Research On DCO-OFDM In Indoor Visible Light Communications

Visible light communication (VLC), enabling illumination and communication simultaneously, emerges as a promising technology to satisfy the huge demand for wireless services. The advantages of VLC include low cost, safety, security, simple implementation, no interference with sensitive electronic systems, etc. Dif-ferent from conventional radio communication systems, VLC systems generally adopt intensity modulation and direct detection (IM/DD), meaning that the transmitted intensity signal must be real and nonnegative. To generate nonnegative real signals, direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) uses an offset followed by clipping. Due to its high spectra) efficiency and simple implemen-tation, DCO-OFDM receives much attention. We mainly focus the thesis on the DCO-OFDM in indoor VLC systems, as well as the related research.Firstly, we give an overview of the indoor VLC systems, point out the differences between VLC and conventional radio frequency communications, and describe the important components of the VLC systems including the optoelectronic devices, the VLC channels, the noise, and the power constraints in common scenarios. We would like to study two types of VLC channels in this thesis:AWGN and dispersive channels. The noise is modeled as a zero-mean signal-independent Gaussian noise, and the optical and electrical power constraints are considered in the thesis.Additionally, we introduce the system model and operating principles of DCO-OFDM, and analyze the nonlinear clipping distortion, and evaluate the system performance from different points of view. Meanwhile, we study another two VLC OFDM systems (ACO-OFDM and PAM-DMT), and investigate the system mod-els, along with their performance. Then, we compare DCO-OFDM with them in AWGN and dispersive channels, respectively. From the simulation results, DCO-OFDM has a higher spectral efficiency and a better performance at high SNR, but is significantly influenced by the direct current (DC) offset.Therefore, we concentrate on the offset and power optimization for DCO-OFDM in the following. We consider the AWGN case first, and take both the optical and/or electrical power constraints into account. We analytically characterize the optimal solutions along with new insights on setting the direct current (DC) offset and information-carrying power in DCO-OFDM systems. We also investigate the relationship and impact of the optical and electrical power constraints, and propose efficient algorithms to obtain the optimal solution. Meanwhile, we address the situation with the constant optical power constraint (instead of the maximum power constraint) to guarantee a constant illumination level. The optimality and superiority of our proposed method are corroborated by numerical results.Finally, we study the optimization for DCO-OFDM over dispersive channels. According to the conclu-sions in AWGN case, we identify the scope of the optimal solution. We propose an accurate algorithm to obtain the globally optimal solution, and also a simplified method with lower complexity. Furthermore, we also con-sider the joint optimization of DC-offset, information-carrying power and subcarrier power. Comprehensive simulation results are provided to demonstrate the optimality and superiority of our proposed methods.