## 2 Structure of the Proposed Handset  
The particular geometry and dimensions of the proposed miniature eight-port antenna array are displayed in Figure 1. In Figure 1a, two small PCBs (printed circuit boards) and a system circuit board form a big U-shaped tridimensional substrate that imitates a handset edge frame with a size of 150 \(\times\) 75 \(\times\) 7 mm\({}^{3}\). The thickness of the FR4 (flame retardant 4) substrate is 0.8 mm, the dielectric constant is 4.4, and the loss tan is 0.02. The outside of the two small PCBs and the back side of the system circuit board are grounded with copper. Each small PCB has four antenna elements. The distances between Ant.1 and the frame edge corner, Ant. 1 and Ant. 2, and Ant. 2 and Ant. 3 are 22.5 mm, 20 mm, and 37.8 mm, respectively. Antenna locations are placed according to the worst isolation situation of the antenna array. The Wang-shaped radiator is on the inside of the small PCB and three split-ring resonators. A square frame with a width of 1 mm and external dimensions of 6.2 mm \(\times\) 6.2 mm encloses the Wang-shaped radiator. The square frame is connected to the Wang-shaped radiator through a short wire at the center of the Wang-shaped feeder unit, facilitating a connection between the inner and outer frames.  
There are three SRRs on the outside of the small PCB. The SSRs are made up of 0.3 mm strips to ensure consistent structure. These strips, as they are bent into various shapes and their sizes change, collaborate with the internal Wang-shaped radiator to achieve functionality. The shapes of the three SRRs are not identical, and their positions correspond to those of the internal Wang-shaped radiating element. The opening positions of each SRR correspond to the internal Wang-shaped radiating element. The width of the feeding line is 1.5 mm, which can achieve the best impedance matching at 3.3-5 GHz. The system circuit board has a cylindrical hole with a radius of 0.6 mm for connecting the SMA (sub-miniature A) probe head from bottom to top. The geometry and detailed dimensions of the proposed wideband antenna element are listed in Figure 1b (inner copper) and Figure 1c (outer copper). The antenna element has a Wang-shaped feed element and an SRR structure on the metal rim, which resonates at 3.5 GHz, 4.3 GHz, and 4.9 GHz, separately. The Wang-shaped feed element is placed at the internal surface of a small PCB. The S-parameter array of the proposed miniature eight-port antenna array is first displayed in Figure 2. The range of reflection coefficients less than \(-10\) dB are shown in Figure 2.  
ranges from 3.3 GHz to 5.02 GHz, while the transmission coefficients are all less than \(-10\) dB within this range. It should be noted that although the MIMO antenna array proposed in this paper has eight antenna elements, each antenna element has identical geometry. According to different antenna element locations, it can be divided into two types, namely Ant. 1 and Ant. 2. Ant. 1, Ant. 4, Ant. 5, and Ant. 8 are all located on the outside of the antenna bezel on each side and, thus, present the same results. Comparatively, Ant. 2, Ant. 3, Ant. 6, and Ant. 7 are located on the inner side of the antenna bezel, which means that these four antennas present the same result. Ant. 1, Ant. 2, Ant. 3, and Ant. 4 are symmetrical about the system. Furthermore, Ant. 5, Ant. 6, Ant. 7, and Ant. 8 are symmetrical with Ant 1, Ant. 2, Ant. 3, and Ant. 4 about the overall system. In addition, the most sensitive isolation can be represented by the couplings between Ant. 1 and Ant. 2, Ant. 2 and Ant. 3, Ant. 1 and Ant. 3, Ant. 1 and Ant. 4, and Ant. 1 and Ant. 8.