DSRC and Cellular Duke It Out for Autonomous Vehicle Connectivity

By Barry Manz for Mouser Electronics

Figure 1. True vehicle autonomy can only be achieved when vehicles have precise situational awareness provided by connecting vehicles to other vehicles and to transportation infrastructure such as traffic signs and signals.
Figure 1.True vehicle autonomy can only be achieved when vehicles have precise situational awareness provided by connecting
vehicles to other vehicles and to transportation
infrastructure such as trafficsigns and

When viewed from on high, the market for“driverless vehicles” is the same as everything else under the huge IoT umbrella, as they rely on sensors, data, and wireless communication to accomplish the goals. It also shares another attribute common to IoT: A fierce battle to determine the technology (or technologies) that will connect vehicles to each other and to roadside sensors. The primary issue is whether a solution first proposed in the 1990s and made an IEEE standard in 2009 should be replaced or at least complemented by cellular networks and their underlying technologies. What’s at stake is a market that will generate hundreds of billions of dollars for electronics manufacturers, the auto industry, and wireless service providers.
The primary goals of achieving vehicle autonomy are to increase traffic safety and reduce congestion, leading to fewer cars on the road, greater transportation efficiency, and potentially reduced energy requirements (Figure 1). To achieve this, vehicles must be truly autonomous, which means they need comprehensive and precise situational awareness to make decisions in real time. This in turn requires wireless communications between vehicles and communication to and from infrastructure, such as sensors mounted on traffic signs, signals, and so forth.

The Long Road to DSRC Development

Although autonomous vehicles are one of today’s Big Things, it’s been on the mind of vehicle planners in the automotive industry for decades. In the 1990s, the Federal Communications Commission in the U.S. and its counterparts in Europe had the foresight to agree on a set of communication requirements that would allow vehicle autonomy to proceed. They set aside 75 MHz of spectrum around 5.9 GHz along with other requirements that collectively are called dedicated short-range communication (DSRC).

This resulted in an enhancement to the IEEE Wi-Fi standard 802.11, called 802.11p finalized in 2009, and a higher-layer standard within it called IEEE 1609, which is also the foundation of a European standard for vehicular communication known as ETSI ITS-G5.
At the time it was envisioned, the approach was deemed capable of providing the framework for addressing all vehicle autonomy scenarios, called V2X, with these variations:

• V2I (vehicle-to-infrastructure)
• V2V (vehicle-to-vehicle)
• V2P (vehicle-to-pedestrian)
• V2D (vehicle-to-device)
• V2G (vehicle-to-grid)
And of course, all of these have somewhat different requirements (Figure 2). But that was 20 years ago, an eternity for wireless
communications, when cellular technology was in its second generation and the primary application was automatic toll collection. Mobile communication technology as we know it today did not exist. It’s not surprising that what once seemed a fait accompli is now the subject of fierce debate in the wireless, automotive, and electronics industries, as well as federal governments and academia.

Figure 2. The various elements of V2X.
Figure 2. The various elements of V2X.
The DSRC Perspective

In one corner, the automotive industry and some electronics manufacturers believe that DSRC is still valid today as it was when conceived. They point out the enormous amounts of money expended and significant technical progress over the last decade, as well as manufacturers’ hardware, software, and even complete systems for DSRC already available.
They believe DSRC is a solution that’s ready today, even though further work remains to address specific problems. U.S. automakers in particular note that they have been developing and testing DSRC for a decade and have no interest in simply scrapping it and “reinventing the wheel” by having to go through the entire process again using cellular technology.
The pro-DSRC camp notes that while the cellular industry makes the case that it is ready to implement V2X today, reality dictates that unlike DSRC it has not been subjected to much testing and its solutions have not been finalized. Once the Third-Generation
Partnership Project (3GPP), which administers cellular standards, agrees on the capabilities required to support all V2X scenarios, a period of development will ensue followed by implementing those capabilities in its existing networks. It would thus take many years for cellular V2X to be widely deployed. Cellular has also demonstrated its increasingly formidable capability for machine-tomachine communications, including telematics systems used for years for remote diagnostics and automatic crash notification.
DSRC manufacturers agree with the cellular industry that scenarios not considered safety-related can be accommodated by LTE today with little or no modification, but argue that there is no consensus on how the technology will perform in highly congested traffic environments. They also counter the cellular industry’s view that DSRC has no funding mechanism, noting that wireless carriers don’t have one either, and that DSRC generally eliminates the issue of subscriber fees. Cellular V2X also requires evolved Multimedia Broadcast/Multicast Service (eMBMS), also called LTE Broadcast, which allows wireless carriers to simultaneously broadcast
messages to all vehicles in range. It was originally designed to serve fixed environments such as stadiums rather than mobile environments, so it’s viability for V2X remains to be verified.
DSRC was also designed to have very low latency, which is a crucial metric for vehicle autonomy where success or failure to deliver data is measured in milliseconds. Cellular networks have low latency but not all the time, although this situation is improving as the industry moves toward 5G. DSRC infrastructure is also less expensive to deploy in a cellular network, which to some degree mitigates it needing to “start from scratch.” As basic vehicle autonomy functions are at least in theory supposed to be free, the cellular industry will have to figure out a way to monetize its other offerings as it would not be able to charge for the basic services. A new business model would have to be created to support the massive amounts of additional data from V2X that their networks would
have to accommodate. Finally, there is the possibly contentious point of cellular network reliability during catastrophes such as hurricanes, which hasn’t proven universally robust. However, the same could probably be said for DSRC, but as it has not yet
been deployed, no one really knows the answer.

