## 3 Spectrum Management Issues
### Small Cell  
The current 5G networks have been characterized by a combination of small cell networks, also known as HetNet, due to mmWave integration (Figure 4) [126; 127]. A straightforward but exceedingly realistic way to maximize the network capacity is by reducing the cell sizes using the concept of network densification [128]. For instance, during the 1 G era of cellular systems in the early 80s, the cell sizes used were basically in the order of hundreds of square km [129]. From 1 G up to the present time, the cell sizes have been progressively decreasing and have recently been reduced to approximately 1 km for the outdoor and 100 m for the indoor scenario [130]. Nevertheless, cell reduction has several benefits, including high chances of frequency reuse across a given geographic area, resulting in a significant decrease in resource contention among users at each BS [131].  
Several new techniques, including the utilization of RN, D2D, and IoT, also come to in design due to the small cell deployment network [126]. However, the small cells based on HetNet also suffer from several challenges due to the necessity of deploying additional BSs in a small geographical area. Consequently, it suffers from the limitation of energy-efficient power sources. In this regard, many BSs are operated entirely on various renewable energy sources, such as solar energy [132]. For example, the main issue that has caused a drawback to the use of small cells in HetNet is the interferences among the pico, macro, micro, and femtocells [133]. Given that the users are in the coverage of more than one cell, the interference causes many severe issues to achieve optimum results [134]. The interference can be of any type, such as intercell, intracell, BS to RN, and D2D interference. Many researchers are recently focusing on new approaches to mitigate this issue [135].  
\begin{table}
\begin{tabular}{p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt}} \hline \hline
**Approach** & **Methodology/Technique** & **Advantages** & **Limitation/Future Work** & **References** \\ \hline \multirow{8}{*}{Spectrum allocation} & The Chaotic & & & \\  & Biogeography-Based Optimization (CBBO) & CBBO performance is higher as compared to other traditional algorithms & Non-linear migration model can be used in future & [120] \\  & &  
Small cell networks face several other challenges, such as requiring additional frequency bands to facilitate each cell user [136]. The capacity of the cell depends on the number of active users and requires high bandwidths to achieve high throughput [137]. Moreover, fairness among cell-edge and cell-center users is also a big challenge [138]. The use of 5G small cells is based on the mmWave frequency band; these high-frequency signals are keen to degrade for several reasons, such as reflection, refraction, and diffraction. These factors are due to trees, buildings, or even the moving people in the coverage area, especially in urban environments [126]. The cost of the small cell network is also a big challenge that requires considerable attention; thus, the end-user should not be affected by this issue [139]. The backhaul connectivity of the small cell-based HetNet is essential in reducing the delay in transmission and in preventing the signal loss during the handover process performed by a user when it moves from one small cell to another [140]. Various major areas require considerable attention, such as interference avoidance, throughput improvement, coverage planning, and capacity enhancement. Several studies have been conducted to address these issues; some of the approaches are discussed in the succeeding subsections.