MIMO Antenna Using a Decoupling Network for Next Generation Mobile Application Min-Seok Han* and Jaehoon Choi* (Corresponding Author) *
Department of Electronics and Computer Engineering Hanyang University, Seoul, 133-791, Korea Tel: +82-2-2220-0376, Fax: +82-2-2293-0377 E-mail:
[email protected]
Abstract— A compact MIMO antenna using a decoupling network for next generation mobile application is proposed. The proposed MIMO antenna consists of two parallel folded monopole antennas with the length of 100 mm and spacing S = 6 mm and a decoupling network. In order to improve the isolation characteristic at the LTE band 13, a decoupling network was added between two antennas spaced close to each other. The decoupling network is simple and compact, which contains two transmission lines, a shunt reactive component and quarter-wavelength jointed shorting structure. The proposed MIMO antenna has the isolation of approximately 15 dB at the LTE band 13 and the ECC value less than 0.2 over the whole LTE band.
I.
close to each other. The required parameters of the decoupling network are derived based on the measured and simulated coupling coefficients between antennas. Details of the design considerations and experimental results of the MIMO antenna using a decoupling network are presented and discussed.
II.
The geometry of the proposed MIMO antenna using a decoupling network is shown in Figure 1. The proposed MIMO antenna consists of two parallel folded monopole antennas with the length of 100 mm (0.25 λ0 at 750 MHz) and
INTRODUCTION
spacing S = 6 mm (0.015 λ0 at 750 MHz) and a decoupling network. The overall size of the proposed MIMO antenna is 48 mm × 12 mm × 6 mm. Two same elements are placed at the two corners of top edge of a FR4 (r = 4.4) substrate having the volume of 48 mm× 108 mm× 0.8 mm, which simulates the ground plane of a practical bar type mobile handset. In order to improve the isolation characteristic at the LTE band 13, a decoupling network is added between the two antennas. The coupling coefficient S21 at 770 MHz has the amplitude α = 5.87 and phase φ = 1060 . The required parameters of the structure can be optimized based on the measured and simulated coupling coefficients between the two antennas. The decoupling network composed of two transmission lines with characteristic impedance Z0 and electrical length θ , a shunt reactive component with admittance jB and a quarter-wavelength jointed shorting structure at 0.77 GHz. In designing decoupling network, two transmission lines (TLs) are individually connected to input ports of two strongly coupled antennas. The length of the TLs is chosen so that the trans-admittance between ports changes from complex one at antenna inputs to a pure imaginary one. A shunt reactive component is then attached in between the TL ends to cancel the resultant imaginary trans-admittance. Finally, quarter-wavelength jointed shorting structure at 0.77 GHz is added to each port for input impedance matching.
Even increasing demand for high quality and high data rate mobile communications calls for the development of multi-antenna system for handsets such as diversity and multiple-input multiple-output (MIMO) handset applications [1, 2]. To realize an effective MIMO system, it is necessary to have a sufficient number of uncorrelated antennas at each end of the link. However, it is usually a big challenge to place multiple antennas within a small and slim mobile handset while maintaining the good isolation between antenna elements since the antennas are strongly coupled with each other and even with the ground plane by sharing the surface currents distributed on it. To enhance the isolation between ports closely located within restricted space in mobile handsets, various methods had been developed by employing protruded T-shaped ground plane [3], quarter wavelength slot on the ground plane [4], and notches on the ground plane as resonators [5]. Although these methods reduce the mutual coupling effectively, it is still quite difficult for antennas to obtain good isolation at long term evolution (LTE) bands in that two antennas are very closely located due to the limited antenna space in a mobile handset. In this paper, we propose the method to improve the isolation performance of two-antenna systems for LTE band 13. The proposed MIMO antenna consists of two parallel folded monopole antennas and a decoupling network. In order to improve the isolation characteristic at the LTE band 13, a decoupling network is added between two antennas spaced
978-1-4244-4522-6/09/$25.00 ©2009 IEEE
MIMO ANTENNA DESIGN
568
ISCIT 2009
(d) Fig. 1. Geometry of the proposed MIMO antenna (a) 3D view, (b) basic concept of decoupling network, (c) structure of radiating element, (d) fabricated MIMO antenna
(a)
III. RESULTS AND DISCUSSIONS The simulated S-parameter characteristics with and without a decoupling network are given in Figure 2. When a decoupling network is added, the resonant frequency is slightly shifted toward the high frequency region at LTE band and the isolation characteristic at LTE band is increased upto 15 dB as shown in Figure 2.
