An FSS-Based Nonplanar Quad-Element UWB-MIMO ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017

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An FSS-Based Nonplanar Quad-Element UWB-MIMO Antenna System Muhammad Bilal, Rashid Saleem, Hammad. H. Abbasi, Muhammad Farhan Shafique, Senior Member, IEEE, and Anthony K. Brown, Senior Member, IEEE

Abstract—In this letter, a low-profile, miniaturized four-element ultrawideband (UWB) antenna for four-port multiple-input– multiple-output (MIMO) configuration is proposed. The MIMO antenna elements are organized in a cuboidal geometry around a polystyrene block. An inverted L-shaped structure provides decoupling among the antenna elements. This structure is frequencyselective-surface-based and has slotted Y-shapes etched in it. In addition to that, a square spiral parasitic structure improves input impedance matching over the desired frequency band. Antenna elements are realized on low-profile FR-4 substrate having compact dimensions of 32 × 36 × 1.5 mm3 . The proposed three-dimensional (3-D) UWB-MIMO system achieves good impedance matching and an effective isolation of 20 dB among antenna elements in most of the band. The reported configuration is suitable for nonplanar/3-D system-in-package applications where a planar arrangement of four elements is not possible due to size limitations. Index Terms—Frequency selective surfaces (FSSs), microstrip antennas, multiple-input–multiple-output (MIMO), mutual coupling, ultrawideband (UWB) antennas.

I. INTRODUCTION VER since the FCC has allocated the unlicensed frequency band of 3.1–10.6 GHz for ultrawideband (UWB) communications [1], it has attracted a lot of interest from industry and academia. However, UWB has limitations of reduced channel capacity and short range mainly because of power limitation of −41 dBm/MHz. Multiple-input–multiple-output (MIMO) systems incorporating UWB technology have potential to address this limitation, resulting in a number of possible applications [2]. In the existing literature, the placement of UWB-MIMO antennas is commonly reported in a single plane [3], [4]. However,

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Manuscript received July 25, 2016; revised September 11, 2016; accepted October 1, 2016. Date of publication October 7, 2016; date of current version April 17, 2017. M. Bilal and R. Saleem are with the Department of Telecommunication Engineering, University of Engineering and Technology, Taxila 47050, Pakistan (e-mail: [email protected]; [email protected]). H. H. Abbasi is with Ericsson, Islamabad 44000, Pakistan (e-mail: [email protected]). M. F. Shafique is with the Center for Advance Studies in Telecommunications, COMSATS Institute of Information Technology, Tarlai Kalan, Islamabad 45550, Pakistan (e-mail: [email protected]). A. K. Brown is with the Microwave and Communication Systems Research Group, School of Electrical and Electronic Engineering, University of Manchester, Manchester M13 9PL, United Kingdom (e-mail: anthony. [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2016.2615884

for many applications, small footprint of a device does not allow such a placement without compromising the overall size. One of the solutions to this limitation is to place antenna elements closely [5]. However, antenna elements in the same plane have significantly reduced isolation among them, consequently reducing the benefits of MIMO communication. Geometrically, MIMO designs can be broadly classified into two categories, i.e., planar and nonplanar. Nonplanar MIMO configurations may have an edge over their planar counterparts when there are constraints on the size of devices. In the existing literature, a number of nonplanar and planar MIMO configurations are reported. In [6], a dual-port nonplanar MIMO system is presented. An F-shaped decoupling structure achieves 20 dB isolation between antenna elements. However, this UWB-MIMO system is only dual-port. An eight-element MIMO planar array is reported in [4]. Closed-loop frequency selective surface (FSS) and quad-strip-connected circular arc decoupling structures achieve an overall isolation of over 20 dB. However, this arrangement compromises on the overall compactness of the system. In [7], a four-element planar MIMO system is presented. A double-layer mushroom wall structure achieves 16 dB isolation, but the system is a narrow band and has relatively large dimensions. A two-element MIMO system is reported in [8]. The two patch antennas are placed in orthogonal configuration. A short ground strip provides an isolation of no more than 15 dB. Although the proposed MIMO system has only two elements, it compromises on the overall compactness. Spline antennas are popular for broadband performance and compactness [9], [10]. The reason behind compactness and performance is their nonconventional shape that is also often optimized using different optimization techniques such as genetic algorithm, particle swarm optimization, etc. These antennas offer better impedance matching over wide frequency band as well as better radiation efficiency. A quad-element UWB-MIMO spline antenna system is reported in this letter. Proposed quad antenna elements are wrapped around a cuboid polystyrene block in MIMO arrangement. A novel inverted-L structure with an array of slotted Yshaped FSSs provides isolation among the antenna ports of the proposed design. Simulated and measured results reveal good isolation of over 20 dB among antenna ports in most of the intended frequency band. This letter is structured as follows. In Section II, design configuration of the proposed system is discussed. Section III details and discusses the decoupling structure and its performance. Simulated and measured results are presented in Section IV including isolation, radiation characteristics, and MIMO parameters. Finally, Section V concludes the letter.

