## 3 Antenna Designs and Configurations  
Figure 1 presents the overall antenna layout o, and the mechanical integration strategy for the proposed sub-6 GHz MIMO antenna system within modern smartphone platforms. In commercial smartphones, the internal mechanical architecture, particularly the placement of the PCB and battery, is crucial in terms of determining the space available for antenna integration. Two common configurations are depicted in Figure 1. In the first [Figure 1a], the principal PCB is centrally positioned and the battery is in the top of the chassis. This ensures access to the top and sides of the metal frame. Radiofrequency modules and antennas, including sub-6 GHz components, may be installed along the lateral or top edges. However, this limits the ground clearance at the bottom of the device; bottom-mounted antennas cannot be readily placed. The second configuration [Figure 1b] features a split-level sub-PCB architecture; the PCB is divided and arranged around the battery. This increases the mechanical clearance at the lower edge of the housing, enabling antenna integration beneath the battery or on the sub-PCB. Although this enhances antenna isolation and increases the integration density, it may compromise the placement of high-band or mmWave components at the top of the device. To accommodate both configurations, the proposed sub-6 GHz antenna uses a flexible frame-coupling design that employs the smartphone metal frame as the primary radiator. This enables both lateral-edge and bottom-edge port deployment with maintenance of a consistent impedance and radiation performance across the architectures. When the current paths are shared along the frame and the coupling gap is tuned, the antenna can be placed in various mechanical layouts with minimal structural changes. Such a dual-compatible strategy facilitates seam \begin{table}
\begin{tabular}{c c c} \hline \hline  & **S-Parameter Method** & **Radiation Pattern Method** \\ \hline Accuracy & \
\begin{tabular}{c} Medium (approximation based on \\ reflection/transmission) \\ \end{tabular} & High (considers the full radiation behavior) \\ \hline Data Required & Two-port S-parameters & Full 3D complex radiation patterns \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Ease of Use \\ \end{tabular} } & High (direct calculation using a vector \\  & network analyzer or a simulator) & Low (post-processing of radiation data required) \\ \hline Application Stage & Early design; estimation is rapid & Final validation or measurement \\ \hline Sensitivity to Losses & High & Low \\ \hline Computational Load & Low & High \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of each ECC extraction method. less integration across diverse smartphone product lines, accelerating the developmental cycles and reducing design complexity. Figure 2 illustrates the conceptual architecture of metal-frame-coupled antennas for sub-6 GHz operation. The design exploits the current distributions and coupling effects along the metal frame. This creates compact multi-port configurations that are spatially very efficient. Figure 2a shows a four-port configuration. Each side of the metal frame accommodates a shared-coupling antenna. Two symmetrical coupling sections each support two feed ports--ANTs #1 and #2 on the left and ANTs #3 and #4 on the right. The diagram highlights the capacitive coupling regions, the slot-fed stubs, and the shared current paths that extend across the frame sections. This efficiently uses the (limited) edge space of metal framed smartphones, leveraging controlled mutual coupling to enhance pattern diversity and enable (potential) reconfigurability. The strong inter-port coupling in shared frame sections, as depicted, provides additional radiation paths but requires careful impedance tuning. To further explore the simplified configurations and reduce design complexity, Figure 2b shows a two-port configuration. A single shared coupling antenna is deployed on either side of the frame.