Study On Precoding Technology Of Visible Light MIMO Communication Systems

In recent years,there has been a rapid development of mobile communication technologies.The 4th generation mobile communication technology has been widely applied.The 5th generation mobile communication technology is about to be commercialized.However,it has also encountered some problems,such as serious electromagnetic interference and spectrum shortage.Visible light communication(VLC),due to a number of merits such as immunity to radio frequency interference,license-free spectrum and energy saving,has become a popular area recently.In this paper,the precoding of VLC systems is studied.The goal is to maximize the achievable sum rate by optimize the precoding matrix design.Firstly,an overview of VLC systems is given.System model,channel and analog devices of a typical single input single output(SISO)VLC system are described.More general ones,multi-user multiple input single output(MISO)VLC systems and multi-color VLC systems are discussed.Considering the low-pass characteristic of the VLC channel,orthogonal frequency division multiplexing(OFDM)is introduced,and its modified form in VLC,direct current biased OFDM(DCO-OFDM)is detailed.Secondly,the zero-forcing(ZF)precoding of VLC multi-user MISO systems is optimized.Based on the considered system model,we derive a lower bound of the achievable sum rate.By incorporating the constraints on the input of light emitting diodes(LEDs),we formulate the rate maximization problem mathematically.Firstly,we employ concave-convex procedure(CCCP)to solve this nonconvex problem.Subsequently,by exploiting high signal to noise ratio approximation(HSNRA),we transform the primal problem into a convex one.Then,the structure of the precoding matrix is limited to channel pseudoinverse to reveal its impact on the system performance.Finally,simulations under different scenarios are conducted to compare the performance of the above algorithms.Thirdly,considering the power loss introduced by ZF precoding,more generalized linear precoding optimization is studied.Firstly,the rate maximization problem with the presence of inter-user interference is formulated.Then,in order to solve this problem,existing numerical algorithms,including sequential parametric convex approximation(SPCA),semi-definite relaxation(SDR),CCCP and gradient projection method(GPM),are modified according to the specific feature of VLC systems.Finally,we perform simulations to test the performance of these algorithms.At last,the hardware implementation of multi-color DCO-OFDM VLC systems' digital baseband processing on field programmable gate array(FPGA)is discussed.A presentation and key parameters of our demo system are given.The 4-color DCO-OFDM VLC system can achieve a transmission rate of 1.0712 Gbps when the distance is 3 m,and the frame error rate(FER)is under 10-6.