# 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
### Sub-Arrays
#### 3.2.2 Non-Symmetric Array  
A non-symmetric array was considered as a (1 \(\times\) n) sub-array in the arrangement. A wideband dual polarization antenna was suggested for 4G/5G communications applications. The antenna element was made up of two open-loop, bipolar dipoles with three different resonant modes pushed closer to one another to create a wide bandwidth. An extra \"U\"-shaped slot that was etched around the feed point made the antenna element's input impedance equal to 50 ohms. The constructed antenna element had high port-to-port  
Figure 23: Front and back plane of the fabricated L-shaped and ground plane with four etched slots for the (4 \(\times\) 4) MIMO antenna [106]. isolation (higher than 25 dB), a 75.90% impedance bandwidth (VSWR \(\leq\) 2), and operated in the 1.8 to 4.0 GHz frequency range. It also offered a beamwidth of 67\({}^{\circ}\)\(\pm\) 1\({}^{\circ}\) in the H plane and 68.7\({}^{\circ}\)\(\pm\) 3.3\({}^{\circ}\) in the V plane, with a gain of 8.5 \(\pm\) 1 dBi in the supported bands. A 6-element (1 \(\times\) 6) dual-polarized array with an electrically down-tilt was also created and measured; this array is preferred for communication applications because it achieves a peak gain of 16.8 dBi, a beam width in the H-plane that was comparable to one antenna element, and an electrically down tilt in the V-plane that ranged between 0\({}^{\circ}\) and 12\({}^{\circ}\). Six antenna elements were used to obtain such results, and an RF phase shifting module was created. In the near future, this antenna array might be utilized for future 5G communications as well as other applications [108; 109]. An innovative linearly dual-polarized antenna array with triple elements (1 \(\times\) 3) has been suggested for beam width reconfigurable base station communications. A single, dual-polarized antenna unit has the ability to obtain a wide bandwidth of 78.40% for SWR \(\leq\) 2, which ranges between (1.63 GHz and 3.73 GHz) for 2G/3G/LTE and 3.5 GHz of C-band (3.40-3.60 GHz) applications if arc-shaped corners on the radiated patches are removed. This can be conducted by cutting off the corners of the radiated patches. Additionally, because of the box-shaped reflector and intrinsic performance of the ME dipole antenna, the dual-polarized antenna element has an approximate gain of 11.5 dBi on average. In the final result, a 3-element antenna array with a widely adjustable beam width in the E-plane as well as the H-plane was investigated. This antenna array could meet the needs of smart applications in the future and was made with a two-stage circuit and numerous power dividers [110]. Four arrays with more than 60 dB isolation in the 5.15-5.925 GHz band were used to illustrate the process of designing a dual-polarized linear antenna array with enhanced port-to-port isolation. The single antenna adopts a dual-polarized electromagnetic coupling microstrip antenna, and the isolation between ports did not exceed 25 dB. Array isolation can be significantly improved by using a dedicated feed network. A mathematical model was established based on theoretical analysis to describe the isolation between ports. Circuit and full-wave simulations were performed to show the effect of electromagnetic coupling between the antennas and/or microstrip lines in the feed network and the effect of the phase shifter/power divider selection on isolation. The manufactured prototype featured a gain of approximately 14 dBi, a minimum polarization purity of \(-\)27 dB within the main lobe, and was close to the expected isolation of \(>\)57 dB over the entire operating range [111]. A compact, highly isolated MIMO antenna system for wireless applications in 5G-connected devices has been considered with the size of 92 mm \(\times\) 88 mm and consisting of two elliptical antennas symmetrically arranged next to one another. To provide the various MIMO antennas proposed, two decoupling methods were applied: neutralization and DGS. The single and MIMO antennas were modeled and analyzed, then built and measured.