On the Safe Road Toward Autonomous Driving Phase noise monitoring in radar sensors for functional safety compliance

The first approaches to improve vehicle safety were so-called passive safety systems, which did not directly interfere with the driving process but protected the occupants during a crash. In contrast, the first assistance system was the antilock braking system (ABS) successfully introduced in the early 1970s. This active system was developed to avoid an accident by automatically intervening in the braking behavior of the car. At about the same time, the first automotive radar prototype was presented. Since the invention of this very unwieldy radar system, organizations all around the world spent significant efforts in pushing the development of automotive radar systems forward. Today, radar sensors together with ultrasound sensors, lidar, and cameras form the backbone of advanced driver assistant systems (ADASs) as well as autonomous driving (AD), which is in the prototype stage. In particular, because of their robustness against adverse lighting and weather conditions, radar sensors are considered a key technology for modem vehicle safety and comfort systems. Along with the trend toward higher automation, more cars will be equipped with radar sensors in the near future. Because ADASs directly influence the vehicle dynamics, new regulating functional safety (FuSa) requirements, such as the ISO 26262 standard, were introduced. These requirements are mandatory to protect the road users. Modern automotive radar systems make use of the frequency-modulated continuous wave (FMCW) principle. Despite many advantages to pulse-based radars, one of the most limiting factors of an FMCW radar is the phase noise (PN) contained in the transmit (Tx) signal, which significantly affects the sensitivity and range. To fulfill the ISO 26262 standard, it is thus of high importance to monitor the PN of a radar system throughout its whole lifecycle. In this article, we present the most common PN measurement and estimation techniques for CW signals. Further, we address the problem of estimating the PN of an FMCW signal, which is of particular relevance for automotive FMCW radars and the aforementioned monitoring to fulfill the FuSa requirements. Finally, we present state-of-the-art methods for PN estimation and monitoring in automotive FMCW radar systems.