Compact 8 × 8 MIMO Antenna Design for 5G Terminals

: In this paper, a compact 8 × 8 MIMO antenna design for 5G terminals is proposed. The 8 × 8 MIMO antenna consists of two quad-element antenna pairs, each of which includes two symmetrical T-shaped monopole mode elements and two symmetrical edge-coupled fed dipole mode elements. The size of the quad-element antenna is 38 × 7 × 0.8 mm 3 . T-shaped monopoles are decoupled by parasitic elements, and dipoles are decoupled by grounding strips. Meanwhile, both T-shaped monopoles and dipoles are also decoupled by the orthogonal mode. The results show that the operating frequency band of each antenna element meets the requirement of 3.4–3.6 GHz, the reﬂection coefﬁcient is less than − 6 dB, and the isolation between any antenna element is more than 10 dB. The antenna radiation efﬁciency is over 50% in the entire operating frequency band for the 8 × 8 MIMO system.


Introduction
With the continuous improvement of mobile communication quality requirements, the fifth generation (5G) mobile communication technology provides a promising solution for high communication rate, low latency, large connection density and high communication capacity. In order to meet the goals of 5G mobile communication and effectively improve the channel capacity of the communication system in a rich scattering environment, MIMO technology has become a key technology in the new generation of wireless communication systems [1]. Generally, the MIMO systems enlarge the channel capacity through multiple independently placed elements, known as spatial diversity. However, due to the narrow space of the terminals, spatial diversity cannot reach its true potential. Thus, other diversity techniques, such as polarization diversity and radiation pattern diversity are applied in MIMO systems. Both the transmitter and the receiver of a MIMO system need to place multiple antennas, and the coupling between the antennas will reduce the performance of the MIMO system [2]. Therefore, the decoupling design of the MIMO antenna has become an important part of the wireless communication system. With the increase in the number of antennas on smart devices and the miniaturization of terminal equipment, the space left for antenna placement is limited. How to reduce mutual coupling between antennas and achieve high isolation in a compact space is a difficult problem.
Most of the initial polarization diversity technologies [19][20][21][22] use the same antenna elements placed orthogonally to achieve decoupling, which undoubtedly increases the occupied area of the antenna. Therefore, researchers use different antenna elements to achieve polarization diversity through their orthogonal current modes, which enhances the isolation and improves the compactness of the antenna.
In this paper, the polarization diversity is combined with the parasitic element to reduce the isolation of the designed compact MIMO antenna. The antenna integrates two T-shaped monopole elements and two edge-coupled dipole mode elements on an area of 28.6 × 7 mm 2 , which greatly improves the integration degree of the antenna design. The orthogonal current modes of two types of elements enhance the isolation. In addition, parasitic elements and grounding strips are used to weaken coupling between elements of the same type. The operating frequency band of each element is 3.4-3.6 GHz, with an S 11 less than −6 dB. The isolation between any elements is above 10 dB. The simulation was performed with the help of Ansys HFSS [23]. This article is organized as follows. Section 2 describes the antenna configuration and decoupling principles. Section 3 presents the experimental results and discussion. Finally, Section 4 is the conclusion of this paper. Figure 1 shows the overall structure and detailed dimensions of the eight-element highly integrated antenna. As shown in Figure 1, the antenna consists of two 4-element highly integrated antenna pairs, and each antenna pair consists of four antenna elements: two symmetrical T-shaped monopole-mode antenna elements and two symmetrical edgecoupled-fed dipole-mode antenna elements, printed on the inner and outer sides of the side plates, respectively. The material of the substrate is FR4. The size of the substrate is 150 × 75 × 0.8 mm 3 , and the area of the ground is 150 × 73 mm 2 . The metal ground is printed on the back of the main board, and the width of the clearance area is only 1 mm. Two small FR4 dielectric side plates with a size of 38 × 7 × 0.8 mm 3 are installed along the left and right sides of the ground plane.

