## 1 Introduction
The mutual coupling interactions between AEs affect the generated radiation patterns, leading to worse performance and failure to meet the operative constraints and requirements [27]. The demand for high-density integrated circuits has propelled research into the innovative designs of compact antennas with high performance [28]. It is a challenge to provide high isolation within closely spaced AEs. And, by introducing any decoupling technique (DT), the overall size of the MIMO antenna may increase. In the case of UMAs, isolation needs to be higher throughout the UWB range. Also, antenna design becomes complex and even high cross-polarization is obtained. Hence, the careful design of a DT in a MIMO antenna that does not degrade its performance is required. There are a number of reasons due to which the UWB antenna is combined with MIMO technology. And, to achieve more than 50 Mb/s data rates, technologies such as MIMO and orthogonal frequency-division multiplexing (OFDM) can be adopted, as recommended in IEEE 802.11n. To reach the target of 1 Gb/s, more advanced techniques should be used. The UWB technology combined with MIMO could provide a solution to this issue [7]. Due to the regulation imposed by the FCC, the permitted power spectral density of the UWB signal is rather limited. This again limits data transmission rates. Incorporating the MIMO technique into UWB provides a viable solution for the restricted access problem of power limitation [29]. The channel capacity of UWB systems is proportional to the number of transmit/receive antennas, and data rates can be significantly further increased through the combination of UWB and MIMO [30]. Some review papers on UMAs were published in the past, as displayed in Table 1. However, in [31; 32], DTs were not explained, and in [33; 34], only a few DTs were discussed. A detailed analysis of DTs was not performed in [35]. The literature survey in [36] was not focused on UMAs. The following are the main highlights of this review article:  
* In the proposed review article, all DTs for UMAs are analyzed in detail. This analysis will help the new researchers to understand all DTs. * Various parameters of the reported UMAs are examined, such as antenna size, substrate used, isolation, bandwidth, and gain. This investigation provides shortcomings of the previously reported UMAs. * A simulated study of all DTs has been carried out through the implementation of various types of DTs on the same two-port UMA, which consists of two microstrip-fed circular-shaped AEs with an open-end-slotted partial ground. This study determines the effect of applying various DTs on antenna parameters, such as impedance matching, isolation, and radiation pattern. * A comparative analysis of various DTs has been carried out both with and without common ground (CG). Using this analysis, one can determine the best DT to obtain better isolation. * This article can save time for the researchers to select appropriate DT without compromising the antenna performance. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ref.** & **Contribution** & **Inferences** \\ \hline
[31] & Provided a review on notched-band characteristics of & No discussion on isolation/DTs \\  & reconfigurable UMAs & \\ \hline
[32] & Provided a review on flexible UWB MIMO antennas for & No discussion on isolation/DTs \\  & wearable applications & \\ \hline
[33] & Discussed DTs like orthogonal polarization, parasitic & \\  & element (PE), decoupling structure (DS), defected ground & Newly reported DTs were not presented \\  & structure (DGS), neutralization line (NL), and metamaterials & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main contributions of previously reported articles and the presented review article.