# Performance Evaluation of Direct-Link Backhaul for UAV-Aided Emergency Networks
## 6 Conclusions and Future Work  
Despite that land-based networks are quite reliable, in case of an emergency, they could suffer from saturation or total failure. Using fast deployable systems mounted on UAVs are a promising solution, but they arise challenges as optimal drone selection and placement, radio network section including the access and backhaul. This paper presented a network architecture based on UAVs to provide wireless connectivity for a disaster scenario and the performance evaluation of the capacity for the proposed architecture. To this end, we developed a novel simulation tool and applied it on a realistic scenario in the city center of Gent in Belgium, accounting for capacity and power restrictions from both the access and backhaul networks. The study focused on the usage of unoccupied frequency bands such as 3.5 GHz and 60 GHz and compared the performance of variables such as the number of demanding users and UABSs' flight altitude. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Scenario I**} & \multicolumn{3}{c}{**Scenario II**} & \multicolumn{3}{c}{**Scenario III**} \\ \cline{2-10}  & **Type 1** & **Type 2** & **Type 3** & **Type 1** & **Type 2** & **Type 3** & **Type 1** & **Type 2** & **Type 3** \\ \hline \multicolumn{10}{c}{**USERS**} \\ \hline Users served (users) & 54 & 54.2 & 54.1 & 188.2 & 187.9 & 190.8 & 177.6 & 178.1 & 177.6 \\ Users served (\%) & 21.6 & 21.7 & 21.7 & 72.4 & 72.3 & 73.4 & 68.3 & 68.5 & 68.3 \\ Users per UABS & 3.36 & 3.2 & 3.4 & 8.1 & 7.9 & 8.1 & 9.4 & 10.1 & 9.6 \\ \hline \multicolumn{10}{c}{**UABSs**} \\ \hline \# UABSs locations & 16.4 & 17.0 & 16.2 & 23.4 & 23.9 & 23.6 & 19.2 & 18.6 & 18.8 \\ \# Used UABS & 75.2 & 18.5 & 16.2 & 110.1 & 26.5 & 23.6 & 76.7 & 18.6 & 18.8 \\ Mean power usage (w) & 34.4 & 33.7 & 35.3 & 36.9 & 36.2 & 36.9 & 24.9 & 24.9 & 24.9 \\ \hline \multicolumn{10}{c}{**CAPACITY**} \\ \hline Total BH capacity (Mbps) & 47.4 & 47.6 & 47.9 & 173.1 & 171.4 & 174.7 & 163.8 & 164.1 & 163.5 \\ BH capacity per UABS (Mbps) & 2.9 & 2.8 & 2.9 & 7.4 & 7.2 & 7.4 & 8.7 & 8.8 & 8.81 \\ Total RB usage (RB) & 100 & 99.9 & 100 & 500 & 499.8 & 499.9 & 1617.8 & 1619.1 & 1620.8 \\ RB usage per UABS (RB) & 6.2 & 5.9 & 6.2 & 21.5 & 21.0 & 21.3 & 85.4 & 87.1 & 87.36 \\ RB capacity (kbps/RB) & 474.0 & 476.5 & 479.0 & 346.2 & 342.9 & 349.5 & 101.2 & 101.3 & 100.8 \\ BH RB efficiency (\%) & 87.0 & 87.2 & 87.1 & 96.2 & 96.4 & 96.1 & 98.4 & 98.7 & 98.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Resume of the critical parameters for 260 users at 80 m drone altitude. The study also introduced the comparison of traditional off-the-shelf drones with hybrid ones and concluded that the last one outperformed in flight time and power usage. Further analysis in the weights of the backhaul equipment needs to be done in the future. Our results showed that for the proposed architecture, 80 m was the optimal flight altitude. The capacity results showed that by using the simultaneous access and backhaul resource allocation a real solution could be achieved, serving up to 17.3%, 72.4% and 68.1% of the users for a 3.5 GHz link, 3.5 GHz with carrier aggregation and 60 GHz network configuration, respectively. For the 60 GHz link, the main limitation of users served was due to the distance as a result of path loss building penetration reducing the covered area down to a radius of 750 m. Contrarily, the 3.5 GHz solutions were mainly limited by the full usage of resource blocks. Future work will include the usage of a multi-frequency backhaul network using 3.5 GHz and 60 GHz bands choosing which link is optimal to serve UABSs with different constraints. Moreover, future work may involve the usage of Multiple-Input Multiple-Output (MIMO) antennas, multi-channeling to reduce the noise floor and beamforming analysis. Investigation when part of the terrestrial network is down, might also be included, where inter-cell interference is evaluated in conjunction with in-band an out-of-band radio resource allocations.