Electronics and Semiconductors

Wireless coexistence challenges in C-V2X

10 November 2021
A rendering of how connected cars may be able to communicate with one another on the road. Source: AdobeStock

The automotive industry has undergone its own digital transformation to the point where electronics and development costs are the major cost drivers of new vehicles. Since this is the case, designers should not be surprised there is a semiconductor shortage, especially now as EMS companies scramble to transform their supply chains from just-in-time to just-in-case inventory systems. Within the automotive industry, semiconductor shortages may only be transitory until new semiconductor manufacturing capacity comes online. However, there has been one theme that has stayed with the industry and has guided its digital transformation: wireless coexistence and advanced connectivity between vehicles.

More wireless protocols, both within a vehicle and outside a vehicle, brings coexistence challenges that are often discussed in the context of IoT and WLAN. Common consumer networking protocols like Wi-Fi and Bluetooth now comfortably operate alongside a mobile connection within some vehicles, but the next wave of challenges focuses on implementing these protocols alongside a wireless standard for inter-vehicle and infrastructure connectivity. Now that dedicated short-range communication (DSRC) is de facto dead in the water, cellular vehicle to everything (C-V2X) is emerging as the new paradigm for implementing wireless connectivity between vehicles and with infrastructure.

From DSRC to C-V2X

The story of connected vehicles spans back as far as December 2003, when the FCC allocated 75 MHz of the ~6 GHz spectrum (5.85 to 5.925 GHz) to the dedicated short-range communications (DSRC) standard, or IEEE 802.11p. This variant of the original Wi-Fi standard was implemented in demonstrations and test units, but it was still too early to implement an 802.11 standard or its supporting technologies at large scale.

In October 2020, the FCC reallocated 5.85 to 5.895 GHz of spectrum to an unlicensed band, while the rest of the original DSRC spectrum was allocated to newer C-V2X, which is standardized in 3GPP Release 14. The 5.9 GHz portion of spectrum allocated to C-V2X is now being designated as the intelligent transportation system (ITS).

This move was partially in response to the implementation of 5 GHz Wi-Fi/WLANs, and the high end of the 5 GHz band unfortunately overlaps with the DSRC allocation. The other driving factor is emerging 5G mobile infrastructure, which now has the capability to provide the data rates required to implement new services for connected vehicles.

Both protocols have their role to play in enabling fully autonomous vehicles, although C-V2X fills a major connectivity gap left by DSRC, the auto industry can be expected to act as the main driver of technologies that implement this protocol. For designers who are unfamiliar with the frequency allocations in C-V2X and 802.11 standards, there is a coexistence challenge to be solved in order to enable comprehensive in-vehicle, inter-vehicle and infrastructure networking over multiple wireless protocols.

The C-V2X coexistence challenge

C-V2X operating in the direct mode must coexist with the upper end of the highest 5.8 GHz Wi-Fi band, as shown in the diagram below. Note that the relevant upper end of the 5.8 GHz region is an unlicensed band with an entire channel’s worth of overlap. The simplest solution is to apply filtering in the relevant channel that will be used in the C-V2X system. Tight roll off is required to ensure a C-V2X communications module will not receive interference from a neighboring channel.

Chipmakers and systems designers now have to go back to the fundamentals of coexistence to ensure multiple signals within a single band or in neighboring bands will be reliably received with low latency and low interference. Due to very tight roll off at the edges of the ITS band, some simple filtering solutions should be implemented on feedline sections, as well as in the RF front end. Multiplexing/multiple-access solutions implemented at the hardware level are also needed.

In addition to chipsets implementing simple solutions like TDMA/FDMA/FHSS communication, overlap in adjacent wireless channels is most easily solved with the right RF bandpass filter operating at high frequencies. For the automotive environment, acoustic wave filters are an ideal solution that can operate at high frequencies and high Q-values, exactly what’s needed to prevent overlap between neighboring bands. In particular, commercially available bulk acoustic wave (BAW) filters are preferred for the types of coexistence problems seen in C-V2X as they have sufficiently broad bandwidth to cover the ITS band. Today’s component options comfortably operate in the 5.9 GHz band with a smaller form factor than a printed filter element, and we can expect newer components to provide filtration into 6 to 7 GHz bands to enable continued coexistence with newer generations of Wi-Fi.

Beyond consumer autos

One of the major (and probably earlier) use cases and implementations of autonomous vehicles will likely be in commercial freight vehicles. Freight operators are relying more on telematics systems to monitor usage, vehicle health, driver behavior and trip progress, all with the goal of increasing vehicle efficiency, ensuring safety, and decreasing delivery times. Multiple wireless options will be implemented in consumer vehicles and in freight telematics systems beyond C-V2X, Wi-Fi, Bluetooth and DSRC. Such additional protocols include:

  • Global satellite navigation protocols (GPS, GLONASS, GNSS)
  • ISM-band protocols for short-range communication, including in-vehicle networking
  • Sub-1 GHz protocols for long-range communication (e.g., LoRaWAN or ZigBee)

Specialty chipsets and highly integrated processors are available that implement the required coexistence in multiple bands, supporting standard Wi-Fi/Bluetooth coexistence and ISM band communication. These systems can handle the bulk of the communication on one board, while cellular functions might be implemented in another package using a standard modem on a dedicated module. Advanced systems-in-package are also available with MCU architecture but with on-board LTE capabilities. Overall, these new chipsets can provide higher levels of integration in smaller packages, leaving room for hardware-level solutions to coexistence problems.

As new technologies like C-V2X proliferate, expect more of the big semiconductor industry players to respond with newer chipsets that target this application. Other MCU options are available from major vendors, and startups are entering the C-V2X arena to offer their own chipsets to help solve these coexistence problems. Designers should expect the same solutions to be scaled into 6 to 7 GHz for C-V2X over 5G.

To contact the author of this article, email GlobalSpecEditors@globalspec.com

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