16-Port Non-Planar MIMO Antenna System With Near-Zero-Index (NZI) Metamaterial Decoupling Structure for 5G Applications

In this article, a low-cost 16-port non-planar Multiple-Input-Multiple Output (MIMO) antenna system is proposed for future 5G applications. The non-planar MIMO antenna system is established around a 3D-octagonal-shape polystyrene block. The MIMO elements are arranged on the eight-sides of octagonal-shape block, whereas bottom and top faces of polystyrene block are left void. The single antenna element comprises of slotted microstrip patch with a stepped chamfered feed line and defected ground plane. Each MIMO element is designed on FR-4 substrate with a size of 22 mm $\times20$ mm, to cover the frequency band of 3.35 GHz to 3.65 GHz for the fifth-generation (5G) applications. The isolation between array elements is improved by using a meander-lines based near-zero-index epsilon-negative (NZI-ENG) metamaterial decoupling structure. The array elements are placed on the top-layer, whereas common connected ground plane and decoupling structure is placed on the bottom-layer. The metamaterial-based decoupling structure offers an isolation of more than 28 dB for antenna elements arranged in across and side-by-side configuration. Moreover, simulated and measured MIMO performance parameters i.e. Total Active Reflection Coefficient (TARC) <−18 dB, Envelop correlation coefficient (ECC) < 0.1 and Channel capacity loss (CCL) < 0.3 are in acceptable limits. The proposed non-planar 3D-MIMO antenna system can be employed for indoor localization systems and wireless personal area network applications, where different 5G devices are wirelessly linked to a centralized server. Moreover, a good agreement between simulated and measured results is achieved for the non-planar MIMO antenna system.


I. INTRODUCTION
The future fifth-generation (5G) communication era will play an important role in the evolution of wireless communication systems [1]. The Multiple-Input-Multiple Output (MIMO) The associate editor coordinating the review of this manuscript and approving it for publication was Jinming Wen . technology integration with 5G communication has the capability to increase the channel capacity, spectral efficiency and transmission rates with minimum delays [2]. Therefore, the microwave research community has put significant efforts to design the 5G-MIMO antenna systems. The integration of multiple MIMO elements in a limited space is a major design challenge in the 5G-MIMO antenna design [3]. In MIMO configuration, each radiating array element should be miniaturized, well-matched, and has a low mutual coupling with neighbouring array elements. However, due to space constraints, closely placed antenna elements produce high mutual coupling, which is undesirable and reduced the performance of the MIMO antenna system. Therefore, an effective decoupling structure is required to reduce the mutual coupling between array elements [4], [5]. In the literature, numerous techniques have been investigated to integrate multiple MIMO elements with acceptable isolation between the array elements. These techniques include but are not limited to Frequency Selective Surfaces (FSSs) [6], neutralization lines [7], Defected ground Structures [8], metamaterial isolators [9] and parasitic elements [10]. Among these decoupling techniques, metamaterial-based decoupling structures have been widely used due to their unique electromagnetic characteristics. The metamaterial-based decoupling structures provide high isolation in closely placed array elements.
A mu-negative (MNG) metamaterial structure in [11], is employed to obtain low mutual coupling among monopole antennas at 2.67 GHz frequency band. Furthermore, a correlation coefficient of less than 0.02 and mutual coupling less than −25 dB is realized for the proposed MIMO antenna configuration. A dual-port microstrip patch antenna with a flower-shaped metamaterial structure is proposed in [12]. The flower-shaped metamaterial decoupling structure provides isolation of about 30 dB, TARC < −12 dB and peak gain of 5 dBi for the WiMAX frequency band. In another study, split-ring resonator-based metamaterial decoupling structure is employed to isolate the multi-band MIMO elements. In this work, a correlation coefficient of less than 0.04 is achieved, and mutual coupling never exceeds −20 dB for a two-port array configuration [13]. A microstrip patch antenna array with dual-polarization and metamaterial slabs is proposed in [14]. In this MIMO configuration, split-ring resonators based metamaterial decoupling structure is employed to reduce the mutual coupling between array elements. In [15], mutual coupling reduction between two antenna elements is achieved by employing coupled split-ring resonators for the 5.3 GHz frequency band. The metamaterial-based decoupling is used to achieve an isolation of more than 25 dB between array elements. A wheel-like metamaterial decoupling structure is proposed in [16]. This metamaterial-based decoupling structure provides a peak gain of 4.62 dBi and isolation of 20 dB in 8−12 GHz frequency band.
