Research On Polar Codes Encoding And Decoding Scheme In Free Space Optical Communication

Free space optical(FSO)communication,as an innovative technology with large capacity,high date rate,good confidentiality and license-free spectrum,has received growing attention in communication field.In near-ground FSO communication link,the laser beam is easily affected by atmospheric turbulence,which would bring random fluctuations in both the amplitude and phase of the received signals,leading to high bit error rate(BER)and worse communication quality.Channel coding technology can effectively mitigate the above-effects,which is an essential technology in FSO communication.Polar codes is the only forward error correction code that has been rigorously proven to reach the Shannon limit,with low encoding and decoding complexity.As a new coding technology with good performance,polar codes have been widely studied and applied.The polar codes can bring high coding gains and improve the performance of FSO communication systems.Firstly,this paper analyzes and studies the atmospheric channels of FSO communications,and then presents a Gamma-Gamma channel model that realistically simulates the characteristics of atmospheric turbulence.in addition,studies two common modulation techniques of FSO communication.Then we research the common polar codes construction methods,encoding and decoding methods.On this basis,we propose a new polar codes construction method and an improvement to decoding method.Except basic research,the detail work of this paper is as follows.(1)Construction of polar codes in atmospheric turbulent channels of FSO.The atmospheric turbulence channel is special because of randomness and variability,and structure polar codes(that is the selection of information bits for polar codes)directly affects the error correction performance.Therefore,how to construct suitable polar codes for turbulence characteristics channel is critical.This paper proposes a method for constructing polar codes in atmospheric turbulent channels.Firstly,based on the general partial ordering characteristics of the polar codes,the reliability of the partially polarized sub-channels that is not affected by the channel is determined.Then,we use Monte Carlo method to analyze the polarization sub-channels in gamma-gamma distribution channels with different turbulence intensities,polarization weights and ?-expansions concepts form a closed formula to evaluate the reliability of the remaining sub-channels in turbulent channels.In this way,the "good or bad" of all polarized sub-channels can be measured in turbulent channels,and then practical polar codes in FSO communication can be constructed.Based on the analysis and research of the polar codes,and considering the trade-off between decoding performance and decoding complexity,a parity-check code-assisted successive cancellation list(PA-SCL)decoding algorithm is proposed.The algorithm is compared with other algorithms on Gaussian channel,simulations show that the algorithm proposed in this paper effectively improves BER performance,and can effectively reduce the computational complexity of decoding when the code length is short.Finally,we apply the polar codes scheme which proposed in this paper to FSO communication,and simulation experiments are performed under different turbulence intensities based on gamma-gamma distribution channels.what's more,we compare our scheme with a commonly used code(LDPC)of FSO communication.Simulations show that the scheme proposed in this paper can effectively improve the performance loss caused by atmospheric turbulence,and has a coding gain of 0.8~1.5dB over the LDPC code under different turbulence intensities.