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Mitigating Spectrum Coexistence of V2X Systems with High-Performance Filters

July 29, 2020
Before we can fully realize the safety and efficiency benefits promised by a shift to autonomous vehicles, designers must first overcome spectrum challenges associated with V2X systems.

Wi-Fi and 5G will both be necessary in realizing a future with autonomous vehicles. The challenge lies in how these wireless connectivity technologies will coexist. Spectrum interference between them and other in-vehicle radio systems can inhibit operation and potentially put passengers at risk. This article highlights how high-selectivity filter solutions enable coexistence of V2X with Wi-Fi and electronic toll collection (ETC) in future self-driving cars. 

The Foundation for Vehicular Connectivity

The next generation of autonomous vehicles will continuously capture and interpret real-time data from the surrounding environment for automotive safety and autonomous driving. To navigate without human intervention, data of all types must be shared in real time with vehicles and the surrounding infrastructure. This communication will happen over vehicle-to-everything (V2X) radio systems. A constant stream of information is exchanged between everything from other cars to traffic lights to pedestrians. Cars will have all of the data needed to brake and accelerate safely without driver assistance.

V2X encompasses vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-network (V2N), and vehicle-to-pedestrian (V2P) communications. These systems will not only reduce traffic accidents and injuries, they will also improve global transportation efficiency and dramatically reduce harmful CO2 emissions. Taken together, the positive impact on public health and private safety will be significant.

V2X technology is based on a 5.9-GHz, dedicated short-range communications (DSRC) standard that’s designed for fast-moving objects. It makes establishing a reliable radio link possible in non-line-of-sight conditions. Anything that interferes with this V2X link will limit an autonomous car’s ability to perceive and react to road hazards. This critical element is complicated by the fact that V2X can be either C-V2X (cellular vehicle-to-everything), which uses cellular technology to create direct communication links; or DSRC, which is based on the IEEE 802.11p standard and was, at one time, the only V2X technology available.

Despite different auto manufacturers and countries supporting one standard over another, both utilize the same spectrum to solve the same problem and they can coexist. This means that both standards need high-performance filtering to mitigate the spectrum interference adversely impacting vehicle operation and passenger safety.

Understanding Connectivity Technologies

To better address these coexistence challenges, it’s helpful to understand the different technologies involved in vehicular connectivity and how they interact. Besides V2X, as many as four different types radios may reside in an autonomous vehicle:

4G/5G Cloud Connectivity

  • Vehicle OEM services: Applications would include remotely diagnosing and monitoring car operations, making over-the-air software updates, performing teleoperation, and operating a fleet of shared, autonomous vehicles.
  • In-vehicle experiences: Drivers and passengers would use this type of connectivity to enjoy new in-vehicle experiences, from augmented-reality-based navigation to rear-seat entertainment and music streaming services.

Wi-Fi

  • In-vehicle experiences: Drivers and passengers would use in-car Wi-Fi for ultra-high-definition (ultra-HD) video streaming to multiple displays and screen mirroring from compatible devices and wireless back-up cameras.
  • Automotive dealer services: Wi-Fi would support enabling automatic check-in, diagnostic data transfer, and software updates.

Bluetooth

  • Drivers and passengers could stream high-fidelity music via Bluetooth, as well as benefit from practical services such as using a smartphone as a key fob.

Satellite Digital Audio Radio Services (SDARS)

  • With connectivity to satellite-based radio services, vehicle occupants can connect to their favorite radio broadcasts anywhere.

In some cases, these radio technologies will use adjacent frequency bands, creating coexistence issues.

As an example, Wi-Fi operates in the 2.4-, 5.2-, and 5.6-GHz spectrums. This means that 2.4-GHz Wi-Fi operates between the LTE B40 and B41 bands. The 5-GHz Wi-Fi delivers higher data rates than 2.4 GHz because more channels can be bundled together due to larger bandwidth. However, this interacts with V2X (Fig. 1) when a passenger uses a 5.6-GHz in-car or mobile hotspot. This means radio designers must use the correct filter products—ones that have enough attenuation in adjacent bands to deliver good receiver sensitivity—to ensure relatively low desense to the receiver.

