# Phase-Modulated Continuous-Wave Coherent Ranging Method and Anti-Interference Evaluation
## 4 Anti-Interference Capability  
With the increasing applications of LiDAR, numerous LiDARs can be made to operate simultaneously, and potential interference is inevitable. For autonomous vehicles, the anti-interference capability is among the most important issues owing to safety requirements [26]. Therefore, exploring the anti-interference capabilities of LiDAR systems is crucial. By measuring the anti-interference capability of the PhMCW, we demonstrated two sets of experiments that can quantitatively analyze the anti-interference capability, which can be used in any other LiDAR system. The setup of an anti-interference experiment against ToF is illustrated in Figure 7. The aggressor light source is a 1550 nm pulsed laser triggered by an AFG. It is aimed at the target by a collimator (COL1). The setup of the PhMCW system (victim) is similar to that shown in Figure 2. The minor difference lies in that, instead of being terminated by a balanced photodetector, each output port of the 2 \(\times\) 2 optical coupler (OC2) is further split by a 50:50 optical coupler (OC3 and OC4), resulting in a total of four ports, where port two  
Figure 5: Precision as a function of the measured distance. The orange line indicates the 1/2-power polynomial fitting. Figure 6: One-dimensional scanning results. (**a**) An image of the experimental scene. The colored boxes indicate the corresponding measurement positions. (**b**) The measured results of targets. The points are color-coded by the distance values. and three are connected to the balanced photodetector to detect the IF signal, and port 4 is connected to a single-ended photodetector to monitor the interfered RX signal. A highly reflective metal plate located at 7.10 m is set as the target. The nominal frequencies of both AFGs are 100 kHz. The peak power of the aggressor light pulse is measured to be 100 W to 160 W and the power of the TX signal is maintained at 4 mW. Figure 8a,b shows typical waveforms of the interfered RX signal and the IF signal, respectively. The pronounced peaking in Figure 8a indicates that the aggressor pulses have been received by the victim aperture and are concurrent with the PhMCW detection. As shown in Figure 8b, the aggressor pulse has negligible influence on the IF signal. The minor peaking in the IF signal, as circled in Figure 8b, is due to a minute imbalance resulting from the optical couplers or the balanced photodetectors. We acquired 50 successive RX signals, as shown in Figure 9a. For better visibility, we only show every other four RX signals. It is obvious that the pulsed interference signals gradually advance in time with respect to the IF signals. This is because the AFGs are asynchronous and there is a minute frequency offset between them, even though their nominal frequencies are both 100 kHz. The pulse widths of the corresponding IF signals are obtained by the oscilloscope, and we can also conclude, from Figure 9a, that the timing relationship between the interference signal and the IF signal does not influence the ranging  
Figure 8: Waveforms of the interfered RX signal **(a)** and IF signal **(b)**. Figure 7: A schematic of the experimental setup demonstrating the anti-interference capability against ToF. AFG: arbitrary function generator; PLD: pulsed laser; COL: collimator; LD: laser; EDFA: erbium-doped fiber amplifier; PM: phase modulator; OC: optical coupler; CIR: circulator; BPD: balance photodetector; PD: photodetector; and OSC: oscilloscope. result. Figure 9b shows the ranging precision as a function of target distance with and without the interference signals. There is no significant difference between the two curves, suggesting the capability of anti-interference against ToF LiDARs. In order to reach a more quantitative conclusion, the Ljung-Box test is performed [34].