High-Isolation UWB MIMO Antenna with Multiple X-Shaped Stubs Loaded between Ground Planes

A miniaturized ultra-wideband multiple-input multiple-output (UWB MIMO) two-port antenna with high isolation based on FR4 is designed in this article. -e size of the antenna is only 18× 28×1.6mm. -e MIMO antenna consists of two identical antenna elements symmetrically placed on the same dielectric substrate in opposite directions. By loading three crossed X-shaped stubs between two unconnected ground planes, high isolation and good impedance matching are achieved. -e working frequency band measured by this UWB MIMO antenna is 1.9–14GHz, and the isolation is kept above 20.2 dB in the whole analysis frequency band. Good radiation characteristics as well as envelope correlation coefficient (ECC, <0.09), mean effective gain (MEG), and channel capacity loss (CCL) in the passband meet the requirements of the application, which can be applied to the UWB wireless communication system. To verify the applicability of the proposed method for enhancing the isolation between antenna elements, the two-port antenna structure was extended to a four-port antenna structure. In the case of loading the X-shaped stubs to connect to the ground plane, the isolation of the antenna is maintained above 15.5 dB within 1.7–14GHz.


Introduction
With the rapid development of the wireless communication field, people's requirements for communication quality and high speed increase accordingly. To improve the system capacity (by applying multiplexing technology) and reliability (by applying diversity technology), the multiple-input multiple-output (MIMO) technology is proposed. By using the advantages of these two aspects, the transmission signal is appropriately encoded and decoded. MIMO is a technology that uses multiple antenna elements at both ends of the transmitter and receiver. To make the MIMO system work normally, it is necessary to reduce the correlation of the antenna elements in the system [1]. For this reason, several methods for reducing the correlation between elements in an antenna array in a MIMO system have been proposed, for example, defect ground structure decoupling [2,3], parasitic stub decoupling [4][5][6], neutralization line decoupling [7][8][9], diversity technology decoupling [10][11][12], and electromagnetic band-gap (EBG) structure decoupling [13,14].
Especially in ultra-wideband (UWB) systems, it is difficult to achieve long-distance transmission due to the disadvantages of low radiation power and multipath fading. erefore, the combination of UWB technology and MIMO technology can transmit multiple paths at the same time. Without increasing the antenna's radiation power, the transmission distance of the UWB system is significantly improved [15]. e ultra-wideband multiple-input multiple-output (UWB MIMO) technology has been deeply studied by many scholars because of its high transmission rate, good communication quality, and large channel capacity.
In the design of UWB MIMO antennas, the ground plane structure has a separate ground plane [16,17] and a common ground plane. In the design process of the same MIMO antenna structure, a separate ground plane is usually better than a shared ground plane in terms of isolation between antenna elements. Considering that the MIMO antenna connected to the ground plane will be better applied to the commercial field [18], the decoupling structure of the MIMO antenna that shares the ground plane is further analyzed.
Reference [19] loads an inverted T-slot at the top of the center of the common ground plane of two rectangular antenna elements so that the isolation of the antenna is maintained above 24.5 dB within 3-5 GHz. Loading a vertical rectangular slot near the bottom of the ground plane reduces the isolation of the antenna in the range of 5-10.6 GHz and keeps the isolation of the antenna above 20 dB within 3.1-10.6 GHz. Reference [20] reduces the coupling between antenna elements by loading slotted rectangular parasitic stubs on the ground plane and keeps the isolation of the antenna above 20 dB within 2-13.7 GHz. Reference [21] connects two circular radiating patches by loading a wideband neutralization line between two antenna elements. e current coupling of the ground plane is reduced so that the isolation of the antenna is maintained above 22 dB within 3.1-5 GHz. Reference [22] designed a four-port MIMO antenna. rough the use of polarization diversity technology, the isolation of the antenna is maintained above 17 dB within 3-13.2 GHz. In addition, loading a set of slots on the ground plane further reduces the envelope correlation coefficient of the MIMO antenna. Reference [23] designed a four-port double-sided UWB MIMO antenna. By loading the EBG structure on the front and back of the antenna, the isolation of the antenna was kept above 20 dB within 3-11 GHz.
is article proposes a two-port UWB MIMO antenna that uses multiple cross X-shaped stubs to achieve high isolation and a large working frequency band between the ground planes of the antenna elements. While connecting the two ground planes, most of the current is concentrated on the loaded branches, reducing the mutual coupling between the antenna elements. rough measurement, it can be known that the isolation in the working frequency band 1.9-14 GHz is all greater than 20.2 dB, and the envelope correlation coefficient (ECC < 0.09), mean effective gain (MEG), and channel capacity loss (CCL) meet the requirements of practical applications [24]. e designed twoport MIMO antenna is extended to a four-port MIMO antenna with adjacent antenna elements placed orthogonally. Similarly, X-shaped stubs are loaded between the ground planes of the antenna elements placed in the opposite direction, and the two intersecting stubs make the antenna structure share the same ground plane. e simulation shows that the isolation of the antenna is maintained above 15.5 dB within 1.7-14 GHz. It is confirmed that the method of improving isolation adopted by the two-port antenna is also applicable to the design of the four-port MIMO antenna. e smaller antenna size (34 × 34 × 1.6 mm 3 ) makes it easier to integrate with modern wireless communication equipment.