Antenna Configuration and Decoupling Principle
achieve polarization diversity through their orthogonal current modes, which enha the isolation and improves the compactness of the antenna.
In this paper, the polarization diversity is combined with the parasitic elemen reduce the isolation of the designed compact MIMO antenna. The antenna integrates T-shaped monopole elements and two edge-coupled dipole mode elements on an ar 28.6 × 7 mm 2 , which greatly improves the integration degree of the antenna design. orthogonal current modes of two types of elements enhance the isolation. In addi parasitic elements and grounding strips are used to weaken coupling between elem of the same type. The operating frequency band of each element is 3.4-3.6 GHz, wit S11 less than −6 dB. The isolation between any elements is above 10 dB. The simula was performed with the help of Ansys HFSS [23]. This article is organized as foll Section 2 describes the antenna configuration and decoupling principles. Section 3 sents the experimental results and discussion. Finally, Section 4 is the conclusion of paper. Figure 1 shows the overall structure and detailed dimensions of the eight-elem highly integrated antenna. As shown in Figure 1, the antenna consists of two 4-elem highly integrated antenna pairs, and each antenna pair consists of four antenna elem two symmetrical T-shaped monopole-mode antenna elements and two symmet edge-coupled-fed dipole-mode antenna elements, printed on the inner and outer sid the side plates, respectively. The material of the substrate is FR4. The size of the subs is 150 × 75 × 0.8 mm 3 , and the area of the ground is 150 × 73 mm 2 . The metal groun printed on the back of the main board, and the width of the clearance area is only 1 Two small FR4 dielectric side plates with a size of 38 × 7 × 0.8 mm 3 are installed along left and right sides of the ground plane. The overall occupied area of the compact antenna pair is 28.6 × 7 mm 2 . Ant1 Ant2 are two symmetrical T-shaped monopole mode antenna elements, and each tenna element occupies an area of 10 × 2.9 mm 2 . A 50 Ω microstrip line is used to dir feed Ant 1 and Ant 2 to form a monopole current mode. Microstrip lines with a leng 2.4 mm are extended at port 1 and port 2, respectively. They are connected to the m ground through the metallized Via1 and Via2 to adjust the operating band of the m pole mode antenna element while improving the impedance matching. A T-type p The overall occupied area of the compact antenna pair is 28.6 × 7 mm 2 . Ant1 and Ant2 are two symmetrical T-shaped monopole mode antenna elements, and each antenna element occupies an area of 10 × 2.9 mm 2 . A 50 Ω microstrip line is used to directly feed Ant 1 and Ant 2 to form a monopole current mode. Microstrip lines with a length of 2.4 mm are extended at port 1 and port 2, respectively. They are connected to the metal ground through the metallized Via1 and Via2 to adjust the operating band of the monopole mode antenna element while improving the impedance matching. A T-type parasitic element is introduced between Ant1 and Ant2. The structure is connected to the metal ground through a microstrip line and Via5. The decoupling of Ant1 and Ant2 is mainly realized through the T-type parasitic element structure.

Antenna Configuration and Decoupling Principle
Ant3 and Ant4 adopt the form of edge-coupling feed to realize the dipole current mode, and each antenna element occupies an area of 13 × 4.3 mm 2 . As shown in Figure 1, a 50 Ω microstrip feed line is connected to the coupled feed branch on the side plate through Via3 and Via4 at points B and C. The decoupling of Ant3 and Ant4 is mainly realized by the ground strip between them. The strip is connected to the metal ground through Via6 on the side plate at point A.
The optimized parameters are as follows: L 1 = 3.7 mm, L 2 = 3.7 mm. Figure 2 shows the current distribution on the antenna and ground when port1 and port3 are excited at 3.5 GHz, respectively. As shown in Figure 2a,b, when the monopole (Ant1) mode is excited, the current on the antenna element is distributed in the same direction along the Z-axis. The current distribution along the X-axis is in the opposite direction, and there is a radiation zero point at the center.

