## 2 Antenna Design
### Quad-Port MIMO Antenna  
The geometric layout of the MIMO antenna is presented in Figure 5a, and its design dimensions are provided in Table 1. The MIMO design consists of microstrip-line fed octagonal-shaped four identical antenna elements, which are arranged in orthogonal directions to attain superior performance. The antenna feeding points are represented as port-1, port-2, port-3, and port-4, in the layout diagram. Enhanced isolation was achieved as a result of the orthogonal field produced between the adjacent octagonal-shaped radiating elements, consequently, a reduction in inter-element coupling was noticed. However, due to close proximity of the resonating elements, the weak coupling was observed at the lower frequency range, which can be reduced by introducing decoupling elements on the ground surface. Therefore, on the back-side of the dielectric substrate, three metal strips were introduced between the ground patches of the antenna elements to enhance inter-element isolation. The metal strips restrained the current radiated to the adjacent excited elements, thereby enhancing the mutual coupling level. For realizing the optimal decoupling response, the size and spacing among the strips were determined by analyzing multiple simulations and surface current distribution. In addition, these metallic strips united the ground patches of the four radiators to confirm the same voltage in the designed antenna. In a real system, the resonating patches should have the same reference plane to understand the signal level properly. The MIMO system will not work efficiently if isolated ground planes are present [20]. The antenna fabricated prototype is displayed in Figure 5b. The total size of the presented four-port MIMO/diversity antenna is \(58\times 58\times 0.8\) mm\({}^{3}\).  
The proposed MIMO antenna design steps are demonstrated in Figure 6. Firstly, as illustrated in Figure 6a, four identical octagonal-shaped resonating elements were arranged orthogonally to each other. Sufficient space was provided between the four radiators to suppress inter-element correlation. In step-2, as shown in Figure 6b, a thin strip was inserted in the middle of the MIMO antenna to reduce mutual coupling further. The thin strip suppresses the inter-element coupling considerably, but more isolation was required between the antenna elements to obtain effective notching and diversity performance. Therefore, as illustrated in Figure 6c, a decoupling structure consisting of two strips was introduced in the center of the MIMO antenna (step-3), which reduces mutual coupling up to \(-15\) dB in the working band.  
Figure 4: Surface current distributions at (**a**) 3.5 GHz; (**b**) 5.5 GHz.  
In addition, to obtain more isolation between the resonating elements, a decoupling element comprised of three metal strips was introduced in the middle of the MIMO antenna (shown in Figure 5). The decoupling element with three metal strips offers isolation greater than 18 dB and also unites the ground planes of the four radiators to confirm the same voltage in the proposed MIMO antenna. The mutual coupling comparison of the three MIMO stages is displayed in Figure 6d.  
Figure 5: Proposed UWB MIMO antenna: (**a**) geometric layout; (**b**) prototype.  
Figure 6: Proposed UWB MIMO antenna design steps: (**a**) step-1; (**b**) step-2; (**c**) step-3; (**d**) mutual coupling comparison.