Eight-Port Metamaterial Loaded UWB-MIMO Antenna System for 3D System-in-Package Applications

In this article, an eight-element ultra-wideband (UWB) Multiple-Input-Multiple Output (MIMO) antenna system is proposed for 3D non-planar applications. The proposed UWB-MIMO antenna is installed around a polystyrene block in the 3D-octagonal arrangement. The eight radiating elements are placed on the sides of the octagonal polystyrene block with top and bottom surfaces left open. The single antenna element consists of a modified Y-shaped radiating patch, epsilon-negative (ENG) metamaterial, and a partial ground plane. A modified pie-shaped decoupling structure is deployed at the back-side of the radiating patch to improve the isolation among array elements. Each antenna element is printed on a low-cost FR-4 substrate with dimensions of 28 mm $\times23$ mm with coverage of the whole UWB spectrum from 3.1 to 10.6 GHz frequency band. The eight-port UWB-MIMO antenna system consists of symmetric and non-symmetric array configurations. Simulated and measured MIMO performance parameters i.e. Channel Capacity Loss (CCL) < 0.35, Envelope Correlation Coefficient (ECC) < 0.0025 and Total Active Reflection Coefficient (TARC) < −11 dB are in acceptable limits for both symmetric and non-symmetric configurations. The proposed MIMO antenna system is suitable for 3D system-in-package, indoor localization systems, and wireless personal area network applications in industries where multiple machines are connected to a central server wirelessly through such kinds of antennas in a rich scattering environment.


I. INTRODUCTION
The microwave research community has put significant efforts in designing antennas in the ultra-wideband spectrum ever since the Federal Communications Commission (FCC) has announced its license free usage. However, the maximum allowable power spectral density in this band is −41dBm/MHz. This limitation has resulted in the reduction of both channel capacity and communication range for UWB The associate editor coordinating the review of this manuscript and approving it for publication was Walid Al-Hussaibi .
technology [1]- [3]. A combination of UWB with MIMO technology has therefore emerged as an effective solution to overcome these limitations and has resulted in the evolution of several application areas. UWB-MIMO antenna systems usually consist of several radiating elements, placed in close proximity of one another, operate simultaneously to provide high diversity and multiplexing gain. In modern wireless communication miniaturized components are usually required [4]- [6] owing to space constraints and hence the closely placed antenna elements create significantly high mutual-coupling causing reduced performance.
Mutual coupling reduction between antenna elements is a major challenge in the design of the UWB-MIMO antenna system. The individual antenna elements must be properly matched to the system impedance also and effectively decoupled from the adjacent MIMO elements [7], [8].
A two elements non-planar UWB-MIMO antenna system is presented in [19], having dimensions of 40 mm × 37.5 mm × 41.5 mm with the isolation of 20 dB for the complete UWB frequency band. Moreover, radiating elements in a square configuration are etched on a low-cost FR-4 substrate, high isolation is achieved by introducing vertical stubs and C-shaped strips on the rear-side of the substrate. In another study, four elements non-planar UWB-MIMO antenna system is discussed [4]. Frequency Selective Surface based decoupling structures offer isolation of more than 20 dB in most of the frequency band. This UWB-MIMO configuration with dimensions of 32 mm × 36 mm is proposed for 3D system-in-package applications. An eight-port non-planar diversity antenna with multiple polarizations is reported in [20]. A set of four antennas are placed horizontally while the rest of the antennas are arranged in the vertical direction. In [21], a dielectric resonator integrated eight-port MIMO system with pattern diversity is presented. This array configuration provides isolation of more than 20 dB, correlation level of 0.25, and TARC less than -15 dB. A compact 3D UWB-MIMO antenna is discussed in [22]. Four antenna elements are placed in a planar configuration while the remaining four elements are placed in a non-planar configuration. Isolation of 20 dB along with ECC less than 0.5 is achieved in the complete UWB frequency band.
