Patent Publication Number: US-8526526-B2

Title: Apparatus for transmission and reception with transmit diversity using cyclic subcarrier shift

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. §371 national stage filing of PCT Application No. PCT/KR2009/003594 filed on Jul. 1, 2009, which claims priority to, and the benefit of, Korean Patent Application No. 10-2008-0064007 filed on Jul. 2, 2008, and Korean Patent Application No. 10-2009-0057692 filed on Jun. 26, 2009. The contents of the aforementioned applications are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a diversity transmission technology using multiple transmit antennas in radio communication systems. 
     BACKGROUND ART 
     Currently, an orthogonal frequency division multiple access (OFDMA) scheme is being applied as a basic transmission scheme for most mobile communication systems. The OFDMA scheme may have a high peak to average power ratio (PAPR), which may be a result of a plurality of symbols being a complex number modulating/multiplexing a plurality of subcarriers in the OFDMA scheme. 
     In order to overcome the above shortcoming, a single carrier frequency division multiple access (SC-FDMA) scheme may spread modulated symbol being a complex number using a discrete Fourier transform (DFT), and after that transmit spread symbols using a ordinary OFDMA scheme. 
     A transmit antenna diversity transmission scheme may enhance a performance by transmitting a symbol using multiple antennas on a transmission side. The transmit antenna diversity scheme is being widely adopted in radio communication systems using the OFDMA scheme, a code division multiple access (CDMA) scheme, and the like. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     An aspect of the present invention provides a transmit diversity that may maintain a low peak to average power ratio (PAPR) while transmitting data using multiple antennas in a mobile communication system adopting a single carrier frequency domain multiple access (SC-FDMA) scheme. 
     Another aspect of the present invention also provides a new transmit diversity that may transmit a modulated symbol via a different frequency domain for each transmit antenna, when transmitting data using multiple antennas in a radio communication system. 
     Technical Solution 
     According to an aspect of the present invention, there is provided a transmission apparatus including: a discrete Fourier transform (DFT) spreader to perform a DFT for a modulated symbol to generate frequency domain symbols; a resource mapper to map the frequency domain symbols and a plurality of frequency domain streams; a cyclic shift unit to perform a cyclic shift for each of the frequency domain streams according to a cyclic shift value that is determined with respect to each of the frequency domain streams; and a transmitter to perform an inverse DFT (IDFT) for the cyclic shifted frequency domain streams to generate a plurality of time domain streams, and to transmit the time domain streams to a reception apparatus using a transmit antenna corresponding to each of the time domain streams. 
     According to another aspect of the present invention, there is provided a transmission apparatus including: a DFT spreader to perform a DFT for a modulated symbol to generate frequency domain symbols; a resource mapper to map the frequency domain symbols and a plurality of frequency domain streams; an IDFT despreader to perform an IDFT for the frequency domain streams to generate a plurality of time domain streams; a phase shift unit to perform a phase shift for each of the time domain streams according to a phase shift value that is determined with respect to each of the time domain streams; and a transmitter to transmit the phase-shifted time domain stream to a reception apparatus using a transmit antenna corresponding to each of the time domain streams. 
     Advantageous Effects 
     According to embodiments of the present invention, in a mobile communication system adopting an SC-FDMA scheme, it is possible to transmit data in a form of providing a transmit diversity using multiple antennas while maintaining a low peak to average power ratio (PAPR). 
     Also, according to embodiments of the present invention, it is possible to provide a new type of a transmit diversity even in a general radio communication system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a structure of a transmission apparatus to perform a cyclic shift for data in a frequency domain according to an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a partial structure of a transmission apparatus to generate a modulated symbol according to an embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating a structure of a transmitter according to an embodiment of the present invention; 
         FIG. 4  is a block diagram illustrating a structure of a transmission apparatus to perform a phase shift for data in a time domain according to an embodiment of the present invention; and 
         FIG. 5  is a block diagram illustrating a structure of a reception apparatus to detect a signal in a reception end according to an embodiment of the present invention. 
     
