# Cooperative Terrestrial-Underwater FSO System: Design and Performance Analysis
## 1 Introduction  
Over the last few years, there has been a constant and growing interest in free-space optical (FSO) technologies mainly due to the straightforward creation of point-to-point high-capacity transmissions in terrestrial and space networks [1; 2; 3; 4]. This is in addition to the fact that FSO technologies offer broadband and high-speed transmissions far beyond those offered by radio frequency systems [5]. The inherent security of FSO technologies makes it difficult to intercept the transmitted optical signal, which has further contributed to the popularity of this technology, not only in traditional networking scenarios but also in mobile networks [6]. On the other hand, given the enormous interest aroused by all aspects of the marine environment and the need to develop wireless communication systems for underwater environments that can support emerging applications such as offshore installation monitoring, underwater robotics, and port security, to name a few, a rational objective has emerged to extend telecommunication networks into the underwater domain. Underwater wireless optical communications (UWOC) can offer high security, low delay and much higher data rates than acoustic communications. As a result, the research community has recently focused its efforts on the research and development of optical wireless communication systems in the underwater environment [7; 8; 9; 10; 11; 12; 13]. Despite the many advantages of wireless optical links, they are also subject to certain limitations due to the adverse nature of the optical channel. The main phenomena that degrade the quality of terrestrial FSO links are atmospheric loss, caused by the absorption and scattering of molecules and particles suspended in the atmosphere; pointing errors, caused by the random misalignment caused by wind gusts or vibrations at the ends of the link; and, finally, atmospheric turbulence, which is due to the small variations in pressure and temperature that occur in the medium and result in random variations of the atmospheric refractive index [14; 15] causing fluctuation of the received optical signal in both phase and intensity. These fluctuations, also known as atmospheric scintillation, are considered to be the most relevant degradation factor in terrestrial links. Extensive research has been carried out by the scientific community to model the random fading of atmospheric channels under variable turbulence conditions. Accordingly, several mathematical models for the probability density function (PDF) of the received irradiance have been proposed to date [16; 17; 18; 19; 20; 21]. The Gamma-Gamma distribution, which is valid for any turbulence regime, stands out to characterize the scintillation originating from terrestrial links. In the case of UWOC links, the limiting factors are similar to those described above, but now taking into account the underwater environment. The transmitted optical beam suffers from absorption and scattering through propagation in seawater. Absorption is caused by the molecules and other particles that form the water and which, when interacting with the photons of the optical beam, cause a loss of energy. Scattering is due to the presence in the water of organic material, mineral salts and other dissolved particles that cause changes in the direction of propagation of the photons of the optical beam. Both phenomena (absorption and scattering) cause path loss and, in the case of scattering, also spatial and temporal dispersion of the transmitted signal. To minimize path loss, UWOC links use the blue/green region of the visible light spectrum for data transmission, since this region of light suffers the least attenuation in natural water [22]. Furthermore, in the underwater environment, the ocean currents that describe the movement of seawater induce differences in temperature and pressure which, together with other characteristics such as turbidity or salinity of the water, lead to changes in its refractive index and thus to fluctuations in the received power.