Decoupling Principle between Ant1 and Ant3
Electronics 2022, 11, x FOR PEER REVIEW 3 of 11 sitic element is introduced between Ant1 and Ant2. The structure is connected to the metal ground through a microstrip line and Via5. The decoupling of Ant1 and Ant2 is mainly realized through the T-type parasitic element structure. Ant3 and Ant4 adopt the form of edge-coupling feed to realize the dipole current mode, and each antenna element occupies an area of 13 × 4.3 mm 2 . As shown in Figure 1, a 50 Ω microstrip feed line is connected to the coupled feed branch on the side plate through Via3 and Via4 at points B and C. The decoupling of Ant3 and Ant4 is mainly realized by the ground strip between them. The strip is connected to the metal ground through Via6 on the side plate at point A.
The optimized parameters are as follows: L1 = 3.7 mm, L2 = 3.7 mm. Figure 2 shows the current distribution on the antenna and ground when port1 and port3 are excited at 3.5 GHz, respectively. As shown in Figure 2a,b, when the monopole (Ant1) mode is excited, the current on the antenna element is distributed in the same direction along the Z-axis. The current distribution along the X-axis is in the opposite direction, and there is a radiation zero point at the center. When the dipole (Ant3) mode is excited, the current on the antenna is distributed in the same direction along the X-axis, and the radiation is the strongest at the center position. In this way, orthogonal antenna current modes are formed, so that there is no mutual coupling in the space between the antennas. There is also a set of quadrature modes, which are the orthogonal currents on the ground. As shown in Figure 2c,d, the currents on the ground are also orthogonal, so the current coupled through the ground is also blocked.

Decoupling Principle between Ant1 and Ant3
For the edge-fed dipole (Ant3), the grounded strip is the key to decoupling not only the two edge-fed dipoles Ant3 and Ant4, but also the monopole Ant1 and the edge-fed dipole Ant3. If the strip is not grounded, it will affect the current balance; the isolation between the two antenna elements Ant1 and Ant3 will deteriorate rapidly. As shown in Figure 3, if the grounded point of Ant3 is removed, the isolation between Ant1 and Ant3 When the dipole (Ant3) mode is excited, the current on the antenna is distributed in the same direction along the X-axis, and the radiation is the strongest at the center position. In this way, orthogonal antenna current modes are formed, so that there is no mutual coupling in the space between the antennas. There is also a set of quadrature modes, which are the orthogonal currents on the ground. As shown in Figure 2c,d, the currents on the ground are also orthogonal, so the current coupled through the ground is also blocked.
For the edge-fed dipole (Ant3), the grounded strip is the key to decoupling not only the two edge-fed dipoles Ant3 and Ant4, but also the monopole Ant1 and the edge-fed dipole Ant3. If the strip is not grounded, it will affect the current balance; the isolation between the two antenna elements Ant1 and Ant3 will deteriorate rapidly. As shown in Figure 3, if the grounded point of Ant3 is removed, the isolation between Ant1 and Ant3 will rapidly deteriorate to below 10 dB. It can be seen in Figure 3 that, through the introduction of Via6, the performances of S 11 and S 33 are somewhat worse, but these still meet the requirement of −6 dB. will rapidly deteriorate to below 10 dB. It can be seen in Figure 3 that, through the introduction of Via6, the performances of S11 and S33 are somewhat worse, but these still meet the requirement of −6 dB.   Figure 4 shows the current distribution on Ant1 and Ant2 with or without the grounded pin Via5 when Ant1 is excited. It can be clearly seen that when Via5 is provided, the T-type parasitic element can effectively excite the current to form a current loop with Ant1 and block the coupling current from entering Ant2.  In order to describe the decoupling mechanism more clearly, the current model diagram is established as shown in Figure 5. In Figures 4 and 5, it can be seen that when there is no Via5, the space coupling current forms a current loop between the two antenna elements, and the T-shaped decoupling branch in the middle cannot effectively excite the current. When there is a grounded pin Via5, the half side of the T-type decoupling branch close to Ant1 can effectively excite the current, forming a current loop with Ant1, and the coupling current intensity on Ant2 is significantly reduced. Figure 6 shows the simulated S-parameter of Ant1 and Ant2 with or without Via5. It can be seen that when Via5 is added, the isolation between antenna elements is significantly improved. However, since the parasitic element will also have a coupling effect on the cur-  Figure 4 shows the current distribution on Ant1 and Ant2 with or without the grounded pin Via5 when Ant1 is excited. It can be clearly seen that when Via5 is provided, the T-type parasitic element can effectively excite the current to form a current loop with Ant1 and block the coupling current from entering Ant2. will rapidly deteriorate to below 10 dB. It can be seen in Figure 3 that, through the introduction of Via6, the performances of S11 and S33 are somewhat worse, but these still meet the requirement of −6 dB.

