# Design, Challenges and Developments for 5G Massive MIMO Antenna Systems at Sub 6-GHz Band: A Review
## 2 5G Massive MIMO at Sub-6 GHz
### 5G Massive MIMO Antennas for BS Applications
#### 2.1.1 2D Massive MIMO  
A rectangular planar lattice array technology is the most straightforward planar mMIMO configuration. For instance, an array of N\(\times\)M is an arrangement of the planar array [27]. The 2D model element is one of the most popular used for the design of mMIMO. For use in 5G base stations, dual-band antenna arrays with dual polarization and common aperture have been introduced. The created antenna array includes a (4 \(\times\) 4) planar MIMO array that operates at the 3.3-5 GHz band (upper band-UB), in addition to one antenna element that works at the 0.69-0.96 GHz band (lower band-LB). The UB antenna array can  
Figure 2: Techniques of applications for 5G massive MIMO at sub 6 GHz. be defined as a large rectangular grid array that has 16 antenna elements that are designed for use in practical applications of MIMO. Figure 3a illustrates how in-band and cross-band mutual coupling amongst the UB and LB antennas are suppressed using three decoupling methods: a ferrite chock ring, a rectangular ring resonator, and a unique baffle design. The UB and LB antenna arrays are efficiently integrated with a small size of (0.93 \(\times\) 0.93 \(\times\) 0.17) \(\lambda\)L, employing decoupling technology. In HB and LB antennas, a dual-band antenna array achieves a bandwidth of 41% and 32.7%, respectively. High cross-band port isolation (more than 30 dB) is provided. With average gains of 7.3 dBi and 8.6 dBi, respectively, the UB and LB antennas also produce stable radiation patterns. All operational ranges have better than 90% radiation efficiency. Compact volumes were used as GSM antenna elements for the first time in large-scale MIMO antenna arrays up to 6 GHz with a shared aperture. Additionally, the antenna array's overall height is 0.17 L, which is considerably shorter than the present dual-band BS antennas. For low-profile, wide-band common-aperture 5G mMIMO BS arrays, the decoupling technique thus offers an effective option [28; 29]. Low cross-polarization (X pol), high gain, and isolation properties could be accomplished with the antenna by combining a dual-polarized differential feeding arrangement with a complementary magneto electric (M.E.) dipole antenna. A modified H-shaped (1-16) differential feed network was created to feed the 16-antenna array to utilize these array properties, as illustrated in Figure 3b. In this case, the vertical cross-sections with dotted slots are positioned between adjacent dipole units to further improve the array isolation. A system of high-capacity MIMO antennas could be created through the removal of an H-shaped differential power feeding network and the underlying substrate. Measurement findings show that an antenna element in the 5G frequency band can achieve a low X-pol of \(-\)35.7 dB and a high gain of over 8.1 dBi (i.e., 3.3 to 5.1 GHz). The antenna array can be used to create a 32-channel capacity MIMO antenna technology, thanks to its high gain of 17.3 dBi and low envelope correlation coefficient (ECC) value of 0.004 [30; 31]. For 5G BS applications, dual polarization (D.P.) mMIMO (32T/32R) wideband tightly spaced big antenna arrays have a whole distinct structure and interconnect (mutual cou  
Figure 3: The fabricated prototypes: (**a**) (4 \(\times\) 4) mMIMO of dual-band, dual-polarized shared aperture rectangular antenna array, top view; with side view, (**b**) (4 \(\times\) 4) transformable dual polarized ME-dipole antenna for 5G array/mMIMO of dual-polarized differential feeding lines [30]. pling) analysis. By aligning two chamfered aerial bowtie dipole antennas in the orthogonal directions and employing 2-coaxial cables for feeding, dual polarization is made possible. The model includes chamfering, which increases the port isolation to port, and 2-arms of the bowtie aerials. Uni-directional radiation is produced through the placement of a metal ground plate at a \(\lambda/4\) distance. The radiating component's expected operating range is between 3.5 and 4.0 GHz.