A Compact and Straightforward Self-Decoupled MIMO Antenna System for 5G Applications

This study investigates a compact and straightforward self-decoupled <inline-formula> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> multiple-input multiple-output (MIMO) antenna set and its applications for current and future 5G terminal devices. The proposed self-decoupled MIMO antennas include a popularly used loop antenna and a compact loop-type ground-radiation antenna without utilizing any supplementary decoupling structures or undergoing complex tuning process. It is revealed that the loop antenna and the ground-radiation antenna can be modeled as an electric-current element and a magnetic-current element, respectively. This orthogonality allows the self-decoupled characteristic of the proposed MIMO antennas even though the antenna elements are tightly arranged and collocated together. An <inline-formula> <tex-math notation="LaTeX">$8\times 8$ </tex-math></inline-formula> MIMO antenna system is further demonstrated for 5G MIMO applications, where both simulation and measurement are conducted to validate the feasibility of the proposed technique. It is concluded that the proposed MIMO antenna system is a smart way to generate high isolation and low correlation characteristics while having advantages of low profile and easy fabrication so that it can be recognized as a promising candidate for 5G applications.


I. INTRODUCTION
Multiple-input multiple-output (MIMO), utilizing multiple antenna elements for simultaneous transmission and reception at the same radio channel without increasing the input power or frequency spectrum, is a critical technology to increase the channel capacity and provide high data rate for 4G Long-Term Evolution (LTE) applications and the next generation wireless communication (5G) [1], [2]. With the popularization of the 4G communication system, LTE and LTE-Advanced systems have been deployed so fast with the ability to support data rates of 100 Mb/s to 1 Gb/s. However, the proliferation of user numbers and an explosion of more powerful cellular devices, demanding high-quality video and multimedia applications, lead to an avalanche of wireless traffic. Therefore, 5G is proposed to support ultra-fast speeds (up to multiple gigabits per second with a minimum user experienced data rate of one gigabit per second), low latency, and excellent reliability while supporting massive The associate editor coordinating the review of this manuscript and approving it for publication was Sudipta Chattopadhyay. machine-type communications [3], [4]. As one of the most innovative and practical solutions to realize the aforementioned 5G vision, the use of a vast amount of underutilized spectrum in the 3-300 GHz has gained significant interest. Of the 5G new radio spectrum, the sub-6 GHz bands can achieve the best compromise between capacity and coverage, not easily being interfered by walls, trees, and other obstacles.
With the allocation of the 3.5 GHz (3.4-3.6 GHz) band for 5G wireless communication [5], the sub-6 GHz MIMO antenna systems have been popularly studied to allow more antenna elements to be integrated into space-scarce terminal devices [6]- [23]. It has been demonstrated that high isolation (i.e., low mutual coupling) and low correlation are essential figure-of-merits for MIMO antenna systems to guarantee excellent radiation performance and diversity property. Moreover, terminal antennas with compact volume, simple installation, and cost-effective fabrication, are of great interest because terminal antennas are often accompanied by the harshest requirements and the worst implementation circumstances. Therefore, integration of large-scale sub-6 GHz antenna elements into the crowd terminal devices while maintaining high performances is the first and foremost issue because their wavelengths are still comparable to those in current 4G systems.
