  This paper reports a detailed experimental characterization of optical
performances of Visible Light Communication (VLC) system using a real traffic
light for ultra-low latency, infrastructure-to-vehicle (I2V) communications for
intelligent transportation systems (ITS) protocols. Despite the implementation
of long sought ITS protocols poses the crucial need to detail how the features
of optical stages influence the overall performances of a VLC system in
realistic configurations, such characterization has rarely been addressed at
present. We carried out an experimental investigation in a realistic
configuration where a regular traffic light (TX), enabled for VLC transmission,
sends digital information towards a receiving stage (RX), composed by an
optical condenser and a dedicated amplified photodiode stage. We performed a
detailed measurements campaign of VLC performances encompassing a broad set of
optical condensers, and for TX-RX distances in the range 3 - 50 m, in terms of
both effective field of view (EFOV) and packet error rate (PER). The results
show several nontrivial behaviors for different lens sets as a function of
position on the measurement grid, highlighting critical aspects as well as
identifying most suitable optical configurations depending on the specific
application and on the required EFOV. In this paper we also provide a
theoretical model for both the signal intensity and the EFOV as a function of
several parameters, such as distance, RX orientation and focal length of the
specific condenser. Our results could be very relevant in the near future to
assess a most suited solution in terms of acceptance angle when designing a VLC
system for real applications, where angle-dependent misalignment effects play a
non-negligible role, and we argue that it could have more general implications
with respect to the pristine I2V case mentioned here.
