## 4 Conclusions  
This work presents a new high-performance MIMO antenna with millimeter-wave (mmWave) 5G wireless communication allocations. The tri-band working frequencies are 28, 35, and 38 GHz. A compact four-port MIMO antenna, measuring 50 \(\times\) 50 mm\({}^{2}\) exhibits good performance, with recorded peak gains of 9.6, 7.8, and 13.7 dBi at 28, 35, and 38 GHz. The bandwidths acquired are 1.1, 2.2, and 3.7 GHz for the tri-band. The performance of the antenna was significantly improved through careful spacing of the elements and a decoupling method using metamaterial cells. This brought about the minimization of coupling  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Reference No.**} & \multirow{2}{*}{\begin{tabular}{c} **Frequency** \\ **(GHz)** \\ \end{tabular} } & \multirow{2}{*}{**Ports**} & \multirow{2}{*}{\begin{tabular}{c} **Dimensions** \\ **(\(\lambda_{\text{o}}\) and mm\({}^{2}\))** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Bandwidth** \\ **(GHz)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Gain** \\ **(dBi)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Isolation** \\ **(dB)** \\ \end{tabular} } \\ \hline
[33] & 28 & 4 &
\begin{tabular}{c} 2.8\(\lambda_{\text{o}}\times 3.3\lambda_{\text{o}}\) \\ \((30\times 35)\) \\ \end{tabular} & 4.1 & 8.3 & 22 \\ \hline
[34] & 38 & 4 &
\begin{tabular}{c} 2.3\(\lambda_{\text{o}}\times 2.3\lambda_{\text{o}}\) \\ \((25\times 25)\) \\ \end{tabular} & 2.0 & 5.0 & 25 \\ \hline
[35] & 38 & 8 &
\begin{tabular}{c} 1.5\(\lambda_{\text{o}}\times 6.5\lambda_{\text{o}}\) \\ \((16\times 70)\) \\ \end{tabular} & 2.6 & 7.5 & 25 \\ \hline
[36] & 28/38 & 2 &
\begin{tabular}{c} 0.7\(\lambda_{\text{o}}\times 0.8\lambda_{\text{o}}\) \\ \((7.5\times 8.8)\) \\ \end{tabular} & 1.23/1.06 & 6.6/5.86 & 20 \\ \hline
[37] & 28/38 & 2 &
\begin{tabular}{c} 5.1\(\lambda_{\text{o}}\times 10.2\lambda_{\text{o}}\) \\ \((55\times 110)\) \\ \end{tabular} & 1.07/1.43 & 7/8 & 27 \\ \hline
[38] & 28/39/48.7 & 2 & \
\begin{tabular}{c} 0.9\(\lambda_{\text{o}}\times 1.5\lambda_{\text{o}}\) \\ \((10\times 16)\) \\ \end{tabular} & 1.8/6.2/3.8 & 9.5/11.5 & 17 \\ \hline This work & 28/35/38 & 4 &
\begin{tabular}{c} 4.7\(\lambda_{\text{o}}\times 4.7\lambda_{\text{o}}\) \\ \((50\times 50)\) \\ \end{tabular} & 1.1/2.2/3.7 & 9.6/7.8/13.7 & 34.5/22/30 \\ \hline \end{tabular}
\end{table}
Table 2: Quantitative comparison between the proposed antenna and contemporary designs across multiple performance metrics.  
Figure 10: (**a**) Simulated and (**b**) measured S-parameters of 4-port MIMO antenna.  
between antenna elements, leading to an efficient MIMO performance with a very low envelope correlation coefficient of 0.00015, a further high diversity gain nearing 10 dB, and high isolation of 34.5, 22, and 30 dB, in the tri-band. Experimental verification has proven the design's effectiveness for broadband mmWave system implementation. Additional experimental characterization--including gain-frequency response and radiation pattern measurements--remains to be conducted for a comprehensive performance validation.  
Conceptualization, N.A. and M.S.; methodology, M.S.; software, N.A.; validation, M.S. and N.A.; formal analysis, M.S.; investigation, N.A.; resources, N.A.; data curation, M.S.; writing--original draft preparation, N.A. and M.S.; writing--review and editing, N.A. and M.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.  
The researchers would like to thank the Deanship of Graduate Studies and Scientific Research at Qassim University for financial support (QU-APC-2025).  
The authors declare no conflicts of interest.