Compact Metamaterial High Isolation MIMO Antenna Subsystem Cheng-Jung Lee, Maha Achour, and Ajay Gummalla Rayspan Corporation, San Diego, CA, USA Cheng@rayspan.com Introduction The use of multiple antennas for diversity, including Multiple Input Multiple Output (MIMO), is one of the most promising wireless technologies for broadband communication applications [1]. The benefits range from improved data throughput, wider network coverage, higher channel capacity and link reliability. However, all wireless devices are becoming smaller as manufacturing technologies for integrated circuits improve over the years. This presents a significant challenge for incorporating multiple antennas in compact consumer devices such as cell phones, wireless adapters and PDAs. In addition to creating a novel antenna that can fit into a small area; a bigger issue emerges that deals with minimizing the interaction between the antennas when they are placed in close proximity. In many occasions, the interaction between the antennas is so high that it negates all potential benefits of using multiple antennas. The minimum coupling between two closely coupled antennas can be achieved by placing antenna elements half-wavelength away from each other. However, this is not practical in commercial products because of the limited space. One of the approaches to improve the isolation for the closely coupled antenna is to integrate microwave directional coupler and antennas into a subsystem to minimize the correlation between antenna elements [2]. However, the size of conventional microwave directional couplers prevents it from the practical usage. In addition, the printed circuit board (PCB) fabrication process makes the conventional microwave directional coupler extremely difficult to achieve more than -8dB coupling. This restriction limits the spacing of the antenna array used in the multi-system to at most one sixth of the wavelength. Metamaterial technology has the advantage of reducing the circuit size while providing equivalent or better performance in both antenna and passive circuit application [3]-[4]. The dispersion engineering used in MTM technology can easily control the propagation constant and the characteristic impedance of the transmission line [3]- [4]. Therefore, the circuit physical size is independent of the operational frequency and can be significantly reduced to fit in a small area. This paper will present a novel MIMO antenna subsystem which incorporates two metamaterial antennas with less than.1λ spacing and a metamaterial directional coupler in a small foot print area while the isolation between two antennas is -28.8dB. In addition, the far field envelope correlation between two antennas is less than.1. High Isolation MIMO Antenna Subsystem Design Concept The high coupling in a multi-antenna system is mainly due to the close spacing between each element. The lack of isolation between antennas will force the radiated signal from one antenna to be terminated at other antennas, thus reducing the radiation efficiency and eliminating the benefit of using MIMO technology. Fig. 1 illustrates an approach of solving the isolation issue for two closely coupled antennas by employing a microwave backward wave coupler. The signal can be transmitted from port1 to port3 through two routes. The first one is path1 which is the backward coupling of the coupler. Signals can also go through path2, path3 and path4 which correspond to coupler transmission (port1 to port2), antenna 978-1-4244-2642-3/8/$25. 28 IEEE
coupling through the air (port1 to port2 ) and another coupler transmission (port4 to port3), respectively. The magnitude and phase of each path can be represented as M 1 θ1, M 2 θ 2, M 3 θ 3 and M 4 θ 4. The signals from two routes will cancel to each other when M1 M 2 M 3 M 4 and θ 2 + θ3 + θ 4 θ1 (2n + 1) π where n is an integer. The above two requirements can be simplified to M1 M 3 and 2 θ 2 + θ3 θ1 (2n + 1) π for a lossless and symmetric coupler where M 2 = M 4 1 and θ 2 = θ 4. Accordingly, any coupling between the two antennas can be significantly reduced when the coupler is carefully designed to satisfy above two conditions. Metamaterial MIMO Antenna Subsystem Design Fig. 2 depicts the structure of a two metamaterial MIMO antenna array. This antenna array is printed on an FR4 substrate with dielectric constant of 4.4 and thickness of 1mm. The substrate size is 25.2mm wide and 5.8mm long. Each antenna element comprises a top path, launch pad, via, via pad and via line. The antenna is excited by coupling an L-shape launch pad to the top patch with a gap. The metallic via connects the top patch on one side of the substrate to the via pad on the other side of the substrate. The via pad is connected to the ground through an L-shape via line. The dimensions of each part are optimized to match this antenna to 5 coplanar waveguide (CPW) at the input port. Fig. 3 shows the measured results of the metamaterial MIMO array which matches well with the simulated results. The bandwidth of S 11 <-1dB is from 2.39GHz to 2.87GHz and the maximum coupling between two antennas is -6.dB. As mentioned before, the strong coupling will degrade the antenna radiation efficiency for each antenna, thus reducing the system performance. Therefore, the