## 5 Massive MIMO
### Open Challenges of mMIMO  
The mMIMO is obviously high-caliber and remarkable to the conventional multiple antenna networks. Although, massive MIMO and 5G technology can be wonders for future wireless networks. However, several hardware issues such as the choice of the material, the limited size of the mobile phone chassis, overall cost, and characteristic parameters (bandwidth, mutual coupling, gain, efficiency, etc.) can be observed for both of the above-discussed applications. The unlimited variety of smartphones, operating in different frequency bands is another problem for antenna designers. Although, the impact of mutual coupling and limited space in the 5G mMIMO system is challenging. However, the decoupling techniques and compact antenna size can be used to enhance isolation among the antenna elements. In this part, we report the most recent advancement of research on challenges in mMIMO frameworks with regard to the most serious problem of mutual coupling by targeting the recent literature published between 2018 to 2023. To implement MIMO in 5G and beyond networks, there are various research challenges that need to be addressed such as mutual coupling, channel estimation, signal detection, energy efficiency, and pilot contamination. The narrow space in smartphones is a major challenge and the effect of coupling between adjacent antenna elements severely decreases \begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multirow{2}{*}{**Technology**} & **Enhanced** & **Energy** & **Increased Channel** & **Enhanced Spectral** & **Substantial Device** \\  & **Data** & **Efficiency** & **Capacity** & **Efficiency** & **Support** \\ \hline mMIMO & ✓ & ✓ & ✓ & ✓ & ✓ \\ FD & ✗ & ✗ & ✓ & ✓ & ✗ \\ mmWave & ✓ & ✓ & ✓ & ✓ & ✓ \\ DSA & ✓ & ✗ & ✓ & ✓ & ✗ \\ UDense Net & ✓ & Partial & ✓ & ✓ & Partial \\ NOMA & ✓ & Partial & ✓ & ✓ & ✓ \\ SCMA & ✓ & ✓ & ✓ & ✓ & ✓ \\ Beamforming & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of mMIMO with other key enabling techniques. the MIMO performance. Hence, the reduction of mutual coupling is highly desirable as it offers high isolation, efficiency, and MIMO diversity performance. Various decoupling techniques have been addressed in Table 6 to reduce mutual coupling. Metal meandering consisting of metal-strip lines is an effective technique to overcome mutual coupling. This solution was proposed in [179] to achieve effective decoupling by meandering strip lines vertically and horizontally among 16 antenna elements. A six-element, three ports antenna is proposed in [180] that demonstrates pattern diversity by loading periodical inter-digital capacitors on the radiating elements. The proposed dual-polarized antenna was found to have an omni- directional radiation pattern and efficient mutual coupling of below \(-20\) dB between any two ports. This design is considered an important milestone in the implementation of the mMIMO network for 5G and B5G due to the low coupling effect. The isolation can be improved by orthogonal geometry of the antenna elements using molecule fractal structure given in [182]. This technique shows improved isolation of higher than 20 dB. Another efficient solution reported in [183], is to reduce mutual coupling by inserting a frequency-selective surface scheme among the array elements. To improve isolation further, a frequency-selective surface decoupling structure in the substrate is used which blocks the propagation of the electromagnetic wave (EM) in the substrate. The effect of both the coupled magnetic and electric fields was removed by using the Frequency-selective surface (FSS) decoupling structure. An embedded decoupling scheme is introduced in [184] by combining parallel reversed C-shaped metal strips with two inverted U-shaped metal strips that produce additional coupling while placed near a pair of closely spaced dipole antennas.