## 3 Antenna Design
### Analysis of the Graphene Plasmonic MIMO Configurations  
The evaluation of the two-element graphene plasmonic MIMO antenna structure is carried out and optimized using CMA for three different configurations. Firstly, in Ant. A structure, the model significance, and the model current distribution curves at the resonance frequency of 3.5 THz are investigated in Figures 10a and 11a, respectively. The model significance curve indicates the first four modes of operation through the entire band. It can be seen that the four modes play a significant role in the desired bandwidth (\(M_{\mathrm{s}}\geq 0.707\)), but modes 1 and 2 give a high contribution in the desired bandwidth rather than modes 3 and 4. Moreover, modes 1 and 2, have the highest values (close to unity). So, it is expected that one of these modes is responsible for the antenna resonation at 3.5 THz and the other one is responsible for coupling between the two-elements' graphene plasmonic MIMO antenna. The model current distribution makes the divergence between the first two modes. From the current distribution depicted in Figure 11a; mode (\(\mathrm{J_{n1}}\)) is the dominant mode that is responsible for the antenna resonation and mode (\(\mathrm{J_{n2}}\)) causes the coupling between the two-radiating elements of the MIMO antenna because of the  
Figure 8: The proposed three different MIMO antenna configurations (**a**) 0\({}^{\circ}\) orientation, (**b**) 90\({}^{\circ}\) orientation, (**c**) 180\({}^{\circ}\)orientation.  
Figure 9: S–parameters (S\({}_{11}\) and S\({}_{21}\)) of the three different MIMO configurations.  
high current density between the elements which, in turn, affects the overall antenna performance negatively. Therefore, it is imperative that the characteristics of this mode need to be tuned again with a certain approach. In addition, mode (Jn3) and mode (Jn4) have no impact on the coupling between the MIMO elements. Secondly, in Ant. B, the model significant curve shown in Figure 10b illustrates the first four modes. It can be seen that all modes have the opportunity to contribute to the radiating bandwidth (\(M_{s}\geq 0.707\)) with different levels of acceptance such as modes 1 and 3, which have the highest values of \(M_{s}\), while modes 2 and 4, have the lowest one. So, the model significant property is not sufficient to specify all the modes and clarify which of them is responsible for the coupling, non-coupling, and resonance effect. The model current distribution in Figure 11b is a good candidate that is used to illustrate this point. This figure reveals that modes (Jn3) and, (Jn4), have a current null between the antenna elements, and mode (Jn1) is the resonance one while mode (Jn2) is the coupling mode that needs to be characterized and tuned again to reduce the mutual coupling between the graphene plasmonic MIMO antenna elements. Finally, in Ant. C configuration, the model significant curve depicted in Figure 10c demonstrates the first four modes that participate in the desired bandwidth (\(M_{s}\geq 0.707\)). It is interesting to note that, modes 1,2, and 3 only contribute to the desired bandwidth of the antenna as they have the highest value of \(M_{s}\). However, mode 4 can be neglected because of its low value of \(M_{s}\) within interested bandwidth. This result can be assured by introducing the model current distribution in Figure 11c. From this curve, we can overlook mode (Jn3) and (Jn4) as the current density concentration is very weak between the patches. So, mode (Jn1) is the resonance mode, and mode (Jn2) is the coupling one. The model significance and model current distribution curves of the proposed three configurations of the MIMO antenna reveal that the configuration of the designed antenna is responsible for the degree of the coupling between the two elements. The model current distribution indicates that the best configuration is Ant. C which gives a low coupling between the patches and low current density concentration in comparison with Ant. A and Ant. B.