Several planar UWB-MIMO geometries are reported in the literature [23]- [27]. The eight-port planar UWB-MIMO antenna for polarization and pattern diversity is discussed in [23]. Isolation of more than 15 dB with envelop correlation coefficient less than 0.1 is achieved for all ports. In another study, an 8-port ultra-wideband antenna system for smartphone applications is reported in [24]. In this configuration, an isolation level of 15 dB and a correlation coefficient better than 0.1 is obtained for smartphone applications. The MIMO design proposed in [25] has two identical half-slot antenna elements with a Y-shaped cut in the ground plane. This compact MIMO antenna has an isolation of 15 dB in the lower frequency band. Whereas, mutual coupling suppression of more than 20 dB is achieved in the higher frequency band of UWB. Another planar MIMO system with an orthogonally dual-polarized antenna system is reported in [26].
This MIMO configuration has eight elements connected with square rings. This configuration provides isolation of 15 dB. Similarly, a two-element planar MIMO antenna system with band notch characteristics is discussed in [27]. This MIMO configuration has an isolation of 20 dB with more than 85% efficiency in the entire UWB band. The metamaterial (MTM) loaded UWB antenna system design is a novel approach to obtain a high gain, improved radiation performance, and large bandwidth with miniaturized antenna size [28], [29]. A compact printed monopole antenna integrated with a single negative metamaterial is proposed in [28]. This UWB antenna configuration has a compact size of 22.5 mm × 14 mm with a peak gain of 6.12 dBi and a radiation efficiency of more than 80%. In [30], a two-layer integrated metamaterial-based antenna system is discussed. The π-shape and cross-shaped slots are used to enhance the frequency band and radiation efficiency of the proposed antenna system. In this article, an eight-element UWB-MIMO antenna system is presented with symmetric as well as non-symmetric configurations, as shown in Fig. 1. The antenna elements are placed in the 3D octagonal arrangement around a polystyrene block. Isolation of more than 20 dB and MIMO performance parameters (ECC, TARC, and CCL) in acceptable limits are achieved for both configurations. A commercially available full-wave 3D simulator, High Frequency Structural Simulator (HFSS), is used to design and analyze the eightport UWB-MIMO system. The proposed 3D non-planar UWB-MIMO system is suitable for system-in-package applications and wireless personal area network applications in industries where multiple machines are connected to a central server wirelessly through these kinds of antennas in a rich scattering environment. The rest of the paper is structured as follows: Section II describes the metamaterial unit cell analysis. Single element UWB antenna design along with eightport UWB-MIMO configuration is discussed in Section III. The simulated and measured results, along with the MIMO performance parameters are described in Section IV. Finally, the manuscript is concluded in Section V.

II. METAMATERIAL DESIGN AND ANALYSIS
The metamaterial (MTM) unit cell is designed and analyzed by using a Finite Element Method (FEM) provided in the sim- VOLUME 8, 2020 ulator. The metamaterial unit cell is designed on a low-cost FR-4 substrate with a dielectric constant of 4.4, a thickness of 1.5 mm, and copper cladding of 0.035 mm. The proposed unit element consists of two square rings. Each ring contains two gaps introduced 1 mm away from the diagonally opposite corners. However, inner split-ring is attached with a meanderline structure by using two horizontal stubs. The metamaterial unit cell configuration, dimensions, and simulation setup are shown in Fig. 2. The metamaterial unit cell was placed between two wave ports on the negative and positive z-axis and excited with an electromagnetic wave in the positive z-axis direction. To achieve negative characteristics, perfect magnetic boundary conditions have been set along the x-axis, and perfect electric boundaries are chosen for the y-axis [31]. The z-axis was chosen for free-space propagation of incoming waves as shown in Fig. 2b. The analysis