# Underwater Wireless Communications for Cooperative Robotics with UWSim-NET
## 3 Results
### Physical Layer  
The experiment shown in this section consisted of two robots moving away and approaching each other. The block diagram of this experiment is shown in Figure 2c. It illustrated the packet loss due to attenuation, and packet collisions when both robots try to transmit a message at the same time. This experiment also showed the modeling of the transmission FIFO stacks. Both robots were equipped with Seathooth S100 RF modems [21], by Wireless For Subsea. Packet loss was due to RF attenuation in sea water. The attenuation model used in this experiment was obtained empirically: \(BER=6\ 10^{-13}d^{13.7}\), being \(d\) the distance between devices (in meters). The maximum and minimum ranges of the device were set to 6 m and 0.5 m, respectively. The motion of the robots during the experiment is shown in Figure 3a. At the beginning of the simulation, the distance between the ROVs was 80 cm. They held the position for 11 s, and then one of the ROVs moved away from the other at a constant speed of 0.25 m/s for 20 s. To ensure that the loss of packets was due to attenuation and to avoid packet collisions, only one ROV transmitted while the other received the packets. The moving ROV returned at the same speed for 5 s and held the position at 4.5 m for 10 s. Then, it moved again towards the fixed robot for 5 s at 0.25 m/s and held the position at 3.3 m. To show the packet collision, the second ROV transmitted during Seconds 37-58 during its return. Figure 3b,c shows transmitted (blue dots), received (green dots) and lost (black, red and orange dots) packets when two transmission fluxes were considered from node 0 to node 1 and from node 1 to node 0, respectively. Different colors are used to illustrate the packet loss mechanism. The packets  
Figure 3: Collision of packets when two ROVs are transmitting at the same time. Transmitted packets (tx), successfully received packets (rx), packets lost due to attenuation (err(PROP)), packets lost due to collision (err(COL)) and packets that have suffered attenuation and collision (err(MULT)) are presented: (**a**) distance between the ROVs during the experiment; (**b**) flux of packets of the ROV that is transmitting continuously; (**c**) flux of packets of the ROV that starts a transmission while the other ROV is transmitting; and (**d**) occupation of the transmission FIFO stack. dropped because of attenuation are marked in black, while red dots correspond to those that suffered collision. The ones that suffered both attenuation and collision are shown in orange. When the second robot started its transmission at Second 37, when the distance between the ROVs was about 4.5 m both, signal attenuation and collision mechanisms occurred. However, as the second ROV approached, no packets suffered from attenuation, and loses were due to packet collision. The results shown in Figure 3b illustrate the continuous transmission, and how most of the packets were successfully transmitted. As the second ROV started to transmit (see Figure 3c), it is appreciated in both figures how many packets suffered collisions. The number of successfully transmitted packets was small in both fluxes. As the second ROV stopped sending packets, the situation of the transmission flux, as shown in Figure 3b, returned to the original situation. Both ROVs were configured to transmit at 1500 bps, which is close to the 1800 bps maximum bitrate of the modems. When both ROVs were transmitting, some packets were stored in the transmission FIFO, as can be seen in Figure 3d. The same experiment shown in Figure 3 was repeated modeling acoustic modems to measure the end-to-end delay, as shown in Figure 4. It accounted for the times required to transmit and receive the packet, and the propagation time. The low bitrate of the modems caused the major contributions to the end-to-end delay to be the transmission and reception times. However, it is appreciated in Figure 4 how the end-to-end delay increased and decreased with the distance between the robots.