## 4 Resource Management in Heterogeneous Network (RM HetNet)
### Mode Selection (MS)
#### 4.3.3 Open Issues and Proposed Solutions
\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
**Characteristics** & **Issue** & **Methodologies** & **Advantages** & **Limitations/ Future Work** & **Ref.** \\ \hline \multirow{8}{*}{Enhance the system EE} & \multirow{2}{*}{Enhance the system EE} & Particle swarm optimization algorithm & Effectively increase system EE by satisfying total transmit power and load balancing & The mobility of the user needs to be considered. & [114] \\ \cline{2-5}  & & Improve EE by focusing on QoS for D2D and CU & A deep deterministic policy gradient algorithm & Outperforms the benchmark algorithms in terms of SE convergence properties and EE & SE needs more attention and consideration. & [115] \\ \cline{2-5}  & & Maximize SE by utilizing a mix of full-duplex and half-duplex modes & Decentralized user scheduling and MS scheme & boost the network sum rate in comparison to the network that only uses half or full duplex & All possible transmission modes do not consider simultaneously. & [116] \\ \cline{2-5}  & & Utilize an effective capacity to evaluate the statistical QoS of a D2D link. & A novel multiple features-based weights performs better than in proportional is not considered in the proposed scenario & & [126] \\ \cline{2-5}  & & Maximize system throughput & Greedy strategy and convex optimization theory. & Significantly improve system throughput & Does not consider the interference between D2D and CU. & [127] \\ \cline{2-5}  & & & & & \\ \cline{2-5}  & & & & & \\ \cline{2-5}  & & & & & \\ \cline{2-5}  & & & & & \\ \hline \multirow{2}{*}{Improve SE in a cellular network.} & \multirow{2}{*}{Search plus concave-procedure algorithm} & Illustrated the interference cancellation capabilities and channel gain with maximum SE. & EE and imperfect channel state information are not considered & [117] \\ \cline{2-5}  & & & Improved network performance and decreased network traffic load. & EE needs more attention and investigation. & [118] \\ \hline \multirow{8}{*}{Improve the network EE} & \multirow{2}{*}{A novel dynamic MS based on fuzzy clustering} & The EE of downlink transmission is improved through the deployment of the D2D users. & The greater physical distance between devices leads to a user being banned. & [119] \\ \cline{2-5}  & & Novel MS approach with multi-hop cellular network communications. & Maximize system throughput, EE, and network capacity & Signaling overhead is not considered. & [120] \\ \cline{2-5}  & & Enhance the performance of cellular networks & Novel MS scheme & Confirmed the importance of the system’s dynamic performance. & The interference between the D2D and CU is not considered in the proposed scheme. & [121] \\ \cline{2-5}  & & Identify and select the optimal mode for each device. & Context-aware MS strategy & Demonstrated the ability to adapt the MS within the cell & User association requires attention and consideration. & [122] \\ \cline{2-5}  & & Improve the throughput of the system while simultaneously maximizing user access. & Probabilistic integrated resource allocation approach & Significantly improve system throughput and user experience, and eliminate user interference. & The equilibrium between user experience and channel utilization in the proposed approach is not considered. & [123] \\ \cline{2-5}  & & Minimize the power consumption of the D2D devices while maximizing the total SE & Distributed AI algorithm and power consumption & Outperformed its related counterparts in terms of SE and power consumption & Signaling overhead due to many relays and users is not considered. & [124] \\ \cline{2-5}  & & & & & \\ \cline{2-5}  & & & & & \\ \cline{2-5}  & & & & & \\ \cline{2-5}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the related work for mode selection.