# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 3 Hybrid VLC/RF Networks in Outdoor Scenarios with Harsh Conditions  
In an outdoor scenario with harsh conditions (e.g., sunlight, rain, dust, wind) the communication channel of hybrid VLC/RF networks becomes very complex and dynamic. These conditions distort the VLC optical beam, causing amplitude and phase fluctuations  
Figure 7: Different hybrid network topologies: (**A**) symmetrical without interference, (**B**) symmetric with interference, (**C**) asymmetric, (**D**) dual transmission, (**E**) separate transmission, (**F**) hybrid transmission. along with optical losses. The level of loss of the VLC signal is higher than the losses suffered by the RF network, as the VLC signal is very sensitive to atmospheric conditions such as sun, rain, and fog [136]. There are several factors (scattering, absorption, non-alignment, and turbulence) that can cause signal loss in both the VLC and RF networks, which complicate the design of the hybrid communication channel model [137]. Among the most important phenomena responsible for signal loss in VLC/RF networks are:  
* Scattering corresponds to an angular spread of the optical field, which is independent of frequency and wavelength in certain cases. It can be caused by solar radiation, rain, or particles and objects in the environment, which create a divergence in the transmission beam and cause the size of the received transmission beam to be larger than the receiver aperture, resulting in part of the signals being lost. * Absorption takes place when a collision occurs between the photons of the propagating beam of the VLC network and the particles or molecules existing in the environment. The absorption coefficient depends on the type of molecules in the air and their concentration. There are areas of minimum absorption in the atmosphere called transmission windows. One way to avoid or decrease this phenomenon is to select the wavelengths of the grid to be within the wavelengths of the transmission window. * The distance between the transmitter and receiver in an outdoor scenario can cause non-alignment of the transmitter and receiver, resulting in a loss of signal power. Non-alignment between the transmitter and receiver can be due to the presence of obstacles, buildings, and trees, as well as changes in the weather. As the distance between the transmitter and receiver increases, these factors must be taken into account. * Ambient turbulence corresponds to a divergence in the optical signal, which can be caused by random temperature variations, solar radiation, increased wind speed, and atmospheric pressure. Divergence caused by turbulence during transmission results in random variations of signal amplitude and phase, causing fading of the received power and decreased performance of hybrid systems. * In an outdoor scenario, multiple sources of noise and interference can affect hybrid VLC/RF networks, including shot noise, thermal noise, and environmental interference. Among these, sunlight is considered the most serious co-channel interference source for the VLC network, and thus for the hybrid network, as it can significantly reduce the SINR of the system. One way to reduce the level of sunlight interference in the VLC network is to use lenses and filters on the receivers; however, this is not an optimal solution. The harsh conditions in outdoor scenarios are important factors to take into account in the design of hybrid VLC/RF networks. A study of the degrading effects of physical phenomena on the quality of VLC communications in an industrial environment, where dust, other particles, and artificial light sources increase signal attenuation and cause optical interference, was carried out in [138]. In such cases, it is necessary to design and implement a robust model capable of considering the existing environmental conditions in order to improve the overall performance of the hybrid system [139].