## 1 Introduction  
Recently, communication systems have come under great pressure from an increasing number of users adopting electronic devices. It is understood that higher data rates are demanded, and great development and integration with wireless technology and broadband are necessary to achieve those speeds. Accordingly, with each new generation of wireless technology, new features are being launched, along with better advantages and overall performance compared to the previous generation. Fifth-generation (5G) wireless technology employs a mix of different frequency bands, each having its unique characteristics. For outdoor users, sub-1 GHz bands have large coverage areas, while mid-band frequencies (2.5, 3.5, 3.7-4.2, and 4.8-5 GHz) provide a balance between coverage and capacity. On the contrary, the millimeter-wave (mmWave) bands, which operate above 24 GHz, will produce a very high data rate, but they have limited coverage at present. Moreover, the Federal Communications Commission (FCC) allocated some bands for 5G, including 24 GHz (N257 and N261), 28 GHz (N261), 37-40 GHz (N260 and N261), and 47 GHz for licensed use [1, 2, 3, 4, 5]. The 64-71 GHz band is also available for unlicensed wireless communication and is the frequency range for Wi-Gig (Wireless and Gigabit) and other high-speed wireless systems [6; 7]. Multiple-input multiple-output (MIMO) antenna technology utilizes multiple antennas at both the transmitter and receiver to improve communication performance across various dimensions. The use of MIMO systems enables the simultaneous transmission and reception of multiple data streams, thereby increasing data throughput, enhancing signal quality, and improving overall wireless communication efficiency. Consequently, the exploration of MIMO antenna design for 5G, particularly in the millimeter-wave (mmWave) frequency range, presents a promising and compelling area for extensive research [8; 9; 10]. The most critical requirements from the point of view of MIMO antenna design are maximizing spectral efficiency, improving interference mitigation, and optimizing bandwidth adaptation--all essential needs in any 5G technology. Thus, depending on network demand, they require spectrum-efficient value chains, effective interference management, and dynamic bandwidth allocation. MIMO antennas have long been considered for maximizing spectral efficiency using spatial multiplexing and beamforming to exploit the advantages of such adaptively planned radiation patterns, polarization diversity, and directional beamforming techniques to reduce radiation interference [11; 12; 13; 14; 15]. The main challenges in MIMO antenna design are minimizing mutual coupling between antenna elements, reducing the size and shape of antennas, maximizing antenna gains, increasing the bandwidth and frequency of use, and ensuring a high antenna efficiency. Therefore, different antenna designs have been suggested in previous research as possible solutions for alleviating the stated challenges and facilitating the MIMO antenna development process. The study presented in reference [16] introduced a MIMO antenna structure designed for 5G applications, operating at a frequency of 28 GHz. The design features three circular rings enclosed within an infinity-shaped shell aimed at enhancing the antenna's performance for high-frequency communication. Simulations were shown to give a gain of 6.1 dBi and an antenna efficiency of 92%. A compact planar MIMO antenna configuration featuring a tree-like structure, as outlined in reference [17], was designed to support a wide bandwidth suitable for 5G applications. It is composed of four curved elements to cater for wideband performance. Radiation patterns were studied at 28 GHz, 33 GHz, and 38 GHz, with the maximum total gains realized being 10.58 dB, 8.87 dB, and 11.45 dB, respectively. In [18], a dual-band MIMO antenna with high gain operates mainly at 28 and 38 GHz with peak realized gains at 4.15 dBi and 7.73 dBi and total efficiencies of 80.13% and 85.44%, respectively.