Time variation in vehicle-to-vehicle visible light communication channels

Vehicular Visible Light Communications (V2LC) utilizes automotive Light Emitting Diode (LED) lighting to transmit information-modulated optical signal in free space. It offers reliable, secure, highly scalable communications, and thus presents itself as a complementary and low-cost solution which can be combined with radio-frequency-(RF-) based Dedicated Short Range Communications (DSRC) to support a wider range of applications. While there exist a large number of studies investigating V2V RF channels and indoor optical channels, there are few works focusing on V2V VLC channels in driving settings. This work aims to empirically characterize the time variation of the V2V VLC channel, and investigate how vehicle mobility and different vehicle behaviors correlate with the time variation. Utilizing OED LED headlamp as the transmitter and a photodiode module as the receiver, we developed a measurement platform consisting of a transmitter car and a receiver car, operated in real-world road environment in a car-following setting. Our key findings include the following. First, cars making turns could contribute to significant change of received power, up to 20 dB. Second, vertical vehicle movements due to road surface irregularity can also contribute to sudden increase of received power as large as 5 dB. Finally, the channel coherence time is found to be at least an order of magnitude larger than that of RF channels, indicating that the V2V VLC channels are much more stable. We hope that these measurement results can be used to generate design guidelines or determine optimal system parameters for future V2LC systems.