12-2018

mmWave Automotive Radar

mmWave Automotive Radar and Antenna System Development As modern vehicle development expands to include more and more sophisticated electronics, automobile manufacturers are equipping their new models with advanced driver-assistance systems (ADAS) to obtain high safety ratings by increasing automotive safety. AWR Application Note National Instruments www.ni.com/awr Figure 1: Different ranges, fields-of-view (FOV), and functions for advanced driver assist systems Most road accidents occur due to human error and ADAS are proven to reduce injuries and fatalities by alerting drivers to and assisting them with a variety of issues, including collision avoidance and low tire pressure using radar technology mostly focused over the 76...81 GHz spectrum. They perform over a range of applications, operating conditions, and object detection challenges in order to provide reliable coverage over the range (distance) and field of view (angle) as dictated by the particular driver assist function. This application note presents some of the challenges behind developing millimeter-wave (mmWave) radar systems and the antenna array technologies for the next generation of smart cars and trucks. Examples will be presented demonstrating how the NI AWR Design Environment platform, specifically the radar design capabilities within Visual System Simulator (VSS) system design software, can be used successfully in ADAS applications. ADAS Technology ADAS is made possible through a network of sensors that perform specific safety functions. Manufacturers are currently implementing these systems based on vision sensor technology and radar systems operating at either 24 and/or 77 GHz. Vision systems detect lane markings and process other visual road information, however, they are susceptible to inadequate performance due to precipitation, particularly snow and fog, as well as distance. On other the hand, long-range radar (LRR) supports multiple functions, comfortably handling distances between 30 and 200 m, and short-range radar (SRR) can detect objects below 30-meter distances. While the 24 GHz frequency band, which addresses SRR detection, is expected to be phased out of new vehicles by 2022, today it is commonly found in hybrid architectures. Meanwhile, the 77 GHz band (from 76 to 81 GHz) supporting LRR is expected to provide both short and long-range detection for all future automotive radars. Figure 1 provides details on short/medium and long-range radar. Technical advantages of the 77 GHz band include smaller antennas (one-third of the size of the current 24 GHz ones), higher permitted transmit power, and, most importantly, wider available bandwidth, which enables higher object resolution. As a result, advances in radar modulation techniques, antenna beam steering, system architecture, and semiconductor technology are driving the rapid adoption of mmWave radar in future ADAS enabled cars and trucks. To manage the adoption of these technologies, radar developers require RF-aware system design software that supports radar simulations with detailed analysis of RF front-end components, including nonlinear RF chains, advanced antenna design, and channel modeling. Co-simulation with circuit and electromagnetic (EM) analysis provides accurate representation of true system performance prior to building and testing costly radar prototypes. NI AWR software provides these capabilities, all within a platform that manages automotive radar product development—from initial architecture and modulation studies through the physical design of 58 hf-praxis 12/2018

RF & Wireless Figure 2: Multiple frequency shift keying the antenna array and front-end electronics based on either III-V or silicon integrated circuit (IC) technologies. The NI AWR Design Environment platform integrates these critical radar simulation technologies while providing the necessary automation to assist the engineering team with the very complex task of managing the physical and electrical design data associated with ADAS electronics. ADAS support includes: Design of waveforms, baseband signal processing, and parameter estimation for radar systems, with specific analyses for radar measurements along with comprehensive behavioral models for RF components and signal processing. Design of transceiver RF/microwave front-end with circuitlevel analyses and modeling (distributed transmission lines and active and passive devices) to address printed circuit board (PCB) and monolithic microwave IC (MMIC)/RFIC design. Planar/3D EM analysis for characterizing the electrical behavior of passive structures, complex interconnects, and housings, as well as antennas and antenna arrays. The connection between simulation software and test and measurement instruments. Radar Architectures and Modulation For adaptive cruise control (ACC), simultaneous target range and velocity measurements require both high resolution and accuracy to manage multi-target scenarios such as highway traffic. Future developments targeting safety applications like collision avoidance (CA) or autonomous driving (AD) call for even greater reliability (extreme low false alarm rate) and significantly faster reaction times compared to current ACC systems, which utilize relatively well-known waveforms with long measurement times (50...100 ms). Important requirements for automotive radar systems include the maximum range of approximately 200 m for ACC, a range resolution of about 1 m and a velocity resolution of 2.5 km/h. To meet all these system requirements, various waveform modulation techniques and architectures have been implemented, including a continuous wave (CW) transmit signal or a classical pulsed waveform with ultra-short pulse length. The main advantages of CW radar systems in comparison with pulsed waveforms are the relatively low measurement time and computation complexity for a fixed high-range resolution system requirement. The two classes of CW waveforms widely reported in literature include linear-frequency modulation (LFMCW) and frequency-shift keying (FSK), which use at least two different discrete transmit frequencies. Table 1 compares the different radar architectures and their advantages and disadvantages. For ACC applications, simultaneous range and relative velocity are of the utmost importance. While LFMCW and FSK fulfill these requirements, LFMCW needs multiple measurement cycles and mathematical solution algorithms to solve ambiguities, while FSK lacks in range resolution. As a result, a technique combining LFMCW and FSK into a single waveform called multiple frequency shift keying (MFSK) is of considerable interest. MFSK was specifically developed to serve radar development for automotive applications and consists of two or more transmit frequencies with an intertwined frequency shift and with a certain bandwidth and duration, as shown in Figure 2 [1]. As previously mentioned, pulsed radars are also widely used in automotive radar systems. Relative velocity can be determined from consecutive pulses using a coherent transmitter and receiver to measure pulse-to-pulse Table 1: Different radar architectures and their technical advantages/disadvantages in target detection, range, robustness, and resolution hf-praxis 12/2018 59