Research And Implementation Of Key Technologies In OFDM Visible Light Communication System

With rich spectrum resources, high security as well as the properties of energy conservation and environmental protection, visible light communication (VLC) is becoming a new wireless communication technology attracting extensive attention. This article made a deep research on OFDM technique, adaptive modulation and the synchronization of sampling clock in VLC system. The design and implementation of DCO-OFDM-based VLC digital baseband system is also elaborated in detail in this article. The main content of this article is as follows:Firstly, the structure differences of DCO-OFDM, ACO-OFDM and PAM-DMT are compared in detail, and the performance differences of the three systems are analyzed by using light bit signal-to-noise ratio. In a DCO-OFDM system, all subcarriers are modulated and non-negative transmit signal is obtained by DC offset. Smaller DC offset may result in larger clipping noise, and larger DC offset may decrease power efficiency. The optimal DC offset of DCO-OFDM system with different QAM modulation order is determined by simulation. By contrast, in the ACO-OFDM and PAM-DMT system, only odd subcarriers and the imaginary part of subcarriers are modulated respectively, and non-negative transmit signal is obtained by zero clipping. Though only unmodulated subcarriers are influenced by clipping noise, half of the transmit power is wasted on unmodulated subcarriers, which causes a 3dB performance loss compared with traditional bipolar OFDM. Simulation results show that:in a low speed communication system, ACO-OFDM and PAM-DMT may help increase the power efficiency, and higher power efficiency may be obtained by DCO-OFDM in a high speed communication system.Secondly, the adaptive modulation performance of DCO-OFDM and ACO-OFDM is compared in detail. In this section, the VLC channel model and its influence factor are first introduced, and the channel can be seen as a low pass channel. The adaptive modulation mechanism and implementation algorithm of DCO-OFDM and ACO-OFDM is then described. In the adaptive modulation mechanism, the modulation order and sending power of different subcarriers is set to be different according to the channel condition, which may make the system performance no longer restricted by subcarriers with small amplitude-frequency response. However, the implementation complexity may be increased. In the simulation, square QAM adaptive modulation is utilized in DCO-OFDM and ACO-OFDM systems. Simulation results show that under low SNR demand, ACO-OFDM can obtain higher spectrum efficiency in contrast with DCO-OFDM, and vice versa under high SNR demand.Thirdly, the synchronization of sampling clock in a DCO-OFDM system is analyzed the frequency domain phase compensation algorithm is improved. In this section, a model for the sampling clock problem is first proposed and the effects of sampling clock offset are analyzed. Initial timing error may cause phase rotation. By contrast, sampling frequency offset may not only cause phase rotation, but can also result in ICI. What’s more, three main improvements to the frequency domain phase compensation algorithm are put forward in this section, and the performance of the improved algorithm is compared with the former one by simulation. Simulation results show that the system performance is improved obviously using the proposed algorithm.At last, the design and implementation of DCO-OFDM-based VLC digital baseband system is elaborated in detail. Experimental results show that a speed of 490Mbps and 300Mbps can be realized with the distance between transmitter and receiver being lm and 3m respectively.