# Compact Sub 6 GHz Dual Band Twelve-Element MIMO Antenna for 5G Metal-Rimmed Smartphone Applications
In [15], the authors proposed a 2 \(\times\) 2 MIMO array covering the entire long-term evolution (LTE) band (698-960 and 1710-2690 MHz) and a 4 \(\times\) 4 MIMO array covering 5G bands. When the radio frequency switch (PIN diode) loaded on the LTE antenna structures was turned off, the resonance was shifted to a lower spectrum of 698-770 MHz. In addition, the LTE antenna structures included a complex matching network of capacitors and inductors that could broaden the bandwidth. In [16], the authors presented a novel dual-port single-dipole MIMO antenna pair, which used a feeding loop to selectively excite the half-wavelength dipole mode and a feeding capacitor to excite the full-wavelength dipole mode. A dual-port single-loop antenna unit between the ground plane and metal frame was proposed in [17]. To isolate the two antenna ports, a basic decoupling capacitor was employed at the center of the loop resonator. As a result, even though the two antenna ports shared a single loop structure, high port-to-port isolation was achieved. All of the MIMO antennas in these studies [14; 15; 16; 17; 18] achieved impedance matching or improved isolation. However, they included capacitors or inductors, which increased both the complexity and cost of the antenna systems. The development of a 12 \(\times\) 12 MIMO antenna for smartphones was able to improve the transmission rate, capacity, anti-interference ability, coverage, and positioning accuracy of wireless communication systems, thereby improving the performance and user experience of wireless communication [19; 20; 21; 22; 23; 24]. A twelve-element inverted-F antenna array for smartphones was constructed in [19]. The array used the hybrid decoupling method. Multiple decoupling capacitors were used to couple two back-to-back inverted-F antenna pairsand T-shaped mode transformers for the other couplings. A hybrid twelve-element MIMO antenna array for 5G smartphones was presented in [21]. The proposed antenna comprised four identical three-element antenna arrays. Two L-shaped monopole slots on the ground were placed symmetrically on both sides of each M-shaped loop antenna. Because these two antenna types had orthogonal polarization, coupling was reduced. In [21], the authors proposed a dual-band twelve-element MIMO antenna system with a T-shaped slot for each antenna. They applied spatial diversity and orthogonal antenna placement (i.e., feeding strips of two antennas are not pointing at each other) to improve isolation. However, none of the proposed twelve-element MIMO antenna systems of these studies [19; 20; 21; 22; 23; 24] have been verified for use in smartphones with metal frames. An antenna configuration must be compact, should be effective in a metal frame, and should achieve high throughput by using multiple antennas. We therefore propose a compact configuration for a 5G dual-band 12 \(\times\) 12 MIMO antenna system that is integrated with the metal frame on a smartphone. The first feature of the proposed compact antenna design is a distance between two elements of only 3 mm. Both sides of each element have short-circuit branches, which reduces the impact of the ground plane current on adjacent antennas. Therefore, the proposed antenna design can achieve good isolation without decoupling elements and has narrow spacing between elements of only 3 mm. The second feature is the structural design of an inverted L-shaped feeding branch and two inverted L-shaped short-circuiting branches in the clearance area. Because the length of the inverted L-shaped feeding branch and the configuration of the two inverted L-shaped short-circuiting branches were adjusted, the operating frequency bands were impedance matched at 3300-3600 MHz and 4800-5000 MHz. The third feature is that the profile height of the antenna is only 3 mm, enabling it to fit in a narrow frame. The resonance path covers part of the metal frame, and the opening position of the metal frame is used to excite the low and high-frequency resonance paths.