Design and Implementation of Quad-Port MIMO Antenna with Dual-Band Elimination Characteristics for Ultra-Wideband Applications

: A planar, microstrip line-fed, quad-port, multiple-input-multiple-output (MIMO) antenna with dual-band rejection features is proposed for ultra-wideband (UWB) applications. The proposed MIMO antenna design consists of four identical octagonal-shaped radiating elements, which are placed orthogonally to each other. The dual-band rejection property (3.5 GHz and 5.5 GHz corresponding to Wi-MAX and WLAN bands) was obtained by introducing a hexagonal-shaped complementary split-ring resonator (HCSRR) in the radiators of the designed antenna. The MIMO antenna was etched on low-cost FR-4 dielectric substrate of size 58 × 58 × 0.8 mm 3 . Isolation higher than 18 dB and envelope correlation coe ﬃ cient (ECC) lesser than 0.07 was observed for the MIMO / diversity antenna in the operating range of 3–16 GHz. The presented four-port UWB MIMO antenna conﬁguration was fabricated, and the experimental results validate the simulation outcomes. and the are anti-parallelly. The an for the operational An in the for and WLAN (3.5 and 5.5 respectively). The simulated and experimental results for gain, S-parameters, isolation, ECC, and radiation patterns were studied. validated that the decoupling metal strips used to reduce the inter-element coupling is a simple and e ﬃ cient approach, and a good diversity response was achieved. The obtained results that be useful It base other


Introduction
The use of ultra-wideband (UWB) technology for wireless applications has seen a surge after the Federal Communications Commission (FCC) specified the 3.1-10.6 GHz band as an unlicensed band [1]. Recently, UWB antennas have attracted the focus of RF engineers and researchers due to their extensive usage in sensing networks, cognitive radios, microwave imaging, wearable devices, military applications, wireless personal area networks, and high data rate communication systems. The monopole-based UWB antennas of various shapes and sizes have been explored by the researchers for various applications [2,3]. The features, such as low-profile, miniature size, light-weight, good radiation efficiency, and simple integration into communication devices, make planar monopole antennas a preferred choice for UWB transceiver systems [4]. However, UWB antennas suffer from the disadvantage that radiation can be transmitted to smaller distances only, which is due to the use of low power for transmission, as mentioned by the Federal Communications Commission (FCC) [5]. The limitations of low power and short-range transmission can be encountered using multiple-input-multiple-output (MIMO) technology along with UWB. It uses multiple radiating elements for transmitting and receiving wireless signals, but the placement of multiple elements in a limited space within a communication device/system is a challenge. The close placement of resonating elements will result in poor isolation and high envelope correlation coefficient (ECC), which deteriorates the functioning of the designed system. An inverse Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 12 utilized for simulation, designing, and implementation of the designed antenna. The dimension details of the octagonal monopole element are provided in Table 1.  The antenna design stages are demonstrated in Figure 2. Firstly, an octagonal-shaped radiator (stage-1), fed by a microstrip line of impedance 50 Ω was considered, as displayed in Figure 2a. The radiator was characterized by high eccentricity, which is required for supporting multiple modes in UWB applications. The surface below the antenna radiating patch acted as an unbalanced impedance as it was not grounded. A small U-shaped slot was carved from the ground surface (beneath the microstrip feed line) for providing the required impedance matching among the patch and the feed line (stage-2), as demonstrated in Figure 2b. In Figure 2c, a hexagonal SRR was implanted in the antenna radiator to remove the interfering 5.5 GHz (WLAN) band from the UWB (stage-3). In the next stage, as shown in Figure 2d, an additional hexagonal SRR (opposite to the hexagonal SRR in stage-3) was loaded on the radiator to eliminate interfering 3.5 GHz (Wi-MAX) band signals (stage-4). The lengths of the implanted SRR (outer and inner) can be computed as [19].  The antenna design stages are demonstrated in Figure 2. Firstly, an octagonal-shaped radiator (stage-1), fed by a microstrip line of impedance 50 Ω was considered, as displayed in Figure 2a. The radiator was characterized by high eccentricity, which is required for supporting multiple modes in UWB applications. The surface below the antenna radiating patch acted as an unbalanced impedance as it was not grounded. A small U-shaped slot was carved from the ground surface (beneath the microstrip feed line) for providing the required impedance matching among the patch and the feed line (stage-2), as demonstrated in Figure 2b. In Figure 2c, a hexagonal SRR was implanted in the antenna radiator to remove the interfering 5.5 GHz (WLAN) band from the UWB (stage-3). In the next stage, as shown in Figure 2d, an additional hexagonal SRR (opposite to the hexagonal SRR in stage-3) was loaded on the radiator to eliminate interfering 3.5 GHz (Wi-MAX) band signals (stage-4). The lengths of the implanted SRR (outer and inner) can be computed as [19].
