# MIMO Relaying UAVs Operating in Public Safety Scenarios
## 3 Results  
Figure 4 shows the allocated positions for users and the relaying UAV during the experiment. The figure highlights the drones' relay, the users' initial and final positions, and the number of users in the simulation. The base station is not represented in the figure but is located 300 m from the relays, as described in the previous section. In the simulation, UAVs establish a relay connection with nearby users and send their data to a remote host, as illustrated in Figure 3. The UAVs move in concentric circles at a radius of 20, 50, and 80 m. Meanwhile, the users move randomly according to the Random Walk principle. The network uses the round-robin (RR) scheduler. The network's topology and dynamics infer that the modulation and coding schemes may adapt according to the channel phenomena (distance, interference, fading, etc.) received in the channel quality indicator (CQI) parameter. The overall throughput is estimated considering the number of relays, bandwidth, number of resource blocks available, and the transmission time interval in LTE [36]. Figure 5 illustrates the percentage of received packets as a function of the relaying UAV's speed. In this experiment, three relaying UAVs are used to establish communication with the user groups allocated to each UAV. For example, UAV 1 supports 4 users, then UAV 2 supports 4 additional users, etc. There are three group scenarios in this simulation, i.e., 4 users, 5 users, and 6 users. \begin{table}
\begin{tabular}{c c} \hline \hline
**General Parameters** & **Values** \\ \hline Relay UAV Height [m] & 20–30 \\ \hline eNB Height [m] & 120 \\ \hline Relay drone radius [m] & 20–400 \\ \hline Relay Speed [m/s] & 5 \\ \hline User’s height [m] & 1.5 \\ \hline Uplink Frequency [MHz] & 3410 \\ \hline Downlink Frequency [MHz] & 3510 \\ \hline Bandwidth [MHz] & 50 \\ \hline Relay Power [dBm] & 20 \\ \hline Users Power [dBm] & 20 \\ \hline eNB Power [dBm] & 40 \\ \hline Simulation Time [s] & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Frequencies bands used in the simulation. The overall throughput exhibits the same behavior. The only exception is group 3, which has 6 users, where the overall packet reception decreases at a speed of 30 m/s instead of increases when compared to its measurements at 20 m/s. The peak that occurs at 10m/s may be related to the physical layer configuration changes that support quick communication. These changes are based on the adaptative modulation and coding schemes that support the simulation and the reserve resource blocks for PSCCH/PSSCH signals for retransmissions. Some improvements might be justified using group communication techniques. Figure 6 presents the effect of increasing the number of UAVs in the network. In this simulation, the number of relaying drones is increased while the number of users per relaying UAV and the speed is fixed at 4 and 10 m/s, respectively. As a relay is included in the network, the number of connected users also increases, and the overall packet reception is affected by that. In this study, we do not consider the D2D connection between users. Figure 4: Users and relaying UAVs positions in the simulated scenario: (**a**) In blue: users’ initial position. In brown: relay UAVs initial position; (**b**) In blue: users’ final position. In brown: relay final position inside 350 \(\times\) 200 m\({}^{2}\) area. Figure 5: Effects of speed on the overall throughput per users’ groups. Relaying drone = 3. MIMO technique: Alamouti. Figure 7 compares MIMO techniques when the speed is configured to 10 m/s, and the number of users is 4 and 8, respectively. Despite the gain diversity and spatial multiplexing included in the NS3 algorithm, no effect is observed in relation to the SISO system. Despite the diversity gain and the spatial multiplexing included in MIMO [41], which provides a one or two-layer gain as defined in [39], in the simulation with 4 users, the throughput was reduced, and the one with 8 users kept constant.