    
    
     MODE FOR THE INVENTION 
     Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Exemplary embodiments are described below to explain the present invention by referring to the figures. 
       FIG. 1  is a block diagram illustrating a structure of a transmission apparatus  100  to perform a cyclic shift for data in a frequency domain according to an embodiment of the present invention. The transmission apparatus  100  may include a discrete Fourier transform (DFT) spreader  110 , a resource mapper  120 , a plurality of cyclic shift units  131 ,  132 ,  133 , and  134 , and a plurality of transmitters  141 ,  142 ,  143 , and  144 . 
     The DFT spreader  110  may receive a predetermined number of symbols being a complex number to perform a DFT for the received symbols being a complex number to generate frequency domain symbols. The number of symbols input into the DFT spreader  110  may be determined based on a transform size of the DFT spreader  110 . 
     The resource mapper  120  may map the frequency domain symbols and a plurality of frequency domain streams. For example, the plurality of frequency domain streams may be allocated to different frequency bands. Also, the plurality of frequency domain streams may be allocated to a single frequency band. 
     The cyclic shift units  131 ,  132 ,  133 , and  134  may perform a cyclic shift for each of the frequency domain streams according to a cyclic shift value that is determined with respect to each of the frequency domain streams. The cyclic shift units  131 ,  132 ,  133 , and  134  may determine the cyclic shift value with respect to each of the frequency domain streams. For example, the cyclic shift units  131 ,  132 ,  133 , and  134  may determine the cyclic shift value to be different with respect to each of the frequency domain streams. 
     The cyclic shift units  131 ,  132 ,  133 , and  134  may determine the cyclic shift value based on a number of transmit antennas  151 ,  152 ,  153 , and  154  of the transmission apparatus  100 , and a transform size of the DFT spreader  110 . For example, the cyclic shift units  131 ,  132 ,  133 , and  134  may determine, as the cyclic shift value, an integer fold increase of a value that is obtained by dividing the transform size of the DFT spreader  110  by the number of transmit antennas  151 ,  152 ,  153 , and  154  or a number of subcarriers. 
     Referring to  FIG. 1 , the cyclic shift unit  131  may determine, as the cyclic shift value, a “zero” fold increase of the value that is obtained by dividing the transform size of the DFT spreader  110  by the number of transmit antennas  151 ,  152 ,  153 , and  154 , that is, “4”. The cyclic shift unit  132  may determine, as the cyclic shift value, a “one” fold increase of the value that is obtained by dividing the transform size of the DFT spreader  110  by “4”. The cyclic shift unit  133  may determine, as the cyclic shift value, a “two” fold increase of the value that is obtained by dividing the transform size of the DFT spreader  110  by “4”. The cyclic shift unit  134  may determine, as the cyclic shift value, a “three” fold increase of the value that is obtained by dividing the transform size of the DFT spreader  110  by “4”. 
     The transmitters  141 ,  142 ,  143 , and  144  may perform an IDFT for the cyclic shifted frequency domain streams to generate a plurality of time domain streams. The transmitters  141 ,  142 ,  143 , and  144  may transmit the time domain streams to a reception apparatus  160  using a transmit antenna  151 ,  152 ,  153 , or  154  corresponding to each of the time domain streams. An operation of the transmitters  141 ,  142 ,  143 , and  144  will be described in detail later with reference to  FIG. 3 . 
       FIG. 2  is a block diagram illustrating a partial structure of a transmission apparatus  200  to generate a modulated symbol according to an embodiment of the present invention. The transmission apparatus  200  may include a channel encoder  210 , an interleaver  220 , a scrambler  230 , and a modulator  240 . 
     The channel encoder  210  may perform a channel coding for data. The channel encoder  210  may perform the channel coding for the data using a convolutional coding scheme, a turbo coding scheme, or a low density parity check (LDPC) coding scheme. 
     The interleaver  220  may perform interleaving for the channel coded data. The interleaver  220  may randomly rearrange a sequence of the channel coded data to thereby recover from a burst error occurring in a radio channel. 
     The scrambler  230  may perform an exclusive operation for the interleaved data and a scrambling sequence to generate scrambled data. 
