Abstract:
Problems of fading in a multi-path environment are ameliorated, and the presence of reflective surfaces is turned from a disadvantage to an advantage, by employing a third polarization direction that effectively creates a third communication channel. This third communication channel can be used to send more information, or to send information with enhanced spatial diversity to thereby improve the overall communication performance. A transmitted signal with three polarization directions is created with a transmitter having, illustratively, three dipole antennas that are spatially orthogonal to each other. To take advantage of the signal with the third polarization direction, the receiver also comprises three mutually orthogonal antenna dipoles.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to wireless communication. More particularly, this invention relates to use of polarized communication signals. 
     Prior art systems accept the long-recognized constraint imposed by Maxwell&#39;s equations that signals which are transmitted from point A to point B over a free space path that directly connects points A and B, and which differ only in their polarization modes, can comprise at most two independent channels. The reason for this constraint lies in the fact that the polarized transmission coefficients between points A and B form a matrix, T, of rank 2. The prior art, therefore, were always of the view that signals can be usefully transmitted from a point A to point B at most with two polarizations, and realizing thereby at most two independent channels of communication. This is demonstrated in the prior art system of FIG. 1, where a transmitter  10  has one dipole antenna  11  and another dipole antenna  12  and a receiver  20  has one dipole antenna  21  and another dipole antenna  22 . Typically, dipole antennas  11  and  12  perpendicular to each other, and so are dipole antennas  21  and  22 . The most efficient transfer of information from the transmitter to the receiver occurs when antennas  11  and  12  are in a plane that is perpendicular to the line connecting points A and B, antennas  21  and  22  are in a plane that is parallel to the plane of antennas  11  and  12 , and antenna dipole  11  is also in a plane that contains antenna  21 . Of course, any other spatial arrangement of antennas  11 ,  12 ,  21  and  22  may be used for communicating information from the transmitter to the receiver, except that the effectiveness of the communication is reduced (a greater portion of the transmitted signal energy cannot be recovered), and the processing burden on the receiver is increased (both antennas  21  and  22  detect a portion of the signal of antenna  11  and of antenna  12 ). 
     Whether a transmitter has a single antenna (polarized or not) or two polarized antennas (as in FIG.  1 ), it remains that multi-pathing presents a problem. Specifically, multiple paths can cause destructive interference in the received signal, and in indoor environments that presents a major problem because there are many reflective surfaces that cause multiple paths, and those reflective surfaces are nearby (which results in the multiple path signals having significant amplitudes). 
     SUMMARY OF THE INVENTION 
     The problems of fading in a multi-path environment are ameliorated, and the presence of reflective surfaces is turned from a disadvantage to an advantage by employing a receiver that accepts and utilizes signals that are polarized to contain energy in the three orthogonal directions of free space. Even more improved operation is obtained when the transmitter transmits information in three independent communication channels with signals that are polarized so that there is transmitted signal energy in the three orthogonal directions of free space, in a third independent communications channel, The third communication channel can be used to send more information, or to send information with enhanced polarization diversity to thereby improve the overall communication efficiency. A transmitted signal with the third polarization direction is created, illustratively, with a transmitter having a third antenna dipole that is orthogonal to the transmitter&#39;s first and second antenna dipoles. To take advantage of the signal with the third polarization direction, the receiver illustratively also comprises three mutually orthogonal antenna dipoles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents a prior art arrangement; 
     FIG. 2 illustrates a condition where the transmitter antenna are not optimally aligned 
     FIG. 3 illustrates a condition if reflective surfaces contributing to the received signal; 
     FIG. 4 presents an arrangement where the receiver has three dipole antennas; 
     FIG. 5 presents an arrangement where the receiver has three dipole antennas; 
     FIG. 6 presents an arrangement where both the transmitter and the receiver have three dipole antennas; and 
     FIG. 7 presents a block diagram of a transceiver in conformance with the principles disclosed herein. 
    
    
     DETAILED DESCRIPTION 
     The arrangement of FIG. 1 is shown to employ antenna dipoles that are orthogonal to each other. The arrangements disclosed in the FIGs. that follow FIG. 1, and described herein, are also depicted with antenna dipoles that are orthogonal to each other. It should be understood, however, that these arrangements are so presented for convenience of the description herein. Use of antenna arrangements that are other than three antenna dipoles that are orthogonal to each other, and other than transmitting effectively from one point is within the scope of this invention. The key attribute of a receiving antenna arrangement is that it can receive signals that are effectively polarized in any and all of three mutually orthogonal directions. It is expected, however, that the transmitting and receiving antennas used will be constructed so as to be associated with a single physical hardware unit (such as a base station, mobile wireless terminal, etc.). 
