# Design and Analysis on Decoupling Techniques for MIMO Wireless Systems in 5G Applications
## 5 Additional Effects of Decoupling Techniques
### Analysis of Internal Decoupling  
The internal decoupling techniques have also been well-established recently. The research focus on internal decoupling was concentrated on polarization diversity. Constructing orthogonal modes was a favored selection to ensure minimal correlation of different signals. The performances of these self-decoupling MIMO systems are given in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Decoupling** & **Decoupling Scheme** & **Frequency (GHz)** & **Isolation (dB)** & **ECC** &
\begin{tabular}{c} **MIMO** \\ **Order** \\ \end{tabular} \\ \hline \multirow{6}{*}{Orthogonal polarization polarization [60]} & Dual orthogonal polarization [58] & 3.4–3.6 & \textgreater{}12.7 & \textless{}0.13 & 8 \\  & Dual orthogonal polarization [59] & 2.2–2.4 & \textgreater{}20 & 0.04 & 8 \\  & Dual orthogonal polarization [60] & 3.5–5.0 & \textgreater{}12 & \textless{}0.11 & \(8\times 8\) \\  & Dual orthogonal polarization [61] & 2.55–2.65 & \textgreater{}11.4 & \textless{}0.1 & 8 \\  & Triple orthogonal polarization and NLs [62] & 3.4–3.6 & \textgreater{}12.5 & \textless{}0.2 & 12 \\  & Triple orthogonal polarization [63] & 2.8–2.9/4.5–5.4 & \textgreater{}48 & - & 3 \\ \hline \multirow{6}{*}{Characteristic modes analysis} & CMA optimization and dual orthogonal polarization [65] & 2–9.5 & \textgreater{}20 & \textless{}0.03 & 2 \\  & CMA optimization [68] & 3.4–3.6/4.8–5.0 & \textgreater{}12.5 & \textless{}0.5 & 12 \\  & CMA optimization and DGS [69] & 3.3–5.95 & \textgreater{}15 & \textless{}0.11 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison among the referenced techniques of internal decoupling. There are four additional effects brought by the internal decoupling application that deserve our attention:  
1. Due to polarization diversity, the internal decoupling can further increase the isolation level within a common radiation element. In addition to the significant decoupling result, multipath effects and fading can be effectively mitigated by polarization diversity. Thus, these self-decoupling MIMO systems exhibit strong multi-directional robustness compared to single-polarized systems in some scattering propagation environments [71], demonstrating it has great potential to deploy in massive MIMO for large scale wireless communication devices;
2. Due to the absence of extra decoupling structures, on the one hand, the MIMO systems using internal decoupling can be allowed for smaller volume, which makes the integration with the communication devices frame become easy to realize, and space utilization is enhanced again. On the other hand, this approach that generating uncorrelated polarization modes by elaborately arranging antenna elements to ensure low isolation will not change the excepted radiation characteristics of the MIMO antenna array;
3. It can be noticed that the popular polarization selection is to produce linear polarization (LP) waves in the exciting literature on internal decoupling. Although LP waves have better diversity characteristics, which benefit the directional propagation of the signal, the direction of the LP wave is easily deflected when reflections are encountered. Compared to the LP antenna, the received signals using circular polarization (CP) waves are more stable regardless of the antenna orientation [72]. The MIMO arrays with orthogonal circular polarization can be considered when the signal transmission environment is complicated, such as [73]. 4. The antenna element with different polarized radiation often works as a compact block. Thus, the potential of building multiple antenna arrays is exhibited. It is well known that the increase in the number of antenna array elements leads to the expansion of the ergodic channel capacity. By briefly copying antenna blocks having polarization diversity, the multi-antenna arrays are convenient to be obtained, which facilitates modular design during the construction of MIMO systems.