# 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 Massive MIMO Antennas for BS Applications
#### 2.1.2 3D Massive MIMO  
The varying channel characteristics (circular, planar, and cylindrical) resulting from various antenna array designs substantially impact the entire system's performance. Generally, array dimension designs that are circular or planar show a considerable reduction, with the beam only being horizontally adjustable. These configurations also fall short of the escalating capacity demands. To address this shortage, it is suggested that 3D massive array configurations are adopted such as hexagons, cylinders, triangles, etc. [39]. Two types of 3D techniques are listed. The first type depends on the arrangements of elements in a three-dimensional array. A compact mMIMO antenna system with 1 \(\times\) 4 (sector) sub-array configurations operating in the sub-6 GHz band for 5G base stations was designed and decomposed into various configurations from numerous array topologies, and cubic and stacked polyhedral arrays (rectangular, triangular, and hexagonal) for array design, as shown in Figure 5. A mMIMO process can enhance a system limit by more than ten times while increasing its energy efficiency by a hundred times. Each sector contains components for a 1 \(\times\) 4 sub-array and can have up to five sectors. The single sector has three layers, with the top layer containing (1 \(\times\) 4) patches, while the bottom and center layers contain a separate ground plane and are responsible for the organization. The complete system could operate in either a massive MIMO show mode with beam steering capability or a single port mode. The frame's 140 MHz intentional data transmission spans the sub-6 GHz spectrum at frequencies between 3.36 GHz and 3.50 GHz. The size of a unit sub-array in terms of its length, width, and height was 280.5 mm \(\times\) 56.1 mm \(\times\) 2 mm, respectively. The gain of a single port was realized at 12.95 dBi, and a single panel with five sectors arranged in a rectangular configuration had a total addition of 19.73 dBi. All of the ports had a mutual coupling of about \(-\)16 dB. The operational frequency of the radio antenna array system was set between 3.3 GHz and 3.8 GHz. This is because the sub-6 GHz spectrum was set aside and concentrated internationally to support 5G [40]. To make it easier to analyze mutual coupling, the 24 antenna units were designated consecutively from 1 to 24 and arranged in three stacked stages with the letters A, B, and C. The mMIMO of the dual-polarized antenna array provides a maximum mutual coupling of \(<\)\(-\)35 dB between each pair of ports in this array. The created array was compensated by 18 low-profile sub-arrays. There were four single units in each sub-array. Every one of the antenna units comprised a single port that was vertically polarized, and a single horizontally polarized port connected to the power splitters served as the feeding network. For a dual port convertible patch antenna of the same size, a stacked patch design that incorporates a feed network offers higher gain and less mutual coupling. The three stacked stages of regular hexagonal walls in the array layout, which depend on the Turning Torso building model, had increasing twist angles of 20 degrees between the adjacent levels. Every stage had six sub-arrays. As a result, 18 sub-arrays were dispersed throughout an area with a 324 mm radius. This arrangement decreased the array's radial size by increasing the longitudinal vertical stack size. The array's overall volume was 648 mm by 258 mm and included 288 patches and 144 ports, three stacked stages, A, B, and C, with a total of 24 antenna units, which were successively designated (1-24) for the facilitation of a mutual coupling study that followed. The mMIMO of a dual-polarized antenna array decreased the maximum mutual coupling between every two array ports to \(<\)\(-\)35 dB [41]. For a future 5G base station with a measured bandwidth of 250 MHz, a relatively high isolation mMIMO antenna with 32 elements is investigated in [42]. This antenna can span the frequency range of 3400-3650 MHz.