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017

Fig. 2. Realized antenna setup. (a) Elements attached to polystyrene block. (b) 3-D view of antenna elements without polystyrene.

Fig. 1. Geometry of the proposed UWB-MIMO antenna. (a) Front view; (b) rear view; (c) square spiral parasitic element (all dimensions are in millimeters).

II. GEOMETRICAL CONFIGURATION The proposed antenna elements are fabricated on a low-cost FR-4 substrate (εr = 4.4, tanδ = 0.02) of 1.5 mm thickness. The antenna elements have compact dimensions of 36 × 32 mm2 . The front and rear views of the proposed MIMO element are shown in Fig. 1. A radiating patch of the antenna elements is formed by a cubic spline-based geometry on the foreside of the substrate, as shown in Fig. 1(a). A number of bandwidth enhancement techniques detailed in [5] are employed to enhance the overall bandwidth of the proposed MIMO system. A smoothly tapered feedline is employed, which gradually decreases in width from 3 to 0.5 mm along the length. This tapered feedline configuration enhances impedance match over lower frequencies [5]. Vertical slits are etched in the radiating element. These slits introduce extra resonances in lower and middle frequency bands. The curves on the upper edge of patch have radii r1 = 3.8 mm and r2 = 2.7 mm. The semicircles on the top edge of elements contribute by introducing extra resonances in the middle frequency band and also achieve miniaturization in the design [4]. A chamfered ground plane is etched on the rear side of the substrate, as shown in Fig. 1(b). A square slot is added to the middle of the upper edge in the ground plane that enhances the impedance matching. In the literature, parasitic structures are commonly reported to improve overall impedance match [5]. As shown in Fig. 1(c), a square spiral is employed as the parasitic element on the rear side of each radiation element to retain the impedance matching over the complete band, which is likely to suffer as a result of nonplanar MIMO configuration. The MIMO system consists of four radiating elements that are individually fed with a 50-Ω tapered feedline. For MIMO applications, the antenna elements are placed around a polystyrene block in orthogonal configuration, as shown in Fig. 2. The polystyrene block has compact dimensions of 34 × 34 × 36 mm3 . However, unwanted coupling arises among antenna elements as a result of this compact arrangement. III. DECOUPLING STRUCTURE CONFIGURATION The proposed decoupling structure is placed on the rear side of each antenna element, as shown in Fig. 1(b). The decoupling

Fig. 3.

FSS analysis. (a) Analysis setup. (b) Transmission loss.

