Research On Key Technologies Of Fountain Codes And Its Applications In Wireless Sensor Networks

Channel coding technology is an indispensably key technology in communication systems as a common solution to achieving coding gain and improving system reliability.As a new class of erasure-correcting codes,fountain codes which have the advantages of limited use of feedback,low codec complexity and rate adaption,are very suitable for multimedia broadcast/multicast service and have drawn increasingly attention from academia and industry.Recently,research effort has been devoted to the theory and practical applications of fountain codes and fruitful achievement have been obtained.On the basis of analyzing and summarizing predecessors' work,in this dissertation,the study is focused on the fountain codes,with emphasis on the fountain codes with feedback,unequal error protection fountain codes and the applications of founta:in codes in wireless sensor networks.The main work and contributions of this dissertation are as follows:(1)An encoding scheme for online fountain codes is proposed with the joint optimization of variable node degree and check node degree.First shift LT codes(SLT),LT codes with index feedback(LT-IF),LT codes with alternative feedback(LT-AF)and online fountain codes are introduced and analyzed.The demonstration that the number of symbols required in the completion phase can be reduced if less connected components are constructed in the build-up phase is derived based on random graph theory.Thus with the exploitation of optimizing the variable node degree in the build-up phase,the degradation of performance caused by the way that the optimal coding strategy is only set in the completion phase of the conventional online fountain codes is improved.The proposed scheme can provide significant performance improvement with respect to both coding overhead and feedback cost.The upper bound on coding overhead is also derived from the analysis of random graph theory.(2)A novel scheme which can produce unequal error protection(UEP)property over Additive White Gaussian Noise(AWGN)channel is proposed.First,weighted UEP fountain codes,UEP fountain codes with intermediate feedback and expanding window fountain codes are introduced.And-or tree lemmas suitable for analysis of conventional UEP fountain codes over binary erasure channel(BEC)are derived.The conventional unequal error fountain codes designed for BEC may not provide promising performance improvement when they are extended to noisy channels.Based on the analysis of the inherent relationship between the error floor performance and the variable node degree,the expanding window technology is applied to increase the expected variable node degree of the more important bits(MIB).Then the avoidance of variable nodes with degrees smaller than the defined threshold can be achieved with the exploitation of regularizing variable node degree.The lower bound on bit error rate for prioritized data is also derived.Compared with the conventional UEP schemes,what is confirmed both theoretically and experimentally is that the proposed scheme can produce UEP property over AWGN channel and provide error floor performance improvement.(3)A transmission scheme based on UEP fountain codes is proposed for image delivery over AWGN channel.Layer segmentation processing based on an improved guided filtering is introduced to decompose the source image into the base layer and detail layer.The proposed UEP fountain codes are applied to encode the base layer and detail layer respectively,which can improve the transmission reliability of the MIB data.Compared with the conventional image transmission schemes based on fountain codes,the proposed approach can achieve higher coding gain and better peak signal to noise ratio(PSNR)performance over AWGN channels.(4)A novel network coding scheme based on UEP rateless codes with feedback is designed for prioritized data transmission over wireless sensor networks(WSNs),where multiple source nodes communicate with a single destination node through multiple relay nodes in a two-hop fashion.In the initialization of the relaying phase,more protection was attached to the more important packets by only sampling the more important packets of the source sensor nodes.In the optimization of the relaying phase,decoding state information is fed back to the encoder through an available feedback channel,then coding strategy at the relay phase is optimized.Simulation results show that the proposed scheme can introduce improvement in terms of the less important packets decoding performance while provide enhanced unequal error protection with the appropriate incorporation of a feedback channel.