Abstract:
With the objective of reducing noise components to thereby avoid degradation of receiving characteristics, by making an antenna arrangement having taken into consideration transfer functions for radio wave propagation in an MIMO system, the present invention provides an OFDM transmission system suitable for use between a transmitting device that performs transmission using first and second transmitting antennas and a receiving device that performs reception using first and second receiving antennas, wherein the first transmitting antenna and the second transmitting antenna have an orthogonal relationship with each other and the first receiving antenna and the second receiving antenna have an orthogonal relationship with each other.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an orthogonal frequency division multiplexing (hereinafter abbreviated as “OFDM”) signal transmission system, and particularly to a layout relationship among antennas employed in an MIMO (Multiple Input Multiple Output) system using a transmitting device provided with a plurality of antennas and a receiving device provided with a plurality of antennas, and a transmission system using the same.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 10  is a block diagram showing the relationship between a transmitting device and a receiving device employed in an MIMO system. The transmitting device  1000  comprises two antennas  1001  and  1002  and transmitters TX 1  and TX 2 . Transmit data T 1  is OFDM-modulated by the transmitter TX 1 , which in turn is transmitted through the antenna  1001  as a transmit signal W 1 . Further, transmit data T 2  is OFDM-modulated by the transmitter TX 2 , which in turn is transmitted through the antenna  1002  as a transmit signal W 2 . On the other hand, the receiving device  1010  comprises two antennas  1011  and  1012 , receivers RX 1  and RX 2  and an MIMO computing processor  1013 . A signal received by the antenna  1011  is subjected to Fourier transformation or the like in the receiver RX 1 , which in turn is subjected to demodulation or the like in the MIMO computing processor  1013 , so that transmit data is obtained. A signal received by the antenna  1012  is subjected to Fourier transformation or the like in the receiver RX 2 , which in turn is subjected to demodulation or the like in the MIMO computing processor  1013 , so that transmit data is obtained. For the convenience of explanation in this case, the transmitting device  1000  and the receiving device  1010  will be explained assuming that they perform transmission/reception using the two antennas respectively.  
         [0005]     Now consider where in  FIG. 10 , the function of transfer of data from the antenna  1001  to the antenna  1011  is assumed to be  a , the function of transfer of data from the antenna  1002  to the antenna  1011  is assumed to be  b , the function of transfer of data from the antenna  1001  to the antenna  1012  is assumed to be  c , and the function of transfer of data from the antenna  1002  to the antenna  1012  is assumed to be  d , respectively. The receiving device  1010  receives receiving signals via the plurality of receiving antennas  1011  and  1012  and extracts necessary signal components from the overlapped received signals, using an inverse matrix of the transfer functions for their propagation paths, respectively. A computing process for extracting the necessary signal components from the overlapped received signals will be explained below.  
         [0006]     In a noise-free ideal state, the signals that overlap each other at the receiving antennas respectively satisfy the following equation:  
               (         R1           R2         )     =       (         a       b           c       d         )     ⁢     (         T1           T2         )               (   1   )             
 
         [0007]     Then, the computing process for extracting the necessary signal components from each of the overlapped signals in the ideal state performs estimation of the transfer functions for radio wave propagation by means of the receiving device through the use of a pilot signal or the like. Next, the following arithmetic operation is performed using estimated values thereof:  
               (         T1           T2         )     =       1       a   *   d     -     b   *   c         ⁢     (         d         -   b               -   c         a         )     ⁢     (         R1           R2         )               (   2   )             
 
         [0008]     Since noise components are contained in a signal to be demodulated upon actual communication, the equation (2) is represented as follows. In the following equation, n1 and n2 indicate noise components respectively.  
               (           T1   ′               T2   ′           )     =       1       a   *   d     -     b   *   c         ⁢     (         d         -   b               -   c         a         )     ⁢     (           R1   +   n1               R2   +   n2           )               (   3   )             
 
         [0009]     The following equations are derived from the equations (1) and (3):  
               T1   ′     =     T1   +       1       a   *   d     -     b   *   c         ⁢     (       d   *   n1     -     b   *   n2       )                 (   4   )                 T2   ′     =     T2   +       1       a   *   d     -     b   *   c         ⁢     (         -   c     *   n1     -     a   *   n2       )                 (   5   )             
 
