## 3 Results and Discussion
### Four-Port MIMO Antenna  
The MIMO arrangement in this design is tailored for a four-port antenna operating at 28.8 GHz resonance frequency. Particular emphasis is placed on optimizing the spacing distance between elements to improve isolation, minimize mutual coupling, and enhance the overall performance of the MIMO system. The structure comprises four individual ports compactly integrated within an area of 68 \(\times\) 80 mm\({}^{2}\), employing the same substrate as that of the single-array antenna. Figure 6a,b display the MIMO antenna structure in top and bottom views, showing SMAs (Sub-Miniature version A) RF connectors for each port. The distance between each array antenna and the MTM cells is 5.68 mm, which is very close to \(\lambda/2\), which is 5.36 mm. The simulation results in Figure 6c depict the S-parameters, specifically the mutual coupling coefficients (S\({}_{12}\), S\({}_{32}\), S\({}_{42}\)) within the MIMO system. Port 2, chosen as the central port, is notably influenced by its adjacent ports. The results indicate that mutual coupling remains below \(-41\) dB across the entire operational bandwidth, underscoring the effectiveness of the proposed decoupling technique in mitigating interference among the MIMO ports. This high level of isolation demonstrates the success of the decoupling strategy, ensuring improved performance and reduced interference in the suggested MIMO configuration. The significant mutual coupling reduction is primarily attributed to the design of the metamaterial (MTM) cell, as illustrated in Figure 7a,b. The extracted parameters--relative permittivity (\(\varepsilon_{\mathrm{r}}\)), relative permeability (\(\upmu_{\mathrm{r}}\)), and refractive index (n)--demonstrate positive and negative values that change with frequency. This unique behavior of the MTM cell plays a critical role in enhancing isolation and minimizing interference within the MIMO system. Specifically, the design output features negative \(\varepsilon_{\mathrm{r}}\) values through the investigated frequency band and demonstrates resonant behavior. This underlines the MTM cell behavior. Furthermore, it enhances the gain as the MTM enables focusing EM energy in specific directions. Moreover, regarding isolation, it absorbs or prevents surface wave propagation in certain directions. The split-ring-like cells operate similar to bandstop filters, filtering out unwanted interference and thus improving the attenuation of mutual coupling. Figure 8 presents the computed frequency-dependent ECC and DG of the proposed MIMO construction. Across the frequency range, the ECC values remain consistently below 0.00012, significantly lower than the conventional threshold of 0.5. This indicates excellent isolation and minimal signal correlation between antenna ports. At the same time, the DG achieved very close level to the 10 dB, reflecting strong signal reception in the desired direction and the effective mitigation of fading effects. Additionally, the CCL values exhibit strong performance. At 28.8 GHz, the CCL is calculated to be 0.147 bit/s/Hz. This value is well below the standard reference threshold of 0.4 bit/s/Hz, demonstrating that the proposed antenna design effectively preserves channel capacity. Such a low CCL reflects strong signal integrity, reduced interference, and the efficient utilization of the available bandwidth, which are critical for high-performance wireless communications. This result underscores the robustness and reliability of the antenna design, making it highly suitable for applications requiring high data rates and reliable connectivity. The simulated radiation pattern of the MIMO structure is depicted in Figure 9. At 28.8 GHz, the antenna demonstrates a gain of 12 dBi in the E-plane, with the major lobe oriented at \(\theta\) = 1\({}^{\circ}\) and a beamwidth of 61.6\({}^{\circ}\). In the H-plane, the major lobe achieves a gain of 12.9 dBi, directed at \(\theta\) = 5\({}^{\circ}\), with a beamwidth of 18.9\({}^{\circ}\).