Research On Polarization Code And Network Coding For Cooperative FSO Communication Systems

Aiming at the problem of severe interference from atmospheric turbulence channels in free space optical communication systems,which leads to a sharp decline in system communication performance,this research is based on cooperative communication technology,introducing polarization code and network coding technology to combat interference caused by atmospheric turbulence effects,and achieving the goal of improving the performance of free space optical communication systems.Point-to-point free space optical communication is subject to fading caused by atmospheric turbulence effects,resulting in short communication distances and poor performance.In order to break through this limitation and achieve high reliability transmission in free space optical communication,cooperative communication methods can be combined with free space optical communication technology,and channel coding technology can be used to achieve reliability improvement.The research content of this article is as follows:⑴ Establish a cooperative FSO communication system,calculate the channel capacity of each branch,and propose a construction scheme of system polarization code in the cooperative FSO communication system based on the channel capacity of each branch.This scheme uses the upper limit of the average bit error rate of the channel as the selection limit for the frozen bits of the system polarization code,and uses the relationship between the S-R link and the set of frozen bits of the S-D link to recover the set of information bits of the S-R link at the destination node D.The decoding and forwarding method is used at the relay node,and the equal gain combining method is used for the received information at the destination node D.The scheme is applied to a cooperative FSO communication system,and the expressions for the upper bit error rate and outage probability of the communication system are derived.Finally,the simulation results of the bit error rate and outage probability performance of the communication system show that this scheme can suppress interference in atmospheric turbulence channels,and exhibits better suppression effects on strong turbulence channels.⑵ Based on the collaborative FSO communication system,introduce physical layer network coding to build a collaborative FSO communication system between users.In an inter user cooperative FSO communication system,a joint decoding scheme of polarization code and physical layer network coding is proposed,and the logarithmic likelihood ratio expression for the decoding of polarization code in this scheme is derived.The joint decoding scheme performs polarization code decoding on the received signal at the user node,and then performs physical layer network coding on the decoded signal and the signal to be transmitted at the local node,before sending it to the destination node D.After receiving the signal,the destination node D performs the decoding operations of polarization code and physical layer network code on the signal,and finally obtains the required data sequence.Finally,the bit error rate performance of the inter user cooperative FSO communication system is simulated,and the bit error rate performance of the joint decoding scheme using polarization code and direct network coding is compared.Simulation results show that this scheme can significantly improve the bit error rate performance of inter user cooperative FSO communication systems,and the longer the polarization code length,the better the effect.