Research On OFDM-NOMA Technology Of Visible Light Indoor Communication

Indoor visible light communication（VLC）technology can use light-emitting diode（LED）as information carriers to communicate in the band of 380nm～780nm.Compared with RF wireless communication,VLC can provide ultra-high spectrum communication bandwidth,which is regarded as the development direction of the new generation of communication technology.Because of its great excavation value,it has been widely concerned by scholars at home and abroad,but at the same time,there are also many key technologies need to be solved.On the one hand,VLC channel is prone to multipath interference,which has great influence on high-speed communication.On the other hand,leds have a narrow modulation bandwidth,which limits the maximum capacity of a VLC system.In view of the above problems,this paper introduces orthogonal frequency division multiplexing（OFDM）technology and non-orthogonal multiple access（NOMA）technology into VLC system respectively,and uses the advantages of OFDM to overcome the multipath interference brought by VLC channel,at the same time,OFDM can be combined with NOMA multiplexing technology to improve the number of multi-user access,so as to improve the maximum capacity of VLC system,this paper proposes to study the OFDM-NOMA technology of visible light indoor communication.The main work of this paper is as follows:（1）An efficient OFDM modulation technique is introduced to overcome multipath interference in VLC system.Firstly,the basic principles and signal characteristics of different OFDM technologies are introduced,and the main problems of using OFDM technologies in visible light communication systems are analyzed.Then,in view of the problems of high complexity,extra delay and error propagation caused by serial interference cancellation（SIC）technology in hybrid asymmetrically clipped optical OFDM（HACO-OFDM）,this paper proposes a method to improve the performance of HACO-OFDM by using predistortion technology,when the bit error rate of HACO-OFDM is also 10^-6,the proposed method improves the signal-to-noise ratio by 4d B compared with the traditional SIC technology.Finally,the algorithm simulation is carried out to compare the performance of different VLC-OFDM,which provides theoretical support for selecting specific modulation mode of OFDM.（2）NOMA multiplexing technology is introduced into VLC system to improve the throughput of VLC system.Firstly,the basic principle,key technology and main problems of VLC-NOMA are introduced.In order to further improve the performance of NOMA system,this paper proposes a predistortion HACO-OFDM-NOMA technology,and then describes the data processing process of the predistortion HACO-OFDM-NOMA downlink and uplink link.Finally,the algorithm simulation is carried out under the indoor visible light channel condition to analyze the performance of the pre-distortion HACO-OFDM-NOMA system,the simulation results show that the signal to noise ratio of the proposed pre-distortion HACO-OFDM-NOMA system is improved by 3d B compared with the traditional HACO-OFDM-NOMA system.（3）A communication system with predistortion HACO-OFDM modulation is designed by FPGA.Firstly,the hardware circuit design and program design of the transmitting and receiving end of the predistortion HACO-OFDM system are described.Then the discrete circuit of the hardware built by the system is packaged and integrated into two different boxes.By using the light source to send out waveforms of different frequencies,the communication bandwidth of LED is tested and the optimal frequency range of communication is obtained.Then,the theoretical model of predistortion HACO-OFDM is established by MATLAB,and the data is compared with the system model built by FPGA to verify the correctness of the predistortion HACO-OFDM system model built by FPGA.Finally,the performance test is carried out in the actual scenario,and the results show that the communication system designed in this paper has better bit error rate performance without any optical devices.