Design of UWB Antenna.
In order not to increase the overall size of the antenna design, the antenna structure is kept compact. UWB characteristics are achieved by using a method of digging grooves on the original microstrip patch antenna structure to change the surface current distribution. We design an improved UWB antenna based on a rectangular radiating patch and microstrip feeder with FR4 material on the dielectric substrate [25]. e structure diagram of the UWB antenna finally designed is shown in Figure 1. Some design parameters of the UWB antenna can be calculated using the wavelength (λ) corresponding to the low frequency (3.7 GHz) of the UWB antenna [26]. e long of the rectangular radiation patch (L P ) satisfies the following equation: e effective length of the antenna is the sum of the long of the rectangular radiating patch (L P ) and the length of the microstrip feeder (L f ), as shown in the following equation: e length of the feed gap is the difference between the length of the microstrip feed line (L f ) and the length of part of the ground plane (L g ), and it satisfies the following equation: e thickness of the dielectric substrate (H) can be calculated by the following equation: rough the use of electromagnetic simulation software to further optimize the antenna structure and calculated parameters, the specific dimensions of the antenna area are marked in Figure 1. e three antenna structures and the simulated S 11 results are shown in Figure 2. It can be seen from Figure 2 that by cutting off the triangular grooves at the edges of the rectangular radiating patch and the microstrip feeder of the antenna, the UWB characteristics with a working frequency band of 3.7-14 GHz are realized.
To study the effect of truncating the long side of the triangular slot on the antenna performance, the other parameters remain unchanged. Figures 3(a) and 3(b) show the S 11 results of the different long sides of the triangular slot truncated by radiation patch (L C ) and the different long sides of the triangular slot truncated by the microstrip feeder (L FC ), respectively. It can be seen from Figure 3(a) that with the increase of L C , the low-frequency point of the antenna in the working frequency band decreases, but the amplitude is smaller. When L C � 6 mm, the performance of the antenna at high frequencies is better than that of the other two values, so L C � 6 mm is selected as the optimal parameter. It can be seen from Figure 3(b) that with the increase of L FC , the working frequency band of the antenna is expanded, and the maximum working frequency band is realized when L FC � 7 mm. erefore, L C � 6 mm and L FC � 7 mm are used as the optimal long-side values of the triangular slot truncated by the antenna's radiating patch and the microstrip feeder.

