## 1 Introduction
Nevertheless, due to the fact that the design adjusts the amplification in accordance with the total light intensity received, strong optical-noise conditions can lead to insufficient amplification levels for the useful signal, and in turn, to limited communication ranges. Thus, combining this method with an adaptive FoV design can further improve its performance. The resilience to optical interference can also be addressed with the help of improved signal-processing techniques. Both analog and digital signal-processing techniques can be used to eliminate unwanted noise components and improve the SNR [19; 20; 25; 26; 36; 37]. In [20], the integration of a digital signal-processing (DSP) technique enabled the system to cope with a 12,000 lx optical interference level and almost double the communication range. Then again, the authors of [36] showed that the use of a DSP block can allow a vehicular VLC system to better deal with thermal turbulence, rain, and fog, reconfirming the benefits associated with a well-designed signal-processing plan. In addition to hardware upgrades, the performance and resilience of vehicular VLC systems can be enhanced with the help of various modulation techniques. This aspect was pointed out by the authors of [38; 39], who intensively investigated the benefits of the direct sequence spread spectrum (DSSS) technique. Their experiments showed that such a system can deal with daylight noise and can enable communication ranges of up to 40 m. The authors of [21] experimentally demonstrated that noise resilience can be improved with the help of binary phase shift keying (BPSK) modulation. This approach enabled the VLC system to better filter the noise, improving the resilience to optical interference by 25% compared to the use of the classical on--off keying (OOK) modulation. Furthermore, in combination with optical filters, the prototype was able to maintain the communication link even when exposed to optical noise levels reaching up to 65,000 \(\upmu\)W/cm\({}^{2}\). On the other hand, the authors of [40; 41; 42] demonstrated the enhanced noise resilience provided by the use of orthogonal frequency division multiplexing (OFDM) modulation. Again, in combination with optical filtering, it was able to cope with optical noise intensities of up to 50,000 lx [40]. In addition to the techniques mentioned above, the effect of optical interference can eventually be mitigated with the help of forward error correcting (FEC) protocols. From this perspective, the FEC protocols included in the IEEE 802.15.7 standard for visible light communication can provide outdoor VLC with a 10 dB SNR improvement [43]. In summary, the resilience of vehicular VLC systems to optical noise can be enhanced by reducing the VLC receiver's FoV; by using optical filters; by improving the design of the transimpedance circuit; or by using various analog and digital signal-processing techniques, improved modulation techniques, and FEC protocols. Nevertheless, none of these solutions alone are able to make a VLC system fully compatible with vehicular applications. On the other hand, the combination of these techniques has the potential to provide superior vehicular VLC prototypes that are suitable for real-life applications. From this perspective, a context-adaptive VLC system seems to be the most adequate approach for developing high-performance vehicular VLC prototypes [12; 37; 44; 45; 46]. Context-adaptive VLC systems represent a promising solution, because they are able to tie together the unique advantages of VLC, such as an immunity to radio frequency interference, low latencies, and the potential for high data rates, with smart adaptations to changing conditions within the environment. In a vehicular scenario, the importance of context-adaptive VLC systems becomes even more obvious. Such flexibility is essential in coping with the variability met in real-world situations, where aspects such as the lighting conditions, obstacles, and interference levels can vary unpredictably.