Designing for V2X Communication: Wireless Protocols and Standards
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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 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).
Transition Away From DSRC
There are two standards that formed the starting point for component makers and systems designers to build V2X modules. The original V2X communication standards being pursued by the automotive and telecom industries were Dedicated Short-Range Communication (DSRC) and Cellular V2X (C-V2X).
The story of DSRC-capable vehicles begins back in 1999, when the US Federal Communications Commission (FCC) allocated 75 MHz of the ~6 GHz spectrum (5.85 to 5.925 GHz) to DSRC-capable hardware. This is codified in IEEE 802.11p, which is a variant of the original WiFi standard. Unforturnately for DSRC, this wireless technology was still too early to implement at scale, despite efforts from semiconductor providers. The lack of infrastructure to support trasnmission/reception and backhaul in this frequency range at scale inhibited widespread usage.
The competing C-V2X standard was codified in 3GPP Release 14. Then in October 2020, the FCC reallocated 45 MHz (5.85 to 5.895 GHz) of the original DSRC spectrum to an unlicensed band (WiFi), and the remaining 30 MHz of the original DSRC spectrum was allocated to C-V2X. Finally, earlier this year, a US court ruled that the FCC can complete its reassignment plan despite objections from two automotive industry groups. The 5.9 GHz portion of spectrum allocated to C-V2X is now being designated as the intelligent transportation system (ITS).
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 original requirements in DSRC systems:
- Frequency: 5850 to 5925 MHz
- Modulation: BPSK/QPSK/16QAM/64QAM
- Data Rate: 27 Mbps
- Maximum Latency: ~150 ms
There are still many roadside units that are equipped with DSRC, but the recent UAS court ruling in favor of the FCC may leave DSRC as a zombie technology that persists with no vendor or developer support. For years, there was confusion around these protocols due to lack of guidance from the FCC and NHTSA, which is in turn due to lack of guidance from the US automotive industry. The NHTSA doesn’t endorse or require DSRC in new vehicles, mentions of the protocol remain scattered throughout the agency’s website.
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
C-V2X is implemented by placing a transceiver module on the PCB (see below for a short list of vendors). The PCB designer must design the board to support a cellular modem as well as any other wireless protocols that will be integrated into the board. A few of the practices used in cellular phones can be implemented to support C-V2X, namely placement of the important RF components away from the digital section in the PCB layout.
In the past, modules supporting a technology like C-V2X would have patch antennas placed on the PCB. These would take up a lot of space, and there are newer antenna modules available that can be placed on a board. Another option is an external modular that connects to the board through a coaxial cable.
Here, we have the typical signal RF integrity challenges involved in routing a wireless protocol on a PCB. 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 into the cellular module.
- Amplifier and antenna matching. The output from the trnasceiver module will include a power amnplifier operating near saturation and the power amplifier needs to be impedance matched to the antenna within the relevant bandwidth (50 Ohms normally). Highest precision may be achieved through load-pull analysis for amplifier impedance matching, although a model for the output RF pin would neeed to be known to do this.
- Antenna placement. The antenna will generally be placed near the edge of the board if it is a printed element or an antenna module. There could also be a coax connector that brings signals to/from an external antenna, such as the shark fin antenna used for GPS reception.
Finally, as is the case with many other RF systems, don't forget the importance of ground. Until 5G-capable C-V2X enters mmWave bands, routing will be fine on the surface layer, which means ground is needed on the next layer to set impedance and isolate board sections. This is best done with a thin laminate on the outer layer. Copper pour region can then be used on the top layer to form coplanar waveguide routing for the main antenna feedline, such as with the printed antenna example shown below.
Components Supporting C-V2X
C-V2X is not the only wireless protocol that is required in vehicles in order to provide connectivity with infrastructure, pedestrians, and other vehicles. New vehicles will likely incorporate multiple wireless protocols in addition to cellular:
- Global satellite navigation (GPS/GNSS)
- 2.4/5 GHz wireless communication for in-vehicle networking
- Sub-1 GHz protocols for long-range communication (e.g., LoRaWAN or ZigBee)
Of course, GPS/GNSS is already used in vehicles for navigation. The other points are less common, except for Bluetooth so that drivers can connect their car to their phone. More advanced systems can use 2.4 GHz or 5 GHz wireless to eliminate cable runs in the vehicle; one of these applications is wireless battery management system (BMS) for electric vehicles.
Some semiconductor vendors are offering cellular modules that can support C-V2X technologies with integrated GPS/GNSS reception. Some of these vendors include:
- Nordic Semiconductor
- Sierra Wireless
The Quectel modules have recently become very popular. These modules are available as add-in cards with M.2 connector form factor, or as land-grid components that are surface-mounted on the a PCB. Alongside a BLE/sub-1 GHz capable MCU, designers can cover the entire range of wireless technologies envisioned in C-V2X.
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’ve finished your design, and you want to release files to your manufacturer, the Altium 365™ platform makes it easy to collaborate and share your projects. Come see the newest feature releases in Altium Designer.
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