The Research Of Visible Light Communication Based On MIMO

Visible light communication (VLC) uses the high-speed modulated light-wave signal emitted by light emitting diodes (LED) to transmit information, photo-detectors (PD) to receive light-wave signal and extract information. The modulation bandwidth of LEDs is usually only a few MHz and it is unable to meet the needs of high-rate communications. To solve this problem, researchers at home and abroad apply the multiple input multiple output (MIMO) technique into VLC and propose the MIMO VLC system. This system improves the communication rate and achieves higher spatial multiplexing gain.By changing the spatial distribution of LEDs and PDs, we can get different antenna layouts, and then analyze the effect of different antenna layouts on the performance of the indoor MIMO VLC system. We first introduce the channel model based on Lambertian radiation, different MIMO techniques and the calculation of channel capacity.Further, we analyze the impact of antenna layouts on the system performance when the VLC systems use Repetition Coding (RC), Spatial Multiplexing (SMP) and Spatial Modulation (SM). The first case is a 4×6 system, which use 4 LEDs at the transmitter side arranged in square layout and 6 detectors at receivers arranged in hexagon, rectangle and triangle layouts. As shown in the simulation results, triangle layout gives the best bit error rate (BER) performance for RC at the center of the room. The BER curves of hexagon and triangle layout are almost coincident for SMP and SM, which both are better than the BER curve of rectangle layout. If the receiver is not at the center of the room, hexagon layout is the best choice. The second case is a 8×8 system with LEDs and PDs using octagon, square layouts respectively. The simulation results show that the system using square layout in the transmitter has better BER performance than the octagon layout.At last, we analyze the performance of various indoor VLC systems with different channel matrix H of various parameters, such as the distance between LEDs dTX, the height between the transmitters plane and receivers plane h. Based on the simulation results, the key factors which are critical to the performance of VLC systems consist of the minimum Euclidean distance dmin and the determinant number Det(H), where s is the transmitted signal vector and dmin is the minimum Euclidean distance of{Hs}. According to these key factors, the antenna layouts are divided into two categories, namely the circular layout and lattice layout. A circular layout means that all LEDs or PDs are uniformly distributed on a circle while a lattice layout means that all LEDs or PDs are distributed on a lattice of two-dimensional plane. As shown in the simulation results, if dTX is variable and h is fixed, Det(H) is critical to the bit error rate of the system with a circular layout while dmin and Det(H) are critical to the BER of a system with a lattice layout. Moreover, dmin is the critical factor if dTX is within a certain range. If h is variable and dTX is fixed, Det(H) is the critical factor for the system performance. In addition, based on the channel capacity curves of different antenna layouts, the channel capacity of lattice layout is larger than circular layout when the signal noise ratio (SNR) is small and the channel capacity of circular layout is larger than lattice layout when the SNR is large.