| Visible light communication(VLC),as its name implies,uses visible light to realize wireless communication.It mainly uses the high-frequency flashing light signal sent by lightemitting diode(LED)to transmit information.VLC can realize the functions of lighting and communication at the same time.It has the advantages of high communication rate,green security and rich spectrum resources.It is considered as a supplementary technology of radio frequency(RF)technology in the 5G wireless communication,and has broad prospects in indoor scene applications.However,the voltage loaded on the LED light source needs to fall on its linear region,otherwise it will cause cut-off distortion or saturation distortion,which greatly reduces the modulation bandwidth that can be used by the high-speed indoor VLC system.Therefore,academia has focused on new modulation technology and advanced digital signal equalization technology.Among them,multiple input multiple output orthogonal frequency division multiplexing(MIMO-OFDM)technology is considered to be one of the key technologies to solve this problem.Although MIMO-OFDM technology can effectively improve the channel capacity and transmission rate of indoor VLC system,the communication performance of indoor MIMOOFDM VLC system largely depends on the accuracy of channel state information(CSI).Therefore,accurate channel estimation(CE)technology can effectively ensure the communication performance of VLC system.In the DC biased optical orthogonal frequency division multiplexing(DCO-OFDM)system,the number of effective carriers will be reduced to half of the traditional RF OFDM system.Therefore,the use of traditional channel estimation algorithms such as least square(LS)in VLC system will further reduce the spectrum utilization of VLC system.In the indoor VLC system,due to the refraction and emission of optical signals,the nonline-of-sight(NLOS)channel is generated in the free space visible light transmission channel.However,the indoor optical power is mainly concentrated in the line-of-sight(LOS)channel,which has obvious sparsity.Therefore,in order to improve the spectral efficiency,this paper applies the compressed sensing(CS)algorithm to the channel estimation of indoor visible light communication system,and studies the channel estimation technology of indoor MIMO-DCOOFDM VLC system based on CS algorithm.The main work of this paper is as follows:(1)This paper expounds the basic principle of indoor VLC system,and introduces its working principle and channel mathematical model.This paper studies the combination of optical MIMO technology and optical DCO-OFDM technology into MIMO-DCO-OFDM technology,and applies it to indoor VLC system to build an indoor 4×4 for MIMO-DCOOFDM VLC system,the optical intensity and optical power at the receiver are simulated and analyzed,and the system is modeled as a compressed sensing problem.(2)The compressed sensing algorithm is introduced in general,and the application of CS in CE is analyzed.On the basis of introducing the channel estimation technology based on OMP algorithm and SWOMP algorithm,the shortcomings of existing OMP algorithm and SWOMP algorithm are analyzed,and proposes an Improved-SWOMP(ISWOMP)algorithm based on SWOMP algorithm.The algorithm improves the accuracy of atomic selection by combining step size based atomic selection and atomic secondary selection.The simulation results show that the ISWOMP algorithm is superior to the traditional compressed sensing algorithm.(3)Analyze the sparse channel model of indoor 4×4 MIMO-DCO-OFDM VLC system,apply compressed sensing technology to CE,and study and compare the performance of CSI reconstruction by LS,OMP,SWOMP and ISWOMP algorithms.Simulation verification was carried out by indoor 4×4 MIMO-DCO-OFDM VLC system.Simulation results show that compared with the other three algorithms,the channel estimation technique based on ISWOMP algorithm proposed in this paper has lower bit error rate and higher frequency spectrum utilization. |
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