (t), a discrete signal form of the base band signals is obtained.                    m   l          (   n   )       =       ∑     k   =   0       N   -   1                         X   k        cos                 2      π        nk   N                
              m   q          (   n   )       =       ∑     k   =   0       N   -   1                         X   k        sin                 2                 π        nk   N                   [     Equation                 2     ]                                
     It can be seen that the equation 2 is a same representation as an N point Inverse Discrete Fourier Transform (IDFT) with the exception of a scaling factor 1/N and j of imaginary term. Therefore, the baseband signal can be implemented by carrying out the IDFT of a symbol to be transmitted, and the same result can be obtained by dividing the result of IDFT into real terms and imaginary terms, transforming the discrete signal into a continuous signal, modulating the result with the frequency fc and carrying out a sum operation thereof. At this time, it should be noted the (−) sign must be put in sum operation of imaginary parts. The Discrete Fourier Transform (DFT) block can be further fast calculated by using a fast Fourier transform (FFT) digital signal processor (DSP). A construction of a Multi Carrier wireless transceiver of Orthogonal Frequency Division Multiplexing (OFDM) method based on the FFT is shown in FIG.  3 . 
     FIG. 3 is an illustrative construction drawing of a conventional Multi Carrier wireless transceiver which comprises a serial/parallel (S/P) conversion section  111 , an Inverse Fast Fourier transformation section (IFFT)  112 , a parallel/serial (P/S) transformation section  113 , a digital/analog (D/A) conversion section  114 , a modulation section  115 , a phase shift section  116 , a synthesizing section  117 , a transceiving channel  118 , a demodulation section  119 , a phase shift section  120 , a low pass filter  121 , an analog/digital (A/D) conversion section  122 , a serial/parallel conversion section  123 , a Fast Fourier Transformation section (FFT)  124  and a parallel/serial conversion section  125 . 
     A description of operation of the conventional Multi Carrier wireless transceiver having a structure described above is given below. 
     The serial/parallel conversion section  111  converts the transmission data into N low speed parallel binary data and transmits the converted data to the IFFT section  112 . 
     The IFFT section  112  inverse Fourier transforms the input data by taking the input data as frequency domain spectrum components and transmits to the parallel/serial conversion section  113  an output composed of a real part data sequence and an imaginary part data sequence. 
     The parallel/serial conversion section  113  divides the Inverse Fourier transformed parallel data into a real part and imaginary part, inserts a guard bit for preventing an adjacent signal interference in a transmitting channel, converts the parallel data into a serial data sequence and transmits it to the digital/analog conversion section  114 . At this time, the output is divided into a real part (Re) data and imaginary part (Im) data. The digital/analog conversion section  114  converts an input Multi-level digital signal into analog signal and transmits the converted signal to the modulation section  115 . The modulation section  115  multiplies the input analog signals with a carrier of cos 2π f c t and a carrier obtained by shifting the phase of the former carrier by 90 degree by the phase shift section  116 , respectively. The synthesizing section  117  inverts the sign of the imaginary part signal from the modulation section  115 , adds the imaginary part signal to the real part signal, and transmits the added signal through the channel  118 . 
     The demodulation section  119  divides the received signal from the channel  118 , restores the divided signals to signals same as the output signals from the digital/analog conversion section  114  by multiplying the divided signals with a signal of cos 2π f c t having same frequency and phase as the carrier and a signal obtained by shifting the phase by 90 degree by the phase shift section  120 , respectively, and transmits them to low pass filters  121 . The low pass filters  121  only passes low frequency band among the demodulated signals, and the analog/digital conversion section  122  converts the analog signal transmitted from the low pass filters  121  into digital and transmits it to the serial/parallel conversion section  123 . 
     The serial/parallel conversion section  123  converts the input real part signal and inverted imaginary part signal into N low speed parallel binary data, removes the guard bit and transmits the data to the FFT section  124 . The FFT section  124  discrete Fourier transforms the input discrete parallel data and transmits the data to the parallel/serial conversion section  125 , and the parallel/serial conversion section  125  converts the Fourier transformed parallel data into serial data stream. 
