## 8 Conclusions  
A comprehensive study on design approaches and applications of MIMO antennas was presented in this paper. Design approaches for ultra-wideband MIMO antennas, dual-band MIMO antennas and circularly polarized MIMO antennas were discussed. The presented study is very useful for researchers working in the field of MIMO antennas. From this study, it is found that the UWB characteristics of MIMO antennas can be achieved by modifying the radiating patches as L-shaped, staircase-shaped, and square-shaped embedded with slits and slots and use of stubs in the patch. Furthermore, creating defects in the GP which includes fence-shaped GPs, different stub loaded designs and tapered slots/slits are some of the methods to improve the antenna bandwidth. Dual-band MIMO antennas can be obtained by incorporating the slots in the radiating patches, U-shaped patches and modification of patches to resonate in the dual/multi frequency band. It is also observed that the uses of SRR-loaded and groove-loaded GP also provide dual- band MIMO antennas. CP-based MIMO antennas are designed by simultaneously optimizing the structure of the patch and the GP. Some basic radiating structures such as cross-loaded patches, G-shaped and Z-shaped patches, and square and circular patches embedded with slots at specific positions and some modified DRAs are used for CP characteristics. The truncated corners and slots itched in the GP with a specific shape, and use of hybrid slots within and at the periphery of the GP are common techniques to improve the CP quality in the MIMO antenna. In addition to that, the isolation techniques in each type of MIMO design are described and presented in tables. In most of the MIMO designs, decoupling parasitic lines are used between the ports to improve the isolation. Moreover, the neutralization line, SRR and EBG structures in the GP as well as between the radiating elements are used for better decoupling. Placing the radiating elements more than the half wavelength of the designed antenna minimizes mutual coupling. Orthogonal orientation of the resonators is also utilized for mutual coupling reduction. The comparison of various antenna parameters including the size, isolation techniques, design methodologies and other characteristics provide an overview of the specific MIMO antenna design. Thus, it is concluded that this review paper will surely help a lot to enhance the quality and performance of MIMO antenna designs which are demanded by the high data transmission rates of present communication systems, as well as the upcoming 6G technology.  
Conceptualization, [PERSON] ([PERSON]), [PERSON] ([PERSON]) and R.N.T.; methodology, [PERSON] ([PERSON]), [PERSON] ([PERSON]) and P.K.; validation, R.N.T. and P.K.; investigation, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); resources, [PERSON] ([PERSON]),B.K.K.; writing--original draft preparation, [PERSON] ([PERSON]) and R.N.T.; writing--review and editing, [PERSON] ([PERSON]) and P.K.; supervision, [PERSON] ([PERSON]), P.K. and B.K.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.