# Wireless Remote Control for Underwater Vehicles
## 2 Underwater Communication Technologies
### Modern Selection and Considerations  
According to the technology comparison presented in this section and summarized in Figure 3, we can conclude that optical technologies are the preferred choice up to a distance of about few tens of meters (100 m in very good conditions) whereas acoustics would be the preferred choice from that point onward. We also note that RF and MI modems are consistently outperformed by optical or acoustic modems, although they have the advantage that their communication is not prone to environmental characteristics (unlike acoustic and optical). The optical modem considered in this paper is BlueComm 200, that has a hemispherical beam pattern and is able to transmit at a speed of 10 Mbps at a range up to 100 m in good channel conditions, such as those considered in this paper. For different scenarios, when choosing which modem to use, \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Manufacturer and Model** & **Max Range** & **Bit Rate** \\ \hline \multirow{4}{*}{**Real**} & \multirow{2}{*}{VLF (3–30) kHz [99; 100]} & 20 m below the sea surface, & 300 bps one way, from \\  & & inland antenna size = 2 km & land to underwater \\ \cline{2-4}  & & & hundreds of meters below the \\  & & & sea surface, inland ground \\  & & & dipole size = [52; 60] km & land to underwater \\ \hline \multirow{4}{*}{**Real**} & \multirow{2}{*}{Dalhousie Univ. Prototype [96]} & 10 m & 8 kbps \\ \cline{2-4}  & & inductive modems for & up to few kilometers from tx \\  & & & using the mooring line, \\  & & & few cm from the mooring line & \\ \cline{2-4}  & & MST Prototype [97] & 40 m & 1 kbps \\ \cline{2-4}  & & CSS/MISL Prototype [98] & [250; 400] m & [153; 40] bps \\ \hline \multirow{4}{*}{**Real**} & \multirow{2}{*}{WiSub Maelstrom [92]} & 5 cm & 100 Mbps \\ \cline{2-4}  & & CoSa WiFi [5] & 10 cm & [10; 50] Mbps \\ \cline{2-4}  & & WFS Seatooth S500 [91] & 10 cm & 10 Mbps \\ \cline{2-4}  & & INESC TEC Dipole [6] & 1 m & 1 Mbps \\ \cline{2-4}  & & CoSa EF Dipole [5] & [1; 8] m & \{0.2, 1\} Mbps \\ \cline{2-4}  & & WFS Seatooth Mark IV SR [4] & [5; 7] m & 2.4 kbps \\ \cline{2-4}  & & WFS Seatooth Mark IV MR [4] & [30; 45] m & 100 bps \\ \hline \hline \end{tabular}
\end{table} 
Table 3: List of performance figures for representative radio frequency (RF)/magneto-inductive (MI) underwater modems. Figure 3: Nominal bit rate vs. range for the best among the technologies presented in Section 2.  
it should be considered that the BlueComm 200 modem is strongly affected by noise due to sunlight and external lights, and for this reason, Sonardyne supplies it together with an ROV lighting system that does not affect the modem performance. Indeed, BlueComm 200 can be used only in deep-water scenarios or during night operations in shallow water. If the ROV needs to be operated in shallow water during daytime, a different model tailored for shorter ranges in these conditions should be selected, like BlueComm 100, that achieves a maximum range of 15 m and a maximum rate of 5 Mbps even in the presence of sunlight, or BlueComm 200 UV, that still suffers from direct sunlight but is robust against ROV lighting systems. One of the most important things to take into account when selecting multiple acoustic modems to be used in the same network is to avoid interference between the different devices. For instance, if both LF and MF acoustic modems need to be used in the same system, the maximum working frequency of the LF modem must be smaller than the minimum working frequency of the MF modem to avoid bandwidth overlap. Moreover, some guard between the bandwidths of the modems should be provided, as the drop of the transducer sensitivity outside the modem bandwidth is usually not vertical: for example, EvoLogics S2C 7/17 should not be used along with EvoLogics S2C 18/34 because the bandwidths of the two modems are spaced apart only by 1 kHz.