Research On Modulation And Demodulation Technology Of Wireless Optical Communication System

Nowadays the spectrum resources have been greatly exploited,which makes the spectrum resources more and more scarce.At the same time,as the enhancement of people's awareness on environment and health,RF radiation hazards to the human body has gradually come into the people's eyesight.All of these prompted people to find a new way of access.At the same time,with the gradual maturity of LED technology and the subsequent application of people's lives,the visible light communication technology is gradually concerned by the researchers because of its rich potential bandwidth,safety and no radiation.In this paper,the modulation techniques in multicolor visible light communication are studied,including CSK constellation design and constellation probability distribution optimization.First of all,this paper introduces the research background of the visible light communication and the re-search progress at home and abroad,which also introduces the research of constellation design in visible light communication.Then,the multi-color visible light communication system is modeled.The characteristics of the visible light channel and the channel noise are analyzed.The modulation and demodulation scheme of color shift keying(CSK)is introduced in detail.Secondly,the problem of CSK constellation design for equal-distribution in visible light communica-tion is studied in this paper.In this problem,in order to ensure the quality of the synthesized white light,the color temperature constraint and color rendering index constraint are taken into consideration,which are transformed into linear constraints.The position of the optimal constellation points is obtained through iter-ative optimizations.Simulation results show that the proposed method can effectively increase the minimum distance between pairwise constellation points.Compared with the three-color CSK modulation,the perfor-mance can be greatly improved.At the same time,a scheme for four-color LED lights achieving the best performance are given according to the simulation results.Then,the probability distribution under regular four-color visible light communication constellation is optimized.In order to reduce the hardware complexity of the transmitter,the communication system usually adopts the regular constellation.In order to reduce the average power,this paper introduces the trellis shaping technique.According to the unique characteristics of the four-color communication system,two shaping methods are proposed:jointly shaping and individually shaping.The jointly shaping regards the transmitted signal as a four-dimensional signal as well as change the probability of each constellation point on the four-dimensional constellation,increasing the probability of the constellation point with less energy and reducing the probability of the constellation point with larger energy.Individually shaping transmits signals in two independent channels based on the characteristics of four-color communication.The data is divided into two separate transmission,and shaped individually.The simulation results show that the two shaping schemes can obviously reduce the transmission power,while the higher the modulation order is,the more gain shaping brings.At the end of this paper,the influence of shot noise on the demodulator in the visible light communication system is researched.In the optical channel,there usually exists shot noise dependent on the signal,which affects the system performance during demodulation.In this paper,we first proposed a method to measure ratio of the signal independent Gaussian noise(SIGN)and signal dependent Gaussian noise(SDGN)in the optical channel.Then the likelihood ratio formula for the 2-PAM signal is improved.Combining the LDPC code,a new method of calculating the likelihood ratio is proposed for the minimum sum algorithm of LDPC.The calculation method is not sensitive to the noise estimation error with low computational complexity.Simulation results show that these two methods both outperform the traditional method,while the improved approach for the MSA is highly robust.