How to address spectrum challenges associated with V2X

1) Basic knowledge of vehicle connectivity

For truly autonomous cars to navigate without human intervention, data of all types must be shared continuously and in real time with other vehicles and the surrounding infrastructure.

This will be achieved through the vehicle-to-everything (V2X) communication system. V2X includes multiple levels of communication such as vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-network (V2N) and vehicle-to-pedestrian (V2P).

V2X is based on 5.9 GHz dedicated short-range communications, designed for fast-moving objects. A reliable radio link can be established even in non-line-of-sight conditions. This trusted link enables drivers to be aware of hazards ahead, reducing potential car crashes, fatalities, and injuries.

Additionally, V2X can improve global transportation efficiency and reduce CO2 emissions by warning of impending traffic jams and suggesting alternative routes, with the added benefit of reduced vehicle maintenance.

Realizing the full potential of autonomous vehicles is relatively complex because V2X can be either C-V2X (Cellular Vehicle-to-Everything), which uses cellular technology to create a direct communication link, or DSRC (Dedicated Short Range Communication) based on the IEEE 802.11p standard, which was once the only available V2X technology.

Different automakers and countries support one standard or another, but all utilize the same spectrum to solve the same problems and the standards can coexist.

2) Understand linking technology

To better understand the challenges of achieving signal coexistence, we must examine the technologies and their functions related to vehicle connectivity ( Figure 1 ). Because each technology has its own characteristics, they must interact without degrading the performance of the other technologies.

Figure 1. Connected vehicle technology

These technologies include:

Based on an understanding of the capabilities/benefits of the various technologies, we can better address coexistence challenges—particularly compatibility with 5G and LTE.

5G, the fifth generation of cellular technology, promises higher data rates, reduced latency, and greater flexibility for wireless services. The 5G spectrum is divided into Sub-6 GHz and millimeter wave.

Wi-Fi operates in the 2.4 GHz, 5.2 GHz, and 5.6 GHz spectrum, and 2.4 GHz Wi-Fi must coexist with LTE B40 and B41 bands. With greater bandwidth, more channels can be bundled together in the 5 GHz band, so 5 GHz Wi-Fi can achieve higher data rates than 2.4 GHz. This means that radio designers must use the right filter products—with enough attenuation in adjacent bands to provide good receiver sensitivity—to take full advantage of the wider bands.

When passengers in autonomous vehicles use 5.6 GHz hotspots, a new challenge arises: the coexistence of 5.6 GHz Wi-Fi and V2X ( Figure 2 ). The only way to have a reliable V2X radio link is to ensure that the receiver's sensitivity degradation is relatively low. This can only be achieved with appropriate filter solutions that provide sufficient out-of-band attenuation for 5.6 GHz Wi-Fi ( Figures 3 and Figure 4 ).

Figure 2 V2X and 5 GHz Wi-Fi coexistence

The increasing number of features has increased the number of different radios in the car, with up to 5 radios in a car today (i.e. V2X, 4G/5G, Wi-Fi, Bluetooth, SDARS). This means multiple radio transceivers operating in different frequency bands in close proximity to each other. If the transmit power of one RF chain exceeds the power level of the signal reaching nearby receivers, it can cause receiver sensitivity issues.

Coexistence Filters help reduce interference from these “attack signals” that can cause receiver sensitivity issues and compliance violations. However, not all filters that claim to have coexistence capabilities are suitable for the job. For example, the curves in Figure 3 compare the performance and system impact of a B47 bulk acoustic wave ( BAW ) filter and a low temperature cofired ceramic (LTCC) wideband filter.

Figure 3 Comparison of QPQ2200Q and LTCC: broadband performance

LTCC filters only broadband frequencies. The B47 BAW filter provides similar insertion loss as the LTCC filter, but also brings high rejection performance to the 5 GHz UNII 1-3 band. The B47 BAW filter can replace the LTCC filter on the Tx/Rx path or can be placed only on the Rx side. Figure 4 shows how the LTCC filter has no rejection on the UNI-3 band and poor rejection on the UNII-2 and UNI-1 bands.

Figure 4. Comparison of QPQ2200Q and LTCC: B47 BAW filter suppression of 5GHz UNII 1-3 band.