     As indicated above in connection with the perspective view presented in FIG. 1, the positioning of antennas  11  and  12  relative to antennas  21  and  22  is critical only when the maximum energy is to be transferred from transmitter  10  to receiver  20 . In such situations, the plane in which antennas  11  and  12  lie should be parallel to the plane in which antennas  21  and  22  lie, and those planes should be perpendicular to line  30  that connects points A and B. Moreover, antennas  11  and  22  should lie in a common (other) plane. Arrow  13  shows the polarized signal in plane x-z, and arrow  14  shows the polarized signal of plane y-z. Illustratively, arrows  13  and  14  depict the same signal strength. 
     Of course, regardless of the orientation of antennas  11  and  12  (relative to antennas  21  and  22 ), all transmitted signals can be expressed in terms of signals that are polarized along the x axis, the y axis, and the z axis of FIG.  1 . An arrangement where the receiver&#39;s antenna are at some arbitrary orientation with respect to the transmitter&#39;s antennas is shown in FIG. 2, where the antenna  11 - 12  arrangement is rotated so that the plane in which antennas  11  and  12  lie is perpendicular to line  31 . Because the drawing is in two dimensions and it may be difficult to perceive the direction of line  31 , assume that point  15  is at a distance R from antennas  11  and  12  along line  30  and the movement of line  30  to coincide with line  31  moves point  15  to point  16 . One has to move along the x, y and z axes to go from point  15  to point  16 . This demonstrates visually that a signal that is polarized orthogonaly to line  31  can be viewed to have signal components along the x, y and z axes, but those signals do not represent three independent signals. 
     Expressed mathematically, we can say                  [           r   1               r   2               r   3           ]     =       [           t   11           t   12               t   21           t   22               t   31           t   32           ]     ·     [           s   1               s   2           ]                       or                     r   =   Ts     ,             (   1   )                                
     where the s 1  and s 2  are the signals sent by antennas  11  and  12 , the matrix T reflects the channel&#39;s transmission coefficients between points A and B with respect to signals polarized in each of three orthogonal directions, and r 1 , r 2 , and r 3  are the signals present at the receiver&#39;s point B in the three orthogonal directions. The rank of a matrix is the largest square array in that matrix whose determinant does not vanish. Hence, the rank of matrix T is 2. 
     Of course, the arrangement of FIG. 2 has only two receiver antennas and, therefore, equation (1) degenerates to                [           r   1               r   2           ]     =       [           t   11           t   12               t   21           t   22           ]     ·     [           s   1               s   2           ]               (   2   )                                
     It can happen that the receiver and the transmitter antennas are aligned in such a way that one of the rows in T contains all zero coefficients, and if the row that contains the all zero coefficients is the first or the second row, then one of the receiver antennas will receive nothing. It can even happen that one of the coefficients in the non-zero row will also be zero, resulting in the situation that one receiving antenna is receiving only one of the sent signals. This is not really any worse than receiving a signal such as r 1 =t 11 s 1 +t 12 s 2  with no means to separate s 1  from s 2 . 
     Consider, however, the arrangement of FIG. 3, where the antennas of transmitter  10  are arranged as in FIG. 2 while receiver  20  includes a third antenna dipole  23  that is orthogonal to antenna dipoles  21  and  22 . The relationship between the transmitted signal and the received signal is then as in equation (1), but now there are three detected signals. Therefore, even if one of the rows in equation (1) degenerates to zero, there are still two signals that are viable. Moreover, since the s 1  and s 2  signals are transmitted at different polarization directions, the coefficients of a column in T cannot be all zero. Hence, it is always possible to detect the transmitted signals s 1  and s 2 . From the above it can be seen that use of the third receiver antenna obviates the need to align the transmitter and receiver antennas. 
