# Minimized Triple-Band Eight-Element Antenna Array for 5G Metal-frame Smartphone Applications
## 1 Introduction  
5G mobile communication technology has plenty of advantages, such as high communication capacity, high mass connection density, and high transmission rate. However, with the limited internal space of the universal smartphones, the number of antenna elements will be influenced. In order to solve this problem, each antenna element should be minimized [1, 2, 3, 4], and operate at dual-band [5, 6, 7, 8, 9, 10, 11, 12] or broadband [13, 14, 15, 16, 17]. By sharing a common grounding branch for the two adjacent antenna elements, a compact self-decoupled antenna was obtained in [1, 2, 3]. In [4], the height of the small printed circuit boards (PCBs) placed vertically to the system PCB is only 3.8 mm, which is a novelty for compact antennas. By using a multi-slot decoupling technique, a dual-band eight-element antenna array is proposed in [5, 12]. A single antenna element consisting of two radiators was presented for dual-band operation [6]. A folded monopole and a gap-coupled loop branch were combined together to obtain two broad bands [7]. By adjusting the impedance ratio of the stepped impedance resonators and the position of the microstrip feed line, a dual-band eight-antenna array is proposed for the 5G mobile application [8]. Two double-branch monopoles and a T-shaped decoupling stub were utilized for dual-band operation [9]. In [10], two different antenna elements, namely the folded L-shaped antenna and the couple-fed U-shaped antenna, were applied for LTE band 42 and 5.8-GHz WLAN band, respectively. A modified E-shaped strip on the corner of the frame was employed to realize a dual-band MIMO antenna [11]. Wideband MIMO antenna array can cover 5G sub-6 GHz bands, which was a good option for smartphones [13, 15, 16, 17]. A systematic design of a high-performance eight-element antenna array was introduced for broadband operation [14].  
On the other hand, isolation between antenna elements should be optimized for better performance [18, 19, 20, 21, 22]. A self-isolated MIMO antenna system was introduced in [18], which was achieved by introducing two vertical stubs into the original antenna element. By using a novel isolation technique named building block, a gap-coupled ion antenna anda loop antenna are isolated from each other [19]. A novel balanced open-slot antenna is designed as an array antenna element generating a balanced slot mode that can enhance the isolation [20]. The parasitic elements and defective ground can be used to obtain an outstanding isolation [21]. Different from the above isolation techniques, the author utilized a pattern diversity that two adjacent ports worked simultaneously but were excited at a relative phase in [22], which was proved to have good isolation.  
This paper proposes a MIMO antenna array for triple-band 5G metal-frame smartphone applications. An S-shaped feeding strip and an L-shaped radiation strip on the frame are utilized to achieve good performance. By using an S-shaped feeding strip, we can improve the bandwidth of the lower frequency band with minimized size. The structure of the antenna element is only 6.5 mm \(\times\) 7 mm, which is the innovative point of this paper. The \(-\)6 dB impedance bandwidth of the proposed eight-antenna array can fully cover 3.3-3.8 GHz, 4.8-5 GHz, and 5.15-5.925 GHz frequency bands.