# Compact Sub 6 GHz Dual Band Twelve-Element MIMO Antenna for 5G Metal-Rimmed Smartphone Applications
## 2 Proposed MIMO Antenna System Design  
Figure 1 presents the overall configuration of the proposed sub-6-GHz MIMO twelve-antenna system. The system was designed for a 6.8-inch smartphone with a thickness of 7 mm and overall dimensions of 165 \(\times\) 85 \(\times\) 7 mm\({}^{3}\). The system circuit board is positioned at the center of the metal frame. The board comprises an FR4 glass fiber substrate with the dimensions of 84.2 \(\times\) 164.2 \(\times\) 0.8 mm\({}^{3}\). The metal frame consists of a 0.4 mm thick copper-coated FR4 glass fiber substrate with a 7 mm width. The FR4 substrate has a relative dielectric constant of 4.4 and a loss tangent of 0.02. The dual-band MIMO twelve-antenna units (Ant 1, Ant 2, Ant 3, Ant 4, Ant 5, Ant 6, Ant 7, Ant 8, Ant 9, Ant 10, Ant 11, and Ant 12) are arranged into two sets of six-antenna units, which are diagonally and mirror-symmetrically arranged on both sides of the system circuit board and centered on the long side edge at a distance of 30 mm from the short edges of the phone. The profile height of each antenna element is only 3 mm, and the dimensions of the antenna elements are 105 \(\times\) 3 \(\times\) 3.1 mm\({}^{3}\). All six-antenna units have the same structure and size and are arranged in parallel at a distance of 3 mm from their neighbors.  
This design not only integrates part of the metal frame as part of the antenna resonance path but also uses a specifically designed short-circuit branch on both sides of each antenna structure, which is coupled with the opening position of the metal frame, to excite the low and high-frequency resonance paths on both sides. By arranging the six-antenna elements of each unit in a parallel and codirectional manner with 3 mm spacing, we isolate the low and high-frequency resonance paths between adjacent antennas. Additionally, regardless of whether the low and high-frequency modes are excited, the short-circuit branches can attract a ground plane current, which effectively reduces the flow of the current to adjacent antennas. This design achieves good isolation performance with a spacing of only 3 mm between the antennas without requiring isolation components.  
Figure 2 presents the detailed structure of a sub-6-GHz antenna element of the proposed system. All of the twelve-antenna units have the same structure and size. The antenna dimensions are 15 \(\times\) 3 \(\times\) 3.1 mm\({}^{3}\), and the antennas are configured on the system circuit. Each antenna has a clearance zone without a ground plane with a size of 15 \(\times\) 3 mm\({}^{2}\). The clearance zone features a reversed L-shaped feed branch and two reversed L-shaped short-circuit branches. It is integrated with the upper metal frame to form multiple resonant paths for dual-frequency band sub-6-GHz operation (i.e., 3300-3600 MHz and 4800-5000 MHz). The metal frame has a 1 mm opening above the clearance zone at 4 mm and 10 mm from the left and right ground planes, respectively. This antenna structure has both low and high-frequency resonance modes covering the dual-frequency band operation of 3300-3600 MHz and 4800-5000 MHz. The low-frequency resonance is enhanced by the dual-coupled loop path formed by the feeding strip (\(\overline{\text{AB}}\)), which is coupled to shorting strip 1 (\(\overline{\text{CD1}}\)) and then coupled to the upper edge of the metal frame. The high-frequency resonance is enhanced by the loop path formed by left shorting strip 2 (\(\overline{\text{FD2}}\)), which is coupled to the upper edge of the metal frame. Point A in Figure 2 is the feeding point, which is connected to the inner conductor of the 50-\(\Omega\) coaxial transmission line (50-\(\Omega\) mini coaxial line), and the outer conductor of the 50-\(\Omega\) coaxial transmission line is connected to ground shorting point G on the metal ground plane of the screen.  
Figure 1: Overall configuration of the sub-6-GHz 5G MIMO twelve-antenna system.