     Alternatively, consider the situation where the antennas of transmitter  10  are aligned for maximum reception by receiver  20  (as in FIG.  1 ), but there exists a second, reflective, path between the transmitter and the receiver. This is illustrated in FIG. 4 with a tilted surface  40 , where the transmitter has the two antennas  11  and  12  and the receiver has the two antennas  21  and  22 . It can be readily observed that there exists a path  41 - 42  that starts at transmitter  10 , bounces off surface  40  and arrives at receiver  20 . The direction of the signal that arrives via path  41 - 42  is not along path  30  (i.e. impinges at an angle other than 90 degrees relative to the plane at which antennas  21  and  22  lie). The signals arriving at point B can be expressed by                [           r   1               r   2               r   3           ]     =         [           t   11           t   12               t   21           t   22               t   31           t   32           ]     ·     [           s   1               s   2           ]       +       [           t   13           t   14               t   23           t   24               t   33           t   34           ]     ·     [           s   1               s   2           ]                 (   3   )             or                           [           r   1               r   2               r   3           ]     =         [             t   11     +     t   13               t   12     +     t   14                   t   21     +     t   23               t   22     +     t   24                   t   31     +     t   33               t   32     +     t   34             ]     ·     [           s   1               s   2           ]       =       [           t   11   ′           t   12   ′               t   21   ′           t   22   ′               t   31   ′           t   32   ′           ]     ·     [           s   1               s   2           ]                 (   4   )             or                         r   =       T   ′        s                                              
     Moreover, in an arrangement that has only two receiver antennas at point B, and equation (4) degenerates to                  [           r   1               r   2           ]     =       [           t   11   ′           t   12   ′               t   21   ′           t   22   ′           ]     ·     [           s   1               s   2           ]         ,           (   6   )                                
     the likelihood of any row having all zero terms is still quite small. Fading can be reduced even in the face of this small likelihood in the arrangement of FIG. 5, where the receiver has antennas  21 ,  22 , and  23 , adapted to receive the signals r 1 , r 2 , and r 3  of equation (5). 
     FIG. 6 depicts an arrangement where both transmitter  10  and receiver  20  employ three mutually orthogonal antennas, in an environment with multipathing. In this case, the transfer finction is represented by r=T′s where                T   ′     =       [             t   11   ′           t   12   ′               t   21   ′           t   22   ′               t   31   ′           t   32   ′                      t   13   ′               t   23   ′               t   33   ′             ]     .             (   7   )                                
     It can be shown that the matrix T′ matrix is of rank  3  and is, therefore, able to sustain three independent channels of information. Therefore, the transmitter  10  of FIG. 6 advantageously is able to transmit three independent signals, making the FIG. 6 arrangement well suited for high data rate transmissions in cellular environments in the presence of multi-paths, such as indoors. The third independent channel can be used to send additional information, it can be used to send the information with additional redundancy, or a combination of the two. 
     FIG. 7 presents in block diagram form the structure of a transceiver unit that employs three dipole antennas that are orthogonal to each other. Antennas  21 ,  22 , and  23  each are connected to a port which receives signals from its antenna, and feeds signals to its antenna. Illustratively in FIG. 7, antenna  22  feeds signals to receiver  30 , and transmitter  31  feeds signals to antenna  11 . Receiver  30  applies its output signal to detector  32 , which detects the signal r 1  and sends it to processor  100 . Similarly, receiver  40  receives the signal of antenna  23 , applies its output signal to detector  42 , and detector  42  detects the signal r 2  and sends it to processor  100 . Likewise, receiver  50  receives the signal of antenna  21 , applies its output signal to detector  52 , and detector  52  detects the signal r 2  and sends it to processor  100 . By conventional means (e.g. involving the reception of known pilot signals, the elements of T′ are known to processor  100 , and processor  100  computes the signals s 1  s 2 , and s 3  by evaluating 
     
       
           s= ( T′ ) −1   r.   
       
     
     To transmit, signals x 1 , x 2 , and x 2  are applied to encoders  33 ,  43 , and  53 , respectively, where they are encoded and applied to transmitters  31 ,  41 , and  51 , respectively. Transmitters  31 ,  41 , and  51  feed their signals to antennas  22 ,  23 , and  21 . 
     The above discloses principles of this invention by means of illustrative embodiments. It should be understood that other embodiments can be employed, and that some of the characteristics of the illustrated embodiments do not necessarily form requirements of a viable design. By way of example, it should be realized that while it may be desirable to have the three dipole antennas spatially orthogonal to each other, an arrangement that does not quite have this orientation will still work. In the context of the this disclosure, therefore, the term “orthogonal,” where appropriate, includes “substantially orthogonal.”