V2X Communication for Autonomous Vehicles

By European Editors

Contributed By Digi-Key's European Editors

The development of driverless cars is highlighting the need for robust, low latency wireless communications to provide a vast range of functions. There are two different approaches being proposed to address this need, 802.11p and C-V2X:PC5.

Wireless communication can be used from vehicle to infrastructure (V2I), vehicle to vehicle (V2V) and vehicle to pedestrian (V2P), all combined as V2X.

Image of driverless vehicles communicating with each other

Figure 1: Driverless vehicles communicating with each other and to roadside infrastructure is a key element of the rollout of the technology over the next few years. (Source: US Department of Transport)

Wireless technology standardization

Standardization has also been a key issue to allow volume production of wireless transceivers to reduce costs and ensure that vehicles can operate safely in many different markets around the world. This has also raised issues for both sets of technologies. 

While the different approaches may seem to be in competition, the application of the technologies depends on the use cases and the level of interaction that is needed for a particular type of autonomous vehicle. For example, an autonomous truck may have a more limited set of interactions from a taxi that would connect directly to a customer’s cellphone.

The more established technology is 802.11p, a variant of 2.4 GHz Wi-Fi technology that runs in the unlicensed ISM band at 5.9 GHz. This spectrum has been allocated in the US and Europe for Digital Short Range Communication (DSRC) links that provide data from roadside units such as traffic lights and traffic management gantries to a vehicle, as well as some information between vehicles. For example, this can help with traffic flow by letting cars further back on the road know there is a problem ahead and to slow down to avoid accidents. The infrastructure is relatively simple to install and does not rely on a particular operator or cellular costs.

There are two categories of messages, emergency and general information. Safety messages, such as the ability to transmit and receive “emergency electronic brake lights,” is a message transmitted by a vehicle in broadcast mode every tenth of a second to signal the event of emergency breaking with a latency of under 50 ms. General information messages such as “traffic light - optimal speed advisory,” would be to improve traffic flow by using periodic broadcasts to recommend an appropriate speed.

Having to operate in a highly dynamic environment with relatively high speeds between transmitters and receivers, they need to support extremely low latency in the safety-related applications. They also need to tolerate the periodic transmission of multiple messages by multiple vehicles due to congested traffic scenarios.

Another consideration is that V2X messages are local in nature, meaning they are most relevant to nearby receivers. For example, a “pre-crash sensing warning message” is extremely important for surrounding vehicles, but irrelevant to ones that are far away.

Image of types of messages delivered on DSRC

Figure 2: Types of messages delivered on DSRC.

The latest generation of cellular technology, 5G, has been designed by the 3GPP standards group with some of these features in mind as well. It can use more frequency bands than 3G and 4G, and has a focus on low latency so that messages can be sent to a vehicle quickly. This has been a key requirement for any smart vehicle system.

The infrastructure is more expensive to install to support millions of driverless cars, but allowing vehicles to communicate directly with smartphones of passengers and pedestrians is a key V2P technology. A single car broadcasting a V2V Cooperative Awareness Message (CAM) or a Basic Safety Message (BSM) generates about 0.5 gigabytes per month, at a peak rate of 2.5 Kbyte/second. That assumes 256 bytes per message, at five messages per second, and four hours of driving per day. On the receiver side, assuming 30 cars are in the area of interest, the infrastructure has to handle roughly 16 gigabytes per month.

Some chip designers are looking at supporting both technologies for V2X direct communications. Both operate without network infrastructure and both can use the 5.9 GHz spectrum. The Physical Layer (PHY) and Media Access Control (MAC) both benefit from the same automotive investments into upper layers developed in standard development organizations, such as the Society for Automotive Engineers (SAE), European Telecommunications Standards Institute – Intelligent Transport Systems (ETSI-ITS), Institute of Electrical and Electronics Engineers (IEEE) and the International Organization for Standardization (ISO).

