# 5G/B5G Internet of Things MIMO Antenna Design
## 2 MIMO Antenna & Design Procedure  
The proposed design is shown in Figure 2, where it has an overall dimension of 120 \(\times\) 60 \(\times\) 10 mm\({}^{3}\), which is comparable to existing designs [18, 19]. The tapered slot and its feeding 50 \(\Omega\) transmission line are printed on the sidewall of the design using a RO-5880 substrate (length (l\({}_{1}\)) = 25 \(\times\) width (h\({}_{1}\)) = 10 mm\({}^{2}\)) with a thickness of 0.254 mm, a loss tangent of 0.0009, and relative permittivity (\(\varepsilon_{r}\)) of 2.2. A detailed view of the tapered slot antenna is shown in Figure 2. The radiating antenna part contains a microstrip feedline of 50 \(\Omega\) and a tapered slot antenna. The tapered slot operates as an open-ended slot antenna at the 3.6 GHz band and, at the same time, as an end-fire Vivaldi antenna at the 28 GHz band, resulting in a shared- aperture and dual-function design. Both the feeding microstrip line and the tapered slot are attached with \(\lambda/4\) (\(\lambda\) is a free space wavelength at 28 GHz) circular stubs [20]. This circular stub is optimized to improve impedance matching. A low pass filter is also added to the 50 \(\Omega\) microstrip line to block the high-frequency signal and to work as a stub at low frequency to improve impedance matching. Hence, the low pass filter provides dual functionality. These features of the proposed design make it attractive for installation in mobile phones and other handheld devices in IoT communications.  
Figure 1: Integrated Sub-6 GHz and mm-wave bands IOT-based communication system and potential utilization of the proposed MIMO antenna design.  
To achieve a pattern of diversity at both Sub-6 GHz and mm-wave bands, the tapered slot antenna design is integrated along the four sides of the substrate, establishing an MIMO antenna design. The performance of Ant. 1 is depicted in Figure 3. It can be seen from the S-parameters curves that Ant. 1 operates at 3.6 GHz in the 3.5 to 3.8 GHz band as an open slot antenna, and at 28 GHz in the 25 GHz to 29 GHz band as a Vivaldi antenna. These observations can be verified by the 3D radiation patterns shown in Figure 3. Ant. 1 provides a wide beam width at the 3.6 GHz band with a 4 dBi realized gain, and a narrower beam width with a higher realized gain of 8 dBi at 28 GHz. Since Ant. 1 is based on a tapered slot structure, the direction of the radiation is end-fire, as shown in Figure 3. The operating band resonances can be tuned by changing the width and length of the tapered slot. A detailed design procedure with all dimensions is provided [20].  
Figure 3: The performance of the proposed MIMO antenna design when only Ant. 1 is excited (operating).  
Figure 2: The proposed MIMO antenna design: (**a**) full configuration; (**b,c**) fabricated prototype.  
Due to large inter-antenna element spacing and antenna placement orientation, the isolation (coupling) between adjacent antenna elements is low, i.e., more than 17 dB between Ant. 1 and Ant. 2. Moreover, the MIMO design has four directional radiation beams pointing to four different locations (\(\varphi=180^{\circ}\), \(\varphi=0^{\circ}\), \(\varphi=-90^{\circ}\), and \(\varphi=90^{\circ}\)) in the azimuth plane (the X-Yplane) at both the 3.6 GHz and the 28 GHz bands. This indicates a low correlation coefficient (\(\varrho\)) value between adjacent antenna radiation beams, i.e., between Ant. 1 & Ant. 2, resulting in a design that is a favorable candidate for MIMO communications [21; 22; 23]. This design is also justified by the 3D radiation patterns shown in Figure 4. The verified simulation and measurement results are presented in Section 3 to validate the design.