An FMCW Radar-Based Active Tag Carried by Vulnerable Road Users for Collision Avoidance

Abstract

Traffic accidents involving pedestrians are a serious problem in Japan, particularly for vulnerable road users (VRUs), including pedestrians and cyclists. Furthermore, road conditions, particularly at intersections without traffic lights and minor intersections in residential areas, contribute to accidents. The development of advanced driver assistance systems (ADASs) for drivers continues to advance in addressing traffic accidents. Various studies have been conducted to develop ADAS using multiple devices, including cameras, LiDAR, radars, and other detection devices. Since radar cannot detect small objects such as VRUs, this study proposes VRU radars as active tags that transmit an extraordinarily fast chirp modulation-based frequency-modulated continuous wave (FCM-FMCW) radar signal with long chirp periods, which generates an impulse signal for the car radar. It can detect the presence of VRUs and determine the number of VRUs by counting the impulses on the car radar. The experimental results confirmed that in realistic environments, the radars rarely identify VRUs because the Radar Cross Sections (RCS) of VRUs are significantly smaller than the vehicle targets and surrounding objects on a road. These facts validate the necessity of the proposed concept. According to the experiments using radar evaluation boards, the proposed system does not cause adverse effects on car radar. In this paper, the range and spatial spectra with and without interference mitigation are compared. A radar evaluation board can generate impulses at a maximum distance of approximately 22 meters away, which is obtained by varying the distances between the car radar and VRUs radars. The violin plots over a certain number of frames can be used as a metric to identify the presence of VRUs and the number of VRUs.