## 3 Antenna Design
### Two Elements Graphene Plasmonic MIMO Antenna Configurations  
The MIMO system's advantage is that it provides data rate augmentation even in the presence of signal fading, multipath fading, and interference. The MIMO system was designed to meet the requirements of acceptable MIMO parameters because of the need to transmit more data over greater distances. Mutual coupling between elements in a MIMO array could alter the performance of a single-element antenna. The distance between the MIMO units regulates this mutual coupling, which is formed via electromagnetic interaction between them. The optimum separation distance between adjacent MIMO elements is the system's most important goal. An antenna with a large distance between elements ensures non-correlation but increases the overall antenna size, whereas a small distance will increase the mutual coupling between the elements. So, mutual coupling is an important parameter that should be studied that faces the MIMO antenna systems. The two-element graphene plasmonic microstrip MIMO patch antennas are illustrated in the following part. In order to create the optimal MIMO antenna configuration, it is necessary to first determine the orientation of the optimal elements to achieve the desired high level of isolation. Figure 8 demonstrates the proposed three configurations that are used in the antenna design. The first configuration (Ant. A) is shown in Figure 8a the two elements have a 0\({}^{\circ}\) orientation side by side with the first element. The second configuration (Ant. B) is shown in Figure 8b where the two elements are orthogonally oriented with a 90\({}^{\circ}\) between them. In the last configuration (Ant. C), the two elements have a 180\({}^{\circ}\) orientation as shown in Figure 8c. The simulated reflection coefficients \(\mid\)S\({}_{11}\mid\) and transmission coefficients \(\mid\)S\({}_{21}\mid\) versus frequency of the three MIMO configurations are investigated in Figure 9. The transmission coefficients \(\mid\)S\({}_{21}\mid\) are an important parameter while designing the MIMO system. It is the measurable parameter of the mutual coupling between the MIMO elements.  
These are three possible orientations that can be used using two elements MIMO antenna. Figure 9 illustrates the \(\mid\)S\({}_{11}\mid\) parameters of the three MIMO configurations that are designed to resonate at 3.5 THz. The \(\mid\)S\({}_{21}\mid\) curves reveal that Ant. C has the lowest mutual coupling of \(-\)25.6 dB within the entire interested frequency band rather than the other configurations of Ant. B that has a \(-\)22.4 dB in addition, the Ant. A has a mutual coupling of \(-\)18 dB The three graphene patch MIMO configurations are printed on Teflon substrate with a relative permittivity (\(\varepsilon_{r}\)) of 2.1 and a thickness of 10 \(\mu\)m. The CMA is used to describe the behavior of the graphene plasmonic MIMO antenna configurations with the help of model significant (Ms), and the model current distribution for the three configurations of Ant. A, Ant. B and Ant. C.  
Figure 7: \(\mid\)S\({}_{11}\mid\) parameters of the graphene plasmonic antenna for different chemical potential.