## 3 Spectrum Management Issues
### M-Milmo  
M-MIMO technology has been utilized to maximize system capacity and bitrate since the evolution of LTE-A frameworks [200, 201]. However, critical work has been conducted in the previous years to help the potential addition of this arrangement of strategies to current 5G networks [202, 203]. Different techniques, such as beamforming, Spatial Division Multiple Access (SDMA), and spatial multiplexing, have been utilized in M-MIMO [204, 205, 206]. It straightforwardly enhances system throughput and bandwidth efficiency [207]. In contrast to the existing conventional MIMO antenna systems, M-MIMO offers more spectrum and higher power utilization efficiency for the future cellular networks by using the massive array gains provided by the low-complex transmission framework [208]. It is also expected to provide flexibility to avoid interference with the adaptive beamforming technique [209, 210]. It is noticed that the performance of MIMO is different for sub-6 GHz and mmWave frequency band. Despite conceptual similarities, the way in which MIMO can be utilized in these bands is fundamentally different due to their transmission behaviors and hardware characteristics. The major differences in M-MIMO design for sub-6 GHz and mmWave frequencies concern to the propagation mechanisms, transceiver design, and signal processing algorithms [211]. However, the data rate in mmWave frequency band is achieved to be high of around 10 Gbps for a single user, whereas, in sub-6 GHz, around 100 Mbps of data rate can be achieved when using 40 MHz of bandwidth [212]. The use of MIMO in sub-6 GHz has several advantages over the mmWave frequency band. Like, the beamforming in sub-6 GHz delivers power-savings and better coverage; however, in mmWave, it is not suitable for low data rate application as it increases the power overhead, especially for IoT and mMTC application [213]. Besides, applications where the requirement of URLLC is necessary, sub-6 GHz M-MIMO improves the network reliability, whereas mmWave M-MIMO faces propagation unreliability due to blockage [214]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Issues/Approach** & **Methodology/Technique** & **Advantages** & **Limitation/Future Work** & **References** \\ \hline \hline \multirow{6}{*}{} & Path loss based on directional & \multirow{2}{*}{} & \multirow{2}{*}{} & \\  & and omnidirectional antennas & & & \\  & for 32 GHz for both line of & & & \\  & Sight (LOS) and Non-Line of & & & \\  & Sight (NLOS) & & & \\  & outdoor environment & & & \\ \hline \hline \multicolumn{5}{l}{Channel characteristics for 28,} \\  & 38, 60, and 73 GHz outdoor & \multirow{2}{*}{} & Accurate values of & \multirow{2}{*}{} & \\  & scenarios using a wideband & & path loss exponent & \\  & sliding correlator channel & & & \\  & sounder with the horn-to horn & (PLE) is achieved & & \\  & antenna configuration & & & \\ \hline \hline \multicolumn{5}{l}{Channel behavior in an indoor} & \multirow{2}{*}{} & \multirow{2}{*}{} & \multirow{2}{*}{} \\  & environment for 28 and & & better network & \\  & 73 GHz using the CI model & & performance & scattering issue \\ \hline \multicolumn{5}{l}{Polarization effect estimation} \\  & for indoor LOS environment & \multirow{2}{*}{} & \multirow{2}{*}{} & \\  & at 60 GHz using & & \\  & ray-tracing simulation & & & \\ \hline \multicolumn{5}{l}{Indoor office environment} \\  & investigation at 28 and 73 GHz & \multirow{2}{*}{} & Calculate delay spread & \multirow{2}{*}{} & \\  & omnidirectional antenna in co- & & & \\  & and cross-polarization & & \\  & antenna settings for both LOS & & \\  & and NLOS environment & & \\ \hline \multicolumn{5}{l}{Indoor effect of material’s} \\  & conductivity and permittivity & \multirow{2}{*}{} & \multirow{2}{*}{} & \\  & at 28, 39, 60, and 73 GHz for & & \\  & LOS and NLOS using 3D & & \\  & ray-tracing wireless & & \\  & insite software & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont_.