Antenna tuning apparatus for a multiport antenna array

Abstract

An apparatus for tuning a plurality of antennas is provided. The apparatus includes a plurality of antenna ports connected to the plurality of antennas; a plurality of user ports; first adjustable impedance elements, each being connected to one of the antenna ports and ground; second adjustable impedance elements, each being connected to one of the user ports and another one of the user ports; and a plurality of connecting elements, each being connected to one of the antenna ports and one of the user ports, wherein the first and second adjustable impedance elements are configured to be adjustable so to tune the plurality of antennas.

Claims

1. An apparatus for tuning a plurality of antennas, the apparatus comprising: a plurality of antenna ports connected to the plurality of antennas; a plurality of user ports; first adjustable impedance elements, each being connected to one of the antenna ports and ground; second adjustable impedance elements, each being connected to one of the user ports and another one of the user ports; and a plurality of connecting elements, each being connected to one of the antenna ports and one of the user ports, wherein the first and second adjustable impedance elements are configured to be adjustable for tuning the plurality of antennas.

2. The apparatus of claim 1, further comprising: third adjustable impedance elements, each being connected to one of the antenna ports and another one of the user ports.

3. The apparatus of claim 2, wherein the number of the plurality of user ports is m, and wherein the third adjustable impedance elements comprise m(m1)/2 adjustable impedance elements, each having a first terminal coupled to a first terminal of one of the antenna ports and each having a second terminal coupled to a second terminal of the one of the antenna ports.

4. The apparatus of claim 3, wherein each of the third adjustable impedance elements is selected from a group consisting of an adjustable impedance device and a passive linear two-terminal circuit element.

5. The apparatus of claim 2, wherein at least one of the third adjustable impedance elements is connected to two of the antenna ports that are not adjacent.

6. The apparatus of claim 2, wherein each of the third adjustable impedance elements is a capacitor.

7. The apparatus of claim 1, wherein the antenna ports seeing, at a frequency in a given frequency band for the plurality of antennas, an impedance matrix referred to as the impedance matrix seen by the antenna ports, the impedance matrix seen by the antenna ports being a complex matrix of size nn (n is the number of the antenna ports), and wherein the user ports presenting, at the frequency in the given frequency band, an impedance matrix referred to as the impedance matrix presented by the user ports, the impedance matrix presented by the user ports being a complex matrix of size mm (m is the number of the user ports).

8. The apparatus of claim 7, wherein the apparatus configured to tune the plurality of antennas by: generating a diagonal impedance matrix referred to as a given diagonal impedance matrix, determining whether the impedance matrix seen by the antenna ports is equal to the given diagonal impedance matrix, determining, when the impedance matrix seen by the antenna ports is equal to the given diagonal impedance matrix, the impedance matrix presented by the user ports depends on the reactance of any one of the second adjustable impedance elements, and at least one non-diagonal entry of the impedance matrix presented by the user ports depends on the reactance of at least one of the second adjustable impedance elements, and controlling, based on the determination, an absolute value and phase of the at least one non-diagonal entry independently from other entries of the impedance matrix presented by the user ports.

9. The apparatus of claim 1, wherein at least one of the second adjustable impedance elements is connected to two of the user ports that are not adjacent.

10. The apparatus of claim 1, wherein the number of the plurality of antenna ports is n, and wherein the second adjustable impedance elements comprise n(n1)/2 adjustable impedance elements, each having a first terminal coupled to a first terminal of one of the user ports and each having a second terminal coupled to a second terminal of the one of the user ports.

11. The apparatus of claim 1, further comprising: fourth adjustable impedance elements, each being connected to one of the user ports and the ground.

12. The apparatus of claim 1, wherein the number of the antenna ports is identical to the number of the user ports.

13. The apparatus of claim 1, wherein each of the connecting elements is a winding, and mutual inductance exists between two or more of the windings.

14. The apparatus of claim 1, wherein each of the second adjustable impedance elements is selected from a group consisting of an adjustable impedance device and a passive linear two-terminal circuit element.

15. The apparatus of claim 1, wherein each of the second adjustable impedance elements is a capacitor.

16. The apparatus of claim 1, wherein the plurality of antennas are tuned simultaneously by adjusting the first and second adjustable impedance elements.

