## 2 Materials and Methods  
### Analysis of the OWC System  
The modulated light is emitted by a transmitter and registered by a receiver (Figure 1). To determine the data link range, the signal into the noise power ratio (determined by the light power registered by the receiver and 'floor' signal generated for no-light conditions) is analyzed. The registered power \(P_{R}\) is calculated by [27]:  
\[P_{R}=P_{L}\frac{D_{d}^{2}}{(D_{0}+\theta_{div}L)^{2}}e^{-\gamma(\lambda)L}, \tag{1}\]  
where \(P_{L}\) is the laser optical power, \(\gamma(\lambda)\) is the extinction coefficient, L is the link distance, \(\theta_{div}\) is the beam divergence, \(D_{0}\) and \(D_{d}\) are the aperture diameters of the transmitter and the receiver. The extinction coefficient usually determines a light attenuation caused by both absorption and scattering effects. The absorption describes the light interaction with air gases. There are some \"spectral windows\" in which this effect is minimal (Figure 2). Analyzing the scattering effects, the radiation losses can be estimated using visibility (\(V\)is) and different models [28; 29]. The visibility is combined with well-defined weather conditions described by the International Visibility Code [17]. Regarding Figure 3, the comparison of the extinction coefficient for two wavelengths (SWIR--1.55 \(\upmu\)m and MWIR--4.5 \(\upmu\)m) and two selected weather seasons is presented. The wavelength of 1.55 \(\upmu\)m is used as a reference in the commercially available OWC systems. The coefficients were determined using PcModwin 6.0 software (Ontar Corporation, North  
Figure 1: Block scheme of the OWC system. Figure 2: Spectral atmospheric transmittance. Andover, MA, USA) for the urban area and two seasons. Weather conditions were defined by temperatures of 20.5 \({}^{\circ}\)C and \(-1.5\) \({}^{\circ}\)C, and water vapors of 13.4 g/m\({}^{3}\) and 3.4 g/m\({}^{3}\) (RH-70%) for summer and winter, respectively. It can be seen that the scattering is decreasing with an increase in visibility. However, the radiation losses can be determined by its absorption for longer wavelengths (MWIR) and by its scattering for shorter ones (SWIR). More complicated analyses should be performed in the case of attenuation by radiation or advection fogs. To compare, advection fog is characterized by a linear attenuation wavelength dependence and the radiation fog by a quadratic one [30]. Additionally, the fog increases the air relative humidity (RH) [31]. The attenuation coefficient of radiation at two wavelengths is listed in Table 3.  
Further scattering attenuation is caused by the presence of rain, snow, and dust. Except for these phenomena, the relation between the size of the compared particles and the analyzed radiation wavelength indicates no spectral benefits [32]. The optical link performance also is modified very often by air turbulence. This phenomenon is based on a random variation of the refractive index with fluctuations in temperature and pressure. Therefore, beam wander, beam spreading, or scintillation can be observed. During these conditions, the beam profile is redistributed. The change in temporal and spatial light direction, and fluctuations in its power and phase, are registered. The origin of turbulence and its optical effects are precisely analyzed in [33]. Therein is described a relation of the refractive index to the light wavelength, pressure, humidity, and temperature. The strength of turbulence, the refractive index structure constant\(-\)C\({}_{n}\)\({}^{2}\), also was defined. The C\({}_{n}\)\({}^{2}\) values range from about 10\({}^{-12}\) to 10\({}^{-16}\) m\({}^{-2/3}\). Values above 10\({}^{-13}\) m\({}^{-2/3}\) most often determine high turbulence in the atmosphere. Regarding [34], the influence of the atmosphere on laser beam propagation for three turbulence effects was analyzed. Concerning the case of beam wander, the generated optical signal fluctuations  
Figure 3: Extinction coefficient for two analyzed wavelengths.