Research On Hybrid Modulation Technology In Visible Light Communication

Compared with traditional radio frequency communication,visible light communication(VLC)has the advantages of abundant spectrum resources,high confidentiality,and low implementation cost.VLC has become a promising technology in the next-generation wireless communication network.Orthogonal frequency division multiplexing(OFDM)technology has been widely used in VLC systems because of its ability to resist multipath fading and reduce inter-symbol interference.Therefore,many optical OFDM(O-OFDM)schemes have been proposed.However,in VLC hybrid modulation technology,O-OFDM schemes face the problems of high system complexity and poor anti-nonlinearity,which limits its promotion and application.In this context,how to optimize the design of the communication system and improve the performance according to the characteristics of the OFDM signal in the VLC system has become an important research direction.This paper mainly focuses on the optimization design and performance improvement of hybrid modulation technology in VLC.The specific research contents and contributions are as follows:Firstly,a hybrid asymmetrically clipped O-OFDM modulation scheme based on three-layer signal transmission is proposed.At the transmitter,asymmetrically clipped O-OFDM(ACO-OFDM)branch is constructed by time-domain processing operation.The traditional pulse amplitude modulated discrete multitone(PAM-DMT)branch is divided into two channels for transmission according to specific rules.At the receiver,the ACO-OFDM branch was recovered by time domain processing.And clipping noise of the ACO-OFDM branch was reconstructed to recover two PAM-DMT branches from the original hybrid signal.Compared with HACO-OFDM,the clipping noise reconstruction in the proposed scheme uses fewer inverse fast Fourier transform operations.Hence,the complexity of the receiver is reduced.The simulation results show that the bir error rate(BER)performance of the proposed scheme is similar to that of HACO-OFDM.However,the peak-to-average-power ratio(PAPR)performance is about 0.6d B better than that of HACO-OFDM.In addition,the proposed scheme outperforms HACO-OFDM in terms of BER performance under nonlinear conditions.Secondly,a low-complexity hybrid PAM-DMT modulation scheme based on time-domain processing is proposed.In the transmitter,only one N/2-point inverse fast Fourier transform operation is required to complete the conversion process from the frequency domain to the time domain in each branch.And the three PAM-DMT signals are constructed to transmit information according to different time-domain symmetry properties.In the receiver,three PAM-DMT signals are reconstructed through time-domain processing operation using specific symmetry properties.Since clipping noise is removed in the signal time-domain processing operation,the noise elimination operation is avoided.Therefore,the complexity of the transmitter and receiver of the proposed scheme is significantly reduced compared with the traditional hybrid O-OFDM schemes.The simulation results show that the BER performance of the proposed scheme is almost the same as that of HACO-OFDM and TimeDomain Clipping Noise Elimination(TD-CNE).In addition,Compared with HACO-OFDM and TD-CNE,the PAPR performance of the proposed scheme is improved by about 0.9d B.Thirdly,an interference-free hybrid PAM-DMT modulation scheme is proposed.At the transmitter,the real and imaginary parts of odd-subcarriers,the imaginary parts of the(4m+2)-th and the 4m-th subcarriers are modulated to form four channel signals to transmit information.Since clipping noise of the four channel signals all fall in the unused subcarrier resources,the hybrid signals do not interfere with each other.Hence,the receiver only needs a standard OFDM signal demodulator,which shows that the receiver’s complexity of the proposed scheme is significantly lower than traditional hybrid O-OFDM schemes.The simulation results show that the PAPR performance of the proposed scheme is significantly improved about 1.6d B and 0.6d B compared with HACO-OFDM and Interference-Free Hybrid Optical OFDM(IFHO-OFDM).In addition,simulation results also show that the BER performance of the proposed scheme is significantly superior to that of HACO-OFDM and IFHO-OFDM under nonlinear conditions.