Abstract
A compact planar multiple-input multiple-output (MIMO) antenna array for 5G band is proposed in this paper. To improve the isolation of compact microstrip antenna array, this paper mainly presents an electromagnetic resonant ring method for MIMO antenna array. The proposed antenna can cover both the 3.3-3.6 GHz and 4.8-5 GHz bands proposed for the 5G band. The antenna proposed in this paper consists of two symmetrical meandered monopole radiators, grid structures, and a Y-shape element. Two different sizes of split-ring resonators (SRRs) are used to suppress the interference of the coupled signal to the antenna system; thereby it can reduce the mutual coupling effect. The experimental results show that the mutual coupling between the two elements is below -25 dB in both of the bands after adding the SRRs. And this antenna is only 23×19 mm2. Its compact size and structure can be used as a mobile terminal antenna.
1. Introduction
With the rapid development of wireless communication technology, mobile communication has been integrated into all aspects of life. Nowadays, fifth generation is becoming a hot spot in the field of global research because of its faster transmission rate and higher performance [1]. At present, the fifth generation of mobile communications (5G) of the band is identified as two frequency bands in 3.3-3.6 GHz and 4.8-5 GHz. Total of 500 MHz bandwidth of these frequencies can be reached. And in communication systems, multiple-input multiple-output (MIMO) techniques can significantly improve spectrum utilization and channel capacity without increasing transmit power and adding additional transmission bandwidth [2, 3]. However, as the miniaturization and portability of wireless devices become mainstream, the available space for antennas is more limited. Therefore, it is very necessary to design a compact MIMO antenna. In order to adapt to the commonly used wireless devices, printing MIMO antennas is a necessary choice. However, there is strong mutual coupling when the distance between MIMO antenna elements is very close. It is contrary to the desire for higher isolation and lower envelope correlation coefficients. Hence, it is indispensable to reduce the mutual coupling between the antenna elements.
From the existing literatures, the researchers have done a lot of work in reducing coupling and have proposed various solutions. In order to suppress the propagation of surface waves and reduce the coupling, a parasitic branching method [4] can usually be used. There are also many methods for improving the electromagnetic wave propagation in a certain frequency band by using an electromagnetic band gap structure [5, 6]; they can form a frequency band gap and a phase band gap characteristic to improve isolation. Changing the current propagation path to restrain coupling multiple often chooses neutralization line techniques [7, 8]. Besides, it is also common to use a defective ground structure [9–11] as a band-stop filter to achieve decoupling. In addition to the traditional methods, the method of using the special properties of metamaterials to achieve the desired effect is also mentioned in [12–14]. But the above-mentioned methods are having insufficient space on a compact MIMO antenna and are difficult to function. Therefore, this paper proposes to reduce the coupling by using two different sizes of split-ring resonators (SRRs).
This paper presents a compact binary array printed MIMO antenna. The antenna can cover the 5G band recently proposed by the Ministry of Industry and Information Technology of China. The proposed antenna has a symmetrical structure. And it uses microstrip feed line. In this paper, electromagnetic waves resonate when they arrive at the SRRs, thereby effectively impeding the propagation of the electromagnetic wave and reducing the mutual coupling between the two antenna elements. The second part introduces the specific structure and size of the antenna. The third part gives the detail of simulated and measured results of the antenna. At the same time, the performance of the antenna and decoupling structure are analyzed and discussed in this section. Finally, the proposed MIMO antenna is summarized.
2. Antenna Design
The proposed antenna is printed on the Rogers 4003C substrate of thickness 0.406 mm with a dielectric constant of 3.38 and loss tangent of 0.0027. The size is only 23×19 mm2, which is considered compact enough. The metal here is a 0.035 mm thickness copper with a conductivity of 5.8 × 107 S/m. The simulated model of the proposed MIMO antenna is shown in Figure 1. This antenna uses the microstrip feed line method. It can be seen from the front view (shows in the Figure 1(a)) that the proposed MIMO antenna is consisted of two monopole radiators and a Y-shape connected to the ground through metal via hole. And the bottom of the antenna constitutes ground and two meandered lines strips connected to the respective radiators, which are connected to the radiators through metal via holes as shown in the Figure 1(b).
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In the MIMO antenna model proposed in this paper, the resonant frequency and the resonant intensity are mainly determined by the parameters of each structure. Different structures and shapes can form different electrical lengths, resulting in different resonant frequencies. The right side of the monopole radiation element at the top of the dielectric slab and the corresponding bend line at the back mainly affect the frequency of 4.9 GHz. The length of the short side of the Y-shaped structure plays a major role at the resonant frequency of 3.52 GHz. By analyzing the structure that affects the resonant frequency, the size of the structure can be adjusted to achieve the change of the resonant frequency.
Although the coverage of the required frequency band can be achieved, the antenna distance is very close, only 0.18; the two array elements have highly coupled. It is a common problem in MIMO applications. Therefore, in order to reduce the mutual coupling between the antenna elements and improve the isolation, the decoupling structures shown in Figures 1(c) and 1(d) are added. Figure 1(c) illustrates that six smaller SRRs are added to the front of the antenna. In Figure 1(d), three larger SRRs are added to the bottom of the antenna. Considering several situations, we finally chose to place SRRs on both sides. The combination of these decoupling structures can well restrain the mutual coupling at the bands of 3.52 GHz and 4.9 GHz.