High-Performance FilteringWhy LTCC Isn’t Enough

These different wireless technologies all contribute to an improved in-car experience, but they require multiple radio transceivers operating in close proximity to one another. If the transmit power of one RF chain exceeds the power level of the signal reaching a nearby receiver, it significantly degrades system performance. The resulting receiver sensitivity issues can prevent vehicles from achieving regulatory compliance for safe operation.

Coexistence filters reduce interference issues from these “aggressor signals,” but not all filters that claim coexistence capabilities work in the unique automotive environment. For example, the plot in Figure 2 compares the performance of a B47 bulk acoustic-wave (BAW) filter with a low-temperature co-fired ceramic (LTCC) broadband filter.

The graph shows that the LTCC is only filtering broadband frequencies. The B47 BAW filter offers similar insertion loss as the LTCC filter, but also provides high rejection of the 5-GHz UNII 1-3 bands. The B47 BAW filter can replace the LTCC filter on the Tx/Rx path or be placed on the Rx side to enable V2X coexistence with 5-GHz Wi-Fi. Figure 3 illustrates how the LTCC filter provides no rejection of the UNII 1-3 band.

It’s also worthwhile to compare LTCC and B47 V2X coexistence filters from a systems and implementation standpoint. Figure 4 compares the V2X and Wi-Fi antenna isolation needed to achieve a 1-km V2X link. The plot to the left shows that a V2X system (TCU and active antenna) with only an LTCC filter on the Tx path requires more than 80 dB of antenna isolation. This is difficult to achieve in practice, let alone in a real-world scenario.

The plot to the right in Figure 4 shows a V2X system with a B47 V2X coexist filter in the TCU; the active antenna requires just 15 dB of antenna isolation to achieve a 1-km V2X link. If systems engineers can achieve >20-dB antenna isolation, they may need only one V2X coexist filter in the active antenna. Even if the vehicle doesn’t feature built-in Wi-Fi capabilities, there are other use cases to consider when deciding on filtering solutions. For example, one might be passengers using Wi-Fi hotspots from their mobile devices.

Some optimized filter products on the market use BAW technology to address complex selectivity requirements, from 1.5 GHz up to 6 GHz in standard footprints. These filters also offer a smaller footprint than ceramic filters, which gives systems engineers greater design flexibility.

Coexistence Mitigation with Notch Filters

Yet, even BAW bandpass filters aren’t a complete solution to coexistence issues in the V2X environment. It’s important to also consider the essential role played by notch filters. While the bandpass filter discussed above provides adequate out-of-band rejection to UNII bands, a notch filter will be needed to “notch out” Rx band noise in the V2X band on the 5-GHz Wi-Fi path, thus preventing Rx band noise from coupling back into the V2X system and causing desense issues.

Figure 5 shows the placement of this filter on the 5-GHz Wi-Fi path, and Figure 6 illustrates that there will be up to 18-dB desense in the V2X receiver if the notch filter is not used on the 5-GHz Wi-Fi path. On the same system, there’s almost zero desense when using a well-designed notch filter to leverage the benefits of BAW technology.

Another critical challenge that needs careful attention is the coexistence issues between V2X systems and ETC due to the narrow spacing between V2X and ETC spectrums across North America, Europe, and China.

One way to address this issue is by notching out the ETC spectrum with a properly designed filter on the V2X path. A notch filter at the input of V2X front-end module (FEM) will limit spectrum emissions from the output of the V2X system, thus allowing it to pass the ETC spec of −65 dBm/MHz.

Choosing the Right Filtering Solution

When design engineers choose a filter for their system, the most important parameters to consider are the two that characterize the resonator qualities: quality factor (Q) and coupling factor (k2). A high Q is necessary to achieve the lowest insertion loss, while a high k2 enables wider bandwidth. Technology advances at the resonator level have helped improve insertion loss and high-selectivity performance with wider-bandwidth filter products at frequencies up to 6 GHz to address the industry’s most complex spectrum challenges.

A combination of high-Q bandpass and notch filters offers the most complete solution to coexistence challenges in the design of autonomous vehicles. The seamless coexistence of all technologies on the connected-car spectrum, enabled by advanced BAW bandpass and notch-filtering solutions, will ensure that our increasingly mobile world is safer, more reliable, and more enjoyable.

Ali Bawangaonwala is Head of Marketing, V2X, at Qorvo Inc.