# 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
MIMO performances such as the ECC and MEG were also determined. Because of how straightforward its form is and how thin it is, it has the potential to serve as the chassis for future handsets. As a result, different user hand scenarios such as single-handed and dual-handed use have been investigated. Figure 16: The eight- and sixteen-antenna MIMO arrays formed using the quad-antenna linear (QAL) array as a building block with front and back views [72]. Along with the SAR, a discussion was held regarding the effects of various hand scenarios on various MIMO parameters. The fact that the proposed system worked well in different situations shows that the proposed structure has a good chance of being used in the next generation of radio smartphones [74]. For 5G applications operating at sub-6 GHz, a new smartphone array antenna design that utilizes new double-fed CPW-fed resonators was comprised of two modified, T-ring radiators that were closely spaced and operated in a frequency band that ranged from 3.3 to 4.4 GHz. An (8 \(\times\) 8) MIMO antenna was formed by placing four pairs of CPW-fed diversity antennas in each of the board's four corners that make up the smartphone. Without needing an additional decoupling structure, it offers excellent isolation of greater than \(-16\) dB. The design of the CPW-fed smart phone antenna array occupies only a very small portion of the board, which is partly due to the fact that the array itself is quite small, and partly because the elements that comprise the array are placed in very particular locations. The proposed MIMO design not only has enough radiation coverage to support all of the main board's sides, but it also has different polarizations [75]. Future smartphones will have a MIMO array with eight unique, balanced, open-slot antennas that operate in the 3.50 GHz band (3.40-3.60 GHz). With less ground design, this antenna's balanced slot mode could improve the isolation between two neighboring input ports. The 3.5 GHz band is where the array operates. By carefully planning the positions related to the eight antenna elements that further reduce the coupling between the elements of the antenna, it is also possible to efficiently reach the desired polarization variety. The positions of antenna elements could be carefully arranged to achieve this. There was good impedance matching (return loss > 10 dB), high total efficiency (>62%), high isolation (>17.5 dB), and a low envelope correlation coefficient (ECC0.05) across the necessary operation bandwidth, according to the tests. To verify the effectiveness of MIMO, calculations regarding the ergodic channel capacity were made with the use of  
Figure 17: Dimensions and specifications in millimeter of the proposed triple-band, eight-element antenna array (**top**) Prospective view; (**bottom**) Side view [73]. the Kronecker channel model. Investigations into the hand phantom's effects were also conducted [76]. For 5G MIMO metal-rimmed smart phones, the combination of the orthogonal mono-pole/dipole modes in a lower band and orthogonal slot/open-slot modes in a higher band was reported. This led to a wideband, orthogonal mode, decoupling property of a dual antenna pair with a shared radiator. For 5G MIMO smartphones, this property was realized. The dual-antenna pair can show a large impedance band width of 3.30-5 GHz, in addition to a strong isolation of \(>\)21.0 dB across the whole band thanks to the orthogonal-mode design method without the need for any additional external decoupling devices. Without the need for any directional couplers, this is accomplished. The simulation and measurement results showed that an 8 \(\times\) 8 MIMO system can fulfill the proposed edges of the smartphone if four of these dual-antenna pairs are arranged on each of the two sides of the smartphone. Both 8 \(\times\) 8 MIMO systems have the potential to provide greater than 12.0 decibels of isolation between all ports and an envelope correlation coefficient of \(<\)0.11.