## 1 Introduction
### Motivation and Major Contributions  
The UD-HetNet-based 5G technology satisfies the demands of the present era by providing faster data rates, lower latency, and higher user density. The higher cell density in UD-HetNet, with a large number of small cells operating at low power, ensures ubiquitous and high-capacity connectivity [26; 27]. These low-power small-cell deployments improve capacity, coverage, and reliability and enhance user performance at the cell edge. However, the introduction of small cells in the 5G network also brings some drawbacks. Due to the limited frequency range and the vast number of small cells, frequency reuse in 5G leads to co-channel interference (CCI). Additionally, small cells utilize the mm-Wave frequency spectrum due to their short range and susceptibility to signal interference. Furthermore, high-speed UEs quickly cross multiple cells due to the small cells' limited coverage area, leading to FHO, which results in HOF and HOPP effects. In the presence of FHO, the UE will consume additional energy to connect with the appropriate cells.  
In HOPP, the UE temporarily connects to a nearby/target cell due to rapid fluctuations in RSS from both the serving and target cells and then bounces back to the original serving cell. Unnecessary HO significantly degrades the performance of future WCS, which also results in inefficient resource usage and excessive power consumption. User mobility is the primary driver of HO. The optimal time for HO can be determined using many HO decision parameters, including the UE's location, the direction and pattern of the user's mobility, the type of cell, the availability of radio resources, the user's behavior and preferences, the battery level, the environment, and the data and traffic volume in a specific cell. The UE switches cells based on these attributes to deliver uninterrupted, secure, and timely services. To ensure seamless and reliable connectivity and minimize frequent and UHOs, the number of handovers (HOs) must be mitigated for 5G UD-HetNet. The main factors that disrupt seamless connectivity in UD-HetNet include mm-Wave, a massive increase in the number of connected UEs, massive deployment of small cells (HetNet), high-speed users, overlapping network deployment (small cells' coverage area coinciding with macro cells' coverage area), and the implementation of dual connectivity (DC), where UEs connect to two BSs simultaneously.  
Figure 1: Frequency spectra and architectures of various cellular networks.  
This study aims to compare alternative HO and mobility management strategies and algorithms in UD-HetNet for 5G cellular systems based on numerous key performance factors (KPIs). The article's major contributions are as follows:  
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2. Thorough analysis of 5G HetNet's setup adjustments concerning HO failures (HOFs) and mobility management issues;
3. Examination of the KPIs of current works, including HO and mobility issues, with the remedies and applied standards;
4. Evaluation of the current model's performance against HOF and mobility management issues based on throughput, energy efficiency (EE), reliability, latency, and scalability;
5. Highlighting the significant challenges associated with HO and mobility management in the current research models in 5G HetNet and discussing potential recommendations for solutions.  
The outline of the paper is given in Figure 2. The rest of the paper is organized as follows: Section 2 covers related work and presents an overview of relevant survey papers. Section 3 discusses 5G HetNet services, architecture, and related technologies. Section 4 explains the HO and mobility process in 5G HetNet, while Section 5 explores the current technologies for addressing HO and MM issues in 5G HetNet. Section 6 focuses on the challenges related to HO and MM, while Section 7 discusses future directions. Finally, the paper is concluded and summarized in Section 8.