In the literature, the previously reported techniques for the sub-6 GHz 5G MIMO antennas can be categorized into the following two categories: decoupled MIMO antenna arrays by utilizing decoupling structures [6]- [12] and self-decoupled MIMO antenna arrays without any additional structures [13]- [23]. For the decoupled MIMO antenna arrays, the frequently adopted decoupling structures include neutralization lines or decoupling networks [6]- [9], and additional resonators [9]- [12], which require additional occupation and complicated tuning efforts. On the contrary, the self-decoupled MIMO antenna arrays can achieve adequate isolation by proper arrangement [13]- [17] and polarization or pattern control [18]- [23], circumventing the case-by-case implementation of extra decoupling structures. However, some of them require vast distances between each two antenna elements [13]- [17], which severely limit the scale of the integrated antennas; in contrast, others suffer from structural complexity, implementation difficulty, or constrained position requirements. Therefore, compact and straightforward self-decoupled MIMO antennas with simple structures and cost-effective implementation is favorable in practical applications.
Furthermore, ground-radiation antennas [24]- [30] are reported as small loop-type resonators, occupying ultra-small ground clearance, and exciting the conducting body for radiation. Although they have been widely applied in terminal devices, such as laptops, smartphones, Internet of Things (IoT) devices, their MIMO applications are not compact at all [27], [30] so that they are not suitable in 5G MIMO scenarios. This motivates this paper to investigate a simple and efficient MIMO technique for ground-radiation antennas to suit current and future terminal devices. In this study, a self-decoupled MIMO antenna set [31] is accomplished by merely integrating a popularly used loop antenna [32]- [34] with a conventional ground-radiation antenna. It is revealed that the loop is equivalent to an array of two parallel electric-current sources, and the ground-radiation antenna is equivalent to a magnetic-current source; this orthogonality contributes to the high isolation property with no need for any additional structural support or tuning efforts. The rest of this manuscript is organized as follows. In Section II, the proposed self-decoupled 2 × 2 MIMO antenna set is described. Additionally, the operation mechanism is explained, and further study of various implementation cases is also discussed. In Section III, an 8 × 8 MIMO antenna array is evaluated in simulation and measurement to verify the proposed technique for 5G applications.  used loop antenna with a conventional loop-type groundradiation antenna. As shown, the proposed MIMO antennas are installed on a 140 mm × 70 mm ground plane, which is etched in a 0.8-mm-thick FR4 substrate (ε r = 4.4, tan δ = 0.02).
The loop antenna (Antenna-1) is vertically constructed in the yz-plane and has a symmetrical structure etched on a 0.8-mm-thick FR4 substrate. It is directly fed by a voltage source at one end and shorted to the ground plane at the other end, thus producing current maximums at its two end portions and current nulls at its center portion [32]- [34], thus the electrical length of the loop antenna is about 0.5 wavelength. The loop antenna is popularly utilized in terminal devices because its input impedance can be varied over a wide range of values to match the characteristic impedance of the transmission lines [35]. The upper horizontal strip and the lower horizontal strip have lengths of l 1 (20.6 mm) and l 2 (9.3 mm), respectively; the edge-to-edge distance between two ends of the antenna structure (d) is 2 mm. Accordingly, the overall dimension of the proposed loop antenna is 4 mm × 20.6 mm × 0.8 mm.
The ground-radiation antenna is designed within a 1 mmwide T-shaped clearance of the ground plane. A voltage source is utilized for excitation and impedance matching of the ground-radiation antenna, and a resonance capacitor C r (0.29 pF) is adopted at the open end of the clearance VOLUME 8, 2020 for convenient control of the antenna's resonant frequency. The resonance capacitor C r not only can dramatically miniaturize the antenna area but also can control the antenna's resonance without modifying the antenna's dimensions. In this way, a miniature loop-type resonator can be formed around the clearance, operating as a magnetic coupling element to the ground plane for radiation [24]- [30]. As can be observed, the two ends of the loop antenna are symmetrically installed on the two sides of the open end of the ground-radiation antenna, establishing the proposed self-decoupled 2 × 2 MIMO antenna set without any additional decoupling structures. Note that the width of all conductor lines is 0.5 mm, and more information can be referred to in Fig. 2.

