Broadband Dual-Polarized 2 × 2 MIMO Antenna for a 5G Wireless Communication System

In this study, we proposed an indoor broadband dual-polarized 2 × 2 MIMO (multipleinput and multiple-output) antenna having dimensions of 240 mm × 200 mm × 40 mm, for application in 5G wireless communication systems. The proposed antenna comprised two vertically polarized circular monopole antennas (CMAs), two horizontally polarized modified rectangular dipole antennas (MRDAs), and a ground plane. The distance between the two MRDAs (MRDA1 and MRDA2) was 70.5 mm and 109.5 mm in the horizontal (x-direction) and 109.5 mm vertical (y-direction) directions, respectively. Conversely, the distance between the two CMAs (CMA1 and CMA2) was 109.5 mm and 70.5 mm in the horizontal (x-direction) and vertical (y-direction) directions, respectively. While the CMAs achieved broadband characteristics owing to the optimal gap between the dielectric and the driven radiator using a parasitic element, the MRDAs achieved broadband owing to the optimal distance between the dipole antennas. The observations in this experiment confirmed that the proposed could operate in the 5G NR n46 (5.15–5.925 GHz), n47 (5.855–5.925 GHz), n77 (3.3–4.2 GHz), n78 (3.3–3.8 GHz), and the n79 (4.4–5 GHz) bands. Moreover, it exhibited a wide impedance bandwidth (dB magnitude of ) of 101% in the 2.3–7 GHz frequency range, high isolation (dB magnitude of ), low envelope coefficient correlation (ECC), gain of over 5 dB, and average radiation efficiency of 87.19%, which verified its suitability for application in sub-6 GHz 5G wireless communication systems.