         [0010]     In the MIMO system as is understood from the above equations (4) and (5), when the noise is assumed to be a normal distribution, the more the denominator (a*d−b*c) decreases, the more the dependence on the noise components increases, and hence the receiving characteristics are degraded.  
         [0011]     Incidentally, a patent document 1 (Japanese Unexamined Patent Publication No. 2002-374224) discloses that in an OFDM signal transmission system, an inverse matrix computation is performed using transfer functions to restore and demodulate a transmit signal.  
         [0012]     Also a patent document 2 (Japanese Unexamined Patent Publication No. 2003-244056) discloses a base station which receives receiving signals using an antenna perpendicular to ground and an antenna parallel to ground in a wireless communication system.  
         [0013]     However, both the patent documents do not have the aim of arranging antennas considering transfer functions for radio wave propagation in an MIMO system to thereby reduce noise components and avoid degradation of receiving characteristics.  
       SUMMARY OF THE INVENTION  
       [0014]     With the foregoing in view, it is an object of the present invention to reduce noise components and avoid degradation of receiving characteristics by arranging antennas having taken into consideration transfer functions for radio wave propagation in an MIMO system.  
         [0015]     According to one aspect of the present invention, for achieving the above object, there is provided an OFDM transmission system suitable for use between transmitters that perform transmission using first and second transmitting antennas and receivers that perform reception using first and second receiving antennas, wherein the first transmitting antenna and the second transmitting antenna have an orthogonal relationship with each other, and the first receiving antenna and the second receiving antenna have an orthogonal relationship with each other.  
         [0016]     According to the OFDM transmission system, it is possible to reduce noise components and avoid degradation of receiving characteristics.  
         [0017]     The above and further objects and novel features of the invention will more fully appear from the following detailed description appended claims and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a block diagram for describing a layout relationship among transmitting antennas and receiving antennas employed in a first embodiment of the present invention;  
         [0019]      FIG. 2  is a block diagram showing a polarity relationship between transmit signals and a first receiving antenna employed in the first embodiment;  
         [0020]      FIG. 3  is a block diagram illustrating a polarity relationship between transmit signals and a second receiving antenna employed in the first embodiment;  
         [0021]      FIG. 4  is a block diagram for describing a layout relationship among transmitting antennas in their entirety, which are employed in a second embodiment of the present invention;  
         [0022]      FIG. 5  is a block diagram for describing a layout relationship among the individual transmitting antennas employed in the second embodiment of the present invention;  
         [0023]      FIG. 6  is a block diagram for describing a layout relationship among receiving antennas in their entirety, which are employed in the second embodiment of the present invention;  
         [0024]      FIG. 7  is a block diagram for describing a layout relationship among the individual receiving antennas employed in the second embodiment of the present invention;  
         [0025]      FIG. 8  is a block diagram for describing a layout relationship among transmitting antennas in their entirety, which are employed in a third embodiment of the present invention;  
         [0026]      FIG. 9  is a block diagram for describing a layout relationship among transmitting antennas in their entirety, which are employed in a fourth embodiment of the present invention; and  
         [0027]      FIG. 10  is a block diagram showing the relationship between a transmitting device and a receiving device employed in an MIMO system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     OFDM transmission systems of the present invention will hereinafter be described with reference to the accompanying drawings.  
       First Preferred Embodiment  
       [0029]     An OFDM transmission system according to a first embodiment of the present invention will first be explained using  FIGS. 1 through 3 .  