     FIG. 4 is a wireless channel frequency spectrum diagram in which the signal transmitted from the conventional Multi Carrier wireless transceiver is presented in a frequency spectrum at the channel. 
     As shown in FIG. 4, the conventional Multi Carrier wireless transceiver occupies a bandwidth of (N+1)Δf, where the Δf is the carrier interval. 
     As shown in FIG. 4, in case of using the conventional Multi Carrier wireless transceiver, since the occupied bandwidth at the wireless or wired channel is large, there are problems that degrades transmission efficiency and is vulnerable to multi-path fading, noise and interference. 
     SUMMARY OF THE INVENTION 
     An object of the present invention invented to solve the problems described above is to provide a Multi Carrier wireless transceiver and a method thereof which can reduce the transmission bandwidth to one half of required bandwidth by using the symmetry of the discrete Fourier transformed signal in a wireless transceiver system. 
     A reduced bandwidth Multi Carrier wireless transceiver of the present invention to accomplish the object described above comprises: a first serial/parallel conversion means for converting externally input serial signals into parallel signals; an Inverse Fast Fourier Transform (IFFT) means for Inverse Fourier Transforming the plurality of parallel signals transmitted from the first serial/parallel conversion means, dividing the signals into data sequences of real part and imaginary part, and outputting the signals by reducing the number of channels; a transmission processing means for converting the output signal of the IFFT means into analog signal and transmitting a modulated signal; a reception processing means for demodulating the signal transmitted from the transmission processing means and thereafter converting the demodulated signal into digital signal; a high frequency signal generation means for receiving the output signal of the reception processing means and recovering the high frequency signal; and a signal recovery means for receiving the output signal of the high frequency signal generation means and outputting the original serial data. 
     A reduced bandwidth Multi Carrier wireless transceiving method which can reduce transmission bandwidth to a half of required bandwidth by using the symmetry of Inverse Fourier Transformed signal comprises the steps of: a first step in which a serial/parallel conversion section converts externally input serial signals into parallel signals; a second step in which an Inverse Fast Fourier Transform (IFFT) section Inverse Fourier Transforms the plurality of parallel signals transmitted from the first serial/parallel conversion means, divides the signals into data sequences of real part and imaginary part, and outputs the signals by reducing the number of high frequency channels; a third step in which a transmission processing section converts the output signal of the IFFT section into analog signal and transmits the signal by modulating the signal; a fourth step in which a reception processing section demodulates the signal transmitted from the transmission processing means and thereafter converts the demodulated signal into digital signal; a fifth step in which a high frequency signal generation section receives the output signal of the reception processing means and generates the high frequency signal; and a sixth step in which a signal recovery section receives the output signal of the high frequency signal generation means and outputs the original serial data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For fuller understanding of the nature and object of the invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a basic structure of a prior art transmitter using the Multi Carrier modulation method; 
     FIG. 2 is a spectrum diagram of the prior art Multi Carrier modulation method; 
     FIG. 3 is a block diagram of a conventional Multi Carrier wireless transceiver; 
     FIG. 4 is a spectrum diagram of the conventional Multi Carrier wireless transceiver: 
     FIG. 5 is a block diagram of one embodiment of a reduced bandwidth Multi Carrier wireless transceiver of the present invention; 
     FIG. 6 is a spectrum diagram of a wireless channel frequency of the reduced bandwidth Multi Carrier wireless transceiver of the present invention; and 
     FIG. 7 is flow diagram showing a performing process of a reduced bandwidth Multi Carrier wireless transceiving method. 
    
    
     Similar reference characters refer to similar parts in the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A prefered embodiment of the present invention will be explained in detail below with reference to the accompanying drawings. 
     FIG. 5 is a block diagram of one embodiment of a reduced bandwidth Multi Carrier wireless transceiver of the present invention. 
     As shown in FIG. 5, the reduced bandwidth Multi Carrier wireless transceiver according to an embodiment of the present invention comprises a serial/parallel conversion section  310  for converting transmission data into low speed N parallel binary data, an Inverse Fast Fourier Transformation section (IFFT)  320  for providing (n/2)+1 outputs by taking the input data as frequency domain spectrum component, a transmission processing section  330  for transmitting the inverse Fourier transformed data, a reception processing section  340  for receiving the transmitted signal, a high frequency signal generator  350  for recovering high frequency band signals from the received signals, and a signal recovery section  360  for serial/parallel converting the entire signal, performing the discrete Fourier transform, and thereafter for generating N serial data stream. 
