## 7 Experimental Results
### The Experimental Evaluation of the Effect of Vehicle Misalignment and of Inter-Vehicle Distance
At short distances (0-2.5 m for a \(\pm\)53\({}^{\circ}\) FOV, respectively 0-8 m for a \(\pm\)20\({}^{\circ}\) FOV), vehicle misalignment generated by the fact that vehicles are located on adjacent lanes, imposes a no-connectivity zone, which is dependent on the VLC receiver FOV. Next to this region (3-5 m for a \(\pm\)53\({}^{\circ}\) FOV, respectively 8-10 m for a \(\pm\)20\({}^{\circ}\) FOV), the wide incidence angle \(\alpha\) at the VLC receiver determines an increased BER value of \(10^{-5}\)-\(10^{-3}\), with respect to the \(10^{-7}\) BER achieved in vehicle-aligned conditions. Further on, the SNR at the VLC receiver located on an adjacent lane with respect to the VLC emitter is still lower (see Figure 10c,d, channel 1), but this SNR difference is compensated by the adequate signal processing. Therefore, the BER remains in the \(10^{-6}\) region. For distances above 15 m, the SNR in both scenarios (i.e., vehicles on the same lane and on adjacent lanes) becomes rather the same, generating similar BER results. As the distance further increases to above 20-25 m for a \(\pm\)53\({}^{\circ}\) FOV and above 50 m for a \(\pm\)20\({}^{\circ}\) FOV, the SNR continues to decrease, generating an abrupt BER increase. As one can see, fitting the optical collimator, which has a narrower FOV, improves the SNR and extends the \(10^{-7}\)-\(10^{-6}\) BER region up to 50 m and the maximum communication distance up to 75 m. Thus, these results demonstrate the performances of the VLC technology, which can provide highly reliable V2V links, with BERs that can go as low as \(10^{-7}\) and communication distances up to 75 m. Nevertheless, as one can see, at 75 m the BER significantly increases, reaching a \(10^{-3}\) value. This shows that due to the low optical power, this is the range limit of the current configuration. Although in some applications the \(10^{-3}\) BER value is adequate, in communication-based vehicle safety applications, this BER limit is rather high. On the other hand, these results have been achieved without the usage of forward error correcting protocols. However, communication-based vehicle applications will integrate FEC codes, which will have the ability to improve the BER performances [67]. The 75 m communication limit is determined by three main causes. First of all, the VLC emitter limited the optical power, which is determined by the optical system main lighting function. In order to be in accordance with the lighting regulations and the necessity to avoid glaring other drivers, the optical power of a lighting device cannot be increased above a certain limit. Nevertheless, there is still room to improve the efficiency of the lighting system, so that a better range can be achieved in the future. Another aspect that determines the communication range is given by the VLC receiver sensibility, which is in turn determined by the photosensitive element responsivity (A/W) and by the signal-collecting area. As the proposed VLC receiver uses a highly sensitive optical receiver and a 2-inch optical lens, it is rather difficult to further improve these aspects. From a different point of view, the communication distance could be slightly enhanced by reducing the optical clock frequency and in turn the data rate. These measures would enable a higher gain at the transimpedance circuit level. Last but not least, the communication range could be further increased by enhancing the VLC receiver ability to process low SNR signals. From this point of view, as the performances in terms of low SNR signal processing algorithms will be further improved, the communication distance of such systems will be improved as well. The results of the experimental investigation performed in night conditions show that the VLC receiver is able to maintain the connectivity, even under exposure to the fluorescent lighting system.