## 1 Introduction
When the pulse repetition period is smaller than the time difference between the reflections, the range ambiguity problem occurs. In order to solve this problem only, increasing the pulse repetition period decreases the range ambiguity by increasing the maximum distance, but increases the idle listening time between the laser pulse transmission and reception and then reduces the number of measurement points, frame refresh rate, and angular resolution per second [9,16,17]. In the pulsed scanning LIDAR system, range ambiguity resolution extends the system's unambiguous range between transmit pulses at a high pulse repetition frequency (PRF). Some studies have focused on solving the range ambiguity of pulse scanning LIDAR by using pulse coding to avoid crosstalk [16, 17, 18, 19, 20] or mutual interference that occurs when two or more LIDARs simultaneously operate [16,17,21]. Such LIDARs measure distances using multiple pulses generated by random sequences [18, 19, 20] or specially designed codes [16,17,21], rather than one pulse per measurement point. By adapting the intensity-modulated direct detection (IM/DD) optical code-division multiple access (OCDMA, a kind of specially designed code) method to the emitted laser pulses of a scanning LIDAR system [16,17], these pulses become orthogonal to other pulses and then they are distinguished from each other [22, 23, 24, 25, 26, 27, 28]. In the receiver, all of the reflected pulses that are received at the same time are individually identified and demodulated into original information by the receiver. Thus, the reflected waves of laser pulses emitted from different measurement points can be received at the same time. As a result, we can ignore the idle listening time and eliminate the range ambiguity problem, and then we can achieve longer maximum distance, high measurement point, high frame refresh rate, and high angular resolution per second at the same time. The proposed scanning LIDAR consists of a transmitter and a receiver, which operate independently of each other, as shown in Figure 1. The transmitter codes the pixel location bit stream through the IM/DD OCDMA technique and generates laser pulses using a laser diode. Subsequently,the transmitter emits the laser pulses generated in a bearing direction using a scanning-based microelectromechanical systems (MEMS) scanning mirror. The receiver digitalizes the received reflected pulses using an analog-to-digital converter (ADC) and demodulates them using the IM/DD OCDMA technique. The receiver then calculates the ToFs of the laser pulses and converts them to distances to the relevant objects based on the emission time of the pixel locations. Even if multiple pulses are used, the parameters that determine the characteristics of the transmitter in the conventional pulse scanning scheme are maintained. To comply with eye safety standards, the pulse peak power is distributed across several pulses so that the energy allocated to a pulse decreases in inverse proportion to the number of pulses, and the time required to transmit pulses at a given measurement point is proportional to the number of pulses [16; 17; 21]. The use of multiple pulses also enhances the accuracy of the distance measurement [17]. In LIDAR with pulse coding [16; 17], the number of pulses used at a measurement point is determined by the modulation method and the spreading code method. The optical channel differs significantly from radio frequency (RF) channels. Unlike RF systems, where the amplitude, frequency, and phase of the carrier signal are modulated, the intensity of the optical carrier is modulated in optical systems. In an optical wireless communication system using unipolar signaling, the numbers and positions of the pulses to be transmitted and empty slots are determined by the modulation and spreading code methods used [24; 27; 29; 30; 31]. To accurately demodulate and despread the signal at the receiver of the code pulse LIDAR, both the numbers and positions of the pulses and the empty slots are used.