Cellular V2X (C-V2X) is an umbrella term that comprises all 3rd Generation Partnership Project (3GPP) V2X technologies for connected mobility and self-driving cars. It includes both direct and cellular network communications and is an alternative to 802.11p, the IEEE specified standard for V2V and other forms of V2X communications.
Cellular V2X uses 3GPP standardised 4G LTE or 5G mobile cellular connectivity to exchange messages between vehicles, pedestrians, and wayside traffic control devices such as traffic signals. It commonly uses the 5.9 GHz frequency band, which is the officially designated intelligent transportation system (ITS) frequency in most countries. C-V2X can function without network assistance and exceeds the range of DSRC by about 25%.
C-V2X was developed within the 3GPP to replace DSRC in the US and C-ITS in Europe.
History.
In 2014, 3GPP Release 13 spurred studies to test the applicability of the then current standards to V2X. This resulted in the 3GPP Release 14 specifications for C-V2X communications, finalised in 2017. 3GPP Release 15 introduced 5G for V2N use-cases and 3GPP Release 16 includes work on 5G NR direct communications for V2V/V2I.
In Europe, the EU announced in July 2019 that it was adopting a technology-neutral approach to C-ITS, leaving the way forward for 4G, 5G and other advanced technologies to be part of V2X applications and services.
In the United States, the Federal Communications Commission proposed late in 2019 that 20 MHz and possibly 30 MHz of the 5.9 GHz band be allocated to C-V2X. In November 2020, this proposal was accepted, and the upper 30 MHz (5.895–5.925 GHz) were allocated to C-V2X.
After a slow start linked to the slower-than-expected roll-out of 5G cellular networks, there were more than 50 C-V2X devices on the market in the first half of 2024. 
Modes.
C-V2X has the following modes:
C-V2X mode 4 communication relies on a distributed resource allocation scheme, namely sensing-based semipersistent scheduling which schedules radio resources in a stand-alone fashion in each user equipment (UE).
Technical limitations.
Although the roll-out of 5G services globally promises a dramatic reduction in latency where a signal is strong as well as an increase in security compared with previous networks, all communications systems based entirely on wireless communication, especially older and rural networks, suffer from limitations inherent to wireless communication, including:
Integrating non-terrestrial network coverage in addition to cellular and direct communications is one potential way to address coverage caps and latency concerns. The 5G Automotive Association and European Space Agency have discussed the role of non-terrestrial networks in the connectivity of the car of the future and concluded that it offers many benefits, such as extending more reliable connectivity to rural areas at a comparatively low cost. This, in turn, would enable better digital services and autonomous driving applications. 
Outlook.
The 5G Automotive Association (5GAA), which comprises companies from the automotive, technology, and telecommunications industries, has published several "roadmaps" that highlight both the potential benefits of C-V2X technologies and the technical, regulatory and market challenges it faces. Most implementations to-date have focused on road safety and improving traffic management, which reduces congestion and pollution. 
Artificial intelligence offers one potential solution for managing the large flow of data that will grow as C-V2X communications applications expand in the market. Doubts in artificial intelligence (AI) and decision making by AI exist. 
Tests.
C-V2X technology is being tested world-wide both at the company and industry level and in publicly funded pilots. For example, ETSI, in partnership with the 5GAA and co-funded by the European Commission, and the European Free Trade Association, has organized several annual C-V2X testing events called "Plugfests". These enable companies manufacturing on-board-C-V2X units, roadside units and public key infrastructure to run interoperability test sessions to assess the level of interoperability of their implementations of C-V2X technology and validate their understanding of the standards. 
In October 2023, the 5GAA organized several live demonstrations of the potential of C-V2X technology to protect drivers, pedestrians, cyclists and other vulnerable road users at the Mcity Test Facility at the University of Michigan in Ann Arbor. 
In June 2024 the U.S. Department of Transportation announced that it is awarding $60 million in grants to advance connected and interoperable vehicle technologies under a program called "Saving Lives with Connectivity: Accelerating V2X Deployment program". It said the grants to recipients in Arizona, Texas and Utah would serve as national models to accelerate and spur new deployments of V2X technologies. 
References.
 ;templatestyles src="Reflist/styles.css" / ;