directional coupler will be integrated with the proposed MIMO antenna array to restore the orthogonality of the receiving and transmitting signals. In order to maintain the compact size of the whole antenna subsystem, the metamaterial directional coupler is inserted between the two antennas. Fig. 4 shows the structure of the proposed MIMO antenna subsystem which includes the metamaterial directional coupler. This backward-wave coupler is built by paralleling two composite right left-handed transmission lines (CRLH-TLs). Each CRLH-TL comprises one unit cell where the righthanded portion is implemented by two sections of 5 microstrip coupled lines and the lefthanded portion is realized by using the chip capacitors (2C L ) and shorted CPW stubs (L L ). An additional capacitor (C m ) is mounted between the two CRLH-TLs to enhance the coupling between the two CRLH lines. The microstrip coupled line length, the capacitor values and the shorted CPW stubs dimensions are optimized to satisfy the magnitude and phase requirement mentioned in the previous section. Fig. 5 plots the simulated and measured results of the proposed MIMO antenna subsystem. The isolation is better than -1dB from 2.52GHz to 2.68GHz and the minimum coupling of - 28.8dB occurs at 2.59GHz. This demonstrates more than 2dB isolation improvement compared to the one without the metamaterial directional coupler. Fig. 6 compares the measured radiation efficiencies between the MIMO antenna array without the coupler and with the coupler which indicates a maximum of 17% increase at 2.59GHz and average of 8% improvement from 2.52GHz to 2.68GHz. The measured radiation patterns are drawn in Fig. 7 where maximum gains for two antenna elements are 2.2dBi and 1.74dBi, respectively. One can find from Fig. 7 (c) and (d) that radiation patterns corresponding to two different ports point at opposite direction. This pattern diversity can also benefit the proposed subsystem using MIMO technology. Besides antenna isolation and radiation efficiency, far-field
envelope correlation is another important factor in MIMO application [5]. The far-field envelope correlation of the proposed MIMO antenna subsystem is.4 at 2.59GHz which is calculated by inputting the measured far-fields into the equation listed in [5]. Conclusion The compact MIMO antenna subsystem is proposed in this paper by integrating the metamaterial antenna array and the metamaterial directional coupler. The coupling between the two closely coupled antenna elements can be significantly reduced, thus boosting the radiation efficiency. The low far-field envelope correlation will help providing independent channel in the MIMO application. These results demonstrate a great potential of using the proposed MIMO antenna subsystem to increase the system performance. References [1] IEEE 82.11n, Joint Proposal: High throughput extension to the 82.11 Standard: PHY, doc.: IEEE 82.11/112r4. [2] K. L. Lau, K. M. Luk and D. Lin, A wide-band dual-polarization patch antenna with directional coupler, IEEE Antennas and Wireless Propagation Letters, vol. 1, pp.186-189, 22. [3] C. Caloz and T. Itoh, "Electromagnetic metamaterials: Transmission line theory and microwave applications, the engineering approach," John wiley & Sons, New York, 25. [4] C. J. Lee, K. M. K. H. Leong, and T. Itoh, Composite right/left-handed transmission line based compact resonant antennas for RF module integration, IEEE Transaction on Antennas and Propagation, col. 54, issue 8, pp. 2283-2291, Aug. 26. [5] R. G. Vaughan, and J. B. Andersen, Antenna diversity in mobile communications, IEEE Transaction on Vehicular Technology, vol. VT-36, no. 4, pp. 149-172, Nov. 1987. top patch 25.2mm path3 8.6mm spacing d Ant1 Ant2 port1 port2 5 CPW feed launch pad via via pad via line port2 path1 port4 42.2mm path2 Microwave Coupler path4 port1 port3 Figure 1. MIMO antenna subsystem. (a) (b) Figure 2. Metamaterial antenna array in (a) top view (b) bottom view.
efficiency MTM cell 25.2mm return loss & coupling (db) -1 launch pad C m 2C L CPW shorted stub via via pad via line 8.6mm -2 simulated return loss simulated coupling measured return loss measured coupling -3 1.5 2 2.5 3 3.5 frequency (GHz) 5 CPW feed z x y 42.2mm Figure 3. Simulated and measured results Figure 4. Structure of metamaterial of MIMO antenna array. MIMO antenna subsystem in (a) top and (b) bottom view..6.5 (a) (b) with coupler -1.4 without coupler return loss & coupling (db) simulated return -2 loss simulated coupling measured return loss measured -3 coupling 1.5 2 2.5 3 3.5 frequency (GHz).3.2.1 P1 with coupler P2 with coupler P1 without coupler P2 without coupler 23 24 25 26 27 28 29 3 frequency (MHz) Figure 5. S-parameters of metamaterial Figure 6. Measured radiation efficiencies MIMO antenna subsystem. of metamaterial MIMO antenna subsystem. 18 9-1 -2-3 -4-3 -2-1 18 9-1 -1 θ θ φ -2-2 -3-4 -3-2 -1 18 9-3 -4-3 -2-1 18 27 27 27 (a) (b) (c) 9-1 -2-3 θ -4-3 -2-1 18 9-1 -2-3 θ -3-2 -1 18 9-1 -2-3 φ -4 - -4-3 -2-1 27 (d) (e) (f) 27 Figure 7. Measured radiation pattern for MIMO antenna subsystem shown in Fig. 4 at (a) x-z plane (port1). (b) y-z plane (port1) (c) x-y plane (port1) (d) x-z plane (port2). (b) y-z plane (port2) (c) x-y plane (port2) 27
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