# Uni-Planar MIMO Antenna for Sub-6 GHz 5G Mobile Phone Applications
## 1 Introduction  
For fifth-generation (5G) communication, most research societies are concentrating on obtaining high throughput and high data rates at a cheap cost [1, 2]. It is predicted that the 5G systems will have a data rate 1000 times faster compared with the fourth-generation (4G) communication systems. Such a high data rate can be achieved with the help of multiple-input multiple-output (MIMO) technology [3, 4]. In MIMO, the data rate can be improved by reducing multipath fading or by utilizing multiple independent channels. One of the major keys to boosting the channel capacity is to design several antennas for each channel [4]. The MIMO antenna must have a low mutual coupling, which is a prerequisite for a 5G mobile communication system [4]. Moreover, the 5G MIMO antennas can offer better diversity and multiplexing gain [5], which leads to enhanced channel capacity [6]. In the literature, several MIMO antennas have been presented for sub-6 GHz applications. In [7], an eight-element MIMO structure was developed for the 3.5 GHz spectrum. A symmetrical open-fork slit antenna with an L-shaped feeding strip was designed. Thisconfiguration offers isolation of \(>\)17.5 dB between antenna elements, but the design suffers because of poor antenna efficiency. The same kind of configuration with a rectangular-shaped slot was presented in Ref. [8]. The MIMO arrays consisted of two kinds of antenna elements for different frequency operations. Two elements of the same configuration were placed on the top and bottom corners of the printed circuit board (PCB) for 2G/3G/4G communication, while eight-elements meant to operate at 3.5 GHz were placed on the left and right sides of the PCB. The authors in [9] designed an eight-port broadband MIMO antenna for sub-6 GHz services. The single antenna of the MIMO system was made up of a 50 \(\Omega\) feed line with an open circuit tuning stub, a slot on the metal frame, and a U-shaped slot on the ground plane. The MIMO antenna provided a broadband response in the frequency range of 3.3-6 GHz. In [10], a monopole slotted MIMO antenna was presented for the 2.6/3.5 GHz bands. The four metal frame-based antennas fed using an L-shaped feed line were positioned in the middle of the PCB, while the other elements were placed on the top and bottom sides of the PCB. In [11], a MIMO antenna array was designed for 3.5 GHz 5G smartphone applications. The array was made up of two distinct ways: one was an L-shaped coupled-fed array and the other was a U-shaped loop array, both of which were mounted on the smartphone's metal frame. For isolation enhancement, the authors created an inverted-I slot and a neutralization line between the antenna components, resulting in 15 dB of isolation in the operational bandwidth. In [12], a dual-polarized diamond ring slot-based MIMO antenna was designed for 5G massive MIMO applications. Four dual-fed slot elements were placed on each corner of the mobile phone PCB. An L-shaped microstrip feed line was used to feed the slot elements. The designed antenna element offered a stable gain of 3 dBi for the band of interest and \(>\)80% radiation and total efficiencies. In addition, an isolation of more than 20 dB was observed within the operating range. In [13], the same kind of configuration was utilized to design MIMO antenna array for sub-6 GHz applications. In the presented configuration, a circular-ring resonator was etched from the ground plane and fed using orthogonally placed microstrip feed lines. The authors also designed a multi-band MIMO antenna by replacing the conventional microstrip feed line with a fork-shaped feed line. In [14], an eight-port MIMO antenna based on an I-shaped element was developed for IoT and 5G technologies. To provide space for additional components, the cellphone's metal frame was constructed to accommodate the antenna components. The antenna was found to resonate effectively in the 3.5 GHz frequency band.