# A Dual-Band Flexible MIMO Array Antenna for Sub-6 GHz 5G Communications
## 4 MIMO Array Antenna
### Results and Discussion  
The suggested MIMO antenna prototype is provided in Figure 19. The antenna holds a measured bandwidth of 3.6-3.8 GHz and 5.65-5.95 GHz with a resonance of 3.75 GHz and 5.9 GHz, respectively. There is a negligible shift in the first resonance of the proposed antenna due to cable losses. Figure 20 displays the proposed MIMO array antenna's measured scattering properties.  
From Figure 20, the presented antenna offers enhanced isolation in the operating bands. The mechanism of the decoupling structure can be inferred from the current vector distribution plots portrayed in Figure 21 at both resonant frequencies. It is investigated by exciting port 1 and terminating port 2. From the current distribution, it is seen that the maximum current is concentrated on elements in the excited port and the decoupling structure. On the other hand, there is a trivial current concentration in the nearest elements. The current concentration in the decoupling structure shows that the current direction is  
Figure 19: Fabricated prototype of the MIMO configuration.  
Figure 20: Simulated and measured scattering parameters of the MIMO configuration.  
Figure 18: Surface current distribution of CM modes: (**a**) first resonance and (**b**) second resonance.  
opposite to each other, indicating that the coupling current cancels out each other, thereby offering enhanced isolation.  
The radiation characteristic measurement of the MIMO array antenna in the chamber is shown in Figure 22. A double-ridged horn is utilized as the reference antenna, and the proposed MIMO array antenna is kept as the antenna under test (AUT), as seen from the figure. The radiation characteristics in both E and H planes (co- and cross-polarization) of the MIMO array antenna are depicted in Figure 23. The radiation pattern gives significant variation between the co- and cross-polarization. The proposed antenna shows the bidirectional and omnidirectional types of characteristics in the E and H planes, respectively. The max gain and radiation efficiency plots of the suggested antenna are provided in Figure 24. The antenna exhibits a gain value of 5.2 dBi/92% and 7.7 dBi/91% at the first and second resonances, as given in the figure.  
The diversity parameter estimation of the suggested antenna is given in Figure 25a-c. The envelope correlation coefficient (ECC) evaluates the correlation of the individual elements of the MIMO antenna. Diversity gain (DG) is the parameter that indicates the transmission capacity of the antenna when the diversity system is utilized. Equation (4) can be used to determine the ECC from the far-field patterns [21]. DG can be derived from ECC using Equation (5). The proposed antenna holds ECC and DG values of <0.05 and >9.99 dB, respectively, for both bands. The mean effective gain (MEG), which  
Figure 21: Vector current distribution: (**a**) 3.75 GHz; (**b**) 5.9 GHz.  
Equation (6), compares the energy obtained by an isotropic antenna to a MIMO antenna in a fading environment. The suggested antenna gives a MEG value of \(-3\) dB over the bands. The proposed antenna's total return loss characteristics can be estimated using the total active reflection coefficient (TARC). Equation (7) computes the TARC of the proposed MIMO array antenna.  
\[\text{ECC}_{\rho}=\frac{\left.\left|\int_{4\pi}[\text{E}_{1}(\theta,\phi). \text{E}_{2}(\theta,\phi)\text{d}\Omega]\right.\right|^{2}}{\iint_{4\pi}\  
Figure 24: Gain and efficiency plots of the MIMO configuration.  
Figure 25: Cont.  
Figure 23: Radiation patterns of MIMO antenna: (**a**) 3.75 GHz; (**b**) 5.9 GHz.