# Design, Challenges and Developments for 5G Massive MIMO Antenna Systems at Sub 6-GHz Band: A Review
## 4 Challenges and Future Directions  
Massive MIMO is clearly superior to conventional multiple antenna systems. Massive MIMO and the most recent 5G technology could work wonders for wireless networking. However, a number of problems continue to prevent mMIMO from being used practically [115]. Several issues for hardware components such as choosing the material, size, cost, and characteristic features (bandwidth, gain, efficiency, mutual coupling, etc.) can be faced for both types of applications. The almost unlimited variety of devices will cause various design issues, which in turn will be exacerbated by the variety of frequencies in 5G. To support the devices operating on diverse spectral bands, the spectrum must be flexible [116]. The sub-6 GHz frequency band is becoming the band of interest for 5G communication to solve these issues. In 5G sub-6 GHz, base station techniques use single and multiband designs through different frequency ranges, which still face some challenges. With multi shapes for single and arrays such as ME dipole-cross patches, patch sub-array, multimode-slotted structures, and other designs, high gain and efficiency can be realized. For smartphone designs with LTE (42/43/46 and 47) frequency bands, the inverted-F stub fed hybrid-loop antenna, T-shaped coupled-fed slot antenna, and planar inverted F-antenna (PIFA) can be used with high performance. Frequencies do not present a wide variety of challenges as the current questions are where to put the antenna, how many are needed, and how to keep them from interfering with each other, especially after the recent demand for a 5G network [117]. In the design of a symmetrical and asymmetrical array, the placement of the antennas is arranged with a specified number of elements. Symmetrical and non-symmetrical planar rectangular arrays such as (4 \(\times\) 4) and (4 \(\times\) 1) models of elements, as an example of base station techniques, arranged the antennas and studied the effect of the single element and arrays on the radiation pattern directivity, gain, and efficiency. The arrangement of antennas in smartphone design is to put them at the edges as symmetrical or non-symmetrical for two sides or one side such as the (8 \(\times\) 8) model of elements, with each four arranged at each edge. These can obtain high results with the specified dimensions between elements and perfect isolation. The effects of mutual coupling due to the close spacing of the MIMO antenna elements and their impact on the correlation, which ECC determines, are challenging [118]. However, the compact size and decoupling techniques for the enhancement of isolation between the antennas has relatively resolved the problem, and the effect on the characteristics' results was positive. The geometry of the antennas was installed in a compact size, which could contribute to the low mutual coupling and high spectral efficiency. It can be noted that the 2D and 3D placement of antenna elements have become the support for mMIMO base stations. Nevertheless, implementing antenna arrays in 2D or 3D can dramatically decrease energy efficiency and enhance coupling effects. In order to dramatically improve the spectral efficiency, it is possible to increase the spacing of the array element by using decoupling techniques. The 3D-model element rectangular planar lattice array uses three decoupling methods: a ferrite chock ring, a rectangular ring resonator, and a unique baffle design [28]. A 3D array massive MIMO antenna with a compact size can be used with decoupling and without decoupling methods such as cylindrical, triangular, and hexagonal models for enhancement results. The size of the smartphone and the requirements for design in the massive technique need decoupling methods, which remains one of the key features of 5G MIMO antennas. Thus, the slot method affects the decoupling component positively to enhance isolation between other elements [58].