         [0030]      FIG. 1  is a block diagram for describing a layout relationship among transmitting antennas and receiving antennas employed in the first embodiment of the present invention. In the OFDM transmission system according to the first embodiment of the present invention, a transmitting device is provided with two transmitting antennas  201  and  202 . The transmitting antenna  201  extends in positive directions of an imaginary X axis and an imaginary Y axis on an imaginary X-Y plane and has an angle θ 1  made with respect to the imaginary X axis. The transmitting antenna  201  transmits a transmit signal having a COSθ waveform. The transmitting antenna  202  extends in a positive direction of an imaginary X axis and a negative direction of an imaginary Y axis and has an angle θ 2  made with respect to the imaginary X axis. The transmitting antenna  202  transmits a transmit signal having a COSθ waveform. Now, the two transmitting antennas  201  and  202  form an orthogonal relationship with each other. That is, they have a relationship of θ 1 +θ 2 =90°. When θ 1  is 45°, for example, θ 2  is also 45°. It is needless to say that although each of the transmitting antennas  201  and  202  sends the transmit signal having the COSθ waveform in the present embodiment, it may transmit a transmit signal having a Sinθ waveform. On the other hand, a receiving device is provided with two receiving antennas  211  and  212 . The receiving antenna  211  is placed on an imaginary X-Y plane and located on an imaginary Y axis extending in a positive direction. The receiving antenna  212  is placed on an imaginary X-Y plane and located on an imaginary X axis extending in a positive direction. Now, the two receiving antennas  211  and  212  form an orthogonal relationship with each other.  
         [0031]     In the present invention, the transmitting antennas  201  and  202  having the orthogonal relationship, and the receiving antennas  211  and  212  having the orthogonal relationship respectively have an offset of 45°. In other words, the transmitting antenna  201  has an angle of 45° with respect to the receiving antenna  211 . Further, the transmitting antenna  202  has an angle of 45° with respect to the receiving antenna  212 .  
         [0032]     A polarity relationship between the transmit signals and receiving antennas will be described below using  FIGS. 2 and 3 .  FIGS. 2 and 3  are diagrams showing the receiving device as viewed from the transmitting device.  FIG. 2  is a block diagram showing a polarity relationship between a transmit signal W 1  outputted from the transmitting antenna  201  and the receiving antenna  211 , and a polarity relationship between a transmit signal W 2  outputted from the transmitting antenna  202  and the receiving antenna  211 .  FIG. 3  is a block diagram showing a polarity relationship between a transmit signal W 1  outputted from the transmitting antenna  201  and the receiving antenna  212  and a polarity relationship between a transmit signal W 2  outputted from the transmitting antenna  202  and the receiving antenna  212 .  
         [0033]     Now consider where the polarity of an electric field of the transmit signal W 1  transmitted from the transmitting antenna  201 , which is received at the receiving antenna  211 , is assumed to be plus. In other words, the distance from the receiving antenna  211  to the transmitting antenna  201  is equivalent to the distance equal to an integral multiple of the radio frequency transmitted from the transmitting antenna  201  in  FIG. 1 . In this case, the polarity of an electric field of the transmit signal W 2  transmitted from the transmitting antenna  202 , which is received at the receiving antenna  211 , is minus as shown in  FIG. 2 . Similarly, as shown in  FIG. 3 , the polarity of an electric field of the transmit signal W 1  sent from the transmitting antenna  201 , which is received at the receiving antenna  212 , is plus, whereas the polarity of an electric field of the transmit signal W 2  transmitted from the transmitting antenna  202 , which is received at the receiving antenna  212 , is plus. Thus, only the electric field of the transmit signal W 2  sent from the transmitting antenna  202 , which is received at the receiving antenna  211 , becomes minus. That is, only a transfer function  b  of transfer functions  a  through  d  has polarity different from other transfer functions  a ,  c  and  d . When the transfer function  b  is of negative polarity, other transfer functions  a ,  c  and  d  are respectively brought to positive polarity. On the other hand, when the transfer function  b  is of positive polarity, other transfer functions  a ,  c  and  d  reach the negative polarity. When the transfer function  b  is of the negative polarity and other transfer functions  a ,  c  and  d  are respectively of the positive polarity, a*d−b*c is represented as a*d+b*c. Thus, the value of the denominator (a*d−b*c) becomes large in the equations (4) and (5). That is, when the value of the denominator (a*d−b*c) increases, the value of  
         1       a   *   d     -     b   *   c         ⁢     (       d   *   n1     -     b   *   n2       )         
 
