## 4 Experimental Setup
### Field Tests  
Field tests were performed in Chioggia, Italy: such place is inside the Venice Lagoon, close however to the lagoon mouths and thus featuring a salinity which can be assumed comparable with the one of the northern Adriatic Sea, around 35 g/L [42]. Tests were performed by placing the transmitter node underwater close to the port quay, in order to achieve a distance between the transmitter and the gateway of 2 m, resembling the experimental setup of the laboratory. Such distance was measured again with a measurement  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **Depth \(d_{\text{L}\text{I}W}\) (cm)** & **Salinity (g/L)** \\ \hline
#1 & 12 & 0 \\
#2 & 12 & 10 \\
#3 & 12 & 20 \\
#4 & 6 & 20 \\
#5 & 6 & 23.3 \\
#6 & 6 & 26.7 \\
#7 & 6 & 30 \\
#8 & 6 & 33.3 \\
#9 & 6 & 36.7 \\
#10 & 6 & 40 \\
#11 & 6 & 43.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The operational conditions used during the laboratory tests.  
Figure 3: Plastic container used for the laboratory tests.  
tape with \(\pm\)0.1 cm of uncertainty: however, due to the movements of the water surface such measure can be assumed with a \(\pm\)5 cm uncertainty.  
Concerning the technological setup, the transmitter node, as well as the firmware and the operating principle, was the same employed for the laboratory tests with the only difference that 400 transmissions for each SF from 7 to 12 were performed. Similarly, the same Gateway and 4G dongle were employed: however, since tests were carried out in the field, a 44 Ah, 12 V lead acid battery connected to an inverter was used for powering the system. The gateway was positioned close to the quay edge, in order to ensure direct line of sight with the transmitter.  
In order to place the transmitter node under water, at a specific depth and at a distance from the quay which was notably larger than the depth itself to avoid lateral waves, a pole was used. The node was provided with a ballast and was tied to the pole by means of a rope. The overall experimental setup can be seen in Figure 4 while a picture of the tests can be seen in Figure 5.  
While for the laboratory tests the positioning of the transmitter node at a specific depth (i.e., 3 cm and 6 cm) was feasible since a flat water surface was easily achievable, this was almost unachievable in the real scenario. Even if the tests were carried out in a sunny day, with a very limited wave motion, some cms of variation have always to be taken into account. Moreover, larger variations were also caused by the passage of boats of different sizes. Together with the actual variations in the sea height, this motion also caused small displacements of the node, which prevented from keeping a constant depth value. For this  
Figure 4: Test setup of the field tests in Chioggia.  
Figure 5: Tests performed in Chioggia harbor.  
reason, the results of these tests are to be evaluated mainly from a qualitative point of view rather than from a quantitative one. Due to the difficulty in positioning the node at a specific depth, we can assume the actual depth value to be 6 \(\pm\) 4 cm. This value is in line with those adopted in the laboratory tests and partially allows a comparison between the two results.