## 5 Simulation and Experiment
### Simulation
#### 5.1.1 Simulation and Analysis of ILEACH and TGA Algorithms
The result shows that, in the combination of clustering algorithm and path optimization algorithm, the combination of ILEACH and TGA has the best optimization effect. Figures 12 and 13 are the shortest paths obtained by ILEACH-GA and ILEACH-TGA from the 30 runs, in which red dots represent cluster heads and blue squares represent take-off points. the mode, in which the UAV can only collect data when hovering over the cluster head [32], as the HOVER collection. The mode in [20] is called the MOVE collection, in which the UAV flies to the current cluster head and collects data at the same time. When the amount of data is large, it needs to hover over the current cluster head to complete the data gathering. After completing data gathering, it flies directly to the next node using fixed flying speed. In our schemes, the MOVING collection mode is defined, in which the flying speed of the UAV can be adjusted to complete the data gathering task. Moreover, as shown in Figure 7, at a fixed data rate, the data gathering time of DR6 is always minimal, regardless of data size. Therefore, we use DR6 for all HOVER and MOVE modes. We compare our joint method TGA-MOVING-DCMDR with GA-HOVER-DR6, TGA-HOVER-DR6, GA-MOVING-DCMDR, and TGA-MOVE-DR6 to evaluate the performance. In scenario 1, we evaluated the influence of the average amount of data in cluster heads on the completion time of data gathering task. The flight speed of the UAV was set to 12 m/s. We evaluated the large amount and small amount of data in cluster heads, respectively. The simulations were implemented with 20 consecutive runs, and the average results are shown in Figure 14. It is obvious that the completion time of the TGA-MOVING-DCMDR method is always the minimum, regardless of the amount of data stored in the cluster heads. The completion time of GA-HOVER-DR6 and TGA-HOVER-DR6 methods increases with the amount of data stored in the cluster heads, showing a linear relationship. Because, in these two methods, the main factor determining the flight time of the UAV is the amount of data in cluster heads. Due to the shorter planned path of TGA, TGA-HOVER-DR6 is always smaller than GA-HOVER-DR6. In small amount of data cases, as shown in Figure 14a, when the amount of data is very small, DCMDR has an obvious effect. With the increase in the amount of data, TGA has an obvious effect. Therefore, with the increase in the amount of data, the completion time of GA-MOVING-DCMDR is obviously more than that of TGA-HOVER-DR6. With the increase in the amount of data, the completion time of TGA-MOVE-DR6 changes slightly; because with DR6 mode, the small amount data can be transmitted rapidly, which has a slight impact on time. With TGA-MOVING-DCMDR, UAV can start gathering data from a long distance with the lower DRs and it has obvious advantages over other methods. In large amount of data cases, as shown in Figure 14b, when the amount of data exceeds 32 Mbits, the completion time of all methods linearly increases with the amount of data obviously, because the UAV has to reduce flight speed or hover within the signal coverage of DR6, to complete the data gathering. The increased time is almost proportional to the increased amount of data. In scenario 2, we evaluated the influence of UAV speed on the completion time of the data collection task. The simulations of two cases were carried out, with the average data amount of cluster heads of 32 and 64 Mbits, respectively. In Figure 15, the completion time of the UAV data collection task decreases with the increase in the UAV speed, and the task completion time of the TGA-MOVING-DCMDR method is always the smallest in two data amount cases. Regarding TGA-MOVING-DCMDR, the difference in the completion time between the 64 Mbits case and 32 Mbits case increases with the increase in speed. This is because, within the signal coverage of DR6, the UAV hardly needs to reduce its speed in the 32 Mbits case, while it has to reduce its speed in the 64 Mbits case to complete the data gathering.