ε r, e f f = ε r + 1 2 where λg and f r denote the guided wavelength and notch-band central frequency, respectively, c denotes the speed of light in vacuum, and ε r,eff represents the effective relative permittivity. Figure 3 displays the S 11 characteristics of the design steps of the proposed UWB antenna element. A magnified view of the HCSRR is given in Figure 1b, which contains two concentric hexagonal split rings of different dimensions.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12 where λg and fr denote the guided wavelength and notch-band central frequency, respectively, c denotes the speed of light in vacuum, and εr,eff represents the effective relative permittivity. Figure 3 displays the S11 characteristics of the design steps of the proposed UWB antenna element. A magnified view of the HCSRR is given in Figure 1b, which contains two concentric hexagonal split rings of different dimensions.  (1) where λg and fr denote the guided wavelength and notch-band central frequency, respectively, c denotes the speed of light in vacuum, and εr,eff represents the effective relative permittivity. Figure 3 displays the S11 characteristics of the design steps of the proposed UWB antenna element. A magnified view of the HCSRR is given in Figure 1b, which contains two concentric hexagonal split rings of different dimensions.   Figure 4a,b display the current behavior at notch frequencies 3.5 GHz and 5.5 GHz, correspondingly. From Figure 4a, it can be observed that the concentration of current was higher near the boundaries of the outer hexagonal split-ring, resulting in suppression of the Wi-MAX band. Similarly, Figure 4b shows that a high magnitude current flowed along the boundaries of the inner hexagonal split-ring, which resulted in the elimination of WLAN signals. Therefore, dual band-notched characteristics were achieved by loading an HCSRR in the octagonal radiating patch.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 Figure 4a,b display the current behavior at notch frequencies 3.5 GHz and 5.5 GHz, correspondingly. From Figure 4a, it can be observed that the concentration of current was higher near the boundaries of the outer hexagonal split-ring, resulting in suppression of the Wi-MAX band. Similarly, Figure 4b shows that a high magnitude current flowed along the boundaries of the inner hexagonal split-ring, which resulted in the elimination of WLAN signals. Therefore, dual bandnotched characteristics were achieved by loading an HCSRR in the octagonal radiating patch.

Quad-Port MIMO Antenna
The geometric layout of the MIMO antenna is presented in Figure 5a, and its design dimensions are provided in Table 1. The MIMO design consists of microstrip-line fed octagonal-shaped four identical antenna elements, which are arranged in orthogonal directions to attain superior performance. The antenna feeding points are represented as port-1, port-2, port-3, and port-4, in the layout diagram. Enhanced isolation was achieved as a result of the orthogonal field produced between the adjacent octagonal-shaped radiating elements, consequently, a reduction in interelement coupling was noticed. However, due to close proximity of the resonating elements, the weak coupling was observed at the lower frequency range, which can be reduced by introducing decoupling elements on the ground surface. Therefore, on the back-side of the dielectric substrate, three metal strips were introduced between the ground patches of the antenna elements to enhance inter-element isolation. The metal strips restrained the current radiated to the adjacent excited elements, thereby enhancing the mutual coupling level. For realizing the optimal decoupling response, the size and spacing among the strips were determined by analyzing multiple simulations and surface current distribution. In addition, these metallic strips united the ground patches of the four radiators to confirm the same voltage in the designed antenna. In a real system, the resonating patches should have the same reference plane to understand the signal level properly. The MIMO system will not work efficiently if isolated ground planes are present [20]. The antenna fabricated prototype is displayed in Figure 5b. The total size of the presented four-port MIMO/diversity antenna is 58 × 58 × 0.8 mm 3 .

