A Dual-Polarized Parasitic Patch Antenna for MIMO Systems Daniele Pinchera#1, Fulvio Schettino#2 #
DAEIMI, Faculty of Engineering, University of Cassino via G.Di Biasio 43, 03043 CASSINO (FR) ITALY 1 2
[email protected] [email protected]
Abstract— A preliminary analysis of a reconfigurable parasitic patch antenna for MIMO applications is presented. The proposed antenna is able to work on two different polarizations, providing a separate control of the antenna pattern of each of the two polarization. A number of numerical results, showing the adaptive capabilities of the antenna, is given.
I. INTRODUCTION MIMO systems can provide a great performance improvement with respect to SISO wireless communication systems [1]; this improvement is mainly due to the capability to spatially multiplex the information over multiple spacetime channels. Recently, a number of antenna architectures, exploiting reconfigurability features, especially designed for MIMO approach has been presented [2][3]. Such antennas exhibit better performances due to their capability to adapt the radiating characteristics to the particular realization of the electromagnetic environment in which they have to work. In particular, the architecture presented in [3] uses an high number of MEMS switches in order to change the radiating characteristics, f.i. the polarization, of each element of the array; the solution presented in [2], instead, is based on a number of parasitic elements surrounding the active ones, and connected to electronically controllable impedances. The latter solution can provide an interesting performance improvement, but is not as easily embeddable as the former in small size communication systems (like laptops) due to its size. Moreover it is not suitable for exploiting dualpolarization capability. As a matter of fact, it has been shown [4] that polarization diversity in conjunction with spatial multiplexing allows a better symbol error rate. In this work a 2-port low-profile adaptive parasitic antenna based on the same idea as in [2], but in planar technology is investigated. In addition, the presented antenna provides two radiative modes with orthogonal polarization, the two ports exhibiting a very low cross talking. In section II some details on the proposed architecture will be given, whereas in section III a number of preliminary numerical results will be presented. All the numerical results have been obtained by means of the CST Microwave Studio full-wave electromagnetic simulation software. II. THE ANTENNA ARCHITECTURE In the following the layout of the parasitic patch antenna is given; the substrate thickness is 1.524 mm, and the material
used for the simulation was FR4 lossy; the top layer is realized by means of a PEC thin surface 35µm high, whereas for the ground plane a PEC boundary condition has been used. The circuit ports are realized by means of discrete ports, thus simulating the feeding of the patch from a coaxial line. As stated in the introduction, the antenna we are interested in should radiate with a different polarization with each of its two ports. This can be done by means of a properly fed cross patch antenna [5]. This solution has the advantage of being more compact, and it is thus the optimal candidate for becoming the core our parasitic antenna. It is interesting to underline that a proper choice of the positions of the feeding points on the cross patch allows to excite currents in orthogonal directions with the two ports, thus having a very low cross-talk among the ports. If we now surround the cross patch with four parasitic square patch antennas, resonating at the same operation frequency of the cross patch, we obtain the architecture depicted in fig. 1. The proper choice of the distance between the cross and the square patches allows us to maximize the coupling between the currents on the cross and the currents on the squares.
D A
C B
Figure 1: geometry of the dual-polarized parasitic patch antenna. The feeding ports are shown in red.
All the dimensions of the antenna depicted in fig. 1 are summarized in Table I. In figure 2 the behaviour of the currents on the patches is shown when the antenna is fed from port 1 (and having a 50Ω load on port 2); it is interesting to observe that the direction on the current on the square patches
is the same as the current on the cross patch. Obviously, the direction of the currents is rotated of 90° when the antenna is fed from port 2. TABLE I GEOMETRY DIMENSIONS
Cross arm length Cross arm width Resonant patch side Patch-cross distance Off-center feeding ports distance
28.8 mm 8.39 mm 27.2 mm 1.5 mm 4.7 mm
architecture proposed in [2], since in that previous work the switching of one of the parasitic elements would always influence the signal received on both ports. It has to be noted that, by means of more sophisticated switching circuits it could be possible to have a deeper control of the currents on the square patches, thus giving the antenna a wider range of states in which it could work and improving its adaptivity capabilities. III. NUMERICAL RESULTS In this Section some numerical results are shown. All the simulations have been performed by means of CST Microwave Studio full-wave simulation software, with a mesh of 20 lines per wavelength. In figure 3 the behaviour of S-parameters when all the switches are in the ON state, and when patches A and/or B are switched of, is plotted. As can be seen in fig. 3b the coupling between the ports is very low in a wide frequencies range in every working condition. As regard to S11 parameter, there is a difference depending on wether all the switches are on, or at least one patch is switched off. Such a behaviour could be avoided by means of a suitable design of the switch.