is introduced through an inverted L-shaped structure; an array of optimized FSS-based slotted Y’s are etched in this structure to suppress undesired coupling among antennas. The analysis and optimization is performed by using a full-wave finite-elementmethod-based electromagnetic solver (Ansys HFSS), as shown in Fig. 3(a). The proposed FSSs are optimized to achieve overall transmission loss over the UWB band, as shown in Fig. 3(b). The vertical arm of the inverted L-shaped structure provides isolation among orthogonal antennas, while the horizontal arm of the structure provides isolation among antennas placed across each other, as shown in Fig. 2(b). The dimensions of the proposed decoupling structures are parameterized in terms of length and width. These parameters of the horizontal and vertical decoupling arms are optimized to achieve better isolation while maintaining a wideband impedance match of each radiation element. The optimized dimensions in Fig. 1(b) are selected by analyzing the variations listed in Table I. It may be noted that an improvement in the isolation is achieved at the cost of impedance match, in particular, in nonplanar MIMO configurations, as also reported in [6]. The dimensions of slotted Y’s are optimized in HFSS optimetrics. The optimized dimensions have width w = 1.6 mm, height h = 2.1 mm, and trace width t = 0.5 mm. IV. SIMULATIONS AND MEASUREMENTS A. Isolation Performance Simulated and measured isolation demonstrate the effectiveness of the decoupling structure, as shown in Fig. 4. From simulated and measured results, it can be observed that without the decoupling structure in place, antennas have low isolation. However, the introduction of the decoupling struc-

BILAL et al.: FSS-BASED NONPLANAR QUAD-ELEMENT UWB-MIMO ANTENNA SYSTEM

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TABLE I OPTIMIZATION OF DECOUPLING PARAMETERS WITH LENGTHS AND WIDTHS Variables

Length of vertical arm (l)

Width of vertical arm (wv )

Width of horizontal arm (wh )

Value (mm)

Impedance Bandwidths (GHz)

Overall isolation (dB)

Match

Mismatch

18.2

3–7

8–10

> 12

19.2 20.2

4–6 3–4

7–10 5–10

> 14 > 15

4.8

4–7

8–10

> 12

5.8 6.8

3–6 4–7

7–10 8–10

> 14 > 14

5.8

6–9

3–5

> 15

6.8 7.8

5–10 3–5

3–4 6–10

> 12 > 14

Fig. 5. Simulated and measured return loss with/without decoupling and spiral structure in the nonplanar geometry. (a) Antenna 1. (b) Antenna 2. (c) Antenna 3. (d) Antenna 4.

Fig. 6.

Surface current density plots at (a) 5 and (b) 10 GHz.

B. Impedance Match

Fig. 4. Isolation performance of proposed system with/without polystyrene (pol. str.) and decoupling (dec.) structure. (a)–(d) orthogonal antennas; (e) and (f) across antennas [ref. Fig. 2(b)].

ture improves isolation among antennas, especially at lower and middle frequencies. At higher frequencies, measured isolation is better due to effect of the polystyrene block since the polystyrene block has some loss, which enhances the isolation by impeding the coupled fields.

The advantage of adding a spiral strip on the back side of antenna elements is clear from Fig. 5. A slight difference between simulated and measured return loss is observed, but the overall return loss is below –12 dB, ensuring sufficient power delivery to antennas. C. Induction Current Suppression To show effectiveness of the decoupling structure, surface currents are plotted in Fig. 6. A strong induced current can be clearly observed in the antenna feed and ground plane when there is no decoupling structure. The decoupling structure suppresses the induced currents significantly, thus improving isolation among antenna ports. Similar effect is noticeable on 10 GHz frequency.

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Fig. 7.

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017

E- and H-plane plots. (a) 5 GHz; (b) 10 GHz.

