# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 5 Challenges and Future Directions for Hybrid VLC/RF Networks
The growing dependence on communication technologies and the need to protect the integrity and confidentiality of the information being transmitted makes security mechanisms in hybrid VLC/RF networks of vital importance today [184]. Multiple vulnerabilitiesand security risks can arise from the use of different communication technologies in hybrid VLC/RF networks. Therefore, it is necessary to develop security mechanisms that protect the privacy of the information transmitted, prevent unauthorized interception of data, and prevent attacks and malicious manipulation. These security mechanisms should include data encryption techniques, user and device authentication, and intrusion detection and prevention. In addition, the integrity of transmitted data must be guaranteed, while access and control policies must be established to ensure that only authorized users can access the network [185; 186]. To date, VLC systems lack acceptable uplink performance; for this reason, hybrid networks are forced to use different direct and return communication paths [93; 111]. For uplinks, these systems require the beam to be in a fixed direction; thus, shifting the MTs can greatly impair system performance and make the link inadequate. One of the main reasons for this is that smart mobile devices have limited power; it has not yet been possible to integrate a light source for communications [115; 116]. Therefore, supporting uplink transmission using VLC is a challenge in the implementation of hybrid systems. Outdoor scenarios are characterized by their very complex and dynamic communication channel. There are several factors that cause fading of the transmission signal, such as scattering, absorption, non-alignment, turbulence, and sunlight. Sunlight is considered to be the most severe co-channel interference source for VLC networks, and coupled with other sources of signal attenuation results in a very low SNR. Another major challenge for the VLC/RF network is the support of mobility in outdoor scenarios where obstacles and time changes can affect the direct link between the transmitter and receiver. To counteract the adverse effects of sunlight on the VLC channel, the receiver FOV can be optimized by adjusting the receiving angle. However, FOV adjustment is usually a compromise between noise reduction and the possible reception angle [155]. To avoid reducing the receiver FOV too much, in [143] the authors proposed using a high-pass filter to filter the sunlight intensity. In addition, it was proposed to coat the entire area from the optical receiver to the photodiode in order to block unwanted light angles [143]. An optimal design of the communication channel of VLC/RF networks in harsh environments capable of adapting to the existing environmental conditions and enhancing overall system performance is required [153; 154]. Hybrid VLC/RF networks could be a key point in long-distance (satellite) communication. This could be achieved through the use of solar panels designed in such a way that they can receive satellite information via sunlight or detecting light from lasers [140]. This is a very interesting open research topic, as it would make sunlight a source of illumination, energy, and data transmission. Optical camera-to-camera communication technology (OCC) is a branch of visible light communication that has emerged in recent years. OCC communication, like VLC, takes place between an optical source and a camera composed of image sensors. OCC can be implemented in any type of camera, smartphone, or any device that has integrated image sensors [187]. No changes are needed to the current infrastructure of imaging sensors that act as signal receivers; therefore, the complexity and installation cost are very low. Several papers have presented OCC technology as a fundamental key for the development of 5G communication systems in both indoor and outdoor scenarios [188; 189; 190]. Therefore, it would be very interesting to combine OCC technology with hybrid VLC/RF networks as a way to improve system performance.