In the existing literature, different MIMO designs are reported for non-planar and planar array configurations. A compact 3D non-planar array with two elements is reported in [10]. The MIMO radiating elements are arranged around a square polystyrene block. A parasitic decoupling structure with C-shape slits and vertical-stubs are used to realize isolation of 20 dB within non-planar array configuration. Another non-planar array with four antenna elements is presented in [5]. FSS-based decoupling is employed to isolate the array elements. The mutual coupling between array elements never exceeds −20 dB for 3D non-planar array configuration.
A compact miniaturized non-planar array with eightelements is proposed in [17]. A modified π-shaped parasitic decoupling structure is used to decrease the mutual coupling in the 3D-octagonal arrangement. Moreover, ECC < 0.0025, TARC < −11 dB, CCL < 0.35 and isolation better than 20 dB is attained for proposed non-planar MIMO antenna system. In [18], antenna array with eight radiating elements is reported for non-planar configuration. The isolation enhancement between array elements is achieved by arranging four array elements in a horizontal configuration while remaining four in a vertical configuration. A MIMO antenna system with eight radiating elements for 3.5 GHz smartphone applications is proposed in [2]. A good impedance matching along with ECC < 0.05 is realized for 5G communication.
In [19], a dual-band ten-elements MIMO antenna system for 5G terminal applications is proposed. The proposed MIMO configuration provides isolation of more than 12 dB, efficiency is greater than 45% and ECC < 0.15 for 5G communication. A planar MIMO antenna system with twelve-ports is proposed for fifth-generation smartphone applications [20]. This array configuration covers the 3.5 GHz frequency band with the isolation of more than 20 dB between array elements. In another study, 8-port MIMO antenna system with slots and stubs is reported in [21], an isolation level better than 15 dB, efficiency more than 70%, and envelop correlation coefficient below 0.05 realized for metal-rimmed smartphones.
In this paper, a 16-port non-planar MIMO antenna array with NZI-ENG metamaterial decoupling structure is proposed for future 5G applications. The fabricated 16-port MIMO antenna system with a polystyrene block is illustrated in Fig. 1. The MIMO elements in the non-planar configuration are placed on the 3D octagonal-shaped polystyrene block. A meander-lines based near-zero-index epsilonnegative (NZI-ENG) metamaterial decoupling structure is employed to achieve an isolation of more than 30 dB for antennas placed in across and side-by-side arrangement. Moreover, performance parameters of the MIMO antenna system: CCL, TARC and ECC are in acceptable limits. The proposed MIMO antenna system is designed and analyzed by using a full-wave ANSYS High-Frequency Structural Simulator (HFSS R ). The proposed high isolated, non-planar MIMO antenna system can be used for the wireless personal area network applications, where different 5G devices are wirelessly linked to a centralized server in a rich scattering environment. Moreover, non-planar MIMO antenna system can be employed for indoor localization systems. Where high accuracy via sophisticated coverage area, higher capacity system, multipath component rejection and less number of reference units are considered as the main features of implementing such type of MIMO antenna system for indoor localization technique due to its capability to cover a 360 • mechanism with a distributed 16-port omnidirectional antennas for indoor localization scenario, each element with higher gain up to 6.5 dBi at 3.5 GHz. This paper is structured as follows: in Section II design and analysis of metamaterial is discussed. The 16-port MIMO antenna design VOLUME 8, 2020  and methodology is described in Section III. The simulated and measured results are described in Section IV, including impedance matching, isolation and far-field radiation characteristics. Finally, Section V concludes the paper.

