The Key Technology Research Of Indoor Visible Light Communications Based On OFDM

The emerging white LEDs based visible light communications technology has aroused a widespread concern due to its special ability of combining the indoor illumination and communication technology together. Since VLC technology is limited to its low modulation bandwidth and multipath effect induced ISI issue, we proposed a series of improved scheme of VLC based on OFDM modulation. The OFDM technology ensures the stability of VLC systems due to its advantages of high spectrum utilization and resistance to ISI effect. Some experimental results are accomplished in Northumbria University' Optical Communication Research Group(NCRG) supervised by professor Zabih(Fary) Ghassemlooy and supported by China Scholarship Council(CSC). The main contributions of this dissertation can be summarized as below:Firsty, a novel O-OFDM system based on cascaded codes and frequency-domain channel compensation is proposed. The simulation results demonstrate that, for 16-QAM ACO-OFDM visible light communication systems, a ~3dB SNR gain can be achieved when the BER is 10-3 below the forward error correction(FEC) limit compared with the traditional OFDM system.Secondly, we propose an adaptive channel estimation algorithm for indoor NLOS visible light communication systems. The simulation results demonstrate that the proposed adaptive algorithm has good performance between high and low SNR suitation in terms of mean square error(MSE) and uncoded bit error rate(BER). In the high SNR range, the OMP based reconstruct scheme improves the MSE up to 10 dB.Thirdly, we propose a novel peak to average power ratio(PAPR) reduction scheme for visible light communication systems based on Gaussian blur(GB). Simulation results show that for the 256-subcarrier ACO-OFDM system under BER limit(10-4), a ~6 dB improvement in PAPR is achieved compared with the original ACO-OFDM in term of the complementary cumulative distribution function at the cost of only about 2 d B SNR penalty and the reconstruct error(RE) < 0.1. We also demonstrated that for DCO-OFDM using BP recovery algorithm under BER limit(10-4), the SNR penalty < 0.1 dB compared with the ideal DCO-OFDM.Lastly, we proposed a novel hybrid optical orthogonal frequency division multiplexing(O-OFDM) based on pulse-width modulation(PWM) scheme for visible light communications. The simulation and experimental results demonstrate that the OFDM-PWM offers significant advantages compared with the traditional ACO-OFDM scheme including lower PAPR, higher illumination level, improved BER performance and enhanced resilient to non-linearity, thus demonstrating the potential of the proposed scheme.