Vehicle-to-everything (V2X) communication is the passing of information from a vehicle to any entity that may affect the vehicle, and vice versa. It is a vehicular communication system that incorporates other more specific types of communication as V2I (Vehicle-to-Infrastructure), V2N (Vehicle-to-network), V2V (Vehicle-to-vehicle), V2P (Vehicle-to-Pedestrian), V2D (Vehicle-to-device) and V2G (Vehicle-to-grid).
The main motivations for V2X are road safety, traffic efficiency, and energy savings. There are two types of V2X communication technology depending on the underlying technology being used: 1) WLAN based, and 2) cellular based. Standardization of WLAN based V2X supersedes that of cellular based V2X system. IEEE first published the specification of WLAN based V2X (IEEE 802.11p) in 2012. It supports direct communication between vehicles (V2V) and between vehicle and infrastructure (V2I). DSRC uses the underlying radio communication provided by 802.11p. In 2016, 3GPP published V2X specifications based on LTE as the underlying technology. It is generally referred to as "cellular V2X" (C-V2X) to differentiate itself from the 802.11p based V2X technology. In addition to the direct communication (V2X, V2I), C-V2X also supports wide area communication over cellular network (V2N). This additional mode of communication and native migration path to 5G are two main advantages over 802.11p based V2X system.
As of December 2017, only a single European automotive manufacture has announced an intention to deploy V2X technology based on 802.11p from 2019. Recent study and analysis in 2017 and 2018 indicate that cellular based C-V2X technology in direct communication mode is superior to 802.11p in multiple aspects, such as performance, communication range, and reliability.
The original V2X communication uses WLAN technology and works directly between vehicles, which form a vehicular ad-hoc network as two V2X senders come within each other’s range. Hence it does not require any infrastructure for vehicles to communicate, which is key to assure safety in remote or little developed areas. WLAN is particularly well-suited for V2X communication, due to its low latency. It transmits messages known as Common Awareness Messages (CAM) and Decentralised Environmental Notification Messages (DENM) or Basic Safety Message (BSM). The data volume of these messages is very low. The radio technology is part of the WLAN IEEE 802.11 family of standards and known in the US as Wireless Access in Vehicular Environments (WAVE) and in Europe as ITS-G5.
More recent V2X communication uses Cellular network and is called Cellular V2X (or C-V2X) to differentiate it from the WLAN based V2X. There has been multiple industry organizations, such as 5G Automotive Association (5GAA) and Automotive Edge Computing Consortium (AECC) promoting C-V2X due to its advantages over WLAN based V2X. C-V2X is initially defined as LTE in 3GPP Release 14 and is designed to operate in several modes: 1) Device-to-device (V2V), 2) Device-to-cell-tower (V2I), and 3) Device-to-network (V2N). In 3GPP Release 15, the V2X functionalities are expanded to support 5G. The main advantage of C-V2X includes support of both direct communication between vehicles (V2V) and traditional cellular-network based communication. Also, C-V2X provides migration path to 5G based systems and services.
The direct communication between vehicle and other devices (V2V, V2I) uses so-called PC5 interface. PC5 refers to a reference point where the User Equipment (UE), i.e. mobile handset, directly communicates with another UE over the direct channel. In this case, the communication with the base station is not required. In system architectural level, proximity service (ProSe) is the feature that specifies the architecture of the direct communication between UEs. In 3GPP RAN specifications, "sidelink" is the terminology to refer to the direct communication over PC5. PC5 interface was originally defined to address the needs of mission critical communication for public safety community (Public Safety-LTE, or PS-LTE) in Release 13. The motivation of the mission critical communication was to allow law enforcement agencies or emergency rescue to use the LTE communication even when the infrastructure is not available, such as natural disaster scenario. In Release 14 onwards, the use of PC5 interface has been expanded to meet various market needs, such as communication involving wearable devices such as smartwatch. In C-V2X, PC5 interface is re-applied to the direct communication in V2V and V2I.
In addition to the direct communication over PC5, C-V2X also allows the C-V2X device to use the cellular network connection in the traditional manner over Uu interface. Uu refers to the logical interface between the UE and the base station. This is generally referred to as vehicle-to-network (V2N). V2N is a unique use case to C-V2X and does not exist in 802.11p based V2X given that the latter supports direct communication only. Combination of the direction communication over PC5 and traditional cellular communication over Uu interface allows wider and more diverse C-ITS usages.
Through its instant communication V2X allows road safety applications such as (non-exhaustive list):
- Forward collision warning
- Lane change warning/blind spot warning
- Emergency Electric Brake Light Warning
- Intersection Movement Assist
- Emergency Vehicle Approaching
- Road Works Warning
The US National Highway Traffic Safety Administration’s (NHTSA) report “Vehicle-to-Vehicle Communications: Readiness of V2V Technology for Application“ lists the initial use cases envisioned for the US. European standardisation body ETSI and SAE published standards on what they see as use cases. Early use cases focus on road safety and efficiency. 