17. The apparatus of claim 1, wherein the plurality of antennas are tuned based on a variation of an impedance matrix presented by the user ports.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics will appear more clearly from the following description of particular embodiments of the invention, given by way of non-limiting examples, with reference to the accompanying drawings in which:

(2) FIG. 1 shows a block diagram of a typical use of an antenna tuning apparatus for tuning a single antenna, and has already been discussed in the section dedicated to the presentation of the prior art;

(3) FIG. 2 shows a schematic diagram of a first antenna tuning apparatus which could be used as shown in FIG. 1 to tune a single antenna, and has already been discussed in the section dedicated to the presentation of the prior art;

(4) FIG. 3 shows a schematic diagram of a second antenna tuning apparatus which could be used as shown in FIG. 1 to tune a single antenna, and has already been discussed in the section dedicated to the presentation of the prior art;

(5) FIG. 4 shows a block diagram of a typical use of a plurality of antenna tuning apparatuses for simultaneously tuning 4 antennas, and has already been discussed in the section dedicated to the presentation of the prior art;

(6) FIG. 5 shows a block diagram of a typical use of an antenna tuning apparatus for simultaneously tuning 4 antennas (first embodiment);

(7) FIG. 6 shows a schematic diagram of an antenna tuning apparatus for simultaneously tuning 4 antennas (third embodiment);

(8) FIG. 7 shows a schematic diagram of an antenna tuning apparatus for simultaneously tuning 4 antennas (fourth embodiment);

(9) FIG. 8 shows a schematic diagram of an antenna tuning apparatus for simultaneously tuning 4 antennas (fifth embodiment).

DETAILED DESCRIPTION OF SOME EMBODIMENTS

First Embodiment

(10) A first embodiment of an apparatus of the invention, for simultaneously tuning n antennas between which a non-negligible interaction exists, given by way of non-limiting example, is an antenna tuning apparatus for a multiport antenna array, characterized in that: the number of user ports is equal to the number of antenna ports, that is n=m; a circuit diagram and the component values of a decoupling and matching network are obtained using the method presented in the paper of J. Weber, C. Volmer, K. Blau, R. Stephan and M. A. Hein, entitled Miniaturized Antenna Arrays Using Decoupling Networks With Realistic Elements, published in IEEE Transactions on Microwave Theory and Techniques, vol. 54, No. 6, pp. 2733-2740, in June 2006; to obtain a circuit diagram and the component values of the antenna tuning apparatus, p=m (m+1) components of the decoupling and matching network are each replaced with an adjustable impedance device, such that said p partial derivatives defined above by the equation (2) are linearly independent in the real vector space E, the reactance of any one of the adjustable impedance devices being adjustable by electrical means.

(11) The specialist understands that, at a frequency at which the antenna tuning apparatus is intended to operate, if the impedance matrix seen by the antenna ports is a diagonal matrix having all its diagonal entries equal to 50, the reactance of any one of the adjustable impedance devices has an influence on the impedance matrix presented by the user ports, and the reactance of one or more of the adjustable impedance devices has an influence on one or more of the non-diagonal entries of the impedance matrix presented by the user ports. The condition if the impedance matrix seen by the antenna ports is a diagonal matrix having all its diagonal entries equal to 50 does not reflect an intended use of the antenna tuning apparatus.

(12) The specialist understands that the antenna tuning apparatus cannot be made up of a plurality of independent and uncoupled antenna tuning apparatuses each having a single antenna port and a single radio port, as shown in FIG. 4, because in this case, if the impedance matrix seen by the antenna ports is equal to any diagonal impedance matrix, then the impedance matrix presented by the radio ports is a diagonal matrix, the non-diagonal entries of which cannot be influenced by anything.

(13) We note that the decoupling and matching network synthesized using the method presented in said paper of J. Weber, C. Volmer, K. Blau, R. Stephan and M. A. Hein comprises m (2 m+1) components, only m (m+1) of which are replaced with said adjustable impedance devices. The specialist understands how he can determine if the partial derivatives are linearly independent in the real vector space E, for a given choice of m (m+1) adjustable impedance devices, to obtain an appropriate choice.

(14) The FIG. 5 shows a block diagram of a typical use of the antenna tuning apparatus (3) for simultaneously tuning 4 antennas (11) (12) (13) (14), the 4 antennas operating in a given frequency band, the 4 antennas forming an antenna array (1). In FIG. 5, the antenna tuning apparatus (3) comprises: n=4 antenna ports (311) (321) (331) (341), each of the antenna ports being coupled to one of the antennas (11) (12) (13) (14) through a feeder (21) (22) (23) (24); m=4 user ports (312) (322) (332) (342), each of the user ports being coupled to the user (5) through an interconnection (41) (42) (43) (44); p=m (m+1)=20 adjustable impedance devices, the reactance of any one of the adjustable impedance devices being adjustable by electrical means.

