Optimized Design Of Fountain Codes

With the rapid development of the Internet and the technology of mobile communication technology, the technology of broadcasting and multicasting techniques is used widely. Recently, fountain codes are adopted by 3GPP MBMS With its own technical advantages, and are applied and promoted in more and more communications field. Focusing on the designing method of fountain codes, we mainly research the degree distribution of fountain codes, the fountain codes based on collaborative relay systems and the unequal error protection fountain codes, and make a certain contribution to the research and application of fountain codes.Firstly, we expound the advantages, the development, the research situation and the applications of fountain code, and point out the existed problems. Then we study the encoding and decoding algorithms and fundamental theory of Tornado code, LT code, Raptor code and fountain code in 3GPP MBMS. And we analyze the merits and drawbacks of these codes.Secondly, as the degree distribution plays a key role in determining the encoding and decoding performance of fountain codes, we build the relationship between degree distribution and bit error rate (BER) from the perspective of the belief propagation decoding algorithm, and then proposed a designing method of degree distribution by differential evolution. Simulation and theoretical results show that fountain codes generated by the proposed distribution have a lower average BER than those generated by the traditional degree distribution.Thirdly, we analyze the affection of relay node to the degree distribution in collaborative relay systems, and present a designing method of degree distribution based on the existed degree distribution with good performance. Simulation results show that the designed fountain codes have a good decoding performance in relay systems, compared to the fountain codes in 3GPP. In the multicast system, in order to improve the decoding performance for all users, we can choose the optimization parameters according to the circumstances of relay system.Finally, aiming at the application of unequal error protection (UEP) codes, we study the optimization design method of UEP fountain codes. We investigate the UEP fountain codes realized by encoding the important data more frequently and then present a designing method to optimize the property of UEP fountain code by adjusting the sending probability of input symbols according to their importance. Simulation results show the proposed fountain codes have a better decoding performance than the UEP fountain codes without optimized.