# Design and Analysis on Decoupling Techniques for MIMO Wireless Systems in 5G Applications
## 5 Additional Effects of Decoupling Techniques
### Analysis of External Decoupling
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Decoupling** & **Decoupling Shape** & **Frequency (GHZ)** & **Isolation (dB)** & **ECC** & **MIMO Order** \\ \hline \multirow{4}{*}{Neutralization lines} & \multirow{2}{*}{meandering NL [32]} & 2.3–2.5/3.2– & \multirow{2}{*}{\(>\)16} & \multirow{2}{*}{\(<\)0.005} & \multirow{2}{*}{2} \\  & & 3.6/5.0–6.7 & & & \\  & Two U-shaped NL and & 2.2–2.4/3.3– & \multirow{2}{*}{\(>\)18/\(>\)32/\(>\)24} & \multirow{2}{*}{\(<\)0.02} & \multirow{2}{*}{2} \\  & symmetrical distribution [33] & 4.5/5.6–5.7 & & & \\  & Strip NL [34] & 3.4–3.6/4.8–5.1 & \multirow{2}{*}{\(>\)11} & \multirow{2}{*}{\(<\)0.08/\(<\)0.05} & \multirow{2}{*}{8} \\  & meandering NL [35] & 3.4–3.6 & & & \\ \hline \multirow{4}{*}{Parasitic decoupling elements} & PDE [36] & 4.58–6.12 & \multirow{2}{*}{\(>\)15.4} & \multirow{2}{*}{\(<\)0.15} & \multirow{2}{*}{4} \\  & PDE [37] & 15.5–17 & & & \\  & PDE [38] & 3.1–10.6 & & & \\  & ADS and DGS [41] & 5.1–6.2 & & & \\ \hline \multirow{4}{*}{Defective ground structures} & Circular DSG and CSRR [42] & 26.5–38.2 & \multirow{2}{*}{\(>\)15} & \multirow{2}{*}{\(<\)0.005} & \multirow{2}{*}{2} \\  & Rectangular DSG [43] & 3.3–3.6/4.8–5 & & & \\  & DGS and hybrid EBG [44] & 4.5–8 & & & \\  & Two zigzag–shaped DGS [45] & 5.8 & & & \\  & Rectangular, circular and & & & & \\  & zigzag shaped DGS and & 25.5–29.6 & \multirow{2}{*}{\(>\)17} & \multirow{2}{*}{\(<\)0.01} & \multirow{2}{*}{4} \\  & polarization diversity [46] & & & & \\ \hline \multirow{4}{*}{Grounding branch structures} & Rectangular grounding branch [47] & 3.4–3.6 & \multirow{2}{*}{\(>\)17} & \multirow{2}{*}{-} & \multirow{2}{*}{4} \\  & Rectangular grounding structures & 3.4–3.6/4.8–5.0 & \multirow{2}{*}{\(>\)17.5/\(>\)20} & \multirow{2}{*}{-} & \multirow{2}{*}{4} \\  & Multi-branch T-shaped grounding branch [49] & 3–11 & & & \\  & T-shaped grounding branch [50] & 3.3–5.6 & \multirow{2}{*}{\(>\)15} & \multirow{2}{*}{\(<\)0.03} & \multirow{2}{*}{8} \\  & meandering grounding branch and NLs [51] & 3.4–3.6/4.8–5.0 & & & \\ \hline \multirow{4}{*}{Metamaterials} & Four-EBG array [53] & 3.6–17.6 & \multirow{2}{*}{\(>\)18} & \multirow{2}{*}{\(<\)0.015} & \multirow{2}{*}{2} \\  & Ten-CSR array [54] & 2.51 & & & \\ \cline{1-1}  & SRR [55] & 2.73–3.12/4.33–4.68 & & & \\ \cline{1-1}  & NFR consists of five split rings & 2.38–2.49 & \multirow{2}{*}{\(>\)20} & \multirow{2}{*}{-} & \multirow{2}{*}{8} \\  & SRR and slot [57] & 2.4/3.5/5.3 & \multirow{2}{*}{\(>\)14} & \multirow{2}{*}{-} & \multirow{2}{*}{4} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison among the referenced techniques of external decoupling. strong decoupling effect, the ADS composed of parasitic patches is indirectly placed above the substrate, which will lead to an increase in the height of the antenna systems;
3. The DGS can be etched with simple crafts and a low profile. Due to expanding the length of the current path by added slots, it can be seen the improvement of efficient bandwidth. The introduced slots with inductance will act as the resonator circuits at the operating frequency, so another resonance frequency will appear. Additionally, the matching impedance had almost no influence by the DGS of the background. Moreover, it has heat dissipation and is friendly to high power MIMO systems such as mm-wave arrays. However, the radiation patterns display a negative effect for the DGS that it will create undesired back radiation due to its defected shape. Additionally, the phenomenon of DGS simulation that the central frequency will shift to the lower point under the impact of the bandgap effect should be noticed;
4. As for metamaterials, they can be found with more significant coupling reduction effects than other techniques due to the effective control of EM wave transmission by those human-made left-hand materials. The metamaterial array is also beneficial for gain enhancement.