# Design, Challenges and Developments for 5G Massive MIMO Antenna Systems at Sub 6-GHz Band: A Review
## 3 5G Antennas as a Candidate for Massive MIMO Technique at Sub-6 GHz
### Single Elements
#### 3.1.1 3D-Model Antennas  
An experimental study was created on a dual-polarized broadband BS antenna with a new feed structure. The suggested antenna includes two crossed pairs of dipoles, two specifically designed feed connectors, a carrier (also known as a balun), two dielectric pads, and a reflector. We created a prototype and evaluated it to ensure the designed antenna worked as intended. The antenna achieved a port-to-port isolation of over 32.5 dB from 3.14 GHz to 5.04 GHz, a bandwidth of roughly 46.5%, and a reflection coefficient of \(<\)\(-\)15 dB. With half power beam width values of 71.8\({}^{\circ}\)\(\pm\) 2.5\({}^{\circ}\) in the horizontal as well as the vertical planes and a gain of roughly 8 dB in in the operating band, it also offers a remarkably stable radiation pattern. The suggested antenna is also appropriate for BS in the sub-6 GHz frequency band of 5G cells, thanks to its properties [81]. For BS applications, a low-profile, differentially fed, dual-polarization slot antenna was suggested. Its radiator is an octagonal patch that has 2-conventional H-shaped slots etched into it. A folded feed line was introduced, as seen in Figure 19, to make room for the differential feeding scheme and to match the impedance. High isolation and a consistent radiation pattern could both be attained by using differential feeding technology. According to the measurement data, the impedance bandwidths for the two polarizations (VSWR 1.5) were 19.3% (3.14-3.81 GHz) and 20.3% (3.10-3.80 GHz), respectively. Over the whole operating band, the suggested antenna had a high isolation of over 43 dB. Within the operating frequencies, the measured gain was more remarkable than 8.1 dBi [82; 83; 84; 85]. Figure 18: 5G antenna design techniques for 5G massive MIMO at sub-6 GHz. Two rectangular patches and a connected feeding arrangement were combined to create a small BS antenna. The antenna dimensions were 80 mm \(\times\) 75 mm \(\times\) 19 mm. The suggested antenna covered the 5G n78-band with a broad bandwidth of 3.15-3.67 GHz for the VSWR < 1.50. Within the whole operating band, the half power beam width (HPBW) was roughly 67\({}^{\circ}\), the FBR was above 20 dB, and the maximal gain was approximately 8.20 dBi [86]. A cross dipole antenna for dual-band sub-6 GHz 5G applications, 8-parasitic patches, 8-radiating patches, and a reflector made up the majority of the suggested antenna presented. The polarization characteristic of the proposed antenna was \(\pm\)45\({}^{\circ}\). The overlapping bandwidth range was 2.77-5.31 GHz, and the gain fluctuated between 7.60 and 8.40 dBi. With higher operating frequencies, the gain rose. The port isolation was greater than 26 dB, while the cross-polarization level was less than \(-\)28 dB [54]. A brand new dual-polarized base station antenna element has been suggested for LTE, 3G, and 5G mobile communication systems where the structure consists of two orthogonal diamond dipoles, one of which has two circular slots carved into it, while the other is a real diamond. A cross pair of two dipoles could produce polarized radiation at a \(\pm\)45\({}^{\circ}\) angle. Every one of the dipoles had two pairs of branches in its diamond patch. Due to its high cross-polarization discrimination (>13 dB), consistent gains of roughly 8.5 dBi in 1.40-2.7 GHz and 5 dBi in 3.80-4.2 GHz, and high front-to-back ratio (>22 dB), the dual-polarized antenna element of the base station has enough bandwidth and very good radiation properties. The designed aspect of the antenna is commonly utilized in the BS antenna array because of its straightforward structure and outstanding performance [87]. A dual-polarized loop-shaped dipole antenna was designed where the suggested antenna had a small enough structure to be used in a 5G BS. It comprised two feeding baluns and a radiator shaped like a cross. To further reduce the size, the antenna's radiating arms were folded downward. The simulation results demonstrated that the suggested antenna's radiation pattern is stable.