Link Adaptive Technology In LDPC Coded Atmospheric Laser Communication

Due to the advantages of high transmission rate,good security and flexibility,atmospheric laser communication(ALC)has great prospects in military and civilian applications.However,because of the complex atmosphere channel environment,as well as the rapidly growing demands for communication capacity,further research and exploration on how to improve the performance of ALC is needed.In this paper,on the basis of fully discussing the related technologies that can be used to increase the effectiveness and reliability of ALC,we apply the Low Density Parity Check Codes(LDPC)and link adaptation technique to ALC,focusing on the problems that analysis of atmosphere channel characteristics,construction of LDPC,model establishment of adaptive modulation and coding(AMC)system for ALC and the cross-layer design of ALC system.The main work of this paper is:Firstly,aiming at the demand of link adaptation techniques,an atmosphere channel model based on instantaneous signal-to-noise ratio(SNR)was proposed.The probability density function of instantaneous SNR was derived and the model was verified by experiments.The results of we study can provide support for subsequent research.Secondly,in order to improve the reliability of ALC,a quasi cyclic(QC)LDPC construction method was presented.Meanwhile,for the need of variable coding rate,a rate compatible algorithm based on the recovery node was presented too.The simulation results have shown that the proposed LDPC not only has good error correction performance,but also has lower coding complexity;compared with the traditional algorithm,the proposed algorithm has better performance.Thirdly,starting from physical layer,an AMC system model for ALC with discrete rate was proposed.The system parameters were designed,and the calculation formulas of bit error rate and spectral efficiency were deduced as well.The results have shown that the AMC can effectively raise the spectral efficiency of the system,and enhance the advantages of ALC.Finally,aiming at the OSI protocol layer,the hybrid automatic repeat request(HARQ)of data link layer was combined with the AMC of physical layer.The cross-layer optimization design for ALC was carried out,and the cross-layer adaptive model was established.The simulation results have shown that extending the physical adaptive system to cross-layer adaptive system can further improve the spectral efficiency of ALC.The innovations of this paper can be seen as followed: the time varying characteristic model of atmospheric channel is established,which provides an important theoretical basis for the application of link adaptation technology;in order to satisfy the performance requirement of ALC adaptive link for channel coding,a construction method of LDPC with good error correction performance,low coding complexity and compatible rate is proposed;the application of link adaptation technology in ALC is studied.The physical layer adaptive and cross-layer adaptive models of ALC are established,as a result,the effectiveness of system is improved.