# 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  
Massive MIMO systems for 5G have one proposed design for an antenna array for mobile phones that uses diamond-ring slot elements. Four double-fed (eight ports/four elements) diamond-ring slot antenna elements make up the design configuration, and they are each placed in a separate corner of the printed circuit board for the mobile phone (PCB). A cheap FR4 dielectric with overall dimensions of 75 mm \(\times\) 150 mm was utilized as the design substrate. The elements of the antenna were fed by L-shaped micro-strip wires with a resistance of 50 ohms. Because of the orthogonal positioning of the micro-strip feedlines, the diamond-ring slot elements were able to display the radiation and polarization pattern diversity characteristics. For each antenna radiator, we were successful in achieving a good impedance bandwidth with a range of 3.2-4 GHz (S11 \(\leq-\)10 dB). Nevertheless, this value was 3-4.2 GHz when S11 is \(\leq-\)6 dB. The design that was proposed offers the necessary radiation coverage for 5G mobile phones. The proposed MIMO smartphone antenna has achieved high levels of isolation and efficiency. Additionally, for the entire band of interest, the TARC and ECC of the antenna elements are incredibly low, which guarantees that multi-antenna systems under consideration could be applied to large MIMO and diversity applications [64]. Even though the number of elements of massive MIMO in smartphones should be equal to or greater than eight elements, many technologies do not support the mMIMO system in smartphones, despite having the same number of mMIMO elements. However, these technologies can be considered as good candidates for the massive MIMO system. Figure 14: Reconfigurable (8 \(\times\) 8) MIMO frame–antenna structure [63]. The performance varies depending on how many antenna elements (AEs) are combined into a single MIMO mobile terminal. This is carried out in free space when a user is holding the mobile terminal in their hand while in data mode. The performance of the MIMO terminal was evaluated with regard to efficiency, ECC, multiplexing efficiency, capacity, and maximal ratio combined with two additional AEs each (as many as 18 AEs). The investigation began with a minimum of two AEs and progressed to 18 AEs (MRC). For use with 5G technology, the integrated MIMO antennas were identical and had a frequency range of between 5 and 6 GHz. According to the findings, the ECC rose along with the total number of adverse events. In spite of this, the ECC for the case of 18 AEs was still lower than 0.32 in both free space and when a user's hand was present. In the meantime, it was observed that the free space efficiency, which was approximately 90% for the two AEs, decreased with the increasing number of AEs, reaching approximately 50% with 18 AEs. However, the effectiveness of the elements shifts depending on the user's hand because each element interacts with the hand. The efficiency of some AEs dropped as low as 5% when the hand was used to block them directly, while the efficiency of other AEs remained at up to 75% throughout the experiment. When a center frequency was 5.5 GHz, the free space capacity became 11.10, 49.50, and 83.2 bit/s/Hz with 2, 10, and 18 AEs, respectively. These levels decreased by 11% (two adverse events), 35% (10 adverse events), and 31%, respectively (18 AEs), when a mobile terminal was held close to the user's hand. The influence of poor AE correlation was shown to be a negligible component, and multiplexing efficiency demonstrated that a decline in the efficiency of the AEs mostly drove the deterioration of the capacity. This was discovered to be the case after it was established that the effect of the AEs' poor correlation was a negligible factor. Additionally, the maximal ratio combining the method's gain, capacity, and diversity gain was examined [65]. A mMIMO array with 10-ports or 14-ports is one of the designs that has been proposed for use in 5G mobile phone applications.