Decoupling Principle between Ant1 and Ant2
Figure 4 shows the current distribution on Ant1 and Ant2 with or without the grounded pin Via5 when Ant1 is excited. It can be clearly seen that when Via5 is provided, the T-type parasitic element can effectively excite the current to form a current loop with Ant1 and block the coupling current from entering Ant2.  In order to describe the decoupling mechanism more clearly, the current model diagram is established as shown in Figure 5. In Figures 4 and 5, it can be seen that when there is no Via5, the space coupling current forms a current loop between the two antenna elements, and the T-shaped decoupling branch in the middle cannot effectively excite the current. When there is a grounded pin Via5, the half side of the T-type decoupling branch close to Ant1 can effectively excite the current, forming a current loop with Ant1, and the coupling current intensity on Ant2 is significantly reduced. Figure 6 shows the simulated S-parameter of Ant1 and Ant2 with or without Via5. It can be seen that when Via5 is added, the isolation between antenna elements is significantly improved. However, since the parasitic element will also have a coupling effect on the cur- In order to describe the decoupling mechanism more clearly, the current model diagram is established as shown in Figure 5. In Figures 4 and 5, it can be seen that when there is no Via5, the space coupling current forms a current loop between the two antenna elements, and the T-shaped decoupling branch in the middle cannot effectively excite the current. When there is a grounded pin Via5, the half side of the T-type decoupling branch close to Ant1 can effectively excite the current, forming a current loop with Ant1, and the coupling current intensity on Ant2 is significantly reduced. Figure 6 shows the simulated S-parameter of Ant1 and Ant2 with or without Via5. It can be seen that when Via5 is added, the isolation between antenna elements is significantly improved. However, since the parasitic element will also have a coupling effect on the current on the excitation antenna Ant1, it will affect the impedance matching of the antenna. From the results in Figure 6, the introduction of Via5 enhances the isolation but sacrifices part of the bandwidth of the antenna. rent on the excitation antenna Ant1, it will affect the impedance matching of the antenna. From the results in Figure 6, the introduction of Via5 enhances the isolation but sacrifices part of the bandwidth of the antenna.

Decoupling Principle between Ant3 and Ant4
For the edge-fed dipoles Ant3 and Ant4, the grounded strip is the key to decoupling between Ant3 and Ant4, and its length L1 is a key parameter. Figure 7 presents the current distribution at two resonance points, 3.45 GHz (resonance point 1) and 4.1 GHz (resonance point 2), when Ant3 is excited. When there is no grounded strip, that is, L1 = 0 mm, as shown in Figure 7, there are two working modes of Ant3; the resonance point of working mode 1 is 3.45 GHz, and the resonance point of working mode 2 is 4.1 GHz. It can be seen that working mode 1 is not desired, because Ant3 and Ant4 are in mixed working mode at this time. The current loop of this mode is formed by the combination of the current on Ant3 and Ant4 and the current flowing through the ground plane; there must be strong coupling in this working mode. The operating mode 2 is generated by Ant3 and Ant4 separately. Ant3 and Ant4 have separate current loops in this mode, but the isolation of the antenna unit is still poor at this time, because there is a coupling current in the space. rent on the excitation antenna Ant1, it will affect the impedance matching of the antenna. From the results in Figure 6, the introduction of Via5 enhances the isolation but sacrifices part of the bandwidth of the antenna.

