Channel Correlation Reduction For Visible Light Communication

With the rapid growth of the mobile Internet industry,a large amount of mobile data that relies high-speed wireless connections have been generated by various information services.This makes higher requirements for the communication rate of traditional radio frequency networks.In recent years,visible light communication(VLC)technology using light emitting diode(LED)for simultaneous illumination and data transmission with unregulated visible light frequency band,has become an effective alternative to existing radio frequency communication networks.However,visible light channel is a deterministic channel with time-invariant,geometric characteristics,and non-negative real number characteristics.These characteristics lead to high correlation between VLC channels,limiting the development of VLC systems.This article is to investigate the channel correlation reduction technology for VLC systems.Based on multiple-input multiple-output direct current-biased optical orthogonal frequency division multiplexing(MIMO DCO-OFDM)VLC systems,a modulation bandwidth grouping method is proposed to be robust against highly correlated channels and improve the bit error rate(BER)performance for MIMO VLC systems.This method increases the channel difference and reduces the influence of channel correlation by grouping the modulation bandwidth of LEDs.The optimal modulation bandwidth combination scheme of LED s can be obtained by minimizing the BER of the system.The simulation results show that the proposed modulation bandwidth combination of LEDs has better BER performance than the existing methods.Since the VLC signals are transmitted with being nonnegative and real,there is no phase information on single-carrier modulation.Therefore,the traditional methods in the literature are to adjust transmission power on the transmitted signals to reduce the impact of highly correlated channels on BER performance.However,the multicarrier modulation makes the complex signals in frequency domain satisfy Hermitian symmetry,and then transform them into real signal in time domain by inverse fast Fourier transform.This makes the channel correlation reduction in phase domain become possible.This is the first work to use DCO-OFDM to realize the phase domain processing of visible light signal to reduce the impact of high channel correlation for VLC MIMO systems.A phase rotation based precoding method is proposed at the transmitter to enhance the channel difference and reduce the influence of high channel correlation.The optimal phase rotation factor can be solved by maximizing the minimum Euclidean squared distance.In order to reduce the complexity of solving the optimal phase rotation factor,an improved precoding method is proposed based on the channel characteristics of the symmetrical layout of LEDs.The simulation results demonstrate that the two proposed methods provide better BER performance than the existing methods.This verifies that the proposed methods can effectively reduce the adverse effects of high channel correlation.