# A Dual-Polarized and Broadband Multiple-Antenna System for 5G Cellular Communications
## 3 Fundamental Characteristics of the Suggested MIMO Antenna  
Figure 8 depicts the schematic of the investigated MIMO antenna design. The structural layout of this multi-feed antenna system is relatively simple and straightforward. The antenna's overall dimension is 75 \(\times\) 150 mm\({}^{2}\), and it includes an array of 8 \(\times\) 8 trapezoidal  
Figure 6: Linear-scaled (**a**) 3D and (**b**) 2D dual-polarized radiation patterns at 4.5 GHz. Figure 7: 3D radiation patterns of the dual-polarized antenna at (**a**) 3.5 and (**b**) 5.5 GHz. slot antennas, each paired with L-shaped stubs and CPW feeds, all positioned in close proximity to one another. It is worth mentioning that multiple similar antenna elements are positioned at PCB's edges. The utilization of the suggested antennas has several advantages. Firstly, it enhances the frequency response and matching characteristics of the antenna. Secondly, it results in symmetrical radiation patterns that cover both/bottom areas of the board. This symmetrical radiation pattern is desirable as it provides better overall coverage and performance for the MIMO system. Overall, the design aims to achieve efficient and reliable performance for smartphone applications, considering the compact size and appropriate radiation characteristics [45; 46]. Figure 9 presents the S-parameters for the designed antenna array, demonstrating that the antenna elements exhibit consistent performance across the frequency range. The reflection coefficients (S\({}_{\text{nn}}\)) are notably low, with values better than \(-15\) dB within the broad operational frequency band from 3.2 to 6 GHz, indicating excellent matching. Additionally, as depicted in Figure 9b, the mutual coupling between the elements is effectively minimized, with isolation levels (S\({}_{\text{nm}}\)) exceeding \(-13\) dB. This indicates that the antenna elements maintain strong isolation from each other, contributing to improved overall performance and reduced interference. Typically, values as low as \(-10\) dB, and in some cases even \(-6\) dB, are considered standard and acceptable isolation levels in smartphone MIMO antenna designs. However, it is worth mentioning that to further enhance isolation between the antenna elements, various decoupling techniques such as applying additional parasitic elements, using metamaterials, introducing neutralization networks, or implementing electromagnetic bandgap (EBG) can be employed [47]. They might, however, increase the complexity and size of the antenna, which is important considering the limited space occupied by antennas in smartphone boards. In Figure 10, the 3D radiation patterns of the multiple elements, along with their corresponding gain values at different frequencies, including 3.5 and 5.5 GHz, are displayed. The figure reveals that the 8-antenna system provides radiation with various polarizations. It is important to note that the observation of polarization diversity stems from the placement and orientation of the antenna elements, as clarified in Figure 6a. In the smartphone MIMO antenna layout, adjacent elements are positioned at 90-degree angles to each other. This polarization diversity applies to all antenna pairs in the array, enabling the system to support multiple polarization states, which is reflected in the overall radiation performance  
Figure 8: Schematic of the introduced MIMO antenna. illustrated in Figure 10. Moreover, the gain levels are sufficient to cover the different regions of the smartphone board effectively. This observation suggests that the investigated array design is robust concerning the holding positions of 5G handheld devices. Regardless of how users hold their smartphones, the antenna array maintains reliable and strong signal reception and transmission due to its multi-antenna configuration and diverse radiation patterns. This characteristic is essential in 5G smartphones to ensure consistent and stable connectivity in various usage scenarios and positions [48; 49; 50].