of surface current distribution is carried out to understand the physical phenomena of Epsilon Negative (ENG) metamaterial structure. The surface current distribution at 3.9 GHz, 5.2 GHz, 7.5 GHz, and 10.15 GHz is plotted in Fig. 3. A large concentration of surface current can be observed at meander-line structure, inner rings gaps/splits, and corners. The current flows in opposite directions in the meander line structure and the split rings. This current direction reversal generates a stop band at 3.9 GHz, 5. GHz and 10.15 GHz frequency bands. Moreover, a strong current distribution at the inner ring, outer ring, and middle of the meander line structure create a wide stopband response at 7.5 GHz frequency band. The splits/gaps in metamaterial structure act like capacitors and metallic rings act as inductors. The unit cell scattering parameters  The outer and inner split ring resonators along with meander-line structure are enough to achieve an effective stop-band operation within the UWB frequency band. The metamaterial structure acts like an inductance-capacitance (LC) resonator. The metallic rings along with meander-line structure act as inductors and the splits/gaps act as capacitors that can control the resonance characteristics of the unit element [32], [33]. The metamaterial unit cell optimized parameters are: a = 7 mm, b = 7 mm, a 1 = 0.24 mm, a 2 = 2.64 mm, a 3 = 1.84 mm, a 4 = 1.84 mm, a 5 = 0.23 mm, a 6 = 0.69 mm, a 7 = 3.3 mm, a 8 = 3.9 mm, a 9 = 6.40 mm and a 10 = 0.24 mm. The metamaterial constitutive parameters (permittivity, permeability, and refractive index) for 2 -12 GHz frequency band are extracted by using a robust method provided in [28], [34].
The permittivity and permeability can be extracted by using the following relations: In Robust Method, equations (1) to (10) are used to retrieve the effective metamaterial characteristics. The metamaterial unit cell reflection and transmission parameters are used to extract the permittivity, permeability, and refractive index characteristics of the metamaterial structure. The real and imaginary parts of permittivity, permeability, and refractive index are shown in Fig. 4.

III. METAMATERIAL LOADED ULTRA-WIDEBAND ANTENNA AND EIGHT ELEMENTS ARRAY
The metamaterial loaded UWB antenna element is designed in the first stage. Secondly, eight-port UWB-MIMO, along with decoupling structure is designed for symmetric and nonsymmetric configurations.

A. UNIT ELEMENT DESIGN AND ANALYSIS
The proposed UWB antenna geometry has been evolved from a rectangular microstrip patch. A low-cost 1.5 mm thick FR-4 substrate is used with relative permittivity of 4.4 and loss tangent of 0.02. A modified Y-shaped rectangular patch is chamfered from the top corners to improve the impedance matching over the whole band of interest. A beveled stepped structure is introduced at the lower edge of the patch, which is attached to a tapered feed-line. This stepped structure and the beveled region act as an impedance transformer. The feedline width is linearly decreased from 2.3 mm to 0.8 mm.
All edges in the radiating patch are beveled with a radius of 1 mm. The partial ground plane is applied at the rear-side along with tapered sides to achieve good impedance matching in the UWB frequency band. The UWB antenna front and back-side views are shown in Fig. 5. Moreover, UWB antenna optimized parameters are shown in Table 1.
The effect of different modifications is shown in Fig. 6(a). A simple microstrip patch shown in stage-A has a narrow bandwidth. A stepped response is introduced in stage-B to achieve a better impedance matching at the lower frequency band. The beveled feed-line along with truncated radiating patch and beveled corners, shown in stage C and D, provides better impedance matching for 3.1 -10.6 GHz frequency band. The effect of ground plane size and shapes is shown in Fig. 6 (b). More stable response with impedance matching at the start band is achieved with a partial ground plane with chamfered sides. Moreover, 1 × 2 elements MTM array is placed at the back-side of the radiating patch to enhance the gain and efficiency of the UWB antenna. The MTM has ENG characteristics within 3.1 -10.6 GHz frequency band.
