## 4 Experiments
### Experiment III: Underwater Communications  
In the current experiment, the communications architecture has been studied to enable the underwater team of robots to communicate in a cooperative manner and to be able to transmit status and sensor data (including compressed images) to the surface.  
As can it be seen in Figure 20, a user interface networking module, including 3D virtual and augmented reality, has been implemented to allow the operator to remotely control an underwater robot using both sonar and radio frequency modems.  
Figure 19: Usability test using 1 operator to control the team using cooperative behaviours: 1 experiment failed on grasping, and the following attempts were successful.  
Figure 20: Underwater radio frequency experiments for robot teams: (**top left**) predictive 3D module to teleoperate the wireless underwater robot, where the opaque robot represents the current position and the transparent robot represents the required one; (**top right**) underwater robot and wireless modem; (**bottom left**) user interface of the network; and (**bottom right**) Region-Of-Interest (ROI) specification.  
Wireless underwater image feedback is expensive in terms of consumed bandwidth and requires advanced compression techniques. As shown in Figure 20, once the robot obtains its position by using for example markers or target localization, it is possible to use the 3D virtual scenario to represent the real position of the robot to the user and to allow the user to specify the next movement. This operation can be seen in Figure 20 as the transparent robot that represents the next position of the robot programmed to the operator, while the opaque 3D model represents its real position. This type of user interface reduces the consumed bandwidth, enhances the information provided to the user, and enables enrichment of the 3D scene with augmented information, such as the projection of the robot position in the pool floor, angles, etc.  
The user interface enables the operator to select the control parameters of the network protocol and compression algorithm, allowing for specification of both the quality and resolution of the transmitted data as input. Also, a Region-Of-Interest (ROI) module has been added, allowing the operator to define multiple layers of compression. Further details about the communication and compression underwater techniques can be found in [8].  
The results of the communication experiment are shown in Table 3. From this, we can conclude that, in CERN tunnels, at least, the leader of the robot team should have a Global System for Mobile communications (GSM)/3G/4G. link to the surface, while the followers or auxiliary robots should connect to the leader using a Wi-Fi ad hoc infrastructure, taking into account that having a low jitter is mandatory for cooperative team actions.  
In addition, for underwater teams, the TWINBOT network architecture has a sonar link to the surface between the leader mobile manipulator and the surface. Also, the three-team robots communicate using radio frequency modems, which have proven to provide 1.9 Kbps of bandwidth at 5 m and can be updated to reach 12 or 30 m of coverage. The sonar link provides 3.2 Kbps, while the propagation and intrinsic delay are too high for cooperative interventions, so it is used to get the operator in the control loop in a supervised manner.  
Acoustic communication for this experiment has proven to have greater fluctuation in the intrinsic delay (i.e., 363.853 standard deviation) due to Media Access Control (MAC). protocol implementation, which uses a burst of packages that require buffers in the modem.  
The underwater communication experiment results have been used as input to model the network behaviour in the UWSim underwater simulator ([[http://www.irs.uj.es/uwsim](http://www.irs.uj.es/uwsim)]([http://www.irs.uj.es/uwsim](http://www.irs.uj.es/uwsim))).