## 3 Scope of This Survey
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Survey** & **Year** & & **Summary** \\ \hline
[67] & 2021 & & * &
\begin{tabular}{c} **Presents resource allocation in ultra-dense LTE-U,** \\ HetNets, CRNs, C-RAN, mmWave, and D2D networks.** \\ \end{tabular} \\
[67] & 2021 & & * &
\begin{tabular}{c} The resource allocation problem have classified according to Approaches, optimization criteria, and methods. \\ New technologies, difficulties, and existing research \\ are described. \\ \end{tabular} \\ \hline
[68] & 2020 & & * &
\begin{tabular}{c} A survey of resource management schemes that optimize spectrum allocation with other mechanisms in HetNet is presented. \\ Power allocation, spectrum allocation, fairness, user association, and complexity are investigated. \\ Approaches and techniques for resolving related issues \\ are described. \\ \end{tabular} \\ \hline
[69] & 2021 & & * &
\begin{tabular}{c} Different network scenarios of HetNets are discussed. \\ Provide a learning-based RA structure and a \\ control-based RA process as two possible 6G communication structures for addressing RA issues in \\ future-generation HetNets. \\ Challenges and future research are investigated. \\ \end{tabular} \\ \hline
[70] & 2022 & & * &
\begin{tabular}{c} Introduces the challenges in 5G HetNets, including user \\ association, resource allocation, power \\ allocation, cross-tier interference, and co-tier interference. \\ Classifications for individual and combined approaches to analyze existing schemes for fairness, QoS, QoE, SE, EE, and coverage probability. \\ Challenges and opportunities in the 5G network are discussed, along with requirements and advanced \\ mechanism solutions. \\ \end{tabular} \\ \hline
[71] & 2019 & * &
\begin{tabular}{c} Introduces enabling techniques such as coordinated \\ scheduling, coordinated multipoint, and inter-cell \\ interference coordination to address interference \\ management challenges in HetNet and D2D communication. \\ The methodologies, benefits, limitations, and future work \\ of several important issues have been reviewed. \\ The 3 rd Generation Partnership Project also utilized \\ interference mitigation strategies as Future directions. \\ \end{tabular} \\ \hline
[72] & 2020 & * &
\begin{tabular}{c} Described connected state inter-radio access network \\ handover methods and integrated mmWave cells of \\ 5G network. \\ Handover issues, power consumption, signaling \\ overhead, latency, and security have been addressed. \\ 5G mobility management solutions have been shown. \\ \end{tabular} \\ \hline
[73] & 2021 & * &
\begin{tabular}{c} Provided a spectrum sensing survey for cognitive radio \\ by explaining the half-duplex and \\ full-duplex approaches. \\ Discuss learning techniques to improve spectrum sensing \\ and cognitive radio for 5G and beyond. \\ Highlight potential future research challenges based on \\ emerging techniques. \\ \end{tabular} \\ \hline
[74] & 2021 & * &
\begin{tabular}{c} Different network scenarios of HetNets are discussed. \\ Provide a learning-based RA structure and a \\ control-based RA process as two possible 6G \\ communication structures for addressing RA issues in \\ future-generation HetNets. \\ Challenges and future research are investigated. \\ \end{tabular} \\ \hline
[70] & 2022 & * &
\begin{tabular}{c} Introduces enabling techniques such as coordinated \\ scheduling, coordinated multipoint, and inter-cell \\ interference coordination to address interference \\ management challenges in HetNet and D2D \\ communication. \\ The methodologies, benefits, limitations, and future work \\ of several important issues have been reviewed. \\ The 3 rd Generation Partnership Project also outlined \\ interference mitigation strategies as Future directions. \\ \end{tabular} \\ \hline
[71] & 2019 & * &
\begin{tabular}{c} Described connected state inter-radio access network \\ handover methods and integrated mmWave cells of \\ 5G network. \\ Handover issues, power consumption, signaling \\ overhead, latency, and security have been addressed. \\ 5G mobility management solutions have been shown. \\ \end{tabular} \\ \hline
[72] & 2020 & * &
\begin{tabular}{c} Described connected state inter-radio access network \\ handover methods and integrated mmWave cells of \\ 5G network. \\ Handover issues, power consumption, signaling \\ overhead, latency, and security have been addressed. \\ 5G mobility management solutions have been shown. \\ \end{tabular} \\ \hline
[73] & 2021 & * &
\begin{tabular}{c} Provided a spectrum sensing survey for cognitive radio \\ by explaining the half-duplex and \\ full-duplex approaches. \\ Discuss learning techniques to improve spectrum sensing \\ and cognitive radio for 5G and beyond. \\ \end{tabular} \\ \hline \end{tabular}
\end{table}