Theoretical And Experimental Study On Orthogonal Frequency Division Multiple Access For Visible Light Communication

In recent years,with the development and maturity of Visible Light Communication(VLC)technology,the application of Light Emitting Diodes(LEDs)-based VLC has involved many scenarios such as indoor and outdoor.In the indoor VLC-based communication system,the information in the downlink is broadcasted,whilst the uplink can generally be completed through infrared or Wi-Fi.However,in an outdoor scenario,such as a typical outdoor traffic system,high-speed Wi-Fi transmission cannot be guaranteed due to long distance.High-speed communication between multiple vehicle nodes based on the visible light communication technology has an inherent advantage of rich spectral resource.However,unlike multi-user systems in the traditional radio frequency communication system,the performance of multi-user systems based on VLC is affected by the limited bandwidth and nonlinearity of LEDs.After the modulated signal passes through nonlinear LEDs,multiple harmonics are generated and the new spectral component is superimposed on the signal,causing various interferences.In this dissertation,based on the OFDMA(Orthogonal Frequency Division Multiple Access)technology,we study the basic theory of multi-user visible light communication,effective method of suppressing inter-user interference and improving system capacity.In view of the LED nonlinearity,the OFDMA system model is established for multi-user VLC.The impacts of background noise and the LED nonlinearity on the transmission performance of multi-user communication systems are theoretically analyzed.Firstly,an OFDMA experimental platform was set up,based on which a net bit rate of 43.1 Mbps for 10m?30m multi-to-one transmissions is demonstrated at outdoor.In order to mitigate the nonlinearity effect among users,the post-compensation and pre-compensation methods are adopted for the compensation of LED nonlinearity,and also the system performance under different number of users is studied.Simulation and experimental results show that an EVM(Error Vector Magnitude)gain of 0.74dB is achieved by post-compensation and 0.15dB is achieved by pre-compensation for two users at a distance of 10m at outdoor.However,the gain by the post-compensation method is reduced to only 0.03dB when the number of users is equal to 4,because the received signal suffers from serious inter-user interference due to the increased number of users.At the same time,the gain remains stable by the pre-compensation method.The research on the resource allocation for the OFDMA systems is given to improve the transmission speed.Therefore,the effects of centralized and distributed subcarrier allocation strategies on transmission performance are studied.The transmission speed is greatly improved by the combination of different subcarrier allocation strategies and bit allocations.Under the premise of guaranteeing BER<3×10-3,maximum bit rates of 138.89 Mbps at the distances of 10m in a four-user system is achieved.