# Underwater Wireless Communications for Cooperative Robotics with UWSim-NET
## 1 Introduction  
Marine environments are not easy scenarios for human activities, especially when the operations have to be performed at depths larger than 50 m. Underwater research is hampered by the requirement of developing waterproof and weight compensated devices, protected against high pressure. Despite these limitations, the range of underwater applications continuously increases. Research on archaeology or marine environment, maintenance and inspection of oil and gas infrastructures, and fish farming are some examples of human activities conducted at sea. Technological advances have enabled the automation of some of these activities, and also the cooperation of several devices in order to conduct complex tasks. The rise of the number of devices employed in underwater activities has motivated intense research in the field of underwater wireless networks to interconnect all these devices. A current trend in robotic intervention is focused on cooperative applications with multiple remotely operated vehicles (ROV) or I-AUVs (Autonomous Underwater Vehicles for Intervention). For this, some robots conduct a given task while others perform additional activities, such as visual surveying, to provide the operator with visual feedback of the progress of the operation. The communication between the ROVs or autonomous underwater vehicles (AUV) and the operator is usually based on umbilical cables or acoustic transducers. While these approaches are valid in experiments with a small number of vehicles, the participation of more and more robots in underwater interventions require novel solutions. In general, acoustic communications are a good solution forlong range transmissions (>1000 m), but having several robots sharing the same acoustic channel might degrade the performance of the communication link. Therefore, alternative solutions based on radio frequency (RF) [1; 2; 3] are also considered for short range communications between ROVs. The problem with RF is the strong attenuation of electromagnetic signals in marine water, which limits the communication range to 15 m [4]. Visual light communication (VLC) [5; 6; 7] is another alternative that has a higher range than RF, but requires that the transmitter and the receiver are aligned. The communication link based on VLC is heavily influenced by water turbidity, being hard to operate in dirty water. The TWINBOT research project [8] is funded by the Spanish Government and led by some of the authors. It extends previous research on autonomous underwater robotic interventions [9; 10]. TWINBOT is aimed at providing experimental evidence about cooperative manipulation with several AUVs. Several wireless approaches for underwater communication will be explored within the TWINBOT project (Figure 1). The communication between the operator on the surface and the AUVs will be based on an umbilical or an acoustic link (Figure 1a). The surface modem will be mounted either on the ship or a buoy via a Wi-Fi connection to the ship. Another goal of the TWINBOT project is to explore the suitability of a VLC or a RF network as possible solutions for communicate the two I-AUV on the seafloor (Figure 1b). The leader AUV will have an acoustic modem for communication with the surface, and will route the commands to the follower via an RF (or VLC) modem. The work presented in this manuscript is the simulation package developed to evaluate the communication technologies to be explored within the TWINBOT project. The poor performance of wireless technologies at sea has a heavy impact on underwater applications. The modeling of the communications is therefore required to account for their high attenuation, large delay, low range, low bandwidth and low bitrate in underwater scenarios, and the limitations they impose on the application. Several software packages aim at modeling underwater wireless sensor networks (UWSN) [11].