Design of the Connection Structure of the Ground Planes.
Two identical antenna elements of the above design are placed in opposite directions on the same dielectric substrate 2 International Journal of Antennas and Propagation   International Journal of Antennas and Propagation 3 to form a two-port MIMO antenna, and the spacing between the elements is 4 mm. e material of the dielectric substrate is FR4, the relative permittivity of the substrate is 4.4, the loss tangent is 0.02, and the thickness is 1.6 mm. Figure 4 shows a schematic diagram of the UWB MIMO antenna structure without any decoupling structure. Figure 5 shows the S-parameter results obtained in the electromagnetic simulation software of the antenna structure and the surface current distribution diagram at 5 GHz. It can be seen from the simulation result graph that when the left port of the antenna is connected to the excitation source and the right port is connected to a 50 Ω load, a large amount of current is coupled to the right antenna element, which reduces the irrelevance between the antenna elements. Since the two ground planes of the antenna element are not connected, there is no ground current coupling. e reason for the formation of this coupling mainly comes from the spatial coupling. At the same time, the isolation of the antenna remains above 13.6 dB in the 3.2-14 GHz working frequency band. e low isolation between antenna elements makes it impossible to use in MIMO systems. Although the working frequency band of the antenna meets the design requirements of the UWB antenna, in practical applications, the MIMO antenna needs to share the ground plane with the system, which requires the antenna elements in the MIMO antenna to share the ground plane. MIMO antennas that do not share the ground plane are not suitable for wireless communication systems [27], so the antenna elements need to be ground-connected. Figure 6 shows a schematic diagram of the connection structure of the three ground planes. To further determine the ground connection structure and obtain better UWB MIMO antenna performance indicators, Figure 7 shows the S-parameter simulation results of three connection structures. It can be seen from Figure 7 that the return loss of A1 connection structure and A2 connection structure at 3.7-6.6 GHz and 3.7-6.9 GHz is greater than −10 dB, and this frequency band cannot work normally, thereby reducing the overall working frequency band of the antenna. Also, the isolation in the passband is greater than 12.9 dB and 16.2 dB, respectively. A3 connection structure achieves good impedance matching within 1.9-14 GHz while achieving isolation greater than 18.4 dB. In summary, loading an X-shaped stub between ground planes can more effectively improve the isolation between antenna elements and the working frequency band. For A1 and A2 connection structures, as the coverage area of the connection stubs increases, the isolation of the antenna in the passband is improved, while the S 11 result hardly changes. is article will specifically discuss the impact of the number of loaded X-shaped stubs on antenna isolation in Section 2.3 to achieve higher isolation between antenna elements.
To further study the influence of different connection structures on antenna performance, Figure 8 shows the surface current distribution diagrams of A1, A2, and A3 at 3 GHz. In the figure, the currents distributed on the feeders and radiating patches in the right antenna element of A3 structure are significantly smaller than those of A1 and A2 structures, and most of the current is concentrated on the loaded X-shaped branch. us, it can be verified that A3 structure loaded with an X-shaped stub in Figure 7 can achieve higher isolation between antenna elements and effectively reduce the mutual coupling between the elements. Different connection structures make the surface current distribution paths of the left antenna element connected to the excitation source different so that the frequency bands at which the antenna achieves impedance matching are different. Figure 9 shows the S-parameter simulation results for different numbers of X-shaped stubs. e width of each stub connecting with the ground is 0.5 mm, and the interval between each stub is 0.5 mm. It can be seen from the figure that with the increase in the number of X-shaped stubs, the working frequency band of the MIMO antenna has hardly changed (1.9-14 GHz), while the isolation of the antenna at low       International Journal of Antennas and Propagation 5 frequencies is gradually improved. Although the isolation at low frequencies is further improved when four X-shaped stubs are loaded, the isolation at high frequencies around 8-10 GHz is less than 20 dB, which makes the antenna isolation greater than 19.3 dB in the working frequency band. erefore, the antenna structure loaded with three X-shaped stubs is used as the two-port UWB MIMO antenna decoupling structure designed in this article. While achieving the miniaturization of the antenna (18 × 28 × 1.6 mm 3 ), the isolation of the antenna is maintained above 20.3 dB within 1.9-14 GHz. Figure 10 shows the surface current distribution diagram of the antenna structure with different numbers of X-shaped stubs at 9 GHz. It can be seen from the figure that as the number of stubs increases, the current coupling between antenna elements increases at this frequency. e current coupling situation corresponds to the value of S 21 at this frequency in Figure 9, which verifies the consistency of the results.

Design of Four-Port UWB MIMO Antenna.
On the basis of the decoupling principle adopted by the designed twoport UWB MIMO antenna, the adjacent antenna elements are placed orthogonally to expand into a four-port UWB MIMO antenna. Similarly, an X-shaped stub is loaded between the ground planes of two antenna elements placed in opposite directions, and the two intersecting stubs are connected to the ground planes so that the antennas share the same ground plane. e structure of the four-port UWB MIMO antenna is shown in Figure 11. e simulated Sparameter results are shown in Figure 12. It can be seen from Figure 12 that the working frequency band of the designed four-port antenna is 1.7-14 GHz, the isolation between antenna elements S 12 , S 14 , S 23 , and S 34 is greater than 15.5 dB, and S 13 and S 24 are greater than 16.1 dB. erefore, the isolation of the designed four-port antenna is greater than 15.5 dB within 1.7-14 GHz, achieving good isolation between antenna elements. It is verified that the X-shaped stub connected to the ground plane of the two-port element described in Section 2.2 achieves better isolation and is also applicable to the four-port antenna design.
To study the influence of the number of different X-shaped stubs on the performance of the four-port antenna, Figures 13 and 14 show the S-parameter simulation results of two, four, and six X-shaped stubs and the current distribution diagram at 5 GHz, respectively. Taking into account the symmetry of the antenna elements, the performance of one element can be derived from the performance of other units, so only the simulation results of S 11 , S 12 , and S 13 are given. To ensure the compactness of the antenna design, the four antenna elements are distributed in a limited space. It can be seen from the figure that as the number of stubs increases, the distance between the stubs and the upper surface antenna element decreases. As a result, the current coupled to the stub is extended to the adjacent antenna elements, thereby reducing the overall isolation of the antenna and changing the working frequency band of the antenna. To this end, two X-shaped stubs will be added as the decoupling structure of the designed four-port antenna.