In the medium term V2X is perceived as a key enabler for autonomous driving, assuming it would be allowed to intervene into the actual driving. In that case vehicles would be able to join platoons, the way HGVs do.
WLAN-based V2X communication is based on a set of standards drafted by the American Society for Testing and Materials (ASTM). The ASTM E 2213 series of standards looks at wireless communication for high-speed information exchange between vehicles themselves as well as road infrastructure. The first standard of this series was published 2002. Here the acronym Wireless Access in Vehicular Environments (WAVE) was first used for V2X communication.
From 2004 onwards the Institute Electrical and Electronics Engineers (IEEE) started to work on wireless access for vehicles under the umbrella of their standards family IEEE 802.11 for Wireless Local Area Networks (WLAN). Their initial standard for wireless communication for vehicles is known as IEEE 802.11p and is based on the work done by the ASTM. Later on in 2012 IEEE 802.11p was incorporated in IEEE 802.11.
Around 2007 when IEEE 802.11p got stable, IEEE started to develop the 1609.x standards family standardising applications and a security framework (IEEE uses the term WAVE), and soon after SAE started to specify standards for V2V communication applications. SAE uses the term DSRC for this technology (this is how the term was coined in the US). In parallel at ETSI the technical committee for Intelligent transportation system (ITS) was founded and started to produce standards for protocols and applications (ETSI coined the term ITS-G5). All these standards are based on IEEE 802.11p technology.
In 2015 ITU published as summary of all V2V and V2I standards that are worldwide in use, comprising the systems specified by ETSI, IEEE, ARIB, and TTA (Republic of Korea, Telecommunication Technology Association).
3GPP started standardization work of cellular V2X (C-V2X) in Release 14 in 2014. It is based on LTE as the underlying technology. Specifications were published in 2016. Because this C-V2X functionalities are based on LTE, it is often referred to as LTE-V2X. The scope of functionalities supported by C-V2X includes both direct communication (V2V, V2I) as well as wide area celluar network communication (V2N).
In Release 15, 3GPP continued its C-V2X standardization to be based on 5G. Specifications are published in 2018 as Release 15 comes to completion. To indicate the underlying technology, the term 5G-V2X is often used in contrast to LTE-based V2X (LTE-V2X). Either case, C-V2X is the generic terminology that refers to the V2X technology using the cellular technology irrespective of the specific generation of technology.
In Release 16, 3GPP further enhances the C-V2X functionality. The work is currently in progress. In this way, C-V2X is inherently future proof by supporting migration path to 5G.
Study and analysis were done   to compare the effectiveness of direct communication technologies between LTE-V2X PC5 and 802.11p from the perspective of accident avoided and reduction in fatal and serious injuries. The study shows that LTE-V2X achieves higher level of accident avoidance and reduction in injury. It also indicates LTE-V2X performs higher percentage of successful packet delivery and communication range. Another link-level and system-level simulation result indicates that, to achieve the same link peformance for both line-of-sight (LOS) and non-line-of-sight (NLOS) scenarios, lower signal-to-noise-ratio (SNR) are achievable by LTE-V2X PC5 interface compared to IEEE 802.11p.
Celllar based V2X solution also leads to the possibility of further protecting other types of road users (e.g. pedestrian, cyclist) by having PC5 interface to be integrated into smartphones, effectively integrating those road users into the overall C-ITS solution. Vehicle-to-person (V2P) includes Vulnerable Road User (VRU) scenarios to detect pedestrians and cyclists to avoid accident and injuries involving those road users.
As both direct communication and wide area cellular network communication are defined in the same standard (3GPP), both modes of communication will likely be integrated into a single chipset. Commercialization of those chipsets further enhances economy of scale and leads to possibilities to wider range of business models and services using both types of communications.
In 1999 the US Federal Communications Commission (FCC) allocated 75 MHz in the spectrum of 5.850-5.925 GHz for intelligent transport systems. Since then the US Department of Transportation (USDOT) has been working with a range of stakeholders on V2X. In 2012 a pre-deployment project was implemented in Ann Arbor, Michigan. 2800 vehicles covering cars, motorcycles, buses and HGV of different brands took part using equipment by different manufacturers. The US National Highway Traffic Safety Administration (NHTSA) saw this model deployment as proof that road safety could be improved and that WAVE standard technology was interoperable. In August 2014 NHTSA published a report arguing vehicle-to-vehicle technology was technically proven as ready for deployment. On 20 August 2014 the NHTSA published an Advance Notice of Proposed Rulemaking (ANPRM) in the Federal Register, arguing that the safety benefits of V2X communication could only be achieved if a significant part of the vehicles fleet was equipped. Because of the lack of an immediate benefit for early adopters, the NHTSA proposed a mandatory introduction. On 25 June 2015 the US House of Representatives held a hearing on the matter, where again the NHTSA, as well as other stakeholders argued the case for V2X.
