# Phase-Modulated Continuous-Wave Coherent Ranging Method and Anti-Interference Evaluation
## 1 Introduction  
Light detection and ranging (LiDAR) has garnered considerable attention because of its excellent performance [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. Compared to its counterpart operating at radio frequencies, LiDAR can achieve long-range, superior accuracy and angular resolution owing to the short wavelength of light [1]. LiDAR devices have been applied in many areas such as autonomous vehicles, robotics, and drones. In particular, autonomous vehicle technology requires an eye-safe, anti-interference, and low-cost ranging method. The ranging mechanism is a core technology of the LiDAR system [11], and the time-of-flight (ToF) ranging method based on pulsed light has been the dominant technology in commercial lidar applications because of its simplicity and ease of implementation [2; 5; 12; 13; 14; 15]. However, the detection range of the ToF is primarily limited by eye safety issues [16]. In addition, to improve the precision of the lidar system to the millimeter scale, the modulation bandwidth must be at the GHz level [6; 17]. Such high-level modulation devices are difficult to use in commercial applications. Therefore, much research interest has moved on to frequency-modulated continuous waves (FMCW) [7; 18; 19; 20]. However, FMCW is plagued by low stability, high complexity, and high insertion loss of the frequency modulation source [21; 22; 23; 24]. Consequently, the use of FMCW is impractical for commercial applications. Thus, LiDAR requires an easily achievable, cost-effective, high-performance ranging method. Moreover, in realistic applications, LiDAR systems are encountering situations wherein numerous LiDARs operate simultaneously and a potential interference is inevitable [25; 26; 27]. Therefore, immunity against interference is paramount in LiDAR systems [26]. However, the ToF is notorious for its lack of anti-interference capability [12; 15; 26]. Several studies have focused on the modification of sensors in ToF ranging systems. [PERSON] et al. [28] used an internal interference light reduction structure on the sensors to reduce the measurement error. [PERSON] et al. [29] used the polarization state of reflected light to reduce the depth measurement error. Moreover, to improve the capability of interference by design, the authors of [25; 30; 31] analyzed different interference modes by modeling the signal interference. However, the problem of being easily interfered with is prevalent in ToF LiDARs. Thus, there is a need for a new ranging method with a much better anti-interference capability than ToF. Meanwhile, in many evaluations of LiDAR's anti-interference capability, the victim LiDAR is placed in an indirect interference environment. [PERSON] et al. [32] performed measurements from a fixed position using multiple LiDARs mounted side-by-side and operated them simultaneously. [PERSON] et al. [31] used a scanner at different angles to modify direct and scattered interference. However, in these reports, the victim and interferential LiDARs were not synchronized in either the time domain or the field of view, implying that the possibility of disturbance caused to the victim LiDAR remains completely uncertain and random. Therefore, the evaluation results were not convincing. To quantitatively analyze the anti-interference capability of LiDARs, a novel test method is required. As for the FMCW, although the coherent ranging method has an intrinsic rejection of interference [19], to the best of our knowledge, there have been no experimental demonstrations. In this study, we demonstrated a ranging method referred to as phase-modulated continuous-wave (PhMCW), which exhibited excellent anti-interference capabilities. First, we proposed a PhMCW ranging mechanism and combined it to set up a ranging system. In addition, we varied the distance of a diffusive reflection board placed at the furthest distance of 50 m, and the ranging error was as low as 2.2 cm. A one-dimensional scanning LiDAR system that is capable of detecting targets at 28 m was built, demonstrating the validation of the PhMCW method.