Research On MIMO Technology In Indoor Visible Light Communication

Visible light communication(VLC)is a new rapidly developing technology.As a supplement to traditional wireless communication,visible light communication has the characteristics of high transmission power,no electromagnetic interference,no electromagnetic radiation,high confidentiality,energy conservation and environment protection,and can realize data transmission when providing illumination.In the visible light communication multi-input multi-output(MIMO)system,several Light emitting diode(LED)arrays are used for transmitting optical signal to make indoor illumination and received power distribution more uniform,several photo-diodes are used for detecting the optical signal,and the initial information is recovered after a series of processing.The indoor VLC system using MIMO technology,can improve the communication coverage,obtain a higher transmission rate,solve the alignment between transmitter and receiver,overcome the communication interruption caused by personnel movement and shadow.This paper will focus on MIMO technology in the indoor visible light communication,including system modeling,diversity reception and space time encoding.Firstly,three modeling methods of VLC-MIMO system are introduced and researched,including direct link model,reflective link model and independent reflecting element interaction characterization(IREIC)model?In IREIC,the reflective link model is divided to three parts,between optical source and reflective surface,among reflective surface,between reflective surface and receiver.The tedious recursive operations in reflection link model are replaced by simple matrix multiplication in IREIC,and the computing complexity of performance indicators(indoor illumination distribution,received power distribution,channel impulse response and signal to noise ratio distribution)has reduced effectively.Then,the diversity reception technologies of MIMO system in indoor visible light communication are studied,including diversity technology,combining technology and system channel capacity.Three main combining technologies are studied.Maximum ratio combining(MRC)is better than the select best(SB)and equal gain combining(EGC)by distributing the gain coefficient in real time using variable gain amplifier.When diversity number is 7,the SNR for MRC increases 4.5 dB than SB,and 1dB than EGC.The channel capacity is researched when channel state information(CSI)is known and unknown.The principle of "water flooring" is applied when CSI is unknown,allocates transmission power according to the channel condition,and can improve the channel capacity effectively.The results shown system capacity is larger by 44.1%with number of transmitter and receiver is 4 and SNR is 30 dB.Finally,the technologies of space time coding in the VLC-MIMO system are studied,including the space time block coding and the layered space time coding.A modified STBC scheme for visible light communication is proposed based on intensity modulation/directive detection principle.The proposed scheme makes best diversity gains and reduce the detective complexity of indoor VLC-MIMO system.The system BER is lower than 10-6 When 2 transmitters,4 receivers and SNR is only 20 dB.The encoding technologies and decoding technologies of Bell Labs layered space time coding(BLAST)technologies are studied.Two decoding schemes combined ML with ZF-OSIC and ML with MMSE-OSIC are proposed in order to trade-off system performance and complexity of decoding.The results show that the BER performance with the proposed decoding scheme is close to ML,and lower about 2 orders of magnitude than ZF and MMSE with SNR is 25 dB.