Decoupling Principle between Ant3 and Ant4
For the edge-fed dipoles Ant3 and Ant4, the grounded strip is the key to decoupling between Ant3 and Ant4, and its length L1 is a key parameter. Figure 7 presents the current distribution at two resonance points, 3.45 GHz (resonance point 1) and 4.1 GHz (resonance point 2), when Ant3 is excited. When there is no grounded strip, that is, L1 = 0 mm, as shown in Figure 7, there are two working modes of Ant3; the resonance point of working mode 1 is 3.45 GHz, and the resonance point of working mode 2 is 4.1 GHz. It can be seen that working mode 1 is not desired, because Ant3 and Ant4 are in mixed working mode at this time. The current loop of this mode is formed by the combination of the current on Ant3 and Ant4 and the current flowing through the ground plane; there must be strong coupling in this working mode. The operating mode 2 is generated by Ant3 and Ant4 separately. Ant3 and Ant4 have separate current loops in this mode, but the isolation of the antenna unit is still poor at this time, because there is a coupling current in the space.

Decoupling Principle between Ant3 and Ant4
For the edge-fed dipoles Ant3 and Ant4, the grounded strip is the key to decoupling between Ant3 and Ant4, and its length L 1 is a key parameter. Figure 7 presents the current distribution at two resonance points, 3.45 GHz (resonance point 1) and 4.1 GHz (resonance point 2), when Ant3 is excited. When there is no grounded strip, that is, L 1 = 0 mm, as shown in Figure 7, there are two working modes of Ant3; the resonance point of working mode 1 is 3.45 GHz, and the resonance point of working mode 2 is 4.1 GHz. It can be seen that working mode 1 is not desired, because Ant3 and Ant4 are in mixed working mode at this time. The current loop of this mode is formed by the combination of the current on Ant3 and Ant4 and the current flowing through the ground plane; there must be strong coupling in this working mode. The operating mode 2 is generated by Ant3 and Ant4 separately. Ant3 and Ant4 have separate current loops in this mode, but the isolation of the antenna unit is still poor at this time, because there is a coupling current in the space.   Figure 8 shows the current distribution on the two antenna elements when Ant3 is excited. In Figure 8a, when L1 = 2 mm, the resonance frequency is 3.6 GHz. In Figure 8b, when L1 = 4 mm, the resonance frequency 3.3 GHz. At these two frequency points, the  Figure 8 shows the current distribution on the two antenna elements when Ant3 is excited. In Figure 8a, when L 1 = 2 mm, the resonance frequency is 3.6 GHz. In Figure 8b, when L 1 = 4 mm, the resonance frequency 3.3 GHz. At these two frequency points, the working mode 1 can no longer be effectively excited. The resonance point shifts to the low frequency band with the increase of L 1 . At the same time, when L 1 = 4 mm, only a small amount of energy is coupled into Ant4 at this time, indicating that this working mode can meet isolation requirements when the length of the ground branch is large enough.  Figure 8 shows the current distribution on the two antenna elements when Ant3 is excited. In Figure 8a, when L1 = 2 mm, the resonance frequency is 3.6 GHz. In Figure 8b, when L1 = 4 mm, the resonance frequency 3.3 GHz. At these two frequency points, the working mode 1 can no longer be effectively excited. The resonance point shifts to the low frequency band with the increase of L1. At the same time, when L1 = 4 mm, only a small amount of energy is coupled into Ant4 at this time, indicating that this working mode can meet isolation requirements when the length of the ground branch is large enough.

