Research Of Distributed LT Codes On Multiple-Layer-Relay Networks

In modern society,while the require for communication is growing rapidly,the structure of communication networks is becoming more and more complex.The receive-and-forward mechanism in traditional routing networks restrains the improvement of the network throughput.Meanwhile,conventional channel codes is not helpful to adaptive with varying channel characteristics.Therefore,it is necessary to develop a code which can both improve the network throughput and achieve channel independence.Network coding is a novel code that allows relay nodes in the network to participate in coding.The basic idea is that the relay nodes in the network re-code the symbols and forward them instead of symplly forwarding them.Through exploiting the computing capability of relays,network coding could help to maximize the throughput of networks.Fountain codes have the basic point that the information symbols to be transmitted are linearly combined into coded symbols at sources,and then transmitted to the destination.The destination could start decoding when it receives sufficient code symbols,regardless of the channel erasure rate.This feature of fountain codes is called rateless,and it leads to the channel-independence and adaptability to varying channels.LT code is the most important realizaion of fountain codes.The distributed LT code is the engagement of network coding and LT codes.For the distributed LT codes,the fetures inherited from network coding can improve the network throughput,and those inherited from fountain codes can achieve channel-independence.The distributed LT code is applied in the multi-access network,in which the sources encode their symbols through the encoding steps of LT codes,the relays perform network coding to the coded symbols from the sources,and the destinations decode the re-coded symbols through the decoding algorithm of LT codes.The challenging fact in designing a distributed LT code is that the network coding operation of the relay will destroy the degree distributions of the LT code,while the degree distributions are vital for the decoding.Therefore,how to ensure that the degree distribution should remain practicable after the network coding of the relays is the focus of research on distributed LT codes.For the distributed LT codes,the larger the number of sources is,the more layers of the relays,the more difficult it is to ensure that the distribution of the ultimate-coded symbols are proper for decoding.In this paper,Multiple Level Distributed LT Codes(MLDLT)are proposed,based on the previous research on distributed LT codes in single-relay networks.MDLLT could contain from hundreds to several of sources,and multiple layers of relays.The biggest challenge of MLDLT is the design of degree distributions of each layer of relays.In order to obtain the asymptotic performance of MLDLT,the And-Or tree analysis is performed.Conclusions of the And-Or tree analysis could be transformed into linear procedures of the degree distributions,and the degree distributions could be estimated through the linear procedures.For the optimized degree distributions,numerical simulations are carried out.Simulation results demonstrated that MDLLT has a very low error floor with the well-estimated degree distributions.Performance of MDLLT on lossy links is analyzed and simulated,too,and the results demonstrate that MDLLT has tough resistance against erasure of links.For MLDLTs on several-sources networks which present poor decoding performance,buffers are imported in their code schemes,and degree distributions are expanded with the help of buffers.The importing of buffers can also reduce the computational complexity of sources and intermediate relays,and the transmission load of the link.Although MLDLT is proposed for the multi-layer relays networks,it can be equalized into a conventional distributed LT code on single-layer relay networks.Its degree distributions can be transplanted into the single-layer relay networks,to obtain a relatively low error floor.In conclusions,in this paper,MLDLT was comprehensively analyzed,designed,simulated and equalized.It can be proved that MLDLT is a robust and superior-performed distributed LT code.