II. DESIGN AND ANALYSIS OF METAMATERIAL
The metamaterial structure is designed on a low-cost FR-4 substrate (relative permittivity = 4.4 and loss tangent = 0.02), thickness of 1.5 mm and copper cladding of 0.035 mm. Metamaterial simulations are performed in the commercially available full-wave HFSS software. The MTM unit element consists of a meander-lines based square ring attached with T-shape stubs and circular split ring. A 0.3 mm split is introduced on the left side of a meandered square ring. Each corner of the square ring is attached with the T-shape stubs. Finally, a circular ring with four splits at 0 • , 90 • , 180 • and 270 • angles is attached at the center of the square ring. The proposed metamaterial design, configuration and analysis setup is illustrated in Fig. 2. Moreover, Table 1 shows the optimized metamaterial unit element parameters. The meanderlines based metamaterial structure is arranged between two wave ports, one along negative and another along the positive z-axis. The proposed metamaterial structure negative characteristics are achieved by placing in perfect magnetic (x-axis) and perfect electric (y-axis) boundaries [22]- [24], as illustrated in Fig. 2b. Furthermore, metamaterial structure physical phenomena is explained by using surface current distribution. Fig. 3 shows the surface current distribution on the metamaterial structure at 3.5 GHz frequency. The high surface current concentration is observed on the three sides of the meandered square ring and T-shape stubs. Whereas, a low surface current distribution is perceived on the splits in the inner circular-ring and left side of meander-lines geometry. Moreover, surface current flows in the opposite direction in the circular split-ring. This current direction reversal, along with strong surface current distribution on the meandered square ring and T-shape stubs, creates a good stop-band at 3.5 GHz frequency band.
The scattering parameters (reflection and transmission coefficient) of the metamaterial unit element are plotted in Fig. 4. The meander-lines based square split-ring, T-shaped stubs and circular ring creates an effective stopband at the 3.5 GHz frequency band. The metamaterial structure usually acts like an inductance-capacitance (LC) resonator. The gaps/splits create a capacitive effect, whereas stubs and metallic rings are responsible for the inductive effect. The metamaterial structure resonant characteristics are typically controlled by gaps, stubs and metallic rings [25].
A robust method in [25], [26] is utilized to obtain the effective permittivity, effective permeability and effective refractive index for 2 -6 GHz frequency band. The S-parameters and metamaterial characteristics are shown in Fig. 4. A good stop-band response is achieved at the 3.5 GHz frequency band. Equations (1) to (10) provided in [25], [26], [38] are employed to extract the effective parameters (permittivity, permeability and refractive index), as shown in Figs. 4b -4d. For 3.4 -3.6 GHz frequency band, NZI-ENG metamaterial characteristics are achieved. The metamaterial structure contains negative permittivity, near-zero permeability and negative refractive index in the intended frequency band.

III. SIXTEEN PORT NON-PLANAR MIMO ANTENNA SYSTEM
Initially, a slotted microstrip patch antenna operating in 3.4 to 3.6 GHz frequency band is designed on a low-cost FR-4 substrate. Secondly, 1 × 2 elements MIMO antenna system is designed. The array elements are decoupled by employing a meander lines-based metamaterial decoupling structure on the flip-side of the substrate. Finally, 16-port MIMO antenna system for the non-planar arrangement is designed around an octagonal-shaped polystyrene block with a common connected ground plane for fifth-generation applications.

A. UNIT ELEMENT AND 1 × 2 ARRAY ANALYSIS
The single antenna element with compact dimensions of 20 mm × 22 mm size is designed on 1.5 mm thick FR-4 substrate (dielectric constant = 4.4, loss tangent = 0.02). The single antenna element comprises of a slotted microstrip patch on the top-layer, whereas defected ground structure is placed on the rear-side of the substrate. The radiating patch consists of T-shape slots and pair of circular splitrings, to obtain a good impedance matching in 3.4 to 3.6 GHz frequency band. A stepped chamfered structure is attached to the feed line and radiating patch. This stepped structure acts as an impedance transformer and provides better impedance matching in the desired frequency band. The defected ground plane structure at the bottom-layer also contributes to control the resonance and impedance matching in 3.4 to 3.6 GHz frequency band. Fig. 5 shows the single antenna element geometry and configuration.
