V2X Communication for ITS - from IEEE 802.11p Towards 5G
by Kees Wevers and Meng Lu
IEEE 5G Tech Focus: Volume 1, Number 2, June 2017
The past decade has seen a substantial development of radio communication technology for ITS (Intelligent Transportation Systems) applications for road transport. Communication of vehicles with each other (V2V), with the infrastructure (V2I) and with vulnerable road users are expected to bring substantial benefits in terms of safety and comfort, and may also contribute to improved and more granular traffic management, provide a better way to prevent or reduce congestion, and bring fuel savings and thereby reduction of emissions . These communication modes are summarised in the term V2X, vehicle to anything (relevant).
An extensive set of ITS-related use cases for V2X have been elaborated, and, based on these, requirements for V2X communication have been drafted. A clear distinction needs to be made between safety applications and non-safety ones. Safety applications clearly set the benchmarks for the functional requirements with respect to the communication, especially in terms of reliability, maximum latency and message frequency . Many of the safety-related use cases aim to avoid collisions, between vehicles, and of vehicles with other road users like bicycles and pedestrians. Examples are vehicles approaching a crossing in a built-up environment or a curve in a road, impaired visibility in a queue or under foggy conditions, or pedestrians crossing the road. Data from sensors like camera, radar and lidar can be used to detect certain potentially hazardous encounters, but continuous and instantaneous exchange and processing of position and velocity data (and some other vehicle status data) can certainly drastically extend the domain of useful applications, in terms of all-weather visibility, field of view and range.
Today, accurate position and velocity data can be produced using GPS and dead-reckoning data in combination with an accurate map of high detail. Reliability of positioning will further improve with the development of high-precision maps for automated driving, and sensing adequate and well-mapped fixed infrastructure objects along the road. Position and velocity data may be used by each vehicle to build and maintain a dynamic map of its surroundings, and permits situational awareness, and extrapolations to identify potential hazards. The existing sensors remain useful, and will be complementary to the communication and add redundancy.
Suitable safety-related V2X communication has low latency (in the range of milliseconds), can cope with the high relative speeds between transceivers (up to 200 km/h and above), high dynamics of the collection of nearby actors involved (dynamic network topology) and substantial network load (by ongoing periodic broadcast of messages by multiple actors and high numbers of transceivers in scenarios of congested traffic), and is able to bridge a substantial distance (several hundred meters up to 1 km) and work under non-line-of-sight (NLOS) conditions.
During the years 2005 to 2010 an adapted version of the IEEE standard 802.11 for WiFi communication was developed, able to meet the mentioned requirements. This 801.11p amendment defines the physical communication layer (in terms of the OSI model), and on top of it the WAVE (wireless access in vehicular environments) and ETSI ITS-G5 standards have been developed, in the US and Europe respectively, for the higher layers. Extensive field-operational testing has taken place in the US and in various countries in Europe, and the technology has proven to meet the requirements, and to be mature. In the mean time transceiver chipsets have been developed by the industry, permitting mass production at low-cost, and the technology is ready for large-scale deployment. Both the US and Europe have allocated bandwidth in the 5.9 GHz band for this type of communication. Relay of messages by multi-hop routing using other mobile nodes, and where appropriate, infrastructure nodes, may solve NLOS issues and also increase communication range.
Another radio communication technology, cellular for personal mobile communication, has seen an incredible development in the past 25 years, since it went digital with the introduction of GSM (Global System for Mobile communications). Although not truly global, it became an enormous success in many parts of the world, and has set the stage for world-wide cooperation in the further development of standards for cellular communication. In 1998 the 3GPP initiative was created, the 3rd Generation Partnership Project, which since then has taken the lead in cellular standardisation . Its standards are published as Releases, the latest version being Release 13 of March 2016. 3GPP first developed 3G (2000) and several later evolutions, followed by 4G (formally named LTE, Long Term Evolution; 2008) and successive later advancements. Given its pervasive presence in society, with billions of users and extensive networks, it is only natural that, especially with the advent of 4G LTE, cellular appeared on the stage (and is being discussed) as a serious candidate for ITS-related communication.
However, major differences exist between the two different communication solutions in view of V2X communication. 802.11p communication is peer to peer while cellular operates with the intervention of a network operator. Cellular messages need to pass the uplink and downlink channels of the cellular network to reach their destination, while 802.11p messages are directly sent to any listening user. As 802.11 messages are just sent into the air, it operates anywhere, while cellular requires network coverage. In the cellular network V2X communication is competing with other uses absorbing a large share of the available bandwidth, while 802.11p is solely dedicated to V2X communication. While in the cellular network the communication bears a variable cost (mobile operator charges), the variable cost of 802.11p communication is zero; it is free of charge. It is evident that current 4G/LTE cellular technology cannot meet the stringent requirements for safety-related V2X communication. Even recent new features of LTE are not going to change this. As an example, take the current status of technology for cellular direct communication, building on the Device-to-Device (D2D) communication protocol. D2D was identified as part of 3GPP Release 12 (March 2015), but is not adequate for support of V2V use cases, as it requires pre-assignment of network resources. V2V also needs to work when there is no cellular network. D2D can work in the absence of a network, but this is only permitted for emergencies. In addition, and more importantly, it uses a slow protocol for device discovery [2, 4, 5, 6, 7].