B. SIMULATION RESULTS AND OPERATION MECHANISM
The simulated scattering parameters (S-parameters) are presented in Fig. 2, where it can be observed that wideband characteristics and high isolation are generated. As shown in the S 11 and S 22 curves, the 3:1 VSWR bandwidths of the loop antenna and the ground-radiation antenna are 430 MHz (from 3.3 to 3.73 GHz) and 310 MHz (from 3.35 to 3.66 GHz), respectively, fully covering the target frequency band. As observed in the S 12 curve, the mutual coupling over the whole frequency band is lower than −28 dB, indicating the high isolation property of the proposed self-decoupled MIMO antenna set. Therefore, the assembly of the loop antenna and the ground-radiation antenna is a smart way that can introduce the self-decoupling effect without any additional structural support and complicated tuning efforts. Figure 3 presents the simulated S 22 curves with the variation of the capacitor values of C r to further clarify the operation principle of the ground-radiation antenna. As shown, the resonant frequency of the ground-radiation antenna is lowered down from 5.5 GHz to 3.5 GHz as the capacitor value of C r is increased to 0.29 pF. This is the case because the ground-radiation antenna can exploit the inductance from the ground body around the clearance and the capacitance from the loaded capacitor to form a loop-type resonant circuit. In this way, the resonant frequency of the ground-radiation antenna can be conveniently controlled only by adjusting the capacitor value without changing the clearance area. More information on the ground-radiation antenna can be referred to [24]- [30]. To better understand the operation mechanism of the proposed self-decoupled MIMO antenna set, the simulated surface current distributions at 3.5 GHz are displayed in Fig. 4 for a clear observation of the operating current modes. As shown in Fig. 4(a), the loop antenna produces current maximums at the two end portions and a current minimum at its center portion (denoted by crosses in the figure), which resembles the fundamental current mode of a loop antenna. Alternatively, a small loop-type current mode, flowing around the clearance and through the resonance capacitor C r , is generated by the ground-radiation antenna, as can be observed in Fig. 4(b). In this way, the ground-radiation antenna is operating as a magnetic coupling element and exciting the ground plane for far-field radiation, just as the name of the ground-radiation antenna implies. Additionally, the induced currents from one port to another are extremely weak, verifying the high isolation property between the two antenna elements. To further understand the operation mechanism of the proposed self-decoupled MIMO antenna set, more explanation is illustrated by the modeled current sources of the antenna elements, as shown in Fig. 5. The loop antenna can be modeled as an array of two parallel electric-current sources placed in the yz-plane, which is then equivalent to a dipole-type electric-current element (J 1 ) along the z-axis. Whereas, the ground-radiation antenna can be modeled as a small looptype electric-current source (J 2 ) in the xy-plane, which is then equivalent to a magnetic-current element (M 2 ) along the z-axis. Accordingly, the radiated electric fields from J 1 are orthogonal to those from M 2 ; this orthogonality characteristic between the two current elements ensures the high isolation characteristic between the two antennas, allowing successful implementation of the proposed self-decoupled MIMO antenna set.

C. PLANAR DESIGN
Alternatively, a planar design of the proposed self-decoupled MIMO antennas is constructed by disposing of the loop antenna in the same plane as the ground-radiation antenna (i.e., the xy-plane), as shown in Fig. 6(a).
The loop antenna (Antenna-1) is horizontally constructed in the xy-plane and has a symmetrical structure etched on a 3 mm × 21 mm × 0.8 mm FR4 substrate. The ground-radiation antenna has the same configuration as the one in Fig. 1, which is designed within a 1 mm-wide T-shaped clearance with a resonance capacitor C r (0.31 pF). As shown, the loop antenna is symmetrically installed outside of the ground-radiation antenna, establishing a planar design of the proposed self-decoupled MIMO antenna set.
The simulated S-parameters are presented in Fig. 6(b), where it can be verified that the planar design produces broadband impedance bandwidths and high isolation property. Therefore, both the original design in Fig. 1 and the planar design in Fig. 6 are suitable for 5G applications. The modeled current sources of the planar design of the proposed self-decoupled MIMO antennas are shown in Fig. 7. In this case, the modeled array of two parallel electric-current sources in the xy-plane is equivalent to an electric-current element (J 1 ) directed to the x-axis. VOLUME 8, 2020 Accordingly, the z-directed magnetic-current element M 2 and the x-directed electric-current element (J 1 ) also have orthogonality property. Accordingly, the loop antenna can be implemented at any plane to tailor various construction requirements, which can be an attractive feature in practical scenarios.

