## 6 Benchmarking  
Table 2 shows the performance comparison of the proposed mutual coupling reduction technique versus that of state-of-the-art designs reported in the literature. The comparison is conducted in terms of the arrangement of the mutual coupling suppression technique, edge-to-edge element distance, and mutual coupling reduction in terms of \(|\)S21\(|\).  
Most available mutual coupling suppression studies consider lower operating frequencies, specifically, the sub-6 GHz range, and only very few papers address the issue at higher frequencies, in particular, at mmWave frequencies. The size of the MIMO antenna system becomes physically smaller at higher frequencies, and the antenna system is considerably more difficult to handle compared to at a lower frequency. Additionally, for massive MIMO antenna systems, the transmitting and receiving antennas are situated close to each other so that mutual coupling effects degrade the overall performance of the system to a great extent. The solution proposed in this work addresses these specific issues, i.e., isolation improvement under challenging scenarios with physically small MIMO systems where the application of a single technique is typically insufficient (recall that the approach presented here combines three methods: FSS unit cells with a metal strip wall and a slot on the common ground plane). Bearing this in mind, the performance of our technique is superior in terms of offering a significant reduction in mutual coupling while operating under a small edge-to-edge radiator spacing of only about \(0.1\lambda\). It is better than what is proposed in, e.g., [27], let alone the many solutions reported for lower operating frequencies, which are not easily transferable to higher bands.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Design**} & **Mutual Coupling** & **Operating Frequency** & **Edge-to-Edge** & **Max. Mutual Coupling** & **Total Size of the** \\  & **Structures** & **(GHz)** & **Spacing** & **Suppression** & **Antenna System** \\  & & & & & 1S21 (dB) & \\ \hline
[8] & EBG & 3.1 to 13.5 & \(\approx\)1.01\(\lambda\) & 23 & \(\approx\)0.87 \(\lambda\)\(\times\) 1.55 \(\lambda\)\(\times\) 0.0008 \(\lambda\) \\
[10] & DGS & 2 to 10 & \(\approx\)0.1\(\lambda\) & 30 & \(\approx\)1.2 \(\lambda\)\(\times\) 0.7\(\lambda\)\(\times\) 0.032 \(\lambda\) \\
[16] & Cascaded decoupling network & 2.3 to 2.4/2 to 2.483 & \(\approx\)0.18\(\lambda\) & 20 & \(\approx\)0.82 \(\lambda\)\(\times\) 0.82 \(\lambda\)\(\times\) 0.25 \(\lambda\) \\
[19] & Neutralization line and decoupling network & 4.68 to 5.5 & \(\approx\)0.083\(\lambda\) & 20 & Not available \\
[22] & EBG and metamaterial & 8.7-11.7, 11.9–14.6, & & & \\  & wall & 29-34.2 & & & \\
[27] & FSS wall & 57 to 63 & \(\approx\)0.5\(\lambda\) & 30 & Not available \\
[28] & Metsuurface & 23.8 to 25.2 & \(\approx\)0.3\(\lambda\) & 25 & \(\approx\)5.8 \(\times\) 1.7 \(\lambda\)\(\times\) 0.28 \(\lambda\) \\
[29] & Metsuurface & 28 and 39 & \(\approx\)0.23\(\lambda\) & 30 and 48 & \(\approx\)0.7 \(\lambda\)\(\times\) 0.63 \(\lambda\)\(\times\) 0.013 \(\lambda\) \\
[30] & EBG & 28 and 39 & \(\approx\)0.5\(\lambda\) & 30 and 25 & \(\approx\)1.43 \(\lambda\)\(\times\) 0.79 \(\lambda\)\(\times\) 0.074 \(\lambda\) \\ This work (Design 1) & FS, metallic strip, and ground slot & 24 to 27 & \(\approx\)0.107\(\lambda\) & 22.5 & \(\approx\)2.57\(\lambda\)\(\times\) 2.57\(\lambda\)\(\times\) 1.43\(\lambda\) \\ This work (Design 2) & FS, metallic strip, and ground slot & 24 to 27 & \(\approx\)0.107\(\lambda\) & 22.5 & \(\approx\)2.57\(\lambda\)\(\times\) 2.57\(\lambda\)\(\times\) 1.43\(\lambda\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance comparison between the proposed mutual coupling reduction technique and state-of-the-art solutions.  
Figure 21: Design 2: simulated (gray) and measured (black) radiation patterns at 25.5 GHz: (**a**) H-plane, (**b**) E-plane.