Research On MC-CDMA System In Indoor Visible Light Communication

With the integration of mobile communication and various industries,especially the development of mobile Internet and Internet of Things,new challenges have been brought to mobile communication technology.The next generation of wireless communication is facing the shortage of spectrum resources and serious interference.Visible light communication(VLC)uses visible light band as information carrier,does not need wired transmission media such as optical fibers,and transmits optical signals directly in the air.It has dual functions of illumination and communication at the same time.Compared with the traditional radio frequency communication,the advantages of visible light communication lie in its low cost,green security,rich spectrum resources,good confidentiality,suitable for electromagnetic sensitive environment,etc.However,to be practical,VLC still faces some technical challenges,such as th e lack of a recognized channel model,multipath interference and serious background light interference,and the non-linearity of light emitting diodes(LED).This paper aims at improving the communication capacity and speed of the system.The specific rese arch contents are as follows:1.Combining direct current-biased optical orthogonal frequency division multiplexing(DCO-OFDM)with code division multiple access(CDMA),a DCO-MC-CDMA system is established.Due to the limitation of system structure,Gold code and m sequence are selected as spread spectrum address codes in DCO-MC-CDMA system,but their cross-correlation is worse than that of orthogonal codes,which will lead to greater multi-user interference(MUI).Using maximum ratio combining(MRC),equal gain combining(EGC)and orthogonal recovery(ORC)combining,the expression of signal-to-noise ratio(SNR)under the influence of additive white Gaussian noise(AWGN),MUI and limiting noise is derived,and the simulation model of Monte Carlo error rate(BER)in MATLAB is established.The results show that with the increase of the number of users,MUI will make the BER performance of the system worse.ORC eliminates MUI caused by different subchannel gain,so BER performance is the best.The cross-correlation of Gold code is better than that of m sequence,and the BER performance of DCO-MC-CDMA system using Gold code is better.2.Different spread spectrum sequences will produce different size MUI,and the MUI produced by spread spectrum sequences with sm all cross-correlation is smaller.Asymmetrically clipped optical orthogonal frequency division multiplexing(ACO-OFDM)has the characteristics of high power efficiency and good bit error rate performance.Combining ACO-OFDM and code division multiple access(CDMA),an ACO-MC-CDMA system is established.In ACO-MC-CDMA system,Walsh code and orthogonal Gold sequence(OG)are selected as spread spectrum address codes.The theoretical bit error rate of the system is deduced and the Monte Carlo simulation model is established.The simulation results show that the cross-correlation of Walsh code is better than that of OG sequence.The BER performance of ACO-MC-CDMA system using Walsh code is better.The longer the spread spectrum sequence is,the better the BER performance of the system is.