# Performance Evaluation of Direct-Link Backhaul for UAV-Aided Emergency Networks
## 5 Results
### Scenario III: LTE with a Millimeter-Wave Backhaul Link  
Although the usage of carrier aggregation as described in the previous scenario, not all the users could be allocated due to the network restrictions. The proposed solution was to use the 60 GHz band to support uncovered users in a disaster scenario. In Figure 6, a comparison of the three backhaul link scenarios was presented. Here, it can be seen that the number of locations and served users for the CA and millimeter-wave was comparable. For lower altitudes, carrier aggregation could serve slightly more users, and for higher elevations, the millimeter backhaul link could serve more. At the optimal altitude of 80 m, CA performed better, serving 188 users compared to the 177 using 60 GHz. It is known that the 60 GHz band has higher path losses, especially when NLoS links are presented, mainly due to building losses. In some simulations runs (up to 16 runs 24.5%), no UABSs could be connected to the facility, particularly in elevations under 40 m due to high path losses through buildings. This can be easily overcome by incrementing the flight altitude of the drone, as shown in Figure 6b. However, despite that 80 m height was the best altitude, only 68.3% of the users were covered. This is because only UABSs locations in a radius of 550 m close to the facility were served. According to the link budget calculations, if a drone is in an NLoS using the 60 GHz band, the maximum distance will be 390 m. However, the UABSs between 390 m and 550 m, are connected through LoS where the angle is high enough to avoid buildings. In Figure 7a, the coverage simulation results in the city of Ghent for the 60 GHz band are presented. In this case, all the users in a distant radius of 750 m from the facility were served due to the coverage extension obtained by the UABSs. Some users far away from the facility were connected to UABSs that had LoS link to the facility. In comparison with CA, 60 GHz connections outperformed the served users in altitudes higher than 100 m, where NLoS connections were minimal. In Figure 7b, the efficiency of the backhaul link as a function of the flight height is shown. It is shown that the performance of 60 GHz (98.7%) was better than CA (96.4%). This is because the MCSs for the 3.5 GHz band could allocate bigger data packets in the RB, leading to a difficult resource allocation that was simplified in the 60 GHz allocation process. This goes with the cost of using up to four times more RB than in the 3.5 GHz band. Furthermore, the 60 GHz backhaul link had only three schemes with higher SNR  
Figure 6: Comparison of network performance in different flight heights. (**a**) Number of users and number of UABS. (**b**) Percentage of served users. values, compared with the seven in the 3.5 GHz band. It used better connections that led to overall better results in resource allocation. Next, we evaluated how the demand of users affected the network (Figure 7c,d). The results were proportional to the number of demanding users because the number of RBs for the 60 GHz network was much higher (45000 RBs) compared to the maximum users in our simulation and backhaul saturation was not achieved. For 500 demanding users, only 3290 RBs were needed, which was 7.3% of the total RBs at 60 GHz. Figure 7c presents the usage of RB for the three scenarios. It shows that for the 3.5 GHz band, the number of the RBs was fully used, but for the 60 GHz there was space for more. On the other hand, the backhaul RB efficiency was stable at around \(98.1\%\pm 0.8\%\). In Figure 7d, the number of users served by each UABS is presented. It is shown that CA and 60 GHz performed quite similar for less than 200 users. From 200 users on, the CA was saturated when having eight users per UABS, but for 60 GHz, it increased more gently. However, using 60 GHz, the percentage of covered users was smaller than 72% due to the propagation restrictions described previously.