 in the equation (4) becomes small. Similarly, when the value of the denominator (a*d−b*c) becomes large, the value of  
         1       a   *   d     -     b   *   c         ⁢     (         -   c     *   n1     -     a   *   n2       )         
 
 in the equation (5) decreases. Incidentally, when the transfer function  b  is of the positive polarity and other transfer functions  a ,  c  and  d  are respectively of the negative polarity, a*d−b*c is represented as −a*d−b*c=−(a*d+b*c). Thus, even in this case, the denominator (a*d+b*c) also increases. It is needless to say that although the polarity of the denominator is brought to the negative polarity in this case, the denominator&#39;s number may become large and hence no problem occurs. 
 
         [0034]     According to the OFDM transmission system showing the first embodiment of the present invention, the value of a*d−b*c increases even in line-of-sight propagation less subject to a multipath and the influence of noise can be reduced. Thus, according to the OFDM transmission system showing the first embodiment of the present invention, an improvement in receiving characteristic is obtained.  
         [0035]     Incidentally, it is needless to say that although the numbers of the transmitting antennas and the receiving antennas are two respectively, they may be three or more respectively.  
       Second Preferred Embodiment  
       [0036]     An OFDM transmission system according to a second embodiment of the present invention will next be described using  FIGS. 4 through 6 .  FIG. 4  is a block diagram for describing a layout relationship among transmitting antennas in their entirety, which are employed in the second embodiment of the present invention.  FIG. 5  is a block diagram for describing a layout relationship among the individual transmitting antennas employed in the second embodiment of the present invention.  FIG. 6  is a block diagram for describing a layout relationship among receiving antennas in their entirety, which are employed in the second embodiment of the present invention.  FIG. 7  is a block diagram for describing a layout relationship among the individual receiving antennas employed in the second embodiment of the present invention.  
         [0037]     In the OFDM transmission system according to the second embodiment of the present invention, a transmitting device is provided with three transmitting antennas  401  through  403 . As shown in  FIGS. 4 and 5 , the transmitting antenna  401  is placed on an imaginary X-Y plane and located on an imaginary X axis extending in a positive direction. Similarly, the transmitting antenna  402  extends in a positive direction of an imaginary Y axis on the imaginary X-Y plane and has an angle θ 3  made with respect to the imaginary X axis. The transmitting antenna  403  extends in a negative direction of the imaginary X axis on the imaginary X-Y plane and has an angle θ 4  made with respect to the imaginary X axis. For convenience of explanation in the present embodiment, they have a relationship of θ 3 =60° and θ 4 =120°. On the other hand, in the OFDM transmission system according to the second embodiment of the present invention, a receiving device is provided with three receiving antennas  601  through  603 . As shown in  FIGS. 6 and 7 , the receiving antenna  601  is placed on an imaginary X-Y plane and located on an imaginary X axis extending in a positive direction. Similarly, the receiving antenna  602  extends in a positive direction of an imaginary Y axis on the imaginary X-Y plane and has an angle θ 5  made with respect to the imaginary X axis. The receiving antenna  603  extends in a negative direction of the imaginary X axis on the imaginary X-Y plane and has an angle θ 6  made with respect to the imaginary X axis. For convenience of explanation in the present embodiment, they have a relationship of θ 5 =60° and θ 6 =120°. In other words, the OFDM transmission system according to the second embodiment of the present invention is placed in a relationship in which two-dimensional spaces are allocated according to the number of the transmitting antennas and the number of the receiving antennas, and they have angular differences different from one another.  
         [0038]     Advantageous effects of the OFDM transmission system according to the second embodiment of the present invention, which has been configured as described above, will be explained below.  
         [0039]     Now consider where for convenience of explanation, the distances among a plurality of transmitting antennas and a plurality of receiving antennas are the same and the radio wave propagation of only a line-of-sight direct wave a is carried out. In doing so, transfer functions for radio wave propagation, which exist among a plurality of transmitting antennas and a plurality of receiving antennas employed in a conventional OFDM transmission system, are expressed in the following equation:  
             (         a       a       a       a           a       a       a       a           a       a       a       a           a       a       a       a         )           (   6   )             
 
         [0040]     Here, it is necessary that the inverse matrix of the equation (6) exists as expressed in the equation (2) in order to obtain a transmit signal at the receiving device. There is generally provided the condition that a determinant is not 0 (zero) in a matrix with 3 rows and 3 columns (generally called “Sarrus theorem”). Determining the determinant of the equation (6) using the present theorem yields an equation expressed as follows:  
                        a       a       a           a       a       a           a       a       a              =         a   3     +     a   3     +     a   3     -     a   3     -     a   3     -     a   3       =   0             (   7   )             
 
         [0041]     As is understood from the equation (7), no inverse matrix exists in the equation (6). Thus, the conventional OFDM transmission system is not capable of performing satisfactory reception.  
         [0042]     Next, transfer functions for radio wave propagation, which exist among the plurality of transmitting antennas and the plurality of receiving antennas employed in the OFDM transmission system according to the second embodiment are expressed as follows. For convenience of explanation, now consider where antenna receiving power relates to angles or tilt angles (each corresponding to the difference between the angles of transmitting and receiving antennas with respect to the ground) of each transmitting antenna and each receiving antenna, and when the angle of each of the transmitting and receiving antennas is expressed in θ, the receiving power is proportional to cosθ. However, actual antennas are different in this relation due to their structures.  
               (           a   *   cos   ⁢           ⁢   0           a   *   cos   ⁢           ⁢     π   3             a   *   cos   ⁢           ⁢       2   ⁢   π     3                 a   *   cos   ⁢       -   π     3             a   *   cos   ⁢           ⁢   0           a   *   cos   ⁢           ⁢     π   3                 a   *   cos   ⁢               ⁢     2   ⁢   π       3             a   *   cos   ⁢       -   π     3             a   *   cos   ⁢           ⁢   0           )     =     (         a           a   ⁢     3       2               -   a     ⁢     3       2                 a   ⁢     3       2         a           a   ⁢     3       2                   -   a     ⁢     3       2             a   ⁢     3       2         a         )             (   8   )             
 
