# 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 Smartphone mMIMO Array Antenna Techniques
#### 2.2.4 8-Element 5G Massive MIMO  
For 5G mMIMO systems, a multi-band antenna array has been studied in the literature as a potential solution. The antenna discussed here is suited for use in multi-mode operations since it creates the feature of polarization diversity and operates on multiple bands. Eight modified planar-inverted F antenna (PIFA) elements make up its configuration, and each one is placed in a distinct location on the smartphone's main board. To simplify the integration process and design, both the ground plane and antenna elements were etched on the same layer. For the S11 \(\leq\)-10 dB performance, the PIFA elements of the MIMO design operated at frequencies of 2.50-2.7 GHz, 3.40-3.8 GHz, and 5.60-6 GHz, spanning the LTE (2600, 42/43/47) operation bands, respectively. The proposed design can support vertical or horizontal polarization thanks to the positioning of the antenna elements. The investigation focused on the fundamental characteristics of the design that was proposed, which possessed sufficient efficiency, good S-parameters, acceptable isolation, and provided coverage for dual-polarized radiation. In addition, the results of the modified PIFAs' TARC and ECC calculations showed that they had a low value across all of the operation bands [62].  
In order to provide coverage for two bands of frequency--an LB for LTE2500 and an HB for the next 3.5 GHz standard with the diversity of the polarization--a multi-band re-configurable structure of the (8 \(\times\) 8) MIMO frame antenna is presented. LTE2500 might make use of the low band. The coming standard would take advantage of the high band. For forthcoming (4G/5G) mMIMO smartphone applications, a switchable antenna array is described. Two antenna array modules were used to create the prototype: a switchable  
Figure 13: Fabricated prototype of the (10 \(\times\) 10) MIMO of T-shaped, coupled fed-slot antenna elements. (**a**) Front view, (**b**) back view [61].  
(6 \(\times\) 6) MIMO 5G antenna module with pattern diversity and a (2 \(\times\) 2) MIMO 4G antenna module with frequency and circular polarization diversities. These two modules together make up the antenna array. A micro-strip line with a tuning slot on the ground plane supplies each element in the 5G and 4G modules. This line is a parasitic element on a non-metal frame that includes a spiral patch as well as a short-ended, C-shape stab. This slot is utilized to change the line's frequency. Each element occupies a space on the ground plane that is 22 mm \(\times\) 9.3 mm in size, as illustrated in Figure 14. These antenna elements employ spatial variety and polarization approaches to decrease the coupling effects and improve isolation. The intention behind doing this is to obtain the desired outcomes. By incorporating the PIN diode into the antenna element's structure, the module regarding a 4G antenna may operate and switch between the two 5G bands and LTE-2500 operating bands. The module of a 5G antenna can operate in a frequency range of 3.4 GHz-3.6 GHz and might be able to meet the needs of a future 5G application. The suggested antenna has an isolation level higher than 15.2 dB in both the OFF and ON states of the PIN diodes, which can be accomplished without the use of decoupling structures [63].