# Adaptive Handover Decision Using Fuzzy Logic for 5G Ultra-Dense Networks
###### Abstract
The ping-pong effect refers to the back-and-forth handover phenomenon between two BSs that is caused by unstable signal strength resulting from the movement of UE close to the edge of a BS's coverage. As summarized in Table 1, the 3 rd Generation Partnership Project (3 GPP) defines the trigger scenarios of handover decision for cellular networks [10]. A-series events correspond to horizontal handovers, that is, handover between two BSs based on the same radio access technology (RAT), whereas B-series events correspond to vertical handovers, that is, handover between two BSs based on different RATs. Figure 1: 5G UDN architecture. Starting with 4G networks, Event A3 has been implemented for handover triggering. 5G networks also use Event A3 for performing handovers. As depicted in Figure 2, when the signal strength of the neighbor BS is greater than that of the serving BS plus a threshold value and an offset, which is designated as the handover margin (HOM), handover is performed after waiting for a time-to-trigger (TTT) duration. HOM and TTT are used to make handover decisions to avoid the ping-pong effect and alleviate the control burden of BSs. The 5G era is characterized by larger bandwidth, lower latency, and smaller cell coverage. In this context, mmWave technology can support high-density BS deployment for hotspots, perform high-precision positioning of UE, and facilitate equipment integration. Therefore, it is beneficial to promote the miniaturization of base stations and user terminals. The UE moving within a 5G UDN may encounter more frequent handover opportunities because the cell coverage of each small BS is not as wide as that of 4G BSs, and handover scenarios in 5G UDNs appear to be more complex. In 4G network environments, handovers are only performed at the cell edge of a BS, and there are fewer target BS candidates. Consequently, the use of Event A3 for making handover decisions can lead to good performance. However, the use of Event A3 with fixed HOM and TTT for making handover decisions is not reliable for 5G UDNs, because UE may frequently move forward into a certain area where the cell coverage of multiple BSs overlaps. Moreover, individual handover parameter values are essential for different UEs to adapt to diverse handover scenarios. For example, when the signal strength of the serving BS is good, a high HOM can prevent unnecessary handovers or the ping-pong effect. In addition, for UE moving at a high speed, small HOM and TTT can lead to more successful handovers in time. Fuzzy logic (FL) [11; 12] is a well-known method for translating the domain knowledge of human experts into a rule base and for experts to formalize uncertainty. FL is considered the best technique to unravel the handover problem in a high-density scenario of small cells for 4G/5G networks [13]. Recently, numerous FL-based handover schemes, which are elaborated in Section 2, have been proposed for making handover decisions more \begin{table}
\begin{tabular}{c c} \hline \hline
**Event** & **Trigger Scenario** \\ \hline A1 & Serving is better than the threshold \\ A2 & Serving is worse than the threshold \\ A3 & Neighbor is offset better than the serving cell \\ A4 & Neighbor is better than the threshold \\ A5 & Serving is worse than threshold\_1, and neighbor is better than threshold\_2 \\ A6 & Neighbor is offset better than the secondary serving cell \\ B1 & Inter-RAT neighbor is better than the threshold \\ B2 & Serving is worse than threshold\_1, and inter-RAT neighbor is better than threshold\_2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measurement report triggering. Figure 2: Event A3 handover procedure. accurately in time. To alleviate unnecessary handovers and avoid a severe ping-pong effect in 5G UDNs, we propose an FL-based handover algorithm with dynamic HOM and dynamic TTT (abbreviated as FLDHDT) by using the signal to interference plus noise ratio (SINR) and horizontal moving speed of UE as inputs to the FL controller.