The View from the Cellular Industry

In the other corner are the cellular industry as well as other electronics manufacturers that believe the best approach is to use current cellular technology and networks that, unlike DSRC, are already massively deployed, have a clear roadmap to greater future performance, and would be far less expensive to implement than DSRC which would require significant new infrastructure. They also stress that federal rulemaking for DSRC is still in process at the speed of government, that the underlying technology is incapable of scaling to accommodate increasing traffic demand, that it lacks the required quality of service, and that it has no coordinated channel access strategy. Perhaps most damaging is that DSRC has no roadmap for future development making it a technological dead end. When it was developed, DSRC was suitable for V2V communications, but does not address all potential use cases that have appeared since. There is no “next-generation” DSRC in development or planned.
DSRC also requires the use of roadside units (RSUs) that must be deployed (Figure 3). RSUs are one of two main components in DSRC networks, the other being on board units (OBUs) that have integral transceivers. An RSU communicates with OBUs to acquire traffic information such as time, speed, vehicle location, and other metrics. Having passed through an area of congestion or an intersection, for example, the RSU calculates travel time and when the congestion began, sending this data to others OBUs within range.

Figure 3. Roadside units (RSUs) are a mandatory requirement for DSRC. (Source: WSP)
Figure 3. Roadside units (RSUs) are a mandatory
requirement for DSRC. (Source: WSP)

However, while RSUs add a major cost element to DSRC, any V2X technology must communicate with roadside sensors ranging from cameras to lidar and radar. The difference between the two is that much of the basic information provided by RSUs is gathered by cellular networks without the need for such units.
Supporters of the cellular approach say that DSRC has no clear business model nor funding source to support its development. If the federal government mandated the use of DSRC (which it may), there is little doubt that consumers would bear some of the financial burden, although it’s likely that government industry partnerships would also be in the mix. It would also require many years to fully deploy DSRC on a nationwide basis, so the logical course is thus to begin using cellular technology for V2X today rather than waiting for perhaps a decade for DSRC.
Then there is the issue of interference between existing and proposed Wi-Fi services and DSRC that have yet to be solved, a problem the FCC is seeking to address through a public notice seeking comments on two band-sharing proposals. In the
first, unlicensed operators would use “detect and avoid” technology where DSRC is not in use, and the second proposes to move the “safety-related” portion of DSRC to between 5.895 and 5.925GHz, leaving the lower portion of the designated spectrum to be shared between non-public safety related DSRC and Wi-Fi.
The cellular camp also observes that it can deal with the potential problem of service outages using device-to-device (D2D) technology that allows communication to take place between users without having to go through a base station. Although data
rates drop precipitously when this is enabled, this shouldn’t be a concern, as almost all core safety centric V2X capabilities need only low speeds to function.
Finally, although U.S. automakers are strongly behind the use of DSRC, this feeling is not universally shared elsewhere. For example, a consortium that includes major German automakers as well as major drivers of cellular, such as Vodafone, Ericsson, Intel, Huawei, Nokia, and Qualcomm, believe cellular technology is the best route. This could result in the use of cellular technology in Europe and DSRC in the U.S., Japan, and perhaps other countries. This would obviously be a burden on U.S. automakers that would need to build country-specific connectivity solutions into their vehicles.

