## 2 Antenna Design  
Designing an antenna for indoor communication is founded on three steps: the first step involves the design of a rectangular microstrip feed unit element, the second step involves the design of the unit element of the MTM and the whole 3 \(\times\) 3 MTM superstrate, and the last step gives the design of a four-port MIMO array with a 3 \(\times\) 3 MTM superstrate above a radiating array. Figure 1 illustrates the systematic design workflow for the four-port MIMO antenna array, detailing the iterative optimization process from the initial patch configuration to the final metamaterial-enhanced implementation. The four-port MIMO configuration, combined with a 3 \(\times\) 3 MTM superstrate, was chosen to balance relatively high gain (4.5-5 dBi) and low mutual coupling (\(<\)\(-\)30 dB), aligning with the requirements of ORAN for multi-operator indoor 5G base stations. A. Unit Antenna Element Design  
Figure 1: MIMO antenna design flow chart. The unit element is designed based on the transmission line model implemented on a commercially available 1.6 mm FR-4 substrate with a relative permittivity of 4.3 and a loss tangent of 0.02. The designed antenna incorporates a full ground plane, chosen to ensure compatibility with the ORU where it is placed on top of other electronic components. A partial ground plane could introduce arbitrary effects on the radiation pattern and other performance parameters due to interference from underlying electronics. Additionally, for ceiling or wall-mounted applications, the full ground plane minimizes backward radiation. The patch antenna was selected for this work due to its ease of integration with radio units and its capability to be camouflaged within the housing of radio units, which is particularly advantageous for discreet 5G base station deployments. The use of FR-4 as the substrate, despite its higher loss tangent (0.02) that may affect efficiency, is justified by its cost-effectiveness. An online analysis reveals that premium substrates, such as Rogers RO3003 or RO5880, are 20-30 times more expensive. For 5G small cell networks requiring hundreds to thousands of antennas, this results in a potential 2000-3000% increase in infrastructure costs at scale, making FR-4 a practical choice for mass deployment. The antenna dimensions obtained from the transmission line model were optimized to achieve optimal impedance matching at the target frequency of 3.65 GHz. The inset-feed mechanism is chosen due to its superior impedance matching capabilities and reduced feed radiation. Three rectangular slits of different lengths are carved on one side of the patch. A good impedance matching is achieved with another rectangular slit carved on the other side of the patch. These modifications adjust the effective electrical length and surface current distribution, reducing the quality factor (\(Q\)) and broadening the antenna's frequency response. The theoretical framework and calculations for incorporating slits and slots in the base design are based on the cavity model and transmission line theory. The resonant frequency of a rectangular patch without slits is given by Equation (1). \[f_{r}=\frac{c}{2L_{eff}\sqrt{\varepsilon_{eff}}} \tag{1}\]  
where:  
\(c=3\times 10^{8}\) m/s (speed of light);  
\(L_{eff}=L_{p}+2\Delta L\) (effective length, including fringing);  
\(\Delta L=0.412h\left(\frac{\left(\varepsilon_{eff}+0.3\right)\left(\frac{W_{p }}{k}+0.264\right)}{\left(\varepsilon_{eff}-0.258\right)\left(\frac{W_{p}}{k} +0.8\right)}\right)\) ([PERSON] fringing field correction formula). The slits and slot disrupt the TM10 mode current, causing it to meander, which increases the path length and excites higher-order modes. Edge slits add asymmetry, potentially tilting the pattern or introducing cross-polarization. The center slot splits the current, creating two parallel paths that enhance coupling and broaden the resonant response.