Research On Fractional Low Order OFDM Visible Optical Communication System

The shortage of wireless band source leads to the need for novel technologies which can help us and make full use of it. Visible light communication(VLC) is one of the promising methods which could cover this urgent situation, due to the 405 THz bandwidth it could provide with wave lengths from 380 to 780 nm. However, there are still a number of issues to study. For example, we need novel efficient modulation to realize fast communication, new synchronized receiver to realize accuracy, and original algorithm which is robust to ISI and time delay caused by multipath transmission. In this paper, OFDM modulation, OFDM parameter estimation and time delay estimation are chosen to be research topics.OFDM synchronization and time delay estimation(TDE) are two important contents in VLC field. The length estimations of cyclic prefix and useful signal are keys to realize synchronization. Many traditional estimation methods are based on the second-order and higher-order statistics and they are verified efficient under Gaussian noise. However, a mount of recent studies show that it is more reasonable to choose α-stable distribution as the noise mode due to the impulsive phenomenon in many practical applications. Classic estimation methods deteriorates since their order is larger than α and this leads to its divergence. Thus, there still remains a void in the study of better estimation algorithms under impulsive noise.In this paper, we focus on the OFDM cyclic prefix length estimation and time delay estimation in the presence of α-stable noise. The fractional lower-order statistics is employed to face the divergence of the second-order statistics under impulsive noise. Meanwhile, we have studied and analyzed the relationship between the methods based on the fractional lower-order and the second-order moments. Simulations verify its advantage and robustness under both Gaussian and non-Gaussian noise. However, as the research progresses, we find its heavy reliance on the priori knowledge of characteristic exponent α especially under strong impulsive noise. Furthermore, we propose novel correntropy-like methods to meet the needs when priori knowledge of the noise is hard to obtain and simulations verify its advantage and robustness.We also realize the practical VLC test systems for files and characters transmission. Furthermore, we use MATLAB Simulink to realize the simulation for OFDM communication systems to implement former VLC system for future practical high speed communication.