## 3 1 \(\times\) 4 mm-Wave Linear MIMO Array Antenna Design and Measurement  
The antenna design is now transformed from a single optimized element to a 1 \(\times\) 4 linear MIMO array antenna, maintaining a compact size with dimensions of 11.0 \(\times\) 44.0 mm\({}^{2}\) (Figure 4a). Each element within the array is spaced precisely 1.0 mm apart, ensuring optimal performance. Additionally, the ground plane features a series of cross-shaped slots, strategically positioned with a 5.0 mm separation between each adjacent pair (Figure 4b). Figure 5 illustrates the prototype of the proposed MIMO antenna, and S-parameter measurements are conducted to confirm the accuracy of the simulated results. In Figure 6, a comparison is drawn between the S-parameter results obtained from simulation and measurement. The simulated and measured resonances are found at 26.89 GHz and 26.82 GHz, 28.07 GHz and 28.34 GHz, 32.51 GHz and 32.68 GHz, and 38.02 GHz and 38.11 GHz, respectively. The slight variance observed between the simulated and measured resonance frequencies may be due to inconsistencies in soldering error and fabrication processes. Furthermore, the proposed frequency bands are part of the mm-wave spectrum, which are increasingly being utilized for various advanced communication applications [22]. The first operating band covering 26.28-27.36 GHz is part of the 5G NR (New Radio) frequency range 2 (FR2). It is used for high-speed data transmission, providing enhanced mobile broadband and supporting ultra-reliable low-latency communications. The second band occupies a 27.94-28.62 GHz frequency range and supports applications like enhanced mobile broadband (eMBB) and fixed wireless access (FWA). FWA provides high-speed internet connectivity to homes and businesses, especially in areas where fiber or cable deployment is not feasible. Further, the third band (32.33-33.08 GHz) is part of the Ka-band, which is widely used for satellite communications, including both commercial and military applications. It supports high-capacity data transmission for internet, television, and other communication services. The last operating band covering 37.59-39.47 GHz is also part of the 5G NR FR2 and is used for very high-speed wireless communication such as augmented reality (AR), virtual reality (VR), and industrial automation. The proposed antenna exhibits excellent isolation values, surpassing 24 dB across all operational frequency bands. This outstanding performance makes it well-suited for employment in both mm-wave applications and mm-wave wearable biomedical applications. The surface current density for the MIMO antenna is evaluated at 28.0 GHz and 38.0 GHz (Figure. 7). The current density around the periphery including the slots in the patch is higher (Figure 7a), which elongates the effective length of the patch leading to resonate the antenna at 28 GHz. At 38 GHz, the current distribution shows null points (Figure 7b), indicating the higher mode generated in the modified triangular patch. From the current distribution graph at 28/38 GHz, it is evident that there is minimal coupling between adjacent ports, even in the absence of decoupling elements. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline Parameter & \(L_{\text{s1}}\) & \(W_{\text{s1}}\) & \(a_{1}\) & \(a_{2}\) & \(a_  
Figure 4: \(1\times 4\) MIMO array antenna. Figure 5: Prototype antenna and S-parameter measurement setup. 1 \(\times\) 4 mm-Wave Linear MIMO Array Antenna Gain, Radiation Efficiency and Radiation Pattern  
The antenna gain measurement setup is shown in Figure 8. Figure 9 (left side) presents the simulated and measured realized gain. The measured peak gains for the antenna at four different frequency bands are 6.12 dBi (26.28-27.36 GHz), 8.06 dBi (27.94-28.62 GHz), 5.58 dBi (32.33-33.08 GHz), and 8.58 dBi (37.59-39.47 GHz). In comparison, the simulated peak values are 6.42 dBi (26.68-27.12 GHz), 8.17 dBi (27.85-28.33 GHz), 6.46 dBi (32.28-32.71 GHz), and 8.72 dBi (37.71-38.10 GHz).