Introduction
Mid-band 5G, also called the mid sub-6 GHz band, is gaining widespread attention owing to the intensive usage by general 5G subscribers. While some countries have considered expanding their 5G frequency band above 4 GHz, other countries have already done so [1]. Among the various mid sub-6 GHz bands, the n77 (3300-4200 MHz) and n78 (3300-3800 MHz) bands have been widely adopted in several countries; this is only feasible by using a sub-6 GHz 5G antenna operating in the n46 (5150-5925 MHz), n47 (5855-5925 MHz), and n79 (4400-5000 MHz) bands. A high channel capacity, dual-polarization, and broadband are essential factors for wireless communication [2].
The most popular method to realize a broadband dipole antenna is using two orthogonal crossed-dipole antennas with a reflector or cavity [3][4][5][6][7]. However, it is difficult to achieve bandwidth over 100% using the conventional crossed-dipole MIMO antenna. The conventional structures for which it is difficult to achieve broadband characteristics cannot cover all 5G bands in the current situation of expanding 5G bands. In addition, crossed-dipole antennas have little space between the ports. The lack of space between the ports makes it difficult to use relatively big N-type connectors which widely use the Marek Bugaj, Leszek Nowosielski and Sotirios K. Goudos wireless industry and wireless modules. However, the hybrid monopole/dipole structure gives sufficient space between the ports. It makes it easier to use a relatively big N-type connector and connect the antenna with a wireless module.
Many studies on broadband monopole antennas with a parasitic element have been reported [8][9][10]. In study [8], a novel dual band notched monopole antenna with increased bandwidth is proposed. It is a radiator on top of the substrate and ground plane with Lshaped slots and a parasitic at the bottom of the substrate to enhance the bandwidth. Similarly, Refs. [9,10] proposed monopole antennas with parasitic elements.
Several MIMO antennas with shared radiator have been studied [11][12][13]. In study [11], a common radiator coplanar waveguide (CPW) fed a four port multiple-input-multipleoutput (MIMO) antenna for 5G sub-1 GHz, sub-6 GHz and Wi-Fi 6 applications with a shared radiator is proposed. This antenna has relatively bad impedance bandwidth (−6 dB). Since the antenna's impedance bandwidth should be under −10 dB of , it is not suitable for 5G wireless communication. The study [12] proposed a wideband compact Yagi-like directional MIMO antenna with a shared radiator. It also has insufficient impedance bandwidth of 28%. In addition, a wearable four port MIMO antenna with a shared radiator in [13] shows insufficient bandwidth of 4.1%. These studies show that a MIMO antenna with a shared radiator can make the antenna size compact, but it is hard to achieve impedance bandwidth over 100%.
If the MIMO antenna does not have a shared radiator and is composed of independent radiators, it must have considered high isolation and mutual coupling between each radiator. The mutual coupling suppression between independent radiators is essential for the high isolation. There are several mutual coupling suppression techniques such as neutralization lines [14,15], defected ground structure [16][17][18], pin or varactor diode [19][20][21][22], electromagnetic bandgap decoupling structure [23], etc. The easiest way to increase the isolation is to increase the distance between the radiators.
In this study, we propose a simple broadband monopole antenna with a parasitic element and dipole antenna composing a dual-polarized 2 × 2 multi-input multi-output (MIMO) antenna. This MIMO antenna comprises two circular monopole antennas (CMAs) and two modified rectangular dipole antennas (MRDAs) in different biased directions for dual polarization in indoor 5G wireless communication systems. The proposed MIMO antenna exhibited a wide impedance bandwidth (dB magnitude of ), high isolation (dB magnitude of ), and low envelope correlation coefficient. Figure 1 shows the CMA used in this study. The antenna had dimensions 80 mm × 80 mm × 33 mm and comprised driven circular and parasitic circular radiators manufactured on a 0.8 mm thick FR4 substrate (ε = 4.4, tan = 0.02) and a ground plane. The driven radiator (R1), having a diameter of 32 mm (≈λ/2 at 2.2 GHz), was used to determine the resonant frequency, and the driven element was the impedance matched with the gap between the dielectric and driven element. Figure 2 shows the simulated and measured values of the voltage standing wave ratio (VSWR) based on the distance between the driven element and the dielectric (D1). An EM simulation tool Ansys HFSS was used to simulate. It was observed that the smaller the gap between the driven element and ground plane, the better the impedance match. The parasitic radiator (R2), having a diameter of 10 mm (≈λ/2 at 7 GHz), was used to analyze the wide impedance bandwidth. Figure 3 shows the simulated and measured CMA values in the presence and absence of the parasitic element.         Figure 7 shows the simulated and measured VSWR values based on the distance between the radiators in the presence and absence of the ground plane. It was observed that the antenna exhibits the widest impedance bandwidth in the absence of the ground plane. However, the ground plane, which acts as a reflector, was introduced to improve the gain of the MRDA.    GHz. It shows that the MIMO antenna with four CMAs is suitable for 5G wireless communication. However, the MIMO antenna with four CMAs size is large. To reduce the size of the MIMO antenna, this study proposed a hybrid structure MIMO antenna with two CMAs and two MRDAs.  The overall dimensions of the antenna were 242 mm × 200 mm × 40 mm. The two CMAs and MRDAs were vertically and horizontally polarized, respectively. To the CMA, the ground plane is an electrical ground plane. However, the ground plane is not connected to the MRDA, and it acts as a reflector. The distances between the two MRDAs (MRDA1 and MRDA2) were 70.5 mm and 109.5 mm in the horizontal (x-direction) and 109.5 mm vertical (y-direction) directions, respectively. Conversely, the distance between the two CMAs (CMA1 and CMA2) was 109.5 mm and 70.5 mm in the horizontal (x-direction) and vertical (y-direction) directions, respectively. Figure 11 shows the prototype of the proposed 2 × 2 MIMO antenna. Figure 12 shows the simulated and measured values of the proposed antenna. The measured impedance bandwidth of CMA1 and CMA2 (dB magnitude ) was 113.2% (1.94-7 GHz), and that of MRDA1 and MRDA2 (dB magnitude of ) was 101% (2.3-7 GHz). The proposed antenna operated successfully the 5G NR n46 (5.15-5.925 GHz), n47 (5.855-5.925 GHz), n77 (3.3-4.2 GHz), n78 (3.3-3.8 GHz), and n79 (4.4-5 GHz) bands. Figure 13 shows the simulated and measured values of the proposed 2 × 2 MIMO antenna. The isolation between CMA1 and CMA2 is relatively low in the frequency range 1.4-2.88 GHz, which can be improved by increasing the distance between them. However, as the distance between CMA1 and CMA2 increases, the gain decreases relatively. Therefore, the distance between CMA1 and CMA2 was traded off to meet the requirements of isolation and gain.   In Equation (1), ⃗ ( , ) is the field radiation pattern of the antenna at port n. Solving this equation is a complex process, and the field radiation pattern is essential. The field radiation pattern can be defined as Equation (2). In Equation (2), is maximum directivity of the antenna. By using Equation (2), the ECC can be calculated as expressed in Equation (3) [24]. Figure 14 shows the envelope correlation coefficient (ECC) measurements based on the simulated and measured S-parameters:  Figure 15 shows the simulated and measured peak gain. The antenna exhibited a gain of over 5 dB in the 2-6 GHz band. The MRDA exhibited a peak gain of 9.6 dB at 5 GHz and the CMA exhibited a peak gain of 7.07 dB at 4.5 GHz. In addition, it exhibited average measured radiation efficiency (gain to directivity ratio) of 87.19%.    Table 1 presents a comparison among various existing wide band MIMO antennas and the proposed broadband MIMO antenna. It is observed that the proposed antenna provides the widest bandwidth. The proposed MIMO antenna has a bandwidth of 2.3-7 GHz (101.07%) and low envelope correlation coefficient under 0.1 at the operating frequency. It also has a peak gain of 9.69 dBi at 5 GHz.

Conclusions
In this study, we proposed a broadband dual-polarized 2 × 2 MIMO antenna for application in 5G wireless communication systems, comprising two vertically polarized circular monopole antennas, two horizontally polarized modified rectangular dipole antennas, and a ground plane. The CMAs achieved broadband characteristics owing to the distance between the driven element and the dielectric, using a parasitic element, whereas the MRDAs achieved broadband characteristics owing to the distance between the radiators. The dimensions of the proposed MIMO antenna were 242 mm×200 mm×40 mm, and it exhibited an impedance bandwidth (dB magnitude of ) of 101% (2.3-7 GHz). The isolation between the antennas was over 20 dB in the 5G NR n46 (5.15-5.925 GHz), n47 (5.855-5.925 GHz), n77 (3.3-4.2 GHz), n78 (3.3-3.8 GHz), and n79 (4.4-5 GHz) bands. The peak gain was observed to be over 5 dB. The results of this study verify that the proposed 2 × 2 MIMO antenna is suitable for application in indoor 5G wireless communication systems.