Research On Modulation Technologies Of Visible Light Communications

With the increasing shortage of traditional wireless communication spectrum resources and the rapid development of LED, visible light communications based on LED have gained more and more attention owing to its advantages of wide frequency band, no electromagnetic interference, environmental protection and low cost. It can carry on communication and lighting at the same time.The topic of this thesis comes from the Huawei Innovation Research Program(HIRP) (No. 20150101). Aiming at the problem that achieving high communication rate and high dimming precision at the same time is the bottleneck of the application of single modulation technique in visible light communications, this paper puts forward two improved schemes.They are based on VPPM (Variable Pulse Position Modulation) and MPPM(Multipulse Pulse Position Modulation), respectively in indoor visible light communication point-to-point model and MIMO system model to solve the problem that the communication rate of VPPM is low and the dimming precision of MPPM is low. Main research contents of this paper are listed as follows:1. This paper comprehensively summarizes the research contents,research status, application scenarios and the challenges of visible light communications. Meanwhile, key technologies in Visible Light Communications are summarized and combed in detail and the modulation technologies are mostly analyzed. Through comparing the characteristics of various modulation techniques, we can come to the conclusion that realizing high communication rate and precise dimming precision simultaneously is the bottleneck for single modulation technology in Visible Light Communications.2. In order to keep the advantage of precise dimming control and overcome the disadvantage of low communication rate of VPPM, this paper starts from increasing the number of possible codewords and puts forward an optimal N-times slot division scheme of VPPM in Visible Light Communications, termed as N-VPPM. This method can achieve the goal of improving the data transmission rate by optimizing the time slot of VPPM while considering three constraints: the communication performance, the brightness level and the bandwidth limit of LED.Theoretical analysis and simulation results show that the proposed scheme solves the low rate shortcoming of VPPM by improving the rate N times,while retaining the advantage of precise dimming control. In addition, this paper takes the nonlinear effects of human eyes into account and conducts corresponding simulations. The nonlinear effect of human eyes and the method of eliminating the influence are obtained.3. In visible light communication systems, aiming at the problem that the communication rate of MPPM is high but the dimming precision is low,a multipulse pulse position modulation scheme based on the number selection of MIMO antennas is proposed. The scheme takes advantage of the natural dimming control function of MIMO that the different choice of the number of LEDs can provide different brightness levels. By selecting the number of MIMO antennas according to different brightness levels and combined with the dimming factor(duty ratio) of MPPM, the dimming error is decreased. Theoretical analysis and simulation results show that the proposed scheme solves the disadvantage of low dimming accuracy while retaining the advantage of high communication rate of MPPM.