# Eight-Port Modified E-Slot MIMO Antenna Array with Enhanced Isolation for 5G Mobile Phone
## 2 Proposed MIMO Antenna Design  
_Design Evolution_  
The proposed MIMO antenna design is shown in Figure 1. Table 1 shows the antenna dimension values. To better comprehend how our proposed antenna originated, we considered five cases as depicted in Figure 2. In case I, a simple E-shape slot was etched on the ground plane, which generated a resonance at 3.54 GHz with an isolation level of 22.2 dB. In case II, the lower side part of the E-shape is cut, it is designated by \(w\) as seen in Figure 1b. For this case (II), the resonance frequency remains unchanged, but the isolation further improved to 22.6 dB. In case III, the structure of case I is modified by adding another bent section in the upper part of the structure pointing toward the inner section of the E-slot. The resonance frequency and isolation values experienced unnoticeable change, except that the S11 has a lower magnitude value when compared to that of case I. The frequency resonance moves to 3.51 GHz for case IV while the isolation drops to 22.1 dB.  
Figure 1: Proposed antenna view (**a**) front (**b**) antenna elements with decoupling element.  
The final proposed E-shaped slot was achieved by combining the concepts from case I-III to come up with the final layout, with the desired center frequency of 3.5 GHz and a superior isolation level of 21 dB. Figure 2b shows the corresponding S-parameters obtained from the evolved E-shaped slot etched on the ground plane. Considering the plotted scattering parameters in Figure 2b, we can conclude that in the case of I to IV, the ground current effect is considerably less, leading to higher isolation. In the final proposed design, it is clear that the introduction of the bent sections resulted in shifting the center frequency to the desired one 3.5 GHz. However, the isolation level is less when compared to the previous cases I to IV. That is to say, the bent section has a direct effect on the S11 result. All the isolation levels are found to be below \(-21\) dB. The result demonstrates that the proposed antenna is a suitable candidate to be employed in smartphone applications in the LTE frequency band 42.  
Figure 3 shows the current distribution on the ground when Ant. 1 is energized while all others are terminated to 50-ohm impedance. A little current spreads across Ant. 2, which is non-desirable and leads to lowering the isolation level. For this reason, we employ an E-shaped resonator to restrict undesirable currents, as illustrated in Figure 3b. With no decoupling structure, the isolation is \(-19\) dB, while the isolation improves to more than \(-21\) dB with the decoupling structure. The advantage of the decoupling structure can further be deduced by looking at Figure 4a,b.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Parameter** & \(a\) & \(b\) & \(d\) & \(g\) & \(h\) & \(I\) & \(L\) & \(m\) & \(p\) & \(t\) & \(w\) \\ \hline Value (mm) & 1.2 & 3.3 & 3 & 5.2 & 5.6 & 20.6 & 17.8 & 1 & 2.2 & 2.8 & 1.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Proposed antenna dimensions.  
Figure 3: Antenna surface current distribution at 3.5 GHz with Ant.1 excited (**a**) with no decoupling element and (**b**) with the decoupling element.  
Figure 2: (**a**) Design evolution (**b**) S-parameters of design evolution.