# 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  
High data speeds and minimal latency are anticipated from fifth-generation mobile networks (5G), which will also boost the power efficiency and spectrum of wireless communication systems. For use in 5G systems, many technologies are being investigated. The mMIMO system is one of the most significant technologies anticipated to guide the development of 5G. One of the technologies that will support fifth-generation (5G) wireless systems is mMIMO [19]. A massive MIMO system is defined as an arrangement of an MU-MIMO system in which a base station and a terminal dispose of many antenna elements. Simultaneous communication with multiple users using the same resource is possible with multiple antennas, which provide higher spectral and power efficiency. In the case of a base station with multiple active antennas, it will be possible to communicate with a terminal (UE) on the same time-frequency resource through spatial multiplexing [20]. Additionally, the beamforming enhancement technology developed for these mMIMO systems can be used to reduce the transmit energy required by upgrading the base station equipment with a huge number of antennas, instead of deploying a new base station site [19; 21].  
Massive MIMO in (sub-6 GHz) 5G NR networks is an advanced antenna technology that improves spectral efficiency, network capacity, coverage, and feasible data rates. Massive MIMO uses multi-antenna elements to support multiple users simultaneously [22; 23].  
The essential parameters of mMIMO antennas can be considered as frequency bands, gain, isolation, radiation pattern, polarization, and beamwidth. One possible way to increase throughput and data transfer rates in current and future generations of mobile and wireless devices is to extend the bandwidth, as a higher bandwidth provides a higher data rate [24].  
Reducing the mutual coupling effects between antenna elements in large-scale arrays with limited space requires several techniques suitable for such a crowded capacity of antenna elements without increasing the space between antennas. The low mutual coupling will offer high isolation and low correlations for the system without limiting the system's performance. Several decoupling techniques have been put forth such as inserting a metamaterial wall to create a spatial band-stop filter or a spatial polarization-rotated wall, or utilizing metal structures between the parts of the antenna to add an additional coupling path [25]. It is challenging to implement such decoupling structures in a large-scale MIMO antenna since they all need to be fairly substantial in size. For BSs and less than \(-10\) dB or lower for smartphone applications, the mutual coupling regarding a mMIMO array should ideally be less than \(-25\) dB or even lower. Using beamforming methods could boost the gain and efficiency of a mMIMO system. To enhance the overall system performance and capacity, beamforming can be defined as a signal processing approach that is utilized with multiple antenna arrays on the receiver side and/or transmitter side for sending or detecting multiple signals from multiple desired terminals at once [26].  
In order to investigate the types of applications, in this study, the types of 5G antenna can be divided into two kinds that support massive MIMO, base station, and smartphone antenna techniques. Figure 2 explains the application techniques used for 5G massive MIMO at sub-6 GHz.