Design Of Expanding Window Fountain Codes Over BIAWGN Channels

With the rapid development of wireless communication technology and the further improvement of network bandwidth, multimedia transmission increasingly becomes the core business of communication service. However, high packet loss rate and non-ideal channel characteristic in the process of network communication seriously decrease the multimedia transmission reliability. Digital fountain coding is a novel forward error correction (FEC) coding technology, unlike traditional fixed-rate schemes, digital foun-tain codes are rateless, can obtain better performance without the need of feedback and have low coding complexity, and also can ensure the effective reliability transmission under the circumstance of bad channels, so such codes have been widely investigated by the coding industry.For some multimedia applications which have an important characteristic of un-equal importance of data that need different degree protection, in this paper, we main-ly discuss and research the encoding and modulation algorithm of Expanding Window Fountain (EWF) codes with unequal error protection (UEP) and propose an efficient scheme to construct fountain codes that can provide UEP property over the binary input additive white Gaussian noise (BIAWGN) channels. The error floor of EWF codes is mainly determined by low degree of input nodes, in order to improve the error floor performance of EWF codes, the encoding process is modified to make Poisson degree distribution of the traditional EWF codes variable node to regular degree distribution which have two or three degrees. Furthermore, EWF codes can generate encoded bits with unequal importance from different windows. Based on this property, we can fur-ther improve UEP property of EWF codes by utilizing BPSK modulation with unequal power allocation (UPA). In order to investigate the asymptotic performance of our pro-posed EWF codes, we have derived density evolution formulation based on extrinsic information transfer (EXIT) functions. We verified that our proposed scheme in which variable nodes have regular degree distribution can lower error floor, and can obtain further performance improvement for more important bits by introducing UPA.Finally, based on the performance of Raptor codes, in order to completely eliminate the error floor of EWF codes, we construct a novel precoded EWF codes by concate-nating the proposed EWF codes with high-rate low-density parity-check (LDPC) codes.Asymptotic analysis and simulation results show that, compared with conventional pre-coded EWF codes, our proposed scheme can obtain better decoding performance and has great design flexibility. We also can get best performance by jointly optimizing encoding parameters and power allocation ratio.