# Broadband Dual-Polarized 2 \(\times\) 2 MIMO Antenna for a 5G Wireless Communication System
## 3 MIMO Antenna  
Figure 8 illustrates the 2 \(\times\) 2 MIMO antenna with four CMAs. The overall dimensions of the MIMO antenna with four CMAs were 250 mm \(\times\) 250 mm \(\times\) 34.6 mm. The CMA1 and CMA3 were vertically polarized and the CMA2 and CMA4 were horizontally polarized. To the CMA, the ground plane is an electrical ground plane. The distances between each CMA were 142.5 mm in the horizontal (x-direction) and 142.5 mm vertical (y-direction) directions, respectively. Figure 9 shows the \(S_{11}\) of the MIMO antenna with four CMAs and Figure 10 shows \(S_{21}\) of the fo MIMO antenna with four CMAs. The MIMO antenna with four CMAs has simulated bandwidth of 118.9% (1.78-7 GHz) and has peak isolation of 15.4 dB at 2.38 GHz. It shows that the MIMO antenna with four CMAs is suitable for 5  
Figure 8: 2 \(\times\) 2 MIMO (multiple-input and multiple-output) antenna with four CMAs. Figure 7: Simulated and measured MRDA VSWR values depending on the height of the ground plane. However, the MIMO antenna with four CMAs size is large. To reduce the size of the MIMO antenna, this study proposed a hybrid structure MIMO antenna with two CMAs and two MRDAs. Figure 10: Proposed \(2\times 2\) MIMO antenna: (**a**) overall view; (**b**) top view. Figure 9: (**a**) Simulated \(S_{11}\) values of the \(2\times 2\) MIMO antenna with four CMAs; (**b**) simulated \(S_{21}\) values of the CMAs \(2\times 2\) MIMO antenna with four CMAs. Figure 10 illustrates the proposed broadband dual-polarized 2 \(\times\) 2 MIMO antenna. The overall dimensions of the antenna were 242 mm \(\times\) 200 mm \(\times\) 40 mm. The two CMAs and MRDAs were vertically and horizontally polarized, respectively. To the CMA, the ground plane is an electrical ground plane. However, the ground plane is not connected to the MRDA, and it acts as a reflector. The distances between the two MRDAs (MRDA1 and MRDA2) were 70.5 mm and 109.5 mm in the horizontal (\(x\)-direction) and 109.5 mm vertical (\(y\)-direction) directions, respectively. Conversely, the distance between the two CMAs (CMA1 and CMA2) was 109.5 mm and 70.5 mm in the horizontal (\(x\)-direction) and vertical (\(y\)-direction) directions, respectively. Figure 11 shows the prototype of the proposed 2 \(\times\) 2 MIMO antenna. Figure 12 shows the simulated and measured \(S_{11}\) values of the proposed antenna. The measured impedance bandwidth of CMA1 and CMA2 (dB magnitude \(S_{11}\)) was 113.2% (1.94-7 GHz), and that of MRDA1 and MRDA2 (dB magnitude of \(S_{11}\)) was 101% (2.3-7 GHz). The proposed antenna operated successfully the 5G NR nd6 (5.15-5.925 GHz), n47 (5.855-5.925 GHz), n77 (3.3-4.2 GHz), n78 (3.3-3.8 GHz), and n79 (4.4-5 GHz) bands. Figure 13 shows the simulated and measured \(S_{21}\) values of the proposed 2 \(\times\) 2 MIMO antenna. The isolation between CMA1 and CMA2 is relatively low in the frequency range 1.4-2.88 GHz, which can be improved by increasing the distance between them. However, as the distance between CMA1 and CMA2 increases, the gain decreases relatively. Therefore, the distance between CMA1 and CMA2 was traded off to meet the requirements of isolation and gain. Figure 11: Prototype of 2 \(\times\) 2 MIMO antenna: (**a**) top view; (**b**) side view. Figure 12: Simulated and measured \(S_{11}\) values of the proposed broadband dual-polarized 2 \(\times\) 2 MIMO antenna. The envelope correlation coefficient (ECC) can be defined by Equation (1):  
\[\rho_{e}=\frac{\left|\iint\overset{\rightarrow}{F_{1}}(\theta,\phi)\overset{ \rightarrow}{F_{2}}(\theta,\phi)d\Omega\right|^{2}}{\iint\left|\overset{ \rightarrow}{F_{1}}(\theta,\phi)\right|^{2}d\Omega\iint\left|\overset{ \rightarrow}{F_{2}}(\theta,\phi)\right|^{2}d\Omega} \tag{1}\]  
In Equation (1), \(\overset{\rightarrow}{F_{n}}(\theta,\phi)\) is the field radiation pattern of the antenna at port n. Solving this equation is a complex process, and the field radiation pattern is essential. The field radiation pattern can be defined as Equation (2). In Equation (2), \(D_{n}\) is maximum directivity of the antenna.