# Wireless Remote Control for Underwater Vehicles
## 2 Underwater Communication Technologies
### Radio Frequency and Magneto-Inductive Communications
Broadband RF modems can also be employed in docking stations to quickly download data from an AUV [94]. For instance, the WFS Seatooth S500 [91] RF modem provides a bit rate up to 100 Mbps up to a range of 10 cm, and the Lubeck University of Applied Science developed the CoSa underwater WiFi [5], with a rate of 10-50 Mbps up to 10 cm. These examples prove how RF communications can achieve high transmission bit rates underwater, although their communication range is very limited. Indeed, RF communications suffer from RF interference and are prone to very strong attenuation in salty waters, where the conductivity of the medium is larger than in fresh waters. A range up to a few meters (SR) can be reached with RF modems, at the price of a lower bit rate. For example, INESC Tec developed a dipole antenna prototype [6] to support 1 Mbps communication at 1 m, and the Lubeck University of Applied Sciences developed a dipole [5] antenna to communicate with a rate of 0.2 to 1 Mbps and a range of 1-8 m, depending on water conditions (i.e., 1 meter in salty water and 8 m in fresh water). Although in air MI communication is outperformed by RF modems, as the latter can achieve a higher bit rate and a longer range, underwater MI modems are almost unaffected by the change of medium while the electrical field is strongly attenuated. Indeed, MI modems are proven to reach a bit rate of a few kbps at tens of meters, both in air and underwater [95]. Dalhousie University developed an MI prototype that achieves 8 kbps at 10 m [96], to perform low-rate low-latency communications. With MI modems, longer distances can be achieved at the price of a lower bit rate. For instance, the authors in [95] established a directional link with a maximum range of 41 m and an omnidirectional link with a range of 26 m. Both links provide data rates of 512 bps. With their new modem design recently presented in [97], they were able to achieve 1 kbps at a 40-m distance with an omnidirectional beam pattern. Nearly 20 years ago, the authors in [98], instead, demonstrated a 153-bps communication link at a distance of 250 m and a 40-bps communication link at a distance of 400 m.  
Very-low (VLF) and extremely-low radio frequency (ELF) signals have been extensively used during the cold war to communicate from inland control stations to submarines [99]. The drawbacks of these systems are the low rate and the need for a very large and high-power-consuming inland antennas. Indeed, VLF can provide a 300 bps one-way communication link from shore to the submarine up to a distance of 20 m below the sea surface and requires a broadcast inland antenna with a size between 300 m and 2 km [99]. For example, the Sweden Grimeton Radio Station [100] uses a set of antennas that are 1.9 km long, each with an RF power peak of 200 kW. ELF can also be used to communicate from land to submarines (one way): they reach up to 1 bps [101] at a range of several hundreds of meters below the sea surface but require a grounded wire inland antenna (ground dipole) with a size of several tens of kilometers and a transmission power in the order of millions of watts. Due to the high cost of deployment, US, Russia, India, and China are the only nations known to have constructed ELF communication facilities. For instance, the US ELF system employed a ground dipole antenna 52 km long [102] while the Russian system used an antenna 60 km long [103]. This system has been typically used to signal one-way coded messages to the submarine's commander to resurface to receive more information via other means. Both VLF and ELF technologies are not applicable to ROVs and AUVs but only to submarines due to their very large size and demanding power consumption. The most representative underwater MI and RF communication systems are summarized in Table 3.