Designing for V2X Communication: Wireless Protocols and Standards
If you’re waiting for truly connected cars on a grand scale, there is still a massive amount of work to be done, both on the hardware and software sides. Connected cars can only become a widespread reality once the automotive industry and telecom carriers can decide which protocol will work best for vehicle-to-everything (V2X) communication. PCB designers will then need to step in to create these systems and fit them into a vehicular environment.
Among the various standards component manufacturers and PCB designers need to concern themselves with, the key wireless standards are defined in IEEE 802.11p and 3GPP standards for cellular communication. Although we’d ideally like to design with a single standard, WLAN-based standards and cellular have different advantages for connected vehicles. There is still some debate as to where each protocol will be best used, or whether everything will just go cellular once 5G is widely rolled out. No matter which protocol wins out and becomes the dominant wireless standard for connected vehicles, we’re here with V2X communication design guidelines for your system.
The V2X Communication Ecosystem and Standards
The current V2X wireless environment is meant to support long-range and short-range communication, which is where cellular and 802.11p will likely become most useful, respectively. Currently, there are municipal-level trials of cellular V2X (C-V2X) networks for interfacing between cars and surrounding infrastructure. Car companies like Toyota and American OEMs are embracing C-V2X as the primary communication channel for establishing vehicular ad-hoc networks (VANETs).
The other V2X communication route can be designed under 802.11p, with dedicated short-range communication (DSRC) being embraced by Volkswagen. Toyota is tepid on this protocol and it will no longer include DSRC chipset in its cars as of 2021, and the FCC still needs to decide whether to allocate more spectrum to DSRC since the previous update in October 1999. Currently, a 75 MHz portion of the 5.9 GHz band has been set aside for DSCR in the US; the EU has also allocated 30 MHz in the same band for intelligent transport systems (ITS).
These standards are the starting point for component makers, which will then determine how designers will need to layout PCBs for V2X systems. Let’s take a look at the requirements in each type of system:
Designing for DSRC
If you’re familiar with 802.11 standards, then you know that these are the standards dedicated to products marketed as WiFi. The 802.11p standards include the following requirements for DSRC systems:
- Frequency: 5850 to 5925 MHz
- Modulation: BPSK/QPSK/16QAM/64QAM
- Data Rate: 27 Mbps
- Maximum Latency: ~150 ms
Another useful feature is power over Ethernet, making these systems easily interoperable with automotive Ethernet. These modules are smaller multiboard RF systems with a digital section for interface with the rest of the in-vehicle network. The RF section with the transceiver/emitter/receiver section can be placed on its own board with a small connector and cable, which is then housed in a radome (this is the approach taken in automotive radar modules).
Here, we have the typical signal integrity challenges involved in routing WiFi boards. These include:
- Isolation. Beamformed systems and mixed-signal systems need to be isolated, both in terms of return paths and crosstalk from the digital section to the analog section, and in terms of antenna-to-antenna isolation (e.g., with a phased array). This is aided by using grounded coplanar waveguide traces or substrate integrated waveguide routing.
- Controlled impedance. Your traces need to be designed with consistent impedance within the relevant bandwidth. It’s a good idea to design for flat bandwidth 25% above and below the 5.9 GHz band edge.
- Amplifier and antenna matching. The Tx power amplifier needs to be carefully matched to the antenna, requiring precise antenna layout and load-pull for amplifier impedance matching.
Placing the transceiver and antenna section on its own board naturally satisfies the isolation requirements as you’ll be taking advantage of at least one ground plane between the antenna section and the digital section. WiFi designers and microwave designers will already be familiar with the appropriate layout choices. Things get more complicated once we look at cellular V2X communication:
Designing for Cellular V2X
Under the C-V2X Rel. 14/15 standards (for up to 4G-LTE) and Rel 16 standards (for 5G NR), we have the following requirements for cellular V2X systems:
- Frequency: Varies by carrier and 5G rollout
- Modulation: QPSK/16QAM/64QAM for 4G-LTE, OFDM (QAM + FDM) for 5G
- Data Rate: 50 Mbps peak download
- Maximum Latency: <50 ms
For the PCB designer, creating a system for cellular V2X communication is all about designing your board to accommodate a cellular modem. Here, you’ll need to worry primarily about isolation as you’ll be placing a powerful RFFE near your digital processing block. In other words, you’re basically designing a PCB for a cell phone to fit in a car, and the same signal integrity challenges in DSRC systems apply here.
Isolation between a modem and the rest of your board is all about gridding with ground pour, adding shielding, or using other isolation structures at higher frequencies, just as is done in a typical cell phone. Once you get to 5G, you’ll be using an antenna array for beamforming to communicate with the network, and isolation between antenna elements and the rest of the system becomes more important.
Different Protocols, Different Uses
One reason for the apparent confusion around these protocols is lack of guidance from the FCC and NHTSA, which is in turn due to lack of guidance from industry. American municipalities and OEMs are watching whether the FCC will allocate more spectrum to DSRC, which will in turn guide what happens overseas. The NHTSA doesn’t endorse or require DSRC in new vehicles, mentions of the protocol remain scattered throughout the agency’s website.
With all the confusion, it’s up to PCB designers to build systems that can prove the viability of each type of protocol. Some component manufacturers (e.g., NXP) are taking a software-defined radio approach, where the RFFE on a single radio unit can be configured to switch between these protocols as needed. This makes new systems adaptable to a range of environments and compliant with multiple municipalities, although it makes PCB design and layout more difficult; I’ll discuss this aspect of designing for software-defined radio in a future article.
Designing for V2X communication is easy when you use the right PCB design and analysis tools. Altium Designer® includes everything you need to design and model V2X systems, and you can quickly create a new PCB layout with integrated schematic capture. When you need to collaborate with others on your design team, you can share and track revisions through the Altium 365® platform, allowing designers to work from home and reach unprecedented levels of efficiency.
We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.