(15) In FIG. 5, the user (5) is a radio receiver or a radio transmitter or a radio transceiver which uses a plurality of antennas simultaneously, in the given frequency band.

(16) The p partial derivatives being linearly independent in E, the specialist understands that a small variation in the impedance matrix of the antenna array, caused by a change in operating frequency or a change in the medium surrounding the antennas, can be compensated with a new adjustment of the adjustable impedance devices, for instance to obtain that the impedance matrix presented by the user ports is a wanted real diagonal matrix. Thus, it is always possible to obtain the best possible performance.

Second Embodiment

(17) A second embodiment of an apparatus of the invention, for simultaneously tuning n antennas between which a non-negligible interaction exists, given by way of non-limiting example, is an antenna tuning apparatus for a multiport antenna array, characterized in that: n adjustable impedance devices are each coupled in parallel with one of the antenna ports and are each adjustable by electrical means; n(n1)/2 adjustable impedance devices each have a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled; m adjustable impedance devices are each coupled in parallel with one of the user ports and are each adjustable by electrical means; m (m1)/2 adjustable impedance devices each have a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled.

(18) In practice, losses are undesirable for signals applied to the antenna ports or the user ports, in a frequency band at which the antenna tuning apparatus is intended to operate. Thus, the antenna tuning apparatus is ideally lossless for signals applied to its antenna ports or user ports, in this frequency band.

(19) The specialist understands that, at a frequency at which the antenna tuning apparatus is intended to operate, if the impedance matrix seen by the antenna ports is a diagonal matrix having all its diagonal entries equal to 50, the reactance of any one of the adjustable impedance devices has an influence on the impedance matrix presented by the user ports, and the reactance of one or more of the adjustable impedance devices has an influence on one or more of the non-diagonal entries of the impedance matrix presented by the user ports.

Third Embodiment (Best Mode)

(20) As a third embodiment of an apparatus of the invention, given by way of non-limiting example and best mode of carrying out the invention, we have represented in FIG. 6 an antenna tuning apparatus for a multiport antenna array, comprising: n=4 antenna ports (311) (321) (331) (341), each of the antenna ports being single-ended; m=4 user ports (312) (322) (332) (342), each of the user ports being single-ended; n adjustable impedance devices (301) each presenting a negative reactance and each being coupled in parallel with one of the antenna ports; n(n1)/2 adjustable impedance devices (302) each presenting a negative reactance and each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled; n=m windings (303) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the user ports; m adjustable impedance devices (304) each presenting a negative reactance and each being coupled in parallel with one of the user ports; m (m1)/2 adjustable impedance devices (305) each presenting a negative reactance and each having a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled.

(21) All adjustable impedance devices (301) (302) (304) (305) are adjustable by electrical means, but the circuits and the control links needed to determine the reactance of each of the adjustable impedance devices are not shown in FIG. 6.

(22) We note that this third embodiment is a special case of the second embodiment, in which n=m, so that it uses p=m (m+1) adjustable impedance devices. We also note that none of the user ports is directly coupled to any one of the antenna ports.

(23) The specialist understands that the antenna tuning apparatus is reciprocal with respect to the antenna ports or user ports. Thus, the antenna tuning apparatus can be used for radio reception (in which case the user behaves as a radio receiver) and for radio emission (in which case the user behaves as a radio transmitter).

(24) The specialist understands that, in practice, because of the properties of antennas, it is possible that Z.sub.A is the matrix of a passive and reciprocal n-port, that is to say a symmetric matrix having a positive semidefinite real part. In this case, since the antenna tuning apparatus shown in FIG. 6 obviously behaves, at any frequency, with respect to its antenna and user ports, substantially as a passive reciprocal linear device, Z.sub.U is the matrix of a passive reciprocal n-port, so that Z.sub.U is a symmetric matrix having a positive semidefinite real part.

