2018 International Symposium on Antennas and Propagation (ISAP 2018) [FrP-48] October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea Two-Port UWB MIMO Antenna with Modified Ground for Isolation Improvement Sungpeel Kim and Jaehoon Choi* Department of Electronics and Computer Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-gu, Seoul 04763, Korea *[email protected] (corresponding author)

Abstract – A printed two-port ultra-wideband (UWB) multiple-input-multiple-output (MIMO) antenna with a compact dimension of 24 mm × 34 mm × 0.787 mm is proposed for UWB communication applications. The proposed antenna consists of two identical planar monopole antennas with a decoupling structure on the ground plane. Isolation between two radiators is improved by suppressing the surface current flow through the ground plane. The simulated impedance bandwidth (S11 < -10 dB) is 8.2 GHz from 2.44 to 10.64 GHz. The proposed antenna has a low mutual coupling of less than -15 dB over the target band (3.1 – 10.6 GHz). Therefore, the proposed antenna is a good candidate for UWB MIMO portable applications. (a) (b) Index Terms — Ultra-wideband (UWB), multiple-input- multiple-output (MIMO), compact, isolation, mutual coupling. Fig. 1. Geometry of the proposed antenna: (a) top view, (b) bottom view.

1. Introduction In this paper, a compact two-port UWB MIMO antenna is proposed. The proposed antenna is comprised of two Ultra-wideband (UWB) is utilized for communication microstrip fed identical planar monopole antennas with a systems requiring high speed transmission rate with low power decoupling structure on the ground plane for improving the consumption [1]. The Federal Communications Commission isolation. The proposed antenna has a compact size of 24 mm (FCC) authorized an unlicensed 3.1 – 10.6 GHz spectrum for × 34 mm which is about 42 % smaller than the one in [6]. The UWB systems [2]. Typically, UWB is applied to indoor short- simulated results show that the proposed compact MIMO range communications characterized by dense multipath antenna is a suitable for UWB portable applications. propagation. Multipath fading of the indoor environment can deteriorate UWB communication performance. Multiple-input- multiple-output (MIMO) technology combined with UWB has 2. Antenna Design and Its Performance been researched as a promising solution for decreasing the (1) Antenna Geometry multipath fading effect and increasing channel capacity [3]. As shown in Fig. 1(a), two identical planar monopole The MIMO antenna means multiple antennas can form on a antennas are printed on upper side of 24 mm × 34 mm × single substrate, but this could increase the mutual coupling 0.787 mm Rogers RO4003 substrate (εr = 3.55, tanδ = 0.0027). and decrease the antenna performance. Various techniques A rectangular slit with a size of 1.2 mm × 3.4 mm is etched on have been studied to enhance the isolation performance of the edge of the ground plane to enhance the impedance MIMO antennas [4-6]. In [4], mutual coupling was reduced by matching as shown in Fig. 1(b). A stair-shaped structure is using defected ground structure (DGS). However, design applied to planar monopole antennas to improve the complexity was increased due to the fractal structure of DGS. impedance matching at high frequencies. Isolation In [5], isolation enhancement was achieved by etching a simple improvement is achieved by adding a T-shaped stub on the resonant slot on the ground plane for suppressing the surface ground plane. This protruded stub operates as a decoupling current flowing through the ground plane. Because the slot structure. T-shaped structure is better than straight-shaped operates as a slot antenna, the mutual coupling power is structure for improving isolation at low frequencies. Two radiated by the slot into the free space. But this strong back- identical rectangular slits with a size of 3 mm × 1 mm are radiation through the slot deteriorates the radiation pattern. etched beside the protruded stub to enhance the isolation Another method inserting a decoupling structure such as performance at high frequencies. protruded ground stub was proposed [6]. Good isolation of more than -16 dB in the whole UWB spectrum was achieved (2) Results of Simulation by adding a decoupling structure, but large antenna size of 35 Figure 2 shows the surface current distributions at 6.85 GHz mm × 40 mm is required. (center frequency of UWB) with and without a decoupling structure. When port 1 is excited, the surface current flowing

