## 4 Handover and Mobility Process in 5G HetNet
### Key HO Parameters in 5G HetNet
#### 4.1.2 HO Control Parameters  
The HO Control Parameters (HCPs) are essential in 5G HetNet for optimizing and controlling the HO process to maintain network connection quality between the serving and target cell. HCPs control when and how a UE needs to HO to ensure connectivity to the best available cell at all times. In 5G HetNet, HO margin (HOM), time-to-trigger (TTT), and Cell Individual Offset (CIO) are commonly used HCPs that significantly contribute to maintaining the connection quality of UEs.  
HOM is measured in dB and represents the difference in RSS or quality between the serving cell and the target cell. In high-speed scenarios, a lower HOM is preferred to prevent too-late HO (TLHO), while in low-speed environments, a higher HOM is suitable to avoid too-early HO (TEHO). In 5G HetNet, the HOM is an important parameter that directly affects HO performance. It should be set lower than in previous generations to achieve the high-speed and low-latency requirements of 5G. Additionally, the HOM should be optimized to balance the HO rate, HOF rate, and HO interruption time.  
TTT is the time that a UE needs to spend below the trigger threshold before a HO is triggered. In 5G HetNet, TTT affects HO performance and overall QoS. A lower TTT can improve the HO rate and user experience but increase HOF rate, while a longer TTT can decrease the HO rate but reduce the QoS and user experience. The value of TTT should be carefully set based on the specific use case and network requirements.  
The CIO is used to measure the signal level of a neighboring cell relative to the serving cell and adjust the HO decision criteria accordingly. In 5G HetNets, where multiple cells of varying sizes and technologies coexist, it is crucial to ensure that the UE is handed over to the best available cell based on the network conditions and the user's location. The value of the CIO is typically set during network design and is based on network requirements.  
To significantly improve the HO process, optimizing only one HCP is insufficient, and all HCPs need to be optimized according to UE requirements to enhance the HO KPIs. To reduce HOF and HOPP rates in 5G HetNets, the values of HCPs, including HOM and TTT, should be low for high-speed UEs and high for UEs moving at a low speed. In HetNets, due to their small coverage area and high-speed UEs, the HCPs are set to be as low as possible to improve HO performance in terms of HOF and HOPP and to achieve high capacity in terms of UE connectivity and reliability.  
Mobility robustness optimization (MRO) and load-balancing optimization (LBO) are key schemes for enhancing HO performance by adjusting HCP values and offloading users to the best cell in the neighborhood, respectively, where they can attain excellent QoS. The primary goal of MRO is to avoid too-early HO (TLHO), too-late HO (TEHO), and unwanted or wrong HOs, while LBO significantly improves UE throughput [84]. In LBO, the UE performs an early HO to cells in the surrounding area that have less traffic load, which provides sufficient resources by adjusting HCP values [85; 86; 87].  
In a 5G HetNet, different types of mobile users, such as pedestrians, vehicular users, and high-speed users, pose unique challenges related to mobility and connectivity. Pedestrian users may experience HOPP due to their slower speed, while high-speed users may experience a higher rate of RLF due to their rapid movement and the limited time available for the network to establish and maintain connections. A wrong cell HO in HetNets can be caused by various factors, including interference from neighboring cells, inadequate signal strength, and network congestion. Figure 7 depicts the 5G HetNet with different types of mobile users and their related issues.