D. Radiation Characteristics E-plane and H-plane radiation patterns at 5 and 10 GHz are shown in Fig. 7. As the reported configuration is nonplanar, there is a chance of misaligned assembly. Second, it is not uncommon to have pattern distortion at higher frequencies due to smaller fabrication tolerance and finer features becoming resonant. Added components such as launchers, mounting assembly, a rigid feeding cable, etc., do affect patterns. The SMA launchers are three dimensional (3-D), and at least two lie in the E-plane radiation path, especially affecting higher frequencies, resulting in E-plane distortion at 10 GHz. However, in simulation, a two-dimensional waveport is used to excite antenna elements; therefore, no effect is noticeable in the simulated pattern. E. MIMO Performance Criteria The diversity performance of the proposed design is analyzed by total active reflection coefficient (TARC), envelope correlation coefficient (ECC), and channel capacity loss (CCL) [11]. It is desirable to have TARC < 0 dB, ECC < 0.5, and CCL < 0.5 bits/s/Hz for acceptable performance of an MIMO system [11]. From Fig. 8, TARC is better than –8 dB, ECC is less than 0.0025, and CCL is less than 0.2 bits/s/Hz when decoupling is deployed. The simulated radiation efficiency varies 65%–70% without the decoupling structure. The decoupling structure slightly reduces the radiation efficiency to vary between 52% and 60%. However, considerable improvement (over 50%) in MIMO performance parameters, including, more importantly, better isolation has been achieved. The radiation efficiency is low primarily because of the lossy nature of FR-4, and it can be increased by using low-loss substrates. V. CONCLUSION In this letter, a four-port UWB-MIMO antenna system is presented. The design finds application in 3-D system-in-package applications where planar arrangement is not possible due to size constraints. In designing the antenna elements, chamfering and defected ground techniques have been employed for bandwidth enhancement. A square spiral structure is optimally placed as a parasitic element to optimize the impedance match. Effective

Fig. 8. MIMO performance criteria. (a) TARC; (b) ECC; (c) CCL; and (d) radiation efficiency.

decoupling between antenna elements is achieved by employing an inverted L-type structure with FSS-based slotted Y’s. Simulated and measured results of the design are in good agreement. The MIMO performance criteria indicate suitability of the proposed system for UWB-MIMO 3-D system-in-package applications. REFERENCES [1] Federal Communications Commissions, “First report and order, Revision of part 15 of the commission’s rules regarding ultra wideband transmission systems,” Federal Communications Commissions, Washington, D.C, USA, Tech. Rep. FCC 02-48, 2002. [2] Y. Song, N. Guo, and R. C. Qiu, “Towards a real-time UWB MIMO test bed for sensing and communications,” in Proc. IEEE Southeastcon., Mar. 2011, pp. 59–63. [3] G. Srivastava and A. Mohan, “Compact MIMO slot antenna for UWB applications,” IEEE Antennas Wireless Propag. Lett., vol. 15, pp. 1057–1060, 2016. [4] R. Saleem, M. Bilal, K. B. Bajwa, and M. F. Shafique, “Eight-element UWB-MIMO array with three distinct isolation mechanisms,” Electron. Lett., vol. 51, no. 4, pp. 311–313, 2015. [5] D. Valderas, J. I. Sancho, D. Puente, C. Ling, and X. Chen, Ultrawideband Antennas: Design and Applications. London, UK: Imperial College Press, 2011. [6] A. Shaikh, R. Saleem, M. F. Shafique, and A. K. Brown, “Reconfigurable dual-port UWB diversity antenna with high port isolation,” Electron. Lett., vol. 50, no. 11, pp. 786–788, 2014. [7] G. Zhai, Z. N. Chen, and X. Qing, “Enhanced isolation of a closely spaced four-element MIMO antenna system using metamaterial mushroom,” IEEE Trans. Antennas Propag., vol. 63, no. 8, pp. 3362–3370, Aug. 2015. [8] L. Liu, S. W. Cheung, and T. I. Yuk, “Compact MIMO antenna for portable devices in UWB applications,” IEEE Trans. Antennas Propag., vol. 61, no. 8, pp. 4257–4264, Aug. 2013. [9] M. John and M. J. Ammann, “Spline-based geometry for printed monopole antennas,” Electron. Lett., vol. 43, no. 6, pp. 7–8, 2007. [10] M. John, M. J. Ammann, and P. McEvoy, “UWB Vivaldi antenna based on a spline geometry with frequency band-notch,” in Proc. IEEE Int. Symp. Antenna Propag. Soc., 2008. pp. 1–4. [11] S. H. Chae, S. K. Oh, and S. O. Park, “Analysis of mutual coupling, correlations, and TARC in WiBro MIMO array antenna,” IEEE Antennas Wireless Propag. Lett., vol. 6, pp. 122–125, 2007.