914 2018 International Symposium on Antennas and Propagation (ISAP 2018) October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

towards port 2 through ground plane is suppressed by the addition of a decoupling structure as shown in Fig. 2(b). Figure 3 shows significant isolation improvement over the UWB spectrum with a decoupling structure. Figure 4 shows the simulated S-parameters of the proposed antenna. The simulated impedance bandwidth (S11 < -10 dB) is 8.2 GHz from 2.44 to 10.64 GHz. Over 6.6 GHz, the simulated mutual coupling (S ) is lower than -20 dB which means that the (a) (b) 21 proposed antenna has a high isolation at that frequency band. Fig. 2. Surface current distributions when port 1 is excited The simulated S21 value is lower than -15 dB from 3.1 GHz to and port 2 is matched with 50 Ω load at 6.85 GHz: (a) without 6.6 GHz, but it is still acceptable because -15 dB is the decoupling structure, (b) with decoupling structure. required threshold for isolation [4]. Figure 5 shows the simulated radiation patterns in the xy, xz, and yz planes at 3.7, 6.85, and 10 GHz when port 1 is excited and port 2 is matched with 50 Ω load. It is hard to achieve a complete omni- directional radiation pattern over the entire UWB spectrum [4]. However, the simulated radiation patterns show that the proposed antenna has quasi omni-directional property at various UWB frequencies.

3. Conclusion

Fig. 3. Simulated S21 characteristics of the proposed antenna In this paper, a compact two-port UWB MIMO antenna with and without decoupling structure. with high isolation is proposed. The simulated impedance bandwidth (S11 < -10 dB) is 8.2 GHz from 2.44 to 10.64 GHz. 0 High isolation characteristic across UWB spectrum is

-10 achieved by suppressing the surface current flow on the ground plane using a decoupling structure. The simulated -20 radiation patterns are quasi omni-directional over the whole frequency range. Therefore, the proposed antenna is a good -30 candidate for UWB MIMO portable applications. S11 S21 -40 S22

23456789101112 Acknowledgment Frequency [GHz] Fig. 4. Simulated S-parameters of the proposed antenna. This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2017R1A2B4002811).

References Gain [dBi] Gain [1] I. Oppermann, M. Hamalainen and J. Iinatti, UWB theory and applications, Wiley: New York, 2004, pp.3-4. [2] Federal Communications Commission (FCC), Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems (a) (b) First Rep. and Order, ET Docket 98-153, FCC 02-48, Adopted: Feb. 2002; Released, Apr. 2002. [3] T. Kaiser, Z. Feng and E. Dimitrov, “An overview of ultra-wideband systems with MIMO,” Proc. IEEE, vol. 97, no. 2, pp. 285–312, Feb. 2009. [4] J. Banerjee, A. Karmakar, R. Ghatak and D. R. Poddar, “Compact CPW-

Gain [dBi] fed UWB MIMO antenna with a novel modified Minkowski fractal defected ground structure (DGS) for high isolation and triple band-notch characteristic,” J. Electromagn.Waves and App, vol. 31, no. 15, pp. 1550-1565, Jul. 2017. [5] J. OuYang, F. Yang and Z. M. Wang, “Reducing mutual coupling of (c) closely spaced microstrip MIMO antennas for WLAN application, ” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 310-313, Apr. 2011. Fig. 5. Simulated radiation patterns at various frequencies [6] S. Zhang, Z. Ying, J. Xiong and S. He, “Ultrawideband MIMO/diversity when port 1 is excited and port 2 is matched with 50 Ω load: antennas with a tree-like structure to enhance wideband isolation,” IEEE (a) xy plane, (b) xz plane, (c) yz plane. Antennas Wireless Propag. Lett., vol. 8, pp. 1279-1282, Nov. 2009.

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