# 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
#### 3.1.2 2D-Model Antennas  
A dual-band, dual-polarization BS antenna was designed for 5G mobile communication. This was made up of two bowtie-shaped cross dipoles that were bent to create two linear polarizations (\(\pm 45^{\circ}\)), as seen in Figure 21. The dual operating frequency values of 700 MHz and 1800 MHz were provided by the bowtie and bent strip-line dipole. At 775 MHz and 1850 MHz, the antenna exhibited an impedance property of approximately 15.80% and 12.0% for S22 \(\leq-\)10 dB (\(-45^{\circ}\) polarization), and S11 less than or equal to \(-\)10 dB (\(+45^{\circ}\) polarization), respectively. In addition, the operational band was given the isolation between the two polarizations, with \(|\)S12\(|\) being approximately \(-33\) dB and \(-30\) dB at 775 MHz and 1850 MHz, respectively. At 775 MHz and 1850 MHz, the antenna's measured peak gain was between 1.26 and 4.76 dBi. Additionally, dual-polarization over the two operating bands as well as the omni-directional radiation patterns were obtained. The antenna was designed proportionally for BS applications for 5G mobile communication [87]. For 5G WLAN applications, a rectangular microstrip patch antenna with an H-shaped slot operating at 4.8 GHz is suggested. The H-shaped slot has been utilized in a rectangular patch of the microstrip patch antenna in order to improve the VSWR, gain, reflection coefficient, and bandwidth of the antenna. The suggested antenna was designed in order to meet the requirements of the 5G WLAN applications. The thin planner profile of the microstrip patch antenna is helping it become more popular as they are easily mountable on the planner surface of space-born applications such as aircraft and missiles. For the frequency of 4.8 GHz, the reflection coefficient and VSWR for this antenna design were \(-25.44\) dB and \(1.11\), respectively. This work developed the suggested antenna with microstrip line feeding [94]. Using a slot feedline and a touch-coupled feeding scheme, a lightweight slotted rectangular microstrip patch antenna for use in 5G wireless applications was proposed, would have high gain and minimal cross-polarization characteristics. The model that has been suggested operates in the sub-6 GHz band and has a frequency span of 3.5 GHz  
Figure 21: Dual-band, dual-polarization antenna with a pair of bowtie and bent strip-line cross dipoles: (**a**) front, (**b**) back [87]. in addition to a bandwidth of 206 MHz. The top layer of the low-k PCB has a compact slotted patch that measures 32 by 25.7 mm\({}^{2}\), and the backside of the second layer has a full ground applied to it. In addition, the middle layer contains several rectangular slots, which are there to improve the impedance bandwidth, gain, and efficiency as well as lower the reflection coefficient. The overall measurements are 43.36 mm by 35 mm by 1.575 mm\({}^{3}\) in volume. In addition, utilizing the slotted tape that was previously mentioned raised the radiative efficiency to 83% with a gain that could reach up to 6.58 dBi, which improved the overall performance. This particular antenna configuration has a reflection coefficient (S11) of 28.774 dB and a cross-polarization of only 70.6 dBi, which is considered low. The design that was proposed is a strong contender for use in 5G massive MIMO applications [95; 96]. For 5G applications, elliptical microstrip patch antennas with and without slots operating at 3.5 GHz have been designed, and their performance examined. For the 3.5 GHz wireless 5G spectrum, three potential designs of small, affordable, wide-band microstrip patch antennas employing FR4 substrate were examined. The suggested antennas have bandwidth values of 1.13363 GHz, 1.0265 GHz, and 2.1562 GHz at \(-\)10 dB return loss with \(-\)41.31 dB, \(-\)43.95 dB, and 20.44 dB, respectively. The resulting gain was 4.45, 4.36, and 4.42 dBi, whereas the obtained directivity was 4.577, 4.906, and 5.0 dBi. The proposed antennas also appear to have the potential to be included in mobile devices [97].