# A Survey of Handover Management in Mobile HetNets: Current Challenges and Future Directions
## 4 Handover Decision Techniques
### UE Velocity-Aware HO Decision Approaches  
In velocity or speed-based HOs, UEs velocity acts as an input parameter for HO decisions. Numerous techniques added UE speed as an input parameter for the HO procedure to reduce the number of HOPPs. The velocity-aware HO approaches work by modifying the HCP parameters in accordance with speed depending on specified threshold values as shown in Figure 6. In [148], they performed an experimental study on the impact of high-speed mobilities on the HOs. The results show that UE speed has a direct impact on HOF and overall handover timing. As the HO phenomena in HetNets is more complex compared to base networks, we can predict and perform more accurate HOs in these networks by using UEs speed as a HO decision parameter. This results in significant improvement in network performance in terms of HOPP and RLFs. Numerous studies based on UE speed have been conducted; a few of them are listed here. In [38] the authors proposed a conflict resolution approach to solve the optimization conflict between MRO and LBO. This conflict is caused by the non-optimal HCP values after optimization which leads to network performance degradation. To overcome the said issue they used UE Velocity, Cell Load, and RSRP values of the serving and targeting BS to get optimal values of HCP. This Optimization of HCP parameters is based on weight functions for each user for each parameter. After optimization in the second stage they adjust the HCP values according to the handover failure type. According to the results presented in the paper, the proposed scheme is compared with MRO, LBO, and a Hybrid of MRO and LBO. Results show that the proposed scheme outperforms the previous schemes by a significant margin. In [85], authors proposed a Velocity Aware Handover Self Optimization algorithm for 4G and 5G dense networks to optimize HOM and TTT values according to the UE Velocity. Moreover, HOP, HOPP, and RLF were used to evaluate the performance. They developed a distributed UE velocity-aware HO optimization algorithm that works in a hybrid way. Initially, the system monitor and get the values of RSRP and Velocity of the UE. HOM and TTT values will be adjusted based on these threshold values (conditions) in the second phase with respect to UE speed st  
Figure 6: Illustration of Velocity Aware Handovers in HetNets. values were categorized on the basis of UE Velocity; however, the velocity range is only from 40 to 160 km/h. Fixed mobility patterns, absence of LB information and use of fixed pattern or jumping values for HCP adjustments can degrade the algorithm's performance. Similarly, some optimization approaches can predict more accurate HCP values instead of manual adjustments in a jumping manner. The paper's [86] authors proposed a robust algorithm for the optimization of HCP parameters in an LTE-A HetNets environment by introducing a new relationship between HOM and TTT values. By doing this, they achieved excellent results compared to the previous literature by comparing RLF, HOPP, and HOFs. Their proposed scheme adaptively selects the HOM and TTT values according to the UE Velocity for each user. They assumed the locations of UE and BS to estimate the velocity and UE's distance from the BS. Using this location and Velocity information, they adaptively predict the HOM Values. They also considered the RSRP and SINR from both serving and targeting cells for predicting HOM according to UE Velocity. TTT is purely a time-dependent phenomenon, but this article has shown a direct relationship between TTT and HOM. It means the TTT values are dependent upon HOM as well as the velocity of the UE, and they derived this relationship with the help of a geometric function to create a base against fixed or unjustified steps [85] for tuning the HCPs. Similarly, the authours in [149; 150] investigates the downlink coverage and handovers by employing stochastic geometry and SINR to enhance coverage and handover performance in 5G HetNets.