Ant4 Ant3
3.6GHz L 1 =2mm In order to illustrate the role of the parameter L1 more clearly, Figure 9a shows the simulated results of the parameter analysis of L1. It can be seen that with the increase of L1, the electrical size of the antenna increases, and S33 shifts to the low frequency band. But the S11 increases, sacrificing some bandwidth. When L1 = 3 mm, S43 starts to be less than −10 dB. In order to illustrate the role of the parameter L 1 more clearly, Figure 9a shows the simulated results of the parameter analysis of L 1 . It can be seen that with the increase of L 1 , the electrical size of the antenna increases, and S 33 shifts to the low frequency band. But the S 11 increases, sacrificing some bandwidth. When L 1 = 3 mm, S 43 starts to be less than −10 dB. Since L1 affects the matching, S33 and S43 will change with L1 at the same time. As shown in Figure 9b, with the complementary changes of L1 and L2, the resonant frequency point hardly changes. When L1 = 3 mm, the isolation of the entire frequency band is above 20 dB. Therefore, for Ant3 and Ant4, the length of the L1 strip must be greater than 3 mm, because the isolation can meet the requirements at this time. Then, the length L2 of the coupling feed branch is adjusted to make the antenna work at the desired frequency band. Table 1 shows the comparison between this work and previous works. Comparing the antenna size and integration, the proposed antenna shows a high compactness and realizes an excellent integration degree, which integrates four elements on an area of only 28.7 × 7 mm 2 .  Since L 1 affects the matching, S 33 and S 43 will change with L 1 at the same time. As shown in Figure 9b, with the complementary changes of L 1 and L 2 , the resonant frequency point hardly changes. When L 1 = 3 mm, the isolation of the entire frequency band is above 20 dB. Therefore, for Ant3 and Ant4, the length of the L 1 strip must be greater than 3 mm, because the isolation can meet the requirements at this time. Then, the length L 2 of the coupling feed branch is adjusted to make the antenna work at the desired frequency band. Table 1 shows the comparison between this work and previous works. Comparing the antenna size and integration, the proposed antenna shows a high compactness and realizes an excellent integration degree, which integrates four elements on an area of only 28.7 × 7 mm 2 .

