## 4 Simulation and Optimization
The metal frame coupling technique supports excellent wideband impedance performance across multiple sub-6 GHz bands. Figure 4: The simulated VSWR characteristics of the sub-6 GHz antennas: (**a**) ANT #1, (**b**) ANT #2, (**c**) ANT #3, (**d**) ANT #4. Figure 3: The geometry and the practical integration of the proposed shared coupling antenna: (**a**) the detailed topology and dimensional configuration, (**b**) physical placement of the antenna along the top metal frame of the smartphone housing. In 5G NR sub-6 GHz systems, particularly within the n78 band (3.3-3.8 GHz), the radiation efficiency of mobile user equipment (UE) antennas plays a pivotal role in terms of overall link performance. Given the inherent integration constraints of compact device platforms, it is common industry practice to allocate distinct radiation efficiency targets to each antenna port. For example, in a typical four-port MIMO configuration, the target efficiencies may be -4.5, -7, -9 and -10 dB, respectively. These values reflect practical tradeoffs between antenna positioning, the internal component layout, and the susceptibility to user hand blockage. To assess the system-level impacts of such port-specific efficiency values, a downlink link budget analysis was performed under the following assumptions: a carrier frequency of 3.5 GHz (center of the n78 band), a bandwidth of 100 MHz, a maximum base station effective isotropic radiated power (EIRP) of 57 dBm in line with the 3 GPP TR 38.901 standard [15], and an urban cell edge path loss of approximately 135 dB. The received power at the mobile UE was calculated as  
\[Rx\ Power=EIRP-PathLoss+Receiver\ Antenna\ Gain \tag{9}\]  
Applying this to each port,  
* For the port with a \(-4.5\) dB gain, \(Rx\ Power=57-135-4.5=-82.5\) dBm
* For the port with a \(-10\) dB gain, \(Rx\ Power=57-135-10=-88\) dBm. Given a 5G UE receiver sensitivity floor of approximately -98 dBm (assuming 100 MHz of bandwidth and a block error rate [BLER] \(\leq\) 10%), all four antenna ports maintain a positive link margin of at least 10 dB. Even when additional fading and propagation losses are considered, these margins are (generally) sufficient to support robust communication. Furthermore, the inherent MIMO diversity and spatial multiplexing techniques can be used to further alleviate any performance degradation associated with individual port inefficiencies. The radiation efficiency targets adopted here from \(-4.5\) to \(-10\) dB per port are consistent with modern design practice and industry-accepted performance thresholds. The validity of such values was confirmed in previous studies on compact \(4\times 4\) MIMO antenna systems for the n78 band, where similar or slightly better efficiency levels (e.g., \(-3\) to \(-6\) dB) were reported and deemed acceptable [7; 9]. In the present study, the target range was based on both a system-level, link budget analysis and the empirical integration constraints, reinforcing the relevance and applicability thereof to real-world smartphone designs. The radiation efficiency performance of each antenna element was next evaluated across the n78 (3.3-3.8 GHz) and n79 (4.4-5.0 GHz) 5G NR bands (Figure 5). A four-port MIMO configuration was simulated, and the radiation efficiency of each antenna port benchmarked against the respective design target. Specifically, ANT #1 achieved a target efficiency (TE) of \(-4.5\) dB, ANT #2 a TE of -7 dB, ANT #3 a TE of \(-9\) dB, and ANT #4 a TE of \(-10\) dB. These per-port specifications were not arbitrarily chosen, being rather derived from empirical studies and validated using system-level link budget analyses tailored to sub-6 GHz 5G NR terminals. Assuming a base station EIRP of 57 dBm, a cell edge path loss of 135 dB (in an urban scenario), and the above antenna efficiencies, the received power at the UE is estimated to be \(-82.5\) dBm to \(-88.0\) dBm across all four ports.