Next, let’s compare the LTCC and B47 V2X coexistence filters from a system and implementation perspective. Figure 5 compares the V2X-Wi-Fi antenna isolation required to create a 1000 m V2X link. The left figure shows a V2X system (TCU + active antenna) with only 1 LTCC filter in the transmit path, requiring >80 dB antenna isolation, which may be difficult to achieve in real applications. The right figure shows a V2X system with a B47 V2X coexistence filter in the TCU and an active antenna that only requires 15 dB antenna isolation to achieve a 1000 m V2X link. If the design/system engineer can achieve >20 dB antenna isolation, they may only need to install 1 V2X coexistence filter in the active antenna. In addition to in-car Wi-Fi, another use case to consider when selecting a filtering solution is whether the car has built-in Wi-Fi capabilities. That is, the antenna isolation is determined by the passengers setting up a Wi-Fi hotspot with their phones.

Figure 5 V2X – Wi-Fi antenna isolation required for reliable V2X links: QPQ2200Q B47 vs. LTCC

Qorvo's filter products use patented BAW technology; the technology is optimized to meet complex selectivity requirements, and standard packages cover the range from (1.5~6) GHz. For example, the Qorvo QPQ2200Q filter is the world's first filter to solve the coexistence between V2X and 5.6 GHz Wi-Fi for autonomous vehicles. Another example is the Qorvo QPQ2254Q 2.4 GHz Wi-Fi filter, which is designed to coexist with LTE B40 and B41. Such filters occupy a smaller board area than ceramic filters, which increases design flexibility.

However, even BAW bandpass filters are not a complete solution to the coexistence issues in a V2X environment, and we must also consider the important role of notch filters. While the bandpass filters discussed above provide adequate out-of-band rejection, the Qorvo QPQ230Q notch filter “notches” the Rx band noise in the V2X band on the 5 GHz Wi-Fi path to prevent the Rx band noise from coupling back into the V2X system and causing sensitivity degradation, as shown in the system calculator ( Figure 6 ). Figure 7 shows that if a notch filter is not used on the 5 GHz Wi-Fi path, the V2X receiver will experience up to 18 dB of sensitivity degradation; in contrast, a carefully designed notch filter based on the advantages of BAW technology can achieve almost zero sensitivity degradation.

Figure 6 Wi-Fi front-end with V2X notch (QPQ2230Q) on 5 GHz path

Figure 7 Noise and sensitivity degradation in the Rx band with and without the QPQ2230Q notch filter

V2X needs to coexist with the Electronic Toll Collection (ETC) system, which is another key challenge that requires special attention. The problem is that the ETC spectrum (5795~5815 MHz in Europe, 5790~5800 MHz UL and 5830~5840 MHz DL in China) is too close to the V2X spectrum (5855~5925 MHz in North America and Europe, 5905~5925 MHz in China).

One way to address this issue is to notch the ETC spectrum using a properly designed filter on the V2X path.

Now, let’s look at the spectrum situation in China ( Figure 8 ). As shown on the left, ETC cannot coexist with V2X unless this issue is addressed in the ETC radio. If a well-designed filter is used in the V2X radio, the ETC specification margin of -65 dBm/MHz is met, as shown on the right.

Figure 8 Global ETC spectrum and V2X coexistence

Two parameters that characterize high-performance filter products are the resonator quality, namely the quality factor (Q) and the coupling factor (k2). High Q is necessary to minimize insertion loss, while high k2 leads to wider bandwidth. Technological advances at the resonator level help improve insertion loss and high selectivity performance, enabling wider bandwidth filter products at frequencies up to 6 GHz.

The combination of high-Q bandpass and notch filters creates the most complete solution to the coexistence challenges in autonomous vehicle design. Based on the data discussed above, LTCC filters are not true coexistence filters and cannot function in the unique driving environment where Wi-Fi and V2X are adjacent to each other.

About the author: Ali Bawangaonwal, Marketing Executive, has an electrical engineering background and nearly 20 years of industry experience in the technology field. He received his Master of Science in Electrical Engineering from Bradley University and his MBA in Strategy from Elon University. He is a recognized expert in RF front-end semiconductors and currently represents Qorvo at 5GAA.

This article is from the scientific journal "Electronic Products World" Issue 8, 2020.