# Design, Challenges and Developments for 5G Massive MIMO Antenna Systems at Sub 6-GHz Band: A Review
## 2 5G Massive MIMO at Sub-6 GHz
### 5G Smartphone mMIMO Array Antenna Techniques
#### 2.2.5 4-Element/8-Port 5G Massive MIMO
The two antenna elements that make up the dual-antenna pair have average efficiencies of 74.7% and 57.8%, respectively, according to the measurements taken. The proposed design has the benefits of a shared radiator, a wide bandwidth, and the ability to work with metal rims and so could be used in 5G phones in the future [77; 78]. A dual-band (8 \(\times\) 8) MIMO array antenna that operates in the 3.5 GHz band (3.4-3.6 GHz) and the 5.5 GHz band (5.150-5.925 GHz) for 5G mobile handsets was made up of a comb-shaped monopole and an L-shaped open slot antenna that were symmetrically positioned on the inner surface of the side-edge frame of the smartphone. It is feasible to achieve pattern diversity, which could lower the ECCs and enhance the functionality of MIMO systems. The results show that the isolation of 15 dB and 10 dB in the high and low bands, respectively, could be accomplished without introducing any additional decoupling element and that the desired bands could be satisfied with an impedance matching of 6 dB. According to the findings, the suggested MIMO antenna will be a fantastic option for 5G services in the near future [79]. MIMO schemes with 4-antennas/8-dual polarized ports were developed on a printed circuit board (PCB) side (67 mm \(\times\) 139 mm), side by side, using an epsilon 4.4, FR-4 dielectric substrate, and the thermal conductivity was 0.025. To improve the radiation characteristics, circular slot radiators were etched into the substrate. In addition, to reduce mutual coupling, two rectangular open-fin parasitic radiators were included into a single square-slotted radiator in the suggested MIMO architecture. The proposed single-antenna model had an impedance bandwidth of \(-\)10 dB and covered the frequency band 5.81-6.66 GHz. However, at \(-\)6 dB, the bandwidth rose to 1.47 MHz (5.48-6.95 GHz). According to the data, a return loss of \(-\)20 dB is feasible, and the isolation of the dual micro-strip lines could reach \(-\)45 dB. The suggested antenna could be included in new smart mobile devices for upcoming 5G wireless communications [80]. Table 3 presents the massive MIMO technique enhancement and supposition for smartphone applications. Table 4 compares the previous references for massive MIMO for smartphone applications. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**Antenna Schemes**} & \multirow{2}{*}{**MIMO Model**} & \multirow{2}{*}{
\begin{tabular}{c} **Supported** \\ **Massive Tech.** \\ \end{tabular} } & \multicolumn{6}{c}{**Enhancements**} \\ \cline{5-8}  & & & **Compact** & **B.W** & **Isolation** & **Gain** & **Eff.** \\ \hline \multirow{3}{*}{[58]} & Slot antenna-open & \multirow{3}{*}{\(18\times 18\)} & \multirow{3}{*}{Yes} & ✓ & ✓ & ✓ & ✓ & ✓ \\  & ended decoupling & & & & & & \\  & slots & & & & & & \\  & Inverted \(\pi\)-, longer & \multirow{3}{*}{\(8\times 8\) (LB)} & \multirow{3}{*}{Yes} & ✓ & ✓ & & & ✓ \\  & and shorter inverted & & & & & & \\  & L-shaped, open slot, & & & & & & \\  & antennas & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Massive MIMO technique enhancement and supposition for smartphone applications. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Ref. ** & \ \begin{tabular}{c} **No. of** \\ **Elements/** \\ **Ports** \\ \end{tabular}  & **Size (mm)** & **B.W (GHz)** & **Isolation** & **ECC** & **Gain (dBi)** & **Eff. (\%)** & \begin{tabular}{c} **PLL** \\ **(bps/Hz)** \\ \end{tabular}  \\ \hline  
Higher antenna efficiency is needed for mobile devices using mMIMO systems. This entails adding additional antennas to the same physical area or increasing the bandwidth of already installed antennas. The suggested system's effectiveness was examined by adjusting critical operating parameters including ECC, MEG, scattering parameters, and channel capacity, and carrying out research such as user hand analysis. The technology is safe to use close to the human body, according to a SAR analysis that was conducted to understand how it interacts with the body.