# Study on Drone Handover Methods Suitable for Multipath Interference Due to Obstacles
## 4 Proposal Method According to the Issue
### Overview of Proposed Method #2  
Handover using proposed method #1 was confirmed to be effective in reducing the time of communication disconnection and the low transmission rate by performing a handover to the optimal AP by following the SINR threshold before the beacon signal is completely disconnected. However, if, for example, an AP on the flight path is locally shielded by a building and cannot receive a high signal until just before it passes, it may perform an unnecessary handover with a very short connection period and may not achieve a stable communication speed. A topographic map of the area is shown in Figure 17 assuming such an environment. This condition is the simulation model of a localized area, as shown in Figure 12. The STA is assumed to fly along a straight path, and the location of AP2, which exists in the vicinity of AP1 to AP3, is subject to a low received power until the STA passes immediately in front of it, due to the surrounding shielding. The STA moves further and obtains high received power from AP2, but immediately after passing AP2, the received power drops again. Using proposed method #1 in such an environment, the connection to AP2, which has a locally high SINR, is immediately disconnected, requiring a handover to another AP, which increases the overhead caused by the switching time connecting to the destination. To solve this problem, proposed method #2 uses the moving average value calculated based on the past SINR values during the flight as a decision index for handover to prevent connection to an AP with a locally high SINR. Figure 18 shows the handover procedure for proposed method #2. Proposed method #1 implements the aforementioned handover timing based on SINR as a threshold and a destination selection method that operates based on the MCS index. On the other hand,  
Figure 16: Evaluation results of proposed method #1. Figure 17: Environmental conditions with frequent handovers. proposed method #2 measures SINR before and calculates the moving average using the latest SINR and statistics referring to the past SINR by Equation (6). Using the proposed method, SINR can be estimated by referring to past values when measuring SINR, such that the locally high SINR can be estimated to be low. \[\text{Moving Average}=\frac{1}{M}\sum_{i=1}^{M}S_{i}, \tag{6}\]  
where \(S_{i}\) indicates SINR at each point by assigning a number \(i\) to each point along the divided travel route, and \(M\) denotes the total number of points referred to in the past. Using the proposed method, the locally high SINR can be estimated as low because past values can be referred to when measuring SINR. The flowchart of proposed method #2 is shown in Figure 19. In proposed method #2, in addition to the procedure of proposed method #1, after calculating the SINR, the moving average value of the SINR is calculated by referring to the past SINRs. This moving average is compared with the threshold value to make a handover decision. An example of the operation of proposed method #2 is illustrated using an example image of the SINR values obtained from each AP in Table 3 and the terrain configuration in Figure 17. In Figure 17, the STA handovers APs from points A to D while flying over the green points. Point C refers to the point at which a high SINR value is obtained from AP2. Table 3 shows the moving average of SINR for points A to D and the SINR for point C. For the case of proposed method #1, which refers only to the SINR of each point in the topography of Figure 17, moving to point C connects to AP2 whose SINR in Table 3 is 30 dB at the highest value. However, by moving to point D, the SINR of AP2 decreases and is immediately connected to that of AP3, which has a higher SINR. In such an environment, where APs with high SINR exist locally, handovers occur frequently, making it difficult to maintain seamless communication. Therefore, proposed method #2 calculates the moving average of SINR at each point and prevents SINR from suddenly becoming high at a certain point.