Research On Performance Of SCMA-based Visible Light Communication System

With the advent of the 5G era,smart phones,tablets,smart homes and various Internet of things devices are becoming more and more popular,and their demand for data rate and bandwidth is becoming higher and higher.In the face of these increasing access users,wireless spectrum resources are increasingly strained.Due to the continuous adjustment of the global lighting market structure in recent years,light emitting diode(LED)with small volume,easy installation,energy saving and long service life has been widely used in indoor daily lighting.LED-based Visible Light Communication(VLC)also emerged,which has the advantages of environmental protection,low power consumption and taking into account lighting and communication.However,the development of high-capacity indoor visible light communication system for multi-user access is still limited by LED modulation bandwidth.Multiple access strategy enables VLC to provide multiple services for multiple users at the same time.Compared with orthogonal multiple access,the visible light communication based on non-orthogonal multiple access introduces inter user interference at the transmitting.At the receiver,the decoding algorithm with high complexity is used to complete multi-user detection,which greatly improves the overload capacity of the system.Therefore,this paper intends to combine non-orthogonal multiple access technology with visible light communication to increase the number of users that can access under the limited system resources.The main work is summarized as follows:(1)In this paper,the non-orthogonal Multiple Access technology Sparse Code Multiple Access(SCMA)technology is introduced into visible light communication,and the SCMA Code book is modified to meet the demands of VLC signal transmission.Specifically,based on the low dimensional constellations 4-PAM and 4-QAM with better performance,the corresponding relation between input bit sequence and Gray mapping is established,and the rotated high dimensional mother constellations are expanded into sparse vector according to the indicator matrix assigned to users,so as to obtain the real codebook of each user.The simulation results show that compared with the traditional VLC multiple access system,the bit error rate and throughput performance of the VLC-SCMA system using the codebooks in this paper have been improved,which could meet the demands of multi-user access.(2)Aiming at the VLC-SCMA system with high peak to average power ratio(PAPR)problem caused by access to OFDM technology,this paper proposes an improved SCMA codebook scheme,which combines the pre-distortion technology and uses the optimized codebook to improve the SCMA coding module.Specifically for the SCMA mother constellation with multiple constellation points,the high peak power constellation points are mapped to the unit circle or on the unit circle in the constellation,which reduces the number of constellation points at the peak power.For the low-dimensional modulation mother constellation,the mapping factor is introduced to reduce the power of constellation points at the peak power.Simulation results show that this scheme limits the dynamic range of the input signal and reduces the peak to average power ratio of the system effectively while maintaining high bit error rate performance.The work of this paper provides a reference for the research of VLC-SCMA system.