S-Parameters.
In the design process of the UWB MIMO antenna, this article uses electromagnetic simulation software to simulate and optimize the antenna structure and combine the measurement results of the vector network analyzer to comprehensively discuss the performance of the antenna. Considering the symmetry of antenna elements, the performance of one element can be derived from the performance of other elements. erefore, only the S-parameters of the left element are given here. Figure 15 shows the fabricated design of the two-port UWB MIMO antenna. Figure 16 shows the measured S-parameter results of the UWB MIMO antenna. e working frequency band of the antenna measurement is 1.9-14 GHz, and the isolation in the whole working frequency band is greater than 20.2 dB, which has excellent isolation performance. e measurement and simulation results are roughly the same. e main reason for the error is that the measurement environment is not completely shielded, and there are many electromagnetic interferences around. e accuracy of antenna processing, the welding of joints, and the quality of dielectric substrate plates can all cause similar deviations. Figure 17 shows the radiation pattern of the two-port UWB MIMO antenna simulation and measurement. Figures 17(a)-17(d) show the radiation patterns at 4 GHz, 6 GHz, 8 GHz, and 10 GHz frequency points. Each figure contains the co-polarization and crosspolarization patterns of the xoz-plane (E-plane) and the yozplane (H-plane). e left port of the antenna is connected to the excitation source, and the right port is connected to a 50 Ω load. It can be seen from the figure that the cross-  International Journal of Antennas and Propagation polarization pattern of the xoz-plane maintains bidirectional radiation. e co-polarization pattern of the yoz-plane maintains good omnidirectional radiation characteristics. As the frequency increases, although the shape of the copolarization pattern on the xoz-plane and the cross-polarization pattern on the yoz-plane is distorted, the antenna generally exhibits better omnidirectional radiation characteristics in the passband range. Figure 18 shows the simulated and measured gain plots of the two-port antenna. In the working frequency band, the fluctuation range of the simulated gain value is 0.6-6.7 dBi, and the fluctuation range of the measured gain value is 0.4-4.8 dBi. Due to errors in physical processing and interference from the measurement environment, the maximum gain of the measurement antenna is weaker than the simulation result.         International Journal of Antennas and Propagation of the angular density function of the incident wave, respectively. Ω is the solid angle of spherical coordinates [28]. Equation (6) uses the calculated ECC to calculate the diversity gain (DG). Generally, the ECC of the MIMO antenna is required to be less than 0.5. e smaller the ECC, the larger the diversity gain (close to 10 dB). It can be seen from Figure 19 that in the entire working frequency band, ECC is less than 0.09, which is far less than the general requirement for MIMO antennas.

Radiation Characteristics.
Mean effective gain (MEG) is an important parameter to measure the diversity performance of MIMO antennas. It can be used to evaluate the ability of antenna elements to receive electromagnetic signals in a multipath fading environment [29]. e measurement result of MEG is shown in Figure 20.  the working frequency band, and the difference between the two is maintained at ±3 dB, which meets the design requirements of MIMO antennas. e channel capacity loss (CCL) can be used to evaluate the maximum limit value of the signal transmission rate. To increase the transmission rate, under normal circumstances, CCL should be less than 0.4 bit/s/Hz [30]. Figure 21 shows the simulated and measured CCL. It can be seen from the figure that the CCL in the working frequency band is lower than 0.4 bit/s/Hz, which meets the design requirements of the MIMO antenna and can be used in the MIMO communication system. Table 1 lists the performance comparison of the single antenna element, the two-port UWB MIMO antenna, and the four-port UWB MIMO antenna designed in this paper with the two-port and four-port MIMO antennas in the reference. rough the comprehensive analysis of the performance of each antenna given in the  wider working frequency band and higher isolation while ensuring the miniaturization of the structure. e better gain and the ECC that meets the application requirements make the MIMO antenna designed in this article more suitable for miniaturized UWB communication equipment.

Conclusions
We designed a miniaturized high-isolation UWB MIMO antenna in this article. e isolation of the MIMO antenna is promoted by loading three crossed X-shaped stubs between the ground planes of the two antenna elements. Further, we expanded the two-port MIMO antenna to a four-port MIMO antenna. e same connection structure as the ground plane of the two-port antenna is adopted. e two X-shaped stubs loaded between the antenna elements placed in the opposite direction are cross-connected to ensure that the antenna structure shares a ground plane. e simulation results show that the four-port antenna can achieve high isolation (greater than 15.5 dB) with a wide working frequency band (1.7-14 GHz). e measurement results show that the isolation of the two-port antenna in the 1.9-14 GHz working frequency band is greater than 20.2 dB, and the ECC is less than 0.09, which is far less than the generally required value. Good MEG and CCL ensure good MIMO antenna performance. erefore, the UWB MIMO antenna has good UWB performance and excellent isolation and can be used in UWB wireless communication systems.

Data Availability
All data generated or analyzed during this study are included within this article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this article.