# 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 Massive MIMO Antennas for BS Applications
#### 2.1.3 Combined (Sub-6 GHz/MM Wave) mMIMO
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**Antenna Schemes**} & \multirow{2}{*}{**Topology**} & \multirow{2}{*}{**MIMO Model**} & \multirow{2}{*}{**Supported Massive Tech.**} & \multicolumn{5}{c}{**Enhancements**} \\ \cline{6-10}  & & & & **Model** & & **Massive Tech.** & **Compact** & **B.W** & **Isolation** & **Gain** & **Eff.** \\ \hline
[28] & UB-FSS-LB & Rectangular & 4 \(\times\) 4 & Yes & ✓ & ✓ & & & ✓ \\  & ME dipole-cross & & & & & & & & \\  & patches-dotted & Rectangular & 4 \(\times\) 4 & Yes & ✓ & ✓ & & & \\  & slots & & & & & & & & \\  & Bowtie dipole & & & & & & & & \\  & aerial & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Massive MIMO techniques: enhancement and support for base station applications. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Ref.** & **Antenna Schemes** & **Topology** & **MIMO Model** & \multicolumn{3}{c}{**Supported Massive Tech.**} & \multicolumn{3}{c}{**Enhancements**} \\ \cline{4-10}
**Model** & **Control-based** & **Model** & **Model** & **Model** & **Model** & **Model** & **Model** & **Model** & **Model** & **Model** \\ \hline
**[**34**]** & **Patch sub-array** & **Rectangular** & \(8\times 8\) & **Yes** & \(\checkmark\) & & & \(\checkmark\) & \(\checkmark\) & \\
**[**35**]** & **Multimode-slotted** & **Rectangular** & \(11\times 11\) & **Yes** & \(\checkmark\) & \(\checkmark\) & \(\checkmark\) & & \(\checkmark\) \\
**[**40**]** & **Patch sub-array** & **Rectangular,** & \(5\times 4\) & **Yes** & \(\checkmark\) & & & \(\checkmark\) & \\
**[**41**]** & **Patch-stacked** & **Ortho-** & & & & & & \(\checkmark\) & \\  & **polyhedron ring** & & & & & & & & \\  & **Tapered slot** & & & & & & & & \\  & **antenna & **Cylindrical** & & & & & & & \\  & **(TSA)** & & & & & & & & \\  & **Square patch with** & & & & & & & & \\  & **two merged** & **Rectangular** & & & & & & & & \\  & **u-slots-dipole** & & & & & & & & & \\  & **Microstrip patch christop** & & & & & & & & & \\  & **Slotted microstrip patch antenna-** & & & & & & & & \\  & **(NZI-ENG) metamaterial** & & & & & & & & & \\  & **Cylindrical decoupling structure** & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Ref.** & **No. of** & **Size (mm)** & **B.W (GHz)** & **Isolation (dB)** & **ECC** & **Gain (dBi)** & **Eff. (\%)** \\ \cline{2-7}
**[**28**]** & **16/32** & **344** \(\times\) **344** \(\times\) **63** & **3.3–5.0 (UB) & \(>\)**30** & \(<\) **0.1** & **7.3 (UB) 8.6 (LB)** & \(>\)**90** \\
**[**30**]** & **16/32** & **90** \(\times\) **90** \(\times\) **19.5** & & & & & & \\  & **(single element)** & **3.25–5.35** & **-** & **0.004** & **-** & **-** \\
**[**32**]** & **16/32** & **200** \(\times\) **200** \(\times\) **32** & **2.8–4.0** & **27** & **-** & **9.1** & **-** \\
**[**34**]** & **64/16** & **333.3** \(\times\) **333.3** \(\times\) **1.6** & **3.45–3.68** & **25** & **-** & **5.4** & **30.887** (one-port)** & **(one-port)** \\  & & & & & & & & \\  & & & & & & & & \\  & & & & & & & & \\
**[**40**]** & **20/5** & **280.5** \(\times\) **56.1** \(\times\) **2** & & & & & & \\  & & & & & & & & & \\  & & & & & & & & & \\  & & & & & & & & & \\  & & & & & & & & & \\
**[**41**]** & **288/144** & **648** \(\times\) **648** \(\times\) **258** & **3.65–3.81** & \(>\)**31** & **-** & **Sub-array** & **-** \\  & & & & & & & & (1\(\times\)4 ant. units) & \\
**[**43**]** & **24/24** & **280** \(\times\) **194.4** \(\times\) **1.6** & **3.0–4.2** & **\(>\)**20** & **\(<\)**0.01** & **-** & **-** \\
**[**50**]** & **32/32** & **440** \(\times\) **440** \(\times\) **1.6** & **2.4–2.62** & & & & & \\  & & & & & & & & (Single Patch) & **-** \\ \hline \hline \end{tabular}  
* UB: upper band; LB: lower band.  
\end{table} 
Table 2: Comparison of sub-6 GHz mMIMO antenna techniques.