## 1 Introduction
In a unipolar optical communication system, on-off keying (OOK), pulse position modulation (PPM), differential PPM (DPPM), multipulse PPM (MPPM), digital pulse interval modulation (DPIM), and dual-header pulse interval modulation (DH-PIM) are widely used as modulation techniques, and prime code (PC) and optical orthogonal code (OOC) are widely used as spreading code techniques [29; 30; 31]. In this paper, we investigate the characteristics of various modulations and spreading code methods that can be used for the prototype LIDAR with pulse coding and compare various characteristics of LIDAR according to the combinations. The prototype LIDAR system uses a unipolar optical digital modulation scheme and spreading code to identify pixel locations and determine the distance to an object [16; 17]. The number of pulses, pulse peak power, average signal-to-noise ratio (SNR), maximum measurable distance, accuracy and precision of the measured distance, and system error probability vary depending on the combination of modulation scheme and spreading code scheme used. At each pixel, the prototype LIDAR system generates pixel information to identify the  
Figure 1: Overall architecture and operation flow of the proposed scanning light detection and ranging (LIDAR) system. A/D: Analog-to-digital; CRC: Cyclic redundancy check; MEMS: Microelectromechanical systems; PIN: Positive-intrinsic-negative; RX: Receiver; ToF: Time-of-flight; TX: Transmitter. measuring point and emission time. Pixel information is represented by a nine-bit stream consisting of a leading or trailing '1', a five-bit column identification number (CID), and a three-bit cyclic redundancy check (CRC) checksum. The CID represents the locations of corresponding pixels for each measurement angle and identifies each of the 30 columns from a 30 \(\times\) 30 range image. The pixel information is converted to a sequence of pulses by the selected optical modulation scheme and spreading code technique. The transmitter adjusts the angle of the MEMS mirror based on the pixel information, emits and deflects the optically modulated and spread laser pulses in the desired bearing direction, and simultaneously records its row identification number (RID), CID, and emission time. The receiver collects and digitizes the reflected pulses, and then despreads and demodulates them to the pixel information. The prototype system has its operation timing adjusted so that CIDs are not overwrapped on reception. Through the receiving process, the CID included in the received reflected pulses can be used to identify the RID and emission time [16]. A prototype LIDAR system comprises commercial off-the-shelf (COTS) products [32], such as an optical modulator module, an amplified photodetector module, an MEMS mirror development kit, an ADC evaluation module, a digital signal processor (DSP) with an ARM processor evaluation kit, and a Windows PC. We used an OPM-LD-D1-C digital high-speed pulsed laser generator as the optical modulator [33], which is designed for systems that require high-speed transmission and operates at up to 1 GHz, with a peak current of 500 mA and a peak optical power of 250 mW. The coded laser pulses were deflected and steered in the desired measurement angle using a two-axis MEMS mirror from Mirrorcle Technologies, Inc. [34; 35] that has a aluminum-coated mirror with a diameter of 1.2 mm. An ET-4000 AF from EOT [36; 37] that operates at frequencies of up to 9 GHz was chosen for the high-speed amplified positive-intrinsic-negative (PIN) gallium arsenide (GaAs) photodiode equipped with a transimpedance amplifier (TIA) that senses light levels as low as 100 nW. We selected an ADC12J4000 from Texas Instruments (TI), which is a 12 bit, 4 GHz radio frequency-sampling ADC with a buffered analog input [38; 39]. The outline of the paper is organized as follows. In Section 2, the popular unipolar optical digital modulation schemes and spreading codes are reviewed.