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Overview: In order to support higher information transmission rate, improve the system error characteristics, solve the problems of frequency selective fading and multipath effect of the wireless optical communication system in complex turbulent environment, improve laser atmospheric channel signal transmission problems, based on orthogonal frequency division multiplexing technology, the FSO-OFDM system was built, the multicarrier modulation, modulation and demodulation, etc used this system were studied. Firstly, the turbulence effect, which is a bottleneck problem restricting the application of atmospheric laser communication, was discussed in detail. Then, on the basis of Kolmogorov spectrum model, the plane wave model was studied. By using the modified Rytov method, the spatial optical communication system model of Gaussian beam under log-normal turbulence channel was established, and the probability density function of the system light wave intensity was derived. Then, the OFDM multi-carrier modulation scheme of the wireless optical communication system was designed, the baseband mode model of the FSO-OFDM system was constructed, and the modulation and demodulation of the signal were studied using this model. When the signal of the binary signal source was sent out, it was encoded and modulated to the series-and-conversion module for series-and-conversion, followed by IFFT exchange. After digital-to-analog conversion and filtering, electro-optical conversion was carried out, which becomes the signal s(t) modulated by OFDM. After passing through the atmospheric channel, the signal was sent to the receiving module of the FSO-OFDM system, which is photoelectric converted, low-pass filtering, and then converted by mode/number to restore the orthogonality of the subcarrier. After FFT and string/combination conversion, the signal was restored to the signal before OFDM modulation. After demodulation and decoding, the original input signal was restored. Finally, MATLAB programming was adopted to realize the FSO-OFDM system. Simulation experiments were carried out on the FSO communication system under multi-path interference. Under the same bandwidth condition, the average bit error rate performance of the single-carrier wireless optical communication system and the FSO-OFDM communication system were simulated, respectively. Compared with the traditional single-carrier system, the OFDM system improves the atmospheric channel transmission of laser signal and the information transmission rate. FSO-OFDM system has a strong ability to resist multi-path interference and spectrum selective fading, as well as a good BER performance. It provides a technical method to break through the bottleneck of optical communication and has a very wide application prospect and value.
Relationship between atmospheric environment and wireless optical system performance
Turbulent vortices in atmospheric channels
Richardson turbulence cascade
Modulation principle of OFDM
Principle diagram of OFDM multi-carrier parallel transmission
Baseband model of the FSO-OFDM system
Signal modulation and demodulation of FSO-OFDM system
Bit error rate characteristics under different carrier numbers
Bit error rate characteristic curves under different protection intervals