論文

Long-range and high-pixel-resolution LiDAR systems, using Time-of-Flight (ToF) information of the reflected photon from the target, are essential upon launching safe and reliable self-driving programs of Level 4 and above. 200m long-range distance measurement (DM) is required to sense proceeding vehicles and obstacles as fast as possible in a highway situation. To realize safe and reliable self-driving in city areas, LiDAR systems uniting wide angle-of-view and high pixel resolution are required to fully perceive surrounding events. Moreover, these performances must be achieved under strong background light (e.g., sunlight), which is the most significant noise source for LiDAR systems. To accomplish a 100m-range DM, an accumulation of the DM results through several pixels is utilized to improve the S/N ratio with 70klux background light [1]. Here, S is the number of photons reflected from the target and N as the number of background light photons. However, if the range is extended to 200m under similar condition of the laser power and frame rate (FPS), 16x more pixel accumulation is required. Such pixel accumulation leads to blurring the range image, and hence, a serious oversight in the surrounding events, such as a flying-out pedestrian, may occur, not suiting self-driving applications. Furthermore, the Time-to-Digital Converter (tDC) based ToF measurement is activated only when 2 or more photons are detected simultaneously [1], and thus, is not suitable for the 200m long-range DM where few photons are reflected from the target. On the other hand, ToF measurements using ADCs, which can continuously quantize the silicon photomultiplier (SiPM) output and can sense single-photon events, suits long-range measuring purposes well [2]. However, a number of accumulations should still be required to accomplish 200m-range DM, and hence, low resolution is inevitable. In addition, the SoC cost is critical. To enhance the short-range DM resolution by using ADCs, the required sampling rate is over 10GS/s; upon realizing a 20ch AFE, such an ADC array alone may occupy an area of over 10mm2 and consume huge power [3].

This article presents a 40-channel high-resolution automotive LiDAR system-on-chip (SoC), which utilizes the world's first dual-data converter (DDC). The proposed DDC consolidates the functions of ADC and TDC into a single circuitry and achieves acquisition of both high-precision time and voltage data from the input, realizing a 5 times smaller analog front-end (AFE) area than prior arts. Such innovations lead us to a 40-channel AFE integration of the SoC without silicon cost increase, which characterizes our LiDAR system with 2 times higher pixel resolution. To enhance the ADC's signal-to-noise and distortion ratio (SNDR) without sacrificing the area efficiency, a calibration-free and variation-tolerant voltage-controlled oscillator (VCO)-based ADC with a multiphase pulse density modulator (MP-PDM)-based feedback is proposed. Our proof-of-concept LiDAR system achieves a measurement range of 225 m at 70-klx direct sunlight, with a resolution of 240 times 192 pixels at 10 frames per second (FPS). To quantitatively measure the effect of the resolution improvement of our LiDAR, we evaluate the LiDAR's detection accuracy of small targets 75 m away. Due to the 2 times higher vertical pixel resolution, our LiDAR can detect targets with 2 times smaller height. The 3-D point cloud of the LiDAR captured outdoor in the wild is presented.

A safe and reliable self-driving system is a key enabling technology for a society without traffic jams or accidents; LiDAR plays an essential role for such systems. To ensure higher levels of safety and comfort, early detection of small objects (e.g., debris/children) is crucial. To achieve this, state-of-the-art LiDARs [1-3] must attain even more finely scaled pixel resolution: for example, a 0.1-degree angle resolution (a 2x finer resolution than [3]) is required to detect a 20×20cm2 object 100m away. However, hybrid LiDAR systems [3] require a pair of TDC/ADC AFEs per pixel to obtain both precise short-range (SR) distance measurement (DM) and 200m long-range (LR) DM. Scaling the pixel resolution will significantly enlarge the SoC area and explode its cost.