(25) The apparatus of this third embodiment may be used as shown in FIG. 5, the array of antennas being for instance made of 4 identical and parallel (hence of the same polarization) antennas, these antennas being close to each other and intended to operate in the frequency band 1850 MHz to 1910 MHz. At the center frequency of 1880 MHz, the impedance matrix Z.sub.A is approximately given by:

(26) $\begin{array}{cc}{Z}_A\left(\begin{array}{cccc}84.4+10.1j& -18.7-32.5j& -17.9+13.5j& -18.7-32.5j\\ -18.7-32.5j& 84.4+10.1j& -18.7-32.5j& -17.9+13.5j\\ -17.9+13.5j& -18.7-32.5j& 84.4+10.1j& -18.7-32.5j\\ -18.7-32.5j& -17.9+13.5j& -18.7-32.5j& 84.4+10.1j\end{array}\right)& \left(3\right)\end{array}$

(27) Here, Z.sub.A given by the equation (3) is the matrix of a passive and reciprocal n-port, that is a symmetric matrix having a positive semidefinite real part. Thus, Z.sub.U is the matrix of a passive and reciprocal n-port, that is a symmetric matrix having a positive semidefinite real part.

(28) The specialist knows how to determine the capacitance of each of the adjustable impedance devices (301) (302) coupled to one of the antenna ports, the inductance of each of the windings (303), the mutual inductance between the windings (303), and the capacitance of each of the adjustable impedance devices (304) (305) coupled to one of the user ports, to obtain a wanted impedance matrix Z.sub.U, at the center frequency. For instance, if C.sub.A is used to denote the capacitance matrix of the adjustable impedance devices (301) (302) coupled to one of the antenna ports, if L is used to denote the inductance matrix of the windings (303) and if C.sub.U is used to denote the capacitance matrix of the adjustable impedance devices (304) (305) coupled to one of the user ports, we find that the approximate values

(29) $\begin{array}{cc}{C}_A\left(\begin{array}{cccc}10.20& -2.10& -1.20& -2.10\\ -2.10& 10.20& -2.10& -1.20\\ -1.20& -2.10& 10.20& -2.10\\ -2.10& -1.20& -2.10& 10.20\end{array}\right)pF& \left(4\right)\\ L\left(\begin{array}{cccc}1.238& 0.282& 0.180& 0.282\\ 0.282& 1.238& 0.282& 0.180\\ 0.180& 0.282& 1.238& 0.282\\ 0.282& 0.180& 0.282& 1.238\end{array}\right)\mathrm{nH}\mathrm{and}& \left(5\right)\\ C_U\left(\begin{array}{cccc}16.23& -4.03& -0.07& -4.03\\ -4.03& 16.23& -4.03& -0.07\\ -0.07& -4.03& 16.23& -4.03\\ -4.03& -0.07& -4.03& 16.23\end{array}\right)pF& \left(6\right)\end{array}$
are suitable to obtain

(30) $\begin{array}{cc}{Z}_U\left(\begin{array}{cccc}50.0& 0.0& 0.0& 0.0\\ 0.0& 50.0& 0.0& 0.0\\ 0.0& 0.0& 50.0& 0.0\\ 0.0& 0.0& 0.0& 50.0\end{array}\right)& \left(7\right)\end{array}$
given by the general formula
Z.sub.U=[[[Z.sub.A.sup.1+jC.sub.A].sup.1+jL].sup.1+jC.sub.U].sup.1(8)

(31) For these values, it is possible to show that the p=20 partial derivatives defined above by the equation (2) are linearly independent in the real vector space of dimension 32 of the complex matrices of size 44, denoted by E. Thus, the span of the p partial derivatives in E is a subspace of dimension 20 equal to the set of the symmetric complex matrices of size 44. Consequently, any diagonal complex matrix of size 44 has the same diagonal entries as at least one element of the span of the p partial derivatives.

(32) The specialist understands that any small variation in the impedance matrix of the n antennas can be compensated with a new adjustment of the adjustable impedance devices. The proof of this statement, for any value of n=m2, is as follows. Here E is a real vector space of dimension 2n.sup.2. We shall use S to denote the set of the symmetric complex matrices of size nn over the field of the real numbers. S is a subspace of E and S is of dimension q=n(n+1). We shall use B to denote a basis of S. We assume that Z.sub.A is the matrix of a reciprocal n-port, so that Z.sub.A lies in S, and we shall use Z.sub.A1, . . . , Z.sub.Aq to denote the coordinates of Z.sub.A with respect to the basis B. We assume that the antenna tuning apparatus behaves, with respect to its antenna and user ports, substantially as a reciprocal linear device, so that Z.sub.U lies in S, and we shall use Z.sub.U1, . . . , Z.sub.Uq to denote the coordinates of Z.sub.U with respect to the basis B. Since Z.sub.U is a function of the complex matrix Z.sub.A and of the p real variables X.sub.1, . . . , X.sub.p, we define q functions g.sub.1, . . . , g.sub.q such that, for any integer i greater than or equal to 1 and less than or equal to q,
g.sub.i(Z.sub.A1, . . . ,Z.sub.Aq,X.sub.1, . . . ,X.sub.p)=Z.sub.Ui(9)

