Research On Synchronization And Modulation Technology Of OCC Communication System

With the rapid development of intelligent transportation system,research on synchronization and modulation of RF-based wireless communication technology makes travel more convenient.However,RF wireless communication occupies a large amount of spectrum resources,and its channel is susceptible to electromagnetic interference,which makes it difficult to meet the growing communication requirements of people.Optical Camera Communication(OCC),as an emerging optical wireless communication technology that uses Light Emitting Diode(LED)to transmit information and image sensor to collect information,has the advantages of low equipment cost,abundant spectrum resources,high channel capacity and antielectromagnetic interference capability.It has great potential for applications in intelligent transportation,indoor and outdoor positioning,etc.As two key technologies of OCC communication system,synchronization technology and modulation technology have acquired wide attention in OCC-related research.However,there are some shortcomings in the current OCC synchronization and demodulation investigations:(1)In most existing research on OCC synchronization techniques,the receiver and transmitter do not use the same clock,so in order to overcome the impact of clock drift and jitter on system synchronization,the OCC receiver image acquisition frequency is set to two times of the transmitter modulation frequency based on Nyquist’s sampling theorem.However,this method requires high camera sampling frequency and high overhead of image processing algorithm,which increases the camera cost and processing time delay and deviates the transmission efficiency.(2)Pulse Width Modulation(PWM)as a common modulation method can effectively improve the system transmission rate,but the existing PWM pre-modulation research is to extract the luminance in ideal indoor scenes to obtain the uneven duty cycle,and lacks the extraction experiments for longer communication distances in outdoor scenarios with complex background light.Besides,the luminance profile at each acquisition distance is not fully utilized to obtain the average luminance profile as well.In this thesis,synchronization and modulation techniques of OCC communication system are investigated to address the above problems,and the main works and contributions are as follows:(1)Synchronization implementation of OCC system based on Bei Dou satellite navigation.Considering that cars are commonly installed with satellite navigation systems that can provide external synchronization clock signals,the OCC bit synchronization scheme based on Bei Dou satellite navigation system is proposed.Firstly,the problems of OCC system synchronization are analyzed;then the Bei Dou satellite timing module is used as the external synchronization clock of OCC system,which achieves high precision OCC system bit synchronization with almost no increase in hardware cost and avoids the error code caused by bit synchronization problem;then the transmitter modulation program and receiver window opening program are optimized.The results show that the optimized OCC system has zero probability of missed synchronization and false synchronization within 500 Hz of modulation and demodulation frequency at the transmitter and receiver,and the maximum frame synchronization establishment time is one frame structure cycle.(2)PWM modulation and its demodulation implementation for OCC systemIn view of the low transmission rate of the current OCC system,a PWM modulation and demodulation scheme based on the average luminance extraction is studied and implemented.Firstly,the luminance extraction area is determined based on the directional projection method and the system LED imaging characteristics;then the maximum luminance extraction and the average luminance extraction are used to obtain the relationship between the transmitter PWM duty cycle and the receiver LED projection area luminance(Duty Cycle-Luminance)under the outdoor environment with complicated background in experimental tests.The DC-L diagram is selected for the average luminance extraction method without adjusting the aperture and is applied in the mobile environment.The average curve is derived by fitting the nonlinear curve in the DC-L diagram of the average luminance extraction based on the least squares method,and the better duty cycle is extracted as the transmitter PWM modulation duty cycle by equating the average curve luminance through PWM pre-modulation;finally,a segmented area threshold judgment demodulation scheme based on the directional projection method is designed.By experimentally testing the transmission performance of PWM pre-modulation technique,the results show that in the case of 0～40m communication distance,compared with the traditional PWM modulation,the PWM modulation scheme using pre-modulation maintains a lower BER,while having a higher transmission rate compared with OOK modulation,which is suitable for medium and short distance communication scenarios with high data transmission rate and BER requirements.