     The transmission processing section  330  comprises a converter  331  for dividing the parallel data transmitted from the IFFT section  320  into a real part and imaginary part, inserting a guard bit for preventing an adjacent signal interference in a transmitting channel and for converting the data into a serial data sequence, a digital/analog converter  332  for converting the input Multi-level digital signal into analog signal, a modulator  333  for multiplying the carrier cos 2π f c t to the input analog signal, a phase shifter  334  for shifting the phase of the carrier by 90 degree, and a synthesizer  335  for synthesizing the two modulated signals into one output signal by converting the sign of the imaginary part signal and by adding the imaginary part to the real part signal. 
     The reception processing section  340  comprises a demodulator  341  for dividing the input signal and for restoring the divided signals to signals same as the signals of the output of the digital/analog converter  332  by multiplying the divided signals with a demodulation signal cos 2π f c t having same frequency and phase as the carrier, a phase shifter  342  for shifting the phase of the demodulation signal by 90 degree, a low pass filter  343  and an analog/digital converter  344 . 
     The high frequency signal generator  350  comprises a duplicator  351  for duplicating the high frequency signal component from (N/2)+1 to N-1 channels corresponding to the high frequency component from the received digital signal, and a synthesizer  352  for summing the duplicated signal component and the original signal. 
     The signal recovery section  360  comprises a serial/parallel converter  361  for converting the input real part signal and the inverted imaginary part signal into low speed N parallel binary data a FFT  362  for discrete Fourier transforming the input discrete parallel data, and a parallel/serial converter  363  for converting the Fourier transformed parallel data into serial data stream. 
     The operation of the reduced bandwidth Multi Carrier wireless transceiver according to the present invention is described in detail below with reference to FIGS. 5 and 7. 
     FIG. 7 is a flow diagram showing a performing process of a reduced bandwidth Multi Carrier wireless transceiving method. 
     The serial/parallel conversion section  310  converts a wide bandwidth signal sequence Xk into low speed N parallel binary data having period of T and transmits the converted data to the IFFT section  320  at step  701 . 
     The IFFT section  320  performs a N point Discrete Fourier Inverse Transformation for the input N data and provides (N/2)+1 data stream to the transmission processing section  330  by using the symmetry of the Discrete Fourier Transformed signal at step  702 . 
     The signal transmitted to the transmission processing section  330  is converted into serial Multi-level signal by being divided into a real part and an imaginary part at the parallel/serial converter  331  and the signal is inserted with guard bits for preventing the adjacent signal interference at the channel and is output. The output signal is transmitted to the modulator  333  by being converted to analog signal at digital/analog converter  332 . The modulator  333  modulates the input analog signals of real part and imaginary part by multiplying the signals with the carrier cos 2π f c t and the carrier obtained by shifting the phase of the former carrier by 90 degree by the phase shifter  334 . The synthesizer  335  inverts the sign of imaginary part signal modulated at the modulator and adds the inverted imaginary part to the modulated real part signal to make one output signal and transmits the signal through the channel at step  703 . 
     The demodulator  341  of the reception processing section  340  restores the real part and imaginary part signals output from the transmission processing section  330  into signals same as the output signals of the digital/analog converter  332  by multiplying the real part and imaginary part signals with a signal cos 2π f c t having the same frequency and phase as the carrier and a signal obtained by shifting the phase of the signal cos 2π f c t by the phase shifter  342 , and transmits the restored signal to the low pass filter  343 . The low pass filter  343  only passes the low frequency band among the signals output from the demodulator  341 . The analog/digital converter  344  converts the analog signal transmitted from the low pass filter  343  into digital signal and transmits the digital signal to the high frequency signal generator  350  at step  704 . 
