# 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 epsilon negative and almost zero refractive index characteristics of the suggested design's operational working principle are intended to concurrently enhance the isolation and overall performance of the MIMO antenna system. To back up this assertion, a specific ENG/NZRI/DNG metamaterial unit cell is also examined. The proposed MTM consists of four small, square-shaped splitted components as shown in Figure 6. The proposed MTM-based technique permits significant decoupling of up to 32 dB between the small array elements with ECC 0.0001 and the proposed radiating MIMO antenna elements, in contrast to conventional isolation solutions [42]. Figure 5: Manufactured prototype of the hexagonal (5 \(\times\) 4) mMIMO of the patch antenna array [40]. A nature-inspired MIMO antenna configuration that can provide lower-bound wireless channels in the lower 5G band (3-4.2 GHz) has been achieved, as shown in Figure 7. In principle, the cross-correlation between antenna elements is decreased by applying the golden angle (137.5\({}^{\circ}\)) concept to the cylindrical configuration of tapered slot antenna arrays. The configuration of a cylindrical array offers high directivity and narrow beam pointing in all spatial directions. The golden angle helps in positioning end-fire radiating tapered slot antennas (TSAs) to prevent spatial overlapping of the radiated individual antenna element fields. TSA elements arranged in 24 cylinders were used to test this theory. The ECC value was <0.01 at 3 GHz-4.25 GHz. According to the simulations and measurements, the above frequency range exhibited extremely excellent impedance matching and mutual coupling between the antenna elements. It is thought that using the golden angle concept with MIMO antennas will enable it to use dense massive MIMO [43]. A 72-port/288-antennas triangular-shaped massive multiple-input/multiple-output (mMIMO) antenna system was achieved for 5G base stations, as shown in Figure 8. Each side of the antenna system contains three layers and 24 ports with an overall size of 44.4 cm \(\times\) 29.6 cm \(\times\) 0.1524 cm. A single-port (sub-array) is composed of a ground plane in the middle, a main with pre-computed phases on the bottom, and a (2 \(\times\) 2) patch on the top layer. Each sub-array (one port) is fed by tilting the beam direction relative to the other to obtain unrelated patterns. For the moment, the antenna system supports two operating modes. The operation uses multiple ports simultaneously (72-port MIMO) and a mMIMO array simultaneously (using beam-switching). All characteristics were measured, and the attained bandwidth was 100 MHz, which covered the band at 3.45-3.55 GHz. The gain of a single port was 9.41 dBi, the envelope correlation coefficient (ECC) was \(\leq\)0.1198, and the efficiency was 64%. Beam steering techniques were introduced and used to direct the beams on each side with 24 ports to various locations in space based on non-uniform port configurations, in contrast to most conventional methods that use identical array elements. The received 13-switchable beams had a center coverage angle of up to 34\({}^{\circ}\) at a gain and height of up to 19.5 dBi [44]. Figure 6: (a) Top view, (b) bottom view of fabricated single-side prototype [42]. Other types of 3-dimensions depend on the full dimension of the beamforming pattern in (3D) space by using active antenna elements with a multi-beam system. With beamforming technology, a MIMO base station massive antenna focuses radio signals directly on the user and their device, as opposed to broadcasting them in all directions, which causes an increase in the efficiency as it reduces interference. Beamforming antenna arrays that improve the throughput of mMIMO systems reduce intra- and inter-cell interference. In a beamforming antenna array, the signal received by each antenna element is adaptively shaped to increase the wireless communication system's overall efficiency and gain. Complex weights are multiplied when signals are detected from various BS antennas.