# Design of Indoor Visible Light Communication PAM4 System
## 1 Introduction  
Visible light communication (VLC) is a new type of wireless communication technology that has emerged in recent years [1; 2]. It uses light emitting diode (LED) light sources for high-speed modulation, uses light as a carrier to transmit signals, and takes into account communication and lighting functions. It has the advantages of energy saving and environmental protection, no electromagnetic radiation, and low cost [3; 4], which has attracted people's attention. In addition to meeting the basic communication, the VLC system must also consider the communication rate and frequency band utilization. With the improvement of the technical level, the demand for high-speed and high-bandwidth transmission systems is gradually increasing, so it is necessary to improve the communication rate while meeting the performance requirements. At present, the indoor VLC system mostly uses on-off keying (OOK), pulse position modulation (PPM), and pulse interval modulation (PIM) [5; 6]; the transmission efficiency of OOK is too low, and it is not suitable for a high-speed transmission system. PPM modulation has good anti-noise ability and has been used in optical communication systems for a long time. The Ke research group at the Xi'an University of Technology systematically analyzed PPM. In recent years, high speed has been the main requirement of the current communication system, and the upper limit of communication rate can be increased by increasing the modulation order. Compared with the above three modulation methods, four-level pulse amplitude modulation (PAM4) can transmit more bit information in the same symbol; its information transmission rate per unit band is high and has higher spectral efficiency [7]. Under the same communication rate conditions, PAM4 can also reduce inter-symbol interference, thereby reducing the bit error rate (BER) [8] and making full use of frequency band resources. Research in the field of communication has achieved results. In 2016, [PERSON] equalization technology implementation of PAM4 signal transmission in 100 m fiber at a transmission rate of 155 Gb/s [9]. In 2017, [PERSON] realized the transmission of laser light in optical fiber at high speed and used PAM4 as the modulation method, and the BER was less than \(10^{-12}\)[10]. In 2019, [PERSON] used PAM4 as the main modulation method for the communication system and combined it with equalization technology. Through experimental verification, a high-speed transmission system based on PAM4 was finally realized [11]. In 2020, [PERSON] designed a dual-mode 32 Gb/s receiver that can simultaneously operate in NRZ and PAM4 modes [12]. In 2021, [PERSON] used the DSP development board to design a VLC system based on PAM4 through the combination of software and hardware. The author successfully completed the communication verification experiment, and the BER met the communication system requirements [13]. The research on PAM4 at home and abroad is mainly developing rapidly in the direction of high speed and high bandwidth, and most of them are used in the field of optical fiber communication. There are relatively few studies on the use of PAM4 technology in indoor VLC. Therefore, the study of the encoding and decoding of the PAM4 signal in indoor VLC systems has certain significance and application value. This paper focuses on the design scheme of the key parts of the system and studies and analyzes the BER performance of the PAM4 signal under different channel noise models. The results show that the BER performance of the PAM4 signal depends on the variance of the noise data. The greater the variance, the worse the BER performance. The BER performance in the case of the multiplicative noise is much larger than the BER in the case of the additive noise. The main reason is that the multiplicative noise is caused by the time-varying and nonlinearity of the system, reflecting the fading of the channel, while the additive noise generally refers to the background noise of the system.