DSRC vs. C-V2X Death Match

There’s a tug-of-war going on between which wireless technology should rule the ADAS roost—the established DRSC or the relatively new C-V2X.


By Lou Frenzel, Contributing Editor

Adding wireless communications to vehicles is the next step toward improving existing advanced driver-assistance systems (ADAS). The ADAS now being built into most new cars and trucks includes safety features like backup cameras, adaptive cruise control, automatic braking, lane keeping, blind-spot detection, and other features.

These ADAS have been widely accepted and are proving to significantly enhance driving safety. Adding radio communications in the mix should improve things even further. These wireless improvements are called vehicle-to-vehicle (V2V) and vehicle-to-everything (V2X) communications.

The industry and government were well along the path to adopting a universally agreed upon technology called Dedicated Short Range Communications (DSRC). Then suddenly another technology designated C-V2X popped up, disrupting the DSRC plans. Now the industry is considering both options and delaying the adoption of a wireless component to ADAS. The battle lines have been drawn and warfare is now erupting. Will the C-V2X newcomer beat the established and proven technology?

Both technologies fill the need for vehicle connectivity. V2V lets vehicles within range of one another exchange data for the purpose of collision avoidance. Data such as GPS location, speed, direction of travel, braking and turning status, and other data is communicated 10 times a second to adjacent vehicles.

V2X, which includes vehicle-to-infrastructure (V2I) communications, enables communications links to nearby roadside units (RSUs). RSUs will provide information on road conditions, construction, and weather and traffic status to passing vehicles (see “Managing the V2V and V2X Data Glut”). When combined with existing ADAS capability, the V2V and V2X augmentation should greatly reduce collisions and improve driving safety.

A Look at the Standards

Both standards would operate in 75 MHz of bandwidth located from 5.850 to 5.925 GHz in the U.S. and in the 5.855- to 5.925-GHz range in Europe. DSRC is a variant of the IEEE standard 802.11a that was the first Wi-Fi standard to use orthogonal frequency-division multiplexing (OFDM) and the 5-GHz band. The DSRC version is called 802.11p. It operates in one of seven 10-MHz channels in the assigned spectrum. Data rates from 3 to 27 Mb/s are possible.

DSRC has been widely heavily developed and tested in real-world conditions and has proven itself. Chips and modules are available. No infrastructure or RSUs have been developed or deployed, and there’s some consideration about who develops, pays for, and operates them.

C-V2X is a version of the 4G cellular standard called Long Term Evolution (LTE). It’s a variation defined in Release 14 of the 3GPP organization that develops cellular radio standards. That added the direct car to car capability. The data rate is comparable to DSRC and it offers much lower latency than DSRC. It implements V2V and uses the cellular network for V2I rather than RSUs. And it provides cloud connectivity as more applications are discovered. Some service fee is involved but it is not clear who pays whom for what.

The big problem is that C-V2X hasn’t been as extensively tested and vetted as DSRC, but trials are ongoing. Chips and modules are available. An upgrade path to 5G connectivity will be beneficial.

Decision Time

So, where are we? In limbo. The NHTSA and DOT are fretting over what to do. As you know, government tends to move slowly and no one seems to know where these agencies stand. The cellular carriers and some chip companies like Qualcomm are lobbying for C-V2X. And now, some of the auto manufacturers previously committed to DSRC have switched sides.

Furthermore, a newly formed advocacy organization 5G Automotive Association (5GAA) is pushing the C-V2X agenda. With over 75 corporate members, eight or more auto manufacturers, and big technology guns like Intel and Qualcomm, this is an influential group.

At this point, the decision is still up in the air, but the momentum clearly seems to be toward a C-V2X outcome. Some say that DSRC is an old technology and standard with no place to go. However, the IEEE recently started a new study group to update the 802.11p standard and improve its capabilities in V2X applications. The goal is to make it faster, provide lower latency, and give it a viable future. This is a great idea, but given the amount of time needed to produce a new standard, the whole effort seems to be too little, too late.

A related issue is the desire of others to get that precious 75-MHz chunk of the 5.9-GHz band. This segment is formally allocated to DSRC by the FCC. Wi-Fi needs more spectrum and would love to move into that space. The Wi-Fi Alliance even advocated that this spectrum be declared unlicensed so that Wi-Fi and V2V/V2X can share the space. That seems unworkable if (and how) interference could compromise safety. The wireless carriers are also lusting after more spectrum and have expressed an interest in this range. For now, the FCC is holding out and keeping that assignment for the automotive communications. But the pressure is on everyone right now.

This competition was probably over when C-V2X was put forth as an option. It’s new technology vs. old technology. The new technology has a path forward that should be adaptable not only to current vehicles, but also to future vehicles including self-driving cars. But DSRC is ready to go right now. The only thing needed is a decision and possibly a mandate to make things happen.

If the winner is C-V2X, we may expect a further delay; the DOT/NHTSA could require years of testing. The auto companies seem to be leaning toward C-V2X but will balk at any delay. Lead time for automotive design is years—not months—so these companies must really be frustrated. It’s time to move forward with this regardless of the standard. Both will work.

Managing the V2V and V2X Data Glut

The proposed vehicle communications systems are going to generate a boatload of data as the vehicle is in operation. Think of all the V2V/V2X-capable vehicles bumper-to-bumper stretched out over a mile and four lanes wide on a freeway at rush hour. The interference has got to be crushing. I keep wondering how this data will be used and will it overwhelm the vehicles’ electronics and the driver.

Just how will all of that data be translated into useful instructions to the driver? Will warnings be by audible signal (voice or tone?), LCD screen, head-up display, or what? Just what is the human-machine interface (HMI)? If visible data is to be presented, won’t that add to driver distraction? Or will the data be translated into actual electronic signals that directly control braking, acceleration, and steering? The vehicle data is going to be immensely useful, but the success of its adoption will no doubt depend on how it’s presented to the driver and the vehicle controls.

In any case, it’s going to take years before the system is useful. Older cars don’t have this communications capability, and these will be on the road for many more years. New cars with V2V/V2X will gradually have a widening communications audience as new cars are added each year. Critical mass is probably a decade or more away.

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