D. FURTHER STUDY AND DISCUSSION
This subsection discusses other implementation cases of the proposed self-decoupled MIMO antenna set to validate its functionality and feasibility. Here, three cases are studied by varying the structures of the loop antenna while maintaining the structural parameters of the ground-radiation antenna.
In Case 1, the edge-to-edge distance between the two ends of the loop antenna (d) is expanded to 6 mm (see Fig. 8(a)). The upper and lower horizontal strips (l 1 and l 2 ) are adjusted as 23.2 and 8.6 mm, respectively, to match the resonant frequency. It can be observed that the isolation between the two antenna elements is over 19 dB while the impedance characteristics are barely changed. Figure 8(b) displays the antenna configurations and simulated results of Case 2, where the edge-to-edge distance (d) is expanded to 1 2 mm. In this case, the upper and lower horizontal strip (l 1 and l 2 ) are correspondingly adjusted as 26.6 and 7.3 mm, respectively. It can be seen that only a slight perturbation is engendered in the reflection coefficients and the transmission coefficient.
Additionally, a particular case is presented in Fig. 8(c), where a simple rectangular loop structure is used to constitutes the loop antenna with a length of 30 mm, i.e., l 1 = 30 mm. Accordingly, the loop antenna is transformed into a 30 mm × 4 mm rectangular loop that is directly fed by a voltage source at the right end. Still, the obtained isolation between the two antenna elements is higher than 19 dB; however, the reflection coefficient of the loop antenna deteriorates significantly due to the over-coupled impedance matching. It is noted that, in all three cases, the ground-radiation antenna is located at the center of the loop antenna, exhibiting almost unchanged impedance characteristics.
The insertion of a shorting pin is a well-known method to manipulate the input impedance characteristics of the loop antenna. Accordingly, an alternative implementation case of the proposed self-decoupled MIMO antennas, evolved from Case 3, is presented in Fig. 9(a). The loop antenna has a dimension of a 32.2 mm × 4 mm, and a shorting pin is adopted at the right side of the feed point for impedance matching. It is noted that the ground-radiation antenna is still located at the center portion of the loop antenna for consideration of an optimized self-decoupled effect. The simulated S-parameters are displayed in Fig. 9(b); as can be verified, both antennas can produce sufficient impedance bandwidths to cover the operating frequency band fully, and the mutual coupling between the two antennas is lower than −15 dB. Therefore, the proposed self-decoupled MIMO antenna set is a simple yet efficient method to accomplish compact MIMO antennas in various forms, thereby suitable for different application scenarios. Furthermore, the unique orthogonality between the loop antenna and the ground radiation antenna also ensures their stable performance regardless of their location with respect to the ground plane, which is not discussed here. Additionally, further study is undergoing, for example, a comprehensive analysis of the interaction  between the ground-radiation antenna and the components in the terminal devices (e.g., the display and the metal frame) is under study.

III. DEMONSTRATION OF 8 × 8 MIMO ANTENNA SYSTEM A. ANTENNA CONFIGURATION AND SIMULATION RESULTS
Generally, the 5G MIMO antennas can be arranged along the long sides of the ground plane to leave space for the implementation of 3G/4G antenna systems. Meanwhile, the proposed self-decoupled 2 × 2 MIMO antenna set can be duplicated into terminal devices to simply accomplish an N × N MIMO antenna system for current and future 5G MIMO applications. Accordingly, an 8 × 8 MIMO antenna system is presented in Fig. 10 as a case study to verify the feasibility of the proposed technique. The proposed 8 × 8 MIMO antenna system is comprised of four symmetrically arranged 2 × 2 MIMO antenna sets, and detailed information can be found by referring to Fig. 10. It is important to note that the edge-to-edge distance between two ground-radiation antenna elements at the same side of the ground plane is larger than one wavelength (86 mm), so self-decoupling effects between each two 2 × 2 MIMO antenna sets are also guaranteed due to spatial separation. VOLUME 8, 2020  The simulated S-parameters of the proposed 8 × 8 MIMO antennas are given in Fig. 11. As shown in Fig. 11(a), the S 11  and S 22 curves indicate that the antennas can fully cover the 3.5 GHz band for 5G applications. Furthermore, the transmission coefficients are presented in Figs. 11(b) and (c), where it can be seen that the isolation between any two antenna elements is over 16 dB. Since the spatial distance between each two MIMO antenna sets is over one wavelength, the isolation performance of the proposed MIMO antenna system is not sensitive to this distance. Therefore, the proposed MIMO antennas can be promisingly employed in current and future terminal devices for 5G applications, having advantages of compact volume, easy fabrication, and high isolation property.

