## 3 Spectrum Management Issues
### High-Spectrum Access  
The channel characteristics of wireless communications mainly depend on several factors, namely, the radio spectrum used, the air-interface design type, and the network architecture [151]. Existing cellular networks, such as 4G, are operated in frequency bands under 6 GHz because of the favorable channel propagation characteristics available in those frequency bands for cellular communications [152]. However, most of the frequency spectra below 3 GHz are nearly used up, efforts to acquire new operating frequency bands for future terrestrial mobile communications have  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Approach** & **Methodology/Technique** & **Advantages** & **Limitation/Future Work** & **References** \\ \hline \multirow{8}{*}{Interference avoidance} & \multirow{4}{*}{A non-orthogonal} & Better results as & \multirow{4}{*}{The results for larger} \\  & & interference-free Spectrum Sharing (SS) approach to form & & orthogonal spectrum \\  & & & & \\  & 3D clusters and less distance among co-channel small cells & & Shared Access (LSA) and Licensed Assisted Access (LAA) method & & \\  & & & & \\ \cline{2-5}  & & It utilizes an Almost Blank Subframe (ABS) scheme to analyze the operation effect between various small cells & & \multirow{4}{*}{Limited transmission} \\  & & & & \\ \hline \multirow{8}{*}{Throughput improvement} & \multirow{4}{*}{A cooperative game theory-based RRM scheme for small cell network} & \multirow{4}{*}{The optimal size} \\  & & & & \\  & & & & \\ \cline{2-5}  & Spectral efficiency and capacity improvement technique for isolated mmWave MU-MIMO small cell users & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline \multirow{8}{*}{Coverage planning} & \multirow{4}{*}{Mitigating the coverage hole issue for a two-layer small network} & \multirow{4}{*}{\begin{tabular}{} \end{tabular} } & \multirow{4}{*}{\
\begin{tabular}{} \end{tabular} } & \multirow{4}{*}{
\begin{tabular}{} \end{tabular} } \\  & & & & \\ \cline{2-5}  & Scheduling technique based on BS density adaptation algorithm and a cell-zooming algorithm & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & To enhance the number of traffic demand nodes based on power limited, bandwidth, and traffic requirement & & & \\ \cline{2-5}  & The file cloud service is used to to offload the mobile user data when the user’s data demand increases & & & \\ \cline{2-5}  & Design of a self-organizing Artificial Immune System (AIS) approach that activates and calculates small cells concerning the traffic load & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of the related work for the small cell-based network.  
been intensified, and the primary focus has been shifted on the frequency spectrum in the mmWave bands [153]. One of the few potential solutions to address the envisaged issue of a capacity explosion in the next-generation radio network is by utilizing the massively untapped frequency resources. It can be divided into three different frequency ranges, i.e., (1) below 6 GHz (sub-6 GHz), (2) NR mmWave (24-100 GHz), and (3) above 100 GHz (100-300 GHz) (as shown in Figure 5) [154; 155].