Research On Superposition OFDM For High Speed Visible Light Communication

Visible light communication(VLC)is a new type of optical wireless communication technology that realizes lighting and communication simultaneously.In VLC systems,light emitting diodes(LEDs)and photodiodes(PDs)can be used as optical transmitter and receiver respectively.In order to achieve high data rate,orthogonal frequency division multiplexing(OFDM)is usually adopted in VLC systems.OFDM signals should meet the requirements of real and non-negative value by means of Hermitian symmetry,which degrades the spectral efficiency and leads to a high peak-to-average power ratio(PAPR).The limited modulation bandwidth and the nonlinearity distortion characteristics of LED are the bottleneck of the high speed VLC.Therefore,the superposition OFDM is considered,and the following is the main research contents and results:1.Aiming at the problem that the spectral efficiency of spectral energy efficient OFDM(SEE-OFDM)is limited when the superposition layer number is finite,superposition asymmetrically clipped optical OFDM(ACO-OFDM)is proposed to improve the spectral efficiency by changing the modulation order of each layer information stream.When the modulation order is variable,there are a variety of modulation order combinations for a given spectral efficiency and superposition layer number.Then subcarrier selection for each layer information stream in the demodulation process is derived.After the analysis of the bit error rate(BER)for the system,an algorithm is given to find the modulation order combination with the least BER performance for a given spectral efficiency and superposition layer number.Simulation comparisons with conventional ACO-OFDM show that superposition ACO-OFDM can achieve a better BER performance with the same spectral efficiency.2.In order to reduce the PAPR of OFDM signal in VLC systems,the unipolar asymmetrically clipped optical OFDM(UACO-OFDM)is proposed,combining the ACO-OFDM and unipolar OFDM(U-OFDM)signals for simultaneous transmission.The paper researches the superposition method of transmitter signal and the demodulation mode of different layer signals at the receiver.The BER of the system is analyzed and the selection method of the combination of the modulation order with the best BER performance is given for the certain spectral efficiency and superposition layer number.The PAPR of the signal is further reduced by optimizing the signal structure of the transmitter.Compared with the superposition ACO-OFDM,UACO-OFDM has a lower PAPR value.3.To improve the transmission performance of VLC systems under the LED nonlinear distortion,the multi-LED UACO-OFDM technology is proposed.According to the multiple LED chips inside a LED lamp and the approximately linear superposition characteristics of optical in the space,an equivalent multi-input single-output transmission system is constructed.Each layer of information in the UACO-OFDM system is transmitted by the corresponding LED chip in parallel.There is only a PD used to detect optical signal.The signal synchronization is analyzed and the feasibility of the proposed system is verified by simulations.Numerical simulations show that the multi-LED UACO-OFDM can obtain a better anti-nonlinearity distortion performance than the conventional single-input single-output transmission system.