The optimized parameters of a single antenna element to achieve resonance at 3.5 GHz frequency band are shown in Table 2. The design methodology of a single antenna element (radiating patch and ground plane) is illustrated in Fig. 6. The different design modifications in the ground plane and radiating patch are shown in Fig. 6. The design amendments effect on the reflection coefficient is observed, to achieve proper impedance matching in 3.4 to 3.6 GHz frequency band. Microstrip patch antenna shown in design-A, provides a resonance at 5 GHz frequency band. A stepped VOLUME 8, 2020  structure is introduced and attached with a feed line, as shown in design-B. This stepped structure acts as an impedance transformer. By decreasing the microstrip patch size, resonance is shifted towards a higher frequency band.
The impedance matching at the lower frequency band is achieved by introducing a pair of circular split-ring resonators, and T-shape slots in the main radiator, as illustrated in design-C and D. Finally, a good impedance matching in 3.4 to 3.6 GHz frequency band is attained for the slotted microstrip patch. The ground plane design methodology and shape variation effect on the reflection coefficient is plotted  in Fig. 6(b). By decreasing the ground plane height, resonance shifts towards a higher frequency band. The defected ground plane structure at the flip-side of the FR-4 laminate helps to achieve a wide-band resonance at 3.5 GHz frequency.
A 1 × 2 MIMO antenna system with radiating elements and metamaterial decoupling structure is illustrated in Fig. 7. This two-port array arrangement has dimensions of L a × W. Besides, two antenna elements are placed on the FR-4 substrate with 180 • angle and 4.2 mm vertical distance between them, as shown in Fig. 7 (a). The metamaterial-based decoupling structure is employed to enhance the isolation among the array elements. A near-zero-index epsilon-negative (NZI-ENG) metamaterial structure is placed on the bottom-side of the main radiating elements, as shown in Fig. 7(b).
The S-parameters of 1 × 2 MIMO antenna system without a decoupling structure is plotted in Fig. 8(a). In the absence of the metamaterial decoupling structure, high mutual coupling is observed for the dual-port MIMO antenna system. The surface distribution is used to show the effectiveness of the metamaterial decoupling structure. High induced currents are observed on antenna 2, without the decoupling structure, which is undesirable, as shown in Fig. 9(a). A metamaterialbased effective decoupling structure is employed for the mutual coupling reduction among MIMO array elements. The S-parameters of the two-element MIMO antenna system with metamaterial decoupling structure is shown in Fig. 8(b). A significant reduction in mutual coupling along with isolation of more than 30 dB is realized with a metamaterial decoupling structure. The surface current distribution in the presence of the decoupling structure is shown in Fig. 9(b). The MTM decoupling structure suppresses the unwanted induced currents on the antenna 2. This suppression of unwanted induced currents helps to improve the isolation between MIMO elements.

B. SIXTEEN PORT 5G MIMO ANTENNA SYSTEM
A 16-port MIMO antenna system is designed to create a strong and reliable communication link in a rich scattering environment. These types of MIMO antenna systems with off devices can be used for the production lines or hospital rooms, where the best reception is required. The array elements are placed on the octagonal-shaped polystyrene block. The polystyrene block has a dielectric constant of 2.6; each face height is 36 mm and width is 23 mm. The array elements are arranged in a 3D octagonal-shape arrangement. Each side of the octagonal shape polystyrene block is loaded with 1 × 2 elements MIMO antenna array. The proposed 16-port MIMO antenna configuration with and without metamaterial decoupling structure is shown in Fig. 10. Each face contains two antenna elements placed at 0 • and 180 • angle (across-arrangement). The antenna elements placed across has a port-to-port distance of 36 mm. A copper sheet of thickness 0.5 mm and width 3.5 mm is used to connect the ground planes of the MIMO elements. All sixteen antenna elements have a common connected ground plane, which is very important in the practical implementation of MIMO antenna systems. A slight change in operating bandwidth and mutual coupling between array elements is observed with a common connected ground plane configuration. Whereas, side-by-side antennas, placed at the adjacent side of the block, have a port-to-port distance of 21.5 mm. The MIMO elements are arranged in a compact octagonal-shaped