Research On Nonlinear Distortion Suppression Method Of Visible Light Communication OFDM System

Combining illumination with communication, visible light communication(VLC) which is based on the ubiquitous green lighting resources has received widespread attention. It provides a high-speed wireless transmission mode of deep coverage, physical security, environmental protection and energy saving. In order to match the equivalent frequency-selective channel and increase the bandwidth efficiency, optical orthogonal frequency division multiplexing(O-OFDM) with high bandwidth efficiency is usually adopted in VLC. However, the conjugate anti-symmetric structure of O-OFDM input sequence is needed to meet the unidirectional conductive feature of lighting terminals. It increases the correlation of input sequences that the peak-to-average power ratio(PAPR) problem becomes more serious. Therefore, it is particularly important to study the nonlinear distortion for VLC in which the terminals have small dynamic range. According to the uniqueness of VLC and O-OFDM, the research is done around the PAPR suppression technologies of O-OFDM signals. In this paper, our main contributions are summarized as follows:1. To solve the problem that the conjugate anti-symmetric structure of the O-OFDM input sequences makes the PAPR worse, the grouping cyclic shift based on discrete cosine transform(DCT-GCS) which effectively reduces the nonlinear distortion is proposed. Due to the unipolar and real signal constraints in VLC transmitter terminal, O-OFDM is constrained to have conjugate anti-symmetric structure which leads to strong correlation of input sequences and a high PAPR. The proposed DCT-GCS firstly makes DCT transform for signal to decrease the correlation of input sequences. Then GCS is used to seek the arrangement with minimum peak power before the IFFT transformation. Theoretical analysis and numerical results show that the proposed DCT-GCS method effectively solves the high PAPR problem caused by the conjugate anti-symmetric structure and has low implementation complexity compared with the existing schemes.2. In VLC, there is a conflict between the safety of lighting requiring static stability of the intensity and high-speed communication requiring large dynamic range of the signal. To reduce the serious clipping distortion brought by the limited dynamic range of LED, we put forward two kinds of companding method suitable for O-OFDM signals. On the one hand, the nonlinear companding based on the cosine probability distribution is proposed to overcome the shortcoming that traditional exponential companding(EC) is sensitive to noise. EC method is usually used to make the output signal of IFFT obey uniform distribution for PAPR reduction. But the companded signal is sensitive to noise which leads to a bad BER performance. On the other hand, according to the deep coupling characteristic of lighting and communication, the partial linear companding with illumination constraint is put forward in VLC. When the optical power constraint of LED is large, the positive part of bipolar signal is linearly compressed at transmitter and the compressed signal would be linearly expanded at receiver. While the optical power constraint of LED is small, the objective of partial linear companding is the negative part of bipolar signal. To enhance the system performance, the optimal compression coefficient is achieved under the criterion of maximum effective SNR.3. To meet the demand of unipolarity, a large DC bias is directly added to the direct current-biased O-OFDM(DCO-OFDM) which leads to low energy efficiency. So the polarity code modulation which transmits the polarity and the amplitude information separately is proposed. DCO-OFDM is a typical O-OFDM scheme. A constant DC bias is needed to make the signal work in the quasi linear range of LED. But it reduces the energy efficiency. In order to improve the energy efficiency of system, we put forward the polarity code modulation. In this method, the polarity and modulus of the bipolar signal are got at first. Then the polarity information and amplitude information of the bipolar signal is transmitted separately. The polarity code modulation needs no extra DC bias so that it has high energy efficiency. Based on the lighting constraint, the optimal power allocation of the time-division transmission signal is given under the criterion of maximum effective SNR. In addition, the attained gain of BER performance is almost 2d B.4. Based on the multiple wicks of LED lighting resources and the space linear superposition of optical signal, the nonlinear distortion reduction method based on the multi-terminal space-frequency collaboration is proposed to take full advantage of the freedom of spatial dimensions provided by the ubiquitous lighting terminals in VLC. The above research work is mainly used in the time-domain. In fact, the potential multi-input spatial dimensions of VLC transmitter can reduce the pressure of time-domain methods for nonlinear distortion reduction. Therefore, we divide the input data of IFFT into several sub-blocks in which the data is loaded into different LED wicks. The nonlinear distortion is effectively restrained by the space-frequency collaboration which improves the modulation index of the optical signal. Based on the criterion of maximizing the output effective SNR, the optimal DC-bias selection scheme of Multi-LEDs OFDM(MLED-OFDM) is given to further optimize the system performance and improve the energy efficiency and BER performance.5. In order to examine the effectiveness of the technical scheme and verify the consistency of simulation and experimental results, we build the off-line experimental OFDM system for VLC. Based on the commercial lighting terminals, the nonlinear distortion suppression methods combined with the proposed O-OFDM schemes have been examined and analyzed. Experimental results show that the proposed method of nonlinear distortion suppression compared with the existing schemes can effectively reduce the PAPR of O-OFDM signal and improve the system transmission performance. It can be seen that the experimental results are consistent with the simulation results.