# Underwater Communications for Video Surveillance Systems at 2.4 GHz
## 1 Introduction  
Over the past 20 years, human activity in aquatic and marine environments has increased. Moreover, the development and application of aquaculture technology has led the creation of new research lines. Consequently, the impact on marine ecosystems has also increased. Human activity generates unwanted wastes in rock reefs, benthic habitats or sensitive sea grass communities, mainly from the food not consumed by the fish and the faeces and waste thereof. In addition, due to food waste and the consequent economic impact, fish farming is often not considered a feasible activity. In recent years, research activity in underwater wireless communications and underwater ad hoc networks has experienced an enormous increase. Most experts agree that the future of the sector needs some improvements in the competitiveness of enterprises to be profitable in the global market, through the incorporation of new technologies. Nowadays, some researchers are developing low-cost sensors with very good features. However, underwater wireless communication remains a challenge, especially when the amount of information sent between devices requires high bandwidth, like video. Current research activity is focused on increasing the distance between devices and increasing the bandwidth. The research community is also trying to increase the network lifetime by reducing the energy consumption of underwater devices [1; 2]. In order to work with underwater wireless systems, we should know and understand how waves can propagate through the water environment. Because of the physicochemical characteristics of water [3] and the constraints of the medium [4; 5], the technology used to propagate the communication will present a set of benefits and drawbacks. In addition, the EM wave's propagation through water is very different from the propagation through air because of water's high permittivity and electrical conductivity. The wave's attenuation is high in the water, compared to the air, and increases rapidly as a function of the frequency. With a relative permittivity of 80, water is one of the materials with the highest permittivity and this has a significant impact on the angle of refraction at the air-water interface. In order to implement an underwater wireless sensor network (UWSN), researchers usually use acoustic waves. This kind of waves presents better behavior in turbid waters and they are less susceptible to being affected by particles suspended in the water. It is the most used method because acoustic waves can reach distances of up to 20 km [6]. Acoustic communication is a very well proven technology. However, it presents several drawbacks such as low data rate (up to 20,000 bps approximately) which is mainly limited by several parameters like strong reflections and low carrier frequency that generates an important attenuation when the communication is carried out in shallow waters [7]. Other parameters such as salinity or temperature also worsen the communication performance. To mitigate these effects and achieve higher data rates, researchers usually work with different modulation and communication protocols. We can also find underwater communication devices based on optical systems. These devices present very high propagation speed, but they are affected by the water turbidity and register a strong backscattering phenomenon due to particles suspended in the water. For this reason, optical signals are not good options for long distances. Finally, EM waves, working in the radio frequency (RF) range, can also be considered as a good option to implement underwater wireless communication systems. EM waves can achieve data transfer rates of up to 100 Mbps for very short distances. They also present substantial immunity to several environmental features. EM waves are less affected than acoustic waves by the refraction and reflection effects in water with little depth. In addition, suspended particles have very little effect on them.