## 4 Resource Management in Heterogeneous Network (RM HetNet)
### Spectrum Allocation (SA)
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Characteristics** & **Issue** & **Methodologies** & **Advantages** & **Limitations/ Future Work** & **Ref.** \\ \hline \multirow{8}{*}{**HetNetNet**} & Improve QoS for the cell center users and cell edge users. & Utility-based SA algorithm & Effective solutions for the real-time application of cell edge users. & A centralized SA method is presented. Hence, the burden on the BS increased. & \multirow{2}{*}{[128]} & \multirow{2}{*}{[129]} & \multirow{2}{*}{[130]} \\  & Improve the network SE & Cooperative bargaining solution. & Attain significant performance improvements over other existing schemes & The tradeoff between SA and the power allocation problem is not considered. & \multirow{2}{*}{[129]} & \multirow{2}{*}{[131]} \\  & Improve the SE & Genetic algorithm-based EE SA scheme & Showed a tradeoff between EE and throughput & The transmitted power increased, and the EE and SE decreased. & \multirow{2}{*}{[132]} & \multirow{2}{*}{[133]} \\  & Match secondary users with randomly distributed SBSs. & Many-to-one college admissions matching game & Evaluate the proposed repeated auction in comparison to the matching theory and the single auction. & The system EE of the proposed scheme is not considered & \multirow{2}{*}{[131]} & \multirow{2}{*}{[131]} \\  & Enhance EE and SE & Lyapunov optimization algorithm & Achieved an outstanding overall performance with low complexity. & User mobility and imperfect CSI need to be considered. & \multirow{2}{*}{[132]} & \multirow{2}{*}{[133]} \\ \hline \multirow{8}{*}{**HetNetNet**} & Maximize the sum-rate & The joint optimization problem of UA, subcarrier allocation, power control, and SA & -Perform better transmission performance in terms of the total rate and power efficiency. & Users’ fairness needs more attention and consideration. & \multirow{2}{*}{[133]} & \multirow{2}{*}{[133]} \\  & Optimize EE by balancing SA and route selection & Stochastic algorithm & -EE and SE are improved by optimizing resource allocation and path selection. & User mobility is not considered. & \multirow{2}{*}{[134]} & \multirow{2}{*}{[135]} \\  & Allocate the entire 28 GHz mmWave spectrum to each BS & Novel licensed SA approach & Provide similar performance improvements in average capacity, SE, and cost efficiency. & \multirow{2}{*}{Load balance is not considered.} & \multirow{2}{*}{[135]} \\  & Enhance spectrum utilization & Static-licensed SA and flexible-licensed SA & Demonstrated that the spectrum reuse method enhanced SE and EE results. & User mobility needs to be considered. & \multirow{2}{*}{[136]} & \multirow{2}{*}{[137]} \\  & Improve HetNet & The centralized background SA algorithm & showed that the proposed algorithm boosts & EE needs more attention and consideration. & \multirow{2}{*}{[137]} & \multirow{2}{*}{[138]} \\  & To enhance the available spectrum utilization & A decentralized resource allocation strategy based on deep reinforcement learning and federated learning & Effectively optimizes network performance in terms of power consumption and throughput. & Realistic mobility models need to be considered. & \multirow{2}{*}{[138]} & \multirow{2}{*}{[138]} \\  & Optimize the effective capacity in the THz band & Energy harvesting-based THz band & Verify and assess the proposed THz band schemes over HetNets. & Network energy consumption needs more consideration and investigation. & \multirow{2}{*}{[139]} & \multirow{2}{*}{[140]} \\  & Optimum the effective capacity in the THz band & Energy harvesting-based THz band & Verify and assess the proposed THz band schemes over HetNets. & Network energy consumption needs more consideration and investigation. & \multirow{2}{*}{[141]} & \multirow{2}{*}{[142]} \\  & Optimum the effective capacity in the THz band & A novel multi-sub band quasi-prelect (MS-QP) sequence & Outperforms the existing methods in terms of communication and sensing performance. & \multirow{2}{*}{[143]} & \multirow{2}{*}{[143]} \\  & Perform high data transmission & A novel multi-sub band quasi-prelect (MS-QP) sequence & Outperforms the existing methods in terms of communication and sensing performance. & \multirow{2}{*}{[143]} & \multirow{2}{*}{[142]} \\  & Determine sub-band bandwidth, sub-band assignment, and optimum transmit power & Iterative algorithms using the successive convex approximation approach & - Increase the network throughput by enabling and optimizing adaptive sub-band bandwidth & The use of adaptive sub-band bandwidth is limited to situations in which the authors have a relatively narrow bandwidth. & \multirow{2}{*}{[141]} \\  & Optimize the SE while simultaneously fulfilling the data rate needs of users. & Long user central window principle and the bipartite graph matching approach. & Achieved greater system spectrum efficiency. & The proposed THz system is limited to a small number of users. & \multirow{2}{*}{[142]} \\  & & & & & \\ \hline \hline \end{tabular}
\end{table}