The specific size and distance of each structure are finally determined by electromagnetic simulation software CST Microwave Studio, as shown in Table 1. In order to have a good impedance matching with a 50-Ω standard SMA connector, the width of the feed line is designed to be 1 mm.
3. Simulation and Experimental Results
In this section, we will discuss the antennas’ simulated and measured results. To verify the simulation, a prototype of the proposed antenna is constructed and measured based on the design and dimensions thereof described in Figure 1. Due to limitations of the manufacture technology, the metal via hole embedded into the dielectric are fabricated as electroplated with a channel diameter of 0.05 mm. The experimental test in this paper uses a vector network analyzer (Agilent E8362B). In experimental measurements, the proposed MIMO antenna is fabricated on a 0.406 mm thick Rogers 4003C substrate. The deposited copper thickness was 0.035 mm. The photograph of the fabricated antenna prototype is shown in Figure 2. The top and the bottom of original antenna are given in Figures 2(a) and 2(b), and Figures 2(c) and 2(d) show the top and bottom of decoupling antenna, respectively. It can be intuitively seen that this MIMO antenna is compact from the fabricated antenna.
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3.1. S-Parameters
The simulated and measured S-parameters of the proposed MIMO antenna are sketched in Figures 3(a) and 3(b), respectively. During the test, the two ports of the antenna are, respectively, connected to the two ports of the vector network analyzer Agilent E8362B. Comparing Figures 3(a) and 3(b); the overall trend of the measured results is consistent with the simulated results, but there is a slight deviation. That is, the resonance points obtained by the test are all biased to the high frequency. It may be due to the frequency limit of the SMA connectors and the printed circuit board manufacture tolerance.
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It can be noted from the S-parameters shown in Figure 3 that the proposed antenna has a good effect in the above-mentioned frequency bands. Each antenna can cover two frequency bands of 5G, and its return loss S11 can basically reach below -10 dB. With the addition of SRRs, the isolation of all corresponding bands is better than 25 dB, improving about 10 dB. At the same time, the resonant frequency of the antenna is substantially not offset. The return loss at 3.52 GHz is also slightly better than the original antenna. It can be noticed from the return loss and isolation that the MIMO antenna proposed in this paper performs well. In the measured results, the centre frequency is slightly shifted to the high frequency due to factors such as dielectric plate loss and test feeder loss. There are some errors between the measured results and the simulated results. This may be the reason for the compact size of the antenna and the workmanship is imprecise. However, after adding the SRRs, the overall performance of the antenna is better than the original one.
The original antenna we designed in this paper has a Y-shape structure in the middle of the two radiation elements. The Y-shape structure in this design mainly serves to enhance the radiation effect of the resonant point. When there is no Y-shape structure, although S21 is lower than S11 at the resonance point, it is apparent that S11 at the resonance frequency is less than -10 dB, as shown in Figure 4. S11 with/without Y-shape structure is plotted in Figure 4(a). And Figure 4(b) shows the curves of S21 with/without Y-shape structure. Comparing the red solid line and the black dash line in Figure 4(a), the resonance of antenna is significantly enhanced in the desired frequency bands after adding the Y-shape structure, and the design requirements are met.
Normally, the ground plane will also participate in the radiation of the antenna, so its size will also affect the S-parameters. Using the control variable method, the other parts remain the same size, only the width of the ground plane is changed (the size of w2), as shown in Figure 5. It can be known from the figure that the resonant frequency will shift to the high frequency as the width of the ground plane increases, both in the low frequency and in the high frequency bands. When w2=5 mm, the performance of S11 is best in the 3.3-3.6 GHz band, but the S11 in the 4.8-5 GHz band is slightly smaller and more biased towards low frequencies. When w2=7 mm, it has a slightly higher frequency in the 4.8-5 GHz band and S11 is slightly smaller, but its resonant frequency is already higher than 3.6 GHz in the band of 3.3-3.6 GHz and does not meet the < -10 dB requirement. Therefore, considering the comprehensive consideration, it is finally determined that the width of the ground plane is 6 mm.
Since the SRR can be equivalent to an LC series resonant circuit, its resonant frequency can be obtained substantially by . L is the full inductance of the SRR. When calculating the SRR’s inductance, it can be equivalent to a full-closed loop inductance. The split of the two rings can be equivalent to the capacitance C, which is the full capacitance of the SRR. The initial size of the SRR is obtained according to the formula mention above and then optimized by using CST Microwave Studio to obtain the final size. Take the distance between the larger SRRs on the bottom of antenna (the size of ub_w1) as an example for discussion. The S-parameter when ub_w1 is changed can be seen from Figure 6. The change of ub_w1 has little effect on S11, but it has a significant impact on S21. When ub_w1=1.22 mm, the frequency of coupling reduction at 3.52 GHz shifts to higher frequency. When ub_w1=0.22 mm, the decoupling effect is not significant at 3.52 GHz. Therefore, ub_w1=0.72 mm is the most ideal choice.
