# A Dual-Band 8-Antenna Array Design for 5G/WiFi 5 Metal-Frame Smartphone Applications
## 1 Introduction  
The advent of the fifth-generation (5G) internet protocol has spurred a substantial demand for high-speed mobile data services. Achieving high transmission efficiency, low latency, and minimal energy consumption poses a significant challenge in current mobile communication systems. The 3 rd Generation Partnership Project (3 GPP) has identified three sub-6 GHz operating bands for 5G New Radio (NR): N77 (3.3-4.2 GHz), N78 (3.3-3.8 GHz), and N79 (4.4-5.0 GHz). In 2017, China allocated spectrum for 5G systems in sub-6 GHz, designating N78 (3.4-3.6 GHz) and N79 (4.8-4.9 GHz) as the operational frequency bands. On one hand, multiple-input multiple-output (MIMO) antenna technology excels in enhancing system capacity and mitigating multipath interference effects. Various MIMO antennas operating in the sub-6 GHz spectrum have been documented [1; 2; 3; 4; 5; 6; 7; 8; 9]. Lower-order systems like 2 \(\times\) 2 and 4 \(\times\) 4 have gained popularity for fourth-generation (4G) smartphones in long-term evolution (LTE) bands [3]. The efficacy of massive MIMO antennas in achieving larger channel capacity and higher transmission rates for 5G systems is evident, garnering attention from both the industry and academia. Noteworthy MIMO antenna designs include folded monopoles and gap-coupled loop branches for dual-band operation [4], double-branch monopoles with a T-shaped decoupling stub for mutual coupling reduction [5], and an eight-port triple-band MIMO antenna for 5G metal-frame smartphones [6]. A multiband 10-antenna array for sub-6 GHz MIMO applications and a 10-port MIMO antenna with two types of antenna modules are presented in [7; 8], respectively. An eighteen-antenna system compatible with a massive MIMO/Diversity 5G smartphone is proposed without external decoupling structures [9]. On the other hand, size constraints in multi-band MIMO systems often lead to strong mutual coupling effects between antenna elements. Decoupling methods such as decoupling branches, defected ground structures (DGS), neutralization lines (NL), parasitic elements, and metamaterials have been proposed to address these challenges. In this paper, a variety of defective ground structures are used for antenna design. The defected ground structures of the microstrip line are implemented by making an artificial defect on the ground and provide a resonance property in transfer characteristics [10]. This structure can change the distribution of the effective dielectric constant of the antenna dielectric substrate, thereby changing the distributed inductance and distributed capacitance of the microstrip line based on the medium, thereby giving such microstrip lines band gap characteristics and slow wave characteristics. According to the circuit theory [11], parallel RLC circuits work as a band-stop filter. By applying the same concept to defected ground structures in microstrip technology, the vertical slot of defected ground structures accumulates charge and increases the effective capacitor of the microstrip line. Two defected areas on both sides and one connecting slot correspond to the equivalently added inductance L and capacitance C, respectively. The defected ground structures can provide additional effective inductance of the transmission line. The increase in effective inductance from insertion of the defected ground structures can provide a longer electrical length of transmission line than that of a conventional line, which enables size reduction of the microwave and millimeter circuit. Compared with other structures, the defected ground structure has a simple structure; it has the advantages of simple production, small size, and easy integration, so it also has a certain competitiveness in the field of antenna applications [12]. Additionally, the metal frame is a distinctive feature of full-screen smartphones, gaining attention in recent years [13; 14; 15; 16].