# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 3 Hybrid VLC/RF Networks in Outdoor Scenarios with Harsh Conditions
The use of transmitters and receivers with complementary metal oxide semiconductor-based cameras and image sensors is a robust way to realize VLC network connection in outdoor scenarios, as complementary metal oxide semiconductors are capable of converting the modulated optical signal into an electrical waveform. As in indoor scenarios, a hybrid VLC/RF network can be implemented through different topologies in outdoor scenarios, mainly depending on the type of VLC network link, i.e., line-of-sight (LOS) or non-line-of-sight (NLOS). A very suitable scheme for an outdoor scenario with a line-of-sight link is the Point-to-Point topology. In this topology, the transmitters and receivers are in a fixed position and oriented towards each other. Otherwise, if the link is NLOS, a fuzzy topology can be used in which the transmitter and receiver are not require to be aligned. In this case, a receiver with a panoramic Lambertian pattern that can rotate within the radiation field of the transmitter is used to receive the signal [140]. One of the main challenges in hybrid VLC/RF networks stems from difficult outdoor conditions, mainly due to the high mobility of background noise caused by sunlight or other artificial light sources. A study of the impact of sunlight noise sources on the packet delivery rate in a VLC network was carried out in [141]. The results showed that when the sun is not in the receiver's field of view (FOV) a packet delivery rate of 100% can be achieved for distances of less than 100 meters. On the other hand, [PERSON] performed a time-domain characterization of the VLC channel by conducting real experiments in a static outdoor scenario during different times of the day [142]. The results showed that sunlight both affects the effective useful bandwidth of the VLC channel and reduces the channel gain when direct sunlight is present in the receiver FOV. To counteract the adverse effects of sunlight on the VLC channel, the receiver FOV can be optimized by adjusting the receiving angle. It should be noted that FOV adjustment is usually a compromise between noise reduction and the possible reception angle. To avoid reducing the receiver FOV too much, in [143] the use of a high-pass filter was proposed to filter the sunlight intensity in a one-channel VLC experiment. This filter was composed of a biconvex lens adaptive optical receiver, allowing high reduction of background noise without affecting the FOV angle. In addition, the authors coated the entire area from the optical receiver to the photodiode in order to block unwanted light angles, as shown in Figure 8. Finally, they added a DC blocker after the photodiode to block strong DC components. Experiments showed that data transmission is possible up to a distance of 40 m under perfect lighting conditions; however, the connection distance decreases in strong sunlight. In visible light communication, due to the propagation and directionality characteristics of light it is important to establish a clear line-of-sight (LOS) link between the transmitter and receiver. However, maintaining this LOS link becomes a challenge when users are in motion. This is especially true in outdoor environments, where light beams can travel long distances and reflected components lose energy, making reliable communication difficult. In [144], the authors suggested that non-line-of-sight (NLOS) communication via ground reflections can be advantageous in VLC. The effectiveness of NLOS communication depends on factors such as weather conditions and ground surface materials, which influence ground reflectivity. Unlike traditional wireless communication, VLC communication does not suffer from multipath fading caused by NLOS reflections. This is due to the inherent spatial diversity resulting from the carrier wavelength of visible light waves, which is significantly smaller compared to the size of typical receivers [43].