## 1 Introduction  
Recent times have shown an ever-increasing number of users for the excess of electronic devices, something that has burdened communication systems, as the users are looking for higher data rates, which must still be achieved through a singular wireless communication technology and broadband integration. Therefore, every new generation of wireless technology has seen a vast amount of upgradation and change from the previous one in terms of features, advantages, and improvements. 5G wireless communication utilizes various frequency bands comprising the sub-1 GHz, mid-band frequencies 2.5/3.5/3.7-4.2/4.8-5 GHz, in addition to the newly allocated millimeter-wave (mmWave) including 24, 28, 37, 38, 39, and 47 GHz [1, 2, 3, 4, 5]. Additionally, the 64-71 GHz band is reserved for unlicensed wireless applications, such as Wireless Gigabit (Wi-Gig) [6, 7]. MIMO (multiple-input-multiple-output) technologies use multiple-port antennas on the transmitting and receiving ends to boost communication characteristics through multiple aspects. MIMO systems can transmit and receive numerous data streams simultaneously, enhancing signal quality, data throughput, and overall wireless communication efficiency. Therefore, exploring the 5G MIMO antenna, especially in mmWave, presents an intriguing avenue for intensive research [8, 9, 10]. This new trend in antenna design aims to enhance spectral efficiency, improve interference mitigation, and optimize bandwidth flexibility. These goals conform to the needs of the 5G technology, which is characterized by the efficient use of the spectrum, proper interference control, and use of bandwidth that can be adjusted according to the requirements. MIMO antennas support spatial multiplexing and beamforming techniques to improve spectral efficiency and simultaneously reduce radiation interference by utilizing diversely adaptive radiation patterns, polarization control, and beamforming directions [11; 12; 13; 14; 15]. The main challenges in the design of MIMO antennas include reducing antenna elements' mutual coupling, miniaturizing size and shape, increasing antenna gains, expanding bandwidth and frequency use, and maintaining high antenna efficiencies. Thus, many antenna design techniques have recently been described as promising developments toward coping with these challenges. Several MIMO antenna designs have been proposed for 5G applications, particularly targeting the 28 GHz and 38 GHz bands. Reference [16] presented a 28 GHz MIMO antenna featuring three circular rings inside an infinity-shaped shell, achieving a simulated gain of 6.1 dBi and 92% efficiency. A compact, tree-like four-element MIMO antenna [17] was designed for wideband 5G operation, exhibiting peak total gains of 10.6/8.9/11.5 dBi for the tri-band 28/33/38 GHz, respectively. In Reference [18], a flexible dual-band MIMO antenna operated at 28/38 GHz, with gains of 4.2/7.7 dBi, respectively. Another dual-band MIMO design reported in [19] utilized a 1 \(\times\) 4 elliptical array with a crescent slot, achieving a 10.4 dBi gain and 7.8 GHz bandwidth. The authors of [20] employed spatial and polarization diversity to attain 23 dB isolation, covering the 28/38 GHz dual-band with gains of 9.5 and 11.7 dBi and bandwidths of 1.75 and 6.5 GHz, respectively. Metamaterials (MTMs) are artificially engineered materials that exhibit unique electromagnetic properties, making them highly effective tools for improving both gain and isolation in MIMO antennas. This can be realized by focusing electromagnetic waves to improve antenna directivity, thereby boosting antenna gain. Additionally, MTM structures act as absorbent barriers or blockers for undesirable radiation between narrowly spaced antenna ports, significantly suppressing mutual coupling and hence minimizing crosstalk and interference in densely packed MIMO structures.