# A Dual-Band 8-Antenna Array Design for 5G/WiFi 5 Metal-Frame Smartphone Applications
## 2 Proposed MIMO Antenna
### Antenna Element  
Each antenna element consists of a radiator, feeding line, and a defected ground plane. The radiator takes the form of a Chinese character \(\mathrm{\SIUnitSymbolMicro E}\)-shaped strip, which consists of three horizontal rectangular microstrip lines and two longitudinal rectangular microstrip lines, precisely printed on the inner surface of the side-edge substrate. The defected ground plane is composed of a rectangular strip connected to an inverted L-shaped strip, and another rectangular strip attached to an inverted L-shaped strip and a C-shaped strip. The feeding line is printed on the top surface of the main substrate, establishing a connection with the radiator. Four antenna elements are printed on the inner surface of each side-edge substrate. To enhance isolation between these elements, two distinct decoupling structures (\(\mathrm{DS_{1}}\) and \(\mathrm{DS_{2}}\)) are employed, as depicted in Figure 1d. Among them, the decoupling structure 1 consists of a T-shaped slot and an I-shaped slot, while the decoupling structure 2 is composed of an inverted T-shaped slot and an I-shaped slot. These structures are etched on the outer surface of the side-edge substrates and the bottom surface of the main substrate, effectively extending the coupling current path and minimizing interaction between different antenna elements. Viewed from different directions, the specific structure of the metal frame is as shown in Figure 1e, which shows the positional relationship of the structures mentioned above. Viewed from the inside of the antenna, the orthogonal structure of the \(\mathrm{\SIUnitSymbolMicro E}\)-shaped strip is located on the inner surface of the side dielectric substrate, which can just cover the defected ground structure. These two structures are interconnected, jointly forming the radiating unit of the antenna during operation. In the figure, the defective ground plane of the antenna unit is located directly outside the \(\mathrm{\SIUnitSymbolMicro E}\)-shaped strip. Looking from the outside of the mobile phone, the defective ground plane, T-shaped slot, and inverted T-shaped slot mentioned above are both located on the outer surface of the metal frame. Although the above two structures have the same principle, their distinct locations endow them with varied functions. The defected ground plane and the orthogonal structure constitute the antenna element, which is used to generate the resonant frequency at which the antenna operates, whereas the slots on the metal frame and the defective slots of the metal ground plane form a decoupling structure to enhance isolation between each antenna port. Due to variations in the location and radiation environment of each antenna unit, the reflection coefficient of each unit differs slightly, despite their identical size. With the antenna elements sharing the same dimensions and structures and being symmetrically placed alongside decoupling structures, the eight units can be categorized into two groups: Ant. 1, Ant. 4, Ant. 5, and Ant. 8 form one category, while the remaining four antennas compose another. Therefore, only the reflection coefficients of Ant. 1, Ant. 2, Ant. 3, and Ant. 4 are given in Figure 2a. The antenna operates within the frequency range of  
Figure 1: (**a**) Geometry of the proposed antenna array, (**b**) detailed structure of the radiator, (**c**) detailed structure of the defected ground plane, (**d**) detailed structure of the decoupling structures, (**e**) the relative position of various structures (all values are in millimeters). 2.55-4.2 GHz and 4.4-6 GHz, effectively covering 5G China Bands N78 (3.4-3.6 GHz), N79 (4.4-5.0 GHz), and WiFi 5 (5.15-5.85 GHz). In this paper, S18 denotes the transmission coefficient between antenna elements 1 and 8, where S18 = S45, S12 = S34 = S56 = S78, S23 = S67, and S14 = S58. Figure 2b showcases the antenna's isolation, revealing that all ports maintain isolation levels exceeding 16.5 dB, ensuring independence between antenna units and preventing mutual interference.