# 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 Smartphone mMIMO Array Antenna Techniques
#### 2.2.5 4-Element/8-Port 5G Massive MIMO
The LTE band 42 (3.4-3.6 GHz) antenna, a LTE band 43 (3.6-3.8 GHz) antenna, and a LTE band 46 antennae are to be covered by a dual-band ring loop antenna (5.15-5.925 GHz). The proposed arrays are designed to change the battery position and exploit the space to add more antennas. To achieve higher isolation, the substrate is formed as the loop antenna elements are printed on a separate dodecagon FR4 substrate with different orientation angles. The proposed designs can achieve an isolation better than \(-26\) dB. The envelope correlation coefficient (ECC) based on S-parameters was found to be better than 0.005 in LTE band 42/43 and 0.006 in LTE band 46. Additionally, the ECC was evaluated based on far-field radiation patterns and was found to be less than 0.2 in LTE band 42/43 and less than 0.12 in LTE band 46. The channel capacities were attained, the 10 \(\times\) 10 MIMO achieved 57.6 bps/Hz, and the (14 \(\times\) 14) MIMO achieved 72 bps/Hz. Additionally, the specific absorption rate (SAR), diversity gain (DG), and the effect of frame insertion on the proposed array were also discussed [66; 67]. The next-generation 5G smartphone offers a small broadband and dual-band antenna array that forms a 12-element MIMO array. Each antenna uses a coupling feed to improve isolation from other antenna elements nearby and impedance matching. With a hybrid loop antenna/IFA, six 5G MIMO antennas were designed to cover the 3.3-4.2 GHz frequency range. To span 2.4-2.5 and 5.1-5.9 GHz, six WiFi MIMO antennas work as a coupled-fed dual-loop antenna. With very little ground clearance, the simulated results showed excellent isolation, impedance matching, and antenna efficiency capabilities [68]. Future MIMO operations in smartphones for 5G will use a tri-polarized 12-antenna array that operates in the 3.5 GHz band (3.40-3.60 GHz). The mutual couplings will be reduced, and the design procedure is made simpler by using the orthogonal polarization technique. A QMSIW antenna and two open-end slots could be combined to create a small (17 mm \(\times\) 17 mm \(\times\) 6 mm) three-antenna tri-polarization block that operates in a 3.5 GHz band. The antenna has an incorporated quarter ode substrate waveguide as a result of this configuration. The three antennas in the block might have low mutual coupling and strong impedance matching due to the presence of orthogonal polarization properties. Next, combining such types of tri-polarization blocks into four different arrays, a 12-antenna MIMOarray was designed for smartphone applications. With just two additional decoupling structures, the presented array might achieve low correlations and good isolation between antennas. Additionally, a tri-polarization feature incorporated into the design is to blame for this. Good antenna performance is conceivable including a return loss of more than 10 dB, isolation of more than 12.5 dB, and antenna efficiencies of over 50%. The channel capacity of the 12-antenna array has been determined to be around 57 bps/Hz in a (12 \(\times\) 12) MIMO system with a 20 dB SNR, showing that the suggested array using the tri-polarization approach is a solid candidate for potential future 5G terminals [69]. In order to facilitate multi-input multi-output (MIMO) functionality in a mobile device, a 10-antenna array that operates in the 3.6 GHz band (3.4-3.8 GHz) has been structured. The proposed antenna array will consist of microstrip line-fed open-slot antennas, each of which will have the same modest dimensions of 3 mm \(\times\) 8 mm. The proposed array is made up of two sets of five antennas, each of which is arranged symmetrically along one of the smartphone's long side edges of the system ground plane. The suggested array is predicted to be positioned in the constrained space between the display screen and longitudinal side borders of a smartphone, as seen in Figure 15. Any two antennas in the suggested array were found to be capable of achieving a satisfactory ECC of 0.1 and an adequate level of isolation (more than 10 dB).