## 1 Introduction  
In recent years, the integration of assisted driving and advanced communication technologies in modern vehicles has significantly increased, aiming at reducing fatalities and injury rates caused by accidents in urban scenarios [1]. Intelligent Transportation Systems (ITS) aim at increasing the safety and efficiency of the transportation system by interconnecting vehicles, humans and infrastructures through new communication technologies, so that vehicular units could readily share information on their dynamical state or information about surrounding environmental conditions [2, 3, 4]. However, according to a recent report by the World Health Organization (WHO) [5], more than 1 million people died annually and up to 50 million were injured in road accidents, despite the conspicuous efforts in introducing assisted reaction capabilities to critical events in new vehicles. Hence, the need for improvements in the modern transportation system safety clearly asks for further developments in advanced low-latency vehicular communication technologies, aimed at increasing the vehicle's capability to promptly detect and respond to critical events through active and proactive safety protocols [6]. As a matter of fact, the fifth-generation (5G) communication standard has introduced, for the first time, a pillar called ultra reliable low latency (URLL), with the aim of providing mission-critical services, including road safety [7]. Before 5G-mobile communication systems, the Vehicle-to-Everything (V2X) communications were standardized by IEEE using wireless access for vehicular environments (WAVE) and the set of protocols of dedicated short range communication (DSRC). In particular, the radio frequency (RF) based physical layer in DSCR is defined in IEEE 802.11p, while the cellular-based competing technology for vehicular application (C-V2X) has been defined within Long-Term Evolution (LTE) technology as the LTE-V2X. Currently, the Third Generation Partnership Project (3 GPP) is working on 5G-V2X standard, but in literature there are only some tests on the 5G network for vehicular applications [8]. Orthogonal frequency division multiplexing (OFDM) is often proposed to provide higher data rate in VLC systems. For example, in [9], a novel optical OFDM scheme for VLC systems is proposed to overcome the problem of power limitation and non-negative transmitted signal constraint of the lighting source. In [10], a hybrid OFDM-pulse time modulation (PTM) scheme where bipolar O-OFDM samples are converted into the digital PTM formats of pulse width modulation (PWM) and pulse position modulation (PPM) for the intensity modulation of light emitting diodes (LEDs) is proposed. In [11], a hybrid pre-coding technique-based peak-to-average power ratio (PAPR) reduction solution is proposed. Vehicle to vehicle communication using VLC is a topic recently quite investigated by the scientific community. In [12], an analytical performance analysis of a car-to-car visible light communications system under different communication geometries during the daytime is reported. In [13], a driving test of a vehicle-to-vehicle (V2V) VLC prototype, with two cars driving on a highway, is performed. OFDM-based system is used to send data from one vehicle to the other (one way). In [14], a path loss expression as a function of distance and different weather conditions for a V2V link is considered. Ray tracing simulations are carried out to verify the accuracy. In [15], a market-weighted headlamp beam pattern model is employed to outline the relationship between the communication distance and bit error rate (BER) performance, considering both the line-of-sight (LOS) and non-line-of-sight (NLOS) links. In [16], a simple VLC system, based on commercial off-the-shelf LED headlamp and taillight, is presented and tested in several configurations in order to evaluate its compatibility with platooning. Both BER and latency are considered. In [17], a VLC-based vehicle localization method is proposed to overcome the problem of a critical brake in car platooning.