Results and Discussion
In order to verify the performance of the antenna, the prototype of the antenna is fabricated, as shown in Figure 10. The prototype is measured with a vector network analyzer Agilent E8363B and a microwave chamber. Figure 11 presents the measured reflection coefficient and transmission coefficient. For simplicity, the reflection coefficient and isolation of only one antenna pair are given. The results show that if −6 dB is used as a criterion of reflection coefficient, the bandwidth of the four antenna elements of one antenna pair can cover 3.4-3.6 GHz, and the isolation between any two elements is greater than 10 dB.  In order to quantitatively evaluate the diversity performance of the proposed MIMO antenna, the ECC of the antenna is calculated based on the radiated far-field using simulation software; the formula is as follows [29]: 22 12 , , where Ei (θ,φ) is the radiation pattern of the antenna when port i is excited. Diversity gain (DG) is an important parameter to measure good diversity characteristics and MIMO system performance. According to [30], DG can be calculated from Formula (2), and the ideal value of DG should be 10 dB.  In order to quantitatively evaluate the diversity performance of the proposed MIMO antenna, the ECC of the antenna is calculated based on the radiated far-field using simulation software; the formula is as follows [29]: where Ei (θ,φ) is the radiation pattern of the antenna when port i is excited. Diversity gain (DG) is an important parameter to measure good diversity charac- In order to quantitatively evaluate the diversity performance of the proposed MIMO antenna, the ECC of the antenna is calculated based on the radiated far-field using simulation software; the formula is as follows [29]: where E i (θ,ϕ) is the radiation pattern of the antenna when port i is excited. Diversity gain (DG) is an important parameter to measure good diversity characteristics and MIMO system performance. According to [30], DG can be calculated from Formula (2), and the ideal value of DG should be 10 dB.
It can be seen from Figure 12 that the ECC in the desired frequency band is less than 0.35, indicating that the presented antenna has good diversity performance. It can be seen from Figure 12 that diversity gain values are all around 10 dB, meeting the MIMO diversity criterion. Due to the symmetry of the antenna, only the antenna efficiencies of Ant1 and Ant3 are calculated. As shown in Figure 13, the efficiencies of Ant1 and Ant3 are 52.2-57.8% and 50.2-64.8%, respectively, over the entire operating frequency band.   Figure 14a shows the measured radiation pattern of Ant1 and Ant3 at 3.5 GHz in the XOY plane. As shown in Figure 14a, the radiation zero point of Ant1 is at the center of the antenna, and the radiated energy is concentrated in the positive and negative directions of the X axis, while the Ant3 has the largest radiation energy at the center, but due to the effect of the metal floor, the energy is concentrated in the +Y direction.    Figure 14a shows the measured radiation pattern of Ant1 and Ant3 at 3.5 GHz in the XOY plane. As shown in Figure 14a, the radiation zero point of Ant1 is at the center of the antenna, and the radiated energy is concentrated in the positive and negative directions of the X axis, while the Ant3 has the largest radiation energy at the center, but due to the effect of the metal floor, the energy is concentrated in the +Y direction.  Figure 14a shows the measured radiation pattern of Ant1 and Ant3 at 3.5 GHz in the XOY plane. As shown in Figure 14a, the radiation zero point of Ant1 is at the center of the antenna, and the radiated energy is concentrated in the positive and negative directions of the X axis, while the Ant3 has the largest radiation energy at the center, but due to the effect of the metal floor, the energy is concentrated in the +Y direction. Figure 14b shows the measured radiation pattern of Ant1 and Ant3 at 3.5 GHz in the XOZ plane. As shown in Figure 14b, the radiation zero point of Ant1 is at the center of the antenna, and the radiated energy is concentrated in the positive and negative directions of the X-axis; Ant3 has the largest radiation energy at the center, but due to the influence of the metal floor and surrounding radiation patches, the energy is not concentrated in the positive and negative directions of the Z axis, but is shifted. It can be known from the radiation patterns of Ant1 and Ant3 that the designed antenna has good diversity characteristics. The simulated 3D radiation patterns of Ant1 and Ant3 at 3.5 GHz are presented in Figure 15.  Figure 14a shows the measured radiation pattern of Ant1 and Ant3 at 3.5 GHz in the XOY plane. As shown in Figure 14a, the radiation zero point of Ant1 is at the center of the antenna, and the radiated energy is concentrated in the positive and negative directions of the X axis, while the Ant3 has the largest radiation energy at the center, but due to the effect of the metal floor, the energy is concentrated in the +Y direction.  Electronics 2022, 11, x FOR PEER REVIEW 10 of 11 Figure 14b shows the measured radiation pattern of Ant1 and Ant3 at 3.5 GHz in the XOZ plane. As shown in Figure 14b, the radiation zero point of Ant1 is at the center of the antenna, and the radiated energy is concentrated in the positive and negative directions of the X-axis; Ant3 has the largest radiation energy at the center, but due to the influence of the metal floor and surrounding radiation patches, the energy is not concentrated in the positive and negative directions of the Z axis, but is shifted. It can be known from the radiation patterns of Ant1 and Ant3 that the designed antenna has good diversity characteristics. The simulated 3D radiation patterns of Ant1 and Ant3 at 3.5 GHz are presented in Figure 15.

Conclusions
This article presents a compact 8 × 8 MIMO antenna for 5G terminals, which consists of a pair of quad-element antennae. The quad-element antenna is composed of two T-shaped monopoles and two edge-coupled dipoles. The quad-element is placed in a compact area. The antenna adopts polarization diversity technology, parasitic elements and grounding strips to realize decoupling between antenna elements. The measured working frequency band of each element satisfies 3.4-3.6 GHz, and the isolation is above 10 dB. The ECC in the desired frequency band is less than 0.35, and the efficiency is over 50% in the operating bandwidth. The measured results agree well with the simulation. Owing to the convenience and flexibility of the self-decoupled design with a wideband performance, the proposed structure will be widely employed in 5G broadband MIMO antennas. This design provides an important reference for future compact MIMO antenna design.

Conflicts of Interest:
The authors declare no conflict of interest.