(33) Since Z.sub.U lies in S for any values of X.sub.1, . . . , X.sub.p, the partial derivatives defined above by the equation (2), which are partial derivatives of the function defined above, lie in S. Let us use J.sub.X to denote the matrix of size qp whose entry of the row i and column j is given by

(34) $\begin{array}{cc}{J}_{\mathrm{Xij}}=\frac{g_i}{X_j}& \left(10\right)\end{array}$

(35) We see that J.sub.X is a Jacobian matrix. If we now assume that the p partial derivatives defined by the equation (2) are linearly independent in E and that p=q, we find that J.sub.Y is a square and invertible matrix of size pp. An arbitrary small variation in the impedance matrix of the antenna array corresponds to small variations dZ.sub.A1, . . . , dZ.sub.Ap in the variables Z.sub.A1, . . . , Z.sub.Ap, respectively. An arbitrary small variation in the reactances of the adjustable impedance devices corresponds to small variations dX.sub.1, . . . , dX.sub.p, in the variables X.sub.1, . . . , X.sub.p, respectively. The arbitrary small variation in the impedance matrix of the antenna array and the arbitrary small variation in the reactances of the adjustable impedance devices produce small variations dZ.sub.U1, . . . , dZ.sub.Up in the variables Z.sub.U1, . . . , Z.sub.Up, respectively. We have

(36) $\begin{array}{cc}{\mathrm{dZ}}_{\mathrm{Ui}}=\underset{j=1}{\overset{p}{.Math.}}\frac{g_i}{Z_{\mathrm{Aj}}}{\mathrm{dZ}}_{\mathrm{Aj}}+\underset{j=1}{\overset{p}{.Math.}}\frac{g_i}{X_j}{\mathrm{dX}}_j& \left(11\right)\end{array}$
which may be cast in the form

(37) $\begin{array}{cc}\left(\begin{array}{c}{\mathrm{dZ}}_{U1}\\ \mathrm{.Math.}\\ {\mathrm{dZ}}_{\mathrm{Up}}\end{array}\right)=J_A\left(\begin{array}{c}{\mathrm{dZ}}_{A1}\\ \mathrm{.Math.}\\ {\mathrm{dZ}}_{\mathrm{Ap}}\end{array}\right)+J_X\left(\begin{array}{c}{\mathrm{dX}}_1\\ \mathrm{.Math.}\\ {\mathrm{dX}}_p\end{array}\right)& \left(12\right)\end{array}$
where we use J.sub.A to denote the matrix of size pp whose entry of the row i and column j is given by

(38) $\begin{array}{cc}{J}_{\mathrm{Aij}}=\frac{g_i}{Z_{\mathrm{Aj}}}& \left(13\right)\end{array}$

(39) A compensation of the small variation in the impedance matrix of the antenna array is obtained with a new adjustment in the reactances of the adjustable impedance devices if and only if we can find dX.sub.1, . . . , dX.sub.p such that dZ.sub.U1= . . . =dZ.sub.Up=0. Since, as explained above, J.sub.X is an invertible matrix, this problem has a unique solution, which is given by

(40) $\begin{array}{cc}\left(\begin{array}{c}{\mathrm{dX}}_1\\ \mathrm{.Math.}\\ {\mathrm{dX}}_p\end{array}\right)=-J_X^{-1}{J}_A\left(\begin{array}{c}{\mathrm{dZ}}_{A1}\\ \mathrm{.Math.}\\ {\mathrm{dZ}}_{\mathrm{Ap}}\end{array}\right)& \left(14\right)\end{array}$

(41) Consequently, we have established that our assumptions lead us to the conclusion that any small variation in the impedance matrix of the antenna array can be compensated with a new adjustment of the adjustable impedance devices, if each of the adjustable impedance devices provides an adequate set of reactance values, for instance a continuous set of reactance values.

