## 2 Decoupling Techniques (DTs) in Ultra-Wideband MIMO Antennas (UMAs)
### Placing AEs at 90\({}^{\circ}\) and 180\({}^{\circ}\)
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & \multicolumn{2}{c}{**Contribution**} & \multicolumn{2}{c}{**Inferences**} \\ \hline
[34] & DTs like DGS, DS, metamaterial, electromagnetic band gap (EBG), NL, PE, and complementary split ring resonator (CSR) were discussed & DTs like carbon black film (CBF), slotted edge substrate (SES), 90\({}^{\circ}\), 180\({}^{\circ}\) position, etc., were not explained & DTs like carbon black film (CBF), slotted edge substrate (SES), 90\({}^{\circ}\), 180\({}^{\circ}\) position, etc., were not explained & DTs like carbon black film (CBF), slotted edge \\ \hline
[35] & Introduced isolation enhancement of the selected antennas & Detailed literature survey on isolation DTs was not presented \\ \hline
[36] & Discussed DTs of MIMO antennas with various frequency bands & Not focused on UMAs \\ \hline This Work & \(\bullet\) & Discussed all DTs reported so far in detail & \(\bullet\) & Almost all DTs are discussed in the proposed review paper \\  & \(\bullet\) & Various parameters of the reported UMAs are analyzed & \(\bullet\) & Performance of the previously reported \\  & such as antenna size, substrate used, isolation, & \(\bullet\) & UMAs has been examined to determine \\  & bandwidth, and gain & & research gaps \\  & Simulated study of all DTs has been carried out by & \(\bullet\) & A simulated study of DTs illustrates which \\  & implementing various types of DTs on the same antenna elements & \(\bullet\) & DT provides high isolation, better matching, \\  & \(\bullet\) & Comparative analysis of various DTs has been carried & and better radiation characteristics \\  & out for both with and without CG & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._radiators with several slots loaded into them, which were placed orthogonally, as displayed in Figure 2h. In [45], the AE consisted of a circular patch that offers better impedance matching over the UWB range without the use of a feeding line, as shown in Figure 2i. The four AEs were arranged across the four edges of a common square-shaped ground to design the MIMO antenna. In [46], a coplanar waveguide (CPW)-fed quasi-elliptical-self complementary antenna exhibiting super-wideband characteristics with dual-band notches was presented. In order to achieve better inter-element isolation and polarization diversity, the four radiating elements of the MIMO antenna were placed orthogonally to each other. Each MIMO antenna element had an elliptical-shaped conductor patch and a slot, which was a complement of similar shape, both present on the same side of the substrate, as displayed in Figure 2j. The isolation can also be improved by placing AEs at a 180\({}^{\circ}\) rotation. In the literature survey, many UMAs were designed with 180\({}^{\circ}\) rotation. In reference [47], the AE had a flower-shaped patch with microstrip feeding, as displayed in Figure 3a. The edges of the ground were chamfered, and an open-end slot was inserted near the feeding point for bandwidth improvement. The AEs were placed at 180\({}^{\circ}\) to each other to obtain high isolation. In reference [48], a two-port UMA was designed, where the first AE consisted of a rectangular patch and the second AE had a tapered slot, which were fed by two microstrip lines, as shown in Figure 3b. To improve impedance matching, a circular coupling design was used to feed the slot antenna. The parasitic resonance was produced by a ground stub, which helped enhance the bandwidth of the monopole AE. High isolation was obtained by just placing AEs in a 180\({}^{\circ}\) location. In the article [49], the MIMO antenna was composed of dual UWB-based AEs. The antipodal Vivaldi and circular-shaped monopole element were positioned on the two opposite edges of the rectangular-shaped ground surface, as presented in Figure 3c. The two AEs were able to stimulate distinct modes on the ground surface, resulting in the required distinct radiation (far-field) patterns and highly isolated AEs.