     The modulator  240  may modulate the scrambled data to generate a modulated symbol. For example, the modulator  240  may modulate the scrambled data using a phase shift keying (PSK) scheme or a quadratic amplitude modulation (QAM) scheme. The modulated symbol may be input into the DFT spreader  110  of  FIG. 1 . 
       FIG. 3  is a block diagram illustrating a structure of a transmitter  300  according to an embodiment of the present invention. The transmitter  300  may include an IDFT despreader  310  and a cyclic prefix (CP) inserter  320 . 
     The IDFT despreader  310  may perform an IDFT for cyclic shifted frequency domain streams to generate time domain streams. The generated time domain streams may be transmission signals of a single carrier frequency division multiple access (SC-FDMA) scheme. 
     When the cyclic shift units  131 ,  132 ,  133 , and  134  performs the IDFT for the cyclic shifted frequency domain streams according to a different cyclic shift value, a peak to average power ratio (PARP) of the time domain streams generated by the IDFT despreader  310  may be similar to a PARP of modulated symbols that are input into the DFT spreader  110 . Specifically, according to an embodiment of the present invention, when generating the transmission signals using the SC-FDMA scheme, it is possible to maintain a low PARP of the transmission signals to be transmitted using a plurality of transmit antennas. 
     The CP inserter  320  may insert a CP signal into each of the time domain streams. 
     Here, it is assumed that a transmission of a pilot signal or a reference signal for a channel estimation, a channel estimation of a reception side, and the like is performed using a separate general scheme. 
     Embodiments of performing a phase shift for data, transmitted via each transmit antenna, in a frequency domain have been described above with reference to  FIGS. 1 through 3 . When considering a principle of a Fourier transform, a cyclic shift in the frequency domain may correspond to a phase shift in a time domain. Hereinafter, an embodiment of performing the phase shift in the time domain to maintain a low PARP of a transmission signal of an SC-FDMA scheme will be described. 
       FIG. 4  is a block diagram illustrating a structure of a transmission apparatus  400  to perform a phase shift for data in a time domain according to an embodiment of the present invention. 
     A DFT spreader  410  may perform a DFT for a modulated symbol to generate frequency domain symbols. According to an embodiment of the present invention, the DFT spreader  410  may perform the DFT for the modulated symbol using the channel encoder  210 , the interleaver  220 , the scrambler  230 , and the modulator  240  of  FIG. 2 . 
     A resource mapper  420  may map the frequency domain symbols and a plurality of frequency domain streams. For example, the plurality of frequency domain streams may be allocated to different frequency bands. Also, the plurality of frequency domain streams may be allocated to a single frequency band. 
     An IDFT despreader  430  may perform an IDFT for the frequency domain streams to generate time domain streams corresponding to the frequency domain streams. Each of the time domain stream may be an SC-FDMA signal that is transmitted via each of multiple transmit antennas  461 ,  462 ,  463 , and  464 . 
     Phase shift units  441 ,  442 ,  443 , and  444  may perform a phase shift for each of the time domain streams according to a phase shift value that is determined with respect to each of the time domain streams. According to an embodiment of the present invention, phase shift values with respect to the time domain streams may be different from each other. 
     The phase shift units  441 ,  442 ,  443 , and  444  may determine a phase shift value. For example, the phase shift units  441 ,  442 ,  443 , and  444  may determine a phase shift value according to each of the time domain streams, based on a number of transmit antennas  461 ,  462 ,  463 , and  464  of the transmission apparatus  400 . For example, the phase shift units  441 ,  442 ,  443 , and  444  may determine, as the phase shift value with respect to each of the time domain streams, an integer fold increase of an inverse number of the number of transmit antennas  461 ,  462 ,  463 , and  464 . 
     Referring to  FIG. 4 , the phase shift unit  441  may determine, as the phase shift value, a “zero” fold increase of the inverse of the number of transmit antennas  461 ,  462 ,  463 , and  464 , that is, “4”. The phase shift unit  442  may determine, as the phase shift value, a “one” fold increase of the inverse of “4”. The phase shift unit  443  may determine, as the phase shift value, a “two” fold increase of the inverse of “4”. The phase shift unit  444  may determine, as the phase shift value, a “three” fold increase of the inverse of “4”. It may be given by the following Equation 1:
 
