BPSK-SIM Free-Space Optical Communication With Reference Continuous Wave Light

Compared with wireless radio frequency communication and optical fibre communication, free space optical(FSO) communication technology has received significant attention because of its advantages, including free spectrum license, secure transmission, high capacity and low deployment cost. However, optical beams carried with information could be absorbed or scattered during transmission through atmosphere, which makes attenuation and fluctuation to be the main challenges for FSO system, and lots of researches have been done to improve the performance. Recently, we have investigated the reference continuous wave light(RCWL) assisted detection technique in BPSK subcarrier intensity modulated(SIM) FSO communication system. In this thesis, we present the simulation results of the bit error rate(BER), and analyze the performance improvement by RCWL in details.Firstly, we introduce the characteristics of the atmospheric channel, including scintillation effect and atmospheric attenuation, which are the main reasons for the signal distortion. Based on the mathematical formula, three kinds of atmospheric turbulence channel models are discussed.Secondly, we compare the OOK intensity modulated communication system with the BPSK-SIM communication system. And the BER floor problem for OOK system employing fixed-threshold detection is analyzed by calculating the limit value of BER when the signal-to-noise-ratio(SNR) is very high. Then we also introduce the power spectral density of the received signals for both systems, which can explain why the BPSK-SIM communication system can resist the scintillation effect.Finally, in order to enhance the performance of BPSK-SIM system, RCWL assisted detection technique is employed in the proposed system. By dividing the RCWL which records the scintillation index, the light with data can mitigate intensity fluctuation under the same atmospheric turbulence channel. In addition, the block diagram model of the proposed system has been described, and its performance improvement has been analyzed with the consideration of SNR, coherent index and turbulence intensity.