Characterizing link asymmetry in vehicle-to-vehicle Visible Light Communications

Recently, it has been found that Vehicular Visible Light Communications (V2LC) can effectively address several challenges faced by conventional Radio-Frequency-based (RF-based) vehicular communications. Utilizing the LED headlamps and taillights as the transmitter and a photodiode as the receiver, V2LC has high reliability even in high vehicle density scenarios, can effectively identify the transmitter of a received message, and has low cost. In this paper, we identify a new problem unique to V2LC - link asymmetry, which is defined as significant difference in incoming and outgoing links and perform the first experimental work to characterize the problem. To this end, we performed extensive optical radiation pattern measurements with off-the-shelf LED headlamps and taillights. We also collected a number of vehicle mobility traces with 2-dimensional locations with centimeter-level accuracy as well as vehicle bearing. Combining these data, we performed an analytical study to obtain an experimental distribution of received power difference of the links in opposite directions. Our results show that in 80% of the cases the received power of the outgoing link using the headlamp of our vehicle is 22 dB or more than the received power of the incoming link using the neighboring vehicle's taillight, and only 8.1% of the cases have less than 10-dB received power difference. This suggests that the link asymmetry problem is very severe in V2LC and new system design or protocol design consideration is required to prevent significant throughput degradation.