# Designed Circularly Polarized Two-Port Microstrip MIMO Antenna for WLAN Applications
## 6 MIMO Antenna System Parameters
### Diversity Gain  
When transmitters receive multiple forms of the transmission stream through distinct channel pathways, the effects of diversity are frequently accomplished. The diversity gain value was found to be near 10 dB, indicating good diversity performance. The value of diversity gain can be computed using Equation (2) from the envelope correlation coefficient [51]. \[DG=10\times\sqrt{1-(ECC)^{2}} \tag{2}\]  
where \(ECC\) denotes the envelope correlation coefficient, and \(DG\) denotes diversity gain. Figure 16b shows the proposed antenna's simulated and measured diversity gain, which is close to 10 dB. The characteristics of the MIMO-CP antenna are compared with those of other state-of-the-art antennas, regarding mutual coupling reduction and circular polarization, as shown in Table 2. The objective of the current work is to improve axial-ratio and reduce mutual coupling, as well as high gain using MIMO-CP antenna. As observed from the comparison, the proposed design is a two-port MIMO-CP antenna. Apart from Table 2, a pair of complementary point symmetric micro-machined meander line (ML) slots was inserted on the ground plane between two patches that are closely spaced, to suppress mutual coupling without any resonant frequency mismatch between S11 and S22 [52]. In [34], for high isolation, F-shaped DGS were used with simple offset feeding for circular polarization, but the design suffers from a narrow axial ratio and a complicated structure. In [53], for improved isolation, defected ground slits were etched between two radiators with the lowest bandwidth and axial ratio. In [54], in the ground, an inverted \(Z\)-shaped slot was etched to produce circular polarization for both the axial ratio and mutual coupling. In [33], the authors presented the design of the L-shaped CP MIMO antenna, using DGS for the WLAN application. However, in order to achieve circular polarization characteristics, they used two complicated slots in the ground, making the design comparatively more complex and requiring a large antenna size. This work is unique in two ways. First, we determined how geometrical parameters (main square patch heights, corner square-cut, and length/width) affect MIMO-CP antenna characteristics and how these can be adjusted to achieve an optimal design without requiring extensive simulations. Second, the final design incorporates a slot in the ground that \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Reference**} & **Central Frequency** & **Peak Isolation** & **Bandwidth** & \multirow{2}{*}{**ABBW CP/LP**} & \multirow{2}{*}{**Gain (dB)**} & \multirow{2}{*}{**Size (mm)**} \\  & **(GHz)** & **(dB)** & **(GHz)** & & & \\ \hline
[23] & 5.7 & 18 & 5.5–6.1 & 5.52–5.64 & 2.85 & 29 \(\times\) 48 \\
[33] & 5.5 & 22.5 & 5.2–6.08 & 5.2–5.58 & 4.0 & 40 \(\times\) 65 \\
[35] & 5.8 & 24.5 & 5.6–5.93 & LP & 4.0 & 44 \(\times\) 37 \\
[36] & 5.7 & 15.4 & 5.6–5.8 & LP & 1.41 & 26 \(\times\) 26 \\
[37] & 5.2 & 15 & 5.05–5.35 & LP & 1.40 & 26 \(\times\) 26 \\
[38] & 5.8 & 35 & 5.4–6.08 & 5.72–5.815 & 5.3 & 27.69 \(\times\) 97 \\
[40] & 6.44 & 15 & 5.71–8.2 & 7.72–8.08 & 3.8 & 80 \(\times\) 80 \\
[41] & 2.44 & 24.6 & 2.43–2.50 & LP & 4.68 & 95.9 \(\times\) 38.2 \\
[42] & 3.7 & 15 & 3.3–4.2 & 3.2–4.25 & 2.5 & 30 \(\times\) 37 \\
[43] & 2.08 & 24 & 1.82–2.57 & 2.04–2.5 & 4.0 & 66 \(\times\) 66 \\
[44] & 5.8 & 20 & 5.5–5.85 & 5.59–5.71 & 3.92 & 29 \(\times\) 34 \\
[45] & 3.5 & 21 & 3.38–3.80 & 3.340–3.57 & 4.91 & 40 \(\times\) 65 \\
[46] & 5.6 & 14 & 5.5–6.08 & 5.5–5.9 & 4.4 & 30 \(\times\) 25 \\
[47] & 5.65 & 25 & 5.2–6.0 & LP & Nil & 100 \(\times\) 50 \\ Proposed work & 5.6 & 37 & 5.2–6.4 & 5.37–5.72 & 6 & 22.5 \(\times\) 50 \\ \hline \end{tabular}
\end{table} 
Table 2: Design parameter of the MIMO antenna circular polarization. improves both the isolation and axial ratio. In addition, an air gap was provided between the ground and the substrate to eliminate dielectric losses, resulting in maximum antenna gain and efficiency.