**4. Conclusions**

In this article, a small-sized four-port UWB MIMO antenna with dual-band rejection features is presented. The proposed design contains four matching octagonal-shaped resonators arranged orthogonally, and the diagonal elements are arranged anti-parallelly. The antenna showed an isolation >18 dB for the entire operational range. An HCSRR was implanted in the radiating element for notching the interfering Wi-MAX and WLAN frequencies (3.5 and 5.5 GHz, respectively). The simulated and experimental results for gain, S-parameters, isolation, ECC, and radiation patterns were studied. The results validated that the decoupling metal strips used to reduce the inter-element coupling is a simple and efficient approach, and a good diversity response was achieved. The obtained results illustrate that the proposed MIMO antenna could be useful for UWB applications. It can serve as a potential candidate for base station terminals and other wireless communication systems.

**Author Contributions:** Conceptualization, P.K. and S.U.; methodology, P.K.; software, P.K.; validation, P.K., S.U., and F.A.; formal analysis, P.K.; investigation, P.K.; resources, S.U.; data curation, P.K.; writing—Original draft preparation, P.K.; writing—Review and editing, S.U.; visualization, P.K.; supervision, S.U.; project administration, S.U.; funding acquisition, S.U. and F.A. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** This research was funded by the Deanship of Scientific Research at Princess Nourah Bint Abdulrahman University, Riyadh, Saudi Arabia, through the Fast-Track Research Funding Program.

**Conflicts of Interest:** The authors declare no conflicts of interest.
