# Advanced Progress of Optical Wireless Technologies for Power Industry: An Overview
## 2 Classification and Progress of Power Industry Optical Wireless Communications
### Channel Characteristics between Power Line and Indoor Wireless Optical Communications
#### 2.1.2 Channel Characteristics of Outdoor Wireless Optical Communications  
Outdoor transmission channel is atmospheric channel, which contains a large amount of gases and scattered particles, which will produce deviation in the transmission process of light. Some components and particles in the atmosphere will also absorb the energy of light, so that the transmitting power of the transmitted signal will be attenuated. These factors have a great negative effect on outdoor wireless optical communications system. After the OWC system equipment receives the signal through the power line, it is received through the optical interface, and connected to the receiving end through the optical cable, and is generally not affected by lightning strikes. The power supply voltage of wireless optical communication equipment is generally \(-48\) V DC or \(220\) V AC, and then converted to \(12\) V working voltage through the internal power module. The interference of the background light has a remarkable influence on the optical receiver. However, under the interference of intense natural light, especially under direct sunlight, the photodetector may enter a saturated state, causing the photodetector to fail to receive weak communication light. Atmospheric turbulence effect is due to the existence of air vortices with different temperature, pressure, density, flow velocity, and size in the atmosphere. These vortices collide with each other and superpose to form irregular atmospheric turbulence motion in the continuous movement, which results in the change of atmospheric refractive index, and makes visible light signal produce light intensity scintillation and central beam drift. Compared with the atmospheric attenuation effect, the atmospheric turbulence effect will randomly lead to the intensity fluctuation of the optical signal, so the influence of atmospheric turbulence is mainly considered in the analysis of outdoor visible optical communication channel. Weather is crucial to communications when it comes to outdoor communications. The transmittance of visible light decreases in rain, snow, fog, and haze. Snowflakes, rain drops, and fog drops scatter and absorb visible light, etc. The optical signal transmitted through the atmosphere attenuates and the attenuation amplitude varies greatly, which seriously affects the transmission of light and the communication quality of the system. Therefore, channel conditions should be fully considered in LED outdoor visible light communication system to reduce the influence of external interference and improve system performance. Optical wireless (OW) transmission quality is very sensitive to weather. Sunny days have the least influence on OW transmission quality, while rainy days, snow days, and fog days have the greatest influence on wireless optical transmission quality. According to the test, the empirical values of optical wireless attenuation affected by weather are as follows: sunny days is 5-15 dB/km, rainy days is 20-50 dB/km, snow day is 50-150 dB/km, and foggy days is 50-300 dB/km [24]. OWC technologies need to overcome the influence of weather on communications quality in order to meet the requirement of all-weather operation of power system communications. Different from PLC, the reasons affected the quality of wireless communications are that the wireless penetration is weak and they propagate in the process of various obstacles. Therefore, the signals will be reflected, diffracted, and scattered or directly absorbed during the transmission process, and then the wireless signals are weakness. The wireless channel model can be divided into large-scale fading and small-scale fading [10]. Large-scale fading is usually divided into two categories: one is the fading of signal caused by the influence of other objects in the environment, which is called shadow fading, and the other is the transmission loss caused by signal communication over long distances, which is called path loss.