# Wireless Remote Control for Underwater Vehicles
## 2 Underwater Communication Technologies
### Optical Communications  
Although acoustic modems are the typical solution for underwater communications, their bandwidth is very limited. The need for high-speed communications in an underwater environment has pushed the realization of optical devices that can transmit data within short distances at a bit rate on the order of one or more Mbps (up to few Gbps at very short ranges, depending on the model and the water conditions). Indeed, unlike acoustic communications, optical communications are more suitable for ranges up to 100 m, especially in deep dark waters, and are not affected by multipath, shipping, and wind noise, as their performance mainly depends on water turbidity and sunlight noise [70]. In fact, high turbidity scatters and attenuates the optical field, whereas ambient light may become a significant source of noise, making transmissions close to the sea surface more difficult. The turbidity coefficient, called the attenuation coefficient, is composed of the sum of scattering and absorption coefficients. The former depends on the quantity of particles, such as plankton, dissolved in the water. The plankton exists due to the chlorophyll effect, that happens only where solar light reaches the medium, i.e., in shallow water, up to a depth of 100 m. The latter, instead, is an inherent optical property of the medium. Blue and green lights, which have wavelengths of 470 and 530 nm, respectively, are the most widely used for underwater optical communication [71], as these wavelengths are the least attenuated in deep and shallow water, respectively. Intuitively, in order to understand which of the two wavelengths is less attenuated in a certain scenario, we just need to observe the color of the water in the presence of white light (e.g., sunlight). If the color of the water is blue, the best wavelength to use is around 470 nm (that is the typical case of a deep water deployment); otherwise, if the water color is green, wavelengths around 530 nm should be employed (that is the typical case of deployment close to the surface). Similar to acoustic modems, optical transceivers are also designed to perform best in some scenarios; therefore, the best optical modem that outperforms all others in all possible conditions does not exist. Specifically, we can divide the optical modems into models composed of a light emitting diode (LED)-based transmitter designed for hemispherical communications [8; 25; 72; 73] and models composed of a high-directional laser-based transmitter [74; 75; 76; 77; 78; 79]. Although the latter achieves a throughput from 10 to 100 times higher than the former, we focus on LED-based modems, as laser-based transmitters require perfect alignment between the transmitter and receiver, a condition that a mobile vehicle can fulfill only during a docking operation or if the modems are equipped with beam-steering capabilities, such as the SA Photonics Neptune modem [80]. The latter, however, is not an off-the-shelf product, i.e., it is custom built to order, with a significantly higher price compared to other commercial products. We can also divide the optical modems into models tailored for dark water medium range (MR) communications (up to 100 m) [7; 25] and devices designed for short-range (SR) communications in high