Research On Fountain Code And Its Cascade Over Atmospheric Laser Communication

Atmospheric laser communication is a point to point and multipoint-to-multipoint communication technology,which transmits voice,data,image information in atmosphere by the use of laser as signal carrier.In recent years,this technology has received noticeable attention due to a variety of applications in overcoming the last-mile problem and emergency communications field.In atmospheric laser communication link,the received light signal energy is attenuated due to a variety of bad weather,and atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal,impairing link performance.Therefore,the study of channel coding technology with high error correction performance for the atmospheric laser communication has become an important issue.All efforts are centered as follows.1.Since the fountain code is a type of coding method with rateless characteristics,the LT code,which is one of fountain codes,can be adapted to the changing atmosphere channels.For the coding process of LT codes,the Poisson Robust Soliton Distribution(PRSD)combines the Robust Soliton Distribution(RSD)with the Poisson distribution.The simulation results show that the average decoding cost of LT codes using PRSD is lower than that of LT codes using RSD(about 10.53 and 12.94),and the average degree of RSD is lower than that of RSD,and the average degree between them is about 5.For the decoding process of LT codes,this paper improves on the basis of the Belier Propagation(BP)decoding algorithm,and obtains the Again BP(ABP)decoding algorithm,and analyzes the performance of the algorithm.The simulation results show that the decoding success rate of LT code using ABP decoding algorithm is higher than that of LT-PC codes using BP decoding algorithm.2.For the problem that the LT code itself has an error diffusion,the LT-PC code is obtained by concatenating LT code with parity check code.The PC code in the concatenated code can locate and delete the error coding packet generated by LT code.There is no effect on the LT decoding process due to the rateless characteristics of LT codes,thus reducing the probability of error spread.At the same time,the error detection failure probability of LT-PC code is analyzed,and the expression of decoding success rate of LT-PC code is deduced.Simulation results show that the success rate of LT codes islower than that of LT-PC codes when the decoding overhead is low.3.The mathematical expressions of average bit error rate(BER),channel capacity and outage probability of atmospheric laser communication system based on OOK modulation are derived.Then the effects of LT codes using PRSD,LT codes using ABP decoding algorithm and LT-PC codes on the performance of atmospheric laser communication systems are analyzed.The simulation results show that the coding gain of different codes is not different under the weak turbulence environment,and compared with other codes,LT-PC codes can improve the performance of atmospheric laser communication system more effectively,and the coding gain is about 2 dB under the strong turbulence environment.