Multiple-input multiple-output (MIMO) Ultra-Wide Band are a popular study topic in wireless communication technology as it can enhance the channel capacity of the antenna system to meet new communication needs while maintaining bandwidth and transmission power. However, increasing the number of antenna elements in a given space can result in mutual coupling effects, reducing transmission quality. As a result, it is critical to build a small MIMO wireless system using decoupling techniques. Several studies to reduce mutual coupling effects have been proposed. However, an extensive review of the characteristics and similarities amongst various decoupling methods is missing. This paper provides a comprehensive review of studies on the mutual coupling effect in MIMO. Then, several decoupling approaches that are important are classified, discussed and full examination of the benefits and drawbacks of each decoupling strategy. The article summarizes the impact of various decoupling designs and discusses their potential usability and flaws.