Research On OFDM-Based Visible Light Communication Systems

In the 21th century, our life is enriched by various multimedia informations. Therefore, the demand of the high speed data transmissioin is increased. With the urgent need of mobile office, mobile social etc., visible light communication (VLC) is deemed as a kind compliment of radio frequency (RF) communication and has drwan much attentions of governments and researchers. In VLC systems, the visible light (VL) is the transmission carrier of the valid infromantion. Different from RF, VL has many advantages, such as no frequency limit, wide coverage, human harmless etc. Meanwhile, the light emitting diode (LED) is used as the transmitter of VLC systems. With the development of semiconductor technology, VLC systems provide low power consumption, high stability, and long service life. In the past ten years, many researchers make deep researchs on the VLC technology, lots of VLC equipments are invented and applied to the daily life. I believe that VLC will become a kind of important short distance wireless communication technology and will be widely used in the indoor communication. In this paper, the VLC system based on the OFDM is investigated. Meanwhile, due to decrese the system nonlinear distortion and increase the system transmission performance, the algorithms of Advaned A-law and Beneficial Clipping are proposed.First of all, the VLC system and orthogonal frequency division multiplexing (OFDM) are studied in this paper. In this context, we research the OFDM-based VLC system. In most VLC systems, the positive signal is modulated by light intensity, i.e. the Intensity Modulation and Directed Detection (IM/DD) method. In order to obtain the positive signal, researchers transform the complex signal to real by the Hermitian Symmetry. Depending on the way the real signal is generated, there are two kinds modulation themes of the OFDM systems based on the VLC:Directed Current Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) and Asymmetrical Clipping Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM).The research in this paper is based on the DCO-OFDM, the main achievements and innovations are presented as follows:(1) The LED which is the transmitter of the VLC system is studied. Meanwhile, the nonlinear transfer model of the LED is built. For the convenience of the theoretical analysis, the LED presented by the nonlinear transfer model is handled by a predistorter. In this way, we can analyse different LEDs in a uniform model. Then an advanced A-law companding algorithm is proposed. It is used to mitigate the nonlinear distoriton derived from the LED nonlinear transfer characteristics. All of the mathematical model, the distribution of the signal amplitude, and the bit error ratio (BER) are used to research the advanced A-law in DCO-OFDM system. Finally, it is proved that the advanced A-law can efficient concentrate the singal amplitude to the linear range and mitigate the system nonlineaer distortion.(2) In order to improve the transmission performance of the VLC system, a Beneficial Clipping method is proposed. In this paper, this method is introduced by the analysis of the system model and signal amplitude distribution, then the mathematical model of the DCO-OFDM signal processed by the beneficial clipping method is built. And the effectiveness of the algorithm is proved by the system signal to noise ratio (SNR) and BER. Finally, the performance of the beneficial clipping is obtained through comparing the relations among the system, modulation depth, and truncation ratio. In a fixed modulation depth, the lower system BER can be obtained by changing the truncation ratio. Meanwhile, in a fixed requirement of the BER performance, the lower modulation depth and faster transmission speed can be obtained by changing the truncation ratio.