## 2 Design of MIMO Antenna  
Four designs were considered to analyze the reflection/transmission coefficient: peak gain, directivity, electric fields, and radiation patterns. Figure 1 characterizes the three-dimensional view of the top side and bottom side. All of the dimensions were considered in the millimeter. The overall dimensions of the design were 50 \(\times\) 35 mm\({}^{2}\). The lumped port was applied to the structure for the excitation. The patch and ground layers were designed by considering them as copper material. The thickness of the patch and ground layers was 0.35 mm. The substrate height was 1.6 mm. The substrate material was chosen to be FR-4 epoxy, as it is inexpensive and easy to fabricate. FR4 has dielectric constant of 4.4 [24]. The novelties of the structure are mentioned below. * The proposed MIMO antenna design incorporates CSRR metamaterial, which offers unique electromagnetic properties, such as negative permeability and permittivity. This integration enables the antenna to exhibit enhanced performance characteristics, including improved bandwidth and radiation efficiency. * The antenna design incorporates a defected ground structure to mitigate surface wave propagation and improve the isolation between antenna elements. * The proposed MIMO antenna design achieves a low profile and compact form factor, making it suitable for various space-constrained applications. In addition, the antenna's reduced size and planar structure contribute to its versatility and ease of integration into modern wireless communication devices. * The antenna design offers wideband operation across the S/C/X/Ka frequency bands, covering various communication applications. In addition, the CSRR metamaterial and DGS implementation contribute to the achieved broad operating bandwidths, ensuring compatibility with multiple frequency bands. Figure 1: The three-dimensional view of the proposed antenna. (**a**) 3D top view. (**b**) 3D lower view of the proposed structure. * A sufficient reflection coefficient, more than 1 GHz of bandwidth, and a healthy gain are achieved for all of the proposed designs. * Design and fabrication were carried out using FR4 dielectric material to check the reliability of the structure. The FR4-based low-profile material was used to achieve targeted goals, which helped with cost reduction. * The four design structures were analyzed based on modifying the DGS and slit around the patch region to check the performance of different forms. * The proposed design is compared with the other existing methods to identify the improvement in the structure. * The simulated and measured results are compared to judge tolerance among them. Lower tolerance among both results is observed. * Overall, the proposed antenna design exhibits excellent characteristics for MIMO applications. It demonstrates low mutual coupling between antenna elements, high isolation, good radiation efficiency, healthy gain, and proper diversity parameters. These attributes contribute to enhanced channel capacity, increased data rates, and improved system performance in MIMO systems. The different antenna structures were examined by inserting CSRR in the patch with slits outside of the patch and defective ground structure. The insertion of a CSRR in a patch antenna can have several effects on the performance of a MIMO antenna, as follows: The CSRR can alter the resonant frequency of the patch antenna. Due to a unique metamaterial property, it is possible to shift the resonant frequency of the patch antenna to a desired frequency. This adjustment can be beneficial in achieving better frequency selectivity and matching with other antennas in the MIMO system. The CSRR can widen the bandwidth. By introducing the CSRR, the effective electrical length of the antenna structure can be modified, resulting in a broader frequency bandwidth. This expanded bandwidth can be advantageous for MIMO systems that require simultaneous operation over multiple frequency bands.