# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 3 Hybrid VLC/RF Networks in Outdoor Scenarios with Harsh Conditions
Based on the conditions of the outdoor VLC channel and the solutions proposed by [141; 142; 143], we propose designing a hybrid outdoor VLC/RF network as shown in Figure 9. This hybrid VLC/RF network can provide service to users through both networks at times when the solar intensity direction does not coincide with the direct FOV angle, thereby taking advantage of the multiple benefits of hybrid networks. At times when it is not possible to connect through the VLC network due to saturation of the photodiode by high background noise caused by sunlight, the service can be provided only by the RF network to ensure that users are not left without connection at any time. At night, the VLC network can be affected by artificial light sources such as lights used by cars and street  
Figure 8: General schematic of a VLC transceiver system with an optical filter. lights. These light sources have much lower luminance values than sunlight, and over long distances they should not affect the VLC network connection [145]. In case the artificial lights are at a relatively short distance from the user connected to the VLC network, they can be blocked by the filter of the optical receiver as long as they have a different direction from the FOV angle. Due to the attractions and advantages of VLC networks, such as offering greater data security, being less susceptible to interference from external signals, offering greater bandwidth for data transmission, and making efficient use of the frequency spectrum, thereby helping to alleviate congestion in the radio frequency spectrum, multiple studies [140; 141; 142; 143; 146; 147] have been conducted on the study of VLC networks in outdoor scenarios regardless of the presence or absence of sunlight. The deployment of outdoor VLC networks aims to extend the multiple advantages of this technology to all users, regardless of their locations and scenario. A study of the impact of sunlight noise sources on the packet delivery rate in a VLC network was carried out in [141]. In [142], a time-domain characterization of the VLC channel was performed by conducting experiments in a static outdoor scenario during different times of the day. In [143], a high pass filter was proposed to filter the sunlight intensity in a one-channel VLC experiment. Outdoor VLC network access points could be powered by solar panels or other renewable energy sources, meaning that there would be no additional energy costs during the day. In addition, VLC technology could use directional LEDs with a small light cone that transmit only the desired information and would not be occupied by unnecessary lighting. On the other hand, the VLC network saves energy at night, as the lighting LEDs act as data transmitters, eliminating the need for separate devices for lighting and communication. However, outstanding issues related to interference caused by solar illumination on VLC devices must be resolved. Consequently, several investigations on outdoor VLC networks have focused on this issue [140; 141; 142; 143]. Other external factors that greatly affect the VLC channel include difficult weather conditions such as fog, rain, and snow. Normally, molecules and particles in the atmosphere interact with the light, deflecting the beams and attenuating the transmitted signal. In adverse weather conditions these effects are much more severe, as fog, rain, and snow are made up of larger particles that have a large impact on the range and reliability of VLC. The study conducted in [148] showed that fog has a greater negative impact on VLC; however,  
Figure 9: VLC/RF hybrid network in an outdoor scenario for different sun positions. the study conducted in [149] showed that dry snow can be even more detrimental to VLC system performance than fog. For this type of scenario in which weather conditions are adverse, it is possible to establish reliable communication up to 15 m using the highest possible modulation scheme [150].