Atmospheric Transmission Theory And Research Of Wireless Laser Communication Link

Wireless optical communication technology combines the advantages of wirelesscommunications and fiber communication, it springs up again along with the development ofthe progress of the semiconductor laser and optoelectronic detectors.The wireless opticalcommunication possesses these advantages of high broadband, large capacity, confidentiality,low-cost, license-free allocation, etc, so it has widely market prospects. However, wirelessoptical communications system’s transmission medium is atmospheric channel, and theatmospheric channel seriously impact the performance of wireless optical communicationsystem. On the basis of the previous study of atmospheric channel, the paper mainly studiesthe performance of atmospheric optical wireless link under the influence of channel coding,modulation and demodulation technology.First of all, the attenuation effect and turbulence effects have an impact on the lasertransmission in the atmosphere, and turbulence channel model based on Kolmogorovspectral is given.Second, the chapter explains in detail the theory of encoding and decoding of Turbo code.The design of interleaver directly determine the performance of decoding. We use Spseudo-random interleaver with the heip of bubble sort method. The chapter makes acomparative analysis of the complexity of the three decoding algorithms in the end.Thirdly, the chapter elaborates the basic of principle of PPM modulation, the powerspectral density of PPM, transmission efficiency and channel capacity. Time synchronizationincludes frame synchronization and slot synchronization. The frame synchronization uses thesliding correlation method, and the slot synchronization can use software PLL algorithm.Finally, I make a study of the performance of atmospheric laser communication link. Thebit rate, the number of iterations, interleave length and decoding algorithm can improve thesystem performance in the simulation of system.When the bit rate is1/2and the SNR is1.5dB,the BER is close to104. When the rate is1/3and the SNR is1.5dB, the BER is below105.The increasement of number of iterations and interleaver length can effectively improve thesystem performance, but it leads to the upgrading of the system complexity. The MAP andLog-MAP algorithm’accuracy is higher than the Max-Log-MAP algorithm. The Log-MAPalgorithm is not like the the MAP algorithm’s complexity, but it is very close to theperformance of the MAP algorithm. In the atmospheric channel, the PPM-BPSK’sperformance gradually decreases with the strengthening of the atmospheric turbulence. Whenthere is no atmospheric turbulence effect, the performance of PPM-BPSK and BPSK isbasicslly the same. Compared with the previous two modulations, the PPM is better. However,in the case of a relatively weak turbulence, the turbulence variance is less than0.3,PPM-BPSK’s performance is better than the BPSK, but it is worse than the PPM. With theincreasing of the atmospheric turbulence effect, the PPM-BPSK has a distinct advantagecompared with the PPM and BPSK. The former is higher about7dB and10dB than the latter.Therefore, the PPM-BPSK scheme is suitable for use in the case of the high turbulence level.