## 2 Related Work: A Review of Relevant Survey Papers  
A number of surveys have been published on HO and mobility management issues in 5G HetNet with applied techniques to mitigate them, which are discussed with limitations as follows. The main concept of HO and mobility management and how to improve their performance through the dual connectivity (DC) approach, where a UE can simultaneously connect to two different radio access technologies (RATs), is reviewed in [28] for future UD-HetNet. Various HO and mobility management techniques, such as machine learning (ML)- and software-defined networking (SDN)-based schemes, are discussed to enhance mobility issues. The key aspects of future HO probabilities over DC in existing and NG-WCS are highlighted. Furthermore, the authors also focused on several HO/mobility management challenges, such as high-speed scenarios and security and privacy issues in UD-HetNet, which highly degrade the performance of the WCS. However, load balancing (LB) and inter-cell interference (ICI) have not been considered during the HO process, and they can severely affect mobility performance and increase the HOF rate in the UD-HetNet environment. Furthermore, the article does not comprehensively review all HO management techniques proposed for 5G networks, further limiting its scope. The basics of mobility management in 5G UD-HetNet, mobility management requirements, architecture, and issues related to mobility management have been extensively reviewed in [29]. Radio resource control (RRC) inactive status, initial access, registration,  
Figure 2: The outline of the paper. and the paging procedure have been discussed in detail. The authors have also examined different kinds of mobility--cell level and beam level mobility--used in 5G UD-HetNet to provide seamless and uninterrupted connectivity. In addition, the survey paper has covered modern methods, such as ML, SDN, and fuzzy-logic controller (FLC), that are utilized for estimating HO and mobility management issues. Further, challenges such as load balancing, signaling overhead, and security and privacy issues followed by effective solutions to overcome these issues in 5G UD-HetNet have been addressed and demonstrated. Although this paper exhaustively reviewed the various aspects of 5G UD-HetNet, it still lacks high-speed scenarios and ICI challenges. High speeds can cause HOF due to FHO, and ICI is produced by the presence of multiple-RATs (Multi-RATs) in HetNet, which can disrupt the HO and mobility performance. In [30], a concise and clear overview of the advancements in mobility management within the evolving 5G architecture has been provided. It explores the current approaches to vertical HO (VHO) in 5G HetNet, considering the impact of new architectural elements such as SDN, NFV, and mobile edge computing (MEC) while maintaining media-independent HO procedures. Moreover, the authors analyzed the requirements of the three main service types and the related proposals for distributed mobility management within 5G HetNet. In addition, the evolutionary steps in mobility management signaling, aligning the QoS requirements for various 5G service scenarios, and vertical use cases have been highlighted. Nevertheless, the authors have only explained the SDN- and D-HCP-based HO management schemes and have yet to address the challenges encountered by UE during the HO process in a UD-HetNet. The emergence of innovative mobile networks, such as IoT, machine-to-machine (M2M), D2D, and vehicle-to-everything (V2X), exacerbates mobility issues, which significantly deteriorates HO performance in terms of HOF, HOPP, and RLF in future UD-HetNet. In reference [31], the authors examine the general concept of mobility management and the key factors that cause HO and mobility issues in 5G HetNet. The work also discusses the existing studies on mobility management and its characterizations in the 5G HetNet, particularly exploring ML- and D-HCP-based approaches.