## 4 Handover and Mobility Process in 5G HetNet
### Key HO Parameters in 5G HetNet
#### 4.1.3 Key Performance Indicators for Handovers  
In a HetNet, Key Performance Indicators (KPIs) refer to a set of measurements used to evaluate network performance related to HO and mobility. These KPIs include HO rate (HOR), HO failure rate (HOFR), HO success rate (HOSR), HO delay (HOD), HOPP, RLF rate, HO execution time (HOET), HO interruption time (HOIT), packet loss, energy consumption, and signaling overhead due to unnecessary FHO. These KPIs are critical for assessing how well the network meets the needs of its users and for identifying areas for improvement. These KPIs also offer many benefits, including enhancing network efficiency, reliability, and user experience. Besides their advantages, these KPIs also have potential limitations that should be taken into consideration. These limitations include a lack of standardization in defining and measuring KPIs, inadequate reflection of network performance or end-user experience, suboptimal selection of KPIs, and incomplete assessment of network performance. To improve 5G performance, particularly HO and mobility performance, the research community must be aware of these limitations and consider and explore alternative schemes to handle these issues efficiently. Each KPI is described in more detail in the following subheadings:  
**HO rate (HOR):** HO rate (HOR) is the frequency or number of HOs that occur within a given time period, such as per second or per hour. A high HOR indicates that the UE is frequently moving from one cell to another or that the network is unable to maintain a stable connection with the UE. Conversely, a low HOR indicates that the network is providing a stable connection to the UE. In 5G HetNet, the HOR tends to increase due to the smaller cell sizes and high-speed UE scenarios. It is generally preferred to maintain a low HOR to ensure a secure and reliable HO process in 5G HetNet. This can be achieved through proper settings of HCPs (such as HOM and TTT). \[KPI_{HOR}=\frac{N_{HO}}{unit} \tag{1}\]  
**HOF rate (HOFR):** HOFR measures the percentage of HO attempts that fail and is defined as the \"total number of failed HOs (HOFs) divided by the total number of HOs (sum of successful HOs and failed HOs)\". HOFs can occur due to various reasons such as poor network planning, heavy cell load or network congestion, ICI, weak RSS, outdated UE, and poor mobility management. Reducing the HOR can help to address HOFR, but there is a tradeoff between minimizing the HOR and HOFR [88]. \[KPI_{HOFR}=\frac{N_{HOF}}{N_{HO(total)}} \tag{2}\]  
**HO success rate (HOSR):** HOSR indicates the percentage of HOs that are successfully completed, which is defined as \"the ratio between the total number of successful HOs and the total number of triggered HOs in a particular time\" [88]. The HO process is considered successful if it is completed before the RSS from the serving cell drops below a specified threshold level (minimum acceptable RSS level) [89]. A high HOSR means that the number of failed HOs is minimized, the HO process is smooth, and the system is providing seamless connectivity to the UE, while a low HOSR indicates the opposite. \[KPI_{HOSR}=\frac{N_{HO}}{N_{HO(total)}} \tag{3}\]  
**HOPP rate (HOPPR):** HO Ping-Pong (HOPP) occurs when a UE frequently connects and disconnects with two different BSs due to rapid fluctuations in signal strength (RSSI) from both stations (serving and target BS) [90]. HOPPR measures the frequency of a UE going back and forth between two cells during the HO process. A high HOPPR indicates that the UE moves frequently between two cells, leading to increased call drop rates, poor user experience, and interruptions in data transfer. A low HOPPR indicates that the UE does not frequently move between two cells and that the HO process is smooth. \[KPI_{HOPPR}=\frac{N_{HOPP}}{N_{HO(total)}} \tag{4}\]  
**HO interruption time (HOIT):** The duration of time during which the HO process is interrupted and the UE is disconnected from the network is called HOIT.