# Adaptive Suppression Method of LiDAR Background Noise Based on Threshold Detection
## 2 Influence of Background Noise  
Noise in the LiDAR system is caused by the superposition of various noises, which can be divided into internal device noise and external noise according to the source of noise [24]. Background noise at the receiving end includes signals irradiated or reflected onto the detector from undesired radiation sources such as the sun and artificial light sources [25]. In daytime photoelectric detection, external background noise is usually far greater than the noise of the internal device of the system, the radiation source is mainly the sun, and the energy is concentrated in the visible and infrared spectrum [26]. According to ASTMG173 solar spectral data, within the spectral response range of LiDAR, the corresponding irradiance of direct sunlight is high [27]. Therefore, when LiDAR scanning works under strong sunlight, its instantaneous receiving field of view changes rapidly, and the background noise in different fields of view varies greatly. As shown in Figure 1, sunlight enters the receiving end through target reflection and atmospheric scattering, and the background light power affects the detection of weak signals by LiDAR. Under uniform atmospheric conditions, the irradiance E\({}_{\lambda}\) of sunlight on the ground can be obtained by integrating the spectrum of sunlight near the ground. Letting the sunlight be a beam of parallel light, the angle between the light and the target surface normal is \(\theta_{i}\), the atmospheric one-way transmission is \(T_{r}\), the target reflection coefficient is \(\rho\), the receiving field angle is \(\theta_{r}\) and the angle between the receiving optical axis and the target surface normal is \(\phi\). Then, the background radiation power received by the detector is as follows [28]:  
\[P_{b}=\frac{\pi}{16}\eta_{r}\theta_{r}^{2}D_{r}^{2}\Delta\lambda[T_{r}\rho E_{ \lambda}\cos\theta_{i}\cos\phi+\pi N_{\lambda}(1-T_{r})] \tag{1}\]  
where \(D_{r}\) is the receiving optical aperture, \(n_{r}\) is the optical transmission of the receiver, \(\Delta\lambda\) is the spectral bandwidth of the filter, and N\({}_{\lambda}\) is the radiance brightness of the solar spectrum scattered by the atmosphere. As shown in Figure 2, a simple building wall is used as an example to simulate Equation (1) to understand the variation in the background noise at each angle at the receiving end in the detection field of view. We can assume that the sunlight is incident on the wall of a building perpendicular to the ground normally at an angle of 60\({}^{\circ}\), and the LiDAR is parallel to the opposite side. The horizontal field of view detected by the laser radar is \(\pm 35^{\circ}\), the vertical field of view is 30\({}^{\circ}\), the optical transmittance is 0.8, the receiving field of view angle is 2 mrad, the receiving optical aperture is 50 mm and the filter bandwidth is 20 nm. The one-way transmittance of the atmosphere is 0.99, the reflection coefficient of the wall is 0.5 and the radiant illuminance of the sunlight on the ground is 1000 W/m\({}^{2}\)/um. The simulated background noise variation of the building wall in the scanning field of view is shown in Figure 3.  
Figure 1: Effect of background radiation on LiDAR. Figure 2: Diagram of background radiation simulation. For LiDAR with APD detector, all noises are independent of each other [24], and the value of the received total noise current \(\mathrm{i_{n}}\) is:  
\[i_{n}^{2}=i_{ns}^{2}+i_{nb}^{2}+i_{nd}^{2}+i_{nt}^{2}+i_{Na}^{2} \tag{2}\]  
where \(\mathrm{i_{ns}}\) is the signal light shot noise current, \(\mathrm{i_{nb}}\) is the background optical shot noise current, \(\mathrm{i_{nd}}\) is the dark current noise, \(\mathrm{i_{nt}}\) is the thermal noise current and \(\mathrm{i_{Na}}\) is the total equivalent input noise current of the amplifier.