B. FABRICATION AND MEASUREMENT
To further validate the proposed technique, the proposed 8 × 8 MIMO antennas were fabricated, and its prototype is pictured in Fig. 12. Moreover, the fabricated 8 × 8 MIMO antennas were then tested using a network analyzer and in a 6 m × 3 m × 3 m three-dimensional (3D) CTIA OTA anechoic chamber to derive the measured impedance characteristics and far-field parameters.
First, the measured reflection coefficients and transmission coefficients are presented in Fig. 13. It can be verified that the measured results are similar to the simulated ones, and the slight difference between measurement and simulation may be attributed to the fabrication error and the soldered semi-rigid cables. Furthermore, the antennas produced sufficient impedance bandwidths that can fully cover the target frequency band (see Fig. 13(a)). Meanwhile, the measured transmission coefficient between each two antenna elements is lower than −17 dB (see Figs. 13(b) and (c)), satisfying the engineering requirement in practical scenarios.
Additionally, the measured total efficiencies are plotted in Fig. 14, where it can be observed that high efficiencies over 56% can be obtained over the frequency range from 3.4 to 3.6 GHz, indicating their high radiation performances and feasibility in practical applications. Furthermore, the far-field radiation patterns at 3.5 GHz are also displayed in the xz-, yz-, and xy-planes, as shown in Fig. 15. The maximum gains produced by Antenna 1 and Antenna 2 direct against each other, and this is an important feature that can guarantee excellent diversity performance of MIMO antennas. Herein, correlation is an essential parameter to identify the diversity performance of MIMO antennas, which can be computed by considering the amplitude, phase, and polarization of the 3D far-field radiation patterns [36]. Therefore, the envelope correlation coefficient (ECC) ρ e of the proposed MIMO antennas are obtained by measuring the 3D far-field radiation pattern and plotted in Fig. 16. It can be observed that all the measured ECC values are below 0.1, which is much lower than the acceptable criterion (ρ e < 0.5) in mobile applications, indicating their extraordinary diversity performances. Therefore, the proposed technique is a simple yet effective method that can achieve high isolation and low correlation simultaneously, suitable for the 5G MIMO applications in current and future terminal devices.

C. COMPARISON
A comparison table with the state-of-the-art MIMO antennas is presented to validate the novelty and advantages of VOLUME 8, 2020 the proposed technique. As shown in Table 1, the selfdecoupling technique has superiority in compactness and simple implementation when compared to the aforementioned decoupling techniques, such as decoupling network, additional resonators, and spatial arraignment. The proposed self-decoupled MIMO set exhibits comparable isolation and bandwidth performance but has a simpler implementation compared to [18] and [22].

IV. CONCLUSION
In this paper, a simple and compact self-decoupled MIMO antenna set is accomplished by merely assembling a loop antenna and a ground-radiation antenna. It has been demonstrated that the high isolation property is obtained due to the orthogonality between the equivalent electric-current element of the loop antenna and the equivalent magnetic-current element of the ground-radiation antenna. This orthogonality between the two antennas is the fundamental feature that allows the successful construction of various implementation cases to suit for different application scenarios.
The 8 × 8 MIMO antennas for 5G applications are investigated in both simulation and measurement. The antennas can produce wide impedance bandwidths and measured total efficiencies over 56%. Meanwhile, the measured ECC values between any two antenna elements are below 0.1, which can also be confirmed from their complementary radiation patterns. Therefore, the proposed MIMO antenna set can be a promising candidate for current and future 5G terminal devices, having advantages of high integration, low profile, easy implementation, as well as excellent radiation performance and diversity performances. Furthermore, further study is now undergoing, such as the multiband self-decoupled antenna sets.

ACKNOWLEDGMENT
(Haiyan Piao and Yunnan Jin contributed equally to this work.)