V2X communications – LTE versus DSRC
V2X refers broadly to a set of standards and technologies that will allow vehicles to interact with public roads and other road users – not necessarily by having to rely on use of cutting-edge electronics hardware, but potentially via tried and tested networking protocols and technologies. Despite its relative simplicity, V2X makes a multitude of interesting applications possible, including higher degrees of driving assistance, more efficient road usage and collision avoidance.
For many years, the leaders of the V2X pack have been the US-developed dedicated short-range communications (DSRC) and the European cooperative intelligent transport systems (C-ITS). The National Highway Traffic Safety Administration (NHTSA) is currently finalising a proposal to make V2X technology a mandatory feature of US vehicles, by perhaps as early as the 2020 timeframe, with DSRC expected to be the solution upon which it is based. However, while V2X has great potential, adoption and finalisation of dedicated regional V2X standards has been surprisingly sluggish, taking well over a decade already. It has been so slow in fact, that the mobile communications industry has recently seized the opportunity to aggressively push its own V2X standard, cellular-V2X (C-V2X).
In January 2017, when the NHTSA issued its proposal to make V2X a legally-required component of all US vehicle designs moving forward, it named DSRC as its preferred choice, but went to considerable lengths to say that it would also look favourably upon other technologies that could match or exceed the capabilities of DSRC, while remaining backwards-compatible with DSRC’s critical safety features. The long V2X marathon has, as a result, suddenly turned into a sprint, and the finishing line is now approaching rapidly. Has the former front-runner delayed too long and will it end up losing out?
The established technologies – C-ITS and DSRC
C-ITS and DSRC both use variants of the familiar IEEE 802.11 Wi-Fi standards as the physical and medium access layers of their protocol stack. For DSRC this protocol layer is defined by the IEEE 802.11p standard, while C-ITS uses the similar ETSI-defined ITS-G5 standard, which is basically a version of 802.11p adapted to suit European regulations. While there are significant differences in the higher-level functioning of C-ITS and DSRC, both can run on broadly-compatible hardware and reside in almost the same frequency range. Both these V2X systems can communicate effectively between fast-moving vehicles over a distance of at least 300m and at data rates ranging from approximately 3-27 Mbps.
In 1999, the US FCC allocated the 5.850-5.925 GHz band for high-priority road safety and traffic management applications, and the European Commission has dedicated 5.875-5.905 GHz for similar purposes. IEEE 802.11p and ITS-G5 are designed to work in these respective ranges. To make them better optimised for the cluttered and fast-moving automotive environment, the standards feature modifications to typical IEEE 802.11 Wi-Fi frequency usage which reduce the impact of Doppler shifts and multipath fading.
One of the key differences between these automotive protocols and other members of the 802.11 family is that the initial handshake and association period can be reduced to a bare minimum. The protocol can send data almost immediately, and can also defer authentication, encryption and full identification to higher level protocols. Therefore, vehicles and infrastructure can begin exchanging essential data on speed and position within tens of milliseconds of detecting each other. This is obviously ideal for applications such as collision avoidance, and a stark difference from the seconds-long handshaking negotiation that are common in other Wi-Fi variants.
While C-ITS and DSRC are very similar at the MAC/PHY access level, above that, they diverge significantly at the networking and transport level. Both support TCP/UDP over IPv6, but for time-critical features they each have their own specialised low-overhead messaging standard. DSRC uses the wave short message protocol (WSMP), which is defined as part of IEEE 1609. However, C-ITS employs a more ambitious multi-hop routing system, the basic transport protocol (BTP) over the GeoNetworking service, as defined by the ETSI EN 302 636 series of standards. GeoNetworking is a geographically aware routing technique, which establishes an ad-hoc network that is efficiently arranged according to the physical locations of nodes.
C-V2X – the mobile industry’s contender
C-V2X is the new name for the V2X standard that was initially launched as LTE-V2X by respected international mobile communications standards body the 3rd Generation Partnership Project (3GPP), in its 3GPP Release 14. The change of name reflects the fact that C-V2X now promises an upgrade path from LTE to the much anticipated 5G mobile standard.
For device-to-device mode direct communications, C-V2X uses the same 5.9 GHz spectrum as DSRC and C-ITS. However, C-V2X devices can also operate in traditional mobile bands to communicate with the mobile network in so-called vehicle-to-network (V2N) mode. C-V2X device-to-device communications do not require a SIM card, a mobile subscription or any mobile network infrastructure.
C-V2X is a much younger technology than either DSRC or CITS, and is still under development, but early tests have shown that at the data rates required for V2X applications C-V2X reaches a range comparable to DSRC’s 300m at urban driving speeds and achieves 20-30 percent better range at faster speeds.
C-V2X versus the Incumbents
Supporters of DSRC and C-ITS will argue that they are far more suitable for V2X applications than C-V2X because they have been carefully constructed from first principles for one clearly-defined purpose, rather than being bolted on to an existing standard. For example, DSRC and C-ITS are designed to offer low networking overheads and low latency, they claim. Latency is a key issue for V2X. Functions like collision avoidance will require round trip latencies below 50ms. The NHTSA believes that “…at this time, DSRC is the only mature communication option that meets the latency requirements to support vehicle communication based crash avoidance…” – although it has clearly kept the door open for other technology platforms. In addition, proponents point out that DSRC and C-ITS have been in development for almost twenty years and this means they are stable and assured. Large-scale tests are underway in various locations. Meanwhile, the 5G Automotive Association (5GAA), main industry body promoting C-V2X, promises that it “can be commercially ready by 2018.”
Opponents of DSRC and C-ITS will say that the very fact that these technologies have been under development for close to two decades without reaching the mass market in any meaningful form clearly suggests that they may never be market-ready. Also that they are grounded in old technology that lacks a viable upgrade path. It may also be pointed out that although LTE and 5G technology might not have originally been designed with V2X in mind, C-V2X can leverage the economies of scale that the huge global mobile telecommunications market present in order to reduce hardware, software and development costs.
Can old and new Standards co-exist?
The 5GAA states that C-V2X technology does not have to be a complete replacement for C-ITS and DSRC. C-V2X is designed to be able to share the available spectrum with 802.11p-based V2X systems by switching to vacant channels within the 5.9 GHz band. In addition, it can support the upper protocol layers of C-ITS and DSRC – effectively replacing only the IEEE 802.11 MAC and PHY radio layer with 3GPP-specified equivalents, while allowing the specialised higher-level DSRC and C-ITS V2X messaging standards to run as normal. With these capabilities, C-V2X appears to meet the NHTSA’s requirements for mandatory V2X devices.
Perhaps the question of which technology is better is not actually as critical as it seems. Automobile OEMs could choose the underlying technology they prefer, or even hedge their bets and provision for supporting both – IC vendor Qualcomm has, for instance, recently announced its 9150 C-V2X chipset which does just this.