# A Survey of Handover Management in Mobile HetNets: Current Challenges and Future Directions
## 3 Background
### Handovers in HetNets  
For next-generation mobile communication networks, HetNets will be a key solution since they will deliver better data rates, more user capacity, and wider coverage. Low-power small cells such as microcells, picocells, and femtocells are incorporated inside the high-power macro cell positioned within the same geographical coverage region to form HetNets which resultantly achieve cost-effective and energy-efficient solutions for users QoE [34; 35; 36]. However, the massive and ultra-dense deployment of these small cells increases the number of HO. This spike in the number of HO is exacerbated in a high-speed mobility environment. When compared to a pedestrian-moving UE, a high-speed UE requires a more smooth and more effective HO transfer to the next cell to ensure connectivity and quality. The incorrect or inefficient HO strategy does have quite a severe impact on communication quality in the form of longer interruption times, higher call dropouts, and throughput deterioration. As a result, in order to offer seamless connectivity and services to mobile customers, HOs must be maintained properly and optimally [30; 37].  
The HO phenomenon refers to a user's radio link process switching from a source cell to a target cell in order to retain connectivity while traversing from one cell to the next [38; 39]. Figure 3 illustrates the basic concept of HOs in mobile communication systems with a number of scenarios. HOs ensure that users receive better QoS and uninterrupted connectivity, as well as maintain good network performance overall. The number of successful/failed HOs, interruption times, and radio link failures all affect QoS and performance. Although HO techniques vary by technology, we may model them in three or four steps [26; 40; 41; 42]:  
* Measurement Phase: The UE constantly collects RSRP values from surrounding cells during the measurement or information collecting phase in order to look for improved signal conditions for HO. This method of continual scanning or information collection assures QoS and network availability. Other parameters, such as RSRQ, SINR, and RSSI, can also influence this measuring and scanning procedure for HO decisions.
* Decision Phase: The HO decision step will proceed after the information acquisition and measurement phase at the serving cell. A list of adjacent cells will be examinedduring this process to determine the target cell that best meets the requirements. Following analysis, the target cell with the highest RSRP value receives the HO request message. The target cell has the option to accept or reject the HO request after it is received. Requests that are not accepted set off the HOF, which eventually results in RLF. As an alternative, the target cell will begin preparing for the UE's arrival.
* Execution and Completion Phase: The execution and completion phase is about the signaling and radio link transfer process for UE mobility to the target cell from the serving cell. In addition, the data transfer process, synchronization, network reconfiguration and authentication have all been completed in the execution phase. Lastly, the UE initiates the completion message to indicate the HO completion process.