Quad-Port MIMO Antenna
The geometric layout of the MIMO antenna is presented in Figure 5a, and its design dimensions are provided in Table 1. The MIMO design consists of microstrip-line fed octagonal-shaped four identical antenna elements, which are arranged in orthogonal directions to attain superior performance. The antenna feeding points are represented as port-1, port-2, port-3, and port-4, in the layout diagram. Enhanced isolation was achieved as a result of the orthogonal field produced between the adjacent octagonal-shaped radiating elements, consequently, a reduction in inter-element coupling was noticed. However, due to close proximity of the resonating elements, the weak coupling was observed at the lower frequency range, which can be reduced by introducing decoupling elements on the ground surface. Therefore, on the back-side of the dielectric substrate, three metal strips were introduced between the ground patches of the antenna elements to enhance inter-element isolation. The metal strips restrained the current radiated to the adjacent excited elements, thereby enhancing the mutual coupling level. For realizing the optimal decoupling response, the size and spacing among the strips were determined by analyzing multiple simulations and surface current distribution. In addition, these metallic strips united the ground patches of the four radiators to confirm the same voltage in the designed antenna. In a real system, the resonating patches should have the same reference plane to understand the signal level properly. The MIMO system will not work efficiently if isolated ground planes are present [20]. The antenna fabricated prototype is displayed in Figure 5b. The total size of the presented four-port MIMO/diversity antenna is 58 × 58 × 0.8 mm 3 .
The proposed MIMO antenna design steps are demonstrated in Figure 6. Firstly, as illustrated in Figure 6a, four identical octagonal-shaped resonating elements were arranged orthogonally to each other. Sufficient space was provided between the four radiators to suppress inter-element correlation. In step-2, as shown in Figure 6b, a thin strip was inserted in the middle of the MIMO antenna to reduce mutual coupling further. The thin strip suppresses the inter-element coupling considerably, but more isolation was required between the antenna elements to obtain effective notching and diversity performance. Therefore, as illustrated in Figure 6c The proposed MIMO antenna design steps are demonstrated in Figure 6. Firstly, as illustrated in Figure 6a, four identical octagonal-shaped resonating elements were arranged orthogonally to each other. Sufficient space was provided between the four radiators to suppress inter-element correlation.
In step-2, as shown in Figure 6b, a thin strip was inserted in the middle of the MIMO antenna to reduce mutual coupling further. The thin strip suppresses the inter-element coupling considerably, but more isolation was required between the antenna elements to obtain effective notching and diversity performance. Therefore, as illustrated in Figure 6c, a decoupling structure consisting of two strips was introduced in the center of the MIMO antenna (step-3), which reduces mutual coupling up to −15 dB in the working band.   The proposed MIMO antenna design steps are demonstrated in Figure 6. Firstly, as illustrated in Figure 6a, four identical octagonal-shaped resonating elements were arranged orthogonally to each other. Sufficient space was provided between the four radiators to suppress inter-element correlation. In step-2, as shown in Figure 6b, a thin strip was inserted in the middle of the MIMO antenna to reduce mutual coupling further. The thin strip suppresses the inter-element coupling considerably, but more isolation was required between the antenna elements to obtain effective notching and diversity performance. Therefore, as illustrated in Figure 6c, a decoupling structure consisting of two strips was introduced in the center of the MIMO antenna (step-3), which reduces mutual coupling up to −15 dB in the working band.  In addition, to obtain more isolation between the resonating elements, a decoupling element comprised of three metal strips was introduced in the middle of the MIMO antenna (shown in Figure 5). The decoupling element with three metal strips offers isolation greater than 18 dB and also unites the ground planes of the four radiators to confirm the same voltage in the proposed MIMO antenna. The mutual coupling comparison of the three MIMO stages is displayed in Figure 6d.

