# Broadband Dual-Polarized 2 \(\times\) 2 MIMO Antenna for a 5G Wireless Communication System
## 1 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 wireless industry and wireless modules. However, the hybrid monopole/dipole structuregives 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 L-shaped 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-multiple-output (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 \(S_{11}\), 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 \(2\times 2\) 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 \(S_{11}\)), high isolation (dB magnitude of \(S_{21}\)), and low envelope correlation coefficient.