###### Abstract
Moreover, there are always some obstacles to the THz wireless communication frequency band such as high attenuation, Multipath fading, absorptions, and path loss. So, the MIMO system is a good candidate to face these problems. In the MIMO antenna, the distance between elements has a very strong impact on the mutual coupling between the antenna elements. Greater mutual coupling occurs as a result of closer separation, which raises the correlation coefficients and reduces efficiency. Thus, it is required to have enough space between elements, which should be taken as \(\lambda/2\), or for even better isolation it can be taken as \(\lambda/4\) of the frequency to have a mutual coupling less than \(-15\) dB [25]. Reduction in mutual coupling leads to reducing the Envelop Correlation Coefficient (ECC), increasing diversity gain (DG), and lowering the Channel Capacity Loss (CCL). It is crucial to note that graphene has inherent benefits in reducing mutual coupling effects because of its short SPP wavelength resulting from the slow propagation speed of SPP waves. As a result, graphene-based antenna arrays with the same spacing between components display fewer mutual coupling effects than metallic antenna arrays of the same distance. Different techniques and strategies are presented for the development of the decoupling methods. A neutralization line is used for isolation improvement between MIMO antenna elements, defected ground structure (DGS), electromagnetic bandgap (EBG), frequency selective surfaces (FSS), and artificial magnetic conductor (AMC) metamaterial structures that are considered effective methods to reduce the isolation between the MIMO antenna elements. The MTM structure is used as a band stop filter to reduce the coupling. Furthermore, the main parameters that the MTM used to enhance the isolation process are the structure form, the position of the unit cell, and the number of elements used. All of these parameters can be analyzed using the characteristic mode analysis (CMA) technique. CMA has become a popular technique for studying and constructing antennas, especially owing to the physical understanding acquired regarding antenna working principles without regard for any specific feeding method. Characteristic mode theory is a method of modal analysis for antennas of any form. In addition, it clearly describes the resonating frequency of certain modes, radiation patterns, and the associated mode current. Fundamental modes are extracted from an eigenvalue issue using the CMA. Calculating Z, the Method of Moments (MOM) impedance matrix, is often required to solve the following eigenvalue equation. \[X(J_{n})=\lambda_{n}R(J_{n}) \tag{1}\]  
where \(X\) and \(R\) are the imaginary and real parts, respectively, of the MOM impedance matrix \(Z\), \(J_{n}^{\prime}s\) are the modes or eigen-currents, and \(\lambda_{n}\) are the eigenvalues. The eigenvalue problem is solved, for each frequency, using the procedure detailed by [PERSON] and [PERSON] [26]. Antenna analysis using the MOM approach requires an understanding of the antenna's structure and its excitation field. In the meantime, just the antenna structure is needed as a known requirement in CMA. The characteristic modes are obtained by solving the following mathematical equations:  
\[\overrightarrow{J}=\sum_{n}a_{n}J_{n} \tag{2}\]  
where \(J_{n}\) is the eigen-current and \(a_{n}\) is the modal weighting coefficient  
\[a_{n}=\frac{V_{n}^{i}}{1+j\lambda_{n}} \tag{3}\]  
where \(\lambda_{n}\) is the eigenvalue and \(V_{n}^{i}\) is the modal excitation coefficient. The modal significance, \(MS_{n}\) represented by the normalized amplitude of the current modes is calculated as follows:  
\[MS_{n}=\left|\frac{1}{1+j\lambda_{n}}\right| \tag{4}\]  
Model significance demonstrated how each mode is near to resonance at each frequency. It reaches the maximum value of \(1\) at its resonance frequency (when \(\lambda_{n}=0\)). The modal significance ranges from \(0<MS_{n}\leq 1\). The resonance mode is associated with a model significance of unity.