# 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
#### 3.2.1 Symmetric Array  
A dual-polarized antenna that occupied one-half the volume of the traditional crossed dipole antenna was developed. It serves as the basic array element and comprises two back-to-back folded dipoles (2 \(\times\) 2 sub-array). For each dipole, a portion of its feeding line and one dipole arm are set along the vertical direction. In contrast, the remainder portion of the feeding line and other dipole arms are formed along the horizontal direction, as shown in Figure 22. The dimensions of the antenna size were 38.4 mm \(\times\) 19 mm \(\times\) 21.7 mm. The orthogonal currents on the two arms of the dipole could synthesize the slant currents, thus generating \(\pm\)45\({}^{\circ}\) polarization. The proposed antenna element was designed to work at 3.4 to 3.6 GHz band with VSWR \(<\) 1.5. Its longitudinal dimension was only 19 mm at 3.5 GHz, about half of the traditional crossed dipole. Then, a sub-array that adopted four sequentially rotated elements and appropriate phase assignment was investigated. The performance of the proposed array was better than that of the regular array. Therefore, the sub-array can be a good candidate for 5G massive MIMO antenna applications [101]. A 4-port wideband cavity-backed antenna for use in indoor BSs is presented. The antenna consists of an X-shaped isolating block and four feeding monopoles that are orthogonally and symmetrically positioned in the cavity's aperture. The cavity has a square aperture. The modes contributing to the coupling were found using a unique technique depending on characteristic modes analysis (CMA). This investigation led to the suggestion of an X-shaped isolating block put in the cavity's center to improve the port isolation. With measured impedance bandwidth (S11 \(<\) -10 dB) spanning from 1.55 GHz to 6 GHz (118%), a wide-band 4-port antenna with unidirectional radiation patterns was created, covering the majority of sub-6 GHz 5G bands. With 16 dB of minimal measured isolation between ports and an efficiency of more than 84%, the suggested antenna offers four independent radiation patterns. A (4 \(\times\) 4) MIMO simulated system with an ECC of less than 0.5 was used to demonstrate the compatibility of MIMO in various propagation scenarios. The antenna, which has dimensions of 129.5 mm \(\times\) 129.5 mm \(\times\) 28.2 mm and operates at a frequency of 1.55 GHz, is simple to construct. The antenna also has the benefit of avoiding complicated feeding mechanisms with directional couplers or baluns [102]. For 5G wireless communication applications, a brand new dual-polarized dual-band shared-aperture antenna array has been suggested. A LB antenna and four high-band antennas were combined to form this new antenna array. An antenna operating in the low band (0.68-0.99 GHz) and four 5G MIMO antennas operating in the high band (from 3.3 to 5.1 GHz) made up the suggested antenna array. Antennas operating in the low band (from 0.68 to 0.99 GHz) and high band (between 3.30 and 5.10 GHz) were both loop-shape radiators supplied by the Y-shape feed, which simplified the design model while achieving broadband. Incorporating four HB antenna elements into the LB antenna allowed for the resolution of coplanar, dual-band, and shared-aperture issues. According to the simulations,  
Figure 22: 3D of a dual-polarized dipole with halved volume (4 \(\times\) 4) sub-MIMO array antenna [101]. the suggested coplanar antenna array achieved an impedance bandwidth of 43% in the LB (between 0.68 GHz and 0.99 GHz) and 37% in the high band (3.30-5.1 GHz) [103; 104]. A sub-6 GHz uni-planar MIMO antenna system for 5G-capable smartphones was designed. Following the idea of pattern diversity, a MIMO antenna was made up of four symmetric loop-shaped radiators that were positioned at every corner of a mobile phone board. A single-antenna element had an impedance bandwidth of 1.28 GHz (3-4.28 GHz) for S11 \(\leq-6\) and equaled 720 MHz (3.18-3.9 GHz) for S11 \(\leq-10\) dB. Its resonance frequency was 3.5 GHz.