Communication employing triply-polarized transmissions

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.

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'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'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.

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's antenna are at some arbitrary
 orientation with respect to the transmitter'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
 ##EQU1##
 where the s.sub.1 and s.sub.2 are the signals sent by antennas 11 and 12,
 the matrix T reflects the channel's transmission coefficients between
 points A and B with respect to signals polarized in each of three
 orthogonal directions, and r.sub.1, r.sub.2, and r.sub.3 are the signals
 present at the receiver'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
 ##EQU2##
 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.sub.1 =t.sub.11 s.sub.1 +t.sub.12 s.sub.2 with no means
 to separate s.sub.1 from s.sub.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.sub.1
 and s.sub.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.sub.1 and s.sub.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
 ##EQU3##
 Moreover, in an arrangement that has only two receiver antennas at point B,
 and equation (4) degenerates to
 ##EQU4##
 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.sub.1, r.sub.2, and r.sub.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
 ##EQU5##
 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.sub.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.sub.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.sub.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.sub.1 s.sub.2, and s.sub.3 by evaluating
EQU s=(T').sup.-1 r.
 To transmit, signals x1, x2, and x2 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."