Research On MIMO Technology In Visible Light Communication System

When Multiple-Input Multiple-Output(MIMO) technique is applied in indoor visible light communication(VLC), the data transmission rate can be increased without the enhancement of bandwidth or transmit power, so it is currently become a research hotspot. However, intensity modulation and direct detection are employed in indoor VLC, resulting in strong correlation in VLC MIMO channels. This causes that the improvement of the data transmission rate will be limited. The current research on MIMO technology in indoor VLC still faces the following problems: 1) the reasonable layout of indoor VLC MIMO system is lack of theoretical guidance; 2) there still hasn’t a practical solution to solve the problem caused by the strong correlation MIMO channels. Aimed at the above problems, the paper launches a special study on the MIMO technology in indoor VLC, which is supported by the National High Technology Research and Development Program of China(863 Program).This study firstly establishes an indoor VLC MIMO channel model based on the Lambert radiation model, and then analyze the effect on the spatial correlation of both transmit and receive channel by different distance parameters in this model. Then, we propose an optical MIMO precoding scheme based on singular value decomposition(SVD) when the transmitter has the channel state information(CSI). After that, when the transmitter doesn’t have the CSI, we propose a MIMO transmission scheme combined spatial modulation and spatial multiplexing. Finally, by using the RGB type LED, a color modulation scheme is proposed to further improve the system capacity. The content of this paper includes:1. We establish a model for indoor VLC MIMO channel based on the Lambert radiation model, and then analyze the effect on the spatial correlation of both transmit and receive channel by different distance parameters in this model. Thus the mathematical expressions of spatial correlation coefficients of both transmit and receive channel are derived, which can provide a theoretical guidance to the layout of the indoor VLC MIMO system.2. We propose a novel optical precoding scheme based on SVD to solve the problem that strong channel correlation causes low spatial multiplexing gain in indoor visible light MIMO communication when the transmitter has the CSI. This scheme first ensures a nonnegative signal after precoding through mapping the transmit bits into a specific symbol constellation. Then it eliminates the inter-channel interference by decomposing the MIMO channel through SVD. Finally, it reduces the BER by optimally allocating power among the sub-channels with the object to make the condition of each sub-channel equal. The simulation results show that this scheme can significantly reduce the BER compared to ZF algorithm, but also increase the channel capacity compared.3. We propose a MIMO transmission scheme combined spatial modulation and spatial multiplexing when the transmitter doesn’t have the CSI. First, we divide the LEDs into groups under the principle that the minimum LED spacing of each group is large enough to transfer information in parallel by spatial multiplexing. Among different LED groups spatial modulation is employed since there correlations are strong. At each time slot only one LED group is active to avoid the interference between different LED groups. The simulation results show that this scheme can effectively enhance the spatial multiplexing gain in MIMO visible light communication system.4. We propose an optical MIMO color modulation scheme based on RGB type LEDs. The white light sending by RGB type LED includes red, green and blue light, so each color light can carry information respectively. The sending bits in each time slot is divided into color information and transmission information. The 3 bits color information is mapped as one color set, and the transmission information is transmitted by the active color LED. The simulation results show that this scheme can improve spectral efficiency by 1.5 bit/s/Hz compared to color multiplexing.