## 5 Current Solutions for HO and Mobility Management Challenges in 5G HetNet  
Effective management of mobility and HO is critical for ensuring high-quality service in NG-WCS. In particular, mobility management poses significant challenges in UD-HetNets in 5G architecture, requiring innovative and enhanced techniques to guarantee secure and uninterrupted mobility. Extensive research has been conducted on mobility management. This article aims to investigate the HO and mobility challenges in 5G HetNet in terms of current mechanisms and propose possible solutions. In previous sections, we provided a detailed background of 5G technology, HO, and mobility management. This section focuses on state-of-the-art techniques used to address HO and mobility management issues in 5G HetNet. While 5G HetNet offers high performance and throughput, HOF and mobility management are key challenges that must be addressed to ensure seamless connectivity among users. Various approaches have been proposed by researchers to address HO and mobility management in 5G HetNet. For instance, in [97; 98; 99], the authors investigate techniques to mitigate HOFs. HO and HOF rates are mathematically evaluated in [100; 101; 102; 103; 104]. The performance of HHO in 5G HetNet is analyzed in terms of outage probability (OP) and HO rate in [105]. The analysis is conducted by estimating UE speed against OP and the total number of HOs. In [106], the authors analyze HO performance for HetNet by considering vehicular user velocity, mobility management parameters, and cell size. They investigate HOFs caused by fading and shadowing, and concentric circles are used to model HO locations at picocells. A balanced 5G model is proposed in [107] to organize traffic during the HO process. The mid, high, and low frequencies are selected to investigate the proposed model in terms of network load, active network slice, and type of services. An HO decision algorithm is proposed in [108] using ML techniques to monitor active time, available radio resources, UE speed, and received power of the reference signal. A simulation model is designed based on small and macro cells, carrier frequency (2 to 2.8 GHz), radius, and shared bandwidth. An SDN-based 5G HetNet model is discussed in [109] to address HO authentication issues, and an algorithm called 'dual constraints chaotic radial movement' is considered for this purpose. An SDN-based 5G HetNet is introduced in [110] to ensure a seamless HO process, and elliptic curve integrated encryption is applied for system privacy and detection of illegal access. A seamless and secure HO process is the focus of [111] with mutual, quick, and strong authentication. The authors developed an SDN model for this purpose, and the paper mainly investigates VHO. [PERSON] et al. [112] presented a 5G SDN-based HetNet using a wireless-link signature scheme to achieve a fast and unified HO process. The model can support macro, small, and femtocells, WiFi, WiMax, and visible light communication. In [113], the authors investigated the increase in interruption time due to HOF and presented a technique called 'zero HOF' with unforced and automatic time. The HOF is minimized with the enhancement of the HOPP rate. The presented model is discussed using a theoretical model and validated with a simulation structure. The work in [114] utilizes received signal power and speed for adapting TTT and HOM, targets ping pong HOs, and attempts to reduce the standard deviation of balanced load. The proposed system is developed based on performance indicators, control parameters, and system matrices. Mobility management is a critical and challenging issue for NG-HetNet. In [115], researchers investigated key mobility management issues such as HOPP, HOF, delays, and HOR using radio resources to enhance the performance of 5G HetNet. To improve resource efficiency and QoS, an advanced mobility management and utilization framework is presented in [116], transforming mobility management from a reactive to a proactive state.