# 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
In the operating bandwidth of 3.68 to 4.05 GHz, the peak gain was greater than 6.7 dBi. The antenna is only 0.145\(\lambda_{0}\) in height, while the radiator is roughly a quarter wavelength in size [88]. For BS applications, a dual-polarized broadband antenna with interference-canceling features is offered. The three parts of the suggested antenna are the feed structure, the main radiator, and the reflector. To achieve \(\pm\)45\({}^{\circ}\) polarization, the central radiator, for instance, contains two crossed dipoles. Between the primary radiator and the feed's reflectors are two vertical boards with \(\Gamma\)-shaped feeders on the front and rectangular patches on the back. A reflector was used underneath the antenna to produce high gain and unidirectional radiation. A C-shaped stub was further positioned next to the lead to filter undesirable frequency ranges. A prototype antenna was built and tested, as illustrated in Figure 20. The findings from the measurements showed that a notched band with a 52.6% bandwidth existed between 2.27 and 2.53 GHz (VSWR 1.5). The HPBW was roughly 60 degrees, the observed isolation was over 25.4 dB, and the overall operating band average gain was 7.57 dBi. The suggested antenna has the following benefits: small size, dual-polarization operation, band suppression properties, excellent impedance matching, sturdy  
Figure 19: Fabrication of dual-polarized slot antennas with octagonal patches and folded feedlines [82]. structure, perfect PCB design, and straightforward process, making it appropriate for next-generation wireless and making it possible for it to be widely utilized in communication systems [89; 90]. The new dual-loop array antenna has four rectangular loops and four trapezoidal loops printed on the front and back of the substrate. These loops are each positioned over a flat square reflector and are intended for use in current and future applications involving base stations. In order to achieve polarization states of less than 45 degrees, all eight loop radiators were energized at the same time by a feed network that had been thoughtfully designed. The behavior of a trapezoidal loop is analogous to that of a folded (electric) dipole. Magnetic dipoles are the primary effect that rectangular loops possess. The combination of these two loop arrays resulted in a magnetoelectric (ME) loop antenna that demonstrated a consistent directional pattern with high cross-polarization discrimination (XPD) values over an operating fractional bandwidth of 45.5% from 1.7 to 2.7 GHz. This magnetoelectric (ME) loop antenna covered the frequency range from 1.7 to 2.7 GHz and it was possible to acquire it. The results of the simulation were confirmed by the fabrication and measurement of a prototype, which showed that the horizontal full width at half maximum (HPBW) varied between 63\({}^{\circ}\) and 70\({}^{\circ}\) and that the XPD values were greater than 20 dB on the central axis and greater than 10 dB on the axis. The total angular range covered by cellular service was \(-60^{\circ}\leq 0\leq 60^{\circ}\)[91]. The main radiator, feeding balun, reflector, and two parasitic loops are the four components that comprise the dual-polarization filter wideband dipole-antenna for base station application. This antenna has a compact size of 50 mm \(\times\) 50 mm \(\times\) 31.8 mm. In order to achieve adequate filter performance while simultaneously expanding its bandwidth, a dual-polarized dipole antenna requires only two parasitic loops instead of more complicated filter circuits. Consequently, two distinct radiation zeros are produced, each of which is controlled independently by two parasitic loops. Simple stubs with open ends can be added to the arms of the dipole so that the selectivity of the upper stopband can be improved even further, as can the bandwidth of the band. Therefore, the bandwidth can be changed from 7.4% to 47.6%, and the gain that can be accomplished can range from 8.6 dBi at 2.7 GHz (in-band) to \(-10\) dBi at 2.9 GHz (out-of-band).