## 6 Discussion and Future Work
### Frequency Shift Between Simulation and Measurement  
The measured results showed a frequency shift exceeding 200 MHz compared to simulations (Figure 4 vs. Figure 7). This discrepancy is primarily attributed to four factors. First, the SMA connector and feed transition were modeled using a discrete port in CST Studio Suite 2024, without accounting for cable losses or parasitic effects, which can significantly impact resonance in compact designs. Second, the FR4 substrate properties (permittivity 4.3,  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Reference** & \begin{tabular}{c} **Freq.** \\ **[GHz]** \\ \end{tabular} & \begin{tabular}{c} **VSWR** \\ **[3:1]** \\ \end{tabular} & \begin{tabular}{c} **Efficiency** \\ **[dB]** \\ \end{tabular} & \begin{tabular}{c} **Isolation** \\ **[dB]** \\ \end{tabular} & **ECC** & \begin{tabular}{c} **Size** \\ **[mm]** \\ \end{tabular} & \begin{tabular}{c} **Substrate** \\ **Type/Thickness** \\ \end{tabular} & **Integration Type** \\ \hline This work & 3.3-3.8 & 3.3-3.8 & \(-4.5\) to \(-10\) & \(>\)12 & \
\begin{tabular}{c} \(<\)0.11 \\ (0.01 max) \\ \end{tabular} &
\begin{tabular}{c} 148 \(\times\) 86 \\ \end{tabular} & FR4, 0.8 mm & Real smartphone \\ \hline \hline
[1] Appl. Sci. 2021 & 3.3-4.2 & 3.2-4.0 & \(-5\) to \(-8\) & \(>\)10-15 & \(<\)0.2 &
\begin{tabular}{c} 150 \(\times\) 78 \\ \end{tabular} & FR4, 1.0 mm & Bare test board \\ \hline
[2] Appl. Sci. 2022 & 3.3-3.8 & 3.3-3.9 & \(-4\) to \(-9\) & \(>\)10-14 & \(<\)0.15 & \(145\times\) 75 & FR4, 1.6 mm & Bare test board \\ \hline
[7] IEEE Access 2021 & 3.3-3.8 & 3.3-3.9 & \(-6\) max & \(>\)10 & \(<\)0.2 & \(140\times\) 70 & FR4, 0.8 mm & Bare test board \\ \hline
[10] IEEE AWPL 2020 & 3.4-3.8 & 3.3-3.8 & \(-4\) to \(-8\) & \(>\)15 & \(<\)0.1 & \(148\times\) 72 & FR4, 1.0 mm & Bare test board \\ \hline
[16] IEEE TAP 2024 & 5.1-7.1 & 4.5-7.5 & \(-2\) to \(-5\) & \(>\)15 & - & \(235\times 150\) &
\begin{tabular}{c} Real \\ Table \\ \end{tabular} \\ \hline
[17] IEEE AWPL 2023 & 2.4-2.485 & 2.4-2.485 & \(-1.5\) to \(-2.2\) & \(>\)30 & - & \(144\times\) 74 & FR4, 4.0 mm & Bare test board \\ \hline
[18] IEEE AWPL 2023 & 3.3-4.0 & 3.3-4.2 & \(-2.8\) to \(-4.2\) & \(>\)11.5 & \(<\)0.2 & \(150\times\) 70 & FR4, 0.8 mm & Bare test board \\ \hline
[19] IEEE AWPL 2023 & 3.3-4.2 & 3.3-4.5 & \(-4\) to \(-10\) & \(>\)15 & \(<\)0.04 & \(158\times\) 73 & FR4, 0.8 mm & Bare test board \\ \hline \hline \end{tabular}
\end{table}
Table 3: A comparison between the performance of the new antenna and prior sub-6 GHz MIMO systems.  
loss tangent 0.025) can vary by \(\pm\)10% due to material tolerances. Additionally, as shown in Figure 6b, the placement of components, the battery, and the full-screen display contribute to further detuning when integrated into real devices. Third, fabrication tolerances of \(\pm\)0.1 mm in slot and gap dimensions can induce resonance shifts between 50 MHz and 200 MHz. Finally, a mesh convergence study confirmed simulation stability, using a time domain solver over 0.1-6 GHz with -40 dB accuracy and approximately 3.3 million mesh cells. To further clarify the impact of individual parameter variations, Table 4 provides a sensitivity analysis summarizing the estimated resonance frequency shifts associated with each factor: