## 6 Focused Challenges in 5G HetNet  
This section discusses various challenges faced by 5G HetNet. Each review article mentioned in the related survey comparison table (Table 2) focuses on specific issues faced by UE during the HO process. This paper discusses problems that degrade the HO performance of 5G HetNet, such as ultra-high speed, load balancing, signaling overhead, ICI, and security and privacy. \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Ref.** & **Technology** & **HODPs** & **KPIs** & **Comments** \\ \hline
[132] & 4G/5G HetNet & Speed, SINR, and cell load & HOPP, and latency & RLF, HO & Cell sectorization approach is utilized; CIO can be used to further enhance HO performance \\ \hline
[133] & Dense Het-Net & Speed and channel quality & HOR, HOF, and HOPP & HO KPIs are evaluated on the basis of the number of SCs deployed. \\ \hline
[134] & LTE & Speed and HO delay & HOR, HOPP and HO delay & The proposed FLC scheme only enhances HO performance in LTE rather than 5G. Target cell load has not been considered for HO \\ \hline
[135] & 5G HetNet load, and RSRP & Speed, cell load, and RSRP & HO delay, throughput and coverage & The article does not consider practical implementation challenges of mm-Wave small cells in HetNet \\ \hline
[136] & LTE-A and 5G HetNet & Cell load, RSRP, and SINR & Load level, CDR, and throughput & The proposed algorithm can be applied to various types of HetNet, including mobile networks and cloud computing environments \\ \hline \hline \end{tabular}
\end{table} 
Table 8: FLC schemes based on various HCP values for improving HO performance in 4G and 5G HetNet. \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Ref.** & **Technology** & **HODPs** & **KPIs** & **Drawbacks/Recommendation** \\ \hline
[137] & 5G UD-HetNet & Speed and network densification & HOF and HO delay & User speed up to 100 Km/h was taken into account; the effects of HOPP, FHO, and UHO on HO delay was not investigated \\ \hline
[138] & 5G UD-HetNet & Speed, location, and direction & HOF and HO delay & Only number of UE variation is considered to measure the HO KPIs; SDN can be utilized for radio resource allocation and management in 5G UD-HetNet \\ \hline
[139] & 5G UD-HetNet & RSRP, SINR, and cell load & HOF and HOPP & Difficult to obtain the optimal HO trigger point in the case of drastic changes in the network \\ \hline
[140] & 5G HetNet & Speed, RSS, and application & HOR, signaling overhead, HO delay, and throughput & Enhances the mobility management of the user within 5G Femto-eNodeB HetNet only \\ \hline
[141] & 5G HetNet and RSRI & RSRI, SNR, and RAN load & HOR, latency, and throughput & Vehicle speed of 36 km/h taken into account; inadequate solutions for SDN-based mobility and HO management in MEC-enabled vehicular networks \\ \hline
[142] & 5G & Cell load and RSSI & HO delay and throughput & Homogeneous network assumption limits proposed scheme’s applicability to HetNet vehicular environments \\ \hline \hline \end{tabular}
\end{table} 
Table 9: SDN-based schemes for improving HO performance in 4G and 5G HetNet. The high-speed challenge in 5G HetNet refers to the difficulty of maintaining stable and smooth communication for fast-moving users, such as trains, vehicles, or aircraft. The issue arises because high-speed movement creates a rapidly changing radio environment and frequent and UHO, which can lead to signal interference and fading. These UHOs increase the probability of HOF and RLF, which consequently degrade the network performance. To address this challenge, 5G HetNet uses advanced techniques such as beamforming, massive MIMO, and predictive algorithms to improve signal strength, reduce interference, and optimize the HO process. These technologies work together to provide users with a seamless and reliable network experience, even in high-speed scenarios, to ensure that they can stay connected while moving.