## 2 Related Work: A Review of Relevant Survey Papers
Furthermore, the study highlights the key issues that UEs face throughout the HO/mobility process and future research directions on mobility management in 5G HetNet and B5G. Despite a thorough discussion on mobility and HO management techniques, the article fails to address critical factors such as high-speed scenarios, ICI, and security and privacy, which significantly affect the performance of HO and mobility in 5G HetNet. In reference [32], the authors discuss the background of 5G, services, different models of 5G architecture, 5G benefits, and their impacts on modern applications. The article also studies 5G key technologies that enhance QoS and meet next-generation WCS (NG-WCS) requirements. Moreover, the authors examine various challenges encountered by UEs in the 5G UD-HetNet environment and existing ML-based solutions applied to overcome 5G challenges. The authors in [32] also discuss ML- and SDN-based schemes to further improve HO and mobility performance in 5G UD-HetNet. In addition, a few HO/mobility management-related issues such as ICI, security, and privacy have been taken into account to enhance HO performance in terms of various KPIs in the 5G system. However, HO in 4G/LTE-Advance systems and other key challenges that significantly degrade the HO and mobility performance of 5G UD-HetNet still need to be addressed. A survey on HO and mobility management in 5G ultra-dense small cell (UDSC) networks using ML-based algorithms is presented in [33]. The article comparatively analyzes various ML- and fuzzy-logic controller (FLC)-based HO and mobility management schemes. The future research directions and challenges for 5G-UDSC networks are also briefly addressed in the paper. Nevertheless, the omission of a discussion on the issue of frequent and UHOs caused by high-speed users in UDSC environments is a limitation of this article. In [34], the authors address HO and mobility management issues in 4G-LTE HetNet using ML and SDN algorithms. The article discusses challenges such as load balancing and ICI that impact HO performance in 4G/LTE. However, a major drawback of this article is that it fails to address the HO/mobility management issue in 5G UD-HetNet, which should also be considered. Article [35] provides an overview of HO and mobility management in future mobile networks, with a focus on optimizing mobility robustness. It covers challenges in managing mobility in HetNet and techniques to enhance mobility management. The potential of emerging technologies such as ML, AI, blockchain, and edge computing is also mentioned. However, its relevance may be limited to those interested in emerging technologies as it lacks practical insights for real-world networks and a wider social and economic context. In [36], the authors comprehensively examined mobility and HO management in LTE and 5G-NR with UD-HetNet wireless networks. They explored various aspects of HO management, including performance metrics, HO types, types of HOF, differences between LTE and NR mobility procedures, and potential mobility enhancers. The paper also discussed research challenges and techniques related to HO and mobility management in 5G-NR, including high-speed scenarios, beam management, and beam mobility. The authors explored ML- and FLC-based techniques as possible solutions to overcome HO and mobility management issues in 5G-NR. However, the paper noted that the analysis did not consider some important HO performance challenges, such as ICI, load balancing, and signaling overhead. The authors in [37] present a comprehensive review of 5G-enabling technologies with UD-HetNet. They also address research challenges and opportunities related to intelligent or ML-based HO and mobility management techniques and backhaul solutions for 5G wireless networks. These challenges include issues such as unnecessary HOs, imbalances in radio resource sharing, high energy consumption, intense ICI, and reduced QoS. However, the authors did not consider user speed, which is a major issue for HO and mobility performance improvement in future UD-HetNet.