To acquire EU-wide spectrum, radio applications require a harmonised standard, in case of ITS-G5 ETSI EN 302 571, first published in 2008. A harmonised standard in turn requires an ETSI System Reference Document, here ETSI TR 101 788. Commission Decision 2008/671/EC harmonises the use of the 5 875-5 905 MHz frequency band for transport safety ITS applications. In 2010 the ITS Directive 2010/40/EU was adopted. It aims to assure that ITS applications are interoperable and can operate across national borders, it defines priority areas for secondary legislation, which cover V2X and requires technologies to be mature. In 2014 the European Commission’s industry stakeholder “C-ITS Deployment Platform” started working on a regulatory framework for V2X in the EU. It identified key approaches to an EU-wide V2X security Public Key infrastructure (PKI) and data protection, as well as facilitating a mitigation standard to prevent radio interference between ITS-G5 based V2X and road charging systems. The European Commission recognised ITS-G5 as the initial communication technology in its 5G Action Plan and the accompanying explanatory document, to form a communication environment consisting of ITS-G5 and cellular communication as envisioned by EU Member States. Various pre-deployment projects exist at EU or EU Member State level, such as SCOOP@F, the Testfeld Telematik, the digital testbed Autobahn, the Rotterdam-Vienna ITS Corridor, Nordic Way, COMPASS4D or C-ROADS.
Spectrum allocation for C-ITS in various countries is shown in the following table. Due to the standardization of V2X in 802.11p preceding C-V2X standardization in 3GPP, spectrum allocation was originally intended for the 802.11p based system. However, the regulations are technology neutral so that the deployment of C-V2X is not excluded.
|Country||Spectrum (MHz)||Allocated bandwidth (MHz)|
|Australia||5855 – 5925||70|
|China||5905 - 5925 (trials)||20|
|Europe||5875 – 5905||30|
|Japan||755.5-764.5 and 5770 – 5850||9 and 80|
|Korea||5855 – 5925||70|
|Singapore||5875 – 5925||50|
Consideration in the transition period
The deployment of V2X technology (either C-V2X or 802.11p based products) will occur gradually over time. New cars will be equipped with either of the two technologies starting around 2020 and its proportion on the road is expected to increase gradually. In the meantime, existing (legacy) vehicles will continue to exist on the road. This implies that the V2X capable vehicles will need to co-exist with non-V2X (legacy) vehicles or with V2X vehicles of incompatible technology.
The main obstacles to its adoption are legal issues and the fact that, unless almost all vehicles adopt it, its effectiveness is limited. British weekly "The Economist" argues that autonomous driving is more driven by regulations than by technology.
However, more recent study indicates that the there are benefits in reducing traffic accidents even during the transitional period in which the technology is being adopted in the market.
- An assessment of LTE - V2X (PC5) and 802.11p direct communications technologies for improved road safety in the EU (http://5gaa.org/wp-content/uploads/2017/12/5GAA-Road-safety-FINAL2017-12-05.pdf)
- White Paper on ITS spectrum utilization in the Asia Pacific Region (http://5gaa.org/wp-content/uploads/2018/07/5GAA_WhitePaper_ITS-spectrum-utilization-in-the-Asia-Pacific-Region_FINAL_160718docx.pdf)
- EN 302 663 Intelligent Transport Systems (ITS); Access layer specification for Intelligent Transport Systems operating in the 5 GHz frequency band (http://www.etsi.org/deliver/etsi_en/302600_302699/302663/01.02.00_20/en_302663v010200a.pdf)
- The Case for Cellular V2X for Safety and Cooperative Driving (http://5gaa.org/wp-content/uploads/2017/10/5GAA-whitepaper-23-Nov-2016.pdf)
- NHTSA: Vehicle-to-Vehicle Communications: Readiness of V2V Technology for Application (http://www.nhtsa.gov/staticfiles/rulemaking/pdf/V2V/Readiness-of-V2V-Technology-for-Application-812014.pdf)
- ETSI TR 102638: Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of Applications; Definitions (http://www.etsi.org/deliver/etsi_tr%5C102600_102699%5C102638%5C01.01.01_60%5Ctr_102638v010101p.pdf)
- SAE J2945/x family of standards: (http://standards.sae.org/wip/j2945/)
- Xie, Xiao-Feng; Wang, Zun-Jing (2018). "SIV-DSS: Smart in-vehicle decision support system for driving at signalized intersections with V2I communication". Transportation Research Part C. 90: 181-197. doi:10.1016/j.trc.2018.03.008.
