## 1 Introduction  
Transportation is entering the new era of cooperative, connected and automated mobility (CCAM), with vehicles largely controlled by advanced driver assistance systems (ADAS) and exchanging messages through direct vehicle-to-everything (V2X) wireless communications. Focusing on connectivity, two families of radio access technology (RAT) standards are available today, one based on IEEE 802.11p (and its enhancement IEEE 802.11 bd) and the other belonging to the 3 GPP cellular ecosystem, including the fourth generation (4G) long term evolution (LTE)-V2X and the fifth generation (5G) new radio (NR)-V2X sidelink (in the paper we will use abbreviations for the sake of conciseness).  
Even if the standards for wireless communications appear mature and have indeed already started to be installed on board the new vehicles [1], one of the issues is the fact that no regulations have been currently set to determine which technology should be used in the limited bandwidth that is allocated to the intelligent transport system (ITS). In many countries worldwide, a spectrum has been reserved for exclusive use for direct V2X communications, mostly around 5.9 GHz [2]. In particular, seven channels of 10 MHz (between 5855 MHz and 5925 MHz) have been reserved in the US since 1999, which was reduced to three in 2021 (between 5895  MHz and 5925 MHz). In the US, the FCC explicitly indicated in 2021 that the three remaining channels will be used for C-V2X; however, the statement is not clear on the timeline and the restriction appears to be not actually enforced. There have been five channels (two for non-safety between 5875 and 5895 MHz and three for safety between 5895 and 5925 MHz) reserved in Europe since 2008, which increased to seven in 2021 (those added, both for safety, are from 5825 to 5845 MHz, although from 5835 to 5845 MHz is subject to urban rail priority). For the moment, the regulations define the use of the channels for cooperative intelligent transport systems (C-ITS), including their applicability for safety and non-safety use cases, but they do not indicate a specific wireless access technology. As a consequence, multiple non-interoperable technologies might share the same channel at the same time in the same area, addressing common or separate use cases, possibly leading to inter-technology interference and reduced performance.  
The issue of co-channel coexistence has been therefore considered a priority by European Conference of Postal and Telecommunications Administrations (CEPT) [3], which asked the European Telecommunications Standards Institute (ETSI) to introduce some solutions for the mitigation of inter-technology interference, focusing on the two main wireless technologies available at that time, which are ITS-G5, the ETSI technology based on IEEE 802.11p, and LTE-V2X. A number of proposals, called co-channel coexistence mitigation methods and labeled with letters from A to F, were then defined and analyzed in [4]. All these proposals have advantages and drawbacks, and they all imply modifications to one standard or the other. At the time of writing, the discussion is still ongoing between the involved stakeholders and no decision has been taken.