Figure 2. Current behaviour when antenna is fed by port 1.
In order to make the antenna “reconfigurable” we need a technique to control the current on the patch surfaces. Such a goal can be achieved by changing the boundary conditions of the resonant patches [5], thus putting the square patches in or out of resonance. In such a way the currents on the patch can be switched on and off. The variation of the boundary conditions can be achieved by a proper microwave switch creating a short circuit condition between the edge of the patch and the ground plane when we want to turn off the currents on the patch; the same circuit should also provide an high impedance between the same two points in the other working condition, in order to allow the presence of a resonating current on the square patch itself. The realization of the switching circuit, that can be engineered by means of classical microwave circuits techniques [6], will not be discussed in this work, since our work is focused on the antenna; consequently, in all the simulations performed, the short circuit needed for the OFF state has been modelled by means of simple low impedance lumped elements, and the ON state has been modelled by means of simple high impedance lumped elements.. Since the square patches exhibit two degenerate and orthogonal resonant modes [5], two switches can be suitably placed on the centre of adjacent borders of the patches in order to switch on and off the currents related to the resonant modes independently. This can be very useful for the control of the antenna since we can independently modify the overall radiating behaviour of the patch array for the two possible polarizations. This is an advantage with respect to the
Figure 3. S parameters in different working conditions. a) S11, b) S21
In Figure 4 an example is shown of the effect of switching a patch off: patch A has been switched off, and its current is negligibly small with respect to patch B current. In Figure 5 the antenna patterns obtained in four working conditions are plotted. As can be seen, the pattern beam inclination can be rotated by switching patches off, so that the effective reconfigurability of the proposed antenna is demonstrated. Finally, all the working conditions and the related beam inclinations are summarized in Table I. It has to be underlined
that no optimizations have been done on the impedance matching; this is evident from the very low value of the reflection coefficient when all the patches are in the ON state with respect to the other cases, so it is possible to have a better matching of the antenna in the working conditions.
BC
6.93
140
15
- 5.93
BD
7.54
207
7
- 4.9
ACD
6.46
343
29
- 8.46
ABC
6.27
162
37
- 8.27
IV. CONCLUSION In the present work a preliminary analysis of an adaptive parasitic patch antenna for MIMO systems has been presented. The presented feasibility study shows that it is possible to achieve in planar technology the reconfigurability features of the antenna analysed in [2], thus making it possible its integration into small terminals, like laptops and PDAs. The antenna has the capability to modify its radiation pattern independently for each polarization. Furthermore, by means of more sophisticated switching circuits it could be possible to modify the behaviour of the current on the patch, thus having more degrees of freedom with respect to the simply switching a square patch on and off. The development and testing of a prototype of the antenna, as well as the switching circuit are already in course.
ABD
6.39
200
36
- 8.3
BDC
6.33
17
29
- 8.4
REFERENCES [1]
A. Paulraj and R. Nabar and D. Gore, Introduction to Space-Time Wireless Communications, Cambridge University Press, 2003
[2]
M. D. Migliore and D. Pinchera and F. Schettino, “Improving channel capacity using adaptive MIMO antennas,” IEEE Trans. Antennas Propag., vol. 54, pp.3481-3489, November 2006.
[3]
B. A. Cetiner, E. Akay, E. Sengul, and E. Ayanoglu, “A MIMO system with multifunctional reconfigurable antennas,” IEEE Antennas and Wireless Communication Letters, vol.5, pp.463-466, 2006
[4]
R. U. Nabar, H. Bölcskei,V. Erceg, D. Gesbert, and A. J. Paulraj, “Performance of Multiantenna Signaling Techniques in the Presence of Polarization Diversity”, IEEE Transactions on Signal Processing, vol. 50, no. 10, october 2002 pp.2553-2562
[5]
C. A. Balanis, Antenna Theory – Analysis and Design, Wiley 1982
[6]
I. Bahl, P. Barthia, Microwave Solid State Circuit Design, Wiley 1988
Figure 4. Example of the capability of switching off the currents on the patch. Patch A has been switched OFF.
TABLE II ANTENNA PARAMETERS FOR DIFFERENT WORKING CONDITIONS
CC(1)
Gain dB
Phimax
Thetamax
S11 dB
/
7.29
178
3
- 26.24
A
7.68
201
26
- 5.88
B
7.55
161
21
- 6.16
C
7.68
44
8
- 4.58
D
7.74
320
9
- 4.6
AC
7.51
165
9
- 5.09
AD
7.07
220
19
- 6.16
CD
7.14
0
35
- 6.56
AB
7.05
181
44
- 5.74
Figure 5. Example of the reconfigurability features of the presented antenna. The switched off patches are (clockwise from the upper left figure) AB, BC, AD, CD.
