# Design, Challenges and Developments for 5G Massive MIMO Antenna Systems at Sub 6-GHz Band: A Review
## 1 Introduction  
As the need for faster data speeds grows every day, Fifth Generation (5G) is set to become the cutting edge of wireless communications. The data rate (as high as 20 Gbit/s) and capacity, high reliability, low latency (1 ms), enhanced device-to-device communication, and increased flexibility are the primary objectives of 5G communication systems [1]. Massive multiple-input multiple-output (mMIMO), a key-enabling technology for next-generation and beyond networks, was created as a reaction to the global bandwidth bottleneck in the wireless communications sector. In order to offer power efficiency and a great spectrum with fairly simple processing, mMIMO uses arrays of antennas at both the receiver and transmitter [2]. mMIMO technology has achieved a major breakthrough in meeting the demand for high-quality mobile communication services that provide better coverage with lower power consumption without additional radio resources, reducing the bandwidth and transmission power. It can remarkably increase data throughput without adding more [3]. Several reviews have summarized the 5G requirements above-mentioned as meeting the benefits and properties of mMIMO, which can be concluded as shown in Figure 1[2; 4; 5]. A key component of the design of the 5G communication system is the massive MIMO antenna. Massive MIMO represents an evolution of the MIMO technology that increases spectral efficiency and throughput by employing a significant number of active communication antennas [5]. In order to design a 5G mMIMO system, the type of element (antenna) and the application for which it is used must be determined. Therefore, the design can be defined as:  
5G mMIMO = 5G antennas + Application  
5G antennas make it possible to have multiple-input and multiple-output (MIMO). They consist of a large number of antenna elements in order to send and receive a large amount of data simultaneously. 5G antennas for base stations and smartphone applications have to be able to cover many frequencies. In this way, it becomes possible to achieve faster download speeds. Furthermore, it creates more capacity and connectivity for different devices. More bandwidth makes more data able to get through. For base stations, mobile phones, and other mobile devices such as connected cars, health monitoring equipment, and even industrial equipment, these antennas are vital for beamforming, steering, and reception [1; 6]. The fundamental elements of wireless communication networks are base station (BS) antennas. At least one directional RF antenna, which might send and receive radio signals, may be part of it. These antennas are BSs that are tower-mounted and give consumers cellular access. Multiband base station mMIMO antennas are frequently contained within a single radome. In this instance, each antenna has a distinct port that can be attached to many BSs or parts of a single BS [7]. This is the main communication point for a single user or multiuser devices. 5G base stations are equipped with beamforming mMIMO antennas. Furthermore, an array of antennas can simultaneously focus and direct multiple beams to different targets on the ground [3]. Figure 1: 5G requirements and benefits of mMIMO. The base station antenna defines an antenna that is used to send out a signal to more receivers and is suitable for indoor and outdoor applications. This type of antenna serves in satellite, GPS, GSM, WIMAX, WAN, LAN communications,...etc., and can be used to both receive and transmit a signal [8; 9; 10; 11; 12]. Mobile antennas have multiple advantages and are best suited for moving devices, from which you need to be able to communicate while moving around. These will need MIMO antennas for high-level performance antennas located on the edges and corners of the phones that support beamforming. A typical smartphone has approximately six antennas for high and low frequencies. MIMO, on the other hand, is a must for 5G, which needs two for the lower band and one for the upper band, of which 4 \(\times\) 2 (or 4T2R) is one common combination.