In the race for improved efficiency, fuel economy and safety, Intelligent Transportation Systems (ITS) play a major role, of which vehicular communication is part of. Different types of vehicular communication exist: vehicle to infrastructure (V2I), vehicle to home (V2H), vehicle to vehicle (V2V), vehicle to everything (V2X), etc. To enable the communication between vehicles and other surrounding devices, a certain communication channel is to be used. A communication channel is a medium through which information can be sent and can be either physical (wire, optical fiber) or wireless. Traditionally, the radio frequency (RF) spectrum is used as wireless information carrier. Think about the now disappearing AM and FM radio signals used for music and entertainment. However, in certain scenarios, such as a congested urban road, RF based methods show poor performance due to interference and wave reflection leading to loss of information. This is to be avoided in an environment where a reliable data link is required. Therefore, alternative communication methods are emerging. The one of interest here is the visible light communication or more specifically for vehicles, the vehicular visible light communication (V-VLC). V-VLC uses the visible part of the spectrum (380-750 nm) as carrier wave. Hence, vehicle headlamps and taillights are used as transmitters, enabling simultaneous illumination and data transfer. Several advantages of V-VLC over its RF based competitors include a license free usage of the spectrum, a very wide bandwidth, a secure connection due to the line-of-sight requirement, possible weight saving, optimal usage of the capabilities of LED light sources, etc. V-VLC will not completely replace the RF based communication methods due to the lack of broadcasting capabilities, i.e. sending messages to all receivers in the surrounding environment, as well as the limited range. Therefore, it is seen as a complementary communication method to the RF based systems, adding a redundant layer, hence increasing safety. It is mainly to be used for very fast communication at short distance, e.g. platooning where vehicles drive autonomously very close to each other to limit the effect of drag.
For a communication technology to be widespread, robust design tools and well-defined operational conditions are to be provided. For this, the spacing in between the transmitter and receiver, called the optical channel, shall be well modelled. Most channel models rely on a simplified representation of the light source, i.e. a point source assumption with a Lambertian emission pattern. However, the complex and regulated radiation pattern of vehicle headlamps prohibit the use of this simplified representation. Furthermore, it is not guaranteed that the point source assumption can be used as this assumption is only valid starting from a certain distance from the light source. If the point source assumption is not valid, the channel modelling becomes much more complex. Hence, we aim to improve the optical channel model for V-VLC using a numerical tool allowing us to verify whether the point source assumption can be used for the modelling of vehicle headlamps with a well-defined radiation pattern . The preliminary performance analysis of this tool is currently under peer review. The experimental validation of it is in progress starting with the restoration of Birdi (Bidirectional Reflectometer for the measurement of reflection and transmission properties), designed by Patrick Rombauts at the VUB in 1994. The Birdi will be used as a homemade goniophotometer to measure the photo- and radiometric quantities of several light sources to be compared with the numerical tool.
It is believed that the Lambertian point source assumption used commonly in the literature underestimates the achievable performance with V-VLC. With a more precise, traceable, channel model, a better idea of the capabilities of V-VLC can be obtained as well as improved designs. Furthermore, the numerical tool is not limited to V-VLC studies but could also be used by the lighting industry and measurement facilities as a reference for calibration, uncertainty analysis or simple preliminary designs. In my PhD, I, Guillaume Dotreppe, under the supervision of Prof. Dr. Valéry Ann Jacobs, will investigate whether the above claims are true.