(42) Thus, the specialist understands that, for the antenna tuning apparatus shown in FIG. 6, any small variation in the impedance matrix of the antenna array, caused by a change in operating frequency or a change in the medium surrounding the antennas, can be compensated with a new adjustment of the adjustable impedance devices, for instance to obtain the real diagonal matrix given by the equation (7). Thus, it is always possible to obtain the best possible performance. In particular, if the multiport antenna array is built in a portable transceiver, for instance a user equipment (UE) of an LTE wireless network, the body of the user has an effect on Z.sub.A, and Z.sub.A depends on the position of the body of the user. This is referred to as user interaction, or hand effect or finger effect. The specialist understands that the antenna tuning apparatus may be used to compensate the user interaction.

(43) More generally, we see that an apparatus for tuning n antennas, where n is an integer greater than or equal to two, the n antennas operating in a given frequency band, the apparatus comprising n antenna ports (311) (321) (331) (341) and m user ports (312) (322) (332) (342), where m is an integer greater than or equal to two, may be characterized in that: the apparatus comprises n adjustable impedance devices (301) each coupled in parallel with one of the antenna ports; the apparatus comprises n(n1)/2 adjustable impedance devices (302) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled; the apparatus comprises m adjustable impedance devices (304) each coupled in parallel with one of the user ports; the apparatus comprises m (m1)/2 adjustable impedance devices (305) each having a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled; none of the user ports is directly coupled to any one of the antenna ports; and each of the adjustable impedance devices (301) (302) (304) (305) has a reactance at a frequency in said given frequency band, the reactance of any one of the adjustable impedance devices being adjustable by electrical means.

(44) The specialist understands that this apparatus for tuning n antennas is such that, at a frequency at which the antenna tuning apparatus is intended to operate, if the impedance matrix seen by the antenna ports is a diagonal matrix having all its diagonal entries equal to 50 S2, the reactance of any one of the adjustable impedance devices has an influence on the impedance matrix presented by the user ports, and the reactance of one or more of the adjustable impedance devices has an influence on one or more of the non-diagonal entries of the impedance matrix presented by the user ports.

(45) The specialist understands that, in the case n=m, such an apparatus for tuning n antennas may be proportioned such that said p=n(n+1) partial derivatives defined above by the equation (2) are linearly independent in the real vector space of the complex matrices of size nn. Consequently, any small variation in the impedance matrix of the n antennas, caused by a change in operating frequency or a change in the medium surrounding the antennas, can be compensated with a new adjustment of the adjustable impedance devices, using only p=n(n+1) adjustable impedance devices.

(46) In the special case n=m=2, it is interesting to note that such an apparatus of the invention for tuning 2 antennas needs only 6 adjustable impedance devices to compensate any small variation in the impedance matrix of the 2 antennas, whereas said connection circuit disclosed in said article of S. M. Ali and J. Warden, if it can provide this result, is such that 12 adjustable parameters are needed, as explained in the prior art section. Consequently, an apparatus of the invention for tuning 2 antennas is much more effective and less expensive than said connection circuit to obtain this wanted result, if said connection circuit can provide this wanted result.

Fourth Embodiment

(47) As a fourth embodiment of an apparatus of the invention, given by way of non-limiting example, we have represented in FIG. 7 an antenna tuning apparatus of the invention, comprising: n=4 antenna ports (311) (321) (331) (341), each of the antenna ports being single-ended; m=4 user ports (312) (322) (332) (342), each of the user ports being single-ended; n adjustable impedance devices (301) each presenting a negative reactance and each being coupled in parallel with one of the antenna ports; n(n1)/2 capacitors (306) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled; n=m windings (303) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the user ports; m adjustable impedance devices (304) each presenting a negative reactance and each being coupled in parallel with one of the user ports; m (m1)/2 adjustable impedance devices (305) each presenting a negative reactance and each having a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled.

(48) It is possible that mutual induction exists between the windings (303). In this case, the inductance matrix of the windings is not a diagonal matrix.

(49) All adjustable impedance devices (301) (304) (305) are adjustable by electrical means, but the circuits and the control links needed to determine the reactance of each of the adjustable impedance devices are not shown in FIG. 7.

