Semi-blind Timing Synchronization For Visible Light Communication DCO-OFDM Systems

With the explosive growth of wireless devices and wireless services,the existing radio frequency spectrum resources are scarce and cannot meet people's demand for high-speed communications.Visible light communication(VLC)has a wide range of visible light spectrum,which uses light emitting diode(LED)to transmit wireless data and becomes one of the potential technologies in the future.However,the limited bandwidth of LED can cause inter-symbol interference(ISI),resulting in bit error rate(BER)performance degradation and data transmission rate limitation.Direct current biased optical-orthogonal frequency division multiplexing(DCO-OFDM)has been considered for VLC systems,as it can effectively combat ISI problem.However,DCOOFDM VLC systems are very sensitive to timing synchronization,and small timing synchronization errors degrade the system performance.Therefore,three timing synchronization algorithms are proposed for DCO-OFDM VLC systems in this paper.Based on the zero correlation code(ZCC)pair,an enhanced zero correlation code(e ZCC)algorithm is proposed to achieve timing synchronization by designing a single DCO-OFDM block,which is robust against the limited bandwidth of LED.We prove that the traditional ZCC algorithm in the literature is seriously disturbed by the limited bandwidth of LED,and can not achieve timing synchronization.Therefore,a new robust timing metric is designed,which uses the impulse-shaped correlation property of the ZCC pair to perform timing synchronization via peak detection in the correlation operation.Compared with the traditional ZCC algorithm in the literature,the proposed e ZCC algorithm is much more robust against the limited bandwidth of LED.Simulation results show that the proposed e ZCC algorithm has better performance of timing synchronization and BER than the traditional ZCC algorithm.This is the first work to propose a direct current bias(DCB)based semi-blind timing synchronization algorithm and a null subcarrier(NS)based semi-blind timing synchronization algorithm for DCO-OFDM VLC systems.The proposed both DCB and NS semi-blind algorithms provide higher spectral efficiency,requiring no pilot than training sequence based timing synchronization methods.The proposed DCB algorithm defines the timing metric with respect to direct current bias in DCO-OFDM systems,and then uses the threshold to refine the timing index.The proposed NS algorithm defines the timing metric with respect to the power of the null subcarrier inherent in DCO-OFDM systems and then refines the timing index by maximizing the ratio of the timing metric.In this paper,the threshold used in the proposed DCB algorithm is analyzed theoretically.The upper and lower bounds of the threshold,the optimal threshold,and between the optimal threshold and the number of DCO-OFDM blocks used for timing synchronization are derived.Simulation results show that the proposed DCB semi-blind timing synchronization and NS semi-blind timing synchronization algorithms outperform the state-of-the-art methods in terms of timing synchronization performance,provide the BER performance close to the ideal case with perfect timing synchronization and perfect channel state information,and provide higher spectral efficiency and lower complexity.