Cellular V2X Particulars

The intention of the cellular V2X camp is not to replace DSRC but to potentially complement it and use the message sets and many of the its other features, such as the IEEE 1609.2 standard and its security mechanisms. This makes sense as the Society of Automotive Engineers (SAE) DSRC technical committee has created a data dictionary (SAE J27355) that defines V2X messages and much of the work conducted on DSRC is applicable to cellular solutions as well. The RSUs required by DSRC could potentially be used within cellular infrastructure to enhance cellular V2X performance with the cellular network acting as a backup.
The cellular V2X community’s goals are to provide service at vehicle speeds up to 100mph with enough range to provide time for drivers to respond, maximum latency of 100ms between two vehicles, and 1000ms for messages sent via a network server.
It is anticipated that these and other performance metrics such as speed, reliability, location accuracy, and message payloads will increase width advances in cellular technology. In 5G for example, round-trip latency between vehicles is expected to be 5ms or
less with 99 percent packet delivery reliability over 260 to 650ft.
The acronym for the technology for V2X is LTE-V (LTE Vehicular), which is an LTE variant being standardized by 3GPP so it is ready when 3GPP Release 14 is finalized. It builds on current LTE capabilities to meet automotive requirements for vehicle-to-vehicle and vehicle-to-infrastructure communication and addresses all use cases. LTE-V includes two modes:
• LTE-V-direct is a decentralized LTE architecture including direct communications, low latency, and reliability improvements for both V2V and V2I. LTE-V-direct and LTE-Vcell coordinate with each other to provide an integrated V2X solution.
• LTE-V-cell optimizes radio resource management for better supporting V2I and offers features for handover enhancement, fast resource allocation, and coverage optimization.
LTE-V is still in its study phase within 3GPP and might be finalized this year. Once that happens, it will take at least a year to produce a commercial chipset, so LTE-V is unlikely to be available for commercial application until 2018 or even later.
When 5G becomes a reality, it will dramatically increase data rates, use advanced antenna techniques, and provide multi-hop capability to extend coverage.

Some Conclusions

DSRC will almost certainly be used in the U.S., as automakers have spent the last decade working to perfect it and incorporate it into their vehicles, and General Motors has already integrated it in the 2017 Cadillac CTS sedan. It won’t have full DSRC functionality because it will only be able to communicate with other CTS models, as RSUs have not been deployed. Nevertheless, it can scan the area for other vehicles and track their positions, directions, and speeds, warning the driver of potential dangers. The system can handle 1,000 messages per second from vehicles up to nearly 1,000ft. away.
The other reason DSRC will have a home court advantage versus cellular is that the National Highway Traffic Safety Administration and Department of Transportation are in the process of establishing Federal Motor Vehicle Safety Standard (FMVSS) No. 150, which would mandate V2V communications in light vehicles. Their Notice of Proposed Rulemaking (NPRM) states that without a mandate to require and standardize V2V communications, manufacturers cannot move forward in an efficient way so a critical mass of equipped vehicles would take many years, if ever, to develop. The NPRM was published in the Federal Register on January 12 and comments were to be submitted by April 12. The rule may go into effect later this year.
However, even with the mandate in place, that’s not the end of the line for cellular V2X in the U.S., as the NPRM leaves some wiggle room for alternate approaches such as hybrid systems that use DSRC as the primary solution with cellular backup. It devoted several pages to why DSRC is a better choice, from cost, to security, coverage, and even hazards to humans caused by exposure to electromagnetic radiation from on-board cellular systems. The latter caution is dubious at best as many vehicles already have integrated cellular capability and the debate about radiation hazards has come to no conclusion. However, it clearly shows how committed the federal government is to DSRC.
Still, it’s difficult to believe that cellular will not ultimately make its way into V2X in the U.S., especially as LTE-V and related technologies continue to advance. In Europe and perhaps other regions, the cellular approach is more highly regarded and has much more support from the automotive industry, so it’s likely the situation might be the opposite of what is occurring in the U.S. In short, it would be foolish to draw any conclusions today about how connectivity in the autonomous vehicle market will develop in the coming years, when true autonomous vehicles are plying the streets.




Share this post