The effect of MTM on the gain and efficiency of the UWB antenna is shown in Fig. 6(c). More than 60% enhancement in gain and efficiency is achieved by employing the metamaterial array at the backside of the radiating patch.

B. EIGHT PORT UWB-MIMO AND DECOUPLING STRUCTURE DESIGN
The eight-port UWB-MIMO antenna system is intended to be used off devices at a convenient location to establish a strong communication link in rich scattered environments like hospital rooms or production lines. The UWB-MIMO antenna elements are installed around an octagonal shaped polystyrene block of permittivity 2.6. The octagonal shape has a radius of 33 mm; each face of the octagon has a width of 25 mm and a height of 32 mm. The eight antenna elements are installed on each face of the octagon resulting in a 45 • angle between adjacent elements. Two adjacent elements have a pitch of 28.5 mm, while the distance between two opposite antenna elements is 60.52 mm as shown in Fig. 7.
The eight-port metamaterial loaded UWB-MIMO antenna system is arranged in two different configurations. In the first setup, referred to as symmetric configuration, all antenna ports are placed on the same side. Whereas in the nonsymmetric configuration adjacent antennas have ports in the opposite directions. The UWB-MIMO antenna elements are categorized in edge-to-edge(E2E) and back-to-back(B2B)  arrangement for the sake of simplicity. The antenna elements (1, 5),(2, 6), (3,7) and (4,8) are in back-to-back (B2B) configuration relative to each other. Whereas, antenna elements (1, 2), (1, 3), and (1, 4) are in edge-to-edge (E2E) configuration in both symmetric and non-symmetric topology. These UWB-MIMO topologies require a very effective decoupling structure since the UWB antenna elements radiate in all directions thus developing a very strong mutual coupling.
A π shaped decoupling structure is designed and optimized for the proposed setup. This structure consists of four vertical stubs, the outer two stubs have zigzag edges while the inner stubs have plane edges. The decoupling structure analysis is performed by using wave ports, perfect electric, and magnetic boundary conditions configured in the simulator. The transmission loss variation with the decoupling structure modifications is shown in Fig. 8. PEC boundary conditions are assigned to the left and right side of the airbox. In contrast,    The four vertical stubs along with slotted middle stub and zigzag pattern in outer stub are used to reduce the mutual coupling between antennas in B2B and E2E configurations.
The final optimized decoupling structure has evolved in three iterations as shown in Fig. 8. The final evolution step involves the introduction of several defects in order to achieve the desired decoupling level for the whole UWB frequency spectrum for both symmetric and non-symmetric arrangements.

IV. RESULTS AND DISCUSSION
The proposed MIMO system is intended for UWB applications. Therefore, wideband impedance matching is imperative. The matching over the whole frequency band is achieved by different modifications which involve the variation of antenna geometry as well as the ground plane and introduction of an ENG metamaterial structure. The simulated and measured reflection coefficient of the symmetric and non-symmetric UWB-MIMO antenna system is presented in Fig. 9. Eight identical UWB antenna elements are used for MIMO synthesis in a non-planar arrangement around an octagonal polystyrene block, as shown in Fig. 7. Therefore, all antenna elements have the same simulated and measured reflection coefficient. The reflection coefficient for an antenna is less than −10 dB in the whole UWB frequency band irrespective of its configuration.
The mutual coupling effect with and without decoupling structure is illustrated in Fig. 10. Without decoupling structure a high mutual coupling is present between array elements. Moreover, the mutual coupling is significantly large among the adjacent elements. The decoupling structure helps to reduce the mutual coupling among array elements. Simulated and measured decoupling performance of the antenna system for both symmetric and non-symmetric configuration is shown in Fig. 11. The isolation for B2B and E2E antenna pairs in the entire UWB is better than 20 dB for symmetric and non-symmetric configurations. Moreover, the addition of polystyrene block does not affect the isolation.
