# 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
Figure 8: Triangular 72 port mMIMO system prototype [44]. Figure 7: Fully-assembled 24-antennas of the MIMO array during testing with VNA for S-parameters [43]. Active antenna systems (AAS) with two-dimensional (2D) planar array structures are used in FD-MIMO. This permits adaptive electron beamforming and fitting numerous antenna elements into a functional base station compact design in three-dimensions (3D) [45]. The array of active transceivers is integrated into an array of passive antenna elements using the advanced BS technology known as AAS. This technology enables adaptive control of the gain, beamwidth, and transmit beam tilt using active electronics that are directly connected to each element. To enable adaptive beamforming for elevation and traditional azimuth measurements, active and functional antenna elements must be positioned in both the vertical and horizontal orientations [27]. On the other hand, a suggested and characterized meta surface lens antenna was fed through a dual polarization plane (\(8\times 8\)) antenna array for sub-6 GHz full-size array MIMO and multi-beam systems, as demonstrated in Figure 9. A lightweight multilayer meta-surface structure built of Jerusalem transverse elements was used to build the planar lens. An (\(8\times 8\)) dual-polarized stacked patch array fed a lens antenna with dimensions of (\(560\) mm \(\times\)\(560\) mm \(\times\)\(266\) mm) to operate at the range of \(5.17\)-\(6.10\) GHz. With a maximal gain of \(22.4\) dBi and a \(3.3\) dB at \(5.6\) GHz gain variation, the scanning range of \(\pm 25^{\circ}\) was attained. The beam coverage characteristics, pattern envelope correlation coefficient (ECC), and port-to-port isolation properties indicate that it can provide full-dimensional access. Additionally, while isolated beams can achieve frequency division multiplexing, beams operating at the same frequency offer a high-gain multi-beam antenna system [46]. A highly integrated active multi-beam antenna system was developed and implemented at \(5.8\) GHz with \(64\) RF channels and \(256\) antenna elements for high-volume \(5\)G MIMO wireless communication applications. The \(64\)-channel highly integrated active multi-beam antenna system provides a testing ground for the digital beamforming algorithms and the mMIMO channel estimation for \(5\)G wireless communication. Eight PCB boards with six layers make up the \(64\)-channel multi-beam antenna system. The \(256\)-element antenna arrays of the \(64\)-channel multibeam antenna system could be assessed with an array gain of \(18\) dB. Each PCB measures \(320\) mm in length and \(215\) mm in width. The beam sweep of \(64\times 4\) antenna arrays is between \(-30^{\circ}\) and \(30^{\circ}\) with \(10^{\circ}\) intervals [47]. The authors in [48] provided a 3D coverage optimization approach after analyzing the structure of a \(128\) elements/\(64\)-channel active mMIMO antenna array. There were \(64\) dual-polarized elements (\(8\times 8\)) in the antenna array, with every one of the elements  
Figure 9: Full-dimensional massive MIMO of a planar (\(8\times 8\)) dual polarization antenna array feeding a meta surface lens antenna [46]. ending in a +45\({}^{\circ}\) and a \(-\)45\({}^{\circ}\) polarized antenna. The total 3D beamforming and coverage optimization regarding the array were realized by integrating and optimizing two major plane patterns of the desired 3D radiation pattern. Additionally, the simulation verified and validated the 3D beamforming optimization approach, demonstrating its effectiveness.