| Free Space Optical Communication(FSO)has the advantages of rich spectrum resources,large communication capacity,and strong anti-interference.In practical applications,it also has strong confidentiality,no spectrum certification,flexible and simple installation as well as lower cost,lower power consumption,and smaller size.So it has attracted widespread attention and is considered to be one of the reliable solutions in the "last mile" problem.Because the performance of the FSO link is susceptible to atmospheric conditions,which will lead to degraded performance,so we need some measures to reduce the interference which is caused during the transmission process.Channel encoding technology is an effective way.Channel encoding technology improves the transmission performance of the entire FSO system by correcting and detecting errors in time at the receiving end.Polar codes,as the only channel coding scheme that has been proven to reach the Shannon limit,have attracted great interest in the communications field.In FSO communications,there are relatively few studies on polar codes,and related studies are mainly based on weak atmospheric turbulence channels.Aiming at the gamma-gamma atmospheric turbulence channel model,which is suitable for weak,moderate and strong turbulence,in this thesis we use polar codes as the channel encoding scheme in FSO.The main work in this thesis includes the following parts:(1)Analyze the channel characteristics of free space optical communication and establish a channel model.By analyzing the impact of atmospheric effects(including atmospheric attenuation effects and atmospheric turbulence effects)during the transmission of optical waves,an atmospheric channel model that can accurately describe weak,moderate and strong turbulences and obey the gamma-gamma distribution was established and verified by simulation.Due to the complexity of the atmospheric turbulence channel,this thesis uses the Monte Carlo method to construct polar codes under various turbulence intensities.(2)Deduce the soft information extraction method of common modulation methods in FSO communication.In order to apply polar codes to atmospheric turbulence channels,the soft information extraction methods which use different modulation methods are deduced.And under the channel of different turbulence intensities,the performance with different modulation methods with soft-decision was simulated.(3)Simulate and analysis under the constructed channel model.We use the Cyclic Redundancy Check(CRC)-Aided Successive Cancellation List(CA-SCL)decoding method.The effects of length,code rate and decoding width of polar codes on its performance are explored.In addition,simulating and analyzing the performance of polar codes with different modulation methods under different turbulence intensities,and comparing polar codes and Low Density Parity Check(LDPC)codes with similar code lengths under different turbulence intensities.The simulation results show that the performance with polar codes has been greatly improved compared with that without encoding.Polar codes which are constructed in this article,have better performance in most intervals than LDPC codes with similar code lengths,and this advantage becomes more obvious with the increase of turbulence intensity.In addition,different with polar codes,LDPC codes have error floor,so it is difficult to improve the signal-to-noise ratio to obtain a significant performance improvement in this case,which proves that polar codes are very competitive in FSO.Polar codes can significantly improve the performance in FSO,so it has good application prospects in complex atmospheric turbulence channels. |
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