Analysis About The Key Technologies Of Visible Light Communications On OFDM

Visible light communication,as an emerging short-range wireless communication tech-nology,is getting more and more attention and research.Visible light communication is rich in spectrum resources,low energy loss,low cost,good security and other advantages.With the popularity of light-emitting diode and the high-speed development of LED's manufacturing process,LED-based lighting and display systems have become increasingly integrated into peo-ple's lives.As with the traditional radio frequency communication,in the VLC implementation technology,orthogonal frequency division multiplexing(OFDM)technologies because of its spectrum utilization,anti-frequency selective fading,modulation and demodulation simplicity,not need complex equalization technology at the receiver and other advantages are also widely used.Based on the OFDM visible-light communication system,this paper will study the key technologies in the realization of the system and start the corresponding systematic research.Firstly,this paper summarizes the working principle of OFDM technology,analyzes the origin of its frequency and orthogonality.The application of inverse fast Fourier transform(IFFT)in OFDM and the significance and principle of cyclic prefix reduction in intersymbol interference(ISI)are also discussed.The advantages and disadvantages of OFDM communica-tion system compared with single carrier communication system are pointed out.Furthermore,the general principle block diagram of the visible light communication system is also outlined in this paper and compared with the traditional RF communication system.The model of shot noise and thermal noise in visible light communication system are also analyzed.Secondly,this paper analyzes the four most widely used OFDM technologies in the visible optical communication system,including DC-baised optical OFDM(DCO-OFDM),Asymmet-rically clipped optical OFDM(ACO-OFDM),pulse amplitude modulated discrete multitone(PAM-DMT),and Flip OFDM(Flip-OFDM).In this paper,the implementation mechanism of each OFDM system is described in detail,and the similarities and differences are compared.This paper focuses on the impact of bilateral clipping on the peak-to-average PAPR distribution of each OFDM system.In this paper,the complementary cumulative distribution function of the peak-to-average power ratio of each OFDM signal after clipping is presented.The complemen-tary cumulative distribution function is found to be a piecewise function,and the discontinuity distribution of the segment function is further revealed.And the relationship between the clip-ping peak and the characteristics of the upper and lower bounds and the clipping size.Finally,the simulation results verify the correctness of the conclusions.Thirdly,in this paper,the problem of clipping in the digital domain or in the analog do-main should be taken into account in the OFDM visible light communication system.This paper shows that the peak-to-average ratio of the clipped signals of ACO-OFDM,PAM-DMT,and Flip-OFDM increases by about 3dB compared to the original signal before clipping.There-fore,in these three systems,The transmit signal should first be passed through the amplifier and clipped in the analog domain.In DCO-OFDM,we first model the clipping,and then optimize the clipping strength of the transmitter from the angle of maximizing the signal-to-interference ratio of the receiver,and consider the digital clipping or the simulated clipping under this con-dition.The simulation results show that the analog domain clipping should be taken under the condition of the maximum signal-to-noise ratio(SNR)of 20dB and below in the receiver;in the case of channel conditions above 25dB,the digital domain clipping should theoretically be taken,clipping in the digital domian and then power amplification.Finally,the DCO-OFDM visible light communication system is as the research object and consider the non-linear characteristic of the actual LED and the limitation of the optical power,the probabilistic problem of the upper clipping is considered from the point of view of the opti-mal system DC bias and signal power,considering a practical application scenario.It is shown that wehn the DCO-OFDM system is adopt the optimal DC offset and signal transmit power,the probability that the upper clipping occurs is monotonically decreasing with the maximum optical signal-to-noise ratio at the receiver.The probability of upper clipping occurring in the normal case is negligible.