| Visible light communication(VLC)uses white light LEDs to realize wireless communication and illumination at the same time,which has advantages such as high transmission rate,energy saving,etc.However,the current white LED has a modulation bandwidth of less than 20 MHz,and there is a nonlinear effect when the transmitting power is high,so the communication rate of VLC is constrained.In order to meet the current growing indoor wireless communication needs,how to improve transmission rate and reliability of VLC under the existing LED device condition is a scientific issue that urgently needs to be solved.This paper uses orthogonal frequency division multiplexing(OFDM)multi-carrier modulation to make full use of the limited modulation bandwidth,uses probabilistic shaping to lower signal optical transmitting power.Based on this technology,channel coding research is carried out.The job of this paper is as follows:(1)The performances of RS and LDPC channel coding in indoor VLC system are studied,the correction capability of LDPC is improved.Build the model of the system after analyzing the characteristics of optical devices and channel,compare the advantages and disadvantages of DC biased optic OFDM(DCO-OFDM)and asymmetrically clipped optic OFDM(ACO-OFDM)using this model,and ACO-OFDM is selected due to the characteristic of VLC.The work of channel estimation and equalization is completed,and the impulse responses of the system at different transmission rates are explored,the problem of inter-symbol interference and carrier interference is solved.Analyze and compare the performances of RS and LDPC at different transmission rates.Further explore the belief-propagation algorithm of LDPC decoding and adjust it,make it achieve better error correction capability with the iteration times as small as possible,the coding gain has increased by more than 1 d B.(2)The LDPC code combined with probabilistic shaping is proposed to be applied to indoor VLC system under ACO-OFDM modulation.From the perspective of information divergence,source entropy,transmitting power and transmission error,the characteristics of probabilistic shaping are analyzed,4.7% information loss can be exchanged for 36.4% of power saving.Combining LDPC code with probabilistic shaping,effectively improve the granularity of the transmission mode in the coded modulation system,the requirement for signal noise ratio is lower in certain transmission rate.Compare probabilistic shaping when lambda equal 1 combining LDPC code when coding rate is 5/6 with only probabilistic shaping when lambda equal 2.85,the spectrum utilization rate is the same,the requirement for signal-to-noise ratio of the former is 2d B less than the latter;compare it with only LDPC code when coding rate is 3/4,the requirements for signal-to-noise ratio are the same when achieving error-free,but the spectrum utilization rate of the former is 0.26bits/sym higher,and the transmitting power is 30.5% lower. |
