## 6 MIMO Antenna Performance Enhancement Techniques
### Reconfigurability  
The reconfigurable antenna structure can provide lower mutual coupling as well as a high value of diversity gain, and high efficiency, wide bandwidth, and front-to-back ratio can also be achieved [30; 33; 73; 74; 81; 82; 88]. These antennas are either a single element or multielement. The main focus of these enhancement techniques is to improve the antenna performance throughout in terms of mean effective gain (MEG), envelop correlation coefficient ECC, and 38.1 bps/Hz channel capacity.  
A comprehensive and summarized detail of MIMO antenna designing techniques as well as performance enhancement techniques in terms of mutual coupling reduction is shown in Table 5. Table 6 elaborates the comparison of different techniques employed for mutual coupling reduction/decoupling. in this summery, the comparison of different techniques employed for mutual coupling reduction/decoupling is presented.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Reference** & **Performance** & **Advantages** & **Disadvantages** \\
**Antennas** & **Enhancement Techniques** & **Andvantages** & **Disadvantages** \\ \hline \multirow{3}{*}{[90]} & \multirow{3}{*}{Neutralization Lines} & This technique is mostly used in & \\  & & compact-size antenna to decouple the & \\  & & closed interelement coupling. & \\ \hline \multirow{3}{*}{[91; 92]} & Dielectric Resonator & Bandwidth Gain and & \\  & Antenna & Efficiency Enhancement. & \\ \hline \multirow{3}{*}{[93; 94]} & \multirow{3}{*}{Neutralization Lines} & This technique is mostly used in & \\  & & compact-size antenna to decouple the & \\  & & closed interelement coupling. & \\ \hline \multirow{3}{*}{[95; 96]} & Frequency Reconfigurable & It provides feasibility for antenna to be in & \\  & Antenna & compact size and excellent provision for & \\  & & compactness dimensionally. & \\ \hline \multirow{3}{*}{[97; 98]} & \multirow{3}{*}{Slot or Parasitic element} & Diversity gain, bandwidth, and efficiency & Designing and decision about \\  & & could excellently be improved. & \\ \cline{1-1}  & & enhancement in the diversity gain, & \\ \cline{1-1}  & Metamaterials & bandwidth and & \\ \cline{1-1}  & Decoupling Network & envelop correlation coefficient (ECC) & \\ \cline{1-1}  & & Diversity gain and impedance matching & \\ \cline{1-1}  & & could be improved effectively. & \\ \hline \end{tabular}
\end{table}
Table 5: Mutual Coupling Reduction/Decoupling Techniques Advantages and Disadvantages.  
Some techniques are used not only for gain and bandwidth enhancement in single element and arrays but also used for mutual coupling reduction or decoupling the correlated closed elements' place on the same single PCB. Like metamaterials, researchers have used metasurface structure for bandwidth enhancement in [31].  
The same metameterial plays a vital role in MIMO as it is used for mutual coupling reduction due to its unique double-negative (DNG) characteristics. Different geometrical shapes are used by different researcher for achieving the goal of wide bandwidth, high gain and mutual coupling reduction. The role of metamaterials could not be ignored as they have excellent characteristics.  
The Antenna in [82] offers 2.6 times a 4 \(\times\) 4 MIMO antenna. All parameters effecting the performance of antenna such as SAR in terms of user hand effect is also taken in consideration.