3.2. Radiation Patterns
To further validate the influence of the SRRs in the MIMO antenna, simulated and measured radiation patterns are plotted in Figures 7 and 8, respectively. Due to the symmetry of the antenna design, the antenna pattern shows good complementarity and orthogonality (the antenna’s maximum radiating area is complementary and the main lobe is orthogonal). Therefore, only given the case when port 1 is fed, and port 2 is symmetric with it, the E planes (yOz) and H planes (xOz) at the two resonance points when port 1 is excited are shown in the figures. Among these, Figures 7(a) and 7(b) show the simulated E planes and H planes at 3.52 GHz. Figures 8(a) and 8(b) are the measured results at 3.52 GHz. And the E planes of the simulated and measured at 4.9 GHz are drawn in Figures 7(c) and 8(c). Figures 7(d) and 8(d) describe the simulated and measured results of the H plane at 4.9 GHz. In these figures, original antenna is drawn with black solid lines, and the improved antenna is drawn with red dashed lines. During the measurement, one port is excited and the other port is connected to the 50-Ω load. From the given results, the radiation omnidirectionality of the antenna is well. The observed radiation patterns show a good agreement with the simulated results and the measured results. Some differences here are possibly due to the factor that the test environment is nonanechoic chamber. It is evident from the figures that loading the SRRs has little notable effect on radiation performance of the antenna. A slight inconsistency did not worsen the radiation performance of the antenna. The overall radiation performances of the antenna, such as gain and efficiency, are consistent with the original one.
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3.3. Surface Current
In order to study the mutual coupling between the two antenna elements more intuitively and analyze the working mechanism of the SRRs at great length, the simulated results of CST are used to observe the surface current distribution of the antenna. Through these results, the influence of the SRRs on the MIMO antenna is discussed. In this paper, the decoupling structure is mainly composed of two different sizes of SRRs on the front and back sides. The surface current distribution of the antenna with or without SRRs at the excitation of port 1 at 3.52 GHz and 4.9 GHz is displayed in Figure 9. By comparing Figures 9(a) and 9(b), it can be seen that after SRRs are added, most of the current is concentrated to the SRRs on the back side, so that less current can reach the side of port 2, thus effectively reducing the mutual coupling. From the comparison of Figures 9(c) and 9(d), it can be concluded that the original antenna is without SRRs; the current can flow unimpeded to the side of port 2, and thus it has a high coupling. After loading the SRRs; the current propagation is limited, and the port 2 and the radiation of the right side cannot be reached directly, so the mutual coupling is suppressed. Therefore, introducing SRRs can reduce the coupling current of the nonexcited antenna elements. This is because when the coupling current passes through the metal resonant ring, it can effectively restrain the current radiated to the nonexcited antenna element, thereby improving the mutual coupling. For achieving the optimal decoupling effect, the size and position of the SRRs are determined through multiple simulations and optimizations. From the simulated results and the measured results, the proposed decoupling structure has a good effect, which provides a thought for MIMO antenna decoupling and has a certain prospect.
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3.4. ECC
The envelop correlation coefficient (ECC) is an important figure of merit for any MIMO enabled antenna systems. It can be calculated from the S-parameters of the antenna [15], as formula (1). In formula (1), and represent the conjugates of S11 and S21, respectively. The calculated ECC less than 0.5 is the most basic requirement for MIMO antenna. A comparison of ECC computed from S-parameters for MIMO antenna with and without SRRs is exhibited in Figure 10. It is clear that after adding the SRRs, ECC of the antenna is significantly reduced and is less than 0.3 in the required operating frequency band. That means the antenna with SRRs ensures good diversity performance.Furthermore, in order to distinguish our results from other existing equivalent antennas, the properties of the geometrical dimensions, the relationship between wavelength and size, type of substrate, the decoupling method, and other properties are compared in Table 2. In Table 2, λ represents the dielectric wavelength at the high frequency resonance point of the antenna. The maximum size of the antenna can be obtained by the relationship between λ and size. It is evidently illustrated from the table that the proposed MIMO antenna has a more compact size. And it can cover 5G frequency bands very well.
4. Conclusions
A compact 5G MIMO antenna with SRRs has been proposed in this paper. Two-element microstrip MIMO antenna with two different sizes of SRRs is studied. Improvement in the antenna mutual coupling has been achieved using this method. The studied indicate that the mutual coupling in the MIMO antenna loaded by SRRs is suppressed effectively, and the improved MIMO antenna has a better performance. The simulated and measured results show a well agreement. It verifies the good performance of the MIMO antenna. The isolation between the antenna elements is better than -25 dB over all required frequency bands. This simple, compact, low-cost design makes it suitable for applications such as mobile phone terminals for the fifth generation of mobile communications.
Data Availability
The dimension parameters of proposed structure data used to support the findings of this study are included in the article. The dimension parameters of the proposed structure are presented in Figure 1 and Table 1.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (no. 41704176, 41574131), the National Key Research and Development Program of China (no. 2017YFF0105704), and the Fundamental Research Funds for the Central Universities from China.