## 6 Conclusions  
In conclusion, this research presents a novel design for an ultra-compact shared-aperture antenna (footprint area equals \(0.018\lambda_{0}^{2}\)) that can operate in the 5G MMw bands, the 6G sub-THz band, and the wireless fidelity microwave band. By incorporating four quarter-mode substrate integrated waveguide (QMSIW) cavity resonators, a close-spaced tri-band 4 \(\times\) 4 multi-input, multi-output (MIMO) antenna is achieved for high-frequency applications. The proposed antenna demonstrates good characteristics regarding scattering parameters, antenna gains, efficiency, radiation pattern, envelope correlation coefficient, and mean effective gain power ratio through simulations, measurement, and co-simulation. The proposed design offers excellent performance and shows promising potential for practical applications in future wireless communication systems. The compact footprint of the antenna makes it very promising for integration into planar circuits in mobile and IoT devices. Thus, continuous research shall be conducted to integrate this new design into smartphone devices.  
Conceptualization, S.S.A. and A.T.A.; methodology, S.S.A. and A.T.A.; software, S.S.A., A.T.A. and E.A.; validation, S.S.A. and A.T.A.; formal analysis, A.M.A.; investigation, S.S.A. and A.M.A.; resources, S.S.A. and E.A.; data curation, S.S.A., A.T.A. and L.A.-T.; writing--original draft preparation, S.S.A., A.T.A., A.M.A., E.A., L.A.-T. and J.Y.; writing--review and editing, S.S.A., A.T.A., A.M.A., E.A., L.A.-T. and J.Y.; visualization, S.S.A., A.T.A. and A.M.A.; supervision, S.S.A.; project administration, S.S.A.; funding acquisition, S.S.A., A.T.A., A.M.A., E.A., L.A.-T. and J.Y. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref**} & \multirow{2}{*}{**Design Technique**} & \multicolumn{5}{c}{**Operation Bands**} \\  & & \multicolumn{3}{c}{**No. of Freq (GHz/THz)**} & \multirow{2}{*}{**MIMO**} & \multirow{2}{*}{**RAU [\%]**} & \multirow{2}{*}{**RF**} & \multirow{2}{*}{**Area [\(\lambda_{0}^{2}\)]**} \\ \cline{3-3} \cline{5-8}  & & **Sub-THz** & & & & & & \\ \hline
[32] & HOM, FMSIW & No & No & Yes, \(3.2/3.8\) & No & 67 & 1.18 & 0.71 \\ \hline
[30] & HOM, FMSIW & No & Yes, \(28/38\) & No & No & 62 & 1.35 & 0.2 \\ \hline
[31] & HOM, FMSIW & No & No & Yes, \(2.6/5.9\) & No & 3 & 2.26 & 0.104 \\ \hline
[33] & HOM, FMSIW & No & Yes, \(28/33/38\) & No & No & 23.6 & 1.35 & 7.22 \\ \hline
[34] & HOM, FMSIW & No & No & Yes, \(9.84/10.2\) & No & 3.7 & 1.03 & 0.348 \\ \hline
[35] & HOM, FMSIW & No & No & Yes, \(9.4/16\) & No & 14.6 & 1.7 & 0.164 \\ \hline
[37] & DMC, FMSIW & No & Yes, \(60\) & Yes, \(2.4/5.4\) & Yes, \(2\)-port & 92 & 25 & 0.464 \\ \hline
[38] & DMC, FMSIW & No & Yes, \(28/38\) & Yes, \(2.3/2.4/2.5/3.5/5\) & No & N.A & 16 & 0.563 \\ \hline
[40] & DMC, FMSIW & No & Yes, \(60\) & Yes, \(3.5\) & No & 72 & 17 & 0.126 \\ \hline
[21] & SMSIW & No & Yes, \(28/38\) & No & No & 90 & 1.35 & 4.622 \\ \hline
[25] & Ant.diplexer, SMSIW & No & No & Yes, \(6.44/7.09\) & No & 38 & 1.11 & 0.476 \\ \hline
[26] & Ant.diplexer, SMSIW & No & No & Yes, \(3.4/3.8\) & Yes & 89 & 1.11 & 0.16 \\ \hline
[27] & Ant.triplexer, SMSIW & No & No & Yes, \(5.57/7.17/7.65\) & No & 78 & 1.37 & 0.435 \\ \hline
[28] & Ant.triplexer, SMSIW & No & No & Yes, \(6.53/7.65/9.09\) & No & 62 & 1.39 & 0.42 \\ \hline
[24] & Ant.quadrplexer, SMSIW & No & Yes, \(28/30\) & Yes, \(4.8/5.4\) & No & N.A & 6.25 & 0.260 \\ \hline
[29] & Ant.quadrplexer, SMSIW & No & No & Yes, \(2.45/3.5/4.9/5.4\) & No & 69 & 2.2 & 0.053 \\ \hline
**[This]** & **HOM, DMC, and SMSIW** & **Yes, \(0.13\)** & **Yes, \(28/38\)** & **Yes, \(5.5\)** & **Yes, \(4\)-port** & **92** & **23** & **0.018** \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the proposed with other shared-aperture antenna designs.