(50) The specialist understands that, at a frequency at which the antenna tuning apparatus is intended to operate, if the impedance matrix seen by the antenna ports is a diagonal matrix having all its diagonal entries equal to 50, the reactance of any one of the adjustable impedance devices has an influence on the impedance matrix presented by the user ports, and the reactance of one or more of the adjustable impedance devices has an influence on one or more of the non-diagonal entries of the impedance matrix presented by the user ports.

(51) At the center frequency of 1880 MHz, for an impedance matrix Z.sub.A approximately given by the equation (3) and suitable component values leading to an impedance matrix Z.sub.U given by the equation (7), it is possible to show that the p=14 partial derivatives defined by the equation (2) are linearly independent in E. Thus, the span of the p partial derivatives in E is of dimension 14. It is also possible to show that any diagonal complex matrix of size 44 has the same diagonal entries as at least one element of the span of the p partial derivatives.

(52) The specialist understands that any small variation in the impedance matrix of the antenna array, caused by a change in operating frequency or a change in the medium surrounding the antennas, can be partially compensated with a new adjustment of the adjustable impedance devices, for instance to obtain that each diagonal entry of Z.sub.U is close to 50 and that some of the non-diagonal entries of Z.sub.U have a sufficiently small absolute value.

Fifth Embodiment

(53) As a fifth embodiment of an apparatus of the invention, given by way of non-limiting example, we have represented in FIG. 8 an antenna tuning apparatus of the invention, comprising: n=4 antenna ports (311) (321) (331) (341), each of the antenna ports being single-ended; m=4 user ports (312) (322) (332) (342), each of the user ports being single-ended; n adjustable impedance devices (301) each presenting a negative reactance and each being coupled in parallel with one of the antenna ports; n(n1)/2 capacitors (306) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled; n=m windings (303) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the user ports; m adjustable impedance devices (304) each presenting a negative reactance and each being coupled in parallel with one of the user ports; m (m1)/2 capacitors (307) each having a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled.

(54) It is possible that mutual induction exists between two or more of the windings (303), so that in this case the inductance matrix of the windings is not a diagonal matrix.

(55) All adjustable impedance devices (301) (304) are adjustable by electrical means, but the circuits and the control links needed to determine the reactance of each of the adjustable impedance devices are not shown in FIG. 8.

(56) The specialist understands that, at a frequency at which the antenna tuning apparatus is intended to operate, if the impedance matrix seen by the antenna ports is a diagonal matrix having all its diagonal entries equal to 50, the reactance of any one of the adjustable impedance devices has an influence on the impedance matrix presented by the user ports, and the reactance of one or more of the adjustable impedance devices has an influence on one or more of the non-diagonal entries of the impedance matrix presented by the user ports.

(57) At the center frequency of 1880 MHz, for an impedance matrix Z.sub.A approximately given by the equation (3) and suitable component values leading to an impedance matrix Z.sub.U given by the equation (7), it is possible to show that the p=8 partial derivatives defined by the equation (2) are linearly independent in E. Thus, the span of the p partial derivatives in E is of dimension 8. It is also possible to show that any diagonal complex matrix of size 44 has the same diagonal entries as at least one element of the span of the p partial derivatives.

(58) The specialist understands that any small variation in the impedance matrix of the antenna array, caused by a change in operating frequency or a change in the medium surrounding the antennas, can be partially compensated with a new adjustment of the adjustable impedance devices, for instance to obtain that each diagonal entry of Z.sub.U is close to 50.

(59) More generally, a specialist understands that, to obtain that any diagonal complex matrix of size mm has the same diagonal entries as at least one element of the span of the p partial derivatives, it is necessary that the dimension of the span of the p partial derivatives considered as a real vector space is greater than or equal to the dimension of the subspace of the diagonal complex matrices of size mm considered as a real vector space. Since the dimension of the span of the p partial derivatives considered as a real vector space is less than or equal to p, and since the dimension of the subspace of the diagonal complex matrices of size mm considered as a real vector space is equal to 2 m, the necessary condition implies that p is an integer greater than or equal to 2 m. This is why the requirement p is an integer greater than or equal to 2 m is an essential characteristic of the invention.