DSRC devices are in their third generation of development since the standard was proposed in 2007. The DSRC Task Group in the Wi-Fi Alliance is responsible for DSRC certification, and companies such as Honda are working on ways to allow DSRC systems to link to consumer smartphones via direct Wi-Fi connections.

Both technologies require a new generation of antenna to handle the 5.9 GHz links from suppliers such as Taoglas.

Image of connecting pedestrians to driverless vehicles

Figure 3: Connecting pedestrians to driverless vehicles is a key challenge for future wireless systems.

5G systems for driverless cars

Leading car makers, chip makers and cellular operators have established the 5G Automotive Association to develop, test, and promote 5G systems for driverless cars, and are starting to work on automotive-specific standards and accelerating commercial development.

This cellular-V2X (C-V2X) is defined as LTE V2X in 3GPP Release 14. An additional element here is the specification of longer range vehicle-to-network (V2N) connections that allow cloud services to be part and parcel of the end-to-end solution. This could be enabled by existing 4G cellular or even 3G data modules, such as those from Sierra Wireless, but 5GAA is looking at how this can be integrated with 5G systems for future proofing.

  C-V2X: PC5 802.11p C-V2X: PC5 Advantage
Synchronization Synchronous Asynchronous Spectral Efficiency. Synchronization enables time division multiplexing (TDM) and lowers channel access overhead.
Resource Multiplexing Across Vehicles FDM and Time Division Multiplexing (TDM) Possible TDM Only Frequency Division Multiplexing allows for larger link budget and therefore longer range - or more reliable performance at the same range.
Channel Coding Turbo Convolutional Coding gain from turbo codes leads to longer range - or more reliable performance at the same range.
Retransmission Hybrid Automatic Repeat Request (HARQ) No HARQ Leads to longer range - or more reliable performance at the same range.
Waveform SC-FDM OFDM Allows for more transmit power with the same power amplifier. Leads to longer range - or more reliable performance at the same range.
Resource Selection Semi-persistent transmission with relative energy-based selection. Carrier Sensr Multiple Access with Collision Avoidance (CSMA-CA) Optimiazes resource selection with selection of close to 'best' resource with no contention overheads. By contrast 802.11p protocol selects the first 'good enough' resource and requires contention overhead.

Figure 4: The advantage of 5G cellular technology for driverless vehicles. (Source: 5GAA)

The European Union sees 5G services being introduced in 2018 with large scale commercial roll out by 2020. This may impact current plans for driverless cars to be launched on the roads in 2018 and 2019 when the infrastructure is not available. There may also be changes to the spectrum bands available for 5G to be agreed upon at the 2019 World Radio Communication Conference (WRC-19), which will likely include 5.9 GHz and possibly bands above 6 GHz. These 5G systems are showing promise in trials across Europe.

Real world outdoor trials of 5G in Stockholm, Sweden, and Tallin, Estonia, used 800 MHz of spectrum in the 15 GHz band. During the test, peak rates of 15 GB/s per user and a latency below 3 milliseconds were achieved. This is more than 40 times faster than the current maximum speeds achievable on 4G, and the manufacturer is aiming to roll out commercial 5G services in 2018. At the same time in France, test equipment has shown wireless communication with peak rates beyond 10 GB/s.

Some trials have shown other uses for a 5G connection to driverless cars. In Japan, a 5G network connection uses the 10 GB/s data rates to transfer high resolution images from HD cameras around the car back to a remote center. This allows the remote operator to monitor the vehicle and assist passengers.

Image of 5G technology for driverless cars

Figure 5: Where 5G technology would be used for driverless cars. (Source: 5GAA)

Conclusion

The argument for automotive system developers is that 802.11p is available today. Four “plug-test” events organized by the ETSI have taken place in the last six years, allowing system developers to be confident that a variety of vehicles can connect to the infrastructure. Further, as more driverless cars roll out, the volume of transceivers in vehicles and roadside units will increase, driving down costs.

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European Editors

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Digi-Key's European Editors