         [0043]     Determining a determinant of the equation (8) using the Sarrus theorem yields an equation expressed as follows:  
                        a           a   ⁢     3       2               -   a     ⁢     3       2                 a   ⁢     3       2         a           a   ⁢     3       2                   -   a     ⁢     3       2             a   ⁢     3       2         a              =         a   3     +     (         a   ⁢     3       2     ×       a   ⁢     3       2     ×         -   a     ⁢     3       2       )     +     (         a   ⁢     3       2     ×       a   ⁢     3       2     ×         -   a     ⁢     3       2       )     -     (           -   a     ⁢     3       2     ×   a   ×         -   a     ⁢     3       2       )     -     (         a   ⁢     3       2     ×       a   ⁢     3       2     ×   a     )       =         a   3     +         -   3     ⁢   a   ⁢     3   3       8     +         -   3     ⁢   a   ⁢     3   3       8     +       3   ⁢     a   3       4     -       3   ⁢     a   3       4     -       3   ⁢     a   3       4       =           -   7     -     3   ⁢     3         4     ⁢     a   3                   (   9   )             
 
         [0044]     Since the equation (9) is not brought to zero, an inverse matrix exists in the equation (8). Thus, since the inverse matrix exists in the matrix based on the transfer functions, the OFDM transmission system according to the second embodiment of the present invention is capable of performing demodulation and obtaining satisfactory reception.  
       Third Preferred Embodiment  
       [0045]     An OFDM transmission system according to a third embodiment of the present invention will next be explained using  FIG. 8 .  FIG. 8  is a block diagram for describing a layout relationship among transmitting antennas in their entirety, which are employed in the third embodiment of the present invention.  
         [0046]     In the OFDM transmission system according to the third embodiment of the present invention, a transmitting device is provided with three transmitting antennas  801  through  803 . As shown in  FIG. 8 , the transmitting antenna  801  is located on an imaginary X axis. The transmitting antenna  802  is located on an imaginary Z axis. The transmitting antenna  803  is located on an imaginary Y axis. That is, in the OFDM transmission system according to the third embodiment, the respective antennas are placed in a three-dimensional layout relationship and orthogonal to one another.  
         [0047]     According to the OFDM transmission system showing the third embodiment of the present invention, different transfer functions for radio wave propagation can be obtained even in line-of-sight propagation less subject to a multipath and an improvement in receiving characteristic is obtained.  
         [0048]     Incidentally, it is needless to say that the above configuration may be applied to receiving antennas. Even in this case, the above advantageous effects are brought about.  
       Fourth Preferred Embodiment  
       [0049]     An OFDM transmission system according to a fourth embodiment of the present invention will next be descried using  FIG. 9 .  FIG. 9  is a block diagram for describing a layout relationship among transmitting antennas in their entirety, which are employed in the fourth embodiment of the present invention.  
         [0050]     In the OFDM transmission system according to the fourth embodiment of the present invention, a transmitting device is equipped with three transmitting antennas  901  through  903 . As shown in  FIG. 9 , a leading end of the transmitting antenna  901  is placed on an imaginary X-Z plane and oriented in its corresponding positive directions of imaginary X and Y axes. A leading end of the transmitting antenna  902  is placed on an imaginary X-Y plane and oriented in its corresponding positive directions of the imaginary X axis and an imaginary Y axis. A leading end of the transmitting antenna  903  is placed on an imaginary X-Y-Z plane and oriented in its corresponding positive directions of the imaginary X axis and imaginary Y axis and imaginary Z axis. In other words, the OFDM transmission system according to the fourth embodiment of the present invention is placed in a relationship in which space is allocated three-dimensionally and the allocated spaces have angular differences different from one another.  
         [0051]     According to the OFDM transmission system showing the fourth embodiment of the present invention, different transfer functions for radio wave propagation can be obtained even in line-of-sight propagation less subject to a multipath and an improvement in receiving characteristic is obtained.  
         [0052]     Incidentally, it is needless to say that the above configuration may be applied to receiving antennas. Even in this case, the above advantageous effects are brought about.  
         [0053]     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention is to be determined solely by the following claims.