(60) More generally, the specialist understands that an apparatus for tuning n antennas, referred to as apparatus C, where n is an integer greater than or equal to two, the n antennas operating in a given frequency band, the apparatus comprising n antenna ports (311) (321) (331) (341) and n user ports (312) (322) (332) (342), may be characterized in that: the apparatus comprises n adjustable impedance devices (301) each coupled in parallel with one of the antenna ports; the apparatus comprises n adjustable impedance devices (304) each coupled in parallel with one of the user ports; the apparatus comprises one or more passive linear two-terminal circuit elements (306) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled, and/or the apparatus comprises one or more passive linear two-terminal circuit elements (307) each having a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled; the apparatus comprises n windings (303) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the user ports; and each of the adjustable impedance devices (301) (304) has a reactance at a frequency in said given frequency band, the reactance of any one of the adjustable impedance devices being adjustable by electrical means.

(61) Also, the specialist understands that an apparatus for tuning n antennas, referred to as apparatus D, where n is an integer greater than or equal to two, the n antennas operating in a given frequency band, the apparatus comprising n antenna ports (311) (321) (331) (341) and n user ports (312) (322) (332) (342), may be characterized in that: the apparatus comprises n adjustable impedance devices (301) each coupled in parallel with one of the antenna ports; the apparatus comprises n adjustable impedance devices (304) each coupled in parallel with one of the user ports; the apparatus comprises n windings (303) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the user ports; mutual induction exists between two or more of the windings (303); and each of the adjustable impedance devices (301) (304) has a reactance at a frequency in said given frequency band, the reactance of any one of the adjustable impedance devices being adjustable by electrical means.

(62) The specialist understands that the apparatus C and the apparatus D are such that, at a frequency at which the antenna tuning apparatus is intended to operate, if the impedance matrix seen by the antenna ports is a diagonal matrix any diagonal entry of which is a strictly positive real number, the reactance of any one of the adjustable impedance devices has an influence on the impedance matrix presented by the user ports, and the reactance of one or more of the adjustable impedance devices has an influence on one or more of the non-diagonal entries of the impedance matrix presented by the user ports.

(63) Additionally, as shown in the third embodiment, any one of the apparatus C or the apparatus D may be characterized in that: the apparatus comprises n(n1)/2 additional adjustable impedance devices (302) each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the antenna ports which is different from the antenna port to which the first terminal is coupled, each of the additional adjustable impedance devices (302) having a reactance at said frequency in said given frequency band, the reactance of any one of the additional adjustable impedance devices being adjustable by electrical means; and/or in that the apparatus comprises n(n1)/2 additional adjustable impedance devices (305) each having a first terminal coupled to one of the user ports and a second terminal coupled to one of the user ports which is different from the user port to which the first terminal is coupled, each of the additional adjustable impedance devices (305) having a reactance at said frequency in said given frequency band, the reactance of any one of the additional adjustable impedance devices being adjustable by electrical means.

INDICATIONS ON INDUSTRIAL APPLICATIONS

(64) The specialist understands that the antenna tuning apparatus of the invention is suitable for compensating the variations in the impedance matrix of an antenna array, using a reduced number of electrical signals to determine the reactance of each of the adjustable impedance devices. The invention is therefore particularly suitable for being used in an automatic antenna tuning system for simultaneously tuning a plurality of antennas.

(65) We note that in the FIGS. 6, 7 and 8 presented in the third, fourth and fifth embodiments, each adjustable impedance device presents a negative reactance. This is not at all a characteristic of the invention, and it is also possible to use adjustable impedance devices presenting a positive reactance. However, the specialist understands that the third, fourth and fifth embodiments use a small number of windings, so that it is possible to obtain low losses in the antenna tuning apparatus.

(66) It should also be noted that we have said several times that an adjustable impedance device has a reactance at a frequency, the reactance being adjustable by electrical means. This does not imply that the impedance of the adjustable impedance device is purely reactive, or equivalently that the resistance of the adjustable impedance device is substantially zero. This might be desirable in some cases, for instance to obtain low losses, but this is not at all a characteristic of the invention. An adjustable impedance device used in the invention may have a resistance which is not substantially zero, and this resistance may vary when the reactance of the adjustable impedance device varies.

(67) The invention may be used in receivers and transmitters for radio communication which use a plurality of antennas simultaneously, in the same frequency band, for instance receivers and transmitters for MIMO radio communication. In particular, the invention provides the best possible characteristics using very close antennas, hence presenting a strong interaction between the antennas. The invention is therefore particularly suitable for mobile receivers and transmitters, for instance those used in portable radiotelephones. The invention is also particularly suitable for high-performance receivers and transmitters using a large number of antennas, for instance those used in the fixed stations of cellular radiotelephony networks.