| Visible light communications (VLC) is a rising technology for communications, and has an irreplaceable role to achieve the target for "green communications and integrative development".VLC system will experience rich multipath fading channel when is employed in indoor environment. And, transmitting terminal light emitting diode (LED) has the problem of limited modulation bandwidth. Therefore, by introducing orthognal frequency division (OFDM), the interference from multipath fading channel will be effectively solved. Also, the bandwidth efficiency is able to be improved, and guarantee the quality of communications. The accuracy of indoor positioning system based on VLC also will be enhanced by employing OFDM, and decrease the range of root mean square error for positioning.On the other hand, VLC system based on multiple input multiple output (MIMO) technology is capable of high capacity and high coverage of illumination. In the real system, human shadowing effect is always the difficulty for VLC system. By adopoting MIMO technology, the anti-jamming problem is able to be soloved. Therefore, VLC system based on MIMO-OFDM is one of the key technology and hot reach area for the goal of effeiciency and low cost communicaitons.The study of this dissertation is supported by the National Natrual Science Foundation of Chnia (NSFC). Addressing key technologies regarding VLC system based on MIMO-OFDM. Special attention has been paid to the following mainly three areas:further study on indoor multiband and human shadowing channel characteristics, performance improvement of the VLC system based on MIMO-OFDM, indoor VLC positioning system based on OFDM.The main innovative ideas are summarized as follows:Fisrtly, since few studies have been reported on multiband channel characteristics for indoor VLC system. An indoor VLC multiband channel model (MRT) is proposed and validated according to extensive measurements. Simulation has been made for waveband from 380nm to 780nm.Secondly, few reaseaches are conducted to investigate the modeling algorithm for hunman shadowing effect on the VLC system. A VLC channel model analyzing the human shadowing effect (SRT) is proposed and validated according to extensive measurements. Human shadowing effect on line of sight (LOS) and non-line of sight (NLOS) VLC system has been investigated, also the effect on the capacity of MIMO VLC system has been simulatied.Thirdly, a discrete fourier transform (DFT) post-processing channel estimation method is proposed to improve the system performance. Simulation results show that the proposed method is able to improve the received signal to noise (SNR) for about 1.5dB when the space between pilot signal Nps=8, BER= 10-2.Fourthly, focus on the human shadowing effect on the indoor VLC system, a joint optimal LED selection and power allocation algorithm is proposed. Simulation results show that for 3×4 MIMO VLC channel, the proposed method is able to improve the channel capacity for about 3bps/Hz. Meanwhile, when employing 4 quadrature amplitude modulation (QAM), the proposed method is able to save about 3dB transmit power than only adopt LED selection algorithm.Lastly, addressing the problem of high resolution for indoor VLC positioning system, a positioning algorithnm based on the pilot signal in VLC ACO-OFDM system is proposed. Simulation results have compared the performance of three kinds of VLC positioning system, high speed OFDM, high speed on-off keying (OOK) and low speed OOK. And show that the proposed algorithm is able to improve the positioning accrucy.
|
PDF Full Text Request