| Limited by the spectrum resources,the existing wireless communication technology is difficult to bear the future high-speed transmission requirement.The visible light communication technology based on light-emitting diode combines wireless communication technology and optical communication technology,and has the advantages such as unoccupied spectrum resources,high transmitting power,and immunity to radio frequency interference.It is a unique supplementary technology to realize indoor high-speed wireless transmission.The limited physical bandwidth of LED restricts the transmission rate of VLC.Despite multiple input multiple output and orthogonal frequency division multiplexing technology can improve the transmission rate,the strong correlation between sub-channel restricts the advantages of MIMO,and the non-linearity of LED can seriously reduce the performance of OFDM.Hence it is necessary to explore effective coding and modulation schemes suitable for indoor visible light system and available methods to reduce the correlation between sub-channel by utilizing the characteristics of LED itself.The main innovations and works are summarized as follows.(1)The distribution model of reflector of random distribution for non-empty indoor environment is studied,the channel characterization parameters are estimated,and the channel transmission characteristics are compared with those in empty indoor environment.The comparison results show that the random distribution reflectors exert a greater influence on line-of-sight than that of non-line-of-sight,the magnitude and scope of root mean square spread is larger than that of empty indoor environment,and the transmission rate is smaller than that of void indoor environment.(2)The discrete sine/discrete cosine transform DC-biased discrete multitone modulation scheme is presented.The complementary cumulative distribution of the peak-to-average power ratio of the modulated signal is theoretically analyzed,and the performance of the proposed scheme is compared with that of the existing multi-carrier modulation schemes.The comparison results show that the peak-to-average power ratio of the DST/DCT DC-DMT signal increases as the subcarrier number increase,which is independent of modulation order.At the same transmission rate,the system bit error rate performance of the proposed scheme is better than that of the existing DMT and OFDM schemes.(3)The metamerism OFDM spatial modulation scheme is proposed.This scheme has high spectral efficiency and power efficiency.The receiver will not produce color flicker.The polarity of the transmitted signal is decided by distinguishing the spectral components of thetarget light.The demodulation process is simple and easy to implement.The comparison results of the system performance between the proposed scheme and the existing spatial modulation OFDM scheme show that the proposed scheme is compatible with the color of visible light band and maintain the stability of the bit error rate performance of the system when the target light color changes.Under the same spectral efficiency,the proposed scheme has better bit error rate performance than other spatial modulation OFDM schemes.(4)The distribution of channel matrix rank,the factors of effect channel correlation,and the influence of correlation on the performance are studied for indoor MIMO channel of vertical detection.Angle diversity detection is used to reduce the channel correlation of indoor visible light MIMO-OFDM system.The channel capacity of correlation channel for indoor visible communication systems is studied,and the system performance of angle diversity detection is compared with that of vertical detection.The comparison results show that angle diversity can effectively separate signals from different light sources under the optimal detection angle,the bit error rate of the system is basically unchanged,and the channel capacity is also better than that of the vertical detection.This work provides new ways to improve the transmission reliability and the transmission capacity of indoor visible light communication systems,lays a theoretical foundation for enriching the application scenarios of visible light communication system,and provides theoretical guidance for the design of real indoor visible light communication systems. |
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