## 1 Introduction
A multi-band non-planar antenna for 5G applications covering the sub-6 GHz (2-7 GHz) band, the Ku-band (13-17.5 GHz), and the mm-wave band (25-39 GHz) is suggested in [18], which demonstrates a wide frequency coverage across multiple 5G bands. The proposed U-shaped element configuration features two arms with three slots and one rectangular-shaped parasitic element in the patch, developed with a partial ground and two parasitic elements, achieving a maximum realized gain of 4.3 dB for the sub-6 GHz band. However, this design suffers from low mutual coupling of up to 21 dB and utilizes a 3D non-planarcomplex structure, a challenge to incorporate with 5G communication devices. A detailed comparison of these studies with the proposed MIMO antenna is presented in Section 3. Metamaterial structures offer unprecedented benefits for applications in optics, acoustics, and telecommunications [19]. These metamaterials can alter the behavior of electromagnetic waves primarily through their structural design rather than through the inherent properties of the materials themselves. Structures possessing negative permittivity and negative permeability exhibit extraordinary electromagnetic behavior, enabling advanced wave manipulation through engineered material properties that deviate from the natural responses of conventional materials. The most famous metamaterials are split-ring resonators (SRRs); coplanar SRRs along radiating elements increase decoupling and improve port isolation. Metamaterials above radiating elements showing lens-type properties increase gain and improve port isolation among adjacent radiating elements [20; 21]. In [22], a unit-cell-based methodology is employed to simplify the design of a four-port cavity-backed MIMO antenna for 5G indoor base stations, achieving wideband operation (1.68-4.15 GHz) with port isolation exceeding 16 dB. However, the design's complexity, reliance on an x-shaped isolating block, and lack of real-world performance metrics limit its practical deployment. This underscores the need for compact, high-isolation antennas with validated field performance for multi-operator 5G ORAN applications. A low-profile omnidirectional antenna with Vivaldi slits, as presented in [23], offers a broad bandwidth of 58.8% (2.8-5.13 GHz) and a gain of 0.6-3.7 dBi for 5G indoor base stations. Its single-port configuration, however, lacks MIMO functionality, and the absence of real-world performance data restricts its suitability for multi-operator ORAN deployments. This highlights the need for compact MIMO designs with improved gain and validated field performance. A 16-port MIMO antenna array, detailed in [24], achieves a wide bandwidth 3.3-6.0 GHz and a gain of 5-6 dBi for indoor 5G base stations, with isolation exceeding 15 dB. Its moderate isolation, alongside the absence of field testing, limits its practicality for MIMO deployments where throughput and capacity are primary concerns. A comparison of sub-6 GHz indoor base station antennas with the proposed one is presented in Table 1.  
This investigation elucidates several groundbreaking advantages derived from the proposed antenna design:  
1. A novel microstrip patch antenna (MPA) was developed, incorporating meticulously optimized slits and slots. These features facilitate impedance matching and significantly enhance the operational bandwidth to 300 MHz, spanning the frequency range of 3.5-3.8 GHz. This bandwidth is crucial for supporting three distinct operators that utilize shared infrastructure. The unit cell was subsequently reconfigured into a four-port, orthogonally arranged multiple-input multiple-output (MIMO) structure. 2. A double-negative metamaterial (MTM) unit cell was engineered, leveraging well-established constitutive effective parameters. This unit cell was optimized for spacing and transformed into a 3 \(\times\) 3 array. By positioning the MTM structure as a superstrate