## 4 Handover and Mobility Process in 5G HetNet  
Handover (HO) refers to the process of a UE switching from the coverage of one cell to another without experiencing any interruption. The smooth and timely HO in any communication system is important for seamless connectivity. In WCS, HO is mainly categorized into Hard HO (H-HO) and Soft HO (S-HO), which are also known as break-before-make (BBM) and make-before-break (MBB) HO, respectively. In the H-HO process, the UE first terminates its connection from the old cell (serving cell) and then connects to the new cell (target cell) without any delays. In contrast, the S-HO process involves establishing a new connection to the next cell before terminating the existing connection [59]. Furthermore, Horizontal HO (HHO) and Vertical HO (VHO) are other types of HO. In the HHO process, the HO is performed within the same network, i.e., switching of UE from one cell to another using the same frequency [60]. As for VHO, it enables seamless UE mobility between different networks [61]. In other words, HHO occurs between homogeneous BSs, while VHO happens between HetNet BSs [79; 80]. The HO process in 5G HetNet is performed based on VHO because of the overlapping deployment scenarios of different RATs (mm-Wave and Sub-6 GHz). LTE-A utilizes the BBM-HO/H-HO technique to manage UE mobility between cells, which can result in increased HO interruption time (HOIT), data packet losses, and delay. The future WCS (5G and B5G) is based on HetNet and Ultra Dense HetNet deployment to enhance system capacity and provide ubiquitous connectivity. In the HetNet environment, the HO process is expected to be faster and more redundant due to the smaller cell radius. The main feature of the utilization of VHO and S-HO techniques is to minimize HOIT, HO delay, and packet losses, which are important factors in 5G and B5G wireless networks. The HO process in 5G new radio (5G-NR) is the same as that of 4G (LTE-A) HO and is composed of four phases: measurement (MR), HO initiation, HO execution, and HO termination or completion [81]. Figure 4: The 5G key technologies. The HO process in 5G-NR involves several steps that enable UEs to switch from the coverage of one cell to another without interruption. The following is a list of steps that describe this process:  
1. The UE receives signals from both the source gNB (s-gNB) and the target gNB (T-gNB) on the downlink, where the s-gNB is the serving cell and the T-gNB is the neighbor cell. 2. The UE measures the signal strength of these signals and creates a measurement report (MR). 3. The UE sends the MR to the s-gNB, and if the signal strength of the T-gNB is better than the s-gNB by a certain threshold during a predetermined time period, an HO event is initiated. 4. Based on the MR, the s-gNB sends an HO request (HO-req) to the T-gNB to initiate the HO process. 5. The T-gNB responds with an HO request Ack-command to indicate the availability of resources for HO execution. 6. After receiving the Ack-command, the s-gNB stops downlink data transmission and sends the HO command (HO-cmd) to the UE to initiate the HO execution process to the T-gNB. 7. In the execution phase, the UE disconnects from the s-gNB and establishes a connection with the T-gNB to synchronize. 8. Finally, in the HO completion phase, the T-gNB sends an HO complete command to the s-gNB to release the resources, and the downlink path is changed from the s-gNB to the T-gNB. The procedure for HO from initiation to completion is illustrated in Figure 5. The HO is typically initiated when signal quality from the serving gNB (S-gNB) is poor and interference from the target gNB (T-gNB) is very high. During the HO initiation phase, the UE may experience high RLF due to FHO, which is the most vulnerable period in the baseline HO process. The HO may fail if the HO control messages fail to be delivered or if RLF occurs during the HO procedure [82; 83].