High-Sensitivity Digital Coherent Detection Technology For Free-Space Optical Communication

Free space optical communication generally refers to long-distance optical wireless communication,which uses the laser as a carrier and the atmosphere as the transmission medium.It is commonly used in satellite communication,UAV communication,emergency communication and other applications.Compared with radio communication,it has the advantages of high bandwidth,high security,low deployment cost and free of spectrum license.However,the propagation of laser light in the atmosphere is easily affected by atmospheric turbulence and weather conditions,causing power loss of the received signal and distort signals,and further degrading the performance of the communication system.In order to resist such interference,on the one hand,the redundancy and reliability of communications can be improved by coding;on the other hand,we can improve the receiving sensitivity of the receiver.After reviewing the atmospheric turbulence,the thesis summarizes the free space communication channel model.To achieve the high-speed and high-sensitivity receivers,we then introduce and carry out experiments of the coherent receiver based on digital signal processing,single-photodetector coherent receiver and multi-aperture receiver.The thesis first introduces the physical mechanism of turbulence and gives important parameters and conclusions.Secondly,the distortion of advanced modulation format optical signals introduced in laser's spatial transmission is analyzed,and the atmospheric channel models including loss,fading and their coherence are analyzed.The thesis draws an important conclusion,that in the high-speed optical communication system,the atmospheric turbulence channel can be regarded as a slowly varying,frequency non-selected channel.Based on the atmospheric channel,the thesis introduces the composition and principle of coherent optical communication system and digital signal processing technology,and then summarizes the channel model of free space coherent optical communication system.The thesis reviews the digital signal processing techniques including clock recovery,channel equalization,frequency offset estimation and phase noise estimation.The experimental results show the effectiveness of digital signal processing in free-space coherent optical communication system.Finally,the thesis proposes a single-photodetector coherent receiver,which has the advantages of low cost,low complexity,high speed,high sensitivity,and comparable performances to commercial homodyne coherent receiver.The thesis first summarizes the available diversity techniques in free space optical communication system and points out that the multi-aperture receiver can improve the performance of the communication system in atmospheric fading channels without increasing the transmitter complexity and remain the same transmitting power.Then a single-photodetector coherent receiver based on Kramers-Kronig relation is studied in detail.Finally,a 1 x2 single-photodetector coherent array receiver is experimental demonstrated and achieves the sensitivity of-47.5 dBm,or 8.4 dB PPB at the bit error rate of 0.01,which shows its great potential for high-sensitivity detection.