Research On OFDM Adaptive Technology In Indoor Visible Light Communications

As an indoor wireless broadband access scheme,visible light communication(VLC)has the dual functions of providing communication while lighting.In transmission,VLC can provide higher data rate and better communication security,with almost unlimited spectrum resources that can be used without permission,and has little harm to human health.Therefore,the visible light communication technology has been paid more and more attention by the researchers at home and abroad.However,when the light signal is transmitted indoors,it is reflected by the wall,roof and furniture,the Inter-symbol interference(ISI)caused by the multipath effect occurs at the receiving end.Therefore,in order to accurately evaluate the effect of multipath effect on system performance,it is necessary to establish multipath channel model for indoor visible light communication.Secondly,optical orthogonal frequency division multiplexing(OFDM)can effectively resist Inter-symbol interference and can reduce the narrow-band interference caused by artificial light source.In addition,VLC channel can be regarded as a channel with stable time and slow change with the location of the receiving and transmitting terminal.Optical OFDM can divide the actual channel into several sub-channels and the gain of each sub-channel is different,thus producing different transmission quality.So it is necessary to introduce adaptive technology into the visible light system.The main work of this dissertation as follows:(1)In the single source indoor visible light communication system,an indoor VLC multipath channel modeling method is proposed.Using iterative method to calculate the impulse response of line-of-sight transmission and reflection transmission,to analyze the relationship between the receiver sampling period and the LED modulation bandwidth,and to define the intersymbol interference.The channel delay of the line of sight is used as the starting point of the modeling,and the sum of all impulse responses between the adjacent sampling intervals of the receiver is used as the path gain of the multipath channel.(2)The performance of DC offset optical orthogonal frequency division multiplexing(DCO-OFDM)and asymmetric limited optical orthogonal frequency division multiplexing(ACO-OFDM)systems is studied.Taking the influence ofnonlinear limiting noise into consideration,the theoretical error rate(BER)of DCO-OFDM and ACO-OFDM systems is derived,and Monte Carlo simulation model is established in indoor VLC multipath channel.The effects of the photodetector(PD)position,LED emission angle,PD field angle and DC bias parameters on the performance of BER is investigated.(3)Chow,Hughes-Hartogs and Fischer adaptive bit-power loading algorithms are applied to ACO-OFDM system.The performance of the adaptive ACO-OFDM system,DCO-OFDM system,and equal bit loading system is compared.The results show that,under the condition of guaranteeing the performance of bit error rate(BER),the adaptive algorithm can save the light power and the electric power than the equal bit loading method,and Fischer algorithm saves the power most.When the information rate is the same,the adaptive ACO-OFDM is more power saved than the DCD-OFDM.