Results
The antenna electrical performance was measured using 50 Ω SMA connectors. The reflection coefficients (simulated and measured) of the designed UWB MIMO antenna at different ports (S 11 , S 22 , S 33 , S 44 ) are shown in Figure 7. While performing measurements at one resonating element, the remaining antenna ports were matched with 50 Ω loads.
In addition, to obtain more isolation between the resonating elements, a decoupling element comprised of three metal strips was introduced in the middle of the MIMO antenna (shown in Figure  5). The decoupling element with three metal strips offers isolation greater than 18 dB and also unites the ground planes of the four radiators to confirm the same voltage in the proposed MIMO antenna. The mutual coupling comparison of the three MIMO stages is displayed in Figure 6d.

Results
The antenna electrical performance was measured using 50 Ω SMA connectors. The reflection coefficients (simulated and measured) of the designed UWB MIMO antenna at different ports (S11, S22, S33, S44) are shown in Figure 7. While performing measurements at one resonating element, the remaining antenna ports were matched with 50 Ω loads. The HCSRR embedded in the antenna radiator eliminated the Wi-MAX and WLAN frequencies.
The frequency of the elimination bands can be altered by varying dimensions of the HCSRR. The mutual coupling among different resonating elements of the MIMO antenna is illustrated in Figure  8a,b. The inter-element coupling was decreased through an orthogonal arrangement of the resonating elements and by introducing metal strips between their ground planes. Isolation greater than 18 dB was attained for impedance bandwidth range. As displayed in Figure 9, a 7 dBi peak gain was realized in the proposed UWB antenna. A sudden fall in the gain was observed at dual-band rejection frequencies. The HCSRR embedded in the antenna radiator eliminated the Wi-MAX and WLAN frequencies.
The frequency of the elimination bands can be altered by varying dimensions of the HCSRR. The mutual coupling among different resonating elements of the MIMO antenna is illustrated in Figure 8a,b. The inter-element coupling was decreased through an orthogonal arrangement of the resonating elements and by introducing metal strips between their ground planes. Isolation greater than 18 dB was attained for impedance bandwidth range. As displayed in Figure 9, a 7 dBi peak gain was realized in the proposed UWB antenna. A sudden fall in the gain was observed at dual-band rejection frequencies.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 12 (a)      The surface-current behaviors at notch frequencies 3.5 GHz and 5.5 GHz are presented in Figure 10a,b, correspondingly, here, all the four elements were excited concurrently. The maximum concentration of current (highlighted in red color) was observed near to the boundary of the outer hexagonal split-ring (Figure 10a), which resulted in Wi-MAX band rejection. Similarly, high current concentration near the boundaries of the inner hexagonal split-ring (Figure 10b), marked the rejection of the WLAN band.
ECC can be used to study the coupling effect between adjacent elements of the antenna. The given expression can be used to calculate ECC values between the first and second ports of a multi-port MIMO system [21].
Appl. Sci. 2020, 10, 1715 9 of 12 In a similar way, the ECC among remaining antenna elements can also be determined. Figure 11 displays the ECC curves among different elements. ECC was observed to remain below 0.07 for the entire UWB band.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 12 The surface-current behaviors at notch frequencies 3.5 GHz and 5.5 GHz are presented in Figure  10a,b, correspondingly, here, all the four elements were excited concurrently. The maximum concentration of current (highlighted in red color) was observed near to the boundary of the outer hexagonal split-ring (Figure 10a), which resulted in Wi-MAX band rejection. Similarly, high current concentration near the boundaries of the inner hexagonal split-ring (Figure 10b), marked the rejection of the WLAN band. ECC can be used to study the coupling effect between adjacent elements of the antenna. The given expression can be used to calculate ECC values between the first and second ports of a multiport MIMO system [21].
