**4. Conclusions**

In this work, we designed and tested a low-cost, low-latency VLC prototype for I2V communications, using a commercial LED-based traffic light as transmitter and a conventional amplified photodiode with low-cost Fresnel lenses as a receiver. Our VLC-based system is based on an open-source microcontroller platform (Arduino Due) and is capable of Active Decode and Relay (ADR) of received information to a further modulation stage to propagate the information to incoming units through, for example, rear car lamps.

We evaluated the PER performance of the system in various configurations for both the 1 and 2 Fresnel lenses. Two baud rates of 115 and 230 kbaud are used for data transmissions. When embedding a 2 Fresnel lens in Optimal configuration, our system attains an error-free transmission (PER < <sup>10</sup>−5) up to 36 m with sub-millisecond (820 μs) latency of the full ADR process at 230 kbaud (50 m for 230 kbaud). For a more realistic Flat system configuration, simulating a car approaching the traffic light without an optical tracking mechanism, the error-free transmission is achieved at 115 kbaud with 2 lens and above 24 m. If only PER = 10−<sup>3</sup> is required, instead, the 18–50 m range can be covered. We also tested the VLC Fresnel-based system for both the beaconing of situational information and event-triggered message broadcast, finding it suitable for both modes of transmission. The noticeable low-latency feature of this prototype makes it integrable with current 5G-based C-V2X Intelligent Transportation Systems.

For the future, we are planning outdoor measurement campaigns for both I2V, V2V and Infrastructure-to-Vehicle-to-Vehicle (I2V2V) communications. In addition, our system will be tested for various environmental conditions.

**Author Contributions:** Conceptualization, T.N., M.S., S.C., L.M. and J.C.; methodology, T.N., M.S., S.C., L.M. and J.C.; software, T.N. and S.C.; validation, L.M. and J.C.; formal analysis, L.M. and S.C.; investigation, T.N., M.S. and J.C.; resources, J.C.; data curation, M.S. and S.C.; original draft preparation, T.N. and J.C.; supervision, J.C.; funding acquisition, L.M. and J.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been carried out under the financial support of Project PON MIUR 2017 "OK-INSAID", of Progetto Premiale MIUR FOE 2015 "OpenLab", and by Project MISE "5G City"—Prato.

**Acknowledgments:** Authors would like to thank the Company ILES srl in Prato (Italy) for providing the traffic light and for support, and Francesco Cataliotti for precious discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
