## 1 Introduction
The crescent-shaped slot elliptical MIMO antenna defined in [19] operates at dual frequencies of 28/38 GHz, which utilizes a 1 \(\times\) 4 linear array with a peak gain of 10.4 dBi and a bandwidth of 7.8 GHz. The implementation in [20] attains isolation of the elements in a MIMO antenna up to 23 dB using spatial and polarization diversity. The constructed design is in the frequency bands of 27.25 to 29 GHz and 34.5 to 41 GHz and gains of 9.5 dBi in 28 GHz and 11.7 dBi in 38 GHz. Metamaterials, engineered materials with distinctive electromagnetic properties, are crucial in enhancing gain and isolation in MIMO systems. They improve gain by efficiently focusing electromagnetic waves, which enhances directivity and radiated power. Furthermore, metamaterial structures create barriers that absorb or block unwanted radiation between closely spaced antennas, thereby improving isolation. This isolation is essential for preventing interference and crosstalk in densely packed MIMO configurations. By strategically integrating metamaterials, the properties of antennas can be tailored to optimize performance, increasing gain and strengthening isolation in MIMO systems, ultimately enhancing communication reliability and efficiency [21; 22; 23]. Studies have been performed on improving isolation in dense arrays of antennas using periodic metamaterial photonic bandgap structures to approach applications such as MIMO systems and Synthetic-Aperture Radar (SAR) for isolation problems [24]. Further investigations aimed to understand the envelope of surface wave effects due to antenna arrays that involve mostly visible contributions from metasurface inclusions in MIMO and SAR systems [25]. Other methodologies use metamaterials as electromagnetic bandgap (EM-BG) decoupling slabs to reduce mutual coupling in antenna arrays, enhancing output in harsh operating environments of communication [26; 27]. The research presented in [28] explored achieving mutual-coupling isolation in densely packed array antennas using an embedded metamaterial EM bandgap decoupling slab. Regarding MIMO antenna compactness, a compact four-element MIMO antenna customized for 5G millimeter-wave applications in the 37-39 GHz frequency range has been introduced in reference [29]. Additionally, reconfigurable MIMO structures operating at 28/38 GHz with wideband coverage and high isolation for advanced mmWave applications has been reported in [30]. More recently, another study investigated the design of a reconfigurable MIMO antenna, aiming to enhance MIMO performance in automotive 24 GHz radar sensors [31]. The proposed design here is of substantial significance for several key reasons:  
* It achieves peak gains of 15.5 dBi at 28 GHz, enhancing signal directivity and extending communication range. * The antenna supports a bandwidth of 2.0 GHz, exhibiting a low reflection coefficient of \(-\)35 dB, facilitating low-loss operation. * It demonstrates more than 40 dB isolation levels across its ports, effectively rejecting interference and ensuring robust, clear signal transmission. * Experimental measurements have confirmed the achieved results, emphasizing the design's reliability and positioning it as a promising option for future 5G millimeter-wave wireless applications.