In a similar way, the ECC among remaining antenna elements can also be determined. Figure 11 displays the ECC curves among different elements. ECC was observed to remain below 0.07 for the entire UWB band.    ECC can be used to study the coupling effect between adjacent elements of the antenna. The given expression can be used to calculate ECC values between the first and second ports of a multiport MIMO system [21].
In a similar way, the ECC among remaining antenna elements can also be determined. Figure 11 displays the ECC curves among different elements. ECC was observed to remain below 0.07 for the entire UWB band.  Figure 12 illustrates the co-polar and cross-polar radiation characteristics (simulated and experimental) of the designed MIMO antenna at frequencies 5 GHz, 9 GHz, and 14 GHz. A difference Figure 11. Envelope correlation coefficient (ECC) of the proposed four-port ultra-wideband (UWB) MIMO antenna. Figure 12 illustrates the co-polar and cross-polar radiation characteristics (simulated and experimental) of the designed MIMO antenna at frequencies 5 GHz, 9 GHz, and 14 GHz. A difference higher than 15 dB was seen among the co-polar and cross-polar pattern curves (in E-and H-planes), which shows the stable functioning of the designed antenna. The co-polar patterns of the E-plane displayed bi-directional behavior while the H-plane co-polar patterns possessed omnidirectional behavior. The simulated and experimental outcomes were found in close proximity. The variations observed between the measured and simulated outcomes were due to high loss tangent, antenna fabrication errors, and soldering of the SMA connectors. Table 2 lists the comparison of the presented dual-band notched UWB MIMO antenna to other existing UWB MIMO antennas in the literature. The comparison table highlights that the designed antenna has numerous advantages over the previous reported notched-band antennas [10][11][12][13][14][15][16][17][18], with reference to size, bandwidth ratio, inter-element isolation, and the number of radiating patches. The three metal ring-based decoupling technique used for reducing mutual coupling is simple and effective. In the presented MIMO antenna, the notch frequencies (3.5 GHz and 5.5 GHz) were removed by introducing HCSRR in the resonating patch, without engaging any other active elements or filter circuitry. Moreover, the orthogonal orientation of the resonating elements offered better isolation and joined ground patches of the monopole elements offered a common reference voltage.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 12 higher than 15 dB was seen among the co-polar and cross-polar pattern curves (in E-and H-planes), which shows the stable functioning of the designed antenna. The co-polar patterns of the E-plane displayed bi-directional behavior while the H-plane co-polar patterns possessed omnidirectional behavior. The simulated and experimental outcomes were found in close proximity. The variations observed between the measured and simulated outcomes were due to high loss tangent, antenna fabrication errors, and soldering of the SMA connectors.

Conclusions
In this article, a small-sized four-port UWB MIMO antenna with dual-band rejection features is presented. The proposed design contains four matching octagonal-shaped resonators arranged orthogonally, and the diagonal elements are arranged anti-parallelly. The antenna showed an isolation >18 dB for the entire operational range. An HCSRR was implanted in the radiating element for notching the interfering Wi-MAX and WLAN frequencies (3.5 and 5.5 GHz, respectively). The simulated and experimental results for gain, S-parameters, isolation, ECC, and radiation patterns were studied. The results validated that the decoupling metal strips used to reduce the inter-element coupling is a simple and efficient approach, and a good diversity response was achieved. The obtained results illustrate that the proposed MIMO antenna could be useful for UWB applications. It can serve as a potential candidate for base station terminals and other wireless communication systems.