## 5 Feasibility and Measurement
real-world integration including the PCB tolerances, parasitic SMA effects, and complex internal coupling. Unlike (idealized) simulation conditions, real-world integration introduces detuning effects because of the mutual proximity of internal components (e.g., the battery, shielding, and flexible interconnects), the non-uniform ground return paths, and changes in the surface current distribution along the metal frame. Such findings underscore the need to optimize the electromagnetic design via detailed mechanical and layout aware modeling. This is essential to ensure the consistent performance of real-world smartphones. Although the shared coupling multi-port structure is both compact and integration-friendly, it is inherently associated with suboptimal isolation and an elevated ECC associated with strong inter-port coupling. To address these limitations, a two-step optimization strategy was implemented as illustrated in Figure 8.  
In Step (a), the original four-port configuration was decomposed into two physically separated two-port antenna modules. Each module was then independently optimized to cover the n78 and n79 bands. This enabled enhanced spatial and polarization diversity associated with good ECC values (ECC < 0.3 across both bands). However, residual coupling along the shared metal frame rendered the measured isolation suboptimal. The worst-case value was approximately \(-4.6\) dB. Such poor isolation poses a significant challenge. MIMO performance is degraded, particularly in terms of maintaining independent signal paths and suppressing inter-port interference. Figure 7: The measured VSWR characteristics of the sub-6 GHz antennas: (a) ANT #1, (b) ANT #2, (c) ANT #3, (d) ANT #4. To overcome this, Step (b) involved additional tuning of both the feed structures and the radiation patterns. Specifically, the excitation points were shifted, the slot geometries were rendered asymmetric, and the coupling gap and ground clearance fine-tuned. These substantially improved the isolation to values exceeding 10 dB, with acceptable VSWR and ECC figures maintained (below 0.4 across all operational bands). The final antenna configuration exhibited a balanced performance that satisfied the system-level MIMO requirements without compromising the compact form factor of modern smartphone platforms. Such results validate the practicality of the new metal frame coupled antenna system. They also underscore the need for iterative co-optimization of the electromagnetic design and mechanical integration during sub-6 GHz MIMO implementation in mobile devices. A two-step implementation strategy was adopted to systematically examine the tradeoffs between port coupling, the ECC, and integration feasibility (Figure 8). Both stages employed a smartphone testbed. The metal frame served as the primary radiating structure. In Step 1 [Figure 8a], a baseline two-port shared coupling configuration was constructed by directly placing two feed points (Ports 1 and 2) on the metal frame. Each port was excited via soldered coaxial lines. The ground points were symmetrically distributed to regulate current flow along the coupling path. This initial configuration enabled foundational assessment of the broadband impedance characteristics and spatial coupling behavior within a realistic chassis environment. However, the results revealed substantial inter-port coupling and elevated ECC levels, underscoring the need for further optimization to meet the performance requirements of a MIMO system. To address the limitations identified in Step 1, Step 2 employed a pattern-tuned antenna featuring geometrically optimized slots and coupling gaps [Figure 8b]. The discrete sections were defined using critical design parameters, including the pattern length \(\mathrm{S_{w}}\), the radiating length \(\mathrm{R_{L}}\), and the coupling gap \(\mathrm{C_{g}}\). A central grounding connection between the PCB and the metal frame was added to ensure a stable current return path and to suppress uncontrolled parasitic resonances.