- 1609.x family of standards (http://odysseus.ieee.org/query.html?qt=1609.&charset=iso-8859-1&style=standard&col=sa)
- ETSI TR 101 607; Intelligent Transport Systems (ITS); Cooperative ITS (C-ITS); Release 1 (http://www.etsi.org/deliver/etsi_tr/101600_101699/101607/01.01.01_60/tr_101607v010101p.pdf)
- ARIB STD-T109; 700 MHz BAND; INTELLIGENT TRANSPORT SYSTEMS: (http://www.arib.or.jp/english/html/overview/doc/5-STD-T109v1_2-E1.pdf)
- Recommendation ITU-R M.2084-0; Radio interface standards of vehicle-to-vehicle and vehicle-to-infrastructure communications for Intelligent Transport System applications (https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.2084-0-201509-I!!PDF-E.pdf)
- Federal Communication Commission -Amendment of Parts 2 and 90 of the Commission's Rules to Allocate the 5.850-5.925 GHz Band to the Mobile Service for Dedicated Short Range Communications of Intelligent Transportation Services ET Docket No. 98-95 (https://apps.fcc.gov/edocs_public/attachmatch/FCC-99-305A1.doc)
- Safety Pilot Model Deployment Technical Fact Sheet (http://www.safercar.gov/staticfiles/safercar/connected/Technical_Fact_Sheet-Model_Deployment.pdf)
- NHTSA: Vehicle-to-Vehicle Communications: Readiness of V2V Technology for Application (http://www.nhtsa.gov/staticfiles/rulemaking/pdf/V2V/Readiness-of-V2V-Technology-for-Application-812014.pdf)
- Federal Motor Vehicle Safety Standards: Vehicle-to-Vehicle (V2V) Communications, Docket No. NHTSA–2014–0022 (http://www.nhtsa.gov/staticfiles/rulemaking/pdf/V2V/V2V-ANPRM_081514.pdf)
- Hearing in US Congress: https://energycommerce.house.gov/hearings-and-votes/hearings/vehicle-vehicle-communications-and-connected-roadways-future
- First version ETSI EN 302 571: Intelligent Transport Systems (ITS); Radiocommunications equipment operating in the 5 855 MHz to 5 925 MHz frequency band; Harmonized EN covering the essential requirements of article 3.2 of the R&TTE Directive (http://www.etsi.org/deliver/etsi_en/302500_302599/302571/01.01.01_60/en_302571v010101p.pdf)
- Here the 2014 version: Electromagnetic compatibility and Radio spectrum Matters (ERM); System Reference document (SRdoc); Technical characteristics for pan European harmonized communications equipment operating in the 5,855 GHz to 5,925 GHz range intended for road safety and traffic management, and for non-safety related ITS applications (http://www.etsi.org/deliver/etsi_tr/103000_103099/103083/01.01.01_60/tr_103083v010101p.pdf)
- Commission Decision 2008/671/EC „on the harmonised use of radio spectrum in the 5875-5905 MHz frequency band for safety-related applications of Intelligent Transport Systems (ITS)“ (http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32008D0671)
- Directive 2010/40/EU on the framework for the deployment of Intelligent Transport Systems in the field of road transport and for interfaces with other modes of transport (http://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX%3A32010L0040)
- C-ITS Deployment Platform – Final Report, January 2016 (http://ec.europa.eu/transport/themes/its/doc/c-its-platform-final-report-january-2016.pdf)
- Intelligent Transport Systems (ITS); Mitigation techniques to avoid interference between European CEN Dedicated Short Range Communication (CEN DSRC) equipment and Intelligent Transport Systems (ITS) operating in the 5 GHz frequency range (http://www.etsi.org/deliver/etsi_ts/102700_102799/102792/01.02.01_60/ts_102792v010201p.pdf)
- 5G for Europe: An Action Plan – COM (2016) 588, footnote 29 (http://ec.europa.eu/newsroom/dae/document.cfm?doc_id=17131)
- 5G Global Developments – SWD (2016) 306, page 9 (http://ec.europa.eu/newsroom/dae/document.cfm?doc_id=17132)
- Amsterdam Declaration – Cooperation in the field of connected and automated driving (https://english.eu2016.nl/binaries/eu2016-en/documents/publications/2016/04/14/declaration-of-amsterdam/2016-04-08-declaration-of-amsterdam-final-format-3.pdf)
- For C-ROADS see: Connecting Europe Facility – Transport 2015 Call for Proposals – Proposal for the Selection of Projects, pages 119-127 (https://ec.europa.eu/inea/sites/inea/files/20160712_cef_tran_brochure_web.pdf)
- "Uberworld". Economist. 3 September 2016.