# Performance Evaluation of Direct-Link Backhaul for UAV-Aided Emergency Networks
## 5 Results
### Scenario I: LTE at 3.5 GHz No Carrier Aggregation  
We evaluated the performance of the network varying the flying height of the UABSs. The values assessed go from 20 m to 200 m above the ground level. We also studied the implication of the facility antenna height and compared an antenna of 25 m against a 60 m antenna using a type 1 drone. In Figure 4a, the required number of UABSs and the served users is presented. It can be seen that if the flight height was low, the number of backhauled UAVs was small due to the buildings that were present in the area. As the altitude increases, the number of line-of-sight (LoS) links increased and the viability increased leading to more suitable connections at 80 m serving around 45 users. This value was optimal and worked under the maximum limitations from the Belgian Civil Aviation Authority, which was 90 m for the considered type of drones [45]. Above this altitude, the connected number of UABSs decreased slightly due to the distance between the facility antenna and the UABS. It is vital to notice that the facility antenna height hardly affected the performance of the number of backhauled UABS only in values less than 60 m. Above that altitude, the UABS-users connectivity defined more the performance. Besides, it can be seen that usually, only 45 users were served. This value could not be increased because all of the 100 RBs were allocated. Figure 4b presents an assessment of the RB efficiency for the backhaul. If all the connection used the best MCS (64 QAM-2/3), the maximum bandwidth achievable would be 72 Mbps. However, the actual bandwidth was 41.3 Mbps, equivalent to 0.4 Mbps per RB (dotted lines in Figure 4b), equal to an MCS of 16 QAM-2/3. Figure 3: Flow diagram of the algorithm implemented for backhaul analysis. Moreover, the maximum of 72 Mbps could hardly be achieved due to the resource block allocation, i.e., a user requesting 1 Mbps will need two RB of 720 kbps each, leading to 1.4 Mbps allocated but only 1 Mbps used (69.4% efficiency). This efficiency could be improved if more user's requests are assigned in the available backhaul RB with divers MCS. Our results show that efficiency was nearly \(81.9\%\pm 3.2\%\), as shown in the straight lines of Figure 4b. Next, we evaluated the performance of users (50-500 users) asking for resources in the network using the three types of drones (see Table 3) at three different altitudes (40, 80 and 120 m). Figure 4c,d,e present these results. In Figure 4c, the number of UABS needed depended on the type of drone used, where the worst case was with the Type 1 drone. Type 2 and hybrid type behaved similarly because the autonomy per drone was more than one hour, which was the considered intervention time. Hence similar results were obtained but with a slight increment from the Type 2 (16 hybrid drones to 18 Type 2 drones) mainly because of the speed of the drone. The average power used by UABSs as a function of the served users is presented in Figure 4d. The power consumption is the sum of the energy used by the flight from the facility to the serving location and return, plus the power for the transmission of the access and the backhaul network [46]. The averaged power consumption was almost independent of the type of drone used, and because the majority of the consumption was used in radio transmissions, it was proportional to the requesting users. At the optimal height of 80 m,  
Figure 4: Evaluation results for the Scenario I. (**a**) Flight performance of UABSs. (**b**) Backhaul resource block performance. (**c**) UABS needed for different types of drones. (**d**) Power usage for different types of drones. (**e**) Backhaul network utilization for different kinds of drones. a lower amount of power was consumed because the UABSs have to transmit less power to achieve the same bit rate due to better LoS links and lower flight times to the serving position. In Figure 4e, we evaluated the backhaul network performance. The results show that the network was stable independent of the number of users.