arrangement, which creates a strong mutual coupling between array placed in all configurations, i.e. antennas placed across at 180 • angle (Ant. 1 and Ant. 2), side-by-side adjacent antenna at 45 • (Ant. 1 and Ant. 3) and antenna elements in diagonal configuration (Ant. 1 and Ant. 4). Therefore, an effective decoupling structure is required to reduce mutual coupling in all configurations. Therefore, an H-shaped near-zeroindex epsilon negative (NZI-ENG) metamaterial-decoupling structure is designed for non-planar array configuration. The physical phenomena of metamaterial-based decoupling metamaterial is analyzed by using the surface current distribution. The MTM elements placed behind the radiating patch have low surface current distribution. In contrast, high surface current distribution is observed on the MTM elements place at the radiator corners and middle of the two MIMO elements. Therefore, MTM elements with low surface current distribution have been removed from the metamaterial array. The modified H-shape decoupling structure provides better isolation, high surface current distribution and suppresses the unwanted strong induction currents, when antenna elements are arranged in 3D-arrangement (side-by-side elements at 45 • , across elements at 180 • and diagonally placed elements). Moreover, by using the modified H-shape decoupling structure simulation time and computational resources significantly reduced for the 16-port non-planar MIMO antenna configuration.

IV. RESULTS AND DISCUSSION
A 16-port no-planar MIMO antenna is proposed for 5G applications. A good impedance matching along with low mutual coupling between array elements is imperative for the 5G-MIMO antenna design. A 1 × 2 MIMO array prototype is placed on each face of the octagonal-shaped polystyrene block. The antenna elements are symmetric and identical in the 3D non-planar arrangement. Therefore, impedance matching for adjacent elements is only considered for the octagonal-shape arrangement. Fig. 11 shows the measured and simulated reflection coefficient of side-by-side and across elements. The MIMO antenna elements in both configurations (side-by-side and across) has a reflection coefficient less than −10 dB at 3.5 GHz frequency band.
The mutual coupling of 16-port MIMO antenna system with and without metamaterial decoupling structure is shown in Fig. 12. High mutual coupling is observed for side-by-side and across elements without the metamaterial decoupling structure, as shown in Fig. 12. Besides, high mutual coupling is also observed on the adjacent elements, and mutual coupling gradually decreases on the MIMO elements placed far way. The mutual coupling reduction between the array elements is achieved by using the NZI-ENG metamaterial decoupling structure. In non-planar 3D-MIMO configurations, mutual coupling between array elements never exceeds −28 dB with H-shape decoupling structure.
Simulated and measured isolation of the proposed MIMO antenna configuration is shown in Fig. 13. In both configurations (side-by-side and across), isolation better than 28 dB is realized for 3.5 GHz frequency band. A lossless polystyrene block was used in the simulator to create a non-planar MIMO assembly. Therefore, additional effect of polystyrene material on the mutual coupling and reflection coefficient is not evident in the simulated results. A slight variation in simulated and measured isolation is observed for the proposed MIMO configuration. These variations are due to the imperfections in the non-planar 3D assembly. Overall, 3D polystyrene block is employed to hold the MIMO elements in non-planar configuration, without any contribution in isolation enhancement. The surface current distribution is employed to show the utility of the metamaterial-based decoupling structure. The surface current distribution on the 3D-MIMO antenna system is illustrated in Fig. 14. In the absence of MTM decoupling array, strong induction currents are present on the MIMO elements placed in the side-byside and across configuration in non-planar configuration, as shown in Fig. 14 (a). The strong induced currents on the surrounding MIMO elements is suppressed by using the H-shape NZI-ENG metamaterial decoupling structure. A significant reduction in mutual coupling is found between MIMO elements by using the decoupling structure.
The surface current distribution in the presence of the MTM decoupling structure is shown in Fig. 14(b). ECC, TARC and CCL are used to evaluate the diversity performance of the 16-port MIMO antenna system. The value of TARC, ECC and CLL should be less than 0 dB, 0.5 and 0.5 bits/s/Hz, respectively, for an efficient non-planar MIMO antenna system [5].
