## 4 Dual-Band MIMO Antenna Designs
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **MIMO** & **Antenna Size** & **Antenna Frequency** & **Bandwidth** & **Isolation (dB)** & \multirow{2}{*}{**Gain (dBi)**} & \multirow{2}{*}{**Efficiency**} & \multirow{2}{*}{**ECC**} & \multirow{2}{*}{**CCL** (**bits/s/Hz)**} \\  & **Element** & **(mm\({}^{3}\))** & & & & & & & \\ \hline
[22] & \(4\times 4\) & \(48\times 34\times 1.6\) & 3.52–10.08 & Modified & \(\leq\)\(-23\) & 0.95–2.91 & 70.01– & \(\leq\)0.039 & \(\leq\)0.29 \\  & & & rectangular patch & Neutralization line & & & & & \\ \hline
[24] & \(2\times 2\) & \(25\times 36\times 1.6\) & 2.78–17.43 & Corner-truncated & \(<\)\(-19\) & & & & \\  & & & rhombus-shaped & Rectangular strip & - & - & \(<\)0.008 & \(<\)0.31 \\  & & & (CTRS) & connected with GP & & & & & \\ \hline
[25] & \(2\times 2\) & \(36\times 22\times 1.6\) & 2.5–2.85/ 4.82–6.1 & Square radiating patch & Flag-shaped stub connected with middle GP & 9.992 & - & \(<\)0.05 & - \\ \hline
[26] & \(2\times 2\) & \(21\times 34\times 1.6\) & 3.52–9.89 & Dome-shaped patch & \(\leq\)\(-22\) & 3.08–5.12 & \(>\)62 & \(\leq\)0.005 & \(<\)0.26 \\  & & & patch & Neutralization line & & & & \\ \hline
[27] & \(2\times 2\) & \(30\times 50\times 1\) & 3–10.9 & F-shaped radiators with & \(\leq\)\(-20\) & & & & \\  & & & L-shaped open-slots & Fork-shaped slots & 1.9–38 & - & \(<\)0.06 & - \\ \hline
[28] & \(2\times 2\) & \(22\times 43.5\times 1\) & 2.45 & L-shaped radiating patch & \(<\)\(-40\) & 1.9 & 81.7 & 0.06 & - \\  & & & radiating patch & Tapered slot & & & & & \\ \hline
[29] & \(4\times 4\) & \(42\times 42\times 1\) & 3.3–4.2 & Four conjoined slots & \(<\)\(-10\) & & & & \\  & & & & & & & & \\ \hline
[30] & \(4\times 4\) & \(45\times 45\times 1.6\) & 4.3–6.45 & Split-shaped radiating patch & Decoupling structure & 4.0–5.0 & 90 & \(<\)0.2 & \(<\)0.018 \\ \hline
[31] & \(4\times 4\) & \(40\times 40\times 1.6\) & 3.1–11 & Circular patch & \(<\)\(-20\) Decoupling structure & 3.28 (avg. gain) & - & \(<\)0.004 & \(<\)0.4 \\ \hline
[32] & \(3\times 3\) & \(45\times 25\times 1.588\) & 3.1–11.5 & Staircase-shaped radiators & \(\leq\)\(-19\) & 5.5 (peak gain) & 61–98 & \(\leq\) 0.2 & - \\ \hline
[33] & \(4\times 4\) & \(\begin{array}{c}110\times 110\times \\ 1.45\end{array}\) & 1.7–7.2 & Kraus technique & \(\begin{array}{c}<\)\(-20\) \\ Electro-magnetic walls \\ \end{array}\) & 3.0–5.2 & 90 & 0.0025 & - \\ \hline
[34] & \(2\times 2\) & \(29.5\times 60\times 1.6\) & 3.05–20 & L-like stubs & L-like stubs & Metallic barriers & 3.36–4.92 & 83 & \(<\)0.00012 & 0.325 \\ \hline
[35] & \(2\times 2\) & \(50\times 35\times 1\) & 3.0–11 & L-shaped parasitic branches & \(\begin{array}{c}<\)\(-25\) \\ Fence-type decoupling structure \\ \end{array}\) & above 3 dB & \(>\)80 & \(<\)0.004 & - \\ \hline
[36] & \(2\times 2\) & \(18\times 28\times 1.6\) & 1.9–14 & Three crossed X-shaped stubs & \(<\)\(-15.5\) X-shaped stubs in ground planes & 0.4–4.8 & - & \(<\)0.09 & \(<\)0.4 \\ \hline
[37] & \(2\times 2\) & \(16\times 26\times 1.6\) & 2.82–14.45 & Circular radiator & \(\begin{array}{c}<\)\(-22\) \\ Stubs and protruded strip \\ \end{array}\) & 0.7–6.86 & \(\geq\)91.7 & \(<\)0.08 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of different UWB MIMO antennas. Additionally, a decoupling network is added between the two patches to improve isolation without increasing the footprint. The dual band characteristics are achieved by changing the length of two monopole arms. An elliptical slot and a rectangular parasitic strip are used to establish isolation over the operating dual bands [44]. A dual-band MIMO antenna with two C- shaped patches is employed and a bent T-shaped structure is implanted between the two monopole radiators to provide good isolation [45]. To decrease inter-element coupling, an open slit in the GP is used in a dual-band MIMO antenna design [46].