Research On High Efficiency Modulation And Demodulation Technology For Visible Light Communication

Under rapid development of"Internet+",the conventional radio frequency(RF)network with increasingly crowded spectrum resources can not meet the requirements of mass communications and low power consumption.With the advantages of new spectrum resources,high data rates,and low power consumption,Visible Light Communication(VLC)based on light-emitting diode(LED)can complement RF communications.The main contradiction in VLC is that high-speed requirements are constrained by the limited modulation bandwidth of LEDs.Optical Orthogonal Frequency Division Multiplexing(O-OFDM)is an efficient modulation and demodulation technology,and its high spectrum efficiency can alleviate this contradiction.However,due to factors such as the intensity modulation method,multipath effect,and LED nonlinearity,there are some problems with existing superimposed O-OFDM,such as high peak-to-average power ratio(PAPR),high complexity,bit error rate is affected by clipping noise and LED nonlinear distortion,the accuracy of channel estimation in the case of multipath propagation is affected by clipping noise,and the spectral efficiency is impaired.This thesis conducts research on the above issues,the main research work completed is as follows:1.The conventional receiver of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing(HACO-OFDM)requires additional processing to regenerate and cancel the clipping noise,which increases the complexity and makes the performance of pulse-amplitude modulated discrete multitone(PAM-DMT)branch dependent on the accuracy of clipping noise estimation.In this thesis,a novel receiver of HACO-OFDM is proposed by fully exploiting the structure of time-domain signals.Through time-domain processing,the clipping noise can be removed and error propagation is avoided.Compared with the conventional receiver,the proposed receiver can achieve 58.3%complexity reduction in the case of a fast Fourier transform with size of 512.Simulation results show that the bit error rate(BER)of the proposed receiver is exactly the same as that of the conventional receiver for the asymmetrically clipped optical OFDM(ACO-OFDM)branch,but when the BER of the ACO-OFDM branch is high in the low-to-medium signal-to-noise ratio region,the BER performance of the PAM-DMT branch is significantly improved over other competing schemes.2.In this thesis,a channel estimation method for hybrid ACO-OFDM(HACO-OFDM)based visible light communications systems is proposed.The channel characteristics carried on the ACO-OFDM subcarriers are estimated by the least square(LS)method,and the channel response on the PAM-DMT subcarriers can be obtained by interpolation of which on the ACO-OFDM subcarriers.While considering the multipath channel in the thesis,the simulation results show that the proposed scheme is able to estimate the channel effectively for HACO-OFDM system.After equalization based on the results of the channel estimation,the average BER on the ACO-OFDM branch will be lower than10^-3 when SNR>18.2d B,the average BER on the PAM-DMT branch will be lower than10^-3 when SNR>19.4d B.3.The HACO-OFDM scheme has an obvious spectral efficiency loss due to the fact that the real part of the even subcarriers are not utilized.This thesis proposes a scheme to augment the spectral efficiency.In this scheme,more spectrum resources are exploited because the real part of selected even subcarrier group is utilized for data modulation at different depths.When the same modulation orders are used,the proposed scheme can enhance the spectral efficiency of HACO-OFDM scheme.When this scheme takes 5 depths,the joint modulation scheme uses 16QAM-4PAM and 64QAM-4PAM,the spectral efficiency of the proposed scheme can be increased by 31.33%and 23.48%,respectively.Moreover,compared with HACO-OFDM,the proposed scheme has advantages of lower PAPR and higher immunity to LED nonlinearity.4.Due to the high PAPR of O-OFDM signals,the inherent nonlinearity of LEDs is a challenge for O-OFDM schemes in visible light communication systems.In order to reduce PAPR to mitigate the effects of the nonlinearity of LEDs,and avoid the asymmetric clipping interference in the conventional hybrid O-OFDM schemes,a novel superimposed O-OFDM scheme is proposed in this thesis,which reduce the PAPR by combining the-law mapping and the superposition modulation.Moreover,at the transmitter,the hybrid O-OFDM components are superimposed in an interference-free way,the receiver does not require the operation of interference cancellation,and the symbols of each branch can be demodulated independently.Simulation results verify that the proposed scheme can significantly reduce the PAPR.At the same time,the comprehensive performance of the proposed scheme outperforms other competing schemes under the constraints of the nonlinearity and limited dynamic range of LEDs.