     The duplicator  351  of the high frequency signal generator  350  duplicates (N/2)−1 high frequency band data, which was not transmitted, by copying the digital signal received from analog/digital converter  344  of the reception processing section  340 . The synthesizer  352  sums the signal component duplicated by the duplicator  351  and the original signal, and at this time, in the imaginary part, sums the duplicated signal and the original signal with the sign inverted at step  705 . The signal summed by the synthesizer  352  is transmitted to the signal recovery section  360 . 
     The serial/parallel converter  361  of the signal recovery section converts the input real part and inverted imaginary part signals into low speed N parallel binary data, removes the guard bits and transmits the converted signal to the FFT  362 . The FFT  362  N point Discrete Fourier Transforms the input N discrete parallel data sequence and transmits the result to the parallel/serial converter  363 . The parallel/serial converter  363  converts the Fourier transformed parallel data into serial data stream at step  706 . This signal, that is, the stream becomes the original user data X k . 
     FIG. 6 shows a spectrum on the channel for the transmitted signal using the present invention. As a conclusion, the transmission efficiency is almost doubled in view of that the conventional method required N+1 subchannels, however, the present invention only occupies (N/2)+1 subchannels. 
     On the other hand, the theory of the present invention is described below in detail. 
     Since the IFFT section  320  generates the Orthogonal Frequency Division Multiplexing (OFDM) symbols, a method is invented for reducing the bandwidth by using the property of Discrete Fourier Transforms. Substituting the m I (n) and M q (n) of equation 1 and equation 2 with n values gives the following equation 3.                  m   l          (   0   )       =       ∑     k   =   0       N   -   1                       X   k               [     Equation                 3     ]                   m   l          (   1   )       =       ∑     k   =   0       N   -   1                         X   k        cos                 2                 π        k   N                                     m   l          (   2   )       =       ∑     k   =   0       N   -   1                         X   k        cos                 2                 π          2      k     N                               …                             m   l          (     N        -        2     )       =         ∑     k   =   0       N   -   1                         X   k        cos                 2                   π        (     k   -       2      k     N       )           =       m   l          (   2   )                                     m   l          (     N        -        1     )       =         ∑     k   =   0       N   -   1                         X   k        cos                 2                   π        (     k   -     k   N       )           =       m   l          (   1   )                                     m   q          (   0   )       =   0                               m   q          (   1   )       =       ∑     k   =   0       N   -   1                         X   k        sin                 2                 π        k   N                                     m   q          (   2   )       =       ∑     k   =   0       N   -   1                         X   k        sin                 2                 π          2      k     N                               …                             m   q          (     N        -        2     )       =         ∑     k   =   0       N   -   1                         X   k        sin                 2                   π        (     k   -       2      k     N       )           =     -       m   q          (   2   )                                       m   q          (     N        -        1     )       =         ∑     k   =   0       N   -   1                         X   k        sin                 2                   π        (     k   -     k   N       )           =     -       m   q          (   1   )                                                    
     In the Inverse Discrete Fourier Transformed signal waveform as described above, the real term is formed from (N/2)+1th carrier by an even function symmetry, and the imaginary term is formed from (N/2)+1th carrier by an odd function symmetry. Therefore, instead of sending N subcarriers, only (N/2)+1 subcarriers are sent and the remaining subcarriers can be duplicated at the reception stage. In this way, only the bandwidth of ((N/2)+2)Δf is required instead of the entire bandwidth (N+1)Δf. For example, when N is 8, the occupied bandwidth is 9Δf. In the present invention, in this case, since only (N/2)+1 subcarriers are used instead of N subcarriers, the occupied bandwidth is 6Δf. The transmission stage transmits the signal having the spectrum shown in FIG. 6, and the reception stage duplicates the (N/2)−1 data after analog/digital conversion thereof the make left-right symmetry and adds the duplicated data to the original data, so that the original transmission data are recovered. 
     Therefore, the present invention effectuates the satisfactory transceiving with only the transmission bandwidth equivalent to the half of conventionally required transmission bandwidth. 
     The present invention described above can efficiently use the transmission bandwidth in the transceiver system using a limited transmission bandwidth, and has an effect of doubling the usage efficiency and the transmission efficiency of wireless frequency resources in a system using a wired or wireless transmission channel by reducing the transmission bandwidth to one half thereof by using the symmetry of DFT.