While 4G still has limitations in comparison to 802.11p, these days proponents of cellular-based V2X communication point to the next or fifth generation wireless mobile, termed 5G (not to be confused with the ETSI ITS-G5 standard), and in this respect also on its huge potential for ITS-related communication, indicating that it will be able to provide a solution that surpasses the capacities of 802.11p. It is still rather vague what exactly 5G is going to be, and different people may have different perceptions and tell different stories about it. Nevertheless, a real debate has developed since early last year between the 802.11p and cellular communities on the best way forward. The 5G Automotive Association (5GAA), a global group of telecommunications and automotive industry, established 27 September 2016 , apparently intends to promote the use of future 5G for the development and introduction of Cellular-V2X (C-V2X). It is stated that "3GPP-based cellular technology offers superior performance and a more futureproof radio access than IEEE 802.11p" .
Some proponents of 5G for V2X argue that high bandwidth is essential to communicate the massive amounts of sensor data that will be generated by vehicles in the near future. Indeed, sensors like camera, radar and lidar can generate huge amounts of data. But it is questionable if there is any need to exchange such data with other vehicles or the infrastructure. However, periodic communication of accurate position, velocity and related vehicle-status data, composing a relatively small packet of data, in combination with the use of up-to-date, accurate and detailed digital maps is sufficient for safety-related use cases. Also specific safety information messages do not require massive data .
A first specification of 5G is foreseen for Release 15 of the 3GPP, which is currently planned for September 2018. One of the features scheduled to be included is support for a V2X service. Taking into account that experience learns that roll-out of new cellular technology and related investments take some six years from the publication of the standard, the 5G cellular V2X service would be ready for large-scale deployment only in 2024. That is a long time to go. While on the other hand 802.11 technology is mature, extensively tested and ready for large-scale introduction, and is meeting the requirements for safety-related V2V use cases. Certainly, it may also have some issues with congestion if the penetration rate becomes substantial, or if the number of use cases will grow. However, further work is ongoing to tackle these issues and to further improve the protocol, and to implement multi-channel communication and improved decentralised congestion control (DCC) schemes.
One of the options is to largely reserve the 802.11p communication capacity for safety-related V2V communication and including some critical I2V communication, such as retransmission (relay) of relevant messages under NLOS conditions, and leave less time-critical I2V communication to the cellular domain. Much of this less time-critical communication would already be possible with current 4G technology.
The United States Department of Transportation has taken a firm stand toward 802.11p introduction, by making a proposal for a rulemaking process to mandate deployment of DSRC-based V2V communications for safety-related use cases . It believes that a mandate will support manufacturers to move forward in an efficient way and will help to develop a critical mass of equipped vehicles much sooner than otherwise would be possible. The European Commission strongly promotes C-ITS implementation, but has not taken a step to mandate implementation . The US mandate proposal also provides a path for vehicles to comply by deploying other technologies that meet certain performance and interoperability requirements, including interoperability with DSRC. Similarly the European Commission has stated that to support all C-ITS services on the vehicle side, the full hybrid communication mix needs to be on-board, including, inter alia, 5G.
Therefore, the planned development of V2X services in 5G makes much sense, and holds promises for the future. But at the same time cooperative ITS based on 802.11p can take off and further develop. Providing experience and insight that in turn can be useful for and support the development of 5G-based V2X services. But especially as in this way high penetration levels of V2X can be reached much sooner, and thereby also the safety benefits from V2V communication can be harvested at a much earlier stage. Like it is not wise to defer purchase of a computer because next-year's model may be more advanced.
 Lu, M. (Ed.) (2016). Evaluation of Intelligent Road Transport Systems: Methods and Results. Publisher: IET (Institution of Engineering and Technology). ISBN 978-1-78561-172-8 (print); ISBN 978-1-78561-173-5 (eBook).
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 National Highway Traffic Safety Administration (NHTSA), Department of Transportation (DOT), "Federal Motor Vehicle Safety Standards; V2V Communications", Notice of Proposed Rulemaking (NPRM), NHTSA-2016-0126, draft version of 13 December 2016.
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Kees Wevers, BrightAngel ITS, Consultant, The Netherlands (2012-present). President, Transport Network ITS Spatial Data Deployment Platform (TN-ITS), Brussels (2013-2016). NAVTEQ (1992-2012) - Involved in international research projects (map databases, traffic information, ADAS, cooperative systems) and standardisation, various duties in international organisations. TISA Fellowship award (Traveller Informations Services Association; May 2012)
Meng Lu, Dynniq, The Netherlands. VP, IEEE Intelligent Transportation Systems Society; Co-Chair WG Industry Engagement, IEEE 5G Initiative. Participation in European initiatives and projects since 2005, as Coordinator, WP Leader and/or Partner. Received PhD at LTH, Lund University, Sweden; Master's title and degree of Engineering in The Netherlands and P.R. China.
Editor: Siming Zhang
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