Research On OFDM Based Indoor Optical Wireless Transmission Technology

Visible Light Communication (VLC) system is an attractive and competitive solution for indoor high-speed data transmission. OFDM (Orthogonal Frequency Division Multiplexing) technique is employed in VLC to meet the needs of high data rate. Since the VLC system uses IM/DD (intensity modulation/direct detection) technique, the OFDM symbols must be a real and positive to modulate LED light intensity. Therefore, this paper introduces the principles of OFDM symbol single-polarizing methods, including DCO-OFDM (DC Biased Optical OFDM)、 ACO-OFDM (Asymmetrically Clipped Optical OFDM、U-OFDM (Unipolar OFDM)、 MACO-OFDM (Modified ACO-OFDM)and Flip-OFDM, and analyzes these unipolar OFDM systems in power efficiency, bandwidth efficiency and bit error rate performance.Dimming control is one of the main challenges for Visible Light Communication (VLC) which is a promising candidate for future short distance wireless communications. To this end, the principles of two dimming control methods, i.e. continuous current reduction (CCR) and pulse width modulation (PWM), employed in optical OFDM system are discussed, and also a new dimming control scheme is proposed in this paper. CCR and PWM are applied in U-OFDM system, which can achieve better performance in terms of both power efficiency and bit error rate.This scheme can make sure that lighting signal will be adjusted continuously and linearly while maintaining a reliable communication link. Simulation results demonstrate that U-OFDM system using dimming control can obtain better error performance in the mean time without sacrificing illumination quality. For indoor high-speed data VLC systems with dimming control function, U-OFDM system based on PWM dimming control is an optimal solution. In addition, this paper presents ZACO-OFDM (Zero-padding ACO-OFDM) system which is more efficient in power for the demand of weak lighting intensity and low power consumption. Simulation results shows that ZACO-OFDM system can improve system performance, dimming depth and flexibility without loss in bit error rate performance.