# 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
#### 2.1.3 Combined (Sub-6 GHz/MM Wave) mMIMO  
There are three scenarios for 5G, where enhanced mobile broadband (eMBB) is one of them. eMBB employs mMIMO with multiple antennas. 5G uses frequency candidates below 6 GHz and below 28 GHz. The principal objective is to model large-scale MIMO antenna systems operating at 3.5 GHz and 26 GHz. The antenna in [49] included a rectangular patch shaped like an array, with 12 patches for 3.5 GHz and 96 patches for 26 GHz, for 108 patches on the antenna. The constructed antenna will be utilized as an internal transmit antenna. This antenna has a 2.2 dielectric constant proximity-coupled feed and a connection. The designed antenna has an s-parameter result of \(<\)\(-\)10.8199 dB, a mutual coupling of \(<\)\(-\)32.6201 dB, and a 7.3 dB gain [49]. In the mMIMO system, increasing the number of elements increases the channel capacity. Various research has explored the effect of the number of antennas while assuming uniform antenna gain across very large arrays, relying mostly on channel properties to investigate the influence of edge effects and mutual coupling on the variation in the gain illustration in a 32-element large-scale MIMO array. The impact analysis focused on contrasting patch and dipole antennas, which are typical of the antennas frequently used in the high-volume MIMO experiments performed today. Compared to a dipole array, the finite patch array has a lower gain pattern variation. The massive MIMO system is affected by a large gain pattern variation in that not all antennas contribute equally to all users, and the number of effective antennas seen for a single user is decreased. As a result, as seen at the system level, the zero-force MIMO detector for all users is reduced in speed by 20% for patch arrays and 35% for dipole arrays. On the other hand, combining maximum ratios creates inequality and injustice in users. Antenna measurements depend on measuring 32 active elements in the array, which can be achieved using a large MIMO test bench installed in an anechoic chamber. In terms of system throughput, patch arrays have proven to be the best option. The 31 mm square patch on the microstrip model had two combined U-slots that were each 1.4 mm wide. Next, the 2.4-2.62 GHz and 3.4-3.6 GHz frequency bands were covered. At 2.6 GHz, the main comparison was performed. It has been demonstrated experimentally that the gain patterns of the various antenna elements in a finite array exhibited significant variation. The edge effect and mutual coupling are both responsible for this beamforming, which is highly dependent on the angle of arrival. The gain variation is greater in a dipole array because the system has more powerful mutual coupling. As a result, the array composed of omni-directional elements is more angle-sensitive than a patch array comprised of directional elements [50]. The two-user MU-MIMO (multi-user MIMO) capacity based on a zero-forcing block diagonalized (ZFBD) scheme, and the distribution of eigenvalues were examined in a viable urban macro (UMa) environment at 3.5 GHz. It covered the topic of expanding the amount of Tx items from 8 to 256. For Tx numbers up to 64 and 128, respectively, ratios of over 63% and 73% were attained in comparison to independent channels with the same distribution (i. i. d), with only a slight improvement seen when the numbers rose quickly. Additionally, when the number of Tx rose, there was a very slight tendency toward a uniform distribution of eigenvalues. Compared to traditional MIMO, mMIMO produces more ordered sub-channels and de-correlated user channels, but (i. i. d.) channel gaps still exist in the measuring environment under consideration. As shown, capacity grows with the number of antennas installed per user, and the performance is influenced by the antenna layout on both the receive and transmit sides. As a result, in a measurement scenario, orthogonal user channel representation is not used [51; 52].