# 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 proposed array's maximum channel capacity in a (10 \(\times\) 10) MIMO system has been estimated to be around 47 bps/Hz at a SNR of 20 dB. This exceeds the upper limit of the ideal (2 \(\times\) 2) MIMO system, which is 11.5 bps/Hz, has 100% antenna efficiency, and has no ECC between the antennas [70]. Two radiators were etched on the outer and inner surfaces of the side-edge frame device to form a dual-band 10-element MIMO array based on dual-mode inverted-F antennas (IFAs). This array was designed for use in 5G terminal applications. An additional one-quarter wavelength mode was radiated by the inner part of the antenna at 4.9 GHz, while the outer part of the antenna was responsible for generating the low-order mode at 3.5 GHz. The IFA can accomplish dual-band operation in this manner despite its compact size of 10.6 mm \(\times\) 5.3 mm \(\times\) 0.8 mm. In order to support 5G terminal applications, a dual-band, ten-element multiple-input multiple-output (MIMO) array was developed. The foundation of this array is the suggested antenna. Combining decoupling branch structures with neutralization line structures helps improve isolations between the elements. The idea for a prototype of a 10-element MIMO array was developed, built, and tested to confirm the practicality of the design concept. The 3.30-3.6 GHz and 4.80-5 GHz frequency  
Figure 15: Fabricated (10 \(\times\) 10) MIMO array with the microstrip line-fed open-slot antenna [70]. bands can both be covered by the suggested antenna with high efficiency and strong isolation, according to the experimental results. To evaluate the effectiveness of MIMO for 5G applications that operate below 6 GHz, the ECC and the channel capacity were also evaluated [71]. The quad-antenna linear (QAL) array can be used as a building block for smartphone 16-antenna and 8-antenna arrays for 3.5 GHz MIMO operation. The QAL array has the dimensions of 50 mm \(\times\) 3 mm. Two QAL arrays are positioned on the system circuit board of the smartphone, either on two different sides or on the same side, to produce an 8-antenna array. The analysis of the 16-antenna array made up of four QAL arrays arranged along two opposing side edges is shown in Figure 16. With a 20-dB SNR, the calculated channel capacity for use in a (16 \(\times\) 16) MIMO system can reach approximately 66-70 bps/Hz. The achieved channel capacity is about 5.70-6.10 times more than the maximum achievable in an ideal (2 \(\times\) 2) MIMO system with antennas operating at 11.5 bps/Hz (100% antenna efficiency) [72]. For applications on triple-band 5G metal-frame smartphones, the author in [73] proposed a MIMO antenna array with good performance whereas the structure use an L-shaped radiation strip and an S-shaped feeding strip (Figure 17). The achieved bandwidth increased for lower frequency band while minimizing size by employing an S-shaped feeding strip. The novel aspect of this investigation is the antenna element's small size with 6.5 mm 7 mm construction. The suggested eight-antenna array's 6 dB impedance bandwidth may completely cover the 3.3-3.8 GHz, 4.8-5 GHz, and 5.15-5.925 GHz frequency bands. A 0.8 mm thick, low-cost, FR-4-substrate with dimensions of 136 mm \(\times\) 68 mm and an inverted L-shaped, eight-element MIMO antenna system was developed. This resonates at 3.50 GHz with a measured bandwidth of 450 MHz, inter-element isolation of over 15 dB, and gain of 4 dBi. The system was developed on an inexpensive FR4 substrate. A total of eight elements in the shape of an inverted L and parasitic L-shaped strips extending from the ground plane make up the proposed design. These short stripes served as tuning stubs for four inverted L-shaped monopole elements on the side of the chassis. To achieve the objective of reaching the necessary frequency range, the electrical length of antennas was increased. The decision was made to create a prototype, and the findings of the investigations showed that there was adequate impedance matching and reasonable measured isolation within the intended frequency range.