(b)
(a)
(c)
(b) Fig. 2. Simulated S-parameter characteristics with and without decoupling network (a) simulated return loss characteristics, (b) simulated isolation characteristics
569
Figure 3 shows the measured S-parameter characteristics and Envelope Correlation Coefficient (ECC) characteristics with decoupling network. From the measured results, the 6-dB return loss impedance bandwidth is 4.15% (from 755 MHz to 787 MHz) for LTE band 13 and the isolation characteristic at LTE band is increased upto 15 dB. Additional study is required to widen the bandwidth at the LTE band. Simulation was carried out with the aid of the commercially available simulation software MWS [6] to optimize the geometric parameters of the proposed antenna.
ρ12 =
path environment, it can be alternatively calculated from its scattering parameters. The ECC of two antennas is given by
(
2
1 − S11 − S 21
2
)(
2
1 − S 22 − S12
2
)
(1)
Figure 3 (b) presents the ECC values computed from scattering parameters with a decoupling network. It can be seen that the ECC of two antennas is always lower than 0.2 over the whole LTE band. Therefore, good performance in terms of diversity is expected. Figure 4 shows the measured radiation patterns of the fabricated MIMO antenna at 770 MHz. Although MIMO antenna elements usually have different directivity for each element, the radiation patterns of the designed antennas resemble to each other. From the H (xz)-plane patterns, it is confirmed that the antenna has patterns close to omnidirectional in LTE band 13.
(a)
2
S11∗ S12 + S12∗ S22
(a)
(b)
Fig. 3. Measured S-parameter characteristics and ECC characteristics with decoupling network (a) measured S-parameter characteristics (b) measured ECC characteristics
For diversity and MIMO applications, the correlation between signals received by the involved antennas at the same side of a wireless link is an important figure of merit for the whole system. Usually, the envelope correlation coefficient is used to evaluate the diversity capability of a multi-antenna system. This parameter should be preferably computed from 3D radiation patterns but this method is actually laborious. Assuming that the antennas will operate in an uniform multi-
(b) Fig. 4. Measured radiation patterns of the fabricated MIMO antennas (a) antenna #1, (b) antenna #2
570
The measured antenna gains and efficiencies are shown in Table. 1. The measured peak gains of two antenna elements are -0.12 dBi and -0.32 dBi at 770 MHz. The measured antenna efficiencies of two antenna elements are 37.2 % and 31.4% at 770 MHz. Table 1: Measured antenna gains and efficiencies
Measured Antenna Gains (dBi) Antenna #1 Antenna #2 Measured Antenna Efficiencies (%) Antenna #1 Antenna #2
f=0.77 GHz -0.12 -0.32 f=0.77 GHz 37.2 31.4
IV. CONCLUSIONS In this paper, a decoupling network for improving the isolation between two closely spaced antennas was proposed. The decoupling network is simple and compact and consists of two transmission lines, a shunt reactive component and quarter-wavelength jointed shorting structure. The isolations between antennas are greatly improved from 6 dB to more than 15 dB at the LTE band 13 while the input return losses remain better than 6 dB. The measured peak gains of two antenna elements are -0.12 dBi and -0.32 dBi at 770 MHz. The suggested isolation technique for two adjacent antenna ports can be used in various areas such as MIMO antennas, two closely positioned antennas for different systems. ACKNOWLEDGMENT This research was supported by the MKE (Ministry of Knowledge Economy), Korea, under the ITRC(Information Technology Research Center) support program supervised by the IITA(Institute of Information Technology Assessment) (IITA-2008-C1090-0801-0019) REFERENCES [1] R. G. Vaughan and J. B. Anderson, “Antenna diversity in mobile communications,” IEEE Trans. Veh. Technol., vol. VT36, no. 4, pp. 147-172, Nov. 1987. [2] D. Gesbert, M. Shafi, D. S. Shiu, P. Smith, and A. Naguib, “From theory to practice: An overview of MIMO space-time coded wireless systems,” IEEE J. Sel. Areas Commun., vol. 21, no.3, pp.281-302, April. 2003. [3] T.-Y. Wu, S.-T. Fang, and K.-L. Wong, “Printed diversity monopole antenna for WLAN operation,” Electron. Lett., vol. 38, no. 25, pp.1625–1626, Dec. 2002. [4] Y. Ge, K. P. Esselle, and T. S. Bird, “Compact diversity antenna for wireless devices,” Electron. Lett., vol. 41, no. 2, pp. 52–53, Jan. 2005. [5] K.-J. Kim and K.-H. Ahn, “The high isolation dual-band inverted F antenna diversity system with the small N-section resonators on the ground plane,” Microw. Opt. Technol. Lett., vol. 49, no. 3, pp. 731–734, Mar. 2007. [6] Computer Simulation Technology (CST) Microwave Studio. Suite 2008 [Online]. Available: http://www.cst.com
571
