The continued global schism between cellular and 802.11p WiFi for vehicle-to-everything (V2X) communications loomed large at the Consumer Electronics Show (CES) earlier this month. The two highlights in this context were Qualcomm’s announcement of the first cellular C-V2X testing in Las Vegas in partnership with Ford, and Visteon Corporation’s launch of the first dual-mode V2X module supporting both options.
Visteon’s module supports both the 3GPP’s Cellular-V2X (C-V2X) based on the earlier LTE-V2X and the IEEE’s somewhat confusingly named ITS-G5 utilizing the 802.11p WiFi, which is often known as DSRC in the US, after the RF band it is used in. The shared letters in 5G and ITS-G5 are a little confusing, because it is C-V2X rather than ITS-G5 that is being aligned with emerging 5G cellular technologies.
The connected vehicle world is split between the two standards both by geography and manufacturer. ITS-G5 came first and as a result has gained traction with early deployments in Europe and Japan, but since its more recent release C-V2X has gained ground elsewhere. The US itself is pivotal because DSRC was the early mover with initial indications its use would be mandated, although that has not happened. Recently there has been a swing in sentiment towards C-V2X there partly as a result of Qualcomm’s powerful push – after all, C-V2X is something of Qualcomm’s baby.
This geographical split is partly mirrored among automobile makers, with Volkswagen and Renault in the ITS-G5 camp. However, three other major European OEMs, Daimler, BMW and the PSA Groupe trading under the Peugeot, Citroën, DS, Opel and Vauxhall brands, have come down on the side of C-V2X, along with Ford and SAIC Motor. Outside Europe ITS-G5 does have two big backers in Toyota and GM. This has helped keep it on the map in the face of growing global momentum behind C-V2X with strong advocacy from many of the big technology hitters outside the automobile industry, including Deutsche Telekom, Telefonica, Vodafone, Ericsson, Huawei, Intel, Nokia, Samsung and Savari, beside Qualcomm.
Against this background though, ITS-G5 received a major endorsement in October 2018 when the European Commission’s decided to approve it after intense lobbying from both camps. The EU was swayed by a paper published by Siemens, arguing ITS-G5 was more mature and met all the key requirements of V2V and V2X communication, while C-V2X did not, at least yet. Such features include ability to operate in a very dynamic environment with high relative speeds between transmitters and receivers and support for extremely low latency – below 50ms in safety-related applications for the so-called pre-crash sensing warning message.
This paper seems to have tilted the EU towards ITS-G5 by arguing it favored road safety. The paper said ITS-G5 technology was tailor-made for road safety applications because of its low latency and also ability, shared with most wireless protocols, to communicate beyond line of sight. Its properties are deemed by the EU to make it suitable for numerous road safety applications, including electronic emergency brake lights, distance-keeping in platoons of trucks and in future for higher levels of autonomous driving, being crucially designed to operate at short range.
Bystanders could be forgiven for being confused though, because shortly after the EU announced its decision the 5G Automotive Association (5GAA) published results of its tests comparing ITS-G5 with C-V2X. Its conclusion was that “C-V2X direct communications technology, consistently – and in many cases overwhelmingly – outperforms 802.11p/DSRC. With a natural evolution path towards the low latency and high bandwidth benefits of 5G NR, C-V2X also demonstrated superior performance in several dimensions, including enhanced reliability over extended communications range, better non-line-of-sight performance and greater resiliency to interference for example from other devices.”
This set of tests was more comprehensive and was probably the first providing a proper match up, since Siemens had admitted that it was not looking at the final version of C-V2X. A key point about C-V2X is that it combines two complementary transmission modes, with a direct short-range mode for communication V2V or between vehicles and nearby roadside infrastructure, or road users such as cyclists and pedestrians.
In this mode, C-V2X is quite independent of cellular networks but designed to dovetail with them. Then the second mode supports network communications over the conventional mobile infrastructure to receive information about road conditions and traffic in the area, or for infotainment generally. This enables the C-V2X camp to argue that it can do everything ITS-G5 can locally while being much better suited to receiving broadcast services without being dependent on nearby Access Points by the roadside.
To some extent these claims were born out by the latest 5GAA tests, which concluded that the application-specific latency requirements can only be met in all conceivable real-world scenarios, including highly congested situations, with C-V2X. As the system load increases, C-V2X continues to meet the latency required by a particular safety application in a predictable manner by virtue of its distributed architecture, while end-to-end latency becomes unpredictable under heavy loading with ITS-G5.
The key factor here is that ITS-G5 relies on CSMA/CA for channel access based on energy sensing on the channel. When the system is lightly loaded, messages can be transmitted with just as low latency as C-2VX, or even lower if Siemens’ results are correct. However, as the system becomes heavily loaded, latency experienced by messages will grow.
C-V2X on the other hand uses a synchronous approach with a distributed scheduling mechanism for packet transmission. This allows delays to be set in advance, for example to 100ms, with devices able to reselect resources subsequently to keep to their latency budget. This also allows latency to be varied to suit the application, so that for example it can be reduced to 40ms for platooning where low delays are essential.
C-2VX also scored in non-line of sight situations which could have advantages for safety and performance at times of poor visibility for example, contradicting the Siemens test results. It looks then as if the EU was premature to base its judgement on Siemens’ results and may regret its decision. For the foreseeable future though the two standards will ride in parallel and that was the motivation behind Visteon’s dual-mode module supporting both.
It is not clear yet though whether there will be much demand for a dual-mode module saddled with extra cost and complexity, even if both standards do continue to fly. There may not be many regions where vehicles will need to embrace both standards, given that it looks like ITS-G5 will prevail in Europe while C-V2X will be preferred elsewhere.
The exception could be the US where there is some traction behind both, but it is beginning to look as if C-V2X has greater momentum there. It does seem though as if ITS-G5 will need further enhancements to keep it in the game despite the EU’s endorsement, in which case the C-V2X camp will be able to argue that it is now the more mature standard.