However, losses in the polystyrene block increase the measured isolation at higher frequencies by impeding the coupled fields. It is observed that measured results have slightly deviated from simulations. The polystyrene used in the simulator was lossless. Therefore, an additional polystyrene effect on the reflection coefficient and isolation is not visible in simulated results. The polystyrene block is used as supporting material to hold the antenna elements. Moreover, 3D assembly imperfections during non-planar array formation and array elements placement contribute to these variations. Overall, polystyrene helps in the non-planar 3D arrangement of the MIMO system with no significant contribution to the results.
The surface currents distribution of the proposed UWB-MIMO antenna system at 4 GHz, 7 GHz, and 10 GHz are shown in Fig. 12. In the absence of a decoupling structure,  a strong induced current is observed. However, metamaterial structure and discontinuities in the decoupling structure in the shape of slots and zigzag patterns capture the electromagnetic field and convert them in induced currents. A significant reduction is thus visible at lower, middle, and higher frequency bands.
The diversity performance of the metamaterial loaded UWB-MIMO antenna system in both configurations (symmetric and non-symmetric) is evaluated by the Envelope Correlation Coefficient (ECC), Channel Capacity Loss (CCL), and Total Active Reflection Coefficient (TARC). For an efficient UWB-MIMO system ECC, CCL and TARC should be below 0.5, 0.5 bits/s/Hz and 0 dB respectively [4]. The value of ECC is calculated from S-parameters with the help of the relation provided in [4], [19] which applies to a rich scattering environment that is under investigation in this research work.
The CCL of the proposed UWB-MIMO system is calculated for both symmetric and non-symmetric configurations. Moreover, ECC and CCL are calculated individually for both the E2E and B2B arrangements for the UWB-MIMO antenna system, whereas TARC is calculated for all of the eight radiating antennas collectively. The simulated and measured ECC, CCL, and TARC of the proposed UWB-MIMO configuration are shown in Fig. 13. The simulated and measured values of performance parameters are within the desired limits.  The fabricated eight-port UWB-MIMO has been measured in Satimo near Field Measurement Lab by using Satimo Passive Measurement (SPM), SatEnv software, and Agilent N5227A vector network analyzer [35]. Simulated and measured radiation patterns for E-plane and H-plane at 4 GHz, 7 GHz, and 10 GHz are shown in Fig. 14. The antenna radiation patterns have shown some variations due to 3D assembly imperfection and measurement setup constraints. The UWB-MIMO measurement setup is shown in Fig. 15. The proposed metamaterial loaded UWB-MIMO system provides a maximum measured gain of 7 dBi along with an efficiency of more than 60% as shown in Fig. 16.
A comparison of the proposed UWB-MIMO design with the existing literature is given in Table 2. It can be noted that the proposed non-planar metamaterial loaded UWB-MIMO system competes very well with planar and non-planar designs. The size of the proposed UWB antenna is highly miniaturized in MIMO configuration, which is extremely difficult to achieve without sacrificing other performance parameters. Moreover, an isolation of 20 dB is achieved with TARC < −8 dB, ECC < 0.0025, CCL < 0.35, which is highly competitive to other proposed UWB-MIMO designs in both planar and non-planar configuration.

V. CONCLUSION
In this study, an eight-port UWB MIMO antenna system is presented. The proposed MIMO system is suitable for applications that require compact size and 3D nonplanar arrangement. The customized design of patch antenna along with tapered feedline and ENG metamaterial ensure the impedance matching over the whole UWB spectrum. The mutual coupling reduction among antenna elements is achieved by introducing parasitic decoupling structure at antenna rear-sides. The metamaterial loaded MIMO system achieves isolation of 20 dB in the whole band. The significance of the proposed system is further established by MIMO performance parameters. The measured and simulated results are in good agreement. Eight-element planar arrangements require more space and cannot exploit spatial diversity as much as this scheme can do. The off-device installation of the antenna module is very helpful by conveniently placing it where the best reception is achieved.