## 2 Hardware Overview and Experimental Setup  
The bidirectional VLC link is established performing an On-Off Keying (OOK) 0-20% modulation of the light intensity of a commercial headlamp (Qililu QL04156, 12 V 160 mA current DC value) and of a tail light (Alchemy Parts, UPC: 5060502564597, 12 V 90 mA current nominal value) of a motorbike for distances up to 25 m with baud rates up to 57 kBaud [29]. Both lamps act as VLC transmission (TX) sources. The experiment has been carried out in a realistic outdoor configuration in the central hour of the day under sunlight exposure with a luminous flux intensity of around 100 klux. Referring to Figure 1a, the lights are placed facing each other, at the same height of 75 cm from the ground, pointing forward (_Flat_ configuration), which is the most realistic situation occurring in a real road scenario. A receiving system (RX) is placed besides each lamp, thus each lamp has one transmitter and one receiver. We named TX1 and RX2 the transmitter and receiver, respectively, of the first motorbike, while we named TX2 and RX1 the transmitter and receiver of the second motorbike, respectively. For a detailed analysis of the luminous pattern emitted in terms of the measured experimental SNR, considering the combination between the radiance map of the light sources and the FoV of the detectors, see [29]. Figure 1b shows the block diagram of our experiment. The first motorbike (TX1/RX2) is equipped with a low-cost, open-source microcontroller board (Arduino DUE), which sends a digital data stream of 10\({}^{4}\) packets (9 byte each), through its head LED-based lights towards the second motorbike (TX2/RX1). The Manchester encoding is assured by a custom analog current modulator. The light collecting unit at the receiver is equipped with a customized version of a commercial variable gain photodetector (Thorlabs PDA100A2). The photodetector is equipped with an aspherical 2\" uncoated lens (Thorlabs ACL50832U, focal length = 32 mm, resulting Field of View (FoV) = 17\({}^{\circ}\)) and a cylindrical shield (height = 75 mm, diameter = 85 mm, FoV = 60\({}^{\circ}\)), which is used to reduce the contribution of the stray light that affects the optical system [33]. The dimensions of the latter have been carefully selected in order not to limit the FoV of the optical system. The choice of the FoV of about 17\({}^{\circ}\) assures a sufficient FoV for the analysed scenario (Figure 1a) and at the same time reduces the contribution of any interference effect given by the contribution of other light sources or the sun that is out of sight. Furthermore, to reject the direct current (DC) sunlight component, a physical current DC-block stage is implanted into the photodetector before the first transimpedance stage. More detail about the receiver circuit is presented in [19]. The RX system is composed by the above mentioned light collecting unit together with a digitizing board and an Arduino based microcontroller, which performs the active decoding. In case all packets of the message have been correctly received by the second motorbike (RX1), a message is generated and sent through the rear light back to the first motorcycle. Here, an analogous RX system, placed onboard to the headlights of the first motorbike, performs an active decoding of the received data stream, retrieving the number of correctly received packets (for further details see [19; 29]). This allows us to measure both the Packet Error Rate (PER) and the latency for the bidirectional VLC link, which is a fundamental parameter for safety application [31]. We record the RX signal after a complete round-trip in the bidirectional VLC chain by using a 1 Gs/s digital oscilloscope (Siglenet SDS1204X-E). Figure 1: (**a**) Sketch of the bidirectional communication. The head-light (HL, TX1/RX2) and the rear light (RL, TX2/RX1) are placed facing each other at the same height. The TX1/RX2 sends a continuous data stream, the TX2/RX1 back replies only if the packet is correctly received. At the end, the TX1/RX2 fulfils a byte-wise comparison.