# Underwater Communications for Video Surveillance Systems at 2.4 GHz
## 1 Introduction
The speed of EM waves is around 150,000 times higher than that of acoustic ones, and it depends on the conductivity (\(\sigma\)), permeability (\(\mu\)), volume charge density (\(\varrho\)) and permittivity (\(\varepsilon\)) of the medium [8]. It is important to know that these parameters usually change depending on the type of water and they are associated with the electrical conductivity value. Seawater and freshwater have different conductivity values. Drinking water can have conductivity values ranging from 0.005-0.05 S/m for, while seawater can have values up to 4 S/m. The conductivity of fresh water presents a typical value of 0.01 S/m. However, the permittivity value of seawater is directly related to changes in salinity, temperature and working frequency. Regarding EM wave propagation, it is hampered by the high attenuation due to the water conductivity. The EM wave propagation speed and absorption coefficient are directly related to the working frequency. If the working frequency increases, the signal attenuation also increases. Hence, the higher frequencies will register greater signal losses [9; 10; 11]. Most of the studies analyze losses and signal attenuation as a function of working frequency. However, there are few studies that provide analysis considering modulation type, data transfer rates and the water characteristics [12]. Because of the limited information about underwater communication based on EM waves and the possibility of using them in many applications, this paper presents a practical study to demonstrate that EM waves are the best option to transmit video through underwater environments. To do this, firstly, we have analyzed the minimum network requirements to transmit a video with a specific format. After that, underwater measurements are performed taking into account all modulation schemes defined in the IEEE 802.11 standard [13] for the 2.4 GHz Industrial, Scientific and Medical (ISM) frequency band. Measurements are carried out in fresh water. In [14], the authors compared the signal transmission in underwater environments for 700 MHz, 2.4 GHz and 5 GHz frequencies. As we noted, when the 5 GHz frequency band is used, the results are bad (less than 12 cm). However, for 700 MHz, the maximum distance achieved is significantly larger (around 200 cm). Therefore, we know that the 915 MHz frequency will achieve larger distances than 2.4 GHz. On the other hand, its available bandwidth for video transmission will decrease considerably. The reason for using devices compatible with the IEEE 802.11 standard is because they are cheap and implement a modulator and demodulator for this frequency. The study shows the network performance for different water conditions when the water temperature is changed. The tests are carried out for different configurations. The modified parameters are the data transfer rates, signal modulation and working frequency. The current paper improves our previous works [15; 16] by providing a more detailed study which includes higher data bit rates, other coding techniques and an analysis of performance depending on the temperature. These new results have shown lower round trip times (RTTs) and larger distances. In order to determine if the analyzed parameters present a relationship with distance and between them, we analyze the results obtained from each case by using the analysis of variance (ANOVA). We use a two-way ANOVA. Finally, we will use these results to estimate the analytical models that describe the operation and behavior of signals in underwater environments, taking into account all analyzed parameters. The rest of the paper is structured as follows: Section 2 reviews some works and studies where researchers analyze the performance of EM waves when working in fresh water. Section 3 explains the preliminary tests to know the minimum requirements needed for an underwater video transmission. It is also presented the scenario where underwater tests have been performed as well as the equipment and the software used in our tests.