Research On The Nonlinearity Of Visible Light Communication

According to the rapid development of wireless communications, it has encountered some problems such as the shortage of spectrum resources and the increasing cost of hardware. Besides, as the media of conventional wireless communications, the radio frequency (RF) signals will cause some side effects on the human health. Moreover, the problem on the indoor radiation has become the major issue in green communi-cations. There have come up some research works on the potential technologies that can take place of the RF communications. Among them, the visible light communication (VLC) systems based on the light emitting diode (LED) have gained comprehensive attention due to their advantages in plenty of available spectrum re-sources, safety and high power conversion efficiency. This thesis focuses on the modelling and the mitigation technology of the nonlinearity in VLC systems.Firstly, the thesis gives the overview of VLC systems including the model and the principle of the wireless light transmission with LED. The method and the application scenarios of modulations mentioned in the VLC standards proposed by IEEE 802.15.7 group such as OOK, PPM and CSK are introduced. A comparison between the VLC systems and the RF systems explains the potential advantages of wireless transmission over light-wave. Considering to overcome the diffuse channel and inter-symbol interference, we investigate three multi-subcarrier modulations (MSM) namely DCO-OFDM, ACO-OFDM and PAM-DMT including the implementation, the application and the spectral efficiency in VLC systems. Except for modulations, two near capacity-achieving error-correction codes, turbo codes and low-density parity-check codes (LDPC) are presented on aspects such as coding theory and applications.In addition, we introduce the lighting and voltage current characteristics of the three typical LEDs in VLC systems. The nonlinearity model is established and verified by the experimental results. A similar problem about high peak to average power ratio (PAPR) raised in RF systems is analyzed in the optical MSM system. The PAPR complementary cumulative density function (CCDF) curves of three modulations are presented in theoretical analysis and simulation, respectively. In order to clarify the nonlinear effects on the signals, the thesis shows the probability density function of double sided clipped signals and defines the clipping noises in VLC systems. Some methods for PAPR reduction along with the nonlinearity mitigation technique through the LED array are discussed in the thesis.Furthermore, a research on the nonlinearity in the coded VLC systems is given. Extrinsic information transfer (EXIT) and protograph based EXIT (PEXIT) are briefly introduced to analyze the overall perfor-mance of the coded systems. We take the coded ACO-OFDM system as an example to show the nonlinearity mitigation by channel coding due to its error correcting capability. An EXIT analysis is performed to obtain the convergence threshold of the LDPC coded system and a BER performance is simulated to verify the re-sults. Besides, a simulation about the joint of demodulator and decoder outer iterative system is conducted to show the performance gain in the double sided clipping system. Moreover, the thesis proposes a direct current (DC) bias optimization for the LDPC coded DCO-OFDM system. Similarly, an EXIT analysis is carried out to calculate the convergence thresholds under different DC biases. The thesis takes the DC bias related to the lowest threshold as the optimized DC bias in the coded system and simulates the BER performance with different modulation orders and channel conditions to verify the results.Finally, we focus on the timing synchronization and frequency offset estimation problems caused by the nonlinearity in VLC system reverse link. A discuss about AM-AM and AM-PM distortion induced by nonlinearity is presented in the thesis. According to those nonlinearity distortions, we analyze the effect of the synchronization error and the frequency offset on the signals at the receiver. Specially, two synchronization methods are investigated in the signals with frequency offset and the comparison between these two methods are performed in the thesis. Additionally, two approaches to estimate the frequency offset and a simulation on the root mean square error along with BER performance are shown in the thesis. The hardware structures on synchronization and frequency offset estimation are proposed which are verified in the hardware platform.