The ECC, TARC and CCL values are extracted by using relations provided in [34]- [37]. Equation (11) is used to calculate the envelope correlation coefficient for antenna elements placed across and side-by-side configuration. The simulated and measured ECC is shown in Fig. 15(a). The CCL and TARC of the proposed MIMO antenna system in both configurations is calculated by using the equation (12) and (13). Simulated and measured CCL and TARC is portrayed in Fig. 15 (b) and (c). For the proposed 16-port nonplanar MIMO configuration with a connected ground plane, ECC, CCL and TARC values are less than 0.1, 0.3 bits/s/Hz and −18 dB respectively for both antenna arrangements (side-by-side at 45 • and antenna elements at 180 • angle). Moreover, simulated and measured performance parameters values are within acceptable limits when decoupling is employed.
CCL = −log 2 det α 11 α 12 α 21 α 22 (12) where i and j = 1, 2 The far-field radiation characteristics of fabricated MIMO antenna system are measured in Satimo measurement lab (UKM-star lab), by using the Agilent N5227A vector analyzer, Satimo passive measurement setup and SatEnv software [27], as shown in Fig. 17. Simulated and measured far-field radiation patterns of Ant. 1 and Ant. 2 in E and H -plane are shown in Fig. 16. During the ant.1 and ant.2 measurements, other MIMO elements are properly terminated with 50 load impedance. The radiation patterns in both planes are almost omnidirectional. However, a little difference in simulated and measured results is observed in both planes due to measurement setup constraints and imperfections in 3D non-planar array assembly. The 16-port MIMO antenna system far-field radiation characteristics (gain and efficiency) are plotted in Fig. 17. A measured gain of more than 6.5 Bi and an efficiency of more than 65% is realized for the non-planar array configuration.
The radiation efficiency and gain of the proposed MIMO antenna system are measured by employing the SATIMO near-field antenna measurement system. The 16-identical measurement probs in SATIMO measurement system are used to obtain the radiated near-field data by using the combination of the 360 • horizontally rotating values of proposed MIMO antenna and 3D scan values of the probes. The near-field data is then converted to far-field data through Fourier transformation (FT) to reveal the radiation patterns. Afterwards, efficiency and gain from the far-field data of the radiation pattern are calculated by using the SatEnv software.
The performance of the non-planar 3D-MIMO antenna system is compared with the recent state-of-the-art MIMO antenna systems in Table 3. It can be observed that high isolated, 3D-MIMO antenna system with NZI-ENG  metamaterial decoupling structure competes well with nonplanar and planar MIMO antenna systems. In a compact nonplanar arrangement, good impedance matching along with high isolation, good gain and radiation efficiency is very challenging to achieve in the rich scattering environment, without scarifying the MIMO performance parameters (CCL, TARC, ECC). Additionally, isolation of more than 28 dB, ECC < 0.1, CCL < 0.3, TARC < −18 dB, gain of 6.5 dBi and radiation efficiency greater than 65% realized for the all MIMO antenna elements, which is very competitive to planar and non-planar MIMO configurations.

V. CONCLUSION
In this paper, a 16-port non-planar MIMO antenna system with a metamaterial-based decoupling structure is proposed for 5 th generation communication. The proposed 3D-MIMO configuration can be employed for the applications which require a miniaturized and non-planar arrangement. The slotted microstrip patch antenna ensures a proper impedance matching at 3.35 to 3.65 GHz frequency band. Moreover, near-zero-index epsilon-negative (NZI-ENG) metamaterial decoupling structure is employed to obtain an isolation of more than 28 dB in 3.35 to 3.65 GHz frequency band. Moreover, performance of the proposed MIMO antenna system is evaluated by ECC, TARC and CCL. A good agreement between simulated and measured results is achieved and proved that the proposed MIMO can be a potential candidate for 5G systems. The 16-port non-planar 5G-MIMO antenna system is very helpful for off device placement where best reception is obtained. Moreover, upcoming research will be foreseen to be concentrated on the implementation of the proposed MIMO antenna system in future wireless indoor localization technique with irregular scenario based on the performance analysis of the received signal strength.