 p   m ( n )= e   −j2πnm/M ,0 ≦m≦M− 1,0 ≦n≦N− 1  [Equation 1]
 
     where P m   (n)  denotes an n th  value of a phase shift values for an m th  phase shift unit, M denotes a number of transmit antennas, and N denotes a transform size of the IDFT despreader  430 . The phase shift value may be multiplied with an output signal string of the IDFT despreader  430  for each symbol to thereby generate a phase shifted output signal of each symbol-wise product. 
     The time domain streams transmitted via the transmit antennas  461 ,  462 ,  463 , and  464  may be phase shifted according to a different phase shift value. Transmitters  451 ,  452 ,  453 , and  454  may transmit the time domain streams to a reception apparatus  470  via the transmit antenna  461 ,  462 ,  463 , or  464  corresponding to each of the time domain streams. 
     The transmitters  451 ,  452 ,  453 , and  454  may insert a CP signal into each of the time domain streams, and transmit the time domain streams with the inserted CP to the reception apparatus  470 . 
     Here, it is assumed that a transmission of a pilot signal or a reference signal for a channel estimation, a channel estimation and a synchronization of a reception side, and the like is performed using a separate general scheme. 
       FIG. 5  is a block diagram illustrating a structure of a reception apparatus  500  according to an embodiment of the present invention. Although a single receive antenna is shown in  FIG. 5 , the present invention may be applicable to a plurality of receive antennas as is. 
     A receiver  510  may receive, from a transmission apparatus (not shown), a plurality of time domain streams that are cyclic shifted according to different cyclic shift values. Each of the time domain streams may be transmitted via a radio channel formed from the transmission apparatus to the reception apparatus  500 . The receiver  510  may remove a CP in each of the time domain streams, and perform a DFT for the time domain streams in which the CP is removed. The receiver  510  may perform a DFT for the time domain streams to generate frequency domain streams. 
     According to an embodiment of the present invention, it is possible to transmit time domain streams using a plurality of transmit antennas. In this case, a difference between cyclic shift values of the time domain streams may be in inverse proportion to the number of transmit antennas. 
     According to an embodiment of the present invention, the receiver may receive time domain streams using the same number of subcarriers as a number of transmit antennas of a transmission apparatus. 
     A frequency domain equalizer  520  may equalize the frequency domain streams using channel information that is estimated for a channel estimation process. Specifically, the frequency domain equalizer  520  may perform an IDFT for an equalized frequency domain stream to generate a time domain symbol. 
     An IDFT despreader  530  may perform an inverse operation of a spread process by the DFT spreader  110  or  410 . In a radio communication system adopting an OFDMA scheme instead of an SC-FDMA scheme, when passing the DFT spreader  110  or  410  and the IDFT despreader  530  as is, the present invention may be applicable as is. 
     According to an embodiment of the present invention, a difference between cyclic shift values of time domain streams may be in proportion to a transform size of the IDFT despreader  530 . 
     A demodulator  540  may demodulate a time domain symbol. According to an embodiment of the present invention, the demodulator may detect a de-spread signal using a symbol, being a complex number, of QPSK, 16QAM, 64QAM, and the like, and may transform the detected signal based on a bit unit. In this instance, the demodulator  540  may receive a soft input and generate a soft output according to a receiver type. 
     A descrambler  550  may descramble the demodulated time domain symbol. 
     A channel decoder  560  may perform a channel decoding for the descrambled time domain symbol. 
     According to an embodiment of the present invention, the frequency domain equalizer  520  may be constructed as follows. Modulated frequency domain symbols may be transmitted by a plurality of subcarriers via a plurality of transmit antennas of a transmission side. Therefore, a reception end may need to perform a frequency domain equalization with respect to the plurality of subcarriers. When the number of transmit antennas of the transmission apparatus is two, the number of receive antennas of the reception apparatus is one, and the transmission apparatus transmits a time domain stream using K subcarriers, a frequency domain stream after the reception apparatus performs a DFT for the time domain stream may be expressed by the following Equation 2: 
     
       
         
           
             
               
                 
                   
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     Here, Y k  denotes a reception value at a k th  subcarrier of a frequency domain stream, H i   k  denotes a frequency response of a k th  subcarrier of an i th  transmit antenna, and X k  denotes a frequency domain signal of a transmission side. Specifically, X k  denotes a k th  symbol among outputs of the DFT spreader  110  of  FIG. 1  or the DFT spreader  410  of  FIG. 4 . N k  denotes a frequency domain noise in the k th  subcarrier. It can be known that a received signal may be expressed by the above Equation 2 due to a predetermined transmission structure of the transmission apparatus. Equation 2 may also be divided into various equations as given by the following Equation 3: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Specifically, as expressed by the above Equation 3, the equalizer  540  may restore a transmission signal from M frequency domain received signals, using a minimum mean square error (MMSE) scheme or a zero forcing scheme, with respect to a transmit diversity transmission scheme by a cyclic subcarrier shift disclosed in the present invention. 
     According to an embodiment of the present invention, each of frequency domain streams received by the receiver  510  may include a plurality of frequency domain symbols. In this case, frequency domain streams simultaneously equalized by the frequency domain equalizer  520  may be spaced apart from each other by a value that is obtained by dividing a transformed magnitude of the IDFT despreader  530  by a number of transmit antennas. 
     Although a few exemplary embodiments of the present invention have been shown and described, the present invention is not limited to the described exemplary embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.