Figure 1 Waymo One Jaguar I-PACE in San Francisco.
Figure 2 (a) Shark fin antenna mounted on an ideal ground plane. (b) Shark fin antenna mounted on a vehicle.
Figure 3 Typical car connectivity module integration.
Connectivity is at the core of the automotive technology revolution, which will enable not only self-driving cars but also new services and features. However, antennas interact with adjacent components and the performance of integrated car transceivers must be evaluated in situ. As a consequence, full-vehicle over-the-air (OTA) testing has become a subject of growing interest in the automotive industry. This article discusses the most recognized measurement methodologies, their implementation and the rationale behind the metrics of interest.
AUTONOMOUS, CONNECTED AND SHARED VEHICLES SHAPE EVOLUTION
A multi-dimensional technological evolution, or revolution, has moved the automotive industry forward in the past decade and is nowhere near completion. A major stir-up for the industry happened early in the 2010s when electric cars were introduced. These vehicles were powerful with enough battery autonomy to satisfy a significant segment of users. Reinforced by the fast-growing costs of fossil fuels and environmental concerns, this boosted the race towards carbon neutrality.
Several prominent advancements in connectivity and autonomous driving already impact the lives of consumers. When walking in San Francisco, it is not rare to see a car from a fully self-driving ride-hailing service, as shown in Figure 1. Also, luxury cars already feature 31 in. 8K screens in response to the needs of premium customers who want flawless data quality. These examples of technical features may seem different, but they are correlated.
As cars become more autonomous, our needs stay the same. We connect to the internet, chat, watch videos, access data or simply entertain ourselves while our self-driving vehicle takes us to our destination. But, how does our vehicle get us to that destination safely? It relies on robust and intelligent sensing devices like radars and lidars, as well as high performance ground communication through cellular and non-cellular services, along with satellite links for navigation and non-terrestrial network connectivity. Connectivity already plays a key role in road safety, with e-call functionality deployed and mandatory in multiple countries.
Connectivity occupies a vital role in the automotive revolution. High performance connectivity in vehicles is a differentiator, creating advantageous features for car makers. In the future, as the number of connected cars increases and embedded RF transceivers perform more safety-critical functions, high performance car connectivity will likely become a matter for network operators and regulators. As a result of this vision, industry efforts are underway to harmonize vehicle connectivity testing methodology that could be part of a homologation process, eventually. Demand for test equipment and solutions is increasing significantly to support these efforts. To address these issues, efforts have focused on vehicle-level OTA measurements in the past five years.
WHY FULL-VEHICLE OTA?
The antenna is a key component of every radio communication transceiver. Car antennas are more integrated, becoming almost invisible for aesthetics, theft prevention or mechanical robustness. Since the antenna radiates and receives electromagnetic energy, coupling mechanisms between the antenna and the rest of the car elements are an unavoidable consequence of tight integration. As a result, a car antenna cannot be designed or validated without considering the actual conditions. Figure 2a shows simulated directivity patterns for a typical shark fin antenna mounted on an ideal ground plane. Figure 2b shows the same antenna top-mounted on a commercial vehicle. Both simulations are far-field patterns at 1.8 GHz calculated with IMST EMPIRE XPU software.1 The peak directivity increases from 5.7 to 7.3 dBi when the antenna is mounted on the roof of the car, indicating the impact of the car/antenna coupling.
This issue involves more than the modification of the radiation properties. Each car antenna is part of an RF system, so qualifying this system without the antenna provides an incomplete, imperfect picture. Conducted testing excludes the influence of coupling between the antenna and the RF boards. It affects the actual operation of the electronics, which typically sees a matched load from the instrumentation instead of the actual antenna impedance. As illustrated in Figure 3, a car has multiple RF systems that perturbate each other and eventually degrade sensitivity. Similar to what the wireless industry adopted more than 20 years ago, major automotive industry stakeholders see OTA measurements in a controlled and repeatable chamber environment as the only approach to accurately evaluate embedded radio module connectivity performance and integration effects. As a benefit, OTA tests replace expensive proving ground tests.
A DE FACTO STANDARD?
Standards, technical specifications or test plans play a critical role in guaranteeing a consistent understanding of test methodologies and interpreting test data. Standards often originate from a growing market need, but long development times create situations where the technology precedes the standard. This describes the full-vehicle OTA measurement standards situation.
The landscape of testing guidances is not an open field, yet there is a dynamic in the industry to rapidly fill the gaps. As cars inherit the communication and localization functions supported by mobile phones, a cornerstone of full-vehicle OTA testing can be found in the CTIA “Certification Test Plan for Wireless Over-the-Air Performance.”2 CTIA has defined the key metrics characterizing OTA performance for wireless devices and put together proven methodologies to assess these metrics for all cellular and non-cellular technologies. CTIA test plans also define metrics and measurement procedures for standalone (GNSS) and assisted (A-GNSS) location-based services. Hardware and software measurement solutions that are compliant with CTIA test plans are state-of-the-art for OTA measurements. This is what all “active” system-level OTA measurements at the vehicle level required: system-level measurements including the transceiver, by opposition to the “passive” pure antenna measurements.
However, CTIA is primarily focused on wireless devices like smartphones, tablets and laptops. Additional considerations are required when measuring a device under test (DUT) as large as a car. These are currently not addressed by the CTIA “Test Plan for Wireless Over-the-Air Performance.”
The 5G Automotive Association (5GAA) identified this lack of vehicle-level testing guidance and worked to produce the “Vehicular Antenna Test Methodology” (VATM) technical report published in August 2021.3 Some of the key elements of this document include:
- Recommended test environments: Anechoic chamber, shielded chamber with a reflective ground plane or open area test sites
- Base methodology: Direct far-field probing or near-field measurements accompanied by near-field to far-field transformation with data measured with spherical, cylindrical or planar scanners with final results translated into spherical coordinates
- Key active OTA system-level metrics
- Required test volume, based on the maximum considered vehicle size and how to validate it using reference antennas
- Measurement methods, including options for conventional or combinational measurements and detailed procedures per wireless technology.
The CTIA “Certification Test Plan” and recommended practices from the 5GAA VATM precisely and thoroughly define how vehicle-level OTA testing should be performed for accurate results. In the absence of a dedicated international standard, this combination of documents is being used as a de facto standard by automotive industry stakeholders to set the requirements of any new full-vehicle OTA test system.
However, China has recognized the strategic importance of defining procedures for full-vehicle OTA evaluations in the competitive race for connected and autonomous cars. In 2022, the National Technical Committee for Auto Standardization initiated a new work item for a local standard using the CTIA and 5GAA documents as input references.4
PRACTICAL IMPLEMENTATION OF VEHICLE OTA
Figure 4 Full-vehicle OTA anechoic test environment.
Even though 5GAA describes alternative test sites, anechoic chamber environments are preferred for the reliability and repeatability of measurements. Figure 4 illustrates a typical setup that meets 5GAA recommendations. In this type of anechoic chamber, the car under test is mounted on a pole rotating in azimuth at a distance from the floor. The measurement antenna is attached at the tip of a gantry arm, rotating in elevation. The combination of the two rotations provides a complete spherical scanning capability.
