# Underwater Wireless Communications for Cooperative Robotics with UWSim-NET
## 4 Conclusions  
Devices for underwater wireless communications suffer from small bandwidth, high attenuation and large delay. The modeling of these systems might contribute to minimize the impact of their limited performance in cooperative robotic applications. On the one hand, software applications used in robotics such as ROS hardly ever account for the modeling of the communications. On the other hand, network simulators such as NS3 require the user to define in advance all the fluxes and the movements of the nodes during the modeling, which hampers their use in HIL experiments. This manuscript introduces UWSim-NET, an extension of UWSim that permits the modeling of underwater communications for wireless sensors and robots (e.g., AUVs). The modeling of the dynamics of the robots is based on UWSim and ROS, while the communication network is modeled using the NS3. A modular architecture is proposed for UWSim-NET. One of the modules is a Generic Link Interface, which acts as an abstraction layer for the devices. This module facilitates the replacement of real and modeled communication devices, and also permits HIL experimentation. Three experiments are presented in this article to demonstrate the capabilities of UWSim-NET in modeling communication impairments, allowing RF and acoustic channels, modeling several MAC protocols and performing HIL experiments (see Table 2). The first one modeled an RF and an acoustic link between two robots, and showed the capabilities of UWSim-NET in modeling losses by signal attenuation or packet collisions. The measurement of end-to-end delay and the modeling of the transmission FIFO stack of the modems were also illustrated in this first experiment. The second experiment was the simulation of a search and recovery intervention with autonomous vehicles. The experiment was composed of two groups of three and four AUVs, respectively. The first group of robots, in charge of the intervention, was equipped with RF modems, while the second group was in charge of the search, communicating with acoustic devices. Efficiency, throughput, end-to-end delay and jitter were evaluated when using teh CS-ALOHA and S-FAMA MAC protocols. The results of this experiment demonstrate the relevance of the MAC protocol, and that, depending on the traffic characteristics, a given protocol or another might be preferable. In the third experiment, one of the four BlueROVs modeled in the second experiment was substituted by a real BlueROV. The performance of the supervised remote control was assessed by measuring the distance between expected and real positions of each robot. This experiment demonstrated the capability of UWSim-NET in performing HIL trials. \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Experiment** & **Description** & **Findings** \\ \hline Physical Layer & Two moving AUVs sending data at the same time & Demonstration of the correct modeling of end-to-end delay, packet loss, packet collisions and the transmission FIFOs. \\ \hline MAC layer & Cooperative intervention with 4 AUVs sharing an acoustic channel and 3 I-AUVs sharing an RF channel. & Demonstration of the capability of UWSim-NET to model acoustic and RF devices, and the possibility to use different MAC protocols. \\ \hline HIL & Teleoperation of 4 AUVs, one of them being a real BlueROV in a water tank & Demonstration of the capability of UWSim-NET to perform HIL experiments in a real application, in which the positions of the AUVs vary arbitrarily during the experiment. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the main findings of each experiment. **Author Contributions:** Conceptualization, R.M. and P.J.S. ; methodology, J.V.M. ; software D.C.; validation, R.M. ; formal analysis, D.C. and A.S.-A. ; investigation, D.C.; resources, J.V.M. ; data curation, D.C.; writing--original draft preparation, D.C. and A.S.-A. ; writing--review and editing, D.C., A.S.-A. and R.M. ; visualization, D.C. and A.S.-A. ; supervision, R.M. ; project administration, P.J.S. ; and funding acquisition, R.M.