Research On Implementation And Key Technologies Of DCO-OFDM System Based On Blue-green LED

With the continuous promotion of strategic position of marine,various complex underwater operations are becoming more and more frequent,which lead to an urgent need for high-speed real-time underwater wireless communication technology.Underwater visible light communication has the advantages of high bandwidth,low latency,which has become a research hotspot.However,the narrow modulation bandwidth of LED has become a bottleneck that limits the transmission rate of underwater visible light communication system.Pre-equalization technology can be used to extend the modulation bandwidth of LED.In addition,DCO-OFDM modulation technology with high communication rate and spectrum utilization can be introduced to make full use of the limited bandwidth.Therefore,this paper studies the pre-equalization technology and FPGA implementation of DCO-OFDM modulation and demodulation based on the underwater visible light communication system using green LED as light source.The main contents are as follows:(1)Firstly,the underwater visible light communication system is introduced in detail,the modulation characteristics of LED and the suitable modulation mode are analyzed.Then the DCO-OFDM modulation mode with the highest frequency utilization is selected as the research object of this paper and its basic principle is analyzed in detail.(2)Research on pre-equalization technology based on DCO-OFDM system.Firstly,the frequency response model of underwater visible light communication system is measured.Secondly,hardware pre-equalization technology and software preequalization technology which extend the LED modulation bandwidth by analog circuit and algorithm are analyzed respectively.FIR pre-equalization filters are designed according to the measured frequency response parameters.Thirdly,the basic principles and problems of digital pre-weighting algorithm are analyzed.Then an improved preweighting algorithm is proposed and its compensation effect is tested,which can avoid the problems of digital pre-weighting caused by over compensation and achieve the similar compensation effect with lower complexity compared with high order FIR filters.Finally,the bandwidth of the system is raised from 7MHz to 20 MHz using the improved pre-weighting algorithm.(3)Implementation of pre-equalization and DCO-OFDM modulation and demodulation system based on FPGA.Firstly,according to the characteristics of different FFT algorithm structures,different implementation schemes of DCO-OFDM modulation and demodulation system and pre-equalization are designed respectively.The timing simulation results are made respectively,which verifies the correctness of the scheme designs.Finally,an appropriate scheme is selected to build an underwater DCO-OFDM visible light communication test platform based on FPGA,then the BER performance of the system under different QAM orders,different distance,different bandwidth and using the improved pre-weighting is tested.Experimental results show that the implementation of modulation,demodulation and pre-equalization using FPGA and MATLAB are basically the same,which fully proves the correctness of the design scheme.