# 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
Massive MIMO is a leading technology contender for the 5G mobile communication system because it can fully utilize the existing space resources, significantly increase the spectrum efficiency, and lessen the current spectrum resource crisis. It is required to record various radio channel characteristics from the current MIMO system to study mMIMO technology. Thus, a 3.33 GHz measurement campaign was carried out outdoors, utilizing a sizable virtual antenna array that included 64 components. According to the measured data, common channel parameters including the azimuth power delay spectrum (PADS), power delay profile (PDP), azimuth power spectrum (PAS), power delay spectrum (PDS), azimuth spread (AS), and delay spread (OS) behaved as expected. In both the latency and spatial domains, the acquired data demonstrate that channels are not static for large array sizes. The outcomes can be used to create mMIMO channel models and construct 5G communication systems [53; 54]. For upcoming 5G applications, a 16-port, low-cost MIMO non-planar antenna system was constructed on a 3D octagonal polystyrene block. The bottom and top of the polystyrene block were visible due to the arrangement of the MIMO elements on the eight sides of the octagonal block. One of the antenna's components was a slotted microstrip patch with a broken ground plane and a step-biased feed line. The frequency range (3.35-3.65 GHz) for every one of the MIMO elements was constructed on an FR4 substrate with dimensions of 22 mm \(\times\) 20 mm for 5G applications. A meander line-based, exponential, near-zero, negative (NZI-ENG) metamaterial decoupling structure was used to increase the isolation between array members. Ground planes and isolation structures for common connections were positioned in the bottom layer, whereas array elements were positioned in the top layer [55]. The antenna elements were isolated by >28 dB in side-by-side configurations thanks to the metamaterial-based decoupling structure. The envelope correlation coefficient (ECC) < 0.10, total active reflection coefficient (TARC) < \(-\)18 dB, and channel capacity loss (CCL) < 0.30 were all within adequate bounds. The suggested non-planar 3D-MIMO antenna system could be utilized for wireless personal area network applications and indoor positioning systems in the case when different 5G devices are connected by wireless means to a central server [56]. The spatial multiplexing feature of a typical 5G MIMO system was realized by combining several antennas at the receiver and transmitter. The 3.3-6.0 GHz band-capable proposed broadband 16-element indoor BS antenna array was made for 5G applications. In order to cover the lower and upper bands (LTE bands 42/43/46-N77-N78-N79), a monopole antenna in the shape of a was used. A printed hexa-kaidecagon polygon, which was close by on a substrate, provided the antenna element. When the antenna elements were in a position that provided good polarization diversity and good isolation, the planned BS array was designed, constructed, and tested by examining the overall outcomes, antenna performance, S-parameters, and radiation patterns. For MIMO performance testing, it also achieved a high antenna efficiency of roughly 82-93.2% and a very low ECC (envelope correlation coefficient) of <0.02. The (16 \(\times\) 16) MIMO system's expected ergodic channel capacity was 85 bps/Hz [57]. In this study, all designs were used in the fabrications, and the values were in measurements. Table 1 shows the enhancement and support of mMIMO techniques for base station applications. Table 2 compares the 5G sub-6 GHz mMIMO antenna techniques.