# Underwater Wireless Communications for Cooperative Robotics with UWSim-NET
## 3 Results
### Remote ROV Team Control in HIL  
The third experiment presented in this paper demonstrated the capability of UWSim-NET in performing HIL experiments. It corresponded to the four exploration AUVs detailed in Section 3.2. The four AUVs were arranged in a squared formation and the distance between adjacent vehicles was 1.5 m. The leader AUV was replaced by a real BlueROV; it is identified with the marker in Figure 8.  
Figure 7: Results for the TWINBOT based scenario without considering any MAC protocol (NOMAC), considering CS-ALOHA MAC protocol (ALOHA) and the S-FAMA MAC protocol (SFAMA), in terms of: (**a**) throughput (bytes per second); (**b**) efficiency (%); (**c**) end-to-end delay (s); and (**d**) jitter (s).  
The operator sent positioning commands to the leader, and the rest of the AUVs followed the movements of the leader trying to maintain the formation of the AUVs. The block diagram with the software and hardware elements used in this experiment are shown in Figure 8c.  
All the AUVs were equipped with acoustic modems; the modems modeled in this experiment are the S2 CR 7/17, by Evologics [24]. Despite the umbilical link between the leader and the operator, this communication was simulated with UWSim-NET as an acoustic channel. The transmission fluxes were the same as those indicated in Section 3.2. Since GPS positioning does not work properly in the lab we have at UJI, we developed a positioning feedback based on Aruco markers [25] (see Figure 8).  
The same experiment was performed considering CS-ALOHA and S-FAMA MAC protocols. Positioning commands were sent from the operator to the leader that moved towards the commanded position, while the other AUVs tried to keep the formation by following the movements of the leader as they noticed them. The comparison among algorithms, as shown in Figure 9, was based on the error distance between real and expected position for each AUV. In the case of the leader, the error was the distance between actual and commanded position (see Figure 9a). A rapid increase of the error is appreciated in Figure 9a every time the leader received a new positioning command. The error for the follower AUVs was the distance between their actual position and their expected position  
Figure 8: Cooperative AUV Control in HIL. (**a**) UWSim visual interface of the HIL experiment with the four robots. The one with the marker is the real BlueROV. The floor of the water tank and the Aruco markers have also been represented in the UWSim visualization. (**b**) Image of the real BlueROV in the water tank at the IRSLab. (**c**) Block diagram of the elements used in this experiment.  
based on the actual position of the leader. As expected, a rapid increase of the error can be appreciated in Figure 9b-d when the leader moved. As appreciated in Figure 8, comparable performance was achieved when CS-ALOHA and S-FAMA MAC protocols were considered.  
Figure 9: Waypoint distance error for each AUV for the CS-ALOHA and S-FAMA MAC protocols. (**a**) Distance between commanded position and actual position of the leader. Distance between actual position and expected position based on leader’s for: (**b**) the AUV on the left of the leader; (**c**) the AUV placed back and left; and (**d**) the AUV placed behind the leader.