**4. Conclusions**

A four-port fabricated compact MIMO antenna with interconnected ground plane and simple decoupling structure is proposed and developed covering different sub-6 GHz bands, including 5G and Wi-Fi bands. The measured results show that the impedance bandwidth of 54% (3.2–5.6 GHz) and approximate peak gain of 2.4 to 4.9 dBi over the entire operational bandwidth is achieved. Simple decoupling structure provided good, measured isolation results, better than 16 dB, for the proposed four-port MIMO antenna system, even though the antenna elements are placed close to each other. Furthermore, the measured results and the radiation patterns ensure that the fabricated MIMO antenna system provides a good solution for compact sub-6 GHz MIMO portable devices with diversity performance and for IoT devices.

**Author Contributions:** Conceptualization, N.S. and H.T.C.; methodology, N.S. and T.K.G.; software, N.S.; validation, H.T.C., B.A.K. and S.K.; formal analysis, H.T.C., N.S. and S.K.; investigation, N.S. and S.K.; resources, S.K. and T.K.G.; data curation, N.S. and H.T.C.; writing—original draft preparation, N.S.; writing—review and editing, H.T.C., B.A.K., S.K. and T.K.G.; visualization, T.K.G.; supervision, H.T.C. and S.K.; project administration, S.K., B.A.K. and T.K.G.; funding acquisition, S.K. and T.K.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to thank the support of Telekom Malaysia Berhad (TM) under project MMUE/220012 and the Universiti Teknologi Malaysia—grants 03M81 and 4C599.

**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.
