## 1 Introduction
Through strategic integration, MTMs enable the customization of antenna properties to optimize performance, enhancing both gain and isolation for improved communication reliability and efficiency [21; 22; 23]. Recent advancements in MIMO antenna design for millimeter-wave (mmWave) applications demonstrate a consistent reliance on metamaterials to achieve high gain, low mutual coupling, and reconfigurable operation. One study presents a four-port MIMO antenna resonating at 28 GHz, using a series-fed array with metamaterial decoupling to achieve a 15.5 dBi gain and mutual coupling below \(-\)40 dB [24; 25]. The trend extends to dual-band reconfigurable MIMO antennas, such as a proposed 28/38 GHz design emphasizing wideband operation and high isolation, utilizing metamaterials [26]. Similarly, another study introduces a frequency-reconfigurable MIMO antenna for 24 GHz ISM and automotive radar applications, employing a varactor diode for tunability and a split-ring resonator (SRR) metamaterial absorber to attain a 16 dBi gain and 38.4 dB isolation [27]. Collectively, these studies underscore metamaterials as a critical solution for optimizing mmWave MIMO systems, enabling high gain, strong isolation, and adaptive functionality through tunable elements. Further research has explored the use of periodic metamaterial-photonic bandgap structures to address isolation challenges in densely packed array antennas, with applications extending to MIMO systems and Synthetic Aperture Radar (SAR) [28]. Additional studies have investigated mitigating surface wave effects in antenna arrays by incorporating metasurface inclusions in MIMO and SAR systems [25]. Moreover, an approach involves integrating metamaterials as a decoupling slab to mitigate mutual coupling in array antennas, thereby enhancing performance in complicated communication media [29; 30]. The multi-band MIMO antenna addresses critical system-level challenges in 5G mmWave deployments beyond conventional performance metrics. Unlike traditional approaches that optimize antennas in isolation, our design explicitly considers the interplay between antenna characteristics and communication system requirements, particularly in challenging propagation environments [31]. The high-isolation topology mitigates mutual coupling effects that could degrade channel estimation accuracy in dense multipath scenarios, while the multi-band operation provides flexibility for carrier aggregation in heterogeneous networks. Furthermore, the antenna's consistent radiation patterns across operational bands ensure reliable beamforming performance, which is crucial for maintaining link quality in mobile mmWave applications. These features collectively enhance the practical viability of our design for real-world 5G systems where antenna performance directly impacts channel estimation complexity and overall network efficiency. Employing multiple ports in a MIMO antenna array offers several merits, including enhanced channel capacity, improved spectral efficiency, and robust multipath resilience, which are crucial for high-speed 5G/6G communications. The independent excitation of antenna elements allows for beamforming and spatial diversity, reducing interference and increasing signal reliability. However, downsides include increased design complexity, mutual coupling between ports degrading isolation, and higher power consumption. Additionally, the physical size of the antenna may grow with more ports, making integration into compact devices challenging. Careful optimization of feeding networks and decoupling techniques is required to mitigate these drawbacks while maintaining performance. From an implementation perspective, the antenna's high gain characteristics compensate for mmWave propagation losses while maintaining a compact form factor suitable for modern user equipment. The design achieves this without resorting to complex feeding networks or external decoupling structures that would increase fabrication complexity and cost [32].